Stable isotopes of clay minerals from autoclave tests of oil sands: Implications for clay formation during steaming of Alberta Clearwater oil sands

In an effort to evaluate mineral-water isotopic exchange during cyclic steam stimulation (CSS), solutions and < 2 mu m berthierine-dominated solids from the Clearwater Formation oil sands of Alberta, Canada were analyzed for stable isotope compositions before and after reaction in autoclaves for 1008 h at 250 degrees C. There was no significant change in solution delta O-18 and delta H-2, which is consistent with the high water/mineral ratio used in the experiments. The solids showed a marked decrease in both d18O and delta H-2 following the experiments. Pre-run solids have delta O-18 of + 9.5 to + 12.9% and delta H-2 of -114 to -113%, whereas post-run solids have delta O-18 of -4.7 to +2.1% and delta H-2 of -147 to -128%. Neither oxygen-nor hydrogen-isotope equilibrium was established between the solids and the solutions. Calculation suggests that oxygen-isotope exchange (44-58%) was greater between the solids and the solutions than was the case for hydrogen isotopes (23-50%). We propose that this behaviour resulted from partial inheritance of the pre-run berthierine structure during formation of the post-run smectite, chlorite-smectite and chlorite. This process confounds the use of clay mineral stable isotope compositions as a temperature indicator of in situ steam/steam condensate interaction with oil-sands reservoirs. The results also suggest an additional mechanism by which new clay minerals can be formed during CSS-related, artificial diagenesis.