South Atlantic Margin Rift Basin Asymmetry and Implications for Pre-salt Exploration

摘要

The South Atlantic margins of West Africa and Eastern South America can be divided into three distinct tectonic provinces and opening segments: 1) southern Jurassic (Namib/Orange-Pelotas), 2) central Aptian Salt Basin (Walvis/Kwanza-Santos/Campos to Douala-Pernambuco), and 3) equatorial Albo-Cenomanian strike-slip segment (Anambra/Benoue-Potiguar to Liberia-Amapa.), each with its own distinct tectonic history. This paper will concentrate primarily on the West African Salt Basin, but employs examples from elsewhere in West and East Africa. The South Atlantic margins of West Africa and South America are comprised of a diverse range of conjugate margin pairs and associated failed rifts (aulocogens), each with its own tectonic and stratigraphic expression. Such diversity is caused by a fundamental underlying structural asymmetry, derived from each basin's 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 the configuration of early syn-rift structural architecture, in which the location and polarity of fundamental rift half graben units are significantly shaped by pre-rift structure (anisotropy). Datasets from active rift zones in East Africa as well as a wide range of passive margins around the world provide evidence that while pre-rift anisotropic fabrics may vary in style, trend, and lithology-crustal type; they will reactivate in a number of predictable styles under the active maximum principal stress state of the rift zone. These links of syn-rift asymmetry to precursor anisotropic roots also ultimately determine the shape, size and polarity of the upper plate-lower plate morphology of the OCS margins. OCS margins are likely driven by crustal-lthospheric delamination mechanisms (sensu Wernicke; Lister et al.) until continental break-up. Thereafter, a combination of delamination and sub-crustal pure shear mechanisms are presumed to dominate, and may offer an explanation for the inherent asymmetry of narrow and wide conjugate margin pairs, which are observed along the South Atlantic margins. This mechanism, acting under the influence of inherent asymmetry guided by strong pre-existing strain guides in the crust, varies along strike in any OCS margin. This behavior is typically expressed by alternating Upper Plate-Lower Plate geometries along strike, as well as with former conjugate margin pairs. The thermal and isostatic history of South Atlantic OCS margins is associated with post rift uplift and subsidence, local hotspot activity, and salt thickness variations (non-allochthonous), and can be tied to this fundamental, underlying tectonic 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 resource prices, the industry is now potentially poised at the beginning of a new age of syn-rift exploration in frontier areas.