ABSTRACT

After primary migration out of source beds, further migration of petroleum is not achieved by its buoyancy alone but also by movement of aqueous fluids in the porous rock through which the petroleum migrates. Temperatures required for the generation of oil and the hydrodynamic conditions during secondary and tertiary migration, therefore, influence the ultimate trapping site for petroleum.

Times of fluid movement have been documented in the Devonian Prairie Evaporite by vertical extent of collapse features, characterization of the properties of fluids in inclusions in halite, and radiometric dating of evaporite minerals. Halites from the potash ore zones of the Prairie Evaporite in the Saskatoon area were recrystallized by migrating fluids and contain in their fluid inclusions a record of the temperature and stable isotopic compositions of the fluids. Three events can be recognized. The first, which occurred shortly after deposition of the Prairie Evaporite, was at a temperature of about 35°C (95°F), and involved water with 8D near -92 and ઠ180 near -8 which are typical of groundwaters from mid-latitudes. Two later events, at about 50°C (120°F) and 80°C (180°F), were important with respect to petroleum migration because of the substantial amount of fluid involved, the maturity of the basin during these events, and the effect of these fluids on the permeability of the reservoir rocks. The lower temperature event involved water with 8D near -70 and ઠ180 near -6, similar in isotopic composition to evolved basin fluids. The higher temperature event involved water with 5D near -141 and ઠ180 near -15 and occurred during maximum burial at the end of the Cretaceous. The stable isotopic compositions of these fluids are similar to brines presently residing within formations overlying the Prairie Evaporite but the temperatures were higher because the basin was deeper and the geothermal gradient was greater.