--> Abstract: Modern Turbidite System in the Campos Basin: Key to Reservoir Heterogeneities, by L. C. R. Machado, R. O. Kowsmann, W. de Almeida Jr., C. Y. Murakami, S. Schreiner, D. J. Miller, and P. O. V. Piauilino; #90933 (1998).

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Abstract: Modern Turbidite System in the Campos Basin: Key to Reservoir Heterogeneities

Machado, Luis Claudio R.; Renato O. Kowsmann; Waldemar de Almeida Jr.; Celso Y. Murakami; Simone Schreiner; Dennis J. Miller; Pedro O.V. Piauilino - Petrobras/E&P/Cenpes

The Carapebus Fm. contains the main turbidite reservoirs of the Campos Basin. It has been studied on the sea floor surface, where it is still actively depositing, using side scan sonar (SSI-SYS09), 3.5 kHz seismic, swath bathymetry, multichannel seismic and piston core data. Better knowledge of its geometry and heterogeneities would lead to an improved reservoir evaluation and production of its ancient counterparts in this same basin. The Campos Basin has a sandy continental shelf and carbonates at its shelf edge. Its muddy slope is being eroded by several immature canyons that end in giant muddy diamictite tongues (30 x 15 km) downslope, forming a debris apron (Fig. 1). Nowadays, Almirante Camara, an isolated mature canyon, is draining the arcosean sand from the shelf to deep water, forming a turbidite system.

This turbidite system forms inside a trough flanked by listric faults, overlaying a salt province. The dynamic interaction between salt tectonics and sedimentation produces the trough shape. The trough is a bathymetric depression of 80 m, 4 km wide, more than 130 km long, which is already filled with 100 m of sediment up to the basal unconformity. Below the modern trough, conventional multichannel seismic shows that the depositional system repeats itself, with little lateral shifting. Inside the trough, the turbidite sedimentation is dominated by a myriad of sinuous channels that braid each other (Fig. 2), composing a network similar to some seismic amplitude maps of turbidite petroleum reservoirs. The sinuous channels are 40 to 600 m wide and 2 to 40 m deep, having low sinuosity (1.05 to 1.2). This system also resembles subaerial braided rivers, having some braiding, low sinuosity, coarse to medium sand as main composition (two kullenbergs of high density turbidites collected) and high sand/ shale ratio (indicated by indirect tools).

Salt-related faults outcrop at the sea floor, forming sediment walls ten times higher than levees of the turbidite channels. Sedimentary processes, mainly channel avulsion, only control the geometry of the system in topographically flat regions. Inside the turbidite trough longitudinal faults produce a relief of grabens and plateaus at the bottom of the sea. Against a fault scarp (10 m in relief) a major channel (700-1000 m wide and 14-18 m deep) develops, possibly by the conjugation and erosional amplification of normal sinuous channels. At one point of the trough, a large debris flow has invaded it, producing a dam that has retained turbidite flows, smoothing the topography and building a depositional lobe. This lobe, formed internally by a myriad of channels (Fig. 2), is also similar to those seen on amplitude maps of many deep water reservoirs of the Campos Basin.

The analysis of this turbidite system still in formation suggests important types of reservoir heterogeneities in buried turbidite systems.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil