Regional Stratigraphy and General Petroleum Geology of the U.S. Portion of the Williston Basin and Adjacent Areas

Paleozoic sedimentary rocks in the northern Great Plains and northern Rocky Mountain region include a sequence of dominantly shallow water marine carbonate, clastic, and evaporite deposits of Middle Cambrian-Early Permian age. The lower part of the Paleozoic section is a Middle CambrianMiddle Ordovician sequence of marine sandstone, shale, and minor limestone with some porous sandstone beds located along the eastern and southern margins of the Williston basin and Central Montana trough. Upper Ordovician-Upper Mississippian rocks are carbonate beds containing numerous evaporite and fine-grained clastic deposits. Carbonate mounds or banks were deposited throughout most of this time in the shallow water areas of the Williston basin and northern Rocky Mountains. Porous units, mainly dolomite or dolomitic limestone, are common but discontinuous and are more widespread along the eastern and southern margins of the basin. Upper Paleozoic beds are dominated by clastic rocks, beginning with marine shales, marine carbonates, red beds, and some evaporites of the Upper Mississippian Big Snowy Group, and terminating with thick marine and eolian sandstones and widespread red bed and evaporite deposits of Pennsylvanian-Permian age. The Big Snowy Group is present only in the Central Montana trough and the central part of the Williston basin. Pennsylvanian-Permian beds unconformably overlie the Big Snowy Group and overlie Mississippian or Devonian rocks in the remainder of the northern Great Plains and northern Rocky Mountains, pinching out northward in central Montana. Thick porous sandstone beds are present in the Upper Pennsylvanian-Lower Permian section in Wyoming, southeastern Montana, and western North and South Dakota. Triassic marine and continental red shale, siltstone, sandstone, and some evaporites overlie Permian rocks. Some discontinuous porous sandstones are present, mostly along the southern border of the Williston basin. Jurassic and Cretaceous rocks are primarily marine clastic beds that were partly removed by Cenozoic erosion except in the basin center. Tertiary lignite- and coal-bearing continental clastic rocks are present in the Williston basin, the main basin areas of Montana, and the central part of the Powder River basin. Important regional unconformities occur in the Paleozoic sequence at the bases of the Cambrian, Middle Ordovician, Devonian, Pennsylvanian, and Permian systems. A major unconformity occurs at the base of the Middle Jurassic, which overlies upper Paleozoic or Triassic rocks in the area. It is particularly prominent in northcentral Montana where pre-Jurassic erosion has removed the upper part of the Madison Limestone and younger Paleozoic beds. Cumulative petroleum production (January 1984) in the United States part of the Williston basin is about 1.14 billion bbl of oil and 1.8 tcfof gas. Estimated remaining recoverable reserves are about 350 million bbl of oil and 0.8 tcfof gas. U.S. Geological Survey 1980 estimates of undiscovered recoverable oil and gas resources are about 900 million bbl of oil and 3.5 tcf of gas.