Geologic and Hydrologic Assessment of Natural Gas from Coal: Greater Green River, Piceance, Powder River, and Raton Basins, Western US

Five coal basins in the Rocky Mountain Foreland of the western United States--San Juan, Greater Green River, Piceance, Powder River, and Raton--contain, by virtue of their tremendous coal tonnage, 522 Tcf (14.7 Tm3) coalbed gas resources, or 77 percent of the nation's total of 675 Tcf (19.1 Tm3), and each is attractive in terms of coalbed methane exploration and development. Only in the San Juan Basin (84 Tcf; 2.38 Tm3), however, are these coalbed resources being extensively exploited to meet the nation's demand for natural gas. In this report we assess the natural gas (coalbed methane) exploration and production potential in the Rocky Mountain Foreland and, using the technology acquired and experience gained from previous studies in the San Juan and Sand Wash Basins, summarize the geologic and hydrologic occurrence of natural gas from coal beds in the Greater Green River, Piceance, Powder River, and Raton Basins. We describe the factors that were found to influence high gas productivity in coal seams in the San Juan and Sand Wash Basins, such as tectonic and structural setting, coal distribution and thickness, coal rank, gas content, and basin hydrodynamics-ground-water flow and permeability. These insights advance the understanding of the controls on and occurrences of areas having high coalbed methane potential and will guide exploration and development in other United States and international coal basins. Of the four basins studied, the Greater Green River Basin has secondary potential for coalbed methane exploration and development. The basin is divided into four subbasins (Green River, Great Divide, Washakie, and Sand Wash Basins) by the Rock Springs Uplift and Wamsutter and Cherokee Arches. Whereas Upper Cretaceous and lower Tertiary strata offer numerous coalbed methane targets, vitrinite-reflectance profiles indicate that the eastern Greater Green River Basin has greatest coalbed methane potential. Coal rank ranges from subbituminous to semianthracite, and typical gas contents range from 50 to 400 scf/ton (1.4 to 12.5 m3/t). Permeable, normally pressured, and artesian coal seams occur at depths to 7,000 ft (2,140 m) above regional overpressure. Although the coalbed methane resource is at least 314 Tcf (8.89 Tm3), this basin is largely an untested, frontier basin, having a low cumulative production of 134 MMcf (3.8 MM3), mostly from Mesaverde and Fort Union coal beds in the Sand Wash Basin. The Piceance Basin, bounded by the Uinta Mountain Uplift, Axial Arch, White River Uplift, and Elk Mountains, has primary potential for coalbed methane exploration and production. Coals in the Cameo coal group (Mesaverde Group) are the major coalbed methane targets, and coal rank ranges from high-volatile C bituminous to semianthracite, whereas gas contents range from 200 to 450 scf/ton (5.8 to 13.12 m3/t). The basin is dominated by underpressure and hydrocarbon-related overpressure, indicating low overall permeability. The Cameo coal group contains 65 Tcf (1.84 Tm3 coalbed methane resources of a total 84 Tcf (2.38 Tm3) in the basin. Cumulative coalbed methane production is 21,022 MMcf (594.9 MMm3). The Powder River Basin has the least potential for coalbed methane exploration and production of the four basins studied. Bounded by the Big Horn, Laramie, and Black Hills Uplifts, the basin has a primary coalbed methane target in the Tongue River Member of the Fort Union Formation, where coal seams are large aquifers, difficult to dewater. Coal rank is lignite to subbituminous and locally to high-volatile C bituminous. Gas contents are less than 100 scf/ton (<2.9 m3/t), and although coalbed methane resource estimates range from 16 to 103 Tcf (0.453 to 2.9 Tm3) cumulative coalbed methane production is only 2,625 MMcf (74.3 MMm3). On the other hand, shallow drilling depths (average completion depth 500 ft [152 m]) are attractive to smaller operators. The Raton Basin has secondary potential for coalbed methane exploration and production. Bounded by the Sangre de Cristo Mountains, the Apishapa and Las Animas Arches, and the Sierra Grande Uplift, the basin's main coal-bearing units are the Vermejo and Raton Formations. Coals of high gas content (200 to 500 scf/ton [5.8 to 14.2 m3/t]) ranging from high-volatile C bituminous to low-volatile bituminous in rank occur at shallow depths. The basin is regionally underpressured, reflecting insulation from recharge and coal-seam lenticularity, and the coalbed methane resource is estimated at between 8.4 and 12.1 Tcf (226 and 340 Bm3). Only 53.9 MMcf (1.5 MMm3) of coalbed methane has been produced because most of the basin’s wells are currently shut-in, pending construction of an interstate gas pipeline. On the basis of controls assessed in this report, we found that the Piceance Basin has the greatest potential for making a significant near-term contribution to the nation's gas supply. This basin has large gas resources, high-rank coals, high gas content, and established coalbed methane production. The Piceance has several Cretaceous coalbed methane targets, coal-seam permeability, and areas that are hydrologically favorable for production, although low coal-seam permeability may limit deep methane exploration. We judged the Greater Green River, Powder River, and Raton Basins to have secondary potential. Low gas production and minimal industrial activity limit the near-term production potential of the Greater Green River and Raton Basins. Furthermore, coal beds in the Greater Green River and Powder River Basins, generally thermally immature, have produced large volumes of water. The Raton Basin's shallow, high-permeability, thermally mature coal beds, moreover, although attractive coalbed methane targets, have a limited pipeline infrastructure, limiting the basin's near-term potential.