Deepwater Exploration in the Offset Margins of the Equatorial South Atlantic

Fainstein, Roberto¹ and Mohriak, Webster
¹[email protected]

The Equatorial Margins of the South Atlantic are predominantly offset margins the result of shear motions that marked the Equatorial Atlantic since early rifting. These margins follows the morphology imprint of the offset shift amongst mid-ocean ridge crests these in the order of 2,000 km. It is expressed in the West Africa coast re-entrance into the Gulf of Guinea, and also remarkably seen along the trace of volcanic extrusions of the North Brazilian Ridge. Regional geological and geophysical data indicate that this entire shear rift system followed closely the morphology of older continental structural trends, mainly of high grade metamorphic rocks.

The regional geophysical appraisal of the Northern Brazil and Guyana’s upper continental margins, when compared with the offshore Ivory Coast and Liberia margins, establish the important concepts concerning the nature of these of sheared margins. Magnetic maps from conjugate margins display trends with similar wavelengths, and these are oriented normal to the respective coast lines. We suggest they represent magnetic sources that have remained intact during the Pan-African, Eburnean and Liberian thermo-tectonic events. During the Early Cretaceous rifting, these anomalies represented Precambrian basement rocks which retained the magnetic fabric from breakup to the drift stage. Post breakup factors accounting for thermal subsidence of these basins in deep waters include volcanic cooling and sediment loading. Large rivers such as the Niger and the Amazon accounts for the great Tertiary sediment accumulation along the continental margins.

Geophysical interpretation of modern seismic data sets have led to remarkable oil and gas discoveries in turbidite reservoirs along the deepwater realm of the Atlantic Equatorial Margin in Western Africa, particularly Nigeria and Ghana. Recent discoveries in the French Guiana and Ceara basins in northern South America indicate a deep-water trend with potential accumulations in the Amazon, Para-Maranhao, Barreirinhas and Potiguar basins offshore Brazil. This paper therefore reviews the petroleum geology of these basins in tandem with seismic image results from recently acquired data for the purpose of a better understanding of the stratigraphic nature of the deeper continental and oceanic crust and in search of hydrocarbon source rocks that may yield new hydrocarbon production.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013