Abstract

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%; N₂ <3%). Methane has δ¹³C₁ values of −70 to −62‰ and δD₁ values of −240 to −220‰, suggesting generation following a CO₂ reduction pathway. Ethane is very light with δ¹³C₂ values of −50 to −44‰; propane δ¹³C₃ 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 (δD and δ¹⁸O) and ³⁶Cl 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.