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.

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 favourable 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 over lain 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.

Figure 2 Simplified geological map of Somaliland (modified from Abbate et al., 1993) showing positions of oil wells drilled and major sedimentary basins in the country Legend of wells are: (1) Zeila-I, (2) Zeila-2, (3) Zeila-3, (4) Zeila-4, (5) Heemal-l (6) Berbera-1, (7) Dagah Shabel-1, (8) Dagah Shabel-2, (9) Dagah Shabel-3, (10) Biyo Dader-1, (11) Las Dureh-I (12) Bur Dab-I, (13) Yaguri-I, (14) Las Anod-1, (15) Burhisso-1, (16) Nogal-1 (17) Faro Hills-I, (18) Hedad-1, (19) Buran-1 (20) Dab Qua-I, and (21) Bandar Harshau-1. Also shown are concessions held by various oil companies in the early I990s and the location of the seismic profile in Fig. 7.

The lack of success in the early exploration wells drilled in Somaliland is primarily due to the complexity of the subsurface geology, and the lack of subsurface control by seismic data. Somaliland is a region of relatively abrupt lateral changes of stratigraphy that is related to the rifting and differential vertical movement during Mesozoic and Tertiary times (Ali, 2005). From post-drill analysis of the failed wells, the majority were as a result of poorly defined trap, thin, or absent reservoir, or due to the seal being either thin or absent. In addition, several of the wells were almost certainly stratigraphic tests. These include Zaila-1, Zaila-2, Las Dureh-1, and Dagah Shabel-3.

Source rock potential

The country contains several good quality source rocks which have potential for hydrocarbon generation (Table 1).

Jurassic

Numerous excellent quality source rocks of Jurassic age are known in outcrops along the coastal margin, including Bihendula area. Gahodleh and Daghani shales are the most important source rocks in the area (Fig. 4). Field investigations in Bihendula area (during July 2005) demonstrated that the Jurassic formations extend approximately 10 km along an east-west striking sequence of limestone ridges and shale valleys that dip generally south at about 17° (Fig. 5). The shales are dark to medium gray kerogen-rich fossiliferous claystones that have fine texture (Fig. 4). It is likely, therefore, that these shales have played an important part in the generation of hydrocarbon in the area. The 28 barrels of 32.2° API oil recovered from the Wanderer limestone in the Dagah Shabel-1 well supports this suggestion. Offshore wells have also indicated good source rocks of Jurassic age. For example, Dab Qua-1 well intersected shales of Daghani formation that had TOCs in the range of 0.53-1.18%.

The results of the hydrocarbon source potential evaluation are summarized in Table 1. As indicated in the table, the composition of the kerogen in samples of Gahodleh and Daghani shales is primarily amorphous (Type I), which are algal remains that have greatest oil source potential.