Abstract: Rifting and Its Bearing on the Hydrocarbon System of the Margin of Angola, West Africa

Jerónimo, Paulino; Fernando de Sousa Gonçalves and Murthy Inkollu - Sonangol

The rift sequence of Angola is a classic asymmetric rift, which is explained by a common rift model that describes the co-genetic relationship of basins on either side of South Atlantic. The observable characteristics of the margin suggest that the locus of Angola?s north and central zone is on a Lower Plate, whereas, the southern portion is on a Upper Plate. The notable features are a complex basement-rift graben style in the Lower Plate and, simple basement-rift geometry in the Upper Plate. Reversal of the Plates on the margin is thought to occur near the coordinates 13 45? E, 10 45? S and is considered to be an inter-plate transfer fault zone. Further, the margin is demarked by a series of intra-plate transfer faults. Along the axis of the rift, major half-grabens developed in the Lower Congo, Kwanza and Namibe areas. The accommodation zones are separated by basement highs. Lower plate segments support the Lower Congo and Kwanza half-grabens, whereas, the Upper plate contains the Namibe half-graben. A schematic perspective of grabens and accommodation zones is indicated in Figure 1. The half-graben configuration is predictable from the available 2D and 3D seismic data and wells penetrating the Lower Congo and Kwanza rift sequence. The graben system is composed of an onshore inner-graben and offshore outer graben. These are separated by a basement ridge, acting as a flexural edge of the rift (Fig. 2).

Regional profiles of the rift sequence for the Lower Congo, Kwanza and Namibe half-grabens settings illustrate the stratigraphy. The Lower Plate grabens are dominated by early and late syn-rift sedimentation. In contrast, late syn-rift sediments dominate the Upper Plate graben and accommodation zones. The early syn-rift sequence is comprised of fluvio-alluvial to continental lithofacies, followed by deepwater lake argillaceous sediments. The late syn-rift sediments were deposited in deltaic and shallow lacustrine to marginal marine environments. The lithologies include deltaic to marine sandstones, shallow water lacustrine carbonates and claystones. The lithofacies of the inner and outer graben widely differs. The inner graben lithologies are dominated by coarse to fine grained siliciclastic sequences. The outer graben is predominantly carbonates, marls, argillaceous clastics and less arenaceous sediment.

Rift setting and climate are key factors that have influenced the lake sediment deposition. These in turn have contributed to the development of reservoir, source, trap and seal. Current exploration efforts have indicated that the north portion of the Lower Congo is a significant petroleum system, relative to other zones. This region contains deep fresh water lake sediments with thick source beds of high TOC. The southern Lower Congo and Kwanza areas of accommodation contain more saline lake influenced sediments with moderate to poor TOC source beds. Late syn-rift source beds are present in accommodation areas at intra-plate transfer fault zones. Scant drilling efforts in this region did not reveal any significant reservoir potential. It will be of interest to note that reservoirs and mature source beds are present on either side of the basement ridge that follows the coastline.

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