--> Abstract: Evolution of Salt-Related Structures and Their Impact on the Post-Salt Petroleum Systems of the Lower Congo Basin, Offshore Angola, by G. Marton, G. Tari, and C. Lehmann; #90933 (1998).

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Abstract: Evolution of Salt-Related Structures and Their Impact on the Post-Salt Petroleum Systems of the Lower Congo Basin, Offshore Angola

Marton, Gyorgy; Gabor Tari; and Christoph Lehmann - Amoco

Three representative cross sections, based on depth-converted 2D seismic reflection profiles, were sequentially restored (using GEOSEC) and analyzed to study the main basin forming processes and the evolution of salt along the westward facing passive margin of Angola, West Africa.

Subsidence analysis suggests a relatively minor crustal extension (beta factor ~1.2) close to the present day shoreline. Seaward increasing thermal subsidence, augmented by sediment load, created the accommodation space for the approximately 5000 m post-rift section in the center part of the basin about 150 km offshore. Here the crust appears to be more extended (beta factor ~2). The beta factor increases to 3.6 in the clearly oceanic domain about 300 km offshore. The oceanic basement to the west is about 1500 m shallower than the depth of the acoustic basement in the central part of the basin. This reversal of the basement slope along the western 100 km segment of the margin results in a "stretched U-shape" configuration for the post-rift surface under the Cretaceous-Recent sedimentary section. On the westward tilted slope, from the present day shoreline to the center of the basin, gravity-driven thin-skinned extensional deformation dominated the evolution of the post-salt succession throughout its history. Small-scale rafting is restricted to the Cretaceous section, while younger large-scale raft structures developed involving Upper Tertiary sediments. This large-scale rafting can be regarded as a response to the increasing sedimentation rates and basement tilt during the Oligo-Miocene period. In the central part of the Angolan margin (from 100 to 200 km offshore), where westward tilting is more subtle, raft tectonics gives way to salt diapirism. In this domain, regional extension is expressed in form of reactive and falling diapirs, although many of the observed diapirs went through different evolutionary stages. Substantial amount of Upper Tertiary sediments were trapped in the intervening salt withdrawal basins. Further outboard, where the reversed basement dip was observed, massive salt dominates the sedimentary section. Most of the massive salt outboard is interpreted to be allochthonous. The extremely thin Upper Tertiary section indicates that the outer salt massive has been thickening since at least Oligo-Miocene times.

Based on the reconstructions we suggest a model in which the original Aptian tabular salt layer (approximately 1.5 km thick) was progressively redistributed. Most of the salt from the rafted domain was squeezed out towards the central diapiric and oceanic domain to the west. In the central diapiric domain salt is largely evacuated from the sub-basins to feed the diapirs, diapiric walls and the outboard salt massive. Detailed seismic facies mapping and attribute analysis revealed a strong influence on the dispersal patterns of Tertiary deep-water sands by synchronously growing adjacent salt-structures. An understanding of this relationship based on systematic 2D/3D kinematic reconstructions appears to be an important element in reducing exploration risk in offshore Angola.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil