Thermal maturation and migration
Maturity levels of the source rocks vary from early to post-mature (Table 1). The Jurassic units (Gahodleh and Daghani shales), and possibly the Eocene units in the offshore areas, which may contain multiple source sequences, are situated within the oil window and are highly prospective for oil and gas. In addition, thermal modelling of the Gulf of Aden has identified a number of offshore kitchen areas that have excellent source potential (Bott et al., 1992). Furthermore, seismic and well data suggest both structural migration and strati-graphic migration are operative in these locations (Fig. 6). However, in general the potential Oligocene-Miocene source rocks are likely to be immature to early-mature for the majority of the onshore areas.
The sedimentary strata of Somaliland contain numerous reservoirs within the pre-syn- and post-rift rocks of the Gulf of Aden. Reservoir rocks for the pre-rift sequence include both carbonates (Eocene and Cretaceous-Jurassic) and sandstones (Cretaceous and basal Jurassic). Reservoir rocks in syn- and post-rift sequences are principally sandstones with secondary carbonate reservoirs.
Two onshore wells, Dagah Shabel-2 and Biyo Dader-1, intersected 191 m and 160 m of sandstones in the Adigrat respectively. Porosity of the sandstones was variable, but as high as 15%. At Dagah Shabel-2 they produced fresh water. The reservoir potential of the Middle-Upper Jurassic deposits of Bihen, Wandere, and Gawan limestones is also good.
In the Upper Cretaceous, good clastic reservoirs have been recognized. Dagah Shabel-1 well intersected very thick (790 m) fine to coarse-graded fluvial sands of the Nubian (Yesomma) formation. The well encountered two highly porous sand units where small quantities (four barrels) of good quality (33.6° API) oil were recovered. In the offshore, the Bandar Harshau-1 well penetrated 536 m of Upper Cretaceous sediments of restricted shallow marine environment that had porosities of up to 14%.
Clastics and carbonates of Eocene-Oligocene age also offer reservoir possibilities in syn- and post-rift traps. In the offshore, the Dab Qua-1 well penetrated a total of 183 m of Auradu limestone that have oil shows. Bandar Harshau-1 well also had minor oil and gas shows.
In Somaliland, there is no evidence of large-scale compressive folding like that of north-eastern Arabia. However, at many localities in the country, minor folds are known to occur which are believed to have been caused either by rejuvenation of old fault blocks or drag along major faults parallel to the Gulf of Aden. Such folds are mainly confined to the Guban region where the intensity of the faulting has been very much greater than in the plateau. One such example is the minor folds recorded in the Dagah Shabel district, close to the Dagah Shabel fault.
Figure 6 Idealized north-south cross section of Biyo Darer area showing the elements of the petroleum systems. The dia gram depicts three potential migration scenarios: (1) Vertical migration from potential Jurassic source rocks into overlying Cretaceous Nubian formation, (2) Lateral migration from Jurassic source rocks into overlying Jurassic reservoirs, (3) Lateral migration from Jurassic source rocks into underlying Lower Jurassic Adigrat formation.
As a result the primary structural traps in the country are rollover anticlines associated with major growth faults that become listric above the pre-rift section. Stratigraphic traps are also important, particularly in the offshore areas where thick successions of Jurassic and Cretaceous sediments were eroded. The seismic profile across the Dagah Shabel and Daban basin, as seen in Fig. 7, provides a sample of the type and density of structures present in the area. The figure illustrates the trapping styles in the area. In addition, the general magnitude of area of uplift of some of the structures might be a guide to possible structural size.
The country has ideal conditions for seal development. Seals are constituted principally by interbedded shales for the Jurassic and Cretaceous sequences, with Eocene anhydrites also forming a regional seal. For the syn-rift sequence of Gulf of Aden Oligocene, anhydrites and interbedded shales are the main seals.
Comparison with Yemen
The geology of Yemen shows many similarities with Somaliland. This can be supported by reconstructing the Arabian plate to the position before the opening of the Gulf of Aden when many basins in southern Yemen seem to extend to Somaliland. For example, the Balhaf graben appears to be a continuation of the Berbera basin, and Masila basin appears to be a continuation of Al Mado basin (Fig. 1). As in the case of the Jurassic basins in Somaliland, drilling within the Balhaf and Masila grabens have indicated the majority of basin infills are Jurassic and Cretaceous in age. Furthermore, significant accumulations of oil and gas have been found in Jurassic grabens in Yemen including the Masila basin. Therefore, exploration interest to date in Somaliland has centred on searching for Jurassic rifts similar to those in Yemen.
Figure 7 Regional seismic profile crossing Dagah Shabel and Daban basin illustrating a graben structure of the Oligocene-Miocene units and the overall structural style of the area.
The available well, seismic, and outcrop data show that the potential for commercial accumulations of hydrocarbons in Somaliland is good. These data show that the hydrocarbons may have accumulated in numerous large tilted fault-blocks and isolated sub-basins. Jurassic rift basins form the main exploration plays. Secondary exploration targets include Oligocene-Miocene rift sequence of the Gulf of Aden together with underlying pre-rift Eocene carbonates. Hence, favourable hydrocarbon plays could exist over many thousands of untested square kilometres along the entire north coast and south-east of the country at various drilling depths. However, much more exploration work and exploratory drilling activities are needed especially in the basins adjacent to the Gulf of Aden to determine whether Oligocene-Miocene successions are mature.
The author wishes to thank Abdirahman Yusuf and the Somaliland Ministry of Minerals and Water Resources for providing the seismic data, evaluation of source rocks, and supporting his visit to Somaliland in July 2005. He also wishes to thank the Petroleum Institute (Abu Dhabi) for supporting this work.
*The Petroleum Institute, PO Box 2533, Abu Dhabi, UAE. E-mail: email@example.com
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Source: First published in first break volume 24, August 2006, © 2006 EAGE
Article layout, text & images formated for Somaliland Times by Rashid M X Noor.
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