重油是原油提取汽油、柴油后的剩余重质油，其特点是分子量大、黏度高。重油的比重一般在0.82～0.95，比热在10,000～11,000kcal/kg左右。其成分主要是碳氢化合物，另外含有部分的（约0.1～4％）的硫黄及微量的无机化合物。主要用于大型蒸汽轮机锅炉及中大型船舶用引擎的燃油，不过由于现在大功率燃气轮机的出现，以及重油燃烧对环境的污染，现在重油锅炉几乎被淘汰了。不过，重油可以用作现代大型燃煤锅炉的启动点火燃料。现在对重油的研究方向主要是长链烃的裂化，目的是通过裂解重油得到轻质的柴油或者煤油。此外，二战其间重油可用于制造燃烧弹。

Heavy crude oil (or extra heavy crude oil) is highly-viscous oil that cannot easily flow to production wells under normal reservoir conditions.(Mai)

It is referred to as "heavy" because its density or specific gravity is higher than that of light crude oil. Heavy crude oil has been defined as any liquid petroleum with an API gravity less than 20°.(Dusseault 2001) Physical properties that differ between heavy crude oils and lighter grades include higher viscosity and specific gravity, as well as heavier molecular composition. In 2010, the World Energy Council defined extra heavy oil as crude oil having a gravity of less than 10° and a reservoir viscosity of no more than 10,000 centipoises. When reservoir viscosity measurements are not available, extra-heavy oil is considered by the WEC to have a lower limit of 4° °API.(WEC 2007) (i.e. with density greater than 1000 kg/m or, equivalently, a specific gravity greater than 1 and a reservoir viscosity of no more than 10,000 centipoises. Heavy oils and asphalt are dense nonaqueous phase liquids (DNAPLs). They have a "low solubility and are with viscosity lower and density higher than water.(2003 & Llamas 118) "Large spills of DNAPL will quickly penetrate the full depth of the aquifer and accumulate on its bottom."(2008 & Vrba 23)