HomeMy WebLinkAboutAPA725UNITED STATES MINERAL RESOURCES
COAL
By PAUL AVERITI
CONTENTS
Abstract of conclusions
Introduction ------------------------------------
Environment of coal accumulation ----------------
Rank of coal ---------------~--------------------
Grade of coal --------------------------------~--
Sulfur --------------------------------------
Research on removal of sulfur ------------
Minor elements in coal -----------------------
United States coal resources ---------------------
Identified resources --------------------------
Distribution by selected categories ---------
Stripping coal .resources -----------------
Hypothetical resources -----------------------
.Speculative resources ------------------------
World coal resources ----------------------------
References cited ---------------------------------
FIGURES
15. Graph showing comparison on moist, mineral-
matter-free basis of heat values and proxi-
mate analyses of coal of different ranks __ _
16. Graph showing approximate percentage distri-
bution of original identified U.S. coal re-
sources by major resource categories _____ _
17. Graph showing probable distribution of total
estimated U.S. coal resources according to
thickness of overburden ------------------
TABLES
26. Distribution of identified United States coal re-
sources according to rank and sulfur content_
27. Total estimated remaining coal resources. of
the United States, January 1, 1972 --------
28. Estimated total original coal resources of the
world, by continents ---------------------
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ABSTRACT OF CONCLUSIONS
The estimated coal resources of the United States remain-
ing in the ground as of January 1, 1972, totaled 3,224 billion
tons. Of this large total, 1,581 billion tons, or 49 percent, is
classed as identified, and 1,643 billion tons, or 51 percent, is
classed as hypothetical.
Stripping coal resources remaining in the ground as of
January 1, 1968, totaled 118 billion tons, or about 7.5 percent
of the identified resources.
World coal resources are estimated to total 16,830 billion
tons, of which 9,500 billion tons is classed as identified, and
7,330 billion tons is classed as hypothetical. The United
States contains about one-fifth of estimated total world re-
sources.
On a uniform Btu basis, U.S. coal resources. are larger
than the combined domestic resources of petroleum, natural
gas, oil shale, and bituminous sandstone. The prolonged fu-
ture need for energy in ever-increasi~g quantities, and the
prospect of decreasing availability of and increased prices
for petroleum and natural gas, have focused very sharp at-
tention on coal as an alternative source of synthetic gas,
liquid fuels, and lubricants.
INTRODUCTION
Coal is widespread and abundant in the United
States. Coal-bearing rocks underlie about 13 percent
of the land area of the 50 States, and are present
in varying amounts in parts of 37 States (Trum-
bull, 1960; Barnes, 1961). The ready availability of
coal has contributed substantially to the growth
and industrial development of the nation.
On any basis of analysis, U.S. resources of coal
are larger than the combined resources of petroleum,
natural gas, oil shale, and bituminous sandstone,
but use of coal lags behind use of both petroleum
and natural gas because these two fuels are cleaner
to burn and easier to handle. In spite of this handi-
cap, annual coal production in the United States
ranges typically from 500 to 600 million tons. About
10 percent of the annual production is exported,
primarily to Japan, Canada, and western Europe.
Of coal consumed annually in the United States,
about 62 percent is used in the production of elec-
U.S. GEOL. SURVEY PROF. PAPER 820
133
134 UNITED STATES MINERAL RESOURCES
tric power, 20 percent is used by the steel industry,
16 percent by the manufacturing industry, and 2
percent for all other purposes. Coal is also of great
future value and importance as a subsidiary source
of synthetic gas, liquid fuels, and lubricants.
ENVIRONMENT OF COAL ACCUMULATION
Coal is the compressed and altered residue of
plants that grew in ancient fresh-or brackish-water
swamps. As the plant remains accumulated they
were transformed into peat; later they were altered
by diagenesis (chemical and physical changes occur,..
ring before they became solidified), and still later
by metamorphism (chemical and physical changes
brought about by pressure and heat after they be-
came solidified). Coal contains widely varying
amounts of sand, silt, and mud that was washed
into the peat swamps, and this admixed sediment
forms the bulk of the ash of burned coal. The physi-
cal and chemical properties of coal and the coalifi-
cation process have been described in considerable
detail by Schopf (1948; 1956) and by Dapples and
Hopkins (1969).
The accumulation of peat requires a humid cli-
mate to support a rich growth of vegetation, and
a high water table to permit prolonged accumula-
tion of plant material in a reducing environment
(See "Peat," this volume). Most of the large peat
deposits of Pennsylvanian age that were the pre-
cursors of coal mined extensively in the Eastern
and Central United States were formed near sea .
level-some in estuaries or coastal lagoons, others
on large deltas or many coalescing deltas, others on
low-lying, broad coastal plains. These features form
characteristically in areas of gentle downwarping of
the sea floor marginal to the edges of an eroding
landmass. This topographically low position in an
area of gentle downwarping permitted periodic
transgressions of the sea. Some thick coal beds of
very wide areal extent required a very large and
wide coastal plain, a prolonged optimum rate of
plant growth and accumulation, a slow rate of sub-
sidence, and an equally slow encroachment of the
sea over periods measured in centuries.
The transgressive sea ultimately covered the peat-
forming swamp and terminated plant growth. The
eroding landmass continued to supply sand, silt,
and mud to the sea, and this material settled in
layers over the submerged peat swamp. In time,
depending in length on the rate of sedimentation,
the depth of the transgressive sea, and the rate of
subsidence, this sedimentary material ·built up new
deltas, lagoons, and coastal plains conducive to the
development of new, younger peat-forming swamps.
This sequence of deposition was repeated ma1 ..
times by intermittent downwarping alone, but the
sequence might have been prolonged, shortened, or
terminated at any time by relatively minor move-
ments of land relative to the sea floor. In the very
delicate balance between sedimentation, subsidence,
and uplift of the land, the sea also regressed from
time to time. Peat swamps obviously formed during
the regressive phase of the cycle, but these were
subject to oxidation and are less commonly pre-
served. These cyclic repetitions of the conditions
allowing the formation of coal are documented in
many of the world's coal fields, but rarely as strik-
ingly as in a sequence of several thousands of feet
of sedimentary rock in West Virginia that contains
117 coal beds of sufficient geologic and economic
interest to have been described and named.
Weight of the overlying sedimentary rock, heat
produced by depth of burial, structural deforma-
tion, and time all contribute to the progressive com-
paction and devolatilization of peat to form the
higher ranks of coal, which are discussed below. A
subsequent major uplift of the land relative to the
sea has raised the U.S. coal fields to their present
positions, exposing them to erosion and to view,
thereby permitting study and mine development.
RANK OF COAL
Coal is classified by rank according to the per-
centage of fixed carbon and heat content, calcu-
lated on a mineral-matter-free basis. As shown in
figure 15, the percentage of fixed carbon and the
heat content increase from lignite to low-volatile
bituminous coal as the percentages of volatile mat-
ter and moisture decrease. These changes are pri-
marily the result of depth and heat of burial,
compaction, time, and structural deformation. Rank
is thus a way of expressing the progressive meta-
morphism of coal. It is quite independent of grade,
which is a way of expressing quality.
As coals of different rank are adapted to differ-
ent uses, rank is a major basis of differentiation in
coal-resource calculations. In accompanying tables
and figures, the coal resources are expressed in
short tons. If arithmetic adjustments were made
for the contained heat values, the distribution pat-
terns would be changed somewhat because of the
lower heat values of lignite and subbituminous coal.
GRADE OF COAL
Coal is classified by grade largely according to the
content of ash, sulfur, and other deleterious con-
stituents. Thus far in work on coal resources, a
preliminary classification on the basis of sulfur
COAL 135
---
............. ------
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] ~ ~ '6 ::E ~ r----1 :;: ,;
-"l: '-1 r-,__
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100 M 0 I s _L J!. _IL .£. -----------------r IlL ' / --r--r--;-/ ---til p.
