GEOLOGICAL SURVEY CIRCULAR 650 

Energy Resources 

of the United States 

Energy Resources 

of the United States 

By P. K. Theobald, 5. P. Schweinfurth, and D. C. Duncan 

GEOLOGICAL SURVEY CIRCULAR 6 50 

Washington J 9 72 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page Page 

Albstract ---------------------------------------- 1 Petroleum liquids and natural gas-Con. 

Introduction ----------------------- _____________ 1 Comparison with other estimates-Con. 

Coal resources ----------------------------------- 3 Individual estimates-Con. 

Sulfur content of coal ----------------------- 4 

Comparison with other recent coal-resource 

Methodology-Con. 

Geologic methods _______________ 19 

estimates ---------------------------------- 4 Mathematical methods___________ 20 

Bituminous coal resources of the Alppala- Discussion of individual estimates ____ 20 

chian basin ______________ _________________ 5 National Petroleum Council_____ 20 

U.S. Geological Survey --------- 21 

M. K. Hubbert__________________ 21 

Low-sulfur coal resources of the Alppalachian ba~n ------------------------------- 5 

Petroleum liquids and natural-gas resources______ 5 L. G. Weeks-------------------- 22 

C. L. Moore -------------------- 22 

M. Al. Elliott and H. R. Linden___ 22 

Comparison with other petroleum liquids and 

natural-gas resource estimates ________ 12 

Terminology ---------------------------- 12 

Known deposits --------------------- 12 

Potential Gas Committee ------- 22 

T. W. Nelson and C. Al. Burk --- 22 

Undiscovered deposits _______________ 19 

Total resources in place_____________ 19 

Individual estimates --------------------- 19 

Uranium resources ------------------------------ 23 

Geothermal resources ---------------------------- 24 

Oil-shale resources ------------------------------ 25 

Methodology References cited --------------------------------- 26 _____ ___________________ 19 

ILLUSTRATIONS 

Pa!le 

FIGURE 1. Conceptual diagram for the classification of mineral resources --------------------------------- 2 

2-12. Scaled diagrams of resources: 

13-15. 

2. Coal in the United States -------------------------------------------------------------- 3 

3. Bituminous coal of the Alppalachian basin -----------------------------------------------

4. Low-sulfur coal of the Alppalachian basin ----------------------------------------------

5. Crude oil and natural-gas liquids in the United States ____ -----------------------------

6. Crude oil and natural-gas liquids in Allaska ------------------------- ------------------

5 

6 

7 

7 

7. Crude oil and natural-gas liquids offshore from the United States---------------------- 8 

8. Crude oil and natural-gas liquids onshore in the conterminous United States------------- 8 

9. Natural gas in the United States ----------------------------------------------------- 9 

10. Natural gas in Allaska ----------------------------------------------------------------- 9 

11. Natural gas offshore from the United States ------------------------------------------- 10 

12. Natural gas onshore in the conterminous United States --------------------------------- 10 

Diagrams showing some estimates of original resources of petroleum liquids: 

13. United States ------------------------------------------------------------------------- 13 

14. United States onshore ----------------------------------------------------------------- 14 

15. United States offshore ______________ --------------------------------------------------- 15 

16-18. Diagrams showing some estimates of original resources of natural gas: 

16. United States ------------------------------------------- _____ ------------------ ___ ___ _ 16 

17. United States onshore ----------------------------------------------------------------- 17 

18. United States offshore ----------------------------------------------------------------- 18 

19, 20. Scaled diagrams of uranium resources in the United States: 

19. Conventional deposits --------------------------------- _____________ ------------------- 23 

20. Phosphate rock ------------------------------------------------------------------------ 24 

21. Diagram of geothermal resources in the United States ------------------- --------------------- 25 

22. Scaled diagram of oil-shale resources in the United States _________ ---------------------------- 26 

III 

Energy Resources of the United States 

By P. K. Theobald, S. P. Schweinfurth, and D. C. Duncan 

ABSTRACT 

Estimates are made of United States resources of 

coal, petroleum liquids, natural gas, uranium, geothermal energy, and oil from oil shale. The estimates, compiled by specialists of the U.S. Geological Survey, are 

generally made on geologic projections of favorable 

rocks and on anticipated frequency of the energy resource in the favorable rocks. Accuracy of the estimates 

probably ranges from 20 to 50 percent for identifiedrecoverable resources to about an order of magnitude 

for undiscovered-submarginal resources. 

The total coal resource base in the United States is 

estimated to be about 3,200 billion tons, of which 200-

390 billion tons can be considered in the category identified and recoverable. More than 70 percent of current 

production comes from the Appalachian basin where 

the resource base, better known than for the United 

States as a whole, is about 330 billion tons, of which 22 

billion tons is identified and recoverable. Coals containing less than 1 percent sulfur are the premium coals. 

These are abundant in the western coal fields, but in 

the Appalachian basin the resource base for low-sulfur 

coal is estimated to be only a little more than 100 

billion tons, of which 12 billion tons is identified and 

recoverable. 

Of the many estimates of petroleum liquids and natural-gas resources, those of the U.S. Geological Survey 

are the largest because, in general, our estimates include the largest proportion of favorable ground for 

exploration. We estimate the total resource base for 

petroleum liquids to be about 2,900 billion barrels, of 

which 52 billion barrels is identified and recoverable. Of 

the total resource base, some 600 billion barrels is in 

Alaska or offshore from Alaska, 1,500 billion barrels is 

offshore from the United States, and 1,300 billion barrels is onshore in the conterminous United States. Identified-recoverable resources of petroleum liquids corresponding to these geographic units are 11, 6, and 36 

billion barrels, respectively. 

The total natural-gas resource of the United States 

is estimated to be about 6,600 trillion cubic feet, of 

which 290 trillion cubic feet is identified and recoverable. In geographic units comparable to those for petroleum liquids, the resource bases are 1,400, 3,400, and 

2,900 trillion cubic feet, and the identified-recoverable 

resources are 31, 40, and 220 trillion cubic feet, respectively. 

1 

Uranium resources in conventional deposits, where 

uranium is the major product, are estimated at 1,600,-

000 tons of UaO,, of which 250,000 tons is identified and 

recoverable. A potential byproduct resource of more 

than 7 million tons of U:O., is estimated for phosphate 

rock, but none of this resource is recoverable under 

present economic conditions. 

The resources of heat in potential geothermal energy 

sources are poorly known. The total resource base for 

the United States is certainly greater than 1022 calories, 

of which only 2.5 X 1018 calories can be considered identified and recoverable at present. 

Oil shale is estimated to contain 26 trillion barrels of 

oil. None of this resource is economic at present, but if 

prices increase moderately, 160-600 billion barrels of 

this oil could be shifted into the identified-recoverable 

category. 

INTRODUCTION 

The accompanying diagrams present the U.S. 

Geological Survey estimates of the United 

States resources of coal, petroleum liquids, natural gas, uranium, geothermal energy, and oil 

from oil shale. The estimates have been compiled by a group of specialists familiar with 

each of the energy sources, each using techniques he considers most useful for estimating 

his particular energy source. The short text 

accompanying each diagram outlines the method of estimation or the source of the estimate 

and defines the characteristics of each estimate. 

Where appropriate, comparisons with other estimates are also given. 

Resources, as used here, include all rocks 

and minerals (including their contained heat 

for geothermal sources) potentially usable by 

man, including currently known and recoverable reserves, undiscovered resources which 

are estimated geologically or mathematically 

and which would be recoverable if found, and 

energy sources (identified and undiscovered) 

whose exploitation will require more favorable 
Identified 

Recov erable Reserves 

Parama rginal 

Sub rna rginal 

({o 

<.,0 

ov 

00 

~ 

-fUndiscovered 

-

1 

::>-. 

)....! 

Q) 

:> 

0 

() 

Q) 

)....! 

() 

•r-l 

s 

0 

c 

0 

:J 

Q) 

-+---------Degree of certainty--------

FIGURE 1.-Conceptual diagram for the classification of mineral resources. From McKelvey ( 1972). 

economic or technologic conditions than those 

of the present. The diagrams are patterned 

after the classification of mineral reserves and 

resources proposed by McKelvey (1972). 

The horizontal scale in figure 1 shows degree of certainty, increasing to the left, with 

a twofold major division, undiscovered to the 

right and identified to the left. This division 

has been used in this report. For some energy 

sources, a subdivision of the undiscovered resources has been added, again based on degree 

of certainty, to estimate to greater depths 

(coal) or to distinguish estimates in known districts from those in undiscovered districts 

(uranium, oil shale). 

The vertical scale is feasibility of economic 

recovery, increasing upward, with a threefold 

division into submarginal, paramarginal, and 

recoverable. We have retained the recoverable 

unit in all our diagrams and have indicated 

the approximate position of the boundary between paramarginal (defined by McKelvey, 

1972, as "recoverable at prices as much as 1.5 

times those prevailing now") and submarginal 

where feasible. Where this latter split cannot 

be made, the entire field below recoverable is 

called submarginal. Whether the undiscovered 

resources may be subdivided according to feasibility of economic recovery depends upon the 

method of estimation. Where this subdivision 

cannot be made, we have assumed that the 

eventual breakdown will be approximately proportional to that for the identified resources. 

The various areas of the diagrams presented 

here have been approximately scaled within 

each commodity; for example, the area of the 

block for identified recoverable resources (reserves) for a commodity is proportional to the 

magnitude of the number in the block relative 

to the magnitude of the total resource base for 

2 
that commodity. The single exception is for 

geothermal energy where the proportion of the 

resource that is identified and recoverable is too 

small to show without exaggeration. In all diagrams, the figures for the total resource base, 

that is, the total of all resources in the diagram, 

are in the units common to that commodity and 

are for material at the well head or mine portal; that part left in the ground is included 

with the submarginal resource. No attempt has 

been made in these estimates to evaluate the 

efficiency of energy-conversion methods or the 

efficiency of future extraction technology. 

The estimates of identified recoverable resources in the upper left corner of the diagram 

are within 20-50 percent of the correct value. 

The estimates of undiscovered submarginal resources in the lower right corner of the diagram reflect only an order of magnitude. To be 

realized, these less known resources, which constitute the bulk of the resource base, will require great advances in the technologies of 

both search and extraction. Similarly, the submarginal identified resources require advances 

in extraction technology before utilzation is 

feasible, whereas the undiscovered recoverable 

Identified resources 

1 2 0 0 

Recoverable 

2 

1 9 0 

Submarginal 1200 

resources require a continuing effort in both 

exploration and exploration research as well as 

economic incentive. 

COAL RESOURCES 

Coal resources are classified in two broad 

categories: identified resources and undiscovered resources (fig. 2). Short tons of coal are 

the units for coal resource estimation. Identified resources include coal beds that have been 

mapped and tonnage estimates prepared on a 

thickness and general-quality basis. Such estimates include, in order of decreasing reliability, measured and indicated reserves as well 

as inferred conservative extensions of individual coal beds. 

Recoverable coal is identified and separated 

from submarginal coal with the field of identified resources. Two estimates of recoverable 

coal have been made, dependent upon the thickness of the coal beds. With a cutoff at 42 inches 

in thickness for bituminous coal and anthracite and at 10 feet for subbituminous coal and 

lignite, approximately 200 billion short tons 

can be recovered within 1,000 feet of the surUndiscovered resources 

l 3000- 0-3000 ft overburden 6000 ft overburdE!r 

0 

1 3 0 0 

3 4 0 

1 Kecoveraole coal 1n :>eds 42 1nches or more 1n th1·~kness for bituminous 

coal and anthracite -ind 10 feet or more in thickness for subbi tul'linous 

coal and li~ni te. 

2 Additional coal recoverable in bed~ 2B-42 inches in t~ickness for 

bituminous coal and .1nthraci te and 3-5 feet in thickness for subbituminous coill 'lnd lignite. 

FIGURE 2.-Scaled diagram of coal resources in the United States (billions of short 

tons). Reliability of estimates decreases downward and to the right. Compiled by 

Paul Averitt, February 1972. 

3 
face at a recovery factor of 50 percent. With 

a cutoff at 28 inches in thickness for bituminous coal and anthracite and at 5 feet for subbituminous coal and lignite, an additional 190 

billion short tons is recoverable within 1,000 

feet of the surface at a recovery factor of 50 

percent. The total recoverable coal at a 28-

inch cutoff is, therefore, 390 billion tons. These 

two estimates of recoverable coal-200 billion 

short tons and 390 billion short tons-appear 

in different editions of "United States Energy, 

a Summary Review" (U.S. Dept. Interior, 1972, 

p. 29). It is assumed that coal that can be mined 

at depths as great as 3,000 feet approximately 

compensates for coal shallower than 1,000 feet 

that cannot be mined for geologic or land-use 

reasons. 

The submarginal coal estimate includes the 

thicker coal left in the ground on first mining 

and thinner coal beds to a cutoff at 14 inches 

thickness for bituminous coal and anthracite 

and at 30 inches for subbituminous coal and 

lignite. This estimate includes coal to a depth 

of 6,000 feet below the surface. 

Undiscovered coal resources include coal deposits that have not been mapped or sampled 

in detail within the known coal fields. (All the 

coal fields are known in some degree.) These 

resources are estimated on expected volumes 

of favorable rocks and expected coal frequency 

within these rocks in unmapped or unexplored 

areas and in basinward projections from the 

outcrop. 

SULFUR CONTENT OF COAL 

In a study of analytical data on coal, DeCarlo, Sheridan, and Murphy (1966, p. 19) 

concluded that the average sulfur content of 

the coal in the identified remaining resources 

of 1,600 billion tons is as shown below. 

As shown in the table, subbituminous coal 

and lignite are conspicuously low in sulfur. The 

large concentration of coals of these ranks in 

the Rocky Mountain and northern Great Plains 

provinces, about 54 percent of total identified 

United States resources, is responsible for the 

substantially large amount of low-sulfur coal 

in the United States as a whole. 

However, production of coal is concentrated 

in the bituminous coal fields of the eastern 

Snl.fn1· content of United States coal according to rank 

L.Mouifil'<l from DPCal'lo, Sheridan, and Murphy, 1!16()] 

Rank Low ~ulfur 'l\1Nlium Hulfur High ;mlfur 

0--1 1.1-3 >3 

Anthradtl' --------- !17.1 :!.!1 

Bituminou~ eoal ---- 2fJ.H :!6 .. '-i -!;{.4 

Suhhitumino11~ c·oal -- !J!J.6 A 

Lignite ------------ !10.7 B.:3 

All rank~ ---------·-- li:i.O 15.0 20.0 

United States (more than 70 percent of the 

production in 1969); this coal, as the above 

table shows, contains substantially more sulfur 

than the national average. In these areas much 

additional low-sulfur coal in individual beds 

and parts of beds can be identified by additional 

geologic study and exploration. This has been 

done with some success, as will be noted under 

the discussion of low-sulfur coal in the Appalachian basin. 

COMPARISON WITH OTHER RECENT COALRESOURCE ESTIMATES 

Estimates of the coal resources of the United 

States appear in a variety of publications; two 

of the more widely used are U.S. Bureau of 

Mines (1971) and Hubbert (1969). These published estimates are all derived from Averitt 

(1969), aithough the figures themselves are 

sometimes manipulated to fit the context in 

which they are used. Apparent contradictions 

among the various figures usually result from 

differences in the application of recovery factors, from a variety of assumptions used to 

estimate ultimate conversion to energy, or from 

restriction of the estimate to particular reliability or quality criteria. 

A Bureau of Mines report entitled, "United 

States Coal Resources and Production-Interim 

Report, June 1971," deals only with resources 

in the "identified" category. With minor 

changes, it draws on the same U.S. Geological 

Survey and State survey source material used 

by Averitt (1969). 

Hubbert (1969) based his mathematical analysis on both "identified" and "total" (identified 

plus undiscovered) coal resources. The figure of 

1,486 x 109 metric tons that appears in both fig4 
ures 8.24 and 8.26 of that article is the total 

resource base from our figure 2, 3,200 x 10!' 

short tons, reduced by a recovery factor of 50 

percent and converted to metric units. 

BITtJMINOUS COAL RESOURCES OF THE APPALACHIAN BASIN 

The units, definitions, and means of estimation for figure 3 are the same as those for the 

United States coal estimate (fig. 2), but have 

been restricted to the single classification of 

coal in the one major coal-producing region. 

The Appalachian basin is defined here as the 

narrow continuous coal-bearing region extending along the Appalachian Mountains from 

northern Alabama to western Pennsylvania. 

This single basin has been selected for discussion because it provides more than 70 percent of the current coal production and because 

it contains a smaller proportion of low-sulfur 

coals (fig. 4) than do the western coal basins. 

LOW-SULFUR COAL RESOtJRCES OF THE APPALACHIAN BASIN 

Low-surful coals (generally less than 1 percent sulfur) of the Appalachian basin (fig. 4) 

are less abundant and more in demand than are 

coals of other qualities or in other locations. 

This is the premium coal, commanding a higher 

price for metallurgical use, domestic heating, 

electric generation, and export. 

The estimates of low-sulfur coal presented 

here are based on the extrapolation of the proportion of low-sulfur coals to the region as a 

whole from the recent study of selected areas 

that have received comprehensive study by the 

U.S. Bureau of Mines (DeCarlo and others, 

1966). The units and boundaries of the diagram are the same as for the preceding diagrams of coal resources, except for the average 

percentage of recovery, which, for this higher 

priced coal, is closer to 60 percent than to the 

50 percent previously used. 

The low-sulfur coal estimates were compiled 

by K. J. Englund, mainly from the data of 

Averitt (1969) and DeCarlo, Sheridan, and 

Murphy (1966). 

PETROLEUM LIQUIDS AND NATURAL-GAS 

RESOURCES 

The quantities of oil and natural-gas liquids 

(NGL) in figures 5-8 are uniformly reported in 

barrels of 42 standard U.S. gallons, whereas 

Undiscovered 

Identified resources resources 

Recoverable 2 2 

70 

Submarginal 240 

FIGURE 3.-Scaled diagram of bituminous coal resources of the Appalachian basin (billions of short tom:). A part of the coal resources of the United States (fig. 2). 

Reliability of estimates decreases downward and to the right. Compiled by Paul 

Averitt, February 1972. 

5 
Identified resources Undiscovered resources 

Recoverable 1 2 

2 5 

Submarginal 70 

FIGURE 4.-Scaled diagram of low-sulfur coal resources of the Appalachian basin (billions 

of short tons). A part of the bituminous coal resources of the Appalachian basin 

(fig. 3) and a part of the coal resources of the United States (fig. 2). Reliability 

of estimates decreases downward and to the right. Compiled by K. J. Englund, 

February 1972. 

the quantities of natural gas are reported in 

cubic feet at a sea-level pressure of 14.73 

pounds per square inch at 60°F. 

The quantities of petroleum and natural-gas 

resources shown in the various categories of 

figures 5-8 and 9-12 were derived from annual 

reports of the American Petroleum Institute 

(API) and the American Gas Association 

(AGA) and estimates prepared by the U.S. 

Geological Survey (USGS). The quantities reported under identified recoverable resources 

in figures 5-8 are the sums of estimates of 

proved reserves and indicated additional reserves of crude oil published by API plus estimates of pt·oved reserves of NGL published by 

AGA (Am. Gas Assoc. and others, 1971). 

(AGA does not estimate indicated additional 

reserves for the NGL.) The quantities reported 

under identified recoverable resources in figures 9-12 are estimates of proved reserves vf 

natural gas published by AGA. These estimates 

assume recovery factors of 35 percent for oil 

and 80 percent for NGL and for gas. 

The API and AGA published estimates of 

identified resources of petroleum liquids and 

natural gas are the only ones available for the 

United States. These estimates are prepared 

annually by local committees made up of petroleum and natural-gas specialists from both private industry and State governments. These 

estimates are made on a field-by-field basis and 

are then agglomerated into a single report giving reserves by States. 

Identified submarginal resources reported in 

figures 5-12 are estimates of quantities of petroleum or natural gas that cannot be recovered 

from known fields under current technologic 

and (or) economic conditions. The quantities 

shown in this category in figures 5-9 are for 

crude oil only and were derived by subtracting 

cumulative production, proved reserves, and 

indicated additional reserves from the API estimate of original oil-in-place. As there is no 

published estimate of original NGL-in-place 

and as the NGL recovery factor is approximately 80 percent of NGL-in-place, no attempt 

was made to calculate a quantity of NGL for 

the identified submarginal category. 

The case of natural gas, in this respect, is 

similar to that of NGL. However, as the identi6 
dentifiec Undiscovered resources 

resources 

Recoverable 52 450 

Submarginal 290 2100 

FIGURE 5.-Scaled diagram of crude oil and natural-gas liquids in the United State:; 

(including the continental margin to 2,500-m water depth) as of Dec. 31, 1970 

(billions of barrels). Reliability of estimates decreases downward and to the right. 

Compiled by S. P. Schweinfurth. 

I I Identified 

resources Undiscovered resources 

Recoverable 11 100 

20 

Submarginal 4 50 

FIGURE 6.-Scaled diagram of crude oil and natural-gas liquids in Alaska (onshore and 

continental margin to 2,500-m water depth) as of Dec. 31, 1970 (billions of barrels). 

Included in resources of figure 5. Reliability of estimates decreases downward and 

to the right. Compiled by S. P. Schweinfurth. 

7 
~~ 

l! 

~ Undiscovered resources 

Recoverable 6 200 

.... 

.... 

. 

-; 

.-< E 

" ... a c:o 17 550 ..... 

·3 6 c: 

0 

u 

.... 

"' 

~ 

~ J> 

~ a. "' 0 

~ E 

0 .... 0 

~ U'). 700 ;~ 

-~ ~ c: 

0 

u 

FIGURE 7.-Scaled diagram of crude oil and natural-gas liquids offshore from the United 

States to 2,500-m water depth) as of Dec. 31, 1970 (billions of barrels). Included in 

resources of figure 5. Reliability of estimates decreases downward and to the right. 

Compiled by S. P. Schweinfurth. 

Identified Undiscovered resources resources 

--

Recoverable 3 6 230 

800 

Submarginal 2 2 0 

FIGURE 8.-Scaled diagram of crude oil and natural-gas liquids onshore in the conterminous United States as of Dec. 31, 1970 (billions of barrels). Included in resources 

of figure 5. Reliability of estimates decreases downward and to the right. Compiled 

by S. P. Schweinfurth. 

8 
Identified 

resources Undiscovered resources 

Recoverable 2 9 0 2100 

170 

Submarginal 4000 

FIGURE 9.-Scaled diagram of natural gas resources in the United States (includes continental margin to 2,500-m water depth) as of Dec. 31, 1970 (trillions of cubic feet). 

Reliability of estimates decreases downward and to the right. Compiled by S. P. 

Schweinfurth. 

Identified Undiscovered resources resources 

Recoverable 3 1 480 

8 

Submarginal 860 

FIGURE 10.-Scaled diagram of natural gas resources in Alaska (onshore and continental margin to 2,500-m water depth) as of Dec. 31, 1970 (trillions of cubic feet). Included in resources of figure 9. Reliability of estimates decreases downward and to 

the right. Compiled by S. P. Schweinfurth. 

9 
Undiscovered resources 

Recoverable 40 850 

14 

'---- Continental shelf 900 G-200 m 

! 

.. 

... 

3 

~ 

~ 

Continental slope 1600 200-2,500 m 

FIGURE 11.-Scaled diagram of natural gas resources offshore from the United States 

(continental margin to 2,500-m water depth) as of Dec. 31, 1970 (trillions of cubic 

feet). Included in resources of figure 9. Reliability of estimates decreases downward 

and to the right. Compiled by S. P. Schweinfurth. 

Identi.fied Undiscovered resources 

resources 

Recoverable 220 1000 

150 

Submarginal 1500 

f 

FIGURE 12.-Scaled diagram of natural gas resources onshore in the conte1·minous United 

States as of Dec. 31, 1970 (trillions of cubic feet). Included in resources of figure 9. 

Reliability of estimates decreases downward and to the right. Compiled by S. P. 

Schweinfurth. 

10 
tied quantity of proved reserves and cumulative production of natural gas is so large, it 

was felt that it would be worthwhile to include 

an estimate of identified submarginal gas resource, some of it might, one day, be recovered 

by application of a supplementary or advanced 

recovery technique. The quantities shown were 

calculated by applying a recovery factor of 80 

percent to the estimates of proved reserves and 

cumulative production reported by AGA. 

Undoubtedly, some part of the quantities reported of identified submarginal liquids and 

gas could become recoverable with an increase 

in well-head price or equivalent advances in 

technology, and, therefore, could now be called 

paramarginal. The data to make such a breakdown meaningful, however, are not available. 

The terms used by API and AGA are defined as follows : 

Proved reserves of crude oil are defined as 

the estimated quantities of crude oil which geological and engineering data demonstrate with 

reasonable certainty to be recoverable from 

know reservoirs under existing economic and 

operating conditions. 

Proved reserves include identified crude oil 

which can be produced through application of 

fluid injection or other recovery techniques. 

Indicated additional reserves of crude oil 

are certain quantities of crude oil that may be 

recovered from known reservoirs expected to 

respond to secondary-recovery techniques when 

installed. 

0Tiginal oil-in-place is the estimated number of barrels of crude oil in known reservoirs 

prior to any production. It includes past production and recoverable and nonrecoverable oil. 

Proved reserves· of natural gas and NGL 

have the same definitions as crude oil. 

The quantities of undiscovered resources reported in figures 5-12 are estimates, considerably round~d, that were prepared by the USGS 

(Hendricks, 1965; Hendricks and Schweinfurth, unpub. data, 1966; McKelvey and others, 

1969; Schweinfurth, unpub. data, 1971). 

Undiscovered resources of crude oil were estimated first by the USGS by extrapolating the 

results of past exploration drilling to the remaining unexplored but favorable rocks of the 

United States. Allowance was made for the expected decline in reward (oil found per well 

drilled) for future drilling. (Details of the 

USGS estimating method are discussed in the 

following section, along with some additional 

estimates of undiscovered petroleum resources.) The undiscovered resources of natural gas and NGL were calculated by applying 

a gas-oil ratio to the crude-oil estimate and 

then by applying a NGL-gas ratio to the calculated quantity of natural gas. 

This method was applied to the total favorable rocks of the United States to a drilling 

depth of 20,000 feet. The resultant estimate 

was then broken down into separate figures for 

the conterminous United States, Alaska, and 

their respective continental margins to water 

depths of 2,500-m (8,250 ft). 

Estimates of undiscovered resources of crude 

oil and NGL are reported together as petroleum liquids in figures 5-8, and estimates of 

natural gas are given separately in figures 

9-12. 

Undiscovered resources are separated into 

recoverable and submarginal categories. Recoverable undiscovered resources are quantities of petroleum liquids and natural gas that 

may be found using existing exploration technology and produced under approximately current conditions of economics and technology. 

The quantities of liquids and gas reported in 

the submarginal undiscovered category are 

those quantities estimated to be present but 

that cannot now be produced if found, or that 

might never be found because of small size or 

remote location. The actual quantities reported 

were calculated by applying first a discovery 

factor (it is assumed that five-eighths of the 

resource in the ground will be found) and then 

a recovery factor to the estimated total undiscovered liquids-or gas-in-place. The estimates 

for the United States continental slope (200-

2,500 m), however, are entirely in the submarginal category because there has been almost 

no attempt to explore for, and no attempt to 

produce, petroleum in waters deeper than 200 

m. As a result all the quantities reported in the 

undiscovered submarginal resources of figures 

5-7 and 9-11 are proportionately much larger 

in relation to the identified submarginal resources than the recoverable undiscovered resources are to the identified recoverable resources in the same figures. 

11 
As for identified submarginal petroleum liquids and natural-gas resources, part of the 

undiscovered submarginal resources could be 

recovered when found. Therefore, this part 

could be considered to be paramarginal if real 

prices were to increase and if technology were 

to advance beyond today's levels, but there is 

no way at this time to tell what the amounts 

of the resources might be. 

COMPARISON WITH OTHER PETROLEUM LIQUIDS 

AND NATURAL-GAS RESOURCE ESTIMATES 

The estimates included were selected on the 

basis of timeliness and broadness of coverage. 

No one of them, however, is directly comparable with any of the others because each was 

made by a different method or methods (although some are quite similar in method) and 

(or) for a larger or smaller part of the total 

ground favorable for petroleum. 

The authors of the estimates chosen are 

listed here; their estimates will be discussed 

in some detail later. The authors are: (a) Miller and others (1970); (b) U.S. Geological 

Survey: Hendricks and Schweinfurth ( unpub. 

data, 1966) ; McKelvey and others (1969) ; 

Schwienfurth (unpub. data, 1971); (c) Hubbert (1969); (d) Weeks (1958, 1959); (e) 

Moore (1970); (f) Elliott and Linden (1968) ; 

(g) Potential Gas Committee (PGC) (Colorado School Mines Found., Potential Gas Agency, 1971); and (h) Nelson and Burk (1966). 

Estimates of these resources have been fitted 

as well as possible into the diagram of McKelvey (1972) for purposes of comparison 

with each other. Figures 13-18 summarize the 

results. For detailed terminology, however, it 

was deemed advisable to use the terminology 

of the U.S. petroleum industry, which describes 

categories of known reserves and potential future reserves, because industry groups (API, 

AGA, PGC, NPC) have produced the most detailed definitions and work on this subject. It 

was relatively easy to relate, at least generally, 

the terminology of the industry to McKelvey's 

terminology. 

TERMINOLOGY 

KNOWN DEPOSITS 

Known resources, cumulative (past) production, proved reserves, and indicated additional 

reserves are terms defined and used by the API 

in its annual report of reserves of crude oil in 

the United States (see, for example, Am. Gas 

Assoc. and others, 1971). The API also reports 

on discovered total original oil-in-place. These 

terms refer to crude oil, and crude oil only, that 

has been discovered as of a certain date (always the end of December of the given year). 

Cumulative production, proved reserves, and 

indicated additional reserves refer to quantities 

that either have been or could be produced under current economic and technologic conditions. The difference between these quantities 

and discovered total original oil-in-place gives 

McKelvey's known paramarginal and submarginal resourees (nonrecoverable under current 

conditions) of crude oil as of the same reporting date. 

The API also reports proved reserves of 

NGL but does not report either cum'ulative production, or indicated additional reserves, or 

total original NGL-in-place for the gas liquids. 

Apparently there is yet no reliable statistical 

series on either total original NGL-in-place or 

cumulative production of NGL, and the category indicated additional reserves is not applicable to gas liquids because they are not generally amenable to secondary-recovery methods, 

whereas crude oil is. Therefore, a quantity for 

past production of NGL was estimated by the 

USGS but no estimate of known para- and submarginal resources of NGL was calculated. 

Known quantities of natural-gas reserves are 

reported as proved reserves by the AGA (see 

Am. Gas Assoc. and others, 1971) as of the end 

of a given year. Cumulative (past) production 

is also reported, whereas discovered total original gas-in-place is not. The report of cumulative production (gas) is at best an estimate, 

because for many years gas produced with oil 

was flared (burned), and no reliable production records were kept. Indicated additional 

reserves are not reported by the AGA for gas 

because it is not very amenable to secondary 

recovery and because initial recovery is high 

(approximately 80 percent of gas-in-place). 

A quantity for known para- and submarginal 

resources of gas was calculated by the USGS, 

however, because the amount is large and because additional or secondary recovery might 

one day become feasible. 

12 
1-' 

~ 

Area 

Conterminous United States 

Alaska 

Conterminous United States 

~ -;;, Alaska (.) c ... 0 

::s ·- ~ ;a Conterminous United States 

... c Alaska Q) 0 - (.) 

~-

~ ~ Conterminous United States > ... o :; and Alaska (.) (.) 

Q)-

c::: 

Do 

Do 

ell 

Q) I 

~-;;,-;"' ::s § "- ~ 

~ :; ] :; Conterminous United States 

~ ] ! ~ Alaska - 0 ~ ... ~ (.) c- ~-·or,~ . ~ ~ ~ .!: Conterminous Umted States 

8 ~ ~ ~Alaska ~ u ~ E g-Q., 

z 

1 Crude oil 

only. 

Cumulative 

production 

105.6 

.44 

Conterminous United States 

Alaska 

Total 

In known deposits 

Proved Indicated Probable 

additional potential reserves reserves reserves 

36.5 5.2 98.3 

I 0.5 .I 

I ~n. 

I 

I 

I 

I 

I 

I 

I 

268 

20 (API) 

I 

I 

I 

I 

I 

I 

I 

J 

In undiscovered deposits 

Possible Speculative 

potential potential 

reserves reserves 

I 29.6 

(Plus 28.2 NGL to NP(') 

353 

106 

54 

19 

307 

276 

I 3J7 

248.4 

30.1 

I ,663 

465 

Estimates of total petroleum liquids originally in place in the United States 

NPCa 

741.8 

___li&_ 

I 824.4 

61.0 NGL 

885.4 

USGSb 

2,431 

602 

3,'033 

Hubbertc 

575 

~ 

660 

Weeksd Mooree 

I ,315 670 

Elliott and 

Linde/ 

I J,286 

Totals 

Potential 

ultimate 

production 

225.4 a 32.5 INPC) 

I Plus 49 NGL to NP(') 

I 500 b 

117 (USGS) 

I 201 c 

30 (Hubbert) 

I 460 (Weeks)d 

I 436 (Moore)e 

I 1 450 (Elliott ~nd 

Linden) 

516.4 

50.1 <NPC)a 

1,931 

485 (USGS)b 

FIGURE 13.-Some estimates of United States original resources of petroleum liquids (crude oil and natural-gas liquids in billions of barrels). 

Data to Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of the 

figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. Lettered footnote symbols are explained in the section "Discussion of Individual Estimates.'" 
j--l 

~ "' 

"' G>-

u "' .... s:: =' 0 

0 ·= "' G>-o ·- .... s:: 

~ 8 .... 

~­s:: G> G> ;> .... 0 .... u =' G> u c::t:::'-' 

G) I 

::= ~ .g "' =' s:: Cll G> 

~ .g -g ~ ... ·- ~ 0 

~-g";j~ .l:l 0 s:: .... .... 

~ u ·-- l:l.O ~ 

G> - ... s:: ;> s:: "' ·- 8 t ~ ~ ~ 8 ~ E §'-'Q., 

z 

1 

Area 

Conterminuus United States 

Alaska 

(Including Cook Inlet 

and Bristol Bay) 

Conterminous United States 

Alaska (Including Cook Inlet) 

Conterminous United States 

and Alaska 

Conterminous United States 

Alaska 

(Including Cook Inlet) 

Conterminous United States 

Alaska 

Crude oil only. 

In known deposits In undiscovered deposits 

Cumulative Proved Indicated Probable Possible Speculative 

production reserves additional potential potential potential 

reserves reserves reserves reserves 

I I I 

100.4 30.8 5.23 

I I I 

75.3 

0.44 10.54 .1 22.0 

I I 

I I 226 

I I 

40 

I I 243 

NA 

20 (API) 30 

(224) (USGS)b 797 

121 

Estimates of total petroleum liquids originally in place in the land areas of the United States 

Conterminous United States 

Alaska 

Total 

NPCa USGSb· Weeksd 

649.8 

82.6 

1

732.4 

1,383 

192 

1,575 

1,114 

1,114 

Totals 

Potential 

ultimate 

production 

I I 

193.4 a 1

32.5 (NPC) 

I 362 b 

Sl (USGS) 

I 390 (Weeks)d 

1 

4156.4 (NPC)a 

50.1 

I 1,021 b 

141 (USGS) 

FIGURE 14.-Some estimates of United States original onshore resources of petroleum liquids (crude oil and natural-gas liquids in billions of 

barrels). Data to Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of 

the figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. N A, not available. Lettered footnote symbols are explained in the section "Discussion of Individual Estimates." 
1-l 

crt 

cu 

:0 

;::! 

cu 

> 0 

u 

~ s:: 

0 

z 

I 

..,_ 

cu "' u s:: ... 0 

:s ·- 0 .-:: "''t:l cu s:: ... 0 

...2 u .o_ 

<'d s:: ... cu cu ... > ... 0 :s u u cu~ 

~ 

·- ~ ~ ~ -g g a 8 0 $;; .0 ... 

~ ~ ;1 ~ s:: Q., 't:l cu cu- s:: ... ::: c <'d-; 

8 .9 Cii .E ~-s::bO 

'b'o 

Area 

2 

Continental shelf 

(Gulf of Mexico 

and Southern 

California) 

3 

Continental shelf 

and slope (excluding Alaska) 

Continental shelf 

Continental shelf 

and slope ( 600-

1,000 ft water 

depth) 

Continental shelf 

(excluding part 

of Alaska shelf) 

Continental 

shelf2 

Continental 

slope3 

2 Crude oil only. 

3 Shelf, 0-200m (0-656 ft) water depth. 

Slope, 200-2,500m (656-8,250 ft) 

water depth. 

Cumulative 

production 

4.2 

In known deposits In undiscovered deposits Totals 

Proved Indicated Probable Possible Speculative Potential 

reserves additional potential potential potential ultimate 

reserves reserves reserves reserves production 

(API) I 5.7 I >0.03 

I I 

I I 

I I I I 

32 (NPC)a I 23 

I I 198 I 208 (USGS)b 

I I 60 I 70 (Weeks)d 

I I 

I 3 I ? 3 to 22 

I 16 to 35 ~~~l~ou~k)h 

NA (API) 

(17) (USGS) b 

I 

560 567 

690 690 (USGS)b 

Estimates of total petroleum liquids originally in place in United States continental margins 

NPC3 USGSb Weeksd Nelson and Burkh 

Shelf 92 77 5 200 46 to I 00 

Slope 690 

I 

Total 92 I ,465 200 46 to 100 

FIGURE 15.-Some estimates of United States original offshore resources of petroleum liquids (crude oil and natural-gas liquids in billions of 

barrels). Data to Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of 

the figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. NA, not available. Lettered footnote symbols are explained in the section "Discussion of Individual Estimates." 
....... 

0') 

"" 

(!),._ "' 

u "" .... t: = 0 

~ Sl ·-.::: ... -o (I) t: 

- 0 .D u <:':j-

.... t: (I) (I) 

> .... 0 .... 

u (I) u = 

Qi::::'-' 

(I) u,-.' a ,. ~ "" 0 § "" (I) 

~ ·= 'e ~ Q)-o<:':lo .... ·- t: = ,.....c:-~ f 8.5 ~ Q) - till <:':j > t: ... t: 0 Q) <:':j ·-

~ ~ ~ r; t: ~ ~ E 0 Q.. 

z 

Area 

Conterminous United States 

Alaska 

Conterminous United States 

Alaska 

Conterminous United States 

Alaska 

Conterminous United States 

and Alaska 

Do 

Do 

Conterminous United States 

Alaska 

In known deposits 

Cumulative Proved 

production reserves 

391.1 

0.3 

(AGA) 

259.6 

31.1 

(163) (USGS)b 

(8) 

I 

I 

I 

I 

I 

I 

I 

Probable 

potential 

reserves 

218 

39 

I 

I 

I 

I 

I 

I 

I 

In undiscovered deposits 

Possible 

potential 

reserves 

326 

67 

1,579 

479 

399 

119 

318 

865 

1,058 

3,297 

862 

Speculative 

potential 

reserves 

307 

227 

Estimates of total natural gas originally in place in the United States 

PGc8 USGSb Hubbertc Weeksd Mooree 

Elliott fnd 

Linden 

Conterminous United States 1,877 5,690 1,312 

Alaska 447 1,380 637 

Total 2,324 7,070 1,949 1,250 1,934 2,175 

I 

I 

I 

I 

I 

I 

Totals 

Potential 

ultimate 

production 

1,502 g 358 (PGC) 

2,230 b 

510 (USGS) 

1,050 c ISO (Hubbert) 

1,000 (Weeks)d 

I ,54 7 (Moore)e 

1, 740 (Elliott pnd 

Linden) 

3,460 (USGS)b 

870 

FIGURE 16.-Some estimates of United States original resources of natural gas (in trillions of cubic feet, S.T.P. (standard temperature and pressure)). Data to Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of the 

figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. Lettered footnote symbols are explained in 

the section "Discussion of Individual Estimates." 
........ 

-.::) 

"' ou_ 

(.) "' ... s:: ::1 0 ~ ·.;::: 41) ·- ... -o 41) s:: - 0 ,r;; (.) ~ .... ... s:: ~ ~ 0 ... (.) ::1 41) (.) 

0::'-' 

41) "' 

(.)-.6 

:; "' ::1 "' ~ s:: "' 41) 

41) .!2 "0 ~ ... ::: s:: ::1 

41) "0 ~ ~ 

- s::- 41) 

~ 8 ~ ... J-t ~- -- 41) - C() ~ 

~ E ~ .s :..~:::: E ~ 41) ::1 ~ ~ 2 ~ ~ E 0 Q.. 

z 

In known deposits In undiscovered deposits 

Area Cumulative Proved Probable Possible Speculative 

potential potential potential production reserves reserves reserves reserves 

(AGA) I I 

Conterminous United States 374.2 220.5 179 I 227 207 

I Alaska .3 31.1 NA 

I I Conterminous United States l ,015 

Alaska 199 

Conterminous United States I I 

and Alaska I I 62 

NA (AGA) 

Conterminous United States ( 149) 1,491 

Alaska (8) (USGS)b 191 

Estimates of total natural gas originally in place in the land areas of the United States 

Conterminous United States 

Alaska 

Total 

PGCg USGSb Weeksd 

I ,51 0 3,250 860 

430 

1,510 3,680 860 

Totals 

Potential 

ultimate 

production 

1 ,208 (PGC)g 

NA 

1,610 (USGS)b 

231 

688 (Weeks)d 

1,640 b 

199 (USGS) 

FIGURE 17.-Some estimates of United States original onshore resources of natural g;as (in trillions of cubic feet, S.T.P.). 

Data to Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of the figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. NA. 

not available. Lettered footnote symbols are explained in the section "Discussion of Individual Estimates." 
........ 

00 

<lJ 

:0 

e 

<lJ 

;;. 

0 

u 

<lJ 

t: 

c 

z 

I 

~ :r. 

<lJ = u 0 

~ ·= 9.] 

~ 0 <lJ u 

::Cc: 

"" <lJ .... .... (l) .... 

;;. ::l 0 u u 

<lJ 

0::: 

"-

-- <lJ :r.. ~ = "'0 .... 0 = ::l 

·= ~ ~ 

:g ~ ~ 

8 '§~ 

:o::l ·-

= (l) E :o::l ~ :o::l 

t: ~ E BQ..-; 

- "':/'. 

Area 

I 

U.S. Continental shelf 

(Gulf of Mexico and 

Southern California) 

Conterminous United States I 

continental shelf 

and slope (part, 656-

1500 ft water depth) 

I 

U.S. Continental shelf 

I 

U.S. Continental shelf 

and slope (part, 656-

l 000 ft water depth) 

U.S. Continental shelf 

(excluding part of 

the Alaska shelt) 

I 

I 

U.S. Continental shelf 

2 

U.S. Continental slope 

Shelf, 0-200m(0-656 ft) 

2 water depth. 

Slope, 200-2,500m (656-

8,250 ft) water depth. 

In known deposits 

Cumulative 

production 

(AGA) 

16.9 

Proved 

reserves 

39.1 

(14) (USGS)b 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

Probable 

potential 

reserves 

39 

27 

I 

I 

I 

I 

I 

I 

I 

I 

') 

I 

_l 

In undiscovered deposits 

Possible 

potential 

reserves 

89 

10 

844 

256 

5 to 85 

886 

I ,590 

Speculative 

potential 

reserves 

72 

28 

Estimates of total natural gas originally in place in United States continental margins 

Shelf 

Slope 

Total 

PGC'g USGSb Wecksd Nelson lind Burkh 

320 1.800 390 ll0-210 

47 1,590 

367 3,390 390 110-210 

Totals 

Potential 

ultimate 

production 

256 

38 <PG(')g 

900 (USGS)b 

312 (Weeks)d 

88 to 168 (Nelson and 

Hurk)h 

900 (USGS)b 

l ,590 ( USGS)b 

FIGURE 18.-Some estimates of United States original offshore resources of natural gas (in trillions of cubic feet, S. T.P.). Data to 

Dec. 31, 1970. The values reported in this figure are not rounded but are simply arithmetic calculations. The accuracy of the figures is largely unevaluated, and the number of significant figures is not indicated in the numbers. Lettered footnote symbols are 

explained in the section "Discussion of Individual Estimates." 
UNDISCOVERED DEPOSITS 

The terms probable-, possible-, and speculative-potential reserves are defined by the Potential Gas Committee (Colorado School Mines 

Found., Potential Gas Agency, 1971, p. 11) to 

categorize estimates of undiscovered but recoverable, if found, resources of natural gas by 

degree of reliability of the estimate. Reliability 

decreases from left to right, probable being the 

most reliable and speculative the least. The 

probable category includes, however, some 

known resources, in the sense of having been 

discovered but not completely developed, and 

some unknown resources, so that this category 

brackets the boundary between the known and 

the unknown. This terminology was adopted 

by the National Petroleum Council ( NPC) 

(Miller and others, 1970) for its report on future petroleum provinces of the United States, 

but estimates were reported as probable-possible combined and speculative. 

Estimates of potential reserves of NG L are 

not made by the PGC. The NPC did estimate 

potential NGL reserves but reported them in a 

lump sum for the United States and not in the 

aforementioned categories. 

The term potential ultimate production under the total columns of figures 13-18 is one 

used here for convenience by the USGS and is 

self explanatory. It is the sum of estimates of 

cumulative production, known recoverable, and 

undiscovered recoverable resources of petroleum. 

The term current conditions refers to the 

current well-head price structure for oil and 

gas and current recovery factors, or, more succinctly, to current economics and technology. 

TOTAL RESOURCES IN PLACE 

The estimates of total petroleum liquids, or 

natural gas, originally in place in the United 

States (lower part of figs. 13-18) are estimates of total original quantities of petroleum 

in discrete deposits in the earth -and consist 

of the sum of the quantities for a given estimate in each of the four categories of resources, that is, known recoverable+known 

para- and submarginal+undiscovered recoverable +undiscovered para- and submarginal. In 

most estimates, the quantities of undiscovered 

para- and submarginal resources had to be calculated from the author's reported estimate of 

ultimate production by applying an appropriate recovery factor (80 percent for gas and 

NGL, 35 percent for oil). 

INDIVIDUAL ESTIMATES 

The API and the AGA have provided the 

only estimates of known resources of oil and 

gas. Therefore, their data alone were used to 

fill in those categories except for known paraand submarginal resources of natural gas, 

which were estimated by the USGS. 

Estimates of undiscovered resources made by 

the NPC and PGC for liquids and gas, respectively, are listed first in figures 13-18 because 

these agencies were most responsible for the 

development and use of the terminology. Other 

estimates are listed in order of the relative 

amount of detail reported. For total United 

States and onshore United States (figs. 13, 14, 

16, 17), it was felt that the conterminous States 

and Alaska should be reported separately insofar as possible because of the distance and 

transportation difficulties associated with 

Alaska. For the United States offshore areas 

(figs. 15 and 18), it was felt that the most important aspect is water depth; division of the 

area into shelf and slope has become customary (by convention, the shelf is the area between 0 and 200 m (0 and 656 ft) and the 

slope is the area between 200 and 2,500 m 

( 656 and 8,250 ft) ) . 

METHODOLOGY 

In general, two basic methods, geologic and 

mathematical were used by the authors reported here; in detail, however, each author 

used a method that was somewhat different 

from the others. 

GEOLOGIC METHODS 

The geologic method was used by the NPC, 

PGC, USGS, and L. G. Weeks, and apparently 

by T. W. Nelson and C. A. Burk; the mathematical method was used by M. K. Hubbert, 

C. L. Moore, and M. A. Elliott and H. R. Linden. Both basic methods employ more or less 

19 
of the statistical records of past oil and gas 

explorations and production; the mathematical 

method is based entirely on those statistics. 

Comparison is difficult between the estimates made by each basic method but, roughly, 

the most similar in terms of method are the 

estimates made by the NPC, PGC, and L. W. 

Weeks. Possibly Nelson and Burk's work also 

belongs in this group, but they also use the 

mathematical methods. 

In terms of area and depth (rock volume) 

of favorable strata, the estimates of the USGS 

were applied to the largest potential area. The 

PGC comes next closest in area; however, as 

it includes potential rocks to drilling depths 

of 30,000 feet, whereas the USGS estimates 

include potential rocks only to drilling depths 

of 20,000 feet, the rock volume used by the 

PGC might be as large as that used by the 

USGS. 

MATHEMATICAL METHODS 

Theoretically, at least, all the mathematical 

estimates cover the same area (and volume), 

which should be the total of all the potential 

oil- and gas-producing rocks in the United 

States. This is so because the mathematical 

methods supposedly take into account all evolutionary trends in the exploration and production of oil and gas, and one very definite 

trend is toward the deeper rocks and more remote areas believed to have oil and gas potential. Consequently, the areas (volumes) taken 

into account should include at least as much 

territory as the USGS estimates do. (Practically speaking, they probably do not, but this 

subject is too complicated for these notes.) 

Other authors have included more or less of 

the potential oil- and gas-producing area..s, for 

various reasons to be discussed later. 

Most estimates of resources of natural gas 

(and NGL) are arrived at by applying a gasoil ratio to estimated oil resources, This method, called the gas-oil ratio method, although 

less satisfactory, was used by the USGS and 

M. K. Hubbert. L. C. Weeks and Nelson and 

Burk have provided little information on their 

methodology, but they may have used some 

form of this technique. 

The ratio of NGL to gas is applied to the 

estimate of gas resources to obtain the estimate of resources of NGL. Estimates of potential future reserves by other than the NPC 

and PGC are in figures 13-18, centered under 

the undiscovered deposits column below the 

possible potential rese1'ves column only for convenience. The estimates actually range from 

the probable to the speculative categories but 

cannot be broken down in that detail because 

of lack of information. 

DISCUSSION OF INDIVIDUAL ESTIMATES 

Figures 13-18 provide a tabular comparison 

of the various estimates according to the category of the resource being estimated. In the 

discussion that follows, the letter designations 

preceding each section serve as footnotes to 

these figures. 

(a) NATIONAL PETROLEUM CouNCIL 

The data used were taken from table 22, p. 

104, and several paragraphs of the report by 

Miller and others (1970). Their table 22 was 

based on a recovery factor for crude oil of 30 

percent. 

The original oil-in-place was estimated by 

NPC for each of 11 regions, in the three categories of probable, possible, and speculative, 

but the estimates for the probable and possible 

categories were reported as one figure. Ultimate recovery estimates were also made by the 

NPC using recovery factors of 42 percent and 

60 percent, but they have not been included in 

figures 13-18. 

A flat estimate of 49 billion barrels of ultimately recoverable NGL was reported in table 

25, p. 108, of the NPC report (Miller and 

others, 1970) without any details as to how 

this quantity should be assigned to the various 

regions. Therefore, it is reported as a single 

number in figure 13. The quantity of original 

NGL-in-place reported under the NPC at the 

bottom of figure 13 was calculated by the 

USGS on the basis of an 80 percent recovery 

factor. 

Future potential gas reserves were also reported in the NPC publication, but as these 

had been received from the PGC, they are reported in figure 16 with PGC as the source. 

A variety of geologic methods were used in 

preparing the NPC report. The methods in20 
elude extrapolation and interpolation along 

trends, comparison of similar areas and strata, 

evaluation of structures determined by geophysical prospecting, evaluation of the undrilled parts of known fields and pools, and 

assignment of average oil incidence to rock 

volumes. The results, though conservative, are 

probably the most detailed estimate of potential crude-oil reserves that has been made for 

the United States to date. 

Some oil in the probable-possible category 

can be considered to have been already discovered but not cited as proved reserve at the 

time the NPC report was prepared. This is oil 

in undeveloped parts of fields and pools that 

have been "discovered" but not drilled out. 

There is no way to tell, from the report, how 

much this might be, but roughly it might be 

assumed to be an amount nearly equal to the 

proved reserves reported by the API. 

The NPC report does not cover all potentially favorable areas. Those left out are onshore and offshore areas of Washington and 

Oregon ; parts of North Dakota, South Dakota, 

Michigan, Montana, and the Great Basin area; 

the offshore areas of Alaska, except for parts 

of Cook Inlet and Bristol Bay; and the offshore 

areas of California north of the Santa Barbara 

Channel. 

(b) THE U.S. GEOLOGICAL SURVEY 

The crude oil estimates made by the USGS 

have been prepared by a method developed by 

A. D. Zapp and T. A. Hendricks (Hendricks, 

1965) . The method is basically geologic and 

involves applying the historical results of past 

exploratory drilling to unexplored rocks on an 

equal basis for all of the favorable strata of 

the United States, including Alaska and the 

continental margins to a water depth of 2,500 

m (8,250 ft). This is, therefore, the only 

geology-based estimating method that is entirely consistent. This method is not necessarily good, however, in estimating potential resources because not all regions are alike in 

their geology and petroleum potential. On the 

other hand, if the method is applied to large 

enough areas, it is felt that errors will average 

out, and at least a valuable first approximation 

will result. 

