Controls on biogenic gas formation in the Qaidam Basin, northwestern China

Factors controlling biogenic gas generation were examined in the Quaternary Qaidam Basin of China, one of the youngest and largest biogenic gas-producing basins in the world. Indicators of microbial activity were found in higher permeability stratigraphic intervals, including high ratios of unresolved complex mixture/resolved compounds (U/R), as well as high ratios of methanogen biomarkers to normal alkanes, such as 2,6,10,14,19-pentamethylicosane/nC22 (ipC25/nC22) and squalane/nC26 (ipC30/nC26), from solvent organic extracts from 86 samples. Samples with the highest concentrations of ipC25/nC22 and ipC30/nC26 were the most highly biodegraded as shown by U/R ratios. Most intervals with high levels of methanogen biomarkers were characterized by groundwater with low total dissolved solids (TDS) and low Cl concentrations, thought to be related to meteoric water recharge. High H2 concentrations were also related to low TDS (and Cl) formation water. Results demonstrate that meteoric water invasion was important in stimulating microbial activity.