OCS Report MMS 86-0033
GeologicReportfortheNortonBasin PlanningArea,BeringSea,Alaska
Ronald F. Turner Gary C. Martin David E. Risley David A. Steffy Tabe 0. Flett Maurice B. Lynch
edited by
Ronald F. Turner
UnitedStatesDepartment of the Interior Minerals Management Service Alaska OCS Region
Anchorage,Alaska 1986
Any use of trade names is fordescriptive purposes only and doesnot constitute endorsement of these products by the Minerals Management Service. CONTENTS
~Page
Introduction, byRonald F. Turner...... 1
Part 1 Regionalgeology 1 Geologicframework,byGary C. Martin...... 5 2 Regionalgeologichistory, byDavid E. Risley ...... 10 3 Structuralgeology,by David E. Risley ...... 13 4 Biostratigraphy, byRonald F. Turner...... 27 5 Lithostratigraphy and seismicstratigraphy, by Gary C. Martin...... 47
Part 2 Petroleum geology 6 Explorationhistory,byMaurice B. Lynch ...... 73 7 Reservoirrocks,by Gary C. Martin...... 77 8 Organicgeochemistry,by Tabe 0. Flett...... 91 9 Playconcepts,by Gary C. Martin, David E. Risley, RonaldF.Turner,andTabe 0. Flett...... 127
Part 3 Shallowgeology,geohazards,andenvironmental conditions 10Shallowgeology,by David A. Steffy ...... 135 11 Geohazards,by David A. Steffy andDavid E. Risley ... 145 12 Environmentalconditions,by David A. Steffy ...... 158
References...... 161
Figures
Figure 1 Location of NortonBasinplanningarea...... 2 2 Generalgeologic map ofeasternSiberia,BeringSea, and westernAlaska...... 3 LateCretaceoustoTertiarytectonic framework of theBeringStraitregion...... 11 4 StructuralprovincesoftheNortonBasinregion ...... 14 5 Generalizedstratigraphiccrosssections across Norton Basin...... 16 6 Structurecontours(intime) on horizon A in NortonBasin...... 19 7 Structurecontours(indepth) on horizon A in NortonBasin...... 21 8 Contoursof Bouguer gravityoverthe Yukon Deltaand offree-airgravity overNortonBasin...... 24
iii Page
9 Relationship between free-air gravity value and depth tobasement...... 25 10 Stratigraphic and paleobathymetric summary of the COST No. 1 well...... 28 11 Stratigraphic and paleobathymetric swmnary of theCOST No. 2 well...... 29 12 Biostratigraphic correlation of the COST wells...... 45 13 Seismic profile, time-stratigraphic column, seismic sequences, lithology, and lithologic zones ofCOST No. 1 well...... 50 14 Seismic profile, time-stratigraphic column, seismic sequences, lithology, and lithologic zones of COST No. 2 well...... 51 15 Pattern of decrease in acoustic interval transit time and increasein bulk density for diatomaceous shales and their diagenetic equivalents, COST No. 1 well...... 52 16 Pattern of decreasein acoustic interval transit time and increasein bulk density for diatomaceous shales and their diagenetic equivalents, COST No. 2 well...... 53 17 Structure contours (in time)on horizon D in Norton Basin...... 57 18 Structure contours(in depth) on horizon D in Norton Basin...... 59 19 Comparison of large- and small-scale vertical wireline log patterns of rock unitTo-2, St. Lawrence subbasin, and characteristic vertical cycles of submarine fan turbiditefacies...... 63 20 Wireline log details of large- and small-scale bedding cycles of the inferred distal facies of a distributary mouth bar sandstone, To-3,zone COST No. 2 well...... 66 21 Locations of sale areas, leased blocks, and COST and exploratory wells...... 74 22 Ternary diagrams showing the range of the principle sandstone framework constituents and the major rock types of lithic fragmentsin sandstones from conventional and sidewall core samples from theCOST No. 1 andNo. 2 wells...... 76 23 Plot of porosity versus permeability for conventional core3 (7,020.6 to 7,047.5 feet), COST No. 2 well...... 83 24 Distribution of sandstone porosity with depth,COST No. 1 and No. 2 wells...... 85 25 Water saturation determinations of a suspected oil zone at12,133 to 12,300 feet, COST No. 2 well..... 88 26 Chart of porosity versus irreducible water saturation for estimating permeability and determining bulk volume water for theCOST No. 2 well...... 89 iv Page
27 Partial compilation of organic geochemistry from Norton Sound and the adjacentarea...... 28 Organic carbon and classification of organic matter, COST No.1 well...... 97 29 Modified Van Krevelen diagram, COST No.1 well...... 99 30 Indicators of thermal maturity, COST No.1 well...... 101 31 Classification of organic matter, COST No. 2 well...... 105 32 Modified Van Krevelen diagram, COST No. 2 well...... 107 33 Indicators of thermal maturity, COST No. 2 well...... 109 34 Hypothetical depositional history, tectonic history, and temperature gradient, plus selected indicatorsof thermal maturity, COST No. 2 well...... 113 35 Correlation of Ro and present dayTTI values, COST No. 2 well...... 119 36 Hydrocarbon generation model for oil and condensate from source rocks containing terrestrial organicmatter...... 124 37 Possible structural and stratigraphic trap configurations associated with major Tertiary sedimentary sequences and pre-Tertiary basement...... ~...... 126 38 Physiographic mapof the Bering Sea region...... 136 39 Bathymetric map of the Norton Sound planning area...... 139 40 Sea level history for the BeringSea...... 141 41 Isopach map of three Holocene sedimentary units in Norton Sound...... 142 42 Locations of epicenters of earthquakes...... 147 43 Surface and near-surface faultsin the Norton Basin planningarea...... 149 44 Map showing seafloor featuresin Norton Sound...... 152
Tables
Table 1 Wireline log analysis of the sandstone interval cut by core 3, COST No. 2 well...... 80 2 Petrographic modal analysisof thin sections and core analysis of conventional core3, COST No. 2 well...... 81
V Page
3 Wireline log analysisof Oligocene sandstone sequence in the COST No. 2 well...... 81 4 Depths and absolute ages of sediments in the COST No. 2 wellused in the Lopatin computation...... 112 5 Random mean vitrinite reflectance values for the oilgeneration zone...... 115 6 Relationship of TTI values to hydrocarbon generation, COST No. 2 well...... 116 7 Maceral classification for northern Alaska coals...... 122 8 Relative compositions of coal maceral groups and theliptinite macerals...... 123 Introduction
Thisreport is a summary oftheregionalgeology,petroleum geology,andenvironmentalcharacteristicsoftheNortonBasin PlanningArea. To some extent,theinterpretationshereinare based on previous detailed studies by the Minerals Management Service (MMS) of two deepstratigraphictest wells, theNortonBasin COST No. 1 and No. 2 wells(Turnerandothers, 1983a,b). However, since the release of these reports, a great deal of thesedatahavebeen reprocessed and reinterpreted. In addition, new data havebeen collected or become publicallyavailable. In particular,the common-depth-point (CDP) seismic reflection data base has been improvedandexpandedby theinclusion of lines generously released to MMS byWesternGeophysical Company.
The NortonBasinPlanningArea(fig. 1) is roughly bounded by theNorton Sound coastline on the east and southeast,the Seward Peninsula on the north, the 63 degree line of north latitude on thesouth, and thedisputed US-USSR 1867 conventionline on the west. The mostprospectivepart of theplanningareaunderlies NortonSound.
The volume, quality, and distribution of reservoirrocks, thepresence of potential seals, and a diversity of trapping configurationsshould combine to ensure further exploration in the NortonBasin. The timing of sourcerockmaturation in relation to trap development alsoappears favorable for hydrocarbonentrapment. The volume, quality, and distributionofsourcerocksappearto representthe most serious constraints on thepotentialforcommercial hydrocarbon accumulations in the basin.
LeaseSale 57, held March 19, 1983, generated $325 million in highbids.Ninety-eightbids were received on 64 tracts; 59 bids wereaccepted, 5 rejected.LeaseSale 100 is scheduledfor March 1986.
Six exploratory wells have been drilled in the NortonBasinsince 1984. All of these wells havebeenplugged andabandoned. No commercial hydrocarbon discoveries have been announced.
1
1
GeologicFramework
The NortonBasin is locatedoffthecoastofwest-central Alaska,approximatelycoincidentwithNorton Sound (fig.1). It is oneof severalmajor Tertiary basins on theBering Sea continental shelf(fig. 2). The basin is approximately125mileslong and ranges from 30 to 60 milesinwidth. It is bounded bythe Seward Peninsula on thenorth,the Yukon-Koyukuk geologicprovince on theeast, the ChukotskPeninsula on thewest, and the Yukon DeltaandSt. Lawrence Island on the south.
The presence of a sedimentary basin beneath Norton Sound was first suggested by Payne(1955)fromonshoreregionaltrends. Subsequentseismicmapping in thearea(Scholl andHopkins,1969) indicatedthepresence of significantaccumulations of relatively young,undeformed sedimentarystratabeneaththesound. More detailed seismic mapping subsequentlyrevealedthattheNortonBasinconsists of two distinctsubbasins, each with a somewhat different geological history. The subbasinscontain up to 24,000 feet ofmarineand nonmarineTertiary strata.
The pre-TertiarybasementcomplexoftheBeringSeashelf, parts of northeastern Siberia, and westernAlaskahasbeensubdivided intothreebroadtectonostratigraphicprovinces(Fisher and others, 1979).Theseprovinces,fromnorthtosouth,aretermedthe miogeoclinal belt, the Okhotsk-Chukotsk volcanic belt, and the forearcbasinbelt. The Tertiarybasins of theBering Sea shelf are superimposed on thesethreeprovinces(fig. 2). The stratigraphic and lithologic characteristics of thesediments contained within these basins have been greatly influenced by the sourceterrane and tectonicstyle of thepre-Tertiarytectono stratigraphic province or belt within whichtheywereformed. The NortonBasin lies within the miogeoclinal belt, near its border with the Okhotsk-Chukotsk volcanic belt, and the strata of the Norton subbasins consequently reflect provenances within both of these tectonostratigraphicprovinces.
MIOGEOCLINAL BELT
The miogeoclinalbelt of Fisher and others(1979)extendsfrom thenorthern ChukotskPeninsulaandWrangel Island to the Seward
5 M.091 M.OL I 0081 !I I I -.ss
008'Zs9'9:I 3lVX S3lIW r I 1 , I 002 051 001 OS 0 I
I .
-.09
.os 9 EXPLANATION FOR FIGURE 2
Tv: undiWarentiated volcanic rocks.
Kv: undifferentiated volcanic rocks.
KJ: lava, tuff, agglomerate, argillite, shale, graywacke, quartzite, and conglomerate. Slightly metamorphosedin places.
KPz: south of 64O N latitude sandatone, siltstone, limestone, chert, and volcaniclastic rocks of Permian through Late Cretacaous age. Locally includes malangeand olistostrome sequences. North of 64O N latitude are sandstone, siltstone, argillite, conglomerate, coal, spilite, and basalt.
POs: sedimentary rocks of Permian and Mississippianage. Includes some Ordovician, Silurian, and Mississippian limestone.
Ope: phyllite, sandstone, siltstone, limestone, chert, and quartzite.
pE: Undifferentiated metaaedimentary and metamorphic rocks. Peninsula and intothe Brooks Range (fig. 2). Thisbelt is composed ofPrecambrianthroughlowerMesozoicmetamorphicandsedimentary rocksoverlyingcontinentalcrust.
In thenorthern ChukotskPeninsula,surfaceexposuresconsist mostly of unmetamorphosedlowerMesozoic carbonatesand clastics. In theeastern part of the Chukotsk Peninsula, gneiss and schist of probablePrecambrianageunconformablyunderliePaleozoicstrata. Miogeoclinal belt strata in the Brooks Range consist of imbricate slabsofPaleozoiccarbonates and early Mesozoicsedimentaryrocks. Inthe western parts of the Seward Peninsula, unmetamorphosed Paleozoic carbonatesconformablyoverlie a thicksuccessionofPrecambrian slate (Sainsburyandothers,1970). On St. Lawrence Island,near thesouthern border of the miogeoclinal belt, these rocks are represented by unmetamorphosed Paleozoic andlowerMesozoiccarbonates and fine-grainedclastics(Dutro,1981).
Miogeoclinal belt rocks in the basement complex penetrated by the two NortonBasin COST wells consist of quartzite, marble, phyllite, and slate which are thoughttobeofPrecambrianto Paleozoicage(Turnerandothers,1983a,1983b). These rocksare similar to the metamorphiccomplexofPrecambrianandPaleozoic schists and marblesthat are exposed in the central and eastern parts of the Seward Peninsula and thewesternBrooks Range. These rocks are inferredto constitute basement over mostof theNorton Basin.
OKHOTSK-CHUKOTSK VOLCANIC BELT
The Okhotsk-Chukotsk volcanic belt (fig. 2) is a continuous. bandofmostlyupperMesozoicvolcaniclastic and volcanic rocks thatbordersthemiogeoclinalbelttothesouth. It canbetraced from the region of the southern Chukotsk Peninsula to western St. Lawrence Island, and is exposed at isolatedlocations on St. Matthew, Nunivak,and thePribilofIslands in theBeringSea. It continues into the Yukon-Koyukuk provinceofwest-centralAlaska(fig. 2).
In western Alaska, the Okhotsk-Chukotsk belt consists of marine andesitic volcanic rocks of Early Cretaceous age overlain by as much as 26,000 feetofmiddleCretaceousvolcaniclasticgraywacke,mudstone, andcoal-bearingstrata(Patton,1973). In easternSiberia,thebelt is composed almostexclusively of volcanicrocks. Rocks ofthis tectonostratigraphic province may compose thebasementoftheNorton Basin near the Yukon Delta.
Paleozoic and early Mesozoicoceaniccrust is thoughtto underlie the Okhotsk-Chukotsk volcanic belt (Patton and Tailleur, 1977).This is in contrasttothePrecambriancrystallinerocks composing the continental crust underlying the miogeoclinal belt.
8 Marlow and others(1976) speculated that the volcanic belt represents a magmatic arc associated with the Late Cretaceousto early Tertiary obliquesubduction of the Kula plateat the Beringian margin. Exposuresofgraniticplutons ofCretaceousage on onshoreareas of thenorthernBeringSearegionareclusteredalongtheboundary betweenthevolcanicandmiogeoclinalbelts.Thesegraniticplutons may be genetically related to magmatic arcprocesses associated with thesubduction of the Kula plate.
Cretaceousgranodiorite is exposed on King Islandnear the northwestedgeoftheNortonBasin(Hudson,1977).Magnetic anomaliesaroundtheisland and alongthewestedge of the basin (Departmentof Commerce, 1969)suggestthatsignificantnearbyareas areunderlain bysubcroppinggranodiorite.Theseplutons,if exhumed, may havecontributed quartz-rich sand to adjacent areas of the basin. QuartzmonzoniteofCretaceousage is exposedneartheeasternedge of the Norton Basin on the Darby Peninsula alongthesoutheastern margin of the Seward Peninsula(Miller and Bunker,1976). There, a magneticanomalyextendsoffshoreinto the east sideoftheNorton Basin and is inferredto delineate a subcrop of similar quartz monzonite(Fisher and others,1982).Althoughthe amount ofsediment shedintoNortonBasin from thesegranite sources is probably minor relative to that shed fromthemiogeoclinalandvolcanicbelt terranes, it may locally constitute a significant percentage of quartz-rich detritusin the basin fill.
FOREARCBASIN BELT
The Okhotsk-Chukotsk volcanic belt is bounded on the south by theforearcbelt. The forearcbeltconsistsofPaleozoic and Mesozoic deepwatermelangeandolistostromes,maficvolcanics,andnonmarine andmarinesediments. Rocksfrom thisbeltprobably formthebasement along the outerBeringianshelf. Becauseof theirremoteness,forearc belt rocks probablycontributed very little sedimenttotheNortonBasin.
9 2
Regional Geologic History
The present geologic configuration of the Norton Basin area is the result of a series ofcomplexMesozoicand early Tertiary tectonicevents.DuringtheLateJurassicandEarlyCretaceous, rocks of the BrooksRangeandSeward Peninsula formed a continental block that was partially subducted to the south beneath an oceanic platethat supported a magmatic arc complex (Fisher and others,1982). Northward-directedoverthrusting, or obduction,oftheoceaniccrust resulted in the emplacementofstackedophiolitebodies in the central Brooks Range (RoederandMull,1978).Evidenceforthis obduction is provided by the presence of klippen of ophiolitic rocksoverlyingPrecambrian and Paleozoic metamorphicrocks in the BrooksRange. At the time the Brooks Range andSeward Peninsula continental crust was subductedbeneath the oceanic crust, the continental rocks were metamorphosed to blueschist facies and strongly deformed (Fisher and others,1982). The Brooks Range andSeward Peninsula continental block continued to be subducted until the subduction zoneceasedtofunction,possiblyduringthe EarlyCretaceous(Fisher and others,1982).Subsequentisostatic. rebound of the subducted continental crust resulted in extensive erosion of the uplifted ophiolite and underlying crust in the BrooksRange, Seward Peninsula,andNorton Sound areas.
Albian andCenomanian conglomerates are widespread in the northern Yukon-Koyukuk province. These conglomeratescontainabundant ophiolite debris and are overlain in part by strata composed of sedimentsourcedfrom a PrecambrianandPaleozoicmetamorphic terrane(Patton,1973). This depositionalrelationshipsuggests that the overthrust Brooks Range ophiolites were the first to be exposed and that the detritus was transportedsouthwardinto a depocenter in thenorthern Yukon-Koyukuk province. This was followed bysubsequenterosion,transport, and deposition of detritus derived from the older Precambrian and Paleozoic metamorphic rocks initially emplaced beneath the ophiolite klippen.
The Yukon-Koyukuk province was a depocenter for much of the erodedsediments aswell as forvolcanogenic debris generated by concurrent volcanism throughout west-central Alaska and eastern Siberia(Patton,1973). A wide,arcuateband of graniticplutons was intrudedinto rocks of the Yukon-Koyukuk province,the eastern Seward Peninsula, St. Lawrence Island, and theChukotsk Peninsulaof Siberia duringthe middleCretaceous(Fisher and others, 1981).
10 3 SEAWAY 0.200 Km
FIGURE 3A. MiddleLateCretaceoustectonic FIGURE 3B. LateLateCretaceoustectonic frameworkoftheBeringStraitregion.Diagram frameworkoftheBeringStraitregion.Diagram modifiedfromHolmesandCreager, 1981. modifiedfromHolmesandCreager. 1981.
tectonicframeworkoftheBeringStraitregion. DiagrammodifiedfromHolmesandCreager,1981 In middle Late Cretaceous time, east-west compression accompanied by oroclinal bending rotated the Seward Peninsula counterclockwiseapproximately 90 degrees(Patton and Tailleur,1977) to its presentposition in relation to the Brooks Range (fig. 3A). Rotationof the peninsula resulted in eastward-directedthrusting of Paleozoic and Precambrian age rocks along the eastern margin ofthe Seward Peninsula. On thebasisoffaunaldistributions, SachsandStrelkov(1961)postulatethatthePacificandArctic basins were connectedby a seaway across the eastern Seward Peninsula duringmiddleLateCretaceous time, then later isolated by closure ofthe seaway in responseto east-west compression. The initial subsidence of the Norton Basin may have been initiated in the Late Cretaceousduringthiseast-westcompressiveperiodbyright lateral strike-slip movement along the newlyformed Kaltag fault (Fisher and others,1982)(fig. 3B).
Compressionceased in latest CretaceousandearlyPaleogene time and was followedby a period of regional extension in west centralAlaska(fig. 3C). Regionalextension in theNortonBasin area triggered major subsidence by block-faulting along master faults, and is believedto be the primary tectonic mechanism for the initialformationofthebasin. Alluvial fans were depositedalong the margins of uplifted fault blocks, which indicates that early in the history of the basin the rate ofsubsidenceexceededthe rate ofsedimentinputfromflankingareas(Fisher and others,1982). The Yukon horst, which separates the two subbasins of theNorton Basin, was activeby early Paleogene time, as evidencedby'the presenceof alluvial deposits along its flanks that predate other Tertiarysequences.Aftermajor,local,fault-controlledbasin subsidenceceased,epeirogenicregionalsubsidence(inferredto stem fromthermalcontractionofthecrust anduppermantle as . well asisostatic compensation for the newlyemplacedsediment load)prevailed in the NortonBasinarea(Fisher and others,1982). Epeirogenicregionalsubsidence of this area has persisted through Holocenetime.
Basaltic volcanism occurred in the northern Bering Sea region during latest Miocene, Pliocene, and Pleistocene time. Volcanic rocks were extruded locally onto land areas along the margin of the Norton Basin(HoareandCoonrad,1980).MioceneandPliocenefloodbasalts locatednearaneast-trending zoneof rifting along the axis of the Seward Peninsulasuggest that the peninsula and immediate areas may haveundergone a period of renewednorth-southextensionbeginning in latest Miocene or earliest Pliocene(Fisher and others,1982). Common-depth-point seismic reflection data in Norton Basin indicate thepresenceofseveral domed structural features that appear to be laccoliths. The agesoftheseintrusives, as tentatively deduced from their structural and stratigraphic relationships with horizons extrapolated fromNortonBasin COSTNo. 1 and No. 2 wells, range from early Miocene to mid-Pliocene.
12 3
Structural Geology
BASIN DEVELOPMENT
The NortonBasin is an extensional basinlocated adjacentto theKaltagfault. The Kaltagfault, a right-lateral strike-slip fault,extendssouthwestwardthroughwest-centralAlaskawhere its tracedisappearsalongthesouthernmarginofNorton Sound.Although the role of theKaltag fault in the development of NortonBasin is unclear,Fisher and others(1982)suggestthat movement along the fault during a periodofregionalextension may haveservedto localizesubsidence.
Interpretation of CDP seismicdata indicates two distinct stages inthe development of theNortonBasin. In thefirst stage, rifting resultedinsubsidencealongmaster andsecondarynormalfaults. The presenceofpossiblePaleocenestrataencounteredintheNorton COST No. 1 and No. 2 wellssuggestsanearly-orpre-Paleoceneage fortherifting. A secondarystageofdevelopmentconsisted of regional,epeirogenic downwarping, beginninginthelateOligocene (figs. 13 and 14, post-horizon C) and continuingintotheHolocene.
These two stagesofbasindevelopmentcorrespondtothesimple extensional model for the evolution of sedimentary basinsenvisioned by McKenzie (1978). McKenzie proposed an initial stagecharacterized by rapid crustal attenuation in response to extensional stress. Thinningof the crust would thenpermitupwellingof hotasthenospheric material.Isostaticadjustmentinresponsetothereplacement,at depth, of the shallow crust by dense asthenosphere then results in fault-controlledinitialsubsidenceatthesurface. A positive thermal anomaly is associatedwith the thinning of thelithosphere and thesubsequentupwelling of thehotasthenosphere (McKenzie, 1978). A secondarystage of cooling of both lithospheric and asthenospheric materialby heat conduction to the surface then results in gradual, regional,non-fault-relatedsubsidence.
McKenzie (1978) illustrates how fault-controlledsubsidence, heatflow, and the resulting thermally controlled subsidence can bepredicted as a function of the factor bywhich thesurface length increasesastheresult of crustalstretching.Calculationsofthe extension factor in the NortonBasinbyHelwigand others(1984) and Y. M. Chang (personal commun., 1985)providedthefollowing values:
13 NORTONBASINLEASESALE 100
NGURE4. Structuralprovinces of theNortonBasinregion. Method Extension Factor
1.9 faultOffset of Kaltag Extension of basementseismicfromrecords <1.4 Mean basinsubsidence (tectonic and isostatic) >1.1
They concludedthatthehigherextensionfactorvalue(1.9) is supported by thestate of maturation and presenttemperature gradient in the basin.
BASIN STRUCTURE
The NortonBasinareacanbesubdividedintofourseparate structural and geographicprovincesbased on thetype and orientation ofmajorgeologicfeatures and thesegmentation of the basin by positive,intrabasinalstructuralfeatures(figs. 4 and 5). Area I, theStuart subbasin, lies east of the Yukon horst andwestand north of StuartIsland. Area 11, theSt. Lawrence subbasin,lies west of the Yukon horst and east and northeast of St. Lawrence Island. The thirdstructuralprovince,area 111, lies tothenorth of St. Lawrence Island,south ofKing Island, and west of a line at approximatelylongitude 168' W. Area IV is bordered on the east by the Seward Peninsula, on thesouth by a shallowplatform extendingnorthwardfromKingIsland,and on the north by Fairway Rock and Little Diomede Island.
Area I, theStuartsubbasin, is bounded on the west by a north- south-trending arcuate structural hightermedthe "Yukon horst" by Fisher and others(1981)(figs. 6 and 7). Seismicprofilesoverthe basementhorst reveal overlying arched strata that display drape or compactionfeatures. The northernsection of thehorst is bounded on the east by a series of westward-dipping rotated fault blocks. These blocks are bordered on their downdip sides by a series of en echelonnormalfaults(fig. 7). Towards thesouth,the Yukon horst is less pronounced and assumes a more northwest-southeastorientation. The absence of seismic data from theshallow-waterareas of the Yukon Delta doesnotpermit definition of thesouthernmarginof theStuartsubbasin. However, gravitydata from thisareaindicate thatonlyabout 3,000 feet ofCenozoicsediment is presentnear thenorthlobe of the delta(Fisherandothers,1980;Barnes, 1977)(fig. 8). Thisestimateofsedimentthickness was calculated fromthe relationship betweendepth-to-basement and free-airgravity values, which in the Norton Basin was determined by Fisher and others(1982)toapproximate a quadraticcurve(fig.9). Low land elevations on the Yukon Delta permit, with negligible error, a direct comparisonbetweenfree-airgravityand Bouguer gravity values,thusallowingtheuse ofBouguer gravity values in the gravity relationship illustrated in figure 9 (Fisher and others,1982)
15 m The Stuart subbasinis bounded on the north and east by a smooth, nearly featureless basement platform coveredby 2,500 feet or less of sediment (fig. 7). An east-west-trending normal fault (or fault zone), informally referred to as the "north-boundary fault" by Fisher and others(1981). marks the termination of the northern basement platform against a deeper, more structurally complex area of the subbasin to the south. The total throwon the north-boundary fault and subordinate faults along the north edge of the subbasin locally exceeds 7,000 feet.
