What happens to the cavities left after oil and gas extraction?

First of all, when the oil is extracted, it's coming out of the pores in the sandy rocks and in most places the framework of that rock remains reasonably intact. You do in the long term get some subsidence. Perhaps the most notorious example is in the Netherlands where they don't really want anymore sinking but there's a very large gas field in the middle of the Netherlands called the Groningen gas field. There's a lot of subsidence associated with that. There's also a lot of subsidence associated with coal fields but that's a little bit different because you're excavating underground. When the oil comes out it's seeping out of the pores, so you're not actually making a big hole down there.