Abstract Located 160 km NE of the Shetland Islands in the East Shetland Basin, the Dunlin Cluster comprises four produced fields, Dunlin, Dunlin SW, Osprey and Merlin, in addition to some near-field satellite discoveries, Skye and Block 6. Dunlin was discovered in July 1973 and production began in August 1978. The field was developed using a concrete gravity-base platform, Dunlin Alpha, which also served as the production facility for the rest of the Dunlin Cluster. Osprey was discovered in 1974 but not tied-in until January 1991. Dunlin SW was discovered in 1973 but not brought onto production until 1996. Merlin was discovered in February 1997 and tied-in later that same year. Fairfield Energy acquired the Dunlin Cluster in 2008, and a programme of investment and facilities improvements, primarily in fuel gas infrastructure and power generation, sought to boost water-injection rates and bolster production, thereby extending the life of the asset. Ultimately, the Dunlin Cluster ceased production on 15 June 2015 after having maximized economic hydrocarbon recovery. The total Dunlin Cluster production exceeded 500 MMbbl of oil (Dunlin and Dunlin SW, 395 MMbbl oil; Osprey, 92 MMbbl oil; and Merlin, 27 MMbbl oil).

Abstract The Penguins Cluster is a group of four oil and gas fields in the northern end of the East Shetland Basin. Its structural complexity is caused by the interaction between two or more fault trend populations, fault reactivation and the impact of faulting on the Brent reservoir architecture. This structural picture is further complicated by a NW–SE trending basement lineament interpreted as a Caledonian shear zone. The present day structural configuration is the result of two Mesozoic rifting episodes and their associated thermal subsidence phases. The Permo-Triassic rifting created a number of north–south-trending tilted fault blocks, and was followed by a period of tectonic quiescence until the Middle Jurassic, when a faulting episode coeval with the Brent Group deposition caused footwall rotation, uplift and erosion of the upper Rannoch Formation prior to the deposition of the Etive Formation across the area. The rifting climaxed in the late Jurassic, when the reactivation of pre-existing faults under oblique-slip conditions in the Penguin C Field created small-scale lozenge-shaped transpressional and transtensional fault blocks. The presence of reverse faults in the area is explained with a continuous kinematic model of structural evolution and oblique-slip fault reactivation rather than positive basin inversion.

Abstract The Statfjord Field, the largest oil field in the Northern North Sea, straddles the Norway/UK boundary and is located on the southwestern part of the Tampen Spur within the East Shetland Basin. The accumulation is trapped in a 6-8° W-NW dipping rotated fault block comprised of Jurassic-Triassic strata sealed by Middle to Upper Jurassic and Cretaceous shales. Reserves are located in three separate reservoirs: Middle Jurassic deltaic sediments of the Brent Group, Lower Jurassic marine-shelf sandstones and siltstones of the Dunlin Group; and Upper Triassic-lowermost Jurassic fluviatile sediments of the Statfjord Formation. The majority of reserves are contained within the Brent Group; and Statfjord Formation sediments which exhibit good to excellent reservoir properties with porosities ranging from 20-30% permeabilities ranging up to several darcies, and an average net-to-gross of 60-75%. The sandstones and siltstones of the Dunlin Group have poorer reservoir properties where the best reservoir unit exhibits an average porosity of 22%, an average permeability 300mD and net-to-gross of 45%. Structurally, the field is subdivided into a main field area characterized by relatively undeformed W-NW dipping strata, and a heavily deformed east flank area characterized by several phases of ‘eastward’ gravitational collapse. Production from the field commenced in 1979 and as of January 2000, 176 wells have been drilled. The oil is undersaturated and no natural gas-cap is present. The drainage strategy has been to develop the Brent and Dunlin Group reservoir with pressure maintenance using water injection and the Statfjord Formation reservoir by miscible gas flood. However, a strategy to improve recovery by implementing water alternating gas (WAG) methods is gradually being implemented for both the Brent and Statfjord reservoirs. Current estimates indicate that by 2015 a total of 666 x 10 6 Sm 3 (4192 MMBBL) of oil will be recovered and 75GSm 3 (2.66 TCF) gas will be exported from the field.

Abstract Cumulative oil production to the end of 2000 from the Don Field was 15.4 MMBBLS, which with an estimated STOIIP of 152 MMBBLS represents a recovery to date of 10%. Don has been producing for over ten years. The field lies 15 km N of the Thistle Field, at the western edge of the Viking Graben in the northern North Sea. The structure of the field is complex, and it comprises several segments, the two largest of which have been developed, Don NE and Don SW. The reservoir sequence is Middle Jurassic Brent Formation, but more deeply buried and of a more distal facies than is typical for other fields in the province. The Don Field is a sub-sea development tied-back to the Thistle platform, and Britoil (BP) is the operator. The field has been developed with five producers, three in NE and two in SW, with a supporting water injection well in each part of the field. All wells have been drill deviated from a seabed manifold located over Don NE.

Abstract The Strathspey Field was the first sub-sea development in the North Sea to be tied back to a third party operator, the Ninian Field now operated by Canadian Natural Resources (CNR). The field was discovered in 1975 by well 3/4-4 and lies wholly within Block 3/4a. The field is a tilted fault block, unconformity trap and consists of two separate reservoirs, a volatile oil and a gas condensate reservoir: the Middle Jurassic, Brent Group and the Lower Jurassic/Upper Triassic, Banks Group respectively. Two 3D seismic surveys cover the field, the most recent being a Vertical Cable Seismic survey recorded in 1996. The Banks Group reservoir is produced under depletion drive by five wells and the Brent Group reservoir by water flooding with 3 water injectors and 6 producing wells. In place volumes arc 290 BCF and 90MMSTB for the Banks Group and 120 MMSTR in the Brent Group Reservoir. Ultimate recoveries are estimated to be 230 BSCF, 22 MMBBL and 70 MMSTB, 88 BSCF respectively. Oil export is via the Ninian pipeline system to Sullom Voe, while gas export is through the Far North Liquids and Gas System (FLAGS) pipeline system to St Fergus.]

