Heavy oil is defined as liquid petroleum of less than 20°API gravity or more than 200 cp viscosity at reservoir conditions.No explicit differentiation is made between heavy oil and oil sands (tar sands)although the criteria of less than 12°API gravity and greater than 10,000 cp are sometimes used to define oil sands.The oil in oil sands is an immobile fluid under existing reservoir conditions, and heavy oils are somewhat mobile fluids under naturally existing pressure gradients. Unconsolidated sandstones (UCSS) are sandstones (or sands) that possess no true tensile strength arising from grain-to-grain mineral cementation. Many heavy oil reservoirs are located in unconsolidated sandstones.