Not just decomposition of organics; hydrocarbons are all over the universe but no one dares claim they all come from living organisms the way they claim for them on earth.
Understand, but access is a bit of a problem from the rest of the universe. You are right, however, as carbon is everywhere but, to the best of my knowledge, the bulk of the marsh gasses, petroleum and coal here are from expired and well-aged carbon based lifeforms.