Organic geochemistry and geohistory of the Triassic succession of Bjørnøya, Barents Sea

~

Org. Geochem. Vol. 24. No. 3, pp. 333-349, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved 0146-6380/96$15.00+ 0.00

Pergamon

PlhS0146-6380(96)00024-1

Organic geochemistry and geohistory of the Triassic succession of Bjarnaya, Barents Sea GARY H. ISAKSEN Exxon Production Research Company, Houston, TX 77252, U.S.A. (Received 24 February 1995; returned to author for revision 11 April 1995; accepted 27 February 1996)

Abstract--The Triassic outcrops of Bjornoya provide an important reference for regional characterization of potential hydrocarbon source rocks of this age in the southwestern Barents Sea. The organic matter in both the Urd and Skuld Formations is dominated by woody and herbaceous material, with elevated contents of algal/bacterial organic matter present in the Urd Formation. Minor amounts of oil is likely to have been generated from these shales. These rocks have been deeply buried and are at a present-day thermal maturity corresponding to vitrinite reflectance of 1.2%, i.e. full realization of the kerogen's oil potential. One-dimensional basin modeling suggests that these Triassic rocks have been buried to near 5 km depth and expelled oil and gas during the Late Cretaceous to Early Tertiary. Bulk geochemical data show good agreement with visual observations of bitumen staining in thin sections. This staining has a biomarker distribution characterized by a homologous series of tricyclic terpanes. GC/MS/MS analyses showed this tricyclic series to extend from C2~ to C46. Copyright © 1996 Elsevier Science Ltd Key words--Bjornoya, Barents Sea, sequence stratigraphy of Bjornoya outcrops, tricyclic hydrocarbons, Triassic of Bjornoya

INTRODUCTION The Triassic succession in the southwestern Barents Sea is exposed in outcrops on Bjornoya (Bear Island) (Fig. 1), representing the southernmost exposures of the Svalbard archipelago. The Triassic outcrops on Bjornoya serve as an important reference point to construct regional depositional facies models of Triassic strata; thus, these outcrops may represent examples of hydrocarbon source or reservoir rocks for Barents Sea petroleum exploration. This study addresses the organic geochemistry and geohistory of the Triassic rocks, as well as comments on their sequence stratigraphy in an attempt to identify intervals with the best regional hydrocarbon potential. Bj~rnoya occupies the crestal position of an uplifted basement block called the Stappen High. The Triassic sedimentary succession (Sassendalen Group) is comprised of the Urd Formation (Scythian), Verdande Bed (Scythian-Anisian), and Skuld Formation (Anisian-Carnian), present only as the uppermost part of the Misery0ellet and representing the youngest rocks on the island. The Triassic succession overlies the Miseryt]ellet Formation, of Artinskian-Kungurian age (Worsley and Edwards, 1976; Worsley and Gjelberg, 1980). This TriassicPermian boundary is very evident in outcrop as alternating shales, silty shales and siltstones of the Triassic which overlie more erosion-resistant calcareous rocks of Permian age. Accumulation of the Permian Kapp Dunrr (Asselian), Hambergt]ellet

(Artinskian), and Miseryfjellet (Kungurian) Formations were characterized by sedimentation within tectonically stable shallow marine platform environments (Agdestein, 1980; Hellem, 1981). A regional marine transgression covered the area throughout the Kungurian and into the Triassic, with the uppermost section of the Miseryt]ellet Formation consisting of sandy biosparites and planar laminated sandstones deposited in a foreshore (swash zone) environment. Marine sedimentation in the area continued throughout the Triassic, under quiescent tectonic conditions (Faleide et al., 1984). During this time period there was a rather drastic change in climate (M~rk et al., 1982). A hot, arid climate prevailed during the Early Triassic, with oxidation of iron-minerals as seen on the exposed uppermost surface of the Misery0ellet Formation. By the Late Triassic, the climate had become less hot and more humid, indicating a northward drift of the Svalbard archipelago from subtropical through temperate climatic zones (Steel and Worsley, 1984). With the onset of the Jurassic, shallow marine conditions prevailed (Johansen et al., 1992), with an emergent Stappen High as indicated by the nearshore sandstones preserved at the summit of Miseryfjellet. Triassic sediments on Bjornoya were first recognized from fossil evidence by the Nathorst expedition of 1898, as reported in Brhm (1899). A palynological study of the greater Barents Sea area by Hochuli et al. (1989) provides a reference point for M~rk et al. (1990) for age-dating palynomorphs recorded in the Urd and Skuld Formations. Specifically, the ' A - P '

333

334

Gary H. lsaksen

74

70

Fig. 1. Location of Bj~rn~ya in the south west Barents Sea between Norway and Svalbard.

palynological assemblages of Hochuli et al. (1989) are applicable to Bjornoya. In their investigation of transgressive-regressive cycles in the circum-arctic, Mork et al. (1989) suggested the presence of at least five such cycles within the Triassic succession of Bjorn~ya. Further stratigraphic studies are reported by Anderson (1900), Pchelina (1972) and M~rk et al. (1982, 1989, 1990). Geochemical studies of the Devonian through Triassic sediments have been documented by Bjor~y et al. (1983) and Isaksen (1985).

Solvent extractions were performed on 100 grams of each sample using an Ytron high speed extractor at 9000 rpm for a duration of 2 × 6 min. The solvent (dichloromethane) was kept near constant at 40°C for the duration of the extraction period. After each 6 min extraction, the sediment was spun down in a Sigma 3E-I centrifuge, and the supernatant transferred. Fresh dichloromethane was then added to the sediment powder and the extraction was repeated until negligible amounts of extractable organic matter (EOM) were recovered. Asphaltenes were precipitated by diluting the EOM 40 times with n-hexane, and refluxing for 24 hours. The n-hexane-soluble portion of the C,5+ EOM was separated into its major compound classes by medium pressure liquid chromatography (MPLC) using a Perkin Elmer ISS-100 automatic injector, a Perkin Elmer Series 4 LC coupled via a time/event interface to a Pye-Unicam LCM2 moving wire detector. The two LC columns used were: a Perkin Elmer alkylnitrilesubstituted secondary amine column (10 #m, 250 mm x 4.6 mm I.D.) and a Merk LiChroprep Si 60 (40-63 /~m, silica column with 240 m m × 10 mm I.D.). Both columns were cleaned and re-activated after each five analyses.

m

200 ~-- Bl15 B36 ~'- BII4 B113 - - B35 ~-- Bl12 ~ " BIll ~ " B34 b--. BI10 B33

150

P-" BI09 ,-- B32 '-- B31 •--- B30 o-- BI08 BI06 &BI07 B29

Analytical methods

Approximately 200 grams of outcrop sample were collected at each site (sample sites listed in Table 1 in meters above the Permian-Triassic unconformity, and shown in Fig. 2). Attempts were made to sample only the least weathered outcrops. All samples were washed under running water and rinsed in dichloromethane to remove possible surface contaminants. Total organic carbon (TOC; Leco EC 12 Carbon Analyzer) and Rock-Eval pyrolysis (Girdel) analyses were carried out on all samples. Aliquots were used for thin sections, visual kerogen analyses (Geochem Laboratories Ltd, U.K.) and vitrinite reflectance measurements (Geo-Optics Ltd, U.K. and Geochem Laboratories Ltd, U.K.). Vitrinite reflectance measurements were performed on both whole rock and kerogen concentrates, using a Zeiss Universal reflecting microscope under oil immersion at a magnification of 220 × . Whenever possible, 40 measurements were made on each sample.

~

B105

~00 •--- B27 B103 •"- B26 B25 ~-- B23 B24 B22

o._!2

B21

50

',-- B20 -,-- BI02 " - - BI9 B18 al01

,,,,-- BIO0

--0 M Si,vf ! m .~ vc. SQ~cis~one

Fig. 2. Sampling locations within the Triassic successionof BjormJya. The stratigraphic section is from Mork et al. (1989).

14 A 0.58 0.47 0.36 0.57 0.83 62

Position Lithology % TOC SI (mgig)

48

32

51

39 0.94

0.46 0.18

443

435

AL-H/-

WI

37 A 0.5 0.6 0.26 0.7 0.86 52

Urd

BlO2

809 9 523

30 A 0.49 0.08 0.07 0.53 0. I5 14

Urd

Bl9

713 8 52s

20 A 0.4 0.08 0.07 0.53 0.15 17

Urd

818

37 I.01 0.98 0.27

55

564

42 A 0.41 0.25 0.17 0.6 0.42 41 1.24 3 WR WI AL-H/3- to3 692 4 658

Urd

B20

48 I .03 2.43 0.16

57

608

ALyA,3- to3 836 5 716

WR

I

50 A 0.48 0.29 0.16 0.64 0.45 33 1.29

Urd

B21

60 A 0.45 0.09 0.07 0.56 0.16 I6

Urd

822

Table 1. Lithological

65 C 0.6 0.07 0.08 0.47 0.15 I6

Verd 65.1 C 0.34 0.03 0.01 0.75 0.04 2

Verd

B24A

and geochemical 824

41 0.93 0.73

13

;:, AL 3- to3 434 4 430

67 A 0.59 0.12 0.09 0.57 0.21 I5 I.2 6

Skuld

823

33 1.04 0.41

24

349

3543 4 408

76 B 1.15 0.13 0.18 0.42 0.3 I 16 I.21 I3 WR W/l/AM

Skuld

B25

28 1.04 0.12

14

87

87 B 0.92 0.05 0.1 I 0.31 0.16 12 I.21 21 WR WI H-I2+to3215 8 130

Skuld

826

B28

I .28 0.04 0.17 0.19 0.2 I I3

IO1

Skuld

829

830

23 I .04 0.16

2

I2 22 I.04 0.13

99

120 A I.1 0.04 0.12 0.25 0.16 11 1.24 31 WR W/l/ H-AM 2+to3I31 6 I25

Skuld

37

2+to388 5 72

A I.2 0.03 0.14 0.18 0.17 12 I.1 40 KC W-I/-/H

Skuld

831

0.14

I .08

24

39

58

2+to3I25 3 122

129 A 1 0.05 0.14 0.26 0.19 14 I.16 30 WR W/I/H

Skuld

832

24 I.01 0.17

8

53

& -/H 2+to3102 5 85

139 A 1.13 0.05 0.16 0.24 0.21 I4 I .23 40

Skuld

833

25 I .08 0.23

9

I04

170 4 I38

I53 A 0.8 0.03 0.16 0.16 0.19 21 n.a.

Skuld

index (scale 1 to 5); PI = production index [S,/(S, + !$)I; HI = hydrogen AL = algal; AM = amorphous; N R. = number of vitrinite reflectance

43 I .06 0.23

I2

163

2+to3272 7 220

W;:H

95 A I.1 0.06 0. I 0.38 0.16 9 1.24 30

Skuld

827

data for the Triassic rocks on Bjerneya

Explanations: A = silty shale; B = shale; C = phosphoritic concretions; WR = whole rock; KC = kerogen concentrate; TAI = thermal alteration index; Position = sample collection site in meters above the Permo-Triassic unconformity; W = woody; H = herbaceous; I = inertinitic; measurements; CPI = carbon preference index.

TAI EOM (ppm) Asph. (ppm) nC6 sol. (ppm) Saturates (ppm) Aromatics (ppm) NSO (ppm) CPI Pr/n-Cl7

PI SI + s2 HI % R, N Ro Sample type Vis. ker.

WI AL-H/-

Urd

Formation

S2 (wig)

BIOI

Sample no.

336

Gary H. Isaksen

Gas chromatographic analyses were performed on a Hewlett-Packard 5880 A Series GC fitted with a 30 m x 0.318 mm J&W Durabond fused silica DB-I capillary column with a film thickness of 0.25 #m. Helium was used as a carrier gas, with a flow rate of 1 ml/min. Saturated hydrocarbon fractions were injected at a concentration of 3 mg/ml, whereas aromatics were at 5 mg/ml. The GC temperature was increased at a rate of 6rC/min for saturated fractions and 2 C / m i n for aromatic fractions. External standard solutions were run after each five samples to monitor variance in GC conditions. GC/MS analyses were performed on a VG Analytical 7070-E double focusing mass spectrometer, linked to a Hewlett-Packard 5790 GC. Samples were introduced by splitless injection onto a 25 m x 0.2 mm I.D. OV-1 capillary column with a film thickness of 0.33/~m. The splitless injection of 1.5 min was followed by a split run of 1:50 for 3 min. Helium was used as carrier gas (0.7 ml/min). A 1 /~1 sample of each mixture was injected at a concentration of approximately 5 mg/ml for both saturated and aromatic hydrocarbon fractions. The GC temperature program rate was 1.5C/min. All analyses were performed in the electron impact (EI) mode at an ionization energy of 70 eV. Optimum MS focusing was ensured by the use of a reference lock mass using perfluorokerosene (PFK). Both voltage selective ion recording (VSIR) and selective metastable ion monitoring (SMIM) were performed. All data were stored and processed with an on-line VG 11/250 data system. GC/MS/MS analyses were made on a VG ProSpec Q operating in a selected ion recording mode (SIR-Vohage) and multiple reaction monitoring quadrupole mode (MRMQ). Gas chromatography was performed on a Hewlett-Packard 5790 GC with a 30 m DB-5 column, 0.25 mm I.D. and 0.25 #m film thickness. The GC temperature rate was 75 200"C at 5 C/min and 200-315 at 3 C/rain and held at 315'~C for 60 rain. The injector temperature was 330"C. Resolution was set at 1000 for both SIR-V and MRMQ.

RESULTS AND DISCUSSION

Geochemical and lithological character o f the Urd Formation Samples from the Urd Formation are characterized by a low content of phytoclasts, oxidation of macerals, strong iron oxide staining, and the presence of decomposed pyrite. Algal organic matter is present in amounts less than 10% (Table 1) (as will be discussed later, the high maturity levels have likely caused a decrease in the amount of identifiable algal organic matter). The samples contain moderate amounts of bitumen wisps. XRD analyses by Mork et al. (1990) and thin-section analyses in this study, both show a lower clay mineral content (about 20% lower) in the Urd Formation. Overall, the organic

matter within Urd and Skuld samples occurs as thin discontinuous to continuous networks of very fine-grained material aligned with the bedding and as thin wisps around quartz grains (Fig. 3). Identifiable microfossils were not observed in thin sections. There is a general parallel fabric to the rock, mostly imparted by alignment of clay minerals and organic macerals during compaction. The sediments are interpreted to have accumulated in proximal settings with silt and very-fine sand concentrated in distinct beds with sharp bases (probably scoured) and distinct grain-size grading even at the thin-section scale. For the Triassic rocks, the total organic carbon is less than 1.5% (Fig. 4a) with hydrogen indices ranging upwards to 60 (Fig. 4b). The amount of free hydrocarbons is greatest within the Urd Formation, with Rock-Eval production indices [S,/(S~ + $2)] up to 0.7 (Fig. 4c). The Urd Formation also has higher yields from solvent extraction (Fig. 4d). Such staining was also observed by Bjoroy et al. (1983), although their data suggest similar levels of staining in both the Urd and Skuld Formations, The data from Bjoroy et al. (1983) have been added to Fig. 4a-c. This bitumen staining is primarily composed of saturated hydrocarbons (Fig. 5), with saturate/aromatic ratios of around 10. As outcrop samples, one can assume preferential removal of aromatic hydrocarbons due to their greater solubility in water. In a study of weathering effects on the outcrops of the Anisian Botneheia Formation on Spitzbergen and Edge-~ya, Forsberg and Bjoroy (1983) noted that aromatic hydrocarbons were more affected by weathering than the saturated compounds. This has also been shown by Clayton and Swetland (1977) in samples from warmer and more arid climates. Gas chromatograms for samples B20 and B21 (see Fig. 2) are from the Urd Formation (Fig. 6) and show a relatively smooth, n-alkane distribution in agreement with the higher maturity levels. Geochemical and lithological character of the Skuld Formation Skuld Formation samples represent a shift to higher TOC contents and lower hydrogen indices (Fig. 4a,b), due to a greater abundance of inertinitic material (Table 1). Like the Urd Formation the Skuld samples show iron oxide staining, decomposed pyrite and wisps of bitumen. Mid-Skuld samples (between 90 and 135 m above the Permian/Triassic boundary) have the highest contents of phytoclasts; mostly woody macerals. Solvent-extractable compounds are in the range of 100 ppm of whole rock, as compared with 400-700 ppm of whole rock for the Urd Formation (Table 1; Fig. 4d). Effective staining of Skuld samples by migrated hydrocarbons may have been lessened by higher percentages of clay minerals, as observed in thin-sections. Most gas chromatograms of the C~5+ saturated hydrocarbon fractions display a thermally mature front-biased n-alkane distribution with pristane/n-C~7 ratios

Fig. 3. Representative thin sections. (a) Sample B20 (Urd Formation) showing bitumen staining and a predominance of quartz grains. (b) Sample B24 (Verdande Bed) showing quartz grains in a sideritic-phosphatic groundmass. (c) B25 (Skuld Formation) shale showing bitumen exhibiting staining. (d) B26 (Skuld Formation) showing fine-grained quartz in a sidertic groundmass. All at magnification 10 x , except B24 at 40 x .

