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.