BIOGENIC AND MIXED BIOGENIC-THERMOGENIC ORIGIN OF COALBED AND SHALE GAS IN EASTERN KANSAS

Middle Pennsylvanian (Desmoinesian) strata in eastern Kansas include gas-bearing coals and black phosphatic shales within mixed carbonate-siliciclastic cyclothems. The majority of the 32 identified coals are high-volatile C to A bituminous in rank.

Carbon-isotope data on gases from coal and shale desorption, coalbed methane and conventional gas production indicate a biogenic and mixed thermogenic-biogenic origin. The most depleted d¹³C(methane) assays were recovered from shale gas, which indicates the least disputable biogenic signature. Due to impermeability, shale likely contains adsorbed gas that is the least mixed with other (likely thermogenic) migrated gas. Thermogenesis dominates at the southern end of the Cherokee basin (comprising one to two counties north of the Oklahoma state line) and in the deeper areas of the Cherokee and Forest City basins. Adsorbed gas contents in eastern Kansas coal and shale generally decrease northward, reflecting decreased heating effects away from the Ouachita orogen in Oklahoma.

WNW dips on the eastern flank of the Cherokee and Forest City basins, and on the Bourbon arch dividing them, are shallow -- 10 to 20 ft per mile. Biogenic gas in the coals and shales may be influenced by interaction of meteoric water (coming from outcrops to the SSE on the flank of the Ozark Dome) with downdip basinal water. Along this interface, which moves with time and varies in each stratum, microbial reduction of dissolved sulfate and associated generation of bicarbonate occurs during biogenic methanogenesis. This interface is expected to create production fairways parallel to the outcrops (trending NNE-SSW).