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Experimental Study on the Influence of Bitumen on Natural Gas Generation


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 was prepared from a low 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 change in the 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 (13C) of gaseous hydrocarbons occurs while less amount of bitumen retains in shales. For example, mature kerogen generated CH4 is about 6.7 per mil heavier than bitumen generated one. 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 (δ13C1, δ13C2, and δ13C3), the difference in carbon isotopic composition between gas components such as ethane and methane (δ13C2 ‒δ13C1) or propane and ethane (δ13C3 ‒δ13C2) are not significantly affected, and therefore these differential values can be potentially used as maturity indicators for overmature natural gases which are absent isotopic reversal in ethane and propane. 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.