Geologic and Hydrologic Assessment of Natural Gas from Coal Seams in the Measaverde Group and Fort Union Formation, Greater Green River Basin, Wyoming and Colorado

The Greater Green River Basin is a fault-bounded, structurally complex intermontane basin that is subdivided into four subbasins. Face cleats strike northeast in most of the greater basin but northwest in the Sand Wash Basin to impose permeability anisotropy on the coal beds. Major coal-bearing units are the Upper Cretaceous Mesaverde Group—primarily the Rock Springs and Williams Fork Formations—and the lower Tertiary Fort Union Formation's lower coal-bearing unit. The combined net coal thickness exceeds 300 ft (91 m); individual seams are as much as 40 ft (12.2 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 200 scf/ton (<6.24 m3/t) at depths of less than 6,000 ft (<1,830 m). Areas of high gas content (350 to 500 scf/ton [10.92 to 15.60 m3/t]) reflect conventional trapping of migrated thermogenic and biogenic gases and correspond to areas of flow orthogonal to flow barriers, pressure transition, and convergent, upward flow of groundwater. Regionally, groundwater flows from the basin margins to discharge eventually basinward along the boundary between hydropressure and hydrocarbon overpressure and along major river valleys. Coal and coalbed methane resources are very large: 1,276 billion short tons (1.158 billion t) and 314 Tcf (8.89 Tm3), respectively. Despite huge resources, coalbed wells drilled to date have yielded little or no gas and large volumes of water, for a basinwide cumulative gas-water ratio of approximately 20 scf/bbl (~3.6 m3/m3). Deeper drilling will be required to penetrate higher rank, higher gas content coals. For example, Mesaverde gas contents between 6,000 and 7,500 ft (1,830 and 2,286 m) are approximately 350 scf/ton (~10.92 m3/t) and exceed 500 scf/ton (15.60 m3/t) below 7,500 ft (2,286 m). Deeper drilling is justified at the northwest end of the Cedar Mountain fault system in the Sand Wash Basin, along the east margin of the Washakie Basin, and around the northeast flank of the Rock Springs Uplift. In the Fort Union Formation, the Sandy Bend Arch and the Big Piney are thought to be prospective, where structural and/or stratigraphic trapping may enhance gas contents in low-rank coals.