Print this pageSouth Atlantic Margin Rift Basin Asymmetry and Implications for Pre-Salt Exploration
The South Atlantic margins of West Africa and South America are comprised of a diverse range of conjugate margin pairs and associated failed rifts (aulocagens), each with its own tectonic and stratigraphic expression. Such diversity is caused by a fundamental underlying structural asymmetry, derived from each basins’ distinct rift, continental margin break-up, and drift phase history. The results are evident in the current day configuration of the Aptian Salt Basin’s outer continental shelf (OCS) margin, where both wide and narrow margin segments can be observed. Wide OCS margins can be reconstructed with their narrow Afro-Brazilian conjugate pair. This observation is also confirmed by examination of the regional geologic, seismic, gravity-magnetic, and other geoscience data.
The asymmetry of these successful rift-to-drift continental margin basins can traced to their early architectural configuration, where the location and polarity of fundamental rift half-graben units are significantly shaped by pre-rift structure (anisotropy). Such inherited asymmetry, when reactivated, also ultimately determines the shape, size and polarity of the upper plate-lower plate morphology of the OCS margins, which are likely driven by crustal-lthospheric delamination mechanisms (sensu Wernicke; Lister et al.) until continental break-up. The thermal and isostatic history associated with post rift uplift or subsidence, along with variable hotspot activity and salt thickness variations (non-allochtonous), can also be tied to this asymmetry.
The maturation, migration, and entrapment of hydrocarbons from or within syn-rift petroleum systems in the Aptian Salt Basin are likely determined by the asymmetry inherent in any particular basin’s early history. Structural and stratigraphic syn-rift play fairways arising from a basin’s syn-rift and post-rift structural and stratigraphic development can also be tied to a margin’s asymmetrical roots. With the advent of deep water to ultra-deepwater drilling capabilities and higher product prices, the industry is now potentially poised at the beginning of a new age of syn-rift exploration in new frontier areas.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil