Hydrocarbon potential of the East Africa continental margin

East Africa Regional Hydrocarbon Study Area, which covered an area of more than 10 million cu km, and involved eight countries.

• Prospective Phanerozoic sedimentary basins in East Africa. Many of the basins have more than 10 km of sediments. [21,783 bytes]
• Phanerozoic Stratigraphy of East Africa showing the correlation and generalized ages of mega-sequences. Marker Horizon 6 marks the approximate end of the Jurassic rifting phase. [37,976 bytes]
• Chart [5,229 bytes]

This paper is the product of a collaborative East Africa Regional Hydrocarbon Study (EARHS) project conducted by Alconsult International of Calgary, Alberta, Canada, (Alconsult) on behalf of the Canadian International Development Agency (CIDA) and the National Oil Corporations of eight countries in East Africa. Alconsult, a geoscience, engineering and environmental consulting firm, was contracted to plan, organize and implement the study.

The EARHS Project area covers the East Africa continental margin and includes a wide onshore belt that extends from Somalia

and Ethiopia, through Kenya, Tanzania, Mozambique, and northeastern South Africa. It also encompasses the islands of Madagascar, Mauritius and the Seychelles, and the adjacent offshore portions of the Western Indian Ocean. The Central Africa Rift System is excluded.

The project reviewed existing information, performed additional analyses where required, integrated and interpreted all pertinent data and prepared individual country-specific reports and a regional synthesis. The country-specific reports were delivered to the respective national oil corporations in mid-1997.

The main objective of the EARHS was to assess the petroleum potential in the sedimentary basins of each of the eight participating countries, with the general aim of encouraging further exploration in the region.

The participating countries allowed the EARHS project full access to their archives, which contained special studies, operators' reports, consultants' reports, well files, seismic lines, potential field reports, geochemical data and rock samples. Well logs from more than 200 wells, and 15% of the available 300,000 km of seismic data, were used for the study.

The project incorporated prior studies by BEICIP in Ethiopia (1985) and Madagascar (1988), NOCK's reports (1993, 1995) on the Lamu Basin of Kenya and ENH's comprehensive work in Mozambique (1985). Robertson Research's (1986) report on the "Hydrocarbon Potential of East Africa" provided extensive geochemical information, and these data were incorporated in the confidential country-specific reports.

A regional synthesis was developed from the individual country reports and presents an up-to-date compilation and interpretation of the hydrocarbon potential of the continental margin of East Africa. Companies or consortiums interested in more detailed technical information are strongly encouraged to access the EARHS country-specific reports and other pertinent data held by the national oil corporations and Alconsult (under agreement with the host countries). The regional report can be ordered from Alconsult for the cost of reproduction.

Historical seismic and drilling activities for the region are recognize a number of exploration cycles terminating in an overall period of relatively little activity during the 1990s. Less than 50 wells have been drilled offshore, which is generally less than 100 km wide to the 200-meter isobath.

Oil seeps, discoveries

Oil seeps have been recorded in the region as follows:

• In the Blue Nile Gorge and Ganale River in Ethiopia
• In a Jurassic outcrop near Tarbaj in Kenya
• On Pemba Island, and at Wingayonqo and Mnazi Bay in coastal Tanzania
• At Nhangela Lake in Mozam bique.

Live oil and gas shows have been recorded in 44 wells in the study area. Nine significant oil and/or gas accumulations have been discovered in this 10 million sq km area during the past 50 years. About 250 new-field wildcats have been drilled and approximately 300,000 km of reflection seismic has been acquired since 1950. Discoveries in excess of 1 Tcf have been made at Calub (gas and condensate), Afgoi (gas), Songo Songo (gas), Mnazi Bay (gas), Pande, Buzi and Temane (gas). Significant resources of tar and heavy oil occur at Bemolanga and Tsimororo, repsectively, in Madagascar.

Rift valleys, Gondwana

For the most part, the Phaner ozoic sedimentary basins of the East African continental margin are the products of diachronous rift tectonics and thermal subsidence during several phases since the early Permian:

• Rift Phases (Permian, Triassic and Early Jurassic, and Early Cretaceous): During this period, predominantly continental sediments were deposited at relatively high sedimentation rates in rift valleys. In Madagascar, there is compelling evidence for N-S rifts of Permo-Triassic age; they are distinct from the younger pull-apart Jurassic rifts with a NE-SW orientation.
• Drift Phases (Mid Jurassic-Cretaceous/ Tertiary): During this period, oceanic crust formed between the physically separated parts of the Gondwana super-continent. Sediments were predominantly marine.

Stratigraphic framework

The tectonic evolution of Phanerozoic Basins in East Africa is reflected in the stratigraphic record where mega-sequences can be recognized and correlated regionally. The following mega-sequences apply (see page 112):

Permian/Triassic, L. Jurassic

This sequence is dominated by arkosic sandstones with reservoir potential. Evaporites occur immediately below the top of the sequence in the Majunga and Morondava Basins (Madagascar), the Mandawa Salt Basin (Tanzania) Salt diapirs occur offshore Majunga Basin, in Kenya, and offshore Somalia. In Ethiopia, the Middle Hamanlei Formation is characterised by the presence of gypsum. In the Mozambique Basin and in northeastern South Africa the sequence is made up exclusively of volcanics (Drakensberg - Lebombo - Letaba basalts and rhyolites) and Karoo sediments.

The Calub and Adigrat formations are proved gas and condensate productive reservoirs capable of commercial production rates in the Ogaden Basin in the Calub Field. In the Morondava Basin the Isalo Formation harbors the huge resources of tar sands (Bemolanga) and heavy oil (Tsimiroro). The Bokh Shale Formation is the source for the gas and condensates in the Calub Gas Field (Ogaden Basin, Ethiopia). The Middle Sakamena Formation in Madagascar is regarded as the source for the tar sands (Bemolanga) and heavy oil (Tsimiroro). The Mikumi Formation in Tanzania is organic rich. The Maji ya Chumvi Formation in Kenya is organic rich.

Palynological analyses conducted by EARHS on these formations in Madagascar, Tanzania, and Ethiopia favor an age close to the Permo-Triassic boundary. Source-rocks of outstanding quality are associated with the evaporites immediately below the top of Sequence 6 in the Majunga Basin, where immature bituminous shales are in outcrop. Geochemical analyses of samples from this outcrop by EARHS confirm that it is a world class source-rock. A potential source-rock interval correlative to Sequence 6 would appear to be present in the basins of the Seychelles.

Middle-Upper Jurassic

Sequence 8 is made up largely of carbonates in the Seychelles, Ethiopia, Somalia, Madagascar, Kenya, and Tanzania. Shales are widespread but subordinate, marine, and reflect a regional flooding event in the Late Jurassic. As a whole, the sequence indicates the onset of a relatively stable marine shelf system with a link to the Tethyan ocean. This interval has not been identified in northeastern South Africa, and is represented by continental sediments in the Nhamura-1 well in Mozambique.

The limestones of the sequence are commonly tight, with occasional development of porosity in dolomitized sections. Porosity is pervasive in oolitic limestones, which are present as a rule, rather than as an exception, in the Seychelles, Madagascar, Tanzania, Kenya, Somalia and Ethiopia. Porous reefal carbonates in the limestones below the Upper Jurassic Shales are also possible.

The following zones are of potential interest as source-rocks for oil and gas:

• Shales of Kimmeridgian-Portlandian age are present over much of the area in, and to the north of the Rovuma Basin in northern Mozambique. They are a prime source-rock in the Ogaden Basin, and have moderate total organic carbons (TOC) in the Seychelles.
• The distal argillaceous facies coeval with the Bemaraha Limestone is an excellent source-rock in the Morondava Basin, and is likely to have a counterpart in those countries where the limestone is developed (e.g. Seychelles).

The hydrocarbon potential in southern Mozambique and northeast South Africa would be enhanced if this sequence, and particularly the marine facies, were present. The only Jurassic intersection in southern Mozambique occurs in the Nhamura-1 well. The Jurassic marine sediments encountered in the Uitenhage, Pletmos, and Bredasdorp basins in South Africa may well indicate such a possibility. These occurrences may have Tethyan or Pacific faunal and floral affinities.

Cretaceous

This sequence is dominated by continental siliciclastics (Anza Basin, Kenya), marine limestones (Seychelles, Ethiopia, Somalia, Lamu Basin in Kenya) and/or by marine shales in the distal, seaward direction (Seychelles, Kenya, Tanzania, Mozambique, Madagascar). Volcanics are common in this sequence in the Seychelles, Madagascar, Ethiopia and Somalia.

Significant quantities of gas have been discovered in the sandstones of this sequence as follows:

• In the Upper Cretaceous Grudja Formation in the Pande Gas Field, and at Buzi and Temane in Mozambique
• In the Coniacian of the Morondava Basin in Madagascar at Manambolo
• In the Lower Cretaceous reservoir of the Songo Songo Gas Field in Tanzania.

The sandstones of Sequence 12 are the most consistent of all intervals in terms of oil and gas shows in East Africa. The source potential of a number of different intervals of the sequence is well documented from the widespread occurrences of oil and gas in the sequence. Anoxic events are thought to be developed regionally near the level of Marker 10 (top Lower Cretaceous).

In the Anza Basin organic-rich shales of the Cretaceous occur in a NW-trending syn-sedimentary rift, which has genetic similarities to the Cretaceous rifts in Sudan.

Moderate values of TOC have been recorded from Sequence 12 in the Seychelles.

Tertiary - Paleogene

Sequence 13 is made up of marine carbonates and shales (Mauritius, Seychelles, Madagascar, Ethiopia, Mozambique) and sand-shale in deltaic depocentres (Kenya, Tanzania, Somalia, Mozambique). A lateral facies change from shallow water carbonates to more deeply deposited carbonates to marine shales has been documented in Madagascar, Kenya and Mozambique.

Porous limestones and dolomites are potential reservoir targets. Shows of oil and gas have been recorded in sandstones of this sequence. There is a pervasive interval of organic-rich shale in this interval in the Lamu Basin (Kenya, Somalia and Tanzania). The shale is immature in the Seychelles.

Undrilled thrust complexes have been observed in this sequence offshore Somalia, Kenya and Tanzania above a decollement horizon close to the Cretaceous-Tertiary boundary. These structures are similar in geometry and style to those on the African west coast, where they have generated considerable industry interest in deep water during the past few years.

Tertiary - Neogene

This sequence is made up mostly of marine limestones (Seychelles, Madagascar, Kenya, Northern Tanzania), often showing facies changes towards shales in a seaward direction. Sands and shales make up most of the interval in South Africa and southern Tanzania. Volcanics of Neogene age are common in Kenya. The sequence is not represented in Ethiopia.

Porous limestones are potential exploration targets, although sands are the more likely reservoirs where they are present. Potentially attractive organic facies are present in Kenya and Tanzania. This mega-sequence is not adequately sampled and may contain potential source beds in other areas. Maturity may be a critical risk where burial diagenesis is inadequate.

Late Tertiary-Quaternary

This sequence consists of sand in the upper reaches of the Anza Basin of Kenya, is dominately shale in the regions' coastal basins and is composed of bank carbonates in the Seychelles and Mauritius. The interval is considered to have little hydrocarbon potential in the EARHS project area.

Hydrocarbon occurrences such as natural seeps, shows of live oil and gas, and discoveries have been recorded in seven of the eight countries in the study area. Coastal basins of the East Africa continental margin containing significant hydrocarbon discoveries extend offshore into the western Indian Ocean.

