Nigel H. Platt
1*, Angus MacLellan
2, Richard K. Morgan
3, Alan J. Armstrong
2and Raquel G. Arzola
11Energean UK Ltd, Accurist House, 44 Baker Street, London W1U 7AL, UK
2CNOOC Petroleum Europe Ltd, Discovery House, Prime Four Business Park, Kingswells Causeway, Aberdeen AB15 9PU, UK
3Rockflow Resources Ltd, Bernay House, Lower Street, Haslemere GU27 2PE, UK NHP,0000-0002-1131-0704; RGA,0009-0009-8945-1273
Present addresses: NHP, Centre for Energy Transition, School of Geosciences, University of Aberdeen, King’s College, Aberdeen AB24 3FX, UK; AM, Equinor, Prime Four Business Park, Kingswells, Aberdeen AB15, 8QG, UK; RKM, Arunside Ltd, Stubbs Copse, Woodland Avenue, Cranleigh, Surrey GU6 7HU, UK; AJA, Dana Petroleum Ltd, 62 Huntly Street, Aberdeen AB10 1RS, UK; RGA, CNOOC Petroleum Europe Ltd, Prospect House, 97 Oxford Road, Uxbridge UB8 1LU, UK
* Correspondence:[email protected]; [email protected]
Abstract: As producing fields enter late life, successful mitigation of subsurface risk is critical in order to justify continued infill well drilling. Despite penetrations by more than 100 wells and the availability of modern 3D seismic across the Scott and Telford fields (Outer Moray Firth, UK Central North Sea), reservoir absence has led to repeated development well failures.
A new model now attributes reservoir attenuation to Late Jurassic footwall uplift and erosion.
The 2015 Scott J40 well encountered a reduced Upper Jurassic reservoir section of Lower Scott sands, with Upper Scott and Piper sands absent. Reinterpretation ascribed reservoir truncation to the effect of Late Jurassic footwall uplift and erosion rather than fault cut-out as previously interpreted. The failed 2016 Telford F6 well was shown to have drilled a thick Kimmeridge Clay hanging-wall section north of the Telford Fault, rather than the southern footwall section targeted. Planning for the Scott J43 and four subsequent wells implemented lessons learned from these failures and mitigated reservoir risk while optimizing reserves.
The Scott and Piper sand distributions are likely to reflect early growth folding before the main Mid-Kimmeridgian–Early Tithonian phase of NE–SW extensional faulting and footwall uplift, which saw the Piper and Scott reservoirs eroded at footwall crests and locally reworked as deep-water Claymore sands. Later Mid-Tithonian–Early Berriasian east–west faulting during the opening of the Witch Ground Graben saw major crestal synsedimentary erosion at Telford and continued erosion at the western crest of Scott Block 1b, while the footwall to the east was partially downfaulted.
Received8 December 2023;revised1 July 2024;accepted3 July 2024
The continued development of mature fields is likely to require the consideration of increasingly challenging drilling locations that carry inherent subsurface risk: for example, in targeting smaller and more structurally complex attic locations or previously undrilled blocks at the margins of proven accumulations.
Despite the drilling of more than 100 wells in the Scott and Telford fields in the Outer Moray Firth (UK Central North Sea) and the availability of multiple generations of towed streamer and ocean- bottom 3D seismic data, a number of development well failures have continued to deliver surprises, with some 25% of wells encounter- ing missing or significantly attenuated reservoir sections. It is therefore critically important to understand the mechanisms responsible for reservoir development and preservation if geological risks are to be managed to an acceptable level and future target selection errors avoided.
Previous interpretations regarded missing reservoir sections as a result of complex cut-out by subseismic faults that were not mappable and which therefore presented an irreducible subsurface risk. Nevertheless, following the depletion of early well stock, updip attic areas may provide the largest remaining incremental reserve targets and these have continued to be prioritized as infill well targets with mixed results.
An initial review of the distribution of failed Scott Field wells showed the reservoir risk to increase towards major faults in some cases while noting complex fault sets in multiple orientations.
Seismic interpretation uncertainty was seen as implicit from the acoustic response of the Upper Jurassic sedimentary units, since neither the Piper nor the Scott reservoir showed a strong seismic response, and a proxy near-top reservoir surface at the Base Kimmeridge Clay Formation is a complex unconformity with truncation below and onlap above.
This paper presents a stratigraphic and structural study of the Scott and Telford area, aiming to identify the controls on reservoir presence and preservation in order to de-risk future drilling while allowing enhanced reservoir modelling to assist efficient reserves recovery and the targeting of water-injection reservoir support.
While the structural history of the Central North Sea is commonly considered in the context of Middle Jurassic regional uplift (Underhill and Partington 1993), interpretations presented here emphasize variations in stratigraphic development as a result of Late Jurassic faulting and footwall uplift, with tectonics influencing the Oxfordian–Kimmeridgian deposition of Scott and Piper reservoir sands and their localized erosion associated with Late Jurassic extensional faulting and footwall crestal erosion.
Materials and methods
Interpretation of multiple regional 2D and field-scale 3D towed streamer and ocean-bottom seismic surveys has been integrated with stratigraphic correlation of historical exploration, appraisal and
© 2024 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/
licenses/by/4.0/). Published by The Geological Society of London for GSL and EAGE. Publishing disclaimer: https://www.lyellcollection.org/publishing-hub/
publishing-ethics
development wells in the Scott and Telford area. Previous interpretations are continuously updated as new seismic and well data become available. Core samples from initial exploration and development drilling formed the subject of several major sedimentological studies of the key reservoir units (Thickpenny and Russell 2000, 2010, 2012), with these results continuously updated and correlated with new data as additional wells are drilled.
These datasets have been used to build static and dynamic reservoir models in Petrel™for the Scott and Telford fields, which again are updated as new seismic and well data become available.
This paper is based on seismic interpretation and well correlation work carried out between 2015 and 2023, incorporating recent results from a 2021 Scott and Telford ocean bottom node (OBN) 3D seismic survey and the Scott J47 well that came on stream in early 2024.
The Scott and Telford fields
The Scott Field (Guscottet al.2003) lies in the Outer Moray Firth of the UK Central North Sea (Fig. 1) within a series of fault blocks forming a large high in the eastern Witch Ground Graben. The Telford Field (Syms et al.1999) lies 9 km to the south, hosted within a major east–west fault block south of the Telford Fault. A small separate closure in the west of this block forms the Marmion sector of the field.
Discovery
The first well on the Scott structure was 15/22-3, drilled in 1977 on a crestal location and encountering a major NE–SW fault beneath the Kimmeridge Clay Formation without finding any reservoir (Guscott et al.2003). The Scott Field was subsequently discovered by well 15/22-4 in 1983, which encountered some 200 ft (approx. 60 m) of Upper Jurassic sandstones in two packages: the Upper Oxfordian Scott Member of the Sgiath Formation and the Lower Kimmeridgian Piper Formation (referred to hereafter as Scott and Piper sands). These reservoir units were separated by a 50 ft (approx.
15 m) shale interval (Harkeret al.1993). The lower Scott sands
were oil bearing, while the Piper sands were water wet. Following the drilling of a disappointing downdip well penetrating only thin and poor-quality water-wet sands at 15/22-5 in 1985, the potential of the field for a major hydrocarbon accumulation did not become clear until the 15/21-15 well in 1987, which encountered almost 400 ft (approx. 120 m) of net pay. This was subsequently followed by a successful field appraisal, and discovery by well 15/22-9 in 1990 of the South Scott accumulation, which in turn was confirmed and appraised by well 15/21a-43 in 1991.
Although the Telford Field was first drilled in 1974 by well 15/
22-1, which encountered 31 ft (10 m) of good-quality gas-bearing sand, a second well drilled at 15/22-2 found over 200 ft (approx.
60 m) of water-wet sands, and the presence of a commercial discovery was accordingly delayed until drilling of the Marmion discovery well 15/21a-44 in 1991.
Development and production history
The Scott Field was developed via a series of subsea and platform wells, supported by water injection and producing via two linked platforms into the Forties Pipeline System and the Scottish Area Gas Evacuation (SAGE) gas export line. Following first oil in September 1993, production peaked at 220 000 bopd in October 1995, declining to 40 000 bopd by 2002 as the water cut rose. The 2023 production was approx. 6000 bopd (Fig. 2) at a water cut of more than 95% and supported by water injection at rates of up to 160 000 bwpd. Total production to July 2024 was 450 MMbbl of oil plus 348 Bcf of gas, with total water injection of 1.84 Bbbl.
The Telford Field was developed as a subsea tieback to the Scott Platform (Jewell and Ward 1997) via a series of injector–producer well pairs, coming on stream in October 1996. Production peaked at approx. 42 000 bopd in October 1997, declining to 10 000 bopd by 2005 as the water cut increased to 90%. The 2023 production was 1500 bopd (Fig. 2) at a water cut of approx. 95% and supported by water injection rates of up to 40 000 bwpd. Total production to July 2024 was 109 MMbbl of oil plus 286 Bcf of gas, with total water injection of 331 MMbbl.
Fig. 1.Location maps: (a) Central and Northern North Sea; and (b) Outer Moray Firth area showing the Scott and Telford fields. BCU, Base Cretaceous Unconformity; KPT, Kimmeridge–Piper Transition. Depths are in ft true vertical depth subsea (TVDss). Source: adapted fromGuscottet al. (2003)and Patrunoet al. (2022).
As production from the Scott and Telford fields has matured over time, the Scott Platform maximum fluid handling capacity has remained broadly constant, with peak total liquid rates in 2023 of approx. 220 000 blpd, which is close to initial peak production rates but with produced water progressively substituting for produced oil.
Structure
NE–SW and east–west faulting in the area was associated with the Jurassic opening of the Central North Sea and Witch Ground Graben, respectively (Guscottet al.2003). A structure map of the area is shown inFigure 3, whileFigure 4provides a representative seismic section.
Scott Field
The main closure of the Scott Field comprises a large composite fault block high at the intersection of the east–west-trending Witch Ground Graben with the NE–SW-trending Theta Graben (Boldy and Brealey 1990;Hibbert and Mackertich 1993). A variety of fault orientations define the structure of the component fault blocks as they wrap around the Scott High. Facies distributions and fault- block rotation geometries indicate that the Scott structure was subject to extensional fault movements through the Oxfordian– Berriasian interval, with several fault sets showing evidence of displacement in different senses during this period.
A number of fault block panels have been mapped (Fig. 3), with multiple development wells drilled in each:
• Block 1b: the updip, crestal sector of the field;
• Block 1: a contiguous fault block located to the south of Block 1b;
• Block 1w: a small, fault-bounded and pressure-isolated compartment to the west of Block 1;
• Block 1s: a hanging wall terrace in the south of the Scott Field and north of the Telford Fault;
• Block 2: a SW-dipping fault block to the west of Block 1b;
• Block 2a: a small fault block north of Block 2; and
• blocks 3, 3A and 4: three small fault blocks located to the north of Block 1b.
Telford Field
The field is located in the crestal area of a major east–west footwall block south of the Telford Fault (Fig. 3). NE–SW faulting is limited to the eastern and western sectors of the Telford Ridge, with segmentation in the west defining a separate closure forming the
Marmion sector of the field. Development wells have been drilled in all sectors of Telford (including in Marmion).
Key units and seismic markers
This subsection describes the main seismic units recognized and mapped in the area. Key reflectors and sequence stratigraphic correlations are listed inTable 1and represented diagrammatically inFigure 5.
A Carboniferous section was penetrated by several wells including 15/22-J17, although the unit is weakly reflective and
Fig. 2.Production history–Scott Field 1993–2023, plus incremental production from the Telford Field 1996–2023 (the data include Marmion). The timing of Scott infill wells drilled since 2005 is shown. kbopd, thousand barrels of oil per day. Source: constructed from the North Sea Transition Authority monthly field production database.
Fig. 3.The Scott and Telford fields: depth map on the Base Kimmeridge Clay Formation (KCF) (‘Top KPT’). Note the variations in the oil–water contact (OWC) in different fault blocks across the fields (seeGuscott et al.2003for a block by block listing). Depths are in ft true vertical depth subsea (TVDss).
not easily mappable on seismic data. By contrast, the Zechstein (Permian)–Carboniferous interface forms a high-amplitude doublet that is laterally persistent on structural highs and forms the acoustic basement across the Scott and Telford area. The acoustic impedance of the Top Zechstein anhydrite is lower where more deeply buried in the hanging walls of major faults. Major overburden thickness changes across faults may see this reflector confused with overlying high-amplitude events formed by the Jurassic Rattray volcanics or Skene Coal (see below).
A Triassic section of Smith Bank Formation shales occurs in the Witch Ground Graben (Harkeret al.1993) and was found in the Scott J17 well. This section is overlain by a series of igneous extrusives of the Middle Jurassic Rattray Member, defined by Richards et al. (1993) as part of the Middle Jurassic Pentland Formation. Variable Rattray thicknesses reflect the development of a complex Bajocian–Bathonian (and potentially basal Callovian) succession of prograding volcanics sourced from the Buchan– Glenn Fissure System, together with associated reworked volcani- clastics (Quirieet al.2019,2020). This package reaches a thickness of 1079 ft (329 m) in the Ivanhoe Field discovery well 15/21a-3,
while thinning to the NE and locally filling erosive relief (as in the north of Scott Block 4).
Figure 6 illustrates the Upper Jurassic–lowermost Cretaceous sequence stratigraphy of the Scott area (after Copestake and Partington 2023b,c, showing correlations with the lithostratigraphic scheme ofRattey and Hayward 1993).
Resting unconformably on the Rattray volcanics (Slateret al.
2024), the Sgiath Formation (Harkeret al.1993) commences with the Skene Member, comprising thinly bedded shales, sandstones and a coal marker that is locally traceable on seismic data. These non-marine facies in the transgressive systems tract (TST) of the lower J52 sequence pass up across a probable transgressive surface into dark grey offshore marine shales of the Saltire Member, which correlate with the upper part of J52 (Copestake and Partington 2023c). Although less evident in downfaulted areas to the west, a weak angular discordance is evident in crestal locations, whereas in central and eastern areas the lower J52 Skene Coal is absent with a northward-dipping/ -prograding Top Rattray reflector directly overlain by laterally continuous upper J52 Saltire Member shales, resulting in subtle variability in the Top Rattray seismic surface.
Fig. 4.Illustrative SW–NE seismic section from the Telford to the Scott areas. The vertical scale is two-way travel time (TWTT) in ms. BCU, Base Cretaceous Unconformity; KPT, Kimmeridge–Piper Transition.
