Geochemical and radiological assessment of source rocks from the Middle Magdalena Valley Basin (Colombia): implications for unconventional hydrocarbon exploration

Abstract

Colombia is exploring unconventional hydrocarbon reservoirs, however after more than 50 years of oil and gas operations, radiological assessments of these oilfields have been lacking. To address this, eight rock samples from the Tablazo and La Luna formations in the Middle Magdalena Valley Basin were analyzed using gamma and alpha spectrometry to quantify natural radionuclides from the 238U and 232Th decay series. Additional ICP-MS and XRD analyses provided elemental and mineralogical data. The mineralogical composition 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 notably enriched in Ca, S, V, and Zn compared to the upper continental crust. The average 238U concentration was 220 ± 80 Bq kg− 1, approximately six times higher than the global average, with two wells showing values near 600 Bq kg− 1, suggesting localized geochemical variation. The average 232Th concentration was 50 ± 4 Bq kg− 1, and 40K ranged from 180 to 350 Bq kg− 1, similar to global soil levels. Significant correlations were found between 238U series radionuclides and elements like V, Cr, Ni, Mo, and Tl, while 232Th series radionuclides correlated with Bi and Al. 40K was linked to K and showed positive correlations with Ti, Li, and Ba. In contrast, elements such as Pb, Hg, Fe, Mg, Mn, and Na displayed negative correlations. These findings highlight the strong relationship between radionuclide activity concentrations and the chemical composition of the rocks, reflecting diverse geological sources.