Hydrous pyrolysis of sediments: Composition and proportions of aromatic hydrocarbons in pyrolysates

Advmces in Organic Geechemistry1985 Org. Geochem.Vol. IO, pp. 1033-1040. 1986 Printed in Great Britain. All rights reserved

Ol46-6380/86 S3.00+ 0.00 Copyright 0 1986 Pergamon Journals Ltd

Hydrous pyrolysis of sediments:Composition and proportions of aromatic hydrocarbonsin pyrolysates S. J. ROWLAND*, K. AARESKJOLDt, Gou XUEMIN and A. G. DOUGLASS Organic Geochemistry Unit, Department of Geology, Drummond Building, University of Newcastle, Newcastle upon Type, NE1 7RU, U.K. (Received

16 September

1985; accepted

18 February

1986)

Abstract-Hydrous pyrolysis (closed vessel autoclaving in the presence of excess water) of organic-rich rocks is said to generate oils which closely resemble natural crude oils in their broad characteristics and composition. However there are only a few accounts of the proportions and compositions of hydrocarbons in hydrous pyrolysates and none of these discuss the aromatic hydrocarbon composition in detail. The present pap& presents some data on the latter. Hvdrous nvrolvsis (3 davs) of a dolomitic siltstone (Penman. Marl Slate) at 280, 300,320, 340 and 360°C produced si&ficant amounts of oils in which the aromatic hydrocarbons were one and a half to two times as abundant as the saturated hydrocarbons. The overall composition of the aromatic hydrocarbons was similar to most crude oils; the major components isolated by our methods from natural oils and from pyrolysates were C,, alkylnaphthalenes. At the lowest pyrolysis temperature (280°C) the distributions of the more minor components of the pyrolysates (e.g. alkylphenanthrenes, aromatic steroids) were also generally similar to those found in natural crudes. However, a number of components (e.g. methylanthracenes, Diels’ hydrocarbon) which are not usually reported in crudes, were also detected and the relative proportions of these increased at the higher temperatures. Hydrous pyrolysis (340°C) of an organic-rich oil shale (Jurassic, Kimmeridge) and an asphaltic-material containing no minerals produced pyrolysates in which many of these unusual compounds were also present. In addition the pyrolysate of the oil-shale contained higher proportions of organic sulphur compounds. It appears that the formation of the unusual compounds is not simply a function of the type of organic matter or mineralogy but rather of the high temperatures or fast heating rates employed. Keywords:

hydrous pyrolysis, aromatic hydrocarbons, GC-MS, Marl Slate, oil shale, source rocks, Diels’

hydrocarbon

MTRODUCHON

Many petroleum geochemists have conducted heating experiments on sediments, and on organic com-

Winters ef al., 1983). Lewan and coworkers (Lewan et al., 1979; Winters et al., 1983) have noted considerable differences in the overall composition of oils recovered from hydrous pyrolysis experiments compared to those recovered from similar experiments with no added water. In particular they pointed out that the former oils did not contain alkenes whereas in the latter, alkenes were present (Lewan et al., 1979). The absence of alkenes in the hydrous experiments has been attributed to the confining effect of the water which allows the free radicals produced during heating to come into close contact with H radicals and hence become saturated (Monthioux et al., 1985). There is considerable evidence that the major hydrocarbons of hydrous pyrolysates are produced by free radical pathways (Almon and John, 1977; Eisma and Jurg, 1969; Hoering, 1983, 1984). Apparently the confining effect of the water in hydrous pyrolysis can be duplicated by pyrolysis in

pounds, in order to investigate the reactions which occur in the petroleum generation process (cf. for example, Eisma and Jurg, 1969; Hunt, 1979; Winters er al., 1983; Hoering, 1983, 1984; Monthioux et al., 1985). The method proposed to generate oils which are most like those that occur naturally, has been termed “hydrous pyrolysis” (Lewan et al., 1979). As described, this method involves heating sediments in a sealed pressurised vessel to temperatures of 280-360°C for periods of a few days in the presence of water. Such temperatures are very much greater than those at which natural petroleum genesis is thought to occur but it has been argued that, to some extent, temperature can be substituted for time in experimental oil generation studies (Connan, 1974; small sealed vessels (with space) without Present addresses: *Department of Environmental Sciences, Plymouth Polytechnic, Drake Circus, Plymouth PL4 8AA. U.K. PIKU, Postbox 1883, 7001 Trondheim, Norway and SPetrobangla Research Laboratory, Gulshan, Dhaka 12, Bangladesh. 1033

a minimum of unfilled added water (e.g. Monthioux et al.,

1985 and references therein). Most investigations of the detailed composition of oils produced by hydrous pyrolysis appear to have concentrated on the non-aromatic hydrocarbons. In this paper we present preliminary results on the

