Carbon Storage


Suitable storage formations can occur in both onshore and offshore settings, and each type of geologic formation presents different opportunities and challenges. The U.S. Department of Energy (DOE) is investigating five types of underground formations for geologic carbon storage:

  • Saline formations
  • Oil and natural gas reservoirs
  • Unmineable coal seams
  • Basalt formations
  • Organic-rich shales

A complete description of these storage types can be found in DOE's Carbon Storage Atlas, Fifth Edition (Atlas V).

Saline formations are porous formations filled with brine, or salty water, and span large volumes deep underground. Carbon capture and storage (CCS) focuses on formations that contain brine with total dissolved solids (TDS) levels greater than 10,000 ppm TDS. Studies show that saline formations have the largest potential volume for storing carbon dioxide (CO2) around the world.

Image depicting the saline storage resources in the United States and portions of Canada. Extensive saline formations exist in the large sedimentary basins located across the country. (click image to enlarge)


Oil and natural gas reservoirs can be found in many places in the United States and around the world. Once the oil and natural gas is extracted from an underground formation, it leaves a permeable and porous volume that can be readily filled with CO2. Oil and natural gas reservoirs are ideal geologic storage sites because they have held hydrocarbons for thousands to millions of years and have conditions suitable for CO2 storage. Injecting CO2 can also enhance oil production by pushing fluids towards producing wells through a process called enhanced oil recovery (EOR).

Images depicting the oil reservoirs, natural gas reservoirs, and unmineable coal storage resources in the United States and portions of Canada. (click image to enlarge)

Coal that is considered unmineable because of geologic, technological, and economic factors (typically too deep, too thin, or lacking the internal continuity to be economically mined) may still serve as locations to store CO2. To be considered for CO2 storage, the ideal coal seam must have sufficient permeability and be considered unmineable. Coal seams may also contain methane (CH4), which can be produced in conjunction with CO2 injection in a process called enhanced coal bed methane (ECBM) recovery (see depiction below). In coal seams, the injected CO2 can be chemically trapped by being adsorbed (or adhered) to the surface of the coal while CH4 is released and produced. This trapping mechanism allows for permanent storage of CO2.

Diagram depicting ECBM and EOR recovery process by which CO2 is injected and used to drive the natural gas or oil towards a recovery well.

Basalt is a type of formation that was deposited when large flows of lava spread from volcanoes, cooled, and then solidified. Over time, thick layers of basalt were built up (with other formation types often layered in between) and have been identified in buried deposits across the United States. The chemical and physical properties of these basalts, as well as the other formation types in between basalt layers, make them good candidates for CO2 storage systems. The chemistry of basalts potentially allows injected CO2 to react with magnesium and calcium in the basalt to form the stable carbonate mineral forms of calcite and dolomite. This mineralization process shows promise to be a valuable tool for CCS because the mineralization process permanently locks carbon in the solid mineral structure, thereby permanently trapping the CO2.

Image depicting Basalt Formations in the United States. Basalts may offer a highly secure method of CO2 storage because of their potential to allow the CO2 to react with the minerals in basalt to form carbonates, thereby permanently trapping the CO2. (click image to enlarge)

Shale formations are found across the United States and are typically low-porosity and low permeability formations best suited as confining zones. However, some shales have similar properties to coal, having the ability to trap CO2 through adsorption (adherence to the surface), subsequently releasing methane and making them potentially attractive for storage.

Image depicting basins containing organic-rich shales in the United States and portions of Canada. (click image to enlarge)

Myth: There are limited options to store CO2 underground, and little is known about these options.
Reality: There are many storage types that can store CO2 and geologic storage of oil, natural gas, and CO2 in the subsurface has been occurring naturally for millions of years.