Geological sequestration refers to the storage of CO underground in depleted oil and gas reservoirs, saline formations, or deep, coal beds unsuitable for mining.
Once CO is captured from a point source, such as a cement factory, it can be compressed to ≈100 bar into a supercritical fluid.  In this form, the CO could be transported via pipeline to the place of storage. The CO could then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years. Under these storage conditions, the density of supercritical CO is 600 to 800 kg/m.
The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, absence of faults, and geometry of rock layers.  The medium in which the CO is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10 Darcy, with a porosity as high as ≈30%. The porous rock must be capped by a layer of low permeability which acts as a seal, or caprock, for the CO.  Shale is an example of a very good caprock, with a permeability of 10 to 10 Darcy. Once injected, the CO plume will rise via buoyant forces, since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault planes near the injection zone, there is a possibility the CO could migrate along the fault to the surface, leaking into the atmosphere, which would be potentially dangerous to life in the surrounding area. Another risk related to carbon sequestration is induced seismicity. If the injection of CO creates pressures underground that are too high, the formation will fracture, potentially causing an earthquake.
Structural trapping is considered the principal storage mechanism, impermeable or low permeability rocks such as mudstone, anhydrite, halite, or tight carbonates act as a barrier to the upward buoyant migration of CO2, resulting in the retention of CO2 within a storage formation. While trapped in a rock formation, CO can be in the supercritical fluid phase or dissolve in groundwater/brine.  It can also react with minerals in the geologic formation to become  carbonates.