Unfortunately, the laws of supply and demand do not wholly apply in the Oil market as a significant amount of oil production is controlled by the cartel OPEC.
This cartel decides amongst itself how much oil it is going to produce. That, coupled with the fact that demand for oil derivatives (petrol, kerosene, etc) are relatively price inelastic (you are going to fill the tank of your car regardless of whther it costs �40 or �50) means that the regular, simplistic, laws of supply and demand cannot apply fully in this case.
I hope this helps.
KHT