Gabor Tari
Mark Kaminski
Jim Molnar
David Valasek
Vanco Energy Co.
Stranded 'nappe' holds exploration promise
Newly acquired seismic data reveal an unexpected and unusual structural history of the marginal ridge of Cote d'Ivoire and Ghana. During the Senonian, several inversional and transpressional structures formed due to the adjacent Romanche Fracture Zone. During the Cenomanian a large-scale, north-vergent nappe system formed with a corresponding foredeep basin.
As the structural geometries are analogous to those found in classical folded belts and foredeep basins, the same play types can be defined, such as en échelon anticlinal structures, stratigraphic updip pinch-outs within the foredeep basin and sub-thrust traps beneath the nappe system. To our knowledge, 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.
The Marginal Ridge of Côte d'Ivoire and Ghana is a prominent structural and topographic feature on the transform margin of equatorial West Africa. The Marginal Ridge and the Tano (or Deep Ivorian) basin landward from it form a deepwater basin that extends over 6,000 sq km with water depths from 200 to 4,000 m.
Some of the earliest oil exploration on the African continent took place along this margin in the late 1890s and early 1900s, when a number of European companies drilled mostly shallow wells in areas of abundant oil seeps and tar sands close to the shore in Ghana and Nigeria. One such site is in western Ghana west of Cape Three Points. Though the onshore wells never established sustained flows, more recent drilling in the offshore portion of the Tano basin has yielded discoveries. The deep and ultra-deepwater part of the Tano Basin, is considered a frontier basin by explorationists.
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Marginal Ridge
Due to strong academic activity for a number of years, the Marginal Ridge along the Romanche Fracture Zone (RFZ) became the unofficial "locus typicus" for a transform margin setting. The structural evolution of the Marginal Ridge was subdivided into four major periods:
- An early rifting and shearing of the southern border along the Romanche Fracture Zone during the Albo-Aptian
- End of rifting and intracontinental transform faulting during the Late Albian
- Continent-to-ocean transform faulting from the Cenomanian until the Late Cretaceous
- Passive margin evolution since the Late Cretaceous.
New model
Reprocessing of the academic seismic data and the acquisition/ interpretation of some 3,000 km of new reflection seismic data over the Marginal Ridge resulted in some observations, that contradict the evolutionary scheme summarized above. Most importantly, the new seismic data clearly image a large landward-verging overthrust system in the Cape Three Points Deep area.
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Interestingly, the seismic reflectors associated with the individual thrust imbrications within this "nappe" were attributed to prograding sediments by previous interpretations. However, the internal geometry of the allochthonous nappe system is identical to those observed at the leading edge of classical folded belts.
Other evidence for compressional deformation is provided by a series of inverted syn-rift half-grabens and a relatively small, but well developed "foredeep basin," which formed due to the load of the incoming folded belt. Whereas the numerous anticlines are undoubtedly the result of repeated transpressional deformation along the transform margin, it is the dominating compressional component that is responsible for the presence of a number of plays identical to those found in folded belts.
- A reprocessed seismic profile across the Marginal Ridge of Ghana shows the reflection geometry north of the Romanche Escarpment and beneath the dark blue horizon. Packages of south-dipping reflectors were interpreted as prograding clinoforms; however, closer inspection reveals the presence of imbricates in a north-vergent, overthrust system. The prominent sedimentary wedge between the light green and purple horizons is interpreted to be a Cenomanian foredeep sequence, slightly redeformed during the Senonian.
Deepwater plays
The new exploration plays that Vanco has identified include from south to north:
- Sub-thrust (nappe) traps, structural and also stratigraphic terminations against the basal detachment
- En échelon anticlines formed during the Senonian (intra-Santonian?)
- Stratigraphic pinchouts within the Cenomanian foredeep basin.
These play types complement, and are independent of, the traditional plays along this transform margin of West Africa. Traditional plays include structural closures within the syn-rift Albian clastic sequence and the subtle stratigraphic/combination traps for Senonian turbidites.
None of the newly defined play types were tested by the few exploration wells drilled to date on the upper and middle slope of the margin.
The landward verging nappe complex and the associated foredeep basin had to form during a Mid-Cretaceous period in which continental crust existed on both sides of the transform margin, and most likely, at a time of major plate reorganization between Africa and South America. It is not clear at present why this unusual set of structures developed along this particular segment of the margin.
One possible explanation is based on the slightly curved trace of the RFZ running into the African continent deviating from the parallel flow-lines of all the other major fracture zones in the Southern Atlantic. This anomaly was inherited from the structural fabric of the pan-African basement prior to the break-up of the Atlantic Ocean.
Another speculation concerns with the anomalously long offset of the RFZ. Reconstructing the late-stage geometry of the rifting between Africa and South America shows that the Cape Three Points area very "sensitive" to tectonic deformation during the opening of the South Atlantic. At this point, where the continental crusts of the two departing plates were still juxtaposed, even a very small (less than 1°) differential rotation of these continental plates could have caused the observed compressional structures.
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Regardless of these speculations, the traditional view of transform margin evolution, based mostly on this particular Marginal Ridge of Côte d'Ivoire and Ghana, needs to be updated to accommodate these new findings. Continued exploration work focusing on detailed seismic stratigraphy in conjunction with biostratigraphic well data should help to pinpoint when this event occurred and improve the overall understanding of the margin. Also, the details of the structural evolution will be better constrained by a future 3D seismic survey.
The folding of the Cenomanian foredeep sequence occurred during the Senonian. We speculate that a regional-scale compressional event was responsible for the formation of the en échelon anticlines beneath the Marginal Ridge.
Analogs
Examples of producing foredeep basins are mostly located onshore and tend to be very prolific. The Colville basin of the North Slope of Alaska, the Aquitanian basin of southern France, the Molasse basin of Austria, and the foredeep basins around the Carpathians, especially in Romania, provide useful analogs that are surprisingly similar to the foredeep basin and folded belt of the Cape Three Points Deep area. These examples show a certain degree of variation in the relative proportion of hydrocarbons found in the folded belt, within the foredeep basin, or in the platform sequence beneath the foredeep. Exactly which of the above mentioned "classical" foredeep basins will provide the closest analog for the anomalous offshore example of the Cape Three Points foredeep basin remains to be seen.
For more information, contact Gabor Tari at tel: 713-386-2701 or e-mail: [email protected].
