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Tetracyclic diterpenoid hydrocarbons in some Australian coals, sediments and crude oils
OOIO-7037/85/13.00 + .W
Geochrmrca n Cosmochlmrca Acro Vol. 49, pp. 2141-2147 0 Pcrgamon PrcsLtd.1985. Printed inU.S.A.
Tetracyclic diterpenoid hydrocarbons in some Australian coals, sediments and crude oils ROHINTONA. NoBLE*‘~, ROBERTALEXANDER-t, ROBERTIAN Ucilt
and JOHNKNox*
*Department of Organic Chemistry, University of Western Australia, Nedlands, 6009, Western Australia tSchoo1 of Applied Chemistry, Western Australian Institute of Technology, Kent Street, Bentley, 6 102, Western Australia (Received March 19, 1985; accepted in revised form July 15, 1985)
Abstract-Tetracyclic diterpenoid hydrocarbons (diterpanes) based on the enf-beyerane, phyllocladane and ent-kaurane skeletons have been identified in the hydrocarbon extracts of some Australian coals, sediments and crude oils. Structures were assigned to the geological diterpanes by comparison with synthetically prepared reference compounds. Studies of a sample suite consisting of low-rank coals and sediments indicate that the ratios of C-16 epimers of phyllocladane and ent-kaurane are maturity dependent, and that the relative proportion of the thermodynamically preferred 168 (H)-compounds increases with increasing thermal maturity. Thermodynamic equilibrium for the interconversion reactions is attained in sediments before the onset of crude oil generation. The most likely natural product precursors for the tetracyclic diterpanes are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Tetracyclic diterpanes have been identified in sediments and coals of Permian age or younger, suggesting that these compounds are markers for both modem and extinct families of conifers. In particular, phyllocladane is proposed as a marker for the Podocarpaceae family of conifers. INTRODUCI’ION
THETETRACYCLIC diterpenoids natural products
form a large group of which occur widely in higher plants.
They have been classified on the basis of structural similarities into groups which include the kaurenephyllocladene class, the stachene class, the gibberellins, the diterpene alkaloids and the aconite alkaloids (HANSON,1968). The parent hydrocarbons of the kaurene-phyllocladene and stachene classes occur as mono-unsaturated alkenes, and are particularly abundant in leaf resins of conifers belonging to the Podocarpaceae, Araucariaceae and Cupressaceae families (APLINet al., 1963; HANSON, 1968; THOMAS,1969). Despite the widespread occurrence of tetracyclic diterpenoids in nature, there are relatively few reports of their occurrence in sedimentary organics. BRICKS ( 1937) summarized the occurrence of the hydrocarbon iosene, which had been isolated from various Iignites. This compound, although of unknown structure at that time, was shown to be identical with “ardihydrophyllocladene”. More recently, the mineral Bombiccite, isolated from lignite deposits near Florence, was chemically identified by crystal structure analysis as I6a( H)-phyllocladane (see Fig. 2) or “a-dihydrophyllocladene” (SERANTONIet al.. 1978). HAGEMANNand HOLLERBACH ( 1980) also report high concentrations of tetracyclic compounds of the kaurane or phyllocladane type in brown coals from West Germany. The reports of tetracyclic diterpenoids in crude oils and sediments are less conclusive. HATCHERet al. ( 1982) reported traces of kaurane in sediments from Mangrove Lake, Bermuda, and SIMONEIT(1977) reported kaurene and iosene in mrne deep-sea sediments. In both reports, no details of the method of identification were presented. LIVESY ef al. (1984) have presented mass spectral data for a tetracyclic diterpane, which they
suggest might have a kaurane skeleton. Compounds with similar mass spectra to that of LIVESYet al. ( 1984) were first observed by PHILPef al. (198 1, 1983). who found these compounds in sediments and crude oils from S.E. Australia. In this study we have examined some Australian coals, sediments and crude oils for the presence of tet-
racyclic diterpenoid hydrocarbons (tetracyclic diterpanes), and have assigned structures to the diterpanes by comparison with synthetically prepared reference compounds. The effect of thermal maturation upon the relative proportions of epimeric diterpanes was assessed by comparing the distribution of compounds in sediments and coals of differing maturity. We also discuss some aspects of the significance of tetracyclic diterpanes as chemical fossils. EXPERIMENTAL Samples studied Three coals, two shales and a crude oil were selected as representative of a larger suite of samples which have been analysed for tetracyclicditerpanes. Details of these six samples are given in Table I. Sample I is Yalloum lignite, a brown coal from the onshore part of the Gippsland Basin, S.E. Australia. There are five basic lithotypes recognizable in this coal
(VERHEYEN et al., 1982),of which we have analysedthe pale lithotype. Sample 2 is a subbituminous coal from the opencut mine in the Muja sub-basin at Collie, Western Australia, and sample 3 is a coal from a deeper section of the neighbouring Cardiff sub-basin. Samples 4 and 5 are shales from offshore sites in the Gippsland Basin, and sample 6 is a Gippsland Basin crude oil from an associated sandstone. The geographical location of these samples is shown in Fig. 1. Isolation of branched and cyclic alkane fractions
Our standard method for the isolation of branched and cyclic alkanes from petroleum extracts has been reported previously (VOLKMAN et al.. 1983). Briefly, sediment samples
2141
R. A. Noble et ai
2142 Table 1. Sample Number : 3 4 5 6
Sample details for coals, shales and a crude 011
Sample Sedimentary name/description Basin va11ourn coal Muja coal Cardiff coal G 3673 m shale G 4536 m shale G 3756-83 m Crude oil
Location
Fomlatlon we
Gippsland Collie Collie Giopsland Gibbsland
Victoria. Australia Western Australia Uestern Australia Victoria. Australia VIctoriai Australia
Miocene Permian Permian Cretsceous Cretaceous
Gippsland
Victona,
Cretaceous
Australia
(1)
(2)
ent-beyerane were crushed and extracted with dichloromethane to afford the soluble organic matter. Aliphatic hydrocarbons were separated by column chromatography on silicic acid using npentane as solvent. The branched and cyclic compounds were then isolated by removal of n-alkanes with 50 nm molecular sieves.
phyllocladar?e a.R,=H b:R,sCHg
ml2
123
R2-CHB R*=H
m/2,23!
Gas chromatography-massspectromelry(GC-MS) K-MS analyses of branched and cyclic aliphatic hydrocarbons were carried out using an HP5895B capillary GC-
quadrupole MScomputer data system. The CC-MS was fitted with either a 50 m X 0.2 mm id. WCOT fused-silica crosslinked methylsilicone column (Hewlett-Packard), or a similar 5% phenylmethylsilicone capillary column (Hewlett-Packard). A 50 m X 0.2 mm i.d. WCOT fused-silica XEdO-S-valine-Sa-P.E.A. capillary column (Chrompak) was used for some analyses. Samples for analysis were dissolved in n-hexane and injected on-column at 50°C. The oven was then heated to 300°C at 4”C/min., using hydrogen carrier gas at a linear flow velocity of 28 cm/set. Samples were analysed either in the data acquisition mode, by scanning from 50 to 450 a.m.u. in 1.3set cycles, or by selected ion monitoring (SIM), using dwell times of 10 msec for each ion monitored. Typical MS operating conditions were: EM voltage 2200 V: electron energy 70 eV; source temperature 250°C.
RESULTS AND DISCUSSION Mass spectra of diterpenoid hydrocarbons
Tetracyclic diterpanes based on the ent-beyerane (I), phyllocladane (2) and ent-kaurane (3) skeletons (Fig. 2) have been synthesized from natural product precursors, details of which have been presented elsewhere (NOBLE et al.,1985). En&eyerane (I), derived from ent-beyerol (JEFFERIESet al., 1963), is identical to the saturated derivative of (-)-hibaene, which in turn is the parent hydrocarbon of the stachene class of tetracyclic diterpenes (HANSON, 1968). The mass spectra for ent-beyerane ( 1), 16~1(H)-phyllocladane (2a), 168 (H)-phyllocladane (2b). ent- 16a (H)-kaurane (3a) and
ant-kaurane a:R,
=H
R2=CH3
b:R,=CH3
RpH
FIG. 2. Molecular structures for tetracyciic diterpanes showing important fragmentation sites observed in mass spectral analysis.
ent-168 (H)-kaurane (3b) are shown in Fig. 3. Each spectrum shows a base peak corresponding to m/z 123, and a molecular ion of m/z 274 (C&,H&. The base peaks result primarily from the rupture of the B-ring. by cleavage of the 5-6, 9-10 bonds (Fig. 2). Characteristic differences in relative ion abundance are apparent in the mass spectra of the diterpanes. Beyerane can be readily distinguished from phyllocladanes and kauranes by the increased abundance of m/ z 245 ions in its mass spectrum. This ion results from cleavage of the 8- 15, 13- 16 bonds, through loss of a CZ unit. The corresponding bond cleavage in phyllocladanes and kauranes results in loss of a C, fragment, producing m/z 231 ions. The m/z 231/259 ion ratio is greater for phyllocladanes than for kauranes, particularly for 1613(H)-phyllocladane (2b). Consequently, phyllocladanes are characterised by an m/z 231/259 ion ratio above 0.50, whereas the corresponding ratio for kauranes is less than 0.35. Kauranes can also be conveniently distinguished from phyllocladanes by the m/z 2591274 ion ratio, for which kauranes have a value greater than 1.20. (XV-MS analwc
COLLIE
BASIN
VALLOU~~v\i._~IPPSLAND
FIG. I. Geographical location of samples.
BASIN
A mixture containing the five synthetic tetracyclic diterpanes was analysed by GC-MS, using selective ion monitoring of m/z 123 ion abundance. A section of the resulting m/z 123 mass fragmentogram is shown
in Fig. 4. The peaks due to the five compounds were readily resolved using apolar capillary columns al-,
Tetracyclic diterpenoids in Australian
2143
cod
a-beyerane
(ii)
,oo
16ol(Hl-phyllocladane RD 40 -
259 274
20 *
16@ (HI-phyllocladane
z 0
(iv) -ent-lGcu(H)-kaurane
loo-
80 _ 60 40 20. o50
206
150
100
250
gnt- 1 G@(H)-kaurane
m/z FIG.
*
3. Mass spectra for tetracyclic diterpanes (i) ent-beyerane (if; (ii) 16a{H~phyll~ladane (2a); (iii} if$
(H)-phyltocladane (Zb); (iv) enf-16a (H)-kaurane (3a); (v) en&t68 (H)-kaurane (3b).
though there was only a small difference in retention times between peak C and peak D, co~es~nding to ent-16p (H)-kaurane (3b) and l&x (~)-phyllocladane (2a), respectively. Kovritf indices were calculated under isothermal (150°C) GC conditions, and these values are given in Table 2. Aliphatic hydrocarbons derived from coals, shales and crude oils were similarly analysed for tetracyclic diterpanes. However, the m/z 123 fmgmento~am proved to be less useful for this purpose, as significant interference was observed in some sampies due to co-
eluting bicyclic and tricyclic diterpenoids. A more specific analysis of tetracyclic compounds in these geological extracts was obtained by monitoring m/z 259 ion abundance. Figure 5 shows an expanded section of the m/z 259 mass fragmentogram for a shale extract from the Gippsland Basin, Australia. The compounds represented by the peaks labelled in this trace were identified as tetracyclic diterpanes, by comparison of mass spectral data with those of the authentic compounds, and by precise coelution with the corresponding standards
2144
K. A. Noble el al.
m/z
mlz
TETRACYCLIC
123
259
Gippsiand
A
3673m
DITERPANES
Bastn shale
E
42
60
Retention
time
(min)
containing synthetic tetracyclicditerpanes. Peak A: enf-beyerane(I ); peak B: 168 (H)-phyllocladane (2b); peak C: enf-168 (H)-kaurane (3b); peak D: 16a (H)-phyllocladane f2a); peak E: en!-lhn (Htkaurane (3a).
Retention
FIG. 4. m/z 123 mass fragmentogram for mixture
predominantly
of phyllocladane
the 16/j (H)-configurations
and ent-kaurane.
The ratios of 16~~(H)/16p (H)-phyllocladanes and ent-kauranes for the six samples examined in this study are given in Table 3. The accompanying matunty data for each sample. based on H/C ratios, vitrinite reflectanes. and sterane and hopane molecular parameters. indicate that the samples are listed in order of increasing maturity, from Yalloum lignite (lowest) to the Gippsland crude oil (highest). The maturity values for the
(i) m/z
.......
0
d,.~._.,.-.,, , ‘_
..,
-‘-*+ ,?.‘%..V
ent-beycrsnc x(H)-phyllocledane 168(H)-phyl10cladane ent-16u(H)-kaurane a-16tl(H)-kaurene
:a 2b
Nesit 1929 1971 1955 1991 1967
a.
5h x 0.2nm i.d. WCOT fused-silica cross-linked methylsilicone coluam (Hewlett-Packard)
b.
5&n x 0.2mn i.d. WCOT fused-silica cross-linked 5% phenylarthylsilicone colunm (Hewlett-Packard)
.,*,,t..
_*,,,
/_,_
..,_,
Cardiff coai
/B
G4536m shale D
__ I_~_~-~-.l
?
krlL_.-de I -....;
?B
+-~
G3756-83m
crude
oil
1 r, I: D ,j, ‘.. A __.___~ i _.... -.___..~ ~../_ ..__ ..__+_,__‘.i CJ:’ ,.~_ A
b 5% Phmesil 1947 1988 1973 2011 1986
. ..
ID
“c’
(iv)
Structure
,
.’
.‘,‘,L’,VJ.
(iii)
Kovits indices (15O'C) for tetracyclic diterprnes
Compound NaiVa
IIgnite
E
(ii)
Table 2.
Va!iourn
259
Maturity eflects in some coals, shales and crude oils Figure 6 shows expanded sections of the m/z 2% mass fmgmentograms of isolates from two coals, a shale, and a crude oil (samples 1, 3, 5 and 6, Table I ). The major tetracyclic diterpane in these two coal samples was identified as 16a (H)-phyllocladane (2a), which is consistent with previous reports of this compound as the mineral Bombiccite in lignites (SERANTONI PI al., 1978). Enfl6cu (H)-kaurane (3a) was also identified as a prominent tetracyclic diterpane in Yallourn lignite. The shale and crude oil samples, on the other hand.
(min)
FIG. 5. Expanded section of m/z 259 mass fragmentogram for 3673 m Gippsland shale. For peak assignments refer to Fig. 4 caption.
contained
on two apolar capillary columns. A third coinjection experiment, using a commercially available chiral capillary column (XE-60-S-Valine-S-a-P.E.A.. Chrompak) also resulted in precise coelution of identical compounds. Although we have not yet been able to demonstrate separation of enantiomeric terpenoid hydrocarbons with this column. we have assigned the geological diterpanes the same absolute stereochemistry as those of our synthetic reference compounds. These assignments remain equivocal for kaurane and beyerane. since their higher plant precursors are known to occur in both enantiomeric forms (HANSON, 1968). On the other hand, the precursors of phyllocladane belong to one enantiomeric series (“normal” A/B ring fusion). and the assignment of absolute stereochemistry is therefore more conclusive.
time
42
60 Retention
time
(min)
FIG. 6. Expanded sections of m/z 259 mass fragmentograms for (i) Yalloum lignite; (ii) Cardiff coal; (iii) Gippsland 4536 m shale; (iv) Gippsland 3756-83 m crude oil. For peak assignments refer to Fig. 4 caption.
2145
Tetracyclic diterpenoids in Australian coal TABLE 3:
Ratios of epimeric diterpanes and maturity parameters for coal, shale and crude oil samples
No
: 3 4 z
a. b. C.
d. e.
Name
Description
va11wm hja Cardiff G 3673m G 4536m G 3756-83m
lignite HVSB coal HVSB coal shale shale crude oil
EPIMER RATIOS
MTURITY
SAMPLES
H/Ca ratio
Vitrinite Reflectance (Ro%)
Steranesd 20Sl20R
1.21 1:; :: ND
ONqD3b 0.63b 0.72' o.8gc NA
N": 0:: 7::
Hopanese
:::
2.0 co.1 co.1 co.1
Phyllocladanesf
1::
12.8 0.3 :::
s-Kauranesg
J
1.6 0.7 co.1 co.1 co.1
Elemental carbon and hydrogen composition determined by Mel Microanalytical Services (5 Australia) Vitrinite reflectance data from K Sappal, School of Physics and Geosciences, W.A.I.T. (unpublished results). Vitrinite reflectance data from Stainforth (1984). The ratio of ZOS/ZOR for 5a(H),14a(H).17a(H)-24-ethylcholestanes measured fran m/z 217 mass fragmentograms (see Mackenzie et al., 1980). The ratio of 176(H),216(H)~H),218(H) for C 10 hopanes measured from m/z 191 mass fragmentograms (see Mackenzie et a1.,1980). The ratio of 16ml66(H) for phyllocladanes measured from m/z 259 mass fragmentograms. The ratio of 16a(H)/166(H) for ent-kauranes measured fran m/z 259 mass fragmentograms. Infinite ratio. since onlv the T&(H)-eoimers of ohvllocladane and ent-kaurane were detected. Not determined. ..~I
.z . .
HVSB.
High volatile sub-bituminous
3673 m Gippsland shale suggest that this sample was of maturity equivalent to that at the onset of crude oil generation, and those of the 4536 m Gippsland shale, suggest that this sample was at peak oil generation. For the three coals, the proportion of ent-16a (Htkaurane relative to the 168 (Htepimer decreases with increasing coal-rank. The corresponding change in ratio of phyllocladane epimers was only evident in the more mature Cardiff coal, suggesting that the processes leading to the formation of 16p (H)-phyllocladane commence at a later stage of maturation than those corresponding to ent- 168 (Hkkaurane. The ratios of diterpane epimers in the 3673 m shale were significantly different from those of the low-rank coals, but were effectively the same as those in the more mature 4536 m shale and in the crude oil. Assuming that the 168 (H)-epimers of phyllocladane and enf-kaurane result from the epimerization of 16a (H)-compounds, our observations suggest that equilibrium for these reactions is reached before the onset of crude oil generation (vittinite reflectance 0.7%; STAN=~RTH, 1984). In summary therefore, studies of this sample suite indicate that the ratios of C- 16 epimers of phyllocladanes and kauranes are maturity dependent, and that the relative proportion of 16p (H)-compounds increases with increasing thermal maturity. The dynamic range of this process appears to encompass regimes of low thermal stress. such as those associated with the early stages of coalification. Thermodynamic equilibrium for the interconversion reactions appears to be attained when the 16a (H)/16@ (H) ratio reaches a value of 0.3 for phyllocladanes, and ~0. I for enr-kauranes. This occurs before the onset of crude oil generation. The observed changes in the ratios of phyllocladane and kaurane epimers can be rationalized in terms of relative thermodynamic stabilities of individual compounds. Force field calculations using the ALLINGER (1977) method have shown that phyllocladanes and enf-kauranes epimeric at C- 16 have significant differences in their free energy of formation, the 160 (H)-
configuration being the thermodynamically more stable. These differences in stability are reflected in the ratios of epimeric compounds in geological samples, in which the relative concentration of less stable 16a (H)-compounds is diminished on maturation with respect to the thermodynamically preferred 168 (H)compounds. The 16a (H)-configuration of phyllocladane and ent-kaurane, found initially in samples of low maturity, was considered to be the configuration attained through early diagenetic reduction of unsaturated diterpene precursors. In support of this. laboratory experiments involving catalytic hydrogenation of phyllocladene (2, Au’,“) and en&kaurene (3, A16.“) produced as their major products, 16a (H)-phyllocladane (2a) and ent-16a (H)-kaurane (3a), respectively (NOBLE et al., 1985, and references therein). We therefore conclude that the 16cu(H)-epimers are the kinetically controlled products, and the 16/3 (H)-epimers the thermodynamically controlled products. Tetracyclic diterpanes as chemical fossils The occurrence of tetracyclic diterpenoids containing the beyerane, phyllocladane and kaurane skeletons in higher plants is well documented (HANSON, 1968. 1972, and references therein). Compounds of this type occur naturally as alkenes, alcohols, ketones. esters, acids and more complex polyfunctional derivatives. The tetracyclic diterpanes found in coals, sediments and crude oils are derived from these higher plant diterpenoids, by diagenetic removal of their functional groups. However, within the large group of tetracyclic diterpenoids, there are probably only a limited number of compounds which would survive diagenetic transformations with complete preservation of skeletal units. The compounds which are least likely to suffer structural alteration during diagenesis are the tetracyclic diterpene hydrocarbons. Compounds such as kaurene, phyllocladene and beyer- I S-ene (hibaene) would be readily converted into saturated derivatives by diage-
2146
R.
A. Noble et ul.
netic reduction of their double bonds. and would therefore appear to be the most likely source of the sedimentary tetracyclic diterpanes. Tetracyclic diterpene hydrocarbons occur abundantly in the leaf resins of conifers (APLIN et al., 1963: THOMAS. 1969; HANSON, 1968). It is therefore likely that the tetracyclic diterpanes are markers for coniferresin contributions to sedimentary organic matter. Furthermore, the taxonomic distribution of tetracyclic diterpene hydrocarbons within the order Con$erales (APLIN ef al.. 1963) may provide a means of identifying contributions made by individual conifer families. For example, phyllocladene only occurs in conifers. and is widely reported in the Podocarpaceae family. The occurrence of phyllocladane in coals and sediments may therefore be developed as a marker for this family of plants. GRANTHAM and DOUGLAS (1980) similarly proposed the use of the sesquiterpenoid cedrane as a chemical marker for the Cupressaceae family of conifers. Our studies of Yalloum lignite (sample 1. Tables I and 3) provide a simple test of this approach. Paleobotanical analyses of the five lithotypes of this Victorian brown coal have been summarized by VERHEYENet al. ( 1982). The fossil assemblage of the pale lithotype is dominated by angiosperm leaves, with less common leaves of the gymnosperm genera Podocarpus. Dactydium and Agarhis. Amber fragments and resin drops are also common. Our analyses of the hydrocarbon extracts of the pale lithotype of Yalloum lignite have shown the presence of 16~~(H)-phyllocladane (2a) and ent-16a (H)-kaurane (3a) [see Fig. 6(i)]. These results are entirely consistent with the paleobotanical studies, since modem plants of the genera Podocarpus and Dacrydium (Podocarpaceae family) are rich in phyllocladene. and those of the Agathis genus (Araucariaceae family) are rich in mt-kaurene. The samples analysed for tetracyclic diterpanes in this study (Tables 1 and 3) were representative of a larger suite of samples consisting of fifty sediments and coals. which ranged in age from Tertiary to Devonian. The results of our analyses showed that twenty-seven samples contained phyllocladane, and at least one of beyerane and kaurane. The samples containing the tetracyclic diterpanes were all of Permian age or younger. This observation suggests that tetracyclic diterpanes are markers not only for the modem conifer families which have evolved since the Mesozoic, but also fat the now extinct groups of Paleozoic conifers which were abundant during the Permian (STEWART, 1983: MILLER, 1977. 1982). The ability to recognize conifer remains in sedimentary environments using chemical methods is of particular significance to petroleum exploration. The resins derived from conifers are often rich in lipids, and when deposited under suitable geological conditions, are considered good sources for crude oil (SNOWWN, 1980; SNOWD~N and POWELL, 1982). Resinites derived from conifers are often readily identified in coals and sediments using microscopic tech-