O

R

o

2

O~

e~

2

©

Gary H. lsaksen

338 (a) 200 180

•~ ~.

(b}

--

--o---o

. . . .

160

.

m~ 140 > o

o

•8

o •

--

-

--

-o

o

_

_

_

~.----•- -

100

. . . . . .

80

•=

o

. . . . . .

•

, ~IIZ_-_, 40 20 ~'

. . . .

o li

0 0

0.5

1

1.5

0

2

20

40

(c) o

-a e=

"-" I-=)

180

•

1SO

-• •'=

m

.= =

I00_.

>

•Q

o

~o

o

o

O- o

-~

_ •

.

.

.

.

•

o

•

.

m m

~

°

=

•

E--o

•

•

i

• 0

0

i.~

20

"r

0

100

o°

-. -. -. .. .

80 60

80

(d)

200

140 120

60

H y d r o g e n Index

% Total Organic C a r b o n

---~:~i-~

+

0

0.5

_~o_

-

....

!

o

!

;

.

1

0

200

Production Index ( $ t l S 1 + $ 2 )

400

600

800

1000

EOM (ppm)

Fig. 4. Bulk geochemical parameters for the Triassic succession. Filled squares represent results from this study, and open circles are data from Bjoroy et al. (1983). (a) % total organic carbon: (b) hydrogen index: (c) production indices [S,/(S, + S~)]; and (d) solvent-extractable organic matter.

i

• Saturates [ ] Aromatics

i

[] Nso B31 ~

[ ] Asphaltenes

[

B30

.29

nBI,

B27 B26

-

100

200

300

400

500

600

700

800

ppm of whole rock

Fig. 5. Compound class types in solvent extracts from the Urd and Skuld Formations. The relatively high contents of saturated hydrocarbons in the Urd Formation are due to staining by hydrocarbons generated in situ or by migrated hydrocarbons. See text for discussion.

Organic geochemistry and geohistory of the Triassic succession

339

Ph

623

Fig. 6. Gs+ gas chromatograms of the saturated hydrocarbon fractions of extracts from the Urd (B20 and B21) and Skuld Formations (B23). The front-ends have been altered by weathering in outcrop.

around 0.2 (Table 1). CPI values are in the range of 1.OS to 1.09. Some samples display a bi-modal n-alkane distribution (local maxima at C,, and C,,) which is likely the result of a higher contribution from terrigenous organic matter. Regular steranes, albeit present in low amounts, show a predominance of C,,cr/?fl isomers, also supporting derivation from a rock with some terrigenous kerogen (Fig. 7). Thermal maturity modeling

assessment

and

1-D geohistory

A one-dimensional geohistory modeling study was undertaken in order to investigate if estimates of burial history and heat flow could account for the maturity levels observed, and to estimate hydrocarbon generation timing and yield volumes. The approach used was the one-dimensional deterministic model developed by Yiikler. More information on this program can be found in Welte and Yiikler (198 1) and Yiikler and Kokesh (1984). The Bjornoya outcrops have undergone significant burial, uplift and

erosion (Isaksen, 1985). Due to the erosion of post-Triassic rocks from the island, their lithology, porosity and thicknesses had to be estimated. These estimates were made by comparing equivalent age rocks on Svalbard and the greater Loppa High/Hammerfest Basin area. For the modeling process a hypothetical well was positioned at the summit of the Urd Mountain in order to include as much of the sedimentary record as possible. Model input data are listed in Table 2. The data are organized in the form sedimentation, erosion, of events, representing non-deposition, uplift, subsidence, etc. Sedimentary interval thickness refers to thicknesses at the hypothetical well location and is not concerned with whether or not the sedimentary unit in question was absent or more fully developed elsewhere. Together with the dominant lithologies (Table 2), the water depth and duration of deposition are input values to allow for sediment compaction. Present-day porosity values for the paleozoic outcrops were obtained from Grranlie et al. (1980), whereas sediment/water

340

Gary H. Isaksen

interface temperatures are from modeling of S v a l b a r d ' s p a l e o l a t i t u d e s t h r o u g h t i m e (Steel a n d W o r s l e y , 1984). Vitrinite reflectance, p r e s e n t - d a y surface temperatures, and thermal maturity information from molecular parameters (steranes and t r i t e r p a n e s ) , s e r v e d as direct c a l i b r a t i o n p a r a m e t e r s . R e s i d u a l h y d r o c a r b o n p o t e n t i a l w a s u s e d to c h e c k h o w well t h e m o d e l e s t i m a t e s g e n e r a t i o n a n d yields. N o a p a t i t e fission t r a c k a n a l y s e s were m a d e for t h e p r e s e n t s t u d y . H o w e v e r , a p a t i t e fission t r a c k d a t a at t h e Svalis D o m e a r e a s u g g e s t a n o n s e t o f uplift a n d c o o l i n g t o o k p l a c e d u r i n g t h e late P a l e o g e n e a n d c o n t i n u e d t h r o u g h o u t t h e N e o g e n e ( L ~ s e t h et al., 1992). T h e s a m e a u t h o r s s t a t e t h a t differential uplift h a s occurred on the Barents Shell with the Stappen High h a v i n g e x p e r i e n c e d t h e g r e a t e s t a m o u n t o f uplift.

Optical maturity parameters. Vitrinite r e f l e c t a n c e m e a s u r e m e n t s were m a d e difficult by o x i d a t i o n , a low p h y t o c l a s t c o n t e n t , a n d a s m a l l particle size o f t h e vitrinite m a c e r a l s . D u e to t h e p r o b l e m o f o x i d a t i o n , special c a r e w a s t a k e n to d i s t i n g u i s h b e t w e e n o x i d i z e d a n d n o n - o x i d i z e d p a r t s o f vitrinite particles. A c c e p t a b l e r e f l e c t a n c e m e a s u r e m e n t s were o b t a i n e d f r o m t h e S k u l d F o r m a t i o n only. Vitrinite reflectance v a l u e s v a r y f r o m 1.1 to 1.3% R,, ( T a b l e 1). T h e r m a l

a l t e r a t i o n indices ( T A I ) m e a s u r e d o n p o l l e n s p o r e s a n d p l a n t cuticles were in t h e r a n g e o f 2 + to 3 - ( o n a scale f r o m 1 to 5), c o r r e s p o n d i n g to a p p r o x i m a t e l y 1.0% Ro. T h e s e d a t a p l a c e t h e T r i a s s i c r o c k s in a late-oil to c o n d e n s a t e g e n e r a t i v e stage. Molecular maturity parameters. M o s t g a s c h r o matograms o f t h e C,~+ s a t u r a t e h y d r o c a r b o n fractions display a thermally mature n-alkane distribution with the abundance of front-end n - a l k a n e s d i m i n i s h e d d u e to e v a p o r a t i o n in o u t c r o p (Fig. 6). C o n s e q u e n t l y , m a t u r i t y a s s e s s m e n t s b a s e d o n pristane/n-C~7 a n d p h y t a n e / n - C ~ s r a t i o s were n o t a t t e m p t e d . S t e r a n e a n d t r i t e r p a n e b i o m a r k e r s are p r e s e n t in relatively low a m o u n t s , d u e to t h e h i g h level o f m a t u r i t y . D e s - m e t h y l s t e r a n e m a t u r i t y p a r a m e t e r s h a v e r e a c h e d e q u i l i b r i u m v a l u e s as e v i d e n c e d by v a l u e s o f a r o u n d 7 5 % for t h e ratio o f C29 ~/~/~ ( 2 0 R + 205)/[C29 0~]~fl ( 2 0 R + 20S) + C29 ~7~ (20S + 20R)] (Seifert a n d M o l d o w a n , 1980; M a c k e n z i e et al., 1980). C27 a n d C29 d i a - s t e r a n e s p r e d o m i n a t e o v e r t h e iso- a n d r e g u l a r d e s - m e t h y l s t e r a n e s , in a g r e e m e n t w i t h t h e h i g h e r t h e r m a l stability o f t h e 13/~(H), 17~(H) s t e r a n e s t r u c t u r e (Fig. 7). T h e effect o f h i g h e r t h e r m a l m a t u r i t i e s o n t h e b i o m a r k e r d i s t r i b u t i o n s is also o b s e r v e d a m o n g C~7

Table 2. Input data tbr one-dimensional geohistory modeling Event no. 1 2

Event

Time interval (mybp)

Time Interval span thickness (year x 10~) (m)

Sediment type

Water depth (m)

Present porosity (%)

Temperature (C)

Heat flow (HFU)

367 360 360-358

7 2

170 75

sst., sh., coal sst., congl.

5 5

18 18

25 25

0.95 0.95

3 4 5

Vesalstranda Kapp Levin Tunheim Nordkapp Landnoringsvika

358-354 354 325 325 308

4 29 17

80 230 145

5 5 5

18 19 II

25 25 23

0.95 0.95 0.95

6 7 8

Kapp KS_re Erosion Erosion

308-294 294-290 290-286

14 4 4

215 215 145

sst.. sh., coal sst., sh., coal sst., congl., sh. limest., sh. limest., sh sst., congl., sh. limest.

60 0 0

11

22 22 22

0.95 0.95 0.95

20

16

22

0.95

9

Kapp 286-277 9 25 Ouner 10 Erosion 277-268 9 25 limest. 11 Erosion 268-265 3 --180 sst., sh.. coal 12 Erosion 265-263 2 105 sst.. sh., coal 13 Erosion 263-261 2 100 sst., sh., coal 14 Miseryfjellet 261 257 4 165 silty limest. 15 Erosion 257 247 10 50 silty limest. 16 Triassic 247-217 30 250 silty sh. 17 Lias 217 177 40 30 silty sh. 18 Baj.-Haut. 177-127 50 300 sh. 19 Brm.-Aptian 127-117 10 800 sst., silt., sh. 20 Aptian-Albian 117 97 20 700 sst., silt., sh. 21 Ceno.-Maas. 97 67 30 500 sst., silt., sh. 22 Paleocene 67-57 10 550 sst., sh. 23 Pal.-Eocene 57M7 10 550 sst., sh. 24 Eoc.-Oligo. 47 36 11 550 sst.. sh. 25 Erosion 36 30 6 --550 sst., sh. 26 Erosion 30-25 5 -550 sst., sh. 27 Erosion 25 20 5 -550 sst., sh. 28 Erosion 2(~16 4 500 sst.. silt., sh. 29 Erosion 16-12 4 -700 sst., silt., sh. 30 Erosion 12-8 4 800 sst., silt., sh. 31 Erosion 8-5 3 300 sh. 32 Erosion 5--4 1 30 silty sh. 33 Erosion 4-2 2 -50 silty sh. Events are named either by rock Formation or Member names, or geological ages. sst. = sandstone; sh. = shale; limest. = limestone; congl. = conglomerate.

0 22 0 21 0 21 0 21 50 I1 21 0 21 250 2 20 200 21 20 200 23 18 100 20 17 100 20 17 50 23 17 250 28 13 250 29 II 250 30 10 0 12 0 12 0 10 0 l0 0 10 0 10 0 I0 0 9 0 5 Abbreviations: mybp = million years before

0.95 0.95 0.95 0.95 0.95 0.95 0.95 I 1.05 1 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 present;

Organic geochemistry and geohistory of the Triassic succession 13+14 6

18

rn/z 2 1 7

8 9

28

29 m/z 218

Fig. 7. Sterane biomarkers present in sample B23 (lowermost Skuld Formation) as seen by monitoring common fragment ions m/z 217 and rn/z 218 for regular des-methyl steranes and rn/z 259 for diasteranes. Labels: 5 = 13 fl(H), 17ct(H)-diacholestane 20S (C27); 6 = 13 fl(H), 17 ~(H)-diacholestane 20R (C_~7); 8 = 13 ct(H), 17 fl(H)diacholestane 20R (C27);9 = 13 fl(H), 17 c~(H)-diaergostane 20S (24S + R) (C28); 13 = 13fl(H), 17 c¢(H)-diastigmastane 20S (C29); 14 = 5 ~(H), 14 fl(H), 17fl(H)-cholestane 20R (C27); 18 = 13 fl(H), 17 ~(H)-diastigmastane 20R (C29); 26 = 5 ct(H), 14 ct(H), 17 ct(H)-stigmastane 20S (C29);28 = 5 ~(H), 14 fl(H), 17 fl(H)-stigmastane 20R (C29);29 = 5 ~t(H), 14 fl(H), 17 fl(H)-stigmastane 20S (C29); 30 = 5ct(H), 14 :t(H), 17 ct(H)-stigmastane 20R (C_,9).

and C29 triterpane isomers. The m/z 370 (parent) to m/z 191 (daughter) transition on GC/MS/MS analyses (Fig. 8a) shows a higher thermal stability for 18 ct(H)-22,29,30-trisnorneohopane (Ts) than the 17 ~(H)-22,29,30 trisnorhopane (Tm). The Ts/Tm ratio is 23. Likewise, the m/z 398-191 transition, monitoring the C29 triterpanes, shows a greater thermal stability for the C29 Ts (18 ct (H)-30norneohopane) as compared with the C29 17 ~(H) norhopane (Fig. 8b). The C29 Ts/(C29 Ts + C29 norhopane) ratio is 0.62. This is in agreement with molecular steric energy calculations (Kolaczkowska et al., 1990) that demonstrate a 3.5 kcal/mole greater stability 0fC29 Ts vs 17 ~t(H)-30-norhopane and a 4.4 kcal/mole greater stability of Ts vs Tm. Similar

341

biomarker distributions have been seen by Sofer (1988) in the Jurassic Smackover trend of the U.S. Gulf Coast, Fowler and Brooks (1990) in their study of the Jeanne d'Arc Basin, in eastern Canada, and Peters and Moldowan (1993) for Oman area oils. Modeling results. Estimates show the heat flow to have been fairly constant around 0.95-1.05 H F U throughout most of the geological history. Maximum heat flow values were estimated for periods of active rifting and crustal thinning during the Jurassic. Figure 9 shows computed burial history, temperature history, and changes in vitrinite reflectance and sterane isomerization, through time, for the Triassic succession. Estimated amounts of hydrocarbons generated through time, from both Type 1I and Type III kerogens, are shown in Fig. 10. Hydrocarbon yields are given in terms of mg hydrocarbons/g organic carbon. Peak generation of liquid hydrocarbons (black oil and condensate) occurred during the late Cretaceous to Early Tertiary, For SW Barents Sea oil and gas exploration, traps in existence at this time in this area could be potential targets. Significant gas generation was also reached during this time, thus competing with oil charge to the traps. In these models, gas has been generated directly from kerogen breakdown, as well as from in-reservoir cracking of pre-generated liquids during exposure to maturities greater than 130°C (equivalent vitrinite reflectance values greater than 1.4% Ro). Peak hydrocarbon generation at equivalent Ro values of 1.1% is about 0.2-0.3% Ro greater than classical peak generations from Type II kerogen (e.g. Tissot and Welte, 1984). This higher level of peak generation indicates a low heating rate. Temperatures estimated for the onset of significant hydrocarbon generation are 58-60°C, whilst peak generation is near 100°C.

Bitumen staining As mentioned earlier, the Urd Formation is stained by bitumen. Within this bitumen, the distributions and relative concentrations of tricyclics and pentacyclic triterpanes (hopanes) are of particular interest. For samples from the Urd Formation (B20, B21 and B23), the major peaks in the m/z 191 mass fragmentograms are short-chain tricyclics, with only minor amounts of hopanes (Fig. 11). This is in agreement with greater thermal stability for tricyclics compared to hopanes (Aquino Neto et al., 1983). GC/MS/MS analyses reveal a homologous series of tricyclics extending from C2, through C4~ (Fig. 12a and b). Note the near absence of C22, and C42 C27, C32 C37 and C42 tricyclics. This is a result of preferential cleavage at specific sites on the isoprenoid side-chain, governed by the position of the methyl substituent at the C22, C27, Cn, C37 and C42 positions (Fig. 13). A hexaprenol (C~0) (cyclization to tricyclohexaprenol) precursor was suggested by Aquino Neto et al. (1983) for the homologous tricyclic series extending up to C30. For the Bjornoya samples an isoprenoid side-chain with at least six isoprene units is required,

Gary H. lsaksen

342

C27 Triterpanes m/z 370.3599 to m/z 191.1800 10oi 95~ 90 85' 8o~

C29 Triterpanes m/z 398.3912 to m/z 191.1800

(a)

(b)

TS

75~ 70~

C29 Ts

C29 NH

65!

60! 55 ~ 50!

45! 401 35 ~ 30" 25" 20151 10' 5O"

Tm

200

400

400

600

800

Fig. 8. GC/MS/MS analyses of parent~taughter transitions for C:7 and C_~9triterpanes. (a) Ts (18 ~(H)-22,29,30-trisnorneohopane) is thermodynamically more stable than Tm (17 ~(H)-22,29,30trisnorhopane. (b) Likewise, the C:~Ts (18 ~(H)-30-norneohopane) is more stable at higher maturities than C~ 17 ¢(H),21 /~(H)-30-norhopane. Thus, the Ts/(Ts + Tin) and C>,Tx/(C_,~Ts + C_~sNH) serve as thermal maturity parameters.

giving a side-chain with 31 carbon atoms. The most abundant compounds in the series are C2, and C24 tricyclics. Tricyclics with extended side-chains have also been observed by a number of other researchers, e.g. Seifert and M o l d o w a n (1978), Moldowan et al. (1983), Peters et al. (1990) and D e G r a n d e et al. (1991).

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In contrast to the Urd samples, samples from the top half of the Skuld Formation (e.g. B31, B34 and B36; of which only B36 is shown in Fig. 14) show a predominance of hopanes, Given the same geohistory for the Urd and Skuld Formations, the differences among the tricyclic and hopanes can not be the result of thermal maturity alone. Thus, the differences may

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Fig. 9. Geohistory analysis of the Devonian through Triassic rocks on Bjornoya. Parameters modeled through time are: vitrinite reflectance (% R,,); temperature ( C); sterane biomarker isomerization at position 20 [C,~ 5 ~<(H), 14 ~(H), 17 ~(H) (22S/22S + 22R)]; and depth (meters). (The Vesalstranda Member is of Late Devonian age, the first bed in the model. With a stratigraphic distance of about 300 m between the Late Devonian and Base Triassic strata, their post-Triassic geohistories are very similar.)

Organic geochemistry and geohistory of the Triassic succession

Carb. Perm. Tr Jurassict

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be due to (a) organic facies variations, or (b) staining by hydrocarbons derived from a different source rock. Work by Ourisson et al. (1982) showed that tricyclic terpanes and hopanes have different precursors in bacterial membranes; probably a C30 hexaprenol for the tricyclics. Visual kerogen analyses on samples from the Triassic section of Bjornoya show an enrichment of algal material in the Urd shales (relative to the Skuld shales). Hydrogen indices are too low to make a reliable assessment on organic facies variations, but they do show elevated values in the Urd Formation (Fig. 4b). The elevated tricyclic contents may, in addition to enhancements from high thermal maturity, be derived from algal and bacterial matter from a stressed depositional environment. Mork et al. (1982) note a change in climate from a hot, arid climate during the Early Triassic to a more humid climate in the Late Triassic. Such a climate change was also noted by Bergan and Knarud (1993) based on clay mineral assemblages (higher smectite contents in the Skuld Formation). A semi-arid climate was also proposed for the Arctic North America by Balkwill et al. (1983) and Embry (1992). Shallow marine environments withinan arid climate may have developed both elevated water-salinity levels and UV radiation levels from sunlight. Under such conditions, bacteria may alter the chemical composition of their cell walls (Nassin and James, 1978). In particular, microorganisms can respond to such extreme environmental conditions by increasing their pigment level (Anderson et al., 1956; Davis

343

et al., 1963). The elevated concentrations of extended

tricyclics in the Urd Formation extracts may, in part, be derived from "metabolic response" compounds. Elevated concentrations of extended tricyclics was also observed by Palacas et al. (1984) in rock extracts from the Sunniland carbonate source rock in the South Florida Basin. Alternatively, the characteristic biomarker pattern, a high tricyclic/hopane ratio, may be the result of hydrocarbon staining from a different source rock, i.e. an allochthonous source. This is suggested by elevated production indices [SJ(St + $2)] and extractable organic matter (EOM) in the Urd Formation relative to the Skuld Formation (Fig. 4c and d). These new data show that the siltstones and shales of the Skuld Formation have not been stained by this oil, possibly due to the Verdande Bed and the higher clay content of the lower Skuld Formation acting as a barrier to petroleum migration. (Note that the production indices reported by Bjoroy et al. (1983) differ from the present results in that they suggest staining of the entire Triassic succession.) Possible source rock candidates for this staining are: • A southern equivalent of the Jurassic/Cretaceous black shales of the Janusfjellet Formation on Spitsbergen or the late Jurassic Hekkingen Formation. Such organic-rich oil-prone shales have been encountered in dredged ocean-floor sediments from the SW Barents Sea (Edwards, 1975; Bjorlykke et al., 1978) and in many of the SW Barents Sea exploration wells drilled to date. Bjoroy et al. (1983) note that since the Triassic sediments accumulated on a structural high (Stappen High), Jurassic shales may have been situated in a lateral or down-faulted position. • Equivalents of the Spathian to Anisian age rocks on the Svalis Dome, located to the southwest of Bjornoya in block 7323/7 (73°15'N, 23°20'E). These rocks are organic-rich with oil-prone marine algal organic matter; TOCs in the 4-12% range, and hydrogen indices between 300 and 460 (Isaksen and Bohacs, 1994). • Equivalents of the Anisian-Ladinian Botneheia Formation on Svalbard (Mork and Bjoroy, 1984). • Organic-rich shales of Permian age. Such facies are present on Svalbard (Skaug et al., 1982) and East Greenland (Surlyk et al., 1984). C o m m e n t s on the sequence stratigraphic f r a m e w o r k

Regional correlations of sequence and parasequence boundaries are limited by the sparse data available within the immediate Western Barents Sea area. The nearest Triassic outcrops are on Svalbard, 480 km to the north. Limited seismic and well data from the Loppa High and Svalis Dome areas are 320 km to the south. Therefore, the sequence stratigraphic framework suggested herein is based on

344

Gary

H. lsaksen

stacking-patterns observed in outcrops at the Urd, Skuld and Verdande mountains, comparisons with Svalbard and Loppa High stratigraphy, published biostratigraphy and lithostratigraphy data, and geochemical of outcrop analyses samples. A stratigraphic column of the Triassic succession has been adapted from Msrk er al. (1990). The proposed sequence stratigraphy is shown in Fig. 15. Urd formation According to Krasilshchikov and Livshits (1974) the Kazanian and Tatarian stages of the Late Permian Miseryfjellet Formation are absent on Bjornarya. Thus, the Permian/Triassic erosional

boundary was formed during Late Permian and (Early Triassic?) uplift. This contact may represent both the 255 and 252 mybp sequence boundary as these surfaces are interpreted to be superimposed on the Stappen High, (Sequence boundaries of these ages are also suggested by the coastal onlap curves of Haq et (II., 1988.) In outcrop, the top surface of the Miseryfjellet Formation is reddish-brown due to oxidation. The same surface at this outcrop location (base Triassic on Miseryfjellet) is also represented as a flooding surface, with the lowermost 9 m of shales interpreted as belonging to the transgressive systems tract. The lowermost beds of the Urd formation were given Griesbachian and Dienerian ages by Pchelina (1972). Msrk (pers. commun.) questions the presence

24

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I

48:00

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1:04:00

1:12:00

1:20:00

1:28:00

1:36:00

1:44:00

1:52:00

Fig. I I. Tricyclics and pentacychcs as monitored by m/z 191 for samples from the Urd (B20 and B21) and the Skuld Formations (B23). The tricyclics are labeled according to the number of carbon atoms in the molecule as determined by GC/MS/MS analyses shown in Fig. 12. The bottom mass chromatogram is that of an external standard with a regular hopane distribution. Note the near absence of Cz2, Cz7. Cj2 and C,, tricyclics, controlled by the position of the methyl substitutes in the side-chain (Fig. 13).

Organic geochemistry and geohistory of the Triassic succession

345

(Q) C31

M/Z 430.4539to 191.1800

C32 M/Z 444.4695to 191.1800

C33 C34 M/Z 458.4851to 191.1800 M/Z 468.4695to 191.1800

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C35 M/Z 486.5164to 191.1800

C3s M/Z 500.5321to 191.1800

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(b) C39 M/Z 542.5790to 191.1800

C40 M/Z 556.5947to 191.1800

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C41 C42 M/Z 570.6104to 191.1800 M/Z 584.6260to 191.1800 1:18:o5 :t13]i i i I l l I IJ/ 1:28:21 1:10:21

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|•jl1:14:09 C43 M/Z 598.64171o 191.1800

C44 M/Z 612.6573to 191.1800 1:23:37

C45 C46 M/Z 626.6730to 191.1800 M/Z 640.6886to 191.1800 1:; ~):35 1:28:47 1:24:39

1:18:45 O)i]~l

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Fig. 12. GC/MS/MS mass fragmentograms monitoring parent to daughter transitions for the tricyclic series from (a) C3~ through to C38, and (b) C39 through to C~.

346

Gary H. Isaksen deposits most often form on continental shelves and oceanic plateau where upwelling of phosphorous-rich bottom waters is prevalent (Blatt et al., 1980). A prerequisite is a very low clastic sediment supply rate; most likely during major landward shifts in coastal onlap. Increased organic productivity in the photic zone may consequently lead to anoxic organic-rich bottom sediments. However, due to low rates of clastic input, long periods of time would be available for the degradation of settling organic matter, leaching of phosphate from organic remains and phosphatization of nonphosphatic sediments. The remanie clasts of the Verdande Bed suggest that the marine transgression was followed by a period of local uplift, with reworking of the phosphatic sediments and their concentration as lag deposits. Mork and Worsley (1979) and Worsley and Gjelberg (1980) suggest that this thin phosphatic horizon represents the reworked remains of a condensed equivalent of the Botneheia Formation of Spitsbergen. The Verdande Bed contains low TOC (0.34 wt%), and essentially no yield upon Rock-Eval pyrolysis. The base of the Verdande Bed is correlated with the 241 mybp condensed interval, whereas the 240.5 and 239 mybp sequence boundaries may define the top of the Verdande Bed (Fig. 15).

Fig. 13. Tricyclic terpane with an extended isoprenoid side-chain. The methyl groups at C~, C27,C~_~,C~7,etc. favors the cleavage of the side-chain at these positions rather than the methyl-branch-sites. of Griesbachian-age rocks, thus favoring a Dienerian first transgression. The base of a prominent 25 cm thick sandstone bed located 7 m above the Permian/Triassic boundary is interpreted as the 245 mybp sequence boundary (Fig. 15). The upper 58 m of the Urd Formation are dominated by silty shales alternating with thin siltstone beds and some cm-thick dolomitic limestone horizons. The thin siltstone beds are most likely storm generated (Mork et al., 1990). The 242 mybp sequence boundary is interpreted at a horizon with slightly coarser (very fine-fine) sandstone 60 m above the base of the Urd Fm. Observation of ammonites Euflemingites sp. around 35 m and Arctoceras blomstrandi about 55 60 m above Urd base-level (Pchelina, 1972), suggest affinity to the romunderi zone of Tozer (1961, 1963) and Tozer and Parker (1968) of Early Smithian age. The lack of fossil evidence for the tardus, pilaticus and subrobustus zones of Tozer (1961, 1963, 1967), may suggest that Late Smithian and Spathian strata are absent. Geochemically, some may argue that organic matter types with slightly higher hydrogen indicies support positioning of the 243 condensed interval age at about 20 m above the Urd Formation base-level (Table 1; Fig. 4b). This is~ however, a weak argument due to the low present-day values (40 60) of the hydrogen index. Biozones developed for the Canadian Arctic by Tozer (1967) suggests that the (Euflemingites) romunderi zone is directly overlain by the Verdande Bed. Thus, the younger (Wasatehites) tardus, (Olenenites) pilaticus, and (Keyserlingites) subrobustus zones of Late Smithian through Spathian may be absent on Bjornoya.

SkuM Formation The Skuld Formation constitutes the uppermost 140 m of the Triassic section on Bjornoya (Mork et al., 1982). Silty shales alternating with thin siltstone beds, comparable to the Urd Formation, constitute the lower 120 m. The 237 mybp sequence boundary is positioned at the first major basinward shift in facies above the Verdande Bed, which is interpreted to be near 84 m. Thin sections (e.g. sample B26~ Fig. 3d) show a predominance of siderite at this horizon. Daonella ~7~. bivalves near 100 m above the Urd Formation base-level suggest a Ladinian age for these strata (Pchelina, 1972). This, together with the stratal stacking patterns and beds representing basinward shift in facies, suggests the positioning of the 237 and 232 mybp sequence boundaries at the base of the coarser-grained siltstone to sandstone beds at 80 and 104 m, respectively. The positioning of the Ladinian/Carnian boundary (231

Verdande bed The Verdande Bed is a thin (20 cm) bed of remanie phosphorite concretions. Sedimentary phosphate 4

5

2

1:0,4:00

1:12:00'

1:26:00

6

1:2l;:00

" 1:3(~:00

1:4'4:00

1:52:00

Fig. 14. m/z 191 mass fragmentogram of sample B36 in the uppermost part of the Skuld Formation. 1 = Ts (18 a(H)-22,29,30-trisnorneohopane); 2 = Tm (17 ~(H)-22,29,30-trisnorhopane); 3 = C,,Ts (18 ~(H)-30-norneohopane); 4 = C~0 diahopane; 5 = C3, 17 ~(H),21 fl(H)-hopane; 6 = C~ homohopane 22S + 22R; 7 = C,_~homohopane 22S + 22R.

Organic geochemistry and geohistory of the Triassic succession MACROFO~SlLS

EPOCH Fm

AGE

~

LIrTHOSTRATIGRAPHY Iv$~l AGE

347 m ~ ~OASTAL ONLAP CURVE AGE LANDWARD IIF-AWARO 224

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Fig. 15. Stratigraphy of the studied section, with a proposed sequence stratigraphic framework based on macrofossils, lithological stacking patterns and organic geochemistry. The lithostratigraphy is that of Mork et al. (1989). An attempt has been made to relate the parasequences to the coastal onlap curves of Haq et al. (1988). HST: highstand systems tract; TST: transgressive systems tract.

mybp) is uncertain. The next youngest age-dating is the presence of Dawsonites (Daxatina) canadensis (B6hm, 1903 and Buchan et al., 1965) between 180 and 156 m above the Urd Formation base-level, thus assigning these beds to the sutherlandi zone of Late Ladinian age (Tozer and Parker, 1968, p. 538). This is overlain by a 20 m thick siltstone to sandstone coarsening upward sequence, with planar lamination and ripple lamination in the sandstones, interpreted to have been deposited in a nearshore environment. This sandstone unit is the Myophoria sandstone of Anderson (1900). A comparison with the Austjfkelen Formation of southern Spitsbergen suggests that the Skuld Formation may have passed up into deltaic deposits only a few meters above the present youngest preserved exposures. Accordingly, the 228 mybp sequence boundary is positioned at 180 m level, as the first major basinward shift in lithofacies. CONCLUSIONS

The Triassic rocks on Bjornoya accumulated within an offshore marine environment which

experienced a series of transgressive-regressive cycles, followed by a final progradation of nearshore marine sediments in the youngest outcrop exposures. The organic matter in both the Urd and Skuld Formations is dominated by woody and herbaceous material, with elevated contents of algal organic matter present in the Urd Formation. Small amounts of oil are likely to have been generated from these shales. No major source rocks for oil were present (pre-maturation), but a significant potential for gas existed. Thermal maturity estimates based on vitrinite reflectance measurements give values of 1.1 to 1.3% Ro, corresponding to the high-maturity part of the oil window. Accordingly, regular sterane and triterpane maturity indicators (% C29 ~c( 20S steranes and %C3z ~/~ 22S hopanes) have reached their maximum values. Geohistory analysis using a one-dimensional basin modeling program showed that the section reached a maximum burial of about 5 kin. Peak generation of liquid hydrocarbons (black oil and condensate) occurred during the Late Cretaceous to Early Tertiary. For SW Barents Sea oil and gas exploration,

Gary H. Isaksen

348

traps in existence at this time in this area could be potential targets. However, significant gas generation was also reached d u r i n g this time, thus competing with oil charge to the traps. A characteristic h o m o l o g o u s series of tricyclic terpanes with extended isoprenoid side-chains up to C46 is d o c u m e n t e d in extracts o f the U r d a n d Miseryt]ellet F o r m a t i o n s . The high tricyclic/hopane ratios are interpreted as resulting from a source rock rich in bacterial organic m a t t e r a n d a high maturity. T h e identity o f this source rock remains uncertain as the staining m a y be from in situ generation within the U r d F o r m a t i o n or from a different source rock with similar organic facies a n d a high t h e r m a l maturity. A sequence stratigraphic f r a m e w o r k for the Triassic strata is p r o p o s e d based o n age-dating from ammonites, bivalves a n d palynological assemblages, stratal stacking patterns, a n d geochemical properties. These d a t a suggest t h a t Early Scythian a n d most of the S p a t h i a n a n d Anisian rocks are absent. Associate E d i t o r I K .

Peters

Acknowledgements--This work is based on field observations and sampling performed during 1984 and 1985 expeditions to Bj~Jrnoya, sponsored by Norsk Hydro and Bergen University. Analyses were performed at Norsk Hydro Research Center, the Geology and Chemistry Departments at the University of Bergen, and Exxon Production Research Company. 1 would like to acknowledge the assistance provided by Gordon C. Speers and Michael Talbot as well as discussions with Eigil Nysaether, Birger Dahl, Nils Telnaes, and Arne Steen at Norsk Hydro. Colleagues at Esso Norge a.s. and Exxon Production Research Company assisted with discussions of the biofacies and the sequence stratigraphic framework. Also thanks to Atle M~rk, Leslie Leith, Kenneth Peters and James Gormly for very helpful revisions to the manuscript. REFERENCES

Agdestein, T. (1980) En stratigrafisk sedimentologisk og diagenetisk undersokelse av Karbon-Perm sedimenter (Kapp Hanna formasjonen og Kapp Duner formasjohen), p~t Bjornoya, Svalbard. Cand. Real. Thesis, University of Oslo, 144 pp. (in Norwegian). Anderson J. G. (1900)/,Jber die Stratigraphie und Tektonik der B~iren Insel (Vorl Mitt.) Upsala. Buul, Geol. Inst. 4, 243-280. Anderson A. W., Nordan H. C., Cain R~ F., Parrish G. and Duggan D. (1956) Studies on a radio-resistant micrococcus. I, Isolation morphology, cultural characteristics and resistance to gamma radiation. Food Technol. 10, 575-578. Aquino Neto F. R., Trendel J. M., Restle A., Connan J. and Albrecht P. A. (1983) Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleums. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 659-676. John Wiley & Sons, New York. Balkwill H. R., Cook D. G., Detterman R. L., Embry A. F., H~kansson E., Miall A. D,, Poluton T. P. and Young F. G. (1983) Arctic North America and Northern Greenland. In The Phanerozoic Geology of the Worm H (Edited by Moullade M. and Nairn A. E. M.), pp. 1-31. The Mesozoic, B, Elsevier, Amsterdam. Bergan M. and Knarud R. (1993) Apparent changes in clastic mineralogy of the Triassic-Jurassic succession,

Norwegian Barents Sea: possible implications for paleodrainage and subsidence. In Arctic Geology and Petroleum Potential, Proceedings of the Norwegian Petroleum Society Conference (Edited by Vorren T. O. et al.), pp. 481~494. 15-17 August 1990, Troms~, Norway, Elsevier. Bj~rlykke K., Bue B. and Elverho A. (1978) Quarternary sediments in the NW part of the Barents Sea and their relation to the underlying Mesozoic bedrock. Sedimentology 25, 227-246. Bjoroy M., M~rk A. and Vigran J. O. (1983) Organic geochemical studies of the Devonian to Triassic succession on Bj~Jrnoya and the implications for the Barents Shelf. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 49 59. John Wiley & Sons Ltd. Blatt H., Middleton G. and Murray R. (1980) Origin of Sedimentary Rocks, 2nd edn. Prentice-Hall Inc., 782 pp. B6hm J. (1899) Ueber Triasfossilien v o n d e r B~iren lnsel. Zs. D. Geol. Ges., Berlin, B. 51, 325-326. B6hm J. (1903) Uber die obertriadische Fauna der B~iren lnsel. Kungliga Svenska Vetenskaps-Akademiens Handlingar 37, 1-76. Buchan S. H., Challinor A., Harland W. B. and Parker J. R. (1965) The Triassic stratigraphy of Svalbard, Norsk Polarinstitutt skrifter, 135, Norsk Polarinstitutt, Oslo 1965, p. 92. Clayton J. L. and Swetland P. J. (1977) Subaerial weathering of sedimentary organic matter. Geochim. Cosmochim. Acta 42, 305-312. Davis N. S., Silverman G. J. and Masurovsky E. B. (1963) Radiation resistant, pigmented coccus isolated from haddock tissue. J. Bacteriology 86, 294-298. DeGrande S. M. B., Aquino Neto F. R. and Mello M. R. (1991) Extended tricyclic triterpanes in sediments and petroleums. Organic Geochemistry: Advances and Applications in Energy and the Natural Environment, 15th Meeting of the European Association qf Organic Geochemists (Edited by Manning D. A. C.), (Abstracts), pp. 181-183. Edwards M. B. (1975) Gravel fraction on the Spitsbergen Bank, NW Barents Shelf. Norges. Geologiske Undersokelse 316, 215-217. Embry A. F. (1992) Crockerland-the northwest source area for the Sverdrup Basin, Canadian Arctic Islands, In Arctic Geology and Petroleum Potential, Proceedings of the Norwegian Petroleum Society Con/erence (Edited by Vorren, T. O. et al.), pp. 205-216. 15 17 August 1990, Troms~, Norway, Elsevier, Amsterdam. Faleide J. I., Gudlaugsson S. T. and Jacquart G. (1984) Evolution of the western Barents Sea. Mar. Petrol. Geol. 1, 123-150. Forsberg A. and Bjoroy M. (1983) A sedimentological and organic geochemical study of the Botneheia Formation, Svalbard, with special emphasis on the effects of weathering on the organic matter in shales. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 60--68. John Wiley & Sons, New York. Fowler M. G. and Brooks P. W. (1990) Organic geochemistry as an aid in the interpretation of the history of oil migration into different reservoirs at the Hibernia K-18 and Ben Nevis I = 45 wells~ Jeanne d'Arc Basin, offshore eastern Canada. Org. Geoehem. 16, 461~,75. Gronlie G., Elverho A. and Kristoffersen Y. (1980) A seismic velocity inversion on Bjorn~ya the western Barents Shelf. Mar. Geol. 35, 17-26. Haq B., Hardenbol J. and Vail P. R. (1988) Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change. In: Wilgus C. W. et al. (eds), Sea Level Changes: An Integrated Approach. Soe. Econ. Paleontol. Mineral., Spec. Publ. 42, 71 108. Hellem T. (1981) En sedimentologisk og diagenetisk undersokelse av utvalgte profiler fra Tempelfjorden

