The U.S. Geological Survey (USGS) last week sharply revised downward its estimate of conventional, undiscovered oil within the National Petroleum Reserve in Alaska (NPRA) after "recent exploration drilling" revealed "gas occurrence rather than oil in much of NPRA."

The new estimates, revised from 2002 numbers, now put undiscovered oil reserves in NPRA and adjacent state waters at 896 million bbl, which is nearly 10% of USGS's estimate eight years ago, which was 10,560 million bbl.

Undiscovered gas reserves also are lower in the latest assessment. The USGS determined that there is an estimated 53 Tcf of "conventional, undiscovered nonassociated gas" within NPRA and nearby state waters. That's about 8 Tcf, or 13% less than the 2002 estimate of 61 Tcf.

"This slight reduction mostly reflects (1) the elimination of four gas-prone plays from quantitative assessment as conventional resources, (2) a substantial shift of resources from oil to gas in Beaufortian stratigraphic plays, and (3) a moderate shift of resources from oil to gas in Brookian stratigraphic plays," said the report.

The updated assessment used the same 24 plays and methodology as the 2002 assessment, USGS stated. The figures are mean estimates of fully risked, undiscovered, technically recoverable oil and gas resources of "priority petroleum basins" in the United States.

"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," said the USGS report. "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."

The Alpine Field, considered the largest onshore oil discovery in the United States in the past 25 years, is in the NPRA. Most of the wells drilled during recent NPRA exploration, whose initial drilling season occurred in 2000, are within 50 miles of the Alpine oilfield and targeted the Alpine sandstone, the field's main reservoir.

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

The largest potential for undiscovered nonassociated gas resources is in structural plays in southern NPRA, the assessment found. "Also, significant potential for nonassociated gas resources is now shifted to Beaufortian and Brookian stratigraphic plays in northern NPRA, which previously were considered primarily oil-prone."

More information on the assessment is available at http://pubs.usgs.gov/fs/2010/3102.

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