ABSTRACT: Characteristics and Development of Mesaverde Group Reservoirs, Grand Valley Field, Northwest Colorado

REINECKE, KURT M., Barrett Resources Corporation, Denver, CO, and DUDLEY D. RICE and RONALD C. JOHNSON, U.S. Geological Survey, Denver, CO

Two major gas reservoirs are being developed in the Upper Cretaceous Mesaverde Group in the area of Parachute, Colorado. Gas is produced by dual-completion techniques from Cameo coalbeds and lenticular, fluvial sandstone reservoirs in the Williams Fork Formation. A good mud log is the best qualitative indicator of the potential of a sandstone reservoir. Gas productivity from low-volatile bituminous Cameo coalbeds is less predictable, and no mappable parameters have been identified to explain the variations.

Gases from coal and overlying (non-Cameo) sandstone reservoirs generally can be distinguished by chemical composition. Gases from Cameo coals are chemically drier and contain a larger proportion of carbon dioxide compared to gases from the sandstones. However, the fact that many gases are mixtures of the two types suggests some contribution to the sandstone reservoirs by expelled coal-generated gases. The greater wetness of coal gases in the area, compared to other areas of production, indicates that, as a result of different maceral and/or chemical composition, the coal may be richer in hydrogen.

Wells produce relatively low volumes of sodium-chloride-type water, which usually is associated with sandstone reservoirs. The occurrence of such water in coalbeds, instead of sodium bicarbonate type, indicates either contamination by treatment fluids or a past hydrologic communication with the sandstones.

Although ultimate reservoir performance can be predicted, coal reservoirs cannot conclusively be distinguished from sandstone reservoirs on the basis of short-term decline characteristics. However, sandstone reservoirs are prone to brief periods of flush production after shut in, whereas coal reservoirs generally are not.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)