genesis of gas at these depths

Gas go's up. So the lime stone or salt layers were hardened eons ago. One would have thought that if gas (and oil for that matter) was produced by delicate, friable organic matter, that process of creation of these substances would be faster than what it takes to sink it down 5 or more km and create a seal ! what I am saying is that I have my doubts about the whole bl**dy theory. One would think that most of it would have permeated up and out and lost (to us) way before a seal was created. But then again I am ignorant. If any1 has a useful link to explain this (if u have understood my predicament) please do share.