U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1438

An Approach to the Classification of Potential

Reserve Additions of Giant Oil Fields of the World
An Approach to the Classification of 

Potential Reserve Additions of Giant 

Oil Fields of the World 

By T.R. Klett and M.E. Tennyson 

Open-File Report 2007–1438 

U. S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2008 

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Suggested citation:

Klett, T.R., and Tennyson, M.E., 2008, An approach to the classification of potential 

reserve additions of giant oil fields of the world: U.S. Geological Survey Open-File 

Report 2007-1438, 28 p. 

2
An Approach to the Classification of 

Potential Reserve Additions of 

Giant Oil Fields of the World 

By T.R. Klett and M.E. Tennyson 

Foreword 

 This report contains notes for slides in a presentation given to the 

Committee on Sustainable Energy and the Ad Hoc Group of Experts on 

Harmonization of Fossil Energy and Mineral Resources Terminology on 17 

October 2007 in Geneva, Switzerland. 

An Approach to the Classification of 

Potential Reserves Additions of Giant 

Oil Fields of the World

T.R. Klett and M.E. Tennyson

U.S. Geological Survey, Box 25046 Denver Federal Center, MS 939, Denver, CO 80225 U.S.A.

United Nations Ad Hoc Group of Experts on Harmonization of Fossil Energy and 

Mineral Resources Terminology

Geneva, Switzerland

17-19 October 2007

U.S. Department of the Interior

U.S. Geological Survey

1

Slide 1. This presentation describes the U.S. Geological Survey (USGS) 

study to characterize and quantify petroleum-reserve additions, and the 

application of this study to help classify the quantities. 

3
Issues

• Classification of quantities in discovered fields 

that can be added to reserves (potentially 

recoverable) is somewhat ambiguous

• U.S. Geological Survey (USGS) broadly 

classifies these quantities

• Analysis of individual field-development 

operations and resulting reserve growth may aid 

in the classification of these potential reserve 

additions

2

Slide 2. The classification of quantities of petroleum that have the potential 

to be added to reserves can be ambiguous. The USGS only broadly classifies 

these quantities. However, analysis of individual field-development 

operations and resulting reserve growth might aid in the classification of 

these potential reserve additions. 

 

4
Outline

• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

3

Slide 3. The presentation starts with a brief introduction, followed by a 

description of the USGS classification of petroleum quantities. Next, the 

approach to the field analysis study is described with some examples of 

reserve additions from actual field operations. Finally, a database of the 

reserve additions, which might be used for classification purposes, is 

demonstrated. 

 

5
• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

4

Introduction

• U.S. Geological Survey (USGS) estimates 

petroleum quantities in BOTH

– Undiscovered accumulations

• Easily classified in UNFC system

– Discovered accumulations, as reserve 

growth

• Somewhat ambiguous in any classification system

5

Slides 4 and 5. The USGS estimates petroleum resources in both 

discovered accumulations, in the form of reserve growth, and in 

undiscovered accumulations. The term “petroleum” is used as a collective 

term for crude oil, natural gas, natural gas liquids, and condensates. 

6
Undiscovered resources can be easily classified in the United 

Nations Framework Classification (UNFC) system. However, resources in 

discovered accumulations that have the potential to be added to reserves, or 

recoverable, are not as easily classified. The USGS attributes these 

additions to reserves as reserve growth and is responsible for estimating 

these quantities. 

Modified from U.S. Geological Survey National Oil and Gas 6

Resource Assessment Team (1995)

Slides 6. The USGS only broadly classifies these quantities. This slide shows 

the classes that contribute to reserve additions. 

 

7
Reserve Growth

Definition

Klett (2004)

• Delineation of additional in-place volumes (geological)

• Increases in recovery efficiency (technological)

• Recalculation of viable reserves in changing conditions (economical)

• Economic, operating, and regulatory

Increases in successive estimates of recoverable volumes of crude 

oil, natural gas, and natural gas liquids in discovered fields

From Klett (2005)

Slide 7. Reserve growth is defined as increases in successive estimates of 

recoverable petroleum quantities in discovered fields. Reserve growth can 

result from delineation of additional in-place volumes, increases in recovery 

efficiency, and recalculation of viable reserves in changing economic, 

operating, and regulatory conditions. 

