HomeMy WebLinkAbout5 2001 Guide to Shallow Gas Potential
Division of Geological & Geophysical Surveys
MISCELLANEOUS PUBLICATION 128
2001 Guide to the petroleum, geology, and shallow gas
potential of the Kenai Peninsula, Alaska
A Field Trip Guideboook
compiled by
T.A. Dallegge
$10.00 (CD format)
November 2003
THIS REPORT HAS NOT BEEN REVIEWED FOR
TECHNICAL CONTENT OR FOR CONFORMITY TO THE
EDITORIAL STANDARDS OF DGGS
Released by
STATE OF ALASKA
DEPARTMENT OF NATURAL RESOURCES
Division of Geological & Geophysical Surveys
794 University Avenue, Suite 200
Fairbanks, Alaska 99709-3645
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
A Field Trip Guidebook Compiled by
Todd Dallegge
U.S. Geological Survey and University of Alaska Fairbanks
Field Trip Leaders
Todd Dallegge, U.S. Geological Survey and University of Alaska, Fairbanks
Robert Swenson, Phillips Alaska, Anchorage
Charles Barker, U.S. Geological Survey, Denver
David Brimberry, Marathon Oil Company, Anchorage
Rod Combellick, Alaska Department of Natural Resources, Fairbanks
Prepared for use during the field trip conducted on April 30 – May 1, 2001 during the
Alaska Coalbed and Shallow Gas Resources workshop sponsored by the West Coast PTTC,
Alaska Department of Natural Resources, and U.S. Geological Survey
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
i
Table of Contents
The penultimate great earthquake in southcentral Alaska: evidence from a
buried forest near Girdwood
by Rodney A. Combellick…………………………………………………...1
Introduction to Tertiary Tectonics and Sedimentation in the Cook Inlet Basin
by Robert “Bob” Swenson…………………………………………………..10
Kenai Field, the Kenai Peninsula's Largest Gas Field
by D.L. Brimberry, P.S. Gardner, M.L. McCullough, and S.E. Trudell….…20
High-resolution Chronostratigraphic Analyses of the Tertiary Kenai Group,
South-central Alaska: Applications to Basin Analysis and Coal-bed
Methane Assessment: An Update
by Todd A. Dallegge and Charles E. Barker………………………………...29
Ongoing coalbed desorption studies, Cook Inlet Basin, Alaska
by Charles E. Barker, Todd A. Dallegge, and Dan Seamount………………40
Road Log for the 2001 Alaska Coalbed and Shallow Gas Resources Field Trip
to the Kenai Peninsula, Cook Inlet Alaska
by Todd A. Dallegge, Robert Swenson, Charles E. Barker, Rod A.
Combellick, and David L. Brimberry…………………………………….56
Production of this guidebook was made possible U.S. Geological Survey, Denver, CO. This
report is preliminary and has not been reviewed for conformity with U.S. Geological Survey
editorial standards and stratigraphic nomenclature. Any use of trade, product, or firm names
is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Cover photo of Beluga and Sterling Formations west of McNeil Canyon, Kachemak Bay
Alaska. Photo taken by T. Dallegge.
ence was substantial and sudden (coseismic), and sug-
gest that this subsidence at Girdwood probably coincided
with subsidence at Portage, Chickaloon Bay, Palmer Hay
Flats, and Goose Bay and with uplift at Copper River Delta.
If such a large area was deformed during a single event, it
must have been a great earthquake similar to the 1964
event.
VERTICAL CHANGES DURING
AND AFTER THE 1964
EARTHQUAKE
The 1964 earthquake released accumulated stresses
on the Alaska-Aleutian subduction zone, where the North
American and Pacific plates converge at about 6 cm/yr
(DeMets and others, 1990). The associated pattern of
uplift and subsidence (fig. 1) resulted from regional crustal
warping and from displacements on subsurface portions
of the northwest-dipping Aleutian megathrust and
subsidiary reverse faults (Plafker, 1969). Downwarp of as
much as 2 m occurred over an elongate region including
Kodiak Island, Kenai Peninsula, most of Cook Inlet, and
Copper River Basin. Uplift as much as 11.3 m occurred
seaward of the subsidence zone in an elongate region
including Middleton Island, Montague Island, most of
Prince William Sound, and Copper River Delta.
Superimposed on this pattern of coseismic deformation is
regional interseismic subsidence (Plafker and others, 1992).
The long-term net vertical displacement is the sum of
coseismic and interseismic displacements. Therefore, some
areas undergo coseismic and long-term subsidence (upper
Cook Inlet), others coseismic and long-term uplift
(Middleton Island), and still others coseismic uplift and
long-term subsidence (Copper River Delta) or coseismic
uplift and long-term stability (Montague Island).
Postearthquake changes have restored much of the
subsided area of Turnagain Arm (fig. 2) to near pre-
earthquake conditions. Subsidence totaled as much as
2.4 m at Portage, including about 1.6 m of regional tectonic
subsidence and 0.8 m of local surficial compaction
(McCulloch and Bonilla, 1970). During the decade following
the earthquake, as much as 0.55 m of rebound occurred
along Turnagain Arm (Brown and others, 1977).
Additionally, intertidal silt deposition began in submerged
areas immediately following the earthquake and by 1980
had nearly restored the tidal flats to pre-earthquake levels
THE PENULTIMATE GREAT EARTHQUAKE IN SOUTHCENTRAL
ALASKA: EVIDENCE FROM A BURIED FOREST NEAR GIRDWOOD
by
Rodney A. Combellick1
INTRODUCTION
Determining the potential for future great earthquakes
(Richter magnitude about 8 or 9) requires knowing how
often they have occurred in the past. In the Anchorage,
Alaska, region, only one great earthquake has occurred
during historic time. This event, the great 1964 Alaska
earthquake (Mw=9.2), was accompanied by tectonic uplift
and subsidence that affected an area of more than
140,000 km2 along 800 km of the Alaska-Aleutian
subduction zone (fig. 1) (Plafker, 1969). Historic records
and instrument monitoring show that no other great
earthquake ruptured this segment, roughly between Kodiak
and Cape Suckling, for at least 180 yr before the 1964 event
(Sykes and others, 1980). However, there is no reason to
doubt the inevitability of future earthquakes similar to the
1964 event. Adjacent areas of the Alaska-Aleutian arc
have ruptured during at least one and, in some cases,
during several great earthquakes throughout historic
time. Considering that about 11 percent of the world’s
earthquakes have occurred in Alaska, including three of
the ten largest events (Davies, 1984), the potential for
future great earthquakes in the region is clearly high.
Given the limitations of instrument and historic records
to resolve the recurrence times of great earthquakes, only
geologic investigation can disclose the long-term record
of sudden tectonic changes and earthquake effects. Re-
cent geologic studies show that recurrence intervals for
great earthquakes in this region range from about 400 to
1,300 yr (Plafker and others, 1992). My purpose in this
paper is to present evidence from a coastal marsh near
Girdwood that the penultimate, or second to last, great
earthquake in the Anchorage region occurred between
about 700 and 900 yr ago. This evidence comes from a
layer of high-marsh peat2 and rooted trees that were
submerged by subsidence, probably killed by salt-water
intake, and buried by postseismic deposition of intertidal
silt and clay. I present preliminary evidence that subsid-
1Alaska Division of Geological & Geophysical Surveys, 794 Uni-
versity Avenue, Suite 200, Fairbanks, Alaska 99709-3645.
2In this paper, high marsh refers to the vegetated upper portion of
the intertidal zone that is primarily influenced by terrestrial
conditions (infrequently flooded by salt water). Low marsh refers
to a topographically lower part of the intertidal zone that is
primarily influenced by marine conditions (flooded at least daily
by salt water).
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 2
Figure 1. Region affected by vertical tectonic displacement during the great Alaska earthquake of 1964 (modified from
Plafker, 1969, fig. 3).
(Bartsch-Winkler and Garrow, 1982). In the Portage area,
Ovenshine and others (1976) mapped this deposit as the
Placer River Silt. An equivalent but thinner deposit is
present at Girdwood. This postearthquake silt overlies
grasses, mosses, herbaceous plants, peat, and root systems
of spruce and cottonwood trees on the high marsh that
was submerged in the Portage and Girdwood areas during
the earthquake.
PALEOSEISMOLOGY AND
RADIOCARBON DATING
Determining the effects and timing of prehistoric
earthquakes depends on (1) correct interpretation of
the geologic evidence of earthquakes and (2) accurate
dating of the affected rocks or sediments (Allen, 1986).
In the case of subduction-zone earthquakes like the 1964
Alaska event, the geologic evidence is usually related to
sudden coastal uplift or subsidence (Plafker and Rubin,
1978; Lajoie, 1986) but may also be related to shaking
effects, such as liquefaction (Obermeier and others, 1985).
Radiocarbon dating can provide a minimum or
maximum age for an earthquake recorded in coastal
sediments if organic material overlies or underlies the
earthquake-related horizon. If organic material predating
and postdating the horizon can be obtained, the age of the
event can be bracketed. Alternatively, if it can be shown
that the dated organisms died as a result of the earthquake,
then the age of the youngest tissue approximates (very
closely postdates) the age of the event.
Several sources of error make radiocarbon dating
a relatively crude method for dating earthquakes and
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 3
even less reliable for determining whether earthquake
effects observed at different locations are attributable
to a single event (Atwater and others, 1991). Bulk
samples of organic material, such as peat, may predate
or postdate the event by many years. Contamination
by younger roots, detrital organic material, or bacterial
decomposition can introduce significant error. Quoted
laboratory errors on reported ages are normally at least
several decades and may be understated because of
unknown analytical errors (Scott and others, 1990).
Different laboratories analyzing splits of the same or
stratigraphically equivalent Holocene samples have
reported ages that fail to overlap even at two standard
deviations and may differ by up to 700 yr (Nelson,
1992). Finally, because of natural variation of 14C con-
tent of atmospheric carbon through time, calibration to
calendar years using dendrochronology is not linear
and can result in a radiocarbon age yielding several
possible calendar ages (Stuiver and Quay, 1980; Stuiver
and Becker, 1986).
Despite these problems, conventional radiocarbon
dating is a useful tool for determining approximate ages of
earthquakes and their average recurrence interval. In
some circumstances, high-precision radiocarbon dating
of very carefully selected and prepared materials can
provide ages with quoted errors on the order of one or two
decades (Atwater and others, 1991).
REGIONAL STUDIES
Numerous studies have documented repeated sud-
den vertical tectonic displacements in southcentral Alaska
during the past 5,000 yr, or late Holocene (Plafker and
Rubin, 1967, 1978; Plafker, 1969; Plafker and others, 1992;
Combellick, 1986, 1991; Bartsch-Winkler and Schmoll,
1987, 1992). Although the timing of all events is not yet
clear, the distribution of prehistoric uplift and subsidence
appears consistent with the pattern of 1964 deformation.
Radiocarbon dating of submerged peat layers in
estuarine sediments of upper Cook Inlet indicates that
the area probably subsided coseismically six to eight
times during the 4,700 yr prior to the 1964 event, imply-
ing recurrence times on the order of 600 to 800 yr
(Combellick, 1991). Elevated terraces on Middleton
Island record five pre-1964 uplifts during the past 4,300
radiocarbon yr (Plafker and Rubin, 1978), or 4,900
calendar yr. The most recent uplift preserved by
Middleton Island terraces was about 1,300 yr ago. At
Copper River Delta, which undergoes net long-term
subsidence punctuated by coseismic uplift, buried
peat and forest layers record four pre-1964 uplifts
during the past 3,000 yr (Plafker and others, 1992). The
most recent event represented in the Copper River
Delta sequence was about 800 yr ago.
Karlstrom (1964) dated and briefly described a buried
forest layer in tidal sediments at Girdwood and recognized
that it recorded a period of lower relative sea level. He
obtained an age of 700 ± 250 radiocarbon yr (510-920
calendar yr) for wood from a rooted stump at about 0.8 m
below the pre-1964 surface and 2,800 ± 180 yr (2,759-3,207
calendar yr) for wood from a peat layer about 4 m lower in
the section. Karlstrom, whose report was prepared prior to
the 1964 earthquake, did not attribute burial of the forest
layer to possible coseismic subsidence.
Plafker (1969) used Karlstrom’s dates to infer a regional
average subsidence rate of 2-30 cm per century between
700 and 2,800 radiocarbon yr ago. He also concluded that
gradual tectonic submergence prevailed in Prince William
Sound as much as 1,180 yr prior to the 1964 earthquake.
Plafker and Rubin (1978) inferred that the lowest elevated
terrace at Middleton Island (fig. 1), dated at 1,360 radio-
carbon yr, represented the last coseismic uplift in the
region prior to 1964. However, Plafker and others (1992),
reported new evidence of widespread coseismic uplift
with a calibrated age of 665-895 yr in the Copper River
Delta.
Figure 2. Anchorage region and portion of upper Cook
Inlet, including study area in vicinity of Turnagain
and Knik Arms (see fig. 1 for map location).
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 4
BURIED FOREST LAYERS
AT GIRDWOOD
Erosion by waves and tidal currents has exposed two
peat layers with rooted tree stumps along coastal banks
at Girdwood (fig. A3). Trees rooted in the uppermost layer
were killed as a result of submergence during the 1964
earthquake, probably by saltwater intake. Many of these
dead trees remain standing but their root systems are
partially or completely buried beneath postearthquake silt
(equivalent to Placer River Silt) up to several tens of cm
thick. Patches of bark remain on above-ground portions
of some of the trees and are commonly well preserved on
buried portions of the trunks.
Stumps rooted in the lower layer are broken or eroded
to heights of less than 1 m and encased in about 1 m of gray
clayey silt between the upper and lower forest layers.
None of the lower stumps protrude through the modern
marsh surface. No bark remains on the specimens I ob-
served from the lower layer except for very few small,
loosely attached patches. The lower stumps are rooted in
a layer of mossy and woody peat 10-15 cm thick, which has
been partly removed by tidal and wave erosion where the
stumps crop out at the base of the bank (fig. 3B). This
lower layer of rooted stumps is probably the same layer
that Karlstrom (1964) dated at 700 ± 250 radiocarbon yr.
Several lines of evidence indicate that the lower forest
layer was submerged and buried as a result of sudden
subsidence: (1) the contact between the peat layer and
overlying mud is very sharp, indicating rapid burial, as
with the 1964 layer; (2) outer growth rings of the buried
stumps are continuous and as wide as or wider than inner
rings, indicating healthy growth until death, comparable
to that observed in coastal Washington by Atwater and
Yamaguchi (1991); and (3) the lower part of the overlying
mud layer contains below-ground stems of the halophytic
plant Triglochin maritimum, indicating submergence to
a lower level in the intertidal zone. In southwestern Wash-
ington, where diurnal tide range is about 3 m, T. maritimum
is dominant only in low-marsh areas that are 0.5-2.0 m or
more below the typical high-marsh surface (Atwater,
1987). In upper Cook Inlet, where diurnal tide range is
about 10 m, T. maritimum may indicate greater than 2-m
depth below the high-marsh elevation. The overlying mud
layer shows a gradual upward increase in roots, grass
stems, and other plant material and grades into the over-
lying peat, suggesting a gradual return to the high-marsh
environment.
Although alternating layers of peat and intertidal
mud can be produced by nonseismic processes (Nelson
and Personius, in press), the sharp peat-mud contact,
presence of T. maritimum in mud above the contact,
apparent sudden death of rooted trees, and gradual mud-
peat transition are strong evidence of coseismic subsid-
ence followed by gradual uplift or sedimentation that
returned the tidal flat to high-marsh conditions. Still lack-
ing, however, is convincing evidence of strong ground
shaking in the form of liquefaction features associated
with burial of the peat. This evidence may be very difficult
to find; a 1991 reconnaissance of several km of tidal- and
river-bank exposures in the Portage area, where liquefac-
tion was extensive during the 1964 earthquake (McCulloch
and Bonilla, 1970), revealed only a few sand dikes pen-
etrating the 1964 soil (B. Atwater and R. Combellick,
unpub. data, 1991). Locating similar evidence of ground
shaking during previous events will be even more diffi-
cult because previous coseismic subsidence is inter-
preted mainly from borehole samples (Combellick, 1991).
RADIOCARBON AGES
My estimate for age of burial of the lower forest layer
at Girdwood is based on radiocarbon dating of three wood
samples and three peat samples. The wood samples were
collected from the outermost 10 to 25 growth rings of three
rooted tree stumps. Because bark was not present on the
sampled stumps, these may not be the last growth rings
added before death. However, the well-preserved condi-
tion of the stumps, nearly continuous outermost rings,
and smoothness of the outer surface suggest that few or
no growth rings have been lost to decomposition. Consid-
ering the potential sources of error in conventional radio-
carbon dating, the exact position of the dated wood
relative to the outermost ring at time of death is probably
insignificant. If my assumption is correct that these trees
were growing normally at the time of submergence and
died quickly as a result of submergence, the average age
of these outer rings should predate the time of the event
by no more than a few years. The potential for contamina-
tion of wood samples with older or younger organic
material is far less than for peat samples. Therefore, the
wood samples should provide a more reliable age estimate
for submergence.
Two of the peat samples were collected from the
upper 2-3 cm of the peat layer exposed near the rooted
stumps, immediately below the abrupt contact between
the peat and overlying tidal mud. The third peat sample
was collected from an equivalent stratigraphic position in
a nearby borehole (Combellick, 1991, table 2, sample TA1-
12.8). If the peat is composed primarily of plant material
that was growing at or shortly before submergence, these
samples should provide a closely limiting maximum age for
the event. However, possible contamination by older
plant matter or younger roots may introduce unknown
errors.
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 5
Figure 3. Erosion has exposed root systems of two generations of dead spruce trees in coastal banks along upper
Turnagain Arm near Girdwood. (7A) View of standing trees that probably died from salt-water intake following
submergence during the 1964 earthquake. The roots of these trees and other contemporaneous vegetation were
buried by postearthquake deposition of intertidal silt. Stumps along the base of the bank are rooted in an older
peat layer, which is buried beneath about 1 m of intertidal silt. (7B) Closeup of older rooted stump showing
intertidal mud that buried the root system and associated peat layer (erosion has removed much of the peat). Death
of these older trees and burial of the peat layer within which they are rooted probably resulted from subsidence
during the previous great earthquake (1991 photographs).
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 6
Two commercial laboratories performed the
radiocarbon dating using standard pretreatment and gas-
proportional counting techniques. Reported radiocarbon
ages (fig. 4) include a correction for natural 13C/12C isotopic
fractionation and are referenced to A.D. 1950. Calibration
to calendar years was based on tree-ring data of Stuiver
and Becker (1986) and was performed using a computer
program by Stuiver and Reimer (1986). The laboratories
did not provide specific error multipliers to account for
non-counting analytical errors, so I used a conservative
value of 2 for all calibrated ages. Although this has the
effect of doubling the quoted standard deviation, the
probability that the true sample age is within the calibrated
age range remains at 68 percent. This is because the
quoted standard deviation may not be large enough to
account for all sources of laboratory error (Stuiver and
Pearson, 1986).
The weighted average age of the wood samples is
926 ± 35 radiocarbon yr, which gives possible calibrated
ages of 799, 810, 830, 856, and 909 yr and an error range
probably within the interval 744-946 yr B.P. at 68 percent
confidence (fig. 4). The calibrated age range is greater
than the uncalibrated range because of multiple intercepts
in the calibration curve and greater rate of change of
calibrated age versus radiocarbon age during this period
(fig. 5).
Ages of the peat samples (fig. 4) are consistent with
the wood ages but show greater variation, probably
because of the longer period spanned by each peat sample
and, in the case of sample 91-1-1, possible contamination
by younger organic material.
REGIONAL CORRELATION
AND DISCUSSION
Buried peat layers with characteristics similar to the
lower Girdwood forest layer, but without observed rooted
stumps, occupy an equivalent stratigraphic position in
estuarine sediments at Portage and Palmer Hay Flats
(Combellick, 1991) and at Chickaloon Bay and Goose Bay
(this study) (fig. 2). Samples from the tops of the peat
layers yielded ages that are consistent with the ages of
wood and peat samples from the lower Girdwood forest
layer (fig. 4). The reported 665-895-yr calibrated age for
coseismic uplift in the Copper River Delta area (Plafker and
others, 1992) is also consistent with the wood and peat
ages in upper Cook Inlet.
These data do not prove that vertical displacement
was coeval in all areas; if the dated deposits were produced
by events separated by only a few years or decades, these
events cannot be resolved by conventional radiocarbon
dating. However, because the current body of paleoseismic
evidence indicates an average recurrence interval of 600-
800 yr for great earthquakes in this region, it is reasonable
to presume that the dated deposits represent a single
event.
