Fractal characteristic of microscopic pore structure of tight sandstone reservoirs in Kalpintag Formation in Shuntuoguole area, Tarim Basin

Abstract

Using the fractal geometry method, the microscopic pore structures of tight sandstone reservoirs in Kalpintag Formation of Shuntuoguole area in Tarim Basin were conducted fractal characterization on the base of test analysis data such as physical property, cast thin section, scanning electron microscope and mercury injection, and the genetic mechanism of pore structure heterogeneity was investigated. The storage spaces are dominated by intergranular dissolved pore, intragranular dissolved pore and residual intergranular pore, and the throat type consists of the necking throat, lamellar throat, curved lamellar throat and tube-shaped throat. The microscopic structure type includes Type I (fractal dimension≤2.350), Type II (2.350<fractal dimension<2.580), Type III (fractal dimension≥2.580) and fracture type. The most favorable reservoirs with Type-I microscopic pore structure are mainly distributed in the Upper Member of Kalpintag Formation, while the reservoirs with Type-II and Type-III microscopic pore structures are mainly in the Lower Member of Kalpintag Formation. The sedimentation controls the heterogeneity of microscopic pore structure, and the differences on composition and particle size of sandstone lead to differentiation of microscopic pore structures. The Lower Member of the Kalpintag Formation experiences stronger compaction and cementation but weaker dissolution than the Upper Member of the Kalpingtag Formation, and thus the microscopic pore structure of Upper Member of the Kalpintag Formation is significantly worse that of the Lower Member o the Kalpingtag Formation. The Upper Member of the Kalpintag Formation with high content of brittle mineral develops microscopic fractures due to tectonic rupture, thus the permeability is improved and the heterogeneity of microscopic pore structures is enhanced; but the Lower Member of Kalpintag Formation is characterized by attrition crushing of particles and strong compaction.