Williston Basin

The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North and South Dakota, and southern Saskatchewan 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 town of Williston, North Dakota lie about 16,000 feet (4,900 m) below the surface.




Stratigraphic chart showing 12 hydrocarbon source beds and 15 reservoir rocks in the Williston Basin
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, siltsones, 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. 



Natural resources


Petroleum


Annual oil production in the US part of the Williston Basin (ND=North Dakota, SD=South Dakota, MT=Montana)
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.
Oil was first found in the Williston Basin along the Cedar Creek Anticline in southeastern Montana, in the 1920s and 1930s. The basin did not become a major oil province until the 1950s when large fields were discovered in North Dakota. Production peaked in 1986, but in the early 2000s significant increases in production began because of application of horizontal drilling techniques, 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.
Field Name Discovery Year Est. ult. oil
(million barrels) Est. ult. gas
(billion cubic feet) Est. ult. nat. gas liquids
(million barrels)
Elm Coulee, Richland Co.,MT[9] 2000 270
Beaver Lodge, N. Dak. 1951 130 115 47
Pine, Mont. 1952 127 20
Pennel, Mont. 1955 115
Cabin Creek, Mont. 1953 115
Little Knife, N. Dak. 1977 96 120 15
Tioga, N. Dak. 1952 77 43 17
Blue Buttes, N. Dak. 1955 53 36 5
Charlson, N. Dak. 1952 52 100 11




Potash


Potash produced from the Williston Basin makes Canada the world's leading producer of that commodity. Major potash-producing companies include the Potash Corporation of Saskatchewan.


Coal


The Williston Basin holds large coal deposits, primarily in the Fort Union Formation of mostly Paleogene age.



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