Lead isotopes as tracers of crude oil migration within deep crustal fluid systems

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Lead isotopes as tracers of crude oil migration within

deep crustal fluid systems

Nadège Fetter, Janne Blichert-Toft, John Ludden, Aivo Lepland, Jorge

Sánchez Borque, Erica Greenhalgh, Bruno Garcia, Dianne Edwards, Philippe

Telouk, Francis Albarède

To cite this version:

Earth and Planetary Science Letters 525 (2019) 115747

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Lead

isotopes

as

tracers

of

crude

oil

migration

within

deep

crustal

fluid

systems

Nadège Fetter

a

,

b

,

Janne Blichert-Toft

a

,

∗

,

John Ludden

b

,

Aivo Lepland

c

,

Jorge Sánchez Borque

d

,

Erica Greenhalgh

b

,

Bruno Garcia

e

,

Dianne Edwards

f

,

Philippe Télouk

a

,

Francis Albarède

a

a_{LaboratoiredeGéologiedeLyon,EcoleNormaleSupérieuredeLyon,CNRSUMR5276,UniversitédeLyon,46Alléed’Italie,69007Lyon,France} b_{TheLyellCentre,BritishGeologicalSurvey,HeriotWattUniversity,ResearchAvenueSouth,EdinburghEH144AP,UK}

c_{GeologicalSurveyofNorway,P.O.Box6315Torgarden,7491Trondheim,Norway}

d_{NorwegianPetroleumDirectorate,ProfessorOlavHanssensvej10,4021Stavanger,Pb.600,4003Stavanger,Norway} e_{IFPEnergiesNouvelles,1-4AvenueduBoisPréau,92500Rueil-Malmaison,France}

f_{ResourcesDivision,GeoscienceAustralia,JerrabomberraAve,Symonston,CanberraACT2609,Australia}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received10April2019

Receivedinrevisedform28July2019 Accepted29July2019

Availableonline13August2019 Editor: L.Derry Keywords: crudeoil blackshales Pbisotopes oilmigration NorthernEurope porous-mediaconvection

AlthoughPb,U,and Thmay befractionatedbetweencrude oiland formation waters,Pbisotopesare not.ThisuniquepropertymakesPbisotopesaparticularlyusefulmarkerofhydrocarbongenerationand migration. Hereweshow thatPb isotopesoffer anew visionoflong-range(secondary)oilmigration relevanttotheformationofoilfields.NorthSeaoilsarelargelygeneratedfromJurassicblackshales,yet theirPbisotopesare mixturesofCenozoictoProterozoicend-members.Thesameobservationismade forcrude oilsfromthe ParisBasin,the BarentsSea, Libya,Kuwait, Kazakhstan, andAustralia. BulkPb incrudeoiltherefore, forthemostpart,isforeigntoitssource rock(s).Ourhigh-precisionPb isotope data on195crudeoilsworldwide,the firstsuchdata setinthepublishedliterature,and17Northern European blackshalesindicate thatdeep-seatedPb components originating beneath the source rocks are ubiquitousincrude oil.Thisimplies thatoilfieldsareembeddedinbasinalconvectivesystemsof hydrous fluidsheated from below. Plumes ofhot fluids risefrom the lower thermal boundarylayer, whichPbisotopesrequiredousethebasement,intothecoreoftheporous-flowconvectivecellwhere theydissolvethenewlyformedhydrocarbonssequesteredinthesourcerocks.Thefluidsfinallyunload unmixed formationwaters and crudeoilatthebase ofthe upper (conductive)boundarylayerwhere theycanbetrappedinfavorablesites.BasedonthesenewinsightswearguethatPbisotopesincrude oilconstituteagoodtracerofoilmigration.

©2019UnitedKingdomResearchandInnovation,asrepresentedbyitscomponentbody,theBritish GeologicalSurvey.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Petroleumformation requires atleast two distinct stages: hy-drocarbon formation from organic compounds present in the source rock and migration from the source rock into reservoirs. Asformulated byMann (1994), however,‘theprecisemechanismof primarypetroleummigrationhasbeenelusivedespiteintensive investi-gationanddiscussion.’Organicgeochemistryandthestableisotopes ofC, S, and Ohave long been used to studythe processes con-trollingoil genesisandevolution, whileinorganictracershaveso farlargelyfailedtodemonstratethesameusefulnessasinigneous

*

Correspondingauthor.

E-mailaddress:jblicher@ens-lyon.fr(J. Blichert-Toft).

and sedimentary geochemistry (e.g., Prinzhoferet al., 2009). The limitedamountofpublisheddataontraceelementsandradiogenic and heavy metal isotopes in crude oil bear out both the ana-lytical challenge and lackof information and first-orderpatterns that could improve the understanding of oil generation. Crude oil is notoriously difficult to mineralize and represents a daunt-ingmatrixproblemformassspectrometrictechniquesthatrequire a high degree of element purity. Data on elemental concentra-tionsareknowntoscatterwidelyandtobenon-reproducible(e.g., Venturaetal.,2015).Water-hydrocarbonunmixingatlow temper-ature(M ˛aczy ´nskietal.,2004) isthemostlikelycauseofthelarge range observed formetal abundances (e.g.,Ventura etal., 2015). Although numbers may vary from one hydrocarbon to the next, typical water solubility in oil is

∼

20 mol% at about 250◦C and reduces to

∼

0.1 mol% below 35◦C (Glandt andChapman, 1995;

https://doi.org/10.1016/j.epsl.2019.115747

2 N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747

Fig. 1. MapoftheNorthSeaandsurroundingregionsdisplayingthelocationsof 93crudeoils.Depthsinexcessof200maredarkgreen.Sampleswithlowmodel Th/Uareshownas squares,whiletherestofthesamplesareshownascircles. MajorstructuralfeaturesoftheNorthSeagrabensaresketchedwithbrownlines. SixcrudeoilsfromtheBarentsSeaplotoffthemap(samples7120/1-3,7121/7-2, 7122/7-1,7128/4-1,7220/11-1,and7222/6-1SinTableS1).AsimilarmapforLibya (Fig.2)isintendedtoshowthecorrelationbetweenoilfindsandvolcanicactivity. (Forinterpretationofthecolorsinthefigure(s),thereaderisreferredtotheweb versionofthisarticle.)

GriswoldandKasch,1942).Thistemperaturedependenceofwater solubilityincrudeoilcreatesamajor issueininterpreting chem-ical and isotopic data obtained on natural hydrocarbons. Water circulatesinsedimentarybasinsby porousflow andthrough frac-tures.It is largely exsolvedduring theadiabatic cooling ofcrude oiluponemplacementinthethermalboundarylayerofthecrust (the upper 2-5 km). From this follows that a significant fraction of metals must be lost in the process, in addition to loss dur-ing pumping andextraction. Overall, water-oil separation during extraction from the underground therefore rules out most trace elements as markers of oil-related processes, leaving only heavy isotopes, which are not readilyfractionated by phase separation, aspotentiallyreliableinorganictracers.Sinceuraniumisknownto correlatepositivelywithorganiccarboncontents ofsediments,in particularcarbon-richblackshales(e.g.,Leventhal,1991),exploring Pbisotopesincrude oilastracersofhydrocarbongenerationand migration seemsto be a worthwhile avenueforfurther research. AmajoradvantageofusingPbisotopesoverstableisotopesof el-ementssuch asC,N,S,V,andNi isthatPbisotopicvariabilityat thepercentleveliscreatedbytheradioactivedecayof238U,235U, and 232Th, which contrasts with the thermodynamic isotope ef-fectatalevelonetotwoordersofmagnitudesmaller.Inaddition, becausePbisotopicvariations are controlledby multiple radioac-tive systems, their interpretation does not depend on measured parent/daughterratios.In thisrespect,Pbisotopes differfromOs isotopes, which cannot be understoodwithout knowledge of the Re/Os ratio, a variable that is subject to fractionation during oil formation andmigration (Mahdaoui etal., 2015; DiMarzio etal., 2018). TheU-Th-Pbisotopesystems,therefore,appearparticularly wellsuitedtothestudyofoilgenerationandmigration.

Fig. 2. MapofNorthernLibyashowing25crudeoilsites.SampleswithlowTh/Uare displayedassquares,therestascircles.Namesinitalicshowthevolcaniccenters oftheNeogeneAlHarujprovince(ElshaafiandGudmundsson,2017).

N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747 3

ThePbisotopedataforthe195crudeoilsand17blackshales an-alyzedhere are listed in Table S1 along with all their pertinent information.Lead,U,andThconcentrationsonasubset(36)ofthe 195crude oilsandallthe blackshalesalsoaregiveninTableS1, whilemajor andtrace elementconcentrations forthe 36sample subsetofcrudeoilsarelistedinTableS2.

2. Methods

Theanalyticalprotocolforhigh-precisionPbisotopicanalysisof crudeoilbyMC-ICP-MSisdescribedinFetteretal. (2019).Itwas designed to extract Pb and Zn from small volumes of crude oil andcondensatesdissolvedin dichloromethaneinthe presenceof dilute HBr, which is known to strongly complex these elements. Given the small targeted sample size (

<

5 ml), inherent sample heterogeneity,andthelackofreliableconcentrationdataon stan-dardreferencematerials,theextractionyieldcannotbeestimated precisely.However, two successive extractionsteps systematically recover

>

95% ofthetotalextractablePb(Fetteretal.,2019). Iso-topefractionationuponextractionisconsistentwithexperimental stableisotopefractionationatambienttemperature(10−4 to10−5) andtoo smalltoaffectthemeasured Pbisotopiccompositionsat thepresentlevel ofprecision (100-200ppm for204-basedratios and50ppmfor206-basedratios)(Fetteretal.,2019).

Forthe 36 crude oil samples for which major andtrace ele-ment concentrations,including U, Th, andPb, were measured in additiontoPbisotopiccompositions,theprotocolfromFetteretal. (2019) wasslightlymodified.Afterarepeateddigestionindistilled concentratedHNO3 and30%H2O2,thesamplesweredissolvedin

1ml distilled0.5 M HNO3 fromwhich 5% aliquots (50 μl) were

takenforelemental concentrationanalyses. Boththe aliquotsand theremaining95%fractionswereevaporatedtodrynessat110◦C. Anion-exchangecolumnchromatographywas usedto separatePb forisotopicanalysisontheremaining95%fractions.

For the black shales, a protocol different from that used for crude oil was followed to allow for elemental analyses of U,Th, andPbona5%aliquotandisotopicanalysisofPbonthe remain-ing95%ofthedissolvedsample.Theblackshalesamplesfirstwere groundinan agatemortar andapproximately1gofpowder was transferred into a PFA Savillex beaker and weighed. The surface layers were removed by aggressive leaching at hightemperature as follows: 4 ml 6 M distilled HCl were added to each sample andtheclosedbeakersleft toreact for30min ona hotplateat 130◦C,then10mininanultrasonicbath,10minat130◦C,5min inan ultrasonic bath,and finally 5 min at 130◦C. The acid was pipettedout andthesamples rinsedtwicein distilledwater.The leachedsample powderwas evaporatedtodrynessat110◦C.The sampleswere thendissolvedinamixtureof3:1:0.5concentrated double-distilled HF:HNO3:HClO4. The beakerswere left overnight

at 130◦C, then dried down, first at 130◦C to get rid of the HF andHNO3,then at210◦Cto eliminatethe HClO4.A last

dissolu-tionstep consistingof5ml6 M distilled HClwas carriedout to bringthesamplesintocompletesolution.Afterleavingtheclosed beakersfor 2-3h at 130◦C, 5% aliquots (250 μl) were taken for elementalconcentrationanalyses,andboththealiquotsandthe re-maining95%fractionswereevaporatedtodrynessat110◦C.Lead waselutedfromtheremaining95%fractionsbythecolumn chro-matographyproceduredescribedinFetteretal. (2019).

AllelementalandPbisotopicanalyses weredone attheEcole Normale Supérieure in Lyon. Concentration measurements were done onan Agilent7500CXQ-ICP-MS (Agilent TechnologiesInc.), whilePb isotopic analyses were carriedout oneither a Neptune PlusHR MC-ICP-MS (Thermo Scientific) or a NuPlasma HR MC-ICP-MS(NuInstrumentsLtd.).Thesample preparationprocedures forICP-MS analyses as well as the instrumentsettings were the sameasthoseinFetteretal. (2019).Precisiononmajorandtrace

Fig. 3. TernaryplotinPb-U-ThspaceshowingtheU/Pb(∼238_U/204_Pb/70=μ/70)

andTh/U(∼κ =232_Th/238_{U)valuescalculatedfor17blackshalesand36crude}

oils.Blackshalesandoilsamplesarenotpaired.Thegreenandreddottedlines indicatetheterrestrialvaluesoftheseratios.Inset:HistogramofPbconcentrations in195crudeoils.

element concentrationson the Agilent 7500CX Q-ICP-MS was of the order of 5%. Crude oils duplicated at

<

4% for Pb concentra-tions (Fetter et al., 2019), while black shale duplicate measure-ments showed a variation of 10-20%. Crude oil and black shale duplicates were 3-20% forTh concentrationsand 30% maximum forUconcentrations.Theexternalreproducibilityofthemeasured Pbisotopiccompositions,estimatedfromrepeatmeasurements of NISTSRM981,was100-200ppm(or0.01-0.02%)forPbisotope ra-tios basedon 204 (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) and50 ppm(or0.005%)for207Pb/206Pb, 208Pb/206Pb,and207Pb/208Pb. In-ternalrunerrorsforbothstandardsandunknowns(samples)were smallerthantheexternalreproducibility(TableS1).

3. Results

As listed in Table S2, the crude oil samples contain large amountsofNaandK(rangingfromafew100ppbto

>

1000ppm) and,toalesserextent,Mg,Al,CaFe,andZn(uptoseveraltensof ppm).Lithium,Ti,V,Cr,Mn,Mo,Ni,andCdabundancesvaryfrom oneoilsampletothenext,withmostconcentrationsbeingofthe orderof0.1-100ppb,withafewexceptions

>

1ppm.

