Evidence for Complex Rift-Related Extension Beneath the Espirito Santo Basin — Petroleum Systems Implications

Scot I. Fraser¹, Rod P. Nourse², Richard Davies³, Mike R. Lentini¹, and Scott Sumner^1
1Cobalt International Energy L.P., Houston, TX
²Shell International E&P, Rijswijk, Netherlands
³Department of Earth Sciences, Durham University Science Labs, Durham, United Kingdom

Two end member rift models have been proposed to explain how continental crust is thinned by extension. Along-strike variability in both syn-rift and post-rift subsidence patterns suggest that thermal stresses experienced by the crust are inconsistent with a single extension model. Whether a simple shear or pure shear mechanism be invoked, there is no unique solution that adequately accounts for all geological and geophysical observations. Interpretation of seismic from the Espirito Santos Basin reveals the presence of a regional intra-crustal reflector. Basement faults do not extend below the continuous seismic event indicating its significance as an important rheological interface. We speculate that this event records the reflection moho or the fossilization of a “palaeo-moho” within the attenuated continental crust. Two basement fault trends constrain the distribution of syn-rift depocentres. The extensional faults in tandem with the mapped subsidence patterns, segment the basin into two thermo-structural provinces. These provinces reflect the brittle crust, lower crust and lithospheric mantle response to Neocomian rifting. The northern basin has evolved via a more uniform, pure shear extension mechanism, whereas the southern basin has formed in a manner more consistent with simple shear deformation. Basin models need to account for alternative rift-related deformation mechanisms and the associated temporal constraints imposed by their contrasting heat-flow anomalies. Pure shear extension implies that the early rift heat-flux is coincident with syn-rift source rock depocentes in contrast to simple shear mechanisms where maximum heat flux is offset. The interpretation of along-strike co-existence of contrasting rift mechanisms has profound implications for presence, effectiveness and maturation of syn-rift source rocks associated with South Atlantic break-up.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery