Abstract: Rock-Water Interactions in the Madison Aquifer, Williston Basin, North Dakota

J.F. Spicer, B. E. Herbert, S. L. Dorobek

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 rock-water interactions in highly saline aquifers. Geochemical speciation, coupled with graphical matrix interpretations, was developed to understand specific rock-water interactions that occur in groundwaters within the 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 150°C and 300 bars, and ionic strengths up to 8M.

Geochemical speciation modeling, using a Pitzer-based model, was used to identify specific precipitation-dissolution reactions occurring in the Madison Aquifer system. Speciation 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 precipitation-dissolution boundaries for these minerals within the aquifer.

These analytical techniques were used to interpret precipitation-dissolution reactions that are influencing the basinal waters in the Madison Aquifer. The maps indicate that dissolution of both calcite and dolomite is occurring toward the center of the basin, an area that coincides with high brine concentrations and greater temperatures and pressures. Halite is precipitating toward the eastern portion of the study area, away from the area of high brine concentration and high temperatures. Results of the speciation modeling will be verified by future studies of core porosity in various fields across North Dakota.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994