system
Given the following post, respond as if you're commenting on Reddit:
human
When we drain oil from wells. Does it make a empty pocket?
gpt
The oil doesn't exist underground in caverns that would otherwise be empty if it wasn't for the oil. Instead of thinking of large empty pockets, think of a wet sponge, where the sponge is rock and it's "wet" with oil. 

But also remember that the oil and gas held underground is under great pressure. It is this pressure that allows the oil to be extracted, but when the pressure is relieved by removing the oil these smaller pockets don't necessarily remain empty, as they're often filled water seeping in from the surrounding rock. And even when they are not filled back in with water from surrounding rock, oil extraction often leaves about 13% porosity, which is about the same as modern concrete. 

And it's not just water that can remain in the well: when an oil well is considered spent, and no more oil can be extracted (using traditional techniques), there may remain as much as 90% of the original deposit left in the ground. Because of the large amount of oil that can be left in an otherwise "spent" well, there are techniques for getting out more of the oil, which involves pumping fluids (mostly water) into the well, increasing the pressure and allowing more oil to be extracted. But this obviously displaces oil with water, and so the "wet sponge" never really dries out. 

So while oil extraction often leaves a well less dense than it was to begin with, it's not a whole lot less dense...it's still mostly rock all the way down!