RE:I wonder It depends on wheter the sands are new or if they are connected. You can have a 2m sand that connected to 50m of pay nearby. The 2m will drain that 50m. Conversely they may not be connected. In these wells its entirely possible to have a depleted zones above lower virgin zones.
pressure when perfing the zones is the only real confirmation, especially seeing they dont run resistivity logs in the wells as far as I know. If a zone is depleted you will often see a lower resistivity as water is more conductive. These are conventional reservoirs so water is present downdip.
hope this helps