Structural and Maceral Controls on Biogenic Methane Generation: Insights from Vitrain-Durain Separation across the First Coalification Jump.

Biogenic coalbed methane resources are widely distributed in high-volatile bituminous coal areas, yet the controlling effect of the first coalification jump on biogas generation remains unclear. To explore the combined controlling effects of coal macerals and metamorphic degree on biogenic methane production potential, five coal samples with different metamorphic grades near the first coalification jump were selected. Vitrain and durain fractions were separated from the samples to conduct microbial degradation and methanogenesis experiments. The results show that the methane production yield of vitrain (11.96-15.75 μmol/g) is generally higher than that of durain (4.14-15.49 μmol/g). The methane production of both vitrain and durain presents a general N-shaped variation trend with increasing coalification degree, suggesting that coalification exerts a notable controlling effect on methane generation. The methane yield of vitrain is negatively correlated with stacking height (L c), while that of durain is negatively correlated with carbon lamellar spacing (d 002). The liptinite-rich characteristic of durain can offset the adverse impact of metamorphism. Essentially, the biogenic methanogenesis of coal refers to the biochemical transformation of aliphatic structures mediated by anaerobic microorganisms, and the gas production scale is directly determined by the abundance of available aliphatic components. The aliphatic structure of vitrain is mainly controlled by coalification, whereas that of durain is jointly regulated by coalification and macerals, with macerals playing a dominant role. Vitrinite serves as the major contributor to methane production, while inertinite has a low contribution rate, which is the primary reason for the superior gas-producing capacity of vitrain over durain.