Geochemistry of Thorium and Uranium During Late-Stage Diagenesis--Potential New Method for Exploration

Miles E. Denham, Thomas T. Tieh

The contrasting geochemistry of uranium and thorium may provide the exploration geologist with a new tool for delineating migration paths and evolution of fluids associated with hydrocarbon generation and migration. Uranium and thorium are lithophilic actinides that commonly occur in sediments at low concentrations (1-50 ppm). Under reducing conditions their geochemical behavior is similar and both are essentially immobile. However, as the oxidation potential of a system increases, uranium is oxidized to the mobile pentavalent and then hexavalent form, while thorium remains immobile. Thus, mobility of uranium relative to thorium, as shown by thorium-uranium ratios, indicates the passage of pore fluids enriched in uranium.

Pore fluids are likely to become enriched in uranium during thermal maturation of kerogen. Uranium shows a strong affinity for oxygen-rich organic matter such as immature kerogen. During the early stages of thermal maturation in the source rock, oxygen is lost from kerogen resulting in the weakening of the uranium-organic association. This uranium is released to late-stage pore fluids to migrate from source to reservoir. Along the migration path, uranium is adsorbed onto clays and organic matter, leaving a trace of the migrating fluids.

Despite their low concentrations, uranium and thorium are readily detected by neutron activation methods. Delayed neutron counting allows determination of these elements in bulk samples, and fission track methods are used to map uranium and thorium occurrence on a microscopic scale. Fission track analysis can also be used to determine thorium-uranium ratios in individual grains in thin section. These techniques may provide a valuable new tool for investigating pore fluids associated with the generation and migration of hydrocarbons.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.