Origin of hydrocarbon fluids and discussion of abnormal carbon isotopic compositions in the Lishui-Jiaojiang Sag,East China Sea Shelf Basin

The hydrocarbon gases in the L1 gas field of the Lishui-Jiaojiang Sag have been commonly interpreted to be an accumulation of pure sapropelic-type thermogenic gas. In this study, chemical components, stable isotopic compositions, and light hydrocarbons were utilized to shed light on the origins of the hydrocarbon fluids in the L1 gas pool. The hydrocarbon fluids in the L1 gas pool are proposed to be a mixture of three unique components: mid-maturity oil from the middle Paleocene coastal marine Lingfeng source rock, oil-associated (late oil window) gas generated from the lower Paleocene lacustrine Yueguifeng source rock, and primary microbial gas from the paralic deposits of the upper Paleocene Mingyuefeng source rock. Here, for the first time, the hydrocarbon gases in the L1 gas pool are diagnosed as mixed oil-associated sapropelic-type gas and microbial gas via four pieces of principal evidence: (1) The abnormal carbon isotopic distributions of all methane homologues from C₁ (CH₄ or methane) to C₅ (C₅H₁₂ or pentane) shown in the Chung plot; (2) the diagnostic 13C-depleted C₁ compared with the thermogenic sapropelic-type gas model, while δ13C₂ (C₂H₆ or ethane) and δ13C₃ (C₃H₈ or propane) both fit perfectly; (3) the excellent agreement of the calculated carbon isotopic compositions of the pure thermogenic gas with the results of the thermal simulated gas from the type-II₁ kerogen-rich Yueguifeng source rock; and (4) the oil-associated gas inferred from various binary genetic diagrams with an abnormally elevated gas oil ratio. Overall, the natural gases of the L1 gas pool were quantified in this study to comprise approximately 13% microbial gas, nearly 48% oil-associated sapropelic-type gas, and 39% of nonhydrocarbon gas. The microbial gas is interpreted to have been codeposited and entrained in the humic-kerogen-rich Mingyuefeng Formation under favorable low-temperature conditions during the late Paleocene-middle Eocene. The microbial gas subsequently leaked into the structurally and stratigraphically complex L1 trap with oil-associated sapropelic-type gas from the Yueguifeng source rock during the late Eocene?Oligocene uplifting event. A small amount of humic-kerogen-generated oil in the L1 gas pool is most likely to be derived from the underlying Lingfeng source rock. The detailed geological and geochemical considerations of source rocks are discussed to explain the accumulation history of hydrocarbon fluids in the L1 gas pool. This paper, therefore, represents an effort to increase the awareness of the pitfalls of various genetic diagrams, and an integrated geochemical and geological approach is required for hydrocarbon-source correlation.