HIGH RESOLUTION CARBONATE RECORDS OF FLUID-FAULT-INTERACTIONS AT THE GREEN RIVER CO2 NATURAL ANALOGUE

Carbon Capture and Storage (CCS) will play a key role in reducing anthropogenic CO₂ emissions. The long-term fate of CO₂ in a geological reservoir (~1,000 to 10,000 years) must be predicted to satisfy both regulatory frameworks and public perception. Timescales of trapping mechanisms within different reservoirs must be quantified, and the consequences of leakage must be understood for planning of remediation strategies. Natural accumulations of CO₂ represent a unique way to investigate processes associated with CO₂ storage over geological time.

The Colorado plateau contains multiple accumulations of CO₂ which have been securely stored for 10⁴-10⁷ years. At Green River (UT) CO₂-enriched fluids leak up the damaged zone of two faults in the core of an anticlinal trap. Fault zone mineralisation takes the form of aragonite, calcite, gypsum and celestine deposits. The focus of this study are the large aragonite veins found in the northern footwalls of the Little Grand and northern Salt Wash faults. These veins provide a >~ 120,000 year record of CO₂ migration up the fault planes. The veins formed periodically as CO₂ leakage is pulsed, and CO₂ exsolution induces fracturing and mineral precipitation. The veins inform us of: 1) timing and location of fluid flux out of the fault system; 2) gas exsolution processes; 3) fault zone mineralisation; and 4) rates of fluid-rock reaction. We present new U-Th and Sr-isotope data across the length of the faults and high resolution transects from within the veins, using recently developed Laser-Ablation MC-ICP-MS techniques.

Carbonate deposit locations migrate with time, suggestive of fault pathway sealing. Individual localities tend to have distinct ⁸⁷Sr/⁸⁶Sr and Uranium (δ²³⁴U_i) isotope signatures suggesting that each pathway has a unique ‘plumbing’. The two isotope systems show loose coupling, with a large variation in δ²³⁴U_i suggesting stagnant regions of fluid or fluid moving through a region previously occupied by a gas cap.