paper2 justwan

241Journal of Petroleum Geology, Vol. 28(3), July 2005, pp 241-268

Source facies and quality of the Late to Middle Jurassic source rock system in the South VikingGraben between 58ºN and 60º15’N are highly variable both regionally and stratigraphically. Inorder to assess the degree of variability and to create a model of source rock quality and potential,isochore maps of the syn- and post-rift sections of the Upper Jurassic Draupne Formation andunderlying Heather Formation were generated from seismic and well data, and maturity-correctedRock-Eval data were used to generate quantitative maps of oil and gas potential. The thin post-riftsection at the top of the Draupne Formation is a rich oil-prone source, while the up to 1,600 mthick syn-rift section contains a mixture of Type III and Type II material with substantial amountsof gas-prone and inert organic matter. The Heather Formation, which reaches modelled thicknessesof up to 930 m, is a lean source and is generally gas-prone.

Detailed analyses and interpretations of biomarker and isotopic characteristics support thisupward increase in oil-prone Type II material. The analytical parameters include increasing relativeamounts of C27 regular steranes; decreasing ratios of C30 moretane relative to C30 hopane; and anincreasing predominance of short chain n-alkanes and progressively lighter isotopic values forsaturate and aromatic fractions of source rock extracts. In addition, increasing amounts of17α(H),21β(H)-28,30-bisnorhopane and decreasing amounts of C34 homohopanes relative toC35 homohopanes, as well as decreasing Pr/Ph ratios, suggest a general decrease in oxygenationupwards. Maps of average Pr/Ph ratios for the syn- and post-rift Draupne Formation and for theHeather Formation are consistent with permanent water column stratification and gradual ascentof the O2:H2S interface from the Callovian to the Ryazanian.

Interpretation of oil and gas potential maps, molecular parameters and estimates of sedimentaccumulation rates in combination suggest that the source facies of the upper, post-rift DraupneFormation is controlled by widespread anoxia, reduced siliciclastic dilution and reduced input ofgas-prone organic and inert material; by contrast, the potential of the lower, syn-rift DraupneFormation is strongly controlled by dilution by gas-prone and inert organic matter resulting frommass flows and also by varying degrees of oxygenation. The oil and gas potential of the HeatherFormation is mainly controlled by the degree of oxygenation and siliciclastic dilution.

(Huc et al., 1985; Isaksen and Ledje, 2001; Kubala etal., 2003; Justwan and Dahl, 2005). In the areabetween 58º N and 60º15’ N in the Norwegian SouthViking Graben, major differences can be observedbetween syn- and post-rift sections of the DraupneFormation (Justwan and Dahl, 2005). As more dataconcerning Jurassic source rocks in the matureexploration area of the South Viking Graben becomeavailable, it is becoming evident that source faciesdistribution can no longer be explained by simplemodels and that it is controlled by numerous factors.The new data has enabled us to construct a more

LATE TO MIDDLE JURASSIC SOURCE FACIESAND QUALITY VARIATIONS, SOUTH VIKINGGRABEN, NORTH SEA

H. Justwan*1, B. Dahl1, G.H. Isaksen2 and I. Meisingset3

1Department of Earth Science, University of Bergen,Allegaten 41, 5007 Bergen, Norway.2 ExxonMobil Exploration Company, 233 Benmar,Houston, Texas 77210, USA.3 Aker Kvaerner Geo AS, PO Box 242, Lilleaker, 0216Oslo, Norway.*corresponding author, email: [email protected]

INTRODUCTION

Source facies and quality of Jurassic source rocks inthe Viking Graben vary both laterally and vertically

242 Late to Middle Jurassic source facies and quality variations, South Viking Graben, North Sea

reliable model of source rock distribution, potentialand quality than was previously possible. We havefocussed on the Upper Jurassic source interval becauseonly a small amount of new data on the MiddleJurassic source rocks in the study area was available.

This paper forms the first part of a larger study ofthe petroleum system in the South Viking Grabenwhich will also include an oil-source correlation studyand a basin modelling project. The objectives of thepresent paper are threefold. The first aim is to producemaps of the thickness and quality of the Upper Jurassicsource rock section which can be used in basinmodelling. Much attention in recent basin modellingprojects has been given to kinetic parameters (e.g.Dieckmann et al., 2004) and phase predictions (e.g.di Primio, 2002), while the issue of source rockvariability as an important input parameter is rarelyaddressed. We hope to shed light on this importantaspect of the petroleum systems of the South VikingGraben by presenting quantitative maps of the oil andgas potential in the area. The second objective of thisstudy is to gain a detailed knowledge of variations insource rock quality (including molecular properties)which will later be used in an oil-source correlationstudy within the South Viking Graben. Finally, weattempt to explain the development of source faciesand oil and gas potential in the area in terms of basindevelopment through time.

GEOLOGICAL SETTING

Evolution of the South Viking GrabenThe present-day structural configuration of the studyarea (Fig. 1) was inherited from two major extensionalphases in the Permo-Triassic and the Late Jurassic(Ziegler, 1992). Permo-Triassic extension was morepronounced in the Stord Basin on the eastern marginof the study area, while Jurassic extension mainlyaffected the Viking Graben in the west and left theStord Basin largely unaffected (Faerseth, 1996). TheSouth Viking Graben is asymmetric; it is bound bythe East Shetland Platform to the west, and steps upto the east with numerous fault blocks (Fig. 1) formingthe Gudrun and Sleipner terraces (Cockings et al.,1992; Sneider et al., 1995). The Utsira High, whichwas a positive feature at least as early as the Permian(Hanslien, 1987), together with the Ling Highconstitute the eastern boundary of the graben. Theoldest sediments encountered in the area are ofPermian age (Isaksen et al., 2002) and are overlainby Triassic continental siliciclastics (Fisher andMudge, 1998). Marine sediments of the LowerJurassic Dunlin Group were deposited during a risein sea level (Skarpnes et al., 1980), but are sparselypreserved south of 59°N. The Brent delta developedin the northern half of the study area in response to

regional doming during the Middle Jurassic. Itssediments pass transgressively southwards into thecoastal plain deposits of the Sleipner Formation andthe shallow-marine to coastal deposits of the HuginFormation. Middle Jurassic sandstones form importantreservoir intervals in the area, while the coals andcoaly shales are source rocks for gas and volatile oils(Isaksen et al., 1998). The organic rich oil- and gas-prone shales of the Heather and Draupne Formationswere deposited during a sea-level rise in the LateJurassic. Intra-Heather and Draupne Formationsandstones also act as reservoirs in the study area,and are best developed along the western margin ofthe South Viking Graben in the UK Sector. FollowingLate Jurassic rifting, there followed a period oftectonic quiescence and subsequent uplift and erosionof the East Shetland Platform, resulting in thedeposition of submarine fan sandstones of Paleoceneto Eocene age which include important reservoirintervals. Continued subsidence with occasionalphases of uplift accompanied by the resulting supplyof sediments from the west resulted in an extensivepost-Eocene sedimentary section (Gregersen et al.,1997).

The Jurassic source rock systemin the South Viking GrabenThe Upper Jurassic Draupne Formation andstratigraphic equivalents are the major source rocksin the North Sea and have been subject of numerousstudies (e.g. Barnard and Cooper, 1981; Goff, 1983;Cooper and Barnard, 1984; Doré et al., 1985; Field,1985; Northam, 1985; Cooper et al., 1993; Cornford,1998; Kubala et al., 2003). In addition, the underlyingHeather Formation together with Middle Jurassic unitsalso have potential for oil and gas generation (Cooperand Barnard, 1984; Field, 1985; Cornford, 1998;Isaksen et al., 1998; Isaksen et al., 2002). In thefollowing paragraphs, we summarise previous studiesconcerning Jurassic source rocks in the study area.

The mid-Jurassic Sleipner and Hugin Formations(Fig. 2) were deposited in a coastal plain to offshoremarine setting. The Sleipner Formation, which is upto 500 m thick (Husmo et al., 2003), consists mainlyof fluvio-deltaic siliciclastics including coals and coalyshales, and ranges in age from Bathonian to earliestOxfordian on the margins of the Utsira High (Vollsetand Doré, 1984; Cockings et al., 1992). It istransgressively overlain by the Hugin Formation oflatest Bathonian to Early Oxfordian age (Cockings etal., 1992; Husmo et al., 2003) which comprisesnearshore shallow-marine sandstones andfluviodeltaic sediments including coals and coalyshales (Vollset and Doré, 1984). These Middle Jurassiccoals and coaly shales have the potential to generategas and volatile oil (Isaksen et al., 1998), and exhibit

243H. Justwan et al.

TOC values of up to 80 wt % and Hydrogen Indicesup to 400 g HC/kg Corg (Isaksen et al., 2002). Shalesand grey silty mudstones of the Callovian to OxfordianHeather Formation have average TOC values up to4% (Goff, 1983; Cooper and Barnard, 1984; Field,1985; Thomas et al., 1985). It is generally thoughtthat this formation is a lean dry gas source, althoughit shows large variations in source potential and hasgood hydrocarbon potential in some small sub-basins(Cooper and Barnard, 1984; Field, 1985; Thomas etal., 1985; Scotchman, 1991; Gormly et al., 1994;Isaksen et al., 2002). Thomas et al. (1985) describeda widespread basal-Heather “hot shale” depositedunder dysoxic to anoxic conditions beneath a stratified

water column, whose deposition was terminated byincreasing oxygenation accompanying a progressivetransgression.

The Heather Formation is diachronously overlainby the Oxfordian to Ryazanian (Kimmeridge ClayEquivalent) Draupne Formation (Vollset and Doré,1984) (Fig. 2). Various depositional models for thisunit have been put forward (e.g. Hallam andBradshaw, 1979; Tyson et al., 1979; Demaison andMoore, 1980; Parrish and Curtis, 1982; Oschmann,1988; Miller, 1990). The well studied onshoreoutcrops are however not easily comparable in age orfacies with the offshore equivalent (Wignall, 1994).It is generally agreed that deposition of the offshore

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Fig.1. General map of the South Viking Graben showing major structural elements (after NorwegianPetroleum Directorate, 2005) and the location of the wells used in quantitative source rock mapping.The schematic SW-NE cross-section (after Isaksen and Ledje, 2001) displays the structure of the South VikingGraben, the flanking highs and the adjacent Stord Basin (line of section A-A’ in main map). The location of thestudy area in the North Sea is shown in the overview map (lower right).


Kimmeridge Clay occurred in a shallow sea with anoxicbottom waters (Cornford, 1998), although the degreeof oxygenation appears to decrease from the lower tothe upper Draupne Formation (Thomas et al., 1985;Justwan and Dahl, 2005). The Draupne Formation hasTOC up to 12 wt% (Thomas et al., 1985) and HydrogenIndices which are well over 600 g HC/kg Corg.However, the formation has large lateral and verticalvariations in source quality (Field, 1985; Huc et al.,1985; Scotchman, 1991; Cooper et al., 1993; Isaksenand Ledje, 2001; Kubala et al. 2003), especially in thenarrow South Viking Graben whose fringing land areaswere the source of terrestrial organic matter. It has beenobserved not only that the terrestrial contributiondecreases upwards (Huc et al., 1985), but also that thereis a distinct difference in source facies and potentialbetween the upper and lower parts of the formation.Thus, the upper section has better source quality andrichness, while the lower part has a higher content ofType III kerogen (Field, 1985; Huc et al., 1985; Isaksenand Ledje, 2001; Isaksen et al., 2002; Kubala et al.,

2003). The upper and lower parts of the DraupneFormation were therefore considered separately inthe present study.

In general, these sections correspond to post- andsyn-rift successions although the identification ofthese intervals is difficult in the study area(Gabrielsen et al., 2001; see also Rawson and Riley,1982; and Rattey and Hayward, 1993). For this study,the base of the J70 sequence of Rattey and Hayward(1993) was chosen as the base of the post-rift section.This corresponds to the base of genetic sequence Dof Middle Volgian age of Fraser et al. (2003), andthe Ti5 sequence boundary of Jacquin et al. (1998)(Fig. 2). Gamma-ray log pattern and geochemical datawere utilized in addition (see below). The lowermostsection of the syn-rift lower Draupne Formation wasdeposited during intensifying rift activity and risingrelative sea level (genetic sequence B, see Fig. 2)with fan systems shedding sands from the surround-ing highs. The South Viking Graben contains a seriesof these basinward-thinning and -fining sandy

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-Only minor extension, thermal subsidence dominates, onset of post-riftphase, abandonment of fan systems

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-Deposition of Lower Draupne Formation with widespread change to dysoxicfacies

-Intensification of rifting activity

-Active fan systems sourced from Utsira High and East Shetland Platform

-Accelerated rifting and deepening

-Continuing southward transgression of Boreal sea along graben axis gives riseto deposition of Heather shales

-Deposition of coastal plain sediments including sands, coals and coaly shales ofthe Sleipner Formation and nearshore sands to coastal plain deposits of theHugin Formation

Major Events(after Rawson & Riley 1982; Cockings 1992; Cornford, 1998; Fraser 2003)et al. et al.

Utsira High

Upper DraupneFormation

Lower DraupneFormation

HeatherFormation

SleipnerFormation

HuginFormation

Intra-Heathersands

Intra-Draupnesands

Intra-Draupnesands

Fig. 2. General stratigraphic chart of the Upper to Middle Jurassic succession in the South Viking Graben witha summary of the major tectonic and depositional events. The Oxfordian to Kimmeridgian DraupneFormation can be divided into an upper post-rift section including genetic sequences D and E (Fraser et al.,2003) and a lower syn-rift section. The Draupne and Heather Formation contain a series of sands sourcedfrom the flanking highs, the East Shetland Platform and the Utsira High.


wedges of Kimmeridgian to Volgian age (Thomas etal., 1985; Sneider et al., 1995; Underhill, 1998;Isaksen and Ledje, 2001) which are interpreted to beproximal deep-water slope apron complexes sourcedfrom the East Shetland Platform and the Utsira High(Underhill, 1998; Fraser et al., 2003). They containlarge amounts of reworked terrestrial organic matter(Cooper et al., 1993; Isaksen and Ledje, 2001; Justwanand Dahl, 2005). Rifting intensified in the EarlyVolgian and widespread dysoxic conditions wereestablished (Fraser et al., 2003). A regressive episodefollowed in the Middle Volgian while fan depositioncontinued.

The upper Draupne Formation represents a claylayer deposited during thermal subsidence after thecessation of rifting. Its lowermost section wasdeposited during the late Middle Volgian when fansystems became less common. The uppermost section,corresponding to genetic sequence E of Fraser et al.(2003), does not show the influence of fans (Fig.2).Mass flows from surrounding highs therefore hadmuch less of an influence in the upper DraupneFormation.

In general, areas surrounding highs and close tothe palaeo-shoreline were dominated by the depositionof Type III and IV organic matter (Thomas et al., 1985;Cooper et al., 1993; Kubala et al., 2003). The deep,basinal sections are richest in Type II organic matter(Thomas et al., 1985; Miller, 1990; Cornford, 1998),with a typical concentric, outward-decreasing TOCdistribution (Huc, 1988; Cooper et al., 1993).Although the basinal sections are generally oil prone,they can potentially contain large amounts of inertinitetransported by mass flow processes (Tyson, 1989;Thomas et al., 1985). Below, using recently-acquireddata, we investigate if these general assumptions arestill applicable to the South Viking Graben, andattempt to determine the controls on the distributionof organic facies in this area.

SAMPLES, DATA, ANALYTICAL METHODS

A large Rock-Eval database was available comprisingsamples from 47 wells in the study area, of whicheight where analysed during this project using a VinciTechnologies Rock-Eval 6 in bulk rock mode. Theremaining data is derived from geochemical servicereports publicly available from the NorwegianPetroleum Directorate or were provided by EssoNorway. Cuttings provided by the NorwegianPetroleum Directorate for this project from the sourcerock sections of eight wells were analysed by gaschromatography (GC) and gas chromatography-massspectrometry (GC-MS). In addition, the isotopiccomposition of 15 samples from well 15/9-18 wereanalysed. Vitrinite reflectance data (36 wells), GC and

GC-MS data (26 wells) and isotope data (21 wells)obtained from the Norwegian Petroleum Directorate,Esso Exploration and Production A/S Norway orprevious projects at the University of Bergen werealso used.

Solvent extraction was carried out with a SOXTECSystem HT2 1045 extraction unit on washed, crushedand ground cutting samples with dichloromethane/methanol (93:7) as solvent. The asphaltene fractionof all samples was precipitated by adding an excessamount of n-pentane, following the proceduresuggested in Weiss et al. (2000). For GC, GC-MS andisotope analysis, preparative group-type separation ofthe maltene fraction was carried out using mediumpressure liquid chromatography (MPLC) followingmethods modified from Radke et al. (1980). Thecarbon isotopic analysis of the saturate and aromaticfractions was carried out on a Carlo Erba NA 1500elementary analyser and a Finnigan MAT Delta Eonline mass spectrometer. All results are reportedagainst PDB. The saturate hydrocarbon fraction ofextracts of all source rock samples from wells 15/9-18, 25/5-5, 25/7-2, 16/7-2 and 25/1-10 were analysedusing a Hewlett Packard GC-MSD, consisting of aHP 6890 GC Plus, a HP 5973 Mass Selective Detectoroperating in SIM mode at the University of Bergen.The GC unit was equipped with two HP 19091Z-105HP-1 methyl siloxane columns each of 50 m nominallength, 200 µm nominal diameter and 0.33 µmnominal film thickness. The oven’s initial temperaturewas 50°C and increased regularly during 20 minutesto an end temperature of 310°C until 80 minutes.Helium was used as the carrier gas. The temperatureof the FID was 320°C and the hydrogen flow was 40ml/min, while the airflow was held at 400 ml/min.

The saturate hydrocarbon fractions of samplesfrom wells 15/3-3, 15/3-5 and 16/1-2 (location seeFig.1) were analysed on the same system, but usingtwo HP 19091J-102 HP-5 5% phenyl methyl siloxanecolumns of 25 m nominal length. The oven’s initialtemperature was 70°C, increased during 20 minutesto an end temperature of 310°C until 60 minutes, andthe airflow was held at 450 ml/min. GC and GCMSratios were calculated from measured peak heights.For all analyses performed, standards were run everytenth sample to ensure analytical quality andcomparability of results.

RESULTS AND DISCUSSION

Thickness, areal extent and accumulation ratesof the Upper Jurassic source rock intervalsFor basin modelling, the quality and potential of thesource rock as well as the thickness of the respectivesource intervals should be investigated. To produceisochore maps of the upper Draupne, lower Draupne


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ckne

sses

of t

he r

espe

ctiv

e un

its

from

wel

l dat

a (s

ee in

set

grap

hs).


and Heather Formation, a pragmatic statistical approachwas chosen because the intra-Upper Jurassic surfacesare not discernable on seismic sections. This method isvery coarse, but gives a general idea about thedistribution of the analysed source rock sections andtheir thicknesses. The information used comprised anUpper Jurassic thickness map which was based on thenear base-Cretaceous and top-Middle Jurassic surfaces,as well as data from 90 well penetrations. In this process,linear regression analysis was performed to investigatethe statistical relationship between the total thicknessof the Upper Jurassic succession and the partialthicknesses of the upper and lower Draupne and HeatherFormations (Fig. 3, insets).

The total Upper Jurassic thickness and the partialthickness of the lower Draupne Formation are stronglycorrelated (R2 = 0.84) (Fig. 3b, insets), while total andpartial thicknesses for the upper Draupne and theHeather Formation have lower correlation coefficientsof 0.59 and 0.50 respectively (Fig. 3a,c, inset). Basedon the observed statistical relationships (equations seeFig. 3, inset), the total Upper Jurassic isochore wassubdivided into three sections, corresponding to theupper and lower Draupne and Heather Formations.

The maximum modelled thickness of the HeatherFormation is approximately 930 m in the deepest partsof the graben, while the maximum observed thicknessis 315 m in well 24/12-2 (Fig. 3c). The thicknessdecreases to the east and although the Heather Formationdoes not cover the entire Utsira High area, it is the mostwidespread of the three analysed source rock units. Thelower Draupne Formation is up to about 1,600 m thickin depocentres in the graben, with a maximum observedthickness of 671 m in well 15/3-1S and decreases rapidlyin thickness towards the Utsira High in the east (Fig.3b). The upper Draupne Formation is considerablythinner, with thicknesses of up to approximately 330 mand a maximum observed thickness of 142 m in well15/3-3 (Fig. 3a); it appears to fill relief in the graben asa draping clay layer deposited after the cessation ofrifting (Fig. 3a). The Utsira High remained partlyemergent during the deposition of the upper DraupneFormation (Fraser et al., 2003) which is less than 10 mthick on the high.

Average compacted linear sediment accumulationrates were determined for the studied section from welldata using Esso’s proprietary sequence stratigraphicframework. While hiatuses are included, the mass flowsand units have been removed because they representvery rapid events which would introduce additionalerror, increasing the accumulation rates. Theaccumulation rate in the Heather Formation ranges from1 m/Ma to over 50 m/Ma, with an average value ofaround 20 m/Ma in the analysed wells. Linear sedimentaccumulation rates in the lower Draupne Formation arebetween 1 and 47 m/Ma with an average of 11.7 m/Ma.

The upper Draupne Formation appears sedimentstarved and has even lower accumulation rates,reaching a maximum of 20m/Ma in the deep grabenand around 1m/Ma for the condensed sections onthe Utsira High, with an average of only 7m/Ma.Sediment starvation in the mid-Volgian toRyazanian has also been observed in the DanishCentral Graben (Ineson et al., 2003).

Similar accumulation rates were reported byMiller (1990) for the Kimmeridge Clay Equivalentin the Northern North Sea. These values are,however, significantly lower than the 40 to 100 m/Ma for the Kimmeridge Clay of the Wessex Basin(Tyson, 2004) which is the approximate time-equivalent of the offshore lower DraupneFormation. Considering the much larger thicknessesin the undrilled sections of the South Viking Grabennot used in the calculations above, values in theSouth Viking Graben might however approach thoseof Tyson (2004).

Source rock thermal maturityand hydrocarbon generationThe analysed sections exhibit a wide range ofthermal maturities as a function of burial depth,ranging from immature on the highs to late maturein the deeper parts of the graben. Maturitydetermination is strongly dependent on kerogen type(Tissot and Welte, 1984; Waples and Marzi, 1998;Kubala et al., 2003). Various studies (e.g. Huc etal., 1985; Isaksen and Ledje, 2001; Kubala et al.,2003; Justwan and Dahl, 2005) have shown thatthe Draupne and Heather Formation containvariable mixtures of terrestrial and marine organicmatter. Source rocks in the Middle Jurassic VestlandGroup, which were deposited in a generallyterrestrial environment, are dominated by Type IIIkerogen. Due to these variations, the source rockthermal maturity was determined for each sectionseparately. The onset of generation was determinedfrom depth plots of the Hydrogen Index (HI),Bitumen Index (BI=S1/TOC*100) and ProductionIndex (PI=S1/(S1+S2)), and cross-checked withhigh-quality vitrinite reflectance and Rock-Eval Tmaxvalues (Fig. 4a-e).

The HI versus depth plot (Fig. 4a) is a goodindicator for the onset of generation (Petersen etal., 2002; Kubala et al., 2003; Justwan and Dahl,2005). In the study area, within the Upper Jurassic,a decline in the HI trend at around 3,500 m coincideswith vitrinite reflectance values of 0.6% and Tmaxvalues of 435 ºC, all indicating the onset ofhydrocarbon generation. Baird (1986) determinedthe onset of generation to be at 3,340 m, whileIsaksen and Ledje (2001) proposed a depth of 3,400m and Kubala et al. (2003) a depth of 3,500 m for


the same area. The onset of hydrocarbon generationappears to be slightly deeper at around 3,800 m forthe Middle Jurassic Vestland Group. Rock-Eval Tmaxand vitrinite reflectance (Fig. 4d, e) are in goodagreement with these observations and confirm adepth of 3,500 m for the top of the oil window in theUpper Jurassic section.

Peak oil generation, or onset of expulsion (Petersenet al., 2002), was estimated to occur at a depth ofaround 4,100 m for the lower Draupne Formation (Fig.4b). Expulsion occurs at a slightly shallower level forthe upper Draupne Formation (around 4,000 m) andat around 4,200 m for the Heather Formation (notshown).

