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TheproductionhistoryfromtheSpearfishFormation(Triassic)inthestateissimilartothedrillinghistoryoftheBakkenFormationandotherunconventionalreservoirs.DrillinginBottineauCountybeganwitharandomwildcat,theZachBrooksDrillingCompany-#1EdwinBerentson(SW¼SE¼,Sec.21,T163N,R79W).Drilledand completed in November 1952, this well was the firstcommercialoilwell inBottineauCountyand thediscoverywellforWesthopeField.EarlywellsinthecountyproducedprimarilyfromtheMadisonGroup(Mississippian)butstrongoilshowsintheoverlyingSpearfishFormationsuggestedthatitmightalsobeatarget. SubsequentdiscoveriesatNewburg (1955)andSouthWesthope(1956)fieldsprovedthistobetrue(fig.1).

Drilling activity reached record highs from 1957 to 1959 andcontinued through 1961 in the region as companies activelypursued theMadison Group and Spearfish Formation (LeFeverandLeFever,1991).Anumberofnewfieldswereaddedatthistime(fig.1).Itisinterestingtonotethatmarketingtheoilinthisareawasdifficultearly in theexplorationhistoryof the region.Prior to 1963, production in the north-central portion of thebasinwasmarketedby rail until1963when thePortalPipelinewas constructed to carry crude to refineries inMinnesota and

Wisconsin (Gerhard et al., 1982). A successfulwaterfloodwasinitiatedbyAmeradaPetroleumCorporationin1967toreversethegradualdeclineofproductioninNewburgField.Drillingactivityslowedbasinwidefrom1966to1974(Gerhardetal.,1982).

AdiscoverybyOmegaHydrocarbonsLtd. in June1980resultedin renewedactivitywhenthecompanyrecompletedaMadisonproducerintheLowerAmaranthFormation(SpearfishFormationequivalent) in Waskada Field, Manitoba (Barchyn, 1982). Thisdiscoverywasfollowedbyasignificantleaseplayinnorth-centralNorthDakota,butthedrillingsuccessratewasvariableandtheplaydidnotdevelopasexpected.TheplayinsouthernManitobacontinued until 1990 when exploratory drilling became non-economic.Productionfromthesefieldscontinuedwithhorizontaldevelopmentdrillingcoupledwithsecondaryrecoverymethods.Earlyin2008,EOGResourcesrevisitedtheWaskadaFieldareawithtechnologyusedtoexplorefortheBakkenFormationinMountrailCounty.Theapplicationofhorizontaldrillingincombinationwith

The Spearfish Formation – Another Unconventional Target

Julie A. LeFever

Figure 1. Structure on the top of theMadison unconformity surface.ThedistributionofSpearfishFormationproducingoilfieldsinBottineauCounty, North Dakota and Canada are also indicated. The fields arecoloredbasedonthediscoverydate(modifiedfromLeFeverandLeFever,1991).

Figure 2. Outcrop photograph of red siltstones, sandstone, andmudstonesof theSpearfishFormationtaken in thetownofSpearfish,SouthDakota(wildturkeysforscale).

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fracturestimulationresultedinwellsthatproduceexponentiallybetter than traditional vertical wells (Redekop, 2010). TheManitobaPetroleumBranchestimatedthat300wellswouldbedrilledintothelowerAmaranthinthatprovincein2010atacostof$1.5millionperwell.

EOG Resources announced in April 2010 that they hadmovedthe Spearfish play across the border into north-central NorthDakota.Thisprospect,referredtoastheWaskadaSouthField,isconsideredtobeasmallaccumulation(20millionbarrels)withaveryhighrateofreturn(Papa,2010).

SpearfishFormationThe Spearfish Formation was originally described by Darton(1898,1899-1900)forasequenceofredsandyclay,orshalewithanhydritebeds thatoutcrop in theBlacksHillsofSouthDakota(fig. 2). The Spearfish Formation in North Dakota occurs onlyin thesubsurface,andconsistsof threemembers, inascendingorder:theBelfield,thePineSalt,andtheSaude.Theformationispresentoverhalfofthestate,reachesamaximumthicknessof750feet,andisequivalenttotheLowerWatrousinSaskatchewanandtheLowerAmaranthinManitoba.

Only the uppermost member of the Spearfish Formation ispresent in the north-central portion of the state at a depthrangingfrom2,900to3,400feet.TheSpearfishFormationonlapstheMadisonunconformitysurfaceinBottineauCountyandintosouthernManitobawhereitisproductive(fig.3).Thisportionoftheformationoverthestudyareaisa125-to220-foot-thickredbedsequenceconsistingprimarilyofquartzosesandstonesthatare occasionally cemented with anhydrite or having anhydritenodules, siltstones, shales, shaly sandstones, and some thinanhydritebeds (figs.4,and5;Dow,1967;LeFeverandLeFever,1991).

Thesequenceofshales,siltstones,andsandstonesthatcomprisetheSpearfishFormationweredepositedinasubtidaltosupratidalenvironment.Thesandstoneswereprobablyderivedfromcoastaldunesthatwereinundatedbytransgressiveseas.Thereworkingandre-depositionofthesesandsproducedawidespreadsheet-like morphology. Periodic emergence resulted in evaporiticconditionsandextensiveoxidation.Inaddition,aseriesofsubtidalchannelscrosscuttheinterval.Thesedepositsarecharacterizedby channel lags (recognizable on logs and highly cemented),troughcross-bedding,andbyplanar-beddedandcurrent-rippledsandstones (the trough cross-bedded and planar-bedded faciesare thereservoir faciesatRussell,StarbuckandSouthStarbuckfields(LeFeverandLeFever,1991;OglesbyandFischer,1991).

ReservoirPropertiesMostoftheavailablereservoirdataarefromthetwolargestfieldswithSpearfishpools,NewburgandSouthWesthope.Additionaldatawasobtained in theearly1980swith thedevelopmentofWaskadaandPiersonfieldsontheManitobasideofthebasinandManorFieldinSaskatchewan.

NewburgandSouthWesthopeFieldsProduction in Newburg and South Westhope is from boththe Berentson beds of the underlying Charles and SpearfishFormations (figs. 5 and 6, Table 1). These fields produce froma stratigraphic trap thatoccurson theupdip limbof a synclineformed from the dissolution of salt in the underlying PrairieFormation (Devonian). Theproductive sectionof theSpearfishFormationisunlikethemajorityoftheoverlyingsectionbecauseitisunoxidized,representedbyapalegray-greencolor.

TheSpearfishpoolrangesinthicknessfrom10to40feetwithinthefields.Theeffectivepaymaybesignificantlylessdepending

Figure3.Schematiccross-sectionshowingtheonlapoftheSpearfishFormationovertheMadisonunconformitysurfaceinBottineauCounty,NorthDakota. The shadesof green represent the variousmemberswithin theMissionCanyon Formation. The shades of blue andpurple representmembersoftheLodgepoleFormation(modifiedfromLeFeverandLeFever,1991).

  Southwestern Manitoba  North‐Central North Dakota 

Formation Depth (ft); TVD  2,935 3,000 ft  Thickness (ft)  78 50 Average porosity  14.5% 14% API Oil Gravity  35  36 Initial Production  208‐337 Bopd 285‐305 Bopd Oil‐in‐Place per Section  9‐15 MMbo 13 MMbo 

 

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on the lithology. The reservoir has measured intergranularporositiesof12to15%withpermeabilitiesrangingfrom<0.1to320millidarcies(md).Thereislittleeffectiveverticalpermeability.Watersaturationcanbeashighas50to60%andoilsaturationsrangefrom0to37%andvaryonawell-to-wellbasis.Thereservoirisundersaturatedwithgas-oilratiosof100:1orless.Productionisaidedbynaturalandsecondarywaterdrives.

RussellandStarbuckfieldsfallalongthesametrendasNewburg-SouthWesthopefields. Thereservoirs forRussellandStarbuckshowsimilarreservoircharacteristicsalthoughtheyresultfromaslightlydifferentdepositionalenvironment.Recoveryefficienciesareverygoodforthesetwofieldsandappeartobeafunctionofreservoirenergy.WellsintheRussellFieldhavehighrecoverieswith abundant water production related to the strong waterdrive.Incontrast,SouthStarbuckhasalowervolume,withnearlywater-freeproduction.

WaskadaandPiersonFieldsThe Lower Amaranth pools (Spearfish equivalent) in Waskadaand Pierson fields are represented by red dolomitic siltstonesand sandstones interbedded with argillaceous siltstones andshale(Barchyn,1982).Barchyn(1982)notedthatthedolomiteisassociatedprimarilywiththefinergrainedsiltymaterial,whereasanhydrite cements the coarser sandy material and occurs asnodulesandlenses.

Again, thetrap is stratigraphicwith thereservoir locatedin the coarser grained lithologies -- the siltstones andsandstones. Porosities average 14.5% with an averagepermeabilityof14.8md.SlightlyhighervalueshavebeennotedfromthesandstonesatPiersonField(Husain,1990).The fields were initially developed with vertical wellsfollowed by secondary recovery through waterflooding.Horizontal drilling has successfully increased productionandreserves.

ManorFieldManorFieldinsoutheasternSaskatchewanisdifferentfromthepreviouslydiscussedfields, inthatpriortohorizontaldrillingtherewasnocommercialproductionfromverticalwells. Theverticaltestswereusedtodelineatethefieldbyidentifyingthedistributionandreservoirqualityofthesandstones.ProductionisfromthebasalsandsoftheLowerWatrousFormation(SpearfishFormationequivalent).TheLowerWatrousisinterpretedasamarginalmarinetotidalflat environment with the reservoir in the interbeddedto bioturbated sandstone and muddy siltstone facies,interpretedassubtidalshoaldeposits.ThesebedsonlaptruncatedMississippianstrata(Musial,1995).AnisopachmapofthetopoftheLowerWatroustotheMississippian

unconformity is a good indicator of the presence of the basalreservoir sandstone. The sand is deposited in areas of lowMississippianrelief.

Porosity,permeabilityandpaythicknessaresimilartoNewburg/South Westhope and Waskada/Pierson. Porosity values rangefrom15 to24%,averaging19%. Permeabilitiesaverage10mdwithanaveragenetpayof16feet.ThereservoiratManorisalsoundersaturated,withagas-oilratioof292.

CurrentActivityAspreviouslystated,thecurrentactivitybeganinsouthwesternManitobawithEOGResources.Applyingthedrillingadvancementacquired in Mountrail County with the Bakken Formation, thecompanyhasbeenhorizontallydrillingandfracturestimulatingtheLowerAmaranthsuccessfully(table1).ReservesfortheManitobaportionoftheplayareestimatedtobe25millionbarrels.

InApril2010,EOGResources,Inc.announcedthatitwasextendingtheplay from southwesternManitoba intoNorthDakota. TwowellshavebeencompletedintheSourisFieldtodate,aspartofitsWaskadaSouthprospect.Wellscurrentlyrequirefourdaystodrillandcompleteatacostofapproximately$1million.ReservesfortheNorthDakotaportionoftheplayhavebeenplacedat20millionbarrels.

Figure 4. Isopach of the Spearfish Formation through Bottineau County andsouthernManitoba.Oilfieldsrepresentedinfigure1areoutlinedinblue.

Table 1. A comparison of the reservoir characteristics of the currentSpearfish play (modified from Papa, 2010; Redekop, 2010; NDIC Case#12076). TVD = true vertical depth, Bopd = barrels of oil per day,MMbo=millionsofbarrels.

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ReferencesBarchyn, D., 1982, Geology and hydrocarbon potential of the Lower

Amaranth Formation Waskada-Pierson area, southwesternManitoba:ManitobaDepartmentofEnergyandMinesGeologicalReport82-6,30p.

Darton,N.H.,1898,JurassicFormationsoftheBlackHillsofSouthDakota:GeologicalSocietyofAmericaBulletin,v.10,p.383-409.

Darton, N.H., 1899-1900, Preliminary description of the geology andwaterresourcesofthesouthernhalfoftheBlackHillsandadjacentregions in SouthDakota andWyoming:USGS 21stAnnual ReportandHydrography,pt.4,p.489-598.

Dow, W.G., 1967, The Spearfish Formation in the Williston Basin ofwesternNorthDakota:NorthDakotaGeologicalSurveyBulletin52,28p.

Gerhard, L.C., Anderson, S.B., LeFever, J.A., and Carlson, C.G., 1982,Geologicaldevelopment,origin,andenergymineralresourcesoftheWillistonBasin,NorthDakota:AmericanAssociationofPetroleumGeologistsBulletin,v.66,p.989-1020.

Husain, M., 1990, Regional geology and petroleum potential of theLower Amaranth Formation, Coulter-Pierson area, SouthwesternManitoba:ManitobaEnergyandMinesPetroleumOpenFileReport11-90,54p.

LeFever, R.D., and LeFever, J.A., 1991, Newburg and SouthWesthopefields,in,Foster,N.H.,andBeaumont,E.A.,(eds.),StratigraphictrapsII, Atlas of oil and gas fields: American Association of PetroleumGeologistsTreatiseofPetroleumGeology,p.161-188.

Musial, R., 1995, An overview of the Lower Watrous Manor Pool insoutheast Saskatchewan, in, Hunter, L.D. and Schalla, R.A., (eds.),Seventh International Williston Basin Symposium: MontanaGeologicalSociety,Billings,MT,p.219-234.

Oglesby, C.A. and Fischer, D.W., 1991, Sedimentology and petroleumgeologyofaTriassicSpearfishsandstonereservoir,SouthStarbuckField, Bottineau County, North Dakota, in, Christopher, J.E. andHaidl, F.M., (eds.), Sixth InternationalWilliston Basin Symposium:Saskatchewan Geological Society Special Publication 11, p. 161-164.

Papa,B.,2010,EOGResources,RockyMountainOil,QuarterlyReporttoInvestors.

Redekop,B., 2010, SouthwestManitobawelcomesoil slick: WinnipegFreePress,February20.

Figure6.TypicallogfromthecurrentplayintheSpearfishFormation(modifiedfromPapa,2010).

Figure5.RepresentativecorephotographsoftheSpearfishFormationfromproducingwellsinBottineauCounty-(a)NewburgFieldSpearfishpaysandstoneatadepthof3,357feet(AmeradaHessCorp.-Newburg-Spearfish-CharlesUnitP-713,NE¼NE¼,Sec.16,T161N,R79W,33-009-01390-0000)(widthinphotographrepresents3.25inchesofcore);(b)TypicalSpearfishFormationshalebedatadepthof2,977feet(NorcenEnergyInc.-Lindstrom34-1,NW¼NE¼,Sec.34,T164N,R78W,33-009-01599-0000)(widthis4inches);(c)SpearfishFormationshalewithanhydritenodulesatadepthof2,987feet(NorcenEnergyInc.-Lindstrom34-1,NW¼NE¼,Sec.34,T164N,R78W,33-009-01599-0000)(widthis4inches);(d)SpearfishFormationsandstonebedwithrip-upclastsatadepthof3,020.5feet(NorcenEnergyInc.-Lindstrom34-1,NW¼NE¼,Sec.34,T164N,R78W,33-009-01599-0000)(widthis3inches).

(a) (b) (c) (d)