October 27, 2010
Recent activity in 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 geologic information that is more indicative of gas than oil. Many of the newly drilled wells show an abrupt transition from oil to gas just 15 to 20 miles west of the giant Alpine field, located just outside the northeastern boundary of NPRA. “These new findings underscore the challenge of predicting whether oil or gas will be found in frontier areas,” explains USGS Director Dr. Marcia McNutt. “It is important to re-evaluate the petroleum potential of an area as new data becomes available.” The new assessment also indicates 8 trillion cubic feet less gas than the 2002 USGS estimate of 61 trillion cubic feet of undiscovered, conventional, non-associated gas. Conventional gas refers to gas in discrete accumulations, while non-associated means there is little to no crude oil in the reservoir. New geologic analysis by the USGS provides an explanation for the unanticipated predominance of gas in much of NPRA. It is likely that oil formed approximately 90 million years ago in many reservoirs in northern NPRA and gas in many reservoirs in southern NPRA. Subsequently, 15-60 million years ago, many parts of NPRA were uplifted and eroded, thereby changing the pressure conditions in the subsurface. In areas of modest uplift and erosion, gas that was dissolved in oil came out of solution, forming gas caps and displacing oil downward into poorer quality reservoir rocks. In areas where uplift and erosion were significant, the gas present in the subsurface expanded so much that it completely displaced oil from reservoirs. The variable nature of the uplift and erosion resulted in some reservoirs retaining their oil, but the majority of the reservoirs instead became gas bearing. NPRA has been the focus of significant oil exploration during the past decade, stimulated by the mid-1990’s discovery of the largest onshore oil discovery in the U.S. during the past 25 years, in the Alpine field. The estimates cited above are mean estimates of fully risked, undiscovered, technically recoverable oil and gas resources. The USGS conducted this assessment as part of a program directed at estimating the undiscovered, technically recoverable oil and gas resources of priority petroleum basins in the United States.
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