A permeable rock that may contain oil or gas in appreciable quantity and through which petroleum may migrate. For rocks to contain oil and gas they must have two important properties: (1) a place to store the oil and gas, namely the pores. The more pores the more oil or gas that can be contained in the rock which is measured in terms of a rock’s porosity. Porosity is expressed as the volume percent of the rock that contains open space and can range from 5 to 30%. (2) Permeability – The pores must be connected so that the oil and gas can move through the rock, which is known as the rock’s permeability. Permeability is measured in thousandths of a darcy or millidarcys. Sandstone and carbonate rocks are generally the most porous and permeable rocks