The U.S. Geological Survey found that there are around 896 million barrels of undiscovered oil in the National Petroleum Reserve in Alaska, about 90 percent less than 2002 estimates.
The USGS said the updated estimate is based on new data from exploratory drilling in Alaska that revealed more gas in the reserve and in adjacent waters.
"These new findings underscore the challenge of predicting whether oil or gas will be found in frontier areas," USGS Director Marcia McNutt said in a statement. "It is important to re-evaluate the petroleum potential of an area as new data becomes available."
The USGS said its surveys of the land found that parts of the reservoir were eroded more than 15 million years ago, which change pressure conditions and displaced much of the oil into poor quality reservoir rocks. This caused natural gas to displace much of the oil.
Gas estimates in the NPRA, however, were revised from 61 trillion cubic feet to 8 trillion cubic feet of undiscovered, conventional and non-associated gas.
The NPRA is federal land that lies west of the Arctic National Wildlife Refuge in the North Slope of Alaska.