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Investigation of three natural bitumens from central England by hydrous pyrolysis and gas chromatography-mass spectrometry
Chemical Geology, 64 (1987) 181-195 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
181
[2]
INVESTIGATION OF THREE NATURAL BITUMENS FROM CENTRAL ENGLAND BY HYDROUS PYROLYSIS AND GAS CHROMATOGRAPHY-MASS SPECTROMETRY GOU X U E M I N 1'.1, M.G. F O W L E R 1'.2, P.A. C O M E T l'*a, D.A.C. M A N N I N G 1, A.G. DOUGLAS 1, J. McEVOY 2'.4 and W. GIGER 2 Organic Geochemistry Unit, Department of Geology, University of Newcastle upo n- Tyne, Newcastle- upon- Tyne NE1 7RU (Great Britain) ~Swiss Federal Institute for Water Resources and Water Pollution Control ( EA WA G), CH-8600 Di~bendor[ (Switzerland) (Received March 10, 1987; revised and accepted May 13, 1987)
Abstract Gou Xuemin, Fowler, M.G., Comet, P.A., Manning, D.A.C., Douglas, A.G., McEvoy, J. and Giger, W., 1987. Investigation of three natural bitumens from central England by hydrous pyrolysis and gas chromatography-mass spectrometry. Chem. Geol., 64: 181-195. Bitumen samples from three hydrothermal mineral deposits (Windy Knoll, Derbyshire; Staunton Harold and Mountsorrel, Leicestershire) have been studied partly in an attempt to resolve the continuing controversy over whether they are biogenic or abiogenic in origin, and partly to investigate a new approach to studying such materials. A more conventional method was first employed, involving analysis of alkane fractions isolated from the bitumens using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). This showed that the Staunton Harold and Mountsorrel bitumens contained abundant biomarkers (tricyclic terpanes, steranes and hopanes) but no nalkanes, acyclic isoprenoids and n-alkylcyclohexanes, whereas the Windy Knoll bitumen lacked biomarkers other than acyclic isoprenoids. Hydrous pyrolysis of the insoluble residue from the Windy Knoll bitumen, and of the asphaltenes from the other two bitumens, produced alkanes in the pyrolysates which were analysed using GC and GC-MS. The pyrolysates contained abundant n-alkanes, n-alkylcyclohexanes and acyclic isoprenoids in addition to the polycyclic alkanes found in the original bitumens. The distributions of steranes and hopanes in the original Staunton Harold and Mountsorrel bitumens and their pyrolysates are briefly discussed. Our results suggest that the bitumens are of biogenic origin and have been subjected to varying degrees of biodegradation and thermal maturation.
Mountsorrel, Leicestershire (Fig. 1 ), added to the controversy because gas chromatograms of
1. I n t r o d u c t i o n
T h e occurrence of natural b i t u m e n s within Carboniferous rocks in central England in association with hydrothermal mineralisation ( Mueller, 1954, 1970; King and Ford, 1968) led to considerable discussion on whether they had a biogenic or abiogenic origin (Sylvester-Bradley and King, 1963 ). Early work on the organic geochemistry of one of these bitumens from 0009-2541/87/$03.50
Permanent addresses: "Research Institute of Petroleum Exploration, Chengdu, People's Republic of China. "2ISPG, 3303, 33rd Street N.W., Calgary, Alta. T2L 2A7, Canada. *:~Core Laboratories Singapore, 24A Lim Teck Boo Road, Singapore. *4School of Ocean Science, Marine Science Labs., University of Bangor, Menai Bridge, Gwynedd, LL59 5EY, U.K.
© 1987 Elsevier Science Publishers B.V.
182 these investigations (including the finding of biomarkers such as acyclic isoprenoids, the carbon isotope ratios and optical studies) suggested that the Windy Knoll bitumens probably did have a biogenic origin (Nooner et al., 1973; Pering, 1973; Khavari-Khorasani and Murchison, 1978). In this present study, samples from three
alkanes isolated from this deposit were found to be dominated by a "hump" of unresolved compounds similar to those produced abiogenically in laboratory experiments (Ponnamperuma and Pering, 1966; Calvin, 1969). Other bitumens from this area that were frequently studied include those from Windy Knoll, Derbyshire ( Mueller, 1954 ). The results of some of I
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183
well-known bitumen occurrences in central England whose origins have been disputed in the past (Sylvester-Bradley and King, 1963; Ponnamperuma and Pering, 1966; Calvin, 1969; Nooner et al., 1973; Pering, 1973) have been reinvestigated. This was primarily to investigate the usefulness of a new approach in the study of such materials. The choice of samples was partly inspired by Gold citing the past controversy over the origins of these particular bitumens in support of his theory of a possible abiogenic origin for petroleum (Gold and Soter, 1982 ). In addition this study contributes to our understanding of the means by which organic matter is included within hydrothermal mineral deposits of essentially Mississippi Valley type, placing further constraints on theories of their genesis. Two different investigative methods were used in the present work. A more conventional method was first employed, i.e. an investigation of biomarkers, using gas chromatography-mass spectrometry ( G C - M S ) , in the alkanes separated from the bitumens. These techniques have been used by previous workers in studies of similar types of samples (Didyk et al., 1983; Robinson et al., 1986). Subsequently, asphaltenes isolated from two of the bitumens, and the insoluble residue from the third bitumen, were pyrolysed in the presence of water (hydrous pyrolysis) and the alkanes isolated from the resulting pyrolysates were analysed using GC-MS. It has been shown previously in this laboratory that hydrous pyrolysis of asphaltenes separated from the natural bitumen Gilsonite generates biomarkers with distributions similar to those thought to occur in its source rock (Telnaes et al., 1985). Workers in other laboratories, using different pyrolysis techniques, have reported similar results (Bandurski, 1982; Rubinstein et al., 1979; Behar et al., 1984). Hydrous pyrolysis was chosen for this work, rather than other forms of pyrolysis, since its products are reported to more closely resemble naturally occurring bitumens and oils
(Lewan et al., 1979; Winters et al., 1983).
2. Geological setting and samples Localities for the samples examined in this study are given in Fig. 1, and the geology of the region has been summarised by SylvesterBradley and Ford (1968). One sample was chosen from a collection made at Windy Knoll near Castleton in Derbyshire (National Grid Reference SK127830), where a number of different bitumens occur within neptunian dykes and hydrothermal veins within Lower Carboniferous limestones (Mueller, 1954, 1970). The sample studied was a greenish-yellow viscous oil which has previously been classified as a "vaselous" oil (Mueller, 1954), an oil (Pering, 1973) and a type-C bitumen (Nooner et al., 1973). The second sample was from material collected by Dr. R.J. King from the Earl Ferrer's lead mine at Staunton Harold, Leicestershire (SK374217; National Museum of Wales accession number 83.41G.M9440). At this locality, hydrothermal veins carrying galena and sphalerite occur within dolomitised Lower Carboniferous limestones overlain unconformably by Namurian shales (King, 1959; King and Ford, 1968). The bitumen occurs within the gangue minerals of the vein which are mostly calcite and dolomite. The third sample is from the former Main Quarry at Mountsorrel, Leicestershire (SK477148; National Museum of Wales accession number 83.41G,M9439) and was again collected by Dr. R.J. King. Here, the Caledonian Mountsorrel Granite (430 + 7 Ma; Hampton and Taylor, 1983) is cut by dolerite dykes which correlate with dykes of Hercynian age within nearby Coal Measures ( King, 1959 ). The granite adjacent to the dykes is extensively hydrothermally altered and vuggy, and is mineralised with a dolomite-calciteclay-pyrite assemblage which contains bitumen; the sample studied is from this assemblage. This mineralisation is considered to
184 predate the current Permo-Trias cover, and may have formed beneath a cover of Carboniferous sediments similar to those exposed at S t a u n t o n Harold (King, 1959). Thus, in summary, the Windy Knoll and S t a u n t o n Harold samples are from closely corresponding geological settings - - within Carboniferous limestones overlain unconformably by N a m u r i a n shales - - whereas the Mountsorrel sample is from mineralised altered granite. In all cases the mineralisation and associated bitumen is hosted by a reactive, competent and relatively easily dissolved rock beneath an actual or inferred impervious cover.
40-60 ° C). Another aliquot of each extract was redissolved in dichloromethane and asphaltenes were precipitated by the addition of a 40fold excess of cold n-heptane. The asphaltenes were purified by reprecipitation twice from clean solvent. Significant amounts of asphaltenes were isolated from the S t a u n t o n Harold and Mountsorrel bitumens (Table I) but none was obtained from the Windy Knoll bitumen extract. Asphaltenes isolated from the S t a u n t o n Harold and Mountsorrel bitumens and the insoluble residue of the W i n d y Knoll sample were hydrously pyrolysed using a method similar to t h a t of Eglinton et al. {1986), which is itself based on one previously described by Lewan et al. (1979) and Winters et al. (1983). The experiments were carried out in small "bomblets", purpose built from 316 grade stainless steel [ 2 in. ( ~ 5 cm) long, ~ in. ( ~ 9.5 ram) i.d., ~ in. ( ~2.22 cm) across flat hexagonal section, internal capacity 3 ml ] each fitted with a steel screw closure and a soft copper sealing gasket. Distilled water (1 ml ) and ~ 300 mg of the material to be pyrolysed were placed in each bomblet which was then purged with nitrogen, to remove air, and tightly sealed. The bomblets were placed in a pressure reactor (1 1,
3. Experimental
Mineral-free bitumen samples were first extracted with dichloromethane for 24 hr. using a Soxhlet apparatus. The Mountsorrel and S t a u n t o n Harold bitumens completely dissolved during this extraction but only 21% of the Windy Knoll bitumen was found to be soluble (Table I). Aliphatic hydrocarbons (fraction A) were separated from a portion of the extracts by thin layer chromatography ( T L C ) using glass plates coated (0.55 mm thick) with silica gel (Merck ® Kieselgel Nach Stahl, type 60G) and developed with light petroleum (b.p. TABLEI Gross geochemicaldata Sample
% extracted
% aliphatics
% aromatics
% resins
% asphaltenes
100.1 100"1 21.2°~
15.3 20.2 23.9
11.0 14.8 7.4
73.7 65.0 68.7
24.8 16.1 0.0
49.4"2 22.6"2
11.3 24.7
13.7
72.5
Bitumens:
Staunton Harold Mountsorrel Windy Knoll Asphaltene pyrolysate:
Staunton Harold Mountsorrel Unextracted residue pyrolysate:
Windy Knoll •1Extractedusing dichloromethane. "~Extractedusing light petroleum ether.
13.8
185 Parr Instrument Co. ) also part-filled with water (to minimise the pressure differential across the bomblet wall and to reduce temperature gradients in the system), which was in turn sealed. The temperature of the reactor was raised by a heating jacket, with temperature control to + 5 ° C. Samples were heated rapidly to 330 ° C and held isothermal for 72 hr. after which time the vessel was allowed to cool prior to opening. The resulting pyrolysates were extracted with light petroleum (b.p. 40-60 ° C) : these extracts were then dried by filtering through short columns of activated alumina. Aliphatic hydrocarbons were separated from the extract using TLC (as described on p.184) to give the " P " fractions. Prior to G C - M S analyses, branched/ cyclic alkanes were separated from straightchain and simply branched alkanes by urea adduction. Gas chromatography was carried out on a Carlo Erba ® 4160 instrument equipped with a fused silica column (50 m × 0.32 m m i.d. ) coated with OV-1 ®, using on-column injection. H2 was used as carrier gas and a temperature programme of 50-290°C at 4°C min. 1 was employed. G C - M S analysis was performed on a Carlo Erba ® 4200 GC directly coupled to a Kratos ® MS80-RF (electron energy 70 eV) under the control of a DS-5M ® data system. The GC was equipped with a fused silica colu m n (25 m × 0.3 m m i.d.) coated with DB5 ® ( J & W Scientific) and the temperature programme employed was from 50 to 325 ° C at 4 ° C m i n . - ~. 4. R e s u l t s
4.1. Aliphatic hydrocarbons extracted from bitumens The gas chromatograms of the A fractions (aliphatic hydrocarbons separated from the original bitumens), were dominated by "humps" of unresolved components on which some small peaks were superimposed (Figs. 2a, 3a and 4a). G C - M S analysis of the Mountsor-
rel and Staunton Harold bitumens indicated t h a t they contained polycyclic alkanes such as tricyclic terpanes, steranes and hopanes but no n-alkanes, acyclic isoprenoids or n-alkylcyclohexanes. The distributions of hopanes and steranes (Figs. 5a, b and 6a, b) in these two samples are those expected from mature organic m a t t e r (MacKenzie, 1984) with, for example, the former series dominated by 17~ ( H ) ,21fl( H ) hopanes and the latter showing almost equal concentrations of 5o~(H),14fl(H),I7fl(H) steranes and 5c~(H),14o~(H),17a(H) steranes (Table II). The Staunton Harold bitumen showed a sterane distribution with some features that could be indicative of more severe biodegradation. These include a greater abundance of diasteranes t h a n unrearranged steranes and apparent lack of C27 steranes ( Fig. 5a). No biomarkers were detected in the Windy Knoll A fraction, except for the previously reported acyclic isoprenoids (Pering and Ponnamperuma, 1969; Nooner et al., 1973; Pering, 1973). The lack of polycyclic alkanes in this bitumen might indicate that it has been subjected to temperatures high enough to have caused their thermal degradation.
4.2. Aliphatic hydrocarbons in pyrolysates In contrast to the gas chromatograms of the A fractions, those of alkanes in the pyrolysates ( P fractions ) of the Mountsorrel and Staunton Harold asphaltenes and the Windy Knoll insoluble fraction all have n-alkanes ranging from about C1~-C3o as their highest peaks (Figs. 2b, 3b and 4b ). Acyclic isoprenoids with 15-20 carbon atoms per molecule are also prominent in these chromatograms. The n-alkane and acyclic isoprenoid peaks are superimposed on "humps" in the chromatograms of the Staunton Harold (Fig. 2b) and Windy Knoll (Fig. 4b) P fractions but there is no prominent hump in the chromatograms of the Mountsorrel P fraction. G C - M S analysis revealed the presence of acyclic isoprenoids (possibly up to C4o) and n-
186
(a)
STAUNTON HAROLD A
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Fig. 2. Gas chromatograms of: (a) alkanes (A) extracted from Staunton Harold bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of asphaltenes isolated from the Staunton Harold bitumen. Peaks labelled in (a) are: x, C j 13fl ( H ) ,17v~ ( H ) 2 0 S + R-diasterane; and y, 17~ (H) ,2Ifl ( H ) -hopane. Peaks labelled in (b) are C15, C~,~and C.~ n-alkanes ( P h = phytane; P r = pristane ).
187
MOUNTSORREL A
s
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J~ ~l'# Fig. 3. Gas chromatograms of: (a) alkanes (A) extracted from Mountsorrel bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of asphaltenes isolated from the Mountsorrel bitumen. Peaks labelled as for Fig. 2.
alkylcyclohexanes (C14-C26) in the Mountsorrel and Staunton Harold P fractions. Hopanes
and steranes are also present in these fractions (Figs. 5c, d and 6c, d) with steranes in much
188
WINDY KNOLL A
J
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WINDY KNOLL P
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Fig. 4. Gas chromatograms of: (a) alkanes (A) extracted from Windy Knoll bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of the unextracted residue of Windy Knoll bitumen.
189 T A B L E II Biomarker data from G C - M S analysis of Staunton Harold and Mountsorrel bitumen fractions Sample
Fraction
C27
C28
C29
a
C29c~o~o~20(R + S )
a ow~20S
~ o~fl fl ~29 o~cL~+ o~flfl
C~,
~
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b
c
d
e
Staunton Harold
A P
n.d. 28
30 27
70 45
0.50 0.48
0.53 0.48
1.88 1.43
0.30 0.26
Mountsorrel
A P
37 37
19 21
44 42
0.52 0.49
0.47 0.42
1.31 0.77
0.23 0.22
a = percentage of 5~ ( H ) ,14fl( H ) ,I 7fl( H ) -steranes ( from m/z 217 )
b= C2~ 5(~ ( H ) ,14c~ ( H ) ,17~ ( H ) 20S/C29 5e~ ( H ) ,14a ( H ) ,17c~ ( H ) 20S + 20R-steranes (from m/z 217) c=C295(~(H),I4fl(H),17fl(H) /C295a(H),14fl(H),17fl(H) + 5 ~ ( H ) , 1 4 ~ ( H ) , 1 7 a ( H ) - s t e r a n e s (from m/z 217) d = C2,13~( H ), 17~ (H) 20S-diasterane/C29-5a ( H ) ,14a ( H ) ,17(~ ( H ) 20R-sterane ( from m/z 217 )
e = 17fl(H ) ,21c~ (H) -moretane/l 7a (H) ,21fl (H) -hopane (from m/z 191 ) n.d. = not determinable.
lower concentrations relative to the hopanes than they were in the A fractions. The distribution of these polycyclic alkanes shows some systematic differences between the A and P fractions of the Staunton Harold bitumen. The P fractions appear to be slightly less mature as indicated by smaller values for the sterane ratios b and c but slightly more mature as indicated in the hopane ratio (see Table II). The C2v hopanes 17~ ( H ) ,22,29,30-trisnorhopane (Tin) and 18c~( H ) ,22,29,30-trisnorhopane ( Ts ) were present in roughly equal amounts in the A fraction, whereas Tm alone was detected in the P fraction (Fig. 5a and c). The absence of T~ in kerogen pyrolysates has also been observed in other studies ( e.g., Eglinton et al., 1986). It was also noted that the relative abundance of gammacerane in the P fraction is greatly reduced from that in the A fraction. Other significant differences in the sterane patterns of fractions A and P (Fig. 5b and d) include the presence, in the latter, of C27 steranes in concentrations about equal to those of the Czs steranes, diasteranes in lower abundance relative to unrearranged steranes ( Table II, ratio d) and greater amounts of steranes with short side-chains (peaks I-5) compared to the C2v-C29 steranes. Most of the differences in the hopane and ste-
rane patterns discussed above were also noted in the Mountsorrel fractions (Fig. 6; Table II). One major difference between the Mountsorrel and Staunton Harold samples was that there was little change in the relative amounts of C2v-C29 steranes in the respective A and P fractions of the former. GC-MS analysis of the Windy Knoll P fraction confirmed the presence of acyclic isoprenoids and n-alkylcyclohexanesbut no polycyclic alkanes were detected. 5. Discussion
Biomarkers are present in all three samples, indicating that these bitumens are at least partially derived from biogenic sources. The unresolved complex mixtures observed in the gas chromatograms of their A fractions suggest that the bitumens have been biodegraded. Previous authors have established the following sequence for the selective removal of compound classes by microorganisms: n-alkanes
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STAUNTON HAROLD P m/z 191
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Fig. 5. m/z 191 and m/z 217 mass fragmentograms of Staunton Harold A and P branched/cyclic alkane fractions. Peaks identified in Tables III and IV.
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192 TABLE III Peaks identified in m/z 191 fragmentograms A
C2~)tricyclic terpane C21 tricyclic terpane C2:~tricyclic terpane C24 tricyclic terpane C2,~tricyclic terpane C26 tricyclic terpane C2s tricyclic terpane C29 tricyclic terpane T~: 18~ (H) ,22,29,30-trisnorhopane Tin: 17a (H) -trisnorhopane 17 ~ ( H ) ,21fl ( H ) ,30-norhopane unknown 17fl(H),21~ (H) ,30-normoretane 17a (H) ,21fl(H)-hopane 17fl( H ) ,21~ (H) -moretane 17 ~ ( H ) ,21fl ( H ) -homohopanes gammacerane 17~ ( H ) ,21fl ( H ) -bishomohopanes 17~ (H) ,21fl ( H ) -trishomohopanes 17~ (H) ,21fl ( H ) -tetrakishomohopanes 17~ (H) 21fl ( H ) -pentakishomohopanes
B
C D E F G H I
J K L M N 0 P Q R S T U
TABLE IV Peaks identified in m/z 217 fragmentograms 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
C,9 sterane C~1 sterane C2l sterane C22 sterane C22 sterane 13,8(H ) ,17a (H) -diacholestane (20S) 13fl (H),17a (H)-diacholestane (20R) 24-methyl-13,8( H ) ,17a ( H ) -diacholestane ( 20S ) 24-methyl-13,8 (H),I 7a ( H ) -diacholestane (20R) 5(~ (H) ,14~ (H) ,17a (H) -cholestane (20S) 5~ ( H ) ,14fl ( H ) ,17fl ( H ) -cholestane (20R) 24-ethyl-13fl(H) ,17a (H) -diacholestane (20S) 5~(H),14fl(H),17fl(H)-cholestane (20S) 5 a ( H ) , 1 4 ~ ( H ) , 1 7 ~ ( H ) - c h o l e s t a n e (20R) 24-ethyl-13fl( H ) ,I 7a ( H ) -diacholestane (2OR) 24-methyl-5a ( H ),14~ (H) ,17a (H) -cholestane (20S) 24-methyl-5~ (H) ,1413(H) ,17,8 (H) -cholestane (2OR) 24-methyl-5~ { H ) ,14fl ( H ) ,17fl ( H ) -cholestane (20S) 2 4-methyl-5~ ( H ) ,14~ ( H ) ,17 ~ ( H ) -cholestane (20R) 24-ethyl-5a ( H ) ,14~ ( H ) ,17~ (H) -cholestane (20S) 24-ethyl-5a ( H ) ,1413(H ) ,17]3( H ) -cholestane (20R ) 2 4-ethyl-5~ ( H ) ,14fl ( H ) ,17,8(H) -cholestane ( 20S ) 24-ethyl-5a (H) ,14~ (H) ,17a (H) -cholestane (20R)
alkylcyclohexanes, acyclic isoprenoids) are the most susceptible to biodegradation. The Windy Knoll A fraction does contain acyclic isoprenoids which were not detected in the A fractions of the other two samples, indicating it to be the least biodegraded. Hydrous pyrolysis of the asphaltenes from the Mountsorrel and Staunton Harold bitumens generated alkanes which gave gas chromatograms similar to those from undegraded or partially degraded oils. This is consistent with the observations that n-alkanes, n-alkylcyclohexanes and isoprenoid moieties in the asphaltene macromolecule are, in some way, protected from biodegradation (Rubinstein et al., 1979; Ekweozor, 1985; Telneas et al., 1985 ). The lower maturity of steranes in the asphaltene pyrolysates is also in agreement with the results of previous workers ( Philp and Gilbert, 1985; Telneas et al., 1985). However, the differences in the ratios b and c between the A and P fractions, and also the increase in maturity as indicated by hopane ratio e, are very small and probably within the limits of experimental error. One puzzling and unexplained result concerns the C27 steranes which are absent from fraction A of the Staunton Harold bitumen, but present in fraction P (Fig. 5; Table II). Because the relative amounts of C~7-C~ steranes between the A and P fractions remain similar for the Mountsorrel bitumen, the occurrence of C27 steranes in the Staunton Harold pyrolysates is unlikely to be due to cracking of the C~s and C29 steranes. However, cracking processes do appear to have occurred, as indicated by the relative increase in the abundance of steranes with short side-chains (C19-C22steranes) in the P fractions of both Staunton Harold and Mountsorrel bitumens. Microbial processes should be considered for causing the differences discussed. It is known that C27 regular steranes are biodegraded in preference to Czs and C29 regular steranes (Goodwin et al., 1983; Volkman et al., 1983) but invoking this for the Staunton Harold bitumen leaves the problem that, preferential
193
removal of the C29 5 a ( H ) , 1 4 a ( H ) , 1 7 a ( H ) 20R-sterane relative to the C29 5o~(H),I4~ ( H ) ,17a ( H ) 20S-sterane ( Volkman et al., 1983; Connan, 1984) in fraction A, is not observed. Also no C27 rearranged steranes were detected in fraction A yet these steranes are thought to be more resistant to biodegradation than C2s and C29 unrearranged steranes (Seifert and Moldowan, 1979; Goodwin et al., 1983; Connan, 1984). Further work is required to resolve these problems. The Windy Knoll sample is dissimilar to the Staunton Harold and Mountsorrel bitumens in a number of ways which suggest a higher-temperature history. H u n t (1979) classified natural bitumens by their solubility in organic solvents, particularly carbon disulphide. Asphalts and asphaltites are soluble and pyrobitumens relatively inso!uble. Whilst the other two bitumens investigated in this study were completely soluble in dichloromethane, only 21% of the Windy Knoll sample dissolved. This puts it into the pyrobitumen class where H u n t (1979) has previously placed "elaterite" from the same locality. A higher-temperature history for the Windy Knoll samples is also supported by the biomarker evidence. No polycyclic alkanes were detected in either fractions A or P, which may indicate their having been thermally degraded, n-Alkanes and n-alkylcyclohexanes were generated from the insoluble residue by hydrous pyrolysis, which further suggests that the absence of these compounds in fraction A is due to microbial action. Khavari-Khorasani and Murchison (1978) have previously concluded from microscopical studies that bitumen at the base of the Windy Knoll deposit was a product of thermal metamorphism. Other independent evidence also supports the conclusion that the Windy Knoll sample has been subjected to higher temperatures than the other two. The Staunton Harold and Windy Knoll samples are from Pb-Zn deposits typical of those within the South Pennine Orefield, whereas the Mountsorrel sample is from a broadly similar style of mineralisa-
tion. Although no fluid inclusion data are available for the two Leicestershire occurrences, they are available for the orefield overall and place constraints on the temperatures at which the mineral deposits (and associated organic matter) formed. In general, the P b - Z n deposits, with characteristic calcite gangue, of the orefield formed in the range 60-100 ° C (Atkinson, 1983), and these temperatures are considered appropriate for both the Staunton Harold and Mountsorrel occurrences (especially, for the latter, in view of the close association between the bitumen and clays/goethite). However, Atkinson (1983) found that the Windy Knoll deposits have anomalously high fluid inclusion homogenisation temperatures of the order of 150°C. This is entirely consistent with the findings of this study which also indicate that relatively high-temperature processes have affected the composition and formation of the Windy Knoll bitumens. 6. C o n c l u s i o n s
This work has shown that three natural bitumens from central England, whose origins have previously been disputed, do contain biomarkers implying a biogenic origin. Biomarker alkanes were found both as free molecules (i.e. within fraction A) and bonded to the asphaltene or pyrobitumen matrices from where they were released by hydrous pyrolysis. The idea that the bitumens in central England might have had an abiogenic origin probably arose due to a combination of their geological situation (e.g., near to hydrothermal mineral deposits or igneous rocks) and their close proximity to the surface which allowed them to be microbially altered. These two factors have affected the three bitumens to varying extents. The Windy Knoll sample appears to have been subjected to higher temperatures but less microbial alteration than the Mountsorrel and Staunton Harold bitumens. Similar results and conclusions have been obtained by other workers investigating the origins of natural bitumens else-
194 w h e r e in the B r i t i s h Isles ( D i d y k et al., 1983; R o b i n s o n et al., 1986). Finally, this w o r k h a s c o n f i r m e d t h a t p y r o l ysis m e t h o d s such as h y d r o u s p y r o l y s i s c a n be useful in assisting in t h e d e t e r m i n a t i o n of t h e sources of n a t u r a l b i t u m e n s a n d o t h e r biodegraded substances.
Acknowledgements We t h a n k Dr. T.D. F o r d ( L e i c e s t e r U n i v e r sity) for his g u i d a n c e in t h e field, Dr. R.J. K i n g for p r o v i d i n g s a m p l e s f r o m t h e N a t i o n a l M u s e u m of Wales, the I S P G for access to G C - M S facilities a n d P r o f e s s o r G. E g l i n t o n a n d Mrs. A. G o w a r ( B r i s t o l U n i v e r s i t y ) for help in o b t a i n ing some initial G C - M S data. We w o u l d also like to t h a n k Dr. D.M. J o n e s for critically reading the m a n u s c r i p t , a n d Mrs. Y. Hall a n d Mrs. A. S u m m e r b e l l for t y p i n g a n d p r e p a r i n g figures.
References Atkinson, P., 1983. A fluid inclusion and geochemical investigation of the fluorite deposits of the Southern Pennine Orefield. Ph.D. Thesis, University of Leicester, Leicester, 284 pp. Bandurski, E., 1982. Structural similarities between oilgenerating kerogens and petroleum asphaltenes. Energy Sources, 6: 47-66. Behar, F.H., Pelet, R. and Roucache, J., 1984. Geochemistry of asphaltenes. Org. Geochem., 6: 587-596. Calvin, M., 1969. Chemical Evolution. Oxford University Press, London, 278 pp. Connan, J., 1984. Biodegradation of crude oils in reservoirs. In: J. Brooks and D. Welte {Editors), Advances in Petroleum Geochemistry, Vol. 1. Academic Press, London, pp. 299-335. Didyk, B.M., Simoneit, B.R.T. and Eglinton, G., 1983. Bitumen from Coalport Tar Tunnel. Org. Geochem., 5: 99-109. Eglinton, T.I., Rowland, S.J., Curtis, C.D. and Douglas, A.G., 1986. Kerogen-mineral reactions at raised temperatures in the presence of water. In: D. Leythaeuser and J. RullkStter (Editors), Advances in Organic Geochemistry 1985. Pergamon, Oxford, pp. 1041-1052. Ekweozor, C.M., 1984. Tricylic terpenoid derivatives from chemical degradation reactions of asphaltenes. Org. Geochem., 6: 51-62. Gold, T. and Soter, S., 1982. Abiogenic methane and the origin of petroleum. Energy Explor. Exploit., 1: 89-104.
Goodwin, N.S., Park, P.J.D. and Rawlinson, A.P., 1983. Crude oil biodegradation under simulated and natural conditions. In: M. Bjoroy, P. Albrecht, C. Cornfbrd et al. (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 650-658. Hampton, C.M. and Taylor, P.M., 1983. The age and nature of the basement of southern Britain: evidence from Sr and Pb isotopes in granites. J. Geol. Soc., 140: 499-509. Hunt, J.M., 1979. Petroleum Geochemistry and Geology. Freeman, San Francisco, Calif., 617 pp. Khavari-Khorasani, G. and Murchison, D.G., 1978. Thermally metamorphosed bitumen from Windy Knoll, Derbyshire, England. Chem. Geol., 22: 91-105. King, R.J., 1959. The mineralisation of the Mountsorrel Granodiorite. Trans. Leicester Lit. Philos. Suc., 53: 18-30. King, R.J. and Ford, T.D., 1968. Mineralization. In: P.C. Sylvester-Bradley and T.D. Ford (Editors), The Geology of the East Midlands, Ch. 7. Leicester University Press, Leicester pp. 112-137. Lewan, M.D., Winters, J.C. and McDonald, J.M., 1979. Generation of oil-like pyrolysates from organic rich shales. Science, 203: 897-899. MacKenzie, A.S., 1984. Applications of biological markers in petroleum geochemistry. In: J. Brooks and D. Welte (Editors), Advances in Petroleum Geochemistry, Vol. 1. Academic Press, London, pp. 115-214. Mueller, G., 1954. The theory of genesis of oil through hydrothermal alteration of coal type substances within certain Carboniferous strata of the British Isles. Int. Geol. Congr., 1952 Sess., Algiers, No. 12, pp. 279-328. Mueller, G., 1970. Indications of high temperature processes in organic geochemistry. In: G.D. Hobson and G.C. Speers (Editors), Advances in Organic Geochemistry 1966. Pergamon, Oxford, pp. 443-467. Nooner, D.W., Updegrove, W.S., Flory, D.A., Oro, J. and Mueller, G., 1973. Isotopic and chemical data of bitumens associated with hydrothermal veins from Windy Knoll, Derbyshire, England. Chem. Geol., 11: 189-202. Pering, K., 1973. Bitumens associated with lead, zinc and fluorite ore minerals in North Derbyshire, England. Geochim. Cosmochim. Acta, 37: 401-417. Pering, K. and Ponnamperuma, C., 1969. Alicyclic hydrocarbons from an unusual deposit in Derbyshire, England - - a study in possible diagenesis. Geochim. Cosmochim. Acta, 33: 528-532. Philp, R.P. and Gilbert, T.D., 1985. Source rock and asphaltene biomarker characterisation by pyrolysis-gas chromatography-mass spectrometry-multiple ion detection. Geochim. Cosmochim. Acta, 49:1421 1432. Ponnamperuma, C. and Pering, K., 1966. Possible abiogenic origin of some naturally occurring hydrocarbons. Nature (London), 209: 979-982. Robinson, N., Eglinton, G., Brassell, S.C., Gowar, A.P. and Parnell, J., 1986. Hydrocarbon compositions of bitu-
195 mens associated with igneous intrusions and hydrothermal deposits in Britain. In: D. Leythaeuser and J. RullkStter (Editors), Advances in Organic Geochemistry 1985. Pergamon, Oxford, pp. 145-152. Rubinstein, I., Spyckerelle, C. and Strausz, O.P., 1979. Pyrolysis of asphaltenes: A source of geochemical information. Geochim. Cosmochim. Acta, 43: 1-6. Seifert, W.K. and Moldowan, J.M., 1979. The effect of biodegradation on steranes and terpanes in crude oils. Geochim. Cosmochim. Acta, 43: 111-126. Sylvester-Bradley, P.C. and Ford, T.D., 1968. The geology of the East Midlands. Leicester University Press, Leicester, 400 pp. Sylvester-Bradley, P.C. and King, R.J., 1963. Evidence for abiogenic hydrocarbons. Nature (London), 198: 728-731.
