--> Abstract: Seismic Sequence Stratigraphy of the Lower Congo, Kwanza, and Benguela Basins, Offshore Angola, Africa, by E. Schollnberger and P. R. Vail; #90923 (1999)

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SCHOLLNBERGER, E., and P.R.VAIL, Rice University, Houston

Abstract: Seismic Sequence Stratigraphy of the Lower Congo, Kwanza, and Benguela Basins, Offshore Angola, Africa

The deposition, preservation, and hydrocarbon charging of sediments depends on many factors including eustasy and tectonics. In the deep waters offshore Angola,West Africa, salt tectonics is a major control on exploration plays. We evaluate the impact of eustasy and salt tectonics on the petroleum system of three basins offshore Angola.

The three basins offshore Angola studied are (from north to south): the Lower Congo, Kwanza, and Benguela basin. Since these basins have varying sedimentary histories, they provide insights to the combined effects of sedimentation and salt tectonics. One variation between the basins is the thickness of sediment above the Aptian salt. The Lower Congo basin has a significantly thicker package of sediment above the salt than the two basins to the south. The amount of sediment load on the salt and the timing of the deposition of the sediments has affected the present distribution of salt.With varying intensities and styles of salt tectonics, the three basins provide an excellent opportunity to investigate the effect of salt movement on the petroleum system.

Using seismic stratigraphic principles, we interpret a regional 2D seismic data set of the basins offshore Angola. Unconformities are traced within and between the basins. Variations in the sequences and their bounding unconformities are examined in relation to salt structures. The relative timing of the deposition of the sequences to the movements of the salt structures is established. Both the impact of the salt movement on the deep-water portion of the sequences and the movement of shallow water sediments to the deep-water setting along the salt detachment are illustrated.

The study of the interplay of sedimentation and salt tectonics provides the following insights for exploration in deep-water offshore Angola. Erosional unconformities created by eustatic changes, salt tectonics, and deep-water currents provide a source of the deep-water sediments. Due to thinned-skinned extension on the salt detachment, rafts containing Albian carbonates moved down the slope. Carbonates formed in a shallow marine environment are in a deep-water setting under Cretaceous to Recent sediments. The extent of deep-water sediment bodies is restricted laterally by salt structures. The lateral restriction can occur during deposition or during later salt withdrawal and diapirism. Salt structures provide traps for hydrocarbons. The movement of salt has changed the migration pathways available to hydrocarbons through time. Hydrocarbons formed by pre-salt source beds may migrate to post-salt sediments through salt windows created by salt withdrawal. Hydrocarbons formed by postsalt sources are restricted in their migration by salt diapirs and other salt structures which could confine them to a minibasin within the larger basin. The interplay of sediment input and salt movement is crucial to understanding the petroleum system.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England