Petroleum Potential of 

Wilderness Lands in 

Montana 

By William J. Perry, Jr., Dudley D. Rice, 

and Edwin K. Maughan 

PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE 

WESTERN UNITED STATES 

GEOLOGICAL SURVEY CIRCULAR 902-G 

This chapter on the petroleum 

geology and resource potential of 

Wilderness Lands in Montana is 

also provided as an accompanying 

pamphlet for Miscellaneous Investigations Series Map 1-1541 

CONTENTS 

Page 

Abstract---------------------------------------------------------------------- G1 

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

Geologic framework ------------------------------------------------------------ 1 

Williston Basin province------------------------------------------------------ 1 

Northern Montana Foreland (Sweetgrass Arch) province---------------------------- 2 

Montana Thrust Belt province------------------------------------------------- 3 

Central Montana Uplift province----------------------------------------------- 7 

Montana portion-Wyoming Structural province---------------------------------- 8 

Stratigraphicsummary --------------------------------------------------------- 8 

Precambrian--------------------------------------------------------------- 10 

Cambrian------------------------------------------------------------------ 10 

Ordovician----------------------------------------------------------------- 10 

Silurian ------------------------------------------------------------------- 11 

Devonian------------------------------------------------------------------ 11 

Mississippian--------------------------------------------------------------- 12 

Pennsylvanian ------------------------------------------------------------- 12 

Permian------------------------------------------------------------------- 13 

Triassic ------------------------------------------------------------------- 13 Jurassic------------------------------------------------------~------------ 14 

Cretaceous----------------------------------------------------------------- 14 

Oil and gas potential------------------------------------------------------------ 14 

Summary--------------------------------------------------------------------- 18 

Glossary---------------------------------------------------------------------- 18 

Selected bibliography ----------------------------------------------------------- 19 

ILLUSTRATIONS 

Page 

FIGURE 1. Index map of Montana showing location of principal geologic and geographic features mentioned in text------- G2 

2. Index map of Federal Wilderness Lands in Montana ------------------------------------------------- 3 

3. A, Correlation chart of Paleozoic rocks for east-central and northeastern Montana. B, Correlation chart of 

Mesozoic rocks for east-central and northeastern Montana ----------------------------------------- 4 

4. Correlation chart of significant pre-Tertiary stratigraphic units, with respect to petroleum geology, of the 

western flank of the Sweetgrass arch and adjacent eastern disturbed belt, northwest Montana------------ 6 

5. Correlation chart of pre-Tertiary formations ofBlacktail-Snowcrest uplift and Centennial basin in southwestern 

Montana --------------------------------------------------------------------------------- 9 

III 

PETROLEUM POTENTIAL OF WILDERNESS LANDS IN THE WESTERN UNITED STATES 

Petroleum Potential of Wilderness Lands in Montana 

By William J. Perry, Jr., Dudley D. Rice, and Edwin K. Maughan 

ABSTRACT 

Wilderness Lands in Montana, totalling 6,930,433 acres, are 

subdivided for this report into 34 clusters covering 5,066,893 

acres for the assessment of petroleum potential based on 

widely varying geologic conditions; 38,193 acres classified as 

having unknown potential; and 1,825,347 acres assessed as 

having zero potential. 

The presence or absence of possible source rock, reservoir 

rock, adequate thermal history, and structural or stratigraphic 

traps for hydrocarbons are the most critical factors used in 

qualitatively assigning a high, medium, low, or low to zero oil 

and (or) gas potential to each of the clusters. Montana encompasses parts of five major geologic provinces. The provinces 

exhibit a wide range of structural and stratigraphic variations, 

summarized in this report. 

INTRODUCTION 

The State of Montana encompasses parts of five 

major geologic provinces (fig. 1). The first is the 

southwestern part of the Williston Basin province 

in eastern Montana. The second is the Northern 

Montana Foreland (Sweetgrass Arch) province, a 

complex of domes and arches in the Paleozoic Milk 

River uplift area of north-central Montana. The 

third is the Montana Thrust Belt province, that 

sector of the Cordilleran fold-and-thrust belt located in western Montana. This province, because 

of its complexity, is subdivided from north to 

south into (1) disturbed belt and adjoining northern sector of the Montana thrust belt to the west, 

(2) Helena salient and adjoining central sector to 

the west, and (3) southwest Montana recess. The 

fourth is the Central Montana Uplift province, loG1 

cated east of the Helena salient and including 

Mesozoic uplifts and basins superimposed on the 

early Paleozoic Central Montana trough and the 

adjacent northern margin of the Paleozoic Wyoming shelf. The fifth is the southern Montana part 

of the Wyoming province, a series of major 

Laramide uplifts and adjoining basins across the 

southern part of Montana from the southwest 

Montana recess eastward to South Dakota. 

In this report we will first briefly discuss (1) the 

geologic, particularly structural, framework of 

these provinces with respect to hydrocarbon accumulations or possible accumulations, (2) Montana stratigraphy, focusing on rocks that have 

generated hydrocarbons (as source rocks) or 

yielded hydrocarbons (as reservoir rocks), and (3) 

the petroleum potential of the clustered Wilderness Lands in Montana (fig. 2). Obscure geologic 

terminology is eliminated wherever possible; some 

scientific terms are defined within the text and in 

the accompanying glossary. 

GEOLOGIC FRAMEWORK 

WILLISTON BASIN PROVINCE 

The Williston basin (fig. 1) is the largest depositional basin of the North American continental interior, preserving the most complete Paleozoic sequence of rocks found in the continental interior 

(Sandberg and Hammond, 1958; Sandberg, 1962), 

as much as 16,000 ft of strata, and is one of the 
110" 

NORTHERN MONTANA 

MONTANA FORELAND PROVINCE (..... 

SAPPHIRE 

THRUST 

PLATE 

CENTRAL SECTOR 

BELT 

.... " 

.,.~,. 

HELENA 

SALIENT 

~<II .. . 

................. ..._, .... 

BLOOD CREEK SYIICLIIIE 

CENTRAL MONT ANA PROVINCE 

BULL MOUNTAIN 

BASIN 

50 100 MILES 

PORCUPINE DOME 

FIGURE 1.-Index map of Montana showing location of principal geologic and geographic features mentioned in text. Boundary 

lines follow province boundaries as used in U.S. Geological Survey Circular 860 (Dolton and others, 1981): Williston Basin 

province-Province 94; Northern Montana Foreland (Sweetgrass Arch) province-Province 96; Montana Thrust Belt-Province 

98; Central Montana Uplift province-Province 97; and Wyoming Structural province-Provinces 99, 101, and 103. 

most important oil-productive basins in North 

America (Dow, 1974; Rice, 1979). The northern 

part of the Williston basin lies in Canada, the 

southwestern part in Montana, and the remainder 

in North and South Dakota. Precambrian to Mississippian paleotectonics of the Williston basin are 

summarized by Sandberg (1964). A composite 

stratigraphic section (fig. 3A and 3B) for northeastern Montana illustrates the stratigraphic units 

involved in the thick Cambrian, Ordovician, Silurian, Devonian, and Mississippian rock sequence 

of the Williston basin, many of which yield oil and 

gas (Glaze, 1971; Rice, 1979). The distribution and 

thickness of many of these rocks, as well as 

Mesozoic rock units, were controlled by intermittent movements of structural features already in 

existence by the close of Precambrian time 

(Sandberg, 1962). The Blood Creek syncline (fig. 

1) is herein included within the Williston Basin 

province. This broad, shallow downwarp extends 

westward from the southwestern edge of the Williston basin, along the older northeastern margin 

of the upper Paleozoic Big Snowy trough. 

G2 

NORTHERN MONTANA FORELAND (SWEETGRASS 

ARCH) PROVINCE 

The Northern Montana Foreland province (fig.1) 

lies between the western part of the Williston 

basin and the Montana thrust belt. During much 

of Paleozoic time, this province was part of the 

broad Milk River uplift, a southern extension of 

the Paleozoic Alberta shelf (Schwartz, 1983, fig. 

3). The Paleozoic Milk River uplift separated the 

Williston basin to the east from the Cordilleran 

miogeoclinal wedge, the eastward-thinning wedge 

of sedimentary rocks deposited on the continental 

margin to the west, remnants of which are preserved west of the disturbed belt in extreme 

northwestern Montana. This broad positive area 

was partitioned in Mesozoic and early Tertiary 

time, from east to west, into the Bowdoin dome, 

igneous intrusive complexes of the Little Rocky 

Mountains, Bearpaw Mountains and Highwood 

Mountains and the Sweetgrass Arch (Alpha, 1955; 

Glaze, 1971; Lorenz, 1983). The Sweetgrass Arch 

and Sweetgrass Hills area just east of the north-
0 50 MILES 

.HIGH 

~MEDIUM 

.LOW 

EXPLANATION 

_/..-

CLUSTER 

BOUNDARIES 

1·-~:_.: ... :i:::l LOW TO ZERO 

DzERO 

~UNKNOWN 

FIGURE 2.-lndex map of Federal Wilderness Lands in Montana. Cluster boundaries shown by dashed lines; numbers refer to 

clusters as described in text. Ratings (high, low, etc.) refer to oil and gas potential. 

ern part of the arch have yielded significant 

amounts of oil and gas, particularly the giant Cutbank oil and gas field on the western flank of the 

Sweetgrass Arch. A stratigraphic summary of the 

western flank of the Sweetgrass Arch is provided 

(fig. 4). Common to the northern MontanaForeland province and disturbed belt to the west is a 

major unconformity between Mississippian and 

Jurassic rocks in which the thick Pennsylvanian 

through Triassic sequence of sedimentary rocks of 

southern Montana and the Williston basin is omitted by erosion or nondeposition (figs. 3 and 4, also 

Rice, 1979). The Bearpaw Mountains laccolith is 

the only major igneous complex in Montana with 

associated hydrocarbon production; the flanking, 

chiefly Cretaceous, sedimentary rocks contain 

G3 

numerous accumulations of natural gas. Both the 

Bowdoin dome and Bearpaw Mountains yield shallow biogenic gas (Rice and Claypool, 1981). In the 

Bearpaw Mountains, gas is trapped by gravity 

slide blocks on the flanks of the uplift. 

MONTANA THRUST BELT PROVINCE 

The Montana sector of the Cordilleran thrust 

belt, commonly designated the Montana thrust 

belt (fig. 1), is geologically more complex than 

either that of adjoining Alberta and British Columbia to the north or the Wyoming-Utah-Idaho 

sector to the south. The complexities are due in 

part to inherited very old (Precambrian) structural elements, which have been reactivated at 
PENNSYLVANIAN 

z 

<( 

a.. 

a.. 

(/) 

(/) 

(/) 

(/) 

~ 

z 

<( 

z 

0 

> 

w 

0 

SILURIAN 

ORDOVICIAN 

CAMBRIAN 

Upper, 

Middle 

Lower 

Upper 

Lower 

Upper 

Middle 

Lower 

Upper 

Middle 

Lower 

Upper 

Middle 

Lower 

Upper 

Middle 

Lower 

PRECAMBRIAN 

/ 

r

! I I.~ I I_,. I I I I I ld I I 

Devils Pocket an 

Alaska ~Bench Fms 

CV¥'yler 

llU!~. Fm Group • 

~... - - - - l;'~ Heath Fm Big e ./~ Otter Fm Snowy 

/~" Kibbey Ss. Group e 

/ 

r Charles Fm • 

-

Madison 

Group 

Mission Canyon Ls. • 

Lodgepole Ls. 

-. - - ~-- Bakken F=m- - u~-n:.L ,_I:! l_ J .1 "IJ.ll_ 

Jefferson 

Group 

........ 

Birdbear Dol e 

Duperow Fm • 

r""-r--r.... 

~r---------------~ ....., 'r--r-- Dawson Bay Fm • 

Souris River 

,........., ..,Winnipegosis 

r""-r-- Fm • r--..., .... , 

'""' Beartooth Butte Fm _..'1 

i- - ~~ ... - ~ ..• Y~ Interlake Fm e Stonewall Fm 

1.1 ........ 

,.~ 

~~stony Mountain Fm e 

Red River Fm • IT winnijl Fm 

.,_, Emerson Fm $Deadwood Fm 

Flathead Ss. 

Undivided 

FIGURE 3A.-Correlation chart of Paleozoic rocks for east-central and northeastern Montana. 

Solid circle in right-hand column indicates oil productive. 

G4 
TERTIARY PALEOCENE Fort Union Fm 

Hell Creek Fm 

Fox Hills Ss. 

Bearpaw Shale 

Judith River Fm ~ Upper 

Claggett Shale 

(j) ......:::::::: 

Eagle Ss. i) 

:::::> ~ammon Shale 0 Telegraph I 

w Creek Fm I (.) 

<( Niobrara Fm 

1-

w Carlile Shale i) a: 

(.) Greenhorn Fm 

~Mosby Ss. Mbr~ i) 

Belle Fourche Shale 

Mowry Shale 

MuddY. Ss. 

Skull Creek Shale 

Lower 

Fall River Ss. 

- - - Kootenai Fm • 

IL - - - Upper Morrison Fm • - - - Swift Fm 

(.) • - - - - -(j) Ellis Group Rierdon Fm 

(j) 

<( 

Middle a: Piper Fm • 

:::::> ........... Nessen Fm ....., r"--r--, 

Lower 

FIGURE 3B.-Correlation chart of Mesozoic rocks for east-central and northeastern Montana. Symbols in 

right-hand column: solid circle--oil productive; open circle with rays-gas productive. 

G5 
... A• 

VIRGELLE SANDSTONE JuR-f ELLIS GROUP AS SIC s 

TELEGRAPH CREEK FM 

(/) 

:::> 

-- SUN RIVER DOLOMITE.lJ R* ....:;;,_ -- - 0 

UJ 

u 

<( 0::: 

1- UJ KEVIN MEMBER 

UJ > s 0::: 0::: z 

u 0 

z MISSION CANYON 

<( R 

il LIMESTONE 

* a. 

u; 

(/) 

(/) 1-

0::: <( <( 

UJ ~ a. 0::: 0::: a. <( 

:::> ~ 0 s LL. FERDIG MEMBER 

u; 

(/) LODGEPOLE ~ 5 

LIMESTONE 

CONE MEMBER R.S. r--- BAKKEN/THREE FORKS FMS. s FLOWEREE MEMBER s BOOTLEGGER MEMBER 

LL. 

<( z 

(/) UJ 0 VAUGHN~ R 

..J ~ MEMBER BOW 

:::> ~ <( ISLAND 

0 

UJ 

u 

<( ~ ~ SANDSlJ ~ 0::: u ..J TAFT HILL MEMBER 

<( m 0 

LL. 1- FLOOD MEMBER s 

UJ 

0::: 

z JEFFERSON R <( 

z FORMATION 

0 

> UJ 

0 

u 

0::: KOOTENAI FORMATION 

UJ 

~ MAYWOOD FORMATION 

0 

..J 

MOULTON MEMBER ~ 

CUT BANK SS. MEMBER * CAMBRIAN UNDIVIDED 

FIGURE 4.-Correlation chart of pre-Tertiary stratigraphic units of the western flank of the Sweetgrass arch and adjacent eastern disturbed belt, northwest Montana. A superscripted 1 denotes informal drillers terms in this area. Abbreviations and 

symbols in right-hand column are as follows: S-known or potential source rock; R-known or potential reservoir rock; solid 

circle-oil productive; open circle with rays-gas productive; and solid circle with rays-both oil and gas productive. Within 

the disturbed belt, Mississippian rocks are divided into the Allan Mountain Limestone, equivalent to the Lodgepole Limestone and overlying Castle Reef Dolomite which includes the Sun River Dolomite Member (Mudge, 1972a). 

various times and which have influenced the 

geologic framework of the Montana thrust belt 

(Harris, 1957; Harrison and others, 1974; and 

many other workers). The Cordilleran thrust belt, 

which extends from Alaska to Mexico (Powers, 

1983), is characterized by west-to-east displacement of sheets of chiefly sedimentary rocks 

(thrust sheets) detached from the underlying crust 

and moved on successively more eastward or 

northeastward located west-dipping thrust faults 

(low-angle, map-scale contraction faults) during 

G6 

Jurassic to early Tertiary time. The total horizontal contraction or shortening (west to east) of the 

sedimentary wedge is estimated by many workers 

to exceed 100 miles. 

