diagenesis = any chemical, physical, biological …...diagenesis = any chemical, physical,...

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Diagenesis=anychemical,physical,biologicalchangea5erdeposi8on.Itdoesnotincludeweatheringandmetamorphism.Diagene8cprocessesoccuratrela8velylowtemperaturesandpressuresandcausechangestooriginalmineralogyandtexture.Theboundaryofpressureandtemperatureswherediagenesispassesintometamorphismisnotclearlydefined.Compac8onwilloccurassedimentsareburied.Somemineralswillberecrystallizedordissolve.Newmineralsmayalsogrow(authigenesis).Grainsbecomecementedbymineralsthatprecipitatefromsolu8onsmovingthroughporespaces.Generallyporositydecreasesduringdiagenesis,apartfromwheredissolu8onofmineralsoccursandduringdolomi8za8on.Anapprecia8onofthediagene8cdevelopmentofasedimentcanalsohelpinourunderstandingofitstectonichistoryandthekindsoffluidsthathavemovedthroughit.ThisisVERYimportantinforma8onforthemineralandhydrocarbonindustriesasitrelatestotheporosity(spaceswithinarockexpressedasΦ)andPermeability(theinterconnectednessofthosespacesexpressedasK).RockswithhighΦandKmakeexcellentreservoirsforoilgasorwater.Diagenesisisalsoimportantinunderstandingthevariousprocessthatconvertorganicmaterialinsedimentsintooilandgas.

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Thestyleofdiagenesiswillvarydependingonthetypeofsedimentandtheenvironmentinwhichthesedimentisundergoingdiagenesiseithermarineorterrestrial.Ifterrestrial,withdepthyoucandividediagene8czones:1.  TheVadosezoneistheposi8onatwhichthegroundwater(thewaterinthe

sedimentspores)isatatmosphericpressure("vadose"isLa8nfor"shallow").

2.  Thephrea8czone(zoneofsatura8on)=por8onofanaquifer,belowthewatertablewhereporesandfracturesaresaturatedwithwater.Thephrea8czonemayfluctuateduringwetanddryperiods.

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Anumberofprocessesmayoccurinthisshallowzoneofburial.1.  Porewatersmay“wash”claygradematerialintoasandstonechangingitsoriginal

texturalcomposi8on.AsandstonethatwasoriginallyahighΦandKquartzarenitemaytakeonthetextureofawackestone.

2.  Unstablecomponents(pyroxenes,amphiboles,plagioclase)maycon8nuetodegradebecomingpi]edandetched.

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3.Growthofauthigenicmineralssuchasilliteandkaolinite.Thelackofclaysatthecontactsbetweenthesandstonegrainsindicatesthattheclaywasnotdepositedwiththesandstonebutgrewa5erdeposi8onofthesandstone.Claysmaygrowasaresultofthedissolu8on/altera8onofmoreunstablemineralogicalcomponentsinthesandstone.4.Diagene8creddeningofsediments.Redrimsofhema8tecanoccurasaprimarydeposi8onfeature(seedesertsandsearlier).Redstainingcanalsooccurduringdiagensiseitherbyhema8tebeingwashedinthesedimentfollowingdeposi8onorfromthealtera8on/dissolu8onofunstablemienralswithinthesedimentleadingtothereleaseofironandtheforma8onofdiagene8chema8te.Diagene8chema8tecanbedis8nguishedfromprimaryhema8terimsbytheabsenceofhema8teatgraincontactsinthecaseofdiagene8chema8te.

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Pressuresolu8onorpressuredissolu8on=dissolu8onofmineralsatgrain-to-graincontactsintoanaqueousporefluid.Thisreleasessilicaintotheporefluidwhichcanthenbere-precipitatedasasilicacement.NOTE:thisisnottheonlysourceofsilicaforcements.Silicadustcanalsobepresentindesertsandstonesandsilicacanalsobereleasedfromsilicatemineralsundergoingdissolu8on.

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Silicacementsarecommonlyprecipitatedinop8calcon8nuitywiththequartzgraintheyaresurroundingcrea8ngaquartzovergrowth.Calcitecementsarecommon(althoughdolomiteandsideritecementscanalsooccur).Calcitecementscanformeitherearlyorlate,earlycementsformingsome8mesverysoona5erthedeposi8onofthesediment.Earlyprecipita8onofcalcitemayinhibittheforma8onofquartzovergrowths.Latercalcitecementscandevelopfromthereleaseofionsduringmineraldissolu8oninlaterdiagenesis.

