Heidemann Field Report
- 1. Heidemann Field Report Kimball County, Nebraska EAS 424 Lori McDonald
- 2. Introduction The J Sandstone inthisfieldof studyislocatedinthe northern andwestern portionsof Nebraskainthe DenverBasin. The area coversa 250 square mile areainthe westernpartof Nebraska. The J sandstone isLowerCretaceousinage and isa middle memberof the DakotaGroup. It isfurther subdividedintothree distinctmembers,the J2,J1,andvalleyfill J. J2, the oldestsedimentatthe bottom of the section, ischaracterized asa lenticularmarine barsandstone thatisinterfingeredwithmudstone and shale units. A regressionandeventual transgressionof the shallow waterseamarksan unconformitybetweenthe J2 andJ1. J1sandstone isalsoashallow watermarine sediment laidontopof the J2. Anotherregressionof the seaisseenasanothererosional unconformitybetweenthe J1and valley-fill thatwasdepositedtothe NEof the J2andJ1. Valleyfill sequencesare characterized by meanderingsandstone bedsthatoverliesthe J1andJ2 sequences,andentrenchthemselveswithinthe oldersequences. Laramide foldingof the areaandanticlinal structurespresentcausedthe entrapment of the mostproductive fieldsof oil,whichare locatedin the upperstreambedsof the valleyfill deposits. Stratigraphyisa more importantfactor inoil trappinginlowermembersof the Jsandstone. The J Sandstone ison average lessthan25 feetthick,and isa mediumgrained,well sorted friable sandstone withgoodporosityandpermeability.(Gabarini,1964) Sedimentarystructures present, texturesandfossilsare all usedtohelp characterize the depositionalenvironmentinthisarea. Oil accumulationinthe JSandstone iscontrolledbythe shape of the sandstone body,suchasstructural highareas. Itis alsocontrolledbystratigraphicchangesfromsandstone toshale,aswell aslocal anticlinesandJSandstone pinchoutstructures. A bentonite layerisdepositedontopof the “J” sandstone sequences,givingaspecificage timeline forwhenthesesedimentswere deposited. PreviousInvestigations J2 sedimentsare characterized asbeingformedfromaninflux of coarse clasticsfromthe West and depositionof sandsina shallowwaternearshore orshoreline environment. Thisunitcoversalarge area inWesternNebraska.(Garbarini,1964) Because itmostlyconsistsof almostplanar,horizontal laminae lessthanahalf-millimeterthick;thisportionof the “J”is interpretedasbeinglaiddowninvery shallowwaterorsubaerial conditions. These fine gradedlaminationsindicate abroad,relatively current-free environment. Oil trappinginthissectionof the “J”sandstone ismostlystratigraphic,with structure not beingmuchof a factor.(Etheridge,1989) The top of thissequence ismarkedbyaroot
- 3. burrowzone and coal deposits. Thisindicatesaregressionof the sea,andisrecordedinthe rock record as an unconformity. The J2 interval thinsfrommore than40 ftthick inthe SE to lessthan 20 ftthick in the N. (Harms,1966) This thinningisthoughttobe a resultof erosionbystreamchannelingafterthe depositionof streamdeposits. The depositionof the J1 sandstoneisinterpretedasthe re-entryof the seawiththe deposition of sand,siltandclay derivedfromthe easterninnearshore oroffshore environments. Mineralogy indicatesthatJ1 wasderivedfromaneasternprovenance.(Etheridge,1989) The coarsermaterial and small scale burrowsindicate amarine environmentthatwasfairlyshallow. The J1 shallow watermarine consistlargelyof shale,interfingeredwithelongatedlenticularsandstonebodies.(Exum,1968) Both the J1 andJ2 are relativelythin,widespreadunitsof sandstone. The producingmembersof the J1 have elongate,lenticularsandstone bodiesthatinterfingerwiththe adjacentmudstone andshale facies. (Harms,1966) The cleansandstone inthe central bar faciesformsthe importantpetroleumreservoirs. The sandstone bodiesinthe J1showadistinctdepositional trendandare elongatedNW-SE.(Harms, 1966) The positionof the oil fieldsisnotspecificallyrelatedtostructural configuration,sotherefore the trappingof the oil mustbe largelystratigraphic.(Etheridge,1989) The mudstone andshale members that encase the sandstone serve asgoodoil trappingbarriers. The majorityof oil reservoirsinthe J1 unitsare withinstratigraphicallyisolatedsandstone. Shale andsandstone of J1andJ2membersprovide up dipbarriersforentrapment.(Harms,1966) Valleyfillsedimentsare limitedtoareaswhere the J1andJ2are greatlythinnedthrougherosion, or absent. These sedimentsare lightgreyincolorwhere theyare unstainedbyoil,withfinetoveryfine grainsize.(Etheridge,1989) Valleyfill sedimentsare limitedtoa narrow,N-Strendingband. The sedimentsinthe valleyfill are determinedtobe fluvial because theyoccurina steep-walled,long narrow depressioncutinto oldersedimentarystructures.(Gabrini,1964) Sandstone thathas burrowed intosandstone andmudrockbodiesof the J1 andJ2 servesasan effectiveoil trappingbarrier.(Harms, 1966) The major oil reservoirsinthisstudyareaof the “J” reservoirsare inthe streamdeposited sandstone. Thisisthe area withthe mostsuccessful oil production,producingover5 millionbarrelsof oil.(Harms,1966) The valleyfill sedimentsare determinedtobe low gradientmeanderingstreams. (Exum,1968) Afterdepositionof the valleyfill sediments,the areaunderwentfoldingduringthe Laramide Orogeny.(Etheridge,1989) Oil staininginthe rocksindicatesthatoil wastrappedinother parts of the valleyandwasredistributedafterthe folding. Now the oil reservoirsare situatedinthe sand streamdepositsatstructural highareas,and structure isessential forthe entrapmentof oil inthis
- 4. specificreservoir. Escape of the oil to the north or southalongthe valleyfill trendispreventedbylocal plunginganticlinesof overlyingshale.(Etheridge,1989) Entrapmentof oil in thisvalleyfillsection requiresacombinationof stratigraphyandstructure. Stratigraphy The sandstone inthe J2isrelativelyall the same lithology,andisverylightgreycolorwithvery fine grainsize. Itis well sortedwithawhite claymatrix. Itconsistsof almosthorizontal laminaethatare lessthana half millimeterthick.(Harms,1966) Betweenthe J2andJ1 members,there are burrowsmade by organisms,whichcausessome intermixingbetweenthe twolayers. The dominantmineralogyof the J2sandstone layerisquartz,chert,andfeldspar. Itisdistinguishedfromthe J1 andvalleyfill sandstone depositsbyhavingalowerquartzto chert ratio,witha lowerfeldsparcontent.(Harms,1966) Heavy mineralsinthe J2 include tourmaline,rutile,zircon,andleuxcoxene. There isalmostacomplete absence of chloritiod(brittle mica) inthe J2,whichisseeninlargerquantitiesinthe J1and valleyfillsediments. (Harms,1966) It alsohas a largerpercentage of kaolinite thanotherJsandstone beds. The laminae of the J2ismarkedby concentrationsof fine siltsizedblackgrainsthathave higherconcentrationsinlower parts of the section. Sandsizedclaypelletsare seenin nearlyall thinsections. Theyare composedof aggregatesof differentclays,andare derivedfromthe reworkingof partlyconsolidatedsiltandmud that wasshapedduringsedimentaryprocesses. KaoliniteThe onlyfossilsfoundinthispartof the J sandstone are microscopicsuchas dinoflagellates,pollenorspores. These fossilsalsoindicate amarine depositional environment.(Exum, 1968) The J1 sandstone iscomposedof veryfine tofine sandparticleswithclaysized materialspresent inthe matrix.(Harms,1966) Thisunitcontainsmore sandstone,andhassedimentarystructuressuchas ripple marks,small scale crossstratification. The mineralogyof the J1sandstone ismostlyquartz,chert, and feldspar. There isahigherpercentage of chertand feldsparinthinsectionbecause manyof these grainsare smallerthan049mm indiameter.(Harms,1966) This unitismediumgreyincolor,witha high siltcontentat the top. It hasverythingraded laminae atthe top as well,butisotherwiseunbeddedin appearance.(Etheridge,1989) Much of the shaleyportionsof the unithave a speckled appearance. Fossilscontainedinthisunitinclude manyforaminifera,andabundantmicrofossilassemblage. Carbonizedplantfragmentsare alsovisible inthe lowershaleyunit. The thicknessof thisunitis approximately5-10feet.Burrowspresentbetweenthe J1andJ2 sandstone membersillustrate the
- 5. unconformitybetweenthe twolayerswhere the stratawere exposedwhenthe searegressed.(Harms, 1966) The uppermemberof the J sandstone unitiscomposedof sandstone,siltstone,andshale. Itisa veryfine grained,lightgreysandstone withdarkgreysiltstone andshale.(Harms,1966) Sedimentary structuresinthe upperlayersof the J valleyfill are complex,withcleaner,thickerbeddedsandstoneas the most commonupperpart of the sequence.(Etheridge,1989) It is composedof largelysiderite and greyshale pebbles,withcoarse crosslaminae thathave dipanglesof greaterthan20 degrees.(Harms, 1966) The texture of the valleyfill sectionscontainssandstonewithinterbeddedsiltstoneandshale. Theylack foraminifera,buthave abundantplantfossilsandpollen.(Harms,1966) Siltstone andshaley unitsinthe valleyfill sedimentsare more complex,beingonlyafew millimetersthickinsome areas. The heavymineral contentisnotdistinctiveexceptforthe presence of the brittle micachlorotiod. For fossil contentithassome foraminifera,withasignificantlylesspercentage of dinoflagellatesthanother J units.(Harms,1966) It also containsmore cypresspollenandcarbonizedplantmaterial iscommonin thisupperlayerof the J unit. Porosityrangesfrom2-17 percent. (Harms,1966) Methods These sedimentsof the “J”Unit have beeninvestigatedandrecordedthroughwell logging, mineralogical investigation,provenance studies,seismicinterpretation,andstratigraphiccorrelationof knownunits. The thicknessof the unitspresenthasalsobeenanalyzedbycreatinganisopachand structure map of the drill sites. The structure andisopachcontourmapsillustrate the topsof the bentonite andJsandto findanystructuresthat may be conducive tooil entrapmentandwouldpromote furtherexploration. Structure mapsare createdbysubtractingthe value of the Kellybushingfromthe top of the bentonite andtopof the J sandstone. The isopachmapcalculatesthe thicknessof the bentonite layerbetweenthe topof the bentoniteandtopof the J sandstone,andthe thicknessof the netJ sand.
- 6. Discussion Baseduponthe structural andstratigraphic isopach mapsof the cross sectional areathatwas investigated;there are bothstructural andstratigraphictrapsthat are present. The structure map illustrateshowdifferentstructural highsandlowsare capable of trappingoil andgas. The stratigraphic map alsoillustratesareasof differentthicknessesthatwouldserve aseffectivetrapsforoil andgas. As seenfrompreviousinvestigations andthe mapsproduced,the Jsand unithaslonglenticular sand bodiesassociatedwithit. Italsohasstructural highsand lowsthatcould create a dome-shapedtrap. The isopachmap isa representationof the thicknessof boththe bentonite andnetsand. The thickness of the netsand isnot uniformacrossthe sectionandisthickerinsome areas. Differentratesof depositionanddifferentenvironmentsof depositionhave leadtothe specificdepositional patternof the netsand. The Bentonite layerisathickvolcanicash layerthatwas depositedontopof the upper memberof the J sand. This clay-richlayerisaneffective barriertoaidin the trappingof oil and gas reservoirs. Conclusions Lookingat the structural and stratigraphicmapsof thisarea,the structural historyof this areais complex andhasvariedovertime. The transgressionandregressioncycle of the seahasleftdepositsof veryunique sandsiltandclayformationsthatare long,lenticularsandbodies. Oil andgasinthisregion isboth trappedthroughstructure and Stratigraphy. Bothstructural and stratigraphicfactorsinfluence the discoverywell. Exploration Basedon the maps producedthroughthisproject,areasthatwouldbe suggestedforadditional drillingwouldbe the SW¼ NW ¼ of Section26, NW ¼ SW ¼ of Section26, and the SW ¼ SW ¼ of section 26. These siteswouldbe recommendedforadditional drilling because of theirclose proximitytoother oil producingwellsinthe area,and alsobecause of the structural featuresof the sandstone reservoir seeninthe structure map. Also,lookingatthe crosssectionof the area, itis likelythatthisareawould containa structural highspot, providingforanotherfactorintrappingoil andgas reserves.
- 7. References Cited Dolsonandothers,1991, Regional paleogeographictrendsandproduction,MuddySandstone (Lower Cretaceous),Central andnorthernRocky Mountains:AAPGBull.,v.75,p. 186-206. Etheridge,FG,andDolson,JC,1989, Unconformitiesandvalley-fill sequences –keyto understanding“J” Sandstone (LowerCretaceous) reservoirsatLonetree andPonchoFields,D-JBasin,Colorado,Rky.Mtn. Assoc. of GeologistGdbk,p.221-223. Exum,FA,and Harms, JC,1968, Comparisonof marine-barwithvalley-fill stratigraphictraps,western Nebraska:AAPGBull.,v.52, p. 1851-1868. Garbarini,GS, 1964, Cedar ValleyandCanal Fields,ScottsBluff County,Nebraska:The Mountain Geologist,v.1,p. 47-53. Harms, JC,1996, Stratigraphictrapsin valleyfill,westernNebraska:AAPGBull.,v.50, p.2119-2149.