Geologic and Hydrologic Controls Critical to Coalbed Methane Producibilty and Resource Assessment: Williams Fork Formation, Piceance Basin, NorthWest Colorado

As predicted from an evolving coalbed methane producibility model, extraordinary coal gas production is precluded in the Piceance Basin by the absence of reservoir continuity and 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 reservoir and hydraulic communication and in areas of vertical flow potential. Synergism between tectonic and structural setting, depositional systems and coal distribution, coal rank, gas content, permeability, and hydrodynamics are controls critical to coalbed methane producibility.

Within the genetically defined, coal-bearing Upper Cretaceous Williams Fork Formation, net coal thickness is typically 80 to 120 ft and is thickest in a north-south belt, behind west-east prograding shoreline sequences. Depositional setting and thrust faults cause coals along the Grand Hogback and in the subsurface to be in modest to poor reservoir and hydraulic communication, restricting meteoric groundwater recharge and flow basinward. 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.

In the Grand Valley/Parachute/Rulison and White River/Pinyon Ridge areas, structure and sandstone development control gas production from Cameo-Wheeler-Fairfield coals and/or sandstones. The most productive wells are on anticlines and structural terraces or correspond to sandstone development, reflecting fracture-enhanced permeability. Total subsurface coal resources are estimated to be 289 billion tons (262 Bt), whereas coal gas resources are approximately 99 Tcf (3.09 Tm^3), although coal gas estimates range between 80 (2.49 Tm^3) and 136 Tcf (4.24 Tm^3), depending on the calculation method.

Based on completion data provided by operators, it appears that production from coalbed methane reservoirs can meet minimum economic requirements in the Grand Valley, Rulison, and Pinyon Ridge fields. To achieve high gas contents or fully gas-saturated coals for consequent high productivity in the Piceance Basin, research, exploration, and development for migrated conventionally and hydrodynamically trapped gases, in situ generated secondary biogenic gases, and solution gases will be required.