East Africa, overlooked in terms of its hydrocarbon potential for many years, is increasingly viewed as an exploration hot spot, with recent discoveries in the Albertine Rift and offshore Tanzania leading to a resurgence in interest. Dominated by the East Africa Rift System (EARS), the region has a complex geological history and provides the potential to bring together modern techniques to aid geological understanding and to help efficiently target hydrocarbon exploration.
One technique particularly suited to frontier exploration is the use of medium resolution Earth Observation data in the form of satellite imagery and Digital Elevation Models (DEMs). These geo-information datasets can contribute in two main ways. Firstly, optical satellite imagery and DEMs can provide information on surface structure, geomorphology and stratigraphy, enabling a consistent regional interpretation of the surface geology to be undertaken. Secondly, radar imagery can assist in the identification of natural oil seeps that have been reported both offshore and on many of the rift lakes such as Albert, Tanganyika, and Nyasa (Malawi).
Astrium has recently completed an ambitious project to interpret the complete EARS, an area of approximately four and a half million square kilometres, at a scale of between 1:100,000 to 1:500,000. The project incorporates a broad range of territories including, from north to south; Eritrea, Djibouti, Somaliland, Eastern Ethiopia, south-east Sudan, Uganda, Kenya, Rwanda, Burundi, eastern border area of Democratic Republic of Congo, Tanzania, Malawi, Mozambique, and the border zones of Zambia, Swaziland and South Africa.
Active Continental Rift Zone
The EARS is an active continental rift zone with an elongate morphology, extending approximately 5,000km from the triple plate junction of the Afar Triangle in Ethiopia to the Inhambane region in Mozambique. Dominated by extensional faulting, the area is characterised by two main rifting trends, defined as the Eastern and Western Branches, with several phases of superimposed rifting having occurred.
When studied in detail, the full complexity of the rift system becomes clear. As well as the main rift structure, smaller grabens are also apparent, running parallel or branching from the main rift feature. The orientations of the rift basin and grabens are controlled by the tectonic regimes the region has experienced since the Late Permian. By far the most significant control on the current rift morphology is the Tertiary to Quaternary rifting, manifested in the general north-south orientation of the rift structure. This dominant trend truncates structures formed during previous rifting events, namely the Late Permian to Early Jurassic ‘Karoo’ events and the Cretaceous event. Karoo rift basins are concentrated in the south of the EARS, predominantly in Mozambique and Tanzania, and include the north-north-east trending Luangwa rift and the Selous graben, trending north-east to south-west.
The East Africa Rift lakes form striking elongate features on the satellite imagery, with well-reported surface oil seeps on many of the larger water bodies. Many of the smaller lakes are also thought to have oil seeps. Radar imagery was used to screen for the presence of natural oil seeps and to try and establish the spatial pattern of oil seepage between individual lakes.
Main Rifting Events
Rifting events took place in the Permian to Jurassic, Cretaceous and Tertiary-Quaternary periods. The older Karoo grabens represent the Early Permian to Early Jurassic phase of rifting trending north-east to north-north-east (Selous Graben, Luangwa Valley, Mpotepote Basin, Metangula Basin) or east-west (Upper Zambezi, Tete Basin), or occasionally north-west (north-west flank of Lake Malawi/Nyasa, the Kalemie Basin off Lake Tanganyika). Late Jurassic to Cretaceous rifting is represented by the north-west to south-east trending Anza Rift in Kenya, and a continuation of the Sudanese Rifts such as the Muglad Basin. North-south trending Tertiary rifting in the Turkana Depression, located between the Afar and Kenya Domes, truncates this Mesozoic rifting event.
The Eastern Branch of the Rift runs from the Afar Triangle triple junction in Ethiopia and Eritrea to the South Tanzania Divergence where the rift meets the Tanzania Craton. In the north, the eastern rift trends north-east to south-west, then north-north-east to south-south-west at the Ethiopia-Kenya border, before deflecting north-south at Lake Baringo. A series of small rifts branch off from the main structure at Lake Turkana, before terminating directly north in southern Ethiopia. A number of oil slicks have been identified on Lake Turkana and there have also been reports of good quality source and reservoir rocks in the adjacent Lokichar and Kerio Basins.
The Western Rift stretches from the border between Uganda and Sudan in the north, to the major rifts of Lake Tanganyika and Lake Nyasa (Malawi) - predominantly trending north-south except where it diverts around the Tanzanian Craton near Lake Tanganyika. The extensive lakes in the Western Branch, including Lakes Albert, Edward, Nyasa (Malawi) and Tanganyika, have a significant number of oil seeps identified from Radar imagery, with the potential for associated good quality source rocks.
In the southern section of the Western Rift, the surface expressions of geological features are dominated by the Tertiary-Quaternary fracture sets associated with the EARS. These mostly trend approximately north-south and cut the older, failed north-east, east-west and north-west trending rifts of the Permo-Triassic Karoo grabens, as well as the multi-temporal, multi-directional tectonic elements of the Precambrian Basement. The intracratonic basins containing the Karoo Supergroup are distinct features on the remote sensing imagery, and can be accurately defined. Further south, the Inhembane area of Southern Mozambique is transacted for over 300 km by Quaternary north-south intersecting narrow graben structures, 9-11 km wide, that are coaxial to the present-day coast and indicate that the extensional forces which formed the system are still active in this region.
Tertiary fluvio-lacustrine deposits within the rift grabens and older Karoo Supergroup deposits are the main onshore Petroleum Systems. In Somaliland, similarities are also thought to be found with the Petroleum Systems of Southern Yemen.
In the northern section of the study area, contrasting characteristics of the Eastern and Western Branches of the rift have been observed. The Western Branch, initiating in the Albertine Graben in northern Kenya, displays a high level of seismic activity, has less active volcanism and generally a greater thickness of sediment in comparison to the Eastern Branch, excluding the rifts of the Turkana Depression. These factors are likely to have a corresponding influence on prospectivity, supported by recent discoveries in the Albertine Graben. However, previous studies of the Turkana Depression region have also emphasized hydrocarbon opportunities in the Northern and Central Kenya Rifts of the Eastern Branch. These are thought to be the oldest and longest-lived sedimentary basins of the Tertiary-Quaternary EARS because they represent an overlap area with the Cretaceous rifts.
The oil seeps identified by this study - in the rift lakes of Lake Tanganyika, Edward and Nyasa (Malawi) - may indicate the presence of a similar petroleum play involving the Tertiary sections, as discovered recently at Lake Albert. Additionally, for the lakes in the south, there is the potential for the seeps to be derived from older Karoo sediments common in the southern part of the EARS. Recent exploration on the Karoo basins such as the Ruhuhu and Upper Zambezi Grabens has revealed large reserves of Gondwana coals within the lower part of the Karoo Supergroup. These may yield commercial amounts of coal-bed methane as well as reasonable quality coals for future exploitation. Other prospective regions include the Ogaden Basin in Eastern Ethiopia. This is an area of proven hydrocarbon reserves with large gas discoveries and frequent oil shows from the Mesozoic.
Similarities exist between the Petroleum Systems in Somaliland and the proven hydrocarbon regions of Yemen, with the Balhaf Graben in Yemen thought to be a continuation of the Berbera Basin in Somaliland. Jurassic shales are the main source rocks in this region and the satellite imagery has identified numerous roll-over anticlines, closely associated with listric fault growth, that are likely to be most significant structural traps.
Incorporating a structural and stratigraphic interpretation this 1:100,000-1:500,000 study has identified evidence of the main rifting events that have defined its current morphology. There is considerable potential to add further detail to the study using higher resolution satellite imagery over specific sub areas.