Molecular forensic geochemistry applied to spilled oils from Arraial do Cabo (RJ) and produced oils from the Campos Basin

The Campos Basin is currently the second largest oil producer in Brazil. In 2023, spilled oil was found in Arraial do Cabo beaches, affecting the local environment and tourism. To identify those responsible for the oil spill, the molecular analyses of eight spilled oils from Arraial do Cabo beaches (ACn), and crude oils from two different companies (Comp A and Comp B), were performed using gas chromatography with flame ionization detector (GC-FID), gas chromatography with mass spectrometry (GC-MS) and high-resolution mass spectrometry (Orbitrap-HRMS) to investigate geochemical biomarkers and polar compounds. Oil samples were extracted by ultrasonic bath, followed by asphaltene precipitation and fractionation into saturated, aromatic, and polar compounds by liquid chromatography. Crude oils were analysed by GC-FID, obtaining as a result for Comp B sample a distribution of n-alkanes ranging from C₈ to C₃₉ and isoprenoids pristane and phytane, similar to spilled oils AC1, AC2, AC6 and AC8. Spilled oils AC3, AC4, AC5 and AC7 do not show significant contribution of n-alkanes, being more similar to Comp A production oils. The saturated and aromatic fraction of oils were analysed by GC-MS to give information about the origin of the spilled oils using geochemical parameters. The geochemical indices calculated for each of the extracts indicate that the oils have a lacustrine origin for the sedimentary organic matter, with low salinity. The level of biodegradation of the oils which occurred at the reservoir may be estimated according to the presence of 25-nor-hopane (25-NH) series. The 25-NH/H30 ratio for Comp A oil (0.22) was comparable to those observed in samples AC3, AC4, AC5, and AC7 (0.15, 0.16, 0.16, and 0.16, respectively). The second group was comprised by a lower biodegradation level, namely Comp B, AC1, AC2, AC6 and AC8 (0.08; 0.07, 0.10, 0.09; and 0.12) samples. Other ratios such as C29[αββ/ααα+αββ)], TPP/DiaC27(R+S), TeT24/Tr26 and (3+2MP)/(9/4 + 1MP) also show the presence of two distinct groups of spilled oils, with group 1 consisting of AC3, AC4, AC5 and AC7 oils, related to Comp A, and group 2 consisting of AC1, AC2, AC6 and AC8, related to Comp B. The Orbitrap-HRMS petroleomic results showed that the dominant classes in positive mode of ionization were N [H], N2 [H] and NO [H] for all samples at different levels. In negative mode, the classes showed the weathering processes suffered by the oil in the environment. Spilled oil samples had the classes O_1-8 [H], O_3-5S [H] and O_1-4N [H]. Chemometric analysis using geochemical parameters through Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) revealed two groups based on sample similarity: the first comprising Comp A, AC3, AC4, AC5, and AC7, and the second comprising Comp B, AC1, AC2, AC6, and AC8. The integrated analytical and chemometric approaches revealed two distinct groups of spilled oils, showing similarities with oils produced by Comp A and Comp B.