Abstract: Propane and shale samples taken from the same well and different depths in the Longmaxi Formation in the Sichuan Basin were selected to carry out different series of simulation experiments in order to investigate the influence of the degradation of small molecular hydrocarbons on the composition of shale gas in the shale during the over-mature stage. The constant temperature thermal simulation experiments of a gold tube-restricted system were carried out on C₃H₈, C₃H₈ + shale, C₃H₈ + shale + water at 360℃ and 50 MPa for 72, 216, 360, 720 h. Other pyrolysis experiments were performed using C₃H₈ and C₃H₈ + shale samples at 400, 450, 500 and 550℃ with a pressure of 50 MPa for 72 h in order to increase the degradation degree of C₃H₈. The results showed that the CH₄, C₂H₆ yields and the CH₄/C₂H₆ ratio of C₃H₈ + shale at 360℃ are higher than those of C₃H₈ alone. The experiments with S1 shale rock which contains more clay minerals generated more CH₄ and C₂H₆ than those with S2 rock sample, and generally showed higher CH₄/C₂H₆ ratios. After increasing the simulation temperature, the conversion of C₃H₈ was significantly increased. The yields of CH₄ and C₂H₆ in the C₃H₈ + shale experiment were all higher than those in the control experiment, and the yield of CH₄ was higher than that of C₂H₆. These results indicated that clay minerals can catalyze C₃H₈ degradation and produce more CH₄. The yields of CH₄ and C₂H₆, and the CH₄/C₂H₆ ratio from the hydrous experiments were higher than those from anhydrous experiments, which indicated that water can promote the cracking of C₃H₈ to produce more CH₄. The positive effect of clay mineral and water in shale on C₃H₈ cracking increased the dryness index of shale gas. The degradation of small hydrocarbon molecules had an important influence on the composition of shale gas particularly in high-to over-mature shale rocks. It seems that water played a significant role and its effect on the evaluation of shale gas reserve should be thoroughly investigated especially for high-to over-mature shale rocks.