ReactionpathmodellingillustratingthefluidhistoryofanaturalCO2-H2Sreservoir 1

CarmenZwahlen,RoyWogelius,CathyHollis,GregHolland2

3

Abstract4

Despite the increasing interest in geologic co-sequestration of CO2 and H2S, the long-term5

consequences of the chemical interactions involved in this process remain largely unknown on a6

reservoir scale.AMississippianagedCO2-H2S reservoir in LaBargeField,Wyoming,USA is an ideal7

studysitetoinvestigatemineralandfluidreactionsrelatedtogaseousH2SandCO2.Weconducted8

tworeactionpathmodelsbasedonmineralogical,fluid,gas,andstableisotopecompositionaldata9

to discern the role of CO2 influx upon the generation of H2S through thermochemical sulphate10

reduction(TSR).Wediscriminatebetweentwomodels-oneinwhichTSRistriggeredbytemperature11

atagivenburialdepthandonewhereTSR istriggeredby ingressofCO2.Thereactionpathmodel12

baseduponburial-controlledTSRand laterCO2 influx isconsistentwithmineralogicalobservations13

andstableisotopemeasurementsfromdrillcores.ThemodelsshowthatCO2influxleadstocalcite14

precipitationwhichisonlylimitedbythecalciumconcentrationinthefluid.Thismodellingapproach15

is useful in constraining the timing of fluid flux in the reservoir and gives further insight into the16

mineralogical consequences of the gas,water, and rock interactions occurring in the reservoir. In17

termsofgeologic co-sequestration this implies that theadditionofCO2 intoa reducingcarbonate18

systemcanresultincalciteprecipitation,insteadofanhydriteaspreviouslythought.Furthermore,it19

isonly limitedby theavailabilityofCa2+ andwill thereforenotdiminish theamountofH2S in the20

system.21

Introduction22

Naturaloil andgas reservoirs cangive insight intokeyprocessesactingovergeological timescales23

(Allis et al., 2001; Bickle et al., 2013; Kampman et al., 2014; Kaszuba et al., 2011). Additionally,24

knowledgeoftheH2Sconcentrationinreservoirsisofinterestfordrillingsafetyandgasdegradation.25

Itisimportanttounderstandthefluidhistoryinthesereservoirsinordertopredictwhichreactions26

canoccurandtowhatextent.Inparticular,thereisincreasinginterestinthefeasibilityofgeologic27

co-sequestrationofCO2andH2Swithincarbonatesystems(Kaszubaetal.,2011;Williams&Paulo,28

2002).Thelongtermconsequences,however,aredifficulttopredictusinglabandfieldexperiments29

orgeochemicalreactionmodellingalone.30

TheCO2-H2SreservoirinLaBargeField,southwesternWyoming,USA,hasbeenclassifiedandstudied31

asanaturalanalogueforgeologicalco-sequestration(Allisetal.,2001;Kaszubaetal.,2011)(Figure32

1).Thegas trapped inLaBargeFieldconsistsonaverageof66%CO2,21%CH4,7%N2,5%H2Sand33

0.6% He (Huang et al., 2007). The H2S is thought to be produced by thermochemical sulphate34

reduction (TSR)whereastheCO2 isbelievedtobe fromamagmaticsource (DeBruin,1991,2001;35

Huangetal.,2007;Stilwell,1989;Zwahlenetal.,2019).However the timingof the fluids remains36

controversial.Hydrocarbonsare thought tohavemigrated into thereservoir approximately84 -7637

Maago(Johnson,2005)(Figure2).Differenttechniquesanddatasetshavebeenemployedtostudy38

theinfluenceofCO2andH2Suponeachother(Huangetal.,2007;Kaszubaetal.,2011;Obidi,2014;39

Zwahlenetal., 2019):diffusionmodellingof theCO2-CH4distribution concluded thathydrocarbon40

andCO2wereintroducedatsimilartimes,around50Maago(Huangetal.,2007)andreactionpath41

modellinghasbeenusedtosuggestthatTSRwastriggeredbytheinfluxofCO2(Kaszubaetal.,2011).42

Incontrast,amorecomplexdiffusionmodel,usingthesameCO2-CH4distribution,suggestsalarger43

timegapof>45MabetweentheCH4andCO2influx,withtheCO2arriving<3Maago(Obidi,2014).44

Mineralogical observations and stable isotope measurements also suggest a two step process in45

whichCO2arrived later (Zwahlenetal.,2019). Inthispaper,webuildageochemicalmodel that is46

informedby petrographical and geochemical observations to further constrain the fluid history in47

LaBarge Field. Thiswill improve our understanding of themineralogical consequences of the CO248

influx,theinitiationofTSRandhencetheH2Sgenerationinthecarbonatereservoir.49

50

Figure1a)Mapofgreatergreenriverbasinwithmarkedfluidsamplelocalitiesb)MapoftheCO2-H2Sreservoirsalongthe51Moxa arch andmain tectonic features in the areamodified after Becker and Lynds, (2012). c) The LaBarge Field with52samplingwellsmodifiedafter(Stilwell,1989).53

We conducted two different reaction path models: Model 1 involved a two step process where54

burial related TSR occurred and CO2 ingress occured later (Figure 3). Model 2 assumed TSR was55

triggered by the CO2 influx (Figure 3). The two different scenarios also occurred at different56

temperatures.TheCO2isthoughttohaveenteredthereservoiratthemaximalburialtemperature,57

estimatedtobe200to215°C(Zwahlenetal.,2019),whereasburialrelatedTSRisthoughttohave58

startedaround175°Cbasedonfluidinclusionmicrothermometrydata(Zwahlenetal.,2019).Using59

asimpletitrationmodelof theorganosulphurcompoundmethionine(aq)andCO2(g)wesimulated60

TSRandCO2inputintothereservoir,respectively(Figure3).Thisapproachcouldbeappliedtoother61

systems inorder to improvetheunderstandingof their fluidhistoryandthe long-termfateof the62

presenceofareactivegassuchasH2S.63

64

Figure2Burialhistoryof theMadisonFormationatLaBargemodifiedafterRobertsetal., (2005)andadaptedto500m65furtherupliftatLaBarge inthe last5Ma.TheMadisonFormation ishighlighted ingrey.Fm.,Formation;Sh.,Shale;Gp.,66Group;Ss.,Sandstone;L.Cret.,LowerCretaceousrocks.67

68

Figure3Schematicofmodel1&2includingreactiontemperatureandaimedmineralprecipitates.69

MineralogyandparagenesisofLaBarge70

TheMississippianMadisonFormation inLaBargeFieldconsistsof limestone,thatwasdeposited in71

shallowwater, interlayeredwithdolomitethatformedduringshallowburial,andformeranhydrite72

nodules thatwere replacedby calcite (Figure 4). A similar successionhas beenobserved in other73

partsofWyoming,USA(Budai&Cummings,1987;Budai,1985;Budaietal.,1984;Buoniconti,2008;74

Katz,2008;Katzetal.,2007;Smithetal.,2004;Sonnenfeld,1996).The following summaryof the75

paragenetic sequence in LaBarge Field has been established by petrographic observations, stable76

isotope and fluid inclusion microthermometric analysis (Zwahlen et al., 2019). The carbon and77

oxygen isotopic signaturesmeasured onwhole rock limestone and dolostone are consistentwith78

other measurements of the Mississippian Madison Formation in Wyoming (e.g. Budai and79

Cummings, 1987; Katz et al., 2006)(Figure 5). Calcite cements fill primary and secondary matrix80

macropores. Thereafter, quartz, fluorite and fracture filling calcite precipitated in sequence from81

hydrothermalfluidsthatcooledduringfluidcirculationandmineralprecipitation.Thisissupported82

by decreasing primary fluid inclusion homogenisation temperatures within single quartz crystals83

fromcoretorim(145-110°C),byprimaryfluidinclusionhomogenisationtemperaturesinfluorite84

of105-110°Candcarbonandoxygenisotopicvaluesofthefracturefillingcalcitephasethatshow85

equilibration with the host rock (Figure 4c & 5). After the fracture filling calcite precipitation,86

hydrocarbonsseepedintothesystemat84-76Maago(Robertsetal.,2005).Thelastprecipitating87

carbonate mineral phase is the calcite that replaces anhydrite, which has primary fluid inclusion88

homogenisation temperatures between 175-200 °C (Figure 4d & e). This is interpreted to have89

occurred during thermochemical sulphate reduction, also leading to precipitation of pyrite,90

elemental sulphurandsolidbitumen (Figure4d& f).There isnoprimaryanhydrite leftexcept for91

micro inclusions in the calcite nodules (Figure 4e). The distinctive, depleted carbon and oxygen92

isotopic values of these samples are in consistent with an isotopically light carbon source, which93

could either be hydrocarbonor carbondioxide, andwith the elevatedprecipitation temperatures94

(Figure5).95

96

Figure4a)Mississippian limestoneandb)dolomitehost rock.c)Fracture fillingcalcited)Formeranhydritenodule that97was replaced by calcite with elemental sulphur precipitated between calcite crystals e) Remaining anhydrite98microinclusionsincalcitethatreplacedanhydritef)pyrite99

100

101

Figure5Carbonandoxygenisotopicdatafromcarbonatesamples.Black,greyandwhitesymbolsrepresentsamplesfrom102drillcoreFC13-10,LR8-11andFC15-28respectively.Dataisfrom(Zwahlenetal.,2019).103

Modelinputdataandconstraints104

Thetwodifferenttitrationmodelsarebasedongas,mineralogicalandfluidcompositionaldatafrom105

fourboreholesatthecentreofthereservoir(Blondesetal.,2016;Huangetal.,2007;Zwahlenetal.,106

2019) (Figure 1). Therefore, the reservoir conditions are calculated for this part of the field. The107

reservoir temperature iscalculatedtobe133°Cbasedonanaveragemaximumdepthof the four108

drill cores, 4612m, and a geothermal gradient of 28.8 °C/km (Roberts et al., 2005). According to109

bottomholepressuresthegaspressureisonaverage450barforthefourboreholesandidenticalto110

hydrostaticpressure(Becker&Lynds,2012).111

Themodelwasfurtherconstrainedbymineralogicaldatafromthreedrillcores(LR8-11,FC15-28and112

FC13-10,Figure1B&C).Themodelissetupfor1kgofwater.Basedonadensityof1kg/dm3anda113

porosity of 9% (Huang 2007) this water would fill a rock volume of 10111 cm3. The host rock is114

composedofdolomite(~60%)andlimestone,however,inthemodelweusedolomiteasasolehost-115

rock condition (10091 cm3) to avoid over-constraining the system (Zwahlen et al., 2019). The116

estimatedvolumeofcalcitethatreplacedanhydriteinthedrillcoresis0.2vol%whichequatesto20117

cm3 (Zwahlen et al., 2019). The pH in the carbonate reservoir is likely governed by calcite and118

dolomite.Theoriginalfluidat133°CwiththepHset inequilibriumwitheitherdolomiteorcalcite119

resultsinapHof5.5to5.7andthereforewechooseastartingpHof5.6.120

H2Sfugacitywasusedasaconstraintfortheoxygenfugacityofthemodel.TheH2Sfugacitycanbe121

calculatedbasedonthegascomposition,thetotalgaspressuresandthefugacitycoefficient:122

𝑓"#$(&)=P*𝑥"#$(&) ∗ φ"#$(&) (1)123

where fH2S(g) is thegas fugacity,P the totalpressure, xH2S(g) themole fractionofH2SandϕH2S(g) the124

fugacitycoefficient.ThefugacitycoefficientofH2Sinthegasmixturewascalculatedtobe0.33with125

thePeng-Robinsonequationofstatewhichhasbeen implemented into theThermosolvesoftware126

(Barnes,2007;Koretsky,2004;Peng&Robinson,1976;Richardetal.,2005).Thepartialpressureof127

H2Sis22barandresultsinafugacityof7.3bar.Thisvalueliesintherangeoffugacitiescalculated128

forcarbonatereservoirsintheAlbertaBasin(Richardetal.,2005).Ithasbeensuggestedthatthese129

fugacities are controlled by metastable equilibrium between hydrocarbons, organic sulphur130

compoundsandelementalsulfur(Richardetal.,2005).Allthesecompoundsarealsopresentinthe131

MadisonFormationintheLaBargeFieldandhenceitislikelythatametastableequilibriumbetween132

thesamecompoundscontrolstheH2Sfugacity.133

Thefluidcomposition inthereservoirhasbeenreportedforwellUnit22-19(Blondesetal.,2016)134

(Figure1).ThisfluidcompositionissimilartothefluidreportedfortheWhiskeyButtesFieldfurther135

southalongtheMoxaArch(Figure1,Table1)(Blondesetal.,2016).Thesefluidsarecharacterized136

byahigherHCO3-andSO4

2-concentrationsbut lowerCa2+concentrationscomparedto fluids from137

wellspenetratingtheMadisonFormationawayfromtheMoxaArchareaintheGreaterGreenRiver138

Basin(Table1)(Blondesetal.,2016).Initialfluidconcentrationsweresettothosemeasuredinwell139

Unit19-22withtheexceptionforHCO3-,SO4

2-andMg2+.TheHCO3-concentrationisexcessivelyhigh140

duetotheequilibrationwiththeCO2inthereservoir.Thereforeweusedtheaverageconcentration141

from wells outside the Moxa Arch (Table 2) (Blondes et al., 2016). The initial Mg2+ and SO42-142

concentrations were set in equilibrium with dolomite and anhydrite, respectively (Table 2). The143

chlorineconcentrationactedasachargebalancethroughoutthereactionpath.144

145

Table1AveragefluidcompositionoftheMadisonFormationwater(Appendix1)indifferentlocationswithintheGreater146GreenRiverBasin(Figure1a).147

148

Table2Fluidcompositionatthebeginningandendofmodel1and2.149

Weconductedthemodellingwith thereactmoduleandtheV8R6themodynamicdatabasewithin150

Geochemist'sWorkbench®(GWB)software(Appendix3).Inmodel1,theburialrelatedTSRprocess151

was simulated as a titration model where methionine (aq) was titrated into the fluid at 175 °C,152

initiallyinequilibriumwithdolomiteandanhydrite,until20cm3ofcalciteprecipitated(Table2).The153

resultingfluidwasthenreactedwith0.5molofCO2(g)at200°Cand1cm3ofanhydrite(Table2).154

The anhydritewas added at the onset of CO2 addition to simulate the reaction of any remaining155

sulphate.Formodel2, theTSR triggeredbyCO2model, the sameamountofmethionine (aq)and156

CO2 (g)were titrated simultaneously into the fluidat200 °C, initially inequilibriumwithdolomite157

andanhydrite(Table2).Wechose200°CasaCO2influxtemperatureinordertonotoverestimateit158

andtobeinagreementwithprimaryfluidinclusionhomogenisationtemperaturedata(Zwahlenet159

al., 2019). In all cases the Eh was kept fixed during the reaction path since it controlled by the160

sulphur species and buffered by the H2S(g) concentration (Kaszuba et al., 2011). We chose161

methionineas anorganic compoundbecause it includes sulphurwhich is in agreementwith solid162

bitumenanalysis (Kingetal.,2014).Partsof theorgano-sulfur in thesolidbitumenare likely from163

theoriginaloil.However in termsof sulphurmassbalance the sulphur inmethionineplaysavery164

minorrolecomparedtothevastamountofanhydritethatgetsconsumed.Methioninealsocontains165

nitrogen,whichresultsinanoverestimationofgaseousnitrogen;however,thedatabasedoesn'tlist166

anorganosulphurcompoundwithoutnitrogensuchascystine.Forthestable isotopefractionation167

modelling, we updated the GWB stable isotope database (Appendix 2). For the sulphate168

fractionationweusedequilibriumandkineticfractionationfactors(Kiyosu&Krouse,1990b;Ohmoto169

&Rye,1979).Wechooseaδ34Scompositionforanhydriteof15‰,inagreementwithMississippian170

sulphate and carbonate-associated sulphate measured in pre-TSR calcite in the LaBarge Field171

(Zwahlen et al., 2019). The sulphur isotopic composition of methionine was set to the average172

phosphoriaoilcompositionof1‰(Kingetal.,2014;Orr,1974).Themineralsweresegregatedfrom173

isotopicexchangewiththefluidunlesstheydissolvedorprecipitated.174

Modellingresults175

Theoxygenfugacityofthemodels,constrainedbythecalculatedH2Sfugacity,resultedintheHm-Mt176

buffer (Figure6).This isclosetooxygenfugacitiescalculatedforothercarbonatereservoirsand is177

muchlargerthanhasbeenestimatedforclasticreservoirs(Figure6)(Helgesonetal.,1993;Richard178

etal.,2005).Itremainsunclearwhetherthedifferenceinoxygenfugacitiesbetweencarbonateand179

clasticreservoirsisaneffectofthermodynamicdatasetinconsistency(Richardetal.,2005).180

181

Figure 6 Oxygen fugacity calculated for the LaBarge reservoir (filled square) compared to other carbonate (triangles)182(Richardetal.,2005)andclastic(circles)reservoirs(Helgesonetal.,1993).ThecurvescorrespondtotheO2(g)fugacities183setbythehematite-magnetite(HM-MT)andthepyrrhotite-pyrite-magnetite(PO-PY-MT)buffers.184

In bothmodels (model 1& 2) the fluid evolved to higher dissolved carbon dioxide and hydrogen185

sulphide and lower calcium concentrations (Table 2). The two models show distinctly different186

mineralogicalresults(Figure7).Intheburial-relatedTSRmodel(model1)anhydritedissolvedwhile187

20 cm3 calcite and elemental sulphur precipitated as a result ofmethionine titration (Figure 7a).188

Pyrite and dolomite remained saturated throughout the model, although pyrite precipitation is189

limitedby the lowamount of iron in the fluid. In the secondpart of thismodel (model 1) calcite190

precipitatedasaconsequenceofCO2influx(Figure7b).WealsotestedtheCO2influxwithoutthe1191

cm3 of anhydrite. This resulted in an equally saturated volume of calcite, however, less calcite192

precipitatedduetothelimitingamountofCa2+inthefluid.Theporosityremainedalmostunchanged193

andincreasedbyonly0.1%throughoutthemodel.194

195

Figure7Mineralogical resultsof the two titrationmodels:a)model1part1withmethionine titration including runsat196165°C,175°Cand185°Cb)model1part2withCO2influxandc)model2withsimultaneousmethioninetitrationandCO2197influx.198

IntheCO2-triggeredTSRmodel(model2)anhydritealsodissolved,alongwithcalciteprecipitation.199

However,withthesameamountofmethionine,morethan1cm3anhydriteremainedundissolvedat200

the end of the reaction path and less than 19 cm3 of calcite precipitated (Figure 7c). A further201

difference to model 1 is that elemental sulphur did not become saturated (Figure 7c). Since202

elementalsulphurprecipitatedinmodel1butnotinmodel2weexploredtheeffectoftemperature203

on the saturation of elemental sulphur and ran the methionine titration of model 1 at different204

temperatures(Figure7a).Theresultsshowedthatelementalsulphursaturatedat165°Cwhereasat205

185°Ccalcitewastheonlyprecipitatingmineral(Figure7a).206

Additionally we investigated the isotopic fractionation of sulphur during the reaction paths, with207

pre-definedδ34Sisotopicvaluesforthedifferentphases,andcomparedittoexistingmineralandgas208

stable isotopedata (Kingetal., 2014;Zwahlenetal., 2019) (Figure8). Therewasno fractionation209

expectedforaqueoussulphateincorporationintocalcite,thereforewecancomparethemeasured210

carbonate associated sulphate (CAS) isotopic composition to the modelled aqueous sulphate211

isotopic composition (Burdett et al., 1989). The modelled extent of the sulphate fractionation212

depends on whether equilibrium or kinetic fractionation factors are used (Figure 8a) (Kiyosu &213

Krouse, 1990a; Ohmoto & Rye, 1979). The modelled sulphate fractionation with the kinetic214

fractionation factor overlaps completely with the extent of fractionation observed in carbonate215

associatedsulphatesmeasuredinanhydrite-replacivecalcite(15.3to22.7‰)(Zwahlenetal.,2019)216

(Figure 8a). Therefore the kinetic fractionation factor is used for the following fractionation217

calculation.Thefractionationbetweenaqueousandgaseoushydrogensulphide isnegligibleabove218

150 °C which enables us to compare the measured gaseous isotopic H2S composition to the219

modelled aqueous isotopic H2S composition (Czarnacki & Hałas, 2012; Eldridge et al., 2016). The220

modelledδ34Svalueoftheaqueoushydrogensulphideincreasedduringthereactionpathtocloseto221

10‰,which is themeasured composition for the gaseous H2S in the reservoir (King et al., 2014)222

(Figure8b).Hencetheremodelledaqueoushydrogensulphideisotopiccompositionisinagreement223

withthereservoirsvalue.224

225

Figure 8 a) Kinetic and equilibrium fractionation of sulphate in model 1 in comparison with measurements from the226reservoir carbonate associated sulphate (15.3 to 22.7‰, grey area) (Zwahlen et al., 2019) and b) Isotopic evolution of227aqueoussulphideinmodel1bycontrastwithgaseoushydrogensulphidedatafromthereservoir(dottedline)(Kingetal.,2282014).229

Discussion230

Intermsofmineralogy,thetwomodelsdiffermainlyinthesaturationofelementalsulphurandthe231

amount of calcite that is precipitated. Model 1 predicts the mineralogy observed in the cores,232

specifically calcite, elemental sulphur and pyrite. Elemental sulphur doesn't saturate when the233

modelisrunabove175°C.ThisindicatesthatTSRlikelyoccurredat≤175°C.Theloweramountof234

dissolvedanhydriteandprecipitatedcalciteinmodel2couldberelatedtothedecreasingsolubility235

of anhydrite with increasing temperature. Both models suggest that calcite precipitates as a236

consequenceofCO2influx,notanhydriteaspreviouslysuggested(Kaszubaetal.,2011).Alltogether237

model1representsthemineralogicaldatabetterthanmodel2.Thisindicatesthattheinititationof238

TSRwasmostlikelyrelatedtotemperature,andoccurredat≤175°C.Thisisinagreementwiththe239

lowest primary fluid inclusion homogenisation temperatures measured in calcite that replaced240

anhydrite(Zwahlenetal.,2019).ThemodelledTSRprocessismoreefficientattemperaturesaround241

175°Ccomparedto200°Cduetothehigheranhydritesolubilityatlowertemperatures.Thismight242

explainwhyTSRhasnotbeenobservedathightemperature(≥200°C) intheLaBargeFieldandin243

otherstudyareas(Biehletal.,2016;Bildsteinetal.,2001;Caietal.,2001;Claypool&Mancini,1989;244

Hao et al., 2015; Heydari&Moore, 1989; Jenden et al., 2015; Jiang et al., 2015; Liu et al., 2013;245

Machel, 2001; Riciputi et al., 1994;Worden et al., 1995;Worden& Smalgeoley, 1996) and could246

indicatethatthereisanuppertemperaturelimitforTSR.247

Model1 isfurthervalidatedbyconsiderationofthefractionationofthesulphurspeciespresentin248

thereservoir.Themodelledfractionationofaqueoussulphateandhydrogensulphide isconsistent249

with mineral and gas data from the reservoir when a kinetic fractionation factor is used. The250

fractionationbetweentheCASsulphateisotopeshasbeenpreviouslyrelatedtofractionationduring251

the reduction step (Meshoulam et al., 2016; Zwahlen et al., 2019). Therefore the modelled252

fractionation during the reduction step in agreementwith the extent of fractionation of the CAS253

samplesalsoconfirms that reduction rather thandiffusion is the rate limitingstep (Meshoulamet254

al.,2016).255

Inbothmodelsthecalciumconcentrationinthefluidlimitedtheextentofcalciteprecipitation.The256

lowcalciumconcentrationinthemodelledfluidisinagreementwiththereportedconcentrationsof257

the fluid from the LaBarge Field (Table 1). The high sulphate concentration in the reported fluid258

analysis suggests that calcium sulphate was not the limiting factor for TSR and that the calcium259

concentration is low due to calcite precipitation. The aqueous hydrogen sulphide concentration260

predictedbythemodelsisextremelyhighbutmightbemuchlowerinrealityduetosorptiononto261

organicmatterorprecipitationofmoremetalsulphides(Kingetal.,2014).262

The insights regarding the initiation andmineralogical consequencesof the TSRprocess basedon263

reactionpathmodellingintheLaBargeFieldcouldhelptopredicttheoccurrenceofTSRelsewhereif264

fluid and mineral data are available. For geologic co-sequestration, these models show that265

anhydritedoesnotprecipitateasaconsequenceofCO2additionintoareducingcarbonatesystem.266

This disagrees with previous predictions (Kaszuba et al., 2011) and indicates that the H2S267

concentration issolelydiminishedbysorptionandsulphideprecipitation.Theseresultshavetobe268

takenintoaccountwhenco-sequestrationstoragesecurityisevaluatedduetothereactivecharacter269

of H2S. However the insignificant porosity change is consistent with previous modelling results270

(Kaszubaetal.,2011),whichisimportantforpressurepredictionsofco-sequestrationscenarios.271

Conclusion272

Based on two simple reaction path models we explored the effect of burial related TSR and273

compared it to TSR triggered by hot CO2 influx.Model 1, which assumed burial related TSR and274

subsequentCO2 influx, correctlypredicts themineralogyobserved in the core.On theotherhand275

model2 fails tosaturateelementalsulphur.ThereforeTSRatLaBargewas likely inducedbyburial276

andtheCO2influxhadnoinfluenceonTSR.FurtherbothreactionpathmodelssuggestthattheCO2277

influxleadstocalciteprecipitationinsteadofanhydriteaspreviouslythought(Kaszubaetal.,2011).278

This modelling approach is further validated by the agreement between sulphate and sulphide279

fractionationandthemeasurementsofthecorrespondingphasesinthereservoir.Theoutputofthe280

modeladdsadditionalconstraintsonthefluidhistoryof thereservoirandcanhelppredictingTSR281

elsewhere. In terms of geologic co-sequestration themodelledmineralogical consequences differ282

from previous predictions but are consistent with little porosity changes observed in previous283

modelsandhavetobetakenintoaccountwhenassessingdifferentco-sequestrationscenarios.284

However,morefluidanalysisfromwellsintheLaBargeFieldcouldimprovethevalidityofthemodel.285

Additionally kinetic rate laws could be included into to the model to compare the rates of the286

differentoccurringprocessessuchasdissolutionandprecipitationtotherateofsulphatereduction.287

Future research could also look at the complex fractionation processes of carbon and oxygen288

isotopes.289

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