the permeability of stylolite-bearing limestone
TRANSCRIPT
1
Thepermeabilityofstylolite-bearinglimestone1
2
MichaelHeap1,ThierryReuschlé1,PatrickBaud1,FrançoisRenard2,3,andGianlucaIezzi43
1GéophysiqueExpérimentale,InstitutdePhysiquedeGlobedeStrasbourg(UMR7516CNRS,Université4
deStrasbourg/EOST),5rueRenéDescartes,67084Strasbourgcedex,France.5
2PGP,TheNjordCentre,DepartmentsofGeosciences&Physics,UniversityofOslo,Norway6
3Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 380007
Grenoble,France8
4Dipartimento di Ingegneria Geologia, IV Piano del Plz. Ex-Rettorato, Università degli Studi “G.9
d’Annunzio”,ViaDeiVestini30,66100Chieti,Italy.10
11
Correspondingauthor:M.Heap([email protected])12
13
Abstract14
Stylolitesareplanarfeaturesthatformduetointergranularpressuresolution.Duetotheir15
planargeometryandrelativeabundanceinlimestonereservoirs,theirimpactonregionalfluidflow16
has attracted considerable interest. We present laboratory permeability data that show that17
stylolites can be considered as conduits for flow in the stylolite-bearing limestonesmeasured. A18
combinationofanalysistechniquesshowsthatthisisduetoazonethatsurroundsthesestylolites19
thatismoreporousandcontainslargerporesthanthehostrock.Ourdataalsoshowthatthewater20
permeability of a sample containing a stylolite parallel to fluid flow is typically lower than its21
permeabilitytogas,explainedhereasaresultoftheexpansionofminoramountsofclayfoundin22
thestylolite,andthat,duetotheirmicrostructuralsimilarities,tectonicandsedimentarystylolites23
affect sample permeability similarly. Finally, we show that the permeability anisotropy that24
2
developsintherockmassduetothepresenceofsedimentarystylolitesmakesitappearasthough25
the stylolites are acting as barriers to fluid flow, and may explain the discrepancy between26
laboratorymeasurementsandfield-scaleobservations.Thisapproachcanprovideestimatesforthe27
equivalent permeability, and permeability anisotropy, for stylolite-bearing limestone reservoirs28
worldwide.29
30
Keywords:Stylolite; limestone;permeability;synchrotronX-raycomputedtomography;scanning31
electronmicroscopy32
33
3
Highlights34
35
• Stylolitesinlimestonesareconduitsforflow,notbarrierstoflow.36
• Stylolitesarecharacterisedbyzoneofhigherporositythanthehostrock.37
• Thehigh-porositystylolitezonecontainslargerporesthaninthehostrock.38
• Poreswithinstylolitesarelessspherical:stylolitescreatethehigh-porosityzoneduring39
theirformation.40
• Stylolitescreateapermeabilityanisotropythatmaymakethemfalselyappearasbarriersto41
flow. 42
4
1Introduction43
Stylolites are planes of insoluble minerals that form in rocks as soluble minerals are44
removedbypressuresolution(e.g.,ParkandSchot,1968;Wanless,1979;NennaandAydin,2011;45
Croizéetal.,2013;Toussaintetal.,2018).Theyarecommoninlimestonesduetotherelativelyhigh46
solubilityofcalcite(e.g.,Tondietal.,2006;FabriciusandBorre,2007;BenedictoandSchultz,2010;47
Smith et al., 2011; Rustichelli et al., 2012; Agosta et al., 2012; Laronne Ben-Itzhak et al., 2014;48
Rustichellietal.,2015;Martín-Martínetal.,2018),butarealsofoundinsandstones(Heald,1955;49
Walderhaug,1996;Bjørkumetal.,1998;WalderhaugandBjørkum,2003;Emmanueletal.,2010).50
Stylolitesformperpendiculartothemajorcompressivestressandarecommonlyfoundsub-parallel51
to bedding (formed by overburden stresses; “sedimentary stylolites”), but can form sub-52
perpendiculartobeddingduetotectonicstresses(“tectonicstylolites”;e.g.,RailsbackandAndrews,53
1995;Ebneretal.,2010a).Althoughmacroscopicallyplanar,stylolitesaremorphologicallyvariable54
onthemeso-andmicroscale(e.g.,KarczandScholz,2003;Renardetal.,2004;Schmittbuhletal.,55
2004;Rollandetal.,2012;LaronneBen-Itzhaketal.,2012;Rollandetal.,2014;Koehnetal.,2016).56
Theirroughnessisthoughttobeafunctionofthemagnitudeofthestressunderwhichtheyformed57
(e.g., Koehn et al., 2007; Ebner et al., 2009a; Koehn et al., 2012), the heterogeneity of the host58
material(e.g.,AndrewsandRailsback,1997;Brousteetal.,2007;Ebneretal.,2009b,2010b;Koehn59
etal.,2012),and/orthecompetitionbetweenlong-rangeelasticredistributionandsurfacetension60
forcesalongtheinterface(e.g.,Schmittbuhletal.,2004;Renardetal.,2004).61
Due to their macroscopically planar form, the influence of stylolites on fluid flow and62
reservoircompartmentalisationhasdrawnconsiderableinterest.Ahandfulofexperimentalstudies63
haveshownthatstylolitescanprovideconduitsforflow(Lindetal.,1994;MallonandSwarbrick,64
1998; Heap et al., 2014; Rustichelli et al., 2015), challenging paradigms that stylolites present65
barrierstofluidflow(Dunnington,1967;Nelson,1981;BurgessandPeter,1985;Koepnick,1987;66
Finkel and Wilkinson, 1990; Dutton and Willis, 1998; Alsharhan and Sadd, 2000). Heap et al.67
5
(2014),forexample,measuredthepermeabilityoflimestonesamplesthatcontainednostylolites,68
onestyloliteperpendiculartothe imposedfluid flow,oronestyloliteparallel to flow.Theyfound69
that samples containing stylolites parallel to flow were about an order of magnitude more70
permeable than the stylolite-freematerial. They concluded that this was likely due to a zone of71
enhanced porosity surrounding the stylolite, a conclusion supported by microstructural72
observations(CarozziandvonBergen,1987;RaynaudandCarrio-Schaffhauser,1992;vanGeetet73
al.,2000;Gringasetal.,2002;Padmanabhanetal.,2015).Thehigherporosityzonesurroundinga74
stylolite has also been shown to reduce the uniaxial compressive strength of a stylolite-bearing75
sample (Baudetal.,2016).However, althoughstylolites themselvesmayactas conduits for fluid76
flow(e.g.,Heapetal.,2014;Rustichellietal.,2015),wehighlightthattheyaretheby-productofa77
processwherebydissolvedmaterialsareoftenprecipitatedintotheporespaceoftheadjacentrock,78
therebyloweringtheporosity,andpresumablypermeability,relativetotheoriginalhostrock(see79
Toussaint et al. (2018) for a review). Therefore, formations containing abundant stylolites will80
likely be characterised by lower porosities and permeabilities than neighbouring stylolite-free81
formations.Indeed,stylolitedensityhasbeenmeasuredtobeinverselyproportionaltoporosityin82
somelimestoneformations(e.g.,AlsharhanandSadd,2000).83
WeextendthestudyofHeapetal.(2014)byprovidingnewporosity-permeabilitydatafor84
stylolite-bearing limestones. We also (1) compare the gas and water permeability of stylolite-85
bearing limestones and (2) compare the permeabilities of limestone samples containing86
sedimentary and tectonic stylolites. Our experimental data are supported by microstructural87
observations(scanningelectronmicroscope,SEM),multi-resolution(voxelsizeof6.27and0.7μm)88
synchrotronX-raycomputed tomography(CT),andestimationsof theaverageporeradiusof the89
flowpathusedbygasparticlesdeterminedusingtheKlinkenbergslipfactor.Finally,andusingour90
experimental data,we consider the “upscaled” permeability of a limestone rockmass containing91
stylolites.92
6
93
2Experimentalmaterialsandmethods94
We selected six stylolite-bearing limestones for this study: two from open quarries in95
Burgundy (France) (Corton limestone andComblanchien limestone, both Jurassic) and four from96
coresdrilledaroundtheANDRAUndergroundResearchLaboratorynearBure(France)(onefrom97
the Middle Jurassic “Dogger” series and three from the Late Jurassic Oxfordian stage). The98
porositiesandgaspermeabilitiesofsomeofthesamplesfromBurewerepreviouslypresentedin99
Heap et al. (2014). We provide here new water permeability data on these samples and new100
porosityandpermeability(gasandwater)dataforadditionalsamplestakenfromoneofthecores101
fromBure(fromtheLateJurassicOxfordianstage)andthesamplesfromBurgundy.102
WefirstquantifiedthemineralcontentofourexperimentalmaterialsusingX-raypowder103
diffraction (XRPD). Powdered samples of each of the limestoneswere ground for 10minutes in104
alcoholusinganagatepestleandmortar.TheXRPDanalyseswereperformedonpowderedmounts105
(usingnominallyzero-backgroundSisampleholdersand10-20mgofmaterial)usingaBrukerD-106
5005θ-2θBragg-BrentanodiffractometerequippedwithNi-filteredCuKαradiation.Theobtained107
XRPD patterns were first checked for their crystalline content using search-match comparisons108
with XRPD standards contained in the inorganic crystal structure database (ICSD). The XRPD109
patternswithmorethanonecrystallinephasewerethenrefinedusingthesoftwareEXPGUI-GSAS110
(Larson and Von Dreele 1994; Toby 2001). EXPGUI-GSAS uses the Rietveld method to derive111
crystallographic parameters and phase abundances (wt. %). More detailed descriptions of the112
RietveldrefinementmethodarereportedinIezzietal.(2004;2010andreferencestherein).XRPD113
analysiswasperformedon(1)stylolite-freematerialand(2)onsamplescuttocontainastylolite,114
butwithaslittlehostrockaspossible(inanattempttoidentifythemineralsformingthestylolite).115
In addition, minerals within the stylolites were also identified using energy-dispersive X-ray116
7
spectroscopy (EDS) during our SEM analyses. The mineral content for the Bure samples was117
previouslypresentedinHeapetal.(2014).118
ThefirstoftheOxfordianlimestones(O1;depth=159m)isaheterogeneousallochemical119
limestone that contains ooids, peloids, shell fragments, and fossil foraminifera within a micrite120
matrix (Figure1a). Theooids are typically0.1-0.25mm indiameter (Figure1a).Thepeloids are121
noticeablylargerthantheooids(Figure1a);somepeloidshavediametersgreaterthan1mm.Shell122
fragments inO1 can bemanymillimetres in length.O1 is predominately calcite (99wt.%)with123
subordinatedolomite(<1wt.%)(Table1).ThesecondOxfordianlimestone(O3;depth=174m)is124
awell-sortedallochemicallimestonethatcontainsooids,typically0.25-0.5mmindiameter,within125
amicritematrix(Figure1b).O3ispredominatelycalcite(99wt.%)withsubordinatedolomite(<1126
wt.%),gypsum(<1wt.%),andpyrite(<<1wt.%)(Table1).ThethirdOxfordian limestone(O6;127
depth = 364 m) is a very heterogeneous allochemical limestone that contains peloids, shell128
fragments (>1 mm), and fossil foraminifera (>1 mm) within a micrite matrix (Figure 1c). O6 is129
predominately calcite (99wt.%)with subordinate dolomite (<1wt.%) and pyrite (<<1wt.%)130
(Table 1). The “Dogger” limestone (D3; depth 747 m) is an orthochemical limestone (micrite)131
(Figure1d)composedof93wt.%calcite,4wt.%dolomite,3wt.%quartz,andsubordinatepyrite132
(<<1 wt. %) (Table 1). Corton limestone is an allochemical limestone that contains peloids133
(typically 0.2-1 mm in diameter) within a micrite matrix (Figure 1e). Corton limestone is134
predominately calcite (99 wt. %) with subordinate quartz (<1 wt. %) (Table 1). Comblanchien135
limestone isaheterogeneousallochemical limestonethatcontainsooids,peloids,shell fragments,136
andfossilforaminiferawithinamicritematrix(Figure1f).Allochemsaretypicallybetween0.1and137
1mmindiameter(Figure1f).Comblanchienlimestoneisessentiallyentirelycalciteincomposition138
(99wt.%)(Table1).139
Examples of the stylolites in these materials are shown in Figures 2, 3, 4, and 5.140
Qualitatively, therearecleardifferencesbetween thesedimentarystylolites in termsof thickness141
8
androughness/tortuosity(Figures2,3,and4).Thethickeststylolites(upto2-3mm)arefoundin142
theD3samples(Figure3a).ThestylolitesinCortonlimestonearethemostrough/tortuous(Figure143
4a);theleastrough/tortuousstylolitesarefoundinsamplesO3(Figure2b)andtheComblanchien144
limestone (Figure 4b). Stylolites found in the most heterogeneous limestone—sample 06—are145
correspondingly anastomosing (Figure 2c).We also provide images of tectonic and sedimentary146
stylolites found in sample O3 (Figure 5). There are no discernable differences between the147
thicknessandroughness/tortuositybetweenthetectonicandsedimentarystylolitesinsampleO3148
(Figure5).A combinationofXRPDandEDSanalyses found that the stylolites typically consistof149
dolomiteand/orquartz,withminorquantitiesofpyriteandorganicmatter/clay(Table1).150
Cylindricalsamples(20mmindiameterandnominally40mminlength)werecoredfrom151
theblocks/cores.Sampleswereprepared tocontain (1)onestyloliteperpendicular to theaxisof152
thecore(i.e.perpendicular to the imposed flowdirection), (2)onestyloliteparallel to theaxisof153
thecore(i.e.parallel to the imposed flowdirection),or (3)nostylolite (wherepossible, stylolite-154
free samples were prepared in two or three orthogonal directions). The samples containing no155
stylolites were typically prepared from material 5-10 cm from the stylolite studied. We also156
prepared samples containing tectonic stylolites either perpendicular or parallel to the core axis157
fromoneoftheOxfordianlimestones(sampleO3).Wenotethat,followingsamplepreparation,our158
samplesdidnotcontainanyobviousstylolite-associatedfractures.Representativephotographsof159
the 20 mm-diameter samples prepared for laboratory testing (stylolite-free, one stylolite160
perpendicular to the sample axis, and one stylolite parallel to the sample axis) are provided in161
Figures6and7.Figure8showsphotographsofsamplesofO3containingtectonicandsedimentary162
stylolites.163
The connected porosity of each sample was determined using the triple weight water164
saturationtechnique(GuéguenandPalciauskas,1994).Gas(argonornitrogen)andwater(distilled165
water) permeabilities were then measured in a hydrostatic pressure vessel under a confining166
9
pressureof2MPa.Allmeasurementsofwaterpermeabilitywereperformedusingthesteady-state167
flowmethod.Followingmicrostructuralequilibrium,apressuredifferentialwasimposedacrossthe168
sampleandtheflowratewasmeasuredusinganelectronicbalance(withaprecision±0.0005g).169
Oncesteady-stateflowhadbeenestablished,thewaterpermeability𝑘!"#$% wasdeterminedusing170
Darcy’srelation:171
172
𝑄𝐴= 𝑘!"#$%𝜂𝐿
𝑃!" − 𝑃!"#$ , (1)
173
where Q is the volumetric flow rate, A is the cross-sectional area of the sample, Pup and Pdown174
represent the upstream and downstream pressure, respectively (wherePdown is the atmospheric175
pressure),Listhelengthofthesample,𝑘!"#$% isthepermeabilitytowater,andηistheviscosityof176
theporefluid(takenhereas1.008×10-3Pas).Apressuredifferential(i.e. 𝑃!" − 𝑃!"#$)of0.5MPa177
wasusedforallmeasurementsreportedherein.178
Gas (argonornitrogen)permeabilitywasmeasuredusingeither the steady-statemethod179
(forhigh-permeabilitysamples)orthepulse-decaymethod(forlow-permeabilitysamples).Forthe180
steady-state method, a pressure differential was imposed across the sample (following181
microstructuralequilibrium)andtheoutlet flowratewasmeasuredusingaflowmeter.Sincethe182
pore fluid is compressible, the raw permeability to gas 𝑘!"#_!"# is expressed as (Scheidegger,183
1974):184
185
𝑄𝐴= 𝑘!"#_!"#𝜂𝐿
(𝑃!")! − (𝑃!"#$)! 2𝑃!"#$
, (2)
186
whereη,theviscosityoftheporefluid,wastakenas2.21×10-5and1.78×10-5Pasforargonand187
nitrogen,respectively.Steady-statevolumetricflowrateQmeasurementsweretakenunderseveral188
10
pore pressure differentials (i.e. 𝑃!" − 𝑃!"#$, where Pdown is the atmospheric pressure) to check189
whether any auxiliary corrections were required.We first plot 1/𝑘!"#_!"# as a function of𝑄 to190
check whether the Forchheimer correction is required (Forchheimer, 1901). The correction is191
necessaryifthesedatacanbewelldescribedbyalinearfitwithapositiveslope.TheForchheimer-192
correctedpermeabilityistakenastheinverseofthey-interceptofthebest-fitlinearregressionin193
the plot of1/𝑘!"#_!"# as a function of𝑄. If the Forchheimer correction is not required,we then194
check whether the Klinkenberg correction is required (Klinkenberg, 1941). To do so, we plot195
𝑘!"#_!"# as a function of the reciprocal mean pressure 1/𝑃! , where Pm is the mean pore fluid196
pressure (i.e. (𝑃!" + 𝑃!"#$)/2). TheKlinkenberg correction is required if these data can bewell197
describedbya linear fitwithapositiveslopeand, if true, theKlinkenberg-correctedpermeability198
canbetakenasthey-interceptofthebest-fitlinearregressionintheplotof𝑘!"#_!"# asafunctionof199
1/𝑃! . The Klinkenberg correctionwas required for all samplesmeasured using the steady-state200
method;theForchheimercorrectionwasnotrequired.201
Weusedthepulse-decaymethod(Braceetal.,1968)tomeasurethegaspermeabilityofthe202
low-permeabilitysamples.Followingmicrostructuralequilibriumatthetargetconfiningpressure,203
the decay of an initial pore pressure differential (𝑃!" − 𝑃!"#$ = 0.5 MPa, where Pdown is the204
atmospheric pressure) was monitored using a pressure transducer following the closure of the205
upstreampressure inlet. The gaspermeability𝑘!"#_!"# was thendeterminedusing the following206
relation:207
208
𝑘!"#_!"# = 2𝜂𝐿𝐴
𝑉!"
𝑃!"! − 𝑃!"#$! 𝑑𝑃!"𝑑𝑡
, (3)
209
whereVup is the volume of the upstream pore pressure circuit (7.8 × 10-6m3) and t is time. As210
before, we checked whether these data required any auxiliary corrections (the Forchheimer or211
11
Klinkenbergcorrection).TheKlinkenbergcorrectionwasrequiredforallsamplesmeasuredusing212
the pulse-decaymethod; the Forchheimer correctionwas not required. A detailed description of213
thesepermeabilitymethodsisavailableinHeapetal.(2017).214
215
3Results216
The gas permeability data for the stylolite-free limestones as a function of connected217
porosity are shown in Figure 9a. The data of Lind et al. (1994), a study that alsomeasured the218
permeability of stylolite-bearing carbonate rocks, are also included on Figure 9 because they219
preserveahigherporosity(porosity>0.2)thanthesamplesmeasuredherein.Ourdatashowthat220
gas permeability increases as connected porosity is increased, in accordance with previously221
published studies on the permeability of limestones (e.g., Ehrenberg et al., 2006; Zinszner and222
Pellerin,2007),and that there isnomeasurablepermeabilityanisotropy in thestudiedmaterials223
(Figure 9a contains data on samples cored in orthogonal directions, seeTable 2). The difference224
between permeability to gas and permeability to water in the stylolite-free samples appears to225
dependontheconnectedporosity:permeabilitytogascanbeafactorof4.5higheratlowporosity226
(porosity < 0.05) and the ratio between gas and water permeability is essentially unity at the227
highesttestedporosity(porosity~0.15)(Figure10a).228
The porosity-permeability data for the stylolite-free and stylolite-bearing (perpendicular229
andparalleltoflow)limestonesareshowninFigure9b,togetherwiththehigh-porosity(porosity>230
0.2)dataofLindetal.(1994).Ourdatashowthat(1)thepermeabilitiesofthesamplescontaining231
stylolitesperpendiculartothedirectionofflowaresimilartothoseofthestylolite-freesamplesand232
(2) the permeabilities of the samples containing stylolites parallel to the direction of flow are233
characterisedbypermeabilitieshigherthanthoseofthestylolite-freesamples(Figure9b).Indetail,234
we notice that larger differences between the permeability of the samples containing stylolites235
parallel to the direction of flow and the stylolite-free samples are observed at lower connected236
12
porosities (Figure 9b). For example, the permeability of stylolite-bearing Corton limestone237
(porosity~0.03) canbe twoor threeorders ofmagnitudehigher than the stylolite-freematerial238
(Figure9b).Theratioofgastowaterpermeabilityforallthesamplestested(includingstylolite-free239
samples and samples containing stylolites perpendicular and parallel to flow) is plotted as a240
functionofconnectedporosityinFigure10b.Asforthestylolite-freelimestones(Figure10a),high-241
porosity (porosity ~0.15) samples containing stylolites show little difference between gas and242
water permeability (Figure 10b). The gas permeabilities of the low-porosity samples containing243
stylolites are higher than their water permeabilities; this is especially true for the low-porosity244
samplescontainingstylolitesparalleltoflow(thedifferenceforonesampleismorethananorder245
ofmagnitude)(Figure10b).246
Our data also show that there is essentially no difference between the influence of247
sedimentaryandtectonicstylolitesonthepermeabilityofour limestonesamples(Figures9band248
10b;Table2).249
250
4Discussion251
252
4.1Barrierstoorconduitsforfluidflow?253
OurnewpermeabilitydataareinagreementwiththeconclusionofHeapetal.(2014)and254
Rustichelli et al. (2015): the stylolites measured are not barriers to flow, but conduits for flow255
(Figure9b).Heapetal.(2014)postulatedthatazoneofhigherporositysurroundsastyloliteand256
thatitisthishigh-porosityzonethatenhancesthecirculationoffluids,assuggestedbyCarozziand257
von Bergen (1987), Raynaud and Carrio-Schaffhauser (1992), and Van Geet et al. (2000). The258
greaterincreaseinpermeabilityinthelow-porositysamples(whencomparingthepermeabilityof259
a stylolite-free sample to a sample containing a stylolite parallel to flow) (Figure 9b) is likely a260
consequenceof their lowmatrixpermeabilities.Conduits for flowhaveamuchgreater impacton261
13
the equivalent permeability of low-porosity samples than on high-porosity samples, since the262
matrixpermeabilityofahigh-porositysampleismuchclosertothepermeabilityofthefracture(as263
observed in variably porous fractured materials; e.g., Heap and Kennedy, 2016; Kushnir et al.,264
2018).265
To image the hypothesised zone of higher porosity, we first provide a backscattered266
scanningelectronmicroscope(BSE) imageofastylolitewithinsampleD3,selecteddueto its low267
matrixporosityandpermeability.Thisimageshowsthatthematrix-styloliteinterfaceispopulated268
bynumerousmicropores, typicallyonlya fewmicrons indiameter (whitearrows;Figure11).To269
betterresolvetheporosity,anddistributionofporosity,aroundastylolite,wealsoprovidemulti-270
resolution(voxelsizeof6.27(beamlineMB05)and0.7μm(beamlineID19)andenergyof35keV)271
three-dimensional X-ray tomography imaging performed on a stylolite within sample D3 at the272
EuropeanSynchrotronRadiationFacility(Grenoble,France).Becauseofthehigh-contrastbetween273
theporosityandthemineralsthatcomprisetherock(primarilycalcite,quartz,anddolomite;Table274
1),itisstraightforwardtosegmenttheporositysothatindividualporescanbeimaged(Figure12).275
Thesegmentedimagesshowthatthestyloliteisassociatedwithazoneofhighporosity(Figure12),276
as previously measured by Baud et al. (2016). In particular, we observe that (1) the pores277
surroundingthestylolitearelargerthanthosewithinthehostrock(Figure12)and(2)someofthe278
pores are aligned with the teeth of the stylolite and are characterised by a “finger-like” shape279
(Figure13). Indeed, analysing theX-ray tomographydata (similar toX-ray tomographicanalyses280
performed on intact porous limestones by Ji et al., 2012; 2014) show that the pores are larger281
insidethestylolite(Figure14a)andthattheporeswithinthestylolitearecharacterisedbylower282
valuesofsphericity(where1.0isaperfectsphere;sphericityisdefinedusingtheThermoScientific283
Avizotoolboxas( !!!!"#!"!!
)!/!where𝑆ℎ𝑎𝑝𝑒!"!! = 𝐴𝑟𝑒𝑎!!!/(36 × 𝜋 × 𝑉𝑜𝑙𝑢𝑚𝑒!!!))(Figure14b).284
Thevolumeof an individualporewithin the stylolite varies froma fewμm3up to>105μm3; the285
volume of the pores outside the stylolite are all <104 μm3 (Figure 14a). The average equivalent286
14
diameterof thepores insideandoutside thestylolite is36.5μm(standarddeviationof26.8μm)287
and11.1μm(standarddeviationof4.7μm),respectively.Sphericityinsideandoutsidethestylolite288
varies from0.2to0.4and0.4and0.9,respectively(Figure14b).Sincetheshapeof theporesare289
sometimes linked to the shape of the stylolite (Figure 13), we additionally conclude that such290
porosity is likely the consequence of stylolite formation (in agreement with the conclusions of291
Raynaud and Carrio-Schaffhauser (1992) and Carozzi and von Bergen (1987)), rather than that292
stylolitesformpreferentiallyinazoneofhigherporosity(ashypothesisedbyBraithwaite,1989).293
Tocomplementthesemicrostructuraldata,weusetheKlinkenbergslipfactor,𝑏(whichhas294
theunitsofpressure;Table2)(Klinkenberg,1941), toprovidean independentassessmentof the295
average pore radius used by the gas molecules. Since the pore radius determined using this296
techniqueusesdatafrompermeabilityexperiments,itwillthereforeyieldtheaverageporethroat297
radius(incontrasttotheCTdata,whichprovides informationonthepores).Sincethemeanfree298
pathis inverselyproportionalto𝑃! ,Poiseuille's lawforgasflowinacylindricaltubeandDarcy's299
lawforflowinporousmediayieldsthefollowingrelation:300
301
𝑘!"# = 𝑘!"#_!"# 1 + 𝑏𝑃!
, (4)
302
where 𝑘!"# is the true (i.e. Klinkenberg-corrected) gas permeability (Klinkenberg, 1941). The303
averageporethroatradiusoftheflowpathfollowedbythegasmolecules,𝑟,canbeestimatedusing304
thefollowingrelation(Civan,2010):305
306
𝑟 = 4𝑏𝜂
𝜋𝑅!𝑇2𝑀!
, (5)
307
15
where𝑅!istheidealgasconstant(takenas8.31Jmol-1K-1),𝑇isthetemperature(takenas293K),308
and𝑀! isthemolarmassoftheporefluid(takenas0.03995and0.02802kgmol-1forargonand309
nitrogen, respectively). The Klinkenberg slip factor has previously been used to examine the310
averageporethroatradiusoftheflowpathinrockssuchasshales(e.g.,Helleretal.,2014;Firouzi311
etal.,2014;LethamandBustin,2016)and,morerecently,volcanicrocks(Heapetal.,2018)using312
thesame,orsimilar,method(i.e.Equation(5)).Wefindthattheaverageporethroatradiusofthe313
flow path followed by the gas molecules in the stylolite-free samples, excluding the Corton314
limestone samples, variesbetween~0.05and0.15μm (Figure15a).ExcludingCorton limestone,315
thesamplescontainingthehighestporosities(porosity~0.15)arecharacterisedbythelargestpore316
throat radii (~0.1 to ~0.15 μm; Figure 15a). The average pore throat radius of the flow path317
followedbythegasmoleculesismuchlargerinCortonlimestone,varyingbetween~0.2and0.35318
μm(Figure15a).Althoughnotobviousfromourmicrostructuralobservations(Figure1e),Corton319
limestone must contain larger pore throats than the other limestones measured herein. The320
averageporethroatradiioftheflowpathsfollowedbythegasmoleculesinthesamplescontaining321
stylolites(togetherwiththestylolite-freesamples)areprovidedinFigure15b.Thedatashowthat322
(1)theporethroatradiiformingtheflowpathinthesamplescontainingstylolitesperpendicularto323
flow are similar to the stylolite-free samples and (2) the pore throat radii along the flow path324
parallel to the stylolite are systematically larger than those of the other samples (Figure 15b).325
Thesedatasuggestthatthestylolitesareassociatedwithporethroatswithlargerradiithanthose326
that typify the host rock. This conclusion is in agreement with our X-ray tomography analysis,327
which shows that the pores are larger inside the stylolite than in the host rock (Figure 14a). As328
expected,theradiipredictedusingEquation(5)aremuchsmaller(typically<1μm;Figure15)than329
therangeofradiipredictedfromtheX-raytomographyanalysis(uptoafewtensofmicrons).This330
is because the Klinkenberg analysis (Equation 5) yields the pore throat radius and the X-ray331
tomographyanalysisyieldstheporeradius.332
16
Weconcludeherethatstylolitespresentconduitsfor,ratherthanbarriersto,flow(Figure333
9b)inlimestonesmeasuredherein.Thiscanbeexplainedbyazoneofelevatedporosity(Figure12)334
that contains pores andpore throatswith larger radii than thehost rock (Figures 14a and15a),335
which we conclude must develop around a stylolite during its formation. The development of336
stylolitic porosity is discussed in detail in Carozzi and von Bergen (1987) and is considered the337
resultofgrainscaleheterogeneitiesintherockduringthedissolutionprocess.338
339
4.2Differencesbetweentectonicandsedimentarystylolites340
Ourpermeabilitydatasuggest,forthematerialsstudiedherein,thatthereisessentiallyno341
difference between the influence of sedimentary and tectonic stylolites on the permeability of a342
stylolite-bearing sample: both sedimentary and tectonic stylolites are conduits for fluid flow343
(Figure9b;Table2).This isperhapsnotsurprisingsince theyareverysimilaron themicroscale344
(Figure 5) and on the sample lengthscale (Figure 8). The fact that tectonic stylolites are also345
conduits for flow(Table2) furthersupports thehypothesis thatstylolitescreateazoneofhigher346
porosity during their formation (e.g., Raynaud and Carrio-Schaffhauser, 1992; Carozzi and von347
Bergen,1987),ratherthanthattheyformpreferentiallyinhigherporositylayers(e.g.,Braithwaite,348
1989).349
350
4.3Differencesbetweengasandwaterpermeability351
Differencesbetweenpermeabilitytogasandwateraretypicallyobservedinthepresenceof352
swellingclays(e.g.,FaulknerandRutter,2000,2003;TanikawaandShimamoto2006;Davyetal.,353
2007;TanikawaandShimamoto2009;BehnsenandFaulkner,2011).OurXRPDanalyseshighlight354
that clays are below the detection limit in the stylolite-free material (Table 1). It is therefore355
perhapssurprisingthatweseeaboutafourfolddifferencebetweengasandwaterpermeabilityin356
the low-porosity limestones (Figure 10a). A recent study found that the permeability to gaswas357
17
higherthanpermeabilitytowater intwovolcanicrocks(basaltandandesite)byafactorofupto358
five (Heap et al., 2018). In the absence of significant physicochemical reactions, these authors359
suggestedthatthedifferenceingasandwaterpermeabilities is likelyduetowateradsorptionon360
thesurfaceofthinmicrostructuralelements.Forthestylolite-freelimestones,wefindthatthereis361
essentiallynodifferencebetweenthegasandwaterpermeabilitiesforthesamplescharacterisedby362
the largest average pore throat radii (~0.1 to ~0.15 μm) (Table 2), as determined using the363
Klinkenbergslipfactor.Sampleswithaverageporethroatradiibetween~0.05and0.1μmaremore364
permeabletogasthantowater(Table2).SimilartotheconclusionsdrawnbyHeapetal.(2018),365
weconcludeherethat,intheabsenceofclaywithintheintactmaterials(Table1),thedifferencein366
gasandwaterpermeabilitiesislikelyduetowateradsorptiononthesurfaceofthin(~0.05to0.1367
μm)microstructuralelements.368
Measurementsofgasandwaterpermeabilityonthesamplescontainingstylolitesshowthat369
samplescontainingstylolitesparalleltoflowareoftenmorepermeabletogasthanwater,byupto370
oneorderofmagnitude(Figure10b).Sinceaverageporethroatradiusoftheflowpathfollowedby371
thegasmoleculesisrelativelyhighforthesesamples(upto~1μm;Figure15b),weconcludethat372
thedifferenceingasandwaterpermeabilitiesinthesesamplesareduetominorquantitiesofclay373
foundwithinthestylolite(identifiedbyEDSduringourSEManalyses;Table1).Theexpansionof374
clay minerals in contact with water constricts pore throats and thus reduces permeability (e.g.,375
FaulknerandRutter,2003).376
377
4.4Implicationsforfluidflowinlimestonereservoirs378
LimestoneformsanimportantcomponentoftheEarth’scontinentalcrust(Ehrenbergetal.,379
2006;FordandWilliams,2013)and,asaresult,thepermeabilityoflimestonereservoirsisnotonly380
important for fluid flow and pore pressure distribution within the crust, but also for the381
exploitationofhydrocarbonreserves.382
18
Ourstudyshowsthatstylolitesinlimestonepresentconduitsforflow(Figure9b)duetoa383
zone of elevated porosity, containing poreswith larger radii than the host rock,which develops384
aroundastyloliteduringitsformation(Figures11,12,14,and15).Inordertoconsiderfluidflowin385
stylolite-bearing limestone reservoirs, we must first upscale our laboratory measurements. One386
method to upscale such laboratory data is to first extract the permeability of a stylolite. The387
permeabilityofastylolite,𝑘!"#$% ,canbedeterminedusingatwo-dimensionalmodelthatconsiders388
flowinparallellayers(thesamemodelusedtodeterminethepermeabilityofcompactionbandsin389
Vajdova et al. (2004) and fractures inHeap andKennedy (2016), Farquharson et al. (2016), and390
Kushniretal.(2018)):391
392
𝑘!"#$% = (𝐴 ∙ 𝑘!) − (𝐴!"#$%# ∙ 𝑘!)
𝐴!"#$% (6)
393
where 𝐴 is the cross-sectional area of the sample, 𝑘! is the equivalent permeability (the394
permeabilityofthestylolite-bearingsample),𝐴!"#$%# istheareaofstylolite-freematerial,𝑘! isthe395
stylolite-freepermeability,and𝐴!"#$%istheareaofthestylolite.Forthepurposeofthisexercise,we396
will consider a core of Dogger limestone (D3) taken from the ANDRA Underground Research397
Laboratory at Bure (Figure 16). To calculate 𝑘!"#$% we use the permeability of the stylolite-free398
sampleofD3(𝑘!=3.69×10-19m2;Table2).Theequivalentpermeability, 𝑘! ,ispermeabilityofthe399
D3samplecontainingastyloliteparalleltothedirectionofflow(𝑘! =5.98×10-18m2;Table2),and400
weuseastylolitethicknessof1mm(areasonableapproximationofthethicknessofthestylolitein401
this sample; Figure6d).Using these values, Equation (6) yields a stylolite permeability, 𝑘!"#$% , of402
8.85 × 10-17 m2.We can nowmodel the equivalent permeability of a rockmass populated with403
stylolitesusingourvaluefor𝑘!"#$%andthefollowingrelation:404
405
19
𝑘! = 𝑤!"#$%# ∙ 𝑘! + (𝑤!"#$% ∙ 𝑘!"#$%)
𝑊, (7)
406
where𝑤!"#$%#isthetotalwidthoftheintactmaterial,𝑤!"#$%isthetotalwidthofthestylolites,and407
𝑊 is the length of rock considered (𝑊 = 𝑤!"#$%# + 𝑤!"#$%). The Dogger limestone core sample408
showsthattherearefivesedimentarystylolitesoveralengthofabout25cm(i.e.astylolitedensity409
of20m-1)(Figure16).Althoughstylolitethicknessvaries(Figure16)wewill,forsimplicity,assume410
thatthethicknessofeachstyloliteis1mm.Therefore,accordingtoourmodel(Equation(7);𝑊=411
250mm;𝑤!"#$% = 5mm;𝑤!"#$%# = 245mm;𝑘! = 3.69 × 10-19m2;𝑘!"#$% = 8.85 × 10-17m2), the412
equivalent permeability parallel and perpendicular to bedding for the limestone core shown in413
Figure16is1.80×10-17and3.69×10-19m2,respectively.Inotherwords,the25cm-longsampleis414
50timesmorepermeableparallel tobedding thanperpendicular tobedding.Therefore,although415
stylolitesareconduitsforflow,theycancreateapermeabilityanisotropyinarockunitorreservoir416
thatmaymakeitappearthattheyformbarrierstofluidflow(becausepermeabilityperpendicular417
to bedding is lower than the permeability parallel to bedding) and may therefore explain the418
discrepancybetweenlaboratorymeasurements(thatsuggestthatstylolitesareconduits)andfield-419
scaleinvestigations(thatsuggestthatstylolitesarebarriers).Itisimportanttohighlightthatthese420
equivalent permeability estimates for a stylolite-bearing rockmass are just one snapshot in the421
porosity-permeabilityevolutionof this limestoneformation.Forexample, it is likelythat,priorto422
pressure-solution and the formation of the stylolites, the host rockwasmuchmore porous and423
more permeable. Although not considered in our simple model, we note that the presence of424
stylolites perpendicular to bedding (i.e. tectonic stylolites) will reduce such permeability425
anisotropy.Indeed,ifthenumberoftectonicstylolitesequalsthenumberofsedimentarystylolites,426
no permeability anisotropy will be observed. Although tectonic stylolites are not uncommon in427
limestonereservoirs(RailsbackandAndrews,1995;Ebneretal.,2010a;Figure8),itisdifficultto428
20
assess their density at theANDRAUndergroundResearch Laboratory atBure due to the drilling429
direction(perpendiculartobedding).Thesimplemethodpresentedabovecanbeeasilyadaptedto430
provide estimates for the equivalent permeability, and permeability anisotropy, for stylolite-431
bearing(bothsedimentaryandtectonic)limestonereservoirsworldwide.432
Althoughweconcludeherethatourstylolitesformconduitsforfluidflow,wecannotrule433
out that some stylolites, different to those measured here, may provide barriers to flow. For434
example, (1) stylolites may provide barriers to flow if they are characterised by thick and435
continuous layers of clay-rich material, (2) an abstract by Corwin et al. (1997) suggests that436
stylolites associatedwith a cemented zone could be of lower permeability than the surrounding437
host rock, and (3) the modelling of Koehn et al. (2016) suggests that stylolites with simple438
geometries(e.g.,“simplewave-liketype”)maybemorelikelytoprovidebarrierstoflow.Wealso439
highlight that, due to differences in mineral composition and microstructure, the influence of440
styloliteson thepermeabilityofsandstonemaydiffer fromtheir influenceon thepermeabilityof441
carbonaterocks(e.g.,WalderhaugandBjørkum,2003;Emmanueletal.,2010).442
Therefore, and although we provide laboratory measurements for the permeability of443
stylolite-bearing limestone fromsix formations collected from two locationswithinFrance,more444
laboratory measurements on stylolites that are characterised by thick and continuous layers of445
clay-richmaterial arenow required to further explore the role of stylolites on the regional-scale446
permeability of limestone reservoirs (as concluded by Bruna et al., 2018). Laboratory447
measurementsonstylolite-bearingsandstonesalsoofferaninterestingavenueforfutureresearch.448
449
5Conclusions450
Thesalientconclusionsofthisstudycanbesummarisedthusly:451
(1) Thestylolitesmeasuredhereinareconduitsforfluidflow,notbarrierstofluidflow.452
21
(2) The permeability of a stylolite-bearing sample is lowerwhenmeasuredwithwater than453
withgas.We interpret thishereas the resultof theexpansionofminorquantitiesof clay454
foundwithinthestylolite.Theexpansionofclaymineralsconstrictsporethroatsandthus455
reducespermeability.456
(3) Sedimentaryandtectonicstylolitesaffectsamplepermeabilitysimilarly.We interpret this457
asaresultoftheirsimilarmicrostructures.458
(4) X-raytomographydatashowthatthestylolitesaresurroundedbyazoneofhigherporosity459
that ischaracterisedbypores largerthanthose foundinthe intactmaterial.Thisexplains460
whythestylolitesmeasuredhereinareconduitsforfluidflow.461
(5) The presence of larger poreswithin the stylolite zone is supported by an analysis of the462
Klinkenberg slip factor, which highlights that the average pore throat radius of the flow463
pathfollowedislargerwhenthesamplecontainsastyloliteparalleltoflow.464
(6) X-raytomographydatashowthattheporeswithinthestylolitearemuchlesssphericalthan465
thoseof thehost rockand that theyaresometimesalignedwith the teethof thestylolite.466
Sincetheshapeoftheporesarelinkedtotheshapeofthestylolite,weconcludethatsuch467
porosity is likely the consequence of stylolite formation, rather than that stylolites form468
preferentiallyinazoneofhigherporosity.469
(7) Upscaling our laboratory measurements using a simple two-dimensional model that470
considers flow in parallel layers shows that the equivalent permeability of a stylolite-471
bearinglimestonerockmassishigherparalleltobeddingthanperpendiculartobedding.472
(8) Thepermeabilityanisotropythatdevelopsintherockmassduetothepresenceofstylolites473
makes it appear as though the stylolites are acting as barriers to fluid flow (since474
permeabilityperpendiculartobedding is lowerthanthepermeabilityparallel tobedding)475
and may explain the discrepancy between laboratory measurements and field-scale476
observations.477
22
478
Acknowledgements479
This research was partly funded by CNRS and ANDRA (FORPRO program) and the480
Norwegian Research Council (project ARGUS, grant 272217).Wewould like to thank Alexandra481
Rolland, SilvioMollo, GillesMorvan, andBertrandRenaudié.We thankGilles Jouillerot (Rocamat482
PierreNaturelle)forhishelpselectingthematerialsatComblanchienandElodieBollerforherhelp483
attheEuropeanSynchrotronRadiationFacility(beamlineID19).ThecommentsofEinatAharonov484
andoneanonymousreviewerhelpedimprovethismanuscript.485
486
Dataavailability487
Mostof thedataused inthisstudyareavailable inTables1and2.TheX-raytomography488
datamaybemadeavailableonrequest(toFrançoisRenard). 489
23
Figurecaptions490
491
Figure 1. Optical microscope images of the stylolite-free (host rock) material for the studied492
limestones.(a)SampleO1–OxfordianlimestonefromBure.(b)SampleO3–Oxfordianlimestone493
fromBure. (c) SampleO6–Oxfordian limestone fromBure. (d) SampleD3– “Dogger” limestone494
fromBure.(e)CortonlimestonefromBurgundy.(f)ComblanchienlimestonefromBurgundy.495
496
24
Figure2.Opticalmicroscopeimagesofthestylolites foundwithinstudiedlimestones.(a)Sample497
O1–Oxfordian limestonefromBure.(b)SampleO3–Oxfordian limestonefromBure.(c)Sample498
O6–OxfordianlimestonefromBure.499
500
25
Figure3.Opticalmicroscopeimagesofthestylolites foundwithinstudiedlimestones.(a)Sample501
O6–OxfordianlimestonefromBure.(b)SampleD3–“Dogger”limestonefromBure.502
503
504
26
Figure4.Opticalmicroscope imagesof thestylolites foundwithinstudied limestones. (a)Corton505
limestonefromBurgundy.(b)ComblanchienlimestonefromBurgundy.506
507
27
Figure5. Opticalmicroscope images of (a) a sedimentary stylolite and (b) a tectonic stylolite in508
sampleO3–OxfordianlimestonefromBure.509
510
511
512
513
28
Figure 6. Photographs of the cylindrical samples prepared for laboratory measurements. Three514
representativesamplesareshownforeachlithology:anintactsample(ontheleft),asamplewitha515
styloliteperpendiculartoflow(inthemiddle),andasamplewithastyloliteparalleltoflow(onthe516
right).(a)SampleO1–OxfordianlimestonefromBure.(b)SampleO3–Oxfordianlimestonefrom517
Bure. (c) SampleO6–Oxfordian limestone fromBure. (d) SampleD3– “Dogger” limestone from518
Bure.519
520
29
Figure 7. Photographs of the cylindrical samples prepared for laboratory measurements. Three521
representativesamplesareshownforeachlithology:anintactsample(ontheleft),asamplewitha522
styloliteperpendiculartoflow(inthemiddle),andasamplewithastyloliteparalleltoflow(onthe523
right).(a)CortonlimestonefromBurgundy.(b)ComblanchienlimestonefromBurgundy.524
525
526
527
30
Figure 8. Photographs of the cylindrical samples containing either a tectonic stylolite (the two528
samplesontheleft)orasedimentarystylolite(thetwosamplesontheright).Allsamplesarefrom529
sampleO3–OxfordianlimestonefromBure.530
531
532
533
534
31
Figure9. (a) Gas permeability (measuredunder a confining pressure of 2MPa) as a function of535
connectedporosityforintact(i.e.stylolite-free)limestone.Alldatapointsaboveaporosityof0.2are536
takenfromLindetal.(1994).(b)Gaspermeability(measuredunderaconfiningpressureof2MPa)537
asa functionof connectedporosity for limestone samples containingeithera styloliteparallel to538
floworastyloliteperpendiculartoflow.Thegaspermeabilitiesoftheintactsamples(i.e.thedataof539
panel(a))arealsoplottedinpanel(b).540
541
542
543
32
Figure10.(a)Theratioofgastowaterpermeabilityasafunctionofconnectedporosityforintact544
(i.e.stylolite-free)limestone.(b)Theratioofgastowaterpermeabilityasafunctionofconnected545
porosity for limestone samples containing either a stylolite parallel to flow or a stylolite546
perpendiculartoflow.Theratiosoftheintactsamples(i.e.thedataofpanel(a))arealsoplottedin547
panel(b).548
549
550
33
Figure11.Backscatteredscanningelectronmicroscopeimageofthestylolite-hostrockboundary551
(sampleD3 – “Dogger” limestone fromBure).Quartz grainswithin the stylolite are labelled. The552
whitearrowspointtoporosity(inblack).553
554
555
556
557
34
Figure12.Multi-resolutionX-raysynchrotronmicrotomographyimagesofastyloliteinsampleD3558
(“Dogger” limestone from Bure). (a) An image of the stylolite at a voxel size of 6.27 μm. The559
coloured shapes are pores; individual pores are allocated different colours. (b) An image of the560
stylolite at a voxel size of 0.7 μm. The coloured shapes are pores; individual pores are allocated561
differentcolours.562
563
564
35
Figure 13. X-ray synchrotron microtomography image showing two pores within a stylolite in565
sampleD3 (“Dogger” limestone fromBure). The pores are “finger-like” in shape and are aligned566
withtheteethofthestylolite.567
568
569
570
571
36
Figure 14. X-ray synchrotronmicrotomography data showing (a) the number of poreswithin a572
subvolume of 0.16mm3 as a function of pore size inside and outside of a stylolite (sample D3 –573
“Dogger”limestonefromBure)and(b)theprobabilitydensityfunctionasafunctionofsphericity574
for thepores inside andoutside of a stylolite (sampleD3 – “Dogger” limestone fromBure). Two575
subvolumes of sample D3 with the same volume of 0.16 mm3, one inside the stylolite and one576
outsideofit,wereusedtoperformthesecalculations.577
578
579
580
37
Figure15.(a)Theaverageporeradiusoftheflowpathfollowedbythegasmolecules(calculated581
usingtheKlinkenbergslipfactor;seeEquation5)asafunctionofconnectedporosityfortheintact582
(i.e. stylolite-free) samples. (a) The average pore radius of the pores used by the gas particles583
(calculatedusingtheKlinkenbergslipfactor;seeEquation5)asafunctionofconnectedporosityfor584
limestonesamplescontainingeitherastyloliteparalleltofloworastyloliteperpendiculartoflow.585
Theaverageporeradiusof the intactsamples(i.e. thedataofpanel(a))arealsoplotted inpanel586
(b).587
588
589
38
Figure16.Aphotographof a78mm-diameter core fromBure (sampleD3– “Dogger” limestone590
fromBure).Arrowsindicatethepositionofsedimentarystylolites.591
592
39
Tables593
594
Table1. X-raypowderdiffraction (XRPD)analysis showingquantitativemineral composition for595
thesixlimestonesstudiedherein.MineralcontentsofO1,O3,O6,andD3weretakenfromHeapet596
al.(2014).597
Sample Stylolite-freecomposition
(wt.%)
Mineralswithinthestylolite
O1(Oxfordianlimestone) 99%calcite;<1%dolomite dolomite,clay
O3(Oxfordianlimestone) 99%calcite,<1%dolomite;
<1%gypsum;<<1%pyrite
dolomite,gypsum,pyrite,clay
O6(Oxfordianlimestone) 99%calcite,<1%dolomite;
<<1%pyrite
dolomite,pyrite,clay
D3(“Dogger”limestone) 93%quartz;4%dolomite;3%
quartz;<<1%pyrite
dolomite,quartz,pyrite,clay
Cortonlimestone 99%calcite;<1%quartz quartz,clay
Comblanchienlimestone 99%calcite clay
598
599
40
Table 2. Summary of the experimental data collected for this study. Porosities were measured600
using the tripleweightwater saturation technique.Gasandwaterpermeabilitiesweremeasured601
under a confining pressure of 2 MPa. Gas permeabilities were measured with either argon or602
nitrogengas.Waterpermeabilitiesweremeasuredwithdeionisedwater.Asteriskindicatesthatthe603
connectedporosityandthegaspermeabilitydataweretakenfromHeapetal.(2014).604
Sample Description Connectedporosity
Gaspermeability
(m2)
Klinkenbergslipfactor(MPa)
Waterpermeability
(m2)
Gas/waterpermeability
*O1 Nostyloperp 0.154 7.77×10-17 0.189 7.22×10-17 1.08
*O1 Sedstylopara 0.168 3.29×10-16 0.102 1.81×10-16 1.82
*O1 Sedstyloperp 0.166 1.09×10-16 0.206 1.07×10-16 1.02
*O3 Nostyloperp 0.150 2.25×10-17 0.309 2.37×10-17 0.95
*O3 Nostylopara 0.159 2.20×10-17 0.286 2.54×10-17 0.87
*O3 Sedstyloperp 0.169 4.10×10-17 0.278 4.51×10-17 0.91
*O3 Sedstylopara 0.175 4.65×10-17 0.233 4.78×10-17 0.97
O3 Nostylopara 0.076 1.60×10-18 0.449 8.72×10-19 1.83O3 Nostylopara 0.088 3.63×10-18 0.458 1.98×10-18 1.83
O3 Nostyloperp 0.084 2.05×10-18 0.568 1.08×10-18 1.90
O3 Tectstyloperp 0.075 2.14×10-18 0.452 1.16×10-18 1.84
O3 Tectstylopara 0.080 2.18×10-17 0.079 9.73×10-18 2.24
O3 Tectstylopara 0.082 2.62×10-17 0.078 6.35×10-18 4.13
O3 Tectstylopara 0.077 2.17×10-17 0.089 7.17×10-18 3.03
O3 Sedstyloperp 0.087 2.32×10-18 0.621 1.12×10-18 2.07
O3 Sedstyloperp 0.087 2.61×10-18 0.537 1.51×10-18 1.73
O3 Sedstylopara 0.096 1.75×10-17 0.196 1.34×10-17 1.31
O3 Sedstylopara 0.095 1.84×10-17 0.195 1.15×10-17 1.60
*O6 Nostyloperp 0.067 3.04×10-18 0.335 7.62×10-19 3.99
*O6 Sedstylopara 0.070 5.36×10-17 0.102 3.72×10-18 14.41
*O6 Sedstylopara 0.092 6.58×10-17 0.111 1.07×10-17 6.15
*O6 Sedstylo 0.084 1.51×10-17 0.241 6.04×10-18 2.50
41
perp
*O6 Sedstyloperp 0.086 1.43×10-17 0.252 4.98×10-18 2.87
*O6 Sedstyloperp 0.085 1.39×10-17 0.217 2.73×10-18 5.09
*D3 Nostyloperp 0.034 4.38×10-19 0.557 1.32×10-19 3.32
*D3 Nostylopara 0.034 3.69×10-19 0.447 8.11×10-20 4.55
*D3 Sedstyloperp 0.037 4.88×10-19 0.398 1.61×10-19 3.03
*D3 Sedstyloperp 0.032 3.44×10-19 0.521 7.55×10-20 4.56
*D3 Sedstyloperp 0.029 1.56×10-19 0.847 6.83×10-20 2.28
*D3 Sedstylopara 0.040 5.98×10-18 0.089 7.01×10-19 8.53
COMB Nostylopara 0.031 1.64×10-18 0.270 - -COMB Nostylopara 0.026 7.20×10-19 0.335 - -
COMB Sedstylopara 0.037 1.54×10-17 0.118 - -
COMB Sedstylopara 0.034 3.20×10-18 0.180 - -
COMB Sedstylopara 0.035 4.64×10-18 0.140 - -
COMB Nostyloperp 0.021 2.18×10-19 0.298 - -
COMB Sedstyloperp 0.027 6.95×10-19 0.164 - -
CORT Nostylopara 0.026 1.28×10-18 0.092 - -CORT Nostylopara 0.028 1.07×10-18 0.099 - -
CORT Sedstylopara 0.033 1.98×10-16 0.024 - -
CORT Sedstylopara 0.030 5.22×10-17 0.040 - -
CORT Sedstylopara 0.031 1.31×10-17 0.078 - -
CORT Nostyloperp 0.026 7.85×10-19 0.079 - -
CORT Nostyloperp 0.029 6.58×10-19 0.142 - -
CORT Sedstyloperp 0.027 7.48×10-19 0.085 - -
CORT Sedstyloperp 0.031 9.45×10-20 0.433 - -
605
606
42
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