Eocene slope and basin depositional systems in the Santos Basin

Abstract

The study of deposits originated by sedimentary gravity flows in continental margin basins has systematically underrated the local stratigraphic context. In consequence, some types of terrigenous depositional systems, from both shallow and deep waters, are improperly described as turbidite deposits. In the same way, sand/shale maps have been elaborated for sedimentary gravity flow deposits without considering their depositional envi-ronment. This fact provides incorrect information about reservoir behaviour and seal rock distribution, with possible prejudice to geological interpretation for petroleum exploration and production. Sedimentary gravity flows form the main deposits in the slope-basin of the lower-middle Eocene deposi-tional sequences to the N of the Santos Basin. The objective of this study is to characterize stratigraphically the main elements of the depositional systems that occur in the Eocene section of the study area on the basis of sequence stratigraphy. As a result, two distinct systems of terrigenous deposits formed by sedimentary gravity flows on a single 3rd order depositional sequence are defined:  A Sandy System and a Muddy System. The Sandy System, deposited during periods of sea level fall, was formed by allochthonous sediments of gravity flows originated by fluvial floods. It is characterized by the following depositional elements: fluvial channels, canyons, submarine channels and basin sandy fans. These sandstone reservoirs have generally high porosity and permeability. A Muddy System, deposited under high-stand sea level conditions, and formed by gravity flows caused by autochthonous sediment slides, can be recognized by the following depositional elements: shelf-margin delta, steep erosional surfaces, accretion wedges and chaotic matrix-supported conglomerate beds. Such conglomerate beds have low permeability and form poor quality reservoirs. The hemipelagic sediments of the late transgressive sea level phase, when preserved from the strong erosive action of sedimentary gravity flows, are the only seal rocks for Sandy System oil accumulation.