Print this pageThe Early Cretaceous Rift and Sag Phases in the Offshore Basins of Brazil and Uruguay: How Much in Common?
The South Atlantic opening is characterized by an initial (Early Cretaceous) extensional phase dominated by asymmetric half-grabens. This early phase is similar to the current structural framework of the East African Rift System (EARS), where recent discoveries have renewed the interest in this type of oil and gas habitats. Recent exploratory success in the South Atlantic has been focused on offshore Brazil, particularly in the sag fill of the pre-salt section of the Santos, Campos and Espirito Santo basins, an emerging world-class oil province. The structural asymmetry of these half-grabens is mainly due to differential, tectonically-induced subsidence that created optimal conditions for the potential development of lacustrine to brackish source rocks. Classic seismic signatures like fanning (strongly divergent internal configuration) on fault borders, thinning (convergent internal configuration) and onlap on flexural margins, and compaction synclines over basement footwall cut off points are common. This half-graben fill phase was followed by a sag phase dominated by thermal subsidence which expanded beyond the rift shoulders. The trap geometries and distribution of facies, particularly in the lower section of the sag fill, have been influenced by highs inherited from the underlying asymmetric half-grabens. New seismic in the basins of offshore Uruguay allows to draw significant analogies between the offshore basins in Brazil and their counterpart further south in Uruguay. In particular, the Punta del Este basin exhibits similar half-graben geometries followed by a laterally extensive sag fill. This configuration has the potential for onlap/pinchout traps at the sag level, indirectly controlled by pre-existing half-graben geometries, which tend to develop on the flexural margins or on shoulders inboard. Differential compaction at the half-graben border fault margin would be a critical factor to create counter-regional dips necessary to form structural (4-way) closures at the sag level. From the point of view of trap efficiency, the main difference between the rift-sag sections of offshore Brazil and Punta del Este basin is the absence of salt in the latter. Marine shales in the drift section can be invoked as potential top seals in light of the absence of a regional salt layer.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil