Read the sections titled "oil basics," "what fuels are made from crude oil? ," and "what is a refinery? " then, answer the questions. how and when oil was formed? (site 1)

Crude oil is made of several components- called hydrocarbons-  mixed together and can be chemically separated.  These components are, majorly, fuels. Examples of these fuels are; diesel, petrol, kerosene, liquefied petroleum gas and heavy oils such as bitumen (used on tarmac roads).

An oil refinery is basically a big plant that utilizes the chemistry of fractional distillation to separate the oil components. This involves boiling the crude oil at different temperatures to evaporate the different components that have different boiling points. Each component is later condensed and recovered separately.

Crude oil is a natural resource that is formed over millions of years from organic matter especially planktons. When the organic matter dies and settles at the bottom of oceans, they are overlaid by layers and layers of sediment. This, over time, subjects the organic matter to high pressure and temperatures  and they metamorphose into crude oil.