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Marine and Petroleum Geology 26 (2009) 1640–1655

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Marine and Petroleum Geology

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A giant (5.3�107 m3) middle Miocene (c. 15 Ma) sediment mound (M1) above theSiri Canyon, Norwegian–Danish Basin: Origin and significance

Katrine Juul Andresen a,*, Ole Rønø Clausen a, Mads Huuse b

a Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs Gade 2, 8000 Århus C, Denmarkb School of Geosciences, University of Aberdeen, Aberdeen AB24 3UF, Scotland, UK

a r t i c l e i n f o

Article history:Received 20 March 2008Received in revised form12 February 2009Accepted 13 February 2009Available online 28 February 2009

Keywords:Norwegian–Danish BasinCenozoicSiri CanyonSand volcanoHydrocarbon migrationLateral pressure transfer

* Corresponding author. Tel.: þ45 8942 9539; fax:E-mail addresses: katrine.andresen@geo.au.dk (K

geo.au.dk (O.R. Clausen), m.huuse@abdn.ac.uk (M. Hu

0264-8172/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.marpetgeo.2009.02.005

a b s t r a c t

A large-scale enigmatic mound structure (M1) has been discovered in middle Miocene strata of theNorwegian–Danish Basin, c. 10 km east and updip of the Central Graben. It is located about 1 km beneaththe seabed and clearly resolved by a 3D seismic data set focused on the deeper, remobilised, sand-filledSiri Canyon. M1 comprises two culminations, up to 80 m high and up to 1400 m long, constitutinga sediment volume of some 5.3� 107 m3. It is characterized by a hard reflection at the top, a softreflection at the base, differential compaction relative to the surrounding sediments, and 10 ms TWTvelocity pull up of underlying reflections, indicating a relatively fast mound fill, attributed to the pres-ence of sand within the mound. Internal seismic reflections are arranged in an asymmetric concentricpattern, suggesting a progressive aggradation to the NW, downstream to a mid-Miocene contour currentsystem. Numerous elongated pockmarks occur in the upper Miocene succession close to the mound andindicate that the study area was influenced by gas expulsion in the mid- and late Miocene.The reflection configuration, velocity, dimensions, regional setting, and isolated location can best beexplained by interpreting the mound as a giant sand volcano extruded >1 km upward from the SiriCanyon during the middle Miocene (c. 15 Ma). The likely causes of this remarkable structure include gascharge and lateral pressure transfer from the Central Graben along the Siri Canyon reservoir. While this isthe first such structure described from this part of the North Sea, similar-aged sand extrudites haverecently been inferred from seismic observations in the North Viking Graben, thus suggesting that themid-Miocene was a time of widespread and intense sediment remobilization and fluid expulsion in theNorth Sea.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Mounded structures comprise a wide range of geneticallydistinct geological features and examples exist from all over theworld. Seismic scale (10’s to 100’s m) examples include siliciclasticsedimentary mounds such as deep-sea channels, fans and con-tourites (Timbrell, 1993; Viana et al., 1998; Jennette et al., 2000),carbonate mounds (Hovland et al., 1994; Feary and James, 1995;Huuse, 1999; Nielsen et al., 2003), and structures related to clasticremobilization, intrusion and extrusion (Brooke et al., 1995; Dixonet al., 1995; Graue, 2000; Huuse et al., 2004; Hurst et al., 2006; Frey-Martınez et al., 2007; Hamberg et al., 2007). The latter category isoften related to the expulsion of thermogenic gas and formationfluids, which results in the entrainment of sediments and in theformation of mud and sand volcanoes and clastic intrusions, which

þ45 8942 9406..J. Andresen), ole.r.clausen@use).

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can reach hundreds of metres to kilometres in extent (Jolly andLonergan, 2002; Hansen et al., 2005; Huuse et al., 2005). Sand-prone sediment mounds represent high-quality hydrocarbonreservoirs in the Paleogene North Sea area (Timbrell, 1993; Dixonet al., 1995; Jennette et al., 2000; Galloway, 2001; Hamberg et al.,2007). Other mound types include hyaloclastite mounds associatedwith igneous vents (e.g. Davies et al., 2002).

The Central Graben in the North Sea is a well known hydro-carbon province, while areas outside the Central Graben, have beendemonstrated to be less prospective. Paleocene sandstones in theSiri Canyon thus form the only economical reservoirs in the Danisharea outside the Central Graben (Hamberg et al., 2005, 2007; Ohmet al., 2006). Studies from the Siri Canyon show that fluid migrationand sediment remobilization, resulting in both large and smallscale sand injection structures, are important factors in locationand compartmentalisation of prospective sand bodies in the canyon(Hamberg et al., 2005, 2007).

The mound (M1) analysed here is the first middle Miocenemound structure observed in the Norwegian–Danish Basin, and its

K.J. Andresen et al. / Marine and Petroleum Geology 26 (2009) 1640–1655 1641

large volume (5.3�107 m3) suggests an unusual explanation. M1 islocated vertically c. 1 km above the Siri Canyon known for itsremobilized fine-grained sand reservoirs, and Neogene hydro-carbon charge. The objective of this study is to use the available 3Dseismic and well data to describe the morphology and internalarchitecture of this unique mound structure in order to constrain itsorigin and possible implications for sediment remobilization andfluid expulsion. The study concludes that the mound is likely torepresent a giant mid-Miocene sand volcano sourced from the SiriCanyon c. 1 km beneath the mound. The large volume and greatdistance to the parent sand make this one of the most remarkablesediment remobilization features described to date and highlightthat remarkable discoveries can still be made in heavily exploredareas such as the North Sea Basin by use of modern 3D seismic data.

2. Geological setting

The Cenozoic North Sea Basin is as an epicontinental basincentred above an aborted Mesozoic rift, part of which includes theCentral Graben. The basin is surrounded by structurally high sedi-ment source areas, and its interior is deformed by halokinesis(Ziegler, 1990). The study area is located in the Norwegian–DanishBasin, approximately 10 km to the east of the Coffee Soil Faultwhich constitutes the eastern margin of the Central Graben (Fig. 1).The Norwegian–Danish Basin is bounded by the Sorgenfrei–Torn-quist Zone to the east, the Central Graben to the west, theNorwegian landmasses to the north and the Ringkøbing-Fyn Highto the south (Ziegler, 1990). No major basement-related faultingtook place during the Cenozoic, but inversion-related pulses ofuplift affected selected parts of the basin during the early Cenozoic(Ziegler, 1990; Vejbæk and Andersen, 2002; Nielsen et al., 2005).Halokinesis-induced faulting developed, especially along thepinch-out line of the Zechstein Group evaporites onto the Ring-købing-Fyn High (Ziegler, 1990; Clausen and Huuse, 1999; Huuse,1999). The upper Paleocene to middle Miocene succession isdeformed basin-wide by polygonal faulting caused by volumetriccontraction of the pelagic and hemi-pelagic mudstones (Cart-wright, 1994, 1996; Cartwright and Lonergan, 1996).

The Siri Canyon was eroded into the upper part of the UpperCretaceous–Danian limestones during middle Paleocene times andwas filled with glauconitic turbidite sandstones derived from theStavanger Platform during the late Paleocene (Hamberg et al., 2005).The canyon fill contains several laterally restricted sand bodies ofvarying age, which were deposited in channels and lobe environ-ments within the canyon (Hamberg et al., 2005, 2007; Schiøler et al.,2007). The sands which were formally named as a series of distinctlithostratigraphic members by Schiøler et al. (2007) are here infor-mally referred to as the ‘‘Siri sands’’. The ‘‘Siri sands’’ form thereservoirs in a number of producing fields in the Siri Canyon.Hydrocarbons were generated from Jurassic source rocks in theCentral Graben from the Paleocene to the present day (Kubala et al.,2003). Vertical migration from the Central Graben area and long-distance lateral migration of hydrocarbons through the Siri Canyonare thought to be responsible for charging the Paleocene fields in theSiri Canyon (Hamberg et al., 2005; Ohm et al., 2006).

Cenozoic deposits in the Norwegian–Danish Basin including theSiri Canyon infill were mainly derived from the eastern basinmargin, and were controlled by tectonically and climaticallycontrolled relative sea level changes and shifts in sediment sourceareas (Michelsen et al., 1998; Clausen et al., 1999; Huuse, 2002;Rasmussen et al., 2005). An overall upward coarsening of theCenozoic succession (Kristofferson and Bang, 1982; Schiøler et al.,2007) reflects progradation of the eastern basin margin whichresulted in a progressive narrowing of the North Sea Basin duringthe Cenozoic (Ziegler, 1990; Huuse, 2002).

The M1 structure studied here immediately overlies the mid-Miocene unconformity (Fig. 2), a prominent sequence stratigraphicboundary and basin-wide seismic marker defined by Michelsen et al.(1998). The pre mid-Miocene succession below the mound is charac-terized by a dense system of polygonal faults (Clausen and Korstgård,1993; Cartwright, 1994). The mid-Miocene unconformity was floodedduring a widespread transgression, the so-called Hodde transgression,in response to a minoreustatic rise c.1 Myr before the onset of the long-term mid-Miocene eustatic lowering at c. 15 Ma (Huuse and Clausen,2001; Huuse, 2002). During deposition of the Hodde Formation,climate was relatively warm and sea level was significantly higher thantoday, with water depths in the Central Graben area being of the orderof 200–500 m (Laursen et al., 1997; Huuse, 2002; Rasmussen et al.,2005). Sedimentation rates during the Hodde transgression were lowand the Hodde Formation is thus composed of transgressive marineclays with only minor sand layers (Schiøler et al., 2007).

Integrated studies of the Miocene succession including onshoreoutcrops, onshore and offshore seismic data, petrophysical well logdata and detailed biostratigraphy, have resulted in a detailedMiocene paleogeography of the eastern North Sea (e.g. Huuse,2002; Rasmussen et al., 2005). Generally, the middle Miocenepaleogeography depicts a gently SW dipping shelf, with a shorelinelocated to the east and a clinoform breakpoint (depositional shelfbreak) trending NW–SE (Fig. 1) (Laursen et al., 1997; Huuse, 2002;Rasmussen et al., 2005).

Several elongated paleo-pockmarks occur in the upper Miocenesuccession near the mound (Andresen et al., 2008) (Figs. 3a and 4a).The pockmarks are interpreted as caused by upward migration ofgas derived from mature Jurassic source rocks (Andresen et al.,2008). These source rocks have been modelled to generate lighthydrocarbons during the Miocene (Kubala et al., 2003; Rasmussenet al., 2005; Ohm et al., 2006), and the widespread occurrence ofpockmarks indicates that much of the gas generated escaped toseafloor during the Miocene (Andresen et al., 2008). In addition,recent studies based on 2D and 3D seismic data have shown thathighly erosive bottom currents (Fig. 1) impinged on the seafloor inlarge parts of the North Sea during the mid-Miocene (Galloway,2001; Huuse and Clausen, 2001; Andresen et al., 2008).

3. Database and methodology

3.1. Database

The study database includes two 3D seismic surveys, several 2Dseismic lines, primarily used for correlation to wells located outsidethe area covered by the 3D seismic surveys, and well data from 10exploration wells (Fig. 3). The 3D seismic surveys cover an area ofabout 1500 km2 along the Norwegian–Danish border NE of theDanish Central Graben, and above the Siri Canyon (Fig. 3). Thesurveys were acquired in 1996 with a bin spacing of 12.5 m. Anacoustic impedance increase (hard reflection) is represented by apositive amplitude (black peak) in the seismic data, while a decreasein acoustic impedance (soft reflections) is represented by a negativeamplitude (white trough). A full stack amplitude volume anda coherency volume were available for both 3D seismic surveys.

The dominant frequency in the interval of study (i.e. 800–1300 ms TWT) is of the order of 45–50 Hz. This results in a verticalresolution (quarter of the dominant wavelength, l/4) of about 10 mand a horizontal resolution (l/2) of about 20 m when an intervalvelocity of 2 km/s is applied as observed in the nearest wells, Elna-1and Cleo-1 (Fig. 3b; Nielsen and Japsen, 1991).

The well data consists of petrophysical well logs includinggamma ray, sonic and resistivity logs for the interval studied.Biostratigraphic and commercial stratigraphic well reports werealso consulted.

Fig. 1. Study area (black box) in the eastern part of the central North Sea and the main structural elements of the North Sea. The TWT (ms) contours of the mid-Mioceneunconformity (compiled from Jordt et al., 1995; Michelsen et al., 1998) show the general SW-ward dipping paleogeography of the area during the middle Miocene. Also shown arethe Siri Canyon outline and the dominating mid-Miocene bottom current direction (Andresen et al., 2008) in the Central Graben area.

