Petroleum or crude oil – a naturally occurring, odorous, yellow-to-black liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in oil reservoirs.  It is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rock and undergo intense heat and pressure.  It varies greatly in appearance depending on its composition. It is usually black or dark brown (although it may be yellowish, reddish, or even greenish), and can be light, medium, heavy, or extra heavy.

Light crude oil – is liquid petroleum that has a low density and flows freely at room temperature.  Light crude oil receives a higher price than heavy crude oil on commodity markets because it produces a higher percentage of gasoline and diesel fuel when converted into products by an oil refinery.

Heavy crude oil or Extra Heavy crude oil – oil that is highly viscous, and cannot easily flow to production wells under normal reservoir conditions and has a molasses-like texture.  It is referred to as “heavy” because its density or specific gravity is higher than that of light crude oil.

Oil Sands – loose sand or partially consolidated sandstone containing naturally occurring mixtures of sand, clay, and water, saturated with a dense and extremely viscous form of petroleum technically referred to as bitumen (or tar due to its similar appearance, odor and color). Oil sands are most commonly found in Canada, Kazakhstan and Russia.

Oil Shale –  an organic-rich fine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds) from which liquid hydrocarbons called shale oil can be produced. It is a substitute for conventional crude oil; but extracting shale oil from oil shale is more costly than the traditional production of conventional crude.