Objective The Longdong area is located in the southwestern part of the Ordos Basin. Fine characterization of the full pore size distribution and fractal characteristics at different pore-throat scales in the Chang 7 dense sandstone reservoirs in this area is conducive to the quantitative prediction of favorable reservoirs.

Methods This study employed the dense sandstone reservoirs from Chang 7 member of Yanchang Formation in Longdong area of Ordos Basin as the object of study, drilled 9 core samples from different wells, and utilized the means of casting thin section, scanning electron microscope, high−pressure mercury compression, and nuclear magnetic resonance (NMR), etc. Based on the analysis of reservoir characteristics, the NMR saturated water T₂ spectrum was converted to a full aperture pore−throat size distribution curve using high−pressure pressurized mercury pore−throat distribution to analyze the fractal dimensions of different pore−throat types and their influencing factors.

Results The results show that the pore types of the Chang 7 reservoir in the study area are mainly feldspar solution pores, intergranular pores and rock chip solution pores. The pore structure of the reservoir can be categorized into three types: Type Ⅰ, Type Ⅱ and Type Ⅲ, corresponding to the deterioration of physical properties, storage performance and seepage capacity in that order, with an increase in the proportion of micro−miniature pore throats, and the enhancement of non−homogeneity.

Conclusions According to the full pore size distribution, the pore throats are categorized into micro pore throats (<0.01 µm), small pore throats (0.01~0.1 µm) and medium pore throats (0.1~1 µm), corresponding to the fractal dimensions increasing in order. As the main contributor to reservoir physical properties and reservoir space, the fractal dimension of the mesopores and small pores correlates well with the pore structure parameters, the mineral composition and content influence the complexity of the pore structure, and the clay minerals exacerbate the pore throat surface roughness and non−homogeneity, all of which correlate well and positively with the fractal dimension of the mesopore. The fractal dimension of tight sandstone reservoirs is correlated with physical properties, pore−throat structure parameters, and movable fluid parameters, and can be used to quantitatively characterize reservoir heterogeneity and establish reservoir classification criteria.