Effects of paleo sedimentary environment in saline lacustrine basin on organic matter accumulation and preservation: A case study from the Dongpu Depression, Bohai Bay Basin, China

The Dongpu Depression has abundant oil and gas resources, but the distribution of hydrocarbon resources and enrichment degrees of organic matter (OM) in the northern and southern Dongpu Depression have significant differences due to the diverse sedimentary environment. To gain deep insight into the effects of sedimentary environment on OM accumulation and preservation, a series of experiments including element geochemistry, organic geochemistry and isotope geochemistry were performed on core samples collected from the fresh, brackish and saline regions of the third member of the Eocene Shahejie Formation (Es-3). The lacustrine closure, paleo-salinity, redox conditions, hydrodynamic conditions, paleo-climate and paleo-productivity were quantitatively evaluated. Controlling factors and OM enrichment models were evaluated and established. The results show that the Es-3 Formation was deposited as the lake experienced closed saline to transitional and then fresh lake stages. The lacustrine deposits were accompanied by climate changes from dry to humid and strongly rifting during Es-3 deposition. The weak hydrodynamic conditions (Zr/Rb) and paleo climate indices (delta O-18, climate index, Fe/Mn, Sr/Cu and Al/Mg) imply that moderate humid and dry climate can encourage OM development and preservation. The paleo productivity indices (delta C-13, P/Al, P/Ti and P content) are positively related to TOC indicating that high productivity promotes OM accumulation. All the paleo salinity indices (Z value, Sr/Ba and B/Ga) have positive correlations with TOC content, suggesting that OM enrichment increases with salinity to some extent. The positive relationships between the redox indices (V/Cr, V/Sc, U/Th, and dU) and TOC indicate that redox conditions are the most important for OM preservation. This study not only advances the theory of OM enrichment mechanism in saline basins, but also provides guidance for predicting the distributions of high-quality source rocks in the Dongpu Depression.