Reserve-Growth Assessment Project

Assessment of Potential Additions to Conventional Oil and 

Gas Resources in Discovered Fields of the United States from 

Reserve Growth, 2012

Introduction

The U.S. Geological Survey (USGS) estimated 

volumes of technically recoverable, conventional oil 

and gas resources that have the potential to be added 

to reserves (called “reserve growth” hereafter) in 

70 discovered accumulations of the United States, 

excluding Federal offshore areas. Most reserve 

growth results from delineation of new reservoirs, 

field extensions, improved technology that enhances 

efficiency, and recalculation of reserves due to 

changing economic and operating conditions.

Unlike past estimates of reserve growth that relied 

entirely on statistical extrapolations of growth trends, 

this assessment is based in part on detailed analysis of 

geology and engineering practices used in the assessed 

producing accumulations. The assessment used published and commercial, proprietary sources of geologic 

information and field production data.

Pumps at Santa Fe Springs field, Los Angeles, California, U.S.A. Photograph by Ken Takahashi, 

U.S. Geological Survey, January 2011.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3108

August 2012

The U.S. Geological Survey estimated volumes of technically recoverable, conventional petroleum 

resources that have the potential to be added to reserves from reserve growth in 70 discovered oil and gas 

accumulations of the United States, excluding Federal offshore areas. The mean estimated volumes are 

32 billion barrels of crude oil, 291 trillion cubic feet of natural gas, and 10 billion barrels of natural gas 

liquids.
Methodology

In the past, the USGS estimated reserve growth 

by statistical curve-fitting, regression methods using 

historical data on known recoverable oil and gas 

(cumulative production plus remaining reserves). 

Those regression methods were based on growth rates 

averaged by the number of years since accumulation 

discovery. Accumulations within mature petroleum 

provinces that exhibit volumetrically significant 

reserve growth, however, cause bias regression models 

of the data. Therefore, accumulations that have grown 

by greater proportions than other accumulations of 

similar age are best analyzed separately from those 

with only minor reserve growth. Reserve growth 

in fields of the United States was analyzed to determine which fields contributed most to reserve growth 

between 1982 and 2006, using field- and reservoirproduction data from a commercial, proprietary 

database.

The assessment methodology used to assess 

reserve growth in conventional accumulations of U.S. 

fields is described in Klett and others (2011). The 

primary data used to estimate reserve growth are original in-place volumes and recovery factors. Identified 

unconventional (continuous) oil and gas accumulations were excluded from the assessment.

Fifty-five large oil fields and 35 large gas fields 

significantly contributed to reserve growth during 

the time of the dataset. Within the 55 oil fields, 68 

individual conventional accumulations (reservoirs or 

groups of reservoirs) were identified and assessed. Of 

the 35 gas fields, only 2 accumulations were individually assessed. Unconventional accumulations contributed to reserve growth in the remaining 33 gas fields, 

which were excluded from the assessment. Once the 

identified unconventional reservoirs were removed, 

many of the 33 gas fields showed little significant 

reserve growth and were assessed by regression 

methods.

Probability distributions were assigned to reported 

original in-place volumes and to reported recovery 

factors. The original in-place and recovery factor 

distributions assigned to a given accumulation were 

statistically combined (multiplied) by Monte Carlo 

simulation, and known recoverable volumes were 

subtracted to provide a probability distribution of 

estimated reserve growth. Estimates of reserve growth 

for each accumulation were aggregated together and 

aggregated with estimates of reserve growth calculated by regression methods.

Reserve growth of oil was assessed for oil accumulations (accumulations having a gas-to-oil ratio 

of less than 20,000 cubic feet of recoverable natural 

gas per barrel of recoverable crude oil), and nonassociated gas was assessed for only gas accumulations 

(20,000 cubic feet of recoverable natural gas per barrel of recoverable crude oil or more). Reserve growth 

of coproducts was calculated by statistical combination (multiplication) using Monte Carlo simulation 

of the reserve growth of the primary commodity (oil 

or gas) with the volumetric coproduct ratios. The 

growth of associated and dissolved gas, along with 

natural gas liquids in the associated and dissolved 

gas, was calculated for oil fields. The growth of oil 

plus natural gas liquids (total liquids) was calculated for nonassociated gas fields. Coproduct reserve 

growth was not estimated for fields having unreported coproduct volumes.

