Hi bosswho,
Hope this helps.
Here, the trapped gas volume is purely structurally trapped in a 4-way, dip closed structure.
What if the reservoir sandstone continues up-dip of the 4-way dip closed structural trap? If gas continues to migrate into the 4-way dip-closed structure, it will completely fill up the 4-way dip closed structural trap, and then spill up-dip (to the left) and begin filling the sandstone reservoir in the stratigraphic component of the trap.
Because the sandstone pinches out up-dip, the migrating gas is trapped in the stratigraphic component and the overall gas volume - structural + stratigraphic increases.
Clear as mud?
Kind regards OldGeo
