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.