Hendershott Thesis

8/13/2019 Hendershott Thesis

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EVALUATIONOFTHEDEPOSITIONALENVIRONMENTOFTHEEAGLEFORD

FORMATIONUSINGWELLLOG,SEISMIC,ANDCOREDATAINTHEHAWKVILLE

TROUGH,LASALLEANDMCMULLENCOUNTIES,SOUTHTEXAS

AThesis

SubmittedtotheGraduateFacultyofthe

LouisianaStateUniversityAgriculturalandMechanicalCollege

inpartialfulfillmentoftherequirementsfordegreeof

MasterofScience

in

TheDepartmentofGeologyandGeophysics

byZacharyPaulHendershott

B.S.,UniversityoftheSouthSewanee,2009December2012


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ACKNOWLEDGEMENTS Iwouldliketothankmycommitteechairandadvisor,Dr.JeffreyNunn,forhisconstant

guidanceandsupportduringmyacademiccareeratLSU.AdditionallyIwouldliketothankDr.

StephenSearsandDr.SamuelBentleyforsupportingmeasmycommitteememberswiththeir

knowledgeableinputandflexibilitywithmyworkschedule.Also,IwouldliketothankDr.Brian

LockoftheUniversityofLouisiana-Lafayettefortakingalookatmyinitialprojectandgivingme

valuablewordsofadviceandencouragement.IamhumblefortheopportunitytheDepartment

ofGeologyandGeophysicsatLouisianaStateUniversitygavemetopursueaMasters

Degreeatsuchareputableuniversity.

SpecialthankstoGreggRobertson,theHawkvilleGeologistsofPetrohawkEnergy

Corporation,aswellasCharlesCusackandDickStoneburnerforthepermissiontousethe

dataandresourcesforthisstudy.Withouttheirinitialeffortsandcontributionsasthepioneers

oftheEagleFordFormationinSouthTexas,muchlesswouldbeknownaboutthisworldclass

resource.TheirencouragementtoturnmyinternshipintoathesistopicissomethingIwillbe

forevergratefulfor,andtheirfriendshipinandoutoftheofficeissomethingIwillneverforget.


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TABLE OF CONTENTSAcknowledgements..i

ListofFigures.iv

Abstract..vi

Introduction..1

GeologicBackground/StudyArea...6

Methods.13

FormationsandUnitDistinctions..15

Results..19

WellLog..19

SeismicAnalysis..33

Discussion...38

CoreAnalysis..38

WellLog/Seismic.41

Conclusions..45

References..47

Vita..52


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LIST OF FIGURESFigure1:MapoftheregionalextentoftheEagleFordShaleinTexas...2Figure2:Stratigraphiccolumnspecifictothestudyarea.....3

Figure3:SummaryofpreviousstudiesoftheEagleFordFormationinoutcrop andinthesubsurface..........5Figure4:PaleogeographicmapoftheGulfCoastregionduringtheLateCenomanian...7Figure5:CoretypelogfromMcMullenCounty,Texas(McM-1)..10Figure6:Northwest-southeastschematiccrosssectionthroughtheHawkvilleTrough..12Figure7:MapofstudiedwellsintheHawkvilleTroughinLaSalleandMcMullenCounties,Texas......14Figure8:LaS-4typelogfromLaSalleCounty,Texas....16Figure9:CoreimagesfromMcM-1......17Figure10:N-SwelllogcorrelationfromLaS-10toLaS-9..21Figure11:IsopachmapoftheentireEagleFordFormationintheHawkvilleTrough.24Figure12:IsopachmapoftheUpperEagleFordFormation......25

Figure13:IsopachmapoftheLowerEagleFordFormation......26Figure14:RegionalEagleFordstructuremapintheHawkvilleTrough.....27Figure15:Southwest-northeastregionalcrosssectionthroughtheHawkvilleTrough.28Figure16:Southwest-northeast(strike-oriented)crosssectioninLaSalleCounty,Texas.29Figure17:Northwest-southeast(dip-oriented)crosssectioninLaSalleCounty,Texas.30Figure18:Southwest-northeast(strike-oriented)crosssectioninMcMullenCounty,

Texas..31Figure19:N-S(dip-oriented)crosssectioninMcMullenCounty,Texas..32Figure20:Northwest-southeastseismicdipsectionacrossLaSalleCounty,Texas..35Figure21:Northwest-southeastseismicdipsectionacrossMcMullenCounty,Texas.36


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Figure22:Northwest-southeastseismicsectioninLaSalleCounty,TexasflattenedontheBudaLimestone(blueline)..37

Figure23:CoreimagefromMcM-1showingunconformablenature

alongthecontactbetweentheAustinChalkandEagleFord39

Figure24:CoreimagefromLaSalleCountywellshowinggradationalnatureofthecontactbetweentheAustinChalkandEagleFord...39

Figure25:Amplitudeextractionprojectedinmapviewfrom

southeastLaSalleCounty,Texas...40


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ABSTRACTTheUpperCretaceousEagleFordFormationofSouthTexasrecordsamixed

siliciclastic/carbonatedepositionalenvironmentacrosstheLateCretaceousPlatformofthe

GulfofMexico.DuringtheLateCretaceous,LaSalleandMcMullenCountieswaspartofthe

HawkvilleTrough,awedgedshapedregionbetweentheEdwardsandSligocarbonatereefs.

Welllogsfrom21wellsandseismicdatawereusedtoconstructstructureandisopachmapsof

theEagleFordFormationthroughouttheHawkvilleTrough.Onlytheunconformablebottom

(Buda-EagleFord)andtop(EagleFord-AustinChalk)boundariesplustheconformable

boundarybetweentheupperandlowerEagleFordcanbeconsistentlycorrelatedinthearea.

TheEagleFord-AustinChalkboundaryisvariable/gradationalduetovariableerosionofthe

EagleFordpriortodepositionoftheAustinChalk.Thisvariabilityisalsoobservedincore

data.TheLowerandUpperEagleFordaretroughshapeddepositsthatstrikenortheast

roughlyparallelwiththeEdwardsreef.MaximumthicknessoftheLowerEagleFordismore

than180ftinLaSalleCountyand140ftinMcMullenCounty.TheUpperEagleFordhasa

maximumthicknessof160ftalongtheLaSalle-McMullenCountyBorder.BoththeLowerand

UpperEagleFordthintohalftheirmaximumthicknesswithin5-6milesoftheaxis.Depthto

thetopoftheEagleFordvariesfrom9600ftto15000ftandstrikesparalleltotheEdwardsand

Sligoreefs.Numerousfaultsarevisible.Mostfaultsarepost-depositionalwithmodest

offsets.Fewfaultsaresyn-depositionalgrowthfaultsandtheEagleFordisthickeronthe

downthrownside.Wellandseismicdatadocumentdramaticdecreasesinthicknessofthe

UpperEagleFordoverafewmiles.InthemostextremecaseinsouthwestLaSalleCounty,

theUpperEagleFordisentirelymissingin2wellsandhasbeenreplacedbyasandunitnot

previouslyreported.Aseismiccrosssection,ortimeslice,justabovethetopoftheEagleFord


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showsachannelstructurerunningwesttoeastalongsouthernLaSalleCounty.Thischannel

islikelythecauseofobservederosionandsanddeposition.


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INTRODUCTIONTheEagleFordFormationisanUpperCretaceousorganic-richcalcareous-rich

mudstonethatoccupiesanortheast-southwestbandacrossSouthTexas(Figure1).TheEagle

FordFormationisanunconventionalshaleoilandgasplayandisstratigraphicallyvariablein

termsofthickness,organiccontent,andcomposition(Locketal,2010).AlthoughtheEagle

FordFormationhasbeenextensivelydrilledforoilandgasproduction,theregional

depositionalanddiagenetichistoryarestillpoorlyunderstood.Inparticular,theHawkville

TroughinLaSalleandMcMullenCountiescontainsastratigraphicallyvariablesectionofthe

EagleFordFormationranginginthicknessfrom50-317feet(thisstudy).Anotabledifferencein

theEagleFordwithintheHawkvilleTroughisthattheTuronianportionofthesectionis

incompleteorinsomecorescompletelymissing(Figure2).Thepresenceofonlythe

CenomanianagedsectionoftheEagleFordisuniquetotheHawkvilleTroughandisbeneficial

tothereservoirduetotheoverlyingAustinChalkandAnacachoformationshavingahighclay

content,lowresistivity,andhighductilitythatcreatesaneffectivesealfortheEagleFord.The

upperunconformityactsasaneffectivetopsealduetoanabsenceofextensivenatural

fracturing,allowingasuccessfultrapforhydrocarbons.

Inthefallof2008,PetrohawkEnergyCorporationacquiredthefirstacreagespecifically

targetingtheEagleFordFormation.Onlyafewpilotholeshadbeendrilledintheareathrough

theEagleFord,butweretargetingtheEdwardsLimestoneorAustinChalk.Thatfirstyearthere

wereonly3wellspermittedanddrilledtargetingtheEagleFordFormationinSouthTexas

(DrillingInfo,2012).Fouryearslater,thereareover120pilotholesover24counties,andjust

under6000horizontalwellsdrilled,makingitoneofthemostactivefieldsintheUnitedStates

(DrillingInfo,2012).


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Figure1:MapoftheregionalextentoftheEagleFordShaleinTexas.TheHawkvilleTrough(blackbox)liesbetweenthe

EdwardsandSligoreefmargins(modifiedfromHentzandRuppel,2010).


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Figure2:Stratigraphiccolumnspecifictostudyarea,showingtheEagleFordFormation

unconformablyboundbyAustinChalk(above)andBudaLimestone(below).Thickwave

linesindicatehiatusesthatarenotwellconstrainedinthesubsurface(SeeDonovanand

Staerker,2010)


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RecentstudieshavefocusedondefiningalithostratigraphicframeworkfortheEagle

FordFormationbyseparatingitintoanumberofdepositionalparasequences(e.g.,Locketal.,

2010;DonovanandStaerker,2010;HentzandRuppel,2010;AdamsandCarr,2010)(Figure

3).Lock(2010)usedsubsurfacedatafromtheHawkvilleTroughtoconcludethattheunstable

slopesedimentsmakingupthelowermemberidentifiedinoutcropareabsentbecausethey

arealocalizedfaciesconfinedtothemarginalslope.DonovanandStaerker(2010)evaluated

thesameoutcropinLozierCanyonalongwithsubsurfacedatawithinthestudyareaand

referredtoitastheRioGrandeSubmarinePlateau.Theyproposedthatthissubmarineplateau

consistedofsubmergedportionsofthe(older)SligoPlatformbetweentheEdwardsandSligo

marginsthatformedaphysiographicbenchontheinnerportionsofthecontinentalslope

(DonovanandStaerker,2010).DonovanandStaerkers(2010)workcorrelatedmultiple

parasequencepackagesfromoutcroptosubsurface,andnotedthatitisgenerallyacceptedto

separatethesubsurfaceintoupperandlowermembersaccordingtoworkdonebyGrabowski

(1995).Finally,DonovanandStaerker(2010)furthercitedanadditional(Langtry)memberasa

transitionfromupperEagleFordtoAustinChalklithologies.TheLangtrymemberisa40-90ft

thickdepositionalsequencethatrecordsasubtleupwardgradationfromtheunderlyingEagle

FordtotheoverlyingAustinChalk(DonovanandStaerker,2010).Thecorrelationsby

GrabowskiidentifythedivisionbetweentheupperandlowerEagleFordasthedivision

betweentheCenomanianandTuroniansections.

Thisstudyusesgammaray,resistivity,anddensitylogsfromtwenty-onewellsthat

penetratetheentirestratigraphicsectionoftheEagleFordacrosstheHawkvilleTroughin

LaSalleandMcMullenCountiestomapvariationsinthicknessandpreservationoftheEagle


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Ford.Corephotosareusedtoidentifythenatureofthelogresponsealongthebounding

unconformities.

Anamplitudemapmadefroma3Dseismiccrosssectionisusedtoshowanerosionalfeature

affectingtotalthicknessalongtheupperEagleFord/AustinChalkboundary.Additionally,this

amplitudemapandotherseismiccrosssectionsshowacomplexnetworkoffaultsspanning

theentirestudyarea.

Figure3:SummaryofpreviousstudiesoftheEagleFordFormationinoutcropandinthe

subsurface(fromDonovanandStaerker,2010).


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GEOLOGIC BACKGROUND / STUDY AREA TheEagleFordFormationinsouthwestTexasrecordsanUpperCretaceous

(CenomaniantoTuronian)mixedsiliciclastic/carbonatedepositionalsystem.TheEagleFord

Formationwasdepositedduringatransgressivecycleinashallowepeiricseawaythatcovered

thesouthernmarginofNorthAmerica(e.g.,LiroandDawson,1994).AcrosstheCretaceous

Platform,theEagleFordFormationtrendsfromsouthwesttonortheastroughlysubparallelto

thepresentdaystrikeoftheGulfCoast.TheEagleFordFormationcropsoutalongabroad

bandextendingfromElPaso,Texas,eastwardtoSanAntonio,Texas,whereitthenfollowsthe

marginoftheEastTexasBasinnorthwardtotheOklahomaStateline(Figure1)(e.g.,Liroand

Dawson,1994).TheEagleFordFormationinsouthTexasdipssouth-southeasttowardsthe

GulfofMexico(Martin,2011).TheEagleFordFormationiscorrelativetotheBoquillas

FormationintheMaverickBasin,northwestTexas,andtheTuscaloosaFormationinLouisiana

andMississippi(LockandPeschier,2006).

TheyoungestandeasternmostdeformationoftheLaramideOrogeny(postEagleFord

deposition)wasastructuralinfluenceontheCretaceousPlatform(e.g.,Scott,2010).Duringa

transgressivecycleintheCenomaniantolateTuronian,theSabineUplift,locatedeastofthe

studyarea(Figure4),wasapositivesalientfeatureandtheOuachitaMountainchain,tothe

north,wasaprimarysiliciclasticsedimentarysource(Scott,2010).Sedimentswerealsoshed

intotheMaverickBasinfromthenorthwestWesternInteriorSeawayandthenintothe

restrictedbasinoftheHawkvilleTrough(Figure4).TheEagleFordFormationandthe

stratigraphicunitsabovewereheavilyinfluencedbybasementfaultsandbasintopography.

LateCretaceoussedimentsdisplaythicknessvariationsacrossthearea.Thissuggeststhat


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basementcontrolplaysamajorroleinthedepositionalandpost-depositionalaccommodation

patternsforalltheseunits(DonovanandStaerker,2010).

Inascendingorder,themajorlithostratigraphicunitsacrosstheUpperCretaceous

PlatformaretheDelRio,Buda,EagleFord,andAustinChalk(Figure2).WithintheHawkville

Trough,theEagleFordisunconformablyboundbytheoverlyingAustinChalkandthe

underlyingearlyCenomanianBudaFormation(Figure2).TheEagleFordFormationwas

Figure4:PaleogeographicmapoftheGulfCoastregionduringtheLateCenomanian

showingthemajortopographicfeaturesandrelativedistancefromsedimentsources

(ImagemodifiedfromBailey[2007]bySalvador[1991],Sageman&Arthur[1994],and

DonovanandStaerker[2010]).


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depositedduringaglobaltransgressionandhighstandofsealevelfollowingtheMiddle

CretaceousUnconformityat96Ma(MCUofWinkerandBuffler,1988)intheGulfofMexico

Basin(Haqetal.,1988;Jiang,1989).Theprecedinglowstandisrepresentedbytheunderlying

carbonatehorizon,theBudalimestone(e.g.,Treadgold,2010).Theinternalcarbonatemarker

oftheEagleFordFormation,theKampRanchMember,isthestratigraphicmarkerbetween

theupperandlowerEagleFordintheregion(e.g.,Donovan,2010).TheKampRanchMember

separatesthelower,organicrichshalememberfromtheuppermorecalcareousmember.The

ConiaciantoSantonianAustinChalkstratigraphicallyoverliestheEagleFordwithamajor

unconformityseparatingthetwoformations.DonovanandStaerker(2010)placetheK69

MaximumFloodingSurface(MFS)asapossiblecontactbetweentheAustinChalkandthetop

oftheEagleFordFormation,citingatransitionzonereaching40feetthickinthesubsurface

(alsocalledtheLangtryMember)(DonovanandStaerker,2010).Insomewellsthecontactis

abrupt,andothersitismoregradational,thusmakingitdifficulttoresolve.Forthisstudythere

isparticularinterestintheuppercontactbetweentheEagleFordandtheAustinChalk

becauseoffluctuatingamountsofmissingsection.Adropinbaselevelandsubsequent

subaerialexposure,aswellaschannelincisionpostEagleForddepositionisthemainsource

ofmissingsectionalongthissurface,andwillbediscussedingreaterdetailbelow(Scott,

2009).

Regionallithofaciespatternsandfossilassemblagesindicateamarginaltoopen

marginalmarinedepositionalenvironmentfortheEagleFordFormation(Passagno,1969;

Surles,1987).Thesestudiesconcludethatsouthwestwardprogradingdeltassupplied

bioclastic-siliciclasticsedimentsnearandbelowstormwave-base(DawsonandAlmon,2010).

Sedimentsdepositedontheshallowshelfrepresenttheproximaldeltaicfacies;sediments


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depositedfurthersouthandsouthwestareinterpretedasthedistaldeltaicfaciesbasinwardof

theshallowshelf(Figure4).

MudstonesintheEagleFordFormationvaryfromslightlytoverysilty,calcareous,

phosphatic,pyritic,glauconitic,bentoniticandcarbonaceousfacies,rangingfrommassiveto

well-laminatedandslightlytoabundantlyfossiliferous(DawsonandAlmon,2010). SixmicrofacieswereidentifiedbyDawson(2000)inacoresamplefromLaSalleCountywithinthe

studyarea:1)pyriticshales;2)phosphaticshales;3)bentoniticshales;4)fossiliferousshales;

5)silty(quartzose)shales;and6)bituminousclaystoneandshales.Thetransgressive(lower)

EagleFordshalesconsistedofmicrofacies1,4,and6;thecondensedintervalconsistedof1,

2,and3,andhighstand(upper)EagleFordmicrofaciesexhibitmicrofacies4and5(Dawson,

2000).CoreimagesillustratethenatureoftheupperandlowerEagleFord(Figure5).Black,

thinlylaminatedshalesdominatetheLowerEagleFord,andtheupperEagleFordcontains

moreabundantcalcareousandmixedsiliciclastic(quartzose)beds(light-gray).Upward-fining

trenddominatesthelowermemberasgammaraydecreases,whileupward-coarseningtrend

characterizestheuppermemberwithgammavaluesincreasing(Dawson,2000).Thelower

EagleFordischaracterizedbyhighgamma-rayvalues(90to135APIunits)andanupward-

coarseningtrend(Figure5).Thelowermemberisdominatedbydark,well-laminatedorganic-

richshale(Figure5)withsubordinatelight-graycalcareousmudstone,marl,andtraceable

amountsoflimestone(i.e.,HentzandRuppel,2010).Thereisacondensedsection,

representingaperiodofsedimentstarvationduringamaximumfloodingeventseparatingthe

transgressiveandearlyhighstandsystemstracts(Loutitetal.,1988).Thiscondensedsection

occursbetweentheupperandlowerEagleFordFormationandtendstofluctuateinthickness

acrosstheregionalextentoftheEagleFord(DonovanandStaerker,2010).Thecondensed


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intervaldevelopedduringcycle2.5oftheUpperZuniA-2(UZA-2)supercycle(Haqetal.,1988;

Ulicnyetal.,1993).TheupperEagleFordFormation,interpretedaspartofthehighstand

systemstract,ischaracterizedbygenerallylowgamma-rayvalues(45to75APIunits)andan

upward-finingtrend.Theuppermemberconsistsofinterbeddeddark-andlight-gray

mudstonesaswellasthinlystratifiedshale,limestone,andcarbonaceousquartzosesiltstone

(Figure6;HentzandRuppel,2010).

Figure5:CoretypelogfromMcMullenCounty,Texas(McM-1).Fromleft-rightcurves

presentaregammaray,resistivity,anddensityporosity.Coreimageisfromthesamewell,

andusedtohighlightnatureofthecontactbetweenupperandlowerEagleFord(Petrohawk

Energy,2009).


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TheLateCenomanian-EarlyTuronianismarkedbyamajoranoxicextinctionevent(Fan

etal.,2011).Thisboundaryischaracterizedbyaworldwidedepositionofhydrocarbon-rich

shalelithofaciesliketheEagleFordFormation.Othernotableoccurrencesoforganic-rich

CretaceousshaledepositsincludetheMowryandPierreShalesoftheWesternInterior

Seawayandmudstone-richformationsinMorocco,Venezuela,Tunisia,Nigeria,Western

Australia,andthePolishCarpathians(Jenkyns,2010;Hallam,1987).Warm,shallowseas

wereprevalentduringthisglobalgreenhouseevent.Thegreenhouseeffectcauseda

significantincreaseinCO2levelsandthusorganicproductivity.Regressiveperiods,orperiods

ofrelativesealevelfall,turnedtheHawkvilleTroughintoarestrictedbasin(Figure6).The

organicproductivitycontinueduntilsedimentandoxygensupplywasdepleted,resultinginan

anoxicenvironment,whichinturnpreservedtheorganicmaterial(MartinandBaihly,2011).

Thedeep(200-400ft)andrestrictedsettingoftheHawkvilleTroughaswellascyclic

sedimentinfluxallowedforananomalouslythicksectionoforganicrichmaterialtoaccumulate

betweentheEdwardsandSligoReefmargins(Figure6)(Lock,2010).TheEdwardsandSligo

reefcomplexeswereformedduringperiodsofrapidsealeveltransgression.Subsequent

regressiveperiodsresultedinanoxicconditionsintherestrictedbasinsduetoalackof

nutrient-richsedimentsupply(Figure6).ThelowersectionoftheEagleFordFormationhasa

highertotalorganiccarbon(TOC)valueasaresultofhighorganicproductivity.Followingan

increaseofsedimentinfluxthereweresubsequentperiodsofhypoxia,ortimesoflocaldysoxic

(oxygen-poor)conditions(AdamsandCarr,2010).Generally,thisdescribesperiodsofpartially

oxygenatedwater.Asaresulttherewaswatercolumnstratificationanddepletionofoxygen

belowthepycnocline.Thepycnoclinedescribestheboundaryseparatingchangesindensity

(inthiscasesalinity)betweentwoliquidlayers.Thedepletionofoxygenbelowthisboundary


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allowsfortheorganicmattertobepreservedatthebottomofthebasin.Acyclicdepositional

modelintheseconditionscanbeusedtoillustratethethickandorganic-richsectionofEagle

Ford:1)relativesealevelrise,2)terrigenoussedimentinflux,3)organicproductivity,4)

organicsedimentation,5)relativesealevelfall,6)localdysoxicandsubsequentanoxic

environment,and7)organicpreservation(personalcommunicationwithScottyTuttle-

PetrohawkGeologist,2011).TheUpperEagleFordismorecarbonaterich,implyinga

shelf/slopedepositionalsetting.TheUppermemberdisplaysaprogradationallogpatternof

finingupward(Figure5).ThisunithaslowerTOCvaluesacrosstheareaandrepresentsthe

midtoinner-shelfdepositswithhighlylaminatedcarbonate-richmarlsandskeletallimestones.

ThelowerTOCoftheuppermembercorrespondstomoreproximalfacieswithhigherenergy,

lessproductivity,andmoreinteractionwiththebottomoftheocean(rip-upclasts).

Figure6:Schematiccross-sectionfromnorthwestTexasthroughtheHawkvilleTroughdepictingestimatedapproximatewaterdepthduringEagleForddeposition.A-Afromfigure

3modifiedfromDonovanandStaerker,2010.


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METHODS Gammaray,resistivity,anddensitylogcurveswereusedtoevaluatethethickness

variationintheEagleFordFormationintheHawkvilleTrough.Twenty-onewellsspanning72

milesacrossLaSalleandMcMullencounties,Texas,wereusedtounderstandthe

Cenomaniandepositionalenvironmentinthestudyarea(Figure7).Thesewellsvertically

penetratedtheentiresectionoftheEagleFordFormationintotheunderlyingBudaFormation.

Mosthorizontalwellswereexcludedbecauseofanincompleteverticalstratigraphic

successionthroughtheEagleFordsection.Wellswithbottomholelocationsusedinthisstudy

weredrilledintotheBudaandcorrectedtoTVDbyusingknownbeddips,lengthoflateral,and

penetrationpointsinordertocalculatethetruestratigraphicthickness(TST).Formationtops

wereprincipallyinterpretedusingthegammaraylog;however,resistivityanddensity-porosity

curveswereusedtosupportcorrelationofwellsincloseproximitytooneanother.Isopach

mapsweregeneratedbylogdatacompiledtoshowtheshapeofthetotalsection,aswellas

upperandlowerEagleFord.Duetoproprietaryobligations,thewellswerelistedbycounty

(LaS=LaSalle;McM=McMullen),andnumberedincreasingfromWesttoEastperrequestof

PetrohawkEnergy.


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Figure7:MapofstudiedwellsintheHawkvilleTroughinLaSalleandMcMullenCounties,TX.Wellnamesareposted(LaS

LaSallewells;McM-McMullenWells).LaS-4andMcM-1arethetypelogsforthearea.Regional,strike,anddiporiented

crosssectionlinesareprovided.Thosewellswithabottomholelocation(labeledhorizontalabove)weredrilledverticallyinto

theBudaLimestoneandcorrectedfortrueverticaldepth(TVD)(ImagecreatedinSMT).


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FORMATIONS AND UNIT DISTINCTIONSThemarkeratthetopoftheEagleFordFormationischaracterizedbyadistinct,high

gammarayresponse(120-145API)andsharpincreaseinresistivity(from1-3ohmsto10-15

ohms)immediatelybelowthebaseoftheAustinChalkFormation(~76API)(Figure8).The

AustinChalkliesunconformablyabovethelowersectionoftheEagleFordFormationinthe

HawkvilleTrough.Asawhole,theuppersectionoftheEagleFordisrepresentedbya

generallylower(45-75API)gammarayresponse(Figure8).AnexaminationoftheLangtry

Member,asdescribedbyDonovanandStaerker(2010)asadistinctlydifferentstratigraphic

unitbetweentheEagleFordandAustinChalk,wasexploredinwelllogsinthestudyareaand

remainsindistinguishable.

TheupperandlowerunitsaredividedbytheK72sequenceboundary(Donovanand

Staerker,2010).Thisdivisionisnotedbyasignificantincreaseinthegammarayresponse,

andadecreaseinresistivity.ThelowerEagleFordhasahigher(90-135API)gamma

response(Figure8).Thisunitrangesfrom72-186feetthickandisdominatedbyinterbedded

marlsandlimestones.Theseunitdistinctions,tops,andbasesrelyprimarilyonpattern

recognitioninthewelllogs.ImagesofcorefromawellintheHawkvilleTroughareusedto

demonstratethenatureofunitchange,theunconformablecontactsatthetopandbaseofthe

EagleFord,andtoexploreapossiblecorrelationtologsignatures(Figure8and9).These

imagescanbeusedtoeithersupportordisprovethenatureoftheunconformablecontactsand

arealsousedforvisualinspectioninternalunitdistinctions.Anattemptwasmadetoidentify

andcorrelateadditionalinternalunitsbasedonlogresponsetocorroboratewithoutcrop

studiesdonebyDonovanandStaerker(2010)andLockandPeschier(2010).


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Figure8:LaS-4typelogfromLaSalleCounty,Texas.Fromlefttorightthemostimportant

logcurvestothisstudywerethegammaray,resistivity,anddensity-porositycurves(color

ofscalebarindicatescolorofcurveused).


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CrosssectionsbasedongammaraytopsthroughtheHawkvilleTroughwere

constructedtohighlightthethicknessvariationsinbothstrike(northeast-southwest)anddip

orientation(southeast).LogsanalysesweredonewiththeIHS-PETRAsoftwarepackage.

PETRAisanintegratedapplicationwithacommondatabaseandinterfaceforprojectanddata

management;wellloganalysis,mapping,cross-sections,seismicintegration,productionand

reservoiranalysis,and3Dvisualization.

Figure9:CoreImagefromMcM-1displaysthenatureoftheabruptcontactbetweenthe

AustinChalkandEagleFordformations.Skeletallagandripupclastsarecommon

featuresalongthiscontact(ImagecourtesyofPetrohawkEnergyEagleFordConsortium).

0ft

2ft


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RESULTSWELL LOG

GammaRay,resistivity,anddensity-porositylogswereusedtoidentifythreeimportant

stratigraphicmarkers.ThemostregionallyconsistentmarkerfortheEagleFordFormationis

thebasalunconformitybetweentheEagleFordFormationandtheunderlyingBudaLimestone.

ThetransitionbetweentheEagleFordandBudaLimestoneisindicatedbyabruptchangesin

rockproperties,whichmadethissurfaceeasilycorrelatableacrosstheregion.Thecalcium

carbonatepercentagegoesfrom15-25%(EFS)to90%(Buda),gammarayvaluesdropfrom

125-130API(EFS)tolessthan15API(Buda),resistivityincreasesfromlessthan10(EFS)to

over50Ohms(Buda),andthedensity-porosityvaluesdropsfrom10-15%(EFS)tolessthan

5%(Buda).

TheconformablecontactbetweenthelowerandtheuppermembersoftheEagleFord

Formationisreliablybasedonadrasticincreaseinthegammaray(120-140API)(Figure8).

AcrosstheHawkvilletrough,thismarkerisoneofthefewthatcanbeconsistentlycorrelated.

Thecharacterofthegammaincreasediffersfromwelltowell,butoccursroughlyinthemiddle

oftheformation.Theresistivityanddensity-porositylogsprovedlesshelpfulforcorrelation

purposesbecauseofthevariationoffluidspresentandporosityvalues.Therewasnodistinct

changeinformationresistivityandporositybetweentheupperandlowerunits.

ThetopofEagleFordisthethirdcorrelativemarkeracrosstheHawkvilletroughthat

canberecognizedwithconfidencebasedongammarayandresistivityresponse.However,

thesectionatthetopoftheEagleFordwasremovedpost-depositioninpartsofthefield,

causingsomeuncertainty.DonovanandStaerker(2010)identifiedatransitionalunit(Langtry

Member)abovetheEagleFordthatwasnotconsistentlyidentifiedwithinthestudyarea.


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Instead,thistransitionalunitwascategorizedastheUpperEagleFord.Thetransitionfromthe

overlyingAustinChalkandEagleFordisvariablyexpressedinloganalysis.However,the

gammaray-signatureisusuallyanotableincrease,whetherabruptorgradational,withAPI

valuesrespectivelyincreasingfrom50-75APIto120-135APIfromUpperEagleFordtoAustin

Chalk,respectively(Figure5and8).

TheHawkvilleTroughinLaSalleandMcMullenCountycontainsanunusuallythick

sectionoftheEagleFordFormationsituatedbetweentheEdwardsandSligoreefmargins.

ThetotalEagleFordintervalthinstothenorthoftheEdwardsreef,southoftheSligoReef,and

totheeastinBeeCounty.However,thicknessacrossthe72-milelongtroughvariesgreatly

overshortlateraldistances.Basedsolelyontrueverticaldepth(TVD)welldata,thetotalEagle

Fordthicknessvariesby245feet(72-317;LaS-12-LaS-7,respectively).FromLaS-9toLaS-

10(4.4Mi)theEagleFordFormationthinsfrom289ftto82ftandcontainsnouppersection

(Figure10).

BasedonTVDwelldatabycounty,LaSalleCountycontainsthethickestEagleFord

sectionforallmappedhorizons(Figures11-13).WelldatafortheEagleFordFormation

indicatesthatthethickestandthinnestsectionsarepresentinLaSalleCounty,withthethickest

totalsection(317;LaS-7)(Figure11),thinnesttotalsection(72;LaS-12),andthickest/thinnest

upperandlowersections(Table1)(Figure11).ThetotalEagleFordSectionthinstonortheast

intoMcMullenCounty.AstructuremapbasedoffoftopsgatheredfromwelldataoftheEagle

Fordshowsasoutheasterlydipdirection(Figure14).Strikeanddiporientedcrosssections,as

wellasaregionalcrosssectionacrosstheHawkvilleTroughhighlightthosevariationsin

separateunits,aswellasoverallthickness(Figures15-19).


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Figure10:N-SwelllogcorrelationfromLaS-10toLaS-9(4.4mi)illustratingthattheupperEagleFordmemberiserodedoutcompletelyinLaS-9.Theredlineistheinterfacebetween

theupperandlowerEagleFordmembers.Asandbody,notfoundanywhereelsein

HawkvilleTrough,isfoundabovetheEagleFordFormationinLaS-9.


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Name Top EFS TVD) Middle Marker TVD) Thickness Upper Top Buda Thickness Lower Total ThicknessLaS-1 10525 10607 113 10697 159 272LaS-2 11035 11107 102 11194 149 251LaS-3 10561 10615 74 10677 93 167LaS-4 10911 11055 144 11179 124 268LaS-5 11240 11374 134 11525 151 285LaS-6 11467 11621 153 11778 157 310LaS-7 11381 11512 131 11699 186 317LaS-8 11039 11181 141 11299 118 259LaS-9 12827 12827 0 12909 82 82LaS-10 11934 12069 137 12220 153 290LaS-11 10972 11090 118 11206 116 234LaS-12 13153 13153 0 13225 72 72LaS-13 11020 11091 128 11139 122 250McM-1 11416 11547 131 11645 98 229McM-2 11866 11956 103 12052 125 228McM-3 12924 13004 128 13057 98 226McM-4 11916 11996 98 12093 119 217McM-5 12909 13024 115 13159 135 250McM-6 12407 12501 137 12562 118 255McM-7 12734 12832 120 12904 109 229McM-8 12855 12981 126 13097 116 242

Table1:TVDwelldatacompiledfrom21wellsintheHawkvilleTrough.WellswithLaSnamesarefromLaSalle

County,andMcMarefromMcMullenCounty.Wellnumbersincreasefromwesttoeastbycounty.


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Figure11:IsopachmapofentireEagleFordFormationobservedintheHawkvilleTrough.Theentiresectionistroughshaped

withanaxisroughlyparalleltobeddingstrike,andshowsdrasticthinningwithin5-6milesoftheaxis.Seismicdatawasused

tofillinareasbetweenknownvaluesfromTable1(HandcontouredinSMT).


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Figure12:IsopachmapoftheupperEagleFordFormationasseenintheHawkvilleTrough.Valuesarebasedprimarilyfrom

welldata,andseismicwasalsousedtofillingapsbetweenwellswithknownvalues.NotethattheupperEagleFord

Formationisabsentin2southernLaSalleCountywells(Hand-contouredinSMT).


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Figure13:IsopachmapoftheLowerEagleFordintheHawkvilleTrough.Valuesarebasedprimarilyfromwelldata,and

seismicwasalsousedtofillingapsbetweenwellswithknownvalues.ItisthickestinLaSalleCountyinLaS-6at153ft.

Similartotheupperandtotalsection,thisunitistrough-shapedandtheaxisofthetroughthinslaterallyfromthecenter.This

sectionthinsdramaticallytowardssouthwestLaSalleCounty(ImagecreatedinSMT).


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Figure14:RegionalEagleFordStructuremapintheHawkvilleTroughbasedonthesub-seatrueverticaltop(SSTVD)data

gatheredfromlogs.Faultswerepickedusingseismicdata.ThereisextensivefaultinginMcMullenCounty.Mostofthefaults

post-datedepositionoftheEagleFordFormation,withtheexceptionofafewgrowthfaultsinMcMullenCountythatcontribute

tothicknessandsectionvariationovershortlateraldistances.(MapcreatedinSMT)


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Figure15:Southwest-northeastcrosssectionthroughtheEagleFordFormationintheHawkvilleTrough.Cross-sectionishungonthetopofEagleFordandverticalaxisisshownwithblacklineinfeet.(crosssectioncreatedinPetra,mapcreatedinSMT).


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Figure16:Southwest-northeast(strike-oriented)crosssection,hungonthetopEagleFord,fromLaS-2toLaS-8inLaSalle

County,TX.Blacklineindicatesverticalaxisinfeet(crosssectioncreatedinPetra,mapimagecreatedinSMT).


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Figure17:Northwest-southeast(dip-oriented)crosssection,hungonthetopEagleFord,fromLaS-3toLaS-6inLaSalle

County,TX.Blacklineindicatesverticalaxisinfeet(crosssectioncreatedinPetra,mapimagecreatedinSMT).


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Figure18:Southwest-northeast(strike-oriented)crosssection,hungonthetopEagleFord,fromMcM-3(southwest)toMcM-

8(northeast)inMcMullenCounty,TX.Blacklineindicatesverticalaxisinfeet(crosssectioncreatedinPetra,mapimage

createdinSMT).


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Figure19:N-S(dip-oriented)crosssection,hungonthetopEagleFord,fromMcM-1(N)toMcM-3(S)inMcMullenCounty,

TX.Blacklineindicatesverticalaxisinfeet(crosssectioncreatedinPetra,mapimagecreatedinSMT).


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SEISMIC ANALYSIS TheavailabilityofseismicinsouthTexas,bothtwo-dimensional(2D)andthree-

dimensional(3D),areplentifulduetotheextensivecontinueddrillingintotheEagleFordand

manyotherproductivezonesstatewide.The3DseismicinHawkvillefieldwasshotand

processedoverthecourseofseveralyearsandbrokenupintophases.ThePatronGrande

surveycoversHawkvilleFieldandencompassesapproximately955squaremiles

(www.globalgeophysical.com).The3Ddatademonstratesacomplexnetworkoffaultsand

erosionalfeaturesthroughouttheHawkvilleTough(Figures14,20,and21).TheEagleFord

andBudaFormationswerepickedbasedontopsseenwhiledrilling,andthentiedintothe

seismic,topfortop(personalcommunicationwithMarieHenry-Geophysicist,Petrohawk

2010).ThemethodmostcommonlyusedhereisaTimeDepthChart,whichisusedtoconvert

TVDvaluesintotimesothewellboreisplottedinseismic.TheBudaFormationhasahigh

velocityanddensity,makingitthemostreliabletopthatcanbepickedintheHawkvilletrough;

thisyieldshighacousticimpedance.Acousticimpedanceindicateshowmuchsoundpressure

isgeneratedbythevibrationofmoleculesinaparticularmediumatagivenfrequency;locally,

thisshowsupasastrongpeak(personalcommunicationJarrettPierce,Geophysicist2012)

(Figure20and21).InFigure20(highlightedinpurple),anextrapeakcanbeseenontheleft

sideofthefigurebetweenthetopofEagleFordandthetopofBuda.Inseismicdatathispeak

onlyoccurswhentotalthicknessreachesgreaterthan150feet.Movingtowardstherightside

ofthefigure,thisstrongpeakdisappearsandtheupperEagleFordbecomestruncated.Figure

21,fromMcMullenCounty,demonstratestheadditionalaccommodationseenonthedownthrownsideofgrowthfaults.Thisimageshowstheappearanceofthestrongpeak,

discussedabove,thatoccurswhentotalthicknessisgreaterthan150feet.Thisthickness

continuestoincreasedowndipasanothergrowthfaultisencountered.Thetotalthickness


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increased95feetovertheextentofthelinethatisindiporientationincentralMcMullen

County.Figure22isaseismiccrosssectionthatwasgeneratedbyflatteningthedataonthe

BudaHorizonandobservingthehorizonjustabovetheEagleFordshowingdrasticlateral

thinningacrosstheprofile.Amplitudeextractionmaps,ortimeslices,takenfrom3Dseismic

volumesrevealhigh-resolutiondispersalpatternsandassociatedsystemstractsongeologic

timesurfaces(Li,2008).FlatteningwasachievedthroughSMTsseismicmoduleand

subtractedtheinfluenceofregionaldipinordertoproperlyimagethefeatureseenabove(solid

yellowlineinFigure22).Thepurposeofastructure-removedtimesliceistobeabletoimage

amplitudevariationsinmapviewaffectingagreaterregionalextentthatoccurredatorneara

geologictime-equivalenthorizon.Thistimeslice,inmapview,revealeda6-7milewide

channelrunningfromsouthwestLaSalleacrosstotheLaSalle/McMullenborder(yellowlinesin

figure25).ThefeaturediscoveredcorrespondstothinningoftheEagleFordinLaSalleCounty

fromover+/-250feettolessthan70feetinlessthanfivemiles.


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Figure20:Northwest-southeastSeismicdipsectionacrossLaSalleCounty,Texas.Thereisminorfaultingassociated

withsectionthickness.FromnorthwesttosoutheastthethicknessoftheEagleFordchangesfromthicktothin(as

notedwheretheextrapeakispresent).TheChannelIncisionisevidentonthissectionof3Ddata.Fieldofviewfor

figures20-22areapproximately7miles.Verticalaxisisrecordedintime(roughly0.5seconds)(Imagecreatedin

SMT).


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Figure22:Northwest-southeastseismicscreenshotthatshowsflatteningontheBudaLimestone(blueline)and

generatingthetimeslicethroughthehorizonabovetheEagleFordFormationinsoutheastLaSalleCounty(yellowline).

Thisimageshowsthatinlessthan5milestheEagleFordthinsby180feet.Verticalaxisisrecordedintime(roughly

0.5seconds)(ImagecreatedinSMT).


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DISCUSSIONCORE ANALYSIS

Basiccoreanalysisillustratesthenatureoftheboundingunconformities,atthebaseof

EagleFord/TopofBudacontact,butmostnotablytheupperunconformitybetweentheEagle

FordFormationandtheoverlyingAustinChalk(Figure23and24).Insomeareasofthe

HawkvilleTroughthecontactisfoundtobeabrupt(Figure23),andinotherareasitismore

gradational(Figure24).However,itisacceptedthatthetwoformationsareseparatedbya

majorunconformityandK72sequenceboundary(Petrohawk,2009;DonovanandStaerker,

2010).Theabruptnatureofthisboundaryconsistsofathin(~.3-.5in)layerofskeletallag,rip-

upclasts,andsomesoft-sedimentdeformation(Figure23;Petrohawk,2009).Therip-up

clasts,skeletallimestones,andsoftsedimentdeformationfeaturesindicatesmoreproximal

facies(Petrohawk,2009).TheproximalfacieshadlowerTOCvaluesandhigheramountsof

silica-bearingminerals.TheLEFcontainshigherTOC,hemipelagicmarlsdepositedinamore

anoxicenvironmentdistaltosedimentsource(Petrohawk,2009).Thegeneraldepositional

modelfortheEagleFordisagentlyinclinedcarbonaterampattheinner/outershelfinterface,

withinreachofstorm-wavebase(Petrohawk,2009).

AnattemptwasmadetocorrelatethelogresponsesattheupperandlowerEagleFord

interface,aswellasinternalparasequenceboundariesidentifiedbyDonovanandStaerker

(2010).WhilethesharpandgradationalnatureoftheAustinChalk/EagleFordboundaryin

corecanbeidentifiedbythesharpandgradualincreaseofgammarayseeninlogs,therewas

nodatasuggestingacorrelationbetweenlogresponseandadditionalindividual

parasequencespackages.Thiscouldbecausedbytheinabilitytocapturethesesmallevents

inlogresponse.Thelikelihoodthattheseparasequencepackagesexistishigh,butremainto

beidentifiedandcorrelatedtospecificlogresponses.


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Figure23:CoreimagedemonstratingabruptandunconformablenatureoftheAustinChalk

andEagleFordFormationfromMcM-1wellinMcMullenCounty,TX(imagecourtesyof

PetrohawkEnergyEagleFordConsortium).

Figure24:Coreimageillustratingthegradationalnatureoftheunconformablecontact

betweentheAustinChalkandEagleFordFormationasseeninaLaSalleCounty,TXwell

(imagecourtesyofPetrohawkEnergyEagleFordConsortium).


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Figure25:AmplitudeextractionprojectedinmapviewfromFigure22insoutheastLaSalleCounty.Thisprojectionis

generatedfromatime-equivalenthorizonjustabovetheEagleFord.Yellowlinesshowtheextent(width)ofchannel

describedinFigure22.Faultscanbeseennorthofthechannel(darklines).Wholeimageisunavailabledueto

proprietaryobligations.(ImagecreatedinSMT).


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WELL LOG/SEISMICTheEagleFordFormationintheHawkvilleTroughvariessignificantlyintermsof

thickness(showninseismic)andinternalstratigraphicframework(showninmappingandin

seismic)overshortlateraldistances.Thepaleo-waterdepthandbasintopographysituated

betweenthestructurallyhighfeaturesoftheEdwardsandSligoReefmargins,allowedfor

moreaccommodationthandistalareas.Thethicknessvariationisduetothebowlshaped

featurethatformedbetweenthetworeefs(Figures15and17).Basedonwelllogandseismic

data,depthtotheEagleFordvariesfrom9600ftto15000ft(subsea)alongaroughlyplanar

surfacethatstrikesparalleltothesereefmargins(Figure14).Post-depositionalforces-suchas

erosionoftheupperand,inextremecases,portionsofthelowerEagleFord-weremajor

controlsonthicknessvariationseenintheHawkvilleTrough.Inthemostextremecasein

southwestLaSalleCounty,theUpperEagleFordisentirelymissingintwowellsandhasbeen

replacedbyachannelsandunitnotpreviouslyreported(Figure10).Thishasbeenidentified

asachannelsandbasedonthegammaraysignatures(20-45API)andresistivityresponse(2-

4ohms)usingthecriteriaofBoothetal.,2003.TheLowerEagleFordwasalsopartially

erodedinthesetwowells(Figure10).Numerousfaultsarevisibleonseismicdata(Figures20

and21).Mostofthesefaultsarepost-depositionalwithmodestoffsets(25-200feet)(Figure

21).Afewofthefaultsaresyn-depositionalgrowthfaults,indicatedbythickersediment

accumulationintheadditionalaccommodationonthedownthrownside(Figures18and20).

Figure18showsathickerLEFsectionduetoasyn-depositionalgrowthfault.Figure20shows

theappearanceanddisappearanceofthepeakonthedownthrownsideofthegrowthfault

(highlightedinpurple),interpretedastheseismicexpressionoftheUEF/LEFinterface.Itis

onlypresentwhentotalthicknessexceeds150ft.


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ParticularlyinterestingisanerosionaleventbestcapturedinLaSalleCounty.Whatis

interpretedasachannel,referredtohereinasthePaleo-NuecesChannel,post-datingAustin

ChalkandAnacachodepositionranthroughsouthernLaSalleCounty(Figure25)(thisstudy).

FirstdescribedbyTreadgold(2010)asagravitationalslump,furtherwelllogandseismic

analysisshowsthatameanderingchannelisanotherpossibleinterpretation.Whenthe

amplitudeisextracted,afterflatteningontheBudahorizon,fromthesurfacejustabovethe

EagleFord(yellowlineinFigure22),a6-7milewidechannelisrevealedrunningthroughthe

HawkvilleTrough(Figure25).Figure20showshighacousticimpedanceabovethetopof

EagleFordthatcannotbecorrelatedthroughthecenterofthefeature,butispresentoneither

side.ThishighacousticimpedanceisinterpretedinthisstudyastheAustinChalkformation,

andallowsageneraltimerelationshiptobedetermined.Proprietaryobligationsanddata

availabilityonlyallowsthisfeaturetobecapturedwithintheboundariesoftheHawkville

Trough,primarilyinLaSalleCountyalthoughrecentseismicanalysisshowsitextendinginto

McMullenCounty.Figure25showsahorizonthatwasextractedfromaseismicline(Figure22

yellowline)projectingitsamplitudevariationsinmapview.TheyellowlinesinFigure25

annotatetheboundariesofthechannel,withthestrikeanddipdirection(oftheEagleFord

Formation)symbolinred.Thewhitelinestothenortheastofthechannelarefaultsystemsthat

propagatethroughoutLaSalleCounty,butdonotintersectthechannelandarethusinterpreted

aspre-incisiondeformation.ThePaleo-NueceschannelisfirstobservedinsouthwestLaSalle

Countyrunningnortheast-southwestalongbeddingstrike.Updip,thechannelisnotpresent

andthetotalthicknessoftheEagleFordremainsbetween250-270feet(LaS-10inFigure10).

Downdip,thechannelispresent,shownbythepresenceofsandabovetheEagleFordand

thenoticeablymissingsectionoftheupperEagleFordentirelyandpartofthelowerEagleFord

(LaS-9infigure10).Whenthechannelmeanderssoutheast(paralleltoEagleFordbeddip),


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thestratadirectlyabovetheEagleFordFormation(AustinChalk),theupperEagleFord,and

someofthelowerEagleFordFormationareerodedoutwithinamatterof4.4miles(LaS-9in

figure10).AdistinctsandbodyisdepositedabovetheEagleFord,andthisparticularsand

bodyisuniquetotheareainbetweentheboundariesofthechannel(Figure10).Mostlikely

thisisachannelandnotaslumpduetothefactthat:1)gravitationalslumpswouldpermeate

alongbeddipinsteadofdisplayingameanderingnature;2)horizonswithinandabovethe

EagleFordFormationwouldnotbemissing,onlytiltedunlesstheslumpdisplacedthe

sedimentsasignificantdistance;3)thestrataoneithersideofthefeatureareundeformedand

presentinexpectedthicknesses;and4)mostimportantlythereisnoexplanationastowhya

sandbodythatdoesnotoccurstratigraphicallyinanyareaoftheEagleFordFormationis

presenthere,solelywithintheboundariesofthisfeature.

Basedontheisopachmaps,LaSallewasmostlikelyasiteofsedimentinfluxasseenin

loganalysis.ApproachingtheEdwardsReefMargin,dipsincreasefrom2-4degrees

north/northwestto7-12degreesnorth/northwest.ApproachingtheEdwardsreef,athinner

EagleFordsectionisencountered,ultimatelyaffectinginternalstratigraphiccorrelation(LaS-3

inFigure17).ThelowerEagleFordisconsistentlyhigherintotalorganiccarbonandinsupport

ofpreviousstudies,thedark,well-laminatedmarlsweredepositedinadeep,oxygen-starved,

outer-shelf/marginalmarinesettingduringaworldwidegreenhouseenvironment(Petrohawk

Energy,2009).Ananoxicenvironmentcoupledwithhighorganicproductivityatthesurface

wouldsupportwhytherearehighertotalorganiccarbonvaluesinthislowermember.Also,the

anoxicenvironmentcausedbytherestrictedsettingbetweenthereefboundarieswouldallow

foralargeaccumulationandpreservationoforganic-richsediments.Duringtimesofincreased

sedimentationratesand/orstormevents,breaksinthereefwouldallowclastic-richsediments


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tofloodintotheHawkvilleTrough.Proofforthistypeofsedimentationcouldbesupportedby

in-depthanalysisofcores.

ThedepositionalmodelfortheupperEagleFordintheHawkvilleTroughismoredifficult

todecipherduetoamorecomplexnetworkoferosionalfeaturesalongtheuppercontact.The

influencethatthePaleoNuecesChannelhadonthiscontactisevidentthroughseismicand

wellloganalysisinsouthernLaSalleCounty.Wherethechannelwascuttingsedimentsabove

theEagleFordalongbeddingstrike,theinfluenceofitserosionontheupperEagleFordis

largelyspeculativewithoutachronostratigraphicreconstructionofthischannel.However,the

erosionalinfluenceontheupperEagleFordisclearwhereitiscompletelymissing,theunique

sandbodyisdepositedabovethelowerEagleFord,andthecharacteristiccoarseningupward

natureofthelowerEagleFordremainsintact.


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CONCLUSIONSWelllogandseismicdatahavebeenusedtoregionallymapthefollowingEagleFord

boundariesintheHawkvillefieldinLaSalleandMcMullenCounties,Texas:(1)Unconformable

boundarybetweenthebaseoftheEagleFordandBuda,whichischaracterizedby9-15API

gammaresponse,atransitionfromastrongpeak(Buda)toastrongtrough(LEF)inseismic,

andtransitionfromlimestonetoorganic-richmarlstonelithology;(2)Conformablecontact

betweentheLEFandUEFcharacterizedbya120-140APIgammaresponse,transitionfrom

troughtopeak(wheregreaterthan150ft)inseismic,andtransitionfrommarlstoneto

interbeddedshale/limestonelithology;(3)UnconformableboundarybetweentheUEFand

AustinChalk,whichischaracterizedbya120-135APIgammaresponse,transitionfromstrong

trough(UEF)tostrongpeak(AustinChalk)inseismic,andtransitionfromtheinterbedded

shale/limestonelithologytothemassivechalk-bearinglimestonelithology.

WelllogandseismicexpressionoftheEagleFord-AustinChalkboundaryis

variable/gradationalduetolaterallyvariableerosionoftheEagleFordpriortodepositionofthe

AustinChalk.Thisvariabilityisalsoobservedincoredata,wherethesharpcontactcontains

skeletallagdepositsandrip-upclasts,andthegradationalcontactonlyshowsminorchanges

infossilcontentandlightercolorduetoincreasedlimestonecontent.Othersubdivisionsofthe

EagleFordsuggestedfromoutcropstudies(e.g.LockandPeschier,2010;Donovanand

Staerker,2010)cannotbeconsistentlyrecognizedintheHawkvilleTroughbecausethe

contrastinpropertiesbetweenlithologiescannotbedecipheredinthecurrentlogandseismic

resolution.BoththeLEFandUEFaretroughshapeddepositsthatstrikenortheastroughly

parallelwiththeEdwardsReefmargin.TheaxisofthetroughisclosertotheEdwardsReef

thantheSligoReef.MaximumthicknessoftheLEFismorethan180ftinLaSalleCountyand


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approximately140ftinMcMullenCounty.TheUEFisthinnerwithamaximumthicknessof160

ftalongtheLaSalle-McMullenCountyborder.BoththeLEFandUEFthinrapidlytohalftheir

maximumthicknesseswithin5-6milesoftheaxis.TheEagleFordalsothinsapproachingthe

reef,andtothenortheastintoMcMullenCounty.Welldatadocumentsdramaticdecreasesin

thicknessoftheUEFoverafewmilesduetoerosion.Inthemostextremecaseinsouthwest

LaSalleCounty,theUEFisentirelymissingin2wellsandhasbeenreplacedbyasandunit

notpreviouslyreported.TheLEFmayhavealsobeenpartiallyerodedinthesetwowells.

Basedonwelllogandseismicdata,depthtotheEagleFordvariesfrom9600ftto

15000ftalongaroughlyplanarsurfacethatstrikesparalleltotheEdwardsandSligoReef

margins.Numerousfaultsarevisibleonseismicdata.Mostofthesefaultsarepost-

depositionalwithmodestoffsets(25-200ft).Afewofthefaultsaresyn-depositionalgrowth

faultsandtheEagleFordisthickeronthedownthrown(Gulfward)side.Seismicdatashows

thattheEagleFordinthisregiondecreasesinthicknessfromover270fttolessthan80ftand

thatthereflector(strongpeakinFigures20-22)interpretedtobetheboundarybetweenthe

LEFandUEFdisappearsastheunitthinstolessthan150ft.Aseismictimeslicejustabove

thetopoftheEagleFordhungontheBudatoremoveeffortsofbasinwarddipshowsa

channelstructurerunningwesttoeastalongsouthernLaSalleCounty.Thischannelislikely

thesourceoferosionandsanddepositionobservedinwellsLaS-9andLaS-12.


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VITA ZachHendershottwasborninHouston,Texasin1987andattendedEpiscopalHigh

SchoolinBellaire,Texas.Aftergraduatingin2005,heleftHoustontopursueabachelors

degreeattheUniversityoftheSouthinSewanee,Tennessee.Duringhisfouryearsat

Sewanee,ZachwasamemberofSigmaAlphaEpsilonFraternityandwasinductedintothe

OrderoftheGownsmanhonorsocietyin2008.Zachworkedasanexplorationgeologyintern

duringthesummersof2007and2008forEtoco,L.P..AftergraduatingfromSewaneein2009,

ZachmovedtoNewOrleansandworkedasalegalassistantwhilestudyingforhisGREand

applyingtoLouisianaStateUniversity.Zachenrolledinthegraduateprogramtopursuehis

MastersofSciencedegreeingeologyundertheguidanceofDr.JeffreyNunnin2010.He

becamethefirstgeologyinternforPetrohawkEnergyCorporationduringtheSummerof2011

inHouston,Texas.Zachacceptedafull-timejobofferwithPetrohawkEnergyCorporationin

HoustonasanOperationsGeologistintheirEagleFordGroup.

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