niques. However, there are cases when the resinous material occurs in a finely dispersed form throughout the coal or sediment, and is not identifiable by optical methods (POWELL, 1985). In these cases. hydrocarbon extracts rich in cyclic compounds have been used to identify the presence of finely dispersed restns. The present study indicates that this approach can bo further
is catried out for tetracyclic diterpanes, and in particular, for compounds containing the phyllocladane and kaurane skeletons, enhanced if specific analysis
CONCLUSIONS Tetracyclic diterpenoid hydrocarbons (diterpanes) have been identified in the hydrocarbon extracts of some coals. shales and crude oils. The compounds arc based on the cnt-beyerane, phyllocladane and ENIkaurane skeletons. The most likely natural product precursors for these compounds are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Diagenetic reduction of phyllocladene and mt-kaurene is expected to produce mainly 16n (H)-phyllocladane and eni- 16n (H)-kaurane. respectively. which are the compounds identified in low-rank coals. The concentration of the 16~~(Htcompounds relative to the thermodynamically more stable 168 (H)-epimers decreases with increasing coal-rank. Thermodynamic equilibrium for the interconversion processes is attained in sediments before the onset of crude oil generation. The tetracyclic diterpanes are proposed as markers for both modern and extinct families of conifers. .4cknonded~emenfs-We thank Professor P. R. Jeheries for helpful advice with aspects of diterpenoid chemistry, and we are grateful to Professor D. L. Kepert for his calculations on relative stabilities of epimeric diterpenoid hydrocarbons. We also express our thanks to Dr. K. Sappal for samples of Collie coal and to Shell Development (Australia) for providing samples from the Gippsland Basin. One of us (RAN) thanks the University of Western Australia for a research scholarship. This project was partially supported by the National Energy Research Development Demonstration Project of Australia. Edr/orru/ hand/kg:
C. Barker
REFERENCES ALLINGER N. L. (1977) Conformational analysis. I30. MM2. A hydrocarbon force field utilizing V, and V, torsional terms. J. llmer. Chem. Stx. 99, 8127-8134. APLIN R. T.. CAMBIE R. C. and RUTLEDGE P. S. (I 963) The taxonomic distribution of some diterpene hydrocarbons. Phvmchem. 2, 205-2 14. BRIGGS L. H. (1937) The identity of a-dihydrophyllociadene with iosene. J. Chem. Sot., 1035-1036. GRANTHAM P. J. and DOUGLAS A. G. (1980)The nature and origin of sesquiterpenoids in some tertiary fossil resins Geochim. Cosmochim. Acta 44, 180 I- 18 IO. HAGEMANN H. W. and HOLLERBACH A. (1980) Relationship between the macropetrographic and organic geochemical compositton of Iignites. In Advances in Organic Gecxhemisrrv, 1979 (eds. A. G. DOUGLAS and J. R. MAXWELL.). pp. 63 I-638. Pergamon Press. HANSON J. R. (1968) The Tetracvclic Duerpenes. Pergamon
Press.
Tetracyclic diterpenoids in Australian coal HANSONJ. R. (1972)The di- and sesterterpenes-part 1. In Chemistry ef Terpenes and Terpenoids (ed. A. A. NEWMAN). pp. 1% 199. Academic Press. HATCHERP. G., SIMONEITB. R. T.. MACKENZIE F. T., NEUMAMV A. G., THORSTENSON D. C. and GERCHAKOVS. M. ( 1982) Organic geochemistry and pore water chemistry of sediments from Mangrove Lake, Bermuda. Org. Geochem. 4,93-l
12.
JE~RIES P. R., ROSICH R. S. and WHITE D. E. (1963) The absolute configuration of beyerol. Terruhedron Left.. 17931799. LIVESY A., DOUGLAS A. G. and CONNANJ. (1984) Diterpenoid hydrocarbons in sediments from an offshore (Labrador) well. Org. Geochem. 6, 73-8 1. MACKENZIE A. S., PATIENCER. L., MAXWELL J. R., VANDENBROUKE M. and DURANDB. (1980) Molecular parameters of maturation in the Toarcian shales, Paris Basin. France-I. Changes in the configurations of acyclic isoprenoid alkanes, steranes and triterpanes. Geochim. Cosmochim. Acta44,
1709-1721.
MILLER C. N. (1977) Mesozoic conifers. Botanical Review 43,2 17-280.
MILLERC. N. (1982) Current status of Paleozoic and Mesozoic conifers. Rev. Paleobotany Pal_vnology37, 99- 1 14. NOBLE R., KNOX J., ALEXANDER R. and KAGI R. (1985) Identification of tetracyclic diterpene hydrocarbons in Australian crude oils and sediments. J. Chem. Sot. Chem. Commun.,
32-33.
PHILPR. P., GILBERTT. D. and FRIEDRICHJ. (I98 I) Bicyclic sesquiterpenoids and diterpenoids in Australian crude oils. Geochim. Cosmochim. Acta 45, 1173-l 180. PHILP R. P., SIMONEITB. R. T. and GILBERTT. D. (1983) Diterpenoids in crude oils and coals of South Eastern Australia. In Advances in Organic Geochemistry, 1981 (eds. M. BJOR~Y et al.) pp. 698-704. Wiley and Sons Ltd.
‘147
POWELLT. G. (1985) Developments in concepts of hydrocarbon generation from terrestrial organic matter. Proc. Beijing Petrol. Symp. Sept. 20-24. I984 (in press). SERANTONIE. F.. KRAJEWSKIA.. MONGIORGIR.. RIVA DI SANSEVERINO L. and SHELDRICKG. M. (1978) The crystal and molecular structure of a mineral diterpene. Bombiccite. ClOH,4. Acta Cyst. B34, I 3 1 I - I 3 16. SIMONEITB. R. T. ( 1977) Diterpenoid compounds and other lipids in deepsea sediments and their geochemical significance. Geochim. Cosmochim. Acta 41, 463-476. SNOWD~N L. R. (1980) Resinite-a potential petroleum source in the Upper Cretaceous/Tertiary of the BeaufortMackenzie Basin. In Facts and Principles yf H ‘orld Oil Occurrence (ed. A. D. MIALL).Can. Sot. Petrol. Geol. Memoir 6. 509-52 I. SNOWEON
L. R. and POWELLT. G. (1982) Immature oil and condensate-Modification of hydrocarbon generation model for terrestrial organic matter. .4mer. Assoc. Petrol Geol. Bull. 66, 775-788.
STAINFORTHJ. G. (1984)Gippsland hydrocarbons-a perspective from the basin edge. APEA Journal 24, 9 I- 100. STEWARTW. N. (1983) Paleobotany and the Evolution of Plants, pp. 324-340. Cambridge University Press. THOMASB. R. (1969) Kauri resins-modern and fossil. In Organic Gc~ockc~~nisrr~-Methods and Results (eds. G. EGLINTONand M. T. J. MURPHY).pp. 599-6 18. SpringerVerlag. VERHEYENT. V.. JOHNS R. B. and BLACKBURND. T. (1982) Structural investigations of Australian Coals II: A “C-NMR study of the humic acids from Victorian brown coal lithotypes. Geochim. Cosmochim. Acta 46, 269-277. VOLKMAN J. K.. ALEXANDER R., KAGI R. I., NOBLE R. A. and WOODHOUSEG. W. ( 1983) A geochemical reconstruction of oil generation in the Barrow Sub-basin of Western Australia. Geochim. Cosmochim. Acta 47. 2091-2 105.
Geochrmrca n Cosmochlmrca Acro Vol. 49, pp. 2141-2147 0 Pcrgamon PrcsLtd.1985. Printed inU.S.A.
Tetracyclic diterpenoid hydrocarbons in some Australian coals, sediments and crude oils ROHINTONA. NoBLE*‘~, ROBERTALEXANDER-t, ROBERTIAN Ucilt
and JOHNKNox*
*Department of Organic Chemistry, University of Western Australia, Nedlands, 6009, Western Australia tSchoo1 of Applied Chemistry, Western Australian Institute of Technology, Kent Street, Bentley, 6 102, Western Australia (Received March 19, 1985; accepted in revised form July 15, 1985)
Abstract-Tetracyclic diterpenoid hydrocarbons (diterpanes) based on the enf-beyerane, phyllocladane and ent-kaurane skeletons have been identified in the hydrocarbon extracts of some Australian coals, sediments and crude oils. Structures were assigned to the geological diterpanes by comparison with synthetically prepared reference compounds. Studies of a sample suite consisting of low-rank coals and sediments indicate that the ratios of C-16 epimers of phyllocladane and ent-kaurane are maturity dependent, and that the relative proportion of the thermodynamically preferred 168 (H)-compounds increases with increasing thermal maturity. Thermodynamic equilibrium for the interconversion reactions is attained in sediments before the onset of crude oil generation. The most likely natural product precursors for the tetracyclic diterpanes are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Tetracyclic diterpanes have been identified in sediments and coals of Permian age or younger, suggesting that these compounds are markers for both modem and extinct families of conifers. In particular, phyllocladane is proposed as a marker for the Podocarpaceae family of conifers. INTRODUCI’ION
THETETRACYCLIC diterpenoids natural products
form a large group of which occur widely in higher plants.
They have been classified on the basis of structural similarities into groups which include the kaurenephyllocladene class, the stachene class, the gibberellins, the diterpene alkaloids and the aconite alkaloids (HANSON,1968). The parent hydrocarbons of the kaurene-phyllocladene and stachene classes occur as mono-unsaturated alkenes, and are particularly abundant in leaf resins of conifers belonging to the Podocarpaceae, Araucariaceae and Cupressaceae families (APLINet al., 1963; HANSON, 1968; THOMAS,1969). Despite the widespread occurrence of tetracyclic diterpenoids in nature, there are relatively few reports of their occurrence in sedimentary organics. BRICKS ( 1937) summarized the occurrence of the hydrocarbon iosene, which had been isolated from various Iignites. This compound, although of unknown structure at that time, was shown to be identical with “ardihydrophyllocladene”. More recently, the mineral Bombiccite, isolated from lignite deposits near Florence, was chemically identified by crystal structure analysis as I6a( H)-phyllocladane (see Fig. 2) or “a-dihydrophyllocladene” (SERANTONIet al.. 1978). HAGEMANNand HOLLERBACH ( 1980) also report high concentrations of tetracyclic compounds of the kaurane or phyllocladane type in brown coals from West Germany. The reports of tetracyclic diterpenoids in crude oils and sediments are less conclusive. HATCHERet al. ( 1982) reported traces of kaurane in sediments from Mangrove Lake, Bermuda, and SIMONEIT(1977) reported kaurene and iosene in mrne deep-sea sediments. In both reports, no details of the method of identification were presented. LIVESY ef al. (1984) have presented mass spectral data for a tetracyclic diterpane, which they
suggest might have a kaurane skeleton. Compounds with similar mass spectra to that of LIVESYet al. ( 1984) were first observed by PHILPef al. (198 1, 1983). who found these compounds in sediments and crude oils from S.E. Australia. In this study we have examined some Australian coals, sediments and crude oils for the presence of tet-
racyclic diterpenoid hydrocarbons (tetracyclic diterpanes), and have assigned structures to the diterpanes by comparison with synthetically prepared reference compounds. The effect of thermal maturation upon the relative proportions of epimeric diterpanes was assessed by comparing the distribution of compounds in sediments and coals of differing maturity. We also discuss some aspects of the significance of tetracyclic diterpanes as chemical fossils. EXPERIMENTAL Samples studied Three coals, two shales and a crude oil were selected as representative of a larger suite of samples which have been analysed for tetracyclicditerpanes. Details of these six samples are given in Table I. Sample I is Yalloum lignite, a brown coal from the onshore part of the Gippsland Basin, S.E. Australia. There are five basic lithotypes recognizable in this coal
(VERHEYEN et al., 1982),of which we have analysedthe pale lithotype. Sample 2 is a subbituminous coal from the opencut mine in the Muja sub-basin at Collie, Western Australia, and sample 3 is a coal from a deeper section of the neighbouring Cardiff sub-basin. Samples 4 and 5 are shales from offshore sites in the Gippsland Basin, and sample 6 is a Gippsland Basin crude oil from an associated sandstone. The geographical location of these samples is shown in Fig. 1. Isolation of branched and cyclic alkane fractions
Our standard method for the isolation of branched and cyclic alkanes from petroleum extracts has been reported previously (VOLKMAN et al.. 1983). Briefly, sediment samples
2141
R. A. Noble et ai
2142 Table 1. Sample Number : 3 4 5 6
Sample details for coals, shales and a crude 011
Sample Sedimentary name/description Basin va11ourn coal Muja coal Cardiff coal G 3673 m shale G 4536 m shale G 3756-83 m Crude oil
Location
Fomlatlon we
Gippsland Collie Collie Giopsland Gibbsland
Victoria. Australia Western Australia Uestern Australia Victoria. Australia VIctoriai Australia
Miocene Permian Permian Cretsceous Cretaceous
Gippsland
Victona,
Cretaceous
Australia
(1)
(2)
ent-beyerane were crushed and extracted with dichloromethane to afford the soluble organic matter. Aliphatic hydrocarbons were separated by column chromatography on silicic acid using npentane as solvent. The branched and cyclic compounds were then isolated by removal of n-alkanes with 50 nm molecular sieves.
phyllocladar?e a.R,=H b:R,sCHg
ml2
123
R2-CHB R*=H
m/2,23!
Gas chromatography-massspectromelry(GC-MS) K-MS analyses of branched and cyclic aliphatic hydrocarbons were carried out using an HP5895B capillary GC-
quadrupole MScomputer data system. The CC-MS was fitted with either a 50 m X 0.2 mm id. WCOT fused-silica crosslinked methylsilicone column (Hewlett-Packard), or a similar 5% phenylmethylsilicone capillary column (Hewlett-Packard). A 50 m X 0.2 mm i.d. WCOT fused-silica XEdO-S-valine-Sa-P.E.A. capillary column (Chrompak) was used for some analyses. Samples for analysis were dissolved in n-hexane and injected on-column at 50°C. The oven was then heated to 300°C at 4”C/min., using hydrogen carrier gas at a linear flow velocity of 28 cm/set. Samples were analysed either in the data acquisition mode, by scanning from 50 to 450 a.m.u. in 1.3set cycles, or by selected ion monitoring (SIM), using dwell times of 10 msec for each ion monitored. Typical MS operating conditions were: EM voltage 2200 V: electron energy 70 eV; source temperature 250°C.
RESULTS AND DISCUSSION Mass spectra of diterpenoid hydrocarbons
Tetracyclic diterpanes based on the ent-beyerane (I), phyllocladane (2) and ent-kaurane (3) skeletons (Fig. 2) have been synthesized from natural product precursors, details of which have been presented elsewhere (NOBLE et al.,1985). En&eyerane (I), derived from ent-beyerol (JEFFERIESet al., 1963), is identical to the saturated derivative of (-)-hibaene, which in turn is the parent hydrocarbon of the stachene class of tetracyclic diterpenes (HANSON, 1968). The mass spectra for ent-beyerane ( 1), 16~1(H)-phyllocladane (2a), 168 (H)-phyllocladane (2b). ent- 16a (H)-kaurane (3a) and
ant-kaurane a:R,
=H
R2=CH3
b:R,=CH3
RpH
FIG. 2. Molecular structures for tetracyciic diterpanes showing important fragmentation sites observed in mass spectral analysis.
ent-168 (H)-kaurane (3b) are shown in Fig. 3. Each spectrum shows a base peak corresponding to m/z 123, and a molecular ion of m/z 274 (C&,H&. The base peaks result primarily from the rupture of the B-ring. by cleavage of the 5-6, 9-10 bonds (Fig. 2). Characteristic differences in relative ion abundance are apparent in the mass spectra of the diterpanes. Beyerane can be readily distinguished from phyllocladanes and kauranes by the increased abundance of m/ z 245 ions in its mass spectrum. This ion results from cleavage of the 8- 15, 13- 16 bonds, through loss of a CZ unit. The corresponding bond cleavage in phyllocladanes and kauranes results in loss of a C, fragment, producing m/z 231 ions. The m/z 231/259 ion ratio is greater for phyllocladanes than for kauranes, particularly for 1613(H)-phyllocladane (2b). Consequently, phyllocladanes are characterised by an m/z 231/259 ion ratio above 0.50, whereas the corresponding ratio for kauranes is less than 0.35. Kauranes can also be conveniently distinguished from phyllocladanes by the m/z 2591274 ion ratio, for which kauranes have a value greater than 1.20. (XV-MS analwc
COLLIE
BASIN
VALLOU~~v\i._~IPPSLAND
FIG. I. Geographical location of samples.
BASIN
A mixture containing the five synthetic tetracyclic diterpanes was analysed by GC-MS, using selective ion monitoring of m/z 123 ion abundance. A section of the resulting m/z 123 mass fragmentogram is shown
in Fig. 4. The peaks due to the five compounds were readily resolved using apolar capillary columns al-,
Tetracyclic diterpenoids in Australian
2143
cod
a-beyerane
(ii)
,oo
16ol(Hl-phyllocladane RD 40 -
259 274
20 *
16@ (HI-phyllocladane
z 0
(iv) -ent-lGcu(H)-kaurane
loo-
80 _ 60 40 20. o50
206
150
100
250
gnt- 1 G@(H)-kaurane
m/z FIG.
*
3. Mass spectra for tetracyclic diterpanes (i) ent-beyerane (if; (ii) 16a{H~phyll~ladane (2a); (iii} if$
(H)-phyltocladane (Zb); (iv) enf-16a (H)-kaurane (3a); (v) en&t68 (H)-kaurane (3b).
though there was only a small difference in retention times between peak C and peak D, co~es~nding to ent-16p (H)-kaurane (3b) and l&x (~)-phyllocladane (2a), respectively. Kovritf indices were calculated under isothermal (150°C) GC conditions, and these values are given in Table 2. Aliphatic hydrocarbons derived from coals, shales and crude oils were similarly analysed for tetracyclic diterpanes. However, the m/z 123 fmgmento~am proved to be less useful for this purpose, as significant interference was observed in some sampies due to co-
eluting bicyclic and tricyclic diterpenoids. A more specific analysis of tetracyclic compounds in these geological extracts was obtained by monitoring m/z 259 ion abundance. Figure 5 shows an expanded section of the m/z 259 mass fragmentogram for a shale extract from the Gippsland Basin, Australia. The compounds represented by the peaks labelled in this trace were identified as tetracyclic diterpanes, by comparison of mass spectral data with those of the authentic compounds, and by precise coelution with the corresponding standards
2144
K. A. Noble el al.
m/z
mlz
TETRACYCLIC
123
259
Gippsiand
A
3673m
DITERPANES
Bastn shale
E
42
60
Retention
time
(min)
containing synthetic tetracyclicditerpanes. Peak A: enf-beyerane(I ); peak B: 168 (H)-phyllocladane (2b); peak C: enf-168 (H)-kaurane (3b); peak D: 16a (H)-phyllocladane f2a); peak E: en!-lhn (Htkaurane (3a).
Retention
FIG. 4. m/z 123 mass fragmentogram for mixture
predominantly
of phyllocladane
the 16/j (H)-configurations
and ent-kaurane.
The ratios of 16~~(H)/16p (H)-phyllocladanes and ent-kauranes for the six samples examined in this study are given in Table 3. The accompanying matunty data for each sample. based on H/C ratios, vitrinite reflectanes. and sterane and hopane molecular parameters. indicate that the samples are listed in order of increasing maturity, from Yalloum lignite (lowest) to the Gippsland crude oil (highest). The maturity values for the
(i) m/z
.......
0
d,.~._.,.-.,, , ‘_
..,
-‘-*+ ,?.‘%..V
ent-beycrsnc x(H)-phyllocledane 168(H)-phyl10cladane ent-16u(H)-kaurane a-16tl(H)-kaurene
:a 2b
Nesit 1929 1971 1955 1991 1967
a.
5h x 0.2nm i.d. WCOT fused-silica cross-linked methylsilicone coluam (Hewlett-Packard)
b.
5&n x 0.2mn i.d. WCOT fused-silica cross-linked 5% phenylarthylsilicone colunm (Hewlett-Packard)
.,*,,t..
_*,,,
/_,_
..,_,
Cardiff coai
/B
G4536m shale D
__ I_~_~-~-.l
?
krlL_.-de I -....;
?B
+-~
G3756-83m
crude
oil
1 r, I: D ,j, ‘.. A __.___~ i _.... -.___..~ ~../_ ..__ ..__+_,__‘.i CJ:’ ,.~_ A
b 5% Phmesil 1947 1988 1973 2011 1986
. ..
ID
“c’
(iv)
Structure
,
.’
.‘,‘,L’,VJ.
(iii)
Kovits indices (15O'C) for tetracyclic diterprnes
Compound NaiVa
IIgnite
E
(ii)
Table 2.
Va!iourn
259
Maturity eflects in some coals, shales and crude oils Figure 6 shows expanded sections of the m/z 2% mass fmgmentograms of isolates from two coals, a shale, and a crude oil (samples 1, 3, 5 and 6, Table I ). The major tetracyclic diterpane in these two coal samples was identified as 16a (H)-phyllocladane (2a), which is consistent with previous reports of this compound as the mineral Bombiccite in lignites (SERANTONI PI al., 1978). Enfl6cu (H)-kaurane (3a) was also identified as a prominent tetracyclic diterpane in Yallourn lignite. The shale and crude oil samples, on the other hand.
(min)
FIG. 5. Expanded section of m/z 259 mass fragmentogram for 3673 m Gippsland shale. For peak assignments refer to Fig. 4 caption.
contained
on two apolar capillary columns. A third coinjection experiment, using a commercially available chiral capillary column (XE-60-S-Valine-S-a-P.E.A.. Chrompak) also resulted in precise coelution of identical compounds. Although we have not yet been able to demonstrate separation of enantiomeric terpenoid hydrocarbons with this column. we have assigned the geological diterpanes the same absolute stereochemistry as those of our synthetic reference compounds. These assignments remain equivocal for kaurane and beyerane. since their higher plant precursors are known to occur in both enantiomeric forms (HANSON, 1968). On the other hand, the precursors of phyllocladane belong to one enantiomeric series (“normal” A/B ring fusion). and the assignment of absolute stereochemistry is therefore more conclusive.
time
42
60 Retention
time
(min)
FIG. 6. Expanded sections of m/z 259 mass fragmentograms for (i) Yalloum lignite; (ii) Cardiff coal; (iii) Gippsland 4536 m shale; (iv) Gippsland 3756-83 m crude oil. For peak assignments refer to Fig. 4 caption.
