(CNN) -- The U.S. Geological Survey says a revised estimate for the amount of conventional, undiscovered oil in the National Petroleum Reserve in Alaska is a fraction of a previous estimate.

The group estimates about 896 million barrels of such oil are in the reserve, about 90 percent less than a 2002 estimate of 10.6 billion barrels.

The new estimate is mainly due to the incorporation of new data from recent exploration drilling revealing gas occurrence rather than oil in much of the area, the geological survey said.

"These new findings underscore the challenge of predicting whether oil or gas will be found in frontier areas," USGS Director Dr. Marcia McNutt said in a statement. "It is important to re-evaluate the petroleum potential of an area as new data becomes available."

The organization also estimates 8 trillion cubic feet less gas than a 2002 estimate of 61 trillion cubic feet of undiscovered, conventional, non-associated gas -- meaning gas found in discrete accumulations with little to no crude oil in the reservoir.

"Recent activity in the NPRA, including 3-D seismic surveys, federal lease sales administered by the Bureau of Land Management and drilling of more than 30 exploration wells in the area provides geological information that is more indicative of gas than oil," the geological survey said.

The petroleum reserve in Alaska has been the focus of significant oil exploration during the past decade, stimulated by the mid-1990s discovery of the largest onshore oil discovery in the U.S. during the past 25 years, the organization said.