Geochemical investigation of organic matter of various occurrences released via sequential treatments of two argillaceous source rock samples from fresh and saline lacustrine environments

Organic matter (OM) in argillaceous source rocks occurs in various forms, and it is common for OM be bounded to minerals forming OM-mineral complexes. In order to geochemically characterize OM of different occurrences, some high quality source rocks were sampled from the lower third member (Es₃) and the upper fourth member (Es₄) of the Shahejie Formation in the Dongying Sag, and were subjected to sequential treatments as follows:solvent extraction-saponification-hydrochloric acid hydrolysis-hydrofluoric acid hydrolysis. The free, clay interlayer absorbed, carbonate-bound, and clay surface-bound OM were successively released and GC-MS analyzed. Results indicated that the mineral-bound OM accounted for approximately 10% of the total soluble OM in source rocks, and there exists distinct geochemical differences between the free and mineral-bound OM, with the latter dominated by polar fractions such as nonhydrocarbons and asphaltene. The former comprised higher proportions of light hydrocarbons, and selectively preserved biomarkers indicative of biogenic origin, like the bicyclic sesquiterpenoids in the carbonate-bound OM, indicative of input from Botryococcus braunii, and squalane in the clay surface-bound OM, indicative of input from archaea. The mineral-bound OM was also sensitive to environmental changes, and exhibited molecular differences between fresh Es³ and saline Es⁴ samples. Thus, it was concluded that comprehensive geochemical investigation of the free and mineral-bound OM in argillaceous source rocks could achieve more authentic geological interpretations. Besides, molecular characterization of hydrocarbons of various occurrences could help further elucidate OM preservation mechanisms, as well as better evaluate source rock potential.