Paleolimnological environments and the formation of high quality source rocks in the Bohai Bay Basin: An integrated geochemical study of biomarkers, stable carbon and oxygen isotopes, and trace elements

Abstract

Fourteen Paleogene rock samples from the Bohai Bay Basin, including samples from the third member of the Oligocene Dongying Formation (Ed3) and the third member of the Eocene Shahejie Formation (Es3), underwent total organic carbon (TOC) analysis, Rock-Eval pyrolysis, stable carbon (δ13Ccarb) and oxygen isotope (δ18Ocarb) analysis of carbonates, stable carbon isotope analysis of organic matter (δ13Com), biomarker and trace element compositional analysis to reconstruct paleolimnological environments and investigate the factors that control the occurrence of high quality source rocks. The Rock-Eval and TOC results revealed that the source rocks meet the standard for good to excellent source rocks. The biomarker parameters and trace elements suggest that warm and humid paleolimnological conditions prevailed during the deposition of Es3. In addition, heavier values of δ13Com and light values of δ18Ocarb indicate the existence of a hydrologically open paleolake with high primary productivity, which is supported by relatively high 4-methyl sterane index values and high barium/calcium (Ba/Ca) ratios. Ed3 rocks formed under less reducing and more acidic conditions, which is supported by high pristane/phytane, C27 diasteranes/C27 regular steranes and kaolinite/illite ratios and low strontium/barium (Sr/Ba) and vanadium/chromium (V/Cr) ratios. Moreover, Ed3 is characterized by a relatively high input of terrestrial organic matter under humid but cooler settings. A long-term positive shift of δ18Ocarb revealed by the Ed3 samples is associated with changes in oxygen isotopic composition of inflow water (higher precipitation) and temperature (colder). The conditions of both high productivity and enhanced preservation controlled the distribution of good source rocks in the Bohai Bay Basin (BBB). Es3 is more likely linked with high primary productivity, which resulted in an overall positive shift in δ13Com, whereas Ed3 may be more related to favourable preservation conditions associated with relatively high sedimentation rates.