Molecular characterisation of kerogen from the Kimmeridge clay formation by mild selective chemical degradation and solid state 13C-NMR

Advances in Organic Geochemistry 1989

Org. Geochem. Vol. 16, Nos 4-6, pp. 951-958, 1990 Printed in Great Britain.All rights reserved

0146-6380/90 $3.00+ 0.00 Copyright© 1990Pergamon Press plc

Molecular characterisation of kerogen from the Kimmeridge clay formation by mild selective chemical degradation and solid state ~3C-NMR R. J. BOUCI-IERj, G. STANDEN 1, R. L. PATIENCE 2 and G. EGLINTON I ~Organic Geochemistry Unit, University of Bristol, School of Chemistry, Cantock's Close, Bristol BS8 ITS, U.K. 2BP Research International, Geochemistry Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TWI6 7LN, U.K. (Received 19 September 1989; accepted 12 December 1989) Abstract--Molecular characterisation of a kerogen from the Kimmeridge clay formation (Upper Jurassic, Dorset, U.K.) has been undertaken by integration of results from a variety of chemical techniques. Basic data have been obtained from elemental analysis, pyrolysis yields, pyrolysis-GC and a study of the solvent-soluble fraction. More detailed molecular information (particularly about the aliphatic structures present) has been gained from selective degradation using ruthenium tetroxide (RuO4), which oxidises aromatic rings predominantly. The average molecular structure was obtained from solid state dipolar dephased I3C-CP/MAS NMR, using a peak synthesis approach based on chemical shifts of standard compounds. The RuO4 oxidation gave a dichloromethane-soluble extract (42%), a solid residue (14%) and CO2 (26%) based on original kerogen weight. The extract contained a series of straight chain monocarboxylic acids (14%) in the range C9~27 and ~t,o~-dicarboxylicacids (51%) in the range C44227, several branched chain mono- and diacids (9%), and particularly large amounts of acyclic isoprenoid acids (16%) (C~4~217, Cl~--C21). The NMR data revealed that the average molecular structure is mainly aliphatic (only 25.8% aromatic carbon) and contains predominantly CH 2 groups in rings or chains, or attached to oxygen. The aromatic species present are protonated (10%) ("benzene"-like), bridgehead or ring junction (8.1%) alkylated (5.8%) and phenolic (1.9%). ~3C-NMR spectroscopy and ruthenium tetroxide oxidation provide complementary information on Kimmeridge clay kerogen, the former on the aromatic and the latter on the aliphatic component. The overal conclusion is that the carbon skeleton of the Kimmeridge clay kerogen contains mainly linear aliphatic chains with only small amounts of cyclic structures, a view which contrasts with some current models for Type II kerogens. Key words--Kimmeridge clay, t3C-NMR, ruthenium tetroxide oxidation, pyrolysis-gas chromatography

INTRODUCTION Molecular characterisation of kerogen has proved difficult due to the insolubility and presumed macromolecular nature of this heteropolymeric material. Various models have been proposed for the structure of kerogen (Behar and Vandenbroucke, 1987) and these have been based on a statistical representation and consequently show an average picture. However, kerogen is more likely to be heterogenous, containing chemical species derived from a range of organisms. A wide variety of methods have been applied (Durand, 1980) to the study of kerogen. "Bulk" methods, e.g. elemental analysis, IR and solid state N M R spectroscopy give useful overall information, while dcgradative techniques, in particular chemical degradation, (Robinson, 1976; Yen, 1976; Chappe et al., 1980; Vitorovic, 1980; Mycke and Michaelis, 1986; Mycke et al., 1987; Barakat and Yen, 1988) provide more detailed molecular information

although this can be difficult to relate back to the original kerogen structure. Ruthenium tetroxide (RuO4) oxidation was chosen as a method for obtaining molecular information concerning the aliphatic and alicyclic portions of the kerogen. It is a mild and selective oxidising agent which preferentially destroys, oxidatively, aromatic rings, converts alcohols into ketones or aldehydes, aldehydes into acids, ethers into esters or lactones and alkenes into aldehydes and ketones, leaving the aliphatic and alicyclic portion intact. It is only required in a catalytic quantity because, once reduced by the organic matter, it may be converted back to the active form by sodium periodate (NaIO4), which is the co-oxidant. The stoichiometry of the oxidation of aromatic substrates is unknown, since RuO4 oxidation of alkyl benzenes gives more than one product (Stock and Tse, 1983; Olson and Diehl, 1984). Isley et al. (1986) showed that phenyl groups are rapidly 951

