Interesting, but what has that got to do with the weight of gases relative to rock and water (in the article) versus the weight of gases relative to air (in your comment on it)?
Gas doesn't tend to migrate downward through sediment. Being less dense than rock and water, it migrates upward.
You and the idiot author don't seem to realize that some gases are heavier than air; and that such gases are quite capable of migrating downward through air, or downward through rocks via holes and crevices, or into liquids via absorbtion.