Medium-deep tight sandstone reservoirs represent a new frontier for hydrocarbon exploration. Great natural gas exploration breakthroughs have been made in the third member of the Lingshui Formation in the Baodao Sag, Qiongdongnan Basin. However, the characteristics of tight sandstone reservoirs and the controlling factors of sweet spots remain poorly understood. Using thin sections, SEM and petrophysical data, this study analyzes reservoir properties and key factors controlling sweet-spot formation, and establishes a pore evolution model. The results show that the reservoirs are dominated by lithic feldspathic quartz sandstones, with feldspar dissolution pores, moldic pores and intergranular pores as major pore types, with average areal porosities of 3.90%, 3.57%, and 1.31%, respectively. Feldspathic quartz sandstones constitute sweet-spot reservoirs. The average porosity is 10.92%, and the average permeability is 6.73 × 10−3 μm2. Grain size shows a positive correlation with reservoir quality. Compaction provides the basis for reservoir densification, resulting in a porosity loss rate of 22.0–28.0%, with an average of 24.0%. Dissolution is critical for sweet-spot development, forming secondary pore zones at 3800–3950 m and 4100–4400 m, with the dissolution-induced porosity increment ranging from 5.77% to 8.68% and averaging 7.20%. Late carbonate cementation further enhances reservoir densification, corresponding to a porosity loss rate of 5.70–10.9% with an average of 8.28%. This study provides a theoretical basis for sweet-spot evaluation and hydrocarbon exploration in deep-water areas of the South China Sea.
Zheng et al. (Thu,) studied this question.