The purpose of this study was to improve the knowledge, increase the quality of the input data for the static and dynamic modeling, and to contribute to define new areas for exploration and production of hydrocarbons in the Cretaceous reservoirs of the Tapi-TTT oilfield, Block 57, northernmost Oriente basin, Ecuador. Specifically, the pursued objectives were: understand the structural context of this oilfield, run sedimentological core descriptions and identify regional sequence stratigraphic surfaces, define major paleoenvironments, describe the paleogeography of the sedimentary area, construct field maps of sediment distribution, run a quality control and interpret the petrophysical data, and characterize the reservoirs. For getting the objectives there were employed two 4-in core samples 102.5-ft in length, well logs, and a conventional study on 122 core plugs of the Tapi-9 well. The study concludes that the Lower T Sandstone reservoir, upper Albian in age, is a good quality clastic reservoir, composed by medium-grained Orthoquarzites, with its petrophysical parameters depending on its sedimentological facies, with the best quality reservoir facies placed in the sandy fluvial- to tide-dominated estuarine realms and the less quality reservoir facies in the medium to upper shoreface clastic realms, and showing a near equal reservoir capacity for fluid flowing in the H & V directions and a great potential of depletion. For the Lower U Sandstone, lower Cenomanian in age, the study concludes that it is a good to excellent reservoir, with its petrophysical parameters depending on its sedimentological facies too, with the best quality reservoir facies occurring in the sandy tide-dominated estuarine realm and the less quality reservoir facies in the silty-sandy (tight) tide-dominated estuarine realm, and showing a different reservoir capacity for fluid flowing in V & H directions and a low potential of depletion but with a high potential of creating stratigraphic traps.