<p>This study takes the shale of the Jurassic Da’anzhai Member in the Sichuan Basin as the research object. The mineral composition, microstructure and physicochemical properties of the shale were systematically analyzed via X-ray diffraction, scanning electron microscopy, basic physical property testing, rolling recovery tests, linear expansion rate tests, wettability tests and triaxial mechanical experiments. The results indicate that the shale has a high clay minerals content (average of 58.52%), which is mainly composed of illite and chlorite. It is characterized by well-developed microfractures, strong oil wettability and easy hydration when in contact with water. After soaking in the JFS water-based drilling fluid, the compressive strength and elastic modulus of the core decrease significantly, with the reduction rates reaching 43.1% and 50.3%, respectively. The mechanical properties further deteriorate as the soaking pressure (1-7 MPa) increases and the soaking time (1-15 days) increases. By optimizing the drilling fluid system, it was found that adding 50% organic salt inhibitor effectively alleviated the strength weakening. Compared with those of the unoptimized system, the compressive strength and elastic modulus increase by 14% and 16.2%, respectively. These findings provide a theoretical basis and technical support for the selection of drilling fluids and the maintenance of wellbore stability in the Da’anzhai Member shale formation.</p>