Research on the microscopic pore-throat structure and reservoir quality of tight sandstone using fractal dimensions.

The pore-throat structure is a crucial parameter for evaluating reservoir characteristics and assessing the potential of oil and gas resources. Understanding the relationship between reservoir pore-throat variations and oil-bearing properties is essential. Through a combination of techniques, including thin-section casting, scanning electron microscopy (SEM), micro-computed tomography (micro-CT), and high-pressure mercury injection (HPMI), we examined the tight sandstone reservoirs from the Chang 4 + 5 members of the Yanchang Formation in the study area. This analysis elucidates the relationship between the pore-throat structure and fractal characteristics of the samples and their oil-bearing properties. The results show that : (1) The tight sandstone reservoirs in the study area mainly develop three types of pores: dissolution pores, residual intergranular pores, and microfractures. Residual intergranular pores are primarily controlled by early compaction processes, while dissolution processes easily form secondary pores, increasing the porosity of the reservoir. Microfractures can significantly enhance both the permeability of the reservoir. (2) Using the characteristic parameters of HPMI, the reservoir is classified into four categories, labeled as type I to type IV. As the categories progress from type I to type IV, pore-throat size decreases, porosity and permeability decrease, and reservoir properties deteriorate. The overall fractal dimension of pores decreases, while the fractal dimensions of individual pore types increase. Pore connectivity becomes more complex, and heterogeneity strengthens. (3) Reservoir porosity shows a strong positive correlation with permeability. As reservoir properties improve, the number of macropores increases, leading to a higher Reservoir Quality Index (RQI) and better oil-bearing characteristics.