Study on the pore structure characteristics and multifractal features of different lamination in chang73 shale, Ordos Basin

The Chang7 3 shale of the Triassic Yanchang Formation in the Ordos Basin is a prospective lacustrine shale oil reservoir in China, yet pervasive laminae create highly heterogeneous pore structures that complicate reservoir-quality evaluation and sweet-spot identification. In this study, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) mineral mapping, low-pressure CO 2 adsorption, low-temperature N 2 adsorption, and nuclear magnetic resonance were integrated to characterize mineral composition and full-scale pore size distributions across distinct lamina types, and applied multifractal theory to quantify pore heterogeneity and its mineralogical controls. Five lamina combinations were identified: organic-tuffaceous, organic-clay, felsic-clay, felsic-tuffaceous, and tuffaceous-felsic. Integrated pore size distribution curves reveal that organic-clay laminae, despite containing the highest organic matter content and the largest proportion of micropores smaller than 2 nm at 28.8%, exhibit the lowest total porosity at 4.15%, indicating the poorest reservoir quality where compaction overrides organic-matter-hosted microporosity. Tuffaceous-felsic and felsic-tuffaceous laminae, dominated by macropores larger than 50 nm with proportions up to 58.0%, display the best petrophysical properties, with the tuffaceous-felsic type being superior. Organic-tuffaceous and felsic-clay laminae are both dominated by mesopores in the 2–50 nm range and share broadly similar pore structures. Multifractal analysis reveals that the singularity spectrum width Δα correlates strongly with dolomite and quartz contents, with a coefficient of determination R 2 of 0.8302, establishing mineral composition as the primary control on pore heterogeneity. These lamina-specific pore characteristics indicate that tuffaceous-felsic intervals represent the optimal sweet-spot targets in the Chang7 3 shale.