The control of astronomical cycles on lacustrine mixed sedimentation and hydrocarbon occurrence: A case study of the Paleogene Shahejie Formation in the Dongying Sag, Bohai Bay Basin

Gravity flows in steep-slope zones of lacustrine rift basins interact with deep-water shales to form heterogeneous mixed sedimentation zones, yet their sedimentary evolution and hydrocarbon occurrence characteristics remain poorly understood. In this study, core and thin section observations, X-ray diffraction, scanning electron microscopy, mercury intrusion porosimetry, low-pressure nitrogen physisorption, total organic carbon testing, step-by-step Rock-Eval pyrolysis, and time series analysis, were conducted to decode sedimentology and hydrocarbon occurrence characteristics based on lithofacies combinations controlled by astronomical cycles. Results reveal five microfacies (calcareous/felsic semi-deep lake, outer fan, channel front, braided channel) and six lithofacies combinations (LC1–LC6) shaped by debris flows, turbidity currents, and felsic mixed floating currents. The 406-kyr long eccentricity cycle governs sediment supply and reservoir heterogeneity. During high eccentricity phases, increased monsoon-driven terrigenous input enhances sandstone deposition, developing braided channel (LC5) and channel front (LC6) microfacies. These intervals exhibit elevated terrigenous quartz/feldspar content and reduced organic carbon in fine-grained sedimentary rocks, with hydrocarbons occurring as quasi-continuous to discontinuous accumulations. Conversely, during low eccentricity phases, felsic mineral layers or isolated sandstone interlayers dominate, forming outer fan (LC3), channel front (LC4), and semi-deep lake (LC1–LC2) microfacies. These intervals show lower terrigenous content and higher TOC in fine-grained sedimentary rocks, with hydrocarbons occurring as continuous to quasi-continuous accumulations. A novel sedimentary evolution and hydrocarbon occurrence model demonstrates the ordered coexistence of conventional and unconventional reservoirs, driven by astronomically forced climate fluctuations. This framework advances theoretical understanding and optimizes exploration strategies for steep-slope lacustrine basins with mixed sedimentation.