NW_AfricaDavison

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Journal of African Earth Sciences 43 (2005) 254–274

Central Atlantic margin basins of North West Africa: Geologyand hydrocarbon potential (Morocco to Guinea)

Ian Davison *

Earthmoves Ltd., Chartley House, 38–42 Upper Park Road, Camberley, Surrey GU15 2EF, United Kingdom

Received 10 March 2004; accepted 18 July 2005Available online 11 November 2005

Abstract

This paper summarises the stratigraphy, structure and petroleum geology of the Central Atlantic margin of NW Africa, from Mor-occo to Guinea. Rifting of the margin began in Late Triassic (Carnian) times and clastic red bed sequences were deposited on both sidesof the Atlantic margins. Red beds were followed by early Jurassic evaporite deposition, with three separate salt basins developed. Amajor magmatic event with dykes, lavas and plutons occurred along the whole Central Atlantic margin at 200 Ma during salt deposition.A carbonate platform developed along the margin in Jurassic to Early Cretaceous times. This consists mainly of carbonate ramp facies,but with rimmed-shelf carbonate platforms developed in Senegal. The deepwater sections of the margin consist of predominantly deep-marine clastic sedimentation from the Jurassic to Recent. Important deltas built out at Tan Tan, Cape Boudjour (Early Cretaceous),Nouakchott (Tertiary) and Casamance (Late Cretaceous). These delta deposits are important for oil exploration, because the rich Ceno-manian–Turonian source rock reaches maturity for hydrocarbon generation in these areas.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Central Atlantic continental margin Africa; Stratigraphy; Hydrocarbon potential

1. Introduction

The NW African Central Atlantic margin extends fromthe northern tip of Morocco southward to the GuineaFracture Zone which bounds the southern margin of theGuinea Plateau (Figs. 1 and 2). There is a marked similar-ity in the stratigraphy along NW African Atlantic margin,with Triassic red bed rift infill, followed by Early Jurassicsalt, Jurassic to Early Cretaceous carbonate platforms,and a marine clastic infill in the Cretaceous and Tertiary(Fig. 3). The margin also has a similar structural historyalong its length, with rifting in the late Triassic to EarlyJurassic followed by oceanic spreading which initiatedaround 180–170 Ma.

NW Africa has seen a recent surge of oil explorationinterest, with the discovery of the Chinguetti Field inMauritania, and a large amount of new seismic data has

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doi:10.1016/j.jafrearsci.2005.07.018

* Tel.: +44 (0)1276 671772; fax: +44 (0)1276 517451.E-mail address: [email protected]

been recently acquired along most of the margin. Thispaper reviews the geological history and petroleum geologyof the Atlantic margin, with a general introduction fol-lowed by a description of the basins ordered from northto south. The onshore marginal basins are brieflydescribed, but the main subject of the paper is the offshorecontinental margin basins (see Pique and Michard, 1989;Le Roy, 1997; Beauchamp et al., 1999; Le Roy and Pique,2001; Pique et al., 2002; Ellouz et al., 2003; Laville et al.,2004; for reviews of the onshore Moroccan basins).

2. Central Atlantic Triassic–Jurassic rifting

and magmatism

2.1. Rifting

Rifting of the Central Atlantic margin began in Late Tri-assic times, at the same time as adjacent onshore rifts devel-oped (Figs. 1 and 2). The first offshore sedimentation ismarked by the deposition of Triassic red beds (Brown,

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Fig. 1. Free Air Gravity map showing the fracture zone pattern in the Central Atlantic Fracture Zone labeled A can be correlated from America to Africaand this has been used in the plate tectonic fit shown in Fig. 2.

I. Davison / Journal of African Earth Sciences 43 (2005) 254–274 255

1980), followedbyEarly Jurassic salt in the late rifting phase.Little is known of the early syn-rift phase offshore, as it hasbeen rarely drilled and poorly dated. Seismic data indicate aseries of syn-rift half-graben are present inMorocco (Fig. 2),but these have not been observed south of the CanaryIslands. Olsen et al. (1996) concluded that the rift phase inthe Newark Basin on the conjugate USA margin (Fig. 1)was deposited from Carnian to Hettangian times. Up to5000 m of red marine to fluvial clastics were deposited inthe onshore southMoroccan Basins (Brown, 1980; Heyman,1989; Le Roy and Pique, 2001), and seismic evidence indi-cates the Triassic sequence has a similar thickness in the off-shore region (Le Roy and Pique, 2001; Le Roy et al., 1997).

The rifting trend closely parallels the old Hercynide foldbelt on the NW African margin, and pre-existing basementstructure probably controls rift orientation and style. Trias-sic basalts of the Central Atlantic Magmatic Province havebeen found below and intercalated with salt in the DoukkalaBasin (e.g. well DA-8 and RFE-1) (Le Roy and Pique, 2001;

Fietchner et al., 1992; Hafid, 1999), and the High PlateauxBasin (Coo de et al., 2003), suggesting that the salt wasdeposited in approximately 1 M yr around 200 Ma. Thethickest salt along the NW African margin is located in thedeepest half-graben formed during the rifting, and salt isabsent from the footwalls of major normal faults. Down-slope sliding of overburden above the salt is limited to withinindividual fault blocks due to the patchy nature of the salt(Tari et al., 2000, 2003). The original thickness of the saltin the offshore Morocco region probably exceeded 1.5 kmjudging by the size and frequency of the salt diapirs, butmay have been around 1 km, in the Mauritanian and Sene-gal–Guinea Bissau Salt Basins. Hence, salt deposition wasa very rapid process which probably averaged 1 mm a�1.

2.2. Central Atlantic Magmatic Province (CAMP)

Mesozoic dykes and sills were injected within a shortspace of time (ca. 1 M yr) along the whole Central Atlantic

Fig. 2. Correlation of the two margins showing the main 200 Ma salt basins and predicted extensions of salt basins. Plate Tectonics Reconstruction byauthor.