2145
Tetracyclic diterpenoids in Australian coal TABLE 3:
Ratios of epimeric diterpanes and maturity parameters for coal, shale and crude oil samples
No
: 3 4 z
a. b. C.
d. e.
Name
Description
va11wm hja Cardiff G 3673m G 4536m G 3756-83m
lignite HVSB coal HVSB coal shale shale crude oil
EPIMER RATIOS
MTURITY
SAMPLES
H/Ca ratio
Vitrinite Reflectance (Ro%)
Steranesd 20Sl20R
1.21 1:; :: ND
ONqD3b 0.63b 0.72' o.8gc NA
N": 0:: 7::
Hopanese
:::
2.0 co.1 co.1 co.1
Phyllocladanesf
1::
12.8 0.3 :::
s-Kauranesg
J
1.6 0.7 co.1 co.1 co.1
Elemental carbon and hydrogen composition determined by Mel Microanalytical Services (5 Australia) Vitrinite reflectance data from K Sappal, School of Physics and Geosciences, W.A.I.T. (unpublished results). Vitrinite reflectance data from Stainforth (1984). The ratio of ZOS/ZOR for 5a(H),14a(H).17a(H)-24-ethylcholestanes measured fran m/z 217 mass fragmentograms (see Mackenzie et al., 1980). The ratio of 176(H),216(H)~H),218(H) for C 10 hopanes measured from m/z 191 mass fragmentograms (see Mackenzie et a1.,1980). The ratio of 16ml66(H) for phyllocladanes measured from m/z 259 mass fragmentograms. The ratio of 16a(H)/166(H) for ent-kauranes measured fran m/z 259 mass fragmentograms. Infinite ratio. since onlv the T&(H)-eoimers of ohvllocladane and ent-kaurane were detected. Not determined. ..~I
.z . .
HVSB.
High volatile sub-bituminous
3673 m Gippsland shale suggest that this sample was of maturity equivalent to that at the onset of crude oil generation, and those of the 4536 m Gippsland shale, suggest that this sample was at peak oil generation. For the three coals, the proportion of ent-16a (Htkaurane relative to the 168 (Htepimer decreases with increasing coal-rank. The corresponding change in ratio of phyllocladane epimers was only evident in the more mature Cardiff coal, suggesting that the processes leading to the formation of 16p (H)-phyllocladane commence at a later stage of maturation than those corresponding to ent- 168 (Hkkaurane. The ratios of diterpane epimers in the 3673 m shale were significantly different from those of the low-rank coals, but were effectively the same as those in the more mature 4536 m shale and in the crude oil. Assuming that the 168 (H)-epimers of phyllocladane and enf-kaurane result from the epimerization of 16a (H)-compounds, our observations suggest that equilibrium for these reactions is reached before the onset of crude oil generation (vittinite reflectance 0.7%; STAN=~RTH, 1984). In summary therefore, studies of this sample suite indicate that the ratios of C- 16 epimers of phyllocladanes and kauranes are maturity dependent, and that the relative proportion of 16p (H)-compounds increases with increasing thermal maturity. The dynamic range of this process appears to encompass regimes of low thermal stress. such as those associated with the early stages of coalification. Thermodynamic equilibrium for the interconversion reactions appears to be attained when the 16a (H)/16@ (H) ratio reaches a value of 0.3 for phyllocladanes, and ~0. I for enr-kauranes. This occurs before the onset of crude oil generation. The observed changes in the ratios of phyllocladane and kaurane epimers can be rationalized in terms of relative thermodynamic stabilities of individual compounds. Force field calculations using the ALLINGER (1977) method have shown that phyllocladanes and enf-kauranes epimeric at C- 16 have significant differences in their free energy of formation, the 160 (H)-
configuration being the thermodynamically more stable. These differences in stability are reflected in the ratios of epimeric compounds in geological samples, in which the relative concentration of less stable 16a (H)-compounds is diminished on maturation with respect to the thermodynamically preferred 168 (H)compounds. The 16a (H)-configuration of phyllocladane and ent-kaurane, found initially in samples of low maturity, was considered to be the configuration attained through early diagenetic reduction of unsaturated diterpene precursors. In support of this. laboratory experiments involving catalytic hydrogenation of phyllocladene (2, Au’,“) and en&kaurene (3, A16.“) produced as their major products, 16a (H)-phyllocladane (2a) and ent-16a (H)-kaurane (3a), respectively (NOBLE et al., 1985, and references therein). We therefore conclude that the 16cu(H)-epimers are the kinetically controlled products, and the 16/3 (H)-epimers the thermodynamically controlled products. Tetracyclic diterpanes as chemical fossils The occurrence of tetracyclic diterpenoids containing the beyerane, phyllocladane and kaurane skeletons in higher plants is well documented (HANSON, 1968. 1972, and references therein). Compounds of this type occur naturally as alkenes, alcohols, ketones. esters, acids and more complex polyfunctional derivatives. The tetracyclic diterpanes found in coals, sediments and crude oils are derived from these higher plant diterpenoids, by diagenetic removal of their functional groups. However, within the large group of tetracyclic diterpenoids, there are probably only a limited number of compounds which would survive diagenetic transformations with complete preservation of skeletal units. The compounds which are least likely to suffer structural alteration during diagenesis are the tetracyclic diterpene hydrocarbons. Compounds such as kaurene, phyllocladene and beyer- I S-ene (hibaene) would be readily converted into saturated derivatives by diage-
2146
R.
A. Noble et ul.
netic reduction of their double bonds. and would therefore appear to be the most likely source of the sedimentary tetracyclic diterpanes. Tetracyclic diterpene hydrocarbons occur abundantly in the leaf resins of conifers (APLIN et al., 1963: THOMAS. 1969; HANSON, 1968). It is therefore likely that the tetracyclic diterpanes are markers for coniferresin contributions to sedimentary organic matter. Furthermore, the taxonomic distribution of tetracyclic diterpene hydrocarbons within the order Con$erales (APLIN ef al.. 1963) may provide a means of identifying contributions made by individual conifer families. For example, phyllocladene only occurs in conifers. and is widely reported in the Podocarpaceae family. The occurrence of phyllocladane in coals and sediments may therefore be developed as a marker for this family of plants. GRANTHAM and DOUGLAS (1980) similarly proposed the use of the sesquiterpenoid cedrane as a chemical marker for the Cupressaceae family of conifers. Our studies of Yalloum lignite (sample 1. Tables I and 3) provide a simple test of this approach. Paleobotanical analyses of the five lithotypes of this Victorian brown coal have been summarized by VERHEYENet al. ( 1982). The fossil assemblage of the pale lithotype is dominated by angiosperm leaves, with less common leaves of the gymnosperm genera Podocarpus. Dactydium and Agarhis. Amber fragments and resin drops are also common. Our analyses of the hydrocarbon extracts of the pale lithotype of Yalloum lignite have shown the presence of 16~~(H)-phyllocladane (2a) and ent-16a (H)-kaurane (3a) [see Fig. 6(i)]. These results are entirely consistent with the paleobotanical studies, since modem plants of the genera Podocarpus and Dacrydium (Podocarpaceae family) are rich in phyllocladene. and those of the Agathis genus (Araucariaceae family) are rich in mt-kaurene. The samples analysed for tetracyclic diterpanes in this study (Tables 1 and 3) were representative of a larger suite of samples consisting of fifty sediments and coals. which ranged in age from Tertiary to Devonian. The results of our analyses showed that twenty-seven samples contained phyllocladane, and at least one of beyerane and kaurane. The samples containing the tetracyclic diterpanes were all of Permian age or younger. This observation suggests that tetracyclic diterpanes are markers not only for the modem conifer families which have evolved since the Mesozoic, but also fat the now extinct groups of Paleozoic conifers which were abundant during the Permian (STEWART, 1983: MILLER, 1977. 1982). The ability to recognize conifer remains in sedimentary environments using chemical methods is of particular significance to petroleum exploration. The resins derived from conifers are often rich in lipids, and when deposited under suitable geological conditions, are considered good sources for crude oil (SNOWWN, 1980; SNOWD~N and POWELL, 1982). Resinites derived from conifers are often readily identified in coals and sediments using microscopic tech-
