Answer:
Well crude oil is less dense than water so it floats to the top.
Fire requires 3 things: Heat, oxygen and fuel.
Now that it's at the top, oxygen is readily available so all it takes is a spark and you have a cloud of black, crude smoke. And it's really hard to put the fires out; since it floats you can't exactly spray water on it.
I tried lighting up some lighter fluid in my sink once, I blasted the tap on it but all the fluid did was to float on top and continue burning. :X