Formation and preservation of high-quality reservoirs in the Sha 4 lower submember (Es4x), lacustrine rift bonan depression, Bohai Bay Basin, China

Abstract

The formation of high-quality reservoirs remains a critical challenge in the exploration and development of deeply buried clastic systems, particularly in lacustrine rift basins where reservoir quality evolution is governed by complex depositional, diagenetic, and structural interactions. This study focuses on deep reservoirs within the lower submember of the fourth member of the Shahejie Formation (Es4x) in the Bonan Depression, Bohai Bay Basin, China. To elucidate the mechanisms controlling reservoir quality, an integrated analytical framework combining core observations, petrographic thin-section analysis, petrophysical measurements, well-log interpretation, and three-dimensional seismic characterization was established. Sedimentary facies analysis indicates that depositional architecture exerted a first-order control on reservoir heterogeneity, with high-energy fan delta front distributary channel and mouth-bar sand bodies providing favorable conditions for primary pore preservation and fluid storage capacity. Reservoir quality evolution is subsequently modified by diagenetic processes, including compaction, cementation, and dissolution, which collectively regulate porosity preservation and secondary pore development. Overpressure evolution and hydrocarbon charging are interpreted to contribute to reservoir preservation, potentially reducing effective compaction and modifying diagenetic pathways. Structural elements, particularly faults and unconformities, are interpreted as fluid migration conduits associated with more efficient hydrocarbon charging and pressure transmission. Evaporative mineral precipitation is interpreted to exert an additional but secondary influence on reservoir physical properties. The formation of high-quality reservoirs is interpreted to result from the coupled effects of sediment supply, transport pathways, diagenetic evolution, and fluid charging processes, which are integrated into a “source-flow-sink” reservoir development framework. Within this framework, favorable depositional conditions, efficient migration systems, short source-to-reservoir distances, and sustained hydrocarbon supply collectively control the distribution and preservation of high-quality reservoirs. This study establishes an integrated genetic model linking depositional processes, diagenetic modification, structural controls, and fluid evolution, providing a predictive framework for reservoir quality evaluation and exploration target identification in deeply buried clastic reservoir systems within lacustrine rift basins.