Timothy O. Nesheim

IntroductionBeginning in2012,variousoperators in theBakken-ThreeForksplaybegan testingandexamining themiddle to lowerportionsof the Three Forks Formation as a prospective unconventionalreservoir (fig. 1) (Petroleum News, 2012; Gaswirth andMarra,2015). The prospective Three Forks reservoir horizons were

informally subdivided into four benches, numbered 1 to 4 indescendingorder(fig.1).Priortothattime,drillingandproductionwas primarily focused on theMiddle Bakken and upper ThreeForks(1stbench).Beginninginearly2015,approximately2yearsaftercommencementofinitialdrillingandtestingofthemiddletolowerThreeForks,theNorthDakotaOilandGasDivisionbegantracking the specific target interval of horizontal Three Forkswells.Sincethattime,over180horizontalwellshavebeendrilled

Figure 1.WirelinelogexampleoftheBakkenandThreeForksFormationswithcoreplugoilandwatersaturationdata(rightside).Notetheconsis-tentoil saturations (typically25-50%) thatextend fromtheBakkendownthrough thebaseof themiddleThreeForks (2ndbench).P=Pronghorn.Depthsareinfeetbelowsurface.

Examination of middle Three Forks (2nd Bench) Hydrocarbon Charge and Production

Figure 2.Productionmapshowingthedistributionofproductivehor-izontaloilwells(blackcircles)thathavebeendrilledandcompletedwithinthemiddleThreeForks(2ndbench).Theyellowstardepictsthelocationofwell#26990(fig.1),andthegreenstarthelocationofwell#22388(figs.3and4).Thelightgreylinesrepresentcountyout-lines.Abasin-scalereferencemapisprovidedinthetopleftcorner.

4 GEO NEWS

and completed in themiddle to lower Three Forks Formation,whichhavecumulativelyproducednearly20millionbarrelsofoiltodate(fig.2).Themajority(~80%)ofthosewellstargetedthe middle Three Forks (2ndbench). A significant number of add- itional, pre-2015 middle to lower Three Forks wells arealsopresent,butthenumberofthoseearlierwellsandtheircumulativeproductionvolumeshaveyettobedetermined.

There is currently limited geological information availableregarding the petroleum geology and economic importance ofthemiddle to lowerThreeForks. Resourceassessmentsof theThree Forks Formation have ranged from approximately 2 to4billionbarrelsof recoverableoil, but theseassessmentshavebeen limited to the upper Three Forks (Nordeng and Helms,2010;Gaswirthetal.,2013).ThisarticlereportsthepreliminaryfindingsofarecentlyinitiatedstudyontheoriginanddistributionofoilsaturationswithinthemiddleThreeForks.Italsoreviewsthepresentproductionfootprintof2ndbenchhorizontaloilwellsandprojectstheirpotentialfuturedistribution.Inaddition,Iamexamining oil saturations and hydrocarbon production in thelowerThreeForks(3rdand4thbenches).

GeologyThe Three Forks Formation was deposited during the UpperDevonian (385 to 359 million years before present) anddirectly underlies the Bakken Formation (Murphy et al., 2009).Hydrocarbons present within the Three Forks are believed tobe sourced from the overlying lower Bakken shale, which isconsideredtobeaworldclasspetroleumsourcerock.

Previous studies have sub-divided the Three Forks in differentschemes, including 1) six sub-units ranging from unit 1 to unit 6 in ascending stratigraphic order (e.g. Christopher, 1961), and2) more simplistic upper, middle, and lower members (e.g. Bottjer et al., 2011)which is utilized in this article. A numberof oil and gas companies have also adopted an informal“bench” terminology systemwhich refers to the various ThreeForks reservoir intervals as bench 1 to bench 4 in descendingstratigraphicorder(e.g.fig.1).

ThemiddleThreeForks (2ndbench)containssimilar lithofacies(rocktypes)totheprolificupperThreeForks(1stbench)includingbeds of tan silty dolostone intercalated with grey to greenclaystone,whichhavebeeninterpretedtorepresentintertidaltotidalflatdeposits(fig.3).Inaddition,themiddleThreeForksisalso comprised partly of siltstone to mudstone beds containing variableamountsofpebble-sizeddolostoneclastsinterpretedasdebrisflows(fig.4)(Droege,2014).

Middle Three Forks (2nd Bench) Oil SaturationsOilandwatersaturationdatameasuredfrommiddleThreeForkscore sampleswere compiled andexamined fromover 50wells(fig. 5). The fluid saturation averages indicate that themiddleThreeForkscanbesubdivided into threeareasbasedprimarilyon oil saturations and secondarily on water saturations. Fluidsaturationswere found to be nearly indistinguishable betweentheupperandmiddleThreeForks(e.g.fig.1)withinthe1sttierarea(highestoilsaturations),whichextendsacrossnortheasternMcKenzie to southeastern Dunn and southwestern Mountrailcounties (dashedoutlined area - fig. 5). There is also a larger,2ndtierareawherewater saturationsarehigher in themiddle

ThreeForks,butnotableoilsaturationsare still present (dotted outline area– fig. 5). Moving laterally outwardfrom beyond the 1st and 2nd tier oilsaturation areas, the 3rd tier areamerely represents where core plugwatersaturationscontinuetoincreaseoverall while oil saturation valuesdecreaseto<10%(fig.5).

Lower Bakken Shale Thermal MaturityThe low porosity and permeabilitythroughout the Bakken-Three Forkssection leads to the understandingthatmostoftheoil inplacehasbeenlocally generated (minimal lateralmigration). Thesourcerockincloseststratigraphic proximity to the middleThreeForks is the lowerBakkenshale(e.g.fig.1).Examiningthelowershale,most of the high oil saturation (1sttier) area correlates with where thelower Bakken shale is both >20 feetthick and has reached the greatestlevelofthermalmaturitywithrespecttooilgeneration(fig.6).The2ndtier,

Figure 3.Corephotographexampleofthemid-dleThreeForksfromWhiting’sSkaarFederal41-3-3H (NDIC: #22388, API: 33-053-03979-00-00,Sec.3,T152N,R97W).Thetancoloredlayersofthecoreconsistofdolomitesiltwhilethegrey-greenportionsaremoreclay-rich.

Figure 4.Corephotographexampleofthemid-dle Three Forks from Whiting’s Skaar Federal41-3-3H (NDIC: #22388, API: 33-053-03979-00-00,Sec.3,T152N,R97W) thatdisplays sand topebble sized clasts of dolostone suspended in thesilttomudmatrix.Thisisanexampleoftheinterpreted debris flow facies common of themiddleThreeForks(2ndbench).

JANUARY 2018 5

Figure 5.Fluidsaturationmapshowingtheaverageoil(white-greendots)andwater(bluecircles)saturationsofthemiddleThreeForksfromcorepluganalysisdata.ThethickdashedoutlinedepictswherethemiddleThreeForksaveragesthehighestoil(20-40%)andlowestwater(<50%)saturations. ThedottedoutlinedepictswherethemiddleThreeForkstypicallycontainsintermediateoil(10-30%)andwater(50-80%)satura-tions. Beyond the intermediate oil saturation area, themiddle ThreeForkscommonlyaverages70-90%coreplugwatersaturationwith<10%oil.Thegrey-coloredcontoursrepresentthethicknessofthePronghornMemberoftheBakkenFormation,whichappearstoinhibithydrocarbonchargefromthelowerBakkenshaletotheThreeForksFormation(Mil-lardandBrinkerhoff,2016).

intermediateoilsaturationareareflectswhereeitherthelowerBakkenshalehasthinnedand/orislessthermallymature,butstill within the peak oil generation window. Therefore, oilsaturationsof themiddleThreeForksappear tobeprimarilya functionofboththethicknessandthermalmaturityofthelowerBakkenshale.Thegreaterthevolumeofoilgeneratedfrom the lower shale, the further hydrocarbons are forceddownwardsintotheunderlyingThreeForksFormation.

Middle Three Forks Current and Future FootprintThe majority of middle Three Forks wells permitted and/ordrilled and completed since early 2015 have been proximalto the area of highest core plug oil saturation (fig. 7). Thistimeframe correlates with the beginning of depressed oilprices. Oil saturations likely correlatewith oil production tosome degree, where drilling and completing wells in areaswith higher oil saturations result in betterwell performance.The immediate future of drilling activity in themiddle ThreeForkswill likelycontinuetofocusontheareawithhighestoilsaturations (1st tier area). Over the course of time, middleThreeForksdevelopmentwill likelyextendmoreintothe2ndtier intermediate oil saturation area through a combinationof commodity price changes, technological advances, and/orcontinueddrilling-completioncostreductions.

Figure 6.Thermalmaturity(hydrogenindex=HI)andisopach(thickness)mapofthelowerBakkenshale.ThecoloredHIcontoursreflectthether-malmaturityofthelowerBakkenshale,whereredisthemostthermallymatureareawithrespecttooilgenerationandthelightgreentoyellowistheleastthermallymature.Thethickblacklinesareisopach(thickness)contoursofthelowerBakkenshaleinfeet.ThedashedwhitelinedepictswherethemiddleThreeForkscontainsthehighestaveragecoreplugoilsaturationsandthedottedlinetheareawithlowtointermediateaverageoilsaturations.

6 GEO NEWS

ReferencesBottjer, R.J., Sterling, R., Grau, A., Dea, P., 2011, Stratigraphic

relationshipsandreservoirqualityattheThreeForks-Bakkenunconformity,WillistonBasin,NorthDakota,inL.Robinson,J.LeFever,andS.Gaswirth,eds.,Bakken-ThreeForksPetroleumSystemintheWillistonBasin:RockyMountainAssociationofGeologistsguidebook,p.173-228.

Christopher, J.E., 1961, Transitional Devonian-Mississippianformations of southern Saskatchewan: SaskatchewanDepartmentofMineralResources,ReportNo.66,103p.

Droege, L.A., 2014, Sedimentology, Facies Architecture, andDiagenesis of the middle Three Forks Formation – NorthDakota, U.S.A.: MS thesis, Colorado State University, FortCollins,Colorado,150p.

Gaswirth,S.B.andMarra,K.R.,2015,U.S.GeologicalSurvey2013assessmentofundiscoveredresourcesintheBakkenandThreeForks Formations of theU.S.Williston Basin Province: AAPGBulletin,v.99,no.4,p.639-660.

Gaswirth,S.B.,Marra,K.R.,Cook,T.A.,Charpentier,R.R.,Gautier,D.L.,Higley,D.K.,Klett,T.R.,Lewan,M.D.,Lillis,P.G.,Schenk,C. J., Tennyson,M.E.,andWhidden,K. J.,2013,Assessment

ofundiscoveredoil resources in theBakkenandThreeForksFormations,WillistonBasinProvince,Montana,NorthDakota,and South Dakota, 2013: U.S. Geological Survey Fact Sheet2013-3013,4p.

Millard,M.,andBrinkerhoff,R.,2016,TheIntegrationofGeochemical,Stratigraphic, and Production Data to Improve GeologicalModelsintheBakken-ThreeForksPetroleumSystem,WillistonBasin,NorthDakota,inDolan,M.,Higley,D.,andLillis,P.,eds.,Hydrocarbon Source Rocks in Unconventional Plays, RockyMountain Region: RockyMountain Association of Geologistsguidebook,p.190-211.

Murphy,E.C.,Nordeng,S.H.,Junker,B.J.,andHoganson,J.W.,2009,NorthDakotaStratigraphicColumn:NorthDakotaGeologicalSurveyMiscellaneousSeries91.

Nordeng,S.H.,andHelms,L.D.,2010,Bakkensourcesystem–ThreeForksFormationassessment:NorthDakotaGeologicalSurvey,accessed April 2011, https://www.dmr.nd.gov/ndgs/bakken/bakkenthree.asp

PetroleumNews,2012,Reservesrocket:PetroleumNewsBakken,v.1,no.4,p.1,14.

Figure 7. Middle Three Forksproduction map with outline of1st tier/high oil saturation area(dashed line) and 2nd tier/inter-mediate oil saturation area (dot-ted line). Green circles show thedistributionofmiddleThreeForkshorizontal oilwell and the yellowcircles are undrilled, permittedwelllocations.Notehowover95%ofthemiddleThreeForkswellsarepresentlylocatedwithintheinter-mediate oil saturation area, themajority of which are positionedwithinorclosetothehighoilsatu-rationarea. Greyshadingdepictsthe thickness (isopach) of thePronghornMemberoftheBakkenFormationwithcontouringrangingfrom5to30feetin5footcontourintervals.

JANUARY 2018 7