HomeMy WebLinkAboutPreliminary Conceptual Report Tanakee Geothermal 2012PRELIMINARY CONCEPTUAL REPORT
For:
TENAKEE INLET GEOTHERMAL
RECONNAISSANCE PROJECT
Prepared for: /EALASKA
(> ENERGY AUTHORITY
813 West Northern Lights Boulevard
Anchorage, Alaska 99503
Prepared by:
FDL HATTENBURG DILLEY & LINNELL
Engineering Consultants
3335 Arctic Boulevard, Suite 100
Anchorage, Alaska 99503
On Behalf of:
a IPEC
INSIDE PASSAGE ELECTRIC COOPERATIVE
P.O. Box 210149
Auke Bay, Alaska 99821
(907) 789-3196
DECEMBER 2012
TABLE OF CONTENTS
1.0 INTRODUCTION...........::cccesecccseccceeeceeeeccesecsseecneeeeueeeueeeueneanseaeeees 1
2.0 SITE AND PROJECT DESCRIPTION. ..........2:ccssccceseceeeeccecceesseeeeeeeeeeees 1
FIELD WORK ........ccccceceesseceeeceseeceeeeesseeenaaaeeeeeeesceceueuseeeeeeeeseeeeeeeusseeeessetececsectseceeeueeeeeeeeetess 3
3.0 REGIONAL CHARACTERISTICS ..........02:cccsseccseecceececeeseseeeeseeceeeseeeeees 4
GENERAL GEOLOGY..........0.ceesceeseseessseessseeeseecsseecsseesseesssecessescseecesececseccrseccsseeceeecsssesresssesentees 4
STRUCTURAL GEOLOGY .......0.cececsesseeeseseeeeceeeseeeseesececsueecesseeeceesueeececsseccecauecesersaseeneetaeesentsaeess 4
Ce ON 0 4
SOIL TEMPERATURE.........ccccccccccssecceeceeseseeeneeeeeeeeeeeeseeeseaeeeeecececsesesesuaeceseeseceeseccsseeesceeeceeeeeeees 4
SOIL DATA 5
WATER DATA 5
CO, GAS SURVEY 9
LINEATIONS 9
LIST OF FIGURES
Figure 1 Location Map
Figure 2 Site Map of Hot Springs Area
Figure 3 Shallow Soil Temperature Contour Map
Figure 4 Soil Chemistry Map
Figure 5 Plots of Water Chemistry
Figure 6 Isotope plot and Geothermometry
Figure 7 Major Lineation Map
Figure 8 Preliminary Conceptual Model
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PRELIMINARY CONCEPTUAL MODEL
TENAKEE INLET GEOTHERMAL RESOURCE
TENAKEE INLET, ALASKA
1.0 INTRODUCTION
The purpose of this report is to present the preliminary conceptual model for Tenakee Inlet
Geothermal Resource located at Tenakee Inlet, Alaska. The preliminary conceptual model
presents our interpretation of the geothermal resource. A conceptual model is developed based
on the available information as oppose to targeting anomalies. The most important part of the
model is a predicted pattern of isotherms. Inferring the isotherm pattern is challenging with
limited subsurface data. As more data is obtained through wells and/or geophysics then the
isotherm pattern can be better constrained in the model. This report presents our
interpretation of the data used to develop the model and identifies the variance possible within
the model. An interim data report was developed for this project titled “Interim Report for Field
Exploration and Laboratory Analyses, Reconnaissance Study of Tenakee Inlet Geothermal
Resource”, dated December 2011. Additional field data was collected through 2012 and was
also used in developing this model.
2.0 SITE AND PROJECT DESCRIPTION
The Tenakee Inlet geothermal resource is located near the head of Tenakee Inlet on Chichagof
Island in Southeast Alaska, approximately 19 miles southwest of Hoonah along an un-named
river we have called Tenakee Creek. Figure 1 presents a location map for the hot springs. The
area is characterized by rugged, steep terrain covered with thick vegetation typical of the
southeastern Alaska rainforest. Topography limited the exploration area to the valley floors and
to the first bench above the river valley. The resource is characterized at the surface by at least
four small hot springs that occur together on the southeast side of the Tenakee Creek located at approximately 57° 59’ 24” N and 135° 56’ 20” W.
