Crude oil is a finite resource formed from ancient organisms over millions of years. It is a mixture of hydrocarbons — compounds containing only carbon and hydrogen.
Fractional distillation separates crude oil: the oil is heated and vapours rise up a fractionating column. Shorter-chain hydrocarbons (lower boiling point) condense near the top; longer chains (higher boiling point) condense near the bottom.
As chain length increases: boiling point increases, viscosity increases, flammability decreases.