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3.2. Methodology

The M1 is defined on 3D seismic data by mapping of the tworeflections constituting its lower and upper boundary (base moundreflection, BMR; and top mound reflection, TMR). In addition fourhorizons within M1 were interpreted (Fig. 4). Another four referencehorizons were also interpreted, two below (Top Chalk Group and TopRogaland Group) and two above the mound (Horizons 1 and 2). Allhorizons were mapped by interpreter-guided 2D horizon trackingfollowed by 3D auto-tracking utilizing specific parameters for eachhorizon. Any picking artifacts were made good and horizons re-tracked where necessary. The external and internal geometry of themound was investigated by 3D visualization of the bounding andinternal surfaces, calibrated to vertical seismic profiles and time-slices to help to elucidate the mound architecture.

Time-structure maps, time-thickness (isochron) maps andseismic attribute maps were used during the analysis anddescription of the mound and in the genetic interpretation of thestructure. The seismic attribute maps include amplitude and

coherency extractions along the interpreted horizons, time-slicesthrough the seismic volumes and RMS amplitude extractions ofintervals around and between interpreted horizons.

M1 is located approximately 7 km from the nearest well Elna-1(Fig. 3) and the sedimentology and age of the mound are inferred bytying the interval including the mound to the Elna-1 and otherwells located in the area by the means of seismic data using stan-dard sequence stratigraphic methodologies.

4. Seismic observations on occurrence, morphology,bounding surfaces and internal reflection configuration of M1

4.1. Occurrence

As previously mentioned M1 is located c. 10 km east of thegraben-bounding Coffee Soil Fault and overlies the mid-Mioceneunconformity (c. 15 Ma) (Fig. 4). Stratigraphic correlation from theElna-1 well and other nearby wells shows that M1 is of Serravallian

Fig. 2. (a) Stratigraphic chart of the Cenozoic succession in the North Sea showing offshore lithostratigraphy, global sea level variations, climatic changes and major sequencestratigraphic units. The mound is found in the lower part of sequence 7.1 (Nordland Group), within the transgressive deposits of the Hodde Formation. (b) Seismic stratigraphy at theElna-1 well, illustrating that M1 is of Serravallian age, while Horizon 1 in the overburden is of middle Serravallian age and Horizon 2 is of Serravallian/Tortonian boundary age. Alsoshown is the gamma ray (GR) from the Elna-1 well and a generated synthetic seismic trace, which assisted in the correlation between the seismic data and the well data. TMR¼ topmound reflection, BMR¼ base mound reflection, mMu¼mid-Miocene unconformity. For location of seismic line see Fig. 3b.

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Fig. 3. (a) Zoom of the study area showing the position of the two 3D seismic surveys, the 2D seismic lines and the well locations in the surrounding area. Well data from 10 of thesewells were used in this study. Also shown are nearby elongated mid-Miocene pockmarks, extent of the Paleocene Siri Canyon and the hydrocarbon fields located within the Canyon.The mound is found vertically above the Siri Canyon in the eastern parts of the western 3D seismic survey. (b) Zoom on the proximal area around the mound giving the location ofseismic lines shown in other figures.

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age, while Horizon 1 in the overburden is of middle Serravallian ageand Horizon 2 is of Serravallian/Tortonian boundary age (Fig. 2b).

The sedimentary succession below the mid-Miocene unconfor-mity is characterized by intense polygonal faulting, whereas thesuccession above in general is unfaulted, with an overall west toSW-ward prograding clinoform architecture.

In seismic sections M1 is seen to occur vertically above a thin-ning of the sand-prone units in the Siri Canyon (Figs. 4a and 5). M1is located above the eastern limit of a local depocenter as illustratedby the TWT thickness variations of sand-prone units within thecanyon (Fig. 6). At the thickness map (Fig. 6) localized depocentersrepresent the different sand bodies (lobes) within the canyon(Hamberg et al., 2005, 2007; Schiøler et al., 2007). The depocenter

below M1 is thus part of a number of stacked sand bodies mutuallyonlapping further to the west and south, thus constitutinga continuous laterally stacked sand body here informally called the‘‘Siri sands’’ from the Central Graben area to the area where M1 islocated (Fig. 6) (Hamberg et al., 2007; Schiøler et al., 2007).

4.2. Morphology of M1

Three-dimensional visualization of the top and base horizons ofM1 clearly elucidates the external form and general geometry ofthe mound (Fig. 7). M1 is seen to consist of two irregular culmi-nations (A and B; Fig. 7b). Culmination A is by far the largest and hasan elongate asymmetric oval geometry with the long axis trending

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Fig. 4. Interpreted horizons used in this study. The seismic line in (a) shows the position of the mound at the mid-Miocene unconformity and above the polygonally faultedsuccession. To the west are the Central Graben, the Coffee Soil Fault and the nearby mid-Miocene pockmarks and vertically below the mound is the Siri Canyon with a local pinch-out shown. Some vertical disturbance in the succession between the mound and the Siri Canyon is indicated by this line. (b) and (c) show in detail the internal reflectionconfiguration of the mound fill. Four internal horizons were interpreted subdividing the mound into five units (I–V). Unit I is comprised between the base of the mound and theIntern 1 horizon, Unit II between the Intern 1 and 2 horizons and so forth. The reflection configuration shows an asymmetric, concentric build up directed NW-ward with downlapof the internal reflections onto the mound base. TMR¼ top mound reflection, BMR¼ base mound reflection. For location of seismic lines see Fig. 3b.

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Fig. 5. Two seismic lines showing some structures associated with M1. (a) An amplitude anomaly is seen right below M1 and vertical disturbance can also be seen. Furthermore themound is located above a local pinch-out in the Siri Canyon. The mound also causes a minor velocity pull up of about 10 ms TWT in the underlying succession (b), indicating that thevelocity within the mound is about 2.3 km/s compared to the general velocity of about 2 km/s in the surrounding mudstones. For location of seismic lines see Fig. 3b.

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70� (ENE–WSW). The asymmetric geometry is also defined by thesteep flank angles (15–20�) to the east and south, with more gentleflanks angle (around 5�) to the west and north (Fig. 7b). Culmina-tion A is 1400 m long and 800 m wide and reaches a maximumTWT thickness of 70 ms (Fig. 7g). Culmination B is smaller andlocated to the north of culmination A. It is also elongated in mapview and strikes east-west. In seismic cross-section, culmination Bis more symmetric than culmination A. Culmination B is 750 m longby 400 m wide and about 30 ms TWT high. In total the moundcomprises a volume of c. 5.3�107 m3, calculated based on themound isochron using SMT software.

4.3. Seismic reflection characteristics of the bounding surfaces

The surface defining the base of the mound (base moundreflection, BMR) is represented by a gently (0.6� present day) SW-ward dipping reflection (Fig. 7). It is characterized by a strongnegative amplitude (soft) reflection event and is generally spatiallycontinuous and concordant with the underlying reflections (Fig. 4).The base reflection is pervasively offset by the polygonal faults inthe underlying succession, and it is deflected downwards to forman elongate, curved depression centred along the line separatingculminations A and B. The base reflection is furthermore slightlydeflected upwards in the area where the mound is at its thickest(Figs. 4b and 7a). This upward deflection along with a small velocitypull up of about 10 ms TWT observed in the reflections below the

mound (Fig. 5b) may be explained by a relatively high-velocity fillof the mound causing velocity pull up where the mound is thickest.

No other similar mounded structures, or channels, or channel-like features spatially related to M1 have been observed along theBMR. Furthermore there are no indications of erosion nor haveother mounds of similar scale been found and the mound thusseems to be unique in this area. Only a NW–SE striking highamplitude area (Fig. 7c) represents an anomaly spatially related tothe mound. The anomaly has a characteristic triangular shape witha lineation continuing north-ward away from the mound. In addi-tion other closely spaced minor circular and/or elongated highamplitude anomalies are seen in the area east of a NW–SE trendingline through the westernmost parts of the mound (Fig. 7c). Some ofthese high amplitude patches are associated with minor local TWTthickening of the succession between BMR and TMR and this couldindicate the presence of smaller mound-like accumulations, whichwould be within the seismic tuning window (�10 m or so) andhence have a high amplitude expression.

Polygonal faults in the underlying succession extend upwardsand deform the mound. This is seen around the elongate depres-sion in the BMR (Figs. 4b and 7e). However, the configuration of thefaults directly beneath the mound is not significantly different tothat of the surrounding areas. It is possible that pervasive polygonalfaulting may, however, hamper the recognition of small irregular-ities genetically related to the mound.

No strong acoustic turbidity or deformed reflections character-istic of a ‘‘gas chimney’’ (e.g. Løseth et al., 2003) have been observed

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Fig. 6. TWT thickness map of the Siri Canyon (Rogaland Group). Localized depocenters represent the varying distribution of sand within the Canyon. The mound is seen to belocated vertically above the eastern pinch-out of a local depocenter, which probably correlates to the Ty Member (Schiøler et al., 2007). This depocenter is in connectivity withanother depocenter further to west which extends into the Central Graben.

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directly beneath the mound. Only subtle vertical disturbances andamplitude anomalies have been observed (Fig. 5a, see also Fig. 4a),possibly indicating acoustic distortion caused by the mound orsome sort of sub-vertical pathways for fluids mowing upwards.

The reflection defining the top of the mound (top moundreflection, TMR) is a prominent positive (hard) amplitude event,which is onlapped by overlying reflections (Figs. 4 and 8). The topreflection is most continuous at the northern and western flanks ofthe mound, while it is less prominent and more discontinuousabove the mound and along the eastern and southern flanks (Figs.4c and 8). The difference in seismic character correlates with thedip angles of the flanks; steeply dipping flanks correlate witha more discontinuous top reflection, whereas shallower dippingflanks correlate to more continuous reflections. The reason for thismay be depositional interaction between the sediments within andsurrounding the mound e.g. interfingering between the moundsediment and the encasing sediments (Fig. 8b). However, theseismic data used here is not of sufficient resolution to fully eluci-date these details. Amplitude variations in the top reflection aredominated by the same triangular pattern as the base reflection,but high amplitudes are also seen across the mound top (Fig. 7d).

The general seismic facies of the units above M1 is characterizedby parallel reflections and overall dimmer amplitudes than seenboth within the mound itself, and in the underlying, polygonalfaulted succession (Fig. 4). The reflections from strata immediatelyoverlying the mound display onlap on the mound flanks. Theonlapping reflections are slightly tilted away from the mound apex.Later deposited units drape the mound (Fig. 8a). As mentionedabove, an element of interfingering may have occurred at theeastern flank (Fig. 8b). Two horizons (Horizons 1 and 2) wereinterpreted in the overburden. Horizon 1 marks the transitionbetween reflections onlapping and reflections draping the mound.Horizon 2 marks the transition from draping to horizontal reflec-tions which are unaffected by the mound.

4.4. Reflection configuration of the mound interior

Where the mound is thickest several internal reflections, oftenhigh amplitude, can be seen within the mound (e.g. Figs. 4 and 7h).In contrast, where the mound is thinner, only one reflection ora chaotic internal reflection configuration is seen (e.g. Fig. 4).Polygonal faults are seen to deform both the internal reflectionsand the top of the mound (Figs. 4 and 8).

Reflections within the mound are generally sub-parallel and, tosome degree, concordant with the top reflection of the mound, assuch, these reflections progressively downlaps onto the basereflection towards the north and west, initiating from a point in thesouth-eastern part of the mound (Figs. 4 and 9a). This pattern isvery clearly imaged on a sequence of horizontal time-slices takenthrough the mound which shows a characteristic concentricpattern (Fig. 9c). Four internal horizons were interpreted sub-dividing the mound fill into five units (I–V) (Figs. 4 and 9). TWTthickness maps of these five units (Fig. 9b) show that the mound isprogressively built up from the SE towards the NW with youngerunits deposited progressively further to the NW (Fig. 9a and b).Units I and II are seen to have very limited symmetrical (circular)extent around the assumed starting point, while Unit III has a widerdistribution and extends to the north and NW. Unit IV is mainlydeposited west-ward of the centre of Units I and II, while Unit Vdrapes the entire mound. Units I–IV have similar thicknesses, whileUnit V is generally thinner but also the most extensive (Fig. 9b).

5. Discussion

The following discussion will focus on the lithology of themound as inferred from seismic observations, the deposition ofthe mound in relation to the embedding sediments and finally thepossible processes which may have interacted to form the mound.

Fig. 7. Figure showing the seismic characteristics of the top and base reflections of the mound. TWT structure of the mound base (a) and top (b) is shown and the downwarddeflection in the base horizon and the configuration of the two culminations are clearly visible. The amplitude maps of the base (c) and top (d) reflections show the triangularamplitude anomaly around M1 and the north-ward extension of this. The anomaly could be a sedimentological effect where sand from M1 is winnowed from and dumped adjacentto M1. In the base reflection (c) other small patches of high amplitude anomalies are seen generally east of a NW–SE trending line shown on the map. The coherency map of the basereflection (e) shows low coherence around the downward deflection but otherwise the mound does not separate strongly from the surroundings. The top reflection generally hashigh coherence (f) except for the area between the two culminations. (g) TWT thickness map of the mound; maximum thickness is 70 ms TWT. (h) RMS amplitude extraction offsetfrom the top and base reflection shows some variation in the internal amplitudes of M1.

