Hughes 2008 Hanifa part 2

Late Oxfordian micropalaeontology, Saudi Arabia

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PLATE 3

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PLATE 4

Plate 4: Biocomponents of the “dasyclad algal biofacies”. Image number followed by field sample or well number, sample number or well depth and width of image given as (mm). (1–24)Pseudoclypeina distomensis BarattoloandCarras, (1–8) Transversesectionsofthallus. (1.RMTN-1:#629:2mm.2.RMTN-1:#651:2mm.3.RMTN-1:

#651:2mm.4.RMTN-1:#637:2mm.5.RMTN-1:#627:2mm.6.RMTN-1:#641:2mm.7.RMTN-1:#635:4mm.8.RMTN-1:#645:1mm);

See facing page for continuation.


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Plate 4 (continued):(9–13) Sagittalsectionofthallus(9.BRRI-24:#41:2mm.10.BRRI-24:#71:2mm.11.BRRI-24:#41:

2mm.12.BRRI-24:#135:1mm.13.BRRI-24:#151:2mm);(14–19) Transversesectionofstem(14.BRRI-24:#289:1mm.15.BRRI-24:#159:1mm.16.BRRI-24:

#143:1mm.17.BRRI-24:#285:1mm.18.BRRI-24:#49:19.RMTN-1:#797:1mm);(20–24) Transversesectionofstemwiththalli(20.RMTN-1:#645:4mm.21.RMTN-1:#627:4mm.

22.RMTN-1:#612:4mm.23.RMTN-1:#625:4mm.24.RMTN-1:#631:4mm).

deeper-marineformswithineachmember,andthesemayrepresentthemaximumfloodingzones,includingthedeeper-marineformssuchasLenticulinacf.sublenticulariswithinthemiddlepartoftheHawtahMember,andaconcentrationofspongespiculesinthelowertomiddlepartoftheUlayyahMember.Thisinterpretationisfurthersupportedbythegamma-raylog,asitdisplaysanincreasedvalue adjacent to the interpreted lower, or transgressive portions of the interpreted Hawtah andUlayyahmembers,andareducingintensitytowardstheupperpartofeachmemberequivalenttotheregressivetract.

Sedimentologicalevidencesupportsthisinterpretation,asthereisaverticaltrendfromwackstoneandpackstonestograinstoneswithinbothmembers.Thedistinctivepolarisationofthedasyclad-bearingsedimentstotheuppermemberishighlysuggestiveofabasinwardshiftofbiofaciescharacteristicofasequenceboundary,wherintheshallowerdasycladlagoonfacieshasrapidlyprogradedovertheunderlyingdeeper,dasyclad-freeassemblageoftheHawtahMember.ThereplacementofLenticulinacf.sublenticularisbyspongespiculesintherespectivemaximumfloodingintervalsisalsoconsistentwiththeuppersequencebeingshallowerthanthelower.

Micropalaeontology and Palaeoenvironmental Interpretation from the Regional Study of Core Samples of the Upper Ulayyah Member of the Hanifa Formation

TheHawtahandUlayyahmembersoftheHanifaFormationmayeachrepresentthird-ordersequences(Mattner andAl-Husseini, 2002,Al-Husseini et al., 2006) of which the upper part of the UlayyahMember,herestudied,representslatehighstandconditions.Palaeoenvironmentalsubdivisionsbasedonthevariousbiofaciesrangeinorderofwater-depthcontrolrelatedtoincreasingdistanceoffshore,from foraminiferal lagoon, Clypeina/Pseudoclypeina lagoon, Cladocoropsis (branched stromatoporoid)back-bank/lagoon, stromatoporoid bank complex to foraminiferal-spicule intra-shelf basin. Withinbasinalareas,highstandconditionswouldbeexpectedtobepoorlyrepresented,exceptforthepossibleincreasedincidenceofstorm-triggered,carbonate-debrisflowsoriginatingfromanadjacentflank.Onthebasinmargin,however,thevarietyofbiofacieswouldbeexpectedtoformatieredsuccessioninWaltherianstyle,progressingfrombasinal,throughdomedstromatoporoid,branchedstromatoporoid,dasycladalgalandforaminiferalbiofacies.SuchaverticalsuccessionisalsorecognisedintheJubailaandArab-Dassociations.Rapidprogradationofabiofacies,asseeninthestudywell,wouldindicateasequenceboundaryforwhichtheresponsiblefallinsealevelwouldhavecausedtherapidbasinwardmigrationof thebiofacies. InFigure13(seeEnclosure), this iswell-definedbythesuddenupholeappearanceofdasycladalgaethatrapidlyprogradedovertheunderlyingdeeperHawtahsequence.

