Petroleum ResOurce 

Assessments of the 

Wilderness Lands in the 

Western United States 

By Betty M. Miller 

PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE 

WESTERN UNITED STATES 

GEOLOGICAL SURVEY CIRCULAR 902-A 

CONTENTS 

Page 

Abstract-----~---------------------------------------------------------------- A1 

Introduction------------------------------------------------------------------- 1 

Background------------------------------------------------------------------- 2 

Purpose and scope of study ------------------------------------------------------ 2 

Commodities assessed------------------------------------..:.___________________ 3 

Region and province locations and boundaries ------------------------------------ 3 

Sourcesofdata-------------------------------------------------------------- 3 

An overview of data processing procedures -------------------------------------- 3 

Analysis ofthe geologic characteristics of the Wilderness Lands------------------------- 6 

Resource appraisal methods and procedures----------------------------------------- 7 

Qualitative assessments ofpetroleum potential----------------------------------- 7 

Quantitative assessments of petroleum potential---------------------------------- 8 

Referencescited---------------------------------------------------------------- 10 

ILLUSTRATIONS 

Page 

FIGURE 1. Petroleum regions of the United States (from Dolton and others, 1981) and the boundary marking the eastern 

limits of the 11 Western States included in this study --------------------------------------------- A4 

2. Index map of lower 48 States showing USGS petroleum provinces (from Dolton and others, 1981)--------- 5 

TABLE 

Page 

TABLE 1. Wilderness cluster input------------------------------------------------------------------------- A9 

III 
PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE WESTERN UNITED STATES 

Petroleum Resource Assessments of the Wilderness Lands 

in the Western United States 

By Betty M. Miller 

ABSTRACT 

The USGS conducted an investigation of the oil and gas resource potential of the designated and proposed Wilderness 

Lands in the Western United States. These assessments are 

based upon a Wilderness System containing approximately 74 

million acres of Wilderness Lands in the States of: Arizona, 

California, Colorado, Idaho, Montana, Nevada, New Mexico, 

Oregon, Utah, Washington, and Wyoming. The Wilderness 

Lands are administered under four Federal agencies: Bureau 

of Land Management (BLM), U.S. Forest Service (USFS), 

National Park Service (NPS), and Fish and Wildlife Service 

(FWS). The Wilderness Lands included in this study are identified under the following categories: Designated Wilderness, 

Administratively Endorsed as Suitable (prior to July 1981), 

Further Planning or Study Areas, BLM Lands Under Appeal, 

BLM Wilderness Inventory Not Completed, and U.S. Forest 

Service RARE II under litigation in California. 

This paper defines the background, scope, and objectives of 

the study and discusses the procedures and methods used to 

combine a variety of geologic and geographic data, along with 

logged information on wells and petroleum statistics, by applying computer and digital cartographic techniques for the purpose of assessing the petroleum resources in the Wilderness 

Lands. 

The methodology briefly described in this paper involves a 

review by geologists of existing information and evaluating the 

geologic characteristics for each wilderness tract to dete.rmine 

the favorability, or lack of favorability, for petroleum occurrence. Basic assumptions incorporated into the resource-appraisal methods are (1) resource potential is not uniformly distributed throughout a petroleum province; (2) total distribution 

of all recoverable petroleum resources is considered, both discovered and undiscovered; and (3) consideration of all the 

geologic characteristics essential for the occurrence of petroleum resources for each wilderness tract. 

A1 

The assessments of the petroleum resources are analyzed in 

two phases: by a designated qualitative rating of the petroleum 

potential for each wilderness area, displayed on State maps; 

and by estimating the amounts of oil and gas within each province or basin that occur in the Wilderness Lands, expressed 

as probability distributions for the quantitative estimates. 

INTRODUCTION 

The U.S. Geological Survey (USGS), in 1975 

and 1981, completed and published two separate 

assessments of the remaining conventional, undiscovered petroleum resources, both onshore and 

offshore, for the United States (Miller and others, 

1975; Dolton and others, 1981). These assessments 

were conducted on a regional basis within the 

geologic framework of the country and the results 

were reported by basin or geologic province for all 

lands regardless of ownership. Petroleum resource 

assessments were not separately determined or 

reported by land ownership, whether on public or 

private lands. Thus a systematic study to assess 

the petroleum resources occurring for all the respective Federal land categories has not been conducted by the USGS. The two basic reasons for 

this have been due (1) to the Geological Survey's 

past philosophy of conducting petroleum assessments on a regional scale (with the exception of 

Outer Continental Shelf (OCS) tract assessments 

by the former Conservation Division, now the 
Minerals Management Service (MMS)); and (2) to 

limitations on the availability of time, workforce, 

budget, and essential data for assessing resource 

potential for tract-sized areas. 

In the last few years, national concerns over 

sources for continuing energy supplies within the 

United States have generated many controversial 

issues centering around the probable occurrence 

of energy and· nonenergy minerals on Federal 

lands with limited access to exploration and development, particularly in Wilderness Lands. In 

light of the fact that the USGS had not assessed 

the Federal onshore lands separately from the 

total province assessments, a pilot program was 

started in the USGS in the spring of 1982 to conduct an investigation, based upon currently available information, of the potential oil and gas resources in the designated and proposed Wilderness Lands for the Western United States. This 

pilot study for the Wilderness Lands of the Western United States will provide a framework for 

conducting similar assessments for the remaining 

Federal lands. 

Future petroleum assessment studies of this nature on the remaining Federal lands of the United 

States will be compatible to application by the 

USGS in its Federal Mineral Land Information 

System (FMLIS). FMLiS is being developed in 

support of the USGS's charter to collect, analyze, 

and disseminate information about the Nation's 

water, mineral, and energy resources. Specifically, FMLIS is designed as a tool for developing 

land management and policy objective regarding 

the Nation's strategic and critical minerals and energy resources. Central to this objective is the 

creation of a data base on Federal surface ownership, subsurface mineral rights, Federal restrictions to mineral development, and resource occurrence and potential input similar to this study for 

the Wilderness Lands. 

BACKGROUND 

The Federal Government owns approximately 

738 million acres of land in the United States, 

nearly one-third of the Nation's entire land area. 

It also retains control over the subsurface mineral 

rights to an additional 66 million acres (Bureau of 

Land Management, 1980). Approximately 49 percent of all the onshore Federal lands lie within the 

borders of the 11 Western States. Thirty-one percent of these Federal lands, classified as designated and proposed Wilderness Lands, are inA2 

eluded in this study (American Petroleum Institute, 1981a). 

The Federal Government also controls the submerged lands under the OCS seaward of State 

ownership. These offshore Federal lands are estimated to cover nearly 1 billion (965.8 million) 

acres (U.S. General Accounting Office, 1981). Of 

this acreage, some 528 million acres lie shoreward 

of the 200-meter water depth. 

Numerous studies by industry, government, 

and academic scientists have been done on various 

segments of these Federal lands; many of these 

studies forecast that large percentages of this N ation's-future energy and mineral resources may lie 

beneath Government-controlled lands. No one can 

be sure of the accuracy of these estimates because 

most of these lands have not been open to any 

kind of exploration and data are limited. However, scattered information does exist within or 

adjacent to many of these Federal lands that is 

useful to the geologist for interpretative purposes 

to appraise the geologic characteristics favorable 

for the occurrence of energy and mineral resources on these lands. 

PURPOSE AND SCOPE OF STUDY 

It is the objective of this study to assemble 

through various means all the available pertinent 

information that can be brought together within 

the USGS and which can be focused upon the 

single issue of reviewing the geological and 

geophysical data to determine the geologic characteristics favorable or unfavorable for the occurrence of petroleum resources in Wilderness 

Lands. 

In this initial effort the scope of the study is 

limited to conventional petroleum resources occurring in all the designated and proposed Wilderness 

Land categories in the Western United States, as 

of July 1981. This includes the Wilderness Lands 

administered under four Federal agencies: Bureau 

of Land Management (BLM), U.S. Forest Service 

(USFS), National Park Service (NPS), and Fish 

and Wildlife Service (FWS). The Wilderness 

Lands included in this study are identified under 

the following categories: Designated Wilderness, 

Administratively Endorsed as Suitable (prior to 

July 1981), Further Planning or Study Areas, 

BLM Lands Under Appeal, BLM Wilderness Inventory Not Completed, and U.S. Forest Service 

RARE II under litigation in California (Bureau of 

Land Management, 1981a-j). 
The area of the study includes approximately 7 4 

million acres of Wilderness Lands in these 11 

Western States: Arizona, California, Colorado, 

Idaho, Montana, Nevada, New Mexico, Oregon, 

Utah, Washington, and Wyoming (fig. 1). 

COMMODITIES ASSESSED 

In the regional petroleum assessments of the 

USGS, published as Open-File Report 82-666-A 

(Varnes and others, 1982), separate estimates 

were provided for crude oil, associated-dissolved 

gas, and nonassociated gas; an estimate for total 

gas, which' is an aggregate of the separate gas estimates, based on the assumption that one or both 

are present, was provided also. 

In this assessment for the Wilderness Lands the 

two commodities assessed are the conventionally 

recoverable crude oil and total natural gas. The 

associated-dissolved gas has not been reported on 

separately in this paper, but is included as a part 

of the total natural gas. 

Definitions for commodity and resource terms 

and a summary of the methods used to derive the 

regional resource estimates are documented in 

USGS Circular 860 (Dolton and others, 1981). 

REGION AND PROVINCE 

LOCATIONS AND BOUNDARIES 

The nationwide petroleum resource appraisals 

reported by the USGS in 1975 and 1981 were conducted on a systematic regional basis. Regional 

boundaries were established to facilitate the study 

and to orient the reader with reference to the geographic distribution of these resources (fig. 1). 

The 11 onshore regions were subsequently divided 

into two or more individual geologic basins or provinces that exhibit certain common geologic features characteristic of the terrane they occupy. 

The same region and province boundaries used for 

the national assessments and for reporting purposes in Circular 860 (Dolton and others, 1981) are 

used in this study (fig. 2). 

Wilderness Lands occur within 38 of the 39 petroleum provinces or parts of petroleum provinces 

which fall within the 11-State area. The names of 

the provinces included in the study are listed in 

the appendix. The regions that cover the total 11-

State area are Region 2, Pacific Coast; Region 3, 

Colorado Plateau and Basin and Range; the major 

part of Region 4, Rocky Mountains and Northern 

Great Plains; and a small part of Region 5, West 

Texas and Eastern New Mexico. 

A3 

SOURCES OF DATA 

The primary sources of geologic data for this 

study were the direct contributions of USGS 

geologists with expertise in their respectively assigned areas; published geologic information, field 

reports, and maps; published statistical data on 

petroleum exploration, reserves, and production; 

and unpublished materials in the USGS. 

The primary sources for estimates of undiscovered petroleum resources on a regional basis 

were taken from the Survey's earlier publications 

on national petroleum assessments, USGS Circular 725 and Circular 860 (Miller and others, 1975; 

and Dolton and others, 1981). 

Subsurface geological information and well production data for wells drilled within or adjacent to 

Wilderness tracts were obtained on contract from 

Petroleum Information Corporation's Well History 

Control System (WHCS). 

Maps used to determine the location and boundaries for the Wilderness Lands in the Western 

States were obtained from the BLM's Wilderness 

Status Map Series, June 1981 (with updated revisions) for 10 of the 11 States (Bureau of Land 

Management, 1981a-j). The Wilderness Lands 

map used for the State of Washington was taken 

from the American Petroleum Institute (API) 

Federal Land Status Map Series (American Petroleum Institute, 1981b). 

AN OVERVIEW OF DATA 

PROCESSING PROCEDURES 

The initial problem that needed addressing was 

a procedure in which multiple types of information 

(multivariate data, in a sense) could be brought together, preferably as a mappable product, to 

analyze the petroleum potential in Wilderness 

Lands. These analyses required the combination 

of the following types of data: locations of the Wilderness Lands, their boundaries and calculated 

acreage; location and interpretative boundaries of 

the various. geologic and tectonic or structural 

units; the location and identification of sedimentary rock units; the location and boundaries of the 

USGS's petroleum provinces; and the location and 

compilation of data on all wells drilled either within or adjacent to all the wilderness tracts. The 

solution to the above phase of the investigations 

was conducted in the USGS as a joint effort between the Geologic Division (GD) and the National 

Mapping Division (NMD) by utilizing NMD's 
Region 1, 

Region lA, 

Region 2, 

Region 2A, 

Region 3, 

Region 4, 

Region 5, 

Region 6, 

1 

The United States has not resolved its offshore 

boundaries with other States concerned. The lines 

on this chart are for purposes of illustration 

only, and do not necessarily reflect the position 

or views of the United States with respect to the 

boundary involved. 

