Sedimentary Characteristics and Controlling Factors of Es₄^U-Es3 Fine-grained Sedimentary Rock Lithofacies in Dongpu Depression

As a rift saline lacustrine basin in the southern Bohai Bay basin, the sedimentary filling characteristics of Dongpu Depression (DD) are “full of sand and generally fine”, and fine-grained sedimentary rocks are generally developed. Exploration practice shows that the fine-grained sedimentary rocks in the upper 4th member-later 3rd member of Shahejie Formation (Es₄^U-Es₃^L) of the Puwei sub-sag in the northern DD are stable and rich in organic matter. Good oil and gas shows exist in the shale well section, which is a potential favorable zone for shale oil. In this study, the fine-grained sedimentary rocks in the Es₄^U-Es₃ were analyzed in detail and 12 types of fine-grained lithofacies were subdivided. Among them, lamellar organic-rich clayey carbonate rocks and lamellar carbonate mixed rocks are favorable lithofacies for shale oil exploration. Combined with the regional tectonic evolution, paleoclimate, ancient provenance characteristics, core analysis, thin section analysis, X-ray diffraction and trace element analysis, the characteristics and controlling factors of fine-grained sedimentation were studied. Changes in mineral composition, micro-paleontological characteristics and trace element content in fine-grained lithofacies indicated the frequent fluctuations in paleo-climate and paleo-water depth in a short cycle. The corresponding lithofacies showed regular changes in the longitudinal and transverse (longitudinal: Sand-mud-carbonate rock formed with carbonate rock-gypsum rock-saline rock rhythmic interbedded. Transverse: Lamellar carbonate lithofacies showed a ring distribution around the saline lake sedimentary center), and a favorable lithofacies development model was established. Results showed that the sedimentation of fine-grained sedimentary rocks in DD is controlled by tectonics, provenance and climate, characterized by typical tectonic-climate-sedimentary coupling sedimentation.