Compositional variation in formation fluids and their relation to fluid flow and water-rock interaction in Devonian to Lower Cretaceous sedimentary rocks of southern Alberta

Variations in the chemical composition of waters in six aquifers ranging in age from Upper Devonian to Lower Cretaceous were examined to determine the chemical and hydrologic controls on the evolution of formation fluids in southern Alberta. The principal factors affecting fluid composition are mixing, water-rock interaction, and bacterial sulfate reduction. Mixing of chemically evolved formation waters with meteoric water has resulted in a complex regional distribution of chemical constituents. Several episodes of meteoric influx are recognized based on isotopic and chemical variations. Overlain on the fluid mixing signature are a number of chemical trends that are a product of water-rock interaction. Mineralogical control of the water chemistry is dominated by calcite-dolomite equilibrium in the carbonate and clastic reservoirs, although in the clastic reservoirs, calcium to magnesium activity ratios appear to be artifacts of earlier equilibria. The Nisku/Arcs formation waters are close to equilibrium with calcite, dolomite and anhydrite. Dissolved silica activities and pH of clastic-hosted waters are strongly influenced by the presence of clay minerals and smectite-aqueous solution metastable equilibrium. Bicarbonate and sulfate content and stable isotopes of sulfur and carbon are interpreted to reflect bacterial sulfate reduction that has occurred in areas affected by meteoric influx.; Fluid flow is dominated by south to north topography-driven flow with recharge in Alberta, Montana and Wyoming. Rapid changes in chemical composition and hydraulic head gradient indicate complex flow patterns that reflect changes in hydraulic conductivity and/or continuity of aquifers, and structural features like the Sweetgrass/Bow Island Arch and the Vulcan Low.