The Oriente, Ucayali, and Madre de Dios basins in the Amazon drainage of Colombia, Ecuador, and Peru are members of a series of large asymmetric depressions between the Andean cordillera and the Guiana and Brazilian shields. They are separated from one another by basement arches and have areas of 458,000, of 200,000, and of 95,000 sq km, respectively. The area is topographically low, covered by heavy rain forest, traversed by many huge tributaries of the Amazon and is 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 deposits. At the beginning of the Tertiary, dominantly 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 with a maximum thickness of 2,500 m, but it thins and becomes sandier toward the east. Although the cycle consists mainly of sands and shales, limestones and sandy limestones are important potential reservoirs. Most prospective structures in the basin are anticlines, generally fau t-bounded and 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 appear to have occurred at various times in the Tertiary.

The state of exploration in the Colombian part of the Oriente basin is well advanced with low-undiscovered potential. In Ecuador, although the peak of exploration activity has been passed, the future potential may be substantial. In Peru exploration drilling in the Oriente basin already has discovered reserves on the order of 400 million bbl of oil. On the basis of these facts and on information from Colombia and Ecuador, the total potential of the Oriente basin is estimated to be 25 to 35 billion bbl.

About 20 wildcats have been drilled in the Ucayali basin with the discovery of two small oil

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fields and an undeveloped, but potentially large gas field. From these results, and estimates for the Oriente basin, the Ucayali basin has an estimated potential of 5 to 10 billion bbl. In addition, Paleozoic sediments also have some potential.

Favorable conditions are known to exist and oil seeps are present in the Madre de Dios basin, the least known of the three. A potential of 3 to 8 billion bbl is estimated. The total estimated potential for the three basins is about 40 billion bbl.

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