Abstract: Mechanical properties of mudstone largely determines both their quality as cap rocks and the quality as shale gas reservoirs.In this study,the typical mudstone in the Silurian Longmaxi Formation in southern Sichuan Basin was selected for triaxial mechanical tests,so as to understand the variations of mechanical properties of the mudstone and their affecting factors.A total of 20 sets of experiments were carried out at confining pressures of 0,10,20,40,and 60 MPa,and at temperatures of 20,50,100,and 130℃,respectively.The relationships between the mechanical properties of mudstone,temperature,and confining pressure were established.The results showed that there is basically a positive linear relationship between the confining pressure and elastic modulus,Poisson's ratio,compressive strength,and residual strength of the mudstone within the confining pressure range of less than 60 MPa.There is no clear correlation between temperature and those mechanical properties of mudstone within the temperature range of lower than 130℃.However,those mechanical parameters of mudstone remained relatively consistent under the same confining pressure and different temperatures,indicating that burial depth (confining pressure),instead of temperature,determines the mechanical parameters of mudstone.The stress-strain curves of mudstone also indicated that even the mudstone at the late diagenetic stage tends to transit from brittle to ductile with the increase of buried depth.According to the relationship between the peak and the residual compressive strengths,the critical burial depth for brittle-ductile transition of the Silurian mudstone in Sichuan Basin is determined approximately at 4200-4400m.Beyond this critical depth,the mudstone is less prone to fracture and thus has greater sealing ability as cap rocks,while has poor fracability as shale gas reservoirs.

Key words: brittleness, ductility, fracability, mudstone, Silurian, shale gas, Sichuan Basin