During the drilling process

the wellbore instability frequently occurres in Baxigai Formation in the Kelasu structural belt of the Tarim Basin

which seriously affects the drilling efficiency. In order to study the physic-chemical and mechanical mechanical properties properties

differences of physical and pechanical characteristics and influence of drilling fluid immersion on the mechanical properties of the sandstone and mudstone in Baxigai Formation

the physic-chemical characteristic tests（such as mineral composition test and cation exchange experiment）

and mechanical test（such as triaxial compression test）have been carried out. The research results show that the clay minerals content of mudstone in Baxigai Formation is high

and the cation exchange capacity is about three times that of sandstone. The mudstone here also has a large number of micro-cracks

and the hydration swelling is obviously stronger than that of sandstone. All these things make it more prone to be damaged. At the same time

the strength decreases under the immersion and lubricating effects of oil-based drilling fluid. The strength decrease of mudstone is about 1.5 times that of sandstone. Under the same confining pressure

the mudstone and sandstone strength

the corresponding axial and radial strain at the critical failure point

and the rock damage mode of mudstone and sandstone in Baxigai Formation are different. The damage to mudstone structure caused by drilling fluid immersion is greater than that of sandstone. The analysis of the mechanical properties of the rock in Baxigai Formation is of great significance to clarify the mechanism of wellbore instability in this formation and formulate prevention and control measures for wellbore instability.