GEOLOGICAL SURVEY CIRCULAR 522 

Resources of Oil, Gas, and 

Natural-Gas Liquids in the 

United States and the World 

Resources of Oil, Gas, and Natural-Gas 

liquids in the United States 

and the World 

By T. A. Hendricks 

Geological Survey Circular 522 

Washington 1965 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

REPRINTED 

1966 

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

Page Page 

Abstract---------------------------- 1 United States resources of natural 

Introduction ____ ----- __ ------------__ 1 gas and natural-gas liquids -------- 10 

Crude oil resources of the United Summary of United States resources 

States ---------------------------- 3 of crude oil, natural gas, and 

Past production and proved natural-gas liquids________________ 12 

reserves------------------------ 5 Potential world resources of oil, gas, 

Crude oil. originally in place in and natural-gas liquids ___ - - - _ - - --- 12 

known reservoirs---------------- 5 Comparison of the relatively highly 

Extensions and revisions to estimates explored areas of the United States 

of oil in place in known pools______ 5 with the remainder of the world -- 13 

Effect of reentering old wells and Relative productivity of various 

new pool discoveries in development areas in the United States _ -- 13 drill~ng _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 5 Application of factors derived 

Future discoveries of crude oil in for the United States to the 

the United States ---------------- 7 remainder of the world------ 15 

Economically producible and sub- Conclusion------------------------- 18 

marginal crude oil_______________ 9 References ------------------------ 19 

ILLUSTRATIONS 

Page 

Figure 1. Diagram illustrating conceptual components of total oil in the ground___________ 4 

2-4. Graph showing2. Effect of crediting extensions and revisions back to year discovery________ 6 

3. Production and proved reserves of crude oil and its equivalent by States 

and the grouping of States in categories of productiveness--------------- 9 

4. Cumulative totals of past production plus proved reserves as compared 

with cumulative total of exploratory footage drilled_____________________ 14 

TABLES 

Page 

Table 1. New fields and new pools discovered without additional exploratory drilling---- 6 

2. Cumulative totals of past production, proved reserves, and exploratory footage drilled ___________________________________ ---- _ ___ _ __ _ _ __ _ _ ___ _ 9 

3. Oil, gas, and natural-gas liquids originally underlying the United States ------- 12 

4. Production and proved reserves of crude oil, natural gas equivalent, and 

natural-gas liquids by States as of January 1, 1962 ___ --------------------- 15 

5. Amount of oil, natural gas, and natural-gas liquids in areas of various degrees 

of potential in the United States ---------------------------~------------- 15 

6. Estimated quantities of petroleum and natural gas originally in place in world areas and bordering continental shelves _______________________ ':"'__________ 17 

7. World oil resourc.es by regions ---.---------------------------------------- 18 

III 

Resources of Oil, Gas, and Natural-Gas Liquids 

the United States and the World 

. 1n 

By T. A. Hendricks 

ABSTRACT 

The amount of crude oil originally in place in the world is 

estimated at slightly less than 10,000 billion barrels, of 

which approximately 1,600 billion barrels was in the United 

States. Natural gas originally in place in the world ir· estimated at more than 30,000 tri IIi on cubic feet, of which about 

4,000 trillion cubic feet was in the United States. Liquids 

originally contained in the natural gas of the world are estimated at more than 800 bi IIi on barrels, of which about 120 

billion barrels was in the United States. 

It is estimated that in the United States about five-eighths 

of the oil and gas originally in place may be discovered. 

Also, it is estimated that one-fourth of the oil originally in 

place and one-half of the gas and associated liquids may be 

produced ultimately. Because of the great differences in 

economic and physical settings, no such breakdown is attempted for the world as a whole. 

The first step was to estimate the quantities of oi I, gas, 

and natural-gas liquids originally in place in the United 

States. This is done more easily for the United States than 

for most other areas of the world because data are more abundant and more accurate. This estimate was based on avai I able 

data on exploratory footage drilled, fraction of the total volume of sedimentary rocks explored, past production record~, 

estimates of proved reserves, and the attractiveness of the 

explored rocks relative to that of the roc~s as yet unexplored. 

The next step was to divide productive areas of the United 

States into four categories according to relative productivity. 

A factor of the quantity in place per thousand square miles 

was derived for each commodity in areas of each category. 

Final steps were to divide the sedimentary areas of the 

world into categories similar to those of the United States, 

to multiply the measured area of each category by the appropriate factor, and to add together the resulting quantities for 

areas of regional or continentwide extent. 

INTRODUCTION 

This study of the resources of petroleum, 

natural gas, and natural-gas liquids of the 

United States and the world was originally 

undertaken jointly by A. D. Zapp (deceased), 

J. F. Pepper (deceased), and T. A. Hendricks. 

Zapp prepared most of the part dealing with 

resources· in the United States. Much of 

Zapp's methodology follows that used in 1962 

in estimating the future petroleum producing 

capacity of the United States (Zapp, 196 21 

Pepper and Hendricks each classified the 

sedimentary basins of the world geologically 

according to their relative potential. They 

then resolved their differences in evaluation 

by specific discussion. Pepper computed 

the area of each basin unit outside the United 

States. Hendricks developed the procedure 

used and computed the quantities of crude 

oil, natural gas, and natural- gas liquids for 

the areas of the world as classified by Pepper and Hendricks. He also consolidated the 

material and wrote the final report. Credit 

for their ideas and methods should go to 

Zapp and Pepper. Responsibility for the 

complete report rests with the author. 

The problems of estimating oil and gas 

potential are markedly different from those 

of estimating other fossil fuels. Coal, oil 

shale, and asphalt occur in solid bodies that 

are extensively exposed at the earth's surface, where their dimensions, character, and 

geologic relations may be observed directly. 

Oil and gas accumulations, by contrast, are 

not exposed; moreover, they occur as fluids 

filling fractures and intergranular spaces in 

the rock in a much wider range of stratigraphic settings than the solid fuels. In 

short, resources of the solid fossil fuels are 

at least partially visible and relatively concentrated, whereas resources of oil and gas 

are entirely unseen and relatively diffuse. 

Because man cannot see oil and gas resources, it is not surprising that he has been 

ultraconservative in estimating yet-undiscovered quantities. Repeatedly, expert estimates of total undiscovered quantities have 

been exceeded within a decade or so by 

quantities actually discovered. This conservatism regarding the unseen is also reflected in estimates of yet-to-be-produced 

quantities in accumulations that have already 

1 
2 RESOURCES OF OIL, CAS, AND NATURAL-GAS LIQUIDS IN THE UNITFD STATES AND THE WORLD 

been outlined by drilling~ that is~ estimates 

of "proved reserves." The record shows that 

estimates of ultimate recovery from drilled 

pools are generally revised sharply upward 

as cumulative production mounts. 

Geological science has been very successful in classifying parts of the earth's crust 

as favorable, unfavorable~ or impossible for 

the occurrence of oil and gas~ but the exact 

location and e~tent of the occurrences can be 

determined only by drilling. By the same 

token~ only extensive drilling and production 

experience can provide the statistical sample 

needed to attempt quantitative estimates of 

the producible quantities in the unexplored 

favorable parts of the earth's crust. Moreover~ much of the purely geologic information needed for intelligent appraisal of unexplored rock can be provided only by drilling. 

At present~ only a tiny part of the geologically favorable rock in the earth's crust has 

been explored by drilling. The amount of exploratory footage drilled in the United States 

dwarfs the total footage in the rest of the 

world~ but even in the United States there is 

•room" in the geologically favorable rocks 

for more than six times as much exploratory 

drilling as has been done so far. Until a far 

larger • sample" has been drilled~ estimates 

of undiscovered quantities will be subject to 

major revision. 

In this report an attempt is made to estimate the potential resources of the United 

States by utilizing the statistical results of 

drilling and to estimate the potential of other 

parts of the world by comparing their geologic characteristics with the United States. 

Consideration is limited to the continents, 

the continental shelves, and island areas. 