TherangeofPbconcentrationsinthecrudeoilextracts(Fig.3, inset; Tables S1 and S2) is very broad (0.1-11,600 ppb, mean

=

21 ppb). Given the uncertainties on elemental yields other than thatofPb,wemeasuredUandThconcentrationsononlyasmall subset of crude oilsamples (Fig. 3; Tables S1 andS2)buton all the blackshales asthesewere analyzed by bulk dissolution.The

238_U/204_{Pbratiosofthecrudeoilextracts(0.01-142,mean}

₌

_27.4)

bracketthenarrowrangeofmantleandcrustvalues(7-10),while most oftheir 232_Th/238_{Uratios (1.5-4.1, mean}

₌

_{3.1) are}

consis-tently lower than the planetary value (3.876; Blichert-Toft etal., 2010).The U,Th,andPbconcentration dataon theNorthern Eu-ropean blackshales also are plottedin Fig. 3. In contrast to the oils, all the black shale samples but two have 232_Th/238_{U ratios}

(2.0-142.6, mean

=

24.2) significantly higher than the planetary value.

The overall structure of the crude oil Pbisotope data is best understood in 3-dimensional space. Since, due to the noisier signal on the smaller 204_{Pb peak,} 204_{Pb-normalized ratios are}

4 N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747

Fig. 4. Three-dimensionalrepresentationofPbisotopecompositionsfor192crudeoilsamplesfromNorthernEurope(topleft-handpanelA),Libya(topright-handpanel B),theMiddleEast(bottomleft-handpanelC),andAustralia(bottomright-handpanelD).Thex-axis(204_Pb/206_{Pb)ishomologoustothemodelagecalculatedfromthe} 206_Pb/204_Pband207_Pb/204_{Pbratios(Albarèdeetal.,2012).Theprojectionontothebottompanelrepresentsthe}204_Pb/206_Pb-207_Pb/206_{Pbisochronplot,whilethe}

projec-tionontothebackpanelrepresents204_Pb/206_Pb-208_Pb/206_{Pb.This3-dimensionalplotshowsthatPbfromtheoilsourceisaccountedforbyamixtureofatleastthree}

N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747 5

Fig. 5.208_Pb/206_Pband204_Pb/206_{PbofNorthernEuropeanblackshalesandNorth}

Seacrudeoils.Thex-coordinate(204_Pb/206_{Pb)approximatelyvariesalongwiththe}

modelage,whiletheinterceptofalignmentswiththey-coordinate(208_Pb/206_Pb)

increaseswith theTh/U ratio.Leadisotopesfrom mostblackshalesplot as ex-tensionsofthe crudeoilarrayat itsradiogenic(young)end and,therefore,are consistentwiththeseformations,notablytheKimmeridgeClayanditslateralUpper Jurassicstratigraphicequivalents,beingthesourcerocksofthecrudeoil.Lead nev-erthelesscontainsother,muchlessradiogeniccomponents,whichmusthavebeen acquiredbyinteractionofconnatewaterswiththebasement.

3-dimensional spaces, we chose to instead represent the over-all isotopic variations using 206_{Pb-normalized ratios. In Fig. 4,}

the2-dimensional 207_Pb/206_Pb-204_Pb/206_{Pbplotoccupies the}

bot-tom panel and the 208Pb/206Pb-204Pb/206Pb plot the back panel. The interceptof a 207Pb/206Pbvs 204Pb/206Pb array gives the ra-diogenic (∗) 207_Pb∗_/206_Pb∗ _{ratio and the model age, while the}

intercept of a 208_Pb/206_{Pb vs} 204_Pb/206_{Pb array gives the}

radio-genic208Pb∗/206Pb∗ ratio,whichitself isproportional toTh/U(or equivalently

κ

=

232_Th/238_{U).IfeithertheageortheTh/Uvalues}

obtainedaregeologically inconsistent,thealignments inquestion representmixingarrays. The intercepts arelisted in Table 1. The

207_Pb/206_Pb-204_Pb/206_{Pbalignments ofthe NorthernEuropean oil}

dataandthe oil datafrom theother regions investigatedare far toooldtorepresenttheageoftheirsourcerocks.Inaddition,the multiplicityofarraysinthecorresponding208_Pb/206_Pb-204_Pb/206_Pb

spacerequiresmixingbetweenthreesourcesofPbormore(Fig.4). Theradiogenic207Pb∗/206Pb∗interceptsdefinedbythedifferentoil suites,therefore,arenotvalidchronometersandtheagesonly ‘ap-parent’,not‘true’meaningfulgeologicalages.

The204Pb/206Pb-208Pb/206Pbdata onNorthernEuropean black shalesoverlapthoseofoilsfromthesamedomainbutextendthe rangetowardsradiogenic(low204Pb/206Pb)Pb(Fig.5).Blackshales may, therefore,be considered end-members of crude oil Pb, but cannot be the onlysource asthey are unable to account forthe unradiogenic(high204Pb/206Pb)Pb.

ThePbisotopecompositionsofoilssampledatdifferentdepths in the same well are distinct and the ages provided by the in-terceptofthe207Pb/206Pb-204Pb/206Pbarraysmucholderthanthe permissiblemigrationages(Tables1andS1).

The three-component Pb isotope systematics of oils from the Ghadamis Basin in Libya (Fig. 4B) show similar systematics to those of the North Sea oil fields(Fig. 4A), withunradiogenic Pb consistent with the pre-Pan-African (

>

540 Ma) basement and a more radiogenic Pb component characteristic of the low-Th/U Phanerozoic sedimentary cover. It is worth noticing that the most unradiogenic samples originate exclusively from

prospec-tion provinces NC1 and NC8, situated near the early Paleozoic mountsNafusahandQarqaf,respectively(HallettandClark-Lowes, 2016).The

κ

(232Th/238U)valuesderivedfromthetime-integrated

208_Pb∗_/206_Pb∗_{interceptsofalltheoilprovincesinvestigatedinthe}

presentwork,excludingKuwait(with

κ

∼

5.69)butincluding Aus-traliaandKazakhstan,requirethatTh/Uissignificantlylowerthan theplanetaryvalue(Table1).

4. Discussion

BeforeexploringthePbisotopesystematics,wefirstbriefly dis-cusstheimplicationsoftheconcentrationdata.ExceptforThand U, the correlations observed for multiple elements between the logarithms of their concentrations disappear oncethe concentra-tiondataarenormalizedtothesumofcations,whichshowsthat they reflect asimple dilutioneffect.We surmisethat the solutes analyzedafteracidextractionfromthe36crudeoilsamplescould have been trapped either as solid suspensions or as microemul-sions of formation water in hydrocarbons (Fetter et al., 2019). In the particular caseof Pb and despite the scatter of the data, the highPb/Al (for a log-normal distribution, average of0.5 and 1

σ

range of0.04-5.2) andPb/Fe (average of0.1and1

σ

rangeof 0.01-1.14)ratiosare toohighforPbtobederived fromrock frag-ments.ThesameobservationholdstrueforThandU.Wetherefore concludethatPb,Th,andUhavebeenintroducedintooilbyfluids emulsifiedwiththeliquidhydrocarbons.

InPb-Th-Uspace, blackshalesandoilfallonoppositesidesof thelinemarkingtheplanetaryTh/Uvalue(Fig.3),whichquestions thesignificanceoftheblackshalesandraisesconcernsabout ele-mentalfractionationbetweenrock,water,andoil.Thelackofmore adequatedetailedsamplingpreventsusfromdeterminingwhether theserocks are an effective sourceof oilor ratherresidues after oil expulsionand migration. Circulation of hot hydroussolutions in the sedimentary layers will both increase water solubility in oil anddecreaseoilviscosity (Glandt andChapman,1995), even-tually favoring emulsification and oil mobility. Fig. 3 showsthat eitherUdepletionofblackshaleswithrespecttoPbandThdates fromoriginalsedimentation,asisthecaseforthefewotherblack shalesthathavebeenanalyzedfromelsewhere(Chenetal.,2009; Jiang et al., 2006), or Uwas lost at a later stage. In the former case, it would argue fordiagenetic remobilizationat thetime of sedimentation(AndersonandFleisher, 1991; BarnesandCochran, 1991), whilein the lattercase, U was remobilized and preferen-tiallydissolved upon interactionofthe blackshaleprotolith with geopressuredhydrousfluidsatthetemperatures ofoil generation andmigration.Pb-Pb evidencefrombitumen(Parnelland Swain-bank,1990) suggeststhatUmaybeentrainedtogetherwith bitu-menoutofthesource rockatthetime ofhydrocarbonmigration but not necessarily by the oil itself. The low U contents of our samples actually require that U was either transported by water migratingwithoilorwaslostduringcrudeoilextraction.

As for Pb isotopes, they offer a new set of constraints on oil migration. Evidence of mixing inferred from the very old

207_Pb∗_/206_Pb∗ _{ages in} 207_Pb/206_Pb-204_Pb/206_{Pbspace (Fig. 4)}

sug-geststhatoilmigrationdoesnotleadtocompleteresettingofthe U-Th-Pbisotopesystems.Theconcentrationdatademonstratethat itisnotpossibletoignoreradiogenicPbingrowthafteroil forma-tionandtwo-stagemodels(StaceyandKramers,1975;Albarèdeet al., 2012) donotstrictlyapply.Itisnevertheless revealingtoplot the datain the coordinates ofthe two-stagemodel age Tmod,

μ

6 N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747

Table 1

Modelagesandκvaluescalculatedfrom,respectively,theinterceptofthelinearregressionina207_Pb/206_Pbvs204_Pb/206_Pbplot,

andtheinterceptina208_Pb/206_Pbvs204_Pb/206_{Pbplot,fortheblackshalesandcrudeoilsamplesofeachregionfeaturinginthis}

study.Whenseveralsamplesinagivenregionclearlyseparatedintolow- andhigh-Th/Ugroups,alinearregressionwascalculated forbothsetsofvalues.

Province 207_Pb/206_{Pb vs}204_Pb/206_Pb 208_Pb/206_{Pb vs}204_Pb/206_Pb

Intercept Inferred age (Ma) Intercept Inferred kappa

Black shales

UK 0.0589±0.0003 561.7±0.1 0.422±0.001 1.689±0.003

Crude oils

Norway Barents Sea 0.0872±0.0011 1364.3±0.2 0.611±0.002 2.446±0.009 Norway North Sea (low Th/U) 0.0883±0.0771 1387.8±12.2 0.548±0.140 2.192±0.560 Norway North Sea (high Th/U) 0.1039±0.0215 1693.6±2.8 0.939±0.031 3.755±0.123 UK North Sea (low Th/U) 0.0938±0.1341 1502.8±19.6 0.874±0.212 3.495±0.848 UK North Sea (high Th/U) 0.1171±0.0090 1911.7±1.0 1.130±0.010 4.519±0.039 UK Onshore 0.0872±0.0150 1364.7±2.4 0.529±0.031 2.116±0.125 Paris Basin 0.0709±0.0250 954.0±5.2 0.738±0.031 2.952±0.125 Libya (low Th/U) 0.1174±0.0431 1916.4±4.8 0.815±0.059 3.258±0.236 Libya (high Th/U) 0.0710±0.0126 957.1±2.6 1.086±0.015 4.342±0.062 Kuwait 0.1339±0.0412 2148.8±3.9 1.423±0.033 5.692±0.131 Kazakhstan 0.0835±0.0174 1279.1±2.9 0.591±0.033 2.362±0.130 Australia 0.0880±0.0004 1380.7±0.1 0.896±0.001 3.583±0.003

previously identifiedin Pboresfromthesame area (Blichert-Toft etal.,2016).EvidencethatPbinoilisamixtureofmultiple com-ponents,mostofthemforeigntothesource rocks,isreminiscent ofthescatteredarrayscommonlyobservedin187Re-187Osisochron plots(SelbyandCreaser,2005;Finlayetal., 2011; Georgiev etal., 2016). But the consensus is that Re and Os originate in source rocks andmassivelyfractionate intooil, whichmakes the uptake mechanismsvery differentfromthose controlling Pb. The role of coexistingasphalteneandmaltene fractionsin preserving Pb iso-topeheterogeneities,asobservedby DiMarzioetal. (2018) forOs isotopes,wouldneverthelessbenefitfromsomeclarification.

Overall,theisotopicarraysofNorthernEuropeanoilsandblack shales are consistent with them beingmixing lines betweenthe three,ormore,sourcesidentifiedabove,withvariableproportions ofeachsourcecontributingtothemixtures.Mixingfurtheris con-firmedby the agesobtainedon differentsubsetsof oil(Table 1). The 207_Pb/206_Pb-204_Pb/206_{Pbisochron agesvaryfrom954}

_±

_{5 Ma}

to2149

±

4 Maandhencearedifficulttoexplainwithina geolog-icalcontextdominatedbyPaleozoicandMesozoictectonicevents. Mixing is also confirmed by the kinked 208_Pb/206_Pb-204_Pb/206_Pb

arrays(Fig. 4). The apparent

κ

(232Th/238U) ofthe samples with

204_Pb/206_{Pbfallingintherangeof0.054-0.056,andtherefore}

asso-ciated withPaleozoicevents,reachesdistinctly lower valuesthan samples associatedwith both Proterozoic (

>

0.057) and Cenozoic (

<

0.054)events(Fig. 4andS1). The low-

κ

group islocated near thelateJurassictriplejunctionoftheViking,MorayFirth,and Cen-tral grabens (Zanella and Coward, 2003) (Fig. 1). A similar Th/U dichotomy can be observed in Australiaand the Ghadamis Basin (Libya) for which the 208_Pb/206_Pb-204_Pb/206_{Pb arrays can be}

ac-counted for by three distinct Pb components (Proterozoic, Pale-ozoic, andCenozoic Pb). All of these observations are consistent withtheabovediscussionofpreferentialUremovalbyfluids asso-ciatedwithoilformation.

Leadisotoperesultssuggest thatthe porous-mediaconvection of hot fluids released by formations underlying the source rocks is a prevalent phenomenon. Osmium isotopes evidence that, in the North Sea, mantle fluids interacted with oil (Finlay et al., 2010). The currentparadigm of hydrocarbon expulsion(primary) andlong-distance(secondary)migrationhasbeenreviewed multi-pletimes(e.g.,TissotandWelte, 1984; Walters, 2017). Itappeals topressuregradientsinducedbysubsidence,compaction,and tec-tonic liberation (England et al., 1987; Mackenzie et al., 1988), andtothevolumeexpansionresultingfromthetransformationof kerogentooilandgas.Noneoftheseconceptsareintrinsically

suf-Fig. 6. Temperature distributioninaporoussystemheatedfrom below(e.g.,by amantleplume).Inthe coreofthe system,advectiveheattransportdominates andtemperaturedistributionisnearlyadiabatic(∂T /∂z∼0).Nexttotheupperand lowerboundaries,advectivefluidtransportdropsandconductionbecomesthe dom-inantmodeofheattransfer.Thebottomboundarylayercontainsexcessheatwhich createsbuoyancyandtriggerstheformationofhotplumes.OldPbpresentinoil demandsthatthebottomboundarylayerreachesintothebasement.Intheupper boundarylayer,thetemperaturedistributionisdeterminedbythelocalgeothermal gradient.Hotfluidsenteringtheupperboundarylayercooldownveryefficiently, whichleadstooil–waterexsolution.

ficienttoaccountfortheubiquity ofPaleozoic andProterozoicPb componentsinoil.

N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747 7

Fig. 7. Mutualsolubilityofwaterandoctane(M ˛aczy ´nskiet al.,2004),oneofthe abundanthydrocarbonsincrudeoil,atambientpressure,cyclohexane(M ˛aczy ´nski etal.,2004),andbenzene(Góraletal.,2004).Dataabove300◦Carelackingand hencethemiscibilityloopcannotbeclosedprecisely.Althoughoilformedintheoil formationwindow(90-180◦C)andwastrappedinthecoldthermalboundarylayer ataround100◦C,itmayneverthelesshavebeentransportedwithoutmajordamage bysignificantlyhottersolutionsrisingfrombelowthesourcerocks.

fluids, orthermals,rise fromthe lowerboundary layer with par-ticularly strong gradients developing at their front (Graham and Steen,1994).Althoughhydrocarbons formfromkerogenwithin a narrow range of temperatures, typically 60-150◦C, experimental evidenceshowsthatthey remainstableuptotemperaturesin ex-cessof250◦C (see references in M ˛aczy ´nski etal., 2004) (Fig. 7). They are therefore readily soluble in hydrous fluids hotter than 150◦C, which facilitates their migration over long distances. At thesetransport temperatures, the oil solubility contrastbetween the main body of convective hydrous fluids and the subsurface boundarylayermayreachtwoordersofmagnitude.Loadinglarge quantities of liquidhydrocarbons fromthe source rock therefore requires large-scale percolation of hot fluids in the sedimentary basin.Wealsoinvestigatedwhetherextractcompositionscouldbe usedforthermometrybyusingtheNa,K,andMgabundances de-terminedontheextractsoftheoilsubsetof36samplesforwhich we havemorecomplete analyses (Table S2). The datawere plot-tedinGiggenbach’s(1988) ternarydiagramK/100–Na/1000–Mg1/2 (Fig.8)andapparentequilibrationtemperaturesassessedfromthe Na-K andK-Mg thermometers.Most samples plotin the field of dissolvedrocks(lowNa),butthehigh-Nasamplesreflectapproach to equilibration with feldspar and chlorites and temperatures of 100-180◦Cconsistent withthe conditionsofthe conductive ther-malboundarylayer.

Sincebothwaterandoilareliquidphases,thepredominant fac-torcontrollingmutualwater-hydrocarbonsolubilityistemperature, not pressure. The fraction xoil of oil in water typically decreases

byan orderofmagnitudefrom150◦Cto50◦C (M ˛aczy ´nski etal., 2004) (Fig. 7) withalkanes being moreinsoluble than aromatics. Atdepthz andtemperatureT ,xoilchangesaccordingto

∂

ln xoil

∂

z

=

∂

ln xoil

∂ (

1

/

T

)

×

∂ (

1

/

T

)

∂

z

≈



hsol R T2

×

∂

T

∂

z

whereR isthegasconstantand



hsol theheatofsolutionof

hy-drocarbons in water. The solubility effect has been explored by Price (1976), reviewed by Tissot and Welte (1984), and eventu-allyconsidered inefficient for short-distance (primary) migration. Largetemperaturegradients,andthereforelittle differential solu-bility,areactuallynotexpectedatdepthinthesedimentarybasin

Fig. 8. TernaryplotK-100-Na/1000-Mg1/2inppbrepresentingtheK-NaandK-Mg thermometers(Giggenbach,1988).Theassumptionmadehereisthatwaterandits solutesweretrapped inoilasmicro-emulsionsandeventuallydissolvedinto oil, andthattheirpresentrelativeabundancesinacidextracts(bluedots)reflectthose ofthetrappedwaters.K-Na-Mgconcentrationsseemtoinitiallyreflectthe compo-sitionofthewall-rock(lowNa)but,astemperaturedecreases,becomesaturatedin albite,K-feldspar,andchlorite.Equilibrationbetweenwaterandsequesteredoilwas neverfullyachievedbuttemperaturesof100-180◦C,likelyrepresentingthe condi-tionsofsecondaryoilmigrationandentrapment,areconsistentwithfluidinclusion evidence(e.g.,Sverjensky,1984).

