1995 National Assessment of 

United States Oil and Gas Resources 

OVERVIEW OF THE 1995 NATIONAL ASSESSMENT OF 

POTENTIAL ADDITIONS TO TECHNICALLY RECOVERABLE RESOURCES OF 

OIL AND GAS-ONSHORE AND STATE WATERS OF THE UNITED STATES 

U.S. GEOLOGICAL SURVEY CIRCULAR 1118 
Cover. Map of the United States showing regions 1 through 8 (see fig. 3) as defined for 

this report, 1995 National Assessment of United States Oil and Gas Resources. 
1995 National Assessment of 

United States Oil and Gas Resources 

By U.S. Geological Survey 

National Oil and Gas Resource Assessment Team 

U.S. GEOLOGICAL SURVEY CIRCULAR 1118 

Overview of the 1995 National Assessment of potential additions to 

technically recoverable resources of oil and gas-onshore and State 

waters of the United States 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

Free on application to U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

1995 National Assessment of United States oil and gas resources I by U.S. Geological 

Survey National Oil and Gas Resource Assessment Team. 

p. em.- (U.S. Geological Survey circular; 1118) 

Includes bibliographical references (p. 20). 

Supt. of docs. no.: I 19.4/2:1118 

1. Petroleum reserves-United States. 2. Natural gas reserves-United States. 

3. Petroleum-Geology-United States-Statistical methods. I. National Oil and 

Gas Resource Assessment Team (U.S.) II. Series. 

TN872.A45 1995 

622 s-dc20 94-45974 

[553.2'8'0973] CIP 
CONTENTS 

Summary of Results ....................................................................................................... . 

Introduction .. .. . . . . . . . . . .. . . . . .. . . . . . . . . . . . ... .. . . . . . . .... .. . . . . . ... . .. . . . . .............. ... . . . . . . ... .. .. . . . . . . . . . .. . . . . . .. .... 1 

Commodities Assessed................................................................................................... 2 

Assessment Categories . . .. . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . ... . . .. . . . . .. . . . . . . . . . . ........... ... . . .. . . . . . . . . ... . . . . ... . . . . 4 

Undiscovered Conventional Accumulations.......................................................... 4 

Inferred Reserves (Reserve Growth) . ........... ............ ..... ......................................... 4 

Continuous-Type (Unconventional) Accumulations.............................................. 4 

Terminology . ........... ............ ......... .. ........... ........... ...... .. . . . . ........... ........ .. . ................. ... . . . . 5 

Data Sources . .. . . . . .. . . . .. . . . . . . . . . . .. ... .. . . . . .. . . . . . . . . . . ... . . . . . . . .. . .. . . . . . . . . ... . .. . . . . .. . . . . . . . . . .. . . . . .. . . . ...... .. . . . . . 6 

Areas of Study . . .. .. . . . .. . . . . . . . . . . .. . . . .. . . . . .. . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . .. . . .. . . . . .. . . . . . . . . . . . .. . ... . . . . .. . . . . . . . . . . 7 

Methods of Assessment.................................................................................................. 8 

Undiscovered Conventional Accumulations.......................................................... 8 

Risking Structure............................................................................................ 8 

Truncated Shifted Pareto Model..................................................................... 8 

Small Field Assessment.................................................................................. 9 

Aggregation and Dependency . . . . . . . . . ..... .. . . . . .............. ......... ..... .. . . . . . . . . . .. . . . . .. ... . . 9 

Reserve Growth (Inferred Reserves) . . . . . . . .. . . . .. . . . . .. . . . . . . . . . ... ... .. . . . . ... . . .. . . . . . . . .. ... . . . .. . . . . . 9 

Continuous-Type Accumulations........................................................................... 9 

Sandstones, Shales, and Chalks...................................................................... 9 

Coal-Bed Gas.................................................................................................. 13 

Results of the Assessment . . . .. . . . .. . . . . ... . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . .. . .. . . . . ... . . . . . . . . .. . . .. .. . . . ............. 13 

Technically Recoverable Conventional Resources................................................ 13 

Oil................................................................................................................... 13 

Gas.................................................................................................................. 13 

NGL ................................................................................................................ 13 

Inferred Reserves (Growth of Conventional Fields).............................................. 16 

Technically Recoverable Resources in Continuous-Type Deposits....................... 16 

Sandstones, Shales, and Chalks...................................................................... 16 

Coal-Bed Gas.................................................................................................. 16 

Discussion and Comparison of Results with the 

Previous USGS/MMS Assessment........................................................... 17 

Oil................................................................................................................... 17 

Gas .................................................................................................................. 19 

Acknowledgments .. . ........... ....................... ........... ............ ...................... ........................ 20 

References Cited............................................................................................................. 20 

FIGURES 

1. Technically recoverable oil resources of the United States, exclusive of Federal offshore....................................... 2 

2. Technically recoverable gas resources of the United States, exclusive of Federal offshore...................................... 2 

3. Petroleum regions and provinces assessed in this report . . . . . . . . . . . .. . . . . . . . . . ..... ... . . . . ..... .. . . . . . .... ... . . . .. . . . . .. . . . ...... .. . . . . .. . . . .. . . . . . 3 

4. Diagrammatic representation of petroleum resource classification........................................................................... 4 

5. Geologic setting of continuous-type gas and oil accumulations relative to discrete 

accumulations in structural or stratigraphic traps .. . . . .. . . . . .. . . . .. . . .. .. . . . . . . . . . .. . . . . ... . . . .. . . . . . . . . . ... . .. . . . . . .. . . . . .. . . . .. . . . . .. . . . ... . . . .. . . . . . 5 

III 
IV CONTENTS 

6. Map of lower 48 States showing location of continuous-type plays for oil and gas in 

sandstones, shales, and chalks.................................................................................................................................... 10 

7. Sketch depicting continuous-type play as a collection of hydrocarbon-containing cells........................................... 11 

8. Map of lower 48 States showing areas of coal-bed gas and locations of plays assessed ........................................... 12 

9. Distribution of undiscovered technically recoverable conventional oil resources, by region, based on 

mean estimates for the regions................................................................................................................................... 16 

10. Distribution of undiscovered technically recoverable conventional gas resources, by region, based on 

mean estimates for the regions................................................................................................................................... 16 

11. Estimated cumulative probability distributions for undiscovered technically recoverable conventional 

oil resources of onshore areas and State waters of the United States......................................................................... 17 

12. Estimated cumulative probability distributions for undiscovered technically recoverable conventional 

gas resources of onshore areas and State waters of the United States........................................................................ 17 

13. Potential reserve additions for non-associated gas in continuous-type plays as a function of 

estimated ultimate recovery (EUR) per well ....... ......... ........................................... ... ......... .......... ........... ... ............... 20 

TABLES 

1. Estimates of national totals for undiscovered technically recoverable conventional 

oil, gas, and NGL resources; growth of reserves in known fields; technically recoverable 

resources in continuous-type (unconventional) accumulations; and measured reserves............................................ 2 

2. Estimates of undiscovered technically recoverable conventional oil, gas, and NGL resources 

by petroleum region and province .. .. .. .. .. ... .. .. . . . . .... . .. .. .. ... .. ... .. .. .. ...... .. .... .. .. .. .. ... .. .. ............ ...... . . . .... .. .. . . . ..................... 14 

3. Estimated future growth (inferred reserves) of conventional fields as of December 31, 1991 .. ...................... .......... 17 

4. Technically recoverable resources estimated for continuous-type plays in sandstones, 

shales, and chalks, onshore United States.................................................................................................................. 18 

5. Technically recoverable resources of gas estimated for continuous-type plays in 

coal beds, onshore United States................................................................................................................................ 19 

METRIC CONVERSION FACTORS 

[For this assessment, 6,000 cubic feet of gas equals 1 barrel of oil equivalent (BOE)] 

Multiply inch-pound unit By To obtain metric unit 

barrel 1.590xi0-1 cubic meter 

cubic foot 2.832xio-2 cubic meter 

mile 1.609x103 meter 

foot 3.048xi0-1 meter 

UNIT ABBREVIATIONS AND ACRONYMS USED IN THIS REPORT 

Unit abbreviations 

BBNGL ................... Billion barrels of natural gas liquids BOE ......................... Barrel of oil equivalent 

BBO ........................ Billion barrels of oil MMBO .................... Million barrels of oil 

BCFG ...................... Billion cubic feet of gas TCFG ...................... Trillion cubic feet of gas 

Acronyms 

ANWR ..................... Arctic National Wildlife Refuge NGL ........................ Natural gas liquids 

API .......................... American Petroleum Institute OGIFF ..................... Oil and Gas Integrated Fidd File 

EIA .......................... Energy Information Administration PI. ............................ Petroleum Information Co!>. 

EUR ......................... Estimated ultimate recovery TSP .......................... Truncated Shifted Pareto (model) 

FERC ....................... Federal Energy Regulatory Commission USGS ...................... U.S. Geological Survey 

GOR ........................ Gas-oil ratio WHCS ..................... Well History Control System 

MMS ....................... Minerals Management Service 
CONTENTS 

MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1995 NATIONAL 

OIL AND GAS RESOURCE ASSESSMENT TEAM 

Project Chief .......................................................................... Donald L. Gautier 

Co-Chief Scientist .................................................................. Gordon L. Dolton 

Methodology ........................................................................... Emil D. Attanasi 

Ronald R. Charpentier 

Gordon L. Dolton 

Lawrence J. Drew 

Donald L. Gautier 

John D. Grace (consultant) 

Caroline M. Isaacs 

Richard F. Mast 

Dudley D. Rice 

David H. Root 

James W. Schmoker 

John H. Schuenemeyer 

Administration ........................................................................ Thomas S.J. Ahlbrandt 

Bonnie Claus 

Shirley M. Person 

David J. Taylor 

Managing Editor ..................................................................... Katharine L. Varnes 

Administrative Assistant.. ....................................................... Barbara J. Johnson 

Janet B. Pascale 

CD-ROM Editor ..................................................................... Kenneth I. Takahashi 

Editor ..................................................................................... Shirley A. Oscarson 

Yolanda L. Clausen 

Editorial Assistant .................................................................. Leslie K. Oliver 

Elaine M. Knodel 

Dorothy R. Malone 

Editorial Consultant. ............................................................... William C. Butler 

Simon M. Cargill 

Richard B. Powers 

Gregory F. Ulmishek 

Mathematics and Statistics ..................................................... Ronald R. Charpentier 

David H. Root 

John H. Schuenemeyer 

Data Management.. ................................................................. Ronald R. Charpentier 

Timothy R. Klett 

Raymond C. Obuch 

David H. Root 

Craig J. Wandrey 

Lois P. Williams 

Computing and Programming .................................................. Ronald R. Charpentier 

Raymond C. Obuch 

David H. Root 

Computer Specialist.. .............................................................. Richard H. Salay 

James D. Brewton 

Digital Mapping and GIS ........................................................ William R. Beeman 

Richard F. Mast 

Christopher J. Schenk 

Roland J. Viger 

Lois P. Williams 

GIS Assistant. ......................................................................... Kerry L. Aggen 

Diane L. Barnett 

v 
VI CONTENTS 

MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1995 NATION/ L 

OIL AND GAS RESOURCE ASSESSMENT TEAM-Continued 

GIS Assistant-Continued 

Scarlet Tang 

Zenon C. Valin 

David Vaughan 

Graphic Data Display ............................................................. Timothy R. Klett 

Graphics ................................................................................. Laura L. Zink 

Video Production 

Alice A. Springfield 

Joseph T. Springfield 

Producer, Director, Writer ............................................ Jonathan Swinchatt 

Co-Producer ................................................................. David G. Howell 

Associate Producer ....................................................... Nick Hardigg 

Computer Graphics and Animation ............................... Jose Vigil 

Geological Coordination 

Alaska Region .............................................................. Kenneth J. Bird 

Pacific Coast Region ..................................................... Caroline M. Isaacs 

Marilyn E. Tennyson 

Colorado Plateau and Basin and Range Region ............. John A. Grow 

James A. Peterson 

Rocky Mountains and Northern Great Plains Region ..... Cornelius M. Molenaar 

Charles W. Spencer 

Midcontinent Region .................................................... Ronald R. Charpentier 

James W. Schmoker 

West Texas and Eastern New Mexico Region ............... Mahlon M. Ball 

Gulf Coast Region ........................................................ Christopher J. Schenk 

Eastern Region ............................................................. Robert T. Ryder 

Coordinator, Continuous-Type Deposits .................................. James W. Schmoker 

Geologic Specialist, Continuous-Type Deposits ............. Thomas D. Fouch 

BenE. Law 

Statistics, Continuous-Type Deposits ............................ Robert A. Crovelli 

Computation, Continuous-Type Deposits ...................... Richard H. Balay 

Petroleum Engineer ...................................................... Lynn E. Boone 

John C. Quinn 

Coordinator, Coal-Bed Gas .................................................... Dudley D. Rice 

Geologic Specialist, Coal-Bed Gas ................................ Ronald. C. Johnson 

William B. Cashion 

Assistant, Coal-Bed Gas .............................................. Thomas M. Finn 

Resource Specialist, Coal-Bed Gas ............................... Advanced Resources 

International, Inc. 

Geologic Specialist, Gas Hydrates ........................................... Timothy S. Collett 

Geologic Specialist, Deep Gas ................................................ Thaddeus S. Dyman 

MichaelS. Wilson 

Geologic Specialist, Heavy Oil. ............................................... Mark J. Pawlewicz 

Christopher J. Schenk 

Geochemical Specialist ........................................................... Jerry L. Clayton 

Joseph R. Hatch 

Michael D. Lewan 

James G. Palacas 

Economics .............................................................................. Emil D. Attanasi 

John D. Grace (consultant) 

Keith R. Long 

Discovery-Process Models ...................................................... Lawrence J. Drew 

Richard F. Mast 
CONTENTS 

MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1995 NATIONAL 

OIL AND GAS RESOURCE ASSESSMENT TEAM-Continued 

Discovery-Process Models-Continued 

John H. Schuenemeyer 

Fractal Methodology ............................................................... Christopher C. Barton 

Assistant, Fractal Methodology ..................................... Steven K. Faust 

Grigori L. Troussov 

Inferred Reserves .................................................................... Emil D. Attanasi 

Richard F. Mast 

David H. Root 

Aggregation Models ................................................................ David H. Root 

John H. Schuenemeyer 

Province Geologists 

Northern Alaska (00 I) .................................................. Kenneth J. Bird 

Central Alaska (002) .................................................... Richard G. Stanley 

David G. Howell 

Southern Alaska (003) .................................................. Leslie B. Magoon ill 

Terry R. Bruns 

Michael A. Fisher 

Cornelius M. Molenaar 

Western Oregon-Washington (004) .............................. Samuel Y. Johnson 

Marilyn E. Tennyson 

Eastern Oregon-Washington (005) ............................... Marilyn E. Tennyson 

Klamath-Sierra Nevada (006) ....................................... Marilyn E. Tennyson 

Northern Coastal (007) ................................................. Richard G. Stanley 

Sonoma-Livermore Basin (008) .................................... Leslie B. Magoon, ill 

Sacramento Basin (009) ................................................ Leslie B. Magoon, ill 

San Joaquin Basin (0 I 0) ............................................... Larry A Beyer 

Central Coastal (Oil) .................................................... Richard G. Stanley 

Santa Maria Basin (OI2) ............................................... Marilyn E. Tennyson 

Ventura Basin (0 13) ..................................................... Margaret A. Keller 

Los Angeles Basin (014) ............................................... Larry A. Beyer 

Salton Trough (OI6) ...................................................... Charles E. Barker 

Idaho-Snake River Downwarp (0 17) ............................. James A. Peterson 

Western Great Basin (OI8) ........................................... Charles E. Barker 

Eastern Great Basin (019) ............................................. James A. Peterson 

John A Grow 

Uinta-Piceance Basin (020) .......................................... Charles W. Spencer 

Thomas D. Fouch 

Paradox Basin (02I) ..................................................... A Curtis Huffinan, Jr. 

San Juan Basin (022) .................................................... A. Curtis Huffinan, Jr. 

Albuquerque-Santa Fe Rift (023) ................................. Cornelius M. Molenaar 

Northern Arizona (024) ................................................. William C. Butler 

James G. Palacas 

Southern Arizona-Southwestern New Mexico (025) ..... William C. Butler 

South-Central New Mexico (026) ................................. William C. Butler 

Montana Thrust Belt (027) ........................................... William J. Perry, Jr. 

North-Central Montana (028) ....................................... Thaddeus S. Dyman 

Southwest Montana (029) ............................................. William J. Perry, Jr. 

Williston Basin (031) ................................................... James A. Peterson 

Sioux Arch (032) .......................................................... James A. Peterson 

Powder River Basin (033) ............................................. Gordon L. Dolton 

James E. Fox 

Big Hom Basin (034) ................................................... James E. Fox 

Gordon L. Dolton 

Wind River Basin (035) ................................................ James E. Fox 

Gordon L. Dolton 

Vll 
VIII CONTENTS 

MEMBERS OF THE U.S. GEOLOGICAL SURVEY 1995 NATIONAL 

OIL AND GAS RESOURCE ASSESSMENT TEAM-Continued 

Province Geologists-Continued 

Wyoming Thrust Belt (036) .......................................... Richard B. Powers 

Southwestern Wyoming (037) ....................................... Ben E. Law 

Park Basins (038) ......................................................... Craig J. Wandrey 

Denver Basin (039) ...................................................... Debra K. Higley 

Las Animas Arch (040) ................................................. C. William Keighin 

Raton Basin-Sierra Grande Uplift (041) ....................... C. William Keighin 

Pedernal Uplift (042) .................................................... Mahlon M. Ball 

Mitchell E. Henry 

Palo Duro Basin (043) .................................................. Mahlon M. Ball 

Mitchell E. Henry 

Pennian Basin (044) ..................................................... Mahlon M. Ball 

Bend Arch-Fort Worth Basin (045) .............................. Mahlon M. Ball 

William J. Perry, Jr. 

Marathon Thrust Belt (046) .......................................... William J. Perry, Jr. 

Western Gulf(047) ....................................................... Christopher J. Schenk 

East Texas Basin (048) ................................................. Christopher J. Schenk 

Louisiana-Mississippi Salt Basins (049) ....................... Christopher J. Schenk 

Florida Peninsula (050) ................................................ Richard M. Pollastro 

Superior (051) .............................................................. James G. Palacas 

Iowa Shelf(052) ........................................................... James G. Palacas 

Cambridge Arch-Central Kansas Uplift (053) ............... Debra K. Higley 

Salina Basin (054) ........................................................ Stephen E. Prensky 

Nemaha Uplift (055) ..................................................... Ronald R. Charpentier 

Forest City Basin (056) ................................................. Ronald R. Charpentier 

Ozark Uplift (057) ........................................................ Joseph R. Hatch 

Anadarko Basin (058) ................................................... Mitchell E. Henry 

Timothy C. Hester 

Sedgwick Basin (059) ................................................... Stephen E. Prensky 

Cherokee Platform (060) ............................................... Ronald R. Charpentier 

Southern Oklahoma (061) ............................................. Mitchell E. Henry 

Timothy C. Hester 

Arkoma Basin (062) ..................................................... William J. Perry, Jr. 

Michigan Basin (063) ................................................... Gordon L. Dolton 

illinois Basin (064) ....................................................... David L. Macke 

Black Warrior Basin (065) ............................................ Robert T. Ryder 

Cincinnati Arch (066) ................................................... Robert T. Ryder 

Appalachian Basin (067) .............................................. Robert T. Ryder 

Blue Ridge Thrust Belt (068) ........................................ Robert C. Milici 

Piedmont (069) ............................................................. Robert C. Milici 

Atlantic Coastal Plain (070) .......................................... Robert C. Milici 

Adirondack (071 ) .......................................................... Robert C. Milici 

New England (072) ....................................................... Robert C. Milici 
1995 NATIONAL ASSESSMENT OF 

UNITED STATES OIL AND GAS RESOURCES 

By U.S. Geological Survey National Oil and Gas Resource Assessment Team 

SUMMARY OF RESULTS 

This report summarizes the results of a 3-year study of 

the oil and gas resources of onshore areas and State waters of 

the United States by the U.S. Geological Survey (USGS). A 

parallel study of the Federal offshore is being conducted by 

the Minerals Management Service (MMS). 

Assuming existing technology, there are approximately 

110 billion barrels of technically recoverable oil onshore and 

in State waters. This includes measured (proved) reserves, 

future additions to reserves in existing fields, and undiscovered resources. 

The technically recoverable conventional resources of 

natural gas in measured reserves, future additions to reserves 

in existing fields, and undiscovered accumulations equal 

approximately 715 trillion cubic feet of gas. 

In addition to conventional gas resources, the USGS 

has made an assessment of technically recoverable resources 

in continuous-type (largely unconventional) accumulations. 

We estimate about 300 TCFG (trillion cubic feet of gas) of 

technically recoverable natural gas in continuous-type 

deposits in sandstones, shales, and chalks, and almost 50 

TCFG of technically recoverable gas in coal beds. 

The total technically recoverable oil and gas resource 

base onshore and in State waters of the United States is listed 

in table 1 and shown on figures 1 and 2. 

INTRODUCTION 

The purpose of the National Oil and Gas Resource 

Assessment Project is to develop a set of scientifically based 

hypotheses concerning the quantities of oil and gas that 

could be added to the measured (proved) reserves of the 

United States. 

The word assessment sometimes has the connotation of 

an inventory. But this is not the case in this study. The quantities being evaluated here are largely unknown. This assessment is an attempt to bound the uncertainties concerning 

potential additions to oil and gas reserves under specified 

conditions. As such, the assessment consists of a set of 

constructs, based on the best information and theory available to the USGS scientists charged with this effort. 

The U.S. Geological Survey has occasionally conducted assessments of the oil and gas resources of the United 

States since shortly after the tum of the century. Each successive assessment is a refinement of previous work. Sy~tematic 

National Assessments have been conducted more regularly 

since 1975. In 1982, the Minerals Management Service was 

formed and given responsibility for resource evaluation in 

the Federal offshore areas of the United States. Th~ USGS 

retained responsibility for onshore areas and State waters. In 

1991, the two organizations (USGS and MMS) began their 

second joint study of the oil and gas resources of the United 

States. This report summarizes the results of the U~l}S part 

of that study and reports estimates of potential add :tions to 

reserves onshore and under State waters of the Unite1 States. 

Documentation for this assessment is available on the 

CD-ROM that supports this report (Gautier and others, 

1995). 

The previous USGS/MMS assessment (Mast and others, 1989) encompassed estimates of both technically recoverable and economically recoverable resources. Th~ present 

report concerns only technically recoverable reso·rrces. A 

parallel study concerns the economic evaluation of the 

resources described in this report. The geological assessment 

of technically recoverable resources makes no ae~mpt to 

predict at what time or what part of potential addW0ns will 

be added to reserves. For the National Assessment, resources 

and potential reserve additions are evaluated rega""dless of 

political, economic, and other considerations. 

The onshore and State water areas of the United States 

were divided into eight regions consisting of 71 vrovinces 

(fig. 3). These regions and provinces are similar, but not 

identical, to those addressed by U.S. Geological Survey Circular 860 (Dolton and others, 1981) and the U.S. De"Jartment 

of the Interior report from the previous National Assessment 

of oil and gas resources (Mast and others, 1989). Within 

these provinces, about 560 plays were assessed, of which 

about 100 were in continuous-type deposits; the remainder 

were hypothetical and confirmed conventional plays. 
2 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

Table 1. Estimates of national totals for undiscovered technically recoverable conventional oil, gas, and NGL resources; growth of 

reserves in known fields; technically recoverable resources in continuous-type (unconventional) accumulations; and mea~ured reserves. 