/ /
/ ' -~ / -0 ~ r-----: r~ r-~ -~ ~ -----j I ~ 0 ---·e e ~ ----r-i :E -f--;;, "' < -.~ '!' -< .!!~ ~~ c
.... 'M"' .. .. '-' ~-~ m E < ~ ---:g ~ :g s! :;g . ~ .:: ::E 0 ~-~ -'! c ~·e OE 0 -~ r-~(.) --~ ~---~ ~~ .;:a r---1" 1"~ .c.'!:: •! :e .. .., ~ ~ i :g .c~ :;: :z: ~ .!!."" i ::; .., :z:
F J! x.i! E "' 0 c A ::E R B 0 N
80
60
40
FIGURE 15.-Comparison on moist, mineral-matter-free basis
of heat values and proximate analyses of coal of different
ranks.
content has been made, but classification on the
basis of ash content has not been made, because ash
is a more highly variable component than sulfur.
In recent years, information on trace elements in
coal has increased somewhat, but classification ac-
cording to trace-element content is not yet possible.
SULFUR
Sulfur is an undesirable element in coal. It lowers
the quality of coke and of the resulting iron and
steel products. It contributes to corrosion, to the
formation of boiler deposits, and to air pollution.
Its presence in spoil banks inhibits the growth of
vegetation. As sulfuric acid, it is the main deleteri-
ous compound in acid mine waters, which contribute
to stream pollution.
The sulfur content of coal in the United States
ranges from 0.2 to about 7.0 percent, but the aver-
age in all coal is 1.0-2.0 percent. Most of the sulfur,
perhaps 40-80 percent, occurs as a constituent of
pyrite and marcasite (FeS2). The remainder occurs
as hydrous ferrous sulfate (FeS4 •7H20), derived
by weathering of pyrite, as gypsum (CaS04 •2H20),
and as organic sulfur in combination with the coal-
forming vegetal material (Walker and Hartner,
1966).
The percentage of sulfur and of pyritic sulfur is
highest in bituminous coals of Pennsylvanian age
in the Appalachian and Interior coal basins. The
percentage is relatively low, generally less than 1
percent, in subbituminous coal and lignite of the
Rocky Mountain and Northern Great Plains regions.
This relation is shown clearly in table 26.
TABLE 26.-Distribution, in percent, of identified 1 United
States coal resources according to rank and sulfur content •
Sulfur content (In pereent)
Rank Low Medium Hieh
0-1 1.1-3.0 3+
Anthracite --------------97.1 2.9
Bituminous coal ---------29.8 26.8 43.4
Subbituminous coal ------99.6 .4
Lignite -----------------90.7 9.3
All ranks ---------------65.0 15.0 20.0
1Identified resources: Specific, identi-fied mineral deposits that may or
may not be evaluated as to extent and grade, and whose contained min-
erals may or may not be profitably recoverable with existin& technolo&Y
and economic conditions.
• From DeCarlo, Sheridan, and Murphy ( 1966) •
The conspicuously large percentage of low-sulfur
coal in the United States, shown on the last line
of table 26, is primarily due to the fact that the
resources of low-sulfur subbituminous coal and lig-
nite concentrated in the Rocky Mountain and North-
ern Great Plains regions represent about 54 percent
of total identified resources.
RESEARCH ON REMOVAL OF SULFUR
Pyrite and marcasite have a high specific gravity,
and most of this material can be removed from coal
by various washing and cleaning procedures. The
other forms of sulfur have lower specific gravities
and are more intimately mixed with the coal, and
consequently are less easily removed. Between 60
and 65 percent of all coal mined in the United States
is cleaned to remove pyritic and inert material be-
fore use. However, in spite of such large-scale clean-
ing, the average sulfur content of all coal used in
the United States is still nearly 2 percent.
Current efforts to reduce the sulfur content of
coal and of flue gas take many forms:
1. Much r~search is in progress on methods to re-
move S02 and SOa from flue gas. '!'his can be
done by several well-known chemical processes,
and the technical problems inherent in the
large-scale commercial application of chemical
processes are likely to be solved in the near
future.
2. Meanwhile, the search for low-sulfur coal has
136 UNITED STATES MINERAL RESOURCES
been intensified, particularly in the Eastern
States, and the use of lower-sulfur coal has
been increased. A few older coal-burning util-
ity plants in the Midwest have converted from
high-sulfur local coal to low-sulfur Rocky
Mountain coal. This substitution has required
payment of transportation costs of $3-$5 per
ton, and acceptance of the lower heat content
of Rocky Mountain coal. Such high transpor-
tation costs obviously will intensify research
efforts mentioned in item 1.
