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Global Exploration Team

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Mega-tectonic, Structural& Palaeogeographic Framework

of East Africa

East Africa Exploration Team,

Shell International E & P


Presentation outline:

• Methodology.

• Structural & tectonic evolution of East Africa.

• Chronostratigraphic framework.

• Palaeogeographic evolution.

• Conclusions.


Multiple orders of magnitude:

Stratigraphic framework

Palaeogeography

Petroleum system

Mega-tectonic & structural framework


Regional Structural Frame (Free air gravity map)

• Gulf of Aden

• Owen Ridge

• Somali Basin

• Mascarene Basin

• Carlsberg Ridge

• Seychelles & Mascarene Ridge

• Lamu Embayment

• Davie Fracture Zone


PliocenePliocene Early EoceneEarly Eocene Late CretaceousLate Cretaceous

Early CretaceousEarly Cretaceous Early JurassicEarly Jurassic Major Tectonic events


Rifting of Gondwana& sea floor spreading in SomaliBasin

Rifting of Antarticafrom India

Ridge jump toMascarene Basin

Ridge jump toCarlsberg Ridge

Opening ofGulf of Aden

Mad. Som.

Jura

ssic

Terti

ary

Tr.

Early

Late

Middle

Cre

tace

ous

Early

Late

Paleo

Eoc

Oligo

Mio

Plio

Late

Major Tectonic events Ken.Tanz.

500 km

East African margin regional chronostratigraphic strike sectionsouth north

Observations:

Variety of reservoir regimes- Carbonate- Siliciclastic (deltaic & turbiditic)

Source rock intervals-Early to Middle Jurassic-Late Cretaceous in Tanzania

Seals- Regional & intra-formational


East Africa region generalised chronostratigraphic dip section

Reservoir units along the basin margin pinch out distally into basinal mudstones.


Chronostratigraphic sections show the main reservoir, source rock & seal time intervals for which palaeogeographic maps have been created


Early Jurassic

• Rift phase.

• Semi-restricted basins.

• Sporadic connection withthe Tethys.

• Restricted marine sourcerock deposited in the depocentres.

Early Jurassic–Toarcian


restricting transform fault

Middle Jurassic- Bajocian Middle Jurassic

• Early drift phase.

• Oceanic crust creation

• Still semi-restricted.

• Oceanic transforms mayhave acted as barriersto ocean circulation,

• Continued marine source rock deposition.


Early Cretaceous

• Spreading centre switchfrom Somali toMascarene basin.

• Drift between Madagascar & India.

• General siliciclasticstarvation

• Carbonate platform developed in north Somalia

Early Cretaceous–Albian


Late Cretaceous

• Source rock deposition along off-shore Tanzania.

• Delta progradation

• Deposition of turbidites

• Carbonate platform in northern Somalia

Late Cretaceous–Campanian


Paleocene

• Spreading centre switch from Mascerene Basin to the Carlsberg Ridge.

• Deltaic deposition along or near the present day margin.

• Turbidites deposited into deeper water.

• Regional seal developed in offshore north Somalia

Tertiary–Paleocene


Early Eocene

• Limited deltaic depositionresulting in siliciclasticstarvation.

• Carbonate deposition in north Somalia & MajungaBasin.

• Regional seal development.

Tertiary–Eocene

Palaeogeographyoutlines different playsbased on reservoir presence:- Persistent reservoir machine

through time- Carbonate machine- Sand machine

• Northern Somalia (KK Carbonates)

•Northern Coriole Basin(KK/TT Clastics)

•Lamu Embayment(KK Clastics)

•Pemba Zanzibar Dw(TT Clastics)

•Majunga(KK Clastics)

•Morondava(KK Clastics)

•Seychelles(JJ/KK/TT Clastics/carbonates)


Conclusions (i)

• Palaeogeographic environments are intrinsically linked to the Mesozoic breakup of Gondwana & the evolution of the Indian Ocean region.

• Multiple deformation phases have affected the region.

• A number of distinct terrane boundaries acted as drainage and sediment conduits into the larger basins.


Conclusions (ii)

• Drainage systems were long lived.

• Sediments were deposited in a range of environments (fluvial - deltaic - turbiditic).

• Northern Somalia was siliciclastic starved & dominated by carbonate deposition.

• Regional source rock intervals were confined to restricted marine environments during rift & early drift stage.

• Numerous intraformational & regional seals occur.

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