0 

I 

0 

I 

500 MILES 

I 

500 KILOMETERS 

EXPLANATION 

Alaska; 

Alaska Offshore; 

Pacific Coast; 

Pacific Coast Offshore; 

Colorado Plateau and Basin and Range; 

Rocky Mountains and Northern Great Plains; 

West Texas and Eastern New Mexico; 

Gulf Coast; 

Region 6A, 

Region 7, 

Region 8, 

Region 9, 

Region 10, 

Region 11, 

Region llA, 

Gulf of Mexico; 

Mid-continent; 

Michigan Basin; 

Eastern Interior; 

Appalachians; 

Atlantic Coast; 

Atlantic Coast Offshore. 

FIGURE I.-Petroleum regions of the United States (from Dolton and others, 1981) and the boundary marking the eastern limits 

of the 11 Western States included in this study. 

A4 
FIGURE 2.-Index map of lower 48 States showing USGS petroleum provinces. Shading denotes offshore shelf areas. Names of 

onshore provinces included in this study are listed numerically in the appendix (froni Dolton and others, 1981). The heavy 

boundary marks the eastern limits of the 11 Western States included in this study. 

capabilities in digital cartographic techniques and 

by assembling the required multivariate information in a computer-based digital cartographic data 

system that could serve a range of applications in 

machine-plotted graphics used in assessing the petroleum resources in Wilderness Lands. Eventually, all public lands could be subject to this type 

of multivariate analysis. 

The location and boundaries of the Wilderness 

Lands were digitized by NMD in a metric coordinate system, which was then converted to 

latitude-longitude coordinate pairs from the BLM 

Wilderness Status Maps and the API Federal 

Land Status Map of Washington at a scale of 

1:1,000,000. The acreage of each wilderness polygon (a closed plane figure (tract or area), especially one with more than four sides and angles) 

was calculated and identified by a four component 

code scheme identifying the State Federal Information Processing Standards (FIPS) code, the 

agency, the wilderness category, and a unique 

number for each polygon. 

A5 

The participating geologists prepared State 

maps at a scale of 1:1,000,000 showing interpretative delineating boundaries for the geologic and 

tectonic or structural provinces and the identification of the sedimentary rock units relative to the 

igneous, metamorphic, and volcanic rock units. 

Each of these maps was digitized by NMD with 

identifying codes indicating geologic or geographic 

province names and identification of rock types. 

The USGS's petroleum province boundaries, as 

published in Circular 860 (Dolton and others, 

1981), were digitized by NMD at a scale of 

1:1,000,000. The petroleum province boundaries 

identify the province framework within which the 

USGS has published its previous regional resource 

assessments. 

The search for the locations and logged well information on any wells drilled either within or adjacent to the boundaries of the wilderness tracts 

was performed under a service contract by Petroleum Information Corporation. All well locations 

and well information was compiled, computerized, 
and identified by the coded boundary coordinates 

for the wilderness polygons. The well search and 

retrievals were conducted within the minimum 

and maximum latitude and longitude coordinates 

of the wilderness polygons and additionally in the 

areas included in a zone extending 2 miles outward from the minimum-maximum latitude-longitude boundaries for each of the designated 

areas. A total of 5,414 wells were retrieved from 

the 11 Western States which met these search requirements. A summary of the number of wells by 

State retrieved in this search are Arizona, 93; 

California, 585; Colorado, 1,275; Idaho, 5; Montana, 695; Nevada, 84; New Mexico, 414; Oregon, 

33; Utah, 1,229; Washington, 10; and Wyoming, 

991. 

Digital cartographic map products produced on 

a Gerber Plotter, Model 44 77, at a scale of 

1:1,000,000 generated in the initial procedures for 

assessing the favorability of petroleum occurrence 

relative to the Wilderness Lands were the location 

and. boundaries of the wilderness polygons; the 

wilderness polygons identified by controlling 

agency and wilderness category; the petroleum 

province boundaries; and the basic geological and 

tectonic units and the identification of major rock 

units. The computer listings for the compiled data 

on wells within or adjacent to the Wilderness 

Lands were cross-referenced by use of the polygon code for rapid referral. Later in the project 

the actual well locations with their respective well 

status, for example, dry hole, oil producer, gas 

producer, geothermal well, etc., were mapped relative to the wilderness polygon boundaries by Petroleum Information Corporation under contract 

to the USGS. 

Detailed procedures of the digital cartographic 

techniques used by NMD to prepare the digital 

data for computer processing and produce 

machine-plotted maps are covered in this circular 

in chapter B discussing digital cartography. 

The mapped and computer-produced products 

discussed above provided the essential input for 

the. geologist in the next phase of the study. 

ANALYSIS OF THE GEOLOGIC 

CHARACTERISTICS OF THE 

WILDERNESS LANDS 

The analysis of the geologic characteristics 

favorable or unfavorable for petroleum occurrence 

in conjunction with the geologic settings for all the 

A6 

Wilderness Lands scattered within the framework 

of the petroleum province boundaries was performed by a team of approximately 11 geologists. 

In their respective areas of expertise, the 

geologists analyzed the basic geologic data and 

provided the geological interpretations for the 

province or provinces within which the individual 

wilderness tracts were located. 

Discussions of the geologic framework for each 

of the 11 Western States and a review of the 

geologic characteristics for the wilderness tracts 

within each of the States have been prepared by 

the geologists and are reported as separate chapters (C through N) in this circular. 

Two basic geologic assumptions were maintained thoroughout this study: (1) that petroleum 

resource potential is not uniformly distributed 

throughout a petroleum province or basin; and (2) 

that the total distribution of all petroleum resources must be considered, both the discovered 

(production and reserves in known fields) and the 

remaining undiscovered resources. The geologists 

examined the geographic locale for each wilderness tract relative to its respective geological setting. Characteristics favorable for the accumulation of oil and gas were determined from geologic 

data by analysis and by geologic interpretation. 

The geologic characteristics were reviewed for 

each wilderness tract to determine the presence 

or absence of the following: 

An adequate thickness of sedimentary rock, 

preferably organically rich marine sediments 

for the generation of oil and associated gas, 

or nonmarine organically rich sediments for 

genesis of nonassociated gas and (or) oil; 

Porous and permeable reservoir rocks; 

Adequate geologic trapping mechanisms; 

A favorable thermal history; 

A suitable geologic environment and timing 

for the maturation, petroleum generation, 

and migration relative to the development of 

traps; 

Oil and gas seeps, or adjacent drilling with 

shows or production; 

Favorable sedimentary rock sections underlying volcanics such as lava flows or underlying faulted and overthrust areas; that is, 

areas in which the surface geology does not 

reflect the subsurface geology. 
In many of the wilderness tracts there has been 

no exploratory drilling and therefore no well data 

relative to the nature of the subsurface geology. 

In situations such as these the geologists interpret 

the known geology in adjacent areas and extrapolate by using their best geologic judgments regarding the probable nature of the geology within 

the locale of the wilderness tract. This of course 

generates a degree of uncertainty regarding the 

probable geology .in some of the tracts. Uncertainties are inherent, however, in estimating undiscovered quantities of oil and gas, and only direct information that comes with drilling can promise 

any greater certainty toward determining the final 

resource estimates. Quantitative resource estimates reported in this study are expressed as 

probability distributions in an attempt to show the 

risks of uncertainty. 

RESOURCE APPRAISAL METHODS 

AND PROCEDURES 

The assessments of the petroleum resources on 

the Wilderness Lands were completed in two separate stages. In the first stage . the geologists 

evaluated the geological characteristics as described above for the favorability or lack of 

favorability for the occurrence of oil and natural 

gas within .each wilderness tract and assigned a 

qualitative rating of each tract's potential for the 

occurrence of oil and gas resources. The second 

stage in evaluating the petroleum potential for the 

wilderness tracts was to attempt a quantitative 

assessment that could be developed upon the foundation of the qualitative assessment procedures 

and results and that could be designed for the 

geologist as a natural progression in the analysis 

of the geologic favorability of the area from a rating of the qualitative assessment to a quantitative 

assessment. This stage requires a more complex 

procedure than the first one and probably induces 

a greater degree of uncertainty in the resulting 

quantitative estimates due to ·a lack of detailed information within the tracts and also due to limitations inherent in the methodology employed. 

A brief account of the qualitative and quantitative methods and procedures used in the two separate stages follows. 

