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BRITAIN'S OFFSHORE OIL & GAS

*Origins of Oil and Gas


Oil and gas are derived almost entirely from decayed plants and bacteria. Energy from the sun, which fuelled the plant growth, has been recycled into useful energy in the form of hydrocarbon compounds - hydrogen and carbon atoms linked together.

Of all the diverse life that has ever existed comparatively little has become, or will become oil and gas. Plant remains must first be trapped and preserved in sediments, then be buried deeply and slowly 'cooked' to yield oil or gas. Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks.

Dead plants usually are dispersed and decay rapidly, but in areas such as swamps, lakes and poorly oxygenated areas of the seafloor, vast amounts of plant material accumulate. Bacteria breaking down this material may use up all available oxygen, producing a stagnant environment which is unfit for larger grazing and scavenging animals. The plant and bacteria remains become buried and preserved in muds. In swamps the remains may form coals on burial.

Whether oil or gas is formed depends partly on the starting materials (F3). Almost all oil forms from the buried remains of minute aquatic algae and bacteria, but gas forms if these remains are deeply buried. The stems and leaves of buried land plants are altered to coals. Generally these yield no oil, but again produce gas on deep burial.

Britain's offshore and gas originates from two sources. Gas from beneath the southern North Sea and the Irish Sea formed from coals which were derived from the lush, tropical rain forests that grew in the Carboniferous Period, about 300 million years ago. Oil and most gas under the central and northern North Sea and west of the Shetland Islands formed from the remains of planktonic algae and bacteria that flourished in tropical seas of the Jurassic and Cretaceous Periods, about 140 to 130 million years ago (a significant amount of the Kimmeridge Clay Formation is Cretaceous in age). They accumulated in muds, which are now the prolific Kimmeridge Clay source rock.

F3: Origins of oil and gas

F4: Oil and gas generation beneath the North sea

On burial the carbohydrates and proteins of the plant remains are soon destroyed. The remaining organic compounds form a material called kerogen. Aquatic plants and bacteria form kerogen of different composition from woody land plants.

The processes of oil and gas formation resemble those of a kitchen where the rocks are slowly cooked. Temperatures within the Earth's crust increase with depth so that sediments (F4), and kerogen which they contain, warm up as they become buried under thick piles of younger sediments.

As a source rock, deposited under the sea or in a lake, becomes hotter (typically >100oC), long chains of hydrogen and carbon atoms break from the kerogen, forming waxy and viscous heavy oil. At higher temperatures, shorter hydrocarbon chains break away to give light oil and then, above about 160oC, gas. Most North Sea oil is the more valuable light oil. The woody kerogen of coals yield mainly methane gas, whose molecules contain only one carbon atom. Gas from the southern North Sea is methane.

Once a source rock has started to generate oil or gas it is said to be mature. The most important products generated are gas, oil, oil containing dissolved gas, and gas containing dissolved oil which is called gas condensate. Condensate is the light oil which is derived from gas condensates which are found at high underground temperatures and pressures; it is the most important product in some North Sea fields.

F5: Open pore spaces in permeable sandstone X 120 F6: Pore spaces filled by mineral growth X 90

In the North Sea, oil forms at 3-4.5km depth, gas at 5-6km. At greater depths any remaining kerogen has become carbonised and no longer yields hydrocarbons. Burial to these depths occurs in areas where the Earth's crust is sagging. In the central and northern North Sea the oil source rock is buried in a deep rift valley. In the southern North Sea, coal-bearing rocks formed the floor of a basin, which filled with younger sediments.



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