Constraining the timing of microbial methane generation in an organic-rich shale using noble gases, Illinois Basin, USA

► Microbial methane in the New Albany Shale is linked with meteoric recharge. ► δ18OH2O and δDH2O values indicate that recharge is largely from paleoprecipitation. ► 4He ages of H2O linked with gas show that Pleistocene recharge affected all groundwater. ► 4He ages of microb. CH4 (0.082-0.79 Ma) are younger than thermo. CH4 (0.80-1.2 Ma). ► In-situ microb. CH4 production rates (10-1000 TCF/Ma) are 104-106× slower than lab.