Lithospheric Extension of the Espirito Santo Basin – Petroleum System Implications

Scot I. Fraser, Kristan Reimann, Rod RP Nourse, and Neil Casson
Shell International Exploration & Production, Inc, Houston, TX

Interpretation of regional 2D-seismic from the Espirito Santo basin reveals the existence of a continuous, strong seismic reflector extending some 1.5-5 seconds TWT beneath the rift unconformity. High-angle extensional faults do not extend below the seismic event reflecting its significance as an important intra-crustal discontinuity. Because of its seismic expression, the event is considered to be an important rheological interface at the base of the brittle upper crust. This would mark the transition to a lower ductile continental crust, below a postulated thin viscous layer. In essence, the seismic data has imaged the fossilisation of an important thermal boundary that records a palaeo-solidus within continental basement. The basement response to Neocomian lithospheric stretching has been accommodated by differential boudinage of the upper-crust brittle layer. Crust anisotropy is evidenced by the spatial variation of basin margin fault geometries and associated patterns of post-rift subsidence. Two resultant major rift fault trends have determined the distribution of syn-rift and post-rift depo-centres. The rift-faults segment the basin margin into discrete thermo-structural provinces that reflect basement heterogeneity. The northern Espirito Santos basin has undergone more extensive crustal attenuation, the distribution of which provides the technical foundation for modified heat flow assumptions. The correspondence of volcanic seismo-facies with this region of maximum crustal-thinning suggests that igneous extrusives would dominate the early syn-rift basin-fill. The predictive regional model for b-stretching factors thus constrains the temporal development of potential active petroleum systems. Careful consideration of these observations will impact exploration of the Espirito Santo basin.