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Offshore and the western part of onshore Guinea-Bissau are underlain by the southern part of the Senegal basin known as the Casamance-Bissau sub-basin (in the north the Dakar-Banjul sub-basin).
The Senegal basin is an Atlantic-type passive margin basin of Middle Jurassic to Recent age which began to form within a Permian to Triassic rift system that developed over an extensive Paleozoic basin during the breakup of North America, Africa and South America. It continued as a passive margin basin after opening of the ocean.
The northern limit of this basin is the Precambrian Reguibate Shield in Morocco, the southern limit is the Bove Basin and the eastern edge is Precambrian rocks of the Mauritanide Mountains, uplifted during the Late Paleozoic Hercynian Orogeny.
Onshore the basin is unproductive and Globalshift believes it probably lacks source rocks and traps. However offshore the Dome Flore and Dome Gea heavy oil discoveries, shared with Senegal, lie on the shelf edge. They have Late Cretaceous sandstones and Oligocene carbonate reservoirs mostly containing heavy oil although these have never been made commercially viable. The Sinapa discovery further offshore found oil in a Cretaceous sandstone associated with a diapir of Triassic salt.
There may also be potential oil and gas accumulations in deeper waters. Recent drilling in Senegal to the north has located petroleum systems associated with Cretaceous basin floor fans and shelf edge to slope clastic deposits. Similar plays have been tested elsewhere along the West Africa margin. Salt deformation in the deep basin also provides potential traps.
Beneath the Senegal basin and the eastern onshore part of Guinea-Bissau is The Bove Palaeozoic basin deposited on the northwestern margin of Gondwana. The Bove basin contains Cambro-Ordovician sediments dominated by continental deposition followed by a marine transgression from the Early Silurian to the Lower Devonian. Some oil seeps have been reported in Guinea and there may be a source rock interval of Silurian shales corresponding to other areas in North Africa but the nearest wells where these have been drilled are in Senegal.