If vertical displacement was coeval, the minimum
magnitude of this earthquake can be roughly estimated
using an empirical relationship between magnitude of
subduction-zone earthquakes and length of measurable
deformation (West and McCrumb, 1988). Considering
that deformation zones are elongated parallel to the trench
axis, the deformation associated with the earthquake must
have exceeded the 300-km distance between upper Cook
Inlet and Copper River Delta because this line is roughly
perpendicular to the trench axis. According to the graphic
relationship of West and McCrumb (1988, fig. 1), the
minimum earthquake magnitude for a 300-km-long zone of
deformation is about 7.8.
High-precision dating involving much longer
counting periods could provide radiocarbon ages with
standard deviations on the order of one or two decades
(Stuiver and Becker, 1986; Atwater and others, 1991).
However, this technique may not improve resolution of
the calendar age of the earthquake because of apparent
wide variability of atmospheric 14C between about 750 and
900 yr B.P. (A.D. 1050-1200). The resulting calibration
curve (fig. 5) shows that radiocarbon ages between about
860 and 960 yr give multiple calibrated ages between 750
and 925 yr. As Atwater and others (1991) have
demonstrated, it may be possible to precisely date older
rings of rooted stumps, thereby obtaining radiocarbon
ages on a steeper portion of the calibration curve (greater
than 960 radiocarbon yr B.P.) where there are fewer multiple
intercepts. Subtracting the number of growth rings between
the sampled portion and the outer surface would then
yield more precise calibrated ages for the samples. This
method could help determine whether sudden vertical
displacements in this region between 700 and 900 yr ago
resulted from multiple great earthquakes.
SUMMARY AND CONCLUSIONS
The last great earthquake that caused tectonic defor-
mation in the Anchorage region prior to 1964 probably
occurred between 700 and 900 yr ago. This inferred
earthquake caused vertical shoreline changes extending
at least from upper Cook Inlet to the Copper River Delta.
Evidence of these changes appears in tidewater banks at
Girdwood, where a layer of peat and rooted tree stumps
exposed about 1.7 m below the modern coastal high marsh
is buried beneath intertidal mud. The abrupt upper contact
of the peat, apparent sudden death of the trees, and
presence of halophytic plant fossils in mud above the
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 7
contact indicate submergence into the low-marsh envi-
ronment more rapidly than would be expected as a result
of nontectonic rise of relative sea level. As intertidal silt
deposition and possibly postseismic rebound restored
the tidal flat to subaerial conditions, the mud above this
forest layer became increasingly rich in plant matter and
a new brackish-water high marsh developed. Subsidence
during the 1964 earthquake submerged this marsh in a
similar fashion and resulted in burial by intertidal silt.
Age control for this penultimate earthquake comes
from radiocarbon dates of outer growth rings from three
rooted trees that were probably killed by salt water as a
result of coseismic submergence. Mathematically
combining these ages gives a calibrated age range probably
within 744-946 calendar yr B.P. at 68 percent confidence.
Peat samples from the top of the soil layer within which
these stumps are rooted yield calibrated ages that are
consistent with the wood ages but show greater variation.
The wood and peat ages at Girdwood closely match ages
obtained from similar buried peat layers at Portage,
Figure 4. Radiocarbon ages of wood and peat samples used in this study. Wood samples were collected from the
outer 10-25 growth rings of rooted stumps (*CI-101 age provided by Gordon Jacoby, written commun.). Peat
samples were collected from the upper 2-3 cm of buried peat. Age in 14C yr B.P. is conventional radiocarbon
age in years before A.D. 1950 with quoted counting error of one laboratory standard deviation. Calibrated
age ranges at right are based on tree-ring data of Stuiver and Becker (1986) and incorporate an error
multiplier of 2 to account for possible non-counting sources of laboratory error. Probable age range shown
represents error limits of the average calibrated age of two wood samples at the Girdwood site, at 68 percent
confidence (fig. 5).
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 8
Chickaloon Bay, Palmer Hay Flats, and Goose Bay and
from deposits formed by coseismic uplift at Copper River
Delta. Although I presume that these vertical displaceîp•,s
occurred during a single great earthquake, multiple
earthquakes may have affected each area separately within
a period that was too brief to resolve with conventional
radiocarbon dating. If the vertical displacements occurred
during a single earthquake, its magnitude was probably
7.8 or larger.
ACKNOWLEDGMENTS
The U.S. Geological Survey, Department of the
Interior, supported this study under National Earthquake
Hazards Reduction Program award 14-08-0001-G1949 to
the Alaska Division of Geological & Geophysical Surveys.
The views and conclusions contained in this document
are those of the author and should not be interpreted as
necessarily representing official policies, either expressed
or implied, of the U.S. Government.
I thank Richard Reger for his assistance in the field
and for his knowledgeable insights. Beta Analytic, Inc.
and Geochron Laboratories Division, Krueger Enterprises,
Inc. performed all radiocarbon-age determinations. Reger,
Brian Atwater, and George Plafker reviewed the manuscript
and provided many valuable suggestions for improvement.
REFERENCES CITED
Allen, C.R., 1986, Seismological and paleoseismo-logical
techniques of research in active tectonics, in Studies
in geophysics: Active tectonics: Washington, D.C.,
National Academy Press, p. 148-154.
Atwater, B.F., 1987, Evidence for great Holocene earth-
quakes along the outer coast of Washington State:
Science, v. 236, p. 942-944.
Atwater, B.F., and Yamaguchi, D.K., 1991, Sudden, prob-
ably coseismic submergence of Holocene trees and
grass in coastal Washington State: Geology, v. 19,
no. 7, p. 706-709.
Atwater, B.F., Stuiver, Minze, and Yamaguchi, D.K., 1991,
Radiocarbon test of earthquake magnitude at the
Cascadia subduction zone: Nature, v. 353, p. 156-158.
Bartsch-Winkler, S.R., and Garrow, H.C., 1982, Deposi-
tional system approaching maturity at Portage Flats,
in Coonrad, W.L., ed., The U.S. Geological Survey in
Alaska: Accomplishments during 1980: U.S. Geologi-
cal Survey Circular 844, p. 115-117.
Bartsch-Winkler, Susan, and Schmoll, H.R., 1987, Earth-
quake-caused sedimentary couplets in the upper
Cook Inlet region, in Hamilton, T.D., and Galloway,
J.P., eds., Geologic Studies in Alaska by the U.S.
Geological Survey during 1986: U.S. Geological Sur-
vey Circular 998, p. 92-95.
Bartsch-Winkler, Susan, and Schmoll, H.R., 1992, Utility of
radiocarbon-dated stratigraphy in determining late
Holocene earthquake recurrence intervals, upper Cook
Inlet region, Alaska: Geological Society of America
Bulletin, v. 104, no. 6, p. 684-694.
Brown, L.D., Reilinger, R.E., Holdahl, S.R., and Balazs, E.I.,
1977, Postseismic crustal uplift near Anchorage,
Alaska: Journal of Geophysical Research, v. 82, no. 23,
p. 3369-3378.
Combellick, R.A., 1986, Chronology of late-Holocene
earthquakes in southcentral Alaska: Evidence from
buried organic soils in upper Turnagain Arm [abs.]:
Geological Society of America Abstracts with
Programs, v. 18, no. 6, p. 569.
__________1991, Paleoseismicity of the Cook Inlet region,
Alaska: Evidence from peat stratigraphy in Turnagain
and Knik Arms: Alaska Division of Geological &
Geophysical Surveys Professional Report 112, 52 p.
Figure 5. Portion of tree-ring calibration curve show-
ing translation of average radiocarbon-age
range for the two wood samples to calibrated
probable range shown in figure 4. Prior to cali-
bration, the error calculated from the labora-
tory standard deviations is increased by an
error multiplier of 2. The center line is the cali-
bration curve and the outer lines represent un-
certainty of one standard deviation in the cali-
bration data set; this standard deviation incor-
porates a laboratory error multiplier of 1.6
(modified from Stuiver and Becker, 1986, fig. 1B).
2001 Guide to the Petroleum, Geology, and Shallow Gas Potential of the Kenai Peninsula, Alaska 9
Davies, J.N., 1984, Alaska’s earthquakes: The Northern
Engineer, v. 16, no. 4, p. 8-13.
DeMets, C., Gordon, R.G., Argus, D.F., and Stein, S., 1990,
Current plate motions: Geophysical Journal Interna-
tional, v. 101, no. 2, p. 425-478.
Karlstrom, T.N.V., 1964, Quaternary geology of the Kenai
lowland and glacial history of the Cook Inlet region,
Alaska: U.S. Geological Survey Professional Paper
443, 69 p.
Lajoie, K.R., 1986, Coastal tectonics, in Wallace, R.E., ed.,
Studies in geophysics: Active tectonics: Washington,
D.C., National Academy Press, p. 95-124.
McCulloch, D.S., and Bonilla, M.G., 1970, Effects of the
earthquake of March 27, 1964, on the Alaska Railroad:
U.S. Geological Survey Professional Paper 545-D,
161 p.
Nelson, A.R., 1992, Discordant 14C ages from buried tidal-
marsh soils in the Cascadia subduction zone, south-
ern Oregon coast: Quaternary Research, v. 38, no. 1,
p. 74-90.
Nelson, A.R., and Personius, S.F., in press, The potential
for great earthquakes in Oregon and Washington: An
overview of recent coastal geologic studies and their
bearing on segmentation of Holocene ruptures, cen-
tral Cascadia subduction zone, in Rogers, A.M.,
Kockelman, W.J., Priest, G., and Walsh, T.J., eds.,
Assessing and reducing earthquake hazards in the
Pacific Northwest: U.S. Geological Survey Profes-
sional Paper 1560.
Obermeier, S.F., Gohn, G.S., Weems, R.E., Gelinas, R.L.,
and Rubin, Meyer, 1985, Geologic evidence for recur-
rent moderate to large earthquakes near Charleston,
South Carolina: Science, v. 227, p. 408-411.
Ovenshine, A.T., Lawson, D.E., and Bartsch-Winkler,
S.R., 1976, The Placer River Silt—An intertidal de-
posit caused by the 1964 Alaska earthquake: Journal
of Research of the U.S. Geological Survey, v. 4, no. 2,
p. 151-162.
Plafker, George, 1969, Tectonics of the March 27, 1964,
Alaska earthquake: U.S. Geological Survey Profes-
sional Paper 543-I, 74 p., 2 sheets, scales 1:2,000,000
and 1:500,000.
Plafker, George, and Rubin, Meyer, 1967, Vertical
tectonic displacements in south-central Alaska
during and prior to the great 1964 earthquake:
Journal of Geosciences, Osaka City University,
v. 10, p. 53-66.
_______1978, Uplift history and earthquake recurrence
as deduced from marine terraces on Middleton Island,
Alaska, in Proceedings of Conference VI,
Methodology for Identifying Seismic Gaps and Soon-
to-break Gaps, National Earthquake Hazards
Reduction Program, 25-27 May, 1978: U.S. Geological
Survey Open File Report 78-943,
p. 687-721.
Plafker, George, Lajoie, K.R., and Rubin, Meyer, 1992,
Determining recurrence intervals of great subduction
zone earthquakes in southern Alaska by radiocarbon
dating, in Taylor, R.E., Long, Austin, and Kra, R.S.,
eds., Radiocarbon after four decades: An interdisci-
plinary perspective: New York, Springer-Verlag, p. 436-
452.
Scott, E.M., Aitchison, T.C., Harkness, D.D., Cook, G.T.,
and Baxter, M.S., 1990, An overview of all three stages
of the international radiocarbon intercomparison:
Radiocarbon, v. 32, no. 3, p. 309-319.
Stuiver, M., and Becker, B., 1986, High-precision decadal
calibration of the radiocarbon time scale, AD 1950-2500
BC: Radiocarbon, v. 28, p. 863-910.
Stuiver, M., and Pearson, G.W., 1986, High-precision
calibration of radiocarbon time scale, AD 1950-500
BC: Radiocarbon, v. 28, p. 805-838.
Stuiver, Minze, and Quay, P.D., 1980, Changes in
atmospheric carbon-14 attributed to a variable sun:
Science, v. 207, p. 11-19.
Stuiver, M., and Reimer, P.J., 1986, A computer program for
radiocarbon age calibration: Radiocarbon, v. 28,
p. 1022-1030.
Sykes, L.R., Kisslinger, J.B., House, Leigh, Davies, J.N.,
and Jacob, K.H., 1980, Rupture zones of great earth-
quakes in the Alaska-Aleutian Arc, 1784 to 1980:
Science, v. 210, no. 19, p. 1343-1345.
West, D.O., and McCrumb, D.R., 1988, Coastal uplift in
Oregon and Washington and the nature of Cascadia
subduction-zone tectonics: Geology, v. 16, no. 2,
p. 169-172.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 10
Introduction to Tertiary Tectonics and
Sedimentation in the Cook Inlet Basin
Robert “Bob” Swenson
Phillips Alaska Inc.
Introduction
Tertiary rocks of the Cook Inlet Basin record the geology of an active plate boundary
and associated cycles of deposition in a forearc setting. Beginning with the subduction of the
Kula oceanic plate and spreading center, Cenozoic tectonics were the driving force of a
complex geologic system that in places accumulated over 25,000 feet of non-marine
stratigraphy. Variation in uplift histories of adjacent tectonic blocks provided both sediment
input and stress needed for formation of structures that accumulated both gas and liquid
hydrocarbons.
The regional Tertiary stratigraphic column is separated into 5 distinct non-marine
lithologic units. These formations are regionally time transgressive and represent laterally
equivalent facies that were deposited in a clastic dominated basin. The current depositional
model suggests that alluvial fans carried sediment off the uplifting margins and provided the
bulk of sediment influx. A migrating axial fluvial system produced an environment for the
thick accumulation of sandstone, siltstone, and coal near the basin center. Plio/Pleistocene
tectonic activity caused dramatic change in the deposystem and contributed to uplift that
exposed the Tertiary stratigraphy.
The ideas presented in this paper come from years of research by many scientists within
the oil industry, government and academia. Much of the recent work contains proprietary
information and cannot be presented here. However, it is the author's hope that this general
outline will provide the reader with a basic understanding of the geology of the Cook Inlet
region. A reference list of research papers is included for more detailed models and data.
Cook Inlet Basin
The Tertiary Cook Inlet Basin can be defined as an elongate, northeast trending, fault-
bounded forearc basin that extends from the Matanuska Valley south along the Alaskan
Peninsula. Although the basin geometry appears relatively straight forward, dramatic geologic
variation is evident along trend. For example, variation in uplift/subsidence rates along the
basin axis has greatly affected thickness of the present Tertiary section. Where preserved in
the Kenai area, Eocene through Pliocene sediments are up to 25,000 feet thick whereas
Tertiary strata over the Seldovia Arch thins to less than 1500 feet in total thickness.
Figure 1 shows the present-day geometry and location of major basin-bounding faults
that have controlled much of the tectonic history. The Bruin Bay and Castle Mountain fault
zones make up the northern and northwestern boundaries and separate the uplifted volcanic arc
complex from the Tertiary depocenter. Much of the deformation along the northwest margin
of the basin was related to motion on these faults and resulted in structural traps for
hydrocarbon accumulation.
2001
2001 Guide to the Petroleum, Geology, and Shadow
Gas Potential of the Kenai Peninsula, Alaska 11
Figure 1. Present day Cook Inlet basin morphology and regional tectonic boundries
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 12
The Border Ranges Fault to the southeast separates the Tertiary basin from the
Chugach Terrane. This tectonic block is an emergent accretionary prism composed of
metasedimentary and metaigneous rocks obducted at the plate boundary. Continued
subduction/obduction processes along the Aleutian Trench caused thermal alteration, rotation,
and uplift of these rocks during the Late Cretaceous (figures 3 & 4).
Both the Chugach and Peninsular/Wrangellia geologic terranes, adjacent to the Tertiary
Basin, provided detritus for the thick sedimentary package. Variation in mineralogy,
depositional style, and accumulation rates of the Tertiary stratigraphy record changes in uplift
of these terranes and understanding the tectonic histories has been critical in deciphering the
geology of the Cook Inlet.
Tectonic History
Evidence for active tectonism in the Cook Inlet region spans back to the Late Triassic
with onset of subduction and change from shelfal carbonates of the Kamishak Formation to
oceanic arc volcanism and sedimentation of the Talkeetna Formation (Wang, 1988). It is not
clear how subduction was initiated, but the change in depositional patterns was dramatic.
Nearly all Cook Inlet sedimentation following this event can be related to deposition in a
foreland/forearc setting with episodic uplift and erosion to the north. Unroofing of the arc-
related highlands provided sediment for Mesozoic forearc deposits which presently make up
the 'basement' for the Tertiary basin.
Figure 2 is a general stratigraphic column relating the tectonic events with stratigraphy.
Periodic uplift and pulses of deformation created numerous unconformities and change of
depositional environments from marine to non-marine. The Late Cretaceous marks the final
emergence of the basin with a marked unconformity at the Paleocene and Early Eocene
boundaries. All subsequent deposition was non-marine to marginal marine and derived from
both the volcanic arc to the northwest and the exposed accretionary prism to the southeast.
The Aleutian Subduction Zone was also undergoing dramatic changes during the Early
Tertiary. Figure 3 is a cartoon depiction of the overall geometry of the plate boundary well
into its evolution. Morphologic and geologic variation of the down-going slab had the greatest
influence on basin evolution, including local and regional tectonism of the overriding plate.
Subduction of the Kula Plate and spreading center in Early Eocene is thought to be the driving
force for the early Tertiary deformation and increased tectonism (Byrne, 1979, Pavlis, 1982).
Thermal effects associated with that event can be observed throughout southern Alaska and
include near-trench plutonism and gold-quartz vein mineralization.
Following Kula spreading ridge subduction, the Cook Inlet basin underwent a phase of
rapid subsidence and deposition resulting in a very thick section of non-marine sandstone, coal,
and siltstone. Although there are numerous unconformities in the section, this subsidence and
depositional environment continued until the end of Pliocene time when the latest phase of
tectonism deformed the basin margins. Many of these recent folds are tight asymmetric
anticlines associated with transpressional strain from right lateral motion on the northern basin-
bounding faults. This lateral stress could be associated with accretion/collision of the Yakutat
continental block (Figure 4) during Late Tertiary.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 13
Figure 2. Tectonostratigraphic correlation chart for the Cook Inlet (from Currey et al.,
1993).
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 14
Figure 4 is a generalized tectonic map which shows the modern elements of the
Southern Alaska tectonic boundary and Cenozoic stratigraphy. Understanding the distribution
of structures, volcanic centers, and non-marine depocenters has been critical in deciphering the
history of the basin as a whole.
Figure 3: General tectonic configuration of Cook Inlet Basin and associated subduction zone
(From Doherty, et al., 1994)
Tertiary Stratigraphy
The Tertiary section is as thick as 25,000 feet near the northern basin axis and thins
radically to both the basin edges, as well as to the south towards the Augustine-Seldovia Arch.
Unit identification, age control, and facies relationships within the units has undergone many
iterations and is based on both outcrop and well data.
Tertiary rocks were first identified as the "Kenai group" by Dall and Harris in 1892 and
further refined by Parkinson in 1962 using well control from the Swanson River Field.
Calderwood and Fackler, 1972, studied five widely separated "type" well logs and elevated the
"Kenai" to Group status. Based on their correlation, they assigned five formation names that
are retained in the present nomenclature. Much work has been done since that time to refine
the stratigraphy and age assignments, and provide depositional models to better understand the
distribution of facies.
As mentioned in the previous section, subduction of the Kula oceanic plate and
spreading center dominated the Early Tertiary tectonics and initiated the final phase of non-
marine clastic deposition within the basin. The regional unconformity (locally angular) at the
base of the Tertiary section separates Mesozoic stratigraphy from overlying Paleocene/ Eocene
volcaniclastic rocks. The amount of stratigraphic section missing at this boundary varies
widely and ranges from Eocene on Jurassic, to Paleocene on Cretaceous. Figure 5 is a
generalized stratigraphic column for the Tertiary Cook Inlet.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 15
Figure 4: Present day tectonic framework and Cenozoic forearc basin deposits (from Doherty
et al., 1994)
The formal lithologic units include the Sterling, Beluga, Tyonek, Hemlock and West
Foreland Formations, all of which are non-marine. Identification of these units has historically
been based on lithologic character and well log correlation. Recent research suggests that many
of the lithologic units are time transgressive, laterally correlative facies related to a dynamic
non-marine depositional system. More details concerning this research will be published at a
later date.
The oldest Tertiary units in the inlet outcrop in the Matanuska Valley and contain four
distinct non-marine facies of Paleocene/Eocene age. These formations are the Tsdaka,
Wishbone, Chickaloon and Arkose Ridge. The lateral extent of these units, and distribution of
Paleocene strata in the subsurface is limited (Magoon and Claypool, 1979) and record initial
Tertiary uplift and cessation of Mesozoic depositional patterns. The Paleocene section is
shown as unnamed in the basin-wide stratigraphic section.
The Eocene/Oligocene West Foreland Formation is tuffaceous sand and conglomerate
that, with the exception of the localized 'unnamed' Paleocene unit, makes up the basal part of
the inlet-wide Tertiary section. The dominant lithic component of the coarse facies is
volcaniclastic which coincides with increased volcanism associated with subduction of the Kula
spreading center. The West Foreland and overlying Hemlock Conglomerate are often hard to
distinguish because of their similar lithology and log character and are primarily distinguished
by mineralogical variation.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 16
Figure 5: Generalized Cook Inlet Tertiary Stratigraphy
The Hemlock Conglomerate overlies the West Foreland and is an important oil
reservoir in the basin. This unit is Oligocene in age and comprised predominantly of fine to
coarse-grained sandstone and conglomerate. Dominant mineralogies within the sands are
quartz, feldspar, and metamorphic/plutonic rock fragments, which explains the increase in
reservoir quality over the volcaniclastic rich West Foreland. The fine-grained facies of this unit
are siltstone and tuffaceous siltstone which locally contain coal beds.
The Tyonek Formation is very similar to the overlying Beluga and is composed of
abundant coal, siltstone, and massive sandstone of Oligocene and Miocene age. Unlike the
Beluga facies however, coal beds within the Tyonek are relatively high quality, sub-bituminous
to bituminous, and often regionally continuous. The similarities in lithology between this unit
and the Beluga Formation can make it difficult to identify a distinct contact. The base of the
Tyonek Formation is gradational with the Hemlock and placed at the first occurrence of thick,
coarse sand and conglomerate with a general lack of coal beds.
The Miocene Beluga formation is a thick (> 3000 ft,) siltstone rich unit with common
interbeds of channelized muddy sandstone, coal, and tuff. Lithic components of the coarser
facies, in contrast with the overlying Sterling, are dominated by metamorphic rock fragments
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 17
and quartz (Curry et al., 1993). Beluga coals tend to be thin (< 5 feet), lignitic to sub-
bituminous, and regionally discontinuous. Much of the gas produced in the Inlet to date has
been from Sterling and Beluga reservoirs. The base of the Beluga Formation can be hard to
consistently identify on logs and is placed at the top of the first thick (> 10 feet) coal.
The Miocene-Pliocene Sterling Formation is the youngest non-glacial unit in the inlet,
and with the exception of the uplifted basin edges, is the predominant submarine outcrop. The
Sterling is a friable, fine to coarse-grained cross-bedded sandstone deposited in stacked
channels with associated mud drapes and siltstone facies with local thin coals. Outcrop of this
unit can contain as much as 80% sand and well logs show a very distinct blocky character.
Mineralogically, the sandstone contains abundant volcaniclastics, common glass shards, quartz
and feldspar. The base of the Sterling Formation is a regional unconformity and picked at the
first occurrence of abundant coal and loss of massive sands.
Depositional Model
A depositional model for the above described units involves a rapidly subsiding non-
marine basin with sediment sources from both the north and south. Figure 6 depicts this model
and shows local and regional aspects of the system (McGowen and Doherty, 1994). The
coarsest grained facies were deposited proximal to the source by an alluvial fan system which
carried sediment out into the basin from both the arc and accretionary complex margins.
Location of these fans was related to uplift on the basin bounding faults.
The distal portions of the fans were later reworked by an axial-fluvial system that
migrated across the basin floor in relation to sediment input and topography. The fluvial
system provided mixing of the various mineralogies, was dominantly fine grained, and moved
sediment out into the flood plain areas. Swamps, marshes and flood basins provided the biotic
material that produced the ubiquitous coal horizons. The final product of this depositional
system is the thick package of clastic sediment and coal that defines the non-marine Tertiary of
the Cook Inlet.
Summary
Tertiary rocks of the Cook Inlet Basin record the geology of an active plate boundary
and associated cycles of deposition in a forearc setting. Beginning with the subduction of the
Kula oceanic plate and spreading center, Cenozoic tectonics were the driving force of a
complex geologic system that in places accumulated over 25,000 feet of non-marine
stratigraphy. Variation in uplift histories of adjacent tectonic blocks provided both sediment
input and stress needed for formation of structures that accumulated both gas and liquid
hydrocarbons maturing at depth.
The Tertiary stratigraphic column is separated into 5 distinct lithologic units. These
formations are regionally time transgressive and represent laterally equivalent facies that were
deposited in a clastic dominated basin. The current depositional model suggests that alluvial
fans carried sediment off the uplifting margins and provided the bulk of sediment influx. A
migrating axial fluvial system produced an environment for the thick accumulation of siltstone
and coal near the basin center. Increased tectonism in Plio/Pleistocene time shut the
deposystem down and helped create the geologic snapshot that is observed today.
Asthenosphere
2001 Guide to the Petroleum, Geology, and Shadow
Gas Potential of the Kenai Peninsula, Alaska
18
Figure 6. Cook Inlet Depositional systems model (From McGowen, et al., 1994)
Acknowledgments
The list of scientists that have worked this basin and provided critical information is much
too extensive to provide here, but special recognition should be given to Joe McGowen, Dave
Doherty, Mike Gardner, Dave Bannan, Bill Grether, Bud Simpson, Richard Curry, Steve Bergman,
David Hite, Kris Meisling, Paul Daggett, Jef Corrigan, Ken Helmold and many others.
Reference List
Dall, W. H., and Harris, G. D., 1892, Correlation Papers–Neocene: U.S. Geological Survey Bulletin 84, 349 p.
Byrne, T., 1979, Late Paleocene demise of the Kula-Pacific spreading center: Geology, v. 7, p. 341-344.
Calerwood, K. W., and Fackler, W. C., 1972, Proposed stratigraphic nomenclature for Kenai Group, Cook Inlet Basin,
Alaska: American Association of Petroleum Geologists Bulletin, v. 56, no. 4, p. 739-754.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 19
Parkinson, L.J.,1962, One field, one giant-The story of the Swanson River: Oil and Gas Journal, v. 60 (13), p.
180-183.
Plavis, T. L., 1982, Origin and age of the Border Ranges Fault of southern Alaska and its bearing on the Late
Mesozoic tectonic evolution of Alaska: Tectonics, v. 1, no. 4, p. 343-368.
Wang, J., Newton C. R., Dunne, L., 1988, Late Triassic transition from biogenic to arc sedimentation on the
Peninsular terrane: Puale Bay, Alaska Peninsula; Geological Society of America, Bulletin, v. 100,
p.1466-1478.
Internal ARCO Reports:
Curry, et al., 1993
Doherty, D. J., 1994
McGowen, J. H., and Doherty, D. J., 1994
Additional References
Adkison, W. L., Kelley, J. S., and Newman, K. R., 1975, Lithology and Palynology of the Beluga and Sterling
Formations exposed near Homer, Kenai Peninsula, Alaska: U.S. Geological Survey Open-file Report
75-383, 239 p..
Burk, C.A., 1965, Geology of the Alaska Peninsula-Island arc and continental margin: Geological Society of
America, Memoir 99, p.19-78.
Detterman, R. L., and Reed, B. L., 1980, Stratigraphy, structure, and economic geology of the Illiamna
quadrangle, Alaska: U.S. Geological Survey Bulletin No. 1368-B, 86 p..
Fisher, M. A., and Magoon, L. B., 1978, Geologic framework of lower Cook Inlet, Alaska: American
Association of Petroleum Geologists Bulletin, v. 62, no. 3, p. 373-402.
Hastings, D. S., Robinson, A. G., and Robinson, N. M., 1983, Stratigraphy, depositional history, and reservoir
potential of Cretaceous and Early Tertiary rocks of lower Cook Inlet, Alaska, American Association of
Petroleum Geologists Bulletin, v. 63, p. 480.
Hite, D. M., 1976, Some sedimentary aspects of the Kenai Group, Cook Inlet, Alaska: in Miller, T. P., ed.,
Recent and ancient sedimentary environments in Alaska: Alaska Geological Society Symposium
Proceedings, p. I1 - I23.
Jones, D. L., and Silberling, N. J., 1979, The key to tectonic analysis of southern and central Alaska: U.S.
Geological Survey Open File Report, 79-1200, p. 1-37.
Kirschner, C. E., and Lyon, C. A., 1973, Stratigraphic and tectonic development of Cook Inlet petroleum
province: American Association of Petroleum Geologists Memoir 19, p. 396-407
Kremer, M. C., and Stadnicky, G., 1985, Tertiary stratigraphy of the Kenai Peninsula-Cook Inlet Region: in
Guide to the Geology of the Kenai Peninsula, Alaska: Alaska Geological Society Field Guide, p. 24-
42.
Magoon, L. B., and Claypool, G. E., 1979, Petroleum geology of Cook Inlet Basin, Alaska--An exploration
model: U.S. Geological Survey Open-file Report 79-548, 23 p..
Moore, J. C., and Connelly, W., 1979, Tectonic History of the continental margin of southwestern Alaska: Late
Triassic to earliest Tertiary, in The relationship of plate tectonics to Alaskan geology and resources:
Alaska Geological Society Symposium Proceedings, H1-H29.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 20
Kenai Field,
the Kenai Peninsula's Largest Gas Field
D.L. Brimberry1, P.S. Gardner1, M.L. McCullough1, and S.E. Trudell1
(REVISED April 2001)
History
In 1959, the Union Kenai Unit 14-6 well identified significant gas reserves in sandstones of
the Tertiary Sterling, Beluga and Tyonek formations. The sandstones were deposited by
streams that flowed through a broad valley which is mostly occupied by the Cook Inlet today.
Kenai field owners (Unocal, Marathon Oil Company, SOCAL and CIRI) have sold natural gas
to south-central Alaska for electricity production, heating and manufacturing since 1961. This
prolific gas field and other Cook Inlet gas fields offered the opportunity to export natural gas
to other markets in 1969. In 1993, Marathon assumed complete ownership and the operations
of the field. The remaining reserve volume will continue to supply the utilities and the export
markets past the year 2010. The following information identifies some specifics of the oil and
gas geology for Kenai field.
Discovery
Well Name and Date:Union Kenai Unit 14-6 (API No. 50-133-10089-00), 1959
Reservoir Discovered:Tertiary Sterling Formation gas.
Total Depth:15,047 feet measured depth.
Bottom Hole Formation:Tertiary West Foreland Formation.
Objective:Tertiary Hemlock Conglomerate oil.
PRODUCTION DATA (12/00)
POOL NAME TOP INITIAL YEAR INITIAL PROD.CUM. PROD
(SSTVD FT.)WELL NAME (MMCF/D)(BCF)
Sterling -3425 KU 14-6 1959 11 1774
Beluga -4450 KBU 13-8 1977 10 158
Upper Tyonek-7100 KDU 5 1978 5.2 7
Deep Tyonek -8200 KDU 1 1967 11.5 31
TOTAL 2170
Reservoir Data
Total completions:69 (Commingled flow, typically dual tubing strings).
Spacing:160 to 1200 acres.
Drive:Gas expansion.
Structure:Simple anticline.
Gas Analysis:99% Methane, 0.5% Nitrogen, 0.2% CO2, 1008 BTU, 0.56
specific gravity
1 Marathon Oil Co., P.O. Box 196168, Anchorage, AK 99519-6168
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 21
Figure 1. Structure map on the top of the Beluga Formation at Kenai field.
Contour interval 100 feet. Sterling Formation accumulation area limit identified
by dashed line. Approximate accumulation area for Beluga and Tyonek
reservoirs identified by crosshatched area.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 22
Structural Geology
The Kenai field structure (fig. 1) is a large simple anticline that stretches over 10 miles
north-south (including the Cannery Loop field fault block) and 4 miles east-west. The only
fault of significance to the structure is the large normal fault on the north end of the structure
which segregates the Kenai field accumulation from the Cannery Loop field accumulation.
Additional faulting is not recognizable with the well control but small NE-SW normal faults are
seen on the seismic sections. Late Tertiary, local compression interrupted Sterling Formation
deposition and created the structural feature for gas to migrate into.
Reservoir Description
The reservoirs for the Kenai field accumulation are sandstones in the Tertiary Kenai Group;
Sterling, Beluga and Tyonek formations. Within the field the Tyonek is subdivided into two
informal intervals referred to as the “upper Tyonek” and the “deep Tyonek”.
Sterling Formation (Plio-Pleistocene)
The gas reservoirs of the Sterling Formation (fig. 2) were deposited in large meandering
streams which originated to the north/northwest. Productive sandstone sequences are
typically 30-60 feet thick, some are more than 100 feet thick. The reservoirs are fine to coarse
grained, moderately well sorted and angular to sub-rounded. Limited matrix material is
evident in the sandstones. Composition of the sandstones varies from primarily quartz-rich
feldspathic litharenites to sub-litharenites. Effective porosity ranges from 25 to 31 percent.
Permeability of more than one darcy is common. The sandstones are semi-consolidated with
slight cementation from calcite, authigenic smectite and kaolinite. Coals and silty shales
separate the sandstones. These characteristics describe the most prolific reservoir at Kenai
field. Sterling production extends along the northern nose of the Kenai field structure to cover
the largest productive area of the productive intervals.
Beluga Formation (Late Miocene)
The approximately 2,600 feet of Beluga Formation section (fig. 3) represents a marked
compositional difference from the underlying Tyonek Formation and the overlying Sterling
Formation. The Beluga lithology is dominated with metasedimentary lithic fragments derived
from the Chugach terrane to the east. The depositional sequence of the Beluga Formation at
Kenai field grades from thin sandstones deposited on an outwash plain in the lower Beluga
Formation to anastamosing streams in the upper Beluga Formation. Average bed thicknesses
are 10-20 feet in the lower and middle Beluga Formation. Thicker, higher quality sandstones
are in generally 20 foot sandstones beds in the upper Beluga Formation. The upper Beluga
sandstones have effective porosity above 15 percent and permeability from five to more than
50 millidarcies. Middle and lower Beluga sandstones occasionally have these reservoir
parameters; but, typically have 9-12% porosity and 0.1-10 milidarcies permeability. More than
85 percent of the gas reserves for the Beluga have flowed from the upper Beluga sandstones.
The rock fabric is very fine to coarse grained, moderately sorted, sub-angular and moderately
to well consolidated. Illite and smectite clays are present within the reservoir beds and serve
with calcite as the cementing agents. Between the sandstone bodies, the Beluga section is
dominated by silty shale with numerous thin, 1-2 foot thick coals. Beluga production does not
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 23
extend onto the northern nose of the Kenai structure, but is present on the small closure of
Cannery Loop field to the north.
“Upper Tyonek” (Miocene)
The “upper Tyonek” sandstones (fig. 4) are the least productive units in Kenai field due to
their thin nature and low reservoir quality. The sandstones were deposited by small tributaries
of meandering and anastomosing streams which carried sand from the north-northwest.
Overbank deposits are common and coals are considerably thicker in the Tyonek Formation
(typically more than 10 feet thick) than in the overlying Beluga or Sterling Formations. The
individual sandstones are very fine- to medium-grained. Fining upward to silt sequences or
interlamination with silt is common in the sandstones. The cleaner, reservoir quality portions
of the sandy intervals range from 20 - 40 feet thick. Sand composition varies laterally and
vertically from poorly sorted feldspathic litharenites to moderately well sorted sub-litharenites.
Grain angularity varies from angular to sub-rounded. Smectite clay dominates the matrix clay
in the sandstone and serves as the consolidation agent. Effective porosity is 12 to 15 percent.
Permeability ranges from 0.1 to 10 millidarcies. Fines migration and complexities associated
with variability in the “upper Tyonek” reservoir sandstones are the primary deterrents to high
flow rates and extensive reserves. Production from the “upper Tyonek” has also come only
from the crestal portion of the field.
“Deep Tyonek” (Miocene)
The sandstone reservoirs of the “deep Tyonek” (fig. 5) are similar in composition to the
“upper Tyonek”, quartz-rich feldspathic litharenites and sub-litharenites. The primary
difference is that these deeper reservoirs represent larger meandering stream sands.
Conglomeratic intervals are common in the thicker sandstones of the “deep Tyonek”. Grain
size varies widely in the sandstone bodies from conglomerate to medium grained. Reservoir
thicknesses exceed 40 feet in the blocky sandstone intervals. Effective porosity of around 12
percent and permeability of more than 50 millidarcies combined to yield initial gas flow rates in
excess of 20 mmcfpd (million cubic feet per day) in the early “deep Tyonek” wells of the Kenai
field. The sandstones are well consolidated with compaction, clay and calcite cement. Coal
beds are thicker in the “deep Tyonek” than any unit above and are often located at the base of
the reservoir bodies. “Deep Tyonek” production is limited to the crestal portion of the field
and the small closure on the Cannery Loop feature to the north.
Hydrocarbons
The high methane content of the gas in the field and isotope markers indicate the gas
reserves were generated by biogenic activity in the sediment. Abundant carbonaceous material
in the form of coal and debris incorporated into the sediment is available for microbial
consumption and methane generation. A very minor amount of condensate, approximately
1000 barrels per year, is produced with the gas from the “deep Tyonek” reservoirs. This may
be related to the thermal cracking of the coals, but is not significant.
The oil seen in numerous other Cook Inlet fields is noticeably absent from the large Kenai
structure. The two deep wells, Kenai Unit 14-6 and Kenai Unit 41-18, have minimal sample
shows. The drill stem tests of the initial objective in the field, the Tertiary Hemlock
Conglomerate, recovered conate water.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 24
Figure 2. Log section for Sterling Formation in the Kenai Tyonek Unit 43-06X (API
No. 50-133-20328-00) at Kenai field. Gas bearing zones identified by cross hatch on the
resistivity log (logrithmic scale: 1 to 100 ohms). Gamma ray scale: 40 to 115 API. SP
scale: -90 to -15 mv. Density (RHOB) scale: 50 to 0 percent.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 25
Figure 3. Partial log section for upper and middle Beluga Formation in the Kenai
Tyonek Unit 43-06X (API No. 50-133-20328-00) at Kenai field. Gas bearing zones
identified by cross hatch on the resistivity log (logrithmic scale: 1 to 100 ohms).
Gamma ray scale: 40 to 115 API. SP scale: -90 to -15 mv. Density (RHOB) scale: 50
to 0 percent.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 26
Figure 4. Log section for the “upper Tyonek” in the Kenai Deep Unit 2 (API No. 50-
133-20121-00) at Kenai field. Gas bearing zones for the field are identified by sandy (SP
response) intervals cross hatched on the resistivity log (logarithmic scale: 1 to 100
ohms). Gamma ray scale: 40 to 115 API. SP scale: -90 to -15 mv. Density (RHOB)
scale: 50 to 0 percent.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 27
Figure 5. Log section for “deep Tyonek” in the Kenai Deep Unit 2 (API No. 50-133-
20121-00) at Kenai field. Gas bearing zones for the field are identified by sandy (SP
response) intervals cross hatched on the resistivity log (logarithmic scale: 1 to 100
ohms). Gamma ray scale: 40 to 115 API. SP scale: -90 to -15 mv. Density (RHOB)
scale: 50 to 0 percent.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 28
Summary
The prolific gas production from Kenai field has proven significant to the regional economy
of the Kenai Peninsula. The production from the extremely high quality reservoirs in the
Sterling Formation has been supported with additional production from the deeper Beluga and
Tyonek formations. The simple anticlinal structure allows for very good recoveries from
each well and efficient reservoir management. The field's production will continue to be a
significant and reliable contributor to the resource base of the area.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 29
High-resolution Chronostratigraphic Analyses of the Tertiary Kenai Group,
South-central Alaska: Applications to Basin Analysis and Coal-bed Methane
Assessment: An Update
Todd A. Dallegge, Department of Geology and Geophysics, University of Alaska Fairbanks,
Fairbanks, AK
Charles E. Barker, U.S. Geological Survey, Denver Federal Center, Denver
Introduction
This article first appeared in last year's Second Alaska Workshop on Coalbed Methane
(Barker et al., 2000). The article is modified and includes an update section covering the
current status of Dallegge’s Ph.D. research. Some early material has cleared proprietary
restrictions and is reported in Barker et al. this volume.
The Kenai Group within the Cook Inlet Basin, south-central Alaska, contains a
substantial record of terrestrial sedimentation and regional volcanism throughout most of
Tertiary time. Due to the abundance of well-preserved plant leaves, the Kenai Group has been
designated the type section of three Neogene provincial paleobotanical stages: the Seldovian,
Homerian, and Clamgulchian (Wolfe et al., 1966; Wolfe and Tanai, 1972). Tertiary plant-
bearing strata from Alaska, Pacific northwest and eastern Russia are correlated on the basis of
these stages (Wolfe and Tanai, 1972; Wolfe, 1994). In addition, these units are a source of oil
and gas production and contain a valuable, under-developed coal resource that has significant
coal-bed methane potential. Thus, understanding the geologic history of the basin is important
from an economic as well as paleontological standpoint. The purpose of this paper is to outline
a proposed method for determining stratigraphic relations and coal-bed methane potential for
the Cook Inlet Basin.
Abundant stratigraphic information exists in the form of cores, well logs and scattered
surface outcrops from the Cook Inlet Basin. Because the surface outcrops are incomplete,
scattered throughout the basin, and covered by heavy vegetation, the construction of
stratigraphic models has been primarily from well data. Correlation of isolated outcrops within
the Cook Inlet Basin has been problematic and poorly documented despite the abundant work
done to date. Over two-thirds of the entire Tertiary section is known only from subsurface well
information. Currently, there is some debate about whether the units in the Kenai Group are
time-transgressive (Swenson, 1997) (Fig. 1). Part of the problem is that type sections for the
formations of the Kenai Group are defined based on cuttings and subsurface electric log
characteristics with some palynologic and heavy mineral analyses (Calderwood and Fackler,
1972; Hite, 1976). These units are then projected to surface exposures (Adkison et al., 1975)
but the lack of diagnostic characteristics of individual units and the lack of age controlling
fossil material hampers clearly defined correlations. Attempts to correlate outcrop and well
data are further hindered by stratigraphic complexity produced by the braided and meandering
fluvial systems that deposited the Kenai Group sediments. Many attempts have been made to
correlate strata locally and regionally but thus far, these studies have had limited success, even
over short intervals.
Deposition of the Kenai Group is believed to have taken ~30 Ma, however the age
control for this is poor. Imprecise K-Ar and fission track ages have been assigned to the upper
parts of the group (Fig. 2; Triplehorn et al., 1977; Turner et al., 1980), representing only 7
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 30
million years of deposition. Much of the older portion of the group, mostly seen in well
samples, has only one limiting radiometric age control point. The remainder is dated by pollen
genera and leaf species with relatively long ranges. No vertebrate mammal material has been
documented within these units (Dorr, 1964; McClellan and Giovannetti, 1979). Therefore, the
best available dates are based on long-ranging botanical fossils and imprecise K-Ar data,
leaving the age of the Tertiary section poorly documented.
Current approaches to chronostratigraphy involve integration of several techniques.
The 40Ar/39Ar method has been used to precisely date type-sections for many litho-, bio-, and
chronostratigraphic units throughout the world and, when coupled with palynology and other
paleobotanical methods, can be used to correlate units across broad regions and identify lateral
variations in geologic units (e.g. Deino et al., 1990; Berggren et al., 1995; Larson and Evonoff,
1998; Dallegge, 1999). The abundant coal-bearing units of the Kenai Group contain ash beds
(partings) within the coal seams (Adkison et al., 1975; Kremer and Stadnicky, 1985; Reinink-
Smith, 1987, 1989, 1990, 1995). Reinink-Smith (1987) has reported over 98 ash beds in coals
from the Kenai Lowland (Fig. 3). The goal of this research is to use these ash layers to
establish a chronostratigraphic framework for a continuous section of the upper Kenai Group
along the northern shore of Kachemak Bay and then apply it to subsurface core material and
other outcrop locations in the Cook Inlet Basin.
Once this chronostratigraphic framework is in place, the thermal history of the basin
can be evaluated. Thermal maturation information can be placed in proper stratigraphic
succession allowing for the creation of basin-wide, maturity isopach maps showing areas of
potential coal-bed methane generation and storage.
Geologic Background
Cook Inlet Basin
The Cook Inlet Basin (Fig. 3) is an elongate (110 km x 320 km), northeast-trending,
fault-bounded forearc basin. The basin begins north of Anchorage in the Matanuska Valley
and trends south along the Alaska Peninsula (Kelley, 1985; Swenson, 1997). The basin is
bounded on the north by the Castle Mountain Fault system and to the northwest by the Bruin
Bay Fault system and the magmatic Aleutian arc. The Chugach Terrane and associated
Chugach Mountains and Border Ranges Fault Zone abut the southeastern side of the basin.
The northwest-southeast trending Augustine Seldovia Arch separates the basin into two
depocenters, upper and lower. Over 8500 meters of Tertiary sediments occupy the basin, 6000
meters of which belong to the Kenai Group (Crick, 1971; Fisher and Magoon, 1978).
Kenai Group
Dall and Harris (1892) first used the term “Kenai Group” for coal-bearing strata in the
Cook Inlet area. Barnes and Cobb (1959) measured multiple sections and described the coal-
bearing units, applying the name Kenai Formation to sediments on the Kenai Peninsula.
Calderwood and Fackler (1972) elevated the Kenai Formation to group status and described
and defined five formations (West Foreland, Hemlock Conglomerate, Tyonek, Beluga, and
Sterling formations, see Fig. 1) based on subsurface type sections. These type sections are
distinguished by electric log characteristics and well cuttings, with supporting palynology and
heavy mineral analyses. Fisher and Magoon (1978) removed the West Foreland Formation
because it did not meet the original description of a “coal-bearing unit.”
COOKINLET
?
W est F
orela
n
d
F or matio
nHemlockConglo
mer ateTyon
e
k
F
m
.
B
e
l
u
g
a
F
m
.
Ste
r
ling
F
m
.
Bell
I
s
. s/s
Swenson, 1997
Unnamed
unnamed - till and
alluviumERASystemTimeMaStageSeriesCOOK INLETLowerUpper MaxThicknessmetersClamgulchian
Homerian
Seldovian
Angoonian
Unnamed
Kummerian
Ravenian
Fultonian
Franklinian
UnnamedPLIOCENEMIOCENEQUATERN-ARYOLIGO-CENEEOCENEPALEO-CENESterling
Fm.
Beluga
Fm.
Tyonek
Fm.
Hemlock
Conglomerate
1. Bell Island
Sandstone
2. Tsadaka Fm.
West
Foreland
Fm.
Wish-
bone
Fm.ArkoseRidge Fm.Chicka-loon Fm.1000
450
2135
1525
1850
K E N A I G R O U P?PleistoceneHoloceneT E R T I A R YC E N O Z O I C60
55
50
45
40
35
25
20
30
5
10
15
4
2
1
3
Reinink-Smith, 1995; Fisher and Magoon, 1978unnamed - till and
alluvium
Figure 1. Time stratigraphic chart of Cenozoic units in Cook Inlet Basin showing three stratigraphic
models. Note - Tertiary Series boundaries have been adjusted to 1999 Geological Society of America
Time Scale; time chart is not to scale for the interval 0-5 Ma.PLIOCENEMIOCENEOLIGO-CENEEOCENEPALEO-CENEPleistoceneHoloceneSeriesSterling
Formation
Beluga Formation
Tyonek Formation
Hemlock
Conglomerate
Early Pliocene
Homerian Stage
Upper half of Miocene
Late early Miocene
or middle Miocene
Seldovian Stage
Early Miocene
Late Oligocene
Clamgulchian Stage
??
??
??
unconformity
unconformity
unconformity
unnamed alluvium
and glacial depositsKENAIPENINSULA Stage
Older Tertiary
Rocks
Flores and Stricker, 1993a
DT75-200
DT75-201
7-13-73-6
7-13-73-9
7-13-73-3
Clamgulchian Type SectionMeters
Above
Base
600
500
400
300
200
100
8.9+1.0
7.0+0.7
8.8+0.5
5.9+0.5
7.4+0.7
5.0+0.8
8.5+1.0
6.6+0.7
Plagioclase
K-Ar Age
Hornblende
K-Ar Age
Zircon
Fission-
Track Age
South of Ninilchik at Happy Creek
to Clam Gulch Composite Section
600
500
400
300
200
100
700
800
900
700
UT
S
R
Q PO
N
M
L K
J I
GFE
DC
B
A
7-13-73-1
7-13-73-4
7-13-73-5
DT75-203
DT75-204DT75-202
DT75-210
DT75-211
DT75-212
DT75-209b
DT75-206
DT75-207
DT75-208
6-25-77-1
Homerian Type SectionClamgulchian Reference SectionMeters
Above
Base
7.2+0.6
6.9+0.5
5.4+0.6
Plagioclase
K-Ar Age
Hornblende
K-Ar Age
Zircon
Fission-
Track Age
11.3+0.7 8.8+1.0
8.8+0.9 8.1+1.0 12.9+5.1
8.1+0.7
7.2+1.3
8.1+0.8
7.6+0.7
8.1+1.0
4.2+1.4 4.7+0.6
4.9+0.8
5.6+0.9
Apatite
Fission-
Track Age
4.6+0.7, 8.4+0.7*
7.5+0.6, 7.6+0.6*
30.1+1.8, 32.2+1.9*
11.7+0.9, 14.1+1.1*
8.2+0.8, 9.1+0.7*
Northern Shore of Kachemak Bay Composite Section
Figure 2. Comparison of all available absolute dates for the Beluga and Sterling Formations, Kenai Peninsula.
Letters on Kachemak Bay section correspond to designated coal bed names of Barnes and Cobb (1959).
Dates with asterisk (*) were attributed to apparent detrital contamination and not used in final interpretation
(Turner et al., 1980). Adapted and modified from Triplehorn et al. (1977) and Turner et al. (1980).?Beluga FormationSterling Formation100Kilometers
Seward
Homer
Kenai
Soldotna
Kalgin I.
Augustine I.
Anchorage
KENAI PENINSULACOOK INLETBasin AxisGULF OF
ALASKA
N
KENAI
LOWLANDCOOKINLET
KA CHE
M
A
K
B
A Y
NINILCHIK
CLAM
GULCH
ANCHOR
POINT
HOMER
Tkb
Tks
Tks
Tks
Tks
Tks
Tks
Tks
TksTkb
Tkb
Tkb
Di
a
m
o
n
d
C
k
.Anchor
R
.
Fritz Ck.
McNeil Ck.
Swift Ck.
Fox Ck.
Deep Ck.
Nin
i
l
c
h
i
k
R
.
Homer Spit
Sterling Formation
Beluga Formation
approximate limits of
Homer Escarpment
N
1520 151030'1510
60015'
600
59045'
fault
anticline
Quaternary deposits
Tks
Tkb
Q
Tks
Tks
Tks
ALASKA Tyonek Susitna
Seldovia
C
D
B
A
Figure 3. Regional and geologic maps of Cook Inlet area, south-
central Alaska. Adapted and modified from Lueck et al., (1987);
Reinink-Smith, (1990); Flores et al., (1997).
Se
ld
o
vi
a
Fa
ul
t
1000Kilometers
W ell location and line
of section for Fig. 4
Basin synclinal axial
City
C
2001 Guide to the Petroleum, Geology, and ShallowGas Potential of the Kenai Peninsula, Alaska31
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 32
Several studies have examined the sedimentology of the Kenai Group. The
depositional setting includes braided, anastomosing, and meandering stream systems on a
broad alluvial plain (Hayes et al., 1976; Hite, 1976, 1985; Rawlinson, 1979, 1984; Kremer and
Stadnicky, 1985; Flores and Stricker, 1992; Flores and Stricker, 1993a, 1993b; Flores et al.,
1997). This fluvial setting produced laterally discontinuous beds of interfingering sandstone,
siltstone, conglomerate, and coal.
Problems to be Addressed
Although the Kenai Group has been the subject of repeated sedimentological and
paleontolological investigations, problems with dating, correlation of outcrops, subsurface
continuity, and paleobotanical assessment are unresolved. Development of an integrated
chronostratigraphic framework for these units is proposed in order to resolve existing
correlation difficulties.
Chronology
Three provincial paleobotanical stages with type sections in the Kenai Group are used
for local and regional correlations (Wolfe et al., 1966; Wolfe, 1994; For’yanova, 1985), but
their ages remain poorly constrained. All but one of the published K-Ar dates occur within the
stratigraphic section equal to the upper Beluga and lower Sterling formations. Thus only one
sixth of the total stratigraphic thickness, or about 7 of the ~30 million years, is represented by
existing radiometric dates. Furthermore, the precision of the K-Ar ages is generally greater
than 0.5 Ma, and reversals occur throughout the dated stratigraphic section (Fig. 2). Several
chronohorizons were disregarded due to apparent detrital contamination (Turner et al., 1980).
These poor dates may be the result of the inherent problems of dating plagioclase using the K-
Ar technique. The deposits along the western edge of the Kenai Peninsula have been assigned
to the Clamgulchian Stage (Fig. 3). However, the radiometric data suggests they belong to the
Homerian Stage, based on the dated Homerian/Clamgulchian boundary in Kachemak Bay (Fig.
2). No radiometric data is currently available for the subsurface.
Apatite and zircon fission-track data have been published in conjunction with the K-Ar
data (Turner et al., 1980). In many cases, these fission-track data disagree with plagioclase
and hornblende K-Ar ages from the same ash bed (Fig. 2).
Additional dates are based on the distribution of fossil leaves, fruits, and pollen.
However, many of the palynomorphs are long-ranging genera found in all three of the
paleobotanical stages (Wolfe et al., 1966). The ranges of fossil leaves are determined by
comparison with distant localities that have better age control. Because plants migrate in
response to climate changes, the first and last appearances of plant species may vary widely
between sites. Hence, ages based upon the ranges of leaf species are approximate at best
(Wolfe et al., 1966; Wiggins and Hill, 1987).
Correlation
“One of the primary problems in the Tertiary of the Cook Inlet is the lack of tools for
rapid and widespread correlation. The absence of marine fossils and rapid lateral facies changes
are the chief deterrents to effective correlations. Gross lithologic characteristics, such as thick
coals in the Tyonek, are the only guide.” Hite (1976, p.13)
12000100008000600040002000140001200010000800060004000200020001000080006000400020001400012000100008000600040002000800060004000200014000120001000080006000400020001400012000100008000600040002000140001200010000800060004000200014000120001000080006000400020001400012000100008000600040002000140001200010000800060004000200080006000400020002000600040002000120001000080006000400020001000080006000400020001000080006000400020001000080006000400020001400012000100008000600040002000120001000080006000400020002001 Guide to the Petroleum, Geology, and ShallowGas Potential of the Kenai Peninsula, Alaska33a
b
c
d
e
f
a Pollen Identification
f Pterocarya, Rugaepollis, 1000-8815 ft., middle
or late Miocene to Pliocene
e Ilex, Nyssa, Tilia, Ulmus-Zelkova, 9262-12,168 ft.,
early to middle Miocene
d Non-distinct assemblage, 1000-8815 ft., Oligocene(?)
to "Seldovian" Miocene
c Tiliaepollenites, Polypodiidites, 13,171-13,705 ft.,
early Tertiary and probable Eocene
b Tiliaepollenites, Alnus, 13,733-13,750 ft., probable
Eocene
a Trudopollis, Dinogymnium, 13,889-14,221 ft., upper
Cretaceous
Halbouty Alaska Oil CO.Fritz Creek No. 1Sec 4, T6S, R12W Occidental Petroleum Corp.South Diamond Gulch No. 1Sec 6, T6S, R14W Standard Oil Co. of CA.Anchor Point No. 1Sec 10, T5S, R15W Pennzoil Co.Starichkof State Unit No. 1Sec 22, T3S, R15W Standard Oil Co. of CA.Deep Creek Unit No. 1Sec 15, T2S, R13W Union Oil Co. of CA.Ninilchik No. 1Sec 6, T1S, R13W Marathon Oil Co.Clam Gulch Gulch State No. 1Sec 3, T1S, R13W Union Oil Co. of CA.Kasilof No. 1Sec 28, T3N, R12W Union Oil Co. of CA.Kenai Unit No. 14-6Sec 6, T4N, R11W Union Oil Co. of CA.Sterling Unit No. 23-15Sec 15, T5N, R10W Standard Oil Co. of CA.Soldotna Creek Unit No. 22-32Sec 32, T7N, R9W Richfield Oil Corp.Swanson River No. 1Sec 10, T8N, R9W14.0 miles 5.8 miles 10.7 miles 14.3 miles 8.1 miles 6.5 miles 9.5 miles 11.0 miles 9.8 miles 10.3 miles 10.0 miles
TD 3793 ft
TD 10,568 ft
TD 14,705 ft
TD 8775 ft
TD 14,221 ft
TD 14,940 ft TD 15,011 ft
TD 16,121 ft
TD 15,047 ft
TD 14,852 ft TD 14,550 ft
TD 12,384 ft
Hemlo
c
k
C
o
n
g
.
W est F
o
r
e
l
a
n
d
F
m
.
Tyonek Formation
Sterling Formation
Beluga Formation
H
e
m
l
o
c
k
C
o
n
g
l
o
m
e
r
at
e
W est F
o
r
e
l
a
n
d
Forma
t
i
o
n
Tyonek Formation
Sterling Formation
Beluga Formation
Tyonek
Formation
SterlingFormation
Beluga
Formation
Mesozoic
Rocks
West F
o
r
e
l
a
n
d
Forma
t
i
o
n
Rocks
o
f
Creta
c
e
o
u
s
Age
Rocks
o
f
Creta
c
e
o
u
s
Age
Rock
s
o
f
C
r
e
t
a
c
e
o
u
s
Age
Roc
k
s
o
f
Jur
a
s
s
i
c
Age
Heml
o
c
k
C
o
n
g
l
o
m
e
r
a
t
e
Tyonek Formation
Sterling Formation
Beluga Formation
Rocks ofQuaternary Age Sea Level DatumSea Level DatumSea Level Datum
Type Section for Sterling FormationType Section forHemlock ConglomerateShell Oil CO.
Kustatan River No. 1
Sec 4, T8N, R15W
Pan American Petr. Corp.
West Foreland No. 1
Sec 21, T8N, R14W
Shell Oil Co.
Forelands Channel No. 1 State
Sec 30, T8N, R13W
Pan American Petr. Corp.
Middle Ground Shoal State 18746 No. 1
Sec 35, T8N, R13W
Halbouty Alaska Oil Co.
Bishop Creek Unit No. 11-11
Sec 11, T7N, R11W
Richfield Oil Co.
Swanson River No. 1
Sec 10, T8N, R9W
Forest Oil Corp.
Sunrise Lake No. 1
Sec 15, T8N, R8W
Standard Oil Co. of CA.
Swan Lake Unit 34-27
Sec 27, T8N, R7W
Sinclair Oil and Gas Co.
Swan Lake Unit No. 2
Sec 3, T7N, R6W
7.0 miles 3.4 miles 4.3 miles 12 miles 12.8 miles 6.2 miles 6.2 miles 6.2 miles
Sea Level DatumSea Level Datum
TD 3960 ft
TD 13,500 ft
TD 11,786 ft
TD 10,298 ft
TD 12,384 ft
TD 9034 ft
TD 14,500 ft
TD 11,984 ft
TD 6931 ft
Heml
ock
Cong
.West ForelandFm.Tyonek
Formation
Sterling Formation
Beluga Formation
Rocks of
Quaternary Age
Rocks of
Jurassic Age
Rocks of
Jurassic Age
W est Foreland
Fm.TyonekFormationSterling FormationBeluga FormationRocks of
Quaternary Age
Rocks of
Jurassic Age
Beluga
Formation
Sterling
Formation
Tyonek
Formation
Sterling FormationBeluga FormationType Section for
West Foreland Fm.
A B
DC
Hemlock ConglomerateHorizontal Scale
Kilometers010
South
North
EastWest
Figure 4. Generalized version of Carter and Adkison (1972,
Plates 1 and 2) well-to-well correlation of the Kenai Group
based on electric well logs, cutting logs, and palynology. Self-
potential, resistivity, and gamma curves were not included on
this small scale modification but are available on original
report. See Figure 3 for location of wells.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 34
Due to the fluvial nature of the deposits, units of the Kenai Group are laterally
discontinuous. Lenticular sand bodies and lateral facies changes complicate attempts to
correlate surface outcrops, even over short distances. Hence correlation of outcrops and
subsurface well logs over long distances has proven extremely problematic, despite the battery
of physical and biological correlation techniques employed by various researchers. Studies of
heavy mineral concentrations in subsurface wells (Kirschner and Lyon, 1973; Hite, 1976)
suggest that assemblages are not correlative across the depositional basin. Furthermore,
formation boundaries identified by heavy mineral assemblages generally do not coincide with
boundaries established by log correlations and palynological analyses (Hite, 1976).
Attempts to correlate coal beds locally and across the Kenai Peninsula (Barnes and
Cobb, 1959; Adkison et al., 1975; Hite, 1976; Reinink-Smith, 1989, 1995) have shown that
most coal beds are lenticular or split into multiple seams. Others have been removed by
erosion. In some sections, erosion has removed overlying units until a resistant coal seam was
reached. The erosional surface then migrates along the surface of that coal bed. The
unconformity created by subsequent deposition is difficult to recognize without careful lateral
inspection of the outcrop (Triplehorn, pers. comm, 1999). Thus, different aged coals can be
placed in apparent stratigraphic continuity, and lateral correlation of coal beds based on
thickness, is suspect.
Ash bed partings from coal seams across the Kenai Peninsula have been analyzed by X-
ray diffraction (XRD), geochemical techniques (Direct Current Plasma spectrometer [DCP],
inductively coupled plasma spectrometer [ICP], X-ray fluorescence [XRF], and electron
microprobe), and petrographic methods (optical and scanning electron microscope [SEM]) in
an attempt to achieve regional correlations (Reinink-Smith 1987, 1989, 1990, 1995). These
analyses of whole-rock, coarse-fraction, trace elements, and glass were combined with
stratigraphic relations, inertinite content of coals, and results of prior studies in order to
correlate between outcrop sections. Reinink-Smith (1989, 1995) successfully correlated local
outcrops, but she was unable to achieve correlation of regional Kenai Peninsula outcrops, with
one possible exception. A diagnostic pumice-fragment ash bed in the Sterling Formation
extends from the southeast shore of Cook Inlet to the northern end of Kachemak Bay (Fig. 3)
(Reinink-Smith, 1989, 1995). Despite the distinctive appearance of this ash bed, whole-rock
elemental analyses and prior published isotopic dates from isolated exposures were found to
disagree (Reinink-Smith, 1995), casting doubt on the reliability of this ash for long-distance
correlations.
Basin-wide correlation has also been attempted by comparison of electric logs (Fig. 4)
(Kelley, 1963; Calderwood and Fackler, 1972; Carter and Adkison, 1972). Such e-log
correlations are successful only at the formation level and they commonly have poor resolution
near the edges of the basin, where facies changes are abrupt. Calderwood and Fackler (1972)
noted that the contact between the Beluga and Sterling formations is difficult to distinguish in
some areas. Along the northeastern margin of the basin, the Beluga Formation is missing
entirely or cannot be distinguished from the Sterling Formation (Kirschner and Lyon, 1973).
Furthermore, Carter and Adkison (1972) noted that electric logs from the upper Sterling
Formation and overlying Quaternary deposits are very similar. Thus the top of the Sterling
Formation is difficult to resolve by means of e-log data.
Structural features within the Kenai Peninsula exacerbate correlation difficulties.
Formation contacts in the subsurface vary several thousand feet between wells separated by
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 35
distances of ten miles or less (Fig. 4; Carter and Adkison, 1972, Plate 2). The east-west
trending synclinal structure, apparent on Figure 4, further complicates interpretation of
subsurface data. This synclinal form parallels mapped anticlines on the Kenai Peninsula (Fig.
3), suggesting a north-south compressional regime. This is not consistent with the current
north-south trending forearc basin interpretation (Fisher and Magoon, 1978; Kelley, 1985;
Magoon and Anders, 1992; Swenson, 1997). Furthermore, exposed surface faults with small
or undetermined amounts of displacement have been noted (Barnes and Cobb, 1959; Adkison
et al., 1975; Reinink-Smith, 1989, 1995). These faults are supported by geophysical and
structural studies (Parkinson, 1962; Kirschner and Lyon, 1973; Beikman, 1974; Fisher and
Magoon, 1978; Flores and Stricker, 1992; Magoon and Anders, 1992; Swenson, 1997). The
Seldovia fault (Fig. 3) is particularly problematic, in that it separates the western side of the
Kenai Peninsula from the eastern portion of Kachemak Bay, rendering physical correlation
across this zone virtually impossible (Beikman, 1974; Reinink-Smith, 1990).
Projection of the subsurface Kenai Group type-sections into outcrop is encumbered by
the myriad problems that attend long-distance correlations. Adkison et al. (1975) projected
type sections of the Beluga and Sterling formations into outcrop at Kachemak Bay. These
correlations were based on previous studies, including lithologic criteria (Calderwood and
Fackler, 1972), paleobotanical stages (Wolfe et al. 1966), isopach maps (Hartman et al., 1972),
and surface maps (Kirschner and Lyon 1973). Adkison et al. (1975) determined the location of
the Beluga/Sterling Formation contact by projecting surface features shown on a geologic map
(Barnes and Cobb, 1959) to depth, without mapping the contact in the field. This method
produces acceptable results, placing the boundary between the Beluga and Sterling formations
at approximately the same level as the boundary between the Homerian and Clamgulchian
stages (Wolfe et al., 1966) (Figs. 1, 2). However, these results are in conflict with radiometric
ages of Sterling Formation outcrops on the west side of the Kenai Peninsula. In this location,
the radiometric ages of these deposits suggest they are partially of Homerian age (Fig. 2).
The abundance of conflicting age data suggests that formations of the Kenai Group are
substantially time-transgressive (Fig. 1). Swenson (1997) supports this hypothesis, citing
several internal ARCO reports. Flores and Stricker (1993a) go so far as to place all of the
Beluga Formation within the Seldovian Stage, contradicting all previous interpretations (Fig.
1). However, the existing chronostratigraphic framework is too inaccurate to confirm or deny
the time-transgressive nature of Kenai Group formations.
Goal of the Project and Research Method
The goal of this project is to determine if the methane generation and storage potential
of Cook Inlet Basin can be modeled by incorporating high-resolution chronostratigraphy,
thermal maturation studies, and burial history reconstruction. In order to complete this study,
two components must be evaluated. (1) Methane stored in coal beds can be modeled if the
burial history, rank of the coals (i.e., volume of gas generated), shallow structure (gas traps),
and depth to the coals (pressure acting to hold gas in) are known. Changes in burial depth,
erosion rates, and geothermal gradient affect the distribution of vitrinite reflectance (Ro)
values, methane formation, and potential storage in coal beds. Therefore, a complete
understanding of the stratigraphic relations is necessary to adequately assess methane
production. (2) The stratigraphic architecture of the Kenai Group is complex and little age
control has been reported (Fig. 2). We propose construction of a detailed chronostratigraphic
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 36
framework for the portion of the Kenai Group exposed along the northern shores of Kachemak
Bay on the Kenai Peninsula (Fig. 3). Multiple ash partings in coal beds have been reported
from this section (Triplehorn et al., 1977; Turner et al., 1980; Reinink-Smith, 1987, 1990). By
using precise 40Ar/39Ar dating, we will be able to obtain dates with a precision of
approximately 0.05 Ma throughout the exposed section. Subsurface samples will be gathered
from cores and cuttings housed at the Alaska Geological Materials Center in Eagle River, AK,
the USGS Core Library in Denver, CO, and from the core libraries of several contributing oil
companies (names withheld at this time due to proprietary agreements). Two cores in
particular, the Deep Creek #1 well and the AK 94CBM-1 have published occurrences of coal
and volcanic ash as well as palynological data and coal quality analyses (Adkison and Newman,
1973; Flores et al., 1997). These samples will be analyzed in order to place the subsurface
sections within the chronostratigraphic framework defined by the outcrop data. Samples from
other areas of the Kenai Lowland and Cook Inlet area will also be correlated with these dated
sections.
40Ar/39Ar dating will focus on ash bed partings located in coal seams. These partings
are often less than a cm in thickness as observed in outcrop. Microscopic examination of the
coal seams from cores will be necessary to find these ash partings. Multiple 40Ar/39Ar age
determinations from several coal seams per well or per outcrop and the published age data
from K-Ar and palynology will be used to establish chronohorizons that will allow for the
creation of high-resolution correlation diagrams. These diagrams will identify shallow
structures that may be potential gas traps. Published seismic and structural information will be
used to further constrain these diagrams across the basin.
A thermal stratigraphic framework will be developed by measuring vitrinite reflectance
values. Samples will be collected from coal beds and coaly fossils found in cores and outcrops.
The vitrinite reflectance data will provide the rank and maximum burial depth of the coals.
Given the published geothermal gradient, current depth to and quality of the coals, and the
rank and maximum burial depth as determined from this study, the potential gas generation will
be assessed using BasinMod software.
Once both detailed high-resolution frameworks are complete, the criteria necessary for
coal bed methane generation and storage are known. The thermal framework will then be
superimposed on the chronostratigraphic framework to determine current areas of potential
gas storage. This available information will then be used to create basin-wide, maturity isopach
maps showing areas of potential coal-bed methane generation and storage.
These factors make the Cook Inlet Basin an ideal setting to document and test a high-
resolution chronostratigraphic and thermal maturation model for coal-bed methane resource
assessment.
Research update
Over the past year gas desorption analyses of coal cuttings, outcrop and core sampling,
and 40Ar/39Ar sample preparation has begun. Coal cuttings were collected from several active
exploration and production wells within the northern Cook Inlet basin and desorption and coal
quality analyses are underway. Most of this data is still under proprietary agreement and can
not be discussed at this time. Several weeks were spent sampling outcrop and core holdings for
40Ar/39Ar dateable material and vitrinite reflectance work. Several samples have been irradiated
and are waiting their turn in the analytical line. Funding continues to be a limiting factor for the
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 37
project and much of last year was spent grant writing with limited success. This summer will
see a return to field collection and continued radiometric dating preparation and analyses.
References Cited
Adkison, W. L., Kelley, J. S., and Newman, K. R., 1975, Lithology and palynology of the Beluga and Sterling
Formations exposed near Homer, Kenai Peninsula: U.S. Geological Survey Open-File Report 75-383, 239 p.
Adkison, W. L., and Newman, K. R., 1973, Lithologic characteristics and palynology of Upper Cretaceous and
Tertiary rocks in the Deep Creek Unit well, Kenai Peninsula, Alaska: U.S. Geological Survey Open-File
report, 271 p.
Barker, C. E., Dallegge, T.A. and Clough, J.C., 2000, Coalbed Methane Prospects of the Upper Cook Inlet:
Field Trip Guidebook, Second Alaska Workshop on Coalbed Methane, Anchorage, AK, March 2,2000,
Published as Alaska Division of Geological and Geophysical Surveys Miscellaneous Publications no. 41,
115 p.
Barnes, F. F., and Cobb, E. H., 1959, Geology and coal resources of the Homer district, Kenai coal field,
Alaska: U.S. Geological Survey Bulletin 1058-F, p. 217-260.
Beikman, H. M., compiler, 1974, Preliminary geologic map of the southeast quadrant of Alaska: U.S.
Geological Survey Map MF612.
Berggren, W. A., Kent, D. V., Aubry, M. P., Hardenbol, J., eds., 1995, Geochronology, time Scales and global
stratigraphic correlation: Society for Sedimentary Geology Special Paper No. 54, 386 p.
Calderwood, K. W., and Fackler, W. C., 1972, Proposed stratigraphic nomenclature for the Kenai Group, Cook
Inlet basin, Alaska: American Association of Petroleum Geologists Bulletin, v. 56, p. 739-754.
Carter, R. D., and Adkison, W. L., 1972, Correlation of subsurface Tertiary rocks, Cook Inlet basin Alaska:
U.S. Geological Survey Open-File Report 72-64, 8 p.
Crick, R. W., 1971, Potential petroleum reserves, Cook Inlet, Alaska, in Cram, I. H., ed., Future petroleum
provinces of the United States – their geology and potential: American Association of Petroleum Geologists
Memoir 15, v. 1, p. 109-119.
Dall, W. H., and Harris, G. D., 1892, Correlation papers – Neogene: U.S. Geological Survey Bulletin 84, p.
234-238.
Dallegge, T. A., 1999, Correlation and chronology of the Miocene-Pliocene Bidahochi Formation, Navajo and
Hopi Nations, northeastern Arizona: Masters Thesis, Northern Arizona University, Flagstaff, Arizona, 304
p.
Deino, A., Tauxe, L., Monaghan, M., Drake, R., 1990, 40Ar/39Ar age calibrations of the litho- and
paleomagnetic stratigraphies of the Ngorora Formation, Kenya: Journal of Geology, V. 98, p. 567-587.
Dorr, J. A., 1964, Tertiary non-marine vertebrates in Alaska – the lack thereof: American Association of
Petroleum Geologists Bulletin, v. 48, p. 1198-1203.
Fisher, M. A., and Magoon, L. B., 1978, Geologic framework of lower Cook inlet, Alaska: American
Association of Petroleum Geologists Bulletin, v. 62, no. 3, p. 373-402.
Flores, R. M., Myers, M. D., Stricker, G. D., and Houle, J. A., 1997, Core lithofacies analysis and fluvio-tidal
environments in the AK 94 CBM-1 well, near Wasilla, Alaska, in Geological Studies in Alaska by the U.S.
Geological Survey, 1997: U.S. Geological Survey Professional Paper 1614, p. 57-72.
Flores, R. M., and Stricker, G. D., 1992, Some facies aspects of the upper part of the Kenai Group, southern
Kenai Peninsula, Alaska, in Gradley, D. C., and Dusel-Bacon, C., eds., Geologic studies in Alaska by the
U.S. Geological Survey, 1991: U.S. Geological Survey Bulletin, 2041, p. 160-170.
Flores, R. M., and Stricker, G. D., 1993a, Interfluve-channel facies models in the Miocene Beluga Formation
near Homer, south Kenai Peninsula, Alaska, in Roa, P. D., and Walsh, D. E., eds., Focus on Alaska’s coal
1993: Mineral Industry Research Laboratory, University of Alaska, Fairbanks, p. 140-166.
Flores, R. M., and Stricker, G. D., 1993b, Reservoir framework architecture in the Clamgulchian (Pliocene)
Sterling Formation, Kenai Peninsula, Alaska, in Dusel-Bacon, C., and Till, A. B., eds., Geologic studies in
Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 118-129.
Flores, R. M., Stricker, G. D., and Bader, L. R., 1997, Stratigraphic architecture of the Tertiary alluvial Beluga
and Sterling Formations, Kenai Peninsula, Alaska, in Karl, S. M., Vaughn, N. R., and Ryherd, T. J., eds.,
1997 guide to the geology of the Kenai Peninsula, Alaska: Anchorage, Geological Society of Alaska, p. 36-
53.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 38
Fot’yanova, L. I., 1985, The Seldovian regional stage in Alaska and its analogs in the Soviet far east:
International Geology Review, v. 27, no. 6, p. 669-677.
Hartman, D. C., Pessel, G. H., and McGee, D. L., 1972, Preliminary report on stratigraphy of Kenai Group of
upper Cook Inlet, Alaska: Alaska Division of Mines and Geology Special Report, v. 5, p. 1-4.
Hayes, J. B., Harms, J. C., and Wilson, T. W., 1976, Contrasts between braided and meandering stream
deposits, Beluga and Sterling Formations (Tertiary), Cook Inlet, Alaska, in Miller, T. P., ed., Recent and
ancient sedimentary environments in Alaska: Alaska Geological Society Symposium, Proceedings, p. J1-
J27.
Hite, D. M., 1976, Some sedimentary aspects of the Kenai Group, Cook Inlet, Alaska, in Miller, T. P., ed.,
Recent and ancient sedimentary environments in Alaska: Alaska Geological Society Symposium,
Proceedings, p. I1-I22.
Hite, D. M., 1985, Some sedimentary aspects of the Kenai Group, Cook Inlet, Alaska, in Sisson, Alexandra,
ed., Guide to the geology of the Kenai Peninsula: Anchorage, Alaska Geological Society, p. 3-19.
Kelly, T. E., 1963, Geology and hydrocarbons in cook Inlet Basin, Alaska, in Backbone of the Americas:
American Association of Petroleum Geologist Memoir 2, p. 278-296.
Kelly, T. E., 1985, Geological setting of the Kenai Peninsula and Cook Inlet Tertiary basin, south-central
Alaska, in Sisson, Alexandra, ed., Guide to the geology of the Kenai Peninsula: Anchorage, Alaska
Geological Society, p. 3-19.
Kirschner, C. E., and Lyon, C. A., 1973, Stratigraphic and tectonic development of the Cook Inlet petroleum
province, in Pitcher, M. G., ed., Arctic geology: American Association of Petroleum Geologist Memoir 19,
p. 396-407.
Kremer, M. C., and Stadnicky, George, 1985, Tertiary stratigraphy of the Kenai Peninsula-Cook Inlet region,
in Sisson, Alexandra, ed., Guide to the geology of the Kenai Peninsula: Anchorage, Alaska Geological
Society, p. 24-42.
Larson, E. E., and Evanoff, E., 1998, Tephrostratigraphy and source of the tuffs of the White River sequence,
in, Terry, D. T. LaGarry, H. E., Hunt, R. M., Jr., eds., Depositional environments, lithostratigraphy, and
biostratigraphy of the White River and Arikaree Groups (late Eocene to early Miocene, North America):
Geological Society of America Special Paper 325, p. 1-14.
Leuck, L., Rawlinson, S. E., Belowich, M. A., Clough, J. G., and Goff, K. M., 1987, Kenai Coal assessment
and mapping project, in Rao, P. D., ed., Focus on Alaska’s Coal ‘86, proceedings of the conference held at
Anchorage Alaska, October 27-30, 1986: Mineral Industry Research Laboratory, University of Alaska
Fairbanks, p. 198-210.
Magoon, L. B., and Anders, D. E., 1992, Oil-to-source-rock correlation using carbon-isotopic data and
biological marker compounds, Cook Inlet-Alaska Peninsula, Alaska, in Moldowan, J. M., Albrecht, P., and
Philp, R. P., eds., Biological Markers in Sediments and Petroleum; a Tribute to Wolfgang K. Seifert:
Prentice Hall, New Jersey, p. 241-274.
McClellan, P. H., and Giovannetti, D. M., 1981, New invertebrate fossils, but still no land vertebrates, from
nonmarine Tertiary rocks of the Kenai Peninsula, Alaska, in Albert, N. R., and Hudson, T., eds., The
United States Geological Survey in Alaska; accomplishments during1979: U.S. Geological Survey Circular
823-B, p. 84-86.
Parkinson, L. J., 1962, One field, one giant – The story of Swanson River: Oil and Gas Journal, v. 60, no. 13,
p. 180-183.
Rawlinson, S. E., 1979, Paleoenvironment of deposition, paleocurrent directions, and the provenance of
Tertiary deposits along Kachemak Bay, Kenai Peninsula, Alaska [M.S. thesis]: Fairbanks Alaska,
University of Alaska-Fairbanks, 162 p.
Rawlinson, S. E., 1984, Environments of deposition, paleocurrents, and provenance of Tertiary deposits along
Kachemak Bay, Kenai Peninsula, Alaska: Sedimentary Geology, v. 38, p. 421-442.
Reinink-Smith, L. M., 1987, The origin, mineralogy and geochemistry of inorganic partings in coal seams near
Homer, Alaska, in Rao, P. D., ed., Focus on Alaska’s Coal ‘86, proceedings of the conference held at
Anchorage Alaska, October 27-30, 1986: Mineral Industry Research Laboratory, University of Alaska
Fairbanks, p. 211-228.
Reinink-Smith, L. M., 1989, Origin, character, application and correlation of tephra partings in Tertiary coal
beds of the Kenai Peninsula, Alaska: Ph.D. Thesis, University of Alaska Fairbanks, Fairbanks, Alaska 131
p.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 39
Reinink-Smith, L. M., 1990, Relative frequency of Neogene volcanic events as recorded in coal partings from
the Kenai lowland, Alaska: A comparison with deep-sea core data: Geological Society of America Bulletin,
v. 102, p. 830-840.
Reinink-Smith, L. M., 1995, Tephra layers as correlation tools of Neogene coal-bearing strata from the Kenai
lowland, Alaska: Geological Society of America Bulletin, v. 107, no. 3, p. 340-353.
