Quantitative characterization of crude oil depositional environments and kerogen source types via FTIR-ATR Organic Absorptance Factor (OAF) indices

This study establishes a rapid, non-destructive numerical framework for the macromolecular characterization of petroleum systems using Attenuated Total Reflectance Fourier-Transform Infrared (FTIR-ATR) spectroscopy. By algebraically transforming raw spectral parameters into normalized Organic Absorbance Factor (OAF) indices within a closed stoichiometric matrix, this protocol systematically quantifies the relative integrated areas of aliphatic C–H, skeletal aromatic C = C, and oxygenated carbonyl C = O functional group domains. A geographically diverse calibration suite of twenty-eight organic samples, assembled from major petroleum-producing provinces worldwide and comprising fifteen global crude oils and thirteen solid kerogen isolates, was processed via baseline-stabilized co-added spectral scans. The multi-variable coordinate cross-plots of the Aliphatic Index (OAFH) versus the Oxygenated Index (OAFO) separate the global fluid petroleum phase into discrete clusters corresponding to Terrestrial Oxic, Marine–Terrestrial Dysoxic, and Carbonate Marine Anoxic organofacies. Specific molecular anomalies in high-sulfur reference fluid phases are identified through baseline-corrected functional group variations linked to heteroatom-rich polar fractions, maintaining analytical rigor without relying on fingerprint regions susceptible to mineral matrix scattering artifacts. Furthermore, within the solid geopolymeric isolate subset, ordinary least-squares linear regression establishes a statistically significant inverse linear relationship between the spectroscopic Aliphatic Index (OAFH) and the laboratory-verified pyrolytic Hydrogen Index (HI). Incorporating the Shangfuchi (SFC) reference standard via the mean OAFH value of 0.4058 derived from eleven independent replicate measurements yields an initial regression model with an R² value of 0.4772 ( n = 10, p < 0.05). Conversely, the selective exclusion of the SFC sample as an extreme compositional anomaly resolves a moderate inverse linear trajectory across the principal non-SFC kerogen cohort that remains significant at the 99% confidence level, yielding an improved R² value of 0.6825 ( n = 9, p = 0.006). This inverse covariance is mathematically governed by a non-linear denominator dilution effect within the closed OAF matrix, where the expansion of multi-directional aliphatic deformation bands flanking 1480 to 1360 cm⁻¹ suppresses the normalized stretching ratio as total hydrogen saturation increases. Compared to the standard sixty-minute thermal tracking cycle required for conventional Rock-Eval pyrolysis, the automated FTIR-ATR computational script delivers reproducible molecular indicators in less than 5 min per sample, functioning as an efficient, non-destructive protocol for regional reservoir characterization and depositional organofacies screening.