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Thediagenesisoforganicrichmudstonesisprobablythemostimportant“economicprocess”thatwewillcover.Itisfromtheprogressivechemicalalterna8onoftheorganicmaterialandreleaseofvariousionsandwaterfrommudstonesthatcontroltheforma8onandini8almigra8onofpetroleum.Diagenesisoccursinanumbersofzonesassedimentgetsprogressivelyburiedover8me:Zone1:0-0.5mThisthinzonecontainsporewatersthatareoxic.Porosityishigh,around80%asthesedimentinthiszoneisrela8velyuncompacted.Organicma]erisoxidizedbybacteriawhichproducesCO2(asthisisbiologicallyproducedthiswillbeisotopicallylight12C).ThisCO2dissolvesintheporewatersproducingHCO3

-mostofwhichislosttoseawaterandassuchgenerallydoesn’treactwithCatoformcalcite.Bioturba8on(mixingofsedimentsbyorganisms)isimportantinthiszoneandintothetopofzone2whichhelpskeepthiszoneintheoxiczone.Zone2:0.5–10mThezoneofbacterialsulfate(SO4

2-)reduc8ongenerallybyDesulphovibrovulgaris.Thesulfate(fromseawater–andul8matelyfromtheerosionofcon8nentalrocks)willeventuallygetexhaustedwithdepth.TheS2-producedbysulfatereduc8onmayformH2SwhichmayreactwithFe3+toproducepyrite(FeS2)givingtherockadarkcolour.TheCO2producedbysulfatereduc8onmayreactwithCatoformcalciteconcre8ons.

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Zone3:10–1000mInthiszoneallthesulfatehasbeenreducedandassuchpyriteandcalciteconcre8onproduc8onstops.ThisisthezoneofOrganicFermenta8on.AsFeisnownotbeingtakenintheforma8onofFeS2itisfreetoreactwiththeavailableCO2toformmineralssuchasSiderite(FeCO3).Furthercompac8oninthiszoneseestheexpulsionofmuchoftheporewatersandanincreaseinthesedimentdensity.Zone4:1–2.5kmRemainingorganicma]erbrokendownfurtherbydecarboxyla8on(breakingdowncarboxylCOOHgroups):R.CO2H!RH+CO2Porewatercon8nuestobeexpelledinthiszone.

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Zone5:2.5–7kmThezoneofhydrocarbondiagenesis.Heretheremainingorganicma]er(calledKerogen)isbrokendownfurther(“cracked”)genera8ngpetroleum.ConversionofclaysandotherunstablecomponentsreleasesH2O(boundwater–watercontainedintheclaymineralcrystalstructure),SiO2,Mg2+andFe2+.Thisfinalreleaseofwaterallowsthesecomponentstomigratetootherforma8onswheretheyareusedinthediagenesisofotherrocks.Thisexpulsionofboundwateralsohelpsmoveanypetroleumformedoutofthesourcerockandintoreservoirs.Astemperatureincreases,gasratherthanliquidpetroleumisproduced.Zone6:>7kmBythe8meyougettothesedepthsyouleavethezoneofdiagenesisandpassintometamorphism.Illiteclayaltersintosericiteandeventuallymuscoviteataround300°C.

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Asyoucansee,togeneratepetroleumorgas,theorganicmaterialcontainedinthemudstonehastosurvivealotofprocesses.

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Duetothelackofsulfateinfreshwatersystems,diagenesisoforganicsskipsthesulfatereducingstageandgoesrapidlyintobacterialfermenta8on.TheCO2producedreactswithFeinthegroundwatertoproduceSiderite(FeCO3).TheMazonCreekfossilbedsareaconserva8onlagerstä]e(anareaofexcep8onalfossilpreserva8on)inIllinois.Thefossilbedsrepresentafreshwaterdeltasystem.Sideriteconcre8onsgrewarounddeadorganismsinthesediment.Thishasallowedforthepreserva8onso58ssuesinaddi8ontothehardpartsofanimalsandplants.Insomeinstancescompletelyso5-bodiedorganisms(thatusuallyrotbeforetheyhaveachancetofossilize-forexamplethe“Tullimonster”above)canbefoundinthesideritenodulesofthisforma8on.

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Thereare5majorprocessesincarbonatediagenesis.1)  Cementa8on:Themineralogyofthecementincarbonatesdependsonanumber

offactorsbutinpar8culartheamountofdissolvedCO2intheporewaters,themagnesium:Calciumra8oandthesupplyofcarbonate

2)  Neomorphism:basically“newgrowth”orrecrystalliza8onprocesses.Two

importantprocessesinneomorphismareaggradingneomorphism,wheretherefinegrainedmicriteisrecrystallizedintoslightlycoarserspariteandcalci8za8onwherearagoniteisrecrystallizedintomorestablecalcite(o5enwithalossofinternaldetailinthecaseofshellmaterial).

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3)Dissolu8on:Inthemeteoriczoneandisgenerallycausedbyslightlyacidicgroundwaterpreferen8allydissolvingcracksinthebedrock.Fractureswidenandeventuallyformanundergrounddrainagesystem,allowingmorewatertopassthroughthearea,andaccelera8ngtheforma8onofundergroundkarstfeatures.Dissolu8oncanALSOoccurindeepburial.Dissolu8onwillaidinthedevelopmentofsecondaryporosityandtheimprovementofthereservoirpoten8alofacarbonaterock.NOTETHOUGH–incarbonatesdissolu8oncanbeaccompaniedbycementa8onresul8ngindissolu8onporositythatisveryrapidlyfilledwithcement.4)Compac8on:Mechanicalcompac8on(duetooverburdenofsediment)isnotthatcommon.Thisisprobablyduetotheearlycementa8onthatmanycarbonatesexperience.Indeeperburialthoughchemicalcompac8onmayoccur.Inthiscasefracture/dissolu8onoccursatgraincontactsduetopressuresolu8on.ThisreleasesCa2+andCO3

2-ionsintotherockwhichlikelyaccountforalotofthecementindeepburialserngs.Insolubleclays,pyriteandmineraloxidesandle5behindinstyloliteswhichcanmaketheseareasmorevisuallyprominent.Achangeintextureoftherockmightalsohighlightthepresenceofstylolites.5)Dolomi8za8on:Theconversionofcalciteintodolomite–seelaterinthislecture.

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Diagene8cenvironmentsofcarbonatesareCOMPLEX.ThemajorzonesareMarine,MeteoricandBurialdiagenesis.Inthisclasswewillconcentratemostlyonthemeteoricserng.1)MeteoricZone–Probablyamoresignificantzoneintermofdiagenesisthanthemarinezone:Significantdiagenesiswilloccurasthesedimentisexposedatthesurfaceandisexposedtometeoric(rainwater)infiltra8on–thisisverysignificantduringperiodsofglacia8onwhenfallingsealevelswouldlowerthewatertable.ThediagenesisintheVadosezoneisrela8velyrapid,commonlytakinglessthana1000years.IngeneralcementsinthemeteoriczonetendtobelowMgCalcite.i.UpperVadoseZone–zoneofinfiltra8on:Theuppervadosezoneisalsoknownasthezoneofinfiltra8on.RainwaterwillbeslightlyacidicduetoCO2dissolvedinthewaterANDduetothepresenceoforganicacidspickedupfromsoils.AragoniteandhighMgcalcitewillbepar8cularlysuscep8bletodissolu8oninthiszone.Manygrainsinthiszonewillbecomepi]edanddissolved:iiLowerVadozeZone–zoneofpercola8onHerethepercola8ngwaterbecomessaturatedwithCaCO3.Inaddi8on,lossofCO2(degassing)helpsprecipitatelowMgcalcitecementsandconversionofaragoniteandlowMgcalciteintolowMgcalcite.InternaldetailofaragoniteandHiMgcalciteshellymaterialwillbelostastheyareconvertedtocoarsecrystallinelowMgcalcite.Typicalcementsaremeniscuscementsthatformwithinthefilmofwaterthatclingsbetweengrainsinthesediment(weares8llabovethewatertablehere).

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iii.Thephrea8czoneiscon8nuallysaturatedinwater.Processeshereareprobablyevenmorerapidthaninthevadosezone.Thisareaseestheprecipita8onofaddi8onallowMgcalcitecementsini8allyasfairlyfinerims(isopachous)cementsandthenlaterascoarserblades.Somecalcitecementsmayalsogrowinop8calcon8nuitywiththegrains(commonlyechinoidfragments)producingsyntaxialovergrowths.

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2)MarineDiagenesisDiagenesiscanstartfairlysoona5eracarbonategrainisdeposited.Micri8za8on,thetransforma8onofcarbonategrainsintofinegrainedmicrite,canbecausedbyabrasionofmaterialontheseafloorbutalsobyorganismssuchasalgaeboringintoshellfragmentsandthelikeandconver8ngthematerialintomicrite.Inaddi8on,earlycementa8onofcarbonatesedimentcanoccurresul8ngintheforma8onofahardground.ThecementsusuallyformaroundtherimsofthegrainsandcanbecomposedofironoxidesandhighMgcalcite.Hardgroundsorbeahcrockformwhenthereisapauseinsedimenta8on,some8meslas8ng1000’sofyearsandarecommonlycolonizedbyencrus8ngorganismssuchasoystersandareo5enboredbyvariouscreaturesincludingbivalves(clams).Thedetailedchemistryofhardgroundforma8onissomethingwewillnotcoverinthiscoursebutyoumaygetpendentcementsinthesedeposits.OthermarinecementswillhaveisopachusbladedandfibrouscementslikeyougetinthemeteoriczoneHOWEVERmarinecementsinshallowburialserngstendtobearagoniteandhighMgCalcite

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3)  DeepBurialDeepburialdiagenesisisnottoowellunderstood.Ionsreleasedfromotherstrataundergoingdiagenesis(par8cularlymuds–seeearlier)canmakethisachemicallycomplicateddiagene8cenvironment.Deepburialdiagenesiswilloccurastemperaturesandpressureincreaseandwillo5en(butnotexclusively)becharacterizedbylargercrystalsizeandanincreaseintheironcontentofthecalcitecements(ferroancalcites).Ferroancalcitecementsareo5enregardedasbeingindica8veofdeepburialdiagenesisinANOXICcondi8ons.Itisthoughtthatthemajorityofthecalcitemadeavailableforthecementsinthiszonecomefromthedissolu8onofothermaterial,par8cularlyfromtheforma8onofstylolites.Micritesinthiszonemaydemonstrateaggradingneomorphismwithpatchesofmicritebeingconvertedintospar.

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DOLOMITIZATIONDolomi8za8onisverypoorlyunderstood.Mostdolomitesareregardedashavingformedbythealtera8onofcalciumcarbonatesoona5erdeposi8on(penecontemporaneous)orthealtera8onoflimestonesindeepburialcondi8onsthatmaytakemillionsofyears.Therehasbeenasugges8onthatsomefinegraineddolomitesthatmayhavebeendirectlyprecipitatedoutofseawater.Atpresentthough,noscien8sthasbeenabletoprecipitatedolomiteinthelaboratoryatEarthstandardsurfacepressuresandtemperatures.Someofthegeneralfactorsthatarethoughttofavordolomiteforma8onarehighMg/Catora8osandhightemperatures.

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A.PENECONTEMPORANEOUSTheseareenvironmentswerecarbonatesedimentsarethoughttobealteredtodolomitesoona5erdeposi8on.Thereareanumberofmodels:1.  Evaporite–BrineResidueModeli.RefluxModel:Evapora8onofseawaterinanearshoreenvironmentcausestheprecipita8onofgypsum,CaSO4.TheremovalofCafromthewaterconcentratesMg.TheBrineformedbyevapora8onisdenseandsinksintothesedimentdisplacingthemarineporewaterandcausingdolomi8za8onasitmovesdownintothelimestone.Althoughaneatandsimplemodelithasyettobeobservedoccurringinnature.

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ii.Evapora8vePumpingAnotherwayinwhichgypsumcanbeprecipitatedinacoastalenvironmentisbyevapora8vepumpingclosetoshore.Hereintenseevapora8ondrawssalinegroundwatersthroughthelimestone.Eventuallytheconcentra8onofthosewatersinthenearsurfacecausestheprecipita8onofgypsumandincreasestheconcentra8onofMgrela8vetoCaandallowsfortheforma8onofdolomite.Wewilldealwiththisenvironmentagainwhenwestudyevaporites.

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2.GroundwaterMixingWhenfreshwatermixedwithupto30%marinewaterbrackishgroundwatersareproduced.Thismayalsobeconducivetotheproduc8onofdolomite.

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3.SeaWaterModelInthismodelseawateriscon8nuallyflushedthroughthesedimentporesbringingthecalciteintocontactwithhigherlevelsofMgthatisbeingcon8nuallyreplenishedbyfreshseawater.Thiscanbeaccomplishedbyrising8des(par8cularlyspring8des)andduringperiodsofsealevelrise.

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B.DEEPBURIALThemajorityofdolomitesprobablyformbythealtera8onoflimestonesindeepburialserngs.Theoriginaltexturesofthelimestonescansome8mesbefaintlyseen.Thedolomi8cDevonianlimestonesintheWesternCanadaSedimentaryBasinprobablyformedatdepthsbetween500–3000movermillionsofyears.Dolomi8za8onintheseserngswouldrequirethecircula8onofMgrichporewatersandalimestonewithsufficientporositytopermitthepassageofthesefluids.

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