 The amount of oil that is recoverable is determined by a number of factors, including the permeability of the rock, the strength of natural drives (the associated gas present, pressure from adjacent water or gravity), porosity of the reservoir rock, i.e.the rock storage capacity, and the viscosity of the oil.When the reservoir rocks are "tight", as in shale, oil generally cannot flow through, but when they are permeable, as in sandstone, oil flows freely.