A physical property of matter, as each element and compound has a unique density associated with it. Density, defined in a qualitative manner, is the measure of the mass of the substance per its unit volume. In petroleum reservoirs, the reservoir fluids (Gas-Oil-Water) are distributed according to their density; gas has the lowest density, therefore it is trapped at the top of the reservoir, while oil is trapped between gas and water, as water has a higher density than oil.