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Abstract

A regional survey, consisting of long lines averaging 350 km in a dip direction and 2,500 km in a strike direction, consisting of nearly 13,000 km of 2D prestack depth migrated (PSDM) seismic data was acquired by GX Technology over the West African salt basins of Angola, Congo and Gabon in 2004 (Congo-Span). This data provide the first consistent acquisition and processing from shallow water (50 m), past the limit of the salt, and into the ultra-deep water (4,000 m or deeper). These profiles are in depth, providing multiple transects of the continental margin, from rifted continental crust, across the crustal transition zone, and onto the interpreted drift volcanic terrain associated with breakup of the South Atlantic. The CongoSpan regional grid provides adequate coverage for observing changes along both strike and dip of the continental margin, avoiding the pitfalls of single line interpretations, and affords insights into rifting process on both sides of the South Atlantic.

The advantage of the PSDM images is that they provide improved reflector geometry by minimizing distortions associated with lateral velocity variations caused by salt and water. These depth images present a new view of the crust beneath the salt and the drift volcanics. These improved images have been used in identifying a consistent set of tectono-stratigraphic units (pre-rift, syn-rift, sag, and drift), which have been correlated over the study area. The sag is an important unit that developed in response to post syn-rift thermal subsidence forming a large basin (1,200 × 200 km, and 2 km thick) containing fluvial, lacustrine and possible marine sediments.

The base of the sag unit in the deeper parts of the basin has been difficult to image prior to the acquisition of this data, which images to a depth of 25 km. Because the sag unit contains rich source rocks currently in the active oil generation window over much of the area, its delineation is an important constraint on models for the rifting of the South Atlantic. The change in the style of salt tectonics, from extension to compression, appears related to sag-unit geometry and the underlying basement structure. The structural and stratigraphic insights provided by the PSDM data are generating new ideas on the petroleum systems and will be useful for developing the new plays that will continue driving exploration on both sides of the South Atlantic.

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