DEVELOPING A CONSERVATIVE TRANSPORT MODEL TO EXAMINE EVOLUTIONARY HISTORY OF DEVONIAN BRINES IN THE WILLISTON BASIN, CANADA

The Williston Basin is structurally simple with extensive regional aquifers guiding flow between Montana, U.S.A, and Manitoba, Canada. Freshwater influx during a period of increased hydraulic heads in the Pleistocene resulted in regional groundwater flow reversal and complex geochemical signatures along the northeastern margin of the Williston Basin. Meteoric water infiltration into Devonian aquifers is responsible for fluid mixing, though the timing of freshwater influx has not been studied in-depth. This investigation focuses on Devonian aquifers situated on the northern edge of the Williston Basin. Solute (Cl, Cl/Br) and isotope (δ¹⁸O, δ²H) geochemistry of formation waters in Devonian aquifers were analyzed to examine the potential sources and evolutionary history of fluids in the Williston Basin. Observations reveal the presence of a paleo-evaporated seawater endmember mixing with a fluid sourced from glacial meltwater. A 1-D numerical transport model was developed to evaluate the evolution of geochemical profiles for various transport mechanisms and paleoevents to predict the origins of multiple endmembers. The results demonstrate that advection in regional groundwater flow systems during Pleistocene glacial cycles in the last 1 Ma has perturbed a long-term profile dominated by diffusive transport. Preliminary results reveal that an incursion of glacial meltwater is estimated to have arrived into Devonian aquifers in the study region between 0.24-0.40 Ma under advective-dispersive conditions along the northeastern flank of the Williston Basin. This model has helped to constrain an age of the first arrival of glacial meltwater to our study area and improve our understanding of large-scale fluid transport within Devonian formations in the Williston Basin.