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Abstract

The clastic fill of the rifted grabens in the central South Atlantic includes several fields containing over 1 billion barrels of oil in place. The largest fields are mainly hosted by continental fluvial and aeolian sandstones deposited near the base of the rift, before rift topography was fully developed.

Footwall erosion progressively eroded back the main fault scarps, and normal faults propagated through the footwall to create terraces along the rift borders. This increased the potential drainage area on the footwall, so eventually, large alluvial fans developed in the hanging wall along the main rift border faults. The basement clast-supported conglomerates scatter seismic energy; and on seismic data, the top of the fan can appear to be the intact crystalline basement. Even when drilled, the top of the fan can still be mistaken for true basement due to the large size of the clasts. Some important oil fields are hidden below these marginal alluvial fans, within prerift and early rift fluvial and eolian sandstones and in lacustrine turbidite sandstone reservoirs. It is suggested that more fields may be found in West Africa using the model of the sub-fan terrace play.

The large amount of footwall denudation of dense basement rocks can lead to unloading of the adjacent basin, as well as the footwall, when the crust has a finite flexural strength. The resulting uplift and erosion produce an ‘End of Rift’ unconformity, and it is suggested this process is likely to have caused the so called ‘Breakup’ unconformity on continental margins, rather than this being due to initiation of ocean spreading.

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