Origin Of Hydrocarbons

For most of their history, oil and natural gas were thought of as minerals, substances formed out of nonliving rock, just as gold, sulfur, and salt were part of the rock. There was little reason to assume otherwise. Although petroleum smelled like something that had died, and although natural gas burned like swamp gas, most of the gas and oil escaping from the ground seemed to come from solid rock deep beneath the surface, where, as everyone knew, nothing lived.

Beginning two centuries ago, however, the geologic insights of Hutton, Lyell, and other scientists showed that the rocks in which oil was found were once loose sediment piling up in shallow coastal waters where fish and algae and plankton and corals lived. Now it seemed possible that oil and gas had something to do with the decay of dead organisms, just as coal, with its leaf and stem imprints, seemed to be the fossilized remains of swamp plants.

Later advances in microscopy revealed that oil-producing and oil-bearing rocks often contain fossilized creatures too small to be seen with the unaided eye. Chemists discovered that the carbon-hydrogen ratios in petroleum are much like those in marine organisms and that certain complex molecules are found in petroleum that are otherwise known to occur only in living cells. But it was the fact that most source rocks could be shown to have originated in an environment rich with life that clinched the organic theory of the origin of petroleum.

Unanswered questions about the occurrence of petroleum remain, and men of science still debate the evidence of its organic origin. Because of the weight of that evidence, however, few scientists doubt that most petroleum originates in the life and death of living things.

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