You need to remember that heat and pressure BREAK DOWN the complex hydrocarbons found in oil into shorter, simpler molecules. We do this in refineries every day.
If you have a basic understanding of enthalpy and entropy, this will help explain:
http://www.chemcases.com/fuels/fuels-b.htm
Both the organic and discreet chemical processes described in the article require high pressures and heat to produce oil.