Organic facies variations in the Triassic shallow marine and deep marine shales of central Spitsbergen, Svalbard

Marine and Petroleum Geology 05 "0888# 356Ð370

Organic facies variations in the Triassic shallow marine and deep marine shales of central Spitsbergen\ Svalbard Wan Hasiah Abdullah Department of Geology\ University of Malaya\ 49592\ Kuala Lumpur\ Malaysia Received 29 April 0886^ received in revised form 05 December 0887^ accepted 19 December 0887

Abstract Shallow marine shales of the Lower Triassic Sticky Keep Member and deep marine shales of the Middle Triassic Botneheia Member of the Barentso ya Formation\ central Spitsbergen have been subjected to detailed organic petrological and organic geochemical study[ An assessment\ based on organic facies characteristics\ has been carried out on these sediments\ in order to distinguish between those deposited in shallow vs deep marine depositional settings[ The methods employed include evaluation of organic carbon content\ RockÐEval pyrolysis\ extractable organic matter\ biomarker distributions and petrographic data[ The maturity assessment of the samples analysed is mainly based on vitrinite re~ectance and Tmax data[ Organic facies criteria such as TOC\ HI\ PP "Petroleum Potential#\ hydrocarbon yield\ kerogen assemblage and ratios of pristane:n!C06\ phytane:n!C07\ Tm:Ts\ C13 tetracyclic:C15 tricyclic\ C23:C18 hopane\ C24:C23 hopane\ hopanes:steranes\ diasteranes:steranes\ C10:C11 abb sterane and C10 baa:C10 abb sterane were found to be useful in di}erentiating between the two depositional environments[ The biomarker ratios in particular\ are thought to re~ect more of the depositional condition\ such as the extent of anoxicity:oxicity\ and less on source input\ probably as a result of enhanced microbial degradation on the organic matter deposited[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] organic facies^ shallow marine^ deep marine

0[ Introduction Spitsbergen is the largest island in the Svalbard archi! pelago\ which is situated in the northwest corner of the Barents Shelf of Europe between 63> and 70> North and 09> and 24> East "Fig[ 0a#[ Although a stratigraphically extensive sedimentary succession occurs in Spitsbergen\ ranging in age from Precambrian to Cenozoic\ this paper is concerned only with the marine Triassic sequence[ The Triassic succession on Spitsbergen is dominated by marine shales\ siltstones and sandstones deposited in environments varying from marginal marine to open marine[ The lithostratigraphical framework for the Triassic in areas of Spitsbergen is mainly based on the initial work of Buchan\ Challinor\ Harland\ and Parker "0854#\ and later modi_ed by Harland\ Cutbill\ Friend\ Gobbett\ Holliday\ Maton\ Parker\ + Wallis "0863#[ This scheme proposes a division for the Triassic succession into two major units\ namely the Sassendalen Group and the Kapp Toscana Group[ The Sassendalen Group\ subject of this paper "Fig[ 0b# comprises the Lower to Middle Triassic succession within the Svalbard archi! pelago and consists predominantly of marine shales\ with subordinate siltstones and sandstones[ According to the lithostratigraphical scheme proposed by Mo rk\ Knarud\

and Worsley "0871#\ the Barentso ya Formation of the Sassendalen Group consists of three members\ i[e[ the Deltadalen\ Sticky Keep and Botneheia members "Fig[ 0b#[ Samples analysed in this study belong to the two latter members from the three localities shown in Fig[ 0a^ Tschermakfjellet "TSCH#\ Botneheia "BOTN# and Sticky Keep "SK#[ The shales of the Barentso ya Formation are considered to have been deposited in open marine environments throughout the Lower and Middle Triassic "Mo rk et al[\ 0871#[ In the early Lower Triassic\ the central Spitsbergen area was covered by a shallow\ but extensive\ shelf area "Fig[ 1a#[ Recurrent transgressive and regressive episodes took place throughout Lower Triassic time[ Thick marine shales\ and subordinate siltstones and sandstones\ con! stitute the lowermost part of the Sassendalen Group "Deltadalen Member#[ These were subsequently overlain by the dark shales of the Sticky Keep Member that typi! cally lack benthic fauna\ possibly indicating distal\ anoxic conditions "Mo rk et al[\ 0871#[ A major transgression in early Anisian time resulted in deposition of the black phosphatic shales found in the lowermost Botneheia Member[ Deep marine conditions prevailed during the Middle Triassic in central and eastern areas of Svalbard "Fig[ 1b#[ The Anisian transgression deposited\ very

9153!7061:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 1 5 3 ! 7 0 6 1 " 8 7 # 9 9 9 7 5 ! 4

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Fig[ 0[ "a# Location map of study area[ "b# Stratigraphic sections of the Barentso ya Formation at Tschermakfjellet "TSCH#\ Botneheia "BOTN# and Sticky Keep "SK#\ central Spitsbergen "after Mo rk et al[\ 0871#[ Sample locations are indicated by arrows[

probably under anoxic conditions\ a non!bioturbated black organic!rich shale facies which constitutes the Mid! dle Triassic Botneheia Member "Mo rk et al[\ 0871#[

This study was undertaken to characterize the organic facies distributions within the shales of the deep marine Botneheia Member and the shallow marine Sticky Keep

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358

Fig[ 1[ "a# Lower Triassic palaeogeography of Spitsbergen "after Mo rk et al[\ 0871#[ "b# Middle Triassic palaeogeography of Spitsbergen "after Mo rk et al[\ 0871#[

Member\ and to determine their hydrocarbon generating potential[ The organic facies de_ned herein have been characterized by biomarker distributions\ TOC "total organic carbon# content\ RockÐEval and petrological data[

1[ Experimental 1[0[ Petrology Crushed samples were mounted in slow!setting resin\ allowed to harden\ then polished[ Microscopic exam! ination and measurement were carried out principally using oil immersion[ The organic matter types were described from whole rock blocks^ _rstly in re~ected\ plane!polarised light and\ secondly\ in {blue light| exci! tation\ using an exciter _lter peaking at 309 nm and a barrier _lter at 429 nm to examine ~uorescing liptinitic material[ Re~ectance measurements on vitrinite ")R9#

were carried out in plane!polarised\ re~ected light using a photometer aperture of 1 um to restrict the measurement _eld[ 1[1[ Geochemistry Approximately 09 g of powdered shale samples were Soxhlet extracted for 61 h using an azeotropic mixture of dichloromethane and methanol "82]6#[ Thin!layer chro! matography was used to fractionate the total extract[ Several standards "n!C04\ l!phenyldodecane and phen! anthrene# were used to assist identi_cation of aliphatic\ 0Ð2 rings aromatic and ×2 rings aromatic hydrocarbon bands[ The recovered aliphatic fractions were analysed by a Hewlett!Packard 4789 gas chromatograph _tted with a 49 m×9[21 mm i[d[ fused silica OV!0 column connected to a ~ame ionization detector[ The oven temperature was programmed from 39 to 189>C at 3>C:min with hydrogen as carrier gas[ The aliphatic fractions of all extracted samples were

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further analysed by combined gas chromatography!mass spectrometry "GC!MS#[ The GC!MS analysis was per! formed on a Hewlett!Packard 4789 GC "split:splitless injection# interfaced to a Hewlett!Packard 4869 MSD[ Separation was performed on a 14 m×9[1 mm i[d[ fused silica column coated with 9[00 um methyl silicon "HP! Ultra l#[ The GC was temperature programmed from 39 to 299 >C at 3>C:min with helium as the carrier gas[ Crushed whole rock samples were analysed by RockÐ Eval pyrolysis "as described by Espitalie\ Madec\ Tissot\ Mennig\ + Leplat\ 0866#[ The total organic carbon "TOC# content of the samples were determined using a LECO instrument[

2[ Results 2[0[ Kerogen assemblage Kerogen typing carried out on whole rock samples revealed that the deep marine Botneheia Member shales consists predominantly of a mixture of type II and III kerogen and are moderately to heavily stained with bitu! men[ The type II kerogen consists predominantly of amorphous organic matter while the type III kerogen is composed mainly of vitrinitic and inertinitic material[ Non!~uorescing\ type III\ bituminite "Fig[ 2a# is common in a number of these Botneheia Member shales[ Minor amounts of bright!yellow ~uorescing type I kerogen\ gen! erally the alga Tasmanites "Fig[ 2b#\ occurs in a few of these deep marine samples[ The kerogen of the shallow marine Sticky Keep Mem! ber shales consists predominantly of a type III vitrinitic component with minor amounts of ~uorescing yellow! orange amorphous type II kerogen[ Tasmanites and bitu! minite\ which are present in the deep marine shales\ are virtually absent in these shallow marine shales[ 2[1[ Biomarker data The gas chromatograms of the aliphatic fractions of all the sediments analysed\ examples of which are shown in Figs[ 3Ð5\ display abundant n!alkanes\ particularly low to medium molecular weight members[ n!C03 or n!C04 is the most dominant peak in the saturate gas chro! matogram of the shallow marine samples\ while a broader range of n!alkane maxima of n!C00 to n!C05 is observed from among the deep marine samples[ Acyclic iso! prenoids are abundant in the shallow marine samples with pristane being the most dominant peak in all of the saturate gas chromatograms of the shallow shelf samples studied^ pristane concentration is always higher than n! C06 and phytane concentration is always higher than n! C07\ thus giving distinctively high pristane:n!C06 and phy! tane:n!C07 ratios of 0[4Ð0[5 and 0[1Ð0[5\ respectively[

Comparatively lower values for these ratios "9[3Ð0[0 and 9[3Ð9[8\ respectively# were displayed by deep marine sam! ples\ which generally possess relatively lower amounts of acyclic isoprenoids "compared to n!alkanes# than the shallow marine samples[ The m:z 080 mass fragmentograms of the aliphatic hydrocarbon fractions "Figs[ 3Ð5# of all the samples ana! lysed show high abundances of pentacyclic triterpanes and tricyclic terpanes with lower amounts of tetracyclic terpanes[ Peaks identi_cation of all these compounds\ including the steranes "in m:z 106#\ are based on their retention times and comparison of mass spectra with those published in previous literature "Philp\ 0874#[ The relative abundance of the C18 hopane is generally half\ or less\ that of the C29 hopane in all the samples studied[ All samples from the Botneheia locality possess similarly low Tm:Ts ratios "06a"H#!11\18\29!trisnorhopane:07a"H#! 11\18\29!trisnorneohopane#[ However\ at the Tscher! makfjellet and Sticky Keep localities\ the deep marine sediments of the Botneheia Member possess higher Tm:Ts ratios compared to the shallow marine sediments "Table 0#[ Low values of 9[2Ð9[5 for the C13 tetracyclic to C15 tricyclic ratios "C13tetra:C15tri# are displayed by the shal! low marine sediments\ whilst deep marine sediments dis! play slightly higher values in the range of 9[4Ð0[9[ The ratios of C23ÐC18 hopane and C24ÐC23 hopane are observed to be slightly higher in the shallow marine shales compared to deep marine shales[ The m:z 106 mass fragmentograms "Figs[ 3Ð5# of all the samples display high abundances of C16\ C17 and C18 regular steranes\ diasteranes and low molecular weight steranes "C10 and C11#[ A particularly high abundance of low molecular weight steranes "relative to C16ÐC18 ster! anes# is present in samples from the Sticky Keep locality compared to the other two localities[ The ratio of diaster! anes to steranes is distinctly higher in the shallow marine shales "values of 0[4Ð0[6# whilst the deep marine shales display lower values in the range of 9[3Ð0[9[ It is also observed that at all localities\ the C10:C11 abb 19R sterane ratios are higher in shallow marine samples\ having values of 0[7Ð1[6\ compared to deep marine samples which pos! sess lower values of 0[3Ð1[9[ It is also interesting to note that the C10 4b"H# to C10 4a"H# ratios "i[e[ C10i:C10f in Table 0# in all the shallow shelf samples are higher than that of deep shelf samples[ The hopane:sterane ratio is generally lower in shallow shelf sediments compared to the deep shelf sediments except for sample SK124 which possesses a relatively higher value[ The signi_cance of all these parameters will be discussed in the organic facies characterization section[ 2[2[ Organic maturation The thermal maturity of dispersed organic matter gov! erns\ in part\ the character of the organic matter and therefore may in~uence interpretation of organic facies[

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Fig[ 2[ "a# Bituminite in silty shale of a Botneheia Member sample] re~ected light\ oil immersion[ "b# Tasmanites alga in a Botneheia Member sample] re~ected light\ oil immersion[

For this reason\ when assessing an organic facies\ one has to be aware of the e}ects of maturation on the organic matter and to take this e}ect into consideration[ All of the deep and shallow marine sediments analysed range from thermally early mature to immature\ based on vitrinite re~ectance measurements "Table 0#[ However\ Tmax values "Table 1#\ in contrast\ suggest all the samples are early to mature for oil generation[ The ratios of 11 S:"11R¦11 S# for C21 06a"H#\10b"H#! hopanes are between 9[5Ð9[6 and the ratios of 19 S:"19 S¦19R# for C18 4a"H#\03a"H#\06a"H#!steranes

CMYK Page 360

are between 9[4Ð9[5\ both suggesting that they have reached equilibrium and therefore support the Tmax data in indicating all samples are at least early mature\ and are likely to be approaching mid!oil window maturity[ The slightly depressed values of the vitrinite re~ectance data\ particularly in samples from the Sticky Keep locality "SK#\ are likely to be attributed to the heavy bitumen staining generally observed in these samples[ Other biomarker ratios normally taken as indicators of maturity\ such as the diasterane to sterane ratio and C10 baa:abb as well as C18 abb:"abb¦aaa# sterane ratios\

)

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Fig[ 3[ Gas chromatogram and mass fragmentograms "m:z 080 and 106# of total alkanes in a shallow marine shale "left# and a deep marine shale "right# at Tschermakfjellet[

do not show any signi_cant variation among the samples nor show correlation with vitrinite re~ectance data[ For example\ samples from the Botneheia locality\ which have )R9 values of approximately 9[5)\ the diaster! ane:sterane and C10 baa:abb ratios show large variation

i[e[ between 9[3Ð0[6 and 9[3Ð9[8\ respectively "see Table 0#[ Thus\ the biomarker _ngerprints presented here are believed to be strongly in~uenced by facies di}erence\ particularly source input and depositional conditions[ This is to be further discussed in the following section[

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Fig[ 4[ Gas chromatogram and mass fragmentograms "m:z 080 and 106# of total alkanes in a shallow marine shale "left# and a deep marine shale "right# at Botneheia[

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Fig[ 5[ Gas chromatogram and mass fragmentograms "m:z 080 and 106# of total alkanes in a shallow marine shale "left# and a deep marine shale "right# at Sticky Keep[

Table 0 Triterpane\ terpane and sterane biomarker ratios\ and vitrinite re~ectance ")R9# data "bold typeSticky Keep Member^ light typeBotneheia Member#a Locality

a

TSCH139 TSCH059 TSCH034 BOTN056 BOTN030 BOTN62 SK304 SK399 SK283 SK241 SK124

Triterpanes and terpanes

Steranes and diasteranes

11S: "11 S¦11R#

Tm:Ts

C18Hop: C29Hop

C13Tetra: C12Tri: C15Tri C29Hop

C23Hop: C23Hop

C24Hop: C23Hop

19S: "19 S¦19R#

abb: "abb¦aaa#

C10f: 18e"R¦S#

C10f: C10f

C10i: C11f

Diasteranes: Hopanes: steranes steranes

)R9

9[5 9[6 9[6 9[5 9[5 9[6 9[5 9[5 9[5 9[5 9[5

9[6 9[5 9[3 9[3 9[2 9[2 9[8 0[0 0[1 9[8 9[4

9[3 9[2 9[3 9[4 9[3 9[4 9[4 9[4 9[4 9[4 9[3

0[9 9[2 9[3 9[5 9[6 9[3 9[4 9[5 9[8 0[9 9[5

9[0 9[1 9[1 9[0 9[1 9[5 9[1 9[1 9[0 9[0 9[3

9[4 9[4 9[4 9[9 9[4 9[5 9[4 9[4 9[3 9[5 9[7

9[5 9[5 9[5 9[4 9[5 9[5 9[4 9[5 9[4 9[5 9[5

9[5 9[5 9[5 9[5 9[5 9[6 9[5 9[5 9[5 9[5 9[5

9[3 9[3 9[5 0[6 0[9 3[9 1[1 1[7 3[9 2[0 1[9

0[5 0[7 0[8 0[3 0[6 1[6 1[9 1[9 0[6 1[9 1[1

9[7 0[9 9[8 9[3 9[5 9[8 9[7 9[6 9[5 9[6 0[9

0[9 0[5 0[4 9[3 9[7 0[6 9[6 9[6 9[5 9[8 0[4

9[49 9[30 9[43 9[59 9[47 9[47 9[38 9[30 9[35 9[30 9[33

9[0 9[3 9[3 9[4 9[2 1[3 9[7 9[7 9[5 9[4 9[4

6 4 3 4 4 2 8 7 00 02 00

See Appendix C for de_nitions and measurement procedures[

Table 1 RockÐEval and extractable organic matter data\ and normal and branched alkane biomarker ratios "bold typeSticky Keep Member^ light typeBotneheia Member#a Locality

Tschermakfjellet Tschermakfjellet Tschermakfjellet Botneheia Botneheia Botneheia Sticky Keep Sticky Keep Sticky Keep Sticky Keep Sticky Keep a

Sample

TSCH139 TSCH059 TSCH034 BOTN056 BOTN030 BOTN62 SK304 SK399 SK283 SK241 SK124

TOC HI ")wt#

3[97 1[28 0[14 6[00 1[85 1[76 3[71 2[27 5[65 2[58 0[65

254 207 116 142 136 083 302 281 398 307 103

PP

04[8 7[8 2[5 08[3 8[0 5[6 19[8 02[8 18[1 05[1 3[7

PI

9[0 9[0 9[1 9[0 9[1 9[1 9[0 9[0 9[0 9[0 9[1

Tmax EOM "ppm of whole rock# EOM "mgEOM:gTOC# "deg C# Total extr Aliph Arom Total extr Aliph Arom Total HC

