Changes

==Rift geometry and geographic limits==

==Rift geometry and geographic limits==

The SVG has a half-graben geometry, with the graben-bounding fault system on the western side being far more pronounced than the boundary on its eastern margin (Figures 2–5). The wider structural context of the SVG is shown in Zanella and Coward (2003, their figure 4.11) and high-quality seismic images of parts of the western margin of the graben and the platform area to the west are shown in Patruno and Reid (2016, 2017). To the south of the North Brae field, the western graben margin trends north-south, and to the north of the Braemar field it trends north-northeast–south-southwest (Figure 3). Between the North Brae and East Brae fields, the graben margin trends northeast–southwest, probably following a Caledonian lineament, and skirts an area of Silurian granite (drilled at the Cairngorm discovery—well U.K. 16/3a-11) at the edge of the footwall (the FGS). Between East Brae and Braemar the graben margin trends to the north-northwest, following the interpreted edge of this granite body, which is assumed to intrude early Paleozoic basement rocks (undrilled in this area). Thick Devonian sandstones overlying these older rocks are present across much of the FGS (Patruno and Reid, 2016, 2017; Patruno et al,, in press). The major fault zone on the western side of the SVG can be defined by three significant lineaments;

The SVG has a half-graben geometry, with the [[graben]]-bounding [[fault]] system on the western side being far more pronounced than the boundary on its eastern margin ([[:file:M115CH02FG02.jpg|Figure 2]], [[:file:M115CH02FG03.jpg|Figure 3]], [[:file:M115CH02FG04.jpg|Figure 4]], [[:file:M115CH02FG05.jpg|Figure 5]]). The wider structural context of the SVG is shown in Zanella and Coward (2003<ref name=Zanellaandcoward2003 />, their figure 4.11) and high-quality [[seismic data|seismic]] images of parts of the western margin of the graben and the platform area to the west are shown in Patruno and Reid (2016<ref name=Patrunoandreid2016>Patruno, S., and W. Reid, 2016, New plays on the Greater East Shetland Platform (UKCS Quadrants 3, 8–9, 14–16)—part 1: Regional setting and a working petroleum system: First Break, v. 34, p. 33–45.</ref>, 2017<ref name=Patrunoandreid2017>Patruno, S., and W. Reid, 2017, New plays on the Greater East Shetland Platform (UKCS Quadrants 3, 8–9, 14–16)—part 2: Newly reported Permo-Triassic intra-platform basins and their influence on the Devonian-Paleogene prospectivity of the area: First Break, v. 35, p. 59–69.</ref>). To the south of the North [[Brae field]], the western graben margin trends north-south, and to the north of the [[Braemar field]] it trends north-northeast–south-southwest ([[:file:M115CH02FG03.jpg|Figure 3]]). Between the North Brae and East Brae fields, the graben margin trends northeast–southwest, probably following a [[Caledonia]]n lineament, and skirts an area of [[Silurian]] [[granite]] (drilled at the [[Cairngorm]] discovery—well U.K. 16/3a-11) at the edge of the [[footwall]] (the FGS). Between East Brae and Braemar the graben margin trends to the north-northwest, following the interpreted edge of this granite body, which is assumed to intrude early [[Paleozoic]] [[basement]] rocks (undrilled in this area). Thick [[Devonian]] [[sandstone]]s overlying these older rocks are present across much of the FGS (Patruno and Reid, 2016<ref name=Patrunoandreid2016 />, 2017<ref name=Patrunoandreid2017 />; Patruno et al., 2018<ref name=Patrunoetal2018>Patruno, S., W. Reid, C. Berndt, and L. Feuilleaubois, in press, Polyphase tectonic inversion and its role in controlling hydrocarbon prospectivity in the Greater East Shetland Platform and Mid North Sea High, UK, in A. A. Monaghan, J. R. Underhill, A. J. Hewett, and J. E. A. Marshall, eds., Paleozoic plays of NW Europe: Geological Society (London) Special Publication 471, [https://doi.org/10.1144/SP471.9 doi:10.1144/SP471.9].</ref>). The major [[fault zone]] on the western side of the SVG can be defined by three significant lineaments;

the erosional eastern edge of sub-Upper Jurassic rocks that comprise the FGS,

the erosional eastern edge of sub-Upper [[Jurassic]] rocks that comprise the FGS,

the abutment of the Top KCF (early Ryazanian) against the fault plane or eroded fault scarp (although thin units of KCF occur in places on the FGS—Figure 3), and

the abutment of the Top KCF (early [[Ryazanian]]) against the fault plane or eroded fault scarp (although thin units of KCF occur in places on the FGS—[[:file:M115CH02FG03.jpg|Figure 3]]), and

the abutment of the (prerift) Top Sleipner Formation (approximately Top Bathonian) against the eastward-dipping graben-margin fault plane (Figures 3–5).

the abutment of the ([[prerift]]) [[Top Sleipner Formation]] (approximately Top Bathonian) against the eastward-dipping graben-margin fault plane ([[:file:M115CH02FG03.jpg|Figure 3]], [[:file:M115CH02FG04.jpg|Figure 4]], [[:file:M115CH02FG05.jpg|Figure 5]]).

The area between the edge of the FGS and the abutment of the KCF is in most parts an eroded slope, against which Cretaceous units onlap (Figure 4). The dip of this slope is about 30–35 degrees, calculated from well penetrations at the top and near the base of the slope. This slope extends a short distance basinward beneath the upper part of the Kimmeridge Clay before grading into the noneroded fault scarp, which has a dip of approximately 60 degrees (estimated from seismic mapping and depth conversion). The total amount of original throw on the western boundary fault is more difficult to estimate due to the erosion on the FGS. The depth of the pre-Upper Jurassic “basement” on the FGS along the edge of the graben is generally between 7750 and 9000 ft (∼2360–2740 m). The deepest parts of the synrift section along the western margin of the graben, represented by the top of the Bathonian Sleipner Formation, lie at approximately 20,000 to 23,000 ft (∼6100–7000 m) in the area shown in Figure 3, implying a minimum throw of 12,000 to 14,000 ft (∼3660–4270 m). However, the total throw on the western boundary fault may have been considerably larger than this difference between these two depth ranges, due to the unknown thicknesses of strata eroded from the FGS during the Late Jurassic. It is probable that at least some Middle Jurassic to Permian sequences overlying Devonian and older rocks (there were probably only minor amounts, if any, of Carboniferous rocks in the area) were originally present on the FGS prior to late Jurassic rifting

The area between the edge of the FGS and the abutment of the KCF is in most parts an eroded slope, against which Cretaceous units onlap (Figure 4). The dip of this slope is about 30–35 degrees, calculated from well penetrations at the top and near the base of the slope. This slope extends a short distance basinward beneath the upper part of the Kimmeridge Clay before grading into the noneroded fault scarp, which has a dip of approximately 60 degrees (estimated from seismic mapping and depth conversion). The total amount of original throw on the western boundary fault is more difficult to estimate due to the erosion on the FGS. The depth of the pre-Upper Jurassic “basement” on the FGS along the edge of the graben is generally between 7750 and 9000 ft (∼2360–2740 m). The deepest parts of the synrift section along the western margin of the graben, represented by the top of the Bathonian Sleipner Formation, lie at approximately 20,000 to 23,000 ft (∼6100–7000 m) in the area shown in Figure 3, implying a minimum throw of 12,000 to 14,000 ft (∼3660–4270 m). However, the total throw on the western boundary fault may have been considerably larger than this difference between these two depth ranges, due to the unknown thicknesses of strata eroded from the FGS during the Late Jurassic. It is probable that at least some Middle Jurassic to Permian sequences overlying Devonian and older rocks (there were probably only minor amounts, if any, of Carboniferous rocks in the area) were originally present on the FGS prior to late Jurassic rifting