Swenson, R. F., 1997, Introduction to Tertiary tectonics and sedimentation in the Cook Inlet Basin, in Karl, S.
M., Vaughn, N. R., and Ryherd, T. J., eds., 1997 guide to the geology of the Kenai Peninsula, Alaska:
Anchorage, Geological Society of Alaska, p. 18-27.
Triplehorn, D. M., Turner, D. L., and Naeser, C. W., 1977, K-Ar and fission-track dating of ash partings in
coal beds from the Kenai Peninsula, Alaska: A revised age for the Homerian Stage - Clamgulchian Stage
boundary: Geological Society of America Bulletin, v. 88, p. 1156-1160.
Turner, D. L., Triplehorn, D. M., Naeser, C. W., and Wolfe, J. A., 1980, Radiometric dating of ash partings in
Alaskan coal beds and upper Tertiary paleobotanical stages: Geology, v. 8, p. 92-96.
Wiggins, V. D., and Hill, J. M., 1987, Stratigraphy of Kenai Group, Cook Inlet, Alaska, and application of
ecological shift plot: American Association of Petroleum Geologists Bulletin, v. 71, no. 5, p. 627.
Wolfe, J. A., 1994, An analysis of Neogene climates in Beringia: Palaeogeography , Palaeoclimatology ,
Palaeoecology, v. 108, p. 207-216.
Wolfe, J. A., Hopkins, D. M., and Leopold, E. R., 1966, Tertiary stratigraphy and paleobotany of the Cook Inlet
region, Alaska: U.S. Geological Survey Professional Paper 398-A, p. A1-A29.
Wolfe, J. A., and Tanai, Toshimasa, 1972,
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
1Slide:
Ongoing Coalbed Desorption Studies, Cook Inlet Basin, AlaskaOngoing Coalbed Desorption Studies, Cook Inlet Basin, Alaska
by Charles E. Barker , Todd A. Dallegge (USGS, Denver and Fairbanks) andby Charles E. Barker , Todd A. Dallegge (USGS, Denver and Fairbanks) and
Dan Seamount (Alaska Oil and Gas Conservation CommissionDan Seamount (Alaska Oil and Gas Conservation Commission ))
Abstract: Analysis of core and cuttings from nine wells around the northern edge of the
Cook Inlet basin, Alaska, indicate 50 to 250 standard ft3 (SCF)/ton of biogenic and
thermogenic coalbed methane (CBM). The apparent CBM potential is large: reports
indicate up to 175 ft net coal thickness in portions of the basin buried at<6000 ft. deep.
The sub-bituminous coalbeds in the central and southern portions of the Cook Inlet
basin contain, an average 60 scf/ton based on desorption of cuttings (dry ash-free basis,
DAF). Reports suggest 750 billion tons of pure coal equivalent in these areas and simple
calculation leads to a geologically indicated 45 trillion cubic feet (TCF) of gas in place
(GIP). Adding a correction of 25% for cuttings gas content data to make it comparable to
core data suggests about 60 TCF GIP. The higher rank coalbeds found in the Matanuska-
Susitna (Mat-Su) Valley area contain about 350 billion tons with a gas content that
averages 230 SCF/ton DAF based on desorption of core and cuttings data corrected to
core equivalents. This geologically indicated resource is about 80 TCF GIP.
The indicated total Cook Inlet CBM resource estimate is about 140 TCF gas in place.
If 10% of this resource is accessible for production and 50% of the accessible resource is
recoverable, then the geologically indicated reserve is about 7 TCF. This is a 30 year
supply to South-Central Alaska based on the current 220 BCF/yr consumption.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
2Slide:
IntroductionIntroduction
A major energy issue in the South-Central Alaska area is whether there is enough natural
gas in coalbeds to justify development. South-Central Alaska is strongly dependent on
natural gas for heating and electricity (slide 3) and faces energy shortages of natural gas
(NG) due to declining production capacity and a lack of storage capacity. The NG supply
problems are influenced by a lack of exploration incentive caused by the nation’s lowest
gas prices (slides 4 and 5). The low price is seemingly controlled by long term contracts
and a relatively non-competitive marketing area limited to local area use, export as LNG,
or as fertilizer made from natural gas. In 1998 there was some 2.15 to 2.95 trillion cubic
feet (TCF) of producible conventional gas reserves. South-Central Alaska gas use is now
about 220 BCF/ year. Thus, these 1998 reserves will apparently be depleted in 10 to 14
years (2008 to 2012) if new sources are not found (slide 6).
Although the reserves of natural gas in conventional traps is well known in the Cook
Inlet, coalbed methane (CBM) reserves associated with the conventional gas fields are
poorly known. Reports indicate Alaska contains widespread coal reserves, especially in
the Cook Inlet and North Slope basins (slide 7), where a conventional oil and gas
infrastructure is in place. Thus, CBM developed in the Cook Inlet basin could possibly use
the existing infrastructure to rapidly develop and deliver new gas (slide 8).
Our project addresses the CBM reserve issue by measuring gas content in just-
retrieved gas-bearing coal samples using canister desorption and the modified Bureau of
Mines methods. We sampled coal cores and cuttings from wells drilled solely for CBM
testing and also gleaned samples from oil and gas tests that drilled through shallow
coalbeds (slide 8).
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
3Slide:
Energy Sources, Alaska, 1997Energy Sources, Alaska, 1997
Data Source: EIA (2001) http://www.eia.doe.gov/emeu/sep/ak/
•At times of peak demand, spot gas shortages could
occur by 2003 to 2005.
• Reports attribute initial shortages to a limited gas storage
capacity
• In the future: shortages due to reduced production capacity
Natural gas mostly used for
power generation, LNG
and Fertilizer production.
Oil mostly for fuel and Oil mostly for fuel and
remote power generationremote power generationTrillion BtuTrillion Btu
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
4Slide:
Cook Inlet Quarterly Gas Prices 1995 to 2001
Year
1995 1996 1997 1998 1999 2000 2001 2002Dollars per 1000 ft31.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9 •Delivered gas price
approximately twice
the City Gate price
• Long term contracts
appear to control price
• Commercial users
first to shut down
during shortages
Data Source: AK Tax Revenue Division
Cook Inlet:Cook Inlet:
Recent Gas Price HistoryRecent Gas Price History
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
5Slide:
Alaska's Energy Prices & State RankingAlaska's Energy Prices & State Ranking
(1997, in Dollars per Million Btu)(1997, in Dollars per Million Btu)
820.1529.57Electricity
41.312.18Coal
51*4.622.07Natural Gas
487.826.93Petroleum
29.7311.91Gasoline
488.826.69All Energy
RankUSAAlaska
Source: EIA (2001)
* * Includes Puerto Rico
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
6Slide:
Cook Inlet Gas Proven Reserves 1998Cook Inlet Gas Proven Reserves 1998
286286
528528
119119
8585
UndevelopedUndeveloped(BCF)(BCF)
9.8 (11.1)9.8 (11.1)21502150ENSTARENSTAR
10.4 (12.7)10.4 (12.7)22772277UNOCALUNOCAL
13.4 (13.9)13.4 (13.9)29472947AK DNRAK DNR
11.3 (11.7)11.3 (11.7)24942494Phillips-Phillips-MarathonMarathon
Years to Depletion*Years to Depletion*DevelopedDeveloped(BCF)(BCF)SourceSource
Data Source: DOE Opinion and Order no. 1473 (1999)
*at the current 220BCF/yr rate of consumption. Value in parentheses includesundeveloped reserves but no reserve growth or new discoveries
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
7Slide:
Cook Inlet
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
8Slide:
Map Source: Map Source:
AK DNRAK DNR
Kustastan 1
Coffee Creek 1
GRI Houston 1
Pioneer Wells
Enstar Gas line
Lone Creek 1
AK-94
KenaiKenai
Gas
Oil
Gas pipeline
Conventional
Wells with
CBM testing
CBM wells
Slide: 8Slide: 8
AnchorageAnchorage
WasillaWasilla
Recent
Exploration
In
Cook Inlet
Numerous
Infill or
step-out wells
In this area
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
9Slide:
Desorption MethodsDesorption Methods
Cuttings: During drilling, potential coalbeds can be detected by monitoring drilling breaks
and gas kicks on the geolograph or mud loggers gas detection equipment. The lag time is
monitored so that coal cuttings can be captured at the shale shaker in a timely manner
and the sampled depth can be calculated. The coal cuttings are washed in screens to
remove fines and drilling mud and then immediately placed in pressure tight canisters.
The gas content is measured over time using a modified Bureau of Mines method. The
procedure now includes putting the canisters into temperature regulated water baths that
match the drilling mud temperature. The gas desorbing from the coal cuttings is then
periodically measured and totaled to determine the amount of gas in the coals. For the
first 4 hours, often repeated measurements are required in order to define the lost gas
curve. The lost gas curve determines a correction for the amount of gas lost as the
cuttings are pumped up the well bore and before they are placed into the canisters.
Because of the crushed nature of the cuttings, the amount of lost gas can be large. Thus,
without this ‘lost gas’ correction, accurate measurements of gas contents in cuttings are
not possible. We do not use a sink-float separation of non-coal materials to correct the gas
content because we do not know how to separate the non-coal chips cut from the coal bed
itself (partings) from the materials mixed in as the chips travel up the well bore.
The canisters used for cuttings can use a pressure plug that slides up inside the
canister cylinder to the base of the cuttings sample (Slide 11). This feature minimizes
headspace volume which can introduce errors in P-V-T gas calculations.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
10Slide:
Core: Coring operations were drilled next to existing wells and therefore the depth to the
coal bed was known in advance. The procedure for measuring gas content on coal core is
similar to coal cuttings, except the lost gas correction is computed from the time the core
is lifted off the bottom. Lost gas corrections in conventional coring can also be large
because of the long time required to retrieve the core. A lost gas correction is also
required for accurate measurements of gas contents in core.
The canister for core is selected to fit the core diameter closely to minimize canister
headspace that like in cuttings can introduce errors in P-V-T calculations.
Cuttings versus core: Samples of coal from core are better quality but relatively costly. Of
course, the best sample for CBM analysis are pressure cores that capture the entire gas
and fluid content along with the coal, making lost gas estimates unnecessary. We prefer
core, but use cuttings as well, to make an estimate of gas in place in a realistic time
frame. Studies of gas contents of related core and cuttings samples indicate that a gas
content based on cuttings is generally 25 to 30% lower than a related core value (Nelson,
1999). However, values as high as 40 to 50% are reported. We conservatively apply only a
25% correction to gas content data to make them more comparable to core-based
measurements.
Whether using cuttings or core samples, the canister headspace is measured after
desorption. Headspace is calculated as volume of water required to fill the canister with
the core or cuttings still filling it. A correction for headspace is made in our spreadsheet.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
11Slide: Slide: 11Slide: 11
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
12Slide:
Coal Chemical AnalysisCoal Chemical Analysis
Coal chemical analysis is performed after canister desorption is completed.
Completing desorption measurements may require 1 to 4 months for the Cook
Inlet coals. Cuttings samples are coned and split into quarters for submission to
various laboratory for analyses. Core samples are usually sliced with a saw to
make composite or canister level samples for laboratory analyses.
We routinely run proximate chemistry (ash yield, moisture, fixed carbon,
volatile matter and Btu content), methane adsorption isotherm, vitrinite
reflectance and maceral petrography on composite samples of each coalbed.
Selected coal samples from individual canisters that contain the lowest and
highest gas content or most and least ash may also be analyzed at assess their
effects on gas content.
We also characterize CH4 and CO2 samples taken during desorption using C,
O, and H isotope analyses. Hydrocarbon composition in the natural gas is
measured by gas chromatography analysis. These data are useful in interpreting
a biogenic or thermogenic origin of the gas using tools like the Bernard diagram
(see slide 23).
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
13Slide:
Cook Inlet Basin CBM GeologyCook Inlet Basin CBM Geology
The Cook Inlet Basin is an elongate, northeast-trending, fault-bounded forearc basin that
contains 28,000 feet of Tertairy terrestrial sediment (Swenson, this volume; Slide 14).
The basin is characterized by marginal alluvial fans feeding an axial fluvial depositional
system. Rapid infilling of the basin is indicated by laterally discontinuous, interfingering
beds of sandstone, siltstone, conglomerate, and coal (Slide 15). Coal bed methane
production is strongly dependent upon the thermal maturity and depth to the coal bed.
Optimal coal bed methane generation, maximum storage capacity occurs at a starting
vitrinite reflectance value of 0.6 %Ro (Slide 16). Coalbeds, because of their plastic nature,
tend to lose permeability and have non-economic production levels below a depth of about
6000 ft. These thermal maturity and depth to coalbed criteria suggest the most
prospective areas of the Cook Inlet basin are to the north in the Mat-Su Valley area and to
western and southern edge of the basin where coals are found at less than 6000 ft depth
(Slide 16).
The Cook Inlet basin contains two basic types of CBM prospects: 1) thick
immature coals of the Beluga and Tyonek Formations in the western and southern
portions of the basin; and 2) thick mature coals of the Mat-Su valley area. So far we have
identified only 1 ft to perhaps 15 ft thick, locally-discontinuous coals in the Sterling
Formation. The generally immature Sterling coal, along with the deeply buried Tyonek
coal onshore or offshore, and any coal remote to existing NG pipelines, are not thought
prospective under current market conditions.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
14Slide:
Tectonic Regime, Cook Inlet, Alaska Tectonic Regime, Cook Inlet, Alaska
Modified from Doherty et al. (1994) and Swenson (2000)
Bruin Bay
Fault Border Ranges
Fault
Forearc
Basin with
8.5 km of
Tertiary fill
Basin
A
xis
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
15Slide:
Modified From Swenson (2000), Smith (1995)
lignite, beds to 10 ft thick
sb, beds to 40 ft thick
sbC to hvBb,
beds >50 ft thick
HvBb to A, beds to 34 ft.
Chickaloon Fm. (Mat-Su Valley)
Cook Inlet Tertiary StratigraphyCook Inlet Tertiary Stratigraphy
SUMMARY:SUMMARY:
Two basic CBM plays: Two basic CBM plays:
1. Onshore immature coals1. Onshore immature coals
2. Onshore mature coals 2. Onshore mature coals
CBM SourcesCBM Sources
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
16Slide:
North Middle Ground
Shoal
••Only NE Cook Inlet Only NE Cook Inlet hashas
bituminous coalsbituminous coals
••Depth to 0.6% Ro at lessDepth to 0.6% Ro at less
than 5000 ft depththan 5000 ft depth
••Existing Gas PipelineExisting Gas Pipeline
••230 SCF/ton DAF Gas230 SCF/ton DAF Gas
contents (incl. all Bit. coalcontents (incl. all Bit. coal
data)data)
••Southern andSouthern and
Southwestern Cook Southwestern Cook InletInlet
may also bemay also be prospectiveprospective
Slide modified from Ocean Energy
& Unocal well permit presentations50005000
~ Depth to 0.6% VR
(Smith 1995)100001500010000100005000Wasilla
Anchorage
Kenai
Homer
Slide: 16
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
17Slide:
A Brief History of Cook Inlet CBM ExplorationA Brief History of Cook Inlet CBM Exploration
The Alaska Department of Natural Resources AK-94 well, cored and desorbed in
cooperation with the USGS, was the first CBM test well drilled in Alaska (Slide 18).
Desorption tests of core from this well used USGS canisters and methods modified for
improved temperature control. The results of this test indicating 40 net feet of coal at less
than 1300 ft depth are averaging about 160 SCF /ton DAF (Slide 19). Documentation of
gas-bearing coals in the Mat-Su valley was seminal in the subsequent CBM developments
at Houston prospect and the Pioneer Unit. The location of these developments was
controlled by apparently gas-bearing coal near the surface and a play area crossed by the
major natural gas pipeline supplying South-Central Alaska (Slides 20 and 21). Analysis of
isotherm and desorption data from Pioneer prospect core indicates that the Tyonek
bituminous coal is saturated (Slide 22) with a mixed thermogenic and biogenic methane
source (Slide 23).
In 1997, measurements by the Forcenergy, UNOCAL and USGS of immature sub-
bituminous CBM potential at the Coffee Creek 1 well indicate that the coals there may not
be saturated (slide 24); possibly due to the effect of uplift and cooling (Slide 25) or to
under-estimating gas content from cuttings. The CBM at Coffee Creek appears to have a
biogenic origin (Slide 23). The concern is that under-saturated coals can require pumping
large volumes of water to make the reservoir pressure decrease to the critical desorption
point (Slide 25) for gas production. Water chemistry and disposal of the large amounts of
produced water would be problematic (Slides 26 and 27).
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
18Slide:
DNR AK-94DNR AK-94
Photo by Smith, AK DNR, 1994Photo by Smith, AK DNR, 1994
USGS USGS
DrillDrill
RigRig Pioneer PeakPioneer Peak
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
19Slide:
Gas Content (scf/ton, DAF)
50 100 150 200 250 300Depth (feet)0
200
400
600
800
1000
1200
1400
AK-94 Desorption DataAK-94 Desorption DataAK-94 Desorption DataAK-94 Desorption Data
Key Results:Key Results:
••40 net feet of coal40 net feet of coal
••< 1300 ft depth< 1300 ft depth
••Mixed BiogenicMixed Biogenic
and Thermogenic and Thermogenicgasgas
••Averages about 160Averages about 160SCF /ton DAFSCF /ton DAF
Data from Smith (1995) and AK Div. O&G files
Bituminous CoalBituminous Coal
Core DataCore Data
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
20Slide:
Enstar Gas Distribution System,Enstar Gas Distribution System,
Northern Cook InletNorthern Cook Inlet
Base pipeline map from Lapp Resources. Inc.Base pipeline map from Lapp Resources. Inc.
~Pioneer UnitHouston Prospect
Wasilla
DNR AK -94
OE/UNOCAL
Pioneer
Vine Rd
Wells
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
21Slide:
••Bounded byBounded bytwo activetwo activereversereversefaultsfaults(seals?)(seals?)
••PittmanPittmanAnticlineAnticlinebisects thebisects theunit (trap?)unit (trap?)
••ExhumationExhumationof Bit. Coalof Bit. Coal
to less thanto less than5000 depth5000 depthPittman Pittman Pittman Pittman AnticlineAnticlineStructure at the Base of the Tyonek FormationStructure at the Base of the Tyonek Formation
Slide modified from Ocean Energy & Unocal public well permit presentations
Vine Rd.
Project
GRI Houston 1
DNR AK-94
Castle M
t
n.
F
a
ul
t
Castle M
t
n.
F
a
ul
t
Vine Rd.
Project
GRI Houston 1
DNR AK-94
Pittman Anticline
Castle Mnt Fault
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
22Slide:
Pioneer Coal SaturationPioneer Coal SaturationPioneer Coal SaturationPioneer Coal Saturation
Pressure (PSIA)02004006008001000120014001600CH4 Content(ft3/ton, DAF)0
100
200
300
400
Canister 296 Pres = 840 psi
Desorbed Gas253 scf/ton DAF
Tisotherm = 24oCTres = > 24oC
Coal sample and Isotherm data Courtesy of Ocean Energy and Unocal
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
23Slide:
8 samples from
Coffee Creek 1
five samples from
GRI Houston 3 &
Pioneer 14 and 15
Slide: 23Slide: 23
8 samples
from
five samples
from
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
24Slide:
Pressure (PSIA)020040060080010001200140016001800CH4 Adsorbed (ft3/ton, DAF)0
20
40
60
80
100
120
140
160
180
Coffee Creek 1 CBM SaturationCoffee Creek 1 CBM SaturationCoffee Creek 1 CBM SaturationCoffee Creek 1 CBM Saturation
Tisotherm = 77oF
Tres est. = 62 and 90oF1856 ft.
3960 ft.Sub bit.Tyonek Fm. coal
Coal cuttings courtesy of Forcenergy and UNOCAL
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
25Slide:Pressure (PSIA) or Depth (km)Gas content (SCF/T)Initially
Saturated
Desorbed
gas content
Reservoir PressureCritical DesorptionPressure (CDP) at Saturation
CDP(Unsaturated)
Gas Contents of Saturated vs. Unsaturated Coals
TT22
TT11 Uplift andCoolingCoalbed sealed,
Pres maintained
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
26Slide:
SUBSURFACE WATERS IN COOK Inlet, AK.SUBSURFACE WATERS IN COOK Inlet, AK.