Telnaes, N., Speers, G.C., Steen, A. and Douglas, A.G., 1985. Hydrous pyrolysis of asphaltenes. In: B.M. Thomas et al. (Editors), Geochemistry in Exploration of the Norwegian Shelf. Graham & Trotman, London, pp. 287-292. Volkman, J.K., Alexander, R., Kagi, R.I. and Woodhouse, G.W., 1983. Demethylated hopanes in crude oils and their application in petroleum geochemistry. Geochim. Cosmochim. Acta, 47: 785-794. Winters, J.C., Williams, J.A. and Lewan, M.D., 1983. A laboratory study of petroleum generation by hydrous pyrolysis. In: M. Bjoroy, P. Albrecht, C. Cornford et al. (Editors), Advances in Organic Geochemistry 1981. Wiley, Chichester, pp. 524-533.
181
[2]
INVESTIGATION OF THREE NATURAL BITUMENS FROM CENTRAL ENGLAND BY HYDROUS PYROLYSIS AND GAS CHROMATOGRAPHY-MASS SPECTROMETRY GOU X U E M I N 1'.1, M.G. F O W L E R 1'.2, P.A. C O M E T l'*a, D.A.C. M A N N I N G 1, A.G. DOUGLAS 1, J. McEVOY 2'.4 and W. GIGER 2 Organic Geochemistry Unit, Department of Geology, University of Newcastle upo n- Tyne, Newcastle- upon- Tyne NE1 7RU (Great Britain) ~Swiss Federal Institute for Water Resources and Water Pollution Control ( EA WA G), CH-8600 Di~bendor[ (Switzerland) (Received March 10, 1987; revised and accepted May 13, 1987)
Abstract Gou Xuemin, Fowler, M.G., Comet, P.A., Manning, D.A.C., Douglas, A.G., McEvoy, J. and Giger, W., 1987. Investigation of three natural bitumens from central England by hydrous pyrolysis and gas chromatography-mass spectrometry. Chem. Geol., 64: 181-195. Bitumen samples from three hydrothermal mineral deposits (Windy Knoll, Derbyshire; Staunton Harold and Mountsorrel, Leicestershire) have been studied partly in an attempt to resolve the continuing controversy over whether they are biogenic or abiogenic in origin, and partly to investigate a new approach to studying such materials. A more conventional method was first employed, involving analysis of alkane fractions isolated from the bitumens using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). This showed that the Staunton Harold and Mountsorrel bitumens contained abundant biomarkers (tricyclic terpanes, steranes and hopanes) but no nalkanes, acyclic isoprenoids and n-alkylcyclohexanes, whereas the Windy Knoll bitumen lacked biomarkers other than acyclic isoprenoids. Hydrous pyrolysis of the insoluble residue from the Windy Knoll bitumen, and of the asphaltenes from the other two bitumens, produced alkanes in the pyrolysates which were analysed using GC and GC-MS. The pyrolysates contained abundant n-alkanes, n-alkylcyclohexanes and acyclic isoprenoids in addition to the polycyclic alkanes found in the original bitumens. The distributions of steranes and hopanes in the original Staunton Harold and Mountsorrel bitumens and their pyrolysates are briefly discussed. Our results suggest that the bitumens are of biogenic origin and have been subjected to varying degrees of biodegradation and thermal maturation.
Mountsorrel, Leicestershire (Fig. 1 ), added to the controversy because gas chromatograms of
1. I n t r o d u c t i o n
T h e occurrence of natural b i t u m e n s within Carboniferous rocks in central England in association with hydrothermal mineralisation ( Mueller, 1954, 1970; King and Ford, 1968) led to considerable discussion on whether they had a biogenic or abiogenic origin (Sylvester-Bradley and King, 1963 ). Early work on the organic geochemistry of one of these bitumens from 0009-2541/87/$03.50
Permanent addresses: "Research Institute of Petroleum Exploration, Chengdu, People's Republic of China. "2ISPG, 3303, 33rd Street N.W., Calgary, Alta. T2L 2A7, Canada. *:~Core Laboratories Singapore, 24A Lim Teck Boo Road, Singapore. *4School of Ocean Science, Marine Science Labs., University of Bangor, Menai Bridge, Gwynedd, LL59 5EY, U.K.
© 1987 Elsevier Science Publishers B.V.
182 these investigations (including the finding of biomarkers such as acyclic isoprenoids, the carbon isotope ratios and optical studies) suggested that the Windy Knoll bitumens probably did have a biogenic origin (Nooner et al., 1973; Pering, 1973; Khavari-Khorasani and Murchison, 1978). In this present study, samples from three
alkanes isolated from this deposit were found to be dominated by a "hump" of unresolved compounds similar to those produced abiogenically in laboratory experiments (Ponnamperuma and Pering, 1966; Calvin, 1969). Other bitumens from this area that were frequently studied include those from Windy Knoll, Derbyshire ( Mueller, 1954 ). The results of some of I
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WINDY KNOLL
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70
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NTON HAROLD
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National Grid coordinates
within lOOkm grid square SK
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183
well-known bitumen occurrences in central England whose origins have been disputed in the past (Sylvester-Bradley and King, 1963; Ponnamperuma and Pering, 1966; Calvin, 1969; Nooner et al., 1973; Pering, 1973) have been reinvestigated. This was primarily to investigate the usefulness of a new approach in the study of such materials. The choice of samples was partly inspired by Gold citing the past controversy over the origins of these particular bitumens in support of his theory of a possible abiogenic origin for petroleum (Gold and Soter, 1982 ). In addition this study contributes to our understanding of the means by which organic matter is included within hydrothermal mineral deposits of essentially Mississippi Valley type, placing further constraints on theories of their genesis. Two different investigative methods were used in the present work. A more conventional method was first employed, i.e. an investigation of biomarkers, using gas chromatography-mass spectrometry ( G C - M S ) , in the alkanes separated from the bitumens. These techniques have been used by previous workers in studies of similar types of samples (Didyk et al., 1983; Robinson et al., 1986). Subsequently, asphaltenes isolated from two of the bitumens, and the insoluble residue from the third bitumen, were pyrolysed in the presence of water (hydrous pyrolysis) and the alkanes isolated from the resulting pyrolysates were analysed using GC-MS. It has been shown previously in this laboratory that hydrous pyrolysis of asphaltenes separated from the natural bitumen Gilsonite generates biomarkers with distributions similar to those thought to occur in its source rock (Telnaes et al., 1985). Workers in other laboratories, using different pyrolysis techniques, have reported similar results (Bandurski, 1982; Rubinstein et al., 1979; Behar et al., 1984). Hydrous pyrolysis was chosen for this work, rather than other forms of pyrolysis, since its products are reported to more closely resemble naturally occurring bitumens and oils
(Lewan et al., 1979; Winters et al., 1983).
2. Geological setting and samples Localities for the samples examined in this study are given in Fig. 1, and the geology of the region has been summarised by SylvesterBradley and Ford (1968). One sample was chosen from a collection made at Windy Knoll near Castleton in Derbyshire (National Grid Reference SK127830), where a number of different bitumens occur within neptunian dykes and hydrothermal veins within Lower Carboniferous limestones (Mueller, 1954, 1970). The sample studied was a greenish-yellow viscous oil which has previously been classified as a "vaselous" oil (Mueller, 1954), an oil (Pering, 1973) and a type-C bitumen (Nooner et al., 1973). The second sample was from material collected by Dr. R.J. King from the Earl Ferrer's lead mine at Staunton Harold, Leicestershire (SK374217; National Museum of Wales accession number 83.41G.M9440). At this locality, hydrothermal veins carrying galena and sphalerite occur within dolomitised Lower Carboniferous limestones overlain unconformably by Namurian shales (King, 1959; King and Ford, 1968). The bitumen occurs within the gangue minerals of the vein which are mostly calcite and dolomite. The third sample is from the former Main Quarry at Mountsorrel, Leicestershire (SK477148; National Museum of Wales accession number 83.41G,M9439) and was again collected by Dr. R.J. King. Here, the Caledonian Mountsorrel Granite (430 + 7 Ma; Hampton and Taylor, 1983) is cut by dolerite dykes which correlate with dykes of Hercynian age within nearby Coal Measures ( King, 1959 ). The granite adjacent to the dykes is extensively hydrothermally altered and vuggy, and is mineralised with a dolomite-calciteclay-pyrite assemblage which contains bitumen; the sample studied is from this assemblage. This mineralisation is considered to
184 predate the current Permo-Trias cover, and may have formed beneath a cover of Carboniferous sediments similar to those exposed at S t a u n t o n Harold (King, 1959). Thus, in summary, the Windy Knoll and S t a u n t o n Harold samples are from closely corresponding geological settings - - within Carboniferous limestones overlain unconformably by N a m u r i a n shales - - whereas the Mountsorrel sample is from mineralised altered granite. In all cases the mineralisation and associated bitumen is hosted by a reactive, competent and relatively easily dissolved rock beneath an actual or inferred impervious cover.
40-60 ° C). Another aliquot of each extract was redissolved in dichloromethane and asphaltenes were precipitated by the addition of a 40fold excess of cold n-heptane. The asphaltenes were purified by reprecipitation twice from clean solvent. Significant amounts of asphaltenes were isolated from the S t a u n t o n Harold and Mountsorrel bitumens (Table I) but none was obtained from the Windy Knoll bitumen extract. Asphaltenes isolated from the S t a u n t o n Harold and Mountsorrel bitumens and the insoluble residue of the W i n d y Knoll sample were hydrously pyrolysed using a method similar to t h a t of Eglinton et al. {1986), which is itself based on one previously described by Lewan et al. (1979) and Winters et al. (1983). The experiments were carried out in small "bomblets", purpose built from 316 grade stainless steel [ 2 in. ( ~ 5 cm) long, ~ in. ( ~ 9.5 ram) i.d., ~ in. ( ~2.22 cm) across flat hexagonal section, internal capacity 3 ml ] each fitted with a steel screw closure and a soft copper sealing gasket. Distilled water (1 ml ) and ~ 300 mg of the material to be pyrolysed were placed in each bomblet which was then purged with nitrogen, to remove air, and tightly sealed. The bomblets were placed in a pressure reactor (1 1,
3. Experimental
Mineral-free bitumen samples were first extracted with dichloromethane for 24 hr. using a Soxhlet apparatus. The Mountsorrel and S t a u n t o n Harold bitumens completely dissolved during this extraction but only 21% of the Windy Knoll bitumen was found to be soluble (Table I). Aliphatic hydrocarbons (fraction A) were separated from a portion of the extracts by thin layer chromatography ( T L C ) using glass plates coated (0.55 mm thick) with silica gel (Merck ® Kieselgel Nach Stahl, type 60G) and developed with light petroleum (b.p. TABLEI Gross geochemicaldata Sample
% extracted
% aliphatics
% aromatics
% resins
% asphaltenes
100.1 100"1 21.2°~
15.3 20.2 23.9
11.0 14.8 7.4
73.7 65.0 68.7
24.8 16.1 0.0
49.4"2 22.6"2
11.3 24.7
13.7
72.5
Bitumens:
Staunton Harold Mountsorrel Windy Knoll Asphaltene pyrolysate:
Staunton Harold Mountsorrel Unextracted residue pyrolysate:
Windy Knoll •1Extractedusing dichloromethane. "~Extractedusing light petroleum ether.
13.8
185 Parr Instrument Co. ) also part-filled with water (to minimise the pressure differential across the bomblet wall and to reduce temperature gradients in the system), which was in turn sealed. The temperature of the reactor was raised by a heating jacket, with temperature control to + 5 ° C. Samples were heated rapidly to 330 ° C and held isothermal for 72 hr. after which time the vessel was allowed to cool prior to opening. The resulting pyrolysates were extracted with light petroleum (b.p. 40-60 ° C) : these extracts were then dried by filtering through short columns of activated alumina. Aliphatic hydrocarbons were separated from the extract using TLC (as described on p.184) to give the " P " fractions. Prior to G C - M S analyses, branched/ cyclic alkanes were separated from straightchain and simply branched alkanes by urea adduction. Gas chromatography was carried out on a Carlo Erba ® 4160 instrument equipped with a fused silica column (50 m × 0.32 m m i.d. ) coated with OV-1 ®, using on-column injection. H2 was used as carrier gas and a temperature programme of 50-290°C at 4°C min. 1 was employed. G C - M S analysis was performed on a Carlo Erba ® 4200 GC directly coupled to a Kratos ® MS80-RF (electron energy 70 eV) under the control of a DS-5M ® data system. The GC was equipped with a fused silica colu m n (25 m × 0.3 m m i.d.) coated with DB5 ® ( J & W Scientific) and the temperature programme employed was from 50 to 325 ° C at 4 ° C m i n . - ~. 4. R e s u l t s
4.1. Aliphatic hydrocarbons extracted from bitumens The gas chromatograms of the A fractions (aliphatic hydrocarbons separated from the original bitumens), were dominated by "humps" of unresolved components on which some small peaks were superimposed (Figs. 2a, 3a and 4a). G C - M S analysis of the Mountsor-
rel and Staunton Harold bitumens indicated t h a t they contained polycyclic alkanes such as tricyclic terpanes, steranes and hopanes but no n-alkanes, acyclic isoprenoids or n-alkylcyclohexanes. The distributions of hopanes and steranes (Figs. 5a, b and 6a, b) in these two samples are those expected from mature organic m a t t e r (MacKenzie, 1984) with, for example, the former series dominated by 17~ ( H ) ,21fl( H ) hopanes and the latter showing almost equal concentrations of 5o~(H),14fl(H),I7fl(H) steranes and 5c~(H),14o~(H),17a(H) steranes (Table II). The Staunton Harold bitumen showed a sterane distribution with some features that could be indicative of more severe biodegradation. These include a greater abundance of diasteranes t h a n unrearranged steranes and apparent lack of C27 steranes ( Fig. 5a). No biomarkers were detected in the Windy Knoll A fraction, except for the previously reported acyclic isoprenoids (Pering and Ponnamperuma, 1969; Nooner et al., 1973; Pering, 1973). The lack of polycyclic alkanes in this bitumen might indicate that it has been subjected to temperatures high enough to have caused their thermal degradation.
