Orbital forcing and marine transgression controls on lacustrine organic matter enrichment during the Turonian: evidence from the lower Qingshankou Formation, Songliao Basin

The lacustrine Qingshankou Formation in the Songliao Basin is a major source rock for shale oil. However, the controls on its organic matter (OM) enrichment, especially the interplay between orbital forcing and marine transgressions, remain unclear. This study focuses on the Lower Member of the Qingshankou Formation (K 2 qn 1 ), which was deposited during the Turonian stage (∼91.8–90.9 Ma). By integrating high-resolution well logging data, cyclostratigraphy, geochemistry, and machine learning (LSTM-SHAP), we developed the astronomical time scale (ATS) for K 2 qn 1 , reconstructed lake-level fluctuations, and quantified the controls on OM enrichment. Results show that Milankovitch cycles controlled sedimentation and lake-level fluctuations, which in turn drove facies-dependent OM enrichment mechanisms. In semi-deep lake facies, OM enrichment was dominantly controlled by redox conditions linked to lake-level fluctuations, whereas in deep lake facies, it was primarily governed by paleoproductivity. Marine transgressions, identified as episodic events at ∼91.5 Ma, enhanced bacterial sulfate reduction, which typically modulated and thereby suppressed OM enrichment across the system. The findings demonstrate that OM enrichment in large lacustrine systems results from the complex interaction of orbitally-driven climate rhythms and episodic geological events, providing a predictive model for identifying heterogeneous “sweet spots” in lacustrine shale systems.