An Analysis of the Coal-Seam Gas Resource of the Piceance Basin, Colorado

Abstract

Summary A detailed geologic analysis of the Piceance basin in northwestern Colorado shows that nearly 84 Tcf [2.4 × 10 M] of coal-seam gas is in place in three target coal groups. The Cameo coal group contains the most coalbed methane with 65 Tcf [1.8 X 10 M ]. The more areally limited Coal Ridge and Black Diamond coal groups contain significantly less gas, 10 and 9 Tcf [280 × 10 and 255 × 10 M], respectively. The areas of highest methane concentration are in the east-central portion of the Piceance basin. These areas coincide with thick deposits of high-rank coals at significant depths and their associated higher gas contents. Also, these areas appear to have been structurally (tectonically) altered, leading to enhanced permeability to gas and water. Thus, the east-central basin area appears favorable for coalbed methane production. This study relied on extensive well data to correlate and map the subsurface extent of the Cretaceous coals of the Piceance basin. Newly derived correlations of coal-rank/depth with gas content were used along with estimates of coal volume to determine gas in place. Introduction Coal-seam gas-natural gas that has evolved from and is adsorbed by coal seams-is a major potential source of domestic natural gas, The majority of this resource is contained in 13 coal basins distributed broadly across the U.S. (Fig. 1). The Piceance basin, located in northwestern Colorado, is documented as one of the gassiest coal regions in the U.S. Currently, the basin is the site of the Gas Research Inst. (GRI) Deep Coal Seam Research Project and, at the end of 1985, contained 32 wells drilled specifically for coalbed methane extraction. A detailed understanding of the regional distribution of the geologic parameters controlling coal-seam gas accumulation and flow is a key factor for economical gas recovery. This paper presents the results of a detailed subsurface geologic analysis of the three major Piceance basin coal groups-the Black Diamond, Cameo, and Coal Ridge of the Iles and Williams Fork formations of the Mesaverde group. From extensive well data, the lateral and vertical extent of the Late Cretaceous Age coals were delineated to estimate coal volumes. These estimates were combined with newly derived correlations of coal-rank/depth with gas content to calculate gas in place for the basin's three major coal groups. The geologic analysis and resource estimate of the Piceance basin coals were sponsored by GRI's Natural Gas Supply Program. The analysis was undertaken as part of a larger resource economics study to determine the economically recoverable natural gas from U.S. coal basins, particularly with advanced extraction technologies. Study Methodology The estimate of coal-seam gas in place for the Piceance basin relied on a basic geologic evaluation of the subsurface geometry of the basin's three major coal groups. The two foundations of the Piceance basin geologic analysis are the regional cross sections correlating coal groups across the basin and the stratigraphic data from 190 well logs.