OCS Report MMS 85-01 11
Geologic Report for the BEAUFORT SEA Planning Area
Alaska i
OCS Report MMS 85-0111
GEOLOGIC REPORT FOR THE
BEAUFORT SEA PLANNING AREA, ALASKA:
RegionalGeology,PetroleumGeology,EnvironmentalGeology
JAMES D. CRAIG KIRK W. SHERWOOD PETER P. JOHNSON
December 1985
UnitedStatesDepartment of the Interior Minerals Management Service Alaska OCS Region Anchorage, A1 aska I
Contents * Abstract ...... 1 Introduction...... 3 Part 1 RegionalGeology 1 GeologicalFramework ...... 7 2 Stratigraphy ...... 14 Franklinian Sequence...... 14 Ellesmerian Sequence ...... 16 Rift Sequence ...... 19 Brookian Sequence...... 20 Pleistocene Sequence ...... 21 Sequence Holocene ...... 22 Stratigraphy 3 Seismic ...... 23 NortheasternChukchi Shelf ...... 23 CentralBeaufort Shelf ...... 33 EasternBeaufort Shelf ...... 37 Part 2 Petroleum Geology 4 ExplorationHistory ...... 47 5 SourceRockson theBeaufortShelf ...... 53 Ellesmerian and Rift Sequences ...... 53 Brookian Sequence...... 57 6 GeothermalGradientsontheBeaufortShelf ...... 65 7 PotentialReservoirFormations ...... 69 Devonian Clastic RocksandCarbonates ...... 69 Kekiktuk Formation (Mississippian) ...... 70 Lisburne Group (Mississippi an to Pennsyl vaniari) . 71 EchookaFormation(Permian) ...... 72 IvishakFormation(Triassic) ...... 72 Sag RiverFormation(TriassictoJurassic) ...... 75 SimpsonandBarrowSandstones(Jurassic) ...... 75 KuparukFormation(EarlyCretaceous) ...... 76 Rift SequenceSandstones (EarlyCretaceous) ..... 77 BrookianProdeltaSandstones(Cretaceous to Tertiary)...... 82 Brooki an F1uvia1 -Deltaic Sandstones (Cretaceous to Tertiary) ...... 82 B PlayConcepts andHydrocarbonTrapConfigurations .. 84 Barrow Arch (IA) ...... 84 Outer Arctic Platform (IB) ...... 89 ChukchiShelf (IC)...... 93 Nuwuk Basin (IIA) ...... 97 Kaktovik Basin ...... 101 Camden Sector (IIB) ...... 101 Demarcation Sector (IIC) ...... 105 “7 b2
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Figure 6 Index map showing a geographicdivision of the Beaufort Sea PlanningAreaintothreesectorsfor detailed discussion...... 25
7 Regional map showingthegeologicalframeworkof the northeasternChukchi shelf...... 26 8 Acousticvelocityanalysis of seismicunitsident ified inthe northeastern Chukchi shelf sector...... 32 9 Regional map showing thegeologic frameworkof the centralBeaufort shelf...... 34
10 Regional map showingthegeologicframeworkofthe easternBeaufortshelf and onshore areas...... 38
11 Areaproposed forinclusioninSale 97 (tentatively scheduledfor1987) inrelation to tracts previously leased inSale BF-79(December 1979),Sale 71 (October 1982),Sal e 87 (August1984), and majorhydrocarbon accumulations innorthern Alaska...... 48
12 Diagram illustrating stratigraphic relationships of significant source bedson theNorth Slope of Alaska... 54
13 GeologicalcrosssectionacrossBeaufortshelf illustrating stratigraphic and structural distribution of potentialsource beds in Brookian, Rift, and Ellesmerian sequences...... 56
14 Van Krevelendiagramsforcompositionsofthemajor sourcebeds for the Prudhoe Bay oils...... 58
15 Van Krevelendiagramsforpotentialsourcebeds in theBrookiandelta and prodeltaseismic sequences in thePoint Thomson No. 2and the West Staines No. 2 wells,NorthSlope, Alaska...... 60
16 Cross plotsfor source bed identification in the West Staines No. 2, Point Thomson No.2, and Point Thomson No. 3 wells...... 61 17 Schematicgeologicalcrosssectionillustrating possible time-transgressive nature of Cenozoicsource bedsand relationship to oil window in the Kaktovik Basin...... 62
18 Geothermal gradient map forArctic plain and coastal areas,northern Alaska...... 67
V Figure19 Isopach map fornet sand in theTriassic Ivishak Formation...... 73 20 Stratigraphiccrosssectionillustratingfacies relationships within the LowerCretaceousPoint Thomson sandstones...... 78
21 Isopachandfacies map for LowerCretaceous depositionalsystem in thePoint Thomson andFlaxman Island areas...... 79
22 Schematic illustrations of play concepts and known or potentialtrapconfigurationsinEllesmerian, Rift, and Brookian strata of the Barrow Arch province (IA)... 86 23 SchematicgeologicalcrosssectionthroughMukluk structure in Barrow Arch province (IA) ...... 88 24 SchematicgeologiccrosssectionacrossDinkum infrarift graben in the OuterArcticPlatform province (18) ...... 91 25 Playconceptsandpotentialtrapconfigurationsin the Chukchi shelf province (IC) ...... 95 26 Schematicgeologicalcrosssectionsummarizing potentialplayconcepts and trapconfigurationsin the NuwukBasin...... 99 27 Schematicstructurecontour map and geologicalcross sectionsillustrating the fundamental geology of the Camden anticline in the Camden sector(IIB) of the Kaktovik Basin...... 103
28 Map illustrating major structural features of Camden andDemarcationsectors of the Kaktovik Basin...... 106
29 Bouguer gravity map fornorthern Arctic National Wildlife Refuge...... 107 30 SchematicgeologicalcrosssectionacrossDemarcation sector(IIC) of the Kaktovik Basin illustrating the geology oftheBartersubbasin and internalfolds within flanking structural highs ...... 111 31 Lithologic and wireline log profiles for the Dome NatsekE-56 well...... ,...... 113 32 Sketch of an idealtrain of concentric or parallel folds...... 115
Vi Figure 33 Map oficezonationintheBeaufort Sea Planning Area...... 123
34A,B Composite maps ofallmajoriceridgesobserved in theBeaufort Sea...... 125
35A.B Side-scansonar and fathometerrecordsoftheice- gouged seafloor offshore of CapeHalkett...... 126
36 Generalizeddistributionof ice-gougedensity in the Beaufort Sea Planning Area...... 127
37 Currentpatterns and regionsofstrudelscour on the AlaskanBeaufort and northeasternChukchi shelves...... 131
38 Isopach map (in meters) of the thickness of the shallow,seismicallytransparentunitinterpretedto beHolocene in age, and ratesof coastal erosion in meters/year...... 133 39 Map showingthe generalized distribution of coarse- grained(sand and gravel) and fine-grained(silt and clay)surfacesediments inthe Beaufort Sea Planning Area...... 136
40 Map showing the generalized distribution of surficial gravel(greaterthan 2mn) intheBeaufort Sea Planning Area...... 137
41 FathometerprofilefromHarrison Bay showing a hydraulic bed formmigratingover an ice-gouged seafloor...... 139
42 Map showingthe distribution of expandable clays in the Beaufort Sea Planning Area...... 140
43 Uniboom profile showing a Pleistocenesubbottomridge probablyformed at the northern edge of the late Wisconsin regression...... 144 44 USGS uniboom line fromtheBarterIslandareashowing shallow dipping bedrock of probable Tertiary age intersecting or nearly intersecting the seafloor...... 145
45 Map showingthe known distributions of high-velocity materialinferredtobeice-bondedsedimentsfrom Harrison Bay to the Canning River...... 147
46 Summary ofthedepthtoice-bondedpermafrostalong the OCSEAP study line near Prudhoe Bay...... 148
v.42 Figure 47 Map showingthe distribution of shallow gas concentrations,the minimum areainferred to be underlainbynatural-gashydrates, and the distributionof diapiric structures in the Beaufort Sea Planning Area...... 150 48 Map showingareaswhereshallowfaultsare mapped or expected inthe Beaufort Sea Planning Areaand the distributionof earthquake epicenters...... 152
49 Map showing the distribution of inferred shallow gas and shallow faults in Harrison Bay...... 153 50 Map showingthe distribution of shallow faults, fold axes,and earthquakeepicentersinthe Camden Bay area. 154
51 Map showingearthquakeepicenters innorthernAlaska locatedbylocalseismographicnetworksbetween 1968 and 1978...... 155
52 Uniboom profilefrom Camden Bay showingshallow faults offsetting Quaternary and older sediments...... 156
53A,BMaps showing thelocation ofthe blockglide terrane and bedding-plane slide terrane on the Beaufort Sea she1 f andslope...... 158 54A,B Uniboom profileacrossthewesternBeaufort block glide terrane and sing1e-channel air gun recordacrosstheoutershelf,Beaufort Ramp, and upperslopeslump terrane...... 159
55 USGS uniboom profile from northwestofDemarcation Bay showing a wipe-out zone inferred to becaused by gas-charged sediments...... 162
Plates
Plate 1 Representativeseismicprofile(courtesyofWestern Geophysical Company) extendingfromtheColvilleBasin totheNortheast ChukchiBasin
2 A representativeseismicprofile (Husky-Geophysical Service,Inc.,LineEl-74-1178) on thenorthernArctic Platform in the vicinity of Barrow Plate 3 Representativeseismicprofile(courtesyofWestern Geophysical Company) acrosstheNortheastChukchi Basin and parallel to the NW-SE strike of the Barrow Arch
4 A representativeseismicprofile(courtesyofWestern Geophysical Company) illustratingthe stratigraphy and structuralcharacter of the Arctic Platform and Nuwuk Basin
5 A representativeseismicprofile(courtesy of Western Geophysical Company) showing the thinning of the Ellesmeriansequenceoverthe Mi kkelsen high and the ‘stratigraphy of the LowerCretaceous Rift sequence in the Dinkumgraben
6 Representativeseismicprofile(courtesy of Western Geophysical Company) illustrating the stratigraphy of thewesternKaktovikBasin (Camden sector) and the structuralcharacter of the Camden anticline
7 Representativeseismicprofile(courtesyofWestern Geophysical Company) illustrating the complex structure and stratigraphyof the eastern Kaktovik Basin(Demarcationsector)
8 Sourcerockgeochemical profilefortheMobil West Staines No. 2 well
9 Sourcerockgeochemicalprofileforthe EXXON Point Thomson No. 2 well
10 Sourcerockgeochemicalprofileforthe EXXON Point Thomson No. 3 well
11Bathymetric map oftheBeaufort Sea PlanningArea
Tab1es
Table 1 Petroleumprovinces of theBeaufort Sea Planning Area.. 10
2 Summary and playanalysisforBarrowArchprovince (IA)...... 85 3 Summary and playanalysisforOuterArcticPlatform province(16) ...... 90
ix Table 4 Sumnary and playanalysisforChukchishelfprovince (IC) ...... 94 5 Sumnary and playanalysisfor Nuwuk Basinprovince (IIA) ...... 98 6 Sumnary and playanalysisfor Camden sector of Kaktovik Basinprovince(IIB) ...... 102 7 Summary and playanalysisforDemarcationsectorof KaktovikBasinprovince(IIC) ...... 110
English-MetricConversion
(The followingtable gives the factors used to convert English units to metricunits.)
~ ~~
multiplyEnglishunits by obtainmetricunits to
inches 2.5400 centimeters feet 0.3048 meters miles(statute) 1.6093 kilometers miles square 2.5899 squarekilometers acres 0.4047 hectares barrels (U.S. petroleum) 158.9828 1iters 0.1589 cubic meters feet cubic 0.0283 cubicmeters knots 1.8520 kilometersper hour
To convertfromFahrenheit (OF) toCelsius("C),subtract 32 then divide by 1.8.
x i
Abstract
The FederalOuterContinentalShelf (OCS) Beaufort Sea Planning Areaextendsapproximately 500 miles along thenorthern continental marginofAlaskafromthe Canadianborder to 162" west longitude, where it meets the Chukchi Sea OCS Planning Area. The planningarea includes continental she1 f , slope, and abyssalplainphysiographic provinces. The Beaufortcontinentalshelf is relativelynarrow, and most oftheplanningarealiesintheabyssalplainoftheArctic Ocean. Forgeological and logisticalreasons,onlythecontinental shelf is thought to haveany realistic potential for economic accumulationsofhydrocarbons.Leaseblocks on theshelfare tentativelyscheduled for public offering at lease Sale 97.
NorthernAlaska is divisible into twomajorgeologicprovinces: (1) a landwardprovincecontainingPaleozoic andMesozoicrocks underlain by continentalcrust and (2) an offshoreprovincecontaining a thick clastic wedge of CretaceousandTertiarysedimentsdeposited on thesubsidingcontinentalmarginunderlain by transitional to oceaniccrust. The gentlysouthward-dippingsurfaceofthecontinental basementcomplex, termedthe"ArcticPlatform," is separatedfrom thepost-Jurassic continentalmarginalong a highlyfaultedflexure termedthe"Hinge Line."
Acoustic andeconomicbasement of the Arctic Platform consists of a metamorphiccomplex(theFrankliniansequence)formed bya regionalorogeny in Devoniantime. The basementcomplex isoverlain by Middle(?)Devonian to LowerCretaceous strata(theEllesmerian sequence)deposited in a stableshelfsetting. The Ellesmerian sequence generallythins northward toward the orogenic terrane which existed before the rifting of the Beaufort continental margin inEarly Cretaceoustime. A structurally anomalous basin(informally termedthe"NortheastChukchiBasin")containinggreaterthan 30,000 feetofPaleozoicsedimentsunderliesthenortheastern Chukchi shelf. These lowerEllesmeriansedimentarydepositsare juxtaposedwiththe basementcomplex ofthe Arctic Platform across a poorlyresolvedfault zonetermedthe"Barrowfault."Ellesmerian sedimentationterminated in Early Cretaceoustime with the uplift of an incipient rift zone in the vicinity of the present continental margin.Thisuplift andassociatederosionproduced a regionally extensiveunconfonnity that truncated the Ellesmerian sequenceon some onshoreandmostoffshorepartsoftheArcticPlatform. Extensionaltectonicsearlyintheriftingepisodeproducedgrabens whichwere filled with LowerCretaceoussediments(the Rift sequence) derivedfromnearbyupliftedblocks. A broad,SE-plunging structuralhigh,the "BarrowArch," was created by thesubsidenceof flankingbasins(Colville and Nuwuk) followingcontinentalbreakup. SubsidenceoftheoffshorecontinentalmargininCretaceous and Tertiarytimecreated deep structuralbasins (Nuwuk andKaktovik Basins)beneaththepresentBeaufortshelf. An immense clastic wedge (theBrookiansequence)progradednorthwardfromtheBrooks Range orogenicbeltintothesedepocenters.
All NorthSlopeproducingoilfieldsoccurwithinthe Ellesmerian sequence,and any areaofnorthernAlaska wherethese rocksoccur is consideredhighlyprospective. The northernmost parts of the Arctic Platform are considered to be lessprospective becauseEllesmerianrocksareabsent asa result of northward pinch-out by onlap and erosionatoverlyingunconformities. Paleozoic strata in theNortheast ChukchiBasinareinvolved in numerous fault and foldstructures, and thepotential for significant hydrocarbonaccumulations isaccordinglyhigh.Untested Rift sequencerocksdeposited in infrarift grabens on thenorthern ArcticPlatform havegood reservoir and sourcerockpotential. Numerous structural and stratigraphic traps may exist within the deep basinsseaward.of the HingeLine filled withBrookianclastic units.Potentialreservoirsinthisprogradingclastic wedge are likely to be deltaic or basinalsandstones,whichsuggeststhat individualreservoirs may be thin and lenticular.Rotational foldsassociated with listric faulting near the basin margins are the most attractiveexploration objectives in the western Beaufort (Nuwuk Basin); compressionalfolds and fault traps form the most prevalentplays inthe eastern Beaufort (Kaktovik Basin).
Geologicfeatures andprocesseswhich may affectpetroleum• related activities in theplanningareainclude a mobile and pervasiveicecover,active seabed scouring by ice and currents, unstableseafloorsediments,massiveslumps on the outer shelf, subseapermafrost,shallow gas accumulations,abnormalformation pressure,near-seafl oor faults , andmodern seismicity.Ice movement may exertgreatstresses on platforms as well as on the seabed. Ice gouging on theshelf may necessitate burial of pipelines andwellheads. Strudelscouringoftheseafloornearthemouthsofriversisactive duringspringfloodperiods. Subsea permafrosthas been confirmed in severalnearshoreareas and presentsuniqueengineeringproblems forfoundations,gravelexcavation, and pipelinerouting.Shallow gas may be trapped in several ways and presents a seriousdrilling hazard,particularlyinareasdissected bynear-seafloorfaults. Abnormallyhighformationpressureshave been encountered in Cenozoic sedimentarybasinsineasternAlaska and theBeaufort Sea. Earthquake activity hasbeendocumented on theeasternBeaufortshelf and may berelated to ongoing tectonism in the northeastern Brooks Range. Surfacefaultdisplacements andmassiveslumps on theouterBeaufort shelf may be triggered by theseshallow-focus,moderate-magnitude earthquakes.
2 Introduction
Thisreport is a smmary ofthe geologic framework,hydrocarbon potential, and environmentalconditions inthe Beaufort Sea Planning Area. It was preparedaspartofthesupportdocumentationrelated to Federal OCS LeaseSale 97, tentativelyscheduledfor 1987. The reportfocuses specifically on thepetroleum geology of this highly prospective area. The discussion of offshore geology is based largely on seismic reflectiondata collected duringregionalsurveys byWesternGeophysical Company (WGC), whichhasgenerouslyallowed us to useselected seismic lines to illustrate the regional geology discussedinthisreport. WGC and otherindustry-acquiredseismic datagenerallydisplayhigherresolution,are more accurately navigated, and are far denser in coveragethanexisting, publicly available surveys used in previousgeologicreportsfor thisplanning area(Grantz and others,1982b).Publiclyreleasedexplorationwells are fairly abundanton theNorthSlope and offshore in Canadian waters, and thesewellsprovideregionalstratigraphiccontrolfor ourseismicinterpretation. However, theBeaufort Sea Planning Area is truly a frontierexplorationprovince.Seismicdatacoverage is sparse in some areas, andonshorestratigraphy may varyconsiderably from that offshore. The few offshore exploratory wells drilled in previous saleareas are proprietary at thepresent time, and there are nodeep stratigraphic test wells (referred to as COST wells) on theBeaufortshelf. Our geologicanalysis,basedalmostentirely on seismicdata, is therefore somewhat speculative and will surely be modifiedasoffshorewellinformationisreleased.
The Beaufort Sea Planning Area encompasses the entire continental marginofnorthern Alaskaandpartoftheabyssal Canada Basin(plate 11). It stretches in longitude for approximately 500 miles from just eastof 141' west, thedisputed border with Canada, to 162" west, where it meetstheChukchi Sea Planning Area. On thebasisof projectedindustryinterest,futuretechnologydevelopment, and economic feasibility,theNationalPetroleumCouncil(1981)concluded that offshore exploration in the Arctic will be confined, in the foreseeablefuture, to the continental shelf wherewaterdepths arelessthanabout 200 feet. The seriouslogisticaldifficulties associatedwiththeArcticicepack,whichcoverstheareamostof theyear,areofprimaryconcern.Sincegeological and logistical considerationssuggestthatonlythecontinentalshelf haseconomic hydrocarbonpotential,mostofthisreport will focus on thegeology oftheshelfarea.IntheChukchi Sea, approximately 60 percent ofthe total planning area lies on thecontinental shelf; in the Beaufort Sea, only 25 percent of the planning area is located on theshelf. The potentialoccurrence andeconomic significanceof nonenergyminerals(gravelandmetalplacers)arenotaddressed.
Despiteitsremote setting, hostile climate, and politically sensitive environmental issues, northern Alaskahasexperienced a greatdealofpetroleum-relatedactivityforseveral decades. The largest andsecond largest oil fields in North America(Prudhoe Bay andKuparuk, respectively)are currently producing over 1.5 million barrels of oil per day throughtheTrans-AlaskaPipelineSystem (TAPS). Several peripheralfields (Endicott andSealIsland)are considered to be of commercialsize and may bedeveloped in the near future.Therehave been threeFederal OCS leasesaleswhichhave receivedover $4 billion in bonus bids. At thepresenttime,there areapproximately 376 Federalleases in the Beaufort Sea Planning Area.
4
I
1
Geological Framework
The northernAlaskaregion, includingthe offshore continental margin, canbedividedintotwo mainpetroleum provinces: an older southernprovincecontainingstratadepositedon a continental basementcomplex (ArcticPlatform,province I), and a younger northernprovince containing strata deposited in deep basins on thecontinentalmargin(Brookianbasins,province 11) (fig.1). The dividing line betweenthesemainprovinces lies offshore along a zone of down-to-the-north basement faultstermed the"HingeLine" byGrantz and others(1982b). The HingeLinemarks thesouthern edge of a major rift zone thatdeveloped in Late Jurassic to Early Cretaceoustime.South oftheHingeLine,the basement surface forms a broadplatform(the"ArcticPlatform")thatdipssouthward away from northern tectonic highlands which existed before continental rifting. The present.structura1 relief of this continental basement complex (fig. 2) islargely the result of coeval compressional tectonics in the Brooks Range and rifting of the Arctic Platform sinceLateJurassicorEarlyCretaceoustime. A prominentstructural ridge (the "BarrowArch")trends NW-SE alongthe Beaufort coastline andacrosstheeasternChukchishelf(fig.1).Thisregional feature,formedafterMesozoiccontinentalrifting,separates 'a continentalforedeep(theColvilleBasin)frombasinsonthesubsiding continentalmargin (Nuwuk and KaktovikBasins). A thirdpetroleum province is located seaward of the present continental she1 f (Canada Basin,province 111). Thismodernoceanicbasincontains a relatively youngsedimentary fill and is thought to have little economic potentialforhydrocarbonsintheforeseeablefuture. The general characteristicsof these petroleum provinces are summarized in table 1.
The stratigraphyofnorthernAlaskacan be dividedinto four mainsequencesbyprominentunconformitieswhichmarkregional tectonicepisodes. From oldesttoyoungest,thesearethe Franklinian,Ellesmerian, Rift, andBrookiansequences. Each ofthese sedimentarysequenceshas a uniquesourcearea,depositional environment,andstructuralcharacter.Additionalangular unconformities were identifiedin the seismic data; however,because theyrepresentlocaltectonic movements orsea-levelfluctuation events, a furthersubdivisionofthemainregionaltectonostratigraphic sequences is not warranted. The mostprominent oftheseunconformities
GeoLogid Ftramewohk, 7 /" NPRA
Figure 1. Petroleumprovinces inthe Beaufort Sea PlanningArea.ArcticPlatformprovinces(Chukchishelf,BarrowArch, andOuterArcticPlatform--province I) aregeologicbasinsformed in mid-Paleozoic to mid-Mesozoictime on a continental basementcomplex. The HingeLine is thecrustalflexurealongthecontinentalmarginformedafter mid-Mesozoic rifting. Post-breakupbasinsalongthesubsidingcontinentalmargin (Nuwuk andKaktovikBasins-• province 11) containthicksectionsof CretaceoustoTertiary clastic sediments beneaththepresentBeaufort shelf. The Canada Basin(province 111) is anoceanicbasinnorthoftheBeaufortshelf edge. These petroleum provincesarealsodescribedintable 1.
I .1 ' i Flgure 2. Structurecontours on acoustic basement, representingthe top of theFrankliniansequence.This basement complex is composed of Middle Devonian and older strata that were structurally deformed and metamorphosed duringseveral earlyPaleozoictectonicepisodes. The basement rocks are separated from overlyingPaleozoicthroughTertiary strata by a regionalangularunconformity. Onshore contours were taken, in part, from a similar map presentedin Jackson and others (1981). Offshorecontours were derived from seismicdata. Contour linesarelabeledin thousands of feet below sealevel. Major fault zones are shown, withhachures toward the downthrown block. Table 1. Petroleumprovinces inthe Beaufort Sea Planning Area.
PetroleumProvincesGeologic PhysiographicSetting Prospective MostSetting Sequence
I. ARCTIC PLATFORM
A. BarrowArch Northern Arctic Platform Inner Beaufort shelf; Ellesmerian south ofzero Ellesmerian water depth <20 m. 1ine.
B. OuterArctic Between Ellesmerianzero MiddleouterBeaufortto Rift P1Hinge(south)shelf;waterdepthsatformandline Line(north). 20-100 m.
C. ChukchiShelfSouth of HingeChukchiLine she1f; water El1 esmeri an westand of Barrowdepths20-100 m. fault.
11. BROOKIAN BASINS
A. Nuwuk HingeouterBasinMiddleBeaufortLineNorth ofto Brookian andsouthpresent of shelf; water depths shelf edge in western 20-100 m. Beaufort Sea.
B. KaktovikBasin-HingeNorthLineof and MiddleouterBeaufortto Brookian Camden Sectorpresentsouthof shelfshelf; water depths edge in easternBeaufort 20-100 m. Sea.
C. KaktovikBasin-presentSouthofshelfEntire continental shelf;Brookian DemarcationSector edgeand east of Barterwater depths 10-100 m. Island.
111. CANAOA BASIN North of present shelf Continentalslope, rise , Unknown edge. abyssalplain;waterdepths 100-3,800 m. are a Permian unconformity (PU) in the westernBeaufort and Chukchi sectors, a Miocene unconformity (MU) intheeasternBeaufort, and a shelf-widePleistoceneunconformity (QUI. A generalized stratigraphic column of the majorrock units foundonshore and their relationshiptooffshoreseismicsequences is shown in figure 3. The followingdescriptionsof rock unit age and lithology are drawn from numerous previousstudies and publjclyavailable NorthSlope wells (Brosgb and Tailleur, 1971; Brosge and Dutro,1973;Detterman and others, 1975; Carter and others, 1977; TetraTech,Inc.,1982; Mol enaar, 1983). InArctic NorthAmerica,an earlyPaleozoicsedimentarysequence (the Franklinian sequence) was severely deformed and mildly metamorphosed during a regional orogeny in Middle(?)toLate Devonian time. This metamorphic complex forms the present-daybasement. The Ellesmeriansequence (Middle(?) Devonian to EarlyCretaceous) unconformably overlies the Franklinian basement complex and represents deposition on a stableepicontinentalshelfthatpersisted until the onset of continental rifting which began in latest Jurassic time (Grantz and May, 1982). The post-Jurassic developmentof the Beaufort continental margin generally foll owed the rift evolution model proposed by Falvey (1974). The "intracratonicbasin"(Ellesmerian sequence) was uplifted in the incipient rift zone prior to continental breakup and truncated by a "rift-onset unconformity" (the Lower Cretaceousunconformity,or LCU). Initial grabenformation and deposition of "rift valley" sediments (the Rift sequence)before continentalfragmentation was followed by the progradation of a thick clastic wedge (the Brookiansequence) intobasinsalong the newly formed continentalmargin. The Riftsequence(EarlyCretaceous) and erosionalremnants of infrarifthighlandsareseparated from the overlying Brookian clastic wedge by a "breakupunconformity" (BU, fig.3). The Brookiansequence(EarlyCretaceousthrough Cenozoic) on the North Slopeconsists of clastic sediments derived from a contemporaneous orogenic belt (the Brooks Range) to the south. These evolutionarystages and their associatedsedimentarydeposits are illustrated by figure 4. a
+CANAOlANBEAUFOf $X- TAGLU (MACKENZIEOELTP
UGNU WESTSAK 0 UMIAT
.PARSONS ~ACKENZIEOELTP * POINTTHOMSON KUPARUK * MILNE PT./ GWYOYR BAY
0. BARROW GAS FIELI
PRUDHOE BAY *SEAL ISLAND
* PRUDHOE BAY
4 ENDICOTT
0 OILFIELD & GAS FIELD EROSIONAL f OILAN0GAS CONGLOMERATE LIMESTONE .FIFI n UNCONFORMITY ARGILLITE @JOOLOMITE Figure 3. Generalizedlithostratigraphic columnshowingtherelationshipofonshore rockunitsinnorthernAlaskatooffshoreseismic sequences in theBeaufort Sea PlanningArea.Significanthydrocarbondiscoveries innorthernAlaska and Canada are shown bytheirreservoirformations. NORTH SOUTH
. EarlyCretaceous
BASINCOLVILLERANGE BROOKS RIFTZONE
:. EarlyCretaceous - Tertiary SLOPEBROOKSNORTH RANGE BEAUFORTSEA BARROWHINGE MODERN
OCEANICCRUST ,,? ,,? ____ F = FRANKLINIAN SEQUENCE 1 SEDIMENT , E - ELLESMERIAN SEQUENCE SOURCE , Y'
R =SEQUENCERIFT I_ SEALEVEL
B = BROOKIAN SEQUENCE
Figure 4. Generalizedgeologicevolution of northern Alaska.Paleogeographicrelief is diagramnatically shown duringthetimeperiodsrepresented by the Ellesmerian, Rift, and Brookianstratigraphic Sequences. Activedepocenters areindicated by shading. 2
Stratigraphy
FRANKLINIAN SEQUENCE
A diverse assemblage of highly deformed,low-grademetamorphic rocks,predominantlysedimentaryandcarbonatelithologies,generally constitutesacoustic basement innorthernAlaska.This basement complex is composed of steeply dipping Middle Devonian and older rocks beneath a regionalangularunconformity (EU, fig. 3). LowerPaleozoic rocks innorthern Alaska are thought to represent sedimentation in a circum-Arcticbasin (referred to as theFranklinianGeosyncline) that generally deepened to the north andhadbroadshelfareas to thesouth(Churkin,1973).Thesedepositshavebeenreferredto as the NeruokpukFormation in the Brooks Range (Dutro andothers, 1972)as well as theFranklinian sequence in the Canadian Arctic Archipelago(Lerand,1973). We will referto the basementcomplex as the Franklinian sequence in conformance with prior usage for the AlaskanBeaufortarea(Grantzandothers, 1982a,1982b; Grantzand May, 1982, 1984).
The internalstratigraphy of theFranklinian sequence in northern Alaska is partially known bothfrom outcrops in the Brooks,Range andfrom we1 1 localities throughout the North Slope (Brosge and Dutro, 1973; Churkin,1973).Originaldepositionalrelationships areusually obscured by later tectonic overprinting, and Franklinian rocksare better known in areas in northern Canada outsidethe influenceof Cordilleran tectonics (Cookand Aitken,1973;Churkin, 1973; Trettin, 1973). Typically,fossiliferouslowerPaleozoicstrata lie unconformably on Precambrian quartzite and schist in the northeasternBrooks Range (Outroandothers,1972). The Cambrian andOrdovicianrocks,particularlyinthe northernmost partsof Canada andAlaska,arecharacterizedbybasinaldeposits,including graptoliticshale and chert.SiluriantoMiddleDevonianrocks typically consist of platformcarbonates, particularly to the south (Brosg6 and Dutro,1973).MiddletoUpperDevonianrocksreflect a majortectonic episode (perhaps correlative to the Ellesmerian orogeny)andare composed ofcoarse clastic deposits and intrusive plutons(Dutro,1981).Sedimentation intheFrankliniangeosyncline was terminatedbythisorogeny, and theselowerPaleozoicdeposits weresubsequentlydeformedanduplifted. The tectonichighland formedby thismid-Paleozoicepisodeserved as thesourceprovince for succeedingEllesmeriandepositsuntilEarlyCretaceoustime.
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Due to similarities of lithology and regionalvariations in tectonic alteration caused by themid-Paleozoicorogeny,the relationship of Devonian rock units to the Franklinian orEllesmerian seismicsequences is ambiguous. Our broad division of seismic units is intendedtoseparatethehighly deformed metamorphicbasement rock (theFranklinian sequence) from unconformably overlying miogeosynclinalstrata (theEllesmeriansequence) withoutimplying strictprovincial age definitions.Grantz and others (1982b) assignedallpre-Mississippian rock totheFrankliniansequence, followingthestratigraphicnomenclaturedeveloped in the Canadian Arctic by Lerand (1973). This is clearlyvalidforthe North Slope of Alaska where a Mississippian clastic unit (the Kekiktuk Conglomerate) unconformably over1 ies metamorphic argil 1i te of Ordovician toSilurianage(Carter and Laufeld,1975). On the northernArcticPlatform,acoustic basement in seismicdataclosely coincides with highly deformed argillite encountered in coastal wells. However, in thecentral and southwestern Brooks Range, an allochthonoussectionof Upper Devonian marine and fluvial rocks (Hunt Fork Shale and KanayutConglomerate, respectively) lies with angular unconformity on Middle Devonian and older strata (Brosg6 and Dutro, 1973, fig.10). These allochthonous clastic deposits are conformably overlain by parautochthonousMississippianstrata of the Endicott Group (Brosg6 and Tailleur, 1971; Nilsen andMoore, 1982b). Clearly, the effects of themid-Paleozoicorogenyareless pronounced on Devonian and older rocks farther to the south, away from the northernfoldbelt, in Canada and Alaska (Bell,1973).Consequently, the distinction between theFranklinian and Ellesmeriansequences, based on a criterion of deformed versus undeformed character, becomes lessapparent in thesesouthernareas. In the Brooks Range, this distinction is further obscured by Mesozoic to Cenozoicorogenic thrusting. Our preliminary conclusion is that the unconformi ty separating the Franklinian basement complex and theEllesmerianmiogeosynclinal sequence is diachronous,ranging in possibleage from Middle Devonian to EarlyMississippian. This is based on thefollowingobservations in northern Alaska: (1) theyoungestreportedagefortectonized rock below the regional basementunconformity (EU) is Middle Devonian or older from the metasedimentary strata in the Topagoruk well (Collins, 1961); and (2) theoldest reported age for strata unconformably overlyinghighly deformed lowerPaleozoic rock is Middle(?) to Late Devonian in the Endicott Group (Dutro,1981). The hiatusrepresented by this unconformity (EU) and the tectonic alteration of underlying rock units generallyincreasetowards the northernfoldbelt. The diachronousnature of the basementunconformity is further enhanced by youngerunconformities which have exhumed basement surface in northernparts of theArcticPlatform. In many places,strataranging in age from Mississippian to Tertiary lie directly on thelower Paleozoic basement complex. Our broad divisionoftectonostratigraphic sequences is based on seismicallyrecognizablecharacteristics, and the bounding unconformities of these units arerecognized to be timetransgressive. The seismicsequences,therefore, may not coincide with strict formationaldefinitionsbased on stratigraphic age. h
ELLESMERIAN SEQUENCE
The Ellesmerian sequence (Middle(?)DevoniantoEarlyCretaceous) is bounded at its base by an angular unconformity on the Frank1 inian basementcomplex (ELI) and at its top by the rift-onset unconformity (LCU, fig. 3). Ellesmeriansedimentswerederivedfrom a terrane composed of deformed Franklinian rocks that existed north of the presentBeaufortcoastline.Coarse-grained,proximalsediments depositedon an epicontinentalshelf(theArcticPlatform)adjacent tothenorthernsourceterranegenerallygradesouthwardinto fine-grainedbasinalfacies. The Ellesmeriandepocenter(alsotermed the"ArcticAlaskaBasin") deepened,to thesouthbeneaththepresent locationoftheBrooks Range (BrosgeandTailleur,1971)(fig.4). Sedimentationthroughoutthisperiodfollowed a generalpattern of progressivemarineonlap(transgression)tothenorthinterrupted bybriefregressivepulseswhichprogradedsouth.Tectonicactivity on the Arctic Platform was largely restricted to post-orogenic block faulting in Late Devonian to Late Mississippian time (Tetra Tech, 1982). Ellesmerian sedimentationterminated in EarlyCretaceous timeas thenorthern sourceterrane was rifted away fromnorthern Alaskaduringtheformation of the modern Canada Basin(Grantzand May, 1982). In comnon withriftingepisodes on othercontinental margins(Falvey,1974), a regional uplift of the incipient rift zone is markedbyanextensiveerosionalunconformity(the LCU) which truncatedsouthward-dippingEllesmerianformations. The present distribution of Ellesmerian units is controlled by northward depositionalthinning as well aserosionaltruncation.Ellesmerian• unitzero-lines generally trend northwest-southeast along the coastal and innershelf areas of the Beaufort Sea (fig. 1).
Ellesmerianformations contain much of the known hydrocarbon reserveson the North Slope and are consequently the mostthoroughly studiedpartofthegeologic section innorthernAlaska(Brosg6 and Tailleur, 1971; Morgridgeand Smith, 1972; Dettermanandothers, 1975; Carter andothers, 1977;Jamisonandothers, 1980; Tetra Tech, 1982). Our briefdescriptionof the lithology and depositional setting of these formations is based largely on theseprevious studies.
The first Ellesmeriandepositional cycle occurred in Late Devonian to Late Mississippian time, when coarse clastic sediment was shedfromtheorogenicfoldbeltwhichlaytothenorth(fig. 4). Fault-boundedbasins,suchastheNortheastChukchiandIkpikpuk Basins,contain thickaccumulationsofthesecoarse clastic deposits. As a consequence ofthe active tectonic setting, the thickness and regional distribution of these lower Ellesmerian rocks are more irregularthanthoseofMesozoic-ageupperEllesmerianunits. The post-orogenicclastic deposits include both marine units (for example, HuntForkand Kayak Shales)and fluvialunits (Kanayut and Kekiktuk Conglomerates)assigned to the Endicott Group(Brosg6andTailleur, 19711. Although somewhat obscured by later northward thrusting in
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theBrooks Range, the deposition of Endicott clastic units seems to follow a generaltime-transgressivetrendinwhicholder(LateDevonian) and thicker units occur to the south, whereasyounger(Mississippian) and thinner units occur on thenorthern Arctic Platform.
An exceptionto this regional stratigraphic trend is observed in seismicdatafromtheChukchishelfarea.There, a thickclastic wedge, tentatively correlated to the Endicott Group, is underlain bya seismic unit inferred from interval velocity data to becomposed of carbonaterock.This isthe only known occurrenceof a thick carbonate unit stratigraphically interposed between theEndicott Groupand acoustic(presumablymetamorphic) basement northof the Brooks Range. The stratigraphicrelationshiptotheunderlying basementcomplex(assumed to beMiddleDevonianorolder)and overlying Endicott-equivalent clastic wedge (LateDevonian to Late Mississippian)suggests that this locally restricted carbonateunit may beage-equivalentto carbonate rocks of the Baird Groupdescribed inthesouthwesternBrooks Range (Tailleur andBrosg6,1970). We include this carbonate unit in the lower Ellesmerian sequence because it is relatively undeformedand is closely associated with inferredEndicott-equivalentstrata. It mustbeemphasized that thelower Ellesmerian sequenceobserved in the Northeast Chukchi Basin is anomalous with respect to thickness andapparent ageas compared with traditional Ellesmerian units on the Arctic Platform to the east.
Deposition of the Endicott Group was followed by a slowmarine transgression andsubsequentaccumulationofplatformcarbonates assigned tothe Lisburne Group. A diverseassortmentofshallow marinecarbonate facies progressively onlapped existing basement highs,withtime-transgressivedepositionprogressingnorthwardfrom LateMississippian to Pennsylvanian time (Armstrong and Bird,1976). AlthoughtheLisburne Group is wider in regional distribution than theunderlying Endicott Group, itsoverall thickness increases significantly in fault-bounded basins that continued to subside throughLateMississippiantime(Tetra Tech, 1982).
In Late Pennsylvanian to Early Permiantime,erosion of Carboniferousand older strata occurred in response tosea-level loweringorepeirogenicelevationoftheArcticPlatform. The effects of this episode are particularly pronounced on positive basement features(ArmstrongandBird, 1976; Carterandothers, 1977; Dutro, 1981; Tetra Tech, 1982)andon thenorthernChukchi shelf, where it dividesthe Ellesmerian section into distinct upper (Permian to Early Cretaceous) andlower(Middle(?) Devonian to Permian) seismic sequences. We refer to this unconformity as thePermian unconformity (PU, fig. 3) and believe it is correlative to the "Echooka unconformity" as definedbyTetra Tech(1982) in NPRA. Grantzandothers(1982a)alsorecognizedthisunconformity in the Chukchishelfregionandtermedtheunderlying,pre-Permiansection "Eo-Ellesmerian." The upperEllesmeriantransgressivecycle began in Late Permian time,withmarinesandstones and shales assigned to the Sadlerochi t Group onlappingstructural highs from southtonorth. The basal transgressivesandstones of the Echooka Formation (LatePermian) . are overlain by thedeepwatershales of the Kavik Formation(Early Triassic). Inthe upper Sadlerochit Group, fluvial-deltaicdeposits of the IvishakFormation(Early to Middle Triassic)represent a progradationalpulse from thesourceterranetothenortheast. In thesoutherndistalportions of thebasin,time-equivalentrocksare composed of a deepwater facies known as the Siksikpuk Formation. InMiddle to Late Triassic time, a secondmarinetransgression extendednorth of thepresentBeaufortcoastline.Limestone, mudstone, sandstone, and phosphatic beds assignedtotheShublik Formation were deposited in a low-energy,moderatelydeepwater environment(Detterman and others,1975). The Shublik Formation is overlain by well-sorted,fine-grained, glauconitic sandstones and interbedded marine shales of the Sag River Formation (LateTriassictoEarlyJurassic). Sag Riversandstone lensesprobablyrepresent a barrier beach complex that prograded to the southoverdeeperwaterShubliksediments(Jones and Speers, 1976). The upper Ellesmeriantransgressionreached its maximum extent in Jurassic time when the Kingak Formation was deposited in a moderatelydeepwater setting throughoutnorthernAlaska and Canada (Poul ton, 1982). Lithologicstudies and organicchemistryanalyses suggest that the Kingak shelfshoaled to the northwest in Alaska, resulting in an overallthinner and more sand-prone facies, while tothe southeast a thick,organic-richshalefaciesaccumulated (Magoon and Claypool,1984). In western NPRA, transgressivemarine shelfsandstonesareinterbedded with Kingak shales. These lenses areshingled above one another, progressively shiftinglocalized sandstonedeposition to the northwest in the Lower to Middle Jurassic section(Tetra Tech, 1982). In Late Jurassic to Early Cretaceous time, the northern part oftheArcticPlatform began toshoal,probably in response to thermal uplift of the incipient rift zone(byanalogy to the Falvey model). Locallyderived,shallowmarinesandstonelensesarelocally common in the uppermost Kingak Formation,particularly above basement highs. These discontinuoussandstonebodies have been informally referredto as "Kuparuk River sands.'' .In the type area(the Kuparuk oilfield), sandstone beds assignedto the Kuparuk Formation (Carman and Hardwick, 1983) lie above and below the regionalrift-onset unconformity (LCU). Inthepresentreport, the usage of "Kuparuk Riversands" isrestrictedtosandstonesimnediately below the LCU in theupperEllesmeriansequence. Lower Cretaceoussandstones above the regional unconformi ty are included in ourRiftsequence. RIFT SEQUENCE
PreviousgeologicstudiesofnorthernAlaskahaveusuallydivided thePhanerozoicsectionintothreemain sequences (Franklinian, Ellesmerian. and Brookian). However, thesestudies have often disagreed on theboundarybetweentheEllesmerianandBrookian sequencesas well as theplacementofLowerCretaceous(generally Neocomian) strata. TheNeocomian section,oftenreferredto as the "PebbleShaleunit,"hasbeenincluded in theEllesmerian sequence (Grantzandothers, 1982b), the Brookian sequence (Tetra Tech, 1982). orconsidered asa separate "Barrovian" sequence (Carman andHardwick, 1983).Because the Neocomian sectionisdistinctlybracketed by regionalunconformities and, inthe offshore area, itsthicknessand depositional setting are uniquely related to the rift zonewhich was located on this part of the Arctic Platform, we havedecided to distinguishthesesediments asa separateseismicsequenceinformally termed the "Rift sequence" (fig.3). We have notadoptedthe Carman andHardwick(1983)terminologyforseveralreasons:(1)their infrarift "Barrovian sequence" includesKuparukRiversandstones and some upperKingakshaleswhichareclearlybeneaththeregional rift-onset unconformity (LCU); (2)the depositional setting for KuparukRiversediments on the Barrow Arch is very different from that of the deep infrarift grabens tothe north; and (3)there may be some confusionbetween a prior usage of"Barrovia"(Brosg6 and Tailleur, 1971)as thenorthern source terrane for Ellesmerian sedimentsand"Barrovian" (CarmanandHardwick,1983) forsediments derivedfrom Ellesmerian highlands in the rift zone.
Two time-transgressiveregional unconformitiesformthe boundaries ofthe Rift sequence. The lower unconfonni ty is a southward-dippingerosionalsurfacethatprogressivelytruncates underlyingEllesmerian rocks until it eventuallyreachesFranklinian basementon northernportions of theArctic Platform. This angular unconformityhasbeentermedthe"LowerCretaceousunconformity" (LCU) by Jamison and others(1980) and the "Pebble Shaleunconformity" by Bird (1982). The LCU formspart of the trap for petroleumpooled in Ellesmerian strata at the Prudhoe Bay field. The upperunconformity forms a southward-dippingseismiccoupletwiththe LCU in the Colville Basin but divergesfromthe LCU on the Beaufort shelf, becoming a gentlynorthward-dippingdepositionaldisconformity that is downlappedbyprogradingBrookianhorizons.Thisunconformity hasbeentermedthe"breakupunconformity" (BU) byGrantzandothers (1982b)andcorresponds to the top of the Pebble Shale unit (Bird, 1982).
As the name implies,thedepositional setting for the Rift sequence is closely related to the locality, time interval, and structural character of the Mesozoic rift zone whichdevelopedon the northern Arctic Platform in latest Jurassic to Early Cretaceous time(fig. 4). Duringtheearly rift stage,Ellesmerianunits wereextensivelyerodedonupliftedblocks,andclasticsediments
Souzcig&phy, 19 shed intolocalfault-boundedbasins. The largestof the infrarift basins is the Dinkumgraben(Grantzand May, 1982),whichcontains a sedimentarysectionatleast 10,000 feetthick.Severalsmaller, but genetically related, infrarift basins are identified farther to thewest(fig.9). As the rift zoneevolvedthrough Neocomiantime, it subsided and the supply of clastic material from highlands to infrariftbasins was eliminated. The upperfaciesofthe Rift sequence may becharacterized as relatively deepwater,starved-basin deposits lithologically similar andage-equivalenttothePebbleShale unit onshore.Duringthistimetectonicactivity began in the Brooks Range tothe south, and thickflysch deposits of the Okpikruak or KongakutFormations were shed northward into the Colville foreland basinfrom the orogenic belt.
BROOKIAN SEQUENCE
The Brookian sequence (EarlyCretaceous to Pliocene) is the thickest andmostwidespreadsedimentarysequence in the planning areaaswellasthemostcomplexstratigraphically and structurally. It is essentially ahuge wedge ofclasticsedimentaryrockwhich progradednorthwardfromtheBrooks Range orogenicbelt (fig. 4). This wedge initially filled the Colville Basin andthenprograded northacrossthenewlyformedcontinentalmarginintothe Nuwuk and KaktovikBasins. The Brookian sequence is bounded at its base bythebreakupunconformity (BU), which is typically a depositional discontinuity, and at its top bya low-angleunconformity(QU) at the base ofthePleistocene sequence (fig.3). The thicknessof theBrookiansectionvariesfromlessthan 3,000 feet on theBarrow Arch(fig.2)togreaterthan 35,000 feet(6-secondseismicrecord length)in the Nuwuk BasinnorthoftheHingeLine.IntheKaktovik BasintheBrookian sequence is over 8 seconds thick (CDP record . length). The lowerportion(approximatelyhalf)ofthisclastic wedge is composed ofsigmoidalclinoform and hummocky (foreset) reflectors indicative of deepwater, prodeltamarineshales(Brown andFisher,1977).Lithologicdescriptionsof numerous North Slopewellsconfirmthisinterpretation. The .lowerBrookianshale facies is overlain by a seriesof parallel, high-amplitude (topset) reflectorsthatgentlydipintothemaindepocenters.Thisseismic faciesistypicalofdepositionalenvironmentsrangingfromcoastal plaintomarineshelf (BrownandFisher,1977).Welldataconfirm thattheupperBrookianfaciesincludesfluvial(nonmarine) as well as neritic(marineshelf)deposits,with a highproportionof sandstoneandabundantcoalbeds.
Although a verydetailed lithostratigraphic nomenclature has beendeveloped forBrookianstrata(Brosg6 and Tailleur, 1971; Molenaar,1983),anabbreviatednomenclature is sufficient for our purposes(fig.3).InthewesternpartoftheNorthSlope and adjacentoffshoreareas,thelower,prodeltashalefacies is referred to as theTorokFormation(AptiantoAlbian) and theupper, fluvial-deltaic facies is referred to as the NanushukGroup
20 (Albianto Cenomanian). InthecentralpartoftheNorthSlopeand adjacentoffshore areas, prograding Brookian strata are identified as the Colville Group(Cenomanian toMaestrichtian age)and the SagavanirktokFormation(Tertiary).
Beginning inLate Cretaceous to early Tertiarytime, Brookian sedimentation was focused in the Kaktovik Basin(fig. 1). West of BarterIsland (the Camden sector),theBrookian sequence consists of a singlemajorprogradationalcycle, wherePaleocene to Eocene marineshalesareoversteppedbyOligoceneandMiocenefluvial-deltaic beds. EastofBarterIsland(theDemarcationsector),theBrookian sequence contains numerous major transgressive and regressive cycles moreanalogous to theMackenzie Delta region of Canada than to the NorthSlopeofAlaska.Fluvial-deltaicsedimentsdepositedduring regressivecycles in the Paleocene (equivalent to Moose Channel/Reindeer Formation),OligocenetomiddleMiocene(equivalenttoKugmallit/Pullen sequence),and late Miocene to Pliocene(BeaufortFormation)are separated by localunconformities and transgressivemarineshales (Youngandothers, 1981; WillumsenandCote,1982). The complex stratigraphyintheeasternBeaufortareaisapparentlyrelatedto contemporarytectonics in thenortheasternBrooks Range as wellas syndepositionaltectonicswithintheeasternKaktovikBasin. The Brookiandepositionalcycle was terminatedbysea-levellowering associatedwith Pleistocene glacial periods.
PLEISTOCENE SEQUENCE
The Pleistocene section is identified as a separateseismic sequencebecause it is boundedby regionalunconformities and it accumulated in a distinctlydifferentdepositionalenvironment thantheunderlyingBrookian wedge. It canbeargued,however, thatthese coastal plain andglaciomarinedepositsshouldbeincluded in the Brookian sequencebecausetheywere also partially derived fromtheBrooks Range. A low-angle,regionalunconformity(QU) separatestheprogradationalBrookian sequencefroman overlying sequence of Pleistocene age (Craig and Thrasher, 1982). This sequence is correlative with the Gubik Formation of the Arctic coastalplain and consistsof a laterallydiscontinuous group of marineandnonmarinerocksdepositedduring numerous glacial and interglacialperiods (further discussion in Part 3, Environmental Geology). The Pleistocenesection may beseveralhundredfeet thick on theouterBeaufortshelf,butisconspicuouslyabsent on thecrestsofshallowfoldsintheKaktovikBasin (Dinter,1982) aswellas on much oftheChukchishelf(Grantz andothers,1982a). Lithologiesrecoveredfromshallowboreholes on the inner shelf varyfrommassivesiltstonetocoarse-beddedgravelswithpeatlayers (Harding-Lawson,1979).Onshore, thePleistocenesection andupper portionsof theBrookian sequence typically contain a permafrost layer up to 1,500 feetthick.Thispermafrost zone isprojected (by refraction velocitystudies)northwardbeneaththe present Beaufort shelf where it grades laterally into unfrozen strata (Neave and Sellmann,1983). The permafrostlayerapparently does notoccur on the Chukchi shelf(Grantz and others,1982a).
HOLOCENE SEQUENCE The Holocenesequence lies above a strong,often irregular reflector at the top of thePleistocenesequence and is generally recognizedas a seaward-thickening,acousticallytransparentlayer (Craig and Thrasher, 1982). Itsrelatively uniformcomposition (sandysi1 t) and unconsolidated state are probably responsible for its acoustictransparency. The surface of the Holocene sequence, the seafloor, is covered by ice gougesand small-scale bed forms, creating a very irregularmicrorelief. The overallthicknessof this unit isuncertain (Briggs, 1983), althoughthethickness of theacousticallytransparentlayerranges from zeronearshore,or above shallowfolds,toseveraltens of meters on theoutershelf (Dinter, 1982;Craig and Thrasher,1982).
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3
Seisr.lic Stratigraphy
Thissectionpresentsseismicprofileswhichrepresentthe generalstratigraphy and structuralcharacteroftheBeaufort Sea Planning Area.Western Geophysical Company(WGC) hasgenerously allowed MMS to releasethesedataforourgeneralregionalanalysis. The nonexclusive WGC database inthe Beaufort Sea Planning Area is shown infigure 5. Where possible,seismicprofiles were tied to onshore,publiclyavailablewellsprovidingdirectcorrelations intooffshoreprovinces.Formationtops and regionalunconformities as tabulatedforNorth Slope wells byWitmerand others(1981) and Bird (1982) wereused inconjunction with numerous Husky-NPR Geologic Reports. Thesedatumswere tiedtoseismichorizonsusingsynthetic seismograms that weregeneratedfrom digitized5-inch,long-spaced soniclogs. We derivedthedepthconversionscalesgiven on the seismicprofilesfromtheDixEquationusingtypicalRoot Mean Square (RMS) velocitiespickedfrom WGC Velans. The regionaltime• depthconversions shownon theseismicplatesareonlyapproximations because of the complex structure and stratigraphy of the geologic provinces.Significantlocalvelocityvariations canbeexpected within eachgeologicprovince. A regionaltime-depthcurve,although perhapspracticalin a reportof regional scope, may not bean applicabletool for detailed mapping.
Our seismicinterpretationfocuses on thestratigraphy and structuralevolution of the major tectonostratigraphic sequences boundedby regionalunconformities. Maps ofseismichorizons representingprospectiverockunits will not be presented inorder toprotect the proprietary interests of lease-holders of Federal OCS tracts. In ordertodescribethe complexgeologichistoryofthisvery largeregion, we found it necessary todivide the planning area intothreegeographicsectors(fig. 6). The sectors may eachcontain one or more petroleumprovinces (fig. 1)whicharedistinct stratigraphically as well as structurally.
NORTHEASTERN CHUKCHI SHELF
The Chukchi shelfsector is quite different in structural character and Paleozoicstratigraphyfromcontiguousareasofthe ArcticPlatform on theNorthSlope. The wellcontrolinadjacent
Sehmic SihaLLghaphq, 23
Figure 7. Geologicframework of theChukchisector. Our geologicinterpretation is illustrated by threeregionally compressedseismic profiles(plate 1, 2, and 3) controlledbyjump-ties to onshoreexplorationwellsin western NPRA. Basement blocks, lnajor fault zones,dndsedimentarybasins arelabeled.
26 I
onshoreareas isisolated from a relativelysparseseismicgrid on thenortheasternChukchishelf(fig. 5) bycomplex structural zones developed in Paleozoictime.Directseismiccorrelationofthe Paleozoicstratigraphyacrossthesestructuraldiscontinuitiesis impossible,therebyrenderingouranalysisofthelowerpartsofthe sei smi c profi1 es somewhat uncertai n.
The structural frameworkandsedimentarybasins in the northeasternChukchi Sea are shown infigure 7. In contrasttothe gentlysouthward-dipping surface of the Arctic Platform to the east, the Chukchi sector hasan irregularnortheast-trending"basin and range"structuralcharacter, wheredeepsedimentarybasinsarebounded bycomplex fault zones. The largestofthesebasins,which we informallytermthe"NortheastChukchiBasin," is structurally isolatedfrom the Arctic Platform to the east by a poorlyresolved, NE-SW-trending fault zone (informallytermedthe"Barrowfault"). The NortheastChukchiBasin is a deep half-grabenfilled with a clastic wedge ofPaleozoic age. The deepestpartofthisPaleozoic basinlies along the Barrow fault, and thebasinfloorrisestowards thenorthwest where it is deformedacross a highlyfaulted structural upliftreferredto as the"North Chukchi high"(fig. 2). This structural uplift is characterized by a dense arrayofextensional blockfaults that offset and tilt theBrookian sequence in the shallowsubsurface. The broad,unfaultedcrestoftheBarrowArch plunges tothesoutheastovertheolderNortheast ChukchiBasin and is disrupted at its northwesterncrest by the faulted uplift oftheNorth Chukchihigh. The northeasternpartofthe Chukchi sectorcontainstheHingeLine,whichtrendsbeneaththeouter continentalshelf. The Nuwuk Basin liesnorthofthe HingeLine. The Chukchisector is flanked to the south by theColville Basin, whichcontainsover 30,000 feetofsedimentaryrocksassigned to theEllesmerian andBrookiansequences(fig. 2). The Northeast ChukchiBasin isstructurally isolated from the Colville Basin by a complexlystructured basement ridge.
Plate 1 is a seismic"dip" line whichillustrates the stratigraphy of the Colville andNortheastChukchiBasins, and thestructural character of theBarrowArch(fig. 7 shows seismicprofile locations). Stratigraphiccontrolforthisseismicinterpretationisprovided byjump-ties tocoastal exploration wells in western NPRA, in particulartheTunalik No. 1 well.This deep well (TD at 20,335 feet)penetrated a nearlycompletesectionofEllesmerian and Brookianstrata,representingdepositionfromlatePaleozoicto Mesozoictime. However, theTunalik No. 1 wellbottomedin PennsylvaniancarbonatesoftheLisburne Group anddoes notprovide directcontrol for older (Endicott Group orFranklinian basement] seismichorizons. The correlationofpre-Lisburnestratigraphy fromtheColvilleBasinnorthwardintotheNortheastChukchiBasin is also obscured byan interveningstructural zone whicheffectively isolatesthesetwomajorbasins. As shown infigure 7, thisstructural zone occurs at theprojectedintersectionofseveralmajorfault trends and may represent a complexlyfaulted basement ridge of pre-Li sburne age.
Sehmic SWgnaphy, 27 4
The interpretation of the upper Ellesmerian and Brookian seismic sequences on plate 1 is relatively simple because of direct we1 1 control and uncomplicatedstructure. The PU, picked at the base of the Sadlerochi t Group, is a smooth, unfaul ted, angular unconformi ty which dips southward from the Barrow Arch. Thisunconformity represents the contact between the upper Ellesmerian and lower Ellesmerianseismicsequences(fig. 3). The Permian and Triassic units (Sadlerochit Group and Shubl ik Formation) in the upper El lesmerian sequence progressivelyonlapthe PU and thin northward. The overlying Triassic and Jurassic units (Sag River and Kingak Formations), which depositionally overstep the olderunits,areprogressivelytruncated at the LCU northwardonto the BarrowArch. On the broad crest of the Barrow Arch, all upper Ellesmerian and PebbleShale (Rift sequenceequivalent)strata have been removed by erosion at the LCU and BU, and a thin layer ofBrookian strata lies directly on lowerEllesmeriandeposits. The Pebble Shaleunit(ageequivalent to the Rift sequence) is easily identified in the ColvilleBasin and on the southernflank of the Barrow Arch as a distinctive seismic couplet formed by the LCU and BU. The Brookian sequence thins considerablynorthward from the Colville Basinover the Barrow Arch, and is reduced to a thickness of less than 3,000 feet on the crest of the arch. Brookian foreset and bottomset beds, correlativeto the Torok Formation, downlap the BU. Brookian topset beds, correlative to the NanushukGroup which originallyprogradednorthward,are progressivelytruncated at a shallowunconformity related to post-Albian uplift of the Barrow Arch in the Chukchi sector. The structuralcharacter of the northernArcticPlatform in the Barrow area is illustrated by the seismicprofile presented in plate 2. Well controlfor this seismicinterpretation is provided by the Walakpa No. 2 and the South Barrow No. 1 wells (fig. 7). At the Wal akpa No. 2 well, the upper Ellesmerian andBrookian . sequences have thinned considerably, and the ShublikFormation (Middle to Late Triassic) lies directly on Franklinianargillite ofOrdovician to Silurian age(Carter and Laufeld,1975). The lowerEllesmeriansequence(Endicott and LisburneGroups) is absent on this part of the Arctic Platformas a result of depositional onlap(on the EU) and perhaps some erosionaltruncation(at the PU). Similarly,the upper Ellesmeriansequence is considerably reducedin thickness as a consequence of depositionalonlap(on the PU) and erosionaltruncation(at the LCU). Between the Walakpa No. 2 and the South Barrow No. 1 wells, the upper Ellesmerian section is completelytruncated at the LCU, and the Lower Cretaceous PebbleShale unit lies directly on the basement complex (Haga and Mickey, 1982). The PebbleShale unit(Rift sequenceequivalent) remains fairlyconstantinoverallthicknesssouth of the Barrow Arch and then abruptly thickens on the northflankof the basement ridge (right side of plate 2). Plate 2 illustrates that the crest of the Barrow Arch defined by a Cretaceoushorizon (the BU) lies south of the basement high in the Barrow area. The Brookian sequence thins to approximately 2,000 feet on the Barrow Arch from over 10,000 feet in the ColvilleBasin(Bird,1982)largelyas the
2s result of erosion on the Barrow high. North of the Hinge Line, these CretaceousBrookian strata thicken to greater than 35,000 feet (6-second CDP record length) inthe Nuwuk Basinbeneath the Beaufort she1f. It is importantto inspect theseismiccharacter of the Frankliniansequenceinplate 2 which is typical of acoustic basement observedelsewhere on the ArcticPlatform. The reflection from the top of the basementcomplex is usuallymoderate to high amplitude, and internal reflectors generally 1ackany lateral continuity. Consequently, few attempts have been made to resolve the seismic stratigraphy of the Franklinian sequence beneaththeArctic Platform. The seismicprofile shown in plate 3 extends from the Arctic Platform,across the Northeast Chukchi Basin, and onto the eastern flank of the North Chukchi high (fig. 7). Subsurfacecontrolfor this interpretation is provided by a jump-tie to the Walakpa No. 2 well. This seismic profilegenerallyparallels the SE-plunging axis of the Barrow Arch. In marked contrastto the acoustic basement beneath the PU on the ArcticPlatformto the east (plate 2), pre-Permian rocks in the Northeast Chukchi Basin exhibit laterally continuoushorizons with a distinctly bedded appearance.Primary depositionalfeatures, such as sigmoidalprogradingclinoforms,are comnon, asarestructuralfeatures, such asfaults and folds. Two distinct seismic units can be recognizedwithin the lowerEllesmerian sequence: an upper clastic wedge characterized by numerous detached folds, and a lower unit, relatively uniform in thickness, characterized by parallel,high-amplitude,laterallycontinuoushorizons. These apparentsedimentary units lie unconformably on acousticbasement, presumably correlative to the Franklinian sequence.
The rocks which we have identified as lower Ellesmerian in plates 1 and 3 areenigmatic:theyclearly have not experienced the same degree of tectonic alteration as lowerPaleozoic(Frankliniansequence) basement rocks of the Arctic Platform, but they also are quite different from Ellesmerian strata distributed throughout Arctic Alaska. Grantz and Eittriem (1979) first recognized these rocks and identified them as"lowerEllesmerianorFranklinian and Pre-Cambrian(?)." Grantz and May (1982) later proposed that these strata (their unit "SOf," p. 84) are correlative to the Ordovician to Silurian argillite of the basement complex in the Barrow area.Subsequent maps depict a shallowFranklinian basementhigh on the eastern Chukchi shelf overlain by a thin Brookian section (Ehm, 1983). However, a comparison of plates 1 and 3 (on the Chukchi shelf) with plate 2 (in the Barrow area) shows that it is unlikely that the metamorphic complex of the Arctic P1 atform is correlative to the sedimentary strata in the Northeast Chukchi Basin. We believethat a major fault zone (the "Barrow fault," fig. 7) must juxtapose the youngersedimentary deposits in the basinwith the basementrockof the ArcticPlatform. The seismicexpression of this fault zone is poorlyresolvedbecause of its high (perhapsreverse)angle and the lackofavailableseismic 4
datainstatewaters. A laterpublication by Grantzand May (1984) abandoned theFrankliniancorrelationtothesePaleozoic sedimentary rocksandprovisionallyassigned them to an informalgrouptermedthe "Eo-Ellesmeriansequence." Our work essentiallyagreeswiththis conclusion; however, we refertothesesedimentaryunitsasthelower El 1esmeri an sequence.
OverlyingthelowerEllesmerian sequenceon thisseismic profile is a highlyabbreviatedsectionof Mesozoicrocks. The upper Ell esmerian, Rift, andBrookiansequencesoccur at shallowsubsurface depths (3,000 feetor less) and aretruncatedatseveralerosional unconformitiesrelatedtoupliftofthe BarrowArch. On theeastern flankof the North Chukchihigh,theupperEllesmeriansequenceand PebbleShaleareentirelytruncated, and LowerCretaceousBrookian strata lie unconformably on Paleozoicrock.
On thenorthwest end of this seismic line, thebroad, extensively faulted North Chukchi high dissectsthe crestal portion of the Barrow Arch. Thisstructuralupliftextendsintotheadjacent Chukchi Sea PlanningArea (D. Thurston,personal commun.). Inthis interpretation, we show Brookianstratalyingunconformably on the lowerEllesmerian sequence. It isalsopossiblethatFranklinian basement rocksoccuratshallowsubsurfacedepthsbeneaththethin, highlyfaultedBrookiansection. An accuratedelineationof individualfault traces is difficult becauseseismicdatacoverage is sparse,butthe structuralcharacter is one ofhigh-angle, basement-involved block'faulting on a north-to-northeasttrend. The timing and origin of the tectonic activity on theNorth Chukchi high is uncertain; however, 1argenear-surfacefaultdisplacementsof Brookianbedssuggestthat it occurred in Late Cretaceous to Tertiary time. The block-faultpatternclearlydeformstheNW-SE-trending BarrowArch.
These seismic profiles clearly illustrate that the "BarrowArch" in the Chukchisector is notcoincidentwith a regional basement ridge. The BarrowArch,whichtrendsroughlyparallel tothe Hinge Line, is a structuralflexure betweentwomajorpost-riftingbasins (Nuwuk and Colville;fig. 2). EarlyCretaceousandyoungerstrata exhibitthebroad,unfaulted symmetry typical of an arch,whereas the structural relief of the basement rockhas a highlyirregular, block-faultedconfiguration. On theArcticPlatform(eastofthe Barrow fault),the Barrow Archcoincidesapproximatelywith a SE-plunging basement ridge (fig. 2). West ofthe Barrow fault, however, theNortheastChukchiBasinandNorthChukchihightrend obliquely to the BarrowArchandrepresententirelydifferenttectonic episodes.
TheseseismicdatahaveillustratedthatMesozoic andyounger stratalie at shallow depths over large portions of the eastern Chukchishelf. The geologyoftheunderlyingsectionis,therefore, ofprimaryconsiderationforthepetroleumpotentialoftheChukchi
30 sector. By tracingseismichorizons, it can be demonstrated that the lowerEllesmeriansequence in theNortheast Chukchi Basin is pre-Permian in age. A lowerage limit cannot be asaccuratelydefined, althoughthesesedimentaryrocksareapparentlyyoungerthanthe Upper Ordovician to Silurianargillites of the ArcticPlatform. These middle to upper Paleozoic deposits areseparatedinto two distinct seismic units which unconformably overlie acoustic basement (presumably representing lower Paleozoic metamorphic rock). The lower seismic unit consists of parallel,high-amplitude,laterally continuoushorizons and has a constantoverallthicknessthroughout the basin(approximately 10,000 feet). The upper seismic unit appears to be a clastic wedgewhich has beendeformed into a series offolds that roughly parallel the Barrow fault zone.These foldsare thousands of feet in amplitude and aredetachedacross a d6collement from the underlyingseismic unit. Restoration of azimuths (by removal of regional dip) forprogradingclinoformhorizonsyields a primary depositional strike of 45 degrees and an initial dip (beforebasin tilting) of 2 degreestothesoutheast. The southeastwardaccretion of sigmoidalseismichorizonsalso suggests that this basinreceived clastic sediment from the northwest. The lithology of theseseismic units can be inferred from their acousticvelocity. Figure 8 sumarizes a velocityanalysis of the two lower El lesmerianseismic units and the Brooki an sequence which unconformably overlies these deposits(plate 3). Interval velocities calculated by using WGC Velan data are plotted with sonic velocities of known rock units from various North Slope wells. Group A (Brookiansequence)velocitiesarecomparabletothoseobtainedfor Lower Cretaceousrocks(TorokFormation) at shallowburial depths. Group B (lowerEllesmerianclastic wedge) velocities are similar to intervalvelocitiesforEndicott Group rocks. The intervalvelocity for Group B is somewhat higher than that of the Torok Formation at equivalentsubsurface depths. This differencecould be explained by a greater abundanceofsandstone in this clastic wedge (compared to the Torok Shale)or a considerable amount of post-depositional up1 ift. The acoustic velocity of Group C (basallower Ellesmerian unit) is typicalofcarbonaterockscharacterized by the Lisburne Group. The seismic velocities of the Lisburne Group are shown forcomparative purposesonly. Our preliminaryconclusions,based entirely on seismic data, are that the lowerEllesmerian sequence in the Northeast Chukchi Basinconsists of a lower carbonate unit and anupper clastic unit of middle to late Paleozoicage. However, these units arenotsimilar in thickness or in lithologic succession to the lowerEllesmerian units known from the central and eastern NorthSlope. There, the Endicott Group ofMississippianage(Kekiktuk and Kayak Formations) is conformablyoverlain by MississippiantoPennsylvanianplatform carbonates of the Lisburne Group (fig. 3). In the Northeast Chukchi Basin, the clastic wedge and basalcarbonateunitsare much thicker, and the carbonate-clasticsuccession is reversed from the Endicott- INTERVAL VELOCITY ( I o3 FEET/ SECOND) 6 7 8 9 10 II 12 13 14 15 16 17 la IS 20 21 1 1 1
.T 2
D GROUP A 5.000 v V W 0 -X I D -i m El " 0 IO.000 ; 4 I h GROUP B ll m m L2 -i m 15.000 p T = TorokFormation; 0 I SouthBarrow # I7 . z Lo T = TorokFormation; m
Tunalik #I D 20.000 r m E = EndicottGroup; < I ColvilleDelta #I m GROUP r C v E = EndicottGroup; 2 lnigok # I Z5.000
L = LisburneGroup; I ColvilleDelta # I
L = LisburneGroup; lniaok #I
Figure 8. Acousticvelocityanalysisofseismic sequences identifiedbeneaththe Chukchishelf.Dixintervalvelocities (shown as lines) were calculated byusing RMS picks fromWestern Geophysical Company Velans. Typical dcoustic velocities (soliddots) Herecomputedas theinverse ofinterval transittimesinsonic logs fromonshorewells.Representativerockunits. in onshorewellsarelisted. Groups A, €3,and C correspondtotheseismic sequences labeled in the box on plate 3.
32 dl3HS ltlOdnW38 lWtllN33 NPRA
Figure 9. Geologic frdmework ofthecentralBeaufortshelf. The structure and stratigraphyoftheArcticPlatform and Nuwuk Basinareillustrated by plate 4, extendingoffshorefromthe Husky J. W. Daltonwell. The structure and stratigraphy of theOuter Arctic Platform are illustrated by plate 5, extendingnorthwardfromtheSohio Niakuk No. 3 wellintothe Dinkurngraben. the LCU. The overlying Rift sequence (Pebble Shaleunit) and Brookiansequence (TorokFormation-NanushukGroup) arerelatively constantin thickness and unfaultedover the Barrow Arch. On the northernmostpart of the ArcticPlatform, the stratigraphy and structuralcharacter change rapidly. Remnantsof the northward- thinning upper Ellesmeriansequence may be preserved locally in downdropped blocksbeneath the LCU. Block faulting in the rift zone prior to the erosion at the LCU influencedthe present distribution of upper Ellesmerian units on the northernArctic P1 atform. The overlyingRiftsequence thickens greatlyinto a grabentypicalof the Outer ArcticPlatform.Thisinfrariftdepocenterisfilled with severalthousands of feet of well-stratified,high-amplitudehorizons.
The Hinge Line (center of plate 4) marks an abruptthickening of the Brookiansequence into the Nuwuk Basinand is identified by large-displacement,down-to-the-northbasement faults and a listric fault system in overlying Brookian strata. The Hinge Line faults characteristically postdate basement faults that bound the infrarift grabens;that is, Brookian horizonsarenotusuallyfaulted above the infrarift grabens as theyarealong the Hinge Line and in the basinsfarthertothenorth. The Hinge Line marks the northern edge of theCretaceouscontinental margin. Post-rift subsidence along the continental margin is marked by the systemof listric growth faults. These growth faultsarenotnecessarilyconnectedwith the underlying basement faults; they seem to be detached from the underlying, pre-Brookiansequencesalong a dkol lement in the lower Brookian shale. The extensive growth fault systemcan be used to trace the approximatesouthernmargin of post-rifting depocenters beneath the Beaufortshelf where the basement complex lies below the 6-secondseismicrecords. The Nuwuk Basin is located north of the Hinge Line and contains over 6 seconds(approximately 35,000 feet) ofBrookian sediment. This CretaceoustoTertiarydepocenter is cut by numerous listric growth faults traceable from shallowsubsurface depths to the base ofcoherentseismicdata. The upper Brookian seismic facies, consistingofparalleltoslightly divergent, high-amplitude, laterallycontinuous,"topset"horizons, is a facies-equivalentto the fluvial-deltaic strata of the Nanushuk (Lower Cretaceous) and Colville (Upper Cretaceous) Groups. The lowerBrookianseismic facies,consistingofvariable-amplitude,discontinuoustosigmoidal "foreset"horizons, is a facies-equivalentto the deepwater,prodelta shales of the Torok (Lower Cretaceous) and Seabee (Upper Cretaceous) Formations. The Rift sequence may be present atgreatsubsurface depths (below 20,000 feet) beneath the Brookian fill of the Nuwuk Basin. Plate 5 is a representative seismic line across the Mikkelsen basement high and the Dinkum graben. Well control is provided by the Sohio Niakuk No. 3 well, which contains a complete, but reduced, section of Ellesmerianunits, the Pebble Shale, and the Brookian sequence. The lower Ellesmerian sequence (the Endicott and Lisburne
Seinmic SLmLLghaphy, 35 Groups) is presentat the well but thins abruptly on thesouthern flankoftheMikkelsenhigh. The abruptthickness change in the lower Ellesmerian sectionoccurs asa result of depositionalonlap (onthe EU) and possibleerosionatthe PU. The northernflankof this local basement high was formedby faulting associated with the Mesozoic rift episode. Upper Ellesmerianstrata(Sadlerochit Group through Kingak Formation) overstepped lower Ellesmerian units on the Arctic Platform but weresubsequentlytruncated at the LCU. Near the crest of the Mikkelsen high, the LCU lies directly on Franklinian basementrockandEllesmerianunitsareprobablyabsent farther north.
In the Dinkumgraben,twoseismicfaciescanbedistinguished in the Rift sequence. The lowerfaciesischaracterized by discontinuous, hummocky, high-amplitudereflectorsandattains a thicknessofover 10,000 feet on thisprofile. It is inferred to represent LowerCretaceous clastic deposits composed ofreworked Ellesmerian and Franklinianrockswhichrapidlyaccumulated in infrarift depocenters. A possibleanalogfortheseclastic deposits may bethe Point Thomson sandstonesfoundsouth of the Mikkelsenhigh. The upper Rift faciesconsistsof a rel-atively thinseries of laterally continuous, high-amplitude reflectors which onlaptothesouth on thelower Rift facies(plate 5). We infer thatthese horizons represent deep basinalshales which are age equivalent to the Pebble Shale unit in onshore North Slope wells. In contrast to the relatively thin PebbleShale onshore, the upper Rift facies reaches Over 1,000 feet in thickness in the Dinkumgraben. The thickness of the entire Rift sequence in the Dinkumgraben cannotbeaccuratelydeterminedbecauseFranklinianbasement is difficult to resolve below about 4 seconds(approximately 20,000 feet).Similarly,theinterpretationofthenorthwallofthe Dinkum grabenandtheblock-faulted basement relief along the Hinge Line. isalsoverysubjectiveatthesesubsurfacedepths,perhapsbecause of the low acoustic impedance contrast betweendeeplyburied sedimentary (Rift sequence)andmetasedimentary(Frank1 inian sequence) rocks. The Brookiansequence(plate 5) lies above the BU andrepresents a northward-progradingclastic wedge ofCretaceous to Tertiary age. On theMikkelsen high, local erosion at the BU occasionallytruncates thePebbleShaleunit andupperEllesmerian sequence. In theseareas, the Brookian prodelta shale lies directly on Franklinian basement rock.Typically, however, the BU is a depositionalunconformity between thestarved-basinshaleoftheupper Rift facies and the more rapidlydeposited prodelta shaleof the prograding Brookian sequence.
As described inthe previous plates, the Brookian sequence consistsof twoseismicfacies: a lower"prodeltafacies" composed of variable-amplitude,sigmoidalclinoformreflectors; andanupper "fluvial-deltaicfacies"consistingofparallel,high-amplitude, laterallycontinuousseismichorizons. The lithologyofthelower
36 Brookianfacies is inferred to bedeepwatershales,whereas the upperBrookianfacies is inferred to represent fluvial, delta plain, andshallow marine shelf deposits with high proportions of sand to shale(Brownand'fisher,1977). The Brookian sequence insouth-central parts of the Beaufort she1 f is probably age equivalent to the Colvi 1le Group (LateCretaceous)andSagavanirktokFormation(Tertiary) (fig. 3). The toplapcontactbetweenthefluvial-deltaicfacies and prodeltafaciesprobablyrepresents a depositionaldisconformity where thesedimentsupplyexceededbasinsubsidenceandclastic materialbypassednearshoreshelfareastodeepwaterdepocenters farthernorth. On thenorthern end ofthisseismicprofile,the interfingering of hi h amplitude"topset"horizonswithlow• amp1 itude cl inoform9-shale) intervals suggests temporary variations intherateofbasinsubsidenceorsedimentsupplyfromtheBrooks Range. Decreasingtherateofsedimentsupply(orincreasingthe basincapacitybysubsidence)couldresultinlocalmarine transgressionsbetweenshiftingdeltalobes on a regionallystationary shoreline.
EASTERN BEAUFORTSHELF
The geologicframework ofthe eastern Beaufort sector is shown in figure 10. Thispartof theplanningareacontainsthe Kaktovik Basin,northofthe.HingeLine, and a narrowportion of the Arctic Platform. Onshore, theactivetectonicfrontoftheBrooks Range orogenandthesoutheastward-plungingBarrowArchformprominent structuralelements. The easternBeaufortsector may befurther subdividedintotwogeologicallydistinctsub-provinces: a western, or Camden, sector(provinceIIB)and an eastern,orDemarcation, sector(province IIC) (Grantzandothers,1982b). As shown infigures 2 and 10, the Arctic Platform extends only a short distance offshore beneath the present Beaufort shelf before it is intersected by 'the HingeLine and is abruptlyfaultedbelowthe6-second CDP records (plate 6). The KaktovikBasin is youngerthanthe Nuwuk Basin in that it is filled largely with Tertiary sediment,whereas the Nuwuk Basin is filled primarily with Cretaceoussediment. The time- transgressiveprogradationoftheBrookiandeltasystemtowardsthe northeastacrosstheBeaufortcontinentalmargin is responsible for this age distribution.Bothofthesepost-breakupdepocenterscontain over 6 seconds(approximately 35,000 feet) of Brookian strata andan extensive NW-SE-trendinggrowth fault system related to active basinal subsidencenorthoftheHingeLine. In theKaktovikBasin,thegrowth fault systemwhich formed contemporaneously with early Tertiary basin subsidence is obliquely intersected by a series of NE-SW-trending compressionalfolds of lateTertiarytoQuaternary age. These structures postdate the deposition of the Brookian clastic wedge.
Wellcontrol for the Camden sectoris provided by the Exxon AlaskaState A-1 well on theeastern end oftheMikkelsenhigh.This welldiscovered oil in Paleocene turbidite sands at the base of the Brookiansequence(Wharton,1981). These oil sands aresiliceous w a'
Figure 10. Geologic framework oftheeasternBeaufortshelf and adjacentonshoreareas.Majorfaultzones,prominent anticlines,ridgeuplifts,subbasins, and the Brooks Range foldbeltarelabeled. The Exxon Alas!ta State in compositionandclearlynot an extension of the LowerCretaceous Point Thomson carbonatesands.found in wells on thesouthsideof theMikkelsenhigh(discussed furtherinchapter 7, PotentialReservoir Formations). The Franklinian basementcomplexon theeastern end ofthe Mikkelsen high is not the massive argillite usually encountered in mostwells on theArctic Platform; instead, basement rockconsists ofinterbeddedquartzite,marble, and phylliticshale. Oil shows in thesefracturedmetasedimentaryrockssuggestthatmigrationhas occurredfromoverlying(Brookian)sourcesintothe basementcomplex.
Plate 6 is a representativeseismic "dip" line illustrating the stratigraphy and structuralfeaturesfound inthe Camden sector (subprovinceIIB). The Brookian sequence inthe Camden sector is similar to areas of the Beaufort margin farther west in that it consists of a lowerseismicfaciesofprograding"foreset"clinoforms andanupperseismicfaciesofcontinuous,high-amplitude"topset" horizons. At theAlaskaState A-1 well,thelowerseismicfacies is correlated to a massiveshaleintervalofPaleoceneto Eocene age (Wharton,1981) inferred to represent deposition in a prodelta marineenvironment. The upperseismicfacies is correlated to a predominantly sandy intervalinferred to represent a nonmarine to marginalmarine she1 f envi ronment.
SouthoftheHingeLine,theBrookian sequence is approximately 12,000 to 15,000 feet thick on the nearly flat-lying Arctic Platform; largefaults offsetting both Brookian and Franklinianrocks are rare. On thisportion of the Arctic Platform, the Ellesmerian sequence is absent asa resultof extensive erosion, and thebreakup unconformitylies directly on Franklinian basement, representing a hiatus between earlyPaleozoic and early Cenozoictime.Local outliers of the Rift sequence may occur in small infrarift grabens.
Northof the Hinge Line, Franklinian basement blocksare abruptlydownfaultedintotheKaktovikBasin. At thebaseofthe Brookian sequence, we identify a group ofhigh-amplitudehorizons possiblycorrelatedtothe Rift sequence. The Rift sequence may occurelsewherebeneaththethickBrookian wedge in fault-bounded basinsgeneticallyrelatedtothe Dinkumgraben. A NW-trending growthfaultsystem is recognized parallel to and north of the basementHingeLine,and listric faults often offset strata throughout theentire Brookian sequence toshallowdepths in thesubsurface. These largegrowthfaultsapparentlyhave influenceddeltaic sedimentation in Tertiary time, asevidencedbysignificantthickening of the Tertiary topset beds in downthrown fault blocks.
The Camden anticline rises as a large-ampli tude compressional foldthat is apparently detached from the extensional basement structurenorthoftheHingeLine. The NE-SW trend of this anticline is structurally anomalous in that it isoblique to the regional NW-SE• trendingmarginofthewesternKaktovikBasin(fig. 10). Rollover antic1 ines formed contemporaneously with growth faults along a passivebasinmarginshouldgenerallytrendparalleltothebasin margin. The lateralcontinuity and constantoverall thickness of the upper Brookian faciesacross the Camden anticline indicate that these fluvial-deltaic strata were depositedpriorto the formation of this compressionalfold.Seismic evidence suggests that the Camden anticline is a lateTertiary,possiblyQuaternary,feature whose origin is related to external ordeep-seated tectonic mechanisms and notto growth faultingwithin the western KaktovikBasin. On the crest of the Camden anticline many of the listric faults extend upward intoPleistocene and perhapsHolocene strata. The extensive unconformity which truncates the top of the Camden anticline and other shallow anticlines in the Camden sector is probablyPleistocene in age. The regional structure of the Demarcation sector(subprovince IIC) isillustrated in figure10. The Beaufortshelfeast of Barter Island contains two 1arge structural up1 ifts (the Jag0 and Demarcation ridges) and two intervening depressions (the Barter and Demarcation subbasins). The NW-SE-trending growth fault system that typically occurs seaward of and parallel to the Hinge Line is obscured by the structuralcomplexitybeneath the easternBeaufort shelf. The Brookiansequence in the Demarcation sector is composed ofseveraldistincttransgressive and regressive depositionalcycles, and attains a thickness of greater than 8 seconds on seismic profiles. Prominent unconformitiesarerecognizedwithin and between these cycles. This is in contrastto the Camden sector and areasto the west, where a single major regressive cycle characterizes the Brookian sequence. The contemporaneous tectonicactivity in the Demarcation sector hasobviouslymodified the generalnortheastwardprogradation of the Brookian clastic wedge. The nearestpubliclyavailable well controlforthisareais the Dome Natsek E-56 well drilled northwest of HerschelIsland on a structural high which may be an easternextensionof the Demarcation ridge. The stratigraphyof this controlwell is summarized infigure 31. The seismicinterpretation shown in plate 7 illustrates the stratigraphy and structuralfeaturesof the Demarcation sector (subprovince IIC). Acousticbasementprobably consistsof a lower Paleozoicmetasedimentary complex identified as the Frankl inian sequence.Incontrastto the flat-lying Arctic Platform of the Camden sector (plate 6), the Frankl inian basement complex is tilted steeplyto the north. The Demarcationsubbasin is floored by Franklinian basementrock and apparently formed as a structural sag between a majorbasement uplift to the south(onshore) and the Demarcation ridgeto the north(offshore).Ellesmerian units were not identified in this offshoreareabeneath an extremely thick Brookian section, but some Ellesmerianformations may occursouthof the Hinge Line in the easternArcticNationalWildlife Refuge (ANWR) (discussed furtherinchapter 8, Play Concepts). Likewise, the Rift sequence was not identified in seismic data in the Demarcation sector. The stratigraphy of the Brookian sequence is inferred from published reports on Canadian geology and on the surface geologyof ANWR (Palmer and others, 1979;Molenaar,1983). The oldest Brookian unit is inferredto be a thickshale of Cretaceousage.Thisunit is
40 represented as a seismically homogeneous interval, and it is identifiedonlyin the cores ofdeep folds or as possible diapiric spines within the Demarcation ridge (plate 7). An overlying stratified unit is provisionally correlated with the Cretaceous to Paleocene fluvial-deltaicrocks which were penetrated by the Natsek well. The parallel, high-amplitudehorizons which comprise this unit are most apparentin deep, possiblythrust-coredfoldswithin the structural ridges. We tentativelycorrelate this seismicunit to the Moose Channel and Reindeer Formations (Youngand others, 1981) in the Mackenzie Deltaorto the Sabbath Creek Formation (Molenaar,1983) in ANWR. The "CretaceoustoPaleocene(?)deltaic unit" (plate 7) is overlain by a second seismically homogeneous intervalinferredto represent a massiveshale. This seismic unit variesgreatly in thicknessover the deep folds and possiblediapiric spines within the structural ridges. This "Eocene(?)mobileshale unit" (plate 7) is thought to be ageequivalentto the Brookian prodeltashale in the Camden sector and to a massivePaleoceneto Eocene shalepenetrated by the Natsek well. We tentativelycorrelate it to the RichardsFormation in the Mackenzie Deltaarea (Youngand others,1981). A prominentlocalunconformityseparates the lower Tertiary seismic units, which form the structural ridges, from the middle to upper Tertiary strata, which onlap the ridges and fill the Barter andDemarcation subbasins.Strata which fill the Demarcationsubbasin are represented by divergent, high-amplitudeseismichorizons presumably deposited irl a shelf environment (Brown and Fisher, 1977). These horizonsare downwarped into the Demarcation subbasin and thin by onlap,aswellas by erosionaltruncation,onto the Demarcation ridge. Numerous localunconformities on the flanks of the Demarcation ridge suggest episodicupliftthroughmid-Tertiary time. Continuous,high-amplitudehorizons were traced between these localsubbasins and westward into the upperBrookian fluvial-deltaic facies in the Camden sector. We tentativelycorrelate these inferred Oligocene to Miocene strata to the Kugmallit and Mackenzie Bay Formations (Young and others,1981)or the Pullen and Akpak Formations (WillumsenandCote,1982) in the Mackenzie Deltaarea.
An upper Tertiary seismic unit lies above a prominent erosional unconformity that has been dated in the Mackenzie Deltaas middle tolate Miocene (Young and others,1981).Inferredfluvial-deltaic strata overlying this regionalunconformity(designated MU in plate 7) are tentatively correlated to the Beaufort and Nuktak Formations (Young and others, 1981;Willumsenand Cote,1982) of late Miocene to Pliocene age. The internalstructure of the Demarcation ridge, althoughhighly deformed, can be partially resolved in some seismic profiles which cross this feature. We offer the interpretation shown in plate 7 to illustrate a possible model for the complex internal structure. According to our preliminary interpretation, parts of the Demarcation ridge may be composed of en echelon(possiblythrust-cored)folds
Seismic SLuLLgtraphy, 41 i
which involve Cretaceous to Paleocene strata andwhichgenerallytrend NE-SW, nearly orthogonal to the NW-SE trend ofthecompositeridge mass (fig.28). Deep-seated thrustfaults (if present)trendparallel tothese en echelon folds, with fault planes dipping to the southeast. The"Eocene mobileshale unit" (plate 7) was apparentlymobilizedby flowageabovethedeeperfolds. The Demarcationridge,including thrust-cored(?)anticlines and diapiric intrusions (if present), was weldedtogether and uplifted asaNW-SE-trending structural mass roughlyparalleltothemarginoftheeasternKaktovikBasin. The shallow,highly faulted crest of the Demarcation ridge covers a wide area of the eastern Beaufort shelf (fig. 101, and the low relief of shallow unconformities contrastsgreatly with the underlying structural complexitywithinthe ridge. A1 thoughthe mechanism responsible for this uplift is uncertain, we believe the timing for the event is contemporaneous with the local subsidenceand deposition of strata in theadjacent Demarcation and Bartersubbasins. Assuming thatour age correlationsareapproximately correct,this structural event is bracketed between earlyOligocene (depositionalonlaponthe "Eocene mobileshale unit") and late Miocene time(extensiveerosionrepresentedbythe MU). We hypothesizethat the structural mechanisms responsible for the uplift of the Demarcation ridge are related to, but do not extend from, the thrust tectonics of thenortheasternBrooks Range. Thispreliminaryconclusionisbased on thefollowingobservations:
1. The internalfoldswithintheoffshoreridge wereformed in middle Tertiary time, with the massive uplift ending by 1ate Miocene time.Incontrast,thrust-foldtectonismonshorein ANWR has apparentlybeenactivethroughoutCenozoictime. We believethat it is unlikely that the effects of a relatively localized, older thrustingepisode would be preserved beyond the presently active tectonicfront.
2.The axialtracesoftheinternalfoldsintheoffshoreridgeare oriented NE-SW, oblique to the more easterly trends in the thrust belt onshore. The orientationoftheinternalfolds and associatedthrust faults within the Demarcation ridge suggest a more northwestward-directedcompressionoffshorecomparedtothe northwardthrusting onshore in ANWR.
We do, however, clearly favor a tectonic mechanism over a syndepositional(diapiric) modelasproposed forthegenesisofthese ridges by WillumsenandCote(1982). The structuralfeatureswithin the Demarcation ridge clearly trend orthogonally to the margin of the easternKaktovikBasin(fig.28).Growthfaultsystemsalongbasin marginstypically control the orientation of localdiapiric spines and usuallytrendpara1le1tothebasinmargin. A wrench fault mechanism may explaintheenecheloncompressionalfold assemblage withintheDemarcationridge. However, theexact location of hypothetical wrenchzonesand theirstructuralrelationship to the
42 i
thrust belt in ANWR or theKaltagshear zone which trends into this area from the Canadian continental margin (Jones, 1982) have yet to be defined.Substantially more mapping, bothonshore and offshore,coupled with biostratigraphic control, is requiredbefore a comprehensive model for the structural evolution of the Beaufort margincan be developed. t
Part 2 Petroleum Geology 4
Exploration History
The followingsection on petroleumexploration in the northern Alaska and Beaufort Sea regions is extracted from summaries by Jamison and others(1980), Lynch and others(1985), Young and others (1981), Meyerhoff(19821,Tetra Tech (19821, Husky Oil NPR Operations,Inc. (1983c), and numerous issuesofPetroleumInformation's"Alaska Report." Geologicinvestigations of northernAlaskawere first conducted in the early1900's by USGS-sponsored fieldparties. The first descriptionofoiloccurrence along the northernAlaskacoast was madeby Leffingwell,ofthe USGS, who reportedoilseepsinthe Cape Simpson area in 1917. Based on the presence of these seeps and conjecturedestimates of resource potential, President Harding established the Naval PetroleumReserve No. 4 (NPR-4) in 1923. Detailed field mapping by the USGS was begun at the request of the Navy Deparment and continues in NPR-4 (now termed theNational PetroleumReserveinAlaska, or NPRA) to thepresenttime. In 1944, the Navy, in cooperation with the USGS, 1aunched a majorexploratory drilling program and by 1953 had drilled 81 holes (45 core tests and 36 test wells). Oil fields were discovered at Umiat, Simpson,and Fish Creek,and gasfields were found at Gubik, South Barrow, Meade, SquareLake, Oumal ik, and Wolf Creek. The largest of these discoveries, 30 to 100 million barrels of oil at Umiat and 370 to 900 billion cubic feet of gas at Gubik were, and still are, uneconomic to produce. The gasfield at Barrowwas used to supply the Naval Arctic Research Laboratory and village of Barrow. In 1953, the Federal Government ceased funding the exploration of NPR-4. In 1959,Alaskachanged from territorialstatus to statehood and, under provisions of the statehoodact,selected as state land the central portion of the North Slope between NPR-4 and the Arctic NationalWildlife Refuge (ANWR). Explorationactivity was centered inthis"corridor"for the next few years. By the mid-1960's exploration activity had largely shifted from surface mapping in the foothills of the Brooks Range toseismicsurveys on the coastal plain. In 1964, theState of Alaska held thefirstcompetitive 1ease sale on the North Slope. A secondcompetitivelease sal e was held in 1965, with interestfocused on a large anticlinal - Proposed Sale 97 Boundary 1Barrow Gas Fields c/ 2 KuparukField 5," Beaufort Sea Planning Area rn, 1 ---- 3 MilnePointField 0,s 51: Approximate Shelf Edge 4 Seal Island 0 50 II Leased Blacks -- Sale BF-79 and Sale 71 5 Prudhoe Bay Field u 2'YX Field 6 Endicott miles t. W. Leased Blocks --Sale87 7 PointThomson structure south of Prudhoe Bay.In 1968, AtlanticRichfield Company and Humble Oil drilled the Prudhoe Bay State No. 1 welland, after drilling an additionalconfirmationwell, announced the discovery of what soon proved to be the largest oil and gas field found in North America. Recoverablereserves from the IvishakFormation(Triassic) and Lisburne Group (Carboniferous) range upwards of 10 bill ion barrels of oil and 26 trillioncubic feet ofgas. A period of intense drilling activity and seismic exploration in the coastal area after the Prudhoe discovery led to discoveries of oil, gas, and condensate in adjacentfields(fig.11).Possible commercial fields were delineated at Kuparuk (1969;1.5 billion barrelsrecoverable), Milne Point and GwydyrBay (1970; 120 million barrelsrecoverable), andPoint Thomson (1977; 350 million barrels, 5 trillion cubic feetgas)fields.Constructionof the Trans-Alaska Pipeline System (TAPS) from Prudhoe Bay to a tanker terminalin Valdez allowedproduction to begin from the Triassic reservoirs in the Prudhoe Bay field in 1977,with Lower Cretaceousreservoirs in the Kuparuk field brought on linein 1981. Federallyfundedexploration in NPR-4 was resumed in 1975 under the direction of the Navy. This responsibility was latertransferred to the Department of the Interior, which renamed it the National PetroleumReserve in Alaska (NPRA) in 1977. A total of 28 exploratory wells were drilled by the USGS contractor, Husky Oil, and 14,770 milesofseismic data were collected over the 7-year life of the program. The structural and stratigraphicanomalies mapped by the USGS and their contractor,Tetra Tech, Inc.,yielded some oil shows but no commercial-sizediscoveries. In 1979, the USGS reduced its estimate of recoverableoil in NPRA from 10 billion barrels to 3 billionbarrels. The first of a series of competitive oil and gas lease sales in the NPRA was he1 d in 1981,and three addition.al leaseofferingsfollowed. The most recent leaseoffering(1984) received no industrybids, and future lease sales are not scheduled at the present time. Only one exploration well was drilled by industry on Federal NPRA acreage,’ and this well (ARCD’s Brontosaurus No. 1) was plugged and abandoned in early 1985. . Explorationin the offshore area of Arctic Alaska began in the early19601s, and during the period of 1964 through 1984,over65,000 line-miles of deep-penetration,multichannel CDP seismic data were collected under permitinFederaloffshore areas. Government-sponsored explorationincluded a reconnaissance survey by the USGS in 1977 (Grantz and others, 1982b) which collectedover5,600 km of24-channel CDP seismic data on the Beaufort and Chukchi shelves. The first offshore lease sale (Sale BF-79) was held jointly by the State ofAlaska and the Federal Government in December 1979 (fig.11). A total of $1.056 billion was collected in high bids for 86 of the 117 tracts offered, with the highest bid of $143 millionfor a tractinthe Sag Delta-Duck Islandarea.Sixteen wells were drilled from natural and artificialgravelislandsin the next 3 yearsto test severalprospects.Potentially commercial discoveries were announced at Sag Delta-Duck Island(1980; approximately 350 millionbarrelsrecoverable from the Kekiktuk Formation) and at Seal Island (1984; approximately 300 million barrelsrecoverable from the Ivishak Formation) (fig. 11). In the Alaskan OCS, the Minerals Management Service (MMS) held its first lease sale (Sale 71) in the Beaufort Sea in October1982. Twenty-fourcompanies participated in activebiddingfor 125 of the 338 tractsoffered(fig.11). High bonus bidstotaled $2.067 billion, with the two highest bids of $227 and $219 million for tracts on the Mukluk structureinHarrison Bay. This 1argesubunconformity trap on the Barrow Arch resembles the supergiant Prudhoe Bay field just to the east (fig. 23). The most expensivewell in OCS history (over $140 million) was drilled from an artificialgravelisland to test a group of tracts receiving over a billion dollars in bonus bids. In early 1984, the drilling partners, led by Sohio, announced that the Mukluk well was a dryhole and they would plug and abandon the well.Additionalwells on the Mukluk structure have not been proposed. Two test wells were drilled farther to the northwest byExxon from a concrete island drilling system (Global Marine’s CIOS) during the 1984-1985 winter. These wells on Exxon’s AntaresProspect were also plugged and abandoned in early 1985. The most recent OCS lease offering in the Beaufort Sea (Sale 87) was he1 d in August 1984 (fig. 11 ) . In contrast to the selected areasincluded in prev$ousleasesales,Sale 87 offered 1,477 tracts covering the entire continental she1f from Point Barrow to the Canadianborder. Twenty-seven companies were awarded 232 tracts with high bidsof $877 million. The highestbids ($55 and$53 million) and most competitivebidding were concentrated on faulted anticline structures in CamdenBay in water depths less than 120 feet. New designs of mobilegravityplatformsorarctic dri!:ships willprobably be used to test these structures in waters beyond the depth limitationforgravelislands(approximately 50 feet). Site-specificshallow-hazardssurveysfor these high-bid tracts in CamdenBay were conducted in the fall of1984, and a series of exploratory wells using the Canmar drillship Explorer I1 began to test these prospects during the summer of1985. The results of exploration activity in the Mackenzie Delta region of Canada are relevant to the petroleumpotential of the AlaskanBeaufortshelfbecausebothregionscontain thick sequences of Brooki an strata equivalent in age, lithofacies, and structural style. Although the largefieldspresentlyproducingoil on the NorthSlopeoccur in the Ellesmerian sequence, the discovery of oil in turbiditesandsat Flaxman Island(AlaskaState A-1, 1975), the vast heavy oildeposits in the West Sakand Ugnu sands(Jamison and others,1980), and well-known oil seeps along the Alaskan coast indicatethathydrocarbons do occur in the Brookian sequence aswell.
50
Given the longlead time required for drilling platform design and construction,exploratory and delineationdrilling,environmental studies, regulatory and productionpermits, and infrastructure development, it is estimatedthat 10 yearswillelapse between a lease sale and first production from the sale area--assuming that discoveries of giant oil fields can' be found by a limited number of very expensive explorationwells. However, the Beaufort Sea Planning Area is highly complex structurally and stratigraphically. Although it may frustratepreliminarygeologicanalyses, the polycyclic tectonic history of this region may eventuallyproveto be favorable forlarge hydrocarbon fields.Explorationplaysareabundant, and offshoreprovincescontain untested potentialreservoir and source rocksranging in age from Devonian toTertiary. All of the offshore petroleumprovincescontainhydrocarbonplays which differ from those tested by previousexplorationonshore.Subsequentsectionsofthis reportwill summarize what is presently known aboutpotentialsource and reservoirrocks on the NorthSlope and their relationship to significant plays and recognizedtraptypes in the offshorepetroleum provinces.
52 5
Source Rocks on the Beaufort Shelf
ELLESMERIAN AND RIFTSEQUENCES The hydrocarbons which occur in the Prudhoe Bay and adjacent fields are generally regarded as having been derived from organic-rich shales in the El 1 esmerian sequence or Rift sequence (the Pebble Shale)orboth(fig. 12). The Lower Cretaceous Pebble Shalecontains an averagetotalorganiccarboncontent (TOC) of5.4percent, with C15+ hydrocarboncontent in excess of 3,000 parts per million (ppm) (Morgridge and Smith,1972). It is thereforeconsidered a very rich, oil-prone,potentialsource bed. The Jurassic Kingak Formation is consideredto be a very good potentialsourcerock,containing an average TOC of 1.9 percent and650 pprn of C1 + (Morgridge and Smith,1972, fig.16; Magoon and Claypool, 19847. The dark-colored, phosphatic,highlyorganicshales and limestones of the Triassic ShublikFormation arealsoregardedasrichpotentialoilsource beds (Seifert and others,1980,table 11). The TriassicSadlerochit Group (Kavik Formation)shales,Carboniferous Li sburne Group shales, and MississippianEndicott Group shalesareregardedas somewhat "lean"(loworganiccarboncontent) and gasprone(Morgridge and Smith,1972, p. 500). Seifert and others(1980, p. 428) provide data which suggest potentialsourceswithincertaindark-colored Endicott Group shales (Kayak Formation),althoughthey do not correlate any Prudhoe oils to this source. Jones and Speers (1976)suggested from crudeoilanalysesthat all oils in the Prudhoe areaaccumulationsaregeochemicallyalike and were derived from a common source or common set of multiple sources. Young and others (1977., p. 594, table10; p. 596, table 12) obtained a set of calculated ages for Prudhoe oils which varied depending on thereservoirhorizon from which they were extracted. Oilsamplesobtained from Triassic and Pennsylvanianreservoirs yieldedgenerationages of 218 million years (m.y.), while oil obtained from an Upper Cretaceousreservoir was found to have an apparentgenerationage of 87 m.y.Magoon andClaypool (1981, p. 644) interpreted this data as indicating that the Prudhoe oils were derived from both Cretaceous and Triassicsources. Magoon and Claypool(1981) identified two principalfamilies of oils on the NorthSlope: (1) a "Barrow-Prudhoe'' typefound in accumulationsalongthe Barrow Arch and (2) a "Simpson-Umiat" type found in the CapeSimpson area and the Umiat areain the northern
Souhce rock^, 53 SEISMIC STRATIGRAPHY SOURCE BEDS ;EQUENCE
SEAFLOOR 2UATER- QUATERNARY I Gubik Formation -0u- NARY NEOGENE BROOKIAN Sagavanirktok TOPSET/ PALEOGENE DELTA z 5 Y SANDS 0 0 [L m BROOKIAN PRODELTA
EARLY - BU - RIFT Pebble Shale Unit -Leu-•
Kingak Formation\ JURASSIC z 5 z 1[L Q w [L TRIASSIC I W u) W I 2 (0 A W w Sadlerochit Group A J PERMIAN W
PENNSYLVANIAN
Baird Gp. ? DEVONIAN (?) ? EU- --- ACOUSTIC Metamorphic Rocks OVERMATURE BASEMENT MIDDLE DEVONIAN -RANK• LINIAN
Figure 12. Stratigraphic relationships of significant source beds on tlie North Slope of Alaska. Shc?ded areas in source bed col ulnn indicate presence and type of source beds. The source potential of the lower Ellesrnerian strata thought to occupythe Northeast Chukchi Basin is unknown. The :netalnorphosed Franklinian sequence on the North Slope locally contains oryanic-rich rocks, but is thermally overmature.
54 foothillsofthe Brooks Range. An informalresearchconsortium conferencerecentlyconcluded (McCloy , 1983,p.14) thatthe"Prudhoe" oils were derivedprincipallyfromTriassic and Jurassicsources, whereas the"Umiat"oils were sourcedfromthe Lower Cretaceous PebbleShaleoryoungerBrookian sequence rocks. The conceptof a Pebble Shalesource for Prudhoe oils was discarded by some participants atthe conference on the basis of the apparentthermal immaturity of these rocks in the vicinity of the Prudhoeareaaccumulations. However, Morgridge and Smith(1972, p. 500) have forcefully argued fromgeochemical and geologicaldatathatthe Lower Cretaceous PebbleShaleformsthemostprobablemajorsourceforthePrudhoe Bay oils.
Few ofthetraditionallyrecognizedPrudhoe-provincesource rocksareconsidered to be present inthe offshore areas north of the Barrow Archprovince (IA, fig. 1). All Ellesmerian sequence source beds areprobablyabsent as a consequence ofdepositional onlaporerosionaltruncationfromareasnorthofthezero-Ellesmerian line traced on figure 1, and, therefore, do notformpotential sourcesover much ofthe OCS planningarea.Jurassic andLower Cretaceouspotentialsource beds arepresentbeneaththeArctic coastalplain of ANWR (Reiser and others,1980), and may extend offshore into that part of the Demarcationsectorwhich is south ofthe HingeLine(Norris and Yorath, 1981, fig.3).Jurassic shalesoftheKingakFormationincrease inorganiccarboncontent to the east (Magoon and Claypool,1984,fig. 6) and may form an excellentpetroleumsource bed in ANWR and adjacentoffshoreareas.
The Pebble Shale, or itsstratigraphic equivalent, probably extendsoffshore (fig. 13) as theupperpart of the Rift sequence. The geochemicalcharacterofthe Rift sequenceshaleswithinoffshore grabens may differ fromthat of time-equivalent rocks to the south, whichaccumulated in the open marineenvironment of the Colville Basin. The Rift sequenceshales inoffshoreareas may haveaccumulated in a more restrictedenvironmentwithincontemporaneouslysubsiding infrarift grabens. Kerogencompositions ofshales within the infrarift grabens may be similar to the geologicallyanalogous North Sea grabens,where highlysapropelic(oil-prone)shalesin thecentral parts of the grabensareringed by leaner,nonsapropelic shalesalongbasinmargins(Reeder andScotchman,1985,p. 142). The recently announced discovery at Seal Island(PetroleumInformation, 1984) is particularlyinteresting because the oil is much lighter (40" API) thantypical "Barrow-Prudhoe'' oils (27" API). These oils may havebeen derivedfrom more matureorgeochemicallydistinct Rift sequencesource beds to the north.
Over much of the Nuwuk and KaktovikBasins, Rift sequence strata lie at depthsexceeding 23,000 feet and thereforeareprobably overmature. Intheseremoteoffshorebasins,potentialsource bedsmustbesought withintheBrookian sequence.
Sowrce Rock, 55 Depth in Feet x I O3 N • - n m o m cn
frP BROOKIAN SEQUENCE
EarlyCreta ce'OUS and youngershaleshave typi cally beenfound to be nonsourceor gas prone,althoughthe A1 bianTorokshales (fig.12) havebeeninvokedas a potentialsourcefortheUmiat-type oil (McCloy, 1983, p. 14). Inthe Prudhoe Bay area,an organic-rich, tuffaceous,radioactiveshaleofConiacian-Turonianage (Carman andHardwick, 1983, fig.4)disconformablyoverliesthePebble Shale. An analysispublishedbySeifertandothers(1980,table II,.p. 428)indicatesthatthis UpperCretaceousshale may be a rich (TOC=3.44 percent),oil-prone,potentialsourcerock. These rocksapparentlyextendnorthwardatthebaseoftheBrookian sequence into offshore areas and may there form an important potential source bed.
UpperCretaceous shalesoftheBrookianprodelta facies disconformablyoverlie Coniacian-Turonianshalesabove thebreakup unconformity (BU) inthe central part of the Arctic Platform. However, BrookianprodeltashalesofAlbian age (TorokFormation) directly overlie the Pebble Shale in the western part of the Arctic Platform.Incontrast,theBrookian sequence intheeasternArctic Platform consists almost entirely of Tertiary strata which overlie a relativelythin sequence of UpperCretaceousrocks. The Tertiary Brookiansequence in the eastern Arctic Platform, unlike the much older Brookian strata found in more western parts of the platform, is probably the most representative of the largely Tertiary Brookian sequence anticipatedoffshore inthe Beaufort Sea Planning Area. The followingdiscussion will thereforefocus upon thegeochemistryof BrookianstrataintheeasternArcticPlatform. The Brookian sequence in this areahasbeenpenetratedbyseveralexploration wells. These wellsformthedatabaseforourspeculationsabout thesourcerockpotentialoftheBrookian sequence in offshore areas.
Geochemicalanalyses of the UpperCretaceous to Tertiary strata of the Brookian sequence in the Camden Bay area(Union Oil, 1983; Harwood, 1983)aresumnarized in a graphicalformatinplates 8, 9, and 10. Theseanalyseswereperformedoncuttingsfrom threewells in the Camden Basin(fig.18):the Exxon Point Thomson No.2, the Exxon Point Thomson No. 3, and theMobil West Staines No. 2 wells. The geochemicaldata show thattheBrookianshalesin this arearange from nonsource to fair source potential(based upon TOC) and yield a low hydrogen index consistent with a dominant populationof gas-pronekerogens. However, in two of the three wells,analyses of certain sample intervalsnearthecontactbetween themarineprodelta and marginalmarine deltaic facies indicate the presence of organic rich beds with hydrogenindices in excess of 200 (plates 9 and10).Thissuggeststhepresenceofliquid-prone potentialsource beds. At presentburialdepthsof 5,000 to 7,000 feet and vitrinite reflectance levels of 0.30 to 0.55, thesepotential sourcebedsarethermallyimmatureor lie just within the lower limit . HYDROGEN INDEX (mg HC/g Org C)
- N (* P Ln 0 0 0 0 0 0 0 0 0 1 I -I-•I TYPE I (OIL)
h 3 m 0 0 N \ 0 z
Ln. 0
40 foroilgeneration. However, atappropriateburialdepthsoffshore in the Kaktovik and Nuwuk Basins,these beds or younger facies-equivalent beds could have undergone sufficient thermal maturationto have formed importantsourcesfor liquid hydrocarbons. The discoveryofpotentialoilsource beds at this stratigraphiclevel is very significant becausethe Brookian sequence is generally regardedasgasprone, and over most of theplanningareanorth of the Hinge Line, the potentialoilsource beds inthedeeperEllesmerian and Riftsequencesareeitherabsentor so deeplyburiedthattheyare probablyovermature. Figure 14 presents a series of modified Van Krevelendiagrams which illustrate the variation in kerogencompositionforthethree formationsbelievedtoconstitute the principalsourcesforthe hydrocarbonsinthe Prudhoe Bay field (Morgridge and Smith, 1972; Magoon and Claypool.1981). All threeformations,includingthe PebbleShale,exhibit a broadspectrum of kerogencompositions, ranging fromType 111 (gasprone)to Type I (oilprone). For comparison, Van Krevelen plots for Brookian strata in the Point ThomsonNo. 2 and West Staines No. 2 wells arepresented in figure 15. The compositional field of the "Prudhoe Bay" oilsource beds synopsized from figure 14 is superimposedas a stippled area upon the Brookian datasets in figure 15. Severalanalyses in the Point Thomson No. 2 wellappear to document a strong Type 1/11 kerogen component that is quitecomparable to the kerogen makeup of the Prudhoe oilsource beds. The depthsofoccurrence of theseliquid• prone strataareannotated on theplot. Most rangebetween6,030 and 6,570 feet, straddling the fluvial-deltaic to prodelta facies boundary in the Point Thomson No. 2 well. In both wells, the bulk oftheBrookian strata aboveand below this facies boundary are compositionallydominated by Type 111 kerogens and are therefore consideredto be unlikelypotentialsourcesforliquidpetroleum. Sample logs for the intervals which contain these apparent oil source beds indicate the presenceof live, heavy hydrocarbons in sandstonecuttings. Itis therefore possible that the geochemical datareflectthepresence of migratedoilratherthanthepresence ofactualoil-pronesourcebeds.Contaminationof this kind is furthersuggested by high bitumen/TOC ratios in theintervalof interest in thePoint ThomsonNo. 3 well (plate 10). With oniy the geochemicaldata at hand, it is difficult to assesstheextent and impact of a possiblecontamination problem in thesedata. Therefore, as a means ofindependentlyevaluatingthepresence of potentialsource beds in this interval,the three wells were analyzed by using a wireline log method devised byMeyer and Nederlof (1984) forsource bed identification. This method is based upon the observation that source shales typically exhibit lower acoustic velocity, lower density, and higherresistivitythanmineralogically similarnonsourceshales. TheMeyer and Nederloftechnique employs bivariate plots of these properties as a means ofdiscriminating between conventionalnonsourceshales and potentialsourceshales.
Sowrce Rocfu, 59 POINT THOMSON #2
0 BROOKIANDELTA SEQUENCE
BROOKIANPRODELTA SEQUENCE
....:.. COMPOSITIONALRANGE :.::.:: OF SOURCEBEDS FOR ..:.. PRUDHOEOILS(FIG. 14)
60 WEST STAINES #2 60 NET FEET
150r THOMSON POINT # 2
70 t
6o t I I I I I 2 3 4 5 6 7 8 910 20
POINT THOMSON # 3 I 57 NET FEET
AT:SHALETRAVELTIME 0 NONSWRCESHALE RT: SHALERESISTIVITY 60 0 SWRCE SHALE
2 3 4 5 6 7 8910 20 RT (75O F)
Figure 16. Cross plotsfor source bed identificationinthe West Staines No. 2, Point ThomsonNo. 2, and Point ThomsonNo. 3 wells. Line separating source and nonsource fields after Meyerand Nederlof (1984, fig. 11). Normalization ofshaleresistivityto 75 "F was done by use of Arp's formula (after Schlumberger, 1972, p. 9).
Sowlce Rochn, 61 CAMDEN BASIN ARCTIC PLATFORM HINGE LINE KAKTOVIK BASIN
OIL WINDOW LOCATIONDIAGRAM TOPSET DELTA SANDS ...... ,...... ,..... V, 0.6 - 1.35 BEAUFORTSEA PLANNINGAREA
SOURCE BEDS AT LINE OF TOP OF PRODELTA -- TIME HORIZON SECTION
Figure 17. Schematicgeological cross section illustratingpossible time-transgressive nature of Cenozoicpotential sourcebedsandrelationshiptotheoil window intheKaktovikBasin. The potential Sourcebedsencountered at the top of theBrookianprodeltafacies in the Camden Basin may extendnorthwardas a faciesofthe northward-progradingBrookiandeltaic system. Inthe vicinity of the Camden anticline,thesepotential sourcebedsprobably lie within the oil windowand there may become activesourcesforliquidhydrocarbons. Potentialsourcesdeeperwithintheprodeltashale sequence orinolder Cretaceous-Jurassicstrata may be overmature in the vicinity of Camden anticline.
I The results of the analysis of the Camden Basinwellsare presented in the cross plots of figure 16. The graphsdisplay shaletraveltimeplottedversusshaleresistivity. The line which separatessourceshales from nonsource shalesis the statistically derived discriminant line obtained from worldwide data by Meyer and Nederlof (1984, fig. 11). The plots show that a considerable number of shale beds, particularly in the Point Thomson No. 2 well,qualifyaspotentialsources. The analysisappearsto confirm thatpotentialsource beds are present inthis interval, although it doesnotaddresswhetherthesesourcebedsareoil prone or gas prone. The aggregatethickness of all beds identified by the graphs in figure 16 aspotentialsource beds is postedwith each plotbeneath the well name.The depthintervalsofoccurrence of alllog-identifiedsource beds arealsopostedin the "Si + S2" columns of plates 8, 9, and 10. As suggestedpreviously from geochemical data, the log-identifiedsource beds areobservedto span the transition zone between the Brookian prodelta and overlying fluvial-deltaicfacies.
The coincidence of the potentialsource beds with the marine to marginalmarine transition in the Brookian deltaic system suggests that the source beds represent a specific facies setting within thistransition zone. Dow (1977, p. D6-D8) notesthatareasof high marineorganicproductivityaretypicallyassociated with areasof high nutrient supply, such as upwellingzones and the mouths of majorrivers. The source beds in the Camden Basin may have formed in interdistributary bays or estuarine settings where moderatesedimentationrates and high nutrient supply were coupled with quiet water and anoxicbottomconditions. If so, then this organic facies might have progradednorthward with the delta complex as it spilledintooffshoresedimentarybasins.This model, summarized in figure 17, impliesthat the source bed facies is timetransgressive and becomes progressivelyyoungerto the north in offshore areas.
As an alternative model, we alsorecognizethatcertainglobal conditions (e.g., climate,sea level, oceaniccirculation) which canpromote the formationofpotentialsource beds may have prevailed at the specifictime(discussed below) that the Brookian sourceshales were deposited in the Point Thomson area. It is possiblethat potentialsource beds may haveaccumulatedsimultaneously in various environents (e.g., interdistributarybay,deltafront,prodelta, abyssalplain) where the chiefsedimentary process was deposition ofshale. If a specific intervalof time and associated global conditions,ratherthanlocalfacies settings, provided the dominant controlsforsource bed accumulation, then source beds in the Brookian sequence may be found to be paleogeographicallywidespread, but confinedto a certain interval of geologictime. The potentialsource bed zone inthePoint Thomson area wells roughlycoincides with an interval that may be earlyOligocene in age,based upon palynologicalevidence (J. Larson,personal commun.,
Souhce Rockb, 63 i
June1985). The mid-Oligocene(30 m.y.1 is thoughttocoincidewith the end of a worldwidecycle of maximum marinetransgression (Vail and others, 1977, fig.2) and relativeglobal warmth(KeigwinandKeller, 1984,p.16; Keigwin, 1980,p.723). EarlyOligocenetime may thus represent a uniqueperiodinthedepositional history oftheBrookian sequence.Demaison andMoore(1980,p.1204)haveemphasized that known oil sourcebedsystems inthe stratigraphic record are not randomly distributed, but coincide with periods of worldwide transgression andoceanicanoxia.Northofthecontrolwells, facies-equivalent but younger strata may havebeendeposited in a climaticsetting or sea-level stand much differentfrom that which prevailedduringearlyOligocenetime.Changingglobalconditions may haveprecludedtheformationofsourcebeds in the transition zone facies setting in younger Tertiary shales in more northern parts oftheBrookiandeltaic system. However, thishypothesiscannot be evaluated in the absence ofoffshorewelldata.Forthepresent, we assume northwardcontinuity of the source beds, either as facies-equivalent,youngerstrata,orastime-equivalentstrataformed indifferentenvironmentalsettings.Ineither case, potentialsource beds are presumed toextendnorthwardwithintheBrookian sequence intooffshore areas of the Kaktovik and, possibly, Nuwuk Basins.
Sourcebedscapable of generatingliquid hydrocarbons may be presentelsewhere in theCenozoicBrookiansedimentary sequence, butremainlargelyundetectedatpresentbyconventionalgeochemical and visualkerogenanalyses.Explorationwells on theCanadian Beaufortshelfhavefoundapparentlyautochthonouslightoiland gascondensate in Cenozoicsedimentswhichexhibitinsufficient levels of thermalmaturity(lessthan 0.6% VR~)for oil generation. These liquid hydrocarbonsarealsoanomalousbecausetheyappear to havebeen derivedfrom sediments containing primarily Type I11 (humic)organicmatter,generallyassocfatedwith gas-phase . hydrocarbons. Snowdon (1980)and Snowdon and Powell(1982)suggest that these anomalous light oils werederived from thermalmaturation of resinite (a maceralderivedfromtreeresfns) or fromdispersed resinousmaterial.Resiniteis a prominentconstituentofthe Eocenebrown coals of Germany and the Tertiary coals of Japan (Stachandothers, 1982, p. 118-1191, but is also found in CarboniferousandCretaceouscoals.Thisorganic component ofthe Brookiansediments may havebeenoverlooked in routine geochemical studies. If it occurs insufficient abundance intheBrookian sequence, Tertiaryrocks at relatively shallow depths and low levelsof thermalmaturity may offer a much greaterpotential for generation of liquid hydrocarbonsthanpreviouslythought.
64 6
Geothermal Gradients on the Beaufort Shelf
The geothermal gradients measured in wells acrosstheArctic coastal plain of northernAlaskavary widely,as illustrated in the geothermal map presented in figure 18. Between 148O and 154' westlongitude,theareas of high heatfluxgenerallyfollowthe axis of the BarrowArch.West of 154", thehighestgeothermal gradients lie south of the BarrowArch (which parallels the coastline) and extendthrough the Husky-NPRA South MeadeNo. 1 well. This well measured the highest reported geothermal gradient (47 Wkm) on the North Slope of Alaska(Blanchard and Tailleur, 1982a, p. 47). East of 148" westlongitude,thenorthwest-trendingaxisofthe Barrow Arch is overprinted by thenortheast-trending Camden Basin. The Camden Basin is a LateCretaceous toTertiaryforedeepdeveloped north of the Sadlerochit Mountains that contains at least 13,000 feet of Tertiarysedimentsonshore. As a result of rapidsubsidence and high sedimentationrates, geothermal gradientsare somewhat lower in the Camden Basin. The compositeaveragegradient from severalwells in thePoint Thomson area is 33 OC/km (fig. 18). Geothermal gradients comparable to those measured alongthe presentcoastlineareinferredtoextendintothegeologicalTy similarnearshoreareas of the ArcticPlatform(fig.18). An averagegradient of approximately 36 "C/km probably characterizes theseareas, with the possible exception of the easternmost parts of province IB in the Camden Basin, where, as noted above, the gradient may be as low as 33 OC/km. Lower geothermal gradientsareanticipated north of the Hinge Line (fig. 18) in the Nuwuk and KaktovikBasins. Both of theseareas were sites ofsubstantialbasinalsubsidence and sedimentationduringCretaceous andCenozoic time. The only information on the thermal structure of these remote provinces is a well in Canadian waters a few tens of miles east of the U.S.-Canadian border. The Dome Natsek E-56 wellmeasured an uncorrected geothermal gradientof 26.7 "C/km (fig. 18). Although insufficientdataare available for the application of Horner-typecorrections to the thermaldata from this well,relativelylongresidencetimesfor uncirculated mud (up to 20 hours)precededmostloggingsurveys. The temperaturedataobtained in routinelogging runs in the Natsek well arethereforeconsideredtoapproximatestatic(true)formation temperatures. The geothermalgradientsinferredabovefortheprincipal petroleum provinces of the Beaufort shelf can be used to estimate the minimum burialdepthsrequiredtoachievesufficientthermal maturationoforganicmatterforhydrocarbongeneration. These minimum depthsfor the top of the ?oil window" aretabulatedbelow. The thermalrangesforoilgeneration and destructionusedhere wereobtained from an organicmaturation chart published by Hunt (1979, figs. 7-42, 7-49) and time-temperaturerelationshipsdescribed by Oow (1977, figs. 13, 15). Because oftheeffectofthermal exposuretime on thematurationofkerogen,youngersedimentsmust beheatedtohighertemperaturesthanoldersediments in order to reachcorresponding levels of thermal maturity, orequalvaluesof vitrinitereflectance. Subsidence in the Arctic Platform,the Camden Basin,andsouthernpartsofthe Nuwuk Basinoccurred primarily duringCretaceoustime,whereassubsidence in the outer Nuwuk Basin andtheKaktovikBasinoccurredprimarily in Tertiary time. Thus, the maturation history of the Arctic Platform, Camden Basin,and perhaps thesouthernmostparts of the Nuwuk Basin is considered to resemble that of the Cretaceous trend of the LouisianaGulfCoast, where the oil window is found to lie between100and 136 'C (0.6 to 1.35 percentvitrinitereflectance). In contrast,areasof Tertiarysubsidence north of the Hinge Line are probably more analogous, for example, to the lower to middl e MioceneGulfCoast trend (Dow, 1977, fig.13), where the oil window is projected to 1ie between131and198 "C (0.6 to 1.35 percent vitrinite reflectance).
Predicted Geothermal GradientProvince Oil Generation Oil Destruction
ArcticPlatform (provinces IA, 36 'Cjkm 9,200 feet 12,300 feet IB, IC) (100 "C) (136 "C)
Camden Basin (easternpartsof 33 'Cjkm 10,200 feet 13,700 feet provinces IA, 19) (100 "C) (136 "C) Kaktovik and outer Nuwuk Basins 27 "C/km 15,400 feet 23,500 feet (provinces IIA, (131 "C) (198 "C) 115, IIC)
Southern27 Nuwuk 'Cjkm 12,200 feet 16,500 feet Basin ( province I IA) (100 "C) (136 "C)
66
The useof a temperature value to define maturity in the rocks of the Nuwuk andKaktovikBasins is probably justified because maturation and oilgeneration are youngandareprobablyrelated to equally young, rapidsubsidence withinthe depocenters (Tissot andWelte,1984,’~.619).Thisassumption may belessvalidfor oldersourcebeds within the Ellesmerian and Rift sequences of the ArcticPlatform.InthePoint Thomson area (Camden Basin),vitrinite reflectancevalues for Rift sequencebedscompare favorablywith maturity 1evels predicted from wellbore geothermal data (plates 8, 9, and10). However, inwellsinnorthwestern NPRA, Ellesmerian and Rift sequencesourcebeds exhibit levels of thermal maturity (Magoon and Bird, 1986)whichexceedthosepredictedbypresent burialdepths andgeothermalregimes.Thissuggests a priorhistory of deeper burialfortheserocks.
Thermalmodels for rift zones (Falvey,1974,p.96)predict elevatedgeothermalgradientsprecedingandaccompanyingtheactual rift event.OldersedimentsontheArcticPlatform may haveaccordingly been subjected to a thermalenvironmentduringthe rift eventwhich was much differentthan that which exists at present. Vitrinite reflectancedatafortheInigok No. 1 well(Lercheandothers, 1984, fig. l), for example, suggest a pasthistoryofelevated heatflow. However, existingdata (Magoon andBird,1986)from coastal we1 1s along the southern margin of the planning area indicate thermalimmaturity to early peak-oil levels of maturity for Rift sequenceand oldersourcebeds (e.g., plates 8, 9, and 10). Isoreflectance maps publishedby Magoon and Bird (1986, figs.12 and 18) for rock units in NPRA document a pattern of increasing thermalmaturitytothesouth, away fromthe rift zone. This does not support an association of elevated heat flow with the rift axisduringthe rift event. However, this map patternprobably more closelyreflectspatternsofburialhistory,ratherthan . arealvariationinheatflow. Because pre-Brookianbeds were at shallowburialdepthsnearthe rift zone at the time of rifting, threshold temperatures for the initiation of vitrinite maturation wereprobablynotreached. The earlyhistory of highheatflow associated with the rift eventprobablyhad little or no effect on the present distribution of thermal maturity within Ellesmerian or Rift sequencesourcebedson the Arctic Platform. 7
Potential Reservoir Formations
In thefollowingparagraphs, a synopticdescription is presented for each potentialreservoirformationwhich may occur in the Beaufort Sea Planning Area (fig. 3). The descriptionsareobtained fromanonshoredata base,and thedistribution,facies setting, and potentialreservoir quality of these rocks in offshore areas canonly be conjectured.Mostoftheformationsdescribedarenot producinghydrocarbons atpresent, and in onshoreareas,severaldo notoccur in sufficient thickness or with adequatereservoir properties to beevenconsideredcapableofsupporting a development facility. It is recognized,however, thatthe thickness and reservoir quality of theserockunits may increase in different facies settings offshore.
The most prolific reservoir formation of the Ellesmerian sequence is theIvishaksandstone,which is the principal reservoir in the Prudhoe Bay field(fig. 3). Development oflessattractivereservoirs in other fields near Prudhoe Bay has only been made possibleby the infrastructure which was constructed to develop theIvishakreservoir in the Prudhoe Bay field. Clearly, theIvishak Formationmust be consideredthe primary reservoir objective within the Barrow Arch province,with all other units forming secondary,lessattractive objectives.
Withinthe Nuwuk and KaktovikBasins,Ellesmerianreservoir strataareeitherabsentorlieatdepthsexceeding 20,000 feet.In theseprovinces, only sandstones of the Brookianfluvial-deltaic facies may occur in sufficient thicknesses and with adequate reservoir qualityto beconsideredattractiveexplorationobjectives. More deeplyburiedBrookiansandstonesthataccumulated in other facies settings,such as turbiditesdeposited in submarinefancomplexes, are much more risky objectives, andcan onlybeconsideredsecondary exploration targets.
DEVONIAN CLASTIC ROCKS AND CARBONATES
ClasticrocksandcarbonatesofDevonian age are known onlyfrom surfaceoutcropstudiesofallochthonousthrustsheetsintheBrooks Range farsouth of theBeaufort Sea Planning Area. No age-equivalent, undeformedrocksare known to occur in any wellswhichhavepenetrated t a
basementneartheArcticcoast. However, stratapossiblyequivalent totheallochthonousDevonian sequenceexposed in the Brooks Range areinferred to occur in the Northeast ChukchiBasinbeneaththe Chukchi shelfprovince.
The KanayutConglomerate(UpperDevonian)ranges in thickness from 2,600 meters inthe east-central Brooks Range toapproximately 300meters inthewesternBrooks Range (Nilsen andMoore,19B2b). InthewesternBrooks Range, thelowerpartoftheKanayut Conglomerategrades into the NoatakSandstone(Nilsenand Moore, 1982b, p.9),estimatedtoreach 1,000 meters inthickness (Tailleur andothers,1967). The KanayutandNoatak sequences consistof sandstones,conglomerates,andshalesdeposited in a fluvial to nearshoremarinesettingwhichflanked a highlandsourceterraneto thenorth and east(Nilsen and Moore, 1982b).Althoughtheserocks arewellinduratedinsurfaceexposures,theirpotentialreservoir qualityinthesubsurface is unknown. No porosityorpermeability dataarepresentlyavailable.
TheUpperDevonian clastic sequence (EndicottGroup)containing theKanayutConglomerate,NoatakSandstone,andHuntForkShale is underlain by platform carbonates of the Baird Group (Nilsen and Moore, 1982b, p. 1)of Middle to LateDevonian age (Tailleur and Brosge, 1970, p. E4). We speculatethattheseDevoniancarbonates may be equivalent to the basal carbonate unit identified in the NortheastChukchiBasin. No informationispresentlyavailable on thereservoirpotentialoftheBaird Group rocks.Deformedsequences oflower Paleozoic carbonates (Katakturuk Dolomite and Nanook Limestone)arealso known fromsurfaceexposures in the northeastern Brooks.Range.Availabledatasuggestthatthereservoirquality of these rocks is poor, with porosities no greaterthan 1.9 percent reportedfor limestones and no greaterthan 5 percent reported for dolomites(Dutro, 1970, p. M2-M3). However, moderatelydeformed toundisturbed Devonian carbonates are known to contain numerous petroleumaccumulations incentralAlbertaProvince, Canada (Barssandothers, 1970; Hemphill and others,1970),and at the Norman Wellsfield,NorthwestTerritories, Canada (Meyerhoff, 1982, p. 525). Devoniancarbonates may, therefore,beregardedaspotentialreservoir formationswherevertheyarethoughttooccur in subsurfacetraps.
KEKIKTUKFORMATION (MISSISSIPPIAN)
The KekiktukFormation,partoftheEndicott Group, is widely exposed in the Brooks Range and is found inthesubsurfaceacross much oftheArcticcoastalplain.WithintheBeaufort Sea Planning AreaeastofBarrow,theKekiktukFormation is confined to the Barrow Archprovince. West of Barrow, intheNortheast ChukchiBasin, many thousands of feet of strata inferred to beage equivalent to the Endicott Group arepresent(fig.7). These strata may formthemost importantobjective in the Chukchi shelf province.
70 At surfaceexposures and in most wells, theprimary porosity' ofKekiktuksandstonesand conglomerates is observed to be completely occludedbysilica cement.For this reason,theformation was not regardedas a significantpotential reservoir until the discovery ofthe Endicott field. At thislocation northeast of Prudhoe Bay, Kekiktuksandstonesare much lessthoroughly cementedandformthe principalreservoirwithinthefield.Reservoirintervalsinthe Endicott field possess an average porosity of 20 percent (Alaska Oil and Gas Conservation Comnission, 1984, p. 57) and the average permeability of some zones may rangeup to 1,100 mill idarcys (Behrman andothers, 1985, p. 656). The studiesof Behrman and others (1985)suggestthattheporenetwork in the Kekiktuk sandstones within the Endicott field hasbeenenhancedbysecondaryleaching. However, even withinthe field, abrupt lateral changes in sandstone thickness and net sand contentare observed within the Kekiktuk Formation. In mostareasacrosstheArcticPlatformtheoverall reservoirqualityoftheKekiktukFormationisquitepoor. The reservoir potential of this formation (or its stratigraphic equivalent)inthe Northeast Chukchi Basin remains unknown.
LISBURNE GROUP (MISSISSIPPIAN TO PENNSYLVANIAN)
The Lisburne Group is widely exposed at surface localities acrosstheBrooks Range andhasbeenpenetratedby many wells alongthecoastalplainofnorthernAlaska. It is consideredto bepresentovermostoftheBarrowArchprovince,but it may pinch out in the southernmostpartoftheChukchishelfprovince.Beneath theArctic coastal plain, the Lisburne Group ranges up to 4,000 feet in thicknessandconsistsfundamentallyof two informallimestone unitsseparatedby a dolomiteunit(Bird andJordan,1977b). The dolomite unit hasbeenconsidered to formtheprimaryreservoir objective within the formation, exhibiting porosities averaging from10 to 15 percent but ranging as highas 27 percent locally (Bird andJordan, 1977b, table 1). The Lisburne Group contains only one known comnercialaccumulation and that is at the Prudhoe Bay field. There, theupperlimestoneunitliesmainlywithinthe Prudhoe oil column,whereas themain dolomite unit lies below the oil-watercontact.Nevertheless,theLisburnepool in the Prudhoe Bay field is thought to contain 2 to 3 billion barrels of oil in place(Edrich,1985).Testsofrelativelythindolomite beds in the upper limestone unit in the Prudhoe oil columnhave yielded flow rates of 1,000 to 1,500 barrels of oil per day (Birdand Jordan, 1977b, p.93).Significantdelineation anddevelopment drilling, however,has only occurred within the past few years, followingdevelopment of the infrastructure for the Ivishak reservoir in the Prudhoe Bay field. Oil hasbeen testedfromLisburnecarbonates at other localities, but no additionalcomnercialaccumulations havebeenfound.Because of its complex reservoirgeologyand generallypoor reservoir quality, the Lisburne Groupcan only be considered a secondaryobjective in theBarrowArchprovince. ECHOOKA FORMATION (PERMIAN)
The Echooka Formationconsistsof very fine- tofine-grained, muddy, glauconitic sandstone. It is presentinthesubsurface acrossthe Arctic coastal plain but thins depositionally to the north. It is probablyabsentovermostoftheBarrowArchprovince. The formationreaches a thickness of at least 250 feet across central NPRA, butthins northward to a zero edge whichprojects offshoreintothe Chukchi shelfatPeard Bay. The Echooka Formation therefore may notextendnorthwardintotheChukchishelfprovince(IC). The formation is quite shaly to the south, but becomes moresandy tothenorth(Tetra Tech,1982, fig. 46). At the USGS-Husky Peard No. 1 well,theformationconsistsof 160 feetof fine-grained sandstoneexhibitingcoreporositiesrangingfrom 10 to 21 percent. However, reservoirqualityisquitepoor:permeabilitiesrange from 0.0 to a maximum of 2.5 millidarcys(Husky Oil NPR Operations, 1982,p.E-1).Poor reservoirqualitytypifiesthe Echooka Formation, andevenwhere it 1ies within the hydrocarbon column at Prudhoe Bay, it does not constitute an attractivereservoir (Jones and Speers, 1976,p.32). On thebasisoftheseobservations,the Echooka Formation is consideredunlikelytoform a potentialreservoir horizon in any part of the Beaufort Sea PlanningArea.
IVISHAK FORMATION (TRIASSIC)
Fluvial-del taic sandstones andconglomerates of the Ivishak Formation form the most significant reservoir unit on the North Slope. The IvishakFormation is theprincipalreservoirinthe Prudhoe Bay field, where porositiescan rise above 30 percentand permeabilities may exceedseveraldarcys(Morgridge and Smith, 1972).These propertiesaccount inpartfortheexceptionalreserves and productivity of this super-giant field.
The distribution of the Ivishak Formation on the NorthSlope is illustrated in figure 19. In the western part of theBarrow Archprovince,theIvishakFormation is absentnorthof a depositional limit shown asthe"ZeroIvishakFormation"lineinfigure 19. AlongthecentralBeaufortcoast,theIvishakdepositional edge is truncated by the LCU. The IvishakFormationtruncation edge lies offshore and roughly parallel to .the present coastline (fig. 19). In the vicinity of PrudhoeBay, it passessouthwardonshorebeneath theArcticcoastalplain.InnortheasternAlaska,surfacegeology (Reiser andothers,1980)suggests a much more northern position (nearthecoastline)forthissubcropline. However, thedistribution oftheIvishakFormationbeneaththecoastalplainofnortheastern Alaskaremains unknown. The map in figure 19 is drawn to suggest thatnortheasternAlaskaisunderlainby a subtlestructural basin or outlier of Early Cretaceous age in whichEllesmerianstrata, includingtheIvishakFormation,havebeenpreservedbeneaththe LCU. However, we recognizethatthe map distributionof the Ivishak
72 \ SEA BEAUFORT PLANNING AREA \ \ \ 72. \ ' \. . Formation and other units in northeastern Alaska may havebeen disturbed by northward-directed thrust faults of Cretaceous and younger age. The possibleimpactof such potentiallysignificant structuraldisplacements, however, cannot be evaluatedwiththe limited data available at present.
Eckelmannand others(1976)and McGowen and Bloch(1985)have recognizedthepresenceof a majorlobe in the Ivishak deltaic system in the vicinity ofthePrudhoe Bay field.Similarconclusions werereachedbyJonesandSpeers(1976, figs.14 and16).Figure 19presents a net sandisopach map fortheIvishakFormation. This map suggeststhat maximum sand deposition in the Ivishak system was focused at two principal localities on theNorthSlope. The westernmostdepocenter(netsandgreaterthan 400 feet) lies nearPrudhoe Bay and is designatedthe"Prudhoe Sand Maximum" in figure 19. Finergrained,distal-faciessandstonesandsiltstones possessingpoorreservoircharacteristicsgenerallydominatethe IvishakFormation at any significant distance from the Prudhoe Bay area. A secondsanddepocenter ordeltaiclobeappearsto be locatedinnortheasternAlaskanearDemarcation Bay, where surface outcropscontain 390 feet of sand in a stratigraphic section abbreviatedby an overlyingunconfonnity(Detterman, 1984, section 6). Thisexposure and other measured sectionsalong the Egaksrak River publishedbyDetterman(1974;1984) show thatthesandstones of theIvishakFormationarethicker andmore conglomeratic in those areasthan in outcropsfarther south andwest,where a more distal faciesappearsto be dominatedbyfine-grainedsandstone,siltstone, andshale. These observationspromptedDetterman(1981, p. 39) to suggesttheexistenceof a secondmajordeltacomplex in the Ivishak depositionalsystem innortheasternAlaska.Thisisdesignated the"Demarcation Sand Maximum" infigure 19. Thiseasterndeltaic lobe may be timeequivalent, as well asfaciesequivalent, to the depocenternearPrudhoe Bay. The existenceof an easterndeltaic lobewhich may have localized the accumulation of a reservoir sequencecomparable to that found at Prudhoe Bay is very significant totheprospectivenessofnortheasternAlaska (ANWR) aswell as OCS waters in the eastern Beaufort Sea.
In the Kavik gas field, 80 milessouthwest of Barter Island, IvishakFormationporositiesrangefrom 5 to 12percent(Mastand others, 1980, fig. 31. Porositiesofonly 2 to 10 percentare reportedfor outcrop specimens ofIvishak sandstone in the northeasternBrooks Range (Palmerandothers,1979). However, on thebasis of the paleogeographic model presented above, we speculate that the reservoir quality of the Ivishak Formation in northeastern Alaska may improve tothenorthbeneaththecoastalplainconcomitant withthedevelopmentof a coarsergrainedfluvial-dominatedfacies asfound at Prudhoe Bay.
Distalfacies of the Ivishak Formationwest andsouthofPrudhoe Bay are composed largelyof fine-grained sandstone closely interbedded withsiltstone and shale.Accordingly,thereservoirqualityofthe IvishakFormation in the Beaufort Sea Planning Area is probablypoor
74 west of 154O west longitude. Because of the pattern of increasing shaliness to the west, the IvishakFormation is not 1ikely to form an attractivereservoirformationin the Chukchi shelfprovince of the Beaufort Sea Planning Area.
SAGRIVER FORMATION (TRIASSIC TO JURASSIC) The Sag River Formation is inferred from geophysical mapping to be presentbeneath mostof the BarrowArch province. It is also thought to be presentbeneath much of the southernhalfof the Chukchi shelfprovince. The Sag River Formationcontains highlybioturbated,glauconitic, muddy, veryfine-tofine-grained sandstone(Jones and Speers,1976, p. 41). The formation ranges in thickness along the BarrowArch from approximately 50 feet at Prudhoe Bay to 70 feet nearPoint Barrow. The presence of abundant detrital matrixcoupled with diagenetic cements and compaction have severely impacted the reservoirqualityof the sandstones(Barnes,1985). Jamison and others(1980, p. 304) report that an averageporosity of 25 percent and permeabilities up to 270 millidarcys are observed where reservoir quality is best developedin the north part ofthe Prudhoe Bay field. The Sag Riversandstonecontainsoil shows at numerous localities and hasyieldedoilindrill stem tests. However, because of the poor overallreservoirquality of the formation and becauseof its limited thickness, the Sag River Formation is not generally considered an attractive objective on the North Slopeor in the Beaufort Sea Planning Area.
SIMPSON AND BARROW SANDSTONES (JURASSIC) Sandstones interbedded with the shales of the Kingak Formation are found in the subsurface in the northwesternQart of the North Slope.In contrastto the regionallywidespread Sag Riversandstone, these younger Jurassic sandstones are known only from wells in western NPRA. The uppermost of the two majorsandstonebodies, informally termed the "Simpson" sand, is approximately130feet thick. The lowersandstone,termed the "Barrow" sand, is approximately 120 feetthick. The Simpson sand is found south and east ofBarrow, but is truncated by the LCU in the vicinity of Barrow. The Barrow sands are found in the subsurface in the vicinity of Point Barrow. Similarfacies-equivalentsandstonesareprobably present throughout the Jurassic section offshore southwest of Barrow in the Chukchi shelfprovince. Because of their limitedeastward extent and because of the truncation of the entire Jurassic section to the north at the LCU, these sands are unlikely to be present offshore in the Barrow Arch province. The Simpson sand is a silty, very fine- to medium-grained glauconiticsandstone. A coreof the most well-developedpartof the Simpson sand body in the USGS-Husky Peard No. 1 wellyielded porositiesranging up to 25 percent but permeabilities no greater than 0.5 millidarcys (Husky Oil NPR Operations,1982, p. E-1). The Barrowsandstonesaretypicallysilty,argillaceous,very fine-to(locally)coarse-grained, and commonly glauconitic(Collins, 1961). Inthe SouthBarrowTest No. 2 well,coresrecoveredfrom the Barrowsand yielded an averageporosityof 17 percent and permeabilitiesrangingfrom 8 to 20 millidarcys(Collins, 1961,p.603). In the SouthBarrowTest No. 3 we1 1, twospecimens ofcoresrecovered fromtheBarrowsandyieldedporositiesof10to 25 percent and permeabilitiesof 7.8 to 9.0 millidarcys(Collins, 1961). IntheEast Barrow gas field, a lowersandstone lens within the Barrowsandstones yielded an average core porosity of 21 percent and an average permeabilityof 552 millidarcys(Gruy and Associates,1978, p. 7). However, the averageporosityofthe sands in the SouthBarrow field is 16.8 percent and theaveragepermeability is only 24 millidarcys (GruyandAssociates,1979, p. 5).
Gas ispresently being produced from the Barrowsandstones as a source of energy for the village of Barrow andnearbygovernment facilities. The productionofthis gas isnot commerciallyviable, but is made possiblethroughsubsidies by the U.S. Government (Lantz,1981, p. 199).
These dataindicate that the overall reservoir quality of the Jurassicsandstones is poor,apparently as a consequence of fine grainsize,highinitialmatrixcontent, and theintroduction of diagenetic cements. Good reservoirqualityislocallyfoundin some minorsandbodies. Because ofthelimitedthicknessofthese sandstonesand their erratic reservoir quality, they are considered torepresent only marginally attractive reservoir objectives, primarily confined to the Chukchi shelfprovince.
KUPARIJK FORMATION(EARLY CRETACEOUS)
The KuparukFormationconsistsof a sequence ofinterbedded shales and veryfine-tocoarse-grained,quartzose,glauconitic sandstoneswhichoverliestheKingakShale (Carman andHardwick, 1983).Sandstonesoccur inindividualbodiesupto150feetthick. The KuparukFormationoccursonly in onshoreareas and in nearshore partsoftheBarrowArchprovince west and north of Prudhoe Bay (Jamisonand others, 1980, fig. 18). Because ofnorthward stratigraphic thinning and truncation at the overlying LCU, the KuparukFormationprobably does notextendfarnorthwardintooffshore OCS waters. However, it may underlie some Federal OCS tracts southof the barrier islands between the deltas of the Colville andSagavanirktokRivers.
The KuparukFormation is generally divided into two memberson thebasisof distinctivelithologies, depositionalsettings, and reservoirproperties (Carman and Hardwick,1983, fig.8).Inthe upper member, porosity(mostlyofsecondaryorigin)averages 23 percent and ranges locally up to 33 percent;permeabilitiesaverage 130 millidarcys,butrange up to 1,500 millidarcys(Eggert,1985).
76 In the sandstones ofthelower member, porosity(chieflyprimary) alsoaverages 23 percent,butranges up to 30percent;permeabilities average100 millidarcys,but may range up to 500 millidarcys(Eggert, 1985).
Oil hasbeen recovered informationtestsfromthe Kuparuk Formation at numerous onshore localities north andwest ofthe Prudhoe Bay field. However, currentproductionislimitedto an area10 to 30 miles west of Prudhoe Bay. The presently mapped Kuparukpoolcovers 300 squaremiles(anareaapproximatelyequal to 30 OCS tracts), with a verticalclosure of about 1,100 feet (Carman and Hardwick,1983, p. 1024).Ultimaterecoverablereserves areestimatedto be between 1.0 and 1.5 billion barrels (Carman and Hardwick,1983, p. 1014),makingtheKuparuk field the second largestintheUnitedStates.Additionalproductionfrom Kuparuk Formationsandstones atthe 100-million-barrel Milne Point field is scheduled tobegin during the first quarter of 1986 (Oil and Gas Journal,1985c, p. 55).
RIFT SEQUENCESANDSTONES (EARLY CRETACEOUS)
At widelyscattered localities on thenorthern ArcticPlatform, explorationwells haveencountered sandstones lying directly upon the LCU. A mixednomenclaturehas been appliedtothesesandstones, generallyreflecting the geographic locale or particular well in whichthesandstones were encountered. Examples includethe Walakpa, Kuyanak, PutRiver,Niakuk, and Point Thomson sandstonesalongthe coast, and the Kemi k sandstones in the Brooks Range foothills. These sandstonesrange inthickness from a few tensof feet at most localities up to 330 feet in the Point Thomson area.
A conceptual model forthepaleogeographicsettingofPoint Thomson sandstonedeposition,asdeveloped inthe following paragraphs, is important because it may provide an analogfordepositional settingsfoundalongthemargin(s)ofinfrariftgrabensdeveloped in the OuterArctic Platform province during Early Cretaceous time.Structurallylocalizedclasticfacies, such as thePoint Thomson sandstones,associatedwiththeevolutionofinfrariftgrabens and highlands may form the basis for the most significant play in the Outer Arctic Platform province. The Point Thomson sandstones were first testedatExxon’sPoint Thomson No. 1 wellin 1977. At thislocality, 330 feetof sandstone andconglomerate lie directly upon metamorphic basement (Franklinian) and areinturnoverlain by the Lower CretaceousPebble Shale. Age inferences based solely upon the stratigraphic position of these coarseclasticsare somewhat equivocal. TheLCU inthe Point Thomson area is commonly locatedat the base ofthePebbleShalesequence (fig. 3).Thisplacementoftheunconformity inthe Point Thomson No. 1 wellonlyconstrainsthestratigraphic age ofthesandstonesas
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L between Middle(?)Devonian (i.e., post-Franklinian) and EarlyCretaceous. However, considerationoftheregionalgeology and detailedcorrelations between a group ofwells in the area (fig. 20)suggestthatthe LCU in the Point Thomson area lies instead at the base ofthesandstone/ conglomerate sequence. The topofthePoint Thomson sandsequence is therefore probablynot a major unconformity,butrather a conformable contactin a faciessuccession. The Early Cretaceous age assignment thusobtainedforthePoint Thomson sandstonessuggeststhatthey areequivalentto(butnotnecessarilycontinuouswith)the Kemik sandstonesexposed tothe south in the Brooks Range (Detterman and others, 1975, fig. 21). The Point Thomson sandstones may thenalso be consideredcorrelativewiththe"PutRiver"sandstones(Jamison and others,1980,fig. 8) known fromthesubsurface in the Prudhoe Bay area, and the Walakpa andKuyanak sandstones in NPRA. All of theserocks, as well as thePebbleShale,areincludedwithinthe Rift sequence.
Faciesrelationshipswithin the LowerCretaceousdepositional system in the Point Thomson area are illustrated in the stratigraphic crosssectionpresentedinfigure 20. The crosssectionisrestored topaleohorizontal, or "hung,"on a thin, high-resistivity bed with excellentlateral traceability which is found within the upper part ofthePebble Shale. Lithologiclogsofthewellssuggestthatthis distinctive marker bed is a well-cementedcoquinoidlimestone.This bed, forreferencepurposes,occursat a log-measureddepth of 12,710 feetin the Exxon Point Thomson No. 1 well. An isopach map forthe stratigraphicinterval between this markerbedand the LCU is presented in figure 21.
As shown in figure 21, LowerCretaceous stratabeneaththe markerbedthickeninto a basinwith a northwest-trendingaxiswhich liesatornearthePoint Thomson No. 1 well. The crosssectionin figure 20 shows thatthesestratathinsouthwestwardbyonlaponto a subtlestructuralhigh. The wellsillustratedinfigure 20 project into a lineofcrosssectionwhichisorientednortheast-southwest, roughly perpendicular to the axis of the basin in whichthePoint Thomson sandstonesaccumulated. Wellsalongthe northeastmarginof thePoint Thomson basinhaveencounteredabundantcoarse-grained sandstones,conglomerates,andbreccias.Detailedcorrelations (fig. 20) show thatwithin a few milesacross depositional strike to thesouthwest,thesecoarse-grainedrocksgradeabruptlyintosiltstones andshalesmoretypicalofthePebbleShale sequence. Withinthe sandcomplex, a northernfaciescharacterized by conglomerateand breccia canbe separatedfrom a southernfaciesdominated by fine• grainedsandstone (fig. 20). Themap distributionofgrainsize facieswithin the Lower Cretaceousdepositionalsystem in the Point Thomson area is superimposed upon theisopach map of figure 21. The faciesdistribution implies that the source terrane for the Point Thomson detritus lay tothe north of the depositional basin.
Breccias and conglomerates inthe Point Thomson areawellsare composed ofangular to rounded clastsof dolomitic marble and
80 I
metaquartzite.Sandstones, and the sand matrix of conglomeratic rocks, are composed largely of .1 ightly abraded, subangular, monocrystalline grains of dolomiticcarbonate. Because suchparticles are highly susceptible to mechanical attrition and chemical dissolution,theirpresenceimpliesrapiderosion,brieftransport, and deposition in a setting verynear thesourcearea.Dolomitic marbles and metaquartzites which aremineralogically identical to the major clast types found within thePoint Thomson sandstones have been encounteredwithin the basement complex penetrated by severalwellsinthearea. In concert with the map distribution of grain size facies within thePoint Thomson sandstones,these observations seem to document thepresenceof a significant, northern sourcefor Point Thomson detritus at subaerial exposures of the basement complex along the Mikkel sen high duringEarlyCretaceous time. The Rift sequence sandstones typically exhibit fair to good reservoirproperties. At its typelocality,theaverageporosity of the Put Riversandstone is about 12 percent.Permeabilities are typically less than 100 millidarcys, but may range up to 404 millidarcys(Jamison and others,1980, p. 304). A coreof a sandstone overlyingthe LCU inthe WalakpaNo. 1 well yielded an average porosity of 18percent and an averagepermeability of 49 millidarcys, with maximum values of 25 percent and 157 millidarcys (Husky Oil NPR Operations,1983a, p. D-1). No coredataarepresentlyavailablefor the Point Thomson sandstones, but a flow test of 87 feet of perforations in the Exxon Point Thomson No. 1 well yielded 2,300 barrels of oil per day and 13 million cubic feet of gasper day (Jamison and others,1980). The Point Thomson pool is estimated tocontainrecoverablereserves of 350 million barrels of condensate and 5 trillion cubic feet of gas (Smith, 1984, p. 4). Inconclusion, with theexception of the Point Thomson sandstones,the thin and discontinuousRiftsequencesandstones arenotconsideredto form significant reservoir formations in mostonshore areas or within offshore parts of the BarrowArch or Chukchi shelfprovinces. However, we speculatethat a significantly thicker sand facies with attractive reservoir properties may have formed in the infrarift grabensoffshore in theOuterArcticPlatform province. In theseareas, Lower Cretaceoussandstonesoverlying the LCU may eventually form the primary target for exploratory drilling. t
BROOKIAN PRODELTA SANDSTONES (CRETACEOUS TO TERTIARY) Shalesand siltstones are presumedon thebasisofonshoredata to compose thebulkoftheRrookianprodeltafaciesthroughout all provinceswithintheBeaufort Sea Planning Area. In onshorewells, sandstones,interpreted to represent turbidite deposits, occur locallywithinthisprimarilyshale sequence. These sandstones occur in beds ranging from severalfeettoseveral tensoffeetin thickness. The prodeltasandstonesarefineto medium grained, silty, and commonly richindetritalclaymatrix.Log-derived porosities are typically high (25 to 35 percent)becauseofthe combined effectsofmatrixclay and undercompaction.Flow testsof oil-bearingintervalsgenerallyreportlowflowrates,apparently a consequence ofpoorformationpermeability. However, Brookian prodeltasandstones in Exxon'sAlaskaState A-1 well onFlaxman Islandform a noteworthyexception to this generalobservation. Inthiswell,sandstonesnearthebaseoftheBrookianprodelta shalefacies flowedoil with associated gas at the rate of 2,507 barrelsper day(Jamison and others,1980, p. 298). Data from onshorewellsindicatethatBrookianprodelta sandstonesarenot likely to form prospective reservoirs within provinceswhich lie southoftheHingeLine. However, prodelta sands may havebeen deposited in thicker andmore extensivebodies in submarinefancomplexesdeveloped at the mouths of submarine canyonsystemswhere thenorthward-progradingBrookiandeltaicsystems metareas ofmajor contemporary subsidence north of the Hinge Line. Stratigraphictraps associated with such sandaccumulationscould formsignificant exploration objectives.
BROOKIAN FLUVIAL-DELTAIC SANDSTONES (CRETACEOUS TO TERTIARY)
Sandstones,shales,and coalsof theBrookian fluvial-deltaic faciesarepresentthroughout allprovinces on the Beaufortshelf. Sandstonesareveryfine to coarsegrained, and conglomeratesare locally abundant. These clasticrocksare commonly friablein coastalwells,butarewellinduratedinsurfaceexposures and in wellstothesouth.Marginalmarinetomarinesandstonesnearthe base of the sequence in coastalwellsarelessconglomeratic and occur in beds rangingfromseveralfeetto100feetinthickness. Fluvialsandstonecomplexeshigher in the sequence are typically more conglomeratic, and individualsandstonebodies may rangeup to 700 feet in thickness.
Poor to fair reservoir quality is typicallyobserved in Brookian deltaic sandstones tothesouthnearthe Brooks Range. Inthe Umiatarea,averageporositiesforBrookiansandstonesrange from 12 to 16percent, and averagepermeabilitiesrangefrom10to167 millidarcys (Fox,1979, p. 53).Diagenetic cementsand deformation ofductileclastsduringcompaction haveadverselyimpactedthe reservoirqualityoftheserocks(Bartsch-Winkler, 1979, p. 69).
82 Reservoirqualitygenerallyappearstoimprove to the north and east infacies-equivalentbutyounger strata of the Brookiandeltaic complex.Along theArcticcoastfromHarrison Bay to the Canning River,sonic-log-derivedporositiesinthemarginalmarineto marinesandstones at the base ofthedeltaicfaciesrangefrom 25 to 33 percent.Intheoil-bearingnearshore-marine West Sak sands, whichoccur at the base ofthe fluvial-deltaic facies, cores have yielded an averageporosityof 29 percent and an averagepermeability of 500 millidarcys(Jamison and others,1980, p. 312).Log-derived porositiesin the fluvial sandstoneshigher intheBrookiandelta complex commonly exceed30percent,possibly due in part to undercompaction.
Oil shows havebeen encountered in Brookianfluvial-deltaic sandstones at numerous welllocalities. Mostcommonly, oil shows occur inthelower,marginalmarine sands nearthe base of the fluvial-deltaicfacies. At present,theonly known potential productioninBrookiandeltaic sands is in small anticlinaltraps in the Umiatarea(northernfoothillsoftheBrooks Range)and in the West Sak and Ugnu pools(westofPrudhoeBay). The Umiat structure is estimated to contain 30 to 100 million barrels of oil (Jamison and others,1980, p. 291) but is notconsideredto be a commercialaccumulation. The known, potentiallyproductivearea ofthe West Sak and Ugnu sands isquitelarge. The totalin-place oilcontained within these accumulations is estimated to beas large as 40 billion barrels (Werner,1985),approximatelytwice thein-place volume ofthe Prudhoe Bay field. However,. thereservoir sands herearelessthan100feetthick(Jamison and others, 1980, fig. 22) and contain a heavy oilranging from 8' to 22" in API gravity (Werner,1985).Because of theseunfavorablereservoir and fluidproperties,oilrecovery,shouldproductionactually occur, is anticipated to be low.
Sandstones ofthe Brookian fluvial-deltaic facies are regarded as theprimary reservoir objectives in the Nuwuk andKaktovik Basins.Thick sequences of poroussandstone and conglomeratewith favorablereservoirproperties have been encountered in this interval by many coastalwells.Significantlocalthickeningofthese deltaic sands may be anticipated on the downthrown blocksof growth faultsin actively subsiding depocenters directly north of theHinge Line.Thissetting is thereforenotonlyfavorableforthestructural entrapment ofmigratinghydrocarbons,but is also a 1ikely area for the localization of thick sandstonereservoir sequences. 8
Play Concepts and Hydrocarbon Trap Configurations
The greatdiversityofstructural styles and stratigraphic historiesofthevariousgeologicalprovincesintheBeaufort Sea Planning Area offers a diverse array ofpotentialhydrocarbonplay concepts.Although it may frustrategeologicalanalysis,this compl exity is viewed as favorablefor the occurrence of commercial accumulationsofhydrocarbons.Complexity and diversityincrease theopportunitiesfortheoccurrenceoftheuniquecombinationof geologicaleventsnecessaryfortheformationof a majorhydrocarbon accumulation.
The discussionofpotentialplayconceptsintheplanningarea is organizedprimarilyaroundthemajortectonostratigraphicprovinces outlinedin figure 1. The discussionofthepetroleumgeologyof the Camden and Demarcationsectors,however,departsfromthisformat in that it is extended toincludeareaswhich lie both north and southoftheHingeLine.Although,as inotherpartsoftheplanning area,theHingeLineseparatesareasofcontrastinggeology,the importanceof these distinctions is diminished in the eastern Beaufort Sea. Inthe Camden and Demarcationsectors, many important . stratigraphic and tectonicelementsoverlap or merge acrossthe HingeLine. Inthesesectors,thekeyaspectsofthepetroleum geologyare more conciselyaddressedwithin an organizational formatbased upon geographicsectors. Synopticdescriptions of theplayconcepts and potentialtrap typesassociatedwitheachof thesemajorprovinces or sectorsare givenin tables 2 through 7.
BARROW ARCH (IA)
WithinthispartoftheBeaufortshelf,Ellesmerianrocksare preserved, at least in part, beneath the Lower Cretaceousunconformity (LCU). The onshoreextensionofprovince IA includesmostofthe majorfieldsdiscoveredtodate on theNorthSlope. Our concepts concerningpotentialplaysoffshore(fig. 22) extendchieflyfrom our knowledge ofthegeoloqyoftheseonshorefields.
Oil andgas containedinonshorefieldsarepooledin a variety oftrap configurations involving numerous reservoirformations. However, mostshare one common feature in that they directly or indirectlylie in contact with the Pebble Shale, a long-recognized Table 2. BarrowArch(province IA): Summary and playanalysis.
SEISMIC PROBABLE PROBABLE SEQUENCE TYPE OF TRAP RESERVOIR SOURCE BEDS AGE OF TRAP SIZE OF TRAP OILIGAS StratigraphicBrookian Upper Lower to Upper Late Areallylarge. 50150 Le Cretaceous Cretaceous Cretaceous Potential pay Or Rift Stratigraphic Lower Lower Early Areally smal 1. 60140 Locally develoDed, Cretaceous Cretaceous Cretaceous Potential pay relativelythin sands GentleEllesmerian folds, CarboniferousTriassic Early to Areallylarge. 40160 Principalreservoirunits (Prudhoe style)closure atfaults, (Endicott andLower CrI?taceous Cretaceous Potential pay at Prudhoe Bay andKupafuk and subunconformity Lirburne), ,100' depending fields have pinchedout Or wedge-outs on Triassic upon reservoir thinnedsignificantlyover Arch Barrow (Ivishak). involved. much of theoffshoreareds Jurassic in the RarrowArch orovince. (Sag River), Cretaceous (Kuparuk River) i A Brookian deltaic sandstones orprodelta turbidite sands chargedwith hydrocarbons whichmigrated up faultsfromdeeperseated sources in Rift orEllesmerian sequences. Diagramadaptedfrom Carman andHardwick (1983, p. 1029). B I 1 Laterallysealed Rift sequence sandstones developedlocallyonthe Lower Cretaceous unconformity (LCU) overshoals possibly relatedtostructuralhighs. C SimpleanticlineinEllesmerianstratawith hydraulicaccessto.sourcebedatthe LCU. D TiltedEllesmerianreservoirformation truncatedupdip at the LCU and overlain by thePebbleShale. Folded or ti1ted El1 esmeri an reservoi r formationtruncatedupdipatfaultin hydrauliccommunication at depth with ..,...... SQC .... potentialsource beds in Rift or Ellesmerian sequences...... Ek...... I cu ...... Figure 22. Playconceptsand known or potentialtrapconfigurationsdevelopedin Ellesmerian, Rift, andBrookianstrataintheBarrowArchprovince (IA). Largearrowsdenotepotentialmigrationpaths. 86 majorsourcebed sequence. It was thisnearlyuniversalrelationship whichpromptedMorgridgeandSmith(1972,p.500-501) to suggest that most of the oil in Prudhoe Bay and satelliteaccumulations was sourcedfromthePebbleShale.Structuresnot indirect communicationwiththePebbleShale,such as the immense Colville anticline, were found to be dry.Thisreinforcedtheconceptof a geneticlink betweencontactwiththePebbleShale and access to migratinghydrocarbons. Known hydrocarbon accumulations on theArctic coastalplain typically combinetwo or more trapping mechanisms. The Prudhoe Bay field, for example, is sealedalong itsnorth margin by a majornormal fault. It is sealedtothesouthbeneathsouth-dippingshaleswhich overliethereservoir sand. A structuralsaddleintheuppersurfaceof thereservoirunit limits thewesternextentofthefield. The crest and eastern flank of the field are sealed where thereservoirsandstones areunconformablytruncated by the LCU and overlain by thePebbleShale. An excellent summary ofthegeologyofthe Prudhoe Bay field is found in MorgridgeandSmith(1972). Must ofthesmalleraccumulationsfound nearthePrudhoe Bay fieldare also composite traps which incorporate more than a single trapping mechanism. PotentialtrapswhichsharefundamentalpropertieswiththePrudhoe Bay field existoffshore inprovince IA. The areally immense Mukluk structure inHarrison Bay containsthe same reservoir unit and partial assemblage oftrapping mechanismsas found at Prudhoe Bay (fig. 23). However, the trap hasbeen tested and found to contain only residual oil. The trap may havebeenbreached by leakagealong a system ofpost- BrookianfaultswhichcrosstheMuklukstructure.Alternatively, a prioraccumulationofhydrocarbons may have spilled into other structures duringregional tilting in Cretaceous or Tertiary time. The most significant reservoir unit within the Ellesmerian sequence istheIvishakFormation,which is theprincipalreservoirat Prudhoe Bay. Inthe Prudhoe Bay field,IvishakFormationporosities may exceed 30 percentandpermeabilitiesareobservedtorise aboveseveraldarcys (JonesandSpeers,1976;MorgridgeandSmith,1972),accountingforthe tremendous productivityof this field. Limestones and dolomitesofthe Lisburne Groupand sandstones ofthe Endicott Group formlessattractive secondaryobjectives.Therefore,thepresenceandreservoirqualityof theIvishaksandstones at localitieswithin province IA are critical to theoverallprospectiveness of structuresinthoseareas. As discussedpreviously,thedepositionofIvishaksandstonesappears to havebeenfocused at twomajordeltaiclobes,ordepocenters,located nearPrudhoe Bay and Demarcation Bay.Much ofwestern NPRA, and presumably the Chukchishelf(province IC), isunderlain by a shalyfaciesofthe IvishakFormation.WithintheBarrowArchprovince,theIvishakFormation occursalong a narrow strip of OCS tracts betweenSmith Bay and the SagavanirktokDelta (fig. 19).Althoughpotentialtrappingstructures arepresent within this area,includingtherecentlydiscovered Seal Islandfield, the largest and mostpromisingfeature,theMuklukstructure, hasbeen tested and found to be dry. N S OUTER ARCTIC PLATFORM (IB) This province is geologicallyseparated from otherArcticPlatform provinces(IA and IC) on thebasis of the apparentabsenceofall Ellesmerianstrata due to a combination of stratigraphiconlap and erosionaltruncation. The basic geologyof the Outer ArcticPlatform province is illustrated in plates 4 and 5 and the schematicgeological crosssection of figure 24. Although structurallypart of the Arctic Platform,province IB possesses a distinctstratigraphy and consequently a somewhat reducedpetroleum potentialrelativeto provinces IA and IC. The major structuralfeature of province IB is the Dinkum graben, firstidentified by Grantz and others(1982b).Thisstructure,as illustratedin plate 5 and schematicallyportrayedinfigure 24, is an asymmetricsimplegraben. The graben was formed by northward tilting and faulting on its south flank and by major normal faulting on its northflank, where it apparently abuts a shelf-edgebasement high. Fault movement on the margins of the grabenappears to have terminatedduringEarlyCretaceous time, and the breakup unconformity (BU), which separates the overlying Brookiansequence from the underlyingstructural complex, is unfaultedover this feature(plate 5). Grantz and May (1982,fig. 11) suggested that the LCU extends across the topof the Dinkum graben and caps a Jurassic to Cretaceous graben fill more or less coeval to the El lesmerian Kingak and Kuparuk Formations. We offer an alternativeinterpretation(fig. 24) which assumes that the formation of the grabenpostdates the regional erosional event on the LCU. This modelimp1 ies that the graben is filled with sediments of the Rift sequence which are time equivalent to the Pebble Shale and to basalsand units that over1 ie the LCU onshore. This interpretation is more consistent with ourseismic mapping and with the rift model advocated in this report for the development of the Beaufortcontinental margin. In accordancewith conceptsforriftevolution proposed by Falvey (1974), the LCU ("rift onsetunconformity") is thought to have formed in response to regional thermal elevation of the crust in the vicinity of the incipient rift. The subsequent onset of actual divergent movement (Falvey's"rift valleystage")apparently produced brittle fracturing and development of infrarift grabens and horstswithin the ArcticPlatformadjacent to the major rift. The structurally negative areas were subsequently filled with sedimentderived from flanking highlands. The Mikkelsen high is an infrarift positive structural block which intervenes between the Dinkum graben and the Arctic Platform tb the south(fig. 24). The Mikkelsen high is flankedalong parts of its southern margin by a thickapron of coarse-grained sandstones and conglomeratesofEarlyCretaceousage,informally termed the Point Thomson sands. These sands have been tested by Exxon as hydrocarbonproductive at several wells and may contain a commercial accumulation. &y Cuncepth and Thap Con&ph&Lonh, 89 Ob109 Ob109 DINKUMINFRARIFTGRABEN S I L, -1 BROOKIANSEOUENCE . \ . -- . .\ \ \ SANDSTONES GRABEN-MARGIN CLASTIC WEDGE 1 BU - BREAKUP UNCONFORMITY LCU - LOWERCRETACEOUS LINE OF UNCONFORMITY ECTlO R - RIFT SEQUENCE E - ELLESMERIAN SEQUENCE F - FRANKLINIAN BASEMENT Ngure 24. SchematicgeologicalcrosssectionacrosstheMikkelsen basement high and Dinkulngraben.Thisdiagram is presented to illustrate how stratigraphic relationshipsobserved within thePoint Thomson sandstones may beextended into a broaderpaleogeographic model forsedimentationalongthemarginsof the Dinkumgraben, a majorinfrarift basin underlying the Beaufort shelf. The model suggeststhatcoarse-grainedclasticdeposits may have been localized alongthe marginsof the Dinkum graben andcontemporaneous features adjacent to highland sourceterranes. Hydrocarbontraps may occurwherethesepotentialreservoirrocks liewithin structural closures againstbasinmarginfaults, as illustrated above. Interiorpartsofthe graben may containhighlyorganicshalessimilartotheequivalentPebble Shale, known fromonshore localities.Faulttrapsalongthe edge ofthe Dinkum graben may have been chargedwithhydrocarbonssourcedfromsuch shales. my Canceph and Tnap Can~igrc.tatiunh, 91 As articulated in a precedingsection, we suggestthat Point Thomson sand deposition may form an analogforEarlyCretaceous sedimentationalongthemarginsofthe Oinkum grabenaswellas otherinfrarift grabens.This is schematicallyillustratedin figure 24.The essentialfeatureofthe model isthe development of a highland-flanking clastic wedge along the faulted margins of the Dinkumgraben.Reservoir sandsand trapsdevelopedalongthese structuralfeatures could haveeasyaccess to hydrocarbonsgenerated bythermallymatureshales in the interior parts of the graben or byonlappingshalesfaciesequivalenttothePebbleShale. As noted byGrantz and others(1982b, p. 17),mostofthesedimentary fill in the Dinkumgraben lies at or beneath the oil window, estimated in this report to lie in the depth interval from 9,200 to 12,300 feet in the westand from 10,200 to 13,700 feetin the east. Strata within the deepernorthernandeasternpartsofthegrabenare probablythermallyovermature. The model outlined aboveforms the basis for the most attractive playwithintheOuterArcticPlatformprovince. The structural setting of this play is somewhat similar to that of the highly faultedHibernia field, which is lodged in LowerCretaceousstrata alongthesouthwestflankoftheAvalonBasin on thecontinental shelfoff Newfoundland(McKenzie,1981). The Hiberniafield is lessthan 4 OCS tracts(24,000 acres) in areal extent (McKenzie,1981) but is estimated to contain between 1.0 and 1.5 billion barrels of recoverable oil in multiple payzones (Oil and Gas Journal,1985b). , It isalsopossiblethatsandstonesofthe LowerCretaceous Rift sequencealongthemargin(s) of the Oinkum grabenweredeposited in a submarinefansettinganalogoustofansystemswhich lie alongthe westernmargin of the Viking graben of the North Sea (Heritier and others, 1980, figs.14,15).Closure on theselatterfeaturesis provided by inheriteddepositionaltopographyorbyfaulting and. dipreversal(Heritier and others, 1980, fig.18). The Fortiesfield of the North Sea is sited in a Paleocenefansystemandcontains recoverablereservesof 1.8 billion barrels of oil (Hill and Wood, 1980,p.81) in a productiveareaof 90squarekilometers(3.9 OCS tracts). Grabens similar to the Dinkum structure, but much smaller in size, arefound in more westernpartsoftheArcticPlatformneartheHinge Line(fig. 9 and plate4).Northwestofthe Oinkum graben,other Early Cretaceous infrarift basins may havebeenoverprintedbythe subsequent phase of faulting and post-rift subsidencealong the HingeLine in the Nuwuk Basin (fig.26). There, disrupted infrarift grabens may occur north of the Hinge Line andperhapsbeneaththe deep partsofthe Nuwuk Basin.Studies ofseismicdatasuggest thatthesedisruptedgrabens may locally preserve thin outliers of Ellesmerian sequencerockswhichwerenotremovedfromtheseparts ofthe Arctic Platfon during the erosional event represented by the LowerCretaceousunconformity. 92 CHUKCHI SHELF (IC) The eastern Chukchi shelf is in some ways a geologicalextension of the BarrowArch province, and some of the plays describedforthe BarrowArch provinceinvolving Mesozoic and Cenozoic rocksalsooccur in province IC. However, thestructural and stratigraphicevolution of thePaleozoicrocks of the Chukchi shelfprovinceappearsto differ in severalsignificant ways from that of the BarrowArch province. The structural blockwest of the Barrow fault zone contains a deep basin,informally termed theNortheast Chukchi Basin, which contains stratified sediments up to 30,000 feet in thicknessbeneaththe axis of the BarrowArch (fig. 2). Our work suggeststhat this stratified sequenceranges in possible age from Middle(?) Devonian to Mississippian and may be correlative, at least in part, with rocks of the Endicott Group and Baird(?) Group exposed in the western Brooks Range. This interpretationcontrasts with previously pub1 ished models for the geology of the Chukchi shelfprovince (Grantz and others, 1982b; Ehm, 1983) which portraytheareaas a large structural high where "Franklinian" basement is mantled by a thinveneer of Mesozoic strata. The factthatthe Chukchi shelf province is underlain by a thick sequenceofmodestly deformed Paleozoicstrata makes this provinceone of the more attractive parts of theBeaufort Sea Planning Area in terms of overall hydrocarbon potenti a1 . Seismic character and intervalvelocities(discussedpreviously in chapter 3, SeismicStratigraphy)suggestthat two distinct seismic units are present within theNortheast Chukchi Basinbeneath the Permian unconformity (PU). The seismicsequence is composed of a lower carbonate unit andan upper unit consisting of clastic sedimentaryrocks. Thetwo seismic units arestructurallydetached in some parts of thebasin where the upper clastic unit contains large folds which do not persist downward intotheunderlyingcarbonate unit (plates 1 and 3). The pre-Permian (lowerEllesmerian) sequence is truncated at major fault-boundedbasement highs alongtheeastern and southernmarginsoftheNortheast Chukchi Basin.Potentialtrap configurations in these'Paleozoicstrata are found inthefollowing settings: 1.folds in the upperdetached. clastic unit of the lowerEllesmerian sequence (plates 1 and 3); 2. faulttrapsalongthenorthwesternflank of the basement ridge (plate 1) thatseparatestheNortheast Chukchi and Colville Basins(fig. 7), where lower Ellesmerianstrataarejuxtaposed against basement; 3. faulttraps or drapestructuresin lower Ellesmerianrocks in the downthrown blockalong the Barrow fault (plate 3); 4. faulttraps and anticlines in lower Ellesmerianstrata along the crest and southeastflank of the North Chukchi high (plate 3). Play concepts and potentialtrapconfigurations in theNortheast Chukchi Basin areschematicallyillustrated in thecrosssections of figure 25a and 25b. Teay Concepfi and Tmp Con&Lgigwratiam, 93 Table 4. Chukchishelf(provinceIC): Summary andplayanalysis SEISMIC PROBABLE PROBABLE SEQUENCE TYPE OF TRAP ~ RESERVOIR SOURCE BEDS - AGE OF TRAP --___SIZE OF TRAP OILIGAS - REMARKS__..-.. Brookian Closureat UpperCretaceous Middlel?) Tertiary Areallysmall 40160 Brookiansandstones in faults to Tertiary Devonian to Lower Potential pay faulttrapsassociated Cretaceous Rift Stratigraphic Lower Middle(?) Early Areallylarge 5015~ Locallydeveloped, Cretaceous Devonian to LowerCretaceous Potential pay relativelvthin Cretaceous Upper EarlyMiddle(?)Stratigraphic to Triassic May be areally 50/50? Stratigraphictraps Ellesmerian Jurassic DevonianLoner toCretaceous larqe.Potential withinPernian to Jurassic Cretaceous pay Lower Pre-Permian DetachedMiddle(?) Middle(?) Areally smal 1 50/50? Confineddetachedto folds Ellesmerian anticlines Devonian to Devonian to Lower Potential pay in Endicott(?)-equivalent Permian Cretaceous ,200 . clastic wedge. Lower ClosureMiddlel?) at Middle(?) Pre-Permian Yay beareally 50/50? Traps withinEndicott(?)• Ellesmerian faults Devonian to Devonian toLover large.Potential equivalentclastic wedge Cretaceous Permian pay,200'. and Raird(?)-equivalent basalcarbonateunit at fault systems bordering NortheastChukchiBasin. I I ~ CHUKCHISHELF (IC) A UPPER ELLESMERIAN NORTHEAST CHUKCHI BASIN: LOWER ELLESMERIANSTRATA Anticlinalclosuresindetachedfoldswithin clastic wedge unit abovebasalcarbonateUnit in NortheastChukchiBasin. B Faulttraps and anticlinalclosuresassociated withfault-bounded basement highswhichenclose theNortheastChukchiBasin. cI BROOKIAN I BARROW ARCH: UPPERELLESMERIANSTRATA- Stratigraphictraps within upperEllesmerian (Permian toJurassic) strata where truncated at the LCU on thesouth flank of the Barrow Arch. ELLESMERIAN I BROOKIAN Dl RIFT AND BRDOKIANSEQUENCES . Laterallysealedsandstonelenses in the Rift sequence (e.g., Put Riveror Walakpasandstones) developedlocallyonthe LCU. E Faulttraps in Cretaceous to Tertiary(?) Brookianstrataincrestalareas of theNorth Chukchihigh. Figure 25. Playconceptsandtrapconfigurationsdeveloped in lowerEllesmerian,upper Ellesmerian, Rift, andBrookianstrataofthe Chukchi shelf(province IC). Largearrowsdenotepotentialmigrationpaths. my COMCC~~Aand Tmp Can6igrylat.ionn, 95 i AcrossthesouthernpartsoftheChukchishelfprovince,upper Ellesmerian(PermiantoJurassic)strataarepreservedbeneaththe LCU. These strataaresuccessivelytruncatednorthward(plate 1) at the overlying LCU with increasing proximity to the crest of the BarrowArch. Where potentialreservoirunits, suchas theIvishak sandstones, Sag Riversandstones,oryoungerJurassicsandstones (suchastheBarrowandSimpsonsands), are sealed updip at the LCU, potentialstratigraphictraps may befound(fig.25c). Discontinuoussandbodieswhichgradelaterallyintoshaleare known to occurwithin the Rift sequence in onshorewells to the eastofthe Chukchi shelfprovince. Examples includethe Walakpa andKuyanaksandstones.Thesesandstones aretypicallylessthan 100 feet thick, however,and are unlikely to househydrocarbon accumulationsofcommercialsize on the Chukchi shelf.Nevertheless, we recognizethatthesesandstonescouldthickenwestward andform widespreadstratigraphictrapsforhydrocarbonsintheChukchi shelfprovince(fig. 25d). The folding and faultingof lower Ellesmerian strata in the NortheastChukchiBasin and alongflankingstructuralhighstook place in most areas prior to erosion onthePermianunconfonity (PU). Except for broad warping on the Barrow Arch,younger strata of the upper Ellesmerian, Rift, and Brookian sequences are virtually undeformedacrossmost ofthe Chukchi shelfprovince. However, a younger(Late Cretaceous to Tertiary) phase of uplift and faulting has affectedbroadareasoftheNorthChukchihighinthenorthwestern partof the Chukchi shelfprovince(plate3).Inthisarea,Brookian and older strata are cut by a dense array of northeast-trending normalfaults. Where thesefaultsjuxtaposeBrookianreservoir bedsupdipagainstimpermeablestrataorbasement-complexrock, a potentialtrapconfigurationisformed(fig. 25e).Althoughfound at relatively shallow depths andperhapsareallysmallbecause of theclose spacing of faults, many such traps appear tobe present, andform the most conspicuous exploration objective in the northwestern partofthe Chukchi shelfprovince. Potentialsource beds intheupperEllesmerian sequence(Kingak andShublikFormations)arepreservedbeneaththe LCU only in the southernpartoftheChukchishelfprovince. Rift sequencesource beds(PebbleShale)arepresentover much of the Chukchi shelf province. However, allof these units lie at depths nogreater than 6,000 feetwithintheplanning area. On thebasisofgeothermal datafromadjacentonshoreareas,theoil window in this area is estimatedto now lie between 9,200 and 12,300 feet. However, upperEllesmerianand Rift sequence sourcebeds in this area appear to havebeenpreviouslyburiedto much greatersubsurfacedepthsthan atpresent.Despitetheirrelativelyshallowdepthsofburial (2,000 to 7,000 feet), in nearby wells in NPRA these strata yield vitrinite reflectance values ranging from 0.6 to 0.8 (Magoon and Bird, 1986, figs. 12and 18),corresponding to the top of the oil window. Offshore(westward)projectionsofisoreflectancecontours mapped in NPRA by Magoon and Bird(1986)indicatethatthese source 96 bedsshouldentertheoil window at thesouthernboundaryofthe Beaufort Sea Planning Areaon theChukchishelf.Onshoredata show thatthethermalmaturity anddepth ofburial of these beds increasetothesouth.Liquidhydrocarbonsgeneratedbythese stratasouthoftheplanningareacouldhavereadilymigrated updipto the north into stratigraphic traps in the Chukchi shelf province.Potentialsource beds may alsooccurwithinthedeeper (lowerEllesmerian) sequence withinthe Chukchi shelfprovince. Hydrocarbonsgeneratedfromtheserockscouldhavemigratedvertically intostructural traps within the sequence or into stratigraphic trapsin the overlying upper Ellesmerian and Rift sequences. However, the complex history of deep burial,folding, tilting, and repeated periods of uplift andexhumation at multiple unconformities probablyhasadverselyaffectedthesourcepotentialofthelower Ellesmerian sequence. Inaddition, little is known aboutthe stratigraphy of the lower Ellesmerian sequence aside from the observation that it apparentlyconsists of a sequence ofcarbonate rocksoverlainby a thickclasticunit. We speculatethatpotential sourcebeds may be foundwithinthelowerEllesmerian sequence if it containsmarineshales(equivalent to theHuntForkShale?) withcompositions and maturitylevels appropriate for hydrocarbon generation. NUWUK BASIN (IIA) The Nuwuk Basinbegan to develop inlate Early Cretaceous time as a consequence of the fragmentation of the Arctic Platform and thesubsidenceofthenewlyformedcontinentalmargintowardthe expanding Canada Basin.Subsidence ofthe outer parts of the continentalmarginoccurredalong a system ofmajor,northward-dipping, listric growthfaults which occur within or parallel to the Hinge Line(plate 4). The structuralstyle of the Nuwuk Basinresembles thatof other passive margins, suchas theNorthAmericanmargin ofthe Gulf of Mexico.The Nuwuk Basin is filled with Cretaceous andCenozoicdepositsoftheBrookian sequence. Potentialreservoir formations in the Nuwuk Basininclude: (1) fluvial-deltaic sands in the upperBrookian sequence; (2) prodeltaslope andbase-of-slope turbiditesorsubmarinefan complexes inthe lower Brookian sequence; and (3) Rift sequencesands in pre-Nuwukgrabens(analogous to the Dinkumgraben)which may locally floor the Nuwuk Basin. The principal mappable traptypes within the Nuwuk Basinare fault truncations of inclined strata or gentle folds associated with northward-dipping listric faults along and north of the Hinge Line. The disrupted pre-Nuwukgrabens which may floorparts of the basin may besealedbeneathoverlyinglowerBrookianprodeltaic shales.PotentialstratigraphictrapsintheBrookian sequence may be very common, butare difficult to recognize in seismic data withoutwellcontrol. The principaltypesofstratigraphictraps shouldconsist of updip pinch-outs of deltaic and prodeltaic sands. Turbidite sandstones deposited in the prodel ta setting may locally Table 5. Nuwuk Basin(province IIAl: Summary andplayanalysis. SElSWlC PROBABLE PROBABLE SEQUENCE TYPE OF TRAP RESERVOIR SOURCE BEDS OF TRAP SIZE OF TRAP ___.-.._...__ AGE __ OILIGAS - .-_.__REMARKS -.-.-._- ._.. Brookian A. Rollover CretaceousUpper Cretaceous LateCretaceous Areallylarge. 40160 Rollover anticlines and fault anticlines and to Tertiary to Tertiary Potential pay trapsin Brookian fluvial• closure at *loo'. deltaicstrata along faults southernmarginof Nuwuk Basin. Stratigraphic 8. LowerCretaceous Cretaceous EarlyCretaceous Areallylarge. 40160 Lenticularprodeltaic and to Tertiary to Tertiary Potential pay deltaicbodies of Brookian Rift at Closure Lo.rer Lower EarlyCretaceous Areally small . 50150 Infrarift grabensalong faults Cretaceous Cretaceous to Tertiary Potential pay southernmarginof Nuwuk >ZOO'. BasinbeneaththickBrookian wedge. EllesmerianatClosure Triassic Lower EarlyCretaceous Areally mal 1 . 50150 Upper Ellesmerianrocks may faults to Jurassic Cretaceous to Tertiary Potential pay he locallypreserved within to Triassic I I I NUWUK BASIN S LINEHINGE BEAUFORT SHELF N BU BreakupUnconformity PLANNING LCU LowerCretaceous Unconformity AREA DIAGRAM EllesmerianRocksPossible E? EU EllesmerianUnconformity Figure 26. Schematicgeologicalcrosssectionsummarizingpotentialplayconcepts and trapconfigurationsinthe Nuwuk Basin. Shown are: (1) trapsdeveloped influvial-deltaic sandsdeformedby rotationalfoldsassociatedwith listric faults; (2) trapswithin reservoir sands deposited in Early Cretaceous infrarift grabens now disrupted by faultingassociated with Nuwuk Basinsubsidence; (3) a varietyof stratigraphic traps involving lenticularbodiesofsandstonedepositedindeltaic,prodeltaslope,or abyssalplainsettings.Ellesmerian sequence rocks (shownas "E?") may be locally preserved in the southern parts of infrarift grabensand may formpotentialreservoirobjectives.Largearrowsdenotepossiblemigration pathsforhydrocarbons,primarilyalongfdults. compositeinto major submarine fan complexes. Major northeast- trending canyonsystems incised into LowerCretaceous rocks and filled with UpperCretaceous rocks have been identified in seismic datanear Barrow and Oease Inletsouth of the Hinge Line. These canyonsprobably formed major submarine(?) sediment transport systems into the Nuwuk Basin,and may have localized the deposition of major submarinefan complexes. In general,however, most stratigraphic traps in the Nuwuk Basin may beexpected to be small inareal extent andvolume. Rotationalfolds associated with listric faults whichdisplace upper Brookian fluvial-deltaic sandstonesremain the most conspicuous exploration objective in the Nuwuk Basin.Play concepts and potentialtraps within the Nuwuk Basinare summarized in the schematicgeological cross section of figure 26. The structuralgeology of the Hinge Line fault system along the southernmargin of the Nuwuk Basin is outwardly similar to the Vicksburgtrend of Texas.The Vicksburgfault zone is a complex system of listric faults up to 300 miles in length which trends para1le1 to the modernGulf Coast of Texas.Displacements resulting in severalthousand feet of stratigraphic throw have occurred across theVicksburg fault system. A hostof hydrocarbon accumulations are localized in rotational folds and fault traps on the downdropped (southeast)side of this fault system(Stanley, 1970). Several giantfields are present in this structural province, and total known recoverablereserves for Oligocene andMiocene strata in the Vicksburgtrend are estimated to be 3 billion barrels of oil and20 trillioncubic feet of gas(Stanley, 1970, p. 301). The largest field, the Tom O’Connor field, contains 500 million barrels (Mills, 1970,p. 292) within 15,000 productiveacres (2.5 OCS tracts). The inferredgeothermal structure of the outer (northern) part of the Nuwuk Basinsuggests a depthrange of 15,400 to 23,500 feet forthe oil window.The oil window may riseto shallower depths (12,200 to 16,500 feet)in the older, primarily Cretaceous, sedimentary wedge which fills the southernmost parts of the Nuwuk Basin. A 1 argevolume of Brookian strata in the Nuwuk Basin 1 ies within either depth interval, but much of the basin fill also lies deeperthan 23,000 feetand is probably overmature. As discussed in a previouschapter, Brookian prodelta shales typically contain insufficient quantities of appropriate kerogens to be considered importantpotential sources for liquid hydrocarbons. However, geochemicalstudies of well samplesfrom the Point Thomson areahave identified potentially important source bedsnear the top of the TertiaryBrookian prodelta sequence. If comparably richsource beds occurat a similarstratigraphic position in Cretaceous or Tertiary rocks in the Nuwuk Basin,they would now be buried at relatively moderatedepths where temperatures are suitable for liquid hydrocarbon generation. The subsidenceof these potential source beds intothe oil window may havemore or less coincided with the development of structuraltraps in the overlying Brookian fluvial-deltaic facies. Thissuggests that the most attractive potential traps wereformed 100 before or at the same time as significant quantities of liquid hydrocarbonsmight have been generated inunderlying source beds. As shown in figure 26, listricfaults probably formed the primary migrationpaths for hydrocarbons moving from deep sourcebeds into shallowerstructural traps. KAKTOVIK BASIN: CAMDEN SECTOR (IIB) AND DEMARCATION SECTOR (IIC) In Camden Bay, theHinge Line (figs. 1 and2) trends southeast andthen swings eastward in a sigmoidalconfiguration, passing north ofBarter Island. Seaward ofthe Hinge Line, over 35,000 feetof Cretaceous(?)and Tertiary sediments have accumulated. This great accumulationhas been termed the Kaktovik Basin by Grantz and others (19B2b, fig. 4). As did Grantzand others (1982b, fig. 16). we further subdivide thebasin andcontiguous areas of the Arctic Platformto the south of the Hinge Line into two sectors(fig. 1) on thebasis of fundamental differences in stratigraphic and structuralhistories. We informallyterm these geographic provinces the Camden andDemarcation sectors (fig. 28). The stratigraphyof the Camden sector is inferred to be similar to that of the Camden Basin,as known from exploratory drilling in thePoint Thomson area. Incontrast, the Demarcation sector is hypothesizedto be underlainby a stratigraphic section more directly analogous to the polycyclic Cenozoic sequence known fromexploratory drillingin the contiguous CanadianBeaufort. Furthermore, unlike the Camden sector,the part of the Demarcation sector south of the HingeLine may containpotential source and reservoirrocks of the Ellesmerian sequence. Inthe Camden sector,the Ellesmerian sequence onthe Arctic Platform has beencompletely removed by erosion at the LCU and BU. Camden Sector(IIB) The principalstructural feature of the Camden sector (IIB) is the Camden anticline.This immense fold(illustrated in fig. 27 and plate 6) can be traced for nearly 60 miles along its northeast- trendingaxis and exhibitsup to several seconds(two-way travel time)of structural relief on some seismicdip lines. Detailed examinationof seismic data reveals that the Camden anticline is not a simplefeature. Seismic panels (plate 6) show thatthe fold is cut bynumerous faults,which in some casesextend to the seafloor. The majority of these faults are part of the Hinge Line fault system and trendnorthwest, or nearly orthogonal to the axis of the Camden anticline.Middle Tertiary strata do notthin toward the crest of the structure and are truncated at or near the seafloor around the perimeterof the fold. This suggests that the Camden anticline is a veryyouthful structure. Modern shallow-crustalseismic activity suggeststhat the fold is still growing (Biswas and Gedney, 1978). The Brookiansequence deformed by the fold contains twoseismic facies asrecognized onshore: a lowerinterval of marine prodelta %q Concepa and Ttrap Con,$igu~caLLo~,101 Table 6. KaktovikBasin, Camden Sector(province 118): Summary andplay analysis. SEISMIC PROBABLE PROBABLE SEQUENCETYPE ._ - OF TRAP ._ RESERVOIRSOURCE BEDS AGE OF TRAP SIZE OF TRAP REMARKS ______- OILIGAS ~ Brookian A. Compressional 01 igocene to Miocenelarge.AreallyTertiaryLate 40160 anticlinesFaulted P I iocene to Pleistocene PotentialPleistocene anticlines to iocene PI pay (e.q.. Camden il00'. anticline1which involveRrookian fluvial-deltaic sandstones. B. Stratigraphic 01Tertiary igocene to 01 igoceneto Areally mal 1 . 40160 Lenticular prodeltdic PI iocene P1 iocene Potential pay and deltaicbodies of I ! I P CAMDEN ANTICLINE LOCATIONDIAGRAM BEAUFORTSEA I PLANNINGAREA L7 MAP VIEW CROSS SECTIONS A 4' // 'PRODELTA SHALES \\, B E' PRDOELTA SHALES '\\ Figure 27. Schematicstructure-contour map and geologicalcross sections illustrating thefundamental geology ofthe Camden anticline. The prodeltashales which appear tocore the anticline are breached at a shallowPleistocene(?) unconformityover much ofthe crestal. area of the structure. Traps develapedalong the northwest and southeastflanks of thefold, where potentialreservoir sediments are truncated at the shallow unconformity, areprobably too shallow for economicproduction. The most attractive potentialtraps (area 1 insection 8-B') areprobably found along the southwesternfaulted nose of the Camden anticline, where the axis of the foldintersects the northwest-trending Hinge Line fault system. Large arrowssuggest potential migration paths along faults for hydrocarbons mobilizedout of prodelta shales into fluvial-deltaic reservoir sands. Many faults a?;" - to beactive at present, and may not form effective seals in some traps. Peay Concepa and T&p Con6ig&on~, 103 c shales andan upper intervalof fluvial-deltaic sediments. The upperfluvial-deltaic facies, which probably contains most of the prospectivereservoir rocks, is breached at the seafloor over most ofthe axial region of the fold. At thecrest of the fold, far offshoreto the northeast, the prodelta shale lies near the seafloor, as illustratedin figure 27. On thesouthwestern nose of the Camden anticline,contemporary faults of the Hinge Line fault systemstructurally isolate numerous blocksof Brookian fluvial-deltaicsediments. Potential reservoir sands inthis sequence may be involved in fault closuresalong the faulted nose of the anticline (fig.27). Many of the faults extendto the seafloor, and their ability to act asseals atshallow subsurface depths may bepoor. Nevertheless, the faulted southwestern nose of the Camden anticline is probably the most prospective part of thestructure. Grantz and others (1982b, p. 18)have stated that the youthful age (late Tertiary to Quaternary) of the Camden structureincreases the risk that it may not havebeen present as a potentialtrap when hydrocarbonswere generated and expelled from deeper strata. As noted in precedingsections, rich potential source bedsapparently occurnear the boundary between the prodeltaic and fluvial-deltaic facies of the Tertiary Brookian sequence in the Point Thomson area. These potentialsource beds may extendoffshore at the same stratigraphiclevel as a continuous,prograding facies within the delta complex. The approximatebase of the fluvial-deltaic sequence, as identified on seismicdata, lies at depths between 6,000 and 15,000 feet (fig. 17) along the flanks of the Camden fold north of theHinge Line. It ispossible that potential source bedsnear thisstratigraphic level began tosubside into the oil window (estimated to lie between15,400 and 23,500 feet) only after significantuplift on the Camden structurehad occurred. These. potentialsource beds in areasflanking the fold continued to move intothe oil windowas thefold grew. If sourcebeds are indeed presentoffshore near the base of the fluvial-deltaic sequence,as appears to be the case in onshore we1 1 s, their thermal evolution may havebeen ideally timed for expulsion of hydrocarbons into the Camden structure.Source beds occurring at much greaterdepths withinthe prodelta facies may havereached thermal overmaturity before the initial uplift of the Camden anticline, andhydrocarbons derivedfrom them may havebeen lost. The geologyof Camden anticline is analogous in many ways to thatof the Teak oilfield of Trinidad, West Indies. TheTeak accumulationoccurs within a northeast-trending anticline of Pleistoceneor younger age dissected bya system of listric normal faultswhich trend at right angles to the anticlinal axis. Numerous individualpetroleum accumulations occur in small,isolated fault blocks.Deltaic sands incontact with the faults werecharged with hydrocarbonsthat migrated upward alongfault surfaces (Baneand Chanpong,1980, p.398). Although thesefaults, like those which cross Camden anticline,extend all the way to the surface, in the Teak fieldthey provide seals for hydrocarbon accumulations lying throughoutthe depth range from 1,000 to 14,000 feet. TheTeak anticline is a much smallerfeature than Camden anticline,with aknown productivearea (Baneand Chanpong,1980, p.387) of 900 acres(one sixth of an OCS tract) and a totalarea under closure of 10,000 acres (1.6 OCS tracts).Nevertheless, within the first 7 yearsof development, the Teak field hasproduced 101 million barrels of oil and107 billion cubic feet of gas(Bane and Chanpong, 1980). DemarcationSector (IIC) The Demarcationsector is divided into two dissimilargeological subprovincesby the Hinge Line fault system. At leastseveral thousandfeet of aggregate stratigraphic throw, down to the north, hasoccurred across the faults which constitute the Hinge Line. Pre-Brookianstrata are generally considered to be absent or to lie below drillabledepths north of the Hinge Line. Much ofthe Brookiansedimentary wedge inthe Kaktovik Basin north of the Hinge Line may rest uponMesozoic oceanic crust. Inthe eastern Beaufort Sea, theHinge Line trends southeast across CamdenBay, crossesthe Camden anticline, and projectstoward the present coast of Alaska southwest ofKaktovik (fig. 28). However, maps ofthe surface geology (Reiser and others,1980) of the Arctic National Wildlife Refuge (ANWR) do notsupport the existence of an onshore extension of the HingeLine fault system. Inaddition, Kososki and others (1978, p.19-20) argue that a gravityhigh centered 20 milessoutheast of Kaktovikalong the trend of Marsh anticline(fig. 29) stemsfrom a structural elevation of dense ArcticPlatform basement rocks in that area. The HingeLine must therefore lie north of this gravity anomaly and northof the present coast of Alaska near Kaktovik. These observations,coupled with studies of seismic data conducted by. the presentauthors, suggest that the Hinge Line deflects eastward roughly 5 milesnorth of Kaktovik anddoes notextend onshore into ANWR. As shown in figure 28, theHinge Line passes along the southern margin of theBarter subbasin andthen turns southeast, trending parallel to the presentAlaska coastline. The HingeLine fault system appears to pass beneathunfaulted Oligocene to Miocene deposits (plate 7 andfig. 28) ofthe Demarcation subbasin. The structure of the Demarcation sector south of the Hinge Line is dominatedby two major anticlinorial features informally termed the "Marsh anticline"and the "Jago anticline." These structures, and a host of minor folds and faults,trend northeast, approximately parallelto the axis of the Camden anticline(fig. 28). Like the youthful Camden fold,these structures appear to deform late Tertiary toQuaternary sediments (Grantz and Mull, 1978,p. 9, 15; Reiser andothers, 1980). The Marshand Jag0 anticlinoriaappear to be truncatedto the northeast approximately 10 milesoffshore, where theyintersect the Hinge Line fault system (fig.28). my Coneep.t, and Tmp Con~igwcation~,105 1 FAULT.HACHURES TERTIARY BROOKIP ONDOWNTHROWN CRETACEOUS 0 20 40 SIDE BROOKIAN 1 I I I ,:?:.'1..7. ELLESMERIAN ~:.:.~..<.:..:I OUTLINEOFMAJOR MILES *.,....,.. -.. TERTIARYBASIN A VILLAGE Figure 28. Majorstructural features of Camden and Demarcationsectors of Kaktovik Basin and adjoiningareas. Map adapted fromReiser and others (1980) andGrantz and Mull (1978). Line X-X' iscross section shown infigure 30. Figure 29. Bouguer gravity map of thenorthern part of theArctic National Wildlife Refuge,adapted from Kososki and others (1978, plate1). The contourinterval is 10 milligals, and all contouredgravity values are negative. Kososki and others (1978, p. 19-20)suggest that the gravity high near Kaktovik indicates an elevated basement platform inthe shallow subsurface. The onlyavailable information on the stratigraphy of the Demarcationsector south of the Hinge Line is that obtained from surfaceoutcrops inthe northeastern Brooks Range and theArctic coastalplain. Fossil-bearing Middle Jurassic shales (Reiser and others,1980) ofthe Kingak Formation are exposed along the axis of Jag0anticline at the locality annotated on figure 28.The MiddleJurassic shales appear to be disconformably overlain by Lower Cretaceousshales above the regionally widespread Lower Cretaceous unconformity (LCU). As recognizedby previous authors (Grantz and Mull, 1978; Mastand others, 19801, theoccurrence of Jurassic rocks atthis locality is extremely significant. Regional stratigraphic relationships documented in nearbyareas to the westimply that if Jurassicrocks have not been strippedfrom the northern coastal plain of ANWR byEarly Cretaceous erosion, then the underlying IvishakFormation (Triassic) logically should also be preserved at depth within or beneath the major fold systemssouth ofthe Hinge Line. However, we recognizethat several processes may haveacted to precludethe important Ivishak strata from the Marsh-Jag0 fold province. The exposure ofJurassic rocks along the axis of the Jag0 structure is anomalous in that it lies 40 miles to the northeast (fig. 28) of the regionally mapped northwest-trendingzero edge where theKingak Shale is completelytruncated by the LCU. We have interpreted this anomalousoutcrop toindicate the presence of a subtle structural basin or outlier of pre-LCUage in whichEllesmerian strata havebeen preserved innorthern ANWR. The northernextent ofthis hypothesized outlier is unknown. As recognizedby Grantz andMull (1978, p. 81, boththe Kingak and olderformations may havebeen stripped by the LCU fromprospective areas a short distance northof the Jag0 outcrop. We also acknowledge thatin northwestern Canada, pre-Kingakstrata are regionally truncated by an unconformity at the base of theKingak Formation (Norris andYorath, 1981). It therefoGemains possible that either Early Cretaceous or Early Jurassicerosional events may haveremoved theIvishak Formation from partsof the Demarcation sector south of the Hinge Line, thereby greatlyreducing the overall hydrocarbon potential of those areas. Lastly, it is not known to whatextent the anomalous distribution of exposures of Ellesmerian rocks in northern ANWR may bethe product of post-LCU large-scale tectonic transport andemplacement by thrust faults.For the present, however, we favorthe most simple model, as outlined above, whichmaintains that Kingak shales exposed in northern ANWR represent the uppermost strata in an outlier of Ellesmerian rocks withinwhich the potential reservoir strata of the Ivishak Formation arepreserved. The paleogeographic model forthe Ivishak Formation presented in precedingsections and in figure 19suggests that reservoir sands,perhaps deposited in a setting similar to that in which the excellent reservoir rocks at Prudhoe Bay accumulated, may be found in thesubsurface beneath the Marsh and Jag0 anticlinoria and relatedoffshore structures. At themost northeasterly point of controlfor Ivishak Formation thickness, 60 milessoutheast of Kaktovik,the formation is 390 feetthick, although somewhat abbreviated by erosionaltruncation at its top (Detterman and others, 1975,p. 12).Thicker accumulations might be anticipated in thesubsurface near Kaktovik. The conceptof the occurrence of a thick sequence of reservoir- qualityIvishak sandstone in structures in the part of the Demarcation sectorsouth of the Hinge Line strongly affects the hydrocarbon potentialof those structures. If sandstonespossessing excellent reservoir properties could be shown to be present in the Jag0 and Marshfeatures, then this area must be regardedas one of the most prospectivewithin the Kaktovik area. In OCS Sale 87, heldin August 1984, industryobtained exploration rights to 9 tracts in theDemarcation sector south of the Hinge Line. Chevron ispresently drilling at anonshore location (K.I.C. No. 1 well)on Kaktovik Village corporate lands 5 miles WSW of this group of offshore tracts(fig. 28). This well and thenearby offshore tracts appear to lie on theMarsh anticlinorial trend. The Chevron well will form a keyevaluation of the geology andhydrocarbon potential of thisarea. However, thedata obtained from this well are expected to remainconfidential for the foreseeable future. The fundamentalstructure of the Demarcation sector north of theHinge Line was portrayedby Grantz and others (1982b, fig. 4) as a seriesof basins Separated by northwest-trending structural highs,which they termed "diapiric shale ridges." Grantz and others 11982b) identified two principalbasins, which they termed theBarter Island and Demarcation subbasins. These basinsappear to be filled with shelf or deltaic sediments inferred to be Oligocene andyounger in age.These strata are upturned or tilted at the basin margins,and deeper strata are truncated at faults which bound thebasins (plate 7 and fig. 30). Shallow strata withinthe basinsare gently upturned and truncated at a shallowunconformity inferredto be lateMiocene(?) in age.The mostattractive potential traps identified within these basins are fault traps associated withbasin-margin faults.(plate 7 andfig. 30). The sedimentary fill inthe deep parts of these basins may exceed 20,000 feet in thickness. Geothermal datafrom wells in adjacent areas predict thatsedimentary rocks below 15,400 feet in this province lie withinthe thermal window foroil generation. The structural and stratigraphic relationships depicted in plate 7 and figure 30 indicate that many basin-marginfault traps formed early in the subsidencehistory of the basins. Theseearly-formed traps would havehad natural access to migrating hydrocarbons expelled later fromthermally mature sediments in the deep interiors of the basins. The seismicpanel in plate 7 and a recordpublished by Grantz and May (1982, fig. 14) show abundantamplitude anomalies, or "bright spots,"and phase reversals along faults at the northern margin of theDemarcation subbasin. These amplitude anomalies suggest the presenceof reservoir beds, possiblycharged with gas. The gas may be associated with accumulations of liquid hydrocarbons. Peay canup& and Tmp Cc--'igwL(Ltiolzd, 109 Table 7. KaktovikBasin, Demarcation Sector (province 1IC): Summary andplay analysis. SfISnK PROBRBLE PROBRBLE SEQUENCE TRRP OF TYPE RESERVOIR SOURCE BEDS TRAPAGE OF SIZETRAP OF 0ILfG.U .... REMARKS ... -. __. Brookian A. Faulttraps igocene 01 to UpperCretaceous Oligocene Areallyto large. 40160 Faulttrapsalongmargins Miocene payPotentialtoMiocene Miocene ofDemarcation and Barter ,100'. subbasins. B. CompressionalUpperCretaceous UpperCretaceous Eocene to Areallylarge. 40160 En echelon,thrust-cored(!) anticlines to Paleocene toMiocene Eocene Potential pay anticlinesin Cretaceous to ,400'. Paleocene(?)fluvial-deltaic sedimentswithin Demarcation Ridge. C. Stratigraphic 01 igocene to Upper CretaceousOligocene Areally smal 1. 40160 Lenticularprodel taic and Miocene to Miocene topayPotevtial Miocene deltaic bodies of Grookian I L/ - 0 ~ \ ACOUSTICALLY-STRATIFIED \ \ ' ' CRETACEOUSTO PALEOCENE (1) SEDIMENTS LOCATIONDIAGRAM LCU LOWERCRETACEOUS UNCONFORMITY PLANNINGAREA E ELLESMERIANSEQUENCE LINE OF SECTION F FRANKLINIANBASEMENT Flgure 30. Schematicgeological cross section across the Demarcation sector (IIC) of theKaktovik Basin illustrating the onlapping Oligocene andyounger fill ofthe Barter subbasin and some internalfeatures of adjacent structural highs,termed "diapiric shale ridges" by Grantz and others (1982b, fig. 4). Potentialtraps exist where basin fill sedimentsare truncated at basin lnarginfaults in area 1 ofthe cross section. Deep foldsinvolving a sequence withstrony acoustic stratification (Cretaceous to Paleocene(?) fluvial-deltaicsandstone and shale(?))are locally observed within the interior of Demarcationridge, and potentialhydrocarbon traps may occur atthe crests of these folds (area 2).Onshore data from theArctic NationalWildlife Refugesuggest that some Ellesmeriansequence rocks may be preservedbeneath the LowerCretaceous unconformity in some offshore areassouth of the Hinge Line. If so, potentialtraps may exist where these strata areinvolved in folds (area 3) whichantedate the formation ofthe Barter subbasin. The locationof the cross section is shown in figure 28. Largearrows suggest possible hydrocarbon migration paths. Eay Concepf~and Ttrslp C olz~igwiatioum, I1I Within the interiors of the major structuralridges which separate the Oligocene-Miocene subbasins,large, poorly defined anticlines and synclinesare observed in seismicdata. These structuresinvolve alower seismic unit of acoustically stratified sediments which is overlain byan interval of variablethickness characterized by discontinuousacoustic reflectors. These two seismicunits are most clearlydefined within the Demarcation ridge. At present, no direct well control is availablefor the internal stratigraphy of the Demarcation ridge. However, some indirectanalogies may be drawn from stratigraphic relationships established by exploratory drilling in contiguousareas. The closest point of offshorestratigraphic control for the Demarcation sector north of the Hinge Line is the Dome Natsek E-56 well, which was drilled at a site roughly 30 miles east of the U.S.-Canadian border(location shown in fig. 28). Two major stratigraphic sequences were penetrated by the Natsek well. The upper half of the well (fig. 31)encountered monotonous marine shalesranging in age from Paleocene to Eocene. These shales overlie asequence of severalthousand feet of nonmarine to marginal marinesandstone, conglomerate, shale, and coal ranging in age from Paleocene to LateCretaceous. The overlyingshale sequence might be expected to generate few coherentreflections, while the underlyingsequence of interbeddedsandstone, shale, and coal would probably form an acoustically well stratifiedinterval on seismicrecords. We hypothesize that the two major seismicsequences observed within the Demarcation ridge may be correlative to the two major sequencespenetrated by the Natsek well.This correlation is significant, because it impliesthe possible presence of substantial thicknesses of reservoir rock within thelower, acoustically . stratified, foldedsequence within the Demarcation ridge. In the Natsek well,sandstone porosities in the lowersequence generally range from 10 to 15percent, and although no shows were encountered, at least several hundred feet of potential reservoirsands are present(fig. 31). Hydrocarbon traps maybe found alongthe crestal areas of folds in this presumed sandstonesequence, as illustrated in figure 30. Geophysical mapping suggests that the folds within the Demarcation ridgetrend northeasterly (fig. 28). subparallel to the somewhat younger Camden, Marsh, and Jag0folds, but nearly perpendicular to theconspicuous northwesterly structural trend of theridge (figs. 10, 28). Northeast-trendingfolds in deep Cretaceous and Paleocene(?)strata within the Demarcation ridge areconcentric in style, linear in axialtrend, and apparently largelyunfaulted. Some folds may be thwst-cored or related to thrust faultdeformation, as suggested by theinterpretation in plate 7. Seismic reflectors do notpersist updip into theaxial regions ofsome anticlines within the Demarcation ridge.This may suggestextreme disruption of strata or diapiric intrusion in the J J2 \ 'I I I 000' I 000'0 000'6 000'8 maws SI --i %O I < AlISOtlOd 000'L V ^D = r DmD 000's Goa DOT 5m< z m OOO'b 000'E Iy_x 000'2 3N3303 AlMV3 -000' I lN3301S131d 01 IN31338 wa4: Hld31 AlI 1.axial steep dips, possibly due tothe existence of mappable reflectors in the fold only at the level where stratadepart from open concentricity anddevelop a cuspateprofile over anticlinalcrests (e.g., fig. 32); 2. gas-chargingor overpressuring of porous sediments inthe crestalparts of the folds, thereby reducing the acoustic impedance contrastat sand-shale interfaces. A pulldowneffect observedbeneath the crests of some ofthese folds suggests crestalvelocity anomalies. TheEocene(?) shale which overlies the deep foldsappears to haveresponded in some areas to the folding of thesubstrate beneath it as a nonrigid or fluid material within which deformation patterns weregoverned by gravitational processes. The typicalresponse of the shale to elevation of the crests of major substrate folds appears to ha:: been listric detachmentand mass movement toward flanking synclinal axes.The more cohesiveOligocene to Miocene shelf(?) sedimentaryrocks which overlie the mobile shale on the Demarcation ridge responded to this movement by fracturing into discrete blocks which rotated and founderedalong listric faults originating from the deepershale section. Gravitational excavation of mobile shale from beneaththe Oligocene to Miocene strata has produced a set of minor basinsand graben structures which are superposed above the axial tracesof deeper anticlines. Numerous and diversepotential trap configurationsare thus associated with the crestal areas of these deep folds. Hydrocarbonsgenerated in shales underlying or overlying the Cretaceous to Paleocene(?) reservoir beds or in the Oligocene to Miocenebasins flanking the major structural ridges may havemigrated tothe crests of the deep foldsor overlying fault traps. The Brookiansequence ofthe nearby Canadian Beaufort has been regarded as somewhatgas prone.However, new conceptsfor organic content andnecessary thermal maturity for liquid hydrocarbon sources (Snowdon, 1980), coupled with the recent significant discoveries of oil at Amauligak, Nipterk, and Pitsiulak (Oil and Gas Journal, 1984; 1985a;1985d) in the MackenzieDelta region, suggest that liquid hydrocarbons are equally likely to be present in the eastern part of the Beaufort Sea Planning Area. 114 t DASHED HORIZON - notresolved in seismic data because of low ----- acousticimpedance SOLID HORIZON - stratigraphicsurface with high acoustic impedance. wellresolved on seismicdata Area of no seismicsignal return due to excessivelysteep dips Figure 32. Sketchof an ideal train of concentric or parallelfolds. This .illustration is presentedto show one possibleexplanation for the cuspate and diapiricallyintruded appearance of anticlinal crests in some of thefolds within the structural ridge complexes. In this model, the only horizons resolvablein seismic data are the strong reflectors at the baseof the foldtrain belowthe sinusoidal surface of maximum shortening. These reflectorsfollow a cuspateprofile, and anticlinal crests contain no reflectors because ofexcessively steep dips and possible diffraction effects. The morerounded anticlinalcrests higher in the fold train (dashed) may not beobserved because thesestrata containreflectors with verylow acoustic impedance contrast and therefore generatepoor signal return.Shortening strain in parallel-fold systemsdecreases upward and downward fromthe SinuSOidal surface, and the folded strata are generally detachedfrom overlying and underlyingundeformed strata across d6collements. Part 3 Environmental Geology 9 Physical Environment PHYSIOGRAPHY The Beaufort Sea Planning Area covers the entire AlaskanBeaufort continentalshelf and slope, the northeastern Chukchi shelf, and part of the Arcticcontinental rise and abyssalplain. The Beaufort shelf is narrow, typically 70 to 120 km wide, with an averagegradient of I m/km. Large-scalerelief features on thesurface of the Beaufort shelfare rare, although on the inner shelf, the seabed is interrupted by shoals a few to several meters high and three sets ofbarrier islands. The northeastern Chukchi shelf is a broad, flat-lying platform,generally 40 to 80 m deep. In the far northwestern end of the planningarea, Hanna Shoal rises to depths of less than 20 m. This shoaloverlies a.structura1 high (the North Chukchi high, fig. 2) on which pre-Quaternarybedrock is exposed at theseafloor (Grantz and Ei ttreim. 1979). The Beaufort and Chukchi shelvesare separated by the Barrow Sea Valley, a relict Pleistocenefeature which was incised by fluvial erosion during Pleistocenelowstands of sea level and by marine currents during interglacialhighstands of sealevel (Grantz aod Eittreim, 1979). The valley is flat bottomed, 200 km long, 2 to B km wide, and100 to 250 m deep. The BarrowSea Valleyleads seaward to the BarrowSea Canyon, which is the largest ofseveral modern submarine canyons.thatincise the continentalslope off northernAlaska. The Beaufortshelf-slope break is a complex zone ofbedding-plane slides and slump blocks(Grantz and Eittreim, 1979).East of 147" west longitude,both an inner andan outershelf break havebeen identified. The inner shelfbreak occurs at the 60-m waterdepth and marks a sharp boundary between the flat-lying shelf and the "Beaufort Ramp" (Grantz and Eittreim,1979). The Beaufort Ramp is an area with a typicalgradient of 16 m/km characterized by bedding- planefaults. The outershelf break lies seaward of the Beaufort Ramp at depthsof 600 to 800 meters. This break marks the top of a chaotic zone oflarge rotational slumps. West of the Beaufort Ramp (147"west longitude), the inner and outershelf breaks merge to form a single steepescarpment which dropsover 1,000 meters from the 60-m isobath(plate 11). The Beaufort coast trends northwesterly from Demarcation Bay to Point Barrow (plate 11). It is punctuatedby numerous shall ow ~~ baysand barrier islands. The beaches arenarrow andoften backed by a low, steep bluff of frozen or partially thawedQuaternary sediments(Hopkins and Hartz, 1978a). The Chukchicoast trends north-northeastto Point Barrow and, unlike much of the Beaufort coast, is unprotectedby islands or bays. The Chukchibeach cliffs are generally taller and contain morecoarse sediment than theBeaufort cliffs (Lewbel, 1984). Hopkinsand Hartz (1978a) divided the is1 ands along the Beaufort coastinto three types: (1) emergentdepositional shoals at the mouthsof rivers; (2) erosionalremnants of the coastal plain; and (3) recentconstructional islands. Type 1 is associatedwith deltas and actsto protect the coast from wave erosion. Gull Island,off Prudhoe Bay, is an example ofthis type. Type 2 includesseveral large is1 ands close to shore, such as F1axman Is1and and Barter Island. The elevationof these erosional remnants is comnonly more than 4 m abovesea level and theyare generally covered with peat,thaw lake deposits, and, in places,Pleistocene lag gravels. The constructional is1 ands (type 3) rise generally less than 3 m abovesea level and areonly sparsely vegetated. These is1ands arefrequently overridden by storm surges and are migrating landward and to the west. Numerous rivers cross the coastal plain to the Beaufort shelf, thelargest being the Colville River. West of Prudhoe Bay, the rivers are generally slow andmeandering, with headwaters in the foothillsof the Brooks Range. Eastof Prudhoe Bay therivers are typicallybraided and flowfrom the Brooks Range itself. These riverstransport predominantly fine sand, silt, and clay to the shelf duringspring runoff. Coarsesediment remains inthe river channels and 1ittle reachesthe coast (Hopkins and Hartz, 1978a). Large deltas at the mouths of the rivers may trap most of the fine sediment (Reimnitzand Bruder, 19721, a1 though some is transported (usually to the west) from thedeltas by longshore marine currents. METEOROLOGY The Beaufort Sea Planning Area lies within the Arctic climatic zone.The mean annualtemperature along the Beaufort coast is -12 "C (10 OF). Typical summer temperaturesrange from -1 "C (30 OF) to 4 "C (40 OF) whilewinter temperatures are typically -23 to -30 OC (-10 to -22 OF) withextreme temperatures to -50 "C (-60 OF) (U.S. Bureau of Land Management, 1979).Cloudy weather prevails most of the year with clear conditions occurring more often during the winter months. Fog is comnon alongthe coast and offshorefrom May to September. ArcticAlaska is verydry. The averageannual precipitationat Barrow is 12.6 cm/yr (4.9 inlyr). Most of this precipitation occurs as rain in August and snow in September (U.S. Bureau of Land Management, 1979). Inthe western Beaufort, the prevailingwinds are persistent in both direction andspeed. There, the wind blows from the east at an averagevelocity of 19 to 21 km/hr (12 to 13 mph) (U.S. Bureau of Land Management, 1979). In theeastern Beaufort,the prevailing wind blows from eitherthe east-northeast orthe west-southwest (Aagaard, 1981). Winds usually blowfrom the west during fallstorms. ICE ZONATION The Beaufort continental she1 f is icecovered much of the year, with a typicalice-free period occurring in August and September only. In thefall, sea ice first forms inlate September toearly October andbecomes continuousnearshore by mid-October. The shelf remains ice coveredthroughout the winter (October through June). The first movementsand openingsin the ice pack occur in late June and only by early August is thenearshore area largely ice free(Barry, 1979, table 1). During the winter months, the offshore ice can be dividedinto three main zones: thelandfast zone, theshear zone (or Stamukhi zone), and the pack ice zone(Reimnitz and Barnes,1974) (fig. 33). The landfastice is seasonal, formingalong theshore and developingseaward in theearly fall. It remains relatively undisturbedthrough the winter until it begins to melt in late June. Small movements relatedto storm frontscause narrow leads and rubblefields in this zone.In latewinter, the fast ice frequentlyextends out to the 25- to 30-m isobath. The stamukhi zone is located between thelandfast ice and the pack ice zone,generally in 18 to 30 m of water (fig. 33). It is a transition zone between the relativelystationary landfast ice and the highlymobile pack ice. Fragments of seasonalice, multiyear ice, and iceridges tens of meters high (fig. 34) aretypically found in this zone.In the stamukhizone thereis an intense interaction between the ice and the seabed as ice-ridge keels plow the seabed todepths ofseveral meters (Reimnitz and Barnes, 1974). During thebrief summer, this is an area of open leads in the ice pack and servesboth as a shipping lane and as a pathway for seasonal whale migrations. On theBeaufort shelf, the constructional islands and theirassociated shoals appear to be importantin controlling thelocation of the stamukhizone and the shorewardadvance of the pack ice. The pack ice zone, seaward of the stamukhi zone, is the shoreward edge of the permanent polarice cap. It consists ofmultiyear ice, ice ridges, and iceisland fragments that migrate westward in response tothe clockwise circumpolar gyre (Reimnitz and Barnes,1974). During the summer, ice movements in excess of 20 km/day are common (Weeks, 1978). Most of theBeaufort Sea Planning Area is covered by theice pack year-round,although the location of theinner edge of the summer ice pack variesgreatly from yearto year (fig. 33). During an atypical summer it may occurtens to hundreds of kilaneters offshore, but it generallyoccurs inside the shelf break (Barnes and Reimnitz, 1974). On theChukchi shelf, ice fons,between the middle of October and early November. Incontrast to the Beaufort Sea, whichremains largelyice covered after freezeup, the Chukchi Sea has a persistent lead,or "polynya," which forms just seaward of the fast ice in thelate winter andspring. This annual lead is thoughtto form in response tothe prevailing easterly (offshore) winds. The polynya is generally wider south of the Beaufort Sea PlanningArea, although it persists as farnorth as Point Barrow. At any timeof the year, however, it canclose rapidly either by freezing or as a result of ice movement in response to changing wind directions (Stringer, 1982a). The polynyaacts as a spring pathway formigrating whales andwaterfowl (Lewbel, 1984). Icemotion in the Chukchi Sea is generally to the northwest in response toprevailing easterly winds and north-flowingcurrents (Pritchard,1978). However, inthe eastern Chukchi Sea, approximately fourtimes a year, a tongue of ice, sometimesextending the length of the Chukchi Sea, moves in mass souththrough the Bering Strait. Thisoccurs in response toseveral poorly understood physical conditions which include current reversals, ice damming at the BeringStrait, and weakening of the ice along the Chukchicoast (Reimerand others, 1981). Inthe planning area, large-scale ice movement andtremendous rubble flows along the coast may result fromthese events. These ice"breakout events" generally last for about 4 days(Lewbel , 1984). 122 ICE ZONATION - N Icezonation in the Beaufort Sea Planning Area showing the location of the stamukhi zone (stippled). Dashed w linesindicate the northernmost (N), southernmost (S), and median (M) position of thesouthern edge of the Arctic pack ice during the period of maximum retreat (September 16 to 30). Based on datacollected from 1954 through 1970 (after Grantz and others, 1982b, and Brower and others,1977). a 10 Surficial Geologic Processes ICE GOUGING Ice gouging is one of the mostimportant agents of sediment reworking on Arctic continental shelves. It isparticularly important atmid-shelf and inner-shelfwater depths. On themid-shelf, ice ridgeswith deep keelsintensely scour the seafloor to depths of severalmeters (figs. 34 and35). Reimnitz andBarnes (1974) found gougesas deep as 5.5 m, withridges 2.7 m high (total relief of 8.2 m), in 39 m ofwater off Smith Bay.The averageice gougeon the Beaufortshelf is 50 cm deep,plows a ridge 40 cm high, and is 7.5 m wide(Barnes, 1981). For planning purposes, ice gouges of between 1 and 10 m ofrelief-may be expected. The maximum incisiondepth of ice gouges tendsto increase with increasing water depth, at least to a water depth of 45 m. The distribution of ice gouge density is shown in fig. 36. Althoughice gouges arefound across the entire shelf, they are concentrated in the stamukhizone, generally between the 18- and 30-m isobaths. The highestintensity gouging occurs on theup-drift'side ofshoals and islandsbordering the stamukhi zone. The shoalsoften show little or no evidenceof ice gouging on their down-drift side (Reimnitzand others, 1982). Off PrudhoeBay, theinner boundary of high-intensity ice gouging is controlled by the location of the islandchains, generally 15 to 20 km fromthe coast. In Harrison Bay, where thereare no barrierislands, two zones of high-intensity icegouging occur: onenear the 10-m isobath and the other in 20 m of waterseaward ofWeller Bank (Reimnitz and others,1978). These zonescorrespond to areasof abundant ice ridge formation (fig. 346). Inshoreof the stamukhi zone (waterdepth less than 18 m), ice gouging is much lesssevere. An averageof 1 or 2 percentof the seafloorper year is gouged(Barnes and others, 19781, and current- relatedhydraulic bedforms dominate over ice gouges(Barnes and Reimnitz,1974). Any ice gougeswhich form are rapidly buried by sand waves orsediment sheets. In addition, nearshore sediments tend to be coarsergrained than those farther offshore, and ice gougesdegrade more rapidly in coarse sediments than in more cohesive, fine-grainedsediments (Barnes and Reimnitz, 1979). 124 \ I 1146. \ 145. \I42 \l40. Figure 34A. Composite map ofall major ice ridges observed in the eastern Beaufort Sea (HerschelIsland to Foggy Island) between1973 and 1981. (Source: Stringer, 1982b.) Figure 348. Composite map of allmajor ice ridges observed in the westernBeaufort Sea (Smith Bay to PrudhoeBay) between 1973and 1977. (Source: Stringer, 1981.) 911 ICE GOUGE DENSITY NUMBER OF ICE GOUGES CROSSED PER KILOMETER OF TRACKLINE ~~~~ ~ Figure 36. Generalizeddistribution of ice gouge densityin the Beaufort Sea PlanningArea. Data inthe Beaufort Sea collected by Barnes(1981). Data inthe Chukchi Sea afterGrantz and others(1982a). Offshoreof the stamukhi zone, waterdepth increases, and the number of ice keels large enough toreach the bottom decreases. However, ice gougeshave been 'reported in wateras deep as 58 m (Reimnitz and others,1982). Canadian workers estimate that at this depth,gouging takes place only once everyfew hundred years (Peter Wadhams, personal commun., citedin Reimnitz and others,1982). Near theouter shelf edge, stronggeostrophic currents probably have acted to erodeand fill older ice gouges (Reimnitzand others, 1982). Ice gouges on theBeaufort shelf are generally oriented east-west, although on the inner shelf whereshoals and other bottom features deflectthe ice, orientations can vary considerably. The east-west orientation reflects the prevailing wind and surfacecurrent directions. On the Chukchi shelf,ice gouge density anddynamics are not as wellunderstood as on theBeaufort she1 f. From thedata available, it is apparentthat ice-gouged'areas are extensive, but more patchy than on theBeaufort shelf. The highestintensity gouging occurs on thenortheastern flank of large shoals, suchas Hanna Shoal,and in areasof steep bathymetric gradient, such as along the Barrow Sea Valley(Toimil, 1978) (fig. 36). Elsewhere,heavily gougedzones occuradjacent to areas only sparsely gouged (Toimil,1978). It is consequently difficult to predict the expected intensity of ice gouging at any givenspot on the Chukchi shelf. On the Chukchi shelf near Point Barrow, ice gouges generallyare oriented parallel to the coast.Elsewhere on theshelf, ice gouge orientationsare highly variable. No ice gougeshave been observed in waterdeeper than 58 m. However,gouges in water 43 m deep show evidence of infilling by recentlytransported sediment, suggesting that these gouges are modern features (Toimil , 1978). ICE PUSH On islands and coastalregions throughout the Beaufort and Chukchi Seas, ice pushand iceoverride events transport anderode significant amounts of sediment.Ice push isthe process whereby iceblocks, forcedonshore by strong winds or currents, push sediment from the coastinto ridges farther inland. Ice push is mostimportant on theouter barrier islands (Narwhal andCross Islands). There, ice push ridges up to 2.5 m high,extending 100 m inshorefrom the beach,have been identified(Hopkins andHartz, 1978al. Ice push rubble is found at least 20 m inland over most of the Arctic coast (Kovacs,1984). Boulders in excess of 1.5 m in diameterare found on some ofthese rubble piles. There are several historic accounts of ice pushevents which have damagedman-made structuresalong the Beaufortcoast. In Januaryof 1984, icepileup overtopped the Kadluk, an8-m-high caisson-retained drilling island located in Mackenzie Bay on the CanadianBeaufort (Kovacs, 1984). 128 I CURRENTS AND CURRENT SCOUR Marine currentsacross the inner shelf of the Beaufort Sea are wind driven and stronglyregulated by thepresence or absence of ice. These currentscause longshore sediment transportalong barrier islands and coastalpromontories. However, because of the short open-waterseason, the annual rate of longshoresediment transport is relatively low. Innershelf currents generally flow to the west inresponse to the prevailingnortheast wind (fig. 37), although currentreversals are common close to shore and duringstorms. On the open shelf,currents average between 7 and 10 cm/s (0.2 knots) (Matthews,1981). During storms,east-flowing currents with peak velocitiesof 95cm/s (2 knots) have been measured,although typical storm current velocities are an order of magnitude lower (Kozo, 1981). During the winter, under-ice currents are generally weak (less than 2 cm/s),although some have been measured up to 25 cm/s (0.5 knots) inrestricted passages aroundgrounded iceblocks (Matthews, 1981). Geostrophic currents with velocities of up to 50 cm/s (1 knot)occur on the outershelf, flowing parall el to the she1 f-slopebreak. Both easterly and westerly directed currents occur there. The tidalrange on the Beaufortshelf is small (15 to 30 cm), and except in confined passages,tidal currents exertonly a minor influence on the sedimentary regime (Matthews,1981). They can be importantscouring agents, however, where waterflow on the shelf is restricted by bottom-fast ice (Reimnitz and Kempema, 1982b) and by narrow passages between barrier is1 ands and shoals. On the Chukchi shelf, northeastward-flowinglongshore currents erode and transportsignificant amounts ofsediment. Nearshore currentsare predominantly wind generated,while farther offshore, northeast-fl owing geostrophic currents (the Alaska Coastal Current) and storm-generated currents predominate.Surface velocities.of 200 cm/s (approximately 4 knots) and mid-depth velocities of70 cm/s have been measured in the AlaskaCoastal Current north of Wainwright. A southwest-flowingcountercurrent of 80 cm/s has been measured near the headof the Barrow Sea Valleynorthwest of Wainwright (Hufford,1977, as cited by Grantz and others,1982a). WAVES AND COASTALEROSION Throughoutmost of the year the wave heights on the Beaufort shelf are low because of the short fetch resulting from the pervasive icecover. However, considerablefetch is developed bothseaward and shorewardof the barrier islands late in the fall open-waterseason. Ouring this time,storm waves up to 6 m high have been observed (Hopkins and Hartz,1978a). These waves can become effectiveerosive agentsboth onshore and alongthe exposed facesof the barrier islands. During storms,wind-induced storm surges force ice and wateronshore and can raise sea level as much as 3 m (Hopkins and Hartz,1978a). Low atmospheric pressures associated with the storms can raise sea level an additionalmeter (Barnes and Reimnitz, 1974). During the mostextreme surges, coastal islands are completely flooded, and majorchanges in the size and shape ofthese islands can occur during veryshort time periods (Reimnitz and Maurer, 1978b). Despitethe short open-water season in the Beaufort Sea, wave action,in combination with the melting of coastal permafrost, causesdramatic rates of coastal erosion (fig. 38). Across the Beaufortcoast, average rates of erosion vary from 1.5 to 4.7 m/yr and short term rates of 30 m/yr havebeen measured (Hopkins and Hartz,1978a). At 01 iktok Point, the coast receded by 11 m during one 2-week period(Hopkins and Hartz, 1978a). The highestrates oferosion occur along coastal promontories where the bluffs are composed offine-grained sediments and ice lenses (fig. 38). Sand andgravel eroded from bluffs cut in coarse-grained deposits form beacheswhich partially isolate those bluffs from wave action. Bluffs cut in fine sedimentare not protected by beaches and tend to erodemore rapidly. Between PointBarrow and Cape Halkett, wherepredominantly fine-grained sediments crop out in coastal bluffs,the coast is receding at an average rate of 4.7 mlyr. East of Harrison Bay, where coarsergrained sediments crop out, the averageretreat rates are between 1.5 and2.5 m/yr. Erosionrates along the Chukchi coast are an orderof magnitude 1ower than rates along the Beaufort coast (fig. 38) (Hopkins and Hartz,1978a). This is probablybecause bluffs along the Chukchi coast contain more coarse-grained sediment and the bases of many of thebluffs are cut in lithified Cretaceoussediments. In addition, while bluffs along the Beaufort coast are typically less than 3 m high andare highly susceptible to erosion and thaw by wave action, bluffs on theChukchi coast are generally higher (10 to 30 m high betweenPeard Bay andBarrow), and most of the bluff is not attacked directly by wave action(Hopkins and Hartr, 1978a). The onlyprograding shoreline areas along the Beaufort or northeastChukchi coasts occur off the deltas of major rivers. In thoseareas, the rate of progradation is slow(averaging 0.4 m/yr on theColville River) (Reimnitz andothers, 1985). BARRIER ISLAND MIGRATION Theconstructional barrier islands on the Beaufort shelf are migratingrapidly westward and landward.Hopkins and Hartz (1978a) determined maximum migration rates of 19 to 30 m/yr westward and3 to 7 m/yr landward. At theserates, it takesonly 30 to 40 years for an islandto cross a givenpoint on the seafloor. Generally theislands are becoming narrower and are breaking up intosmaller segmentsas they migrate. Between1950 and 1978, Reindeer Island split in two.Cross, Argo, and Narwhal Islands have a1 so brokenup in the recent past, andchannels between the island fragments appear to bedeepening (Reimnitz, Kempema, and others,1979). Other islands 130 NPRA CURRENT PATTERNSAND \ ANWR ', REGIONS OF STRUDEL. SCOUR AL AS KA COASTAL CURRENT COASTAL 1 ALASKA \ 5-~?&:20s,.,",. M,hi k BOTTOMCURRENTS 5&~$.!L&m~.v3,,omtl.rr 20 40 AREAS AFFECTEDBYSTRUDEL SCOUR 5g-: N2"I# (adapted'from Grantz and others, 1982b). have gone throughmarked changes in morphologysince studies began in the 1950's.Presumably the sediment derived from these islands is beingredeposited as shoals andsand ridges. Dinkumsands, a shallowshoal between Narwhal and Cross Islands, is probably a remnant of a barrierisland. In 1950, it was exposed 1 m above mean high water. Because of subsequenterosion, it has not beenexposed since 1975 (Reimnitz, Ross,and Barnes, 1979). Ice push,storm surges,and longshore drift occurring during theopen-water season contribute to the rapid migration and breakup of the barrier islands. Grain size analysis andcobble 1 ithologies on the constructional is1ands indicate that most are isolated from their original depositionalsource (Hopkins and Hartz, 1978a). This implies that removingmaterial from these islands may permanentlyaffect their size and influence on coastalprocesses. STRUDELSCOUR Duringspring runoff, the landfast sea ice is inundated by the riverflood waters. Extensive areas of the fast ice are covered as far as30 km fromthe river mouths to depths of up to 1.5 m. When theflood water reaches holes or leads in the ice, it rushesthrough with enough forceto scour the bottom to depths of several meters by theprocess of "strudel scour" (Reimnitz and others,1974). On theBeaufort shelf, strudel scour craters 6 m deepand 20 m across havebeen mapped byshallow bathymetric surveys and scuba diving observations(fig. 37)(Reimnitz and others,1974). Sheltered coastalareas andbays off major rivers, such as the Colville, Sagavanirktok,and Canning, are particularly susceptible to strudel scouring. In theseareas, deltas can be totally reworked by strudel scouring in severalthousand years (Reimnitz and Kempema, 1982a). 132 I I \ I NOISOH3 lVlSVO3 A0 S31VH aNv dWY H3VdOSI 3N33010H 11 Quaternary Geology The lithology of Quaternary sediments on the Beaufort she1 f is known from 20 boreholescollected in the BF-79 salearea by the USGS (Harding-Lawson, 1979), from 8 boreholescollected in the Prudhoe Bay area by the USGS and the U.S.Army Corp of Engineers Cold RegionsResearch and Engineering Laboratory (CRREL) (Hopkins and Hartz,1978b3, and from surficialgeologic samples collected across the Beaufortshelf by the USGS between 1972 and 1983. The distribution of Quaternary sediments is inferred from data collected during numerous high-resolutiongeophysical cruises which occurred between 1970 and 1980. These datainclude 5,600 km of uniboom high- resolution seismic datacollected in 1977 alongreconnaissance lines on the shelf and upper continentalslope by the USGS RV SP Lee (Dinter, 1982); a large nunber ofhigh-resolution seismiF-TiKs collectednearshore by the USGS GeologicDivision (over 7,000 km of uniboom, bathymetry, and sidescansonar records were collected on the RV Karluk between 1975 and 1978)(Barnes and others,1984); and a 1,600-km gridded high-resolutionseismic survey conducted in Harrison Bayby the USGS ConservationDivision (now U.S. Minerals Management Service)(Craig and Thrasher,1982). In additionto these, industry contractors have collected over 6,000 km ofpermitted proprietaryhigh-resolution data regionally on the Beaufortshelf and numerous site-specificgridded surveys over proposed drilling sites. On the Beaufortshelf, the thicknessof Quaternary sediments ranges from nearzero over structural highs offshore of Barter Islandto at least 100 meters elsewhere (Dinter,1985). In contrast, on the Chukchi shelf, the Quaternarysequence is generallyless than 5 meters thick and thickens to a maximum ofonly 15 meters nearshore(Grantz and others,1982a). SURFICIALSEDIMENTS The distribution of modern sediments on the Beaufort and northern Chukchi shelves reflects the originaldistribution of sediments on the subaerialPleistocene coastal plain, the depositional environment during the early Holocene period of continentalice sheet breakup and sea-level rise, and the environental and oceanographicconditions I34 on the modern Beaufortshelf. In thepresent sedimentary regime, the intensity of icegouging, the wave and current activity, and the composition of sedimentdelivered from rivers and from coastalbluffs are the most importantfactors affecting sediment composition and texture. Surface sedimenttextures reported by Barnes and Reimnitz (1974) and Rodeick (1979)are shown infigure 39.In general,surface sediments east of OliktokPoint contain a greatercoarse-grained fractionthan those to the west. Most of this sediment is derived frcm coastalbluffs and reflects the character of sediments on the adjacentcoastal plain. In the westernArctic slope, the coastal plain is broad (the Brooks Range sedimentsource is over 150 km south of the present coast), and riverscrossing the coastalplain are characteristically slow and meandering. The coastalplain sediments are predominantlyfine-grained fluvial and thaw lakedeposits. East of OliktokPoint, the coastalplain is narrower and higher inaverage gradient. There, coastalplain sediments are composed ofcoarse sediment derived from coalescingalluvial fans and braided river systems. Barnes and Reimnitz(1974) divided the shelf into three zones based on surficial sedimenttextures and the sedimentaryenvironment: the inner shelf, from the coast to the 20-m isobath; the central shel f, from the 20-m isobath to the shel f break (the 60-m i sobath) ; and the shelfbreak, between the 60-m and 200-m isobaths. The inner shelf is characterized by moderately- to well-sorted silts and fine sand, which are actively transported by waves and currentsduring the open-waterseason. This area lies in the fastice zoneand is relativelyunaffected by ice gouging.Hydraulic bed formsdominate overice gouge features. The sediments here are derived primarily from coastalerosion and river effluents. The central-shelfsediments arepredominantly gravelly muds. These sedimentsare highly disrupted by ice gouging and few sedimentarystructures are preserved. The coarse clasts in the muds are angular and frequently striated, suggesting that they wereemplaced asice-rafted debris. The shelf break facies is characterized by a 5- to 20-cm-thick unit of muddy graveloverlying a clayey silt unit. The surface unit generally becomes coarsergrained to the east.It contains abundantfauna and is bioturbated. In the lower unit, coarsesediment and bioturbation are uncommon. Ice gouging is less pronounced here than on the midshelf becausewater depths exceedthe depth attained by all but the largest ice keel s. Superimposed on these general sediment zonesare numerous areas of coarse-grained surface sediments on the Beaufort shelf (fig. 40). These aregenerally thin and discontinuous. However, 1 argebodies ofcoarse sediment are located on the shelf asconstructional islands (discussedabove) and submerged shoals. The mostprominent of the shoals is the Reindeer-CrossIslands ridge, which extends several kilometers northwestof Reindeer Island (located in the Midway Islands, plate 11) (Rodeick,1979). Another prominent shoal, between Spy 9E I BEAUFORT PLANNING SEA AREA i ANWR PERCENTGRAVEL IN SURFACESEDIMENTS \ 5 4 70 no 611 -3 ~~ stalule Mmltr 54Lu.%o<,,om*,e,s 60 5s- A?&- - N.",,ra,M~l*. BATHYMETRY IN METERS Figure 40. Generalizeddistribution of surficial gravel (particle size greater than 2 mm) inthe Beaufort Sea Planning Area(adapted from Briggs, 1983). and ThetisIslands offshore of Oliktok Pt., plate ll),is estimated tocontain 10,000 m $ ofclean gravel (Hopkins, 1981). In Harrison Bay,two low sandy shoals ofcoalescing sandwaves occur. These each may contain 100,000 m3 of sand(Briggs, 1983). High-resolution seismic profiles indicate that at least some ofthese shoals andsand waves aremigrating over ice-gouged sediments (fig. 41). Inouter Harrison Bay, a series of shoals lies in 15 to 20 m of water.Located atthe shoreward edge of the stamukhizone, they are evidentlyrelated to physical processes within this zone(Reimnitz andMaurer, 1978a). These shoals include Weller Bank, inouter Harrison Bay, andStamukhi Shoal, northof the JonesIslands. The surfaceof these features is covered by coarse sand and gravel. However,sandy mud found inripple troughs on Weller Bank (Barnes andReiss, 1981) indicates that finer material may underliethe surface of these features. Graveland boulder lag deposits are present at the surface of many of the barrier islands and in patches in Stefansson Sound. The largestof these, known as the"Boulder Patch," contains boulders up to 2 m in diameter and supports an abundantfauna (Reimnitz and Ross, 1979).The Boulder Patch is probably a remnantPleistocene island or coastalplain fragment, similar to Flaxman Island or BrownlowPoint, from which the fines have beenwinnowed (Reimnitz andRoss, 1979). Fine-grainedsediments on the Beaufort shelf include illite clay and minor amounts ofsmectite, kaolinite, and chlorite. The distributionof these clays reflects their detrital source and not insitu diagenetic alterations (Naidu andMowatt, 1983). Relatively highconcentrations (20 to >30percent) of expandable clays (smectite) occur at the mouth of the Colville River and in isolated areas on theouter shelf (fig. 42).The expandable clays at the mouth of the ColvilleRiver reflect the high concentration of smectite in sediments transportedby the river. Seaward ofthe Colville Delta, a linear decrease in the concentration of smectite relative to illite has been reported(Naidu andMowatt, 1983). On theouter shelf, there is noobvious modern source for the smectite. These clays may be relict,derived from Pleistocene or oldersediments. If so, this implies that there hasbeen little or nomodern sedimentation on these parts of the shelf. On thenortheastern Chukchi shelf, surface sediments are primarily muds, althoughin nearshore areas andon Hanna Shoal,sandy sedimentspredominate (fig. 39). Nearshore, an extensivegravel bed occursbetween Wainwright and Barrow, and sandy sediments form migrating sand wave fields andsand ridgesup to 0.5 m high. The nearshoresediments are probably derived from coastal beach cliffs, many of whichcontain high percentages of coarse material (Lewbel, 1984).Toimil and Grantz(1976) suggest that the coarse sediments on _d Hanna Shoal arepossibly ice-rafted, glaciomarine erratics (dropstones) thathave beenwinnowed and concentratedby the combined action of currents and ice gouging. 138 0) m Depth (meters) ru r - m IU c 0 C (n D P m 01 a 0 c a ID a VI (D a 2 0 0 7 Overconsolidatedsurface sediment (sediment that is consolidated beyond thatexpected from presentoverburden pressure) is widespread on theBeaufort shelf (Chamberlain, 1978; Reimnitz and others, 1982). Overconsolidationoccurs in varioussedimentary facies, although it is most prominentin fine-grained sediments. Two principal mechanisms have been proposed which couldoverconsolidate submarine surfacesediments: (1) freeze-thawaction (Chamberlain, 1978) and (2) compaction by ice gouging(Reimnitz and others,1980). The formernecessitates freezing of thesediment after deposition. It hasnot been demonstrated, however, that bottomwater temperatures on theshelf are lowenough tofreeze sediments in greater than 1 to 2 metersof water (Hunter and Hobson, 1974). Evidence for overconsolidation by ice gouging is equivocal(Reimnitz and others, 1982). Intuitively,ice gouging would remold ratherthan consolidate surfacesediments. Some other mechanism may be responsiblefor this phenomenon. HOLOCENE SEQUENCE On high-resolutionseismic profiles, the Holocene sequence is characterized as a thin transparent layer which overlies a generally strong humnocky reflector. The transparentnature of the surficial layer indicates that coherent beds, which would produce reflectors, aregenerally absent in this layer. Reimni tz and others(1982) concluded that ice gouging on the shelf has destroyed most of the primary stratification'inthe Holocene sequence. The underlying hummocky horizonhas been interpreted to represent a lithologic change at the top of thePleistocene sequence or other changes in the physicalproperties of theshallow sediments, such as the surface of ice bonding (Craig and Thrasher, 1982). On the inner shelf, between Harrison Bay and F1axman Is1and, the acoustically transparent layer is generally less than 10 m thick(fig. 38) (assuming a two-way velocity of 1.6 km/s). USGS boreholes (Harding-Lawson,1979) confirm this estimateof the Holocene thickness on thi.s part of theinner she1 f. Dinter (1982) reports that the Holocenesequence is either very thin (bel ow the resolution limits of the uniboom system)or absent close to shore. In Harrison Bay, thetransparent layer thickens from less than 2 m off the Colville Delta to greater than 20 m offshore (Craig and Thrasher,1982). In outerHarrison Bay, it thickensabruptly along a featuresuggested by Craig and Thrasher(1982) to represent a Pleistocenepaleo-shoreline. It is possiblethat the deeper sediment within thetransparent layer north of this boundary is Pleistocene in age. If this isthe case, then this change in the thickness of the transparent layer may be due to a change in the depth tothe surface of ice-bonding and not to a significant 1 ithologic break between Holocene marine and P1 eistocene nonmarine sediments. The thicknessof Holocene sediments on the outer shelf of the Beaufort Sea is unknowndue to a lackof geologic data there. Uniboom lines,interpreted by Dinter (19821, indicatethat the transparentlayer (Holocene sequence?) is wedge shaped, thickening to over 45 m atthe shelf edge off Camden Bay (fig.38). Reimnitz and others(1982) collected grab samplesfrom the outer shelf in the same areaand reported the occurrence of relict surficial gravels. They suggestthat much of whatDinter (19821 identified as Holocene in age isactually Pleistocene. Because theouter shelf andupper slopeare characterized by massive slump blocks and gravity faults, data on the age ofthe sediment involved in these slumps areimportant in determiningwhether ornot they are presently active. Mostworkers agree that the Holocene sequence is thin or absentover the anticlines north of Barter Island where historic seismicity andshallow faults indicatethe occurrence of recent tectonic activity (fig. 38). On the Chukchi shelf,unconsolidated sediments are thin (averaging 2 to 5 m thick). The thickestaccumulations fill channels cutin pre-Pleistocene units (Grantz andothers, 1982a). These sediments may include both Pleistocene andHolocene material. PLEISTOCENE SEQUENCE The Pleistocene stratigraphy of the Beaufort shelf is extrapolatedfrom onshore geologic mapping and shallow borings offshore.Publicly available geologic data offshore are sparse outside of the BF-79 salearea (Harding-Lawson, 1979; Hopkins and Hartz,1978b). There is one publiclyavailable borehole collected westof Cape Halkett(Harrison andOsterkamp, 1981). The Pleistocene stratigraphy on the rest of the Beaufort Sea PlanningArea has been inferredfrom high-resolution seismic surveys only (Barnes and Reimnitz,various reports; Dinter, 1982,1985; Craig andThrasher, 1982).Data quality varies with the different systemsdeployed, but is generally better in deeperwater. Unfortunately, because of heavy ice cover and rapid ice incursions during the short open-waterseason, mostof these high-resolution data have been collected on the inner she1 f only. The Pleistocene sequencecan be characterized as cycles of marine andnonmarine deposition related to alternating glacial and interglacialepisodes. Onshore, Pleistocenerocks are grouped into theGubik Formation (Black, 1964), which sits unconfonnably on Cretaceousor Tertiary sediments of the Brookian sequence. It consists of sticky marine muds, poorlysorted marine sandsand cobbles,clean quartz beachsands and gravels, nonmarine thaw lake muds, eoliansilts and fine sands,and fluvialgravels. Ice locally constitutes morethan half of the volumeof theGubik sediments, andorganic matter is abundant. 142 Inthe offshore boreholes in Stefansson Sound, thePleistocene sequence is composed of stiff silty clay overlying coarse sand and gravel. The thicknessof the silty clay and thedepth to the coarse-grained unit vary from boreholeto borehole. In several of theboreholes, the top of the Pleistocene sequence occurs at or nearthe seafloor. Pollen collected from this unitgive dates of up to 50,000years B.P. (Miller and Bruggers,1980). indicating that 1 ittle or no Holocenesediment has accumulated there.Elsewhere in Stefansson Sound, thePleistocene sequence underlies up to 13 m of Holocene sediment(generally soft silt and clay). The depth to thesurface of thecoarse-grained Pleistocene unit generally increases offshore and tothe east in Stefansson Sound. Dinter (1985) has identifiedseveral Quaternary transgressive and regressivecycles on the Beaufort outer shelf from uniboom and 3.5-kHz seismicrecords. These cycles consist of transparent to well- laminated units, interpreted to be marinetransgressive deposits, unconformably overlain by thin, hummocky-surfaced units, interpreted to be nonmarine regressivedeposits (fig. 43). On the eastern Beaufortshelf, these units form two northward-thickeningsedimentary wedges separated by thestructural high offshore of BarterIsland (fig. 38). The age relationship between these transgressive and regressive cycles and the cycles recorded in the Gubik Formation on theadjacent coastal plain is uncertain. However, based on the depths below presentsea level to each unit, Dinter(1985) has tentatively correlated these units with pub1 ishedsea-level curves. His conclusions suggest that a relatively complete Pleistocenesection may be preserved on partsof the Beaufort shelf. The youngest presumed nonmarine Pleistocenesequence terminates on the outer shelfas a ridge 8 to 18 m high (fig.43). This ridge may represent the northernmost extent of the latest Wisconsinmarine regression on theBeaufort shelf (Dinter, 1982). Within the Pleistocene section, seismicdata suggest that buried paleovalleys exist on the Beaufort and Chukchi shelves (Dinter, 1985; Grantz and others, 19B2b). These paleochannelsoccur offshore of the SagavanirktokRiver (Hartz and Hopkins,1980);the Canning River (Hopkins and Hartz,1978a; Ointer, 1982). and possiblythe Colville River (Hopkins,19811. Some of thePleistocene gravel cored in Stefansson Sound between Prudhoe Bay and ReindeerIsland may occur in theSagavanirktok River paleoval ley. On the Chukchi shel f, where theQuaternary section is generally thin, paleovalleysare cut in Tertiary and olderbedrock. The thickestaccumulation of Quaternary sedimentoccurs in those valleys. Consolidated bedrock is probablyexposed at or near the seafloor on theBeaufort shelf north ofBarter Island where Dinter(1982) has outlined an areaof zero Holocene thickness. In this area, CDP and uniboom seismicrecords show shallow dipping reflectors of probable Tertiaryage approaching the seafloor (Reimnitz and others, 1982) (plate 6, fig. 44). On the northeast Chukchi shelf, bedrock may cropout in the vicinity of Hanna Shoal,along the walls of the Barrow submarinecanyon, and in local sites across the inner shel f betweenWainwright and Barrow (Grantz and others,1982a). J USGS LINE 81 - 32 I I I I I I 0.5 0 kilometers Figure 44. E USGS uniboom line fromthe Barter Island area (for location, see figure 50) showingshallow dipping bedrock Of probableTertiary age intersectingor nearly intersecting the seafloor. The roughseafloor surface may be E caused by icegouging or differential weathering of the bedrock units. ua ‘c- 4 u1 12 Subsurface Geologic Features PERMAFROST The Beaufort shelf was subaerially exposed to the Arctic climate duringseveral Pleistocene lowstands of sea level (Wang andothers, 1982).During this time, well-bonded permafrost formed to depths ofseveral hundred meters beneath the exposed shelf (Hunter and Hobson, 1974).During subsequent highstands of sea level,the permafrost has in part melted by salineadvection from theseawater into the underlyingsediment -and by geothermal heating. Numerous refraction, borehole, and conductivity surveys indicate that permafrost is widespreadbeneath the Beaufort inner shelf (fig. 45). Seismic -refractionsurveys were performed in Harrison Bay byRogers and Morack(1981) and Neaveand Sellmann (19831, in SimpsonLagoon by Neave andSellmann (19831, on the barrier islands byRogers and Morack(19811, and on the CanadianBeaufort shelf by Morackand others(1983). Further data have been obtained from boreholes (Harding-Lawson,1979) and thermal probes in the BF-79 salearea (Rogersand Morack, 1981; Hopkins and Hartz, 1978b) and offshore of Cape Simpson (Harrison andOsterkamp, 1981). On theCanadian' Beaufort,permafrost has been cored as far offshore as32 km north of Cape Bathurst (Hunter and Hobson, 19741. Seismic refraction workby Sellmann and others(1981) indicates that on theAlaskan Beaufortshelf, a high-velocitylayer interpreted to represent permafrost is present at least 15 km northof Reindeer Island and at least 25 km offshore of Harrison Bay (fig. 45). The depth to the surface of subseapermafrost is highly variable (fig.46), reflecting varying degrees ofice-bonding prior to the Holocenemarine transgression aswell as thedegree of subsequent thawing due tothe advection of saline groundwater. In Stefansson Sound, USGS boreholes(Harding-Lawson, 1979) comnonly encountered ice (presumed to bepermafrost) at depthsshallower than 15 m. In theseholes, the depth to the surface of bondedpermafrost varies greatly from less than 9 m to greater than 30 m over a distance of lessthan 12 km (Harding-Lawson,1979). Some ofthe boreholes encountered a transition zone of partially bondedsediments between theunfrozen surface sediments anddeeper, well-bonded sediments (Harrison andOsterkamp, 1981). This transition zone makes it difficult to accurately interpret the depth to the permafrost surfacefrom both borehole logs andseismic refraction data. Frozen 146 + + + 2- + + + . BEAUFORT SEA + + + 10 + + + + + + INFERRED SHALLOW PERMAFROST Figure 45. Known distributions of high-velocitymaterial inferred to beice-bonded sedimentsfrom Harrison Bay tothe Canning River. Data east of the Colville Riverare from Neaveand Sellmann (1983). Datawest of theColville River arefrom Sellmann and others (1981). 0 Refraction data (Rogers and Morack.1978) A Reflection data Rqrsand Morack, 1978) Refraction data (Rogers. in Barnes and Hopkins,l978) a Drilling, sampling, temperature and chemistry data (CRREL-USGS) b Drilling. sampling. temperature and chemistry data (Osterkav and HarrisOn.1976) Figure 46. Summary ofthe depth to ice-bonded permafrost along an OCSEAP studyline near Prudhoe Bay, asreported by Sellmannand Chamberlain (1979). For the location ofthe study line, see figure 45. The twolayers of permafrostdetected in the Humble C-1 well may alternativelybe interpreted as a deep relictpermafrost layer(below 91 m) formedduring a Pleistocenelowstand of sea level and a surficialpermafrost layer (above19 m) formedunder modern Arcticconditions since Reindeer Island migrated to its present site. I t- sedimentencountered in boreholes and interpreted to be well-bonded permafrost, may in fact be lenses of ice-bondedmaterial in the transition zone. Similarly,high-velocity refractors may represent physicalchanges in the permafrostlayer and may lie below the permafrostsurface in thetransition zone. As a result, in the BF-79 sale area, there are differing interpretations of the depth to ice-bondedmaterial between the USGS boreholes (Harding-Lawson, 1979) and the seismicrefraction data ofRogers and Morack (1981). Hopkins and Hartz(1978a) estimate that it takesonly 40 to 50 years for well-bonded permafrost to form in a subaerial arctic environment.Permafrost istherefore expected to occur in the . core of some barrier islands which migrateacross the seafloor. On ReindeerIsland, the Humble Oil C-1 wellencountered two layers ofpermafrost at depths of 0 to 18.9 m and 91 to 128 m (Sellmann and Chamberlain,1979) (fig. 46). The deeperlayer is probably relict Pleistocene permafrost, while theshallow layer may have formed under modern arctic conditions since the islandmigrated to its present site. The thickness of permafrost on the Beaufortshelf cannot be determined from seismic refractiondata or shallow boreholes. However, the thickness of the permafrostlayer beneath the coastal plain has been measured from numerous onshorewells in Arctic Alaska and Canada. Onshore wellsnear Harrison Bay indicatethat the permafrostlayer thins to the west. Eastof Oliktok Point it is 500 m thick, whereaswest of the Colville River it is 300 to 400 m thick (Osterkamp and Payne, 1981). The depth to the surface and overall thickness of permafrost on the easternBeaufort (off ANWR) and northern Chukchi shelves is not known. It has been suggested that in areasof relatively slow coastalretreat, such as the Chukchi coast,permafrost would be degraded and found deeper than on a rapidly retreating coast such as the Beaufortcoast (Grantz and others, 1982a). NATURAL-GAS HYDRATES Natural-gas hydrates(solids composed of light gasescaged in the interstices of an expanded ice crystal lattice) commonly occur in deepwaterareas of continentalmargins under low-temperature, high-pressure conditions(Macleod, 1982). On Arcticshelf areas, gashydrates may form at shallow depths associated with permafrost (Kvenvolden and McMenamin, 1980). In the Alaskan Arctic,gas hydratesare known to occur at shallowdepths onshore at Prudhoe Bay (Kvenvolden and McMenamin, 19801, and hydrates may occur under similarconditions beneath the Beaufortinner shelf in areasunderlain by permafrost(Sellmann and others,1981). Beneath the Beaufort continentalslope, a gashydrate horizon is identified where water depthsexceed 300 m (fig. 47) (Grantz and others,1982b). Subnun6ace GeoLogic F&un~b, 149 S HALLOW GAS AND GAS HYDRATES GAS AND GAS SHALLOW ANWR AREAUNDERLAIN 81 GAS-ENHANCEDREFLECTOR HlGH SHALLOW GAS CONCENTRAITION INFERRED FROM ACOUSTlCATTENUATION ON UNl0OOU RECORDS -3 SElSUlC PROFILECROSSING OF 014PIRIC STRUCTURE 5&: L-AL20 Na",,cill Mllar BATHVUETRV IN Em I Figure 47. Map showingthe distribution of shallowgas concentrations, the minimum areainferred to be underlain by natural-gashydrates, and the distribution of diapiric structures in the Beaufort Sea PlanningArea (modifiedfrom Grantz and others, 1982b). I \ FAULTING AND SEISMICITY Severaltypes of shallow faults are identified on theBeaufort shelf:high-angle, basement-involved normal faults (mapped principallyalong the Barrow Arch inHarrison Bay), listric growth faults (mapped seaward of the Hinge Line), and down-to-the-north gravityfaults (mapped alongthe shelf-slope break) (fig. 48) (Grantz and others,1982b). Locally two or more types may occur in close proximity . High-angle faults occur along the Barrow Arch and are geneticallyrelated to basement tectonics of theArctic Platform. In Harrison Bay, they offset Tertiary and older units (fig. 49) (Craig and Thrasher,1982). There is little evidence of Quaternary movementand no recentseismicity associated with these faults. They may actas conduits for gas migration, however, for "bright spot" anomaliesare commonly identifiedadjacent to the fault traces (Craig and Thrasher,1982). The shallow faults seaward of the Hinge Lineinclude upper extensionsof detached listric growth faults that sole deep inthe Brookian section (plates 4 and 6), some of which may have been reactivated in late Cenozoic time. The distribution of these growth faults is only partially known because of a lackof high-resolution seismiccoverage on the outerBeaufort shelf, especially in the west. These faults are mapped ingreatest detail in the Camden Bay area where the Hinge LineaDproaches the Beaufort coast (fig. 50) andwhere USGS high-resolutionseismic coverage isrelatively dense.Shallow faults have also been mapped beneaththe outer shelf west of Cape Halkett and are reported to show 3 to 10 m of Quaternaryoffset (fig. 48) (Grantz and others, 1983). No seismicity has been recorded in this area in 10 yearsof monitoring (fig. 51) (Biswas and Gedney. 1979). In the CamdenBay area,near-surface faults have severaltens of meters ofQuaternary offset (fig. 52) (Grantz and others,1983), and, in contrast to the rest ofthe Beaufort shelf, CamdenBay is seismicallyactive (fig. 51). This tectonicarea is locatedat the northern end of a north-northeast-trending band of seismicity that extends north from east central Alaska(Biswas andGedney, 1979). The largestearthquake recorded in northeast Alaska was a magnitude5.3 quake located 30 km (18 mi) north of BarterIsland (Biswas and Gedney, 1979).Since monitoring began in 1978, a large number ofearthquakes, ranging in magnitude from 1 to 4, have been recorded in this area. These eventscluster along the axis of the Camden anticline(fig. 50). Tertiary and Quaternaryunits dip away from and are truncated at the top of this fold, indicating that it has been growing in recentgeologic time. In contrastto the northeast-southwest trend of the Camden anticline and of the roughlyparallel zone of earthquakeepicenters, the faults in CamdenBay trendnorthwest-southeast, parallel to the Hinge Line Subnun6ace GeoLogic FeatwLu, 751 Figure 48. Map showingareas where shallow faults are happed or expected in the Beaufort Sea PlanningArea, and the distributionof earthquake epicenters. The HingeLine and Barrow Arch are plotted to show therelationship between shallowfaults and majorstructural trends. Fault data are from Grant,?and others (1982b) andCraig and Thrasher (1982). , i >.I. Figure 44 . . . 9;: . 9 SHALLOW STRUCTURESANDEARTHQUAKE ERCENTERS NEAR CAMDEN BAY Jt-" SHALLOWFOLD ..x. SWLLOW FAULT (DASHED WHERE INFERRED) HEADWALL SCARP OF LARGE ROTATIONAL SLIDES . MAGNITUDE 3-5 EARTHQUAKE EPICENTER e MAGNITUDE 5.3 EARTHQUAKE EPICENTER - Figure 50. Distribution of shallowfaults, fold axes,and earthquake epicenters in the Camden Bay area.Note the northeast trend of thefolds and earthquake epicenters,in contrast to the northwest trend of the faults. For the location of the map, seefigure 48. J 54 i 9s 1 -arn+mcn~w~-o 0000000000 0 (fig. 50). As theyapproach and intersect the axis of the fold, theyoffset progressively younger units. This relationship suggests thatthese faults are older Hinge Line-related structures that were reactivated in 1 ate Tertiary and Quaternary time by the up1 ift of the Camden anticline.Grantz and Ointer(1980) mapped faultscarps alongtwo fault segments in Camden Bay, wherethey observed 6 m of seafloordisplacement. The evidence ofseafloor scarps in this area is equivocal,however, because scarp heights are of the same magnitude as ice gouge relief. In addition,the ice-gouging process shouldquickly smooth scarpsformed on theseafloor. Therefore, active near-surface faults may be much morenumerous in Camden Bay thanindicated by the number ofseafloor scarps previously reported. Faults on theouter Beaufort shelf andupper slope are gravity faultsrelated to large rotational slump blocks(Grantz and Ointer, 1980). On theeastern Alaskan Beaufort shelf, these slumps bound the seawardedge ofthe Beaufort Ramp (figs. 53B, 548).Shoreward of the Ramp, faults havesurface offsets that usually range from 15 to 20 m and, at one site,possibly as high as 70 m (Grantzand others,1982b). The Beaufort Ramp itself may be a gigantic slump blockwhich is boundedby thesegravity faults. Theage ofthe shelf edge faultsis uncertain. If Grantzand others (1982b) are correct in assuming thatsediments on theouter shelf are Holocene in age, thenthese faults have been active in Recent geologic time. If thesurface sediments on the outer shelf are relict Pleistocene deposits, as suggestedby Reimnitz and others (1982),then these large gravity faults may. have heen quiescent throughoutHolocene time(12,000 years B.P. topresent). These faults posean extreme hazard to bottom-founded structures on the outer Beaufort shelf and s1 opebecause they could result in large downslopedisplacements. Eventhough there has been no historic seismicity associated with this type of fault on theBeaufort shelf, they may bemoving by slow,asei smiccreep. Large-scale gravity slumping of blocks on the outer shelf could be triggered by shallow-focusearthquakes centered in CamdenBay or in the Brooks Range. SEDIMENT SLIDES A chaotic sediment slide terrane occurs along the length of the Beaufort outer shelf andupper slope seaward of the 50- to 60-m isobath(figs. 53, 54).Grantz and others (1982b) have mapped severaldistinct lands1 ide types, including large bedding-plane slides(figs. 538,54B) andblock glides (figs. 53A, 54A). The bedding-planeslides are most extensive on the Beaufort Ramp between 148' westlongitude and theMackenzie Sea Valley (figs. 538,548) (Grantz and Eittreim,1979). These slidesare 10 to 43 km long and 70 to 230 m thick.Pull-apart grabens andscarps are comnonon thelandward margin of the slide terrane. Horizontal displacements Subhwr~aceGeoLogic Featwren, 157 i i 0 50 Km I I I I 1 I Figure53A. Blockglide terrane on theouter Beaufort shelf in the westernpart of the Beaufort Sea PlanningArea (Grantz and Eittreim, 1979). Figure 538. Zone ofbedding-plane slides on theBeaufort shelf east of 14.7' westlongitude (Grantz and Eittreim, 1979). Because of thelack of a long-termdata base, the size of the maximum earthquaketo be anticipated in engineering design is uncertain. Modern earthquakes with magnitudes of 5.3 on the Richterscale havebeen measured in this area (Biswas and Gedney, 1979).Seafloor installations constructed on the easternBeaufort shelf should be designedto withstand moderate to strong,shallow- focusearthquakes with significantsurface offsets along faults. Movement of slump blocks on the outerBeaufort shelf and continental slopecould result in major damage toexploration or development structures. The age and activity of the large,easily recognized slump features on the outershelf is unknown, but should be determined beforesiting exploration or production platforms in this area. In 15 years of extensive studies, much has been learned about the physicalenvironment in the Beaufort Sea Planning Area. However, it is still largely a frontierregion, and the origin and distribution of many of the physicalprocesses and features that may affect offshorepetroleum development areonly partially understood. Most of our knowledge is based on data collected on the inner shelf, particularlyin Stefansson Sound(BF-79 leasesale area) and Harrison Bay (OCS LeaseSale 71 area). Becauseof severeice conditions and the remoteness of the central and outer shelf, very little work has beendone in areasincluded in OCS Sal e 87 or proposed for inclusion in OCS Sale 97. From datacollected in Stefansson Sound, it is evident that the type and extent of shallow geologicfeatures varygreatly over very shortdistances. It is, therefore,difficult to predict the occurrence and distribution of features such as shallowgas concentrations, subsea permafrost, surficial sediment type, and near-surfacefaults on a site-specificbasis from regional reconnaissancesurveys. Detailed high-resolution seismic surveys and shallow coring are now routinelyconducted at proposed drilling locationsto fill this site-specificdata gap. Integrationof .these data with regionalsurveys will expand ourdata base and greatly improve ourunderstanding of the Arctic offshoreenvironment. Summy 06 €nv&wwncntd Factom, 167 I Conclusion This report summarizes ourobservations and preliminary conclusionsconcerning the environmental and petroleumgeology of the Beaufort Sea Planning Area. It is the latest in a series of regionalreports on the geologyof the Alaskan OCS published by the Minerals Management Service. This geologicreport was prepared as part of the supportdocumentation related to Federal OCS Lease Sale 97. Sale 97 will be the secondarea-wide lease sale in the Beaufort Sea and the fourthFederal OCS sale in this planningarea. Because of high industry interest in the previouslease sales, approximately 376 tractsare now under lease. The most prospectivetracts are located in relativelyshallow water, near onshore we1 1 control, and overlarge structures. Two possible commercial discoveries have been reported in the BF-79 salearea (Endicott Field and SealIsland). Unleased tracts which will be offered in Sale 97 generally lie in deeper and more remote areas of the Beaufort and northeastern Chukchi shelves. These tractswill be more expensive and logistically difficultto explore. At present, wellcontrol is absent and seismic datacoverage is sparse in these remote areas. Only tracts on the continentalshelf are thought to have any potentialfor commercial hydrocarbondevelopment in the foreseeable future based on geological and logisticalconsiderations. Of the untested tracts on the continentalshelf, structural traps involving Brookian strata in the KaktovikBasin are the most prospective. 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Listof Abbreviations for Control Wells on Plates 1-7 Abbreviation Geologic Unit b!? ?” Qs GubicFormation Quaternary Ts SagavanirktokFormation Tertiary Kc Colville Group LateCretaceous Kn NanushukGroup EarlyCretaceous Kt TorokFormation EarlyCretaceous KPS Pebble Shal e EarlyCretaceous JRk KingakFormation Jurassic TR-JRsr Sag RiverFormation LateTriassic to Early Jurassic TRs h ShublikFormation Triassic TRsrfshfsd Sag Riverto Sadlerochit Triassic P-TRs Sad1 erochi t Group Permian to Triassic PP1 LisburneGroup Pennsylvanian M1 LisburneGroup Mississippian B metamorphicbasement MiddleDevonian and older (NeruokpukFormation; Frankliniansequence) Additionalinformation on lithostratigraphic column (fig. 3). 7 92 (r U S. GOVERNMENT PRINTING OFFICE 1986693-0571U.000REGION NO. 8 w United States Department of the Interior ... Minerals Management Service - Alaska Outer Continental Shelf Region -~ .. .. -