|EAST AFRICAN RIFT SYSTEM REGIONAL GEOLOGICAL INTERPRETATION STUDY, UTILISING EARTH OBSERVATION DATA|
|Exploration Hot Spots|
This paper presents a summary of the results of a geological interpretation study of the entire East Africa Rift System (EARS) based on Earth Observation (EO) data. Incorporating both a structural and stratigraphic interpretation together with lake and offshore oil seeps. The study represents a unique, integrated and consistent interpretation over the entire EARS (Figure 1) at a regional scale of 1:500,000 to aid exploration for energy resources (oil, gas, coal, hydrothermal) and mineral resources. Findings from this study are discussed in relation to the hydrocarbon prospectivity of the area.
Carried out at between 1:100,000 to 1:500,000 scale, the interpretation incorporates territories, from north to south; Eritrea, Djibouti, Somaliland, E Ethiopia, SE border area of Sudan, Uganda, Kenya, Rwanda, Burundi, E border area of Democratic Republic of Congo, Tanzania, Malawi, border area of SE Sudan, Mozambique, and the border zones of Zambia, Swaziland and South Africa, covering an area of approximately four and a half million square kilometres.
The EARS is an elongate system of extensional faults, extending 5000km in a general N-S direction, and is characterised by two rifting trends defined as the Eastern and Western Branches. Several phases of rifting are apparent with significant overprinting between events.
Remotely sensed imagery offers a cost effective method for rapid geological interpretation over extensive onshore areas. Two main remote sensing datasets are used in this study; orthorectified Landsat 7 ETM + mosaics in a 742 (RGB) pseudo-colour composite band combination with a resolution of 15m and the Shuttle Radar Topographic Mission (SRTM) 90m Digital Elevation Model (DEM).
Landsat 7 ETM+ satellite imagery is the standard imagery dataset used for regional scale geological mapping down to 1:50,000 scale, typically in a 742 (RGB) band combination. Utilising the Short Wave InfraRed (SWIR), this 742 band combination offers maximum lithological discrimination. By artificially shading the SRTM elevation model from multiple angles and colour slicing at narrow elevation intervals, subtle structurally controlled landform features can be identified. Interpretation of the satellite and shuttle (EO) data and assessment of other spatially referenced information was completed digitally in the ArcMap environment.
The structural interpretation was carried out at scales ranging from 1:100,000-1:250,000 with the stratigraphic interpretation at 1:250,000-1:500,000 scale. Existing geological mapping was integrated into the study including a USGS Stratigraphic Map of Africa (Persits et al. 1997) and a number of published geological maps and papers.
Additional datasets have also been incorporated into this study including Infoterra’s global oil seeps dataset, which includes oil slicks derived from natural lake bed oil seeps in the lakes of the EARS and surrounding offshore areas. Identified using Envisat and ERS Radar Imagery the slick study includes the slick locations as points, vectors of slick outlines and polygons defining the utilised radar scenes. The offshore areas include the Gulf of Aden, Seychelles, Madagascar, Somalia, Tanzania, Mozambique and Kenya, with over 800 radar scenes assessed and over 500 separate slick points recognised. Published data on recent earthquake activity (1973-2007) and from the location of known mineral resources (USGS) were also incorporated.
Tertiary fluvio-lacustrine deposits within the rift grabens (ENVOI, 2010) and older Karoo Supergroup deposits are the main onshore Petroleum Systems. In Somaliland Similarities are thought to be found with the Petroleum Systems of Southern Yemen (Ali. 2006). Rifting events took place in the Permian to Jurassic, Cretaceous and Tertiary-Quaternary. The older, Karoo grabens represent the Early Permian to Early Jurassic phase of rifting trending NE to NNE (Selous Graben, Luangwa Valley, Mpotepote Basin, Metangula Basin) or E-W (Upper Zambezi, Tete Basin) or occasionally to the NW (NW flank of Lake Nyasa (Malawi), and Kalemie Basin off Lake Tanganyika).
Late Jurassic to Cretaceous rifting is represented by the NW-SE trending Anza Rift, Kenya, a continuation of the Sudanese Rifts such as the Muglad Basin. This Mesozoic rifting event is truncated by N-S trending Tertiary rifting in the Turkana Depression, located between the Afar and Kenya Domes.
