--> Abstract: Structural Development of the Continental Margin of Congo and Northern Angola, by D. A. Hartman, W. A. Swanson, P. R. Smith, F. J. Goulding, and C. A. Kelly; #90933 (1998).

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Abstract: Structural Development of the Continental Margin of Congo and Northern Angola

Hartman, Donald A.; William A. Swanson; Phillip R. Smith; Frank J. Goulding and Catherine A. Kelly - Exxon Exploration Company

Multiple episodes of salt-related deformation have formed abundant structural traps favorable for exploration offshore Congo and northern Angola. These traps are coupled with the presence of high-quality Tertiary deepwater sandstone reservoirs and an active hydrocarbon source. Recent license and exploration activities offshore Congo and Angola have focused in the highly deformed areas in deep water on the continental slope. The number and size of recent discoveries herald the start of a new play in this part of the West Africa continental margin.

A relatively wide continental margin underlain by rifted and attenuated continental crust is interpreted offshore Angola. Thick synrift sediments were deposited in grabens which locally contain rich, lacustrine source rocks. Thick, widespread salt was deposited during marine incursion and subsequent restriction of the initial basins in the Aptian. The salt was deposited on a relatively gentle surface unlike the previous higher relief, rifted terrain. The local slope of the presalt surface, underlying rigid basement highs and compactable thick synrift sediments played subsequent roles in structural deformation as the continental margin was loaded by younger sediments.

An Albian carbonate platform and margin developed overlying the salt along the eastern shoreline of the South Atlantic. Rapid, late and post-Albian seafloor spreading resulted in subsidence and westward tilting of the continental margin. The tilting initiated regional, gravity-driven extension which was associated downdip with contraction, downdip increase in salt thickness or vertical salt structures. Albian and late Cretaceous sediments filled the lows created by extension and salt withdrawal.

A second stage of gravity-driven extension with associated contraction and salt deformation occurred during the Tertiary uplift of western Africa. This was coupled with extensive middle and late Tertiary loading by depositional systems associated with the ancestral Congo and other local river systems. Sea level fluctuations in the late Tertiary also influenced the distribution and focus of deposition. Thick deposits of middle and late Tertiary sediments are preserved in extensional troughs beneath the present continental shelf.

The Aptian salt distribution and deformation style controlled the major Tertiary structural trends. The combination of vertical and horizontal deformation processes resulted in a relatively complex distribution of Tertiary structures and structural styles. Extensional areas are linked to loading and salt withdrawal on the shelf and upper slope. Contractional trends develop in the lower slope and rise and are often reflected in the water bottom topography. Belts of extensional structures, vertical salt structures, and contractional structures can occur from shallow to deep water. The distribution of structures and structural styles are generally not homogeneous within the belts due to the multiple episodes of salt deformation and the complex interplay between structure and sedimentation.

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