The Oriente Basin of Ecuador is one of the most important hydrocarbon-bearing basins in the Andean Foreland Basin Group of South America. The Main-M1 submember of the Upper Cretaceous Napo Formation,as one of the main exploration and developement formations,has been controversial in terms of its depositional environment,and its depositional system and evolutionary process remain unclear. Based on core,well logs and borehole cuttings,the main sedimentary microfacies types,depositional systems and evolutionary processes of Main-M1 submember were investigated in this study. The results show that the study area is located in a marine-continental transition environment with complex hydrodynamic conditions,and a tidal delta develops in the east,which gradually transitions into a shallow shelf environment in the west. In the study area,eight types of sedimentary microfaces including subaqueous distributary channel,subaqueous inter-channel,tidal channel,tidal sand bar,tidal flat,and tidal inter-channel,are recognized. According to the petrographic characteristics,the Main-M1 submember is further divided into three sub-layers. By analyzing the lithological assemblages of the three submembers,it is determined that during depositional of the Main-M1 submember,the Auca paleoarchelon gradually formed,and meanwhile the relative sea level of the study area rose due to the flexural effect. The three submembers overall show a gradual deepening upward trend and the tidal delta continuously retreated landward. Based on the reconstruction of the tidal delta-shelf depositional system during deposition of the Main-M1 submember,the subaqueous distributary channel and the leading sand bar in the eastern part of the study area are considered to have certain exploration potential for lithologic reservoirs.