An arcuate anticlinal structure informally referred astothe "Nome horst" by Fisher and others (1981)is present in the north-central section of the Stuart subbasin (fig.7). Along the crest of the "horst," the seismically determined depth to basementis approximately 4,000 feet. The seismically derived depth to basementis confirmed by a range of free-air gravity values from +2 to -5 milligal (mgal), which corresponds to a sediment thickness of 3,700 to 4,400 feet (fig. 9). Northeast of the Nome horst and adjacent to the north- boundary fault, lies a basinlow characterized by a free-air gravity value of-36 mgal (fig. 8), which is unusually low for most structural depressions within the Stuart subbasin, and indicates a maximum depth of 17,000 feet. The depth to basement,as determined from seismic reflection data,is 16,400 feet.
The deepest area of the Norton Basinis in the central partof the Stuart subbasin south of the Nome horst. This basementlow is bounded to the northwest and southeastby northeast-southwest- striking normal faults. The COST No. well2 was drilledon the southwest flank of this structural depression, where it encountered metamorphic rocks of the Precambrian to Paleozoic miogeoclinal belt at a depth of 14,460 feet. Seismic data indicate that the. sedimentary fillin the central area of the subbasin approaches a maximum thickness of 23,000 feet. Free-air gravity values over this central structural low only reach a minimum of -34 mgal (Fisher and others, 1980). which corresponds toan apparent sediment thickness of 15,800 feet. This discrepancyin the calculated depths to basement may involve a number of factors, including the possible occurrence of local high-density and/or overcompacted Paleogene strata. The presence of underlying high-density (oceanic?) type crustal material could alsobe in part responsible for this anomalous situation.
Area 11, the St. Lawrence subbasin, lies west of the Yukon horst and east of a shallow basement platform extending northeast across Norton Basin from St. Lawrence Island to the Seward Peninsula4).(fig. The predominant strike of faultsin the St. Lawrence subbasinis to the northwest (figs.6 and 7). A north-south-trending fau1.t zone in the southeastern part of the subbasin bounds a half-graben to the east, where it rises northeastward onto the southwestern flank of the Yukon horst. This half-graben contains up to 14,000 feet of sediment above acoustic basement andan hasassociated free- air gravity value of -24 mgal, which correspondsan apparent to sediment thickness of 10,750 feet.
18
li ca (D 3 m a
0 0 X cn U m
V -V-% 3 U Y -.1 D 3 D 5 a -0 D m 0 N 0-. 0 U D W m 3 m 3 rc
0- 0 X W W m U -m 0 r P D D rc 3 D a
lu Y The central area of the St. Lawrence subbasin is occupied by a large northwestwardly elongated structural low that is primarily the result of minor faulting and major regional tilting and subsidence of the basement. Time-depth calculations using stacking velocities derived from seismic reflection data indicatea maximum sediment thickness of more than16,000 feet in the centerof the subbasin. To the southeast, the COST No.1 well was drilled along the axis of the St. Lawrence subbasin and encountered cataclastically deformed and metamorphosed sedimentary rocks of probable Precambrian age at a depth of 12,545 feet (fig. 7).
Several large normal faults extend northwestward from the northwest corner ofthe St. Lawrence subbasin into area111. Area 111 consists of a relatively shallow basement platform cut by predominantly northwest-southeast-trending grabens and half-grabens. The depthto this basement platform throughout the mapped area averages between 1,500 and 3,500 feet (fig. 7). Sediment thickness in the grabens locally exceeds7,500 feet. The western boundary of the subbasin isan extremely shallow, featureless platform with less than 2,500 feet of sediment cover. A small, faulted low just north of St. Lawrence Island approachesa basement depth of9,000 feet.
An airborne high-sensitivity magnetic surveyconducted in the western part of the Norton Basin by Aero ServiceDivision, Western Geophysical Company of America, revealed several groups of high- amplitude magnetic anomalies, many of which exceed200 gamma, along with several isolated low-amplitude anomalies (Vixo andPrucha, 1983). Based on similarities ofthe magnetic disturbances,the high-amplitude magnetic anomalieshave been interpreted Vixoby and Prucha to represent Cretaceous silicic plutonic rocks analogousin composition and genesis to those foundon the Darby Peninsulaof southeastern Seward Peninsula, and to the Cretaceous intrusions in the Yukon-Koyukuk province. Magnetite hasbeen found to be a common accessory mineralin the Cretaceous silicic plutonic rocks in the Norton Basin region (Miller and others,1966; Miller and Bunker, 1976) and is, in large part, responsible for producing the magnetic anomalies associated with the plutons (Decker andKarl, 1977). The low-amplitude magnetic anomalies observed in the western area of the Norton Basin are considered to stem from two separate magnetic horizons (Vixo and Prucha,1983): the weakly magnetic metamorphic and sedimentary rocks of the basement aand weakly magnetic suprabasement horizon, probably containing local Donovan anomalies, which are thought to result from concentrationsof diagenetic magnetite associated with hydrocarbon microseepage (Donovan and others,1979).
A shallow, relatively featureless basement shelf separates areas 111 and IV. Area IV, located north of King Island, consists of a small, isolated structural graben termed the "Bering Strait depression" (BSD) by Greene andPerry ( "pub., cited in Johnson and Holmes, 1980) that is bounded by major east-southeast-striking normal faults (fig.7). Johnson and Holmes(1980) concluded that the east-southeast-striking faultsof area IVappear to intersect and offset the northwest-trendingfaults and structures that extend
23 I67O 166" I64O - Shiptrackline ' c--Gravity Contour.dashed whwe inferred. contowintervai 5 mgal N c - Q Gravity low -20 5 Extrsmevalue within closedcontour *I- -8- Bougusr gravity. Barnes( 1377). contour interval IO mgal
64
63
FlGURE8. ContoursofBouguergravityovertheYukonDelta(Barnes,1077)andof free-airgravityoverNortonBasin(Fisherandothers, 1080). From Fisherandothers,fo82. I I I I
I
R2= 0.73 0 '0
Z =Depth in kilometers mgal in valueGravity Ag= "\ Points notO\ in used O-/Ieast-squares calculations \o 7' I I I , IO 0 -10 -20 -30 -40 Free-air Anomaly (mgal)
FlGURE 9. Relationshipbetweenfree-airgravityvalueanddepthto basement.DiagramfromFisherandothers, 1982.
25 from area 111. This impliesthatthestructural development of the BSD postdates earlier deformation in the mainNortonBasin (Johnsonand Holmes, 1980).Sedimentthicknessnearthecenter of the BSD approaches 7,000 feet. The northernperimeter of the BSD is defined byamajoreast-trendingnormalfault,labeledthe "Bering Strait fault" by Hopkins(unpub.,cited in Johnsonand Holmes, 1980). The lack of seismiccoverageprecludesabetter definition of theBeringStraitfaultfurthertotheeast, where it may mergewithastructure known as the Port Clarence rift (Hopkins,unpub., cited in JohnsonandHolmes,1980).
26 4
Biostratigraphy
A fine-scalestratigraphiczonationoftheNortonBasinbased on microfossilranges is difficult at this juncture because of the paucity of data and the imperfect state of biostratigraphic knowledge oftheBeringSeaarea. In addition,therehasbeenconsiderable controversyconcerningtheseveralzonal schemes so farproposed, most particularly,the nature and extentoftheOligocenesection. In theNortonBasin,high-resolutionzonationsaredifficultto constructbecause of the poor representation of key microfossil groupssuchasplanktonicforaminifers and calcareousnannoplankton. Even when thesetaxa are present there is some uncertaintyinvolved in the extrapolation of theirranges fromlower to higher latitudes. Forthese reasons, the biostratigraphy described in this report should beconsideredpreliminary.Nevertheless,reinvestigationsofthe COST well data have already resulted in tighter and better substantiatedzonations. In general,the new datasupportour previousinterpretations(Turner and others,1983a,b).
Paleoecologic and biostratigraphic determinations in the Norton Basin COST wells are based on detailed analyses of microfossil assemblagescontainingForaminifera,ostracodes.silicoflagellates anddiatoms,calcareousnannoplankton, and marineand terrestrial palynomorphs (chieflyspores,pollen, and dinoflagellates).Rotary drill-bit cuttings wereexamined at 30-footintervals from the first samplestakentothetotaldepths of the wells. Data fromconventional andsidewallcoreswerealso examinedand utilized.Slides,processed samples,and reports prepared for the participants by consultants (Anderson,WarrenandAssociates,1980;Biostratigraphics,1982) wereexamined,interpreted, and integratedinto this report.
The COSTNo. 2 well was reprocessed andreexamined for palynomorphsbyJonathan P. BujakoftheBujakDavies Groupand the preliminaryresults (J. Bujak,personal commun., 1985)integrated intothis report. The COST No. 1 well is currentlybeingrestudied. In addition, both wells were reprocessed forostracodes which were subsequentlyidentified byElisabeth Brouwersof the USGS (E. Brouwers, written commun., 1985). Discrepancies between Minerals Management Service and consultant interpretations,principally the location of biostratigraphictops, for the most partcan be attributed to sample
27 PALEOBATHYMETRY -_0
Pleistocene I80 180'- 1320' IO00 Pliocene late 1320'- 1608' 2000 middle1608'-2327' early2327'-2639' 3000 Miocene late2639'-3464' early3464'-4493' 4000
5000
6000
7000
Oligocene 8000 late4740'-8600' early8600'-9690' 9000
I0,OOO I Oligoceneorolder (probablemarine) (Probable Eocene) 9690'-12,235' I I .ooo f I I Eoceneorolder I (Possible~Paleocene) 1 2.000 12,235'-12,545'
ProbablePaleozoic 13.000 metamorphic Precambrian to basement 12,545'-14,683' I4.000
TD 14,683'
FIGURE IO. Stratigraphicandpaleobathymetricsummary of NortonBasin COST No. 1 well. 28 PALEOBATHYMETRY - .o ” - m .y_ .G. *m c L.C c .I? m 0: g ,? 2e-c ._ * L22L +J c -Torn c m coo* 0 =-- .- 3 0 + -EZo Pleistocene 450 450’-1320’
Pliocene 1000 late 1320’- 1846’ middle1846’-2228’ early 2228’-2580’ 2000
Miocene 3000 late 2580’-3 120’ -- early 3 120’-3524’ 4000 c 500.0 -
6000 Oligocene late 3524’-6850’ 7000 early 6850’-10,160’
8000
9000
I0,OOO
Eocene I I.000 10.160’-12,700’ - 12,000 *-
.rP Eocene or older 13,000 (PossiblePaleocene) 12,700’- 14,460’ 14,000 ProbablePaleozoic 14,460’-14,889’ TD 14,889’ metamorphic basement
FIGURE 11. Stratigraphicandpaleobathyrnetricsummary of
NortonBasin COST No. 2 well.
29 preparation techniques. Foraminiferal analysis, interpretation, and synthesis of other data were done by F.Ronald Turner ofMMS. Siliceous microfossil analysis was done by DonaldL. Olson of MMS.
Strata are discussed in the order that they were penetrated. The biostratigraphic units delineated often represent a synthesis of data derived from various subdisciplines that often do not inagree every particular. Following convention, fossil occurrences are listed as highest and lowest rather than the potentially confusing first and last. Sample depths were measured from the kelly bushing. Data obtained from conventional cores are given somewhat more weight than those from cuttings.
Paleoenvironmental determinations, chiefly water depth and energy, are basedon the entire microfossil and macrofossil suites. Paleoclimatological interpretations are basedon spore and pollen assemblages and, to a lesser extent,on diatoms, silicoflagellates, and Foraminifera. Fluvial, lacustrine, and paludal environments are classified as continental or nonmarine. Transitional environments include brackish estuaries, marshes, and lagoons. For sediments deposited in marine environments, the paleoenvironment is expressed in terms of bathymetry. Paleobathymetric determinations are primarily based on foraminiferal criteria, but dinoflagellates and other marine organisms suchas bryozoans, echinoids, ophiuroids, and cirripeds were also utilized. The marine environment is divided into inner neritic (0 to 60 feet), middle neritic (60 to 300 feet), outer neritic (300 to 600 feet), and upper bathyal (600 to 1,500 feet). The paleontology of the two Norton Basin COST wells is discussed separately (figs. 10 and 11). then correlated at the end of the chapter (fig. 12). In the absence of onshore outcrop data or other well . control, these interpretations are the basis for most of the other correlations, for example, figures13 and14 in the Lithostratigraphy and Seismic Stratigraphy chapter ofthis report.
COST NO. 1 WELL
Pleistocene
The interval from180 to 1.320 feet is Pleistocene in aee on the
A moderately diverse. though svarse. ostracodefauna containineI
1,985).
30 Although the siliceous microfossil assemblagesin this interval contain a high percentage of reworked material, the presence of the diatoms Melosira sulcata, Coscinodiscus marginatus, Actinocyclus curvatulus, and Biddulphia aurita in association with the silicoflagellatesDisephanus~-0ctangulatus and Distephanus octanarius also indicatesa Pleistocene age.
Environment
The foraminiferal fauna and diatom flora indicate an inner neritic (0 to 60 feet) cold-water environment for the Pleistocene interval. Abundant molluscan fragments, barnacle plates, echinoid plates and spines, and fragments of erect bryozoan colonies (adeoniform and vinculariiform) generally support this interpretation, although the bryozoans suggest possible middle neritic water depths(60 to 300 feet). The ostracode fauna indicates that these environments were characterized by periods of reduced or fluctuating salinities. The admixture of stenohaline and euryhaline forms becan explained by the complex interplay of glacio-eustatically controlled fluvial and marine processes.
Pliocene
The interval from 1,320 to 2,639 feet is Pliocene in age. Although the Pliocene siliceous microfossilassemblage containsa substantial number of older, reworked forms wellas as Pleistocenespecies caved from uphole, the interval becan provisionally subdivided into late, middle, and earlyon the basisof diatom and silicoflagellate distributions. The late Pliocene, 1,320 to1,608 feet, is identified by the highest occurrence of the diatoms Coscinodiscus marginatus fossilis, Stephanopyxis inermis, Actinocyclus ehrenbergii, Thalassiosira usatchevii, and Nitzschia fossilis. The middle Pliocene,1,608 to 2,327 feet, is defined by the first occurrence of Thalassionema robusta, Thalassionema convexa aspinosa, Cosmiodiscus intersectus, Denticulopsis kamtschatica, and Distephanus boliviensis boliviensis. The early Pliocene, 2,327to 2,639 feet, is definedby the lowest occurrences of Actinocyclus ochotensis and Ammodochium rectangulare. Siliceous microfossil zonations for all parts of the well areonbased Koizumi (1973), Schrader (1973). and Barron (1980).
The foraminiferal fauna is essentially the same as that of the overlying Pleistocene with the exceptionof the highest occurrence of Pseudopolymorphina cf.-P. suboblonga at1,500 feet.
The ostracode fauna is also quite similar to that of the overlying Pleistocene, with the addition of Rabilimis paramirabilis, Cytheretta teshekpukensis, Cytherura sp.A, Robertsonites tuberculata, Heterocyprideis fascis, Elofsonella concinna concinna, and Eucytheridea punctillata. Most of these species have heretoforebeen considered Pleistocene with the exception of Rabilimis paramirabilis(1,380 to 1,410 feet), which has been reported only from Beringian-age sediments (late Pliocene-early Pleistocene).
31 The sparse terrestrial palynoflora contains Alnipollinites sp. and rare specimens of Betulaceae, Polypodiaceae, Polmoniaceae, and Chenopodiaceae. The marine palynoflora is characterizedby common specimens of Tasmanaceae and the dinoflagellates Lejeunia spp., Paralecanicella indentata, and Operculodinium sp.2. The latter species first occurs in a sidewall core taken at 1,494 feet and marks the approximate position of the Pliocene-Pleistocene boundary in Alaska according to Hideyo Haga (Anderson, Warren and Associates, 1980).
Environment
The Pliocene microfossil and macrofossil assemblages are indicative of cold-water, inner neritic deposition.
Miocene
The interval from 2,639 to 4,740 feet represents the Miocene section of the well. This interval canbe further subdivided into late and early Miocene sectionson the basis of siliceous microfossils. The absence of definitive middle Miocene siliceous microfossil taxain conjunction with the relatively good late and early Miocene assemblages suggests that this timebe mayrepresented by a hiatus, although the foraminiferal data appear to contradict this interpretation. In fact, on the basis of preliminary palynological investigations, Bujak (personal commun., 1985) believes that the top of the middle Miocene is at 3,120 feet. Much of the "missing section" maybe the result of the diageneticloss of the opaline silica of diatom frustules (see discussionin the Lithostratigraphy and Seismic Stratigraphy chapter).
Several distinctive foraminiferal species previously reported from the Miocene of Sakhalin Island,U.S.S.R., are present in the well at 3,630 feet. The strata from which these forms were reported initially were considered to be late Miocene in age (Voloshinova and others, 1970). More recently, these strata were assigned to the middle Miocene (Serova,1976; Menner and others 1977; Gladenkov, 1977). The agedeterminations of these units arestill unsettled, and it is quitepossible that they may prove beto older than middle Miocene (L. Marincovich, personal commun., 1983). The late Miocene, definedon the basis of siliceous microfossil assemblages recovered from sidewall cores over the interval from 2,639 to 3,464feet, is recognized by the lowest continuous occurrence of Melosira sulcata; the lowest occurrences of Thalassiosira zabelinae, Thalassiosira convexa aspinosa, Pseudopyxilla americana, and Ebriopsis antiqua antiqua; the highest occurrences of Coscinodiscus temperei, Coscinodiscus vetustissimus, and Goniothecum tenue; and the ubiquitous presence of Actinocyclus ingens.
32 The interval from3,464 to 4,493 feet is early Miocene in age on the basis of the lowest occurrences of Thalassionema nitzschioides, Stephanopyxisturris,Stephanopyxisschenckii, andActinocyclus ingens.
The Miocene foraminiferalfaunafirstencounteredat3,630feet is characterized by Porosorotalia clarki,-______Elphidiella katangliensis, Elphidiellacf. E. tenera,Elphidiellacf. E. crassorugosa,Elphidiella cf . E. nagaoi , CTibxidmvulgaTe, Cribroelphidiv -C. pzromaense, Cribroelphidium cras-ium, Ellipsoglandulina cf. E. subobesa,Pseudoglandulina sp., Glandulinacf. G. japonica,Buccella frigida, B-mansfieldi, - Bulimineliacf. -~B. curta, andPyrgowilliamsoni.
The verysparse ostracode fauna in this interval consists of pyritizedmolds,carapacefragments, and rare juvenilecytherideids.
The terrestrialpalynoflora over this interval is quitesimilar tothatofthePliocenesection. New elementsincludeUlmipollenites sp.. Tiliaepollenites sp., Juglanspollinitessp.,Pterocaryapollenites sp., and Bosiduvaliasp.
The dinoflagellateassemblage includes Spiniferites cingulatus, Spiniferites cf. -S. crassipellis, Spiniferites cf. S. incertus, Impagidiniumspp.,Lejeuniaspp.,andTuberculodiniumvancampoae. Althoughthelatterspecies is oftenconsidered diagnostic of the Miocene, it alsooccurs in Oligocene strata in theBeringSea area and in younger strataelsewhere.Overall,boththespore-pollen and dinoflagellateassemblagessuggest a Mioceneage for this interval.
Environment
Evidencefrom all of themicrofossilgroupssuggestsdeposition insublittoraltomiddleshelf (300 feet)depths.Paleotemperatures werecolderduringthelate Miocenethan inthe generally temperate early Miocene.
Oligocene
The interval from4,740 to 8,600feet is lateOligocene in age; the interval from8,600 to 9,690 feet represents the early Oligocenesection. The base of theOligocene is somewhat arbitrary. The unconformableboundarybetweentheearlyandlateOligocene (8,600feet) is placed at the top of seismichorizon C, which is alsotheboundarybetweenlithologiczones To-2 and To-3. However, a singlespecimenofthepollenSaxonipollissaxonicusat 7,950 feet, if in place,suggeststhat the early Oligocene may be somewhat higherthan 8,600 feet, as does thepresence of a specimen'ofthe dinoflagellatePhthanoperidinium sp.from a conventionalcore at 7,937.7 feet (J. Bujak,personal commun., 1985). The topofthe
33 Oligocene is marked by the lowest occurrence of several speciesof the dinoflagellate Im agidinium and the highest occurrenceof Heteraulacysta campanula-E-7-- J. Bujak, personal commun., 1985). The previously assigned top of the Oligocene section (Turner and others. 19858) was placed at the highest occurrence of the dinoflagellate Achomosphaera aff. A. alcicornu. According to Hideyo Haga (Anderson, Warren and Associates, 1980), this species hasbeen previously recorded in Oligocene strata in western Alaska. Achomosphaera alcicornu ranges from middle Eoceneto middle Miocenein Europe and from late Paleocene through the Eocenein eastern Canada. The "old top" of4,493 feet (from a sidewall core) may beenhave based on a reworkedspecimen or mudcake-contamination from downhole. It is also possible that 4,493 feetis the actual top, although the "new top" is supported by better paleontological evidence and fits the lithologic, well log, and seismic data better. Other dinoflagellates present include Tenua cf.T. decorata, Systematophora placacantha (early Eocenethroughe Miocene), and Distatodinium ellipticum (middle Eocene through early Oligocene), and several species ofMillioudodinium. These all tend to support toan Oligocene age for theinterval.
The calcareous nannoplankton recovered from the well are rare. poorly preserved, andon the whole relatively nondiagnostic. None are present above 5,100 feet. The interval from 5,101 to 10,590 feet is characterized by sporadic appearances of the long-ranging species Braarudosphaera bigelowi associated with rare coccoliths and placoliths of a small, somewhat problematical form with affinities to species in the Coccolithus miopelagicus plexus. These forms suggest a middle Miocene to late Oligocene age. Braarudosphaera bigelowiis often presentin times characterized by geologic crises (Prothero, 1985). The presence of Thoracosphaera eat 9,150 feet suggests. an age within the Sphenolithus ciperoensis zone. The lower part of this zone correlates with NF'the 24 zone of Martini (1971), the base of whichis late early Oligocene. This somewhat scant evidence lends support to the subdivisionof the Oligocene section at seismic horizon C, our "mid"-Oligocene unconformity.
The foraminiferal assemblage supportsan Oligocene age, although a number of the species present range up into the Miocene and down into the Eocene. The upper partof the interval (4,493 to 5,400 feet) is characterized by a fauna that contains dominantly shallow-water forms suchas Elphidiella katangliensis. Elphidiella nagaoi, Elphidium spp., Cribroelphidium spp., Buccella frigida, --* Miliammina fusca Rotalia cf. R. katangliensis, Rotalia japonica, and Rotalia japonica varianta. A-deeper water assemblage, present from 5,400 to9,690 feet, contains most of the above listed species as well as Psammosphaera carnata, Ammodiscus tenuis, Ammodiscus sakhalinicus. Libusella laevigata, Martinottiella cf. M. communis, Martinottiella bradyana, Bippocrepinella variabilis, Ha%lophragmoides spp., Haplophragmoides tortuosus, Gaudryina quadrangularis, Plectina sp., Dorothia sp., Rhabdamminina aspera, Reophax spp., Bathysiphon edurus, ____Bathysiphon sp., Tritaxilina aff. --T. colei, Pulleniasp., Cyclammina
34 5E assemblages are presentbetween8,130and9,690feet. The terrestrially derived spore-pollen assemblage indicates that warm-temperate climaticconditions prevailed during mostofOligocene time.
Oligocene or Older(ProbableEocene)
Microfossil recovery and preservation from 9,690 to 12,235 feet was quitepoor. Most oftherareforaminiferaloccurrencesappear torepresentcavedmaterial. Most ofthe in situ palynomorphs are of terrestrialorigin. Abundant plantmaterial is disseminated through the massive sandstone units and as subparallel partings. Mica flakes and plant material are common in the fine-grained laminae. The age,boundaries, andenvironmenthavenotyetbeen unequivocally determined.
JonathanBujak(personal commun., 1985)indicatedthat hehad recovered a relatively rich marine microflora from this section of the well. Unfortunately,thesampleshe had at the time were too widely scattered to allow the picking of a definitive top. Nevertheless, the preliminary reinvestigation yielded a late Eocene agefrom a sample at 12,150feet.Thisage is based on thepresence of the dinoflagellates Trinovantedinium boreale andHystrichokolpoma salacium,andto some extentfollowsthezonationBujak(1984)erected fortheBeringSeafrom DSDP leg19coredata.Althoughthe new Trinovantediniumborealeconcurrentrangezone may be valid, it is at variancewithcalcareousnannoplanktondata(Worsley,1973)that suggest an early Oligocene age forthe same BeringSeacores. Bujak (personal commun., 1985) now believes that Trinovantedinium borealerangesintotheearlyOligocene. Much oftheuncertainty will probably be eliminated when the restudy of this interval is completed.
It is probable that this section is in part equivalent to the late andmiddleEocenesectionsoftheNortonBasin COST No. 2 well. Sedimentaryfeaturessuch as gradedbeddingandflamestructures seen in cores 8 and 9 (11,960 to 11,988and12,071 to 12,091 feet) suggestdeposition by turbiditycurrents.Bioturbation is neither extensivenordiagnostic;theinfrequentburrowspresentinthe massive sandstone units cannot be unequivocally related to a particularichnofacies. However, rare tracesobserved on bedding planesinthelaminatedsequencesofcore 8 resembleto some degree theichnogenusPlanolites and the"scribblinggrazingtraces" of a tracefossilassemblagetypical of distalturbidites. The dinocyst assemblage at 12,150 feet tends to rule out a lacustrine origin for thissection.
Eocene orOlder(PossiblePaleocene)
No agediagnosticmicrofossilswererecovered from the interval between12,235and12,545 feet. Rare, poorlypreservedsporesand pollenarepresent. The 310-foot-thicksection, boundedaboveand
36 below by unconformities, appears to be roughly correlative with a much thicker coal-bearing sequencein the nearby Norton Basin COST No. 2 well. Helwig and others (1984) suggest that this sectionis, at least in part, Paleocene in age.
Environment
The presence of terrestrial spores and pollen in association with abundant coal indicates that the sediments are continental (fluvial and paludal) in nature. The paleoclimate was probably tropical to subtropical.
Metamorphic Basement
The No. 1 well penetrated a 2,135-foot-thick sectionof cataclastically deformed pelitic and psammitic metasedimentary rocks of undetermined age. Several lines of evidence suggest that these rocks maybe related to rocks exposed in theYork Mountains in the western part of the Seward Peninsula that are considered to be Precambrian to Paleozoic (Sainsbury and others,1970; A. Till, J. Dumoulin, personal commun.,1985).
COST NO. 2 WELL
Pleistocene
The interval from450 to 1,320 feet is consideredbeto Pleistocene in ageon the basis of a foraminiferal fauna characterized by Elphidium clavatum,Elphidium bartletti, Protoelphidium orbiculare, Elphidiella gorbunovi,Elphidiella oregonense, Elphidiella Hannai, Buccella frigida, andQuinqueloculina akneriana. Rare, poorly preserved and broken ostracodes assignable to Paracyprideis pseudopunctillata, "Acanthocythereis" dunelmensis, Heterocyprideis sorbyana, and Rabilimis septentrionalis substantiate a Pleistocene age.
The diatom assemblage is quite sparse, but the presenceof Melosira sulcata to some degree supports a Pleistocene age. Spores and pollen are also rare, but an assemblage composedof Sphagnumsporites spp., Alnipollenites sp., and Betulaceae and Compositae (Helianthus type) is consistent with a Pleistocene age. No calcareous nannoplankton or radiolarians were recovered in this interval.
Environment
The microfossil assemblage indicates that the Pleistocene sediments were deposited in cold water at inner neritic depths (0 to 60 feet). Salinity varied from normal marine to brackish.
37 Pliocene
The interval from 1,320 to 2,580 feet represents the Pliocene section of the well. The siliceous microfossil zonation utilized hereafter follows that of Koizumi (1973), Schrader(1973). and Barron (1980). The Pliocene section canbe only provisionally further subdivided owing to the poorlyknown biostratigraphy of the areaand to complications causedby extensive reworking and downhole sample contamination.
The late Pliocene(1.320 to 1,846 feet)is defined by the highest occurrences of the diatoms Coscinodiscus marginatus fossilis, Coscinodiscus pustulatus, Stephanopyxis horridus, .Thalassiosiraand zabelinae. Rare specimens of Denticulopsis kamtschatica recovered from a sidewall core 1,846at feet mark the top of the middle Pliocene. The base of this intervalis tentatively placed at 2,228 feeton the basis of the highest occurrence of the early Pliocene forms Cosmiodiscus insignis and Thalassiosira punctata. The early Pliocene section (2,228 to 2,580 feet)is based on the highest occurrences of the aforementioned diatom species and an assemblage characterized by Coscinodiscus temperei and the silicoflagellate Ebriopsis antiqua.
The terrestrial palynoflora contains relatively abundant specimens of Alnipollenites sp., Osmundacites sp., and rare to frequent Betulaceae, Polypodiaceae, Polemoniaceae, Compositae, and Malvaceae.
The marine component of the Pliocene palynoflora includes the dinoflagellates Lejeunia spp., Spiniferites spp., and Cannosphaeropsis aff. C. sp. A Williams and Brideaux 1975. The highest occurrence of the latter species, 2,310 feet,is considered tobe a possible Miocene marker by Biostratigraphics (1982) on the basis of the . occurrence of Cannosphaeropsis sp.A Williams and Brideaux 1975in the Miocene of eastern Canada. They also identify a Tasmanaceae zonule between 2,040 and 2,670 feet that they considerbe Plioceneto to Miocenein age. The No. 2 wellwas recently reprocessed and reexamined utilizing fluorescence microscopy. This analysisyielded abundant specimens of the algal cyst Leisophaeridia and the brackish- water dinoflagellate Peridinium sp. (J.B Bujak, personal commun., 1985) and tends to support a Pliocene age.
The foraminiferal fauna contains all of the species presentin the overlying Pleistocene sectionas wellas specimens of Pseudopolymorphina sp., Dentalinasp., Quinqueloculina seminulum, Globobulimina sp., Elphidiella sibirica, Elphidiella cf.E. brunescens, Elphidium incertum, Cassidulina cf.- C. minuta, PulleniaSF., and Oolina melo.
Environment
Deposition took place at inner neritic depthsin a cold-water environment characterized by fluctuating salinities. The cheilostome bryozoan fragments recovered include both encrusting and erect colonies. The presence of cellariiform and catenicelliform zoarial
38 types (erect with flexible internodes) suggestsa depositional environment characterizedby moderately strong currents and a relatively high sedimentation rate. Strongly stenohaline forms such as echinoids and ophiuroids are present throughout the interval but are neither diverse nor numerous.
Miocene
The interval from2,580 to 3,524 feet is Miocene in age. The section is subdivided into late and early Mioceneon the basis of siliceous microfossil assemblages. The late Miocene (2,580 to 3,120 feet) is based on the lowest occurrence of Thalassiosira zabelinae and the highest occurrencesof Rhaphoneis surirella and Goniothecum __tenue associated with Coscinodiscus vetustissimus, and Coscinodiscus temperei. No in situ middle Miocene forms are present and it is possible that this timeis represented by a hiatus. The early Miocene (3,120 to 3,524 feet) is characterizedby Thalassiothrix longissima, Rhaphoneis miocenica, Rhaphoneis cf.-R. fossilis, and Actinocyclus ingens.
The spore-pollen assemblage is quite similar to that identified in the overlying Pliocene section with the addition of the pollen Tsuga veridifluminipites (J. Bujak, personal commun., 1985). The dinoflagellate assemblage contains Lejeunia paratenella, Lejeunia spp., Spiniferites cingulatus, Spiniferites ramosus, Tuberculodinium vancampoae, and Hystrichosphaeropsis sp. The dinocyst stratigraphy supports that derived from diatoms and silicoflagellates.
The foraminiferal assemblage is similar to the Elphidium- Elphidiella-dominated faunas seen higher in the well and contains all of the same species. New taxa include Cribroelphidium crassum, Quinqueloculina sachalinica, Dentalina aff.D. nasuta, Elphidiella simplex, Elphidiella katangliensis, Pseudogl&dulina sp., Pseudoglandulina aff. P. nallpeensis, Ellipsoglandulina cf. E. subobesa, Sigmoidella pacifica,and Porosorotalia clarki.This assemblage, here considered torepresent middle to earlyMiocene, is best developed from 3,200 3,540to feet. Species such as Elphidiella katangliensis, Porosorotalia clarki, and Quinqueloculina sachalinica were described from deposits of supposed late MioceneaEe- on Sakhalin Island, U.S.S.R. (Voloshinova, and others, 1970). Subsequent stratigraphic revisions (Serova, 1976; Gladenkov, 1977; Menner and others, 1977) place these strata in the middle Miocene and it is quite possible that they may prove beto older. Several of the species appear to range through the Oligocene section and may extend into the Eocene as well.
Environment
The late Miocene interval was deposited in inner to middle neritic depths ina cold climate. The early Miocene interval represents a shallower (inner neritic) and warmer (warm-temperate?) environment.
39 Oligocene
The interval from3,524 to 6,850 feet is late Oligocene age;in the interval from6,850 to 10,160 feet is early Oligocene. The palynofloras over this interval are abundant, diverse, and relatively diagnostic. The uppermost coal-bearing portion of the sequence (3,524 to 3,930 feet) is characterized by an assemblage composedof Alnipollenites sp., Betulaceae, and rare to frequent Ulmipollenites sp., Pterocaryapollenites sp., and Momipites sp. From3,930 to 6,520 feet, the spore-pollen and dinocyst assemblages are much more diverse and age diagnostic. Additional terrestrial palynomorphs include common Caryapollenitessp., Juglanspollenites sp., Tiliaepollenites sp., and lessabundant specimens ofFaguspollenites sp., Ilexpollenites sp., and Liquidamberpollenites sp. The dinocyst assemblage contains Tenua cf.T. decorata, Distatodinium ellipticum, Deflandrea sp., and mecanizella indentata. A reprocessing and reexamination of the cuttings from3,750 to 4,910 feet yielded a rich, relatively long-ranging dinoflagellate assemblage containing Lingulodinium machaerophorum, Systematophara ancyrea, Oliogsphaeridium centrocarpum, and Reticulatosphaera stellata(J. Bujak, personal commun., 1985).
Below 6,890 feet, the presence ofan abundant palynoflora characterized by the heath pollen Ericipites indicatesan early Oligocene age (J. Bujak, personal commun., 1985). The unconformable boundary between early and late Oligocene is placed6,850atfeet on the basis of this paleontological evidenceas well as well log, lithological, and seismic data. Fungal palynomorphs, particularly species of Striadiporites, arean increasingly important floral element below 7,900 feet. Biostratigraphics (1982) suggests that some of the fungal taxa recovered below9,300 feet may haveEocene. affinities.
Calcareous nannoplankton are represented in No.the 2 well by a single, incomplete placolith of Coccolithus pelagicus 7,250from to 7,340 feet. The morphology of the specimen suggests that it is older than Miocene.
The foraminiferal assemblage contains Elphidiella katangliensis, Elphidiella cf. E. problematica, Elphidiella cf.E. tenera, Elphidiella cf. -E. californica, Cribroelphidi-rassum, Cribroelphidium cf . C. vulgare, Buccella mansfieldi, Porosorotalia clarki, Rotalia japoGica, Rotalia japonica varianta, Quinqueloculina sp., Miliamnia fusca, Buliminella curta, Caucasina eocenica kamchatica, Caucasina bullata, Caucasina schwageri, Reophax spp., Plectina sp., Haplophragmoides spp., Cyclammina cf.-C. pacifica, Sigmomorphina suspects, Sigmoidella pacifica, Pseudoglandulina inflata, Martinottiella sp., Pyrgo williamsoni, Trichyohyalus bartletti, Saccammina sp., and Psammosphaera carnata.
40 Shallow-water forms such as Elphidiella katangliensis and Porosorotalia clarki dominate over much of the interval. Shelf forms such as Caucasina and deeper water forms such as Cyclammina are far less common. The_ Caucasina eocenica_ kamchatica_ Zone is~ ~ restricted to the late Eocene in theU.S.S.R. and defines the Eocene-Oligocene boundaryon the Kamchatka and Ilpinsky Peninsulas (Serova, 1976). However, the several speciesof Caucasina recovered from theNo. 2 well range wellup into the Oligocene.
Environment
The climateof the Oligocene was at least warm-temperate and the bathymetry fluctuated from possible outer neritic to continental. Coal-bearing strata are present in the upper partof the section from 3,524 to 4,570 feet. This continental to transitional sequence contains a predominantly inner neritic section from3,930 to 4,250 feet containing shallow-water foraminifers that are commonin hyposalineenvironments. Inner to middle neritic conditions prevailed 4,570from to 5,150 feet. Middle to outer neritic conditions obtained from5,150 to 6,770 feet. Although rare, isolated specimens of Cyclammina C.cf. pacifica and Martinottiella sp. are present, they are not associated awith definitive deep-water faunal assemblage. The presence ofa dominantly shallow-water fauna and minor amounts of coal in drill cuttings suggest that outer shelf-upper slope depositional depths were not obtained. Below 6,770 feet, the environment is characterized by numerous fluctuations from continental and transitional to inner neritic. There are numerous thin coal seams,as well as coal beds as much as 5 feet thick, between6,850 and 8,450 feet. There is some indication that the unconformity 6,850at feet is bracketed by middle neritic pulses. Overall, the interval from6,770 to 10,160 feet appears to represent deltaic to inner shelf paralic deposits.
Eocene
The interval from 10,160 to 12,700 feet is Eocene in age. A suite of distinctive and diagnostic fungal palynomorphs, including Striadiporites sp., Ctenosporites wolfei, Dicellaesporites sp.A Rouse 1977, and Pesavis tagluensis is present. Published ranges suRRest that these species generally occur earlierin the Canadian ArEiic than in British Columbia. The known rangesin the Norton Sound area are somewhat intermediate. On the basis of fungal palynomorph ranges, the interval above11,960 feet is no older than early to middle Eocene and maybe late Eocenein age. The spore- pollen assemblage is essentially the same as that in the overlying early Oligocene. Marine palynomorphs are rare in the samples and most were probably caved.
Foraminifera are quite sparse over this entire interval and occur most frequently between10,160 and 12,000 feet. Many are probably caved from uphole. The assemblage contains rare specimens
41 ofPorosorotaliaclarki,Elphidiumspp.,Elphidiellakatangliensis, Elphidiella cf. -E. californica, Psammosphaera cf. E. carnata, Saccannnina sp., Ammodiscus sp., Loxostomum sp.,andfragmentary polymorphinids.
A dermalscutefrom a juvenile sturgeon was recoveredfrom cuttingsat 10,160feet.Comparisonswithmaterial in the fossil collections of the University of California at Berkeley indicatethat it is a speciesof Acipenser that has affinities with an unnamed species from the early Tertiary ofMontana (Patrick McClellan,personal commun., 1983).
Environment
The Eocenedepositionalenvironmentsfluctuatedbetween continental(predominantlyfluvialandpaludal),transitional (marshes,bays,andestuaries),andinnerneritic.Depositional environments in theupperpartofthesection(10,160to 12.000 feet) may haveoccasionallybeenasdeepasmiddleneritic.Thick coalbedsarenotpresentabove12,700feet. The presenceof euryhaline Foraminifera associated with terrestrial pollen and fungalpalynomorphs,pelecypodshards,sturgeonscutes,and scatteredcoalsuggests a transitionalenvironment.Although sturgeon are anadromous, thejuveniles commonly inhabitsloughs near river mouths. The interval fromabout12,000 to 12,700feet (lithologic zone Te-2) is dominantlycontinental in aspect. The climate in the Eocene was at least subtropical.
Eocene orOlder(PossiblePaleocene)
The interval from12,700 to 14,460feet is Eocene or older. Helwigand others (1984)consider this section to be in part Paleocene in age,probablyonthebasisofpalynologicaldata. Palynomorphrecoveryfromcuttings is poor in this part of the well and thespecimensarepoorlypreserved.Seismic andgeochemical investigations indicate an unconformity at approximately 11,960 feet. Anotherunconformity is placed at 12,700 feet on the basis of lithologic and dipmeter criteria.
Eocene fungalpalynomorphswererecoveredfromsidewallcores inthis interval but not fromconventionalcores.Thisleaves open the possibility that the sidewall cores may havesampled contaminated mud cake.
Environment
The thickcoal sequences indicate a continentalenvironment. The paleoclimate was probably no coolerthanthatoftheoverlying Eocenesection.
42 MetamorphicBasement
The interval from14,460 to14,889feetconsistsofphyllite, quartzite, andmarblesimilartorocks of probablePaleozoicage exposed on the Seward Peninsula. No in situ fossils wererecovered from this part of the well.
CORRELATION
The strata identified in the Norton Sound COST No. 1 and No. 2 wells can be biostratigraphically correlated (fig. 12) despite thefact that they are located approximately 49 nautical miles apart andweredeposited in geographically distinct and tectonically independentsubbasinscharacterized by different depositional environments(figs. 10 and 11). The depositionalenvironmentsofthe St. Lawrence subbasin,thesite of the No. 1 well,are far more marinethanthoseoftheStuartsubbasin,thesiteofthe No. 2 well.Nevertheless,similaritiesbetweenthe two wells are more pronouncedthandifferences,particularlyinthemarinesequences seenaboveseismichorizon C (figs.13 and 14). Below thishorizon, which is the unconformable boundarybetween early and lateOligocene, correlations are somewhat more difficultbecausethenonmarine and transitional strata of the No. 2 well must be compared withthe predominantlyshelfandslopedeposits of the No. 1 well. With theexceptionoftheEoceneorolder(possiblePaleocene)section, which is farthicker in the No. 2 well, time-equivalentunitsare ofroughlyequivalentthicknessesinthe two wells.Sedimentation rates werenotcalculatedbecauseofthetentativenature of some of the biostratigraphic boundaries, particularly below 9,660 feet in the No. 1 well.
Pleistocene
The first sample in each COST well is Pleistocene in age, although it is probable that a thin Holocene section was penetrated. Sample quality is poor. The base of thePleistocene was placedat 1,320feetinboth wells. Microfossilassemblages,lithology, and depositional environments are essentially identical in both wells. Shallowseismicevidenceindicatesthatthere may be a slight unconformitybetweenthePlioceneandthePleistocenesedimentary sections.
Pliocene
The top of thePliocene is at 1,320 feet in each well, the base ofthePliocene is at 2,639 feet in the No. 1 well and 2,580 feet in the No. 2 well. The Pliocene was subdividedintoearly,middle, and late in both wells on the basis of siliceous microfossil assemblages,butthechaoticmixtureofreworked andcavedforms renderssuch a subdivisionprovisionalatbest.Ingeneral,the microfossil assemblages in both wells reflect similar paleo environments.
43 Miocene
The topofthe Miocene is at 2,639 feet in the No. 1 well and at 2,580 feet in the No. 2 well. There may be a middle Miocene hiatus in both wells, possibly in part correlative with the NH4 hiatus of Barronand Keller (1982). This intervalappearstoberepresented byanunconformity (seismic horizonE) on the basin flanks and a zone of biogenic silica dissolution and diagenesis in the subbasins (seeLithostratigraphyandSeismicStratigraphychapter,figs. 13 and 14). The base of theearly Miocene is at 4,740 feet in the No. 1 well and at 3,524 feet in the No. 2 well. There may be a middle Miocene toppresent at 3,120 feet in the No. 1 well (J. Bujak,personal comm., 1985). Microfossilassemblagesandlithologiesarequite similar in both wells, althoughthere are some indications that deposition was at shallowerdepths in the No. 2 well and that the Stuartsubbasin was subjectto greater fresh-water influence.
Oligocene
StrataassignedtotheOligoceneepochaccountforroughlyhalf of the sedimentary section penetrated by the wells, approximately 5,000 feet in the No. 1 well and 6,644 feet in the No. 2 well. In the No. 1well, theOligocene section (4,740 to 9,690 feet) is represented almost entirely by marine deposition, much of it outer shelfandupperslope. By way ofcontrast,intheOligocenesection ofthe No. 2 well (3,524 to 10,160feet) almost halfofthesediments are coal bearing and were deposited under transitional, nearshore to nonmarineconditions.
The boundary between the early and late Oligocene in both wells (8,600 feet in the No. 1 well, 6,850 in the No. 2) correspondsto the. top of seismic horizon C and the boundary between lithologic zones To-2 and To-3. Thisunconformity may representthemajor "mid" Oligocene sea leveldrop (event TO 2.1 of Vail and others, 1977) approximately 30 millionyearsago. The ageoftheunconformity is generally corroborated by the available palynological and nannoplankton data.
There is a small but pronounced coal-bearing transitional environmentpresentnearthetopoftheOligocenesectioninthe No. 1 well (4,740 to 4,980 feet) that is correlativewith the coal- bearing strata seen in the No. 2 well at 3,524 to 4,570 feet.
Eocene
Definite late to middle Eocene strata are presentfrom 10,160 to 12,700 feet in the No. 2 well. Some late Eocenepalynomorphs were found in the No. 1 well in a sample at 12,150 feet. It is probable that part of theproblematicOligoceneoroldersection (9,690 to 12,235 feet) is correlative with the Eocene section in the No. 2 well.
44 NORTONBASIN NORTONBASIN COSTNo. 1 WELL COSTNo.2WELL -0 PLEISTOCENE First Sample - 450 - 1000 1320 PLIOCENE - ZOO0 2580 - 3000 ? MIOCENE ?-- 3 I20 *- 3524 - 4000
50001 - 5000 I
6000 4 16000 LATEOLIGOCENE - 6850 7000- - 7000
8000- - 8000 8600- OLIGOCENEEARLY 1gooo gOOOL9690 7 I0,OOO ?-7 I0,OOO 1 IO, I60 or OLDEREOCENE I1,OOO EOCENE) t 12,000- I 1,960 I2,235--7 I2.000
-I 3.000
-I 4,000 -I 4,460 PROBABLEPALEOZOIC 14.683 to PRECAMBRIAN 14,889 TD TD
FlGURE 72. Biostratigraphic-correlation of NortonBasin COST wells.
45 Eocene orOlder(PossiblePaleocene)
In both wells, a coal-bearing section unconformably overlies theregional early Cenozoic to late Mesozoic erosionalsurface (seismichorizon A). This continentalsection, 310 feetthick in the No. 1 well and 1,760feetthick in the No. 2 well, is truncated byanunconformity at 12,235feet in the No. 1 well and at 11,960 feet in the No. 2 well. Because of thevariablelithologiccharacter of thestrata underlying this surface in two subbasinsthat are separated by a largepositive tectonic element, this unconformity is notcharacterizedby acontinuousseismicreflector.Nevertheless, it is reasonableto assume thattheunconformitiesareapproximately coeval.Likewise, on thebasis of lithology,depositionalenvironment, stratigraphicposition, and thesimilarpreservationalstateof thepalynomorphs, it seems likely that the Eocene or oldersections in the two wells are in partcorrelative. Helwigand others(1984) consider this section to be in partPaleocene in age.
Basement Complex (Probable Late Precambrian to Paleozoic)
Both wells penetrated metasedimentary sections below the regionalunconformitythat marks acousticbasement. The 2,135-foot- thick sequence of cataclastic rocks in the No. 1 well appears quite similar to slate of late PrecambriantoearlyPaleozoicage describedfrom the York Mountains of the Seward Peninsula;the 429 feet of quartzite, phyllite, and marbleidentified in the No. 2 well appears to be quite similar to metamorphicrocksofprobablePaleozoic agedescribed from the central and easternparts of the Seward Peninsula. At present, it is notpossibleto more closelyrelate the metamorphic sections of the two wells on the basis of either ageorgenesis.Severalsamples from the No. 1 wellwererecently reprocessed for acritarchs and chitinozoansbutyieldednegative results.
46 5
Lithostratigraphy and Seismic Stratigraphy
The offshore lithostratigraphy of the Norton Basin is based on the integration of well logandlithologicdatafromtheNorton Basin C.OST No. 1 and No. 2 wells withpaleontologicand CDP seismic data. A stratigraphicsectionof 12,500 to 14,400 feet ofQuaternary, Neogene,and Paleogeneclasticsediments was penetratedbytheNorton Basin COST wells. At bothwells,theTertiarybasin fillunconformably overlies a Precambrian to Paleozoic metasedimentary section of slate, schist, quartzite, andmarblesimilartooutcrops on the Seward Peninsula.Thesemetasedimentaryrocksareincluded inthe miogeoclinalbeltofFisher and others (1979) andappeartoform thebasementcomplexbeneath much ofNortonBasin.
TheNortonBasin is dividedinto two structuralsubbasins by a northwest-trendinganticlinal structure termed the Yukon horst (Fisher and others, 1982). The COST No. 1 wellsampled a stratigraphic section west of the horst in the St. Lawrence subbasin and the COST No. 2 well penetrated a section east of the horst in the Stuart subbasin(fig. 7). Both wells were located on basementlows near subbasindepocenters. Over 16,000 feet of Tertiarysediment is present nearthe COST well site in the St. Lawrencesubbasinandover 24,000 feet is presentnear the well site in the Stuart subbasin.
The Neogene strata in both subbasins are similar and consist primarilyofmarineshelfdeposits.Paleogenestratapenetrated in the COST No. 1 well (St. Lawrence subbasin) are generally finer grainedand more marine in character than those encountered in the COSTNo. 2 well (Stuartsubbasin).Thissedimentsizedistribution probably reflects the closer proximity of the Stuart subbasin to sedimentsourceareas. Most ofthepotentialreservoirsandstones encounteredinthe COSTNo. 2 well represent alluvial, deltaic, and shallowshelfdeposits,whereas mostof thecoeval sandstones in the St. Lawrencesubbasin COST No. 1 well appeartobeoutershelfto upperslopeturbiditedeposits.
The strata encounteredinthe two NortonBasin COST wells are heredivided into informal stratigraphic units or lithologic zones that can be correlated on the basis of microfossil assemblages, seismicstratigraphy,lithology,inferreddepositionalenvironments, large-scalepatterns o� regression and transgression,andwireline logpetrophysicalcharacteristics(figs. 13 and 14).
47
os
I Y I:
61
4 I i J i I5
BulkDensity (g/cm3) I .oo I .40 I .80 2.20 2.60 1 1 I I I 1 1 1 1 IntervalTransit Time (microseconds/foot) I80 I60 I40 120 100 80
2,001 . . 0 . 0 . 0 . Bulk -0 . Density ," . n- 0 Diatomaceous . 0 0 Shales . 0 . 0 (abundantopal-A) . 0 . 0 0 .. 0 . 0 . 0 3,001 . 0 . 0 . 0 . 0 0 Diagenetic Interval 0 .* 0 0 Transit a** Zone 0 4 7. Time . 0 Y . 0 m . 0 m + 0 v . 0 r . 0 Y a e 0 0) 0 04,001 . D 0 0 . 0 0 . . Diatom-poor e 0 Shales 0 : . 0 . 0 . 0 . 0 . 0 0 . 0 5,001 . 0 . 0 . 0 . 0 . 0 . 0 . 0
FlGURE 16. Patternofdecreaseinacousticintervaltransit timeandincreaseinbulkdensityfordiatomaceous shalesandtheirdiageneticequivalents,NortonBasin COST No. 2 well, Stuartsubbasin.
53 function of temperature and requires a range of95 to 125 OF toinitiatelarge-scaleconversion(Hein and others,1978). Petrophysicalchangesassociatedwiththetransformation of opal-A in diatomaceoussedimentsinclude marked increasesincompaction, cementation,bulkdensity,hardness,cohesion,andbrittleness. Thesechanges are primarily due to the reprecipitation, recrystallization, andcollapse of the biogenic silica fraction (Isaacs and others, 1983;Heinand others,1978). In both COST wells in theNortonBasin,changes in lithology anddiatomabundance, andcorrespondingwirelinelogresponses,occur at depths that would be within the requisite temperature range to initiate opal-A instability.
In some partsoftheBeringSea, a bottom-simulatingreflection (BSR) attributed to a silica diageneticzoneoccurs at a depth between 1.0 and2.0seconds two-way travel time (Hammond and Gaither, 1983).Fisherandothers(1982)suggestedthat in theNortonBasin (St. Lawrence subbasin),horizon E mightrepresent a BSR. Horizon E consistentlyproduces refracted arrivals, has a lowerfrequencythan adjacent reflections,and, in the central part of the basin,occurs between 1.0 and1.5seconds. However, the definitive seismic characteristic of a BSR is thatbecause it mimics the sea floor topography it appearsdiscordantwithdippingreflectors. A BSR, if present in theNortonBasinarea, is extremelysubtleandcould not be unequivocally identified on seismic data.
Fisher and others(1982)speculated thatterrigenoussediment inputinto the basin might have diluted the diatom fraction of the sediment to the point that a diagenetic boundary would not be detectable. This doesnotappeartobethe case. The diagenetic zone in theStuart subbasin (at the COSTNo. 2 well)spansan interval which includes the upper part of a deltaic sequence in whichterrigenousinput was high, yet the paleontological and petrophysicalchangesthere and in the St. Lawrencesubbasin COST well, where thediagenetic zonespansanintervaloffine-grained shelfdeposits, are quite similar.
In shallower parts of the Norton Basin, seismic horizon E may coincidewith a late Mioceneunconformity. The discordant reflections aboveandbelowhorizon E in these areas suggest the presenceofanunconformity. In thesestructurallyshallower areas, horizon E is probablytooshallow(above1.0second)tobe within the necessary temperature range for opal-A transformation tooccur. Some interpretativedifficulties in deeperpartsofthe basin, and at the COST wells, may be due to the approximate coincidence of an unconformable stratigraphic surface and a diagenetic boundary. This relationship would probably generate a familyof seismic reflections caused in great part by constructive interference. Similar interpretative difficulties were encounteredin the Navarin Basin(Turnerandothers,1984)andotherBeringSea areas where the zone of biogenic silica diagenesis often generates a strong regional reflector which may obscure structural interpretation of seismic horizons in theshallowsubsurface.
54 Similarities between the diagenetically altered Miocene diatomites of theBeringSeabasins and those of theMontereyFormation,which is animportanthydrocarbonsourcerock and reservoir in California, suggestthatseismicsequence I1 couldbe a prospectiveexploration target.Iffractured,thesiliceousrocksinsequence I1 could functionas a reservoir;if unfractured, the same rockscouldserve as a sealforunderlyingreservoirs.
OLIGOCENE
Strataofprobable Oligoceneageaccountfor much of the Tertiary sectionintheNortonBasin.Sedimentarystrata ofOligoceneage reflect several regressive and transgressiveepisodeswhichare consideredin the subdivision of these strata into three stratigraphic unitsdesignated, fromtoptobottom, To-1 to To-3.
Lithologic Zone To-1
Zone To-1 (4,700 to 5,580feet in the COST No. 1 well;3,524to 4,850feetinthe COST No. 2 well) is composed ofinterbeddeddeltaic toshallow-shelfsandstones, siltstones,mudstones, and coals. In bothsubbasins,thesequence consists of a lowersandyunit, a middle fine-grainedunit of siltstone andmudstone,andanuppersandyunit. The zone consistsofanoverallregressivesequencethatreflects theprogradationofdeltaicdepositsintothesubbasins. The fine- grained middle unitprobably records a minor transgressiveepisode which interrupteddeltaicprogradation.
Sandstonesfromthiszone fall into the feldspathic litharenite category of Folk'sclassification(1974). Framework grainsconsist of45 to 60 percentquartz,15to25percentfeldspar(primarily plagioclase), and25 to 30percentlithicfragments.Sandstone compositionsfromthe two subbasinsdiffer primarily in the composition of theirlithic components. LithicfragmentsintheSt. Lawrence subbasin (COST No. 1 well)are composed chiefly of ductile metamorphicgrains(mica and quartz-micaschist),whereasintheStuart subbasin (COSTNo. 2 well),the lithic clasts include grains of feldspars,carbonates,micas,shale,andschistthatwerederived from volcanic,metamorphic, and sedimentarysourceterranes.
Zone To-1 sedimentsin the Stuart subbasin (COSTNo. 2 well) containnearly equal contributions from volcanic,metamorphic,and sedimentarysourceterranes (AGAT, 1982, D-281-PI,),whereasequivalent sedimentsdepositedinthe St. Lawrence subbasin (No. 1 well) were derivedfrom a metamorphicsourceterrane and reflect only a minor inputfromvolcanicsources (AGAT, 1980, p. 9). Thissuggeststhat duringOligocene time, theStuartsubbasinreceivedsedimentfrom two major lithostratigraphic provinces, the miogeoclinal belt and the Okhotsk-Chukotsk volcanicbelt. The St. Lawrence subbasin, however,receivedterrigenoussedimentprimarilyfrom a single province,themiogeoclinalbelt.
55 Nonmarine deltaic facies are more prevalent in strata of the StuartsubbasinthanintheSt. Lawrence subbasin.Lithologiczone To-1 in the COST No. 2 well contains a number of relatively thick coalbeds. In contrast,the COST No. 1 well contains a more marine facies andhasonly a few thin,shaley coals near the top of the zone.Sandstonebeds in the COST No. 2 well are coarsergrained, thicker, and morenumerous thanthose in the COST No. 1 well. Zone To-1 microfossilassemblages indicate a greater freshwater influence in the COST No. 2 well, as compared to the more marine section in the COST No. 1 well.
Lithologic zone To-1 comprisestheupperpartof seismic sequence 111 (figs. 13 and 14). Sequence 111 containsbothmedium-amplitude, moderatelycontinuous reflections andhigh-amplitude,discontinuous reflections. These seismic characteristics are oftenassociatedwith strata formed in marginal marine and flwiodeltaic environments. Seismicsequence I11 is bounded at the top by seismic horizon D, which is anunconformity mapped at or near the top of theOligocene sectionacrosstheNortonBasin(figs. 17 and 18). Horizon D and seismic sequence I11 are laterallycontinuous throughout the basin and onlapbasement on structuralhighs.Syndepositionalfaulting is common in sequence 111.
Lithologic Zone To-2
In the COST No. 1 well, zone To-2 extendsfrom 5,580 to 8,600 feet; in the COST No. 2 well, from 4,850 to 6,850 feet. The sediments of lithologic zone To-2 consistofmarinemudstones,siltstones, andsandstones. In theSt. Lawrence subbasin,the COST No. 1 well penetrated a section of predominantly deepwater marine mudstones, siltstones, and muddy sandstones; in theStuartsubbasin, the COST No. 2 well penetrated an equivalent sequencecharacterized by shelfsandstones and siltstones. The strata of zone To-2 comprise roughlythelowertwo-thirdsof seismic sequence 111 (figs. 13 and 14). In the St. Lawrence subbasin,sequence I11 reflections are more discontinuousandhavelowerand more variable amplitudes than equivalent strata totheeast in theStuartsubbasin.Sequence I11 reflectorstypically onlap underlying strata over pre-existing structures and along basin margins.
Seismichorizon C is mapped near the base of lithologic zone To-2 and representstheunconformable,lowerboundary of seismic sequence I11 (figs. 13 and 14). Horizon C formstheapproximatelyflat summits ofhorsts in the Norton Basin, which are almost all at aboutthe same depth.Strataabovehorizon C are laterallycontinuousbetween subbasins and overthe Yukon horst,whereas strata below this horizonarenot.Horizon C may reflect a sealeveldropduring the "mid"-Oligocenewhichexposed intrabasin highs to erosion. Erosivetruncation of these highs eventually allowed the expansion of more open-marine conditions across the Yukon horst and into the Stuartsubbasin.
56
n m
rc 5 m a. 0 0 -0 a. 5 m -0 I m 0 0 0 3 m
VI U
l-J 0 3 3 0
cn u) The upper partof zone To-2 in the COST No. 1 well consists of probableprodelta mud and silt deposits.Thisinterpretation is in partbased on the stratigraphic position of the zonesubjacent to a deltaicsequence. The dark-graytoblackcolorofthemudstones is typical of organicallyrichprodeltasediments(Fisher and others,1974). A conventionalcore from this intervalrecovered 25 feet of bioturbated,olive-gray, silty andsandymudstone containingfossilfragments,carbonaceousdebris,mica,pyrite, and tracesofglauconite. Some of thebeddingexhibitssmall-scale soft-sediment deformation structures that suggest deposition on a slope.
The lowerpart of zone To-2 is a sequenceofrhythmiccyclesof thin (5 to 10 feet), coarsening-upwardshaleysandstoneswhich grade upward intothick(20to 110 feet),blocky,stacked,shaley sandstones(fig. 19). The depositionalenvironmentofthesesandstones is uncertain. On a largescale,the upward-thickening, upward coarseningsequence appearsanalogous to the basin-plain faciesof Mutti and RicciLucchi(1972)for a progradationalsubmarinefan (fig.19). The logsignaturesofthecyclic,thin,coarsening-upward sandstonesare similar to those of turbiditic distal-fan sand lobes; thethickerblocky sandswithabruptbasessuggestmiddle-toinner fanchanneldeposits(Selley,1978,figs. 54and 55). The hypothesis that zone To-2 was depositedindeepwater is supportedbythe presence ofupperbathyalmicrofossils and the close relationship withtheunderlyingmarineshale ofzone To-3.
The lithic componentsof the sandfraction ofzone To-2 in the COST No. 1 well (St. Lawrence subbasin)suggest a similarprovenance to that of the overlying deltaic deposits of zone To-1, that is, chiefly metamorphic grainsderived from older miogeoclinal belt rocks. The mudstones of zone To-2 consistofabout 60 percentsmectite, illite, andmica,withtheremaining 40 percent composed ofequal parts of chlorite and kaolinite.
In the COST No. 2 well,thesandstone framework grains consist of 50 to 60 percentquartz, 10 to 20 percentfeldspar,and 25 to 35 percentlithicfragments. Thesesandstonesareclassified as lithic arkoses (AGAT, 1982, D-281-2). The lithicfractioncontainsabout twice as many plutonic and volcanic grains as metamorphicgrains. This suggests that terrigenous input from the Okhotsk-Chukotsk volcanic belt predominated in theStuart subbasin in the late Oligocene.
Wirelinelogsfromthe COST No. 2 wellindicatethatzone To-2 represents a generallyfining-upwardandthinning-upwardsequence. Sandstonebedsarethicker and morenumerous inthe lower half of thesequence;thinbedsofsiltstone,siltysandstone, andmudstone predominate intheupperhalf.Thispatternsuggestsanoverall transgressivesequence.
The thicknesses of individualsandstonebedsaregenerally 7 to 20 feet,withoneatleast 65 feetthick.Althoughcoal is reportedinsamplesfromthissequence, no coal beds thick enough tobedetectedbywirelineloggingtoolswereobserved. The coal
61 present in drill cuttings appears to be predominantly disseminated carbonaceousdetritus from terrestrial plants. The presenceofan innerneritic microfossil assemblage, abundant shell shards, bioturbation traces, and glauconite supports a dominantlymarine origin for zone To-2.
Lithologic Zone To-3
In theStuartsubbasin (COST No. 2 well), zone To-3 consists offluviodeltaic deposits of interbedded sandstone, siltstone,coal, and prodelta mudstone. In theSt. Lawrence subbasin (COST No. 1 well), the inferred equivalent Oligocene section consists of basinal marine shaledeposits. ZoneTo-3 is separatedfromthesuperjacent zone To-2 by the "mid"-Oligoceneunconformity,which is representedby horizon C (figs. 13 and 14).
The strata of zone To-3 comprisetheupperthirdtotwo-thirds ofseismicsequence IV. These strata donotappeartobecontinuous acrossthe Yukon horstthatseparatesthe two subbasins.There are marine fossils in the Stuart subbasin, however,whichsuggest a marine connectionbetween the subbasins at the time of deposition ofzone To-3.
St. LawrenceSubbasin
In the COST No. 1 well, zone To-3 (8,600 to 9,685 feet) is characterized by a uniform sequence of light- to dark-gray pyritiferous mudstones containing a deepwater microfossil assemblage (Turnerandothers,1983a). The gamma-ray, SP, and resistivity logs all display a monotonous shale response with few deflections from theshale baseline except near the base of the interval. . Deposition probably took place in a deepwater basin-plain environment. The seismicreflections that correspond to these strata are discontinuousand weak. These strata overliewhat, on a regional scale, appear to be alluvial fan deposits that grade basinward intosubmarinefandeposits. These seismicallydefinedsedimentary packages display onlapping relationships with basement rocks on horsts and other structural highs.
StuartSubbasin
In the COST No. 2 well, zone To-3 extendsfrom 6,850 to 10,280 feet. The zoneconsists of a sequence of threegeneticallyrelated units of strata representing a singledepositionalsystem. These units are interpreted(fromtop to bottom) to be largely delta- plain,delta-front, and prodeltadepositsthatprogradedinto a shallow-water marine basin.
The sediments in theupperunitofzone To-3 in the COST No. 2 well (6,850 to 8,450 feet) consist of a complexlyinterbeddeddeltaic sequenceofsandstone,siltstone,shale, and coal.Samplesfrom this interval yielded a continental to marginal marine microfossil assemblage. The lithic-clastfractionofthe framework grainsof
62 Schematic Stratigraphic Section, SubmarineFan Model (MuttiandRicciLucchi, 1972; Nilsen, 1984)
FIOURE 79. Comparison of largeandamall-scaleverticalwlrellnelogpatterns ofrockunit To-2, St.Lawrencesubbasin.andcharacteristicvertical cycles of submarinefanturbiditefacies.(Logheadingabbreviations: VSH-shalevolume,GRGammaRay,SFL=SphericailyFocusedResistivityLog)
63 these sandstones indicates a dominantlymetamorphicsourceterrane (AGAT, 1982).Thissuggeststhat a major shift from a metamorphic to a volcanicsourceareaoccurredsubsequenttothedepositionof zone To-3.
Wirelinelogs of theupper unit indicate numerous thin coal beds (5 feetthickor less) throughouttheinterval.Sandstonebeds are generally 5 to 20 feet thick andappear to be relatively clean based upon large SP and gamma-ray log deflections as well as core andpetrographicdata. The depositionalenvironmentsrepresentedby two cores from this interval were interpreted as pointbar, channel fill, levee, overbank and swamp deposits on thebasis of abundant carbonaceousdebris and coal,plantimpressions,fining-upward character,moderatesandsorting,thepresence of ripple to parallel laminations,andtheabsence ofmarine bioturbation traces (AGAT, 1980).Thissedimentaryunitprobablyrepresents a fluvially dominateddelta-plainsystem.Thisinterpretation is based on the preponderanceoffluviallyinfluencedaggradationaldelta-plain facies and the relative paucity ofstrongly marinefacies. Notably absent are the thick shoreface toforeshore delta-frontsands (smooth,funnel-shapedlogprofiles)thataretypicallydeveloped in wave-dominated destructional marine settings (Fisher and others, 1974;BalsleyandParker,1983).
Seismicreflections corresponding to the strata of theupper unitofzone To-3 in the Stuart subbasin display moderate to highamplitudesand are relativelycontinuous.Laterally,these reflections become divergent away frombasementhorsts.These divergent seismic reflections may represent alluvial fan deposits (Fisher and others, 1982) genetically related to the fluviodeltaic sandstonespenetrated in the COST No. 2 well.
Below thefluviodeltaic sequence, the middle unit ofzone To-3 (from8,450 to 9,230feet) is characterizedbythin-bedded,fining upward andthinning-upwardsequencesofveryfinegrainedsandstone, siltstone, and shale. The dipmeterlogindicatesuniformdipswith little variation in magnitude and direction, which suggests deposition in quiet water below wave base(Schlumberger,1981, p. 36). The sparsemicrofossilassemblages,chieflyshallow-waterForaminifera, indicate an inner neritic to transitional (estuarine) depositional environment(Turnerandothers,1983b). The micaceousmudstone,shale, and siltstone recovered from drill cuttings and cores are chocolate brown to dark gray or black and contain abundant coaly andcarbonaceous material. Seismicreflectionscorrespondingtothesestrataare generally weakerand less continuousthanthoseoftheoverlying fluviodeltaicsequence. A conventionalcorefromthemiddleunit contained very fine grained, massive to fining-upward bedding sets ofcarbonaceoussandstonewhichgraded upward intothin, interbedded carbonaceoussandstoneandmudstone,cappedbymottledcarbonaceous mudstone.Sedimentarystructuresincludedcurrentripple, wavy, parallel, andmudstone-inclinedlaminations,disturbedbedding (soft-sedimentdeformation), and load casts (AGAT, D-281-5, 1982). Thesetypesofsedimentarystructures andsequences, as well as
64 patternsobserved on thewirelinelogs(fig.20),suggestdeposition on a slopebyturbiditycurrents (Bouma, 1962;Walker,1965). However, thedominantlyshallow-waterforaminiferalfaunacontains no in situdeeperwaterelementsthat would support a downslope transportoriginforthisfossilassemblage.Ifthesefacies represent deposits generated by density currents, then deposition may have occurred in relatively shallow water.
Fisher and others(1974) and BalsleyandParker(1983)characterize distal distributary mouth bardeposits as consisting of fine-grained sedimentsin graded beds that resulted from rapidsedimentinfluxes duringfloods.Thesesedimentarystructuresincludeparallel and ripplelamination,gradedtomassivebedding,thin Bouma sequences, sole marks and loadcasts, and localburrowingin bedtops. The facies profiles from the COST welllogs and thesedimentaryfeatures describedinthecore from this middle unit compare closelywiththe diagnostic characteristics of ancient andmodern distal mouth bar deposits. It is reasonabletoconcludethatzone To-3 was probably depositedbysimilarprocesses.
Stratain the lower unit of lithologic zone To-3 (9,230 to 10,300feet in the COSTNo. 2 well)consistofthin-beddedshale, siltstone, andminoramountsof siltysandstone that contain a sparse,shallow-waterforaminiferalassemblage. The dipmeterlog indicates uniform dips with little variation in magnitude or direction, whichsuggestsquiet-waterdeposition(Schlumberger, 1981, p. 36). Drill cuttings fromtheserocks consistof carbonaceous, chocolate-brown to dark-gray or black micaceous mudstone,shale,andsiltstone. A conventionalcore from the COST No. 2 well, whichsampled thebasalpartofthelowerunit, containsthinlyinterbeddedcarbonaceousmudstones,siltstones, and siltysandstonesin coarsening-upwardsequences.Sedimentary structuresincluderipplelamination,high-angleplanarcross lamination, andabundantcontortedbedding,suggestingdeposition on a slope. The faciescharacteristics of thislowerpackageare typical of prodeltadeposits(Fisher and others,1974;Sheriff, 1980).
EOCENE
Over 2,400 feet of strata of Eocene or probableEoceneage are presentat both COST well locations. The uppercontactofthese strata with the overlying Oligocene in both COST wells is marked by a largeshift on the gamma-ray logs. An increaseinradioactivity of30 to 50 API units occurs in the siltstones and shales ofEocene orprobable Eocene age relative to those of the overlying Oligocene. Offsets in trends of thethermalmaturityoforganicmatter(fig.30) and indipmeterdata at or near this gamma-ray log shift in the No. 1 well,suggestthepossibility ofanunconformity. However, in the No. 2 well, no suchoffsets(fig.33)wereobserved at theupper contactwherethe gamma-ray log shift occurs.
65 Natural Shallow Intorval Gamma-Ray Log eslstivity Log Transit Time Depth (us/Pt) 8o (feet) I o*----
(API) 40 I 3 3 9100
9200
FlOURE 20. Wirelinelogdetailsoflargeandsmall-scale beddingcycles of the inferred distal facies of a distributarymouthbarsandstone,zoneTo-3,Norton COST No. 2 well.Small-scalefining-andthinning-upward bedcyclesare well-defined on the resistivity andsonic logsand are interpreted to reflect Bouma Ta-e sequences.EachsequenceIsinferredtorepresent a turbidityeventfedbydistributarymouthsduringstorm andfloodevents.
66 The probableEoceneagesection in the St. Lawrence subbasin is represented by a singlelithologic sequence at the COST No. 1 well(fig.13). In theStuartsubbasin (COST No. 2 well), however, the Eocene section is dividedinto two distinct lithologic units separated by anunconformity(fig.14). The unconformity is mainly indicated by a large offset in the vitrinite reflectance trends withdepth(fig.33).
St. LawrenceSubbasin:Lithologic Zone Te
Lithologic zone Te strata in the COST No. 1 wellextendfrom 9,685to 12,235 feet and consist of verymicaceous,fine-grained sandstoneswhichareinterbeddedwith brown todark-gray,sparsely burrowed,micaceousshale and siltstone. The sandstone framework grainsconsist of40 to 60percentquartz,30to55percentmicas, and 2 to 12 percentfragments ofmicaceousschist.Thesesandstones areclassified as litharenites (AGAT, 1980)and were probably derived frommetamorphicrocksofthemiogeoclinalbelt. In the upperthird of this zone there are three basaltic sills thatrange in thickness from 6 toover140feet.Igneousrockssuchasthese probablyaccountfor many of thestrong discontinuous reflections inseismicsequence IV.
Sandstonebedsinzone Te, whichvaryfrom a few inchesto over 100feet in thickness,generallyincrease in frequency of occurrence, thickness, and overallgrainsize downward. Sedimentarystructures and featuresobservedinsandstonesinfiveconventionalcores(which collectivelyrecoveredover90feet of section)includesharp erosional bed bases with occasional groove casts and flame structures (AGAT, 1980). Bedding stylesinclude thin,gradedbeds, fining-upward Bouma cycles, and massive bedding in thethicker units. Burrows are present in thetops of some beds.Althoughonlyraremarine palynomorphswererecoveredfromzone Te (at 12,150feet),the sedimentarystructuressuggestdeposition as marineturbidites.
Seismicreflections in sequence IV correspondingtozone Te make up most of the reflectionsbetween horizon B2 and A at the COST No. 1 well (fig.13). Away from the COST No. 1 well, reflections from this interval vary in amplitude and continuity and dipbasinward from the Yukon horst and otherpositivebasementstructures. Near thebasementhighs,thesedippingreflectionsprobablyrepresent alluvialfandeposits(Fisher,1982). The possibleturbiditesin the COST No. 1 well may represent a distal-basin facies genetically related to an alluvial fan and deltaic depositional system.
StuartSubbasin
Lithologic Zone Te-1
In the COST No. 2 well,lithologic zone Te-1 (10,280 to11,958 feet) consists of a sequence of thinlyinterbeddedmicaceoussandstones, siltstones, and shalesderivedmainlyfrom a metamorphicterrane (AGAT, 1980).Theserocksunconformablyoverlietheremainderof
67 the Eocene section. Zones Te-1 and Te-2 areseparatedby seismic horizon B-3, which occurs in thelowerpartofseismicsequence IV (fig. 14). Reflections from thisinterval are generallyof low amplitude,withpoortofairlateralcontinuity.Strata from this interval terminate against the flanks of the Yukon horst and other positive basement structures.
Sandstones in this zone are chieflyfine- tovery fine-grained, poorlytomoderatelysorted, andmicaceous,althoughthin, well- sorted, medium- tocoarse-grainedsandstones are locally present. Wireline logs indicate that the individual beds are generally less than 5 feet thick and that fining-upwardsequencespredominate. Sedimentarystructuresobserved in coresinclude thin, graded beds, small-scale hummocky(?) crossbedding, and ripplecross-lamination. Microfossilassemblagesindicate a paleobathymetryfromtransitional (estuarine?)tomiddle neritic. The gradedbeddingandlackof bioturbationsuggestrapiddeposition in quietwater.Dipmeterdata show little variation in dip magnitude and direction, which also suggests generally quiet water deposition (Schlumberger,1981,p.36). The hummocky(?), cross-stratified,well-sorted sands are suggestive ofstrongepisodiccurrents (Harms and others,1975) andimply that the depositional environment was shallow enough to be affected bystorm-generatedcurrents.
Lithologic Zone Te-2
In the COST No. 2 well, lithologic zone Te-2 extendsfrom11,958 to 12,700feet. The upper 420 feetof this zone consists of a sequence ofthick,fining-upward,conglomeraticsandstones. Most ofthe pebblesandgranules are volcanic in origin, which indicates a significant detrital influx from the Okhotsk-Chukotsk volcanic beJt. The sequence is composed of a 180-foot-thick basalbed with an abrupt erosional base and a gradational fining-upward profile, overlain by a series of similar beds varying from 25 to 60 feet thick.
A conventional core from the upper 30 feet of the thick basal bedcontains a conglomerateofmatrix-supported,randomlyoriented, subangular volcanic clasts; the conglomerategrades upward into medium- toveryfine-grained,poorlysortedshaleysandstone. The sedimentary structures present are defined by laminae ofcoaly detritus and organic-rich shale clasts along bedding surfaces and includehigh-angle (25 to 40 degrees) inclined stratification and slightlycontortedbedding.Shellfragments are reported in drill cuttingsfromthesandstonesequenceabovethecoredinterval. A sidewallcorefrom s thin mudstoneinterbeddisplayed a mottled texturesuggestive of bioturbation (AGAT, 1980).
Significant amounts ofcoal are present in cuttings from the conglomeraticsandstonesequenceandfrom the underlying shale and siltstone whichcomprisethelower322 feet of the zone in the No. 2 well. However, coalbeds thick enough to be detected on wireline logs are notpresent. The coal is probablyderivedfromeitherthin laminae,coaly detritus, or both.
68 The depositionalenvironmentrepresented byzone Te-2 is uncertain. The presence of coal,conglomerate, and a dominantly continentalmicrofossilassemblagesuggeststhatthissection represents a nonmarine, fluvialdeposit (AGAT 1980;Turnerand others,1983b). However, the presence of shell fragments and bioturbatedshaleinterbeds are inconsistentwith a fluvialdeposit. The sedimentarystructures andbeddingsequencesobservedinthe conglomeraticsandstonecouldhaveresulted from either fluvial or turbiditedeposition. The presence of coal,particularlyif it is detrital,doesnotexcludemarinedeposition.Largeamounts of plant material are common in modern turbidite deposits (Nelson and Nilsen,1984) and coaly detritus is reported from ancientturbidite depositsas well (Balsley and Parker,1983).
POSSIBLE PALEOCENE: LITHOLOGIC ZONE TP
Lithologiczone Tp (12,235 to 12,545feet in the COST No. 1 well; 12,700 to 14,460feet in the COSTNo. 2 well) is composed ofinterbedded sandstone,siltstone,coal, and shale. The zoneunconformably overliesmetamorphicbasement and is unconformablyoverlainbyEocene sedimentarystrata. Zone Tp contains numerous coalbeds(frequently up to 10 feetthick). The sandstonesaregenerally medium tocoarse grained,poorlysorted, and containabundant detrital matrix and carbonaceousdebris.Coresfromthiszoneexhibittypicalfluvial faciescharacteristicsincludingpebblelags, trough to ripple cross-stratification, andsmall-andlarge-scalefining-upward beddingsequences. Abundant leafimpressionsarepreservedinthe interbeddedshales.Sandstoneclastcompositionssuggest a source area dominatedby schist andmarblerock.
Inthe COST No. 2 well,severallayersofigneousrock(varying in thickness from 5 to 25 feet)are intercalated with zone Tp strata. The drill cuttingscontained a possible welded tuff and a single sidewallcorefromanigneousinterval was classified as basalt (AGAT, 1982). The igneousintervalsarecharacterized on welllogs by high resistivities, large gamma-ray deflections (low radiation), high sonic velocities, and high bulk densities.
Strata in zone Tp make up the basal part of seismic sequence IV in bothsubbasins(figs.13 and 14).Reflections from this intervalarediscontinuous,highlyvariableinamplitude,and,in places,nonparallel. The basalpart of seismicsequence IV is represented by reflections that are present in wedge-shaped packages of dipping reflections that diverge away from horsts on the downthrown sides. Basinwardfrom these wedges, thereflectorsconvergewith anddownlapbasementrocks.Stratafromthiszoneappeartorepresent allwial fans deposited adjacent to faulted basement horsts early inthe formation ofNortonBasin.
69
During the summer of 1985, three more exploratory wells were drilled in the Norton Basin(fig. 21). Exxon was the operatorfor allthree wells. The Exxon OCS-Y-0398 No. 1 well,located at lat 63'53'28.76" N., long 164°03'56.16" W., was drilled to a depthof 6,913 feetin 58 feet of water. The Exxon OCS-Y-0407 No. 1 well, located at lat 63O47'15.79" N., long 164O25'56.92" W., was drilled to a depth of 7,867 feet in 58 feet of water. The Exxon OCS-Y-0425 No. 1 well,locatedat lat 63O36'06.11" N., long 164°09'33.40" W., was drilled to a depthof 6,093 feet in 42 feetofwater. All of these wells were plugged andabandoned. No discoveries were announced. The data from thesethree wells are proprietary.
LeaseSale 100 is scheduled to takeplace in March of 1986 (fig. 1). Tracts that werenot leased duringSale 57, as well as additional tracts to the west, will be offered.
75 SUBBASINSTUARTSUBBASINLAWRENCE ST. NORTONNO.COST I WELLNORTONWELL COSTN0.2 Q CLASSIFICATIONQ SANDSTONE (after Folk. 1974)
I. QUARTZARENITE 2. SUBARKOSE 3. SUBLITHARENITE 4. ARKOSE 5. LITHICARKOSE 6. FELDSPATHICLITHARENITE 7. LITHARENITE
Q-QUARTZ F-FELDSPARS L-LITHICS
S S
V-VOLCANIC M-METAMORPHIC S-SEDIMENTARY
FlGUffE 22. Ternarydiagramsshowingtherange of the principal sandstoneframeworkConstituents andthemajorrocktypesoflithicfragmentsinsandstonesfromconventionalandsidewall coresamplesfromNortonBasinCOSTNo.1andNo.2wells.(Modified from AGAT COnSUltantS, Inc. ,1980, 1982.) 7
Reservoir Rocks
The reservoir quality of a sandstone in any given basin is primarily a function ofprovenance,depositionalenvironment,and tectonicsetting. These factorsdeterminethesandtexture(grain size,packing, and angularity) and composition,aswell as most aspects of the burial and diagenetichistory(Hayes, 1984). Sandstones in the NortonBasin COST wells are composed of metamorphic detritus shed from uplifted horsts in and aroundthe basin, and from volcanic and volcaniclasticdebris shed from the volcanicbelttothesouth and east of thebasin. Thesesandstones contain significant fractions of chemically and mechanicallyunstable components. As a result,reservoirqualitydiminishesrapidlywith increasing burial depth in theNortonBasin.
The distributionof quartz, feldspar, and lithic grains in these sands and themajorrocktypes of thesourceterranesfromwhichthey werederivedare shown infigure 22. The majorityofthesandstones areeitherlitharenitesorfeldspathiclitharenites. Most ofthe feldspathic litharenites and thefewlithicarkosesoccur id lithologic zones Tmp, To-1, and To-2 (figs. 13 and 14). Sandstonesbelowthe Oligoceneunconformitytendtobelargelylitharenites. This mineralogicaldifference is probablydue to an increase in the input of volcanicdetritus containing abundant plagioclase feldspars and tothe progressive diagenesis and dissolution of thesefeldspars with depth.
Most of theprospectivereservoirrockseenthusfarinthe NortonBasin is in theOligocenesectionofthe COST No. 2 well (lithologiczones To-1, To-2, and To-3; fig. 14). Thesesandstones representfluvialtoshallow-marinedeposition. The relatively highdepositionalenergyoftheseenvironments winnowed and reworkedthemetamorphicandvolcanicdetritus and ultimately depositedsandstoneswithsufficientquartzframeworkclasts to haveretained significant effective porosity to considerable burial depths. These sandstonesareprobablyrepresentative ofmostof thepotentialreservoirs,includingpossiblePaleocene and Eocene alluvial fan deposits, that ate likely to be encountered in bothsubbasins.Althoughsignificantthicknesses of probable deepwaterturbiditicsandstones wereencountered in the COST No. 1
77 well (in theSt. Lawrence subbasin),thesesandstonesappear less likely to represent significant reservoirs because they may be restricted to the central parts of the subbasin at burial depths where effective porosity and permeability are anticipated to be very low.
OLIGOCENE SANDSTONES
Potentialnet sandstone intervals were determinedforthe Oligocenesandstonesection in the Stuart subbasin by wireline well log analysis. Net sandsweredefined as intervalswith a minimum of 30 percentquartzose components and an SP log deflection fromtheshalebaselineofatleast 10 millivolts (mv). An SP deflection of this magnitude indicates that at least minimal permeability is present.Effectiveporosity was determinedby calculating the volumeofshale in thesandstoneandsubtracting the componentof shale microporosity from the apparent total porosity of thesandstone. The component of shale and mica in thesandstones was determinedfromthe gamma-ray log (technique from Dresser Atlas, 1979)andfromneutron-densitycrossplots(techniquefrom Poupoun and others,1970).Effectiveporositieswerecalculated from acoustic,density, and neutron-densitylogs and the results wereaveraged.
The relative volumeof quartz in the sand was determinedby subtracting from the sand the total volumeof shalecalculated from neutron-densitystatisticalcrossplots.Thisanalysis assumes that quartz is themajor frameworkcomponent remaining after the deletion ofmicroporousgranularandintergranular components detected as shale bytheneutron-densitycrossplots.Althoughquartz is the . majormechanicallystable frameworkcomponent, significant amounts of feldspar(mostlyplagioclase) are alsopresent.Thissimplified analysisof quartz content, while yielding a good general estimate, also includes feldspars and othermechanically stable grains.
The shaley component detected by wireline logs can be distributed in the sand as intergranular matrix (dispersed shale), as shaley lithic framework grains(structuralshale), and asshalelaminae.Dispersed shale probably has the most adverse effect on reservoir quality at depths where compaction and deformation of ductile lithic grains are nottoosevere. The simplifieddispersed-shale modelofAlgerand others(1963) and Tixier and others (1968) was employed to obtain an estimateofthistype of shale.
The results of the log evaluation of reservoirpotential were checked against petrographic and laboratory measurements of reservoir parametersobtainedfromcore 3 of the COST No. 2 well. The three otherconventionalcores in theOligocenesandsequence(cores 1, 2, and 4) didnotpenetrateintervalsofpotentialnetsand.Table 1 summarizes the wireline log analysis of the interval cut by core 3 and table 2 summarizes thelaboratory and petrographicanalysis of thecore.
78 Table 1. Wirelineloganalysis of thesandstoneintervalcutbycore3,NortonBasin COST No. 2 well.
Effective Porosity Quartz Q Factor DepthThickness Content1 (Dispersed Shale)2SonicDensity Neutron-Density Average (feet) (feet) (X) (X) (‘6) (X) Crossplot (X) (X)
7030 3 21.9 41.6 13.5 9.8 2.1 8.5
20.5 45.8 7033. ~~~ 1.5 19.7 17.8 9.9~ 15.8 7034.5 3 49.7 23.2 25.9 21.4 13.6 20.3 7037.5 2.5 33.2 43.4 17.3 10.6 1.6 9.8 13.1 704022.1 2 38.1 44.2 5.0 13.4 7042 2 11.6 26.1 1.7 48.011.7 21.6 15.9 24.1 7044 38.42 43.9 7.8 15.9 12.1 7046 -3 19.6 37.7 43.6 -- -4.3 12.0 19 Gross 13.9 36.620.3 38.2 5.7 13.3
15.013.5 7.2Net3 15.4 40.522.3 36.5
1) Determinedfromneutron-densitycrossplots.Includesfeldspars(chieflyplagioclase).
2) Indicates the amountof porespaceoccluded by shale and lithic detritus.
3) Indicatessandstone with a quartzcontent greater than 30 percentand effective porosity averagingmorethan 10 percent.
80 Table 2. Petrographicmodal analysis of thinsectionsand core analysisofconventional core 3, Norton Basin COSTNo. 2 well (petrographicanalysisfrom AGAT, 1982,p. D-281-3).
Analysis Petrographic Core Analysis
Framework IntergranularGrains (Boyle’s Law - Porosity)Helium
DepthQuartz Feldspar Lithics ClaysSiderite Porosity PorosityPermeability (feet)(%) (%) (%) (%) (%) (%) (a) (millidarcies)
7030.7 31 5 41 5 8 9 20.5 11 27 7032.5 14 37 14 5 2 14.4 0.74 32 7034.3 9 10 376 7 21.0 7.64 __7036.5 __ -- __ -_ __ 543 28.5 7038.6 28 8 37 10 12 51.27 17.7 7041.3 33 9 41 7 8 2 20.0 7.98 39 7044.4 6 21 11 5 18 26.3 183 7047.5 -43 -8 -22 -10 __4 __14 21.2 -~40._ Average .33 9 34 9 7 8 99 21.2
Table 3. Wireline log analysisofOligocenesandstone sequence in theNortonBasin COST No. 2 well.
QuartzContentEffectiveDispersed Sand of ContentShaleSand Porositv
0.65 285 To-1 34-83 57 5-26 15 26 13-36 .68 To-2 400 44-74 54 2-30 17 10-25 18 To-3 .83 230 34-70 53 17-44 27 12-21 16
81 12 percent.Thisassumptioncanbeappliedwith some confidenceto the sandstones of lithologic zone To-3, where core 3 was cut, but is less certain in zones To-1 and To-2, in which no poroussands were cut by conventional cores.
The results of the wireline log analysis of the Oligocene sandsequence are shown intable 3. The analysisindicatesthat significant reservoir potential exists in each lithologic zone. Lithologic zone To-1 contains a total of 285 feet of sandstone in 14 beds greater than 5 feet thick (as defined by minimumSP log deflections of 10 mv). The average bed thickness is about 20 feet; the zonecontainsthreebedsfrom45to 50 feetthick. Of thesesandstones, intervals with a minimum quartz (and feldspar) content of 30 percent and a minimum effective porosity of 10 percent were consideredpotentialreservoirrockornetsandstone. On this basis, zone To-1 contains a net to gross sandstone ratio of 65 percent. Net sandintervalsaverage 26 percenteffectiveporosity and rangefrom13to36percent.
Lithologiczone To-2 containsanaggregatesandthicknessof 400 feetin16bedsgreaterthan 5 feetthick. The averagebed thickness is about 25 feet, with onebed of 45andoneof 70 feet inthickness. The nettogrosssandratio is 68 percentwithnet intervals averaging 18 percent effective porosity and rangingfrom 10 to 25percent.
Lithologic zone To-3 contains 230 feet of sandstonein 18 beds greaterthan 5 feetthick. The averagebedthickness is about13 feetwith a maximum thicknessof 30 feet. The nettogrosssand ratio in zone To-3 is 83percent. Net intervalsdisplayanaverage effective porosity of 16 percent and rangefrom12 to21percent..
EOCENE OR OLDER SANDSTONES
In both COST wells, the Eocene or older sandstones occur at depths where reservoir quality has been severely reduced by the compactionofductilegrains. At depthsbelow9,000feetinthe COSTNo. 2 well, visual estimates ofeffective porosity (mesoporosity) frompetrographic analyses of conventionalcores are less than 2 percent (AGAT, 1982,p. D-281-PI).Measured permeabilitiesfrom conventional cores from both COST wells do notexceed 10 millidarcies belowabout9,000feet. The rapidreductionofporositywith depth is shown in figure 24. The lowerrightenvelope of the porosity trend, which represents the maximum porosities that are likely to be encountered, passes below 20 percent at about 8,500 feet. As indicatedintheanalysisofporosityversuspermeability in figure 23, thisprobablyrepresentsthe point below whichreservoir quality is likely to be poor.
However, the bulge in the porosity trend below 12,700 feet, and the anomalouspoint at 10,900 feet, suggestthatthere may beexceptionstothisporosity-depthtrend. Such offsetsin
82 I I I I I I
5 IO 15 20 25 30
Porosity(percent)
FIGURE~~.Plot of porosityversuspermeability for conventionalcore 3 (7020.6to7047.5 feet) , NortonBasin COST No.2well.
83 porosity-depth trends are generally a result of secondary porosity enhancement.Suchenhancement is thoughttobeattributableto the dissolution of chemically unstable grains and cements by organic acids generated by decarboxylation during the thermal maturation ofkerogen (Surdam and others,1984).Alternately,thehigher apparent porosities seen below 12,700 feet may be in part due to overpressuring,spuriousreadings fromwashed-outzones in the borehole,andpossiblegaseffects frommethanegeneratedfrom interbeddedcoal.Coremeasurementsandpetrographicanalyses of sandstones from two conventionalcorescut in this interval actually indicated very low porosity andpermeability (AGAT, 1982, p. D-281-10, 11).
The 20-percent-porosity point at 10,900 feet appears to represent a 10-foot-thicksandstone with significant reservoir quality. A good drillingbreak was observedthrough the interval and thesandstone was described on the drill cuttings log as being "very well sorted"with"very good porosity." The SP logdisplayed a deflection through the interval whichsuggeststhatthesandstone is permeable.Petrographicanalysesofsidewallcoresfromthis sandindicatedfairamounts (6 to 8 percent)ofeffectiveporosity, much ofwhich was the result of the dissolution of volcanic rock fragments (AGAT, 1982,p. D-281-SC-5). Measurements on these samplesindicated porosities of 23 to 25 percent and permeabilities of 48 to 147millidarcies.Ifthesesidewallcoremeasurements are valid, as thepetrographic analysis suggests, andnotdue to fabric disruption from thecoringprocess,thenthere is fair to good reservoir rock present at a depth over 2,000 feet below that predictedbythegeneraltrendoffigure 24. Ifthese same conditions obtain elsewhere in Norton Basin, particularly in areas where more ofthiscleanersandstone is present,thensignificant volumesof prospective reservoir rock could be present at depths within or in closer proximity to the oil window.
HYDROCARBON SHOWS
Tracesof free oil in the drilling mud were reported in the COSTNo. 2 well between11,800and 13,000 feet.In this same interval, an increase in backgroundgasand numerous gaspeaks were recorded on theExlogFormationEvaluation Log (mud log)and yellow-orangefluorescence"pops" were reported in cuttingsbetween 11,820and11,830feet. In thepossibleturbiditesequenceofthe COSTNo. 1 well, bright-yellowto gold fluorescence and fluorescence cuts were reportedfromabout 5 percent of the drill cuttings in severalintervalsbetween10,200 and11,200feet.
Themost interesting show consisted of residual oil saturations of 14 to 18 percent in conventionalcore 9 (12,212to12,241.5feet) fromthe COSTNo. 2 well. This coresampledthevolcaniclastic conglomerateandsandstonesequenceoflithologiczone Te. Two drilling breaks and gaspeaksanorderof magnitudelarger thanpeaks elsewhere in the well were recordedbetween12,180and12,290feet.
84 SANDSTONE POROSITY (percent) IO 20 30 40 50 60
+I \ * \ I e;.. . I . . '\
I . . '/ 1,000 I 1 */
FIGURE24.Distribution of sandstoneporositywithdepth,NortonBasin COST No.1 andNo. 2 wells.PorositycalculatedfromCompensatedFormationDensity log dataandconventiovalcoremeasurements. m Ln /
+ EensityPorosity e/ Norton COST No. I well I . @ CorePorosity DensityPorosity Norton COST No.2well I I CorePorosity 3 I
I
1 .\*\ ++ .. +%+I -’
tribution of sandstonePorositywithdepth,NortonBasin COST ~0.1 wells.PorositycalculatedfromCompensatedFormationDensity log Dnventiovalcoremeasurements. These indicationsof good porosity and permeabilitywerenot corroboratedbythecoreanalysis,whichmeasuredpermeabilities of less than 0.1 millidarcy and porosities of only 7 to12percent. However, thewirelinelogssuggestthatthecore may havesampled an interval of less prospectiverockbetweentheintervalswhere the drilling breaks occurred.
These two prospectivezoneswereevaluatedbywellloganalysis. Each ofthe intervals displays an annulus invasion profile on the DualInduction Log.The annuluseffect is detectedby a higher resistivity reading on thedeepinductioncurvethan on the medium induction curve, which is significant because it generallydenotes hydrocarbonzones (Asquith,1982,p. 4).
A Hinglecrossplot(Hingle,1959) was constructedoverthe intervalcontainingthese two prospectivezones(fig.25).Thistype of crossp~ot is usefulfor determining the water saturation of a reservoir (SW) where theformationwaterresistivity (RW) is unknown. An RW of 0.045ohm-metersand a sandstone matrix density of about 2.72 to 2.73grams percubiccentimeterweredeterminedfromtheplot. The sandstone matrix density obtained from the plot agrees well with several grain density values of 2.71 to 2.72gramspercubiccentimeter obtainedfromananalysisofcore 9 from theinterval. The close correspondenceobtainedfromthese two disparatetechniquesprovides a positive check on the validity of the data from the crossplot.
Water saturationsoflessthan40percent (60 percent oil saturation) are indicated by theHingle plot for the upper parts of theprospectivezones. The lowestwatersaturations on theplot (thosebetween 15 and25percent)areprobablynotvalid,however, becausethesevaluesweredetermined from a badlywashed-outinterval in theborehole(which may alsorepresent a coalyinterval). The porosities in the washed-out interval weredeterminedfromthe Borehole Compensated Sonic Log,which is less affected bybadhole conditionsthan the density log, but nevertheless probably could notcompensateforthegreaterthan20-inchhole size of thecaved interval.
Permeabilities in theprospectivezones were estimated by plotting porosity versus water saturation on theSchlumbergerchart (1972a) shown in figure 26. The permeabilityvaluesplotapproximately parallel to the hyperbolic, bulk-volumewaterlines,whichindicates that the reservoir intervals are at irreducible water saturation. Irreducible water saturation, a condition in which all water is adsorbed on grains or held by capillary pressure, is necessaryfor obtaining valid permeability values from the chart (Schlumberger, 1972a). A few of thechart-derivedpermeabilityvaluesforgas approximatethepermeabilitiesmeasuredfromcoresobtained at correspondingdepths(upperleftpoints,fig.26). However, the points plottingto the right ofthe 5-millidarcy gas and 50- millidarcy oil permeability line are a washed-outzone,andthese highpermeabilityvaluesareprobablynotvalid. The singlecore
07 RESISTIVITY
I I I I
Porosity 0% IO% 20% 30%
FlGURE 25. Watersaturationdeterminations ofa suspectedoilzoneat 12,133to12.300feet.NortonBasin COST No.2well.Hingle(1959) crossplotofdeepinductionresistivity (RILD) versus density porosity (chartafterHelander,1983,fig.9-25;cementationexponentm=2.0).
88 PERMEABILITY:SANDSTONES,SHALYSANDS *
0
FIGURE26. Chartofporosity ($) versusirreduciblewatersaturation (swi)forestimatingpermeabilityanddeterminingbulkvolume water(c=sw x 8) forNortonBasinCOSTNo. 2 well(depths 12,182to12.218feet).Numbersinparenthesesadjacenttoplotted pointsindicatemeasuredpermeability(forgas)fromconventional cores(chartafterSchlumberger,l972a).
89 permeabilityfrom thiszone was much smaller (0.02 millidarcy) thanthechartvalue(greaterthan 400 millidarcies).Nevertheless, the remaining values on the chart appear to be reasonably accurate and suggestpermeabilities (for oil) as high as 40 millidarcies. If this analysis is valid, it notonlyindicates a significant show of liquid hydrocarbons, but also documents the occurrence of fair quality reservoir rock at depthswherefigure 24 predicts the occurrence of only very poor reservoirs.
90 8
Organic Geochemistry
SURFACE HYDROCARBONS AND INITIAL EXPLORATION
Traces of hydrocarbons have been reported in the vicinity of Norton Sound sincetheturn of thecentury(fig.27). Two test wells were drilled in 1906 in the coastal plain east of Nome at HastingsCreek,and two more were drilledin 1908. They werereported tohavebeendrilled on an oilseep. One oftheearlierholes encounteredgas at 122 feet which nearlyblew the drill stemout of theborehole, and theother produced a traceof oil. None of these wells were drilled much deeperthan 200 feet. The gasencountered was believed to havebeenmarshgas.
Oil seepshavebeen reportedintheSinukValleynearthe junction of theSinuk and StewartRivers,about 20 milesnorthwest of Nome, and nearthe mouth of the Inglutalik River, which drains into Norton Bay. Severalexploratorywellsaresaidtohavebeen drilledin the Sinuk Valley, but there is no record of the results. Miller and others(1959) state that oil shale has been reported to occur on BesboroIsland,northwest of Unalakleetin Norton Sound. These earlyreports remain largely unconfirmed.
In 1976, a submarineseepofnaturalgas was reported approximately 24 milessouth of Nome by Cline and Holmes (1977). They described a plume of C2 to C4 alkane-richwater advecting north from a pointsource on theseafloor. This plume reachedthe coast as far east as Cape Nome, butgenerally spread westward past Sledge Island to at least 168' W longitude(fig.27).
Clineand Holmes (1977)suggestedthatthesegaseswereof thermogenicorigin on the basis of thehighly localized nature of the hydrocarbonsource, the relatively lowethane-to-propane ratio (C2/C3), theproximitytounconformable,truncatedstratathatdip basinwardfromtheseeplocus,andthepresence of seismicallydefined acousticanomalies and numerous steeplydippingfaultsintheimmediate vicinity. Subsequentanalysesofseafloorsedimentsrecovered from vibracores in the vicinity of theseepplaced its locationabout 30 milessouthof Nome (Kvenvolden and others,1979;Kvenvolden and Claypool,1980).KvenvoldenandClaypool(1980)concludedthatthe
91 W N
EWARDPENINSU
H-W.C 2.S(Rp0.92%)
OVERMATURE CELLULOSIC KEROGEN
I .36% C 5(R,=I .S6%) 64 +
+ COST Claypool. 1980) No. I N 0 ,q T 0 N A 0. 38% W-C.H 3.~to 3 No. 2 .0.37% H--W-C 3- to 3
+ Dry hole.abandoned 0.10% W-";-jl 2 to 2+ 0 Well with gas Shows.abandoned 0 Reportedindication OP seep a Reportedoil Shale or petroliferous POI -I Source rock samoies - MMS (TOC. KEROGEN'TYPE. TAI) H HERBACEOUS W WOODY C COAL OP COALY v Source rack samples -~USGS (FromFi.sher.1982)
FlGURE27. A partialcompilation of organicgeochemistry from NortonSoundandthe adjacentarea. hydrocarbon compounds present wereprobablythermogenicbased upon the followingevidence:
1) The ratioof Cl/(C2+C3) is lessthan 20 at sedimentdepths greaterthan 50 centimeters.
2) 613C PD for onemethanesample is about -36 partsper thousan3(seeBernardandothers(1977)forinterpretative geochemical model).
3) About50 gasoline-rangehydrocarbon compounds (C5 to C8) were positivelyor tentatively identified and three of five sampleshave C7 compositionsthatresembletheimmature, nonmarineKenaicondensate. The other two samplesyielded C7 compositionsthatresembleboth immaturenonmarine condensate andbiodegraded oil from theGulfofMexico.
The hydrocarbon componentscomprisedonly 0.1percentofthe above methanesample. theremainderconsisted of about 98 percent carbon dioxidewith 6i3C PDB equal to -2.7 partsperthousand(per mil). Kvenvolden and Claypool(1980)suggestthatthe C02 may havebeen producedby the decarbonization of carbonate in basementrocksby heat or fluids, and thatthe C02 actedasanextractionagentas it migratedthroughthesedimentary columnand transportedhydrocarbons tothesurface.
SURFACEEXPOSURES OF POTENTIAL SOURCE ROCK
Samplesof potentialsource rock fromexposedsediments surroundingNorton Sound havebeencollectedbythe USGS (Fisher, 1982)andtheMinerals Management Service and AlaskaDepartment of NaturalResources(Lyleandothers,1982).
FivePaleozoic to MesozoicagesamplesfromSt.LawrenceIsland (Fisher,1982)yieldedthermalalterationindices(TAI)rangingfrom 1.6 to 3.6,whichcanbeinterpretedas a maturationrangeofimmature toverymature(Bayliss andSmith,1980).Fisher(1982)suggeststhat local heating by igneous activity may have produced a complex thermal historyforthese rocks. Only two shales containing woody-herbaceous kerogenweresampledandtheycontainedabout 2 percentorganiccarbon andyielded no more than 708 parts per million (ppm) total C15+ extractablebitumen,probably from coal macerals. Two thermally immaturelimestonesamplescontainedlessthan 0.3 percentorganic carbon and a single mature dolomite sample contained 0.4 percent organiccarbon and439 ppm C15+ total extract.
ThreeofthefourTertiarysamples from St. Lawrence Island containedcoalwithtotalorganiccarboncontents (TOC) ashighas 82percent(Fisher,1982). The maximum amount of CIS+ bitumen from thesesamples, 489 ppm, constitutes a pooranomaly (Baylissand Smith,1980). A calcareoussandstonesample was immatureand
93 containedonly 0.4 percenttotalorganiccarbon. The Tertiarycoal samples tended to be moderately mature, with a mean vitrinite reflectance (R,) ofabout0.6percentand TAI values no greater than 2.
The limitedpublicinformationfromSt. Lawrence Island indicates that the organic material is composed predominantlyofhumic,gas prone kerogen characterized by lowamountsoforganiccarbonexcept where coal is present. The thermalmaturity is variable,depending, at least to some extent in theolder rocks, upon theproximityof igneousactivity.
o -samples of probable Tertiary age from the Seward Peninsula
/~in the vicinity~~ of the Sinuk River were analyzed by USGSthe (Fisher, 1982). One was a sandstonewith a TOC of 0.37 percent and an Ro of0.86percent,theother a coalsamplewith 29 percent TOC andan % of0.92percent. The organicmaterial in thesesamplescanbe classified as mature humic kerogen, possibly capable of generating gas.
The USGS sampled a Paleozoiclimestone at Cape Denbigh that contained 1.36 percent TOC andproduced 105 ppm C15+ total extract. It containedcoaly material withan Ro of1.86percent.Several OligocenecoalsamplesfromnearUnalakleetyielded Ro values of about0.3percent(Fisher,1982).Thesecoalsappeartoberather insignificant in total volumeand are so poorlyexposedthatthey are often difficult to locate in the field.
Considerably more samplinghasbeenperformed on the east coast ofNorton Sound, particularly fromsequencesof late Earlyandearly Late Cretaceousrocks(Patton,1973)which are exposed in the vicinity of Unalakleet. Woody andherbaceousgas-pronekerogenand small amounts of coal arepresent in the clastic sediments. The USGS analyses (Fisher, 1982)produced totalorganic carbon values from0.31 to 1.87 percent,excludingcoalandcoalyshale. MMS analysesfor TOC from similar samples in the same area rangefrom 0.05 to 7.88 percent, with coaly inertinite present in a fewsamples (Lyle and others,1982;unpublished MMS data). Almost all samples collected from this region contain less than 1.0 percent TOC and mosthave less than 0.5 percentorganiccarbon.Samplesprocessed forsoxhlet extraction produced less than300 ppm total extract when coal was present. The USGS evaluatedthe thermal maturityof theoutcropsamplesfromthe east coastofNorton Sound using both Ro and TAI values and also evaluated a fewsamplesby Rock-Eva1 pyrolysis.Minerals Management Servicesamplesfromthe same area were evaluatedusing TAI. Samplesrangedfromimmature toseverely altered on the basis of TAI values, but mostcouldbeconsidered capableofproducingeither "wet" or"dry"hydrocarbongasdepending upon thekerogentype. However, Fisher(1982)noted a discrepancy between Ro values and TAI values for some samples collected along theeastcoast ofNorton Sound.Most of the USGS valuesranged from about 2 to 3 percent(very mature to severely altered kerogen),
94 in contrast with TAI values ofapproximately 2 to 3 (moderately maturetomaturekerogen). The USGS checkedsevenof theirsamples by Rock-Eva1 pyrolysis and the results comparedmost favorablywith the Ro data. The temperatureat which maximum pyrolytichydrocarbon generationoccurs, T2-max 'C, rangedfrom475 to 530 "C. Ifthese highervalues are correct, it is likelythat only dry gascould surviveintheserocks.
During1959and1960, a test well was drilled by Benedum and Associates in the Yukon-Koyukuk clastics at Nulato,about 80 miles east of Norton Bay on the Yukon River. The sedimentaryrocks of thisprovinceweredescribedbyPatton(1973)aspredominately graywackeandmudstone. The visualkerogen and maturationanalyses of the wellsampleslaterconducted byMobil ExplorationandProducing Services,Inc., are available fromtheAlaska Oil andGas Conservation Commission. All well cuttings to a depth of 11,700 feet below the kelly bushing (elevation 865 feet) contained largely "cellulosic" gas-pronekerogen.Nearly all vitrinite reflectance measurements are in excess of 3 percent and theorganicmaterial is considered tobeovermature. No oil or gas was reported.
Clasticrocks of the Yukon-Koyukuk provinceareexposedeast of Norton Sound. Seismicstudieshavenotresolvedtherelationshipof these deformedrocks to the metamorphic"basement"complexof either of the NortonBasin COST wells.Cretaceous and Tertiarysediments whichcropout on thecoast may actuallyhavebeenexposedtohigher temperaturesthanequivalentrocks in the NortonBasin COST wells. The location and apparentshapeofthe Yukon-Koyukuk province,as wellasthetrend(220degrees) and plunge(16degrees) of folds in the vicinity of Unalakleet(unpublished MMS data),suggest that substantial amountsof Yukon-Koyukuk sediments are notpresent beneathNortonSound,exceptperhapsintheextremesoutheast corner.Moreover,theapparentlack of organicmaterial and the high level of thermalmaturity on the marginof the sound make these sedimentsanunlikelysourceofoilfortheNortonBasin.
OFFSHORE STRATIGRAPHICTEST WELLS
In 1980,the first NortonBasin COST well was drilled in the St. Lawrence subbasin. This was followed in 1982by a second COST well in theStuartsubbasin.Thesedeepstratigraphictestsprovided the first opportunity for a detailedexaminationofthesubsurface geology of Norton Sound and for a reasonableassessmentofthe petroleumpotential of thebasin. The locationsofthese wells are indicated on figure 27.
COST No. 1 Well,St. LawrenceSubbasin
Inthe COST No. 1 wellin the St. Lawrence subbasin,geochemical analyses wereperformedbyCoreLaboratories,Inc.,andby Geochem Laboratories,Inc. From thefirst sample (240 feet)toabout12,000
95 feetbelow the kelly bushing (elevation 98 feet), the total organic carboncontent (TOC) generallyrangedbetweenabout0.5and 2.0 percent(fig.28). TOC values in excessof 2.0 percentoccurmost frequently below 9,000 feet and are generally associated with the occurrence of coal. The kerogen is composed mostlyofgas-prone woody, coaly,andherbaceous materials. Examinations of theorganic material by petrographic microscope in reflected light indicate that at least 50 percent of thekerogen in mostsamplesbelongs to thevitrinitegroup(fig.28). The hydrogenindex (S2/TOC) from pyrolysis does not exceed 300 milligramshydrocarbonper gram TOC and is generally less than100milligramsper gram. haces of coal are generallypresentinsamplesproducingthehigherhydrogen indicesinfigure 29. Between 12,000and13,000feet,there is an increase in the coal content of thesamplesand a corresponding increase in TOC and the proportion of inertinite (up to 80 percent). At 13,000feet,thehydrogenindexdropssharply,presumably in responsetothe lowhydrogencontentoftheinertinite. From 13,000 feet to14,683 feet (T.D.), the TOC is less than1.0 percent and most TOC values in the interval are less than 0.5 percent. Vitrinite and inertinite compose at least 90percentofthekerogenandthehydrogen index is negligible(fig.28). Metamorphic phyllite and schistare presentin samples from depths greater than approximately 14,500 feet.
The contact between theprobable Eocene and the possible Paleocene occurs at about 12,200feet in the COST No. 1 well. As shown in figure 30, this depth also corresponds to an abrupt, discontinuous increase in the Ro profile (random mean vitrinite reflectanceversusdepth). The increaseintheage of therocks, thesuddenappearanceofcoal,thechangeinthecompositionofthe kerogen,andthe marked discontinuity in the Ro profile (fig. 30) combine to suggest the existence of a significantunconformity at. this depth.
Thereappears to be fair agreement among the several indicators of thermalmaturityinthe well. On the basis ofthe criteria of Tissot and Welte (1984)and Hunt (1979),the current oil generative zone in the COST No. 1 well extendsfromapproximately9,500feetto 12,500feet. The random mean vitrinite reflectance varies fromabout 0.6 to 1.3or 1.4 percentand Tz-max "C from Rock-Eva1 pyrolysis rangesbetweenabout 435and460 'C (fig.30). Below 12,600feet, however, there is no senseofaccord among thevarious indicators ofthermalmaturity. Random mean vitrinitereflectance values, in particular,tendtobehigh and erratic. It is probablethat sediments are approachingmetagenicgradeby13,500feet(anof approximately 2.0 percentaccordingtoTissot and Welte, 1984).
In the COST No. 1 well, theindicators of maturity based on kerogen analysis do not agree perfectly with those based on heavy orlight-hydrocarbonanalysis. Radke and others(1980)observed that in coals, the odd-evenpredominanceofn-alkanesfrom C15+ extracts, as expressedbythecarbonpreferenceindex (CPI), exhibits
96 ~~
I 900-
800.
700
600
F v vo x I- mcn 500 E
X u 0 c -c 400 u 0) Cuttings 0 L 0 A Sidewall Core a I 0 Conventional Core 300 Note:Depth of 8smples in feet.
200 9158 A 20 I
2420 I880 10740 IO0 0 .I 1280 )o,6tz6O 02460 16821
-A 0 I I I 1 25 50 75 I bo I25 I 50 Oxygen Index mg COz 9 TOC
FIGURE 29. ModifiedVanKrevelendiagram,COST No. 1 well,St.Lawrence subbasin.(Pyrolysisanalysesperformeduponrepresentativesidewallcore samplesbyGeochemLaboratories,Inc.)
99 a gradient change between vitrinite reflectances of 0.9 and 1.0 percent.Theirdataactually become asymptoticat a vitrinite reflectanceslightly above1.0percent. The peakof oilgeneration for a type 111, humic kerogenshouldoccurataboutthis Ro level (Tissot andWelte,1984). As shown infigure 30, the CPI is becoming asymptoticatabout9,500feet,whichcorrespondstoan Q of roughly 0.6 percent. The ratio of C15+ extractablehydrocarbonsto TOC begins to increase at a depth ofaround9,000feet and thewetness of theheadspacegasbeginstoincreaseataround 8,000 feet.This suggeststhatthesehydrocarbons may havemigrated,atleast from thecoals within the catagenetic zone, if not from some more distant source.
Briefly,the COST No. 1 well in the St. Lawrence subbasin penetrated a sedimentarysection that is dominatedbyhumic,type 111, gas-pronekerogens. The totalorganiccarboncontent is generally low. At depthsgreaterthan13,000feetthere is no indication that sufficient hydrogenand reactive carbon have been available to generatesignificant quantities of hydrocarbons. The current oil window occurs between approximately 9,500 and 12,500 feet, encompassinganOligocenetopossiblePaleocenestratigraphicsection. Biogenicmethaneappearstobepresentabove 6,000 feet and traces ofthermogenichydrocarbonsarepresentatdepth,but no significant oil shows wereencounteredinthisdeepstratigraphictestwell.
COST No. 2 Well,StuartSubbasin
Analyseswereperformed on samplesfromthe COST No. 2 well by RobertsonResearch (U.S.), Inc. As inthesection on the COST No. 1 well, all depths referred to in this chapter weremeasuredfrom the kellybushing(elevation105feet).
Organiccarbon is generallyabundantthroughout mostof the sedimentarysection penetrated by this well, especially in the coal- bearing intervals from3,500 to 4,600feet, 6,400 to 8,600 feet, and 12,200 to 14,000 feet(fig. 31).
The kerogenrecoveredthroughoutthe well is composed predominantlyofhumic,type 111, gas-pronemaceralsprobablyderived from terrestrialsources(fig.32).Examinationofthiskerogenin reflectedlight with the petrographic microscope revealed a zone between 9,500 and 12,000 feet that contained relatively higher percentages ofamorphous organicmaterial(fig.31). However, the hydrogenindexfrom Rock-Eva1 pyrolysisdoesnotcorrespondtothe petrographicanalysisforthisinterval.Elementalanalysisofthe kerogentendstosupport the pyrolysisdata(Turner and others, 1983b).
There areseveralpossibleexplanationsforthehigher petrographicestimatesof amorphous organicmaterial. Amorphous kerogen is generally assumed to be composed ofsapropelic,
103 hydrogen-richorganicmaterial(Hunt, 1979). Such kerogenshould havehydrogenindicesgreaterthan 100 milligramsper gram(mg/g) provided it hasnotbeenexposed to high temperatures for any significant period of time. Humic material can be altered mechanically orchemicallytothe point that it may resemble amorphous kerogen. Dow (1982) suggeststhat at least part of the "amorphous" fraction ofthekerogenfromthe well is actuallydegraded, or finely divided, vitriniteor low-yield, oxidized amorphous material withverylow oil-generatingcapability.Tissot and Welte (1984) pointoutthat amorphous facies can result from severe alteration of the original constituents, whichthenlosetheiridentity.Flocculated humic acids derived from the colloidal state havean amorphous appearance underthemicroscope. The elementalanalysis of such material would fall along the type 111 evolutionpath, but its appearance underthemicroscopecouldlead it to be classified as sapropelic materialwith good oil potential.Finally,thetransformationratio, Sl/(Sl+S2) (Tissot and Welte, 1984). which indicates the extentto which the full genetic potentialof the original kerogen has been realized,providesanadditionalpartialexplanationforthereduced hydrogenindices(fig. 31). At around 10,000 feet, some ofthe availablehydrogen is beginningtooccur in the formof light, volatilehydrocarbonsasthecatagenetic zone is approached. Rock-Eva1 pyrolysisdetects this hydrogen as S1 ratherthan as S2, fromwhich thehydrogenindex is calculated (H.I. = S2/TOC). It seems reasonable toconclude,therefore,thattheorganicmaterialanalyzedfromthis well is nearly all type I11 humickerogenandcoal,ratherthan amorphous, sapropelic material.
From 12,000 to about 14,000 feet, vitrinite group macerals become thedominantkerogentype and thehydrogenindex is low (less than 100 mg/g). TOC values in thisinterval are erratic, andrange as high as 55 percentwherecoal is present in a sample(fig. 31). From 14,400 to 14,889 feet (T.D.),metamorphic lithologies such as schist andmarble are present in thesamples. The organiccarbon ofthemetasediments is very low. The geneticpotential,that is the amount of hydrocarbons that can be generated during burial, is also very low.
The indicators of thermalmaturity (fig. 33) are in better agreementwithoneanother in the COST No. 2 well than in the No. 1 well; nevertheless,thereremaininconsistenciesthatrequire explanation.Becauseofthelargeamountsofvitrinite in the No. 2 well, R,, valuesoughttobeveryreliable. They suggestthatthe onsetof oil generation for type 111 kerogenshouldoccur at about 10,700 feet and that the oil window extendsto perhaps 14,000 feet. The othermeasures of thermalmaturityimplythatthethresholdfor oil generation may benearer to 10,000 feet (fig. 33). When Ro valuesexceed 2.0 percent,dry gas becomes the most likelyhydrocarbon product(Hunt, 1979; Tissot and Welte, 1984). The profileprojects to 2.0 percentatabout 15,300 feet. It hasnotbeenprecisely determinedto what depthsdrygascansurvive,butthemetamorphic basement at approximately 14,500 feet in this well clearly sets a limit at this location.
104 000
IndividualData Points 800 Gl Two CongruentDataPoints
7 00
0 600 0 I
W x 500 m P -C C m 400 0 L P a. I 300
8
200 I.. I...... 100 I.." k 0
107 The Ro gradientbetween 8,000 and 12,100 feet appears to be anomalouslyshallow. That is, thevalues do notincreasewith depth at the same rate at whichtheyincreasethroughouttheremainder of the well, exceptfor a few samplesabove 3,000 feet which seem tocontainreworkedmaterial. It is possiblethatcoal, which is extremelyfriable,mighthavecontaminatedsamples in the 8,000- to 12,100-footinterval andreducedthe mean values of the vitrinite reflectance.Althoughthis may appearunlikelyinviewofthefact that some of theanalyseswereperformed on sidewall and conventional coresamples, Hunt (1979)cautionsthatsidewallcoresaregenerally taken after a well is drilled to total depth and the formationshave beensubjected to the high pressures of circulated drilling mud. Drilling mud with diesel or crude oil in it, orpossibly even fine coal particles in this instance, may haveintroducedcontaminants that were thensampledbythesidewallcores.
There is also a discontinuousincrease(fromaround0.7to1.0 percent) in the Ro profileatapproximately12,100feet. An apparently correlative anomaly is present on a profile of thespore colorationindexatabout11,900feet(Turner and others, 1983b; DOW, 1982). Dow (1982)suggestedthatthis anomaly may represent the Mesozoic-Cenozoicunconformity firstpostulated by Fisher (1982). andmight entailseveralthousandfeet of erosion.Paleontological evidence,however,indicatesthatTertiarysedimentsoccurtoabout 14,500 feet at this site.
The second drilling run was completed at 12,000feetofdepth and thehole was cased.Thisprobablystoppedcontaminationfrom lithologiesabove 12,000 feet and may explain a part of the discontinuousincrease in Ro values,but it doesnotappeartoaccount fortheentireincrease.Inaddition, the F&, gradient,orrate of of Ro increase with depth, is greaterfor the lithology below 12,100 feetthanforanyotherpartofthestratigraphictest.
The caving hypotheses can be tested in a preliminary way by constructing a Lopatin model to compare calculated levels of maturity based upon the current temperature gradient with observed Ro values from the COST No. 2 well. Lopatin (1971) suggested a method for applyingthe Arrhenius equation to calculate a coefficient he called thetime-temperature index (TTI),whichserves as a measureof thermal maturity. It corresponds to random vitrinite reflectance (R,) or to Staplin's(1969)thermalalterationindex (TAT). It canbe demonstratedfromtheArrheniusequationthattherateof a chemical reactiondoublesforevery 10 OC increaseintemperatureif the activationenergy and thetemperature of the reaction are relatively low (Tissot and Welte,1984).Becausesediments become progressively hotter upon burial, an interval maturity can be calculated by multiplyingthe time ofresidence,AT,in a temperatureintervalof 10 OC by a factorof 2", whichchangesto 2"+l,2n+2...2"max every 10 OC. Lopatinarbitrarilyassigned a value of 0 to n forthe temperaturerangefrom 100 to 110 OC. The total sum of theinterval maturities is termed thetime-temperatureindex,or TTI.
111 nmax
TTI = (AT,) 2" %in
Tissot and Welte (1984) feel that this model does not adequately take into account the consequences of higher temperatures and greater activation energies that are believedbetoinvolved after the peak of oil generation has been reached, the point after which thermal cracking becomes the predominate mechanism. However, uncertainties regarding the correct temperature gradient, the possibility that the temperature gradient has changed through time, the absolute ages of the sediments, the timing and amount of tectonic activity,and the composition of the organicmaterial probably present muchmore formidable problems than do chemical kinetics. For example, there is frequentlyno adequate drilling history from which a correction can be computed forraw temperature data taken from a drill hole. Waples (1984a) has pointed out that despite theoretical objections, the Lopatin modelis simple to work with and thatthe results of many applicationsto exploration have demonstratedthat it generally works well (Falvey and Deighton, 1982; Magoon and Claypool, 1983; Middleton and Falvey, 1983; Ibrahim, 1983; Bachman and others, 1983; Issler, 1984).
To apply the Lopatin model it is necessary toreconstruct the depositional and tectonic history for the sedimentsbeing studied. This has been done for four horizons located at 3,600,6,900, 10,700, and 14,000 feetin the COSTNo. 2 well (fig.34). The estimated absolute ages (in millions of years before present) are given in4. table
Table 4. Depths and absolute ages of sedimentsin the Norton Basin COST No. 2 well used in the Lopatin computation.
Depth (feet)
3,600 24 (late Oligocene) 6,900 30 (early Oligocene) 10,700 40 (late Eocene) 14,000 60 (late Paleocene)
Waples (1984b) considers the greatest source of errorin time- temperature modeling tobe poor temperature data. Because drilling fluids alter the ambient rock temperature,a correction mustbe made to the observed temperature measurements. Bottom hole temperatures taken at the end of each drillingrun were corrected using a technique based upon the observation that the temperature rise after circulation has stoppedis similar to static pressure buildup and thus bemay analyzed in a similar manner (Fertl and Wichmann,1977). In practice, this technique yields good estimates of true static formation temperature except when circulation times are in excess24 hours.of After static bottom hole temperatures were calculated from each logging run, aplot of the temperature versus depth was constructed
112 c - ; (DQ. m>,D - I- I-
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O 8 8 8 8 @ 9 9 9 9 9 9 9 9 9 ^. ... A I- ,,. L - ". D - N "l .+ usingboth corrected static bottom hole temperatures and two fluid temperaturesproducedby drill stem tests.Thisyielded a temperature gradient of 2.38 "F per100feet(43 OC/km) below5,000feet. At depths less than5,000feet,temperaturemeasurementstendtobe somewhat erratic,probably due toinhomogeneities in theless-compacted sediments.Uncorrectedhigh-resolutionthermometerobservations were usedfromthesurfaceto 5,000 feet. The correctedtemperature gradient computed forthe COST No. 1 well was 2.39 OF per 100 feet (44 OC/km). Conjecturesconcerningthetemperaturesatdepthsless than 5,000 feet are academic with respect to this Lopatin modelbecause thetemperaturesare less than 60 OC (140 OF), withtheresultthat 2" is lessthan0.03, a value that results in TTI contributionsof no more than 0.5.
Waples(1980and1984b)hascompared calculated TTI valueswith measured Ro data from many worldwidesamplesrepresenting a variety of ages and lithologies.Initialcomputations,based upon a temperature gradient of2.38 "F per 100 feet that was assumed to haveremained relatively constant for the past 60 millionyears of earth history, were compared with Ro values from the COST No. 2 well. These initial calculations resulted in TTI values that were much greater thanWaples'correlationssuggestedtheyshouldbe.Fisher(1982) believesthat the average temperature gradient in theNortonBasin hasvariedbetween 35 and45 T/km sincethebasin formed. If it is assumed thatFisher's minimaltemperaturegradient,1.93 OF per 100feet(35 "C/km) existed after the most active period of tectonic activity,that it increasedto its presentvalue of about2.38 OF per100feet(43 OC/km) rathersuddenlyduringthelast 5 or 10 million years, and thatsurfacetemperatureshavenotchanged radically, one can then construct the depositional model illustrated in figure34.
The mainzoneof oil generation,also termedthe oil generation window, has been identified by various authors using random vitrinite reflectance (Ro) (table5).
Table5. Randommean vitrinitereflectancevalues (Ro) forthe oil generationzone.
Author R, Range
Dow, 1977 0.6 to 1.35%
1977 Hunt, 0.6 to 1.35%
TissotWelte, and19840.5 to 0.77% Althoughthe limits for the oil generation zone vary slightly, there is a generalconsensusthatthelowerthresholdoccursatabout 0.5to 0.6 percent,orperhapsslightlylowerifresinite is present (Snowdon andPowell,1982). The maximum limit of the oil window is generallyconsideredtobeabout1.3or1.35percent. When Ro values 115 r r m Tablo 6. Relationship of TTI valuestohydrocarbon generation, COST NO. 2 Well (adaptedfromWaples, 1084b). Ro( %> Stages of Ro( %> Norton Sound TTI Waples, l984b COST No. 2 Well, 1982 HydrocarbonGeneration I 0.40 0.39 (6.900 feet) Resinite fromCondensate -3 0.50Kerogen S-Rich From IO' 0.60 0.54 i Early 15 0.65 20 0.70 10.700 feet) 0.5B2( I 50 0.90 Peak 75 I .oo -I 80' I .35 I .30' -32 I I .35 (I 4.000 feet WetGas C.UU DryGas ' Extrapolatedvalues.Seefigure 36. * The observed Ro value. 0.58%. appears to rellect coal Contamination from approximately 8,000 feet. The projected Ro value on figure 36 is nearer to 0.64% at a depth of 10.700 feet. exceed2.0percent,onlydrygascanbeanticipated.Thisvalue was notobservedinthe COST No. 2 well,but it canbeprojectedto a depthofabout15,300feet(fig.35).Thisimpliesthatevenif sedimentswerethickerelsewhereinthebasin,thethermalmaturity of thesesediments would probablyprecludethepreservationofliquid petroleum at these depths. Lopatin(1971) originally proposed that specific TTI values correspondtovariousstagesofhydrocarbongeneration. Waples (1980,1984b)modifiedthesethresholdvalues.Table 6 is a compilation of some ofWaples' (1984b) Ro measurementscorrelated with modified TTI values based uponnumerousworldwide geologic reconstructions. The TTI values from the COST No. 2 well (Stewart subbasin)areincludedforcomparison. The TTI values computed forthefourhorizonsinthe COST No. 2 well are 0.07 (3,600 feet), 0.98(6,900feet), 20 (10,700feet)and 321(14,000feet).Additionalextrapolatedvaluesfromfigure35 appear on table 6 toaid ininterpretation. Observed Ro valuesare generally somewhat lowerthan the correlative TTI valuesof Waples suggesttheyoughttobe. However, R,, valuestakenfromthe interpretive projection (fig. 35) are in much betteragreement though still slightly low. A TTI of 10 fromtheprojection correspondstoan Ro value of0.57 percent(9,700feet), and a TTI of 20 to 0.64percent.Thislends some supporttothethesis that caving may havedepressedthe Ro values between 8,000 or 9,000feet and12,000feet.Ifthis is correct,thethreshold for the oil generation zone would benearer to 10,000 feet than 10,700 feet and theoil generation zones indicated on figure 35 by the TTI and Ro values would be in relatively good agreementwith one another . The Lopatin model doesnotprovethatanunconformity exists at 12,100feet,butthepresenceofsuchanunconformity would not violatethethermalconstraints imposedby the model. Finally,the Lopatin model that produces TTI values which agree most satisfactorily with observedmeasures of thermal maturity such as vitrinite reflectancesuggeststhatthecurrenttemperaturegradienthas developed inthe last 5 or10 million years and that it may have beensignificantlylowerpriortothat time. Minoramountsofhydrocarbonswereobserved in Eocene siltstones and sandstonesbetweenapproximately10,000and13,000feetinthe COST No. 2 well. Dow (1982),reporting on analysesperformedby RobertsonResearch (U.S.) Inc.,statedthatthere was little direct evidence of migrated oil or gas, or ofsignificantpetroleum shows in anysampleanalyzed. A siltstone samplefromcore 9 (12,212.6feet) produced nearly4,000partsper million of saturate-richorganic extract resemblingnormalcrude oil in composition. A secondsample ofveryfinegrainedsandstonesuspected of containingliquid hydrocarbonsfromcore 9 (12,213.6feet)containedsolidbituminous materialidentified byRobertsonResearch (U.S.), Inc., asepi-impsonite 117 on thebasis of its Ro andelementalanalysis. Dow observedthat two samplesfromcore 10 (12,963.6and12,972.3feet)andonesamplefrom core 11 (13,404.9 feet) producedgrosscompositions,saturate-fraction gas chromatograms,andkey ratios similar to the samples fromcore9, buttheirlithologies were predominantlyshale. Their totalextract contents and extract to TOC ratios are markedlylowerthanthe sandstonesamplesfromcore 9 (fig. 30).Sample descriptionsindicate that cores 10 and 11 also contained significant amountsof coal (up to 25 percent). Dow (1982)concludedthattheextractablebitumen andhydrocarbonsoccurring in sandstones at around12.200 feet probablyoriginated in thedeeper(12,960to13,405feet),thermally matureshales. In the COST No. 2 well, thewetness ratio, (C+C+C&) x 100, (c1+c2+c3*4) ofheadspace gas begins to increase at a depth of10,000 feet; it reaches its maximum value(75.9percent) at about11,750 feet (fig.32). TheC15+ hydrocarbon/TOC ratioalsobeginstoincrease at about 10,000 feet andpeaks(0.088) at 12,212.9feet(core9). Predictably,thetransformationratio, S1/(S1+S2), alsobegins to increase at 10,000feet, and reaches its maximum value (0.882) in a sidewallcorefrom12,208feet(fig.31).Tracesof"free oil" were observed in the drilling mud between11,800and13,000feet and residual oil was reported from core 9 (seeReservoir Rocks chapter). Anomalously greater amountsof gas were recorded on the mud logbetween12,180 and12,290feet. Fluorescence of cuttings andof solvent cuts was observed between 10,200and11,820feet. Althoughminoramountsofgas, oil, and solidbitumen are present, the oil-generating capability of the potential source rock penetrated by COST No. 2 well doesnotappeartobeverygreat.Furthermore, there is little geochemicalevidencetosuggestthatthetracesof observedhydrocarbonshavemigratedtothis site from some unobserved source. Snowdon (1978)and Snowdon andPowell(1982)havereported naphthenic oils and condensates they believe to have been generated from terrestrially derived organic matter at relatively low levels of thermalmaturity.Specifically,theysuggestthattheresinite present in Tertiary coals of theBeaufort-Mackenzie Basin has produced hydrocarbons at an Ro level ofbetween 0.4 and 0.6 percent. Snowdon andPowell(1982) state thatbetween 5 and 15 percent resinite has beenobserved in samplesfromtheBeaufort-MackenzieBasinandthey rate sampleswith a minimum of 10 percent resinite as excellent source rocks on the basis of the criteria ofPowell(1978). In the COST No. 2 well, valuesincrease from approximately 0.54 percent at 10,000 feet to 0.74 percent at 11,900feetand jump discontinuously to about 1.0 percentbetween11,950and12,200 feet.Thesemeasurementsrepresent a rangeofthermalmaturities thatextend fromroughlythethreshold of oil generation to the 118 I c maximum or peakoil-generatingcapabilityfortype 111 kerogen (Tissot and Welte,1984; Radkeand others,1980). Random vitrinite reflectancevalues, however, arepredominantly low (0.5 to 0.7 percent)to a depth of 11,952feet. Ninecuttingssamplesfromthe COST No. 2 well thatcontained obviousvisualcoal were sentto P. DharmaRao of theMineralIndustry ResearchLaboratory,UniversityofAlaska,forpetrographicanalysis. With the few exceptionsnotedintable 7, Rao'spetrological descriptionsfollow the International Handbook of CoalPetrology (International CommitteeForCoalPetrology,1963, 1971, and1976) and Stach'sTextbookofCoalPetrology(Stachandothers,1982). The results of thesepointcounts on a mineral-freebasis are given in table 8. Five of theninesamplescontainanomalouslyhighresinite contents and a sixthcontains a significant amount of sporinite. Snowdon and Powell(1982)proposed a hydrocarbongeneration model which would include resinite in addition to liptinite (waxy oils) and vitrinite (methane)groupmaceralsfromterrestrialorganic matter.This model is reproducedinfigure36.Additional work is necessaryto identify the source of the hydrocarbonspresentinthe Stuartsubbasin.Becausetype 111 vitrinitickerogen and coaltend to be associated with dry gas rather than with condensate or oil, andbecausetheliquidhydrocarbons at this site occurinsediments exhibiting a relatively low level ofthermalmaturity, it is possible that resinites derived from coal,eitheraloneorincombination withsubjacentthermallymatureshales, may havegeneratedthe hydrocarbonsobserved in COST No. 2 well. SUMMARY The COST No. 1 wellin the St. Lawrencesubbasinencountered type 111, humic,gas-pronekerogenandlesseramountsofcoal. Biogenicmethane is presentin near-surfacesediments,butthere is little evidencetosuggestthatthermogenichydrocarbonshave formed in significant amounts at thewell site. The COST No. 2 wellintheStuartsubbasinpenetrated14,889feet ofsedimentscontainingpredominantlytype 111, humic,gas-prone kerogenandcoalwithanomalousamountsofresiniteand some other liptiniticconstituents.Significant amountsoforganiccarbon are present but are generally associatedwithcoal-bearingsamples. Minoramounts of gaseousand liquidhydrocarbonsplusbituminous materialare present which are thought to be derived locally, either frommature,humickerogenorfrom less mature resinite and liptiniticcoalmacerals. The currentoil window occursbetween 10,000 and 14,000feet. Methane,probablyofbiogenicorigin, is present in near-surface sandstones. Both COST wells penetrated significant thicknesses ofthermallymaturesediments. Tracesofhydrocarbonsand oil shale havebeenreported at various localities on themargins ofNortonSound, butexploratory drilling at these sites has not resultedin any significantpetroleum discoveriestodate.Tertiarysurfaceexposuresarerare andtend 121 Table 7. Maceralclassifi'cation for northern Alaska coals (from Rao, 1983). N N Low Rank Bituminous Coal Classification Coal Classification I Macrinite occursasmrphous gelified material Macera Maceral MaceralClass Maceral Maceral binding such macerals as sporinite enclosed Grour, SubprouDIbfacera11for this stud4 TYPe I within it. Macrinite can also occur as isc- I I I angularlated irreg- withorparticlesrounded ular shapes and distinct toundaries. lminit vltro- detm.. 2 Globular macrinite occurs as isolated spher elo- I ical particles or as an agglomeration of particles, that are usually associated with vitrinite and frequently display oxidation rim, dessicationcracks or differential ccmpaction.Theyarealsoassociatedwith semifusinite and can be found filling cell lumens. 3 Exsudatinite occurs as fillings of small cracks or partings within the bedding planes of the vitrinite, or as yell lumen fillings L in semifusinite and fusinlte. Exsudatinite in fluorescence light exhibitsa variety of color Classification applicable to all Coals from pale yellowto a bright goldor an orange Maceral Maceral Class Maceral !kcera1 Class gold. (;roup for this study Group for this study 4Thick cutinite occurs as wide, banded lenses with thick cuticular ledges, and are usually strongly folded. Sane thick cutinite exhibits multiple lavers. In flwrescent lisht thick a exsudatinite 3 cuticles emit a bright yellowcolor similar -u .d to thecolor of fluorescing alginite. F gouar .d 4J .-a G d .d inertodetrinite 6 Other liptinites include liptcdetrinites and sclerotinite other liptiniticmterials such as waxes, fats suberinite and oils thatcanmt be identified under one micrinite of the other liptinite classes. Table 8. Relativecompositions of coalmaceralgroupsandtheliptinitemacerals (mineral-freepercent). Maceral Group CuttingsSamples and Liptinite (depthbelow kelly bushing in feet) Group Macerals 3,040-7,070-8,060-8,090-9,170-11,600-11,700-12,250-12,910- 3,0707,1008,0908,1209,20011,62011,710 12,260 12,920 1.4 Inertinite2.6 3.6 Group4.7 7.8 ...... 8.7 .... 83.3 75.9 69.2Vitrinite 62.5 61.3 78.4Group 79.8 78.1 76.4 16.7 15.4 30.8 Liptinite37.5 37.3 19.0Group 16.6 17.1 15.8 25.0 10.7 5.0 Sporinite6.5 3.0 9.5 ...... 1.5 18.9 7.4 Resinite6.1 10.8 1.3 .... 23.1 14.0 13.6 Exsudatinite 2.54.9 2.02.5 1.8 ...... 1.0 0.6 Cutinite 0.3 0.2 ...... Alginite .... 0.3 ...... 0.2 ...... Suberinite 0.6 ...... 1.0 0.4 7.7 12.5 2.6 4.1 Liptodetrinite2.2 0.7 1.7 Vitrinitereflectance (%R&) .4 .5 6 .7 .9 1.0 1.1 1.2 1.3 *1.4 1.5 as-naphthenic condensate dry gas * gas as-paraffinic dry gas condensate LlPTlNlTERICH RESlNlTE POOR RICH VlTRiNlTE gas -dissolution in gas phase )) cracking FIGURE36. Hydrocarbongenerationmodelforoilandcondensatefrom sourcerocks containing terrestrial organic matter. Quantity and natureofultimateproduct is a functionofrelativequantitiesof resinite,liptinite,vitrinite.andfusinite in sourceorganicmatter, and Of level of thermalalteration(fromSnowdonandpowell, 1982). 124 tobe composed of coal when present. The Tertiaryoutcropsare thermally immature tomature. The CretaceousrockssurroundingNorton Sound tendtobeover-maturetometamorphicgradeandaregenerally wellindurated. Gas seepshaveproduced an extensive C2 to C4 alkane plume along thenorthcoastofNorton Sound. The plume originatesin the vicinity of the Yukon horst and is composed largely of C02 withminoramounts of lighthydrocarbonsat its source. Continuedexploration will benecessary to evaluate the true petroleumpotential of theNortonBasinarea. However, thesix exploratory wells drilled thus far have been disappointing. 125 -N u. >v) a 1 a FIGURE 37. Possiblestructuralandstratigraphictrapconfigurationsassociated withmajorTertiarysedimentarysequencesandpre-Tertiarybasement.Seismic horizons A through E aredepicted.PotentialtrapsintheGRABENFILLPLAYinclude (1) Clastic wedgesalonghorsts; (2) truncation of porousstratabyanunconformity; (31 faulttraps;and (4) compactionordrapeoverigneousmounds.IntheINTERIORSAGPLAYpotentialtrapsinclude(5) archinganddrapeorcompactionoverhorsts; (61 stratalappingontobasementorunconformities; (7) roll-oversassociatedwithnormalfaulting;and (81 porousstratasealedupdipbyimpermeable silicadiagenesiszone.PREGRABENBASEMENTPLAYSincludepotentialtrapsin (91 weathered carbonateorfracturedquartzite or othermetasedimentaryorcrystallinebasementrock. 9 Play Concepts Hydrocarbonaccumulations known from basins geologically analogoustotheNortonBasinprovide a basis for the prediction of thetypesofplaysinNortonBasin.Harding(1984)defined a conceptual framework for majorhydrocarbonoccurrences in extensional basins from a synthesis of theSirteBasinofLibya,the Suezand VikinggrabensofEgyptandtheNorthSea,andotherblock-faulted areas.Hydrocarbonaccumulationsinextensionalbasinscanbe placed in oneofthreemajorcharacteristicsedimentarysequences: pregraben, graben fill, and interior sag(fig. 37). Trapping mechanisms in all buttheupperpart of the interior sagsequence areprimarily a resultof structural, depositional, erosional, and compactionalprocesses associated with the multidirectional faults that bound horst-graben fault blocks. PREGRABEN BASEMENT PLAYS Rocks of the miogeoclinal belt, whichwere penetrated in the lowerpartsofbothNortonBasin COST wells, probably represent the lithostratigraphic sequence that would beencountered in pregraben plays. This sectionlargelyconsistsofschist,phyllite,slate, quartzite, andmarbleofPrecambrian toPaleozoicage. As these rockscontain no effectiveintergranular porosity, any reservoir porosity would betheresultoffracturingorweathering.These rocks are most likelythermallyovermature;anyhydrocarbonspresent would have to havebeensourcedfromTertiarysediments in deeper parts of the basin and migrated along faults or unconformities into higher basement structures. Secondaryporosityand karst featuresdeveloped in carbonates during subaerial exposure prior to deposition of the graben fill constitute onepotentialplay(fig. 37). Cataclasticmarbleswere encountered in both COST wells, and genetically similar carbonate rocks crop out in the YorkMountains on the Seward Peninsula(Hudson, 1977) and St. Lawrence Island(Patton and Csejtey,1980). The Renqiu oil field in the Bohai Bay BasinofeasternChinarepresents an exampleof thistypeoftrap. The reservoirconsists of solution- enlargedfractures and karst features in a Precambriancarbonate fault block (Qi andXie-Pei,1984). 127 Another pregrabenplay involvesreservoirs in fractured crystalline or metasedimentary basement rock (fig. 37).Fractured quartzite was encounteredinthe COSTNo. 2 well. The presenceof graniticoutcropsaroundNorton Sound,and magneticanomalies offshore, suggest that weathered subcrops of granitic rock may be presentinthebasin.Hydrocarbonstrapped in fracturedquartzite and granite on the crests of fault blocks in the Sirte Basin, Libya(ConantandGoudarzi,1967),provideananalogforthistype ofaccumulation. An importantconstraint on pregrabenplays is the structural complexityofthepregrabentectonostratigraphicsequence(Harding, 1984). Complex pregrabenstructuretendstodisruptthereservoir continuityof fault-block structures and thuspreventlargehydrocarbon accumulations. The complex geologichistoryofthemiogeoclinal belt rocks whichapparentlyunderliemostoftheNortonBasinsuggests thatpregrabenbasementplays are less prospectivethanplays in thegraben fill or interior sag sequences. GRABEN FILL PLAYS The graben fill sequence in the NortonBasin is represented by the Tertiary section below seismic horizon C (figs. 13 and14). This sedimentarysequenceincludestheearlyOligocene,Eocene, andpossiblePaleocenerocksoflithologiczones To-3, Te, and Tp. The best potential reservoir rock encountered in the COST wells in this sequenceconsistsof 230 feetof Oligocene fluviodeltaic sandstone in lithologic zone To-3 (table3).Significantthicknesses ofalluvial sandstones, as well as possible turbidites, were also encountered in the Eoceneand possiblePaleocenesections,but these sandstones were generallyimpermeablebecauseoftheir mineralogicalimmaturityandgreaterburialdepths. However, on basementhighsandalongtheflanksofhorsts,thesesandscould retainsignificantreservoirpotential. At the COST wells, several anomalies in porosity-depthtrends, as well as hydrocarbonshows, suggestthatunderfavorableconditionsthesesandsmightform attractivepotential reservoirs even wheredeeplyburied. The two COST wells representonly a small sampleofthe strata in this large basin and it is likelythat cleaner, quartz-rich sands occur elsewhere.Potentialsourcesofquartz-richdetritusinclude basementrock composed ofCretaceous granitic intrusives and Precambrian to Paleozoic quartzites, both of whichcouldhaveshed significant quantities of quartz sand into the basin. Possibleplays in thegraben fill sequence include traps and structuresdeveloped in conjunctionwith faulting, such as tilted ordipping strata that havebeen sealed updip by faults (fig. 37). Thesetypes of traps are found in fields in the GulfofSuez(Thiebaud andRobson,1981)and in the Reconavo BasinofBrazil(Ghignone and De Andrade,1968).Stratigraphictrapsandtrapsassociated withunconformities include clastic wedges on the flanks of horsts, submarinefansinbasindepocenters,strataonlappinghorsts and 128 otherpositivebasementstructures, and truncatedstratabeneath unconformities(fig.37). Examplesofmostofthesecanbefound in theSirte Basin ofLibya,theVikinggrabenintheNorthSea, and in theEgyptian Gulf of Suez(Harding,1984). Otherpossibletrapsinclude thoseassociated withigneous featurespresent within the graben fill sequence. Severalfeatures that appear to be igneous intrusive bodies havebeenseismically identified, as have several mound-shaped features of probablevolcanic originthatrest on basement (fig.37).Arching,draping, and differential compaction of strata overtheseigneous mounds, or lapouts on theirflanks,representpotentialtraps.Similar domal featurescontaining oil have been discovered in the Sado Basin in the SeaofJapan(Suzuki,1983). Clastic wedges thatappeartobealluvialtosubmarinefan-delta complexes thatonlapbasementhorsts (or aretruncated by unconformitiesalonghorstflanks)areprobablythe most prospective explorationtargets in theNortonBasin.Analogousplaysarefound in thesouthern Brae area of theNorthSea,where a field with an oil columnof about1,500feet is contained in alluvialfan conglomerates and sandstonesthatwereshedfromanupliftedfaultblock (Harms andothers,1980). INTERIOR SAG PLAYS The stratigraphicsectioninvolved in theinteriorsagplay includesthelateOligocene, Miocene,Pliocene,andearlyPleistocene strata above seismic horizon C (lithologiczones To-2,To-1, and Tmp) (figs. 13 and 14). Potentialreservoirrocksencountered in this interval include the fluviodeltaic andmarginalmarineshelfsands oftheupperOligocenesection. In the COST No. 2 well, 685 feet ofsandstone displaying potential reservoir quality was encountered in lithologic zones To-1 and To-2 (table 3). Thesesandsprobably representmarginalmarineequivalents of the alluvial sands that are inferred to represent the bulk of the late Oligocene section in the basin. Potentialinteriorsag plays include archedstrata formed by drapingorcompactionover horsts, and to a lesserextent,fault, stratigraphic, andunconformitytraps(fig.37).Strataarched overigneousbodies(laccoliths?)alsooccur in this section and constituteanotherpotentialplay. A more speculativeplayinvolves potentialreservoirsandsthataresealedupdipbythesubregional Miocene silica diageneticzone (fig. 37). Probablythe most prospectiveinterior sag plays involve archordrapefeaturesoverhorsts and horstshoulders. Examples ofthesetypesoftrapsarepresent in theSirteBasin,Libya (Parsons and others,1980;Harding,1984), and in theVikinggraben of theNorthSea(Blair,1975;Harding,1984). 129 TIMING OF OIL GENERATION AND TRAP FORMATION Potential source rocks occur within the oil window in the Norton Basingraben fill sequence below burial depths ofabout9,500to 10,700feet. The pregrabenPaleozoicmetasedimentsareovermature; thelateOligocenethroughearlyPleistoceneinteriorsagsequence is thermallyimmature. The relative timing ofthermalmaturity derived fromLopatin modeling of the COST No. 2 wellsuggests that the basal Tertiary sequence (now at about13,500 feet)enteredtheoilgeneration window about 20 millionyearsago(earlyMiocene). Becausemostof the prospective traps in the Norton Basin are associated with major normal faulting that was mainly active prior to mid-oligocene time (seismic horizon C),most of the potential hydrocarbontraps should haveformed prior to oil generation. Some exceptions may be the drapeorcompactionfeaturespresentinthesectionaboveseismic horizon C, mostofwhichformed in the Neogene during, and perhaps after, the main phaseofhydrocarbongeneration. Potential migration conduits for hydrocarbons generated at depth includeregional and localunconformities,faults, and theporous facies of clastic wedgeswhich interfinger at their distal endswith thermallymaturepotentialsourcerocks. Minor reactivationoffaults extending upward intothethermally immature interiorsagsequence mightprovidemigrationroutesintotheshallowerarched-drapeor compaction structures. Becausethe dominant typeof organic matterpresent in the potentialsource rocks is humic, gas-prone kerogen derived from terrestrialsources,the most likelyhydrocarbonstobefound in . thebasinaregas and gascondensate. However, oil shows in the COST No. 2 wellbetween11,800 and 13,000feet, and oil-saturated zones in aconglomeraticsandstonebetween12,180and12,290feet, indicatethatthesediments in theNortonBasinarecapable of generatingoil. The oil in these shows is thoughttohavebeen derived chiefly from resinite and lignitic coal macerals (Turner and others, 1983a ,b) . There are numerousexamples of oil accumulations, some very large,that were generatedfrom continentalsediments. In the GippslandBasinofAustralia,forinstance, 3 billion barrels of recoverable oil havebeendiscovered in a humic coal-bearing fluviodeltaicsequence that is boththe reservoir and thesource forthe oil (Shanmugam, 1985). The oilthere is thoughttobe derived from coal and resin. The giant Daqing oil field in northeastern China is another example of oil generated in a basin containingonlynonmarinesediments. The oil was derived from freshwateralgae and higher terrestrial plant debris deposited in a largelakebasin(Wanli,1985). The Mahakam Delta ofIndonesia contains oil and gas that were derived from coal and associated shale (Durand and Oudin, 1979). Snowdon and Powell(1982)describe 130 oils in the Tertiary oftheBeaufort-MackenzieBasinthoughttohave beengeneratedfrom terrestriallyderivedorganicmatter.Gibling and others(1985)describe many smallCenozoicintermontanebasins in northern Thailand that contain distinctive nonmarinesequences of oil shale and coalthathavethepotentialforoil generation. SUMMARY The volumeand distribution of reservoirrock, seals, and trappingconfigurations do notappeartoposesignificantproblems to hydrocarbonaccumulationintheNortonBasin. The timingof thermalmaturity and trapformation also generally appears favorable. However, Fisher (1982) suggestedthatthermallymaturesedimentin the basin composes, at most, 11 percent of the total basin volume, and that thesesediments are restricted to isolated structural lows or grabens. He alsosuggestedthatthe numerous faults and fault blocks may have disrupted lateral migration paths from these lows.Theseconstraints, and thepredominance of humic,gas-prone kerogen,areprobablythemostimportantlimitingfactorsfor commercialhydrocarbonaccumulations inthebasin.Despitethe poorexploration results thus far and thehigh probability of gas or gascondensatebeingthetypeofhydrocarbonsmostlikelytobefound, oil shows in the COST No. 2 well, taken with thecited instances oflargeaccumulationsof oil sourcedfromsimilarnonmarinesediments in otherbasins,indicatethatthepotentialforsignificantpetroleum accumulations in the Norton Basin cannot be ruled out without furtherexploration. 131 Part 3 Shallow Geology, Geohazards, and Environmental Conditions 10 Shallow Geology InvestigationsoftheshallowgeologyofthenorthernBering Seahavebeenongoingsincethe late 1940's. Academic studies on variousaspects of theBering Land Bridge,investigationsconcerning placergoldmining,and detailed studies of potentialgeohazards relatedtopetroleumexploration and developmenthave all contributed to the knowledgeof thisarea. As a resultoftheserelatively comprehensivestudies,thephysicalattributesoftheseafloor and theQuaternaryhistoryoftheBering Sea area are relatively well understood. This sectionincludesan assessment ofthepetroleum relatedenvironmentalgeologyofthe NortonBasin area and a brief synopsisoftheQuaternaryhistoryof theBering Sea area. DATA BASE The surface and near-surfacegeology of thenorthernBeringSea has been elucidated by integrating high-resolution seismic reflection surveyswith data obtainedfrombottomsamples,boxcores,and vibracores. Much of thedatabaseforthisassessment is from USGS investigations, whichincludetheConservationDivision's (now Minerals Management Service)high-densityseismicsurveyof theSale 57 areaofNorton Sound. Alsoincluded is the 1979Tetra Tech,Inc.,surveyoftheshallowgeologiccharacteristics of the COST No. 1 wellsite.Thissurvey, a partoftheapplicationfor permission to drill (APD), included a geotechnicalstudy of the upper 25 feetofsedimentand a high-resolutionseismic-reflection surveyoftheseafloor and its near-surfacefeatures. A similar study,also utilized, was conducted in 1980byNekton,Inc., at the COST No. 2 well site. PHYSIOGRAPHY The northernBering Sea (fig.38)includestheNortonBasin planningarea,which is boundedbytheNorton Sound coastline on theeast and southeast,the Seward Peninsula on thenorth, the 63 degreenorth latitude on thesouthernborder, and the US-USSR conventionline of1867 on thewest. 135 I70° I 70° Bristol Bay Zhernchug ', Canyon I.. PRlBlLOF BERING SEA '7 - ._-_. .--.>ISLANDS Aleutian Basin \.- ., . Lcale I: 10.58 I, I20 miles I I FlGURE 38. Physiographicmap of theBeringSearegion. AdaptedfromNelson,HopkinsandScholl (1974). 