NORTH SEA OIL

'North Sea oil' refers to oil and natural gas (hydrocarbons) produced from oil reservoirs beneath the North Sea. In the oil industry, "North Sea" often refers to a larger geographical set, including areas such as the Norwegian Sea and the UK "Atlantic Margin" (west of Shetland) not, strictly speaking, part of the North Sea.

Contents

History

Licensing

Reserves and production

List of areas ("plays")

Devonian

Carboniferous and Permian

Triassic

Middle Jurassic

Upper Jurassic

Lower Cretaceous

Upper Cretaceous

Palaeocene

See also

External links

References

History

North Sea oil was discovered in the early 1960s,^[1] with the first well coming on line in 1971 and being piped ashore at Teesside, England, from 1975, but the fields were not intensively exploited until rising oil prices in the 1980s made exploitation economically feasible. Volatile weather conditions in Europe's North Sea have made drilling particularly hazardous, claiming many lives. The conditions also make extraction a costly process, by the 1980s costs for developing new methods and technologies to make the process both efficient and safe, far exceeded NASAs budget to land a man on the moon. ^[2]
In reality, oil seeps had been known from coal beds on either side of the North Sea, but only a limited amount of development had occurred (Eakring oil field, Nottinghamshire, England; Edinburgh Oil Shales (which seem unrelated to later discoveries); and small discoveries in the Netherlands and Northern Germany). A "demonstration well" was sunk in 1938 in association with the World Petroleum Congress at The Hague. After the Second World War a small number of onshore gas and oil fields were found in 1959, an academic well drilled at Ten Boer near Groningen, Netherlands was deepened and discovered a significant gas deposit. Appraisal and development wells over the next few years brought the realisation in 1963 that the Groningen field was not just "economic", nor even "big", or "large", or "giant", but was an "elephant" field of huge potential. Given that, extending exploration into adjacent areas was an obvious decision.
The exploration of the North Sea has been a story of continually pushing the edges of the technology of exploitation (in terms of what can be produced) and later the technologies of discovery and evaluation (2-D seismic, followed by 3-D and 4-D seismic; sub-salt seismic; immersive display and analysis suites and supercomputing to handle the flood of computation required).

Licensing

There are five countries with North Sea Production. All operate a tax and Royalty licensing regime. The respective sectors are divided by median lines agreed in the late 1960s:

★ 'United Kingdom' - licences are administered by the 'DTI' (Department of Trade and Industry- Website). The UKCS (United Kingdom Continental Shelf) is divided into quadrants of 1 degree latitude and one degree longitude. Each quadrant is divided into 30 blocks measuring 10 minutes of latitude and 12 minutes of longitude. Some blocks are divided further into part blocks where some areas are relinquished by previous licensees. For example block 13/24a is located in quad 13 and is the 24th block and is the a part block of this block. The UK government has traditionally issued licences via periodic (now annual) licensing rounds. Blocks are awarded on the basis of the work programme bid by the participants. The UK DTI has been very active in attracting new entrants to the UKCS via Promote licensing rounds (less demanding terms) and the fallow acarage initiative where non-active licences have had to be relinquished.

★ 'Norway' - licences are administered by the 'NPD' (Norwegian Petroleum Directorate Website in English ). The NCS is also divided into quads of 1 degree by 1 degree. Norwegian licence blocks are larger than British blocks, being 15 minutes of latitude by 20 minutes of longitude (12 blocks in a quad). Like Britain there are numerous part blocks formed by relicensing relinquished land.

★ 'Denmark' - The Danish sector is administered by the 'Danish Energy Authority' (website in English). The Danes also divide their sector of the North Sea into 1 degree by 1 degree quadrants, their blocks however are 10 minutes latitude by 15 minutes longitude. Part blocks exist where partial relinquishments have taken place.

★ 'Germany' - Germany and the Netherlands share a quadrant and block grid - quadrants are given letters rather than numbers. The blocks are 10 minutes latitude by 20 minutes longitude. Germany has the smallest sector in the North Sea.

★ 'Netherlands' - The Dutch sector is located in the Southern Gas Basin and shares a grid pattern with Germany.

Reserves and production

Most oil companies have investments in the North Sea. Peaking in 1999, production of North Sea oil was nearly 6 million barrels (950,000 m³) per day. Natural gas production was nearly 10 trillion cubic feet (280,000,000,000 m³) in 2001 and continues to increase.
Brent crude (one of the earliest crude oils produced in the North Sea) is still used today as a standard reference for pricing oil.
North Sea oil production fell ten percent (230,000 barrels) in 2004, and fell an additional 12.8% in 2005. This was the largest decrease of any other oil exporting nation in the world, and has led to Britain becoming a net importer of crude for the first time in decades, as recognized by the energy policy of the United Kingdom. [3]. The production is expected to fall to one-third of its peak by 2020 .
In Spring 1999, a prospected oil field, later named Buzzard, came in as the largest discovered in the past 25 years, with producible reserves of almost 500 million barrels. While significant, this is less than the amount of oil consumed globally in a single week.^[3]

List of areas ("plays")

A play is a collection of fields or structures with common features of source rock, thermal history, trap style and structure that make lessons from the discovery and development of one field on a play closely applicable to other similar structures.
In stratigraphic order:

Devonian

★ Devonian Old Red Sandstone - see Buchan oil field

Carboniferous and Permian

★ Sub-Zechstein Gas - Gas fields sourced from the Carboniferous Coal Measures and reservoired in the Permian Rotliegend sandstone or Carboniferous sandstones in the Northern part of the Southern Gas basin. Zechstein dolomite reservoirs also exist (e.g. Auk oilfield) . Seals are from Zechstein (also of Permian age) halite beds. Structures are mostly due to rifting in the pre-salt section. Example field - the discovery for the entire province at Groningen, Netherlands.