1034

S.

effects of hydrous carbons.

pyrolysis

on the aromatic

J. ROWLAND

hydro-

EXPERIMENTAL

Samples Marl

Slate

(Permian)

was a borehole

sample

collected

from NE England and provided by the Institute of Geological Sciences, Newcastle, U.K. Kimmeridge Oil Shale (Jurassic) was obtained from the Blackstone band, Dorset (Bet& 1981). Both sediments were chosen because they are organic-rich and of low thermal maturity. The Marl Slate contained 7.3% total organic carbon and the Kimmeridge at least 25%. Vitrinite reflectance data indicate both samples have experienced only a low degree of heating (e.g. Dungworth, 1972 for Marl Slate; Winters et al., 1983); the Marl Slate sample had a T,,, of 416°C. A mineral-free Wurtzilite-like asphaltic material was collected from Ralen Mine, Duchesue Country, Utah, U.S.A. and was soxhlet extracted with dichloromethane (41 hr) prior to use. Pyrolysis Sediment samples (100 g Marl Slate or Kimmeridge oil shale) were crushed and sieved and the 9.5-3.35 mm fraction was hydrously pyrolysed at varying temperatures for 3 days using a 2.5” High Pressure 1OOOml “bomb” (Parr Instrument Co.). One sample of Marl Slate siltstone was pyrolysed for 24 days at 290°C. After pyrolysis the oil floating on top of the water was collected and the remaining expelled pyrolysate extracted with dichloromethane. Residual pyrolysed sediment was dried at 25”C, crushed and solvent extracted by soxhlet for 24 hr with dichloromethane: methanol 93 : 7. The extracted residue of the asphaltic material was also hydrously pyrolysed at 330°C for 3 days using the apparatus and procedure above. After pyrolysis of this sample the oil floating on the water surface was collected. The residue remaining in the vessel was also collected and was completely soluble in dichloromethane and when examined by gas chromatography both oils appeared to be very similar in composition. Therefore only the floating oil was examined further. All of the pyrolysates (Marl Slate, Kimmeridge oil-shale, asphaltic material) were further fractionated by column chromatography on silica gel:alumina (3: 1). The “aliphatic”, “aromatic”, “NSO”, and “polar” fractions were obtained by eluting with light petroleum, benzene, dichloromethane and methanol, respectively. Aliquots of the benzene eluate were examined by gas chromatography (GC) and GC-mass spectrometry (GC-MS). Aliquots of the benzene fractions were separated further by thin layer chromatography TLC, silica gel G, 0.3 mm; hexane: diethyl ether 95:s developer) and a pale-yellow aromatic fraction (R, > phenanthrene) was recovered by desorbing with dichloromethane. These fractions were then examined by GC and GC-MS.

A second

aliquot

of the benzene

eluate

from

et al.

column chromatography was separated further by TLC on activated alumina (hexane developer) giving “diaromatic” (R, = naphthalene) and “triaromatic” (R, = phenanthrene) fractions, each of which was examined by GC and GC-MS. GC and CC-MS GC was performed using a Carlo Erba Fractovap 2160 chromatograph fitted with a 30 m x 0.3 mm i.d. fused silica capillary column coated with DB-5 (J. & W. Scientific Inc.): thk oven temperature was programmed from 50-300°C a; 4°C min-‘. Hydrogen was used as carrier gas and injections were made in the splitless mode. Equimolar mixtures of n-G428 alkanes, examined on a daily basis, were used to calibrate peak responses. GC-MS was performed on two instruments: (1) a Varian 1400 gas chromatograph coupled to a VG Micromass 12B mass spectrometer; data were acquired and processed using a VG data system 2000; injections were made in a splitless mode onto a 30 m x 0.3 mm i.d. 5% phenylmethylsilicone (cross-bonded) fused silica column (Hewlett-Packard) with helium as carrier gas. Oven temperature was programmed from 70 to 280°C at 4” min-‘. The mass spectrometer scanned a O-460 a.m.u. mass range every c 2 sec. 0.2 mA emission current, 50eV electron voltage, 200°C source temperature. (2) Two fractions were also examined on a Finnigan 4000 mass spectrometer with “on-line” INCOS 2000 data system, coupled to a Carlo Erba “Mega” gas chromatograph. Chromatographic conditions were as above and the same column was used. Injections were made “on-column”. Mass spectrometer conditions were: 50-600 a.m.u. scanned every 1 set, 0.35 mA emission current, 40eV electron voltage, 250°C source temperature. Compounds discussed in the text were identified by co-chromatography with synthesised compounds and by comparison of their mass spectra with those of authentic compounds (trimethylnaphthalenes, phenanthrene, anthracene, methylphenanthrenes, methylanthracenes, Diels’ hydrocarbon; Rowland et al., 1984; Hoffmann, 1984) or by comparison of selected mass fragmentograms with previously authenticated distributions (triaromatic steroids; Wardroper et al., 1984).

RESULTS

AND

DISCUSSION

Hydrous pyrolysis of the Marl Slate produced variable amounts of hydrocarbons depending on temperature (Table 1). The amount of hydrocarbons produced increased with temperature up to 340°C and then decreased at 360°C probably due to “cracking” to more volatile material which was not examined by our methods. The amount of aromatic hydrocarbons produced at each temperature was higher than that of the

Table I. Amounts of hydrocarbons produced by hydrous pyrolysis of Marl Slate (3 days) Amount

temperature (“C) Unheated shale 280 300 320 340 360

of

aromatic

pyrolysis

hydrocarbons (mg/g dry wt sediment)

Amount of saturated . hydrocarbons (mg/g dry wt sediment)

I .34 3.00 5.12

0.94 1.69 3.79

12.25

6.12

12.77 6.85

7.89 4.50

*Aromatic/ saturate ratio 1.4 1.4 1.4 2.0 1.6 1.5

*Hydrous pyrolysis of the asphaltic material (3 days, 340°C) produced a pyrolysate in which the aromatic:saturate ratio was 5: I. and hydrous pyrolysis of Kimmeridge oil shale (3 days, 330°C) produced

a pyrolysate

in which

the aromatic:saturate

ratio was 6: I.

Aromatic hydrocarbonsfrom hydrous pyrolysis Table I. 2. 3. 4. 5. 6. 7. 8. 9. IO. I I. 12. 13. 14. IS. 16. 17.

IS. 19. 20. 21. 22. 23. 24. 25. 26. 27.

2. Compounds identified in pyrolysales Slate 340°C. 3 days)

graphic data shown by Lewan et al. (1979) and Winters ef al. (1983) for a variety of organic-rich sediments. For instance, pyrolysis of Woodford Shale produced mainly saturated hydrocarbons (Lewan et al. (1979) whereas increased proportions of aromatics were present when Phosphoria and Kimmeridge shales were pyrolysed, judging from chromatograms shown by Winters et al. (1983). For a given sediment the temperature of pyrolysis also alters the aromatics: saturates ratio (Table 1) and in natural crudes migrational factors may also have an effect. The hydrous pyrolysis conditions needed in order to most closely reproduce formation of crudes depend on the particular sediment examined. Nonetheless, our preliminary data do suggest that the high temperatures employed in hydrous pyrolysis tend to produce oils with unusually high proportions of aromatic hydrocarbons.

(Marl

2-methylnaphthalene I-methylnaphthalene Biphenyl 2-ethyl+ I-ethylnaphthalene 2.6 + 2,7-dimethylnaphthalene (DMN) C,-benzothiophene I.3 + I,7-DMN I,6-DMN C,-benzothiophene 2.3 + I.S-DMN I ,2-DMN C,-tetralin + I ,3,7-trimethylnaphthalene 1,3.6-trimethylnaphthalene (TMN) C,-benzothiophene I ,4.6 + I ,3,5-TMN 1.2.6 + 1.2.7 + 1,6,7-TMN I ,2,5-TMN C,-naphthalene C,-naphthalene C,-naphthalene C,-naphthalene + phenanthrene 3-methylphenanthrene + I-methyldibenzothiophene 2-methylanthracene 9 + Cmethylphenanlhrene + I-methylanthracene Unknown C,-phenanthrene Diels’ hydrocarbon

Peak numbers

are for chromatogram

1035

Composition

in Fig. I.

saturated hydrocarbons (Table I). This dominance of aromatics probably partly reflects the particular type of immature organic matter present in the unheated Marl Slate. However, this is unlikely to be the sole influence since high proportions of aromatics were also formed when the oil-shale and asphaltic material were pyrolysed (Table 1). Variations in amount of aromatic components with organic matter type were also apparent from quantitative and chromato-

of aromatic fractions

GC and GC-MS of aromatic hydrocarbon fractions isolated from pyrolysates of each sample (siltstone, 280, 320, 340, 360°C; oil shale, 330°C; asphaltic material. 340°C) revealed a complex distribution of components (e.g. Fig. l), the major members of which were C,_, alkylnaphthalenes (e.g. structure I; peaks 1, 2, 5, 7, 8 and 13); C2 and C, benzothiophenes (e.g. II; peaks 6, 9 and 14); C,_, alkylphenanthrenes (e.g. III; peak 24) and C, and C, dibenzothiophenes (e.g. V). Compound structures are given in Roman numerals and shown in Fig. 6. These compounds are all common constituents of crude oils. The overall appearance of the chromatograms of

17

27

RIC

3000 l6:40

33~20

Jo:00

4000 66:40

Sean Time

Fig. 1. GC-MS reconstructedion chromatogram of aromatic hydrocarbonsisolatedfrom pyrolysateof Marl Slate(34O”C,3 days). For peak identificationsseeTable 2. For conditions see Experimental.

1036

S.

J.

ROWLAND

et al.

(a) P

I

m/r

(b)

(c)

p

17%

m/z

192

m/r

178

A

i,i;

Fig. 2. GC-MS mass fragmentograms (m/z 178, 192) showing distributions of phenanthrene (P), anthracene (A) and methyl isomers (e.g. 2’ = 2-methylanthracene; 2 = 2-methylphenanthrene etc.) in pyrolysates of Marl Slate. (a) and (b) 280°C 3 days. (c) and (d) 340°C 3 days.

the siltstone and asphaltic-material pyrolysate fractions was very similar to that of similar fractions of many crude oils (e.g. Overton er al., 1979; Fedorak and Westlake, 1981; Oudot et al., 1981; Alexander et al., 1983; Jones ef al., 1983) but the pyrolysate of the oil shale contained somewhat higher proportions of S-aromatics than most natural crudes. The aromatics probably originate (in both naturally-formed crudes and in the pyrolysates) from a number of biologically derived precursors, including many isoprenoids (e.g. Douglas and Mair, 1965; Ishiwatari and Fukushima, 1979; Mackenzie et al., 1983). In addition to the above components a number of aromatic hydrocarbons which are not normally reported in natural crudes were detected in the pyrolysates. These included anthracene (IV) l- and 2methylanthracene (peak 24; Fig. 1) and Diels’ hydrocarbon (VI; peak 27, Fig. 1; see also Figs 2 and 3). The presence of these compounds in pyrolysates of

the sediments and asphaltic-material suggests that their formation may be quite general and not simply a function of mineralogy or organic matter type. Indeed when the siltstone was pyrolysed at a variety of temperatures (280, 300, 320, 340 and 360°C; 3 days) the proportions of these unusual hydrocarbons were found to increase suggesting that they are produced as a result of the high pyrolysis temperatures..For example Figs 4a,b show the increases in the relative proportions of antracene to phenanthrene and Diels’ hydrocarbon to phenanthrene respectively. Values are given for expelled pyrolysate (E) which is that material which floats on the water after pyrolysis (plus what is obtained by washing the grains and the equipment with solvent) or for adsorbed bitumen (B) which is material extracted by soxhlet extraction from the crushed rock after pyrolysis. Values are also given for the sample treated for 24 days for comparison.

Aromatic hydrocarbons from hydrous pyrolysis

1037

100

12032

l

50

2%

57

108 189 I

70I. I

M/E

95I .?A I I, 100

. I

126 I

165 ‘7e I I ,

I,, 150

I

,, 200

250

300

Fig. 3. Mass Spectrum (electron impact) of component in pyrolysate of Marl Slate (28O”C, 3 days) identified as Diels’ hydrocarbon. For conditions see Experimental.

High proportions of anthracene and 2- and I-methylanthracene relative to phenanthrene were also observed when various coals were pyrolysed (35OT, 1 day; Radke et al., 1982). Amongst the possible explanations given for this was thermocatalytic rearrangement of partially hydrogenated precursors. Some partially hydrogenated aromatics

(e.g. a C4 tetralin, peak 12, Fig. 1) were present in some of the pyrolysates in the present study. Anthracenes have been found in crude oils (e.g. Carruthers, 1956; Douglas, 1963) but are commonly only very minor components relative to phenanthrenes. Diels’ hydrocarbon has rarely been reported in crude oils (reviewed by Hoffman, 1984) although it

1

0.f

la) 89 *E

0:

1 / Xi

D/P

04 /

A/P

O.!

0.2

0.1

0.c

260

300 Temprrturr

320 PC)

, 340

, 360

0

9.

’ 260

I 300 Temperature

I 320 PC

I 340

I 360

1

Fig. 4. (a) Proportion of anthracene: phenanthrene produced by hydrous pyrolysis B = bitumen, E = expelled pyrolysate; superscript indicates proportions (%) of B and E relative to each other. 0 = 24 days pyrolysis. (b) as (a) above but showing proportion of Diels’ Hydrocarbon:phenanthrene.

S. J. ROWLAND er

1038

al.

(a)

Fig. 5. (a) GC-MS reconstructed ion current chromatogram showing relative proportions of Diels’ Hydrocarbon (C,,) and C26-28triaromatic steroidal hydrocarbons in hydrous pyrolysates of Marl Slate. (a) 280°C 3 days. (b) 340°C 3 days. For conditions see Experimental.

was found in Ponca City crude (Mair and MartinezPica, 1962). It is a common catalytic dehydrogenation product of steroids (Robinson, 1964). At the highest pyrolysis temperature the proportion of Diels’ hydrocarbon decreased. Degradation of Diels’ hydrocarbon by thermal cracking of the D ring yields alkyl phenanthrenes (Martinez-Pica, 1962) and several C, and C3 phenanthrenes (or anthracenes) were abundant in the pyrolysate at this temperature.

II

I 0 00 09

c@$Q

III

Q-Jy

I” V

R-Y

Minor components present in the 280°C siltstone pyrolysate (but not in pyrolysates produced at the higher temperatures) were a series of triaromatic steroidal hydrocarbons (e.g. VII; seeannotated peaks in Fig. Sa). These compounds are common in natural crude oils (e.g. Mackenzie et al., 1981; Rullkotter et al., 1985) where they are thought to originate from aromatisation of monoaromatic steroids. Eventually, given a severeenough temperature history, the Czaezs triaromatic steroids are thought to undergo homolysis of the side chain to give C,, and C*, counterparts (Mackenzie et al., 1981). However, this was not found to be duplicated by our experiments since C, and C,, compounds were not abundant in the pyrolysates. In fact at 340°C the only triaromatic ‘steroid’ present in the siltstone pyrolysate was Diels’ hydrocarbon (Fig. 5b). These results suggest the possibility of a product-precursor relationship between the CzGz8 aromatics and Diels’ hydrocarbon (C,,) in our pyrolysis experiments but it is likely that at least some of the latter originates from degradation of other functional&d steroids which may have been present in the siltstone prior to pyrolysis. Alternatively it may be that Diels’ hydrocarbon survives pyrolysis whereas the other components are degraded to lower molecular weight hydrocarbons (Fig. 5). CONCLUSIONS

VI

Fig. 6. Compounds structures (I-VII)

VII

discussed in text.

Hydrous pyrolysis of organic-rich sediments (Marl Slate, Kimmeridge oil shale) and asphaltic material at 280-360°C produces oils with higher proportions of aromatic-to-saturate hydrocarbons than natural crude oils. Whilst

both the type of organic

matter

Aromatic hydrocarbons from hydrous pyrolysis present and the pyrolysis temperature probably influence this ratio it is concluded that the high proportion of aromatics is partly a result of the high temperatures or fast heating rate employed in the experiments. The overall distributions of aromatic hydrocarbons produced by hydrous pyrolysis were similar to those observed in many natural crudes and alkylnaphthalenes and alkylated sulphur-containing aromatics were the major components isolated. However a number of aromatic compounds which are not abundant in natural crudes were also produced. These included anthracene, methylanthracenes and Diels’ hydrocarbon. The proportions of each of these relative to phenanthrene increased with increased pyrolysis temperature. It is concluded that these compounds also result from the high temperatures used in the pyrolysis experiments compared to the temperatures associated with natural oil generation. Acknowledgemenfs-We are grateful to Drs J. R. Maxwell and C. F. Hoffman (Bristol University) for a sample of synthetic Diels’ hydrocarbon and to Professor G. Eglinton and staff (Bristol-University) for use of the NERC funded GC-MS (Finniean). We thank Mr P. Donohoe and Mr I. Harrison ‘for &hnical assistance and Drs P. Comet, A. Mann and A. Barwise for useful discussions. Part of this work was sponsored by a British Petroleum EMRA award (S. J. Rowland). Financial support from the Continental Shelf Institute, Norway (IKU); -Norwegian Council of Science and Technology (NTNF) and NATO Double Jump Programme is gratefully acknowledged (K. Aareskjold).

Hoering T. C. (1984) Thermal reactions of kerogen with added water, heavy water and pure organic substances. Org.

Geochem.

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