Growth of oil reserves in the 68 assessed individual oil accumulations accounts for about 70 percent of the potential reserve growth of the United 

States; the other 30 percent attributed to smaller 

accumulations was estimated by regression methods. 

Almost all of the reserve growth of nonassociated gas 

and liquids in gas accumulations was estimated by 

regression methods. Although identified unconventional accumulations were not included in the assessment, unrecognized unconventional accumulations, 

particularly gas in tight reservoirs, might have been 

included. Inclusion of unconventional accumulations 

greatly affects the assessment results by providing 

unrealistically large estimates.

Assessment Results

Estimated mean volumes of technically recoverable, conventional oil and gas resources of the United 

States that have the potential to be added to reserves 

from reserve growth are listed in table 1. The estimates are about 32 billion barrels (BB) of crude oil, 

291 trillion cubic feet (TCF) of natural gas (50 TCF 

of associated and dissolved natural gas and 241 TCF 

of nonassociated natural gas), and 10 BB of natural 

gas liquids (3 BB of natural gas liquids in oil accumulations and 7 BB of total liquids in nonassociated 

gas accumulations). No attempt was made to estimate 

economically recoverable resources. Continuous, or 

unconventional, oil and gas accumulations, such as 

shale gas, tight gas, tight oil, and tar sands, were not 

included in this study.
Estimated mean volumes of technically recoverable, conventional oil and gas resources of U.S. 

regions that have the potential to be added to reserves 

from reserve growth are listed in table 2. They are as 

follows:

1. For the combined Alaska and the Pacific 

regions, about 20,210 million barrels (MMB) of 

crude oil, 31,921 billion cubic feet (BCF) of natural 

gas (23,717 BCF of associated and dissolved natural 

gas and 8,204 BCF of nonassociated natural gas), and 

1,466 MMB of natural gas liquids (1,443 MMB of 

natural gas liquids in oil accumulations and 23 MMB 

of total liquids in nonassociated gas accumulations).

2. For the Colorado Plateau and Basin and Range 

region, which was combined with the Rocky Mountains and Northern Great Plains region, about 1,730 

MMB of crude oil, 18,434 BCF of natural gas (2,359 

BCF of associated and dissolved natural gas and 

16,075 BCF of nonassociated natural gas), and 561 

MMB of natural gas liquids (149 MMB of natural 

gas liquids in oil accumulations and 412 MMB of 

total liquids in nonassociated gas accumulations).

3. For the West Texas and Eastern New Mexico 

region, about 4,855 MMB of crude oil, 51,721 BCF 

of natural gas (13,601 BCF of associated and dissolved natural gas and 38,120 BCF of nonassociated 

natural gas), and 2,699 MMB of natural gas liquids 

(881 MMB of natural gas liquids in oil accumulations and 1,818 MMB of total liquids in nonassociated gas accumulations).

4. For the Gulf Coast region, about 3,448 MMB 

of crude oil, 143,557 BCF of natural gas (8,576 BCF 

of associated and dissolved natural gas and 134,981 

BCF of nonassociated natural gas), and 4,814 MMB 

of natural gas liquids (412 MMB of natural gas 

liquids in oil accumulations and 4,402 MMB of total 

liquids in nonassociated gas accumulations).

5. For the combined Midcontinent and Eastern 

regions, about 1,522 MMB of crude oil, 45,318 BCF 

of natural gas (1,561 BCF of associated and dissolved natural gas and 43,757 BCF of nonassociated 

natural gas), and 788 MMB of natural gas liquids 

(89 MMB of natural gas liquids in oil accumulations 

and 699 MMB of total liquids in nonassociated gas 

accumulations).

The combined Alaska and the Pacific Coast 

regions should have the greatest amount of oil reserve 

growth, whereas the Gulf Coast region should have 

the greatest amount of gas reserve growth.

For Further Information

Supporting geologic studies of total petroleum 

systems and assessment units, and reports on the 

methodology used in this assessment of the world 

outside the United States, as well as the assessment 

results, are available at the USGS Energy website: 

http://energy.usgs.gov/.

