Why does heavy oil need heating before it can flow more easily?

Heavy oil and the bitumen found in tar sands possess a high viscosity, which creates a significant resistance to flow at natural reservoir temperatures. Because these substances are naturally thick and immobile, they cannot be easily extracted or transported through pipelines using standard methods.1234

Heating is necessary because it lowers the viscosity of the petroleum, markedly improving its mobility. In the context of reservoir recovery, heat is introduced through methods such as steam injection or in situ combustion, which allows the oil to flow toward a production well bore. Similarly, when transporting heavy oil via pipeline, direct heating is often required to ensure the substance moves at an acceptable rate. In the processing of tar sands, steam and hot water are used to separate the bitumen from the sand, facilitating its further upgrading into synthetic crude oil.315462