Development Characteristics and Influencing Factors of Sweet Spot Reservoirs in Deep and Middle-Layer Tight Sandstone of the Lingshui Formation, Northern Slope of Baodao Depression, Qiongdongnan Basin

Deep and middle-layer tight sandstone reservoirs represent an emerging frontier in oil and gas exploration and development. Significant breakthroughs have recently been achieved in the northern deepwater region of the Qiongdongnan Basin, particularly within the Oligocene Lingshui Formation in the Baodao Depression. However, the petrophysical characteristics of these tight sandstone reservoirs and the controlling factors influencing sweet spot development remain poorly understood. This study integrates comprehensive datasets—including thin section petrography, cathodoluminescence, scanning electron microscopy (SEM), X-ray diffraction (XRD), and conventional reservoir property analyses—to systematically investigate the reservoir characteristics and key controls on sweet spot formation in the third member of the Lingshui Formation along the northern slope of the Baodao Depression. A pore evolution model for sweet spot reservoir development is subsequently proposed. The results indicate that: 1) The tight sandstones are predominantly lithic feldspathic quartzarenite, feldspathic quartzarenite, and feldspathic litharenite, with primary pore types including feldspar dissolution pores, moldic pores, and residual intergranular pores. 2) Among these, feldspathic quartzarenite and lithic feldspathic quartzarenite exhibit superior reservoir quality and constitute the main sweet spots; high quartz and feldspar content coupled with low lithic fragment abundance are critical compositional controls on sweet spot formation in deep to middle-depth settings. 3) Grain size demonstrates a positive correlation with reservoir physical properties. Compaction has led to porosity reduction by 22.0%~28.0%, establishing the fundamental basis for reservoir tightness. 4) Dissolution processes play a pivotal role in enhancing reservoir quality. Secondary porosity zones developed at depths of 3800~3950 m and 4100~4400 m due to dissolution significantly improve porosity and permeability. Conversely, during the late stage of mesodiagenesis (Stage B), extensive carbonate cementation contributes to further reservoir compaction. This research provides a theoretical foundation for the evaluation and prediction of sweet spot reservoirs in deepwater tight sandstone systems in the South China Sea, offering guidance for hydrocarbon exploration, field development planning, and the selection of favorable drilling targets. Furthermore, it advances the understanding of the formation mechanisms and evolutionary pathways of different types of sweet spots in deep and middle-layer tight sandstone reservoirs.