Many oils are liquid and gaseous hydrocarbons of geological origin.
While some oils are naturally occurring, a significant proportion of those in the marine and coastal environment have been derived from anthropogenic sources. Most oils from land-based sources are refined petroleum products or their derivatives. Some oils are volatile or easily degraded and disappear rapidly from aquatic systems, but some may persist in the water column or in sediments. Oils may be toxic to aquatic life when ingested or absorbed through skin or gills, interfere with respiratory systems, foul fur and feathers, smother aquatic communities, habitats and bathing beaches, taint seafood and contaminate water supplies.