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Release of bound aliphatic biomarkers via hydropyrolysis from Type II kerogen at high maturity
Organic Geochemistry 39 (2008) 1119–1124
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Release of bound aliphatic biomarkers via hydropyrolysis from Type II kerogen at high maturity Robert S. Lockhart a, Will Meredith a, Gordon D. Love b, Colin E. Snape a,* a b
Nottingham Fuel and Energy Centre, School of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Department of Earth Sciences, University of California, Riverside, CA, USA
a r t i c l e
i n f o
Article history: Received 27 September 2007 Received in revised form 14 March 2008 Accepted 18 March 2008 Available online 1 April 2008
a b s t r a c t At relatively low maturity, kerogen-bound aliphatic biomarkers invariably display retarded maturity compared to their ‘‘free” counterparts as a result of the protection afforded by the macromolecular structure. This study investigates the release of hopanes and steranes at elevated (post oil window) maturity from Type II kerogen. Results for a suite of North Sea petroleum source rocks (Kimmeridge Clay Formation) indicate that, below a hydrogen index (HI) value of ca. 300, differences between the ‘‘free” and ‘‘bound” phases for hopane and sterane side chain isomerisation are small. However, ring isomerisation within the kerogen-bound phase is still more retarded. For a corresponding series of kerogen samples artificially matured via hydrous pyrolysis, both ‘‘free” and ‘‘bound” phase side chain isomerisation reactions are significantly retarded relative to the naturally occurring samples. This is consistent with earlier studies, but the extent of retardation for ring isomerisation in the ‘‘bound” phase is even more pronounced than for the ‘‘free” phase. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fixed-bed hydropyrolysis (Hypy) refers to pyrolysis assisted by high hydrogen pressure (>10 MPa) and a dispersed sulfided molybdenum catalyst, which permits covalent bond cleavage at relatively low temperature (<450 °C; Love et al., 1995). In comparison with conventional pyrolysis techniques, Hypy has been proven to possess the unique ability to release high yields of bound biomarkers (solvent soluble product) from petroleum source rocks and heavy oil fractions. Overall conversion reaches >85 wt% of the total organic matter, while alteration of their isomeric distributions is crucially minimised (Love et al., 1995). Kerogen-bound hopanes and steranes undergo the same epimerisation reaction pathways as their solvent extractable, ‘‘free” counterparts within the bitumen. However, despite tending towards the same point of equilibrium, they are generally less mature in terms of isomerisation at both ring and side chain chiral centres, suggesting a much great-
er sensitivity to relatively small change in maturity (with the former appearing more strongly retarded). Of particular interest are kerogen-bound aliphatic biomarker distributions from samples at elevated, post oil window maturity, where those of the solvent extractable ‘‘free” phase appear in much lower concentration and have often long since reached equilibrium (Van Graas, 1990). As such, these biomarkers have limited application and in some extreme cases show inversion at elevated maturity (Zhang et al., 2005). For such kerogens, the ability of Hypy to provide meaningful information at high thermal maturity has still to be demonstrated fully. In this study, hopane and sterane maturity parameters are reported for a suite of Type II kerogen Kimmeridge Clay source rocks from the North Sea, with hydrogen index (HI) values as low as ca. 50 mg HC/g TOC, comparison being made to artificially matured kerogen samples prepared using hydrous pyrolysis.
2. Methods * Corresponding author. Tel.: +44 (0)115 951 4166; fax: +44 (0)115 951 4115. E-mail address: [email protected] (C.E. Snape). 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.03.016
Table 1 lists the suite of naturally occurring Kimmeridge Clay source rocks investigated, together with their
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R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
Table 1 Bulk geochemical data for a suite of North Sea Kimmeridge Clay source rocks (Okiongbo et al., 2005) Well
Depth (m)
TOC (wt%)
Tmax (°C)
HI
Dorset outcrop 204/28-2 16/7b-20 34/8-6 205/26a-3 16/7b-20 16/7b-28 16/7b-20 211/12A-M16 211/12A-M16 3/29a-4 3/29a-4 3/29a-4 3/29-2
0 2330 3932 3578 2414 4157 4132 4134 3376 3401 4781 4707 4742 4608
8.22 9.98 7.71 9.04 7.64 8.80 9.63 8.47 8.71 8.32 6.18 5.11 5.62 6.07
422 407 430 437 423 430 438 430 421 425 464 454 455 452
525 406 393 315 303 259 250 216 138 121 65 48 38 35
bulk geochemical data. The samples, all containing Type II kerogen, have been selected from a variety of different localities and depths, and collectively span the entire oil generation window, from early through to late mature (Okiongbo et al., 2005). Although HI has been selected as the maturity parameter in the absence of vitrinite reflectance data, instances are discussed where biomarker maturity correlations do not hold and where no significant increase in Tmax occurs with decreasing HI. Fig. 1 shows the overall relationship between HI and Tmax, where such outliers can probably be explained by facies variation within the sample suite. While Tmax could also have been used as the primary measure of maturity, in this instance the greater range in measured HI values has been used to provide higher resolution. The fixed bed Hypy tests were conducted on the pre-extracted source rocks using apparatus described in detail previously (Love et al., 1995; Roberts et al., 1995). The samples were pyrolysed with resistive heating from 50 °C to 250 °C at 250 °C min 1, then from 250 °C to 350 °C at 8 °C min 1. With all residual bitumen removed from the sample, the trap is replaced and the second heating stage is employed, from 50 °C to 350 °C at 250 °C min 1, and fi-
600
HI
400
200
0 400
420
440
460
480
Tmax (ºC) Fig. 1. Plot of HI vs. Tmax for a suite of North Sea, Kimmeridge Clay source rocks.
nally 350 °C to 500 °C at 8 °C min 1, when the kerogenbound products are released. The system is maintained under a constant H2 pressure of 15 MPa and the sweep gas flow of 5 l min 1 ensures that all products are quickly removed from the reactor and are collected in a silica-filled trap, cooled with dry ice (Meredith et al., 2004). Hydrous pyrolysis experiments were conducted on a known weight of powdered (<220 lg) immature (TOC 25%, HI 597) Kimmeridge Clay shale from the Dorset coast (SW England). Samples were pyrolysed 24 h using a 22 ml stainless steel autoclave, pressure gauge-rated to 550 bar, heated in a fluidised sand bath, to the desired temperature (310 °C, 320 °C and 350 °C). The hydrous pyrolysis experiments were all performed with 10 ml of distilled water added to the reactor vessel. Pyrolysed samples were dried and extracted with an azeotropic mixture (200 ml) of 93% CH2Cl2/7% MeOH. The aliphatic fractions separated on silica gel were analysed via gas chromatography–mass spectrometry (GC– MS) using a Varian Instruments CP 3800 gas chromatograph interfaced to a 1200 Quadrupole mass spectrometer (ionising energy 70 eV, source temperature 280 °C). Separation was performed with a fused silica capillary column (50 m 0.32 mm i.d.) coated with BPX5 phase (0.25 lm thickness). He was the carrier gas and the temperature programme was: 50 °C (2 min.) to 300 °C (28 min.) at 5 °C min 1. Ions monitored included m/z 191 (hopanes) and m/z 217 (steranes).
3. Results and discussion 3.1. North sea source rocks Fig. 2 shows the m/z 191 and 217 single ion chromatograms for both the ‘‘free” and kerogen-‘‘bound” phases of a typical high maturity sample (HI 65). Fig. 3 compares the C30ba/C30ab (moretane/hopane) and C31ab 22S/(S + R) ratio values, respectively for the solvent extractable ‘‘free” phase and the kerogen-bound phases. For the ‘‘free” phase, the moretane/hopane ratio displays little variation with increasing maturity but for the ‘‘bound” phase it approaches the minimum value of 0.25 at a HI value of 300 but some of the more mature samples have significantly lower values. The two samples in question that consistently detract from the overall trend have HI values of 138 and 121 and display low values for Tmax (Table 1). Thus, facies variation probably accounts for these samples plotting with the less mature samples with higher HI values of 300–400 (Fig. 3). Overall, the ‘‘bound” phase hopane ring isomerisation is more retarded than that for the alkyl side chain. At HI above 400, differences between the ‘‘free” and‘‘bound” phase C31ab 22S/(S + R) ratio values are relatively small but those for the ‘‘free” phase do not exceed those for the ‘‘bound” phase. Within the free phase, as would be expected, the aaa and abb steranes, along with abundant diasteranes, dominate throughout the samples of oil window maturity (Fig. 2). However, within the bound phase there is little evidence of any diasteranes and there are lower relative levels of the abb steranes, with the 20R isomers most prominent.