HC:non!HC

327 333 334 341 335 338 339 326 331 332 332

9[1 9[1 9[3 9[2 9[6 2[9 9[0 9[1 9[0 9[2 9[2

4925 5159 2526 5573 3193 4842 7216 7906 01096 3534 6758

583 659 641 850 0489 1501 426 597 573 792 0652

176 88 116 404 70 0736 15 454 778 271 109

012 150 180 83 039 194 063 125 067 015 336

06 21 59 03 42 89 00 07 09 11 87

6 3 07 6 2 53 0 06 02 09 01

13 25 67 10 45 043 01 24 12 21 009

n!alkane CPI maximum

Pr:Ph Pr:nC06 Ph:nC07

01 04 04 01 05 04 05 01 03 00 03

0[3 0[1 0[4 0[3 0[5 0[4 0[3 0[6 0[8 1[9 0[5

0[0 0[9 0[0 0[0 0[0 0[0 0[9 0[0 0[9 0[0 0[0

0[9 0[5 0[5 9[3 9[8 0[4 0[0 9[8 0[0 9[5 0[5

9[8 0[5 0[2 9[3 9[5 0[1 9[8 9[6 9[6 9[3 0[2

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Tschermakfjellet Tschermakfjellet Tschermakfjellet Botneheia Botneheia Botneheia Sticky Keep Sticky Keep Sticky Keep Sticky Keep Sticky Keep

Sample

See Appendix B for de_nitions and measurement procedures[

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3[ Discussion 3[0[ Organic facies characterization The results indicate that the deep marine sediments contain more terrestrially!derived organic matter which was deposited in a more oxic environment compared to shallow marine sediments[ This is suggested by higher Tm:Ts ratios\ higher C13tetra:C15tri ratios and supported by the generally higher hopane:sterane ratios[ This\ in turn\ suggests that the shallow marine sediments were deposited in a more reducing environment than the deep marine sediments[ In support\ relatively higher C23:C18 and C24:C23 hopane ratios were obtained for shallow mar! ine compared to the deep marine samples[ High C24:C23 hopane ratios have been reported in highly reducing mar! ine oils and source rock extracts "Moldowan\ Sun! dararaman\ + Schoell\ 0875^ Peters + Moldowan\ 0880#[ In this study\ although no major elevation of C24 hopane homologs were observed\ the ratios of C24:C23 and C23:C18 hopanes are generally slightly higher in the shallow mar! ine sediments[ This possibly suggests the extended hopanes are preferentially preserved in this shallow shelf environment and:or that a speci_c precursor of these extended hopanes is more abundant in shallow marine compared to deep marine environments[ The abundance of the C13 tetracyclic terpane found in oils and rock extracts has on numerous occasions been associated with microbial origin "e[g[ Aquino Neto\ Tren! del\ Restle\ Connan\ + Albrecht\ 0872^ Connan\ Bour! oullec\ Dessort\ + Albrecht\ 0875^ Clark\ + Philp\ 0878#[ Wielens\ von der Dick\ Fowler\ Brooks\ and Monnier "0889# however\ associated the abundance of C13 tetra! cyclic with an alginite source while Philp\ and Gilbert "0875# reported high concentration of this compound in Australian oils with a probable terrestrial source[ Thus it seems likely that there may be more than one origin for this compound and:or it may be re~ecting higher levels of oxicity of a depositional environment as a result of enhanced microbial activity which is believed to be the case here "high C13tetra:C15tri supported by high hopane: sterane ratio#[ High Tm:Ts ratios have been observed in ~uvio!deltaic oils "Robinson\ 0876# and furthermore\ higher Tm:Ts ratios have been observed in oxic sediments compared to anoxic sediments "Moldowan et al[\ 0875#[ It is di.cult to determine with certainty here whether the Tm:Ts ratio for the samples under study is in~uenced by source input or by depositional condition\ or both[ It seems that where there is accumulation of terrestrial! derived organic matter\ the depositional condition tends to be more oxic\ even in deep marine environment[ The Tm:Ts ratio observed here as with the C13tetra:C15tri ratio\ and possibly to some extent\ the hopane:sterane ratio too\ are thought to be re~ecting the extent of anox! icity:oxicity of the depositional environment as a result of enhanced microbial activity within the sedimentary column[

Being more proximal\ the shallow shelf sediments can be expected to have received a greater abundance of terrigenous organic matter\ supplied mainly during the regressive episodes of Lower Triassic times[ However\ biomarker distributions suggest it is the deep marine sedi! ments\ deposited mainly during transgressive episodes of Middle Triassic times "Mo rk et al[\ 0871#\ which possess more terrestrial in~uence[ The existence of a deltaic sys! tem in close proximity to the study area is considered here to be the main contributor to the terrigenous organic matter input during the Middle Triassic time[ However\ based on petrographic study and supported by HI!Tmax diagram "Fig[ 7#\ the shallow marine shales contain more terrestrially!derived organic matter compared to the deep marine shales[ This again suggests that the generally con! sidered terrestrial markers such as Tm:Ts ratio\ hopane: sterane ratio and the high abundance of tetracyclic terpanes "which are higher in the deep marine shales# may not be re~ecting source input but are instead re~ecting enhanced microbial degradation in the prevailing bottom water conditions[ The greater marine in~uence experienced by the more distal shales at the Sticky Keep and Botneheia localities compared to the more proximal shales at Tscher! makfjellet " for both deep marine and shallow marine setting# is re~ected by the higher abundance of low molec! ular weight steranes relative to high molecular weight steranes in the distal shales "represented by C10 abb:C18 aaa"R¦S# sterane ratio^ Table 0#[ This is supported by the generally higher concentration of tricyclic terpanes in these distal samples "represented by C12tricy! clic:C29hopane^ Table 0#[ The enrichment of tricyclic ter! panes relative to pentacyclic terpanes and low molecular weight steranes relative to high molecular weight steranes as observed in sample BOTN 62 "Fig[ 4# are unlikely to be due to a maturity e}ect[ This assumption is supported by the fact that the concentration of isoprenoids relative to n!alkanes is higher in the BOTN 62 sample compared to\ for example\ sample BOTN 030 "Fig[ 4# both of which have R9 of 9[47)[ Wan Hasiah "0883# noted that for relatively low maturity samples possessing )R9 of less than 0[9)\ the ratio of low molecular weight steranes to higher molecular weight steranes seemed to be facies! dependent and could be used to di}erentiate between marine!derived and terrestrially!derived organic matter within the sedimentary rock sequence of central Spits! bergen[ Sediments rich in Tasmanites have been reported to contain large amounts of tricyclic terpanes "Aquino Neto\ Triguis\ Azevedo\ Rodrigues\ + Simoneit\ 0878^ Volkman\ Banks\ Denwer\ + Aquino Neto\ 0878#\ but no obvious correlation between the concentration of tricyclic terpanes and the optically!determined abundance of Tas! manites was observed in the samples currently studied[ No signi_cant variation is observed in Pr:Ph "pri! stane:phytane# ratios within the samples studied[ However\ distinctly higher values of pristane:n!C06 and

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Fig[ 6[ Variation with TOC content of diasteranes:steranes\ C10:C11 abb sterane and C10 baa:C10 abb sterane ratios for Sticky Keep and Botneheia member shales[

phytane:n!C07 ratios are recorded for shallow shelf sedi! ments which is thought to re~ect variation in depositional conditions rather than source input[ Under di}erent environmental conditions a series of reaction pathways for alteration of the phytol sidechain of chlorophyll can take place "e[g[ Cox\ Maxwell\ Aukman\ + Hooper\ 0861^ De Leeuw\ Corriea\ + Schenck\ 0863^ Ikan\ Aizenshtat\ Baedecker\ + Kaplan\ 0864# and also there are other known sources for pristane and phytane besides chloro! phyll "Blumer\ Mullin\ + Thomas\ 0852^ Nissenbaum\ Baedecker\ + Kaplan\ 0861^ Brassell\ Wardroper\ Thompson\ Maxwell\ + Eglinton\ 0870^ Chappe\ Albre! cht\ + Michaelis\ 0871^ Goossens\ De Leeuw\ Schenck\ + Brassell\ 0873^ Albaiges\ Borbon\ + Walker\ 0874#[ As a consequence of this\ the use of the Pr:Ph ratio as palaeoenvironment indicator has been very much ques! tioned of late "see ten Haven\ de Leeuw\ Rullkotter\ + Sinninghe!Damste\ 0876^ Peters\ + Moldowan\ 0882#[ At all three locations\ sediments of shallow shelf origin possess relatively higher values of the C10:C11 abb 19R sterane ratio compared to the deep shelf sediments[ Simi! larly\ it is observed that the ratios of C10 baa to C10 abb steranes is also higher in shallow shelf sediments\ the signi_cance of which is\ however\ unclear[ It is considered more likely that these ratios re~ect variations in diag!

enetic conditions rather than source input[ This deduct! ion is made based on the positive correlation observed between the diasterane:sterane ratio to the C10:C11 abb sterane ratio and between the diasterane:sterane ratio to the C10 baa:C10abb sterane ratio "see Fig[ 6#[ This suggests a possible control by clay!catalysed rearrangement reac! tions under acidic conditions such as that experienced by high molecular weight steranes as initially proposed by Rubinstein\ Sieskind\ + Albrecht "0864#[ The higher diasterane:sterane ratios in the shallow shelf sediments compared to deep shelf sediments correspond to the higher clay contents in the former\ as suggested by the relatively lower TOC values obtained for these shallow shelf sediments[ Within the samples analysed\ it is noted that the relative stability of isomers "a"H# and b"H#\ the former mainly taken to be more stable^ Mackenzie\ 0873# at the C!4 chiral centre for C10 abb and C10 baa does not seem to be in~uenced by maturity[ For example\ samples from Botneheia possessing similar vitrinite re~ectance values gave a rather signi_cant variation in the C10baa:abb ratio "see Table 0#[ Besides biomarker data\ TOC and HI values may also be used as organic facies indicators[ The origin of organic!rich sediments has commonly been associated with their formation under conditions of water!column

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Fig[ 7[ HI!Tmax diagram for Sticky Keep and Botneheia member shales[

anoxia "Demaison\ + Moore\ 0879#[ However\ recent research based on studies of modern oceans\ suggests that high primary production and not water!column anoxia provides the _rst!order control on the accumulation of organic!rich sediments "Calvert\ 0876^ Pedersen\ + Calvert\ 0889#[ The preferential preservation of organic matter under anoxic conditions was thought to be of secondary importance in governing the accumulation of carbon in marine deposits[ In the samples currently stud! ied\ relatively lower TOC and HI values were obtained for samples deposited in more reducing environment as suggested by biomarker data[ Considering this\ it is poss! ible to suggest that the supply of organic matter and:or rate of primary production within the environments of depositions of the Botneheia Member during the Middle Triassic is greater than that of the Sticky Keep Member in the Lower Triassic time[ This greater supply of organic matter can be attributed to the close proximity to a deltaic depositional setting to the west of the study area during the Middle Triassic time "see Fig[ 1b#[

3[1[ Hydrocarbon potential Evaluation of the hydrocarbon source rock potential of the Triassic shales of Spitsbergen has been carried out by a number of workers such as Mo rk and Bjoro y "0873#[

Their studies have shown that organic!rich shales occur repeatedly\ but their hydrocarbon potential varies greatly between the individual units[ Shales of the Botneheia Member of deep shelf origin analysed in this study con! tain the richest organic carbon content\ possessing TOC values of up to 6) wt\ and yield the highest extractable organic matter "EOM# with yields of over 01\999 ppm[ They also possess excellent petroleum potential "PP# of up to 18 mgHc:gRock and hydrogen indices "HI# in the range of 149Ð399 mgHc:gTOC[ The shallow shelf shales of the Sticky Keep Member possess relatively lower TOC\ PP and HI values compared to the deep shelf shales "see Table 1 and Fig[ 7#[ The EOM recovered from the shallow shelf shales varies from approximately 2599Ð6899 ppm[ However\ on a per gram TOC basis\ generally higher EOM levels were recovered from the shallow marine shales possessing values between 194Ð336 mgE! OM:gTOC with much higher yields of aliphatic and total hydrocarbons recovered[ With the exception of sample SK283\ the proportion of aliphatic hydrocarbons is higher than that of the aromatic hydrocarbons in all of the Triassic samples analysed[ It is interesting to note that the deep shelf sediments which contain type I kerogen Tasmanites algae\ yielded lower aliphatic hydrocarbon fractions compared to the shallow shelf sediments which is barren in Tasmanites\ suggesting that these algae have not signi_cantly contributed to the bitumen fraction of the deep marine shales at this early maturation stage[