[Mean value totals may not be equal to the sums of the component means given elsewhere because numbers have been independently rounded. Gas includ ~s both non-associated 

and associated-dissolved gas. Fractile values (F95 , Fs) are not additive. F95 represents a 19 in 20 chance and Fs represents a 1 in 20 chance of the occurrence of at least the amount 

tabulated. NGL, natural gas liquids. NA, not applicable] 

Crude oil 

Category (billion barrels) 

F9s Fs 

Undiscovered conventional 

resources 23.5 39.6 

Reserve growth in conventional fields 

Continuous-type accumulations in 

sandstones, shales, and chalks 

Continuous-type accumulations 

in coal beds 

Measured (proved) reserves 

TOTAL, onshore areas and State 

waters of the United States 

NA 

1.5 

NA 

NA 

Undiscovered Small 

Fields ( <1 MMBO) 

(6.3 BBO) 

Continuous-Type 

Accumulations 

(Unconventional) 

(2.1 BBO) 

1.9 o/o 

NA 

2.7 

NA 

NA 

Mean 

30.3 

60.0 

2.1 

NA 

20.2 

112.6 

Undiscovered Large 

Accumulations (~1 MMBO) 

(24.0 BBO) 

Reserve Growth 

(60.0 BBO) 

53.3 o/o 

21.3 o/o 

Figure 1. Technically recoverable oil resources of the United 

States. exclusive of Federal offshore. BBO, billion barrels of oil; 

MMBO, million barrels of oil. 

The estimates presented in this document reflect USGS 

understanding as of January 1, 1994, and are intended to capture the range of uncertainty, to provide indicators of the relative potential of various petroleum provinces, and to 

provide a guide useful in considering possible effects of 

future oil- and gas-related activities within the United States. 

Gas 

(trillion cubic feet) 

F95 Fs Mean 

207.1 329.1 258.7 

NA NA 322.0 

219.4 416.6 308.1 

42.9 57.6 49.9 

NA NA 135.1 

1,073.8 

Continuous-Type 

Accumulations 

(Unconventional) 

(358TCFG) 

Undiscovered Small 

Fields (<6 BCFG) 

(45 TCFG) 

NGL 

(billion barrels) 

F95 fs Mean 

5.8 8.9 7.2 

NA NA 13.4 

1.1 3.5 2.1 

NA NA NA 

NA NA 6.6 

29.3 

Measur~d Reserves 

(13~ TCFG) 

RE.~erve Growth 

(322 TCFG) 

30.0 o/o 

4.2 o/o Undiscovered Large 

Accumulations (~6 ec~~::G) 

(213 TCFG) 

19.9 o/o 

Figure 2. Technically recoverable gas resour\.es of the United 

States, exclusive of Federal offshore. TCFG, tri1lion cubic feet of 

gas; BCFG, billion cubic feet of gas. 

COMMODITIES ASSESSED 

The commodities considered in this st''dy were crude 

oil, natural gas, and natural gas liquids that can be expected 

to be produced from the subsurface through a well. Most 

heavy oil deposits were assessed as convent:'Jnal resources. 

Specifically excluded from consideration we~e gas dissolved 
COMMODITIES ASSESSED 3 

NO. REGION NAME 

1 Alaska 

2 Patific Coast 

3 Colorado Plateau and Basin and Range 

4 Rocky Mountains and Northern Great Plains 

5 West Texas and Eastern New Mexico 

6 Gulf Coast 

7 Midcontinent 

8 Eastern 

0 500 MILES 

wll~l\*111~1 ~~~~~~~~~--L-~1 

600 KILOMETERS 

Figure 3. Petroleum regions and provinces assessed in this report. Heavy lines, region boundaries; lighter lines, province 

boundaries. Maritime boundaries represent the approximate position of State offshore boundaries. 

in geopressured brines and resources in tar deposits and oil 

shales. Gas in clathrate structures (gas hydrates) were not 

assessed as technically recoverable resources; however, a 

chapter concerning these in-place volumes of gas is included 

in the supporting CD-ROM (Gautier and others, 1995). Specifically included in this assessment were technically recoverable gas from low-permeability "tight" sandstone 

reservoirs, gas and oil from fractured shale reservoirs, and 

coal-bed gas. The systematic inclusion of unconventional 

resources marks a significant departure from previous USGS 

assessments. 

Crude oil, as considered in this assessment, is a natural 

liquid consisting mainly of a mixture of complex hyd-ocarbon molecules. Natural gas is a mixture of hydrocarbon 

gases, mainly methane, and certain non-hydrocarbon gases 

such as carbon dioxide, hydrogen sulfide, nitroger. and 
4 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

helium. This analysis assessed hydrocarbon gases, although 

minor amounts of non-hydrocarbon gases may be included. 

Natural gas liquids (NGL) are the heavierhomologs of methane, which are in the gas phase under reservoir pressure and 

temperature conditions. NGL includes those portions of the 

reservoir gas that are liquefied at the surface in various field 

facilities and in gas-processing plants. NGL commonly 

includes propane, ethane, butane, pentane, natural gasoline, 

and condensate. 

ASSESSMENT CATEGORIES 

The resource classification used in this study is illustrated in figure 4, a modified '"McKelvey box." Resources 

can be classified along two axes: geologic assurance and 

economic feasibility. The degree of geologic certainty 

increases to the left from undiscovered resources, through 

inferred reserves (reserve growth 1) to measured (proved) 

reserves. Degree of economic feasibility increases vertically 

upward from subeconomic to economic resources. 

Previous USGS assessments focused on undiscovered 

conventional accumulations of oil and gas and additions to 

reserves in known fields. This assessment is broader in scope 

because it considers three categories of resources: (1) undiscovered conventional accumulations of oil and gas, (2) 

future additions to reserves of known fields, and (3) oil and 

gas in continuous-type accumulations (largely equivalent to 

""unconventional" categories of other analysts). 

UNDISCOVERED CONVENTIONAL 

ACCUMULATIONS 

Undiscovered conventional accumulations of oil and 

gas are the traditional fare of the oil and gas industry and 

have been the focus of most previous USGS oil and gas 

assessments. These resources include those postulated to 

exist outside known fields or accumulations and that, if 

found, could be extracted using traditional development 

practices. These accumulations generally exist as discrete 

accumulations, which are usually, but not invariably, 

defined, controlled, or limited by hydrocarbon/water contacts. Undiscovered accumulations are shown in the right 

third ofthe McKelvey box (fig. 4 ). Undiscovered technically 

recoverable accumulations, those assessed in this report, are 

within the hachured area shown on figure 4. 

1 Throughout this report. the terms "reserve growth" and "field 

growth" are used interchangeably. Both terms indicate growth of reserves in 

known fields. 

0 

i 

0 

z 

0 

0 

w 

0 

i 

0 

z 

0 

0 

w 

al 

:::l 

en 

..... ....._ __ INCREASING DEGREE OF GEOLOGIC ASSURANCE 

Figure 4. Diagrammatic representation of petroleum resource 

classification (modified from the U.S. Bureau of Mines and U.S. 

Geological Survey, 1980). The figure ("'McKelvey box") represents 

total resource endowment. Area inside heavy bord ::r in upper center 

and right represents technically recoverable resources that are estimated in this study. 

INFERRED RESERVES (RESERVE GROWTH) 

Inferred reserves (reserve growth) include those 

resources expected to be added to reserv~s as a consequence of extension of known fields, through revisions of 

reserve estimates, and by additions of new pools in discovered fields. Also included in this category are resources 

expected to be added to reserves through application of 

improved recovery techniques. This category thus includes 

both the "indicated reserves" and the "inferred reserves" 

described in earlier USGS assessment publications (e.g., 

Mast and others, 1989). Predictions of reserve growth refer 

to fields found before 1992 (the date of most reserve data 

used in this report). The analysis of reserve growth in discrete conventional accumulations is based on the proprietary Oil and Gas Integrated Field File (OGIFF) of the 

Energy Information Administration (EIA). Inferred 

reserves are shown in the stippled area of the middle third 

of the McKelvey box (fig. 4). 

CONTINUOUS-TYPE (UNCONVENTIONAL) 

ACCUMULATIONS 

Continuous-type (unconventional) acc·1mulations are, 

for the purposes of this assessment, defined to include those 

oil and gas resources that exist as geograph;r;ally extensive 

accumulations that generally lack well-defined oil/water or 

gas/water contacts (fig. 5). This geologically grounded definition provides a set of consistent criteria to 1,~ applied in the 

determination of whether a specific accumulation is or is not 

conventional. This geologic definition intentionally avoids 

the regulatory criteria of the Federal Ene-:-gy Regulatory 

Commission (FERC) designations and does not rely on any 

specific permeability as a defining criterion. Included in this 
TERMINOLOGY 5 

category are coal-bed gas, gas in many of the so-called "tight 

sandstone" reservoirs, and auto-sourced oil- and gas-shale 

reservoirs. Because of the immense quantities of oil and gas 

that can be included in this category, only those resources 

that, in our judgment, are technically recoverable and could 

be added to U.S. oil and gas reserves were reported in this 

study. Those resources judged to be potential additions to 

reserves are further subdivided into undiscovered and 

reserve growth categories. Existing technology and development practices as of the date of this assessment (January 

1994) were assumed. In this study, inferred reserves and 

undiscovered resources in continuous-type accumulations 

were not differentiated. Therefore, the volumes of resources 

estimated for continuous-type deposits occur over both the 

hachured and stippled areas of the McKelvey box (fig. 4 ). 

Each of the three broad categories of resources (undiscovered conventional accumulations, inferred reserves, and 

continuous-type accumulations) requires a different technique for evaluation. Each of these resources is thus 

described and considered in separate sections of this report. 

The methods for assessment of the undiscovered recoverable 

discrete conventional accumulations is discussed further in 

the CD-ROM chapter on methodology by Gautier and Dolton (Gautier and others, 1995). The techniques used for evaluation of various continuous-type resources are discussed in 

CD-ROM chapters by Schmoker and by Rice (Gautier and 

others, 1995). Results of the assessment of these various 

resources are generally reported in separate categories. 

TERMINOLOGY 

The terminology used in this report is intended to represent standard definitions and usage of the oil and natural 

gas industry and the resource-assessment community. No 

attempt has been made to include a detailed listing of common industry definitions; however, several definitions that 

are essential to the proper understanding of this report are 

Land surface 

Discrete accumulations 

I \ Structural Stratigraphic 

accumulation accumulation 

!-----Tens of miles (kilometers) ---~-1 

Figure 5. Geologic setting of continuous-type gas and oil accumulations relative to discrete accumulations in structural or stratigraphic traps. 

presented. The definitions are intended to be generally 

explanatory rather than strictly technical. 

Undiscovered resources.-Resources postulated from 

geologic information and theory to exist outside of known 

oil and (or) gas fields. 

Technically recoverable resources.- Resources in 

accumulations producible using current recovery technology 

but without reference to economic profitability. These are oil 

and natural gas resources that may be produced at the surface 

from a well as a consequence of natural pressure within the 

subsurface reservoir, artificial lifting of oil from the reservoir to the surface, and the maintenance of reservoir pressure 

by fluid injection. (This definition is modified from that of 

the National Petroleum Council.) These resources are generally conceived as existing in accumulations of sufficient size 

to be amenable to the application of existing recovery 

technology. 

Measured (proved) reserves.- That part of the identified economic resource that is estimated from geologic evidence supported directly by engineering data. Measured 

reserves are demonstrated with reasonable certainty to be 

recoverable in future years from known reservoirs under 

existing economic and operating conditions. Resources in 

this category are not a principal focus of this assessment. 

Data reported are from the Energy Information Administration (Energy Information Administration, 1994 ). 

Conventional accumulation.-A discrete deposit, usually bounded by a downdip water contact, from which oil, 

gas, or NGL can be extracted using traditional development 

practices, including production at the surface from a well as 

a consequence of natural pressure within the subsurface reservoir, artificial lifting of oil from the reservoir to the surface 

where applicable, and the maintenance of reservoir pressure 

by means of water or gas injection. 

Continuous-type deposit.-A hydrocarbon accumulation that is pervasive throughout a large area, that is not significantly affected by hydrodynamic influences, and for 

which the standard methodology for assessment of sizes and 

numbers of discrete accumulations is not appropriate. 

Unconventional accumulation.-A broad class of 

hydrocarbon deposits of a type (such as gas in "tight" sandstones, gas shales, and coal-bed gas) that historically has not 

been produced using traditional development practices. Such 

accumulations include most continuous-type deposits. 

