Abstract

A favorable combination of multiple geological elements in the Lower Cretaceous (Barremian to Aptian), such as organic-rich source rock, porous reservoirs, synrift structures, and very effective evaporite seal, was responsible for forming giant oil accumulations in the pre-salt section of the Santos Basin. The Aptian pre-salt reservoirs, in the Barra Velha Formation (BVF), purpose of this work, consist of layers, which are centimeter-to-decimeter thick of lacustrine carbonates. The sedimentary facies are the products of chemical (e.g., crystal shrubs and spherulites), microbial, and hydrothermal precipitation that commonly appear mixed with reworked grains. Each facies, with greater or lesser presence, depends on the structural framework in which it was deposited. The knowledge of the genesis and geologic history of BVF is essential to understand the formation of the largest deep-water oil reserves in Brazil. The BVF was divided, from base to top, into three cycles: (1) upper-rift, (2) lower-sag, and (3) upper-sag. These cycles make up a second order sequence with flooding-shallowing upward cycles. The association of calcitic spherulites with hydrated talc and stevensite indicates precipitation in an evaporitic-alkaline lake, rich in magnesium and calcium, oversaturated in calcite and with a salinity greater than 3500 ppm but less than 35,000 ppm. The complexity of the facies arrangement in the lake reflects deposition in a proximal environment influenced by evaporation; hydrothermal activity, with complex water chemistry; oscillating groundwater; and frequent lake-level fluctuations.

The initial rifting of the Santos Basin was accompanied by extensive volcanic activity that lasted throughout the whole rifting phase, up to the upper-sag phase, influencing both the geological evolution and paleophysiography as well as the chemical characteristics of the lake system.