4[ Conclusions The organic geochemical and organic petrological approach used here has been able to clearly di}erentiate between the subtle organic facies variations of a shallow shelf setting and a deep shelf setting of an open marine succession[ Organic facies parameters such as Tm:Ts\ C13tetra:C15tri and hopane:sterane ratios re~ect more on the anoxicity:oxicity of the depositional environment and are less a re~ection of terrestrial in~uence[ Parameters such as pristane:n!C06\ phytane:n!C07 and C24:C23 hopane ratios may\ however\ re~ect variation in depositional con! ditions and:or source input[ Based on these parameters\ the shallow marine sediments are considered to be deposited in more reducing conditions than the deep mar! ine sediments[ The greater oxicity experienced by the deep marine shales was likely to be due to oxygenated water currents associated with a ~uvial system within a deltaic setting during the Middle Triassic time[ Parameters such as C10:C11 abb steranes and baa:abb C10 steranes also show some potential for use in assessing palaeo! depositional environments and could indicate a clay!cat! alysed rearrangement reaction under acidic conditions which would give rise to anoxic bottom conditions of deposition due to hindrance of microbial activity[ Thus\

368

W[ Hasiah Abdullah : Marine and Petroleum Geolo`y 05 "0888# 356Ð370

these biomarker ratios can be taken to re~ect the relative extent of anoxicity "i[e[ more reducing or less reducing# as a consequence of microbial alteration[ The deep and shallow marine sediments under study have both experi! enced anoxic bottom conditions at some stage during their deposition and they have been distinctly di}er! entiated based on these organic facies characterisations[ In this study\ the extent of marine in~uence "i[e[ the proximity to the land mass# can be re~ected by the ratio of C12 tricyclic terpanes to C29 hopane and by the ratio of C10 abb to C18 aaa "R¦S# steranes[ Although tricyclic terpanes are known to be of probable microbial origin "e[g[ Aquino Neto et al[\ 0872#\ however\ this in~uence is not distinct in the studied samples\ suggesting other fac! tors besides source input and microbial alteration govern the distribution of tricyclic terpanes[ The organic facies parameters in this study are believed not to be in~uenced by organic maturation\ as the samples analysed are of approximately equal maturity "early mature#[ Other organic facies parameters such as high TOC and HI may not necessarily re~ect greater anoxicity in depositional setting[ Relatively lower TOC and HI values were obtained for samples deposited in more reducing environments "based on biomarker data# compared to those deposited in less reducing environments[ In terms of oil!generating potential\ both the shallow marine shales of the Sticky Keep Member and the deep marine shales of the Botneheia Member possess reason! ably good oil!generating potential[ However\ at greater depth of burial\ at mid!oil!window maturity\ the Bot! neheia Member shale would be expected to be a better source rock for oil as suggested from its higher abundance of type II kerogen and very high TOC and HI values compared to the Sticky Keep Member[ At a much higher maturation level\ shales of Botneheia and Sticky Keep members will both have good gas!generating potential[ Acknowledgements The author wishes to acknowledge the assistance of Norsk Hydro Research Centre "Bergen# for supplying TOC and RockÐEval data\ Newcastle University for GC and GCMS analyses and University of Malaya for _n! ancial support[ Special thanks are o}ered to Professor RP Philp and Mr P Abolins for their very helpful com! ments on an earlier version of the manuscript[ The author is also very grateful to Professor DG Roberts and an anonymous referee who constructively reviewed the paper[

Appendix A Peak assignments for alkane hydrocarbons in the gas chromatograms of the aliphatic fractions] "I# in the m:z

080 mass fragmentograms and "II# in the m:z 106 mass fragmentograms

Compound

Abbrevia! tion

"I# Peak No 08AÐ18A 13BÐ16B Ts Tm 18 29

C08 to C18 tricyclic terpanes C13 to C16 tetracyclic terpanes 07a"H#\11\18\29!trisnorneohopane 06a"H#\11\18\29!trisnorhopane 06a"H#\10b"H#!norhopane 06a"H#\10b"H#!hopane

20 S 20R 2M

06a"H#\10b"H#!homohopane "11 S# 06a"H#\10b"H#!homohopane "11R# 06b"H#\10a"H#!moretane

tri tetra Ts Tm C18hop hopane or hop C2011 S C2011R C29Mor

"II# The numbers on the chromatograms correspond to the carbon number of the molecules\ and the letters identify the stereochemistry a b c d e f g h i

02b"H#\06a"H#!diasteranes 19 S 02b"H#\06a"H#!diasteranes 19R 02a"H#\06b"H#!diasteranes 19 S 02a"H#\06b"H#!diasteranes 19R 4a"H#\03a"H#\06a"H#!steranes 19 S 4a"H#\03b"H#\06b"H#!steranes 19R 4a"H#\03b"H#\06b"H#!steranes 19 S 4a"H#\03a"H#\06a"H#!steranes 19R 4b"H#\03a"H#\06a"H#!steranes

diasteranes diasteranes diasteranes diasteranes aaa19 S abb19R abb19 S aaa19R baa

Appendix B De_nitions and measurement procedures for data in Table 1

TOC HI PP PI Tmax EOM Total Extr Aliph Arom HC CPI

pr:ph pr:n!C06 ph:n!C07

Total Organic Carbon ") wt# Hydrogen Index "S1:TOC×099 where S1pyrolyzable hydrocarbons# "mgHC:gTOC# Petroleum Potential "Genetic Potential] S0¦S1 where S0free hydrocarbons# "mgHC:gRock# Production Index "Transformation ratio] S0:"S0¦S1# Temperature of maximum S1 generation "C# Extractable Organic Matter Total Extract Aliphatic Aromatic Hydrocarbons Carbon Preference Index] 0:1"ðC14¦C16 [ [ [ ¦C22:C13¦C15 [ [ [ ¦C21Ł ¦ðC14¦C16 [ [ [ ¦C22:C15¦C17 [ [ [ ¦C23Ł# pristane:phytane pristane:n!C06 phytane:n!C07

379

W[ Hasiah Abdullah : Marine and Petroleum Geolo`y 05 "0888# 356Ð370

Appendix C De_nitions and measurement procedures for data in Table 0 11 S:"11R¦11 S#

C2106a"H#\10b"H#11 S:ðC2106a"H#\10b"H# 11"R¦S#Ł Tm:Ts 06a"H#!11\18\29!trisnorhopane:07a"H#! 11\18\29!trisnorneohopane C18hop:C29hop C1806a"H#\10b"H#! hopane:C2906a"H#\10b"H#!hopane C13tetra:C15tri C13tetracyclic terpane:C15 tricyclic terpane C12tri:C29hop C12 tricyclic terpane:C2906a"H#\10b"H#!hopane C2306a"H#\10b"H#11"R¦S#! C23hop:C18hop hopane:C1806a"H#\10b"H#!hopane C24hop:C23hop C2406a"H#\10b"H#11"R¦S#:C2306a"H#\10b"H# 11"R¦S#!hopane 19 S:"19 S¦19R# C184a"H#\03a"H#\06a"H#19 S:ðC184a"H#\03a"H#\ 06a"H#19"S¦R#Ł abb:"abb¦aaa# ð4a"H#\03b"H#\06b"H#"19R¦19 S#C18steraneŁ: ð4a"H#\03b"H#\06b"H#"19R¦19 S#¦4a"H#\ 03a"H#\06a"H#"19R¦19 S#ŁC18 steranes Diasteranes:steranes C1802b"H#06a"H#19"R¦S# diasterane:C18aaa¦abb19"R¦S# steranes C10f:18e"R¦S# C104a"H#\03b"H#\06b"H#19R:C184a"H#\03a"H#\ 06a"H#19"S¦R# C104a"H#\03b"H#\06b"H#19R:C114a"H#\03b"H#\ C10f:C11f 06b"H#19R steranes C10i:C10f C104b"H#\03a"H#\06a"H#:C114a"H#\03b"H#\ 06b"H#19R steranes Hopanes:steranes Relative abundance of hopanes in m:z 080:relative abundance of steranes in m:z 106 )R9 Percent re~ectance of vitrinite in oil

References Albaiges\ J[\ Borbon\ J[\ + Walker\ W[ "0874#[ Petroleum isoprenoid hydrocarbons derived from catagenetic degradation of archae! bacterial lipids[ Organic Geochemistry\ 7\ 182Ð186[ Aquino Neto\ F[ R[\ Trendel\ J[ M[\ Restle\ A[\ Connan\ J[\ + Albrecht\ P[ A[ "0872#[ Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums[ In Bjoroy\ et al[ "Eds[#\ Advances in organic geochemistry 0870 "pp[ 548Ð556#[ Chichester] John Wiley[ Aquino Neto F[R[\ Triguis J[\ Azevedo D[A[\ Rodrigues R[\ Simoneit B[R[T[ 0878 Organic geochemistry of geographically unrelated Tas! manites[ 03th International Meeting on Organic Geochemistry\ Paris\ 07Ð11 September 0878\ Abstract No 078 Blumer\ M[\ Mullin\ M[ M[\ + Thomas\ D[ W[ "0852#[ Pristane in zooplankton[ Science\ 039\ 863[ Brassell\ S[ C[\ Wardroper\ A[ M[ K[\ Thompson\ I[ D[\ Maxwell\ J[ R[\ + Eglinton\ G[ "0870#[ Speci_c acyclic isoprenoids as biological markers of methanogenic bacteria in marine sediments[ Nature\ 189\ 582Ð585[ Buchan\ S[ H[\ Challinor\ A[\ Harland\ W[ B[\ + Parker\ J[ R[ "0854#[ The Triassic stratigraphy of Svalbard[ Norsk Polarinst[ Skr[\ 024\ 0Ð83[ Calvert\ S[ E[ "0876#[ Oceanographic controls on the accumulation of organic matter in marine sediments[ In J[ Brooks\ A[J[ Fleet "Eds[#\ Marine Petroleum Source Rocks\ Geol[ Soc[ Spec[ Publ[ No[ 15 "pp[ 026Ð040#[ Chappe\ B[\ Albrecht\ P[\ + Michaelis\ W[ "0871#[ Polar lipids of archae! bacteria in sediments and petroleum[ Science\ 106\ 54Ð55[

