Geologic and Hydrologic Controls on Coalbed Methane Producibility, Williams Fork Formation, Piceance Basin, Colorado

Roger Tyler, A. R. Scott, W. R. Kaiser, H. S. Nance, and R. G. McMurry

Structural and depositional setting, coal rank, gas content, permeability, hydrodynamics, and reservoir heterogeneity control the producibility of coalbed methane in the Piceance Basin. The coal-rich Upper Cretaceous, Williams Fork Formation is genetically defined and regionally correlated to the genetic sequences in the Sand Wash Basin, to the north. Net coal is thickest in north-south oriented belts which accumulated on a coastal plain, behind west-east prograding shoreline sequences. Face cleats of Late Cretaceous age strike E-NE and W-NW in the southern and northern parts of the basin, respectively, normal to the Grand Hogback thrust front Parallelism between face-cleat strike and present-day maximum horizontal stresses may enhance or inhibit coal permeability in the orth and south, respectively. Geopressure and hydropressure are both present in the basin with regional hydrocarbon overpressure dominant in the central part of the basin and hydropressure limited to the basin margins.

The most productive gas wells in the basin are associated with structural terraces, anticlines, and/or correspond to Cameo-Wheeler-Fairfield coal-sandstone development, reflecting basement detached thrust-faulting, fracture-enhanced permeability, and reservoir heterogeneity. Depositional heterogeneities and thrust faults isolate coal reservoirs along the Grand Hogback from the subsurface by restricting meteoric recharge and basinward flow of ground water. An evolving coalbed methane producibility model predicts that in the Piceance Basin extraordinary coalbed methane production is precluded by low permeability and by the absence of dynamic ground-water flow. Best potential for coalbed methane production may lie at the transition from hydropressure to hydrocarbon overpressure or in are s basinward of where outcrop and subsurface coals are in good hydraulic communication for consequent generation of secondary biogenic gas, advective gathering and transport of gas, and subsequent basinward resorption and conventional trapping, which should promote fully gas-saturated coals and high productivity in the Piceance Basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California