Oil and gas are formed from the remains of marine and plant life, mixed with water and silt millions of years ago. On the seabed layers of rock covered the mixture as the earth's plates moved. High pressure and heat from the earth's core transformed the mixture into the complex chemicals collectively known as hydrocarbons.

As this process progresses the oil and gas rise through cracks in the rocks and gradually seep naturally into the environment. Oil and gas reservoirs are created when the rising hydrocarbons are trapped by a layer of rock through which they cannot pass (impervious). They then collect and remain in the porous rocks, allowing oil companies to drill through the impervious layers and into the reservoir.

Specially equipped airplanes have been used to detect tiny variations in the earth's gravitational and magnetic pull caused by underground hydrocarbons. However, this method is only useful for detecting massive reservoirs.

The most common method of detecting potential hydrocarbon reservoirs is the seismic survey. Time differences in shock wave echoes are measured, giving information on rock formations below the surface. Powerful computer systems analyse the data and help to decide on the best places to drill. This greatly reduces the risk of drilling a dry well.

After the decision on where the best place to drill is taken, exploration-drilling vessels such as the Semi-submersible are chartered to drill test wells. If oil is found there are various tests to assess the size and shape of the discovery. They can also find out how the hydrocarbon will flow. All this gives information on how the well will perform now and also 20 years in the future.