Large drilling costs from where I am in the south due to ground being mostly (lime?)stone: makes it cost-infeasbile. Other areas might have it easier though.
It can vary over the micro-scale. Where I live in NM, we could go down very deep and never hit rock, because we're sitting on the soft sandy soil(ish) alluvial deposits at the bottom of large (20 mile diameter) basin. But neighbors who live less than 1 mile away are sitting on metamorphic rock just inches below the surface.
