Rapid microbial methanogenesis during CO₂ storage in hydrocarbon reservoirs

Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO₂) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO₂ geological sequestration^1,2. Depleted hydrocarbon reservoirs have substantial CO₂ storage potential¹,³, and numerous hydrocarbon reservoirs have undergone CO₂ injection as a means of enhanced oil recovery (CO₂-EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon. Here we present noble gas, stable isotope, clumped isotope and gene-sequencing analyses from a CO₂-EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13-19% of the injected CO₂ to methane (CH₄) and up to an additional 74% of CO₂ was dissolved in the groundwater. We calculate an in situ microbial methanogenesis rate from within a natural system of 73-109 millimoles of CH₄ per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO₂ fields suggest that microbial methanogenesis may be an important subsurface sink of CO₂ globally. For CO₂ sequestration sites within the environmental window for microbial methanogenesis, conversion to CH₄ should be considered in site selection.