Boron isotope ratios of reservoir minerals and fluids can be a useful geothermometer and monitor of fluid-rock interactions. In Cold Lake oil sands of northern Alberta, there is a large variation in 611B of the produced waters generated during steam injection and recovery of oil and water. The higher temperature waters (~ 200 °C) have isotopically light 511B values (+3%o to +14%o) and high B contents (~150p.p.m.). It is inferred that the range of 811B values of the hydrothermal fluids results from reaction with the reservoir rock, and is a function of the temperature of the fluid-rock interaction. The distinct B geochemistry of the produced waters can be used to show that there is no detectable mixing of the oil recovery waters with the regional formation waters or shallow groundwater aquifers containing potable water. Examination of B isotope ratios of reservoir minerals, before and after steam injection, allows the evaluation of sources of B in the reservoir. The only significant phase containing B is pumice. It shows generally positive 511 B values before steam injection and negative values after steam, with 511B as low as -28%o. Other possibly reactive phases include clay minerals and organic matter, but their abundance is not great enough to impact on the isotopic composition of the produced waters. This information makes it possible to evaluate the boron isotope fractionation equation derived from experimental data (Williams LB (2000) Boron isotope geochemistry during burial diagenesis. PhD Dissertation. The University of Calgary, Alberta, Canada; Williams LB, Hervig RL, Holloway JR, Hutcheon I (2001 a) Boron isotope geochemistry during diagenesis: Part 1. Experimental determination of fractionation during illitization of smectite. Geoclnmica et Cosmocbimica Acta, in press). The results show that the fractionation curve predicts the difference between 511B of the pumice and hydrothermal fluids in the Cold Lake reservoir. This not only indicates that the reservoir fluids have approached boron isotope equilibrium with the reservoir rock, but also shows that B isotopes provide a useful geothermometer for hydrothermally stimulated oil reservoirs.
- Boron isotope ratios
- Fluid-rock interactions
- Oil reservoir
ASJC Scopus subject areas
- General Earth and Planetary Sciences