Abstract:The types and the causes of fractures in the Xujiahe Formation tight sandstone reservoir, in the BazhongTongnanba area of northeastern Sichuan basin, were studied based on the core and outcrop observations, analysis and testing, loggings and other data which was combined with inclusion analysis to identify the fracture formation stages. Based on this, the relative control of lithology combination, stratum thickness, fault and fold deformation strength on fracture development is discussed. The results show that the fractures of the Xujiahe Formation in the northeastern Sichuan Basin were triggered by various causes, formed in multiple stages, and show differential distribution. Tectonic fractures, mainly developed and distributed in tight sandstone reservoirs, can be divided into 3 stages: the NNW and NWW directional tectonic fractures in the middle Yanshanian (Middle Jurassic), and filled by finegrained calcite; the NE trending fractures associated with fault and fold were formed in the late period of Yanshan movement and were filled with quartz and coarse calcite; and the NW and SN trending fractures associated with fault were formed in the early period of Himalaya tectonic movement, and are either empty or partially filled with coarse grained calcite. The diagenetic fractures and overpressure fractures are related either to water shrinkage of clay minerals or the fluid pressurization caused by massive hydrocarbon generation in source rocks and formed in Late JurassicEarly Cretaceous. The development of fractures is mainly affected by lithology combination, stratum thickness, fault and fold deformation strength. The middle and fine sandstone reservoirs with small stratum thickness, closer location to fault, and greater strength of fold deformation have developed more fractures.