Storing CO2 in geologic formations deep underground is a well-established industrial carbon storage process. For decades, enhanced oil recovery projects have injected CO2 underground to push remaining oil out of the pore spaces for oil extraction, leaving the oil reservoir full of CO2. Earth’s subsurface is a natural vault with some geological formations able to provide secure places to store CO2.

  While not a CDR process on its own, storing CO2 deep underground can fulfill the storage component of a direct air capture with carbon storage or bioenergy with carbon capture and storage system. After being captured from the atmosphere, CO2 is typically injected over a kilometer below the surface and trapped underground by a variety of mechanisms depending on the rock formation.  

  • What makes a good CO2 storage location? 

    In sedimentary rocks:  

    In order to leverage natural trapping mechanisms to keep CO2 securely stored, ideal underground rock formations in sedimentary rocks need to have several key characteristics: 

    • The formation needs to be deep: At depths of 800 metres underground or more, the temperature and pressure are high enough to keep the injected CO2 as a supercritical fluid, a state that is neither gas nor liquid. In this state, CO2 is much denser than it is as a gas, which increases how much can be stored within a certain space and decreases the chance of the CO2 migrating out of the storage space. 

    • High porosity and permeability of rock: Rock with these characteristics allows the CO2 to spread through the rock, where some of it gets trapped in the pore space and remains there, like water in a sponge. 

    • The formation needs to be below one or more others made from relatively impermeable rock: Some of the CO2 might continue to spread throughout the rock and eventually migrate upwards. That is why it is essential for there to be impermeable layers of caprock above the storage formation. This acts as a seal and prevents the CO2 from leaving the storage site and potentially returning to the atmosphere.  

    Types of sedimentary rock: 

    • Saline aquifers, which are underground rock formations that are filled with saltwater, are a common type of geological formation for CO2 storage. In these, some CO2 dissolves into the salty water within the pore spaces, which is denser than the supercritical CO2 and therefore unlikely to migrate upwards. e.g., the Shell Quest CCS facility stores CO2 in the Basal Cambrian Sandstone, an underground saline aquifer that spans central and southern Alberta.  

    • Depleted oil and gas reservoirs are also commonly used formations for CO2 storage. These rocks once held oil and gas, but the liquid or gaseous fuels have since been extracted, leaving behind available pore space for CO2. e.g., Enhance Energy’s Clive project injects CO2 into declining oil fields in central Alberta.  

    Inigneous or metamorphic rocks: 

    Mafic and ultramafic rocks — high in metal content — are well-suited formations to store the CO2 as a solid via a process called in-situ mineralization. With this method, concentrated CO2 and water are injected and reacted with minerals in the rock to form solid carbonates that keep the CO2 within the underground formation. This means that these projects are less reliant on a cap rock to prevent CO2 migration. Additionally, shallower depths for storage volumes can also be considered, since maintaining sufficient temperature and pressures to keep the CO2 as a supercritical fluid are not as relevant. Carbfix, a company in Iceland, CO2 dissolved in carbonated water and injected 400-800m underground into young basaltic rock, which is porous and suitable for mineralization, was found to mostly mineralize into solid carbonates within two years. 

    For any deep underground storage project, fulsome assessments and rigorous monitoring minimizes the risk of leakage. 

Last Edited - July 22, 2026

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