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Comment on “the kinetics of sterane biological marker release and degradation processes during the hydrous pyrolysis of vitrinite kerogen” by G. D. Abbott, G. Y. Wang, T. I. Eglinton, A. K. Home and G. S. Petch
00 16-7037/92/%3.00 + .OO
Geochimca ef Cosmockrmica Actn Vol. 56, pp. 533-534 Copyright 0 1992 Pergamon Press pk. Printed in U.S.A.
COMMENT
Comment on “The kinetics of sterane biological marker release and degradation processes during the hydrous pyrolysis of vitrinite kerogen” by G. D. Abbott, G. Y. Wang, T. I. Eglinton, A. K. Home, and G. S. Petch R. MARZI* Centre for Petroleum and Environmental Organic Geochemistry, GPO Box U 1987, Perth 600 1, Western Australia (Received May 24, 199 1; accepted in revised form October 4, I99 1)
MARZI, 1988, 1989). All these kinetic data treat the observed change in the sterane isomer ratios as a simple unidirectional isomerization reaction. Since it has been observed by a number of people ( RULLK~TTER and MARZI, 1989; LEWAN et al., 1986) that the so-called sterane isomerization is not a unidirectional reaction, an alternative reaction scheme would be most welcome. Although ABBOTTand coworkers do not claim that the published kinetic data hold true in the natural environments, the question arises as to whether the kinetic data obtained from these hydrous pyrolysis experiments are applicable to sedimentary processes which occur over geological time. As a broad guideline, pairs of values of activation energy and frequency factor for a geochemical reaction must fall into a defined band in a plot of Ea vs. log (A) in order to be geochemically meaningful (e.g., MARZI et al., 1990). The pairs of values of Ea and log (A) reported by ABBOTTet al. ( 1990) based on their hydrous pyrolysis experiments lie well above this band (Fig. 1). This indicates that these parameters describe processes which would occur very rapidly on a geological time scale.
IN ORDER TO FURTHER develop understanding of the observable changes in sterane isomer ratios, ABBOTT et al. ( 1990) conducted a series of hydrous pyrolysis experiments on mineral-free vitrinite kerogen. Using a deuterated model compound, they found that direct chiral isomerization appears to be relatively unimportant. They derived a kinetic model comprising consecutive release and degradation processes for both isomers and, using a bi-exponential concentration-time function, they were able to determine Arrhenius parameters for four individual reactions, namely CZ91401(H ) , 17o1(H) (20R) and CZ9 14c~(H), 17c~(H) (20s) release and degradation. ABBOTT et al. ( 1990) claimed that these four reactions account for the change in sterane diastereomer ratios in hydrous pyrolysis experiments. Quite a range of kinetic data for the sterane epimerization has been published so far (MACKENZIEand MCKENZIE, 1983; SUZUKI, 1984; ALEXANDERet al., 1990; RULLK~TTER and
* Present address..Amoco Production Company, ResearchCenter, P.O. Box 3385, Tulsa, OK 74102, USA.
LO
20
5-
Calculation based on: QO’C isothermal temoerature and 50 Ma isothermal time
A
20%Degradation 20R-DegradatlonX
0
i
100
150
200
250
Ea(kJ/mol)
FIG. 1. Calculated band of valid activation energy/frequency factor pairs for geochemical reactions which have proceeded to extent-of-reaction values between 10 and 9070, assuming isothermal heating at 90°C for a period of 50 Ma (modified after MARZI et al., 1990, Courtesy of B. Krooss, KFA Jiilich). 533
534
R. Marzi
To illustrate this consequence, the kinetic data Of ABBOTT et al. ( 1990) were used to calculate the extent of isomerization in a hypothetical well using a constant heating rate of 3.3”C/ Ma and a geothermal gradient of approximately 33’C/km together with a surface temperature of 14°C. This was done by calculating the individual reactions as a sequence of isothermal intervals (MACKENZIE and MCKENZIE, 1983) generated by a basin modelling package (PDIPC, IES GmbH Jiilich). The result of this calculation shows that release of 20R- and 2OS- is fast. The reactions reach completion at 1500 m and 2500 m, respectively. Sterane degradation reactions are even faster. Destruction is already complete at ca. 800 m (20S-sterane) and ca. 150 m (20 R-sterane), respectively. This means that under the given conditions, degradation of 20 R-steranes is complete below a depth of 150 m and all 20R-isomers released are degraded immediately, while the relative concentration of the 20S-isomer increases. This means that the ( 20 S/ 20 S + 20 R ) epimer ratio formula is completely 20Sisomer dominated and the ratio would soon become unity. Below 800 m depth the degradation of the 204isomer is also complete, i.e., all steranes have been thermally destroyed and cannot be detected anymore. Alternatively, if one carries out an isothermal interval calculation, the following isothermal intervals for completion of the reactions would be found: degradation of 20R-isomers-l Ma at 4O”C, degradation of 20Sisomers-1 Ma at 6O”C, release of 20 R-isomers- 1 Ma at 90°C release of 20S-isomers-l Ma at 80°C. This calculation indicates that no sediment that has been heated to 60°C for more than 1 Ma should contain any CZ9 LY(Y(Y steranes (20 R or 20s). This is in contrast to what is observed in nature. Thus, the kinetic data measured by ABBOTTet al. ( 1990) do not explain the observations in the natural sedimentary record, although the reaction scheme proposed by ABBOTTet al. ( 1990) may still be correct. Empirically it appears that the change in sterane isomer ratios is better calculated using “pseudo’‘-kinetic data. For a number of geological situations this approach has proven to simulate the behavior of sterane isomer ratios reasonably well (MACKENZIE and MCKENZIE, 1983; SAJGO and LEFLER, 1986; RULLKOTTERet al., 199 1; ALEXANDERet al., 1990). Generally speaking, when it comes to maturity modelling one has to be very wary which kinetic data are being used
and how meaningful they are in the natural environment. An aid for this judgment has been published by MARZI et al. ( 1990), and a modified version is presented here (Fig. 1). This figure indicates an approximate range of possible activation energy/frequency factor pairs which could give meaningful results in the geological system. Editorial
handling:
G.
Faure REFERENCES
ABBOTTG. D., WANG G. Y., EGLINTONT. I., HOMEA. K., and PETCH G. S. ( 1990)The kinetics of sterane biologicalmarker release and degradation processes during hydrous pyrolysis of vitrinite kerogen. Geochim.
Cosmochim.
Acta 54, 245 l-246 1.
ALEXANDER R., MARZIR., and KAGIR. I. (1990) A new method for assessing the thermal history of sediments: A case study from the Exmouth Plateau in northwestern Australia. APEA J. 1990, 364-372. LEWANM. D., BJ@ROYM., and DOLCATERD. L. (1986) Effectsof thermal maturation on steroid hydrocarbons as determined by hydrous pyrolysis of Phosphorin Retort Shale. Geochim. Cosmochim. Acta 50, 1977-1987.
MACKENZIEA. S. and MCKENZIE D. P. ( 1983) Aromatization and isomerization of hydrocarbons in sedimentary basins formed by extension. Geol. Msg. 120, 4 17-470. MARZI R.,RULLK~TTER J., and PERRIMANW. S. ( 1990) Application of the change of sterane isomer ratios to the reconstruction of geothermal histories: Implications of the results from hydrous pyrolysis experiments. Org. Geochem. 16, 9 1- 102. RLJLLK~TTERJ. and MARZI R. (1988) Natural and artificial maturation of biological markers in a Toarcion shale from northern Germany. In Advances in Organic Geochemistry 1987 (ed. L. MATTAVELLIand L. NOVELL]),pp. 639-645. Pergamon Press. RULLKOTTERJ. and MARZIR. (1989) New aspect of the application of sterane isomerisation and steroid aromatization to petroleum exploration and the reconstruction of geothermal histories of sedimentary basins. Amer. Chem. Sot., Div. Petrol. Chem. 34(I), 126131. RULLK~TTERJ., MARZIR., and MEYERSP. ( 1991) Biological markers in Paleozoic sedimentary rocks and crude oils from the Michigan Basin: Reassessment of sources and thermal history of organic matter, In Early Organic Evolution: Implications for Mineral and Energy Resources (ed. M. SCHIDLOWSKI et al.), Springer-Verlag (in press). SAJGOC. and LEFLERJ. ( 1986) A reaction kinetic approach to the temperature-time history of sedimentary basins. In Lecture Notes in Earth Science 5, (eds. G. BUNTEBARTand L. STEGENA),pp. 119-15 I. Springer-Verlag.
SUZUKIN. (I 984) Estimation of maximum temperature of mudstone by two kinetic parameters epimerisation of sterane and hopane. Geochim. Cosmochim. Acta 48,2273-2282.
Geochimca ef Cosmockrmica Actn Vol. 56, pp. 533-534 Copyright 0 1992 Pergamon Press pk. Printed in U.S.A.
COMMENT
Comment on “The kinetics of sterane biological marker release and degradation processes during the hydrous pyrolysis of vitrinite kerogen” by G. D. Abbott, G. Y. Wang, T. I. Eglinton, A. K. Home, and G. S. Petch R. MARZI* Centre for Petroleum and Environmental Organic Geochemistry, GPO Box U 1987, Perth 600 1, Western Australia (Received May 24, 199 1; accepted in revised form October 4, I99 1)
MARZI, 1988, 1989). All these kinetic data treat the observed change in the sterane isomer ratios as a simple unidirectional isomerization reaction. Since it has been observed by a number of people ( RULLK~TTER and MARZI, 1989; LEWAN et al., 1986) that the so-called sterane isomerization is not a unidirectional reaction, an alternative reaction scheme would be most welcome. Although ABBOTTand coworkers do not claim that the published kinetic data hold true in the natural environments, the question arises as to whether the kinetic data obtained from these hydrous pyrolysis experiments are applicable to sedimentary processes which occur over geological time. As a broad guideline, pairs of values of activation energy and frequency factor for a geochemical reaction must fall into a defined band in a plot of Ea vs. log (A) in order to be geochemically meaningful (e.g., MARZI et al., 1990). The pairs of values of Ea and log (A) reported by ABBOTTet al. ( 1990) based on their hydrous pyrolysis experiments lie well above this band (Fig. 1). This indicates that these parameters describe processes which would occur very rapidly on a geological time scale.
IN ORDER TO FURTHER develop understanding of the observable changes in sterane isomer ratios, ABBOTT et al. ( 1990) conducted a series of hydrous pyrolysis experiments on mineral-free vitrinite kerogen. Using a deuterated model compound, they found that direct chiral isomerization appears to be relatively unimportant. They derived a kinetic model comprising consecutive release and degradation processes for both isomers and, using a bi-exponential concentration-time function, they were able to determine Arrhenius parameters for four individual reactions, namely CZ91401(H ) , 17o1(H) (20R) and CZ9 14c~(H), 17c~(H) (20s) release and degradation. ABBOTT et al. ( 1990) claimed that these four reactions account for the change in sterane diastereomer ratios in hydrous pyrolysis experiments. Quite a range of kinetic data for the sterane epimerization has been published so far (MACKENZIEand MCKENZIE, 1983; SUZUKI, 1984; ALEXANDERet al., 1990; RULLK~TTER and
* Present address..Amoco Production Company, ResearchCenter, P.O. Box 3385, Tulsa, OK 74102, USA.
LO
20
5-
Calculation based on: QO’C isothermal temoerature and 50 Ma isothermal time
A
20%Degradation 20R-DegradatlonX
0
i
100
150
200
250
Ea(kJ/mol)
FIG. 1. Calculated band of valid activation energy/frequency factor pairs for geochemical reactions which have proceeded to extent-of-reaction values between 10 and 9070, assuming isothermal heating at 90°C for a period of 50 Ma (modified after MARZI et al., 1990, Courtesy of B. Krooss, KFA Jiilich). 533
534
R. Marzi
To illustrate this consequence, the kinetic data Of ABBOTT et al. ( 1990) were used to calculate the extent of isomerization in a hypothetical well using a constant heating rate of 3.3”C/ Ma and a geothermal gradient of approximately 33’C/km together with a surface temperature of 14°C. This was done by calculating the individual reactions as a sequence of isothermal intervals (MACKENZIE and MCKENZIE, 1983) generated by a basin modelling package (PDIPC, IES GmbH Jiilich). The result of this calculation shows that release of 20R- and 2OS- is fast. The reactions reach completion at 1500 m and 2500 m, respectively. Sterane degradation reactions are even faster. Destruction is already complete at ca. 800 m (20S-sterane) and ca. 150 m (20 R-sterane), respectively. This means that under the given conditions, degradation of 20 R-steranes is complete below a depth of 150 m and all 20R-isomers released are degraded immediately, while the relative concentration of the 20S-isomer increases. This means that the ( 20 S/ 20 S + 20 R ) epimer ratio formula is completely 20Sisomer dominated and the ratio would soon become unity. Below 800 m depth the degradation of the 204isomer is also complete, i.e., all steranes have been thermally destroyed and cannot be detected anymore. Alternatively, if one carries out an isothermal interval calculation, the following isothermal intervals for completion of the reactions would be found: degradation of 20R-isomers-l Ma at 4O”C, degradation of 20Sisomers-1 Ma at 6O”C, release of 20 R-isomers- 1 Ma at 90°C release of 20S-isomers-l Ma at 80°C. This calculation indicates that no sediment that has been heated to 60°C for more than 1 Ma should contain any CZ9 LY(Y(Y steranes (20 R or 20s). This is in contrast to what is observed in nature. Thus, the kinetic data measured by ABBOTTet al. ( 1990) do not explain the observations in the natural sedimentary record, although the reaction scheme proposed by ABBOTTet al. ( 1990) may still be correct. Empirically it appears that the change in sterane isomer ratios is better calculated using “pseudo’‘-kinetic data. For a number of geological situations this approach has proven to simulate the behavior of sterane isomer ratios reasonably well (MACKENZIE and MCKENZIE, 1983; SAJGO and LEFLER, 1986; RULLKOTTERet al., 199 1; ALEXANDERet al., 1990). Generally speaking, when it comes to maturity modelling one has to be very wary which kinetic data are being used
and how meaningful they are in the natural environment. An aid for this judgment has been published by MARZI et al. ( 1990), and a modified version is presented here (Fig. 1). This figure indicates an approximate range of possible activation energy/frequency factor pairs which could give meaningful results in the geological system. Editorial
handling:
G.
Faure REFERENCES
ABBOTTG. D., WANG G. Y., EGLINTONT. I., HOMEA. K., and PETCH G. S. ( 1990)The kinetics of sterane biologicalmarker release and degradation processes during hydrous pyrolysis of vitrinite kerogen. Geochim.
Cosmochim.
Acta 54, 245 l-246 1.
ALEXANDER R., MARZIR., and KAGIR. I. (1990) A new method for assessing the thermal history of sediments: A case study from the Exmouth Plateau in northwestern Australia. APEA J. 1990, 364-372. LEWANM. D., BJ@ROYM., and DOLCATERD. L. (1986) Effectsof thermal maturation on steroid hydrocarbons as determined by hydrous pyrolysis of Phosphorin Retort Shale. Geochim. Cosmochim. Acta 50, 1977-1987.
MACKENZIEA. S. and MCKENZIE D. P. ( 1983) Aromatization and isomerization of hydrocarbons in sedimentary basins formed by extension. Geol. Msg. 120, 4 17-470. MARZI R.,RULLK~TTER J., and PERRIMANW. S. ( 1990) Application of the change of sterane isomer ratios to the reconstruction of geothermal histories: Implications of the results from hydrous pyrolysis experiments. Org. Geochem. 16, 9 1- 102. RLJLLK~TTERJ. and MARZI R. (1988) Natural and artificial maturation of biological markers in a Toarcion shale from northern Germany. In Advances in Organic Geochemistry 1987 (ed. L. MATTAVELLIand L. NOVELL]),pp. 639-645. Pergamon Press. RULLKOTTERJ. and MARZIR. (1989) New aspect of the application of sterane isomerisation and steroid aromatization to petroleum exploration and the reconstruction of geothermal histories of sedimentary basins. Amer. Chem. Sot., Div. Petrol. Chem. 34(I), 126131. RULLK~TTERJ., MARZIR., and MEYERSP. ( 1991) Biological markers in Paleozoic sedimentary rocks and crude oils from the Michigan Basin: Reassessment of sources and thermal history of organic matter, In Early Organic Evolution: Implications for Mineral and Energy Resources (ed. M. SCHIDLOWSKI et al.), Springer-Verlag (in press). SAJGOC. and LEFLERJ. ( 1986) A reaction kinetic approach to the temperature-time history of sedimentary basins. In Lecture Notes in Earth Science 5, (eds. G. BUNTEBARTand L. STEGENA),pp. 119-15 I. Springer-Verlag.
SUZUKIN. (I 984) Estimation of maximum temperature of mudstone by two kinetic parameters epimerisation of sterane and hopane. Geochim. Cosmochim. Acta 48,2273-2282.