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

At least 20% of the world's 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 2154mM) and δ¹⁸O values (-0.14 to -7.25‰) penetrating ~1km 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. For the 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 1.2Ma. Thermogenic methane is associated with older fluids (average 1.0Ma), as compared to microbial methane (average 0.33Ma), 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 1000TCF/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.