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.