Biodegradation of early thermogenic gas and generation of secondary microbial gas in the Tieliekedong region of the northern Tarim Basin, NW China

Abstract

Some important progress has been made in coalbed gas (CBG) exploration in the Kuqa-Bay coalfield in the northern Tarim Basin, where a demonstration base of CBG development and utilization has been established in the Tieliekedong (TD) region. Currently, there has been no detailed study of the formation mechanism of the CBG in the TD region, which has restricted the next phase of the CBG exploration. In this study, gas and water samples in the TD region were collected and analyzed to determine the genesis of the CBG, the formation pathways of the microbial gas, the source of the coalbed water, the composition of the microbial community, and the accumulation process of the CBG. The results indicate that the CBG in the TD region is a mixture of biodegraded thermogenic gas and secondary microbial gas. The geochemical characteristics of the water samples indicate that the coalbed water actively receives recharge from surface water, and it exhibits the characteristics of a weak-runoff water environment (i.e., a relatively closed system). Based on the stable isotope compositions of the gas-water samples, CO2 reduction is the main metabolic pathway for generating the microbial gas. That is, in the relatively closed system, initial CO2 from the different sources was converted to CH4 by hydrogenotrophic methanogens (e.g., Methanobacterium), and the residual CO2 became progressively enriched in 13C (i.e., high positive δ13C-CO2 and δ13C-dissolved inorganic carbon values). However, the microbial community composition indicates that acetogenic bacteria (e.g., Acetobacterium, and Lentimicrobium) and acetoclastic methanogens (i.e., Methanosaeta) are widely developed in the coalbed water, so acetoclastic methanogenesis should also take place. According to the priority of the biodegradation, the microbial oxidation of thermogenic C2+ components (e.g., ethane and propane) mainly occurred in the early stage and lasted for a short period of time, primarily producing CH4 enriched in 12C. After these components were consumed, the bacteria began to degrade the organic matter in the coal, and the methanogens continued to generate secondary microbial gas, finally forming a 13C-enriched pool of carbon species. Based on these results, the CBG accumulation in the TD region was determined. The results of this case study provide an obvious supplement to the geological theory of CBG accumulation, and have a guiding significance for CBG exploration in the TD region.