Oil Spills

Petroleum is a naturally occurring mixture of organic chemicals, the most abundant of which are hydrocarbons (molecules containing only hydrogen and carbon atoms). Petroleum is synthesized from biomass by complex, anaerobic reactions occurring at high pressure and temperature over long periods of time deep in sedimentary geological formations. Petroleum can occur as a liquid known as crude oil, which may also contain natural gas, and also as a semi-solid tar or asphalt in oil sands and shales. There are hundreds of molecular species in petroleum, ranging from gaseous methane with only 16 g/mole, to very complex substances weighing more than 20,000 g/mole.

Petroleum differs in its physical and chemical characteristics from deposit to deposit. Some crude oils are extremely thick and viscous, while others are light and volatile. The lighter fractions of petroleum evaporate relatively quickly when spilled into the environment. This leaves behind residues of relatively heavy molecules that are more persistent in terrestrial or aquatic habitats, and cause longer-lasting effects.