The Oriente, Ucayali, and Madre de Dios basins, located in the Amazon drainage of Colombia, Ecuador, and Peru, are part of a series of large asymmetric depressions between the Andes Cordillera and the Guyana and Brazilian shields. They are separated by basement arches and have areas of 458,000, 200,000, and 95,000 km², respectively. The whole area is topographically low, covered by heavy rain forest, traversed by numerous huge tributaries of the Amazon, and very sparsely populated.

From early Paleozoic time until the Maestrichtian, seas repeatedly invaded the area, depositing a variety of sediments, but mostly calcareous and silicate clastic material, over wide areas. At the beginning of the Tertiary, predominantly marine deposition gave way to nonmarine deposition, reflecting the Andean orogeny and topographic development of the Andes mountains. The depositional cycle of major importance for hydrocarbons took place in the Cretaceous. A complete marine cycle of miogeosynclinal sedimentation is represented; the maximum thickness is 2,500 m, but the sequence thins consistently and becomes sandier toward the east. Although the sequence consists mainly of sandstones and shales, limestones and sandy limestones are important potential reservoirs. Most prospective stru tures in the basin are anticlines, generally fault-bounded and somewhat steeper on the east. Salt domes and other diapiric structures are also present. Amplitude of structures and intensity of deformation decrease eastward. The formation of structures and the migration and entrapment of hydrocarbons in them appear to have occurred at various times in the Tertiary.

The Colombian part of the Oriente basin is in an advanced state of exploration and has little potential for future significant discoveries. In Ecuador, although the peak of exploration activity has been passed, the future potential is thought to be substantial. In Peru, exploratory drilling in the Oriente basin has already discovered reserves on the order of 800 million bbl of oil. Based on these facts and on information from Colombia and Ecuador, the total potential of the Oriente basin is estimated to be 25-35 billion bbl. About 20 wildcats have been drilled in the Ucayali basin; two small oil fields and an undeveloped, but potentially large, gas field have been discovered.

On the basis of these results and estimates for the Oriente basin, the Ucayali basin has an estimated potential of 5-10 billion bbl. In addition, Paleozoic sedimentary rocks have some potential. The Madre de Dios is the least explored of the three basins. Favorable conditions are known to exist and oil seeps are present. A potential of 6-12 billion bbl is estimated. The total estimated potential for the three basins is about 45 billion bbl.