The resulting estimate is of total original 

oil-in-place from which the various other categories can be calculated by applying suitable 

finding and recovery factors. It is assumed 

that five-eighths of the oil-in-place could be 

found with current exploration technology and 

that 40 percent of the oil found could be produced ; this defines the proportion of the original oil-in-place that would be recoverable. 

(Forty percent was used to take into account 

projected near-term increases in crude-oil recovery.) Potential resources of natural gas and 

NGL are determined by applying a gas-oil 

ratio to estimated quantities of oil and a NGLgas ratio to the resultant gas estimate. The 

application of gas-oil and NGL-gas ratios differs from the usual in that the ratios used are 

for gas-in-place to oil-in-place, whereas the 

usual method is to use only ratios of proved 

reserves of gas to oil or ratios of cumulative 

production of gas to oil. The result is original 

gas and NGL-in-place to which a finding factor 

of five-eighths is applied, as for oil. In this case, 

however, a recovery factor of 80 percent is 

used to reflect the average experience in gas 

and NGL production. 

Para- and submarginal resources are those 

either not recoverable, or not found and not 

recoverable because of size and location or recovery technology and low price. Future advances in recovery technology or increases in 

price could make some of these known and undiscovered para- and submarginal resources 

recoverable. 

Resource estimates have been prepared by 

the USGS for the conterminous United States 

and Alaska and for two zones of the continental margins, from 0 to 200 m (shelf) and 

200 to 2,500 m (slope). The resources estimated for the continental slope are believed 

to be generally out of reach both technologically and economically at this time and therefore have been placed entirely in the undisco?)-

ered para- and submar·ginal categories. 

(C) M. K. HUBBERT 

Hubbert prepared his estimate of ultimate 

production of crude oil mathematically from 

historical data on reserve additions for the 

conterminous United States and its contiguous 

21 
shelves. Ultimate gas production was calculated from a gas-oil ratio and NGL from a 

NGL-gas ratio. For Alaska, Hubbert used geologic analogy to estimate ultimate crude-oil 

production and gas-oil, NGL-gas ratios for the 

ultimate production of gas and NGL. The estimate of original resources in place shown on 

figures 14, 15, 17 and 18 were calculated by 

the USGS. There is no way to subdivide Hubbert's estimates for the land and continental 

margins. 

(d) L. G. WEEKS 

Weeks (1958, 1959) prepared his estimate 

of ultimate production by geologic methods but 

never fully described the methods nor whether 

or not his estimate of ultimate gas production 

was based on a gas-oil ratio. 

He made an estimate of ultimate production 

for the conterminous United States and 

Alaska combined land and offshore areas, but 

areas such as the Atlantic Coast and offshore, 

where there had been very little or no petroleum experience, were not included, and the 

offshore areas were carried only to a water 

depth of 1,000 feet (1,000 feet is only slightly 

deeper than the seaward edge of the continental shelf). 

(e) C. L. MooRE 

Moore based his resource estimates on mathematical calculations of the historic patterns 

of discovery and recovery of each commodity 

separately. Consequently, there are independent estimates for crude oil, natural gas, and 

NGL. For oil, Moore calculated total original 

oil-in-place and applied a calculated ultimate 

recovery factor to that to get ultimate oil production; this was not done for either gas or 

NGL, however, so the USGS calculated original in-place values for them. 

The quantity for ultimate crude-oil production reported by Moore is based on a much 

higher recovery factor than any of the other 

estimates used. There was no way, however, 

to back up an ultimate production at either 30, 

35, or 40 percent, so Moore's estimate was reported just as given by him. By Moore's calculation, much future oil production will have 

to come from both the known and unknown 

para- and submarginal resource category, so 

the numbers reported cannot be broken down 

into known and undiscovered pa~ra- and submarginal categories with any meaning. 

(f) M.A. ELLIOTT AND H. R. LINDEN 

Elliott and Linden used a mathematical 

method based on the historical statistics of 

cumulative production and reserve additions 

to arrive at independent estimates of the ultimate production (called ultimately economically recoverable by them) both of crude oil 

and natural gas. They did not separate these 

estimates into regional subdivisions nor did 

they estimate NGL resources. Estimates of 

total original resources in place were calculated by the USGS. 

(g) PoTENTIAL GAs CoMMITTEE 

The estimates of potential supply of natural 

gas in the United States reported by the PGC 

(Colorado School of Mines Foundation, Inc., 

Potential Gas Agency) are made by a variety 

of geologic methods (these were outlined above 

under the NPC). These estimates are made 

for the largest possible area of favorable 

ground after those of the USGS and are made 

independently of a gas-oil ratio. 

Gas resources are reported by three categories of reliability and for 12 areas of the 

United States. Within the 12 areas, except for 

Alaska, resources are reported by two drilling 

depths on land and by two water depth zones 

offshore. The drilling zones are 0-15,000 feet 

and 15,000--30,000 feet, and the offshore water 

depth zones are 0-600 feet and 600-1,500 feet. 

Estimates for Alaska are not further broken 

down by depths or areas. 

Part of the quantities estimated in the probable potential reserve category can be considered as discovered for the reasons discussed 

above under the NPC. 

The PGC does not estimate total original 

gas-in-place, so the estimate shown was prepared by the USGS. 

(h) T. W. NELSON AND C. A. BURK 

Nelson and Burk's estimates were only for 

"undeveloped and unfound" petroleum reserves of the United States continental mar22 
gins. Presumably these estimates were prepared by geologic methods, but this is not explained by the authors. The estimates do not 

include all of the Alaska shelves. 

The estimated quantities were reported as 

prospective and speculative and include petroleum liquids and natural gas. The prospec~ 

tive quantities were equated in figure 18 with 

the probable potential category but with a 

question as to how much might actually fall 

under the possible potential category. The 

speculative quantities are given as a range by 

the authors and were placed herein in figure 

18 under the possible and speculatire categories. Potential ultimate production and original resources-in-place were calculated by the 

USGS. 

From the tone of Nelson and Burk's report, 

it is assumed that they were dealing only with 

the continental shelf, or the zone 0-200 m 

deep. 

URANIUM RESOURCES 

The resources are presented in two diagrams. The first is for conventional deposits 

where uranium is the major product of value 

(fig. 19). The second is for deposits of other 

!dent if ied resources 

Recoverable 250 

Submarginal 200 

materials from which uranium may be recovered as a byproduct (fig. 20). Short tons of 

uranium oxide (U::O") in the ground are the 

units for estimation of uranium resources. 

All the identified recoverable resources are 

in conventional deposits. The estimate, 250,000 

tons, is rounded from the AEC estimate of 

246,000 tons as of January 1971 (U.S. Atomic 

Energy Comm., 1971), based on a cutoff price of 

$8 per pound of U::O". Mining loss is not significant. The identified recoverable resources 

estimate has an accuracy of about plus or 

minus 20 percent. 

The identified submarginal resource is estimated to a cutoff at $15 per pound of U::O,_. 

About a quarter of the submarginal material 

could be recovered at $10 per pound of U::O,_. 

The submarginal resource is less than the recoverable resource because the halo of lower 

grade material surrounding the ore is generally small. 

Conventional undiscovered resources in 

known uranium districts are estimated to be 

twice the amount of the identified resource. 

This estimate is based on the productivity of 

favorable rocks along their outcrop, measured 

in tons per linear mile of exposure. This measure is then extrapolated to the third dimenUndiscovered resources 

In unknown In known districts districts 

500 

250 

400 

FIGURE 19.-Scaled diagram of uranium resom·ces of conventional deposits in the United 

States (thousands of short tons of U,O,). Heliability of estimates decreases downward 

and to the right. Compiled by A. P. Butler, Jr., and W. I. Finch, February 1972. 

23 
Undiscovered resources 

ldentif ied resources 

Recoverable 

Paramarginal 1000 

Submarginal 5000 

2 50 

1 0 0 0 

Not 

estimated 

FIGURE 20.-Scaled diagram of uranium resources in phosphate rock in the United States 

(thousands of short tons of U:O,). Reliability of estimates decreases downward and to 

the right. Compiled by A. P. Butler, Jr., and W. I. Finch. 

sion, down the dip of the favorable rocks beneath the basin, to obtain an approximation 

of the total amount of uranium that may be 

present. 

Most of the sedimentary basins favorable for 

the accumulation of conventional uranium deposits have been tested, so it is not likely that 

the resource in undiscovered deposits in undiscovered districts will be large. This last 

figure is estimated to be about the same as 

the identified resource. The estimate of undiscovered resources is thought to be within about 

a factor of 2 of the correct value. The total 

resource base is estimated to be about 

1,600,000 tons. 

Byproduct uranium resources are dominated 

by the potential resource in phosphate rock. 

At present, all this resource is submarginal. 

Two grades of material are considered in the 

diagram. The paramarginal resource is rock 

containing at least 31 percent P:!Oii and approximately 0.012 percent U::O,. Recovery as 

a byproduct would be feasible at $10 per pound 

of U:{Os. The submarginal resource is in rock 

containing 18-30 percent P :!Oii and 0.006-0.012 

percent U::O.~. Recovery as a byproduct would 

be feasible at about $20 per pound of U::O,. 