Field growth (inferred reserves).-That part of the 

identified resources over and above measured (proved) 

reserves that will be added to existing fields through extension, revision, improved recovery efficiency, and the addition of new pools or reservoirs. 

Inferred reserves.-For this report, inferred reserves is 

the difference between proved reserves in known fields and 

the remaining recoverable resources in known fields-this 

definition of inferred reserves includes two resource categories used in previous USGS oil and gas assessment docu-
6 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

ments (e.g., Mast and others, 1989): "indicated reserves" and 

"inferred reserves." 

Indicated reserves.-That part of identified oil 

resources in known productive teservoirs in existing fields in 

addition to measured reserves that are expected to respond to 

improved recovery techniques. For this report, indicated 

reserves are included as part of inferred reserves. 

Barrels of oil equivalent (BOE).-Gas volume that is 

expressed in terms of its energy equivalent in barrels of oil. 

For this assessment, 6,000 cubic feet of gas equals 1 barrel 

of oil equivalent (BOE). 

Gas-oil ratio ( GOR).-A verage ratio of associated-dissolved gas to oil; a point estimate of the volume of gas (in 

cubic feet) dissolved in oil or otherwise associated with a 

barrel of oil in known or postulated oil accumulations. As in 

the most recent National Assessment (Mast and others, 

1989), an accumulation with a GORin excess of 20,000 is 

considered a gas accumulation. 

NGL to non-associated gas ratio.-The volume of natural gas liquids (in barrels) contained in 1 million cubic feet 

of gas in a known or postulated gas accumulation. 

NGL to associated-dissolved gas ratio.-The volume 

of natural gas liquids (in barrels) contained in 1 million cubic 

feet of associated-dissolved gas in a known or postulated oil 

accumulation. 

Field.-An individual producing unit consisting of a 

single pool or multiple pools of hydrocarbons grouped on, or 

related to, a single structural or stratigraphic feature. 

Accumulation.-A single oil or gas deposit as defined 

by the trap, charge, and reservoir characteristics of the play. 

Play.-A play is a set of known or postulated oil and 

(or) gas accumulations sharing similar geologic, geographic, 

and temporal properties, such as source rock, migration pathway, timing, trapping mechanism, and hydrocarbon type. 

Play area.-The two-dimensional plan extent over 

which a play concept is considered to be valid and within 

which all known accumulations and potential for undiscovered accumulations or other additions to reserves within the 

play exist. 

Play attributes.-Geologic characteristics that describe 

principal properties of and necessary conditions for the 

occurrence of oil and (or) gas accumulations of the minimum 

size (1 MMBO [million barrels of oil] or 6 BCFG [billion 

cubic feet of gas]) within the defined parameters of a play. 

Although many combinations of individual underlying elements are possible, three attributes were considered in the 

evaluation of play risk in this assessment. These attributes 

are as follows: 

1. Charge.-The occurrence of conditions of hydrocarbon generation and migration adequate to cause an 

accumulation of the minimum size. Included in this 

attribute are subsidiary elements, including existence 

of source rocks with sufficient organic matter of the 

appropriate composition, appropriate temperature 

and duration of heating to generate and expel 

sufficient quantities of oil and (or) gas, and timing of 

expulsion of oil and gas from source rocks appropriate for filling available traps. 

2. Reservoir.-The occurrence of reservoir rocks of 

sufficient quantity and quality to permit the containment of oil and (or) gas in volumes sufficient for an 

accumulation of the minimum size. 

3. Trap.-The occurrence of those structures, 

pinch-outs, permeability changes, and similar features necessary for the entrapment of oil and (or) gas 

in at least one accumulation of the minimum size. 

Included in this attribute are existence of seals sufficient for entrapping hydrocarbons and capable of 

holding oil and gas accumulations during appropriate 

ranges of geologic time. 

Play probability.-Play probability represents the 

product of the probabilities of the three play attributes considered in this assessment (charge, reservoir, and trap). It is 

an estimate, expressed as a decimal fraction, of the chance 

that oil or natural gas exist within the particular play. For 

recoverable resources, the play probability represents the 

likelihood that technically recoverable quantities of oil or 

natural gas exist in at least one accumulation of the minimum 

size (1 MMBO or 6 BCFG) in the area being assessed. 

Conditional estimates.-Sizes, numbers, or volumes of 

oil or natural gas that are estimated to exist in an area, assuming that they are present. Conditional estimates, therefore, do 

not incorporate the risk that the area may be devoid of oil or 

natural gas. 

Risked (unconditional) estimates.-Resources that are 

estimated to exist, including the possibility that the area may 

be devoid of oil or natural gas. Statistically, the risked mean 

may be determined through multiplication of the mean of a 

conditional distribution by the related probability of occurrence. Resource estimates presented in this report are risked 

estimates. 

Cumulative probability distributions for resource estimates.-Graphical depictions of estimated resource volumes presented with associated cumulative probabilities of 

occurrence. These distributions are used to derive the 95 percent, 5 percent, and mean resource levels reported in this 

publication: a low case, with a 95 percent probability of that 

amount or more occurring (a 19 in 20 chance); a high case, 

with a 5 percent probability of that amount or more occurring 

(a 1 in 20 chance); and a mean case representing an arithmetic average of all possible resource outcomes weighted by 

their probabilities. 

DATA SOURCES 

The USGS portion of the National Assessment Project 

relies largely on data that are either published or commercially available. Some USGS geologic data are from 

in-progress studies and have not necessarily been published. 
AREAS OF STUDY 7 

In several areas. drilling and production information was 

especially sparse or unreliable. Seven major data sources 

were used in this assessment: 

1. USGS geologic data. both published and unpublished, were used in the development of play definitions, play boundaries, and in the analysis of geologic 

information concerning undiscovered conventional 

oil and gas accumulations and possible future developments in continuous-type oil and gas accumulations. 

2. The Significant Oil and Gas Fields of the United 

States file (NRG) is a database commercially available from NRG Associates, Inc., which includes 

reserves, cumulative production, and various other 

types of information for most oil and gas fields of the 

United States larger than 1 million BOE (NRG Associates, Inc., 1993 and 1994 ). The NRG release current as of December 31, 1992 (NRG Associates, Inc., 

1993 ), was a major source of reservoir-level information for this assessment. 

3. The Well History Control System (WHCS) is a commercially available database of computerized drilling 

and completion data from almost 2.5 million exploratory and development wells available from Petroleum Information Corp. (PI). Data were used to 

construct various exploration- and development-intensity maps and plots and statistical analyses of drilling and discovery. For most of the areas 

assessed, the 1993 and 1994 versions of WHCS were 

used (Petroleum Information Corp., 1993 and 1994). 

In most provinces, the WHCS contains essentially all 

wells drilled. However, in certain areas, especially 

the Eastern Region, California, and parts of Oklahoma and Louisiana, drilling information is incomplete. 

4. Petroleum Information Corp. production data files, 

including monthly. yearly, and cumulative production information from numerous recent wells in the 

United States, were employed to construct decline 

curves and estimated ultimate recovery (EUR) distributions used in the analysis of potential additions to 

reserves from continuous-type deposits (Petroleum 

Information Corp., 1994). 

5. Energy Information Administration (EIA) Oil and 

Gas Integrated Field File ( OGIFF) is a proprietary 

file of field-level reserves and production information. The data in OGIFF are collected according to 

legal mandate by the Department of Energy from 

operators of oil and gas fields of the United States. 

This file, which includes yearly estimates of reserves 

from fields in the United States was used mainly as a 

database for the prediction of potential additions to 

reserves of known fields. In a few areas of sparse 

data, especially Oklahoma and the Appalachian 

region, the OGIFF was used to supplement NRG for 

estimation of field sizes. Because of the sensitivity of 

the OGIFF data, however, the output provided in this 

report has been generalized, rounded, or elirrinated 

to avoid releasing any of those data. This is p"'rticularly apparent in the output for provinces 055 (Nemaha Uplift), 056 (Forest City Basin), 060 (Ch~rokee 

Platform), and 067 (Appalachian Basin). 

6. The Energy Information Administration 1993 

Annual Report (Energy Information Administration, 

1994) is the basis of all measured (proved) reserve 

information reported here. 

7. Other data, including publications, State records, 

proprietary energy company reports, and other 

sources, were used by individual province geohgists. 

Contributions of time, information, and insight by 

numerous individuals working in the U.S. oil and gas 

industry and State geological surveys were pmicularly helpful in play definition. In certain area~ of the 

country where drilling, completion, reserve, or production data are sparse, absent, or unreliable, province geologists devoted significant effort to 

compiling original databases for reservoir- and 

field-level information. This was particularly the 

case for Oklahoma, the States of the Appalachian 

Basin, Louisiana, and California. 

AREAS OF STUDY 

The oil and gas resources of the United State2 were 

evaluated on the basis of interpretation of the geolOK' of its 

petroleum provinces. For this study, the United Stat~s was 

divided into eight regions, which, in tum, encompas~ed 71 

separate provinces. Regional and provincial boundaries are 

illustrated in figure 3. The regions are basically geographic 

but are intended to provide broad geologic groupings of 

provinces. The provinces themselves are based on natural 

geologic entities and may include a single dominant structural element or a number of contiguous elements. The provinces are named for structural or geographic features within 

their boundaries. 

The regions and provinces used in this study are generally similar to those used in recent USGS assessments, with 

a few changes. Notable among these changes are the consolidation of the provinces of Alaska into a simpler three-province scheme, the merging of the Atlantic Coast and Eastern 

Interior into a single region, the inclusion and assessrent of 

Florida with the rest of the Gulf Coast, and the movenent of 

the boundary between the Pacific Coast Region and tl'~ Colorado Plateau and Basin and Range Region along mo .. e geologically defined boundaries, such as the San Andreas fault 

in southern California. A few other smaller changes have 

also been made and are described in the supporting 

play-level documentation included in the CD-ROM (Gautier 

and others, 1995). 
8 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

METHODS OF ASSESSMENT 

Distinct methodologies were used for assessment of 

large and small conventional accumulations, continuous-type (unconventional) accumulations, and field growth. 

The following brief summaries of methodology are provided 

for convenience, but for a more detailed treatment, the interested reader should refer to the supporting CD-ROM 

(Gautier and others, 1995). 

UNDISCOVERED CONVENTIONAL 

ACCUMULATIONS 

The assessment of undiscovered conventional 

resources was conducted at the play level. The methodology 

employed required estimation of the sizes, numbers, and 

types of undiscovered conventional accumulations of oil and 

gas and estimation of play risk. Numerous techniques were 

employed to make these estimates. These include reservoir-simulation modeling, discovery-process modeling, 

application of analogs, and spatial analysis. The method provides for a systematic integration and analysis of the geologic factors essential for the occurrence of oil and gas, a 

thorough documentation of the analysis, and an assessment 

containing information on the size, depth distribution, and 

number of hydrocarbon accumulations, as well as the quantity of estimated resources. Two principal categories of conventional plays were assessed: confirmed plays and 

hypothetical plays. 

A play was considered confirmed if one or more accumulations of the minimum size (1 MMBO or 6 BCFG) had 

been discovered in the play. Confirmed plays were commonly assessed by extrapolation or approximation based on 

sizes, numbers, depths, drilling history, and other properties 

of known accumulations. 

Hypothetical plays were those that were identified and 

defined based on geologic information but for which no 

accumulations of the minimum size had, as yet, been discovered. In contrast to confirmed plays, these hypothetical plays 

cannot, of course, be analyzed based on trends of known 

accumulations. Rather, properties of undiscovered accumulations must be postulated based on other types of information, including reservoir simulation and application of 

analog data sets from areas of similar geologic properties 

and known oil and (or) gas accumulations. Hypothetical 

plays characteristically carry a much broader degree of 

uncertainty, as recorded in the range of possible resources 

reported, than do confirmed plays. In addition to the greater 

range of reported resources, virtually all hypothetical plays 

carry a play-level probability of less than one. 

RISKING STRUCTURE 

It is by no means certain that any given play will contain 

an undiscovered accumulation. In order to express this 

uncertainty, a risking structure was develop~d based on the 

three geologic play attributes of charge, rese--voir, and trap. 