Based on thermal maturity data, the original sourcerock properties were restored to compensate formaturity related differences. The empirical methodused here is based on studies by Espitalié et al. (1987)and Justwan and Dahl (2005), and uses depth plots ofHI, PI and BI to establish regional generation curveswhich are then used to restore the original source rockproperties (TOC0, S20 and resulting HI0 from Rock-Eval). The data set for the respective source intervalswas filtered to remove contaminated samples. Thelarge spread of data in the immature range above 3,500m, which reflects the fact that the data originates frommultiple wells, makes the determination of the originalpre-maturation Hydrogen Index (HI0) difficult. HI0was taken as the 3rd quartile value of the HI distributionbetween 0 and 3,500 m — for example, 396 g HC/kgCorg for the lower Draupne Formation. This choice isin good agreement with the shape and curvature ofthe trend-line which is an envelope in the maturesection. The construction of trend-lines for the upper

and lower Draupne Formation and the HeatherFormation was made possible by the large amount ofavailable data, but was difficult for the MiddleJurassic. The empirically determined transformationratios were compared with transformation ratios basedon standard Lawrence Livermore National Labs(LLNL) kinetics as used in Cornford et al. (1998) (Fig.5). As Fig. 5 shows, the Upper Jurassic source rocksplot between the transformation ratios for Type II andType III using LLNL kinetics, and give a fairly goodmatch for a mixture of Type II and III kerogens. Thecurve of the upper Draupne Formation with highestType II kerogen content is most similar to the LLNLType II curve, while the curve for the HeatherFormation consisting of Type III material is nearlyidentical to the LLNL Type III curve. Thetransformation ratios obtained by empiricaldetermination, however, seem slightly to overestimatetransformation at depths down to 4,000 m TVDssrelative to the transformation ratios based on thekinetic model.

The transformation ratios for the Middle JurassicVestland Group are also shown in Fig.5, but, asmentioned above, the determination of the ratio washampered due to the small amount of data available.The Vestland Group curve is similar to the LLNL TypeIII curve down to 4,400 m, but it is considerably lowerbelow this depth.

Variations in source rock potentialBulk parametersThe studied dataset from the upper Draupne Formationshows an average measured Hydrogen Index of 267g HC/kg Corg. After restoration, the Hydrogen Index

0 200 400 600 800

HI (kg HC/t Corg)

5000

4000

3000

2000

TVDssinm

0 40 80 120 160 200

BI (100*S1/TOC)

0 0.2 0.4 0.6 0.8

PI

400 440 480

Tmax (oC)

0 0.4 0.8 1.2 1.6

Ro in %

HI =396 kg HC/t C0 org

Top ExpulsionWindow

Top OilWindow

435Co

a b c d e

PI (S1/(S1+S2))

Fig. 4. Depth plots of (a) Hydrogen Index (HI), (b) Bitumen Index (BI), (c) Production Index (PI), (d) Rock-EvalTmax and (e) vitrinite reflectance for the lower Draupne Formation. These depth plots were used to determinethe transformation ratio which was then used to restore TOC and Hydrogen Index to their prematurationvalues.


0 0.2 0.4 0.6 0.8 1Tr in %

5000

4000

3000

TVDssinm

LLNL TypeIILLNL TypeIIIUpper Draupne FmLower Draupne FmHeather FmVestland Group

shows a bimodal distribution (Fig. 6) with peaks ataround 300 and 450 g HC/kg Corg. This derives fromthe fact that only the mature samples are corrected,which form a second peak. The distribution suggeststhat the upper Draupne Formation contains a mixtureof Type II and Type III kerogen with a highercontribution of Type II source material. Averagerestored TOC is 5.3 wt% and the samples show a TOCrange of up to 12 wt%. The lower Draupne Formationshows a considerable lower average restoredHydrogen Index of 234.2 g HC/kg Corg and a moreunimodal distribution, indicating a greater proportionof terrestrial dominated kerogen of Type III to II (Fig.6). The average restored TOC is 4.1 wt%. Even loweraverage restored TOC and Hydrogen Index values of3.6 wt% and 184.2 g HC/kg Corg respectively, aretypical for the Heather Formation. The MiddleJurassic Vestland Group is difficult to evaluate andaverage values for TOC and Hydrogen Index are oflittle use as no lithological differentiation betweencoals and non-coaly samples has been made. TOCranges up to 80 wt%, while restored Hydrogen Indicesreach up to 400 g HC/kg Corg (Fig. 6).

Gamma-ray log character and vertical changesin HI and TOCCorrelation of the Upper Jurassic section using welllogs is difficult in the South Viking Graben. Whilethe basinal sections are thick, are often expanded bymass flow deposits and span a long time interval, thesections on the high are thin and incomplete due toerosion and non-deposition. The upper DraupneFormation is often found to rest directly on the HeatherFormation on highs in the area. However, a numberof general observations can be made. Thus, a generalincrease in gamma-ray values up to the top of geneticsequence C2 or the base of sequence D (Fraser et al.,2003) in the Middle Volgian can be observed, withgamma-ray API values up to 300 (Fig. 7). This peak

appears to be of basinwide extent and is only missingwhere the section is eroded. Overlying this “hot”interval, which corresponds to the hot shale of Kubalaet al. (2003), is one unit with slightly lower gamma-ray values and another unit with elevated values ontop of the section which is not present everywhere.Kubala et al. (2003) made similar observations in theEast Shetland Basin and defined a hot shale unitfollowed by an upper cool shale and an upper hot shale.

The peak of the gamma-ray log coincides with theboundary chosen for the syn- and post-rift sections inthis study, for example in wells 15/2-1, 15/3-3 and15/9-18 (Fig. 7). In other wells, for example 15/3-1Sand 15/3-5, this peak occurs slightly earlier or later.The time of this gamma-ray peak coincides roughlywith a regressive period in the mid-Volgian (Fig. 2)(Fraser et al., 2003).

These observations were used to divide the UpperJurassic into syn- and post-rift packages, incombination with the Pr/Ph ratio which tends to changefrom a general upward decrease to a slight increase atthe same time (Fig. 7). Kubala et al. (2003) used asimilar subdivision and grouped their “hot shale” andthe overlying units as an “upper facies” and theunderlying units as a “lower facies” of the KimmeridgeClay Equivalent. TOC values generally tend to followthe gamma-ray trend and are overall higher in theupper Draupne Formation. In addition, a generalupwards increase of the Hydrogen Index was observed(Fig. 7) as previously described by Dahl and Speers(1985), Huc et al. (1985) and Justwan and Dahl (2005).Hydrogen Index values were all maturity corrected;therefore, this trend must be related to the sourcefacies. The topmost section may, however, show aslight decrease in Hydrogen Index, as was alsoobserved by Justwan and Dahl (2005).

The similarity between the East Shetland Basin andthe South Viking Graben suggests the rather uniformdevelopment of the Kimmeridge Clay Equivalent in

Fig. 5. Comparison of empirically-determinedtransformation ratios for the upper and lowerDraupne Formation, the Heather Formation andthe Middle Jurassic Vestland Group, withtransformation ratios for Type II and III kerogenbased on standard Lawrence Livermore NationalLab kinetics (as used in Cornford et al., 1998). Ratiosare plotted versus depth (not vitrinite reflectance),based on a regional vitrinite reflectance-depthrelationship.


the North Sea area, extending south into the CentralGraben (pers. comm. Iain Scotchman).

Oil and gas potentialThe source rock potential of the Upper Jurassic sourcerock intervals was quantified using the methoddescribed by Dahl et al. (2004) and Justwan and Dahl

(2005). This method is a practical and rapid means toassess the average amount of oil- and gas-producingconstituents in a source rock when only Rock-Evaland TOC data are available, as is the case for thisstudy. A cross-plot of maturity-corrected Rock-EvalS2 and TOC was used to determine the amount ofinert organic carbon represented by the intercept of

Fig. 6. Histograms for measured and maturity-corrected TOC and Hydrogen Index for the Upper and MiddleJurassic source rock sections. The histograms for the Vestland Group include samples from the Hugin andSleipner Formations. Hydrogen Indices are much more affected by the maturity correction than TOC values.

0 10 20 30 0 200 400 600 800

0 10 20 30 0 200 400 600 800

0 10 20 30 0 200 400 600 800

0 10 20 30 0 200 400 600 800

Upper Draupne Formation (210 samples, 32 wells)

Lower Draupne Formation (520 samples, 33 wells)

Heather Formation (334 samples, 36 wells)

Vestland Group (152 samples, 24 wells)

Average Values:

TOC: 4.9 wt%

TOC : 5.3 wt%0

Average Values:

TOC: 3.9 wt%

TOC : 4.1 wt%0

Average Values:

TOC: 3.5 wt%

TOC : 3.6 wt%0

Average Values:TOC: 9.2 wt%

TOC : 9.4 wt%0

Average Values:

HI: 266.6 g HC/kg C

HI : 340.1 g HC/kg C

org

org0

Average Values:

HI: 191.7 g HC/kg C


org

org0

Average Values:

HI: 162.7 g HC/kg C


org

org0

Average Values:

HI: 204.2 g HC/kg C


org

org0

0

10

20

30

%

TOC in wt%

0

8

12

16

%

4

HI in g HC/kg Corg

0

10

20

30

%

0

10

20

30

%

0

10

20

30

%

TOC in wt%

TOC in wt%

TOC in wt%

0

8

12

16

%

4

0

8

12

16

%

4

0

15

20

25

%

HI in g HC/kg Corg

HI in g HC/kg Corg

HI in g HC/kg Corg

10

540 60 80

1.2

0.8%

wt%

Maturity corrected values Measured values


the regression line with the TOC axis, and todeconvolute the remaining active TOC into oil- andgas-prone fractions. The active TOC represents amixture of two end-member kerogens with HydrogenIndex values of 675 and 150 g HC/kg Corg,respectively. The end-member values have beenchosen to fit the dataset. The oil-prone part of theactive TOC is referred to as TOCII, while the gas-prone TOC is termed TOCIII. Both are determinedfor a suite of samples and do not represent individualsample values. Table 1 shows general source rockproperties, including TOCII, III and IV (inert TOC)for the Upper Jurassic source rock section in all thewells studied.

The upper Draupne Formation generally shows thehighest oil-prone TOCII values with an average of2.8 wt%, while the lower Draupne contains less oil-prone material with an average 1.75 wt%; the Heather

Formation has the lowest oil-prone TOC of 1.05 wt%(Fig. 8). This supports the general observation thatthe upper Draupne Formation has the highest oilpotential. The content of gas-prone organic matterincreases slightly from the upper Draupne Formationto the lower Draupne and Heather Formation from1.11 wt% to 1.29 and 1.27 wt%, respectively. Valuesfor TOCIV between 1.11 and 1.65 wt% are inagreement with those recorded by Barnard et al.(1981), indicating that inertinite is a persistentcomponent in the basin. Ramanampisoa et al. (1994)observed a constant contribution of approximately 1wt% inert organic matter in their study of a thin coresection of the onshore Kimmeridge Clay in the UK.Absolute values and percentages of TOCII increaseupwards, while inert and gas-prone matter show fairlyconstant absolute values and decreasing proportionsupwards (Fig. 8). This suggests that degree of dilution

Depth(mRKB)

Gamma Ray (API Units) Pr/Ph Ratio Gamma Ray (API Units) Pr/Ph Ratio

GeneticSequence*

Generalised

Lithology

TOC, maturity corrected (wt%)

HI, maturity corrected (g HC/kg C )org

TOC, maturity corrected (wt%)

HI, maturity corrected (g HC/kg C )org

Depth(mRKB)

Formation

GeneticSequence*

Formation

UpperDraupne

LowerDraupne

Heather

E

D

C1

C2

B1

A

E

D

C1

C2

B2

B1

APre A

UpperDraupne

LowerDraupne

Heather

Generalised

Lithology

Well NOCS 15/2-1 Well NOCS 15/9-18

Dolomite Stringers

Shale

Sandstone

Carbonate Stringers

* after Fraser (2003)et al.

0 2 4 6 8 10

0 200 400 600

0 50 100 150 200

0.8 1.2 1.6 2 2.4

4300

4200

4100

4000

3900

0.8 1.2 1.6 2 2.4

0 200 400 600

0 2 4 6 8 10

0 100 200 300 400

3200

3160

3120

0.8 1.2 1.6 2 2.4

0.8 1.2 1.6 2 2.4

Fig. 7. Gamma-ray log profile, maturity-corrected TOC (black triangles) and Hydrogen Index (open circles)together with Pr/Ph ratio (black squares) for wells 15/2-1 and 15/9-18 (for well locations, see Fig. 1).The gamma-ray peak coincides roughly with the boundary between the upper and lower Draupne Formation.A maturity-independent upward increase of the Hydrogen Index can be observed. Although a generaldecrease of the Pr/Ph ratio can be observed, the trend changes at the boundary between the upper and lowerDraupne Formation from a decrease to a slight increase upwards. This observation and the gamma-rayprofile can be used to divide the Draupne Formation.


Table 1. Maturity-corrected values for oil- and gas-prone and inert organic matter and average values of theHydrogen Index for the upper and lower Draupne and Heather Formations for all wells used in the sourcerock mapping process (for well locations, see Fig. 1). Hydrogen Index (HI) in g HC/kg Corg, all TOC values inwt%.

Wel

lII

IIIIV

nH

I0n

IIIII

IVn

nII

IIIIV

nn

1.94

03.

1754

262

540.

550.

463.

5274

102.

277

01.

61.

5654

74.4

5515/03-1S

3.5

3.23

0.17

1044

5.4

120.

812.

81.

126

190.

226

02.

330.

982

824

1.55

1.13

0.92

730

87

0.57

2.45

1.38

714

6.4

70

1.13

0.9

462

.74

3.49

0.06

2.99

2535

327

0.77

2.2

2.1

4311

6.7

450.

175.

560.

275

177.

45

0.29

0.41

1.2

413

5.1

40.

291.

631.

2814

135.

714

00.

470.

7910

3510

1.76

1.56

0.98

427

6.7

43.

020

4.79

528

2.9

527

5.3

142

4.8

120

02

1.41

0.8

2.59

1720

9.8

160.

010.

70.

7315

57.4

1550

5.4

24.

21.

052.

553

370.

93

1.45

1.44

2.11

1120

9.3

113.

480

2.09

934

8.8

100.

321.

361.

122

167.

63

15/06- - - -8S

15/08-1

5.57

00.

594

585.

44

2.25

0.14

1.11

541

9.1

52.

860

2.56

236

5.24

21.

362.

420.

625

293.

85

15/09-2

1.09

2.79

1.92

816

4.9

815/093

3.54

3.13

0.13

241

7.5

215/0911

399

115/0918

1.55

0.66

0.39

834

9.8

92.

141.

890.

7832

353.

433

0.78

2.23

0.39

824

5.8

815/12-10S

0.23

0.51

0.46

2218

3.7

2215/12-11S

2.19

2.03

0.78

935

8.4

90.

362.

920.

2214

188.

815

16/012

0.02

0.18

0.4

210

6.6

31.

090

1.1

5627

8.2

580.

670.

761.

2714

187.

115

16/013

0.49

1.39

0.12

217

5.8

231

0.1

216/072

1.16

0.54

1.3

2825

4.3

3016/102

6.18

01.

115

590.

45

16/103

3.28

0.63

0.59

545

6.1

516/104

5.61

1.87

0.51

351

6.4

316/112

3.07

03.

422

409

24.

360.

181.

362

498.

82

24/061

1.38

02.

473

280.

83

1.04

01.

8617

265.

617

24/091

3.16

1.86

0.98

2333

7.6

232.

152.

740.

6134

341.

834

24/12- - - - - - - - - - -1R

1.77

2.9

2.03

422

8.2

50.

831.

682.

699

157.

19

176

225/0110

1.66

3.68

1.86

241

4.3

30

0.8

0.36

874

.68

0.55

1.13

1.14

3212

0.9

3225/0212

0.13

0.66

1.11

584

.67

25/021 4

2.9

03.

633

313.

63

1.15

0.11

0.34

540

2.6

625/031

389.

43

399.

91

1.38

2.24

1.58

521

4.9

525/041

03.

961.

592

118.

33

25/052

4.04

1.73

0.83

546

0.7

52.

391.

593.

327

252.

17

2.34

03.

544

288.

64

25/053

1.21

0.77

1.12

227

8.1

225/061

5.21

02.

645

492.

35

2.71

02.

018

308.

98

25/062

5.16

01.

432

585.

62

619

148

7.6

225/072

3.15

1.77

2.08

533

75

0.84

1.52

0.91

7124

1.8

730.

480.

661.

3122

174.

626

25/08-- -5S

72.7

143

125/087

2.9

1.77

0.13

344

3.8

325/091

636.

81

1.36

0.1

0.74

629

7.4

525/10-6S

127

112

0.9

225/10- - - - -8A

1.47

01.

098

401.

18

0.59

0.71

0.3

1032

010

25/1115

0.53

0.72

0.55

316

8.6

325/1117

34.5

126/041

4.88

0.31

1.41

750

8.6

734

5.5

30.

251.

530.

325

173

530/121

433

12.

181.

280.

844

322.

54

0.56

2.76

0.18

222

32

not p

rese

ntno

t pre

sent

not p

rese

ntno

dat

ano

t pen

etra

ted

not p

rese

ntno

t pre

sent

not p

rese

nt

no d

ata

not p

rese

ntno

dat

a

not p

rese

ntno

dat

a

not p

rese

ntno

t pre

sent

not p

enet

rate

d

no d

ata

no d

ata

no d

ata

not p

rese

ntno

t pre

sent

not p

rese

ntno

t pre

sent

not p

rese

ntno

t pre

sent

not p

rese

ntno

t pre

sent

not p

rese

nt

not p

rese

ntno

t pre

sent

not p

rese

nt

not p

rese

ntno

dat

a

not p

rese

ntno

t pre

sent

no d

ata

not p

rese

ntno

dat

ano

t pre

sent

no d

ata

no d

ata

not p

rese

nt

no d

ata

not p

rese

nt

Upp

er D

raup

ne F

mLo

wer

Dra

upne

Fm

Hea

ther

Fm

TOC

TOC

TOC

HI0

HI0

15/02-1

15/03-2

15/03-3

15/03-5

15/05-1

15/05-2

15/05-3

15/06-2

15/06-7 - - - - - - - - -


by gas-prone and inert material decreases upwards,and increased input and/or preservation of oil-pronematerial dominates the upper Draupne Formation.

In order to evaluate the regional variations in sourcepotential and to investigate the effect of dilution bymass transport from surrounding highs on thedistribution of organofacies and source potential, mapsof the oil- and gas-prone TOC as well as the inert TOCwere generated from well data by convergent trendgridding in IRAP. Approximate outlines of mass flowsands after Fraser et al. (2003) are added for visualcomparison.

Heather FormationThe least oil-prone Upper Jurassic unit in the SouthViking Graben is the Heather Formation. Oil-proneTOC values rarely exceed 2 wt% and are below 1 wt%in Blocks 24/9, 24/12 and 15/3. Interestingly, TOCIIvalues increase towards the High in the east (Fig. 9g).This could be interpreted as an effect either ofenhanced productivity or of reduced siliciclasticdilution. Maximum values are encountered in Blocks25/5, 25/8, 25/6 and in the Ling Graben. The valuesfor gas-prone matter decrease towards the east andhigh values are only encountered in the depocentresin the west where sediments were concentrated (Fig.9h). Although the influence of mass flows is lesspronounced in the Heather Formation (Fraser et al.,2003), regions with elevated values for TOCIII (Fig.9h) coincide with areas which received mass flows.High values of TOCIII on the western side of the studyarea appear to be related to transport of gas-prone andreworked material by mass flows.

High values of TOCIV on the northern edge ofthe Utsira High may be explained in terms of reworkedand gas-prone material being swept from the nearbyhigh, while elevated TOCIV values in Block 15/5could be related to mass flow processes (Fig. 9i).

Lower Draupne FormationThe thick lower Draupne Formation also showsgenerally low oil-prone TOC values, particularly inthe currently mature graben area in Blocks 15/3, 24/12 and 24/9 (Fig. 9d). The Sleipner Terrace and theLing Depression show the highest values, but arecurrently immature. Similar to the Heather Formation,the lower Draupne Formation also shows an eastwarddecrease in TOCIII (Fig. 9e). The main contributionof gas-prone material appears to originate from thewest, from the East Shetland Platform. Exceptionallyhigh values of TOCIII are encountered in Blocks 24/12, 15/3 and 15/2, areas which received mass flowsfrom the East Shetland Platform and the Utsira Highduring deposition of the lower Draupne Formation(Fig. 9d). The correlation of highly gas-prone areaswith mass flows (Fig. 9d) indicates that gas-proneorganic matter has been transported from the EastShetland Platform via these mass flows.

Another highly gas-prone area is the northern edgeof the Utsira High in Block 25/4. This area also showselevated amounts of inert organic matter, probablyderived from the nearby Utsira High. Remarkablyhigh values of TOCIV are also encountered in Blocks15/8, 15/5 and 15/2 where mass flows from the EastShetland Platform shed reworked organic matter tothe east (Fig. 9f).

1.22

1.11

2.80

1.65

1.29

1.75

1.11

1.27

1.05

0.00

1.00

2.00

3.00

4.00

5.00

6.00

TOC(wt%)

UpperDraupne Fm

LowerDraupne Fm

Heather Fm

Oil Prone TOC II

Gas Prone TOC III

Inert TOC IV

(54.6%)(37.3%)

(30.6%)

(21.6%)

(27.5%)

(37.0%)

(23.8%)(35.2%)

(32.4%)

Fig. 8. Average distribution of oil- and gas-prone and inert organic matter in the Upper Jurassic sourceintervals in wt% (in brackets: proportion in percent of total TOC). Increasing amounts of marine organicmatter from the Heather to the upper Draupne Formation are added to near constant values of inertorganic matter. Terrestrial, gas-prone organic matter is least abundant in the upper Draupne Formation.


0.50

0.50

1

1

1

1

1 . 50

1.50

1.50

2

2

2

2

2

2 . 503

3 .50

25/1225/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/1116/10

16/816/7

16/516/4

16/216/1

15/12

15/915/8

15/6

15/3

1

1 S

2

3

5

1

8 S

18

2

3

4

1

1 R

10

2

12

2

8

15

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/1

26/4

26/7

26/10

16/3

16/6 17/4

17/1

16/9 17/7

16/12 17/10

3/28 3/29 3/30

9/59/3

9/8 9/9 9/10

9/13 9/14 9/15

9/18 9/19

9/23 9/24

9/28

16/29

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

0.50

0. 501

1

1. 50

1.50

2

2

2

2. 50

2.50

3

3

3.50

3.504

4.5055.506

26/7

25/1225/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/10

16/7

16/4

16/216/1

15/12

15/915/8

15/615/5

15/3

10/6

1

1 S

2

3

5

18 S

18

2

3

4

1

1 R

10

2

12

2

8

15

1

58°00'

59°00'

60°00'

2°00' 3°00'3/303/293/28

9/3 9/5

9/8 9/9 9/10

9/13 9/14 9/15

9/249/23

9/18 9/19

9/28 9/29

17/7

17/416/616/5

16/8 16/9

17/1016/1216/11

26/10

16/3 17/1

26/1

26/4

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

. 50

0. 50

0.50

1

1

1

1. 50 1 . 50

1.50

2

2

2

2

2.50

2.50

2.50

3

3

3

3.50

3. 5

0

3.50

4

4

4

4.50

4.50

4.50

5

5

5

5.50

5. 50

5.50

6 6. 50 6.50

26/7

25/1225/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/1116/10

16/8

16/516/4

16/216/1

15/12

15/8

15/6

15/3

1

1 S

2

3

5

1

8 S

18

2

3

4

1

1 R

10

2

12

2

8

15

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/1

26/4

26/10

16/3 17/1

16/6 17/4

16/9 17/7

16/12 17/10

3/28 3/29 3/30

9/5

9/8 9/9 9/10

9/149/13

9/18 9/19

9/28

9/23

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

0.50

0.50

1

1

1.50

1.50

2

2

2

2.50

3

3

3

3.50

3.50

4

4

4.50 5

25/1125/10

25/825/7

25/525/4

25/325/225/1

24/12

24/9

16/1116/10

16/816/7

16/516/4

16/216/1

15/12

15/915/8

15/6

15/3

3/293/28

1

1 S

2

3

5

1

3

27

1

2

3

18

11 S

2

3

2

22

1

1

1 R

10

14

1

2

2

8

1

58°00'

59°00'

60°00'

2°00' 3°00'3/30

9/5

9/8 9/9 9/10

9/13 9/14

9/18 9/19

9/23 9/24

9/28

26/1

26/425/6

25/9 26/7

25/12 26/10

16/3 17/1

16/6 17/4

16/9 17/7

17/1016/12

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

0.50

0.50

1

1

1

1.50

1.50

2

2

2

2.50

2.50

25/1125/10

25/825/7

25/525/4

25/225/1

24/12

24/9

16/1116/10

16/7

16/516/4

16/216/1

15/12

15/915/8

15/6

15/3

1

1 S

2

3

5

1

3

27

1

2

3

18

11 S

2

3

2

22

1

1

1 R

10

14

1

2

2

8

1

58°00'

59°00'

60°00'

2°00' 3°00'

25/3

25/6

16/12 17/10

16/8 16/9 17/7

26/1

26/4

25/9 26/7

25/12 26/10

16/3 17/1

16/6 17/4

9/8 9/9 9/10

9/59/3

3/28 3/29 3/30

9/13 9/14

9/23 9/24

9/28

9/18 9/19

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

0.50

1

1

1

1

1.50

1.50

1.50

1.50

2

2

2

2

2

2.50

2.50

2.50

3

3

3.50

3.50

4

4

25/1125/10

25/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/10

16/7

16/516/4

16/216/1

15/12

15/915/8

15/6

15/3

16/29

1

1 S

2

3

5

1

3

27

1

2

3

18

11 S

2

3

2

22

1

1

1 R

10

14

1

2

2

8

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/4

26/1

25/9 26/7

25/12 26/10

16/3 17/1

16/6 17/4

16/9 17/7

17/1016/1216/11

16/8

3/28 3/29 3/30

9/3 9/5

9/8 9/9 9/10

9/13 9/14

9/18 9/19

9/23 9/24

9/28

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

(a) TOCII (b) TOCIII (c) TOCIV

(d) TOCII (e) TOCIII (f) TOCIV

Upper Draupne Formation

Lower Draupne Formation

Fig. 9. Quantitative source rock potential maps (maturity-corrected) of the Upper Jurassic section in theSouth Viking Graben.(a-c) oil-prone, gas-prone and inert organic matter for the upper Draupne Formation.(d-f) oil-prone, gas-prone and inert organic matter for the lower Draupne Formation.(g-i) (facing page) oil-prone, gas-prone and inert organic matter for the Heather Formation.Approximate maximum extent of mass flow sands after Fraser et al. (2003).


The maps in Fig. 9 clearly show the influence ofgas-prone and inert organic matter transported by massflows and their dilution effect. In the fairly narrowSouth Viking Graben, gas-prone as well as reworkedmaterial may have reached the deepest part of thegraben from both flanks. This led to a complex mixingof oil- and gas-prone organic matter.

Upper Draupne FormationThe maps for the upper Draupne Formation illustratethe significantly different organic facies within thissection. Oil-prone organic matter reaches 3.5 wt % inmost of the study area (Fig. 9a). Maxima occur on thewestern flank of the Stord Basin in Blocks 25/6 and26/4, on the southern edge of the Utsira High in Block15/6 and in the Ling Depression, areas which are allcurrently immature. Minimum values are encounteredon the Utsira High in Blocks 16/1 and 25/11. A furtherzone of low oil potential is located in Blocks 24/12,15/3 and 15/5, where mass flows of the Brae trendremained active (Fraser et al., 2003).

The amount of gas-prone organic matter isgenerally lower than of oil-prone material and againshows an eastward decrease towards the Utsira High(Fig. 9b). In contrast to the lower Draupne Formation,mass flows are scarce and are only present in thelowermost section of the post-rift upper Draupne

Formation (Fraser et al., 2003). The input of gas-proneand inert organic matter resulting from mass flows istherefore greatly reduced.

Another factor that may contribute to smallamounts of gas-prone material is that during late phaserifting and subsequent quiescence, the greater depthand the elongated basin shape favoured waterstratification and algal productivity, while the upliftof the shoulders restricted terrestrial input (Huc,1990). Wignall and Ruffell (1990) related the reducedterrestrial input in the Wessex Basin to climatic changetowards arid conditions from the Early Volgianonwards. This may also be applicable to the study area.Elevated values for TOCIII are shown in Block 25/1and can be interpreted as run-off from the EastShetland Platform, as this area shows an additionalregional low in oil-prone organic matter content. Inert,reworked organic matter (Fig. 9c) ranges from 0.5 toaround 2 wt% in the graben area in the upper DraupneFormation. The maximum values encountered inBlocks 15/3 and 15/2 as well as Block 25/7 appear tobe associated with mass flows originating from theEast Shetland Platform and the Utsira High (Fig. 9c).

Another area showing elevated values of TOCIV,which can be related to transport of reworked organicmatter from the nearby Utsira High, is located in Block25/6 on the western flank of the Stord Basin.

00.5

11.5

22.5

33.5

44.5

55.5

66.5

77.5

8Oil-, gas-prone and inert TOC in wt%

0 10 20 30 40 50km

Approximate outline of mass flowunits (after Fraser 2003)et al.

Major structuralelements

0.50

0.50

0. 50

. 50

1

1

1

1

1. 50

1.50

1 . 5

0

1.50

1.

50

2

2

2

2

2 . 50

2. 50

33 .

50

25/1125/10

25/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/1116/10

16/816/7

16/4

16/216/1

15/12

15/915/8

15/6

15/3

16/29

9/15

1

1 S

2

3

5

27

8 S

1

18

10 S

11 S

2

2

1

10 12

14

1

2

3

1

2

71

1

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/1

26/4

26/7

26/1025/12

17/116/3

16/5 17/416/6

17/716/9

16/12 17/10

3/28 3/29 3/30

9/5

9/109/99/8

9/14

9/199/18

9/23 9/24

9/28

9/13

4

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben0.50

1

1

1

1.50

1.50

1.50

2

2

2

2.50

2.50

3

3.50

26/4

25/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/10

16/816/7

16/516/4

16/216/1

15/12

15/915/8

15/6

15/3

1

1 S

2

3

5

27

8 S

1 18

10 S

11 S

2

2

1

10 12

14

1

2

3

1

2

71

1

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/1

26/7

26/1025/12

16/3 17/1

17/416/6

17/716/9

16/11 16/12 17/10

3/28 3/29 3/30

9/3 9/5

9/109/99/8

9/13 9/14 9/15

9/18 9/19

9/28

9/23 9/24

3

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

1

1

11

1.50

1.50

2

26/4

25/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

17/10

17/7

16/12

17/4

16/1116/10

16/8 16/916/7

16/516/4 16/6

16/216/1

15/12

15/915/8

15/6

15/3

16/29

1

1 S

2

3

5

278 S

1

18

10 S

11 S

2

2

1

10 12

14

1

2

3 1

2

71

1

1

58°00'

59°00'

60°00'

2°00' 3°00'

26/7

26/1025/12

17/116/3

9/28

9/23 9/24

9/18 9/19

9/149/13

9/8 9/9 9/10

9/3 9/5

3/28 3/29 3/30

26/1

16/6 17/4

16/9 17/7

16/12 17/10

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin


Viking Graben

(g) TOCII (h) TOCIII (i) TOCIV

Heather Formation


Tabl

e 2.

Mat

urit

y-co

rrec

ted

TO

C a

nd H

ydro

gen

Indi

ces,

GC

and

GC

MS

rati

os fo

r se

lect

ed s

ourc

e ro

ck s

ampl

es fr

om t

he u

pper

and

low

er D

raup

ne F

orm

atio

n as

wel

l as

the

Hea

ther

, Hug

in a

nd S

leip

ner

Form

atio

ns. F

or d

escr

ipti

ons

of t

he p

aram

eter

s, se

e Ta

ble

3.

Dep

thW

ell

m R

KB

Form

atio

nTO

C0

HI0

AB

CD

EF

GH

IJ

KL

15/3- 3

4065

Upp

er D

raup

ne19

.64

246.

51.

270.

550.

490.

721.

170.

221.

430.

580.

340.

420.

961.

2615/33

4070

Upp

er D

raup

ne10

.43

305.

21.

310.

540.

470.

761.

020.

252.

290.

580.

10.

370.

911.

3115/33

4090

Upp

er D

raup

ne8.

1530

3.3

1.27

0.53

0.49

0.76

1.02

0.24

2.76

0.59

0.08

0.35

1.05

1.2

15/33

4095

Upp

er D

raup

ne5.

6234

7.2

1.17

0.59

0.55

0.72

0.98

0.24

3.33

0.6

0.08

0.36

11.

3125/72

4058

Upp

er D

raup

ne5.

5129

9.8

1.21

0.4

0.41

0.86

0.99

0.24

6.31

0.56

0.13

0.42

1.11

1.4

25/72

4070

Upp

er D

raup

ne8.

1734

4.6

0.97

0.35

0.42

0.83

1.02

0.3

5.83

0.57

0.14

0.39

1.06

1.51

15/918

3117

.5U

pper

Dra

upne

4.88

484

1.23

0.57

0.56

0.77

0.82

0.27

0.49

0.6

0.17

0.61

0.82

1.21

15/918

3127

.5U

pper

Dra

upne

0.87

296.

61.

070.

670.

610.

660.

820.

290.

550.

540.

170.

960.

831.

2115/33

4175

Low

er D

raup

ne9.

9423

2.6

1.46

0.36

0.36

0.92

1.05

0.09

3.68

0.6

0.09

0.48

1.63

1.05

15/33

4325

Low

er D

raup

ne5.

1412

5.8

1.43

0.4

0.34

0.79

1.05

0.07

2.81

0.59

0.06

0.43

1.48

1.36

15/33

4416

Low

er D

raup

ne5.

7116

41.

870.

640.

430.

781.

040.

082

0.6

0.12

0.71

21.

1815/35

3840

Low

er D

raup

ne4.

0419

5.7

1.43

0.66

0.64

0.73

1.12

0.12

10.

590.

130.

411.

271.

116/72

2584

Low

er D

raup

ne3.

4537

81.

680.

960.

790.

881.

150.

040.

330.

460.

190.

490.

830.

5725/110

4305

Low

er D

raup

ne1.

1472

.91.

940.

360.

240.

861.

010.

131

0.53

0.15

0.9

1.67

1.12

25/72

4118

Low

er D

raup

ne5.

4915

5.1

1.46

0.54

0.53

0.91

10.

165.

060.

590.

10.

71.

331.

5215/918

3182

.5Lo

wer

Dra

upne

4.05

329.

41.

450.

780.

60.

680.

940.

120.

40.

550.

210.

672.

161.

1725/52

3150

Hea

ther

3.71

101.

12.

831.

40.

350.

661.

30.

350.

180.

60.

260.

791.

60.

9625/52

3160

Hea

ther

6.23

465.

32.

040.

890.

520.

751.

250.

040.

450.

60.

180.

641.

641

25/110

4320

Hea

ther

1.04

61.2

1.86

0.39

0.27

0.91

1.06

0.07

1.25

0.61

0.14

0.94

11.

325/110

4330

Hea

ther

1.21

69.3

2.19

0.42

0.26

0.89

1.04

0.06

1.6

0.54

0.15

0.86

71.

4425/110

4340

Hea

ther

0.95

502.

110.

480.

320.

881.

080.

071.

040.

60.

131.

041.

41.

2125/110

4380

Hea

ther

2.67

170.

11.

870.

430.

250.

791.

380.

071.

020.

580.

140.

891.

61.

1225/72

4400

Hea

ther

4.67

114.

51.

730.

890.

730.

911.

030.

161.

770.

640.

240.

952.

251.

5715/918

3232

.5H

eath

er4.

8728

1.5

1.43

0.61

0.49

0.64

0.95

0.17

0.68

0.58

0.18

0.63

1.27

1.48

15/918

3280

Hug

in58

.65

121.

65.

868.

791.

110.

240.

930.

40.

050.

580.

351.

398.

180.

1316/72

2674

Hug

in9.

7118

1.7

2.4

0.75

0.54

0.91

0.94

0.65

0.24

0.42

0.32

0.56

11.

2525/72

4433

Hug

in1.

5483

1.56

0.82

0.66

0.93

1.14

0.1

1.24

0.6

0.13

0.95

11.

1315/35

3940

Slei

pner

2.82

116.

33.

690.

870.

220.

481.

190.

020.

210.

60.

120.

672.

90.

6715/918

3430

Slei

pner

4.11

217

2.27

0.99

0.41

0.52

1.04

0.11

0.08

0.51

0.34

0.85

1.36

0.89

15/918

3560

Slei

pner

1.81

131.

51.

910.

70.

410.

490.

850.

160.

230.

510.

280.

881.

570.

99

Roc

k-Ev

alG

C R

atio

sG

CM

S R

atio

sM 1.

21.

181.

151.

161.

161.

131.

471.

441.

01 1 0.88

0.95

0.86

1.17

1.07

1.24

0.74

1.22

1.42

1.23

1.13

1.19 1.3

1.5

0.24

0.81

1.13

0.36

0.79

0.95

- - - - - - - - - - - - - - - - - - - - - - - - - - - --


The distribution of oil- and gas-prone organicmatter does not follow the commonly observed patternwhereby the highest potential is in the deepest partsof the graben and there is increasing terrestrialdominance towards the highs (Fisher and Miles, 1983;Thomas et al., 1985). Rather, the deeper graben areas,acting as sediment traps, show the highest content ofterrestrial material.

Molecular source faciesand isotopic characteristicsThe molecular and carbon isotopic characteristics ofthe Upper and Middle Jurassic source rocks wereinvestigated to support the source potential mappingand to gain a better understanding of the regional andstratigraphic trends in geochemical properties. Thecarbon isotopic composition of the saturate andaromatic fraction of source rock extracts reflectsvariations in kerogen type and source maturity, butmight also be affected by factors such as temporalvariations in carbon cycling or local changes inpaleoproductivity (Tyson, 2004). The distribution oftriterpanes and steranes analysed by GCMS also givesvaluable information on source rock depositionalenvironment and development of the Jurassic sourcerock system. Biomarker ratios for selected samplesare listed in Table 2.

Geochemical characteristicsof Middle Jurassic source rocksMolecular and isotopic characteristics of MiddleJurassic source rocks show a great deal of variationas the samples represent different lithofacies rangingfrom mudrocks to coaly shales and coals. Middle

Jurassic source rocks in the Hugin and SleipnerFormation are isotopically heaviest with δ13C valuesranging from -24 to -28 ‰PDB for saturate andaromatic fractions (Fig. 10). Coaly samples show theheaviest ratio values while the shaly and sandysamples are generally lighter. This can be interpretedin terms of kerogen type (Cooper et al., 1993) as TypeIII facies for the coaly samples and a higher Type IIcontribution for the shaly samples.

The extracts from the Middle Jurassic section aredominated by higher molecular weight compounds inthe range of n-C23 to n-C30 (Fig. 11); the MiddleJurassic source rocks therefore show in general a lown-C17/(n-C17 + n-C27) ratio. The presence of these highmolecular weight n-alkanes in the Hugin and SleipnerFormation can be attributed to terrestrial plant input(c.f. Tissot and Welte, 1984), while the short chain n-alkanes are reported to be constituents of algae (Gelpiet al., 1970). The dominance of high molecular weightcompounds is more pronounced in the SleipnerFormation and the coaly samples, while the shales ofthe Hugin Formation show a higher proportion of shortchain compounds.

The distribution of C30 17β(H),21α(H)-moretaneand the corresponding C30 17α(H),21β(H)-hopanealso shows high variability. Middle Jurassic samplescommonly exhibit the highest moretane contents,indicating the highest terrestrial contribution to thekerogen (Isaksen and Bohacs, 1995) (Fig. 12d). Theratio of rearranged (dia-) steranes to regular steranesalso differs in the various sections. Middle Jurassicsamples show values approaching unity, whileoverlying sections show values above unity. Thedistribution of C27 to C29 regular steranes was

Table 3. Description of selected facies-indicating GC and GCMS parameters given in Table 2.

A Pristane/Phytane ratio Brooks et al. (1969); Didyk et al. (1978)

B Pristane/n-C17 -

C Phytane/n-C18 -

D Waxiness: n-C17/(n-C17+n-C27) -

E Carbon Preference Index (CPI) Bray and Evans (1961)

F Isaksen et al. (1998)

G Ts/Tm Seifert and Moldowan (1985)

H Seifert and Moldowan (1986)

I Isaksen and Bohacs (1995)

J C29 versus C30 hopanes after Peters and Moldowan (1993)

K C34/C35 : Ratio of C34 versus C35 homohopanes after Peters and Moldowan (1991)

L Ratio of C27-C29 diasteranes versus C27-C29 regular steranes after Peters and Moldowan (1993)

MC27/C29: Ratio of C27 ββ(R+S) steranes versus C29

regular steranes, measured in m/z 218Huang and Meinschein (1979)

BNH%: 17α(H),21β(H) -28,30-bisnorhopane versus 17α(H),21β(H)-28,30-bisnorhopane + C30 hopane

22S: Ratio of 17α(H),21β(H), 22(S) bishomohopane versus 17α(H),21β(H),22(S) + (R) bishomohopanes

30βα/30αβ: ratio of C30 moretane versus C30 hopane

ββ(R+S)


-32 -30 -28 -26 -2413C SAT

-32

-28

-24

-20

13CARO

Upper Draupne FmLower Draupne FmHeather FmHugin FmSleipner Fm

-32 -30 -28 -26 -2413C SAT

-32

-28

-24

-20

13CARO

Upper Draupne FmLower Draupne FmHeather FmHugin FmSleipner Fm

Entire data set Well 15/9-18

Increasing marineinfluence

3410m

3390m

3280m

3232.5m3175m

3145m3137.5m

3155m

3195m3202.5m

3112.5m

3117.5m

3125m3132.5m

a b

δ

δδ

δ

determined in the m/z 218 fragmentograms. Accordingto Huang and Meinschein (1979), the dominant sourceof C27 sterols is zooplankton while C29 sterols occurmainly in higher plants. It has been found that theratio of C27 versus C29 regular steranes increases onaverage upwards from the Sleipner to the upperDraupne Formation, suggesting an increase in marine,algal influence. Coaly shales and coals show ratiovalues as low as 0.2 while the ratio in shaly samplescan reach 1.1 (Table 2). In general however, theMiddle Jurassic extracts show a dominance by C29steranes, indicating a higher input of terrestrialmaterial.

The distribution of acyclic isoprenoids can be usedto distinguish the source intervals. Pr/Ph ratios rangefrom ca 1.5 for shaly samples in the Hugin Formationto over 6 for coals and coaly shales (Fig. 12c). ThePr/Ph ratio is considered to indicate the degree ofoxygenation in the depositional environment (Brookset al., 1969; Didyk et al., 1978), although itssignificance as an indicator of palaeoenvironmentalconditions is still debated (Brassell et al., 1987; tenHaven et al., 1987). High values of the Pr/Ph ratiofor Middle Jurassic samples are consistent with thefact that the coaly shales and coals were deposited inan oxygenated fluvio-deltaic environment with a highterrestrial OM input.

Geochemical characteristicsof the Heather FormationThe samples of the Upper Jurassic Heather Formationare isotopically slightly lighter than the MiddleJurassic samples with δ13C for saturate fractionsranging from -26.5 to -28.5 ‰, and for aromaticfractions from -25 to -28.5 ‰PDB (Fig. 10). This canbe interpreted using the approach of Cooper et al.

(1993) in terms of an increased Type II contributionrelative to the Middle Jurassic samples. The samplesof the Heather Formation often show a bimodal n-alkane distribution with maxima from n-C14 to n-C18and n-C21 to n-C29 and an odd-over-even carbonnumber dominance (Fig. 11). Similar to the MiddleJurassic samples, the Heather Formation samplesexhibit a high C30 moretane content relative to C30hopane (Fig. 12d). The ratio of C29 hopane to C30hopane is similar to that in the Middle Jurassic samplesand distinctively higher than that in the DraupneFormation samples. C34 homohopanes dominate overthe C35 homologues in the Heather Formation (Fig.12b), indicating oxidising conditions (Peters andMoldowan, 1993). The Pr/Ph ratio varies significantlybetween 1.16 and 3.29; its average (1.98) is lowerthan the average for the Middle Jurassic (Fig.12c),indicating more reducing conditions in the HeatherFormation.

Geochemical characteristicsof the Draupne FormationThe molecular and isotopic characteristics of the syn-and post-rift sections of the Draupne Formation arevery similar. However, certain parameters, such as thePr/Ph ratio, carbon isotopic composition and thecontent of 17α(H),21β(H)-28,30-bisnorhopane, maydistinguish the syn- and postrift facies.

The samples derived from the upper DraupneFormation are on average isotopically lightercompared to those from the lower Draupne Formation,although there is a large variation in the analysed data(Fig. 10). The bulk of the upper Draupne data showsδ13C saturate values from -29.5 to 32 ‰PDB andaromatic values between -28.5 and -32 ‰, indicatinga relatively high algal contribution to the kerogen

Fig. 10. Stable carbon isotopic composition of aromatic and saturate fraction of source rock extracts.(a) Entire data set; (b) well 15/9-18, displaying a trend of progressively lighter isotopic compositions from theMiddle Jurassic upwards.


4045

5055

6065

7040

4550

556045

5055

6010

2030

4050

6070

4045

5055

6065

7045

5055

60

PrPh

PrPh

1718

23

17

18

23

(1) (2)

(3)(4)

(5)

(6)(7) (8) (9) (10)

(15) (16)

(14)

(13)

(12)

(11)

(1) (2)(3)(4)

(5) (6)(7)(8) (9) (10)

(15) (16)

(14)

(13)

(12)

(11)

(27) (28)

(26)

(25)

(24)

(23)

(27)(28)

(26)

(25)

(24)(23)

(21)

(20)

(19)(22)

(18)

(17)

(21)

(20)

(19)(22)

(18)

(17)

4045

5055

6065

7010

2030

4050

6070

4045

5055

6045

5055

6010

2030

4050

6070

4045

5055

6065

7045

5055

6045

5055

60

1020

3040

5060

7040

4550

5560

6570

4550

556045

5055

60

Well15/3-3,4095m

TOC:5.62wt%

HI:

347gHC/kgC

Pr/Ph:1.17

0

0org

m/z191

Well15/3-3,4315-4320m

TOC:2.72wt%

HI:

203gHC/kgC

Pr/Ph:1.52

0

0org

GCFID

GCFID

m/z217

m/z218

m/z191

m/z217

m/z218

UpperDraupneFormation LowerDraupneFormation

1020

3040

5060

7080

4550

5560

6570

758040

4550

5560

657050

5560

6570

PrPh

1718

23

(1) (2)(3)(4) (5) (6)(7) (8) (9) (10)

(14)

(13)

(12)

(11)

(27) (28)

(26)

(25)

(24)

(23)

(21)

(20)

(19)

(22)

1020

3040

5060

7080

4550

5560

6570

7580

4550

5560

657050

5560

6570

Well15/9-18,3280m

TOC:58.65wt%

HI:

207gHC/kgC

Pr/Ph:5.86

0

0org

GCFID

m/z191

m/z217

m/z218

Retentiontime(min.)

Retentiontime(min.)

Retentiontime(min.)

Retentiontime(min.)

MiddleJurassicHuginFormation

4045

5055

6010

2030

4050

6070

PrPh

1718

23

(1)(2) (3

)(4)(5) (6)(7) (8) (9) (10)

(15) (16)

(14)

(13)

(12)

(11)

(27)(28)

(25)

(24)

(23)

(26)

(21)

(20)

(19)(22)

(18)

(17)

Well15/3-5,3930-3935m

TOC:6.69wt%

HI:117gHC/kgC

Pr/Ph:3.29or

g

0

0

GCFID

m/z191

m/z217

m/z218

HeatherFormation

1020

3040

5060

7040

4550

5560

6570

4550

556045

5055

60

Fig.

11.

Sel

ecte

d ga

s ch

rom

atog

ram

s an

d m

/z 1

91, 2

17 a

nd 2

18 fr

agm

ento

gram

s fo

r th

e up

per

and

low

er D

raup

ne, H

eath

er a

nd H

ugin

For

mat

ions

.C

ompo

und

iden

tific

atio

n: n

-alk

anes

are

mar

ked

wit

h th

eir

carb

on n

umbe

r, P

r =

Pri

stan

e, P

h =

Phy

tane

; (1)

C27

18α

(H)-

22,2

9,30

-tri

snor

neoh

opan

e (T

s),

(2)

C27

17α

(H)-

22,2

9,30

-tri

snor

hopa

ne (

Tm

), (

3) C

28 1

7α(H

),21β(

H)-

28,3

0-bi

snor

hopa

ne (

BN

H),

(4)

C29

17α

(H),

21β(

H)-

30-n

orho

pane

(29αβ)

, (5)

C30

17α

(H),

21β(

H)-

hopa

ne (

30αβ)

, (6)

C30

17β

(H),

21α

(H)-

hopa

ne (

30βα

), (7

)-(1

6) C

31 t

o C

35 1

7α(H

),21β(

H),

22(S

)+(R

) ho

moh

opan

es, (

17)

C27

13β

(H),

17α

(H),

20(S

)-ch

oles

tane

,(1

8) C

27 1

3β(H

),17α

(H),

20(R

)-ch

oles

tane

, (19

) C

29 2

4-et

hyl-5α

(H),

14α

(H),

17α

(H),

20(S

)-ch

oles

tane

, (20

) C

29 2

4-et

hyl-5α

(H),

14β(

H),

17β(

H),

20(R

)-ch

oles

tane

,(2

1) C

29 2

4-et

hyl-5α

(H),

14β(

H),

17β(

H),

20(S

)-ch

oles

tane

, (22

) 24

-eth

yl-C

29 5α

(H),

14α

(H),

17α

(H),

20(R

)-ch

oles

tane

, (23

) C

27 5α

(H),

14β(

H),

17β(

H),

20(R

)-ch

oles

tane

,(2

4) C

27 5α

(H),

14β(

H),

17β(

H),

20(S

)-ch

oles

tane

, (25

) C

28 2

4-m

ethy

l-5α

(H),

14β(

H),

17β(

H),

20(R

)-ch

oles

tane

, (26

) C

28 2

4-m

ethy

l-5α

(H),

14β(

H),

17β(

H),

20(S

)-ch

oles

tane

, (2

7) C

29 2

4-et

hyl-5α

(H),

14β

(H),

17β(

H),

20(R

)-ch

oles

tane

, (28

) C

29 2

4-et

hyl-5α

(H),

14β

(H),

17β(

H),

20(S

)-ch

oles

tane

.


(Cooper et al., 1993). Similar values were observedby Kubala et al. (2003) for the “upper facies” in theEast Shetland Basin, while Thomas et al. (1985)reported values between -26.5 and -31.5 ‰ for theKimmeridge Clay. The bulk of the lower DraupneFormation samples had δ13C ratios of -27.5 to -30.5‰ for the saturate fraction and -26 to -31 ‰ for thearomatic fraction, and can therefore be assigned to amixture of Type II and III kerogens. The large variationin the lower Draupne Formation relative to the upperDraupne Formation (Fig. 10a) is indicative of the moreuniform organic facies of the latter unit. Fig. 10ashows the general distribution of carbon isotope ratios

of rock extracts in the area, while Fig. 10b highlightsthe upward trend towards lighter isotopiccompositions for the Jurassic section in well 15/9-18.These samples are all currently immature and showthe same upwards trend towards lighter isotopiccompositions as the entire data set, suggesting thatthis trend is facies related.

The gas chromatograms of the saturate fractionare dominated by compounds ranging from n-C15 ton-C17 for both the upper and the lower DraupneFormations (Fig. 11). This is interpreted to indicatean input of algal organic matter (Tissot and Welte,1984). The CPI values point to differences between

0

0.2

0.4

0.6

0.8

BNH%

0

2

4

6

8

10

C34/C

35homohopanes

0

2

4

6

8

Pr/Ph

0

0.2

0.4

0.6

0.8

C30ba/C

30ab

a

c d

b

UpperDraupne

LowerDraupne

Heather MiddleJurassic

UpperDraupne

LowerDraupne


UpperDraupne

LowerDraupne


UpperDraupne

LowerDraupne


n=37

n=87

n=27

n=17

n=38

n=88

n=27

n=17

n=38

n=87

n=25 n=17

n=37n=87

n=25n=17

C/C

3030

αβ

βα

Fig. 12. “Box-and-whisker” plots for selected biomarker parameters:(a) BNH%: 17α(H),21β(H)-28,30-bisnorhopane vs. 17α(H),21β(H)-28,30-bisnorhopane + C30 hopane;(b) Ratio of C34 to C35 homohopanes;(c) Pristane/Phytane ratio; (d) Ratio of C30 moretane to C30 hopane.Second and third quartiles are displayed by the box, while the median value is indicated by the line across thebox. Outliers shown as * represent values 1.5 times the interquartile range below or above the central 50% ofthe distribution. The changes in specific wells are more pronounced as the overall statistical distributionshows.


the source rock intervals, with the upper DraupneFormation showing the lowest values. Ratios of C34to C35 homohopanes close to unity for the upperDraupne Formation indicate anoxic depositionalconditions (Peters and Moldowan, 1991). Thedominance of C34 homohopanes over C35homohopanes decreases from the Middle Jurassictowards the Upper Jurassic supporting the trend ofdecreasing degree of oxygenation upwards observedby Justwan and Dahl (2005). The upper DraupneFormation samples show a clear predominance of C30hopanes relative to C29 hopanes (Fig. 11), while thedistribution of these compounds varies greatly in thelower Draupne Formation, but is higher with ratiosbetween 0.35 and 0.85. This can be interpreted interms of facies variability indicating a more uniformfacies for the upper Draupne. Also, the distributionsof C30 17β(H),21α(H)-moretane and thecorresponding C30 17α(H),21β(H)-hopane show largevariability in the Draupne Formation (Fig. 12d). Theuniformly low amounts of moretane in the upperDraupne Formation can be attributed to a small amountof terrestrial organic matter in the kerogen, while thelarge variation in the lower Draupne Formationsignifies a mixed input.

The analysed Upper and Middle Jurassic sourcerock samples generally contain small amounts oftricyclic terpanes relative to pentacyclic terpanes. TheDraupne Formation however often exhibits a highabundance of C19 relative to C21 tricyclics with valuesabove 1, rarely encountered in Heather or MiddleJurassic samples. The upper Draupne Formation isalso characterised by the presence of 17α(H),21β(H)-28,30-bisnorhopane which is commonly absent or onlypresent in small amounts in the lower Draupne andHeather Formation (Fig. 12a). This was also observedby Kubala et al. (2003) in the East Shetland Basin,Ineson et al. (2003) in the Volgian to Ryazanian shalesof the Danish Central Graben, and Justwan and Dahl(2005) in the Greater Balder area. The occurrence ofthis compound has been associated with anoxicenvironments (e.g. Grantham et al., 1980; Katz andElrod, 1983; Peters and Moldowan, 1993), and itspresence is a further indication of widespread anoxiain the upper Draupne Formation in addition to the Pr/Ph ratio and the ratio of C34 to C35 homohopanes.

The stratigraphic distribution of bisnorhopane inthe South Viking Graben appears to be different fromits distribution in the North Viking Graben as shownby Dahl (2004). In the Oseberg area, bisnorhopane isabsent in the post-rift section of the DraupneFormation. Huc et al. (1985) also noted the absenceof bisnorhopane in the “hot shales” in their study area.It is however important to note that elevated amountsof bisnorhopane were encountered in shales from theMiddle Jurassic Hugin Formation in wells 16/7-2 and

15/9-18 (Fig. 11). Upper Jurassic samples show higherratios of dia- versus regular steranes than MiddleJurassic samples, although it is difficult to differentiatebetween the Upper Jurassic sections.

The upper Draupne Formation has a narrowdistribution of Pr/Ph ratio values with an average of1.38, while the lower Draupne Formation shows amuch broader distribution and a slightly higheraverage of 1.69. The Pr/Ph ratio decreases upwardswithin the Upper Jurassic up to genetic sequence C2or the base of sequence D (sensu Fraser et al., 2003),where for most wells a change to increasing valuescan be observed (Fig. 7). This shift from decreasingto increasing values occurs, as mentioned before,approximately at the same time as the peak of thegamma-ray values. Considering all the analysedformations, a significant decrease in Pr/Ph ratio fromthe Middle Jurassic to the Upper Jurassic can beobserved.

Regional and stratigraphic variationsThe large variations in source facies and potentialrevealed by the mapping are also reflected by theirmolecular and isotopic characteristics. The amountof dispersion shows strong facies variations, andsignificant overlap in the ranges for geochemicalparameters complicates the unambiguousidentification of the source intervals. Interestingly, thevariation in molecular properties is less pronouncedin the post-rift upper Draupne Formation (Fig. 12).This is in agreement with the source rock potentialmaps presented earlier, suggesting a more uniform oil-prone facies in the upper Draupne Formation. Theincreasing dominance of C27 over C29 regular steranesupwards, as well as the trend towards lighter isotopicvalues upwards, the ratio of C30 moretane to C30hopane and the predominance of low molecular weightalkanes up to C17 in the Draupne Formation, supportsthe conclusion that marine, oil-prone Type II materialincreases upwards in abundance. Another importantconclusion is that the degree of oxygenation decreasesupwards from the Middle Jurassic, as supported byPr/Ph ratios, C34/C35 homohopane ratios and theoccurrence of bisnorhopane. Fig.12c shows thegeneral decrease in Pr/Ph ratio upwards, and also thelarge variations in this parameter.

We tried to examine the significance of thevariation by studying the geographical distribution ofPr/Ph ratio values as indices for anoxic conditions. Inorder to increase the number of data points for regionalmapping, GC data from geochemical service reportswere used as a supplement. Interlaboratory variationwas regarded as insignificant for the Pr/Ph ratio.Average Pr/Ph ratios were mapped for the Heather,lower and upper Draupne Formations (Fig. 13). It canbe observed that anoxic conditions spread in the


1

1

1.20

1.20

1.40

1.60

1.80

1.80

2

2

2.50

2.50

3

3

3.50 4

4

25/1225/1125/10

25/925/825/7

25/625/525/4

25/325/225/1

24/12

24/9

16/10

16/816/7

16/516/4

16/216/1

15/12

15/915/8

15/615/5

15/3

16/29

16/24

9/15

1

1 S3

5

1 7

8 S

118

2

23

1

1 R

10

2

2

2

1

15

1

1

58°00'

59°00'

60°00'

2°00'

2°00'

3°00'

3°00'

16/11 16/12 17/10

17/716/9

16/6 17/4

16/3 17/1

26/10

26/7

26/4

26/1

9/28

9/23 9/24

9/199/18

9/13 9/14

9/8 9/9 9/10

9/3 9/4 9/5

3/28 3/29 3/301

1.20

1.40

1.40

1.40

1.60

2

2

2.50

3

25/1125/10

25/925/825/7

25/6

25/525/4

25/325/225/1

24/12

24/9

16/10

16/7

16/516/4

16/216/1

15/12

15/915/8

15/615/5

15/3

16/29

10/6

1 S

2

3

45

18

2

2

1

1 R

10

2

2

9

6 S

1

2

58°00' 58°00'

59°00' 59°00'

60°00' 60°00'

2°00' 3°00'

2°00' 3°00'

3/28 3/29 3/30

9/3 9/4 9/5

9/109/99/8

9/13 9/14 9/15

9/199/18

9/23 9/24

9/28

26/1

26/4

25/12 26/10

26/7

17/116/3

16/6 17/4

16/8 17/716/9

16/11 16/12 17/10

Utsira High

SleipnerTerrace

Ling Graben

Stord Basin

BjørgvinArch

BjørgvinArch

Utsira High

Ling Graben

Stord Basin

11.20

1.40

1.60

1.80

1.80

2

2

2.50

3

3.50

3.50

4

4

25/1225/1125/10

25/925/825/7

25/625/525/4

25/225/1

24/12

24/9

17/1016/10

16/7

16/516/4

16/216/1

15/12

15/915/8

15/615/5

15/3

16/29

10/6

9/29

4/26

1 S

2

3

5

8 S

18

11 S

2

2

10 12

2

2

2

79 1

1

58°00'

59°00'

60°00'

2°00' 3°00'

2°00' 3°00'

25/3 26/1

26/4

26/7

26/10

17/116/3

16/6 17/4

16/8 16/9 17/7

3/303/293/28

9/59/49/3

9/109/99/8

9/159/149/13

9/18 9/19

9/23 9/24

9/28

16/11 17/1016/12

Utsira High

SleipnerTerrace Ling Graben

Stord Basin

BjørgvinArch

0

0.8

1

1.2

1.4

1.6

1.8

2

2.5

3

3.5

4

4.5 0 10 20 30 40 50kmAverage Pr/Ph ratio

Upper Draupne Fm Lower Draupne Fm Heather Fm

a b c

SleipnerTerrace

Fig. 13. Maps of average Pr/Ph ratios for (a) the upper Draupne (b) the lower Draupne and (c) the HeatherFormation, contoured in IRAP using convergent trend gridding.The large variation in individual values observed in the data set can here be interpreted in map view.Decreasing values upwards can be interpreted in terms of the spreading of anoxia in the area during the LateJurassic transgression.

graben centre during the transgression in the UpperJurassic from local patches in the Heather Formation(Fig. 13c) to a more continuous north-south orientedarea in the lower Draupne Formation (Fig. 13b), to agenerally anoxic graben system in the upper DraupneFormation (Fig.13a). Depositional conditions in theUtsira High in Blocks 25/11 and 16/1 appear to havebeen oxygenated throughout the Late Jurassic. Theobservations support the generally accepted model thatthe Upper Jurassic Draupne Formation was depositedin anoxic bottom waters; however, they demonstratethe development of anoxic conditions through timein the Viking Graben.

The data presented here support a model of watercolumn stratification with gradual ascent of the O2:H2Sinterface. Such a mechanism was proposed on asmaller scale for the cycles in the Kimmeridge ClayFormation in the Wessex Basin by Tyson (1979).Conditions for source rock formation were mostfavourable during the mid-Volgian to Late Ryazanianwhen dysoxic to anoxic conditions were widelyestablished in the South Viking Graben.

Controls on source potentialand source facies distribution

As mentioned above, the source facies distribution ofJurassic source rocks in the South Viking Graben iscontrolled by a complex interplay of factors (Cooperet al., 1993), including platform width (Barnard etal., 1981), mass transport mechanisms (Huc et al.,1985), dilution effects (Justwan and Dahl, 2005) andpreservation (Huc et al., 1985). Intensive mixing ofdifferent types of organic matter may have occurredin the fairly narrow graben, where terrestrial andreworked organic matter could have been transportedfrom surrounding highs by mass flows.

In the following paragraphs, we attempt tohighlight the influence of the various factors and todetermine the most important control on source faciesdistribution in the Upper Jurassic source rock section.The crudeness of the methods used to determine theamount of oil- and gas-prone organic matter, and thefact that averages for bulk geochemical and molecularparameters as well as the linear sediment accumulation


rates were used, does not allow detailed discussion ofproductivity and organic matter accumulation models;however, the most significant controls on the faciescan be determined.

The highest overall amount of organic matter canbe observed in depocentres in the graben due tosediment focusing and sea-floor topography. Oil-proneorganic matter, however, is not restricted to the basinalarea, but occurs also on the flanks of the highs andeven on the highs. One important example is the flankof the Stord Basin which shows high TOCII values.

Figs 8 and 9 illustrate the role of dilution by gas-prone and inert material on the source faciesdistribution. Average values for oil- and gas-proneTOC as well as inert organic matter show that organicmatter capable of hydrocarbon generation is mixedin variable amounts with a near-constant input of inertorganic matter; the maps of the lower DraupneFormation show that dilution of oil-prone material bygas-prone material locally plays an important role. Theinput of inert organic matter is higher in the vicinityof mass flow sands as was also observed by Tyson(1989). The absolute amount of gas-prone organicmatter is fairly constant, with slightly elevated levelsin the lower Draupne Formation indicating theinfluence of mass flows which transported largevolumes of reworked and gas-prone organic matterfrom the East Shetland Platform and the Utsira High.As discussed earlier, the upper Draupne Formationshows a reduction in the proportion of gas-pronematerial. This is related to the decreased occurrenceof mass flows and the generally decreased input ofterrestrial organic matter. This reduced input ofterrestrial organic matter is probably related to an arid

climatic episode from the early Volgian to the lateRyazanian (c.f. Wignall and Ruffell, 1990).

Both absolute amount and proportion of oil-proneorganic matter increase from the Heather to the upperDraupne Formation. This is supported by moleculardata including C27 to C29 regular steranes and GCparameters. The oil-prone nature of the upper DraupneFormation must therefore be explained by factors otherthan reduced dilution by terrestrial and reworkedorganic matter (Justwan and Dahl, 2005).

The effect of anoxia on the preservation of organicmatter was therefore evaluated. The increasingimportance of anoxia from the Middle Jurassic to theupper Draupne Formation is shown by a series offactors including increasing Pr/Ph ratios, increasingrelative amounts of bisnorhopane and C34/C35homohopane indices. Cross-plots of TOC and Pr/Phindicate that TOC values increase with decreasing Pr/Ph ratios. Both plots of measured Pr/Ph ratios versusrestored TOC values (Fig. 14a), as well as a plot ofaverage Pr/Ph ratios versus TOCII values used in themapping (Fig. 14b), show negative correlations. Thereis a stronger dependency of TOC and TOCII on thePr/Ph ratio in the lower Draupne and HeatherFormation. The large variation in TOC values in theupper Draupne Formation, however, cannot beattributed to variations in oxygenation, as anoxia iswidespread from the Middle Volgian onwards.

In order to investigate further the cause of thevariations in TOCII in the upper Draupne Formation,the relationship between the amount of oil-pronematerial and the average compacted linear sedimentaccumulation rate for the wells used in Fig. 9 wasexamined using a log-log cross-plot (Müller and Süss,

0 5 10 15 20 25TOC in wt% (restored)

0

1

2

3

4

5

6

Pr/Ph

0 2 4 6TOC II (wt%)

0

2

4

6

AveragePr/Ph

a b Upper Draupne FmLower Draupne FmHeather Fm

Upper Draupne FmLower Draupne FmHeather Fm

Fig. 14. Cross-plots of TOC and Pr/Ph ratios. (a) Measured Pr/Ph ratios and restored TOC for individualsamples. (b) Average Pr/Ph ratios and TOCII (as shown on maps in Figs 9 and 13).The amount of oil-prone organic mater appears to increase with decreasing Pr/Ph ratios in the lowerDraupne and Heather Formation. No direct relationship is evident in the upper Draupne Formation, low Pr/Ph values are common. This relation can be interpreted as an effect of increased preservation of organicmatter in increasingly more anoxic environments.


1979). Sand intervals have been excluded and thedurations used in the calculation have been taken fromEsso’s proprietary sequence stratigraphic fameworkusing the timescale of Hardenbol et al. (1998). Fullsequences without gaps have been assumed forsimplification. Although not very pronounced, TOCIIvalues increase up to a linear sediment accumulationrate of around 3 m/Ma and decrease from this pointonwards with increasing rates due to siliciclasticdilution (Fig. 15). This phenomenon was previouslydescribed by Ibach (1982) using a large database ofDSDP samples. Ibach however determined a muchhigher critical sedimentation rate of 40.8 m/Ma. Thevariations in the upper Draupne Formation’s oilpotential can thus be attributed to variations insiliciclastic dilution. The maxima on the flank of theStord Basin in wells 25/6-1, 25/6-2 and 26/4-1 andthe southern edge of the Utsira High in well 15/6-8 Swere probably caused by reduced siliciclastic inputat sedimentation rates ideal for the preservation oforganic matter, and not (as previously proposed) byenhanced preservation in local pool-like areas withstagnant water (Justwan and Dahl, 2005). The lowerDraupne and the Heather Formations have generallyhigher accumulation rates above the critical value, andshow decreasing TOC values with increasingsiliciclastic input (Fig.15). There is no correlationbetween TOCIV and linear sediment accumulationrate, but there is a weak positive correlation betweenTOC and linear sediment accumulation rate for theupper, lower Draupne and Heather Formations (notshown), indicating that a higher amount of gas-pronematerial is transported to the basin with highersiliciclastic input.

The source facies of the upper Draupne Formationtherefore appears to be controlled principally by thereduced input of gas-prone organic matter, thegenerally good preservation in a widely anoxic

environment, and the reduced siliciclastic dilution.The facies and potential of the lower DraupneFormation, on the other hand, appear to depend to agreater extent on dilution by reworked and gas-proneorganic matter, derived by mass flow processes fromsurrounding highs. Preservation effects andsiliciclastic dilution still play a role in the control onthe facies distribution, but to a lesser degree than inthe upper Draupne Formation. The degree ofoxygenation and thus preservation, as well as theamount of siliciclastic dilution, are the key factorsdetermining the facies of the Heather Formation.Dilution by gas-prone and reworked organic matteris only of local importance.

CONCLUSIONS

(1) The upper section of the Draupne Formationof mid-Volgian to Ryazanian age is up to 330 m thickin the South Viking Graben, and represents a post-riftclay drape deposited with low sediment accumulationrates averaging 7 m/Ma in the analysed wells. Thesyn-rift section of Oxfordian to mid-Volgian age isup to 1,600 m thick in the graben centre and has ahigher average linear sediment accumulation rate of12 m/Ma in the analysed wells. The up to 930 m thickHeather Formation was deposited more rapidly thanthe Draupne Formation (20 m/Ma on average, in theanalysed wells).

(2) The mid-Volgian section is characterised by abasin-wide peak in gamma-ray values. This peakcoincides with a shift in the trend of Pr/Ph ratio andthe transition from syn- to post-rift sedimentation; itcan therefore be used to divide the Upper Jurassicsection where sequence stratigraphic orbiostratigraphic data are not available.

(3) The top of the oil window for the UpperJurassic source rock section from Rock-Eval and

Fig. 15. Plot of average compacted linear sedimentation rates (LSAR) versus TOCII (log-log scale) for theupper and lower Draupne and Heather Formations. Values of oil-prone TOC increase with increasing LSARup to an optimum value of 3 m/Ma and decrease thereafter due to siliciclastic dilution. Excellent oil potentialin the upper Draupne Formation in wells 16/10-4, 15/6-8S, 25/6-1, 25/6-2, 26/4-1 and 25/7-2 is associated withsediment accumulation rates ideal for the preservation of organic matter.

0.1 1 10 100 1000Average compacted LSAR (m/Ma)

0.01

0.1

1

10

TOCIIinwt%

Upper Draupne FmLower Draupne FmHeather Fm

25/6-225/6-225/6-125/6-1

15/6-8S15/6-8S

25/7-225/7-2

16/10-416/10-426/4-126/4-1


vitrinite reflectance data occurs at 3,500 m, while theMiddle Jurassic source rocks enter the oil window ataround 3,800 m. Significant expulsion ofhydrocarbons occurs at approximately 4,000 m fromthe upper Draupne Formation, and at 4,100 m and4,200 m, respectively, from the lower Draupne andHeather Formations.

(4) Quantitative oil- and gas-potential maps basedon maturity-corrected Rock-Eval data point to the veryhigh oil potential for the thin upper DraupneFormation with maxima on the southern flank of theUtsira High and the flank of the Stord Basin. This isnot consistent with previous assumptions that oilpotential on highs and flanks is reduced. The lowerDraupne Formation has predominantly gas potential,with the strong influence of mass flow processeswhich transported gas-prone and inert organic matterto the deep basin. The gas potential of all sectionsdecreases to the east, suggesting the main sedimentsource was the East Shetland Platform. The HeatherFormation is the least oil-prone section and exhibitsmainly gas potential.

(5) Source potential and molecular and isotopicproperties show major variations in the Middle toUpper Jurassic section. Biomarker and isotope dataindicate changes in depositional environment from theMiddle Jurassic to the upper Draupne Formationwhich support the results of Rock-Eval data.

(6) Decreasing Pr/Ph ratios, decreasing dominanceof C34 homohopanes over C35 homohopanes as wellas increasing relative amounts of 17α(H),21β(H)-28,30-bisnorhopane indicate a decreasing degree ofoxygenation from the Heather to the upper DraupneFormation. Maps of the Pr/Ph ratio show that thisdevelopment is consistent with a permanentlystratified water column with an ascending O2:H2Sinterface.

(7) The distribution of organofacies and sourcepotential in the South Viking Graben is the result ofthe interplay of factors including siliciclastic dilution,dilution by gas-prone and inert material, preservationand basin geometry. The highest overall TOC valuesare encountered in depocentres in the graben, and thenarrow shape of the graben allowed terrestrial andreworked organic matter to reach the graben centrefrom the surrounding highs on both eastern andwestern flanks. While the source facies of the upperDraupne Formation is controlled by preservationunder anoxic to dysoxic conditions together withreduced dilution by siliciclastic, terrestrial andreworked material, that in the lower Draupne is mainlycontrolled by dilution by terrestrial and reworkedorganic matter transported by mass flows. The HeatherFormation’s facies can be explained by siliciclasticdilution and preservation effects. The exceptionallyhigh oil potential in the upper Draupne Formation on

the flank of the Stord Basin and the Utsira High isrelated to deposition under sedimentation ratespromoting preservation of organic matter.

ACKNOWLEDGEMENTS

Financial support for this project was provided by EssoExploration and Production Norway A/S and theResearch Council of Norway (Grant NFR 157825/432). We are indebted to Norsk Hydro A/S for theRock-Eval analyses. Jørgen Bojesen-Koefoed andPeter Nytoft at GEUS (Denmark) are thanked for theuse of the MPLC system and technical assistance inthe laboratory. The authors would also like to thankthe Norwegian Petroleum Directorate for the releaseof sample material, and Esso Exploration andProduction Norway A/S for all the data received andvaluable technical discussions. Aurélie Nowinski isthanked for fruitful comments on earlier versions ofthe manuscript. Reviews by Iain Scotchman andRichard Tyson on a previous version of the manuscriptare acknowledged with thanks.

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