QUALITATIVE ASSESSMENTS OF 

PETROLEUM POTENTIAL 

The assessment procedures followed by the 

geologists to conduct the geologic analysis to determine a qualitative assessment of the petroleum 

potential for each wilderness tract provide the 

foundation for the entire study. The geologic interpretations made by the geologists at this stage 

are carried through into the quantitative assessments. 

The location of each wilderness tract was established· relative to its regional and local geologic 

setting. An analysis of the known geologic characteristics favorable or unfavorable for the occurrence of conventional petroleum resources was 

made for each of the \\oilderness tracts. Using his 

or her best judgments regarding the probable nature of the geology within the locale of each wilderness tract, the geologist made the qualitative 

assessment relative to the probable potential of 

the tract as having favorable or unfavorable 

geologic conditions for the occurrence of petroleum resources. 

A qualitative rating was assigned to each wilderness tract. The rating scale used in the initial 

qualitative assessments was as follows: high, 

medium to high, medium, low to medium, low, low 

to zero, zero, and unknown. 

The qualitative ratings of petroleum potential 

used in this evaluation are generally defined as follows: 

High potential-Geologic environment highly 

favor~ble for occurrence of oil and gas accumulations. Area is near or on trend with existing production from structural and (or) stratigraphic 

traps. 

Medium potential-Geologic environment favorable for the occurrence of oil and gas accumulations. Contains known reservoir rocks and hydrocarbon source beds. Includes some areas of sparse 

subsurface control or areas where expected field 

size will be small. 

T.Jow potential-Geologic environment interpreted 

to have low potential for the occurrence of oil and 

gas accumulations. Includes areas of poor or unknown hydrocarbon source bed and (or) reservoir 

quality. May include areas of sparse or no well 

control and expected thin sequence of sedimentary 

rocks. 

Zero potential-Areas generally with exposed 

Precambrian rocks or with very thin sedimentary 

section with no potential for occurrence of sealed 

structural or stratigraphic traps with hydrocarbons. 

Unknown potential-Generally includes areas 

A7 
with no well control where Tertiary volcanic intru- · 

sions and volcaniclastic rocks are present on the 

surface. This cover, plus lack of subsurface control, makes prediction of hydrocarbon potential 

extremely difficult. Includes some areas where 

Precambrian igneous and metamorphic rocks are 

thrust over Phanerozoic (Cambrian and younger) 

sedimentary rocks of unknown potentia~. Lack of 

subsurface control does not mean that no oil and 

gas potential exists, but only that the hydrocarbon 

potential cannot reasonably be determined with 

present data. 

These ratings were assigned to the wilderness . 

tracts relative to their basin and regional petroleum potential. These ratings do not include the 

potential mineral resources within a tract. Ratings, for example, for areas of low to zero or zero 

petroleum potential may, in many instances, due 

to their geologic settings within igneous and 

metamorphic terranes, have high mineral potential. The rating of unknown is used in some wilderness areas where the geology is almost completely unknown or not enough information is 

available currently for the 1 geologist to make any 

judgment on the favorability of the area for petroleum potential. These are frequently areas that 

have not been penetrated in the subsurface, which 

underlie. massive volcanics, such as the Y ellowstone Park area, or areas in which sedimentary rocks underlie overthrust and faulted sections. 

The qualitative rating scale used for the potential petroleum resources . in the wilderness tracts . 

can be easily adapted to the computer-based digital cartographic data system with machine-plotted, color-coded maps produced for the qualitative 

petroleum potential in all the wilderness tracts for 

the 11 Western States. These maps are discussed 

in more detail later in this circular. 

QUANTITATIVE ASSESSMENTS OF 

PETROLEUM POTENTIAL 

To complete this study expeditiously and to 

keep the petroleum assessments consistent within 

the framework of the USGS's latest published resource estimates (Dolton and others, 1981), the 

geologists assumed that the resources of all the 

lands, including the Wilderness Lands, Within 

each of the USGS's petroleum provinces were included as a part of the total USGS assessment of 

the remaining undiscovered petroleum resources 

A8 

for that province. The obJective then was to determine what part of the total probability distribution for the petroleum assessment within each petroleum province occurs in the Wilderness Lands. 

First, the Wilderness Lands within a basin were 

identified and grouped by the geologists as "clusters" of wilderness tracts that occur within 

geologically similar areas. The homogeneity of the 

geology for these areas determines which of the 

Wilderness Lands are included for assessment 

within each of the clusters. In this manner the 

geologic characteristics evaluated for the favorability of petroleum occurrence within each of the 

clusters were the determining factors for the assessment of the petroleum potential for the wilderness tracts occurring within the respective 

clusters. The basic method considers the resource 

potential of the wilderness clusters relative to the 

USGS resource estimates for the respective basin 

or province within which the wilderness clusters 

occur. 

The resource estimates for the basins or provinces for oil and total gas within the 11 Western 

States are taken from Circular 860 (Dolton and 

others, 1981) and Open-File Report 82-666-A 

(Varnes and others, 1982). In Circular 860 the resource estimates are presented as unconditional 

estimates that incorporate the risk that oil or gas 

may not be present in the areas assessed. Those 

estimates were derived from an assessment of (1) 

the likelihood of the particular recoverable resource being present, called the marginal probability, and (2) a conditional estimate of the amount 

present given that at least some of the recover-

. able resource occurs, reported as a probability distribution (95 percent, 5 percent, and most likely 

estimates) .. 

Using the above resource information and marginal probabilities for the basins, the geologists 

made the following subjective estimates: (1) a 

marginal probability for each wilderness cluster 

given the marginal probability assigned for the 

basin, and (2) the assignment of a rating or richness factor for the wilderness cluster as to its relative potential when compared to an evenly distributed resource throughout the basin, that is, compared to the average (mean) quantity of resource 

per unit area in the basin. The mean estimates are 

the means of the probability distribution for the 

oil and gas resource estimates in a particular 

basin. 

The first estimate, the marginal probability, is a . . 
subjective probability of the condition that the resource actually will be present in recoverable 

quantities within each wilderness cluster. The second estimate, the rating or richness factor (r), 

consists of assigning a range of estimates for the 

richness factor-a minimum (95 percent) and a 

maximum (5 percent) value as to the relative potential per unit area in ·each wilderness cluster if 

compared to an assumed mean quantity of resource distributed evenly per unit area. The 

geologists used a general rating scale for the richness factor based upon the concept that if the 

qualitative assessment were given a medium (or 

average) rating, the quantitative assessment 

would have a range around the mean value of 

r= 1, which implies the resource estimate would 

be as if the resource were evenly distributed 

throughout the basin, that is, the wilderness cluster would have an average potential compared to 

the basin on the average. The minimum and 

maximum value might range from r= 0.5 tor= 1.5 

times the richness compared to a mean quantity 

distributed evenly per unit area in the basin. 

A high rating for a wilderness cluster might be 

assigned a richness range of 2 to 4 times the average resouce potential for the basin per unit area. 

The rating for the richness range is a subjective 

estimate based upon the geologist's evaluation of 

the local geology for each of the wilderness clusters. Thus the local geology is assessed for each 

cluster and the assessments are based upon the 

premise of nonuniform hydrocarbon distributions 

within a basin. 

The area of the wilderness cluster being assessed is the sum of the areas of each of the wilderness tracts that fall within that cluster. The area 

for the cluster of the Wilderness Lands relative to 

the area of the petroleum province is the sum of 

the areas of all the wilderness tracts within the 

cluster relative to the total sedimentary area of 

the basin or province. A hypothetical example of 

the previous exercise by a geologist might be recorded as in table 1. 

The exercise is repeated by the geologist to assess 

separately the same . wilderness clusters for their 

natural gas resources. The potential richness ratings for natural gas in these tracts may or may 

not be similar to those for the occurrence of oil. A 

wilderness cluster may be more oil prone, more 

gas prone, or equally favorable for the occurrence 

of oil and gas. 

