From a film. http://en.wikipedia.org/wiki/Lessons_of_darkness
Stashed in: Imgur!, Some men just want to watch the world burn., Upvote! Downvote!
They're burning the natural gases that are extremely flammable and unsafe to work around, so that they can get to the oil beneath.
You burn the top level of the gases being released so that instead of concentrated methane, it becomes more CO2 - which is less harmful.