(a)(i) FREE Hopanes
34 aß (S&R)
33 aß (S&R)
1121
(ii) HYPY Generated Hopanes 34 aß (S&R)
33 aß (S&R)
31 aß (S&R)
32 aß (S&R)
30 ßa
32 aß (S&R)
31 aß (S&R)
30 aß 29 ßa 30 aß 30 ßa
/Z 191
29 aß
M
Tm
Relative response
29 ßa
/Z 191
Tm
M
29 aß
R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
29aaa (S) 29aßß (R+S)
28aaa (R)
29aaa (R) 29aaa (R)
217
29aaa (S) 29aßß (R+S)
Z
28aaa (R)
M/
217
27aaa (R)
Z
27aaa (R)
Relative response
M/
27aaa (S)
Diasteranes
27aaa (S)
Retention time
(b)(i) FREE Steranes
(ii) HYPY Generated Steranes
Retention time Fig. 2. (a) m/z 191 and (b) 217 single ion chromatograms for (i) ‘‘free” and (ii) kerogen-‘‘bound” phases of a typical high maturity sample [Well 3/29a-4 (4781 m), HI 65 mg HC/g 1 TOC].
Bound sterane profiles are dominated by aaa steranes while baa (20R) may co-elute with abb (20R and 20S) steranes, which may affect the ring isomerisation parameters at the very lowest levels of maturity (Fig. 4). However, these compositional differences and the isomerisation maturity parameter, C29abb 20S + 20R/C29abb(S + R) + C29aaa(S + R) ratio, plotted in Fig. 4, clearly indicate that sterane ring isomerisation for the ‘‘bound” phase is retarded significantly in comparison with that for the ‘‘free” phase, although facies variation is again possible. Further, as for the hopanes, ring isomerisation within the bound phase appears more strongly retarded than the side chain isomerisa-
tion, illustrated by the 20S/(S + R) ratio for the C29 5a,14a, 17a(H) steranes (Fig. 4). The relatively small degree of retardation observed for the kerogen-bound 20S/(S + R) steranes is comparable to that of the 22S/(S + R) hopanes (cf. Figs. 3 and 4). 3.2. Hydrous pyrolysis As suggested in the previous section, using source rocks collected from a variety of localities and depths within a unit like the Kimmeridge Clay Formation means that facies variation can affect thermal maturity parameters. Thus, to
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R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
0
0 Free Free
Bound
Bound
200
HI
HI
200
400
400
600
600 1.5
1.0
0.5
0.0
0.2
C30βα/C30αβ
0.4
0.6
C31αβ 22S/(22S+22R)%
Fig. 3. Hopane ring and side chain isomerisation maturity parameters for a naturally occurring suite of Kimmeridge Clay source rocks.
0
0 Free Bound
200
200
Free
HI
HI
Bound
400
400
600 0.1
600 0.3
0.5
C29αββ 20S+20R / (C29αββ (20S+20R) + C29ααα
0.0
0.2
0.4
0.6
C29ααα 20S/(20S+20R)%
(20S+20R))% Fig. 4. Sterane ring and side chain isomerisation maturity parameters for a naturally occurring suite of Kimmeridge Clay source rocks.
avoid this possibility, hydrous pyrolysis was introduced. Previous hydrous pyrolysis studies (Eglinton et al., 1988; Michels et al., 1995; Lewan, 1997) have indicated that thermal maturity parameters are generally retarded relative to natural samples, possibly due to the associated rapid rates of heating, when compared to natural maturation. ‘‘Bound” and ‘‘free” hopanes and steranes from a Type II kerogen in its unaltered state (HI 597) and following hydrous pyrolysis at 310 °C (HI 263), 320 °C (HI 186) and 350 °C (HI 81) were analysed. Fig. 5 shows the same hopane maturity parameters for ring and side chain isomerisation as used for the natural samples. Again, retardation is most pronounced for the moretane/hopane ratio where the values for the ‘‘free” phase show little variation with increasing maturity and the ‘‘bound” phase values are significantly higher (pertaining to lower relative maturity) than those for natural maturation (Fig. 2). The ‘‘free” and ‘‘bound” phase C31ab 22S/
(S + R) values are very similar, but the values are again much lower for a given maturity level than within the natural maturation series (Fig. 3). The artificially matured steranes (Fig. 6) display similar trends to the hopanes, with ring isomers from the ‘‘bound” phase being much more strongly retarded that the corresponding side chain isomers. The C29abb20S + 20R/ (C29abb(S + R) + C29aaa(S + R) values are all significantly lower than for the natural maturation series (cf. Figs. 4 and 6). The ‘‘free” and ‘‘bound” phase 20S/(S + R) values are generally very similar at low maturity, not increasing until a HI value of ca. 300 is reached. However, for the most mature sample, the ‘‘bound” phase value is still significantly lower than that for the ‘‘free” phase. Overall, these findings demonstrate, for the suite of Type II kerogen samples investigated, that kerogen-bound aliphatic biomarkers are still generally more immature than their free counterparts at high maturity. In particular,
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R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
0
0 Free Bound
200
HI
HI
200
400
Free
400 Bound
600
600
800
800 1.6
1.2
0.8
0.4
0.1
C30βα/C30αβ
0.3
0.5
C31 αβ 22S/(22S+22R)%
Fig. 5. Hopane ring and side chain isomerisation maturity parameters for an artificially matured Type II immature Kimmeridge Clay source rock.
