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The Yingjisu Depression in the Tarim Basin produces gas with high wetness, nitrogen content, and ethane carbon isotope is abnormally depleted. The hydrocarbon generation capability of the Jurassic coal strata in this area has been controversial. Anomalies in the geochemical characteristics and the actual geological conditions have resulted in challenges in identifying the source and genetic type of natural gas in the Yingjisu Depression. In the present study, the composition and carbon isotope of natural gas are investigated. In addition, the controversial Jurassic coal strata are comprehensively evaluated, and the biomarkers are analyzed. The actual geological conditions of the Yingjisu Depression are studied combined with geochemical analysis. The components and carbon isotope suggest that the natural gas in the Yingjisu Depression is oil-associated gas generated from marine source rocks in the late mature stage. The formation temperature of the natural gas was higher than 185 °C, which represents secondary cracking oil at high thermal maturity. The Jurassic coal strata has low T max (lower than 435 °C) and % R o = 0.3–0.7. These strata cannot be the source rocks of natural gas in the Yingjisu Depression with high content of N 2 , typical of marine sediment. However, the biomarkers confirmed that the oil and gas in the Yingjisu Depression were derived from the Cambrian–Ordovician source rocks rather than the Jurassic. Therefore, the natural gas in the Yingjisu Depression is the secondary cracking of oil from the Cambrian–Ordovician paleo-reservoir. According to the actual geological conditions, the cracked gas migrated upward along deep faults to accumulate in the Silurian and Jurassic reservoirs, eventually forming the Yingjisu Depression gas field. • Geochemical and geological characteristics were combined to study the natural gas. • Jurassic coal strata have low thermal maturity and could not produce industrial gas. • Natural gas was generated from the Cambrian–Ordovician paleo-reservoir cracking gas. [ABSTRACT FROM AUTHOR]

Samples for inclusion analysis were collected to investigate reasons for carbon isotope rollover in the Yan'an gas field (YAGF). Homogenization temperature of fluid inclusions, constituents and carbon isotope of gases in fluid inclusions were measured. Geochemical parameters of gas in wells and fluid inclusions in the northern gas fields (NGF) and YAGF were compared. The results show that geochemical characteristics of well and fluid inclusions gas in the NGF are almost the same, suggesting that it was originated from one set of source rocks. The direct reason for carbon isotope rollover in the YAGF is the depleted ethane carbon isotopes, while the methane carbon isotopes do not change much compared with the primary gas. There was a period of high temperature fluid filling and accumulation (140–160 °C) in the YAGF. The higher thermal maturity has disparate effects on methane and ethane. Higher thermal maturity is an important inducement of carbon isotope rollover. Mixture of oil-derived gas generated from the marine Ordovician Majiagou Formation will also result in carbon isotope rollover. • Gas in fluid inclusions was analyzed and compared with well gas. • Carbon isotope rollover was studied by geochemical analysis. • Primary gas in fluid inclusions of the Yan'an gas field was first reported. • High thermal maturity is the principal prerequisite for carbon isotope rollover. [ABSTRACT FROM AUTHOR]