Origins of Oil & Gas

Britain's offshore oil 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.

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.

Usually, dead plants 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. 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.