This paper presents the initial results of a scientific drilling project to recover core and pressurized fluid samples from a natural CO₂ reservoir, near the town of Green River, Utah. The drilling targeted a stacked sequence of CO₂-charged Jurassic sandstone reservoirs and caprocks, situated adjacent to a CO₂-degassing normal fault. This site has actively leaked CO₂ from deep supercritical CO₂ reservoirs at depth > 2 km within the basin for over 400,000 years. The project objectives were to gather samples to examine reactive fluid flow in the reservoirs, caprocks and faults, during migration of CO₂ through the geological overburden from the deep supercritical CO₂ reservoirs. Downhole fluid sampling and fluid element and isotope geochemistry show that the shallow reservoirs are being actively fed by inflow of CO₂-saturated brines through the faults. Comparisons of shallow and deep fluid geochemistry suggests that: (i) CO₂ and CO₂-charged brines co-migrated from the deep reservoirs, (ii) the CO₂-saturated brines migrating from depth interact with significant volumes of meteoric groundwater in aquifers in the shallower Permian and Jurassic sandstones, diluting the brine composition, and (iii) that a significant fraction of the CO₂ migrating from depth is dissolved in these brine-meteoric water mixtures, with > 99% of the CO₂ in fluids sampled from the shallow reservoirs being derived during fluid migration, after the fluids left their source reservoir. The ⁸⁷Sr/⁸⁶Sr ratio of the brine flowing through the faults is significantly elevated due to the addition of Sr from silicate mineral dissolution during fluid migration.

The association of bleached sandstones in the core with CO₂-rich fluids supports interpretations from elsewhere that CO₂-charged brines with CH₄ or H₂S reductants can dissolve hematite present within the sediment. Analysis of fluid geochemistry and sandstone petrology suggest that the CO₂-rich fluids dissolve carbonate, hematite and gypsum in the reservoirs, as they flow away from the faults.

Element and isotope geochemistry of fluid samples from the drillhole and Crystal Geyser constrain mixing models which show that, within the Navajo Sandstone, the reservoir fluids are undergoing complex mixing of: (i) CO₂-saturated brine inflowing from the fault, (ii) CO₂-undersaturated meteoric groundwater flowing through the reservoir and (iii) reacted CO₂-charged brines flow through fracture zones in the overlying Carmel Formation caprock, into the formations above. Such multi-scale mixing processes may significantly improve the efficiency with which groundwaters dissolve the migrating CO₂.