Fractal Characteristics and Influencing Factors of Pore Structure in Tight Sandstone: A Case Study from Chang 6 Member of the Southwestern Yishan Slope

Fractal dimension analysis provides a quantitative approach to characterizing the heterogeneity of pore structures in reservoirs. In this study, casting thin sections, scanning electron microscopy (SEM), and high-pressure mercury intrusion porosimetry (MIP) were integrated with fractal theory to determine the fractal dimensions of different pore types and investigate the controlling factors of reservoir pore structure heterogeneity. This study identifies three primary pore types—residual intergranular, dissolution, and intergranular pores—and classifies the reservoir into three distinct types based on their mercury intrusion curves and pore-throat radius distributions. The fractal analysis of pore structures reveals three segments corresponding to macropores, mesopores, and transition pores, with average fractal dimensions of 2.28, 3.67, and 2.43, respectively. Furthermore, the overall fractal dimensions for Type I, II, and III reservoirs are 2.69, 2.72, and 2.92, indicating an increasing trend in heterogeneity from Type I to Type III. The fractal dimension shows a negative correlation with reservoir properties, median pore-throat radius, maximum mercury saturation, and the content of quartz and feldspar, while it is positively correlated with displacement pressure. No significant correlation is observed with clay mineral content. These findings offer valuable insights into the heterogeneity of reservoir pore structures and provide a basis for evaluating reservoir quality.