Deformation structures in organic-rich shales

Deformation structures in organic-rich shales

Helge L?seth,Lars Wensaas,and Marita Gading

ABSTRACT

Seismic images of Upper Jurassic organic-rich siliciclastic rocks are studied along the2000-km(1243-mi)Norwegian margin. These rocks are considered the main source rock for most of the large oil and gas fields in the North Sea and Norwegian Sea. We report characteristic seismic expressions of thin-skinned gravitational gliding structures that are strata bound to the organic-rich formations.The most characteristic structures are listric faults that offset and rotate the upper part and sole out near the base of the organic-rich zone.These may be present in large areas(10,000km2[3861mi2]),but are usually restricted to tilted areas.The strike of the faults was perpendicular to the downdip direction of the movement of mass,and fault direc-tions can therefore be used as paleodip indicators.Several types of contraction structures are observed,and all are formed at a maximum of a few hundred meters of burial.Although they are not limited to organic-rich shales,such strata-bound struc-tures may help identify organic-rich intervals in basins where their presence is unknown.

We suggest that adding organic material to clay leads to reduced https://www.wendangku.net/doc/842707955.html,paction-related vertical fluid flow may cause fluid overpressure to build up at the base of a less permeable organic-rich layer during early burial.This high fluid pressure zone becomes a low-friction decollement surface on which overlying sediments slide to form characteristic thin-skinned deformation structures.

INTRODUCTION

The primary targets for Norwegian wells6407/5-1and34/ 8-6disappointingly turned out to be organic-rich claystones (NPD,2010a).Well6407/5-1was designed to drill a Lower AUTHORS

Helge L?seth Arkitekt Ebbells Veg10, NO-7053Ranheim,Norway;heloe@https://www.wendangku.net/doc/842707955.html, Helge L?seth received his candidate scientific degree in structural geology from the University of Bergen,Norway,in1985.He worked for Total Norsk A/S and SINTEF(Stiftelsen for Industriell og Teknisk Forskning)Petroleum Research before joining Statoil in1997.In his present position as a specialist in structural geology,he works with petroleum exploration research on seismic inter-pretation of hydrocarbon leakage,sand injectites, and organic-rich claystones.

Lars Wensaas Arkitekt Ebbells Veg10, NO-7053Ranheim,Norway;lawe@https://www.wendangku.net/doc/842707955.html, Lars Wensaas received his candidate scientific degree in sedimentology from the University of Oslo,Norway,in1987.He joined Statoil in1994, where he is currently working as a principal researcher at the Statoil Research Centre in Trondheim,Norway.His research efforts have focused on rock physics,pore-fluid pressure, and sealing properties of argillaceous rocks in petroleum exploration.

Marita Gading Arkitekt Ebbells Veg10, NO-7053Ranheim,Norway;mgad@https://www.wendangku.net/doc/842707955.html, Marita Gading received her Ph.D.in seismic stratigraphy from NTNU(Norges Teknisk-Naturvitenskapelige Universitet,also Norwegian University of Science and Tecnology),Trondheim, Norway,in1994.She worked with seismic in-terpretation in regional geologic projects at SINTEF (Stiftelsen for Industriell og Teknisk Forskning) Petroleum Research from1992until1997when she joined Statoil and is currently working as a specialist in geophysics,at the Research Centre, Trondheim,focusing on rock properties and seismic interpretation of organic-rich rocks. ACKNOWLEDGEMENTS

We thank Statoil for allowing us to present the results of internal research.We also thank Statoil and partners in various licenses for permission to publish the seismic data.We thank Steven Davis, Peter W.Baillie,and an anonymous reviewer for their valuable comments on a previous version of the manuscript.

The AAPG Editor thanks the following reviewers for their work on this paper:Peter W.Baillie,J.Steven Davis,and Martin J.Evans.

Copyright?2011.The American Association of Petroleum Geologists.All rights reserved. Manuscript received March30,2010;provisional acceptance May24,2010;revised manuscript received July2,2010;final acceptance September27,2010.

DOI:10.1306/09271010052

AAPG Bulletin,v.95,no.5(May2011),pp.729–747729

Cretaceous submarine fan,but the well encoun-tered an allochthonous slide-block comprising Up-per Jurassic organic-rich shale.Similarly,there was no sand,but Upper Jurassic organic-rich claystone penetrated in the flat-topped structure tested by well34/8-6.Here,an interpreted listric intra–source rock fault appears to have rotated the beds.Seismic data reveal that similar thin-skinned deformation structures occur regionally,strata bound to the Upper Jurassic organic-rich claystone along the 2000-km(1243-mi)Norwegian margin.The struc-tures are found only in organic-rich claystones,and we question if the organic matter is the reason for their formation.If so,such structures may occur in any organic-rich shale,as they do in the organic-rich HRZ(highly radioactive zone)in Alaska(Homza, 2004).Although not limited to organic-rich shales, such strata-bound structures may help identify organic-rich intervals in basins where their pres-ence is unknown.

The observed structures on the Norwegian margin resemble gravitational gliding structures. Thin-skinned gravitational gliding structures form in sediments by a downdip movement of mass that slips along a low-friction zone that often is caused by a high fluid overpressure(Hubbert and Rubey, 1959;Weimer and Buffler,1992;Cobbold et al., 2004;Cobbold et al.,2009;Mourgues et al.,2009). The structures have an uppermost domain of down-dip extension,an intermediate domain of rigid glid-ing,and a lowermost domain of downdip contrac-tion(Cobbold and Szatmari,1991).Gravitational gliding structures are well documented on many scales from major deltas where several kilometer-thick sediment packages commonly glide on salt (Fort et al.,2004;Rowan et al.,2004;Loncke et al., 2006),overpressured shales(Weimer and Buffler, 1992;Cobbold et al.,2004;Cobbold et al.,2009, Mourgues et al.,2009)through major submarine slides(Bugge,1983;Bull et al.,2009),to thin slides that can be studied in outcrops(Martinsen,1989; Maltman,1992).

The gravitational gliding structures reported here are all strata bound to the Draupne Forma-tion in the North Sea(Vollset and Dore,1984; Keym et al.,2006),the Spekk Formation in the Norwegian Sea(Dalland et al.,1988),and the Hekkingen Formation in the Barents Sea(Dalland et al.,1988;Leith et al.,1993).The syntectonic organic-rich sediments are normally less than 100m(328ft)thick on platforms and on terraces but often more than300m(984ft)in the deeper grabens(Fraser et al.,2003).The total organic car-bon(TOC)content in both the Spekk and Draupne formations is normally less than10%,whereas it is as much as16to25%in the lower part of the Hekkingen Formation(Leith et al.,1993;Smelror et al.,2001;Bugge et al.,2002).The kerogen largely consists of amorphous debris with variable input of terrestrial plant material(Leith et al.,1993).The major oil fields in the North Sea and Norwegian Sea are mostly sourced from these organic-rich Upper Jurassic siliciclastic shales(Odden et al., 1998;Johannesen et al.,2002;Kubala et al., 2003).

In this study,we report seismic expressions of a variety of thin-skinned deformation structures found in the Upper Jurassic source rocks at six lo-calities along the Norwegian margin and discuss how and why they preferentially are formed in organic-rich sediments.

WELL AND SEISMIC DATA

We have studied the seismic expressions of the Up-per Jurassic organic-rich shales along the2000-km (1243-mi)Norwegian margin and selected six ex-amples showing various types of deformation struc-tures(Figure1).Zero-phased three-dimensional (3-D)seismic data of good quality were mainly used.Except for one area,wells are located within the3-D survey areas,and the well-seismic tie is considered good.In all areas,well-log data tie the top of the organic-rich shale to a reduction in acoustic impedance and the base to an increase in acoustic impedance,corresponding to the yellow trough and the blue-turquoise peak on the seismic images, respectively.The depths in the interpreted profiles are calculated using an average velocity similar to that found in the organic-rich claystone in the nearest well(Table1).The stratigraphic Jurassic and Cretaceous nomenclature is shown in Figure2.

730Deformation Structures in Organic-Rich Shales

Figure1.(A)Structural index map from NPD(2010b)showing positions of the study areas:(B)Pancake Basin,northern North Sea;

(C)Norwegian Sea with four study areas:southeast Halten Terrace,northern Halten Terrace,Helgeland Basin,D?nna Terrace(Blystad et al.,1995);(D)Hammerfest Basin,Barents Sea(Gabrielsen et al.,1990).The inserted maximum trough amplitude map shows the east-west–striking fault and high-amplitude(yellow)top source rock reflections.

L?seth et al.731

SEISMIC OBSERVATIONS OF DEFORMATION STRUCTURES IN ORGANIC-RICH CLAYSTONE Listric Faults

On the northern Halten Terrace,Norwegian Sea,well 6506/12-7on the ?sgard field penetrated 72.5m (236ft)of the Spekk Formation.Here,the Spekk Formation is relatively flat and both the top and base source rock reflections are smooth and continuous.However,6km (3.7mi)northeast of this well,the thickness of the Spekk Formation has increased to 100to 110m (328–361ft)before dip-ping into a graben.A few kilometers along the southern flank of the graben,the top source rock reflection is segmented whereas the base reflection is continuous (Figure 3).The top Spekk Formation reflection in the individual segments describes con-tinuous surfaces where the dip varies from one segment to the next.The offset of the top Spekk Formation reflection between the segments nor-mally does not exceed 50ms (two-way traveltime),and the rotation is normally less than 10°.The first good intra-Cretaceous seismic reflection,approxi-mately 100ms above the top Spekk Formation reflection,is not segmented.On amplitude maps,bounding faults between the segments can be fol-lowed as low-amplitude zones that strike subpar-allel to the graben.Many of the faults are only approximately 1km (0.62mi)long.Individual seg-ments are normally less than 700m (2297ft)wide.We interpret these structures to be listric faults that offset and rotate the upper part and sole out in the lower part of the Spekk Formation.

In contrast to the Halten Terrace,where no in-ternal faults are observed in the source rock that is lying flat,the Spekk Formation on the Tr?ndelag platform,160km (99mi)to east-northeast,is per-vasively segmented into numerous fault blocks.An edge detachment map from 3-D seismic data images the strike and distribution of the faults (Figure 4A ).Each segment is separated by normal faults,and the top reflection is tilted.The offsets between segments are normally 20ms (two-way traveltime)or less.Here,400-m to 1000-m (1312-to 3281-ft)-wide segments are common where the fault spacing is dense.Locally,the faults show no

T a b l e 1.S o u r c e R o c k D a t a C o l l e c t e d f r o m W e l l s C l o s e t o o r o n t h e S e i s m i c S e c t i o n s *

F i g u r e

L o c a t i o n /S t r u c t u r e

B l o c k (s )

S o u r c e R o c k (S R )F o r m a t i o n

N e a r e s t W e l l (d i s t a n c e )

S R T h i c k n e s s i n W e l l

S R A v e r a g e V e l o c i t y

i n W e l l

T O C i n W e l l M a x i m u m /A v e r a g e

M a t u r a t i o n (V R )

3

N o r w e g i a n S e a /H a l t e n T e r r a c e n o r t h 6506/12

S p e k k

6506/12-7(6k m [3.7m i ])72.5m (238f t )

2770m /s (110m s /f t )

/6.0–6.5(0.7–0.85)

4N o r w e g i a n S e a /H e l g e l a n d B a s i n 6609/9,6610/7,6609/12,6610/10S p e k k

6610/7-1(20k m [12.4m i ]N o f B )44m (144f t )

2650m /s (115m s /f t )

14.5**/

I m m a t u r e

6,7,8N o r t h S e a /P a n c a k e B a s i n 34/8D r a u p n e 34/8-6(0k m )104m (341f t )2770m /s (110m s /f t )/5–6E a r l y m a t u r e (0.8)?

9B a r e n t s S e a 7121/7H e k k i n g e n 7121/7-2(0k m )74m (243f t )2792m /s (109m s /f t )10.5/2–10.5I m m a t u r e (0.6)10,11N o r w e g i a n S e a /H a l t e n T e r r a c e s o u t h 6407/5

S p e k k

6407/5-1(0k m )

S l i d e :25m (82f t );i n s i t u :248m (814f t )S l i d e :2960m /s (103m s /f t );i n s i t u :3012m /s (101m s /f t )S l i d e :/4.6**;i n s i t u :3–5/7I n s i t u :M a t u r e (0.7–1.1)

12,13

N o r w e g i a n S e a /D ?n n a T e r r a c e

6507/3S p e k k

6507/3-3(0k m )

80m (262f t )

2596m /s (117m s /f t )8.3/6.1–8.3

I m m a t u r e (0.45)

*S R =s o u r c e r o c k ;T O C =t o t a l o r g a n i c c a r b o n ;V R =v i t r i n i t e r e f l e c t a n c e .**F r o m s i d e w a l l c o r e .?M e a s u r e d 25m (82f t )a b o v e t h e t o p o f t h e D r a u p e F o r m a t i o n .

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Deformation Structures in Organic-Rich Shales

preferred strike direction,and the pattern resembles that of polygonal faults (Cartwright and Dewhurst,1998).In most areas,the longest intra –source rock faults have a preferred strike direction,with shorter connecting faults striking perpendicularly (±30°).The most persistent parallel intra –Spekk Forma-tion fault direction is observed in a 3-km (1.9-mi)-wide belt at the crest of the footwall of a larger basement-involved fault (Figure 4B ).The Spekk Formation thins toward the edge of the footwall block.Intra –source rock faults in the associated down-faulted grabens strike nearly perpendicu-larly to the basement-involved faults.Such a fault pattern is only observed where the Spekk Forma-tion and the overlying Cretaceous unit are signif-icantly thicker in the graben than on the footwall block,showing active basement-involved faulting during and immediately after deposition of the Spekk Formation.We interpret that the segments in the Upper Jurassic Spekk Formation are sepa-rated by listric faults that offset and rotate the up-per part and detach close to the base of the Spekk Formation.Part of the source rock thinning ob-served on the footwall block of the basement-involved faults may be fault-related thinning and is therefore postdepositional (Figure 5

).

Figure 2.Lithostratigraphic scheme for the studied Upper Jurassic source rocks and the main surrounding formations and groups as defined by Vollset and Dore (1984),Dalland et al.(1988),Isaksen and Tonstad (1989),and Keym et al.(2006).The dotted line indicates the approximate position of the decollement surface.

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From Listric Normal Faults to Contraction Structures

Well 34/8-6is located on the eastern flank of the Pancake Basin in the Tampen Spur area,North Sea,between the crests of the rotated Jurassic fault blocks of the Visund and Snorre fields (Figure 6).As mentioned in the Introduction,the primary well target was to test hydrocarbon-bearing sands within a flat-topped top Draupne Formation structure (Figure 7).Therefore,a 12-m (39-ft)core was col-lected from the lower part of the 104-m (341-ft)-thick Draupne Formation.No sand was encoun-tered in the Draupne Formation,but deformation structures were observed in the lower 2.5m (6.6ft)of the core (Figure 8).Here,the laminae are dipping between 0and 35°,shifts in dip occur repeatedly over 10-cm (3.9-in.)intervals,and folded and trun-cated laminae are observed locally.No slickensides were identified.In contrast,the laminae are almost lying flat and parallel with no sign of disturbance in the overlying 9.5m (29.5ft)of the core.Similar to the two seismic examples previously described,the top source rock reflection is segmented whereas the

base reflection is more continuous.From the seis-mic data,we interpret listric faults that offset and rotate in the upper part and detach close to the base of the Draupne Formation.The interpreted sole-out level for one of the listric faults coincides with the position of the deformation structure in the core.The absence of slickensides in the core sug-gests that the clays were not lithified when defor-mation took place.Also,5km (3.1mi)downdip from the well,we observe a segment transition with apparent reverse fault movement.In contrast to the listric normal faults with lateral extent as much as 3km (1.9mi),this reverse fault can be followed continuously for 15km (9.3mi)in the lower part of the Pancake Basin (Figure 6B ).

Subtle top Hekkingen Formation deformation structures were observed on the seismic data above the Sn?hvit field in the Barents Sea (Figure 9).The amplitude of the top source rock reflections is quite high at the crest of the east-west –trending footwall,where well 7121/7-2penetrated 74m (243ft)of the Hekkingen Formation.The amplitude of the reflec-tion decreases southward on the tilted fault block as the rugosity increases.The base of the

Hekkingen

Figure 3.(A)Seismic expression of the Upper Jurassic Spekk Formation from a full-stack seismic section located 6km (3.7mi)northeast of the Sm?rbukk discovery,Norwegian Sea.The top and base source rock is confirmed by good well ties.We interpret listric faults that offset and rotate the upper part and sole out in the lower part of the Spekk Formation as shown in the interpreted depth profile (B)(2:1vertical exaggeration)to explain the segmented top Spekk Formation reflection and the continuous base Spekk Formation reflection.(C)Amplitude map extracted from the top Spekk Formation reflection.Faults,expressed as low-amplitude (black-blue)lineaments,are located at the southern flank of the graben.

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Deformation Structures in Organic-Rich Shales

Formation reflection describes a smooth surface in the entire fault block.The rugose character of the top Hekkingen reflection is interpreted to be formed by a combination of updip listric faults and downdip compression folding.The listric faults are inter-preted to detach near the base of the organic-rich Alge Member and offset the top of the organic-rich succession.The downdip moving sediments must have been shortened by folding as further south-ward movements were stopped at the footwall of the next fault block.Possibly,also some of the sed-iments in the footwall,including the fault zone itself,may have had slight deformation.Slide and Glide-Out Blocks

Well 6407/5-1was surprisingly drilled through 50m (164ft)of allochthonous Spekk (25m [82ft])and Lyr (25m [82ft])formations in autochtho-nous Aptian claystones.The well is located in the Gimsan Basin in the southeastern part of Halten Terrace,Norwegian Sea,which was downfaulted from the Tr?ndelag platform along the Bremstein fault complex during Late Jurassic and Late Cre-taceous (Figure 10).The lowermost part of the Spekk Formation is older (late Oxfordian)in the autochthonous succession than in the allochthonous part (late Kimmeridgian)according to the biostrati-graphic ages in the completion log.The youngest part of the allochthonous succession is the Barre-mian Lyr Formation.The claystones immediately below and above the allochthonous succession are early and late Aptian,respectively.Well 6407/5-1ties the top of the allochthonous Spekk Formation to a high-amplitude trough reflection (Figure 11).The lateral extent of these high-amplitude reflec-tions has been mapped over a 150-km 2(58-mi 2)area.The bright amplitude segment onlaps the autochthonous top Spekk Formation reflection to the east along the Bremstein fault complex

and

Figure 4.(A)Edge detection map extracted from the top Spekk Formation reflection in the Helgeland Basin.Black corresponds to detected fault edges and illustrates the strike and distribution of faults.The intra –source rock faults have a preferred northwest –southeast to north-northwest –south-southeast strike in most of the area.Seismic section (B)(interpretation on depth profile C)is located on the footwall block of a basement-involved fault that terminates southwestward at the green arrow in (A).The intra –source rock faults strike parallel to the basement-involved fault on the footwall but perpendicularly to it in the downfaulted grabens (blue arrows in A).Seismic section (D)(interpretation on depth profile (E)shows listric normal faults strata bound to the Spekk Formation in an area where the fault pattern resembles that of polygonal faults.

L?seth et al.

735

terminates quite abruptly as much as 10km (6.2mi)west of the onlap.The bright amplitude area is segmented.Some segments are as much as 2km (1.2mi)long;most are smaller and their long axes are elongated parallel to the Bremstein fault com-plex.The dips of individual segments show local variations.We interpret that a slide moved along the base of the Spekk Formation from its former au-tochthonous position in the Bremstein fault com-plex.When the movement of the mass started,the front ramped up at the seabed and continued down-dip until it reached the flat area,where the speed

was reduced and it came to rest.The gliding block broke up into segments,with long axes perpendic-ular to the direction of motion.In parts of the slide area,the frontal slide blocks first came to rest and the succeeding slide blocks collided and were ramped up onto the block in front.This took place during the mid-Aptian period,more than 20m.y.after the Spekk Formation initially was deposited.We speculate that the slide was triggered by in-creased dip at the base source rock surface because of fault movements along the Bremstein fault

complex.

Figure 5.Schematic evolution of the lis-tric faults at the footwall block in Figure 4B .(The main stresses are labeled s 1(ver-tical)[largest],s 2,and s 3.)(A)The highest fluid overpressure builds up at the base of the organic-rich unit.The horizontal stresses in the organic-rich unit are almost uniform (s 2≈s 3).(B)Basement faulting exposes the headwall and resets the horizontal stress so s 1>s 2>s 3,causing the sediments above the low-friction base to move toward the exposed head-wall.(C)Listric faults,which sole out in the decollement zone,form because of lateral movements of mass.At the ex-posed headwall,the loosely compacted organic-rich sediments decompose as they glide down into the graben.(D)Lateral movements continue until the sediments stabilize.A fault-related thinning of the source rock layer can be observed on the footwall block.

736

Deformation Structures in Organic-Rich Shales

Farther north on the mid-Norwegian shelf,in Block 6507/3,we observed stacked segments of high-amplitude reflections in the Cretaceous succession (Figure 12).They are located along the steeply dipping Revfallet fault complex.The D?nna Terrace was downfaulted from the Nord-land Ridge along this zone during the Late Jurassic and Late Cretaceous.The segments have not been penetrated by wells,but 80m (262ft)of Spekk Formation was penetrated in the nearest well (well 6507/3-3on the D?nna Terrace).The high-amplitude top Spekk Formation reflection is con-tinuous on parts of the D?nna Terrace but is seg-mented where it is dragged along the Revfallet fault complex.The segments are commonly less than 1km (0.6mi)wide.Individual segments can be traced along the fault complex where they curve and split and they often stack on top of each other (Figure 13).On single sections,the segments could easily be interpreted as fans of coarse clastic mate-rial building out into the basin west of the Nordland Ridge.On the 3-D seismic data,we can observe how the top Spekk Formation surface was contin-uously dragged up along the Revfallet fault com-plex in one area and split into segments along the strike.Therefore,we interpret that slices of Spekk Formation glided down the dipping slope along the Revfallet fault complex.During the late Jurassic period,the eastern edge of the D?nna Terrace was located east of its present position.The Nordland Ridge migrated westward during Cretaceous fault-ing,and the thick Spekk Formation was

incorporated

Figure 6.(A)Seismic section from the Visund field in the east,through location of wells 34/8-6and 34/7-15S in the Pancake Basin to the Snorre field in the west.(B)Edge detection map at the top of the Draupne For-mation.Several short faults strike perpendicularly to the west-northwest –dipping eastern flank of the Pancake Basin.An inter-preted thin-skinned thrust fault strikes continuously through the lower part of the basin (green arrows).

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in the steeply dipping Revfallet fault complex where segments glided out.

DISCUSSION

The presented structures are strata bound to the organic-rich clay stones,and the decollement zone is located near the base.The examples reveal that gravitational gliding structures formed when the organic-rich clays were buried at a maximum of a

few hundred meters,and therefore,the degree of compaction was low.This is based on (1)only 25m (82ft)of Lower Cretaceous allochthonous sedi-ments covering the Spekk Formation in the slide block in the southern Halten Terrace (Figure 11);(2)the listric fault terminating upward imme-diately above the top of the source rock reflection (Figures 3,4,7);and (3)the cored deformation zone in which no slickensides were observed and therefore must have formed before the sediments were lithified (Figure 8).Also,the sediments in all the observed structures appear to have

moved

Figure 7.(A)Seismic section through well 34/8-6on the eastern flank of the Pancake Basin (location in Figure 6).(B)Interpreted depth profile of section (A).Listric faults offset and rotate the top and detach close to the base of the Draupne Formation.In the western part of the section,we interpret a thrust fault (green arrow).(C)The oblique dipping seismic cube seen from north-northeast,which is sculpted along the top Draupne Formation horizon,reveals the flat-top structure that was the target of the well.The sculpted surface is colored by root-mean-square amplitudes.Same color scale as in (A).(D)Gamma-ray log from the Draupne Formation in well 34/8-6(depths in meter).(E)Core log showing deformation structures in the lower 2.5m (8.2ft)of the core.Positions of core photos in Figure 8are indicated.

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Deformation Structures in Organic-Rich Shales

downdip,parallel to s 3.The decollement zones of most of the observed structures were dipping with very low angles when deformation took place,in-dicating a zone of low friction.We questioned if

there were significant differences other than a high organic content in the strata-bound layer compared with the underlying claystones but found none.X-ray diffraction analyses from organic-rich and underlying nonorganic (or low-organic)claystones are mineralogically very similar (Smelror et al.,2001;Langrock and Stein,2004).Subsidence rates and tectonic activity also appear to be relatively similar.Therefore,we first discuss why adding or-ganic matter to clays can cause the formation of gravitational gliding structures.Then,the observed structures are compared with other types of grav-itational gliding structures before we discuss why such structures have been misinterpreted as plays.

Relation between Organic Content,

Overpressure,and Formation of Gravitational Gliding Structures

Generally,organic matter is found distributed as either well-individualized particles,such as ligneous continental fragments and planktonic algal bodies,or as diffuse unfigured organic matter in organic-rich clays (Bertrand et al.,1990).Together with wa-ter,salt,and free hydrocarbons,the organic matter fills the pore volumes between the solid grains.Booth and Dahl (1986)added increasing amounts of organic matter to fine-grained sediments and studied the changes in the physical properties.There was an increase in the liquid and plastic limits,which provide a measure of the ability of a sediment par-ticle to attract and retain water on its surface,when organic content was increasing while the grain spe-cific gravity decreased.Keller (1982)analyzed organic-rich (2–20%TOC),fine-grained sediments offshore Peru on or just below the sea floor and found similar relationships as Booth and Dahl (1986).The highly organic-rich Peru samples showed that organic matter significantly alters the geotechnical properties and stability characteristics of the sedi-ment,that is,very high water content (as much as 853%of the dry weight of the sediment),low bulk density,high natural shear strength,and low re-molded strength,giving unusually high sensitivity and a very high overconsolidation ratio (i.e.,a mean of 13compared with 1–4in nonorganic

sediments).

Figure 8.Panel A is taken from the upper 9.5m (31ft)of the core in well 34/8-6.The laminae lie almost flat and parallel,with no signs of disturbance.Panel B is from the lower 2.5m (8.2ft)of the core.No slickensides but disrupted laminations with folding,truncation,and repeated shifts in lamina are observed.Core photos are also available on Norwegian Petroleum Direc-torate Web site (NPD,2010a).

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739

Organic matter forms gel complexes with physico-chemical coupling between organic matter and mineral surfaces (Pusch,1973),which give the organic-rich sediment a shear strength that is higher than normal.Similar to smectite (Mondol et al.,2007),organic matter holds a lot of bound water in the sediments.If the several orders of magni-tude lower permeability in smectite-rich clay rela-tive to illite rich clay (Mondol et al.,2007)is related to the bound water,it is possible that organic-rich sediments with large amounts of bound water also have significantly lower permeability relative to equivalent nonorganic sediments.But why should high fluid pressure build up at the base of a low-permeability layer?

The most significant compaction-related po-rosity loss occurs during early burial (Baldwin and

Butler,1985;Mondol et al.,2007).The excess fluid will move upward through the pores of the sedi-ments at a steady rate if the permeability is suffi-cient and the (nonhydrostatic)pressure gradient is constant.This seeping fluid will transmit forces to every solid grain (Mourgues and Cobbold,2003).Mourgues and Cobbold (2003,p.79)state,“De-spite its name,the seepage force does not depend on fluid velocity through the porous medium,but only on the gradient of fluid pressure.For a given pressure gradient,each grain will be subject to an invariant seepage force no matter what the values of permeability or fluid viscosity are.”Laboratory experiments show that if a low-permeability layer is added to the sediment column,the highest fluid overpressure will build up and survive at the base of this low-permeability layer (Mourgues

and

Figure 9.(A)Seismic section showing the expression of the Hekkingen Formation source rock above the Sn?hvit field,Barents Sea.The top Hekkingen Formation reflection has high amplitudes near the footwall,where it is smooth.Downward in the graben,the surface becomes more rugose and the amplitudes are dimming (Figure 1D ).The downward increasing gamma-ray log readings (red curve)correspond to increasing total organic carbon content in the Hekkingen Formation.(B)We interpreted listric faults in the middle part of the section and folding in the lower southern part of the graben (scale ~2:1).

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Deformation Structures in Organic-Rich Shales

Cobbold,2003;Mourgues and Cobbold,2006;Cobbold and Rodrigues,2007).

Based on these arguments,we suggest that or-ganic matter added to a clay matrix will cause a sig-nificant reduction in permeability.Excess water released because of compaction will seep upward through the sediments and enter the base of the low-permeability organic-rich layer,where high fluid pressure can build up if the low permeability does not allow enough water volume to pass.A fluid pressure gradient will build up through the organic-rich layer such that the fluid pressure will be closest to the lithostatic pressure near the base (Figure 14).This high fluid pressure will reduce the grain-to-grain friction because parts of the overburden weight are carried by the fluid pressure.Therefore,a decolle-ment zone may form at the base of organic-rich clay layers.Anisotropic horizontal stress,either caused by tilted layers or faulting-related stress release,can result in the lateral movement of sediments above the low-friction zone and the formation of the observed structures.

Comparing Strata-Bound Deformation

Structures in Organic-Rich Clays with Other Related Structures

Discriminating strata-bound structures formed in organic-rich clays during early burial from other simi-lar structures can be challenging.In major deltas,sev-eral kilometer-thick sediment packages often glide on salt (Fort et al.,2004;Rowan et al.,2004;Loncke et al.,2006)or overpressured shales (Weimer and Buffler,1992;Cobbold et al.,2009;Mourgues et al.,2009).One such slide is even interpreted to have a decollement zone in organic-rich shale where hy-drocarbon generation is assumed to have generated fluid overpressure (Cobbold et al.,2004).These types of movement of mass are much thicker than those strata bound to organic-rich clays,which sel-dom are more than 200m (656ft)thick.

Submarine slide structures resemble some of the structures described in this article.In the sub-marine Storegga slide offshore Norway,which af-fected an area of 90,000km 2(34,749mi 2),

the

Figure 10.(A)The green area shows the extent of the 150-km 2(58-mi 2)large slide in the Halten Terrace,Norwegian Sea.The dotted line shows the southern limit of 3-D seismic data,and a grid of 2-D seismic lines has been used south of this area.The locations of seismic sections in Figures 10C and 11A,C are indicated.(B)Part of the stratigraphy penetrated by well 6407/5-1.The 50-m (164-ft)-thick allochthonous slide block consisting of Spekk and Lyr formations is indicated.(C)The seismic 2-D section shows the structural position of the slide block on the downfaulted Halten Terrace and the assumed preslide location.

L?seth et al.

741

decollement zone was located as much as 700m (2297ft)below the paleoseabed.In the decollement clay zone,an excess pore pressure ratio of 0.9or higher was interpreted (Bryn et al.,2005;Kvalstad et al.,2005).Smaller scale submarine slides in the Norwegian fjords also appear to have slid on over-pressured clay zones (L ’Heureux et al.,2009).In contrast to organic-rich slides,some submarine

slides often glide on more than one surface (Kvalstad et al.,2005;Bull et al.,2009;L ’Heureux et al.,2009)but can otherwise be comparable in both size and structure.

In some areas,the fault pattern seen in the Helgeland Basin (Figure 4)resembles that of polyg-onal faults (Lonergan and Cartwright,1999).Al-though faults in organic-rich clays occur

strictly

Figure 11.Seismic sections (A,three-dimensional data;C,two-dimensional data)with interpreted depth profiles (B,D;~2:1vertical exaggeration),respectively,from the southeastern part of Halten Terrace.Locations are given in Figure 10A .The top of the highly radioactive Spekk Formation (red curve in A)in Well 6407/5-1ties to the bright amplitude interpreted as a slide block.This bright-amplitude anomaly was interpreted as Cretaceous sand and was the main target for the well.The slide consists of several separate segments.Identification of such segments can discriminate a slide block from sandy fan deposits.

742

Deformation Structures in Organic-Rich Shales

strata bound,the polygonal faults are located in tiers where the upper and lower stratigraphic levels of fault termination sometimes vary from one fault to the next.Polygonal faults are developed through-out a basin,also in the flat-lying parts,whereas faults in organic-rich sediments mainly occur in tilted re-gions.Polygonal faults often have a random fault pattern,but dominant strike directions are also re-ported (Clausen et al.,1999).A parallel fault pat-tern on the footwall of deeper basement-involved faults is typical for organic-rich clays (Figure 4B ).

We consider the strata-bound listric faults that sole out near the base of the organic-rich zone to be the most characteristic intra –source rock defor-mation structure.These listric faults may occur widespread over large areas as in the Helgeland Basin,where they occur more or less continuously over a 10,000-km 2(3861-mi 2)area (Figure 4),but most commonly,they develop in more restricted areas (Figures 3,7,9).The downdip movement of mass causes the strike of the faults to be perpen-dicular to the dip of the basin,and fault directions can therefore be used as paleodip indicators.

The higher the fluid pressure,the lower the dip angle that is required for sediment slip (Kvalstad et al.,2005).We speculate that increasing

TOC

Figure 12.(A)Well 6507/3-3is located on the D?nna Terrace,Norwegian Sea.The location of the seismic section is shown on the top Spekk Formation time-depth index map (red =3.3s,blue =2.0s).The high-amplitude top Spekk Formation reflection describes a continuous surface on the D?nna Terrace but is segmented along the steeply dipping Revfallet fault complex.(B)The segments are interpreted as glide-out Spekk Formation segments,which stack on top of each other.

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743

content will reduce the permeability in clays.The highest fluid pressure will therefore build up at the base of the most organic-rich layer,given otherwise comparable conditions.This may explain why per-vasive faults developed in the Helgeland Basin,where the TOC content is as much as 14.5%com-pared with the Halten Terrace,where the TOC content is around 6to 7%,and more restricted zones of intra –source rock faults are https://www.wendangku.net/doc/842707955.html,teral basinward shifts of layer-bound fault patterns in source rocks may therefore be linked to the rich-ness of organic

clays.

Figure 13.The extracted yellow and orange top Spekk Formation trough reflection is shown within the yellow-framed box.The segments can be traced from the seismic section and along the Revfallet fault complex,where they curve and

split.

Figure 14.Schematic illustration explaining why the decollement zone forms at the base of an organic-rich layer.Fluids flow vertically through (1)nonorganic claystone and (2)organic-rich claystone of lower permeability,here represented by electron microscope images from Heather Formation and Kimmeridge Clay,respectively (dark mass in the former is organic matter,white line is 100m m,both images are on a similar scale).The high flow rate causes fluid overpressure to build up at the base of a less permeable layer.Fluid overpressure decreases linearly downward through a less permeable layer.The decollement zone develops where grain-to-grain friction is lowest,which is at the base of a less permeable layer,where fluid overpressure is closest to lithostatic pressure.744

Deformation Structures in Organic-Rich Shales

Discriminating Strata-Bound Organic Structures from Gas-Charged Sands

As mentioned in the Introduction,both wells6407/ 5-1and34/8-6had primary targets that turned out to be organic-rich claystones.A source rock slide block may indeed look similar to a deep-marine fan at first glance on seismic data.The reduced acoustic impedance contrast at the top of the source rock interval may be mistaken for the top of a gas-filled sand reservoir(Figures10,11).In contrast to sub-marine sand deposits,organic-rich slides comprise several individual blocks that are expressed on seis-mic data as segments with the long axis perpen-dicular to the downdip gravitational gliding direc-tion.Moreover,stacked segments are common in the frontal part of organic-rich slides,whereas in-ternal channellike depositional patterns are not ex-pected.The particular type of glide-out structures interpreted in Figure12could easily be interpreted as fans of coarse clastic sands.On3-D seismic data, it may be possible to laterally trace the top source rock reflection from nonfaulted areas and into seg-ments.The observations and arguments presented in this article have documented strata-bound struc-tures in the organic-rich claystones.This knowledge improves our ability to interpret particular features such as the flat-topped zone in Figure7as defor-mation structures and not as primary depositional features.

CONCLUSIONS

This study describes characteristic seismic expres-sions of thin-skinned gravitational gliding structures observed strata bound in organic-rich siliciclastic rocks along the2000-km(1243-mi)Norwegian margin.Strata-bound listric faults that offset and rotate the upper part and sole out near the base of the organic-rich zone are the most characteristic structures.They may be present over large areas as in the Helgeland Basin(10,000km2[3861mi2]),but usually,they develop more in restricted locations in tilted areas or close to basement-involved faults. The strike of strata-bound faults is perpendicular to the downdip direction of the movement of mass,and fault directions can therefore be used as paleo-dip indicators.Several types of contraction struc-tures are observed:(1)thin-skinned thrust faults, (2)small-scale folds with an irregular rugose top source rock reflection,(3)allochthonous slides comprising elongated segments and frontal stack-ing,(4)stacked glide blocks.All reported structures were formed at a maximum of a few hundred me-ters of burial.Although they are not limited to organic-rich shales,such strata-bound structures may help identify organic-rich intervals in basins where their presence is unknown.

We claim that adding organic matter to a clay matrix will cause reduced https://www.wendangku.net/doc/842707955.html,paction-related vertical fluid flow may cause the highest fluid overpressure to build up at the base of a less per-meable organic-rich layer.The high fluid pressure will reduce the grain-to-grain friction because parts of the overburden weight are carried by the fluid pressure.This highest fluid pressure zone becomes a decollement surface on which overlying sediments slide.Any anisotropy in the horizontal stress,either because of tilted layers or faulting-related stress release,may trigger mass movement along the de-collement and the formation of the characteristic strata-bound deformation structures.

The observations and arguments presented in this article have documented strata-bound structures in organic-rich claystones.Hopefully,this knowl-edge may prevent future misinterpretation of plays in such structures as was the case in the two wells reported here.

REFERENCES CITED

Baldwin,B.,and C.O.Butler,1985,Compaction curves: AAPG Bulletin,v.69,no.4,p.622–626.

Bertrand,Ph.,https://www.wendangku.net/doc/842707955.html,llier-Vergès,L.Martinez,B.Pradier,P.

Tremblay,A.Huc,R.Jouhannel,and J.P.Tricart,1990, Examples of spatial relationships between organic mat-ter and mineral groundmass in the microstructure of the organic-rich Dorset Formation rocks:Great Britain Or-ganic Geochemistry,v.16,no.4–6,p.661–675,doi:10 .1016/0146-6380(90)90108-C.

Blystad,P.,H.Brekke,R.B.F?rseth,https://www.wendangku.net/doc/842707955.html,rsen,J.Skogseid, and B.T?rudbakken,1995,Structural elements of the Norwegian continental shelf.Part II:The Norwegian Sea region:Norwegian Petroleum Directorate Bulletin,v.8, p.1–45.

L?seth et al.745

Booth,J.S.,and A.G.Dahl,1986,A note on the relationships between organic matter and some geotechnical proper-ties of a marine sediment:Marine Geotechnology,v.6, no.3,p.281–297,doi:10.1080/10641198609388191. Bryn,P.,K.Berg,M.S.Stoker,and A.Solheim,2005,Con-touritic deposition and its relevance for the mass wasting record along the mid-Norwegian margin:Marine and Petro-leum Geology,v.22,p.11–19,doi:10.1016/j.marpetgeo .2004.12.003.

Bugge,T.,1983,Submarine slides on the Norwegian conti-nental margin with special emphasis on the Storegga slide:IKU(Continental Shelf Institute)report110, p.1–152.

Bugge,T.,G.Elvebakk,S.Fanavoll,G.Mangerud,M.Smelror,

H.M.Weiss,J.Gjelberg,S.E.Kristensen,and K.Nilsen,

2002,Shallow stratigraphic drilling applied in hydrocar-bon exploration of the Nordkap Basin,Barents Sea:Ma-rine and Petroleum Geology,v.19,no.1,p.13–37,doi:10 .1016/S0264-8172(01)00051-4.

Bull,S.,J.Cartwright,and M.Huuse,2009,A review of ki-nematic indicators from mass-transport complexes using 3D seismic data:Marine and Petroleum Geology,v.26, p.1132–1151,doi:10.1016/j.marpetgeo.2008.09.011. Cartwright,J.,and D.N.Dewhurst,1998,Layer-bounded compaction faults in fine-grained sediments:Bulletin of the Geological Society of America,v.110,p.1141–1166.

Clausen,J.A.,R.H.Gabrielsen,P.A.Reksnes,and E.Nys?ther, 1999,Development of intraformational(Oligocene–Miocene)faults in the northern North Sea:Influence of remote stresses and doming of Fennoscandia:Journal of Structural Geology,v.21,p.1457–1475,doi:10.1016 /S0191-8141(99)00083-8.

Cobbold,P.R.,and N.Rodrigues,2007,Seepage forces,im-portant factors in the formation of horizontal hydraulic fractures and bedding-parallel fibrous veins(“beef”and “cone-in-coneft”):Geofluids,v.7,p.313–332,doi:10 .1111/j.1468-8123.2007.00183.x.

Cobbold,P.R.,and P.Szatmari,1991,Radial gravitational gliding on passive margins:Tectonophysics,v.188, p.249–289,doi:10.1016/0040-1951(91)90459-6. Cobbold,P.R.,R.Mourgues,and K.Boyd,2004,Mechanism of thin-skinned detachment in the Amazon Fan:Assessing the importance of fluid overpressure and hydrocarbon generation:Marine and Petroleum Geology,v.21,no.8, p.1013–1025,doi:10.1016/j.marpetgeo.2004.05.003. Cobbold,P.R.,B.J.Clarke,and H.L?seth,2009,Structural consequences of fluid overpressure and seepage forces in the outer thrust belt of the Niger Delta:Petroleum Geoscience,v.15,no.1,p.3–15,doi:10.1144/1354 -079309-784.

Dalland,A.,D.Worsley,and K.Ofstad,1988,A lithostrati-graphic scheme for the Mesozoic and Cenozoic succes-sion offshore mid-and northern Norway:Norwegian Petroleum Directorate Bulletin,v.4,p.5–42.

Fort,X.,J.P.Brun,and F.Chauvel,2004,Salt tectonics on the Angola margin,synsedimentary deformation pro-cesses:AAPG Bulletin,v.88,no.11,p.1523–1544, doi:10.1306/06010403012.

Fraser,S.I.,A.M.Robinson,H.D.Johnson,J.R.Underhill,

D.G.A.Kadolsky,R.Connell,

E.P.Johannesen,and R.

Ravn?s,2003,Upper Jurassic,in D.Evans,C.Graham,

A.Armour,and P.Bathurst,eds.,The millennium atlas:

Petroleum geology of the central and northern North Sea:The Geological Society(London)Special Publica-tion,p.157–189.

Gabrielsen,R.H.,R.B.F?rseth,L.N.Jensen,J.E.Kalheim, and F.Riis,1990,Structural elements of the Norwegian continental shelf,Part I:The Barents Sea region:Norwe-gian Petroleum Directorate Bulletin,v.6,p.1–33. Homza,T.,2004,A structural interpretation of the Fish Creek Slide(Lower Cretaceous),northern Alaska: AAPG Bulletin,v.88,p.265–278.

Hubbert,M.K.,and W.W.Rubey,1959,Role of fluid pres-sure in mechanics of overthrust faulting,1:Mechanics of fluid-filled porous solids and its application to overthrust faulting:Geological Society of America Bulletin,v.70, p.115–166,doi:10.1130/0016-7606(1959)70[115 :ROFPIM]2.0.CO;2.

Isaksen,D.,and K.Tonstad,1989,A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwe-gian North Sea:NPD Bulletin5,59p. Johannesen,J.,S.J.Hay,https://www.wendangku.net/doc/842707955.html,ne,C.Jebsen,S.C.

Gunnesdal,and A.Vayssaire,2002,3D oil migration modeling of the Jurassic petroleum system of the Stat-fjord area,Norwegian North Sea:Petroleum Geoscience, v.8,p.37–50.

Keller,G.H.,1982,Organic matter and the geotechnical properties of submarine sediments:Geo-Marine Letters, v.2,no.3–4,p.191–198,doi:10.1007/BF02462762. Keym,M.,V.Dieckmann,B.Horsfield,M.Erdmann,R.

Galimberti,L.-C.Kuo,L.Leith,and O.Podlaha,2006, Source rock heterogeneity of the Upper Jurassic Draupne Formation,North Viking graben,and its relevance to pe-troleum generation studies:Organic Geochemistry,v.37, p.220–243,doi:10.1016/https://www.wendangku.net/doc/842707955.html,geochem.2005.08.023. Kubala,M.,M.Bastow,S.Thomson,I.Scotchman,and K.

?ygard,2003,Geothermal regime,Petroleum genera-tion and migration,in D.Evans,C.Graham,A.Armour, and P.Bathurst,eds.,The millennium atlas:Petroleum geology of the central and northern North Sea:The Geo-logical Society(London)Special Publication,p.289–315.

Kvalstad,T.J.,L.Andresen,C.F.Forsberg,K.Berg,P.Bryn, and M.Wangen,2005,The Storegga slide:Evaluation of triggering sources and slide mechanics:Marine and Pe-troleum Geology,v.22,p.245–256,doi:10.1016/j .marpetgeo.2004.10.019.

Langrock,U.,and R.Stein,2004,Origin of marine petroleum source rocks from the Late Jurassic to Early Cretaceous Norwegian Greenland Seaway:Evidence for stagnation and upwelling:Marine and Petroleum Geology,v.21, p.157–176,doi:10.1016/j.marpetgeo.2003.11.011. Leith,T.L.,et al.,1993,Mesozoic hydrocarbon source-rocks of the Arctic region,in T.O.Vorren et al.,eds.,Arctic geology and petroleum potential:Norwegian Petroleum Society Special Publication2,Elsevier Scientific Publi-cations,p.1–25.

L’Heureux,J-S.,L.Hansen,and O.Longva,2009,Develop-ment of the submarine channel in front of the Nidelva

746Deformation Structures in Organic-Rich Shales

River,Trondheimsfjorden,Norway:Marine Geology, v.260,p.30–44,doi:10.1016/j.margeo.2009.01.005. Loncke,L.,V.Gaullier,J.Mascle,B.Vendeville,and L.

Camera,2006,The Nile deep-sea fan:An example of in-teracting sedimentation,salt tectonics,and inherited sub-salt paleotopographic features:Marine and Petroleum Geology,v.23,no.3,p.297–315,doi:10.1016/j.marpetgeo .2006.01.001.

Lonergan,L.,and J.A.Cartwright,1999,Polygonal faults and their influence on reservoir geometries,Alba field, United Kingdom central North Sea:AAPG Bulletin, v.83,p.410–432.

Maltman,A.,1992,The geological deformation of sedi-ments:London,Chapman and Hall,362p. Martinsen,O.J.,1989,Styles of soft sediment deformation on a Namurian(Carboniferous)delta slope,western Irish Namurian Basin,Ireland,in M.K.G.Whateley and K.T.

Pickering,eds.,Deltas:Sites and traps for fossil fuels: Geological Society(London)Special Publication41, p.167–177.

Mondol,N.H.,K.Bj?rlykke,J.Jahren,and K.Hoeg,2007, Experimental mechanical compaction of clay mineral ag-gregates:Changes in physical properties of mudstones during burial:Marine and Petroleum Geology,v.24, p.289–311,doi:10.1016/j.marpetgeo.2007.03.006. Mourgues,R.,and P.R.Cobbold,2003,Some tectonic consequences of fluid overpressures and seepage forces as demonstrated by sandbox modelling:Tectonophys-ics,v.376,p.75–97,doi:10.1016/S0040-1951(03) 00348-2.

Mourgues,R.,and P.R.Cobbold,2006,Sandbox experi-ments on gravitational spreading and gliding in the pres-ence of fluid overpressures:Journal of Structural Geol-ogy,v.28,p.887–901,doi:10.1016/j.jsg.2005.12.013. Mourgues,R.,E.Lecomte,B.Vendeville,and S.Raillard,

2009,An experimental investigation of gravity-driven shale tectonics in progradational delta:Tectonophysics, v.474,p.643–656,doi:10.1016/j.tecto.2009.05.003. NPD,2010a,Source NPD08.2010,http://www.npd.no(ac-cessed June15,2010).

NPD,2010b,Source NPD08.2010,https://npdmap1.npd .no/website/NPDGIS/viewer.htm(accessed June15, 2010).

Odden,W.,R.L.Patience,and G.W.Van Graas,1998,Appli-cation of light hydrocarbons(C4-C13)to oil/source rock correlations:A study of the light hydrocarbon composi-tions of source rocks and test fluids from offshore mid-Norway:Organic Geochemistry,v.28,no.12,p.823–847,doi:10.1016/S0146-6380(98)00039-4.

Pusch,R.,1973,Influence of organic matter on the geotech-nical properties of clays:Statens Institut for Bygnads For-sking,Stockholm,Document D11,64p.

Rowan,M.G.,F.J.Peel,and B.C.Vendeville,2004,Gravity-driven fold belts on passive margins,in K.R.McClay, ed.,Thrust tectonics and hydrocarbon systems:AAPG Memoir82,p.157–182.

Smelror,M.,M.B.E.M?rk,A.M?rk,H.L?seth,and H.M.Weiss, 2001,Middle Jurassic–Lower Cretaceous transgressive-regressive sequences and facies distribution off Troms, northern Norway,in O.J.Martinsen and T.Dreyer,eds., Sedimentary environments offshore Norway–Palaeozoic to recent:Norwegian Petroleum Society Special Publica-tion10,p.211–232.

Vollset,J.,and A.G.Dòre,1984,A revised Triassic and Ju-rassic lithostratigraphic nomenclature for the Norwegian North Sea:Norwegian Petroleum Directorate Bulletin, v.3,p.1–53.

Weimer,P.,and R.Buffler,1992,Structural geology and evo-lution of the Mississippi Fan fold belt,deep Gulf of Mex-ico:AAPG Bulletin,v.76,p.224–251.

L?seth et al.747