East African Rift Valley

Introduction
East African Rift Valley is one of the most extensive rifts on the Earth’s surface; the valley is about 6,400 kilometers long, averages 48 to 64 kilometers wide and has a depth from a few hundred to several thousand meters. The huge, brittle tectonic plates that make up Earth’s crust normally move only a few centimeters per year, not fast enough to be noticeable in a human lifetime. However, in the East African Rift Valley, this tectonic motion is happening with remarkable speed. The East African Rift valley is an example of an active divergent rift, one of the few areas on Earth where a continent is being actively separated (rifted) by the ongoing forces of plate tectonics.

East Africa Rift System (EARS)
The East African rift system is widely recognized as the classical example of a continental rift system, which is part of the Afro Arabian rift system that extends from the Red Sea to Mozambique in the south. The rift is assumed to mark the incipient plate boundary between the Somali and Nubian micro-plates and linked to the Afar- Red Sea- Gulf of Aden rift systems (Figure 1). As the rift extends from the Ethiopian segment southwards it splits at about 5°N into the Eastern and Western branches. The two branches of the rift skirts around the Tanzania craton and formed within the Late Proterozoic belts adjacent to the margins of the craton. However, the Eastern Branch that comprises the Afar, Ethiopian, Turkana and Kenya rifts is older and relatively more volcanically active than the western branch that comprises Albert, Kivu, Tanganyika, Rukwa and Malawi rifts. The SW branch comprises

Luangwa-Kariba-Okavango rifts.

FIGURE 1: Structural map showing the East
African Rift System
(Modified from Atekwana et al., 2004)

The rift valleys are a system of normal faults bordering a 40-60 km wide trough, funneling out toward north in the Afar region. The Kenya Rift diverges into splays towards north (Turkana) and south Tanzania (Figure 2 and 3) (Baker et al). Domal uplift and extension causes the brittle crust to fracture into a series of normal faults giving the classic horst and graben structure of rift valleys. They are generally interpreted to be listric. Listric faults may be open in their upper part, and the gap is then filled with sedimentary breccias. Geologists today still debate why and how rifting comes about, but East Africa displays this process extremely clearly.

Subterranean forces broke the earth’s crust apart and large chunks of the crust sunk between parallel fault lines and forced up molten rock in volcanic eruptions. Many boiling hot springs along the rift prove that volcanic activity is extremely high along the rift system. The geothermal areas of the East-African Rift System are not all of the same type. Even though related to the rift the geological settings are different. Some are volcanic, others are not. Parts of the rift zones are highly volcanic but large segments of them are sediment or lake filled grabens.

FIGURE 2: Fault pattern in northern Kenya
(From: Baker, et al., 1972)

FIGURE 3: Fault pattern in southern Kenya and northern Tanzania (From: Baker, et al., 1972)

The East African Rift is one of the most important zones of the world where the heat energy of the interior of the earth escapes to the surface in the form of volcanic eruptions, earthquakes and the upward transport of heat by hot springs and natural vapor emanations (fumaroles).

In Ethiopia Exploration work peaked during the early to mid-1980’s when exploration drilling was carried out at the Aluto-Langano geothermal field (Lakes District). Eight deep exploratory wells were drilled to a maximum depth of about 2,500 m, and four were found to be productive with a maximum geothermal reservoir temperature of about 350ºC. During the early 1990’s exploration drilling was also carried out at Tendaho (Northern Afar). Three deep (2,100 m) and three shallow wells (500 m) confirmed the existence of a high temperature (270ºC) reservoir.

FIGURE 6: Closest analogue to Lake Albert in EARS in terms of age and structural (development of rift)

Tertiary fluvial-lacustrine deposits within the rift grabens and older Karoo Supergroup deposits are the main onshore Petroleum Systems. The Karoo Supergroup has many potential reservoir horizons of coarse sandstones (but volcanic material found within the sandstones may limit their viability as reservoir rocks) and contains potential source rocks, but lacks regional seals in places. Jurassic shales are the main source rocks in the Berbera in Somaliland) 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.

