Reservoir quality evaluation of tight carbonate gas reservoirs using fractal–multifractal pore-structure metrics: a case study from the Taiyuan Formation, Ordos Basin, China

Abstract

Abstract Tight carbonate gas reservoirs commonly contain appreciable storage space but highly variable flow capacity, making reservoir-quality evaluation and favorable-interval screening challenging in exploration and development. This study investigates 10 representative limestone core samples from the Taiyuan Formation, Ordos Basin, China, using high-pressure mercury intrusion data integrated with segmented single-fractal and multifractal analyses to identify pore-structure metrics relevant to reservoir evaluation. The results show that the pore systems exhibit segmented fractal characteristics: the low-pressure fractal dimension $${D}_{1}$$ is more sensitive to the development of fracture-related macropore pathways and may therefore be associated with seepage potential, whereas the high-pressure fractal dimension $${D}_{2}$$ mainly reflects the roughness and complexity of matrix-controlled storage pores. Furthermore, multifractal parameters provide a quantitative description of the overall heterogeneity of the pore system. The spectral width ( $$\Delta \alpha$$ ) reflects the intensity of heterogeneity, while the asymmetry index ( $$R$$ ) helps distinguish whether the pore structure is more strongly influenced by large seepage pores or small storage pores. When combined with pore-throat attributes and multivariate classification, these metrics divide the reservoir into three resource-quality classes and suggest that intervals with high $${D}_{1}$$ , positive $$R$$ , and moderate $$\Delta \alpha$$ are more favorable pore-structure conditions for fluid mobility. The proposed workflow therefore provides a preliminary pore-structure-based framework for reservoir-quality evaluation and favorable-interval screening in tight carbonate gas reservoirs.