Mechanism Affecting the Pore Differentiation Characteristics of Fine-Grained Tight Sandstones: A Case Study of Permian Shanxi Formation in Ordos Basin

Abstract

In this study, the pore-throat structure and lithological and fractal characteristics of fine-grained sandstone reservoirs of Permian Shanxi Formation in Ordos Basin are investigated using various experiments, including casting thin section, scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), and nuclear magnetic resonance (NMR). The relation between rock type difference and pore structure difference is explored, and the controlling effect of pore-throat structure on physical properties of fine-grained sandstone reservoir is studied. The results show that the sandstone sample can be categorized into quartz arenites, sublitharenite, and lithic sandstone. The average porosity and permeability are, respectively, 4.46% and 0.129 mD. The pores are mainly of intergranular and intercrystalline types, and the throats are lamellar and tubular types. Furthermore, the fine-grained sandstone is typically characterized by a binary pore structure. Micropore-throat, as the main reservoir space, is the dominant factor of the reservoir physical property difference. In addition, the correlation between fractal dimension and rock composition is studied with pore-throat morphology being comprehensively analyzed. The results indicate that the quartz arenite is dominated by quartz intergranular lamellar throats, and there are numerous intergranular pores and tubular throats connecting them in litharenite and sublitharenite. Moreover, the content of pores and throats play major roles in the change of fractal dimension. The characteristics, causes, and main controlling factors of micropore and throat types should be emphatically analyzed to predict the physical properties of fine-grained sandstone.