The distribution of this resource into identified 

and undiscovered categories reflects our knowledge of phosphate resources; that is, about 

five-sixths of the anticipated resource is identified. As a potential resource for uranium, this 

part of the resource base diminishes with time 

as the rock is mined for its phosphate content. 

The principal references are U.S. Atomic 

Energy Commission ( 1971) and Bieniewski, 

Persse, and Brauch (1971). 

GEOTHERMAL RESOURCES 

The units of geothermal energy used in figure 21 are calories of heat in the ground. 

These are calories in excess of normal surface 

heat--taken to be about 15° C. 

The identified recoverable resources of geothermal energy are such a small part of the 

total energy resource that subdivision of the 

remainder of the resources base is unrealistic 

at this time. The Geysers, Calif., is the only 

operating geothermal system in the United 

States and provides the only domestic base for 

estimation of the recoverable resource. Measured and indicated resources at The Geysers 

are estimated to be 2 x 101 " and 3 x 10 ·~ calories, 

respectively, in the ground. (Gram-calories, 

equivalent to 4 x 1o-:: Btu, are used throughout 

this discussion.) With current technology, only 

24 
Identified Undiscovered resources resources 

Recoverable 2.5 

> 1 0,0 0 0 

Submarginal 

FIGURE 21.-Diagram of geothermal resources in the United States ( lOh calories). Reliability of estimates decreases downward and to the right. Compiled by L. J. P. 

Muffler and D. E. White, February 1972. 

about 15 percent of this energy is delivered to 

the well head, so the actual recoverable resource in the proved and probable category is 

about 0.8 X 1018 calories. An additional 5 x 10 ~ 

calories in the ground can be inferred for The 

Geysers area, and at least another 5 X 1018 

calories in the ground in other parts of the 

country are reasonably inferred from geologic 

knowledge and from limited exploration drilling. The inferred reserve for the United States 

is, therefore, at least 10 x 1018 calories in the 

ground, or about 1.5 X 1018 calories recoverable 

with current technology. The total identified 

recoverable resource for the United States is, 

therefore, about 2.5 x 1018 calories. Geothermal 

energy in the National Parks System is excluded from the recoverable estimate. 

The estimate of undiscovered resources 

shown in the figure as 10,000 x 1018 calories is 

based on White's (1965) estimate of 1022 calories in the ground in all hydrothermal systems 

of the world. The United States resource is 

about 10 percent of this, and the estimate is 

increased by a factor of 10 to include abnormally hot, or possibly molten, rocks of low 

permeability, and deep sedimentary basins of 

near-normal conductive gradient. The estimate 

includes resources only to a depth of 10 km 

and is conservative. 

The principal references are White (1965) 

and Muffler and White (1972). 

OIL-SHALE RESOURCES 

Resources of oil shale (fig. 22) are considered submarginal, although the best grade and 

most accessible deposits would yield oil at a 

moderate increase in price of oil into the range 

$4 to $5 per barrel, according to a recent 

study (Natl. Petroleum Council, 1971). 

The units shown are barrels ( 42 gallons) of 

oil that could be obtained from the shale if it 

were treated in a retort to convert part of the 

solid organic matter to oil. Hydrogenation of 

the oil would be required to produce a marketable high-quality synthetic crude. 

The identified resources include deposits 

that have been mapped and sampled sufficiently to determine size and oil-yielding potential 

with moderate accuracy, certainly within plus 

or minus 50 percent. 

The identfiied paramarginal resources have 

recently been estimated by the NPC to contain 

about 80 billion barrels of oil recoverable at 

$4-$5 per barrel. These deposits include the 

more accessible oil shale in Colorado and Utah, 

yielding 30 gallons or more oil per ton. An 

additional 520 billion barrels has been identi25 
Undiscovered resources 

ldent if ied resources 

Recoverable 

Paramarginal 160-600 850 500 

Submarginal 1 6.0 0 2 50 0 2 0,0 0 0 

FIGURE 22.-Scaled diagram of oil-shale resources in the United States (billions of barrels). Reliability of estimates decreases downward and to the right. Compiled by 

D. S. Duncan, February 1972. 

fled in less accessible shale of similar grade. 

Undiscovered resources of this quality within 

3,000 feet of the surface are included in the 

paramarginal category. 

The undi.scove1·ed resources include: ( 1) 

possible extensions of identified deposits, in 

areas where they are inadequately sampled for 

reliable resource estimation; (2) deposits that 

have not been sampled sufficiently to make reliable estimates of oil content; and (3) a statistical estimate of shale containing 10 percent 

or more organic matter capable of yielding 10 

gallons or more oil per ton of shale in sedimentary rocks of the land areas of the United 

States. Deposits extending to a maximum 

depth of 20,000 feet are included. The twofold 

subdivision of the undiscovered resources on 

the diagram falls between category 1 and categories 2 and 3. 

The oil-shale resource estimates were derived mainly from Duncan and Swanson 

(1965). 

REFERENCES CITED 

American Gas Association, American Petroleum Institute, and Canadian Petroleum Association, 1971. 

Reserves of crude oil, natural gas liquids, and 

natural gas in the United States and Canada as of 

December 31, 1970: Am. Gas Assoc., Am. Petroleum Inst., and Canadian Petroleum Assoc., 256 

p. ( v. 25 of an annual publication). 

Averitt, Paul, 1969, Coal resources of the United 

States, January 1, 1967: U.S. Geol. Survey Bull. 

1275, 116 p. 

Bieniewski, C. L., Persse, F. H., and Brauch, E. F., 

1971, Availability of uranium at various prices 

from resources in the United States: U.S. Bur. 

Mines Inf. Circ. 8501, 91 p. 

Colorado School of Mines, Golden, Foundation, Inc., 

Potential Gas Agency, 1971, Potential supply of 

natural gas in the United States (as of December 31, 1970) : Golden, Colo., 41 p. 

DeCarlo, J. A., Sheridan, E. T., and Murphy, Z. E., 

1966, Sulfur content of United States coals: U.S. 

Bur. Mines Inf. Circ. 8312, 44 p. 

Duncan, D. C., and Swanson, V. E., 1965, Organic-rich 

shale of the United States and world land areas: 

U.S. Geol. Survey Cir. 523, 30 p. 

Elliott, M. A., and Linden, H. R., 1968, A new analysis 

of U.S. natural gas supplies: Jour. Petroleum 

Technology, v. 20, p. 135-141. 

Hendricks, T. A., 1965, Resources of oil, gas, and natural-gas liquids in the United States and the 

world: U.S. Geol. Survey Cir. 522, 20 p. 

Hubbert, M. K., 1969, Energy resources, in Resom·ces 

and man : N atl. Acad. Sci.-N atl. Research Council, 

Comm. on Resources and Man, p. 157-242. 

McKelvey, V. E., and others, 1969, Potential mineral 

resources of the United States outer continental 

26 
shelf, in Public Land Law Review Commission 

Study of outer continental shelf lands of the 

United States, Volume IV (Appendices): available from: Natl. Tech. Inf. Service, U.S. Dept. of 

Commerce, Springfield, Va. 22151. 

--- 1972, Mineral resource estimates and public 

policy: Am. Scientist, v. 60, no. 1, p. 32-40. 

Miller, 0. N., and others, 1970, Future petroleum provinces of the United States: Washington, D.C., 

N atl. Petroleum Council, 138 p. 

Moore, C. L., 1970, Analysis and projection of historic 

patterns of U.S. crude oil and natural gas, in 

Miller, 0. N., and others, Future petroleum provinces of the United States: Washington, D.C., National Petroleum Council, p. 133-138. 

Mufller, L. J. P., and White, D. E., 1972, Geothermal 

energy: The Science Teacher, v. 39, no. 3, p. 40-43. 

National Petroleum Council, Committee on U.S. Energy Outlook, 1971, U.S. energy outlook: Washington, D.C., v. 1, 75 p., v. 2, 195 p. 

Nelson, T. W., and Burk, C. A., 1966, Petroleum resources of the continental margins of the United 

States, in Exploiting the Ocean-MTS Conf. and 

Exhibit, 2d Ann., 1966, Trans.: Washington, D.C., 

Marine Technology Soc., p. 116-133. 

U.S. Atomic Energy Commission, 1971, Statistical data 

of the uranium industry, Jan. 1, 1971: Grand 

Junction, Colo., 54 p. 

U.S. Bureau of Mines, 1971, United States coal resources and production-an interim report, June 

1971: Washington, D.C., 47 p. 

U.S. Dept. Interior, 1972, United States energy, a summary review: Washington, D.C., 72 p. 

Weeks, L. G., 1958, Fuel reserves of the future: Am. 

Assoc. Petroleum Geologists Bull., v. 42, no. 2, p. 

431-441. 

--- 1959, Where will energy come from in 2059: 

The Petroleum Engineer, v. 31, p. A-25. 

White, D. E., 1965, Geothermal energy: U.S. Geol. 

Survey Circ. 519, 17 p. 

27 GP 0 929·447