Fractal Dimension Characterization of Pore Systems Within Different Pore Size Intervals in Shale Reservoirs

Pore systems within different pore size intervals in shale reservoirs exhibit heterogeneity in surface roughness and structure, which can be quantitatively characterized by fractal dimensions. To quantitatively characterize the pore structure characteristics, six Longmaxi shale samples were collected from the southern Sichuan Basin. Mercury intrusion, low-pressure N2 adsorption, and low-pressure CO2 adsorption experiments were conducted to obtain the pore size distributions of macropores, mesopores, and micropores, respectively. Fractal dimensions of micropores (D1), mesopores (D2), and macropores (D3) were calculated using the volume-specific surface area (V-S), Frenkel–Halsey–Hill (FHH), and Menger models, and the controls of total organic carbon (TOC) and clay content on these fractal dimensions were investigated. The results show that surface roughness and structural irregularity decrease in the order of macropores > mesopores > micropores, with D3 (2.89–2.98) > D2 (2.777–2.812) > D1 (2.187–2.304). These differences in fractal dimensions are mainly attributed to the development of small pores within the walls of larger pores, as well as the presence of bubble-like, honeycomb-like, and beaded organic matter pores observed under scanning electron microscopy. Specifically, D1 is predominantly controlled by TOC owing to the extensive development of micropores within organic matter, whereas D2 and D3 are governed by clay content, which hosts abundant intraparticle and interparticle mesopores and macropores. These findings provide a quantitative characterization of the pore structure of the Longmaxi Formation across multiple pore size scales and may serve as a reference for future studies on pore structure characterization of organic-rich shales.