The Bakken shale in North Dakota, Montana and nearby states holds twice as much oil as was projected five years ago.
That new assessment was released today by the U.S. Geological Survey.
The formation holds an estimated 7.4 billion barrels, the agency said.
In 2008, the agency had projected oil reserves in the Bakken at 3 billion to 4.3 billion barrels, Bloomberg News reported.
The new review included the deeper-below-ground Three Forks formation for the first time and it is estimated to include 3.73 billion barrels.
"The Three Forks is up and coming," said USGS staffer Brenda Pierce. The formation, she said, "was the big unknown."
North Dakota is the No. 2 oil state behind Texas.
The Bakken has produced about 450 million barrels of oil since 2008, the federal agency said.
Chesapeake Energy Corp,the Oklahoma-based firm is the No. 1 driller in Ohio.
Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.