Organism–sediment interactions in shale-hydrocarbon reservoir facies — Three-dimensional reconstruction of complex ichnofabric geometries and pore-networks

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The lithological and mineralogical characteristics of mudstones and siltstones—and their stress–strain behavior at the meter to nanometer scale—can play a critical role in the exploitation of unconventional shale reservoirs. Shale fabrics that result
  Organism – sediment interactions in shale-hydrocarbon reservoirfacies  —  Three-dimensional reconstruction of complex ichnofabricgeometries and pore-networks Ma ł  gorzata Bednarz ⁎ , Duncan McIlroy Memorial University of Newfoundland, Department of Earth Sciences, St. John's, NL A1B 3X5, Canada a b s t r a c ta r t i c l e i n f o  Article history: Received 9 November 2014Received in revised form 3 September 2015Accepted 3 September 2015Available online 9 September 2015 Keywords: Shale gasIchnofabricPorosityPermeabilityFracture spacingFluid  fl ow Thelithologicalandmineralogicalcharacteristicsofmudstonesandsiltstones — andtheirstress – strainbehavioratthemetertonanometerscale — canplayacriticalroleintheexploitationofunconventionalshalereservoirs.Shalefabricsthatresultfrombioturbationcanproduceextensiveinterconnectednetworksofbiologicallyredistributedsediment grains within reservoir mudstone facies. The presence of biologically generated heterogeneities maysubstantially affect reservoir stimulation and thus production from shale facies. This study presents volumetricevaluation of ichnofabrics dominated by  Phycosiphon -like and aff.  Chondrites , and provides insights into the im-pactoftracefossils on therheologicaland petrophysical characteristics of mudstones. It iscalculatedthat, inad-dition to creating signi fi cant volumes of silty (clay-poor) zones of enhanced porosity and permeability, tracefossils create interpenetrating frameworks of brittle material that reduce communication distances from thelow-permeability matrix to the higher permeability silt-rich burrows. Reducing communication distances toless than 1 cm increases the potential for diffusive transport of hydrocarbon molecules from the  “ tight ”  matrixto the wellbore-connected volumes. This is because shale ichnofabrics create abundant fracture-prone planesof weakness, and increase the surface area of the interface between the hydrocarbon-rich matrix and porousburrow  fi lls, thereby promoting  fl uid exchange. Understanding of the three-dimensional characteristics of ichnofabrics may form the basis of future modeling of fracture spacing and complexity that is critical to shalegas reservoir characterization.© 2015 Elsevier B.V. All rights reserved. 1. Introduction Gas-andoil-bearingshalesarelithologicallydiverse,includinginter-bedded very  fi ne grained sediments e.g., mudstones, siltstones andlimestones. Because of the lack of a universal classi fi cation system forthese lithologically heterogeneous deposits, the word  ‘ shale ’  is used inthis study accordingtocurrentconvention in thehydrocarbon industryratherthanthestrictlithologicalusage(e.g.,Bustin,2012).Thelitholog-ical and mineralogical characteristics of shales and their stress – strainbehavioratthemetertonanometerscaleplayacriticalroleintheexploi-tationofunconventionalshalereservoirs(e.g.,Bustinetal.,2008a,b;Rossand Bustin, 2009; Bustin and Bustin, 2012; Chalmers et al., 2012; Dinget al., 2012; Josh et al., 2012; Spaw, 2013a,b).The macroscopic and microscopic sedimentary fabrics (includingichnofabrics) determine the distribution of mineral grains and organicmatter particles in mudstones that control the porosity, permeabilityand brittleness of mudstones (e.g. Josh et al., 2012). Individualburrowswithin an ichnofabric can redistribute sediment grains, therebyin fl uencing both the bulk and small-scale petrophysical properties of the host sediment (e.g., Pemberton and Gingras, 2005; Spila et al.,2007; Tonkin et al., 2010; Lemiski et al., 2011; Bednarz and McIlroy,2012;Gingrasetal.,2012,2013).Ichnofabricinmudstonesorsiltstonescan