Abstract: The LU Sub-member (LUSM) of the Cretaceous Napo Formation in the northern part of Oriente Basin, Ecuador, is extremely rich in hydrocarbon resources and has a large potential for hydrocarbon exploration. However, the depositional pattern of the LUSM remains poorly understood, which had largely limited the further exploration and development of the area. Based on the core observation, logging, maturity analysis, and thin-section study, the petrological and sedimentary features were described; the biological relics and fossils, sedimentary structure and rhythm, and the logging phase signatures were analyzed. Results show that the LUSM strata were developed in a transitional environment between sea and land, in which tidal action was dominated. Referring to previous studies, we proposed that there was a transitional zone between tide-dominated estuary and a continental shelf, forming a tide-dominated estuary - transitional zone - continental shelf sedimentary system, showing seven sedimentary microfacies: tidal channel, tidal sand bar, sand flat, glauconite sand sheet, mixed flat, shelf mud, and calcareous beach. Among them, the tidal channel and tidal sand bar are sweet reservoir facies, followed by sand flat. By analyzing the depositional system, a tidal-dominated estuarine - transition zone - shelf depositional model was established for the study area. This study provided a favorable scientific basis for oil and gas exploration in the Cretaceous Napo Formation in the Oriente Basin.