| Popis: |
Carbon dioxide, produced at low temperatures, is the dominant gaseous species evolved during steam-assisted thermal recovery of bitumen at the Tucker Lake pilot, Cold Lake, Alberta. Two possible sources for the produced CO2 are considered: pyrolysis of bitumen and dissolution of carbonate minerals. Data from natural systems and experiments by other authors suggest that clay-carbonate reactions are the dominant source of CO2. Carbon isotope and chemical analyses of produced gas and water reveal that CO2 and HCO3− become more enriched in 13C, and concentrations of Ca2+ and HCO3− rise during production. Temperatures in the producing zone are estimated to be between 70 and 220°C using the Na K and chalcedony chemical geothermometers, and show that production of 13C-enriched CO2 and HCO3− occurs as temperature decreases during production. This extends the temperature range at which CO2 is known to be produced from carbonates to lower levels than previously noted. The δ13C of calcite in the reservoir is enriched in δ13C, giving values as high as + 17.4%. When the calcite isotopic compositions are combined with temperature data and temperature fractionation equations for calcite, CO2, and HCO3−, they show calcite to be the only viable source of CO2. Bitumen pyrolysis may contribute small amounts of CO2, probably by decarboxylation, early in the production cycle but cannot contribute significant volumes. The recognition of production of CO2 by reactive calcite destruction at temperatures between 70 and 220°C suggests that this process may be responsible for the production of large quantities of CO2 in natural systems, particularly in lithofeldspathic sands and shales with high carbonate content and abundant clays. Organic acids have been suggested to be the source of CO2 in diagenetic fluids, but the results presented here suggest that this hypothesis requires more complete investigation. |