Clark\ J[ P[\ + Philp\ R[ P[ "0878#[ Geochemical characterization of evaporite and carbonate depositional environments and correlation of associated crude oils in the Black Creek Basin\ Alberta[ Bulletin of Canadian Petroleum Geology\ 26\ 390Ð305[ Connan\ J[\ Bouroullec\ J[\ Dessort\ D[\ + Albrecht\ P[ "0875#[ The microbial input in carbonateÐanhydrite facies of a sabkha palaeo! environment from Guatemala] a molecular approach[ In D[ Ley! thaeuser\ J[ Rullkotter "Eds[#\ Advances in Organic Geochemistry 0874 "pp[ 18Ð49#[ Oxford] Pergamon Press[ Cox\ R[ E[\ Maxwell\ J[ R[\ Aukman\ R[ G[\ + Hooper\ S[ N[ "0861#[ The isolation of a series of acyclic isoprenoid alcohols from an ancient sediment] approaches to a study of the diagenesis and matu! ration of phytol[ In H[R[ Von Gaertner\ H[ Wehner "Eds[#\ Advances in Geochemistry 0860 "pp[ 152Ð160#[ Oxford] Pergamon Press[ De Leeuw\ J[ W[\ Corriea\ V[ A[\ + Schenck\ P[ A[ "0863#[ On the decomposition of phytol under simulated geological conditions and in the top layer of natural sediments[ In B[ Tissot\ F[ Bienner "Eds[#\ Advances in Organic Geochemistry 0862 "pp[ 772Ð0993#[ Paris] Edi! tions Technip[ Demaison\ G[ J[\ + More\ G[ T[ "0879#[ Anoxic environments and oil source bed genesis[ Am[ Assoc[ Pet[ Geol[\ 53\ 0068Ð0198[ Espitalie J[\ Madec M[\ Tissot B[\ Mennig J[J[\ Leplat P[ "0866#[ Source rock characterization methods for petroleum exploration[ 8th Annual Offshore Technology Conference\ pp[ 328Ð333[ Houston[ Goossens\ H[\ De Leeuw\ J[ W[\ Schenck\ D[ A[\ + Brassell\ S[ C[ "0873#[ Tocopherols as likely precursors of pristane in ancient sediments and crude oils[ Nature\ 201\ 339[ Harland\ W[ B[\ Cutbill\ J[ L[\ Friend\ P[ F[\ Gobbett\ D[ J[\ Holliday\ D[ W[\ Maton\ P[ I[\ Parker\ J[ R[\ + Wallis\ R[ H[ "0863#[ The Billefjorden Fault Zones\ Spitsbergen*the long history of a major tectonic lineament[ Norsk Polarinst[ Skr[\ 050\ 0Ð61[ Ikan\ R[\ Aizenshtat\ Z[\ Baedecker\ M[ J[\ + Kaplan\ I[ R[ "0864#[ Thermal alteration experiments on organic matter in recent marine sediments 0*pigments[ Geochim[ Cosmochim[ Acta\ 28\ 072Ð074[ MacKenzie\ A[ S[ "0873#[ Applications of biological markers in pet! roleum geochemistry[ In "pp[ 004Ð103#[ J[ Brooks\ D[ Welte "Eds[#\ Advances in Petroleum Geochemistry\ Vol[ 0[ London] Academic Press[ Moldowan\ J[ M[\ Sundararaman\ P[\ + Schoell\ M[ "0875#[ Sensitivity of biomarker properties to depositional environment and:or source input in the lower Toarcian of SW Germany[ In D[ Leythaeuser\ J[ Rullkotter "Eds[#\ Advances in Organic Geochemistry 0874 "pp[ 804Ð 815#[ Oxford] Pergamon Press[ Mo rk\ A[\ + Bjoro y\ M[ "0873#[ Mesozoic source rocks on Svalbard[ In A[M[ Spencer\ et al[ "Eds[#\ Petroleum Geology of the North European Margin\ Norwegian Petroleum Society\ Trondheim\ 0872 "pp[ 260Ð271#[ London] Graham + Trotman Ltd[ Mo rk\ A[\ Knarud\ R[\ + Worsley\ D[ "0871#[ Depositional and diag! enetic environments of the Triassic and Lower Jurassic succession of Svalbard[ Arctic Geology and Geophysics\ Mem[ Can[ Soc[ Petrol[ Geol[\ 7\ 260Ð287[ Nissenbaum\ A[\ Baedecker\ M[ J[\ + Kaplan\ I[ "0861#[ Organic geo! chemistry of Dead Sea sediments[ Geochim[ Cosmochim[ Acta\ 25\ 698Ð618[ Pedersen\ T[ F[\ + Calvert\ S[ E[ "0889#[ Anoxia vs productivity] what controls the formation of organic!carbon!rich sediments and sedi! mentary rocks<[ Am[ Assoc[ Pet[ Geol[ Bull[\ 63"3#\ 343Ð355[ Peters\ K[ E[\ + Moldowan\ J[ M[ "0880#[ E}ects of source\ thermal maturity\ and biodegradation on the distribution and isomerization of homohopanes in petroleum[ Organic Geochemistry\ 06\ 36Ð50[ Peters\ K[ E[\ + Moldowan\ J[ M[ "0882#[ In The biomarker guide* interpreting molecular fossils in petroleum and ancient sediments "pp[ 038Ð040#[ Englewood Cli}s\ NJ] Prentice Hall[ Philp\ R[ P[ "0874#[ Fossil fuel biomarkers] applications and spectra[ In "pp[ 183#\ Methods in Geochemistry and Geophysics\ Vol[ 12[ Amsterdam] Elsevier[ Philp\ R[ P[\ + Gilbert\ T[ D[ "0875#[ Biomarker distributions in Aus!

W[ Hasiah Abdullah : Marine and Petroleum Geolo`y 05 "0888# 356Ð370 tralian oils predominantly derived from terrigenous source material[ In D[ Leythaeuser\ J[ Rullkotter "Eds[#\ Advances in Organic Geo! chemistry 0874 "pp[ 62Ð73#[ Oxford] Pergamon Press[ Robinson K[M[ "0876#[ An overview of source rocks and oils in Indone! sia[ In Proceedings of the Indonesian Petroleum Association\ Six! teenth Annual Convention "vol[ 0\ "pp[ 86Ð011#[ Indonesia Petroleum Association\ Jakarta[ Rubinstein I[\ Sieskind O[\ Albrecht P[ "0864#[ Rearranged steranes in a shale\ occurrence and simulated formulation[ J[ Chem[ Soc[\ Perkin Trans[ 0722Ð0725[ ten Haven\ H[ L[\ de Leeuw\ J[ W[\ Rullkotter\ J[\ + Sinninghe!Damste\ J[ S[ "0876#[ Restricted utility of the pristane:phytane ratio as a palaeoenvironmental indicator[ Nature\ 229\ 530Ð532[

370

Volkman J[K[\ Banks M[R[\ Denwer K[\ Aquino Neto F[R[ "0878#[ Biomarker composition and depositional setting of Tasmanite oil shale from northern Tasmania\ Australia[ 03th International Meet! ing on Organic Geochemistry\ Paris\ 07Ð11 September\ 0878\ Abstract No[ 057[ Wan Hasiah A[ "0883#[ A regional organic geochemical evaluation of sedimentary rocks of Spitsbergen and their hydrocarbon potential[ Ph[D[ Thesis\ University of Newcastle upon Tyne[ Wielens\ J[ B[ W[\ von der Dick\ H[\ Fowler\ M[ G[\ Brooks\ P[ W[\ + Monnier\ F[ "0889#[ Geochemical comparison of a Cambrian algi! nite potential source rock\ and hydrocarbons from the Col! ville:Tweed Lake area\ Northwest Territories[ Bulletin of Canadian Petroleum Geology\ 27\ 125Ð134[