Abstract: Lower Cretaceous Stratigraphy and Source Rock Distribution in Pre-salt Basins of the South Atlantic: Comparison of Angola and Southern Brazil

Pasley, Mark A. and Edith Newton Wilson - Amoco; Vitor S. Abreu - Rice University; Mário G. P. Brandão - Sonangol; and Augusto Silva Telles - Petrobras/E&P

Generalized Lower Cretaceous stratigraphic columns for Brazilian and Angolan pre-salt sediments indicate that the sections representing the early formation of the South Atlantic are roughly correlative (Fig. 1). Early Neocomian volcanics and coarse continental clastics are overlain by fluvial and lacustrine sands, shales, and carbonates of the Bucomazi (Angola) or Recôncavo (Brazil) section. This Neocomian section was deposited in half-grabens during the phase of rifting when crustal extension rates were high (Syn-Rift I) and may actually represent more than one phase of crustal extension. The top of Syn-Rift I is marked by an unconformity which signals the end of half-graben development. The Barremian to lower Aptian section consists of lacustrine shales and carbonates which were deposited late in the rifting sequence when thermal subsidence became the dominant regional subsidence mechanism (Syn-Rift II). This interval contains the Coqueiras member of the Lagoa Feia in Brazil which is correlative to the Toca/Upper Bucomazi in Angola. Syn-Rift II, or the "pre-salt wedge" in Angola, exhibits a distinctive basinward thickening and shelf-ward pinchout on reflection seismic profiles. The top of the Barremian section is marked by an increase in carbonate content and the first appearance of marine fauna in the uppermost Jiquiá in Brazil. The Syn-Rift II section is capped by a thick sandstone package of lowermost Aptian age (Chela / Cuvo sandstones along the African margin) and finally by the Aptian salt deposits (Retiro Member in Brazil and Loeme in Angola).

Petroleum source rock distribution within the pre-salt section on both sides of the South Atlantic is also similar. The Syn-Rift I section typically contains thick, organic-rich, lacustrine shales which have abundant Type I kerogen and are prolific petroleum sources. These source rocks can be limited in areal extent because of their deposition within the half graben setting. In contrast, the Syn-Rift II section contains source rocks that are more widespread because they were deposited during the thermal sag phase of rifting. Shales rich in Type I kerogen are common at the base of the Syn-Rift II section and biomarker analysis of Angolan samples indicates that marine organic matter may be present in the upper, carbonate-rich portion. Source rocks deposited near the top of Syn-Rift II contain kerogen which are more accurately characterized as Type I/II. Information on facies relationships and maturation histories of source intervals within the Syn-Rift II section is important to deepwater exploration on both sides of the South Atlantic margin.

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