420,000 year assessment of fault leakage rates shows geological carbon storage is secure

Carbon capture and storage (CCS) technology is routinely cited as a cost effective tool for climate change mitigation. CCS can directly reduce industrial CO₂ emissions and is essential for the retention of CO₂ extracted from the atmosphere. To be effective as a climate change mitigation tool, CO₂ must be securely retained for 10,000 years (10 ka) with a leakage rate of below 0.01% per year of the total amount of CO₂ injected. Migration of CO₂ back to the atmosphere via leakage through geological faults is a potential high impact risk to CO₂ storage integrity. Here, we calculate for the first time natural leakage rates from a 420 ka paleo-record of CO₂ leakage above a naturally occurring, faulted, CO₂ reservoir in Arizona, USA. Surface travertine (CaCO₃) deposits provide evidence of vertical CO₂ leakage linked to known faults. U-Th dating of travertine deposits shows leakage varies along a single fault and that individual seeps have lifespans of up to 200 ka. Whilst the total volumes of CO₂ required to form the travertine deposits are high, time-averaged leakage equates to a linear rate of less than 0.01%/yr. Hence, even this natural geological storage site, which would be deemed to be of too high risk to be selected for engineered geologic storage, is adequate to store CO₂ for climate mitigation purposes.