Quaternary biogenic gases in the Qaidam Basin, Western China

The Quaternary Qigequan Formation, a continental clastic sedimentation system in Sanhu Depression of eastern Qaidam Basin, west-central China, contains abundant biogenic gas resources. Favorable characteristics of the Sanhu Depression's geological history and framework responsible for biogenic gas production include rapid sedimentation, shallow burial depth, organic-rich sediments, high porosity/permeability, and a saline depositional environment. Gases are dominated by methane (>99%), with trace ethane/propane (C2+3<0.5%) and minor non-hydrocarbon gases (CO2 <0.5%; N-2 <3%). Methane has delta C-13(1) values of -70 to -62% and delta D-1 values of -240 to -220%, suggesting generation following a CO2 reduction pathway. Ethane is very light with delta C-13(2) values of -50 to -44%; propane delta C-13(3) values range from -34 to -32%. Ethane and propane are inferred to have the same thermocatalytic origin under low organic maturity levels and are unrelated to the biogenic origin of methane. Biogenic methane is equilibrated with the saline formation waters. The formation water geochemistry data, including stable isotope values (delta D and delta O-18) and Cl-36 age, suggest recent dilution by meteoric waters. Methanogenesis is currently active, as indicated by the presence of both abundant hydrogen and microbes. The recent and ongoing biogenic gas generation model proposed explains why biogenic gas accumulations are so abundant in a shallow geological setting where the conditions for accumulation and preservation are otherwise considered relatively poor.