256 I. Davison / Journal of African Earth Sciences 43 (2005) 254–274

Fig. 3. Map of the Central Atlantic Magmatic Province and Triassic–Jurassic Rifts. Ages of igneous events are from Sebai et al. (1991); Hames et al.(2000); Marzoli et al. (1999).



margin (Olsen et al., 2003, Fig. 2). Thismagmatism is knownas the Central Atlantic Magmatic Province (CAMP)(McHone, 2000; Hames et al., 2000; Hames et al., 2003;Wilson, 1997; Wilson, 1992). The age range of the magmaticevents in Mali and Morocco is 197–203 Ma (Sebai et al.,1991; Fietchner et al., 1992; Knight et al., 2003), and dykesare dated between 198 and 203 Ma along the USA margin(Fig. 2,Hames et al., 2000).However, themost recent studiessuggest the basaltic magmatism is restricted to a 1 M yr per-iod centred around 200 Ma (Sebastien, 2001; Hames et al.,2003). The magma volume is estimated to be approximately2 · 106 km3, which ranks as one of the largest magmaticevents of the Phanerozoic, and it has been suggested it wasresponsible for the massive extinction at the end of the Tri-assic (Courtillot et al., 1996). The origin of the magmaticprovince is not clear, but a simple plume does not explainthe complex variation in magma batches which extend inan elongateN–S trending zone over 6000 km (Fig. 2). Amul-tiple source origin, or an asymmetric sub-horizontal plumehas been suggested (e.g. Oyarzun et al., 1997; McHone,2000). The main centre of the magmatism appears to be lo-cated in Guinea Bissau, Guinea, Liberia and southern Mali.

CAMP magmatism occurred approximately 25 M yrafter the initiation of rifting (Schlische et al., 2003), andan elongate zone of thinned lithosphere may have chan-neled plume material to follow the pre-existing rift. Thereis no obvious phase of uplift associated with volcanismalong the zone that was later to become the Central Atlan-tic ocean. Evaporites occur within and above the basalts,which suggests there has not been any major uplift prioror during volcanism.

2.3. Onset of drift

The exact age of the onset of ocean spreading is not welldefined because the oldest oceanic crust does not containidentifiable dated magnetic anomalies. Assuming a con-stant spreading rate in the Jurassic, a Middle Jurassic agewas estimated for the southern Central Atlantic spreadingbased on extrapolation from the earliest magnetic anomalyM 25 (Klitgord and Schouten, 1986). Ocean spreading ap-pears to have initiated first in the southern part of the mar-gin between the Western Sahara–Mauritania segment andthe Blake Plateau–Baltimore Canyon segment (Fig. 3).An older ocean fragment approximately 200 km in widthis preserved here, and an eastward ridge jump of 100 kmoccurred, so that there is a wider ocean crust stranded onthe American plate compared to the African plate (Owen,1983; Klitgord and Schouten, 1986, Fig. 1). The earliestspreading on the NW African margin is defined by middleAalenian to Bajocian age (169–178 Ma) sediment lyingunconformably on oceanic basalts on Fuerteventura Island(Steiner et al., 1998). This outcrop lies approximately30 km westward from the estimated position of theocean–continent boundary (Fig. 3). Using a spreading rateof 43 mm a�1 at this locality (Time Trek v 4.14, 2003) theoldest ocean crust would have formed 0.7 M yr before the

ocean crust on eastern Fuerteventura. Hence, the oldestpossible age of the oceanic crust would be 178.7 Ma, andthe youngest would be ca. 170 Ma.

The earliest phase of spreading of the oceanic crust pro-duced seaward-dipping reflectors, which are imaged alongmost of the N. American margin (Fig. 2, e.g. Talwaniand Abreu, 2000). The seaward-dipping reflectors (SDRs)reach up to 12 km in thickness in Baltimore Canyon andare interpreted to be sub-aerial basaltic flows and interbed-ded sediments. Jurassic carbonate platforms can be foundgrowing directly on top of the SDRs in the Baltimore Can-yon, which supports a sub-aerial spreading ridge hypothe-sis (Talwani and Abreu, 2000). The seaward dippingreflectors have not been identified on seismic reflection pro-files across the NW African margin. This can be explainedby the mid-ocean ridge jump, where the African seaward-dipping reflectors became stranded on the North Americanplate, and now form the Blake Spur Magnetic Anomaly(Figs. 2 and 3). The southern limit of the Central AtlanticOcean is defined by the Guinea Fracture Zone which crosscuts the gravity anomalies in the South Atlantic Ocean dueto a change in opening direction (Fig. 1).

3. Drift phase

During the drift phase (Jurassic to Recent) severalabrupt changes in drift direction occurred which affectedthe African plate. These were caused by South Atlanticopening, rotation and collision of Iberia, and Tethys clo-sure. The NW Moroccan margin was most profoundlyaffected by the collision of Africa and Iberia during theRif-Betic orogeny in Oligocene to Miocene times.

3.1. Jurassic–Cretaceous sedimentation

Northern Mauritania was situated close to the Equatorwhen the carbonate deposition initiated in Late Triassictimes (Fig. 3). Jurassic to Early Cretaceous carbonate plat-forms extend over a NE-SW distance of 6000 km from Por-tugal in the North to Guinea Bissau in the South (Jansa,1981). The platform becomes thicker and wider toward thesouth. The carbonates range from Late Triassic to Cenoma-nian in age on the Morrocan margin. In the shallower, mostproximal sections, the carbonate platform is dominated bydolomite in the Dogger and Upper Lias, giving way to lime-stones, dolomites and anhydrite in the Upper Jurassic andLower Cretaceous onshore (Fig. 4). Offshore Morocco, dol-omites and anhydrites dominate the Lower and MiddleJurassic and give way to limestones in the Upper Jurassic(e.g. well ESW-1bis, Hafid, 1999). In deep half-graben,shales dominate over carbonates (e.g. TAT-1 well, Hafid,1999). The Lower Jurassic carbonates and shales reach upto 600 m in thickness onshore in the Essaouira Basin area,and at least 2000 m offshore. Seismic profiles indicate theplatform is mainly a ramp type facies with occasionalrimmed platform development at Cap Juby (near Tarfaya)and onshore Senegal Theis block (Petrosen, 2003).

Fig. 4. Summary diagrams showing the common stratigraphic and tectonic features which are present along most of the NW African margin.



In general, the Lower Jurassic carbonate platforms arebroader than the Upper Jurassic platforms. During theEarly Cretaceous a major regression took place and thecarbonate platforms were drowned during the Valanginianand covered by prograding delta clastics (e.g. Boudjourand Tan Tan deltas).

Lagoonal shales developed landward of rimmed carbon-ate platform edges and these may constitute good hydro-carbon source rocks and are believed to have sourced theCap Juby Field in the Tarfaya Basin (Fig. 6c), which hasbeen reported to contain up to 400 million barrels of heavyoil in place (Jarvis et al., 1999). Middle Jurassic to Creta-ceous deepwater shale and sandstone turbidites (1600 mthick) are exposed on Fuerteventura Island (Steiner et al.,1998). Similar lithologies are expected in all the deepwaterbasins of the NW African margin.

4. Morocco

4.1. Introduction

Hercynian deformation affected most of Morocco.Palaeozoic basins were folded and thrusted, with themajor collision dated as late Devonian to Westphalianin age. In the south the Anti-Atlas was gently folded(Pique and Michard, 1989). The northern edge of theMoroccan salt basin is unknown, because the salt is off-set by later wrench faulting and thrusting associated withthe Rif-Betic orogeny (Fig. 5; Flinch, 1993; Flinch et al.,1996).

The Atlas Mountains are an intracontinental fold belt,which were produced by the inversion of a previousMesozoic Rift system (Beauchamp, 1998). Triassic–Juras-sic pull-apart basins contain clastics, evaporites and thickcarbonates up to 7 km in thickness and were formed duringthe Central Atlantic rifting along the High and MiddleAtlas Mountain Belts. The extension direction is inter-preted to be NW–SE, which is oblique to the ENE–WSWtrend of the Atlas Mountains (Beauchamp, 1998). Bay ofBiscay rifting commenced in Upper Jurassic times, acceler-ating with rotational drift from Albo-Aptian timesonwards. This process produced anticlockwise rotation ofIberia which began to collide with the African margin fromLate Cretaceous times onward.

A thick Cretaceous sedimentary sequence was developedin the Atlas sedimentary trough. Inversion and right lateralshear of the Mesozoic rifts and formation of the AtlasMountains occurred in the Early Tertiary (Morabetet al., 1998). The High Atlas Mountains reach up to4000 m above sea level. However, the observed upper crus-tal shortening is estimated to reach only 36 km, and lowercrustal shortening may be responsible for the large topog-raphy still present (Beauchamp, 1998). Palaeocene toEocene marine clastics were overlain by Oligocene conti-nental clastics associated with the Betic-Rif orogensis.

Individual basins are now described from north to southin the rest of the paper.

4.2. Rharb and Pre-Rif basins

The Rif Mountains were produced during the Alpineorogeny in the Oligocene–Miocene period. A Miocene ageforedeep is developed in the Rabat area, south of the RharbBasin, which extends offshore into the Atlantic (Flinch,1993; Pratsch, 1996). The arcuate fold and thrust beltextends some 450 km westward from the Straits ofGibraltar, and there is general younging of the thrust sheetswestward (Fig. 5, Flinch, 1993). This basin is associated withthe Rif-Betic orogeny. The Mesozoic age Central Atlanticmargin is difficult to image on the seismic data, as it liesbelow and is involved in the thrust sheets (Flinch, 1993).

4.3. Doukkala basin

The southern offshore part of this basin has also beentermed the Safi Basin (Tari et al., 2003), and the northernoffshore area is known as the Casablanca Offshore Basin(Morabet et al., 1998). The Mazaghan Plateau area in thenorth is a zone of shallow basement capped by Jurassic car-bonates (Fig. 5, Ruellan and Auzende, 1985). This plateauextends westward to the edge of the salt basin, which is nar-rower in this area (Fig. 5). Seaward of the Jurassic carbon-ate bank the basin contains many salt structures withallochthonous salt sheets (Fig. 6a). Salt-cored anticlinesaffect most of the 4–5 km thick stratigraphic sequence ofJurassic to Pliocene age, and were produced during theBetic-Rif orogeny (Fig. 6a). Most of the salt basin lies ingreater than 2000 m of water and no wells have been drilledin the deep offshore Doukkala Basin.

4.4. Essaouira–Hana basin

The Essaouira–Hana Basin is the most important oil-producing basin in Morocco. Seven fields have been dis-covered onshore, with six producing from Jurassic, andone from Triassic reservoirs. The basin extends from theAtlantic margin eastward into the High Atlas. To thenorth, it is separated from the Doukkala Basin by the Safistrike-slip fault, and to the south from the Souss Basin bythe Agadir Canyon system, and the South Atlasic Fault (LeRoy and Pique, 2001, Fig. 5). The continental marginbecomes broader in this area, and the bathymetry and freeair gravity indicate shallow basement (Ras TafalneyPlateau). Total sediment thickness reaches up to 8 km nearto the present-day coastline (Fig. 6b).

4.4.1. Stratigraphy

The following description refers to the onshore and shal-low shelf and is based on descriptions by Brown (1980),Jansa and Weidmann (1982), Broughton and Trepanier(1993) and Hafid (1999). This marginal basin has a com-pletely exposed Mesozoic sequence onshore and is the bestknown rift basin in Morocco. In the Argana graben(Fig. 5), the metamorphic Palaeozoic basement isdisconformably overlain by up to 5 km of continental red

Fig. 5. Tectonic map of the Moroccan Atlantic margin. Partly based on ONAREP (2003); Tari et al. (2003).

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Fig. 6. (a) Cross section of the Doukkala Basin. Based on industry seismic data. (b) Cross section of the Agadir–Essaouira–Hana Basin. Based on sections from Hafid, 1999. (c) Schematic cross sectionof the Tarfaya Basin extrapolated out to Fuertaventura. Based on section in ONAREP (2003).

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conglomerates, sandstones and claystones of Late Triassicage (Tixeront, 1973; Brown, 1980).

In the main Essaouira Basin, the most complete stratig-raphy of the Triassic was encountered in well JRP-1(Fig. 6b). CAMP basalts have been found below, within,and above the Triassic evaporite sequence (Fig. 6b, Hafid,1999), which dates the salt as Early Jurassic at 200 ± 1 Ma.Reefal limestones overlie and interfinger with the Arganared beds. This transitional transgressive sequence (Amsit-tene Formation) is mid to Late Liassic age and correspondsto the Iroquois Formation in the conjugate Scotian Basin.The Amsittene Formation is overlain by 220 m of con-glomerate, red sandstone, claystone and dolomite of earlyto mid-Dogger age (Ameskhoud Formation). A carbonateplatform developed in middle to late Jurassic time (Imouz-zer Formation) which reaches up to 800 m in thickness.Regressive sequences of red beds and evaporites are presentin the Late Jurassic, with 2000 m of sediments deposited inthe Neknafa syncline. Clastic sedimentation dominated inthe early Cretaceous associated with regional tectonic up-lift of the Atlas. Fine-grained marine clastics predominatethrough the early Cretaceous, with the exception of regres-sive red beds in the mid-Hauterivian. Silty marls and thickoyster beds, reaching 1100 m in thickness, were depositedin the Albian to Cenomanian period (Addouz Formation).Black shales, micritic limestone and cherts of the Turonianage Imi Mekki Formation overly the Addouz Formationand attain a thickness of 130 m. Regression followed inthe Coniacian to Santonian period with 300 m of shellbeds, marls, and deltaic sandstones. A final Cretaceoustransgression took place in the Campanian to Maastrich-tian with marls, dolomites, limestones and chalk (AouineFormation) reaching 500 m in thickness. Phosphatic sand-stones are found at the Cretaceous-Tertiary boundary. Pal-aeocene and Eocene sedimentation is represented by marland sandstone passing up into dolomite, sandstone andred marl of Eocene age (Imi n�Tanout Formation). Thisis overlain by a thin disconformable conglomerate ofUpper Oligocene age (Agadir Formation). The conglomer-ate marks the onset of the Betic-Rif orogeny. The Oligo-Miocene uplift was accompanied by important erosion,and as much as 3 km of Jurassic and Cretaceous sectionhave been removed from the southern margin of the basin.Pliocene shallow-marine marl and sandstone complete thestratigraphic succession (El Mhasseur Formation).

4.4.2. Source rocks

Onshore oil fields are believed to be sourced by Oxfor-dian shale, Carboniferous coals, and Silurian shales(Broughton and Trepanier, 1993). The Oxfordian shalesare Type II sapropelic kerogen and are richer in theNeknafa syncline, reaching up to 4.3% total organic carbon(TOC) over at least a 10 m thick interval. This shale ismature for oil generation in the centre of the syncline. Peakliquid generation of hydrocarbons began around 100–40 Ma. Carboniferous and Silurian source rocks arethought to source the highly mature gas in the eastern part

of the basin, which also contains significant nitrogen andhydrogen sulphide. The Cenomanian–Turonian intervalalso contains good source rocks (Wiedmann et al., 1982).

4.4.3. Reservoir rocks

Jurassic (Upper Oxfordian) fractured sandy dolomitesare the main reservoir target onshore, which have beendeformed by salt diapirism, or karstified along horst blocksalong the offshore carbonate shelf edge. Deepwater turbi-dites of Palaeogene and Cretaceous age are the main targetin deeper water (Truempy, 2001).

4.4.4. Structural history

Two main episodes of tectonic evolution occurred in theEssaouira Basin; an initial phase of rifting during the LateTriassic to Jurassic followed by compressional tectonicsfrom the Late Cretaceous to the Plio-Quaternary. The riftfaults onshore are mainly eastward dipping (Fig. 6b; Hafid,1999, 2000; Hafid et al., 2000). Early Jurassic salt reachesits maximum thickness in this basin. An important ENE–WSW branch of the salt basin extends onshore in this areaand continues in to the Middle Atlas and Tunisian Trias-sic–Jurassic basin trend (Figs. 2 and 5). Large diapirs andallochthonous salt sheets have been imaged offshore inthe water depths greater than 500 m (Fig.6b, Hafid, 1999;Tari et al., 2000). Attractive fold structures are present inthe deepwater, with thick synclines of Cretaceous stratadeposited between the salt structures (Fig. 6b). Salt is thinor absent on the shallow offshore shelf. Detachments withlistric faults occur in the Cenomanian–Turonian intervalprobably associated with the main source rock interval.

The offshore extension of the South Atlasic Fault hasproduced a series of important positive flower structures.One unsuccessful well has been drilled on such a structure(AGM-2, Heyman, 1989), but other prospective structuresmay be present along this trend in deeper waters.

4.5. Tarfaya–Aaiun Basin

The Tarfaya–Aaiun (Laayoune) Basin in Morocco is thenorthern continuation of the Dakhla–Laayoune–TarfayaBasin and stretches from Tarfaya to Ifni along the westernmargin of the Sahara.

4.5.1. Stratigraphy

The syn-rift sediments are Upper Triassic to LowerJurassic red beds and evaporites, and thin bedded shalesand carbonates. These were probably deposited in conti-nental to restricted marine conditions, and reach up to3 km in thickness in the Chebeika well (Fig. 6c, El Khatib,1995).The evaporites are thin onshore, and then becomethicker and diapiric offshore.

The Jurassic sequence contains carbonates and lagoonalrestricted shales, which provide good source rocks, and arebelieved to have sourced the Cap Juby Field.

The Lower Cretaceous rests unconformably on theUpper Jurassic carbonates, and is up to 1300 m thick in


the Puerto Cansado well (Fig. 6c). It is predominantly com-posed of fine-grained continental clastics. The UpperAlbian to Lower Cenomanian sequence consists of clay-stone, marl, siltstone and dolomitic limestone (Wiedmannet al., 1982). The Upper Cenomanian–Turonian andConiacian strata contain deeper-water shale and limestone,followed by shallower-water oyster shell beds present in theSantonian. An erosional unconformity truncates all thePalaeocene, Upper Cretaceous and part of the LowerCretaceous at the shelf edge (Fig. 6c). This erosion proba-bly took place in Santonian to Palaeocene times. ThinEocene and Oligocene units are overlain by a thickerMiocene sequence which reaches up to approximately1 km in thickness.

4.5.2. Source rocks

The Cap Juby MO-2 and MO-8 discoveries are believedto be sourced by restricted carbonate Liassic rocks (ONA-REP, 2002). The Tan Tan 1 well had 60 m of Jurassicsource rocks with TOC varying from 1.47-2.5 %, andhydrogen index greater than 200. Jurassic source rocksmatured during early Cretaceous times in the offshore area.Aptian–Albian source rocks are also present (Morabetet al., 1998; Tissot et al., 1980). However, details of theirquality are not known. Cenomanian–Turonian sourcerocks are present onshore and are approximately 50 mthick, with TOC varying from 6% to 10% (Wiedmannet al., 1982; Herbin et al., 1986; Kuhnt et al., 1990).

4.6. Canary islands

This chain of volcanic islands probably developed in theLate Cretaceous to Early Tertiary. The onset of volcanismcannot be easily dated as the oldest igneous rocks are dee-ply buried under younger lavas. The oldest sub-aerial lavaswhich have been dated are approximately 22 Ma on Fuert-eventura. These lie unconformably on submarine lavaswhich may be as old as Cretaceous. Previous workers havesuggested that the Canaries may be situated on continentalcrust (Dietz and Sproll, 1970), but more recent studiesshows Jurassic age deep-water sedimentary rocks lieunconformably on basaltic ocean crust on Fuerteventura(Fig. 6c, Steiner et al., 1998). The geochemistry of the bas-alts does not indicate any crustal contamination, suggest-ing that the Canaries were erupted onto Jurassic oceancrust (Thirlwall et al., 2000). There is no set pattern of agesthrough the islands, so they are not a simple hot spot trail.

4.7. Dakhla–Laayoune–Tarfaya

4.7.1. Introduction

The onshore Dakhla–Laayoune (Aaiun)–Tarfaya Basinand its offshore extension of the West Saharan MarginalBasin extends from the Mauritania border in the south tothe Canary Islands in the north. Total sediment thicknessranges up to 9 km in the northern part of the basinonshore, and decreases southward to Ad Dakhla, where

only 1 km of sediment is present onshore (Fig. 7). Thebroadest part of the margin is located between Boujdourand Ad Dakhla, where the shelf reaches up to 150 km widein water depths less than 200 m. Two DSDP wells (site 369and 397) were drilled NW of Cape Boujdour (Fig. 7). Threewells have reached total depths in Lower Cretaceous strataoffshore and sixteen wells have been drilled in the onshorebasin (Ranke et al., 1982; Heyman, 1989).

4.7.2. Stratigraphy

Correlation across the Atlantic indicates that theGeorges Bank Basin contains syn-rift Early Jurassic saltwhich is located farther south than the mapped salt basin,offshore Morocco (Fig. 2). If the syn-rift salt can beexpected on both conjugate margins of similar width, thensalt may be present in the deepwater Dakhla–Laayoune–Tarfaya Basin (Fig. 2). Salt has been previously indicatedbetween Ad Dakhla and Boudjour (Uchupi et al., 1976;Brown, 2003). However, Von Rad and Wissmann (1982)have also suggested that mud diapirs are present. A smallsalt basin has been described farther south on the USAconjugate margin in the Baltimore Canyon Trough, whichcorrelates with the conjugate Ad Dakhla to Pointe Noirsegment of the Western Saharan margin (Figs. 2 and 7a).Syn-rift deposits have been penetrated in several wells inthe central onshore Laayoune–Tarfaya Basin. These areconglomerates, red shales and sandstones, evaporites andvolcanics (Ranke et al., 1982). Lower to Middle Jurassicstrata are only present in the northern part of the basinnear Tarfaya.

Jurassic–Early Cretaceous carbonates (Puerto CansadoFormation) reach up to 1–2 km in thickness. However,the carbonate bank is only sporadically developed in thisbasin, approximately along the 200 m isobath (Fig. 7a).During the early Cretaceous 1–4 km of continental to mar-ine deltaic sediments were deposited on the shelf (Von Radand Wissmann, 1982; Ranke et al., 1982, Fig. 7b).

