The government estimates up to 4.3 billion barrels of oil can be recovered from the Bakken shale formation in North Dakota and Montana, using current technology.
The U.S. Geological Survey calls it the largest continuous oil accumulation it has ever assessed.
An assessment by USGS in 1999 found the Alaska National Wildlife Refuge had 10.3 billion barrels of recoverable oil, said Brenda Pierce, a geologist for the agency
The Bakken Formation encompasses some 25,000 square miles in North Dakota, Montana, Saskatchewan and Manitoba.
About two-thirds of the acreage is in western North Dakota, where the oil is trapped in a thin layer of dense rock nearly two miles beneath the surface.
Companies use pressurized fluid and sand to break pores in the rock and prop them open to recover the oil.
Donald Kessel, vice president of Houston-based Murex Petroleum Corp., said he believes the Geological Survey's assessment of how much oil can be recovered in the Bakken may be a little on the high side.
"That's a lot of zeros," Kessel said Thursday.
Kessel said his company was the first to get a producing well in the Bakken in North Dakota three years ago. The company now has about 20 producing wells.
The report released Thursday by USGS was done at the request of Sen. Byron Dorgan, D-N.D., over the past 18 months.
A study by the USGS in 1995 found 151 million barrels of oil could be recovered from the Bakken using technology at that time.
"This is great news," Dorgan said of the new report. "This is 25 times the amount of the previous assessment."