Regional Investigations and Hydrocarbon Exploration History of the South Atlantic Rifted Continental Margins: Development of the Salt Basins and Transform Margin Basins Without Salt
Published:December 01, 2015
Roberto Fainstein, Webster U. Mohriak, 2015. "Regional Investigations and Hydrocarbon Exploration History of the South Atlantic Rifted Continental Margins: Development of the Salt Basins and Transform Margin Basins Without Salt", Petroleum Systems in “Rift” Basins, Paul J. Post, James Coleman, Jr., Norman C. Rosen, David E. Brown, Tina Roberts-Ashby, Peter Kahn, Mark Rowan
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The rifted continental margins off Brazil and West Africa encompass several morphological distinct regions that resulted from the plate separation and subsequent drift of South America and Africa. The main building blocks that controlled the development of the Atlantic-type continental margin basins consist of prerift, synrift, and postrift tectonic stages, and these events determined the basin infill. Oceanic fractures created by transform faults that indent the continental margins form basement highs that ultimately define the tectonic edges of the continental margin basins. These boundaries are involved in the marginal plateaus, marginal banks, and characteristic marginal volcanic ridges.
Major petroleum producing provinces are situated in the rifted margin salt basins and also in the equatorial transform margin basins without salt. In the salt basins, the continental slope and rise are characterized by the development of massive salt walls that delineate minibasins that were in-filled with deep-water sediments. The hydrocarbon production from these deep-water reservoirs are mainly from postsalt Tertiary and Upper Cretaceous turbidite sands, plus added production from the cluster trends of presalt carbonate microbialite reservoirs. These reservoirs are all mainly sourced by Lower Cretaceous synrift lacustrine strata, but Upper Cretaceous source rocks have also been identified in the South Atlantic salt basins. The equatorial conjugate transform margin basins are also characterized by minor salt deposition in some regions (such as the Ceará basin). These basins produce hydrocarbons from combination traps of Tertiary and Upper Cretaceous turbidite reservoirs. All of the deep-water basins are influenced either by salt or shale tectonics and related to episodic volcanism. In the southernmost South Atlantic, volcanism dominates the conjugate margins, as indicated by thick wedges of seaward-dipping reflectors.
There are two main types of South Atlantic continental margins:
The transform margins shaped by the large offset equatorial fracture zones and in which the transverse structural lineaments are predominant, except for preexisting structuring that is not related to the Cretaceous transform directions. In these basins, where salt is absent, the typical exploratory play includes combination traps with turbidite reservoirs, generally exhibiting remarkable bright-spots amplitudes that reflect the associated deep-water channels and stratigraphic pinch-outs.
The rifted conjugate margins shaped by salt tectonics, which extend from Sergipe-Alagoas to the Santos basin in Brazil and the corresponding conjugate margin basins from Cameroon to Angola. Here, the typical plays are predominantly associated with autochthonous salt. The salt basins account for most of the South Atlantic’s offshore petroleum production. The postsalt and presalt petroleum yields are explained by lacustrine source rock maturation during Tertiary times. Hydrocarbon migration is either into the synrift reservoirs proper or through salt windows into the postrift/postsalt reservoirs of mid-Cretaceous to Miocene. In the presalt plays, the synrift source rock and the carbonate reservoirs in the sag basin are capped either by massive salt or by a thick layered highly mobile evaporite sequence. Future exploration will need to tackle the ultra-deepwater provinces near the continent-ocean boundary, where there are several potential tectonic, structural and stratigraphic targets.