It seems like every day there are more and more resources for information on the Bakken Oil Formation, a large oil reserve found in western North Dakota, Montana and Manitoba.  One of the most recent is a study published by U.S. Geological Survey.  The fact sheet can be viewed here. 

Another good resource for information on the geology of the Bakken formation can be found in an Oil and Gas Tidbits newsletter published by the North Dakota Petroleum Council.