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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Traditional and New Play
Types
of the Offshore
Tano Basin of Côte d’Ivoire and Ghana, West Africa
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By
Vanco Energy Company
The onshore Tano Basin (named after the border river between Côte d’Ivoire and Ghana) includes a small area between the coastline and the outcrops of the metamorphic Pan-African basement. The Marginal Ridge and the Deep Ivorian basin can be considered as the broader offshore Tano Basin, forming a large deepwater basin with present-day water depths from 200 m to 4,000 m (Fig. 1). To illustrate the basin-scale structure and stratigraphy of the Tano Basin downdip from Cape Three Points, a regional seismic transect is shown as a line drawing in Fig. 2.
The Marginal Ridge is a prominent structural and bathymetric feature separating the deep-water Tano Basin from the East Atlantic abyssal plain. As to its origin, traditionally, ridge development was subdivided into four major periods of structural evolution. These periods are a) early rifting and shearing of the southern border along the Romanche Fracture Zone during the Albo-Aptian, b) end of rifting and intracontinental transform faulting during the Late Albian, c) continent to ocean transform faulting from the Cenomanian until the Late Cretaceous(?) and d) passive margin evolution since the Late Cretaceous.
A different look on the existing data along strike, however, suggests a more specific structural scenario that has important implications for the exploration potential of the basin.Whereas the internal structure of the Marginal Ridge is very poorly imaged on the regional
Figure 1. Index Map of the Tano Basin of Eastern Côte d'Ivoire and Western Ghana.
End_Page 27---------------
seismic transect, a reprocessed subregional seismic section some 50 km to the east reveals the nature of this significant feature (Fig. 3). Both the reprocessed and new 2D seismic data clearly image a large landward-verging overthrust system in the Cape Three Points Deep area.
On closer inspection (see inset), the seismic reflectors associated with the individual thrust imbrications within this “nappe” were attributed to southwardprograding sediments by previous interpretations. However, the internal geometry of the allochthonous nappe system is identical to that observed at the leading edge of classic fold belts.
Other evidence for compressional
deformation is provided by the series of inverted syn-rift halfgrabens
and a well
-developed “foredeep basin” that formed due
to the load of the incoming fold belt. The map-view isopach
of this sedimentary sequence shows a triangular basin with a
maximum thickness of more than 4,000 m just in front of the
north-verging nappe system. Note that the regional transect
shown in Fig. 2 runs at the perimeter of the foredeep basin, and
therefore it fails to document the foredeep basin as the key to
understanding the Marginal Ridge.
The exploration history of the onshore Tano Basin began with
initial drilling in the late 1890s immediately adjacent to the
extensive oil seepages in both Ghana and eastern Côte d’Ivoire.
By the l970s exploration efforts had moved to the offshore shelf,
resulting in a number of oil and gas discoveries in Lower
Cretaceous reservoirs charged from lacustrine and deltaic syn-rift
as well
as earliest marine source rocks. More recent drilling has
occurred at and beyond the shelf break and has resulted in oil
discoveries in Senonian reservoirs charged from Turonian-
Cenomanian marine source rocks, a situation analogous to many
discoveries made in Côte d’Ivoire.
Therefore, the traditional play types
of the Tano Basin include
syn-rift Albian fault blocks with en échelon map-view geometry
and Senonian fan complexes beneath the present-day slope
(Fig. 4). The Albian syn-rift fault blocks can be traced from the
shelf area into the deep-water along subregional hinge zones. The
Senonian fan complexes cover a fairly large area in the center of
the Tano Basin.
Recently acquired 2D and 3D seismic data reveal unexpected
new play types
associated with the unusual structural evolution
of the Marginal Ridge of Côte d’Ivoire and Ghana (Fig. 4).
Because the structural geometries are analogous to those found
in classic fold belts and foredeep basins, the same play
types
can
be defined, such as transpressional en échelon anticlines, stratigraphic
updip pinch-outs within the foredeep basin and
sub-thrust traps beneath the north-verging nappe system. The
Cape Three Points Deep area provides the very first case for this
set of play
types
in the hydrocarbon exploration history of
offshore Africa.
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Figure 4. Summary of Traditional and New Play
Types
in the Cape Three Points Segment of the Tano Basin, Ghana.
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