As they accumulate over millions of years they gradually infiltrate the microscopic cavities of the sea floor sediment and rock where they decay. The resulting oil remains trapped in these spaces, forming oil reserves which can be extracted through large drilling platforms. The use of oil increased significantly after the Second World War. Improved energy efficiency has caused oil consumption to decline in many developed, industrialised countries, as well as shifts to other fuels such as natural gas and nuclear energy. Decreasing use of oil is also resulting from tougher environmental restrictions concerning its use in some regions.
Natural gas is formed in the same way as oil, from the remains of marine microorganisms.
Its main constituent is methane , and when burnt also releases carbon dioxide , although in lower quantities than coal and oil. In addition, distribution leakages are a significant source of methane to the atmosphere. From the mid-1960s, up until the present day, there has been a dramatic increase in the amount of proven reserves of natural gas. Consequently, natural gas has become the fastest-growing energy resource. Natural gas provides an alternative to oil or coal in the provision of energy, and in terms of greenhouse pollution it is a slightly more efficient fuel, with waste emissions contributing less to the enhancement of the greenhouse effect and global warming.