4.2. Aliphatic hydrocarbons in pyrolysates In contrast to the gas chromatograms of the A fractions, those of alkanes in the pyrolysates ( P fractions ) of the Mountsorrel and Staunton Harold asphaltenes and the Windy Knoll insoluble fraction all have n-alkanes ranging from about C1~-C3o as their highest peaks (Figs. 2b, 3b and 4b ). Acyclic isoprenoids with 15-20 carbon atoms per molecule are also prominent in these chromatograms. The n-alkane and acyclic isoprenoid peaks are superimposed on "humps" in the chromatograms of the Staunton Harold (Fig. 2b) and Windy Knoll (Fig. 4b) P fractions but there is no prominent hump in the chromatograms of the Mountsorrel P fraction. G C - M S analysis revealed the presence of acyclic isoprenoids (possibly up to C4o) and n-
186
(a)
STAUNTON HAROLD A
i
iJ,i~
.... .Y STAUNTON HAROLD P
IICI5 i
nC2o
p~
Pril '¸¸
Fig. 2. Gas chromatograms of: (a) alkanes (A) extracted from Staunton Harold bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of asphaltenes isolated from the Staunton Harold bitumen. Peaks labelled in (a) are: x, C j 13fl ( H ) ,17v~ ( H ) 2 0 S + R-diasterane; and y, 17~ (H) ,2Ifl ( H ) -hopane. Peaks labelled in (b) are C15, C~,~and C.~ n-alkanes ( P h = phytane; P r = pristane ).
187
MOUNTSORREL A
s
J
MOLINTSORREL P
nCls
nC2o nC2s
I I
I
I I
jl P'
Ph
J~ ~l'# Fig. 3. Gas chromatograms of: (a) alkanes (A) extracted from Mountsorrel bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of asphaltenes isolated from the Mountsorrel bitumen. Peaks labelled as for Fig. 2.
alkylcyclohexanes (C14-C26) in the Mountsorrel and Staunton Harold P fractions. Hopanes
and steranes are also present in these fractions (Figs. 5c, d and 6c, d) with steranes in much
188
WINDY KNOLL A
J
1__
WINDY KNOLL P
nC2o
nC25
nC15
nC3o
I i' i
L1
i Ph
J
,J~tj,lJ!,/ 1 , '~:~,%t~!I~,1,4, Et,~tl I, ~LI Pr
I
Fig. 4. Gas chromatograms of: (a) alkanes (A) extracted from Windy Knoll bitumen; and (b) alkanes (P) obtained from the hydrous pyrolysis of the unextracted residue of Windy Knoll bitumen.
189 T A B L E II Biomarker data from G C - M S analysis of Staunton Harold and Mountsorrel bitumen fractions Sample
Fraction
C27
C28
C29
a
C29c~o~o~20(R + S )
a ow~20S
~ o~fl fl ~29 o~cL~+ o~flfl
C~,
~
[3o~ ~
b
c
d
e
Staunton Harold
A P
n.d. 28
30 27
70 45
0.50 0.48
0.53 0.48
1.88 1.43
0.30 0.26
Mountsorrel
A P
37 37
19 21
44 42
0.52 0.49
0.47 0.42
1.31 0.77
0.23 0.22
a = percentage of 5~ ( H ) ,14fl( H ) ,I 7fl( H ) -steranes ( from m/z 217 )
b= C2~ 5(~ ( H ) ,14c~ ( H ) ,17~ ( H ) 20S/C29 5e~ ( H ) ,14a ( H ) ,17c~ ( H ) 20S + 20R-steranes (from m/z 217) c=C295(~(H),I4fl(H),17fl(H) /C295a(H),14fl(H),17fl(H) + 5 ~ ( H ) , 1 4 ~ ( H ) , 1 7 a ( H ) - s t e r a n e s (from m/z 217) d = C2,13~( H ), 17~ (H) 20S-diasterane/C29-5a ( H ) ,14a ( H ) ,17(~ ( H ) 20R-sterane ( from m/z 217 )
e = 17fl(H ) ,21c~ (H) -moretane/l 7a (H) ,21fl (H) -hopane (from m/z 191 ) n.d. = not determinable.
lower concentrations relative to the hopanes than they were in the A fractions. The distribution of these polycyclic alkanes shows some systematic differences between the A and P fractions of the Staunton Harold bitumen. The P fractions appear to be slightly less mature as indicated by smaller values for the sterane ratios b and c but slightly more mature as indicated in the hopane ratio (see Table II). The C2v hopanes 17~ ( H ) ,22,29,30-trisnorhopane (Tin) and 18c~( H ) ,22,29,30-trisnorhopane ( Ts ) were present in roughly equal amounts in the A fraction, whereas Tm alone was detected in the P fraction (Fig. 5a and c). The absence of T~ in kerogen pyrolysates has also been observed in other studies ( e.g., Eglinton et al., 1986). It was also noted that the relative abundance of gammacerane in the P fraction is greatly reduced from that in the A fraction. Other significant differences in the sterane patterns of fractions A and P (Fig. 5b and d) include the presence, in the latter, of C27 steranes in concentrations about equal to those of the Czs steranes, diasteranes in lower abundance relative to unrearranged steranes ( Table II, ratio d) and greater amounts of steranes with short side-chains (peaks I-5) compared to the C2v-C29 steranes. Most of the differences in the hopane and ste-
rane patterns discussed above were also noted in the Mountsorrel fractions (Fig. 6; Table II). One major difference between the Mountsorrel and Staunton Harold samples was that there was little change in the relative amounts of C2v-C29 steranes in the respective A and P fractions of the former. GC-MS analysis of the Windy Knoll P fraction confirmed the presence of acyclic isoprenoids and n-alkylcyclohexanesbut no polycyclic alkanes were detected. 5. Discussion
Biomarkers are present in all three samples, indicating that these bitumens are at least partially derived from biogenic sources. The unresolved complex mixtures observed in the gas chromatograms of their A fractions suggest that the bitumens have been biodegraded. Previous authors have established the following sequence for the selective removal of compound classes by microorganisms: n-alkanes
1
•0••
F
3
It -4
2
m/Z 217
(b) STAUNTON HAROLD A
180•
I o
C
m/z ~9~
2200
G
'15
12
21+22
240•
•60•
P L R
i
m/z 217
(d) STAUNTON HAROLD P
STAUNTON HAROLD P m/z 191
6
11+12
21 23
IN
P
Fig. 5. m/z 191 and m/z 217 mass fragmentograms of Staunton Harold A and P branched/cyclic alkane fractions. Peaks identified in Tables III and IV.
5~
6@
?g
Bg
(a) STAUNTON HAROLD A
(C)
191
~-[ ~'-
~
i
i
~Z
©
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2
L.
E
b
T
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-
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~
0
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-
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ii~---~
192 TABLE III Peaks identified in m/z 191 fragmentograms A
C2~)tricyclic terpane C21 tricyclic terpane C2:~tricyclic terpane C24 tricyclic terpane C2,~tricyclic terpane C26 tricyclic terpane C2s tricyclic terpane C29 tricyclic terpane T~: 18~ (H) ,22,29,30-trisnorhopane Tin: 17a (H) -trisnorhopane 17 ~ ( H ) ,21fl ( H ) ,30-norhopane unknown 17fl(H),21~ (H) ,30-normoretane 17a (H) ,21fl(H)-hopane 17fl( H ) ,21~ (H) -moretane 17 ~ ( H ) ,21fl ( H ) -homohopanes gammacerane 17~ ( H ) ,21fl ( H ) -bishomohopanes 17~ (H) ,21fl ( H ) -trishomohopanes 17~ (H) ,21fl ( H ) -tetrakishomohopanes 17~ (H) 21fl ( H ) -pentakishomohopanes
B
C D E F G H I
J K L M N 0 P Q R S T U
TABLE IV Peaks identified in m/z 217 fragmentograms 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
C,9 sterane C~1 sterane C2l sterane C22 sterane C22 sterane 13,8(H ) ,17a (H) -diacholestane (20S) 13fl (H),17a (H)-diacholestane (20R) 24-methyl-13,8( H ) ,17a ( H ) -diacholestane ( 20S ) 24-methyl-13,8 (H),I 7a ( H ) -diacholestane (20R) 5(~ (H) ,14~ (H) ,17a (H) -cholestane (20S) 5~ ( H ) ,14fl ( H ) ,17fl ( H ) -cholestane (20R) 24-ethyl-13fl(H) ,17a (H) -diacholestane (20S) 5~(H),14fl(H),17fl(H)-cholestane (20S) 5 a ( H ) , 1 4 ~ ( H ) , 1 7 ~ ( H ) - c h o l e s t a n e (20R) 24-ethyl-13fl( H ) ,I 7a ( H ) -diacholestane (2OR) 24-methyl-5a ( H ),14~ (H) ,17a (H) -cholestane (20S) 24-methyl-5~ (H) ,1413(H) ,17,8 (H) -cholestane (2OR) 24-methyl-5~ { H ) ,14fl ( H ) ,17fl ( H ) -cholestane (20S) 2 4-methyl-5~ ( H ) ,14~ ( H ) ,17 ~ ( H ) -cholestane (20R) 24-ethyl-5a ( H ) ,14~ ( H ) ,17~ (H) -cholestane (20S) 24-ethyl-5a ( H ) ,1413(H ) ,17]3( H ) -cholestane (20R ) 2 4-ethyl-5~ ( H ) ,14fl ( H ) ,17,8(H) -cholestane ( 20S ) 24-ethyl-5a (H) ,14~ (H) ,17a (H) -cholestane (20R)
alkylcyclohexanes, acyclic isoprenoids) are the most susceptible to biodegradation. The Windy Knoll A fraction does contain acyclic isoprenoids which were not detected in the A fractions of the other two samples, indicating it to be the least biodegraded. Hydrous pyrolysis of the asphaltenes from the Mountsorrel and Staunton Harold bitumens generated alkanes which gave gas chromatograms similar to those from undegraded or partially degraded oils. This is consistent with the observations that n-alkanes, n-alkylcyclohexanes and isoprenoid moieties in the asphaltene macromolecule are, in some way, protected from biodegradation (Rubinstein et al., 1979; Ekweozor, 1985; Telneas et al., 1985 ). The lower maturity of steranes in the asphaltene pyrolysates is also in agreement with the results of previous workers ( Philp and Gilbert, 1985; Telneas et al., 1985). However, the differences in the ratios b and c between the A and P fractions, and also the increase in maturity as indicated by hopane ratio e, are very small and probably within the limits of experimental error. One puzzling and unexplained result concerns the C27 steranes which are absent from fraction A of the Staunton Harold bitumen, but present in fraction P (Fig. 5; Table II). Because the relative amounts of C~7-C~ steranes between the A and P fractions remain similar for the Mountsorrel bitumen, the occurrence of C27 steranes in the Staunton Harold pyrolysates is unlikely to be due to cracking of the C~s and C29 steranes. However, cracking processes do appear to have occurred, as indicated by the relative increase in the abundance of steranes with short side-chains (C19-C22steranes) in the P fractions of both Staunton Harold and Mountsorrel bitumens. Microbial processes should be considered for causing the differences discussed. It is known that C27 regular steranes are biodegraded in preference to Czs and C29 regular steranes (Goodwin et al., 1983; Volkman et al., 1983) but invoking this for the Staunton Harold bitumen leaves the problem that, preferential
193
removal of the C29 5 a ( H ) , 1 4 a ( H ) , 1 7 a ( H ) 20R-sterane relative to the C29 5o~(H),I4~ ( H ) ,17a ( H ) 20S-sterane ( Volkman et al., 1983; Connan, 1984) in fraction A, is not observed. Also no C27 rearranged steranes were detected in fraction A yet these steranes are thought to be more resistant to biodegradation than C2s and C29 unrearranged steranes (Seifert and Moldowan, 1979; Goodwin et al., 1983; Connan, 1984). Further work is required to resolve these problems. The Windy Knoll sample is dissimilar to the Staunton Harold and Mountsorrel bitumens in a number of ways which suggest a higher-temperature history. H u n t (1979) classified natural bitumens by their solubility in organic solvents, particularly carbon disulphide. Asphalts and asphaltites are soluble and pyrobitumens relatively inso!uble. Whilst the other two bitumens investigated in this study were completely soluble in dichloromethane, only 21% of the Windy Knoll sample dissolved. This puts it into the pyrobitumen class where H u n t (1979) has previously placed "elaterite" from the same locality. A higher-temperature history for the Windy Knoll samples is also supported by the biomarker evidence. No polycyclic alkanes were detected in either fractions A or P, which may indicate their having been thermally degraded, n-Alkanes and n-alkylcyclohexanes were generated from the insoluble residue by hydrous pyrolysis, which further suggests that the absence of these compounds in fraction A is due to microbial action. Khavari-Khorasani and Murchison (1978) have previously concluded from microscopical studies that bitumen at the base of the Windy Knoll deposit was a product of thermal metamorphism. Other independent evidence also supports the conclusion that the Windy Knoll sample has been subjected to higher temperatures than the other two. The Staunton Harold and Windy Knoll samples are from Pb-Zn deposits typical of those within the South Pennine Orefield, whereas the Mountsorrel sample is from a broadly similar style of mineralisa-
tion. Although no fluid inclusion data are available for the two Leicestershire occurrences, they are available for the orefield overall and place constraints on the temperatures at which the mineral deposits (and associated organic matter) formed. In general, the P b - Z n deposits, with characteristic calcite gangue, of the orefield formed in the range 60-100 ° C (Atkinson, 1983), and these temperatures are considered appropriate for both the Staunton Harold and Mountsorrel occurrences (especially, for the latter, in view of the close association between the bitumen and clays/goethite). However, Atkinson (1983) found that the Windy Knoll deposits have anomalously high fluid inclusion homogenisation temperatures of the order of 150°C. This is entirely consistent with the findings of this study which also indicate that relatively high-temperature processes have affected the composition and formation of the Windy Knoll bitumens. 6. C o n c l u s i o n s
This work has shown that three natural bitumens from central England, whose origins have previously been disputed, do contain biomarkers implying a biogenic origin. Biomarker alkanes were found both as free molecules (i.e. within fraction A) and bonded to the asphaltene or pyrobitumen matrices from where they were released by hydrous pyrolysis. The idea that the bitumens in central England might have had an abiogenic origin probably arose due to a combination of their geological situation (e.g., near to hydrothermal mineral deposits or igneous rocks) and their close proximity to the surface which allowed them to be microbially altered. These two factors have affected the three bitumens to varying extents. The Windy Knoll sample appears to have been subjected to higher temperatures but less microbial alteration than the Mountsorrel and Staunton Harold bitumens. Similar results and conclusions have been obtained by other workers investigating the origins of natural bitumens else-
194 w h e r e in the B r i t i s h Isles ( D i d y k et al., 1983; R o b i n s o n et al., 1986). Finally, this w o r k h a s c o n f i r m e d t h a t p y r o l ysis m e t h o d s such as h y d r o u s p y r o l y s i s c a n be useful in assisting in t h e d e t e r m i n a t i o n of t h e sources of n a t u r a l b i t u m e n s a n d o t h e r biodegraded substances.
Acknowledgements We t h a n k Dr. T.D. F o r d ( L e i c e s t e r U n i v e r sity) for his g u i d a n c e in t h e field, Dr. R.J. K i n g for p r o v i d i n g s a m p l e s f r o m t h e N a t i o n a l M u s e u m of Wales, the I S P G for access to G C - M S facilities a n d P r o f e s s o r G. E g l i n t o n a n d Mrs. A. G o w a r ( B r i s t o l U n i v e r s i t y ) for help in o b t a i n ing some initial G C - M S data. We w o u l d also like to t h a n k Dr. D.M. J o n e s for critically reading the m a n u s c r i p t , a n d Mrs. Y. Hall a n d Mrs. A. S u m m e r b e l l for t y p i n g a n d p r e p a r i n g figures.
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