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.
Shipyard workers must KNOW this fact, because industrial gases (refrigerants, in particular) released inside a ship's hull might drop and push out all air inside the hull; thereby killing the workers.
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?
Yep!
I was hauled off to the hospital with CO2 poisoning I got whle working in the very bottom of a hydroelectric power plant underground cavern. CO2 is heavier than air (O2 & N2) and had accumulated down there.