Rock-water interactions of the Madison Aquifer, Mission Canyon Formation, Williston Basin, North Dakota

Abstract

The Williston Basin is located in the northern Great Plains of the United States. This area includes eastern Montana, northwestern South Dakota, and western North Dakota. The stratigraphy and geologic history of this basin are well understood and provide an excellent framework in which to study rockwater interactions in highly saline aquifers. Geochemical speciation was coupled with data visualization interpretations in order to understand specific rock-water interactions that occur in groundwaters within this basin. Results from the speciation model were used to predict regions where secondary porosity might be forming. Paleozoic and Mesozoic strata within the Williston Basin comprise several aquifer horizons. The Mississippian Madison Aquifer contains both fresh meteoric waters and high density brines that have concentrations up to 300,000 mg/L. Flow of fresh meteoric water in the Madison Aquifer is diverted around the brines. The physiochemical parameters of the basinal waters encompass temperature and pressure regimes up to 1300C and 300 bars, and ionic strengths up to 5 M. Geochemical modeling, based on a Pitzer-based model, was used to identify potential precipitation-dissolution reactions occurring in-the Madison Aquifer system. Results (i.e., degree of saturation with respect to calcite, dolomite, halite, gypsum, and anhydrite) were integrated with a graphical matrix analysis program to produce color-coded maps that depict potential precipitation-dissolution boundaries within the aquifer. These analytical techniques were used to infer chemical interactions influencing the basinal waters in the Madison Aquifer. These inferences were then used to predict regions of porosity alteration. Carbonate distribution seems to be controlled by ground water chemistries of high PCO2 and low pH irrespective of the effect by temperature. Based on maps of calculated saturation indices, calcite and dolomite dissolution should be occurring toward the center of the basin, which coincides with areas of high brine concentrations, temperatures, and pressures. In the case for both, halite and anhydrite, the subsurface areal extent of evaporate strata apparently controls the ground water saturation state of the evaporate species. Halite probably is precipitating toward the eastern portion of the study area, away from the area of high brine concentration and high temperatures.