Possible origins for inverse stable carbon isotopes of gaseous alkanes from the Xujiaweizi fault depression

Gaseous alkanes from the Xujiaweizi fault depression in the Songliao Basin are characterized by heavier stable carbon isotopes,mostly showing a inverse order or a reverse order.It is still in debate whether these gases are biogenic or abiogenic.To determine the potential origin of these natural gases,a series of experiments concerning the generation and synthesis of gaseous alkanes have been conducted.Mantle-derived carbon dioxide sampled from the Xujiaweizi fault depression was used to synthesize hydrocarbon gas through the Fishcher-Tropsch reaction,and the produced gaseous alkanes yielded a normal stable carbon isotope trend.Meanwhile,some source rocks with low maturity were sampled from the Shahezi Formation to conduct pyrolysis experiments,the produced gases were then determined for chemical components and stable carbon isotopes.Based on the possible end-member gases in the Xujiaweizi fault depression and geological conditions,the produced gases generated by shales and coals at different heating rate were mixed to determine the trend of stable carbon isotopes.The results showed that the mixed biogenic end-member gases could also lead to an inverse stable carbon isotopic order,whereas the mixed abiogenic gases are not necessarily reversal.Integrated with the results of pyrolysis experiments discussed in this paper,comprehensive analyses both on gases released from modern volcanic springs and on source-rock characteristics of the Xujiaweizi fault depression have suggested that natural gases from the Xujiaweizi fault depression are mainly biogenic.