Organic geochemistry and geohistory of the Triassic succession Gruppen, Perm, ! Isfjorden omr~det, Spitsbergen. Can. Real. Thesis, University of Oslo 214, (in Norwegian). Hochuli P. A., Colin J. P. and Vigran J. (1989) Triassic biostratigraphy of the Barents Sea area. In Correlation in Hydrocarbon Exploration (Edited by Collinson .I.D.), pp. 131-153. NPF, Graham and Trotman, London. lsaksen G. H. (1985) Petroleum geochemistry of some pre-Jurassic sediments in the western Barents Shelf, Vol. I and II. Can. Scient. Thesis, Inst. of Geology, University of Bergen, Norway, 1985. lsaksen G. H. and Bohacs K. M. (1994) Geological controls of source rock geochemistry through relative sea level; Triassic, Barents Sea. In Petroleum Source Rocks (Edited by Katz B.J.), pp. 25-50. Springer. Johansen S. E., Ostisty B.K., Birkeland O., Fedorovsky Y. F., Martirosjan V. N., Bruun Christensen O., Cheredeev S. I., lgnatenko E. A. and Margulis L. S. (1992) Hydrocarbon potential in the Barents Sea region: play distribution and potential. In Arctic Geology and Petroleum Potential (Edited by Vorren T. O. et al.), pp. 273-320. NPF Special Publication 2. Elsevier, Amsterdam. Kolaczkowska E., Slougui N. E., Watt D. S., Marcura R. E. and Moldowan J. M. (1990) Thermodynamic stability of various alkylated, dealkylated, and rearranged 17ct- and 17/~-hopane isomers using molecular mechanics calculations. Org. Geochem. 16, 1033-1038. Krasilshchikov A. A. and Livshits J. ,i. (1974) Tektonika ostrova Medvezy (Tectonica of Bjornoya). Geotektonika 4, 39-51. L~seth H., Lippard S. J., Sa~ttem J., Fanavoll S., Fjerdingstad V., Leith T. L., Ritter U., Smelror M. and Sylta (1992) Cenozoic uplift and erosion of the Barents Sea--evidence from the Svalis Dome area. Arctic Geology and Petroleum Potential (Edited by Vorren T. O. et al.), pp. 643-6564. NPF Special Publication 2. Elsevier, Amsterdam. Mackenzie A., Patience R. L., Maxwell ,i. R., Vandenbroucke M. and Durand B. (1980) Molecular parameters of maturation in the Toarcian shales, Paris Basin, France-I, Changes in the configuration of acyclic isoprenoid alkanes, steranes, and triterpanes. Geochim. Cosmochim. Acta 44, 1709-1721. Moldowan ,i. M., Seifert W. K. and Gallegos E. J. (1983) Identification of an extended series of tricyclic terpanes in petroleum. Geochim. Cosmochim. Acta 47, 1531 1534. M~rk A. and Bjor~y M. (1984) Mesozoic source rocks on Svalbar. In Petroleum Geology of the North European Margin (Edited by Spencer A. M. et al.), pp. 371-382. Norwegian Petroleum Society (Graham and Trotman, 1984). M~rk A. and Worsley D. (1979) The Triassic and lower Jurassic succession of Svalbard: A review, Nora,. Sea. Symp, Tromso, Norwegian Petroleum Society, Oslo, NSS/29, 22 pp. M~rk A., Knarud R. and Worsley D. (1982) Depositional and diagenetic environments of the Triassic and Lower Jurassic succession of Svalbard. In Arctic Geology and Geophysics Canadian Soc. Petrol. Geol. Mere. (Edited by Embry A. F. and Balkwill H. R.), 8, 371-398. M~rk A., Embrey A. F. and Weistschat W. (1989) Triassic transgressive-regressive cycles in the Sverdrup Basin, Svalbard and the Barents Shelf. In Correlation in Hydrocarbon Exploration (Edited by Collinson J. D.), Proceedings from the Norwegian Petroleum Society, Bergen, Norway, 3-5 October, 1988, pp. 113-130. M~rk A., Vigran ,i. O. and Hochuli P. A. (1990) Geology and palynology of the Triassic succession of Bjornoya. Polar Res. 8, 141-163. Nassin A. and James A. P. (1978) Life under conditions of high irradiation. Microbial Life in Extreme Environments (Edited by Kushner D. J.), pp. 409439. Academic Press.

349

Ourisson G., Albrecht P. and Rohmer M. (1982) Predictive microbial biochemistry, from molecular fossils to procaryotic membranes. Trends Biochem. Sci. 7, 236-239. Palacas J. G., Anders D. E. and King J. D. (1984) South Florida Basin - - a prime example of carbonate source rocks of petroleum, In Palacas J. G. (ed.), Petroleum Geochemistry and Source Potential of Carbonate Source Rocks. AAPG Studies Geol. 18, 71-96. Pchelina T. M. (1972) Trias deposits of Bj~rn~ya. In Mesozoic deposits of Svalbard (Edited by Sokolov V. N. and Vasilevskaja N. D.), pp. 5-20. NIIGA, Leningrad (in Russian). Peters K. E., Moldowan J. M. and Sundararaman P. (1990) Effects of hydrous pyrolysis on biomarker thermal maturity parameters: Monterey Phosphatic and Siliceous Members. Org. Geochem. 15, 249-265. Peters K. E. and Moldowan ,i. M. (1993) The Biomarker Guide. Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, New Jersey. Seifert W. K. and Moldowan J. M. (1978) Applications of steranes, terpanes, and monoaromatics to the maturation, migration, and source or crude oils. Geochim. Cosmochim. Acta 42, 77-95. Seifert W. K. and Moldowan J. M. (1980) The effect of thermal stress on source-rock quality as measured by hopane stereochemistry. Phys. Chem. Earth 12, 229-237. Skaug M., Dons C., Lauritzen, O- and Worsley D. (1982) Lower Permian palaeoaplysinid and associated sediments from central Spitsbergen. Polar Res. 2, 57-75. Sofer Z. (1988) Biomarkers and carbon isotopes of oils in the Jurassic Smackover Trend of the Gulf Coast States, USA. Org. Geochem. 12, 421~t32. Steel R. ,i. and Worsley D. (1984) Svalbards postCaledonian strata-an atlas of sedimentational patterns and palaeogeographic evolution, In Spencer A. M. et al. (eds), Petroleum Geology of the North European Margin. Proceedings of the North European Margin Symposium (NEMS 83), Norwegian Institute of Technology (NTH), Trondheim, Norway, 9-11 May 1983, pp. 109-135, Graham and Trotman. Surlyk F., Piasecki S., Rolle F., Stemmerik L., Thomsen E. and Wrang P. (1984) The Permian base of East Greenland. In Petroleum Geology of the North European Margin (Edited by Spencer A. M. et aL), pp. 303-315. Norwegian Petroleum Society (Graham and Trotman, 1984). Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence. Springer, Berlin. Tozer E. T. (1961) Triassic stratigraphy and faunas, Queen Elizabeth Islands Arctic Archipelago. Geol. Surv. Can. Mere. 316, 116. Tozer E. T. (1963) Mesozoic and Tertiary stratigraphy, western Ellesmere Island and Axel Heiberg Island, District of Franklin. Geol. Surv. Can., Pap., 63-30, p. 38. Tozer E. T. (1967) A standard for Triassic time. Bull. Geol. Survey Can. 156, 1-103. Tozer E. T. and Parker J. R. (1968) Notes on the Triassic biostratigraphy of Svalbard. Geol. Mag. 105, 526-542. Welte D. H. and Yiikler M. A. (1981) Petroleum origin and accumulation in basin evolution - - a quantitative model. AAPG Bull. 65, 1387-1396. Worsley D. and Gjelberg J. (1980) Excursion guide to Bjornoya, Svalbard, Norsk Hydro Research Center, Bergen, Norway, 33. Worsley D. and Edwards M. B. (1976) The Upper Paleozoic succession of Bj~rnoya. Norsk Polarinstitutt Aarbok 1974, 17-34. Yiikler M. A. and Kokesh F. (1984) A review of models used in petroleum resource estimation and organic geochemistry. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), 1, 69-113.