We identified a previously unknown carbonate buildup in the Cretaceous M1 limestone in the Bermejo field, northwestern Oriente Basin of Ecuador. Thinner carbonate builduplike features are also seen in the deeper A limestone, which has produced small volumes of oil in the northern Bermejo field. The carbonate buildups can represent a significant new play type in the western Oriente Basin, where limestones have been known principally as a source rock. The M1 carbonate buildup, imaged as a lenslike event in three-dimensional seismic data, forms a circular feature in the isopach, amplitude, and dip maps. It is characterized by generally high but variable seismic amplitude, and its thicknesses (20 to 40 m; 66 to 131 ft) exhibit a complicated pattern, suggesting weathering and/or localized carbonate growth. The dark and rough top surface and irregular edges of the M1 carbonate buildup, revealed by the dip map, probably suggest the formation of fracture-cavity-karst–type reservoirs. The Late Cretaceous convergence along the western Oriente Basin may have further fractured the carbonates in the area. The M1 carbonate buildup, assuming a porosity of 10%, can contain more than 4.0 10⁷ bbl of liquid, which is particularly significant in the Bermejo field where production has been marginal.