The interactions between shales and supercritical carbon dioxide (ScCO2) as well as the influence of these interactions on the pore structure and the surface functional groups play a key role in long-term CO2 geological sequestration. To investigate the effect of ScCO2 treatment on the performance of high-pressure CO2 adsorption, three shale samples selected from different formations were treated with ScCO2 in a geochemical reactor to simulate the in situ geological sequestration process. The high-pressure adsorption behaviors of CO2 on untreated and ScCO2-exposed shale samples were measured by gravimetric method at 45 degrees C and pressures up to 18 MPa. Characterization methods included scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), low-pressure gases adsorption (LP-GA), element analysis (EA) and X-ray photoelectron spectroscopy (XPS). The experimental results indicated that ScCO2 fluid could significantly alter the structural and physicochemical properties of shales, leading to the variations in high-pressure CO2 adsorption capacity of shales. The moisture and oxygen contents in the ScCO2-exposed samples were lower than those in the untreated samples. XPS analysis showed that the relative content of the organic oxygen-containing functional groups (C-O, C=O, O-C-O and COO-) in the raw shale samples decreased after the ScCO2 treatment. After long-term ScCO2 immersion, the maximum CO2 adsorption capacity of the shale samples exhibited a decreasing trend when compared to the raw samples, which was linked to the changes in the specific surface area of the micro- and mesopores as well as the oxygen-containing functional groups. The results obtained in this paper provide a basis for further evaluation of CO2 storage mechanisms.