Gas is produced deeper, longer with a higher pressure. Hydrocarbon molecules get smaller, that means gas. They still can be found in the same trap: Gas on the top as a gas cap just below the cap rock, generally oil is always below the gas. Take a look on this video clips there is whole series of them. They are made by Richard A. Landau, former Schlumberger General Field Engineer now teaching oil students in UAE.

But start with these two of them:


Porosity and permeability

Anticline/Trap/Cap rock/Gas cap etc...