Micropore structure and fractal characteristics of shale in Midwest China

To further clarify the optimal target layer in the horizontal section of Wufeng–Longmaxi shale, we selected 126 shale samples from the Wufeng–Longmaxi Formations (Long111 to Long114 and Long12 submember) as the primary study objects from the perspective of the microscopic pore structure. We conducted this experimental study on the pore structure of shale samples according to various methods, including low-temperature nitrogen adsorption and carbon dioxide adsorption experiments, atomic force microscopy, and nano-computed tomography scanning. We calculated the fractal dimensions of samples at different horizons through the study of the characteristics of adsorption–desorption isotherms using the Frenkel–Halsey–Hill model. These results indicated that the shale samples had a complex pore structure (with a higher fractal dimension). The fractal dimensions were positively correlated with the specific surface area and total organic carbon content of the shale samples, weakly positively correlated with the micropore pore volume, and negatively correlated with the average pore radius. The samples from the Wufeng Formation and the Long111 and Long112 submembers had large fractal dimensions, whereas the samples from Long12 had the smallest fractal dimension. The pores of the shale samples generally were type H2/3, which were characterized by wedge, tabular, and ink-bottle shapes. Only three H3-type pore samples corresponded to the three smallest fractal dimension values. The Wufeng Formation’s Long 111 and Long 112 layers should be the target layers for horizontal wells. In this study, we identified the optimal target layer of the horizontal shale profile of Wufeng–Longmaxi Formation, which holds great significance for shale oil and gas development in this formation.