A large Lower Cretaceous fan occurs offshore CapeBoudjour, which developed because of uplift and erosionof the Reguibat Shield and Anti-Atlas Mountains at thistime. Carbonates are present in the Upper Cretaceousand Palaeocene on the shelf. The maximum thickness ofUpper Cretaceous marine rocks reaches approximately1 km in the region of well Spansah 51A-1 (Fig. 7b). Amarked erosion surface is present in the northern part ofthe basin between Cap Juby and Cap Boujdour whichhas removed most of the Upper Cretaceous and EarlyTertiary. Thin Palaeogene (Samlat Formation) overliesthe Cretaceous, and consists of marine siliceous chalk.The Eocene Gueran Member is mainly clastic and isexposed over large areas in the onshore basin (Fig. 7).The Oligocene Morcba member of the Samlat Formationreaches up to 300 m in thickness and is mainly continentalsandstone and conglomerate. The Oligocene is missing inthe north due to erosion during a regressive period. Inthe centre of the onshore Laayoune–Tarfaya Basin thePalaeogene reaches a maximum thickness of 1000 m, and

Fig. 7. (a) Map of the Aaiun–Tarfaya Basin. Based on ONAREP (2003). (b) Cross section of the Aaiun–Tarfaya Basin. From Heyman (1989).



then decreases seaward to 200 m at DSDP site 369 (Rankeet al., 1982). Neogene is generally thin (<100 m) and is onlyexposed onshore in the western part of the basin. Sandylimestone and oyster beds are the main lithologies. Muchof the continental shelf break is dissected by submarinecanyons, which reach up to 900 m in depth (Von Radand Wissmann, 1982).

4.7.3. Source rocks

Restricted carbonate source rocks of Early Jurassic ageare thought to be present in the area between Ad Dakhlaand Pointe Noir. The Cenomanian to Turonian intervalcontains oil-prone sources farther south in Mauritania,but in the Western Sahara this interval is predicted to beless organic rich (Tissot et al., 1980). However, this maybe due to dilution of organic material by clastics in deltaicareas.

4.7.4. Reservoirs

The Jurassic carbonate bank is a possible reservoir.However, no structural closures have been documented atthis level. Lower Cretaceous age sandstones may beexpected in a deep-water fan located off Cape Boujdour(Ranke et al., 1982; Von Rad and Sarti, 1986; FusionOil, 2003).

4.7.5. Structure

The shallow shelf area is fairly unstructured (Fig. 7b).Some Albian roll-over structures have been described asgenerated by listric faulting and these will constitute themain exploration targets (Morabet et al., 1998; Heyman,1989). Seals will be a problem in the northern part of thebasin as the Lower Cretaceous sequence is sand rich.

5. Mauritania

5.1. Introduction

The Mauritanian Basin extends from the southern tip ofthe Mauritanian salt basin to the Ras Al Beida High(Fig. 8). This high produces an important gravity anomaly,which extends E–W out to 200 km from the Mauritaniancoastline. The Mauritanian Salt Basin extends 300 km ina N–S direction and is approximately 60 km wide(Fig. 8a). Salt may be present on the platform below theJurassic-carbonate platform, but this has not been drilledyet. Two wells were drilled in 2001 by Woodside Petroleum(Chinguetti-1 and Courbine-1) with the former being alight oil discovery, and the latter having a small gas col-umn. Several development wells have now been drilled onChinguetti and it is now a commercially viable oilfield.Subsequently, Woodside discovered the Banda and TiofFields in Tertiary sandstones, and the Pelican discoverycontained gas in Upper Cretaceous sandstone reservoirs.Many similar structures are present in the Mauritanian SaltBasin.

5.2. Stratigraphy

Late Triassic to Early Jurassic salt and clastics are pres-ent in the basin. However, these have only been imaged onthe seismic data and have not been drilled. An unconfor-mity separates the overlying Jurassic carbonates from theunderlying rift sequence. The carbonates reach up to sev-eral kilometers in thickness (Fig. 8b). These are followedby Cretaceous shales with shallow marine sandstones onthe shelf grading to deep water shales farther offshore.Early Cretaceous clastics were encountered in the Autru-che-1, OCT-1B and OCT-2 wells. The later Aptian to EarlyAlbian section in Autruche-1 well consisted of a carbonatebank. Offshore in Loup de Mer-1 and MT-2 this intervalconsists of condensed marine shales (Ministere de Mineset de l�Industrie, Mauritanie, 2000). Fine-grained clasticdeposition persists throughout the late Cretaceous and Ter-tiary with turbidite sandstones expected in the deepwater.

Prominent unconformities have been identified on theshelf in the Maastrichtian and the Oligocene–Miocene.This part of the margin was situated in a humid climatefor most of the Cretaceous and Early Tertiary period. Highrates of sedimentation occurred in Oligocene to Recent per-iod in Central Mauritania to produce the Nouakchottdelta, and in the Cretaceous in the Ras Al Beida area(Fig. 8a).

5.3. Source rocks

The DSDP well 368 encountered a 100 m thick intervalof high quality source in the Cenomanian–Turonian inter-val. The Turonian source was early mature at 3500 m in V-1 well (Ministere de Mines et de L�Industrie, Mauritanie2000). Early Jurassic Type 2 kerogen has been tested inV-1, and Coppolani-1 with good source potential up to27 kg/t (Reymond and Negroni, 1989). These source rocksmay be important for the shelfal carbonate play.

5.4. Reservoirs

Principal reservoirs are Miocene age to Late Cretaceousturbidites sealed by deepwater shales. Most of these playslie in greater than 500 m of water depth. Shallow marineand fluvial sandstones of Albian age, and the Jurassic car-bonates may also constitute a reasonable reservoir on theshallow shelf.

5.5. Structure

The most prominent structures are created by salt move-ment and downslope sliding with toe thrusts and compres-sional anticlines detached on the salt (Fig. 8b).Compressional structures are also created by sliding on adetachment at the Turonian source rock level (Fig. 8b).Lack of migration pathways, or lack of maturity fromthe late Cretaceous source can be a problem in some foldtraps.

Fig. 8. (a) Map of the Mauritanian Salt Basin. Based on BRGM (1968) and Tari et al. (2003). (b) Cross section through the central Mauritanian SaltBasin. From Fusion Oil Website (2003).