3. Much research is in progress on methods to
produce a high-Btu, sulfur-free gas from coal.
This is also a technical possibility soon to be
realized. It has the multiple advantages of
lowering the costs of long-distance transpor-
tation of energy, of eliminating the sulfur
problem, of augmenting declining resources
of natural gas, of reducing dependence on
foreign sources of oil and gas, and ultimately
permitting use of high-sulfur eastern coal.
4. Research on improved methods of producing
electric power by nuclear fission and fusion
is continuing.
These varied avenues of approach suggest that
the amount of sulfur.released to the atmosphere by
the burning of coal will soon be greatly reduced.
MINOR ELEMENTS IN COAL
Coal contains small quantities of virtually all
metallic and nonmetallic elements, that were intro-
duced into the coal bed in one or all of four differ-
ent ways:
1. As inert material washed into the coal swamp
at the time of plant accumulation.
2. As a biochemical precipitate from the swamp
water.
3. As a minor constituent of the original plant
cells.
4. As a later addition, introduced after coal forma-
tion, primarily by ground water moving down-
ward and laterally.
When coal is burned, most of these elements are
concentrated in the coal ash, but a few of the more
volatile elements are emitted into the atmosphere.
Coal ash is composed largely of the oxides of Si, AI,
Fe, Ca, Mg, K, N a, and S, which typically make up
93-98 percent of the total weight of the ash (Aber-
nethy and others, 1969a). The remaining few per-
cent of coal ash is made up of small individual
amounts of many other elements, which differ in
variety and quantity in different areas and beds.
These elements are, generally measured in parts per
million or billion, and for this reason are termed
minor elements, although they may not be minor
elements in other contexts.
The minor elements in coal are of considerable
interest because some may become of future re-
source importance, and others may be pollutants.
Most of the minor elements occur in coal in about
the same concentration as their estimated concen-
tration in the earth's crust, but 25-30 elements occur
locally in greater concentration and these have re-
ceived the most study. A few elements, notably U,
Ge, As, B, and Be, occur locally in vastly greater
concentrations than their estimated concentration
in the earth's crust; others, including Ba, Bi, Co,
Cu, Ga, La, Pb, L, Hg, Mo, Ni, Sc, Se, Ag, Sr, Sn,
V, Y, Zn, and Zr, occur locally in appreciably greater
concentrations. Other elements of interest that gen-
erally occur in lower concentrations than those
listed above include Cr, Mn, P, Te, Tl, Ti, and W.
It should be noted that the concentration of an
element in excess of the estimated concentration in
the earth's crust, although of great interest and
geologic significance, does not necessarily imply an
economic or paramarginal concentration, because
that is determined by the concentration in typical
commercial sources of the respective element.
Reports by Abernethy and Gibson (1963); Aber-
nethy, Peterson, and Gibson (1969a, b) ; Zubovic
(1966a, b); Zubovic, Sheffey, and Stadnichenko
(1967); Zubovic, Stadnichenko, and Sheffey (1960a,
b, c; 1961a, b; 1964; and 1966) ; and by Sun,
Vasquez-Rosas, and Augenstein (1971) summarize
available information concerning minor elements in
coal. A selected bibliography on trace elements in
coal, applicable primarily to U.S. coals, has been
compiled by Averitt, Breger, Gluskoter, Swanson,
and Zubovic (1972).
UNITED STATES COAL RESOURCES
The remaining coal resources of the United States
as of January 1, 1972, are estimated to total 3;224
billion tons. Of this large total, 1,581 billion tons,
or 49 percent, has been identified on the basis of
mapping and exploration, and the remainder of
1,643 billion tons, or 51 percent, is classed as hypo-
thetical because it has been determined by extrapo-
lation of the data on identified resources into un-
mapped and unexplored areas. The distribution of
this tonnage by State is given in table 27.
The figures in table 27, and in subsequent tables
and figures, express resources in the ground. The
recoverability in coal mining ranges from 40 to 90
percent, depending largely on the method of mining,
but it is influenced by many other diverse factors
such as the nature of the roof rock, joints, faults,
:<::oAL 137
TABLE 27.-Total estimated remaining coal resources of the United States, January 1, 1972
[In millions of short tons. Figures are for resources in the ground, about half of which may be considered recoverable. Includes beds of bituminous
coal and anthracite 14 in. or more thick and beds of subbituminous coal and lignite 2'h ft or more thick]
State
.Alabama
.Alaska
Arizona
Arkansas
Colorado
Ge?r~ia
lllmoJS -------
Indiana -------
Iowa ---------
Kansas --------Kentucky
Maryland
Michigan
Missouri -------
Montana ------
New Mexico
North Carolina _
North Dakota __
Ohio ---------
Oklahoma -----
Oregon
Pennsylvania
Rhode Island ---
South Dakota __
Tennessee -----
Texas
Utah
Virginia ------
Washington -----
West Virginia __
Wyoming
Other States ---
Total ----
Identified resources 1
Overburden H,OOO ft
Estimated identified resources remaining in the ground, Jan. 1, 1972
Sub-
Bitumi-bitumi-Lignite
nous coal nous coal
13,342
19,413
• 21,246
1,638
62,339
24
139,124
34,573
6,609
18,674
64.842
1,168
206
31,014
2,299
10,762
110
0
4i,358
3,281
50
56,769
0
0
2,572
6,048
1123,641
9,352
1,867
100,628
12,705
18610
686,033
0
110,668
(6)
0
18,242
0
0
0
0
0
0
0
0
0
131,856
50,671
0
0
0
0
284
0
0
0
0
0
11 180
0
4,190
0
107,961
14 32
424,073
2,000
(')
0
350
0
0
0
0
0
(8)
0
0
0
0
87,521
0
0
~50,630
0
(8)
0
0
0
2,031
0
6,824
0
0
117
0
(2)
154,6
449,619
Anthra-
cite and
semi-
anthra-
cite
0
(6)
0
430
78
0
0
0
0
0
0
0
0
0
0
4
0
0
0
0
0
20,510
0
0
0
0
335
6
0
0
0
21,362
ToW
15,342
130,081
21,246
2,418
80,669
24
139,124
34,573
6,609
18,674
64,842
1,158
205
31,014
221,675
61,427
110
350,630
41,368
3,281
334
77,269
2,031
2,572
12,872
11 23,721
9,687
6,179
100,628
120,666
688
1,680,987
Source of estimate
Culbertson (1964); T.
A. Simpson (written
commun., 1972) .
Barnes (1951; 1967)
Peirce and others
(1970)7.
Haley (1960) --------
Landffi (1959) --------
Johnson (1946) 7 ------
Simon (1965)' -------
Spencer (1953) -------
Landis (1965) -------
Schoewe (1952; 1958) 7 _
Huddle and others
(1963).
Averitt (1969) -------
Cohee and others
(1950).
Robertson (1971)7 ___ _
Combo and others
(1949; 1950)7.
Read and others (1950)_
Reinemund (1949;
1965).
Brant (1953) --------
Brant and DeLong
(1960).
Trumbull (1967) -----
R. S. Mason (written
commun., 1966)7.
Reese and Sisler (1928) ;
Arndt and others
(1968)7.
Toenges and others
(1948).
D. M. Brown (19112)
Luther ( 1969; written
commun., 1966).
Mapel (1967); Perkins
and Lonsdale (1965).
Doelling 1970, 1971a,
b, e, d, e, f, Doelling
and Graham, (1970;
1971); H. H. Doelling
(written commun.,
1971).
Brown and others
(1962).
Beikman and others
(1961).
Headlee and Nolting
(1940).
Berryhill and others
( 1950; 1951).
Total
Hypothetical resources • resources
Over-Over-Over-Over-
burd'en burden burden burden
H,OOO 3,000-0-6,000 0-6,000
ft 6,000 ft ft
ft
Total
Estimated estimated
hypo the-identified
tical re-and hypo-
sources in thetical
unmapped Estimated resources
and un-hypo the-remain-
explored tical re-Total ing in
areas rea-sourees in estimated the
sonably deeper hypo the-ground
near the structural tical re-Jan. 1,
surface 3 basins 3 sources 1972
20,000 6,000 26,000 41,342
i30,000 5,000 135,000 265,081
0 0 0 21,246
4,000 0 4,000 6,418
146,000 145,000 291,000 371,669
60 0 60 84
100,000 0 100,000 239,124
22,000 0 22,000 56,673
14,000 0 14,000 20,509
4,000 0 4,000 22,674
52,000 0 52,000 116,842
400 0 400 1,558
500 0 500 706
18,200 0 18,200 49,214
157,000 0 167,000 378,676
27,000 21,000 48,000 109,427
20 5 26 135
180,000 0 180,000 530,630
2,000 0 2,000 43,368
20,000 10,000 30,000 33,281
100 0 100 434
•1o,ooo 0 10,000 87,269
0 0 0
1,000 0 1,000 3,031
2,000 0 2,000 4,672
14,000 0 14,000 26,872
12 21,000 36,000 56,000 79,721
6,000 100 6,100 14,787
30,000 16,000 45,000 51,179
0 0 0 100,628-
325,000 100,000 425,000 545·,656
1,000 0 1,000 1,688
1.306,280 337,106 1,643,386 3,224,372
1 Identified resources: Specific, identified mineral deposits that may or may not be evaluated as to extent and grade, and whose contained minerals
may or may not be profitably recoverab~e with existing, technology and economic conditions.
2 Hypothetical resources: Undiscovered mineral deposits, whether of recoverable or subeconomtc grade, that are geologically predictable as exist-
ing in known dmtrlct.
• Estimates by H. M. Beikman (Washington), H. L. Berryhill, Jr. (Wyoming), R. A. Brant (Ohio and North Dakota), W. C. Culbertson
(Alabama), H. H. Doel!ing (Utah), K J. Englund (Kentuckv and Virginia), B. R. Haley (Arkansas), E. R. Landis (Colorado and Iowa), E. T.
Luther (Tennessee), R. S. Mason (Oregon), C. E Robinson (Mmsouri), J. A. Simon (lllinois), J. V. A. Trumbull (Oklaltoma), C. E. Wier (Indiana),
and the author for the remaining States.
• Small resources of lignite included under subbituminous coal.
6 Small resources of anthracite in the Bering River field believed to be too badly crushed and faulted to be economically recoverable (Barnes, 1961).
6 Includes coal in the Dakota Formation of the Black Mesa field, some of which may be of subbituminous rank. Does not include small resources of
thin and impure coal in the Deer Creek and Pinedale fields.
7 See other summary reports on coal resources in individual States as follows: Arizona (Averitt and O'Sullivan, 1969); Georgia (Butts and
Gildersleeve, 1948; Sullivan, 1942); Illinois (Cady, 1962); Kansas (Abernathy and others, 1947); Mis~ouri (Hinds, 1913; Searight, 1967); eastern
Montana (Averitt, 1965); Ohio (Struble and others, 1971); Oregon (Mason and Erwin, 1965; Mason, 1969); PennSYlVania anthracite (Ashley, 1946;
Ashmead, 1926; Rothrock, 1950); and Utah (Averitt, 1964).
8 Small resources of lignite in beds generally less than 30 in. thick.
9 From Ashley ( 1944).
10 Small resources of meta-anthracite in the Narragansett basin believed to be too graphitic and too badly crushed and faulted to be economically
recoverable as fuel.
11 Excludes coal in beds less than 4 ft thick.
12 Includes coal in beds 14 in. or more thick, of which 14,000 million tons is in beds 4 ft or more thick.
18 California, Idaho, Nebraska, and Nevada.
u California and Idaho.
11i California, Idaho, Louffiiana, and Mississippi.
138 UNITED STATES MINERAL RESOURCES
and the need to protect oil and gas wells and fields.
From the long-term national point of view, average
recoverability is probably about 50 percent. How-
ever, it is not desirable to report coal resource data
on an arbitrary recoverable basis, because experi-
ence with most commodities has shown very signifi-
cant long-term changes in what is regarded as
economically recoverable. Coal in the ground is a
more certain value that can be modified now or in
the future by any recoverability factor deemed
appropriate.
