This paper presents an overview of the North Sea area. In addition, it provides a history of the exploration of the area, petroleum occurrence and location of the North Sea. Consequently, it provides a description of the Stratigraphy of the southern Northern Sea, Central North Sea and the Northern North Sea. A brief description of the tectonic history of the area is provided taking note of the four major tectonic events that occurred. These were the Caledonian collision, rifting and basin formation as identified in the Carboniferous, Mesozoic rifting and graben formation and the inversion.
Province Geology (North Sea)
Discovery of gas in the North Sea was in the 1960 (Alexander, 2004). In addition, in 1970, the first oil field was discovered by in the northern part of the Norwegian North Sea waters. The field was called Ekofisk. Investigations revealed that this field had huge oil reserves. After this discovery, well drilling in the area increased (North Sea Task Force, 1993).
Covering an area of 625,000 km2, the North Sea Basin is located between Norway and Great Britain. The basin is bounded by a continental shelf edge on the north side. The area is characterized by active tectonic history with deposition being different among its sub-basins.
The North Sea Stratrigraphy can be analyzed in different regions. These regions include the Southern North Sea, Central North Sea and the Northern North Sea. In the Southern North Sea, the reservoirs are composed of Carboniferous coals and shales. In most areas of the Southern North Sea, most of the Mesozoic is absent. In the Central North Sea, the main rock found is the Kimmeridge Clay. This clay is developed in similar facies as is with the Northern North Sea. Reservoirs are present in the Cretaceous section and the pre-Cretaceous section. The upper Jurassic Piper and the Fulmar Formations, The middle Jurassic Hugin and Pentland Formation and the Triassic Skagerrak Formation are located in the pre-Cretaceous reservoir. In the Cretaceous reservoir, there exists the Chalk group reservoir in the Norwegian Ekofisk area. The Northern North area main source rock is the organic dark Kimmeridge Shale. This rock is suitable for both oil and gas production. In addition, it is a source rock for production of hydrocarbons. The middle Jurassic Hugin is suitable for production of gas. The Triassic rock does not contain any significant rocks that can be used for gas or oil production.
According to Lyngsie, Thybo and Rasmussen (2006), the North Sea area has undergone four major tectonic events. These include the Caledonian collision, rifting and basin formation as identified in the Carboniferous, Mesozoic rifting and graben formation and the inversion. In the Caledonian collision, two continents were involved, Laurentia to the west and Baltica to the east. In addition, Avalonia, a micro-continent to the south was also involved. Furthermore, the Caledonian collision resulted to the development of a deep foreland on the Baltica plate, which is indicated by the existence of lower Paleozoic sedimentary rocks. Consequently, the tectonic regime changed from a general extension and subsidence to a strike-slip regime at the late stages of the Carboniferous to early Permian.
In the late Variscan cycle, a system of conjugate shear faults transected northwestern Europe. This resulted to the formation of faults. During the late Carboniferous, the Variscan orogen collapsed resulting to an uplift that caused truncation of the Devonian-Carboniferous successions. Extensional wrench tectonics of the late Carboniferous and early Permian caused crustal thinning and subsidence of the Southern and Northern Permian Basins. Additionally, the crustal thinning was linked to magma activity as evidenced by Rotliegendes volcanic deposits and related sedimentary rocks.
Extensive, normal faulting developed from the Triassic to the Jurassic in the basin areas. In the Late Cretaceous Laramide phase of the Alpine orogeny compression stresses caused deformation and inversion throughout the North Sea area. Geological evolution following the Early Tertiary has been exemplified by subsidence in the North Sea area.
According to Homewood, Mitchell and Eberli (1998) the North Sea area contains a range of petroleum that is derived from the Kimmeridge Clay source rock. In some of the faults, in this rock, there is the presence of high, volatile oil, whereas, in other faults, there is the presence of gas condensate. The main principle cement is quartz containing paleofluids that carry liquid petroleum. The main gas producing area is the southern North Sea located in northwestern Europe continental shelf (Ziegler, 1997). Located in the Late Permian Rotliegend reservoir rocks, the gas is obtained from a Carboniferous source. Gas fields occur along the Southern Permian Basin covering a distance of 1200 km. Small oil reserves are located in Lower Cretaceous reservoirs and Zechstein. This is mainly on land.
History of Exploration in the North Sea
According to Gluyas and Hichens (2003), offshore petroleum exploration in Europe gained popularity because of an experience of well drilling in the Netherlands in 1959. The Groningen gas field discovery became the largest gas field with an estimated 85 trillion cubic feet of gas. This resulted to further exploration by geologists in the southern North Sea area of the Groningen reservoir province. The challenge experienced during the exploration was that there was no form of legislation that controlled any form of drilling in the area. In addition, lack of offshore boundaries between countries created a serious problem for exploitation of the reservoirs. Because of the Geneva Convention, Britain was able to offer licenses to most parts of the North Sea that were under its jurisdiction (Gluyas and Hichens, 2003). This was based on 1 degree of latitude and 1 degree of longitude divided into 30 blocks. These blocks were 10 minutes of latitude by 12 minutes of longitude. In 1965, Norway provided blocks that were larder compared to those of Britain.
Offshore drilling in Britain began in 1964. Early exploration wells were done by Amoseas and shell (Ziegler, 1997). These wells were located in the southern flank of the largest structure in the North Sea and the Mid North Sea High. Gulf drilled the third well in Rotliegend rock, but it was discovered to be water-wet (Ziegler, 1997). The fourth exploratory well was done by BP produced the West Sole gas field in 1965. The first dozen exploratory well formed three commercial fields. These were included West Sole, South Viking and Leman. In addition, there was one non-commercial field referred to as Ann and one gas show known as 49/13-1. Consequently, Indefatigable and Hewett were established in 1966.
