Inorganic origin of carbon dioxide during low temperature thermal recovery of bitumen: Chemical and isotopic evidence

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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 CO 2 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 CO 2 . Carbon isotope and chemical analyses of produced gas and water reveal that CO 2 and HCO 3 - become more enriched in 13 C, and concentrations of Ca 2+ and HCO 3 - 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 13 C-enriched CO 2 and HCO 3 - occurs as temperature decreases during production. This extends the temperature range at which CO 2 is known to be produced from carbonates to lower levels than previously noted. The 13 C of calcite in the reservoir is enriched in 13 C, giving values as high as + 17.4%. When the calcite isotopic compositions are combined with temperature data and temperature fractionation equations for calcite, CO 2 , and HCO 3 - , they show calcite to be the only viable source of CO 2 . Bitumen pyrolysis may contribute small amounts of CO 2 , probably by decarboxylation, early in the production cycle but cannot contribute significant volumes. The recognition of production of CO 2 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 CO 2 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 CO 2 in diagenetic fluids, but the results presented here suggest that this hypothesis requires more complete investigation.

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