Tertiary strata:
Evolved meteoric Na-HCO3 brine with 3,755 ppm TDS
Mesozoic strata:
Evolved seawater NA-Ca-Cl brine with 19,725 ppm TDS
Reference Waters: Reference Waters: TDS ppmTDS ppm
River water: River water: 100100
Max drinking water: Max drinking water: 1500 1500
Poor irrigation water: Poor irrigation water: 3000 3000
EPA limit, fresh waterEPA limit, fresh water :: 10000 10000
Sea Water: Sea Water: 3600036000
Disposal Scenarios:Disposal Scenarios: Reinjection-likely
Reinjection into zones of < 3000 ppm not allowed by AK State.
Reinjection into zones of 3000 ppm to 10000 ppm by exemption.
Water chemistry based on AGU poster now published as Bruhn and others (2000).
Reference waters mostly from Davis and DeWiest (1966)
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
27Slide:
GRI/GRI/LappiLappi Well Completion Concept Well Completion Concept
Produced Produced
Water InjectedWater Injected
Coal SeamCoal Seam
Well assembly
+ pump drive
SandstoneSandstone
Packer
Annulus gas outAnnulus gas out
Downward Pumping
Progressive Cavity Pump
Ground Level
Gas flow
Water flow
Mudstone seal ?
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
28Slide:
Discussion:Discussion:
Cuttings vs. Core to Estimate CBM Gas in Place?Cuttings vs. Core to Estimate CBM Gas in Place?
Gas contents based on coal cuttings are typically (always?) low compared to
values obtained from core or pressure core samples (Nelson, 1999; among
others). The gas content appears to be lowered primarily because of sample
contamination with drilling additives and admixture with caving materials as the
cuttings are pumped up-hole. We also believe a major problem with cuttings is
that the finer sizes of coal grains cut from the relevant seam are so small that
complete gas diffusion is a rapid process and they lose all of their gas content
while in the hole. These fine bits of now of dead coal (all gas lost on the trip up
the well) plus the well contamination, when placed in the canister do not
contribute gas; but they are included in the coal mass measured in the canister.
Because gas content values are reported normalized to coal mass, those values
that include dead coal or well contamination will be too low.
Unreleased data from Cook Inlet basin on gas contents from core with
cuttings data that include a 25% correction show that the data fields overlap. The
overlap suggests that the corrected cuttings data are comparable with the core
data and apparently useful in estimating CBM gas content.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
29Slide:
2 months2 months
(cuttings)(cuttings)
3 months3 months
(core)(core)
weeksweeks
??
??
??
weeks
weeks
months
Typical
Desorption
Time
SbSb
Hvb - AnHvb - An
SbSb
Hvb - LvbHvb - Lvb
Hvb - LvbHvb - Lvb
HvbHvb
Sb - Hvb
hvCb -mvb
hvb - Lvb
Coal
Rank
None?None?
NoneNone
(Yet)(Yet)
250250
120120
100100
100100
690
140
2000
Avg. Prod.(Mcfd/well)
2002004466Cherokee
35035025252020Black Warrior
250250111155CentralAppalachian
80 sb80 sb100’s (30)100’s (30)115115 Cook Inlet
(Subbit)
230 bit.230 bit.
100’s (30)100’s (30)8080Cook Inlet
(Bit.)
303075754040Powder River
4002410Uinta
2-50040-70(<10)99Raton
43070:(50)84San Juan
Typical GasContent(scf/ton)
Typical NetCoal:Thickestbed (in ft)
Gas in
Place
(TCF)Basin
Base table from Nelson (1999); Cook Inlet data from unpublished sources
Discussion: Comparison of CBM BasinsDiscussion: Comparison of CBM Basins
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
30Slide:
•Gas contents and net coal thickness are similar toproducing CBM basins
•However, some sb coal beds appear to be undersaturated
•350 billion tons of pure bituminous coal so far averaging ~230 SCF/ton DAF of thermogenic gas = 80 TCF
(Note pure coal = in situ coal reserves minus an estimated mean 25% ash yield)
•750 billion tons of pure subbituminous coal averaging ~60SCF/ton DAF of biogenic gas = 45 TCF and corrected tocore equivalents = 60 TCF
•Total geologically indicated CBM ~140 TCF gas in place
•Production and reinjection infrastructure in place
•In 1998, 2.15 to 2.95 TCF of conventional gas reserves.
•At 220 BCF/ year consumption, conventional gas reservesin south-central Alaska will apparently be depleted in 10to 14 years (2008 to 2012).
Cook Inlet CBM: SummaryCook Inlet CBM: SummaryCook Inlet CBM: SummaryCook Inlet CBM: Summary
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
31Slide:
AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements
Government CollaboratorsGovernment Collaborators
Corporate CollaboratorsCorporate Collaborators
Ocean Energy, Forcenergy (now Forest),
Unocal, Phillips, Marathon
Independent Geologists and EngineersIndependent Geologists and Engineers
R. Downey, N. Waechter, D. Lappi and M. Belowicz
J. Clough (AK DNR-DGGS), R. Tingook (USGS) &
V. Webb (AK DNR-DGGS)
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska
32Slide:
ReferencesReferences
Bruhn, R.L., Parry, W.T., and Bunds, M.P., 2000, Tectonics and fluid pressure distribution
in a deformed forearc basin, Cook Inlet, Alaska. Geol. Soc. Amer. Bull., V. 112, no. 4,
p. 550-563.
Davis, S.N. and DeWiest, R.J.M., Hydrogeology: J. Wiley and Sons, New York, 463 p.
Doherty, D.J., 1994, Internal ARCO Report referenced in Swenson, 2000 (see below).
Merritt, R.D. and Hawley, C.C. 1986, Map of Alaska’s Coal Resources: Alaska Department
Of Natural Resources, Special Report 37. 1 Sheet.
Nelson, C.R., 1999, Advances in Coalbed Reservoir Gas-In-Place Analysis: Gas Tips, V. 5,
p.
Smith, T.N., 1995, Coalbed methane potential for Alaska and drilling results for the upper
Cook Inlet basin: Intergas ‘95, Proceedings of Symposium, May 15-19, 1995,
Tuscaloosa, Alabama, 21 p.
Swenson, R. F., 2000, Introduction to Tertiary tectonics and sedimentation in the Cook
Inlet Basin, in Barker, C. E., Dallegge, T.A. and J.C. Clough, 2000, Coalbed Methane
Prospects of the Upper Cook Inlet: Field Trip Guidebook, Second Alaska Workshop on
Coalbed Methane, Anchorage, AK, March 2,2000. Published as Alaska Division of
Geological and Geophysical Surveys Miscellaneous Publications no. 41, 115 p.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 56
Road Log for the 2001 Alaska Coalbed and Shallow Gas Resources Field
Trip to the Kenai Peninsula, Cook Inlet Alaska
by
Todd A. Dallegge, U.S. Geological Survey and University of Alaska, Fairbanks
Robert F. Swenson, Phillips Alaska, Anchorage
Charles E. Barker, U.S. Geological Survey, Denver
Rodney A. Combellick, Alaska Department of Natural Resources, Fairbanks
David L. Brimberry, Marathon Oil Company, Anchorage
Introduction
This field trip covers geologic and economic features related to oil and gas production
in Cook Inlet and on the Kenai Peninsula. We have emphasized new discoveries or insights
into the potential for shallow gas exploration of these areas. Previous field guides contain
considerable additional geologic information that is not reprinted here (Clark, 1981; Winkler et
al., 1984; Triplehorn et al., 1985; Karl et al., 1997). For additional stops and geologic details,
we suggest the 1997 AGS road log (Karl et al., 1997) as a supplement to this guide.
This log is not the traditional mileage marker road log based on odometer readings.
Constant road construction and inconsistencies in vehicle odometers has limited the usefulness
and accuracy of previous logs. We are trying a new technology approach due to the increased
availability and accuracy of the Global Positioning Satellite (GPS) system. This GPS road log
gives waypoint information at all points of interest and intersections on the trip. We hope that
future expeditions to the area can upload these waypoints into their GPS equipment and thus
have a very accurate system to follow this guide. Many features from the 1997 AGS guide
have been cited with GPS information so correlation with this log can be accomplished.
The Kenai Peninsula is a popular recreational getaway for many Alaskans and is a key
tourist destination. Other than great geologic exposures of late Tertiary rocks, the Kenai
Peninsula offers abundant fishing, hiking, boating, and site-seeing activities. Rubber boats and
tide charts are recommended for the beach outcrops and several stops on this trip. Most of the
stops are on state or federal lands but access to some areas off the road right-away are on
private land and permission should be acquired prior to access. Weather in Alaska can be
unpredictable and one should be prepared for freezing conditions and/or wet weather,
especially in the early spring and late summer months (April-May, August-September).
Road Log
This field trip starts from the Anchorage Marriot Downtown Hotel but road log
information does not start until the commercial weigh station on the Seward Highway in
Turnagain Arm. Most roads leading south will eventually intersect this Seward Highway. We
suggest following the New Seward Highway out of Anchorage by any of the multiple east-
west roads that intersect this major north-south thoroughfare. The New Seward Highway will
merge with the Seward Highway.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 57
Day1
From the Anchorage Marriot Downtown take 6th Avenue eastbound (towards the Chugach
Mountains) to Gambell Street. Turn right (south) on Gambell. The road merges with the New
Seward Highway, continue south to the Seward Highway and beginning of road log.
Waypoint 1 Weigh-station on Seward Highway in Turnagain Arm: N61.03641° W149.77579°
Modified Scene acquired on August 8, 1992 with permission from European
Space Agency. Use approved only for educational purposes.
Waypoint 2 Beluga Point: N61.00726° W149.69180°, There is a easily accessible roadside
pullout on the south side of the highway with an information kiosk. Across the highway the
road cut exposes a boulder conglomerate of the McHugh Complex, part of the Mesozoic
accretionary wedge. Clasts in the conglomerate consist of greenstone, argillite, chert,
limestone, siltstone, and gabbro (Karl et al., 1997). Limited age control for the McHugh
Complex has been reported from nearby locales that include a plutonic clast K-Ar age of
146±7 Ma (M.A. Lanphere, quoted in Clark, 1981) and limestone clast conodont age of
Weigh
Station,
Turnagain
Arm
Anchorage
Airport
Cook Inlet
S
Kenai Peninsula
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 58
Meramecian to Morrowan (Late Mississippian to Early Pennsylvanian; Nelson et al., 1986).
About 4.6 miles east of here we cross a low angle thrust and enter the Valdez Group in the
footwall of the thrust (Karl et al., 1997). The Valdez Group is composed of well-bedded,
slightly deformed, Late Cretaceous turbidites. We will be in outcrops of the Valdez Group
for the next ~100 miles. See Bradley et al. (1997) for more detail.
Waypoint 3 Bird Creek: N60.97364° W149.46521°, A popular salmon fishing area during the
summer run.
Stop 1
Waypoint 4 Girdwood Tidal Marsh, AK: N60. 92943° W149.14708°, Nearly all of the trees
visible on this marsh were killed by saltwater intake as a result of subsidence during the
great 1964 earthquake. Most of the 1964-era buildings in the part of Girdwood near the
present intersection of Seward Highway and Alyeska Road were inundated, and many were
subsequently moved to higher ground up Alyeska Road.
The most extensive tidal flooding occurred about two weeks after the earthquake
during the next high spring tides. During the following two decades, repeated tidal flooding
resulted in deposition of several tens of centimeters of silt, restoring the flats to near pre-
earthquake levels. Salt-tolerant grasses now dominate the vegetation on Girdwood flats,
and the marsh surface is flooded by seawater only at extreme high tides.
Visible in tidal-bank exposures at the edge of the marsh is a stratigraphic record of the
1964 event, where postearthquake marine silt overlies peat containing freshwater plants that
were killed by saltwater flooding. The peat is 10-15 cm thick and contains the roots of dead
trees, many of which are still standing. Also visible about 1 m below the 1964 peat layer is a
second layer of stumps rooted in peat, yielding radiocarbon ages with an average age range
of 730-900 cal yr B.P. This forest layer was probably buried as a result of the previous
1964-style great earthquake. Depending on exposure and accessibility, an older buried peat
layer is visible 1-2 m beneath the 730-900-yr layer in the western part of the marsh. This
older layer has a radiocarbon age of 1,170-1,360 yr B.P. and probably records another
previous coseismic-submergence event. Still older peat layers, ranging to a maximum age of
about 4,000 B.P. are visible at low tide in various portions of the marsh and in samples from
a 19-m-deep borehole. Evidence of as many as five pre-1964 great earthquakes appears in
the tidal-marsh stratigraphy at Girdwood and roughly correlate with the ages of uplift
associated with these prehistoric earthquakes along the Gulf of Alaska margin.
PORTAGE TOWNSITE (no stop)
The abandoned townsite marks the location of the main route through the mountains to
Prince William Sound used by native Alaskans and developed by miners in 1902. The
Alaska Central Railway surveyed the route in 1904. The buildings west of the road were
destroyed by high water resulting from subsidence during the 1964 earthquake and partially
buried by tidal silt during the following two decades. The dead trees and remaining old
buildings are protected artifacts of the earthquake.
Stop 2
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 59
Waypoint 5 Portage Creek stop: N60.82734° W148.97679°, At this location, near the axis of
maximum subsidence during the 1964 earthquake, the postearthquake Placer River Silt is up
to 2 m thick. Numerous abandoned buildings in the vicinity are partially filled with silt and,
as at Girdwood, most of the trees on Portage flats were killed by saltwater intake during
high tides following the earthquake. The pre-1964 ground surface, associated peat layer,
and numerous artifacts such as milled wood, cables, and pallets are visible in the bank
exposures downstream from the bridge. Also visible are clastic dikes of sand and gravel that
erupted to the former ground surface as a result of earthquake-induced liquefaction.
Postearthquake tidal flooding eroded most of these dikes to a depth of about 0.5 m below
the 1964 ground surface and replaced the eroded portion with silt. Consequently, few sand
boils were preserved. However, nearly every dike is associated with a break in the peat that
does not extend into the overlying silt, indicating that the sand erupted after the peat formed
but before the silt was deposited. These stratigraphic relationships effectively place the dike
formation at the time of the 1964 earthquake and serve as an excellent analog in the search
for paleoseismic evidence in other areas of the world.
No evidence of pre-1964 coseismic subsidence is visible in the tidal banks at Portage
because any remaining older peat layers are buried under modern tidal-channel deposits.
Additionally, most boreholes drilled in the vicinity of Portage show that tidal sediments have
been eroded by streams and replaced with alluvium over much of the area. However,
continuous core samples from one borehole in unreworked tidal deposits south of Portage
show evidence of as many as seven pre-1964 events during the past 5,000 yr. The
stratigraphic record at Portage, Girdwood, and other tidal marshes along upper Cook Inlet
suggests that 1964-style (Mw 8-9) earthquakes have occurred in the region an average of
every 600-800 yr. This generally agrees with the record of uplift events preserved at Copper
River Delta and Middleton Island along the Gulf of Alaska coast.
Waypoint 6 Portage Glacier Access Road Intersection: N60.81966° W149.46521°, Continue
past this intersection staying on the Seward Highway. The Portage Glacier road headed east
(left) leads to Begich, Boggs Visitor Center maintained by the Forest Service. Portage Lake
and Portage Glacier are popular tourist destinations.
Waypoint 7 Turnagain Pass: N60.78807° W149.21002°, The pass enters a broad glacial valley
that has numerous visible landforms from the late Wisconsin and early Holocene glaciation of
the area, including terminal moraines, lateral moraines, kame terraces, progalcial-lake basins,
and ice-marginal channels and benches (Comebelick, 1984).
Waypoint 8 Hope Junction: N60.78062° W149.43183°, This junction leads to the small
community of Hope, AK. Continue past this intersection remaining on the Seward
Highway.
Waypoint 9 Sterling Highway/Seward Highway Junction: N60.54189° W149.56228°, Take
exit to the right for the Sterling highway. This intersection has changed considerably in the
last two years. The return point is not the same as the exit and comes out a few tenths of a
mile further south of this exit.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 60
Waypoint 10 Cooper Landing: N60.49052° W149.82383°, This waypoint is located at the
Tesoro gas station on the north side of the road (right) in this small community.
Waypoint 11 Kenai-Russian River Ferry Access: N60.48790° W150.00058°, This is another
popular fishing destination. A Forest Service campground is located near here and there are
several cultural and historical sites in this area.
Waypoint 12 Sterling, AK: N60.53783° W150.77445°, Waypoint located at the weigh station.
This community expanded after the discovery of oil and gas at Swanson River north of the
highway. This was the first economic discovery in Alaska with production from this and
surrounding plays continuing today.
Waypoint 13 Sterling Highway/Kenai Spur Road intersection, Soldotna, AK: N60.48717°
W151.05488°, Turn right (north) on Kenai Spur Road. Follow out of town towards Kenai,
AK.
Waypoint 14 Kenai Spur Road/Warren Ames Bridge Access Road, Kenai, AK: N60.55834°
W151.23941°, Continue through on the Kenai Spur Road through town and head north
towards Nikiski.
Waypoint 15 Agrium Agriculture Fertilizer Plant: N60.67393° W151.37236°, This plant is a
major consumer of natural gas from Cook Inlet.
-Of the roughly 8.3 TCF of gas that has been discovered in the Inlet, over 90 % was
discovered prior to 1968 (the year Prudhoe Bay was discovered).
- All of the major fields, which account for over 90 % of the original reserves, were
discovered while drilling for deeper oil objectives.
- Present recoverable reserves (behind pipe) are around 2.7 TCF.
- Present consumption is around 215 bcf/yr.
- Major consumers include: Utilities @ 60 bcf/yr (Enstar, Chugach Electric, ML&P), Urea
Plant @ 54 bcf/yr, LNG plant @ 78 bcf/yr, Field operations and other @ 23 bcf/yr.
- Present contracts call for interruption in industrial supply to satisfy deliverability to local
consumers if the need arises. Enstar's requirements follow a distinct seasonal swing
(obviously based on the weather).
Waypoint 16 Tesoro Road: N60.68679° W151.38211°, This road leads to the main
production facility for the Tesoro Alaskan Refinery. Cook Inlet production supplies most of
the oil refined by Tesoro (72,000 bbls/day total production capacity). A small portion of oil
is tankered in from Valdez. Cook Inlet’s contribution to the total volume of throughput
should improve with production from Forrest’s Osprey Platform.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 61
Historical and Projected Alaska Oil Production
1975 -2021
-
500,000
1,000,000
1,500,000
2,000,000
2,500,000
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
YearBarrels per DayOther NS
Northstar
Colville R
Badami
Duck Island
GPt. McIntyre
Milne Pt
Kuparuk R
PBU.IPA+Sat
Cook Inlet
Waypoint 17 Salamatof Road Junction: N60.68437° W151.38061°, We won’t be taking this
route on this field due to lack of bus turnaround space. This ~0.5 mile drive west (left) leads
to the shores of Cook Inlet and a view of the offloading docks where oil and LNG are
loaded on tankers. The end of the road is at N60.68329° W151.39096°.
Waypoint 18 Nikiski Beach Road: N60.73100° W151.29943°, Turn left (west) onto Nikiski
Beach Road.
Stop 3
Waypoint 19 Nikiski Docks overlook: N60.74162° W151.30343°, At this location we get a
fine view (weather permitting) of Cook Inlet. On the other side of the inlet, the Bruin Bay
Fault bounds the northwestern edge of this forearc basin. Several active arc-volcanoes line
this side of the Cook Inlet. On a good day, over a dozen offshore platforms can be seen in
the inlet. North of Redoubt Volcano, onshore exploration and production are occurring at
the West McArthur River Unit by Forcenergy and Unocal. The sea cliff exposures here are
mapped as Quaternary surficial deposits (Magoon et al., 1976) mostly of glacial origin
(Reger et al., 1995; Reger and Pinney, 1996). Age control its limited to a few radiocarbon
dates of peat at the very top of the section and organic material from an estuarine silt near
the mouth of the Kenai River (Combelick and Reger, 1994; Reger et al., 1996).
Phillips operates several fields in Cook Inlet, North Cook Inlet Gas Field, Beluga Gas
Field, and Lone Creek/Moquawkie Field. The Tyonek platform taps the North Cook Inlet
Gas Field with the entire feed going to the Kenai LNG plant. In July 2000, the Beluga Gas
Field produced 21 net mmcfd for customers in south central Alaska. Based on the Lone
Creek gas discovery well, Phillips and a co-venturer completed an offset delineation well
near the existing Beluga field on the west side of Cook Inlet. This well was determined to
From AK DNR
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 62
be non-commercial, however gas sales from the Lone Creek discovery well are planned for
January 2002. Recent area-wide lease sales by the state of Alaska have allowed Phillips to
increase its undeveloped exploration acreage in the Cook Inlet area by 41,800 acres to a
total of 48,800 acres. Information in the previous paragraph is from the web page:
(http://www.phillips66.com/newsroom/operations/alaska.html#Alaska, 4-17-01)
Alaska
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 63
Return to waypoint 18 and turn right (south) onto Kenai Spur Highway.
