Pore–throat structure and fractal characteristics of tight sandstones in Yanchang Formation, Ordos Basin

Abstract

Pore–throat structure is a key factor that influences the storage and fluid flow capacity of tight sandstone reservoirs. By performing petro-physical characteristic tests, scanning electron microscopy, and casting thin sections, the main pore types of tight sandstone reservoir samples collected from the Upper Triassic Yanchang Formation in the Baibao area of the Southwest Ordos Basin in China were identified to be dissolved pores, intergranular pores, intercrystalline pores, and micro-cracks. Meanwhile, the size of the pore–throat was widely distributed, whereas its shape varied. Given the advantages and disadvantages of various technical methods, high-pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) technology were used to study the distribution characteristics of a full-size pore–throat. All 10 samples showed bimodal and unimodal distribution characteristics, and their distribution characteristics were consistent with the results of the petro-physical characteristics, high-pressure mercury injection, and movable fluid saturation tests. The throat distribution characteristics of the reservoir were studied by applying constant-rate mercury injection (CRMI), and the throat radius ranged from 0.43 μm to 9.76 μm with an average of 3.34 μm.The fractal features of the pore–throat and throat were quantitatively characterized by CRMI. The fractal curves of the pore–throat showed self-similarities, and the fractal dimension D ranged between 2.6297 and 2.9529 with an average of 2.7865. The fractal curves of the throats were broken down into two segments, and the fractal dimensions corresponded to two ranges, namely, the small (Dt1) and large throats (Dt2) fractal dimensions. The average of fractal dimension Dt1 was 2.9170, and the average of fractal dimension Dt2 was higher than that of Dt1 with an average of 2.9828, thereby indicating the complex structure and strong heterogeneity of the former. The fractal dimensions D, Dt1, and Dt2 were negatively correlated with permeability, movable fluid saturation, average throat radius, clay mineral content, and laumontite content, positively correlated with quartz content, and showed poor correlation with feldspar. Compared with Dt2, the fractal dimension Dt1 had a higher correlation with pore–throat parameters and mineral content. Given the higher self-similarity of small throats, their distribution is considered highly homogeneous.