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Quantitative determination of thermal maturity in sedimentary organic matter by diffuse reflectance infrared spectroscopy of asphaltenes
Org. Geochem. Vol. 14, No, 1, pp. 77-81, 1989 Printed in Great Britain. All rights reserved
0146-6380/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press pie
Quantitative determination of thermal maturity in sedimentary organic matter by diffuse reflectance infrared spectroscopy of asphaitenes ALFRED A. CHRISTY, ANNE LISE HOPLAND, TANJA BARTH a n d OLAV M . KVALHEIM*
Department of Chemistry, University of Bergen, N-5007 Bergen, Norway (Received 10 November 1987; accepted 30 September 1988) Abstract--lnfrared spectroscopic analysis of the asphalthenes extracted from an artificially matured Kimmeridge clay sediment (kerogen type II) reveals an excellent correlation (0.997) between temperature and a maturity parameter based on two peaks at approximately 1600 and 1700cm ~. The change in intensity of the two peaks is discussed in terms of chemical changes of kerogen during the maturity range investigated in the present work (late diagenesis and early catagenis, i.e. °/oJ~o increasing from approximately 0.5 to 0.85%). Comparison with a maturity index based on the methylphenanthrene distribution shows that the spectroscopic determined parameter correlates best with temperature in the maturity range examined in this work. Key words--spectroscopic maturity index, hydrous pyrolysis, kerogen transformation, infrared spectroscopy, asphaltenes, spectral interpretation, methylphenanthrene distribution fraction (MPDF)
Quantitative assessment of thermal maturity of sedimentary organic matter is important for oil-oil and oil-source rock correlation and for basin modelling. The most commonly used parameter for maturity determination is vitrinite reflectance (Tissot and Welte, 1984). Although the accuracy of the technique is reduced for higher ranking samples, vitrinite reflectance has the advantage over many other proposed maturity parameters of covering the complete maturity range found in sediments and coals. However, accurate determination of vitrinite reflectance is time-consuming and the method is sensitive to physical in addition to chemical changes during kerogen degradation (Beny-Bassez and Rouzaud, 1985). Thus, alternative parameters directly related to the chemical changes of kerogen have been proposed. Biomarker ratios represent a large proportion of these parameters (Philp, 1985; Johns, 1986). Systematic investigations of several biomarker fractions reveal that few biomarkers can be expected to be useful maturity indicators for a large range of samples (Kvalheim and Telnaes, 1986). Thus, biomarker ratios are probably most useful as correlation parameters, e.g. for the correlation of migrated sedimentary organic matter to source rocks, and for the reconstruction of the depositional palaeoenvironments. Spectroscopic methods such as infrared and nuclear magnetic resonance techniques represent another chemical approach to maturity deter-
mination. Instead of looking at changes in a single molecule or a particular type of molecule, whole samples (or at least a major part) are characterized spectroscopically with respect to their chemical functionalities. In this way, the gross chemical changes during maturation are recorded. Maturity characterization based on infrared spectroscopy and similar techniques might thus be useful for a larger range of samples than particular biomarker ratios, embracing, e.g. sedimentary organic matter of different maceral composition. Thirty-five years of experience with IR spectroscopy in the field of coal and kerogen characterization has not given the technique a strong position within organic geochemistry. Thus, Tissot and Welte (1984) have given infrared spectroscopy the label "low or limitation of use" for the purpose of type and maturity determination of organic matter. The major reason for this pessimistic view of infrared analysis is that infrared spectra of whole rock samples and extracts appear as profiles of very broad bands. However, Christy et al. (1987a) have shown that the information about maturity in coals can still be obtained from the infrared profiles by use of proper data-analytical methods. Following earlier work of Robin and Rouxhet (1978a, b) and others (Painter et al., 1981; Snyder et al., 1983), Ganz and Kalkreuth (1987) defined a maturity parameter (called the C factor) using the intensities of two broad peaks at 1710 and 1630cm -~ in transmittance IR spectroscopy of kerogen concentrates:
*Author to whom correspondence should be addressed.
C Factor = Imo/(llTtO + Ii630)
INTRODUCTION
77
(1)
ALFRED A. CHRISTY et al.
