Correct. The mantle is on average about 35km thick. Most (all?) oil wells are less than 5km deep, so this is roughly an order of magnitude deeper. The world'

Correct. The mantle is on average about 35km thick. Most (all?) oil wells are less than 5km deep, so this is roughly an order of magnitude deeper. The world's deepest borehole https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole https://en.wikipedia.org/wiki/Kola_Superdeep_Borehole is 12km deep. But even before this, downhole temperatures reach 200C which apparently caused the rock to act like plastic, causing the drill stem to get stuck. A well with 200C temperatures would yield more energy, but you're still limited by how fast heat energy can migrate towards the borehole, followed by transporting it for 12km without it cooling off. A similar situation is apparently happening with the London tube, but in reverse. Heat from the trains is being absorbed by the rock underground, but it can't migrate away fast enough. As a result, the tube tunnels are heating up decade by decade, to the point where temperatures underground reach the mid-30s in summer. Eventually they will have to do something about it, but that probably won't happen until enough people get killed by heat exposure in a stuck train.