Oil~ gas, and associated liquids may be 

present beneath the continental slopes and 

oceanic basins, but no information exists on 

which to base estimates. Also, consideration is focused on these quantities originally 

in place in the rocks and on the amounts that 

may reasonably be expected to be discovered 

by exploration and development. Far larger 

quantities of hydrocarbons than those here 

considered are present in disseminated form 

in the rocks and fluids of the earth's crust. 

For example, Buckley, Hocott, and Taggart 

(1958, p. 882) have shown that the quantity of 

hydrocarbon gas dissolved in the subsurface 

waters beneath the Gulf Coastal Plain probably exceeds the producible gas known in that 

province at the time of their study. Similarly, it is well known that virtually all coal deposits contain appreciable quantities of natural gas, but only rarely have they served as 

commercial gas reservoirs. Lasky and others (1962) discussed this subject and concluded that the potentially usable methane 

contained in the coal beds of the United States 

might be 35D-400 trillion cubic feet. The Oil 

and Gas Journal (Apr. 20, 1959, v. 57, no. 17, 

p. 82; and Mar. 12, 1962, v. 60, no. 11, p. 83) 

reported that more than 2 trillion cubic feet 

of methane is dissolved in the water below a 

depth of 1,000 feet in the 526 square miles of 

Lake Kivu in Ruanda in central Africa. 

Many estiinates of ultimate crude oil resources have been made by well-informed 

students of the subject. The amounts estimated have ranged widely. Hubbert (1962) 

cited 14 estimates made by 12 individuals or 

groups between 1948 and 1962. These range 

from 110 billion to 590 billion barrels. However, no two of the estimates were made on 

exactly the same basis, and much of the dif-. 

ference between them may be more apparent 

than real. For example, three of the estimates made by L. G. Weeks, a widely recognized student of the subject, are 110 billion 

barrels (Weeks, 1948), 204 billion barrels 

(Weeks, 1958), and 391 billion barrels (Weeks, 

1959, table 1). However, the 1948 estimate of 

110 billion was restricted to resources recoverable by conventional primary methods 

in terms of then current economics. The estimate of 204 billion made in 1958 included 

recoverable resources offshore that were not 

considered in 1948. The increase of Weeks' 

estimate to 391 billion in 1959 was largely 

due to the inclusion of 190 billion barrels 

producible by secondary recovery. The highest estimate by Weeks did not include Alaska. 

Hubbert (1962, table 5) cited Zapp as having 

estimated in an unpublished report to the 

Natural Resources Subcommittee of the National Research Council"ultimate U.S. crudeoil reserves" at 590 billion barrels. Zapp's 

breakdown of that estimate, which was not 

cited by Hubbert, consisted of two parts: (1) 

ultimate resources (recoverable under) cur-

.rent economics and technology of 300 billion 

·barrels and (2) submarginal ultimate resources of 290 billion barrels. Submarginal 

resources were defined in that unpublished 

report as being within two orders of magnitude of (currently) minable deposits in cost 

and quality. 
RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORLD 3 

In that same report, and by statistical 

methods, Hubbert (1962, p. 72) estimated the 

total ultimate production of crude oil in the 

United States at 175 billion barrels. His estimate included production offshore .and by 

improved recovery methods but did not include Alaska. 

Moore (1962), in a thoughtful statistical 

analysis and projection of available data, 

concluded that the ultimate cumulative dis-. 

coveries of crude oil in place in the United 

States will be 486 billion barrels of which 

364 billion barrels will be recoverable. However, his estimates did not include Alaska 

and, because of the early stage of its development, the potential of offshore production 

probably entered into the projection to only 

a limited extent. The estimates probably do 

not make ·allowance for all discoveries of 

crude. Because of the lack of data on discoveries abandoned without any recorded 

production data, it is probable that no allowance is made for oil that may be producible 

from such deposits by techniques unknown at 

the time of discovery. 

McKelvey and Duncan (1963), in a study 

conducted as· part of a report of the Energy 

Policy Staff of the United States Department 

of the Interior, recast Zapp's estimates, 

supplemented by those of the Interstate Oil 

Compact Commission of oil recoverable by 

secondary methods already in practice, as 

follows: 

Billion 

barrels 

Known recoverable reserves......................... 48 

Undiscovered recoverable resources.............. 200 

Known marginal resources........................... 40 

Undiscovered marginal resources................... 300 

These estimates total 588 billion barrels and 

do not include past production. The quantity 

of marginal resources that may be produced 

ultimately was not estimated. 

Each of these estimates, as well as others, 

was made by a stated. methodology, and most 

were made for a specific purpose. They are 

cited here in order to give the reader an idea 

of the variation in the amounts as estimated 

by knowledgeable scientists, some of the 

reasons for variation, and the general basis 

for some of the estimates. 

CRUDE OIL RESOURCES OF THE UNITED STATES 

The United States has been drilled for oil 

and gas far more intensively than any other 

large and geologically diversified area in the 

world. Consequently the results of this drilling should provide the most meaningful statistical sample for appraising possible ultimate potential. An estimate of the original 

total crude oil in the ground, for example, 

can be made by simple calculation if the following factors are known or can be estimated: 

1. The quantity of crude oil found by exploration to date (1961). 

2. The fraction of the potentially productive rocks that has been explored to date. 

3. The estimated incidence of oil in the 

unexplored part of favorable rock as determined by comparison with the explored part. 

As stated, the foregoing information is 

pertinent to estimation of oil in the groundnot to estimation of producible quantities. 

Estimation of what fraction of the total oil in 

the ground may ultimately be produced involves additional assumptions or estimates: 

(1) the future economic incentive to explore 

for and produce oil and (2)the extent to which 

progress in petroleum technology will increase the amount of oil that can be profitably recovered. Inability to predict the magnitude and effect of these factors introduces 

a weakness common to all estimates of ultimate discovery and production. 

The statistical data from drilling in the 

United States are utilized to estimate the 

minimum total quantities originally underlying the United States and its adjoining continental shelves. As will be seen, none of the 

factors that enter into such a computation 

can be actually determined; many are purely 

matters Qf opinion. However, the analysis is 

given in such a manner that any reader who 

disagrees with any of the values assigned, or 

assumptions made, may substitute other values or other assumptions and derive his own 

estimates. 
4 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORLD 

A. Future revfsiQns resulting 

from net increase in esti~ 

mates of oil in place in 

drilled acreage 

8. Future extensions to known 

pools, through development drilling 

N £ X P L 0 

·D Economically 

recoverable 

C. In pools to be discovered in 

the course of development 

drilling of known pools 

D. In originally subcommerciel 

deposits and in drilled but 

undetected deposits that 

will be brought into production in the future 

Submarginal 

Figure 1. -Diagram illustratin& conceptual components of total oil in the ground. Not drawn to scale. 
CRUDE OIL RESOURCES OF THE UNITFD STATES 5 

The order of magnitude of the total world 

amount of oil, gas, and natural-gas liquids 

originally in place is estimated by comparing the figures derived for various parts of 

the United States with those from areas considered geologically similar in other parts 

of the world. 

The amount of crude oil in the ground consists of a number of increments which are 

summarized on figure 1. Only two of these 

quantities are known with relative accuracy, 

past production and proved reserves. The 

others can only be estimated. 

PAST PRODUCTION AND PROVED RESERVES 

As of December 31, 1964, past production 

of crude oil in the United States totaled 

75,231,533,000 barrels. The American Petroleum Institute (1964) estimated the proved 

reserves of crude oil recoverable by methods already in operation in known pools to be 

30,990,510,000 barrels. The two quantities 

together total 106,222,043,000 barrels. 

CRUDE OIL ORIGINALLY IN PLACE IN KNOWN 

RESERVOIRS 

The Secondary Recovery Committee of the 

Interstate Oil Compact Commission estimated that, as of January 1, 1960, there was 

a total of 328.4 billion barrels of oil origi~ 

nally in place in known reservoirs (Torrey, 

1960, p. 42). This estimate includes most of 

the oil in the explored part of the favorable 

rocks b~1t does not include all of the oil even 

for these areas. It does not include: 

1. Future revisions resulting from net increase in estimates of oil in place in drilled 

acreage. 

