Geologic carbon sequestration is the process of storing carbon dioxide (CO2) in underground geologic formations. The CO2 is usually pressurized until it becomes a liquid, and then it is injected into porous rock formations in geologic basins. This method of carbon storage is also sometimes a part of enhanced oil recovery, otherwise known as tertiary recovery, because it is typically used later in the life of a producing oil well. In enhanced oil recovery, the liquid CO2 is injected into the oil-bearing formation in order to reduce the viscosity of the oil and allow it to flow more easily to the oil well.
Biologic carbon sequestration refers to storage of atmospheric carbon in vegetation, soils, woody products, and aquatic environments. For example, by encouraging the growth of plants—particularly larger plants like trees—advocates of biologic sequestration hope to help remove CO2 from the atmosphere.