Pauline Watts, of Sunnyvale, asks :-

Karen Phillips, of Dannevirke, asked:-

When oil is extracted from the earth does that leave a void? If so would that allow the earth to subside and cause an earthquake?

Andrew Gorman, a geophysicist at the University of Otago, responded.

The simple answer to your question is no. Although terms like "oil pools" and "reservoirs" conjure up images of big underground lakes lying in caves beneath the surface, most petroleum (oil of all sorts and natural gas) in fact is extracted from solid rock units that lie deep in the Earth. The rock units usually consist of sedimentary rocks like sandstone or limestone that are full of small pores often less than a millimetre in diameter. In good reservoir rocks, these pores will take up only 10 per cent or so of the volume - and it is in these pores that petroleum can be stored.

When a well is drilled down into such a reservoir, the petroleum flows up through the well bore to the surface - either under its own pressure or by pumping it up.

When the petroleum is extracted, it is rare for the reservoir rock to collapse. That is because petroleum usually is associated with other subterranean fluids such as ground water. Since oil and gas are lighter than water, they will float to the top of the reservoir. When the petroleum is produced, the water rises to fill the space that the petroleum previously occupied.

The reservoir rock is only one part of the geological system that results in accumulations of oil and gas. First, organic material sourced from living organisms is required. This is buried to great depths (sometimes many kilometres) in a sedimentary basin where temperature and pressure acting over a period of time modify (cook) the organic structures into hydrocarbon molecules. The petroleum then needs to migrate from its source rock to be concentrated in a suitable reservoir rock. Finally, the reservoir rock needs to be positioned within a trapping structure so that the petroleum cannot escape to the surface where it would naturally be oxidised by the atmosphere.

All that said, the production of oil and gas has been known on some rare occasions to result in small earthquakes cause by the drop in pressure and subsequent collapse of reservoir rocks. Usually this will be associated with intensive production in a highly porous or inadequately solid rock formation.

Likewise, an oil production technique known as "fracturing" intentionally increases formation pressures in order to crack the rock and form more pathways for the petroleum to escape into the well bore.