Lithospheric Stretching and South Atlantic Pre-Salt Petroleum Systems - Important Insights from the Espirito Santo Basin

Scot I. Fraser¹, Richard J. Davies², and Rod P. Nourse^3
1Cobalt International Energy L.P., Houston, TX
²Department of Earth Sciences, University of Durham, Durham, United Kingdom
³Shell International Exploration and Production, Rijswijk, Netherlands

Depth dependent lithospheric stretching may account for the observed asymmetry in passive margin subsidence patterns. This introduces significant complexity when describing the maturation of important Lower Cretaceous source rocks. Petroleum systems models need to account for alternative rift-related deformation mechanisms and the associated temporal constraints imposed by contrasting heat-flow anomalies during basin evolution. Two basement fault trends dominate and determine the distribution of syn-rift depocentres and facies. The intreplay of fault trends and contrasting lithospheric stretching mechanisms segment the basin into two thermo-structural provinces that emphsise crustal and lithospheric mantle heterogeneity. End-member rift models have been proposed to explain the mechanism by which continental crust is thinned by extension. There is no unique model that adequately accounts for all geological and geophysical observations along passive margins, however along-strike coexistence of these models may better reflect South Atlantic rift processes. The northern Espirito Santo Basin has extended via a more uniform, pure shear extension model, whereas the southern Espirito Santo Basin has formed in a manner more consistent with simple shear deformation styles. The interpreted presence of rift-related volcanics within the northern Espirito Santo Basin syn-rift sequence reflects the contrasting rift-related mechanisms. The correspondence of volcanic seismo-facies within the region of maximum “necking” of the whole crust suggests that igneous extrusives would dominate the pre-salt facies preserved within the northerly early syn-rift basin fill. The interpretation of along-strike co-existence of contrasting rift mechanisms has profound implications for presence, effectiveness and maturation of pre-salt source rocks associated with the South Atlantic passive margin.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas