Cumella, Stephen¹
(1) Williams Production, Evergreen, CO

ABSTRACT: Geology of the Basin-Centered Gas Accumulation, Piceance Basin, Colorado

A very large basin-centered gas accumulation in the Williams Fork Formation of the Mesa Verde Group is currently being actively developed at 10-acre density. Ten-acre density is necessary to develop a reasonable amount of the gas-in-place due to the very low (microdarcy) permeability and the highly lenticular nature of the fluvial sandstone reservoirs. Within the area of commercial gas production, gas is produced from a continuously gas-saturated interval of 1,500-3,000 feet. A transition zone of mixed gas- and water-saturated sandstones overlies the continuously gas-saturated interval. Pressure gradients, which can be as high as 0.8 psi/ft in the lower part of the Williams Fork in the structurally deeper part of the basin, decrease upward to hydrostatic gradients near the top of the continuously gas-saturated interval. Pervasive natural fracturing provides sufficient reservoir permeability to allow commercial production over 14-township area. This area is continually expanding as a result of current active exploration for this basin-centered resource. Overpressuring due to generation of large volumes of gas from interbedded coals and carbonaceous shales may have been important in fracturing the sandstones. During maximum burial and peak gas generation, overpressuring may have been maintained beneath a regionally extensive top seal in the upper part of the Williams Fork Formation.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.