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THE PETROLEUM POTENTIAL OF THE SOMALI COASTAL BASIN AND ITS OFFSHORE-

Geologic evolution, petroleum exploration well results; stratigraphy and structure; surface geology and hydrocarbon potential 

Abdulkadir Abiikar; Petrosom
22 Windrush Road, London NW10 8NL
E-mail: moolkaal@aol.com 

• Somali Version: AKHRI HALKAN...

Introduction:

 Location:

The Somali Coastal Basin lies to the southeast of “Buur” basement outcrop in Somalia. Geographically this area encompasses the coastal strip from Adale, Warsheikh, Mogadishu, Afgoye, Marka, Baraawa, Jilib, Jammaama, Kismayo, Bula Haji, Kudha, Badhaadhe, Hoola Wajeer, Buur Gaabo, Maanaraani and Ras Kiyamboni. In terms of strict definition, the Somali Coastal Basin is all that area along the south-eastern coast, lying south of Mogadishu to the Somali border with Kenya at Ras Kiyamboni. Away from Buur Basement outcrop lies younger sediments that thicken. Its southeastern boundary with the Lamu Embayment is ill-defined, although a dramatic increase in Cenozoic sediment thickness towards the south of Somalia near the Alimo Addo (Halima Addey) well (S17 in Fig. 1 may be taken for the transition. The Lamu Embayment proper extends from Oddo Alimo south through Kenya to the Tanzanian border, and inland from the Kenyan coast to Precambrian basement outcrop.  

Pre-Jurassic

No pre-Jurassic sedimentary rock has been reported from outcrop in the Somali Coastal basin. However, the Brava 1 Well (S13 in Fig.1) bottomed in quartz sandstone 120 m thick, which bears resemblance to the continental Triassic – Lower Jurassic Adigrat Formation described in other wells. Both Kamen-Kaye (1978) and Kamen-Kaye and Barnes (1978) noted that palynomorphs of Permo-Triassic age were found in the shale overlying the quartz sandstone in the Brava 1 Well. Nevertheless, Beltrandi and Pyre (1973) considered both units to be Jurassic in age. Until more documentation becomes available, it is considered that the basal sandstone to be the top of Adigrat Formation, the lower part of which is Triassic in age.

Lower Jurassic

Deposition of the Adigrat Formation continued uninterrupted from Triassic through Pliensbachian time. The formation consists of as much as 130 m of quartz sandstone with intercalations of gypsum and dark shale. Interestingly, the Qoryoley 1 Well (S9 in Fig.1) bottomed in extrusive igneous rock after penetrating at least part of the Adigrat Formation. This rock may be the manifestation of the rifting process that ultimately led to the separation of Madagascar and Africa in the Middle Jurassic time (Segoufin and Patriat, 1980; Parson and others, 1981; and Rabinowitz and others, 1983). Overlying this sandstone, and extending to the middle of the top of the Middle Jurassic are basinal dark grey shale and dark grey argillaceous fossiliferous limestone that grade to pure limestone seaward. At the Marai Asha Well (S1 in Fig. 1) this section – the Hamanlei Formation – is at least 1,525 m thick; just to the north at the Hobyo 1 Well (not on this map), the thickness is at least 2, 175 m. 

Upper Jurassic and Lower Cretaceous

The Oxfordian – Kimmeridgian Warandab Formation consists of yellowish, marly limestone containing belemnites and ammonites in Southern Somalia. The formation is represented in the boreholes by basinal dark grey marly limestone stringers, and in the Marai Asha Well (S1 in Fig.1) a total thickness of 538 m was recorded. The remainder of the Upper Jurassic  (Late Kimmerdgian – Tithonian) section is expressed as basinal dark gray and dark brown shale, with some gray, finely crystalline foraminifera – bearing limestone, of the Garbaharrey Formation, which attains a maximum thickness of 350 m. 

In south-central Somalia, the lower Cretaceous crops out as a series of gypsum and limestone with inter-bedded shale. In the subsurface a fore-reef limestone and medium-depth neritic shale make up the Cotton Formation, which is entirely of Early Cretaceous age (dated by foraminifera). At Marai Asha (S1 in Fig. 1), the section is 130 m thick and is in unconformable contact with both the Upper Jurassic and Upper Cretaceous rock. 

