Why there is fire on oil fields?

Answer 1

Its not actually the oil that burns. Its the vapor that rises off it.

Oil is composed of hydrocarbons of various lengths. Heavier ones sink to the bottom and usually go on to become tar for roads or plastics. Lighter ones become fuels and rise as a vapor when revealed. Generally, they're burned gradually so that there's no build-up of pressure or explosion, which would damage equipment since the loss of yield (captured useful stuff) is negotiable.

Why?

Well, you can break the longer chains of hydrocarbons up into thinner ones via a chemical process called cracking or breaking. The lighter hydrocarbons react faster (just like the vapor does) and they're what become jet-fuel and petrolium in cars. The middle-length ones become your everyday plastics.

What's worth noting (and never shared) is that an oil spill can actually be controlled by using micillium or fungus, which break down hydrocarbons and convert them into carbohydrates - fungal sugars - which can then safely disperse into an environment. Unfortunately, not a lot of research has gone into this because oil companies want to sell their dispersant agents or plasticizers which work similarly to cracking, breaking oil up into smaller and smaller pieces so it disperses over a larger area with a much weaker yield. Unfortunately, this process is almost entirely cosmetic and the dispersant agent itself and the combined product is typically extremely dangerous to the environment.

Sorry to go all hippy on you like that. Its one of the next logical steps to your question. :)