Lower Silurian Longmaxi Shale and Lower Cambrian Niutitang Shale are two significant shale horizons being studied for unconventional development in China. In this work, we focused on 25 core samples collected from five wells in southeastern and southern margin around Sichuan Basin, to better understand pore structure and its impact on methane adsorption capacity of high-maturity marine shales. Pore structure of shales are obtained from complementary tests of low-pressure gas (N₂ and CO₂) sorption isotherm and mercury intrusion porosimetry. The organic matter is a main controlling factor on CH₄ adsorption capacity, shown by a positive correlation between TOC (total organic carbon) and Langmuir volume. The clays contents show a slightly positive correlation with the TOC-normalized Langmuir volume for both shales. Micropores (<2 nm) have a greater impact on CH₄ adsorption capacity than mesopores (2-50 nm) and macropores (>50 nm). The CH₄ adsorption capacity will not always increase with an increase of organic matter maturity. A negative relationship between the TOC-normalized Langmuir volume and R_o is observed in Niutitang samples. Due to its higher TOC content, the Niutitang Shale has a stronger Langmuir CH₄ adsorption capacity than Longmaxi Shale. The higher meso-macropore volume of Longmaxi Shale makes it to have a higher free-gas storage capacity than Niutitang Shale. Fractal dimensions were obtained from nitrogen sorption isotherm and mercury intrusion capillary pressure data. The surface fractals (determined by nitrogen adsorption) results show that the shales with a higher fractal dimension D_n2 (P/P₀<0.5) means more surface area and then a stronger adsorption capability. The fractal dimension D_n1 (P/P₀>0.5) is significant in describing pore structures, especially for average pore diameter. The surface fractals (determined by mercury intrusion) results suggest that the shales with high fractal dimension D_m1 (P≤1 MPa) have a weak gas flow capability. Moreover, the shales with a high fractal dimension D_m2 (1 MPa<P≤100 MPa) would have a lower L_e/L ratio of tortuosity, is not conducive to the preservation of shale gas.