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This report details why the Cygnus field in block 44 of the Southern North Sea represents a viable gas play. The field overlies the Permian basin with gas being generated by the Westphalian Coal Measures a formation of Carboniferous age. The Leman sandstone is the main reservoir target which consists of Aeolian and Fluvial sand sealed by the Silver Pit formation. Above this lies a thick sequence of Zechstein evaporites the largest of which is the Stassfurt halite. The field consists of 8 tilted fault blocks providing good trapping structures (figure 2).
The Southern Permian Basin is part of a Permo-Triassic basin which extends from England east to Poland (figure 1). The whole basin is underlain by Carboniferous Westphalian Coal Measures which are a prolific gas source. The bulk of the United Kingdom's gas reserves are contained within the Southern Gas Basin with production mainly from the clastic sandstone reservoirs of the Triassic Bunter Sand, the Lower Permian Rotliegendes Sand and the Carboniferous Westphalian sands. Some gas and oil production comes from Upper Permian Zechstein dolomites. Some of the largest UK gas fields have been discovered in this basin - Leman (6tcf), Indefatigable (5tcf) and Hewett (5tcf).
1.2 Cygnus Field History
The Cygnus field is a composite structure that is made up of a series of terraced, tilted fault blocks (figure 2). Cygnus was discovered in 1988 by the 44/12â€‘1 discovery well (Fault Block 1) which was drilled by Marathon. In the 44/11-2 well the Leman formation had a gross thickness of 48 meters with a net to gross percentage of 42.3% and a porosity of 8.9%. The carboniferous Westphalian A lower coal measures were 143 meters thick with a net to gross percentage of 27.4% and a porosity of 10.9% (Ritchie J.S & P Pratsides 1993)
In 1989 the 44/11-2 well (Fault Block 5a) also drilled by Marathon. In the 44/11-2 well the Leman formation had a gross thickness of 66 meters with a net to gross percentage of 30.6% and a porosity of 8.1%. The carboniferous Westphalian A lower coal measures were 121 meters thick with a net to gross percentage of 39.6% and a porosity of 12.7%.
In 2006, another appraisal well, 44/12-2, drilled in the original discovery fault block confirmed the reservoir held gas
Further appraisal drilling was conducted in February 2009 to prove the presence of the Leman reservoir in the eastern part of the field, to assess reservoir quality and fluid content of the primary Leman reservoir and to test the deeper Carboniferous reservoir as a secondary target. The well, 44/12a-3, was drilled by Venture, confirming the presence of gas in both intervals of the reservoir. Three months later, a fourth well was drilled - 44/12/a-4 - to determine the size of the field. The well was drilled to a total depth of 12,284 feet (3,744 meters) and flow tested at a rate of 32 MMcf/d from the targeted Rotliegendes Leman reservoir. The field contains dry gas with a low condensate-gas ratio i.e. less than 1 barrel of condensate per million standard cubic feet of gas. (Venture Production plc and GDF Suez Metoc Report 2010)
The source rock in the Southern North Sea is the Carboniferous Westphalian Coal measures. This source rock was formed at the end of the Carboniferous period some 300 million years ago when the area of Britain and the North Sea lay over the equator. During this period it was covered by swampy rain forest. These were the Carboniferous coal forests, which subsequently came to provide much of Britain's coal and natural gas. "The northward drift of Laurussia carried NW Europe from the equatorial zone of Carboniferous coal deposition to the latitude of a northern hemisphere trade-wind desert, where it lay in the rain shadow of the Variscan Mountains" (K W Glennie 1990).
As the land sank and sea level fluctuated the vegetation was submerged in a cyclical manner forming a series of differing layers. The carboniferous material was deposited in an anoxic environment and with continued deposition of sediments became buried. This allowed the organic material in sediments to be converted into coal. Thickness variations within the Westphalian are minimal.
"Phases of crustal extension (rifting), followed by phases of relaxation (sag), played an important role in the evolution of the basin". (Gerhard H. Bachmann 2009). D G Quirk and J F Aitken in their extract in (Zeigler et al1997, Petroleum Geology of the Southern North Sea) describe how the area of the Permian basin was subjected to this rifting which was known as the Salian event. This rifting led to a series of E-W and NW-SE faults that tilted and offset the Westphalian formation. The Upper Rotliegend was deposited above the Westphalian during a period of thermal sag after the rifting stopped.
The carboniferous succession consists of the following formations within block 44/11 and 12.
Westphalian B - Westoe coal formation which is mainly coal bearing with small amounts of sandstone.
Westphalian A - Caister coal formation which is predominately sandstone with thin coal bearing layers.
Basal Murdoch - Massive sandstone formation.
Maturation / Migration
During initial deposition the organic matter went through a process of biogenic decay. This area is referred to as the zone of digenesis. Methane, carbon dioxide and water are given off during this process leaving the carbon and hydrogen largely as they were. This process forms kerogen, in this case type 3 kerogen which is particularly associated with gas production.
As they the depth of the plant layers increased over time they were converted to coal layers interbedded with claystone and sand. Sinking due to sediment loading and thermal sag, particularly in the area of the North Sea, continued and once the deposits were around 4,000 m deep the increased pressure and temperature allowed the maturation of the organic material into gas to take place. This is the zone of catagenesis. The formation of oil required temperatures of between 60° and 120° C. Above 120°C only dry gas was produced until the temperatures reached about 180° - 225°C. Above 225°C no further hydrocarbons are produced and only graphite remains.
This process is called the oil window or oil kitchen and requires the above temperature ranges and a burial depth of between 2,500 and 5,000 m. All the above criteria were met in the Permian basin.
From regional studies it appears that gas generation commenced as early as mid-Jurassic times in the northern part of the Permian Basin, being most prolific in late Cretaceous times within the Southern North Sea area and undoubtedly continues to this day. Other minor source rocks comprise the shales of the Carboniferous Namurian which are known to be the source of the oil in the onshore fields to the west-north-west, and the thin, highly organic shale of the basal Zechstein Kupferschiefer Formation but the Westphalian coals constitute the predominant source (Ithaca Energy 2007).
As the gas matures it migrates into the porous sandstone formations displacing the water within the pore spaces. These migration routes can be short or long and have many mechanisms. Within the Cygnus field it is likely that vertical migration along fault conduits occurred with gas then entering the sandstone formation and migrating within the tilted fault block until trapped by the overlying halite seal.
The primary target reservoir is the Lower Permian, Rotliegend Group, Leman Sandstone formation that comprises of predominantly Aeolian dune deposits which range in thickness from 100m to 250m in this area.
In addition to the principle Permian sandstone target the Westphalian 'A', Caister sands form a potential secondary target for the well. The Carboniferous has provided proven reservoirs in the northern part of the Southern North Sea basin in quadrants 43 and 44 (Ithaca Energy 2007).
The Leman sandstone formation was formed during early Permian times, around 270 million years ago. A desert lake was bordered by massive sand dunes in arid conditions of the times. Wind blown dessert sands formed dunes that migrated with the direction of the wind. The continued tectonic plate movement resulted in the collision between Laurussia and Gondwana and the creation of the Variscan mountain chain. These mountains became "the main source of Rotliegend fluvial sediment over successor basins to the former back-arc foreland" (K W Glennie 1990).
Reservoir quality depends greatly on the environment at the time the sands were deposited. Most commercial gas accumulations are found in Aeolian dune deposits where grains have had extensive reworking by wind and are well sorted, similar sized grains. As a result, these formations have good permeability and porosity and some of these dune sands form the most permeable parts of the Rotliegend Sandstone Group.
The Base of the Rotliegendes is marked by the base Permian Unconformity.
The ultimate top seal is provided by the Silverpit formation. The Silverpit formation is thick and laterally continuous in the block 44 area and forms a good seal. Additional seals are provided aabove this by the Zechstein Formation which overlies the Silverpit with a 7,000 ft thick sequence of evaporites. This sequence was formed 250 million years ago when salt layers were deposited in the inland Zechstein Sea over the dune sands. The Zechstein consists of five depositional cycles of evaporite rocks, Z1 to Z5 respectively. The lithologies found within these layers are halite, anhydrite, and dolomite. The stratigraphic detail of the Zechstein in the Cygnus field is detailed in figure 3.
The Stassfurt halite has been the most important formation in the long term sealing of the field due to the mobile nature of the salt. As faults have developed across the area the Stassfurt has flowed into these are and sealed it again. Seismic interpretation has predicted that the Z3 Plattendolomite does not extend into the field. This is an important factor in the economic development of the field as many severe drilling problems are attributed to the Plattendolomite such as high pressure diapers and rafts.
The key area for petroleum discovery is the finding the correct trapping structure. Although almost of the entire Permian basin produces gas from the coal measures unless suitable trapping structures are in place commercially viable quantities will not accumulate. There have probably been more dry wells drilled in the Southern North Sea than there have been productive ones. These trapping structures can be formed by a variety of different methods and are identified from seismic data. The trapping mechanisms for the Cygnus field are a series of tilted faults sealed against the overlying Silverpit and Zechstein evaporites. There is gentle folding across the area. A simplified cartoon diagram of this type of trapping mechanism is seen in figure 4. The lateral seal in these blocks is through porosity occlusion due to halite cementation. Figure 5 shows the full stratigraphic column for the most recent well drilled on Cygnus.
The Cygnus field has all the necessary structural and stratigraphic attributes required for hydrocarbon accumulation.
The Westphalian coal measures are a proven source of gas across the entire Permian basin.
These are overlain by the Rotliegend formation which has a consistent thickness across 3 of the 8 blocks drilled to date. This is an Aeolian dune system is typically highly permeable and with good porosity. Permeabilities in the Leman sandstone are in the region of 300 to 1200 mD with porosity of between 10 and 20%.
Seismic data has shown good trapping structures and isolation within the fault blocks.
The seal is provided by the Silverpit shales and the Zechstein evaporite sequence across the entire field.
"With an estimated 1,000 Bcf of gas in place (gross P50) from the Leman reservoir alone, Cygnus is one of the largest undeveloped discoveries in the southern North Sea" (www.oilvoice.com)
Cygnus is well named. It is set to become a rising star in the gas business.
Zeigler K, Turner P & Daines SR (eds) 1997, Petroleum Geology of the Southern North Sea, Geological Society Special Publication 123, 2008, pp. 235 - 236,
Gerhard H. Bachmann, Martin-Luther-Universität Halle-Wittenberg (Germany)
K W Glennie, Geological Society, London, Special Publications; 1990; v. 55; p. 127-138.
GEUS, Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen NV, Denmark
www.investegate.co.uk/articlePrint.aspx?id=200902100700070415N Venture Production plc, RNS Number : 0415N,10 February 2009
Ithaca Energy 2007
GDF Suez METOC Environmental report 2010
Ritchie, J.S & P. Pratsides (1993) The Caister Fields Block 44/23a, UK North Sea- In: from Petroleum Geology of North West Europe: pp. 762