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Effect of maturity on 13C12C ratios of individual compounds in North Sea oils
Pergamon
01,~-~S003)E0014-D
Org. Geochem. Vol. 21, No. 6/7, pp. 737-750, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0146-6380/94 $7.00 + 0.00
Effect of maturity on I3C/nC ratios of individual compounds in North Sea oils C. J. CLAYTON1. and M. BJOROY2 IBP Research Centre, Chertsey Road, Sunbury-on-Thames TWI6 7LN, England and 2Geolab Nor A/S, Homebergveien 5, 7038 Trondbeim, Norway Abstract--Carbon isotope variations in individual compounds are reported for North Sea oils of variable thermal maturity. Using the pristane/phytane ratio of the oils as an indication of source facies variation, and molecular maturity ratios to determine maturity, it is possible to differentiate the two effects. In these samples, maturity accounts for between 50 and 90% of the observed isotopic variation, all values becoming heavier with increasing maturity, typically by between 2 and 3%o. However, source differences are also apparent with a terrestrially influenced source, possibly the Heather Formation, contributing material to the Kimmeridge Clay sourced oils. The "Heather" contribution is isotopically heavier than the Kimmeridge oil for most compounds, but is isotopicaUylighter for the isoprenoid hydrocarbons and some other branched compounds. It is also more important for lower molecular weight hydrocarbons than for higher homologues. Key words---carbon isotopes, Compound Specific Isotope Analysis (CSIA), source influence, maturity effects, North Sea, Kimmeridge Clay Formation, Heather Formation
INTRODUCTION
In order to use stable carbon isotope ratios for correlation of oils it is necessary to allow for the effects of thermal maturity, and to differentiate these from facies effects which may betray a different source. The effects of maturity on 6 13C of whole oils and bulk fractions (i.e. saturated hydrocarbons, aromatic hydrocarbons, NSO and asphaltene fractions) are now reasonably well established (e.g. Clayton, 1991). Oil generation results in a small (approx. 1%o) increase in 6 13C, possibly caused by mixing of generated oil with the initial immature soluble extract. Subsequent oil to gas cracking results in a more substantial increase in 6 ~3C, typically about 3.5%o caused by kinetic isotopic fractionation as t2C passes preferentially into gas leaving residual oil enriched in "C. At the compound-specific level, maturity effects are less well understood. Soluble extracts from immature samples reveal a wide variation in 6 13C between different biomarkers, reflecting the diversity of biochemical processes that contributed organic matter to the sediment (see for example Hayes et al., 1987; Freeman et al., 1990 and discussions in Galimov, 1985). However this variation is generally absent in oils, which appear to be more homogeneous (e.g. Bjoroy et al., 1990; Collister et al., 1992). Beyond that however, things are less clear. Bjor~y et al. (1992)
*Present address: School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, England.
present data for laboratory pyrolysis products which suggest that 613C of individual compounds increases with increasing maturity. In addition, Northam (1985) shows similar maturity induced increases in '3C of boiling point fractions of oils from the North Sea Beryl Field. Such variations are consistent with the general trend towards heavier isotope values observed in bulk fractions. Here we present data for isotope ratios of the common individual compounds in four oils that define a maturity trend through the bulk of the oil generation window, and in which it is possible to differentiate these from changes which result from lateral source variations. SAMPLES STUDIED For this study, four oils from the United Kingdom Continental Shelf were chosen which define a maturity sequence: Buchan, Forties, Argyll and a Bruce oil. The locations of these fields are shown in Fig. 1. All the fields lie to the west of the main graben structures (and hence source kitchens). Argyll in the south lies on the flank of the Central Graben, Buchan and Forties lie close together further north, at the N.W. end of the Central Graben and the southern end of the Viking Graben, and Bruce lies near the northern end of the South Viking Graben. These fields were chosen so that any source facies variations could be distinguished from the maturity effects, since the geographical trend of the samples, which may cause systematic source variations, does not match the maturity trend (see below).
737
738
C.J. CLAYTONand M. BJOROY
Fig. 1. Location of oil fields sampled. Numbers in circles refer to relative maturity (1 = lowest), dashed lines correspond to international boundaries.
In addition, we include data for oils from the Jurassic Bridport Sands and Triassic S h e r w o o d Sandstone reservoirs o f the W y t c h F a r m oilfield, onshore U . K . These were sourced by m u d s t o n e s o f the same lithofacies as the K i m m e r i d g e Clay but o f different age (Lower Lias). These provide some degree o f additional control on the isotopic character o f oils f r o m this facies type.
Chemical data for the N o r t h Sea oils are presented in Table 1. The oils increase in maturity from Buchan, the least mature, t h r o u g h Forties, Argyll to Bruce. The m e t h y l p h e n a n t h r e n e indices for the latter samples (approx. 0.77) are equivalent to a vitrinite reflectance o f 0.86%, a little above that for the end o f oil generation in clastic marine source rocks (Quigley et al., 1987). These maturity variations are a consequence o f the m a x i m u m temperature attained in the source kitchen and are reflected in a relative increase in the saturated h y d r o c a r b o n c o m p o n e n t s , a general decrease in sulphur and nitrogen content, and increases in the aromatic and, where available, the saturated molecular maturity parameters (Fig. 2). In terms o f absolute maturity, Buchan is in the early generation stage, Forties is near the end o f generation but with no oil to gas cracking, and Argyll and Bruce are in the very early stages o f cracking to gas. The samples thus span the main part o f the conventional "oil window". Besides the maturity differences, there are geographical variations in the source facies o f the oils. Some source-influenced bulk chemical parameters and molecular ratios do not follow the sequence expected for pure maturity differences. Nitrogen and sulphur content should decrease progressively with maturity but there are minor, but consistent, discrepancies to the simple sequence. Similarly, the pristane/phytane ratio and c a r b o n preference index show deviations from a simple trend (Fig. 3). Higher pristane/phytane ratios are associated with organic matter deposition u n d e r m o r e oxidising conditions,
Table 1. Chemical composition and molecular maturity data for North Sea samples analysed % % % %
Saturated hydrocarbons Aromatic hydrocarbons NSO Asphaltenes
Buchan 54.9 9.1 28.2 7.8
Pristane/phytane CPIt %S ppm N
0.99 1.07 0.82 1330
6 t3Cwhol~oil
-- 29.8
MNR2 DMNR3 BphR4 MPI-I 5 MPI-25 C29 steranes6 Triaromatic steranes7
Forties 55.8 9.4 30.0 4.8
Argyll 57.9 10.3 26.3 5.5
1.43 1.11 0.30 830
1.33 1.03 0.18 899
-- 28.9
-- 28.6
Bruce 61.1 7.4 29.1 2.4 1.84 1.18 0.20 460 -- 28.5
0.78 1.16 0.19 0.62 0.25
1.19 2.19 0.17 0.72 0.80
1.23 2.35 0.28 0.77 0.83
1.26 2.69 0.42 0.76 0.86
0.64 0.18
0.71 0.48
---
0.74 0.59
~Carbon PreferenceIndex--ratio of odd/even Cz4-C33n-alkanes. 22-/l-methyl naphthalene (Radke, 1987). 32,6 + 2,7/1,5 dimethyl naphthalene (Radke, 1987). 4Biphynyl 1,6 dimethylnaphthalene(Hall et al., 1985). 5MPI, Methyl Phenanthrene Index (Radke, 1987); MPI-I = 1.5 × (3 MeP+ 2 MeP)/(P + 9 MeP + 1 MeP); MPI-2= 3 x (2 MeP)/(P+ 9 MeP + 1 MeP); P, phenanthrene; MeP, methylphenanthrene. %tflfl/(ctflfl + ~ctot) 20(R + S)Cz9 sterane ratio. 7C2o/(C20+ C2s) 20R methyl triaromatic sterane ratio. --, Not determined.
Effect of maturity on North Sea oils
739
• Buchan ~
I
[] Forties [] Argyll
MNR
DMNR
BphR
MPI-1
MPI-2
Fig. 2. Aromatic molecular maturity parameters for the North Sea oils.
possibly dysaerobic or intermittently anoxic. Under these conditions, terrestrially derived components make up a larger proportion of the bulk organic matter as these are more resistant to oxidation during early diagenesis. The higher Pr/Ph ratio oils therefore probably represent a more terrestrially influenced source. The variations in terrestrial vs marine contribution to the source rock may reflect either lateral variations within the Kimmeridge Clay or a contribution of oil from another, more terrestrially influenced, source rock. Northam (1985) attributed similar variations in Pr/Ph and carbon isotope "profile" between the Central and Viking Grabens to changes in the Kimmeridge Clay source rock. Using isotope profiles he identified a regular progression from "Ekofisk" type oils in the south to "Statl]ord" type oils in the north. Notice that this differs from the maturity trend
present in the samples used for this study in which the lowest maturity samples (Buchan and Forties) lie between the higher maturity samples (Argyll and Bruce). The biggest step in maturity (between Buchan and Forties) actually occurs in the two samples closest together geographically. In this way it should be possible to differentiate isotope variations that are controlled by source variations rather than just maturity. In contrast, Chung et al. (1992) demonstrate that in the North Viking Graben, the Heather Formation, which occurs immediately beneath the Kimmeridge Clay, is also a viable source of oils with a more terrestrial nature. This source is characterized by lower hydrogen indices and Pr/Ph ratios of 2-3, compared with about 1.5 for the Kimmeridge Clay (named the "Draupne" Formation here: Chung et al., 1992). It also has a carbon isotope ratio of - 2 8 to
1.5
0.5
Buchan
Forties
Argyll
Bruce
Fig. 3. Pristane/phytane ratio and carbon preference index for the North Sea oils.
740
C.J.
CLAYTON and
-26%o compared to about -29%0 for the Kimmeridge. Thus the variations observed by Northam (1985) may be due to a variable contribution from the Heather Formation, rather than lateral variations in the Kimmeridge Clay itself. It is worth noting however, that the range of Pr/Ph ratios and bulk oil carbon isotopes of our samples both fall entirely within the "Draupne" end member as defined by Chung et al. (1992), suggesting that any contribution from the Heather is minor. In summary, therefore, there is evidence that the oils discussed here are influenced by both source variations and maturity. The source variations can be recognized in the Pr/Ph ratio, and the maturity differences are apparent in the molecular maturity ratios. The observed facies variations probably reflect a minor contribution of oil from the more terrestrially influenced Heather Formation in the north. Geographically, the maturity sequence does not match
M. BJogov
that expected for a simple facies variation and we are thus able to differentiate the two. ANALYTICAL
METHODS
Asphaltenes were removed from the oils by precipitation in n-pentane in a plastic column containing a small amount of activated silica. The concentration of asphaltenes was calculated by weight difference of the column before and after elution of the asphaltenefree sample. After removal of asphaltenes, the oil/solvent mixture was rotary evaporated at 35°C and the samples stored in a freezer until ready for use. Further separation into saturated hydrocarbons, aromatic hydrocarbons and NSO fractions was by MPLC using hexane as an eluent following the method of Radke et al. (1980). Sample fractions were evaporated at 35°C and 200 mb to remove the hexane solvent.
Table 2. Compound specific isotope ratios for saturated hydrocarbon components. All values in per mil (Yo.) relative to From whole oil
n -Alkanes 6 7 8 9 l0 ll 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 iCl4 iCl5 iCl6 iCl8
Buchan
Forties
Argyll
Bruce
Sherwood
Bridport
-34.64 -33.92 -33.04 - 32.08 -31.57 -31.47 -31.19 -31.10 -30.59 -30.65 -30.69 -30.52 -30.51 -30.02 -30.16 -30.05 -29.95 -29.66 -29.69 -29.71 -29.72 -28.82 -28.86 --28.95 .
--30.78 -31.00 --30.86 - 30.25 -29.84 -30.04 --29.70 --29.65 -29.07 --29.33 --29.40 -29.15 -29.31 --28.96 --28.74 --28.64 -28.66 -28.49 --28.64 -28.59 -29.02 --27.82 --28.17 --28.28 .
--29.54 -29.89 --29.89 -- 2 9 . 4 7 --29.41 -29.35 --29.46 --29.22 --28.42 --28.86 -28.91 -28.85 -28.79 --28.45 --28.46 --28.36 --28.43 --28.10 -28.54 -28.75 --28.63 --28.47 -29.09 -29.50
-28.44 -28.63 -28.64 - 28.04 -28.28 -28.45 -28.45 -28.19 -27.49 -27.94 -28.35 -28.24 -28.19 -28.31 -28.02 -28.10 -27.90 -27.91 -27.96 -27.87 -27.83 -27.74 -27.84 --28.00 28.51
--31.19 -31.59 -31.68 -- 30.98 --31.22 --31.24 --31.30 -31.26 --30.90 --30.91 -31.16 --31.11 -30.87 --30.71 --30.63 -30.70 --30.52 -30.46 -30.31 -30.16 --30.21 --29.34 --29.56 --29.73 .
--31.10 --31.22 -31.22 - 30.69 -30.70 -30.64 --30.58 -30.40 -30.09 -30.08 -30.06 -29.96 --29.83 -29.68 -29.43 -29.40 -29.35 -29.18 -29.29 -29.26 -29.27 -28.48 -28.63 -28.56 .
.
.
PDB
From saturated hydrocarbon fraction
.
Buchan
Sherwood
Bridport
-----31.62 --31.84 --31.56 -31.59 -31.14 -31.50 -31.30 -31.60 -31.45 -31.54 -31.52 -31.66 -31.73 -31.49 -31.75 -----. .
------30.97 -31.34 -31.39 -31.46 -31.50 -31.59 -31.81 -31.86 -31.81 -31.70 --31.82 -32.01 ---------
--------30.83 --30.74 --30.52 -30.63 --30.63 -30.72 -30.72 --30.63 --30.76 --30.80 -30.71 -30.75 --30.82 -30.69 --30.92 --30.90 --30.86 --30.76 30.99
-28.35 -28.47 -28.09 -28.56 -30.19 -30.00
-27.47 -27.70 -27.03 -27.72 -28.96 -28.87
---24.00 -25.18 -27.51 -26.62
-25.08 -27.46 -27.31 -27.28 -28.92 -28.07
-28.56 -26.39 -29.03 -28.96 -30.35 -29.43
-27.63 --28.51 -28.34 -29.67 -28.75
--30.25 -30.19 -30.46 -32.20 -31.67
--30.00 -31.12 --31.12 -32.28 -31.58
--29.86 -30.64 -30.43 -31.34 -30.86
Br CI 1 2,6 D i m e t h y l u n d e c a n e
- 31.73 - 31.35 -27.03 -31.11 - 30.41 -29.21 -28.95
- 29.07 - 28.42 --29.76 -28.88 -28.23 -27.66
-- 2 8 . 6 4 - 28.36 -25.41 -28.43 -28.05 -25.72 -25.35
- 28.48 - 26.15 -24.04 -28.69 -26.92 -27.17 -27.54
-- 30.27 - 30.27 -26.75 - 30.10 -29.35 -28.28 -27.29
-- 29.99 - 29.27 -25.34 -29.47 -28.62 -27.99 -26.92
------30.34 -30.58
------29.57 -29.84
--------
Me-cyclopentane Et-cyclopentane Cyclohexane Me-cyclohexane
-29.30 -31.23 - 31.49 - 30.43
-28.12 -29.51 - 29.52 - 28.15
-27.01 -27.92 - 28.83 - 28.10
-26.66 -27.52 - 28.45 - 26.63
-28.29 -29.20 - 30.54 - 29.05
-27.90 -28.53 - 30.13 .
----
----
----
Pristane Phytane 2-Methylpentane 3-Methylpentane 2-Methylhexane Br C 8 Br C 9
--,
Not determined (peak too small, absent, or not resolved).
.
.
.
741
Effect of maturity on North Sea oils Table 3. Compound specific isotope ratios for aromatic hydrocarbon components. All values in per rail (%0) relative to PDB Buchan
Forties
Argyll
Bruce
Sherwood
Bridport
Benzene Toluene Xylene
- 31.01 -29.29 -30.55
- 28.84 - 28.47 . . . -29.02 -25.81
- 26.86
- 29.28 28.12 -29.77
- 28.63 -27.45 -28.77
2MN IMN BPH 2,6 + 2,7 DMN 1,6 DMN 2,3 + 1,4 DMN
-29.01 -29.61
-26.74 -27.20 -28.42 -27.60 -28.31 -27.39
-27.60 -27.77
-27.50 -27.76 -28.98 -28.65 -29.20 -28.50
-29.98 -31.73 -30.19
. -24.19
-27.40 -27.30 -28.69 -27.70 -27.58 -27.54
-26.33 -26.26 -28.10 -27.38 -27.27 -26.95
-28.43 -28.79 -28.35
From whole oil
From aromatic hydrocarbon fraction
P --27.89 -28.35 -27.85 -28.56 -28.57 3 MeP -30.55 -28.06 -28.42 -27.92 -28.76 -29.06 2 MeP ---28.75 --28.88 -28.90 9 MeP -30.17 -28.00 -28.41 -27.92 -28.76 -28.88 1 MeP . . . . . 28.84 -29.02 --, Not determined (peak too small, absent, or not resolved). MN, methyl naphthalenes; BPH, byphenyl; DMN, dimethyl naphthalenes; P, phenanthrene; MeP, methylphenantbrenes.
ISOTOPIC RESULTS
I s o t o p i c m e a s u r e m e n t s were p e r f o r m e d o n a c o m b i n e d gas c h r o m a t o g r a p h y - i s o t o p e ratio m a s s spect r o m e t r y system, c o n s i s t i n g o f a V G i s o c h r o m II s y s t e m i n t e r f a c e d to a D a n i 8510 gas c h r o m a t o g r a p h ( B j o r o y et al., 1990). T h e G C was fitted with a fused silica OV-1 c o l u m n ( 2 5 m x 0 . 2 2 m m i.d.) leading directly into t h e c o m b u s t i o n furnace. H e l i u m (0.8 bar) was used as the carrier gas a n d injections were m a d e in split m o d e . T h e G C was p r o g r a m m e d f r o m - 10 to 300°C at 4 ° C / m i n a n d held i s o t h e r m a l l y at 300°C for 20 min. R e s u l t s are r e p o r t e d in the usual " d e l " n o t a t i o n relative to P D B . All s a m p l e s were r u n at least twice a n d generally t h r e e times. Replicate m e a s u r e m e n t s generally p r o d u c e d differences b e t w e e n 0.1 a n d 0.39/00 with a m e a n o f 0.2039/00.
T h e c o m p o u n d specific i s o t o p e d a t a are p r e s e n t e d in Tables 2 a n d 3 a n d are s u m m a r i z e d in Fig. 4. In general, there is a r e g u l a r p r o g r e s s i o n o f 6 13C for individual c o m p o u n d s , increasing f r o m Buchan---~ F o r t i e s ~ Argyll ---* Bruce. This reflects the m a t u r i t y s e q u e n c e a n d suggests that, like t h e bulk fractions, individual c o m p o u n d s b e c o m e isotopically heavier with increasing m a t u r i t y . T h e r e are h o w e v e r devia t i o n s f r o m t h e m a t u r i t y sequence, for e x a m p l e in p r i s t a n e a n d p h y t a n e a n d in s o m e o f t h e a r o m a t i c h y d r o c a r b o n species. T h e s e m u s t indicate s o m e degree o f source c o n t r o l o n 6 ]3C s u p e r i m p o s e d o n the m a t u r i t y effect. It is i n f o r m a t i v e to e x a m i n e e a c h o f the m a j o r c o m p o u n d g r o u p s in m o r e detail.
other branched
-24
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:" -26 0
n-alkanes
t
" ,'V* "/
-28
~-32
aromatics
:i¢
cyclics
;
:.;.t .i. ;
"~, :'
;'".
lsoprenoids
-34 -36
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,
,
,
,
l
l
l
l
l
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l
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, l
. l
. l
, l
. l
l l
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l
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,
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l
l
l
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l
l
t l
l l
l l
, l
t l
l l
l l
l l
l l
l l
l l
l l
l l
l l
Fig. 4. Overview of isotopic data for the North Sea oils. Notice that trends are consistent between samples, and most compounds become systematically heavier for the higher maturity oils.
l l
l l
l
C . J.
742
and M. BJoaov
CLAYTON
-26 -27
."*",,.
.
-28
•
""e..
...,..-*--X--,--,,-'~'"P~-.,
"'0 °'4"''9"'-
-29
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_
.-~
-
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, . . . , ~ . , . . , e . . ,. " ~ "
._~-31
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--.e~
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- - ' - * - - - " Argyll
-34
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l .
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4----
Bruce
, .
I I I I
n-~k~e~nnum~
Fig. 5. Isotopic data for normal alkanes measured on whole oil. minor variation in the C 2 5 - C 3 0 range where there may be some additional interference from co-elution with other compounds. In contrast, Fig. 6 shows how measurements made on the whole oils compare with those made on the separated saturated hydrocarbon fraction. For all three oils the saturated hydrocarbon fractions are isotopically more homogeneous, and lighter, than the whole oils, but converge towards them at low molecular weights (approx. C~0-C]2). This suggests that the trend to increasing 6 ~3C with increasing molecular weight shown on Fig. 5 is an analytical problem, presumably related to the poor correction for the unresolved complex mixture that co-elutes with the alkanes (Bjoro~j et al., 1990). We note with interest, however, that the "spurious" trends in the whole oil
Normal alkanes Isotopic ratios for the normal alkanes were measured on whole oils for all samples and on the separated saturated hydrocarbon fraction for Buchan and the two Wytch Farm oils. The saturated hydrocarbon measurements were made to determine the effects of the unresolved material that co-elutes with the n-alkanes from the GC column. The whole oils, in contrast, produce less reliable measurements but enable us to determine 6 ~3C on the low molecular weight compounds also. Results of the measurements on North Sea whole oils are shown in Fig. 5. Two main trends are apparent: an increase in 6 ~3C with increasing molecular weight; and an increase in 6 13C of each component with increasing maturity, excluding some
-28 -29 -30 ..
•
~-31
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._~-32
-34
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. . . . e - - - - Sherwood . . . . D - - - - Brk:lport
saturates
|
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l
,
l
,
t W
, ,
| ,
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saturates
, ,
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Fig. 6. Comparison of n-alkane data measured on whole oil (solid lines) and separated saturated hydrocarbon fractions (broken lines).
743
Effect of maturity on North Sea oils are systematic with respect to both molecular weight and maturity of the oil to a remarkable degree, and are also in qualitative agreement with trends found for distillation cuts of the same samples (Clayton, 1991). For any individual compound, the relative enrichment in t$13C between the oils (i.e. Bridport> Sherwood > Buchan) is observable in both the whole oil and the saturated hydrocarbon measurements. Hence we believe the trend of isotopically light, low maturity, Buchan oil to isotopically heavy, high maturity Bruce oil to be a true maturity effect. Similarly, the trends in the very low molecular weight fraction (C6-C~2) are probably real, since here any errors due to background subtraction are minimal. Similar trends, both in the overall maturity shift and in the isotopic depletion of the low molecular weight components, have been reported for separate distillation cuts of oils by Northam (1985) and Clayton (1991).
Isoprenoid hydrocarbons Isotopic data for these are shown in Fig. 7, based on measurements from the whole oil. The i-Ct4 to i-C16 data for Argyll and Bruce appear to be a little erratic, possibly due to measurement problems (see above for n-alkanes). However, in general there appears to be a systematic trend in ¢5~3C from Buchan ---, Forties ~ Bruce ~ Argyll. This is not the maturity trend described above, suggesting that the isotope ratios are controlled in part by source facies variations or mixing of two or more sources. Nevertheless the pattern is not just a source effect; the trend does not match that of Pr/Ph or CPI either, suggesting that both maturity and source variations are important. In terms of absolute isotope ratios, the isoprenoid hydrocarbons are relatively homogeneous, although
there is a tendency for pristane, and to some extent phytane to be isotopically a little lighter. This is also apparent for the Wytch Farm oils (Fig. 8) and may indicate that at least some of the pristane and phytane is derived from a different source to the other isoprenoid hydrocarbons. Generally, the isoprenoid hydrocarbons have isotope ratios very close to or slightly heavier than that of the corresponding carbon number n-alkane. This is consistent with a common biological source of both n-alkanes and isoprenoids, and differs from the results of Hayes et al. (1989) from the Greenhorn Formation where separate sources appear to have contributed. In all samples, phytane is a little heavier than pristane. Both pristane and phytane are thought to be derived dominantly from the phytol side chain of chlorophyll; phytane by hydrogenation of the terminal carbon atom, and pristane by decarboxylation and resulting loss of the terminal carbon atom. The 0.5-1%o isotopic difference between pristane and phytane can be explained if the terminal carbon atoms in phytol were enriched in ~3C by between 10 and 20?/00 relative to the rest of the molecule. This would require intramolecular variations in t513C of phytol analogous to those proposed for a number of lipids (e.g. Vogler and Hayes, 1979; Monson and Hayes, 1980, 1982).
Branched/cyclic alkanes 6 13C measurements for the branched and cyclic fractions were determined from the whole oil and are summarized in Fig. 9. The branched fraction consists of isomers of pentane and hexane, three uncharacterized branched compounds with carbon numbers of 8, 9 and 11, and 2,6-dimethylundecane, an isoprenoidlike structure lacking the side branch on carbon atom 10. The cyclic compounds are represented by isomers of cyclopentane and cyclohexane.
-23 -24 -25
• -
.::'_'2"" -31 -32
I
I
I
I
I
I
-
•
--~--.
I
~. Fig. 7. Isotopic ratios of isoprenoid hydrocarbons in North Sea oils.
Buchsn - -
Forties
Am~l
C. J.
744
and M. BJoRov
CLAYTON
-23 -24 -25
Sherwood
-28
-28
[~ ,,,,
- - o- -
i . .,°
Bridport
.3O ,31 ,32 `33
I
I
I
tO
tO
I
I
I
I
¢0
Q)
G
"g.
f~
Fig. 8. Isotopic ratios of isoprenoid hydrocarbons in Wytch Farm oils.
There is a general trend in 6 13C from low maturity to high maturity for most components, although variations do occur, notable in the branched Cs, CH and C13 compounds. It is not clear to what extent this is an analytical artefact reflecting the small size of the peaks. However, co-elution with other compounds is unlikely to be the cause at such low molecular weights. The cyclic compounds reflect a more consistent trend of increasing 6 ~3C with increasing maturity. Source related variations appear to be minor in these compounds, with a possible exception in methylcyclohexane. Within individual oils, the branched compounds define a general trend towards increasing ~ 13C with increasing molecular weight, although 2-methylhexane
-24
is a clear anomaly here. This anomaly is also present in the Wytch Farm oils (Fig. 10) suggesting that this may be a general feature of such oils. Why this compound should be so different is unclear, although it must represent a different biological precursor. Interestingly, there is also quite a spread in 6 )3C within each oil for the four cyclic compounds measured, even though they are structurally very similar. For example, methylcyclopentane is almost 2%0 heavier than ethylcyclopentane, and methylcyclohexane is consistently about I%o heavier than cyclohexane. This suggests that, even for such structurally similar compounds, we are seeing differences in biological source for each compound, preserved through the maturity variations.
#
•"
K
d /" '\", ..'
I
.4k
/
/
....e..'~
l\'v-;
•
..
"',~'e.
,,"
--.e--
", "':--:".,"
. . . . e - - - - Bruce
-31 `32
i
I
i
I
Forties
- - ~ - - - Argyll
-30
`33
Buchan
i
I
I
i
i
I
I
I
Fig. 9. Isotopic ratios of branched and cyclic components in North Sea oils.
745
Effect of maturity on North Sea oils -23 -24 -25 -25
! i
-27
I .---.o
//X',
-28 -29
--o--
Sherwood
I~'ldport
-30 -31 -32 -33
I
I
I
I
I
I
I
I
I
I
I
I
N
Fig. 10. Isotopic ratios of branched and cyclic components in Wytch Farm oils.
Aromatic hydrocarbons
Within the monoaromatics, 6 '3C of toluene (where measured) is consistently heavier than benzene or xylene, suggesting a different organic precursor. Similarly, the methylnaphthalenes are consistently isotopically heavier than the dimethylnaphthalenes. Phenanthrene and the methylphenanthrenes all have essentially the same 6 ]3C however, consistent with a common precursor, strengthening the idea that variations in the ratio of isomers of methylphenanthrene are the result of maturity rather than source (Radke, 1987).
The isotopic data for the aromatic hydrocarbons are summarized in Fig. 11. For the lowest maturity oil, Buchan, the values are fairly homogeneous with a spread of about 3%o. With increasing maturity there is a tendency for the diaromatic compounds to become isotopically heavier than the triaromatic compounds, a feature also seen in the Wytch Farm oils (Fig. 12). Within the monoaromatic compounds there is a trend towards heavier isotopic values with increasing maturity, although the relative increase in values do not match the relative increase in maturity (i.e. the isotopic difference between Forties and Argyll and Argyll and Bruce is out of proportion to the maturity difference between these oils). For the diaromatic and triaromatic hydrocarbons the sequence also differs from a simple maturity trend. This suggests that ~ ]3C of these compounds is influenced by facies variations as well as maturity.
MATURITY VS SOURCE EFFECTS
Differentiating source influence from maturity In this paper we have considered the major components of four oils in which both maturity and lateral source variations influence ~ ~3C of individual compounds. Whether the source effects are due to
-23 -24 -25
-25
~
---o--
-27 I"-N!
/Q%
/ e' -32
~'"0
~
x
x
Xg/
/
•
Buchan
- - .-t - .. Forties .... t----
Argyll
.... e----
Bruce
s
Fig. 11. Isotopic ratios of aromatic hydrocarbon components in North Sea oils.
746
C. J. CLAYTONand M. BJOROY -23 -24 -25 .o
== -26 -27
Sherwood
-28 --
i
-29
-
o
-
--
Bridport
-30 -31 -32 -33
Fig. 12. Isotopic ratios of aromatic hydrocarbon components in Wytch Farm oils. lateral variation within the Kimmeridge Clay or due to differing contributions from another source cannot be demonstrated. However, it seems likely that the latter is the case, with the second source represented by the terrestrial kerogen-rich Heather Formation (Chung et al., 1992). The proportion of the isotopic variation which results from maturity can be approximated by using Pr/Ph as a source indicator (cf. Chung et al., 1992) and the aromatic molecular ratios to determine maturity. Multiple regression of 6 13C for each compound, using these parameters as the independent variables will give an approximate estimate of the relative importance of source versus maturity in dictating ~3C: t5 ~3Ccom~ond x = a + b • (Pr/Ph) + c • maturity ratio then: c/([b ] + c). 100 is the percentage of the variation which is attributed to maturity. In practice, the magnitude of the coefficients b and c are dependant on the magnitude of the Pr/Ph and maturity ratios, and are hence essentially arbitrary. We have therefore renormalized these ratios to vary from 0 to 1 over the range represented by the samples analysed for this study. The resulting parameters thus relate to the relative source and maturity influence over the ranges represented by our sample set. It is important to note, however, that this method is only an approximation since we have data for only four samples. We regard this approach as a convenient method with which to compare source and maturity effects between compounds rather than as a rigorous statistical treatment which must await more data. In addition, there are a number of further assumptions inherent in its use: (1) The pristane/phytane ratio is dictated only by source variations and is not significantly affected by maturity. This is probably a reasonable assumption since our samples are not substantially affected by oil to gas cracking.
(2) The molecular maturity parameters are independent of source. This again is probably reasonable since both source end-members are dominated by marine organic matter, even though one has a contribution from terrestrial organic matter. Also, we have used only molecular ratios, avoiding more sensitive bulk parameters such as the saturated hydrocarbon content. (3) 6 13C is related linearly to source input, as revealed by Pr/Ph, and to maturity as reflected in the molecular maturity ratios. This is probably not strictly the case and certainly cannot be demonstrated from these data. However, assuming linear relationships will at least give us an approximation of the relative importance of source and maturity. Non linear effects will alter the magnitude of the resulting coefficients but not the relative values between compounds. The results of this exercise are given in Table 4 and plotted in Fig. 13. Data for n-alkanes are not available since we analysed only one North Sea saturated hydrocarbon fraction. In defining maturity we have used all the available molecular maturity parameters, excluding the biphenyl ratio which appears to conflict with all the others. In this way we can also gain some idea of the uncertainty involved, shown in Fig. 13 by the error bars. In general, the use of the dimethylnaphthalene ratio resulted in significantly different results to the other parameters and so must be treated with caution. The correlation coefficient R : in Table 4 indicates the percentage of the variability explained by this simple model. The isoprenoid alkanes appear to be controlled more by maturity (53-64%) than by facies variations although there is considerable uncertainty on the true value depending on which maturity parameter is used. If the dubious values for D M N R are ignored, the range reduces considerably, and the mean value would be higher. Regardless of this, however, the
Effect of maturity on North Sea oils data from using each maturity parameter in isolation show a systematic increase in the importance of maturity with increasing molecular weight. In other words, the low molecular weight compounds are
747
more prone to source variations than are the higher molecular weight components. The branched and q, cli¢ components are considerably more variable, with between 35 and 90% of the
Table 4. Results of regression analysis to determine proportion of variation with results from maturity Compound
Mat ratio
a
b
c
R2 ( % ) ) b l / c
% mat.
Mean
Isoprenoid Alkanes iCI6
iCl8
Pristane
Phytane
MNR DMNR MPI-I MPI-2
-28.22 -28.24 -28.20 -28.17
-5.49 -9.07 -4.87 -5.04
6.09 9.77 6.04 5.59
80.6 94.6 98.3 74.3
0.90 0.93 0.81 0.90
53 52 55 53
MNR DMNR MPI-1 MPI-2
-28.68 -28.73 -28.71 -28.64
-3.34 -6.16 -3.05 -2.96
4.45 7.35 4.60 4.03
72.6 89.0 94.5 65.9
0.75 0.84 0.66 0.73
57 54 60 58
MNR DMNR MPI-1 MPI-2
- 30.25 -30.24 -30.21 -30.23
- 2.42 3.61 -4.41 5.66 - 1.96 3.46 - 2 . 1 8 3.34
89.1 98.3 99.9 84.4
0.67 0.78 0.56 0.65
60 56 64 61
MNR DMNR MPI-1 MPI-2
-30.11 -30.16 -30.15 -30.07
-2.26 -4,83 -2.01 -I.91
4.03 6.68 4.19 3.65
75.7 90.0 94.8 69.6
0.56 0.72 0.48 0.52
64 58 68 66
MNR DMNR MPI-I MPI-2
-31.74 -31.68 -31.63 - 31.74
-0.01 - 1.49 0.65 0.14
3.25 4.76 2.83 3.08
99.8 99.3 97.9 99.3
0.00 0.31 0.23 0.05
100 76 81 96
MNR DMNR MPI-I MPI-2
-31.38 -31.37 -31.36 -31.37
3.66 2.82 3.85 3.76
1.50 2.37 1.45 1.39
99.5 99.7 99.9 99.2
2.43 1.19 2.65 2.70
29 46 27 27
MNR DMNR MPI-I MPI-2
-31.18 -31.22 -31.20 -31.16
-0.33 -2.02 -0.17 -0.11
2.65 4.39 2.75 2.40
86.0 94.3 97.1 82.7
0.13 0.46 0.06 0.04
89 68 94 96
MNR DMNR MPI-I MPI-2
-30.48 -30.53 -30.54 -30.45
2.09 1.03 1.99 2.25
1.31 2.40 1.57 1.13
91.9 94.9 96.2 90.6
1.60 0.43 1.27 1.99
39 70 44 33
MNR DMNR MPI-I MPI-2
-29.34 -29.42 -29.41 -29.29
-2.16 -4.84 -2.03 - 1.78
4.01 6.81 4.30 3.60
70.0 85.7 91.5 74.5
0.54 0.71 0.47 0.50
65 58 68 67
MNR DMNR MPI-I MPI-2
-29.05 -29.07 -29.03 -29.02
-3.64 -6.51 -3.13 -3.28
4.91 7.86 4.85 4.51
82.8 95.4 98.7 77.0
0.74 0.83 0.64 0.73
57 55 61 58
MNR DMNR MPI-I MPI-2
-29.37 -29.42 -29.43 -29.34
0.77 -0.53 0.74 0.96
1.77 3.11 2.00 1.56
85.6 92.0 94.5 83.4
0.44 0.17 0.37 0.61
70 85 73 62
MNR DMNR MPI-1 MPI-2
-31.33 -31.40 -31.40 -31.29
0.91 2.66 - 0 . 9 9 4.62 0.91 2.95 1.18 2.36
86.0 92.5 95.1 83.6
0.34 0.21 0.31 0.50
75 82 76 67
MNR DMNR MPI-1 MPI-2
-31.53 -31.52 -31.50 -31.52
0.58 -0.74 0.90 0.74
2.40 3.76 2.30 2.23
97.0 99.6 100.0 95.6
0.24 0.20 0.39 0.33
81 84 72 75
MNR DMNR MPI-I MPI-2
-30.46 - 30.43 -30.42 -30.44
2.38 1.67 2.61 2.47
1.40 2.13 1.29 1.31
99.7 100.0 100.0 99.4
1.71 0.78 2.03 1.89
37 56 33 35
53
57
60
64
Branched AIkanes 2-Methylpentane
3-Methylpentane
Br C8
Br C9
Br C I I
2,6 dimethyl undecane
88
32
87
46
65
58
Cyclic Alkanes Me-cyclopentane
Eth-cyclopentane
Cyclohexane
Me-cyclohexane
72
75
78
40
---continued overleaf 0 0 2t-6/7--M
748
C . J . CLAYTON and M. BJOgOY Table
Compound
Mat ratio
Benzene
Xylene
1MN
1,6 DMN
3 MeP
9 MeP
4--continued
a
b
c
R2 (%) Ibl/c
% mat. Mean
Aromatic Hydrocarbons MNR -31.05 2.75 1 . 3 4 DMNR -31.07 1.87 2.25 MPI-I -31.07 2.82 1.42 MPI-2 -31.04 2.87 1.21
97.8 99.0 99.5 97.4
2.05 0.83 1.99 2.37
33 55 33 30
MNR DMNR MPI-I MPI-2
- 30.79 - 30.03 -31.10 -30.69
3.66 2.36 0.89 5.24 2.81 3.63 4.14 1.84
73.8 79.5 82.7 72.2
1.55 0.17 0.77 2.25
39 86 56 31
MNR DMNR MPI-I MP[-2
-29.60 -29.56 -29.52 -29.61
1.52 0.77 1.96 1.58
1.84 99.9 2.61 99.0 1.52 98.2 1.76 100.0
0.83 0.29 1.29 0.90
55 77 44 53
MNR DMNR MPI-I MPI-2
-31.76 -31.64 -31.75
0.23 -1.79 1.01 0.45
4.19 6.26 3.75 3.95
99.4 99.8 99.1 98.5
0.05 0.29 0.27 0.11
95 78 79 90
MNR DMNR MPI-1 MPI-2
-30.51 -30.42 -30.37 -30.53
0.43 -0.29 1.13 0.47
2.25 2.03 1.67 2.21
96.7 92.0 89.4 97.9
0.19 0.14 0.68 0.21
84 88 60 83
MNR DMNR MPI-I MPI-2
-30.13 -30.05 -30.00 - 30.15
0.45 -0.09 1.07 0.47
1.85 2.40 1.33 1.83
95.4 90.2 87.5 96.8
0.24 0.04 0.80 0.26
80 96 55 80
-31.70
38
53
57
85
78
78
13Cc0mp0un
d ---- a + b ' (Pr/Ph) + c • (maturity). "mat. ratio" refers to the maturity ratio used for the regression, and "% mat." is the percentage of the isotopic variation which is attributed to maturity (% mat ~ c/(Ib I + c).
