Resolving subtle geochemical differences among pre-salt crude oils using advanced molecular and carbon isotopic analyses of light hydrocarbons

Geochemical fingerprinting of light hydrocarbons (LH) is widely applied in reservoir surveillance, fluid connectivity assessment, compartmentalization studies, and production allocation. Advanced analytical techniques, such as comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-TOFMS), provide accurate molecular fingerprints due to their enhanced peak capacity and mass spectral deconvolution capabilities. In addition, compound-specific isotope analysis (CSIA) of LH offers complementary geochemical information related to petroleum source, thermal maturity, and in-reservoir alteration processes. The integration of molecular and isotopic fingerprints is therefore a powerful approach for distinguishing crude oils with highly similar geochemical characteristics.

In this study, compositional profiling of 47 pre-salt crude oils from a giant field in the Santos basin was performed using GC×GC-TOFMS focusing on the LH fraction (C₆-C₁₁). The aim was to detect slight differences in these crude oils that are known to exhibit related geochemical characteristics and overall compositional homogeneity. Analyses were carried out on a Pegasus 4D system using a non-polar x mid-polar column configuration. Untargeted chemometrics evaluation of the chromatographic data using Principal Component Analysis (PCA) revealed that, despite the overall compositional homogeneity of the dataset, three samples (#7, #9, and #43) were discriminated from the main group along PC1. Methylcyclohexane and C₇–C₁₁ n-alkanes were the main contributors to sample differentiation, together with cyclohexane, alkylcyclohexanes, branched alkanes, and alkylbenzenes.

A broader evaluation of the dataset revealed that LH abundances generally increased toward shallower reservoir intervals within the same well, particularly for samples #7, #9, and #43 relative to their paired deeper samples. To further investigate these variations, six selected crude oils were analyzed by GC-IRMS using direct whole-oil injections and a 50 m HP-PONA column. The δ¹³C signatures of individual LH compounds showed highly consistent isotopic profiles among these samples, including paired samples collected at different depths within the same wells.

The integrated molecular and isotopic results indicate that the studied crude oils belong to a connected reservoir system, despite subtle compositional variations. While molecular differences were identified within the LH fraction, the absence of significant isotopic variability suggests that the crude oils were derived from a common source. The molecular fractionation patterns observed may reflect reservoir-scale physicochemical processes. These results highlight the potential of GC×GC-TOFMS and CSIA approaches for petroleum geochemistry, providing valuable insights for resolving subtle variations in highly similar pre-salt crude oils from the Santos Basin.