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==Introduction==

==Introduction==

The South Viking Graben (SVG), in the northern part of the North Sea, is a well-studied Middle to Upper Jurassic rift basin containing a major hydrocarbon province that extends across the border between the U.K. and Norway. Thermal doming during the Toarcian–Aalenian, centered on the area south of the southern end of the SVG, resulted in the deposition of thick Middle Jurassic sequences along a north-trending basin that is now the SVG (Underhill and Partington, 1993; and see Hoth et al., 2018). Subsequent deflation of the dome was followed by significant rifting in the SVG, which began in the Callovian. However, the most active phases of rifting occurred during the Oxfordian to early Volgian (early Tithonian), when very thick synrift sequences were deposited in the graben. The graben is bounded on the west by the Fladen Ground Spur (FGS) and on the east by the Utsira High (Figure 1). A large proportion of the hydrocarbon reserves is found in synrift to early postrift, deep-marine clastic sediments of the Brae Formation; these accumulations are informally termed the Brae Play. The Brae Formation interfingers with, and is overlain by, the Kimmeridge Clay Formation (KCF; termed Draupne Formation in Norway). The KCF-Draupne Formation is the major source rock and is the most significant seal for trapped Brae Play hydrocarbons. Several previous publications have shown the outlines and general internal structure of this buried half-graben (e.g., Harris and Fowler, 1987; Johnson et al., 1993; Sneider et al., 1995; Thomas and Coward, 1996; Fraser et al., 2003; Zanella and Coward, 2003) and numerous papers have dealt with various other aspects of the graben (see Connell, 2018, for a full reference list). However, more accurate depictions of the structure, particularly the western fault-bounded margin and the internal structure of the graben fill, are now possible due to the complete coverage of the area by high-quality 3-D seismic data. A 3-D image of the south and central parts of the Viking Graben and its flanks, at Base Cretaceous level, is shown in Figure 2. In addition, extensive released well-data sets are available in this mature hydrocarbon province that allow comprehensive analysis of the stratigraphy and a clearer understanding of the extent of submarine fans containing hydrocarbon reservoirs.

The South Viking Graben (SVG), in the northern part of the [[North Sea]], is a well-studied Middle to Upper [[Jurassic]] [[rift basin]] containing a major [[hydrocarbon province]] that extends across the border between the [[United Kingdom]] and [[Norway]]. Thermal doming during the [[Toarcian]]–[[Aalenian]], centered on the area south of the southern end of the SVG, resulted in the deposition of thick Middle Jurassic sequences along a north-trending basin that is now the SVG (Underhill and Partington, 1993<ref name=Underhillandpartington1993>Underhill, J. R., and M. A. Partington, 1993, Jurassic thermal doming and deflation in the North Sea: Implications of the sequence stratigraphic evidence, in J. R. Parker, ed., Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference: Geological Society (London) Conference Series 4, p. 337–345.</ref>; and see Hoth et al., 2018<ref name=Hothetal201>Hoth, S., D. Knaust, A. Sánchez-López, S. Kassold, and S. Sviland-Østre, 2018, [http://archives.datapages.com/data/specpubs/memoir115/data/387_aapg-sp2050387.htm The Gudrun field: Gravity-flow deposition during rifting and inversion]], in C. C. Turner, and B. T. Cronin, eds., Rift-related coarse-grained submarine fan reservoirs; the Brae Play, South Viking Graben, North Sea: [http://store.aapg.org/detail.aspx?id=1301 AAPG Memoir 115], p. 387–422.</ref>). Subsequent deflation of the dome was followed by significant [[rift]]ing in the SVG, which began in the [[Callovian]]. However, the most active phases of rifting occurred during the [[Oxfordian]] to early [[Volgian]] (early [[Tithonian]]), when very thick [[synrift]] sequences were deposited in the graben. The graben is bounded on the west by the Fladen Ground Spur (FGS) and on the east by the Utsira High ([[:file:M115CH02FG01.jpg|Figure 1]]). A large proportion of the [[hydrocarbon]] reserves is found in synrift to early [[postrift]], deep-marine [[clastic]] [[sediment]]s of the [[Brae Formation]]; these accumulations are informally termed the [[Brae Play]]. The Brae Formation [[interfinger]]s with, and is overlain by, the [[Kimmeridge Clay Formation]] (KCF; termed [[Draupne Formation]] in Norway). The KCF-Draupne Formation is the major [[source rock]] and is the most significant [[seal]] for trapped Brae Play hydrocarbons. Several previous publications have shown the outlines and general internal structure of this buried half-graben (e.g., Harris and Fowler, 1987<ref name=Harrisandfowler1987>Harris, J. P., and R. M. Fowler, 1987, Enhanced prospectivity of the Mid-Late Jurassic sediments of the South Viking Graben, northern North Sea, in J. Brooks and K. Glennie, eds., Petroleum Geology of North West Europe: Bath, U.K., Graham and Trotman, p. 879–898.</ref>; Johnson et al., 1993<ref name=Johnsonetal1993>Johnson, H., P. C. Richards, D. Long, and C. C. Graham, 1993, United Kingdom offshore regional report; the geology of the northern North Sea: London, HMSO, for the British Geological Survey.</ref>; Sneider et al., 1995<ref name=Sneideretal1995>Sneider, J. S., P. de Clarens, and P. R. Vail, 1995, Sequence stratigraphy of the Middle to Upper Jurassic, Viking Graben, North Sea, in R. J. Steel, V. L. Felt, E. P. Johannessen, and C. Mathieu, eds., Sequence stratigraphy on the Northwest European margin: Norwegian Petroleum Society Special Publication 5, p. 167–197.</ref>; Thomas and Coward, 1996<ref name=Thomasandcoward1996>Thomas, D. W., and M. P. Coward, 1996, Mesozoic regional tectonics and South Viking Graben formation: Evidence for localized thin-skinned detachments during rift development and inversion: Marine and Petroleum Geology, v. 13, p. 149–177.</ref>; Fraser et al., 2003<ref name=Fraseretal2003 />; Zanella and Coward, 2003<ref name=Zanellaandcoward2003>Zanella, E., and M. P. Coward, 2003, Structural framework, in D. Evans, C. Graham, A. Armour, and P. Bathurst, eds., The Millennium Atlas: Petroleum geology of the central and northern North Sea: Geological Society (London), p. 45–59.</ref>) and numerous papers have dealt with various other aspects of the graben (see Connell, 2018<ref name=Connell2018>Connell, E. R., 2018, [http://archives.datapages.com/data/specpubs/memoir115/data/619_aapg-sp2050619.htm Bibliography of geological publications and Ph.D. theses from the South Viking Graben area, North Sea], in C. C. Turner, and B. T. Cronin, eds., Rift-related coarse-grained submarine fan reservoirs; the Brae Play, South Viking Graben, North Sea: [http://store.aapg.org/detail.aspx?id=1301 AAPG Memoir 115], p. 619–628.</ref>, for a full reference list). However, more accurate depictions of the [[structure]], particularly the western fault-bounded [[margin]] and the internal [[structure]] of the graben fill, are now possible due to the complete coverage of the area by high-quality 3-D [[seismic data]]. A 3-D image of the south and central parts of the Viking Graben and its flanks, at Base [[Cretaceous]] level, is shown in [[:file:M115CH02FG02.jpg|Figure 2]]. In addition, extensive released well-data sets are available in this mature hydrocarbon province that allow comprehensive analysis of the [[stratigraphy]] and a clearer understanding of the extent of [[submarine fan]]s containing [[hydrocarbon reservoir]]s.

[[file:M115CH02FG03.jpg|300px|thumb|{{figure number|3}}Main structural elements of the South Viking Graben and its flanks. The extent of the thick Middle-Upper [[Jurassic]] [[graben]] fill is defined by the limits of the [[KCF-Draupne Formation]]. Unshaded areas on the Fladen Ground Spur and Utsira High have pre-Upper Jurassic rocks beneath [[Cenozoic]] or [[Cretaceous]] strata (with very thin, <approximately 2 m [6.6 ft], KCF-Draupne Formation in a few wells). The locations of Upper Jurassic [[oil]] and [[gas]] [[condensate]] fields within the graben are shown with solid fill. Fields with Cenozoic [[reservoir]]s are shown with black outlines; [[field]]s and discoveries with Middle Jurassic and older reservoirs are shown with orange outlines. Most faults shown are derived from mapping by some of the authors of the [http://archives.datapages.com/data/specpubs/memoir115/data/9_aapg-sp2050009.htm full paper] associated with this Wiki article, but a few in the southeast portion of the map are from Zanella et al. (2003<ref name=Zanellaetal2003>Zanella, E., M. P. Coward, and A. McGrandle, 2003, Crustal structure, in D. Evans, C. Graham, A. Armour, and P. Bathurst, eds., The millennium atlas: Petroleum geology of the central and northern North Sea: Geological Society (London), p. 35–43.</ref>). Bi = Birch; Br = Braemar; CB = Central Brae; D = Devenick; EB = East Brae; G = Gudrun; JS = John Sverdrup; K = Kingfisher; L = Larch; M = Miller; NB = North Brae; SB = South Brae; S = Sycamore; Th = Thelma; Ti = Tiffany; To = Toni.]]

