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-km² 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.

An 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.