Abstract:The Lower Silurian Longmaxi Formation is an important shale gas exploration and development stratum in China. The shale gas is mainly produced from clay-bearing siliceous marine shale in the lower part. In this paper, shale samples are divided into silicate-bearing clayey shale and clay-bearing siliceous shale by mineral composition, combined with organic carbon content(TOC) test, vitrinite reflectance(Ro) test, and X-ray diffraction analysis(XRD), Low-pressure nitrogen adsorption and high-pressure mercury injection(MIP) experiments, analyzed the pore structure characteristics of two different lithofacies shales in the Longmaxi Formation of YC-6. Based on the Frenkel-Halsey-Hill(FHH) model, the pore fractal dimensions of shales in different lithofacies were calculated. The results show that the gas content of YC-6 is low, the average TOC of the clay-bearing siliceous shale in the lower part is 4.31%, which is higher than that in the upper silicate-bearing clayey shale (1.13%). At the same time, the total pore volume and specific surface area are also higher than the latter. The fractal dimension D2 of shale in different lithofacies is greater than D1, indicating that the complexity of the pore structure is greater than the pore surface. The fractal dimension has a positive correlation with the total pore volume and specific surface area, indicating that the pore structure with large specific surface area and large pore volume is more complicated. TOC and mineral content are important factors affecting the fractal dimension of shale pores. TOC is positively related to the fractal dimension. The higher the content of organic matter, the more pores will be generated during the thermal evolution process, which will make the pore structure more complex. The more the clay mineral content, the lower the fractal dimension will be. The reason is clay minerals subjected to compaction during the diagenesis, the clay mineral particles are arranged more closely, the pores are more regular and the homogeneity is stronger, so the fractal dimension is lower; Most of the quartz in marine shale is of biogenesis. Its irregularity will complicate the pore structure and increase the fractal dimension.