Binary pore structure characteristics of tight sandstone reservoirs

Abstract

The pore structure and its influence on physical properties and oil saturation of the Triassic Chang 7 sandstones, Ordos Basin were discussed using thin sections, physical properties, oil saturation and mercury intrusion data. The results show that the tight sandstone has a binary pore structure: when the pore throat radius is larger than the peak radius, the pore radius is significantly larger than throat size, the pore structure is similar to the bead-string model with no fractal feature, and the pore throat volume is determined by the pore volume. When the pore throat radius is smaller than the peak radius, the pore structure is close to the capillary model and shows fractal features, the pore size is close to the throat size, and the pore throat volume is determined by the throat radius. The development of pore throats larger than the peak radius provides most of the oil storage space and is the major controlling factor for the porosity and permeability variation of tight sandstone. The pore throat smaller than the peak radius (including throats with no mercury invaded) contributes major reservoir space, it shows limited variation and has little effect on the change of physical properties which is lack of correlation with oil saturation. The pore throat larger than the peak radius is mainly composed of secondary and intergranular pores. Therefore genesis and main controlling factors of large pores such as intergranular and dissolved pores should be emphasized when predicting the tight sandstones quality. Key words: tight sandstone, pore structure, reservoir, fractal, Triassic Yanchang Formation, Ordos Basin