Sedimentary Characteristics and Formation Mechanisms of Fine-Grained Sediment Lamination: A Case Study from Well A in the Lijin Sub-Sag, Dongying Sag

Lacustrine fine-grained sediments commonly exhibit well-developed laminations, with significant variations in structural characteristics such as thickness and continuity, which are closely related to depositional environments and genetic processes. This paper focuses on the characteristics and formation mechanisms of the upper Es4 to lower Es3 members of the Shahejie Formation in the Dongying Sag. Through polarized light microscopy, field-emission environmental scanning electron microscopy (FE-SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD), we systematically analyzed the types, characteristics, and genetic mechanisms of laminations in fine-grained sedimentary rocks. Results indicate that the mineral composition of these rocks is dominated by carbonates and clay minerals, allowing classification into calcareous and argillaceous mudstones. The types of laminae include calcareous laminae, argillaceous laminae, and silty laminae, which are formed by chemical precipitation, suspension settling, and low-density turbidity currents, respectively. The primary lamination associations are argillaceous–calcareous interbeds and argillaceous–silty interbeds, exhibiting rhythmic cyclicity. In the upper Es4 member, variations in climate, sediment supply, and seasonal factors caused fine-grained sediments to transition from flocculent suspension settling to chemical precipitation, forming periodic intercalations of argillaceous and calcareous laminae. In the lower Es3 member, seasonal turbidity currents triggered the deposition of normally graded silty layers and fine-silty laminae, followed by a return to suspension deposition, resulting in argillaceous–silty interbeds. This study reveals diverse transport and depositional mechanisms of fine-grained sediments under varying hydrodynamic conditions. It provides a new case for understanding the genesis of fine-grained sedimentary rocks and offers geological insights for shale oil exploration and development in the Dongying Sag.