U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3052

April 2012

Reserve-Growth Assessment Project

Assessment of Potential Additions to Conventional Oil 

and Gas Resources of the World (Outside the United States) 

from Reserve Growth, 2012

Printed on recycled paper

The U.S. Geological Survey 

estimated volumes of technically 

recoverable, conventional petroleum 

resources resulting from reserve 

growth for discovered fields outside 

the United States that have reported 

in-place oil and gas volumes of 

500 million barrels of oil equivalent 

or greater. The mean volumes were 

estimated at 665 billion barrels of 

crude oil, 1,429 trillion cubic feet of 

natural gas, and 16 billion barrels of 

natural gas liquids. These volumes 

constitute a significant portion of the 

world’s oil and gas resources.

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 discovered oil and gas fields of 

the world outside the United States with 

reported in-place oil and gas volumes 

of 500 million barrels of oil equivalent 

(MMBOE) [0.5 billion barrels (BB) of oil 

or 3 trillion cubic feet (TCF) of gas] or 

greater. Unlike past assessments of reserve 

growth that relied on statistical extrapolations of growth trends, this methodology 

includes detailed analysis of geology and 

engineering practices observed in developed fields. Because of the paucity of data 

for many fields outside the United States, 

data acquired from U.S. fields undergoing 

reserve growth were used as analogs in 

this study. The assessment was based on 

published geologic information, data from 

commercial sources on petroleum volumes 

of oil and gas wells and fields, and field 

production records.

Methodology

The primary data used to estimate 

reserve growth are original in-place 

volumes and recovery factors. Studies 

of 68 large oil accumulations and 2 large 

gas accumulations in the United States 

provided probability distributions for 

uncertainty of original (initial) in-place 

volumes, as well as a probability distribution of oil recovery factors. These distributions were used as analogs for large fields 

outside the United States. The assessment 

methodology used to assess reserve 

growth of U.S. fields is described in Klett 

and others (2011).

Probability distributions of original 

in-place volumes for U.S. fields were 

assigned to reported original in-place volumes for fields outside the United States. 

Probability distributions of recovery factors for the 68 U.S. oil accumulations were 

assumed for oil fields outside the United 

States. A uniform probability distribution 

of the mean reported recovery factor of 

nonassociated gas fields outside the United 

States was assumed for recovery factors 

in gas fields. The original in-place and 

recovery factor distributions assigned to 

a given field were statistically combined 

(multiplied) by Monte Carlo simulation 

to provide a probability distribution of 

estimated reserve growth.

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, and 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.

Reserve growth was estimated for 

fields having reported original in-place oil 

or gas of 500 MMBOE or greater. Continuous oil and gas (unconventional) accumulations and tar sands were excluded. 

The reserve growth estimated in this study 

accounts for about 80 percent of the potential reserve growth of the world outside the 

United States; the other 20 percent is in 

fields with in-place volumes smaller than 

500 MMBOE. Large fields were chosen 

Pump jacks photograph by Ken Takahashi, U.S. Geological Survey, January 2011.
for this study because they are most likely 

to experience reserve growth, and their 

oil and gas volume data are relatively 

well reported and accurate. In addition, 

most of the world’s in-place volumes and 

reserves, outside the United States, reside 

in 1,814 fields having reported original 

in-place oil or gas of 500 MMBOE or 

greater. Approximately 85 percent of the 

world’s original in-place oil outside the 

United States resides in 1,467 oil fields 

(about 9 percent of the total number of oil 

fields). Approximately 79 percent of the 

original in-place nonassociated gas resides 

in 347 gas fields (about 3 percent of the 

total number of nonassociated gas fields).

Table 1. Estimated reserve growth of discovered fields outside the United States with reported in-place volumes of 500 million barrels of oil 

equivalent or greater (technically recoverable, conventional petroleum resources).

[MMBOE, million barrels of oil equivalent; BB, billion barrels; TCF, trillion 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. Gray shading indicates not applicable]

World (outside the United States):

Discovered fields with in-place

volumes of 500 MMBOE 

or greater

Field

type

Number

of

fields

Estimated reserve growth

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

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

Reserve growth by commodity

Oil 1,467 329 598 1,205 665 592 956 1,766 1,043 –16 –2 18 –1

Gas 347 –469 210 1,781 386 –14 11 65 17

Total reserve growth 1,814 665 1,429 16

Assessment Results

Estimates of volumes of technically recoverable, conventional oil 

and gas resources of the world outside 

the United States from reserve growth 

are listed in table 1. No attempt was 

made to estimate economically recoverable resources. The estimated mean 

volumes of oil and gas are about 665 BB 

of crude oil, 1,429 TCF of natural gas 

(1,043 TCF of associated and dissolved 

natural gas and 386 TCF of nonassociated 

natural gas), and 16 BB of natural 

gas liquids (–1 BB of natural gas liquids in oil accumulations and 17 BB 

of total liquids in nonassociated gas 

accumulations). Negative values 

indicate the possibility that reported 

reserves could decrease rather than 

increase (growth).

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, 9 p., available at 

http://pubs.usgs.gov/sir/2011/5163/

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 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, Christopher J. Schenk.