You need to include the Three Forks as much of the new production is coming from that layer as well. It has been raised again to 7.4 billion barrels.
Bakken, Three Forks resources rise twofold in new USGS estimate
http://www.ogj.com/articles/print/volume-111/issue-5/general-interest/bakken-three-forks-resources-rise-twofold.html
05/06/2013
The latest assessment, which includes the Three Forks for the first time, found the Bakken has a 3.65 billion bbl estimated mean resource—unchanged from 5 years ago—and Three Forks has an estimated mean 3.73 billion bbl. The formations’ combined estimate ranges from 4.42 million bbl, with a 95% chance of production, to 11.43 billion bbl, with a 5% chance.
That’s why I used the low 3.6 billion number, and added the codicils about new deposits, improved tech, and improved efficiencies. =)