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
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.
Map label |
Exploration Wells |
Coordinates |
Total Depth (m) |
Oldest strata penetrated |
Year |
Operator |
S1 |
Marai Asha 1 |
4°31΄00.0"N
47°26΄00.0"E |
4,115 |
Middle Jurassic |
1958 |
Sinclair |
S2 |
El Qabobe 1 |
4°14΄48.0"N
47°40΄42.0"E |
4,428 |
Upper Triassic |
1980 |
Arco |
S3 |
Mareg 1 |
3°43΄11.6"N
47°32΄05.4"E |
4,303 |
Middle Jurassic |
1982 |
Esso |
S4 |
Gal Tardho 1 |
3°10΄00.0"N
45°50΄50.0"E |
2,438 |
Jurassic |
1967 |
Sinclair |
S5 |
Duddumai 1 |
2°37΄14.0"N
44°53΄57.0"E |
3,380 |
Jurassic |
1960 |
Sinclair |
S6 |
Warsheikh 1 |
2°14΄00.0"N
45°30΄00.0"E |
4,101 |
? |
1968 |
Sinclair |
S7 |
Afgoye 1 |
2°06΄52.0"N
45°04΄10.0"E |
4,164 |
Jurassic |
1966 |
Sinclair |
S20 |
Afgoye 2 |
2°05΄20.0"N
45°04΄52.0"E |
3,353 |
(?) |
1985 |
Somali Government |
S21 |
Afgoye 3 |
Vicinity of Afgoye 1 & 2 |
4,359 |
(?) |
1985 |
Somali Government |
S8 |
Marka 1 |
1°52΄21.0"N
44°53΄28.0"E |
3,998 |
Upper Cretaceous |
1959 |
Sinclair |
S9 |
Qoryoley 1 |
1°50΄39.0"N
44°33΄16.0"E |
3,518 |
Lower Jurassic (?) |
1961 |
Sinclair |
S10 |
Qoryoley 2 |
1°49΄43.0"N
44°33΄52.0"E |
4,069 |
? |
1965 |
Sinclair |
S11 |
Dhobey 1 |
1°48΄31.0"N
44°31΄29.0"E |
2,122 |
? |
1961 |
Sinclair |
S12 |
Dhobey 2 |
2°42΄44.0"N
44°28΄25.0"E |
3,830 |
? |
1961 |
Sinclair |
S13 |
Brava 1 |
1°04΄00.0"N
43°31΄00.0"E |
3,810 |
Triassic (?) |
1963 |
Sinclair |
S14 |
Lag Bissiq 1 |
0°49΄54.0"N
41°21΄07.0"E |
3,086 |
Tertiary |
1965 |
Gulf |
S15 |
Lag Dhera 1 |
0°29΄48.0"N
41°35΄34.0"E |
2,867 |
Tertiary |
1965 |
Gulf |
S16 |
Jammama 1 |
0°06΄09.0"N
42°49΄13.0"E |
4,126 |
(?) |
1965 |
Sinclair |
S17 |
Halimo Addey 1
(Oddo Alimo) 1 |
0°04΄16.0"N
42°25΄08.0"E |
4,465 |
Tertiary |
1964 |
Sinclair |
S18 |
Obbe 1 |
0°39΄11.0"N
41°31΄07.0"E |
4,865 |
Upper Jurassic |
1982 |
Deutsche Texaco |
S19 |
Kudha 1 |
0°56΄27.5"N
41°53΄00.8"E |
4,972 |
Upper Cretaceous |
1982 |
Deutsche Texaco |
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.
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transect of the Somali continental margin, Proceedings of the Offshore
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421-428.
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potential of the East Africa continental margin (from Somalia to South
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246.
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report; Sinclair Somali Corporation; unpublished, Mogadishu.
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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
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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,
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15. St. John, Bill, 2005, Eastern Africa Offshore; 25th Annual GCSSEMP
Foundation Bob F. Perkins Research Conference, Dec. 4-7, 2005, Houston,
TX, USA
SOMALI VERSION: AKHRI HALKAN
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