Together with natural gas, it makes up petroleum, which is Latin for “rock oil”. Petroleum is basically a mix of naturally occurring organic compounds from within the earth that contain primarily hydrogen, carbon and oxygen. When petroleum comes straight out of the ground as a liquid it is called crude oil if dark and viscous, and condensate if clear and volatile. When solid it is asphalt, and when semi-solid it is tar. There is also natural gas, which can be associated with oil or found alone.
Crude oil comes in many forms. Usually it is black, but green, red or brown oils are not uncommon. Thin and volatile oils are called “light”, whereas thick and viscous ones are “heavy”. Light oils have an API gravity of 30 to 40 degrees, which means that the density is much less than 1.0 g/cc. These oils float easily on water. By contrast, some heavy oils have an API gravity of less than 12 degrees and are so dense that they sink, rather than float, in water.
Most oils are mixtures of many different compounds, most of which are hydrocarbons. There are four main hydrocarbon groups in petroleum. The saturates are hydrocarbons consisting of straight chains of carbon atoms. Aromatics are hydrocarbons consisting of rings of carbon. Asphlatenes are complex polycylic hydrocarbons that contain many complicated carbon rings, and NSO compounds are mostly nitrogen, sulfur, and oxygen.
In most oils, the saturate fraction is the largest and is made up of two subgroups called paraffins and isoprenoids. Paraffins are simple straight-chain hydrocarbons, whereas isoprenoids are hydrocarbon chains with branches. Waxes are long-chain paraffins that are solid at surface temperatures and may contain as many as 50 carbon atoms. Waxy oils tend to thick and viscous, whereas aromatic oils tend to be light and volatile.
Petroleum in ancient times was called bitumen, and mankind for centuries was not at all sure what bitumen was made of or where it came from. Two ideas developed in ancient times to explain the composition and origin of bitumen. One held that bitumen was inorganic and bore no relation to living things, whereas the other theorized that it somehow formed from once-living plants or animals.
Jean B. Senteur de Boue.(2015,October 16). What is Oil. Retrieved from. http://www.sjvgeology.org/oil/oil.html