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 through Early Permian age. The lower part of the Paleozoic section is a sequence of marine sandstone, shale, and minor limestone, ranging in age from Middle Cambrian through Middle Ordovician. Some porous sandstone beds occur in this section, mainly in the eastern and southern bordering areas of the Williston basin and Central Montana trough. Upper Ordovician through middle Upper Mississippian rocks are primarily carbonate beds, which contain numerous widespread cyclic interbeds of evaporite and fine-grained clastic deposits. Carbonate mounds or banks were deposited through most of this time in the shallowwater areas of the Williston basin and northern Rocky Mountains. Porous units, mainly dolomite or dolomitic limestone, are common but discontinuous in most of this sequence, and are more widespread in the eastern and southern margins of the Williston basin.
The upper Paleozoic beds are dominated by clastic rocks, beginning with the green and gray marine shales, marine carbonates, red beds, and some evaporites of the Upper Mississippian Big Snowy Group, and terminating with relatively thick marine and eolian sandstones and widespread red bed and evaporite facies of Pennsylvanian and Permian age. The Big Snowy Group is present only in the Central Montana trough and the central part of the Williston basin. Pennsylvanian and Permian beds, where present, unconformably overlie the Big Snowy Group, and overlie Mississippian or Devonian rocks in most of the remainder of the Northern Great Plains and northern Rocky Mountains, pinching out the Upper Pennsylvanian and Lower Permian section in Wyoming, southeastern Montana, northwestern South Dakota, and southwestern North Dakota.
Cumulative petroleum production (January 1982) in the United States part of the Williston basin was about 1.1 billion bbl of oil and 1.6 tcf gas. Estimated remaining recoverable reserves are about 400 million bbl of oil and 0.8 tcf 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 gas.