Formations that have already been tapped of all their recoverable natural gas and which leaves an underground formation, geologically capable of holding natural gas. Using an already developed reservoir for storage purposes allows the use of the extraction and distribution equipment left over from when the field was productive. Of the three types of underground storage, depleted reservoirs, on average, are the cheapest and easiest to develop, operate, and maintain. The factors that determine whether or not a depleted reservoir will make a suitable storage facility are both geographic and geologic. Geographically, depleted reservoirs must be relatively close to consuming regions. They must also be close to transportation infrastructure. Geologically, depleted reservoir formations must have high permeability and porosity. The porosity of the formation determines the amount of natural gas that it may hold, while its permeability determines the rate at which natural gas flows through the formation, which in turn determines the rate of injection and withdrawal of working gas. In order to maintain pressure in depleted reservoirs, about 30-50 percent of the natural gas in the original formation must be kept as cushion gas.