The Atrato sub-basin constitutes the northern portion of the Chocó Basin (sensu ANH, 2007) and it is bounded to the north by the international border with Panama, to the east by the western foothills of Western Cordillera, to the west by the Baudó Range and the Pacific Ocean, and to the south by the Istmina Deformed Belt.
The hydrocarbons prospective sedimentary sequence is composed of Cenozoic rocks accumulated on a Cretaceous basement, composed of oceanic basalts and sedimentary rocks (limestones, dark shales and cherts) deposited in distal shelf and outer slopes (Figure 3). It has been postulated that the oceanic terrain that forms the basement migrated eastwards since the Late Cretaceous and collided, docking on the continental margin of Northwest South America and closing the communication between Pacific Ocean and Caribbean Sea during Plio- Pleistocene times. During Cenozoic, times the Istmina Deformed Belt behaved as a shallow platform, on which organic matter rich sediments accumulated, mainly black cherts and possible reef limestones of apparently refill character.
The Atrato sub-basin has been interpreted as a former forearc region. It is bounded to the west by strike-slip faults, whose directions slightly diverge from those of the Baudó Complex. To the east, the Atrato sub-basin is limited by different igneous units of Mandé Magmatic Arc, the transition zone is represented by a straight fault system with N-S preferential orientation. Southwards it bounds with Istmina-Condoto Structural High and northwards the sub-basin ends at the Quirá low ranges that mark the international border with Panama and separate the Atrato sub-basin from an independent Chucunaque- Tuira Basin.