--> ABSTRACT: Deep-Water Sands of the Brazilian Offshore Basins

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Deep-Water Sands of the Brazilian Offshore Basins

Mutti, Emiliano 1; Carminatti, Mario 2
(1) University of Parma, Parma, Italy. (2) PETROBRAS-Exploration, RIO DE JANEIRO, Brazil.

During the last three decades Petrobras continuously developed aggressive exploration and production activities in deep and ultra deep waters of the Brazilian Marginal basins. As a result, giant oil fields with original oil reserves of up to 2.7 Bn bbl were discovered (e.g. Marlim, Albacora and Roncador oil fields) leading to a very fast growth of Brazilian proved reserves.. Around 88% of these proved reserves and more than 80% of the 2 million boe daily production come from Cretaceous and Tertiary deep-water sandstones in Campos Basin until recently interpreted as turbiditic depositional systems. In this basin, a continuous search for the best geological model has been carried out in order to optimize the production systems which generated an impressive data base composed of 3D and 4D seismic surveys, well logs, cores and fluids samples. A study of these deep-water sands, based mainly on extensive core analysis, reveals their depositional complexity. Bottom currents played a major role since the late Cretaceous in reworking and redistributing turbidite fine sands derived from basin margins, thus generating mixed turbidite-contourite depositional systems. Fine-grained and current-laminated contourite sands form sizeable, high-quality and prolific reservoirs in many oil fields. As a consequence of this sedimentological constraint to manage reservoir uncertainties, most high-value economical decisions were systematically taken after monitoring extended well tests and/or pilot production projects. Contourite sandstone bodies can be seismically recognized and mapped at the limit of their vertical and lateral resolution to tie each of them to its corresponding source area. Through 3D-seismic amplitude maps it is possible to characterize depositional geometries, to infer facies and to determine bottom current directions and their relationship with the underlying topography. Clearly, this kind of depositional setting has no analogs in exposed ancient deep-water systems dominated by turbidity currents. External and internal geometry and facies distribution patterns of mixed turbidite-contourite systems will require a considerable research effort in future years because of their economic importance. The application of new production technologies to improve recovery factors and reduce the number of producing wells, by enhanced well productivities, is completely dependent on the physical and petrophysical characteristics of sedimentary facies.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.