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Fig. 8. Generally the reflection configuration between M1 and the overburden is a clear onlap (slightly tilted) onto the top mound reflection (TMR) (a) followed by draping of theentire mound starting at Horizon 1. However in some orientations (b) TMR is non-continuous which could indicate an interfingering relationship with the overburden. BMR¼ basemound reflection. For location of seismic lines see Fig. 3b.

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5.1. Lithology of the mound

No wells penetrate the mound and, therefore, no direct infor-mation (e.g. from cores or well logs) is available regarding thelithology of the mound. A number of approaches have been used toinfer the lithology, and they are discussed below. Interpretation ofpetrophysical logs in the nearest well, Elna-1, confirms the generallithology of the surrounding sediments with velocities around2 km/s and gamma readings typical of the mudstones above andbelow the mound (Fig. 2b), but otherwise gives no more specificinformation regarding the lithology within the mound.

A wide range of rock-types, including salt, mud, sand, gravel,igneous rocks and carbonates form mounded structures around theworld (Thomas and Connell, 1985; Hovland et al., 1994; Graue,2000; Davies et al., 2002; Nielsen et al., 2003; Hansen et al., 2005;Gwang et al., 2006; Cartwright et al., 2007; Frey-Martınez et al.,2007). Without direct evidence all may be considered candidatesfor the lithology of the observed mound. However, no igneousmaterial, apart from the widely distributed Paleogene tuffs (Heil-mann-Clausen, 1995), is described from the Cenozoic succession inthe eastern North Sea and therefore, an igneous origin for themound is discarded. Candidates for the mound lithology are thusthought to be of sedimentary origin.

The TMR and BMR both display high but not abnormal ampli-tudes indicative of moderately strong acoustic impedance contrastbetween the mound fill and the mud-rich sediments above and

below the mound. The TMR is a ‘‘hard’’ event, indicating an increasein acoustic impedance from the sediments above, while the BMR isa ‘‘soft’’ event, indicating a decrease in acoustic impedance down-wards into the sediments below M1. These relationships could becaused by either higher density and/or higher velocity within themound fill compared to the enclosing mud-rich sediments. Suchhigh impedance sedimentary materials (relative to the mudstones)may include sandstone, carbonates, and other more exotic lithol-ogies such as halite or igneous materials. Halite and igneousmaterials (as mentioned above) are considered unlikely in this areaand within this stratigraphic interval, which is dominated by sili-ciclastic sediments with minor carbonate stringers (Kristoffersonand Bang, 1982; Michelsen et al., 1998; Schiøler et al., 2007).

The reflections above the mound (i.e. TMR to Horizon 1) displayslightly inclined onlap onto the mound. Apparent ‘‘draping’’ ofcontinuous reflections is seen higher above the mound (i.e. Horizon1 to Horizon 2 (Fig. 8a)). These relationships can be explained bydifferential compaction between the mound and the encasingsediments. The clay-dominated Nordland Group would haveundergone a large degree of compaction during early burial, andthe seismic reflection configuration suggests that the mound itselfunderwent less compaction than the enchasing mudstones. Thissuggests a less compactable mound fill such as sand, cementedsediments or the presence of carbonates within the mound. Thedifferential compaction across the mound initiated as soon as coversediments were deposited onto the flanks of the mound. The

Fig. 9. (a) The internal parts of M1 are illustrated by TWT structure maps of the four internal horizons, (b) isochron maps of the five units, and (c) three time-slices through themound. The time-slices very clearly illustrate the asymmetric concentric geometry of the mound fill, while the TWT structure maps and the isochrons illustrate that M1 wasprogressively built up in a generally NW-ward direction emanating from a point located within the SE parts of the mound.

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influence of differential compaction decreased with time, and itceased at Horizon 2, which marks the transition to reflectionsunaffected by differential compaction across the mound.

As stated in Section 4.3, the seismic reflections below the mounddisplay about 10 ms TWT pull up effect (Fig. 5b), suggestive of a high-velocity mound fill. Using a background velocity of 2 km/s and a 10 mspull due to a structure with 70 ms TWT thickness, the mound velocitycan be estimated at c. 2.3 km/s. Such a velocity is characteristic ofslightly cemented sand whereas carbonates and salt usually havesubstantially higher velocities (Rider, 1996; Hilterman, 2001).

The discussion above thus concludes that M1 is a sedimentaryfeature and that the fill of the mound is of higher velocity, possibly

higher density material, and composed of less compactable mate-rial than the surrounding sediments. The most plausible explana-tion is that the mound is composed of sand rich, possibly slightlycemented material; the many high amplitude internal reflections ofthe mound may be the result of early diagenesis.

5.2. Timing of mound formation

The absolute age of M1 is Serravallian, but its exact age and theduration of mound formation are poorly constrained, mainly due tothe ambiguity of the relations with the surrounding sedimentswhich may be either onlapping, interfingering or both (Fig. 8). The

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onset of mound formation clearly post-dates the BMR, which seemsto coincide with the mid-Miocene unconformity dates at c. 15 Ma inthis area (Huuse and Clausen, 2001), and pre-dates the Horizon 1.The maximum duration of mound formation is obtained byassuming interfingering relations and constant sedimentation ratesduring the Serravallian in which case the Horizon 1 is c. 13 Ma(Fig. 2b) and the maximum duration is c. 2 Myr. The minimumduration of formation is given by the onlap-scenario in which all ofthe onlapping strata are younger than the mound fill. The absoluteminimum duration is however difficult to establish with certaintydue to seismic resolution limitations. Given the seismic resolutionof c. 10 m and the duration and thickness of the Serravallian of3.6 Myr and 175 m at the Elna-1 well location (Fig. 2b), one mayinfer a maximum duration of mound formation of c. 200 kyr(3.6 Myr� 10 m/175 m¼ 0.2 Myr), based on the assumption thatall of the seismically resolvable strata are onlapping the mound. Asstated, 200 kyr would be the maximum duration in the onlap-scenario and the true duration may be much shorter. The M1 hasa maximum height of 70 ms TWT which correlates to a maximumheight of 80 m when using the interval velocity of 2.3 km/s for thesediments within the mound. Therefore in the interfingeringscenario, the mound accretion rate would equate to some 40 m/Myr whereas the onlap-scenario implies an accretion rate of at least40 cm/kyr. Given the clear onlap on most sides of the mound andthe possibility of seismic interference phenomena causing apparentinterfingering with surrounding reflections, it is suggested that thebalance of the evidence is in favour of the onlap-scenario.

