Types of Oil and Their Properties

The term ‘petroleum’ which means ‘oil from rock’, includes both crude oil and natural gases containing hydrocarbons. They are also known as petrogenic hydrocarbons. Petroleum was formed some 100 to 200 million years ago from microscopic marine plants and animals that became incorporated in the sediments and rocks formed at the bottom of the sea.

Crude oils are liquids made up of mixtures of many compounds. These compounds are mainly hydrocarbons which vary considerably in structural arrangement. Crude oil contains three main groups of hydrocarbons - alkanes, cycloalkanes and aromatics. However non-hydrocarbons such as sulphur, vanadium, oxygen and nitrogen compounds may represent up to “25% of the oil” (Clark 2001).

Oil and petrochemicals exert impacts on the environment through both physical and chemical (toxic) means. Long-chain aliphatic hydrocarbons are effectively solids and exert their effects by physical means, coating surfaces and smothering organisms. Short-chain, low boiling point compounds, unsaturated compounds and aromatic hydrocarbons exert their effects by primarily chemical (toxic) means. Therefore the overall impact of a discharge of oil or petrochemical on the environment is dependent on the distribution and composition of the petroleum hydrocarbons, especially their weathering, persistence and consequently their bioavailability (GESAMP 1993).

Effects differ according to the physical conditions of the area. In enclosed systems, such as estuaries, the potential for dispersion is not so great and, on shorelines, a number of factors determine its persistence: properties of the oil, porosity of sediments, presence of animal burrows, wave action and type of vegetation.

Oils differ from each other in their viscosity, volatility and toxicity:

Although the nature of oils differs considerably, they can be generalised and classified into categories depending on their general toxicity, physical state, and changes with time and weathering.