--> Paleolimnological Environments and the Formation of Organic-Matter-Rich Rocks in the Bohai Bay Basin: An Integrated Geochemical Study of Biomarkers, Stable Carbon and Oxygen Isotope, and Trace Elements

AAPG ACE 2018

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Paleolimnological Environments and the Formation of Organic-Matter-Rich Rocks in the Bohai Bay Basin: An Integrated Geochemical Study of Biomarkers, Stable Carbon and Oxygen Isotope, and Trace Elements

Abstract

Nineteen Paleogene rock samples in the Bohai Bay Basin, including Oligocene the Dongying Formation (Ed) and the first (Es1) and third members (Es3) of the Eocene Shahejie Formation, were analyzed for Total organic carbon (TOC), Rock-Eval, stable carbon and oxygen isotope of carbonate, stable carbon isotope of organic matter, biomarker compositions and trace elements (eighteen samples) to reconstruct the paleolimnological environments and investigate the controlling factors for the occurrence of high quality source rocks in this research. Rock-Eval and TOC results reveal these source rocks samples meet the standard of good to excellent source rocks. Biomarker parameters and trace elements suggest that warm and humid paleolimnological conditions prevailed during Es3 deposition. In addition, higher values of δ13Com and low ratios of δ18Ocarb indicate a hydrological open paleolake with high primary productivity, which is supported by relatively high 4MSI and high Ba/Ca ratios. Es1 samples generally display high values of G/H and ETR and low values of Pr/Ph, suggesting a strong reducing and high salinity environment (hot and arid paleoclimate) during Es1 deposition, which is in agreement with heavy δ18Ocarb and δ13Ccarb and relatively high Sr/Ba and V/Sc ratios. Ed rocks formed under a more oxic and acid environment evidenced by high values of Pr/Ph, C27Dia/C27st and kaolinite/illite and low ratios of Sr/Ba and V/Cr, moreover, the Ed is characterized by relatively high input of terrestrial matter input under humid and colder settings. The long-term positive shift of δ18Ocarb for Ed samples may be an indication of paleoclimate changes with significant influence of westerly climate.

In fact, both the high productivity and enhanced preservation conditions control the distribution of good to excellent source rock in Bohai Bay Basin. Es3 is more linked with high primary productivity which resulted in an overall positive shift of δ13Com, whereas Es1 and Ed may be more related to favorable preservation conditions. The differences in constraints are supported by various geochemical indicators.