Seismic-stratigraphic study of the Oligocene-Miocene shelf-fed turbidite systems of the Campos Basin, Brazil.

摘要

The discovery of two giant oil fields (Marlim and Albacora) in the deep-water of the Campos Basin, offshore Brazil, not only represented a tremendous improvement in the country's oil reserves but also resulted in the discovery, in the Oligocene and Miocene sedimentary unit, of a new reservoir facies that had not been tapped before. These reservoirs consist of extremely clean and predominantly massive sands, and have porosity and permeability that range from 25 to 30% and from 1.7 to 5.4 darcies, respectively. They are relatively thick, locally exceed 150 m, and cover an area larger than 6000 km;The development and basic framework of the shelf-fed turbidite system is dependent on (1) the input of sediment into the basin during the build-up of a sand-rich shelfal sedimentary unit, which, in a later stage, constitutes the principal source for the deep-water system; (2) a tectonic pulse (e.g., halokinesis) that significantly modifies the outer-shelf declivity, triggering mass-flows; and (3) the relative sea-level variation, which exposes the shelfal sediments to the action of shallow and high energy marine shelf reworking.;The complete depositional cycle of the shelf-fed turbidite system is composed of progradational, aggradational, and retrogradational phases. These phases result predominantly from changing patterns of subaqueous reworking of the shelfal sediments and their transportation by turbidity currents down the deep-water during a cycle of relative sea-level lowering. Each phase can be overdeveloped or aborted as a consequence of localized relative sea-level modification, which can be induced by tectonism on the shelfal source area.;Two families of submarine canyons were scoured during shelf-fed turbidite system evolution in the Campos Basin. One occupies the outer shelf and the other the continental rise environment. They are separated by a sediment-bypass zone on the slope, across which the turbidity currents travel as unconfined flows. Within this unconfined flow regime, the remaining fines of the flow load can be winnowed away by geostrophic flow stripping, resulting in deposition of relatively clean sandstones on the basin floor.