UNUSUAL STABLE ISOTOPIC COMPOSITIONS OF FORMATION-WATERS FROM THE INTERLAKE FORMATION (SILURIAN) IN THE SOUTHEASTERN SASKATCHEWAN AREA OF THE WILLISTON BASIN

The Williston Basin is well known for its large regional groundwater flow-system containing saline formation-waters, and also known for the multitude of water-energy industries active in the basin (e.g., conventional/unconventional oil and gas, geothermal energy, potash mining, geological carbon storage, etc.). Stable isotopes of formation-waters (oxygen and hydrogen) and isotopes of dissolved solutes (strontium, carbon, bromine, chlorine, boron, etc.) have proven very useful both for operational issues (e.g., fingerprinting formation waters for oil and gas) and to understand the present- and paleo-hydrogeology of the basin.

However, oxygen and hydrogen stable isotopic compositions of formation waters from the Interlake Formation (Silurian) in southeastern Saskatchewan are anomalous compared to all other Paleozoic formation-waters in that portion of the basin. Isotopic compositions of 12 samples obtained from different producing wells and valid well-tests from the Interlake Formation in southeastern Saskatchewan range from approximately -16 to -8 per mil (delta 18-oxygen) and -133 to -92 per mil (delta 2-hydrogen). These values are approximately 15 per mil and 60 per mil lower, for oxygen and hydrogen respectively, than the formation-waters in Paleozoic formations directly above and below the Interlake Formation. What is most unusual is that the major and minor dissolved chemical species (e.g., TDS, chloride, sodium, calcium, bromine, etc.) from the Interlake Formation are similar to over/underlying formations. Distributions of dissolved chemical species in the Interlake Formation fit within well-established patterns and end-members of the Williston Basin. The oxygen and hydrogen isotopic compositions to not match established distribution patterns in the basin.

The origin of these anomalous isotopic compositions remains unclear. One possible explanation is recharge of very cold (ice-age?) meteoric water that caused local halite dissolution/mixing, creating a dense brine that was hydraulically isolated from the surrounding flow system.

This talk will present these new isotopic data from the Interlake Formation in southeastern Saskatchewan and provide a discussion of their possible origin and implications for paleo-hydrogeology of the basin.