Interest in the origin and migrational history of methane in low permeability shales reaches beyond that of resource potential to include the evaluation of such formations as geological barriers for the purposes of isolating surface environments from the impacts of deep exploitation and the deep disposal of waste. Here, we present detailed isotope and geochemical profiles of porewaters and methane from an Ordovician shale and carbonate aquiclude on the eastern flank of the Michigan Basin, where a deep geological repository for low and intermediate level nuclear waste is proposed. The solute concentrations and stable isotopes of water (δ18O and δD) indicate that this aquiclude hosts saline brine (39% TDS), originating as evaporated seawater. Methane concentrations are < 4 mmol/g in the shale section, with negative δ13C and δD signatures that are consistent with archaeal methanogenesis and are accompanied by a well defined positive excursion in δ13C of CO2. Below the aquiclude, porewaters contain lower concentrations of methane with a thermocatalytic signature, consistent with regional methane elsewhere in the Michigan Basin. No evidence for archaea was identified by microcosm experiments or through PCR rRNA analysis, suggesting that the microbial methane present in these sediments is not the result of recent microbial activity. Based on these results and considering that archaeal activity has not been observed in such hypersaline brines with low water activities (< 0.75), it is concluded that the methane likely formed prior to the infiltration of hypersaline Silurian seawater. The methane has since remained in place, making it, perhaps, the oldest documented occurrence of biogenic methane. This is consistent with helium isotope data, which also suggests authigenic production and accumulation in this aquiclude since the Paleozoic.

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