Development Mechanism of Organic-rich Shales in the Eocene under the Influence of Paleoclimatic Conditions in the Weixi’nan Sag, Beibuwan Basin

The Eocene was a critical period of global climate transition from the warm “greenhouse” to the cooling “icehouse” state, during which high-quality source rocks developed in several coastal basins in eastern China, with paleoclimatic conditions playing a significant controlling role. During the deposition of the Second Member of the Liushagang Formation in the Weixi’nan Sag of the Beibuwan Basin, the rifting process reached its peak; however, organic-rich shales only developed in the lower submember, suggesting that paleoclimatic conditions played a key role in their formation. This study integrates organic and inorganic geochemical analyses to investigate the development mechanism of organic-rich shales in the Eocene Liushagang Formation driven by paleoclimatic conditions, providing insights into source rock development in the Eocene coastal basins of eastern China. The results show that: (1) The organic-rich shales have total organic carbon (TOC) contents ranging from 2.7% to 10.3% (average 5.66%), significantly higher than conventional mudstones (0.59%–10.34%, average 3.01%) and shales (1.26%–6.03%, average 2.40%). (2) The Beibuwan Basin, located in the tropical monsoon region, experienced increased precipitation and a more humid environment during periods of temperature rise. The chemical weathering index indicates that the source rocks of the Liushagang Formation underwent intense weathering, corresponding to a hot and humid paleoclimate. The high Sr/Cu ratios in the lower submember of the Second Member suggest a hotter and more humid climate during the deposition of organic-rich shales, consistent with the Early Eocene Climatic Optimum (EECO). (3) During the deposition of organic-rich shales, the hot and humid climate intensified the weathering of parent rocks, supplying abundant nutrients to the lake basin and promoting algal proliferation, which led to high paleoproductivity. Meanwhile, strong evaporation created a freshwater to brackish environment that enhanced bottom-water anoxia, providing favorable conditions for organic matter preservation. The combination of high paleoproductivity and superior preservation conditions jointly controlled the development of organic-rich shales.