--> Controls of Asymmetrical Rifting on Giant Oil Habitats Within Conjugate, Pre-Salt Carbonate Sag Basins of Brazil and West Africa

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Controls of Asymmetrical Rifting on Giant Oil Habitats Within Conjugate, Pre-Salt Carbonate Sag Basins of Brazil and West Africa

Abstract

The largest deepwater oil discoveries of the past 10 years were found in carbonate-filled, sag basins of the Equatorial and South Atlantic Ocean. We explain the asymmetrical distribution and thickness variations in the areas of pre-salt carbonate sag basins in Brazil and West Africa by, first, isostatically correcting the top of oceanic crust in the area of the Santos to Espirto basins of Brazil and their conjugates in the Namibe and Kwanza basins of Namibia and Angola to improve the location of the continent-ocean boundaries in these areas; and, second, using bathymetric, gravity, magnetic and 1,700 km of regional seismic transects to define the footwall versus hanging wall of the asymmetrical rift margins for both conjugates. For the Santos-Namibe conjugate, we propose Santos to be the hanging wall of an asymmetrical rift system with a 200-km-wide rifted margin and overlying sag basin with carbonate reservoir facies and sloping bathymetric profile; and the Namibe to be the footwall with a 125-km-wide rift and sag basin and steeper bathymetric profile. For the Campos-Kwanza conjugate 400 km to the north, we propose Campos to be the footwall with a 150-km-wide rift zone and overlying sag basin and sloping bathymetric profile. Well data shows that thicker the carbonate sag (135- 325 m) and its overlying salt basin (up to 2 km) are associated with the hanging wall blocks of Kwanza and Santos and the thinner carbonate sag (15–75 m) and its overlying salt basins (up to 1.5 km) are associated with the footwall blocks in accord with predictions based on recent analog modeling. Reservoirs within the sag phase of these conjugate margins include high porosity and permeability lacustrine carbonates deposited in high-energy ooid and oncoid beds along with highly porous travertine hot-spring deposits that include very porous tufa mounds sealed by 1 km or more of overlying salt. These reservoirs are sourced by brackish-lacustrine shales deposited during the sag phase, interbedded with the carbonates. Based on these correlations we predict more potential for larger discoveries in thicker sags/carbonate reservoirs associated with hanging walls underlying Santos and Kwanza.