Most Williston basin oils belong to one of three basic types: (1) a lower Paleozoic type believed to have originated in Winnipeg shales and found predominantly in Ordovician and Silurian reservoirs; (2) a Mississippian type expelled from Bakken shales and produced mostly from Madison reservoirs; and (3) a Pennsylvanian type restricted to the Tyler Formation. The three types are isolated vertically by evaporites in basin-margin areas.

All three oil types can be correlated to their source facies. Lithofacies maps of these source sequences, when combined with thermal alteration measurements, provide effective oil-source-area definition. The time of expulsion and amount of expelled oil from each source rock can be estimated when its volume, organic content, and thermal history are known. Paleostructure maps and carrier-bed isopach maps indicate the direction and extent of secondary oil migration, both vertically and horizontally from the source facies. Ultimately, the subsurface distribution of each oil type is predicted in order to define high-grade areas in which to concentrate exploration activity.