Methane CO2 Ethane Propane i-Butane n-Butane Methane CO2 Ethane Propane i-Butane n-Butane

l. Vo l. Major gas components of coalbed gases in SE Illinois Basin. Note the logarithmic Y-axes. Biogenic and thermogenic gases are compositionally very different. Coalbed gases and waters from exploratory and production gas wells in the southeastern Illinois Basin were sampled to assess the origin of coalbed gases, with emphasis on Springfield and Seelyville coal members that are commercially targeted for coalbed methane production. On-line analyses of hydrocarbon gases (methane to butanes: C1, C2, C3, n-C4, i-C4) and CO2 yielded chemical concentrations, δD, and δ C values. The low thermal maturity of Indiana coals (vitrinite reflectance Ro ~ 0.6%) is in agreement with an overwhelmingly biogenic isotopic signature of coalbed gas that has ≥ 96 % methane generated via bacterial CO2-reduction. In contrast, thermogenic gas was generated by the stratigraphically equivalent coalbeds in western Kentucky’s Rough Creek Graben zone where higher maturities of up to Ro ~ 0.8 % are reached due to tectonic and hydrothermal activity. No secondary biogenic methane was observed in Kentucky coalbed gases, probably due to greater burial depths and limited recharge of meteoric water. Biogenic and thermogenic coalbed gases represent two end-members that are compositionally and isotopically distinct