UNDERSTANDING THE MECHANISMS OF OXYGEN AND HYDROGEN ISOTOPE FRACTIONATION OF WATER IN SHALES

Oxygen and hydrogen isotopes of water undergo fractionation upon interaction with clay-rich shales. This is mainly due to the high rock to water ratio and the high cation exchange capacity (CEC) of the clays present in the interacting media. Water molecules undergo structural rearrangement in shale nanopores as the clays act as semi-permeable membranes, thus rendering an isotopic signature different from that of the bulk water. The goal of this experimental study is to investigate the role of major cations adsorbed to phyllosilicates such as Ca-Montmorillonite (MMR) (STX1-b) and Na-MMR (SWy1-z) source clays in controlling the nature and extent of isotopic fractionation when they are allowed to react with pure water and cation chloride solutions of varying strengths. Our working hypothesis is that clays containing cations with higher ionic potential (Ca or Mg) would preferentially retain heavier isotopes in their hydration spheres, thereby depleting the reacted water. However, cations with lower ionic potential (K or Na) tend to show less or no preference for heavier isotopes, thus depleting the reacted water to a lesser extent.

Several aliquots of homoionic clay powders were prepared by washing the source clays with 0.05 M solutions of NaCl, KCl, CaCl₂, MgCl₂. This step was done to ensure the adsorption sites of clays were occupied by Na, K, Ca, and Mg only. Pure water and salt solutions were added to the powdered samples in the rock to water ratio of 19:1. The samples were then purged with CO₂-He mixed gas and allowed to equilibrate for ca. 10 days. The isotopic composition was measured in the Isotope Ratio Mass Spectrometer at the IsoBioGeM Lab in West Virginia University.

This study is relevant to recent findings on the effect of nanopore confinement that have emphasized that nanogeochemical properties of shale can significantly modify the chemistry of interacting fluids. The results from these experiments will help in developing better understanding of the mechanisms of isotopic fractionation in produced waters upon interaction with clay-rich shales from shale plays.