Using a geology-based assessment methodology, the U.S. Geological Survey estimated mean volumes of 896 million barrels of oil (MMBO) and about 53 trillion cubic feet (TCFG) of nonassociated natural gas in conventional, undiscovered accumulations within the National Petroleum Reserve in Alaska and adjacent State waters. The estimated volume of undiscovered oil is significantly lower than estimates released in 2002, owing primarily to recent exploration drilling that revealed an abrupt transition from oil to gas and reduced reservoir quality in the Alpine sandstone 15-20 miles west of the giant Alpine oil field. The National Petroleum Reserve in Alaska (NPRA) has been the focus of oil exploration during the past decade, stimulated by the mid-1990s discovery of the adjacent Alpine field-the largest onshore oil discovery in the United States during the past 25 years. Recent activities in NPRA, including extensive 3-D seismic surveys, six Federal lease sales totaling more than $250 million in bonus bids, and completion of more than 30 exploration wells on Federal and Native lands, indicate in key formations more gas than oil and poorer reservoir quality than anticipated. In the absence of a gas pipeline from northern Alaska, exploration has waned and several petroleum companies have relinquished assets in the NPRA. This fact sheet updates U.S. Geological Survey (USGS) estimates of undiscovered oil and gas in NPRA, based on publicly released information from exploration wells completed during the past decade and on the results of research that documents significant Cenozoic uplift and erosion in NPRA. The results included in this fact sheet-released in October 2010-supersede those of a previous assessment completed by the USGS in 2002.