The Fractal Characteristics of Pore Networks in Tight Sandstones: A Case Study of Nanpu Sag in Bohai Bay Basin, NE China

In the exploration of unconventional petroleum resources in the Nanpu Sag of China, several tight oil sandstone reservoirs have been identified; however, their physical properties display pronounced heterogeneity. Using methods such as scanning electron microscopy (SEM), thin-section petrography, X-ray Diffraction (XRD), and high-pressure mercury intrusion, this study analyzed the mineralogical, petrological, and reservoir characteristics of the tight oil sandstone reservoirs in the second member of the Dongying Formation in the Nanpu Sag. This study also examined the relationship between the heterogeneity of the pore networks in the tight oil sandstone reservoirs and their fractal dimensions. The results indicate that as the fractal dimension (Df) of the tight oil sandstone reservoirs increases, their permeability decreases exponentially. The Df is strongly linked to pore morphology: larger Df values correspond to smaller pore sizes, more complex pore shapes, and greater pore heterogeneity. Additionally, variations in Df are closely linked to mineralogy: lower quartz content and higher clay content, particularly abundant illite–smectite mixed layers and illite along with reduced kaolinite, are associated with higher Df values. These findings highlight the complex, irregular nature of pore structures in tight sandstones and demonstrate that integrating high-pressure mercury intrusion analysis with fractal theory provides an effective approach for quantitatively characterizing their heterogeneity.