The Tenakee Inlet springs are comprised of four small springs that flow from the base of a rock
cliff approximately 40 to 50 feet in height. The hot springs area is small about 50 feet long by 20
feet wide occurring on a gravel bar that is heavily vegetated with alders, willows, and spruce
trees. The gravel bar is approximately 800 feet long and 100 feet wide. The hot spring site and
the location of the four hot springs are shown in Figure 2. There is an outflow creek from the
spring site that leads to Tenakee Creek. A stream named the Stairway to Heaven Creek cascades
down the slope and mixes with the outflow near the spring sites. Seeps occur along the shore of
the gravel bar and are periodically inundated by Tenakee Creek.
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UL pa ed
AEROMETRIC 2010 ORTHO PHOTO, rd
aoara ASP ZONE Mee
- FOUND HOT-SPRING ~ LAT: 57°59'20" N= LONG; 135°56'24" W
Figure 1: Location map for Tenakee Inlet hot springs. Hot springs located approximately 19
miles southwest of Hoonah Alaska in southeast Alaska. There is a number of hot springs on
Chichagof Island as shown by black circles on the vicinity map.
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DK STAIRWAY TO HEAVEN CREEK A“.
Figure 2: Site map of hot springs area. Note the location of the four hot springs, the seeps at the
edge of Tenakee Creek and the outflow from the hot springs. The first bench located above the
hot springs is approximately 40 to 50 feet higher than the base of the slope. The field sampling
grid is partially drawn for reference. The hot springs occur at grid point A4.
Field Work
The initial fieldwork in 2011 consisted of collecting shallow soil temperature data, as well as soil,
water and rock samples from various locations surrounding the hot springs and the immediate
vicinity. A grid was established to systematically collect temperature data and soil samples.
Water and rock samples were more varied and were dependent upon their location with respect
to the hot spring. Follow-on fieldwork in 2012 consisted of additional shallow soil temperature
surveys, CO gas survey, thermal infrared imaging, and stream gaging activities.
The interim report discusses in depth the methodology of how the various data was collected.
Over the course of all field work, HDL collected 63 soil samples, 37 water samples, 20 CO,
samples, and over 120 temperature data points.
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3.0 REGIONAL CHARACTERISTICS
General Geology
The Tenakee Inlet area is composed of Devonian argillite, greywacke and limestones that were
subsequently intruded by a wide variety of igneous rocks (Loney, et al 1975). These rocks
outcrop near the study area and north of it. The intrusives vary in age, but are primarily
Cretaceous in the study area and are mainly diorite to granodiorite in nature. These rocks are
widely distributed on Chichagof Island. To the south of the study area there is a large body of
Tertiary intrusives consisting of hornblende leuoconorite and troctolite. The Devonian
sedimentary rocks have undergone extensive regional and contact metamorphism. The
intrusives have metamorphosed the Devonian sedimentary rocks into hornfels, and marbles.
The rocks are intensely folded and faulted. The fold axes trend northwest.
Structural Geology
The geologic structure of the area is dominated by the Queen Charlotte-Fairweather (QCF) fault
system and the Chatham Strait Fault. The QCF fault system lies to the immediate west of
Chichagof Island and the Chatham Strait Fault defines the Chatham strait between Chichagof
Island and Admiralty Island to the east. The faults of the QCF system are active right-lateral
structures with large displacements. The Chatham Strait Fault offsets rocks as young as middle
Tertiary and by as much as 90 miles. (Gehrels and Berg 1994).
The QCF fault system defines the boundary between the Pacific and North American plates. In
the middle Mesozoic prior to and/or concurrent with the intrusion of the igneous rocks in the
study area, southeast Alaska was involved in the subduction of the Pacific Plate beneath the
North American Plate, which over time evolved into the dominant transform plate boundary
seen today. This tectonic activity has resulted in a complicated pattern of thrust, oblique slip,
and strike-slip faults on Chichagof Island. The rocks in the study area are part of the Alexander
Terrane, which is inferred to have continental origins (Karl, 1999). The rocks are interpreted to
represent intermittent volcanic arc activity, similar to the modern day Aleutian Islands.
Modern earthquake activity occurs along the QCF fault system. The most recent large
magnitude earthquakes in the area of the hot springs occurred in 1927 and 1939. The epicenter
of the 1927 magnitude 7.1 event occurred at latitude 57.69 and longitude -136.07. The 1939
magnitude 6 event occurred at latitude 58.00 and longitude -136.0. The hot springs are located
at latitude 57.99 and longitude -135.939. The occurrence of other hot springs on Chichagof
Island may also be due to this structural framework that has produced numerous faults and
permeable rocks.
