ABSTRACT

Hydrogen and oxygen isotopic compositions of formation waters from the Williston Basin are reported from drill stem tests and produced waters for a stratigraphic section near Midale, southeastern Saskatchewan. Above the Prairie aquitard, the Birdbear and Duperow aquifers exhibit the highest SD (-17%>) and δ18O (6.9‰) values known from the Williston Basin. They are substantially more D and 18O enriched than formation waters from equivalent aquifers 160 to 360 km farther north and northeast in stratigraphic sections above three potash mines. Below the Prairie aquitard, the three deepest aquifers at the Midale site yielded lower δD and δ18O values than the Birdbear and Duperow aquifers and are relatively uniform in isotopic composition down to the Precambrian basement. The new data, in conjunction with data from the literature, show that Williston Basin formation waters vary substantially in isotopic composition between aquifers in stratigraphic section and within the same aquifer regionally. These large isotopic variations reveal the potential of H and O isotopes in Williston Basin formation waters for tracing petroleum migration pathways and for fingerprinting sources of contaminant water encountered during drilling and production of hydrocarbons.