Abstract: To further understand shale adsorption mechanism and adsorption model, the surface fractal characteristics of shale nanopores must be clarified. Using the shales from the Wufeng group/Longmaxi group and the Yanchang group as study objects, the surface fractal dimension of the shale nanopores was calculated based on the pore structure parameters obtained from the nitrogen adsorption method. It is then compared with the fractal dimension of mudstone and dense sandstone to reveal the effect of organic matter, clay mineral, and other factors on the fractal characteristics of the pore surface. The results show that the surface fractal dimension of shale nanopores in the Wufeng group/Longmaxi group varies between 2.7~2.9, and that for the Yanchang group shale varies between 2.3~2.7, and the fractal dimension depends on the cumulative surface area of pores below 10 nm. The value of the fractal dimensions of the shale has a positive correlation with the developmental stages of the maturation of organic matter. Its relation with organic matter content is however complicated. The surfaces of pores having organic matter and clay mineral crystal contents have obvious fractal characteristics. The development of these pores is the fundamental reason behind the shale having surface fractal characteristics; however, the fractal dimension value of the former is significantly greater than that of the latter. The surface of organic matter and clay minerals is highly rough and seriously deviates from smoothness. Therefore, the adsorption of methane in shale nanopores is a typical heterogeneous solid adsorption process.

Key words: fractal surface, nanopore, shale, organic matter, clay mineral