Petroleum, along with oil and coal, is classified as a fossil fuel. Fossil fuels are formed when sea plants and animals die, and the remains become buried under several thousand feet of silt, sand or mud. Fossil fuels take millions of years to form and therefore petroleum is also considered to be a non-renewable energy source.
The world has a limited supply of petroleum, and current estimations tell us that within the next few decades mankind will have completely depleted this valuable natural resource. Although measures have been taken to ensure that there are cheap, renewable fuel options in place for the eventuality it is still obvious that mankind faces a serious problem when petroleum supplies finally run out.