8
• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

8

USGS Classification

• USGS evaluates and characterizes geologic 

assurance and economic feasibility, as in the 

UNFC, during the course of the petroleumresource assessments 

• However, USGS does not have data or a 

systematic knowledge base of commercial 

development plans, degree of commitment, and 

technically feasible projects for the feasibility 

axis of the UNFC 

9

Slides 8 and 9. The USGS evaluates and characterizes geologic assurance 

and economic feasibility, as provided in the UNFC, during the course of 

petroleum-resource assessments. However, the USGS does not have data or 

a systematic knowledge base for using the feasibility axis of the UNFC, nor 

9
does it have access to commercial development plans, degree of 

commitment, or information on technically feasible projects. The use of the 

feasibility axis is not required, but is indeed offered in the UNFC. However, 

the UNFC does allow classes to be grouped or expanded. 

 

10

Modified from Ahlbrandt and others (2004)

Modified from SPE and others (2007)

Slide 10. This slide compares three different resource-classification 

systems. A portion of the McKelvey classification system (U.S. Bureau of 

Mines and U.S. Geological Survey, 1980) is shown in the upper left, a portion 

of the Petroleum Resources Management System (PRMS; Society of 

Petroleum Engineers, World Petroleum Council, American Association of 

Petroleum Geologists, and Society of Petroleum Evaluation Engineers, 2007) 

is shown in the upper right, and the UNFC (from Ahlbrandt and others, 2004) 

is shown below. 

The USGS groups the classes on the feasibility axis (lower right). 

Once grouped, the UNFC cube becomes a two-dimensional system of 

economic viability versus geologic knowledge (lower left)—similar to the 

McKelvey classification system. 

Categories in the McKelvey system loosely correspond to those of 

the PRMS and UNFC. Measured reserves correspond to proved reserves 

(PRMS) and category 111 (UNFC). Likewise, inferred reserves correspond to 

proved plus probable plus possible reserves of the PRMS and category 113 

of the UNFC. 

10
Classes loosely correspond as well. Reserves correspond to 

reserves of the PRMS and the three categories of the E1 class of the UNFC. 

Marginally and subeconomic classes correspond to contingent resources of 

the PRMS and E2 and E3 classes of the UNFC. Likewise, measured, 

indicated, and inferred reserves correspond to proved, proved plus probable, 

and proved plus probable plus possible reserves of the PRMS, respectively. 

These reserves also correspond to the G1, G2, and G3 classes of the UNFC. 

UNFC

11

Slide 11. Potential reserve additions are sourced from most of these other 

reserve and resource classes. These additions might involve increasing 

geologic knowledge. 

11
UNFC

12

Slide 12. Or, these additions might involve increasing economic viability. 

The path to recovery can be quite complex. 

 

12
• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

13

Approach

• Compile and analyze field-history data for 

large oil and gas fields of the world

– World’s largest oil fields

• 54 of the 56 oil fields containing = 5X109 barrels of 

recoverable oil

– Giant U.S. oil fields

• = 500X106 barrels of recoverable oil

14

Slides 13. and 14. As a first step to resource classification, one can try to 

track the paths by examining individual fields. The USGS is compiling and 

analyzing field-history data for large oil and gas fields of the world, starting 

with the largest oil fields. So far, 54 of the 56 oil fields containing more than 5 

13
billion barrels of recoverable crude oil were studied. The other two fields are 

aggregations of many smaller fields and are analyzed at this time. 

Many U.S. oil fields containing more than 500 million barrels (MMB) 

of recoverable crude oil, as well as several other smaller oil fields, were also 

studied. 

Data from IHS Energy (2006); NRG Associates (2007)

All Fields

15

Slide 15. In order to prioritize the fields for this study, the oil and gas fields 

of the world were ranked by recoverable petroleum quantities. Recoverable 

quantity is used as a measure of field size. Fields are analyzed in order of 

decreasing size. This slide shows the rank of the world’s fields, except for 

Canadian fields whereby only 165 fields are represented in this graph. In this 

and the next three slides, the vertical axis represents the cumulative percent 

of the world’s known recoverable crude oil and natural gas quantities. The 

horizontal axis represents the fields ranked in order of decreasing 

recoverable quantity. The red curve represents gas fields (fields containing 

greater than 20,000 cubic feet of natural gas per barrel of crude oil). The 

green curve represents oil fields (fields containing less than 20,000 cubic 

feet of natural gas per barrel of crude oil). Our database contains 

approximately 15, 300 gas fields and 20,800 oil fields. 

14
Data from IHS Energy (2006); NRG Associates (2007)

Fields = 5 X 109 Barrels Oil Equivalent

16

Slide 16. This slide shows the world’s fields that contain greater than or 

equal to 5 billion barrels of crude oil equivalent (BBOE) and thus shows the 

oil fields we analyzed so far. The slide also shows gas fields of comparable 

size. The 56 oil fields shown contain approximately 45 percent of the world’s 

known crude oil, and 25 gas fields contain approximately 48 percent of the 

world’s known natural gas. 

 

15
Data from IHS Energy (2006); NRG Associates (2007)

Fields = 1 X 109 Barrels Oil Equivalent

17

Slide 17. This slide shows fields greater than or equal to 1 BBOE. The next 

group of fields planned for analysis (another 206 fields) is in this population. 

Two hundred and sixty-two oil fields contain approximately 64 percent of the 

world’s known crude oil and 123 gas fields contain approximately 64 percent 

of the world’s known natural gas. 

 

16
Data from IHS Energy (2006); NRG Associates (2007)

Fields = 0.5 X 109 Barrels Oil Equivalent

18

Slide 18. This slide shows fields greater than or equal to 0.5 BBOE—fields 

considered as giants. To evaluate this population, another 311 oil fields in 

addition to the 56 fields in the first group and the 206 fields in the second 

group will be studied. Five hundred and seventeen oil fields and 266 gas 

fields contain almost three-fourths of the world’s known crude oil and 

natural gas. 

 

17
Approach

• Database

– Individual fields

– Field operations

– Absolute and relative additions to reserves

19

Slide 19. A database can be constructed of individual fields with field 

operations and associated reserve additions, as both absolute and relative 

quantities. Data for this study are obtained by analyzing historic estimates of 

production and reserves and relating these estimates to field operations. 

 

18
Field Analysis

• Field definition

• Annual production

• In-place quantities

• Cumulative production

• Remaining reserves

• Recoverable quantities

• Field-operation history

• Cumulative production = Sum annual production 

• Recoverable quantity = Cumulative production + 

Remaining reserves 

20

Slide 20. Production and reserve quantities are from published reports and 

proprietary commercial databases. These quantities include (1) annual 

production, (2) in-place quantities, (3) cumulative production, (4) remaining 

reserves, (5) recoverable quantities, and (6) field-operation history. Some 

quantities can be calculated from others, such as cumulative production, 

which is a summation of annual production. Also, recoverable quantities can 

be calculated by adding cumulative production and remaining reserves. 

19
Analysis of Field Data

Recovery Efficiency Percent = (Recoverable / In-Place)*100

21

Analysis of Field Data

22

Slides 21 and 22. When plotted by time, a set of curves is generated that 

characterize field-development history. Data are obtained by simply 

measuring increases in recoverable quantities and remaining reserves, and 

relating these increases to an operation. 

20
 The concept of reserve growth occurs between the recoverable and 

in-place quantities. Recoverable quantities are used as the reference for 

reserve growth because reserves are components of recoverable quantities. 

• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

23

21
Yates (1926)

USA

Data from various sources 24

Yates (1926)

USA

25 Data from various sources

22
Yates (1926)

USA

Water and Gas 

Injection

+948 MMB

+145%

Unitization

+350 MMB

+22%

26 Data from various sources

Yates (1926)

USA

Water and Gas 

Injection

+916 MMB

+692%

Unitization

+313 MMB

+33%

27 Data from various sources

Slides 23 through 27. The next several slides show an example of fieldhistory analysis. The field-development history of the giant Yates oil field of 

western Texas is shown. This field was discovered in 1926. Raw data are 

shown to illustrate the complexity of field histories and inconsistencies 

23
among different data sources, for example, published and commercial data. 

Not all increases in recoverable quantities can be related to field operations. 

 Annual production, cumulative production, remaining reserves, 

recoverable quantities, and in-place quantities are plotted. When in-place 

quantities are removed, the remaining quantities can be replotted at a larger 

scale to show more detail. Several increases in recoverable quantities and 

remaining reserves can be seen. Two of these increases can be related to 

the implementation of field-development operations. These are water and 

gas injection in the late 1960s and unitization in the mid-1970s. Recoverable 

crude oil increased by 948 MMB (145 percent) and 350 MMB (22 percent), 

respectively. Likewise, remaining crude oil reserves increased by 916 MMB 

(692 percent) and 313 MMB (33 percent), respectively. Similar observations 

are made on other large oil fields of the world. 

Future Reserve Growth >3500MMB

Field Years Operation Increase 

(MMB)

Increase 

(percent)

Marun (1964) 1986-1987 Gas injection 300 2

Umm Shaif (1958) 1993-1996 Gas injection 620 11

Zuluf (1965) 1992-1993 Capacity expansion 3360 32

Marjan (1967) 1991-1992 Capacity expansion 1774 80

Priobskoye (1982) 1992-1993 Water injection 3025 4033

Rumaila (1953) 1996-2001 Water injection 1800 8

Yates (1926) 1968-1971 Water and gas inj. 948 145

Wasson (1936) 1969-1971 In-fill drilling 830 128

Wasson (1936) 1983-1985 Carbon dioxide inj. 250 12

Slaughter-L (1936) 1975-1977 Carbon dioxide inj. 293 26 28

Slide 28. Data from these field analyses are tabulated and various statistics 

can be calculated, such as the range and mean of increases in recoverable 

quantities. For example, statistics can be obtained on the relative increases 

of gas injection versus water injection, or gas injection during the 1980s 

versus the 1990s. 

24
• Introduction

• USGS Classification

• Field Analysis

• Examples

• Concluding Remarks

29

Classification of Quantities

• Quantities are assets

• Infill or development drilling – G2 and/or G3?

• Extensions and new reservoirs – E2 or E3?

• Improved recovery operations (floods, 

recompletions) – Combination of categories?

• USGS does not have data to answer these 

questions at this point in time

30

Slides 29 and 30. The volumes of petroleum that contribute to reserve 

additions or increases in recoverable quantities are assets. But how should 

these quantities be classified? Given enough information, one might 

presume that quantities added by infill or development drilling are from the 

25
G2 and G3 classes (112 and 113 categories of the UNFC or possible and 

probable reserves of the PRMS). Likewise, one might presume that 

quantities added by extensions and new reservoirs are from the E2 and E3 

classes (221, 222, 223, 311, 312, 313, 321, 322, and 323 categories of the UNFC 

or contingent resources of the PRMS). Also, one might presume that added 

recoverable quantities by improved recovery operations (floods, 

recompletions) are from a combination of the other reserve and resource 

categories and classes. 

 

Concluding Remarks

• USGS estimates quantities added to 

reserves

• Individual field analysis may be a first step 

to aid in the classification of these 

additional quantities

31

Slide 31. Although the USGS only broadly classifies the sources of potential 

reserve additions, it is responsible for estimating these quantities. Relating 

increases in reserve and recoverable quantities to field operations might be 

a first step to aid in the classification of these additional quantities. 

26
32

Thank You for Your Interest

tklett@usgs.gov

http://energy.usgs.gov/

Slide 32. Thank you for your interest. 

References Cited 

Ahlbrandt, T.S., Blaise, J.R., Blystad, P., Kelter, D., Gabrielyants, G., 

Heiberg, S., Martinéz, A., Ross, J.G., Slavov, S., Subelj, A., and 

Young, E.D., 2004, Updated United Nations Framework 

Classification for reserves and resources of extractive industries: 

Society of Petroleum Engineers Paper SPE 90839, presented at SPE 

Annual Technical Conference and Exhibition, Houston, Texas, 26-29 

September, 7 p. 

IHS Energy, 2006, [includes data current through December, 2006], 

International petroleum exploration and production database: IHS 

Energy; database available from IHS Energy, 15 Inverness Way East, 

Englewood, Colorado 80112 USA. 

Klett, T.R., 2005, United States Geological Survey’s reserve-growth models 

and their implementation: Natural Resources Research, v. 14, no. 3 

(September 2005), p. 249-264. 

27
28

NRG Associates, 2007, [includes data current through December, 2005], 

The Significant Oil and Gas Fields of the United States: NRG 

Associates, Inc.; database available from NRG Associates, Inc., P.O. 

Box 1655, Colorado Springs, CO 80901, U.S.A. 

Society of Petroleum Engineers, World Petroleum Council, American 

Association of Petroleum Geologists, and Society of Petroleum 

Evaluation Engineers, 2007, Petroleum Resources Management 

System: available on the Worldwide Web at www.spe.org. 

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

 Reserve/Resource Classification for Minerals: U.S. Geological 

Survey Circular 831, 5 p. 

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

1995, 1995 National assessment of United States oil and gas resources: 

U.S. Geological Survey Circular 1118, 20 p.