The question below consists of a set of labelled sentences. Out of the four options given, select the most logical order of the sentences to form a coherent paragraph.

P: Most of them are combinations of hydrogen and carbon in varying proportions.
Q: Crude mineral oil comes out of the earth as a thick brown or black liquid with a strong smell.
R: It is a complex mixture of many different substances, each with it's own individual qualities.
S: Such hydrocarbons are also found in other forms such as bitumen, asphalt and natural gas.