niques. However, there are cases when the resinous material occurs in a finely dispersed form throughout the coal or sediment, and is not identifiable by optical methods (POWELL, 1985). In these cases. hydrocarbon extracts rich in cyclic compounds have been used to identify the presence of finely dispersed restns. The present study indicates that this approach can bo further
is catried out for tetracyclic diterpanes, and in particular, for compounds containing the phyllocladane and kaurane skeletons, enhanced if specific analysis
CONCLUSIONS Tetracyclic diterpenoid hydrocarbons (diterpanes) have been identified in the hydrocarbon extracts of some coals. shales and crude oils. The compounds arc based on the cnt-beyerane, phyllocladane and ENIkaurane skeletons. The most likely natural product precursors for these compounds are considered to be the tetracyclic diterpene hydrocarbons which occur widely in the leaf resins of conifers. Diagenetic reduction of phyllocladene and mt-kaurene is expected to produce mainly 16n (H)-phyllocladane and eni- 16n (H)-kaurane. respectively. which are the compounds identified in low-rank coals. The concentration of the 16~~(Htcompounds relative to the thermodynamically more stable 168 (H)-epimers decreases with increasing coal-rank. Thermodynamic equilibrium for the interconversion processes is attained in sediments before the onset of crude oil generation. The tetracyclic diterpanes are proposed as markers for both modern and extinct families of conifers. .4cknonded~emenfs-We thank Professor P. R. Jeheries for helpful advice with aspects of diterpenoid chemistry, and we are grateful to Professor D. L. Kepert for his calculations on relative stabilities of epimeric diterpenoid hydrocarbons. We also express our thanks to Dr. K. Sappal for samples of Collie coal and to Shell Development (Australia) for providing samples from the Gippsland Basin. One of us (RAN) thanks the University of Western Australia for a research scholarship. This project was partially supported by the National Energy Research Development Demonstration Project of Australia. Edr/orru/ hand/kg:
C. Barker
REFERENCES ALLINGER N. L. (1977) Conformational analysis. I30. MM2. A hydrocarbon force field utilizing V, and V, torsional terms. J. llmer. Chem. Stx. 99, 8127-8134. APLIN R. T.. CAMBIE R. C. and RUTLEDGE P. S. (I 963) The taxonomic distribution of some diterpene hydrocarbons. Phvmchem. 2, 205-2 14. BRIGGS L. H. (1937) The identity of a-dihydrophyllociadene with iosene. J. Chem. Sot., 1035-1036. GRANTHAM P. J. and DOUGLAS A. G. (1980)The nature and origin of sesquiterpenoids in some tertiary fossil resins Geochim. Cosmochim. Acta 44, 180 I- 18 IO. HAGEMANN H. W. and HOLLERBACH A. (1980) Relationship between the macropetrographic and organic geochemical compositton of Iignites. In Advances in Organic Gecxhemisrrv, 1979 (eds. A. G. DOUGLAS and J. R. MAXWELL.). pp. 63 I-638. Pergamon Press. HANSON J. R. (1968) The Tetracvclic Duerpenes. Pergamon
Press.
Tetracyclic diterpenoids in Australian coal HANSONJ. R. (1972)The di- and sesterterpenes-part 1. In Chemistry ef Terpenes and Terpenoids (ed. A. A. NEWMAN). pp. 1% 199. Academic Press. HATCHERP. G., SIMONEITB. R. T.. MACKENZIE F. T., NEUMAMV A. G., THORSTENSON D. C. and GERCHAKOVS. M. ( 1982) Organic geochemistry and pore water chemistry of sediments from Mangrove Lake, Bermuda. Org. Geochem. 4,93-l
12.
JE~RIES P. R., ROSICH R. S. and WHITE D. E. (1963) The absolute configuration of beyerol. Terruhedron Left.. 17931799. LIVESY A., DOUGLAS A. G. and CONNANJ. (1984) Diterpenoid hydrocarbons in sediments from an offshore (Labrador) well. Org. Geochem. 6, 73-8 1. MACKENZIE A. S., PATIENCER. L., MAXWELL J. R., VANDENBROUKE M. and DURANDB. (1980) Molecular parameters of maturation in the Toarcian shales, Paris Basin. France-I. Changes in the configurations of acyclic isoprenoid alkanes, steranes and triterpanes. Geochim. Cosmochim. Acta44,
1709-1721.
MILLER C. N. (1977) Mesozoic conifers. Botanical Review 43,2 17-280.
MILLERC. N. (1982) Current status of Paleozoic and Mesozoic conifers. Rev. Paleobotany Pal_vnology37, 99- 1 14. NOBLE R., KNOX J., ALEXANDER R. and KAGI R. (1985) Identification of tetracyclic diterpene hydrocarbons in Australian crude oils and sediments. J. Chem. Sot. Chem. Commun.,
32-33.
PHILPR. P., GILBERTT. D. and FRIEDRICHJ. (I98 I) Bicyclic sesquiterpenoids and diterpenoids in Australian crude oils. Geochim. Cosmochim. Acta 45, 1173-l 180. PHILP R. P., SIMONEITB. R. T. and GILBERTT. D. (1983) Diterpenoids in crude oils and coals of South Eastern Australia. In Advances in Organic Geochemistry, 1981 (eds. M. BJOR~Y et al.) pp. 698-704. Wiley and Sons Ltd.
‘147
POWELLT. G. (1985) Developments in concepts of hydrocarbon generation from terrestrial organic matter. Proc. Beijing Petrol. Symp. Sept. 20-24. I984 (in press). SERANTONIE. F.. KRAJEWSKIA.. MONGIORGIR.. RIVA DI SANSEVERINO L. and SHELDRICKG. M. (1978) The crystal and molecular structure of a mineral diterpene. Bombiccite. ClOH,4. Acta Cyst. B34, I 3 1 I - I 3 16. SIMONEITB. R. T. ( 1977) Diterpenoid compounds and other lipids in deepsea sediments and their geochemical significance. Geochim. Cosmochim. Acta 41, 463-476. SNOWD~N L. R. (1980) Resinite-a potential petroleum source in the Upper Cretaceous/Tertiary of the BeaufortMackenzie Basin. In Facts and Principles yf H ‘orld Oil Occurrence (ed. A. D. MIALL).Can. Sot. Petrol. Geol. Memoir 6. 509-52 I. SNOWEON
L. R. and POWELLT. G. (1982) Immature oil and condensate-Modification of hydrocarbon generation model for terrestrial organic matter. .4mer. Assoc. Petrol Geol. Bull. 66, 775-788.
STAINFORTHJ. G. (1984)Gippsland hydrocarbons-a perspective from the basin edge. APEA Journal 24, 9 I- 100. STEWARTW. N. (1983) Paleobotany and the Evolution of Plants, pp. 324-340. Cambridge University Press. THOMASB. R. (1969) Kauri resins-modern and fossil. In Organic Gc~ockc~~nisrr~-Methods and Results (eds. G. EGLINTONand M. T. J. MURPHY).pp. 599-6 18. SpringerVerlag. VERHEYENT. V.. JOHNS R. B. and BLACKBURND. T. (1982) Structural investigations of Australian Coals II: A “C-NMR study of the humic acids from Victorian brown coal lithotypes. Geochim. Cosmochim. Acta 46, 269-277. VOLKMAN J. K.. ALEXANDER R., KAGI R. I., NOBLE R. A. and WOODHOUSEG. W. ( 1983) A geochemical reconstruction of oil generation in the Barrow Sub-basin of Western Australia. Geochim. Cosmochim. Acta 47. 2091-2 105.