Abstract: Coal and Coalbed Methane Resources of Cretaceous Rocks of the Black Mesa Basin, Northeastern Arizona

J. Dale Nations, Henry W. Haven, Robert L. Swift

Cretaceous rocks of the Black Mesa basin of northeastern Arizona were deposited from Late Cenomanian to Santonian time near the western depositional limit of the transgressive-regressive Greenhorn and Niobrara cyclothems. Coal-bering rocks of the sequence (including the Dakota Formation, Toreva Formation and Wepo Formation) are preserved in the Black Mesa structural basin of Laramide age. These rocks of the Black Mesa basin are the object of an NSF-sponsored research project to evaluate their coal resources and potential for coalbed methane occurrence.

Forty-five measured sections and descriptions of Cretaceous rocks around the eastern margin of the basin have been incorporated into a data base in the software program LOGGER, and have been used to construct a series of stratigraphic cross-sections across the basin using the software program MacSection. The formations mentioned above, as well as the Mancos Shale, can be correlated across the basin.

However, intraformational lithofacies changes occur between most measured sections. These include coal beds and carbonaceous shales that are potential sources of methane as well as reservoirs, and sandstone bodies that are potential secondary reservoirs of methane (and possibly petroleum hydrocarbons from the Mancos Shale). In order to trace the facies changes between measured sections, aerial videography and 35 mm slides have been taken along the entire eastern margin of Black Mesa, a distance of 70 miles. These were digitized to provide detailed lithofacies analyses of the stratigraphic sections along the north-trending cliff line.

Limited subsurface data from three oil tests and numerous Peabody Coal Company coreholes and water well logs within the basin, are used to extend the stratigraphic cross-sections westward into the basin, and to extrapolate the structural dip and depth below surface to the coal-bearing formations. Coal, carbonaceous shale and sandstone facies in the Dakota, Toreva and Wepo formations are interpreted in the subsurface sections. Trends of thickness variations within these facies are interpreted from isopach maps and known shoreline trends, in order to predict the areas of greatest probability of coalbed methane generation and accumulation. These predictions are also based on the locations of anticlinal and monoclinal fold axes where they cross the thickest coal trends, due to likely enha cement of permeability by cleat and butt fractures in the coal and shale reservoir rocks.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada