Character and distribution of hybrid sediment gravity flow deposits from the outer Forties Fan, Palaeocene Central North Sea, UKCS

Marine and Petroleum Geology 26 (2009) 1919–1939

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Character and distribution of hybrid sediment gravity flow deposits from the outer Forties Fan, Palaeocene Central North Sea, UKCS Christopher Davis a, *, Peter Haughton a, William McCaffrey b, Erik Scott c, Nicholas Hogg d, David Kitching e a

School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland Institute of Geological Sciences, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK Marathon Oil, 5555 San Felipe, Houston, TX 77056, USA d Shell UK Limited, Altens Farm Road, Nigg, Aberdeen, AB12 3FY, UK e BP Exploration – Aberdeen, Farburn Industrial Estate, Dyce, Aberdeen, AB21 7PB, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2008 Accepted 9 December 2008 Available online 1 April 2009

Seven categories of event bed (1–7) are recognised in cores from hydrocarbon fields in the outer part of the Palaeocene Forties Fan, a large mixed sand-mud, deep-water fan system in the UK and Norwegian Central North Sea. Bed Types 1, 6 and 7 resemble conventional high-density turbidite, debrite and low-density turbidite, respectively. However the cores are dominated by distinctive hybrid event beds (Types 2–5; 81% by thickness) that comprise an erosively-based graded and structureless and/or banded sandstone overlain by an argillaceous sandstone or sandy-mudstone unit containing mudstone-clasts and common carbonaceous fragments. Many of the hybrid beds are capped by a thin laminated sandstone–mudstone couplet (the deposit of a dilute wake behind the head of the turbidity current). Different types of hybrid event bed Types are defined on the basis of the ratio of sandier lower part to upper argillaceous part of the bed, and the internal structure, particularly the presence of banding. Although the argillaceous and clast-rich upper divisions could reflect post-depositional mixing, sand injection or substrate deformation, they can be shown to be dominantly primary depositional features and record both a temporal (and by implication) spatial change from turbidite to debrite deposition beneath rheologically complex hybrid flows. Where banding occurs between lower sandy and upper argillaceous divisions, the flow may have passed through a transitional flow regime. Significantly, the often soft-sediment sheared and partly sand-injected argillaceous divisions are present in cores both close to and remote from salt diapirs and hence are not a local product of remobilisation around salt-cored topography. Lateral correlations between wells establish that sandy hybrid beds (Types 2, 3S) pass down-dip and laterally into packages dominated by muddier hybrid beds (Types 3M, 4) over relatively short distances (several km). Type 5 beds have minimal or no lower sandier divisions, implying that the debritic component outran the sandier component of the flow. The Forties hybrid beds are thought to record flow transformations affecting fluidal flows following erosion and bulking with mudstone clasts and clays that suppressed near-bed turbulence and induced a change to plastic flow. Hybrid beds dominate the muddier parts of sandying-upward, muddying-upward and sandying to muddying-upward successions, interpreted to record splay growth and abandonment, overall fan progradation, and local non-uniformity effects that either delayed or promoted the onset of flow transformations. The dominance of hybrid event beds in the outer Forties Fan may reflect very rapid delivery of sand to the basin, an uneven substrate that promoted flow non-uniformity, tilting as a consequence of source area uplift and extensive inner-fan erosion to create deep fan valleys. This combination of factors could have promoted erosion and bulking, and hence transformations leading to the predominance of hybrid beds in the outer parts of the fan. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Forties Fan Turbidite Hybrid Flow Linked Debrite

1. Introduction * Corresponding author. E-mail address: [email protected] (C. Davis). 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.02.015

Sediment gravity flows play an important role in delivering sediment to deep-water basins (Normark, 1970; Normark and

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Piper, 1972; Mulder and Alexander, 2001). In the past the deposits of these flows have been attributed to two end-member transport mechanisms: debrites emplaced by cohesive debris flows characterised by high-sediment concentrations, generally significant clay contents and laminar to weakly turbulent flow; and turbidites left by variable but generally lower sediment concentration, partly or wholly turbulent, non-cohesive turbidity currents. Such a distinction is important from a hydrocarbon perspective because the clay-prone debrites are generally non-reservoir, whereas the often-sandy turbidites can form moderate to excellent reservoir rocks. Some have suggested that various intermediate (hyperconcentrated) flow types can also be recognised (e.g. Mulder and Alexander, 2001) or that sandstones attributed to higher concentration turbidity currents are in fact sandy debris flows (Shanmugam et al., 1995). Conventionally, debrites and the mass transport complexes they form are associated with deposition at the base of slope in proximal positions, with turbidity currents dominating transport and deposition deeper in the basin (e.g. Mutti and Ricci Lucchi, 1978; Walker, 1978). However, observations from some deep-water systems e.g., the syn-rift Upper Jurassic Penguin and Miller fans, North Sea, (Haughton et al., 2003); the Miocene Marnoso Arenacea Formation, northern Italian Apennines and Silurian Aberystwyth Grits, Cardigan Bay, Wales, (Talling et al., 2004) and the Palaeocene Forties Fan, Central North Sea, (this paper) establish that muddy debrites can be common in distal settings remote from

the slope occurring interleaved with turbidite sandstones. These debrites appear to have been emplaced as part of the same transport event that deposited the turbidite sandstones, suggesting deposition from either co-generated, but independent flows, or from flows that evolved (transformed) from less cohesive to an increasingly cohesive rheology as they ran out (e.g., Haughton et al., 2009). The latter rheologically complex flows are termed hybrid flows and their deposits hybrid event beds. The cohesive component of the bed is termed as ‘‘linked’’ debrite because it is interpreted to be genetically linked to the associated underlying sandstone (see Haughton et al., 2003). Understanding hybrid event beds is important because they involve interlayering of reservoir and nonreservoir lithologies at bed scale (generally dm to m), resulting in complex, highly layered hydrocarbon flow units with potentially very poor vertical communication. In this paper, we document the character and distribution of previously unrecognised hybrid event beds in the outer part of the Palaeocene Forties Fan. This relatively large (260 km long, 80 km wide), mixed mud-sand, ramp-fed fan system (sensu Reading and Richards, 1994) extends into the Central Graben of the UK North Sea (Fig. 1a). Almost 1060 m of released subsurface core and matching wireline logs from hydrocarbon fields located along the medial lateral and distal lateral eastern margin of the Forties Fan have been examined. The eastern fan margin was chosen as the focus of the study as the western part of the fan system is complicated by the

Fig. 1. Geological setting of the Forties Fan (A) Sketch map of the Forties Fan (taken from Hempton et al., 2005) showing the main Forties Fan and lateral fans sourced from the west. Some of key hydrocarbon fields in the fan system are shown, including Everest Lomond and Pierce on the eastern margin of the fan that are the subject of this study. (B) Cartoon cross-section of the Everest field (taken from Thompson and Butcher, 1991). Everest onlaps onto a structural high and there is no active halokinesis. (C) Plan view of the Everest field (modified from Thompson and Butcher, 1991) showing position of cored wells used in this study. (D) Cartoon cross-section of the Lomond Field. (E) Top FSM depth map (ft sub-sea) of the Pierce field contours are in 200 ft intervals (taken from Ahmadi et al., 2003), modified to show the main sediment fairway.

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presence of a number of point-sourced lateral subsidiary fans that interfinger with the main Forties Fan and obscure larger scale proximal-distal trends. Complex bed-scale juxtaposition of clean (reservoir) and remobilised muddy sandstone facies (non-reservoir) in the outer Forties Fan succession have been recognised in earlier studies, and attributed both to sand injection processes (Anderton, 1995) and to local secondary remobilisation, related spatially to coeval salt diapirism and sea floor deformation (Hempton et al., 2005). The outer Forties Fan system extended distally over an area of active salt diapirs, and many of the Forties hydrocarbon fields are in diapirrelated traps (Davidson et al., 2000). A key issue in terms of reservoir heterogeneity is the distinction between locally developed bed-scale remobilisation around the diapirs and hybrid flow development recording flow evolution at longer ‘fan’ length scales. This will dictate how heterogeneity is populated between wells away from the diapirs. As both processes may potentially occur, it is important to develop criteria to distinguish between them. The presence of the diapirs and local sea floor topography may also influence the primary distribution of hybrid beds and turbidites. The aims of the paper are thus to: (1) document the style and proportions of event bed types along the eastern edge of the Forties Fan; (2) compare the facies close to diapirs with those further away so as to assess the local impact of diapir-related sea floor destabilisation; and (3) to document and account for vertical and lateral interwell facies variations, particularly in the proportion of clean

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sandstone to argillaceous sandstone facies within hybrid flow deposits. 2. Geological setting The upper Palaeocene Forties Fan forms the lowermost of three sandstone members within the Sele Formation of the Moray Group in the northern and central North Sea (Knox and Cordey, 1992; Vining et al., 1993; Bowman, 1998). The regional stratigraphy has been established by a number of authors (e.g., Deegan and Scull, 1977; Isaksen and Tonstad, 1989; Mudge and Copestake, 1992a,b; Mudge and Jones, 2004). Biostratigraphic analysis of the Forties Fan itself is complicated by rapid fan aggradation and basin wide anoxia that limited the fauna and which persisted until the Early Eocene (Bowman, 1998). Most authors assign the Forties Sandstone Member (FSM) to the Upper Thanetian (58.8–56.8  0.2 Ma, e.g., Mudge and Copestake, 1992a), although it has more recently been interpreted as spanning the Thanetian to Lower Eocene Ypresian (56.2–55.2 Ma; Mudge and Jones, 2004; Fig. 2). Although structural highs along the eastern margin of the Forties Fan were established prior to fan deposition (Jones and Milton, 1994; Jones et al., 1999; Bowman, 1998; Kosˇa, 2007), preForties Palaeocene reactivation of Palaeozoic and Mesozoic faults resulted in significant differential subsidence on the flanks of the Jaeren High, Fisher Bank High, Forties-Montrose High and also the Marnock Terrace. The Lisa Formation below the Sele Formation was

Fig. 2. Stratigraphic context of the Forties Sandstone Member (FSM). (A) Geological time-scale for the Upper Palaeocene Lower Eocene, after Mudge and Jones (2004). (B) ‘T’ sequence Maximum Flooding surfaces, after Hempton et al. (2005) modified to produce regional correlation scheme between Pierce and Everest fields. (C) Proprietary field specific biostratigraphic scheme used to subdivide the FSM within the Everest field this has been modified to roughly show the regional seismic derived sequence stratigraphic scheme.

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characterised by syn-depositional faulting and associated mass wasting, which, combined with localised erosion and sediment deposition, complicated the reactivated structural topography (Kosˇa, 2007). The Forties Fan system then compensationally-filled this pre-existing mounded topography. Thermal doming associated with Palaeocene rifting of the Greenland and European plates uplifted the East Shetland platform more than 2 km and tilted it towards the south-east (Den Hartog Jager et al., 1993), causing an 800þ m drop in relative sea-level and a 90 þ km basinwards shift in the coastline (Jones and Milton, 1994). The resulting narrower, shallower shelf rapidly filled with sediment, which was remobilised down the Outer Moray Firth to build the deep-water Forties Fan system (Jennette et al., 2000). Secondary conduits also fed terrigenous sediment into the south and west of the fan (Fig. 1c). Within the Forties Fan, slumps and debris flows dominate basal and proximal areas (Den Hartog Jager et al., 1993). These are overlain by large channel complexes that dominate the central part of the fan. The latter are 50–100 m thick and 2.5–3 km wide, and are separated by mud-prone inter-channel areas approximately 500 m wide in the medial part of the fan (Den Hartog Jager et al., 1993). At the margins and in the more distal parts of the fan, deposition was characterised by the development of sheet-like sandstone bodies. At its eastern lateral edge the Forties Fan dramatically thins and onlaps onto the chain of intra-basinal highs (Thompson and Butcher, 1991; Galloway et al., 1993; Den Hartog Jager et al., 1993; O’Connor and Walker, 1993; Kosˇa, 2007). In the south, the fan advanced over a series of syn-depositionallyactive salt-withdrawal mini-basins and faulted salt-cored diapirs that locally controlled the direction and behaviour of sediment gravity flows on the outer fan (e.g. Hempton et al., 2005). 2.1. Studied cores The data discussed below are drawn from three producing hydrocarbon fields located along the eastern margin of the Forties Fan (Fig. 1); all the studied cores are released. Everest field (six studied cored wells, Fig. 1c) is located on the medial lateral margin of the fan, 225 km due east of Aberdeen and spans UKCS blocks 22/ 5a, 22/9, 22/10a and 22/14a. The Everest field exploits a bifurcated sandstone fairway on the flanks of the Jaeren High comprising two lobate features separated by an NW–SE trending palaeohigh (Fig. 1c). The north-easterly lobate body (Lobe 1 here, North Everest in Thompson and Butcher, 1991), is penetrated by wells 22/9-2, 22/ 9-3, 22/10a-4 and 22/10a-T6 and is located to the north-east of the high, terminating against the eastern margin of the Central Graben. The second body (Lobe 2 here, South Everest in Thompson and Butcher, 1991), sampled by wells 22/9-4 and 22/14a-2, is located to the south-west of the palaeohigh, and is interpreted as a zone of sediment bypass by Thompson and Butcher (1991). Sand deposition around Everest was strongly controlled by substrate topography related to the mounded underlying Lista Formation (Kosˇa, 2007). This led to rapid lateral facies changes and the development of elongate fan lobes (Thompson and Butcher, 1991); there was neither active halokinesis nor active tectonics at the time of Forties Fan deposition in this area. The Lomond Field is in block 23/21 on the flanks of a salt diapir that was active at the time of the Forties deposition. It is located 46 km down-dip and to the south-east of the Everest field (Fig. 1a and d), again close to the eastern lateral fan margin. Two wells were studied in the field: 23/21-T1 and 23/21-T5. Pierce field (6 studied wells, Fig. 1e) is located in the distal eastern part of the fan, approximately 67 km to the south-east of the Everest field; it spans blocks 23/22a and 23/27 and formed around twin (northern and southern) salt diapirs that again were active at the time of deposition (Fig. 1e). Unlike Everest, turbidity

currents and debris flows in Pierce encountered a syn-depositionally active basin floor that resulted in the formation of a number of withdrawal mini-basins around the diapirs in addition to the underlying mounded Lista topography. Net-to-gross variations imply flows were focussed through a main sediment fairway which ran to the west of, and between the salt diapirs, comprising a series of nested sand-filled shallow channels (Fig. 1e). Three of the studied wells are located in this main sediment fairway (23/22a-2, 23/27-8 and 23/27-9). The other three are located in lower net-to-gross settings outside of the fairway; well 23/22a-3 is located to the east of the northern diapir, 23/27-6 in a lobate flow expansion zone, and 23/27-5 to the south-east on a high associated with a fault shoulder marginal to the flow expansion zone (Fig. 1e). 2.2. Intra and inter-field correlations Correlation within the Forties Sandstone Member is not straightforward due to the aforementioned anoxia. Most biostratigraphic samples comprise shelf and freshwater biota transported into the basin. The biostratigraphic picks used to compare wells and to establish lateral facies transitions are therefore field specific and difficult to apply regionally (Fig. 2). Accordingly, longer length-scale correlation is based on maximum flooding surface picks derived from regional seismic surveys which allow a crude correlation of the studied Everest (Fig. 3) and Pierce (Fig. 4) fields (no data were available for Lomond Field). 3. Facies Cores from the three studied fields were described using a facies scheme that recognises 10 main facies types (to facilitate Markov Chain Analysis) grouped into three lithological categories: (1) relatively clean sandstones (forming potential reservoir); (2) argillaceous sandstones (poor/non-reservoir); and (3) mudstone (non-reservoir) and are summarised in Table 1. Facies can be identified to category level using wireline logs in uncored sections. Core-based characterization of facies sub-divisions was supported by petrographical analysis and point-counting on a suite of 19 core-chip samples from well 22/09-4. 3.1. Relatively clean sandstone facies These are pale yellow to beige coloured sandstones (except when oil stained) with a framework mineralogy consisting of quartz (>90%), lithic fragments (>5%) and feldspar (<5%). The grain-size can vary from very-fine lower sandstone (>62 mm) to coarse sandstone (<1000 mm) with modal grain-sizes in the fine-lower (>125 mm) to fine-upper (<250 m) range. When the grain-size is finer than 500 mm, the grains tend to be subrounded and moderately to well sorted. Where coarser, the grains are more angular and the assemblage moderately to poorly sorted. Clay contents are relatively low (<25%, generally 19% of matrix is composed of dispersed clay or detrital mud aggregates). Five cleaner sandstone facies are identified (Table 1a). Facies Sl is massive, whereas facies S2 is characterised by dewatering fabrics consisting predominantly of shallow, sub-horizontal dish structures and sub-vertical pipes (0.5–20 cm long). Where facies S2 occurs towards the base of a sandstone bed, it may also exhibit soft sediment folding and convolute lamination. Facies S3 comprises clean sandstone with floating clasts of either mudstone or argillaceous sandstone. The clasts are generally rounded to subrounded, ranging in size from c.10 mm in size to those that are interpreted to span the 11 cm width of the core. Facies S4 comprises sandstones with a banded fabric (sensu Lowe and Guy, 2000) composed of an alternating series of paired subtly darker (1–2 mm thick) and

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Fig. 3. Well correlation, facies and bed type breakdown for the Everest field utilising the biostratigraphic framework (Zones A–D) shown in Fig. 2. The small pie charts summarize the relative abundance based on thickness of complete event beds recognised within core from each zone in the different wells. For descriptions of Bed Types 1–7, see text. The larger pie charts report the facies breakdown (both complete and incomplete beds) at facies association level and the proportion of cored section in each zone. In addition, the proportion of complete event beds (EB) recorded in the small pie chart is shown by the green dashed segment. NC is the proportion of the zone where no core was recovered. Facies codes are M (mudstone), Sst (isolated clean sandstone unit), ARG (argillaceous sandstone), AS (cleaner sandstone unit truncated by a clean sandstone) and AA (argillaceous sandstone unit truncated by an overlying argillaceous sandstone unit) that do not correspond to a complete event bed. Extracts of logged sections for wells 9-3, 10a-4 and 10a-T6 can be found in Fig. 11 and for wells 9-4 and 14a-2 in Fig. 12.

lighter (3–4 mm thick) bands (Fig. 5b). The banding observed within the Forties Fan is similar to the meso- or micro-banding described by Lowe and Guy (2000), Blackbourn and Thomson (2000) and Lowe et al. (2003) from the Britannia Field of the Central North Sea, although its expression in the Forties core is typically more subtle than in Britannia. The light bands characteristically have planar tops and irregular or loaded bases. Darker bands contain millimetre-scale mudstone-flecks and coarser floating lithic grains. Within a banded interval the thickness of both darker and lighter bands generally decreases upwards (Fig. 5a and b). Facies S4 often occurs at the transition between the clean sandstone facies (S1–3) and overlying argillaceous sandstones (facies A1–4, see below). Facies S5 consists of rippled and parallel laminations, locally with convolutions, and either present at the top of hybrid event beds or as isolated 1–10 cm thick beds. 3.2. Argillaceous sandstones facies These sandstones are similar in framework mineralogy to the cleaner sandstones but have a significantly higher clay matrix content (>30%) that makes them dark grey to brown in colour. Apart from occasional outsized grains (medium lower to coarse

upper sand) that can give the lithology a so-called ‘‘starry night’’ appearance, the upper grain-size is limited to less than medium sand (<500 mm). The grains tend to be moderately to poorly-sorted and subangular. Four argillaceous sandstone facies are recognised (Table 1b). Facies A1 is similar to the massive clean sandstone described above (S1), albeit darker grey in colour, moderately to poorly-sorted and of grain sizes of very-fine-upper to fine medium grained sand (100–180 mm). The facies can also contain partially-abraded clasts of one of four types: (1) rounded to subrounded, grey, laminated mudstone resembling typical Sele mudrocks; (2) subrounded/subangular to angular light to dark-green coloured mudstone fragments resembling the underlying Lista Formation; (3) carbonaceous fragments, and (4) rounded, internally deformed clasts of argillaceous sandstone. These clasts range in size from 1–2 mm to >11 cm (i.e., core spanning). In intervals thicker than 10 cm, the clast-size within facies A1 decreases with height. When facies A1 occurs at top of an event bed otherwise dominated by relatively clean sandstone, carbonaceous fragments 1–4 mm long by 1–2 mm wide may predominate. Facies A2 comprises mudprone sandstones hosting sheared sand patches and sand injection features comprising cleaner sandstone. The injection structures can

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Fig. 4. Subdivision of event beds within the Pierce field using the seismic derived sequence stratigraphic framework. MFS ¼ maximum flooding surface, TFSM ¼ Top Forties Sandstone Member. For an explanation of the pie charts see caption for Fig. 3. Detailed logged sections can be found in Fig. 13.

form discrete pipes, rounded pseudonodules or irregular patches of cleaner sandstone surrounded by the mud-prone host (Fig. 6). In some cores the 3D nature of the clean sand patches can be seen, showing they are pipes or dykes that can be traced to an underlying cleaner sandstone bed with which the facies is in contact. In other cases, sand patches have formed from starved ripple sets that foundered into their substrate. The injected sand pipes and dykes are most common at the base of intervals of facies A2, with the irregular sheared/deformed patches occurring centrally and rounded pseudonodules at the top. Facies A3 is characterised by a banded texture similar in appearance to S4 but with the banding being darker overall (Fig. 5). When banding occurs towards the top of an event bed, it may grade upwards into a crude stratification in which darker bands are composed of concentrated layers of millimetre scale (2–3 mm) carbonaceous fragments (Facies A4; Fig. 5c). 3.3. Mudstone facies Just a single mudstone facies is distinguished (M1) as the aim was to focus on the structure and facies transitions within the sandy event beds. This also reduced the total number of facies recognised, simplifying Markov Chain Analysis. The mudstones are grey in colour and can be either massive or laminated and has a locally variably silt content with floating grains of very-fine lower sand. The massive and laminated mudstone facies are interpreted

as background hemipelagic and small volume, very low concentration turbidity currents, respectively. The silty mudstones with coarser outsized grains are interpreted as the deposits of distal cohesive sediment gravity flows. 4. Event bed organisation An event bed is here taken to be the deposit of a single gravity-driven emplacement event. If not amalgamated, it will be bounded base and top by an M1 mudstone representing background hemipelagic sedimentation. A total of 1133 complete beds were identified (Fig. 7). Complete event beds in the studied fields range between 0.01 m and 4.08 m in thickness, with a mean thickness of 0.39 m. Recognising event bed boundaries can be difficult where there is no intervening mudstone between flow events. In this case step changes in grain size or distinct facies dislocations (e.g. rippled fine-grained sandstones or fine-grained argillaceous sandstone overlain by erosive-based, coarse-grained clean sandstone) are taken to indicate bed boundaries. Uncertainty in bed definition is also introduced by variable core integrity due to removal of preserved samples and un- or poorlyrecovered core intervals. Event beds have been grouped into seven commonly occurring types (described in detail below) using: (1) the proportion of cleaner to argillaceous facies making up the bed and (2) internal

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Table 1 Summary descriptions of the facies scheme. Scale bars in all photographs are in centimetres.

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Fig. 5. Examples of the banded textures seen in the Everest and Pierce cores. (A) Subtle banding developed in cleaner sandstone facies in well 23/27-6 (9769.50 metric). (B) Example of banding spanning the transition between a clean sandstone unit and an overlying argillaceous sandstone unit (dashed line) found in well 22/09-2 (8563.10 metric). (C) Example of the pseudobanding seen at the top of some argillaceous sandstone beds in well 23/27-9 (81370 metric). This banding is a ‘‘dark’’ band with a diffuse base of millimetre scale flecks of carbonaceous fragments that become more concentrated towards the top of the band. In the upper part of the bed the ‘‘dark’’ bands are replaced by thinner mm thick horizontal lamination of carbonaceous fragments.

sedimentary structures (Fig. 7a). The bed classification draws on the combined inventory of completely cored beds in all three studied fields. Although Markov chain analysis was carried out using a facies inventory for complete event beds, this showed only weak linkages between facies due to the wide range of possible vertical facies transitions (Fig. 8) compared, for example with the simpler transitions in Jurassic beds reported in Haughton et al. (2003).

4.1. Type 1 beds: massive cleaner sandstone-dominated event beds Type 1 event beds are dominated by facies S1 (43% of all Type 1 event beds) and S2 (39%) and they range from 0.02 m to 1.78 m in thickness, with an average of 0.41 m (Fig. 9a). Either a finergrained rippled or laminated S5 unit (5%) or an increase in scattered carbonaceous fragments (Facies A4, 2%) marks the event bed top. Type 1 beds and their associated facies (S1–S3) are interpreted as the deposits of conventional sandy high-density turbidity currents (HDTC), sensu Lowe (1982) on account of the dominance of massive/dewatered facies, evidence for well developed normal grading, local evidence for basal erosion and the lack of internal structures suggesting they are dominated by deposition under high sediment fallout rates (Lowe, 1982). The Type 1 beds are typically capped with parallel and ripple-laminated sandstones which are interpreted as the deposits from the dilute tails of the concentrated flows.

4.2. Type 2 beds: sandstone-dominated hybrid event beds Type 2 event beds range from 0.02 m to 3.60 m in thickness, with an average thickness of 0.69 m. They are dominated by facies S1 and S2 (41% and 30%, respectively, see Fig. 9b). In contrast to Type 1 beds, a subordinate argillaceous sandstone division (typically either the A1 or A2 facies) succeeds the clean sandstone division. The boundary between the two divisions can range from 1) a sharp contact between clean and argillaceous sandstone that can be planar or irregular in shape and occasionally associated with injection, loading, foundering and other soft sediment deformation along the contact (Fig. 9b) and 2) a more gradual upward transition distinguished by a progressive colour change resulting from an increase in matrix mud content. The top of the argillaceous division can either rapidly fine into the background M1 facies or is capped by a thin ripple-laminated S5 or A4 unit. The basal clean sandstone is interpreted as the deposit of a high concentration turbidity current as evidenced by an erosive base, grading reflecting turbulent transport, and the lack of sedimentary structures and presence of dewatering indicating rapid suspension fallout. In contrast, the upper argillaceous sandstone division is interpreted as the deposit of matrix-supported, laminar to weakly turbulent debris flow that formed as either the rearward part of the flow underwent turbulence suppression and en masse freezing due to erosional bulking of the turbulent flow by mudstone clasts, or as a debris flow sourced up-dip overran the deposit of a just-deposited

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Fig. 6. Examples of sand injections cross-cutting argillaceous sandstone facies. (A) Example from well 23/22a-3 (2801 m) of injection pipes (arrows) typically found at the base of the argillaceous sandstone unit. (B) Example from well 22/09-2 (8577.70 metric) of the irregular shaped patches of cleaner sandstone seen in argillaceous sandstone units (arrows). (C) Example of pseudonodules (arrows) found in well 22/10a-T6 (104780 ). (D) An example from well 23/22a-3 (2829 m) showing that some pseudonodules can be formed when material is sheared/loaded from a clean sand deposited on top of the debrite unit.

vanguard turbidity current. The welded sharp to gradual internal contact between cleaner and argillaceous sandstone with evidence for occasional loading/foundering suggests deposition as a single ‘hybrid’ flow event. Where the upward transition is gradational, this is consistent with a progressive change in rheology driven by clay incorporated into the flow. In the scenario where an up-dip sourced debris flow overtakes and mixes completely with the rearward part of a vanguard turbidity, a gradational longitudinal change within the composite flow- and hence vertically within the deposit – might also be created. Clay particles are inferred to have segregated to the rear of the flows or to have been released due to disintegration of near-bed mudstone clasts (see Haughton et al., 2003) in sufficient concentration that inter-particle electrostatic attractions dampened turbulence (e.g., Baas and Best, 2002), forcing a change to cohesive debris flow behaviour. The lack of mudstone clasts within the clean sandstone facies component of Type 2 hybrid event beds could be due to: 1) the site of erosion relative to the head of the current – if erosion occurs behind the head, deposits from the frontal part of the flow will be mudstoneclast poor – or 2) regardless of where clasts originally entered the current, fractionation of clasts to the rear of the current will deplete the frontal part of the flow in mudstone clasts. In some Type 2 event beds mudstone clasts occur towards the top of the clean sand interval, reflecting the arrival of parts of the flow where mudstone clasts may not have been sufficiently abundant to modify the flow behaviour. The loaded internal contacts suggest that the linked debrite was emplaced whilst the underlying cleaner sandstone was still water-saturated. However, such deformed contacts are not as

abundant in Forties examples as in the Jurassic hybrid flow deposits described by Haughton et al. (2003). A possible explanation is that the emplacement of the debritic component generally occurred after the underlying interval had dewatered, the longer lag time being consistent with the idea that long runout flows become increasingly stretched out over time, and thus take longer to pass in distal locations compared to proximal locations (McCaffery et al., 2003; Kneller and McCaffrey, 2003). 4.3. Type 3 beds: hybrid event beds with banding Banded fabrics (Facies S4 and A3) can occur anywhere vertically within the studied Forties event beds. They are most common, however, at either event bed bases or at the transition between clean and argillaceous sandstone facies. Type 3 event beds are most commonly characterised by banded fabrics occurring at the transition between the cleaner and argillaceous sandstone divisions (Figs. 5b and 9c) This is the case for both beds that are cleaner sandstone-dominated (>50%, 3S) or argillaceous sandstone-dominated (>51%, 3M – see Fig. 9c). The average thickness of Type 3S beds is 0.72 m and they are dominated by S1 (27%) and S2 (33%) facies. Type 3M event beds are on average 0.59 m thick and are dominated by facies A2 (49%). As Type 3 event beds include both cleaner and clay-prone facies, they are interpreted as a hybrid flow deposit variant. The lower cleaner sandstone is interpreted to form beneath a more turbulent frontal part of the flow as before. Progressive turbulence damping is interpreted to occur towards the rear. The occurrence of banded

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Fig. 7. Summary of distribution of event beds types found in Everest, Lomond and Pierce fields, taken from Haughton et al. (2009) (A) The number of complete event beds in Everest, Lomond and Pierce in terms of the frequency of occurrence and total thickness. (B) The proportion of clay-prone facies vs. event bed thickness shows that there is no distinct thickness variation between clean and argillaceous sandstone-dominated hybrid event bed. Average event bed thickness and range of event bed thickness are also plotted.

fabrics suggest the development of transitional flow conditions before the arrival of the cohesive linked debris flow (Lowe and Guy, 2000; Blackbourn and Thomson, 2000; Lowe et al., 2003; Barker et al., 2008). Thus the incorporation, abrasion and break-up of mudstone clasts in the near-bed part of the flow are interpreted to result in an increase in the concentration of clay particles and the consequent development of a clay-prone traction carpet, which collapses and freezes to form a muddy sand (dark band) deposit. Upon freezing and removing clay particles from the near-bed part of the flow, this near-bed zone reverts back to more turbulent conditions (depositing the light band from turbulent suspension) until the near-bed clay content builds up once more, initiating another cycle of dark and light band deposition occurs. This interpretation is consistent with the textural analysis of the banded fabrics, which shows that a higher proportion of mud in the darker band compared to the lighter band (Fig. 5a, Lowe and Guy, 2000; Blackbourn and Thomson, 2000; Lowe et al., 2003; Barker et al., 2008). As previously stated banding is not restricted to the transition between clean and argillaceous sandstone divisions. Where banding occurs at the base of an event bed, erosion and uptake of mud or fine-grained sediment may have temporarily suppressed fluid turbulence.

4.4. Type 4 beds: clay-prone hybrid event beds Type 4 event beds range between 0.08 m and 2.55 m in thickness, averaging 0.57 m. These event beds are dominated by argillaceous sandstone and sandy-mudstone facies (predominantly A1 and A2 accounting for 40% and 30% of the total bed profile, respectively) overlying a subordinate interval of cleaner sandstone (typically S1, 18%). Like Type 2 beds, the contact between clean and argillaceous divisions are sharp and can vary from planar to irregular, with injection, loading and foundering (the majority of contacts, estimated at 75%, are irregular). The transition between cleaner sandstone and the overlying argillaceous division can have injected dykes of clean sand and/or clasts. In the thicker Type 4 beds where a significant proportion of the event bed is composed of argillaceous sandstone, the upper debrite shows a crude vertical organisation of components (see Type 5 beds below). Carbonaceous fragments are particularly common in the upper part of the argillaceous debrite division (Fig. 10a). The top of the event bed is defined by either the upper argillaceous facies grading into background mud directly or via a rippled/laminated cleaner sandstone or a thin (millimetre scale) interval of fractionated carbonaceous grains that can be stratified (Fig. 5c).

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Fig. 8. Results of vertical transition analysis for two wells located within Lobe 2 of the Everest field, Forties Fan, central North Sea. (A) Upward transition frequency and probability (in brackets) matrix. (B) Transition tree constructed using Markov chain analysis and identifying those transitions that occur more commonly than random (bold lines), the numbers on the arrows are the ‘observed minus random’ transition probabilities. Although transitions between some facies are more common than random, these transitions are circular in nature and movement up through the transition tree is limited.

Type 4 event beds are a mud-rich variant of Type 2 hybrid beds and are interpreted to have been deposited in a similar manner, albeit with a thinner lower cleaner sandstone component. This could be due to the depositing flow having a lower proportion of cleaner sand and a shorter turbulent front, or that the depositional location is relatively distal, with the flow having already deposited a significant proportion of its sand up-dip. The implication is that the debritic component of the event bed thickens at the expense of the lower cleaner sandstone component in a distal direction. Similar down-dip facies transitions are seen in outcrops where individual beds can be traced down-flow (Amy and Talling, 2006).

4.5. Type 5 beds: fractionated hybrid debrites Type 5 event beds consist mostly of argillaceous sandstone facies (>90%), but sometimes they have a very thin (a few cm at most) basal massive or dewatered, cleaner sandstone (although not as clean as in Type 4 beds) (Fig. 10b). Type 5 beds range from 0.01 m to 0.99 m in thickness, and average 0.25 m (i.e. they are thinner than the other hybrid flow types; Fig. 7). They are dominated by facies A1 (58%) and A2 (32%). These event beds are not classed as true (i.e. stand-alone) debris flows (i.e. Type 6 event beds; see below) because they have a vertical and internal organisation of clasts and internal injection, dewatering and shear features that are

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Fig. 9. Core examples of clean sandstone-dominated event beds. (A) Type 1 event beds massive clean sandstone: Well 22/09-2 (8584-85920 ) close up of massive clean sandstone (A1) and rippled/laminated finer-grained cap (A2 and 3). (B) Type 2 event beds clean sandstone dominated (>50%) hybrid event bed: Well 23/22a-3 (2800.2–2802.8 m), the massive clean sandstone overlain by an argillaceous cap (B1). Close up of irregular contact between clean sand and debrite with foundered clasts (B2). (C) Type 3 event beds banded transition event beds: Well 22/09-2 (8563-85690 ) banding at the transition between clean and argillaceous sandstone lithologies (C1), Argillaceous sandstone unit is capped by a ripple-laminated unit (C2). More obvious banded transition (C3). Note depths are in metres and metric feet.

identical to the argillaceous component of Type 2–4 hybrid event described above. They are interpreted as a hybrid bed variant in which the clean sand that normally forms the basal part of the bed has pinched out up-dip. The implication is that the sand-bearing part of the flow is less efficient than the argillaceous linked debris flow. The colour of the debrite event bed changes from light grey at the base to dark grey at the top representing a change in matrix texture to increasingly mud-prone. Injection pipes and flames, composed of clean sandstone, can be present at the base of the bed, but these become more irregular in shape and resemble pseudonodules towards the top of the bed. Any rip-up clasts of mudstone, sandstone and/or reworked debrite are often rounded and are largest at the base of the bed, decreasing in size vertically, with common 1–2 mm scale, angular, carbonaceous fragments concentrated at the bed top. The vertical organisation within what is interpreted as a debrite is interesting and serves to distinguish these divisions from disorganised ‘stand-alone’ debrites (Bed Type 6). Segregation of the matrix and clast components suggests these may have been processed in a turbulent current before becoming cohesive and behaving as a debris flow. The formation of linked debrites by internal fractionation within a flow (rather than runout of the original source failure following a turbidity current) was probably a progressive rather than instantaneous process. The debrites are inferred to have

bulked up through progressive accretion onto their upper surface from increasingly rearward (and hence progressively more fractionated) portions of the associated turbulent or transitionallyturbulent flow. This resulted in a vertically fractionated plastic flow which ultimately froze to preserve a cryptic record of the longitudinal structure of the original hybrid flow (Fig. 11). In the case of Type 5 beds, the linked debris flow is interpreted to have eventually outrun the sandier deposit from the head of the hybrid flow; these beds are therefore interpreted as the downstream and/or lateral fringe of the hybrid deposit.

4.6. Type 6 bed: stand-alone debrites (multi-bed remobilisation) Type 6 event beds are composed of argillaceous sandstone and sandy mud facies, generally with common mudstone clasts. Facies A1 (59%) and A2 (36%) dominate but these beds lack a basal cleaner sand division seen in some Type 5 beds. They lack the vertical organisation seen in Type 5 beds and the matrix has a pronounced fine silt to mud component. Type 6 beds range in thickness from a few centimetres to over 4 m thick, so overall they are thicker than Type 5 beds. Blocks and mudstone rip-up clasts are angular and may be large in size (spanning the core; Fig. 10c). Clean sandstone occurs only as deformed clasts and blocks; in some cases blocks of

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Fig. 10. Core examples of argillaceous sandstone-dominated event beds (A) Type 4 event beds argillaceous sandstone dominated (>51%) hybrid event beds: Well 22/09-4 (878897960 ) transition between clean sandstone and argillaceous sandstone (dashed line) is cryptic (A1). Within the argillaceous caps mudstone clasts (solid arrows) are replaced by carbonaceous fragments (dashed arrows, see photo A2). (B) Type 5 event beds >90% argillaceous sandstone: Well 22/9-4 (8628-86340 ) showing a rough vertical organisation, the mud content increases vertically (B1). An example of banding within the debrite cap (B2). (C) Type 6 event beds ‘true’ debris flows: Well 23/21-T1 (9653-96550 ) distinguished from Type 5 beds by very fine-grained matrix, large angular clasts of mudstone or argillaceous sandstone and exotic clasts (green Lista mudstone clasts highlighted by arrows in C2). In the example shown the bed is nearly 3 m thick with no distinct segregation or organisation of clasts or injection fabrics. (D) Type 7 event beds: Well 23/27-6 (9653.2–96540 ) Ripple and horizontal laminated thin event beds interpreted as the deposit of a low-density turbidity current (LDT). For the key see Fig. 9. Note depths are in metres and metric feet.

remobilised debrite can be found in the event beds which contain clean sandstone injection features. These event beds are interpreted as either long runout mass transport complexes or locally sourced debris flows sourced from areas of localised remobilisation active at the time of FSM deposition, i.e. off the flanks of salt highs, other structural highs, channel margins or steep slopes onto which sediment was deposited. Type 6 event beds are not thought to be coupled to the emplacement of underlying sandstones, and they formed by multi-bed remobilisation rather than processes operating within a single evolving hybrid flow. 4.7. Type 7 beds: laminated and graded sandstones Type 7 event beds comprise isolated, parallel- and ripple-laminated, graded sandstone beds that are typically a few centimetres thick: the average thickness being 0.05 m (Fig. 10d). They occur throughout the FSM, although they are more commonly preserved within thick mudstone intervals in the lowermost Forties interval. Type 7 beds are interpreted as the deposits of low-density, fully turbulent turbidity currents (surge-like flow events of Mulder and Alexander, 2001), or the distal diluted trailing tails that have

detached or runout from high-density turbidity currents, debris flows or hybrid flows. 5. Overall proportions of event bed Types The seven bed types identified in the cores from the eastern lateral and distal Forties Fan margin can be divided into two groups: (1) the deposits of conventional ‘end-member’ flow types including high- and low-density turbidites (Bed Types 1 and 7 respectively) and debris flows produced by multi-bed remobilisation (Bed Type 6); and (2) a spectrum of hybrid event beds (Bed Types 2–5) emplaced by more complex flows characterised by variable rheology (both fluidal and plastic) and turbulence state (turbulent, transitional and laminar) along their length. The inventory of 1133 complete event beds assembled from the studied cores can be used to analyse the relative importance of ‘conventional’ versus hybrid event beds in this system (Fig. 7a). Significantly w67% by number of complete event beds are of a hybrid character, corresponding to w81% by bed thicknesses. The preponderance of hybrid beds is surprising given the marginal/distal position within a large mixed mud-sand fan where low-density turbidites might intuitively be expected to be dominant. Similar

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styles of hybrid bed are seen in wells close to salt diapirs in the Pierce and Lomond Fields as in the Everest wells remote from salt highs. This suggests that the hybrid bed development is related to the intrinsic behaviour of the gravity currents in the outer Forties Fan, rather than to local secondary salt-induced remobilisation effects. 6. Stratigraphic distribution of event bed Types A series of sub-regional maximum flooding surfaces allows a first order correlation of the wells and cored sections within the Everest and Pierce fields (Fig.. 3 and 4) and a general comparison of the lateral and stratigraphic distribution of event bed types within and between the two fields. A proprietary field specific biostratigraphical study was also used to tie the Everest wells (Fig. 3). As hybrid Bed Types 2 through to 5 involve significant proportions of non-reservoir clay-prone facies, their distribution and relationship to sand-prone Type 1 beds and sections of amalgamated cleaner sandstone is important to understanding reservoir heterogeneity. The proportions of bed types discussed below relate to the population of complete event beds (i.e. those not truncated by erosion or interrupted by samples or poor coring). Bed thicknesses reported are based on true vertical depths (corrected for bed dip). Depths within specific wells are cited in either ft or m depending on the units used when the wells were drilled and correspond to the driller’s depth (d.d.) relative to the kelly’s bushing (RKB). 6.1. Everest area Forties deposition within the Everest field occurred within two ‘lobes’, Lobe 1 to the east appears to shale out distally (to the SSE) interpreted here as a frontal splay complex (sensu Posamentier and Kola, 2003), whereas the south-westerly Lobe 2 lay on a throughgoing sand-fairway and is sandier overall. The Everest succession is informally divided into 4 zones (zones A–D from base to top) on the basis of the limited biostratigraphy. The lithology and bed types in Lobe 1 (Fig. 12) define an overall upward-sandying (zones A–C) and upward muddying (zone D) motif interpreted to reflect the growth, aggradation and subsequent retreat of the splay complex. Wells 22/9-2 (only 20 m of core) and 22/9-3 are in the proximal splay; 22/10a-4 in the distal splay and 22/10a-T6 in the lateral fringe. Zone A comprises background mudstone alternating with isolated Type 5 event beds with subordinate Type 6 and 7 beds in wells 10a-4 and 10a-T6. The Type 5 beds are thicker in the proximal splay well. Zone B is sandier overall and becomes sandier upwards with Bed Types 1 and 2 becoming more common in the proximal splay well accompanied by a reduction in thickness of the interbedded mudstones. The zone thins and is muddier down-dip with Bed Types 2 through 5. The equivalent fringe well is significantly muddier with Bed Types 5–7 dominating, and Type 4 beds towards the top of the zone. Zone C represents the acme of splay growth and is sandiest overall; the proximal 9-3 well is dominated by amalgamated Type 1 and Type 2 beds with thin debrite caps (<10 cm) whereas the equivalent section in well 10a-T6 is made up to Type 2 beds with thicker debrite components (>20 cm). Zone C in the distal splay 10a-4 well

Fig. 11. Theoretical model to explain the development of a linked debrite cap from progressive accretion during flow runout resulting in the crude vertical organisation of clasts and fabric observed in the debrite component of a hybrid event bed. T ¼ 0 start with an undifferentiated fully-turbulent sediment gravity current. T ¼ 1 the current undergoes longitudinal flow segregation of clay, mud and clasts (based on hydrodynamic properties) which become concentrated within the flow, suppressing turbulence at the base causing flow freezing and the formation of a debrite, resulting in a hybrid

flow. T ¼ 2 and T ¼ 3 debrite formation is not by en masse freezing and collapse of the flow, but via freezing at the base of the turbulent flow, whenever fluid turbulence is suppressed. As the debrite component of the hybrid flow has a slower velocity than the fully turbulent part of the flow, the distance between the head of the hybrid flow and debrite component increases. The debrite is therefore formed from progressively more distal parts of a segregated turbulent flows; resulting in the increase in clay content and decrease in the size of mudstone clasts (being replaced by the more hydrodynamically buoyant carbonaceous fragments at the top of a debrite cap) that gives the debrite caps of the Forties hybrid beds a crude vertical organisation.

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Fig. 12. Examples of event beds types and vertical stacking for the different stratigraphic zones in Lobe 1 of the Everest field. See Fig. 3 for the location of the examples shown and Fig. 9 for the key to the sedimentary logs.

consists of stacked Type 2, 4 and 5 event beds. Those Type 2 beds that are present are thinner and have thicker debrite divisions than the up-dip well (3 km away). Zone D is characterised by a reduction in overall sand content and a return to thinner event beds. A diverse range of bed types (1–5) is present in the 9-3 well, with a change to debrite-dominated (Type 3M, 4, 5) beds down dip and possibly in the lateral fringe, although this is poorly cored. Only two wells, 4 km apart, are available from the Lobe 2 in Everest where the cores are restricted to the upper part of the thicker Forties Formation interval developed here. Well 22/9-4 lies to the north and is slightly off axis with respect to the sand fairway sampled by the 22/14a-2 well in the south. The overall vertical organisation differs from Lobe 1 in that the lower Forties succession (zone A) is much sandier and the succession becomes less sandy upwards, particularly in the case of the 22/14a-2 well (Fig. 13). The same zonation used in Lobe 1 can be extended to Lobe 2. Only the upper part of a thick Zone A Forties section can be characterised. It is higher net-to-gross (89% cleaner sandstone) in the 14a-2 well where it is composed of Type 2 and 3S beds with minor Type 4 interleaved with thick (>3 m) amalgamated massive and dewatered sandstones (bed type uncertain due to cut-out of bed top; Fig. 13). The equivalent section in the 9-4 well is less sandy (53% net-to-gross ratio) and generally thinner bedded with fewer amalgamated clean sandstone sections (although one reaches 4.7 m thick) and there are more interbedded Type 4 and 5 event beds together with background mudstone intervals up to 0.4 m thick. The overlying Zones B and C show a fall in net-to-gross

(where the argillaceous sandstone is classed as non-reservoir) in the 14a-2 well (Fig. 13), but a lateral change to lower net-to-gross is still maintained in the equivalent intervals in the 9-4 well where there are more Type 3M and 4 beds overall, and fewer amalgamated sand-on-sand contacts. Zone D is incompletely cored but appears to show a reversal in the lateral trends with higher net-togross and thicker succession in the 9-4 well where Type 2 beds are well developed (up to 1.4 m thick) and amalgamated massive and dewatered sandstone sections up to 2 m thick. These are separated by thinner Type 4 and 5 event beds (0.3–0.7 m). 6.2. Pierce area Regional correlation, via the maximum flooding surfaces or MFS, suggests that Forties deposition at Pierce was primarily during zone MFS3 (57%) with MFS2 and Top Forties (TF) Zones being less important (together 34% – see Fig. 4). This contrasts with the up-dip Everest field where most deposition was in the MFS2 and TF zones (82% for the studied cores – see Fig. 3). The six Pierce wells studied (Fig. 14) have a common vertical motif in that isolated event beds are more common in the generally finer grained, lower net-to-gross lower part of the Forties Fan succession (Zones MFS2, MFS3 lower). Where cored, these comprise common hybrid event beds of Types 4–7 with occasional Type 2 beds. Locally they are affected by multi-bed sliding (e.g. well 22a-3, 2830.45–2830.9 m d.d. RKB). Bedding is thinnest in the MFS3 division in well 27-6 (15–60 cm thick), located in the

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Fig. 13. Example of event beds types and vertical stacking for the different stratigraphic zones in Lobe 2 of the Everest field. See Fig. 3 for the location of the examples shown and Fig. 9 for the key to the sedimentary logs.

down-dip splay, where 63% of complete event beds comprise Types 3M, 4 and 5. The mid- to upper parts of the Forties succession (upper MFS3 and TFSM zones) are generally sandier on account of the presence of discrete amalgamated sandstone sections (4–8 m thick) capped by either a Type 2 or 3S event beds (0.8–1.6 m thick) with poorly developed argillaceous divisions. The amalgamated sandstone intervals are associated with stacked beds with coarse granule lags at their base (Fig. 14). The base of the amalgamated sections lack associated mudstone-clast breccias or evidence for tractional structures, and the overlying sandstone beds do not have systematic vertical trends in preserved bed thickness, bed type or overall grain size. The bodies of amalgamated sandstone are vertically separated by 1–2 m thick stacks of isolated, thinner bedded (0.15–1.2 m thick) Type 2, 4 and 5 event beds with intervening mudstone intervals 2–10 cm thick. The overall upwardsandying trend is bucked by wells 22a-3 and 27-9 which show a return to more heterogeneous, less amalgamated event beds in the upper TF package, lateral to wells with much higher overall sand contents and amalgamated sandstone sections as before. Significantly, these two wells are closest to the salt domes that penetrate the Forties sandstone, and in the case of the 27-9 well the section is relatively condensed. 6.3. Interpretation of vertical and lateral trends The studied wells reveal a range of overall vertical organisation within the outer lateral parts of the Forties Fan system which can be

summarised into three general trends 1) The proportion of clean sandstone within event beds increases vertically upwards through the sequence (producing a sandying-upwards trend e.g. Pierce fairway and splay wells away from salt diapirs). 2) The proportion of argillaceous sandstone within event beds increases vertically upwards (i.e. a muddying-upwards trend e.g. Everest Lobe 2 axis). 3) A combination of these two profiles (i.e. a sandying-upward then muddying-upward trend e.g. Everest Lobe 1, and the salt diapiradjacent wells in Pierce). A significant observation is that where the net sand content of the succession falls, there is a concomitant change to event beds with increasing proportions of argillaceous sandstone facies, and a tendency to preserve more thick mudstones between event beds (Fig. 15). This is irrespective of the overall vertical organisation. Whilst a lack of distinguishing marker beds within the studied hydrocarbon fields makes it impossible to determine the temporal and spatial evolution of facies within individual event beds, a comparison of the proportions and thickness of bed types within stratigraphically equivalent sections of wells located in different areas can be used to infer these distal and lateral facies transitions. Equivalent sections from the Everest Lobe 1 wells demonstrate that Type 1 and Type 2 beds must pass both down-dip and laterally into sections dominated by Bed Types 3–5 over distances of a few (up to 8) km; i.e. on a scale that is relatively short compared to the overall scale of the fan system (Fig. 15). Petrographic data show that compositionally the sand component of the different bed types is the same, therefore it is unlikely that the lateral transitions are due to interfingering between texturally

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Fig. 14. Example of event beds types and vertical stacking for the different stratigraphic zones within the Pierce field. See Fig. 4 for the location of the examples shown and Fig. 9 for the key to the sedimentary logs.

different dispersal systems characterised by different flow types. The simplest explanation is that the lateral variations reveal changes in the character of single flows as they reached their downdip and lateral runout limit, with the deposit recording a progressive change from fluidal to plastic flow (cf. Amy and Talling, 2006; Barker et al., 2008; Ito, 2008). Beds with Type 2 or 3S profiles up-dip are inferred to pass into 3M or 4 beds down-dip and eventually Type 5 or 7 beds (Fig. 15). Similar transitions have been inferred between outcrop sections where individual beds can be correlated in more continuous basin plain successions (Amy et al., 2005; Amy and Talling, 2006). The sandying- then muddyingupward motif of Everest Lobe 1 wells is consequently interpreted to reflect the down-dip and then up-dip migration of the point where plastic flow deposits appear in the bed. This could simply reflect splay-outbuilding and subsequent retreat involving the progressive shifting of argillaceous sandstone-dominated event beds down and then back up the depositional profile as the fan advances and then retreats; alternatively it may reflect a change in flow character (e.g. flow volume and momentum) that delayed then promoted flow transformation (splay growth and contraction). Distinction between these two models requires better lateral control on the width of the argillaceous facies dominated fringe. Thompson and Butcher (1991) inferred that the Everest Lobe 2 flows were initially focused through a laterally-restricted palaeodepression generated by the underlying Lista Formation (see also Kosˇa, 2007). This may explain why the vertical succession is different to Lobe 1, with the lower zones (A and B) in well 14a-2 and

to a lesser extent 9-4 being dominated by metre thick beds of amalgamated sandstones characterised by a high proportion of Type 1 and Type 2 event beds (at least where cored; Fig. 3). The focusing of flows through a topographic restriction may explain the absence or poor development of argillaceous sandstone caps at the base of these wells. The constricted flows would have accelerated due to the non-uniformity effect producing accumulative flows (sensu Kneller, 1995) resulting in enhanced flow turbulence (McCaffrey and Kneller, 2004), delaying any tendency of the flow to transform from fluidal to plastic rheology. The constriction of the flow exhibiting hysterisis upon the flows properties where upon existing the constriction the flow has reverted back to either a fully turbulent unsegregated flow or has been modified in such a way that the flow behaves in some other way. Increased energy would also have promoted bed erosion that removed heterogeneous bed caps and deposits of background mud. When bathymetric irregularities were healed and overall gradients reduced through deposition, flow expansion and deceleration occurred, resulting in increased preservation of argillaceous sandstone facies seen in the upper part of Lobe 2. Lateral variation in net sandstone here may indicate compensation due to accretion with offset stacking of Type 1 and 2 beds dominated proximal splay deposits. The lower Forties interval in the Pierce wells (zones MFS2 and 3) shows a sandying-upward trend reflecting a change from Type 3–5 beds to increasing proportions of Type 1 and 2 beds accompanied by the appearance of 4–8 m thick amalgamated sandstone packages. This is consistent with either progradation or growth of the

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Fig. 15. Cartoon (not to scale) illustrating the inferred context and distribution of different bed types in the distal and lateral margin of the Forties Fan system, informed by the study of the Everest, Lomond and Pierce cores. Bed Types 1, 2 and 3S occur in sandier fairways, these are areas where flows are confined resulting in a delaying/limiting the onset and extent of flow transformations. Note the up-fan incision and fan valleys, a potential source of mud for flow transformations down-fan. Muddy hybrid bed Types 3M, 4 and 5 occur in splay fringes (both down-dip and lateral), flow expansion zones, and close to salt diapirs and mounded highs (reflecting the location of pre-Forties mass-wasting deposits) where subtle gradients steered away the turbulent core of flows. Multi-bed remobilisation (Type 6 beds) is identified close to diapirs and slopes and is distinct from bed-scale hybrid flow fabrics. Bed Type 7 (not shown) occurs widely in finer-grained parts of most settings due to spillover and stripping of dilute clouds from the tops of conventional turbidity currents and hybrid flows. The vertical distribution of event beds (shown schematically) identifies sandying/thickening upward splays and channel–splay complexes reflecting fan build-out, muddying-upward cycles reflecting early confinement subsequently overtopped, and symmetrical cycles recording splay growth and abandonment and diapir-induced steering of flows.

fan through time. The wells south of the diapirs (particularly the -6 well) show thinner and more argillaceous event beds at this level, perhaps due to flow expansion as flows left the confinement introduced by subtle diapir relief and pre-existing Lista topography. The amalgamated sandstones could be interpreted as the fill to shallow distributive channels embedded in sheets, but the lack of obvious basal bounding erosion surfaces suggests that these could also represent zones of amalgamation immediately down-dip from channels (cf. ZOAs of Gardner and Borer, 2000; Gardner et al., 2003). Whilst the uppermost Forties interval remains relatively sandy, two wells revert to more argillaceous facies. As these are closest to the diapirs, it is possible a phase of salt movement produced sufficient relief to force the main sandy flows away from the diapirs. The diapir adjacent area was then swept just by the thinner lateral margins of the flows which were more susceptible to turbulence suppression and flow transformation (cf. Barker et al., 2008), resulting in the deposition of the argillaceous sandstone facies dominated event beds.

sandstone facies (Fig. 7b) for the different bed types (see below). This shows that event beds dominated by argillaceous sandstone facies (Types 3M and 4) have broadly similar thicknesses to cleaner sandstone-dominated Type 1 and Type 2 beds. However, Type 5 beds are thinner. The implication is that the bed thickness remains fairly constant as the cleaner sand facies is replaced pro-rata internally by argillaceous sandstone. This pattern is supported by observations from outcropping basin floor sheets in the Marnoso Arenacea Formation, Italian Apennines (Ricchi Lucchi and Valmori, 1980; Talling et al., 2004) and the Oligocene–Miocene, West Crocker Formation, Sabah, North West Borneo, Jackson et al., 2009) and from subsurface studies of the Late Jurassic sand-rich deepwater fans in the northern North Sea (Haughton et al., 2003). In the case of the Forties event beds, the beds thin once the basal cleaner sand is lost (i.e., Type 5 beds). These relationships suggest that thickness of the plastic component of flow is somehow mediated by the wider flow behaviour, an issue we return to below. 7. Origin and significance of Forties hybrid event beds

6.4. Bed shape Longitudinal bed shape cannot be constrained directly as it is impossible to follow individual beds between wells. However, broad relationships can be inferred by cross-plotting complete event bed thicknesses against the proportion of argillaceous

Hybrid event beds are evidently an important component of the outer Forties Fan succession in the studied wells. Flows reaching the lateral and distal fan repeatedly evolved from fluidal to plastic rheology. Secondary remobilisation from local slopes and autoinjection leading to rip-down can be ruled out (see above). Large

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numbers of Type 5 beds imply that the debris flows commonly extended farther than the sandstones to which they were linked, but that they thinned once the basal cleaner sand was lost. A key question is why so many of the Forties flows behaved in this way. A number of mechanisms have been proposed to account for the origin of distal linked debrites and the hybrid beds that host them. These are (1) continued runout of a debris flow that had partly transformed up-slope to release a faster moving turbidity current (Haughton et al., 2003); (2) simultaneous or retrogressive release of co-genetic debris flows and turbidity currents, with the faster moving turbidity current always arriving at the fan fringe first (Haughton et al., 2003, this volume); (3) transformation from turbulent to more cohesive flow driven by mudstone clasts and clays incorporated in a turbidity current by erosional bulking (Ricchi Lucchi and Valmori, 1980; Haughton et al., 2003; Talling et al., 2004, 2007); and 4) transformation from turbulent to more cohesive occurs when a turbulent flow slows e.g. due to a decrease in slope angle (Talling et al., 2007). The scale of the system, texture and organisation of the debrites, lateral facies and thickness trends and the common gradational contacts between cleaner and argillaceous sandstone components favours the third mechanism in the case of the Forties examples. The Forties Fan system was a relatively large system, extending >250 km into the Central Graben. However, well correlations imply that the flows in the outer fan evolved rapidly over relatively short distances. Type 1 and 2 beds change down dip to Type 4 and 5 over distances of up to 8 km in Lobe 1 of the Everest field. If the debritic component of the bed were to come from source as a remnant of the original failure, it would have to bypass some 90–95% of the fan surface, before leaving a deposit. Whilst bypass of debris flows over distances up to 200 km has recently been suggested (Talling et al., 2007), this is for large-volume flows at the base of continental slopes that must have carried significant momentum. It is harder to envisage such long distance bypass operating in flows released from lowstand delta fronts in a shallower post-rift setting like the Palaeocene of the North Sea. Gradational contacts between the sandy part of the bed inferred to represent the fluidal part of the flow and the upper argillaceous sandstone representing the plastic flow suggest that the debritic component of the flow evolved from the fluidal part; two discrete flows (turbidity current and untransformed debris flow) are not represented in these examples. The occurrence of banded facies in Type 3 beds spanning the contact between the two suggests that in some cases the flow behaviour changed systematically from fluidal to plastic via a transitional state characterised by episodic turbulence suppression. This may be explained by progressive change in flow behaviour along its length, due to systematic hydraulic segregation of clasts and clay that can suppress turbulence (Baas and Best, 2002). Debris flows coming from source might, in addition, be expected to transport coarse extrabasinal grains from proximal settings without hydraulic segregation. Many of the linked debrites in the Forties system carry a relatively fine sand component (compared to proximal sandstones) suggesting the debris flows formed in an area where the coarse fraction had already been removed. In addition, the vertical textural trends within a linked debrite are taken to indicate progressive accretion of the parental debris flow from a turbulencesuppressed overpassing fluid flow that was continuing to evolve (Fig. 11). It is also possible that post-depositional foundering of clasts in a deposit that was still fluidal could produce similar trends – although in this case the deposit would also be prone to loading from above from the deposits of the turbulent tail. As a majority (65%) of such upward contacts are unloaded, this alternative is thought to be unlikely. Lastly, the bed thickness data and evidence for debritedominated beds of a similar scale to sandstone-dominated beds (Fig. 7b) suggests that the fluidal flow in some way mediated

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deposition of the following plastic flow, which is more likely in the case where they are co-genetic than in the case where they are unrelated. Taken together, the above features suggest that the change to plastic flow may be due to turbulence dampening through incorporation of clays into the flows. To impact deposition, the clays need to be concentrated near the bed and this is most easily achieved by disintegration of mudstone clasts caught up in the flows and carried as bedload layers. Thus, the common occurrence of hybrid beds throughout the distal and lateral Forties Fan record suggests that the flows were prone to incorporate mudstone clasts via erosion during fan growth, aggradation and retreat. Several factors are consistent with a system that was strongly erosional up-dip: (1) the very short time interval of Forties deposition, perhaps as small as 200,000 years suggesting rapid delivery of sand into the basin; (2) a very uneven surface across which the fan advanced due, to earlier Lista-aged slumping and salt-related topography. This topography would have produced non-uniform flow effects due to flow constriction and expansion, triggering localised areas of erosion and deposition, respectively; (3) tilting as a consequence of source area uplift due to plume inflation and rapid shoreline regression; and (4) evidence for extensive inner-fan erosion to create deep fan valleys infilled with deep-water sandstones in the Forties and Nelson fields. Much of the clay incorporated in the flows may thus have come from intra-basinal slope/inner-fan erosion during erosional bulking of turbulent flows. Although carbonaceous material commonly occurs within, or towards the top of the debritic intervals, this does not mean the associated clasts and mud also came from the shelf edge. Slow settling velocities of carbonaceous clasts suggests they would have been fractionated to the rear of the fluidal flow where they were mixed with entrained components further down slope. Incorporation of clay suppressed turbulence down dip, particularly in zones of flow expansion (Everest Lobe 1, southern splay in Pierce) and laterally (eastern edge of Everest Lobe 1) or in areas elevated around salt highs (Pierce diapir adjacent wells). Flow constriction due to topography (inherited in Everest Lobe 2 and salt west of Pierce) delayed turbulence suppression and resulted in the deposition of higher net sand. 8. Conclusions The main conclusions arising from this re-assessment of facies and bed-scale organisation in the outer Forties Fan are as follows: 1. Hybrid event beds dominate in the outer and distal Forties Fan in the Everest, Lomond and Pierce fields; conventional turbidites are relatively rare. The argillaceous upper parts to hybrid beds introduce significant heterogeneity and impact on the net clean sandstone preserved distally in the system. 2. A range of bed types are recognised; conventional high- and low-density turbidites, stand-alone debrites and hybrid event beds. The latter are subdivided according to whether they are dominated by cleaner sandstone or argillaceous sandstone facies associations, and whether or not banded facies intervene between these associations. 3. The hybrid beds are interpreted as products of flows that were characterised by an initial fluid, or turbulent suspension followed by a change to a more clay rich plastic flow, and capped by a minor dilute fluidal flow. Where internal transitions are more gradational, and banded facies developed, the flows are thought to have evolved progressively from fluidal to plastic behaviour along their length as turbulence was suppressed and cohesion became increasingly important. 4. The bed character close to diapirs resembles that cored in fields away from diapirs and hence the typically deformed

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argillaceous caps to beds are not produced by secondary sliding off local salt-cored slopes; autoinjection and sand rip-down can also be ruled out as unique explanations for argillaceous cap formation. The caps are instead interpreted as an inherent component of the flow evolution in distal and lateral parts of the fan. Similar facies trends have been inferred in other systems. 5. Lateral correlation of bed packages between wells demonstrate changes in bed types with cleaner sandstone-dominated event Bed Types 1, 2 and 3A passing down-dip and laterally into argillaceous sandstone-dominated event beds (Types 3B, 4 and 5). Transitions take place over relatively short distances (compared to the fan scale), and bed thickness statistics suggest the beds become dominated by argillaceous facies without thinning significantly. The argillaceous sandstone-dominated event beds are common in the muddier parts of sandying-upwards, sandying-muddying upward and muddying-upward cycles. 6. Although hybrid beds can potentially arise in several ways, the Forties examples are thought to record flow transformations affecting fluidal flows caused by erosion and bulking with clays that can suppress near-bed turbulence and induce a change to cohesive plastic flow. Propagation of the plastic flows from the source failure is thought to be less likely in this case because of the length of the bypass zone relative to the short length at which the lateral facies transitions are expressed, and the vertical character of the beds, particularly the gradation internal contacts suggesting a gradual evolution of the flow. The relatively uniform composite bed thicknesses, in which the debritic and turbiditic co-vary, also suggest likely co-genetic development. 7. The dominance of hybrid event beds may reflect the setting of the Forties sandstone with very rapid delivery of sand to the basin, a very uneven substrate promoting non-uniformity, tilting as a consequence of source area uplift and extensive inner-fan erosion to create deep fans. This combination of factors would have promoted erosion and flow bulking, and hence transformations leading to formation of hybrid flows in distal settings. Acknowledgements This work forms part of Christopher Davis’s PhD research funded, jointly under the auspices of Phase 5 of the Turbidites Research Group (Anadarko, BG-Group, BHP Billiton, BP, Chevron, ConocoPhillips, Kerr McGee, Devon, Maersk, Norsk Hydro, Shell, Statoil and Woodside) and by a UCD, Dublin research demonstratorship. Additional help was also give by the Shell Pierce Asset Team and also the BP based Everest Lomond partnership in accessing proprietary biostratigraphic datasets and regional correlation panels used in this paper. Support from the DTI in the forms of an ACHARR JIP award, access to released core and use of core logging facilities is gratefully acknowledged. Reviews by Dr Young Kwan Sohn, Dr. D.G. Roberts, Dr. Rufus Brunt and Dr. Lawrence Amy were gratefully appreciated in helping to refine early drafts of this paper. References Ahmadi, Z., Sawyers, M., Kenyon-Roberts, S., Stanworth, B., Kugler, K., Kristensen, J., Fugelli, E., 2003. Paleocene. In: Evans, D., Graham, C., Armour, A., Bathurst, P. (Eds.), The Millennium Atlas: Petroleum Geology of the Central and Northern North Sea. Geological Society, London. Amy, L.A., Talling, P.J., 2006. Anatomy of turbidites and linked debrites based on long distance (120 x 30 km) bed correlation, Marnoso Arenacea Formation, Northern Apennines, Italy. Sedimentology 53, 161–212. Amy, L.A., Talling, P.J., Peakall, J., Wynn, R.B., Arzola Thynne, R.G., 2005. Bed geometry used to test recognition criteria of turbidites and (sandy) debrites. Sedimentary Geology 179, 163–174. Anderton, R., 1995. Sequences, cycles and other nonsense: are submarine fan models any use in reservoir geology? In: Hartley, A.J., Prosser, D.J. (Eds.), Characterization of Deep Marine Clastic Systems. Geological Society, London, Special Publication, vol. 94, pp. 5–11.

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