Although the idea of intentionally storing CO₂ underground for extended periods seems new, nature has in fact been doing it for millions of years. There are numerous natural CO₂ reservoirs throughout the Rocky Mountain states in geologic “domes” and “traps,” suggesting these types of formations and other similar geologic structures will be excellent places for storing CO₂ captured from industrial facilities.

Natural gas reservoirs also demonstrate the effectiveness of long-term geologic storage, as they have trapped methane — which is more buoyant than CO₂ — for millions of years. Once depleted, such reservoirs are excellent candidates for "refilling" with CO₂. In general, CO₂ storage will be successful wherever a porous (high permeability) rock such as sandstone is sealed by a continuous layer of low-permeability rock such as shale. Various "trapping" mechanisms also help keep subsurface as CO₂ in place.

The practice of injecting gases and liquids underground for temporary or long-term storage is well established. For example, natural gas companies routinely use depleted gas reservoirs to store gas received via pipeline, prior to its distribution to consumers.

Although health, safety, and environmental problems aren’t expected from geologic storage of CO₂, technology validation tests and large-volume storage tests are needed to ascertain real-world site management needs and costs.

To date, most experience with CO₂ injection comes from the oil industry, which uses it to “loosen” residual oil in mature producing fields. Known as enhanced oil recovery (EOR), this practice represents an excellent opportunity for initial geologic sequestration projects because of its economic value and established technical and regulatory procedures. However, EOR sites are ultimately too few and too geographically isolated to accommodate the quantity of CO₂ expected to be captured at large-scale industrial operations.

The geologic formations with the greatest potential CO₂ storage capacity, both nationally and in the West Coast region, are sedimentary basins containing well-sealed strata of porous rock filled with salt water. These "saline formations" may lie beneath land or beneath the sea on the continental shelf. The potential capacity of these formations for CO₂ storage is quite large, equivalent to hundreds of years of the region’s man-made CO₂ emissions.

Despite their similarity to oil and gas reservoirs (except for the presence of hydrocarbons), saline formations have been little studied and are generally not used commercially. However, a major demonstration of CO₂ injection into a saline formation for sequestration has been under way for more than a decade at an offshore gas platform in the North Sea near Norway. Results from this project (and others) suggest that saline formations will be reliable, long-term geologic sequestration sites.

Other geologic formations in the West Coast region also warrant investigation as candidates for CO₂ storage. Nevada's mafic and ultra-mafic rock formations are an example; they contain magnesium and iron that would chemically react with injected CO₂ to form minerals deep underground. WESTCARB continues to expand its studies in these areas.