Implications of Structural Inheritance and Post-Rift Deformation on the Petroleum Plays of the Namibian Passive Margin

There is an increasing recognition that post-rift passive margin architectures are not simply controlled by eustatic fluctuations superimposed upon a simple thermal subsidence phase. This study considers the role of additional mechanisms, such as structural inheritance from rift/break-up configuration and post-rift tectonic deformation, in modifying the tectonostratigraphic evolution and petroleum plays of the Namibian South Atlantic passive margin. The Namibian margin represents an ideal location for such a study because it has a relatively simple structure and it is uncomplicated by sectors of transform margin deformation. Also, the margin lacks the salt deposits found north of the Walvis Ridge where it is difficult to distinguish between gravity-driven salt tectonics and deformation associated with post-rift margin tectonics.

The tectonstratigraphic evolution of the Namibian passive margin is divided into three main phases (Rift, Transition and Post-Rift) which are separated by regional unconformities. The Syn-Rift megasequence is characterised by seaward dipping reflectors (SDRs) and continental and lacustrine deposits within the rift basins. The Post-Rift evolution of the Namibian margin has been divided into two main megasequences (Cretaceous & Tertiary) that are separated by a regional unconformity. The lower part of the Cretaceous megasequence is characterised by a progradiational and aggradational geometry that reflects uniform thermal subsidence of the margin. We present results that demonstrate that this Cretaceous architecture is influenced to a large extent by the geometry of the underlying late Syn-Rift megasequence. The upper part of the Cretaceous megasequence is characterised by erosional truncation which indicates that the margin underwent an uplift event during the Upper Cretaceous that affected the inner and middle parts of the margin. Hence the margin is influenced by post-rift tectonic deformation.

Unravelling the tectonostratigraphic evolution of this extensional margin therefore requires a reappraisal of existing models of margin development. Furthermore, this reappraisal has wide reaching implications for understanding hydrocarbon plays in such settings, including impacts upon the timing and distribution of gravity-driven, coupled toe-thrust and extensional systems, structural controls on the location and migration of base of slope deposition, and the distribution of overburden deposition.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California