Methanogenic pathway of the Quaternary biogenic gases in the Qaidam Basin, China

The Quaternary biogenic gas province in the east of the Qaidam basin is the largest biogenic gas province in China. We collected 21 gas samples from the different gas pay beds of the Sebei 1 gas field and the Sebei 2 gas field, measured their chemical compositions and carbon isotopes, and emphatically discuss their methanogenic pathway and significance. The CH4 and CO2 of the biogenic gases were enriched in 13C with increasing depth, which indicates that the gases are CO2 reduction-derived gases. The δ13C1, δ13CCO2 and δD values of the biogenic gases approximate the range of the corresponding values of CO2 reduction gases. The values fall in the CO2 reduction field on genetic diagrams based on δ13C1, δ13CCO2 and δD values of natural gases. The fractionation coefficients (αc) of the carbon isotope of the CH4 and CO2 of the biogenic gases are more than 1.005, characteristic of a CO2 reduction pathway. During the Quaternary due to the dry climate, low temperature, high sedimentation rate and a high content of sulfate in the east of the Qaidam basin, methane bacteria could not propagate until sediments were buried at deeper depth. These conditions benefit a CO2 reduction pathway of the gases. These results are of importance in discussing the biogenic gas origin and formation conditions, determining the methods and conditions for biogenic gas simulation, accessing biogenic gas reserves, establishing a pool-forming model and selecting targets for natural gas exploration.