Therefore, reserves from the first five discoveries were approximately 20 trillion cubic feet of gas. However, these explorations used analog seismic data that limited the detection of structures below the top Zechstein salt (Ziegler, 1997). Presence of large Triassic structures overlying the Zechstein salt located in quadrants 43 and 44 did not produce the expected outcome. The Gordon field (well 43/20-1) stumbled upon gas-bearing Bunter sandstone at 249 feet in a reservoir that was estimated to be 477 feet thick (Ziegler, 1997).
Around 1969, exploration had shifted towards the central North Sea. This is where the Ekofisk field was discovered in the Norwegian waters. This field contained approximately a billion barrels of oil. The following year, a subsequent oil field, Forties, was discovered having almost the same capacity as Ekofisk. However, exploration of the southern North Sea stalled because of development and purchase of gas from both Norway and Britain.
Explorations in the Dutch offshore took place in the Schoonebeek field that contained Lower Cretaceous Bentheim Sandstone reservoir. The first offshore discovery of oil was reported at F/18 field. In addition, oil was also discovered in F/13 field. Other exploration wells included Haven, Helm, Helder, Kotter, Logger and Hoorn fields. Oil was mainly extracted from the Liassic Posidonia Shale (Ziegler, 1997). Exploration in the North Sea near Germany was not a success because of the presence of carbon dioxide and nitrogen in the reservoir rich in shale.
According to Warrender (1991), the Murchison oil field is located in the Northern North Sea, East Shetland and is part of the Brent oil province. Murchison field was discovered in 1975 and spans the British-Norway international boundary. The field was named after Sir Roderick Impey Murchison who contributed a lot in the field of Paleozoic stratigraphy. Oil production in this field started in 1980 and was undertaken by Conoco Ltd a company based in Britain. Oil used to be extracted from Middle Jurassic Brent Group sandstone. This forms the reservoir rock. This Brent Group has an average thickness of 425 feet (Warrender, 1991). For extraction, oil must be sourced from a fault block of Jurassic-Triassic age that is sealed by Upper Jurassic Shales. As of 1991, the Murchison field occupied an area of 7 square miles and contained approximately 790 million barrels of oil.
Warrender (1991) indicates that the productive reservoir in the Murchison field is comprised of coastal deltaic sandstones of the Middle Jurassic Brent group. This reservoir rock lies in between the Upper Jurassic Humber Group and the Lower Jurassic Dunlin Group.
Since Murchison field is located in the northern North Sea, the main source rock is the organic rich dark Kimmeridge Shale. This rock is established for oil and gas production. The Kimmeridge Shale serves as a top seal for the reservoir rock.
Lithology of a rock is a collective description of its physical attributed that can be seen at an outcrop. It is a science used in the subdivision of rock sequences into constituent lithostratigraphic units so that mapping can be done and areas correlated. In Richards et al (1993), the Fladen and Brent Groups are of the Middle Jurassic rocks. These groups are subdivided into constituent formations. The Brora Arenaceous and Brora Argillaceous formations are exposed along the Inner Moray coast. The Brent Group has the most significant hydrocarbon sequence in the North Sea Basin. It has in descending order the Tarbet, Ness, Etive, Rannoch and Broom formations. According to Budding and Ingling (1981), these formations have a widely regressive-transgressive wedge with shallow marine and diachronous coastal sediment. This mirrors a wave-dominated delta that is fed largely from the South.
The Tarbet formation.
This is made up of 75m of sandstone with subordinate mudstone, coal and siltstone seams. These were formed in a shallow marine setting that was transgressive in nature (Underhill et al., 1997).
The Ness Formation.
It is of the Bajocan age and has 180 m of interbedded siltstone, mudstone and sandstone and coal seams created in a kind of delta-top setting. This formation is divided into three parts namely the Upper interbedded unit, the Mid-Ness shale and the lower interbedded unit.
The Broom Formation.
This is made up of 50m of mudstone clasts, conglomeratic sandstone and marine sandstone. It is a fan delta deposit (Hausen et al., 1987).
Trapping mechanism and reservoir heterogeneity.
Traps form when the hydrocarbons migrate upward under buoyant forces via a permeable rock and are unable to overcome a sealing medium’s capillary forces. When trap formation coincides with petroleum generation, a reservoir forms. Traps can be classified into categories according to geological characteristics. These categories include the stratigraphic trap, the structural trap and the hydrodynamic trap. Most petroleum reservoirs, however, have characteristics from more than one categories and are the combination traps. Gas and oil fields linked to the Middle-Jurassic tilted, pre-rift fault blocks have proven to be the most productive. Evidence suggests that the area is mature’ and the remaining potential are in hanging wall closures.
Reservoir drive mechanisms.
Usually, the energy required to produce oil and gas is provided by nature. Pressure on the hydrocarbon fluids increases with the increase in depth. Water and gas in the petroleum reservoirs helps move the oil to the point where the wellbore is located. Reservoir characteristics determine the type of driving energy to be adopted.
Solution Gas Drive Reservoirs.
When a new reservoir pressure is at a point below bubble point, free bubbles of gas escape the reservoir’s oil phase. The pressure at the reservoir decreases with continued production. This caused emergence and expansion of bubbles which create extra energy inside the reservoir. These types of reservoirs are known as the solution gas drive reservoirs.
Gas Cap Drive Reservoirs.
When the reservoir is pressure lower than the bubble point pressure, there is more gas inside the reservoir than in the oil can retain in solution. Due to density difference, this extra gas accumulates at the reservoir top, forming a cap. This kind of reservoir is known as a gas cap drive reservoir.
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