On the basis of field profile and core observation, multiple methods such as thin section identification, scanning electron microscopy, and electron probe were used to conduct a detailed analysis of the occurrence characteristics and formation mechanism of authigenic chlorite in sandstone reservoir of the Qigu Formation in the Fudong slope area, providing the adjustment direction for the search for oil and gas reservoirs and development plans. The results indicate that the authigenic chlorite in the sandstone of the Qigu Formation reservoir in the study area exhibits three occurrence states under the microscope: liner-type, membrane-type, and filling-type, with liner-type being the main occurrence, and the corresponding reservoir properties are good; Authigenic chlorite is iron-rich chlorite, and the dissolution of tuffaceous debris and feldspar minerals, the transformation between clay minerals, and the flocculent precipitation formed by Fe dissolved in rivers are the important material basis for the formation of authigenic chlorite; Sandstones with large grain size and well sorting have high content of authigenic chlorites, which is helpful for forming and protecting of authigenic chlorite; Strong fluid dynamic is also an important controlling factor of authigenic chlorite formation; The occurrence states of chlorite in the Qigu Formation are similar, and different content of authigenic chlorite have a certain affection on the reservoir property and sensitivity.