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Where Should We Drill in the Deep Waters of the Pelotas Basin, Southern Brazil and Uruguay?

Abstract

The Pelotas Basin is a very large strictly offshore marginal basin straddling Southern Brazil and Uruguay. It comprises one of the best examples of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The Pelotas Basin is underlain by an extremely deformed continental crust (highly stretched/thinned/faulted), covered by SDR-filled grabens that can achieve thicknesses of more than 20 km. The width of the hyper-extended crust varies from slightly less than 100 km in Uruguay up to 350 km close to the boundary with the Santos Basin. Covering the Barremian-Aptian volcanic syn-rift formations (SDR) lies a typical post-rift sag basin formed during the thermal subsidence phase. This late Aptian to Recent Drift Sequence may attain thicknesses of up to 8 km of absolutely undisturbed sub-horizontal beds. The sag geometry of the Drift Sequence is attained by maximum subsidence upon the volcanic rifts and gradual thinning of the strata towards the hingeline in the continental shelf and towards the oceanic crust. The essentially volcanic composition of the Rift Sequence and the dominant shale-sandstone composition of the Drift Sequence drive the focus of the exploration towards the thickest parts of the thermal sag basin, in the deep and ultra-deep waters. Rich and mature marine source rocks (Cretaceous anoxic shales) have already been proved in the Late Aptian, Albian and Turonian. A secondary deltaic/marine Paleocene source rock can also be considered. Thick packages of reservoir rocks under the form of turbidite sandstones have recently been proved in Uruguay. The absolutely unfaulted and undisturbed nature of the drift strata and their upward thinning towards the continent and the ocean point to essentially stratigraphic traps as the targets of future exploratory wells. Furthermore, the absence of visible migration routes of tectonic nature clearly indicates that the submarine fans to be tested by ultra-deep wells should be the ones in direct contact with the source rocks, that is, preferably the Albian-Turonian fans, Campanian-Maastrichtian at most. This exploratory model is analogous to the successful model applied in the deep Ghanian and Ivorian waters in the Equatorial Atlantic. The search should be focused towards packages of rocks displaying classical seismic-facies of submarine fans and channels, pinching updip towards both margins of the sag basin. Bright spots should not be expected because of the deep burial of the targets.