Gas doesn't tend to migrate downward through sediment. Being less dense than rock and water, it migrates upward.
The fact is that gases, as with all other elements and elememt combinations, have atomic weights; and those gases lighter than air will rise in air ... and vice versa.
What does the weight of gases relative to air have to do with their weight relative to rock and water?
From a Cal-Tech entrance exam ...
Picture a gas filled balloon suspended in midair inside a constant velocity moving train boxcar. When the train slows down, will the ballon move forward or backward?