Implications of Early Rift Morphology on Development of Pre-Salt Sequences in the Kwanza Basin, Offshore Angola
Abu, Clara; Herbst, Nora; Francis, Malcolm; Milne, Graham; Brown, Gill; Inkollu, Murthy
The Kwanza basin is located offshore Angola, West Africa and has great potential as a frontier exploration province. It is part of the Aptian salt basin created by the opening of the South Atlantic. The geology of the pre-salt play is related to tectonic movements that led to the break-up of the Gondwana supercontinent in the Early Jurassic, about 180 million years ago. Large discoveries on the Brazilian side of the Atlantic and recent discoveries in Angola highlight the hydrocarbon potential along the conjugate margins. There are several articles on the prospectivity of the conjugate margins, but this paper will focus on the development of the pre-salt sequence and its prospectivity.
The Lula (formerly Tupi) discovery in Brazil in October, 2006, proved a gigantic field beneath salt. Recent exploration for similar fields in Angola’s Kwanza basin has been successful. There is increased exploration effort in search of similar structures in the pre-salt in unexplored blocks in this province.
The study area covers both the extensional and compressional domains in the Kwanza basin. Interpretation of regional 2D and 3D depth seismic sections in the Kwanza basin highlights three distinct stratigraphic zones:
- Post-salt clastic minibasins with shallow channels.
- Massive evaporate sequence.
- Pre-salt layered carbonate and or clastic section.
Pre-salt structures in the Kwanza basin consist of asymmetric half grabens created during the initial rifting phase. The pre-salt structure is dominated by northeast-to-southwest trending highs that terminate against the northeast-to-southwest trending transfer fault zones.This provided the basic geometry that created the initial synrift basins that were narrow deep lakes flanked by high relief margins.
An analysis of the pre-salt sequences allows us to identify different stages of rift development and facies deposition. We recognize two major units, a Neocomian synrift phase and a sag phase. We observe a series of tilted fault blocks within the synrift and observe thick sequences of continental, lacustrine to fluvial sediments within these synrift basins. We also observe thickening of this package into the basin. We observe a thick sag basin that overlies the synrift structure. High energy reflectors in the sag basin could point to the development of good carbonate rocks. Analogs from Brazil’s Santos basin suggest the possibility of microbialite carbonate reservoirs. The overlying massive evaporate section varies in thickness and shows high variability in internal image quality.
New seismic depth imaging technology has significantly improved the visualization and interpretation of the pre-salt section in the Kwanza Basin. The tilted fault blocks are well defined in the pre-salt and the overlying sedimentary wedge is clearly imaged. This helps in the location of structural features that may contain exploitable hydrocarbon resources.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013