andthereforelocaltransportanddeposition(primarymigration)is unlikely.Theconclusionmaybedifferentforsecondarymigration. Whentakingintoaccountthelargedifferencesinmutualsolubility ofwaterandoilbetweendeepsedimentarylayers andan overly-ingthermalboundarylayer,dissolutionofhydrocarbonspresentin thesource rockandexsolutionandstorageoftheresultingoil in shallowerreservoirscanbeachievedonaregionalscale.Thisis in-dependentofthepetrophysicalpropertiesoftheenvironmentand hothydroussolutionsmaybeeitherheatedbyanunderlying mag-matichotspotorremobilizedfromgreaterdepths andpercolating throughthePaleozoicandProterozoicbasement(Fig.9).

Such situations should be common above mantle thermal anomalies, typically magmatic provinces, such as that in Libya (Fig.2),andtectonicgrabensassociatedwithmagmaticprovinces, such as those in the North Sea (Fig. 1). Additionally, steep ther-mal gradientsalso are associatedwithregional unloading in tec-tonicforelandbasins,such asinAustraliawheretherearenoreal hotspots.

5. Conclusions

Our high-precision Pb isotope data on 195 crude oils world-wide, the first such data set in the published literature, and 17 NorthernEuropeanblackshales,aswell asU,Th,andPb concen-tration dataon36ofthe crudeoils andall17blackshales,shed newlight onoilmigrationprocesses.The Pbisotopedatarequire thatPbincrudeoilssystematicallyisamixtureofcomponentsof different agesranging fromCenozoic to Proterozoic, andthat in-growth of radiogenic 206_Pb, 207_{Pb, and} 208_{Pb is significant since}

8 N. Fetter et al. / Earth and Planetary Science Letters 525 (2019) 115747

Fig. 9. Cartoonofoilfieldgenesis.WatercontainingPb,includingPaleozoicand Pro-terozoiccomponents,i.e.,mucholderthanthelowerPaleozoicorMesozoicsource rocks,percolatesthroughthebasement(bottomdarkgreylayerwithfolding pat-tern)andrisesbyporousflowthroughtheoverlyingsedimentarybasin,presumably atthetopofadeepthermalsource.Thereddashedlinerepresentssuchan un-definedisotherm. Waterisrepresentedas lightgrey drops.Hotwatersdissolve organicmaterialasthey passthroughcarbon-richblackshales(blacklayerwith whitedashedlines),possiblyalongfaults(darkgreydashedlines).Waterunloads itshydrocarboncomponent(blackdrops)asthefluidsmigratelaterallyandupwards throughporoussedimentarylayers(purplearrows)andoilsolubilityinwaterdrops exponentiallywithtemperature.Bothwaterandoilgettrappedasseparatephases belowasealinglayer(showninred).

sourcerocks,dissolvehydrocarbonssequesteredwithin thesource rocksandalong their migration pathsandredistributetheseinto the cooler shallowertectonic traps of the upperthermal bound-arylayer.Such aprocessbearsconsiderationby thoseworkingin oilgenesisasthedatapresentedherehighlightstheimportanceof anunderestimatedroleofmutualoil-watersolubilitywhichallows long-distance (secondary) migration of hydrocarbons. This inter-pretationofthe presentPbisotopedata showsthat oilfieldsare embeddedinlarge-scalebasin-widefluxesofdeep-seatedhot flu-ids.

Acknowledgements

We thanktwo anonymous referees andthe Editor,Lou Derry, for constructiveand insightful reviews. We further thank Gareth Harriman from GHGeochem, Lundin Norway AS, the Norwegian PetroleumDirectorate,IFPEnergiesNouvelles,andGeoscience Aus-tralia,thelatterthroughAndrewOwen,forprovidingthecrudeoil samplesanalyzedinthisstudy.TheBritishGeologicalSurvey(BGS) viaTraceyGallagheristhankedforprovidingtheblackshalesand Olivier Donard for providing the NIST 1634c standard reference material. The British Geological Survey (NERC-UKRI) further pro-vided funding for the analytical work to JB-T and NF as part of the BGSUniversities FundingInitiative (BUFI). We are gratefulto FlorentArnaud-Godetforhelpwithcleanlaboratorymaintenance, aciddistillation,andoccasionalinstrumentassistance.

Appendix A. Supplementarymaterial

Supplementarymaterialrelatedtothisarticlecanbefound on-lineathttps://doi.org/10.1016/j.epsl.2019.115747.

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