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

Microbiology

Fermentation Technology

Author(s): Melissa E Schlegel, Zheng Zhou

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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 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.

This article was published in Chemical Geology and referenced in Fermentation Technology

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