Introduction

Somaliland (Northern Somalia) 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 north-west (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.

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

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.

Eocene sequences