Originally Posted By: trophymac
The Energy Information Administration (EIA) estimates it at 503 billion barrels.....

However, a recent technological breakthrough has opened up the Bakken's massive reserves..... and we now have access of up to 500 billion barrels.


From your USGS link:
3 to 4.3 Billion Barrels of Technically Recoverable Oil Assessed in North Dakota and Montana’s Bakken Formation

A little short of 500 billion barrels. There's also more on snopes.com
http://www.snopes.com/politics/gasoline/bakken.asp