The discoveries of gas and condensate in the Permo-Triassic sediments at the Calub Gas Field (Ethiopia), the Lower Cretaceous in the Songo Songo Gas Field (Tanzania), the Upper Cretaceous in the Pande-Temane-Buzi Gas Fields (Mozambique) testify to the presence of gas accumulations in various parts of the stratigraphic column, and of resources in excess of 28 billion cu meters (1 tcf) per field.

The resource of heavy oil in the Tsimiroro Field in Madagascar may exceed 27 million cu meters (170 million bbl) original oil in place, and the tar sand resource at Bemolanga may exceed 36 million tons. These accumulations occur in the Permo-Triassic succession.

Live oil and gas shows have been recorded in 44 wells in the study area. Hydrocarbon shows in the Cretaceous and Lower Tertiary sediments are more common than in other parts of the succession, but this may merely reflect a higher incidence of drilling penetrations of the shallower part of the stratigraphic column. The hydrocarbon habitat was sculptured by two main phases of rifting, which is conducive to the presence of structural traps

An early marine flooding event recorded is manifest as the marine limestones of the Vohitolia Formation in Madagascar, and is time equivalent to the Whitehill Formation in South Africa and the Irati Shales of Brazil; a later flooding event is close to the Permo-Triassic boundary and is present as the Middle Sakamena Shales in Madagascar, the Maji ya Chumvi Formation in Kenya, the Mikumi Formation in Tanzania, and the Bokh Shale in Ethiopia.

It is present in outcrop in the Majunga Basin. Geochemical work conducted by the Geological Survey of Canada, as part of the EARHS, proved that this argillaceous unit sourced the deposits of tar and heavy oil in the Morondava Basin, and the gas and condensates in the Calub Gas Field in Ethiopia. Recognition of these flooding events offers considerable scope for regionally developed source-rock systems in the East Africa continental margin.

The rift sequences are dominated by arenaceous sediments, and are proven to contain productive reservoirs in the Calub Gas Field. However, the clastics in the rift sequence often have a tendency to being arkosic and clay-rich.

The EARHS has identified large thrust-faulted complexes in the late Cretaceous-Early Tertiary succession, akin to those on the African west coast, where they have generated significant industry interest during the past few years. The thrust faulted areas are linked to growth faults in a shoreward direction, and a decollement horizon near the top of the Cretaceous (Tanzania), and near the top of the Mid-Jurassic salt, which is close to the base of the Tertiary (Kenya and Somalia).

Towards the end of the rift period, restricted marine conditions resulted in the deposition of Liassic evaporites in Madagascar, Tanzania, Kenya, Ethiopia and Somalia. Geochemical work by the EARHS identified an outstanding regional source-rock in association with these evaporites in Madagascar and Tanzania.

Mega-sequence 12 (Cretaceous) would appear to be the most persistent in the study area in terms of shows, reservoirs and indigenous source systems, and its prospectivity hinges to a large extent on the presence and effectiveness of traps. Rich oil-prone source-rocks occur in the Sakamena Formation (Madagascar), and the Bokh Shale (Ethiopia); the Tsimiroro heavy oil and Bemolanga Tar Sands are sourced by the Sakamena Shales, and the gas and condensate at Calub by the Bokh Shale. These source-rocks have potential for regional distribution. Another excellent source-rock interval has been identified in Tanzania and Madagascar in association with the evaporites at the top of Sequence 6 (Toarcian-Aalenian).

Excellent oil source-rocks have been identified in Sequence 8 in Madagascar, Tanzania and Ethiopia. These source-rocks are of Late Jurassic age and have Tethyan affinities. Sequence 10 and 12 (Cretaceous) have good oil source potential in Kenya, Madagascar and appear to have regional distribution. Sequence 13 (Lower Tertiary) is a potential source for oil in the Lamu Basin (marine) and the Anza Basin (lacustrine).

References are available from the authors or Alconsult.

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