Abstract: Tectonic Habitat of Petroleum Along the South Atlantic Margins

Szatmari, Peter - Petrobras/Cenpes

On a pre-drift reconstruction of their facing margins, almost all of Brazil's and SW-Africa's petroleum reserves occur within a nearly continuous N-S trending strip, the South Atlantic Mega-Trend, whose formation was controlled by continental breakup. It took almost 40 m.y., from earliest Valanginian to late Albian times, for Africa and South America to separate as they rotated relative to each other about a pole located in NE Brazil.

The opening of the Atlantic divided the Mega-Trend into segments that now alternate between the Brazilian and the African margins: a productive segment on one margin is faced by a relatively sterile segment on the other. This distribution may reflect alternating polarity of rifting by simple shear. In the productive segments, both oil richness and the tectonic habitat of the oil are defined by distance from the pole of relative rotation in NE Brazil. Far from that pole, as in the Lower Congo and especially in the Campos basin, both syn-rift extension and oil reserves are large, rifted continental crust now extends far offshore and the largest petroleum accumulations occur offshore. In the Campos basin, part of the petroleum is in early Aptian sub-salt sediments overlying thick syn-rift volcanics, but most of it is in late Cretaceous to early Tertiary turbidites structured by salt flow. Conversely, near the pole, in the Recôncavo, Sergipe-Alagoas, and Potiguar basins, both total extension and petroleum reserves are moderate and petroleum occurs mostly on land, along rift and cross-rift faults. Fig. 1 shows a clear increase in oil richness from north to south, away from the pole of relative rotation, accompanying the increase in total extension. Major discoveries that would fit the graph may be expected in the relatively unexplored Espirito Santo, Cuanza and Santos basins.

At the southern end of the South Atlantic Mega-trend, in the Santos basin, the pre-salt rift and subsequent salt basin was sealed from the euxinic marine basin between Argentina and South Africa by the volcanic complex of the Ponta Grossa dike swarm, created by the Tristan da Cunha hotspot. Near the hotspot, in the Campos and Santos basins, where total extension was high, the pre-Aptian rift sequence consisted mostly of volcanics (basalt flows and tuffs) which correspond to, but are somewhat younger than, the continental flood basalts of the Paraná basin. (Similar increase of hotspot-related volcanics accompanies increasing extension in the East African rift, both reaching maxima in Ethiopia and the Afar triangle). Total extension, and the thickness of pre-Aptian volcanics, decrease together to the north, so that north of the latitude of the Paraná basin most of the rift sequence is composed of sedimentary rocks.

To the north, both the rift and the salt basin tapered off near Recife. Farther to the north, between João Pessoa and Natal, the clockwise rotation of South America relative to Africa took place by left-lateral transpression, without major rifting until Albian times. Movement of the tip of South America away from Nigeria but still pressed against Africa in the east produced E-W compression and N-S extension, creating the ENE-trending right-lateral transtensional graben of the Pendéncia rift in the Potiguar basin (Françolin et al, 1994; Szatmari et al., 1995).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil