LONG-TERM BEHAVIOR OF CO2-RICH FLUID IN THE LITTLE GRAND WASH FAULT, UTAH, USA

The major difficulties in the assessment of the seal integrity majorly include two aspects: lack of available samples (especially for saline aquifers), and the difficulty to study over geological time and spatial scales. The analysis of natural analogue enables to overcome these difficulties. Crystal Geyser, the natural CO₂ spring located at Green River, Utah, is used to study how the long-term CO₂ charge triggers chemical reactions and migrates in shales that associated with the local big fault. The leakage of CO₂ from the Green River region has been demonstrated to be last for at least 400 kyrs. The leaked CO₂ (majorly crust originated) dissolves in the brine and rises upward through the main fault zone to enter the overlying aquifers and mixed with the re-charged meteoric fluid in the Jurassic formations. The deposit of travertines distributed alongside the foot wall of the Little Grand Wash Fault suggest the CO₂ migrates through the fault or the fault related fractures.

The petrographic features of the Mancos shale in the hanging wall of the fault have been studied and compared with the unaltered Mancos shale that not influenced by the CO₂-rich fluid. A small-scale travertine at the junction of the main fault and a small relay fault was discovered was deposited by the CO₂-rich fluid that penetrating the deformed Mancos shales. The stable isotope coupled with petrographic methods were used to distinguish the depositional/early diagenetic carbonates and the CO₂-rich fluid derived carbonates. The result shows the CO₂-rich fluid could precipitate up to 24% of calcite against the whole rock (%weight) at the damaged zone of the Mancos shale that 15m away from the Little Grand Wash fault, while the Mancos further than 17m from the main fault contains no CO₂-rich fluid induced precipitation. The water/rock interaction model has been established to quantitatively illustrate reaction pathways under different water-rock contact schemes. The final conclusion for the natural analogue is, the CO₂-rich fluid can only penetrate and interact with the deformed shales that associated with fault. No evidence show the intact Mancos has been affected by the CO₂-rich fluid. The result has implications for the future siting for the potential geological storage sites, especially for the assessment of integrity of the fault-associated seals.