We identified a previously unknown carbonate buildup in the Cretaceous MI limestone in the Bermejo field northwestern Oriente Thinner carbonate builduplike features are also Basin of Ecuador which has produced small volumes seen in the deeper A limestone, of oil in the northern Bermejo field. The carbonate buildups can represent a significant new play type in the western Oriente Basin, where The M I limestones have been known principally as a source rock. 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 MI 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 M I carbonate buildup, assuming a porosity of 10%, can contain more than 4.0 x 10(7) bbl of liquid, which is particularly significant in the Bermejo field where production has been marginal.