136 The northernBeringSeaoccupies a relatively flat epicontinentalshelfareathatcovers more than77,200square miles. The presentbathymetryandgeomorphologicalconfiguration arethe result of glacial, fluvial, and littoralsedimentological processeswhichoccurredprimarilyduringPleistocene low stands of sealevel (fig. 39).Thesegenerallyregressivefeatureshave beensubsequently modifiedby transgressive (erosional and sedimentological) processes. FederalwatersintheNorton Sound arearange in depth from 19 toover164feet(fig.39). The seafloorslopesgenerally westward toward theinternationalboundarywiththeSoviet Union.Superimposed on this westward slopeare subtle but distinct topographic features thathavebeengroupedintoover 20 physiographicprovincesbyHopkins and others(1976).Larsen and others(1980) list fourmorphologic provinces,eachtheresult of differentgeologicalprocesses: (1) a westernarea of hummocky relief composed of glacial gravel and a transgressive-marinesubstrate,(2) a southeasternareacharacterized by a featureless plain composed of a transgressive-marinesubstrate, (3) a northeasternprovince ofsandridgesandshoalswith a transgressive-marinesubstrate, and (4) aneasternprovince characterizedby a flat, marinereentrant(thepresentNorton Sound) flooredbythe silt and silty sandofthepresent Yukon River prodelta. A portion of the Yukon Riverdelta-front is also present in the southernpart ofNorton Sound. The delta-front is a seawardextension of nearshoreHolocenesanddepositsand is characterizedby a 1 to 2 degreeseawardslopingseafloor.Seaward,thedelta-frontgradesinto theprodelta, which representsthepresent limit of deltaic sedimentation. The prodeltaslopeslessthan 1 degree and is.present undermarinewater52 to89feetdeep. The portion of the Yukon River Deltaanddelta-frontlocatedinFederalwaters is separatedfromthe presentlyprogradingshorelineby a sub-iceplatform(Larsen and others,1980).Thisplatform is 3 to 12 mileswidealongthe southernboundaryofNorton Sound and occursin waterdepthsof less than 32 feet. The character and distribution ofmodern sedimentsin the Norton Sound are affected to a greatextentbyicegouging,storm surging,tidal andbottomcurrents, and thereleaseofbiogenicgas. Thesedynamic processes areresponsible for theformationoftransient bedformssuch asscour marks,longitudinal currentlineations, megaripples,icegouges, and biogenicgascraters (Hooseand others, 1981;Steffy andHoose,1981; Steffy andLybeck, 1981; Steffy,Turner, Lybeck,andRoe,1981; Steffy,Turner, andLybeck,1981). QUATERNARY GEOLOGY Three early to middle Pleistocene glaciations encroached upon thenorthernBeringSeashelf from theeasternSiberia-Chukotsk Peninsulaarea and from thewestern Alaska-Seward Peninsulaarea (Grimand McManus, 1970;Nelsonandothers,1974;Hopkins,1979and 137 1982).Siberianglaciersextendedover90 miles beyond theChukotsk Peninsula and intothewesternChirikov Basin and westernSt. Lawrence Islandareas.Theseglacialadvances are recordedby morainalridges(expressed as gravelbars) that extendnorthward from theisland.Localvalleyglaciations on the Seward Peninsula extended somewhatmore than a mile offshore.Morainesandoutwash deposits were encountered by drillholes just offshore of Nome (NelsonandHopkins,1972). Between 20,000and 14,000 yearsago, the last worldwide glaciation (Wisconsin) lowered sea level by nearly 280 feet, exposingthenorthernBeringSeaandtheBeringStraittoboth subaerialerosion and deposition(Hopkins,1982).Threemajor continental glaciers covered western North America, northeastern Siberia, andthe Brooks Range in Alaska. The BeringSeashelf, however,remained largelyunglaciated. Hopkins(1982)envisions the unglaciated shelf area as a low-lying periglacial plain covered by arctic steppe type vegetation. During theWisconsinlowstandof sea level, the ancestral Yukon,Kuskokwim, and Anadyr Riversdrainedsouthwardoverthe subaeriallyexposedBeringSeacontinentalshelf and deposited sediment on thecontinental slope and into the abyssal Aleutian Basin(Nelsonandothers,1974). Grim and McManus (1970)recognized theburiedchannelsof streams that oncedrainedsouthwestwardfrom the Seward PeninsulaintotheChirikovBasin.KnebelandCreager (1973)recognized similar channelfeaturesbetweenSt. Lawrenceand St. Matthew Islands on thecentralcontinentalshelf. The ancestral Anadyr RiverflowedsouthwardfromtheChukotskPeninsulaviathe Gulf of Anadyr andthroughthesubmarinePenrenets Canyon (Nelson and others,1974). The ancestral Kuskokwim Riverflowedsouthward . intoBristol Bay. Between20,000and2,500yearsago,the Yukon River migrated a distanceofover185 miles from a debouchment south of Nunivak Islandto its presentposition in theNorton Sound (Knebel andCreager,1973;Duprg,1978).Boththedistributionand accumulationof late PleistoceneandHolocenesediment in the Norton Sound were greatly influenced by migration of the Yukon River Delta and its channels. The migrationofthe Yukon River,the last deglaciation, and the latest marinetransgressionovertheBering Sea shelfoccurred essentiallysynchronously. A sealevelcurvefortheBeringSea regiongiven byHopkins(1982) (fig. 40) depicts a rising sea level thatfollowedthemeasured maximum sea level low(-280 feet) that occurredapproximately20,000yearsago. This sea level rise commenced about16,000yearsago,but was interrupted by several stillstands. These stillstandsresultedinthe developmentof a series of shorelines,theremnants ofwhich are present as submerged sandridgesinthewesternChirikovBasin(Nelson, Dupre', Field,and Howard, 1980). In many areas, a generallythinveneer ofHolocene sedimentshasallowedthesurfaceexpressionof relict gravel deposits,streamvalleys,outwashfans, and hummocky topography (Grimand McManus, 1970). 138 Interval 2 meters I, Turner and Lybeck (1981). 139 + 25 I I PRESENT PRESENT n v) n a -25 c W -100 w c W w -50 U. 5 -200 1 c W W -75 > > W w -300 J J -1 a a W W THOUSANDS OF YEARS v) v) F/GURE 40 SealevelhistoryfortheBeringSea.PBindicates wherethesealevelcurve is based on dataextrapolatedfrom PrudhoeBay.AdaptedfromHopkins(l982). 141 167 166 165 164 163 I I I 64- HOHIZON I I I I 166 167 165 164 163 FlGURE 41. Isopachmapofth reeHolocene sedimentaryunitsinNortonSound. OS1, the youngestunit,averages 5 meterst.hickandonlyitsdistributionisshownbythe shadedarea. OS1 obscuresthedistributionoftheunderlying OS2 and OS3 units. AdaptedfromSteffy.Turner,Lybeck,and Roe (1981). Several glacioeustatic stillstands were in phase with the oscillatoryadvances and retreatsoftheglaciers. A rising sea levelasearlyas15,500yearsagobreachedtheBering Land Bridge at -125 feetelevation and connectedtheBeringandChukchiSeas viatheBeringStrait(Hopkins,1982).Deglaciationaccelerated about14,000yearsago(Hopkins,1982).Approximately12,000 yearsago,sealevelrose to -98 feet,floodingtheShpanbergStrait and isolating St. Lawrence IslandfrommainlandAlaska(Hopkins, 1979). From 10,000 to9,500yearsago, a rapidtransgressionin easternNorton Sound buriedtundrapeatdeposits(Nelson,1980; Steffy,Turner,Lybeck, andRoe,1981). Two subsurfacepeatlayers in Norton Sound were mapped using high-resolutionseismic reflection data.Figure 41 is anisopach map of threeQuaternarysedimentary units (Qsl, Qs2,andQs3)which are boundedby the two peatlayers and theseafloor.Theseorganic-richhorizons may representthe remainsof two subaerially exposedsurfacesthatdevelopedpeat-rich coversduringstillstands10,000 and9,000yearsago, at elevations of -82and -52 feet,respectively.Nearshore, Qs2 overlaps Qs3, which indicatesanoscillatingshoreline. Qs3 is a progradational wedge (upto35feetthick)ofHolocenemarine silt containing interbeddedstorm-sandlayers.Subsequenttothestillstand representedbytheyoungestpeatlayer, up to 18 feet of additional marine silt andsand (Qs2) weredeposited.This wedge rapidlythins seawardandpinchsout50milesnorthwestofthe Yukon RiverDelta (fig.41). A layer ofHolocene silt less than 6 feet thick is present in the northern and eastern parts ofNorton Sound. In theChirikovBasin,these same transgressionsdeposited severaltensoffeetofcoarse-grainedsandaswellas a laggravel derived frombedrock and olderglacialsediments(Nelson,1980). In some areas, a veneerofcoarse- to mediunrgrainedsand was deposited overlatePleistocenefreshwater silt and tundradeposits. The Yukon River Delta reached its presentpositionabout2,500 yearsago(McDougall,1980). Over thepast 2,500 years, a 5-foot- thick wedge offinesand(Qsl)hasprogradedseawardfromthedelta (fig.41). The bathymetrictroughinthenorthernhalf ofNorton Sound (fig. 39) is essentially a site ofnon-deposition. Relict beach ridges, outwashfans,stream valleys, and glacial depositsare common in the northernBeringSea area.Although the Yukon Riversupplies 90 percentofthesedimententeringtheBering Sea(an estimated 70 to 90 millionmetric tons per year according to McManus and others,1977),thepresenceofrelicttopographical features and theabsence of thick deposits ofHolocenesedimentsuggest thatbottom currents in the Norton Sound and theBering Strait are strong enough topreventsignificantsedimentaccumulations.Since theopening of theShpanbergStrait12,000years ago, sedimentfrom the Yukon RiverhasbeencarriednorthwardthroughtheBeringStrait intothesouthernChukchiSea(KnebelandCreager,1973). 143 Beforetheopening of the Shpanberg Strait, the northwardflow of theBering Sea was through the Anadyr Strait seaway and into the southern ChukchiSea viatheBeringStrait. The strongbottomcurrents associated with this flow not only prohibited the deposition ofHolocene sediments,but winnowed the fines fromthe existing glacial deposits aswell,leavinglaggraveldepositsbehind. A north-trending bathymetrictrough and thelinear sandbodiesfound in thewestern Chirikov Basin area off Point Clarence may reflect this northward paleo-flow(fig. 39). SUMMARY The seafloor geomorphologyof thenorthernBering Sea is the result of three Pleistocene glaciations and associated low stands of sealevel. The migration of the Yukon River,thelast major deglaciation, and thelatestmarinetransgressionovertheBering Sea shelfOccurredalmostsynchronouslyabout 20,000 yearsago. LimitedHolocenesedimentation,partiallyduetoStrongbottom currents,hasallowedrelictglacialfeaturestoremainexposed in the Chirikov Basinandthe westernNorton Basin. A progradational wedge (up to 58 feet thick) of Holocene muds, silts, sands, and organic-rich interbedsextends seaward from the Yukon RiverDelta intothecentral and easternparts of the Norton Basin. Active sedimenttransport mechanisms continuallyreworktheupperseveral feet of sediment. 144 11 Geohazards Seafloorinstability,stormsurging, and floating ice are potential hazards to oil and gasexploration anddevelopment in theNortonBasinplanningarea. A geologicalprocess or environmentalcondition is considered a potentialhazardif it could threaten the structural integrity of a drill rig, pipeline foundation, or thesafety of men and equipmentworking in the area. High-resolution seismic reflection data, geotechnical borehole information, and seafloorsampleanalyseswereusedinthe identification of types and characteristics of potentially hazardous conditions. The ConservationDivision'shigh-resolutionseismic reflection survey of theSale 57 area identified and locatedareas and conditionsthatcouldbehazardoustopetroleumexploration and development.Outsideofthe Sale 57 area,thelimited amount ofpublic data prohibits lease-block-specific identification of geohazardlocations. SEAFLOORINSTABILITY Hazards related to seafloor instability that may affect bottom foundedstructuresincludefaulting,gas-chargedsediments,seismicity, substrateliquefaction, and erosion.Foundationsupportfailure results from a reduction in shear strength of the supporting substrate or byremovalof thesurroundingsedimentbyerosion. Seismicity and near-surfacefaultingcancause a loss of foundation supportby differential displacement, shaking, and substrate liquefactioninducedbyvibration.Substrateliquefaction may alsobeinducedby storm-wave cyclicloading(Clukey and others, 1980). Seismicity Seismicitystudies of west-centralAlaska are essential for the safeplanning and designof structures involved in petroleum exploration anddevelopment inthe Norton Sound area. A 5-year studybyBiswas and others(1983) utilized a localseismographic network in the Seward Peninsularegiontomonitorandrecordseismic activity.Thisinvestigationrevealed a significantlyhigherlevel ofseismicity,bothonshore and offshore,than had beenpreviously recognizedforthearea. 145 Earthquakeslocatedbythelocalseismographicnetworkwere foundwidelydistributedacrossthe Seward Peninsula andNorton Sound region.In many instances,however,theepicenters were found to cluster along or parallel to mapped faults or linear structuraltrends(fig.42). On the Seward Peninsula,concentrations of epicenters werenotedalongtheKuzitrinand Darby Mountain fault systems,the PennyRiver-Engstrom-AnvilCreek fault set, and along the Kigluaik fault, whichmarks the abrupt northern boundary of the KigluaikMountains. The Bendeleben fault,recognized by Hudson and Plafker(1978)ashavingmajorHolocenedisplacementsalong its surfacetrace,appearstoberelativelyinactiveatpresent.Although the distribution ofearthquakeepicentersinNorton Sound exhibits a wide scatter, there is a weak concentrationalongthebasementfault system(fig.42).Littleseismicactivity was detectedalongthe westernsection of theKaltag fault or along its projectedtrace in Norton Sound. Themaximum magnitudeofanyearthquakerecordedby thelocal seismographic network in Norton Sound measured 4.2 on the RichterScale. This particularearthquakeoccurrednorthof St. Lawrence Island in thewesternpart of theplanningarea. Attempts havebeen made to deduce the regional stress pattern oftheNorton Sound andSeward Peninsula areas from focal mechanisms ofearthquakeslocated in thoseareas(Biswas and others, 1980; Biswas and others,1983). Studies of faultplanesolutionsfor selectedearthquakes and clusters ofsmallearthquakes show normal faulting asthe principal mode of strain-energy release in this region. The dominance of normal faults,as evidenced by seismic reflection data fromNorton Sound as well as by regional mapping on the Seward Peninsula (Hudson,1977).alsoindicatesthattension is theprimarystressoperativeinthisarea. A study of stress trajectories for the Alaska-Aleutian region based on the distribution ofpost-Miocenevolcanos,dike swarm patterns, and Quaternaryfaults (Nakamuraand others,1980)reveals a westerly orientation for the Norton Sound area; the stress orientation calculated from fault planesolutions is northwesterly. From the results of regional stress pattern studies, Btswas and others(1983)postulatedthatthearea in andaroundNorton Sound and the Seward Peninsula represents the active back-arc regionof the Aleutiansubductioncomplex.Earthquakeslocated in this back-arc region are then considered to be a direct result of southwardspreading of therigid back-arc lithosphere plate. Fault s High-resolution seismic reflection surveys have located surface andnear-surfacefaults in thenorthernBering Sea area(Johnson andHolmes,1981; Steffy andHoose,1981). The majorityofthese shallow faults parallel, and may be controlled by, the structure of the underlying rock. Faults in thewestern part of Norton Sound and intheChirikov Basin trend northwest; in the eastern Norton Sound, faultstrend nearlyeast-west(fig. 43). 146 -.3 P I I I I I Surface fault density is highest andmostcomplex in the area westofPortClarence(Johnsonand Holmes, 1981).Severalyounger, west-trendingfaultsintersect and offsetthedominantnorthwest trendingfaults.West-trendingfaults bound theBeringStrait depression (BSD) and its eastwardextension,thePortClarence rift. These faults havemeasured seafloorscarps of 15 and 27 feet.Otherwest-trendingfaultshavescarps of up to 57 feet. Seafloorfaultscarps may indicateeither recent movement or a lack of erosion by bottom currents. A bathymetrictroughparallelingthenortherncoast of St. Lawrence Island is theseafloorexpressionof a significantfault zone.Johnson and Holmes (1981)statethat movement alongthisfault zone is relatedtothetranscurrent,right-lateralKaltagfault, which is known todisplace Pleistocene sediments in westernAlaska. An irregularsea bottom apparently obscures scarps associated with this fault zone. Numerous surface and near-surfacefaults were mapped in the Sale 57 area by Steffyand Hoose (1981).Shallowgrowthfaults continuedto move throughthe Pleistocene, but apparently do not displace Holocenesediments. Only a few faultscouldbe mapped in thesouthernhalf of theSale 57 areabecausewidespreadshallow, gas-chargedsediments mask theunderlying structure on seismic reflectiondata. It canbeassumed,however, that shallowfaulting is present. Shallowfaulting represents a potentialhazard in the Norton Sound planningarea. The seismicity of theareaindicatesactive fault movement, which could result in seafloor displacements that couldendangerbottom-foundedstructures. SedimentTransportProcesses The seafloortopographyofthenorthernBering Sea is the result of theinteraction ofwind,water,ice,andvarious sedimentologicalprocesses.Unconsolidatedsurfacesedimentsare continually reworkedby icegouging, bottomcurrents, storm surging, and thereleaseofbiogenicgas.Activeerosion and redistribution ofsedimentcouldreducethesupport of bottom-founded structures. Such processesareexpressedinthenorthernBering Sea as sand waves,longitudinalcurrentlineations,megaripples,currentscour, icegouges, and gascraters.Figure 44 shows thelocation of these bedformfeatures intheNorton Sound area,exceptfor the sand waves, which are found west ofthePortClarencearea. Activesand waves are found on the crests and flanks of large, linear sandridgeslyingwest of PortClarence.Theseasymmetric bedformsareorientedtransverse to the dominantnorthwardcurrent directions andhaveamplitudes of 3 to 6 feet and wavelengthsof 30 to 650 feet(CacchioneandDrake, 1979). 151 167 166 165 164 163 I I I scale 1:1,267,200 ICEGOUGETREND u AREA 3 kilorn.1.r. TOTAL NUMBER OF OBSERVATIONS-8090 w@E \ . S TOTAL NUMBER OF OBSERVATIONS-131 -64 ICEGOUGETREND --.-. L NUMBER OF OB S I I 167 $ 1 I 185 184 163 FlGURE 44. MapshowingseafloorfeaturesinNortonSound. AdaptedfromSteffyand Lybeck (1981). 0<1 ICEGOUGE/KM 1 TO 10 ICEGOUGES/KM a 10 TO 30 ICEGOUGES/KM >30 ICEGOUGES/KM CURRENTSCOURDEPRESSIONS LONGITUDINALCURRENTLINEATIONS ))I MEGARIPPLES BOUNDARYSEPARATING -.-- AREAS OF COMMON ICEGOUGINGTREND FIGURE 44 fcontinuedl Megaripples are present in the northern part ofNorton Sound in a bathymetric trough characterized by water depths greater than 68 feet (fig.44). The strikeofthemegaripple crests is northerly, which is normal tothe prevailing current direction (Nelson, Dupr6, Field, and Howard, 1980).Megaripplesgenerallyhaveamplitudesof less than 2 feet andwavelengthsbetween65and 165 feet. Bottom samplescollectedbythe USGS indicate that the local surface sediment consistsof a silty,finesand(Larsen,Nelson, andThor,1981). Longitudinal current lineations are present as a series of furrowsalong the southern flank of a bathymetric trough in the northernpartofNorton Sound (fig.44). The strikeofthelineation crestsparallelstheprevailingwesterlycurrentdirection. These lineationshave amplitudes of less than 2 feet andwavelengthsof 32 to 98 feet. The localsedimentconsistsofsilty,finesand (Larsen,Nelson,andThor,1981). Currentscourcauses many ofthebroad,flat-bottomed,elongated depressionsthat are presentthroughoutNorton Sound (fig.44). The twa most extensive areas of scourare located just west ofthe Yukon RiverDelta and in the flat prodelta area 50 miles north of the delta. The area north of the delta is characterized by elongated depressions 330 to 500 feetlong, 115 to 260 feet wide, that are less than 3 feet deep. These depressionstrend N. 100" W. and arelocated on the southernflankof a bathymetrictroughin 62 feet of water. The trend is parallel to the prevailing westerly current direction. The localsurfacesediment is a silty,fine sand. The area west of the delta is characterized by elongated depressions 460 to 520 feet long, 260 to 320 feet wide,and less than 6 feetdeep.These depressionstrend N. 15O W. andoccur in 18 feet of water on the flat,shallowsub-iceplatform. This trend is obliquetothe prevailing northeasterly current direction but is aligned with the dominantbottom-currentflow. The localbottomsediment is a silty, fine sand(Larsen,Nelson,andThor,1981). Gas-Charged Sediments The distribution of acoustic anomalies in Norton Sound suggests that gas-chargedsediments are probablyubiquitous in theplanning area. Shallowoccurrences ofgas-chargedsedimentscancauseincreases in porepressurewhichcauses a decrease in theshear strength of the sediment.Suchconditionscouldleadtounstablefoundationconditions. Gas-charged sediments can often be identified on analog high-resolution seismic reflection data as anomalous acoustic events characterized by polarityreversal, amplitude increase, reflection "wipe out," and reflection"pull down." Gas-chargedsedimentsaredistributed throughoutNorton Sound but are absent in theChirikov Basin area. Steffy and Hoose (1981) mapped extensiveareasofgas-chargedsediments in Norton Sound by using high-resolution seismic reflection data. Nelsonand others(1979) state that most of theshallowgas in Norton Sound is probably biogenically generated from buried Quaternary peat 154 layers,thentrappedinoverlyingcohesive silts and silty, fine sands. These thinpeatylayers commonly havemeasuredorganic carboncontentsof 2 to 8 percent and containabundantbiogenic methane. The degassing of sediments may beeither continuous or episodic. Storm-induced cyclicloading of unconsolidatedsurfacesedimentscan causerapidchangesinporepressurethatreduceshearstrength and permitgastoescape(Fisher and others,1979).Degassing is often evidencedbyseafloorcratering,which is prevalentin the central and easternpartsofNorton Sound (Holmes,1979).These cratersare typicallyassociatedwithnear-surfaceacousticanomalies,shallow depositsoforganic-rich mud, andgas-chargedsediments.Biogenic gas-generatedcratersrangefrom 3 to 30 feet in diameter and are usuallylessthan1.5feetdeep. The absenceofgas-charged sediments and crateringinthenon-cohesive,coarse-grainedseafloor materialoftheChirikovBasin is attributable to the gradual diffusionofbiogenicgasthroughthesepermeablesediments(Larsen, Nelson,andThor,1981). A well-documentednear-surfacegasaccumulationthat is seeping into the water column is locatedapproximately 25 milessouthof Nome (Holmesand others, 1978;Nelsonandothers,1978).Thisgas accumulation,whichcoversanarea of about19squaremiles, is identified by a shallow acoustic anomaly on high-resolution seismic data. The gasconsistsprimarily of C02 with a minorcomponent of hydrocarbongasesandgasoline-rangehydrocarbons;the C02 is presentin the free state in sediment interstices (Kvenvoldenand Claypool,1980).Specificcharacteristicsofthegasoline-range hydrocarbonssuggestthat themixture may beanimmaturecondensate oflowertemperatureoriginthannormalcrudeoil. It is possible that these gases are migrating up faults that offset Tertiary strata. If so, thissuggeststhepresence ofthermogenicgasaccumulations in the NortonBasin.Becauseofthevolatilenature of suchgases, specific safety preparations are necessary when drilling in the Norton Basin. Substrate Liquefaction Substrate liquefaction is the fluidization of granular, non-cohesivesediments.Thisfluidizationresultsin a reduction ofsedimentshearstrengthand its ability to supportbottom-founded structures.Cyclicloadingcaused by vibratingmachinery,earthquake shaking, and storm waves caninducesubstrateliquefaction. The build-up of porepressurebycyclicloadingincohesionless,fine grainedmaterialtemporarilyorpermanentlyreducesshearstrength (Sowers,1979).Thishappens ifthepermeabilityofthematerial is insufficienttodissipateincreasingporepressures.Increasedpore pressureenablesthesubstratetowithstandlargestrains up to the time thatfailureensues. Whether ornotsubstrateliquefaction will occurdependsupon a number of interrelated factors such as the ability of thefine-grainedmaterialtodissipateporepressure,theinitial densityof the material, and the amount ofapplied stress. 155 The upper 6 feet of silt andfine-grainedsandcoverinthe prodelta area north and west of the Yukon River is considered to havethe greatest potential for substrate liquefaction (Clukey and others,1980). In thelate fall, this area is subjectedtolarge stormswithlarge-amplitude,low-frequencywavesthattravelinto Norton Sound fromthesouthernBeringSea. The shallowwaterdepths (commonly less than 60 feet) render this region particularly susceptibleto wave-induced stresses. nOATING ICE Ice gouges are furrows in the seafloor caused by single- or multi-keeledicefloesdriven by windand watercurrents.Single keeled ice gouges in the Norton Sound area range from 15 to 165 feet wide and appeartobeinfilledtovaryingdegreesbysediment.Ice gougeswere mapped andgrouped into 3 areas on the basis ofgouge density andgouge-trend(Steffy andLybeck,1981) (fig. 44). The maximum ice gouge density occurs intheshallow-waterareas of NortonSound, especiallyaroundthe Yukon RiverDelta. The gouges occurinwaterdepths of 21 to 79 feet and are most common in depths of 32 to 56 feet. Most of thegougesarefound in area 3, thedelta frontnorth of the Yukon RiverDelta(fig. 44). Thor andNelson(1981) attribute this concentration to the westward-moving ice packofNorton Sound shearingagainst the shorefast ice offshore fromthedelta. This shearingcausesthenormally thin (less than 6 feet) annual ice to formcompositeiceridgesthickenoughtogougetheseafloor. Thesepressureridgesproducemulti-keeledicegouges.Smaller amountsof icegougingoccurinareas 1 and 2 (fig. 44). Icegouging in area 1 may be caused by the Bering Sea pack ice shearing against shorefasticethatextends westwardfrom thedelta (Dupre,1978). . Ice gouging in area 2 results fromaninteraction of shorefast ice with bothNorton Sound and BeringSeapackice(ThorandNelson, 1981). In allthreeareas,themajortrendsparallelthebathymetric contours. STORM SURGING Shorelineareas of thenorthernBering Sea are vulnerable to stormsurging, a processthat causes drastic changes in seafloor sedimentation(Larsenandothers,1980).Bottom-foundedstructures used in theNortonBasin will require careful design to limit the potentialeffects ofstormsurges.Stormsurgesaremostlikely tooccur in Norton Sound during the late summer or early fall. Strongwindsfromthesouthwestdrivewaterintotheshallow embaymentsof the soundand sealevel rises as high as 10 feet abovethe mean waterlevel may occur.Stormsandlayersupto 8 inches thick have been deposited in the Yukon River Delta area (Larson and others,1980). 156 A well-documentedstorm in Norton Sound occurred from November 10 to 17, 1974. The storm, a once-in-30-yearsevent, generated a surgethat was 20 feet abovethenormal tidal range of 4 feet. Maximum sustainedwindsof38knots with gusts of 70 knotswererecorded(Sharma,1979). SUMMARY A broadspectrum of potential geohazards that could result in seafloorinstability,such as substrateliquefaction,surface or near-surfacefaulting,thepresence of gas-chargedsediments,and variouseffects of seismicity, impose engineeringconstraints on oil and gasexploration anddevelopment in thenorthernBeringSea area.Additionalconstraintsaredictated by thepresenceof floating ice and thelikelihoodofstormsurging. 157 12 Environmental Conditions METEOROLOGY AND OCEANOGRAPHY Most ofthemeteorologic andoceanographicinformation on the northernBering Sea area summarized here is containedintheClimatic Atlas oftheOuterContinentalShelf Waters and CoastalRegionsof Alaska(Browerandothers,1977)and The AlaskaMarine Ice Atlas (Labelle and others,1983). Data compiledbytheNationalWeather Service and otherFederal agencies and data compiledbythe Arctic EnvironmentalInformationand Data CenteroftheStateofAlaska were alsoused. The Norton Sound region is climatically characterized by a subarctic,semi-aridweatherregime.Cyclonicatmospheric circulationpatternsdominateintheregion. Annualweatherpatterns are controlledbytheHonolulu, Arctic, and Siberianhighs, and theAleutianlows. The Norton Sound region is affectedbyboth a moderatingmarine climate to the west and anextremeonshore climate totheeast. Cloudy skies andstrongsurfacewindscharacterize . thesound'smarineweather.Stormsare more frequentin the fall andwinter(Sharma,1979).Temperaturesvaryfrom a January mean of 5 "F to a July mean of 55 OF. Recorded temperatureshave dropped as low as -55 "F. Maximum recordedwinds in the region have reached 75 miles per hour.Inwinter, maximum windsaccompany thestormsthatapproach Norton Sound from thesouthwest,althoughthepredominantwinter wind direction is from thenortheast and averages 12 to 17 miles perhour. Summer windsblowfrom a southerly direction and average 8 to 12 miles perhour. There are two dominantmarinesurfacecurrentpatterns in the Norton Sound area: a northwardflowthatpasseseast of St. Lawrence Island and a counterclockwise system in NortonSound. Tidal variations in Norton Sound are relatively small, but exertan important influence on thesurface current circulation pattern.Tides move northwardthroughthenorthernBeringSeaand proceedthroughthesoundin a counterclockwisedirection.Within thesound, tidal heights range from 1.6 to 6.8 feet. 158 Wave heightsgreater than 8 feet are common lessthan 10 percent of thetimein AugustandSeptemberand less than 20 percent of the timeinOctober. SEA ICE The freeze-upperiodinNorton Sound extends from November throughearly December. At Nome, the mean datesofseaicebreak-up and freeze-overare May 29 and November 12, respectively.Ice spreadssouth from the ChukchiSea tothe western portion ofNorton Sound andaround St. Lawrence Island.IcealsoformsinNorton Bay and spreadssouthwestward.Packicegenerallybeginstoform in Norton Sound in mid- tolateOctober. Some areasin andaround the sound arecompletelyicecoveredby mid-November. After mid- December,pack icegenerallycompletelycoversthesound. By mid-March the ice pack at the head of the sound beginsto thin, butdoesnot show appreciablemeltinguntil mid-May. By mid-June the sound is completelyice-free. Sea iceconditionsvarythroughoutNorton Sound. Ice movement is controlledby a combinationofgeography and theprevailing northeastwinterwinds. The prevailingwindscause a generally east-northeasttowest-southwestevacuationoficethroughoutthe winter.Exceptfortheshorefastice,seaiceinthisarea is entirelyreplaced by this process several times during a season. 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