I'm happy to stand corrected, but I believe you have two types of water mixed togeather here.... I don't think there is such thing as perched water drive.

Perched water is usually referred to isolated pockets of water encountered above where your local FWL lies - think of a lens of sealing material mid-reservoir shaped like a cup or saucer... As the reservoir charged with hydrocarbons (hydrocarbons percolating up from beneath), the water in the 'cup' or 'saucer' would not have been displaced. By it's very nature though it is NOT pervasive, and as such would not have enough volume to provide any significant drive effect

If I'm wrong, I'd be very interested in finding out why....