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 producing a gas reservoir, the gas expands within, and this is usually sufficient to "drive" production until you have got nearly all the gas (say 80 or 90%) out of the ground and the pressure has dropped to near surface conditions. It helps that gas is much more moblie than oil!

Because most oil and gas fields are found several kms deep, the effects of production are rarely seen at surface, although on occasions, subsidence has occured, as in the Ekofisk Field, North Sea.

It won't filled with all water, as the location of oil/gas is on top of the water level.

As gas/oil is extracted, the formation pressure will decrease and overburden pressure from layers above will take place and cause the "void" to close.

That's why on certain oil field area, subsidence happened.

they just leave behind big holes in the earth. currently nobody makes any effort to rebuild an ecosystem after it's been ripped arpart by mining, and it they bother to fill in the holes they have made, they just use slag; the toxic waste left over from porcessing the oil out of the earth. 20 years from now (if we can survive that long) you will start to see some serious problems created by this rape the earth and then abandon it philosophy. it will be the environmental crisis of the day, because there will be nowhere to farm, nowhere to live, just a bunch of glowing green holes of death seeping through the world. it is a problem for the next generation we are creating right now. we will claim ignorance and say "how could we know this would be the outcome?" but it is just willful ignorance and greed, really. just like the polluters of the past were being willfully ignorant. mankind in his natural state is not oblivious to the consequences if his actions. we just like to play the ignorance card to escape blame and consequences for our greed.

I believe that water is pressurised down into the well.

Water sinks to the bottom and forces the oil to the surface.

automatically leaving the water behind.

What void?

Gas and oil aren't in big lakes or holes under the ground - they are IN rocks. Petroleum comes from two words - petro - rock, and oleum - oil. The oil is inside tiny pores in the rock - it is NOT in some big bubble of oil underground.

There are companies out there that supply water to drilling companies to place in their wells.

Water and air