The Williston basin is an elliptical cratonic basin located in central and western North Dakota, eastern Montana, and northwestern South Dakota in the United States and in southern Manitoba and Saskatchewan in Canada. Formed in Late Cambrian or Early Ordovician time, the Williston basin was first transgressed by Phanerozoic sediments during Upper Cambrian time. Structures range from sharp fault folds to gentle, broad arches and clusters of small closed structures. Present sedimentary rock thickness approximates 16,000 ft (4900 m). All of the major faults and fault folds have been related to a hypothetical Proterozoic wrench-fault system.

Basin rocks are characterized by pre-Pennsylvanian carbonate deposits arranged in sequence packages of varying scales and by post-Mississippian detrital and evaporite deposits. Large volumes of evaporites are associated with the carbonates. A few siliciclastic deposits are also present in the pre-Pennsylvanian stratigraphic section.

Petroleum production started in Montana in the 1920s. Saskatchewan has produced the most oil, approximately 1 billion barrels to date, followed by North Dakota (840 MBO), Montana (622 MBO), Manitoba (166 MBO), and South Dakota (16.5 MBO). Mississippian rocks form the major producing reservoirs. Traps are mostly combination and structural.