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Contourites and Turbidites of the Brazilian Marginal Basins


Abstract For the last three decades Petrobras continuously developed aggressive exploration and production activity in deep and ultra deep waters of the Brazilian marginal basins. As a result, giant oil fields were discovered (e.g. Marlim, Albacora and Roncador oil fields) leading to a very fast growth of the Brazilian proved reserves. The main daily production comes from Cretaceous and Tertiary deep-water sandstones in Campos Basin until recently interpreted as turbidite depositional systems. The search for the best geological model to optimize production systems has generated over time an impressive data base of 3D and 4D seismic surveys, well logs, cores and fluid samples. Extensive core analysis of these sands, integrated with seismic data, reveals their depositional complexity. The purpose of this paper is to show that bottom currents played a fundamental role in reworking and redistributing turbidite sands. Marginally-derived coarse-grained and thick-bedded turbidite sands were trapped in delta-fed slope channels, structurally-controlled depressions (intraslope basins) and channel-terminus lobate features. Fine-grained sediment transported in suspension by turbidity currents was almost entirely removed by bottom currents during and immediately after deposition and transported northward, away from their turbidite feeder systems, to form spectacular fields of fine-grained and very well-sorted contourite low-relief sand drifts, characterized by very distinctive facies in cores, which are highly prolific oil reservoirs (e.g. Barracuda, Marlim sul, Albacora leste). Amplitude maps show both erosional and depositional features associated with varying degrees of bottom-current current intensity in space and time, resulting in channels, sand ribbons, barchan dunes and sandwaves. Clearly, this kind of depositional settings has no analogs in deep-water sedimentation dominated by turbidity currents, i.e. in classic deep-sea fan models with channels, levees, and lobe (frontal splay) elements. External geometry and facies distribution patterns of sandy contourite systems will require a considerable research effort in future years due to the proven economic importance of these reservoirs. The application of new production technologies to improve recovery factors and reduce the number of producing wells by enhancing well productivity is completely dependent on the physical and petrophysical characteristics of sedimentary facies.