A Fractal Characteristics Analysis of the Pore Throat Structure in Low-Permeability Sandstone Reservoirs: A Case Study of the Yanchang Formation, Southeast Ordos Basin

In the Southeastern Ordos Basin, the Chang 2 low-permeability sandstone reservoir of the Triassic Yanchang Formation is a typical heterogeneous reservoir. Quantitatively characterizing and analyzing its complex pore throat structure has become crucial for enhancing storage and production in the study area. The pore throat structure is a key factor influencing reservoir properties. To achieve this, a comprehensive suite of analytical techniques was employed, including cast thin section (CTS), scanning electron microscopy (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), and mercury intrusion capillary pressure (MICP). This study quantitatively characterizes the pore size distribution of reservoirs in the Southeast Ordos Basin. Based on fractal theory, it clarifies the complexity of the pore throat structure and the degree of microscopic heterogeneity at different scales. Finally, this study reveals the correlation between fractal dimensions and storage and permeability capacities and analyzes the controlling factors. The findings indicate that the predominant lithotype in the study area is fine-grained feldspar sandstone, which develops pore types such as intergranular pores, dissolution pores, and microfractures. Based on the shapes of mercury injection curves and pore throat structural parameters, and in conjunction with SEM images, the samples are categorized into three types. Type I samples exhibit good pore throat connectivity and are characterized by a lattice model. Type II samples are characterized by a tubular pore throat model. Type III samples have poor pore throat connectivity and are characterized by an isolated model. The pore throat network of low-permeability sandstone is primarily composed of micropores (pore throat radius r < 0.1 μm), mesopores (0.1 < r < 1.0 μm), and macropores (r > 1.0 μm). The complexity of the reservoir pore throat structure was quantitatively characterized by fractal theory. The total fractal dimension (D) of all the samples is between 2 and 3, which indicates that the reservoir has capillary fractal characteristics. The average fractal dimension of micropores (D1) is 2.57, while that for mesopores (D2) and macropores (D3) is slightly higher, at an average of 2.68. This suggests that micropores have higher self-similarity and homogeneity. The fractal dimensions D1, D2, and D3 of the three types of reservoirs all exhibit a negative correlation with porosity and permeability. This shows that the more complex the pore throat structure is, the worse the storage and seepage capacity of the reservoir. For type I samples, the correlation of D3 with pore throat structural parameters such as entry pressure, skewness, and maximum mercury saturation is better than that of D2 and D1. For type II and type III samples, D2 shows a significant correlation with pore throat structural parameters. This indicates that the heterogeneity and complexity of mesopores are key factors influencing the pore throat structure of poor-quality reservoirs. Different mineral compositions have varying effects on the fractal characteristics of pore structures. Quartz, feldspar, and clay exert both negative and positive dual impacts on reservoir quality by altering the pore throat structure and the diagenetic processes. The mineral content exhibits a complex quadratic relationship with the fractal dimension. Moreover, micropores are more significantly influenced by the mineral content. The study of the relationship between the fractal dimension and physical properties, pore throat structural parameters, and mineral composition can improve the understanding of the reservoir quality of low-permeability reservoirs. This provides a theoretical basis for exploration and improving the recovery rate in the study area.