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The geochemical characteristics of natural gases from Lower Paleozoic Jingbian gas field are abnormal and the carbon isotope of most natural gases has reversed, which led to a dispute over the source of natural gas in the Lower Paleozoic. The Upper Paleozoic Yulin gas field is adjacent to the Jingbian gas field, but the geochemical characteristics of the Yulin gas field did not appear abnormal. Besides, the ethane carbon isotope of Upper Paleozoic Yulin gas field is much higher than that of Lower Paleozoic Jingbian gas field. In order to identify the reasons for the carbon isotope reversal of natural gas in Lower Paleozoic Jingbian gas field and its relationship with natural gas source, We conducted the components and stable carbon isotopes of natural gas from Upper Paleozoic Yulin gas field and Lower Paleozoic Jingbian gas field. On the basis of these experiments, we compared the natural gas components and carbon isotope of 100 samples from both Upper Paleozoic Yulin gas field and Lower Paleozoic Jingbian gas field. The results indicate that higher maturity will break the conventional rule, when maturity reaches a certain threshold; δ 13 C ethane value will decrease with maturity. Although δ 13 C methane value will also increase with maturity, the extent will be relatively smaller. This leads to unusual δ 13 C methane  > δ 13 C ethane in the region with higher maturity. In the process of downward migration of coal-derived gas from Carboniferous-Permian coal measures, component fractionation and carbon isotope fractionation might occur, resulting the ethane components and carbon isotope value of Lower Paleozoic less than those of Upper Paleozoic, which will lead to the carbon isotope reversal. The mixing of oil-derived gas and coal-derived gas at different ratios may also lead to the reversal of carbon isotopes. The natural gas types of Jingbian gas field are mainly coal-derived gas generated from Carboniferous-Permian coal measure strata, with a small amount of oil-derived gas generated from Carboniferous-Permian marine-terrigenous limestone. [ABSTRACT FROM AUTHOR]

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]

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]