Source rock characteristics and depositional environment of Lower Cretaceous Dsunbayan oil shales, Mongolia.

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Elemental analyses, Fischer assay pyrolysis, Rock-Eval pyrolysis and biomarker analyses were conducted for twenty-six lacustrine oil shales from the Lower Cretaceous Dsunbayan Group, Mongolia.The Dsunbayan oil shales show large variations of organic carbon content (1.7-21.3wt%) and oil yield (0.3-11.8 wt%). A Rock-Eval pyrolysis shows that immature Type I kerogen dominates in these samples. Biomarker compositions show that major source organisms of the Dsunbayan oil shale were an autotrophic prokaryote such as a cyanobacterium, nonmarine algae, terrestrial higher plants, heterotrophic bacteria and bacterivorous ciliates.Good correlations exist among organic carbon content, oil yield, bitumen content and n-heptadecane anomaly {2×n-C17/(n-C16+n-C18)n-alkane ratio}. The samples with high n-heptadecane anomaly are rich in β-carotane, which suggests that the degree of bottom water anoxia controlled organic carbon content and oil yield, and was related to the contribution of an autotrophic prokaryote which sourced dominant n-heptadecane.High C/S ratio and little sulfur compound in the Dsunbayan oil shales indicate that they were deposited in a freshwater environment. The tropical-subtropical climate promoted thermal stratification and bottom water anoxia in the freshwater lakes. The degree of development of bottom-water anoxia was likely controlled by nutrient supply from the surrounding area.
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