Petroleum Potential of 

Wilderness Lands in 

Idaho

By Charles A. Sandberg

PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE 

WESTERN UNITED STATES

GEOLOGICAL SURVEY CIRCULAR 902-F

This chapter on the petroleum 

geology and resource potential of 

Wilderness Lands in Idaho is also 

provided as an accompanying 

pamphlet for Miscellaneous Investigations Series Map 1-1540

CONTENTS

Page

Abstract ~ Fl

Introduction 1

Geologic framework - 1

Petroleum geology 3

Source rocks 3

Maturation 3

Evaluation of Wilderness Lands by clusters 4

Summary- 5

References cited 5

ILLUSTRATION

Page

FIGURE 1. Index map of Idaho showing Wilderness Lands, wilderness clusters, and their petroleum potential; location of 

Idaho batholith and Snake River volcanic plain; leading edges of Sevier and Roberts Mountains thrust systems; and postulated Snake River strike-slip fault (Poole and Sandberg, 1977; Sandberg and Poole, 1977) F2

III

PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE WESTERN UNITED STATES

Petroleum Potential of Wilderness Lands in Idaho

By Charles A. Sandberg

ABSTRACT

Idaho Wilderness Lands having oil and gas potential were 

grouped into seven clusters. Of these, two clusters, with potentials many times greater than those of the other clusters, 

are situated in the Overthrust belt of southeastern Idaho. 

They are discussed in detail in a separate chapter on the 

Wyoming-Utah-Idaho thrust belt. Of the remaining five clusters, situated in widely separated and geologically distinct 

areas of the State, two have low to zero potential, two have 

low potential, and one has medium potential. Because of scarcity of analogs due to lack of production data, these five clusters are evaluated mainly according to the presence, type, and 

maturation of possible petroleum source rocks. All tracts outside these clusters are considered to have zero potential, 

except for one tract, formed by the westernmost part of 

Yellowstone National Park, which has an unknown potential.

INTRODUCTION

Idaho Wilderness Lands, which comprise 53 

tracts totaling a little more than 2 million acres, 

are grouped into seven clusters. A single tract of 

36.7 thousand acres with unknown potential comprises a narrow strip of easternmost Idaho within 

Yellowstone National Park (fig. 1). With this exception, all wilderness tracts lying outside the 

clusters are considered to have zero petroleum potential. The geology of clusters 6 and 7, which 

contain three tracts totaling 149,389 acres, is discussed separately and in more detail under the 

Wyoming-Utah-Idaho thrust belt (Powers, chapter N in this circular). The remaining five clusters 

lie in geologically distinct frontier provinces. Cluster 1, situated in the panhandle of northern Idaho, 

has mainly Late Precambrian metasedimentary 

rocks at the surface. Such terrane ordinarily 

would be considered to lack petroleum potential, 

but here the Precambrian rocks occupy nearly 

horizontal thrust plates that may be underlain by 

Paleozoic sedimentary rocks at depth. Cluster 2 is 

underlain by gently dipping Paleozoic sedimentary 

rocks between the Centennial Range and Yellowstone National Park. Cluster 3, lying east of 

the Idaho batholith and north of the Snake River 

volcanic plain, comprises mainly mountain ranges 

composed of complexly folded and faulted rocks. 

Cluster 4, situated in the southwest corner of the 

State, is underlain by sedimentary rocks of the 

Tertiary Lake Bruneau basin. These rocVs are 

capped by an extensive cover of younger Tertiary 

volcanic rocks. Cluster 5 is underlain by Paleozoic 

sedimentary rocks along the crest of the Bear 

River and Portneuf Ranges at the eastern edge of 

the Eastern Basin and Range province.

GEOLOGIC FRAMEWORK

To help relate the seven dusters that are 

evaluated herein to their geologic settir^s, a 

geologic map of Idaho (Bond, 1978) is availrble at 

a scale of 1:500,000. This, scale is twice as large as 

the 1:1,000,000 scale of the Miscellaneous Investigations Series Map 1-1540 (in press) showing the 

color-coded qualitative petroleum evaluat'on of 

Idaho's Wilderness Lands. The locations of the 

leading edges of two major thrust fault systems 

that disrupt geologic outcrop patterns ard the 

offset of these thrust systems by a Paleozoic 

strike-slip fault, postulated by Poole and 

Sandberg (1977) and Sandberg and Poole (1977), 

are shown on figure 1. A detailed tectonic map by 

Blackstone (1980) shows the complex pattern of 

thrust faults and the oil and gas development in 

the Overthrust belt of southeastern Idaho. Distribution, thickness, and correlation of Devonian and 

Silurian rocks are shown by Poole, Sandberr, and 

Boucot (1977) and Sandberg and Poole (197V), and 

of Mississippian rocks by Poole and Sardberg 

(1977) and Gutschick, Sandberg, and Sando (1980). 

A correlation chart (Isaacson and others, 1983) 

shows the age, thickness, and nomenclature of 

Paleozoic and Mesozoic rocks in mountain ranges 

throughout Idaho.

Fl
EXPLANATION

HIGH POTENTIAL 

MEDIUM POTENTIAL 

LOW POTENTIAL 

LOW TO ZERO POTENTIAL 

| | ZERO POTENTIAL

UNKNOWN POTENTIAL

2 CLUSTER AND 

NUMBER

LEADING EDGE OF

ROBERTS MOUNTAINS

OR SEVIER THRUST SYSTEM

5O MILES

FIGURE 1. Index map of Idaho showing outlines of Wilderness Lands, designated wilderness dusters, 

and their petroleum potential; location of Idaho batholith and Snake River volcanic plain; leading 

edges of Sevier and Roberts Mountains thrust systems; and postulated Snake River strike slip fault 

(Poole and Sandberg, 1977; Sandberg and Poole, 1977).

F2
PETROLEUM GEOLOGY

The petroleum resource appraisal of Wilderness 

Lands in Idaho, outside the Overthrust belt, is dependent on the four major parameters that govern 

accumulation the presence of source rocks, maturation history, reservoir rocks, and traps. Because of the complex shuffling of many different 

rock types by several types of faulting, reservoir 

rocks and traps are considered to be broadly distributed. The types of structural traps and the 

ages and types of reservoirs encountered in the 

folded leading edges of thrust plates in that part 

of Idaho situated in the Wyoming-Utah-Idaho 

thrust belt are discussed in greater detail in this circular in chapter N by R. B. Powers. Possible Mississippian reservoir rocks and stratigraphic traps 

that extend from the Overthrust belt westward in 

southeastern Idaho were discussed by Sando, 

Sandberg, and Gutschick (1981). Consequently, 

the prime parameter devolves to the presence or 

absence of source rocks and their maturation.

SOURCE ROCKS

Published discussions of source rocks deal 

mainly with the eastern part of the State. Mississippian source rocks and the problems inherent in 

making evaluations based on outcrop samples 

were discussed by Sandberg, Grogan, and 

Clisham (1979). Organic carbon in shale beds of 

the Permian Phosphoria Formation was treated 

by Maughan (1975). Source beds in the Jurassic 

Twin Creek Limestone were evaluated by Swetland, Patterson, and Claypool (1978). Elsewhere 

in Idaho, the argillaceous and once organic-rich 

(but now thermally postmature) Devonian Milligen 

Formation, which previously had been considered 

Mississippian, is present in the Wood River area 

of central Idaho (Sandberg and others, 1975). In 

east-central Idaho, once organic-rich (but now 

thermally postmature) source beds are present in 

the flysch sequence of the Mississippian McGowan 

Creek Formation and in the Devonian, lower part 

of the Sappington Member of the Three Forks 

Formation (Sandberg, 1975). In southwestern 

Idaho, Tertiary lacustrine beds described by 

McDonald (1973) and Warner (1977, 1980) also apparently contain source rocks. Thus, source rocks 

capable of generating hydrocarbons have been 

documented in the areas of clusters 2, 3, 4, and 5.

MATURATION

Because of the widespread availability of source

rocks, the next important consideration is their 

maturation. Maturation can be approximately determined from the study of ancient and present 

geothermal temperatures, which affect mt only 

the ability of source rocks to generate hydrocarbons but also the preservation of oil and fas that 

has already migrated from them into reservoir 

rocks. A general indication of geothermal temperatures, although not the precise degree of maturation, commonly can be obtained from geothermal 

maps. Unfortunately, geothermal-gradient and 

subsurface-temperature maps of North /merica 

(American Association of Petroleum Geologists 

and U.S. Geological Survey, 1976a, 1976b) do not 

interpret much of Idaho except for the southeast 

corner and the west-central edge along tho Snake 

River. More precise measurements of maturation 

are made from studies of microfossils contr.ined in 

source rocks or rocks interbedded with source 

rocks. In Tertiary rocks, such as those in southwestern Idaho (cluster 4), maturation can H measured by study of temperature-induced color 

changes in contained spores and pollen. Elsewhere 

in Idaho, possible source rocks are all of Paleozoic 

age, so an important key to determining their 

maturation is the examination of conodonts, a 

unique group of microfossils. Conodonts are the 

fossilized toothlike, phosphatic hard parts of softbodied organisms that lived in most marir ? environments during Paleozoic and Triassic tines but 

are now extinct. They serve as virtual g^othermometers that readily indicate by their temperature-induced color changes or conodont color-alteration index (CAI) values on a scale of 1 (cold) to 

8 (hot) the maximum temperature to whfoh containing rocks have been subjected. The C.A I scale 

was developed by Epstein, Epstein, and Harris 

(1977). The utility of conodonts to petrole^im exploration in Idaho and adjacent States has been 

discussed by Sandberg and Poole (1977), Sandberg 

(1980), and Sweet and others (1981). A symposium 

volume edited by Sandberg and Clark (197^) presents conodont zonations and distribution7* on a 

systemic basis for Idaho and adjacent States. 

Maps by Harris and others (1980) and Sando, 

Sandberg, and Gutschick (1981) provide CAI 

values that are useful in evaluating the mat'iration 

of source rocks in Idaho. A more complete discussion of CAI values and other economically significant uses of conodonts is given in this c'^cular, 

chapter H, in a companion paper on Nevada by C. 

A. Sandberg.

F3
EVALUATION OF WILDERNESS 

LANDS BY CLUSTERS

Idaho wilderness tracts in the five clusters that 

are evaluated herein are situated mainly in two 

petroleum provinces that were used byDolton and others (1981) for convenience in making 

resource appraisals of the entire United States. 

The tracts are here evaluated or ranked with respect to the total evaluation of their respective petroleum provinces. Clusters 1, 2, 3, and 4 lie in 

the Idaho-Snake River Downwarp province of 

Dolton and others (1981), except that one tract in 

cluster 3 extends eastward slightly into the south 

end of their Montana Overthrust Belt province, 

which barely enters Idaho. Cluster 5 is bisected 

by the boundary between the Eastern Basin and 

Range province and the Wyoming-Utah-Idaho 

Overthrust Belt province. Because this arbitrary 

province boundary is a county line that follows the 

crest of the Bear River and Portneuf Ranges and 

the geologic setting is identical on either side, the 

entire cluster is evaluated as if it were in the 

Eastern Basin and Range province. Clusters 6 and 

7 lie in the part of the Wyoming-Utah-Idaho Overthrust Belt province that barely enters the southeastern part of Idaho from Wyoming; they are 

evaluated in independent treatment of that province in chapter N in this circular by R. B. Powers and their evaluations are briefly summarized 

here.

Cluster 1. Very little is known about the 

Paleozoic sedimentary rocks that are believed to 

underlie the nearly horizontal thrust plates of 

Late Precambrian metasedimentary rocks in the 

panhandle of Idaho (fig. 1). However, the petroleum industry currently is exhibiting great interest in exploring the adjacent part of northwestern 

Montana. Moreover, a rock sample was recently 

collected by W. J. Perry, Jr., from near Roosville 

Custom Station, Montana, just south of the Canadian border and 48 miles (77 km) east of the northeast corner of Idaho, and submitted to me. This 

sample yielded conodonts that I interpret to have 

come from the Upper Devonian Palliser Formation and to have CAI values of 2 to 3. This range 

of CAI values indicates optimum maturation of 

hydrocarbons for oil and gas generation. The identification suggests that if Paleozoic rocks do extend beneath the Idaho panhandle, they would belong to a sequence similar to that of the Canadian

Rocky Mountains. The rocks from which the Montana sample came are on a downthrown block, 

however, and may not be representative of rocks 

in the lower plate in Idaho (W. J. F irry, Jr., written common., June 1983). Therefore, this frontierprovince cluster, instead of being arbitrarily disregarded, is evaluated to have a low to zero potential. However, any results of nearby drilling might 

eliminate or substantially increase this evaluation.

Cluster 2. This cluster lies at the east end of 

the Centennial Range and near Trrghee Peak to 

the east. Conodont CAI values of 1.5 to 2 indicate 

optimum maturation of hydrocarbons for oil and 

gas generation in this area. However, because of 

a lack of favorable results from nearby drilling activity in southwestern Montana, the cluster is 

evaluated to have only a low potent**!.

Cluster 5. This large cluster occupies, from 

east to west, parts of the Boulc^r Mountains, 

Pioneer Mountains, White Knob Mountains, Lost 

River Range, Lemhi Range, Bea^erhead Mountains, and Snake River volcanic pMn. Hundreds 

of samples, personally collected from Ordovician, 

Silurian, Devonian, Mississippian, and Pennsylvanian rocks of this area, have almost universal conodont CAI values of 5, an indication of pervasive 

thermal degradation of hydrocarbons. However, 

the cluster contains extremely thick sequences of 

Devonian source rocks (Sandberz and others, 

1975) and Mississippian source rocks (Sandberg, 

1975). Some of these conceivably might be found 

to be less heated in isolated windows at the surface or in thrust plates at depth. Because of the 

richness of these potential source rocks and because of the similar geologic setting to that of the 

Eastern Basin and Range province between the 

Roberts Mountains and Sevier thrust systems in 

Nevada, discussed in this circu'su* by C. A. 

Sandberg, the cluster cannot be arbitrarily disregarded. It is here evaluated to hav> a low to zero 

potential. This rating could be reduced or substantially increased, depending on the results of any 

future drilling activity among the tracts of this 

cluster.

Cluster 4. This cluster is ic^ntical in its 

geologic setting to the Nevada wilderness cluster 

1 discussed by C. A. Sandberg in this circular. 

Both clusters are underlain by Ternary lacustrine 

beds of the Lake Bruneau basin (Warner, 1977, 

1980), but are covered at the surface mainly by 

slightly younger Tertiary volcanic rocks. Little is

F4
known about the few small windows of presumably postmature Paleozoic rocks that poke out from 

the extensive volcanic cover. Because the heating 

by volcanic rocks and minor areas of intrusive 

rocks may have raised the temperatures of some 

of the Tertiary source beds to optimum maturity, 

this cluster and Nevada cluster 1 are rated to 

have a low potential.

Cluster 5. The three wilderness tracts in this 

cluster are underlain by Paleozoic sedimentary 

rocks along the continuous crest of the Bear River 

and Portneuf Ranges. A regional geologic map 

(Oriel and Platt, 1980) reveals that all three tracts 

have the same geologic setting and are contained 

in a large southward-plunging syncline that also 

contains the eastern tract of wilderness cluster 25 

in Utah discussed in this circular by C. M. 

Molenaar and C. A. Sandberg. Conodont CAI 

values of 3 encountered in Idaho cluster 5 are 

slightly lower, possibly because of lesser sedimentary load, than CAI values of 3.5 in Utah cluster 

25 (which is rated to have a low potential). CAI 

values of 3 indicate that hydrocarbons are at optimum maturation for both oil and gas generation. 

Although little likelihood exists of finding structural traps in the fault plate at the surface, traps 

may be found in underlying thrust plates at depth. 

Hence this cluster is rated to have a medium potential. The quantitative resource estimates, discussed in chapter A of this circular by B. M. Miller, were made as if all three tracts in cluster 5 

were part of the Eastern Basin and Range province.

Clusters 6 and 7. Three wilderness tracts lie 

in the northern part of the Wyoming-Utah-Idaho 

Overthrust Belt province that barely enters the 

southeastern part of Idaho. Cluster 6, in eastern 

Bonneville County, is rated to have a medium oil 

and gas potential. The surface area of this tract 

consists of volcanic cover (not prospective for oil 

and gas); however, several old, shallow wildcat 

wells drilled nearby indicate that sedimentary 

rocks having petroleum potential are present in 

the subsurface.

The two tracts in cluster 7 in Idaho and one 

tract in cluster 16 in Wyoming (see C. W. 

Spencer, Wyoming, chapter M) make up tract 4- 

613 (U.S. Bureau of Land Management, 1981), 

which straddles the Wyoming-Idaho State line in 

the vicinity of Alpine, Wyoming. These three 

tracts are grouped together for this discussion on 

the basis of geologic similarity and are rated to

have a high oil and gas potential on the basis of 

their possessing geologic characteristics ft vorable 

for the occurrence of petroleum. The Darby thrust 

trends northwest into the eastern part of the 

tracts and the Absaroka thrust parallels the 

Darby trend in the central part of the tracts in the 

northern part of the Overthrust Belt province. 

A wildcat well, Allday No. 1 Government, was 

drilled in 1966 in the western part of the r orthern 

Idaho tract in cluster 7 to a depth of 5,7^ feet. 

Live oil and gas shows were encountered in porous and fractured Ordovician limestones ir the interval from 1,252 through 1,375 feet in the well.

The same formations, reservoir rocks, source 

rocks, and trapping structures are present here in 

the northern part of the Overthrust Belt, as well 

as a favorable maturation history, a'1 in a 

framework similar to that in the productive southern area of the Overthrust Belt (R. B. Powers, 

chapter N, figs. 2-4).

SUMMARY

Of the 7,559,186 acres involved in this s*udy for 

the assessment of the petroleum potential of the 

Wilderness Lands in Idaho, the potential acreage 

can be summarized as follows: high potentH, 115.1 

thousand acres; medium potential, 63.4 thousand 

acres; low potential, 804.7 thousand acrer low 

to zero potential, 1,023.7 thousand acres; zero 

potential 5,515.6 thousand acres; and unknown 

potential, 36.7 thousand acres. The high potential 

acreage occurs in the Overthrust Belt province, 

while the acreage with unknown potential forms 

the westernmost part of the Yellowstone rational 

Park and occurs in volcanic terrane. The petroleum potential by acreage of all Wilderness Land 

categories in the Western United States is shown 

in this circular by B. M. Miller in table 1, chapter 

P.

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North America Series, scale 1:5,000,000.

 1976b, Subsurface temperature map of North America: 

U.S. Geological Survey Geologic Maps of North America 

Series, scale 1:5,000,000.

Bond, J. G., compiler, 1978, Geologic map of Idaho: Faho Bureau of Mines and Geology, scale 1:500,000.

F5
Blackstone, D. L., Jr., 1980, Tectonic map of the Overthrust 

Belt, Wyoming, southeastern Idaho and northeastern 

Utah, showing current oil and gas drilling and development: Wyoming Geological Survey Map 8A, 3rd edition, 

updated through July 31, 1980, scale 1:316,800 (1 inch=5 

miles).

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Conodont color alteration an index to organic 

metamorphism: U.S. Geological Survey Professional Paper 

995, 27 p.

Gutschick, R. C., Sandberg, C. A., and Sando, W. J., 1980, 

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Montana to Nevada, in Fouch, T. D., and Magathan, E. 

R., eds., Paleozoic paleogeography of the west-central 

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K., 1980, Maps for assessing thermal maturity (conodont 

color alteration index maps) in Ordovician through Triassic 

rocks in Nevada and Utah and adjacent parts of Idaho and 

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Isaacson, P. E., assisted by Bachtel, S. L., and McFaddan, M. 

D., 1983, Stratigraphic correlation of the Paleozoic and 

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107-115.

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132-134.

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paleogeography and tectonics of the Western United 

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pt. 3, 190 p.

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Claus, 1975, Stratigraphy, conodont df ting, and paleotectonic interpretation of type Milligen Formation (Devonian), Wood River area, Idaho: U.S. Geological Survey 

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sequence at North Georgetown Canyon, Idaho: American 

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p. 1433-1443.

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B. R., 1981, Conodonts; guides to biostratigraphy and 

geothermometry in the western United States, in Montana Geological Society Field Conference and Symposium 

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1075-1080.

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of Idaho, in Wyoming Geological Association Guidebook, 

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prime exploration target: Oil and Gas Journal, v. 78, no. 

18, p. 325/341 (not inclusive).

F6