Tectonic Evolution and Source Rock Distribution Along the East African Margin

Irene Gomez Perez¹, Ricardo Veiga¹, and Fabio Pallottini^2
1RepsolYPF, Madrid, Spain
²Schlumberger, Villahermosa, Mexico

The evolution and petroleum systems of the East African margin are closely linked to polyphase Permian to Early Jurassic Karoo continental rifting which led to the break-up of Gondwana in the Mid Jurassic, and to a subsequent passive margin setting. One of the keys to understand the geology and hydrocarbon systems of East Africa is to study the tectonic setting and distribution of Permo-Triassic and Jurassic rift basins (Karoo Supergroup), as they contain some of the best potential source rocks along this margin, and they influenced sediment distribution in the area both during rift and post-rift stages.

Using tectonic and plate reconstructions as background, the evolution of the East African margin is described from initial Late Paleozoic rifting, which followed Gondwana glaciation, to the Cretaceous, when the opening of the Mozambique Channel and Somali Basin were completed and deposition along the coastal basins was largely controlled by sediment flux and relative sea level changes. Understanding the tectonic patterns of rifting has helped predicting the distribution of these source rocks and potential associated reservoirs.

Source rocks deposited during Karoo rifting include continental coaly shales in the Late Permian-Early Triassic and lacustrine shales in the Mid Triassic. These are expected in interior Karoo basins and coastal basins of northern East Africa. A more marine signature is possible towards the NE. Early Jurassic transitional oil-prone shales are however only expected in coastal basins of northern East Africa, as marine transgression did not progress in interior Karoo rifts, and volcanism was widespread in southern East Africa. Post-rift, marine source rocks deposited in the Late Jurassic and Late Cretaceous may be present along the length of the passive of East African margin, although is quality is variable.

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