GeologicAnalysisof NaturallyFractured Reservoirs This Page Intentionally Left BlankGeologicAnalysisof NaturallyFractured Reservoirs SECONDEDITION ByR.A. Nelson BPAmoco Houston, TX GP PO Gulf Professional Publishing AnI mpr i nt of El sevi erBostonOxfordAucklandJohannesburgMelbourneNewDelhi Gulf ProfessionalPublishmgisanimprintof Elsevier. Copyright 92001byButterworth-Heinemann ~ , Amemberof theReedElsevier group Allrightsreserved. Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,or transmittedinanyformorbyanymeans,electronic,mechanical,photocopying, recording,orotherwise,withoutthepriorwrittenpermissionof thepublisher. Permissionsmay besoughtdirectlyfromElsevler's Scienceand TechnologyRtghtsDepartmentm Oxford,UKPhone(44)1865 843830,Fax(44)1865853333,e-madpermlsslons@elsevlerco uk Youmay also completeyour request on-hnevia theElsevier homepagehttp//www elsevter comby selecting"Customer Support"andthen"Obtaining Perrmssions" ~_~Recognizingthe~mportance of preservingwhathasbeenwritten,Elsevier printsItsbooksonacid-freepaperwheneverpossible. q Elseviersupportstheeffortsof AmericanForestsandthe GlobalReLeaf programInitscampaignforthebetterment of trees, forests,andourenvironment. Libraryof CongressCataloging-in-PublicationData Nelson,RonaldA. Geologic analysisof naturallyfracturedreservoirs /byR.A.Nelson. --2nded. p.cm. Includesbibliographicalreferencesandindex. ISBN-13: 978-0-88415-317-7(alk.Paper)ISBN-10: 0-88415-317-7(alk.Paper) 1.Rocks--Fracture. 2.Rocks,Sedimentary.3.Hydrocarbonreservoirs.I.Title. QE431.6.P5N452001 553.2' 8----dc212001017058 ISBN-13: 978-0-88415-317-7 ISBN-10: 0-88415-317-7 BritishLibraryCataloguing-in-PublicationData A cataloguerecordforthisbookisavailablefromtheBritishLibrary. Thepubhsheroffersspeclaldiscountsonbulkordersof thisbook. Formformat~on,pleasecontact: Managerof SpecialSales Butterworth-Heinemann 225Wildwood Avenue Woburn,MA01801-2041 Tel:781-904-2500 Fax:781-904-2620 ForinformationonallGulfProfessionalPublishingpublicationsavailable,contactour World WideWebhomepageat:http://www.gulfpp.com 1 0 9 8 7 6 5 4 3 2PrintedintheUnitedStatesof America TomycolleaguesRobertoAguilera,Mel Friedman, andDaveStearns,andtomy f ri endsintheindustry whohavetaughtmesomuchovertheyears. This Page Intentionally Left BlankCont ent s A c k n o w l e d g m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x iF o r e w o r d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x i i iP r e f a c e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x v i iN o t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x i x1 . E v a l u a t i n g F r a c t u r e d R e s e r v o i r s " I n t r o d u c t i o n . . . . . . . . . . . 1 AvoidFractureDenial,1 Problems,2 Definitions,3 TheEvaluationSequence,4 BasicTypesofEvaluation,4 EarlyExplorationEvaluations,5 Evaluationsof EconomicPotential,5 EvaluationsforRecovery PlanningandModeling,6 GeneralSequenceofStudy,7 FractureSystemOrigin,7 GenericClassification,9 GeologicClassification,10 FracturedPropertiesAffectingReservoirPerformance, 32 Introduction,32 FractureMorphology,37 FractureWidthandPermeability, 64 FractureSpacing,79 FractureandMatrixPorosityCommunication, 82 Introduction,82 vii Basicsof Fractureand MatrixPorosity,83 Cross-Flowina Two-PorositySystem,95 Examplesof Cross-Flowin ThinSection,96 InhibitedCross-Flow,96 Estimationof PorosityInteraction,100 2. Re s e r v o i r M a n a g e m e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Classificationof FracturedReservoirs,101 Reservoir Types,101 PositiveReservoirAttributes,107 PotentialProblems,109 StrategiesofFracturedReservoirManagement,110 ReservoirDescription,113 ThoughtsonRiskAnalysisinFracturedReservoirs,123 3.D e t e c t i ngandP r e di c t i ngFr a c t ur eO c c u r r e n c e andI nt ensi t y. . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 5Detection,125 DirectDetection,125 IndirectDetection,127 Applicationof DirectandIndirectTechniques,135 PredictionofSubsurfaceFractureSpacing,135 Composition,137 Porosity,137 GrainSize,141 BedThickness,141 StructuralPosition,146 Summary,150 PickingWell Locationsand Well PathsinFolded FracturedReservoirs,152 FractureIntensity,153 Well Trajectory,160 vii~ 4. An a l y s i s of An i s o t r o p i c Re s e r v oi r s . . . . . . . . . . . . . . . . . . 1 6 3Stylolites,163 Stylolitesand TheirContributiontoReservoir Anisotropy,165 Effectof StyloliteZonesonPorosityandPermeability,172 StylolitesasanIndicatorof MechanicalProperties,173 DevelopmentofPermeabilityTensorsforAnisotropicReservoirs,185 Crossbedding,186 Fractures,189 OtherFactors,193 PermeabilityTensorsfor CrossbeddingandFractures,198 RelativeEffect of RockParameters,202 Permeability AnisotropyandStylolites,204 CombinedTensors,206 StatisticalDatainReservoirModeling, 207 ReservoirDomainsor Compartments,207 Averaging Techniquesin ThreeDimensions,212 Three-DimensionalCorrelationof Reservoir PropertiesinFracturedReservoirs,215 StatisticalCharacterizationof BlockSizes,217 StimulationinFracturedReservoirs,217 5. An a l y s i s P r o c e d u r e s inFr a c t ur e dRe s e r v oi r s . . . . . . . . . 2 2 3ScreeningToolsinDefiningaFracturedReservoir,223 DataTypesandConstraintsasaFunctionof WhentheFracturedReservoirisDiscovered, 226 CoreandOutcropAnalysis,229 FractureStratigraphyandtheInterrelationof Deformation, Petrology,andPetrophysics,229 DeterminingNaturalVersusInducedFractures,230 Data Acquisition,239 CoringinFracturedReservoirs,240 UsefulChecklists,246 DataPresentation,248 PressureandProductionAnalysisfor QuantifyingFractureSystemProperties,251 Logging Techniques, 251 Well Testing, 251 NumericalModelinginGeology,252 A p p e n d i x A: Li s t o f D o c u m e n t e d F r a c t u r e d Re s e r v o i r s . . . . 255 A p p e n d i x B:P r o c e d u r e s Che c k l i s t . . . . . . . . . . . . . . . . . . . . 2 7 7A p p e n d i x C:Av e r a g i n g T e c h n i q u e s . . . . . . . . . . . . . . . . . . . 2 7 9Gl o s s a r y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Co n v e r s i o n Fa c t or s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 85Re f e r e n c e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 8 7I ndex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 3Acknowledgments I wishtoacknowledgethesupportandguidanceof mycolleaguesDavid W.Stearns,RobertoAguilera,andMelvinFriedmanthatmadethissecond editionpossible.I alsoacknowledgethemanagementof B P Amocoandthe formerAmocoProductionCo.withoutwhoseencouragementandpermis- sionthismanuscriptwouldnothavebeenpublished.Iamgratefultothe AmericanAssociationofPetroleumGeologists(AAPG),theAmerican GeophysicalUnion,theInternationalSocietyofRockMechanics,the SocietyofPetroleumEngineers,andtheCanadianSocietyofPetroleum Engineersforpermissiontodrawmaterialfromtheirpublications.Lastly,I thankallofthoseattendeesofthefracturedreservoircoursesthatIhave taughtovertheyearswhohavetrulytaughtmetheartandpracticeof frac- turedreservoiranalysis. xi This Page Intentionally Left BlankForeword Fracturesareauniversalelementinsedimentaryrocklayers,somuch thattheyarevirtuallyomnipresentinoutcropsofsedimentaryrocks.Think ofalltheoutcropsofsedimentaryrocksthatyouhaveeverseenandtryto recallalayerthatwascompletelyunfractured,withthepossibleexception of extremelyductilerock,suchassalt or certainshales,youwillnot beable torecallanyunfracturedrockssimplybecausetheydonotexist.Further,it hasbeendemonstratedoverandoveragainthatthevastmajorityoffrac- turesobservedinoutcroparenotsolelytheresultofsurfaceconditions.In otherwords,thefracturesseeninoutcropalsoexistinthesubsurface. Therefore,itfollowsthathydrocarbonreservoirsinsedimentaryrockall containfracturesandmostofthemarefracturedenoughtobetreatedas fracturedreservoirs. Thoughthegeologicalfracturesnecessarytoconcludethatfracturesare commoninthesubsurfacehavebeenknownforatleastthelasthalfcen- tury,thepracticeoftreatingreservoirsasfracturedrockmasseshasbeen extremelyslowinbecomingastandardindustrypractice.Whyisthisso? Probablythegreatestcontributortothewidespreadreluctancetofacethe realityoffracturedreservoirsisbecausefracturedreservoirsareextremely complexandtherefore,muchmoredifficulttodealwiththanareunfrac- turedreservoirs.Thecomplexitycomesfromthevastnumberofbothde- pendentandindependentvariablesthatdictatefinalreservoirresponse. Considerforaminutejustafewoftheobvious,straightforwardreservoir variables,andtheirinteractions,thatmustbeincludedinareservoiranaly- sis.Calculatingreservoirstorage dependsonknowingbothmatrixandfrac- tureporosities.Fracturepermeability,matrixpermeability,andespecially theirinteraction,allcontributetothebehavior of agivenreservoir.Fracture geometry,fracturespacing,fracturesurfacearea,andfractureopeningall combinewithfracturemorphologyandporespacedistributiontocreate truereservoirpermeabilityand/orpermeabilityanisotropy.Fluidpressure declinewithtimechangesthevalueofsomevariablesbutnotthevalueof others.Therefore,initialcalculationsdonotapplythroughoutthelifeof the reservoirandsome parametersmust be recalculatedatseveralintervalsdur- ingthelifeof thereservoir. xii~ Anyonewhohasdealtwithfracturedreservoirsrealizesthatthesevari- ablesare onlya fewof the numerousvariablesthat haveto beevaluatedand properlycombinedinordertopredictreservoirperformance.Isthereany doubt,then,thatreservoircomplexityisamajorcontributortothereluc- tanceeventoattemptsystematictreatmentofreservoirsasfracturedrock masses? Anotherfactorthatisadeterrenttodoingsystematicfracturedreservoir analysisisthatalmostallfracturedreservoirsrespondinamannerunique tothatspecificreservoir.Thatis,despitetheexistenceofagood,working fracturedreservoirclassification,eachfracturedreservoirrespondsinits owndistinctiveway.Asaconsequence,applyinggeneralrulesof thumbto specificfracturedreservoirscanbedangerouslymisleading. Boththecomplexityandindividualityoffracturedreservoirs,then, stronglyarguefor theneedfor a referencebookthat dealsina practicalway withprovenmethodsof dealingwithfracturedreservoirs.Thisbookserves thatfunction.Inthefirsteditionof thisbook,RonNelsonshortenedtheef- fortittooksomeonenewtogetintoreservoiranalysis.Hedidthisbygoing beyondhislucidacademicdiscussionoftheimportantgeologicandengi- neeringfactorsthatmustbeconsideredinfracturedreservoiranalysis.In additiontotechnicaldetails,he alsousedhisvast experiencetodiscusshow to organize,collect,anddealwithfracturedreservoirdatawhileatthesame timestayingwithinthe practicallimitsdemandedbymost corporations.His coveragein1985wascompleteforthestateoftheartthatexistedatthat time.However,theinfluencethatthefirsteditionhadonindustryisnobet- ter demonstratedthanbythefactthatsince1985theindustry-widelevelof sophisticationintreatingfracturedreservoirshasacceleratedenormously. In addition,sincethepublicationof thefirstedition,Nelsonhasundertaken theorganizationandpresentationofnumerousAmericanAssociationof PetroleumGeology(AAPG)fracturedreservoirworkshopsalloverthe