|BRITAIN'S OFFSHORE OIL & GAS
A Closer Look at Some North Sea Fields
Brent Field, discovered in the far north of the area in 1971, contains oil and gas within tilted layers of sandy rock. 170 million years ago, these layers were part of a river delta. Since then, the tilting movements, associated with the rifting Viking Graben have been followed by a long period of sagging. Muddy sediments - including Kimmeridge Clay, the source of the oil - have draped across the titled blocks (F33), filling the subsiding troughs between them, and sealing the eroded upper edges of the sandstone layers (F41) to form traps. Much later, oil was expelled downwards into the sandstones from the thick mudrock, now deeply buried within the troughs. The oil migrated up the tilted sandstone layers to collect in the crests. Some of the gas came from coal within the delta sediments. Oil is still migrating through the area. The sandstone layers, each more than 200 metres thick, have held over 500 billion litres of oil, for millions of years, within an area of 17 by 5 km.
Piper Field, discovered in 1973, lies at the edge of an arm of the buried rift valley. Oil in this field is trapped within a tilted sandstone layer cut by faults. The sandstone was deposited 145 million years ago, during late Jurassic times, as sand bars around a series of river deltas. Kimmeridge Clay source rock overlies the oil-filled sandstone, acting as part of the seal. However, the trapped oil is mostly derived from within the rift valley, on the south side of the field, where source rock is thicker and hotter. The oil has migrated to the field area at some time after a mudrock seal was laid down 70 million years after the sand. Oil is prevented from leaking out at the faults, or from the eroded edges of the sandstone, by this seal. About one cubic kilometre of the sand is filled with 150 billion litres of oil over an area of 30 square kilometres.
South Brae Field contains oil and gas in the sandy debris which accumulated at the foot of a steep submarine slope. This coarse sediment was deposited along the western edge of the buried rift valley, in the southern part of the Viking Graben. At this time, 140 million years ago, the organic mud of the Kimmeridge Clay source rock was being deposited across the area. During episodes of instability, fan deposits of rock fragments and sand spread out from the submerged, rifting edge of the graben, while organic mud deposition was confined to the floor of the undersea rift valley away from the steep margins. Thus the reservoir rocks are now found as sand sheets and wedges of conglomerate - pebbles and boulders in sand - interlayered with black, oily mudrock. Here, therefore, the reservoir rock is the same age as the source rock. South Brae Field was discovered in 1977, its oil being found deeper down than in most North Sea fields, within a maximum of thickness of over 500 metres of reservoir rock. The oil is hot, gassy and corrosive.
Forties Field, discovered in 1970, has held well over 500 billion litres of oil in its sandy reservoir rock. This rock was deposited as a submarine fan sediment 55 million years ago (F37 and 38). At Forties Field, these sandy layers have draped and sagged across a 'hump' in the underlying rocks. The Main Sand reservoir is composed mostly of sandy sediment deposited as a submarine fan, while much of the separate Charlie Sand accumulated within feeder channels of sand flowing across the sea floor. Oil has migrated upwards from Kimmeridge Clay source rock within the buried rift valley, and has then travelled along the sand layers. Some of the migrating oil has then been trapped within the dome-shaped beds of sandstone above the underlying hump.
Shearwater Field, discovered in 1988, lies in the Central North Sea some 225 km (138 miles) east of Aberdeen. The gas condensate reservoir depth is 4,545 metres (15,000 ft) and is characterised by its extremely high pressure and high temperature - 1,000 bar and approximately 180 degrees C. The pressure is equivalent to three elephants standing on a postage stamp. The geological structures date back to the Jurassic Age. The field was brought into production in 2000 with the use of cutting edge technologies. One day's production from Shearwater would fuel a family car for 1690 years or heat the water for 4.5 million showers.
Argyll Field, first discovered in 1971 (and now being re-developed as the Ardmore Field), lies on the western edge of the buried rift valley - the Central Graben - in the central area of the North Sea. Its oil lies within reservoir rock, which is much older than the source rock. Oil migrated along the rift fault zone from the depths of the graben; the reservoir lies within a spur-like block of rock which is fractured and gently folded. The block is also faulted on its western side, preventing oil leakage. The whole block is draped by a seal of much more recent shale and chalk, trapping the oil within an area of thirteen square kilometres. The reservoir rocks are mainly layers of desert sediments. The lower layers are Devonian rocks around 360 million years old. Above them, Permian dune sands with alluvium and lake-bed sediments are overlain by limestone, all around 250 million years old. Deposited as limy mud-flats around an inland salt sea, the limestone makes an unusual reservoir for this area. Some of its minerals have been dissolved out, leaving cavities which now hold oil.
Oil and gas were discovered at Ekofisk Field, in the Norwegian sector, in 1969. They are contained within chalk which is not normally a good reservoir rock. Much of the chalk in this area, however, is free of clay impurities and, moreover, was slumped and re-deposited across the deeper sea floor above the Central Graben area. This left chalk layers with a porous, open texture. The pores were later filled with high-pressure oil and gas, preventing minerals from subsequently blocking the pores. Thus the pores remain open and the rock stays permeable to the flow of oil and gas. Salt movements in the area fractured and domed the chalk, creating both pathways for hydrocarbons to migrate up to the reservoir, and the structure to trap them. Gas has leaked from the trap into the shales overlying the reservoir.
Leman Field was discovered in 1966. It is the largest gas field in the southern North Sea, underlying an area of more than 30 by 10 km off the Norfolk coast. The Leman Bank area is one of a number of places across this part of the North Sea beneath which the chance factors for the origin, migration, containment, sealing and trapping of gas have come together in the circumstances described in pages 18 and 19. The gas originates from the coal-bearing shales of the 300 million year-old Coal Measures. The thick, Late Permian salt beds in this area form a very efficient seal above the excellent reservoir rock of dune sands laid down in Early Permian times, around 270 million years ago.
By 140 million years ago, certain areas of the Coal Measures source rock would have reached a temperature of around 150°C, at depth of about 4.5 km, a sufficient level of heat to generate gas. Beneath Leman Field, however, the crust has since been pushed upwards, lifting the Coal Measures back out of the gas-generating zone. It is possible that the gas has either migrated from flank areas into the resulting dome-shaped trap or it may already have migrated into the reservoir rock. At nearby Hewett Field these same upward movements allowed gas to escape past the Permian salt seal. It then migrated towards the surface but was trapped again in two sandstone reservoirs of Early Triassic age, around 240 million years old. These have seals of shale and a thin salt layer.
The main Permian reservoir sandstone was deposited in a great desert basin which covered the area between the eastern margin of England and the Russian-Polish border. Known as Leman Sandstone in the UK and Slochteren Sandstone in the Netherlands, this rock is part of the Rotliegend Sandstone Group. It is the most important gas reservoir rock in the southern North Sea. However, as this sandstone becomes more deeply buried its reservoir capacity diminishes because minerals precipitate in the pores between the grains. Uplift has not restored the gas-reservoir capacity of the rock.
In 1959, gas was discovered in Rotliegend Sandstone at Groningen in the Netherlands. The field later proved to be very large. With a thick salt seal above it, the sandstone was found to contain highly permeable dune sands which hold much of the gas. Such an important discovery led exploration geologists to follow the dune sands westwards along the direction of the prevailing wind which formed those dunes millions of years ago. This brought their attention into the area now covered by the southern North Sea - to a new region of the Earth's crust waiting to be searched, its structural pattern and geology discovered and its past environments deduced - the era of North Sea exploration had begun!