The Bakken formation, initially described by geologist J.W. Nordquist in 1953, is a rock unit from the Late Devonian to Early Mississippian age occupying about 200,000 square miles of the subsurface of the Williston Basin, covering parts of Montana, North Dakota, and Saskatchewan,

There are significant reservoirs of oil in the Bakken shale.  Oil was first discovered in the Bakken in 1951, but efforts to extract it have historically met with difficulties.  An April 2008 USGS report estimated the amount of technically recoverable oil in the Bakken Formation at 3.0 to 4.3 Billion Barrels.

(Introduction Courtesy of Wikipedia)

Read all About Bakken Geology, Production, etc:  Click Here