REVISITING THE MOLECULAR CHARACTERIZATION OF HYDROCARBONS TRAPPED IN FLUID INCLUSIONS FROM THE BRAZILIAN PRE SALT RESERVOIR, SANTOS BASIN

Although recent studies have advanced the understanding of Brazilian pre-salt carbonate reservoirs through petrographic and microthermometric investigations, the molecular characterization of hydrocarbons preserved in fluid inclusions still offers opportunities to further investigate paleofluid composition and petroleum system evolution. In this study, three core samples from the same stratigraphic interval within a Brazilian pre-salt carbonate reservoir in the Santos Basin were investigated to further characterize the molecular composition of fluid inclusion oil. Mineralogical analysis revealed varying degrees of silicification among the samples. As the host mineral of the fluid inclusions was not known beforehand, the samples were separated into carbonate- and quartz-rich fractions for subsequent analyses. Initially, hydrocarbons present in the rock samples were extracted using organic solvents to obtain oil extracts (EXT). Subsequently, each mineral fraction was subjected to specific cleaning procedures to remove residual organic contaminants. Finally, the samples were crushed in the presence of an organic solvent to recover fluid inclusion oil (FIO). The EXT and FIO obtained from all samples were analyzed by GC-MS and interpreted based on geochemical ratios related to organic matter source [Pr/Ph, Pr/n-C₁₇, HOP/(HOP+STER), GAM/H₃₀, TRI/HOP, ₂₃TRI/₂₁TRI, H₂₉/H₃₀, and TET₂₄/₂₆TRI] and thermal maturity [Ts/(Ts+Tm), C₂₉ αββ/(αββ+ααα), C₂₉ 20S/(20S+20R), and H₃₂ 22S/(22S+22R)]. Chromatograms showed well-preserved terpane and sterane distributions, indicating a high degree of compositional similarity between FIO and EXT. The predominance of C₂₇ steranes, together with the terpane distribution, is consistent with oils generated predominantly from algal organic matter deposited in a moderately restricted lacustrine environment, with minor terrestrial organic matter input. The thermal maturity parameters Ts/(Ts+Tm), H₃₂ 22S/(22S+22R), C₂₉ αββ/(αββ+ααα), and C₂₉ 20S/(20S+20R) yielded values consistent with oils within the main oil generation window and showed close agreement between FIO and EXT. Integration of the data using Principal Component Analysis, hierarchical cluster analysis, and radar plots confirmed the strong geochemical affinity between FIO and EXT. Differences were primarily associated with the Pr/Ph, Pr/n-C₁₇, ₂₃TRI/₂₁TRI, and TET₂₄/₂₆TRI ratios, which distinguished FIO recovered from quartz-rich and carbonate-rich fractions without altering the interpretation of a common geochemical origin. These results demonstrate that hydrocarbons trapped in fluid inclusions preserve molecular signatures representative of the petroleum system, highlighting the potential of this approach as a complementary tool for geochemical correlation and for reconstructing the history of petroleum generation, migration, and accumulation.