Triassic

The Triassic play is located in the Central North Sea. The reservoir is composed of fluvial and lacustrine sandstones of the Skaggerak formation. Fields with Triassic reservoirs include J-Block (Judy, Jade, Joshephine) and Marnock
Triassic reservoirs are also important in the East Irish Sea (Morecambe gas field, Sherwood Sandstone) and Southern North Sea (Hewett gasfield complex - Bunter sandstone)

Middle Jurassic

Northern North Sea tilted fault blocks; Brent Series deltaic sands of Bathonian age. In 1971 the Shell/ Esso joint venture drilled a poorly resolved structure in block 211 and found a classical prograding delta sand sequence (the Brent Sands overlain by a good source rock (the Kimmeridge Clay Formation, broke up into tilted fault blocks due to crustal extension, and the whole sealed by the lateral equivalent of the Chalk, the muddy Shetland Group. Several billion barrels of oil later, the Brent Field is being depressurised in the mid 2000s with shutdown looming on the horizon. Economically, this is the major play in the North Sea.

Upper Jurassic

★ Upper Jurassic Central North Sea, Fulmar Play - The Oxfordian Age Fulmar sandstone is a shallow marine unit present in the southern part of the Central Graben. It is also present in the Norwegian sector where it is called the Ula formation. This reservoir is very good quality with high porosities which can be preserved under deep (6000m) burial in high pressure high temperature (HPHT) fields such as Elgin, Franklin and Shearwater.

★ Upper Jurassic Turbidites - These Kimmeridgian/Volgian sandstone reservoirs are found in the Outer Moray Firth. The Buzzard Field, the largest discovery in recent years has an Ettrick formation turbidite reservoir.

★ Fanglomerates against faults. In the late 1980s seismic along some of the major "bounding faults" of the main basins of the North Sea revealed wedges of sediment which evidently piled up while the faults were still moving (syndepositional faulting). Magnus, Tiffany, Toni and Thelma follow this play, with many smaller developments being developed along-strike since as sub-sea tie-back technology develops

Lower Cretaceous

The Lower Cretacous play developed in the 1980s and 1990s. The reservoir rocks are Aptian / Albian turbidites. Most discoveries are stratigraphic traps . Examples include the Britania gas field and the Blake oil field

Upper Cretaceous

Chalk oil - The first major oil discovery was in 1969 by Phillips in the Ekofisk field of the Central North Sea (56°30'N 3°10'E). Source is Kimmeridge Clay Formation. Oil and gas generated early after deposition invaded the Cretaceous Chalk sediments and displaced the water necessary for compactional diagenesis (with later consequences). Structures are broad anticlines cored either by salt diapirs or Pre-Cretaceous Horsts.

Palaeocene

Central North Sea Tertiary turbidite sand fans - the Forties field (57°45'N 01°E) targeted a series of sand fans in mudstone surrounds and capped by Eocene Sheles. Source was Kimmeridge Clay Formation again. The Forties was discovered the year after Ekofisk, following several years of frustration in failing to adequately resolve these structures and find a major field.

★ Over-salt Sands. In the Central North Sea a large number of salt domes could early be seen to penetrate from the Zechstein up into the Tertiary cover rocks, but without adequate seismic they weren't considered good targets for development. Better seismic allows the development of numerous small fields around or above these structures. Individual fields are mostly small, but can be accommodated by tie-ing the fields back to existing facilities. Tie-in pipelines and control umbilicals of over 10 km are not unknown, despite serious technical difficulties (such as plugging of lines with methane hydrates or the rock stresses associated with piercement diapirs.)

See also

★ 'List of oil and gas fields of the North Sea'

★ List of oil fields

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★ Economy of Norway

★ UK topics

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★ Economy of the United Kingdom

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★ Geology of the United Kingdom

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★ United Kingdom Climate Change Programme

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★ Energy use and conservation in the United Kingdom

★ Oil platform

★ Proposed oil phase-out in Sweden

External links

★ North Sea oil at the (US) Energy Information Administration

★ Shell UK

★ OLF Norwegian Operators association

★ Petroleum Exploration Society of Great Britain

★ United Kingdom Offshore Operators Association

★ Danish North Sea oil and gas production, Danish Energy Authority

References

1. Norfolk Museum and Archaeology service [1]
2. ''TIME'': High costs, high stakes on the North Sea [2]
3. [David Strahan, 2007 - The Last Oil Shock; Pages 61 & 62.]