IDENTIFIED RESOURCES
DISTRIBUTION BY· SELECTED CATEGORIES
In addition to the distribution by rank of the
identified resources of 1,581 billion tons as pre-
sented in table 27, about 60 percent of this total
has been classified into additional categories accord-
ing to the thickness of overburden, degree of relia-
bility of estimates, and thickness of beds as shown
in figure 16. This classified tonnage is fairly large
and is widely distributed in 21 States ; it is likely
to be reasonably representative of the total identi-
fied tonnage.
Overburden.-Figure 16 clearly shows the pro-
nounced concentration of identified resources in the
0-1,000-foot overburden category. This concentra-
tion results in part from the fact that coal-bearing
rocks are near the surface in most parts of the
United States, and in part from the fact that pro-
gressively less information is available for the more
deeply buried 'beds. Much of the tonnage classed as
hypothetical in figure 17 is in the 1,000-2,000-foot
and the 2,000-3,000-foot overburden categories. As
exploration and development are carried to greater
depth it is certain that the identified resources will
be considerably increased by addition of tonnage in
the deeper overburden categories.
Degree of reliability of estimates.-Figure 16 also
shows the progressive increase in tonnage from the
measured to the inferred categories. In the 0-1,000-.
foot overburden category, for example, 8 percent of
the tonnage is classified as measured, 23 percent
as indicated, and 58 percent as inferred. The same
relation can be observed in the deeper overburden
categories. The large percentage of inferred coal
reflects merely distance from points of known infor-
mation. Resources classified as "inferred" obviously
exist, but the locations of .such tonnage may differ
slightly from those assumed to make the calcula-
tions. As mapping and exploration continue, the
100r--------------,---------------------------,
o; o;., 0
8=5 u
~ !!! E .... ·;: 0 "~ ~ "" o£; ~ !!! cc .. ·-.. , .... .. ~-g c.> c ..
CO tV
Thin 14-28 in 2V2-5 It
28-42 in 5-10 It
More than More than
42 in 10 It
Thin
34
FIGURE 16.-Approxim.ate percentage distribution of original identified U.S. coal resources by major resource categories.
COAL 139
percentage of coal classified as measured and indi-
cated will surely increase.
Thickness of beds.-Coal in thick beds, 0-1,000
feet below the surface comprises 4 percent measured,
8 percent indicated, and 13 percent inferred, for a
total of 25 percent of the identified resources shown
in figure 16. This percentage, when applied to the
total of 1,581 billion tons, is equivalent to nearly
400 billion tons. This choice tonnage is in a thick-
ness and overburden category comparable to that of
coal now being mined, and is therefore of current
and near-current economic interest.
Coal in beds of intermediate thickness, 0-1,000
feet below the surface, makes up 23 percent of the
identified resources, and is equivalent to 350 billion
tons. This tonnage is of less immediate economic
interest than tonnage in the thicker beds. However,
some coal in this thickness and overburden category
is currently being mined, and the total must be
considered a paramarginal resource that will be-
come of increasing economic interest and importance
in the future.
Coal in thin beds, 0-1,000 feet below the surface,
makes up 41 percent of the identified resourcesf and
coal in all thickness categories, 1,000-3,000 feet
below the surface, makes up the remaining 11 per-
cent. This coal is of little current economic interest.
The amount in any category or combination of
categories can be derived from figure 16 by the
procedure used above.
STRIPPING COAL RESOURCES
In a recently published study, the U.S. Bureau of
Mines (1971) concluded that the remaining strip-
ping coal resources of the United States as of Janu-
ary 1, 1968, totaled 118 billion tons. Of this total,
about 90 billion tons, or 80 percent, is within reach
by present machinery and methods of mining, but
only 45 billion tons is both available for use and
economically recoverable.
For purpose of comparison, the larger total of
118 billion tons of stripping coal resources is 7.5
percent of the total of 1,581 billion tons of remain-
ing identified resources as reported in table 27.
The 45 billion tons of potentially recoverable
stripping coal includes 32 billion tons of low-sulfur
coal (less than 1 percent), 4 billion tons of medium-
sulfur coal (1-2 percent), and 9 billion tons of
high-sulfur coal (more than 2 percent).
