Oil and gas

Some 50,000 oil and gas fields have been identified world-wide, however about half of the known reserves were originally concentrated in less than 40 ?supergiant? oil fields (each with over 5 billion gallons of recoverable oil). Most of these fields are found in the Middle East, where they are located both below ground and below the shallow waters of the Arabian Gulf. Other major oil fields are located in Russia, Mexico, the USA, China, Libya, Venezuela and Algeria.

Methods of oil and gas extraction vary considerably depending on the form and depth of deposits and the local environmental conditions. Leakage of hydrocarbons and associated pollutants into the oceans occurs in many ways. There is some leakage at the point of extraction, and so marine-based extraction causes some immediate issues of pollution. Refineries also release small amounts of hydrocarbons in their wastewater. The major source of crude and partially refined oil loss to the environment, however, comes from accidental spillage in the form of leakage from pipelines, tanker spillage and damage to drilling platforms. In addition there is considerable loss of hydrocarbons to the environment subsequent to their refinement. This may come from release from ships (deliberate or accidental) or from various forms of terrestrial runoff. Oil-based hydrocarbons are a fairly significant constituent of urban runoff.

Despite the considerable input from marine spillage it is considered likely that land-based sources of oil pollution are of greater significance. Even in the Black Sea where there is some oil extraction and considerable tanker traffic, an analysis showed that 48% of the total input of 110,000 tonnes came from the Danube River, with a further 30,000 tonnes from domestic sources and 15,400 from industrial sources.