world. Theseactivitiesnotonlypermittedhimtheopportunitytopromulgate theideasexpressedinthefirstedition,theyalsopermittedhimtheoppor- tunitytolistento othersconcerningtheirneedsrelativeto dealingwithfrac- turedreservoirs.Nowintheupdatedsecondedition,Nelsonhasextended thefirsteditionbycouplinghisownresearchandexperiencewiththewide exposurehereceivedfromlisteningtotheproblemsofothergeoscientists duringthelast15yearsofgivingannualAAPGseminars. Thesecondeditionnotonlystillincludesthecardinalaspectsoffrac- turedreservoirsthatwerecontainedinthefirstedition,butitalsoincorpo- rateswhatisnewin15yearsof progressintreatingfracturedreservoirs.In additiontotherudimentsofthefirstedition,Nelsonhasincludedsixen- tirelynewsections,whichrangefrom"howtoavoidfracturedenial"to "positivereservoirattributes"to"screeningtoolsindefiningafractured xiv reservoir."Also,inthesecondeditionnewadvancesindirectionaldrilling areintegratedintofracturedreservoirtreatment. IwasfortunateenoughtoparticipatepersonallywithNelsonin10or12 oftheAAPGfracturedreservoirseminars,andthereisnodoubtthatthe mostfrequentrequestfordatanotpresentedintheseseminarswastoin- cludeasummaryof fracturedreservoircasehistories.Nelsonhasremedied thisrequestbyincludingatotallynewappendixinthesecondeditionin whichhepresentshistoricproductionchartsfor25fracturedreservoirson whichhehaspersonallyworked. Anothernewfeatureincludedinthesecondeditionwillaidinsolvinga frequentcommunicationproblem.Itisnotunusualfortheperson(s)with themosttechnicalbackgroundinfracturedreservoirstopresentahighly shortenedversionofaproposedprojecttopeoplewithmuchlesstechnical background.Toaidinthistask,acompanionwebsiteisalsoofferedwith thesecondeditionsothatanytableorillustrationinthebookcanbedown- loadedandprojectedaspartofacommunicationeffort. Justasthefirsteditionwas,theneweditionisequallyindispensableasa shelf referenceforanypersonworkingwithfracturedreservoirs,eventhose whoownafirstedition. DavidW.Stearns UniversityofOklahoma XV This Page Intentionally Left BlankPref ace Muchhashappenedinthefieldoffracturedreservoiranalysissincethe publicationofthefirsteditionofthistextin1985.Manymorereservoirs havebeenidentifiedasbeingfracture-controlledandgreatstrideshave beenmadeintheintegrationoftheworkandapproachesofthemanydis- ciplinesneededtoworksuccessfullywiththesereservoirs.Indeed,fromex- plorationthroughblow-down, theeffectivemanagementof thesereservoirs requirestheapplicationofmulti-disciplinaryapproachesmorethanvirtu- allyanyothertypeof petroleumreservoir. In thissecondedition,I havetriedtoretaintheemphasisonrockdata ap- proachestothestudyofthesereservoirswhileaddingmorematerialon theirproductionhistoriesandcharacteristics.Inaddition,practicalcheck- listshavebeenaddedtohelpdetermineifyouaredealingwithafractured reservoir ornotaswellasproceduresfor howtoapproachthestudyof frac- turedreservoirsdependingonwheninitshistorywe"discover"thatitis fractured.Ibelievethatthematerialincludedinthissecondeditionwill allowustomoveawayfromthehistorical"fracturedenial"thatourreser- voirworkershavesufferedfromformanyyears. Anadditionhasbeenmadeto thiseditionintheformof companionweb- site(http:/ / www. bh. com/ companions/ 0884153177). Thissiteincludes.pdf filesoftheslidesthatIhaveusedinthevariousAAPGfracturedreservoir coursesthatIhavetaughtoverthelast20years.Theseincludemanymore illustrationsthanareusedinthetextandcanbeusedbythepurchaseras trainingresourcematerial. xvii This Page Intentionally Left BlankNot at i on a, b, n A B1 B 2 B 3 BI1,BIz,B33 B U B. II d D c E F 1 F~ F 3 FI I,F22,F33 F ij F. U g h k Bk kf kfr k r K kll, kzz, k33 kH90 =variousconstants =cross-sectionalarea =plugpermeabi l i t yparalleltobeddi ng =plugpermeabi l i t y45 ~ tobeddi ng =plugpermeabi l i t y90 ~ tobeddi ng - maxi mum, intermediate, andmi ni mumprincipalbeddi ng permeabi l i t ytensorcomponent s =beddi ngpermeabi l i t ytensor =rotatedbeddingpermeabi l i t ytensor =averageconstitutivegraindi amet er oftherock =averagefracturespacing( averagedistancebet weenparallel fractures) =fracturewidth =Young' selasticmodul us =plugpermeabi l i t yparalleltofracture =plugpermeabi l i t y45 ~ tofracture =plugpermeabi l i t y90 ~ tofracture - max i mum, intermediate, andmi ni mumprincipalfracture permeabi l i t ytensorcomponent s =fracturepermeabilitytensor =rotatedfracturepermeabi l i t ytensor =accelerationofgravity =hydraulichead =intrinsicpermeabi l i t y =meanpermeabi l i t y =fracturepermeabi l i t y =totalpermeabi l i t y(rockplusfracturesyst em)=rockormatrixpermeabi l i t y =hydraul i cconductivity - max i mum, intermediate, andmi ni mumprincipalperme-abilitytensorcomponent s =horizontal90 ~frommaxi mumhorizontalwhol e- coreper- meability xix kHmax kv 1 M U M,lj N NB NF P p Q R S11, 822, 833 S 11 S. Ij Sv S h V~ V P o~,13 E CY! CY3 I~"h l~' nlea n O v * r =max i mumhori zont al whol e- cor eper meabi l i t y =verticalintrinsicwhol e- cor eper meabi l i t y =length =mat ri xper meabi l i t y =rot at edmat ri xper meabi l i t yt ensor =di mensi onl esscoeffi ci ent charact eri st i cofthemedi um =per meabi l i t ypl ugwithnovisiblebeddi ng =per meabi l i t ypl ugwi t hnovisiblefractures =fluidpressure =poreorfor mat i onpressure =fl owrate =ar andoml ytakenper meabi l i t yplug - ma x i mu m, i nt er medi at e, andmi ni mumtotalpr i nci palstresses =styloliteper meabi l i t yt ensor =rotatedstyloliteper meabi l i t yt ensor =totalverticalstress(s v +Pp) =totalhori zont al stress(S h +Pp) =bulkvol ume =porevol ume =anglesbet weenfractureplanesandpressuregr adi ent=strainc omponent=Poi sson' sratio =vi scosi t y =densi t y =stress =max i mumprincipaleffectivestressc omponent=i nt ermedi at eeffectiveprincipalstressc omponent=mi ni mumprincipaleffectivestressc omponent=hori zont al effectivestressc omponent=meanstress =verticaleffectivestressc omponent=porosi t y =fractureporosi t y( porevol umetototalvol ume)=matrixorrockporosi t y( porevol umetototalvol ume)XX 1 EvaluatingFractured Reservoirs:I ntroduction AVOI DFRACTUREDENI AL Fracturedreservoirsmakeupalargeandincreasingpercentageofthe world' shydrocarbonreserves.InB P Amocoalone,currentandfuturefields invarioustypesof fracturedreservoirsare estimatedtoaccountforsome21 billionbarrelsofoilequivalent(BBOE).However,inspiteoftheimpor- tanceoffracturedreservoirs,weintheindustrytendtodenythepresence of fracturesinourreservoirs.Thisfracturedenialisprobablyduetoourde- siretoavoidcomplicationinourtechnicalworkandreductionofcycle timesinourexplorationandproduction(E&P)efforts.Indeed,fractured reservoirsaremorecomplicatedthanmatrixreservoirs,andtheydorequire moretimeandmoneytobeevaluatedcorrectly.Thetendencyistoignore thepresenceandeffectof naturalfracturesforaslonginthefieldhistoryas possible.Theproblemswiththisdenialoravoidanceinclude:1)oftenir- reparablelossofrecoveryfactor;2)primaryrecoverypatternsthatarein- appropriateforsecondaryrecovery;3)inefficientcapitalexpenditure duringdevelopment;4)drillingofunnecessaryin-fillwells;and5)im- properassessmentof economicopportunities. Itisimportanttodeterminetheeffectofnaturalfracturesinourreser- voirsasearlyaspossiblesothatourevaluationsandplanningcanbedone correctlyfromdayone.Fracturedenialdoesnothingpositiveforourex- plorationanddevelopmentactivitiesandcanonlyleadtopoorertechnical andeconomicperformance. Remember : Finding fracturesisnotenough. 2GeologicAnalysisofNaturallyFracturedReservoirs Fracturedreservoirsareverycomplicatedanddifficulttoevaluate. Effectiveevaluation,prediction,andplanninginthesereservoirsrequirean earlyrecognitionoftheroleof thenaturalfracturesystemandasystematic approachtothegatheringandanalysisofpertinentdata.However,care shouldalwaysbetakentomakesurethatthedegreeofanalysisandevalu- ationiscommensuratewiththeparticularproblembeingaddressed.Itis easytoget lostin detailanddataacquisition,andlosesightof theeconomic questions. Pr obl ems Interestinnaturalfracturestudiesinsurfaceandsubsurfaceformations hasincreaseddramaticallyinthepastyears.Thishasbeenbroughtaboutby greaterindustryknowledgeoftheeffectoffracturesonsubsurfacefluid flowandbyasignificantandeverincreasingpercentageofoilandgasdis- coverieswherenaturalfracturesplayasignificantroleinproduction. Fracturedreservoirspossessmanyinherentobstaclestoproperanalysisdue todifficultiesinprediction,evaluation,andcharacterization,butpossess verypositiveattributesaswell.Severalobstaclesstemfrom: 1.Agenerallackofin-depthquantitativeapproachestodescriptionand characterizationofhighlyanisotropicreservoirs. 2.Failureofgeologistsandengineerstorecognizefracturesand/orthe regularityoftheirdistribution. 3.Over-simplisticapproachesinthedescriptionoffracturedistributions andmorphologies.4.Theneedforadeterministicsolutiontomodelingfluidflowinfrac- turedporousmedia,whileunderstandingthatourdatalimitations forceustowardstochasticsolutions,atbest. Theseobstaclesarecompoundedbytheimproperuseornonuseofthe manytechniquesavailabletodetectnaturalsubsurfacefractures.While mostofthesetechniquesdowork,seldomaretheysignificantbythem- selves,andoftentheymayevencloudtherealissuesofevaluation. Remember: Finding fracturesisnotenough. Detectingsubsurfacefracturesorpredictingtheiroccurrenceisindeed onlythefirst,mostbasicstepinfullyevaluatingafracturedreservoir.The keytoeconomicallyproducingthesereservoirsliesin: 1.Evaluatingrecoverablereservesasafunctionofwellcosts. 2.Predictingoptimumwelllocationsandwellperformancewithtime underavarietyofpotentialcompletionanddevelopmentscenarios. EvaluatingFracturedReservoirs:Introduction3 3.Obtainingsufficientrockandfracturedatatomakethesecalculations possible. Ingeneral,thisbookwillemphasizetechniquesaddressingthelasttwo keyissues.Thebookwillshowthebreadthof rockdataandproductiondata thatcanbeusedinevaluatingfracturedreservoirs.Thesedataarerequired toaddressallof theseissuesatvarioustimesduringthehistoryof thefield. Therefore,thismaterialshouldbeconsideredthedatabasenecessaryto makethemajoreconomicandengineeringdecisionsatvariousdecision pointsfromexploration,toproduction,toharvest.Whilesomeof thisdatais notfullyuseduntillatertimesduringfieldhistory,muchofthestatic(ver- susdynamic)datacanonlybeobtainedearly,inworkinglifewiththefield. Def i ni t i ons Theword"fracture"hasbeendefinedinvariousways.Somedefinitions arepurelydescriptive(Dennis,1967)whileothersaremechanical(Ranalli andGale,1976).Therangeindefinitionsgenerallyreflectsthedifferentin- terestsoftheauthors.Becausethisbookaddressestheeffectnaturallyoc- curringfractureshaveonreservoirrock,thedefinitionwillberestricted heretoareservoircontext. Areservoir fractureisanaturallyoccurringmacroscopicplanardiscon- tinuityinrockduetodeformationorphysicaldiagenesis.If relatedtobrit- tlefailure,itwasprobablyinitiallyopen,butmayhavebeensubsequently alteredormineralized.Ifrelatedtomoreductilefailure,itmayexistasa bandofhighlydeformedcountryrock.Asaresult,naturalreservoirfrac- turesmayhaveeitherapositiveornegativeeffectonfluidflowwithinthe rock.Thisbroaddefinitionallowsthistexttoaddressfluidflowanisotropy createdbynumerousfeaturesregardlessofanymechanicaldifferencesin theirgenerationandpropagation(extensionversusshear,mode1versus mode2,fractureversusmicrofault,etc.).Thisdefinitionalsomakesitpos- sibletotreateffectsof variousfracturemorphologiesonfluidflow.Forex- ample,onecanlookattheeffectofhighlypermeableopenfractureson reservoirbehavior,butcanalsoconsiderthestronganisotropyinrockper- meabilitycreatedbylow-permeabilitydeformedfractures. Thedefinitionof a reservoirfractureisabroadone,andthedefinitionof a"fracturedreservoir"evenmoreso.Becausenaturalfracturesystemscan haveavarietyofeffectsonreservoirperformanceinprimary,secondary, andtertiaryrecovery,andbecausetheseeffectsmustoftenbepredicted longbeforetheyareevidencedinproductiondata,anoperationaldefinition ofafracturedreservoirbecomesanecessity.Afracturedreservoirisde- finedasa reservoir in whichnaturallyoccurringfractureseither have,or are 4Geologic AnalysisofNaturallyFracturedReservoirs predictedtohave,asignificanteffectonreservoirfluidfloweitherinthe formofincreasedreservoirpermeabilityand/orreservesorincreasedper- meabilityanisotropy.Thequalifier,or"arepredictedtohaveasignificant effect,"isimportantoperationallybecausethedatanecessarytoquantifya fracturedreservoirmustbecollectedveryearlyinthelifeof areservoir.We must often,therefore,predictthe"significanteffect"andtreat theformation asafracturedreservoirpriortotruesubstantiationbyproductionhistory. THEEVALUATI ONSEQUENCE Theremainderof thischapterpresentsthecriticalattributesthatmustbe evaluatedtoquantifyfracturedreservoirsinalogical,workablesequence: origin,properties,fracture/matrixinteraction,reservoirtypingand,eventu- ally,wellplacementandcompletion. BASI C TYPESOFEVALUATI ON Explorationandproductioncannotbeseparatedfromevaluationinfrac- turedreservoirs.Itisofparamountimportancetoknowwhatwearelook- ingforandwhatwehavefoundintermsofreservoirproperties.Thereare threebasictypesof evaluationtobeaddressedinfracturedreservoiranaly- sis(Nelson,1982).Theyarelistedinorderofincreasingcomplexity, amountofdata,andtimetocompletion: I.Earlyexplorationevaluationstodetermineorpredictgrossreservoir quality. 2.Evaluationsofeconomicpotential(reserves,flowrates,etc.). 3.Evaluationsforrecoveryplanninganddetailedreservoirmodeling. Thesearedistinctlydifferenttypesofevaluation,requiringvarious amountsofbothqualitativeandquantitativedata.Theywereperformedin thepastatdifferenttimeswithinthehistoryofafieldorprospect.Today, however,workcyclesintheindustryaremuchmorecompressed, forcing ustoaddresssomeofthemoredetailedmodelingaspectsearlyinthe "prospectphase." EvaluatingFracturedReservoirs:Introduction5 EarlyExplorationEvaluations Economically, themostfrequentandoftenmostcriticalfractureevalua- tionisthatperformedearlyintheexplorationphaseof ahydrocarbonplay. Thepurposeistobetter definethepropertiesof interestandtodetermineor predictthegrossreservoirqualityofadiscovery.Theseevaluationsonly dealwithageneralknowledgeofthestructureandstratigraphicsequence (petrophysicalandmechanicalattributes),logsuitesthatarenotdesigned specificallyfornaturalfractureevaluation,andminimalcoreandwell-test data.Evaluationsperformedatthistimearequalitativeatbest,andare probablymorelikespeculationsthantrueevaluations. However,theseevaluationsoftenwill"makeorbreak"aplayinits drillinginfancy.Forexample, coresfromanearlywellcuttingintothe CambriansectioninAmalFieldinLibyawouldhaveshownapermeable fracturesystemwithnosignificantrockmatrixcontributiontoreservoir floworstorage(fracturedreservoirType1,seeChapter2).Becauseweare alwaysskepticalofsuchreservoirs,extremecautionwouldhavebeenad- vised,includingthepossibilityofabandoningtheplay.However, knowl- edgethatthefracturesystempresentisfold-related(tectonicinorigin)and should,therefore,bedevelopedovertheentire100,000acresofstructural closure,andthattheentirequartzitepackage,whichis800ft.thick,and shouldfractureasaunit,wouldhaveallowedworkerstopredicttheenor- mouspotentialofthisdiscovery(1,044millionbarrelsofoil[MMBO]).Theearlyexplorationevaluationdatamostoftenusedare: 1.Generalgeological/ geophysicaldataonstructuralforms. 2.Agoodlithologicdescriptionofthestratigraphicsection. 3.Mechanicaldataontheparticularrocksofinterestoronsimilar lithologies. 4.Matrixpropertiesfromlogsorasinterpretedfromnearbyareas. 5.Drillstemtest(DST)orinitialpotential(IP)flowrates. 6.Coreanalysis(standardorwholecore). 7.Boreholeimaginglogs. 8.Insitustressdata. EvaluationsofEconomicPotential Afterithasbeenproventhatfracturesareanintegralportionofthetotal reservoirqualityandmorequantitativedataareavailable,evaluationsof 6Geologic AnalysisofNaturallyFracturedReservoirs economicpotentialaremade.Thepurposeistoestimatereservesandflow ratestomoreaccuratelydeterminethepotentialworthofthereservoir. Estimatesoffracturespacingandwidthbecomemoreimportantaswell asknowledgeoffracture-matrixporosityinteraction.Alsoimportantare laboratoryestimatesofrelativeflowwithinfracturesandmatrixatsimu- lateddepth. Inadditiontoearlyexplorationdata,otherinformationshouldinclude: 1.Extendedtimepressuretests. 2.3-Dwhole-corepermeabilityanalyses(orientedif possible),borehole imaginglogs. 3.Laboratorydataonmatrixandfracturepropertiesundersimulated depthanddepletionconditions. 4.Estimationsoffracture/matrixinteraction. EvaluationsforRecoveryPlanningandModeling Duringfulldevelopmentofamajorfield,severaldepletionschemes mustbeevaluatedtooptimizerecoveryand/oreconomicfactors.Anim- portanttoolisreservoirmodeling:usingcomputer-assistedmathematical modelstoinvestigatecompositionalbehaviorandrelativeflowratesunder zhangingreservoirpressureandtemperatureconditions.Increatingsuch modelsforfracturedreservoirs,themostdetailedquantitativefracture analysesarerequired.Theseinvolvenotonlystatisticalanalysesoffracture propertiesandpatterns,butalsodetailedknowledgeof3-Ddistributionsof fractureswithinthereservoir.Thisrequiresafoot-by-footdescriptionand Jocumentationofnumerouscoresandorimagelogs.Suchin-depthanaly- sesarecostlyandtimeconsuming, andaredeemedappropriateinonlythe larger,complicatedreservoirs. Thetypesofdatamostoftenusedinrecoveryplanningevaluationsare: 1.Detailedstructuremapscoveringseveralhorizonsaboveandbelow theproducingformation. 2.Detailedcoredescriptionsincludinglithology,mineralogy,textures, andafoot-by-footdocumentationoffractureoccurrence,orientation, andmorphology. 3.Interpretedboreholeimagerylogsinallwells,especiallythosethat areuncored. 4.3-Dwhole-coreanalyseswithatleastoneorientedcoreinthefield. 5.Mechanicaldataderivedfromcoresamplesofinterest. EvaluatingFracturedReservoirs:Introduction7 6.Long-termflowtestsandmultiplewelltests. 7.Estimationofinitialinsitustressstateinthereservoir. 8.Laboratorydataonbothmatrixandfracturepropertiesundersimu- lateddepthanddepletionconditions. 9.Laboratorydataonfracture/matrixinteraction. GE NE RALSEQUENCEOFSTUDY Theorderofinvestigationinfracturedreservoirsisimportantinthat studycanbesuspendedatanytimeifthereservoirqualityappearstobe poor.If,for example, thefracturenetworkinitially detectedwasinterpreted, becauseoftheorigin,tobeoflimitedaerialextent,furtherevaluationand data generationmay beconsideredunnecessary.Thenextthreesectionsdis- cussthefirstthreephasesofthisevaluationsequence.Thefourth (Classificationof Reservoir Type)andfifth(OptimumLocationsandPaths) phaseswillbediscussedinlaterchapters. FRACTURESYSTEMORI GI N Theoriginofthefracturesystemispostulatedfromdataonfracturedip, morphology,strike(ifavailable),relativeabundance, andtheangularrela- tionshipsbetweenfracturesets.Thesedatacanbeobtainedfromfull-diam- etercore(orientedorconventional),boreholeimaginglogoutput,orother lessorientedloggingtools,andappliedtoempiricalmodelsof fracturegen- eration.Availablefracturemodelsrangefromtectonictoothersofprima- rilydiageneticorigin(StearnsandFriedman, 1972;andNelson,1979).Itis onlybyaproperfitof fracturedatatooneofthesegeneticmodelsthatany effectiveextrapolationorinterpolationof fracturedistributioncanbemade. Theinterpretationoffracturesystemorigininvolvesacombinedgeo- logical/rockmechanicsapproachtotheproblem.Itisassumedthatnatural fracturepatternsdepictthelocalstateofstressatthetimeof fracturing,and thatsubsurfacerocksfractureinamannerqualitativelysimilartoequiva- lentrocksinlaboratorytestsperformedatanalogousenvironmentalcondi- tions.Naturalfracturepatternsareinterpretedinlightof laboratory-derived fracturepatterns(HandinandHager,1957)andintermsofpostulated 8Geologic AnalysisofNaturallyFracturedReservoirs paleo-stressfieldsandstraindistributionsatthetimeof fracturing.Ingen- eral,anyphysicalormathematicalmodelof deformationthatdepictsstress orstrainfieldscan,byvariouslevelsof extrapolation,beusedasafracture distributionmodel(Hafner,1951 ; OdE,1957;andLorenzandothers,1993). Ageneticclassificationschemefornaturalfracturesystems,whichisan expansionof thatfoundinStearnsandFriedman(1972),permitsseparation ofcomplicatednaturalfracturesystemsintosuperimposedcomponentsof differentorigin.Suchpartitioningcanmakedelineationofstructure (Friedman,1969;andFriedmanandStearns,1971)andpredictionofin- creasedfracture-relatedreservoirquality(McCalebandWillingham,1967; andStearnsandFriedman,1972)fromfracturedatamoretractable.Stearns andFriedman(1972)classify fracturesintothoseobservedinlaboratoryex- perimentsandthoseobservedinoutcropandsubsurfacesettings.Their classificationscheme,togetherwithmodificationssuggestedbythisbook, formsausefulbasisforfracturemodels(Table1-1).Themajormodifica- tiontoStearns' andFriedman' sschemeistheadditionof twocategoriesof naturallyoccurringfractures:contractionalfracturesandsurface-related fractures.Aminormodificationtotheexperimentalfractureclassification istheadditionofacategorysimilartoextensionfracturesinmorphology andorientation,buthavingadifferentstressstateatgenerationandrock strength:tensionfractures. Tabl e1 - 1E x per i ment al andNat ur al Fr act ur eCl assi f i cat i on ExperimentalFractureClassification 1.Shear fractures 2.Extensionfractures 3.Tensilefractures NaturallyOccurringFractureClassification 1.Tectonicfractures(duetosurfaceforces) 2.Regionalfractures(dueto surfaceforcesor bodyforces) 3.Contractionalfractures(duetobodyforces) 4.Surface-relatedfractures(duetobodyforces) EvaluatingFracturedReservoirs:Introduction9 Gener i cCl assi f i cat i on Threefracturetypesareobservedtoformatconsistentandpredictable anglestothethreeprincipalstressdirectionsduringlaboratorycompres- sion,extension,andtensiletests.Allbrittlefractureinrockmustconform tooneofthesebasicfracturetypes:shear,extension,andtensionfractures. ShearFractures Shearfractureshaveasenseofdisplacementparalleltothefracture plane.Theyformatsomeacuteangletothemaxi mumcompressiveprinci- palstressdirection(cy~) andat anobtuseangletothemi ni mum compressive stressdirection(cy3)withintherocksample.Potentially,twoshearfracture orientationscandevelopineverylaboratoryfractureexperiment, oneonei- thersideof,andorientedatthesameangleto,cy~. Inlaboratoryexperi- ments,thesefracturesformparallelto(3"2andatanobtuseangletocy3 (Figure1-1).Shearfracturesformwhenallthreeprincipalstressesare compressive(compressivestressesareconsideredpositiveforthiswork). Theacuteanglebetweenshearfracturesiscalledtheconjugateangleandis dependentprimarilyon: 1.Themechanicalpropertiesofthematerial. 2.Theabsolutemagnitudeofthemi ni mumprincipalstress(c~3). 3.Themagnitudeof theintermediateprincipalstress(cy2) relativetoboth themaxi mum(cy l)andmi ni mum(cy3)principalstresses(asG2ap- proachesc~ ltheanglebetweenc~andthefractureplanedecreases). o"1 A O"2 0"3~~O"3 0" 1 Figure1-1.Potential fractureplanesdevelopedinlaboratorycompressiontests. Extension fractures(A)and shear fractures(B and C) are shown. 10Geologic AnalysisofNaturallyFracturedReservoirs ExtensionFractures Extensionfractureshaveasenseofdisplacementperpendiculartoand awayfromthefractureplane.Theyformparalleltocy~ and(Y2 andperpen- diculartoo 3 (Figure1-1).Thesefracturesalsoformwhenallthreeprinci- palstressesarecompressive.Inlaboratoryfractureexperiments, extension fracturescanandoftendoformsynchronouslywithshearfractures. TensionFractures Tensionfracturesalsohaveasenseof displacementperpendiculartoand awayfromthefractureplaneandformparalleltocy~ ando 2.Intermsof ori- entationofcy~andsenseofdisplacement, thesefracturesresembleexten- sionfractures.However,toformatensionfracture,atleastoneprincipal stress(cy3)mustbenegative(tensile).Toformanextensionfracture,all threeprincipalstressesmustbepositive(compressive).Thedistinctionbe- tweenthetwoisimportantbecauserockshaveamuchlower(10to50times lower)fracturestrengthintensionteststhantheydoinextensiontests.This becomesimportantinmathematicalpredictionofsubsurfacefracturing. Also,itislikelythattruetensilefracturesonlyoccurinnearsubsurfaceen- vironment,whileextensionfracturescanoccurinalllowmeanstresssub- ~urfaceconditions.Ingeneral,Iwillcallextensionfracturesthosethatare paralleltooI andperpendiculartoo 3 wheno 3 iscompressive(positive)or whenitssignisunknown;tensilefractureswillbereferredtoonlywhenev- idencesuggestso 3 isnegative. Geol ogi cCl assi f i cat i on ThegeneticnaturalfractureclassificationpresentedinSteamsand Friedman(1972)andexpandedhereisbuiltontwofundamentalassumptions: 1.Naturalfracturepatterns(conjugateshearandextensionortensile fractures)faithfullydepictthelocalstateofstressatthetimeoffrac- turing. 2.Subsurfacerocksfractureinamannerqualitativelysimilartoequiva- lentrocksinlaboratorytestsperformedatanalogousenvironmental conditions. Thus,weassumethatnaturalfracturepatternsreflectthesamegeometry withrespecttoappliedloadsasdofracturesgeneratedinlaboratoryexper- EvaluatingFracturedReservoirs:Introduction11 iments.Iftheseassumptionsarecorrect,thennaturallyoccurringfractures canbeclassifiedonthebasisoftheoriginoftheircausativeforcesasde- terminedfromlaboratorydataandfracturesystemgeometry(Table1-1). Therefore,thisclassificationreliesheavilyonthepreviouslypresentedex- perimentalorgenericfractureclassification. Therearetwoschoolsofthoughtonthebestmeanstoobserveandde- scribecomplexnaturalfracturesystemsinoutcrop.Oneassumesthatfrac- turedatamustbehandledstatisticallytobemeaningful.Thus,by combininglargeamountsof datafrommanyoutcropstogetherandsearch- ingforpreferredorientations,it isbelievedthatobjectivityininterpretation canbeobtained(CurrieandReik,1977).Whilethiscombiningofdatais necessaryatsomestageof afracturestudy,Ibelievethisapproachtobein- efficient dueto the greatlossof interpretiveprecisionwhendataarelumped togetherpriortointerpretation.Forexample,anorientationplotcontaining 10,000fracturemeasurementsfrommanyplacesonafoldwilldisplay grosstrendsinthedatabutwillnotallowdescriptionofsubtlechangesin orientationandinferredstressstatesfromoutcroptooutcrop. Asecondapproachinvolvestheinterpretationofindividualoutcropdata withrespecttothemodeof originpriortostatisticaltreatment(Stearnsand Friedman,1972).Theseinterpreteddatasetscanthenbeaddedtogetherse- quentiallytoarriveatacombineddescription.Thecombineddatasetwill havemorestatisticalmeaningandisalsomoreeasilyinterpretedforstress analysisduetopriorinterpretationofthestatisticallylesssignificantindi- vidualdatasets. Thisapproachtofractureinterpretationnecessitatestheuseofagenetic naturalfractureclassificationsuchasthatusedinthisbook.Determining theoriginofloadsthatcausedfracturingattheoutcropscaleincreasesthe precisionof structuralinterpretationonallscales.Thiscanbeaccomplished becausefracturesforminaconsistentgeometrywithrespecttothethree principalstressdirections,thusdelineatingthepaleo-stressfieldatthetime of fracture(comparefigures1-Iand1-2). Thegeologicclassificationdescribedbelowhasimportantramifica- tionstopervasiveness, orthedegreetowhichthefracturesystemisde- velopedovermultiplescalesofsize.Forexample,tectonicfractures relatedtofoldingarepervasivebecausethesamefracturetypesandori- entationsareseenfromaerialphotographsoftheoutcrop,tohandsam- plesfromtheoutcrop,tothinsectionstakenfromtheoutcroporcore.On theotherhand,regionalfracturesarenonpervasivebecausetheycanusu- allybeseenononlyalimitednumberof scales,i.e.,downtooutcropscale only.Ageneralizationof thepervasivenessof thevariousgeologicalclas- sificationsisgiveninTable1-2. 12Geol ogi cAnal ysi sofNatural l yFracturedReservoi rs Table1 - 2ScalesofNaturalFractureDevel opmentfortheGeol ogi cCl assi fi cati on OrdersofMagnitudeinSizeSpanned TectonicFractures RegionalFractures ContractionalFractures Surface-RelatedFractures 9-10Orders 5 2 4-5 Figure 1-2aProbableconjugateshear fracturesin outcrop fromTnnJdad, courtesy ofSSerra andD.BFello. Figure1-2bConjugatefold-relatedfracturesexpressedonabeddingplaneincarbonate rocks~n theWesternWyomingThrustBeltFieldofvIewisabout3ft.PhotocourtesyofS. Serra EvaluatingFracturedReservoirs:Introduction13 Tectoni cFractures Tectonicfracturesarethosewhoseorigincan,onthe basisof orientation, distribution,andmorphology,beattributedtoorassociatedwith alocaltec- tonicevent.Theyareformedbytheapplicationofsurfaceforces.Thisau- thorhasobservedthatthemajorityoftectonicfracturesinoutcroptendto beshearfractures.However,locallyIhaveseenexamplesof foldsincom- pressiveenvironmentswherethedeformationisdominatedbyextension fractures.Tectonicfracturesforminnetworkswithspecificspatialrelation- shipstofoldsandfaults. Fault-Related FractureSystems Faultplanesare,bydefinition,planesofshearmotion.Themajorityof fracturesdevelopedin the vicinity of faultsareshear fractures parallelto the fault,shearfracturesconjugatetothefault,orextensionfracturesbisecting theacuteanglebetweenthesetwosheardirections(thezoneof faultslipor gougeiscomplex,andhasitsowninternaldeformationmorphology). Thesethreeorientations(Figure1-3)correspondtothethreepotentialfrac- turedirectionsduringlaboratoryfractureexperiments(Figure1-1)andare developedrelativetothelocalstateofstresscausingthefault.Thefaultis a result of the samestressfield that causedthe fractures.Thefractureswarm predatesthe through-goingfaultandactsasaprocesszoneconditioningthe rockmassfortheeventualfaultoffset.Therearecaseswherelarge-scale slipdidnotoccur,leavingonlytheprecursivefractureswarm.Inthese cases,theorientationof theswarmitself,aswellastheinternalfractureori- entationsareneededtoascribeafault-relatedorigin.Severalauthorshave notedanddocumentedthefault-fracturerelationship:Stearns(1964), Yamaguchi(1965),Norris(1966),Stearns(1968a,1968b,1972),Skehan (1968),Friedman(1969,1975),TchalenkoandAmbraseys(1970),Stearns andFriedman(1972),andFreund(1974). Becauseoftherelationshipbetweenfaultingandfracturing,itispossi- bletodeterminethedirectionoftheprincipalstressesorloadsatthetime offormation.Also,knowingtheorientationofafaultplaneandthefrac- turesassociatedwithit,thesenseofmovementofthefaultcanbedeter- mined(Figure1-4).Therelationshipoffracturestofaultsexistsonall scales.Indeed,Friedman(1969)wasabletousetheorientationofmicro- scopicfracturesfromorientedcoresintheSaticoyFieldofCaliforniato determinetheorientationanddipof anearbyfault.Anoutcropexampleof fracturesassociatedwithanormalfaultintheSinaiinEgyptisshownin Figure1-5. 14GeologicAnalysisofNaturallyFracturedReservoirs THEORETICAL POSITION OFFAULT CONJUGATE ,~..,'~o IFORFAULI T \ \ \ \ ATTITUDE OFFAULT MEASURED INFIELD O lFigure1-3Rose diagram ofshearfracturesassociated w~th normal defaultAfterStearns (1968b) ~! AC o l A&C I d e a l i z e d f r a c t u r e pa t t e r n 02060~0B0 f or n o r ~l f a u l t s ande x t e n - AnEl ec ot ea x l e t o f r a c t u r e pl ane s i v a p a r t of f ol d~3 i e lAkC Gh I d e a l i z e d f r a c t u r e pa t t e r nf o r r ev er s ef a u l t and020406080 compr essi vepa r t of 8f o l d Ansl ec or ea x i s t o f r a c t u r e pl ane Q! ,(c Or i e n t a t i o n of s t r e s s I d e a l i z e d f r a c t u r e pa t t e r n 020406080 f i e | d e vhent hev a r i o u s f o r r ev er s ef a u l t AnBl ecor ea x l e t o f r a c t u r e pl ane St oupsof f a u l t s v e r ei n i t i a t e dFigure1-4Relattonshtps between stress states, thefaultandfractureonentationsdenved from those stressstates, andtheresultant diph~stograms subsequentlyobtainedfromcore analysesAfterPrice (1966) andFriedman (1969), courtesyofPergamon Press, Ltd., and the American AssociationofPetroleum Geologists (AAPG) Eval uat i ngFr act ur edReser voi r s: I nt r oduct i on15 Fi gure1-5A normalfault~n theM~ocene clast~c sectionof theGulfofSuez.Thefaultts downto theright(west)andoccursontheS~na= sideof theGulf.Thewidthof theoutcropis about100ft. Note theconjugateshear andextensionfractures~n the footwall(left s=de) of the fault. Thesepre- datedthefaultdisplacementandarerelatedtothesamestressstatethatcausedthefault. While,underidealconditions,itisnowpossibletodeterminetheorienta- tionandsenseof displacementof anearbyfaultbytheanalysisof fractures, itisdifficulttodeterminetheproximityofthefault(Skehan,1968;Pohn, 1981;Shepherdandothers,1982).Theintensityoffracturingassociated withfaultingappearstobeafunctionoflithology,distancefromfaultplane, amountofdisplacementalongthefault,totalstrainintherockmass,depth of burial,andpossiblythetypeof fault(thrust,growth,etc.).Whichof these parameterswilldominatefractureintensityvariesfromfaulttofault. Thereareotherlessfrequentfractureorientationsassociatedwithfault- ingofvariousscales.Onegroupof grain-sizedfracturesoccursatacutean- glestothefaultplaneandiscalledmicroscopicfeatherfractures(Friedman andLogan,1970),Conrad(1974)relatesthesetodisplacementalongthe faultandthenormalstressacrossthefaultplane.Whilethesefeatherfrac- turesareimportantindeterminingafaultingoriginandinmicroscopicex- aminationoffaultplanesforthesenseofshearmotion,theirimportancein macroscopicfractureproductionofhydrocarbonsisprobablyminimal. Otherfracturesassociatedwithfaultsoccurwithintheslipzoneitself. Thesereflectcomplexandchangingstressandstrainstatesinherentinthe slipzoneormylonitezoneitself.Adescriptionofthesecanbefoundin Higgs( 1981).