Why must heavy oil sometimes be heated or diluted before pipeline transport?

Heavy oil and bitumen (found in tar sands) possess high viscosity and density due to their chemical composition, which is rich in large hydrocarbon molecules like asphaltenes and resins. At the temperatures typically encountered in natural deposits or during transport, these substances are essentially immobile and will not flow through a pipeline at an acceptable rate.12

To facilitate movement through pipelines, these hydrocarbons must be heated or diluted with lighter oils to improve their mobility. Heating reduces the viscous resistance to flow, though this process often necessitates the combustion of additional fuel, which adds to the operational costs. In some cases, dilution is used as an alternative or supplementary method to ensure the substance can be transported efficiently.123