Britain has abundant reserves of offshore oil and gas. These valuable fossil fuels owe their existence to a precise sequence of environmental conditions that occurred over millions of years. First there had to be a source of suitable raw material to create the oil and gas. Next, the environment had to produce a suitable reservoir rock to store the oil and gas, which tend to migrate upwards. And finally, there had to be a rock seal, to prevent the fuel deposits from escaping. Large deposits would only occur if a suitable rock structure existed in the form of a trap.
 

Click picture for full page view

How are oil and gas formed?

Almost all oil and gas is derived from tiny decayed plants, algae and bacteria. Large quantities of this material can accumulate in swampy environments or on the seabed, where there is no oxygen to destroy the organic matter. Gradually, more recent sediments settle on top, burying the material deeper and deeper. As this happens, both the temperature and pressure increase so that the material is slowly cooked and altered, like food in a pressure cooker. Energy first given to the plants by the sun is then transferred into the useful form of oil or gas, which contain chemicals called hydrocarbon compounds -- that is hydrogen and carbon atoms joined together. Oil forms first and then, as the temperature and pressure continue to increase at greater depth, gas begins to form. Loose sediments that harden into rock and contain sufficient organic material to form oil are then called source rocks. 

Deep heat

At depth, the buried algae, bacteria, spores and cuticles (leaf skin) join their wax, fat and oil to form dark specks called kerogen. Then, as the source rocks get hotter, chains of hydrocarbon chemicals use this heat energy to break away from the kerogen to form oil. At greater depth, the temperature rises. At higher temperatures the chains of hydrocarbon become shorter. Short chains produce lighter oil, while long chains produce heavier oil because of the lower temperatures. Most of the oil under the North Sea was formed at great depth and is therefore very light and of the highest economic value. Beneath the North Sea, oil forms at a depth of 3 - 4.5 km and gas at a depth of 4 - 6 km. 

On the move

Once formed, the oil and gas tend to migrate through the rock that contains them. This movement occurs for two reasons. First, the oil and gas expand to take up more room than their source material of algae, bacteria and leaf skins. Therefore their pressure on the rock increases and they try to escape. Second, being less dense than the surrounding rock and water, they tend to rise upwards. This migration is a slow process, taking millions of years for the oil and gas to rise a few kilometres. 

What happens to the migrating oil and gas?

If there were no seal above the source rock, then the oil and gas would slowly rise and escape at the surface. Thus no hydrocarbon deposits would build up. This has certainly happened on many occasions. However, geological time spans millions of years, allowing new environments to come into existence. For example, a desert could develop whose sands could compact to form a layer of sandstone over the source rock before any or all of the oil and gas had formed or escaped. If this were the case, then the migrating oil and gas could rise up into the sandstone, which would act as a reservoir rock. 

Reservoir rocks

Sandstone can store oil and gas because it is both porous and permeable. Porous means that liquids and gas can be stored within the pores of the rock. Pores (the spaces between all of the individual particles in the rock) exist because the particles are irregularly shaped and don't fit closely together. A permeable rock has pores which are connected and allow oil or gas to flow through. Of course, the oil and gas could eventually migrate all the way through the sandstone layer too, so the deposits will only remain in this reservoir of rock if another layer forms on top which is impermeable i.e. does not allow the liquids or gases to pass through and escape. This impermeable layer is called a seal. 

How a seal forms -- one method

Over immense geological time a new sea could flood an area of desert. Then salt from this sea could be deposited to form an impermeable layer over the sandstone, thus forming a seal which would trap the gas and oil. The fossil fuels could accumulate in large quantities if the powerful tectonic (geological) forces in the Earth's crust pushed these sand and salt layers up into a huge arch-shaped dome to create a vast trap. Millions of years later on a cold and windy day, this oil might eventually be released by a rapidly spinning drill under the North Sea.