isotopic variability controlled by maturity. Again, the u s e o f a n y i n d i v i d u a l m a t u r i t y p a r a m e t e r r e s u l t s in t h e s a m e s e q u e n c e , albeit w i t h slightly d i f f e r e n t a b s o l u t e v a l u e s . E x c e p t for m e t h y l c y c l o h e x a n e , c5 '3C o f t h e cyclic a l k a n e s is d o m i n a t e d b y m a t u r i t y v a r i a t i o n s . T h e l a t t e r c o m p o u n d is a p p a r e n t l y i n f l u e n c e d as m u c h b y s o u r c e facies as it is by m a t u r i t y . O f p a r t i c u l a r i n t e r e s t in t h e b r a n c h e d c o m p o n e n t s is t h e 3 - m e t h y l p e n t a n e , w h i c h a p p e a r s to s h o w a
s i g n i f i c a n t l y g r e a t e r r e s p o n s e to facies v a r i a t i o n t h a n does 2-methylpentane. Combined with the absolute difference in t5 '3C o f t h e 2 - m e t h y l h e x a n e c o m p a r e d w i t h t h e s e c o m p o u n d s (see a b o v e ) , t h i s s u g g e s t s t h a t all o f t h e s e l o w m o l e c u l a r w e i g h t b r a n c h e d a l k a n e s have different biological precursors. Aromatic compounds show a systematic trend towards greater maturity dominance with increasing m o l e c u l a r w e i g h t . T h e effect h e r e is m o r e d r a m a t i c
100 90 80
'-1 70 E .9o 60 G)
e.O "r(U
50 40 30
20 10 0 -
-
o
ii &
~
-
|
~-
IlaJ l
~
~
z
°
z
o.
o.
w
Fig. 13. Relative proportion (%) of isotopic shift caused by maturity rather than source facies. Error bars represent variation determined using different molecular maturity parameters.
Effect of maturity on North Sea oils however, suggesting that the more terrestrially dominated source contributes proportionately more of the light aromatic hydrocarbon compounds. As observed above, the low molecular weight aromatic hydrocarbons are also isotopically the heaviest, consistent with a greater contribution from the Heather Formation. DISCUSSION The regression coefficients b in Table 4 reflect how the isotopic ratios vary in response to changes in the pristane/phytane ratio. The absolute magnitude of the coefficients is unlikely to be reliable since the regressions are based on only four samples which results in only one degree of freedom with which to constrain the curves. However, qualitatively, these give us an idea of the relative response of different compounds to source and facies variations. The cyclic, aromatic and some branched compounds have positive or near zero coefficients, indicating that higher Pr/Ph, and hence a greater terrestrial influence in the source, causes isotopically heavier values. This is consistent with mixing of oils from the high Pr/Ph and isotopically heavy Heather Formation, with low Pr/Ph isotopically lighter Kimmeridge Clay. The decrease in magnitude of this effect with increasing molecular weight in the aromatic hydrocarbon fraction suggests that the Heather has most influence in the low molecular weight compounds. In contrast, the isoprenoid hydrocarbons, 2,6dimethylundecane and the Ca and Ell branched compounds show a negative source coefficient. This is surprising, and indicates that these hydrocarbons in the Heather source are isotopically lighter than those in the Kimmeridge Clay, even though the bulk kerogen is isotopically heavier (Chung et al., 1992). This possibly explains the observation of Bjor~y et al. (1991) that, in the Viking Graben, isoprenoid hydrocarbons are isotopically lighter than the normal alkanes, while in the Central Graben they are isotopically heavier, regardless of maturity. It also implies that in the "coaly" Heather formation, the isoprenoid hydrocarbons are not derived from the same biological precursors as are the n-alkanes, aromatic and cyclic compounds. A similar situation was found by Hayes et al. (1989) for the Greenhorn Formation, suggesting that their observed differences between 6 ~3C behaviour of porphyrins and of bulk organic matter may reflect mixing of marine and terrestrial organic sources, rather than )3C enrichment during bacterial reworking as they suggested. The regression coefficient c in Table 4 reflects the sensitivity of the carbon isotope ratios to increasing maturity. All values are positive, indicating that values become heavier with increasing maturity, consistent with what is known of bulk and boiling point fractions (Northam, 1985; Clayton, 1991). However, there is considerable variation in their relative sensitivity to maturity and apparently no systematic vari-
749
ations between molecular weights or compound types except within the isoprenoid hydrocarbons. Indeed, the trend in the isoprenoid hydrocarbons themselves may be an artefact of the regression owing to the small data set, since large negative "source" coefficients must be balanced by large positive "maturity" effects. Thus we are not seeing simple kinetic isotope fractionation associated with progressive thermal destruction of individual compounds. As higher molecular weight compounds are broken down to smaller products they bias the isotopic ratio of the products away from simple trends. The net effect is to scramble, and homogenize, the isotopic ratios inherited from the source material. SUMMARY
(1) Maturity variations are distinguishable from facies differences by combining compound specific isotopic ratios with conventional molecular ratios. (2) The main cause of variation in 6 ~3C of the limited number of oils studied here is maturity. This typically accounts for between 50 and 90% of the observed variation, and produces an increase in ~ ~3C generally between 2 and 3%o. (3) Superimposed on the maturity variations there is a source effect, best interpreted as a variable contribution of oil from the "coaly" Heather Formation to the bulk of the oil derived from the Kimmeridge Clay. Within both the isoprenoid hydrocarbons and the aromatic hydrocarbon compounds this contribution is most pronounced in the lower molecular weight components. (4) For the majority of compounds, the Heather contributes isotopically heavier or similar material. However, isoprenoid hydrocarbons and some other branched compounds are isotopically lighter than those derived from the Kimmeridge Clay. (5) The maturity dependence of ~ 13C in individual compounds is highly variable, suggesting that there is considerable homogenisation of 6 ~3C during maturation. As complex molecules break down to progressively simpler compounds they mix with the products from other cracking reactions to progressively obliterate source-derived isotopic signatures. Acknowledgements--We thank the staff of Geolab Nor and
particularly Rita Moe for their efforts in producing the data, Arnd Wilhelms for a critical review which improved the earlier text and Martin Schoell who invented the title. Financial support was provided by BP Norway and this paper is published with the kind permission of BP Exploration. REFERENCES
Bjorvy M., Hall K. and Jumeau J. (1990) Stable carbon isotope ratio analysis on single components in crude oils by direct GC-isotope analysis. Trends Anal. Chem. 9, 331-337. Bjor~y M., Hall K., Gillyon P. and Jumeau J. (1991) Carbon isotope variations in n-alkanes and isoprenoids of whole oils. Chem. Geol. 93, 13-20.
750
C. J. CLAYTON and M. BJo~v
Bjoray M., Hall P. B., Hustad E. and Williams J. A. (1992) Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity. In Advances in Organic Geochemistry 1991 (Edited by Eckardt C. B. et al.). Org. Geochem. 19, 89-105. Pergamon Press, Oxford. Chung H. M., Wingert W. S. and Claypool G. E. (1992) Geochemistry of oils in the Northern Viking Graben. In Giant Oil and Gas Fields of the Decade: 1978-1988 (Edited by Halbouty M.). AAPG Memoir 54. Clayton C. J. (1991) Effect of maturity on carbon isotope ratios of oils and condensates. Org. Geochem. 17, 887-899. Collister J. W., Summons R. E., Lichtfouse E. and Hayes J. M. (1992) An isotopic study of the Green River oil shale. In Advances in Organic Geochemistry 1991 (Edited by Eckardt C. B. et al.). Org. Geochem. 19, 265-276. Pergamon Press, Oxford. Freeman K. H., Hayes J. M., Trendel J.-M. and Albrecht P. (1990) Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons. Nature 343, 254-256. Galimov E. M. (1985) The Biological Fractionation of Isotopes. Academic Press, Orlando, Fla. Hall P. B., Schou L. and Bjoray M. (1985) Aromatic hydrocarbon variations in North Sea wells. In Petroleum Geochemistry of the Norwegian Shelf(Edited by Thomas B. M. et al.), pp. 293-301. Graham and Trotman, London. Hayes J. M., Popp B. N., Takigiku R. and Johnson M. W. (1989) An isotopic study of biogeochemical relationships between carbonates and organic carbon in the
Greenhorn Formation. Geochim. Cosmochim. Acta 53, 2961-2972. Hayes J. M., Takiglku R., Ocampo R., Callot H. J. and Albrecht P. (1987) Isotopic compositions and probable origins of organic molecules in the Eocene Messel shale. Nature 329, 48-51. Monson K. D. and Hayes J. M. (1980) Biosyntheticcontrol of the natural abundance of carbon 13 at specificpositionswithin fattyacids in Escherichiacoll.J. Biol. Chem. 255, 11435-I 1441. Monson K. D. and Hayes J. M. (1982) Carbon isotopic fractionation in the biosynthesis of bacterial fatty acids. Ozonolysis of unsaturated fatty acids as a means of determining the intramolecular distribution of carbon isotopes. Geochim. Cosmochim. Acta 46, 139-149. Northam M. A. (1985) Correlation of North Sea oils:the different fades of their Jurassic source. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (Edited by Thomas B. M. et al.), pp. 93-99. Graham & Trotman, London. Quigley T. M., Mackenzie A. S. and Gray J. R. (1987) Kinetic theory of petroleum generation. In Migration of Hydrocarbons in Sedimentary Basins (Edited by Doligez B.), pp. 649-665. Editions Technip, Paris. Radke M. (1987) Organic geochemistry of aromatic hydrocarbons. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), Vol. 2, pp. 141-208. Academic Press, London. Vogler E. A. and Hayes J. M. (1979) Carbon isotope compositions of carboxyl groups of biosynthesised fatty acids. In Advances in Organic Geochemistry 1979 (Edited by Douglas A. G. and Maxwell J. R.), pp. 697-704. Pergamon Press, Oxford.
01,~-~S003)E0014-D
Org. Geochem. Vol. 21, No. 6/7, pp. 737-750, 1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0146-6380/94 $7.00 + 0.00
Effect of maturity on I3C/nC ratios of individual compounds in North Sea oils C. J. CLAYTON1. and M. BJOROY2 IBP Research Centre, Chertsey Road, Sunbury-on-Thames TWI6 7LN, England and 2Geolab Nor A/S, Homebergveien 5, 7038 Trondbeim, Norway Abstract--Carbon isotope variations in individual compounds are reported for North Sea oils of variable thermal maturity. Using the pristane/phytane ratio of the oils as an indication of source facies variation, and molecular maturity ratios to determine maturity, it is possible to differentiate the two effects. In these samples, maturity accounts for between 50 and 90% of the observed isotopic variation, all values becoming heavier with increasing maturity, typically by between 2 and 3%o. However, source differences are also apparent with a terrestrially influenced source, possibly the Heather Formation, contributing material to the Kimmeridge Clay sourced oils. The "Heather" contribution is isotopically heavier than the Kimmeridge oil for most compounds, but is isotopicaUylighter for the isoprenoid hydrocarbons and some other branched compounds. It is also more important for lower molecular weight hydrocarbons than for higher homologues. Key words---carbon isotopes, Compound Specific Isotope Analysis (CSIA), source influence, maturity effects, North Sea, Kimmeridge Clay Formation, Heather Formation
INTRODUCTION
In order to use stable carbon isotope ratios for correlation of oils it is necessary to allow for the effects of thermal maturity, and to differentiate these from facies effects which may betray a different source. The effects of maturity on 6 13C of whole oils and bulk fractions (i.e. saturated hydrocarbons, aromatic hydrocarbons, NSO and asphaltene fractions) are now reasonably well established (e.g. Clayton, 1991). Oil generation results in a small (approx. 1%o) increase in 6 13C, possibly caused by mixing of generated oil with the initial immature soluble extract. Subsequent oil to gas cracking results in a more substantial increase in 6 ~3C, typically about 3.5%o caused by kinetic isotopic fractionation as t2C passes preferentially into gas leaving residual oil enriched in "C. At the compound-specific level, maturity effects are less well understood. Soluble extracts from immature samples reveal a wide variation in 6 13C between different biomarkers, reflecting the diversity of biochemical processes that contributed organic matter to the sediment (see for example Hayes et al., 1987; Freeman et al., 1990 and discussions in Galimov, 1985). However this variation is generally absent in oils, which appear to be more homogeneous (e.g. Bjoroy et al., 1990; Collister et al., 1992). Beyond that however, things are less clear. Bjor~y et al. (1992)
*Present address: School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, England.
present data for laboratory pyrolysis products which suggest that 613C of individual compounds increases with increasing maturity. In addition, Northam (1985) shows similar maturity induced increases in '3C of boiling point fractions of oils from the North Sea Beryl Field. Such variations are consistent with the general trend towards heavier isotope values observed in bulk fractions. Here we present data for isotope ratios of the common individual compounds in four oils that define a maturity trend through the bulk of the oil generation window, and in which it is possible to differentiate these from changes which result from lateral source variations. SAMPLES STUDIED For this study, four oils from the United Kingdom Continental Shelf were chosen which define a maturity sequence: Buchan, Forties, Argyll and a Bruce oil. The locations of these fields are shown in Fig. 1. All the fields lie to the west of the main graben structures (and hence source kitchens). Argyll in the south lies on the flank of the Central Graben, Buchan and Forties lie close together further north, at the N.W. end of the Central Graben and the southern end of the Viking Graben, and Bruce lies near the northern end of the South Viking Graben. These fields were chosen so that any source facies variations could be distinguished from the maturity effects, since the geographical trend of the samples, which may cause systematic source variations, does not match the maturity trend (see below).
737
738
C.J. CLAYTONand M. BJOROY
Fig. 1. Location of oil fields sampled. Numbers in circles refer to relative maturity (1 = lowest), dashed lines correspond to international boundaries.
In addition, we include data for oils from the Jurassic Bridport Sands and Triassic S h e r w o o d Sandstone reservoirs o f the W y t c h F a r m oilfield, onshore U . K . These were sourced by m u d s t o n e s o f the same lithofacies as the K i m m e r i d g e Clay but o f different age (Lower Lias). These provide some degree o f additional control on the isotopic character o f oils f r o m this facies type.
Chemical data for the N o r t h Sea oils are presented in Table 1. The oils increase in maturity from Buchan, the least mature, t h r o u g h Forties, Argyll to Bruce. The m e t h y l p h e n a n t h r e n e indices for the latter samples (approx. 0.77) are equivalent to a vitrinite reflectance o f 0.86%, a little above that for the end o f oil generation in clastic marine source rocks (Quigley et al., 1987). These maturity variations are a consequence o f the m a x i m u m temperature attained in the source kitchen and are reflected in a relative increase in the saturated h y d r o c a r b o n c o m p o n e n t s , a general decrease in sulphur and nitrogen content, and increases in the aromatic and, where available, the saturated molecular maturity parameters (Fig. 2). In terms o f absolute maturity, Buchan is in the early generation stage, Forties is near the end o f generation but with no oil to gas cracking, and Argyll and Bruce are in the very early stages o f cracking to gas. The samples thus span the main part o f the conventional "oil window". Besides the maturity differences, there are geographical variations in the source facies o f the oils. Some source-influenced bulk chemical parameters and molecular ratios do not follow the sequence expected for pure maturity differences. Nitrogen and sulphur content should decrease progressively with maturity but there are minor, but consistent, discrepancies to the simple sequence. Similarly, the pristane/phytane ratio and c a r b o n preference index show deviations from a simple trend (Fig. 3). Higher pristane/phytane ratios are associated with organic matter deposition u n d e r m o r e oxidising conditions,
Table 1. Chemical composition and molecular maturity data for North Sea samples analysed % % % %
Saturated hydrocarbons Aromatic hydrocarbons NSO Asphaltenes
Buchan 54.9 9.1 28.2 7.8
Pristane/phytane CPIt %S ppm N
0.99 1.07 0.82 1330
6 t3Cwhol~oil
-- 29.8
MNR2 DMNR3 BphR4 MPI-I 5 MPI-25 C29 steranes6 Triaromatic steranes7
Forties 55.8 9.4 30.0 4.8
Argyll 57.9 10.3 26.3 5.5
1.43 1.11 0.30 830
1.33 1.03 0.18 899
-- 28.9
-- 28.6
Bruce 61.1 7.4 29.1 2.4 1.84 1.18 0.20 460 -- 28.5
0.78 1.16 0.19 0.62 0.25
1.19 2.19 0.17 0.72 0.80
1.23 2.35 0.28 0.77 0.83
1.26 2.69 0.42 0.76 0.86
0.64 0.18
0.71 0.48
---
0.74 0.59
~Carbon PreferenceIndex--ratio of odd/even Cz4-C33n-alkanes. 22-/l-methyl naphthalene (Radke, 1987). 32,6 + 2,7/1,5 dimethyl naphthalene (Radke, 1987). 4Biphynyl 1,6 dimethylnaphthalene(Hall et al., 1985). 5MPI, Methyl Phenanthrene Index (Radke, 1987); MPI-I = 1.5 × (3 MeP+ 2 MeP)/(P + 9 MeP + 1 MeP); MPI-2= 3 x (2 MeP)/(P+ 9 MeP + 1 MeP); P, phenanthrene; MeP, methylphenanthrene. %tflfl/(ctflfl + ~ctot) 20(R + S)Cz9 sterane ratio. 7C2o/(C20+ C2s) 20R methyl triaromatic sterane ratio. --, Not determined.
Effect of maturity on North Sea oils
739
• Buchan ~
I
[] Forties [] Argyll
MNR
DMNR
BphR
MPI-1
MPI-2
Fig. 2. Aromatic molecular maturity parameters for the North Sea oils.
possibly dysaerobic or intermittently anoxic. Under these conditions, terrestrially derived components make up a larger proportion of the bulk organic matter as these are more resistant to oxidation during early diagenesis. The higher Pr/Ph ratio oils therefore probably represent a more terrestrially influenced source. The variations in terrestrial vs marine contribution to the source rock may reflect either lateral variations within the Kimmeridge Clay or a contribution of oil from another, more terrestrially influenced, source rock. Northam (1985) attributed similar variations in Pr/Ph and carbon isotope "profile" between the Central and Viking Grabens to changes in the Kimmeridge Clay source rock. Using isotope profiles he identified a regular progression from "Ekofisk" type oils in the south to "Statl]ord" type oils in the north. Notice that this differs from the maturity trend
present in the samples used for this study in which the lowest maturity samples (Buchan and Forties) lie between the higher maturity samples (Argyll and Bruce). The biggest step in maturity (between Buchan and Forties) actually occurs in the two samples closest together geographically. In this way it should be possible to differentiate isotope variations that are controlled by source variations rather than just maturity. In contrast, Chung et al. (1992) demonstrate that in the North Viking Graben, the Heather Formation, which occurs immediately beneath the Kimmeridge Clay, is also a viable source of oils with a more terrestrial nature. This source is characterized by lower hydrogen indices and Pr/Ph ratios of 2-3, compared with about 1.5 for the Kimmeridge Clay (named the "Draupne" Formation here: Chung et al., 1992). It also has a carbon isotope ratio of - 2 8 to
1.5
0.5
Buchan
Forties
Argyll
Bruce
Fig. 3. Pristane/phytane ratio and carbon preference index for the North Sea oils.
740
C.J.
CLAYTON and
-26%o compared to about -29%0 for the Kimmeridge. Thus the variations observed by Northam (1985) may be due to a variable contribution from the Heather Formation, rather than lateral variations in the Kimmeridge Clay itself. It is worth noting however, that the range of Pr/Ph ratios and bulk oil carbon isotopes of our samples both fall entirely within the "Draupne" end member as defined by Chung et al. (1992), suggesting that any contribution from the Heather is minor. In summary, therefore, there is evidence that the oils discussed here are influenced by both source variations and maturity. The source variations can be recognized in the Pr/Ph ratio, and the maturity differences are apparent in the molecular maturity ratios. The observed facies variations probably reflect a minor contribution of oil from the more terrestrially influenced Heather Formation in the north. Geographically, the maturity sequence does not match
M. BJogov
that expected for a simple facies variation and we are thus able to differentiate the two. ANALYTICAL
METHODS
Asphaltenes were removed from the oils by precipitation in n-pentane in a plastic column containing a small amount of activated silica. The concentration of asphaltenes was calculated by weight difference of the column before and after elution of the asphaltenefree sample. After removal of asphaltenes, the oil/solvent mixture was rotary evaporated at 35°C and the samples stored in a freezer until ready for use. Further separation into saturated hydrocarbons, aromatic hydrocarbons and NSO fractions was by MPLC using hexane as an eluent following the method of Radke et al. (1980). Sample fractions were evaporated at 35°C and 200 mb to remove the hexane solvent.
Table 2. Compound specific isotope ratios for saturated hydrocarbon components. All values in per mil (Yo.) relative to From whole oil
n -Alkanes 6 7 8 9 l0 ll 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 iCl4 iCl5 iCl6 iCl8
Buchan
Forties
Argyll
Bruce
Sherwood
Bridport
-34.64 -33.92 -33.04 - 32.08 -31.57 -31.47 -31.19 -31.10 -30.59 -30.65 -30.69 -30.52 -30.51 -30.02 -30.16 -30.05 -29.95 -29.66 -29.69 -29.71 -29.72 -28.82 -28.86 --28.95 .
--30.78 -31.00 --30.86 - 30.25 -29.84 -30.04 --29.70 --29.65 -29.07 --29.33 --29.40 -29.15 -29.31 --28.96 --28.74 --28.64 -28.66 -28.49 --28.64 -28.59 -29.02 --27.82 --28.17 --28.28 .
--29.54 -29.89 --29.89 -- 2 9 . 4 7 --29.41 -29.35 --29.46 --29.22 --28.42 --28.86 -28.91 -28.85 -28.79 --28.45 --28.46 --28.36 --28.43 --28.10 -28.54 -28.75 --28.63 --28.47 -29.09 -29.50
-28.44 -28.63 -28.64 - 28.04 -28.28 -28.45 -28.45 -28.19 -27.49 -27.94 -28.35 -28.24 -28.19 -28.31 -28.02 -28.10 -27.90 -27.91 -27.96 -27.87 -27.83 -27.74 -27.84 --28.00 28.51
--31.19 -31.59 -31.68 -- 30.98 --31.22 --31.24 --31.30 -31.26 --30.90 --30.91 -31.16 --31.11 -30.87 --30.71 --30.63 -30.70 --30.52 -30.46 -30.31 -30.16 --30.21 --29.34 --29.56 --29.73 .
--31.10 --31.22 -31.22 - 30.69 -30.70 -30.64 --30.58 -30.40 -30.09 -30.08 -30.06 -29.96 --29.83 -29.68 -29.43 -29.40 -29.35 -29.18 -29.29 -29.26 -29.27 -28.48 -28.63 -28.56 .
.
.
PDB
From saturated hydrocarbon fraction
.
Buchan
Sherwood
Bridport
-----31.62 --31.84 --31.56 -31.59 -31.14 -31.50 -31.30 -31.60 -31.45 -31.54 -31.52 -31.66 -31.73 -31.49 -31.75 -----. .
------30.97 -31.34 -31.39 -31.46 -31.50 -31.59 -31.81 -31.86 -31.81 -31.70 --31.82 -32.01 ---------
--------30.83 --30.74 --30.52 -30.63 --30.63 -30.72 -30.72 --30.63 --30.76 --30.80 -30.71 -30.75 --30.82 -30.69 --30.92 --30.90 --30.86 --30.76 30.99
-28.35 -28.47 -28.09 -28.56 -30.19 -30.00
-27.47 -27.70 -27.03 -27.72 -28.96 -28.87
---24.00 -25.18 -27.51 -26.62
-25.08 -27.46 -27.31 -27.28 -28.92 -28.07
-28.56 -26.39 -29.03 -28.96 -30.35 -29.43
-27.63 --28.51 -28.34 -29.67 -28.75
--30.25 -30.19 -30.46 -32.20 -31.67
--30.00 -31.12 --31.12 -32.28 -31.58
--29.86 -30.64 -30.43 -31.34 -30.86
Br CI 1 2,6 D i m e t h y l u n d e c a n e
- 31.73 - 31.35 -27.03 -31.11 - 30.41 -29.21 -28.95
- 29.07 - 28.42 --29.76 -28.88 -28.23 -27.66
-- 2 8 . 6 4 - 28.36 -25.41 -28.43 -28.05 -25.72 -25.35
- 28.48 - 26.15 -24.04 -28.69 -26.92 -27.17 -27.54
-- 30.27 - 30.27 -26.75 - 30.10 -29.35 -28.28 -27.29
-- 29.99 - 29.27 -25.34 -29.47 -28.62 -27.99 -26.92
------30.34 -30.58
------29.57 -29.84
--------
Me-cyclopentane Et-cyclopentane Cyclohexane Me-cyclohexane
-29.30 -31.23 - 31.49 - 30.43
-28.12 -29.51 - 29.52 - 28.15
-27.01 -27.92 - 28.83 - 28.10
-26.66 -27.52 - 28.45 - 26.63
-28.29 -29.20 - 30.54 - 29.05
-27.90 -28.53 - 30.13 .
----
----
----
Pristane Phytane 2-Methylpentane 3-Methylpentane 2-Methylhexane Br C 8 Br C 9
--,
Not determined (peak too small, absent, or not resolved).
.
.
.
741
Effect of maturity on North Sea oils Table 3. Compound specific isotope ratios for aromatic hydrocarbon components. All values in per rail (%0) relative to PDB Buchan
Forties
Argyll
Bruce
Sherwood
Bridport
Benzene Toluene Xylene
- 31.01 -29.29 -30.55
- 28.84 - 28.47 . . . -29.02 -25.81
- 26.86
- 29.28 28.12 -29.77
- 28.63 -27.45 -28.77
2MN IMN BPH 2,6 + 2,7 DMN 1,6 DMN 2,3 + 1,4 DMN
-29.01 -29.61
-26.74 -27.20 -28.42 -27.60 -28.31 -27.39
-27.60 -27.77
-27.50 -27.76 -28.98 -28.65 -29.20 -28.50
-29.98 -31.73 -30.19
. -24.19
-27.40 -27.30 -28.69 -27.70 -27.58 -27.54
-26.33 -26.26 -28.10 -27.38 -27.27 -26.95
-28.43 -28.79 -28.35
From whole oil
From aromatic hydrocarbon fraction
P --27.89 -28.35 -27.85 -28.56 -28.57 3 MeP -30.55 -28.06 -28.42 -27.92 -28.76 -29.06 2 MeP ---28.75 --28.88 -28.90 9 MeP -30.17 -28.00 -28.41 -27.92 -28.76 -28.88 1 MeP . . . . . 28.84 -29.02 --, Not determined (peak too small, absent, or not resolved). MN, methyl naphthalenes; BPH, byphenyl; DMN, dimethyl naphthalenes; P, phenanthrene; MeP, methylphenantbrenes.
ISOTOPIC RESULTS
I s o t o p i c m e a s u r e m e n t s were p e r f o r m e d o n a c o m b i n e d gas c h r o m a t o g r a p h y - i s o t o p e ratio m a s s spect r o m e t r y system, c o n s i s t i n g o f a V G i s o c h r o m II s y s t e m i n t e r f a c e d to a D a n i 8510 gas c h r o m a t o g r a p h ( B j o r o y et al., 1990). T h e G C was fitted with a fused silica OV-1 c o l u m n ( 2 5 m x 0 . 2 2 m m i.d.) leading directly into t h e c o m b u s t i o n furnace. H e l i u m (0.8 bar) was used as the carrier gas a n d injections were m a d e in split m o d e . T h e G C was p r o g r a m m e d f r o m - 10 to 300°C at 4 ° C / m i n a n d held i s o t h e r m a l l y at 300°C for 20 min. R e s u l t s are r e p o r t e d in the usual " d e l " n o t a t i o n relative to P D B . All s a m p l e s were r u n at least twice a n d generally t h r e e times. Replicate m e a s u r e m e n t s generally p r o d u c e d differences b e t w e e n 0.1 a n d 0.39/00 with a m e a n o f 0.2039/00.
T h e c o m p o u n d specific i s o t o p e d a t a are p r e s e n t e d in Tables 2 a n d 3 a n d are s u m m a r i z e d in Fig. 4. In general, there is a r e g u l a r p r o g r e s s i o n o f 6 13C for individual c o m p o u n d s , increasing f r o m Buchan---~ F o r t i e s ~ Argyll ---* Bruce. This reflects the m a t u r i t y s e q u e n c e a n d suggests that, like t h e bulk fractions, individual c o m p o u n d s b e c o m e isotopically heavier with increasing m a t u r i t y . T h e r e are h o w e v e r devia t i o n s f r o m t h e m a t u r i t y sequence, for e x a m p l e in p r i s t a n e a n d p h y t a n e a n d in s o m e o f t h e a r o m a t i c h y d r o c a r b o n species. T h e s e m u s t indicate s o m e degree o f source c o n t r o l o n 6 ]3C s u p e r i m p o s e d o n the m a t u r i t y effect. It is i n f o r m a t i v e to e x a m i n e e a c h o f the m a j o r c o m p o u n d g r o u p s in m o r e detail.
other branched
-24
~J
:" -26 0
n-alkanes
t
" ,'V* "/
-28
~-32
aromatics
:i¢
cyclics
;
:.;.t .i. ;
"~, :'
;'".
lsoprenoids
-34 -36
I
,
,
,
,
,
l
l
l
l
l
I
I
I
l
~
l
l
l
l
l
, l
l l
l l
l l
l l
l l
l l
l l
l l
. l
' l
* l
, l
. l
. l
, l
. l
l l
l l
l l
l l
l l
l l
l l
l
l
l
l
l
l
l
l
l
l
l
l
,
l
l
l
l
l
l
l
l
l
l
l
l
t l
l l
l l
, l
t l
l l
l l
l l
l l
l l
l l
l l
l l
l l
Fig. 4. Overview of isotopic data for the North Sea oils. Notice that trends are consistent between samples, and most compounds become systematically heavier for the higher maturity oils.
l l
l l
l
C . J.
742
and M. BJoaov
CLAYTON
-26 -27
."*",,.
.
-28
•
""e..
...,..-*--X--,--,,-'~'"P~-.,
"'0 °'4"''9"'-
-29
o" j).. ".~.--&~'
_
.-~
-
~-- -at"" ~"
~..A-
*
/
~
,,,e"
"m""e%.
"~
, . . . , ~ . , . . , e . . ,. " ~ "
._~-31
~
f
-32
=
Buchan
--.e~
Forties
- - ' - * - - - " Argyll
-34
....
| .
-36
, .
l .
, .
, .
, .
l .
t .
l .
l .
t .
, .
t .
, .
| .
, .
l .
|
.
J .
, .
4----
Bruce
, .
I I I I
n-~k~e~nnum~
Fig. 5. Isotopic data for normal alkanes measured on whole oil. minor variation in the C 2 5 - C 3 0 range where there may be some additional interference from co-elution with other compounds. In contrast, Fig. 6 shows how measurements made on the whole oils compare with those made on the separated saturated hydrocarbon fraction. For all three oils the saturated hydrocarbon fractions are isotopically more homogeneous, and lighter, than the whole oils, but converge towards them at low molecular weights (approx. C~0-C]2). This suggests that the trend to increasing 6 ~3C with increasing molecular weight shown on Fig. 5 is an analytical problem, presumably related to the poor correction for the unresolved complex mixture that co-elutes with the alkanes (Bjoro~j et al., 1990). We note with interest, however, that the "spurious" trends in the whole oil
Normal alkanes Isotopic ratios for the normal alkanes were measured on whole oils for all samples and on the separated saturated hydrocarbon fraction for Buchan and the two Wytch Farm oils. The saturated hydrocarbon measurements were made to determine the effects of the unresolved material that co-elutes with the n-alkanes from the GC column. The whole oils, in contrast, produce less reliable measurements but enable us to determine 6 ~3C on the low molecular weight compounds also. Results of the measurements on North Sea whole oils are shown in Fig. 5. Two main trends are apparent: an increase in 6 ~3C with increasing molecular weight; and an increase in 6 13C of each component with increasing maturity, excluding some
-28 -29 -30 ..
•
~-31
""
-
--
°"
- __°
°.,D..~
""
°°
°"
"'O
._~-32
-34
.........
,
,
,
,
m
,
l
,
|
,
t
,
,
,
|
i
t
,
,
Buchan utumte$
,
l
,
l
i
,
,
. . . . e - - - - Sherwood . . . . D - - - - Brk:lport
saturates
|
,
l
~k~e ~n
l
,
l
,
t W
, ,
| ,
, ,
saturates
, ,
, i
, ,
, ,
, ,
num~
Fig. 6. Comparison of n-alkane data measured on whole oil (solid lines) and separated saturated hydrocarbon fractions (broken lines).
743
Effect of maturity on North Sea oils are systematic with respect to both molecular weight and maturity of the oil to a remarkable degree, and are also in qualitative agreement with trends found for distillation cuts of the same samples (Clayton, 1991). For any individual compound, the relative enrichment in t$13C between the oils (i.e. Bridport> Sherwood > Buchan) is observable in both the whole oil and the saturated hydrocarbon measurements. Hence we believe the trend of isotopically light, low maturity, Buchan oil to isotopically heavy, high maturity Bruce oil to be a true maturity effect. Similarly, the trends in the very low molecular weight fraction (C6-C~2) are probably real, since here any errors due to background subtraction are minimal. Similar trends, both in the overall maturity shift and in the isotopic depletion of the low molecular weight components, have been reported for separate distillation cuts of oils by Northam (1985) and Clayton (1991).
Isoprenoid hydrocarbons Isotopic data for these are shown in Fig. 7, based on measurements from the whole oil. The i-Ct4 to i-C16 data for Argyll and Bruce appear to be a little erratic, possibly due to measurement problems (see above for n-alkanes). However, in general there appears to be a systematic trend in ¢5~3C from Buchan ---, Forties ~ Bruce ~ Argyll. This is not the maturity trend described above, suggesting that the isotope ratios are controlled in part by source facies variations or mixing of two or more sources. Nevertheless the pattern is not just a source effect; the trend does not match that of Pr/Ph or CPI either, suggesting that both maturity and source variations are important. In terms of absolute isotope ratios, the isoprenoid hydrocarbons are relatively homogeneous, although
there is a tendency for pristane, and to some extent phytane to be isotopically a little lighter. This is also apparent for the Wytch Farm oils (Fig. 8) and may indicate that at least some of the pristane and phytane is derived from a different source to the other isoprenoid hydrocarbons. Generally, the isoprenoid hydrocarbons have isotope ratios very close to or slightly heavier than that of the corresponding carbon number n-alkane. This is consistent with a common biological source of both n-alkanes and isoprenoids, and differs from the results of Hayes et al. (1989) from the Greenhorn Formation where separate sources appear to have contributed. In all samples, phytane is a little heavier than pristane. Both pristane and phytane are thought to be derived dominantly from the phytol side chain of chlorophyll; phytane by hydrogenation of the terminal carbon atom, and pristane by decarboxylation and resulting loss of the terminal carbon atom. The 0.5-1%o isotopic difference between pristane and phytane can be explained if the terminal carbon atoms in phytol were enriched in ~3C by between 10 and 20?/00 relative to the rest of the molecule. This would require intramolecular variations in t513C of phytol analogous to those proposed for a number of lipids (e.g. Vogler and Hayes, 1979; Monson and Hayes, 1980, 1982).
Branched/cyclic alkanes 6 13C measurements for the branched and cyclic fractions were determined from the whole oil and are summarized in Fig. 9. The branched fraction consists of isomers of pentane and hexane, three uncharacterized branched compounds with carbon numbers of 8, 9 and 11, and 2,6-dimethylundecane, an isoprenoidlike structure lacking the side branch on carbon atom 10. The cyclic compounds are represented by isomers of cyclopentane and cyclohexane.
-23 -24 -25
• -
.::'_'2"" -31 -32
I
I
I
I
I
I
-
•
--~--.
I
~. Fig. 7. Isotopic ratios of isoprenoid hydrocarbons in North Sea oils.
Buchsn - -
Forties
Am~l
C. J.
744
and M. BJoRov
CLAYTON
-23 -24 -25
Sherwood
-28
-28
[~ ,,,,
- - o- -
i . .,°
Bridport
.3O ,31 ,32 `33
I
I
I
tO
tO
I
I
I
I
¢0
Q)
G
"g.
f~
Fig. 8. Isotopic ratios of isoprenoid hydrocarbons in Wytch Farm oils.
There is a general trend in 6 13C from low maturity to high maturity for most components, although variations do occur, notable in the branched Cs, CH and C13 compounds. It is not clear to what extent this is an analytical artefact reflecting the small size of the peaks. However, co-elution with other compounds is unlikely to be the cause at such low molecular weights. The cyclic compounds reflect a more consistent trend of increasing 6 ~3C with increasing maturity. Source related variations appear to be minor in these compounds, with a possible exception in methylcyclohexane. Within individual oils, the branched compounds define a general trend towards increasing ~ 13C with increasing molecular weight, although 2-methylhexane
-24
is a clear anomaly here. This anomaly is also present in the Wytch Farm oils (Fig. 10) suggesting that this may be a general feature of such oils. Why this compound should be so different is unclear, although it must represent a different biological precursor. Interestingly, there is also quite a spread in 6 )3C within each oil for the four cyclic compounds measured, even though they are structurally very similar. For example, methylcyclopentane is almost 2%0 heavier than ethylcyclopentane, and methylcyclohexane is consistently about I%o heavier than cyclohexane. This suggests that, even for such structurally similar compounds, we are seeing differences in biological source for each compound, preserved through the maturity variations.
#
•"
K
d /" '\", ..'
I
.4k
/
/
....e..'~
l\'v-;
•
..
"',~'e.
,,"
--.e--
", "':--:".,"
. . . . e - - - - Bruce
-31 `32
i
I
i
I
Forties
- - ~ - - - Argyll
-30
`33
Buchan
i
I
I
i
i
I
I
I
Fig. 9. Isotopic ratios of branched and cyclic components in North Sea oils.
745
Effect of maturity on North Sea oils -23 -24 -25 -25
! i
-27
I .---.o
//X',
-28 -29
--o--
Sherwood
I~'ldport
-30 -31 -32 -33
I
I
I
I
I
I
I
I
I
I
I
I
N
Fig. 10. Isotopic ratios of branched and cyclic components in Wytch Farm oils.
Aromatic hydrocarbons
Within the monoaromatics, 6 '3C of toluene (where measured) is consistently heavier than benzene or xylene, suggesting a different organic precursor. Similarly, the methylnaphthalenes are consistently isotopically heavier than the dimethylnaphthalenes. Phenanthrene and the methylphenanthrenes all have essentially the same 6 ]3C however, consistent with a common precursor, strengthening the idea that variations in the ratio of isomers of methylphenanthrene are the result of maturity rather than source (Radke, 1987).
The isotopic data for the aromatic hydrocarbons are summarized in Fig. 11. For the lowest maturity oil, Buchan, the values are fairly homogeneous with a spread of about 3%o. With increasing maturity there is a tendency for the diaromatic compounds to become isotopically heavier than the triaromatic compounds, a feature also seen in the Wytch Farm oils (Fig. 12). Within the monoaromatic compounds there is a trend towards heavier isotopic values with increasing maturity, although the relative increase in values do not match the relative increase in maturity (i.e. the isotopic difference between Forties and Argyll and Argyll and Bruce is out of proportion to the maturity difference between these oils). For the diaromatic and triaromatic hydrocarbons the sequence also differs from a simple maturity trend. This suggests that ~ ]3C of these compounds is influenced by facies variations as well as maturity.
MATURITY VS SOURCE EFFECTS
Differentiating source influence from maturity In this paper we have considered the major components of four oils in which both maturity and lateral source variations influence ~ ~3C of individual compounds. Whether the source effects are due to
-23 -24 -25
-25
~
---o--
-27 I"-N!
/Q%
/ e' -32
~'"0
~
x
x
Xg/
/
•
Buchan
- - .-t - .. Forties .... t----
Argyll
.... e----
Bruce
s
Fig. 11. Isotopic ratios of aromatic hydrocarbon components in North Sea oils.
746
C. J. CLAYTONand M. BJOROY -23 -24 -25 .o
== -26 -27
Sherwood
-28 --
i
-29
-
o
-
--
Bridport
-30 -31 -32 -33
Fig. 12. Isotopic ratios of aromatic hydrocarbon components in Wytch Farm oils. lateral variation within the Kimmeridge Clay or due to differing contributions from another source cannot be demonstrated. However, it seems likely that the latter is the case, with the second source represented by the terrestrial kerogen-rich Heather Formation (Chung et al., 1992). The proportion of the isotopic variation which results from maturity can be approximated by using Pr/Ph as a source indicator (cf. Chung et al., 1992) and the aromatic molecular ratios to determine maturity. Multiple regression of 6 13C for each compound, using these parameters as the independent variables will give an approximate estimate of the relative importance of source versus maturity in dictating ~3C: t5 ~3Ccom~ond x = a + b • (Pr/Ph) + c • maturity ratio then: c/([b ] + c). 100 is the percentage of the variation which is attributed to maturity. In practice, the magnitude of the coefficients b and c are dependant on the magnitude of the Pr/Ph and maturity ratios, and are hence essentially arbitrary. We have therefore renormalized these ratios to vary from 0 to 1 over the range represented by the samples analysed for this study. The resulting parameters thus relate to the relative source and maturity influence over the ranges represented by our sample set. It is important to note, however, that this method is only an approximation since we have data for only four samples. We regard this approach as a convenient method with which to compare source and maturity effects between compounds rather than as a rigorous statistical treatment which must await more data. In addition, there are a number of further assumptions inherent in its use: (1) The pristane/phytane ratio is dictated only by source variations and is not significantly affected by maturity. This is probably a reasonable assumption since our samples are not substantially affected by oil to gas cracking.
(2) The molecular maturity parameters are independent of source. This again is probably reasonable since both source end-members are dominated by marine organic matter, even though one has a contribution from terrestrial organic matter. Also, we have used only molecular ratios, avoiding more sensitive bulk parameters such as the saturated hydrocarbon content. (3) 6 13C is related linearly to source input, as revealed by Pr/Ph, and to maturity as reflected in the molecular maturity ratios. This is probably not strictly the case and certainly cannot be demonstrated from these data. However, assuming linear relationships will at least give us an approximation of the relative importance of source and maturity. Non linear effects will alter the magnitude of the resulting coefficients but not the relative values between compounds. The results of this exercise are given in Table 4 and plotted in Fig. 13. Data for n-alkanes are not available since we analysed only one North Sea saturated hydrocarbon fraction. In defining maturity we have used all the available molecular maturity parameters, excluding the biphenyl ratio which appears to conflict with all the others. In this way we can also gain some idea of the uncertainty involved, shown in Fig. 13 by the error bars. In general, the use of the dimethylnaphthalene ratio resulted in significantly different results to the other parameters and so must be treated with caution. The correlation coefficient R : in Table 4 indicates the percentage of the variability explained by this simple model. The isoprenoid alkanes appear to be controlled more by maturity (53-64%) than by facies variations although there is considerable uncertainty on the true value depending on which maturity parameter is used. If the dubious values for D M N R are ignored, the range reduces considerably, and the mean value would be higher. Regardless of this, however, the
Effect of maturity on North Sea oils data from using each maturity parameter in isolation show a systematic increase in the importance of maturity with increasing molecular weight. In other words, the low molecular weight compounds are
747
more prone to source variations than are the higher molecular weight components. The branched and q, cli¢ components are considerably more variable, with between 35 and 90% of the
Table 4. Results of regression analysis to determine proportion of variation with results from maturity Compound
Mat ratio
a
b
c
R2 ( % ) ) b l / c
% mat.
Mean
Isoprenoid Alkanes iCI6
iCl8
Pristane
Phytane
MNR DMNR MPI-I MPI-2
-28.22 -28.24 -28.20 -28.17
-5.49 -9.07 -4.87 -5.04
6.09 9.77 6.04 5.59
80.6 94.6 98.3 74.3
0.90 0.93 0.81 0.90
53 52 55 53
MNR DMNR MPI-1 MPI-2
-28.68 -28.73 -28.71 -28.64
-3.34 -6.16 -3.05 -2.96
4.45 7.35 4.60 4.03
72.6 89.0 94.5 65.9
0.75 0.84 0.66 0.73
57 54 60 58
MNR DMNR MPI-1 MPI-2
- 30.25 -30.24 -30.21 -30.23
- 2.42 3.61 -4.41 5.66 - 1.96 3.46 - 2 . 1 8 3.34
89.1 98.3 99.9 84.4
0.67 0.78 0.56 0.65
60 56 64 61
MNR DMNR MPI-1 MPI-2
-30.11 -30.16 -30.15 -30.07
-2.26 -4,83 -2.01 -I.91
4.03 6.68 4.19 3.65
75.7 90.0 94.8 69.6
0.56 0.72 0.48 0.52
64 58 68 66
MNR DMNR MPI-I MPI-2
-31.74 -31.68 -31.63 - 31.74
-0.01 - 1.49 0.65 0.14
3.25 4.76 2.83 3.08
99.8 99.3 97.9 99.3
0.00 0.31 0.23 0.05
100 76 81 96
MNR DMNR MPI-I MPI-2
-31.38 -31.37 -31.36 -31.37
3.66 2.82 3.85 3.76
1.50 2.37 1.45 1.39
99.5 99.7 99.9 99.2
2.43 1.19 2.65 2.70
29 46 27 27
MNR DMNR MPI-I MPI-2
-31.18 -31.22 -31.20 -31.16
-0.33 -2.02 -0.17 -0.11
2.65 4.39 2.75 2.40
86.0 94.3 97.1 82.7
0.13 0.46 0.06 0.04
89 68 94 96
MNR DMNR MPI-I MPI-2
-30.48 -30.53 -30.54 -30.45
2.09 1.03 1.99 2.25
1.31 2.40 1.57 1.13
91.9 94.9 96.2 90.6
1.60 0.43 1.27 1.99
39 70 44 33
MNR DMNR MPI-I MPI-2
-29.34 -29.42 -29.41 -29.29
-2.16 -4.84 -2.03 - 1.78
4.01 6.81 4.30 3.60
70.0 85.7 91.5 74.5
0.54 0.71 0.47 0.50
65 58 68 67
MNR DMNR MPI-I MPI-2
-29.05 -29.07 -29.03 -29.02
-3.64 -6.51 -3.13 -3.28
4.91 7.86 4.85 4.51
82.8 95.4 98.7 77.0
0.74 0.83 0.64 0.73
57 55 61 58
MNR DMNR MPI-I MPI-2
-29.37 -29.42 -29.43 -29.34
0.77 -0.53 0.74 0.96
1.77 3.11 2.00 1.56
85.6 92.0 94.5 83.4
0.44 0.17 0.37 0.61
70 85 73 62
MNR DMNR MPI-1 MPI-2
-31.33 -31.40 -31.40 -31.29
0.91 2.66 - 0 . 9 9 4.62 0.91 2.95 1.18 2.36
86.0 92.5 95.1 83.6
0.34 0.21 0.31 0.50
75 82 76 67
MNR DMNR MPI-1 MPI-2
-31.53 -31.52 -31.50 -31.52
0.58 -0.74 0.90 0.74
2.40 3.76 2.30 2.23
97.0 99.6 100.0 95.6
0.24 0.20 0.39 0.33
81 84 72 75
MNR DMNR MPI-I MPI-2
-30.46 - 30.43 -30.42 -30.44
2.38 1.67 2.61 2.47
1.40 2.13 1.29 1.31
99.7 100.0 100.0 99.4
1.71 0.78 2.03 1.89
37 56 33 35
53
57
60
64
Branched AIkanes 2-Methylpentane
3-Methylpentane
Br C8
Br C9
Br C I I
2,6 dimethyl undecane
88
32
87
46
65
58
Cyclic Alkanes Me-cyclopentane
Eth-cyclopentane
Cyclohexane
Me-cyclohexane
72
75
78
40
---continued overleaf 0 0 2t-6/7--M
748
C . J . CLAYTON and M. BJOgOY Table
Compound
Mat ratio
Benzene
Xylene
1MN
1,6 DMN
3 MeP
9 MeP
4--continued
a
b
c
R2 (%) Ibl/c
% mat. Mean
Aromatic Hydrocarbons MNR -31.05 2.75 1 . 3 4 DMNR -31.07 1.87 2.25 MPI-I -31.07 2.82 1.42 MPI-2 -31.04 2.87 1.21
97.8 99.0 99.5 97.4
2.05 0.83 1.99 2.37
33 55 33 30
MNR DMNR MPI-I MPI-2
- 30.79 - 30.03 -31.10 -30.69
3.66 2.36 0.89 5.24 2.81 3.63 4.14 1.84
73.8 79.5 82.7 72.2
1.55 0.17 0.77 2.25
39 86 56 31
MNR DMNR MPI-I MP[-2
-29.60 -29.56 -29.52 -29.61
1.52 0.77 1.96 1.58
1.84 99.9 2.61 99.0 1.52 98.2 1.76 100.0
0.83 0.29 1.29 0.90
55 77 44 53
MNR DMNR MPI-I MPI-2
-31.76 -31.64 -31.75
0.23 -1.79 1.01 0.45
4.19 6.26 3.75 3.95
99.4 99.8 99.1 98.5
0.05 0.29 0.27 0.11
95 78 79 90
MNR DMNR MPI-1 MPI-2
-30.51 -30.42 -30.37 -30.53
0.43 -0.29 1.13 0.47
2.25 2.03 1.67 2.21
96.7 92.0 89.4 97.9
0.19 0.14 0.68 0.21
84 88 60 83
MNR DMNR MPI-I MPI-2
-30.13 -30.05 -30.00 - 30.15
0.45 -0.09 1.07 0.47
1.85 2.40 1.33 1.83
95.4 90.2 87.5 96.8
0.24 0.04 0.80 0.26
80 96 55 80
-31.70
38
53
57
85
78
78
13Cc0mp0un
d ---- a + b ' (Pr/Ph) + c • (maturity). "mat. ratio" refers to the maturity ratio used for the regression, and "% mat." is the percentage of the isotopic variation which is attributed to maturity (% mat ~ c/(Ib I + c).
