ABSTRACT

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, δ¹³C_CO2 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, δ¹³C_CO2 and δD values of natural gases. The fractionation coefficients (α_c) 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.