Petroleum is a naturally occurring, flammable liquid found in the Earth's crust. It is composed of a complex mixture of hydrocarbons, which are molecules that contain hydrogen and carbon atoms. These hydrocarbons are formed from the remains of ancient plants and animals that lived millions of years ago.

The composition of petroleum can vary depending on its source, but typically it contains:

1. Alkanes: These are straight-chain hydrocarbons that are relatively unreactive.

2. Cycloalkanes: These are cyclic hydrocarbons that are also relatively unreactive.

3. Aromatic hydrocarbons: These are hydrocarbons that contain benzene rings. Aromatic hydrocarbons are more reactive than alkanes and cycloalkanes and are responsible for many of the properties of petroleum, such as its combustibility.

4. Naphthenes: These are saturated cyclic hydrocarbons that have at least one carbon-carbon double bond.

5. Olefins: These are unsaturated hydrocarbons that have at least one carbon-carbon double bond.

6. Sulphur compounds: These are compounds that contain sulphur atoms. Sulphur compounds can be corrosive and can pollute the environment.

7. Nitrogen compounds: These are compounds that contain nitrogen atoms. Nitrogen compounds can be corrosive and can pollute the environment.

8. Oxygen compounds: These are compounds that contain oxygen atoms. Oxygen compounds can be corrosive and can pollute the environment.

The proportions of these components can vary significantly depending on the source of the petroleum.