Geologic and Hydrologic Controls on Coalbed Methane Resources, Greater Green River Basin, Southwestern Wyoming and Northwestern Colorado

Roger Tyler, W. R. Kaiser, A. R. Scott, D. S. Hamilton

Coal (1,158 billion t) and coalbed methane (8.89 Tm³) resources are large in the Greater Green River Basin, but coalbed methane production, to date, has yielded minor gas and large volumes of water. Deeper exploration and improved technology, integrated with an understanding of geologic and hydrologic controls, will be required to penetrate exploration fairways with higher-rank, higher-gas-content coals. The spatial relationships among tectonics, structure, depositional facies, coal distribution, coal rank, gas content, permeability, and ground water flow determine the occurrence and producibility of methane within this structurally complex basin.

Face cleats generally strike northeastward, but toward the northeast and northwest in the Sand Wash Basin, to impose permeability anisotropy on the coal beds. Coalbed methane targets occur in the Upper Cretaceous Mesaverde Group and lower Tertiary Fort Union Formation; their combined net coal thickness exceeds 90 m, and individual seams are as much as 12 m thick. Coal rank ranges from subbituminous to high-volatile A bituminous, except in deeper subbasins, where rank is medium-volatile bituminous and higher. Gas contents average less than 6.24 m³/t at depths of less than 1,800 m and exceed 15.60 m³/t below 2,300 m. Areas having higher gas contents reflect conventional trapping of migrated thermogenic and/or secondary biogenic gases and correspond to areas of pres ure transition and convergent, upward flow of ground water.

Regionally, ground water flows basinward for eventual discharge along the boundary between hydropressure and hydrocarbon overpressure. Mesaverde Group exploration fairways, therefore, exist basinward in the Sand Wash and Washakie Basins, and around the northeast flank of the Rock Springs Uplift. The Sandy Bend Arch and the Big Piney area are also prospective where structural and/or stratigraphic trapping may enhance gas contents in low-rank Fort Union Formation coals.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995