Crude oil is a liquid found naturally in rock, containing mostly complex hydrocarbons, with some additional organic material. It is the major fuel used on the planet, and is used in the production of many synthetic materials like plastics as well.
This oil can come in many different weights and colors, and can differ greatly in its composition. As little as half of the composition of heavy oils can be made up of hydrocarbons, while the lightest oils can be up to 97% hydrocarbons. There are four main hydrocarbons found in crude oil, in varying amounts depending on the oil. Around half of the hydrocarbons in most unrefined oil are naphthenes, one-third are paraffins, one-sixth are aromatics, and the rest are asphaltics. The color can range from pure black or dark brown to greenish or yellowish, depending on the composition.
Crude oil is considered light if the level of hydrocarbons relative to organics and metals is high, making its density low, and it is considered heavy if the level of hydrocarbons relative to organics and metals is low, making its density as high. Additionally, unrefined oil is classified as sweet if it has very little sulfur in it, and is classified as sour if it has a great deal of sulfur in it. So a crude oil will usually be called something like a sweet, light oil, or a sour, heavy oil. Sweeter oils are more valuable than sour oils, because most countries have sulfur regulations for environmental reasons, and sweet oils require less treatment to remove the sulfur. Light oils are more valuable than heavy oils, because more gasoline can be created from a smaller amount.
Different regions on earth tend to have different types of oil, so unrefined oil is often classified based on where it comes from. Certain regions will act as a sample of a broader region, since they are seen as relatively representative of that broad region. For example, Dubai-Oman oil is a sour crude oil, and is used to benchmark most sour crude from the Middle East; West Texas Intermediate is a sweet, light oil; and the OPEC Reference Basket is a composite oil sample that averages oils from all over the Organization of the Petroleum Exporting Countries.
While conventional unrefined oil is currently the major source of petroleum on the planet, it actually makes up a minority of crude oil currently in reserve. A bit less than one-third of the unrefined oil known on the planet is in conventional form. Another one-sixth is a heavy oil, and a quarter is extra-heavy oil. Another one-third, roughly equal to the amount of conventional oil, is in the form of oil sands, or crude bitumen. This is not a liquid form of crude oil, but is mixed with sand into a somewhat solid form. Huge reserves of bitumen can be found in Venezuela and Canada, which also contain large amount of extra-heavy oil, making the two countries’ reserves equal to about twice the known reserves of conventional oil.