oa Alkylotrophic Methanogenesis Forms Biogenic Methane with δ13C Values Comparable to Thermogenic Gas

Methane, a primary component of natural gas, is mainly derived from biogenic and thermogenic origins. Biogenic methane exhibits lower d13C-CH4 (<-55‰) due to microbial preferential utilization of 12C. However, secondary microbial methane (SMM), an important biogenic gas, was characterized to have d13C-CH4 values overlapping with thermogenic. This deviation cannot be adequately explained by conventional methanogenetic pathways, indicating alternative methanogenesis producing methane with less13C-depleted. Alkylotrophic methanogenesis differs from previous methanogenic metabolism and is widespread in oil reservoirs. Therefore, we incubated the enrichment culture of alkylotrophic methanogens with the eicosane and several crude oils as substrates (d13C-values from -35 to -27‰). The accumulated CH4 exhibited d13C values of -48‰ to -41‰, CO2 from -7‰ to +14‰. It exhibited smaller carbon isotopic fractionation than other pathways, ranging from -10 to -17‰, which can be attributed to its unique metabolic pathway. Since about 1/3 of methane derives from CO2 reduction, the only likely process with substantial fractionation, the overfall fractionation of alkylotrophic methanogenesis will be rather low. Its d13C value departs from canonical methanogenesis and plots in the interface between thermogenic and SMM. Given the prevalence of alkylotrophic methanogenesis in subsurface reservoirs, biogenic methane in reservoirs has been previously underestimated.