AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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The East Africa Rift System as an Analog for South Atlantic Rifting and Hydrocarbon Occurrence

Abstract

During the last decade, several significant oil and gas discoveries have been made along the South Atlantic Passive Margin offshore West Africa and Brasil. The largest new discoveries and developments have been in the Brasil pre-salt section. The East African Rift system, stretching from the Afar Triangle to Malawi and initiated in the Miocene, is the southern leg of a 3-rift triple junction, with each rift in a different stage of evolution. Together, the system provides a reasonable analogy for the beginning of the rift geometry seen in the Atlantic margin plays. Correlations can be made with the source rock and reservoir sections along both Atlantic margins, which have been excellently imaged with 3D seismic data, and can be tied back to equivalent sections in the petroliferous East African Rift. The basic geometry includes pre-rift and stretched continental crust, overlain by syn-rift, usually lacustrine, facies which are often topped by an angular unconformity (the breakup unconformity). The unconformity is easily seen on properly processed 3D seismic. Often, the syn-rift section has a working petroleum system consisting of lacustrine source rocks and deltaic or carbonate reservoir facies. Elongate structural traps, semi-parallel to shore, can be formed on the rift half horsts. However, these only become visible after correcting for the seaward thickening wedge of water and submarine channels, usually via pre-stack depth migration of the 3D seismic. This is then overlain by the first incursion of marine waters after thermal subsidence, sometimes called the sag sequence, in many places represented by pelagic shales or thick evaporite deposits, which form a seal to the syn-rift petroleum system. Finally, thick seaward dipping drift sequences are deposited as the passive margins move away from each other, with new oceanic crust emplaced seaward. The sag and drift sequences in many places have a petroleum system separate from the syn-rift. This talk will review rifting examples from the East African rift, correlations to the south Atlantic margins, reconstructions of the South Atlantic opening, and field examples for the Atlantic margin petroleum systems within the various rift sequences.