Source rock potential, main depocenters, and CO2 occurrence in the pre-salt section of Santos Basin, southeast Brazil

Abstract

The absence of expressive hydrocarbon columns and high CO2 content observed in the pre-salt of Santos Basin reservoirs over the last decade reveal the risk of hydrocarbon charge and contamination to be a relevant concern in exploration activities in the basin. Therefore, this study was focused on elucidation of the source potential, basin depocenters, and CO2 occurrence in the pre-salt of the Santos Basin, aimed at reducing risk of upcoming exploration activities. Based on geochemical analysis, our study revealed that Late Barremian-Early Aptian fine-grained rock intervals, rich in organic matter, from the Itapema Formation and secondarily, from the Piçarras Formation, are likely the main source rocks of the Santos Basin. Seismic interpretation revealed five wide pre-salt depocenters. The Main Depocenter, followed by the Central and North depocenters, could be the major hydrocarbon sources for pre-salt reservoirs. Based on the CO2 distribution data, low amounts of the contaminant (under 5%) were detected in the post-salt and proximal pre-salt reservoirs, with intermediate values (from 10 to 50%) in the main pre-salt accumulations. High CO2 amounts (over 50%) were encountered mainly in a northeast to southwest trend, linking Peroba-Júpiter-Libra areas, and could be related to a deep-seated fault system associated with mantle contamination.Accordingly, the northern and central parts of the Central Santos sub-basin have a low risk of hydrocarbon charge and contamination, whereas its eastern portion has a high risk because of the absence of an expressive hydrocarbon column and elevated CO2 content. The Southwest Santos sub-basin presented a high risk for pre-salt discoveries because of salt windows which limit the top seal for pre-salt-sourced hydrocarbons. In the Southeast Santos sub-basin, the risk of hydrocarbon charge and contamination is significant because of the absence of mapped depocenters and its location on a crustal domain favorable to mantle CO2 access.