gas size theory, page-4

I know live oil contains gas, but I would think that generally in an accumulation one would find gas at the top of an accumulation, oil in the middle and water on the bottom?

Am I correct in thinking that hydrocarbon traps are generally domed structures, even if cut into by shears?

So if gas/oil/water spills from one structure to the next, where is this likely to come from in the source accumulation:

If an unsheared structure - surely from the bottom, hence oil (if present) is more likely to migrate first?

In a sheared structure - could there be migration at any point along the shear?

Can anyone explain this to me or point me to a reference?