Crude oil is the liquid component of petroleum and it varies drastically in its composition. Although the specific amounts of different hydrocarbons varies, it is always composed of a series of different hydrocarbons. Generally crude oil is about 85% carbon by weight, and most of the rest is hydrogen. Within crude oil there are also significant quantities of sulfur, oxygen, and nitrogen.
Crude oil was formed hundreds of millions of years ago in ancient oceans. In these oceans, dead organic matter fell to the ocean floor and was mixed with inorganic material. After much time passed, bacteria decomposed the organic material in the absence of oxygen and the temperature of the material increased with the increased pressure on it from sediment above. As pressure and temperature increased, the organic material slowly turned into crude oil and natural gas.
There are several different general types of crude oil. Light crude oil contains a significant amount of natural gas liquids, whereas heavy crude oil has little or no natural gas liquids and instead contains a large number of heavy hydrocarbons. Since crude oil can vary so much in its composition, it can have different viscosity. Light to medium crude oils can pour easy, slightly more viscous than water, and this type of crude oil can flow through porous rock more easily. Thus this type of crude oil is more easy to extract. Conversely, heavier crude oils flow less easily through pores in rock and are thus difficult to extract. Crude oil can also been characterized using pour point - the lowest temperature at which the liquid will flow just before it becomes solid. A lower pour point is better than a higher pour point as it indicates fewer paraffins in the crude oil and the oil will flow better. As well, the colour of crude oils can vary from nearly colourless - light crude oils - to greenish yellow to reddish to black. Black crude oils are very heavy whereas the other colours likely have other non-hydrocarbon components - such as sulfur - changing their colour. Finally, crude oils can smell different depending on their composition. Sweet crude oils have a more gasoline-like smell, whereas sour crude oils contaminated with high levels of sulfur smell of rotting eggs.
Crude oil itself is not extremely useful, so it is refined to produce a variety of useful products. These products include heavy oils for industrial boilers, fuel oil, diesel, kerosene, and gasoline. The main refining process used for crude oil involves fractional distillation. In this process, crude oil is heated and vaporized and rises up through a vertical column, with different fractions condensing at different heights due to their different condensing temperatures. The remaining products can undergo catalytic cracking to break up large hydrocarbon chains in order to get a larger yield of lighter components - such as gasoline.