Unlike the gasoline that is distilled from it, crude oil has a relatively high flash point of 140°F. The flash point is the temperature at which a substance can vaporize to form a ignitable mixture in air. In order to sustain combustion the crude oil must be maintained at or above the flash point. Unfortunately the sea surface temperature near the oil spill is in the mid to low 70's and the air temperature is only slightly higher. This means that the additional heat to maintain the combustion needs to come from the combustion itself. Since the oil slick is spread out and quite thin, this surface layer does not contain enough chemical potential energy per square foot to sufficiently heat the oil adjacent to it.
This just looks uglier and uglier every day.