Microfacies and environmental study of the lower c

ORIGINALPAPERMicrofaciesandenvironmentalstudyofthelowercretaceousyamamaformationinRatawifieldAfrahH.SalehReceived:3March2013/Accepted:28May2013/Publishedonline:14July2013#SaudiSocietyforGeosciences2013AbstractTheYamamaFormationisthemainLowerCretaceous(lateBerriasian–Valangenian)carbonatereservoirinsouthernIraq.Petrographicstudyfromthin-sectionexam-inationshowsthattheskeletalgrainsincludedcalcareousalgaefrombothredandgreenalgae.RedalgaeisconcentratedintheupperpartoftheFormation,andthemostimportantofthisalgaespeciesisPermocalculusssp.Greenalgaeislesscommon,anditsconcentrationisinthemiddlepartoftheFormation.ThemostspeciesfoundintheYamamaFormationisdasycladeans,andbothsmallandlargespeciesofbenthonicforaminiferasuchasNautiloculina,Textularia,Trocholina,Pseudocyclammina,andEverticyclamminaarealsopresent.Thenon-skeletongrainsincludedoolites,pellets,andmicrite.SixcyclictypemicrofacieshavebeenrecognizedforYamamaFormationinRatawi-3(Rt-3)andRatawi-4(Rt-4)Wells,namelypeloidalpackstone–grainstone,algalwackestone–packstone,oolitic–peloidalgrainstone,bioclasticwackestone–packstone,foraminiferalwackestone,andmudstonemicrofacies.Thelatterhasbeendividedintotwosubmicrofacies:argillaceouslimemudstoneandfossiliferouslimemudstone.Thelateralextensionofthesemicrofacieshasbeenidentifiedbyintegratingthethin-sectiondataandwelllogs’charactervariationswithsimilarcharacteristicformicrofacies.TheYamamaFormationwasaffectedbyfivediageneticprocesses,whicharemicritization,cementation,recrystalli-zation,silicification,andstylolites.TheYamamaFormationwasdepositedduringaregressiveperiodwithintheouterramp,shoal,andinnerrampsetting.KeywordsMicrofacies.RatawiOilField.SouthIraqIntroductionTheYamamaFormationisconsideredasoneoftheimpor-tantcarbonatereservoirsinsouthernIraqwithintheLowerCretaceoussediments.Itconsistsofshoalsedimentationcontainingcomponentsinthepeloidallimestone,ooliticandpseudo-ooliticlimestone,andfragmentandorganicfos-silsofcalcareousalgaeandforaminifera.YamamaFormationbelongstothelate(Berriasian–Aptian)cycle.ThecycleisrepresentedfromshoretodeepbasinbyZubair,Ratawi,Garagu,Yamama,Shuiaba,andSarmord(Buday1980).YamamaFormationwasfirstdescribedbySteinekeandBramkamp(1952)inabstractabouttheMesozoicrocksinSaudiArabia.TheymentionedthattheYamamaFormationbelongstotheThamamaGroup,alongwiththeBuaibandSulaiyFormations.Dunnington(1959)describedtheYamamaFormationandcombineditwithSulaiyFormation.YamamaFormationisdividedintofivelithologicunits:threereservoirunits,designatedfromtopasYR-A,YR-B,andYR-C,separatedbytwopermeabilitybarrierunits,YB-1andYB-2.Thesereservoirunitsarethoughttobeatleastpartiallyisolatedfromeachother(Al-Siddiki1978a,b).Thebestoilprospectsarewithintheooliteshoalsandthepatchreefbuildupsinthecrestalpartsofthestructures(Sadooni1993).TheareaofstudyTheRatawiFieldislocatedabout70kmnorthofBasraCityandwestoftheNorthRumailaField(Fig.1)intheflatsemi-desert.TheYamamaFormationislocatedatadepthofabout3,499mbelowthesealevelinaRatawiwell(Rt-3).ThestudiedsectionsincludedfivewellsofthefieldthatpenetratetheFormation,namelyRt-3,Rt-4,Rt-5,Rt-6,andRt-7.A.H.Saleh(*)UniversityofBaghdad,Baghdad,Iraqe-mail:[e-mail](2014)7:3175–3190DOI10.1007/s12517-013-0991-5YamamaboundariesTheYamamaFormationisunderlainconformablybytheSulaiyFormation,whichmadeupofmud-supportedargilla-ceouslimestonewithcalcispheresandsmallbenthonicfora-minifera.TheYamamagradesupwardintotheRatawiFormation,whichisaheterogeneoussuiteoflimestone,shale,siltstone,andsandstone(Fig.2).Inthisstudy,thelogshavebeendependedontodelineatethetopandbottomoftheYamamaFormation;YamamaFormationhasbeendelimitedatthefirstappearanceofthecleanlimestoneunitoccurringbelowtheshaleandtheargil-laceouslimestoneoftheRatawiFormation.Thelowercontacthasbeenplacedatthefirstappearancedownwardofargilla-ceouslimestonecharacteristicoftheSulaiyFormation.Theboundaryhasbeendelimitedaccordingtothespontaneouspotential(SP)logandthegamma-raylogatthedepthsshowninTable1forthewellsofthestudy(Figs.3and4),showingaresponseofthelowgamma-raylogatappearanceofYamamaFormation,withpositivedeflectionforSPlogduetotheresistivityofmudfiltrate(rmf)thatislessthantheresistivityoftheformationwater(Rw).Thelowercontactplacedatthefirstappearancedown-wardofargillaceouslimestonecharacteristichavebeendis-tinctiveatlogpatternonthegamma-raylogandSPlog.PetrographicdescriptionThroughthethin-sectionexaminationforYamamaFormationintheRatawiFieldfortwowells(Rt-3andRt-4),thepetrog-raphyhasbeenstudiedasitshowsthefollowing:Fig.1Thedistributionofthewellsandthestudiedcross-sections3176ArabJGeosci(2014)7:3175–3190SkeletalgrainsThecharacteristicofYamamaFormationcontainsalargenumberofskeletalgrainsrepresentingtheredandgreenalgae,foraminifera,andsomespiculesofsponges.CalcareousalgaeCalcareousalgaecomprisemostofthefossilsoftheYamamaFormation.Algaeareaquaticphotosyntheticbenthonicandplanktonicplantsrangingfrommicron-sizedunicellularformstogiantkelpsseveralmeterslong.Controlsonthedistributionofrecentbenthonicalgaecom-prise(a)physicalfactors:intensityandqualityof