Radium disequilibria dating of sulfates associated with crude oil spills

CAGNONI, MARIAMA; CABALERI, NORA

Medellín

Congreso; VIII South American Symposium on Isotope Geology (SSAGI); 2012

Universidad de Medellín, Colombia

This study attempt to use disequilibrium methods (radium and daughter isotope activities) to estimate the age of formation of gypsum/anhydrite associated with an oil spill. When a reservoir is exploited, formation water is brought to the surface along with oil or gas and becomes produced water. When an oil spill happen, common associated precipitates related with production water include sulfates, chlorides and carbonates. Naturally occurring radioactive materials (NORM) originate in geological formations and can be brought to the surface with produced water. The most abundant NORM compounds in produced water are 226Ra and 228Ra, which are derived from the radioactive decay of uranium and thorium associated with host rocks in the hydrocarbon reservoir. Ra- bearing minerals include gypsum, anhydrite, calcite, aragonite and barite. The oil spill age can be estimated by comparing the present 228Ra/226Ra activity ratio of the mineral samples to the original activity ratio (228Ra/226Ra)0 of the produced water at the time of mineral precipitation. Alternative methods, useful for dating young discharges, measure the concentration of radium daughters (228Th, 210Pb) that built-up from an initial zero concentration. The mineral system has to remain chemically closed for the radium isotopes since its formation. In order to date the spill, gypsum/anhydrite crystals were separated from an old oil spill on soil. The mineral fibers were analyzed along with a sample of the salts obtained from the production water after evaporation. Activity concentrations were determined by low energy gamma ray spectrometry. 210Pb was directly measured, 226Ra was determined indirectly by measuring its daughter activities (214Pb and 214Bi), 228Ra was measured by its immediate daughter 228Ac and 228Th, using the strong gamma ray emission from 212Pb.  We obtained a minimum age of 14 years using the radium isotope activity ratios and a maximum age of 52 years using the 210Pb/226Ra ratio. The 228Th/228Ra method gave an age too young (1 year). It was inferred that the gypsum/anhydrite crystals were very small and  gaseous intermediates in the radioactive decay series (220Rn and 222Rn) diffused out of the crystal lattice decreasing the activity concentrations of 212Pb (228Th), 214Pb,214Bi (226Ra) and 210Pb, therefore affecting the estimate ages. We conclude that crystal size and composition are decisive conditions for the successful and accurate application of these dating methods.