Multifractal Characteristics and Controlling Factors of Tight Sandstone Reservoirs Across Lithofacies in the Benxi Formation, Ordos Basin, China

The relationship between pore structure heterogeneity in tight sandstone reservoirs and their fractal characteristics is well documented. However, the impact of differential diagenesis across lithofacies on pore-throat structure and fractal properties remains unclear. In this study, we investigate the Carboniferous Benxi Formation in the Ordos Basin using a suite of experiments to characterize pore-throat structure and multifractal behavior, and to assess the influence of diagenesis. The results reveal significant differences among lithofacies in mineral composition, pore types, pore throat structure, fractal dimensions, and petrophysical properties, primarily attributed to variations in sedimentary environments and diagenesis. Fractal characteristics were quantified by converting the T2 spectra into pore-throat size distributions. Macropores exhibit the highest fractal dimensions, indicating the greatest structural complexity and heterogeneity, followed by mesopores, whereas micropores show the lowest heterogeneity (D3 > D2 > D1). Quartz content mainly controls the fractal properties of macropores by enhancing structural stability, whereas clay minerals govern the fractal behavior of micropores and mesopores by increasing pore-throat complexity. High-energy depositional conditions promote sediment transportation and sorting, leading to quartzarenite lithofacies (QL) and sublitharenite lithofacies (SL) with lower fractal dimensions, more uniform pore structures, and better connectivity. In contrast, feldspathic litharenite lithofacies (FL) and litharenite lithofacies (LL) exhibit higher fractal dimensions due to stronger compaction, reduced primary porosity, and higher clay content, resulting in poorer reservoir quality. This study improves understanding of pore structure heterogeneity in tight sandstones and provides useful insights for predicting high-quality reservoirs in similar geological settings.