Coalbed methane reservoirs of the Black Warrior Basin in Alabama are highly prolific, having produced more than 69 × 109 m3 of gas and 254 × 106 m3 of water since 1980. These reservoirs have long been thought to contain a mixture of thermogenic and late-stage biogenic gases, but relatively little is known about geochemical dynamics of coalbed methane generation in the Black Warrior basin. Sampling and geochemical analysis of water and gas from these reservoirs provides crucial insight into the mechanisms of coalbed gas generation and how to prospect for areas where significant resources may lie hidden.
Thermogenic gas was generated principally during the Alleghanian orogeny, when Pennsylvanian coal-bearing strata approached maximum burial depth. During Mesozoic-Cenozoic unroofing, a meteoric recharge system developed along the southeast basin margin, which fed fresh water directly into the metallurgical coal, thus setting the stage for late-stage bacterial methanogenesis. Microbial CO2 reduction is the dominant metabolic pathway for late-stage methanogenesis in Black Warrior coalbed methane reservoirs, and the produced gases become progressively depleted in 13C as coal rank decreases away from the basin margin. Much of the microbial gas lies in a part of the Black Warrior basin that was overlooked by early exploration efforts, which focused on coal thickness, thermal maturity, and absolute gas content. Integrated analysis of subsurface geochemical systems that also consider gas saturation, gas mobility, and basin hydrodynamics can help unlock new reserve areas in mature exploration provinces and can help identify attractive prospects for unconventional gas development in frontier areas that may not otherwise be obvious.