U.S. Geological Survey Assessment Model for

Undiscovered Conventional Oil, Gas, and NGL Resources—

The Seventh Approximation 

By James W. Schmoker and Timothy R. Klett

U.S. Geological Survey Bulletin 2165

U.S. Department of the Interior

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

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This report is only available on-line at:

http://greenwood.cr.usgs.gov/pub/bulletins/b2165/b2165.html

Any use of trade, product or firm names is for

descriptive purposes only and does not imply

endorsement by the U.S. Government.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication May 3, 1999

Graphics by authors and Carol Quesenberry

Photocomposition by Carol Quesenberrry 

Edited by Lorna Carter
III

Contents

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

Key Aspects of Seventh Approximation.......................................................................................... 1

Assessment Units Versus Plays ............................................................................................... 1

Petroleum Volumes Assessed .................................................................................................. 1

Assessment Time Frame (Forecast Span) .............................................................................. 1

Allocations of Assessed Resources ........................................................................................ 3

Probabilistic Approach to Resource Assessment................................................................. 3

Overview of Assessment Procedure ............................................................................................... 3

Input Data Form.................................................................................................................................... 3

Identification Information ......................................................................................................... 3

Characteristics of Assessment Unit ........................................................................................ 3

Undiscovered Fields ................................................................................................................... 5

Average Coproduct Ratios ........................................................................................................ 5

Selected Ancillary Data ............................................................................................................. 5

Allocation of Undiscovered Resources................................................................................... 5

Summary................................................................................................................................................ 5

References Cited ................................................................................................................................. 5

Figures

1. Two-dimensional depiction of a petroleum resource pyramid............................................. 2

2. Simplified flow diagram emphasizing key steps of the Seventh Approximation

 assessment procedure for undiscovered conventional resources................................. 4

3. Basic input data form for the Seventh Approximation ........................................................... 6
1

Introduction

The U.S. Geological Survey (USGS) periodically conducts 

assessments of the oil, gas, and natural-gas liquids (NGL) 

resources of areas within the United States and in other regions 

of the world. The purpose of these assessments is to develop scientifically based estimates of the quantities of petroleum having 

the potential to be added to proved reserves within some future 

time frame. Over the years, USGS assessment procedures have 

not remained static but rather have evolved, as databases, computing power, and geologic knowledge have advanced. 

The primary purpose of this report is to document a 

geology-based assessment model developed by the USGS in 

1998 for undiscovered oil, gas, and NGL resources that reside in 

conventional accumulations. Undiscovered resources are those 

resources postulated from geologic knowledge and theory to 

exist outside of known fields. Conventional accumulations, for 

the purpose of this discussion, are defined as discrete, countable 

deposits commonly delineated by down-dip water contacts. This 

definition does not incorporate criteria involving factors such as 

water depth, regulatory status, or engineering techniques.

As a shorthand notation, the assessment model developed 

in 1998 is called here the “Seventh Approximation,” which is a 

phrase that expresses the evolution of the model as well as the 

idea that an exact analysis of undiscovered resources cannot be 

achieved. The Seventh Approximation has an antecedent in the 

geology-based assessment model for undiscovered conventional 

accumulations used in the USGS 1995 National Assessment of 

United States Oil and Gas Resources (Gautier and others, 1995; 

U.S. Geological Survey National Oil and Gas Resource Assessment Team, 1995).

Key Aspects of Seventh Approximation

Assessment Units Versus Plays

The USGS 1995 National Assessment (Gautier and others, 

1995) used plays as the basic level of assessment. A play consists of a group of geologically related petroleum accumulations. 

Particular emphasis in play analysis is placed on the similarities 

of the rocks in which the accumulations occur. 

Some current USGS petroleum resource assessments are 

being conducted using subdivisions of the total petroleum system (termed assessment units) as the basic level of assessment. 

A total petroleum system consists of all genetically related 

petroleum generated by a pod or closely related pods of mature 

source rocks. Particular emphasis is placed on the similarities of 

the fluids of the accumulations. A total petroleum system might 

equate to a single assessment unit or, if necessary to achieve sufficient homogeneity, be subdivided into two or more assessment 

units. An assessment unit is thus a mappable volume of rock 

within the total petroleum system.

The Seventh Approximation serves equally well for the 

assessment of plays or of assessment units. For consistency, this 

report is written in terms of assessment units.

Petroleum Volumes Assessed

The objective of the Seventh Approximation assessment 

model is to provide a means to estimate quantities of undiscovered conventional oil, gas, and natural-gas liquids that have the 

potential to be added to reserves (proved and inferred) in some 

specified future time span. These estimated petroleum volumes 

reside in fields whose sizes exceed a stated minimum cutoff 

value for the assessment unit. Terms used in this report such as 

“undiscovered conventional resources” are understood to be 

short approximations of this objective.

Note that the geology-based Seventh Approximation does 

not attempt to predict volumes of conventional resources that 

will actually be discovered in a given assessment time frame. To 

do so would require full knowledge of future petroleum economics, exploration technology, and exploration effort at the assessment-unit level. Rather, the Seventh Approximation estimates 

volumes of conventional resources having the potential to be 

discovered in the stated time frame.

Computer programs are used in conjunction with the Seventh Approximation to calculate resource estimates. However, 

assessment results are controlled by geology-based input parameters supplied by assessment geologists who are regional 

experts, and not by computer-generated statistical projections of 

historical trends.

Assessment Time Frame (Forecast Span)

For the USGS 1995 National Assessment, forecasts were 

developed for undiscovered conventional technically recoverable 

resources without regard for the time span that might be required 

to realize them. Because the forecast span of the 1995 National 

Assessment was unlimited, the question of “when?” was not 

directly addressed. However, the issue of forecast span is incorporated into the Seventh Approximation.

At one extreme, forecast spans in the range of 5–10 years 

are rather short for geology-based assessments. For such limited 

time frames, statistical projections of historical data trends might 

be the better assessment approach. At the other extreme, forecast 

spans exceeding 50 years or so seem to be too long when applied 

U.S. Geological Survey Assessment Model for 

Undiscovered Conventional Oil, Gas, and NGL Resources—

The Seventh Approximation

By James W. Schmoker and Timothy R. Klett
2 USGS Assessment Model for Undiscovered Conventional Oil, Gas, and NGL Resources—The Seventh Approximation

Resource Pyramid Higher quality

accumulation

categories

Slice depicting a

particular quality level

Increasing

resource

volumes

to the highly technical petroleum industry. The economic and 

technical foundations of the industry at that future time will 

likely be decoupled from the paradigms of the present day.

For the current USGS World Energy petroleum assessment, 

a forecast span of 30 years—approximately one generation—has 

been adopted. For the World Energy assessment, a 30-year forecast span appears to represent a suitable balance among factors 

such as assessment reliability, long-term planning requirements, 

financial decisions, and relevance to the human condition.

A particular forecast span, such as 30 years, should not be 

interpreted too literally. An algorithm does not exist for calculating conventional resources having the potential to be discovered 

in the next 30 years, as opposed to the next 29 or 31 years, for 

example. It is more appropriate to equate forecast span to the 

idea of a societally relevant resource inventory.

In a given time frame, access to a particular assessment unit 

for petroleum-related activities might not be certain, for political 

(for example, an ecologically sensitive area) or physical (for 

example, extreme water depths) reasons. In such cases, undiscovered accumulations hypothesized on the basis of geologic 

considerations might not actually be available to society. In the 

Seventh Approximation, a probability for access during the forecast span is assigned to the assessment unit. Such an access 

probability was not necessary in the 1995 National Assessment 

because, in an unlimited time frame, all areas are considered eligible for petroleum-related activities.

The specification of a forecast span also impacts estimates 

of undiscovered conventional resources through the initial 

choice of petroleum accumulation categories to be assessed (as 

examples, gas hydrates, heavy oil, small fields, and fields in deep 

Figure 1. Two-dimensional depiction of a petroleum resource pyramid. A given resource-assessment forecast span can be visualized 

as a slice through the resource pyramid at some quality level. Accumulation categories below the slice are excluded from assessment 

consideration. (Modified from McCabe, 1998.)
Overview of Assessment Procedure 3

water). Petroleum resources can be subjectively visualized as 

forming a pyramid, with a relatively small volume of resources 

in high-quality accumulation categories near the top of the pyramid and increasing volumes of resources in progressively lower 

quality accumulation categories towards its base (fig. 1). As discussed by McCabe (1998), a particular forecast span can be 

depicted as a slice through the resource pyramid at some quality 

level (fig. 1). Accumulation categories below the slice are not 

considered to be viable in the time frame of the forecast span and 

are excluded from assessment consideration. 

Allocations of Assessed Resources

An assessment unit can include within its boundaries a variety of land entities, such as countries, States, surface and mineral 

ownerships, parks, or wetlands. As part of the Seventh Approximation, undiscovered conventional resources can be allocated by 

volume percent among various land entities within the assessment unit, as well as to the offshore portion of each entity, if 

applicable. This allocation component permits customized 

resource assessments to be compiled. For example, an estimate 

of undiscovered conventional resources underlying Federal 

lands in the western United States could be compiled from allocations made at the assessment-unit level.

Probabilistic Approach to Resource Assessment

Many of the numerical parameters that make up the input 

data set of the Seventh Approximation are represented by probability distributions rather than by single (point) values. Estimates 

of undiscovered conventional resources derived from these input 

data are probability distributions as well.

The probability distributions for some input parameters 

represent the uncertainty of a fixed value (such as the probability 

distribution for number of undiscovered fields). In other cases, 

input probability distributions represent values that are inherently variable (such as the probability distribution for sizes of 

undiscovered fields).

Minimum (F100), maximum (Fo), and median (F50) fractiles 

are supplied for all input parameters represented by probability 

distributions. These input fractiles are not specifically linked to a 

particular type of probability distribution (lognormal, for example). The choice of probability-distribution type is an operational 

decision that is not constrained by the Seventh Approximation.

Overview of Assessment Procedure

To begin an assessment using the Seventh Approximation, a geologically defined total petroleum system is divided 

into reasonably homogeneous assessment units. Assessment 

units are then considered individually, as outlined by the flow 

diagram of figure 2. 

The essence of the assessment procedure is as follows:

A minimum field size, which relates in part to the forecast 

span, is chosen for the assessment unit. Resources in fields 

smaller than the minimum size are excluded from the 

assessment.

Probabilities for the occurrence of adequate charge, adequate rocks, and adequate timing for at least one undiscovered field of minimum size or greater are assigned to the 

assessment unit (geologic risk). The probability that petroleum-related activities necessary for the discovery of at 

least one field of sufficient size will be possible in the 

assessment unit during the forecast span is also assigned 

(access risk).

The number of undiscovered oil fields and gas fields in the 

assessment unit greater than or equal to the minimum field 

size and the grown sizes of these fields are estimated. 

Grown undiscovered field size equals reserves estimated at 

the time of discovery plus any reserves anticipated to be 

added (or subtracted) as a field is exploited after discovery.

Ratios of gas/oil and NGL/gas for undiscovered oil fields 

and of NGL/gas and oil/gas (or alternatively a single ratio 

of liquids/gas) for undiscovered gas fields are estimated. 

These ratios are used to assess the coproducts associated 

with oil in oil fields and gas in gas fields.

The combination of geologic and access probabilities, number of undiscovered fields, sizes of undiscovered fields, and 

coproduct ratios yields probability distributions for undiscovered conventional oil, gas, and NGL resources.

Undiscovered conventional resources are partitioned 

among various land entities within the assessment unit, and 

also their offshore areas, using allocation percentages estimated by the assessment geologist.

Input Data Form 

As a means of providing additional information about the 

Seventh Approximation, the basic input data form is shown in 

figure 3 (p. 6–7). This data form can be modified for specific 

assessments, but its fundamental elements are fixed. 

Identification Information

The first section of the data form (fig. 3, p. 6) is for identification information and brief notes relevant to the assessment. 

Names and numbers can be established for a hierarchy of areas 

that range downward in size to the total petroleum system and 

then to the assessment unit. 

Characteristics of Assessment Unit

Minimum field size and assessment-unit probabilities (both 

discussed in the preceding section) are essential elements of the 

assessment model and must be specified in the second section of 

the data form (fig. 3, p. 6). The additional information requested 

in this section falls into the category of useful information for 

the resource-assessment process:

Classification of the assessment unit as oil prone or gas prone.

Number of discovered fields in the database(s) exceeding 

minimum size, and classification of the assessment unit 

according to exploration maturity.
4 USGS Assessment Model for Undiscovered Conventional Oil, Gas, and NGL Resources—The Seventh Approximation

Represent total petroleum system

by assessment units

Select minimum field size

Assign geologic risk

Assign access risk

Probability distributions

for undiscovered

conventional resources

Coproduct ratios

for each assessment unit

Probability distribution

for sizes of

undiscovered fields

Probability distribution

for number of

undiscovered fields

Allocations of

assessed resources by

land entity and

by offshore

4 USGS Assessment Model for Undiscovered Conventional Oil, Gas, and NGL Resources—The Seventh Approximation

Figure 2. Simplified flow diagram emphasizing key steps of the 

Seventh Approximation assessment procedure for undiscovered conventional resources.
Summary 5

Median sizes of discovered fields exceeding minimum size, 

subdivided into three groups according to date of discovery 

(discovery thirds).

Established (>13 fields discovered) and frontier (1–13 fields 

discovered) assessment units are confirmed in that the geologic 

elements of the total petroleum system are known to be effectual. 

Established assessment units have enough discovered fields for 

historic field-level data to be of help in postulating properties of 

undiscovered fields. For example, the median field sizes of the 

discovery thirds and their trends through time are factors to consider when estimating the sizes of undiscovered fields. 

Undiscovered Fields

The third section of the data form (bottom of p. 6) represents the core of the Seventh Approximation. This section captures the assessment geologist’s hypotheses regarding the 

number and sizes of undiscovered fields in the assessment unit 

that are of minimum size and larger. A number of techniques can 

be employed to assist in making the required estimates of F100, 

F50, and F0 for the number and sizes of undiscovered fields. 

These techniques include reservoir-simulation modeling, discovery-process modeling, application of geologic analogs, spatial 

analysis such as sequence stratigraphy and global tectonics, and 

study of the total petroleum system. 

Average Coproduct Ratios

The next part of the data form (top of p. 7) is for the ratios 

necessary to assess coproducts associated with oil in oil fields 

and gas in gas fields. For oil fields, the necessary ratios are those 

of gas/oil and NGL/gas. For gas fields, the necessary ratios are 

those of liquids/gas, or alternatively, NGL/gas and oil/gas. 

Ratios are expressed as three fractiles (F100, F50, and F0) in recognition of the uncertainty inherent in estimating average properties of undiscovered fields.

Selected Ancillary Data

The fifth section of the data form (fig. 3, p. 7) establishes a modest set of ancillary data useful for economic and 

environmental analysis of assessment results. The ancillary 

data for undiscovered oil fields are API gravity of oil, sulfur content of oil, drilling depth, and water depth. The 

ancillary data for undiscovered gas fields are inert-gas 

content, carbon dioxide content, hydrogen sulfide content, 

drilling depth, and water depth. 

Allocation of Undiscovered Resources

The final section of the data form (fig. 3, p. 7) is for information necessary to allocate estimates of undiscovered conventional resources to land entities of interest in the assessment unit, 

and to the offshore portions of these entities. The volume percent 

of assessed resources assigned to an entity need not match the 

areal percent of that entity. Because allocated resources should 

add up to the assessment-unit total, operational considerations 

might dictate that allocation percentages be supplied as point 

estimates rather than as fractiles of a probability distribution.

Summary

The Seventh Approximation is a resource-assessment 

model that provides a strategy for estimating volumes of undiscovered conventional oil, gas, and natural-gas liquids that have 

the potential to be added to reserves (proved and inferred) within 

a specified future time frame. These potential resources reside 

in fields having sizes greater than or equal to a stated minimum 

field size.

The Seventh Approximation is a geology-based assessment 

model. The information required for estimation of undiscovered 

conventional resources is supplied by earth scientists who are 

knowledgeable about the petroleum geology of the area under 

consideration. These regional experts complete a form (fig. 3) 

for each assessment unit of a total petroleum system (or alternatively, for each play of an area). This form is the source of data 

necessary for assessment calculations.

References Cited

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 DDS-30 [1 CD-ROM].

McCabe, P.J., 1998, Energy resources—Cornucopia or empty barrel?: 

American Association of Petroleum Geologists Bulletin, v. 82, no. 11, 

p. 2110–2134.

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.
6 USGS Assessment Model for Undiscovered Conventional Oil, Gas, and NGL Resources—The Seventh Approximation

SEVENTH APPROXIMATION--BASIC INPUT DATA FORM

IDENTIFICATION INFORMATION

Date:................................

Assessment Geologist:........

Region:............................. Number:

Province:........................... Number:

Total Petroleum System:...... Number:

Assessment Unit:............... Number:

Notes from Assessor:.........

CHARACTERISTICS OF ASSESSMENT UNIT

What is the minimum field size?......... mmboe grown (6000 cfg = 1 bo)

Oil (<20,000 cfg/bo overall) or Gas (>20,000 cfg/bo overall):...

Number of discovered fields exceeding minimum size:............ Oil: Gas:

 Established (>13 fields) Frontier (1-13 fields) Hypothetical (no fields)

Median size (grown) of discovered oil fields (mmbo):

 1st 3rd discovered 2nd 3rd 3rd 3rd

Median size (grown) of discovered gas fields (bcfg):

 1st 3rd discovered 2nd 3rd 3rd 3rd

Assessment-Unit Probabilities:

 Attribute Probability of occurrence (0-1.0)

1. CHARGE: Adequate petroleum charge for an undiscovered field > minimum size................

2. ROCKS: Adequate reservoirs, traps, and seals for an undiscovered field > minimum size.....

3. TIMING: Favorable geologic timing for an undiscovered field > minimum size.....................

Assessment-Unit GEOLOGIC Probability (Product of 1, 2, and 3):.....................

4. ACCESS: Adequate location for necessary petroleum-related activities...........................

UNDISCOVERED FIELDS

Number of Undiscovered Fields: How many undiscovered fields exist that are > minimum size?: 

 (uncertainty of estimating fixed but unknown values)

Oil fields:....................................min. no. (>0) median no. max no.

Gas fields:..................................min. no. (>0) median no. max no.

Sizes of Undiscovered Fields: What are the anticipated sizes (grown) of the above fields?: 

 (inherent natural variability in the sizes of undiscovered fields)

Oil in oil fields (mmbo).......................... min. size median size max. size

Gas in gas fields (bcfg):........................ min. size median size max. size

Figure 3. Basic input data form for the Seventh Approximation.
References Cited 7

Assessment Unit (name, no.)

AVERAGE COPRODUCT RATIOS FOR UNDISCOVERED FIELDS

(uncertainty of estimating fixed but unknown values)

Oil fields: minimum median maximum

 Gas/oil ratio (cfg/bo).......................................

 NGL/gas ratio (bngl/mmcfg).............................

Gas fields:

 Liquids/gas ratio (bliq/mmcfg)..........................

 OR

 NGL/gas ratio (bngl/mmcfg)..........................

 Oil/gas ratio (bo/mmcfg)..................................

SELECTED ANCILLARY DATA FOR UNDISCOVERED FIELDS

(inherent natural variability in the properties of undiscovered fields)

Oil fields: minimum median maximum

 API gravity of oil (degrees).............................

 Sulfur content of oil (%)...................................

 Drilling depth (m) ..........................................

 Depth (m) of water (if applicable).......................

Gas fields:

 Inert-gas content (%)......................................

 CO2 content (%)............................................

 Hydrogen-sulfide content (%).........................

 Drilling depth (m)..........................................

 Depth (m) of water (if applicable)......................

ALLOCATION OF UNDISCOVERED RESOURCES TO LAND ENTITIES

(uncertainty of estimating fixed but unknown values)

1. ______________________represents areal % of the assessment unit

Oil in oil fields: minimum median maximum

 Volume % in entity......................................

 Portion of volume % that is offshore (0-100%)......

Gas in gas fields:

 Volume % in entity......................................

 Portion of volume % that is offshore (0-100%)......

 (repeat above sequence as necessary to include all land entities of interest)

Figure 3—Continued. Basic input data form for the Seventh Approximation.