also create permeability isotropy by local destruction of sedimentlaminae (e.g., Schieber, 2003; Pemberton and Gingras, 2005; Lemiskiet al., 2011; Bednarz and McIlroy, 2012; Gingras et al., 2012).The spatial geometry of ichnofabrics re fl ects the cumulative effectsof organism – sediment interactions after deposition (McIlroy, 2004,2007, 2008) including bioturbation spatial re-distribution of   fi ne-grained minerals and changes in bulk mineralogy (McIlroy et al.,2003; Harazim, 2013). The tortuosity, connectivity, surface area,volume and spatial distribution of the burrows in an ichnofabric areamong the most signi fi cant factors determining response of the bio-turbated mudstone or siltstone to reservoir stimulation techniques(e.g., Pemberton and Gingras, 2005; Gingras et al., 2007, 2012; Bednarzand McIlroy, 2012).In this study, the potential in fl uence of trace fossils on thepetrophysical and rheological properties of hydrocarbon-bearing shale International Journal of Coal Geology 150 – 151 (2015) 238 – 251 ⁎  Corresponding author at: University of Leicester, Department of Geology, UniversityRoad, Leicester LE1 7RH, UK. E-mail address: (M. Bednarz).© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect International Journal of Coal Geology  journal homepage:  facies is presented. In contrast to our earlier work, we here considerthecompleteichnofabricpresentineachstudiedsampleratherthanfo-cusing on the reconstruction and quanti fi cation of isolated burrows(Bednarz and McIlroy, 2009, 2012). The term  “ aff.  Chondrites ”  is usedherein to encompass all  Chondrites  sensu stricto (s.s.) and other tracefossils closely resembling  Chondrites  isp. (cf. Bromley and Ekdale,1984;WetzelandWijayananda,1990;FuandWerner,1994).Theinter-changeable terms  “ aff.  Phycosiphon ” , and  “ phycosiphoniform burrows ” relate to an informal grouping of ichnofossils similar to  Phycosiphon isp. (including  Nereites ) that are not identi fi ed to the generic level inichnofabricsdue to unresolvedtaxonomic issues. All phycosiphoniformburrows are considered to have similar effects on sediment fabricand reservoir quality in shale-hydrocarbon facies (cf. Bednarz andMcIlroy, 2012).Ichnofabrics rich in aff.  Phycosiphon  and aff.  Chondrites  are here re-constructed in three dimensions in order to understand the spatial ge-ometry and distribution of biologically redistributed mineral grainsand reservoir properties of shale-hydrocarbon facies. In this study, wedo not attempt to remove compaction of the sediment and its impactonthegeometryoftheburrows.Compactionaffectsthegeometricrela-tionships within ichnofabrics of gas-shales in a heterogeneous manner,but is beyond the scope of this study. The rock volumes reconstructedarenotfromproducinghydrocarbon fi elds,astheanalysesthatweper-form are destructive of large volumes of rock. As such we have chosenichnofabricscomparabletothoseinproducing fi eldsasmodelexamplesfor analogue study.The computer-modeled deterministic three-dimensional recon-structionsallowvolumetricconsiderationofthebiogenicporenetworksin the studied ichnofabric. The potential impact of aff.  Chondrites ichnofabrics is addressed herein for the  fi rst time, and involves thesameprinciplesasusedinourrecentconsiderationofphycosiphoniformburrow volumetrics and morphometrics (see Bednarz and McIlroy,2012). The surface area, density and distribution of three-dimensionalarchitecture of aff.  Phycosiphon  and aff.  Chondrites  ichnofabrics is alsoassessedhereinthroughthegenerationofthree-dimensionaldetermin-isticmodelsofaff. Chondrites ,andaff. Phycosiphon burrowsinhighlybio-turbated sediments. 2.Mainichnofabric-formingtracefossilsinhydrocarbonshalefacies The productive lithologies within shale-gas reservoirs commonlyhave inter-bedded layers of dark organic-rich very  fi ne-grained sedi-ments typically marine mudstones, inter-bedded with siltstones. It hasbeen often considered that the black organic rich mudstones that formthe basis of shale hydrocarbon plays were deposited in associationwith anoxia or severe dysoxia (e.g., Tyson, 1995; Bohacs, 1998; Katz,2005). The necessity for anoxia for black shale deposition has how-ever been challenged (e.g., Schieber, 1994b, 2003, 2011; Wetzel andUchman, 1998b; Macquaker et al., 1999; Macquaker and Bohacs, 2007;Schieber et al., 2007; Rodríguez-Tovar and Uchman, 2010; Ghadeerand Macquaker, 2012). A number of recent petrographic studies havedemonstrated the presence of bioturbation in shale-hydrocarbonreservoir facies that previously seemed to be devoid of ichnofabric(e.g., Schieber, 2003; Ghadeer and Macquaker, 2012; Egenhoff andFishman, 2013). It may commonly be the case that bioturbation ispresentatthemicroscopicscale, andthatbothcoreand outcrop studieslack the resolution to determine such small structures (cf. Wetzel andUchman, 1998a; Schieber, 2003; Egenhoff and Fishman, 2013).While dysoxic basins are generally considered to be hostile tomacrobenthic organisms, the organic-rich sediments that are depositedhost abundantsmall endobenthic organismsthat are tolerant of dysoxicto anoxic pore waters (e.g., Bromley and Ekdale, 1984; Savrda andBottjer, 1991; Middelburg and Levin, 2009). Such organisms withextreme tolerance to low-oxygen content (e.g., foraminifera) and/orsmall benthic organisms tolerant even to episodic total anoxia (e.g.,nematodes,polychaetes,pogonophores,sipunculidwormsandbivalves)are responsible for bioturbation of organic-rich muds (e.g., Seilacher,1990; Savrda and Bottjer, 1991; Dufour and Felbeck, 2003; Stewartet al., 2005; Arndt-Sullivan et al., 2008; Dando et al., 2008; Dubilieret al., 2008; Middelburg and Levin, 2009). The resulting trace fossil as-semblages are typical of stressed ecosystems, in having low diversity,but commonly high abundance (e.g., Goldring et al., 1991; Bottjer andSavrda,1993;AnguloandBuatois,2012a,b).Thechemosymbioticorgan-isms (i.e. organisms having microbial symbionts capable of anaerobicrespiration) from among the abovementioned phyla are the most likelytobecandidateproducers of  Chondrites and Trichichnus (e.g.,Swinbanksand Shirayama, 1984; Seilacher, 1990, 2007; Fu, 1991; Zuschin et al.,2001). Both  Chondrites  and  Trichichnus  are deep tier trace fossils, mostcommonlyrecordedfromanoxicmudstones,wheretheyareoftenpres-ent in monospeci fi c assemblages (e.g., Romero-Wetzel, 1987; McBrideand Picard, 1991; Fu and Werner, 1994; Rodríguez-Tovar and Uchman,2010). Of these two, only  Chondrites  is a common ichnofabric-formingtrace fossil (Callow and McIlroy, 2011). The producer of   Chondrites  isusuallyinferredtobeachemosymbioticorganismandisusedasanindi-cator of anoxic or dysoxic settings (e.g., Bromley and Ekdale, 1984;Seilacher, 1990, 2007; Fu, 1991).Phycosiphoniform trace fossils and  Chondrites -like burrows that arethe focus of this study, are among the most frequent ichnofabric-forming trace fossils observed in organic- and clay-rich siliciclasticmarine deposits including both conventional and unconventional res-ervoir facies (e.g., Cluff, 1980; Wetzel and Bromley, 1994; Pembertonand Gingras, 2005; Callow and McIlroy, 2011; Lemiski et al., 2011;Bednarz and McIlroy, 2012; La Croix et al., 2013; Table 1). Other prominent trace fossils in organic-rich shale intervals are  Planolites ,  Zoophycos ,  Trichichnus ,  Helminthopsis ,  Palaeophycus  and  Teichichnus (e.g., Cluff, 1980; Wetzel and Werner, 1980; Callow and McIlroy,2011).  2.1. Chondrites ichnofabricsChondrites burrowsarecomplexroot-likesystemsofbranchingtun-nels penetrating down with more or less vertical tunnel(s) from anopening at the sediment – water interface (e.g., Osgood, 1970; Wetzel,1983; Wetzel et al., 2011; Löwemark, et al. 2006; Wetzel and Reisdorf,2007; Pemberton et al., 2009).  Chondrites  s.s. and aff.  Chondrites  arecommoninvery fi ne-grainedsediments,suchasorganic-richdarkmud-stones. In vertical cross section, the tunnels of   Chondrites  range from afraction of a millimeter up to several millimeters in diameter, formingabundantcircularspots.Inorganic-matterrichshale-hydrocarbonfacies Chondrites  burrows are generally  fi lled by coarser-grained silty or very fi nesandymaterial,dependingonthelithologyofthesedimentoverlay-ing the burrowed deposit. Where the in fi ll is clay-rich, there is usuallysomecolorcontrast(e.g.,Schieber,2003).Sincethe Chondrites producerwas probably chemosymbiotic (thiotrophic and/or methanotrophic), itwould likely have been able to survive and prosper in sediment withsul fi dic pore waters, but would have had to have been connected tothesediment – waterinterfacewhereatleastsomeoxygenwasavailable(cf. Bromley and Ekdale, 1984; Seilacher, 1990; Fu, 1991; Stewart et al.,2005;Dandoetal.,2008). Chondrites iscommonlytheonlymacroscopictrace fossil found in black mudstones (e.g., Bromley and Ekdale, 1984;Bottjer and Savrda, 1993; cf. Schieber, 2003).  2.2. Phycosiphoniform ichnofabrics Phycosiphon-like burrows are produced by grain-selective depositfeeders and are most common in comparatively less organic-rich silt-stones and silty mudstones than those with monotypic assemblagesof   Chondrites  (e.g., Goldring et al., 1991; Wetzel and Bromley, 1994,Bromley, 1996; Wetzel, 2002; Wetzel et al., 2011; Bednarz and McIlroy,2009, 2012; cf. Egenhoff and Fishman, 2013). These phycosiphoniform trace fossils have a mudstone-rich fecal core surrounded by a silt-gradelight-colored quartzose halo (e.g., Bromley, 1996; Wetzel and Bromley, 239 M. Bednarz, D. McIlroy / International Journal of Coal Geology 150 – 151 (2015) 238 –  251  1994; Wetzel, 2002; Bednarz and McIlroy, 2009, 2012; Callow et al.,2013a,b). The host sediment most probably had oxygenated or at leastdysoxicinterstitialwaterstoallowcontinuousburrowingwithoutmain-tenance of a connection to the sediment – water interface (e.g., Wetzeland Uchman, 1998b, 2001; Wetzel, 2002; Bednarz and McIlroy, 2009,2012). Phycosiphoniform-dominated ichnofabrics are common insilty hydrocarbon-bearing facies (e.g., Spila et al., 2007; Lemiski et al.,2011; Bednarz and McIlroy, 2012 and reference therein; Egenhoff andFishman, 2013). In organic-rich shale settings phycosiphoniform bur-rowsmaycommonlyformmonostaxicichnofabrics,althoughlowdiver-sity  Phycosiphon – Nereites  or  Phycosiphon – Chondrites  or  Phycosiphon – Nereites – Chondrites  ichnofabrics are common locally (e.g., Goldringet al., 1991; Bottjer and Savrda, 1993, Wetzel and Uchman, 2001;Angulo and Buatois, 2012a,b; Callow et al., 2013a,b). 3. Methods Samples of rocks containing  Chondrites -like and phycosiphoniformburrowswere serialgroundwith computer-controlledmillingmachineHAASVF-3CNCVerticalMachiningCenter.Serialgrindingcreatesregu-larly spaced parallel surfaces that are photographed. The photographsweregraphicallyprocessedinordertoobtainconsecutivedigitalimagesfrom which the burrows can be selected. The set of prepared imagesform the data from which a computer-based three-dimensional recon-struction can be created (cf. Sutton et al., 2001; Naruse and Nifuku,2008;BednarzandMcIlroy,2009,2012,Bednarzetal.,2015).Fivespatialreconstructions of ichnofabrics were prepared and measured. All mea-surements were done on deterministic models of the ichnofabrics withno compensation for compaction.  Table 1 Examplesofblack/gray,organic-richshaleintervals,currentlyproducingandpotentialshale-gasreservoirswithrecognizedpresenceoftracefossils.WCSB=WesternCanadianSedimen-tary Basin.Organic-rich shale succession  Chondrites Phycosiphon -liketraces Helminthopsis Palaeophycus Planolites Rhizocorallium Teichichnus Trichichnus Terebellina Zoophycos New Albany Shales, Illinois Basin, USA(Cluff, 1980; Schieber, 2003; Lazarand Schieber, 2004)X X X XFayetteville Shale, Arkoma Basin (AR,OK), USA (Ceron and Slatt, 2012)X XWoodford Shale, Anadarko Basin (OK)(Spaw, 2013a,b)XMarcellus Shale, Appalachian Basin,USA (Spaw, 2012)XOhio Shales, Appalachian Basin, USA(Lazar and Schieber, 2004)X X X X XChattanooga Shale, Black Warrior Basin(TN), USA (Schieber, 1994a,b)X X XBakken Formation, Williston Basin,USA/Canada (Kasper, 1992;Pemberton, 1992; Sonnenberg andPramudito, 2009; Angulo andBuatois, 2011; Egenhoff andFishman, 2013; Gingras et al., 2013)X X X X XExshaw Formation, WCSB, Canada(Caplan and Bustin, 2001, Anguloand Buatois, 2011)X X X X X XBarnett Shale, Fort Worth Basin, USA(Loucks and Ruppel, 2007; Ottmannand Bohacs, 2010)X X X X X X XMancos Shale, Uinta Basin, USA(Macquaker et al., 2007;Bhattacharya and MacEachern, 2009;Bednarz and McIlroy, 2012)X X X X X X XMowry Shale, USA (WY) (Bohacs,1998; Bohacs et al., 2005)X X X X X X X XNiobrara Shale, Denver-Julesburg Basin(KS), USA ( Jackson and Hasiotis,2013)X X X XKimmeridge Clay Formation, UK(Macquaker and Gawthorpe, 1993;Morgans-Bell et al., 2001)X X XCleveland Ironstone and WhitbyMudstone Formations, UK (Ghadeer,2011; Ghadeer and Macquaker, 2012)X X X XAlderson Member, WCSB (SK), Canada(Hovikoski et al., 2008; Lemiski et al.,2011)X X X X XMedicine Hat Member, WCSB (AB),Canada (La Croix et al., 2013)X X X X X X X XMontney Formation, Dawson CreekRegion (BC), Canada (Proverbset al., 2010)X X X X X X XPosidonia Shale, Germany (Savrda andBottjer, 1989; Seilacher, 2007)XSilurian shales of the East EuropeanPlatform, Lublin Basin, Poland(Por ę bski et al., 2013)X X X XRosario Formation, Baja California,Mexico (Bednarz and McIlroy, 2009,2012; Callow et al., 2013a,b)X X X X X240  M. Bednarz, D. McIlroy / International Journal of Coal Geology 150 – 151 (2015) 238 –  251  Selected discrete burrows as well as speci fi cally distinctive parts of spatial models enclosed in digital interactive 3D  fi les were arti fi ciallycoloredforclarityandtoexamineburrowinterrelations(AppendixA1).  3.1. Volumetrics The volumetric calculations provided herein are deterministic asthey were made on the basis of the three-dimensional burrows attheir natural scale. It should be noted that, because of algorithms usedto optimizethemeshof theobjects,thecalculationsinclude asmallde-gree of error. These errors mostly give volume underestimates due toshrinking the mesh in the  “ mesh decimation ”  process (cf. Bednarz andMcIlroy, 2012; Bednarz et al., 2015).Quantitative analysis of the reconstructed burrow associations in-volves investigation of the three-dimensional models. The followingvariables are used herein to characterize the ichnofabrics consideredherein (following Platt et al., 2010; Bednarz and McIlroy, 2012;Bednarz et al., 2015): Volume available  ( VA ) is the volume of the smallest rectangularprism(width=  x ,height=  y andlength=  z  )thatenclosestheburrowor burrow association.It representsthe totalvolume of thesample thatthe examined burrows are enclosed within (Fig. 1A). Volume utilized  ( VU  ) is the volume of the sediment reworked by thetrace maker(s) and it is expressed in cubic units. Volume exploited  ( VE  ) is thevolume utilized presented as a percent-ageofthevolumeavailabletobebioturbated.Itdescribestheef  fi ciencyof volume usage by the trace maker(s).In the case of phycosiphoniform burrows, volume utilized can besubdivided into the mineralogically-different component parts of thetrace fossil (silty halo and clayey fecal core; Fig. 2C, D). These areexpressed as volume of halo (% Vh ) and volume of core (% Vc  ). Thesemeasurements allowed for calculation of   core multiplicand  ( CM  )  —  a Fig. 1. Explanation of variables usedtoassess volumetric characteristicsofthe examined ichnofabrics. A.Surface areaindex explained. Surface area index illustrates how manytimes thesurface area of ichnofabric is larger than the area that is shadowed by the ichnofabric (the horizontal section of the block). Surface area index additionally considers the volume of thebioturbated block through the value of the space diagonal of the block that is incorporated in the block section calculation. Example built on 3D model of a single aff.  Chondrites  burrowfrom Staithes. B.Burrowspacingexplained. Distribution clusterisa box bounded bythe edgesof themodeled regular square-shapegridcomposedof idealizedcylinderswhich summa-rizedlengthcorrespondstothetotallengthoftheichnofabrictubeswithinthegivensizeofvolumeavailable. BS  — burrowspacing(spacingbetweenburrowtubes) — describesthepathlength of hydrocarbon molecule transport through the matrix. Top 3D models illustrate representative individual burrows of   Nereites  from Craster and aff.  Chondrites  from Staithes.241 M. Bednarz, D. McIlroy / International Journal of Coal Geology 150 – 151 (2015) 238 –  251  variable that captures the relative volumes of the silty burrow halorelative to that of the clay-rich fecal burrow core. Surfacearea ( SA )isameasurementofthetotalsurfaceareaofexam-ined 3D models of ichnofabric. It is calculated by the visualizing soft-ware, and is given in square units. Surface area index  ( SA i ) is a unit-less variable illustrating the ratio of burrow or ichnofabric surface area to the surface area of the rectanglethatisdiagonaltotheboxenclosingtheichnofabric( SAdr  ). SAdr  re fl ectstherectanglesurfaceareathatisshadowedbytheichnofabricorburrowmodel(surfaceareaofhorizontalsectionofthesampleblock)andaddi-tionally it takes into consideration the vertical dimension of the blockthrough its diagonal (Fig. 1A). Surface area index ( SA i ) illustrates howmany times the surface area of the ichnofabric or burrow is largerthan the surface area of a shadowed horizontal interface such as mud-stone bed or lamina. Distributioncluster  ( Dc  )isanindividualsegmentoftheunbioturbatedrock matrix within a sample block which was cut by a square gridcreated with idealized cylinders representing extricated ichnofabricbuilt by tubular burrows (Fig. 1B).Distribution cluster is presented as a volume ( VDc  ) in cubic unitsand/or as a box of a calculated edge length ( a − 2 r  ; Fig. 1B). Except forthe most external clusters, each distribution cluster is bounded by thetubescomposingthedistributiongrid( Dg  ).Thetotallengthofthedistri-bution grid ( L  g  ) approximately equals the total length of the extricatedtubular burrows ( L t  ) constituting the ichnofabric volume (AppendixA2).Distributiongridanddistributionclustersillustrateburrowspacing. Burrow spacing   ( BS  ) i.e. spacing between burrows (Fig. 1B). It isthe length of the distribution cluster edge. Burrow spacing illus-trates the approximate distance between the closest permeable fl uid  fl ow paths (burrow tunnels) to be reached by the hydrocarbonmolecule traveling through the unbioturbated matrix (distributioncluster). The quantitative values of regular burrow spacing calcu-lated in this study are intended for comparison in order to gradethe signi fi cance of burrow network present in gas- or oil-bearingshale, and do not re fl ect irregularity of the ichnofabric that is pre-sented herein graphically as three-dimensional interactive models(Appendix A1). Because of the irregularity of the ichnofabric, calcu-lations of values of burrow spacing are intended to be considered asapproximations. 4. Results Samplesofichnofabricscontainingaff. Chondrites andaff. Phycosiphon were collected from deep-marine sedimentary facies that are compara-ble in grain size to many shale hydrocarbon reservoirs including theMancos Shale studied from  fi eld-samples herein (see Bednarz andMcIlroy, 2012 and references therein).The material was collected from the following localities:1. Upper Cretaceous (Turonian) Ferron Sandstone Member of theMancos Shale Formation, Muddy Creek, Utah (aff.  Chondrites ichnofabric); Fig. 2.  Examined  Phycosiphon -like and  Chondrites -like ichnofabric. A. Aff.  Chondrites  from Upper Cretaceous Mancos Shale, Muddy Creek, Utah; B. aff.  Chondrites  from the Lower JurassicStaithes Sandstone Formation, Yorkshire coast, UK; C. phycosiphoniform burrows from the Upper Cretaceous Rosario Formation, Baja California, Mexico; D. microscopy image showingclay-composed burrow core (c) and silty quartz-enriched halo (h); E.  Phycosiphon  s.s. from the Lower Jurassic Staithes Sandstone Formation, Yorkshire coast, UK; F.  Nereites  from theMississippian Yoredale Sandstone Formation, Northumberland, UK; and G. cross-cutting relation of two burrows of   Nereites  form Craster. c  —  core; h  —  halo; Ch  —  Chondrites ; andPh  —  Phycosiphon .242  M. Bednarz, D. McIlroy / International Journal of Coal Geology 150 – 151 (2015) 238 –  251
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