Table 1.List of key seismic reflector picks (note: this is not a stratigraphic column) in the Scott and Telford area (standard negative polarity)
Period Stages Age Sequence stratigraphic
unit (bases)
Name Seismic pick (tops)
Neogene Danian–Maastrichtian Top Chalk Trough
Cretaceous Albian Base Chalk/Top Lower Cretaceous Peak
Aptian Top Valhall Formation Trough
Berriasian J73–J76 Base Cretaceous Unconformity Peak
Jurassic Tithonian Early (Hudlestoni) J65 Base Kimmeridge Clay Formation 2 (KCF2)/
(Top Claymore sands where present)
Peak Kimmeridgian Early (Eudoxus) J62 (‘J61’) Base Kimmeridge Clay Formation 1 (KCF1)/
Top Kimmeridge Piper Transition (KPT)/
Top Upper Piper Formation
Trough
Basal J56 Top Lower Piper Formation (Piper sands) Variable/none
Oxfordian Late J54 Top Scott Member (Scott sands) Variable/none
Middle–Late J52 (upper) Top Saltire Member Variable/peak/
trough to zero crossing
Middle J52 (lower) Top Skene Coal Peak
Bathonian–Bajocian? J32–J36 Top Rattray Member (Pentland Formation) Trough
Triassic Top Smith Bank Formation Variable/none
Permian Top Zechstein Trough
Sequence stratigraphy divisions adopted fromCopestake and Partington (2023a),Rattey and Hayward (1993)(see alsoPatrunoet al.2022) are used here in preference to alternatives proposed byRichardset al.(1993).
Harkeret al.(1993)correlated the Skene and Saltire members with the Alness Spiculite Member developed in the Inner Moray Firth Basin, an interpretation supported by Copestake and Partington (2023c).
The Saltire Member is in turn overlain by two shallow-marine reservoir packages of Late Jurassic age: the Scott and Piper sands (Fig. 7). The Scott Member of the Sgiath Formation (‘Scott sands’; basal Upper Oxfordian, J54) is present across the Scott Field and locally in west Telford. Lower and Upper Scott units are commonly separated by a regionally persistent intraformational mudstone termed here the‘Intra-Scott Shale’, alternatively referred to as the G Shale (Guscottet al. 2003). The Scott sands are subdivided into correlatable units designated as Lower Scott A, B and C, and Upper Scott A and B.
The Scott sands are overlain by the‘Mid Shale’, a regionally correlatable horizon corresponding to theRosenkrantzimaximum flooding surface (MFS) of latest Oxfordian age, J55 (Hesketh and Underhill 2002;Copestake and Partington 2023c) (Figs 6and7), which is in turn overlain by a further package of shallow-water sandstones assigned to the Piper Formation (‘Piper sands’; Basal Kimmeridgian, J56). The Piper sands are reduced or missing in some Scott Field wells and on the crest of the Telford Field (Syms et al.1999). Lower and Upper Piper Formation units are commonly differentiated in thicker sections where they are separated by the
‘Intra-Piper Shale’.
The upper, shalier part of the Piper Formation ofCopestake and Partington (2023c) comprises siltstones and shales previously described as the Kimmeridge–Piper Transition unit or ‘KPT’ (Guscottet al.2003) and displays marked thickness variations. The
‘Top KPT’ surface provides an acoustically hard, low-amplitude and laterally discontinuous seismic reflection at the base of the acoustically soft Kimmeridge Clay Formation (KCF; seeFig. 5).
Neither the Top Piper sands nor the Top Scott sands provide significant seismic reflections but the Top KPT presents a mappable near proxy horizon for a ‘Top Reservoir’ pick across the area, although it is critical to recognize that this reflector provides no evidence of the lithologies beneath it. Guscott et al. (2003) postulated correlation of the KPT with the‘Transgressive Unit’in the Rob Roy Field (Boldy and Brealey 1990) and the‘Hot Sand Unit’in the Tartan Field (Cowardet al.1991).
In places, the lowermost 200 ft (60 m) of the KCF shows moderately faster velocities, while locally generating a higher- amplitude seismic reflector that passes laterally into and merges with the Top KPT reflector, creating complex seismic tuning effects across the contact. This configuration can result in a mispick of the KPT.
The overlying KCF section is dominated by acoustically soft mudstones (Kadolskyet al. 1999,2005), and usually displays a slow seismic velocity and complex, low-amplitude internal reflections. Cemented silts and carbonate layers punctuate the succession, together with a unit of cemented turbidite sands associated with the Hudlestoni MFS. This sand unit creates a prominent mappable seismic reflector and a useful time marker in the Theta Graben area to the west of the Scott Field. The KCF can therefore be broadly divided above and below this level into two seismic units: a lower‘KCF1’package, which is present within, and thickens into, structural lows, and an overlying and passively onlapping‘KCF2’.
Although not seismically mappable across structural highs, continuation of the KCF1 unit in condensed sections updip and onlapping onto footwall relief is suggested by the presence of a distinctive gamma-ray spike observed on well logs within the lowermost horizons resting on the Top KPT unconformity (Coward et al.1991). Detailed log correlation allows mapping of this package
Fig. 5.Seismic markers from the Scott and Telford areas. (a) Logs and synthetic seismograms from the 15/21a-A1 and 15/
21a-35 wells showing the key horizons and seismic picks. Red ( peak) and blue (trough) picks in the reservoir interval are not fully resolved on the seismic in all parts of the area. Depths are in ft measured depth (MD) and ft true vertical depth subsea (TVDss). (b) Sketch geoseismic section showing typical relationships observed on seismic data.
Scott, Saltire and Skene are members of the Sgiath Formation (Harkeret al.1993;
seeFig. 6below). BCU, Base Cretaceous Unconformity; KCF, Kimmeridge Clay Formation; KPT, Kimmeridge–Piper Transition.
and reveals its absence towards the margins of Scott Blocks 1 and 2 facing the Theta Graben (seeFig. 3). A Top KCF1 (Hudlestoni) reflector is seismically mappable and has been dated as Early Tithonian byCopestake and Partington (2023c).
The overlying KCF2 package locally commences with deep- water turbidites of the Claymore sands (seePartingtonet al.1993).
These sands and the package overlying them appear to onlap footwall relief on the Top KPT surface. The base of this KCF2 package is assigned a later Early Tithonian age byCopestake and Partington (2023c). The Top KCF2 forms a regionally mappable seismic marker that is more commonly described as the Base Cretaceous Unconformity (BCU), characterized by truncation below it and onlap above. This event, dated as being of Late Berriasian age, is unconformably overlain by onlapping strata of the Lower Cretaceous Cromer Knoll Group (Copestake and Partington 2023c) in basinal areas and by the Chalk on structural highs.
Lateral variations in stratigraphy
While the Scott Field has major reserves in both the Scott and Piper sands, Telford reserves are mostly hosted in the Piper sands.
Broadly, Scott sands thin to the east and north, while Piper sands thin to the west and south. Even with good well control and modern 3D seismic data, mapping the stratigraphy in detail is challenging, noting as above that neither the Scott nor Piper sands present a clear acoustic reflection.
Marked differences in sequences drilled across Scott and Telford partly reflect two distinct unconformities:
• a Middle Oxfordian unconformity (Harker et al. 1993) evident from variable onlap of the Sgiath Formation onto Rattray volcanics (Fig. 7); and
• a Mid-Kimmeridgian unconformity that corresponds with the complex and composite Top KPT reflector seen on seismic data. This surface shows significant erosional truncation relief, with onlap of the KCF above (seeFig. 5).
