Biomarker distributions in asphaltenes and kerogens analysed by flash pyrolysis-gas chromatography-mass spectrometry

Advances in Organic Geochemistry 1985 Org. Geochem. Vol. 10, pp. 1127-1135, 1986 Printed in Greal Britain. All rights reserved

Copyright

Biomarker distributions by flash pyrolysis-gas

0

Ol46-6380/86 1986 Pergamon

$3.00 + 0.00 Journals Ltd

in aspbaltenes and kerogens analysed chromatography-mass spectrometry GER VAN GRAAS*

Organic Geochemistry Section, Continental Shelf and Petroleum Technology, Research Institute (IKU), P.O. Box 1883, 7001 Trondheim, Norway (Received

19 September

1985; accepted 17 February

1986)

Abstract-Biomarker distributions in a suite of asphaltenes and kerogens have been analysed by flash pyrolysis directly coupled to a GCMS system. Attention has been focussed on biomarkers of the sterane and triterpane types. The sample suite under investigation consists of sediments with different kerogen types and some crude oils. Biomarker distributions in the pyrolysates have been compared with the “free” biomarkers in the corresponding saturated hydrocarbon fractions. The analyses show significant differences between the distributions of the free biomarkers and those in the pyrolysates. The latter have lower amounts of steranes while diasteranes are absent or present at low concentrations only. In the triterpane traces a shift of maximum intensity from C,, (free compounds) to C,,/C, is observed. Furthermore, the pyrolysates contain a set of triterpenes (not present among the free compounds), and there is a selective loss of “non-regular” triterpanes that are present in the saturated hydrocarbon fractions. The observed differences between pyrolysates and free hydrocarbons can be explained partly by the processes occurring during pyrolysis such as bond rupture and subsequent stabilisation of primary pyrolysis products. To a certain extent these differences also show that maturation processes occurring in sediments have effects on free biomarker molecules different from those on molecules that are enclosed in a macromolecular matrix (kerogen or aiphaltenes). Differences between biomarker distributions of asphaltene and kerogen pyrolysates are relatively small. A comparison with the pyrolysates from extracted whole sediments suggests that these differences are mainly caused by interactions between the organic material and the mineral matrix during pyrolysis. Oil asphaltenes behave differently from sediment asphaltenes as their pyrolysates are more similar to the corresponding saturated hydrocarbon fractions, i.e. the differences described above are observed to a much smaller extent. This different behaviour appears to be the result of coprecipitation of a part of the maltene fraction with the oil asphaltenes. Key

words: biological markers, flash pyrolysis, GC-MS,

INTRODUCTION

Biological marker (often abbreviated as: biomarker) distributions are today one of the key tools in oil-oil and oil-source rock correlations (Mackenzie, 1984; Seifert and Moldowan, 1981; Shi Jiyang et al., 1982). This is the result of much work during the past decade that has greatly increased our understanding of the behaviour of biomarker molecules in sediments and crude oils as a function of organic matter origin, transformations during maturation, biodegradation etc. (Ensminger et al., 1974; Mackenzie et al., 1980; Seifert and Moldowan, 1979, 1980). However, even with today’s knowledge not all correlation problems can be solved adequately. Specific combinations of geological and geochemical conditions regularly provide the researcher with problems with no or too many solutions (Magoon and Claypool, 1985). One process that affects the biomarker distributions is the selective loss of certain compounds from an oil during migration or the extraction of certain compounds from sediments along the migration route (Philp and Gilbert, 1982). These processes also *Present address: Koninklijke/Shell Exploratie en Produktie Laboratorium, P.O. Box 60, 2280 AB Rijswijk, The Netherlands. 00 IO,,,--EE

asphaltenes, kerogen, oil, source rock

change the biomarker distributions in the extracts of potential source rocks. The problem of source rock contamination with migrated material can be avoided by studying the biomarker distributions in the insoluble organic material (kerogen). It has been known for several years that kerogen

contains

biomarker

molecules-either

chemically bound or physically trapped-and that these can be released by pyrolysis without major structural changes. Following this approach Seifert (1978) was able to prove the indigenous nature of source rock bitumens by comparison with kerogen pyrolysates.

Moving

one step further in this direction

one can study the biomarkers released by pyrolysis of asphaltenes with the aim of using them as a correlation parameter. This approach appears valid as more and more evidence suggests that asphaltenes can be regarded as kerogen fragments and that they are an intermediate stage in the formation of oil and gas (Bandurski, 1982; Tissot and Welte, 1984). Asphaltene pyrolysates have been used successfully in correlating a heavily biodegraded crude oil with the original oil (Curiale et al., 1983; Rubinstein et al., 1979). The research described in this paper aims at investigating

1127

the usefulness of biomarker

distributions

1128

GER VAN GRAAS

in pyrolysates of asphaltenes and kerogens as an additional tool in oil-oil and oil-source rock correlations. Whereas the examples mentioned above concern batch-wise pyrolysis with a rather complicated and time-consuming analytical program, this work involves the use of flash pyrolysis coupled to a GCMS system. This gives a simple analytical procedure and a short analysis time. Only a few papers have been published describing direct pyrolysisGCMS of insoluble organic material followed by analysis of the biomarkers (Gallegos, 1975, 1978; Larter, 1978; Philp and Gilbert, 1984, 1985). Our approach is similar to that recently published by Philp and Gilbert (1985) and our results largely correspond with theirs. In this paper the initial results obtained from the pyrolysis GCMS analysis of a representative sample set containing sediments of different organic matter type and maturity level, and some North Sea crude oils are presented. Comparisons are made between the biomarker distributions in various fractions of the samples: namely the saturated hydrocarbon fraction, pyrolysates of asphaltenes, kerogen concentrates and extracted sediments. EXPERIMENTAL

Sediments were Soxhlet extracted with dichloromethane. After evaporation of the solvent the extracts were mixed with an equal weight of toluene and asphahenes were precipitated by addition of a 30 times excess of n-pentane. The mixture was shaken vigorously and left overnight at room temperature. Asphaltenes were obtained by filtration over a glass filter and washed 5 times with pentane. Flash pyrolysis was performed using a CDS pyroprobe at 600°C. The nyrolvsates were transferred onto a GC column (30 m fused -silica, coated with DB-I) which was programmed from 120 to 280°C at 4”C/min. The gas chromatograph was interfaced to a VG Micromass 70-70H mass spectrometer. The instrument was operated in the MID mode. The common biomarker fragment ions (m/z 191, 217). plus a number of molecular ions, were monitored.

Table I. Sediments used in this study with basic organic geochemical data

IKU No.

Description

Vitrinite reflectance (%)

TOC (“/.)

Typet

-

24.0

I

0.6

6.3

II

0.6

4.4

II

0.4a 0.7

6.9 66.4

I::,,

O.Sa

I.5

Sediments

B3023 ZONE 2 ZONE 3 M801.5 A5708 K1572 K1634

Green River Shale (USA) Kimmeridge Clay Fm. Well 25/2-6 3100/3126m Kimmeridge Clay Fm. Well 2512-6 3I2613 I52m Humber Group (U. Jur.) Well 2/l-3 3783/3795m Coal, Svalbard (Tert.) Sticky Keep Fm. (Trias) Tsjermakfj., Svalbard Sticky Keep Fm. (Trias) Stensiofj., Svalbard

0.3a I.8 l Vitrinite reflectance unreliable due to bitumen staining. tKerogen type as determined by Rock-Eva1 pyrolysis.

111 111

distribution to C,, and Czs relative to C,,, and higher in the extracts. Another difference is that a number of non-regular hopanes (peaks 1, 2, 5, 8, 9) that are present in the extracts are missing in the pyrolysates. A number of characteristic peak ratios from the fragmentograms are listed in Table 3. Steranes

The sterane fragmentograms (m/z 217) of the pyrolysates are of varying quality. In some samples (Green River Shale, coal) the quality is good, whereas in other samples the quality is poor because of a low intensity of the sterane peaks and a high background level. Diasteranes are usually absent, or present in small amounts in the pyrolysates. Asphaltene pyrolyhigh in 5a,14z,17a,20Rsates are often ethylcholestane but this may be due to the coelution of an unknown compound. DISCUSSION

Comparison

of pyrolysates

with extracts

The triterpane traces of the pyrolysates differ from those of the extracts because of the occurrence of The sediments that were used in this study are hopenes and the shift in hopane distribution from C,, listed in Table 1 together with some basic geochemi- and higher to C2, and Cry (see Fig. 3). This shift has cal parameters. During this study the attention has also been observed in other studies both with batch been focussed on biomarkers of the sterane and pyrolysis (Rullkotter et al., 1984; Seifert, 1978; Seifert and Moldowan, 1980) and with flash pyrolysis (Galtriterpane types. The results demonstrate that our legos, 1975; Philp and Gilbert, 1984, 1985). The analytical procedure with an on-line pyrolysis-GCMS occurrence of hopenes was reported only in studies system is sensitive enough to analyse biomarker distributions in pyrolysates of sediments and as- where flash pyrolysis was used. Both observations phaltenes. The general trends in the triterpane and can be explained from the effects of pyrolytic prosterane fragmentograms will be briefly discussed. cesses.Hopane structures are probably bound to the Examples of the results obtained are given in Figs 1 macromolecular matrix via functional groups in the and 2. Peak identifications can be found in Table 2. side-chain. Bond cleavage at this location as a result of pyrolysis will then potentially be followed by the Triterpanes loss of side-chain fragments and secondly by disThe triterpane fragmentograms (m/z 191) of all proportioning reactions producing hopenes and hopsamples are of good analytical quality. The pyroly- anes. The fact that hopenes are not found in batch sates are characterised by the occurrence of un- pyrolysis studies (closed system) is similar to the saturated compounds and by the shift in C-number situation for the alkanes/alkenes where the latter are RESULTS

Biomarker Table

distributions

in pyrolysates

2a. Identifications

of triterpanes

Identifications of peaks in m/z 191 fragmentograms. the hvdroaen at C-17 and C-21 respectively 1. la. 2. 2% 3. 4. 5.

6. 7. a.

9. 9a. 10. 10% II. 12. 12a.

C,,-triterpane C,,-tnterpene l&z-22,29,30-trisnomeohopane 22,29,30-trisnorhop-17(21)-ene 17a-22,29,30-trisnorhopane 17/3-22.29.30-trisnorhopane 28,30-bisnorhopane C,-triterpane ab-30-norhopane C,-triterpane C,-triterpane C,-triterpene pa-30-norhopane C,-triterpane afi-hopane C,,-triterpane

Table

2. 3. 4. 5. 6. 7. 8.

(Tm)

C,-triterpene

Identifications the hydrogen

1.

(Ts)

12b. 13. 13n. 13b. 14. IS. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

2b. Identifications

OLand p refer to the configuration (unless indicated otherwise).

1129

of

C,,-triterpene BK-hopane triterpene C,,-triterpene 22%a/3-homohopane 22R-ab-homohopane C,,-triterpane pa-homohopane 22S-afl-bishomohopane 22R-ufl-bishomohopane /Ia-bishomohopane 22%ab-trishomohopane 22R-aj-trishomohopane 22%L$-tetrakishomohopane 22R-ap-tetrakishomohopane 22S-orb-pentakishomohopane 22R-a~qxntakishomohopane

of steranes

of peaks in m/r 217 fragmentograms. OLand B refer lo the configuration of atoms at C-5, C-14 and C-17 in regular steranes and C-13 and C-17 in diasteranes.

20%bar-diacholestane 20R+a-diacholestane ZOS-up-diacholestane ZOR-ap-diacholestane C,s-sterane 20S-ja-24-methyl-diacholestane 20R-pa-24-methyl-diacholestane ZOS-q5’-24-methyl-diacholestane + 20S-aaa-cholestane ZOR-q9fi-cholestane ZOS-flol-24-ethyl-diacholestane SOS-aflfi-cholestane ZOR-olaacholestane ZOR-/Ia-24&hyl-diacholestane C+terane

also only encountered when flash pyrolysis (open system) is used (van Graas et al., 1981; Rubinstein ef al., 1979).

There is an overall trend for lower sterane concentrations relative to triterpanes in the pyrolysates. This is exemplified by the triterpane/sterane ratios in Table 3. A similar observation was made by Gallegos (1975) who reported 3-4 times higher terpane/sterane ratios in pyrolysates of Green River Shale kerogen than in the free hydrocarbons. Larter (1978) and Philp and Gilbert (1984) did not report any steranes at all in their pyrolysates. The lower abundance of steranes in the pyrolysates can be due to the lower abundance of these structures in the kerogen/asphaltene macromolecule as suggested by Philp and Gilbert (1984). On the other hand, steroids are most likely bound to the macromolecular network via the functional group at C-3. Upon pyrolysis, this could lead to production of several degradation products (e.g. after ring-A cleavage) that are not detected by the routine m/z 217 scan. In this way the sterane/triterpane ratios would be lower in pyrolysates. A specific search for several known sterane derivatives (van Graas et al., 1982a, b) did not reveal the presence of these compounds in the pyrolysates but other degradation products are possible.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

20%aaa-24-methyl-cholestane 20R-a@-24-methyl-cholestane +ZOR-afi-24-ethyl-diacholestane ZOS-a/3/3-24-methyl-cholestane C,,-sterane ZOR-aaa-24-methyl-cholestane ZOS-aaa-2Cethylcholestane ZOR-c@-24-ethyl-cholestane 20S-c&9-24cthyLcholestane ZOR-olaol-24-ethyl-cholestane C,-sterane C,,-sterane C,,-sterane C,-sterane

Another important difference between the pyrolysates and the free hydrocarbons is the absence or very low intensity of non-regular hopanes and diasteranes in the pyrolysates. For the diasteranes this can be explained by the fact that they are late-stage diagenetic products (Ensminger et al., 1978; Rubinstein et al., 1975) and are therefore not incorporated into the kerogen/asphaltene macromolecule. Furthermore, sterane structures already present in the macromolecule are likely to be protected against the effects of the mineral catalysts that promote the rearrangement reactions (Sieskind et al., 1979). Other biomarker isomerisations that do not depend so heavily on the presence of mineral catalysts have been reported to proceed slower in kerogens than in the corresponding extracts (Rullkiitter et al., 1984; Seifert, 1978; Seifert and Moldowan, 1980). The absence of the non-regular hopanes can similarly be due to the fact that these compounds are also the product of late diagenetic processes. A similar explanation was suggested by Moldowan et al. (1984) for the absence of 28,30-bisnorhopane in batch pyrolysates of kerogens. Another explanation could be that these compounds are produced biosynthetically as hydrocarbons. The lack of a functional groups would limit their possibility to be incorporated in kerogen.

GER VAN GRAAS

1130

Comparison of pyrolysates from kerogens and extracted sediments

asphaltenes

with

Differences between biomarker traces in pyrolysates of asphaltenes, kerogen concentrates and extracted sediments are greatest in those samples with the lowest organic carbon content. This suggests that they are caused by effects of the mineral matrix. In general asphaltenes display strongest the characteristics listed above for pyrolysates, i.e. a shift towards C2, in the hopanes (Fig. 3) and the occurrence of hopenes. Extraction residues display lower C2, peaks, and kerogens have an intermediate behaviour. In general, it seems advisable to use kerogen concentrates for analysis as this will give the best correspondence with the asphaltenes.

