CARROLL, RICHARD E., and PASHIN, JACK C., Geological Survey of Alabama, Tuscaloosa, Alabama 35486-9780.

Coalbed methane production and coal mining in the Warrior and Cahaba coal fields are strongly influenced by regional variations of coal rank. Therefore, understanding how various thermal maturation processes contributed to the final rank pattern enables development of better predictive models of coalbed methane generation and production. To analyze the thermal and burial history of Pennsylvanian coal-bearing strata in the Warrior and Cahaba coal fields, structure maps, coal-rank maps, cross sections, vitrinite reflectance profiles, and Lopatin models were constructed.
Lopatin modeling suggests that hydrocarbon generation began at or shortly after maximum burial during the Alleghany orogeny and continued into Mesozoic post-orogenic unroofing of the basin. An elliptical area of high-rank coal occurs near the southeastern edge of the Warrior field. Here, low volatile bituminous coal is mined at depth and is the cradle of the modern coalbed methane industry. At the southeast margin of the high-rank area, isovols cut across the forelimb of the Blue Creek anticline, indicating a post-tectonic component to coalification. In the Cahaba coal field, isovols are folded, indicating a pre- to syntectonic component. Additionally, isovols cut across the Tacoa anticline revealing a post-tectonic component interpreted to be related to diversion of fluids around the structure. Patterns of coalification and gas generation in the Lower Pennsylvanian Pottsville Formation in the Warrior and Cahaba coal fields suggest that, in addition to simple burial coalification, growth of structural features, differences in regional temperature gradients, and basin hydrology were also contributing factors.

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