78
In the present work, we have used diffuse reflectance IR spectroscopy to investigate the temperature dependence of a ratio based on the corresponding peaks in asphaltenes extracted from a Kimmeridge clay sediment (kerogen type II). The sediment was artificially matured by hydrous pyrolysis covering the temperature range 200-350°C. Comparison with the methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987), a maturity parameter derived from the methylphenanthrene distribution, suggests that infrared analysis provides a useful alternative to the commonly used techniques for maturity determination. EXPERIMENTAL
Pyrolysis and isolation of asphaltenes and phenanthrenes A Kimmeridge clay oil shale (kerogen type II) from Upper Jurassic North Sea was subjected to hydrous pyrolysis at fourteen temperatures covering the range from 2 0 0 C up to 350"C. Five grams of the pre-extracted oil shale was placed in a steelautoclave. Distilled water was added to the sample so that the total amount of liquid water in the autoclave at each pyrolysis temperature was estimated to 5 ml. The system was purged with nitrogen and quickly heated to the selected temperature. After 72 h the autoclave was air cooled to room temperature, the gas phase quantitatively collected, and the sediment and water phase removed from the autoclave. The water phase was filtered off and the residue Soxhlet extracted with dichloromethane for 24h using a water separator. The solvent was evaporated under nitrogen and the asphaltenes isolated by refluxing the extract with 40 vol of hexane for 8 h. The precipitated asphaltenes were retained by a 0.2~tm filter and subsequently redissolved in dichloromethane. The oils were separated into aliphatics and aromatics by liquid chromatography on a silica column, and the aromatics further separated on an amino modified silica column. The phenanthrene fraction was analyzed on a 50m BP-5 column. A total of 15 samples for the asphaltenes was obtained, the experiment at 320°C being replicated to check the repeatability of the experimental procedure. For the phenanthrenes, thirteen samples were analyzed, covering the temperature range 230-350~'C.
Infrared analysis Infrared analysis was performed using a PE 1700 FTIR spectrometer (Perkin-Elmer, Norwalk, CT, U.S.A.) equipped with a DTGS detector and a diffuse reflectance accessory (Harrick Scientific, Ossining, NY, U.S.A.). Finely ground (60-80 pm) KBr was packed in a sample cup using a sample packing unit designed and manufactured by Christy et al. (1988). The diffuse reflectance IR spectrum of KBr was recorded from 4000 to 600 cm-I using 44 scans at
a resolution of 4cm -~. Ten #l of the asphaltenes dissolved in dichloromethane was dropped gently on the KBr using a 10/~1 gas chromatographic syringe. The sample cup was then placed in an oven at 50°C for 2min to evaporate the solvent. Diffuse reflectance IR spectra of asphaltenes deposited on KBr were recorded, and the difference spectra calculated by subtracting the background spectrum of KBr from each sample spectrum. Each spectrum, consisting of 3401 data points, was subsequently transformed into Kubelka-Munk format [Kubelka and Munk, 1931; see also Christy et al. (1987) for a short description of the ideas and assumptions behind this transformation]. Finally, the height of the peaks at approximately 1700 and 1600 cm -1 (Fig. 1)(at 1710 and 1630 cm-~ in spectra of kerogen concentrates) were determined. The baseline was chosen as shown in the work of Robin and Rouxhet (1978a). RESULTS AND DISCUSSION
Maturi O, determined by infrared analysis Figure l shows asphaltene spectra obtained at temperatures 200, 230, 280, 330 and 350°C, respectively. A decrease in the peak at 1700 cm -~ and a corresponding increase in the peak at 1600cm with increasing temperature is obvious. Figure 2 shows the ratio between the intensity at 1600cm and the summed intensities at 1700 and 1600cm -~, i.e. 116oo/(116oo+117oo), plotted vs pyrolysis temperature. An excellent correlation is observed between temperature and the IR determined maturity parameter, the correlation coefficient being 0.997, which is close to the theoretical limit of perfect correlation. Note that this maturity parameter is inversely related to the C-factor [equation (1)] introduced by Ganz and Kalkreuth (1987). Our choice has the advantage of giving an index that increases with increasing maturation, while the index of Ganz and Kalkreuth is preferable if an IR analog of the van Krevelen diagram is wanted (Ganze and Kalkreuth, 1987). The experimental replicate at 320°C and the result of rerunning the IR procedure (at 350°C) show the good repeatability of the experimental and analytical procedure.
Comparison with the MPDF maturity parameter The methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987) for the thirteen samples covering the temperature range 230-350°C was calculated and plotted vs temperature (Fig. 3). The correlation of 0.93 implies that the linear relationship between maturity and the distribution of the methylphenanthrenes observed in coals (Kvalheim et al., 1987) and crude oils produced from source rocks of kerogen type II (Telnaes et al., 1987) is valid also for samples matured artificially by hydrous pyrolysis. This observation together with the excellent correlation between temperature and the IR based parameter implies that the IR parameters
Quantitative determination of thermal maturity
79
J f
S
I
I
400(
J
I I I 3000
I
I
i
I
I
200"C
I
1200
2000
600
I /crn
Fig. 1. Asphaltene spectra at selected pyrolysis temperatures.
.s-I /
o 0.7 0 r~ 0.6 4-
/.~'/" /
o.~
0,6
~'*''~/°
0.5 l
0.4 ~ 0.3
el""
•
~
~-
J
v
e---~
~; 0.3 0.2
0.2 0
i
0.4
s
0.1 I
200
I
1
250 300 Pyrotysis temperature (*C)
I
3.50
Fig. 2. Ratio between the intensity at 1600cm -~ and the summed intensities at 1700 and 1600cm ~ plotted vs pyrolysis temperature. The correlation between pyrolysis temperature and IR ratio is 0.997.
I
I
I
250 500 350 PyroLysis temperature [*C)
Fig. 3. The methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987) plotted vs pyrolysis temperature. The correlation between pyrolysis temperature and MPDF is 0.93.
80
ALFRED A. CHRISTY et al.
advocated in this work and by Ganz and Kalkreuth (1987) may be superior to more conventionally used maturity parameters also for natural samples. By using a linear regression model for predicting vitrinite reflectance from M P D F given by Kvalheim et al. (1987), the maturity range covered by the pyrolysis experiments is estimated to 0.5-0.85. Although the regression model was established for coal extracts (kerogen type III) and thus cannot be expected a priori to predict accurately vitrinite reflectance for a kerogen type II sediment extract, the above calculation indicates that the IR parameter is valid for at least maturities corresponding to the lower half of the oil window. Work of Robin and Rouxhet (1978a, b), Boudou et aL (1984) and Ganz and Kalkreuth (1987) suggests that the parameter extends to regions of higher maturity (see also discussion below). Chemical interpretation 1600 cm - 1
o f the p e a k s at
1700 and
An understanding of the chemical changes responsible for the intensity changes in the IR spectra is necessary to evaluate the validity range of the IR determined maturity parameter. Furthermore, the spectra (Fig. 1) reveal a pattern that might throw some new light on previous disputes about possible sources of contributions to the peak at 1600cm -j (Painter et al., 1981). Studies of the evolution in natural samples (Robin and Rouxhet, 1978a, b; Bondou et al., 1984) and of samples artificially matured (Brown, 1955b; Tissot and Welte, 1984) have proved beyond reasonable doubt that the strong peak at approx. 1700cm -~ (1710cm 1 in spectra of kerogen concentrates) corresponds to stretching of the carbonyl group in ketonic, aldehydic and carboxylic surroundings. Thus, the reduction in the carbonyl peak during late diagenesis and early catagenis has been explained in terms of decarboxylation and removal of the carbonyl group in ketones and aldehydes. This interpretation is supported by the simultaneous production of carbon dioxide observed in several artificial maturation experiments (Tissot and Welte, 1984; Barth et al., 1987) and by studies of early maturation in coals by ~3C nuclear magnetic resonance (NMR) spectrometry (Boudou et al., 1984). The origin of the peak at approx. 1600cm -~ (1630cm ~ in spectra of kerogen concentrates) is more uncertain. Heating experiments of coals (Brown 1955b) and work by Painter et al. (1981) suggest that aromatic ring stretching is responsible for this peak. The strong intensity of this band in coals can be explained in terms of the presence of phenolic groups (Brown, 1955b). This explanation is supported by work of Painter et al. (1981). They showed that a synthesized phenolic resin produced an IR spectrum with the same general features as a subbituminous coal.
As shown by Brown (1955b) and later by Boudou et al. (1984) phenolic groups seem to be preserved
during early maturation. Thus, a relative increase in the peak at 1600cm -~ compared to the peak at 1700 cm-~ can be expected. This is in fact confirmed for the maturity range investigated in the present study. Figure 1 implies a relative increase in the broad phenolic - - O H peak (the interval from 3100 to 3500cm -~) parallelling the relative increase in the peak at 1600cm 1. The work of Boudou et al. (1984) using ~3C N M R spectroscopy also showed an increased aromatization and condensation during maturation. Such processes would further increase the intensity of the IR peak at 1600cm J. The IR spectra (Fig. 1) show an increase in aromatic C - - H stretch (at approx. 3050cm -~) compared to aliphatic C - - H stretch (the three sharp peaks between approx. 2850 and 3000cm ~) with increasing temperature. Thus, the present data suggest that the increase in the peak at 1600cm -~ with increasing temperature cannot be attributed to a single factor, but must be explained as contributions from both increased aromatization and a relative increase in phenolic compounds with increasing temperature. If a relative increase in phenolic groups should represent the main reason for the excellent correlation to temperature, the work of Brown (1955b) and others would imply that the IR parameter would be useful only for organic matter of low rank (up to approximately 0.90-1.0 %/~o). If, on the other hand, increased aromatization and condensation contribute most to the increase in the peak at 1600 cm ~ as implied by the work of Boudou et al. (1984), the IR determined parameter might be more reliable as a maturity indicator than any of the conventionally used maturity indices. Yen et al. (1984) have unambiguously assigned a band at 1735 cm ~to open chain carbonyl compounds. Ring-closure and aromatization of such compounds with increasing temperature would account fully for the pattern observed in the present data. CONCLUSION
Further work on natural and artificially matured samples is necessary to explore the usefulness of IR for maturity determination. For natural samples we anticipate problems related to differences in source input and changes in organic facies. However, we have previously shown that such problems can be overcomed by use of proper data-analytical methods (Christy et al., 1987). Another important result of this and previous studies on coals (Christy et al., 1987) is the demonstration of the quantitative maturity information carried by the IR profiles. Thus, the present work implies that IR analysis may, in addition to providing accurate assessment of maturity for, e.g. basin modelling and correlation studies, provide
Quantitative determination of thermal maturity useful i n f o r m a t i o n a b o u t the m a i n chemical transf o r m a t i o n s d u r i n g m a t u r a t i o n of organic matter. Acknowledgements--Norsk Hydro Research Centre (Bergen) is thanked for use of their IR facilities. T.B. and O.M.K. thank the Norwegian Research Council for Science and Humanities (NAVF) for financial support. Dr J. Kister, Marseille University (France) is thanked for many useful comments that improved the manuscript.
REFERENCES
Barth T., Borgund A. E. and Hopland A. L. (1989) Generation of organic compounds by hydrous pyrolysis of a Kimmeridge oil shale---Bulk results and activation energy calculations. Org. Geochem. 14, 69-76. Beny-Basses C. and Rouzaud J. N. (1985) Characterization of carbonaceous materials by correlated electron and optical microscopy and Raman spectroscopy. Scanning Electron Microsc. 1, 119-132. Boudou J.-P., Durand B. and Oudin J.-L. (1984) Diagenetic trends of a low-rank coal series. Geochim. Cosmochim. Acta 48, 2005-2010. Brown J. K. (1955a) The infrared spectra of coals. J. Chem. Soc. 744~752. Brown J. K. (1955b) Infrared studies of carbonised coals. J. Chem. Soc. 752-757. Christy A. A., Velapoldi R. A., Karstang T. V., Kvalheim O. M., Sletten E. and Telnaes N. (1987) Multivariate calibration of diffuse reflectance infrared spectra of coals as an alternative to rank determination by vitrinite reflectance. Chemometrics and Intelligent Lab. Systems 2, 199-207. Christy A. A., Tvedt J. E., Karstang T. V. and Velapoldi R. A. (1988) Low-pressure automated sample packing unit for diffuse reflectance infrared spectrometry. Rev. Sci. Instrum. 59, 423-426. Ganz H. and Kalkreuth W. (1987) Application of infrared spectroscopy to the classification of kerogen-types and the evaluation of source rock and oil shale potential. Fuel 66, 708-711.
O.G. 14/I--F
81
Johns R. B. (Ed.) (1986) Biological Markers in the Sedimentary Records. Meth. Geochem. Geophys. 24, 364 pp. Elsevier, Amsterdam. Kubelka P. and Munk F. (1931) Ein Beitrag zur Optik der Farbanstriche. Z. Tech. Phys. lla, 593-601. Kvalheim O. M. and Telnaes N. (1986) Visualizing information in multivariate data: Applications to petroleum chemistry. Part 2. Interpretation and correlation of North Sea oils by using three different biomarker fractions. Anal. Chim. Acta 191, 97 110. Kvalheim O. M., Christy A. A., Telnaes N. and Bjorseth A. (1987) Maturity determination of organic matter in coals using the methylphenanthrene distribution. Geochim. Cosmochim. Acta 51, 1883-1888. Painter P. C., Snyder R. W., Starsinic M., Coleman M. M., Kuehn D. W. and Davis A. (1981) Concerning the application of FT-IR to the study of coal: A critical assessment of band assignments and the application of spectral analysis program. Appl. Spectrosc. 35, 475-485. Philp R. P. (1985) Fossil Fuel Biomarkers: Applications and Spectra. Meth. Geochem. Geophys. 23, 294 pp. Elsevier, Amsterdam. Robin P. L. and Rouxhet P. G. (1978a) Characterization of kerogens and study of their evolution by infrared spectroscopy: carbonyl and carboxyl groups. Geochim. Cosmochim. Acta 42, 1341-1349. Rouxhet P. G. and Robin P. L. (1978b) Infrared study of the evolution of kerogens of different origins during catagenesis and pyrolysis. Fuel 57, 533 540. Snyder R. W., Painter P. C. and Cronauer D. C. (1983) Development of FT-i.r. procedures for the characterization of oil shale. Fuel 62, 1205 1214. Telnaes N., Bjorseth A., Christy A. A. and Kvalheim O. M. (1987) Interpretation of multivariate data: Relationship between phenanthrenes in crude oils. Chemometrics and Intelligent Lab. Systems 2, 149-153. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, 2nd edn, 699 pp. Springer, Berlin. Yen T. F., Wu W. H. and Chilingar G. V. (1984) A study of the structure of petroleum asphaltene and related substances by infrared spectroscopy. Energy Sources 7, 203-235.
0146-6380/89 $3.00 + 0.00 Copyright © 1989 Pergamon Press pie
Quantitative determination of thermal maturity in sedimentary organic matter by diffuse reflectance infrared spectroscopy of asphaitenes ALFRED A. CHRISTY, ANNE LISE HOPLAND, TANJA BARTH a n d OLAV M . KVALHEIM*
Department of Chemistry, University of Bergen, N-5007 Bergen, Norway (Received 10 November 1987; accepted 30 September 1988) Abstract--lnfrared spectroscopic analysis of the asphalthenes extracted from an artificially matured Kimmeridge clay sediment (kerogen type II) reveals an excellent correlation (0.997) between temperature and a maturity parameter based on two peaks at approximately 1600 and 1700cm ~. The change in intensity of the two peaks is discussed in terms of chemical changes of kerogen during the maturity range investigated in the present work (late diagenesis and early catagenis, i.e. °/oJ~o increasing from approximately 0.5 to 0.85%). Comparison with a maturity index based on the methylphenanthrene distribution shows that the spectroscopic determined parameter correlates best with temperature in the maturity range examined in this work. Key words--spectroscopic maturity index, hydrous pyrolysis, kerogen transformation, infrared spectroscopy, asphaltenes, spectral interpretation, methylphenanthrene distribution fraction (MPDF)
Quantitative assessment of thermal maturity of sedimentary organic matter is important for oil-oil and oil-source rock correlation and for basin modelling. The most commonly used parameter for maturity determination is vitrinite reflectance (Tissot and Welte, 1984). Although the accuracy of the technique is reduced for higher ranking samples, vitrinite reflectance has the advantage over many other proposed maturity parameters of covering the complete maturity range found in sediments and coals. However, accurate determination of vitrinite reflectance is time-consuming and the method is sensitive to physical in addition to chemical changes during kerogen degradation (Beny-Bassez and Rouzaud, 1985). Thus, alternative parameters directly related to the chemical changes of kerogen have been proposed. Biomarker ratios represent a large proportion of these parameters (Philp, 1985; Johns, 1986). Systematic investigations of several biomarker fractions reveal that few biomarkers can be expected to be useful maturity indicators for a large range of samples (Kvalheim and Telnaes, 1986). Thus, biomarker ratios are probably most useful as correlation parameters, e.g. for the correlation of migrated sedimentary organic matter to source rocks, and for the reconstruction of the depositional palaeoenvironments. Spectroscopic methods such as infrared and nuclear magnetic resonance techniques represent another chemical approach to maturity deter-
mination. Instead of looking at changes in a single molecule or a particular type of molecule, whole samples (or at least a major part) are characterized spectroscopically with respect to their chemical functionalities. In this way, the gross chemical changes during maturation are recorded. Maturity characterization based on infrared spectroscopy and similar techniques might thus be useful for a larger range of samples than particular biomarker ratios, embracing, e.g. sedimentary organic matter of different maceral composition. Thirty-five years of experience with IR spectroscopy in the field of coal and kerogen characterization has not given the technique a strong position within organic geochemistry. Thus, Tissot and Welte (1984) have given infrared spectroscopy the label "low or limitation of use" for the purpose of type and maturity determination of organic matter. The major reason for this pessimistic view of infrared analysis is that infrared spectra of whole rock samples and extracts appear as profiles of very broad bands. However, Christy et al. (1987a) have shown that the information about maturity in coals can still be obtained from the infrared profiles by use of proper data-analytical methods. Following earlier work of Robin and Rouxhet (1978a, b) and others (Painter et al., 1981; Snyder et al., 1983), Ganz and Kalkreuth (1987) defined a maturity parameter (called the C factor) using the intensities of two broad peaks at 1710 and 1630cm -~ in transmittance IR spectroscopy of kerogen concentrates:
*Author to whom correspondence should be addressed.
C Factor = Imo/(llTtO + Ii630)
INTRODUCTION
77
(1)
ALFRED A. CHRISTY et al.
78
In the present work, we have used diffuse reflectance IR spectroscopy to investigate the temperature dependence of a ratio based on the corresponding peaks in asphaltenes extracted from a Kimmeridge clay sediment (kerogen type II). The sediment was artificially matured by hydrous pyrolysis covering the temperature range 200-350°C. Comparison with the methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987), a maturity parameter derived from the methylphenanthrene distribution, suggests that infrared analysis provides a useful alternative to the commonly used techniques for maturity determination. EXPERIMENTAL
Pyrolysis and isolation of asphaltenes and phenanthrenes A Kimmeridge clay oil shale (kerogen type II) from Upper Jurassic North Sea was subjected to hydrous pyrolysis at fourteen temperatures covering the range from 2 0 0 C up to 350"C. Five grams of the pre-extracted oil shale was placed in a steelautoclave. Distilled water was added to the sample so that the total amount of liquid water in the autoclave at each pyrolysis temperature was estimated to 5 ml. The system was purged with nitrogen and quickly heated to the selected temperature. After 72 h the autoclave was air cooled to room temperature, the gas phase quantitatively collected, and the sediment and water phase removed from the autoclave. The water phase was filtered off and the residue Soxhlet extracted with dichloromethane for 24h using a water separator. The solvent was evaporated under nitrogen and the asphaltenes isolated by refluxing the extract with 40 vol of hexane for 8 h. The precipitated asphaltenes were retained by a 0.2~tm filter and subsequently redissolved in dichloromethane. The oils were separated into aliphatics and aromatics by liquid chromatography on a silica column, and the aromatics further separated on an amino modified silica column. The phenanthrene fraction was analyzed on a 50m BP-5 column. A total of 15 samples for the asphaltenes was obtained, the experiment at 320°C being replicated to check the repeatability of the experimental procedure. For the phenanthrenes, thirteen samples were analyzed, covering the temperature range 230-350~'C.
Infrared analysis Infrared analysis was performed using a PE 1700 FTIR spectrometer (Perkin-Elmer, Norwalk, CT, U.S.A.) equipped with a DTGS detector and a diffuse reflectance accessory (Harrick Scientific, Ossining, NY, U.S.A.). Finely ground (60-80 pm) KBr was packed in a sample cup using a sample packing unit designed and manufactured by Christy et al. (1988). The diffuse reflectance IR spectrum of KBr was recorded from 4000 to 600 cm-I using 44 scans at
a resolution of 4cm -~. Ten #l of the asphaltenes dissolved in dichloromethane was dropped gently on the KBr using a 10/~1 gas chromatographic syringe. The sample cup was then placed in an oven at 50°C for 2min to evaporate the solvent. Diffuse reflectance IR spectra of asphaltenes deposited on KBr were recorded, and the difference spectra calculated by subtracting the background spectrum of KBr from each sample spectrum. Each spectrum, consisting of 3401 data points, was subsequently transformed into Kubelka-Munk format [Kubelka and Munk, 1931; see also Christy et al. (1987) for a short description of the ideas and assumptions behind this transformation]. Finally, the height of the peaks at approximately 1700 and 1600 cm -1 (Fig. 1)(at 1710 and 1630 cm-~ in spectra of kerogen concentrates) were determined. The baseline was chosen as shown in the work of Robin and Rouxhet (1978a). RESULTS AND DISCUSSION
Maturi O, determined by infrared analysis Figure l shows asphaltene spectra obtained at temperatures 200, 230, 280, 330 and 350°C, respectively. A decrease in the peak at 1700 cm -~ and a corresponding increase in the peak at 1600cm with increasing temperature is obvious. Figure 2 shows the ratio between the intensity at 1600cm and the summed intensities at 1700 and 1600cm -~, i.e. 116oo/(116oo+117oo), plotted vs pyrolysis temperature. An excellent correlation is observed between temperature and the IR determined maturity parameter, the correlation coefficient being 0.997, which is close to the theoretical limit of perfect correlation. Note that this maturity parameter is inversely related to the C-factor [equation (1)] introduced by Ganz and Kalkreuth (1987). Our choice has the advantage of giving an index that increases with increasing maturation, while the index of Ganz and Kalkreuth is preferable if an IR analog of the van Krevelen diagram is wanted (Ganze and Kalkreuth, 1987). The experimental replicate at 320°C and the result of rerunning the IR procedure (at 350°C) show the good repeatability of the experimental and analytical procedure.
Comparison with the MPDF maturity parameter The methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987) for the thirteen samples covering the temperature range 230-350°C was calculated and plotted vs temperature (Fig. 3). The correlation of 0.93 implies that the linear relationship between maturity and the distribution of the methylphenanthrenes observed in coals (Kvalheim et al., 1987) and crude oils produced from source rocks of kerogen type II (Telnaes et al., 1987) is valid also for samples matured artificially by hydrous pyrolysis. This observation together with the excellent correlation between temperature and the IR based parameter implies that the IR parameters
Quantitative determination of thermal maturity
79
J f
S
I
I
400(
J
I I I 3000
I
I
i
I
I
200"C
I
1200
2000
600
I /crn
Fig. 1. Asphaltene spectra at selected pyrolysis temperatures.
.s-I /
o 0.7 0 r~ 0.6 4-
/.~'/" /
o.~
0,6
~'*''~/°
0.5 l
0.4 ~ 0.3
el""
•
~
~-
J
v
e---~
~; 0.3 0.2
0.2 0
i
0.4
s
0.1 I
200
I
1
250 300 Pyrotysis temperature (*C)
I
3.50
Fig. 2. Ratio between the intensity at 1600cm -~ and the summed intensities at 1700 and 1600cm ~ plotted vs pyrolysis temperature. The correlation between pyrolysis temperature and IR ratio is 0.997.
I
I
I
250 500 350 PyroLysis temperature [*C)
Fig. 3. The methylphenanthrene distribution fraction (MPDF) (Kvalheim et al., 1987) plotted vs pyrolysis temperature. The correlation between pyrolysis temperature and MPDF is 0.93.
80
ALFRED A. CHRISTY et al.
