Geometry of the proximal part of the modern turbidite depositional system of the Carapebus Formation, Campos Basin: a model for reservoir heterogeneities

Abstract

The deep-water marine sedimentary environment of the Cenozoic of the Campos Basin is examined at the modern seafloor, where it can be better understood. This environment is responsible for the genesis of the turbidite systems of the Carapebus Formation, the reservoirs that hold more than 90% of Brazil’s petroleum reserves. The study was developed with the records of regional side-scan sonar, swath bathymetry, sub-bottom profiler data, standard multichannel 3D seismic surveys and piston cores covering almost the entire basin. After leaving the Almirante Câmara Canyon, the turbiditic flows erode the muddy debris apron surrounding the continental slope and begin to deposit thick layers of clean sand into a big trough in water depths of 1800m to 3000m. The trough is 3.5 km wide, 150 m deep, 150 km long, and is formed by a chain of salt withdrawal mini-basins. In some places the seafloor is flat enough to develop today a depositional lobe, in all  aspects analogous to the best, geologically ancient petroleum reservoirs in the basin. Aspects of the system: 1) the arcosean sands are brought by the river, cross the shelf, the incised valley, the canyon, and deposit as turbidites - they do not originate from a collapse of the continental slope; 2) a wide muddy debris apron surrounds the continental slope (slope apron), and represents a huge volume of sediment in the Campos Basin ; 3) the turbidites do not develop a submarine fan, but are deposited in an elongated trough formed by salt tectonics; 4) the turbidite beds, both in the lobe or in the trough, are not deposited during a single episode, but in multiple events over significant geologic time in which small channels which brought the turbidites avulse and meander along the entire depositional area, building a single amalgamated bed; 5) classic channel-levees are not present because this system comprises a sandy braidplain and the levees are as sandy as