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Four-dimensional analysis of the Sembo relay system, offshore Angola: Implications for fault growth in salt-detached settings

¹ Nexen Petroleum UK Ltd., Charter Place, Vine Street, Uxbridge, Middlesex UB8 1JG, United Kingdom; david.dutton15{at}ntlworld.com
² Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado, 80401-1877; btrudgil{at}mines.edu

David Dutton is currently working for Nexen as a geophysicist on the North Sea Core Areas team. Prior to this, he worked extensively on play concepts and prospect generation in a variety of basins from the Norwegian and west African margins. For his Ph.D. studies, David researched the genesis, evolution, and breaching of relay ramps, offshore Angola.

Bruce Trudgill is an associate professor in the Department of Geology and Geological Engineering at the Colorado School of Mines. His research interests are in structural controls on depositional systems and the evolution of geological structures through time. He teaches undergraduate and graduate courses in structural geology, seismic interpretation, and basin evolution.

ABSTRACT

Subsurface mapping of several relay ramps from a raft-related fault array in the lower Congo Basin, offshore Angola, reveals a full spectrum of fault linkage styles. A comprehensive three-dimensional geometric and kinematic appraisal of a more complex relay system within the fault array, the Sembo relay system (SRS) highlights differences with current structural models of relay ramp genesis, evolution, and breaching. The SRS is a rearward (upper ramp) breached relay system with a maximum fault overlap and spacing of 9500 and 3600 m (~31,000 and 11,800 ft), respectively. This system is characterized by a structural geometry that becomes increasingly complex with depth as the relay system is gradually assimilated into the structural architecture of a separate and structurally deeper fault system.

A detailed kinematic appraisal of the SRS indicates that the throw patterns on the frontal and rearward segments are intrinsically different. Throw backstripping reveals different initiation ages for the rearward and frontal segments of about 5.5 and 2.5 Ma, respectively. Mutual overlap is estimated to first occur about 2.5 Ma at the 5.0 Ma structural level, with fault linkage and ramp breaching occurring subsequently.

The SRS therefore represents a complex amalgamation of faults that have each developed independently at different times. The genesis and evolution of the SRS have been governed through time by both salt withdrawal and associated fault detachment histories, in conjunction with increased Congo Fan progradation and sedimentation rates and phases of tectonic tilting of the underlying salt detachment surface.