To date, there have only been 21 wells drilled in Somaliland (19 onshore and two offshore), many of which were only stratigraphic tests. In fact, few of the wells evaluated the hydrocarbon potential of the country and the type of prospects in the drilled basins. In addition, modern seismic reflection surveying has had very limited application in Somaliland.
By M. Y. Ali
Hydrocarbon Potential Of Somaliland
By M.Y. Ali
The Petroleum Institute,
PO Box 2533,
Abu Dhabi, UAE.
E-mail: mali@pi.ac.ae.
Source: First Break, Volume 24, Issue 8, Aug 2006,
DOI: https://doi.org/10.3997/1365-2397.2006020
Published online: 01 Aug 2006
Article Type: Research Article
Introduction
Somaliland is situated on the northern side of the Horn of Africa with the Gulf of Aden to the north, Somalia to the east, Ethiopia to the south and west, and Djibouti to the northwest (Fig.1).
The morphology of the country is typical of areas in extension, with basins and mountains of up to 2000 m. There is little folding, but much normal faulting, some of which has very great throws. These strong vertical movements have controlled the accommodation space available for sediment deposition since the Lower Jurassic.
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To date, there have only been 21 wells drilled in Somaliland (19 onshore and two offshore), many of which were only stratigraphic tests (Fig. 2). In fact, few of the wells evaluated the hydrocarbon potential of the country and the type of prospects in the drilled basins. In addition, modern seismic reflection surveying has had very limited application in Somaliland. Therefore, many prospective petroleum systems in the onshore and offshore regions of the country remain relatively unexplored.
In this paper, seismic, well, and outcrop data have been used to determine the petroleum systems of Somaliland. These data demonstrate that the country has favorable stratigraphy, structure, oil shows, and hydrocarbon source rocks.
In addition, the results show that the Upper Jurassic and Cretaceous units, and possibly Oligocene-Miocene units, show potential for hydrocarbon generation. Traps are provided by rollover anticlines associated with listric growth faults and rotated basement faults which are controlled by Upper Jurassic to Lower Cretaceous tensional stresses.
Stratigraphy and depositional setting
The sedimentary section of Somaliland can be divided into four parts (Fig. 2):
Jurassic rift sequences
The Jurassic deposits in Somaliland consist of a thick sequence of continental deposits (basal sandstone formation) resting directly on the peneplain basement rocks and overlain by a succession of limestones (Bihendula group) generally with some marl and shale intervals. Jurassic sediments were deposited in NW-SE trending grabens (e.g., Bihendula graben) that were formed as a result of tensional forces associated with the rifting of India from Africa.
The Jurassic outcrop at Bihendula, 35 km south of Berbera, was the first to be recognized in the country and has since been the most extensively studied. It is where the greatest thickness (more than 1200 m) of fossiliferous marine Jurassic beds are exposed in the country.
Cretaceous sequences
The Cretaceous sedimentation of Somaliland is characterized by lateral lithologic variability resulting from transgressing seas from the east. As a consequence, thick carbonate and shale sections were deposited in the eastern part of the country and equally thick sequences of sandstones were deposited in the west. The widely used term ‘Nubian sandstone’ has been applied by several workers to describe the entire Cretaceous clastic sequences seen in outcrop (MacFadyen, 1933). However, some authors (Bosellini, 1992) divided the Cretaceous into Yesomma sandstone and Tisje carbonates, although it is difficult to distinguish the different formations due to the transitional nature of the interfingering clastic and carbonate facies.
Eocene sequences
Eocene deposits of Somaliland consist of massive limestones of Auradu and Karkar formations separated by massive to banded gypsum and anhydrites. This is the result of Eocene seas transgressing from the east depositing Auradu limestones. Periodic regression in the Middle Eocene resulted in evaporatic conditions, which led to the deposition of the Anhydrite (Taleh) series. Further transgression in the Upper Eocene caused the deposition of marine cherty limestone of Karkar formation.
Oligocene-Miocene rift sequences
Oligocene and Miocene sediments are mostly restricted to narrow and isolated sub-basins along the coastal belt bordering the Ethiopia to the South, and Somalia to the East. Somaliland has a coastline with the majority lying along the Gulf of Aden, occasionally extending inland in low-lying regions. They deposited in localized grabens caused by the rifting of the Ethiopia to the South, and Somalia to the East. Somaliland has a coastline with the majority lying along the Gulf of Aden. They consist of a thick (up to 2500 m) synrift sequence of red-brown, green sand, silts, and gypsiferous sandstone. These sediments are almost entirely terrigenous deposited in lagoon, delta, and alluvial environments. The best outcrops of Oligocene-Miocene sediments occur in Daban basin (southeast of Berbera) which is a down-faulted rotated block bordering the Somaliland plateau (Fig. 2).
A summary of the stratigraphy of Somaliland is given in Fig. 3. This figure shows the major formations, general thicknesses, and known occurrences of oil shows in these rocks.
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