When the well reaches a pool, oil usually rises up the well because of its density difference with water beneath it or because of the pressure of expanding gas trapped above it.

Both oil and gas are less dense than water, so they generally tend to rise upward through water-saturated rock and sediment.

For many countries, though, the first booty from any newly acquired seabed will be either oil or gas, both of which can now be extracted fairly easily from deep water.