Effects of differential hydrocarbon expulsion on the characteristics of saturated and aromatic compounds during hydrous pyrolysis of organic-rich shale

Abstract

Primary migration and expulsion of hydrocarbons from organic-rich shales can remarkably alter the group compositions of shale oil, while the impacts on specific molecular constituents are still unclear. Here, a lacustrine Chang-7 organic-rich shale sample from the Ordos Basin was subjected to semi-closed and closed pyrolysis, and the characteristics and ratios of commonly used saturated and aromatic constituents of the collected oil and rock extracts were discussed. The results indicated that low-molecular-weight (<C₂₀) normal alkanes and pristane (Pr) are preferentially expelled from the shale before oil-cracking stages. In contrast, high-molecular-weight normal alkanes and phytane (Ph) are retained within the shale due to the relatively more robust interactions with kerogen, and polarity dominantly affects the composition fractionation during the expulsion of aromatic compounds. This led to significant discrepancies in ratios of saturated and aromatic hydrocarbon compounds between the collected oil and rock extracts, such as (nC₂₁+nC₂₂)/(nC₂₈+nC₂₉), Pr/Ph, and indicators related to methylnaphthalene, methylphenanthrene, methyldibenzothiophene, and trifluorene series. Trends of those molecular ratios reverse due to intensive thermal cracking at higher maturities. The fractionation heavily affects the accuracy of maturity comparison of aromatic hydrocarbon ratios in shale source rocks and related hydrocarbon accumulations, ultimately casting doubts on oil-source correlations. Moreover, the fractionation can misguide inferences regarding paleoenvironmental conditions based on the trifluorene series and the ternary diagram of the Ph/nC₁₈, Pr/Ph, and Pr/nC₁₇. Finally, we propose that some proxies previously applied to maturity and paleoenvironmental evaluation can be used to indicate hydrocarbon expulsion.