Operation – Exploration – Geological Overview of the Sierra Leone Basin

There is one Off-Shore basin in Sierra Leone which is referred to as the Sierra Leone basin. The Sierra Leone basin forms part of The West African Transform Margin and is made up of complex strike-slip faulting. Basin consist of a continental shelf and a steep slope (Sulima slope plateau) with rotated fault blocks seen on the slope and gradually stepping down into the deep water basin. The Sulima slope seen on the northern part of the area is an expression of a series of horsts and grabens formed by wrench tectonics as the Sierra Leone transform system intersect the African continental mass. The shelf break is characterized by a steep slope with several submarine canyons which cut deeply into the continental shelf down to the abyssal plain. Series of transform fault systems exist along the margin but the main transform fault system that affected the basin is the right-lateral Sierra Leone transform fault system which forms a series of an echelon strike-slip and dip-slip faults with associated folds. In the deep-water part further from the coast, normal faulting during the Atlantic rifting event produced grabens and half-grabens of probably Albian-Aptian age.

The structural architecture of the basin is described in three phases namely the pre-rift, rift and post-rift (passive margin phase). The pre-rift phase mainly encompassed the faulting of the pre-rift strata throughout the Palaeozoic to Jurassic times with associated volcanic activities.

The rift phase started in late Jurassic/early Cretaceous time, coincident with tectonic subsidence of the stretched continental crust, caused by upwelling of the asthenosphere. Extensional faulting is associated with extrusive igneous activity and continental siliclastic deposition took place in the graben. This Phase was active from Aptian to Turonian with fluvial/lacustrine conditions trending to shallow marine as Africa was finally separated from South America.

Since the timing of the onset of seafloor spreading in the basin is not well-known, a hypothesis has been postulated that this event is initiated during the late Albian, hence the beginning of the passive margin phase. Immediately after the onset of seafloor spreading, there was a marine transgression which reached the inner shelf and slope. Seafloor spreading accelerated probably during the early Cenomanian time causing ocean-ward deepening rotation of rift fault blocks and uninterrupted marine deposition on the new crust in the proto-Atlantic basin.

During the mid-Cretaceous, significant thicknesses of alluvial, fluvial and lacustrine sediments were deposited and shallow marine incursions flooded the more subsided parts of the rifted terrain in the mid Albian time. Also, thick sediments occur beneath the continental slope where they are disrupted by faults and gravity slides which are particularly common near the Guinea and Sierra Leone fracture zones. Slumps and canyons are also observed on seismic data occurring mostly on the continental slope.

Sub-aqueous and sub-aerial erosion took place on the shelf and slope in the mid- Cretaceous time and is evident on seismic data by the occurrence of a regional unconformity which persisted as a surface of erosion until the Santonian time. The progression of seafloor spreading introduced a late Cretaceous transgression which advanced shoreward over the mid-Cretaceous erosional surface. Marine deposition was continuous ocean-ward from late Albian to the end of Cretaceous time. Onlap of marine silicilastics and some carbonate deposits are seen on the mid-Cretaceous unconformity. The hydrocarbon potential of the basin was known when the Venus B-1 discovery well was drilled. Series of other discovery wells confirmed that there is an established petroleum system within the basin.