When gas/oil is extracted by drilling, etc – what fills the void the extracted substance comes from?
As has been already pointed out, the vast majority of oil and gas reservoirs are formed of sandstone, and are not undergound caverns or lakes (with the possible exception of some reservoirs formed of karstified limestone). The oil and gas is trapped within the small pore spaces between the individual sand grains; it’s held down there by overlying impermeable cap rocks such as shale or evaporite. When you produce an oil reservoir, the oil may reach the surface assuming there is sufficient pressure in the reservoir. Sometimes, the underlying aquifer (water-filled rock) is big enough to keep the pressure high enough naturally. If not, the oil companies will drill down into the aquifer and pump water in, to maintain the reservoir pressure and flow-rates, and attempt to “sweep” the oil out of the resrevoir. Sometimes 40 or 50% of the oil in the reservoir can be produced in this way (the rest is unavoidably left behind in the pore spaces). Gas reservoirs are a bit different – when you start