952

R.J. BOUC~R et al.

oxidised to carbon dioxide when the ring is activated by electron donating substituents, but slowly or not at all when an electron withdrawing group is present. Ruthenium tetroxide has been used in the analysis of coals (Stock and Tse, 1983; Stock and Wang, 1986; Mallya and Zingaro, 1984; Olson and Diehl, 1984), asphaltenes (Mojelsky et al., 1985; Trifilieff et al., 1987) and kerogens (Boucher et al., 1991). The value of the ruthenium tetroxide oxidation lies in its oxidative specificity for aromatic rings and functionalised carbon atoms. The atiphatic and acyclic structures liberated appear as carboxylic acids, marking their points of attachment in the kerogen. In order to assess the compositional information derived from ruthenium tetroxide oxidation, it was compared with pyrolysis-gas chromatography (py-GC) (van de Meent et al., 1980); Larter and Douglas, 1982) as this is a common method used for estimating and comparing the proportion of discrete structural elements in kerogen and has been used to acquire fingerprint data, which enables variations between kerogen types to be determined. To compare the molecular information derived from degradative technique with that of one of the "bulk" methods solid state t3C-CP (cross-polarisation)/MAS (magic angle spinning) nuclear magnetic resonance (NMR) spectroscopy was chosen to provide information on the average molecular structure. It has been applied before to a number of types of sedimentary organic matter, particularly oil shales (Resing et al., 1987; Maciel et al., 1979) and coals (Bartuska et al., 1977). The most common type of information provided by ~3C-CP/MAS N M R studies is the aromaticity (fa), however in addition it is possible to subdivide the total aromatic and aliphatic carbon peaks in terms of the relative abundance of carbon functional groups (Trewhella et al., 1986). The kerogen is from the Upper Jurassic Kimmeridge clay formation which is mainly composed of argillaceous sediments. In the Dorset area the Kimmeridge clay reaches a thickness of approximately 500 m, of which a maximum 335 m is exposed. From previous organic geochemistry studies (Williams and Douglas, 1980, 1981, 1983; Farrimond et al., 1984) it has been concluded that the sediments are immature (Dorset type area), but rich in organic matter and were deposited under reducing conditions in a marine environment. The aim of this paper was to use solid state ~3C-NMR to investigate the average molecular structure of Kimmeridge clay kerogen and integrate this with the compositional information from the ruthenium tetroxide oxidation and py-GC.

EXPERIMENTAL

content of 22.8% of the dried sample and 64.4% of the dried, extracted kerogen by weight. The kerogen concentrate (obtained from 5-10 g of original shale) was prepared (employing P.T.F.E. equipment) initially using hydrochloric acid (8 M, 100 ml) and stirring for 24 h at room temperature, followed by a further 8 h at 50°C. After washing and centrifuging several times the decarbonated samples were digested in hydrofluoric acid (48% w/v, 100 ml) and hydrochloric acid (5 ml). Neutralisation of the acid using saturated boric acid solution and stirring (24 h) was then executed with subsequent removal of the boric acid via washing and centrifuging. Alternate washings with saturated ammonium carbonate solution and hot hydrochloric acid (4M) were performed to remove fluorides. The kerogen was then washed/centrifuged (3-4 times) until a pH neutral aqueous layer was obtained. Solvent extraction then gave the "pure" kerogen. Materials

All solvents were either "distilled in glass" grade solvents or high purity solvents. All other chemicals were reagent grade. The water used was distilled in all-glass apparatus after deionisation. Extract analysis

The solvent extract was subjected to routine geochemical analysis-automated de-asphaltening, high-performance liquid chromatography (HPLC) separation of the saturates, aromatics and residues, gas chromatography (GC) and GC-mass spectroscopy of the saturates and aromatic fraction. Ruthenium tetroxide oxidation

The oxidation was carried using a modification of the Sharpless et al. (1981) procedure. A flask was charged with 0.1 g of substrate, acetonitrile (4 ml), chloroform (4 ml) and water (6 ml). Sodium periodate (2 g) was added to establish the co-oxidant ratio of 20:1. RuC13"3H20 (0.025 g) was added and the reaction mixture sonicated for 1 h and then stirred for 24 h. The reaction products were filtered, the solid residue washed with DCM (3 × 10ml) and the organic phase separated. Extraction of the aqueous phase (2 x 10 ml DCM and 2 × 10 ml Et20) was then executed. The organic extracts were combined, dried (MgSO4) and filtered through Celite ~ to remove the drying agent and the precipitated ruthenium species. After taking-off the solvent under vacuum, the acids were esterified using boron trifluoride-methanol (14% w/v; Metcalf and Schmitz, 1961). The solid residue was washed with H20 to remove any inorganic material, dried in a desiccator for 24 h and then in a vacuum oven for a similar period of time at 80°C before being submitted for microanalysis (C 23.06%, H 3.00%, N 0.66%).

Sample preparation

Carbon dioxide analysis

The oil shale from the Blackstone, Clavells Hard, near Kimmeridge, Dorset has a total organic carbon

The apparatus used was that of Patchornik and Shalitin (1961), suitably modified for the ruthenium

Molecular characterisation of kerogen tetroxide process. Any CO2 produced was absorbed into standardised (approx. 0.05 M) barium hydroxide solution over the 24 h-period. Initially, excess barium hydroxide was titrated with 0.1 M HC1 using a phenolphthalein (0.5%) indicator solution; the titration was continued using a screened methyl orange indicator solution and 0.1 M HCi to release the carbon dioxide from the carbonate formed. Blanks were also run over identical time periods, permitting subsequent corrections in the calculations. Instrumental methods GC was carried out on a Varian 3400 gas chromatograph fitted with on-column injection and flameionisation detection (FID). A 50m OV-1 capillary column was employed with hydrogen as the carrier gas, and the oven programmed from 50 to 300°C at 4°C/min. The FID signals were acquired and processed with a VG Minichrom Data System. Quantitation was achieved by peak area measurement against known amounts of internal standards (C36 and C40 alkanes). Equal FID response factors were assumed for the standards and samples in quantitation. GC-MS was performed using a Carlo Erba Mega Series 5160 gas chromatograph linked to a Finnigan4500 mass spectrometer, operated in EI mode (ionising energy 40 eV; ion-source temperature 250°C; scan time 1 s for m/z 50-550). INCOS 2300 data system was exployed for data acquisition and processing. Pyrolysis and pyrolysis-gas chromatography The whole ground sediment was heated at 250°C for 3 rain (S1) followed by heating at 30°C/rain from 250-550°C (S2). Detection by FID. For pyrolysis--GC the above conditions were used except the heating rate was at 60°C/min. Chromatography was performed on an aluminium clad BP-1 fused silica column (25 m; 0.53 mm i.d.; SGE). Column temperature was programmed from - 8 0 to 0°C at 16°C/min followed by 0-300°C at 8°C/rain. Helium was the carrier gas and detection was by FID.

Basic geochemical data for Kimmeridge clay formation TOC P2 HI GOGI*

22.8% 115.2 kg/t 504 mg HC/g TOC 0.13

*Gas (C~-Cs)/Oil (C6+) Generation Index from pyrolysis-GC. Solid state IsC.CP/MAS N M R The detailed description of this technique is given elsewhere (Mann et al., 1991). Briefly, average molecular structures were obtained from a carbon type analysis technique involving the fitting of the simulated peak spectrum to the dipolar dephased spectrum, and then the deconvolution of the stan-

Table 1. Biomarkermaturity parameters C32Hopanes 22S/(22S+ 22R) Q9 Steranes 20S/(20S+ 20R) C,o Steranes ',~ l(aB~ + ~ ) MPI Phenanthrenes (3Me+ 2Me)/ (9Me+ 1Me+ Phen)* 1.5 ARO Steranes Czs 20R TRI/ (SAME+ C2920R SA + B MONO) TRI ARO Steranes C~o/(C2o+ C2s 20R)

