Micropore inhomogeneity of continental shale under different side length and resolution: a case study of Chang 7² shale from well Yaoke 1 in Ordos Basin

Continental shale is different from marine shale due to its inhomogeneity, which is apparent mainly in the inhomogeneity of pore structure. The Upper Triassic Yanchang black shale (Chang 7²) samples were collected from the well Yaoke 1 in the Ordos Basin. Micro and nano CT were used to observe and analyze the pores and cracks of 7 different side lengths (100-700 μm) and 4 different side lengths (10-39 μm) at 1 μm and 65 nm resolutions, respectively. The spatial inhomogeneity index of different side lengths and resolutions based on 3D image reconstruction was established. It was found that the number and volume of pores increased exponentially with the increase of side lengths, while the plane density of the pores decreased with the increase of side lengths. The average length, width and their ratio remained basically unchanged. Coordination number, as an indicator of connectivity, is closely related to the average width of pores.