Somalia has been sparsely explored and several exploration plays with world-class potential remain to be tested. Just prior to the onset of the civil war in December of 1990, several concessions were held by major international petroleum companies and at least three key wells were scheduled to be drilled. In addition giant oil companies became keen and interested on different sedimentary basins of Somalia. These basins include:
1. The Somali Coastal Basin, from Adale to Ras Kiyamboni (joins the Lamu Embayment at Oddo Alimo or Halima Addey, south of Kismayo)
2. The Luuq-Mandhera Basin
3. The Somali Embayment or Galmudug Basin
4. The Nugal Uplift Basin
5. Borama –Bihindula Basin or the Northern Coastal Basin
6. The Aal-Madow /Daroor and Sagaleh Basin
Geographical location of the Somali Embayment
The Somali Embayment covers the regions of South Mudug and Galgaduud and extends west across the border into the Ogaden Basin. In terms of coastal towns, it is from Gara’ad to Mareeg on the Indian Ocean coast. To the east, the continental shelf ranges from 25 – 50 km in width offshore Galmudug, and it is terminated seaward by a shelf/slope break. The continental slope is steeper than normal, and the continental rise is mature. Beyond the rise is the margin of the Western Somali Basin that forms the seafloor for western part of the Indian Ocean.
Exploration history
In the Mudugh Basin (or the Somali Embayment) of the Central Regions of Gal-Mudug, exploration geologists regard it as one of the highly prospective sedimentary basins of Somalia. Knowing that, in 1987 Shell (Pectin) signed a production sharing agreement with the then Ministry of Water and Mineral Resources of Somalia. Shell got a concession covering the whole of the continental shelf from Mudug through Galgaduud through Middle Shabelle to South of Mogadishu in Lower Shabelle region. This runs from the north-east, at a point just south of Conoco's Block 34, down to the south of Mogadishu. The concession consists of five offshore blocks, from north to south, M-5, M-4, M-3, M-6 which ends off Mogadishu, and M-7. Agip, Arco, Sinclair, and others drilled and explored the area.
Interesting gas and condensate discoveries were made near Mogadishu before the civil war broke out. Western geologists had indicated since the 1950s that the continental shelf could be an area of high prospectivity. This region is believed to be potentially rich in hydrocarbons.
Further to the South, in the Somali Coastal Basin, Amoco has the three equally interesting Blocks 6, 9 and 12 onshore to the west of Mogadishu. These areas are in the Afgoi region and are said to contain gas and condensates. In one area 30 km west of Mogadishu it was said in 1984 that about 200 billion cubic feet of gas had been found. The coastal Block 6, the southernmost of the three, is said to have good oil possibilities. If more investment, exploration and development are carried out, the region could earn petro-revenue which is needed for development in all its aspects. Furthermore, deforestation and charcoal production could stop in a country where there are scarse resources for wood and domestic energy.
Geological Setting and stratigraphy
The first geological studies on the stratigraphic sequence of Central Somalia were carried out by G. Stephanini and were published from 1920 – 1938., with a rich series of works in “Paleontograhia Italica”. A. Silvestri soon followed by studying the Formaminifera in 1938 – 1948. Then in 1948 -1958 A. Azzaroli studied the stratigraphy in relation to the fauna fossils in the sediments. However, the best information comes from the well logs during the drilling of this area and its neighbourhood, specifically in the Coastal Basin. Among other important documents are those of Agip Mineraria – Agip Somalia; 1957- 1977; P. Lyons, A. Bennison and Barnes, S.U of Sinclair Somali Corporation (1960; 1976); Rompetrol (1975), the publication of the Geological Survey Team of the People’s Republic of China (1972), S.U. Barnes (1976), Beltrandi and Pyre (1973), and most recently by Michael J. Brady (1989; 2002); Mike Coffin (1988 – 2002); Du Toit, S.R. and Slind, OL (Alconsult International) and Bill St. John (2005) on offshore East Africa. Alconsult completed the East Africa Hydrocarbon Study (EARHS) funded by the Canadian International Development Agency [CIDA] in the late 1990's and through data storage agreements with the participating countries maintained detailed, country-specific reports and additional seismic and well data for 8 of the original 9 countries of EARHS. The report covered eight countries of East Africa and the Western Indian Ocean.
The geology of the basin
Precambrian granite and metamorphic rocks dominate the basement rocks of Northern Somalia and southern “Bur” basements. Then the present sea-margins of Somalia began developing in Permo-Carboniferous Time as rift and pull-apart basins formed, and these basins evolved intermittently over 150 million years until seafloor spreading commenced in the Late Jurassic. A salient aspect of the basin’s break-up stratigraphy is salt in isolated grabens and presumed salt diapirs offshore Southern Somalia.
At the initiation of seafloor spreading between West Gondwana [Africa] and East Gondwana [Madagascar, Seychelles, Greater India, Australia and Antarctica] at about 165 Ma, sediment facies changed throughout the basins from dominantly continental to marine. Volcanism and normal faulting occurred at the same time. Thermal subsidence and mechanical (sediment) loading dominated margin evolution following margin breakup, and seafloor spreading ceased in the Western Somali Basin in Neocomian /Aptian Time. Vigorous ocean currents along the East African margin probably commenced in Mid-cretaceous Time, and widespread regional volcanism occurred in the late Cretaceous.
By the close of the Middle Jurassic, oceanic crust separated Eastern Africa from Madagascar-Seychelles, and the respective shorelines began to subside, leading to Middle Jurassic-Early Cretaceous marine transgression. The Middle Cretaceous was a period of alternating transgression and regression phases. Late Cretaceous-Early Cretaceous transgression followed. The Early Oligocene was a quiet period of gentle regression marked by the absence of Oligocene sediments in some areas. Late Oligocene-Miocene transgression with accompanying tectonic movement followed. Subsequent regression established the present-day coastlines.
Pre-Jurassic
No pre-Jurassic sedimentary rock has been reported from outcrop in the Somali Coastal basin. However, the Brava 1 Well 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 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 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 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 fore-reef limestone and medium-depth neritic shale make up the Cotton Formation, that is entirely of Early Cretaceous age (dated by foraminifera). At Marai Asha 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, 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, 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 Embayment Basin. In the Marai Asha Well, 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, 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 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. 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 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: 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 Embayment Basin.
Prospectivity
The Mudug contains very thick Jurassic to Tertiary sedimentary succession that reaches more than 5 km in the deeper parts and includes non-marine to deep marine clastics, shallow-to-deep marine carbonates and evaporates. This sedimentary succession has no proven petroleum potential. Inland, the Jurassic to Tertiary section consists primarily of inter-bedded platform limestones and shales, locally with evaporites. This section is transitional eastward to deeper water shelf and basinal deposits along the coastal margin, on the continental shelf. The Hobyo 1 and El-Qabobe 1 both drilled up to Hamanlei Formation to depths of
5,253 m and 4,426 m respectively and did not reach the basement.
Conclusion
Mature, oil-prone source beds combined with potential reservoir rocks and structures occur in a variety of geological settings. Viable exploration plays include rift basins, regional arches, carbonate platform margins, and faulted basin margins. Galmudug basin is under-explored, and 5 wells in 60 years of “exploration” is noting to mention, and the basin needs exploration and investment. The region is considered highly prospective and relatively unexplored and untested.
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
Abdulkadir Abiikar
The Petroleum (Oil and gas) Potential of the Central Regions
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This General Forum is for general discussions from daily chitchat to more serious discussions among Somalinet Forums members. Please do not use it as your Personal Message center (PM). If you want to contact a particular person or a group of people, please use the PM feature. If you want to contact the moderators, pls PM them. If you insist leaving a public message for the mods or other members, it will be deleted.
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