Vocabulary. fossil, molecule, hydrogen, carbon, sulfur.

　　Petroleum has been important since ancient times.

　　In Latin, the name means "rock oil".

　　Petroleum is a fossil fuel.

　　The liquid comes from the remains of plants and animals that died millions of years ago.

　　These remains were buried deep bellow levels of rock over time and under great pressure.

　　This geological process created complex molecules of hydrogen and carbon.

　　Oil can also contain other elements.

　　Crude oil or unprocessed petroleum is called sour when it contains a lot of sulfur and impurity.

　　Sour crude requires more refining than sweet crude which is often more valuable.