Petroleum potential of wilderness lands in Montana

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 Petroleum Potential of Wilderness Lands in Montana

GEOLOGIC FRAMEWORK

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FIGURE 4.-Correlation chart of pre-Tertiary stratigraphic units of the western flank of the Sweetgrass arch and adjacent east- ern 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 Limes- tone and overlying Castle Reef Dolomite which includes the Sun River Dolomite Member (Mudge, 1972a).

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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 em-

G7 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.

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

STRATIGRAPHIC SUMMARY

CENTENNIAL J3LACKT AIL -SNOWCREST BASIN UPLIFT

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z R 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, 1974) 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 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.

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

G11 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). 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).

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).

G13 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).

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.

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).

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~s gressive 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 de-

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).

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 Yellowstone 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 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 Birdbear 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 post-Paleozoic 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

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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