Relating Changes in the Angolan Deep-Water Folds and Thrusts to Pre-Salt Structure

Lumen P. Sebastiao², Steven G. Henry¹, Severino Cardoso², Al Danforth⁴, Sujata Venkatraman³, and Mvesi Maziano^2
1RIFTT, Las Cruces, NM
²Sonangol, Luanda, Angola
³GX Technology, Houston, TX
⁴Consultant, Houston, TX

Deep-water fold and thrust belts are being established as world class provinces for the discovery and production of hydrocarbons. In this paper we will present examples of deep-water discoveries in the Angolan fold and thrust belt of the Congo delta, and propose new plays in the southern fold belts of the Kwanza and Benguela basins. We will illustrate the changes in styles of folding along strike from northern to southern Angola. These style changes have previously been related to salt and sediment load, but are fundamentally related to the geometry of the basement, which has been imaged on pre-stacked depth migrated (PSDM) regional (CongoSPAN) seismic data.

PSDM data has been crucial for imaging beneath the salt, and providing insights on how the pre-salt structure influenced original salt deposition, and the subsequent styles of post salt folding. In northern Angola (Lower Congo Basin), deep-water discoveries in the late 1990’s (Girasol) consisted of relatively simple salt cored anticlines that developed over the structurally simple western flank of the sag basin. The recent discoveries in the 30’s Blocks, are more complicated due to salt having been deposited in “break-up grabens”, which were active during both the late rift and continuing into the early stages of drift.

The Angolan fold belts continue to the south into the Kwanza and Benguela Basin which has similar settings of a smooth sag basin transitioning to the west into break-up grabens and a structurally high rift flank horst. The thickness of the original salt is significantly higher in these basins, with up-dip extension resulting in down-dip inflation and thrusting over the horst. New plays have been developed for the fold belts in these basins, which are based on present day active Albo-Aptian, and pre-salt sag basin (Falcao) source rocks, with reservoirs in Cretaceous carbonates and clastics.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery