About 300 million years ago the conditions for the subsequent formation of petroleum were established in shallow coastal waters by the terminating tiny creatures and plants that lived and died in vast numbers. The ooze formed o the bottom by the remains of these organisms was unable to decompose because of lack of oxygen. As a result of climatic changes, these coastal areas became buried under layers of earth, and the organic remains were subjected to high pressure and temperatures over periods of millions of years.

The fats carbohydrates and proteins were thereby subjected to conditions in which they were decomposed and underwent extensive chemical changes. As a result of these changes, a large number of compounds were formed which all enter into the composition of petroleum. As the conditions of decomposition varied from one region to another, petroleum found in different parts of the world tends to vary considerably in composition.

In some places the decomposition was very intensive process, with the result that natural gas and petroleum particularly rich in aromatic compounds were formed. In addition to pressure and heat, it is certain that catalysts, ferments and especially bacteria have affected the chemical composition of petroleum crude oils.

The world's petroleum reserves are estimates at upward of 40 milliard tons and production is running at well over 1 milliard tons per year. Unless extensive new deposits are found, it seems likely that the world's recoverable reserves of petroleum will be exhausted in something like half a century from now.

A vast variety of products is obtained from petroleum : petrol (gasoline) for aircraft and vehicles, fuel oil for heating and stream-raising, diesel oil etc. also numerous organic chemicals are manufactured from petroleum and are processes into a wide range of products : synthetic rubber, pesticides, synthetic fibers, solvents drugs etc. large scale exploitation of the world's petroleum reserves started about a hundred years ago and is now a major factor in the power resources available to the modern man.