Mapping Sandstones of the Inyan Kara Formation for Saltwater Disposal in North Dakota

Jeffrey W. Bader

IntroductionThe North Dakota Geological Survey (NDGS) is performing adetailed state-wide investigation of the Inyan Kara Formation(Dakota Group) to support industry needs for undergrounddisposalofproducedwater.Sandstonesoftheformationaretheprimary subsurface injection zone forproducedwater inNorthDakota. Produced water is the salty formation water that isgeneratedasaby-productofoilandgasproduction.Overone-millionbarrelsofco-producedwateraregenerateddailyduringproductionoperations inNorthDakota. Subsurface injection isthe industry-preferred alternative for producedwater disposal.Therefore, because of the prodigious volumes of saltwatergenerated, and the economics of suchdisposal, it is importantto fullyunderstand thevariables involved in selecting locationsfor saltwater disposal wells. These variables would includeaccess; proximity to producing/future exploration wells; andmost importantly, geology. To assist with well placement, theNDGSispreparingdetailedisopach(thickness)mapsatscalesof1:100,000(100K),utilizingdatafromthousandsofwellsper100Ksheettohelpindustrybetterunderstandsandstonetrendsoftheformation.SuchdetailisnecessarybecauseInyanKarasandstonethicknessescanchangequickly,going fromhundredsof feetofcontinuoussandstonetovirtuallynosandstoneoveraveryshortdistance (< 0.5 mile). Cross-sections arealso included to help operators visualizethe lateral complexity of the sandstonegeometry.

Inyan Kara Depositional SettingBeforeanymappingprogramcanbegin,onemustunderstandthegeology,especiallyforsubsurface mapping where the rocks may notbeavailableforstudy.Thisisparticularlytrue for the Inyan Kara, because thedepositional environment is complex andonlyafewsubsurfacecoresareavailable.

Inyan Kara sediments were deposited in a coastal setting adjacent to the CretaceousSeaway from approximately 115 to 105millionyearsago(fig.1).Theunitmarksthebeginningofthe initialsea-levelriseoftheEarlyCretaceous.Atthistime,NorthDakotawasatlatitudesthatweremuchmorehumidand tropical, and dinosaurs roamed thebroad/lowreliefcoastlines.Numerousriversflowed across the coastal plain to the sea(fig.1).Sea-levelvariationscausedshoreline

shifts;withthecoastlinemovinglandwardduringtransgression,and seaward during regression. Major sea-level fluctuationsoccurduetotectoniceventssuchasupliftofmountainranges,orglobalsea-levelchangesbasedonwatervolumeintheoceans,orboth;combiningforanetrelativechangeinsealevel.InyanKarasediments were deposited over western North Dakota duringtwoofthesetransgressive/regressivecyclesasrelativesea-levelfluctuated. Sedimentation along the shoreline was complex,consisting of estuaries (drowned river valleys), that formed astheencroachingseafilledvalleysthatwerecutbyriversflowingtowardstheretreatingseaduringthepreviousfallinrelativesea-level.

Sequence StratigraphySequence stratigraphy, the study of stacking packages withinsedimentary rocks and the surfaces that separate them, wasdeveloped for just such coastal, cyclic depositional settingsas described above. The sequence stratigraphic surfaces aregeneratedbychangesinrelativesea-level,andcanbeconsistentlyrecognizedinrocks,bothincoreandongeophysicallogsmakingthemveryusefulinidentifyingthebestsandstonesforproducedwaterinjection.

Figure 1.PaleogeographicmapoftheNorthDakotaareaduringInyanKaratime(c.a.,106Ma).ModifiedfromBlakey(2014).

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One typical evolution cyclefor an incised valleydeposit isshownonfigure2a.TheInyanKarawasdeposited,alongwiththe lowerportionof theover-lying Skull Creek Formationin two of these cycles (fig. 3).Each cycle records the move-ment of the shoreline through regressive, transgressive, andregressiveevents. The follow-ing discussion is for one cycle that takes place over a verylongtimeperiodofmillionsofyears, as shown on figure 2a.Early in the initial regressivephase, sea-level is droppingrapidly;therefore,riversonthecoastlinewillmovequickly to-wardstheretreatingsea,erod-ing the shoreline and the now exposedsea-floorshelf(fig.2a-A/B). Sea-level thenbegins torise late in the first regressivephase and the newly cut val-leysarefilledwithsand(fig.2a-C). As transgressionproceeds,sea-level continues to rise andthevalleysarefilledwithmoresanduntil fully drowned form-inganestuary(fig.2a-D/E).Theseaeventuallyfloodstheentirearea at the peak of transgres-sion(fig.2a-F),andtheprocessthen starts over with anotherregression(fig.2a-G). Thecor-responding sedimentary depos-itsareshowninacross-sectionthroughthelowerportionofanInyanKara incisedvalley foundat subsurface depths of ap-proximately5,000feet(fig.2b),as interpreted from core and logs. The cross-section showsthevarioussedimentarydepos-itsofsand(LayersCandD)thatare mapped for produced water injection,andthe locationofarock core that was taken from theMathIverson#1welllocat-edinWilliamsCounty.

The logs for the Math Iverson#1well includeboth sequencestratigraphic cycles for the In-yan Kara/lower Skull Creek in-terval(fig3).Becauseidentify-

Figure 2. Blockdiagramsshowingevolutionofatypicalincisedvalleyatatransgressiverivermouth(2a)andassociatedcross-section(2b).RSLF=RelativeSea-LevelFall,RSLR=RelativeSea-LevelRise.

Figure 3. GeophysicalwelllogsfromtheMathIverson#1well,WilliamsCounty,NorthDakota,showingin-cisedvalleyandvalleyfilldepositsoftheInyanKaraFormation.Boldletters(A-F)correspondtofigure2a.MTS=MaximumTransgressiveSurface,MRS=MaximumRegressiveSurface,Perm.=Permeability,TES=Transgres-siveErosionSurface

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ingsandstoneswithgoodporosity/permeabilityforinjectionisthegoalof themappingprogram,thegammaray log ismostuseful.Thegammaraylogisagoodsandstone/shaleindicator;itgivesastrongradioactivesignalforclayeysediments(highinradioactiveminerals)andaweaksignalforcleansands(lowinradioactivemin-erals).Thecorrespondingneutronlogisagoodporosityindicator,withincreasingintensityforgreaterporosity.Inaddition,figure3showsInyanKaracores/photographstakenfromtheMathIverson#1well,withCore-2thatisalsoshownonfigure2b.Themainse-quencestratigraphicsurfacesshownonthelogarethemaximumregressivesurface (MRS)andthemaximumtransgressivesurface(MTS)thatarealsoshownonfigure2b.Theinflection(righttoleft/redand lefttoright/blue)ofthegammaray logchangesateachof these surfaces and represents the regressive/transgressive ortransgressive/regressivetransition.Therefore,ifonlylogsareavail-able,as is thecaseformostofNorthDakota,onecanstilleasilydeterminethenatureofthesedimentaryenvironment/sandstonequality (i.e.,sandporosity/permeability),asdescribed inthenar-rativeon the rightportionof the logand shownonfigs.2aand

2b.Thus,identificationofsequencestratigraphicsurfacesonlogsallowsforaccuratemappingofsandstonebodies intheWillistonBasinofNorthDakota.

Sowhyisallofthissoimportant?Estuariesoccupyingincisedval-leysareubiquitousalongtransgressivecoastlinesinmodernenvi-ronmentsandwerelikelysointhepast(Dalrympleetal.,1992).Es-tuariesandincisedvalleydepositsarecomplexbecauseofthewiderangingandfluctuatingdepositionalenvironmentsandprocessesthatoccurinacoastalsetting.Theyareexcellentsedimenttrapsand,becausedepositionoccursinpaleovalleys,theyarecommonlypreserved.Thisisespeciallyuseful,notonlyforpotentialproducedwaterreservoirs,but foroilandgasreservoirs inothersedimen-tarybasinsthroughouttheworld.However,suchdepositshavenotbeenwellrecognizedintherockrecorduntilthelastfewdecades.

Mapping ProcessMapsandcross-sectionsarepreparedutilizingseverallogs(gammaray,resistivity,andneutrondensity)andcoretoidentifypotential

Figure 4. Generalized map showing Inyan Kara incised valley trends, Watford City 100K Sheet (Bader, 2015), Parshall 100K Sheet (Baderand Nesheim, 2016), Williston 100K Sheet (Bader and others, 2016a), and Stanley 100K Sheet (Bader and others, 2016b), North Dakota. Yellow=thickersandstone;brown=thinnersandstone.BlackdotsareoperatingsaltwaterdisposalwellsasofJanuary2017.

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InyanKarainjectablesandstonebodiesthatcanbemapped(fig.3).Thousandsofindividualwelllogsarereviewedinadatabaseandindividualinjectablesandstonebodiesareidentified.Acon-sistent method with a 50% baseline is used so that each well is evaluatedthesameandbiasesareminimized.Sandstonesabovethe 50% baseline are considered injectable sandstones (yellowzonesonfigure3).Totalsarecomputedfortheentireformationand an isopachmap is created for each 100K. Cross-sectionsare thenprepared, so that sandstone trends canbe accuratelydepictedintwodimensions.Understandingthedepositionalen-vironment iscritical inthemappingstage,because lackofsuchknowledgecaneasily leadtocompletemisinterpretationofthedata.Four100Kmaps(Parshall,Stanley,WatfordCity,andWillis-ton)havebeenpublishedtodate,withanother4-6mapsplannedfor2017;thus,coveringtheentirecoreareaforBakkendrillinginwesternNorthDakota.

ResultsInyan Kara sandstones deposited in incised valleys are thick,porous (20-30%porosity), and permeable (Darcy level) (fig. 3),especiallyalongvalleytrends.Severaldistinctnorth-northwest-trendingvalleys(yellow)canbeidentifiedintheParshall,Stanley,Watford City, and Williston, 100K Sheets, where thicker sand-stoneshavebeendeposited (fig. 4). Between these valleys, intheinterfluvearea,sandstonesarethin(brown),muchlesscon-tinuous,andhavelowerporosity/permeabilitythanincisedvalleysandstones,sotheyarenotoptimalforinjectionofproducedwa-ter.Evenwithintheestuarine/valley-fillcomplex,estuarinesand-stonesthatoverlievalley-fillsandstonesmayhavearelativelylowgammaresponseindicatinggoodsandstoneononelog,butfur-thercorrelationwillshowthatsuchsandstoneisnotlaterallycon-tinuousorporous/permeableasincisedvalleysandstones(fig.3).Therefore,theuseofafewlogstoidentifysandstoneswithoutanunderstandingofsequencestratigraphyprinciplescanberisky.

Thereare251activedisposalwellsinthemaparea(fig.4).Manysaltwater disposal wells are drilled in areas of thinner to margin-allythicksandstonesandareasofthickersandstoneshaveonlybeen locallyutilized,withsandstonesgreaterthan200feetun-derutilized.Thisindicatesthatotherfactorssuchasaccessmayhavebeenmoreofaninfluencethangeologywhenselectingsalt-waterdisposalwelldrillinglocationsinthepast.Althoughaccessandproximityarevery important, thegeologyof the saltwaterdisposalwellwillobviouslybethemainfactorinhavingagoodinjectionwell, and should be given careful thought because ofthe prodigious volumes of water being injected into the InyanKara. Detail of an incised valley and the interfluve area in thenorthernportionoftheWatfordCity100Kisshownonfigure5.Dailysaltwaterdisposalratesareshownforcomparison.Chan-nelsandstoneswithinthevalleytakeonthousandsofbarrelsperday,whereassandstoneoutsidethevalleyintheinterfluveareareceiveswellunder1,000barrelsperday.Historically,only10%ofsaltwaterdisposalwellshavebeenplacedinthickerchannel-izedareas.

TheproductionoftheNDGS100Kseriesmapswillallowopera-torstousesandstonethicknessesandtrends,alongwithsupport-

ingdataand roadmaps, to identifypotentialwell locations foryearstocome.Thiswillhelpminimizepotentialproblemsfromdevelopingduringtheinjectionprocess,suchasthewellnottak-ingonwaterand/ordevelopmentofover-pressuredzonesintheInyanKarathatcanhavesignificantadverseeffectsonnewexplo-rationwellsinthearea.Accuratedatageneratedfromthemap-pingprocessisalsofundamentalinputfordevelopingmodelsthatsimulateinjectionprocessesandvolumesinthreedimensionsforfutureplanningandexplorationpurposes.

ReferencesBader, J.W., 2015, Inyan Kara sandstone isopachmap,Watford

City 100K Sheet,NorthDakota: NorthDakotaGeologicalSurveyInvestigationsno.189.

Bader, J.W., and Nesheim, T.O., 2016, Inyan Kara sandstoneisopachmap, Parshall 100K Sheet, North Dakota: NorthDakotaGeologicalSurveyInvestigationsno.194.

Bader, J.W., Nesheim, T.O., and Ternes, S.A., 2016a, Inyan Karasandstone isopach map, Williston 100K Sheet, NorthDakota:NorthDakotaGeologicalSurveyInvestigationsno.198.

Bader, J.W., Nesheim, T.O., and Ternes, S.A., 2016b, Inyan Karasandstoneisopachmap,Stanley100KSheet,NorthDakota:NorthDakotaGeologicalSurveyInvestigationsno.199.

Blakey, R.C., 2014, History of Western Interior Seaway, NorthAmerica (Jurassic-Cretaceous): Colorado PlateauGeosystems, Inc., http://cpgeosystems.com/index.html,(retrievedMay4,2015).

Dalrymple,R.W.,Zaitlin,B.A.,andBoyd,R.,1992,Estuarinefaciesmodels – conceptual basis and stratigraphic implications:JournalofSedimentaryPetrology,v.62,no.6,p.1130-1146.

Figure 5. Detailofincisedvalleysandstonedepositswithdailysaltwaterdisposalrates,northernWatfordCity100K.Circlesareoperatingsaltwa-terdisposalwells.bbls=barrels.

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