Gambia's hunt for black gold

Mar 01, 2004 01:00 AM

Commercial production has been achieved from Maastrichtian sands on the Dakar peninsula at shallow depths within immature sediments above the oil window. The Dome Flores heavy oil accumulation is associated with carbonate reservoirs above the oil window Oligocene, Eocene, and Palaeocene strata.
Light oil was encountered in Palaeocene and Maastrichtian rocks flanking the salt diaper. These reservoir positions suggest that hydrocarbon migration pathways formed along faults and fractures extending to source rocks at depth. This is an important consideration in a basin of generally low geothermal gradients, where peak oil generation is expected at depths greater than 2,800 metres.

The PGO-3 well drilled in Guinea-Bissau tested light oil from the top of a thick Albian sand sequence. This test is significant because the well was drilled down dip from a large "turtleback structure" resulting from salt withdrawal.
The Aaiun-Tarfaya basin in Morocco developed contemporaneously with the Senegal basin. There, the MO-2 well tested 2,377 bpd from fractured Malm limestones along the Jurassic to Lower Cretaceous carbonate shelf edge. Although not commercial, this accumulation suggests the possibility of economic discoveries in other shelf edge prospects in the Senegal basin.

Nearly 4,800 km of seismic lines across the Southern Senegal offshore basin were interpreted and integrated with subsurface well data and additional geological surveys in order to determine the hydrocarbon potential of the region. The data cover offshore, Gambia as well as the Sangomar and Casamance areas of offshore Senegal. A series of 1:50,000 and 1:100,000 scale time-structure, depth-structure and isochron maps were prepared. Geological models were generated to evaluate the structural evolution of the basin and identify potential hydrocarbon traps.
Structural modifications of this portion of the Senegal continental passive margin basin can be largely attributed to several styles of salt tectonics including dissolution and halokinesis. Syndepositional troughs were important structural features extending across the southern Sangomar and Gambia inner continental shelf. They are interpreted as the consequence of Albian and Maastrichtian dissolution of Upper Triassic and Lower Liassic salt beds.

These overlying Cretaceous clastic filled troughs offset each other as their depositional axe migrated higher in the section with continued deposition above the lateral shifts of the underlying salt dissolution front. Vertical fracture zones throughout the Jurassic and Cretaceous section developed along margins of the syndepositional troughs. These zones may provide important hydrocarbon migration pathways from Cretaceous and Jurassic source beds into the immature Middle ad Upper Cretaceous reservoir rock shallower than 2,800 metres.
Additional important structural features of the area resulted from Late Maastrichtian to Early Tertiary movements along a wide transform fault zone. These movements result in the definition of the upper reaches of salt dissolution fracture zones as shallow seaward dipping normal faults. These extensional tectonics are observed across the eastern Gambia continental shelf and northward into southern Sangomar offshore.

The faults are only associated with areas of relatively intense salt dissolution. In contrast, compressional structures dominate southward in the Casamance offshore region. The intervening area, along the Casamance-Gambia international boundary, was the scene of major Late Maastrichtian basement block down wrap.
Stable carbonate platforms are important features of this study area. A deeply buried Neocomian shelf extends northward across the onshore from Casamance across Gambia and into the Sangomar area of Senegal. Drape effects can be recognized in concert with the salt dissolution structures in the overlying Middle and Upper Cretaceous clastics. The Cenomanian carbonate platform is markedly thickened parallel to the seaward margin of the salt dissolution zone, and may represent a quiescent between Albian and Maastrichtian pulses of salt dissolution and subsidence of overlying clastic depocentres.

Salt withdrawal structures are situated to the southwest of the salt dissolution areas especially along outer continental shelf margin. Salt diapers and adjacent rim synclines are important across the northern Casamance offshore. Large dome structures, possibly overlying titled fault blocks or extensive salt withdrawal structures, are recognized seaward in Gambia offshore.
Of more than 135 structural anomalies identified within the study area, the two largest areas considered to be particularly prospective for hydrocarbon exploration. The Casamance No. 1 prospect is a compressional structure with closure recognized on all Jurassic and Cretaceous horizons. The vertical relief on each mapped horizon varies from 40 to 50 metres.

Four zones of Maastrichtian of Aptian strata are recognized for potential reservoir development at depths of 4,000 metres. A single exploration well may effectively test all of these interpreted reservoirs. The prospect is also attractive because of the shallow water depth (30 metres) at the proposed location.
Gambia No. 9 prospect is a giant structural down that may overlie a deeply buried salt pillow or tilted fault block. A flanking rim syncline with thick Maastrichtian sediments suggest partial tectonic adjustments late in the basin history, probably in association with Late Maastrichtian seaward transform fault movements. The prospect is situated at the outer margin of the continental shelf, with most of the prospect in less than 200 metres of water. The Cenomanian and Albian target horizons have approximately 130 sq km of structure closure at less than 26,000 metres depth.

Three unrisked reservoir development scenarios are presented for each prospect. Casamance No. 1 targets 50 to 100 sq km of Maastrichtian, Cenomanian, Albian and Aptian structural closure with 44 to 55 metres of net pay. Unrisked recoverable reserves are estimated as 400 mm barrels for the intermediate case to a maximum of 1.3 bn barrels for the optimistic case.
Gambia No. 9 prospect targets 120 sq km of structural closure in Cenomanian and Albian strata having a possible 40 to 80 metres of net pay. Recoverable reserves have been estimated as 1.2 to 2.4 bn barrels.

Source: The Independent
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