East meets West - East Africa as an oil source

Materials World magazine
,
9 Apr 2013

East Africa is becoming a major exploration play for oil and gas companies. Dr Roger Key, Chief Geologist at Kalahari Key Ltd, tells Michael Forrest how the east coast’s geological background has shaped its potential as a source of significant oil and gas reserves.

For many years, East Africa’s complex geology was a disincentive to exploration, which has largely focused on West Africa and its passive margins where offshore exploration and production continues to accelerate. But over the past 15 years, oil and gas finds have encouraged oil companies to review the potential of East Africa, including its offshore sedimentary basins. It was a discovery in 2006 by UK Tullow Oil in the Lake Albert region, on the border between Uganda and the Democratic Republic of the Congo, that changed perceptions of regional exploration. According to the Ugandan Ministry of Energy, Lake Albert is a proven petroleum province, while Tullow has discovered more than one billion barrels of oil in the basin.

And it is not just Uganda that has oil resources. Drilling has proved hydrocarbons in neighbouring Tanzania, Kenya, Ethiopia and Sudan, with major companies carrying out extensive exploration programmes over the region. The China National Offshore Oil Corporation, for instance, recently drilled a 5,000m well in northern Kenya and was rewarded with natural gas flows. Offshore exploration has also proven fruitful, with Anadarko Petroleum Corporation making the largest gas discovery of the decade off Mozambique in the Rovuma Basin. Six of the largest global gas finds of 2012 were discovered here, while other companies have found gas off the Kenyan and Tanzanian coasts.

The complex geology of East Africa is now revealing its potential to host significant oil and gas resources in what was previously regarded as a difficult region for the preservation of hydrocarbon reservoirs. The region has a long history of continent formation and destruction, including ongoing regional rifting and volcanism along the East African Rift system. This contrasts with the passive sedimentary environments of West Africa and the Gulf region, although major sedimentary basins are also found both along and off the East African coast.

Africa: a geological history
Africa once formed a central part of the Gondwana supercontinent that dominated the southern hemisphere. Its creation was completed by endPrecambrian-Ordovician (560–480Ma) orogenesis, involving multiple collisions between different continental plates that created major linear mountain belts. One of the most prominent is the East Africa– Antarctica orogenic belt (EAAOB), which now comprises the crystalline basement underpinning East Africa from Egypt to Mozambique. Uplift and extension of the EAAOB during the late Palaeozoic Era created a number of rift basins that followed pre-existing structural weaknesses in the crystalline basement, including the southern trans-Africa shear system (STASS), which bisected the whole of Gondwana.

These rift basins are infilled by Karoo Supergroup sediments, comprising Carboniferous–early Permian periglacial sediments, Permian coal measures and later Permo-Triassic red beds. Westerly currents recorded in the rift basins of Mozambique and Zimbabwe indicate that the EAAOB mountain belt formed a source region of these sediments, which are locally overlain by Jurassic (182Ma) lavas – predominantly tholeiitic flood basalts. Smaller Triassic to early Jurassic half-grabens discordant to the Karoo rift basins are filled with up to 2.5km of predominantly continental sediments with intercalations of marine sediments, including the N’Gapa and Rio Mecole formations of coastal northern Mozambique.

New marine basins started to develop during the Jurassic Period (about 167Ma onwards) in proto-oceans, as East Gondwana (Antarctica, Australia, India and Madagascar) broke away from the African continent. The sedimentary basins along and off East Africa’s coastline include the Rovuma Basin, which straddles the region around the Tanzania–Mozambique border and contains sedimentary sequences recording the east coast’s development. This begins with sedimentation in a south-facing marine gulf, which was fed by rivers flowing off the rising and eroding continental landmass of Eastern Africa that formed the western side of the gulf.

The Late Cretaceous was typified by a widespread marine transgression in Eastern Africa, with uniform marls and clays (including the Mifume formation) deposited along the length of the continental margin and prograding onto the continental slope. The greatest accumulation of sediments occurred during Cretaceous times, with about 3,000m of sediment recorded in the Pemba formation (155–100Ma), around 500m in the Macomia formation (125– 100Ma) and up to 810m in the Mifume formation (100–65Ma). Subsequent sedimentation rates slowed during early Cenozoic (60Ma) times, coinciding with an order of magnitude reduction in the erosion of Africa’s interior. Shallow-water carbonate sediments (of the Alto Jingone and Quissanga formations) were deposited during the Paleocene and Eocene with a reefal facies along the outer edge of the shelf, while periods of relatively low sea levels led to the formation of low-standing submarine fans. The relative thinness of the Paleocene and Eocene sediments in the Rovuma Basin suggests a reduction of sediment supply from the African mainland, which most likely reflects a relative decrease in regional fault activity.

Delta progradation followed rift initiation in the East African Rift system in 35Ma, which has continued intermittently to the present day with deposition of mostly marine and deltaic sediments (of the Mikindani and possibly Chinda formations). Prior to formation of this rift, uplift of East Africa in the Oligocene (35–25Ma) initiated the Rovuma River Delta system and the associated marine regression. A Miocene transgression led to shallow water marine sedimentation during progradation of the Delta, concurrent with rift-related onshore sedimentation in the East African Rift system. Today, outcrop of the Miocene sediments on northeast Mozambique’s mainland is the result of Cenozoic uplift of much of southern Africa.

Although Africa’s east coast is regarded as a passive margin, it is clear from the preserved sedimentary record in the onshore part of the Rovuma Basin that faulting (mostly with vertical displacement) still has major control over sedimentation. With northern Mozambique’s coastline remaining fault controlled, intermittent movement on north–south trending faults often results in earthquakes in the region.

Great expectations
This background underpins the development of oil and gas resources in East Africa. From the Permian onwards, increasing thicknesses of organic rich rocks (up to 17%) throughout the geological column provide the kerogen required for oil and gas formation. The seal rocks are intraformational shales, while reservoirs are associated with fault-related structures, deepwater fans, turbidites, channels and sandstones. These are typified by Anadarko Petroleum Corp’s discovery of the Atum exploration well in June 2012, which identified 10–30 trillion cubic feet in the Offshore Area 1 block in two Oligocene fan systems. Together with the Golfinho discovery 10km north west of the area, estimates of total discoveries on the block are approaching 100 trillion cubic feet.

Together with increasing understanding of the region’s geological background, such discoveries look set to put East Africa firmly on the oil and gas map.

Geo-glossary
Orogeny Folding/deformation of a section of Earth’s crust via lateral compression, forming a mountain range.
Shear zone Structural discontinuity surface in Earth’s crust and upper mantle, comprising rocks more highly strained than those adjacent to the zone. Can measure a few centimetres or up to several kilometres wide.
Rift Major fault separating blocks of Earth’s surface.
Fault Planar fracture or discontinuity in a rock formation, as a result of earth movement.
Kerogen Fossilised, organic material found in sedimentary rock (including oil shale). Distillation of some types yields hydrocarbons.
Submarine fan (also termed abyssal fan, deepwater fan, underwater delta) Largescale underwater structure characterised by sediment deposition resulting from turbidity currents.
Turbidite A sediment or rock deposited by a turbidity current.
Tholeiitic One of two main magma series in igneous rocks, characterised by reduced, sub-alkaline rocks.
Half-graben Depressed block of land bounded by a fault along one side.
Full graben Depressed block of land bordered by parallel faults.
Marine transgression Flooding due to rising sea levels, often a result of orogenesis.
Reefal facies Underwater body rock with specified characteristics that reflect the conditions under which it was formed (for example, sedimentation or a particular environment).

For further information, contact Dr Roger Key kalaharikey@gmail.com