Geological control on carbon isotope equilibrium and kinetic fractionation of CH4-CO2-HCO3− in microbial coalbed and shale gas systems

Microbial coalbed gas (CBG) and shale gas (SG), predominately composed of methane (CH ₄) and carbon dioxide (CO ₂), are important economic resources and potent greenhouse gases. Although isotopic equilibrium of CH ₄ and CO ₂ has been observed in microbial CBG and SG basins, it is difficult to judge under what geological conditions equilibrium is achieved. Moreover, the effects of CO ₂ dissolution on the isotopic fractionation process need to be considered. We use data from eight microbial CBG and SG basins to discuss the geological conditions in which equilibrium and kinetic isotopic fractionation in CH ₄-CO ₂-HCO ₃ ⁻ system is achieved. Based on isotopic equilibrium temperatures calculated using computer codes developed in MatLab software, we show that, in deep and closed reservoirs, the CH ₄-CO ₂ and CH ₄-HCO ₃ ⁻ are close to carbon isotope equilibrium. In contrast, in shallow and open reservoirs, they are in disequilibrium. The CO ₂-HCO ₃ ⁻ is in disequilibrium in most reservoirs. We propose that both low free energy gradients and long coexisting time of CH ₄ and CO ₂/HCO ₃ ⁻ are necessary to attain isotopic equilibrium. However, it is difficult to accurately estimate the timescale for attaining isotope equilibrium among them. In general, a closed and deep CBG/SG reservoir is likely to be geologically and geochemically stable over long timescales, favoring isotopic equilibrium of CH ₄-CO ₂ and CH ₄-HCO ₃ ⁻. However, a shallow and open reservoir is unfavourable for their isotopic equilibrium due to shorter timescales for the coexistence of CH ₄-CO ₂-HCO ₃ ⁻. Using data from systems close to equilibrium, we estimated the percentage of CO ₂ in total CH ₄ and CO ₂ in CBG reservoirs in various basins to be from 27% to 50%, where methanogenesis is mainly by CO ₂ reduction. This is significantly higher than the CO ₂ content (1% to 15%) in gaseous CH ₄ and CO ₂ in these basins but is consistent with those (36% to 48%) from culture experiments for coal conversion by methanogenesis. Further study shows that 53–99% of the CO ₂ formed during CBG generation has dissolved into groundwaters to form dissolved inorganic carbon (DIC) in CBG reservoirs. We propose that CO ₂ dissolution likely has significantly affect the abundance and isotopic compositions of gaseous CO ₂ in subsurface.