0
0 Free
200
Bound
200
HI
HI
Free
400
600
800 0.0
400
Bound
600
800 0.2
0.4
0.1
0.3
0.5
C29ααα 20S/(20S+20R)%
C29αββ 20S+20R / (C29αββ (20S+20R) + C29ααα (20S+20R))%
Fig. 6. Sterane ring and side chain isomerisation maturity parameters for an artificially matured Type II immature Kimmeridge Clay source rock.
ring isomerisation rates for ‘‘bound” biomarkers remain sensitive to small changes in maturity long after their ‘‘free” counterparts have reached equilibrium. Further work is currently being conducted into the behaviour of biomarkers bound into Type I and III kerogens with, once again, particular emphasis towards elevated maturity. 4. Conclusions Within the Kimmeridge Clay samples investigated, the isomerisation reactions of the kerogen-bound biomarkers are retarded with respect to the ‘‘free” phase until a HI of approximately 300 is reached. Beyond this point, ring isomerisation within the kerogen-bound phase is still retarded, whilst the side chain isomerisation for both the hopanes and steranes is similar in both the free and bound phases. For the artificially matured kerogens, both ‘‘free” and ‘‘bound” phase side chain maturation is significantly
retarded, consistent with earlier studies, but the extent of retardation for ring isomerisation in the ‘‘bound” phase is even more pronounced than for the ‘‘free” phase. Acknowledgements RSL acknowledges the financial support provided by a NERC studentship. The authors also thank J. Zumberge and an anonymous reviewer for constructive comments, which greatly improved the paper. Guest Associate Editor—P. Farrimond References Eglinton, T.I., Douglas, A.G., Rowland, S.J., 1988. Release of aliphatic, aromatic and sulphur compounds from Kimmeridge kerogen by hydrous pyrolysis: a quantitative study. Organic Geochemistry 13, 655–663.
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Lewan, M.D., 1997. Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta 61, 3691–3723. Love, G.D., Snape, C.E., Carr, A.D., Houghton, R.C., 1995. Release of covalently-bound alkane biomarkers in high yields from kerogen via catalytic hydropyrolysis. Organic Geochemistry 23, 981–986. Meredith, W., Russell, C.A., Cooper, M., Snape, C.E., Love, G.D., Fabbri, D., Vane, C.H., 2004. Trapping hydropyrolysates on silica and their subsequent thermal desorption to facilitate rapid fingerprinting by GC–MS. Organic Geochemistry 35, 73–89. Michels, R., Landais, P., Torkelson, B.E., Philp, R.P., 1995. Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis. Geochimica et Cosmochimica Acta 59, 1589–1604.
Okiongbo, K.S., Aplin, A.C., Larter, S.R., 2005. Changes in Type II kerogen density as a function of maturity: evidence from the Kimmeridge lay formation. Energy & Fuels 19, 2495–2499. Roberts, M.J., Snape, C.E., Mitchell, S.C., 1995. Hydropyrolysis: fundamentals. Two-stage processing and PDU operation. In: Snape, C.E. (Ed.), Geochemistry Characterisation and Conversion of Oil Shales. Kluwer, Dordrecht. NATO ASI Series C. Van Graas, G.W., 1990. Biomarker maturity parameters for high maturities: calibration of the working range up to the oil/ condensate threshold. Organic Geochemistry 16, 1025–1032. Zhang, S., Huang, H., Xiao, Z., Liang, D., 2005. Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China. Part 2: maturity assessment. Organic Geochemistry 36, 1215–1225.
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Release of bound aliphatic biomarkers via hydropyrolysis from Type II kerogen at high maturity Robert S. Lockhart a, Will Meredith a, Gordon D. Love b, Colin E. Snape a,* a b
Nottingham Fuel and Energy Centre, School of Chemical and Environmental Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK Department of Earth Sciences, University of California, Riverside, CA, USA
a r t i c l e
i n f o
Article history: Received 27 September 2007 Received in revised form 14 March 2008 Accepted 18 March 2008 Available online 1 April 2008
a b s t r a c t At relatively low maturity, kerogen-bound aliphatic biomarkers invariably display retarded maturity compared to their ‘‘free” counterparts as a result of the protection afforded by the macromolecular structure. This study investigates the release of hopanes and steranes at elevated (post oil window) maturity from Type II kerogen. Results for a suite of North Sea petroleum source rocks (Kimmeridge Clay Formation) indicate that, below a hydrogen index (HI) value of ca. 300, differences between the ‘‘free” and ‘‘bound” phases for hopane and sterane side chain isomerisation are small. However, ring isomerisation within the kerogen-bound phase is still more retarded. For a corresponding series of kerogen samples artificially matured via hydrous pyrolysis, both ‘‘free” and ‘‘bound” phase side chain isomerisation reactions are significantly retarded relative to the naturally occurring samples. This is consistent with earlier studies, but the extent of retardation for ring isomerisation in the ‘‘bound” phase is even more pronounced than for the ‘‘free” phase. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Fixed-bed hydropyrolysis (Hypy) refers to pyrolysis assisted by high hydrogen pressure (>10 MPa) and a dispersed sulfided molybdenum catalyst, which permits covalent bond cleavage at relatively low temperature (<450 °C; Love et al., 1995). In comparison with conventional pyrolysis techniques, Hypy has been proven to possess the unique ability to release high yields of bound biomarkers (solvent soluble product) from petroleum source rocks and heavy oil fractions. Overall conversion reaches >85 wt% of the total organic matter, while alteration of their isomeric distributions is crucially minimised (Love et al., 1995). Kerogen-bound hopanes and steranes undergo the same epimerisation reaction pathways as their solvent extractable, ‘‘free” counterparts within the bitumen. However, despite tending towards the same point of equilibrium, they are generally less mature in terms of isomerisation at both ring and side chain chiral centres, suggesting a much great-
er sensitivity to relatively small change in maturity (with the former appearing more strongly retarded). Of particular interest are kerogen-bound aliphatic biomarker distributions from samples at elevated, post oil window maturity, where those of the solvent extractable ‘‘free” phase appear in much lower concentration and have often long since reached equilibrium (Van Graas, 1990). As such, these biomarkers have limited application and in some extreme cases show inversion at elevated maturity (Zhang et al., 2005). For such kerogens, the ability of Hypy to provide meaningful information at high thermal maturity has still to be demonstrated fully. In this study, hopane and sterane maturity parameters are reported for a suite of Type II kerogen Kimmeridge Clay source rocks from the North Sea, with hydrogen index (HI) values as low as ca. 50 mg HC/g TOC, comparison being made to artificially matured kerogen samples prepared using hydrous pyrolysis.
2. Methods * Corresponding author. Tel.: +44 (0)115 951 4166; fax: +44 (0)115 951 4115. E-mail address: [email protected] (C.E. Snape). 0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.03.016
Table 1 lists the suite of naturally occurring Kimmeridge Clay source rocks investigated, together with their
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R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
Table 1 Bulk geochemical data for a suite of North Sea Kimmeridge Clay source rocks (Okiongbo et al., 2005) Well
Depth (m)
TOC (wt%)
Tmax (°C)
HI
Dorset outcrop 204/28-2 16/7b-20 34/8-6 205/26a-3 16/7b-20 16/7b-28 16/7b-20 211/12A-M16 211/12A-M16 3/29a-4 3/29a-4 3/29a-4 3/29-2
0 2330 3932 3578 2414 4157 4132 4134 3376 3401 4781 4707 4742 4608
8.22 9.98 7.71 9.04 7.64 8.80 9.63 8.47 8.71 8.32 6.18 5.11 5.62 6.07
422 407 430 437 423 430 438 430 421 425 464 454 455 452
525 406 393 315 303 259 250 216 138 121 65 48 38 35
bulk geochemical data. The samples, all containing Type II kerogen, have been selected from a variety of different localities and depths, and collectively span the entire oil generation window, from early through to late mature (Okiongbo et al., 2005). Although HI has been selected as the maturity parameter in the absence of vitrinite reflectance data, instances are discussed where biomarker maturity correlations do not hold and where no significant increase in Tmax occurs with decreasing HI. Fig. 1 shows the overall relationship between HI and Tmax, where such outliers can probably be explained by facies variation within the sample suite. While Tmax could also have been used as the primary measure of maturity, in this instance the greater range in measured HI values has been used to provide higher resolution. The fixed bed Hypy tests were conducted on the pre-extracted source rocks using apparatus described in detail previously (Love et al., 1995; Roberts et al., 1995). The samples were pyrolysed with resistive heating from 50 °C to 250 °C at 250 °C min 1, then from 250 °C to 350 °C at 8 °C min 1. With all residual bitumen removed from the sample, the trap is replaced and the second heating stage is employed, from 50 °C to 350 °C at 250 °C min 1, and fi-
600
HI
400
200
0 400
420
440
460
480
Tmax (ºC) Fig. 1. Plot of HI vs. Tmax for a suite of North Sea, Kimmeridge Clay source rocks.
nally 350 °C to 500 °C at 8 °C min 1, when the kerogenbound products are released. The system is maintained under a constant H2 pressure of 15 MPa and the sweep gas flow of 5 l min 1 ensures that all products are quickly removed from the reactor and are collected in a silica-filled trap, cooled with dry ice (Meredith et al., 2004). Hydrous pyrolysis experiments were conducted on a known weight of powdered (<220 lg) immature (TOC 25%, HI 597) Kimmeridge Clay shale from the Dorset coast (SW England). Samples were pyrolysed 24 h using a 22 ml stainless steel autoclave, pressure gauge-rated to 550 bar, heated in a fluidised sand bath, to the desired temperature (310 °C, 320 °C and 350 °C). The hydrous pyrolysis experiments were all performed with 10 ml of distilled water added to the reactor vessel. Pyrolysed samples were dried and extracted with an azeotropic mixture (200 ml) of 93% CH2Cl2/7% MeOH. The aliphatic fractions separated on silica gel were analysed via gas chromatography–mass spectrometry (GC– MS) using a Varian Instruments CP 3800 gas chromatograph interfaced to a 1200 Quadrupole mass spectrometer (ionising energy 70 eV, source temperature 280 °C). Separation was performed with a fused silica capillary column (50 m 0.32 mm i.d.) coated with BPX5 phase (0.25 lm thickness). He was the carrier gas and the temperature programme was: 50 °C (2 min.) to 300 °C (28 min.) at 5 °C min 1. Ions monitored included m/z 191 (hopanes) and m/z 217 (steranes).
3. Results and discussion 3.1. North sea source rocks Fig. 2 shows the m/z 191 and 217 single ion chromatograms for both the ‘‘free” and kerogen-‘‘bound” phases of a typical high maturity sample (HI 65). Fig. 3 compares the C30ba/C30ab (moretane/hopane) and C31ab 22S/(S + R) ratio values, respectively for the solvent extractable ‘‘free” phase and the kerogen-bound phases. For the ‘‘free” phase, the moretane/hopane ratio displays little variation with increasing maturity but for the ‘‘bound” phase it approaches the minimum value of 0.25 at a HI value of 300 but some of the more mature samples have significantly lower values. The two samples in question that consistently detract from the overall trend have HI values of 138 and 121 and display low values for Tmax (Table 1). Thus, facies variation probably accounts for these samples plotting with the less mature samples with higher HI values of 300–400 (Fig. 3). Overall, the ‘‘bound” phase hopane ring isomerisation is more retarded than that for the alkyl side chain. At HI above 400, differences between the ‘‘free” and‘‘bound” phase C31ab 22S/(S + R) ratio values are relatively small but those for the ‘‘free” phase do not exceed those for the ‘‘bound” phase. Within the free phase, as would be expected, the aaa and abb steranes, along with abundant diasteranes, dominate throughout the samples of oil window maturity (Fig. 2). However, within the bound phase there is little evidence of any diasteranes and there are lower relative levels of the abb steranes, with the 20R isomers most prominent.
(a)(i) FREE Hopanes
34 aß (S&R)
33 aß (S&R)
1121
(ii) HYPY Generated Hopanes 34 aß (S&R)
33 aß (S&R)
31 aß (S&R)
32 aß (S&R)
30 ßa
32 aß (S&R)
31 aß (S&R)
30 aß 29 ßa 30 aß 30 ßa
/Z 191
29 aß
M
Tm
Relative response
29 ßa
/Z 191
Tm
M
29 aß
R.S. Lockhart et al. / Organic Geochemistry 39 (2008) 1119–1124
29aaa (S) 29aßß (R+S)
28aaa (R)
29aaa (R) 29aaa (R)
217
29aaa (S) 29aßß (R+S)
Z
28aaa (R)
M/
217
27aaa (R)
Z
27aaa (R)
Relative response
M/
27aaa (S)
Diasteranes
27aaa (S)
Retention time
(b)(i) FREE Steranes
(ii) HYPY Generated Steranes
Retention time Fig. 2. (a) m/z 191 and (b) 217 single ion chromatograms for (i) ‘‘free” and (ii) kerogen-‘‘bound” phases of a typical high maturity sample [Well 3/29a-4 (4781 m), HI 65 mg HC/g 1 TOC].
Bound sterane profiles are dominated by aaa steranes while baa (20R) may co-elute with abb (20R and 20S) steranes, which may affect the ring isomerisation parameters at the very lowest levels of maturity (Fig. 4). However, these compositional differences and the isomerisation maturity parameter, C29abb 20S + 20R/C29abb(S + R) + C29aaa(S + R) ratio, plotted in Fig. 4, clearly indicate that sterane ring isomerisation for the ‘‘bound” phase is retarded significantly in comparison with that for the ‘‘free” phase, although facies variation is again possible. Further, as for the hopanes, ring isomerisation within the bound phase appears more strongly retarded than the side chain isomerisa-
tion, illustrated by the 20S/(S + R) ratio for the C29 5a,14a, 17a(H) steranes (Fig. 4). The relatively small degree of retardation observed for the kerogen-bound 20S/(S + R) steranes is comparable to that of the 22S/(S + R) hopanes (cf. Figs. 3 and 4). 3.2. Hydrous pyrolysis As suggested in the previous section, using source rocks collected from a variety of localities and depths within a unit like the Kimmeridge Clay Formation means that facies variation can affect thermal maturity parameters. Thus, to
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0
0 Free Free
Bound
Bound
200
HI
HI
200
400
400
600
600 1.5
1.0
0.5
0.0
0.2
C30βα/C30αβ
0.4
0.6
C31αβ 22S/(22S+22R)%
Fig. 3. Hopane ring and side chain isomerisation maturity parameters for a naturally occurring suite of Kimmeridge Clay source rocks.
0
0 Free Bound
200
200
Free
HI
HI
Bound
400
400
600 0.1
600 0.3
0.5
C29αββ 20S+20R / (C29αββ (20S+20R) + C29ααα
0.0
0.2
0.4
0.6
C29ααα 20S/(20S+20R)%
(20S+20R))% Fig. 4. Sterane ring and side chain isomerisation maturity parameters for a naturally occurring suite of Kimmeridge Clay source rocks.
avoid this possibility, hydrous pyrolysis was introduced. Previous hydrous pyrolysis studies (Eglinton et al., 1988; Michels et al., 1995; Lewan, 1997) have indicated that thermal maturity parameters are generally retarded relative to natural samples, possibly due to the associated rapid rates of heating, when compared to natural maturation. ‘‘Bound” and ‘‘free” hopanes and steranes from a Type II kerogen in its unaltered state (HI 597) and following hydrous pyrolysis at 310 °C (HI 263), 320 °C (HI 186) and 350 °C (HI 81) were analysed. Fig. 5 shows the same hopane maturity parameters for ring and side chain isomerisation as used for the natural samples. Again, retardation is most pronounced for the moretane/hopane ratio where the values for the ‘‘free” phase show little variation with increasing maturity and the ‘‘bound” phase values are significantly higher (pertaining to lower relative maturity) than those for natural maturation (Fig. 2). The ‘‘free” and ‘‘bound” phase C31ab 22S/
(S + R) values are very similar, but the values are again much lower for a given maturity level than within the natural maturation series (Fig. 3). The artificially matured steranes (Fig. 6) display similar trends to the hopanes, with ring isomers from the ‘‘bound” phase being much more strongly retarded that the corresponding side chain isomers. The C29abb20S + 20R/ (C29abb(S + R) + C29aaa(S + R) values are all significantly lower than for the natural maturation series (cf. Figs. 4 and 6). The ‘‘free” and ‘‘bound” phase 20S/(S + R) values are generally very similar at low maturity, not increasing until a HI value of ca. 300 is reached. However, for the most mature sample, the ‘‘bound” phase value is still significantly lower than that for the ‘‘free” phase. Overall, these findings demonstrate, for the suite of Type II kerogen samples investigated, that kerogen-bound aliphatic biomarkers are still generally more immature than their free counterparts at high maturity. In particular,
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0
0 Free Bound
200
HI
HI
200
400
Free
400 Bound
600
600
800
800 1.6
1.2
0.8
0.4
0.1
C30βα/C30αβ
0.3
0.5
C31 αβ 22S/(22S+22R)%
Fig. 5. Hopane ring and side chain isomerisation maturity parameters for an artificially matured Type II immature Kimmeridge Clay source rock.
