Senegal Basin has Paleozoic prospects similar to Sahara to the east

From Mauritania to Guinea Bissau

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Diam Niado field is in the central MSGBC Basin offshore Senegal.
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The Senegal Basin is the central part of the MSGBC basin which stretches from Mauritania to Guinea Bissau along the coast of West Africa. The recently discovered onshore gas fields, the Dome Flore heavy oil accumulations, the new potential in deepwater offshore areas, and the huge undrilled prospects in the Paleozoic basin, comparable to those of the Sahara, should provide incentives to the future search for hydrocarbons in this proven, but under-explored, petroleum province.

Exploration for hydrocarbons in Senegal started in the 1950s and was highlighted by the discovery of several small onshore oil and gas accumulations and a major heavy oil deposit offshore. Since the 1950's, exploration by the international oil industry, including major companies, has shown the existence in the basin of mature source rock and reservoirs with potential structural and stratigraphic traps.

An analysis of well locations demonstrates that the majority of exploratory boreholes, particularly those based on seismic shot before 1962, were poorly located and many of them were shallow with depths less than 1,200 meters, limited to exploration of the Tertiary to Senonian series.

The largest discovered reserve in the basin is the 1 billion bbl heavy-oil (10° API) accumulation at Dome Flore, where the shallow Oligocene reservoir is trapped over a deep Triassic salt structure which also contains light oil shows. The Oligocene is at a depth of 700 meters.

Commercial gas is present east of Dakar at Diam Niadio, where over 20 Bcf of Upper Cretaceous gas has been produced, along with significant amounts of oil, from faulted structures on the western flank of the Jurassic-Lower Cretaceous shelf margin. The gas is piped for electrical power generation for the city of Dakar.

Thick Palaeozoic to Tertiary sediments capable of generating and holding hydrocarbons are found over most of the onshore and all the offshore areas. Offshore, two major sub-basins are defined by two eastwest transform faults that extend from the midAtlantic Ridge. The northern sub-basin extends from about the 16th parallel south to the Gambia River, where it is replaced by the Casamance sub-basin, itself extending south to GuineaBissau. The northern sub-basin is characterized by numerous volcanic intrusions and absence of the salt tectonics that are present in the Casamance basin to the south and in the Mauritanian sub-basin to the north.

Two domains

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Most of the Senegal Basin has a compressional tectonic style formed by Caledonian and Hercynian orogenies (left.) Tiled fault blocks are the main tectonic style in Eastern Casamance (right.)
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Two domains can be distinguished, delimited by the shelf edge. East of the shelf edge is a gently westward dipping Mesozoic and Cenozoic platform characterized by parallel bedding. The section thins eastward so that the Paleozoic sequence is accessible over a large area. The Mesozoic section has not undergone any orogenic or compressional stress. Normal faults generally strike north-south and are predominantly downthrown to the west, reflecting the predominant tensional structural style during the Mesozoic and Cenozoic. -

Salt diapirs in the Casamance-Guinea Bissau area which have pierced the Mesozoic section are prominent structural features. West of the shelf edge ( greater than 200 meters water depth), the regional structural style is dominated by gravitational features such as listric faulting and slumping, reflecting a slope environment and the influence of the Atlantic opening. A sedimentary depocenter is located in water depths of 1000-2000 meters.

The pre-rift and synrift sequences are shaped by the direct action of basement tectonics. The resulting structures are fault blocks, often tilted and defined by normal multi-directional faults. Onshore, seismic shows that the Palaeozoic sediments are in fact structured by normal faults. The drift sequence is best described as a gently structured westward dipping monocline. In the northern subbasin, deep volcanic activity has given rise to several dome shaped structures, onshore and offshore. Two systems of down-to-the-ocean normal faults, oriented northeast southwest affect Tertiary to Senonian, and possibly deeper sediments, along the inner shelf north of the Dakar Peninsula. These faults are shown to extend across the peninsula at Diam Niadio, where they contribute to the entrapment of hydrocarbons reservoired in Maestrichtian marine sands.

To the south, they are replaced by another similar system that extends into Gambian waters at the level of the 17th meridian. By comparison, the Casamance sub-basin is tectonically more quiescent and is characterized by the presence of several salt domes located along the axis of the depocenter. Movement of the salt was initiated during the Maestrichtian and, in some cases, possibly as early as Cenomanian. Along the continental slope, a major unconformity of PreCarnpanian age truncates all the older units down to the level of the Aptian.This is best seen at the Rufisque structure, in the offshore south of Dakar.

Two tectonic styles

In the Palaeozoic basin two main tectonic styles have been recognized: an extensional system in eastern Casamance, in which a Pre-Hercynian structural style of horsts and grabens and tilted fault blocks was preserved; and a compressional regime in the rest of the Senegal basin, resulting from the combined effect of Caledonian and Hercynian orogenies.

Sandstone reservoirs are abundant following the sections measured in the Bove basin, from the DM-1 and KO-1 wells in Casamance onshore and from the regional geology and range from Ordovician to Permo-Carboniferous. The Silurian shales are the best regional source rocks known. The measurements carried out in the DM-1 well and in the Bove basin show these rocks are very rich in organic matter (TOC 1-3%).

The interpretation and reprocessing of older seismic lines have identified anomalies distributed along Casamance, Diourbel-Saloum, and Louga blocks. The traps and prospective plays that have been identified in formations of Ordovician to Carboniferous age, consist of thrusted anticlines, complex faulted anticlines, four way closures, and tilted fault blocks.

The prospective surface for Paleozoic targets extends from north to south, east of 16° meridian, along a 100 km wide belt, with most of the objectives at drillable depths. The Senegal Paleozoic basin is large in area (60,000 sq km) and constitutes one of the prime targets of exploration the near future.

The Republic of Senegal has requested a credit from the International Development Agency, IDA, for funding of the "Senegal Onshore Paleozoic Seismic Investigation and Promotion Project," which consist of acquisition of 2,000 km of 2D, from North Gambia to the boundary with Mauritania. The bidding documents have been sent to several companies and the contractor will be selected by the end of March, 2000. The survey will be conducted probably during the second half of this year.

Deepwater

Deep water exploration has increased significantly during the last years and led to important oil field discoveries in the Gulf of Mexico, Angola, and other areas in the world. The Government of Senegal has decided to set new deepwater block boundaries that match the needs of today's industry. Vanco Energy, a Texas based company was awarded the Rufisque (Dakar) Deep Offshore Block in October 1999 based on a production sharing contract with Senegal. The Australian ROC Oil received the Casamance shallow offshore blocks, also on the basis of a production sharing contract in November 1999.

Seven blocks situated in deepwater and the shallow Dome Flore area are presently being promoted to the oil industry. Western Geophysical shot a speculative seismic survey in the deep offshore blocks in 1997, followed by a licensing round covering these blocks. Enough companies showed interest, mainly in the Casamance deep offshore and a new shooting program was decided on by Western and Petrosen in 1998. The 3,100 km of newly acquired 2D seismic lines are presently available for Casamance deep offshore and a licensing round covering the area is open until April 2000.

Casamance offshore

The most significant hydrocarbon shows within the Senegal basin are found in Casamance offshore and the Dakar peninsula onshore wells. In Casamance offshore, the diapiric salt has induced a modification of maturation gradients. The best source rocks are encountered in Cenomanian-Turonian and Albian sediments. The general trends observed in the basin show the maturity conditions are clearly improving southward. The oil windows occur at about 1,500 meters with peak oil generation around 2,900 meters in the Albian and 2,550 meters in the Turonian. Very good generating levels exist particularly between 2,550-2,900 meters with an excellent Cenomanian level at 2,608 meters (TOC of 8.72% and an HI of 660 mg/g).

Onshore, the Sebikhotane block was first granted in 1986 to the Petrosen-Tullow-Saim group and since then many seismic lines have been acquired and several wells have been drilled. At present, a one year extension of the terms of the permit has been introduced in order to finalize the work program which consists of the drilling of two wells.

The recent 3D survey covering 120 sq km around Diam Niadio area in 1995, and the 150 km of 2D data at the same period north of the block, resulted in the definition of new prospects which have now been successfully drilled. The results confirm the good potential of the Lower Maastrichtian. The Thies block was first granted to Petrosen in 1993.

The interpretation of various seismic studies resulted in the identification of four potential prospects. The first well was drilled in 1997 to a total depth of 2,180 meters in the Lower Senonian. Several hydrocarbon zones were encountered in the Senonian section with two levels in the Campanian giving commercial gas. Since January 1998, Petrosen has entered into a partnership agreement with Fortesa Corporation, a Houston-based company, for development of the potential in the Thies block. The other onshore blocks - Louga, Diourbel, and Saloum - will be investigated by the imminent 2,000 km survey of Petrosen's Paleozoic investigation project.

The last comprehensive review of the basin was completed by Atlantic Resources Ltd, BEICIP, and Petroconsultants in 1987, under the sponsorship of the World Bank. Since that time, many studies have been completed by Alconsult International and others, but a basin-wide review similar to the East Africa Regional Hydrocarbon Study completed in 1997 by Alconsult is urgently required, using newly acquired seismic and modern geochemical methods of basin analysis.

Author's Note: The East Africa Study was the basis of the article on East Africa published in Offshore Magazine in February 1999. References to previous studies on the Senegal Basin are available from Petrosen in Dakar or Alconsult in Calgary, Canada.

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