953

0.23 0.07 0.12 0.87 0.08 0.13

dard CP/MAS spectrum into individual carbon type resonances, using chemical shift data obtained from standard compounds. RESULTS AND DISCUSSION The basic geochemical data are indicative of a good oil-prone source rock (HI generally 400-550mg HC/g TOC) that is quite immature. Biomarker parameters were calculated from GC and GC-MS analysis of the solvent-soluble fraction in order to assess the maturity of the sample. The main maturity parameters are shown in Table l and indicate low maturity as illustrated by low abundances of (20S)-steranes and (22S)-hopanes and reduced amounts of triaromatic steroids (Mackenzie, 1984). Ruthenium tetroxide oxidation The results of the ruthenium tetroxide oxidation are shown in Table 2. The experiment yielded 42% of a dichloromethane soluble extract and 14% of a solid residue. The amount of CO 2 generated by oxidation of the kerogen was also monitored in a separate experiment and gave a value of 26% by weight of original kerogen. This provides an indication of the aromaticity of the sample as the majority of aromatic rings are oxidatively destroyed by RuO4 oxidation. The products from the RuO4 oxidation are dominated by five homologous series of acids [Fig. l(a)]: (1) Normal monocarboxylic acids (14%) in the range C9-C27. (2) ~t,~-dicarboxylic acids (51%) C4-C24. (3) Acyclic isoprenoid acids (16%) C14--C21,not

including C18. (4) Branched dicarboxylic acids (7%) C5--C12. (5) A series of cyclic acids (10%). Lipid moieties more volatile than the Cj4 fatty acid are preferentially lost by evaporation during work up. These more volatile fatty acids are quantified, but it is important to realise that these values may have been diminished by evaporation. Distribution and quantitation details of mono-, diacids and acyclic isoprenoids are shown in Fig. 2. Table 2. Yields (wt%) of products of oxidation with RuO, Organicsoluble extract Solid Total (%) residue CO2 (%) Kimmeridgeclay kerogen 42 14 26 82

R. J. BoucrmRet al.

954 (a)

0 MONO'CARBOXYLIC ACID C~ MONO-CARBOXYLICAC;O (@RANCHED) • DI'C&RBCO
9 1~

II

4

13

t4

~

t@ @

10

15

20

25

30

35

40

45

50

w

1

55

_

60

65

70

75

RETENTION TIME (MINUTES)

(b) GOGT

. :13

14

~Ph 3

KIMME~IOGE

WHOLE

OUTCROP

ROCK

PYROLYSATE

SAMPLE

87~IOWT03@

(P2)

OISTR~BUTION

Fig. 1. (a) Gas chromatogram of the esterified products (methyl esters) of the RuO4 of Kimmeridge clay kerogen. The analysis was performed using a 50 m OV-I column at 50-300°C at 4°C/min. (b) Pyrolysis-gas chromatography trace of Kimmeridge clay. Numbers refer to carbon number. B = benzene, T = toluene, X = m e t a - and para-xylenes, Pr = pristane and Ph = phytane. A pie chart for the main classes of compounds is shown in Fig. 3. The n-monocarboxylic acids show a maximum at n-CtT, and this was also found to be the major monoacid in the less selective permanganate oxidation of Kimmeridge clay (Palmer et al., 1987). Generally the monoacids contain less than 20 carbon atoms, which is indicative of a major input of lower plant material, probably of algal origin. The ~t,o~-dicarboxylic acids show a maximum at Ca, with large amount (81.5%) of short chain acids
functionalities. The permanganate oxidation of algal lipids, algal cell was material and algal kerogen produced ct,co-dicarboxylic acids in the C4--C12range (Ishiwatari and Machihara, 1982a, b). Hence, the =,to-dicarboxylic acids obtained in the present work may be degradation products from algal remains. Another possibility is that the alkyl chains are substituents of aromatic structures which on oxidation liberate them as dicarboxylic acids. Major amounts of isoprenoid acids (16%) were identified with a maximum at C,~; the C,8 isoprenoid acid was absent. Analysis of solvent extracts of Kimmeridge clay samples has shown large variations in relative abundance of the isoprenoids in samples

