As it turns out, the main factor is the boiling point. Oil is purified by the process of fractional distillation, or repeatedly attempting to remove impurities by increasing the intensity of the removal process. By analyzing a sample and repeatedly attempting to “evaporate” it, the sample will slowly become more concentrated with complex hydrocarbons, which are better for industrial production because of their energetic capacity. Less complex HC’s will be filtered out at lower boiling points.