Fig. 8 (continued)


6. Senegal and Gambia

6.1. Introduction

The Senegal Basin is part of a continuous basin thatruns southward from Cap Verte (Dakar) in Senegal intoGuinea Bissau and to the Guinea fracture zone in thesouth (Fig. 9). Seismic data and well analysis have alsorevealed the presence of a potentially important Palaeo-zoic Basin underlying the thick Meso-Cenozoic strata(Fig. 9). The Palaeozoic Formations crop out in south-eastern Senegal and in the Bove Basin in Guinea and Gui-nea Bissau (Petrosen, 2003). Since 1953, a total of 144wells have been drilled in the Senegal Sedimentary Basin.Forty nine of these wells are located offshore. The major-ity of the exploratory wells are concentrated in two mainareas, the Cap-Vert Peninsula and Casamance Offshore(Fig. 9). The remainder of the Senegal Basin remainsunder-explored, and there has been no deep-water drillingto date.

The Casamance–Bissau sub-basin has been lightly ex-plored since the 1960s. A total of 16 wells have been drilledin the shallow water part of the sub-basin, resulting in threediscovery wells and 5 wells with significant shows. The

Dome Flore and Dome Gea discoveries have been identi-fied as significant oil accumulations within the basin, withpossibly over a billion barrels in place (Brown, 2003;Fig. 10a). However, reservoir is shallow, the oil is mostlybiodegraded and heavy, but smaller volumes of light oilhave also been discovered, and the fields still awaitdevelopment.

Petroleum exploration activity in Senegal has concen-trated primarily on the Mesozoic–Cenozoic Basin, whilethe Palaeozoic Basin remains essentially unexplored.Several wells have been drilled in Guinea Bissau on saltdiapirs, but no commercial hydrocarbons have been dis-covered to date.

6.2. Stratigraphy

The most complete pre-rift Palaeozoic sections in theSenegal Basin have been identified in the DM-1 andKO-1 wells, where Ordovician to Devonian Formationshave been penetrated.

The syn-rift section (including the salt section) does notcrop out in the Senegal Basin and salt has been encoun-tered only where wells have been drilled into diapirs.

Fig. 9. Map of the Senegal, Gambia, Guinea Bissau Atlantic margin. Salt structures modified from Tari et al., 2003. Onshore geology taken fromMinistere des Travaux Publiques etc. (1962).


A Jurassic to Lower Cretaceous carbonate platform, a Cre-taceous clastic wedge, and Tertiary carbonates and shalesconstitute the post-rift section of the Senegal Basin(Fig. 10). The middle/late Cenomanian to early TuronianBanc du Large Formation consists of alternating shales,limestones, clays and sandstones, while the Early TuronianBrikama Formation comprises bituminous shales (Herbinet al., 1986). Cenomanian sediments have been encoun-tered in wells CM-2 and CM-4, where glauconitic whitelimestones, fine-grained sandstones and green to grey clayspredominate. Turonian deposits in the wells consist mainlyof shales with a pelagic fauna.

An important magmatic event occurred in the Cape VertPeninsula and the Theis Plateau to the east of Dakar from

Miocene (35 Ma) to Pleistocene (0.8 Ma) (Goumbo Loet al., 1992). This probably succeeds pulses of magmatismin the area during the early Cretaceous and late Creta-ceous. The age of the Cayar Sea Mount is probably mid-Cretaceous, as Maastrichtian sands sit unconformably onmetamorphosed Lower Cretaceous carbonates above theintrusion (Goumbo Lo et al., 1992).

6.3. Source rocks

The most notable hydrocarbon occurrences within theSenegal Sedimentary Basin are located in the Casamanceoffshore and Cape Vert peninsula onshore. The Cenoma-nian–Turonian and Albian–Aptian sediments in the

Fig. 10. (a) Cross section AB of the Senegal Basin. From Fusion Oil Website (2003). (b) Cross section CD of the Guinea Plateau. Locations shown in Fig. 9.

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Casamance offshore present the best source rock potential.Albian age source rocks were detected in the uppermost100 m of the Jammah-1 well in Gambia (Fig. 9) withTOC averaging approximately 2%. The Cenomanian–Turonian shales north of the Dakar Peninsula have hydro-carbon source potential ranging from 3 to 21 kg/t andthicknesses up to 350 m. South of Dakar the Cenoma-nian–Turonian source is very rich in the CM-4 and CM-1wells. Excellent source rocks in the Cenomanian were pro-ven in the DSDP well at Site 367 with a Turonian intervalover a 30 m thick, and TOC levels averaging over 20 %(Herbin et al., 1986). The Late Cretaceous source rocksprobably reached the oil window in the late Tertiary(Reymond and Negroni, 1989).

Lower Jurassic to Eocene intervals have poor to moder-ate organic matter contents. The Jurassic interval isbelieved to have a restricted carbonate-shale sequencewhich may contain oil-prone source rocks. In deep water,the Lower Cretaceous may contain shales with oil pronesources. However, the Jurassic and Lower Cretaceoussource rocks may be generating gas in northern Senegaldue to their burial depth.

Within the Senegal Palaeozoic Basin, the Silurian shalesare the best regional source rock. These shales have organicmatter content (TOC) of 1–3% and are relatively mature(Vo profiles between 0.95 and 1.3). Geothermal gradientsdetermined from wells from the Mesozoic have beenextrapolated to the Bove Basin. Oil generation commencedfrom 350 to 125 Ma and continues to the present. The bur-ial history models suggest that Silurian source rocks aregenerating oil (Petrosen, 2003).