Upper Cretaceous

At Marai Asha (S1 in Fig 1), the Upper Cretaceous is represented by 1,025 m of deep-water grey shale and marl (Sagaleh Formation) and the section thins seaward. Farther to the south, at the Marka Well (S8 in Fig 1), the entire Upper Cretaceous column was not penetrated, but 360 m of dark gray shale of that age with interbedded splitic basalt flows were recovered. In Southern Somalia, the Upper Cretaceous section is approximately the same thickness as at Marai Asha, but consists of open marine sandstone and siltstone with shale intercalations. The Sageleh Formation  is rich in foraminifera. 

Paleocene

Paleocene rock is abundant in boreholes of the Somali Coastal Basin. In the Marai Asha Well (S1 in Fig 1), the Sagaleh Formation – a foraminifera-bearing deep-water gray shale and marl – continues into the Paleocene from the Upper Cretaceous; Paleocene thickness of the unit is ~100 m. Overlying the shale is a 200-m thick transitional zone, the fossiliferous (foraminifera) Marai Asha Formation, between the shale and the overlying Auradu limestone. The Auradu Formation, 320 m thick in the Marai Asha borehole, is a finely crystalline, compact, hard, tan to light brown limestone with local thin gray shale horizons; the unit grades to a deeper water facies towards the Somali continental margin. Rich in foraminifera, the formation continues into the Eocene. At Marka 1 Well (S8 in Fig 1), the Paleocene section consists of 960 m of dark gray to brown , fine to medium grained, calcareous well-cemented quartz sandstone beds. Sills of spilitic basalt intrude this section. Farther to the south, the epoch (plus the early Eocene) is represented by 2,745 m (Halima Addey, S17 in Fig1) of predominantly terrigenous quartz sandstone interbedded with shale, mudstone, and some anhydrite.  

Eocene

Deposition of the Auradu Formation, a finely crystalline, compact hard tan to light brown, foraminifera-bearing limestone with local thin, gray shale horizons, continued uninterrupted from Paleocene through Ypresian time. This Formation grades seaward to a deeper water shale facies. At Marai Asha, the undifferentiated Paleocene – Lower Eocene is represented by 320 m of Aurado Formation. The Taleh Formation consisting of 117 m of pink, very fine-grained, hard, calcareous quartz sandstone,   immediately overlies the Aurado. To the South, the Marka 1 borehole (S8 in Fig.1) records 430 m of Lower Eocene dark gray to brown shale of the Aurado Formation. The shale contains some dark gray to brown limestone-layers and also some light gray to brown, fine to medium grained, calcareous well-cemented quartz sandstone beds. Overlying the Aurado are 174 m of Taleh Formation, consisting of dark gray to dark green calcareous finely micaceous shale-containing glauconite and pyrite, and a few thin sandstone beds. A very similar fossiliferous shale (Karkar Formation), 268 m thick, lies above the Taleh Formation and extends to the top of the Eocene section. As previously mentioned, 2,745 m of undifferentiated Lower Tertiary clastic sediments, extending through the lower Eocene are preserved in the Oddo Alimo (Halimo Addey) borehole (S17 in Fig.1) in extreme south-eastern Somalia.

Miocene

The undifferentiated Miocene section recovered from Marka 1 Well (S8 in Fig.1) is 837 m thick. At the base are 91 m of multicoloured shale, overlain by 213 m of sandstone. Red, green and gray silt-shale totalling 107 m succeeds the sandstone, and at the top of the section are 426 m of white to gray, hard, fine-t0-medium grained, calcareous sandstone with some gray-green and brown soft clay layers in the lower 91 m, along with one cream to white, finely crystalline, gypsum-bearing fossiliferous limestone bed. At Brava 1 Well (S13 in Fig. 1) 914 m of Miocene marly limestone interbedded with calcareous shale, and a few sandstone beds were encountered, unconformably overlying lower Cretaceous rock. The Halimo Addey (S17 in Fig.1) borehole records Miocene calcareous shale, farther to the south, the facies changes to marine limestone several hundreds metres thick. 

Pliocene

At Marka 1 Well (S8 in Fig. 1), 11 metres of Pliocene clastic and carbonate deposits were recovered. In the south, more than 500 m of limestone, clay and sandstone are present just to the north of the Lamu Embayment. 

Quaternary

The Quaternary period is marked by reef development and clastic (alluvial and dune formation) deposition in the Somali coastal basins. 