isotopic variability controlled by maturity. Again, the u s e o f a n y i n d i v i d u a l m a t u r i t y p a r a m e t e r r e s u l t s in t h e s a m e s e q u e n c e , albeit w i t h slightly d i f f e r e n t a b s o l u t e v a l u e s . E x c e p t for m e t h y l c y c l o h e x a n e , c5 '3C o f t h e cyclic a l k a n e s is d o m i n a t e d b y m a t u r i t y v a r i a t i o n s . T h e l a t t e r c o m p o u n d is a p p a r e n t l y i n f l u e n c e d as m u c h b y s o u r c e facies as it is by m a t u r i t y . O f p a r t i c u l a r i n t e r e s t in t h e b r a n c h e d c o m p o n e n t s is t h e 3 - m e t h y l p e n t a n e , w h i c h a p p e a r s to s h o w a
s i g n i f i c a n t l y g r e a t e r r e s p o n s e to facies v a r i a t i o n t h a n does 2-methylpentane. Combined with the absolute difference in t5 '3C o f t h e 2 - m e t h y l h e x a n e c o m p a r e d w i t h t h e s e c o m p o u n d s (see a b o v e ) , t h i s s u g g e s t s t h a t all o f t h e s e l o w m o l e c u l a r w e i g h t b r a n c h e d a l k a n e s have different biological precursors. Aromatic compounds show a systematic trend towards greater maturity dominance with increasing m o l e c u l a r w e i g h t . T h e effect h e r e is m o r e d r a m a t i c
100 90 80
'-1 70 E .9o 60 G)
e.O "r(U
50 40 30
20 10 0 -
-
o
ii &
~
-
|
~-
IlaJ l
~
~
z
°
z
o.
o.
w
Fig. 13. Relative proportion (%) of isotopic shift caused by maturity rather than source facies. Error bars represent variation determined using different molecular maturity parameters.
Effect of maturity on North Sea oils however, suggesting that the more terrestrially dominated source contributes proportionately more of the light aromatic hydrocarbon compounds. As observed above, the low molecular weight aromatic hydrocarbons are also isotopically the heaviest, consistent with a greater contribution from the Heather Formation. DISCUSSION The regression coefficients b in Table 4 reflect how the isotopic ratios vary in response to changes in the pristane/phytane ratio. The absolute magnitude of the coefficients is unlikely to be reliable since the regressions are based on only four samples which results in only one degree of freedom with which to constrain the curves. However, qualitatively, these give us an idea of the relative response of different compounds to source and facies variations. The cyclic, aromatic and some branched compounds have positive or near zero coefficients, indicating that higher Pr/Ph, and hence a greater terrestrial influence in the source, causes isotopically heavier values. This is consistent with mixing of oils from the high Pr/Ph and isotopically heavy Heather Formation, with low Pr/Ph isotopically lighter Kimmeridge Clay. The decrease in magnitude of this effect with increasing molecular weight in the aromatic hydrocarbon fraction suggests that the Heather has most influence in the low molecular weight compounds. In contrast, the isoprenoid hydrocarbons, 2,6dimethylundecane and the Ca and Ell branched compounds show a negative source coefficient. This is surprising, and indicates that these hydrocarbons in the Heather source are isotopically lighter than those in the Kimmeridge Clay, even though the bulk kerogen is isotopically heavier (Chung et al., 1992). This possibly explains the observation of Bjor~y et al. (1991) that, in the Viking Graben, isoprenoid hydrocarbons are isotopically lighter than the normal alkanes, while in the Central Graben they are isotopically heavier, regardless of maturity. It also implies that in the "coaly" Heather formation, the isoprenoid hydrocarbons are not derived from the same biological precursors as are the n-alkanes, aromatic and cyclic compounds. A similar situation was found by Hayes et al. (1989) for the Greenhorn Formation, suggesting that their observed differences between 6 ~3C behaviour of porphyrins and of bulk organic matter may reflect mixing of marine and terrestrial organic sources, rather than )3C enrichment during bacterial reworking as they suggested. The regression coefficient c in Table 4 reflects the sensitivity of the carbon isotope ratios to increasing maturity. All values are positive, indicating that values become heavier with increasing maturity, consistent with what is known of bulk and boiling point fractions (Northam, 1985; Clayton, 1991). However, there is considerable variation in their relative sensitivity to maturity and apparently no systematic vari-
749
ations between molecular weights or compound types except within the isoprenoid hydrocarbons. Indeed, the trend in the isoprenoid hydrocarbons themselves may be an artefact of the regression owing to the small data set, since large negative "source" coefficients must be balanced by large positive "maturity" effects. Thus we are not seeing simple kinetic isotope fractionation associated with progressive thermal destruction of individual compounds. As higher molecular weight compounds are broken down to smaller products they bias the isotopic ratio of the products away from simple trends. The net effect is to scramble, and homogenize, the isotopic ratios inherited from the source material. SUMMARY
(1) Maturity variations are distinguishable from facies differences by combining compound specific isotopic ratios with conventional molecular ratios. (2) The main cause of variation in 6 ~3C of the limited number of oils studied here is maturity. This typically accounts for between 50 and 90% of the observed variation, and produces an increase in ~ ~3C generally between 2 and 3%o. (3) Superimposed on the maturity variations there is a source effect, best interpreted as a variable contribution of oil from the "coaly" Heather Formation to the bulk of the oil derived from the Kimmeridge Clay. Within both the isoprenoid hydrocarbons and the aromatic hydrocarbon compounds this contribution is most pronounced in the lower molecular weight components. (4) For the majority of compounds, the Heather contributes isotopically heavier or similar material. However, isoprenoid hydrocarbons and some other branched compounds are isotopically lighter than those derived from the Kimmeridge Clay. (5) The maturity dependence of ~ 13C in individual compounds is highly variable, suggesting that there is considerable homogenisation of 6 ~3C during maturation. As complex molecules break down to progressively simpler compounds they mix with the products from other cracking reactions to progressively obliterate source-derived isotopic signatures. Acknowledgements--We thank the staff of Geolab Nor and
particularly Rita Moe for their efforts in producing the data, Arnd Wilhelms for a critical review which improved the earlier text and Martin Schoell who invented the title. Financial support was provided by BP Norway and this paper is published with the kind permission of BP Exploration. REFERENCES
Bjorvy M., Hall K. and Jumeau J. (1990) Stable carbon isotope ratio analysis on single components in crude oils by direct GC-isotope analysis. Trends Anal. Chem. 9, 331-337. Bjor~y M., Hall K., Gillyon P. and Jumeau J. (1991) Carbon isotope variations in n-alkanes and isoprenoids of whole oils. Chem. Geol. 93, 13-20.
750
C. J. CLAYTON and M. BJo~v
Bjoray M., Hall P. B., Hustad E. and Williams J. A. (1992) Variation in stable carbon isotope ratios of individual hydrocarbons as a function of artificial maturity. In Advances in Organic Geochemistry 1991 (Edited by Eckardt C. B. et al.). Org. Geochem. 19, 89-105. Pergamon Press, Oxford. Chung H. M., Wingert W. S. and Claypool G. E. (1992) Geochemistry of oils in the Northern Viking Graben. In Giant Oil and Gas Fields of the Decade: 1978-1988 (Edited by Halbouty M.). AAPG Memoir 54. Clayton C. J. (1991) Effect of maturity on carbon isotope ratios of oils and condensates. Org. Geochem. 17, 887-899. Collister J. W., Summons R. E., Lichtfouse E. and Hayes J. M. (1992) An isotopic study of the Green River oil shale. In Advances in Organic Geochemistry 1991 (Edited by Eckardt C. B. et al.). Org. Geochem. 19, 265-276. Pergamon Press, Oxford. Freeman K. H., Hayes J. M., Trendel J.-M. and Albrecht P. (1990) Evidence from carbon isotope measurements for diverse origins of sedimentary hydrocarbons. Nature 343, 254-256. Galimov E. M. (1985) The Biological Fractionation of Isotopes. Academic Press, Orlando, Fla. Hall P. B., Schou L. and Bjoray M. (1985) Aromatic hydrocarbon variations in North Sea wells. In Petroleum Geochemistry of the Norwegian Shelf(Edited by Thomas B. M. et al.), pp. 293-301. Graham and Trotman, London. Hayes J. M., Popp B. N., Takigiku R. and Johnson M. W. (1989) An isotopic study of biogeochemical relationships between carbonates and organic carbon in the
Greenhorn Formation. Geochim. Cosmochim. Acta 53, 2961-2972. Hayes J. M., Takiglku R., Ocampo R., Callot H. J. and Albrecht P. (1987) Isotopic compositions and probable origins of organic molecules in the Eocene Messel shale. Nature 329, 48-51. Monson K. D. and Hayes J. M. (1980) Biosyntheticcontrol of the natural abundance of carbon 13 at specificpositionswithin fattyacids in Escherichiacoll.J. Biol. Chem. 255, 11435-I 1441. Monson K. D. and Hayes J. M. (1982) Carbon isotopic fractionation in the biosynthesis of bacterial fatty acids. Ozonolysis of unsaturated fatty acids as a means of determining the intramolecular distribution of carbon isotopes. Geochim. Cosmochim. Acta 46, 139-149. Northam M. A. (1985) Correlation of North Sea oils:the different fades of their Jurassic source. In Petroleum Geochemistry in Exploration of the Norwegian Shelf (Edited by Thomas B. M. et al.), pp. 93-99. Graham & Trotman, London. Quigley T. M., Mackenzie A. S. and Gray J. R. (1987) Kinetic theory of petroleum generation. In Migration of Hydrocarbons in Sedimentary Basins (Edited by Doligez B.), pp. 649-665. Editions Technip, Paris. Radke M. (1987) Organic geochemistry of aromatic hydrocarbons. In Advances in Petroleum Geochemistry (Edited by Brooks J. and Welte D.), Vol. 2, pp. 141-208. Academic Press, London. Vogler E. A. and Hayes J. M. (1979) Carbon isotope compositions of carboxyl groups of biosynthesised fatty acids. In Advances in Organic Geochemistry 1979 (Edited by Douglas A. G. and Maxwell J. R.), pp. 697-704. Pergamon Press, Oxford.