Multifractal Characteristics and Factors Associated with Pore Structure in Taiyuan Formation Transitional Shales Based on LTNA and NMR

Transitional shales are characterized by complex multiscale pore systems. A comprehensive understanding of the pore distribution across different scales and its multifractal characteristics is essential for evaluating the reservoir properties and gas-bearing potential of these shales. Based on low-temperature nitrogen adsorption (LTNA) and nuclear magnetic resonance (NMR) experiments, this study investigates the pore structure, heterogeneity and its factors associated with transitional shale from the Upper Carboniferous Taiyuan Formation on the eastern Ordos Basin. The main results are as follows: (1) LTNA results reveal Type II adsorption isotherms and H3 hysteresis loops, suggesting the widespread occurrence of slit-shaped and wedge-shaped pores. The specific surface area (SSA) and pore volume (PV) range from 0.30 to 2.16 m2/g and from 0.000235 to 0.005163 cm3/g, respectively, and are lower than those reported for representative organic-rich marine shales. (2) The NMR T2 spectra exhibit multiple peaks with variable degrees of overlap, indicating distinct relaxation-time populations and heterogeneous pore-related signals. (3) Comparison of the multifractal spectra derived from NMR and LTNA shows that the widths of the generalized fractal dimension spectrum (ΔD) and multifractal spectrum (Δα) obtained from NMR are both significantly greater than those derived from LTNA. In contrast, the information dimension (D1) and correlation dimension (D2) are significantly lower for the NMR results. These differences may reflect variations in measurement principles, characterization ranges, and sample states. (4) Exploratory correlation analysis reveals that pore structure heterogeneity is associated with organic–geochemical properties and mineral composition. TOC is positively correlated with LTNA-derived α1 (r = 0.80), while the hydrocarbon index correlates positively with LTNA-derived ΔD (r = 0.53) and NMR-derived α2 (r = 0.54). The correlations between mineral components and multifractal parameters vary among minerals, with relatively weak correlations observed for quartz.