0
0 Free
200
Bound
200
HI
HI
Free
400
600
800 0.0
400
Bound
600
800 0.2
0.4
0.1
0.3
0.5
C29ααα 20S/(20S+20R)%
C29αββ 20S+20R / (C29αββ (20S+20R) + C29ααα (20S+20R))%
Fig. 6. Sterane ring and side chain isomerisation maturity parameters for an artificially matured Type II immature Kimmeridge Clay source rock.
ring isomerisation rates for ‘‘bound” biomarkers remain sensitive to small changes in maturity long after their ‘‘free” counterparts have reached equilibrium. Further work is currently being conducted into the behaviour of biomarkers bound into Type I and III kerogens with, once again, particular emphasis towards elevated maturity. 4. Conclusions Within the Kimmeridge Clay samples investigated, the isomerisation reactions of the kerogen-bound biomarkers are retarded with respect to the ‘‘free” phase until a HI of approximately 300 is reached. Beyond this point, ring isomerisation within the kerogen-bound phase is still retarded, whilst the side chain isomerisation for both the hopanes and steranes is similar in both the free and bound phases. For the artificially matured kerogens, both ‘‘free” and ‘‘bound” phase side chain maturation is significantly
retarded, consistent with earlier studies, but the extent of retardation for ring isomerisation in the ‘‘bound” phase is even more pronounced than for the ‘‘free” phase. Acknowledgements RSL acknowledges the financial support provided by a NERC studentship. The authors also thank J. Zumberge and an anonymous reviewer for constructive comments, which greatly improved the paper. Guest Associate Editor—P. Farrimond References Eglinton, T.I., Douglas, A.G., Rowland, S.J., 1988. Release of aliphatic, aromatic and sulphur compounds from Kimmeridge kerogen by hydrous pyrolysis: a quantitative study. Organic Geochemistry 13, 655–663.
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Lewan, M.D., 1997. Experiments on the role of water in petroleum formation. Geochimica et Cosmochimica Acta 61, 3691–3723. Love, G.D., Snape, C.E., Carr, A.D., Houghton, R.C., 1995. Release of covalently-bound alkane biomarkers in high yields from kerogen via catalytic hydropyrolysis. Organic Geochemistry 23, 981–986. Meredith, W., Russell, C.A., Cooper, M., Snape, C.E., Love, G.D., Fabbri, D., Vane, C.H., 2004. Trapping hydropyrolysates on silica and their subsequent thermal desorption to facilitate rapid fingerprinting by GC–MS. Organic Geochemistry 35, 73–89. Michels, R., Landais, P., Torkelson, B.E., Philp, R.P., 1995. Effects of effluents and water pressure on oil generation during confined pyrolysis and high-pressure hydrous pyrolysis. Geochimica et Cosmochimica Acta 59, 1589–1604.
Okiongbo, K.S., Aplin, A.C., Larter, S.R., 2005. Changes in Type II kerogen density as a function of maturity: evidence from the Kimmeridge lay formation. Energy & Fuels 19, 2495–2499. Roberts, M.J., Snape, C.E., Mitchell, S.C., 1995. Hydropyrolysis: fundamentals. Two-stage processing and PDU operation. In: Snape, C.E. (Ed.), Geochemistry Characterisation and Conversion of Oil Shales. Kluwer, Dordrecht. NATO ASI Series C. Van Graas, G.W., 1990. Biomarker maturity parameters for high maturities: calibration of the working range up to the oil/ condensate threshold. Organic Geochemistry 16, 1025–1032. Zhang, S., Huang, H., Xiao, Z., Liang, D., 2005. Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China. Part 2: maturity assessment. Organic Geochemistry 36, 1215–1225.