2012-09-06_REVISION - M2008070 (29)

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3.1 Basin Geology E v a luation of Impacts to Underground Source of Drinking Water by Hydraulic Fracturing of C albed Methane Reservoirs ® ' 1 Attachment 3 The Piceance Basin The Piceance Coal Basin is entirely within the northwest corner of Colorado. (Figure A3- 1). The coalbed methane reservoirs are found in the Upper Cretaceous Mesaverde Group, which covers about 7,225 square miles and ranges in thickness from about 2,000 feet on the west to about 6,500 feet on the east side of the basin (Johnson, 1989). It is estimated that 80 trillion to 136 trillion cubic feet (Tcf) of gas are contained in coalbeds within the basin (Tyler et al., 1998). Total coalbed methane production was 1.2 billion cubic feet in 2000 (GTI, 2002). The Piceance is a northwest trending asymmetrical, Laramide -age basin in the Rocky Mountain foreland with gently dipping western and southwestern flanks and a sharply upturned eastern flank (Figure A3 -1) (Tremain and Tyler, 1997). The Douglas Creek Arch bounds the basin on the northwest, and separates it from the Uinta Coal Basin, which lies almost entirely in Utah. The Mesaverde Group is sharply upturned to near vertical along the Grand Hogback, which forms the eastern boundary of the basin and separates the basin from the White River uplift to the east. Most of the Piceance Basin's coal deposits are contained in the Iles and Williams Fork Formations of the Late Cretaceous Age Mesaverde Group, which are approximately 100 to 65 million years in age (McFall et al., 1986). These formations composed of sandstone and shale, were deposited in a series of regressive marine environments (McFall et al., 1986; Johnson, 1989). It is believed that the coals were deposited in marine transitional, brackish, interdistributary marshes and freshwater deltaic swamps (Collins, 1976 in McFall et al., 1986). Figure A3 -2 presents a stratigraphic section shown with a gamma ray - induction log from the Barrett 1 -27 Arco Deep well (Reinecke et al., 1991). The Mesaverde Group is underlain by the marine Mancos Shale and overlain by the lower Tertiary Age Fort Union and Wasatch Formations, which consist of fluvial sandstones and shales. The Mancos Shale, Fort Union, and Wasatch Formations are essentially barren of coals (McFall et al., 1986). Depths to the coal - bearing sediments vary from outcrops around the margins of the basin (Figure A3 -1) to more than 12,000 feet in the deepest part of the basin (Tyler et al., 1996). The major fold structure of the Piceance Basin is the Grand Hogback Monocline, formed as the White River Uplift was uplifted and thrust westward during the Laramide Orogeny in Late Cretaceous through Eocene time (McFall et al., 1986). Broad folds, such as the Crystal Creek and Rangley Syncline, trend northwest to southeast, and generally parallel to the axis of the basin (Figure A3 -1). Intrusions occur throughout the southeast part of the basin, locally elevating coal ranks to as high as anthracite grade. A buried laccolith Gtr EPA- q_ June 2004 A3 -1