Oil and gas (hydrocarbons)

Crude oil and natural gas occur in sedimentary rocks around the world. They were formed millions of years ago by the anaerobic breakdown of phytoplankton within the sediments. Typically, crude oil consists of a complex mix of thousands of compounds. The majority are carbon-based, and are predominantly straight-chain hydrocarbons (the paraffin series), saturated hydrocarbon rings (the napthene series) and unsaturated hydrocarbon rings (the aromatic series).
The importance of these fossil fuels to human industrial life is unparalleled by any other natural material and their extraction, transport and refinement have become a critical component of the global economy.
In the oceans, hydrocarbons have both toxic effects and also physical smothering effects on natural life. All are hydrophilic, and most float on water. They bind strongly to many solid substances, sediments and biological surfaces such as fur and feathers. The sequence of events that follows the release of hydrocarbons into the marine environment varies. Typically the more volatile (and toxic) elements evaporate and are not incorporated into the food- chain in large quantities. Over hours and days, further evaporation may make the remaining spill increasingly thick, or viscous. Weathering by waves and wind may break the spill up into smaller particles, some of which may sink, or the oil may drift onto the coastline and stick to rocks or become bound in with sediments. In some cases the oil may combine with water to form a thick mousse, which is sticky and slow to break down. Human responses to oil spills include physical removal from the water surface or beaches, or the use of chemical dispersants to aid the break-up oil and its incorporation into the water column (see Responses).

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