Which two rock types are the most common reservoir rocks?

• A. Clay and shale
• B. Dolostone and limestone
• C. Sandstone and Carbonate rocks
• D. Granite and sandstone

Answer: (D)

Reservoir rocks are defined by having enough interconnected pore space to store fluids and enough pathways for them to flow. Sandstone is a classic example because its grain packing leaves well-connected pores, giving high porosity and permeability. Carbonate rocks like limestone and dolostone also make excellent reservoirs, often hosting substantial porosity, which can be enhanced by dissolution and fracturing, creating extensive, permeable networks. Together, sandstone and carbonate rocks cover a wide range of reservoir types found in many basins around the world. Clays and shales, by contrast, have very low permeability and mostly act as seals rather than primary reservoirs, while granite is typically dense with little pore space, making it a poor reservoir.