Deconvolution of post- and pre-salt petroleum sources in southeastern offshore Brazilian basins

Abstract

The Campos, Santos and Espírito Santo Basins are part of the southeastern offshore Brazilian basins (SEOBB) where pre-salt reserves are located. This represents the most important oil province in Brazil. Constant growth in the discoveries and production of the country’s oil reserves has shown the importance of the pre-salt discoveries; thus, proper oil constituent characterization at the molecular level is needed for this type of petroleum. Six oils, collected in pre- and post-salt reservoirs from SEOBB, were evaluated using GC-FID, GC–MS, GC-C-IRMS, GC × GC-TOFMS, and ESI(±)-Orbitrap HRMS. The biomarker parameters obtained indicated a lacustrine input for all the oils, except for the TAB-4G post-salt oil. All the oils reached the peak of the oil generation window, except the TAB-4G oil, which reached the late stage. The carbon isotope profiles suggested a lacustrine depositional environment for the organic matter (OM) with δ13C isotopic amplitudes ranging from 3.2‰ to 6.9‰. The profile of the TAB-4G oil suggested a contribution from terrestrial OM. This work is one of the first to apply ESI(±)-Orbitrap HRMS for geochemical characterization. In both (±) ionization modes, the data showed that the oils had high percentages of Nx classes, especially N1 compounds, strengthening the interpretation of a lacustrine origin for the OM. ESI(-) HRMS showed that the TAB-4G oil contained a high relative percentage of Ox classes, especially O1 with 8–16 double bond equivalents (DBE) species, suggesting a marine contribution. The combination of the conventional geochemical approach with advanced chromatographic and mass spectrometric techniques allowed for a comprehensive molecular characterization of the post- and pre-salt oils, contributing to a better understanding of the prolific SEOBB petroleum systems.