6.4. Reservoirs

Wells that have penetrated the Jurassic–Lower Creta-ceous carbonate platform demonstrate locally good poros-ities in the dolomites (23%), and fair porosities (10%) in thelimestones. Reefal targets on the shelf edge remain to betested, but attractive structures are identified on seismicdata (Petrosen, 2003). The Casamance River has been themain input point for Albian fluvial and deltaic sandstonesinto southern Senegal, and the Senegal River is the maininput point north of the Dakar Peninsula (Fig. 9). ThickMaastrichtian sandstone intervals with 20–30% porositywere encountered in the Diam Niadio and Dome FloreFields. The sandstone intervals become much thinner atthe shelf break, and over the crests of salt structures. Porouscarbonate sandstones are found in the Palaeocene–Eocene,and good Oligocene foraminiferal limestone reservoirs arepresent in the Dome Flore Field due to shoaling on top ofgrowing salt domes (Reymond and Negroni, 1989). Turbi-dite sandstone reservoirs may occur locally within theMiocene, and these will be best developed in lows betweenthe salt structures and toe thrusts.

The Senegal Palaeozoic Basin contains abundant sand-stone reservoirs and they are located in the following strati-graphic units: (a) Well-cemented quartzite sandstones

occur within the Ordovician. While these sandstones haveno primary porosity, they are intensely fractured and there-fore constitute good secondary reservoirs. (b) Fine-coarsegrained sandstone units occur in the Devonian, with poros-ities ranging between 15% and 20%.

6.5. Structure

In the eastern part of the Senegal Basin the faulted pre-Mesozoic basement dips gradually towards the west. Thisstructure is complicated by horsts and folds near Dakar,whereas in the south it is dominated by a prominent lineof salt diapirs in the Casamance offshore, and folds devel-oped in deep water. The Lower Jurassic salt penetrates theTertiary Formations in the northern part, and theMaastrichtian in its southern part. An important unconfor-mity has developed at the shelf break and Upper Creta-ceous strata lie directly on Jurassic strata (Chanut andMicholet, 1988; Petrosen, 2003) (Fig. 10). This unconfor-mity represents a scarp slope on the shelf which wasbypassed by sediment for a long period in Mid-Cretaceousto Early Tertiary times.

7. Guinea Bissau and Guinea

7.1. Introduction

The Casamance salt Basin extends southward intoGuinea Bissau and appears to terminate against theBissau–Kidira–Kayes Shear Zone (Fig. 9). The main fea-ture in offshore Guinea Bissau is a large marginal plateauclearly seen on the free air gravity and bathymetry maps,termed here the Guinea Plateau (Figs. 9 and 10). Thenorthern half of the Plateau lies in Guinea Bissau and thesouthern half in Guinea.

Several wells have tested salt domes in this area (Figs. 9and 10b). Well PGO-3 recovered a small amount of 33�–36� API oil from two Albian sandstones, and Sheeps-head-1 also encountered good oil shows. Premier Oildrilled two holes on the Sinapa diapir in 2002 and 2004,and the second well encountered sub-commercial hydrocar-bons. Only one well (GU2 B-1) has been drilled offshoreGuinea, which reached the Barremian (Bungener, 1995).

7.2. Stratigraphy

A thick syn-rift section of Triassic-early Jurassic sedi-ments is predicted (Dombrowski et al., 2002). The GuineaPlateau comprises a thick sequence of carbonates and clas-tics (Fig. 10b). An unconformity truncates 1–2 km of stratafrom the outer margin of the bank (Dombrowski, 2002;Dombrowski et al., 2002). A delta with gravitational tec-tonics is present within a WNW-ESE trending bathymetrichigh outboard of the carbonate bank, and the structuresappear to indicate a N vergence of thrusting that may indi-cate that the sediment source of the delta was from thesouth (Dombrowski et al., 2002; Fig. 9). The delta onlaps


the Jurassic carbonate bank and the structuring predatesthe Santonian unconformity, and therefore the delta isbelieved to be Early Cretaceous in age.

7.3. Source rocks

Turonian–Cenomanian sources are expected (Bungener,1995). The depth to the oil window is about 2–2.5 km onthe shelf. The Turonian source rock is probably too shal-low to be mature over most of the Guinea Plateau, andan Albian or deeper source would be required to generatehydrocarbons.

7.4. Structure

The Guinea Plateau is a major continental promontory,which is estimated to be underlain by 20 km thick continen-tal crust (gravity modelling Mark Longacre, pers. comm.2003). A large magmatic zone of Late Triassic–EarlyJurassic age (200 Ma) is expected to extend offshore fromConakry. There is not much structuring of the Late Creta-ceous to Recent strata over the shallow part of the Plateau,but at the shelf break there is a zone of compressional fold-ing along the Guinea Fracture Zone (Fig. 10b). At leasteight salt domes have been mapped in the offshore GuineaBissau (Fig. 9). The salt structures appear to be similar tothe examples in Casamance. They cut through the layeredCretaceous section with very little disturbance, and thereis no marked rim syncline development.

7.5. Hydrocarbon potential

The salt structures in Guinea Bissau have the best poten-tial for hydrocarbon traps in folds or drapes over salt dia-pirs with Cretaceous sandstone reservoirs. The GuineaPlateau does not have any obvious structural traps.

8. Conclusions

There is a marked similarity in the stratigraphy of theNW African Central Atlantic margin, with the Triassicred bed rift infill, followed by Early Jurassic salt, Jurassicto Early Cretaceous carbonate platforms, and a marineclastic infill in the Cretaceous and Tertiary (Fig. 4a). Thewhole margin also has a similar tectonic history with riftingoccurring in the late Triassic to Early Jurassic around 225–200 Ma, followed by basaltic magmatism and salt deposi-tion at 200 Ma, and oceanic spreading starting around180–170 Ma (Fig. 4b). Oil exploration is in its infancy still,but there have been several notable hydrocarbon discover-ies at Cap Juby (Morocco), Chinguetti, Banda Pelican andTiof (Mauritania), Dome Flore and Dome Gea (Senegal).

Acknowledgements

I would like to thank Matthew Taylor, Pedro Baptista,and Mark Longacre, for useful discussions on NW Africa

and for help with preparing diagrams. Nick Cameronand Roger Key made useful editorial and reviewers com-ments. Laurie Brown kindly provided Fig. 8b.

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