Depositional Characteristics and Reservoir Formation Mechanism of Subaqueous Coarse Clastic Rocks along Steep Slopes in a Continental Lacustrine Rift Basin: A Case Study of Northern Bonan Depression

  The subaqueous coarse clastic rock fans in the Northern Bonan Depression of the Bohai Bay Basin are one of the typical reservoirs for oil and gas accumulation in the Mesozoic-Cenozoic continental rift basin. With the comprehensive exploration of oil and gas fields in eastern China transitioning from the concealed exploration phase to the refined exploration phase, the lack of understanding of the internal structure and distribution law of favorable zones within the subaqueous coarse clastic rock bodies has seriously hindered the exploration and development of such oil and gas reservoirs. This paper focuses on the subaqueous coarse clastic rocks of the third member - fourth member of the Shahejie Formation（Es3–Es4） of Northern Bonan Depression as the research objects. By integrating seismic, logging, core, and analysis tests, investigating the genetic types, sedimentary structures, and transport processes of subaqueous coarse clastic rocks under differential tectonic activities. Furthermore, a comparative analysis of reservoir characteristics and the main controlling factors for reservoir properties are carried out for different types of subaqueous coarse clastic rock bodies, aiming to provide a clear understanding of the development mechanisms of high-quality reservoirs. The following understandings have been obtained.

（1）A sedimentary evolution sequence of subaqueous coarse clastic rocks controlled by differential tectonic activity is proposed. In Es4, thesegments of the boundary faults experience a low tectonic subsidence rate, relatively gentle topography, and sufficient sediment supply, fan deltas form. The subaqueous distributary channels cut each other, migrate and swing frequently, resulting in a wide lateral distribution of the fans.In Es3,the segments of the boundary faults experience high tectonic subsidence rates and large drops, which contribute to the development of nearshore subaqueous fans. The internal migration and superposition of multi-stage fan bodies occur, while the lateral migration capacity of braided channels is weak, and vertical accretion dominates the overall deposition.

（2）This study has identified the differences in lithology, reservoir characteristics, main authigenic mineral types and contents of subaqueous coarse clastic fans during the third member - fourth member of the Shahejie Formation of Northern Bonan Depression。The parent rock of fan delta is mainly composed of metamorphic rocks, while the nearshore subaqueous fans consists of metamorphic rocks and mixed parent rock. The reservoir space type is dominated by feldspar and igneous rock fragment dissolution pore. The highest pore surface porosity is found in the fan delta, followed by the nearshore subaqueous fans。

（3）Quantitatively evaluated the differences in diagenesis subaqueous coarse clastic rock reservoirs during the third member - fourth member of the Shahejie Formation of Northern Bonan Depression，and clarified the types of favorable sedimentary facies belts. The sedimentary environment of the fan delta shows medium compaction, weak cementation, and strong dissolution, medium to strong compaction, medium to strong cementation, and weak to strong dissolution for the nearshore subaqueous fans。 The subaqueous distributary channel of fan delta, braided channels in the nearshore subaqueous fan are the most favorable sedimentary facies。