UNDERSTANDING LEAKAGE FROM A FAULT-SEALED CO2 RESERVOIR IN EAST-CENTRAL UTAH: A NATURAL ANALOGUE APPLICABLE TO CO2 SEQUESTRATION

Assessment of fault seal integrity for sequestered CO₂ reservoirs can be tested by analysing naturally occurring CO₂ systems. We investigate a natural CO₂ reservoir in the Colorado Plateau region of east-central Utah that clearly failed in the past and is currently leaking CO₂ along the Little Grand Wash fault and Salt Wash graben. The reservoir consists of an open, gently north-plunging anticline of siliclastic rocks cut by the west-north-west trending, clay-rich faults. Aligned along the immediate footwall of both faults are CO₂-charged geysers and springs, gas seeps and travertine deposits. Research has focussed on the travertine deposits, as they are products of current and past surficial leakage of CO₂ along both fault systems. Actively forming and ancient travertine deposits occur along both faults with older deposits capping buttes that are up to 50m above the active seepage sites. δ¹³C and δ¹⁸O indicate that all deposits are isotopically similar signifying that the source of the fluid has been relatively constant through time. However, the isotopic signatures of the travertines are different from other carbonate precipitates sampled in the fault zone and hanging wall of both faults indicating that the CO₂-rich fluids have not crossed either fault zone. These observations demonstrate a long lived history of CO₂ leakage from the reservoir that was restricted to up-dip fault parallel flow in the footwall. Dating of the travertine deposits is currently being employed to constrain the timing and possible variations of CO₂ leakage through time along both faults. Used in combination with the travertine analyses, a three dimensional geometric model of the region has been developed to investigate various properties of the leaking CO₂ system that include fault seal, fluid flow and volumetric analyses. Ultimately, this research will aim to understand the factors causing the reservoir to leak, define the migration pathways of CO₂ from reservoir to surface and quantify the leakage rates of CO₂ from the reservoir through time.