4 - The world according to Gold: disputes about the origins of oil

Summary

Every schoolchild sooner or later learns the standard story of the origins of oil; it runs something like this. Once upon a time, hundreds of millions of years ago, the earth was covered by vast oceans. Animals, plants and micro-organisms in the seas lived and died by the billion, their remains sinking to the bottom and mixing with sand and mud to form marine sediment. As the ages passed, the mud turned to rock and eventually the organic mass became buried deep under layers of rock. The oceans receded and the earth's crust heaved and buckled. Compressed under this vast weight of rock, decomposition occurred and the layers of biomass underwent a chemical change to form hydrocarbons (compounds composed only of hydrogen and carbon atoms) – coal, oil, and natural gas.

Special geological conditions are needed to keep the oil trapped underground. The organic material has to be covered by porous rocks and these, in turn, have to be covered by an impermeable layer which acts as a cap to prevent the oil and gas escaping. Oil is consequently found only in places where these geological conditions are met.

Although this crude, even mythical, account has become greatly more refined in modern petroleum geology, the underlying tenet that oil is formed by biological decay is the starting point for any exploration of the subject.