Estimates of the probability of occurrence for each of 

the three attributes were expressed as de~imal fract~ons 

between zero and one. The product of the three values is the 

play probability (risk= 1 -probability). Because the three 

play attributes are not necessarily indeperdent, care was 

taken not to apply multiple risks resulting from a single 

cause or event. 

In addition to the strictly hypothetical p1ays, the risking 

structure was also occasionally applied to intensely explored 

and largely exhausted plays within which the existence of 

yet another accumulation of the minimum size was uncertain. When calculating resources for hypothetical and largely 

exhausted plays, the play probability is app1

.ied against the 

product of the size and number of undiscovered accumulations estimated to exist in the play. For confirmed plays, 

other than those that were nearly exhausted, the play probability was one. Plays were not quantitatively assessed when 

the play probability was 0.10 or less. 

TRUNCATED SHIFTED PARETO MODEL 

For the purposes of this assessment, as in the previous 

USGS assessment (Mast and others, 1989), a model of the 

size-frequency distribution of the populatior of oil and (or) 

gas accumulations was assumed. The Truncated Shifted 

Pareto (TSP) model describes a "J -shaped" distribution in 

which ever-increasing numbers of accumulations occur in 

successively smaller size classes. The distribution is called 

shifted because it has been statistically moved to have its origin at the minimum accumulation size, in thi~ case 1 MMBO 

or 6 BCFG. The TSP distribution is referred to as truncated 

because, for the purposes of analysis, the distribution is cut 

off at the size of the largest accumulation in the distribution. 

For a detailed discussion of the TSP distribution, see Houghton and others (1993 ). 

An important use of the TSP distribution in this assessment was to provide a guide to province geologists in their 

development of estimates of undiscovered ac~umulations. A 

TSP distribution was fit to the population of accumulations 

known from each play and, in chronological order of discovery, to the first third of the accumulations discovered, the 

second third discovered, and the last third. The results of 

these fitted populations were provided to province geologists 

and review panels as source information regarding the 

changing size distribution of accumulations within the play 

as a function of time. 

The TSP distribution was also commonly used to model 

the field-size distribution of the undiscovered population. 

Unless the province geologist had another sp~cific model in 

mind, a TSP was fit to the estimated median size and to the 

estimated largest accumulation expected at a 5 percent probability within the postulated population of undiscovered 
METHODS OF ASSESSMENT 9 

accumulations, also considering the estimated limiting maximum size. The resulting TSP distribution was used to determine the remaining fractiles of the size distribution of the 

undiscovered population. 

Based on sizes and numbers of accumulations of oil and 

(or) non-associated gas estimated as undiscovered in each 

play, resources of each of these commodities were calculated 

using a Monte Carlo simulation technique and application of 

play risk. Estimates of undiscovered resources are presented 

as a range of values corresponding to probabilities of occurrence in order to express the uncertainty inherent in assessment of unknown quantities. The input variables of 

accumulation sizes and numbers are themselves expressed as 

density functions of uncertain quantities. The resulting 

cumulative probability distributions represent the estimated 

quantity of undiscovered resources-from these distributions, various fractiles (including the low (Fg5), the high 

(Fs), and the mean estimates) are obtained. 

Resources of gas associated with or dissolved in oil 

(associated-dissolved gas) were derived through use of estimated GOR's as applied to the calculated oil. Similarly, estimates of NGL were separately calculated for associated and 

non-associated gas by applying ratios provided by the estimators. Total gas and NGL at the play level were determined 

through summation. 

SMALL. FIELD ASSESSMENT 

Probabilistic estimates of recoverable oil and gas in 

accumulations smaller than 1 MMBO or 6 BCFG were 

made separately. The method for small field estimation in 

this assessment is essentially the same as that used by Mast 

and others ( 1989) and described by Root and Attanasi 

(1993). The estimates were based on extrapolations of 

numbers of fields in field-size classes smaller than the 

play-analysis cutoffs (1 MMBO, 6 BCFG) using a 

log-geometric model. The minimum size estimated was 

32,000 barrels of oil and 192,000 cubic feet of gas. Estimates were made for provinces. 

AGGREGATION AND DEPENDENCY 

To arrive at the estimated quantity of undiscovered 

resources for large areas, such as provinces, regions, or the 

Nation as a whole, distributions estimated for basic assessment units were progressively aggregated, with geological 

dependency incorporated at each level. In order to aggregate 

plays within provinces, geologic dependencies between 

plays were established for the three basic attributes of 

charge, reservoir, and trap. Province geologists determined 

for each pair of plays in their province whether the correlation was high (0.9).- moderate (0.5), or low (0.1) for each 

attribute. Thus, to determine the degree of dependency of 

plays A and B, if highly correlated with respect to charge 

(0.9), poorly correlated with respect to reservoirs (0.1), and 

moderately correlated with respect to trap (0.5), the mean 

correlation value was calculated to be (0.9+0.1 +0.5)/3, or 

0.5. This value of dependency would be used in aggregating 

plays A and B. For the aggregation of province-level estimates, the provinces within each region were assigned a 

dependency of 0.5. In aggregation of regions for a national 

total, regions were considered to be independent. 

RESERVE GROWTH (INFERRED RESERVES) 

Measured reserves of oil or gas are the estimated quantities that analysis of geological and engineering data demonstrate with reasonable certainty to be recoverable in future 

years from known fields under existing economic and operating conditions. This definition of reserves more often leads 

to underestimates than to overestimates of the remaining 

resources in a known field. The difference between proved 

reserves in known fields and remaining recoverable 

resources in known fields is here called inferred reserves. 

In the onshore areas and State water areas of the lower 

48 States, the reestimation of reserves in old fields each 

year has added far more to measured (proved) reserves than 

have new discoveries. Therefore, the future growth of discovered fields will be an important source of additions to 

reserves. The estimate of this growth for conventional 

fields is provided as inferred reserves. Growth of reserves 

in continuous-type deposits is included within the estimates 

of technically recoverable resources from those types of 

deposits. 

The Energy Information Administration has created the 

Oil and Gas Integrated Field File (OGIFF), which lists the 

estimated size (cumulative production plus proved reserves) 

for each oil and gas field in the United States. Fifteen estimates of size, as estimated in each of the 15 years from 1977 

through 1991, are given for each field. These are the basic 

data from which the pattern of field growth is calculated. 

For the purpose of estimating inferred reserves, the 

lower 48 States were divided into five areas, and the fields in 

these areas were divided into oil fields and gas fields. The 

five areas consisted of the assessment Regions 2, 3 and 4, 5 

and 7, 6, and 8 (see fig. 3). Growth functions were calculated 

for each area for the primary commodities, i.e., oil in oil 

fields and gas in gas fields. The secondary commodities 

(associated-dissolved gas and NGL) were assumed to grow 

proportionally to the primary commodities. Alaska growth 

was calculated using national growth functions for oil and 

gas because there were inadequate data to construct specific 

growth functions for that region. 

CONTINUOUS-TYPE ACCUMULATIONS 

SANDSTONES, SHALES, AND CHALKS 

Continuous-type accumulations may have spatial 

dimensions approaching those of plays and cannot be 

represented in terms of discrete, countable entities delineated 
0 

'C! 

'C! 

-

Vl 

z 

:J> 

-l 

~ :J> 

r 

:J> 

C/) 

C/) 

tTl 

C/) 

C/) 

3:: 

tTl 

z 

-l 

0 

'Tl 

c::: 

z 

::j 

tTl 

0 

C/) 

-l 

:J> 

-l 

m 

0 

r 

:J> 

z 

0 

a 

:J> 

C/) 

;:tl 

tTl 

C/) 

0 500 MILES 0 

c::: 

;:tl 

() 

tTl 0 500 KILOMETERS C/) 

Yj . /'-

Figure 6. Map of lower 48 States showing location of continuous-type plays for oil and gas in sandstones, shales, and chalks. 
METHODS OF ASSESSMENT 11 

by downdip hydrocarbon/water contacts, as are conventional 

accumulations. The identification of a continuous-type 

hydrocarbon accumulation is based on its geologic setting 

and does not incorporate somewhat ephemeral criteria, such 

as specified low API gravity2, low permeability ("tight"), 

special regulatory status, or need for unusual engineering 

techniques. A low-permeability reservoir may or may not be 

a continuous-type accumulation. 

The geologic setting typical of continuous-type accumulations is illustrated by figure 5. Common geologic characteristics of a continuous-type accumulation include 

occurrence downdip from water-saturated rocks, lack of 

obvious trap and seal, crosscutting of lithologic boundaries, 

large areal extent, relatively low matrix permeability, abnormal pressure (high or low), and close association with source 

rocks. Aspects of hydrocarbon production common to a continuous-type accumulation include a large in-place hydrocarbon volume, a low recovery factor, and a heterogeneous 

"hit-or-miss" character for production rates and ultimate 

recoveries of wells. 

In the case of continuous-type accumulations, the distinction between undiscovered resources and inferred 

reserves is blurred. The locations of continuous-type accumulations are commonly well known (implying inferred 

reserves) (fig. 6), but hydrocarbon estimates may be broadly 

dependent on geologic knowledge and theory (implying 

undiscovered resources). 

The first step of the assessment procedure is to represent the continuous-type accumulation by a play or plays. As 

in the case of conventional accumulations, geologic risk is 

assigned to each play. A gas-to-oil ratio of 20,000 cubic feet 

of gas per barrel of oil separates gas plays from oil plays. 

It is advantageous to envision the hydrocarbons of a 

continuous-type accumulation as residing areally in cells. A 

play is then regarded as a collection of cells (fig. 7). The cell 

area or size is equal to the median spacing, as dictated by 

drainage area, expected for wells of the play. Virtually all 

cells in a continuous-type accumulation are capable of producing some hydrocarbons. For purposes of this assessment, 

however, a productive cell is one for which production from 

the play is formally reported. An untested cell is one in 

which the play in question has not been evaluated by a well. 

The second step of the assessment procedure is to estimate the number of untested cells in a play and the fraction 

of untested cells expected to become productive (success 

ratio). Realistic consideration of the uncertainties associated 

with the number of untested cells in a play usually leads to a 

substantial range between the minimum and maximum number of untested cells. Therefore, the number of untested cells 

is treated as a probability distribution. 

2 A standard adopted by the American Petroleum Institute (API) for expressing the specific weight of oils. 

• • 0 

0 

• • • 0 

0 • 0 

Play boundary 

Figure 7. Sketch depicting continuous-type play as a col'ection of 

hydrocarbon-containing cells. Circles represent cells that tave been 

evaluated by wells; evaluated cells are either productive (solid circles) or nonproductive (open circles). Remaining cells are untested. 

The third step of the assessment procedure is tc establish a probability distribution for estimated ultimate recovery 

(EUR) for untested cells of the play that are expe,~ted to 

become productive. This distribution provides a reference 

model for production from cells yet to be drilled. Of course, 

this statistical model provides no insight as to which u"'tested 

cells are expected to become productive. 

Finally, the combination of play probability, ~uccess 

ratio, number of untested cells, and EUR probability distribution yields the potential additions to reserves expected for 

the continuous-type play. The in-place hydrocarbon volume 

is not us~d in this assessment procedure. 

A salient aspect of the assessment method is th!'l.t production and development patterns of the past are projected 

into the future. No assumptions regarding technology or economics are incorporated into the model. 

The information required for the assessment of continuous-type accumulations is supplied by earth scientists who 

are knowledgeable about the petroleum geology and engineering of the province under consideration. These regional 

experts complete a data form for each play, which is the 

source of the input data required for assessment -compntation 

programs. In those few cases in which there were seriously 

discordant views regarding a continuous-type play, the opinion of the province geologist has been used. 

In order to aggregate continuous-type accumul!'l.tions, 

a slight modification of the procedure used for conventional 

accumulations was employed: ( 1) the three basic attibutes 

of geologic dependency of the assessment units con~idered 

were charge, reservoir, and areal extent (note that the 

dependency associated with areal extent could be negative), 
0 500 MILES 

I I I I 1

1 1

1 

0 500 KILOMETERS 

Figure 8. Map of lower 48 States showing areas of coal-bed gas and locations of plays assessed. 

u 

EXPLANATION 

Area of assessed plays 

Area of potential for 

coal-bed gas 

·-"'"' 

-!'>,) 

-\0 

\0 

Vl 

z 

> ::j 

~ > l' 

> en 

en 

trl 

en 

en 

~ ~ c 

z 

::::3 

tTl 

0 

en 

~ 

~ en 

0 

l' 

-

~ 0 

S2 

en 

~ en 

~ tTl 

en 
RESULTS OF THE ASSESSMENT 13 

(2) correlations could assume any value between 1.0 and 

-1.0, and (3) dependencies at all aggregation levels were 

estimated in the same manner as that described for 

conventional plays. 

COAL-BED GAS 

The unit of assessment of potential additions to reserves 

of coal-bed gas was the play. Coal-bed gas plays were 

defined as areas within widespread, commonly basin-wide, 

accumulations that have similar conditions of generation, 

accumulation, and production of gas (fig. 8). The factors that 

define the plays include coal-bed thickness, heterogeneity, 

depth, and composition; seals; gas content; gas composition; 

permeability; pressure regime; structural setting; and hydrology; as well as conventional trapping mechanisms. It is postulated that recoverable coal-bed gas reserves are generally 

restricted to present-day depths of burial of 500 to 6,000 ft 

because of gas content and formation permeability. 

The assessment of potential additions to reserves of 

coal-bed gas was based on the estimation of the number and 

estimated ultimate recoveries (EUR's) of untested cells 

within each assessed play. The procedure is similar, in part, 

to that used in assessment of continuous-type accumulations 

in sandstones, shales, and chalks that is described above and 

by Schmoker in the supporting CD-ROM (Gautier and others, 1995). However, the coal-bed gas assessment relied 

heavily on production forecasting using a reservoir simulator. A range of EUR's and production rates of both gas and 

water were projected on a "per-well" and "per-foot-of-coal" 

basis for each play. The reservoir simulator was used 

because: (1) coal-bed gas accumulations are in early stages 

of development, and long-term production histories are generally not available, and (2) other methods, such as decline 

curve analysis and material balance are not adequate for 

expressing the complex movement of gas and water in coal. 

Input parameters for modeling in this study were based 

on actual data, analog information, and judgments of geologists and engineers. To resolve some of the data uncertainty, 

particularly for key reservoir parameters such as gas content 

and permeability, well production was compared to that predicted by the simulator for selected wells. This process is 

known as "history matching" because the initial data estimates commonly are adjusted to obtain simulated results that 

are characteristic of actual well performance. 

For most plays, long-term production from vertical 

wells, with a variety of completion techniques, was forecasted. In mining areas, production from wells was modeled. 

The EUR's predicted by reservoir simulation were used in 

conjunction with coal thicknesses to establish an EUR probability distribution for potentially productive, untested cells 

in each play. Seven fractiles (lOOth, 95th, 75th, 50th, 25th, 

5th, and Oth) were provided for the computational model, 

and the distribution was assumed to be lognormal. For plays 

in which no reservoir simulation was performed, EUR's on 

a per-foot-of-coal basis from analog plays were scalect and a 

similar procedure was used. The assessment of coal-b~d gas 

is based on existing technology. 

RESULTS OF THE ASSESSMENT 

TECHNICALLY RECOVERABLE 

CONVENTIONAL RESOURCES 

Approximately 460 conventional plays were defined 

for the 1995 National Assessment, of which 373 were 

assessed. Of these assessed plays, 290 were confirmed plays 

and 83 were hypothetical plays. 

OIL 

We estimate the undiscovered technically recoverable 

conventional oil resources of the United States to rang~ from 

23.5 BBO (billion barrels of oil) at a 95 percent prob<~bility 

to as much as 39.6 BBO at a 5 percent probability. The mean 

estimate of undiscovered conventional oil is 30.3 BE 0. Of 

this amount, approximately 6.4 BBO exist in accumulations 

smaller than 1 MMBO. Estimated conventional oil res')urces 

are listed by region and by province in table 2 and illustrated 

in figure 9. 

GAS 

Undiscovered technically recoverable conventional gas 

resources, including both non-associated gas and associated-dissolved gas, range from 207.1 TCFG at a 95 rercent 

probability to as much as 329.1 TCFG at a 5 percent probability. The mean estimate of undiscovered convention1.l natural gas is 258.7 TCFG. Of this amount, approximately 45.2 

TCFG exist in accumulations smaller than 6 BCFG. Estimated conventional gas resources are tabulated by region 

and by province in table 2 and illustrated in figure 10. 

NGL 

We estimate the undiscovered technically recoverable 

resources of natural gas liquids in conventional accumulations to range from 5.8 BBNGL (billion barrels of natu~al gas 

liquids) at a 95 percent probability to as much as 8.9 BBNGL 

at a 5 percent probability. The mean estimate of undiscovered NGL is 7.2 BBNGL. Estimated NGL resources a~e tabulated by region and by province in table 2. 

Figures 9 and 10 show, by use of mean values, the distribution of undiscovered resources of oil and gas by region. 

We estimate that approximately 28 percent of the undiscovered oil resources and 26 percent of the undiscoven~d gas 

resources exist in Alaska (Region 1-see fig. 3 ); arproximately 13 percent of undiscovered oil resources and 5 
14 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

Table 2. Estimates of undiscovered technically recoverable conventional oil, gas, and NGL resources by petroleum region and province. 

[Mean value totals may not be equal to the sums of the component means given elsewhere because numbers have been independently rounded. Gas includes both non-associated 

and associated-dissolved gas. Fractile values (F95, Fs) are not additive. F95 represents a 19 in 20 chance and F5 represents a I in 20 chance of the occurrenc,~ of at least the amount 

tabulated. NGL, natural gas liquids] 

Crude oil Gas NGL 

Province number and name (billion barrels) (trillion cubic feet) (bil':on barrels) 

F95 Fs Mean F9s Fs Mean F95 Fs Mean 

Region 1-Alaska 

001, Northern Alaska 2.34 15.43 7.41 23.27 124.33 63.50 0.44 2.08 1.12 

002, Central Alaska 0.00 0.32 0.06 0.51 7.31 2.76 0.00 0.00 0.00 

003, Southern Alaska 0.19 2.20 0.97 0.68 4.34 2.14 0.00 0.00 0.00 

Total, Region 1 3.19 16.76 8.44 27.89 129.21 68.41 0.44 2.08 1.12 

Region 2-Pacific Coast 

004, Western Oregon-Wash. 0.00 0.12 0.03 0.10 1.94 0.79 0.00 0.01 <0.01 

005, Eastern Oregon-Wash. 0.00 0.00 0.00 0.00 1.61 0.39 0.00 <0.01 <0.01 

006, Klamath-Sierra Nevada1 

007, Northern Coastal <0.01 0.09 0.03 0.35 2.33 1.09 0.00 <0.01 <0.01 

008, Sonoma-Livermore Basin 0.00 0.06 0.01 0.00 0.42 0.06 0.00 0.00 0.00 

009, Sacramento Basin 0.00 0.00 <0.01 0.62 7.83 3.32 <0.01 0.02 0.01 

010, San Joaquin Basin 0.51 2.15 1.21 1.08 4.61 2.57 0.04 0.20 0.11 

011, Central Coastal 0.10 1.17 0.49 0.03 0.37 0.15 <0.01 0.01 0.01 

012, Santa Maria Basin 0.02 0.60 0.21 0.01 0.35 0.12 <0.01 0.03 0.01 

013, Ventura Basin 0.28 2.27 1.06 0.66 3.66 1.90 0.02 0.12 0.06 

014, Los Angeles Basin 0.41 1.78 0.98 0.61 3.08 1.61 0.02 0.11 0.06 

015, San Diego-Oceanside2 

016, Salton Trough1 

Total, Region 2 2.55 5.93 4.02 7.66 17.67 12.00 0.15 0.38 0.25 

Region 3-Colorado Plateau and Basin and Range 

017, Idaho-Snake River Downwarp 0.00 0.01 <0.01 0.00 0.09 0.01 0.00 0.00 0.00 

018, Western Great Basin 0.00 0.37 0.06 0.00 2.49 0.43 0.00 0.00 0.00 

019, Eastern Great Basin 0.06 1.34 0.49 0.01 1.12 0.33 0.00 0.02 0.01 

020, Uinta-Piceance Basin 0.04 0.59 0.21 1.94 9.56 4.54 0.01 0.21 0.07 

021, Paradox Basin 0.11 0.61 0.31 0.93 3.48 2.02 0.03 0.17 0.09 

022, San Juan Basin 0.07 0.29 0.16 0.52 1.53 0.98 0.01 0.04 0.02 

023, Albuquerque-Santa Fe Rift 0.00 0.15 0.05 0.00 1.29 0.36 0.00 0.06 0.02 

024, Northern Arizona 0.00 0.32 0.07 0.00 0.96 0.17 0.00 0.08 0.01 

025, S. Ariz.-S.W. New Mexico 0.00 0.06 0.02 <0.01 0.53 0.20 <0.01 0.05 0.02 

026, South-Central New Mexico 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

Total, Region 3 0.66 2.46 1.36 5.48 15.27 9.05 0.12 0.41 0.23 

Region 4-Rocky Mountains and Northern Great Plains 

027, Montana Thrust Belt 0.00 0.01 <0.01 0.00 8.53 1.93 0.00 0.03 0.01 

028,Centra1Montana 0.13 0.42 0.27 0.40 1.37 0.84 <0.01 <0.01 <0.01 

029, Southwest Montana 0.00 0.13 0.03 0.12 0.76 0.40 <0.01 0.01 <0.01 

031, Williston Basin 0.25 1.17 0.65 0.89 2.61 1.69 0.07 0.22 0.14 

032, Sioux Arch 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

033, Powder River Basin 0.70 3.88 1.94 0.66 2.88 1.60 0.04 0.17 0.09 

034, Big Hom Basin 0.08 0.87 0.39 0.24 1.19 0.62 <0.01 0.02 0.01 

035, Wind River Basin 0.05 0.32 0.16 0.57 2.21 1.24 0.01 0.02 0.01 

036, Wyoming Thrust Belt 0.21 1.16 0.63 5.55 16.60 10.68 0.51 1.84 1.13 

037, S.W. Wyoming 0.04 0.40 0.17 0.70 2.86 1.58 0.01 0.04 0.02 

038, Park Basins <0.01 0.11 0.03 <0.01 0.07 0.02 0.00 <0.01 0.00 

039, Denver Basin 0.09 0.42 0.23 0.34 1.38 0.75 0.01 0.04 0.02 

040,LasAnimasArch 0.04 0.28 0.14 0.20 1.07 0.53 0.01 0.03 0.01 

041, Raton Basin-Sierra Grande Uplift 0.00 0.00 0.00 0.00 0.11 0.04 0.00 <0.01 <0.01 

Total, Region 4 3.06 6.84 4.63 15.22 31.10 21.91 0.83 2.18 1.45 
RESULTS OF THE ASSESSMENT 15 

Table 2. Estimates of undiscovered technically recoverable conventional oil, gas, and NGL resources by petroleum regio" and 

province-Continued. 

