Pore Structure Monofractal and Multifractal Characteristics of High-Mature Organic-Rich Shale Using N2 Adsorption–Desorption Measurements

Abstract

High-mature organic-rich shale (HMORS) has substantial resource potential, and its reservoir heterogeneity is essential for shale gas resource evaluation and exploration. In this research, to characterize quantitatively the complex pore structure of HMORS in detail, we conducted monofractal and multifractal analyses using N₂ adsorption–desorption data from the Lower Permian (LP) HMORS in the Lower Yangtze South Yellow Sea, which is a prospective target for shale gas exploration. We also aimed to discuss the correlation, controlling factors, and application effects, to provide a new scientific analytical tool for characterizing the pore structure heterogeneity (PSH) of HMORS. The upper, middle, and lower sublayers of the LP are dominated by siliceous shale, clay shale (ClS), and clay shale and clay-mixed shale (ClS–ClMS), respectively. The monofractal dimensions D1 and D2 calculated by the Frenkel–Halsey–Hill model were not notably correlated, indicating that they are independent. The D1 of H3-type HMORS was significantly higher than its D2, while D1 and D2 of the H2 type were similar, indicating that slit-shaped pores have higher surface roughness than the internal structural complexity, whereas ink-bottle pores do not differ substantially. The monofractal study revealed that the overall PSH of HMORS is controlled primarily by calcareous minerals, and that of the ClS is also influenced by total organic carbon. The multifractal analysis revealed that the low-probability measure areas controlled the full-size pore size distribution heterogeneity of HMORS. The monofractal model can characterize ClS–ClMS with ink-bottle pores, and the multifractal model can characterize ClS with slit-shaped pores. In addition, D1 and the multifractal parameters were not significantly correlated [a_-10- a₁₀, Hurst index (H), a₀- a₁₀ and a_-10- a₀], whereas D2 correlated negatively with a₀-a₁₀, which had opposite a_-10-a₀ and H, indicating that the pore connectivity of the internal PSH of HMORS can be improved. Compared to monofractal analysis, the multifractal model has enhanced applicability in characterizing the PSH of HMORS quantitatively, which is of great significance for the study of widely developed HMORS with huge shale gas exploration potential in South China.