[[file:M115CH02FG03.jpg|300px|thumb|{{figure number|3}}Main structural elements of the South Viking Graben and its flanks. The extent of the thick Middle-Upper [[Jurassic]] [[graben]] fill is defined by the limits of the [[KCF-Draupne Formation]]. Unshaded areas on the Fladen Ground Spur and Utsira High have pre-Upper Jurassic rocks beneath [[Cenozoic]] or [[Cretaceous]] strata (with very thin, <approximately 2 m [6.6 ft], KCF-Draupne Formation in a few wells). The locations of Upper Jurassic [[oil]] and [[gas]] [[condensate]] fields within the graben are shown with solid fill. Fields with Cenozoic [[reservoir]]s are shown with black outlines; [[field]]s and discoveries with Middle Jurassic and older reservoirs are shown with orange outlines. Most faults shown are derived from mapping by some of the authors of the [http://archives.datapages.com/data/specpubs/memoir115/data/9_aapg-sp2050009.htm full paper] associated with this Wiki article, but a few in the southeast portion of the map are from Zanella et al. (2003<ref name=Zanellaetal2003>Zanella, E., M. P. Coward, and A. McGrandle, 2003, Crustal structure, in D. Evans, C. Graham, A. Armour, and P. Bathurst, eds., The millennium atlas: Petroleum geology of the central and northern North Sea: Geological Society (London), p. 35–43.</ref>). Bi = Birch; Br = Braemar; CB = Central Brae; D = Devenick; EB = East Brae; G = Gudrun; JS = John Sverdrup; K = Kingfisher; L = Larch; M = Miller; NB = North Brae; SB = South Brae; S = Sycamore; Th = Thelma; Ti = Tiffany; To = Toni.]]

[[file:M115CH02FG04.jpg|300px|thumb|{{figure number|4}}West-east [[seismic]] section across the South Viking Graben, through Sycamore field. Location shown on [[:file:M115CH02FG01.jpg|Figure 1]], [[:file:M115CH02FG02.jpg|Figure 2]], and [[:file:M115CH02FG03.jpg|Figure 3]]. Courtesy of PGS.]]

[[file:M115CH02FG04.jpg|300px|thumb|{{figure number|4}}West-east [[seismic]] section across the South Viking Graben, through Sycamore field. Location shown on [[:file:M115CH02FG01.jpg|Figure 1]], [[:file:M115CH02FG02.jpg|Figure 2]], and [[:file:M115CH02FG03.jpg|Figure 3]]. Courtesy of PGS.]]

[[file:M115CH02FG05.jpg|

[[file:M115CH02FG05.jpg|300px|thumb|{{figure number|5}}West-northwest–east-southeast seismic section across the South Viking Graben, through the [[East Brae field|East Brae]] and [[Gudrun field]]s. Location shown on [[:file:M115CH02FG01.jpg|Figure 1]], [[:file:M115CH02FG02.jpg|Figure 2]], and [[:file:M115CH02FG03.jpg|Figure 3]]. Courtesy of PGS.]]

==Rift geometry and geographic limits==

==Rift geometry and geographic limits==

The difference between these minimum and maximum estimates for late Jurassic throw on the western margin of the SVG is considerable and reflects the continued uncertainty of the post-Variscan, pre-late Jurassic history of the FGS area. From considerations of the thicknesses of Permian to Triassic sequences farther to the north, Turner and Connell (2018) estimate that several thousand feet (several hundred meters) of Permian to Middle Jurassic strata may have been eroded from the FGS in the Brae area. However, Patruno and Reid (2017) consider that there may have been only minor amounts of such strata present prior to late Jurassic erosion. These differences in interpretation result mainly from whether existing fault-bounded Permo-Triassic successions on the FGS are regarded as having been deposited in active grabens with only minor deposition beyond the present basin limits (e.g., Patruno and Reid, 2017), or whether they are interpreted as remnants of much more widespread deposits (albeit probably in part syn-tectonic) that were downfaulted during the Jurassic (as implied by Turner and Connell, 2018).

