Reservoir
The porous, permeable subsurface rock that holds oil and gas and can yield it to a well — defined by its porosity, permeability, and trapped hydrocarbons.
A reservoir is the underground rock that actually stores oil and gas and is capable of giving it up to a well. To be a reservoir, rock must have permeability (connected spaces that let fluids flow) and porosity (open space to hold them), plus a trap and seal that kept the hydrocarbons from escaping.
A reservoir is not the same thing as a formation. A formation is a named rock layer; the reservoir is the specific producing interval within it that holds movable hydrocarbons. One formation can contain several reservoirs, or none at all.
For owners, reservoir quality drives how much your minerals are worth. Better porosity and permeability mean stronger wells, higher reserves, and bigger royalty checks. In conventional plays, oil, gas, and water naturally separate by weight inside the reservoir; in a shale play, the source rock and reservoir are the same tight rock, which is why those wells need fracturing to produce.