High TOC shale’s have been identified beneath a number of small East African Lakes such as Tanganyika, Baringo and Albert. Oil seeps and present day anoxic conditions in Lakes Tanganyika and Malawi are allowing the formation of source rock, and showing the possibility of mature source rock development at depth in these and other East African Rift Lakes. Our current state of knowledge is thus, that the Loperot and Albert Basins have proven subsurface source rocks, while the Tanganyika and Malawi Basins have proven current source rock deposition. Highest heat flow values are located along the rift valley in Kenya (Figure 7), and they are originally from magmatic convection in the crust, volcanism and hydrothermalism at the surface.

FIGURE 7: Kenya heat flow vs. latitude

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(Figure 8) below shows Lokichar-Turkana basin, which is situated in the heart of the dusty and arid Turkana basin (desert) in the Rift Valley and (figure 9) show its development through ages.

FIGURE 8: Lokichar-Turkana basin

FIGURE 9: Lokichar-Turkana basin development

And (figure 10) shows Cratonic rift basin, as a potential petroleum systems, which represented the very first stage of basin formation in an extensive stress regime will follow rifts with sag, and later passive continental
margins.

FIGURE 10: Cratonic rift basin

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Conclusion: Potential petroleum systems within rifts:

Rift basin may contain significant amount of type III lacustrine source rocks, rapid subsidence and high heat flow may speed up early maturation of these source rocks. Oil or gas can be trapped in tilted blocs. But, the large amount of volcanic rocks may seriously reduce the quality of clastic reservoirs, mature source rocks may have a limited extend and active faults bounding tilted blocks may be efficient conduits for HC escape to the surface. However, East African Rift Valley has a significant role and importance in the petroleum play. Oil found so far in such basins like Lake Albert (Mobutu)(Figure 11), Uganda which Has multiple oil bearing reservoirs in stacked channel system and three wells drilled there and all are oil producers (light, waxy crude) and the test results on them shows excellent reservoir

FIGURE 11: Lake Albert

permeability and good oil flows with 100-300 mmbbls oil in place.

References:
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EAST AFRICAN RIFT SYSTEM – AN OVERVIEW
Kristján Saemundsson, ISOR – Iceland GeoSurvey
THE GEOTHERMAL ACTIVITY OF THE EAST AFRICAN RIFT
Peter A. Omenda, Kenya Electricity Generating Company Ltd. (KenGen) The East African rift system
Jean Chorowicz
EAST-AFRICAN RIFT SYSTEM GEOLOGICAL SETTINGS OF GEOTHERMAL RESOURCES AND
THEIR PROSPECTS Kristján Saemundsson, ISOR – Iceland GeoSurvey
OVERVIEW OF GEOTHERMAL RESOURCE UTILIZATION AND POTENTIAL IN THE EAST AFRICAN RIFT SYSTEM Meseret Teklemariam, Geological Survey of Ethiopia Addis Ababa The East African Rift System – The Contribution of Earth Observation to Hydrocarbon Exploration Michael Hall – Astrium Geo-Information Services

Petroleum Systems in East African Rift Lakes: Past and Present C.C. Saunders1
1Colin Saunders, Neftex Petroleum Consultants Ltd. Abingdon OX14 4RY

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The East African Rift System – A View from Space
Michael Hall and John Diggens; Astrium Geo-Information Services

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Presentation The East African Rift, Alain Mascle, IFP School

10- Presentation THE PROGRESSIVE DEVELOPMENT OF THE EAST AFRICAN RIFT SYSTEM AND ITS IMPACT ON PETROLEUM SYSTEMS
Duncan Macgregor
11- WENDY MILLER-EAST AFRICAN RIFT VALLEY PROJECT-GEOLOGY 306-WINTER, 2009 12- China Daily 08/03/2005 page2
13- http://en.wikipedia.org/wiki/Lokichar
14- 15- https://suite101.com/a/dramatic-geologic-activity-in-east-africa-a207013 16- http://disc.sci.gsfc.nasa.gov/education-and-outreach/additional/science-focus/ocean-color/Ethiopia.shtml