According to the biogenic theory, petroleum is a non-renewable fossil fuel consisting primarily of hydrocarbons, derived from the decomposition of plant and animal organisms occurred in the anaerobic environment, following their continued accumulation in the underground for millions of years in rocks which gradually are formed. The oil has not always the same chemical-physical composition but varies depending on the field of origin.
The crude oil is what is extracted from the reservoir, while the products are the fuels and fuel oils obtained by refining crude oil. The commodity classification API (American Petroleum Institute), accepted internationally, divides crude oils into four classes according to their density °API. Combining the classification API to the empirical concept of persistence of oils at sea, these are mainly divided into persistent (crude oils, fuel oils and asphalt) and not persistent (gasoline, kerosene and diesel). On the base of this classification we distinguish 4 main groups of crude oils and products: