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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. They 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 deep waters. Drilling in Senegal to the north located petroleum systems associated with Cretaceous basin floor fans and shelf edge to slope clastic deposits. Similar plays have been tested along the West Africa margin. Salt deformation in the deep basin also provides potential traps.
The Bove Basin - lies beneath the Senegal basin along the eastern onshore part of Guinea-Bissau. It is a Palaeozoic basin deposited on the northwestern margin of Gondwana.
The 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. The nearest wells where these have been drilled are in Senegal.
Guinea-Bissau has no history of production but a number of oil discoveries have been made offshore, the largest in an area straddling the boundary with Senegal. AGC was created in 1993 between the governments of the 2 countries to jointly administer this maritime zone between the two countries.
The exploration of the AGC area began in 1958 when it was separately awarded to Total in Senegal and Exxon in Guinea-Bissau. Initial exploration focused on salt diapir-related structural traps and resulted in the discovery of the Dome Flore heavy oil field in 1967 and the Dome Gea heavy oil field in 1971.
ln 1991, Casamance Petroleum took the area and acquired 3D seismic. The licence expired in 1994 and the area was subsequently relicensed to Pecten who, in 1996, drilled a shallow exploration well on the eastern flank of Dome Gea (Baobab-1). This well encountered many levels with oil shows.Two permits for hydrocarbon exploration in the deep water areas (AGC Central and AGC Profon) were awarded in 2001. After seismic surveys one dry well was drilled in AGC Profond in 2011.
In Guinea-Bissau outside the AGC the shallow water Sinapa oil field in Block 2 was discovered in 2004 when Sinapa-2 drilled by Premier Oil intersected an oil column associated with a salt structure. Two appraisal wells were successful but the small size of the accumulation has so far prevented development.
Although there is an active petroleum system in Guinea-Bissau with numerous discoveries, the fact that no commercial developments have been possible leads Globalshift to forecast no oil or gas production for at least the short and medium term. A few wells were drilled in the early 1960s in the onshore part of the basin but all were dry and no oil or gas production is forecast onshore.
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