The Tano Basin developed in an area of relative tectonic quiescence between the Saint Paul’s and Romanche Fracture Zones, resulting in pull-apart grabens with a thick clastic fill. The St Paul’s transform terminates into a curved coastal fault in the eastern Ivorian offshore and evolves into a horsetail splay structure at the connection with the Romanche Fracture Zone, offshore Ghana. Generally, this has created a broad shelf to deep water profile, underpinned by thick continental crust. In comparison, the Côte d’Ivoire Basin has a narrow shelf to deep water profile, characterized by transform faulting. The continental to oceanic crustal transition is more abrupt here as observed on the Bouguer-corrected gravity data shown below.

Two main plays have been the focus of exploration in the region; the syn-transform Lower Cretaceous and the post-transform Upper Cretaceous. The Upper Cretaceous system is most successful around and outboard of the shelf edge due to the thick clastic section allowing for the sufficient burial of shallower Cenomanian-Turonian aged source rocks. These type II source rocks have mixed oil and gas potential. Complementing this widespread source kitchen are Albian to Santonian aged shallow to deep marine sandstone reservoirs deposited as turbidite channel and fan systems. 

The distribution of Upper Cretaceous sediments can be illustrated using an RMS amplitude extraction highlighting the implications for prospectivity from west to east across the margin, see image below.

In the Tano Basin we observe the influence that structural development has on reservoir distribution. Sands are segregated within terraces as frontal splay complexes before bypassing the subsequent down-slope ramp and ponding in the next terrace (see the final image). This low gradient, paleo-bathymetry allows for the development of combination stratigraphic/structural traps at multiple levels on the shelf slope. 

To the west, the steep single-faulted margin of the Saint Paul’s Fracture Zone results in clastic sediment largely bypassing the slope and being deposited in the deep-water basin floor. This deep-water area in both the Côte d’Ivoire and Tano basins is relatively underexplored although basin floor fans are clearly observed to be widely present and discoveries such as Calao have proven the play fairway in this area.

However, recent 2022 exploration success offshore Namibia with TotalEnergies’ Venus-1 discovery has suggested that Upper Cretaceous turbidite reservoirs at the base of slope can prove successful by utilizing outer highs and counter-regional dip to provide trapping mechanisms. Similar concepts can be applied to the outboard area of the Côte d’Ivoire and Tano Basins where deep-water sediments are observed to drape over oceanic crustal highs in four-way dip closures.

 Drawing further on the Namibia play analogs, Cretaceous aged source rocks deposited over transitional to oceanic crust in the deep-water Orange Basin are oil mature in outboard areas. The concept for hydrocarbon generation and expulsion from source rocks subjected to increased heat flux from the mantle may also be applied to the deep water offshore of Côte d’Ivoire and Ghana, expanding prospectivity beyond the area underpinned by continental crust. 

This regional-scale understanding of petroleum systems highlights the exciting opportunities remaining over oceanic crust in the Côte d’Ivoire and Tano Basins, allowing exploration potential to be extended from analogs and existing in-basin discoveries into underexplored areas.