Re-Os Isotope Geochemistry and Its Potential Use in Basin Fluid Analysis

Ruiz, Joaquin¹, John Chesley², Fernando Barra² (1) University of Arizona, Tucson, AZ (2) University of Arizona, Tucson, AZ

Timing and sourcing of fluids, including hydrocarbons, are a critical component of understanding the evolution of basins and may have significant consequences for exploration models for oilfields. An obvious approach to this question is through radiogenic isotopes, which have been a very powerful tool in addressing other questions in geosciences. Previous work using Sr isotopes yielded ambiguous results and the use of radiogenic isotopes for fluid migration was mostly abandoned until a recent study attempted to date the oil fields of western Canada using Re-Os isotopes.

Rhenium has two isotopes – 185Re and 187Re. Of these two isotopes only 187Re is radioactive. Improvements in the chemical separation of Os and Re from geological samples, and mass spectrometry have allowed us to use Re-Os systematics to study the evolution of the solid Earth, the oceans and hydrothermal systems. This isotopic system has been particularly useful in dating sulfides, such as pyrite, which is a common product of fluid movement in basins. Typical concentrations of pyrite for Os are between 7 and 100ppt, although some samples may reach a few ppb. Re can be as high as 100 ppb.

Although dating sulfides is now common using Re-Os it has not been extensively applied to basin fluid evolution. Similarly very little has been done with hydrocarbons. However the use of Re-Os isotopes in hydrocarbon systems seems very promising and work is underway to prove the concept.