As a component of geoscientific investigations for a deep geological repository in Middle Ordovician strata of the Michigan Basin near Tiverton, Ontario, methane from hypersaline pore waters (Cl– > 6 molal) in shale and limestone core samples was analysed for concentration, 13C and D. CH4 is highly depleted in 13C and 2H from the top of the Queenston Formation down through the Cobourg Formation. Based on the location of the isotopes on a discrimination diagram, this methane is interpreted to be of biogenic origin. An associated zone of 13C-enriched CO2 strengthens this interpretation, and appears to constrain biogenic methane production to the lower Georgian Bay and upper Blue Mountain formations. The δ13C and D signatures of methane from the limestones below the Cobourg Formation increase sharply to values consistent with methane of thermocatalytic origin. In an attempt to resolve whether methanogenic archaea may still be active in these strata, environmental DNA was extracted from core samples and PCR amplified using 16S rDNA primers. PCR performed with archaeal 16S rDNA and methanogen-specific (mcrA) primers yielded no DNA, whereas low, but measurable, yields were obtained for bacterial primers. Gene analysis indicated that bacterial sequences similar to Proteobacteria, Cyanobacteria, Firmicutes, and Actinobacteria were present. The majority of identified bacterial species were uncultured extremophilic heterotrophs. We believe that identified bacteria have survived in the rocks for a prolonged period of time by utilizing available energy sources that were trapped during deposition or by assuming a state of dormancy. In contrast, methanogenic archaea are considered to be halophobic and their absence is not unexpected. We hypothesize that the biogenic methane in the Upper Ordovician shales is of para-diagenetic origin, and has been preserved in-situ for some 400 My.

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