It's not a bad hypothesis, but methane (the simplest of all hydrocarbons at CH4 and the major component of natural gas) results from many different biochemical metabolic pathways. If anything, more organisms results in faster breakdown of complex organic matter into simpler molecules. But I don't know if there truly are more organisms now than a million or even a hundred million years ago. The Cambrian Explosion was more than half a billion years ago.