Tracking Crude Oil Weathering Through FT-ICR MS Petroleomics: Development of Molecular Indicators and Exposure-Time Models

Estimating the timing of petroleum exposure in environmental settings remains a major challenge in environmental and forensic geochemistry. Although conventional molecular biomarkers are widely used to assess weathering processes, their ability to resolve specific temporal scales is often limited by the complex compositional evolution of crude oils after environmental exposure. In this study, an FT-ICR MS petroleomic workflow was developed to transform molecular weathering signatures into predictive models capable of estimating weathering-equivalent exposure time. Three Colombian crude oils (light, medium, and heavy) were subjected to two weathering scenarios: direct solar exposure and sedimentation in contact with soil. Samples were monitored for up to 102 days and characterized by FT-ICR MS using complementary APPI(+) and ESI(−) ionization modes. Thousands of molecular formulas were consolidated into approximately ten heteroatom-based chemical families and expressed as relative abundance changes relative to the initial sample. Temporal indicators were identified through logarithmic transformations of compositional changes and exposure time and subsequently used to develop Partial Least Squares (PLS) models. Weathering produced molecular transformations characterized by progressive decreases in hydrocarbon-, sulfur-, and nitrogen-rich families and increases in oxygenated, polyoxygenated, and oxygen-sulfur species. Nitrogen-containing compounds detected by ESI(−) decreased from approximately 68% to 26% of the total relative abundance after 64 days of exposure, whereas oxygen-sulfur species increased from nearly absent levels to about 30%, revealing extensive molecular reorganization and progressive oxygenation. The resulting models demonstrated that weathering-equivalent time can be estimated from FT-ICR MS molecular proxies but that predictive performance depends on both crude oil type and weathering conditions. Under direct solar exposure, predictive windows reached approximately 32 days for light crude oils, 64 days for medium crude oils, and up to 102 days for heavy crude oils. Under sedimentation conditions, predictive windows decreased to approximately 32 days for light and medium crude oils and 47 days for heavy crude oils, indicating a significant influence of petroleum–soil interactions on molecular evolution. Application of the models to samples collected from the “Dos Bocas” natural oil seep (Santander, Colombia) demonstrated their ability to differentiate relative weathering levels and support the interpretation of spatial alteration patterns and potential migration pathways. These results show that FT-ICR MS can be extended beyond molecular characterization and used as a quantitative framework for estimating weathering-equivalent time and supporting petroleum forensic investigations.