Angolan Margin from Up-Dip Extension to Ultra-Deepwater Compression—a Balancing Act

Tim S. Buddin¹, Jeff Milliken², and Kent England^2
1 Midland Valley US Inc, Golden, CO
² Marathon Oil Company, Houston,

Regional cross-sections have been constructed and restored through the Lower Congo Basin of the Angolan margin from the limit of the up-dip extensional regime into the ultra-deepwater. These sections, compiled from regional 2D and 3D depth and time seismic data, show that linkages between extension and down-dip compression in this gravitationally driven system are not straightforward.

The early structure and thickness of the basal detachment (evaporite) layer play a key role in determining the structural style of the system. In particular, evolving salt geometry establishes the means by which the up-dip extension is 'balanced' or absorbed down-dip during gravitational failure. Up-dip extension can be balanced by the inflation of autochthonous salt, squeezing of pre-existing salt-walls and diapirs, emplacement of salt nappes/canopies or by the development of buckle-fold trains, duplexes and imbricate fans.

Identifying the structural styles and their distribution plays a critical role in driving interpretations in the often poorly-imaged sub-salt section. Examples of direct application to reducing risk in exploration include: predicting structural style, assessing salt body thickness, defining weld locations and timings and understanding trap evolution.

Several regional sections will be shown which illustrate the structural linkage across the entire margin as well as the variety of structural styles developed in the deepwater part of this developing exploration trend. Sequential restorations will be shown which re-construct the geologic evolution of the margin and provide a framework for understanding the range of structural styles and timings that can be developed in a complex linked gravitational system involving salt.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005