Natural gas is a colorless, odorless, gaseous hydrocarbon that is often stored in underground facilities under pressure. Underground natural gas storage in California is stored in depleted natural gas or oil fields that are close to consumption areas. Conversion of a field from production to storage may take advantage of existing wells, gathering systems, and pipeline connections. Principal operators of underground natural gas storage facilities in California are local distribution companies (utilities) and independent storage service providers. Operators of the underground storage facilities are not necessarily the owners of the natural gas held in storage. Most working gas held in storage facilities is held under lease with shippers, utilities, or end users who own the gas. Total natural gas storage capacity is the maximum volume of natural gas that can be stored in an underground facility in accordance with its design, which includes the physical characteristics of the reservoir, installed equipment, and operating procedures particular to the facility as approved by state and federal agencies. Base or cushion gas is the volume of natural gas intended as a permanent inventory in the storage reservoir to maintain adequate pressure and deliverability rates throughout the withdrawal process. Working gas is the volume of natural gas in the reservoir above the base or cushion gas level that is available to the marketplace. Deliverability is generally expressed as the measure of the amount of natural gas that can be delivered (withdrawn) from a storage facility on a daily basis, usually expressed in terms of millions of cubic feet of gas per day (MMcf/d). Injection capacity (or rate) is the complement of the deliverability or withdrawal rate, the amount of natural gas that can be injected into a facility on a daily basis.