When oil is extracted from the earth it does not leave behind hollow voids, the oil is contained within the rock itself.As to what replaces it, in most circumstances it will water that seeps up from beneath the oil that fills the rocks. In some circumstances Carbon Dioxide can be re-injected into the wells that can both help force more oil out, and prevent it being released to the atmosphere.As for long term effects of the extraction, the science behind oil drilling is very similar to that of drilling water wells. There is no structural instability left behind, as you are not leaving any voids.The long term effects of oil are more to do with the emissions it produces when burnt, or the potential damage done above ground by the general infrastructure required to support exploration.
