Physical modelling of sand injectites

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<ul><li><p>anc</p><p>Intrusion</p><p>OverpressureFluidization</p><p>es tarers octure men</p><p>s, whiccturesions of, 1827)</p><p>Tectonophysics 474 (2009) 610632</p><p>Contents lists available at ScienceDirect</p><p>Tectonop</p><p>.e linjectites have a worldwide distribution (Hurst and Cartwright, 2007)and occur in sediments of all ages, from Neoproterozoic (Williams,2001) to Holocene (Obermeier, 1989). Good examples occur at outcropin California (BoehmandMoore, 2002; Schwartz et al., 2003; Thompsonet al., 2007; Vigorito et al., 2008), France (Parize and Fris, 2003; Parizeet al., 2007), Greenland (Surlyk and Noe-Nygaard, 2001; Surlyk et al.,2007) and Chile (Winslow, 1983; Hubbard et al., 2007). In contrast, thebest seismic examples are probably those from theNorth Sea (Jenssen et</p><p>based cones tend to be larger, i.e. 0.8 to 2.2 kmwide, and 130 to 290 mhigh (Cartwright et al., 2008). Laccoliths are typicallyelliptical to circularinplanview, 0.5 to 2 kmwide, and75 to 400mhigh (Hansen et al., 2005;Frey-Martnez et al., 2007). All these injectites are remarkably similar inshape to volcanic igneous bodies. For example, at-based sand injectitesare similar to saucer-shaped magmatic sills (Polteau et al., 2008).</p><p>The sand injectites of the North Sea and Faeroe-Shetland basinshave intruded hemipelagic smectite-richmudstones of the Palaeoceneal.,1993; Dixon et al.,1995;Molyneux et al., 20Duranti et al., 2002; Lseth et al., 2003; HuusMickelson, 2004; Shoulders et al., 2007). Frsurveys, the shapes of the intrusive bodies andtheir host rocks are becoming clearer (Hurs</p><p> Corresponding author.E-mail address: peter.cobbold@univ-rennes1.fr (P.R.</p><p>0040-1951/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.tecto.2009.04.032. In the last few decades,attention, as the number(Huuse et al., 2007). The</p><p>(at-based). In contrast, the anks are discordant to bedding. Apicalcones may be 0.5 to 2 kmwide, 50 to 300 m high, and 10 to 50 m thick(Molyneux et al., 2002; Huuse et al., 2007; Cartwright et al., 2008). Flat-manyexamples of sand injectites have come toand quality of seismic surveys have increased1. Introduction</p><p>Sand injectites are intrusive bodiebilization and injection of sand into fraare in sedimentary strata. Early descriptdate from the 19th century (Murchisonair, formed within cohesive and least permeable layers. Heterogeneities in material properties and layerthicknesses were responsible for localizing fracture networks. When any one network broke through to thesurface, rapid ow of air through the fractures uidized the underlying mobile materials and even depletedsome of the layers. Some of the uidized material extruded at the surface through vents, forming volcanoesand sheets. The remainder lodged at depth, forming sand injectites or laccoliths. Conical sand injectitesformed preferentially, where layers had high resistance to bending. Laccoliths formed nearer the surface,where overlying layers had low resistance to bending. The experimental sand injectites were broadly similarto those in the Tampen Spur area of the North Sea, as well as other areas.</p><p> 2009 Elsevier B.V. All rights reserved.</p><p>h result from the remo-. Typically, such fracturessand injectites at outcrop</p><p>Huuseet al., 2007). Like igneous intrusive bodies, sand injectites occur asdykes, sills or laccoliths. In the North Sea, conical bodies and laccolithsare common (Cosgrove and Hillier, 2000; Lseth et al., 2003; Huuseet al., 2007). The apical zone of a conical injectite is typically concordantto bedding (Cartwright et al., 2008) andmay be narrow (apical) orwideSandPhysical modelling</p><p>increased, until it attained and surpassed the weight of overburden. Flat-lying hydraulic fractures, containing</p><p>Injectite a square box, 1 m1 m wide, resting on a grid of uid diffusers. During the experiments, the uid pressureKeywords: equilibrium was static. Whscaling was approximate and the corresponding Reynolds numbers differed. The experimental apparatus wasPhysical modelling of sand injectites</p><p>N. Rodrigues a, P.R. Cobbold a,, H. Lseth b</p><p>a Gosciences-Rennes (UMR6118), CNRS et Universit de Rennes 1, 35042 Rennes Cedex, Frb StatoilHydro Research Centre, Trondheim, Norway</p><p>a b s t r a c ta r t i c l e i n f o</p><p>Article history:Received 21 October 2008Received in revised form 20 March 2009Accepted 28 April 2009Available online 5 May 2009</p><p>Sand injectites are structurthem in the Tampen Spurcompressed air through layeand capable of hydraulic fratherefore able to uidize. Th</p><p>j ourna l homepage: www02; Lonergan et al., 2000;e et al., 2004; Huuse andom 2D and 3D seismictheir relationships witht and Cartwright, 2007;</p><p>Cobbold).</p><p>ll rights reserved.e</p><p>hat result from intrusion of uidized sand into fractures. We have studieda of the North Sea, and have reproduced them experimentally, by drivingf sand, glass microspheres, and silica powder. The silica powder was cohesiveing, whereas the sand and glass microspheres were almost non-cohesive andodels were dynamically similar to their natural counterparts, for as long as</p><p>the processes became dynamic, so that inertial forces were signicant, the</p><p>hysics</p><p>sev ie r.com/ locate / tectoto Miocene Hordaland Group (Thyberg et al., 2000). In both basins,sand injectites are most common in Eocene mudstones (Molyneuxet al., 2002; Lseth et al., 2003; Huuse andMickelson, 2004; Shoulderset al., 2007). Other examples on the Norwegian continental margin arein Upper Cretaceous mudstones (Jackson, 2007). All these mudstonesare of very low permeability and form efcient seals (Wensaas et al.,1998; Jones et al., 2003). Today, the mudstones tend to be slightlyoverpressured (Teige et al., 1999).</p></li><li><p>At rst glance, sand injection and magmatic intrusion are broadlysimilar processes. At a large scale of observation, dykes, sills and lacco-liths result from hydraulic fracturing by overpressured uids (Phillips,1972; Pollard and Johnson, 1973; Cosgrove, 2001; Jolly and Lonergan,2002). To fail in tension, the host rocks must be cohesive. Tensilehydraulic fractures form perpendicular to the least compressive stress(Hubbert and Willis, 1957; Secor, 1965). However, at a smaller scale ofobservation, sand injection and magmatic intrusion may be somewhatdifferent. Whatever its viscosity, magma can migrate no more than ashort distance through pore space, before it freezes. In contrast, aqueousuids, which are responsible for sand injectites (Jonk et al., 2003, 2005),can migrate over much longer distances. Also, uid ow through porespace, in response to pressure gradients, imparts seepage forces to thesolid framework (Mandl and Crans, 1981; Dahlen, 1990; Mourgues andCobbold, 2003; Cobbold and Rodrigues, 2007). Thus in a homogenouselastic material, under lithostatic conditions, uid owing verticallyupward through pore spacemay result in horizontal hydraulic fractures(Cobbold and Rodrigues, 2007).</p><p>Shallow domes are common above conical sand injectites, as aresult of bending (Shoulders and Cartwright, 2004; Shoulders et al.,2007). Such forced folds are similar to the ones above igneouslaccoliths (Pollard and Johnson, 1973). On a smaller scale, dykes andsills may be common in the roof of a laccolith, forming an intrusivehalo (Huuse et al., 2005, 2007; de Boer et al., 2007). For this to happen,stretching of the roof may be necessary (Huuse et al., 2007).</p><p>Pre-existing fractures, if suitably oriented, may reactivate as uidoverpressure increases (Phillips, 1972; Jolly and Sanderson, 1997).Examples are polygonal faults in Eocene mudstones of the North Sea.These are extensional faults of modest throw that intersect, forming apolygonal network in map view (Cartwright, 1996; Cartwright andLonergan, 1996; Cartwright et al., 2003; Cartwright and Dewhurst,</p><p>1998). Lonergan and Cartwright (1999) identied some sand injectitesalong polygonal faults. However, Huuse et al. (2004) argued thatpolygonal faults are rarely connected and do not form conical structures.Shoulders et al. (2007) showed that polygonal faults are much steeperthan the anks of conical intrusions and that sand injectites commonlycrosscut the polygonal faults.</p><p>The sand thatwas the source for the injectitesmusthaveuidizedandremobilized (Nichols et al., 1994; Nichols, 1995; Jolly and Lonergan,2002). A moving uid entrains sand grains, when the viscous dragexceeds the effective weight of the grains; as well as the cohesive orfrictional stresses that keep them together. The uid velocity at whichthis occurs is known as the minimum uidisation velocity (Richardson,1971; Nichols et al., 1994; Nichols, 1995). To our knowledge, nobody hasyet described evidence for depletion of source layers. Even proving asource can be a major difculty. In theMagellan Basin of southern Chile,Winslow (1983) inferred from fossil evidence that sand in dykes hadmigratedvertically through several kilometres. Shoulders andCartwright(2004) inferred a connection between source and injected sand in theFaeroe-Shetland basin. Rosales-Domnguez et al. (2005) foundOligoceneplanktonic foraminifera in sand dykes intruding Miocene sliciclasticrocks and estimated that the sand had moved upwards some 900 m.</p><p>In theNorth Sea, there is some evidence from seismic data that sandshave extruded at the sea bottom (Huuse et al., 2004, 2005; Shouldersand Cartwright, 2004; Hurst et al., 2005) The sand layers thin and dipaway from vents, typically forming low-angle laminae or thin beds(Hurst et al., 2006).</p><p>We are not aware of any previous attempts at producing sandinjectites experimentally. To do so has been a technical challenge for us.In this paper,wedescribe someof the difculties of proper scaling,whenuid ow is turbulent within open fractures. We explain what criteriaguided us in the choice of model materials. Thenwe describe some new</p><p>nella</p><p>611N. Rodrigues et al. / Tectonophysics 474 (2009) 610632Fig. 1.Map of North Viking Graben, North Sea, showingmainMesozoic rift faults (after Za</p><p>see Fig. 2. (For interpretation of the references to colour in this gure legend, the reader is rand Coward, 2003). Study area (red box) is on Tampen Spur. For seismic section (AB),</p><p>eferred to the web version of this article.)</p></li><li><p>Fig. 2. Sand injectites in Hordaland Group, Tampen Spur. For approximate location of seismic section, see Fig. 1. Vertical scale on seismic section is in milliseconds (two-way travel time). Velocity in Hordaland Group is about 2000 m/s (1 msrepresenting 1 m). Two of three wells (A, B and C) penetrated cemented sand (red traces are gamma-ray logs). Line drawing (top) shows interpreted bedding reections, mobile sand (yellow) and mounds above laccoliths. Notice verticaloffsets of beds across V-shaped reectors (V-brights). Injectites appear to terminate in upper part of Hordaland Group. Sand laccoliths in upper part of Hordaland Group may have fed extruded sand. For details, see text. (For interpretation ofthe references to colour in this gure legend, the reader is referred to the web version of this article.)</p><p>612N.Rodrigues</p><p>etal./</p><p>Tectonophysics474</p><p>(2009)610</p><p>632</p></li><li><p>experimental apparatus, which avoids unwanted boundary effects.During upward ow of compressed air through layers of quartz sand,glass microspheres and silica powder, we obtained sand injectites,which seemed to have realistic shapes and distributions. In the lastsection, we compare these experimental structures with possiblenatural counterparts from the Tampen Spur area of the North Sea basin.</p><p>2. Sand injectites in the Tampen Spur area</p><p>In this section, we describe some examples of sand injectites fromthe Tampen Spur area of the North Sea basin, west of the North VikingGraben (Fig. 1). Our aim is to summarize some of the salient features ofthese sand injectites, in two or three dimensions, and their relationshipto sedimentary strata. These structures served as templates for ourexperimental models.</p><p>The Tampen Spur area is a prolic oil province, comprising severalrotated Jurassic fault blocks (Fig. 1). Above the tilted and eroded crestof one such block, the Balder Fmconsists of (1) approximately 700mofCretaceous to Palaeocene clays, which are relatively at lying, partly</p><p>consolidated and contain minor stringers of silt, sand and carbonate,and (2) Late Palaeocene tuff-rich muds. The overlying Eocene toOligocene Hordaland Group (Deegan and Scull, 1977; Isaksen andTonstad,1989) comprises 500600m of smectite-richmuds (Wensaaset al., 1998; Thyberg et al., 2000) and minor sand intervals (Grid andSkade formations). The sands were originally thought to be openmarine deposits (Isaksen and Tonstad, 1989), but lately it has becomeclear that many are injectites (Lseth et al., 2003; Huuse andMickelson, 2004). Between the Oligocene strata of the HordalandGroup and the overlying Utsira Fm is a marked unconformity (Deeganand Scull, 1977; Isaksen and Tonstad, 1989). Upper Miocene to LowerPliocene glauconitic sands partly overly the unconformity in the studyarea (Eidvin and Rundberg, 2001). The sands may correlate with theUtsira Fm (Eidvin and Rundberg, 2001; Gregersen and Johannessen,2007). Seismic and well data reveal that the Utsira Fm consists ofseveral sub-units of sands and clays, which have had a complexgeological history (Gregersen and Johannessen, 2007). Overlying theUtsira Formation are approximately 600 m of prograding Pleistoceneglaciomarine strata.</p><p>ns oabov</p><p>613N. Rodrigues et al. / Tectonophysics 474 (2009) 610632Fig. 3. Structures at top of Balder Fm and distribution of high-amplitude cones. Divisio(yellows) strikes NNE. Deeper areas (blues) are to east. Apices of cones are either just</p><p>Segmented line (AB) is of seismic section (Fig. 2). (For interpretation of the references ton horizontal scale bars are every kilometre. North is at top. Ridge at top of Balder Fme top of Balder Fm (red clusters), or higher within Hordaland Group (violet clusters).</p><p>colour in this gure legend, the reader is referred to the web version of this article.)</p></li><li><p>According to basin modelling, the deeper parts of an upper Jurassicsource rock (Draupne Formation) entered the oil window during theLate Cretaceous (Johannesen et al., 2002). As a result of furthersubsidence during the Cenozoic, the deeper part of the North VikingGraben is now in thegaswindow. This gas formationmaybe responsiblefor some of the deep overpressure in the area.</p><p>Huuse and Mickelson (2004) and Huuse (2008) described andmapped high-amplitude discordant seismic anomalies in the TampenSpur area and interpreted them as conical sand injectites. They also(1) reported that non-bright discordant anomalies of this type arerare, although their existence cannot be ruled out, (2) suggested thatmounds in the upper part of the Hordaland Group were of mobilizedmud, and (3) speculated that large earthquakes, possibly assisted bypre-existing uid overpressure, triggered sand intrusion.</p><p>In the westernmost part of the area, the at-lying Hordaland Groupshows up exceptionally well on 3D seismic P-wave data. Explorationwells have encountered sandy intervals throughout the HordalandGroup. The sands are either non-cemented or partly cemented bycarbonate. Those that are partly cemented have high acoustic imped-ance...</p></li></ul>