HYPOTHETICAL RESOURCES
The preceding analysis of the distribution of
identified coal resources provides convincing evi-
dence that unmapped and unexplored areas in
known coal fields contain substantial additional re-
sources that must be classed as hypothetical. The
approximate magnitude of the additional hypo-
thetical resources has been estimated by a process
of extrapolation from nearby areas of identified
resources, and estimates for each State are pre-
sented in separate columns in table 27. The total
tonnage of hypothetical resources actually exceeds
by a small amount the tonnage of identified re-
sources. Figure 17 shows the percentage relation
between identified and hypothetical resources in
four overburden categories.
100,----------------------------------,
32.5
1::::::::::::1
~lHHHHH:
llli!!li!!i!.
~H1111mH:
~11111111111!
EXPLANATION -· Mined out
~
Identified resources
~~:!!:~~
~::::::::::l li.,:i;,:.:;:.:.;;,;
Hypothetical resources
FIGURE 17.-Probable distribution of total estimated U.S.
coal resources according to thickness of overburden.
Although large, the hypothetical resources are,
for the most part, relatively inaccessible for mining
at present, and a more exact delineation of the
magnitude, distribution, and future utility of such
resources will require a substantial amount of de-
tailed geologic mapping, exploration, and study over
a long period. Nevertheless, the estimated hypo-
thetical resources constitute an important part of
the total resource that needs to be considered in
140 UNITED STATES MINERAL. RESOURCES
future planning for the utilization of all energy
resources.
SPECULATIVE RESOURCES
The resources presented in table 27 and discussed
under the headings of identified and hypothetical
resources represent total resources within limits
established by the minimum thickness of coal beds
and the maximum thickness of overburden. The
major geologic features of the United States are
known well enough to justify the statement that,
in all probability, no major coal fields remain to be
discovered. Hence the coal resources of the United
States are all either identified or hypothetical. Be-
cause there are no undiscovered districts, there are
no speculative resources.
WORLD COAL RESOURCES
The original identified coal resources of the world
total about 9,500 billion tons, the additional hypo-
thetical resources total about 7,330 billion tons,
and the two categories combined total 16,830 billion
tons. The distribution of this tonnage by continents
is shown in table 28.
TABLE 28.-Estimated total original coal resources of the
world, }Y continents 1
[In billions of short tons]
Hypo the-Estimated
Continent Identified tical re-total
resources 2 sources a resources
Asia' -----------------• 7,000 4,000 "11,000
North America ---------1,720 2,880 4,600
Europe ---------------620 210 830
Africa ----------------80 160 240
Oceania ---------------60 70 130
South and Central
America ------------20 10 30
Total -----------• 9,500 7,330 "16,830
1 Original resources in the ground in beds 12 in. or more thick and gen-
erally less than 4,000 ft below the surface, but includes small amounts
between 4,000 and 6,000 ft.
2 Identified resources: Specific, identified mineral deposits that may
or may not be evaluated as to extent and grade; and whose contained
minerals may or may not· be profitably recoverable with existing tech-
nology and economic conditions.
3 Hypothetical resources: Undiscovered mineral deposits, whether of
recoverable or subeconomic grade, that are geologically predictable as
existing in known districts.
' Includes European U.S.S.R
• Includes about 6,500 billion short tons in the U.S.S.R.
8 Includes about 9,500 bi!lion short tons in the U.S.S.R. (Hodgkins,
1961, p. 6).
The figures for the United States as shown in
table 27 are included in the total for North America
in table 28. On the basis of identified resources, the
United States contains about one-sixth of world
resources; on the basis of total resources, the United
States contains about one-fifth of world resources.
Table 28 shows clearly that Asia contains most
of the world's total coal resources. This tonnage is
concentrated in the U.S.S.R. and China, both of
which are important coal-producing countries. The
table also shows that the coal resources of Europe
have been well established by mapping and explora-
tion, and that estimates will not be greatly increased
by future work. Finally, table 28 shows that Africa,
Oceania, and South America contain small re-
sources as compared with the rest of the world, but
that the quantities assumed to be present are suffi-
cient to justify continued exploration and develop-
ment. (See Averitt, 1969, p. 81-85.)
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UNITED
STATES
MINERAL
RESOURCES
""" DONALD A. BROBST and WALDEN P. PRATT, Editors
00
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U') ..-
N
g GEOLOGICAL SURVEY PROFESSIONAL PAPER 820
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UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1973
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