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 NE-SW, then NNE-SSW at the Ethiopia-Kenya border before deflecting N-S 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 small number of oil slicks have been identified on Turkana. Talbot et al. (2003) suggest good quality source (Miocene) 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 the SE part of Lake Tanganyika and Lake Nyasa (Malawi), predominantly trending N-S and locally NNW-SSE as it diverts around the Tanzanian Craton near Lake Tanganyika. The extensive lakes in the Western Branch including Lake Albert, Edward and Nyasa (Malawi) and Tanganyika have a significant number of oil seeps identified from Radar imagery and their presence corresponds to Katz (1995) who believed the quality of potential source rocks in some rift lakes is good.
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 East African Rift System (EARS). These mostly trend circa N-S and cut the older, failed NE, E-W and NW 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 The Inhembane area of S Mozambique is transacted for over 300km by Quaternary N-S intersecting narrow (9-11km wide) graben structures which are coaxial to the present-day coast. These structures were mapped, but not discussed in earlier publications (e.g. Tikku et al., 2002). They indicate that the E-W extensional stress vectors causing formation of the EARS are still active in this region.
In comparison to the Eastern Branch, the Western branch displays high levels of sediment accumulation, suitable source, reservoirs and seals and lower levels of volcanism. This conclusion is supported by finds by recent discoveries in the Albertine Graben. However studies of the Turkana Depression region (Tiercelin, 2009) have stressed the hydrocarbon prospectively of the Northern and Central Kenya rifts as they are the offer the oldest and longest-lived sedimentary basins of the Cenozoic EARS and because they represent a crossover area between rifts of Cretaceous and Cenozoic age.
Earth Observation data has allowed a consistent geological remote sensing study to be completed for the entire EARS. Utilising a range of optical, and radar derived datasets the emphasis on remote sensing data has given a unique perspective, allowing both the large regional structures to be identified and smaller geological features to be identified and placed in context. Incorporating a structural and stratigraphic interpretation this 1:100,000-1:500,000 study has identified evidence of the main rifting events; the Permian to Jurassic, Cretaceous and Tertiary-Quaternary which 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.
The extensive lakes in the Western Branch including Lake Albert, Edward and Tanganyika have a significant number of oil seeps identified from Radar imagery and supported by previous observations. 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) indicating that the Western Branch is in general likely to be more prospective.
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Ali, M.Y., 2006. Hydrocarbon Potential of Somaliland. First Break. EAGE. Volume 24.
ENVOI (Energy Venture Opportunities International). 2010 Uganda, Block EA5, Rhino Basin, Albertine Graben, East Africa Rift. Tower Resources. http://envoi.co.uk/P183Tower(Uganda)Syn.pdf.
Katz, B.J., 1995. A survey of rift basin source rocks. In Lambiase, J.J. (Ed) Hydrocarbon Habitat in Rift Basins. Geological Society Special Publication. 80.
Persits, F., Tuttle, M., Charpentier, R, Brownfield, M., and Takahashi, K., 1997. Maps showing geology, oil and gas fields and geological provinces of Africa: U.S. Geological Survey Open-File Report 97-470A, U.S. Geological Survey, Denver, Colorado.
Talbot, M.R., Morley C.K., Tiercelin, J. Le Herisse, A., Potdevin, L. and Le Gall, B., 2004. Hydrocarbon potential of the Meso-Cenozoic Turkana Depression, northern Kenya. II. Source rocks: quality, maturation, depositional environments and structural control. Marine and Petroleum Geology, 21 (1).
Tiercelin, J.J and Thuo, P. Nalpas, T. and Potdevin, J., 2009 Hydrocarbon Prospectivity in Mesozoic and Early Cenozoic Rift Basins in Central/Northern Kenya, Search and Discovery Article 10188, http://www.searchanddiscovery.net/documents/2009/10188tiercelin/images/tiercelin_ppt.pdf.
Tikku, A.A., Marks, K.M. and Kovacs,L.C., 2002. An early Cretaceous extinct spreading centre in the northern Natal valley. Tectonophysics 347 (2002) 87-108.