The disturbed belt, the northern subdivision of 

the Montana thrust belt, is an extension of the 

Foothills and Front Range structural subdivisions 

of the Cordilleran thrust belt in southern Alberta 

(Willis, 1902; Mudge and Earhart, 1980; Mudge, 

1983). The Foothills region of Canada has yielded 

prolific amounts of gas and condensate (Rice, 
1~)77). Eiploration has not yet discovered such 

giant accumulations in the Montana thrust belt, 

but the possibility of the existence of such accumulations, particularly within the disturbed 

belt, should not be discounted. Oil and gas exploration of the disturbed belt, particularly west of 

the mountain front, has been restricted from exploration. Glacier National Park is located chiefly 

on the major Lewis thrust sheet in the northern 

part of the disturbed belt (Mudge and Earhart, 

1980) and is probably underlain by imbricate 

thrust slices of Mississippian carbonate rocks, beneath the Lewis thrust sheet, similar to and coextensive with hydrocarbon-producing structures in 

southwestern Alberta (Kulik, 1983). The Bob Marshall Wilderness and associated Wilderness Study 

Areas occupy much of the disturbed belt south of 

Glacier Park, and these areas are considered to 

have significant hydrocarbon potential (Mudge and 

others, 1977; Rice, 1977; Mudge and others, 1978). 

The stratigraphy and structure of much of this 

area were carefully detailed by Mudge (1965, 

1966, 1967, 1972a, 1972b) and excellent regional 

geologic maps are available (Mudge and others, 

1982; Mudge and Earhart, 1983). Principal stratigraphic units east of the mountain front are illustrated (fig. 4). The region is characterized by a 

complex of imbricated thrust sheets, folded thrust 

faults, and other structural features closely similar to those north of the International Boundary. 

West of the disturbed belt the northern sector 

may be subdivided from east to west into the 

Rocky Mountain trench, the Purcell anticlinorium, 

and Purcell trench along the Idaho-Montana State 

boundary. Both the disturbed belt and northern 

sector to the west are bounded on the south by 

the Lewis and Clark line, a linear zone of crustal 

weakness described by Reynolds (1979). 

The central sector of the Montana thrust belt, 

located south of the Lewis and Clark line and west 

of the Helena salient, provided the locus for large 

Mesozoic igneous intrusions (Pioneer, Phillipsburg, and Boulder batholiths) and appears to 

be a region of relatively thin crystalline crust and 

high heat flow. This sector is also the locus of 

most of the hot springs in Montana (Chadwick and 

Leonard, 1979). All of these factors are negative 

with respect to oil and gas possibilities. The 

Helena salient, east of the Boulder batholith (fig. 

1), parallels and structurally overprints the Precambrian Belt embayment and the western part of 

the Paleozoic Central Montana trough (Harris, 

1957; Bregman, 1976). The Precambrian Belt emG7 

bayment is probably not a depositional embayment of Belt age but a later pre-Middle Cambrian 

structural depression (M. K. Reynolds, oral commun., 1983). The Helena salient of the thrust belt, 

comprehensively discussed by Woodward (1981), 

is bounded to the southeast by the Battle Ridge 

monocline (Garrett, 1972), a northeast-trending 

linear structural feature intermittently active 

since Mississippian time, and to the south by the 

southwest Montana transverse zone (Schmidt and 

O'Neill, 1983). 

The structure and petroleum potential of the 

southwest Montana recess is discussed by Perry 

and others (1981, 1983)! Here the intersection of 

foreland and thrust belt structural trends (Scholten, 1967; Beutner, 1977) has generated a complex 

structural style that is currently being deciphered 

(Perry, 1982; Perry and Sando, 1983). Middle Tertiary to recent extension faulting and formation of 

extension-fault-bounded basins (grabens and halfgrabens) have profoundly modified much of western 

Montana and are responsible for much of the present topographic relief in southwestern Montana. 

The presence of the three principal requirements 

for hydrocarbon accumulations-thermally mature, kerogen-rich source beds (possible source 

rock) for hydrocarbons, possible reservoir rocks, 

and structural traps in this sector of the Montana 

thrust belt (Perry and others, 1983)-offers many 

potential exploration targets. 

CENTRAL MONTANA UPLIFT PROVINCE 

The Central Montana Uplift province includes 

from west to east, the Crazy Mountains basin, 

Bull Mountain basin, Lake Basin, intervening uplifts, anticlines, synclines, and Porcupine dome on 

the east (fig. 1). The Crazy Mountains basin (Garrett, 1972) has been modified by intrusion of a 

major early Tertiary laccolith (Bonini and others, 

1972) and associated radial dike swarms, forming 

the Crazy Mountains. The Little Belt Mountains, 

Big Snowy Mountains, and Cat Creek anticline 

(from west to east) lie along the northern boundary of this province (Thorn, 1923). The southern 

boundary is the northern limit of the Beartooth 

uplift and Big Horn basin and uplift-major structural features of the Wyoming province to the 

south. Numerous small oil fields, mainly in structurally enhanced stratigraphic traps, have been 

found in the Central Montana Uplift province, 

along the eastward extension of the Laramide 

(Late Cretaceous to early Tertiary) Big Snowy 
uplift. The northern part of this province is within 

the area of the upper Paleozoic Big Snowy trough, 

an area previously uplifted during Devonian time. 

MONTANA PORTION-WYOMING 

STRUCTURAL PROVINCE 

Between the southwest Montana recess of the 

thrust belt and the southeastern corner of Montana lies a complex of structurally yoked 

Laramide uplifts and basins that may have developed sequentially from west to east. Parts of 

the Archean (early Precambrian) basement-cored 

uplifts have been thrust over adjacent basins 

above thrust faults that involve the entire upper 

part of the earth's crust (Gries, 1981, 1983; Perry 

and Kulik, 1982). Of note are relatively or completely untested basins and thrust-fault traps associated with the Blacktail-Snowcrest and Madison-Gravelly uplifts northwest of Yellowstone 

Park as well as the northern and eastern margin 

of the Beartooth uplift just west of the northern 

Big Horn basin (fig. 1) to the east. The southeastern margin of the Blacktail-Snowcrest uplift incorporates the sequence of rocks deposited in the 

axial part of the Upper Paleozoic Snowcrest 

trough (fig. 5), much thicker than that of the Centennial (upper Tertiary to Holocene) basin immediately to the south. It is likely that the 

Paleozoic Snowcrest trough was separated from 

the Wyoming shelf Paleozoic sequence of the Centennial basin (figs. 1 and 5) by a system of downto-the-northwest extension faults along which displacement occurred during the period of more 

rapid deposition in the trough than on the adjacent shelf. 

Tertiary, possibly Laramide, faults with associated drape folds are present along the Beartooth and Bighorn Mountain fronts, similar to 

other areas of the Wyoming Structural province. 

This province is typified by major northeast- and 

northwest-trending lineaments (Maughan, 1983a) 

that are also present farther north (Thomas, 

1974). These features are linear zones of crustal 

fracturing or faulting, active during the late 

Paleozoic and selectively reactivated during late 

Mesozoic to early Tertiary thick-plate thrust faulting of the Rocky Mountain foreland (Gries, 1983; 

Kulik and others, 1983). The Montana portions of 

the Powder River and Big Horn basins have not 

yielded hydrocarbons as prolifically as those portions in adjacent Wyoming, but exploration for petroleum in these areas is still active. The stratigG8 

raphic sequence in these basins is simi1ar to that 

in adjoining Wyoming and not greatly different 

from that of the Centennial basin (fig. 5). Possible 

structural traps for hydrocarbons, associated with 

thick-plate thrust faulting within the Wyoming 

structural province, are among the most controversial exploration targets currently being drilled. 

STRATIGRAPHIC SUMMARY 

Strata ranging in age from Precambrian to 

Holocene are present in Montana. The most complete stratigraphic section is within the portion of 

the Williston basin that lies in the northeastern 

part of the State. Thick Precambrian and 

Paleozoic sedimentary rock accumulations, which 

were deposited on the ancient continental shelf, 

occur in western Montana. Montana was repeatedly inundated by shallow inland seas 

throughout much of the Phanerozoic, until Late 

Cretaceous time. Major gaps are present in the 

stratigraphic record, particularly the Late Cambrian to Late Devonian hiatus in central and western Montana and Late Mississippian to Jurassic 

hiatus in the disturbed belt and Northern Montana Foreland province (the Paleozoic Milk River 

uplift area). Thick Cretaceous deposits occur locally within the thrust belt, in the foreland of central and southwestern Montana, and in various 

Laramide basins. Thick Tertiary deposits, locally 

up to 16,000 ft thick, occur in extension-faultbounded basins in western Montana (Fields and 

Petkewich, 1967; Reynolds, 1979). Marine 

Paleozoic rocks, enriched in kerogen, are generally thermally mature throughout the State, 

whereas thermally mature Cretaceous source 

rocks are primarily present only in western Montana. Tertiary source rocks for hydrocarbons have 

not yet been identified but may be present in the 

deeper parts of Tertiary basins in western Montana. Reservoir or possible reservoir rocks for petroleum are widely distributed throughout the 

stratigraphic column. 

The names of principal pre-Tertiary rock units 

and their relative placement in the stratigraphic 

sequence are shown for the Williston basin and 

north-central Montana (fig., 3A and 3B), the 

Sweetgrass Arch and eastern disturbed belt (fig. 

4), and southwestern Montana (fig. 5). Correlations between geological regions are shown in 

figures 3A, 3B, and 5. We briefly describe the 

sedimentary rocks that compose each system, beginning with the oldest and omitting the younger, 

post-Cretaceous rocks and sediments. 
J3LACKT AIL -SNOWCREST 

UPLIFT 

BEAVERHEAD 

FM 

R? 

BLACKLEAF FM SG 

KOOTENAI FORMATION 

TRIASSIC ROCKS 

PHOSPHORIA 

FORMATION 

QUADRANT 

SANDSTONE 

CRETACEOUS 

JURASSIC 

TRIASSIC 

PERMIAN 

MISSISSIPPIAN 

l 

UPPER DEVONIAN 

WOLSEY SHALE 

FLATHEAD SANDSTONE 

SILUAIAM, ORDOVICIAN, 

AND MOST OF DEVONIAN ABSENT 

CENTENNIAL 

BASIN 

FRONTIER 

AND YOUNGER 

CRETACEOUS 

Sl LICICLASTIC 

ROCKS 

UPPER 

MEMBER 

TRIASSIC ROCKS R? 

z 

oa.. 

(1)::::) 

MISSION CANYON 

LIMESTONE 

-o 

c a: ~----:-~-----.....j ~ CJ LODGEPOLE 

LIMESTONE 

THREE FORKS FM 

so 

SG 

R 

0 

FIGURE 5.-Correlation chart of pre-Tertiary formations of Blacktail-Snowcrest uplift and Centennial basin 

in southwestern Montana. R?-possible reservoir rocks, S?-possible source rocks, SG-indicated gasprone source rock, SO-indicated oil-prone source rock (from Perry and others, 1983). 

G9 
PRECAMBRIAN 

Sedimentary rocks of the Belt Supergroup in 

northwestern Montana were deposited more than 

1 billion years ago in a major basin near the outer 

edge of the ancient continental margin and in a 

narrow embayment or aulacogen, which extended 

eastward (landward) from this basin. This embayment is now the area of the Helena salient of the 

thrust belt and extends through much of the 

Central Montana Uplift province (Harris, 1957) to 

the Porcupine dome (fig. 1). The Belt sequence is 

composed dominantly of clastic rocks, although 

carbonate rocks are important components low 

within this sequence. The rocks thicken westward; 

an aggregate thickness of over 30,000 ft has been 

estimated for far northwestern Montana (Harrison 

and others, 1974). Coarse conglomeratic rocks 

occur on the southern margin of the Precambrian 

Belt embayment (McMannis, 1963) or pre-Middle 

Cambrian structural depression, along the southern margin of the Helena salient. The Belt Supergroup locally contains very thin beds of carbon-enriched dark gray shale or siltstone. These beds are 

not presently known to contain source rock for hydrocarbons, but they have not been thoroughly 

studied. Fractured Precambrian rocks may locally 

provide fractured reservoirs for hydrocarbons, but 

such reservoirs have not yet been identified in 

Montana. 

CAMBRIAN 

Rocks deposited during the Cambrian PBriod 

record the progressive eastward inundation of 

Montana by the sea over an irregular erosion surface (Sandberg, 1962; Peterson, 1981a, 1981b). 

Cambrian rocks are as much as 2,000 ft thick in 

western Montana and 1,200 ft thick northeast of 

the Bearpaw Mountains (Glaze, 1971). These rocks 

thin eastward toward the continental interior from 

a thickness of more than 1,100 ft on the west side 

of the Williston Basin province to less than 600 ft 

in northeasternmost Montana (Sandberg, 1962). 

An east- to northeast-trending zone of thicker 

Cambrian rocks extends into the Williston basin 

from the central Montana uplift area, from the 

area of the Precambrian Belt embayment 

(Sandberg, 1962). The oldest sedimentary rocks 

are sandstone and, locally, conglomerate deposited 

as the sea transgressed eastward toward the continental interior. These coarse-grained rocks 

grade upward into finer-grained siliciclastic rocks 

G10 

(siltstone and shale), which grade upward in turn 

into limestone and dolomite, the dominant components of the Cambrian rocks in western Montana 

(Lochman-Balk, 1972). Mudstone, siltstone, and 

claystone are the dominant components of the 

rock strata in central Montana where the inland 

sea was shallow. Sandstone is the dominant component in the east. Although possible reservoir 

rocks, both sandstone and dolomite, of Cambrian 

age are widespread, petroleum production from 

reservoirs of Cambrian age are restricted to the 

North Dakota part of the Williston basin. Substantial thicknesses of porous dolomite of Cambrian age are known to be present in western Montana (Peterson, 1981b). Cambrian source rocks, 

however, are unknown in Montana. 

ORDOVICIAN 

In Montana, the Ordovician System comprises 

parts of two depositional sequences (Sandberg, 

1962; Foster, 1972) that are generally restricted to 

the Williston basin. The older Ordovician strata 

comprise fine clastic and carbonate rocks in Montana and are a continuation of the Upper Cambrian depositional sequence. These Lower Ordovician rocks have a maximum thickness of about 600 

ft in the Williston basin in northeastern Montana, 

but they were erosionally thinned west of the Williston basin and removed west of a roughly northsouth line between the Bearpaw Mountains and 

the western part of the Big Horn basin (Foster, 

1972). 

A second depositional cycle, involving Middle 

and Upper Ordovician strata, extends farther 

west but pinches out between the Bearpaw Mountains and the Sweetgrass Arch (fig. 1). Farther 

south, the irregular line of pinchout swings eastward into the ancestral central Montana uplift 

and then southwestward through the Crazy Mountain basin (fig. 1) into southwest Montana as 

shown by Sandberg and Mapel (1967). This cycle 

consists of a basal sandstone near the Bearpaw 

Mountains in the west and sandstones and shales 

(Winnipeg Formation) in the Williston basin overlain by dolomites or limestones of the oil-productive Red River Formation and younger Ordovician 

carbonate units (Sandberg, 1962). Dow (1974) 

identified the medial shale of the Winnipeg Formation as the source rock for most lower Paleozoic 

oil in the Williston basin. 

In southern Montana, Ordovician rocks are over 

600 ft thick in the northeastern Powder River 
basin (Khmison, 1971), 400 ft thick in the northern 

Big Horn basin, and pinch out westward beneath 

the Crazy Mountain basin (Stauffer, 1971). Locally 

they persist as far west as the Madison-Gravelly 

uplift (Hadley, 1960). Kerogen-rich Ordovician 

source rock of the Winnipeg Formation (Dow, 

197 4) and important Ordovician reservoir rocks 

for petroleum are present in, but generally restricted to, the Williston basin in Montana. Important hydrocarbon production has been established 

from structural and stratigraphic traps, principally in the Red River Formation of the Williston 

basin (Ballard, 1969; Kohm and Louden, 1978). 

SILURIAN 

Silurian rocks, primarily dolomite, in the Williston basin have a maximum thickness of 700 ft in 

northeastern Montana (Gibbs, 1972). They thin 

westward to an erosional edge in the eastern part 

of the Northern Montana Foreland province, -onto 

the Paleozoic Milk River uplift, and -thin southward into the eastern part of the Wyoming province, where they pinch out in the northern Powder River basin. During latest Ordovician and 

Silurian time, the Williston basin became a deeply 

subsiding subcircular depression surrounded by a 

broad, stable shelf (Sandberg, 1962). Silurian 

rocks are absent in central and western Montana. 

Production from reservoirs of Silurian age is restricted to limited areas of the Williston basin. 

DEVONIAN 

Deposition during the Devonian Period resulted 

in five sequences, of which the middle three are 

principally dolomitic rocks (Sandberg and others, 

1983). A summary report describing these rocks in 

Montana and adjacent areas by Sandberg and 

Mapel (1967) was incorporated into the work of 

Baars (1972). The earliest Devonian rocks consist 

of conglomerates and sandstones deposited in 

channels and associated red beds, probably deposited as soils on an upland surface formed in Early 

Devonian time when all of Montana was elevated 

above sea level (Sandberg, 1961; Glaze, 1971). 

Some of these channels are located along fracture 

zones in the underlying Silurian rocks (Sandberg, 

oral commun., 1983). 

In northeastern Montana, the second Devonian 

sequence occurs only in the Williston basin (fig. 1) 

where it contains an evaporite-bearing section 

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over 500 ft thick (Sandberg and Mapel, 1967). 

Evaporites (anhydrite, salt, and sylvite) in the 

basinal sequence are fringed by carbonate 

bioherms or reefs that are excellent reservoirs for 

petroleum (Sandberg and Hammond, 1958). 

The third Devonian sequence (latest Middle and 

lower Upper Devonian) occurs throughout Montana but thins to an edge at the southeast corner 

of the State and oil the flanks of the Paleozoic 

Central Montana uplift within the Central Montana province. (fig. 1). It attains a thickness exceeding 1,100 ft in parts of western Montana 

(Sandberg and Mapel, 1967). This sequence comprises the Souris River Formation, equivalent 

Maywood Formation, and overlying Jefferson 

Group of the Williston basin (fig. 3A), cyclically 

de posited dolomitized carbonate rocks overlain 

by anhydritic dolomite and anhydrite. Bioherms 

(reeflike mounds) occur locally in this sequence in 

Montana (Sandberg and Poole, 1977, p. 156-157) 

but are larger north of the International Boundary 

(Glaze, 1971) and are significant stratigraphic 

traps for oil in the Williston basin of that area. 

The fourth Devonian sequence, late Late Devonian in age (Sandberg and Mapel, 1967) comprises 

the lower two-thirds of the Three Forks Formation. This sequence is composed of a lower dolomite and evaporite unit representing restricted 

marine conditions and an upper predominantly 

shale unit representing open marine conditions. 

This sequence, absent through much of southeastern Montana, is best developed (over 800 ft thick) 

in the disturbed belt of northwestern Montana 

(Sandberg and Mapel, 1967). Good reservoir rocks 

are commonly absent in this sequence due to generally low porosity and permeability, although 

sandstone at the top of this sequence is a minor 

productive reservoir in North Dakota. 

The youngest Devonian rocks are the initial 

part of a depositional sequence that continued into 

the Mississippian Period (Sandberg and Poole, 

1977). The latest Devonian and earliest Mississippian strata are predominantly carbonaceous 

mudstone, although sandstone or carbonate rocks 

are locally abundant (Macqueen and Sandberg, 

1970). These strata are generally less than 100 ft 

thick and they are absent in parts of central and 

southeastern Montana. The youngest Devonian 

rocks contain kerogen-enriched black shale, excellent source rock, and potential source rock in 

many parts of Montana (Dow, 1974; Meissner, 

1978). 
Devonian oil reservoirs are important in the 

Williston basin of northeastern Montana. Devonian rocks yield natural gas in a limited area on 

the Sweetgrass Arch. Elsewhere in Montana, Devonian rocks are nonproductive. The Bakken Formation is identified as the principal source rock for 

Mississippian oil reservoirs in the Williston basin 

(Dow, 1974). Lateral equivalents of the Bakken in 

western Montana have been identified as potential 

source rock, especially for possible Paleozoic reservoirs. 

MISSISSIPPIAN 

Two depositional sequences comprise Mississippian rocks in Montana (Sando, 1967; Craig, 1972; 

Peterson, 1978; Roberts, 1979). The lower sequence, which began in latest Devonian with the 

widespread deposition of kerogen-enriched mud as 

noted above, culminated in the thick carbonate 

bank of the Madison Group (Sando, 1967, 1976; 

Sando and others, 1976; Rose, 1976; Gutschick and 

others, 1980). The sequence of transgressive and 

regressive events, which controlled its deposition, 

has been analyzed and dated by Sandberg and 

others (1983). The uppermost strata of this sequence include dolomite and evaporite rocks in 

most areas across the State, although the evaporite rocks, which are assigned to the Charles Formation in the Williston basin, comprise as much as 

one-third of the total sequence in northeastern 

Montana where the maximum thickness of the 

Madison Group is about 2,200 ft. Sandberg (1962) 

pointed out that episodically renewed uplift of the 

Cedar Creek anticline, which trends north-northwest along the southwestern margin of the Williston basin in Montana, provided a barrier that 

caused salt precipitation to the east in the Williston basin whereas only anhydrite precipitation occurred to the west. The Madison Group thins depositionally to a minimum of about 600 ft in the vicinity of the Wyoming border in southeastern and 

south-central Montana. It has been thinned by 

erosion on the Milk River uplift in north-central 

Montana to about 600 ft. The Madison Group in 

southwestern Montana is about 1,600 ft thick. 

Equivalent rocks occur in northwestern Montana 

west of Glacier National Park (Johns, 1970), but 

the thickness there is not accurately known. The 

Madison Group contains the most productive 

Paleozoic reservoir rocks for petroleum in both 

eastern and western Montana. 

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The younger Mississippian sequence that developed throughout Montana succeeding Madison 

deposition is assigned to the Big Snowy Group. 

The Big Snowy includes at its base a karst plainsabkha complex that locally lies unconformably 

upon the Madison (Roberts, 1979; Sandberg and 

others, 1983). The sequence is composed of 

mudstone, sandstone, and evaporite beds, which 

grade upward into limestone, carbonaceous 

mudstone, and anhydrite that compose another 

transgressive-regressive depositional cycle. The 

Big Snowy Group, originally deposited over all or 

most of Montana (Maughan and Roberts, 1967), 

has been preserved from erosion only in the 

Paleozoic Big Snowy trough in the Central Montana Uplift province, the Williston basin, and in 

the Paleozoic Snowcrest trough along the southeastern margin of the Blacktail-Snowcrest uplift in 

southwestern Montana (figs. 1 and 5). Equivalent 

strata have been identified locally in far northwestern Montana west of the disturbed belt 

(Johns, 1970), but their extent and thickness are 

unknown. Thickness of the Big Snowy Group is as 

much as 1,200 ft in central Montana and it is, at 

maximum, about half as thick in the Williston 

basin. Thickness of equivatent rocks in southwestern Montana locally: exceeds 1, 700 ft within the 

Paleozoic Snowcrest trough, and these rocks are 

generally absent on the adjacent Wyoming shelf to 

the southeast. Oil is produced from reservoir 

rocks within the Big Snowy Group in the Central 

Montana Uplift province and Williston Basin province. Black shales in the upper part of the Big 

Snowy Group are important source rocks in this 

same province (Kranzler, 1966). 

PENNSYLVANIAN 

Two depositional sequences comprise the 

Pennsylvanian rocks in Montana. Initial deposits 

of the lower sequence are restricted in their distribution to the Paleozoic Snowcrest trough in southwestern Montana, to the Paleozoic Big Snowy 

trough in central Montana, and to the Williston 

basin in eastern Montana. These rocks of earliest 

Pennsylvanian, and possibly youngest Mississippian age, are dominantly dark gray, moderately 

carbonaceous mudstone; but locally they include 

sandstone, conglomeratic sandstone, limestone, 

and very thin coal beds (Harris, 1972; Jenson and 

Carlson, 1972; Fanshawe, 1978). Fluvially deposited sandstones grade upward into red mudstone 
beds overlain by limestone at the top of this transgressive sequence. This sequence of rocks, assigned to the Amsden Group in central Montana, 

is as much as 800 ft thick in the Big Snowy 

trough. Equivalent rocks are as much as 1,000 ft 

thick in the Snowcrest trough, but the Amsden is 

generally less than 200 ft in south-central and 

northeastern Montana. Equivalent rocks in southeastern Montana are included in the lower member of the Minnelusa Formation, which is no more 

than about 150 ft thick. Rocks of this sequence are 

thinned or locally absent at many places in Montana owing to postdepositional erosion (Maughan, 

1975), particularly in the area of the Paleozoic 

Milk River uplift. 

The second of the Pennsylvanian sequences 

comprises mostly sandstone of the Quadrant 

Sandstone in the western half of the State and 

mostly sandy dolomite and interbedded sandstone 

of the Devils Pocket Formation in central Montana and of the middle member of the Minnelusa 

Formation in eastern Montana. These rocks are 

350 ft thick or less, except in the southwest Montana recess where the maximum thickness is 

about 2,500 ft. Rocks of this age have been removed by erosion from the Milk River uplift in 

north-central Montana, but probably occur in far 

northwestern Montana as indicated by outcrops of 

lithologically similar rocks in the northern 

Whitefish Range (Johns, 1970, Maughan, 1975). 

Reservoir rock of Pennsylvanian age yields hydrocarbons locally in central Montana (Kranzler, 

1966) and the adjacent Big Horn basin to the 

south. The Tyler Formation (fig.3A) is the principal 

oil reservoir rock of the Central Montana Uplift 

province (Maughan, 1983b). 

PERMIAN 

Two Permian depositional sequences are present in Montana. In southeastern Montana and in 

the Williston basin the lower sequence is included 

in the upper member of the Minnelusa Formation. 

The rocks of this lower sequence are dominantly 

sandstone in the Snowcrest trough of southwest 

Montana but include interbedded dolomite and 

sandy dolomite in eastern Montana. A thickness of 

about 350 ft has been estimated for Permian 

sandstones of the lower sequence in the Snowcrest 

trough (Sheldon and others, 1967); the upper 

member of the Minnelusa is about 300 ft or less in 

southeastern Montana (Maughan, 1967). 

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The upper sequence of the Permian is dominantly red mudstone, dolomite, and evaporite beds 

that include salt deposits in southeastern Montana 

and the central part of the Williston basin. The sequence is no more than about 300 ft thick in the 

eastern part of the Central Montana Uplift province, but it thickens eastward. Equivalent rocks 

in the southwestern part of the Central Montana 

Uplift province consist of sandstone adjacent to the 

Milk River uplift and these strata intertongue 

southwestward into dominantly carbonate rocks of 

the Park City Formation and into kerogen-enriched mudstone, chert, and phosphorite beds of 

the Phosphoria Formation (Peterson, 1972a). 

These rocks are about 600 ft thick adjacent to the 

Idaho border but thin depositionally northward toward an erosional edge in the vicinity of the Lewis 

and Clark line and northeastward onto the flank of 

the Milk River uplift. 

Known Permian reservoir rocks for hydrocarbons are rare in Montana. However, kerogen-enriched phosphorite and mudstone of the Phosphoria Formation in southwest Montana provide 

abundant possible source rock. Similar rocks are 

considered to be the principal Paleozoic source 

rocks of the Big Horn basin (Stauffer, 1971) and 

other parts of Wyoming and southern Montana 

(Claypool and others, 1978). 

TRIASSIC 

In south-central and southeastern Montana, 

Triassic rocks are composed entirely of red 

mudstone, siltstone, and sandstone. Maximum 

thickness of these rocks is about 500 ft along the 

Wyoming border in south-central Montana 

(McKee and others, 1959, pl. 3). The Triassic 

strata are bevelled northward by erosion beneath 

the pre-Middle Jurassic unconformity and extend 

only into the southern part of the Williston basin 

(MacLachlan, 1972). Triassic rocks are absent in 

northern Montana, thin to absent in the Central 

Montana province, and pinch out northward in the 

Helena salient of the thrust belt. In southwestern 

Montana, the Triassic rocks are chiefly mudstone 

and clayey to silty limestone. Maximum thickness 

is about 2, 000 ft in the vicinity of the Idaho border 

in southwest Montana (Scholten and others, 1955), 

but the strata thin northward, probably by erosional bevelling, and are not known to occur north 

of the Pioneer batholith. Triassic rocks do not contain petroleum reservoir beds or known source 

rock in Montana. 
JURASSIC 

Peterson (1972b) re~ognized four marine transgressive-regressive depositional cycles within the 

Jurassic sequence in Montana and nearby areas. 

An earlier Late Triassic or probable Early Jurassic cycle is represented in eastern Montana by red 

mudstone, siltstone, sandstone, and gypsum 

(Saude Formation of Zieglar, 1955) which lies unconformably upon northward bevelled strata of 

the underlying Triassic and Permian Spearfish 

Formation in southeastern Montana and upon successively older strata farther north in the Williston basin. Jurassic strata in eastern Montana consist mostly of red beds, evaporites, and carbonates in the lower part, overlain by marine 

mudstone and some sandstone. In western Montana, equivalent marine Jurassic strata consist of 

dark shales with interstratified thin limestone an(i 

sandstone beds of the Sawtooth and Rierdon Formations of the Ellis Group (figs. 3B, 4). Local uplifts in southwestern Montana and adjacent areas 

of the Helena salient (fig. 1) in Late Jurassic time 

led to the bevelling and removal of the older 

Jurassic rocks from these proto-Laramide uplifts 

and the deposition of nearshore sandstone of the 

Swift Formation. Equivalent strata in central and 

eastern Montana consist chiefly of shale and sandy 

shale. The nonmarine Morrison Formation represents the final Jurassic deposits in Montana. The 

Morrison consists of coastal plain deposits, chiefly 

sandstone, mudstone, limestone, and coal. Jurassic rocks are as much as 1, 400 ft thick in the Williston basin in northeasternmost Montanc1. The 

rocks thin to about 750 ft in southeastern Montana 

and to less than 250 ft adjacent to the Belt arch 

and the Sweetgrass Arch and are locally absent 

because of Late Jurassic erosion and (or) nondeposition in some areas of western Montana. Jurassic rocks are thermally mature only in western 

Montana. The richest Jurassic source rock, that of 

the Sawtooth Formation (Clayton· and others, 

1983) is the principal source rock for the Cutbank 

oil fi~ld on the Sweetgrass Arch (D. D. Rice, 

unpub. data). Jurassic rocks are hydrocarbon reservoirs only locally on the eastern flank of the 

Sweetgrass Arch and on the Cat Creek anticline 

in the northeastern part of the Central Montana 

Uplift province (fig. 1). 

CRETACEOUS 

Earliest Cretaceous rocks throughout Montana 

are characterized by nonmarine sandstone, conglomerate, mudstone, and limestone. These rocks 

G14 

are about 100 ft thick in the northern Powder 

River basin in southeastern Montana, about 200 ft 

in northeastern Montana, and generally thicken 

westward to over 1,500 ft in western Montana according to McGookey and others (1972). Oil and 

gas are produced from the Kootenai Formation ~n 

the Sweetgrass Arch, with a source from Jurassic 

rocks, and in central Montana, with a source from 

Paleozoic rocks, and with migration of hydrocarbons from source to reservoir along faults. The 

thick, younger Cretaceous rocks consis~ .of tra~sgressive and regressive cycles of deposi~IOn, With 

eastward thinning deposits of nonmanne rocks 

and shoreline sandstones (representing regression 

of the sea) alternating with westward thinning deposits of marine shale (represent~ng transgressi~n 

of the sea). Volcanism and erosiOn of uplands m 

western Montana and adjacent Idaho provided an 

abundance of clastic debris that was distributed 

eastward across Montana from these western sediment sources during regressive cycles. Many 

shale units, particularly those deposited during 

transgressions, such as the Skull Creek, Mowry, 

and Marias River Shales, are kerogen-rich but 

generally are immature except in the western part 

of the State. Cretaceous rocks have yielded significant amounts of biogenic gas throughout the 

central and eastern part of the State and are the 

principal reservoirs of the Bowdoin do~e, Bearpaw Mountains area, Sweetgrass Hills, Lake 

basin, and northern Big Horn basin. Thic~ess of 

the Cretaceous sequence ranges from about 3,500 

ft of dominantly marine rocks in northeastern 

Montana to more than 10,000 ft of nonmarine 

rocks locally in southwestern Montana (Ryder and 

Scholten, 1973). Lower Cretaceous sandstones are 

the principal oil reservoirs of the giant Cutbank oil 

field and smaller fields on the Sweetgrass Arch. 

Younger Cretaceous sandstones are the principal 

resevoirs of biogenic gas in central and eastern 

Montana (fig. 3B). 

OIL AND GAS POTENTIAL 

The principal references for oil and gas potent~al 

of western Montana are Rice (1977) for the disturbed belt, Perry and others (1983) for southwestern Montana, and Peterson (1981b) for the 

Montana thrust belt as a whole. For eastern Montana we refer to Rice (1979). American Association 'of Petroleum Geologists Memoir 15 is also 

useful, particularly articles by Glaze (1971), Kinnison (1971), and Stauffer (1971). 
For purposes of this report and its accompanying map (Miscellaneous Investigations Series Map 

I-1541, in press), Federal Wilderness Lands in 

Montana with oil and gas potential are grouped or 

subdivided into 34 clusters of geologically related 

appraisal areas. Presence or absence of possible 

source rock, reservoir rock, and structural or 

stratigraphic traps are the most critical factors 

used in qualitatively assigning a zero, low, 

medium, or high oil or gas potential to each of 

these clusters. The structural and thermal history 

of each of the clusters was also carefully considered for those areas where such data are available (fig 2). 

Cluster 1 tracts lie near the eastern margins of 

the Pioneer and Phillipsburg batholiths and southern margin of the Boulder batholith within the 

Helena salient of the Montana thrust belt. These 

are assigned a low to zero oil potential and low gas 

potential on the basis of proximity to these major 

intrusive bodies, absence of hydrocarbon shows in 

nearby drill holes, and intense thermal alteration 

of possible source rocks, which probably resulted 

in thermal destruction of hydrocarbons. 

All but one tract in cluster 2 are located north 

and northwest of the Pioneer batholith, within the 

vicinity of the Grasshopper and Sapphire thrust 

plates of Ruppel and others (1981). These areas 

are assigned a zero oil potential and, optimistically, a low to zero gas potential on the basis of 

the possibility of thermogenic gas in generally 

low-grade Precambrian metamorphic rocks of this 

cluster. The Italian Peaks area, near the southwestern corner of Montana, is also grouped with 

this cluster. 'This area contains a stack of thrust 

sheets, involving chiefly Paleozoic rocks (Skipp 

and Hait, 1977; Skipp, 1979), with perhaps a 

somewhat higher oil or gas potential than the remainder of the cluster. 

Cluster 3 includes tracts along the Idaho bord~r 

in and adjacent to the Centennial Mountains. This 

cluster was assigned a low to zero oil potential and 

low gas potential in part on the basis of data presented by Perry and others (1983). The cluster lies 

near the Snake River volcanic plain and is an area 

that has been subjected to the thermal effects of 

major igneous activity. The timing of presumed 

hydrocarbon generation in Cretaceous and older 

rocks in this cluster predates development of the 

structural traps within the area. The Centennial 

fault, mountains, and adjacent topographic basin 

are late Tertiary to recent geologic features (Witkind, 1975, 1977) coincident with Snake River volcanism. 

G15 

Cluster 4 lies within the Blacktail-Snowcrest 

and Madison-Gravelly uplifts of the southern Montana part of the Wyoming Structural province. 

This cluster was assigned a low oil and gas potential on the basis of the broad distribution of Archean (Precambrian) crystalline and Tertiary volcanic rocks at the surface. Although work in progress suggests that a major Laramide range-front 

thrust system lies within a part of this cluster, 

this structure overrides a Laramide basin that 

contains igneous intrusives and intercalated volcanics and lacks known source beds for hydrocarbons. 

Clusters 5 and 6 lie within the Beartooth uplift, 

north of Yellowstone Park and are assigned a. zero 

oil potential and perhaps optimistically a low to 

zero gas potential. The entire cluster is associated 

with Precambrian metamorphic and Tertiary volcanic rocks of the topographically elevated internal part of the Beartooth uplift. However, the 

northern tract in this cluster lies near the edge of 

this uplift and has remote possibilities for gas, depending on the untested subsurface structural 

configuration of this edge of the Beartooth uplift. 

Cluster 7 consists of tracts along the northeastern margin of the Madison-Gallatin uplift and 

southern margin of the Crazy Mountains basin. It 

is assigned a low oil and gas potential. Drill holes 

in the region of these tracts have not yielded evidence of hydrocarbons, but sedimentary rocks, including possible reservoirs and source beds for hydrocarbons, are present. 

Cluster 8 includes that part of Yellowstone N ationa! Park in Montana west of the Wyoming 

State boundary and adjacent to Idaho. This cluster is assigned an unknown oil and gas potential 

based on the poorly exposed Paleozoic and 

Mesozoic stratigraphic sequence and obscured 

structural relationships, hidden beneath the Tertiary volcanic rocks, and on the lack of any nearby 

deep drill holes. This cluster lies near the Y ellowstone igneous caldera and Snake River volcanic plain, an area of high heat flow and hot 

springs, with the probable thermal destruction of 

any generated hydrocarbons (Chadwick and 

Leonard, 1979; Chadwick, 1981). 

Cluster 9 includes a partial tract on the northeast flank of the Beartooth uplift. Like the northern part of cluster 5, it lies near the front of the 

uplift and is assigned a zero oil and low gas potential on the basis of the remote possibility of deep 

structural traps associated with possible mountain 

flank thrust faulting. 
Clusters 10 and 11 occupy the crest of the Big 

Snowy Mountains uplift within the Central Montana Uplift province. This cluster was assigned a 

low to zero petroleum potential, as most of the 

possible source and reservoir rocks of this area 

are exposed on the breached anticline represented 

by this structure, and generated hydrocarbons 

would have escaped to the surface. 

Clusters 12 and 13 lie on the northeast flank of 

the Little Belt Mountains. Cluster 12 was assigned a low oil potential and low to zero gas potential, as some potential reservoir rocks are present in the subsurface farther down on the flanks 

of this uplift and possible, though unlikely, structural traps and seals may be present. Cluster 13 

was assigned a low to zero petroleum potential, as 

Precambrian rocks are exposed in the area. Surrounding Paleozoic rocks, otherwise containing 

potential source rock and reservoir rocks, are 

also exposed at the surface. 

Cluster 14, in the northwestern part of the Big 

Belt Mountains, was assigned a zero oil potential 

and low to zero gas potential. This cluster is at the 

southern edge of the Adel Mountain Volcanics 

area and the northern edge of extensive exposures 

of Precambrian Belt sedimentary rocks. Important parts of the sedimentary section, including 

possible source rock and reservoir rocks, are absent by erosion or exposed along the margins of 

this area. Structural traps are either absent or 

obscure. 

Cluster 15 lies in the frontal part of the Montana thrust belt within the southwest Montana recess. It is assigned a medium oil potential and 

medium to high gas potential on the basis of the 

possibility of structural traps in thermally mature 

strata expected to contain both source and reservoir rocks, beneath the thermally immature strata 

of the Tendoy thrust sheet (Perry and others, 

1983). Possible source beds include kerogen-rich 

mudrock of the Permian Phosphoria Formation 

and kerogen-rich Upper Mississippian shale. Possible reservoir rocks include the Mississippian 

Madison Group and Pennsylvanian Quadrant 

Sandstone. 

Cluster 16 lies in the western part of the 

Wyoming Structural province adjacent to the 

southwest Montana recess, on or associated with 

the Blacktail-Snowcrest uplift. This cluster was 

assigned a medium oil and medium to high gas potential on the basis of the presence of source 

rocks, possible reservoir rock, and structural 

traps associated with thrust faulting within this 

G16 

uplift and along the southeast margin of the uplift.'-

The petroleum potential is discussed in more detail by Perry and others (1983). 

Cluster 17 includes the eastern part of the Bob 

Marshall Wilderness and associated Wilderness 

Study Areas evaluated by Rice (1977). It is assigned a medium oil and high gas potential on the 

basis of the presence of kerogen-rich source rock, 

reservoir rocks, and numerous structural traps. 

The area is assigned a higher gas than oil potential as a result of organic geochemical studies by 

Clayton and others (1983) and because of the nature of nearby discovered hydrocarbon accumulations. 

Cluster 18, just west of cluster 17, is assigned 

a low to zero oil potential and medium gas potential. In cluster 18, source rock and potential reservoirs are, or were, more deeply buried than in 

cluster 17, reducing porosity, permeability, and 

causing possible thermal destruction of heavier 

hydrocarbons to methane. Structural traps are 

similar to those of cluster 18 and are discussed by 

Rice (1977). 

Cluster 19 occupies the western part of the Bob 

Marshall Wilderness and associated Wilderness 

Study Areas. This cluster is assigned a zero oil potential and low gas potential. This evaluation is 

based on the predominance of Precambrian 

sedimentary rock, which lacks any documented 

hydrocarbon potential, and the presence of large 

extension faults, along which hydrocarbons have 

likely escaped from inferred deeply buried 

Paleozoic rocks (beneath the Lewis and associated 

thrusts). 

Cluster 20 includes the eastern part of Glacier 

National Park. This area is assigned a low oil potential and medium to high gas potential. It is the 

site of the first producing oil field in Montana. Oil 

possibilities appear restricted to fracture production from the Cone Member of the Cretaceous 

Marias River Shale (fig. 4). Limited fracture porosity and extremely low intragranular porosity 

within the Marias River in the adjacent Blackfeet 

Indian Reservation indicate a low potential for oil 

farther west. The giant Waterton gas field, north 

of the International Boundary, may project under 

the northern edge of cluster 20. Analogous structures are most likely present in this area beneath 

the Lewis thrust plate (Kulik, 1983). 

Cluster 21 occupies the western part of Glacier 

National Park and adjacent northern extension of 

the Bob Marshall Wilderness. This cluster was assigned a low to zero oil potential and medium to 
lo~ gas potential. Structurally this cluster is similar to cluster 19. However the presence of a deep 

Tertiary basin and associated hydrocarbon seepages along the western margin of Glacier National 

Park leads us to assign a slightly higher qualitative assessment than that assigned to cluster 19. 

Cluster 22 occupies the eastern part of the 

northern Whitefish Range. We have assigned it a 

zero oil potential and low gas potential on the 

basis of the abundance of Precambrian Belt 

sedimentary rocks at the surface. Possible Mississippian and Pennsylvanian source and reservoir 

rocks occur east of this area beneath the eastern 

part of the major thrust system that underlies 

cluster 22. The assignment of potential is based on 

available surface geologic data and reasonable extrapolations of these data collected by J. W. Whipple and W. J. Perry, Jr. 

Cluster 23, west and southwest of cluster 22, 

lies within the Proterozoic Belt basin of northwestern Montana, as does cluster 22 (Harrison 

and others, 1974). Major thrust sheets in this area 

are composed almost entirely of Precambrian 

rocks, although Paleozoic rocks could be present 

beneath some of these thrust sheets. The possibility of oil or gas production from such Paleozoic 

rocks and geophysical interpretations of subsurface structure have generated exploration interest 

in the region of clusters 22 and 23. On the basis 

of reasonable extrapolations of the surface structure, such rocks appear unlikely to be present in 

structural traps within the area of cluster 23. 

Cluster 23 is assigned a zero oil potential and low 

to zero gas potential, as rocks with demonstrated 

oil and gas potential are not yet known to occupy 

structural traps in this area. 

Clusters 24 and 25 lie respectively south and 

southeast of the Bearpaw Mountains laccolith in a 

shallow gas-producing region of north-central 

Montana. Cluster 24 is within an area of natural 

gas production. It is assigned a medium to low oil 

potential and a high gas potential. Cluster 25 is 

within and adjacent to the Little Rocky Mountains 

igneous intrusives complex, an unfavorable area 

for hydrocarbons. It is assigned a zero oil and low 

to zero gas potential. 

Clusters 26- and 27 lie within and adjacent to the 

Charles M. Russell Wildlife Refuge, an area of 

which the petroleum geology was intensively 

studied by Rice (1979). The Refuge is assigned a 

medium oil and gas potential. Because the area is 

situated on the northwest flank of the Blood Creek 

syncline, traps for Paleozoic oil will be chiefly 

G17 

stratigraphic in nature with local structural enhancement (Rice, 1979). Significant biogenic gas 

resources may be present in Upper Cretaceous reservoirs, stratigraphically controlled accumulations similar to those in adjacent areas (Rice, 

1979). 

Cluster 28 lies within the southwestern part of 

the Williston basin, an area of active hydrocarbon 

exploration and extensive production. This cluster 

is assigned a high oil and medium gas potential. 

The principal Paleozoic oil reservoirs in this province in Montana lie in the Upper Devonian 

Bird bear and Upper Mississippian Charles and 

Kiobey Formations (Rice, 1979). Recent oil discoveries have been made in limestones of the Mississippian Madison Group. Most of the oil fields 

are structurally controlled hydrocarbon accumulations with stratigraphic enhancement (Rice, 1979). 

The most recently discovered petroleum accumulations tend to lie in stratigraphic traps with 

structural enhancement. 

Cluster 29 lies within the northern part of the 

Powder River basin in the area of the Tongue 

River syncline. It is assigned a low oil and 

medium gas potential. The cluster lies in a structural low near the margin of a basin that has not 

yielded abundant hydrocarbons in Montana. Although a significant number of possible reservoir 

rocks are present and possible stratigraphic traps 

may be present, possible source beds for hydrocarbons are generally immature. 

Cluster 30 lies near the southeastern margin of 

the Montana portion of the Powder River basin. It 

is assigned a moderate oil and high gas potential 

on the basis of nearby hydrocarbon production in 

Wyoming and also on the basis of a more favorable structural location than cluster 29. 

Cluster 31lies near the northeastern margin of 

the Big Horn basin and is assigned a low to 

medium oil potential and a medium gas potential. 

The Montana portion of the Big Horn basin has 

historically yielded smaller amounts of hydrocarbons than that portion in Wyoming. Possible reservoir rocks are present in this area and stratigraphic traps may be present, but possible postPaleozoic source rocks are generally immature. 

Cluster 32 lies in the southern part of the Madison-Gravelly uplift adjacent to the Idaho-Montana 

State boundary and Snake River volcanic plain. It 

is assigned a low to zero oil and gas potential. The 

thermal effects of both Snake River volcanism and 

that of nearby Yellowstone Park decrease the hydrocarbon potential of this area with respect to 
cluster 4 to the north, an area structurally similar 

to cluster 32. 

Cluster 33 lies within a small basin developed 

by Tertiary to recent extension faulting within the 

core of the Laramide Madison-Gravelly uplift. 

This cluster is assigned a medium oil potential and 

medium to high gas potential. Although other Tertiary basins in western Montana contain oil shale, 

a potential source rock, and possible reservoir 

beds, none of these Tertiary basins have yet 

yielded commercial quantities of hydrocarbons. If 

such kerogen-rich deposits are present in this 

basin, the thermal effects of nearby Tertiary volcanism and high heat flow may have led to thermal generation of hydrocarbons. 

Cluster 34 lies east of the Bowdoin dome on the 

western margin of the Williston basin in northeastern Montana. It is assigned a low to medium 

oil potential and medium to high gas potential. 

This cluster lies near shallow gas accumulations in 

stratigraphic traps on the Bowdoin Dome. These 

biogenic gas accumulations are in Upper Cretaceous mudstones. Stratigraphic traps in Paleozoic 

and Cretaceous rocks may localize hydrocarbon 

accumulations within the area of cluster 34. 

SUMMARY 

Of the 6,930,433 acres of Wilderness Lands in 

Montana the potential acreage can be summarized 

as follows: high potential, 1,374.2 thousand acres; 

medium potential, 1,154.8 thousand acres; low potential, 1,325.9 thousand acres; low to zero potential, 1,212.1 thousand acres; zero potential, 1,825.4 

thousand acres; and 38.2 thousand acres classified 

as having unknown potential. 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. 

GLOSSARY 

aulacogen A tectonic trough developed in the 

continental crust, bounded by extension faults. 

Aulacogens are nearly perpendicular to continental margins and open outward toward the adjacent 

continental margin (from Bates and Jackson, 

1980). 

biogenic gas Methane-rich gas formed by living 

organisms, particularly microorganisms. 

G18 

contraction fault A fault in sedimentary rocks 

across which there has been bed-parallel shortening, giving rise to tectonic thickening (from Bates 

and Jackson, 1980); now in international use to 

mean a fault that shortens an arbitrary datum 

plane (McClay, 1981); thrust fault is a partial 

synonym. 

extension fault A fault in sedimentary rocks 

across which there has been bed-parallel extension, giving rise to tectonic thinning (from Bates 

and Jackson, 1980); now in international use to 

mean a fault which extends an arbitrary datum 

plane (McClay, 1981); normal fault is a partial 

synonym. 

foreland A stable area marginal to an orogenic 

belt (thrust belt), toward which the rocks of the 

belt were thrust (from Bates and Jackson, 1980). 

graben An elongate, relatively depressed crustal 

unit or block that is bounded by faults on its long 

sides (Bates and Jackson, 1980). 

karst A type of topography that is formed in 

limestone, gypsum, and other rocks by dissolution, and that is characterized by sinkholes, caves, 

and underground drainage (Bates and Jackson, 

1980). 

kerogen The disseminated organic material in 

sedimentary rocks that is insoluble in nonoxidizing 

acids, bases, and organic solvents (Hunt, 1979). 

miogeocline A prograding wedge of shallowwater sediment at the continental margin (from 

Bates and Jackson, 1980). 

sabkha An environment of sedimentation of 

evaporite-salt, tidal-flood, and wind-sorted deposits just above normal high tide level, occurring 

under arid to semiarid very hot climatic conditions 

on restricted coastal plains (from Bates and 

Jackson, 1980). 

shelf A stable area that was periodically flooded 

by shallow marine waters and received a relatively thin cover of sediments (from Bates and 

Jackson, 1980). 

thermally mature That attribute of source rock 

which pertains to its thermal history; geothermally heated such that oil generation has occurred. 

thermogenic Generated by thermal cracking 

processes that result from increased temperature 

and geologic time. 
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Price, N.J., and McClay, K. R., eds., Thrust and nappe 

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G19 

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G20 

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G21 

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