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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 Inves­ tigations 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 cated east of the Helena salient and including Wilderness Lands in Montana, totalling 6,930,433 acres, are Mesozoic uplifts and basins superimposed on the subdivided for this report into 34 clusters covering 5,066,893 early Paleozoic Central Montana trough and the acres for the assessment of petroleum potential based on adjacent northern margin of the Paleozoic Wyom­ widely varying geologic conditions; 38,193 acres classified as ing shelf. The fifth is the southern Montana part having unknown potential; and 1,825,347 acres assessed as having zero potential. of the Wyoming province, a series of major The presence or absence of possible source rock, reservoir Laramide uplifts and adjoining basins across the rock, adequate thermal history, and structural or stratigraphic southern part of Montana from the southwest traps for hydrocarbons are the most critical factors used in Montana recess eastward to South Dakota. qualitatively assigning a high, medium, low, or low to zero oil In this report we will first briefly discuss (1) the and (or) gas potential to each of the clusters. Montana encom­ passes parts of five major geologic provinces. The provinces geologic, particularly structural, framework of exhibit a wide range of structural and stratigraphic variations, these provinces with respect to hydrocarbon ac­ summarized in this report. cumulations or possible accumulations, (2) Mon­ tana stratigraphy, focusing on rocks that have INTRODUCTION generated hydrocarbons (as source rocks) or yielded hydrocarbons (as reservoir rocks), and (3) The State of Montana encompasses parts of five the petroleum potential of the clustered Wilder­ major geologic provinces (fig. 1). The first is the ness Lands in Montana (fig. 2). Obscure geologic southwestern part of the Williston Basin province terminology is eliminated wherever possible; some in eastern Montana. The second is the Northern scientific terms are defined within the text and in Montana Foreland (Sweetgrass Arch) province, a the accompanying glossary. complex of domes and arches in the Paleozoic Milk River uplift area of north-central Montana. The GEOLOGIC FRAMEWORK third is the Montana Thrust Belt province, that sector of the Cordilleran fold-and-thrust belt lo­ WILLISTON BASIN PROVINCE cated in western Montana. This province, because of its complexity, is subdivided from north to The Williston basin (fig. 1) is the largest deposi­ south into (1) disturbed belt and adjoining north­ tional basin of the North American continental in­ ern sector of the Montana thrust belt to the west, terior, preserving the most complete Paleozoic se­ (2) Helena salient and adjoining central sector to quence of rocks found in the continental interior the west, and (3) southwest Montana recess. The (Sandberg and Hammond, 1958; Sandberg, 1962), fourth is the Central Montana Uplift province, lo- as much as 16,000 ft of strata, and is one of the G1 110" NORTHERN MONTANA FORELAND PROVINCE MONTANA (... .... ~<II.. ................. ..._,.... BLOOD CREEK SYIICLIIIE ...." .,.~,. PORCUPINE DOME SAPPHIRE HELENA THRUST SALIENT CENTRAL MONT ANA PROVINCE PLATE BULL MOUNTAIN BASIN CENTRAL SECTOR BELT 50 100 MILES 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 NORTHERN MONTANA FORELAND (SWEETGRASS America (Dow, 1974; Rice, 1979). The northern ARCH) PROVINCE part of the Williston basin lies in Canada, the southwestern part in Montana, and the remainder The Northern Montana Foreland province (fig.1) in North and South Dakota. Precambrian to Mis­ lies between the western part of the Williston sissippian paleotectonics of the Williston basin are basin and the Montana thrust belt. During much summarized by Sandberg (1964). A composite of Paleozoic time, this province was part of the stratigraphic section (fig. 3A and 3B) for north­ broad Milk River uplift, a southern extension of eastern Montana illustrates the stratigraphic units the Paleozoic Alberta shelf (Schwartz, 1983, fig. involved in the thick Cambrian, Ordovician, Silu­ 3). The Paleozoic Milk River uplift separated the rian, Devonian, and Mississippian rock sequence Williston basin to the east from the Cordilleran of the Williston basin, many of which yield oil and miogeoclinal wedge, the eastward-thinning wedge gas (Glaze, 1971; Rice, 1979). The distribution and of sedimentary rocks deposited on the continental thickness of many of these rocks, as well as margin to the west, remnants of which are pre­ Mesozoic rock units, were controlled by intermit­ served west of the disturbed belt in extreme tent movements of structural features already in northwestern Montana. This broad positive area existence by the close of Precambrian time was partitioned in Mesozoic and early Tertiary (Sandberg, 1962). The Blood Creek syncline (fig. time, from east to west, into the Bowdoin dome, 1) is herein included within the Williston Basin igneous intrusive complexes of the Little Rocky province. This broad, shallow downwarp extends Mountains, Bearpaw Mountains and Highwood westward from the southwestern edge of the Wil­ Mountains and the Sweetgrass Arch (Alpha, 1955; liston basin, along the older northeastern margin Glaze, 1971; Lorenz, 1983). The Sweetgrass Arch of the upper Paleozoic Big Snowy trough. and Sweetgrass Hills area just east of the north- G2 0 50 MILES EXPLANATION .HIGH 1·-~:_.:... :i:::l LOW TO ZERO _/..- ~MEDIUM DzERO CLUSTER .LOW BOUNDARIES ~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 numerous accumulations of natural gas.

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