in geology, naturally occurring storage area, characteristically a folded rock formation such as an anticline, that traps and holds natural gas. The reservoir rock must be permeable and porous to contain the gas, and it has to be capped by impervious rock in order to form an effective seal that prevents the gas from escaping upward or laterally. Typical reservoir rocks are sedimentary in origin and include sands, sandstones, arkoses, and fissured limestones and dolomites. The natural gas migrates into these from the compact and less permeable source rock (e.g., clays and fine-grained limestones) because of the pressure difference between the source rocks, which are compressed by the weight of overlying rocks, and the reservoir rocks, which are at lower pressures. Recovery from the reservoir is effected principally by the natural expansion of the gas.

In the United States and certain other countries, artificial gas reservoirs are being created from depleted oil and gas fields, particularly near salt domes and in sedimentary basins, to store gas during periods of low consumption for later use.