Reserve growth by assessment Estimated reserve growth

method for discovered fields of the

United States

Field

type

Oil (BB) Gas (TCF) NGL (BB)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Oil

Individual accumulation analysis

22 32 2

0.3 -0.2

68

2

Number

of

accums.

Oil

Regression method

10 18 1

241 7

9,168

5,169

Gas

Aggregated reserve growth Oil

 by commodity

23 31 41 32 39 50 62 50 2 3 4 3

187 240 298 241 6 7 9 7

9,236

5,171

Total reserve growth 32 291 10

 

Gas

Gas

14,407

Table 1. Estimated reserve growth of discovered accumulations of the United States (technically recoverable, conventional 

petroleum resources).

[BB, billion barrels; TCF, trillion cubic feet. For gas fields, all liquids are included under the natural gas liquids (NGL) category. Accums., 

accumulations. F95 denotes a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Negative values 

indicate the possibility that reported reserves could decrease. Fractiles are not additive except under the assumption of perfect positive 

correlation. Gray shading indicates not applicable]
Reserve Growth Assessment Team

Timothy R. Klett (tklett@usgs.gov), Troy A. 

Cook, Ronald R. Charpentier, Marilyn E. Tennyson, 

Emil D. Attanasi, Phil A. Freeman, Robert T. Ryder, 

Donald L. Gautier, Mahendra K. Verma, Phuong A. 

Le, and Christopher J. Schenk.

Reference Cited

Klett, T.R., Attanasi, E.D., Charpentier, R.R., Cook, 

T.A., Freeman, P.A., Gautier, D.L., Le, P.A., Ryder, 

R.T., Schenk, C.J., Tennyson, M.E., and Verma, 

M.K., 2011, New U.S. Geological Survey method 

for the assessment of reserve growth: U.S. Geological Survey Scientific Investigations Report 2011-

5163, 8 p., accessed August 1, 2012, at http://pubs.

usgs.gov/sir/2011/5163/. 

Estimated reserve growth

Regions of the United States Field

type

Oil (MMB) Gas (BCF) NGL (MMB)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Gas

Oil

Alaska and Pacific Coast

12,849 19,894 28,603 20,210 15,347 23,439 33,016 23,717 900 1,426 2,043 1,443

4,021 7,867 13,479 8,204 11 22 38 23

296

154

Total 20,210 31,921 1,466

Number

of

fields

450

Gas

Colorado Plateau and Basin and Oil

 Range, and Rocky Mountains

 and Northern Great Plains

1,287 1,712 2,223 1,730 1,823 2,335 2,961 2,359 116 148 186 149

6,541 14,810 29,529 16,075 168 379 756 412

1,402

243

Total 1,645 1,730 18,434 561

Gas

West Texas and Eastern Oil

 New Mexico

2,952 4,786 6,983 4,855 10,181 13,521 17,319 13,601 607 873 1,184 881

25,407 37,256 53,613 38,120 1,212 1,777 2,557 1,818

2,511

774

Total 3,285 4,855 51,721 2,699

Gas

Oil Gulf Coast 2,671 3,422 4,310 3,448 6,766 8,515 10,555 8,576 320 409 514 412

98,050 134,783 172,103 134,981 3,195 4,395 5,612 4,402

2,239

2,712

Total 4,951 3,448 143,557 4,814

Gas

Oil

Midcontinent and Eastern

1,146 1,503 1,962 1,522 1,228 1,546 1,944 1,561 -22 79 230 89

28,738 43,055 61,280 43,757 409 687 1,032 699

2,788

1,288

Total 4,076 1,522 45,318 788

Table 2. Estimated reserve growth of discovered accumulations in regions of the United States (technically recoverable,

conventional petroleum resources).

[MMB, million barrels; BCF, billion cubic feet. For gas fields, all liquids are included under the natural gas liquids (NGL) category. F95 

denotes a 95-percent chance of at least the amount tabulated. Other fractiles are defined similarly. Negative values indicate the possibility 

that reported reserves could decrease. Fractiles are not additive except under the assumption of perfect positive correlation. Gray shading 

indicates not applicable]