Geologic Characterization and Coalbed Methane Occurrence: Williams Fork Formation, Piceance Basin, Northwest Colorado

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The coal-bearing Upper Cretaceous Williams Fork Formation, 1,200 to 2,500 ft thick, is operationally defined on the basis of correlation with the Sand Wash Basin. Net coal thickness is typically 80 to 120 ft and is thickest in a north-south belt west of the Divide Creek Anticline. Depositional setting and thrust faults cause coals along the Grand Hogback and in the subsurface to be in modest to poor hydraulic communication. Thus, meteoric recharge and flow basinward is restricted. Face cleats of Late Cretaceous age strike east-northeast and west-northwest in the southern and northern parts of the basin, respectively, normal to the Hogback thrust front. Parallelism between face-cleat strike and present-day maximum horizontal stress direction may enhance coal permeability in the north. Lineament azimuths lie between 20 to 40° and 280 to 310°; they are not a reliable indicator of subsurface fracture attributes nor of gas production. In the Grand Valley/Rulison and White River/Pinyon Ridge areas, structure and sandstone development control gas production from Cameo coals and/or sandstones. The most productive wells are on structural terraces and anticlines or correspond to Cameo sandstone development, reflecting fracture-enhanced permeability. As predicted, from an evolving coalbed methane producibility model, extraordinary coal-gas production is precluded by the absence of dynamic groundwater flow. The best potential for coal-gas production may lie in conventional traps basinward of where outcrop and subsurface coals are in good hydraulic communication.


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