Mapping Sandstones of the Inyan Kara Formation for Saltwater Disposal in … · 2016-12-21 ·...
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Mapping Sandstones of the Inyan Kara Formation for Saltwater Disposal in North Dakota Jeffrey W. Bader Introducon The North Dakota Geological Survey (NDGS) is performing a detailed state-wide invesgaon of the Inyan Kara Formaon (Dakota Group) to support industry needs for underground disposal of produced water. Sandstones of the formaon are the primary subsurface injecon zone for produced water in North Dakota. Produced water is the salty formaon water that is generated as a by-product of oil and gas producon. Over one- million barrels of co-produced water are generated daily during producon operaons in North Dakota. Subsurface injecon is the industry-preferred alternave for produced water disposal. Therefore, because of the prodigious volumes of saltwater generated, and the economics of such disposal, it is important to fully understand the variables involved in selecng locaons for saltwater disposal wells. These variables would include access; proximity to producing/future exploraon wells; and most importantly, geology. To assist with well placement, the NDGS is preparing detailed isopach (thickness) maps at scales of 1:100,000 (100K), ulizing data from thousands of wells per 100K sheet to help industry beer understand sandstone trends of the formaon. Such detail is necessary because Inyan Kara sandstone thicknesses can change quickly, going from hundreds of feet of connuous sandstone to virtually no sandstone over a very short distance (< 0.5 mile). Cross-secons are also included to help operators visualize the lateral complexity of the sandstone geometry. Inyan Kara Deposional Seng Before any mapping program can begin, one must understand the geology, especially for subsurface mapping where the rocks may not be available for study. This is parcularly true for the Inyan Kara, because the deposional environment is complex and only a few subsurface cores are available. Inyan Kara sediments were deposited in a coastal seng adjacent to the Cretaceous Seaway from approximately 115 to 105 million years ago (fig. 1). The unit marks the beginning of the inial sea-level rise of the Early Cretaceous. At this me, North Dakota was at latudes that were much more humid and tropical, and dinosaurs roamed the broad/low relief coastlines. Numerous rivers flowed across the coastal plain to the sea (fig. 1). Sea-level variaons caused shoreline shiſts; with the coastline moving landward during transgression, and seaward during regression. Major sea-level fluctuaons occur due to tectonic events such as upliſt of mountain ranges, or global sea-level changes based on water volume in the oceans, or both; combining for a net relave change in sea level. Inyan Kara sediments were deposited over western North Dakota during two of these transgressive/regressive cycles as relave sea-level fluctuated. Sedimentaon along the shoreline was complex, consisng of estuaries (drowned river valleys), that formed as the encroaching sea filled valleys that were cut by rivers flowing towards the retreang sea during the previous fall in relave sea- level. Sequence Stragraphy Sequence stragraphy, the study of stacking packages within sedimentary rocks and the surfaces that separate them, was developed for just such coastal, cyclic deposional sengs as described above. The sequence stragraphic surfaces are generated by changes in relave sea-level, and can be consistently recognized in rocks, both in core and on geophysical logs making them very useful in idenfying the best sandstones for produced water injecon. Figure 1. Paleogeographic map of the North Dakota area during Inyan Kara me (c.a., 106 Ma). Modified from Blakey (2014). 6 GEO NEWS
Transcript of Mapping Sandstones of the Inyan Kara Formation for Saltwater Disposal in … · 2016-12-21 ·...
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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