Abstract: The composition and content of clay minerals in 395 rock samples from sandstone reservoir in Xujiahe Formation, Sichuan Basin are examined by a combination of X-ray diffraction, thin section casting, scanning electron microscopy, energy spectrum analysis, and trace element assays. Results show that illite and chlorite are the major clay minerals in sandstone reservoir in Xujiahe Formation. Chlorite accounts for 33% of total content of clay minerals and has a positive correlation with porosity and permeability of reservoir. The authigenic chlorite is iron-magnesium transition type, which contains abundant iron and is mainly developed on grain surface of sandstone skeleton in the form of foliated and needle-shaped lining. Authigenic chlorite is formed in alkaline aqueous solution at pH 7-9, whereas quartz overgrowth occurs in weak acidic aqueous solution at pH 5-6. Different symbiotic relationships between authigenic chlorite and quartz are resulted from variations of aqueous medium condition. The presence of authigenic pore-lining chlorite could not restrain quartz overgrowth significantly, and authigenic chlorite preferentially grows in sandbodies with macropore spaces.