Delta 37Cl variations with depth in Mount Simon Sandstone basinal brines

The Mount Simon Sandstone (MSS) is the deepest sedimentary formation of the Illinois Basin (USA). This formation was chosen by the Midwest Geological Sequestration Consortium to demonstrate the potential for carbon dioxide (CO2) geological storage. Chlorine isotopes were used to investigate saline fluids from MSS to better understand their origin and present day hydrodynamic regime. Basinal brines were sampled at 10 different depths (from -1499 m to -2128 m) using a Westbay* multilevel groundwater characterization and monitoring system in September 2011. For all samples (n=10), Cl isotope compositions were measured using the classical method and were duplicated. The delta 37Cl data exhibit a clear upward trend within the MSS, ranging from 0.2+ or -0.03 ppm in the deepest part of the formation to -0.8+ or -0.05 ppm in the upper part. The two samples from the Ironton-Galesville Sandstone (the unit above the MSS and the Eau Claire Formation caprock) were determined to be non-representative. We interpret the delta 37Cl trend observed in the MSS in terms of known geological processes that fractionate chlorine isotopes: (a) evaporation of ancient seawater that leads primary brine to form; and (b) fluid transport processes occurring during and after burial diagenesis, which are known to be able to further modify the Cl isotopic signature of evaporation, such as chemical diffusion or ion filtration. However, isotope compositions expected for primary brines are lower than observed, thus not explaining the full range of the MSS trend. The likelihood of these hypotheses must now be investigated with respect to other geochemical tracers. This on-going work clearly illustrates that Cl isotopes have a strong potential to characterize important aspects of long-term hydrology for formation waters.