Print this pageOverview of a Giant Tight-Gas Accumulation, Mesaverde Group, Piceance Basin, Colorado
A regionally extensive tight-gas accumulation in the Mesaverde Group of the Piceance Basin is currently being actively developed. Daily production has increased from under 200 MMCFD in the year 2000 to over 1 BCFD currently. Most gas production in the Piceance Basin is from discontinuous fluvial sandstones of the Williams Fork Formation of the Mesaverde Group. In some areas of the southern Piceance Basin, 10-acre well density has proven successful. Typical wells’ EURs in these areas range from 1 to 2 BCF per well, resulting in reserves of about 60-120 BCF per section. The depth limits to the commercial gas accumulation are poorly defined, but it is possible that much of the deeper part of the basin may have commercial gas reserves. Within the area of commercial gas production, most gas is produced from a continuously gas-saturated interval in the Williams Fork. Productive intervals can attain gross thicknesses of over 3000 ft (900 m). The gas-saturated interval thins toward the basin margins where the Williams Fork gas reserves become sub-economic. This tremendous gas resource exists because of several important geologic circumstances. Large volumes of gas were generated from thick Mesaverde coals as they achieved high thermal maturity. Migration of this gas was inhibited by the very low permeability and discontinuous nature of the Mesaverde sandstone reservoirs. The rate at which gas was generated and accumulated in the reservoirs outpaced by the rate at which gas could escape, resulting in overpressure. Eventually, the pressure of the gas phase in the pore system exceeded the capillary pressure of the water-wet pores, and water was expelled from the pore system, resulting in the development of an overpressured, gas-saturated reservoir with little movable water.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009