AsthepurposeofthisstudywastodefinethearealdistributionoftheuppermostHanifabiofaciesandinterpretedpalaeoenvironmentsofpotentialHanifareservoirfacies,amodelbasedonbiofaciesassociations and their tiered relationships (both within this study and numerous intra-reservoirmicropalaeontologicalstudiescarriedoutbySaudiAramco)hasledtotheassociationsummarisedin Figure 1b. Because of the absence of extensively cored sections across the area, the regionalpalaeoenvironmental interpretationhasbeenbasedontheyoungestHanifaFormation,essentiallyequivalent to the uppermost 40 ft of the Ulayyah Member. Future work could utilise cuttings-samplemicropalaeontologytoextendthebiofaciescharacterisationtolowerlevelswithintheHanifaFormation.Environmentally-controlledbiofaciesboundarieshavebeententativelydrawnbetweenwellsthathavedifferentbiofacies,withadegreeofconfidencethatdecreasesrapidlywithintheareasoflowwelldensity.Neverthless,eventhewellsinthesouthernpartofthestudyregionprovidepointsourcesofdatathatcanbeinterpretedwithinthedepositionalmodelestablishedfortheHanifa.Anorderlybiofaciesprogressionisevident,gradingfromproximallagoon,todeeperdasycladlagoon,to

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Plate 5: Biocomponents of the “Cladocoropsis and encrusting stromatoporoid, coral biofacies”. Image number followed by field sample or well number, sample number or well depth and width of image given as (mm). (1–12)Cladocoropsis mirabilisFelix(1–5) Obliqueaxialsectionsofbranch(1:BRRI-24:#131:8mm.2:BRRI-24:#67:4mm.3.RMTN-1:

#839:8mm.4.RMTN-1:#843:8mm.5.ST-3:3179’:4mm);See facing page for continuation.

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back-bankCladocoropsislagoonandthehigher-energydomedstromatoporoid-ooidgrainstoneshoals.Thestromatoporoid-bankcomplexisdistallyflankedbythedistalaspectbydeep-marinebiofaciesrepresentativeofanintra-shelfbasin.

Biofacies A: Foraminiferal BiofaciesBiofaciesA (Plates 1 and 2) is characterised by high foraminiferal species diversity and recovery,in which the main component species include Kurnubia palastiniensis (Henson), Redmondoides lugeoni (Redmond), Alveosepta jaccardi (Redmond), Levantinella egyptiaca Fourcade, Mouty andTeherani, previously mis-assigned to the Cretaceous species Mangashtia viennoti (Kaminski, 2004),Quinqueloculina spp., Palaeofenderina salernitana (Sartori and Crescenti) and Nautiloculina oolithica (Mohler) with scattered coarsely agglutinated forms that resemble Reophax horridus following thetaxonomyofKuznetsovaetal.(1996). Itshouldbenotedthatrare,robustspecimensofLenticulinaspp.arefoundwithinthisbiofacies,andtestifytothemoderatelywidepalaeobathymetrictoleranceof this speciesduring theLate Jurassic.Thisgenus isalsopresentwithin the low-diversity,deep-marinebiofaciesdescribedbelow,where itsassociationwithtriaxonandtetraxonspicules,Bositra buchi(Roemer)andcalcareousdinocystsconfirmadeep-marinesetting.

Foraminiferaarewell-represented inmost samples,withhigh-recoveryandhigh-diversity,exceptfor those deposited in the basinal setting where diversity is very low.Agglutinated foraminiferapredominateovermiliolidandrotalidcalcareousspecies.Theforaminiferahavebeenthemostuseful,incombinationwiththestromatoporoidsandcalcareousalgae,fordiscriminatingsubtledifferencesintheinterpretedpalaeoenvironment.ForaminiferalaspectsoftheHanifaFormationinwellsamplesand from outcrop have been published by Hughes (2004a,b,c, 2005) and presented by Dhubeeband Hughes (2005b). Brachiopod and echinoid fragments are common components of the Hanifacarbonates,ofwhichthelatterarepresentbothasplatefragmentsanddiscretespines.Thesmaller,high-spokedspinesseemtobepreferentiallyconcentratedwithinthemudstonesandwackestones.Ostracodsarerare.

Monaxon sponge spicules are present within certain wackestones and packstones of the lagoonalbiofacies,butarealsowell-represented in themudstonesandwackestones thatare interpreted torepresentthebasinalsediments.Inthebasinalsetting,howevermonaxonspiculesareaccompaniedbytriaxonandtetraxonformsprobablyderivedfromhexactinellidsponges,andaretypicallyassociatedwith sparse microfaunal assemblages. Brachiopod and echinoid debris are common throughout.Stromatoporoidsanddasycladalgaeareabsent,buttheabovespeciesarealsofoundtogetherwithsuchformsinthedeeper-lagoonandback-bankfacies.Atcertainlocalities,suchasinthenorthwest,quartzgrainsandfragmentsofwoodarepresentthat,togetherwithcharophyteoogonia,provideevidenceforproximitytoasourceofterrestrially-derivedsediment(Plate3).

Biofacies B: Foraminiferal-Dasyclad Algae BiofaciesBiofaciesB(Plates3and4)consistsofacombinationofmostofthebiofaciesdescribedforBiofaciesA,butaccompaniedbydasycladalgae.Thewell-preservedformsindicatethattherehasbeenlittlesediment transport and disturbance, as these fragile forms are easily disarticulated. Moderatelydeeperorprotectedconditionswithinthelagoonareinterpretedfromthelowerenergyconditions,possiblyclosetoorbelowfairweatherwavebase.ThecalciticformPseudoclypeina distomensis Barattoloand Carras is well-represented, and very well-preserved when compared with the moulds of theprimarilyaragoniticSalpingoporellaspp.Dasycladalgaearewell-representedinmanyofthestudiedsections,andaremostlyassignabletoPseudoclypeina distomensis BarattoloandCarras(Hughes,2005).

Plate 5 (continued):(6–12) Transversesectionofbranch(6.MZLJ-1:#395:4mm.7.BRRI-24:#233:8mm.8.BRRI-24:

#171: 4 mm. 9. BRRI-24: #69: 8 mm. 10. HABA-1: #729: 4 mm. 11. JHHM-1: #393: 8 mm.12.BRRI-24:#79:8mm);

(13–20) Domedstromatoporoid(13.BRRI-24:#243:8mm.13.BRRI-24:#243:8mm.13.BRRI-24:#243:8mm.14.BRRI-24:#243:8mm.15.BRRI-24:#219:8mm.16.BRRI-24:#237:8mm.17.BRRI-24:#63:8mm.18.BRRI-24:#257:2mm.19.BRRI-24:#163:8mm.20.BRRI-24:#209:4mm);

(21–24) Corals(21.ST-3:3179:4mm.22.BRRI-24:#309:8mm.23.BRRI-24:#217:8mm.24.UDYN-1:#307:8mm).

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Plate 6: Biocomponents of the “Lenticulina and spicule biofacies”. Image number followed by field sample or well number, sample number or well depth and width of image given as (mm). (1–14)Lenticulina sublenticularis (Schwager) (1. KHRS-52: #173: 1 mm. 2. HRDH-604: #9: 1 mm. 3.

WLBN-1:#187:1mm.4.WLBN-1:#197:1mm.5.WLBN-1:#22:1mm.6.WLBN-1:#92:1mm.7.WLBN-1:#20:1mm.8.TMYS-1:#71:1mm.9.WLBN-1:#255:1mm.10.WLBN-1:#17:1mm.11.WLBN-1:#212:1mm.12.SUHL-1:#129:1mm.13.WLBN-1:#197:1mm.14.SHYB-700:#110:1mm);

See facing page for continuation.

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Plate 6 (continued):(15–16) Astacolus cf. vacillantesEspitalieandSigale(15.WLBN-1:#12:1mm.16.SUHL-1:#129:1mm);(17–23)Nodosariaspp.Note20-22specimensdisplayrimmedsuture)(17.KHRS-52:#225:1mm.18.

TMYS-1:#139:1mm.19.SHYB-700:#92:1mm.20.SHYB-700:#108:1mm.21.QTIF-67:#421:1mm.22.SUHL-1:#127:1mm.23.WLBN-1:#157:1mm);

(24) Bolivinasp.(SHYB-700:#104:1mm).

Thesedelicateformsareinterpretedtooccupymoderatelydeep,normalsalinitypartsofthelagoon,wherelow-energyconditionspredominate(BannerandSimmons,1994).Theseformsaretypicallypreservedasdisaggregatedfronds,exceptinthenorthwesternpartofthestudyarea,whereentirestemsandbranchesarepreserved.MicrobialitesarepresentwithinthisbiofaciesandincludeRivularia piae.TheencrustingalgaThaumatoporella parvovesiculifera (Raineri)isalsowell-represented.

Biofacies C: Branched Stromatoporoid (Cladocoropsis) BiofaciesBiofacies C (Plate 5) is characterised by the presence of the branched stromatoporoid assigned toCladocoropsis mirabilis FelixtogetherwiththeencrustingalgalformThaumatoporella parvovesiculifera (Raineri), and rare branched corals. Branched stromatoporoids are considered to have requiredmoderately low-energy conditions in order to avoid breakage, and are considered to have best-developedinthedistalpartofthelagoon,intheleeofabank,wherethehigherwaveenergywouldbe dampened. Branched corals occupy a similar niche today, and their distribution has providedsupporttothispalaeoenvironmentalinterpretationoftheseextinctstromatoporoids.Thebranchingformprobablyrepresentsaresponsetotheneedforacceleratedverticalgrowthwithinareaswherethe sedimentation rate was relatively high and this was, therefore, a survival strategy. It is notedherethatthebranchingtendencyofmanycoralsappearstobeastrategyforrapidregrowthinanareas where fragments may be prone to breakage due to periodically elevated energy conditions(Leinfelder,oralcommunication2005;Lirman,2000;Wallace,2004).BiocomponentsofBiofaciesAandBarealsopresentwithinthisfacies.

StromatoporoidspresentwithintheHanifaFormationalsoincludeunspeciatedencrusting,dome-shaped forms that resembleBurgundia ramosaPfender,basedon the criteriaofWood (1987).Bothstromatoporoid morphotypes are present within the Saudi Arabian Middle and Upper Jurassiccarbonates(Hughes,2004c),andtogetherwiththelimitedpublisheddataonTethyanstromatoporoidcarbonates(Leinfelder,2001;Leinfelderetal.,2005)suggestaresponsetovariationsinenvironmentalenergy levels. The existing model proposes that the domed stromatoporoids adapted to higher-energyconditionssuchaswouldbeexpectedontheoceanwardflanksofabankmargin.AswithextantbranchedcoralssuchasAcropora cervicornis, A. palmata andspeciesof PoritesandGoniolithum,the branched stromatoporoids would be expected to have occupied the relatively lower energy,not necessarily deep, relatively sheltered region within the lagoon, on the leeward side of thestromatoporoidbank.A“back-reef”environmentisalsosuggestedforbranchedstromatoporoidsbyTurnseketal.(1981).

Biofacies D: Encrusting / Domed Stromatoporoid and Coral BiofaciesBiofacies D (Plate 5) is characterised by the presence of the massive, domed and encrustingstromatoporoid forms, with comparatively rare corals. These forms are considered to have best-developedinassociationwithhigh-energygrainstonebankswithooids,wheretheelevatedwaveenergy would be effective in inhibiting most biological activity. Domed encrusting corals occupya similar niche today, and their distribution has provided support to the palaeoenvironmentalinterpretationoftheseextinctstromatoporoids.

Corals are, when compared to stromatoporoids, poorly represented in the Hanifa. The relativelow abundance of corals has been attributed by Leinfelder et al. (2005) to significantly highertemperaturesoverArabia,basedonthepalaeoclimatemodelofSellwoodetal.(2000).Theinferredpresence of considerably warmer waters within a climatic belt with annual mean surface watertemperaturesexceeding28°Cmayexplain theoccurrenceofpurestromatoporoidassemblages,asthey are considered to have a greater tolerance to warmer waters; their tolerance to hypersalineconditionsisnotfullyunderstood.Asfilterfeederswithminimalphotosyntheticrequirements,the

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Plate 7: Biocomponents of the “Lenticulina and spicule biofacies” with microbialite. Image number followed by field sample or well number, sample number or well depth and width of image given as (mm).(1–3) Tetraxonspongespicules(1.KHRS-52:#309:1mm.2.DHYN-1:#443:2mm.3.WLBN-1:#175:

2mm);(4–8) Triaxonspongespicules(4.FieldsampleHL-4:1mm.5.KHRS-52:#145:1mm.6.Fieldsample

UH-18:1mm.7.WLBN-1:#264:2mm.8.WLBN-1:#511:2mm);See facing page for continuation.

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inferredabilityofstromatoporoidspongestooccupymuddierwatersbeyondthetolerancerangeofcoralsmaybeanothersignificantfactorworthyofconsideration.Itisofinteresttospeculateontheeffectsofthemid-Oxfordianthermalminimumandthewarmingstageof3–4°Cduringmid-toLateOxfordian,asdescribedbyLecuyeretal.(2003).SuchdatarequirereconciliationwiththeevidenceforglacialconditionsintheLateCallovianassuggestedbyDromartetal.(2003),Ceccaetal.(2005)andTremoladaetal.(2006).

Biofacies E: Lenticulina-Spicule BiofaciesBiofaciesE(Plates6and7)ischaracterisedbythepresenceofsmallerforaminiferaspeciesthatarenormally considered to occupy deeper-marine environments. These include species resemblingLenticulina sublenticularis (Schwager) and Astacolus vacillantes Espitalie and Sigale, Nodosaria spp., variouspolymorphinidsandagglutinatedformssuchasBigenerinaspp.andtheubiquitousKurnubia palastiniensis (Henson). Rare valves of the pelagic bivalve Bositra buchi (Roemer) are also presentwithinthisbiofacies.

Inaddition,thisfine-grained,mudstoneandwackestonelithofaciesischaracterisedbythepresenceofcommontolocallyabundantspongespiculesthatincludemonaxon,triaxonandtetraxontypes(Shrock and Twenhofel, 1953; Laubenfels, 1955), of which the triaxon forms belong to the ClassHexactinellidae and are preferentially representative of deeper-marine spicule-secreting sponges.MonaxonandtetraxonformsaretypicalofClassDemospongiathatoccupyawidepalaeobathymetricrange. This biofacies is confined to the central part of the study area. Barren mudstones are alsoincludedwithinthisbiofacies.

REGIONAL INTERPRETATION OF THE BIOFACIES

Eachofthefivebiofacies(AtoE)representedinFigure1ahavebeenidentifiedfromthevariousstudywells,andtheirdistributioniscontrolledbycoloureddotsdepictingeachbiofacies.Withthisveryscattereddistributionofreferencepointsforeachbiofacies,delimitationofthefivepalaeoenvironmentalprovinces associated with each biofacies must remain tentative. Where no evidence exists for aparticularbiofacies,thepresenceofitsneighbouringbiofacies,assuggestedbyFigure1b,hasbeenused toguide thepositionof thepalaeoenvironmentalprovince.This isespecially thecase in thesouthwesternpartofthestudyarea.Theeastflankofthebasinispoorlyconstrained,andcontrolrelieduponthepublishedworkbyAyoubandEnNadi(2000)anddeMatosandHolstrand(2005)andStrohmengeretal.(2004).FortheregionadjacenttoQatar,Droste(1990)describedtheHanifaasconsistingpredominantlyoforganic-richmudstoneswithminorevaporitesthatsuggeststhatdeep,restricted-marineconditionsprevailed.Forthenorth,thebasinmargininterpretationbyMcGuireetal.(1993)wasused.

Theresultingmap(Figure1a)displaysaninvertedanvil-likeshapefortheintra-shelfbasinthatwasflankedbyastromatoporoidbankcomplexandextendedtotheopen-marinesettingtothenortheastintheregionofQatar.Thestromatoporoidbankandback-bankcomplexwasrathernarrowalongthesouthernandsouthwesternbasinmargin,buttendedtowidenalongthenorthwesternflankwhereitswidthseemstoincreasetothenorth.ThewidthofthestromatoporoidcomplexisafunctionofthecombinationofBiofaciesCandD,becausetheseparationofthedomed-stromatoporoidgrainstonefaciesandtheback-bankCladocoropsis grainstoneandpackstonefaciesisexpectedtobetransitional

Plate 7 (Continued):(9–10) Sphericalspiculeclusters(9.HABA-1:#603:1mm.10.WLBN-1:#255:1mm);(11–12) Barrenmudstone(11.HABA-1:#103:4mm.12.WLBN-1:#255:1mm);(13–16) Calcareousdinocysts(13.TMYS-1:#139:1mm.14.HRDH-604:#25:1mm.15.HRDH-604:

#27:1mm.16.HRDH-604:#13:1mm);(17–19) Bositra buchi(Roemer)(17.HRDH-604:#27:2mm.18.HRDH-604:#21:2mm.19.Fieldsample.

HL-5:2mm);(20) Juvenilebrachiopod(TMYS-1:#139:1mm);(21–24) Encrusting, lamellar microbialites (21. BRRI-24: #243: 2 mm. 22. BRRI-24: #234: 2 mm. 23.

MZLJ-1:#347:4mm).24.Microbialitecrust(BRRI-24:#83:4mm).

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PLATE 8

Plate 8: Calcareous nannofossils from the Hanifa Formation in Wadi Laban-1 (WLBN-1) well. See Plate 9 for scale bar. (1) Cyclagelosphaera argoensis WLBN-1,500’; (2) Cyclagelosphaera margerelii WLBN 1,500’;(3)Watznaueria barnesiaeWLBN-1,500’;(4)Ellipsagelosphaera britannicaWLBN1,500’;(5) Podorhabdus spp. WLBN-1,500’; (6) Podorhabdus spp. WLBN-1,500’; (7) Ellipsagelosphaera britannica WLBN-1,500’; (8) Cyclagelosphaera riyadhensis WLBN-1,500’; (9) Cyclagelosphaera riyadhensis WLBN-1,500’;(10) Zeugrhabdotus erectus BRRI-229,8575.20’ (Plug no 775); (11) Paleodidemnum galbulus ABHD-31,10352.00’ (Plug no 67); (12) Paleodidemnum metaxy WLBN-1,500’; (13) Paleodidemnum saudicus WLBN-1,500’;(14)Paleodidemnum saudicus WLBN-1,500’;(15)Paleodidemnum metaxyWLBN-1,500’.

anddifficulttolocatewithoutcloserwellspacing.Thewidthofthecombinedbranchedanddomed/encrusting-stromatoporoidbeltisremarkablywidealongthenorthwestflankofthebasincomplex,andcontrastsmarkedlywiththeinterpretednarrowbeltthatflanksthesouthwesterlycorneroftheinterpretedelongatearmofthebasin.

CONCLUSIONS AND EXPLORATION POTENTIAL

Forthefirsttime,abroadinterpretationofthepalaeoenvironmentalvariationsoftheupperpartoftheOxfordianHanifaFormationwithinSaudiArabiahasbeengeneratedusingrock-basedevidence.There is, of course, much room for future refinement of this depositional model by insertion ofadditional wells and possibly by using micropalaeontology of cuttings samples. The depositionalmodelprovidesconsiderableinformationforthepalaeogeographersaswellasforthehydrocarbonexplorationists.


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PLATE 9

Plate 9: Calcareous nannofossils from the Hanifa Formation of Saudi Arabia. (1) Bactrolithus delicatus BRRI-24 8695.00’ (Plug no 411); (2) Bactrolithus delicatus BRRI-24 8695.00’(Plug no 411); (3) Bactrolithus delicatus WLBN-1,500’; (4) Acinodidemnum lineola WLBN-1,500’;(5)Acinodidemnum lineola WLBN-1,500’;(6)Disechinatus bicaudatus WLBN-1,500’;(7)Acinodidemnum lineola WLBN-1,500’; (8) Paleodidemnum acutus BRRI-24 8695.00’ (Plug no 411); (9) Velasquezia praegothica WLBN-1,500’;(10)Fusellinus insolutus BRRI-229,8575.20’(Plugno775);(11)Fusellinus brevis WLBN-1,500’;(12)Fusellinus elongatus WLBN-1,500’.

Theoverallpictureisthatthestudyareaconsistedofanintra-shelfbasinalcomplexthatwasflankedbyshallowlagoonsonthewesternandsouthernsides(Figure1a).Themarginoftheintra-shelfbasincomplexisdefinedbythechangefromdeep-marinewackestonesandmudstonestostromatoporoidgrainstonesthataccumulatedonthehigh-energyshoalcomplex.Thismapcanbecautiouslyusedto delimit the location of stromatoporoid-bank grainstones that would be expected to have goodporosityandpermeabilityandmayprovidegoodhydrocarbonreservoirs.Theirjuxtapositiontoanintra-shelfbasinwithknownsource-rockfacies,andthepresenceofanoverlyingmud-richsuccessionofthebasalJubailaFormation,addstothehighlyadvantageouscombinationoftherequiredelementsnecessarytohigh-gradetheselocationsforhydrocarbonexploration.Storm-derivedallochthonousshallow-marinegrainstoneswithintheupperHanifaandtheoverlyingJubailaFormationwouldbeexpectedtoflankthebasinsidesofthesebanks,andmayprovidestratigraphically-trappeddebris-flowreservoirswithinthebasinalmuds.

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The map could be improved by additional wells, and a similar exercise could be carried out foreachSaudiArabianlithostratigraphicunit.Significantinsightstotheregionalbiofacies, lithofaciesand palaeoenvironmental distribution would be realised with potentially tremendous leads forreconnaisanceseismicandexplorationtargetdelimitation.

Subsequenttothisstudy,3-Dseismicsurveyshavebeencompletedoverpartsofthestudyareatotestthevalidityofthebiofacies-derivedmap.Theobjectivewastomapandconfirmthesuggestedexistenceofbasin-marginstromatoporoidbankcomplexesofpotentialreservoirvalue.Thepreliminarydataisencouragingasamplitudedifferenceshavebeenseenthatcanberelateddirectlytothesuggestedpalaeoenvironments(Figure14).Suchseismicdelineationofthegrainstonefaciesbeltwithreservoirpotentialcannowguidethesearchforexplorationtargets.Additionalpotentialtargetsareofferedby the possibility of stratigraphically trapped, allochthonous, basin-margin grainstones along thedeeperflanksofthe Hanifaintra-shelfbasins,asdescribedbyMullinsandCook(1986).LocatingtheHanifabasinmarginalsoprovidesaguidetothepossiblelocationofbasin-margingrainstonesoftheunderlyingUpperFadhiliandDhrumareservoirfaciesoftheTuwaiqMountainLimestone.Thepresenceofmud-dominatedintra-shelfbasinfacieswillallowgeochemiststodelimitpossiblesource-rockaccumulationlocations,astheHanifaFormationandtheunderlyingTuwaiqMountainLimestoneareknowntohaveconsiderablesource-rockpotential(Carriganetal.,1995).

