in the Piceance Basin, Colorado: Controls on Coalbed Methane Producibility

Gas production from coals in the Piceance Basin, which was approximately 29 BCF by the end of 1995, is dependent upon fracture-related permeability. Natural fracture sets in sandstones, shales, and coals of the Upper Cretaceous Mesaverde Group and Tertiary Wasatch and Green River Formations form domains of uniformly oriented fractures in the basin. Face cleats in Mesaverde coals form an eastnortheast trending domain in the southern Piceance Basin and a west-northwest trending domain in the northern part of the basin. Face cleats within these domains are perpendicular to the basin fold axis and the Grand Hogback thrust front and parallel to ancient maximum horizontal compressive stresses. In general, face cleats of the east-northeast trending domain in the south half of the basin are oblique to current maximum horizontal stress. Face cleats in the northern domain parallel current maximum horizontal stress; therefore, these cleats may provide more permeable pathways for gas production. Stratigraphic and structural evidence suggests that Mesaverde face cleat domains are sequential products of the same period of compression (-72 to 40 Ma) that gave rise to the Grand Hogback and associated intrabasin folds. Similar style and orientation of fractures in the different lithologic and stratigraphic intervals studied do not necessarily indicate equivalent fracture age or genesis. Coal face cleat orientations were within 0" to 15" of the first formed systematic sandstone joint trends in only 13 of 23 measured outcrop stations. This lithologic shift can be explained by the nature of fracture genesis in the different rocks. Cleats, a function of coal's maturity, form early in coalification as peat undergoes metamorphism to coal. They occur in coals as low rank as lignite. Sandstone fracturing, however, depends less upon the age of the strata than on the sandstone's degree of cementation and lithification. When geologic formational constraints are added to cleat data, the east-northeast trend to face cleats in lower Mesaverde coals in the southern Piceance shifts to the northwest in Tertiary coals. Therefore, Mesaverde coal fractures need not be Eocene or younger in age, as suggested in earlier studies.