Rapid evaluation of the hydrochemistry of a sedimentary basin using only ‘standard’ formation water analyses: example from the Canadian portion of the Williston Basin

The Canadian portion of the Williston Basin is used as an example of how > 15,000 ‘standard’ formation water analyses in about 15 aquifers can be evaluated rapidly using easily available culling and data distribution display software. Techniques and software used include a program for culling erroneous and missing data in ‘standard’ formation water analyses, electronic grids of stratigraphic surfaces and unit boundaries from the new Geological Atlas of the Western Canada Sedimentary Basin, grid manipulation software, and a geochemical modelling program (SOLMINEQ.88).Intra- and inter-basinal hydrochemical continuity at the regional scale was matched visually by comparing salinity maps on the Canadian side of the basin with salinity maps from the American side, and with the adjacent Alberta Basin. Inter-aquifer continuity to identify aquifer systems and the relative strength of aquitards was determined using a combination of. (1) salinity distribution maps; (2) maps showing salinity differences between aquifers and across intervening aquitards; and (3) the relation among SO₄ in formation waters, the distribution of anhydrite, and the saturation of the formation waters with respect to anhydrite. Evaporites strongly influence the composition of adjacent formation waters, even to the extent that in one aquifer the normal increase of salinity with depth is reversed. The process of freshwater influx into the western part of the basin is demonstrated by a series of salinity maps; they suggest that the process is current and has included removal of halite from the major basinal aquitard. This regional-scale evaluation required an elapsed time of only about 10 working days, hence a fairly rapid study.