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Petroleum and natural oil

Most petroleum and natural gas have been derived from organic remains deposited in a marine sedimentary environments a modern example of the conditions is believed to have been most favourable is the Black Sea, where there islittle circulation of water, and where bottom sediments have been found to contain as much as 35 percent of organic matter, in contrast with the 25 percent which is normal for marine sediments.

Petroleum and natural gas are believed to develop from these organic materials througha series of transformation not unlike those by which coal develops from peat.

The most important source beds are generally believedto be marine shales.

The present location of accumulation of petroleum and natural gas obviously depends on the laws that govern their migration. Unfortunately, these laws are not known, althoughanumber of empirical relationships have been established.

A simple gravitational theory seems to explain many known occurrences. According to this theory, if oil, gas and water are present in a reservoir bed, the oil and gas, which are lighter than water, will rise to the highest point, with the gas being on top of the oil.



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Origins of oil and gas.

Off all the fossil fuels, oil is the one that tends to make the headlines. It has become so important to the global economy that wars have been fought over it. Oil and natural gas have the same origin, and are often found in association. Moreover they are both hydrocarbons, so we will consider them together.

The raw material for hydrocarbons is dead organic matter, chiefly the remains of microorganisms and land plants. For enough of this material to collect, it is necessary to have an environment where there is a large supply of dead organic matter coupled with a lack of oxygen to stop it decaying to water and carbon dioxide These conditions are met in those areas of the sea floor where the surface waters have a high rate of production (and mortality) of planktonic plants, which upon death sink to the bottom to be buried in fine-grained, shaley, sediment. More rarely, the floors of lakes provide a suitable environment.

Once an organic-rich shale has accumulated, it is a potential source rock for hydrocarbons. For this potential to be realized, burial must occur to allow heat, pressure and chemical conditions to cause diagenetic changes in the organic matter. The conversion from dead organic matter to a hydrocarbon is described as maturation.



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