Geochemical characteristics comparison of primary gas with producing gas and its significance for depleted ethane carbon isotopic values in the Southeastern gas field of the Ordos Basin, China

Abstract

To identify the reasons for more depleted ethane carbon isotopic value (MD-δ13C2) 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-δ13C2.