Astronomical forcing controls on lacustrine geochemistry and organic matter enrichment in the Middle Eocene Bohai Bay Basin, China

A continuous succession of organic-rich mudstones developed from the upper sub-member of the 4th member of the Shahejie Formation (E s 4U ) to the lower sub-member of the 3rd member of the Shahejie Formation (E s 3L ) in the Dongying Sag, Bohai Bay Basin. Organic matter enrichment is generally influenced by the interplay of depositional environment and climate variability, modulated by orbital-scale astronomical forcing, though their coupling mechanisms remain poorly understood. This study focuses on core (approximately 390 m in length) data from well Fanye-1 and astronomical cycle analysis. Gamma-ray (GR) logging, along with mineral composition and geochemical proxies, is applied to explore the relationship between orbital forcing, climate variability, and organic matter enrichment. The analytical results show that the E s 4U and E s 3L sub-members exhibiting distinct geochemical signatures follow consistent cyclic patterns. From bottom to top, their total organic carbon ( TOC ) contents decrease slightly, redox indicators (δ 15 N org , V EF , pyrite) decline, chemical indices of alteration (CIA) increase, Ti contents fluctuate upward, and salinity proxies (Sr/Ba and dolomite) decrease. The E s 4U features higher redox indicators, lower CIA and Ti content, and elevated salinity proxies compared to the E s 3L . Astronomical analysis reveals orbital cycles of eccentricity (405 kyr and 100 kyr), obliquity (41 kyr), and precession (20 kyr) in both sub-members. These signals are more pronounced in the lower sections of the E s 4U and E s 3L sub-members, where the maximum amplitude enhanced climatic fluctuations. This led to increased evaporation, elevated salinity, stronger water-column stratification, and persistent anoxia, all of which favored organic matter preservation. In contrast, the minima of orbital amplitude in the upper sections induced a wetter climate, freshwater dilution, and reduced anoxia, resulting in poorer preservation. This study establishes a framework linking astronomical forcing, climate variability, water-column stratification, and organic matter enrichment, providing new insights into shale oil accumulation in continental basins.