Molecular and isotopic evaluation of the paleo-coupling between methanogenesis and oil biodegradation in Ordovician rocks (Michigan Basin).

Recent work on a low permeability (10-15≤Kh≤10-13 m.s-1) and high salinity (>5 M) aquiclude situated in the upper Ordovician in the Michigan Basin suggests a microbial origin for CH4 and CO2 based on their stable isotope values. CH4 is believed to have been produced during the early stages of burial and trapped since the Paleozoic in a relatively discrete and high organic matter (OM) horizon at the shale/carbonate transition. Questions remain regarding the origin of the substrate used by the methanogenic microbial community and the relative timing of this natural gas generation. Here we present detailed isotopic and geochemical stratigraphic profiles of OM and biomarkers, with the objective of understanding the origin of CH4. δ13C analysis on different fractions of OM revealed a 13C enrichment anomaly in the aliphatics relative to the OM above the horizon where microbial CH4 is trapped. This isotopic excursion is concomitant with a drastic change in both n-alkane concentrations and profiles. The n-alkanes shift from a distribution centered on short chain length (i.e., characteristic of marine OM sources prevailing during the Ordovician) to a distribution centered on longer chain length. This cannot be interpreted as a change of OM sources towards more terrestrial OM, as land plants were only starting to emerge during the late Ordovician. Diagnostic ratios of polycyclic aromatic hydrocarbons, relative abundances of hopanes and degraded hopanes and compound specific carbon and hydrogen isotope ratios of n-alkanes suggest a secondary microbial methanogenesis generation through biodegradation of oil within the Ordovician shales. Our results provide new insight into the stratigraphic origin of the substrate associated with microbial CH4; which includes the development of an oil biodegradation zone which fed methanogenesis in the aquiclude. This work provides essential field empirical evidence of the direct relationship between methanogenesis and crude oil biodegradation.