Leigh Price, a USGS scientist, authored a study before his death in 2000 estimating that the entire formation, which extends into Saskatchewan and Manitoba, may hold up to 500 billion barrels of recoverable oil, an amount that dwarf's the 16 billion barrels in the Arctic National Wildlife Refuge (ANWR).
No, it was 200 to 400 billion barrels in place.
His work was included in a USGS report released the year after his death, 2001.
Diagenesis and Fracture Development in the Bakken Formation, Williston Basin: Implications for Reservoir Quality in the Middle Member
http://pubs.usgs.gov/pp/p1653/p1653.pdf
U.S. Geological Survey Professional
By Janet K. Pitman, Leigh C. Price, and Julie A. LeFever
First printing November 2001
Technically recoverable oil is a lot less.
Report says up to 4.3 billion barrels of oil in Bakken shale
http://www.freerepublic.com/focus/f-news/1999840/posts
ANWR is expected to have techically recoverable oil up to 16 billion barrels, a mean of 10.4 billion.
Arctic National Wildlife Refuge, 1002 Area,
Petroleum Assessment, 1998, Including Economic Analysis
http://pubs.usgs.gov/fs/fs-0028-01/fs-0028-01.pdf