advocated in this work and by Ganz and Kalkreuth (1987) may be superior to more conventionally used maturity parameters also for natural samples. By using a linear regression model for predicting vitrinite reflectance from M P D F given by Kvalheim et al. (1987), the maturity range covered by the pyrolysis experiments is estimated to 0.5-0.85. Although the regression model was established for coal extracts (kerogen type III) and thus cannot be expected a priori to predict accurately vitrinite reflectance for a kerogen type II sediment extract, the above calculation indicates that the IR parameter is valid for at least maturities corresponding to the lower half of the oil window. Work of Robin and Rouxhet (1978a, b), Boudou et aL (1984) and Ganz and Kalkreuth (1987) suggests that the parameter extends to regions of higher maturity (see also discussion below). Chemical interpretation 1600 cm - 1
o f the p e a k s at
1700 and
An understanding of the chemical changes responsible for the intensity changes in the IR spectra is necessary to evaluate the validity range of the IR determined maturity parameter. Furthermore, the spectra (Fig. 1) reveal a pattern that might throw some new light on previous disputes about possible sources of contributions to the peak at 1600cm -j (Painter et al., 1981). Studies of the evolution in natural samples (Robin and Rouxhet, 1978a, b; Bondou et al., 1984) and of samples artificially matured (Brown, 1955b; Tissot and Welte, 1984) have proved beyond reasonable doubt that the strong peak at approx. 1700cm -~ (1710cm 1 in spectra of kerogen concentrates) corresponds to stretching of the carbonyl group in ketonic, aldehydic and carboxylic surroundings. Thus, the reduction in the carbonyl peak during late diagenesis and early catagenis has been explained in terms of decarboxylation and removal of the carbonyl group in ketones and aldehydes. This interpretation is supported by the simultaneous production of carbon dioxide observed in several artificial maturation experiments (Tissot and Welte, 1984; Barth et al., 1987) and by studies of early maturation in coals by ~3C nuclear magnetic resonance (NMR) spectrometry (Boudou et al., 1984). The origin of the peak at approx. 1600cm -~ (1630cm ~ in spectra of kerogen concentrates) is more uncertain. Heating experiments of coals (Brown 1955b) and work by Painter et al. (1981) suggest that aromatic ring stretching is responsible for this peak. The strong intensity of this band in coals can be explained in terms of the presence of phenolic groups (Brown, 1955b). This explanation is supported by work of Painter et al. (1981). They showed that a synthesized phenolic resin produced an IR spectrum with the same general features as a subbituminous coal.
As shown by Brown (1955b) and later by Boudou et al. (1984) phenolic groups seem to be preserved
during early maturation. Thus, a relative increase in the peak at 1600cm -~ compared to the peak at 1700 cm-~ can be expected. This is in fact confirmed for the maturity range investigated in the present study. Figure 1 implies a relative increase in the broad phenolic - - O H peak (the interval from 3100 to 3500cm -~) parallelling the relative increase in the peak at 1600cm 1. The work of Boudou et al. (1984) using ~3C N M R spectroscopy also showed an increased aromatization and condensation during maturation. Such processes would further increase the intensity of the IR peak at 1600cm J. The IR spectra (Fig. 1) show an increase in aromatic C - - H stretch (at approx. 3050cm -~) compared to aliphatic C - - H stretch (the three sharp peaks between approx. 2850 and 3000cm ~) with increasing temperature. Thus, the present data suggest that the increase in the peak at 1600cm -~ with increasing temperature cannot be attributed to a single factor, but must be explained as contributions from both increased aromatization and a relative increase in phenolic compounds with increasing temperature. If a relative increase in phenolic groups should represent the main reason for the excellent correlation to temperature, the work of Brown (1955b) and others would imply that the IR parameter would be useful only for organic matter of low rank (up to approximately 0.90-1.0 %/~o). If, on the other hand, increased aromatization and condensation contribute most to the increase in the peak at 1600 cm ~ as implied by the work of Boudou et al. (1984), the IR determined parameter might be more reliable as a maturity indicator than any of the conventionally used maturity indices. Yen et al. (1984) have unambiguously assigned a band at 1735 cm ~to open chain carbonyl compounds. Ring-closure and aromatization of such compounds with increasing temperature would account fully for the pattern observed in the present data. CONCLUSION
Further work on natural and artificially matured samples is necessary to explore the usefulness of IR for maturity determination. For natural samples we anticipate problems related to differences in source input and changes in organic facies. However, we have previously shown that such problems can be overcomed by use of proper data-analytical methods (Christy et al., 1987). Another important result of this and previous studies on coals (Christy et al., 1987) is the demonstration of the quantitative maturity information carried by the IR profiles. Thus, the present work implies that IR analysis may, in addition to providing accurate assessment of maturity for, e.g. basin modelling and correlation studies, provide
Quantitative determination of thermal maturity useful i n f o r m a t i o n a b o u t the m a i n chemical transf o r m a t i o n s d u r i n g m a t u r a t i o n of organic matter. Acknowledgements--Norsk Hydro Research Centre (Bergen) is thanked for use of their IR facilities. T.B. and O.M.K. thank the Norwegian Research Council for Science and Humanities (NAVF) for financial support. Dr J. Kister, Marseille University (France) is thanked for many useful comments that improved the manuscript.
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