I saw it explained on one of the early “Oil Spill” threads (where a similar concern was raised) that underground oil does not take the form of giant lakes or bubbles - but rather resides in capillary-sized voids in otherwise solid rock - so an ‘implosion’ such as concern you is unlikely in the extreme...
To: library user
if left alone it would run out eventually. Perhaps in several months, perhaps a lot longer—the worlds first oil well, over 140 years old, is still producing, albeit at a very slow rate. The oil is in porous rock, so there’s no empty space. Sometimes the ground subsides a bit in old oilfields, but there’s no big cavern under there that will collapse.
Exactly! It is rare to find large cavities (caves, basically) full of oil, and those tend to be self-supporting and fill with water after the oil is gone.