2. Future extensions to known pools 

through development drilling. 

3. New pools to be discovered in the 

course of development drilling of known 

pools. 

4. Oil in originally subcommercial deposits and in drilled but undetected deposits that 

will be brought into production in the future. 

EXTENSIONS AND REVISIONS TO ESTIMATES OF OIL IN PLACE IN KNOWN POOLS 

The principal additions to reserves, as estimated yearly by the American Petroleum 

Institute (API), consist of extensions and revisions of the estimated reserves of pools 

discovered in previous years. Sixty-three 

percent of these quantities over the past 15 

years have resulted from extensions. Some 

idea of the magnitude of extensions and revisions, as compared with older reserve estimates, may be had from figure 2. This shows 

that by 1960 extensions and revisions applicable to fields discovered through 1934 and 

1936, respectively, had already reached a 

total about twice as great as the reserve estimates -made at the end of those years. 

Doubtless, decades will pass before the magnitude of post-1960 extensions and revisions 

to estimates of pre-1960 discoveries is 

known fully, but the historical record suggests that they should be twice as great as 

the API reserve estimate at the end of 1959. 

Therefore, post-1960 extensions and revisions should eventually reach 64 billion barrels. Again, judging from the past, extensions 

alone should account for about 6 3 percent of 

this total, or about 40 billion barrels. All the 

reserve figures just discussed refer to producible oil, which at present is only about 

one-third of the oil in place. It is assumed 

that the average recovery will increase in 

the future to a level of 40 percent in those 

pools already discovered even if recovery is 

restricted to currently proved techniques. 

This means that the 40 billion barrels of recoverable oil in post-1960 extensions would 

represent about 100 billion barrels of oil in 

place. 

EFFECT OF REENTERING OLD WELLS AND NEW POOL 

DISCOVERIES IN DEVELOPMENT DRILLING 

The past record indicates that a number of 

fields and pools would be discovered even if 

there were no further exploratory drilling. 

Some of these will be found in the course of 

development drilling of pre-1960 discoveries; 

others will be found by reentering and retesting old wells previously abandoned as dry 

holes or as subcommercial wells. To illustrate, the data in table 1 were compiled from 

the statistics on exploratory drilling as summarized in the Bulletin of the American Association of Petroleum. Geologists. 

No way is known to the writer of estinlating, on the basis of presently available data, 

the quantity that will be discovered in the future by retesting pre-1960 wells and by development drilling of pre-1960 discoveries, 

but certainly there will be such discoverie~. 
6 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITFD STATES AND THE WORLD 

50 

40 

10 

0 

t-r-rLO 

(Y) 

en 

1""'4 

t-r-rco 

(Y) 

en 

1""'4 

en 

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Figure 2. -Effect of crediting extensions and revisions back to year of discovery. Lower, unshaded bars are American Petroleum Institute 

yearly estimates of proved reserves of crude oil in the United States at the end of each year. Diagonally ruled bars are the quantities added 

by crediting extensions and revisions, as known in 1945, back to years of discovery. (Adapted from Buckley, 1951, chart 1, p. 21.) 

Vertically ruled bars at top are the quantities similarly added by crediting back further extensions and revisions as known in 1960. (Adapted 

from National Petroleum Council, 1961, table 1, by subtracting cumulative production from cumulative discoveries for each year.) 

Table 1.-New fields and new pools discovered without additional exploratory drilling 

Number of new fields or new pools discovered, 

oil and (or) gas 

1958 1959 1960 

(Blanpied, 1959, (Blanpied, 1960, (Carsey. 1961, 

p. 1125) p. 66 5) p. 704, 708) 

By reentering and retesting old wells: 

New fields ____ ----- _________ ---- ____ 28 31 35 

New pools-------------------------- 21 32 24 

New pools discovered in the drilling of 

field development wells ______________ 260 184 349 
CRUDE OIL RESOURCES OF THE UNITED STATES 7 

To summarize, the oil originally in place 

in known pools as of January 1, 1960, has been 

estimated at 328.4 billion barrels. Although 

some additioaal quantities of oil that will be 

developed within and near tested acreage 

cannot be estimated, the most important 

quantity-that from extensions of pre-1960 

discoveries-presumably will reach 100 billion barrels of oil in place eventually. The 

total oil originally in place in explored acreage is probably between 400 and 450 billion 

barrels. Because of the tenuous nature of 

this estimate, the lower figure of 400 billion 

barrels is used in further discussion in this 

report. 

FUTURE DISCOVERIES OF CRUDE OIL IN THE UNITED STATES 

The exact location and extent of the undiscovered occurrences of oil and gas can be 

determined only by drilling. The total footage already drilled in exploratory wells, 

compared with the total footage of such drilling that would be required to find all the important pools, is perhaps the best measure of 

the fraction of the potentially productive 

rocks that has been explored so far. 

Estimation of the total exploratory footage 

that would be required for complete exploration involves estimates of(1)the total volume 

of geologically favorable rocks and (2) the 

average exploratory well density that would 

be required. 

The National Petroleum Council (1952, p. 

54), in a summary of a symposium conducted 

by the American Association of Petroleum 

Geologists, placed the area of the United 

States and adjacent continental shelves that 

has favorable prospects for the discovery of 

oil and gas at 1, 860,000 square miles. Since 

that estimate was made, the addition of Alaska to the Union increased the total to slightly 

more than 2 million square miles. 

The thickness of rocks favorable for the 

occurrence of oil and gas ranges generally 

from 1, 000 feet to 40,000 feet, but the average thickness has not been determined. However, in only one basin (southern Louisiana) 

has a well been completed below 20,000 feet 

as an oil well (Oil and Gas Journal, Jan. 27, 

1964, v. 62, no. 4, p. 151). In the 11 basins 

where production has been found below 15,000 

feet, 9 of the deepest wells have been completed as gas wells and only 2 as oil wells. 

Most basins containing rocks favorable for 

occurrence of oil and gas increase progressively and gradually in depth from their 

margins to the deepest part. These factors 

indicate that an average thickness of 7, 000 

feet for rocks favorable for the occurrence 

of oil and gas is reasonable. As drilling has 

not reached the maximum depth of many basins, this figure seems consistent with the 

average depth of new field exploratory wells 

in the past which has increased from 4,415 

feet in 1945 to 5,108 feet in 1963 (Newfarmer 

and Dillon, 1964, p. 769). 

Economic factors doubtless will determine 

the density of exploratory wells that will be 

drilled through the section of rocks favorable 

for the occurrence of oil and gas. Opinions 

as to what density would constitute reasonably full exploration are likely to vary considerably, but the following facts are pertinent: 

1. The median areal extent of pools discovered thus far in the United States is considerably less than 1 square mile. 

2. Although the large pools have accounted 

for most past production and <!Urrent reserves, important pools need not be of great 

areal extent. The Oil and Gas Journal (Jan. 

26, 1959, v. 57, no. 4, p. 141-147) listed the 

most productive fields in the United States in 

1958. More than 22 percent of these had an 

areal extent of less than 2 square miles; 

many less than 1 square mile. 

3. Some productive areas in the United 

States already have an exploratory-well density greater than one well per 2 square 

miles, though generally the full sedimentary 

column has not been penetrated. A good modern example is an 8,906 square mile area in 

the Denver-Julesburg basin of Colorado and 

Nebraska that was the subject of a recent 

statistical study by Arps and Roberts (1958). 

This study included data to January 1, 1958, 

at which time 3, 705 exploratory holes had 

been drilled. Through February 196 2 an additional 1,877 exploratory wells had been 

drilled, bringing the density to one exploratory well per 1.6 square miles. Exploration 

is still active in the area. 

This example is notable because the average productivity of the pools found is small. 

In the far more prolific Los Angeles Basin 

of California, ·exploratory-well density has 
8 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITFD STATES AND THE WORLD 

reached 2 wells per square mile, with the 

shallower parts having a density of more than 

11 wells per square mile (Barbat, 1958, p. 76), 

and new pools are still being discovered 

(Doell, 1964, p. 1004-1005). 

4. The presence of a dry exploratory hole 

does not mean that a commercial pool is not 

present in the immediate vicinity in the 

rocks penetrated. Many times development 

drilling has shown pools to extend to within a 

few hundred feet of a preexisting "dry hole• 

that penetrated the productive horizon. 

In view of the foregoing, it is difficult to 

see how an average exploratory-well density 

of less than one well per 2 square miles 

could be considered total exploration, and 

that density is assumed here. To explore the 

entire body of favorable sedimentary rock to 

this average density would require, then, 

more than 7 billion feet of exploratory drilling. 

Total exploratory footage drilled in the 

United States through 1958 has been estimated at 980 million feet (Zapp, 1962, p. H23). 

Exploratory drilling during 1959 raised this 

total to just over 1 billion feet by January 1, 

1960. Thus, the amount of unexplored favorable rocks is about six times as great as the 

amount already explored. 

-" 

It should be emphasized that this estimate 

of the relative quantity of unexplored rock is 

rather conservative. The average thickness 

of the' favorable rocks probably exceeds 

7,000 feet, and average exploratory-well density of one well per 2 square miles would not 

be adequate to find even all the important 

pools. The device of using exploratory- drilling footage as a measure of extent of exploration does not make full allowance for the 

fact that the unexplored rocks are deeper, on 

the average, than the rocks already explored. 

Just how to allow for this factor is difficult 

to judge. Much exploration of deeper rocks 

is done by reentering old wells and deepening them; in such explorations no allowance 

need be made for the greater depth of the unexplored rocks. It was concluded that the 

simplest way to make allowance for this factor is through conservatism in estimating 

the relative quantity of unexplored rocks, as 

has been done above. 

In the foregoing sections it was concluded 

that the unexplored part of the favorable 

rocks is at least six times greater than the 

explored part and that the explored part 

originally contained at least 400 billion barrels of crude oil. However, much of the most 

productive part of the favorable rocks has 

been drilled. That this factor is already significant is suggested by a tabulation of cumulative past produ~tion and proved reserves 

as compared with the cumulative exploratory 

footage drilled (table 2 and fig. 4), which 

shows- that the cumulative total of past production and proved reserves per million feet 

of exploratory drilling decreased from the 

decade of 1950-60 through the 1960-63 period, but increased in 1964 to a figure slightly 

better than the average in the 1950's. This 

does not take into account that much of the 

addition to proved reserves in any one year 

or several years is from development drilling and secondary recovery improvements in 

exploratory discoveries in previous years, 

but it does suggest a definite decrease in the 

incidence of discoveries. In consideration of 

this factor, the incidence of crude oil in the 

unexplored rocks is assumed to range uniformly from an incidence equal to the average in the explored rocks for the richest unexplored fraction to nil for the poorest fraction. '!'he average incidence, therefore, would 

be only one-half that of the explored part and 

the estimate of crude oil originally in place 

in both explored and unexplored rocks would 

400 X 6 

be 400 + 2 ' or 1, 600 billion barrels. 

The foregoing assumption regarding incidence of crude oil in unexplored rock seems 

to be the most conservative possible without 

becoming unreasonable. Time after time in 

the past, newly explored favorable rocks have 

proved to be more productive than predicted, 

and this probably will continue to be true in 

the future. A considerable part of the unexplored favorable rock lies on the Continental 

Shelf contiguous to prolific producing areas 

and where major discoveries have already 

been made with little exploration. The Continental Shelf off the coasts of Louisiana and 

California appears to be particularly promising. Recent discoveries in Alaska suggest 

that this large area may have considerable 

favorable acreage. Much acreage is held by 

shallow or minor production in areas where 

prolific production has been developed in 

deeper rocks on acreage nearby. In addition, 

much favorable land is owned by oil companies in fee simple and has been withheld 

from exploration to date. An the factors 
CRUDE OIL RESOURCES OF THE UNITED STATES 9 

Table 2.-Cumulative totals of past production, proved reserves, and exploratory footage drilled 

[Figures are as of Jan. 1 for year indicated] 

(1) (2) (1) + (2) Exploratory 

Proved reserves Past production 1 million footage 

(1,000 bbl) 

1945 ___________________ 19,941,846 1950 ___________________ 25,268,398 1955 ___________________ 30,012,170 1960 ___________________ 31,613,211 1964 ___________________ 30,969,990 1965 ___________________ 30,990,510 

INDIVIDUAL STATES 

Figure 3. -Production and proved reserves of crude oil and its 

equivalent by States and the grouping of States in categories of 

of productiveness. 

indicate that much favorable acreage has yet 

to be explored. 

(1,000 bbl) barrels drilled 

31,691,904 51,634 308,701,735 

41,033,721 66,302 465,007,387 

52,493,082 82,505 759,423,088 

64,931,492 96,544 1, 083, 110,606 

72,587,286 103,557 1, 244,6 54,08 3 

75,231,533 106,221 1, 297,763,355 

ECONOMICALLY PRODUCIBLE AND SUBMARGINAL 

CRUDE OIL 

To calculate how much of the estimated 

increase of 1, 200 billion barrels of oil in 

place will prove to be economically producible, it is necessary to estimate further (1) 

what fraction can be found profitably and (2) 

what average percentage of the oil in place 

in discovered pools can be produced ultimately at a profit. 

In view of the general trend shown in table 

2 and figure 4 of an increase in the footage 

of exploratory drilling required to discover 

a unit amount of oil, it is obvious that at 

some stage the finding rate must decrease to 

a point where exploratory drilling will be 

unprofitable, even though geologic guidance 

is successful in concentrating exploration 

within the best of the remaining unexplored 

rocks. It is assumed here that economic incentive will justify exploratory drilling of at 

least one-third of the unexplored rocks; that 

is, 2 billion feet of additional exploratory 

drilling. It is further assumed that sound 

geological and geophysical guidance will concentrate future drilling within the better half 

of the unexplored rocks. Under the previously stated assumption as to incidence of 

oil, this better half would have an average 

incidence three-fourths as great as the explored rocks. Consequently this additional 

2 billion feet of exploratory drilling (twice 

as much as that done prior to 196 0) should 

discover 600 billion barrels of crude oil in 

place (one and one-half times the amount already found). This would bring the total of oil 

in place which can reasonably be expected 
10 RESOURCES OF Oll., GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORLD 

to be discovered to an estimated 1, 000 billion 

barrels. 

Constant improvements in production technology that conserve the primary energy in 

reservoirs and the development and use of 

secondary-recovery techniques are steadily 

increasing the percentage of oil in place that 

can be economically recovered. Torrey 

(1960, p. 42-43) estimated that a National 

average recovery of slightly more than 40 · 

percent is already technologically feasible, 

and it is assumed here that average recovery 

of 40 percent will ultimately be attained 

economically. This would mean that 400 billion barrels of the estimated 1, 000 billion 

barrels will be recoverable after it is found. 

Approximately 68 billion barrels of this had 

already been produced through 1961. 

Subtracting the 400-billion-barrel estimate for economically recoverable oil from 

the 1,600-billion-barrel estimate for total 

oil in place gives an estimated 1,200 billion 

barrels of submarginal oil, partly as residual' 

oil in produced reservoirs and partly in undiscovered pools too costly to find (fig. 1). 

The foregoing estimate of economically 

recoverable oil assumes that the petroleum 

industry can continue to explore for some 

time with a decreasing finding rate before 

reaching the limit of profitability. There are 

many reasons for believing this to be true; 

some of the more important reasons are 

enumerated below: 

1. In the past the bulk of the industry's 

drilling cost has been for development wells. 

It has long been realized that in most pools 

many more development wells were drilled 

than were needed to produce the reservoir 

effectively. There has been a long-term 

trend toward wider spacing of development 

wells, and there has been an acceleration of 

this trend in recent years. Moreover, development of successful techniques for multiple completion of both exploratory and development wells results in lower exploration 

and development costs. Wells have been 

completed from as many as seven different 

reservoirs; in the past, wells were rarely 

completed in more than one producing zone, 

and separate wells were drilled to each of 

the producing zones. Improvements in drilling equipment and techniques are constantly 

reducing the constant dollar-per-foot cost of 

drilling. The effect of these trends is that a 

continually increasing footage of exploratory 

drilling can be done without increasing the 

total drilling budget. 

2. Through most of the past, exploratory 

drilling was financed from revenues from 

·

1

the .sale of crude oil; the na~!'al gas that was 

:found was largely unmarketable. In the last 

decade the market for natural gas and natural-gas liquids has expanded enormously, 

and there has been a substantial increase on 

the average well-head price of natural gas. 

Revenues from natural gas and natural-gas 

liquids can now support a substantial part of 

the cost of exploration, and the trend is 

likely to continue to be upward in the future. 

3. Considerable economic tolerance for a 

lower finding rate seemingly exists already, 

as indicated by the record of the recent past. 

Since World War II the finding rate for crude 

oil has been such that a crude-oil producing 

capacity of 10,585,000 barrels daily (National 

Petroleum Council, 1961, p. 4)--almost one 

and a half times the average production rate 

of about 7 million barrels daily-had been 

achieved by January 1, 1960. Shut-in capacity, of course, generates no income; the indush·y can maintain maximum economic 

health only if the finding rate is adequate to 

maintain capacity slightly greater than the 

rate of production. 

Other factors could be listed, but the foregoing trends alone suggest that more of the 

rocks in partly developed basins can be explored, even in the face of a declining finciing 

rate. 

The ultimate average recovery factor of 

40 percent assumed above is probably also 

very conservative. Moore (1962, p. 5) pointed 

out that "most estimates of the ultimate percentage recovery of crude oil originally in 

place range from 65 to 85 percent• and used 

the median figure of 75 percent in his computations. 

UNITEr. STATES RESOURCES OF NATURAL GAS 

AND NATURAL-GAS LIQUIDS 

Estimates of ultimate potential for natural 

gas and natural-gas liquids (NGL) could be 

derived in much the same manner as has 

been done above for crude oil, but the statistical record for these commodities is not 

nearly as complete as for crude oil. During 

most of the history of the petroleum industry 
UNITED STATES RESOURCES OF NATURAL GAS AND NATURAL-GAS LIQUIDS 11 

in the United States, natural gas and its contained liquids found during exploration for 

crude oil were unwanted commodities, and ungauged large amounts were vented into the 

atmosphere or flared during production of 

associated crude oil. Discoveries of natural 

gas without associated crude oil, if remote 

from market, were left undeveloped. In the 

last decade or so there has been an enormous 

expansion in transportation facilities and 

market for natural gas and its contained liquids; and very recently the production of 

these commodities, on an energy basis, 

caught up with and passed domestically produced crude oil. Deliberate exploration for 

natural gas is now becoming widespread, and 

henceforth the search for natural gas and the 

rate of development of gas discoveries will 

probably be comparable to those for crude 

oil. However,as of now, the statistical record 

of the past is much poorer for natural gas 

than for crude oil. 

Natural gas and NGL are usually geneti.-

cally associated with crude oil and commonly 

occur with it. Some reservoirs contain only 

crude oil, and some contain only gas, with or 

without a significant liquid hydrocarbon content. Some of the gas that occurs alone contains virtually no heavy hydrocarbons and is 

considered "dry• or "nonassociated• gas. 

However, average ratios of occurrence for a 

large and geologically diversified part of the 

earth's crust, such as the United States, are 

likely to be relatively uniform. To determine from statistics just what the average 

ratio of occurrence is, the statistics should 

be for a period in which effort expended in 

searching for and developing each commodity was comparable. As discussed above, 

that stage has not yet been reached but is 

being approached rapidly. Accordingly, recent statistics and trends should give at 

least a fair idea of the average ratios of occurrence of gas and NGL to oil that may be 

expected ultimately. Such average ratios of 

occurrence, applied to the estimates for 

crude oil, are considered to be the most realistic basis for arriving at estimates for 

natural gas and NGL. 

The gross additions to reserves, as estimated yearly by the American Petroleum 

Institute and the American Gas Association, 

afford perhaps the best information on average ratio of occurrence of natural gas and 

oil. Their statistics for the period 1952-56 

showed a ratio of 6,182 cubic feet of natural 

gas per barrel of oil; for the period 1957-62 

the ratio was 6,637 cubic feet per barrel. 

The rising trend results from the increasing 

effort toward finding and development of gas 

reserves; it may also result in part from the 

greater average depth of drilling. The ratio. 

will doubtless continue to increase, but in 

this report a conservative figure of 6,667 

cubic feet of natural gas per barrel of crude 

oil is used. The foregoing data relate to economically producible quantities; the current 

average recovery factor for crude oil is 

about 30 percent, whereas for more mobile 

natural gas it is about 80 percent. On the 

basis of these factors, the 6.,667-cubic-feetper-barrel ratio for economically recoverable quantities is equivalent to a 2, 500-cubicfeet-per-barrel ratio for the commodities 

estimated as being originally in place. 

Applying this ratio of 2.,500 cubic feet per 

barrel to the previous 1.,600-billion-barrel 

estimate for ultimate crude oil in place gives 

an estimate of 4, 000 trillion cubic feet for 

natural gas. Applying the ratio to the 1,000-

billion-barrel estimate of the total crude oil 

that will be found ultimately indicates that 

2., 500 trillion cubic feet of natural gas will be 

found. At the 80 percent recovery factor, 

2, 000 trillion cubic feet would be producible. 

Subtracting this from the estimated total in 

the ground gives an estimated 2.,000 trillion 

cubic feet for the submarginal quantity that 

will either not be found or cannot be produced 

economically if found. No increase in the 

average recovery factor is assumed, although 

greater recovery is known to be technologically feasible. 

The quantity of this natural gas that has 

already been produced is not accurately 

known, for no one knows how much gas was 

wasted in the early years of the industry, and 

only estimates rather than measurements 

are made of more recent wastage. However, 

. with a rough allowance for wastage, production through 1961 has probably been of the 

general magnitude of 230 trillion cubic feet. 

Because NGL occur in the same system 

with natural gas, recent production figures 

should give a good indication of the average 

ratio of occurrence. The p-:-oduction figures 

of the American Gas Association for the past 

10 years indicate a ratio of about 32 barrels 

. of NGL per million cubic feet of natural gas; 

during the same period gross additions to 

reserves indicate about the same ratio. In 

this report, this is rounded off to 30 barrels 

per million cubic feet for the purpose of deriving estimates. Applying this factor to the 
12 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORLD 

foregoing estimates for natural gas yields 

estimates for NGL of 120 billion barrels as 

the total in the ground, of which 6 0 billion 

would be producible and 6 0 billion submarginal. More of the natural gas that is produced could be processed for NGL, and it is 

technologically feasible to increase the average amount recovered from the processed 

gas; but as no net increase in the combined 

heat value of natural gas and NGL would result, it seems unnecessary to estimate the 

amounts. 

As is true for natural gas, and for the 

same reasons, total past production of NGL 

from reservoirs cannot be accurately determined, but through 1961 such production has 

probably totaled between 6 and· 7 billion barrels. 

SUMMARY OF UNITED STATES RESOURCES OF 

CRUDE OIL, NATURAL GAS, AND 

NATURAL-GAS LIQUIDS 

The estimated quantities of petroleum, 

natural gas, and NGL originally underlying 

the United States and adjacent continental 

shelves, as derived in the preceding section 

of this report, are summarized in table 3. 

It should be emphasized that the quantities 

in the table below are minimum quantities, 

as conservatism was used throughout in deriving them. They are probably particularly 

conservative as regards natural gas and 

NGL, as there is a growing body of evidence 

that these commodities occur in greater relative quantities with increasing depth. As 

has been pointed out previously, most of the 

production thus far found at depths of more 

than 15,000 feet has been gas or gas condensate. All the estimates will probably be 

revised upward in the future when more information is available. The total future exploratory drilling estimated in this report, 2 

billion feet, will be completed by the end of 

the current century if such drilling continues 

at present rates. 

It should also be emphasized that underground quantities, however large, are relatively meaningless until they are found and 

brought into production, and this will be done 

only if incentive to do so is adequate. Moreover, underground quanttties do not in any 

way reflect the rate at which the commodities can be produced. To be efficient, exploration and development must proceed 

slowly, making full use of each increment of 

knowledge from previous drilling. Furthermore, once a reservoir has been drilled, 

there is a definite limit to the rate at which 

its contained fluids can be withdrawn--generally many decades are required to exhaust 

a reservoir. Total resources in the ground 

and the amounts that ultimately will be discovered and produced are important, but for 

economic and political considerations in the 

next few decades the current capacity for 

production and the outlook for maintaining 

that capacity are of greater importance. 

POTENTIAL WORLD RESOURCES OF OIL, GAS, 

AND NATURAL-GAS LIQUIDS 

At present it is impossible to relate resources of oil, gas, and NGL of the United 

States to those of the world in terms of 

United States categories of producing, marginal and submarginal. Many areas of the 

world are not well known geologically, and 

vast areas have not been explored by drilling. Many potential producing areas of the 

world are remote from markets and difficult 

Table 3.-0il, gas, and natural-gas liquids originally underlying the United States 

1 2 3 4 

Crude oil 

Natural NGL 

Total liquid 

(billions gas (billions hydrocarbons 

(trillions (1 + 3) of of cu of (billions of 

ba.:.•rels) ft) barrels) barrels) 

Total in place------------------------------- 1,600 4,000 120 1, 720 

Total in place to be found by exploration------- 1,000 2,500 75 1,075 

Economically recoverable __ ---_------------ __ 400 2,000 60 460 

Submarginal-------------------------------- 1,200 2,000 60 1,260 

Approximate production through 1961, for 

comparison------------------------------- 68 230 6-7 74-75 
POTENTIAL WORLD RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS 13 

of excess to the facilities required for exploration and production. The economic setting and absence of geological and well data 

do not permit the determination of marginal 

and submarginal categories. However~ the 

amount of on~· natural gas~ and NGL originally in place in the world can }be estimated 

by comparing by area the relative attractiveness for hydrocarbon abundance of the 

sedimentary basins of the remainder of the 

world with similar basins in the United 

States. 

COMPARISON OF THE RELATIVELY HIGHLY EXPLORED 

AREAS OF THE UNITED STATES WITH THE 

REMAINDER OF THE WORLD 

No completely satisfactory method can be 

used to establish categories of the relative 

incidence of deposits of oil., gas., and NGL in 

the various provinces of the United States. 

If data were available., the amount of each in 

place might be related to the volume of sedimentary rock by provinces. If more detailed 

data were available~ the amounts might be 

further related to rock types on the basis of 

an arbitrarily assumed or statistically derived relative incidence of deposits of oil~ 

gas~ and NG L in each rock type. 

RELATIVE PRODUCTIVITY OF VARIOUS AREAS IN THE 

UNITED STATES 

A method of classifying the area of sedime!ltary rocks of the United States as to their 

productivity was arrived at by using easily 

obtainable figures. The figures are available 

by States rather than by geologic or petroliferous provinces~ which would have been 

preferable. The method is described below: 

1. Past production of crude oil and natural 

gas to January 1., 1962~ was added to proved 

reserves as of January 1~ 1962~ by States. 

2. The amount of NGL produced and wasted 

was computed on the basis of 1 barrel for 

each 20., 000 cubic feet of gas produced except for areas of dry gas where the factor 

was arbitrarily reduced by approximately 

half, which brings the average approximately 

to 30 barrels per million cubic feet assumed 

for the United States as a whole. · Reserves 

of NGL were related to reserves of gas on 

the same basis. The resulting figures were 

checked for general validity against the rate 

of production of NGL as compared with those 

of natural gas in recent yea'rs. 

3. Natural gas was converted to crude-oil 

equivalent by considering each 5.6 MCF 

eguivalent to a barrel of oil. 

4. Crude oil and NGL past production and 

reserves were combined for each State and 

the natural gas equivalent was added thereto~ 

5. The area of sedimentary basins in 

which oil and gas has been found was outlined 

and measured by planimetry. 

6. The combined past production and 

proved reserves in millions of barrels was 

divided by the area in units of 1, 000 square 

miles for each State. 

7. The State factors of millions of barrels 

per thousand square miles of productive 

area were tabulated in decreasing order. 

Fairly sharp breaks occurred at two points 

and served to group States into three classes 

of productivity (table 4., col. 1., and fig. 3). 

8. For each of the three classes, production and proved reserves for all States in the 

class were combined for crude oil, for natural gas, and for NGL. 

9. The total production and reserve figure 

for each commodity in all States in each 

class was divided by the combined productive area in States in each class to obtain a 

figure of production and reserves per thousand square miles (table 4~ col. 2). 

10. One-sixteenth of the total resources of 

crude oil, natural gas~ and NGL was arbitrarily assigned to currently nonproductive 

areas of sedimentary rocks which was estimated as 1 million square miles. 

11. Past production and reserves of each 

commodity were totaled~ and the percentage 

of the total in each class was determined. 

12. Past production and the proving of reserves were assumed to have been in direct 

ratio to the amount of each commodity originally in place. Therefore the total re-

. source figure for each commodity in the 

United States was multiplied by factors thus 

derived for areas of each class to obtain a 

total resource figure for the area in the 

United States in each class. 

13. The total resource figure was divided 

by the area in the United States in each class 

separately for crude oil, natural gas, and NGL 
14 

~ rz.. 

0 

~ 0 

H 

....:I 

1:=1 

~ 

~ 

"' § 

~ gj 

~ 0 

0 

~ 

f:il 

~ 

1300 

1200 

1100 

1000 

900 

800 

700 

600 

500 

400 

300 

50 

RESOURCES OF on., GAS, AND NATURAL-GAS LIQUIDS IN THE UNITID STATES AND THE WORlD 

~·965 4 

1/ 

J 

I /1960 

I 

v955 

I 

v 

I v5U 

/ 

60 70 80 90 100 llO 

PAST PRODUCTION + PROVED RESERVES, IN MILLIONS OF BARRELS 

Figure 4. -cumulative totals of past production plus proved reserves as compared with cumulative total of exploratory footage drilled. 
POTENTIAL WORLD RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS 15 

Table 4.-Production and proved reserves of crude oil, naturakgas equivalent, and natural~gas liquids by States as of 

] anuary 1 , 1962 

Production and Unit of Production and 

proved reserves 1,·ooo square proved reserves Average by categories State (millions of (millions of miles of barrels per 

of favorableness 

barrels) productive 1,000 sq mi) area 

1 2 

California ________ 21,554 28 770 

Louisiana-------- 31,543 58 544 430 (161, 988 million barrels, Texas_ _____ ------ 83,585 207 404 

New Mexico------ 7,686 20 384 area 377,000 sq mi). 

Oklahoma-------- 17,620 64 272 

Arkansas -------- 1,868 10 187 

Pennsylvania----- 2,717 18 150 146 (20,609 million barrels, 

Kansas ----------- 10.,055 68 148 area 141,000 sq mi). 

Illinois ---------- 2,928 22 133 

West Virginia ............ 3,041 23 132 

Wyoming _________ 4,848 65 75 

Mississippi ------ 2,196 31 71 

Nebraska -------- 310 6 52 Utah _____________ 843 22 38 Kentucky _________ 1,236 37 33 Colorado _________ 1,663 56 30 32 (16,863 million barrels, 

Miscellaneous ____ 1_,857 61 30 area 530,-000 sq mi). Ohio _____________ 1,089 42 26 Michigan _________ 725 35 21 

North Dakota _____ 836 54 16 

Indiana ---------- 414 29 14 

Montana --------- 846 92 9 

Table 5.-Amount of oil, natural gas, and natural--gas liquids in areas of various degrees of potential in the United States 

Crude oil-million barrels per 1, 000 sq mL ______ 

Natural gas-trillion cubic feet per 1,000 sq mi_ __ 

NGL--million barrels per 1,000 sq mi ----~------

to dete::.,mine a figure for total resources per 

thousand square miles· for each comm.odity 

in each class. 

The four categories so established for each 

of th~ three commodities estimated as being 

originally in place were rounded off as shown 

in table 5. Category 5 was added to account 

for areas yielding only dry unassociated gas. 

APPLICATION OF FACTORS DERIVED FOR THE UNITED 

STATES TO THE REMAINDER OF THE WORLD 

The categories were applied to the sedimentary basins ana areas of the world as 

Category 

1 2 3 4 5 

3,000 1,000 250 100 ------- 8 3 .5 .25 0.025 

250 80 20 10 -------

judged by a comparison with those of the 

United States. It is realized that no two geologists could be expected to agree on the 

classification of all areas, but it is probable 

that the differences average out when large 

regions are considered. As a first step_, a 

broad survey was made_, country by country., 

of the geologic literature of the world_, to determine the location and extent of sedimentary basins and areas, the results of drilled 

wells, and past and present production and 

reserves if known or estimated. Next the 

sedimentary areas were plotted on a world 

map_, together with known pools of oil and 
16 RESOURCES OF OIL1 GAS1 AND NATURAL-GAS LIQUIDS IN THE UNITID STATES AND THE WORLD 

gas~ and then divided into four categories 

that were judged to be similar in prospective 

production per 1~ 000 square miles to those 

determined for the United States. A fifth 

category was added to cover possible incidence of natural gas in sedimentary rocks at 

least 1,000 feet thick but considered unfavorable for occurrence of oil. For this category an estimate of 0. 25 trillion cubic feet of 

natural gas per 1, 000 square miles was arbitrarily assumed. Each land area in a particular category was planimetered~ and the 

number of square miles was determined, 

which was then multiplied by the factors that 

were derived for the United States for oil~ 

gas, and NGL for that category. The totals 

of all categories for each commodity were 

then combined for a continent or other ge-

. ologically related area. 

The continental shelves bordering sedimentary areas or basin margins on land 

were also outlined to a water depth of 600 

feet on the world map. They were assigned 

categories similar to those of the bordering 

coast (or lower if considered less favorable 

then the coast), and their area was determined by planimetry. The total amounts of 

oil, gas, and NGL in place were then estimated for the shelves by the same method 

as for the land areas. The categories assigned the shelf are at many places undoubtedly conservative. Only those shelves bordering sedimentary areas were evaluated, 

whereas many shelves that border areas of 

igneous or metamorphic rocks may consist 

large~y of detrital sediments of great thickness that contain organic material and interfinger with marine sediments. Some of these 

sediments may contain accumulations of oil 

or gas. Furthermore, many areas il'l water 

deeper than 600 feet probably will be productive ultimately. 

The combined totals for the land and shelf 

areas as shown in table 6 are believed to 

give reasonable estimates of total oil, gas~ 

and NGL originally in place as compared to 

estimates of these totals for the United 

States. Figures for individual areas may be 

too low or too high, but overall the figures· 

are believed to be of the right order of magnitude. The possible future projected drilli.ng 

in the United States is based on the total exploratory footage drilled and the necessary 

additional footage required to bring the incidence of exploration to the density of one 

well per 2 square miles assumed as reasonable to test adequately the total sedimentary 

rocks of the United States. This would' require at least six times as much footage as 

has been drilled in the past. These assumptions of future requirements for total exploration in the United States to test adequately 

the known sediments~ as well as the projected 

incidence of oil, natural gas, and NGL based 

thereon~ have been extrapolated throughout 

the world's sedimentary areas. The resulting figures of commodity incidence for many 

untested world areas may be extremely conservative, however, because of relatively 

low favorability assumed where geologic information is lacking or sparse. In other 

areas where drilling has been at a low 

rate-for example one well per several 

thousand square miles-without finding commercial accumulations, a category may have 

been assigned that future exploration may 

show is far too conservative. 

The estimate of the world resources of 

natural gas "originally in place• may seem 

astronomical~ but the categories as determined attribute only very small amounts of 

gas to large areas in the world that may 

contain only nonassociated natural gas. Many 

sedimentary areas are more favorable for 

occurrence of natural gas than for oil. In 

the past the discoveries of gas have largely 

been incidental to the search for oil, and intensive exploration has been confined mainly 

to those sedimentary areas with more·obvious structural features and the best prospect for oil. Large stratigraphic traps, for 

which little search has been made so far, 

are more likely to contain gas or mixed 

fluids than oil. In the future, vast areas of 

sediments that were considered unfavorable 

for oil or that have not been explored will be 

prospected for natural gas-for ·example, 

sediments of continental origin similar to 

those of the Kenai Formation in Alaska, or 

those of recent age as in the gas field of 

Pleistocene age in the St. Lawrence valley 

of Quebec, or in wells of great depth where 

the ratio of gas and gas condensate to oil 

rises~ as in the Gulf Coast of the United 

States where the ratio rises from 54 percent 

at depths of more than 8, 000 feet to 70 percent at depths of more than 12~000 feet. 

How much of the natural gas that was originally in place will be recovered will depend largely on the economic needs~ both industrial and domestic, of the region in which 

they are discovered and on new developments 
POTENTIAL WORLD RESOURCES OF OIL. GAS0 AND NATURAL-GAS LIQUIDS 17 

·Table 6.-Estimated quantities of petroleum and natural gas originally in place in world areas and bordering 

continental shelves 

Crude oil Natural gas NGL 

(bill ions of barrels) (trillions of cubic feet) (billions of barrels) 

1 1·2 1 22 1 32 

~anada~ Mexico, Central 

America, and Caribbean _____ 500 300 3,500 2~200 40 25 

South America--------------- 800 500 2,500 1~600 70 45 Europe ______________________ 500 300 1,300 800 40 25 

Africa---------------------- 1,800 1,100 5,400 3,400 150 95 

Middle East ----------------- 1,400 900 3,600 2,200 120 75 South Asia ___________________ 200 100 600 400 20 10 

USSR, China, and Mongolia _____ 2,900 1,800 8,500 5,300 240 150 

Australia~ East Indies, and 

Pacific Islands _____________ 300 200 1,100 700 30 20 

Total: 

World excluding United States _________________ 8,400 5,200 26~500 16,600 710 445 

United States ------------ 1,600 1,000 4,000 2,590 120 75 

World------------------- 10,000 6,200 30,500 19,100 830 520 

1Figures rounded off to the nearest 50 billion barrels. 

2Figures rounded off to the nearest 100 trillion cu ft. 

3Figures rounded off to the nearest 5 billion barrels. 

1. Total in place. 

2. Possible ultimate discoveries. How much might be economically recoverable is dependent 

upon many factors related to the geologic occurrence, geographic accessibility, and economic developments and would be variable between the United States and other countries as 

well as between countries. In many places outside the United States, the economically recoverable part of the possible ultimate discoveries may exceed the conservative percentage 

assumed in this paper for the several commodities as economically recoverable in the 

United States. 

in methods of transportation. Gas is becoming of incr-2asing importance in the 

world's economy. In the past th,e search for 

it was confined largely to those areas that 

lay relatively close to large i.lldustrial and 

domestic outlets or close to existing pipelines. The increased demand within the 

North American continent is being met by 

the discovery and development of vast new 

deposits, particularly in western Canada. 

For Europe the fuel requirements of an increasing industrial development are being 

met increasingly by the discoveries of new 

gas reserves in the Netherlands~ France~ 

and beneath the North Sea. In the future~ 

gas from the large fields in North Africa 

will likely be transported to Europe. The 

discoveries of natural gas in North Africa 

and their possible uses there and abroad 

are only in the early stages of prospecting 

and development. 

Weeks (1963) and Torrey, Moore~ and 

Weber (1963) estimated the reserves of discovered reserves of crude oil in the world 

by various categories. These estimates and 

ratios based on them are given in table 7 

together with estimates of total oil originally in the ground and the amount expected 

to be discovered. 
18 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORLD 

Table 7.-World oil resources by regions 

[The basic unit used is barrels of 42 U.S. gallons. Columns 1 through 6 are in billions of barrels~ 

and columns 1 and 2 are rounded off to the closest 50 billion. Columns 7 through 9 are in 

millions of barrels because the amounts can be computed accurately} 

11 12 23 24 25 26 

United States ___________ 1,600 1~000 346 31 16 

Canada~ Mexico~ Central 

America~ and Caribbean---------------- 500 300 50 8 2 

South America --------- 800 500 214 21 7 

Total Western Bernisphere _____ ~------- 2,900 1,800 610 60 25 

Europe ________________ 500 300 21 3 1 Africa _________________ 1,800 1~ 100 56 10 ~--- Middle East ____________ 1,400 900 794 190 97 

South Asia _____________ 200 100 3 1 0 

USSR~ China~ and 

Mongolia------------ 2~900 1~800 89 25 8 

Indonesia, Australia, and 

Pacific Islands _______ 300 200 27 9 0 

Total Eastern Berni- sphere _____________ 7,100 4~400 990 238 106 

Total world ___________ 10~ 000 6~200 1~600 298 131 

1 Figures rounded off to the nearest 50 billion barrels. 

2Figures after Torrey, Moore, and Weber (1963). 

3Figures after Weeks (1963). 

1. Total oil estimated to be in the ground. 

2. Amount of oil that can be expected to be discovered. 

3. Estimate of original oil content of known reservoirs. 

4. Estimate of primary oil reserves. 

47 

10 

28 

85 

4 

10 

287 

1 

33 

9 

344 

429 

37 28 

37~000 2, 511 

8~300 374 

22~937 1,273 

68,237 4,158 

3,111 224 

12~ 125 180 

208~ 8 70 2,060 

575 10 

33,500 1~225 

11~ 800 194 

269~981 3,893 

338,218 8,051 

5. Additional oil reserves from conventional fluid injection operations. 

6. Combined primary + fluid injection reserves (col. 4 + col. 5). 

7. Reserves producible by methods now in operation. 

8. Oil production in 1961. 

9. Total cumulative oil production to Jan. 1~ 1962. 

10. Percentage of 1961 oil production by regions. 

11. Ratio of oil to be discovered to cumulative reserves (col. 2 I col. 6 ). 

29 10 11 

67~657 31.2 21 

5~945 4.7 30 

17,703 15.8 8 

91,305 51.7 21 

3,487 2.8 75 

618 2.2 110 

18~ 328 25.6 3 

421 .1 100 

14,106 15.2 55 

3,122 2.4 22 

40,082 48.3 13 

131,387 100.0 15 

12. Ratio of oil to be discovered to past production +cumulative reserves (col. 2 I col. 6 + 9). 

12 

9 

19 

11 

10 

43 

69 

3 

100 

38 

17 

11 

12 

CONCLUSION 

Crude oil originally in place in the world 

is considered to be of the order of magnitude 

of 10,000 billion barrels; about 6,000 billion 

barrels may be expected to be discovered. 

Natural gas originally in place is of the order 

of magnitude of 30,000 trillion cubic feet, of 

which about 20,000 trillion may be discovered 

and with which will be associated more than 

800 billion barrels of NGL of which about 500 

billion barrels may be associated with the 

gas to be discovered. Table 6 summarizes 

the estimates for the world derived by the 

method described in the foregoing text. 

The data, estimates, and computations in 

table 7 indicate clearly that the United States, 

the Middle East, South America (principally 

Venezuela), and the USSR will dominate production until 1970. The large proved reserves. 
REFERENCES 19 

of the Middle East indicate that the importance of that region is certain to grow signifcantly at least until 19 7 5. The large potential 

for additional discovery of oil in favorable 

but unexplored areas in Africa and the USSR 

suggests that the impact of those regions on 

oil economics will grow greatly by 1980. 

It is realized that the data available only 

permit estimates of world resources of oil# 

gas, and NGL of gross accuracy to be made. 

Individual elements used in preparing the 

estimates unquestionably contain errors. 

However, the method of considering elements 

of computation in extremely comprehensive 

units tends to average out individual discrepancies# and the consistent selection of the 

most conservative of possible alternatives 

provides a reasonable insurance that the estimates are of a magnitude that can be attained. 

Much of the undiscovered and undeveloped 

resources of crude oil, natural gas, and NGL 

are in the deep parts of partly explored 

basins, beneath the waters of the Continental 

Shelf, in remote areas of Alaska, and in 

foreign countries. In such areas exploration, 

development# production, and transportation 

costs are high, and large-scale operation is 

essential for successful operation. An increasing amount of oil produced is associated 

with natural gas and NGL, a fact that greatly 

complicates marketing. 

The potential world supply of oil# gas# and 

NGL in place in the ground is large# the major 

part of the supply can be discovered, and a 

large part of that discovered can be produced 

when economic and political factors warrant 

the effort and other expenditures necessary 

to win it. 

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Am. Petroleum Inst. 

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flank of Denver-Julesburg basin: Am. 

Assoc. Petroleum Geologists Bull.# v. 42, 

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Barbat, W. F.# 1958, The Los Angeles Basin 

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Buckley, S. E., Hocott, C. R., and Taggard, 

M. S., Jr., 1958, Distribution of dissolved 

hydrocarbons in subsurface waters, in 

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Blanpied, B. W., 1959, Exploratory drilling in 

1958: Am. Assoc. Petroleum Geologists 

Bull., v. 43, no. 6, p. 1117-1138. 

1960, Exploratory drilling in 1959: Am. 

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Carsey, J. B., 1961# Exploratory drilling in 

1960: Am .. Assoc. Petroleum Geologists 

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Doell# E. C., 1964# Developments in west coast 

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1000-D. 

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Senate Committee on Interior and Insular 

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McKelvey, V. E.# and Duncan, D. C., 1963, 

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table 1, in Supplies, costs and uses of fossil 

fuels: Energy Policy Staff, U.S. Dept. of the 

Interior, Washington, D.C. 

Moore, C. L., 1962, Method for evaluating U.S. 

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crude oil availability: U.S. Dept. of the 

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196~ Proved discoveries and productive 

capacity of crude oil, natural gas# and natural gas-liquids in the United States: Washington Natl. Petroleum Council# 40 p., 7 

tables. 

Netshcert# B. C., 1958, The future supply of 

oil and gas: · Baltimore, Md.# The Johns 

Hopkins Press, 134 p. 

Newfarmer, L. R., and Dillon, E. L., 1964, 

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p. 769. 

Torrey, P. D., 1960, Evaluation of United States 

oil resources as of January 1, 1960: The 
20 RESOURCES OF OIL, GAS, AND NATURAL-GAS LIQUIDS IN THE UNITED STATES AND THE WORlD 

Oil and Gas Compact Bull., v. 19, no. 1, 

p. 41-52. 

Torrey, P. D., Moore, C. L., and Weber, G. H., 

1963, World oil resources: Frankfurt A/ 

Main, preprint 6th Internat. Petroleum 

Cong., June 1963, p. 16-28.' 

Weeks, L. G., 1948, Highlights ori 1947 developments in foreign petroleum fields: Am. 

Assoc. Petroleum Geologists Bull., v. 32, 

p. 1094. 

___ 1958, Fuel reserves of the future: Am. 

Assoc. Petroleum Geologists Bull., v. 42, 

p. 434. 

Weeks, L. G., 1959, Where will energy come 

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1963, Worldwide review of petroleum 

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6th Internat. Petroleum Cong., June 1963, 

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INT .DUP. ,D.C .65- 7 0 6 56