Biodegradation sequence of coal organic matter and mechanism of biomethane formation in secondary biogenic gas accumulation areas

• In a coal matrix, the biodegradation susceptibility of aromatics exceeds saturates. • The degradation susceptibility of saturated hydrocarbons is dependent on chemical structure. • Biodegradation susceptibility order of coal is: n -alkane > sterane > hopane. • The ratios of methyldibenzothiophene (MDR) and methylphenanthrene (MPR) can been used as molecular indices of coalification degree. Laboratory biodegradation experiments of organic matter (OM) in coal provide a theoretical foundation for comprehending the formation of secondary biogenic gas (SBG). However, limited research has been conducted on the microbial degradation extent and sequence of OM degradation experienced by underground coal seams throughout geological history. This study analyzes underground coal samples from the typical SBG accumulation areas in the Ordos Basin to investigate the origin and composition of OM in coal as well as their contributions to SBG formation. The findings indicate that terrigenous input is the primary source of OM in coal in these regions, which has undergone varying degrees of biodegradation. Notably, there are significant differences in the degree of biodegradation between saturated and aromatic hydrocarbons. Saturated hydrocarbons show PM grade 4–5 degradation while aromatics demonstrate PM grade 5–7 degradation. Aromatics display a higher susceptibility to biodegradation compared to saturated hydrocarbons, which differs significantly from crude oil. The degradation susceptibility sequence for saturated hydrocarbons is as follows: n -alkane > sterane > hopane. The biodegradation susceptibility of aromatic hydrocarbons depends on both the number of aromatic rings and alkyl-substituents; those with fewer rings and alkyl-substituents degrade more rapidly. Biodegradation of OM in coal plays a crucial role in SBG generation; however, a high degree of OM biodegradation does not necessarily correlate with greater accumulation of SBG due to other factors such as tectonic fractures and hydrogeological conditions that affect its preservation.