Influence of retained bitumen in oil-prone shales on the chemical and carbon isotopic compositions of natural gases: Implications from pyrolysis experiments

Retained bitumen in oil-prone source rocks has a significant impact on late gas generation but such influence is still poorly quantified because it is quite difficult to restore the bitumen content retained in a mature source rock. In this study, a suite of artificially-matured samples were prepared from a moderate maturity shale sample to represent shales with different amount of retained bitumen. These samples were then pyrolyzed in sealed gold tubes to investigate how the amount of retained bitumen affects the changes in chemical and carbon isotopic compositions of generated gases, and the uncertainty of the evaluation of gas maturity and source identification. The results indicate that the decrease in yields of hydrocarbon gases is proportional to the reduction of retained bitumen content, however, a progressive enrichment in heavier carbon isotope (C-13) of gaseous hydrocarbons occurs while less amount of bitumen is retained in shales. This experimental observation partly explains the heterogeneity of methane carbon isotopes for shale gases that have identical thermal maturity levels. Although the amount of retained bitumen can significantly change the carbon isotopic ratios of individual gas (delta C-13(1), delta C-13(2), and delta C-13(3)), the difference in carbon isotopic composition between gaseous components such as ethane and methane (delta C-13(2)-delta C-13(1)) or propane and ethane (delta C-13(3)-delta C-13(2)) are not significantly affected, and therefore these differential values can be potentially used as maturity indicators for overmature natural gases exhibiting no carbon isotopic reversals. Geochemical and carbon isotopic diagrams that are widely used for the identification of oil - and kerogen cracking gases are also found to be affected by the retained bitumen content. These results illustrate that it is necessary to take into account the retained bitumen content of shales when thermal maturity and genetic origin of natural gases derived from oil-prone source rocks are being evaluated.