ICANT, the way that deep oil gets pressurized to 22,000 psi is by being around five miles beneath the surface. Rock is typically sort of heavy - like two and a half times as heavy as water or more - and that weight is borne by whatever is beneath this rock. If what’s beneath is more rock with pore spaces containing fluid, the fluid can feel that weight. A foot’s depth of rock translates into approximately one pound per square inch. So 22,000 feet of rock can supply about 22,000 psi. Leaks from the reservoir will, of course, lower the fluid pressure. But there are reservoirs that get close to that ideal - Mahakam and Arun in Indonesia come to mind.