Soudan2

Sierra Leone / Liberia Offshore

On this tentative interpretation of a manual autotrace of a Sierra Leone offshore not only the divergent margin (Atlantic-type) is evident, but the pre-breakup Lower Cretaceous rift-type basins. The Atlantic-Type divergent margin, which is quit affect by faults associated with the Sierra Leone fracture zone, starts with the emplacement of lava-flows over the lengthened Gondwana lithosphere (basement and rift-type basin). The lava-flows come from the sub-aerial expansion centers (generally, volcanoes in rift valley), thin continentward and dip seaward due to the weight of their stacking. The rift-type basin sediments are deposited in half-grabens looking continentward (bordering fault dipping seaward) with an internal configuration parallel or dipping toward the bordering fault (continentward). Consequently, geoscientists avoid to interpret as rift-type basins not only seismic intervals dipping seaward, but depocenters postdating the breakup of the Gondwana lithosphere (see Page 54).

On this tentative geological interpretation, because the reasons given in the previous plate, contrariwise to John Flinch (2009), the seaward dipping reflectors onlapping on the western flank of the basement buried hill, are interpreted as lava flows (SDRs) and not as Lower Cretaceous rift-type sediments. The dip of the rift-type sediments is, strongly, affected by not only by the seaward tilting of the margin, but by seismic artifact induced by the changing in water depth as well. In fact, at the deposition time, the rift-type sediments dip landward with a well marked continentward divergent internal configuration in the lower part. At the petroleum exploration point of view it is interesting to notice: (i) The thickness of the margin seems, relatively, small to assure the maturation of the organic matter of potential organic rich sedimentary intervals, taking into account that the water depth does not play any role in the maturation of the organic mater ; (ii) The rift-type basins are quite small and have a divergent internal configuration, which is not too propitious to the development of organic rich sediments, since the depositional water depth is practically zero ; (iii) Several deep reservoir-entrapment petroleum subsystems are possible in the divergent margin (see next plate).

On this tentative interpretation of a seismic line of the Sierra Leone offshore, in which just the post-Gondwana rocks were taken into account (the margin sub-aerial volcanic rocks, i.e, the SDRs, although belonging to the divergent margin were included in the Gondwana rocks) potential deep water reservoir-entrapment petroleum sub-systems are, easily, recognized, particularly, in the regressive phase of the post Pangea continental encroachment stratigraphic cycle, in association with submarine basin and slope fans. Potential generating petroleum sub-systems are possible in the transgressive phase in association with maximum flooding surfaces. Such a reservoirs-entrapment sub-systems ( turbidite fans) are considered as the main petroleum exploration targets, as illustrated in next plates.

The submarine turbidite fans (A, B and C Fan) recognized in this Canvas autotrace of an old seismic line illustrate the principal petroleum exploration targets of this offshore, assuming that a generating petroleum sub-systems exist in association with the marine ingressions of the transgressive phase of the post Pangea continental encroachment stratigraphic cycle, although the maturation of the organic matter of the transgressive sediments (Cenomanian-Turonian and Late Albian) can be a problem. The presence of abundant Lower Cretaceous rift-type sediments is problematic, since the lowermost seismic intervals can be interpreted as lava flows (SDRs) as discussed in Page 54.