Western Tight Gas Reservoirs--Resources for Future

Charles W. Spencer, Ben E. Law, Ronald C. Johnson, Robert A. Crovelli

Unconventional, low-permeability (tight) gas reservoirs in the western United States are currently producing about 1 trillion ft³ of gas per year. This volume supplies only about 5% of the United States market. Gas from tight reservoirs is relatively high cost because of completion costs (hydraulic fracturing) and relatively low daily production rates per well. Presently depressed wellhead gas prices have caused a recent decrease in tight gas exploration and development. However, some companies with forward-looking management policies are positioning themselves for coming future increases in the demand for gas. Several recent estimates of United States undiscovered conventional gas indicate decreased volumes of gas from these sources. These reduced estimates un erscore the future importance of unconventional gas sources.

Recent detailed analyses of the gas potential of the tight sandstone reservoirs in the Piceance and Greater Green River basins of Colorado and Wyoming by the U.S. Geological Survey (USGS) indicate a more optimistic assessment of the volume of gas in place in these basins than had previously been estimated by the National Petroleum Council in 1980. The gas resource assessment procedure used by the USGS consisted of a modified play and aggregation analysis using a reservoir engineering model and an analytic probabilistic methodology. Basin-center gas-bearing intervals were subdivided into plays, and the following input data were prepared for each play: (1) depths to top and bottom of gas-bearing interval, (2) isopachs of gas reservoir thickness, (3) ranges of porosity and gas saturation, and (4) pressure and temperature gradients. Percent recoveries of gas in place were estimated using various price and technology scenarios. The input data were developed by the USGS after several years of cooperative research with the U.S. Department of Energy and industry.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.