Benguela and Namibe Basins: Syn-Rift Architecture and Sediment Thickness From Integrated Analysis of Aeromagnetic Data

Abstract

Detailed analysis of the syn-rift structure of the Benguela and Namibe Basins from seismic data alone is often hampered by the attenuation of seismic signals by thick Aptian salt in the Benguela Basin and by the interference effect of widespread volcanic rocks in the Namibe Basin. Therefore, the integration and interpretation of potential field data is vital for producing a viable structural model and, hence, reducing exploration risk. Sonangol acquired extensive high-resolution aeromagnetic surveys over the offshore Benguela and Namibe Basins which have been reprocessed to yield a detailed 500-m grid of TMI. These data, when combined with satellite gravity and seismic data, have enabled a new interpretation of the basement structure by analysing lateral changes in magnetic susceptibility and character. Depth to crystalline basement was validated through 2D modelling of gravity and magnetic anomalies constrained by regional 2D seismic data, which allowed syn-rift sediment thickness to be determined. The geophysical modelling results document an essential difference in structural style between the Benguela and Namibe Basins: a consequence of the change in the syn-rift extension mode. The Benguela Basin is characterised by moderate, widely distributed stretching, whereas the Namibe Basin displays higher extension that is localised in a relatively narrow zone. Higher stretching is confirmed by the increased syn-rift sediment thickness in the Namibe Basin and a reduced thickness of transitional crust. Deeper basement and resultant sedimentary underfilling of the basin during the rift stage means that the Namibe Basin was not at suitably shallow depths for salt deposition. A key feature of syn-rift architecture is the presence of failed N–S-oriented rifts in the Namibe Basin that represent a southward continuation of the N–S-oriented section of the COB in the Benguela Basin. Since early rifting was only successful along the Benguela section of the continental margin, the N–S-oriented rifts were aborted farther south and succeeded by the NNE–SSW-oriented rift that is parallel to the position of the COB. The structural framework observed explains why the pre-salt succession is widely distributed but relatively thin in the Benguela Basin, whereas it is thicker but localised in elongated depocentres in the Namibe Basin. A new model of syn-rift basin structure highlights areas for more focused exploration and provides a basis for further work such as basin modelling.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016