Measurements Of CO2 and CH4 soil gas concentrations and gas exchange with the atmosphere at a large-scale CO2-enhanced oil recovery (EOR) operation at Rangely, Colorado, United States allowed assessment of the microseepage potential. Shallow and deep soil gas concentrations and direct transport Of CO2 and CH4 complemented by carbon isotopes have demonstrated an estimated microseepage rate to the atmosphere of approximately 400 t CH4/yr from the 78-km(2) area of the Rangely field. Preliminary estimates of deep-sourced CO2 losses are in the range of 170 to less than 3800 t/yr. Several holes as much as 9 m deep for nested sampling of gas composition, stable carbon isotopic ratios for CO2 and CH4, and carbon-14 measurements on CO2 indicate that deep-sourced CO2 microseepage loss was detected. Methanotrophic oxidation of microseeping CH4 to CO2 in soils demonstrates significant contribution to soil gas CO2 in high CH4 flux areas necessitating a revision of the estimated direct, CO2 microseepage rate to less than 170 t/yr over the Rangely field.Am evaluation of produced water quality from pre-CO2-EOR to 1999 demonstrates an increase in some parameters particularly of, Ca2+ and HCO3- indicating dissolution of ferroan calcite and ferroan dolomite in the Weber Formation. Inverse computer modeling suggested carbonate mineral sequestration was possible within the constraints of evolution of produced water quality X-ray analyses. of well scales, however, do not support the presence of significant mineral sequestration. Instead, modeling indicates that the bulk of the CO2 that has been injected since 1986 is sequestered as dissolved CO2.