PASHIN, JACK C. and RICHARD E. CARROLL, Geological Survey of Alabama, Tuscaloosa, AL

ABSTRACT: Depositional Environments of Coalbed Methane Reservoirs in the Pottsville Formation (Lower Pennsylvanian), Cahaba Synclinorium, Alabama: Comparison with the Black Warrior Basin

The Pottsville Formation in the Cahaba synclinorium contains 55 named coal beds of high-volatile A bituminous rank with estimated coalbed-methane resources of 1.8 tcf. The synclinorium is an Alleghanian structure of the Appalachian foreland and is one of the oldest coal mining areas in Alabama. Despite economic significance, the depositional architecture of coal-bearing strata in the synclinorium is unknown. This study interprets the depositional environments of these strata and compares them with coeval deposits in the nearby Black Warrior basin, a preeminent coalbed-methane region.

Pottsville sedimentation in the Cahaba synclinorium began with deposition of fining-upward quartzarenite cycles that represent transitions from tide-dominated shelf environments to intertidal mudflats with localized coastal swamps. Most of the Pottsville section is composed of coarsening-upward mudstone-litharenite cycles representing tide-influenced deltaic systems. Coal beds in these cycles are among the thickest and most continuous in the synclinorium and are thus the most desirable completion targets. The upper part of the section contains poorly organized conglomerate-coal cycles. The conglomerate represents bedload-dominated fluvial systems, whereas coal is interbedded with mudstone and litharenite that accumulated in anastomosed interfluves. Continuity, thickness, and geometry f coal beds in these cycles are variable.

Comparing lithologic trends in the Cahaba synclinorium and the Black Warrior basin establishes differences in the sedimentologic response to eustasy and tectonism. Vertical trends in the Cahaba synclinorium include decreasing frequency of marine rocks and increasing frequency of coal, sandstone, and extraformational conglomerate. No significant vertical trends in the distribution of these rock types were observed in the Black Warrior basin. Eustasy is interpreted to have favored deposition of stacked mudstone-litharenite cycles with regionally extensive coal zones and marine zones in the Black Warrior basin. In contrast, high sediment flux from evolving tectonic highlands apparently suppressed the eustatic signal in the Cahaba synclinorium by favoring progression from marine-influence mudstone-litharenite cycles to fully terrestrial conglomerate-coal cycles.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.