2012-09-06_REVISION - M2008070 (29)

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intrusion is thought to be present under a coal basin anticline along the southeast margin of the basin (Figure A3 -1) where high quality coking coal was mined since the 1800s (Collins, 1976). Coalbed methane reservoirs occur exclusively in the Upper Cretaceous Mesaverde Group (Figure A3 -2), which covers an area of approximately 7,255 square miles (Tremain and Tyler, 1997). Depths to the Mesaverde Group range from outcrop to greater than 12,000 feet along the axis of the basin (Tyler et al., 1996; Tremain and Tyler, 1997). Two - thirds of the coalbed methane occurs in coals deeper than 5,000 feet, making the Piceance Basin one of the deepest coalbed methane areas in the United States (Quarterly Review, August 1993). The major coalbed methane target, the Cameo - Wheeler - Fairfield coal zone (Figure A3- 3), is contained within the Williams Fork Formation of the Mesaverde Group and holds approximately 80 to 136 Tcf of coalbed methane (Tyler et al., 1998). This coal zone ranges in thickness from 300 to 600 feet, and lies more than 6,000 feet below the ground surface over a large portion of the basin (Tyler et al., 1998). Individual coal seams of up to 20 to 35 feet thick can be found within the group, with net coal thickness of the Williams Fork Formation averaging 80 to 150 feet thick. In 1991, at the Grand Valley field (Figure A3 -4), there were 23 coalbed methane wells and 18 conventional gas wells (Reinecke et al., 1991). However, in 1984, most wells at the Rulison field (Figure A3 -4) were conventional gas wells. Initially, it was anticipated that coalbed methane wells in the sandstones and coals of the Cameo Zone would have high production rates of water. However, testing later showed that they produced very little water (Reinecke et al., 1991). Both the sandstones and coalbeds are tight, poorly permeable, and are generally saturated with gas rather than water or a mixture of water and gas. The dynamic flow of a hydrologic system enhances the collection of gas in traps, but in much of the Piceance Basin that flow is not present because of the over - pressuring and saturation with gas. Consequently, the conventional models for coalbed methane accumulation developed for other basins do not apply well for exploration and development in the Piceance Basin. Tyler et al. (1996) concluded, "very low permeability and extensive hydrocarbon overpressure indicate that meteoric recharge, and, hence, hydropressure, is limited to the basin margins and that long - distance migration of groundwater is controlled by fault systems." Recharge is limited along the eastern and northeastern margins of the basin because of offsetting faults, but zones of transition between hydropressure and hydrocarbon overpressure in the western part of the basin and on the flanks of the Divide Creek Anticline in the southeastern part of the basin may possess better coalbed methane potential, as indicated by the exploration targets delineated in Tyler et al. (1998) (Figure A3 -5). Evaluation of Impacts to Underground Sources of Drinking Water by Hydraulic Fracturing of Coalbed Methane Reservoirs June 2004 A3 -2