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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.