Geologic and Hydrologic Controls on Coalbed Methane: Sand Wash Basin, Colorado and Wyoming

Coalbed methane production has been established in the Sand Wash Basin. Large coal resources, gas shows during drilling of coal beds, and high gas contents in some coals triggered initial development along the basin margins. Results to date have been disappointing. Coalbed wells have yielded little gas and large volumes of water. In the absence of a regional analysis, neither production data nor the basin's ultimate coalbed methane potential could be fully evaluated. Thus, the need arose for an integrated geologic and hydrologic study of the basin to provide the framework for evaluating development properties and the rationale for future exploration.Large coal resources occur in the Upper Cretaceous Williams Fork Formation and lower Tertiary Fort Union Formation in the eastern part of the Sand Wash Basin. These coals are mainly subbituminous to high-volatile B bituminous and have average gas contents of less than 200 ft3/ton. (>6.24 m3/t). Coalbed methane resources total 101 Tci (2.86 Tm3) and are 24 Tcf (680 Bm3) at shallow drilling depths of less than 6,000 ft (>1.830 m). More than 87 percent of them are in the Williams Fork. The basin's cumulative gas:water ratio is approximately 15 ft3/bbl (2.7 m3/m3). To date, low gas content and high water production have limited coalbed methane activity in the basin. Steep structural dip and coal distribution have restricted exploration to the eastern margins of the basin. Prospective Williams Fork and Fort Union coals, respectively, lie basinward in association with the Cedar Mountain fault system and westward along Cherokee Arch into the Powder Wash field area. High productivity requires that permeability, ground-water flow direction, coal distribution and rank, gas content, and structural grain be synergistically combined. That synergism explains prolific and marginal production in the San Juan and Sand Wash Basins, respectively. On the basis of a comparison between the basins, a basin-scale coalbed methane producibility model is proposed whose essential elements are: ground-water flow through thick coals of high rank and high gas content orthogonally toward no-flow boundaries and conventional trapping of gas along them.