New insight into abiotic or biotic gas in deep reservoirs of the CL-I gas field in Songliao Basin (China) by light hydrocarbons associated with natural gas

Abiotic synthesis through carbon dioxide (CO2) reduction may provide the prebiotic organic compounds needed for the emergence of life, whereas petroleum geologists pay more attention to whether abiotic gas generated by purely chemical reactions plays a prominent role in the formation of industrial gas pools like biotic gas. The Cretaceous reservoir associated with volcanic rocks of the Songliao Basin in northeastern China is suspected of possessing abiotic gas due to some abnormal geochemical characteristics, such as abundant 13C-enriched CO2, 13C-enriched CH4, 13C-depleted C2H6, and an isotopic trend of decreasing 13C content with growing carbon number. To understand further the genesis of deep natural gas, we examined the molecular and isotopic compositions of C1–C4 alkanes, CO2, and light hydrocarbons (LHs) of equal number (four) of gas samples collected from the Yingcheng (K1yc) volcanic and Denglouku (K1d) clastic reservoirs in the Changling (CL-I) gas field. Compared with the products of abiotic polymerization composed of 13C-depleted straight-chain alkanes, various 13C-enriched LH isomers that are highly compatible with the molecular and isotopic compositions of coal-derived thermogenic gas are confirmed in our samples, such as 2-methylhexane (2-MH), 2,3-dimethylpentane (2,3-DMP), cyclohexane (CH), methylcyclohexane (MCH), and other branched-chain alkanes or cycloalkanes. The striking invariable K1 ratio and resemblance of isotopic curves of LHs further indicate that the natural gas in different reservoirs is derived from underlying homologous source rocks, regardless of the variation of CO2 content (K1yc > 15% and K1d < 1%). Combining geological conditions, we propose that C1–C4 and LHs that have no inherent correlation with the concentration of CO2, which is predominantly derived from mantle degassing, are generated from the thermal decomposition of 13C-enriched type III organic matter during burial and diagenesis rather than sourced from mantle degassing, abiotic polymerization, or mixing of thermogenic gas. In addition, we also suggest that the geochemical anomalies consisting of 13C-enriched methane and 13C-depleted ethane are the products of the chemical mechanisms controlled by high temperature, and mantle-derived non-hydrocarbon gas (e.g., CO2 and He) are simply added to thermogenic gas.