From this stage on, the input shown in the table 

is mathematically and statistically processed by 

means of the. computer. Briefly, the fraction of the 

conditional quantity of the undiscovered basin resource occurring in the wilderness clusters is dependent upon the following: (1) the basin or province estimates for (a) the marginal probability for 

resources in the basin and (b) the conditional probability distribution for the remaining undiscovered 

resources in the basin; and (2) the wilderness area 

or cluster estimates for (a) the conditional marginal probability for the wilderness cluster, (b) the 

rating or richness factor of the wilderness cluster 

as to its relative potential when compared to an 

evenly distributed resource throughout the basin 

(expressed as a minimum and a maximum), and (c) 

the fraction of the total basin area that is the area 

of the wilderness cluster. Applying the marginal 

probability for the cluster area and its range of 

richness factors to the basin probability distribution for total oil and gas resources, one derives 

the proportionally smaller unconditional probability distribution of remaining undiscovered resources for the individual wilderness clusters. The 

probability distributions of the oil and gas resources for the individual wilderness clusters are 

aggregated by using a Monte Carlo simulation 

program to determine the total resource distributions. The aggregations for both the oil and gas 

resources are reported by individual States and 

for the total11 Western States by B. M. Miller in 

chapter Pin this circular. A detailed discussion of 

the mathematical and statistical procedures is presented in chapter 0 in this circular on the statistical methodology for probabilistic assessments. 

TABLE 1.-Wilderness cluster input 

[Oil assessment) 

Petroleum Cluster ConditionaP Qualitative Ratings or 

province Cluster marginal petroleum richness factor 

identifier area for oil number (acres) probability potential (min.-max.) 

100 1 40,000 1.00 high 2-3 

100 2 25,000 .80 medium-high 0.9-2.5 

100 3 10,000 .50 low .2-.5 

100 4 20,000 .25 low-zero 0-.3 

'Conditional probability that the resource is present in the area or wilderness cluster given that the resource is present in 

the basin. 

A9 
REFERENCES CITED 

American Petroleum Institute, 1981a, Access to Federal 

Lands: The key to supplies of domestic crude oil and natural gas: American Petroleum Institute, Washington, D.C., 

47 p. 

--1981b, Federal Land Status of Washington, from API 

Federal Land Status Map Series for the western United 

States, American Petroleum Institute, Washington, D.C., 

scale 1:1,000,000 (1 sheet). 

Bureau of Land Management, 1980, Public Land Statistics, 

U.S. Department of the Interior, 191 p. 

Bureau of Land Management, 1981a, Wilderness Status Map, 

State of Arizona, U.S. Department of the Interior, 

1:1,000,000 (1 sheet). 

--1981b, Wilderness Status Map, State of California, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

--1981c, Wilderness Status ¥ap, State of Colorado, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

--1981d, Wilderness Status Map, State of Idaho, U.S. Department of the Interior, 1:1,000,000 (1 sheet). 

--1981e, Wilderness Status Map, State of Montana, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

--1981f, Wilderness Status Map, State of Nevada, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

--1981g, Wilderness Status Map, State of New Mexico, 

U.S. Department of the Interior, 1:1,000,000 (1 sheet). 

--1981h, Wilderness Status Map, State of Oregon, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

--1981i, Wilderness Status Map, State of Utah, U.S. Department ofthe Interior, 1:1,000,000 (1 sheet). · 

--1981j, Wilderness Status Map, State of Wyoming, U.S. 

Department of the Interior, 1:1,000,000 (1 sheet). 

Dolton, G. L., Carlson, K. H., Charpentier, R. R., Coury, A. 

B., Crovelli, R. A., Frezon, S. E., Khan, H. S. Lister, J. 

H., McMullin, R. H., Pike, R. S., Powers, R. B., Scott, 

E. W., and Varnes, K. L., 1981, Estimates of undiscovered recoverable conventional resources of oil and gas in 

the United States, U.S. Geological Survey Circular 860, 

87p. 

Miller, B. M., Thomsen, H. L., Dolton, G. L., Coury, A. B., 

Hendricks, T. A., Lennartz, F. E., Powers, R. B., Sable, 

E. G., and Varnes, K. L., 1975, Geological estimates of 

undiscovered recoverable oil and gas resources in the 

United States, U.S. Geological Survey Circular 725, 78 p. 

U.S. General Accounting Office, 1981, Issues in leasing 

offshore lands for oil and gas development, Report to the 

Congress of the United States by the Comptroller General, EMD-81-59, Washington, D.C., p. 3. 

Varnes, K. L., Charpentier, R. R., and Dolton, G. L., 1982, 

Conditional estimates and marginal probabilities for undiscovered recoverable oil and gas resources by provincestatistical background data for U.S. Geological Survey 

Circular 860, USGS Open-File Report 8~A, USGS, 

Denver, Colorado, 29 p. 

AlO