Oil and gas accumulation

OIL AND GAS ACCUMULATION

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 scientist doubt that most petroleum originates in the life and death of living things.

ORIGIN

Chemical Factors

Petroleum is both simple and complex. It is composed almost entirely of carbon and hydrogen; but the number of ways that carbon and hydrogen can combine is astronomical, and most petroleum contains hundreds or different kinds of hydrocarbons. It occurs in forms as diverse as thick black asphalt or pitch, oily black heavy crude, clear yellow light crude, and petroleum gas. These variations are due mainly to differences in molecular weight, that is, the sizes of the molecules, and the types of impurities.

Despite the differences in molecular weights, however, the proportions of carbon and hydrogen do not vary appreciably among the different varieties of petroleum; carbon comprises 82 to 87 percent and hydrogen 12 to 15 percent.

Petroleum is almost insoluble in pure water and only slightly soluble in salt water or water containing other organic substances. It is lighter than, and therefore floats on, water; but it is often found in an oil-water emulsion, that is, dispersed in small droplets suspended in water.

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