How long is enough: CO2 -H2 O equilibration for δ(18) O analysis in saline formation waters?

The equilibration of H2 -H2 O and CO2 -H2 O is a commonly used method for δ(2) H and δ(18) O analyses in water. Problems with these analyses of formation water include the presence of petroleum contaminants and the slow exchange of oxygen isotopes between CO2 gas and the saline waters collected from petroleum exploration and production wells. Therefore, it is crucial to understand whether isotopic equilibrium between water and CO2 has been reached, how long it takes, and how to deal with contaminants. This study investigates how long it takes to reach isotopic equilibrium between gas and hypersaline brine, and how to deal with petroleum-contaminated formation water, in order to produce good quality δ(2) H and δ(18) O data.

The δ(2) H and δ(18) O values of six natural waters with a variety of salinities were determined by isotope ratio mass spectrometry after different periods of H2 and CO2 equilibration with the samples. Hydrocarbon and non-hydrocarbon gases present in formation brines were pre-treated prior to the H2 - and CO2 -H2 O interaction to minimize their interference with the isotope analyses.

Water samples with different salinities required different times for the oxygen isotopes to reach equilibrium. Salinity does not affect the equilibration of H2 with saline brines. Hydrocarbon gases present in formation waters can interfere with the isotope analysis, but this can be minimized with pre-analysis treatment.

The described procedures should be applied for stable hydrogen and oxygen isotope analyses in waters with high salinities and petroleum contaminants. In particular, waters collected from petroleum exploration and production wells, and from evaporative environments, should go through these procedures. Copyright © 2016 John Wiley & Sons, Ltd.