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

At least 20% of the worldʹs natural gas originates from methanogens subsisting on organic-rich coals and shales; however in-situ microbial methane production rates are unknown. Methanogens in the Upper Devonian New Albany Shale in the Illinois Basin extract hydrogen from low salinity formation water to form economic quantities of natural gas. Because of this association, constraining the source and timing of groundwater recharge will enable estimation of minimum in-situ metabolic rates. Thirty-four formation water and gas samples were analyzed for stable isotopes (oxygen and hydrogen), chloride, tritium, 14C, and noble gases. Chloride and δ18O spatial patterns reveal a plume of water with low salinity (0.7 to 2154 mM) and δ18O values (− 0.14 to − 7.25‰) penetrating ~ 1 km depth into evapo-concentrated brines parallel to terminal moraines of the Laurentide Ice Sheet, suggesting glacial mediated recharge. However, isotopic mixing trends indicate that the recharge endmember (~− 7‰ δ18O) is higher than the assumed bulk ice sheet value (<−15‰ δ18O), and similar to modern local precipitation (− 7.5 to − 4.5‰ δ18O). Continental paleoprecipitation records reveal that throughout the Pleistocene, δ18O of precipitation in the region ranged from − 10 to − 5‰, suggesting that the dilute groundwater was primarily sourced from paleoprecipitation with minor contributions from glacial meltwater. e majority of samples the atmosphere derived 4He contribution is negligible, and the 4He is dominated by a crustal radiogenic source, with near complete transfer of dissolved noble gases to the gas phase. In addition, mantle derived helium is negligible for all samples (< 1%). Helium-4 ages of formation waters associated with natural gas accumulations range from 0.082 to 1.2 Ma. Thermogenic methane is associated with older fluids (average 1.0 Ma), as compared to microbial methane (average 0.33 Ma), consistent with chloride and δ18O data. However, all groundwater in the study area was influenced by Pleistocene recharge. Estimated in-situ microbial methane production rates range from 10 to 1000 TCF/Ma — ~ 104 to 106 times slower than average laboratory rates from coals. Findings from this study have implications for targeting undeveloped microbial gas accumulations, improving natural gas reservoir estimates, the potential of in-situ methanogen stimulation, and understanding biologic cycling of carbon in subsurface reservoirs.