The difference between these minimum and maximum estimates for late Jurassic throw on the western margin of the SVG is considerable and reflects the continued uncertainty of the post-Variscan, pre-late Jurassic history of the FGS area. From considerations of the thicknesses of Permian to Triassic sequences farther to the north, Turner and Connell (2018) estimate that several thousand feet (several hundred meters) of Permian to Middle Jurassic strata may have been eroded from the FGS in the Brae area. However, Patruno and Reid (2017) consider that there may have been only minor amounts of such strata present prior to late Jurassic erosion. These differences in interpretation result mainly from whether existing fault-bounded Permo-Triassic successions on the FGS are regarded as having been deposited in active grabens with only minor deposition beyond the present basin limits (e.g., Patruno and Reid, 2017), or whether they are interpreted as remnants of much more widespread deposits (albeit probably in part syn-tectonic) that were downfaulted during the Jurassic (as implied by Turner and Connell, 2018).

[[file:M114CH02FG06.jpg|

[[file:M115CH02FG06.jpg|300px|thumb|{{figure number|6}}West-east seismic section across the Central Viking Graben. Location shown on [[:file:M115CH02FG01.jpg|Figure 1]] and [[:file:M115CH02FG02.jpg|Figure 2]]. Courtesy of PGS.]]

To the south of approximately 59°N, the base of the synrift fill rises eastward in a series of fault terraces toward the Utsira High (Figures 4, 5) where Devonian and older rocks are present beneath a thin Cretaceous (and in places Jurassic) sequence. Between approximately 59°N and 59° 30’N, the deepest part of the graben is somewhat more symmetrical, with a large fault defining its eastern boundary as well as its western boundary but with the western margin also stepping back westward to include the Beryl embayment. Farther north still, in the Central Viking Graben, it reverts to true half-graben geometry (Figures 2, 6). The eastern margin of the southern part of the SVG is, therefore, less precisely defined than its western margin. The thickest section of Upper Jurassic strata in the graben is bounded by several down-to-the-west normal faults (Figure 3), but the true eastern margin of the graben is difficult to define as flexural downbending was the dominant mechanism here, rather than faulting. An approximation of the easterly limit of the late Jurassic graben can be taken at the eastern termination of seismically-definitive Top KCF-Draupne Formation (Figure 3). However, the uppermost KCF-Draupne Formation was deposited after rifting had ceased and extends in places beyond the original graben limits. Although some areas of the Utsira High are devoid of Draupne Formation (Figure 3), relatively thin units (generally <100 m [∼330 ft]) of Draupne Formation are present on parts of the high, particularly on its eastern flank, where it deepens toward the Stord Basin and southward toward the Jaeren High. Thin (<40 m [∼130 ft]) intra-Draupne sandstones also occur on the eastern flank of the high, notably forming the main reservoir of the extensive Johan Sverdrup field (Scott and Ottesen, 2018). An area of KCF, in part overlying thin Upper Jurassic shallow-marine sandstones, also occurs on the FGS to the west of the Brae fields (Figure 3).

To the south of approximately 59°N, the base of the synrift fill rises eastward in a series of fault terraces toward the Utsira High (Figures 4, 5) where Devonian and older rocks are present beneath a thin Cretaceous (and in places Jurassic) sequence. Between approximately 59°N and 59° 30’N, the deepest part of the graben is somewhat more symmetrical, with a large fault defining its eastern boundary as well as its western boundary but with the western margin also stepping back westward to include the Beryl embayment. Farther north still, in the Central Viking Graben, it reverts to true half-graben geometry (Figures 2, 6). The eastern margin of the southern part of the SVG is, therefore, less precisely defined than its western margin. The thickest section of Upper Jurassic strata in the graben is bounded by several down-to-the-west normal faults (Figure 3), but the true eastern margin of the graben is difficult to define as flexural downbending was the dominant mechanism here, rather than faulting. An approximation of the easterly limit of the late Jurassic graben can be taken at the eastern termination of seismically-definitive Top KCF-Draupne Formation (Figure 3). However, the uppermost KCF-Draupne Formation was deposited after rifting had ceased and extends in places beyond the original graben limits. Although some areas of the Utsira High are devoid of Draupne Formation (Figure 3), relatively thin units (generally <100 m [∼330 ft]) of Draupne Formation are present on parts of the high, particularly on its eastern flank, where it deepens toward the Stord Basin and southward toward the Jaeren High. Thin (<40 m [∼130 ft]) intra-Draupne sandstones also occur on the eastern flank of the high, notably forming the main reservoir of the extensive Johan Sverdrup field (Scott and Ottesen, 2018). An area of KCF, in part overlying thin Upper Jurassic shallow-marine sandstones, also occurs on the FGS to the west of the Brae fields (Figure 3).