Williston Basin

The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North Dakota, South Dakota, southern Saskatchewan, and south-western Manitoba that is known for its rich deposits of petroleum and potash. The basin is a geologic structural basin but not a topographic depression; it is transected by the Missouri River. The oval-shaped depression extends approximately 475 miles (764 km) north-south and 300 miles (480 km) east-west.

The Williston Basin lies above an ancient Precambrian geologic basement feature, the Trans-Hudson Orogenic Belt that developed in this area about 1.8-1.9 billion years ago, and that created a weak zone that later led to sagging to produce the basin. The Precambrian basement rocks in the center of the basin beneath the city of Williston, North Dakota lie about 16,000 feet (4,900 m) below the surface.

Deposition of sediments began in the Williston area during Cambrian time, but subsidence and basin filling were most intense during the Ordovician, Silurian, and Devonian Periods, when thick accumulations of limestone and dolomite, with lesser thicknesses of sandstones, siltstones, shales, and evaporites were laid down. Subsidence continued on a reduced scale into the Mississippian and was largely ended by Pennsylvanian time. Regional subsidence returned during the Mesozoic Era, although total sediment thicknesses were much less than during the Paleozoic. Near the end of the Cretaceous, tectonic activity during the Laramide Orogeny rejuvenated several basement structures in the Williston Basin to produce anticlines that serve as oil traps today.

The long history of sedimentary deposition in the Williston Basin included deposition of rocks well suited to serve as hydrocarbon source and reservoir rocks. The basin's oil and gas fields are found in a wide range of geologic ages, as indicated by the generalized stratigraphic column.

In 1913, shallow gas was discovered in an Upper Cretaceous sandstone on the Cedar Creek Anticline, and oil was discovered on the same anticline in 1936. The basin did not become a major oil province until the 1950s when large fields were discovered in North Dakota. Amerada Corporation began the search in 1946. After four years of testing and mapping they started drilling at a promising lease 30 miles north-east of Williston, North Dakota, and on April 4, 1951, discovered oil on the Nesson Anticline, with the Amerada Hess Corporation Clarence Iverson #1 well. Other oil firms rushed in to buy up leases on farm land to explore for oil and by 1954 80% of the possible oil producing areas were under lease. Royal Dutch Shell at that time had leases on over 8 million acres. Many local farmers and area speculators became instant millionaires, leasing land at an average of $25 an acre and then selling those leases back at a much higher cost per acre.

Production peaked in 1986, and tehn decreased. In the early 2000s significant increases in production began because of horizontal drilling techniques (fracking), especially in the Bakken Formation.

Cumulative basin production totals about 3.8 billion barrels (600,000,000 m³) of oil and 470 billion cubic feet (1.3×10¹⁰ m³) of natural gas. The largest oil fields are listed in the following table, showing estimated ultimate recoveries.

Potash produced from the Williston Basin makes Canada the world's leading producer of that commodity. The deposits occur in the Middle Devonian Prairie Formation. Major potash-producing companies include Nutrien and Mosaic.