Abstract

Petroleum potential of the Triassic synrift Silves Formation of the Lusitanian basin, Portugal, was long considered poor because of the perceived overmaturity of prerift Paleozoic source rocks involved in Hercynian deformation. Field sampling of Silurian graptolitic, black shales along the eastern margin of the Lusitanian basin has revealed equivalent Ro maturity levels as low as 1.04% (late oil window), however, indicating that locally, the Silurian had significant generative potential (primarily gas) in post-Hercynian time. Current data do not permit realistic modeling of the first phase of petroleum generation from the Silurian. A second phase of hydrocarbon generation was initiated by reburial during the Mesozoic development of the Lusitanian basin. Oil extracts from transitional dolomite reservoirs at the base of the evaporite seal overlying the Silves Formation have been correlated with a lower Paleozoic source rock based on biomarker analysis. Hence, a subsalt petroleum system (Silurian source, Triassic reservoir, Jurassic seal) has been documented. A depositional model has been constructed to predict fairways for optimal synrift reservoir sand development that can be used to high-grade structural traps defined by seismic data. Traps (fault blocks and wrench-related folds) in place by the Early Cretaceous should have received gas charge from Silurian source rocks that entered the gas window in the Late Jurassic. Additionally, Tertiary regional inversion south of the Nazaré wrench zone may locally juxtapose presently mature Lias source rocks against potential Silves reservoirs in trap geometries along the wrench.

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