A review of the literature indicates that the stratigraphic and structural history of the oil-producing areas of North Dakota has been well documented. Approximately 92 percent of original oil in place that has been discovered has been in carbonate reservoirs, 70 percent in structural traps, and more than 90 percent in rocks of Paleozoic age. The Williston basin was an active structural element from mid-Ordovician time until at least Late Cretaceous time. Commercial production has been established from 13 separate formations in the Cambrian, Ordovician, Silurian, Devonian, Pennsylvanian, Triassic, and Cretaceous Systems. Approximately 20 percent of the fields produce from more than one formation. Sedimentary rocks range in thickness from approximately 1,000 ft (305 m) in he eastern part of the study area to more than 15,000 ft (4,570 m) in the deepest part of the basin, and carbonate rocks are the dominant type. Structures are of low relief and amounts of closure are small. Recovery rates exceed original estimates, and secondary recovery projects have been very successful. Much of the area is unexplored. A proper combination of good geologic information, structural data, and imagination should lead to the discovery of much more oil.