Abstract: Giant Fields of the Late 1980s associated with Type "A" Subduction in South America

Bernard C. Duval, Carlos Cramez, Gustavo Valdes

About 10 billion bbl of recoverable oil have been found in 4 fields, Cano Limon/Guafita, Ceuta/Tomoporo, Cusiana and El Furrial, discovered in the late 1980s, in the northern part of South America. They are associated with the major type "A" subduction, which is part of the large South American Ceno-Mesozoic megastructure and links the Brasilian plate to both the Caribbean and the Nasca plates. Studying the similarities and differences helps us to understand the petroleum system involved in each case and to establish practical guidelines for exploration.

The petroleum generating subsystem of all the fields is very similar, and they are now universally recognized as being mainly charged from the same source-rocks: the organic sediments of La Luna or equivalent formations, associated with the major downlap surface of the post-Pangean continental encroachment sedimentary cycle (i.e., MFS 91.5 Ma). However, the subsequent geological histories and the subsystems that concentrated the petroleum are not the same for all fields.

Cano Limon, Ceuta and Cusiana are associated with a Tertiary foredeep basin developed over an older generating back-arc basin and, particularly for Ceuta and Cusiana, the majority of compressional structures were created by tectonic inversions.

In El Furrial, which is associated with a Tertiary foredeep basin overlying an older generating passive margin basin, decollement surfaces and their associated thrusts are predominant.

These tectonic settings created different petroleum systems: normally charged with high impedance and vertical drainage in Ceuta and Cusiana; supercharged, with low impedance and lateral drainage in El Furrial. Each case requires appropriate exploration approaches.

For Ceuta, Cusiana and Cano Limon, the existence of Eocene high-angle faults subject to later reactivation, and the proximity of structural highs are key factors for a successful exploration, whereas recent positive features, overpressured seals and lateral ramps are essential in the Furrial area.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994