Timing of biogenic gas formation in the eastern Qaidam Basin, NW China

Abstract

Quaternary biogenic gas from the Qaidam Basin, NW China is regarded as one of the representative examples of early generation biogenic gas (gases formed by microbial activity during sedimentation of source rocks). However, new age determinations of natural gas and formation waters make this generally accepted assumption questionable. Gas in the Qaidam Basin is dominantly methane (>99.0%), and has δ13CCH4 values in the range of −68‰ to −65‰, reflecting the microbial origin. Carbon and hydrogen isotope values of methane, coupled with carbon isotope values of carbon dioxide and dissolved inorganic carbon (DIC), reflect water derived hydrogen in the methane and a dominant CO2 reduction metabolic pathway for the formation of biogenic methane. Coproduced formation waters have hydrogen and oxygen isotope values similar to surface water from the Golmud and Ortmuren rivers (T-test=0.75), but different from the original formation water (T-test<0.01) on the basis of the δD values of the long-chain n-alkanes, indicating open hydrologic communication between meteoric water and the reservoir. The analyzed waters that appear to be the source of hydrogen during methanogenesis are at least 1 to 2Ma younger than the host reservoir sediments, inconsistent with an early gas generation hypothesis. The apparent age of 1.34Ma to 0.35MaBP for formation waters suggests that the earliest accumulated biogenic gas was formed after deposition of the K1 sequence, around 1.35MaBP. This corresponds to a regional uplift, erosion, and water influx event. Economic accumulations of biogenic gas were thus most likely generated “late” in the Qaidam Basin, rather than at the same time as sediment deposition. Fresh water recharge may be critical to enhancing biogenic methane generation by introducing nutrients and/or microbes into the reservoir.