APPLICATION OF HIGH-RESOLUTION PROFILING OF NATURAL TRACERS TO DEFINE THE HYDROGEOLOGY OF CRETACEOUS SHALES ACROSS THE WILLISTON BASIN, CANADA

Understanding the hydrogeology of sedimentary basins is necessary because they contain much of the world’s mineral, energy, and water resources. Furthermore, low permeability units within these basins are being considered as repositories for radioactive and other hazardous wastes as well as seals for carbon capture and sequestration facilities. The hydrogeology of these basins has been defined using hydraulic, chemical, and isotopic data collected from water-bearing units. However, few detailed studies have been conducted on the very low permeability units (aquitards) that form much of the sediments in these basins. The low permeability of these aquitards makes hydraulic studies difficult and, as a result, experiments are often conducted on core samples in the laboratory. The applicability of these lab-derived data to the field scale is, however, often questioned. The hydrogeology of aquitards can alternatively be derived from the interpretation and numerical modeling of vertical profiles of naturally occurring geochemical and isotopic tracers of pore water and dissolved gases (e.g., δ¹⁸O, δ²H, Cl^-, Br^-, ⁴He, CH₄). These tracers not only help define and constrain long-term transport mechanisms at the aquitard scale, but can preserve a historical record of major paleo-hydrogeologic changes that occurred in the basin. This study presents natural tracer profiles of δ¹⁸O and δ²H of porewater, Cl^-, and CH₄ and its stable isotopes δ¹³C and δ²H from thick Cretaceous shales across the Williston Basin, Canada. Results show distinct and consistent geochemical and isotopic profiles that develop over vertical distances of several tens to hundreds of meters and over time scales of several thousand to millions of years, with molecular diffusion being the dominant solute transport mechanism.