955

Molecular characterisation of kerogen (a)

KIMMERIDGE CLAY KEROGEN(20:I) MONOCARBOXYLIC ACIDS mgkg KEROGEN CONCENTRATE

'via

0.08

0.06

0.04

0"01

P 8 9 10 11 12 13 14 15 18 17 18 19 20 21 22 23 24 26 2e 27 28

CARBON NUMBER

BMA

(b)

BDA KIMMERIDGE CLAY KEROGEN(20:I)

OYCLI C

Fig. 3. A pie chart showing main classes of compounds normalised for the RuO4 oxidation products of Kimmeridge clay kerogen (20:1). MA=normal saturated monocarboxylic acids, DA = ct¢o-dicarboxylic acids, BMA = branched monocarboxylic acids, BDA = branched dicarboxylic acids, IP = isoprenoid acids, and cyclic=cyclic acids.

DICARBOXYLIC ACIDS mo~g KEROGEN CONCENTRATE 0.25 0.2 0.16 0.1

&O I 4 6 8 7 8 9 10 11 121314 15 1 6 1 7 1 8 1 9 2 0 2 1 2 2 2 3 2 4 2 5

CARBON NUMBER

(c) KIMMERIDGE CLAY KEROGEN(20:I) ISOPRENOID ACIDS m~\g KEROGEN CONCENTRATE

141a

151p

161p

17'11:)

181p

191p

201p

211p

CARBON NUMBER

Fig. 2. (a) Straight chain monocarboxylic acids, (b) ~,og-dicarboxylic acids and (c) isoprenoid acids identified as products (as methyl esters) from RuO4 oxidation of Kimmeridge clay kerogen. Histograms display mg/g of kerogen versus carbon number.

collected over short intervals (Williams and Douglas, 1980) indicating a variation in input. A series of branched diacids (7%) were identified in the range C5~212, including 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids which have been suggested as markers for carotenoid pigments (Machihara and Ishiwatari, 1983) being derived from them by oxidation. No steroids or hopanoids were observed in the ruthenium tetroxide products, even though the solvent extract of the original kerogen does show a large OG 16/4/~T

abundance of biomarkers in the sterane/triterpane region. However, hydrous pyrolysis results for Kimmeridge clay kerogen parallel our oxidation data in that the pyrolysates also show negligible contents of biomarkers (Eglinton and Douglas, 1987). Hence, Kimmeridge clay kerogen does not apparently contain significant quantities of steroidal or hopanoidal moieties in its heteropolymeric structure. Pyrolysis-gas chromatography The py-GC trace [Fig. l(b)] shows a homologous series of alkene/alkane pairs ranging from C4 to C30, with the intensity of the doublets being markedly reduced after C20. The py-GC trace also shows a high abundance of compounds eluting between the nalkene/n-alkane doublets, partly identified as aromatics and unsaturated/saturated isoprenoids (Burnham et al., 1982). It has been generally concluded that type II kerogens generate isoprenoids during pyrolysis. Py43C characteristics of kerogens have been found to change with increasing maturation, i.e. the abundance of aromatics and isoprenoids is reduced and the aliphatic n-alkene/n-alkane pairs become more dominant with increasing depth of burial (van Gras et al., 1981). The fingerprint observed here is quite characteristic of an immature type II kerogen (Douglas et al., 1983). The largest peaks in the py-GC trace occur between n-C1,, and n-C13, n-Cl3 and n-C~4, n-Cl4 and n-C~5, and n-C~5 and n-Cl6. These may be isoprenoids and therefore correspond to those found in the products of ruthenium tetroxide oxidation of Kimmeridge clay kerogen. Hydrous pyrolysis of Kimmeridge clay kerogen (Winters et al., 1983) indicates an abundance of isoprenoids during the first stage of maturation. Incidentally, partial RuO4 oxidation of Kimmeridge clay using a smaller amoun of co-oxidant, resulted in isoprenoids being produced in

956

R.J. BOUCrlERet al.

correspondingly greater relative proportions indicating that the isoprenoids occur at the periphery of the kerogen macromolecular matrix.

Table 3 Structure

Symbol f,

13C-CP/MAS NMR

Detailed structural information on Kimmeridge clay kerogen was obtained by carbon type analysis (see Experimental) and this is shown in Table 3 and a pictorial representation is given in Fig. 4. The value for aromaticity (f,) is 25.8% which is in good agreement with previous results for type II kerogens (Barwise et al., 1984). The aliphatic and acyclic carbon is expressed in terms of structural types C, CH, CH2 (methylenes in rings or chains, which are indistinguishable, and those adjacent to a methyl group), CH 3 (attached to aliphatic or aromatic C) and carbon attached to "oxygen" ("oxygen" indicated in CHO, CH20, CH30 can be either oxygen or sulphur; the N M R method cannot distinguish between them). The abundance of CH 2 groups in the Kimmeridge clay kerogen is high (41.9% of TOC) and therefore they constitute a major part of the kerogen. The CH component (14.1%) is also important and represents branch points in isoprenoids or ring junctions e.g. in steranes and hopanes. This indicates a large contribution from isoprenoids or cyclic material. The acid/ester component is low (0.9%), therefore the majority of the oxygen is present in the form of either alcohols or ethers (9.9%). The aromatic carbon present is divided into protonated, bridgehead or ring-junction, alkylated and phenolic. The dominant species is the protonated aromatic carbon (10.0%), which is "benzene" like (i.e. unsubstituted), slightly less is the bridgehead or ring-junction (8.1%) aromatic carbon followed by the

ArCC ArC'C

% TOC*

[~

25.8

[~c

5.8

[ ~

8.1

ArCH

~,,H

10.0

ArCO

(~.O

1.9

OOR/H

COOR

0.9 ¢/H

c___--o

c

0.4

c

CHO

c ~

CH20

c@ o - -

~ CH30

--

-°-c/H

2.8

c/,

6.9

O--

C

0.2

¢

CH

c+c c c.~c

cn2

c c@c

CH2(C2)

C@CH~

4.6

CH3 aromatic

[~

4.4

CH3 aliphatic

~cH-~

2.3

c

0.5 14.1 37.3

*Total organiccarbon.

CH

ArCH CH2

ArC^C

ArOC Ar CO

CH3

Fig. 4. A pie chart showing the main groups obtained from 13C-NMR of Kimmeridge clay kerogen.

Molecular characterisation of kerogen alkylated (5.8%). There is a low value (1.9%) for the phenolic aromatic carbon, which is probably ligninderived, indicating that lignin can only be a minor contributor to this kerogen. COMPARATIVE EVALUATIONOF RESULTS P y - G C and the ruthenium tetroxide oxidation both provide information on the distributions and relative abundances of aliphatic chain lengths, which are found to be mainly below C20 in both cases. Both techniques also show the presence of large amounts of isoprenoids in agreement with the significant proportion of CH structures seen in the ~3C-NMR spectrum. The p y - G C trace and the oxidation products from ruthenium tetroxide both show an absence of triterpenoids. In other words, the p y - G C and ruthenium tetroxide methods are giving comparative results and this is evidence that pyrograms may be a good representation of kerogen structure. The majority of the aromatic rings in the ~3C-NMR spectrum do not have electron-withdrawing groups attached and therefore should be oxidised by the ruthenium tetroxide method. This is in fact the case as practically no aromatic compounds are seen in the products. The small amount of ester/acid linkages inferred from the ~3C-NMR spectrum, as compared with the high yield of acids from the ruthenium tetroxide oxidation, leads to the interpretation that the majority of the latter arise from the oxidation of benzene, alkenes, alcohols and ethers etc. The high proportion of diacids almost certainly arises from the existence of extensive crosslinking within the kerogen matrix. ~3C-NMR spectroscopy generates the most useful structural information on the aromatic portion of the kerogen whereas ruthenium tetroxide oxidation provides detailed molecular information on the aliphatic and alicyclic part. Therefore, using both methods provides a greater insight to the kerogen structure than is possible by using only one method. CONCLUSIONS Oxidative degradation with ruthenium tetroxide of Kimmeridge clay kerogen (Dorset type) provides valuable compositional information on the aliphatic and alicyclic portion of the kerogen. Of the oxidation products, straight chain carboxylic acids (65%) are predominant, reflecting a major content of polymethylene chains in the kerogen, a conclusion which is also supported by the p y - G C and 13C-NMR data. Also, acyclic isoprenoid moieties are major components of oxidation and the p y - G C products. The oxidation process only provides information on the aliphatic and alicyclic portion of the kerogen, so solid-state CP/MAS ~3C-NMR is a method which supplies complementary data about both the total aliphatic and total aromatic content of the kerogen.

