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