[Mean value totals may not be equal to the sums of the component means given elsewhere because numbers have been independently rounded. Gas includes both non-associated 

and associated-dissolved gas. Fractile values (F95, Fs) are not additive. F95 represents a 19 in 20 chance and Fs represents a 1 in 20 chance of the occurrence of at least the amount 

tabulated. NGL, natural gas liquids] 

Crude oil Gas NGL 

Province number and name (billion barrels) (trillion cubic feet) (billion barrels) 

Fgs Fs Mean Fgs Fs Mean Fgs Fs M~an 

Region 5--West Texas and Eastern New Mexico 

042, Pedemal Uplift 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

043, Palo Duro Basin 0.01 0.07 0.03 <0.01 0.01 <0.01 0.00 0.00 0.00 

044, Permian Basin 1.59 4.52 2.89 10.56 23.32 16.47 0.40 0.88 0.61 

045, Bend Arch-Fort Worth Basin 0.29 1.13 0.64 1.19 3.36 2.15 0.07 0.20 0.13 

046, Marathon Thrust Belt <0.01 0.04 0.02 0.06 0.28 0.14 <0.01 0.02 0.01 

Total, Region 5 2.22 5.27 3.58 12.91 25.76 18.77 0.52 1.02 0.75 

Region 6-Gulf Coast 

04 7, Western Gulf 0.73 4.56 2.29 44.11 96.18 68.09 1.04 2.20 1.57 

048, East Texas Basin3 

049, Louisiana-Mississippi Salt Basins 0.88 5.39 2.76 18.81 42.34 29.49 0.57 2.10 1.21 

050, Florida Peninsula 0.05 1.20 0.42 <0.01 0.11 0.04 0.00 0.00 0.00 

Total, Region 6 2.69 8.94 5.48 70.93 130.24 97.62 1.91 3.89 2.78 

Region 7-Midcontinent 

051, Superior 0.00 0.44 0.05 0.00 2.95 0.42 0.00 <0.01 <0.01 

052, Iowa Shelf 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

053, Cambridge Arch-Central Kansas 0.04 0.43 0.20 0.08 1.04 0.41 <0.01 0.04 0.02 

054, Salina Basin4 

055, Nemaha Uplift 0.03 0.29 0.12 0.17 0.96 0.47 0.01 0.06 0.03 

056, Forest City Basin 0.00 0.06 0.02 0.00 0.19 0.07 0.00 0.01 <0.01 

057, Ozark Uplift 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

058, Anadarko Basin 0.21 0.66 0.40 8.72 

I 21.27 14.20 0.13 0.28 0.19 

059, Sedgwick Basin 0.02 0.12 0.07 0.14 0.51 0.32 <0.01 0.03 0.02 

060, Cherokee Platform 0.02 0.17 0.08 0.07 0.34 0.18 <0.01 0.02 0.01 

061, Southern Oklahoma 0.05 0.57 0.24 0.47 1.79 1.02 0.01 0.05 0.02 

062, Arkoma Basin 0.00 0.07 0.01 1.17 4.22 2.48 0.04 0.13 0.08 

Total, Region 7 0.77 1.85 1.20 13.60 27.49 19.56 0.28 0.48 0.37 

Region ~Eastern 

063, Michigan Basin 0.50 2.02 1.14 3.33 9.33 5.98 0.14 0.36 0.24 

064, lllinois Basin 0.05 0.57 0.26 0.01 3.24 0.49 0.00 0.00 0.00 

065, Black Warrior Basin 0.01 0.07 0.03 0.97 3.34 2.04 <0.01 0.01 0.01 

066, Cincinnati Arch <0.01 0.04 0.02 <0.01 0.04 0.02 0.00 <0.01 <0.01 

067, Appalachian Basin 0.03 0.24 0.10 1.55 3.43 2.42 <0.01 0.01 <0.01 

068, Blue Ridge Thrust Belt 0.00 0.00 0.00 0.00 0.15 0.03 0.00 0.00 0.00 

069, Piedmont 0.00 0.00 0.00 0.00 1.19 0.39 0.00 0.00 0.00 

Total, Region 8 0.85 2.46 1.55 7.91 15.83 11.38 0.15 0.37 0.25 

TOTAL, United States 23.52 39.63 30.25 207.12 329.13 258.69 5.82 8.87 7.20 

1

No resources assessed. 

2 Assessed by Minerals Management Service. 

3 Assessed with Province 049. 

4Assessed with Province 059. 
16 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

Region 7 

(1.2 BBO) 

Region 8 

(1.6 BBO) 

5.1% 

Region 5 

(3.6 BBO) 

11.8% Region 4 

(4.6 BBO) 

15.3% 

Region 3 

(1.4 BBO) 

4.5% 

Figure 9. Distribution of undiscovered technically recoverable 

conventional oil resources, by region (see fig. 3 ). Based on mean estimates for the regions. BBO, billion barrels of oil. 

percent of undiscovered gas resources exist in California and 

the remainder of the Pacific Coast (Region 2-see fig. 3 ); 

and 18 percent of the undiscovered oil resources and 38 percent of the undiscovered gas resources exist in the Gulf 

Coast (Region 6-see fig. 3). Of the remaining undiscovered 

resources, approximately 41 percent of the oil resources and 

31 percent of the natural gas resources are distributed among 

the remaining five regions (Regions 3, 4, 5, 7, and 8-see 

fig. 3). 

Undiscovered conventional oil and gas resources are 

estimated within ranges of probability. Estimated national 

totals for undiscovered conventional oil and gas resources 

of the onshore and State waters of the United States are 

listed in table 1 and illustrated in figures 11 and 12. Ranges 

of probability for resources by region and province are 

listed in table 2. 

INFERRED RESERVES (GROWTH OF 

CONVENTIONAL FIELDS) 

Growth functions were calculated and applied to the 

1992 estimates from the OGIFF data for oil and gas for each 

year of discovery for each region and commodity. The 

results are summarized in table 3. It is estimated that 60.0 

BBO will be added to oil reserves and 322.0 TCFG to gas 

reserves during the 80 years following 1991. 

TECHNICALLY RECOVERABLE RESOURCES 

IN CONTINUOUS-TYPE DEPOSITS 

SANDSTONES, SHALES, AND C:ClALKS 

Sixty-one continuous-type plays wer1~ defined for the 

1995 National Assessment, of which 47 w~re assessed. Of 

the assessed plays, 34 were gas plays and 13 were oil plays. 

The predominant reservoir rock is sandstone for 32 plays, 

shale for 20 plays, and carbonate for 9 plays. Although the 

plays are geographically diverse, none are in Alaska and 

none extend into State offshore waters (fig. 6). 

Technically recoverable hydrocarbor resources from 

continuous-type accumulations are substantial (table 4). 

Estimated natural gas resources range bet'veen 219 TCFG 

(95th fractile) and 417 TCFG (5th fractile), with a mean of 

308 TCFG; those for crude oil range betvreen 1.5 and 2. 7 

BBO, with a mean of 2.1 BBO; those for n:ttural gas liquids 

range between 1.1 and 3.5 BBNGL, with a mean of 2.1 

BBNGL. 

COAL-BED GAS 

Major coal-bearing areas in the lower 48 States are 

shown in figure 8. The in-place resources of coal-bed gas are 

determined by the product of the coal tonn"'ge and gas content. Although the in-place resources are very large for this 

Region 8 

(11.4 TCFG) 

4.4% 

Region 5 

(18.8 TCFG) 

7.3% 

Region 2 

·-(12.0 TCFG) 

4.6% ., 

R~gion 3 

(9.1 TCFG) 

3.5% 

Figure 10. Distribution of undiscovered techrically recoverable 

conventional gas resources, by region (see fig. 3). Based on mean 

estimates for the regions. TCFG, trillion cubic f~et of gas. 
RESULTS OF THE ASSESSMENT 17 

Table 3. Estimated future growth (inferred reserves) of conventional fields as of December 31, 

1991. 

Crude oil Gas NGL 

Region (billion barrels) (trillion cubic feet) (billion barrels) 

~ 0.8 

~ w 

a: 

g 0.6 

IL 

0 

> 

~ 04 

iii 

ct 

ID 

0 If 0.2 

Region 1, Alaska 

Regions 2-8, lower 48 States 

TOTAL, onshore areas and State 

waters of the United States 

Mean Estimate= 30.3 

04-----~----~----~----r===--~----~--~ 

0 10 20 30 40 50 60 70 

BILLION BARRELS 

Figure 11. Estimated cumulative probability distributions for undiscovered technically recoverable conventional oil resources of 

onshore areas and State waters of the United States. 

type of accumulation, the main concern is recoverability. 

Figure 8 also shows the location of areas within which potential additions to reserves of coal-bed gas were quantitatively 

assessed for 39 plays. For this assessment, technically recoverable resources of coal-bed gas for the lower 48 States are 

estimated to range from 42.9 TCFG to 57.6 TCFG, with a 

mean estimate of 49.9 TCFG. Estimates for individual provinces and regions are presented in table 5. 

DISCUSSION AND COMPARISON OF 

RESULTS WITH THE PREVIOUS 

USGS/MMS ASSESSMENT 

OIL 

On the basis of existing technology and geologic concepts, there are, in total, approximately 110 billion barrels of 

technically recoverable oil, largely in existing and undiscovered conventional accumulations onshore and in State waters 

of the United States-this includes measured reserves and 

inferred reserves in existing fields as well as undiscovered 

accumulations. This number is significantly larger than the 

comparable number of about 78 billion barrels of technically 

recoverable oil recognized at the time of the previous assessment (Mast and others, 1989). In 1993, the United States 

13.0 

47.0 

60.0 

~ 0.8 J: 

1-

w 

a: 

g 0.6 

IL 

0 

~ ::J 0.4 

iii 

ct 

ID 

0 If 02 

32.0 

290.0 

322.0 

0 100 

0.5 

12.9 

13.4 

Mean Estimate = 258.7 

200 300 400 500 600 

TRILLION CUBIC FEET 

Figure 12. Estimated cumulative probability distributions for undiscovered technically recoverable conventional gas resources of 

onshore areas and State waters of the United States. 

produced about 2.4 BBO, approximately 50 percent of 

national consumption. At the time of the last Na6:mal 

Assessment (1989), yearly production stood at app'"oximately 2.5 BBO. The significant increase in technisally 

recoverable oil reported in this assessment largely reflects 

anticipated increases in reserves of known fields. 

Estimated mean amounts of undiscovered convenfonal 

oil resources onshore and in State waters of the United States 

are about 10 percent lower than those reported in the 1989 

National Assessment of oil and gas resources (Mast and others, 1989) (30.3 BBO vs. 33.3 BBO, respectively). Sine~ the 

last assessment, more than 2 BBO has been discovered in 

new fields, thereby reducing previous undiscovered qmmtities by that amount. The change in estimated conventional 

oil resources is also due to a reduction in our estimates of the 

undiscovered oil resources in part of northern Alaska. 

Our understanding of the thermal history and geoc1,~mical makeup of the rocks of northern Alaska leads us to 

expect more natural gas and less oil in the foothills regio'"', an 

area that includes most of the Arctic National Wildlife Refuge (ANWR) as well as the southern part of the National 

Petroleum Reserve in Alaska. This information comes from 

results of drilling the Tenneco Aurora well, located just offshore from the ANWR; results of a major USGS study that 

summarized all thermal data in Alaska; and USGS studi ~s in 

the foothills region (which combine thermal-hiftory 
18 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

Table 4. Technically recoverable resources estimated for continuous-type plays in sandstones, shales, and chalks, ons'lore United States. 

[Gas includes both non-associated and associated-dissolved gas. Fractile values (F95, Fs} are not additive. F95 represents a 19 in 20 chance and Fs repres<"nts a I in 20 chance of 

the occurrence of at least the amount tabulated. NGL. natural gas liquids. Leaders ( --) indicate less than 0.5 million barrels I 

Crude oil Gas rTGL 

Province number and name (million barrels) (trillion cubic feet) (millicn barrels) 

F95 Fs Mean F95 Fs Mean F95 Fs Mean 

Region 2-Pacific Coast 

005, Oregon-Washington 0 0 0 2.80 30.87 12.20 28 309 122 

Total, Region 2 0 0 0 2.80 30.87 12.20 28 ?~9 122 

Region ~olorado Plateau and Basin and Range 

020, Uinta-Piceance Basin 59 139 94 11.55 23.38 16.74 63 139 96 

021, Paradox Basin 61 597 242 0.05 0.48 0.19 0 0 0 

022, San Juan Basin 68 394 189 10.66 36.84 21.15 2 1 

Total, Region 3 249 940 525 24.88 55.02 38.09 63 140 96 

Region 4--Rocky Mountains and Northern Great Plains 

028, Central Montana 0 0 0 19.92 79.03 43.16 0 0 0 

031, Williston Basin 97 283 167 0.08 0.24 0.14 0 0 0 

037, S.W. Wyoming 0 0 0 55.95 213.51 119.30 810 3104 1733 

039, Denver Basin 139 502 285 1.49 5.69 3.16 

Total, Region 4 271 695 452 91.86 268.89 165.76 810 3,104 1,733 

Region 6--Gulf Coast 

04 7, Western Gulf 752 1516 1089 1.82 3.67 2.63 0 0 0 

049, East Texas Basin 0 0 0 3.55 9.40 6.03 89 235 151 

Total, Region 6 752 1,516 1,089 5.91 12.13 8.67 89 .::35 151 

Region 8-Eastem 

063, Michigan Basin 0 0 0 

064, illinois Basin 0 0 0 

067, Appalachian Basin 0 0 0 

Total, Region 8 0 0 0 

TOTAL, onshore United States 1,539 2,695 2,066 

information with that of the time of origin of rock structure 

and reveal an unfavorable relationship for the development 

of hydrocarbon traps). This unfavorable relation has resulted 

in the downgrading of oil resource potential in the foothills 

region but not in the coastal plain to the north. 

