MOLECULAR AND ISOTOPIC EVIDENCE FOR ABIOGENIC METHANOGENESIS LINKED TO GEOCATALYSIS IN THE DEVONIAN NEW ALBANY SHALE (Invited Presentation)

The conventional paradigm for petroleum generation invokes thermal cleavage of C-C bonds. However, evidence for in situabiogenic methane production in thermally immature shale sequences precludes involvement of catagenetic processes, thereby prompting assessment of the role of catalytic methanogenesis and its significance in formation of unconventional gas reserves. Our studies examine the molecular and isotopic composition of organic matter (OM) in the Devonian New Albany Shale (NAS) to evaluate the influence of geocatalysis on the paths and rates of transformations of sedimentary OM during diagenesis. One focus is the role of clay minerals (e.g., montmorillonite, illite) in catalyzing diagenetic reactions that result in structural rearrangement and isomerization of key biomarkers has utilized a suite of ten NAS samples chosen to encompass a broad maturity spectrum (vitrinite reflectance, R_o, from 0.39 to 1.42 %). For the two least mature samples, several biomarker indices based on diasterenes, diasteranes, steranes, and hopanes all confirm the substantive influence of diagenetic reactions involving clay-catalyzed molecular transformations. The sample suite also shows a progressive decrease in the chain-length of isorenieratane homologs with increasing maturity, a trend consistent with the expected effects of heterogenous catalysis reactions on biomarker distributions. Compositional differences in biomarkers along the maturity transect serve as a baseline for evaluating results from low-temperature laboratory simulations (60-100 °C) that attempt to emulate natural conditions for abiogenic methanogenesis in shales. We aim to assess how heating an immature sample (original R_o= 0.50 %) at either 80 °C or 100 °C for over one year affects the composition and isotopic signatures (δ¹³C, δ²H) of gases generated (CO₂, methane, ethane, and higher hydrocarbons), and biomarker distributions. Differences between the characteristics of biomarker ratios, R_o, gas yields, and isotopic characteristics of the newly-generated fluids versussource materials provide evidence of the likely mechanisms and rates of geocatalytic reactions advancing understanding of the processes involved in shale gas generation.