Comparison of samples with different type and maturity

A preliminary comparison of biomarker distributions in pyrolysates from samples with different organic matter type and maturity can be made from these data. Owing to the loss of the non-regular hopanes and the absence of diasteranes, many of the typical source characteristics of free biomarker distributions are lost in the pyrolysates. The use of steranes, as source indicators is severely limited by the low intensity of these compounds in several of our samples. However, in the samples with good sterane traces they show a definite source dependence due to their close relationship with the free compounds. If the analytical quality of the sterane traces in our

Saturated

32

23

h.c. fraction

48

40

Asphaltenes

loo-

32

0

organic matter

pyrolysate

48

40

Kerogen

cont.

pyrolysate

so-

32

48

40

Retention Fig. l(a)

time (min.) -

Biomarker distributions in pyrolysates

1131

Saturated

11

h.c. fraction

32

Asphaltenes

32

100

pyrolysate

40

48

Kerogen

cont.

pyrolysate

21 3

50 4

55

L

+

32

48

40 Retention

time (min.)

-

Fig. I(b) Fig. 1. Biomarker distributions in various fractions of sample Zone 2, Kimmeridge Clay Fm. (a) m/z 191, triterpanes. (b) m/z 217, steranes.

other samples were improved it should be possible to use them for source characterisation as shown by Philp and Gilbert (1985). The application of biomarker traces in maturity evaluations also meets with several difficulties. The established hopane parameters (Seifert and Moldowan, 1980) are difficult to use due either to the absence or low intensity of the compounds involved, or to coelution of other compounds. Philp and Gilbert (1984, 1985) reported that the ratio of hopenes over hopanes is maturity dependent and our samples tend to follow this trend (Table 3). The low intensity of the sterane traces limits their use here as

maturity indicators. Philp and Gilbert (1985) report significant changes in the distribution of steranes in pyrolysates at a maturity level of 0.9% vitrinite reflectance. All our samples are less mature than this, therefore this observation cannot be confirmed. Pyrolysates

of oil

asphaltenes

A brief remark with regard to the pyrolysates of oil asphaltenes can be made here. The biomarker distributions in pyrolysates of oil asphaltenes appear to be fairly similar to the distribution in the free hydrocarbons (Table 3). It is likely that this is due to coprecipitation of a part of the maltene fraction with

GER VAN G~nns

1132

Saturated

h.c. fraction

14

b

loo

1

I

*-

32

100

25: 48

--

LI

Asphaltenes

I

7

pyrolysate

48

9

Kerogen

cont.

pyrolysate

32

48

14 Extraction

lOO-

residue

50-

. ..-.A.-40

32

Retention Fig.

2(a).

48 time (mid

-

Biomarker distributions in pyrolysates

'O"-

1133

Saturated

10 1

h.c. fraction

in, l

23

40

48

Asphaltenes

32

48

40

Kerogen

32 100

__ II

32

pyrolysate

48

h

Extraction residue pyrolysate

48

40 Retention

Fig. 2. Biomarker distribution

cont.

40 23

1

pyrolysate

time (min.)

-

Fig. 2(b) in various fractions of sample K-1572, Sticky Keep Fm. (a) m/z 191, triterpanes. (b) m/z 217, steranes.

1134

GER VAN GRAAS Table

Sample

3. Peak ratios

Hopene/hopane c27 c30 (1) (2)

from /3a/@ c30 (3)

Sediments

B3023SA-f B3023ASF B3023KK B3023ER ZONEZSAT ZONEZASF ZONElKK ZONE3SAT ZONE3ASF ZONE3KK M8015SAT M8015ASF M8015KK M8015ER A5708SAT A5708ASF A5708KK A5708ER Kl573SAT Kl572ASF K I572KK K1572ER Kl634SAT Kl634ASF K I634KK K I634ER Oils

E4992SAT E4992ASF El 19SAT El I9ASF BIZSISAT B125lASF

I .43 2.13 2.77 I.81 I .30 I .23 0.42 1.26 1.26 0.78 I .77 0.22 0.16 I .40 0.97 0.64 I .42 I .47 I .67 0.88 I.21 I.51

0;6 I .a9 I .73 2.00 0.97 2.41 0.62 0.80 I.12 0.06 0.06 I .54 0.50 0.24 I .a9 I .05 I .57

0.68 0.47 3.67 2.45 0.07 0.33 0.07 0.08 0.15 0.16 0.1 I 0.15 0.23 0.15 0.19 0.67 0.14 0.13 0.10 0.08 0.33 0.23 0.17 0.22 2.14 3.36

