Most people presume petroleum to be similar to gasoline or petrol, simply a less pure form, which needs to be refined. In actuality the chemical composition of petroleum in its raw state can vary extremely.
This variation is the reason why petroleum composition differs so much in colour and viscosity between crude oil fields and geographical areas.
Petroleum, or crude oil as it is now usually referred too when raw, contains several chemical compounds, the most prolific being the hydrocarbons themselves which give thepetroleum composition its combustible nature.
Although the composition of petroleum will contain many trace elements the key compounds are carbon (93% – 97%), hydrogen (10% - 14%), nitrogen (0.1% - 2%), oxygen (01.% - 1.5%) and sulphur (0.5% - 6%) with a few trace metals making up a very small percentage of the petroleum composition.
The actual overall properties of each different petroleum source are defined by the percentage of the four main hydrocarbons found within petroleum as part of the petroleum composition.
The percentages for these hydrocarbons can vary greatly, giving the crude oil a quite distinct compound personality depending upon geographic region. These hydrocarbons are typically present in petroleum at the following percentages: paraffins (15% - 60%), napthenes (30% - 60%), aromatics (3% to 30%), with asphaltics making up the remainder.
The composition of petroleum is defined as laid out above, and it is this composition which gives the crude oil its properties.
Raw petroleum is usually dark brown or almost black although some fields deliver a greenish or sometimes yellow petroleum. Depending upon the field and the way the petroleum composition was formed the crude oil will also differ in viscosity.
At the extreme ranges petroleum can be almost solid, and required a significant investment of resources to refine into a useable state as anything other than bitumen. At the other end of the scale the petroleum composition can be a clear fluid resembling kerosene or gasoline, needing very little refining to be useable as a fuel.
When discussing the composition of petroleum it is important to note that the compound of the raw petroleum tends to dictate the usage of the refined product. Petroleum is generally measured in volume, and for some composition of petroleum it is not cost effective to refine these into fuel.
A lighter, less dense raw petroleum composition with a compound that contains higher percentages of hydrocarbons is much more profitable as a fuel source. Whereas other, denser petroleum composition with a less flammable level of hydrocarbons and sulphur are expensive to refine into a fuel and are therefore more suitable for plastics manufacturing and other uses.
Unfortunately the worlds reserves of light petroleum (light crude oil) are severely depleted and refineries are forced to refine and process more and more heavy crude oil and bitumen.
In some cases the refining process will need to remove carbon and add hydrogen, adding an extra, costly step to the refining process. This change in compound of the world's energy producing petroleum and the associated rise in refining costs has directly affected the price of gasoline across the world.