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The Sinian Dengying Formation is a key target for deep oil and gas exploration in the Sichuan Basin, China. Natural gas was mainly distributed in the Ziyang–Weiyuan area (ZWA) and the Gaoshiti–Moxi area (GMA). At present, the genetic types and sources of natural gas are still unclear and controversial. The chemical composition, stable carbon isotopes, and biomarkers were used to analyze geochemical characteristics, genetic types, and gas–source relations. The results showed that the composition and content of hydrocarbon gas in the ZWA and GMA were similar. Natural gas was a typical dry gas with an average dry coefficient (C 1 /C 1 +) of 0.9991, which reflected higher methane (CH 4) content and lower heavy hydrocarbon gas (C 2 +) content. For nonhydrocarbon gas, CO 2 (0.01%–26.34%), N 2 (0.03%–24.95%), H 2 S (0.10%–3.19%), and He (0.01%–0.37%) were mainly distributed. Compared with the GMA, the ZWA was characterized by higher N 2 content and lower CO 2 content. Although the carbon isotopes had not been reversed, the values of δ13C 2 –δ13C 1 were different between the ZWA and GMA, with the GMA having the higher value. The assemblage relation between the natural gas composition and carbon isotopes indicated that the natural gas type was dominated by oil-cracking gas. Mud-shale in the high maturity stage of thermal evolution greatly influenced on the content of N 2 and He. H 2 S mainly belonged to thermochemical sulfate reduction (TSR), and most of CO 2 was formed by deep carbonate metamorphism and TSR. The gas–source relation showed that natural gas in the ZWA was derived from the Cambrian Qiongzhusi Formation and that the gas in the GMA mainly originated from the Sinian Dengying Formation and the Cambrian Qiongzhusi Formation. The established hydrocarbon generation model indicated that the Sinian Dengying Formation is potentially gas-rich. • The Sinian Dengying Formation natural gas was a typical dry gas, dominated by oil-cracking gas. • Source rocks of the Qiongzhusi Formation and Dengying Formation were the mainly gas source for gas reservoirs. • The resource potential was qualitatively evaluated by the established hydrocarbon generation model. [ABSTRACT FROM AUTHOR]

• Major elements trace hydrocarbon sources in over-mature petroleum systems. • The Sinian Dengying Formation gas systems in the Sichuan Basin were investigated. • Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P have variable occurrence states. • Mn/Fe ratio and ternary 10Mn–Fe–P diagram are used to trace hydrocarbon sources. • Lower Cambrian shales are the main sources for natural gases. Identifying hydrocarbon sources in over-mature petroleum systems is important and challenging. Here we present a pilot study using inorganic geochemistry to trace hydrocarbon sources in the giant natural gas accumulations within the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin, SW China. We investigated the major elements (Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) as tracers of hydrocarbon sources for the first time. Our results show that the over-mature solid bitumen in the reservoirs contains parts per million amounts of major elements in addition to carbon (i.e., organic matter). Na and K in the bitumen occur mainly in the V–Ni porphyrin form. Al, Fe, and Mn are present in the Fe porphyrin form. The behavior of P is relatively complex, and it may reside mainly in a non-porphyrin organic form. The other elements (Ti, Mg, and Ca) may be present as emulsified "invisible" nano-minerals. The Mn/Fe ratio and ternary 10Mn–Fe–P diagram can be used for hydrocarbon–source rock correlations. Based on the proxies, it is suggested that the lower Cambrian Qiongzhusi Formation was the main source of hydrocarbons in the Dengying Formation. Our data suggest that inorganic geochemistry is an effective method for the study of over-mature hydrocarbons and complements traditional organic geochemical approaches. [ABSTRACT FROM AUTHOR]