4.0 DATA
Soil Temperature
The shallow soil temperature data obtained are presented in Figure 3. The hottest
temperatures occurred near the hot springs and at the seeps found at the edge of Tenakee
Creek. Temperatures near the hot spring range from 81.2 to 108.9 °F. The hot springs outflow
had soil temperatures of between 58.3 and 86.1°F. Seeps were observed when the water level
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in Tenakee Creek was lowered during a few days of no rain. The one seep had a nearby soil
temperature of 130.5 °F. Temperatures on the hillside above the spring ranged from 49.9 to
46.4 °F. A relatively cool temperature of 44.3 °F was measured upstream of the hot springs
located near the edge of the gravel bar that hosts the hot springs. The temperature readings in
the 40’s were considered background soil temperatures.
Additional temperatures above background were encountered at several spots across Tenakee
Creek at the base of the slope. The hottest shallow soil temperatures across the creek from the
hot springs were 88.8 °F and 59.5°F. There were several points across the creek above 50 °F
with one (56.5 °F) occurring about 1,200 feet downstream of the hot springs. These
temperatures do not appear to be the result of outflow from the hot springs. The temperature
of the water in Tenakee Creek was approximately 40°F.
Soil Data
The soil chemistry was plotted for six elements: Arsenic (As); Cobalt (Co); Gold (Au); Manganese
(Mn); Titanium (Ti) and Vanadium (V). These chemical species had orders of magnitude changes
in concentrations across the sampling area. Data were contoured using roughly the standard
deviation in a particular elements concentration. Mercury is usually used in geothermal
exploration; however, the results did not indicate a large variation in mercury concentration.
Figure 4 presents the soil chemistry.
The species plotted indicated anomalous concentrations generally near the hot springs and
along the outflow but also across Tenakee Creek where the concentrations were higher in areas
of elevated soil temperatures. The highest concentration of gold was near the confluence of the
hot spring outflow and Tenakee Creek. The highest concentration for arsenic was across the
river from the hot springs at the grid point that recorded the highest temperature on that side
of the river. In addition, vanadium had higher concentrations along the ridge above the hot
springs perhaps indicating a fracture or fault.
Water Data
The chemical concentrations for the hot spring, seep, and surface water samples were analyzed
and the temperature of the fluids obtained. The temperature of the hot spring waters averaged
170°F with Hot Spring #1 having the hottest temperature of 177 °F on two sampling events and
Hot Spring #4 having the coldest at 161°F. The average water temperature for hot springs #1
through #3 was 172°F. The average surface water temperature in adjacent Tenakee Creek was
40°F.
a
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LEGEND
@ = SAMPLE POINT
196 SAMPLED TEMPERATURE
—10— CONTOUR INTERVAL
pec ann /
%; 1 *., t oo me 7.
eg
Figure 3. Shallow soil temperature contour map. Note higher temperatures occur at the hot springs, along the seeps, and across the creek.
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LEGEND
@ SAMPLE POINT
196 SAMPLED TEMPERATURE
—10— CONTOUR INTERVAL
--'7 EDGE OF BRUSH - \. oe EPA ra “Ae
i @ ve “ tog ¢
e #9
Arsenic ® Gold &
Manganese &
Cobalt 3
Vanadium @
Titanium @
HOT SPRINGS
e @ @ @. °, e “ gv 5 M5 5 25
Figure 4: Soil chemistry data with color circles indicating location of samples with significant concentrations of that particular element.
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A spreadsheet developed by Powell and Cummings (2010) was used to evaluate the chemistry of
the water samples. Laboratory data were entered into the spreadsheet and a series of standard
geothermal plots were developed. Geothermometers were calculated and ternary plots were
produced. The chlorine-fluorine-boron (CL-F-B) plot shown in Figure 5 indicates that the
collected hot spring waters (HS) and the surface water (SW) samples are from different
populations. This is important in that the two waters clearly represent separate types of fluids.
The often-used chlorine-sulfate-bicarbonate (Cl-SO,-HCOs) ternary plot illustrates the amounts
of major anions present in the geothermal waters (Figure 5). This plot indicates that the hot
spring waters are low in chlorine (Cl) and bicarbonate (HCOs) and high in sulfate (SO,). It also
indicates that the hot spring waters are associated with volcanic waters and perhaps heated by
steam from a deeper reservoir. A high sulfate spring is typically associated with deeper boiling
zones.
cl
iia
= S10.
25F 258 Stoam Heated Waters
Figure S: Plots of water chemistry data. The hot springs water (HS) is clearly different from the
surface water (SW) samples collected. The hot springs waters are high in SO, and low in HCO;
and Cl indicating possibly waters associated with volcanic waters.
The isotope plot (Figure 6) indicates that both the hot springs and surface waters are primarily
meteoric and have not mixed with other fluids. The chalcedony geothermometer provides a
more accurate temperature for the hot spring fluid at depth based on the concentrations of
silica and potassium/magnesium (Figure 6); it shows that the hot spring fluids have been heated to 260°F.
LL
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HCO3
log (K*7Mg)
100
200
wo
| ‘i nies \
600
Delta Deuterium - per mil SiO, mg/kg -120
140
-160 4 22 -20 18 -16 -14 12 40 8 6 4 2 0 2 4 6 8 10
Delta Oxygen 18 - per mil
Figure 6: Isotope plot on the left indicates that the hot springs waters are primarily meteoric.
The geothermometry was based on the chalcedony geothermometry due to the concentrations
of silica and potassium/magnesium as shown in the plot on the right.
CO, Gas Survey
The carbon dioxide (CO2) gas survey consisted of collecting soil gases at a number of locations
near both the hot springs and across the creek. Carbon dioxide gas is typically associated with
geothermal systems and faults that leak the gas upwards. Twenty gas samples were collected.
Four of the samples had carbon dioxide concentrations on the order of 10,000 parts per million
or greater with the highest concentration of 16,100 parts per million. The remaining samples
had carbon dioxide concentrations ranging from 643 to 8,650 parts per million. Three of the
four high concentrations occurred along the base of the bluff on the northwest side of creek
across the creek from the hot springs. The fourth high concentration occurred near the hot
springs.
Lineations
Lineations were determined from stereographic aerial photographs and may represent faults or
joints. The lineations were not observed on the ground due to the dense vegetation, however
during the helicopter flights over the area, many of the lineations could be seen on a regional
scale. Figure 7 presents the more notable lineations and the course of Tenakee Creek. The
lineations are typically aligned northwesterly with some cross lineations. This alignment is
typical over the entire southeast region and is due to the large QCF fault system and regional
tectonics. Particularly interesting is the offset in Tenakee Creek near the hot springs. There is a
set of lineations that occur northwest and the creek is offset on east-west lineations. The
measurements obtained from geological maps indicated steeply dipping lineations.
a
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!
a Figure 7: Major
: lineations in the ~ Tis ; study area. Note aa f the offset of
INFERRED REGIONAL FAULT a } Tenakee Creek é Ve , a near the hot
XN /: % he springs possibly iS We \ a indicating a
AEH hw Ne i ~: | wrenching effect
\ \ nm Ved 1 7 NORTHEAST BOUNDARY STREAM creating
\ nh Dl #.. a permeability for
‘ eA ST \ the springs.
7 AS - <i \
X 7 N \
\ Vd \ \ \ Z . vi iti Nl : LEGEND i \\ Bounty STREAM \. === tnesnons
Va \ fh £ STRIKE DIP \
5.0 PRELIMINARY CONCEPTUAL MODEL
In the Tenakee Inlet Area, based on shallow temperature probe and soil analysis data there
appears to be additional thermal areas across Tenakee Creek from the known four hot springs.
These thermal areas would suggest that the geothermal source is larger than just the known
four hot springs. The occurrence of chemical anomalies in the soil in the hotter areas across
Tenakee Creek also suggests that the hot fluids are circulating near the surface indicating
permeability.
The lineations and general tectonics of the region suggest that the hot springs were developed
due to the wrenching of the cross cutting lineations near the hot springs which led to the
fracturing of the rocks. Also given the high angle nature of many of the lineations, it is
reasonable to assume that high angle faults bring the geothermal fluid to/near the surface. The
earthquake data suggest that the study area is tectonically active and that the igneous intrusives
are permeable.
Based on the water chemistry, the hot springs fluids are most likely associated with volcanic
waters and perhaps heated by steam from a deeper reservoir. The chalcedony geothermometer
indicates that the hot spring fluids have encountered temperatures on the order of 260°F. The
average surface temperature of the hot spring waters is 170°F. These surface and subsurface
temperatures are in the range that binary geothermal power plants operate. Much like Chena
the site benefits from having cool waters at approximately 40°F as a sink. The geochemical
ee
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analysis of the spring has yielded a possible maximum temperature of the source water at a
depth of 260° F (127° C).
Based on our reconnaissance efforts we have developed the conceptual model of the resource
shown in Figure 8. The upper limit on isotherm values is the boiling point of water versus depth.
The minimum depth for 260° F (127° C) would be fairly shallow at less than couple of hundred
feet. Depending upon the localized geothermal gradient the 260° F could occur at a deeper
depth. A high angle fault has allowed for the hotter, deeper waters to move upward creating
the hot springs. The ultimate source of heat is fluid up flow in fractures. The hot spots occur
due to splays in the primary fault that either reach the surface (the case of the seeps) or come
close to the surface (the hot zone across the creek). The carbon dioxide results seem to suggest
faults that connect to the deeper system across the creek from the hot springs. Tenakee Creek
as a source of cold water may cool the system near the surface but does not appear at this point
to cool the overall system. There does not seem to be a significant change in the flow regime of
Tenakee Creek downstream of the resource compared to upstream of the resource. The outflow
of the system is downstream towards the north following the general strike of the lineaments in
the region and along the creek. The heat source is not a typical magma body as seen in places
like Akutan or Mount Spurr but rather hotter deeper fluids associated with deep crustal
materials. The Queen Charlotte/Fairweather fault system is a major transform plate boundary
with high angle faults that cut through the crust. The Cretaceous igneous rocks provided heat
during their emplacements and are still cooling as indicated by high heat flows in the region
(SMU maps)
A temperature gradient reported by Economides in 1982 for the separate resource
(investigated by shallow wells) approximately 30 miles away at Tenakee Springs indicates a
temperature gradient of 13° C/100 feet. If we assume a similar gradient with surface
temperatures at about 45° F (7° C), then at the same gradient, the temperature of 260° F (127°
C) would be reached in less than 1,000 feet. This is probably over optimistic; however it
suggests a shallower resource than a deep resource.
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EXISTING GROUND
HOT SPRING
176°
TENAKEE CREEK ASSUMED HORSE-TAIL 90° FAULT SYSTEM
110°—__—_———
130°—<—<—$<—<—
157 ee
$F
190° —<—————
FO
NOTES 260°
1. TEMPERATURES IN FAHRENHEIT
2. TEMPERATURE CONTOUR INTERVALS
20° FAHRENHEIT
LEGEND:
== COLD WATER RECHARGE
=e HOT WATER UPFLOW
Figure 8: Preliminary conceptual model of the Tenakee Inlet Geothermal Resource. Note the
high-angle fault allowing for upward flow of fluid.
6.0 CLOSURE AND LIMITATIONS
The analysis and conclusions included in this report are based on conditions as they exist in the
literature and from the gathered field data. The conceptual model presents our understanding
of the system at this time. With limited subsurface data, the model is preliminary. As more
subsurface data is obtained, the model will most likely change to reflect the new data.
i
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This preliminary assessment is based on the scientific studies conducted to date and is specific
to the purpose of positioning a possible power plant for a potential geothermal resource. Use of
this report for a purpose other than its intent should be limited.
Prepared by: Reviewed By:
Hattenburg Dilley and Linnell, LLC Hattenburg Dilley and Linnell, LLC
Lorie M. Dilley, PhD, PE/CPG John Fritz
Principal Geologist Senior Geologist
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7.0 REFERENCES
Cumming W. (2009) Geothermal Resource Conceptual Models Using Surface Exploration Data.
Proceedings Thirty-fourth Workshop on Geothermal Reservoir Engineering Stanford University.
Stanford California
Gehrels G.E. and H.C. Berg (1994) Geology of Southeastern Alaska. The Geology of North
America Vol. G-1. The Geological Society of America.
Karl, S.M. (1999). Preliminary Geologic Map of Northeast Chichagof Island, Alaska. US Geological
Survey Open File Report 96-53.
Loney, R.A, D.A. Brew, L.J.P. Muffler and J.S. Pomeroy. (1975) Reconnaissance Geology of
Chichagof, Baranof, and Kruzof Islands, Southeastern Alaska. US Geological Survey Professional
Paper 792.
Motyka, R.J., M.A. Moorman, and S.A. Liss, (1983), Geothermal Resources of Alaska: Alaska
Department of Geology and Geophysical Survey, Miscellaneous Publication 8, 1 sheet scale
1:2,500,000.
Powell, T and W. Cummings, (2010). Spreadsheets for Geothermal Water and Gas Geochemistry.
Proceedings Thirty-fifth Workshop on Geothermal Reservoir Engineering Stanford University.
Stanford California
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