The reduction in Alaska is, in part, offset in the national 

total by small increases in a number of other regions of the 

United States (table 2). Estimates of undiscovered conventional oil resources in most other regions of the United States 

are, in general, similar to those published in the past, 

although they differ somewhat in detail. For further information, refer to play-level documentation in the supporting 

CD-ROM (Gautier and others, 1995). It should be emphasized that substantial overlap exists in the resource-range 

values estimated in the two studies. 

Estimates of anticipated inferred reserves are significantly greater than those reported in 1989 (60 BBO vs. 21 

BBO). This increase reflects our employment of an entirely 

different and newer set of field-level reserves data in this 

assessment. The last National Assessment (Mast and others, 

5.82 42.60 18.87 0 0 0 

0.91 7.59 3.28 1 4 2 

43.12 83.66 61.21 9 23 15 

56.08 118.70 83.36 11 25 17 

219.36 416.55 308.08 1,122 3,542 2,119 

1989) relied on the American Petroleum In<:-titute-American 

Gas Association data collected during th~ 10-year period 

1969-79, whereas, in this assessment, we had access to the 

last 15 years of data collected by the En~rgy Information 

Administration (EIA) in its Oil and Gas Integrated Field File 

(OGIFF). The OGIFF file was collected during a period of 

extraordinary variations of activity in the U.S. oil and gas 

industry, including significant changes in oil and gas prices, 

drilling activity, and development efficienc.y. 

For the first time, the USGS has assessed technically 

recoverable resources in continuous-type (unconventional) 

accumulations. Included are about 2 fBO in continuous-type deposits, mostly in fractured shale reservoirs of the 

Bakken, Niobrara, Austin, and similar formations. These 

resources in unconventional reservoirs may have been 

partially accounted for as undiscovered resources in the previous National Assessment (Mast and others, 1989). 

Proved reserves of the United State~ onshore and in 

State waters, at the time of this assessm~nt, amounted to 

approximately 20 BBO, according to EIA. These values are 
RESULTS OF THE ASSESSMENT 19 

Table 5. Technically recoverable resources of gas estimated for 

continuous-type plays in coal beds, onshore United States. 

[Gas includes both non-associated and associated-dissolved gas. Fractile values (F95, 

Fs) are not additive. F95 represents a 19 in 20 chance and Fs represents a 1 in 20 chance 

of the occurrence of at least the amount tabulated] 

Gas 

Province number and name (trillion cubic feet) 

F95 F5 Mean 

Region 2-Pacific Coast 

004, Bellingham 0 0.09 0.04 

004, West Cascade 0 1.20 0.66 

Total, Region 2 0.26 1.30 0.70 

Region 3-Colorado Plateau and Basin and Range 

020, Uinta Basin 1.86 4.82 3.21 

020, Piceance Basin 5.47 10.09 7.49 

022, San Juan Basin 5.76 9.67 7.53 

Total, Region 3 15.00 21.88 18.24 

Region 4-Rocky Mountains and Northern Great Plains 

033, Powder River Basin 0.32 2.90 1.11 

035, Wind River Basin 0.22 0.72 0.43 

037, S.W. Wyoming 0.83 7.66 3.89 

041, Raton Basin 1.39 2.23 1.78 

Total, Region 4 3.97 11.71 7.20 

Region 7-Midcontinent 

056, Forest City Basin 0 1.44 0.45 

060, Cherokee Platform 1.07 3.08 1.91 

062, Arkoma Basin 1.87 3.58 2.64 

Total, Region 7 3.57 6.76 5.01 

Region 8--Eastem 

064, lllinois Basin 0.84 2.77 1.63 

065, Black Warrior Basin 1.49 3.43 2.30 

067, North Appalachian 7.68 16.36 11.48 

067, Central Appalachian 1.88 4.64 3.07 

067, Cahaba 0.14 0.54 0.29 

Total, Region 8 14.34 24.00 18.78 

TOTAL, lower 48 States 42.89 57.63 49.91 

significantly lower than those reported in 1989, when they 

stood at 24 BBO. 

GAS 

The technically recoverable conventional resources of 

natural gas from both growth of reserves in existing fields 

and from undiscovered accumulations onshore and in State 

waters, as of this assessment, is approximately 580 TCFG, 

compared with 347 TCFG at the time of the previous 

National Assessment (1989). Proved reserves of natural gas 

in the United States stand at approximately 135 TCFG, compared to 157 TCFG in 1989. Natural gas annual production 

has increased significantly in the intervening years from 17.0 

TCFG in 1989 to about 17.8 TCFG in 1993. 

Estimated mean amounts of undiscovered techni~ally 

recoverable conventional gas resources onshore and in State 

waters are approximately the same as those reported in the 

previous National Assessment (Mast and others, 1989) (259 

vs. 254 TCFG, respectively). Although estimates of conventional natural gas have actually been reduced in a few significant areas, such as the Anadarko Basin, estimates have been 

raised in a number of others (table 2). The overall ctange 

probably reflects the discovery of about 26 TCFG during the 

past 7 years and movement of certain resources previously 

estimated under conventional categories to plays in cortinuous-type deposits for this assessment. 

Estimates of future growth of gas reserves in known 

fields are up significantly for this assessment, having 

increased from 93 TCFG in 1989 to approximately 322 

TCFG for this assessment. As with oil, this increase ref ~cts, 

more than anything else, the use of the EIA OGIFF da•a set 

rather than American Petroleum Institute-American Gas 

Association data. 

In addition to conventional gas resources, the USGS 

has, for the first time, made a systematic assessme1.t of 

potential additions to technically recoverable reso'~rces 

deriving from continuous-type, largely unconventionaL reservoirs of natural gas. Resources in this category were not 

evaluated in the previous assessment'" (Mast and others, 

1989) because of the difficulties in developing adequate 

methodologies and data. Historically, these resources have 

contributed little to the national energy supply. However, we 

estimate there exists, at a mean value, 308 TCFG of te~hnically recoverable natural gas in continuous-type deposits in 

sandstones, shales, and chalks, and almost 50 TCFG of technically recoverable gas in coal-bed deposits. These reso'~rces 

are thus comparable in magnitude to conventional resot' ... ces, 

although their anticipated deliverability and development 

economics will be very different than gas in conventional 

accumulations. 

The 1995 National Assessment documents large, technically recoverable resources of non-associated gas in continuous-type deposits. Significant extraction effort w::ll be 

required to obtain this gas. Based on existing technolog:r, the 

assessment indicates that approximately 960,000 productive 

wells will be required to recover potential reserve additions 

of 300 TCFG, based on the distribution of EUR's shown in 

figure 13. Furthermore, extrapolation of present -day su -;cess 

ratios implies that roughly 570,000 "dry" holes would have 

to be drilled along with the productive wells. By way of perspective, the most oil and gas wells of all kinds drilled in the 

United States in 1 year is about 92,000, and from 1986 to the 

present the total has been less than 40,000 wells per ye1r. In 

the case of discrete (conventional) fields, most reso':trces 

have been recovered from relatively few, large fields. Analogously, in the case of continuous-type gas accumulations, 

most gas is expected to be recovered from a relatively small 

subset of productive wells. The assessment data show that 

one-half of the mean potential recoverable resources o~ 300 
20 1995 NATIONAL ASSESSMENT OF UNITED STATES OIL AND GAS RESOURCES 

140 

~ ~ 120 

~ (!) 

0 ll.. 

i= e 100 

i5 w c w <( ll.. 

w (.) 80 >iii a: ::::) ~ (.) 

~ ~ 60 

..J en <( z 

~ 3 40 w ..J 

~ ~ ~ 20 

0 

828,600 

Wells 

0 

Total Potential Reserve 

Additions = 303 TCFG 

60,700 

Wells 

28,800 

Wells 

34,400 

Wells 

0.5 1 1.5 2 

EUR PER PRODUCTIVE WELL (MEAN 

FOR EACH PLAY), IN BILLIONS 

OF CUBIC FEET OF GAS 

5,800 

Wells 

>2 

Figure 13. Potential reserve additions for non-associated gas in 

continuous-type plays as a function of estimated ultimate recovery 

(EUR) per well. 

TCFG will be produced by about 100,000 wells, 25 percent 

will be produced by an additional 150,000 wells, and the 

remaining 25 percent will require some 700,000 producing 

wells (fig. 13). 

ACKNOWLEDGMENTS 

The USGS wishes to thank numerous indi victuals in 

industry, academia, and State geological organizations for 

their help. The American Association of Petroleum Geologists, as an organization, was particularly helpful in identifying individuals who were willing to contribute time and 

expertise to this project. 

REFERENCES CITED 

Dolton, G.L., Carlson, K.H., Charpentier, R.R.. Coury, A.B., Crovelli, R.A., Frezon, S.E., Khan, A.S., Lister, J.H., McMullin, 

R.H., Pike, R.S., Powers, R.B., Scott, E.W., and Varnes, K.L., 

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication December 16, 1994 

Edited by Richard W. Scott, Jr. 

Photocomposition by Joan G. Nadeau 

1981, Estimates of undiscovered recoverable conventional 

resources of oil and gas in the United State~: U.S. Geological 

Survey Circular 860, 87 p. 

Energy Information Administration, 1994, U.S. Crude Oil, Natural 

Gas, and Natural Gas Liquids Reserves: Washington, D.C., 

1993 Annual Report, 155 p. 

Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., 

eds., 1995, 1995 National Assessment of United States oil and 

gas resources-Results, methodology, and supporting data: 

U.S. Geological Survey Digital Data Series 30 [CD-ROM]. 

Houghton, J.C., Dolton, G.L., Mast, R.F., Maste~s, C.D., and Root, 

D.H., 1993, U.S. Geological Survey estimation procedure for 

accumulation size distributions by play: Bulletin of the American Association of Petroleum Geologists, v. 77, no. 3, p. 

454-466. 

Mast, R.F., Dolton, G.L., Crovelli, R.A., Root, D.H., Attanasi, 

E.D., Martin, P.E., Cooke, L.W., Carpenter, G.B., Pecora, 

W.C., and Rose, M.B., 1989, Estimates of undiscovered conventional oil and gas resources in the United States-A part of 

the Nation's energy endowment: U.S. Deprrtment of the Interior, U.S. Geological Survey and Minerals Management Service, 44 p. 

NRG Associates, Inc., 1993 and 1994 [include~ data current as of 

December 31, 1992, and December 31, 1993, respectively], 

The Significant Oil and Gas Fields of the United States: Colorado Springs, Colo., NRG Associates, Inc., [database available 

from NRG Associates, Inc., P.O. Box 1655, Colorado Springs, 

co 80901]. 

Petroleum Information Corp., 1993 and 1994 [includes data current 

as of December 31, 1992, and December 31, 1993, respectively], Well History Control System: Littletor. Colo., Petroleum 

Information Corp., [database available from Petroleum Information Corp., 4100 East Dry Creek Read, Littleton, CO 

80122]. 

Petroleum Information Corp., 1994, Petro-ROM Production Data, 

August 1993-December 1993: Littleton, Colo., Petroleum 

Information Corp., [database available from Petroleum Information Corp., 4100 East Dry Creek Road, Littleton, CO 

80122]. 

Root, D.H., and Attanasi, E.D., 1993, Small fields in the National 

Oil and Gas Assessment: Bulletin of the American Association 

of Petroleum Geologists, v. 77, no. 3, p. 485-490. 

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, 5 p.