GEOCHEMICAL EVOLUTION OF DEEP, SALINE BRINES FROM PALEOZOIC RESERVOIRS IN SOUTHWEST, WYOMING; IMPLICATIONS FOR POTENTIAL CO2 SEQUESTRATION

Two sets of formation brines were collected at depths greater than 11,000 ft below the surface from a stratigraphic test well on the northeast flank of the Rock Springs Uplift (RSU) in southwest Wyoming. The well was drilled to assist in the geologic and geochemical characterization of a potential CO₂ sequestration site. The brines were collected from Paleozoic reservoirs, the Weber and Madison formations, and were analyzed for geochemistry and dissolved gas compositions. Few data exist on the composition of brines from these reservoirs on the RSU: they are limited to one 60 year old sample from the crest of the RSU and the work reported here, which consists of two sets of samples collected via the test well: the first in 2011 and the second in 2012.

The results indicate that the brines have measured salinities that are greater than most Wyoming groundwater samples. Samples from both formations are Na-Cl type waters, with TDS between 75,000 and 109,000. The Weber Sandstone has higher TDS than the Madison Limestone, and conservative element analysis suggests a greater influence from halite dissolution. Conservative element analysis shows that both formations have enriched concentrations of analytes relative to evaporative seawater, indicating post-burial input from water-rock reactions. Dissolved gas compositions are dissimilar by formation; dissolved gases in the Weber are predominantly nitrogen with a higher concentration of alkanes, whereas dissolved gases in the Madison are predominantly carbon dioxide with minute concentrations of alkanes. There are also distinct geochemical differences between the two sample sets: the second sample set, retrieved nearly a year after the well was completed, displays evidence of sulfate reduction which is likely a byproduct of downhole testing and well completion practices.

The high measured salinity and evidence of water-rock reactions suggests these brines have a relatively long residence time, increasing the likelihood of long-term storage in the advent of CO₂ sequestration. Geochemical differences between the formation fluids suggest these reservoirs are isolated from each other, and that interlying strata act as seals. In addition, these samples highlight the geochemical impact of in-situ reservoir tests and well site practices.