Reservoir Rock
Reservoir rock is the porous, permeable rock — usually sandstone, limestone, or fractured shale — that actually holds the oil and gas a well produces, as distinct from the source rock that generated it.
Reservoir rock is the subsurface rock that stores producible oil and gas. For rock to serve as a reservoir it needs two properties: porosity (enough pore space to hold hydrocarbons) and permeability (interconnected pores that let those hydrocarbons flow toward a wellbore). Without permeability, oil and gas may be present but won't move — which is exactly the problem hydraulic fracturing was developed to solve.
Reservoir rock is one leg of the classic petroleum system, and it's easy to mix up with the others. The source rock is the organic-rich shale that generated the hydrocarbons; the reservoir rock is the porous rock they later migrated into; and a sealing trap is what keeps them from escaping. In conventional plays these are different rocks. In unconventional shale plays, one tight formation can act as source, reservoir, and seal all at once — which is why those wells must be fractured.
Typical reservoir rocks are sandstones and carbonates (limestone and dolomite). The quality of the reservoir rock under your acreage is a primary driver of how much a well recovers — and therefore of what a mineral interest there is worth.