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)
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Overview of the Bakken (Courtesy of David L. Debertin): Click Here