At the crest of the Scott Field, the Base KCF–Top KPT surface forms a high-amplitude seismic event that is recognizable above an erosionally truncated stratigraphy, ranging from Piper, Scott, Saltire and Rattray to the Zechstein and Carboniferous beneath.
Sedimentation and tectonics
The Top Rattray Member forms an important unconformity in the Scott and Telford area, and shows a variable and commonly poor seismic response associated with the onlap of overlying shales onto a range of Rattray lithologies. Despite some evidence of erosional truncation at this surface, there is no evidence of volcaniclastic material in the Scott and Piper sands, consistent with a principal sediment source area for these deposits from the Fladen Ground Spur to the NE (seeFig. 1b) and with more minor input from the Halibut Horst to the west (Harker et al. 1993). Figure 8 is a schematic sketch showing the sedimentary architecture of the Scott and Piper sands across the Scott and Telford area.
Figure 9 presents facies maps for the Scott and Piper sands, compiled from detailed sedimentological core analysis (Thickpenny and Russell 2000,2012;Guscottet al.2003). Based on analogy with modern coastal clastic systems in Texas and North Carolina, USA (e.g. seeSusman and Heron 1979;Heronet al.1984), this earlier work interpreted the Scott sands as sand-rich barrier island and tidal facies in the NW, with protected back-barrier, lagoonal and washover facies towards the SE. Shallow-water sands are developed in the Lower Piper in the north of the field, while the Upper Piper comprises shallow-water sands passing progressively offshore towards the NW.
Piper sands are absent at the crest of the field and, whileFigure 9 is reconstructed to show the deposition of Scott sands across the entire field, their absence from the crest of Block 1b with shallow- Fig. 6.Upper Jurassic–basal Cretaceous (Oxfordian–Berriasian)
lithostratigraphy and chronostratigraphy of the Scott and Telford areas (afterCopestake and Partington 2023b,c). BCU,‘Base Cretaceous’ Unconformity; FS, Flooding Surface; IPS, Intra-Piper Shale; ISS, Intra- Scott Shale; KCF1 and KCF2, Lower and Upper Kimmeridge Clay Formation, respectively; KPT, Kimmeridge–Piper Transition; SC, Skene Coal; TS, Transgressive Surface. Extrusion of the Rattray volcanics is thought to be of Bathonian–Bajocian age, potentially extending into the basal Callovian. Source: modified afterCopestake and Partington (2023a);
see also discussions inQuirieet al. (2019,2020).
water facies surrounding it means that partial emergence of this area during Scott deposition cannot be entirely excluded.
The primary sediment supply in coastal and shallow-marine sedimentary environments is mainly from rivers, with clastic material redistributed effectively along coastlines over considerable distances by longshore drift. Modern barrier islands in the Saloum Delta of Senegal migrate at rates of up to 120 m a−1(Wentet al.
2013) and accordingly the preservation potential of barrier island structures may be limited. Reflecting the mobile dynamics of
shoreface environments, ancient barrier island facies commonly show a complex sedimentary architecture reflecting changes in sea level and climate, as well as subsequent pervasive reworking (Mulhern et al. 2021). Although fossil barrier islands may be difficult to identify, their existence is therefore interpreted from facies associations containing back-barrier fill, lower- and upper- shoreface, proximal upper-shoreface and tidal-channel deposits. The deposition of stacked and overstepping successions typically occurs in transgressive intervals during periods of relative sea-level rise.
Fig. 7.Sequence stratigraphy in the area of the Scott and Telford fields (modified from the operator’s technical
documentation). Note: this diagram has no specific orientation and serves only to illustrate stratigraphic onlap onto the Scott and Telford highs (footwall erosion effects are not included in this
illustration). KCF1 and KCF2, Lower and Upper Kimmeridge Clay Formation, respectively.
Fig. 8.Schematic diagram illustrating the sedimentary architecture of key reservoir units within the Scott and Telford fields.
Fig. 9.Distribution of the Scott Member and Piper Formation reservoir facies across the Scott Field over time. The outline of Scott field fault blocks and the Scott Platform location are shown for orientation reference. Source: modified and adapted afterGuscottet al. (2003).
Scott and Piper facies distributions inFigure 9indicate a greater marine influence in the NW, suggesting synsedimentary control through early NE–SW fault movements from Mid-Oxfordian times onwards. Subtle variations in accommodation space, with greater subsidence to the NW and relative uplift to the SE, are likely to have controlled the location of barrier island (Scott) and shoreface (Piper)
environments, as well as the locations of thin and condensed sections in crestal areas. A viable interpretation is therefore that the stratigraphic architecture (Fig. 8) and facies distributions (Fig. 9) of Scott and Piper sands may reflect sedimentation during early growth folding prior to the onset of surface faulting (cf.Gawthorpeet al.
1997).
Fig. 10.Map of the Scott Field on the Top KPT surface. Scott infill wells from 2016 to early 2024 are shown with estimated reserves and first production/
injection dates. (The location of Telford infill well F6 is shown inFig. 12.) boepd, barrels of oil equivalent per day; bopd, barrels of oil per day; bwpd, barrels of water per day; KPT, Kimmeridge–Piper Transition; MMbbl, million barrels; TVDss, true vertical depth subsea.
Table 2.Infill drilling campaign Q4 2015–Q1 2024
Well Field Area Result Reserves (estimated) (MMbbl) On stream Initial rate
J40 Scott Block 1w Producer 4.4 December 2015 25 000 bopd
F6 Telford West Telford Dry
J41z Scott Block 1w Injector 1.0 August 2017 10 000 bwpd
J42 Scott Block 2 Producer 3.0 October 2017 13 000 bopd
J43 Scott Block 3 Producer 2.9 March 2018 10 000 bopd
J44 Scott Block 1s Producer 2.5 June 2019 10 000 bopd
J45z Scott Block 1b Injector 2.3 November 2019 10 000 bwpd
J46 Scott Block 2 Producer 1.0 February 2022 1600 bopd
J47 Scott Block 2 Producer 1.6 January 2024 9250 bopd
Wells are listed in chronological order. The Telford F6 well reached TD in January 2016 while finding only a minimal reservoir section of Scott sands.
Fig. 11.(a) Depth seismic line through Scott infill well J40. Note the contrast between the successions in the Scott A3 and J40 wells. Note also the local dimming of the Top Zechstein reflector beneath the Rattray volcanics. Depths are in ft true vertical depth subsea (TVDss).
(b) Well prognosis and actual sequence drilled. Depths are in ft measured depth (MD). BCU, Base Cretaceous Unconformity; KCF, Kimmeridge Clay Formation; KPT, Kimmeridge–Piper Transition.
Infill drilling campaigns
As initial high production rates from the Scott and Telford fields declined in the late 1990s (seeFig. 2), infill drilling began to be considered. The Scott Platform has 28 well slots and the strategy adopted was to reutilize these by sidetracking high water cut platform wellbores to areas where dry oil remained, either in previously undrilled panels or in updip and previously undrained attic locations. The latter approach implicitly carries increased subsurface risk in targeting smaller volumes in more structurally complex areas.
2003–04
A new OBC seismic survey was acquired in 2001 and reprocessed in 2003 to integrate with an existing 1996 towed streamer dataset.
Following seismic reinterpretation, a drilling campaign in 2003–04 targeting reserves towards the margins of the field saw mixed success (Brook et al. 2010). Structural complexities were encountered in marginal areas of the seismic volume where migration was suboptimal.
2005–07
Further seismic interpretation in 2005 focused on improved fault mapping, ahead of an infill campaign that drilled new producer wells (J30, J32z and J34). These wells targeted unswept areas in Blocks 1 and 1b, away from the margins of the field. In addition, the J31 well was drilled as a downdip injector supporting J16 in Block 1w and the J33 well aimed to drain updip attic reserves in Block
2. The J35 well was another updip attic producer in Block 1b, encountering thinner than expected Scott sands overlain by Piper sands, with this configuration thought to reflect faulting through the wellbore at this location.
This selection of updip well targets proved effective in boosting the Scott Field production rates, which recovered from 15 000 bopd back towards 35 000 bopd. However, early water breakthrough followed in wells J30, J32z and J35, and the overall production uplift from the new wells was relatively brief and below forecasts (Brooket al.2010).
The J36 well was drilled in 2007 from the Scott Platform to the Marmion sector of the Telford Field. At the time, this was the longest well in terms of measured depth, and the well with the longest step-out on the asset. The initial borehole J36 and first sidetrack J36z both failed in the overburden, owing to the shallow drilling angle through the overburden that was required to achieve such a step-out. A second sidetrack J36y then encountered a dry attic Piper reservoir in the Marmion block in the Telford Field.
2009–10
The next Scott infill campaign comprised wells J37, J38 and J39.
Results were mixed and production uplift was modest. The 2009 Scott J37 well and sidetrack J37z targeted attic volumes in Block 4;
however, both wells failed to find any reservoir. A second sidetrack J37y was then drilled as a production accelerator from Piper sands in Block 1b. The well location was downdip of the existing production offtake and a swept reservoir was encountered with only 0.03 MMbbl of incremental production achieved.
Fig. 12.Telford F6 well: alternative seismic interpretations, as shown on the 2010 towed streamer: (a)–(c) TGS seismic data and (d) 2021 OBN survey. BCU, Base Cretaceous Unconformity; KPT, Kimmeridge–Piper Transition; MB, Marmion Block; TWTT, two-way travel time. Source: (a)–(c) reproduced with kind permission of TGS.
The Scott J38 well (also 2009) targeted attic volumes in Block 1 and failed to find any reservoir before being sidetracked downdip as J38z and successfully encountering hydrocarbon-bearing sands.
This well produced 1.95 MMbbl, yielding production acceleration but without adding material new reserves.
Finally, the Scott J39 well (also 2009) targeted by-passed hydrocarbons mapped against the main intrablock fault in Block 1b.
The well encountered a full reservoir sequence but water cut rose swiftly. The well produced 0.25 MMbbl before being sidetracked as the Scott J40 well in the 2015 drilling campaign.
2015–24
After acquisition of a new towed streamer seismic survey in 2010 and refurbishment of the Scott drilling rig in 2014, a further infill
well campaign began in 2015. Target selection aimed to strike a balance between low- and higher-risk targets, including twins of some wells that had been abandoned early when water cuts reached 50% and now presented low-risk targets, as well as structurally complex, and therefore higher-risk, attic locations.
Figure 10andTable 2show the location and result of wells drilled in the Scott Field during this time, with the Telford F6 well result also included (discussion of F6 follows below).
Scott J40 well
This first well in the new campaign targeted an area updip of the J16 producer in Block 1w. Pre-drill prognosis was for 198 ft (60 m) of Piper sands and 507 ft (154 m) of Scott sands. However, the well encountered only 373 ft (114 m) of predominantly Lower Scott Fig. 13.Top KPT depth map of the Scott and Telford area showing the distribution of footwall erosion. The interpreted timing of this erosion is presented inFigure 14.
Depths are in feet true vertical depth subsea (ft TVDss). GR, gamma ray; KPT, Kimmeridge–Piper Transition.
sands. Nevertheless, when brought online in December 2015, the well produced at high initial rates of up to 25 000 bopd before declining relatively rapidly due to a lack of reservoir support following a mechanical failure in the downdip injector well J31.
Telford F6 well
This well aimed to provide an infill producer in west Telford and reached total depth (TD) in early 2016, targeting attic reserves updip of the Telford F5 well, itself an updip sidetrack of the previous producer F3z. The F6 well was sited based on a combination of reprocessed 2001 OBC seismic data and a spectrally whitened volume from the 2010 towed streamer survey. The section drilled did not match pre- drill prognosis, instead comprising a thicker than expected KCF section and only 8 ft (2.4 m) of basal Lower Scott sands above Saltire Formation shales. The well was suspended to allow reuse of the top hole as a donor well for a future sidetrack if required.
Following the surprise results of these two wells, a choice of conservative locations for the next two infill targets provided time for failure analysis before further drilling.
Scott J41z well
J41 was conceived as a downdip injector in Block 1w. After a mechanical sidetrack, the J41z injector was successful in sustaining
production from the J40 well, which had produced 4 MMbbl of oil by July 2023.
Scott J42 well
J42 was designed as an updip twin of the successful Block 2 well J33, which had ceased production following a failed workover in late 2015.
The new well successfully delivered initial rates of 13 000 bopd.
Geological re-evaluation of reservoir absence
Scott J40 wellThe J40 well result revealed a fault-block structure in Block 1w that was more complex than anticipated before drilling. The absence of Upper Scott sands in the well was initially thought to show the borehole trajectory crossing briefly into a hanging-wall KCF section before drilling a thinner Lower Scott section in a second downthrown block. An alternative interpretation could see the deviated well trajectory as having grazed intra-KCF Claymore sands in the hanging wall before entering Scott sands reservoir in the footwall (Fig. 11).
Telford F6 well
A range of alternative structural scenarios was considered in order to explain the absence of almost the entire reservoir section in the Telford F6 well (Fig. 12).
Fig. 14.Top KPT depth maps of the Scott and Telford area showing structural evolution over time. Depths are in ft true vertical depth subsea (TVDss). The depth colour bars are the same as inFigure 13.
(a) Early movement of NE–SW faults was associated with facies and thickness changes during deposition of the Scott sands in the late Oxfordian, as well as of the Piper sands in the early
Kimmeridgian. (b) Major NE–SW faulting during the Early–Middle Kimmeridgian resulted in significant uplift and erosion of footwall crests. (c) A second phase of east–west faulting from the Middle Kimmeridgian to the Early Tithonian led to the displacement and fragmentation of earlier footwall crestal highs, with uplifted areas at key fault intersections subject to further crestal erosion, while some footwall areas were downfaulted and now lie in the hanging wall of these later faults.
Fault section cut-out
The interpretations of several previous wells, such as the Scott J35 well described above, invoked fault cut-outs of various portions of the Scott and Piper sands, and this was the first scenario considered.
In the case of the Telford F6 well, the Piper reservoir interpreted at the well location (Fig. 12a) proved to be missing and a complex two- fault configuration was initially proposed to explain this (Fig. 12b).
However, robust seismic interpretations could not be constructed on this basis and the hypothesis was rejected as unlikely.
Sedimentary slumping
Previous studies of the Northern North Sea fields Statfjord (Hesthammer and Fossen 1999), Gullfaks (Yieldinget al.1999), Snorre (Berger and Roberts 1999), Brent (McLeod and Underhill 1999) and Ninian (Underhillet al.1997) demonstrated sedimentary slumping from the footwall crests of major fault blocks. A scenario was constructed envisaging transport of the key reservoir target section from the footwall to the hanging wall (Fig. 12c). However, with the seismic data showing continuous reflectivity in the hanging-wall section, there was no clear evidence of a displaced block and this hypothesis was also seen as unlikely.
Seismic processing artefacts and related interpretation issues
Mapping of the F6 target area had made use of a 2012 reprocessing of a 2010 towed streamer survey across the west Telford area.
However, the original seismic data as acquired contained relatively high noise. The data had been reprocessed using a spectral whitening routine with the aim of improving reflection continuity, with apparently positive results obtained. It was on this dataset that the F6 well was sited.
A post-drill review of multiple seismic volumes allowed comparison of this reprocessed towed streamer seismic volume with a 2001 OBC seismic survey, which displayed a lower
frequency range and lower noise. Near the Telford Fault, the 2012 towed streamer reprocessing showed footwall reflectors extending for a significant distance further north than their equivalent events on the 2001 OBC data, as subsequently confirmed in a 2021 OBN re-shoot (Fig. 12d). The previous interpretation had therefore implied a larger gross rock volume (GRV), while misleadingly extending the reservoir target area further to the north.
Interpretation of the OBC and OBN data at the F6 location now suggested that the northward continuations of seismic reflectors on the reprocessed towed streamer data were seismic artefacts, with the well initially penetrating the hanging wall of the Telford Fault up to 200 m off structure to the north (Fig. 12d). This explained the presence of only a thin reservoir section, directly beneath a much thicker than expected KCF.
Findings
While earlier interpretations had suggested that failed wells had drilled into fault planes that were not identifiable on seismic data, the number of wells where fault cut-out was inferred to explain incomplete stratigraphy appeared unreasonably high. Accordingly, the following two alternative explanations were put forward.
Misplaced well target due to inaccurate seismic imaging
As outlined above, the Telford F6 well was reinterpreted as a structural near miss, drilling the hanging wall rather than the footwall of the Marmion to West Telford Fault. A comparison of imaging results from the towed streamer and OBC seismic surveys served to confirm that the well location was incorrectly placed.Reservoir truncation due to footwall erosion
Reinterpretation of the Scott J40 well and several earlier well failures suggested that erosion at fault-block crests was a key cause of reservoir truncation. An accurate description of the Base KCF unconformity was therefore critical to define the preservation of the
Fig. 15.Reservoir truncation: (a) Scott well correlation from Block 2 (left) to Block 1b (right), flattened on the Base Cretaceous Unconformity (BCU). Depths are in ft true vertical depth subsea (TVDss). (b) West–east seismic traverse from Block 2 (left) to Block 1b (right).
Pronounced relief on the footwall erosion surface is observed as it is onlapped primarily by the KCF2 unit above the Hudlestoni(Top Claymore sandstone equivalent) reflector, with further onlapping reflectors within the younger KCF2 succession above as shown. Note that as the wells are shown projected onto the seismic section in (b), the total depths are in some cases deeper than indicated on the seismic line.‘Proj.’, nearby well not intersected directly by the seismic line shown; BCU, Base Cretaceous Unconformity; KCF, Kimmeridge Clay Formation; KCF1 and KCF2, Lower and Upper Kimmeridge Clay Formation, respectively; Top KPT, Top Kimmeridge– Piper Transition; TWTT, two-way travel time. The fault offset of 1150 ft at the top reservoir is equivalent to approx. 350 m).
Scott and Piper sands at any location in the area. With neither the Scott nor Piper sands presenting clear seismic reflectors, structure mapping is commonly carried out on the Base KCF–Top KPT event, with a proxy‘Top Reservoir’horizon created by contouring down below it. However, the Base KCF–Top KPT surface forms a significant unconformity that carries a risk of reservoir erosion on structural highs at the frontal culmination of footwall blocks.
Accurate mapping of this unconformity and understanding of its structural and stratigraphic controls is therefore essential in order to reduce the risk of well failure due to reservoir truncation or removal.
Footwall erosion of the reservoir along fault block crests is described in detail in the following subsections.
Depth of erosion. Well correlations and seismic mapping demonstrate that a range of strata are truncated at the Base KCF– Top KPT surface across the area. On the leading edge of footwall crests east and west of the Theta Graben, the KCF may rest on Middle Jurassic Rattray volcanics (e.g. in Scott wells J13 and J12:
Figs 13and14) or on the Saltire Formation (e.g. in well 15/22-3 and Scott wells J38, J37z, J5 and J3). The Scott J37 well found the KCF resting directly on the Carboniferous. In each of these locations, the Scott, Piper and KPT sections are entirely absent, while downslope from footwall crests there is variable truncation with the Scott, Piper and then KPT sections progressively preserved.
It is likely that this erosional truncation was largely subaerial, with the fault-block crests forming‘footwall islands’during part of the Early–Mid-Kimmeridgian interval. A similar evolution of Late Jurassic rift-margin emergence was explored in the Northern North Sea byRobertset al.(2019).
Onlap onto the unconformity. While a relatively complete section of the KCF is present downdip, progressive onlap onto footwall crests sees only a thin and condensed basal KCF section draping the unconformity in updip settings. In more crestal footwall locations, a condensed KCF1 section may be present, although not resolved seismically, with seismic sections appearing to show the unconformity overlain by an onlapping KCF2 section.
Controls on reservoir preservation.A west–east transect and well correlation (Fig. 15) from Block 2 to Block 1b across en echelon fault segments in the Theta Graben illustrates the variation in stratigraphy between the hanging wall in the west and the eroded footwall in the east. The Base Saltire–Top KPT thickness reduces towards the footwall crest from 420 ft (128 m) in the Scott D4 well to zero in the Scott J3 well, where a condensed KCF interval with a prominent basal gamma-ray spike overlies a truncated section of Rattray volcanics. Meanwhile, the Top KPT is more than 2500 ft (760 m) deeper in the downdip hanging-wall well A1z than in Scott wells J11 and J17, which were drilled on the footwall.
Fig. 16.Reservoir truncation: (a) well correlation over the crestal Scott Block 1b and Block 1. Depths are in ft true vertical depth subsea (TVDss). (b) South–north seismic traverse over the crestal Scott Block 1b and Block 1. Note the unusual fining-upward log motif of the Scott sands in the J35 well in Block 1. At the time of deposition, J35 was located upslope of the other three wells, and this change in log character is consistent with the presence of fluvial channel facies in the Lower Scott C at J35 rather than shoreface deposits at locations further downslope. The wells are deviated while projected onto the line of seismic section, and the total depth of well A3 is deeper than appears in the seismic line (b). KCF, Kimmeridge Clay Formation; Top KPT, Top Kimmeridge– Piper Transition; TWTT, two-way travel time.
These changes reflect footwall uplift and erosion of the Piper, Scott and Saltire units at the crest of Block 1b in association with NE–SW fault-controlled subsidence of the hanging wall in Block 2.
Although present and forming a prominent reflector in the hanging wall, the Skene Coal is not recognized in the footwall, reflecting eastward onlap onto the Top Rattray surface (see Fig. 7). This configuration potentially provides the conditions for a seismic mispick between the Skene Coal in the west and the Top Zechstein/
Top Carboniferous in the east (Fig. 15b).
Figure 16 provides a south–north well correlation and seismic line from Block 1 into Block 1b, showing progressive truncation towards the north at the KPT unconformity. Block 1 appears to have developed within a fault relay zone where maximum displacement on the NE–SW fault bounding Block 1b was transferred westwards to a fault extending northeastwards from the northern limit of Block 1 (seeFig. 3).
Case study – Scott J43 well planning and results
Scott J43 wellFollowing a reassessment of subsurface risks, the Scott J43 well was conceived to target an infill location in Block 3 (Fig. 17). A structural model was developed to assess the timing of fault
movements and reservoir erosion (Fig. 18) in order to define a suitable target that would optimize updip volume while assuring drilling success.
Structural model
The Scott J43 well area is compartmentalized by multiple fault sets (seeFig. 17) trending NE–SW, parallel to the Theta Graben, east– west, parallel to the Witch Ground Graben, and NW–SE, stepping down from the Scott Field culmination in Block 1b to the south. The well was sited as far updip as possible in order to maximize reserves while simultaneously mitigating against reservoir erosion risk at the footwall crest.
Detailed block-specific structural, stratigraphic and reservoir models were constructed before drilling, paying particular attention to the correlation of key Upper Jurassic marker horizons within and above the reservoir package (Figs 19and20). Seismic picks were tied to offset Scott wells J9 (which encountered a reduced reservoir section of Lower Scott only) and J15 (which penetrated a full Piper and Scott succession; seeFig. 17for well locations).
Earlier interpretations had shown Upper Scott and Piper sands faulted out at the J9 well location (Fig. 20a). Following the drilling Fig. 17.(a) South–north seismic section through the Scott J43 well. (b) Block 3 structure map with a superimposed KPT to Top Rattray isochore. The black dashed line is the line of the section shown in (a); orange lines are the south–north and NW–SE lines of the stratigraphic and reservoir models inFigure 20a, b and c. Depths shown in (b) are to the KPT (Top Reservoir) in ft true vertical depth subsea (TVDss). The colour bar in (b) shows the KPT to Top Rattray thickness in ft TVDss. KCF, Kimmeridge Clay Formation; Top KPT, Top Kimmeridge–Piper Transition; TWTT, two-way travel time.
Fig. 18.Scott J43 well area (Block 3):
structural models of fault evolution in (top) the Late Jurassic and (bottom) Early Cretaceous. KCF1 and KCF2, Lower and Upper Kimmeridge Clay Formation, respectively.
of the Scott J40 and Telford F6 wells, this interpretation was revised, now attributing the reduced reservoir section in the Scott J9 well to footwall erosion (Fig. 20b).
An assessment of the footwall erosional geometry shown in Figure 20b provided critical de-risking inputs for the decision to drill at this location– not only in order to mitigate the risk of reservoir attenuation but also because, as shown in Figure 20c, the target volumetrics and reservoir offtake rate predictions were dependent on the detailed distribution of reservoir units which was incorporated into the reservoir model for the structure.
Scott J43 well result
On drilling, the Scott J43 well penetrated a near complete and mildly truncated Scott and Piper reservoir package, preserving all but the top 2 ft (0.6 m) of the KPT below the Base KCF unconformity. This successful result verified the geological model while vindicating the rigorous approach to well planning.
Following the mixed results of earlier infill drilling, the positive result of the Scott J43 well provided confidence that the reasons for previous well failures could now be better understood, therefore reducing subsurface risk in future wells.
Fig. 19.Detailed structural reconstruction model used for the Scott J43 well planning and footwall erosion risk mitigation.
Fig. 20.Stratigraphic models for Scott J43 well ( pre-drill) in Block 3. Note that panel (a) has a different orientation from panels (b) and (c) below. The lines of section are shown inFigure 17babove. (a) south– north line with 2015 interpretation ( prior to drilling of Scott J40 well and Telford F6 well) showing assumed cut-out of reservoir across an east–west fault intersecting the J9 well bore. (b) NW–SE line showing revised 2017 interpretation (after drilling of Scott J40 well and Telford F6 well) indicating footwall erosion progressively increasing to the NW and truncating the reservoir in J9 well bore. Line tied to proposed J43 well location and offset well J15. (c) Application of stratigraphic model to the pre-drill reservoir model. Note that the erosional geometry of the fault block forms an important control on the volume of the structure and the predicted distribution of higher quality reservoir units, with these in turn determining the target volumetrics and predicted offtake rates from the well location selected.
Depths are in feet (TVDss).
Subsequent infill drilling and future approach
The Scott Joint Venture planned and drilled a further four infill wells between 2018 and 2023, although progress was delayed by an extended rig recertification operation in 2019 and by the COVID-19 pandemic of 2020–21, as well as by power generation issues on the Scott Platform during 2022–23. A summary of results from this subsequent infill drilling programme is given below.
Scott J44 well
Drilled in 2019–20, this relatively aggressive step-out well targeted undrained Scott sands in Block 1s. The well followed the positive outcome of the Scott J43 well and was also successful, with reservoir thicknesses on prognosis.
Scott J45z well
This 2020 injector well was sited in a low-risk downdip location in central Block 2. While requiring a sidetrack following mud losses during drilling, the well was successful and encountered reservoir thicknesses on prognosis, providing additional water injection to Block 1.
Scott J46 well
This downdip Block 2 producer well successfully drilled a full Scott and Piper section and came online in 2021.
Scott J47 well
This well targeted attic reserves near the crest of Block 2.
Subsurface risk was seen as high in this updip location and a fallback downdip target was selected for a geological sidetrack in the event of failure.
The KPT and Upper Piper sands were absent but the well found Lower Piper sands as prognosed, above a thin Mid Shale and a condensed interval of Scott sands that displayed a high net/gross consistent with sedimentary onlap towards the east of Block 2. The Intra- Scott Shale appeared to be thin or absent at this updip location (Fig. 21).
The positive result of the Scott J47 well confirmed the eighth consecutive successful Scott infill well, raising confidence that subsurface risk could be effectively managed. The future strategy is again likely to use a combination of low risk near ‘twin’ well opportunities with moderately higher-risk attic locations mapped on the new seismic survey acquired in 2021 and step-outs towards the margins of the field development area (FDA).
Timing and style of faulting
The mapping of the Scott and Telford field area showed two key fault orientations (seeFig. 22).
NE
–SW
This fault trend delineates the western edges of Scott blocks 1 and 1b (Fig. 22), and defines Block 2 as an intermediate relay structure stepping down towards the Theta Graben (Boldy and Brealey 1990).
NE–SW faults show no expression at the Top KCF2–BCU, indicating that displacement had ended by the Berriasian. Facies and stratigraphic changes across the area (see above) suggest that this fault trend saw extensional synsedimentary movement from Mid-Oxfordian times onwards, consistent with general evidence from the seismic data that suggest a degree of Oxfordian growth folding on blind NE–SW faults (e.g. seeFig. 15).
During an initial phase of deposition, the Skene Coal was laid down in a defined area in the west of the Scott Field, consistent with the infill of subaerially exposed topography at the Top Rattray surface (Quirieet al.2019). Subsequently, facies trends in both the Scott and Piper sands record deeper-water conditions towards the NW; with shallower-water facies deposited towards the footwall crest of Block 1 (seeFig. 9).
Truncated reservoir sections at the western edges of blocks 1b, 1, 1w, 3 and 4 (Figs 11and15) are interpreted as recording erosion during subaerial exposure (see Fig. 19). Reservoir sections are missing in the NW of Block 1b, reflecting strong footwall erosion.
Piper and partial Upper Scott sections are eroded on crestal Block 1 in Scott wells A3, J38z and J4, and the absence of Piper and Upper Scott sections in the Scott J40 well suggests that blocks 1 and 1w were subaerially emergent as footwall islands and eroded to a similar level before Block 1w was downthrown to its current position.
The KCF shows a marked thickness increase across the NE–SW fault bounding Block 1 to the east and Block 2 to the west (Fig. 23).
TheHudlestoniMFS between KCF1 and KCF2 provides a useful seismic marker, showing that most synsedimentary growth occurred during the deposition of KCF1. By contrast, KCF2 shows only differential subsidence while passively onlapping footwall relief.
Sands within the KCF succession appear to onlap Lower Scott sand in the Scott J40 well in the footwall and are revealed on an amplitude map (Fig. 23a) as filling a re-entrant cut into the footwall crest of Block 1w, while forming fan-shaped anomalies in the hanging wall to the NW. It therefore appears likely that these intra- KCF Claymore sands are at least locally derived as second-cycle deposits from the erosion of Scott and Piper reservoir sands in the footwall followed by sedimentation in the hanging wall, a conclusion supported by onlap of the Hudlestoni reflector onto the KPT in several locations (e.g. seeFigs 9and15).
East
–west
A second set of faults parallel to the Witch Ground Graben includes those which separate blocks 1b, 1, 3 and 4. These faults are
Fig. 21.(a) Scott J47 well geological model, (b) encountered lithology and (c) location map. KPT, Kimmeridge–Piper Transition; N:G, net/gross.
downthrown to the north in each case, with the KCF2 unit onlapping footwall relief and thickening markedly downdip (Fig. 24).
Differential subsidence continued locally into the Early Cretaceous, with relief across the major fault separating Block 1b and Block 4 being onlapped by Valhall Formation deposits. This fault and the large-offset east–west fault defining the Telford Field structure show displacement at all levels from Top Carboniferous to the KCF, although offset is variable along discrete fault segments and linked splays that are discontinuously connected along strike.
Figure 24shows substantial erosion at the Top KPT horizon at the crest of Block 1b where this area forms the footwall to NE–SW faults at the edge of the Theta Graben (seeFig. 22a). This erosion began in the Mid-Kimmeridgian and may have continued locally
during the Tithonian on the northwestern crestal flank of Block 1b, which remained uplifted during later east–west faulting. Fault traces in this area show segments of differing orientations, suggesting formation via the linking of multiple fault splays.
The geometries shown in Figure 24also allow an approximate estimate of the amount of Late Jurassic footwall erosion. The two- way travel time (TWTT) offset between the reconstructed and eroded crests of Block 1b and Block 1 is approx. 128 ms, equivalent at a seismic velocity of 3000 m s−1to a vertical offset of 192 m (630 ft).
The presence of local highs at the BCU horizon (see the central part of seismic line B–B′ in Figs 22b; see also 23b) might initially be considered to provide evidence of structural
Fig. 23.(a) TopHudlestoniamplitude plotted on a time structure map of the Top Rattray volcanics, showing the morphology of Claymore sandstone submarine fans. Note the re-entrants (incised valleys) cut into the footwall crest inboard of the NE–SW fault on the western edge of Block 1w, with the Claymore fans tending to turn axially downslope (shown by orange arrows. (b) NW–SE seismic line from Theta Graben to Scott Block 1b showing the imaging of KCF1 in the hanging wall only (the section between the green and yellow picks). TheHudlestonipick is shown to continue to the NW in the hanging wall in (b); however, note that the Claymore sands at this horizon appear to pinch out downdip a short distance to the east of the Scott A1 well (seeFig. 15). BCU, Base Cretaceous Unconformity; KCF1 and KCF2, Lower and Upper Kimmeridge Clay Formation, respectively; KPT, Kimmeridge–Piper Transition; TWTT, two-way travel time.
Fig. 22.(a) Top KPT time structure map of the Scott Field showing the principal fault elements; the locations of example seismic lines B–B′and C–C′are shown. (b) Seismic line B–B′. (c) Seismic line C–C′. BCU, Base Cretaceous Unconformity; KPT, Kimmeridge–Piper Transition; TWTT, two-way travel time.
inversion. However, these features coincide with footwall crests below, which present a high at every stratigraphic level from the Zechstein through to the KPT. The presence of these highs at the BCU level is therefore interpreted as reflecting continued post-tectonic differential subsidence during the deposition of Lower Cretaceous shales.
Discussion
Footwall uplift and subaerial erosion
The concept that the footwalls of large extensional fault blocks could experience relative uplift and crestal erosion was first developed from models of basin formation (Jackson and McKenzie 1983; Barr 1987a; Yielding 1990; Roberts et al. 1993). A combination of extensional fault displacement and high regional heat flow during rifting (McKenzie 1978;Friedmann and Burbank 1995) can see the faulted margins of rift basins raised above the regional base level due to flexural uplift of the lithosphere (Fernández-Blancoet al.2019) and isostatic effects following ductile crustal thinning. During periods of sea-level fall, emergent footwall fault blocks may be subject to subaerial erosion, resulting in highly complex sedimentary and stratigraphic architecture around ‘footwall islands’ (Barr 1987b;
Robertset al.1993;Elliott et al.2011). Shallow marine erosional
processes may also result in the peneplanation of uplifted fault blocks (Martinezet al.in press).
Synsedimentary fault-block movement, footwall uplift and erosion may all exert a strong influence on sedimentation in coastal depositional systems, with ancient analogues including Middle Jurassic clastic deposits in the Hebrides Basin of NW Scotland (Archeret al.2019) and mixed carbonate–clastic sediments in the Neogene of the Gulf of Corinth (Gawthorpeet al.2018) and the Gulf of Suez (Azabi 2024), where outcrop studies illustrate the morphology and scale of footwall relief created (Sharpet al.2000).
Another modern outcrop analogue illustrating rotated fault-block geometries comes from the southern Black Mountains of Death Valley, USA (Fig. 25a). In this example, an extensional normal fault cuts a series of laterally continuous stratigraphic units. Extension is associated with rotation, relative uplift and crestal erosion of the footwall fault block. Although the section has been eroded to the right of the fault, the relative elevation of the footwall crest is higher and this area has seen erosion to a deeper stratigraphic level than on the downdip flanks, causing a characteristic erosional sculpting of the footwall crest similar to that recognized on many regional seismic lines from the Northern North Sea (e.g. seePlatt 1995).
This pattern of crestal sculpting results in an unconformity with significant erosional relief and across which the stratigraphic separation following later flooding is greatest immediately adjacent
Fig. 25.(a) Footwall erosion in the southern Black Mountains, Death Valley, California, USA, provides an analogue for construction of the structural model shown inFigure 20. The vertical relief shown is approx. 1600 m from the valley floor to the top of the ridge. (b) A comparison with Scott and Telford. KPT,
Kimmeridge–Piper Transition. Source: (a) image © Geotripper Images (http://
geotripperimages.com/), reproduced with the kind permission of Garry and Susan Heyes.
Fig. 24.South–north seismic line crossing east–west normal faults parallel to the Witch Ground Graben and separating blocks 1, 1b and 3. Blue and yellow shading shows the interpreted erosion of reservoir and Rattray volcanics on the footwall crest of Block 1b. BCU, Base Cretaceous Unconformity; KCF, Kimmeridge Clay Formation; KPT, Kimmeridge–Piper Transition; TWTT, two-way travel time.
