Conventional gas is stored in porous and permeable sedimentary rocks, such as sandstones or limestones, in a rock structure known as a trap. Impermeable rocks directly above the structure trap the gas. A well drilled into the structure intersects the porous reservoir and gas flows into the well.
Gas (and/or oil) is generated from a source rock (an organic-rich rock) that has been buried and heated within the Earth’s crust. The gas moves into porous and permeable rocks and travels via these rocks into a structure (a trap).
The gas is held in between the grains of porous sandstone in structures like a dome or upside down dish. This is called a reservoir. Directly over the top of the reservoir is an impermeable layer of much finer grained rock (claystone) that provides a seal over the gas.
It is this combination of naturally occurring features that defines conventional gas.