It is generally believed that the tight sandstone reservoir has strong heterogeneity and poor physical properties, but the tight sandstone reservoir in the Jimsar Sag, Junggar Basin, has good oil and gas production. Through the core data photos, it is found that the reservoir fractures of the Jinjinzigou Formation (P2jj) in the study area are developed, most of which are filled with crude oil and calcite. Based on the microscopic thin sections and scanning electron microscope photos, it is found that most samples develop micro- fractures. Through the observation, measurement and calculation of micro- fractures under the microscope, combined with the core and regional tectonic background, the development characteristics, main controlling factors and genetic mechanism of micro- fractures were systematically analyzed, and the formation stages of micro- fractures were discussed. The research results have guiding significance for the genesis of micro- fractures in tight sandstone reservoirs and the evaluation of high- quality reservoirs in the Junggar Basin, and can be used as a reference for oil and gas exploration in tight reservoirs with similar geological conditions.Methods: In order to study the distribution characteristics and genesis of micro- fractures in tight sandstone reservoirs of the Jinjinzigou Formation (P2jj) , cast thin sections, scanning electron microscopy (SEM) and Nano Measure software were used to observe and measure the reservoirs of the Jinjinzigou Formation (P2jj) in PetroChina Xinjiang Oilfield Company. Through the observation of 226 cast thin sections and SEM images, the characteristics and causes of micro- fractures were analyzed. Through the cutting relationship, development characteristics and regional tectonic background between micro- fractures, the formation stages of micro- fractures were discussed.Results: The results show that the micro- fractures in tight sandstone reservoirs of the P2jj in the Jimsar sag can be divided into three genetic types: tectonic origin, diagenetic origin and abnormal high- pressure origin. Based on the scale of micro- fractures and the relationship between micro- fractures and particles, the micro- fractures in the study area are divided into three basic types: intra- granular fractures, grain- edge fractures and penetrating fractures. Among them, intra- granular fractures are the most developed and the most important micro- fracture type in the reservoir. The second is the most developed grain- penetrating fracture. The development scale of grain- penetrating fracture in the study area is large, cutting through particles, and most of them are effective fractures. The grain boundary seam is less developed, which is mainly developed between the particles, and the connectivity is better. According to the microscopic analysis, the reservoir micro- fractures in the study area are effective, and the open micro- fractures account for more than 60%. Most of the filled fractures are filled with organic matter and crude oil.Conclusions: Based on the analysis, the micro- fractures in the reservoir of Jinjinzigou Formation (P2jj) are mainly affected by diagenesis and tectonism, and their development degree is mainly controlled by internal factors (sandstone grain size, rock debris composition, interstitial content and contact relationship between particles and particles) and external factors (tectonism). Based on the analysis of tectonic stages, there are at least 3 stages of micro- fractures in the study area, which are primary fractures, dissolution fractures, early filling fractures re- split fractures and late cutting fractures. In general, micro- fractures are not developed in tight sandstone reservoirs in the study area. In medium- fine sandstone, micro- fractures are developed in siltstone, and micro- fractures are more developed. In argillaceous siltstone, the development of micro- fractures is weak, and in mudstone, micro- fractures are basically not developed.