A review of alkane gas geochemistry in the Xujiaweizi fault-depression, Songliao Basin

A large suite (172) of gases from the Yingcheng Fm. in the Xujiaweizi fault-depression has been studied chemically and isotopically. The results show that they are mainly type-III kerogen/coal-type gas and have been generated and accumulated under the influence of hydrothermal fluids. Both measured and calculated vitrinite reflectance data show that they have been generated mainly at source maturity higher than 2.0%Ro. Many gases show carbon isotopic reversals (typically, δ13C1 > δ13C2 and δ13C2 > δ13C3). The origin of carbon isotopic reversals is discussed in this paper and the results show that previous interpretations, including (a) mixing and (b) the occurrence of abiogenic gases, are incorrect. Based on iso-butane/n-butane data, wet-gas cracking may have occurred but it is not the main cause of carbon isotopic reversals here. We propose that a process of abiogenic polymerization of C2+ gases with methane is the main cause of the carbon isotopic reversals in this basin. ► Alkane gases in the Xujiaweizi fault-depression are coal-type gas. ► Carbon isotopic reversals mainly occur at thermal maturity >2.0%Ro. ► Abnormally lighter δ13C2 values appear with more content of C2+. ► Wet-gas cracking may contribute carbon isotopically lighter δ13C2+. ► Abiogenic polymerization of alkane gases is the main cause of carbon isotopic reversals.