Correlations between FTIR Spectral Indices and Geochemical Parameters for Differentiating Brazilian Crude Oils

Fourier transform infrared spectroscopy in the mid-infrared region (FTIR) was investigated as a rapid screening tool for the geochemical differentiation of five Brazilian crude oils (A, C, D, E, and F), using gas chromatography coupled to mass spectrometry (GC–MS), elemental analysis, and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry [ESI(−) FT–ICR MS] data as reference. Spectral indices related to aromaticity (C1), oxidation (C2), branching (C3), aliphaticity (C4), sulfurization (C5), and long-chain alkyl structures (C6) were evaluated. The FTIR indices followed trends observed by the reference techniques. Aromaticity (C1) showed an inverse association with H/C (Spearman ρ = −1.00), whereas aliphaticity (C4) was positively correlated with H/C (ρ = 0.90), API gravity (ρ = 1.00), and saturated fraction (ρ = 0.90), distinguishing more aliphatic and hydrogen-rich oils from more aromatic and unsaturated ones. The oxidation index (C2) showed a positive association with total acid number (ρ = 0.90) and polar fraction (ρ = 1.00), indicating a relationship between the spectral response and enrichment in polar and oxygenated components. C6 showed a positive association with the ratio between terrigenous and aquatic n-alkanes (ρ = 0.80), suggesting sensitivity to the abundance or preservation of long alkyl chains, potentially related to terrigenous input. C6 also increased with the O-class MA₁ descriptor (ρ = 0.90) and with the ratio between acyclic and cyclic O₂ species (A/C; ρ = 0.80, n = 4), linking greater aliphatic preservation to relatively more preserved polar molecular profiles. Oil E showed the greatest compositional alteration, with the highest aromaticity and oxidation indices and the lowest aliphaticity and C6 values, in agreement with the pronounced n-alkane depletion observed by GC–MS, the lowest H/C ratio (1.726), the highest O/C ratio (0.015), O₂ enrichment (23.17%), and a low A/C ratio (0.072). In contrast, D showed the highest aliphaticity and H/C ratio (1.916), although its O₂ profile remained intermediate (A/C = 0.703), indicating that bulk composition and polar species may respond differently to geochemical controls. Thus, although FTIR does not provide the molecular specificity of GC–MS and FT–ICR MS, its indices reproduced trends associated with aromaticity, oxidation, aliphatic preservation, and polar composition, supporting its use as a rapid and low-cost tool for geochemical screening and sample selection for more detailed analyses.