Paleotopography, Palaeoclimate and Terminal Fan Evolution System of the Lower Paleogene of the Shahejie Formation in Bonan sag: Insights into Paleogeographic Reconstruction

The fourth member of the Shahejie Formation (Es4x) represents a critical unit within the Bohai Bay Basin (BBB), not only as a significant oil reservoir but also for understanding depositional environments and evolution during the late Paleocene to early Eocene, particularly in the Bonan Sag. This study emphasizes the influence of paleotopography and paleoclimate on the spatial and temporal distributions of sedimentary facies within Es4x, focusing on reconstructing and analysing the evolution of the fan system through paleogeographic modelling. A multidisciplinary approach integrates seismic data, well logs, core samples, petrographic analyses, and geochemical data to provide a comprehensive understanding of depositional processes influenced by tectonic activity, climate, and sedimentary systems. These findings underscore the profound impact of geomorphological and paleotopographic variations on the distributions of terminal fans and their sandstone reservoirs within Es4x. Proximal, middle, and distal fan systems were identified as products of regional topography and sediment transport pathways. Additionally, paleoenvironmental and paleoclimatic reconstructions based on sedimentary features and climate proxies reveal that arid to semiarid conditions prevailed in the region, significantly influencing the sediment supply. A key discovery is the evolution of the spatial and temporal framework of terminal fan systems, marked by progradation indices during arid to hot humid intervals, followed by evaporite deposition, total organic carbon (TOC) accumulation, and mudstone color variations linked to the relative expansion of water bodies. In addition to providing a comprehensive understanding of the interplay among paleoclimate, paleotopography, tectonics, and terminal fan sedimentation during this critical period of complex evolution in the Bohai Bay Basin, this study also offers valuable perspectives for petroleum exploration. By elucidating the evolution of terminal fan depositional systems and their responses to environmental changes, the findings can enhance predictions of reservoir distribution and quality within similar deposits.