ABSTRACT Accurate characterisation of microscopic pore‐throat structures and their influence on seepage capacity is critical to improving the evaluation and permeability prediction of ultra‐low permeability reservoirs. Accordingly, nine core samples from the Chang 6 in the southeastern Yishan Slope, Ordos Basin were analysed through a series of experiments to systematically characterise mineralogy and pore‐throat structure. The results indicate that the reservoir is dominated by arkose, with intergranular and dissolution pores prevailing, accompanied by minor intercrystalline pores and microfractures. Throats are primarily curved sheet‐like or constricted types. Based on HPMI data, fractal theory was introduced to quantitatively characterise the heterogeneity of the pore‐throat structure. Two distinct fractal transition points ( R f1 and R f2 ) are identified in the fractal curves, allowing the pore‐throat system to be segmented into three segments (Large pore‐throat segment, Medium pore‐throat segment and Small pore‐throat segment). Variations of the R f1 and R f2 among three types of samples result in significant differences in the structural characteristics of the corresponding domains. The Medium pore‐throat segment is the dominant pore‐throat system in controlling ultra‐low permeability reservoir physical properties, with its pore‐throat volume contribution and distribution span critically affecting storage and seepage capacities, respectively. In contrast, although the Large pore‐throat segment features the largest pore‐throat radii, it fails to provide effective seepage pathways. Instead, the higher fractal dimension and broader distribution span of the Large pore‐throat segment induce structural disruptions within the Medium pore‐throat segment, weakening its connectivity and representing a key mechanism underlying permeability reduction identified in this study. A new parameter was proposed to quantitatively analyse the spatial reorganisation relationship between Large and Medium pore‐throat segments. We enhance the understanding and prediction of permeability variations in ultra‐low permeability reservoirs from a structural perspective.
Liu et al. (2026) studied this question.