stockwatch man, I'm not quite sure how I can explain it differently, as I have said I am no expert, I'm still learning. My understanding is that gas reservoirs need a seal all around {like a box} and the sides of the box compose of impermeable rock which prevents the gas leaking away. The top and bottom can be of rock strata laid horizontally, and the sides formed where this strata has faulted and moved. If any water is present before this seal is made it will also be trapped inside. If that is not the case then any water that appears after the seal is formed cannot get inside, unless further faulting takes place. These traps are vast in size and vary in shape. The bottom line as I see it is that water could already be present, but the same risks occur with any exploration. You might know all this already, if so, sorry to have bored you!
Off to bed now.
Regards, bs.