5.3. Origin of the mound

The reflection configuration within the mound i.e. the down-lapping internal reflections, radiating asymmetrically from a singlenucleation point defines a large NW-ward accreting sedimentarybody (Fig. 9, Section 4.4). Furthermore the external onlap onto theTMR strongly suggest that the mound was a significant topographicelement on the paleo-seafloor. A variety of geological processesmay result in mound shaped morphological features on theseafloor. These include: (i) iceberg dumping of sediments (Thomasand Connell, 1985), (ii) mud volcanism (Kopf, 2002; Hansen et al.,2005) and sand extrusion and shallow-level intrusion (Huuseet al., 2005; Hurst et al., 2006; Frey-Martınez et al., 2007; Pringleet al., 2007), (iii) carbonate mound growth (e.g. Hovland et al., 1994;Feary and James, 1995; Huuse, 1999), (iv) gravity flow deposition(Mitchum et al., 1977; Stow et al., 1996), and (v) contour currentdeposition (e.g. Viana et al., 1998; Faugeres et al., 1999; Stow, 2001).Iceberg dumping, carbonate mound accretion and mud volcanismcan be discarded based on the preceding discussion of thesystematically accreting, sand-prone mound fill, internal structureand sheer size.

Shallow-level forceful intrusions and near-surface sedimentremobilization are known to give similar mound shaped featureson the seafloor without sediment extrusion (e.g. Løseth et al., 2003;Hansen et al., 2005; Frey-Martınez et al., 2007). However thestratigraphic relations (basal downlap, external onlap), the internalstructure of M1, in contrary shallow-level intrusions often havemore scattered internal structures (e.g. Frey-Martınez et al., 2007;Hansen et al., 2005), and the lack of jacked-up reflections imme-diately above the mound, are more compatible with a depositionalfeature (i.e. extrusion rather than intrusion) and therefore shallow-level intrusion is not considered as a likely origin of the mound.

This leaves the options of gravity flow deposits, contourites andsand extrusions and these will be discussed further below. More-over a carbonate mound origin will be discussed in more detail. Inthis discussion, the uniqueness and the specific geographical and

stratigraphical location of the mound must also be accounted for bythe causal mechanism.

5.3.1. Gravity flow depositsThe concordant reflection pattern of the internal reflections of

the mound resembles geometries seen in gravity flow deposits suchas turbidites or debrites (Mitchum et al., 1977; Lowe, 1982).However, the steep flank angles (>15�), the paleogeographic loca-tion, lack of seismically resolvable supply system, and oval planform geometry seem incompatible with a turbidite origin.

A more cohesive debris-flow might have generated a moundstructure with steep flanks. Such an origin has been invoked forsteep-sided, internally layered sandstone bodies in the Tay Fansystem of the UK Central Graben (Jennette et al., 2000). Thesedeposits are rather similar in cross-section to M1 (cf. Fig. 13 ofJennette et al., 2000), suggesting a possibility of an overall simi-larity in origin. However, the Tay system is highly organised withclearly defined supply channels and fan lobes whereas M1completely lacks any such related features, thus making a debriteorigin difficult to argue.

5.3.2. Carbonate mound accretionSeismic scale biogenic carbonates may form mound-like struc-

tures with a concordant internal seismic reflection configuration ona flat base (Hovland et al., 1994; Feary and James, 1995; Huuse,1999; Nielsen et al., 2003). Their often asymmetrical geometryreflects the progressive build up or accretion of the carbonatemound. The development of reefs or bioherms is controlled bywater depth, nutrition level, and the amount of light and temper-ature in the water. Biogenic build-ups will often be directedupstream into any prevailing nutrition carrying current (Bjeragerand Surlyk, 2007). Typical growth rates of biogenic inducedcarbonate mounds are in the range of 10–200 cm/kyr (Schmid et al.,2001; Bjerager and Surlyk, 2007); therefore biogenic carbonategrowth is not incompatible with the relatively large dimensions(5.3�107 m3) of M1. Bryozoan carbonate mounds on a similar scaleto M1 have previously been described from the Danish North Sea,within the underlying, Danian part of the Chalk Group (Huuse,1999). The preceding discussion on mound lithology showed,however, that a carbonate composition for the mound is unlikely assuch a fill would be expected to cause more acoustic attenuationand pull up than is observed in the seismic data (e.g. Hovland et al.,1994). Additionally the M1 mound shows accretion towards theNW, which is inferred to be the downstream direction of the pre-vailing bottom currents in the mid-Miocene (Andresen et al., 2008).Finally, if M1 had been a biogenic build up it would be expected thatmore than one such structure would occur at the same stratigraphiclevel, which is not the case as far as the data coverage extends. Forthese reasons, a carbonate mound origin for M1 is rejected.

5.3.3. ContouritesContourites are sedimentary deposits formed under the influ-

ence of contour parallel currents. Contourites may be created bydirect submarine erosion of the seafloor and subsequent re-depo-sition (no sediment input from e.g. rivers or shorelines) or by re-deposition of sediments arriving from a source outside the area(e.g. rivers, unstable slopes or shorelines) for instance bottomcurrent reworked turbidites (Viana et al., 1998; Stow, 2001). Erosionat the base of the contourites takes place during both processesalthough the erosional imprint may not be located in the imme-diate vicinity of the contourite itself. Contour currents have previ-ously been invoked to explain elongate erosional features in theMiocene of the eastern North Sea (Huuse and Clausen, 2001;Andresen et al., 2008), and it is thus conceivable that contourcurrents may have played a role in the formation of M1. Contourites

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typically display an asymmetrical geometry reflecting the win-nowing and deposition under a prevailing current (Stow, 2001),often resulting in highly elongate mounds with long axes parallel tothe current (i.e. the contours). The elongation, the asymmetry ofthe internal geometry in the M1 (i.e. vertical and NW-wardmigrating accretion) and the north-ward elongated triangularamplitude anomaly seen in both BMR and TMR (Fig. 7c and d) couldthus resemble evidence of a somewhat ‘plump’ contourite althoughit may be more appropriate to consider the geometry as the resultof another primary emplacement mechanism affected by a pre-vailing current to the NW.

5.3.4. Sand extrusionSeveral examples of sand extrusions or sand volcanoes have

been described in the literature (Boehm and Moore, 2002; Huuseet al., 2005; Hurst et al., 2006; Pringle et al., 2007). These studiesillustrate that sand extrusion may produce mounded structures ofsimilar scale and with similar reflection configuration (or stratalarchitecture at outcrop), as the one observed in this study. For M1to represent a sand extrusion requires interaction with a suffi-ciently large parent sand body from where a high sand flux to thesurface could be remobilized. As previously mentioned the moundis underlain by about 1 km polygonally faulted mudstone overlyingthe sand-filled Siri Canyon, an extensive, compartmentalised andpervasively remobilised submarine sandstone fairway (Hamberget al., 2005, 2007). Core studies from wells in the Siri Canyon haveillustrated numerous sandstone intrusions, often observed alongthe upper boundary of the main sand body (Hamberg et al., 2005).Seismic studies have furthermore provided examples of upwardconcave saucer-shaped injectites within the Paleogene succession(Hamberg et al., 2007). The geographical location of M1 withrespect to the Siri Canyon, and the well-documented extensiveremobilization of the ‘‘Siri sands’’ make it relevant, therefore, todiscuss whether M1 could be a sand extrusion with the mound fillderived from the ‘‘Siri sands’’ located within the Siri Canyon. Suchan extrusion model could explain the uniqueness and specificposition of the mound.

However, some observations seem to argue against this model:(i) seismic sections do not show any obvious disturbance, otherthan slight velocity pull up, in the sedimentary succession betweenM1 and the Siri Canyon itself. Occasional amplitude anomalies(Fig. 5a) may suggest the presence of gas and/or cemented intervalswithin the intervening interval. (ii) The seismic data reveals nodistinct evacuation lows or depleted source areas. Whilst theabsence of conduit-related disturbances and evacuation areas mayseem to discredit the extrusion model, it should be noted that this isalso the case for many known clastic intrusions of km3-scaledescribed from the North Sea, where no such depletion areas orconduits have been documented, perhaps because the sand hasbeen sourced from a relatively large area of the parent sand body(e.g. Huuse et al., 2004, 2007). The absence of a conduit in seismicdata may also be due to inability to image sub-vertical structures ofrelatively small area extent (Huuse et al., 2007). Similarly for M1, itmay be argued that the lack of distinct depletion zones may beexplained by distributed sourcing of the extruded material and thegreat burial depth which would tend to make depletion areas lowamplitude and long wavelength and thus difficult to interpretunequivocally. This along with a high strength of the mid-Mioceneunconformity could also explain the relatively flat base and conicgeometry of M1 as compared to other known extrusion features e.g.mud volcanoes which often have biconic geometry due to differ-ential compaction and collapse of the underlying stratigraphy (e.g.Davies and Stewart, 2005).

Sand extrusions (sand volcanoes) are often seen to produceperfectly symmetric features (e.g. Pringle et al., 2007), but

secondary modifications e.g. erosion by seafloor currents canmodify the initial shape (hybrid sand volcano/contourite model). Asdiscussed previously some of the asymmetry of the mound may beexplained by bottom current modification. The steep flank angles tothe east and south may also be a result of current erosion or win-nowing of fines on the upstream side.

On the balance of the available observations, an origin as a sandvolcano is thus considered the most probable. Additional data suchas core and well log data directly through the mound could help toconstrain the mound fill lithology and hence the interpretation ofM1.

Large-scale sediment volcanoes require the remobilization oflarge volumes of sand and their displacement through the over-burden, a process which requires large volumes of fluid to drive thesediment expulsion (Maltman, 1994). Possible causes for genera-tion of the overpressure and fluid flow which is necessary forgeneration of such a large-scale sediment volcano are discussed inthe following.

Sediment remobilization is often suggested, although rarelyproven, to be associated with hydrocarbon migration (Jolly andLonergan, 2002; Duranti and Mazzini, 2005; Jonk et al., 2005). Theonset of gas maturation and migration has been independentlymodelled to occur during mid-Miocene time, thus coinciding withthe timing of mound formation. An additional source of fluid maybe provided by dewatering and volumetric contraction of thePaleogene to lower Miocene mudstones. This dewatering couldhave resulted in volume loss of as much as 10% and this processmay thus represent a significant source of fluids in sedimentarybasins where polygonal faulting occurs (e.g. Cartwright and Lone-rgan, 1996).

Another possible mechanism for generating the overpressure isby disequilibrium compaction and lateral transfer of pressure(Fig. 10a) (Swarbrick and Osborne, 1998; Yardley and Swarbrick,2000). During the middle Miocene, the Top Chalk Group surfacewas dipping gently to the west (Clausen and Huuse, 1999) and theoverburden thickness was greater towards the Coffee Soil Fault (SiriCanyon mouth, position X, Fig. 10a) than directly beneath M1(pinch-out of internal sand body, position Y, Fig. 10a). The differencein overburden thickness could have caused a lateral pressuregradient along the axis of the Siri Canyon leading to transfer ofpressure between locations X and Y and perhaps from locationsdeeper in the Central Graben across the Coffee Soil Fault. The lateraltransfer of pressure takes place through the ‘‘Siri sands’’ located inthe Siri Channel, because of the higher permeability in the sandscompared to the embedding clays. The lateral transfer of pressureupdip from position X to Y could have caused hydrofracturing andexpulsion of excess pressure (fluid) at Y, which in turn may havetriggered sand remobilization. This could then have resulted insand being injected upward and extruded at the surface thusleading to the formation of M1 as a sand volcano deposited froma fountain of sand extruded from c. 1 km beneath the paleo-seafloorunder the influence of NW-directed bottom currents (Fig. 10b). Theformation of the sand volcano would have continued until fluidoverpressure at Y had decreased sufficiently to allow flow rates tofall below those necessary for continued sand entrainment andtransport. The duration of mound formation is difficult to constraindue to limited stratigraphic and seismic resolution. However,assuming the onlap-scenario discussed above, it is likely thatformation of the entire mound took less than 200 kyr, and possiblya lot less. The number of expulsion events is equally difficult toascertain, although the mapping of 5 discrete layers of the extru-sion may provide a minimum estimate of expulsion pulses, perhapsseparated by background deposition or perhaps just diluted slightlybetween peak expulsion events. Assuming that M1 was exposed atthe seafloor for 2 Myr during deposition of the mMu-Horizon 1