Stop 4
Waypoint 20 Phillips/Marathon LNG Plant: N60.67842° W151.37575°, For this field trip we
will be given a short tour of this research facility. Phillips and Marathon are 70/30 partners
in the LNG facility. Both companies provide gas to the plant. Phillips’ supplies gas
exclusively from the Tyonek Platform for this project and operates the plant while Marathon
runs the tankers that deliver LNG to markets in Japan. The facility has operated since 1969
with over 1000 tanker loads delivered to date. Tankers make the round trip voyage every
20 days. The facility processes 1.5 million tons per year of liquefied natural gas.
Photo from http://www.phillips66.com/photolibrary/images/93-57-31.gif
Polar Eagle tanker loading at Kenai Plant
Continue south on Kenai Spur Highway back towards Kenai, AK.
Waypoint 21 Kenai Spur Road/Warren Ames Bridge Access Road, Kenai, AK: same waypoint
as 18, N60.55834° W151.23941°, Turn right (south) onto Warren Ames Bridge Access
Road and cross the Kenai River.
Waypoint 22 Warren Ames Bridge Access Road/Kalifornski Beach Road intersection:
N60.51840° W151.20237°, Turn right (west) onto Kalifornski Beach Road.
Waypoint 23 Kenai Gas Field, intersection into production/office area: N60.47534°
W151.27695° Marathon began operating the Kenai Gas Field in 1993. Gas from Sterling,
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 64
Beluga and Tyonek reservoirs flows to local gas markets and the LNG facility. The field has
produced since 1961 and still has a solid future. Marathon is currently drilling additional
development wells within the field to develop new reserves identified in recent years. The
Glacier Rig #1 (owned by Marathon and operated by Inlet Drilling) has been active in the
field since May 2000. The made for purpose rig is truck mounted, has a significantly
reduced foot print and takes far fewer loads to move between locations. The rig has already
exceeded operational expectations. Drilling projects in its brief history have included
horizontal and extended reach wells at Kenai Gas Field and other Marathon projects.
Photo by T. Dallegge of Rig setup at Beaver Creek
Waypoint 24 Kalifornski Beach Road/Sterling Highway intersection: N60.32429°
W151.25511°, Turn right (west) onto the Sterling Highway towards Homer, AK.
Waypoint 25 Clam Gulch State Recreation Area intersection: N60.23351° W151.38572°,
Beach access available here as well as a nice state campground. We will visit this location
on Day 2.
Waypoint 26 Corea Creek: N60.17211° W151.44353°, At the mouth of Corea Creek in the
beach cliffs a fault is noticeable near the creek.
Stop 5
Waypoint 27 Photo stop: N60.15427° W151.49661°, Nice overlook of Cook Inlet and
volcanoes Redoubt and Illiamna.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 65
Waypoint 28 Crossing of Ninilchik River: N60.05564° W151.64885°
Waypoint 29 Crossing of Deep Creek: N60.03029° W151.68116°
Waypoint 30 Anchor Point community: N59.78439° W151.83179°
Waypoint 31 Diamond Gulch Bridge over Diamond Creek: N59.66973° W151.66712°, Over
550 meters of Beluga Formation is exposed in the beach cliffs at the mouth of Diamond
Creek. Rocks north of the creek dip to the southwest while rocks south of creek are nearly
horizontal. A concealed fault with unknown displacement has been reported here (Karl et
al., 1997). Composite sections of the Beluga Formation in these beach cliffs make up the
Homerian paleobotanical stage designated by Wolfe et al. (1966). Other than a few reported
fish fossils, leaves and pollen are the only paleontological evidence ever reported for the
Kenai Group.
Stop 6
Waypoint 32 Homer overlook: N59.65500° W151.62584°, This is a popular photo stop
overlooking Kachemak Bay and the Kenai Mountains. Kachemak Bay is a deep fiord that is
40 kilometers long and 39 kilometers wide at the mouth. At the end of the Bay is the
commercially developed Homer Spit. Most boating activity, including large commercial
barging comes and goes from the docks on the Homer Spit. This spit formed as a submarine
end-moraine complex by the partial grounding of a tidewater glacier that once occupied
Kachemak Bay (Karl et al., 1997). Icefields and alpine glaciers cover the Kenai Mountains
across the bay. These mountains are part of the same accretionary wedge that we drove past
in Turnagain Arm and Cooper Landing area. The Border Ranges Fault runs along the edge
of the mountains and Kachemak Bay separating the accretionary wedge from the forearc
basin that accumulated the Tertiary Kenai Group. The Beluga Formation is exposed in the
cliffs below and is predominately composed of large sandstone beds with minor mudstone
and subbituminous coal. We will visit these outcrops the following day. On a clear day the
symmetrical cone of Augustine Volcano can be seen at the southern limits of Cook Inlet
some 100 kilometers to the southwest. Across the bay on the southern shores near
Seldovia, AK and continuing further to the west, are limited exposures of the Tyonek
Formation. These deposits are thin and discontinuous and are juxtaposed up against the
Border Ranges Fault system. Fossil plant specimens from these locations were used to
define the Seldovian paleobotanical stage (Wolfe et al., 1966).
Waypoint 33 Homer Business District junction: N59.64297° W151.52283°, Continue straight
through the intersection, the road changes names to Homer Spit Road. A left turn would
put you in the business district of Homer. Follow Homer Spit Road to the “lands end.”
Waypoint 34 Lands End Resort: N59.60130° W151.40797°, This is our destination for the
first day. We will be staying at this resort and enjoying the lovely views of Kachemak Bay
and the Kenai Mountains.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 66
Day 2
From the northeastern edge of the resort we have a view of the hills above Homer and can see
beach cliffs extending up the northern shores of Kachemak Bay. In the hills above Homer,
outcrops of Sterling Formation are visible in stream and road cuts. At beach level the
outcrops toward the mouth of Kachemak Bay are composed of Beluga Formation deposits.
The boundary between the Beluga and Sterling Formations has been proposed near the
mouth of McNeil Canyon up the northeast end of the bay based on projections from the
subsurface (Adkisson et al, 1972). The boundary between the Homerian and Clamgulchian
paleobotanical stage was coincidentally placed near this location (Wolfe et al., 1966) at the
B coal of Barnes and Cobb (1959). Triplehorn et al. (1977) recorded several K-Ar ages
from volcanic ash partings in coal above and below the B coal. They assigned an average
age of 8 Ma for the boundary between the two paleobotanical stages. The depositional
environment for the Homerian age rocks were interpreted as braided-meandering fluvial
systems while the Clamgulchian age rocks were considered meandering fluvial systems
(Rawlinson, 1984). Rawlinson (1979, 1984) determined from provenance and paleocurrent
studies that the source area for the Beluga and Sterling Formations in Kachemak Bay was
from the Kenai Mountains.
Proceed back up Homer Spit Road past waypoint 33 through Homer staying on the Sterling
Highway.
Stop 7
Waypoint 35 Ocean Shores Hotel/Alaska National Wildlife Refuge Visitor Center:
N59.64271° W151.55337°, We will be talking a short walk down the beach to look at the
Beluga Formation and the Cooper coalbed. Access to this beach is across private property
owned by the Ocean Shores Motel, permission is required. Checking a local tide table is
advised prior to attempting this trek. Walk ~0.5 km (25 minutes) down the beach to the
west to at least N59.64218 W151.57886.
The Beluga Formation at this location consists primarily of thick (4.5-25 m), stacked
(3-5 individual units), sharp based, laterally extensive, lenticular sandstone packages that are
interbedded with thin fine-grained units consisting of coal and carbonaceous shale (Flores et
al., 1997). Formation of these sandstone packages is interpreted as deposition from
channelized flows of a composite braided and low-sinuosity meandering stream system
(Flores and Stricker, 1992, 1993a, 1993b).
From this location, several coal horizons can be seen interbedded within the large
channel sandstone complexes of the upper Beluga Formation. The 1.2-1.5 meter thick coal
in the cliffs above is the Cooper coal bed of Barnes and Cobb (1959). This coal bed extends
around to the west toward the Bluff Point-Diamond Gulch areas (Barnes and Cobb, 1959;
Flores et al., 1997). Laterally extensive thick coals such as the Cooper coal bed would make
good shallow coalbed methane targets on the Kenai Peninsula.
Recently the state of Alaska has opened areas across the state for shallow gas
exploration. The requirements for these leases are not as restricted and expense as
conventional leases thus allow for smaller companies to get involved in the search for
coalbed methane (CBM) and other shallow gas resources.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 67
Alaska State Shallow Gas Leases
Non-competitive leases for natural gas and CBM within 3000 feet of the surface
No bonus, only application fee of $500 and annual rental of $0.50 per acre
The royalty is set at 6.25%
The term of the lease is limited to 3 years
Lease is renewable with production
A shallow gas lease may consist of up to 5,760 acres
A lessee may not hold more than 46,080 acres (two townships) within this program
Applications for Shallow Natural Gas Leases:Applications for Shallow Natural Gas Leases:
Arctic National
Wildlife Refuge
N. P. R. A.TAlaskaDillingham
Fort Yukon
Kotzebue
Nome
Mc Grath
Bethel
Talkeetna
Fairbanks
Kenai
Anchorage
Valdezlillve
eier
Townships Containing
SNG Applications
Homer
Seward
Red Dog Mine
Base Map From Alaska DNR (2001)
KenaiKenai
HomerHomer
FairbanksFairbanks
Red Dog MineRed Dog Mine
Wasilla
T
A
P
S
Update on Coalbed and Conventional Shallow Gas Exploration – Homer Area,
(Reference: modified from http://home.gci.net/~lapres/, 4-17-01)
…LAPP Resources, Inc., among others, recently applied for Alaska State Shallow Natural Gas
leases to explore for CBM and natural gas in the Homer area. Homer is not currently
supplied with natural gas and local residents use a variety of fuels including beach coal,
wood, propane, oil, and electricity to provide space heating. The Homer area is not on a gas
pipeline system and the area's 10,000 residents are not thought to be a large enough
economic market to extend the existing gas pipeline system from the Kenai area. If
successful in finding producible gas in shallow wells, it will be possible to provide a cleaner
low-cost energy source for the community. This development plan has the advantage of not
requiring a costly pipeline to the community from the gas fields to the north. Rather, the gas
will be supplied from under or near the town and supply the local market only.
Return to Ocean Shores Hotel
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 68
Waypoint 36 Go north on the Sterling Highway to Diamond Ridge Road: N59.67442°
W151.67464°, Turn right (east) on Diamond Ridge Road and follow it to waypoint 37.
Waypoint 37 Skyline Road junction: N59.66471° W151.56973°, Turn left (north) at the stop
sign on Skyline Road and follow it to waypoint 38.
Waypoint 38 Sterling quarry: N59.66943° W151.56236°, The quarry is located on private
property, permission is advised. This quarry cuts a sandstone-dominated section of the
middle to upper Sterling Formation. The sandstone ranges from volcaniclastic to arkosic
with abundant wood clasts, leaf imprints, and mudstone rip-up clasts noted (Karl et al.,
1997). Net to gross ratios in the Sterling are 60 to 70% with porosity and permeability in
Sterling sands that can get as high as 28% porosity and 2 Darcies of permeability.
Retrace path back through waypoint 37 to waypoint 36 at the junction of the Sterling Highway
and Diamond Ridge Road. Go north (right) on the Sterling Highway headed back toward
Seldotna, AK to waypoint 25 (mile 117.8) at the Clam Gulch Recreational Area. Turn left
(west) at the brown park state sign and follow road to the campground and waypoint 39.
Stop 8
Waypoint 39 Clam Gulch Campground: N60.23918° W151.39433°, take the right into the
campground area, the road continues down to the beach but is narrow and has little area to
turnaround in on the sandy beach. Four-wheel drive is recommended for beach access.
During the off-season, the campground is unattended and we will pull around the loop and
park in the extra parking area near the exit. There is an additional overflow parking area
along the road if needed. Walk down the road to the beach (N60.23990 W151.39909).
Prominently lying in the intertidal zone are several large boulders of igneous rocks
deposited as glacial dropstones probably derived from Alaska-Aleutian Range on the west
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 69
side of Cook Inlet (Reger and Pinney, 1997). Extending north and south from this location
we can see beach cliff outcrops of the lower Sterling Formation. The Sterling here is
composed of ~70% sandstone, 25% siltstone, and 5% coal-shale (Flores et al., 1997) that
has been interpreted to represent meandering stream deposits (Hayes et al., 1976; Flores et
al., 1997).
Wolfe et al. (1966) designated this locality the type section for the Clamgulchian stage.
K-Ar dating of volcanic ash from the coals here places imprecise ages between 9 and 5 Ma
with several ages out of stratigraphic continuity (see Figure 2, Dallegge and Barker, this
volume). Using the more precise 40Ar/39Ar method at the University of Alaska Fairbanks, T.
A. Dallegge hopes to better define the age and lateral and vertical relations of the Kenai
Group stratigraphy within Cook Inlet Basin. Several volcanic ash beds have been collected
from this locality and dateable materials are currently awaiting analysis.
To the south where the beach cliffs bend back toward the west, a black band can be
noted near the beach level. This black band is two coal seams separated by an altered
volcanic ash bed. The strata containing the coal beds are slightly folded into a synclinal
form. The coal beds are ~0.5 meter in total thickness and continue some distance laterally
before they disappear beneath the recent colluvium and vegetation cover along the beach.
Further to the south, near the mouth of Corea Creek (just around the point), fisherman have
mentioned that at extreme low tides, extensive coal beds are exposed by the waves offshore
and large volumes of coal can be ripped up by storms and strewn along the beach. This area
apparently has fairly extensive beds of coal and may be a potential area for exploring for
shallow coalbed methane.
Return to the parking area and proceed back to the junction with the Sterling Highway. Go
north and follow the Sterling Highway past waypoint 24 and on into Soldotna. On the other
side of Soldotna, you will go through the junction at waypoint 13. Continue on the Sterling
Highway back to the Seward Highway and on to Anchorage.
References Cited
Adkison, W. L., Kelley, J. S., and Newman, K. R., 1975, Lithology and palynology of the Beluga and Sterling
Formations exposed near Homer, Kenai Peninsula: U.S. Geological Survey Open-File Report 75-383,
239 p.
Barnes, F. F., and Cobb, E. H., 1959, Geology and coal resources of the Homer district, Kenai coal field,
Alaska: U.S. Geological Survey Bulletin 1058-F, p. 217-260.
Bradley, D.C., Kusky, T.M., Karl, S.M., Haeussler, P.J., 1997, Field Guide to the Mesozoic accretionary
complex along Turnagain Arm and Kachemak Bay, south-central Alaska, in: Karl, S.M., Ryherd, T.J.,
and Vaughn, N.R., eds., 1997 Guide to the geology of the Kenai Peninsula, Alaska: Alaska Geological
Society, Field Trip Guidebook May 8-10, Anchorage Alaska, p. 1-12.
Bradley, D.C., Kusky1, T.M., Haeussler, P.J., Karl, S.M. and Donley, D.T., 1999, Geologic map of the
Seldovia quadrangle, south-central Alaska: U.S. Geological Survey Open-File Report 99-18, map
1:250,000.
Clark, S.H.B., 1981, Guide to bedrock geology along the Seward Highway north of Turnagain Arm: Alaska
Geological Society, Publication no. 1, 36 p.
Comebellick, R.A., 1984, Surficial-geologic map of the Seward D-6 quadrangle, Alaska: Alaska Division of
Geological and Geophysical Surveys Report of Investigations 84-15, scale 1:63,360.
2001 Guide to the Petroleum, Geology, and Shallow
Gas Potential of the Kenai Peninsula, Alaska 70
Combellick, R.A., and Reger, R.D., 1994, Sedimentological and radiocarbon-age data for tidal marshes along
eastern and upper Cook Inlet, Alaska: Alaska Division of Geological and Geophysical Surveys Report
of Investigations 94-6, 60 p.
Flores, R. M., and Stricker, G. D., 1992, Some facies aspects of the upper part of the Kenai Group, southern
Kenai Peninsula, Alaska, in: Gradley, D. C., and Dusel-Bacon, C., eds., Geologic studies in Alaska by
the U.S. Geological Survey, 1991: U.S. Geological Survey Bulletin, 2041, p. 160-170.
Flores, R. M., and Stricker, G. D., 1993a, Interfluve-channel facies models in the Miocene Beluga Formation
near Homer, south Kenai Peninsula, Alaska, in Roa, P. D., and Walsh, D. E., eds., Focus on Alaska’s
coal 1993: Mineral Industry Research Laboratory, University of Alaska, Fairbanks, p. 140-166.
Flores, R. M., and Stricker, G. D., 1993b, Reservoir framework architecture in the Clamgulchian (Pliocene)
Sterling Formation, Kenai Peninsula, Alaska, in Dusel-Bacon, C., and Till, A. B., eds., Geologic
studies in Alaska by the U.S. Geological Survey, 1992: U.S. Geological Survey Bulletin 2068, p. 118-
129.
Flores, R.M., Stricker, G.D., and Bader, L.R., 1997, Stratigraphic architecture of the Tertiary alluvial Beluga
and Sterling Formations, Kenai Peninsula, Alaska, in: Karl, S.M., Ryherd, T.J., and Vaughn, N.R.,
eds., 1997 Guide to the geology of the Kenai Peninsula, Alaska: Alaska Geological Society, Field Trip
Guidebook May 8-10, Anchorage Alaska, p. 36-53.
Hayes, J. B., Harms, J. C., and Wilson, T. W., 1976, Contrasts between braided and meandering stream
deposits, Beluga and Sterling Formations (Tertiary), Cook Inlet, Alaska, in Miller, T. P., ed., Recent
and ancient sedimentary environments in Alaska: Alaska Geological Society Symposium, Proceedings,
p. J1-J27.
Karl, S.M., Ryherd, T.J., and Vaughn, N.R., 1997, 1997 Guide to the geology of the Kenai Peninsula, Alaska:
Alaska Geological Society, Field Trip Guidebook May 8-10, Anchorage Alaska, 128 p.
Nelson, S.W., Blome, C.D., Harris, A.G., Reed, K.M., and Wilson, F.H., 1986, Late Paleozoic and Early
Jurassic fossil ages from the McHugh Complex, in: Bartsch-Winkler, S., and Reed, K.M., eds.,
Geologic Studies in Alaska by the U.S. Geological Survey during 1985: U.S. Geological Survey
Circular 978, p. 60-64.
Rawlinson, S. E., 1979, Paleoenvironment of deposition, paleocurrent directions, and the provenance of
Tertiary deposits along Kachemak Bay, Kenai Peninsula, Alaska [M.S. thesis]: Fairbanks Alaska,
University of Alaska-Fairbanks, 162 p.
Rawlinson, S. E., 1984, Environments of deposition, paleocurrents, and provenance of Tertiary deposits along
Kachemak Bay, Kenai Peninsula, Alaska: Sedimentary Geology, v. 38, p. 421-442.
Reger, R.D., Combelick, R.A., and Brigham-Crette, J., 1995, Update of latest Wisconsin events in the upper
Cook Inlet region, southcentral Alaska, in: Combelick, R.A., and Tannian, F., eds., Short notes on
Alaska geology 1995: Alaska Division of Geological and Geophysical Surverys Professional Report
117, p. 33-45.
Reger, R.D., and Pinney, D.D., 1996, Late Wisconsin galciation of the Cook Inlet region with emphasis on
Kenai Lowland and implications for early peopling, in: Davis, N.Y., and Davis, W.E., eds.,
Adventures through time: Readings in the anthropology of Cook Inlet, Alaska: Anchorage, Cook Inlet
Historical Society, p. 15-35.
Reger, R.D., and Pinney, D.D., 1997, Last major glaciation of Kenai Lowland, in: Karl, S.M., Ryherd, T.J., and
Vaughn, N.R., eds., 1997 Guide to the geology of the Kenai Peninsula, Alaska: Alaska Geological
Society, Field Trip Guidebook May 8-10, Anchorage Alaska, p. 54-67.
Wolfe, J. A., Hopkins, D. M., and Leopold, E. R., 1966, Tertiary stratigraphy and paleobotany of the Cook Inlet
region, Alaska: U.S. Geological Survey Professional Paper 398-A, p. A1-A29.