Flies and Natural Gas

So i work in a Gas plant and over the past few weeks while at work i usually noticed that once there is a Little release of gas from the any of the gas valves from no where you will see some huge flies hovering over where the gas came from. It has always blaffed me, whats the relationship between natural Gas and flies like it doesn't make any sense. So i decided to go and ask Google and to my greatest surprise even google didn't have a concrete answer to my flies and gas question.
So i don't know if anyone knows why flies are attracted to gas while i still embark on my curious research.