0.58 0.92

0.11 0.11 0.08 0.09 0.09 0.14

triterpane

fragmentograms

Non-regular c27 c28 (4) (5)

triterp C29 C29 (6) (7)

-

-

-

0.63 0.50 2.13 0.3 I 0.50 0.06 2.10 0.14 I .oo I .25 -

1.74 0.07 1.59 0.18 0.13 -

0.1 I 0.06 0.09 0.07 -

053 -

I .46 0.55 0.13 I .22 0.24

0.04 2.03 0.4 I 0.57 0.16

0.06 0.60 0.12

-

OTO 0.10 -

0.44 0.69 I.10 0.44 0.51 0.69 0.45 0.60

Trit Ster

(8)

> I.2 II.1 7.1 8.1 4.7 9.8 17.9 5.6 9.0 16.7 5.5 4.2 6.8 7.5 25. I 8.7 22.0 16.6 6.8 a.2 9.9 18.6 10.0 23.0 15.8 20.2 7.1 21.1 4.6 7.2 4.6 9.6

(I) C27-hopene/aC27-hopane (peak 2a/3 in Table 2a); (2) C30-hopene/a/3C30-hopane (peak lOa/l I in Table 2a); (3) j?aC3O/a/?C30 (peak 13/l I in Table 2a); (4) Ts/Tm (peak 2/3 in Table 2a); (5) C28/aflC29 (Peak 5/l in Table 2a); (6) C29/abC29 (peak 6/7 in Table 2a); (7) C29/aflC29 (peak 8/7 in Table 2a); (8) intensity ratio of triterpanes over steranes as measured by MID GCMS. SAT=saturated hydrocarbon fraction, ASF = asphaltene pyrolysate, KK = pyrolysate of kerogen concentrate, ER = pyrolysate of extraction residue.

the asphaltenes. Indeed, after Soxhlet extraction of the oil asphaltenes they showed characteristics similar to the sediment asphaltenes. It is not clear why coprecipitation is a problem only in case of oils and not of sediment extracts because both types of sample were treated identically.

CONCLUSIONS

Biomarkers that are enclosed in kerogens and asphaltenes can be studied by flash pyrolysis directly coupled to a GCMS system. The relatively short analysis time and small sample size (< 1 mg for asphaltenes) make this method potentially suitable for routine analyses unlike batch pyrolysis techniques which require significantly more time and sample material. Biomarker distributions in pyrolysates are different from those in the corresponding free hydrocarbons. The differences arise partly from thermochemical processes during pyrolysis and partly from the fact that several extractable components are formed by reactions occurring in the sediments after the main phase of formation of kerogen and asphaltenes has been completed. Such data can help to understand the origin of several extractable components. Pyrolysates from different fractions of the same sample (asphaltenes, kerogen concentrates, extraction residues) differ in a way that to a large extent can 100% cm loo%c~ Fig. 3. Triangular plot of C,,, C;, and C,, 17a21p-hopane be explained by the effects of the mineral matrix distributions in saturated hydrocarbon fractions and during pyrolysis. The limited data set shows that a number of pyrolysates.

Biomarker distributions in pyrolysates organic matter type indicators are lost in the pyrolysates. The best remaining source characteristics seem to be the sterane distributions, especially when the quality of these traces can be improved. The traditional maturity indicators are also difficult to use in the pyrolysates. Here the pyrolysates offer a new parameter in the form of the ratio of hopenes over hopanes which decreases with increasing maturity. Oil asphaltenes must be thoroughly extracted after precipitation to avoid the inclusion of significant amounts of maltenes. When this is done their pyrolysates are generally similar to those of sediment asphaltenes. This indicates the possibility of using these pyrolysates in oil-source rock correlations.

Acknowledgemenls-This research has been financially supported by Elf Aquitaine Norge A/S, Fina Exploration Norway U/A, Mobil Exploration Norway U/A., Norsk Hydro A/S and Saga Petroleum A/S. The managements of these companies are gratefully acknowledged for permission to publish these results.

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