Radiological assessment of petroleum source rocks from the Middle Magdalena Valley basin in Colombia

Colombia is currently assessing unconventional hydrocarbon reservoirs; however, despite over five decades of conventional oil and gas operations, systematic radiological studies of petroleum formations remain scarce. To address this gap, eight source rock samples from five oil wells in the Tablazo and La Luna formations of the Middle Magdalena Valley Basin (MMVB) were analyzed by gamma- and alpha-spectrometry to quantify naturally occurring radionuclides from the ²³⁸U and ²³² Th decay series and ⁴⁰K. Complementary ICP-MS and XRD analyses determined elemental and mineralogical composition. The mineralogy varied, with quartz, kaolinite, pyrite, calcite, illite, and gypsum identified. Major elements included Ca, Fe, Al, and S, with V and Zn as prominent trace elements. The source rocks were enriched in Ca, S, V, and Zn relative to the upper continental crust. Mean activity concentrations were 220 ± 80 Bq kg^-1 for ²³⁸U, 52 ± 4 Bq kg^-1 for ²³²Th, and 270 ± 20 Bq kg^-1 for ⁴⁰K. The ²³⁸U activities are approximately one order of magnitude higher than values reported for the upper crust, other oilfields, and local soils, reaching up to ≈600 Bq kg^-1 (≈20 × reference levels). ²³²Th values fall within natural ranges, whereas ⁴⁰K concentrations are lower, reflecting the source rocks' geochemistry. Due to uranium enrichment, accurate quantification is essential for reliable dose assessment and safe management of Naturally Occurring Radioactive Materials (NORM) residues and by-products. Hierarchical Cluster Analysis (HCA) revealed distinct geochemical associations: uranium grouped with Ca, S, Cd, Ni, Zn, and Mo, indicating enrichment under reducing, organic- and sulfate-rich conditions, whereas thorium and potassium clustered with lithophile elements (Al, Ti), reflecting their occurrence in silicate phases. This study provides the first radiological baseline for Colombian petroleum source rocks, evidencing localized uranium enrichment and underscoring the need for systematic monitoring of NORM in future unconventional oil production.