The refining process begins with crude oil. Crude oil is unrefined liquid petroleum, which ranges in color from yellow to black, and may have a paraffin, asphalt or mixed base. Crude oil is composed of thousands of different chemical compounds called hydrocarbons, all with different boiling points. For example, a typical crude oil may begin to boil at 104°F to produce petroleum gas used for heating and making plastics, and finish boiling at greater than 1112°F to produce residuals such as petroleum coke, asphalt and tar.
Crude oil is generally described as sweet or sour according to its sulfur content, and heavy or light according to its API Gravity. The API Gravity index is a relative measure of weight-the lower the number, the heavier the material; the higher the number, the lighter the material.