Geologic and Hydrologic Controls on Coalbed Methane: Sand Wash Basin, Colorado and Wyoming - August 1993

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Coal resources occur mainly in the Upper Cretaceous Williams Fork Formation and Paleocene Fort Union Formation in the eastern part of the Sand Wash Basin. These coals are mostly subbituminous to high-volatile B bituminous rank and have average gas contents of less than 200 ft3/ton (<6.24 m3/t). Coalbed methane resources total 101 Tcf (2.86 Tm3). The basin's cumulative gas/water ratio is approximately 15 ft-1/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 and southeastern margins of the basin. Prospective Williams Fork and Fort Union coals lie basinward, northwest of Craig, Colorado, in association with the Cedar Mountain fault system and westward along the Cherokee Arch, respectively. 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. High productivity is thought to require ground-water flow through coals of high gas content orthogonally toward no-flow boundaries and conventional trapping of gas along them.


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