Carbon and hydrogen isotope fractionation during methanogenesis: A laboratory study using coal and formation water

Abstract

Carbon and hydrogen isotope compositions of CH4 generated via methanogenesis in cultures of South Sumatra Basin (SSB) coalbed methane (CBM) formation waters grown on coal, acetate and H2+CO2 were investigated. CH4 production and molecular analysis confirmed the presence of active microbial communities that are able to convert coal into CH4 using both acetoclastic and hydrogenotrophic pathways. The representative bacterial sequences were dominated by Bacteroidetes, Firmicutes and Deltaproteobacteria, while Methanosaeta and Methanosarcina were the most prevalent archaeal methanogens present in the cultures.CH4 produced in this study's culturing experiments has δ13C values in the range of −50 ‰ to −20 ‰, with most values falling outside the current understanding of the carbon isotopic boundaries for biogenic CH4 (−110 ‰ to −30 ‰). However, the corresponding apparent carbon isotopic α factor (αc=1.02±0.006), and isotopic effect (εc=−20.1 ‰±15.3) showed that CH4 in SSB cultures was predominantly produced by acetoclastic methanogenesis, which is consistent with the results of molecular DNA analysis. In addition, the calculated contribution of CO2 reduction from the δ13C values of coal-treated cultures was overall <50 %, further confirming the high contribution of the acetoclastic pathway to CH4 production in the SSB cultures. The outcome of this experimental study also suggests that δ2H-CH4 values may not provide a reliable basis for distinguishing methanogenic pathways, while apparent carbon isotopic fractionation factor (αc) and isotope effect (εc) are considered more useful indicators of the methanogenic pathway.The high δ13C-CH4 values (≥30 ‰) and the dominance of Methanosaeta over Methanosarcina indicate that methanogens within the SSB cultures were operating at low substrate concentrations. An unusually positive δ13C-CH4 suggests a substrate depletion effect, which is thought to be related to a decrease in the relative abundance of key bacterial coal degraders with formation water inoculum storage time. Closer observation of δ13C-CH4 values during the growth of cultures within a single experiment also showed a 13C-enrichment trend over time. At log phase of growth, the CH4 produced was 13C-depleted when compared to the stationary phase that also indicates substrate depletion effects. Finally, the δ13C-CH4 values encountered in this study (as high as −20 ‰) highlight the possible positive extension of δ13C-CH4 values of acetoclastic methanogenesis from those currently reported in the literature for natural and experimental samples (as high as −30 ‰).