Despite the very limited database, use of detailed, core-based micropalaeontology and modernpalaeoenvironmentalinterpretationsofthebiofacies,combinedwithpublishedpalaeoenvironentaldataofadjacentregions,hasenableddefinitionofthebasinmargingrainstonebelttrendandlocationofintra-shelfbasins.Relativelylow-cost,high-definitionmicropalaeontologicalanalysishasprovedits ability to support exploration and significantly extends the application of micropalaeontologyfor regional and local intra-reservoir studies, in addition to the conventional use for age control.Microfaunalvariationsaresufficientlyvariedthatconsiderationshouldbegivenfortheirusetoassistbiosteeringinreservoirswhereunder-balancedcoiledtubedevelopmentactivitiesareplanned.

km

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Figure 14: Seismic amplitude map to test the micropalaeontologically-derived environmental interpretation in the central part of the study area. The grain- and mud-dominated belts, based on seismic data, fit well with the intra-shelf basin boundary suggested by the micropalaeontology (highlighted in yellow frame).


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ACKNOWLEDGMENTS

The results presented here are based on a SaudiAramco project, and were presented as a posterdisplay at the GEO 2006 Conference, Bahrain, 2006, at the International Congress on the JurassicSystem,Krakow,6–18September,2006,andat the7thSaudiSociety forEarthSciencesConferenceinRiyadh,May12–13,2007.ValuablegeologicaldiscussionswithRaviSingh,SaudiAramcoAreaExploration Department, are acknowledged. Figure 14 is based on data provided by P. LawrenceandA.Gregory,SaudiAramco.ThanksaregiventoSaudiAramcoandtheSaudiArabianMinistryof Petroleum for their kind permission to publish this paper. Valuable editorial comments byA. Afifi and M. Miller, Saudi Aramco, are gratefully acknowledged, in addition to those madeby GeoArabia’s reviewers. The final design and drafting by GeoArabia Graphic DesignerNestorNiñoBuhayisappreciated.

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ABOUT THE AUTHORS

Geraint Wyn ap Gwilym Hughes is Senior Geological Consultant in the Carbonate Systems and Micropalaeontology Unit in Saudi Aramco’s Geological Technical Services Division. He gained BSc, MSc, PhD and DSc degrees from Prifysgol Cymru (University of Wales) Aberystwyth and in 2000 he received the Saudi Aramco Exploration Professional Contribution award, in 2004 the best paper award, and in 2006 the GEO 2006 best poster award. His biostratigraphic experience, prior to joining Saudi Aramco in 1991, includes 10 years with the Solomon Islands Geological Survey, and 10 years as Unit Head of the North Africa-Middle East-India region for Robertson Research International. Wyn’s professional activities are focused on integrating micropaleontology with sedimentology to enhance the sequence stratigraphically-influenced understanding of Saudi Arabian hydrocarbon intra-reservoir bio- and lithofacies distribution. He maintains links with academic research as an Adjunct Professor of the King Fahd University of Petroleum and Minerals, Dhahran. He is an editor for the AAPG, GeoArabia reviewer, and a member of the British Micropaleontological Society, the Grzybowski Agglutinated Foraminiferal Society, the Dhahran Geoscience Society, the International Fossil Alga Association and the Cushman Foundation for Foraminiferal Research.

[email protected]

Osman Varol gained his BSc and MSc from the University of Istanbul, followed by a PhD in calcareous nannopalaeontology from the University College London. He was a nannopalaeontological biostratigrapher for international projects in the Singapore and North Wales offices of Robertson Research from 1981 until 1991. In 1991, he formed Varol Research, a biostratigraphic consultancy specialising in the use of calcareous nannofossils, ascidian spicules and siliceous fossils for world-wide application for exploration and development. Osman has published extensively in both the taxonomic and applied use of nannofossils, and has developed new regional nannofossil-based zonation schemes to improve sequence stratigraphic interpretations.

[email protected]

Mokhtar Al-Khalid graduated with a BSc in geology from King Abdul Aziz University, Jeddah, and joined Saudi Aramco in 1995. He is a carbonate sedimentologist with the Geological Technical Services Division and currently studying towards his MSc in petroleum geology at King Fahd University of Petroleum and Minerals.

[email protected]

Manuscript Received October 19, 2006

Revised June 23, 2007

Accepted July 3, 2007

Press version proofread by the authors February 11, 2008

Hughes 2008 Hanifa part 2

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