Discussion:

Southern Somalia was subjected to epeirogenic uplift at the end of the Cretaceous time, and has remained above sea level ever since except in the coastal areas. The rift faulting of the Middle and Late Tertiary time uplifted the northern half of the country with the result that post-Eocene marine sedimentary rocks are known only along a narrow strip of the coast. That narrow strip is the Somali Coastal Basin. 

Many structural features created by these movements are capable of trapping oil and gas. Petroleum systems of Somalia present areas with combination of source rock, reservoir traps and seals; structures that have not been adequately tested by previous exploration. 

Conclusion

A review of previous exploration indicates the vast majority of the wells drilled to date have been onshore in areas of predominantly continental sediment deposition. These wells into continental sediments have produced abundant shows of gas but very little content of liquid content. Those wells with liquids have invariably been close to the coastline, and at the edge of the marine depositional environments. It is reasonable to assume that as exploration extends into marine depositional environment present in the deeper water, liquids will begin to dominate, and commercial quantities of liquid hydrocarbons will result. Eastern Africa remains distant from the major oil markets of the world; however, the region is considered highly prospective and relatively unexplored and untested.

Abdulkadir Abiikar; London
E-mail: moolkaal@aol.com

References:

1. Abdi Salah Hussein, 1978. Ricerche and Prospettive Petrolifere nel Bacino di Mogadishu; the Somali National University; Dept. of Geology, unpublished graduation work; Mogadishu; Somalia.

2. Agip Mineraria – Agip Somalia; 1957 – 1977. Reports; unpublished, Mogadishu, Somalia.

3. Andrews S.M., 1968; Final Reports; Sinclair Somali Oil Corporation; unpublished; Mogadishu, Somalia.

4. Barnes, S.U. (Sinclair - Somalia Oil Corporation); 1976, Geology and oil prospects of Somalia, East Africa; The American Association of Petroleum Geologists Bulletin [AAPG]; Vol. 60, No. 3 (March 1976); pp. 389 – 413; 10 fig. and 3 tables

5. Beltrandi M. and Pyre A. , 1973; Geological Evolution of Southwest Somalia in Blant, G., ed., Sedimentary Basins of the African Coasts; Part 2- South and East Coast: Paris; Association of African Geological Surveys; p. 159 – 178.

6. Coffin, M.F., and Rabinowitz, P.D., 1988, Evolution of the conjugate East African-Madagascan margins and the Western Somali Basin, Geological Society of America Special Paper, published by Geological Society of America, vol. 226, p. 78.

7. Coffin, M.F., and Rabinowitz, P.D., 1982, Multichannel seismic transect of the Somali continental margin, Proceedings of the Offshore Technology Conference, Houston, Texas, Report OTC, vol. 4259, p. 421-428.

8. Do Toit, S.R. and Slind, O.L. (Al-consult Int.); Hydrocarbon potential of the East Africa continental margin (from Somalia to South Africa), Offshore Journal; v.59, no.2; 1999.

9. Kamen-Kaye, M and Barnes, SU; 1978, Exploration outlook for Somalia, coastal Kenya and Tazania: Oil and Gas Journal; July 24, 1978; p. 80 – 246.

10. Lyons P and Beninson A.; 1960. Somalia: Geological and geophysical report; Sinclair Somali Corporation; unpublished, Mogadishu.

11. Michael J. Brady; Harms & Brady, Inc. Exploration History and Hydrocarbon Potential of Somalia; Presentation for Kansas Geological Society: Technical talk 2002.

12. Michael J. Brady et al. The Hydrocarbon Potential of Somalia; 1989. Unpublished. Ministry of Water and Mineral Resources. Mogadishu, Somalia

13. Mohamud Arush, Andrew Miall and Schalk Du Toit. Petroleum potential of Somalia. Canadian Society of Petroleum Geologists; Rock the Foundation Convention 2001; Abstracts June 18 -22, 2001.

14. Rabinowitz, P.D., Coffin, M.F., and Falvey, D.A., 1982, Salt diapirs bordering the continental margin of northern Kenya - southern Somalia, Science, vol. 215, p. 663-665.

15. St. John, Bill, 2005, Eastern Africa Offshore; 25th Annual GCSSEMP Foundation Bob F. Perkins Research Conference, Dec. 4-7, 2005, Houston, TX, USA

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