Kwanza Basin: Sub-salt Basin Structure and Sediment Thickness from Integrated Analysis of High Resolution Aeromagnetic Data

Detailed analysis of the pre-salt structure of the Kwanza Basin from seismic data is often hampered by the attenuation of seismic signals by the widespread Aptian salt units. Therefore, the integration and interpretation of other supporting data is key to producing a viable sub-salt structural model for the basin, and hence reducing the exploration uncertainties.

Sonangol acquired a high resolution aeromagnetic survey over the offshore part of the basin to add to existing data for the onshore and near shore parts to yield a detailed 250m grid of total magnetic intensity. These data, when combined with regional gravity data and existing seismic and well data, have enabled a new interpretation of the basement structures by analysing lateral changes in sub-surface magnetic susceptibility and data character. In addition, several innovative depth to magnetic source techniques have been applied to assess depth to crystalline basement and hence sub-salt sediment thickness. These have been further validated by 2D modelling of gravity and magnetic anomalies constrained by 2D seismic and well data.

The resulting structural depth to basement interpretation highlights two major rifting phases in the Kwanza Basin that produced: (1) N-S trending rifts succeeded by (2) NW-SE trending rifts. N-S trending rifts were formed in response to E-W extension when the site of future break-up had not yet been defined. They are widely distributed across the Angolan passive margin and, as confirmed by depth to basement analysis, contain the thickest occurrences of pre-salt sediments with in excess of 5 km observed in places. The NW-SE trending rift closely follows the COB and the boundary with transitional crust. It was formed due to the anti-clockwise rotation of the stress field and represents the phase of stretching immediately preceding the final break-up. Local pull-apart depocentres and displacements of the COB are the result of post break-up right-lateral strike-slip faulting along an ENE-WSW trend. We hypothesise that right-lateral faulting may have been contemporaneous with the emplacement of the Sumbe Volcanics in the Late Cretaceous.

The sub-salt interpretation highlights areas for more focused exploration and provides constraint for additional work such as source rock modelling. In addition, comparison of the interpreted structures with seismic data also highlights relationships between basement and post-salt structures and hence potential migration pathways.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California