ABSTRACT

The mobilization and redistribution of uranium in a sedimentary basin during the late stages of diagenesis suggests a relationship to hydrocarbon maturation and migration. Pore fluids expelled from black shales and other hydrocarbon source rocks during their thermal maturation may have elevated uranium concentrations resulting from the thermal destruction of urano-organic compounds. As these pore fluids migrate into reservoir rocks, traces of adsorbed uranium are left along the migration pathways.

Fission track mapping and neutron activation analyses for uranium in the upper Smackover Formation tend to support this model of uranium behavior. U/Th ratios are much higher in the reservoir rocks than in the black shales of the "Gray" sands suggesting uranium redistribution. This redistributed uranium is concentrated in stylolites and unidentified fine-grained opaque material. Spatial variations in uranium concentration throughout the study area correlate to variations in aluminum content. This suggests that a critical factor controlling uranium retention in the reservoir rocks is the amount of adsorbent clay.

These results have implications for the exploration of hydrocarbons and the interpretation of spectral logs.