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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]

To identify the reasons for more depleted ethane carbon isotopic value (MD-δ13C 2) in the Southeastern gas field (SEGF), fluid inclusions samples from northern and the SEGF were collected. Homogenization temperature and salinity of fluid inclusions, components and carbon isotopic values of gases released from the crushed inclusions were tested. Geochemical fingerprints information of producing gas in wells and primary gas in inclusions were contrasted. The comparison suggests that geochemical parameters of primary and producing gas in the NGF are similar, whereas those in the SEGF are completely different. The ethane carbon isotopic values of producing gas became more depleted compared with the primary gas, whereas the methane carbon isotopic values are not much different from the primary gas. This indicates that the secondary effect caused the more depleted ethane carbon isotopic values. There was one more period of high temperature fluid filling (140–160 °C) in the SEGF in comparison with the NGF. Higher thermal maturity is the key inducement of the abnormal MD-δ13C 2. • Geochemical parameters comparison of primary gas with producing gas was conducted. • More depleted ethane carbon isotopic values were studied based on geochemistry. • Fluid inclusions and natural gas parameters were combined to study the mechanism. • Higher thermal maturity is the key inducement of the abnormal carbon isotopic values. [ABSTRACT FROM AUTHOR]