What Control Biogenic Gas Formation in Qaidam Basin, China
Shuai, Yanhua; Zhang, Shuichang; Grasby, Stephen E.; Chen, Zhuoheng
Biogenic gases are widely distributed in the shallow portion of many sedimentary basins. Such accumulations account for as much as 20% of the worlds discovered gas reserves, and more economic reserves of biogenic gas are expected to be discovered in the future. However, many of the processes that drive active microbial methanogenesis in the subsurface, forming these gas reserves, are still uncertain.
With about 320 billion cubic metres of proven biogenic gas reserves, the rapidly buried Qaidam Basin, located north of the Qinghai-Tibetan Plateau in northwest China, forms an ideal location to investigate primary controls on methanogenesis. Previous work suggested that this biogenic gas was generated early in the basin history, during host rock sedimentation. We examined both formation water and organic matter geochemistry to evaluate the extent and controls on microbial activity. Indicators of microbial activity (ratios of ipC25/nC22, ipC30/nC26, and U/R) in the biogenic gas producing region of the Sanhu Depression, Qaidam Basin, were found to vary. Different levels of microbial activity were determined as a function of formation water geochemistry. High levels of microbial activity, with high ratios of ipC25/nC22, ipC30/nC26, and U/R, occurred in layers influenced by fresh water recharge, as indicated by low TDS and Cl concentrations, while the sections containing much more primary formation water, with high TDS and Cl concentrations, showed low levels of microbial activity corresponding to low values of ratios of ipC25/nC22, ipC30/nC26, and U/R. Microbial activity is higher in areas of more diluted waters, related to higher permeability zones. Therefore, meteoric water influx appears to have played an important role in formation of significant economic reserves of biogenic gas in the Qaidam Basin. This result suggests also that the deep biosphere can be greatly affected in semi-closed hydrologic systems that can influence the geochemistry of the pore-space environment microbes inhabit. A relatively closed environment could confine the quantity and activity of the microbes in the deep biosphere.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013