--> Abiogenic methanogenesis associated with geocatalysis: evidence from biomarker transformations and isotopic variations across a maturity transect of the Devonian New Albany Shale

2019 AAPG Eastern Section Meeting:
Energy from the Heartland

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Abiogenic methanogenesis associated with geocatalysis: evidence from biomarker transformations and isotopic variations across a maturity transect of the Devonian New Albany Shale

Abstract

The conventional paradigm for petroleum generation requires thermal cleavage of carbon-carbon bonds. However, evidence for in situ abiogenic methane production within thermally immature shale sequences precludes the involvement of catagenetic processes, which prompts evaluation of the role of catalytic methanogenesis and assessment of its significance in the formation of unconventional gas reserves. Our studies have focused on characterization of the molecular and isotopic composition of organic matter (OM) in the Devonian New Albany Shale (NAS) to evaluate the potential influence of geocatalysis in determining the paths and rates of transformations of sedimentary OM during burial. In particular, the role of clay minerals (e.g., montmorillonite, illite) in catalyzing specific diagenetic reactions that result in the structural rearrangement and isomerization of key biomarkers has been investigated utilizing a suite of ten New Albany Shale samples that cover a broad maturity spectrum (vitrinite reflectance, Ro, 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 NAS sample suite also shows a progressive decrease in the chain-length of isorenieratane homologs with increasing maturity, which represents a trend in biomarker distributions consistent with the expected products of heterogenous catalysis reactions. Determination of such compositional differences in biomarkers along the maturity transect serves as a baseline for evaluating results from low-temperature laboratory simulations (60-100 °C) that aim to emulate the natural conditions for generation of abiogenic methane in shales. In particular, we assess the changes of heating a sample with an original Ro of 0.50 % at either 80°C or 100 °C for over a year, determining the composition and isotopic signatures (δ13C, δ2H) of the gases generated (CO2, methane, ethane, and higher hydrocarbons), and evaluating the distributions of associated biomarkers. Differences between the characteristics of biomarker ratios, Ro, gas yields, and isotopic characteristics for the newly-generated fluids versus source materials provide evidence of the likely mechanisms and rates of geocatalytic reactions and thereby aid understanding of the processes involved in shale gas generation