The Isotopic Tracer and Resource Value of Microbial Gas Production in Coalbeds—A Case Study of Coalbed Gas in Enhong, China

Abstract

From the new perspective of the dynamic isotopic fractionation of microbes and the mixture of secondary biogenic gas and thermogenic gas in coalbeds, this study evaluates the geochemical characteristics of mixed coalbed gas in Enhong. Additionally, by using isotopes, the study traces the microbial activities and the results of those activities, such as the methane production of bacteria, and reveals the formation mechanism and genetic types of coalbed gas. The measured δ13C1 values of the coalbed gas samples are from −54.5‰ to −32.0‰. Based on the values of Ro and δ13C2, by calculation, the δ13C1 values of thermogenic methane range from −30.6‰ to −28.1‰ and from −30.7‰ to −28.3‰, respectively. The results of the two calculations are highly consistent, and both are approximately 20‰ higher than the measured values. The measured δDCH4 values are from −217‰ to −196‰, being between thermogenic methane and microbiogenic methane; the δ13CCO2 values are from −30.5‰ to −23.9‰, growing heavier with respect to the original thermogenic gas; the measured δ13C2 values are from −25.7‰ to −22.6‰, and the estimated δ13C2 values are from −21.8‰ to −21.2‰, also indicating growing heavier. The δ13C1 and δ13C2 values are negatively correlated; both the Δδ13CC2–C1 and Δδ13CCO2–C1 values are increasing. All of the above characteristics indicate that CO2 is reduced into microbial genetic methane by methane-producing bacteria and it mixes with thermogenic gas, which is new evidence showing the existence of secondary biogenic gas. Via calculations using a variety of data, such as the values of δ13C1, δ13C2 and Ro, it is found that thermogenic methane accounts for approximately 38% to 58% of the total amount, and microbial genetic methane accounts for approximately 42% to 62%. The proportion of micro-biogenic methane reduced from the top down, which occupied more than 50% of that in the coalbed buried within 1000 m deep. It increased the content of coalbed gas by more than 1 times. Within 1000 m deep, coalbed temperatures are generally lower than 40 °C, which is the most appropriate section for methanogenic bacteria activity and secondary biogenic gas generation. Coalbed uplift to the shallow parts in the late stage is the basic geological condition for the formation of secondary biogenic gas, which has significant resource value.