A large set of thick sand bodies is developed in the Second Member of the Cretaceous Bashijiqike Formation (the Ba- 2 Mem.) (K1bs2) in the Bozi—Dabei area of Kelasu structural belt, but there are few detailed studies on the difference of lithofacies and reservoir formation mechanism under the control of sedimentary microfacies, which seriously limits the success rate of exploration. Therefore, this paper systematically carried out the study of favorable lithofacies characteristics and high- quality reservoirs formation mechanism of the Ba- 2 Mem., which is conducive to improving the success rate of natural gas exploration and development.Methods:In order to clarify the favorable lithofacies characteristics and reservoir formation mechanism of the deep sandy braided river facies containing muddy gravel in the Ba- 2 Mem., the lithofacies types, distribution rules, reservoir characteristics and formation mechanism of high- quality reservoirs in the study area are analyzed in combination with core observation, laser particle size analysis, whole rock and clay X- ray diffraction, casting thin sections, scanning electron microscopy, routine rock analysis (porosity and permeability), high pressure mercury injection, nuclear magnetic resonance and logging data. Results: ① The sedimentary system of the underwater distributary channel in the front of the sandy braided river delta with muddy gravel is widely developed in the study area, and the reservoir is dominated by feldspathic lithic sandstone. Vertically, the Ba- 2 Mem. can be divided into 5 sand groups, and the single sand group develops successively from bottom to top of the muddy gravel- rich massive medium—fine sandstone lithofacies, muddy gravel—medium massive medium—fine sandstone lithofacies, massive medium—fine sandstone lithofacies, stratified silt—fine sandstone lithofacies and massive mudstone lithofacies. ② The Ba- 2 Mem. sand body is characterized by low porosity and low permeability reservoir, and the lithofacies types developed in the microfacies of the underwater interdistributary bay are poor in physical properties, pore structure and fluid mobility, while the ones controlled by the microfacies of the underwater distributary channel are better. The reservoir mainly develops primary intergranular pores and intergranular dissolution pores, with local micro- fractures and intragranular dissolution pores. ③ The large set of medium—fine sandstone formed in the microfacies of the underwater distributional channels is the basis for the formation of the Ba- 2 Mem. high- quality reservoirs, and the diagenetic transformation of different structural locations and different types of reservoirs is different. Among them, late rapid burial compaction is the main factor of reservoir densification, followed by large cementation in early and middle period. The key factors for the formation of high- quality reservoirs are the retention of pores in the overlying thick layer of gypsum—salt rock, the early atmospheric freshwater dissolution, the late organic acid dissolution and the micro- fractures associated with intense tectonic movement. Conclusions: Based on a variety of macro and micro analysis and test data, this study systematically analyzed sedimentary microfacies types, lithofacies characteristics, pore structure differences among different lithofacies, and the effects of sedimentation, diagenesis and tectonic processes on the formation of high- quality reservoirs of the Ba- 2 Mem. （K1bs2）, which has certain guiding significance for deep—ultradeep oil and gas exploration.