Pore-throat structure and fractal characteristics of tight gas sandstone reservoirs: A case study of the second member of the Upper Triassic Xujiahe Formation in Zhongba area, western Sichuan depression, China

Pore-throat structure is a key factor that influences the storage and fluid flow capacity of tight sandstone reservoirs. Taking the tight sandstone reservoir of Xu2 Fm in Zhongba area as an example, the reservoir quality, pore-throat type and pore size distribution of tight sandstone in the study area were described by casting thin section, scanning electron microscope and high-pressure mercury injection test. In order to quantitatively characterize the complexity and heterogeneity of pore-throat structure, fractal analysis was performed using mercury saturation and pore size data. This study mainly reveals the relationship between the geometric shape characteristics and fractal dimension of the binary pore structure of ultra-low permeability tight sandstone and clarifies the influence of different scale pore throats on reservoir physical properties. The results indicate that the physical properties of the reservoir in the study area are poor, the pores are mainly intergranular pores and dissolution pores, the throat is flake and necked and the pore size distribution range is large. The fractal curve obtained by the mercury saturation method shows a significant turning point, and the pore-throat system is divided into two types: small-scale and large-scale. The fractal dimension of large-scale pore throat is greater than the three-dimensional Euclidean space dimension, which does not conform to the fractal theory. The fractal dimension of small-scale pore throat is closely related to the pore-throat structure and has obvious fractal characteristics. The geometric shape of binary pore-throat structure in tight sandstone is the main factor affecting the difference of fractal dimension. The development of small pores in sandstone is positively correlated with the total porosity, but its contribution to permeability is relatively low. The physical properties of tight sandstone are mainly controlled by the development degree of large-scale pore throat.