For this study, the South Atlantic
margin was divided into approximately forty segments with
the salient features of each segment presented in common format
data sheets. Segment boundaries have been defined largely
on the basis of free-air gravity patterns, although in some
cases the shapes of coastlines and the limits of drainage
basins have also been important in defining them. Geologically,
the segments are controlled by the interplay of original rift
faulting and subsequent volcanism and sedimentation. Their
boundaries are in some cases parallel to fracture zones and
in others at right angles to coastlines, but in many cases
neither of these simple relationships applies. Post-breakup
volcanism has been particularly important in the segmentation
of some regions. Conjugate segments on the far side of the
Atlantic are identified on the data sheets but except in rare
cases, segment boundaries are different on opposing sides
of the ocean and conjugate segments are not necessarily of
similar type.
Brazil-Angola Basins and Gravity
Segments
The data sheets
include small-scale bathymetric maps that show the locations
of the seaward edge of the shelf-edge Free-air high (which
is often but not always very close to the shelf-break) and
of the continent-ocean boundary (COB), as indicated by the
region of steepest Bouguer gravity gradient. Segments are
further classified by type, the most important categories
being fan delta (where there is an obvious young deltaic wedge
associated with a major drainage), parallel (where sediments
are transported from a number of drainage basins and build
a shelf roughly parallel to the coastline), fracture-controlled
(where oceanic fracture zones converging on the coasts at
small angles have controlled sediment distribution in deep
water) and intruded (where the margin has been significantly
modified by post break-up igneous activity. Shelf-breaks and
COBs are further categorized in terms of gradients. Regions
where a COB gives rise to only moderate gradients of Bouguer
gravity, and/or where the sea floor seawards of a COB remains
at depths of less than 2500 m over significant areas, are
identified as having significant potential for deep water
discoveries. Bathymetric gradients in the upper and lower
continental slopes provide guides to the potential of the
slopes to retain sediment. Shallow slopes not only have significant
potential for retaining sediment close to the shelf-break
but also are often indicators of thick sediment accumulations
further offshore.
Absolute magnitudes
of the peaks of the shelf-break Free-air highs are also recorded.
Where these are strong, the shelf break location is likely
to have been stable over a considerable period, so that the
gravitational effects of recent sediments reinforce those
from previous phases of sedimentation. Where they are weak,
an earlier shelf edge may have been partly eroded prior to
the most recent sedimentation phase or younger sediments may
have prograded across an older shelf. The parameters listed
in the data sheets are necessarily approximate or average.
They do, nonetheless, provide basic criteria for quantitative
as well as qualitative comparisons between different marginal
segments.
The Abrohlos segment (approx. 18°S to 20°S) has
been heavily modified by post-break up volcanism, and its
margins
are volcanically defined, being set in the south at the Trinidade
volcanic chain and in the north by the lesser volcanics of
the Charlotte segment. As a result of the igneous activity,
the shelf-break in the Abrolhos segment is situated some
200 km offshore, where water depths increase rapidly to 3000m.
This shelf break is marked by a steep free-air gravity gradient,
the positive effect of the rising sea floor being enhanced
by the generally high density of the igneous rocks making
up the plateau. The position of the Continent-Ocean Boundary
in this region is uncertain. The steep bathymetric slope
coincides with a margin-parallel Bouguer high, outboard of
which there is a parallel belt of lower Bouguer gravity.
The gradient from this low to the high values characteristic
of the thin crust of the ocean basin could be considered
to mark the present-day COB, implying that the igneous activity
has effectively extended the continent seawards. Rifting
in this segment was evidently a far from simple process and
the outer Bouguer low may well mark the location of a minor
rifted sub-basin, perhaps analogous to the Danakil region
at the Red Sea margins. A main and original COB, prior to
underplating, is interpreted as underlying the region of
complex Bouguer gradients but generally low field values
within 100 km of the coast. The local distortions in this
pattern can be interpreted in terms of basement structure
and intra-basement density contrasts.
Deepwater and Established prospectivity
There
are two types of new, deepwater play:
1) Base-of-slope sand sheets and their up-dip feeder channels.
2) The oceanward continuation of plays known from shelf and
onshore settings.
The sand plays fall into two classes, either a) primarily
stratigraphic traps as in the Marlim Field in the Campos
Basin is the type example or b) structured traps. These are
associated with either salt tectonics or clay diapirism.
Three established
reservoir plays have been recognized. The first is Neocomian
rift sandstones charged and sealed by the overlying Jiquia
source rocks in fault-related traps. Secondly, Albian-Cenomanian
sandstones and carbonates sourced by Alagoas shales, sealed
by U. Cretaceous shales and trapped by either fault- or salt-related
structuring form an important, but limited play due to eastward
over-maturation. Lastly, U. Cretaceous to Paleocene carbonates
are sourced by Jiquia shales that either lie under an adjacent
unconformity or next to salt evacuation windows. Principal
risks are related to proximity to source, over-maturation,
and biodegradation of shallow reservoirs.
Technical Details
Conjugate (African) segment(s): (part
of) Lubango Type: (Shelf/Fan/Intruded/Transform controlled):
Intruded Defined by (to south): Edge of volcanic plateau Defined by (to
north): Edge of volcanic plateau Coastline length: 115 km Coast - shelf break distance (average): 200 km Shelf break type (sharp/gradual/intermediate): sharp, Comment:
Very steep slope to 3000m Shelf break – COB distance
(average): 25 km COB type (sharp/gradual/intermediate/complex): Complex,
Comment: The COB is disrupted by post-break-up volcanic
events but
appears to be very steep. Bouguer gradient at COB: 14 mGal/km COB to 4.5 km isobath (average): 150 km Bathymetric gradient,
upper continental slope (1000-2000m): 18o Bathymetric gradient,
lower continental slope (3000-4000m): 0.4o Shelf break free-air
anomaly: maximum: 35 mGal, Minimum: -85 mGal Shelf break anomaly: Positive free-air anomalies are also
associated with subsidiary detached volcanic plateaux Area of
drainage basins supplying sediment: 138,229 Km2 Current hydrocarbon
reserves: 169 MBO/462 BCF (42 fields) Prospectivity: Deepwater
prospectivity is compromised by volcanic succession. Key references: Deepwater targets of the South Atlantic, 1999. Unpublished
NPA Report
