Differential Accumulation Dynamics of Organic Matter in the Central and Eastern Bohai Bay Basin (Eastern China): Implications for Tectonic Activity

Abundant aquatic environments and sufficient material supplies in lacustrine rift basins favor organic matter (OM) accumulation. However, how multistage faulting intensity, tectonic styles, and resultant paleogeomorphological evolution jointly regulate OM enrichment remains unclear, particularly the spatial heterogeneity of such controls across different sag settings. Taking three sags across the central-eastern Bohai Bay Basin (Bozhong Sag (BZS), southern subsag of the Liaozhongnan Sag (SLZS), and Bodong Sag (BDS)) as the target, this study conducted integrated analysis of organic and elemental geochemistry of the organic matter. The spatiotemporal dynamics of organic matter distribution was revealed, and sedimentary environments were reconstructed. By integration with paleogeomorphological analysis, this study elucidates the control of tectonic dynamics on OM enrichment. Results show that during the tectonically stable period, wide-shallow, high-salinity waters dominated by intrabasinal material inputs preserve abundant Type I OM. During the tectonically active stage, spatial variations in fault activity intensity and tectonic styles shaped the basin's geomorphology with alternating sags and uplifts, increasing extra-basinal material input and leading to mixed OM sources. The BZS (basin center) was mainly controlled by extensional activities, resulting in deep, extensive waters. Gentle topography facilitated unimpeded clastic input; however, the concomitant inflow of riverine freshwater was unfavorable to OM preservation. The SLZS, affected by strike-slip extensional activity, formed a local topographic high with shallow waters, where continuous hydrodynamic disturbance and limited terrestrial OM input inhibited OM accumulation. The BDS (marginal sag), influenced by multidirectional overlapping strike-slip/extensional activities, had enclosed deep saline waters due to peripheral uplifts and is the optimal condition among coeval sags. A conceptual model for OM enrichment was established, which aids in predicting high-quality source rocks and in guiding shale oil/gas exploration in continental lacustrine rift basins.