Abstract The Cormorant Field was discovered by exploration well 211/26-1 in 1972; the fifth field to be discovered in the Northern North Sea. It straddles blocks 211/21 a and 211/26a and is made up of four discrete accumulations spread along a major N-S trending fault terrace. Oil is produced from the sandstones belonging to the Brent Group. The sedimentary rocks comprising the Brent Group were deposited in a fluvial-wave dominated delta system during the Middle Jurassic. The field is developed from two fixed platforms and an underwater manifold centre and the oil is exported through the Brent system to Sullom Voe in the Shetland Islands. For development purposes the field is split in half; north and south, and it is the northern part, developed by the North Cormorant platform, that is the subject of this review.

Abstract The Dunbar, Ellon and Grant oil and gas fields (also known as the Alwyn South area) are located in the southeastern part of the East Shetland Basin, approximately 140 km E of the Shetland Islands. Most of the accumulations lie in Blocks 3/9, 3/14 and 3/15, which are parts of Licence P090 operated by Total Oil Marine plc (33.33%) with Elf Exploration UK PLC as sole partner (66.67%). Ellon was discovered in 1972, Dunbar in 1973 and Grant in 1977. Dunbar consists of a number of generally N-S trending, westerly dipping Mesozoic fault blocks with variable amounts of crestal erosion. Reservoir is provided by fluvial, deltaic and shallow marine sandstones of the Middle Jurassic Brent Group, Lower Jurassic Statfjord Formation and Upper Triassic Upper Lunde Formation. The Brent oil composition of Dunbar varies with depth and evolves from volatile oil at the base of the column to gas condensate at the top without a discontinuity of composition. In addition there is a small gas accumulation within a Paleocene submarine fan reservoir in a compactional structure. Ellon consists of two westerly dipping fault blocks with gas condensate contained within the Brent Group. Grant is one westerly dipping fault block with gas condensate in the Brent Group. In both the Ellon panels and also in Grant, thin waxy oil ‘rims’ are found below the gas. The depth of the shallowest structural crest within the Alwyn South complex is 3100m TVDSS, with the deepest proven hydrocarbon at around 3800m TVDSS. Sealing for the Alwyn South accumulations is provided by various combinations of Cretaceous, Upper Jurassic (Heather and Kimmeridge Clay Formations) and Lower Jurassic (Dunlin Group) mudstones. The source rock for the hydrocarbons is the Upper Jurassic Kimmeridge Clay Formation, which is mature and adjacent to the fields. These accumulations are being developed from a tender-assisted minimally manned fixed platform with a total of 28 well slots located over the Dunbar Field, in a water depth of 145 m. The Ellon and Grant Fields are produced as sub-sea satellites to Dunbar from a well-head cluster located between Ellon and Grant, in a water depth of 135 m. First oil and gas production from Dunbar and Ellon was in December 1994 and gas production commenced from Grant in July 1998. The time lag between discovery and development reflects the complex geology (structure, compartmentalization, reservoir thickness variations, diagenesis and differing hydrocarbon compositions) with a total of 28 exploration and appraisal wells being drilled in the Alwyn South area between 1971 and 1998. Total oil and gas initially in place is in the order of 850 MMBBL and 2.62 TCF respectively, with the current estimate for ultimate recoverable reserves being 200 MMBBL liquids and 1.28 TCF gas.

Abstract The Heather oil field is located in Block 2/5 in the Northern North Sea. Oil is produced from sandstones of the Middle Jurassic Brent Group, at depths between 9500 ft and 11 600 ft below sea level. The field has been in production for over 20 years, and to date 120 MMSTB of oil have been produced out of an oil-in-place volume of 464 MMSTB. Although approaching noncommercial flow rates, an ambitious programme of field re-evaluation has been conducted since 1997 to identify remaining infill potential, and to investigate the development potential of satellite accumulations. New 3D seismic surveys shot in 1995 and 1997 have been combined to produce a continuous top reservoir map of the main field and adjacent satellite structures. The new mapping and an updated reservoir description were integrated with reservoir simulation models to identify zones of unswept oil. Main field infill projects have been identified which target the unswept oil, allowing extension of field life to be planned in conjunction with satellite field development. An additional 57 MMSTB of recoverable oil are expected to be produced from Middle Jurassic reservoirs in the main field and in satellite areas.

Abstract This chapter describes Lower Jurassic second-order sequences J00 and J10, and their component third-order sequences J1–J6 and J12–J18. Two sequences (J1 and J3) are new, four sequences (J2, J4, J12 and J16) are amended and one sequence (J17) is renamed. A significant unconformity at the base of the J12 sequence (Upper Sinemurian) is present near the base of the Dunlin Group in the North Viking Graben–East Shetland Platform and in the Danish Central Graben, and correlates with an equivalent unconformity around the margins of the London Platform, onshore UK. A marked unconformity at the base of the J16 sequence is recognized in the North Viking Graben and onshore UK, where it is related to structural movements on the Market Weighton High, eastern England. Several levels of carbon enrichment (carbon isotope excursions (CIEs)) and associated geochemical changes tie to J sequences defining maximum flooding surfaces: the Upper Sinemurian CIE equates to the base J6 maximum flooding surface (MFS), the basal Pliensbachian CIE ties to the base J13 MFS, the basal Toarcian CIE relates to the base J17 MFS and the Toarcian Ocean Anoxic Event corresponds with the base J18 MFS.