Fig. 10. The formation of M1 can be divided into four stages (a–d). (a) Establishment of overpressure and remobilization of the Siri sand: during late Paleogene to early Neogenehydrocarbon migration from the Jurassic source rocks in the Central Graben started. The migration was primarily vertically directed and the Coffee Soil Fault probably acted as a sub-vertical escape conduit. Reaching the Siri Canyon mouth (point X) lateral migration updip through the sand bodies within the canyon continued along with vertical migrationthrough the Cenozoic succession (probably resulting in formation of the nearby mid-Miocene pockmarks). The lateral migration caused overpressure build up and probablyfacilitated remobilization at point Y. In addition to hydrocarbon migration the general west-ward increase in overburden thickness caused disequilibrium compaction and anassociated pressure gradient between X and Y. Lateral transfer of pressure from X to Y caused excess bleed off pressure at Y which also facilitated remobilization at this point. (b)Formation of the mound: from this unstable situation pressure equilibrium could have been established by extrusion of sand (and excess fluids) to the surface and the formation ofthe mound (sand volcano). Formation of the mound (<2 Myr duration) would continue until pressure was equilibrated and all excess fluid had bled off. The internal reflectionconfiguration suggests that the formation of the mound was a progressive build up with five units deposited above each other (extending further and further) from a nucleationpoint in the SE parts of the mound. Furthermore the NW-directed bottom current influenced the deposition giving a general NW-ward biased build up. (c) Onlap period: subsequentsedimentation caused onlap onto the top of the mound and eventually (at time of Horizon 1) embedded the mound entirely. (d) Burial of the mound: differential compaction beganafter deposition of Horizon 1 and increased with increased burial, giving rise to the observed slightly tilted onlaps and apparent draping. Polygonal faulting in the underlyingsuccession occurred both before and after mound formation and affected the present-day mound topography, but not its thickness distribution.

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interval there would have been ample time for re-deposition andwinnowing of the mound, perhaps stripping it of fines and perhapsallowing for some degree of interfingering. During this time, M1may have acted as the seafloor vent for any hydrocarbons migratinginto the Siri Canyon. This may have continued until the system wasfinally sealed by the overburden sediments or possibly by severingof the supply conduits by polygonal faults, which were active after(and before) the formation of M1, as seen by the offset of both BMRand TMR (e.g. Fig. 8). During and after formation, bottom current

winnowing and re-deposition may have controlled the distributionpattern and final geometry of the sand volcano (Fig. 10b). Eventu-ally the sand volcano would have been buried by the Serravallianmudstones (Fig. 10c and d).

6. Implications

The interpretation of the mound as a sand volcano impliesa major (5.3�107 m3) remobilization event from the Siri Canyon

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sands in mid-Miocene time in addition to the Late Eocene–EarlyOligocene event inferred by Hamberg et al. (2007), and also addsa mid-Miocene event to the general three Paleogene remobilizationevents in the North Sea previously described by Huuse et al. (2007).A mid-Miocene phase of remobilization and possible extrusion (ofUtsira sands) has also recently been inferred from the Tampen Spurarea of the North Viking Graben (Huuse, 2008). Together the twostudies suggest that the mid-Miocene was a time of significant sandremobilization in large parts of the North Sea, thus making it likelythat more injectites and extrudites of this age will be found in thefuture. This could potentially have implications for the use of theupper Miocene–Pliocene Utsira sand for CO2 storage.

The sand volcano identifies a fully linked injection/extrusionsystem and demonstrates that overpressure at the updip end ofa compartment within the Siri Canyon was sufficient to causeextrusion through about 1 km of overburden mudstones from theSiri Canyon to the mid-Miocene seafloor. This scale of sand injec-tion and extrusion is comparable to those required to feed conicalsand intrusion fields in the middle Eocene of the South VikingGraben (Huuse et al., 2004). Furthermore M1 is one of few exam-ples where interaction between the prevailing contour current andthe extruded parts of the system can be identified.

The implication of hydrocarbon migration and lateral transfer ofpressure through the Siri Canyon supports the notion of the canyonas an efficient carrier of hydrocarbons sourced from the CentralGraben (Hamberg et al., 2005; Ohm et al., 2006).

A sand volcano as the one observed could constitute a significanthydrocarbon reservoir if charged and sealed (Huuse et al., 2005;Hurst et al., 2006). This is, however, unlikely to be the case for thesand volcano described in this study as it shows no evidence forhydrocarbon charge at the present day. This may be due to long-termleakage from its top and due to supply cut off caused by polygonalfaulting, part of which occurred after mound formation (Fig. 10d).

7. Conclusions

- This study presents a large-scale (5.3�107 m3) sedimentmound (M1) found in the eastern North Sea.

- Given the available observations the most likely interpretationof the mound is a sediment volcano, but other origins have alsobeen considered such as contourites, gravity flow deposits andcarbonate mounds.

- Mound deposition took place under the influence of NW-directed bottom currents causing an asymmetrical plan formwith accretion biased towards the NW from a well definednucleation point.

- The most likely mound fill is sand derived from the PaleoceneSiri Canyon, c. 1 km beneath the mound. Other data such ascore or well log data through the mound would help toconstrain the interpretation of the mound fill and the origin ofM1.

- Sand extrusion was focused above an updip pinch-out of theSiri Canyon, thus explaining the location and uniqueness of themound.

- A mid-Miocene age and short (<200 kyr) duration of forma-tion is inferred on the basis of seismic stratigraphic correlationand interpretation of onlap relations on the mound top.

- Sand remobilization and injection/extrusion may have beencaused by hydrocarbon migration and lateral transfer of pres-sure from the Central Graben through the Siri Canyon fairway,although water expelled due to polygonal faulting may alsohave been involved.

- This study, as many others before, highlights the importance of3D seismic studies in identifying new geological phenomenaeven in heavily explored basins such as the North Sea.

Acknowledgements

The research is part of the PhD project of K.J. Andresen, which issponsored by the Faculty of Science, University of Aarhus. The workhas been carried out at Seislab Aarhus, where interpretation soft-ware is kindly provided by Seismic Micro-Technology. Mærsk Oiland Gas, GEUS, and DONG Energy are thanked for providing andsponsoring well data and seismic data. Erik Skovbjerg Rasmussen isthanked for providing absolute ages for key horizons and Jan Pio-trowski is thanked for a useful discussion regarding the origin ofthe mound. Finally we want to thank the reviewers: ChristopherJackson, Richard Davies and Jose Frey-Martinez for their construc-tive and very helpful reviews.

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