Oil is formed under the sea from the remains of marine life. The oil formation process is similar to that of coal. The remains become buried under layers of sediment, and the resulting high pressure and temperature cause them to turn into a black liquid with a high carbon content. Often, gas forms in the same place as the oil. The liquid oil moves towards the surface and becomes trapped under layers of impermeable rock. The pressure builds up as more liquid moves upwards.
Although oil is formed under the sea, many of the world’s oil fields are located on land. This is a result of changes in sea levels and movements in the Earth that have ‘uplifted’ rocks, causing them to lie above water.
The oil and gas is reached by drilling wells, sometimes several kilometres long, through the overlying rock. When the drill breaks through the rock above the oil, pressure is released, and the oil spurts out of the well. To prevent this from happening, engineers place special valves at the top of the well. When there is not enough pressure to push oil to the surface, water or gas can be pumped into the well to force the oil out. The crude oil that comes out of the ground has to be taken to refineries to be processed before it can be used.
Oil companies are now searching for new oil and gas fields in some of the world’s most inhospitable places, such as the frozen ice fields of Siberia and Alaska and the Arctic Ocean. Once all the oil fields are depleted, the energy companies may turn their attention to oil shales. These are rocks that have oil inside them. However, removing oil from rock is very expensive and produces a lot of waste.