957

Kimmeridge clay kerogen is immature with an aromaticity of 25.8%. The straight chains are generally less than 20 carbon atoms in length, indicating a substantial input from algae. The majority of the straight chain material from the ruthenium tetroxide oxidation was ~t,m-dicarboxylic acids (51%) pointing to the possibility of significant crosslinking. There is substantial branching of the carbon chains, characterised by the presence of large amounts of isoprenoids. The presence of steroids and triterpenoids are not obvious in the products and have yet to be recognised. This is presently under investigation. Acknowledgements--The authors wish to thank British Petroleum plc for permission to publish this work and for financial support (EMRA award). We thank the Natural Environment Research Council for a Case studentship (GS) and for GC-MS facilities (GR3/2951 and GR3/3758). We are also indebted to I. J. F. Poplett (BP) for obtaining the J3C-NMR spectra, to C. J. Dowries (BP) for the kerogen isolation, pyrolysis43C data and the geochemical analysis. Thanks to J. F. Carter (OGU) for assistance with GC-MS analyses. REFERENCES

Barakat A. O. and Yen T. F. (1988) Preliminary analysis of Monterey kerogen by mild stepwise oxidation with sodium dichromate in glacial acetic acid. Geochim. Cosmochim. Acta 52, 359-363. Bartuska V. J., Maciel G. E., Schaefer J. and Stejskal E. O. (1977) Prospects for carbon-13 nuclear magnetic resonance analysis of solid fossil-fuel materials. Fuel 56, 354-358. Barwise A. J. G., Mann A. L., Eglinton G., Gowar A. P., Wardroper A. M. K. and Gutteridg¢ C. S. (1984) Kerogen characterisation by 13C-NMR spectroscopy and pyrolysis-mass spectrometry. Org. Geochem. 6, 343-349. Behar F. H. and Vandenbroucke M. (1987) Chemical modelling of kerogens. Org. Geochem. 11, 15-24. Boucher R. J., Rafalska-Bloch J., Eglinton G. and Shaw P. M. (1991) Oxidation of natural organic material I: messel kerogen. Chem. Geol. Submitted. Burnham A. K., Clarkson J. E., Singleton M. F., Wong C. M. and Crawford R. W. (1982) Biological markers from the Green River Kerogen decomposition. Geochim. Cosmochim. Acta 46, 1243-1251. Chappe B., Michaelis W. and Albrecht P. (1980) Molecular fossils of archaebacteria as selective degradation products of kerogen. In Advances in Organic Geochemistry 1979(Edited by Douglas A. G. and Maxwell J. R.), pp. 265-274. Pergamon Press, Oxford. Douglas A. G., Hall P. B. and Solli H. (1983) Comparative organic geochemistry of some European Oil Shales. In ACS Symposium Series, No. 230, Geochemistry and Chemistry of Oil Shales (Edited by Miknis F. P. and Mackay J. F.), pp. 59-84. Durand B. (Ed.) (1980) Kerogen, 519 pp. Editions Technip, Paris. Eglinton T. I. and Douglas A. G. (1988) Quantitative study of biomarker hydrocarbons released from kerogen during hydrous pyrolysis. Energy Fuels 2, 81-88. Farrimond P., Comet P., Eglinton G., Evershed R. P., Hall M. A., Park D. W. and Wardroper A. M. K. (1984) Organic geochemical study of the Upper Kimmeridge Clay of the Dorset type area. Mar. Pet. Geol. 1, 340-354. van Graas G., de Leeuw J. W., Schenck P. A. and Haverkamp J. (1981) Kerogen of Toarcian Shales of the Paris Basin. A study of its maturation using flash pyrolysis techniques. Geochim. Cosmochim. Acta 45, 2465-2474.

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