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

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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 100 to 200 ft3/ton (<3.12 to 6.24 m3/t). Coalbed methane resources total xx Tcf (x.xx Tm3) and are xx.xx Tcf (xxx Bm3) at drilling depths of less than 6,000 ft (<1,830 m). More than 85 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 productivity model is proposed whose essential elements are: ground-water flow through thick coals of high rank, perpendicular to no-flow boundaries and conventional trapping along them.


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