Orbital-Scale Climate Control on Facies Architecture and Reservoir Heterogeneity: Evidence from the Eocene Fourth Member of the Shahejie Formation, Bonan Depression, China

The Eocene fourth member of the Shahejie formation (Es4x) in the Bonan Depression, Bohai Bay Basin, records syn-rift sedimentation under alternating arid and humid climates. It provides insight into how orbital-scale climatic fluctuations influenced tectonics, facies patterns, and reservoir distribution. This study integrates 406 m of core data, 92 thin sections, 450 km2 of 3D seismic data, and multiple geochemical proxies, leading to the recognition of five facies associations (LFA): (1) alluvial fans, (2) braided rivers, (3) floodplain mudstones, (4) fan deltas, and (5) saline lacustrine evaporites. Three major depositional cycles are defined within the Es4x. Seismic reflections, well-log patterns, and thickness trends suggest that these cycles represent fourth-order lake-level fluctuations (0.8–1.1 Myr) rather than short 21-kyr precession rhythms. This implies long-term climate and tectonic modulation, likely linked to eccentricity-scale monsoon variability. Hyperarid phases are marked by Sr/Ba > 4, δ18O > +4‰, and thick evaporite accumulations. In contrast, Sr/Ba < 1 and δ18O < −8‰ reflect humid conditions with larger lakes and enhanced fluvial input. During wet periods, rivers produced sand bodies nearly 40 times thicker than in dry intervals. Reservoir quality is highest in braided-river sandstones (LFA 2) with 12%–19% porosity, preserved by chlorite coatings that limit quartz cement. Fan-delta sands (LFA 4) have <8% porosity due to calcite cementation, though fractures (10–50 mm) improve permeability. Floodplain mudstones (LFA 3) and evaporites (LFA 5) act as seals. This work presents a predictive depositional and reservoir model for arid–humid rift systems and highlights braided-river targets as promising exploration zones in climate-sensitive basins worldwide.