Energy & Fuels 2001, 15, 653-658
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Maturity Trends in Raman Spectra from Kerogen and Coal S. R. Kelemen* and H. L. Fang Exxon Mobil Research & Engineering Company, Annandale, New Jersey 08801 Received September 18, 2000. Revised Manuscript Received February 14, 2001
The present work explores the potential of Raman spectroscopy to provide maturity information about catagenesis stage kerogens and coals. The first-order Raman spectra of coals and kerogens show a broad amorphous A band between 1310 and 1360 cm-1 and a graphite-like G band near 1580-1600 cm-1. As vitrinite reflectance (R0) increases, the Raman A band becomes narrower and shifts to lower frequencies while the absorption strength of the A band relative to the G band decreases. The area ratio of the A band to G band (R ) fA/fG) decreases from 3.2 to 2.4 in going from the least mature sample up to R0 ) 2.0%. Similar trends were observed for maturation suites of Type II and other Type III kerogens. Laboratory thermolysis produced a range of samples with well-defined laboratory R0. The changes in the Raman A and G bands from the laboratory matured samples directionally paralled the natural samples. These results demonstrate that the Raman spectra of catagenesis stage samples vary in regular ways with sample maturity. The underlying chemical and structural changes associated with changes in the Raman A and G bands are discussed.
I. Introduction In material science applications, Raman spectroscopy has been used to determine the average domain size of graphitic crystallites in carbonaceous materials. Natural graphite only shows a single first-order Raman band at 1578 cm-1. For less ordered carbons, this band is shifted to higher frequencies (1580-600 cm-1) and an additional Raman band appears near 1350 cm-1. This partially Raman active feature at 1350 cm-1 arises from crystalline disorder where the Raman selection rules are broken by lattice discontinuities or structural defects. Similar features are seen in the Raman spectra from coal1-6 The chemical structure of organic matter is significantly altered during catagenesis and metagenesis. During catagenesis the aromatic organic matter in coal and kerogen is composed mostly of small 1- to 4-ring aromatic structures.7-12 It is generally accepted that graphitization does not begin until the later stages of metagenesis. However, there is evidence from Raman spectroscopy that traces of graphite-like domains exist in less mature kerogen and coal.1-6 The first-order Raman spectrum of coal shows carbon bands near 1590 cm-1 labeled graphitic (G) and near 1350 cm-1 labeled amorphous (A).1-6,13-21 Differences in the appearance of these bands were associated with the geologic history of a sample.1-6 It is generally found that during meta* Author to whom correspondence should be addressed (1) Roberts, S.; Tricker, P. M.; Marshall, J. E. A. Org. Geochem. 1995, 23, 223. (2) Spotl, C.; Houseknecht, D. W.; Jaques, R. C. Org. Geochem. 1998, 28, 535. (3) Jehlicka, J.; Beny, C. Org. Geochem. 1992, 18, 211. (4) Pasteris, J. D.; Wopenka, B. Can. Mineral. 1991, 29, 1. (5) Wopenka, B.; Pasteris, J. D. Am. Mineral. 1993, 78, 533. (6) Yui, T. F.; Huang, E.; Xu, J. J. Metamorphic Geol. 1996, 14, 115.
genesis (R0 > 2.0) and beyond, the intensity of the G band increases while the A band decreases with increasing sample maturity. The Raman spectra of less mature samples have not been examined in any detail. The maturity level of sedimentary organic matter is an important geological parameter for coal and kerogen. The reflectance of vitrinite in a sample (vitrinite reflectance, % R0) has been used to define the level of organic metamorphism (LOM) scale.7,8 The peak maximum temperature during Rock-Eval pyrolysis (Tmax)8 is another established thermal maturity indicator. For small amounts of organic material in a sample rock matrix,22,23 where it may not be possible to use vitrinite (7) Durand, B. Kerogen, Insoluble Organic Matter from Sedimentary Rocks; Technip: Paris, 1980. (8) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (9) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187. (10) Orendt, A. M.; Solum, M. S.; Sethi, N. K.; Pugmire, R. J.; Grant, D. M. In Advances in Coal Spectroscopy; Plenum Press: New York, 1992; p 215. (11) Meuzelaar, H. L. C. Ed. Plenum Press: New York, 1992; pp 215-254. (12) Fletcher, T. H.; Kerstein, A. R.; Pugmire, R. J.; Solum, M. S.; Grant, D. M. Energy Fuels 1992, 6, 414. (13) Tuinstra, F.; Koenig, J. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53, 1126. (14) Vidano, R.; Fischbach, D. B. J. Am. Ceram. Soc. 1978, 61, 13. (15) Gruber, T.; Waldek Zerda, T.; Gerspacher, M. Carbon 1994, 32, 1377. (16) Young, J. A.; Koppel, J. U. J. Chem. Phys. 1965, 42, 1377. (17) Lespade, P.; Al-Jishi, R.; Dresselhaus, M. S. Carbon 1982, 20, 427. (18) Lespade, P.; Marchand, A.; Couzi, M.; Cruege, F. Carbon 1984, 22, 375. (19) Nakamizo, M.; Honda, H.; Inagaki, M. Carbon 1978, 16, 281. (20) Katagiri, G.; Ishida, H.; Ishitani, A. Carbon 1988, 26, 565. (21) Kokaji, K.; Oya, A.; Maruyama, K.; Yamada, Y.; Shiraishi, M. Carbon 1997, 35, 253. (22) Fluid inclusions in materials: method and applications. De Viv, B., Frezzotti, M. L., Eds.; Virginia Tech, Blacksburg: VA, 24061.
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reflectance or Rock-Eval pyrolysis (Tmax), Raman spectroscopy24 may provide a viable alternative to estimate sample maturity. The present work explores what kind of maturity related information can be derived from the Raman G and A bands for catagenesis and metagenesis stage samples. Laboratory matured kerogens and coals were examined to see if their Raman response is similar to natural samples. II. Experimental Section The Raman data were collected with a Renishaw Ramascope equipped with two holographic filters and electrically cooled CCD camera. The excitation source was a HeNe laser with a wavelength of 632.8 nm and was focused on the sample through the microscope. The choice of a red excitation instead of Ar+ laser lines was to avoid the fluorescence background from the sample. Data were collected using an 80× or 100× long working distance objective lens. The main benefits using this micro Raman technique were confocal rejection of the fluorescence background and the capability for depth profiling of the sample. Raman bandwidth analysis was done using the standard curve resolution algorithm and a multiple point correction for background subtraction. This gave the Raman A band area (fA) and G band area (fG). The graphite crystal was ZYA graphite monochromator grade obtained from Union Carbide Advanced Ceramics. Coal samples were obtained from the Argonne Premium coal sample program25 and the Penn State coal sample program.26 Maturation suites of type II kerogen (Duvernay Formation) and type III kerogen (Fruitland Formation) were studied. A type I kerogen (Green River) and a type II kerogen (Bakken) were also studied. Samples of types I and II organic matter concentrates were prepared by treatment with HCl/HF at 0 °C and 23 °C, respectively, to remove the inorganic carbonate and alumino-silicate mineral components. The high fluorescence background, from the type I and II organic matter concentrates, interferes with the much weaker Raman signal. Solvent extraction was used to reduce the bitumen content of the sample. Briefly, THF and pyridine were successively used. Each solvent was repeated until the supernatant solution turned colorless. The kerogen samples were dried and stored under the vacuum prior to the Raman measurement. All Argonne premium coals were extracted using THF. The coals were dried and stored under vacuum. Laboratory pyrolysis was performed under well-defined time and temperature conditions in order to simulate natural thermal maturation process. Laboratory maturation was done using both isothermal and nonisothermal conditions. Nonisothermal laboratory maturation of coal and kerogen samples was done in ultrahigh vacuum using a temperature-programmed decomposition apparatus.27 A linear heating rate of 0.23 °C/s up to the final temperature was used to produce the pyrolysis chars. Isothermal laboratory maturation of kerogen and coal was done in a quartz-lined reactor in helium at 1 atm.28 The conversion of thermolysis time and temperature parameters into the R0 scale was done on the basis of a vitrinite maturation model.29 (23) Fluid Inclusions. Reviews in Mineralogy, Vol. 12; Roedder, E., Ed.; 1984. (24) Touret, J. Fluid Phase Petrology; Kluwer Academic Press: Dordrecht, The Netherlands, 250 pp. (25) The Users Handbook for the Argonne Premium Coal SampleProgram; Vorres, K. S., Ed.; Argonne National Laboratory: Argonne IL, 1989; ANL-PCSP-89-1; Energy Fuels 1990, 4, 420. (26) Glick, D. C.; Davis, A. Operation and Composition of the Penn State Coal Sample Bank and Data Base. Org. Geochem. 1991, 17, 421430. (27) Kelemen, S. R.; Vaughn, S. N.; Gorbaty, M. L.; Kwiatek, P. J. Fuel 1993, 72, 645. (28) Freund, H.; Kelemen, S. R. Am. Assoc. Pet. Geol. Bull. 1989, 73, 1011.
Figure 1. Raman spectrum of graphite before and after grinding.
III. Results A. Graphite. The Raman spectrum from graphite and graphitic samples provides a basis for interpreting the Raman spectrum of coals and kerogens. Figure 1 shows the Raman spectrum of graphite before and after grinding. Graphite shows a single first-order Raman band at 1578 cm-1 with a full width at half-maximum (fwhm) bandwidth of 16 cm-1. Grinding the graphite crystal produces a band at 1335 cm-1 with a bandwidth of 40 cm-1. Additional grinding causes an increase in the intensity of the 1335 cm-1. Three second-order Raman features for a graphite crystal are found at 2457, 2686, and 3245 cm-1. These second-order features can be attributed to the overtones and combinations of the first-order bands. Their intensities are much weaker and their bandwidths are significantly broader than the corresponding first-order peaks. For the ground graphite powder, an additional weak band is observed at 3050 cm-1 that is assigned to a defect disorder-induced Raman feature. For less ordered carbons, the graphitic band is shifted to higher frequencies near 1580-1600 cm-1. Another band is found near 1350 cm-1 which has been assigned to a defect-induced vibration.13-21 After thermal treatment of carbon black it is generally observed that the 1350 cm-1 band narrows and becomes less intense, indicating the growth of the hexagonal layer as well as possible stacks of layer-to-layer graphitic structures. Many excellent references have reviewed and discussed fundamental aspects of the first-order Raman spectra from graphitic materials.13-17 Various phonon theoretical models have been developed and studied.15,16,30,32 The Raman ratio between the disordered and the graphitic band (f1350/f1580) has been used as a measure for the growth of microcrystallites. The ratio is normally 1/50 or less for graphitic materials. B. Coals. Raman spectra of the Argonne Premium and Penn State coal samples used in this study show a (29) Sweeney, J. J.; Burnham, A. K. Am. Assoc. Pet. Geol. Bull. 1990, 74, 1559. (30) Nemanich, R. J.; Solin, S. A. Phys. Rev. B 1979, 20, 392. (31) Nicklow, R.; Wakabayashi, N.; Smith, H. G. Phys. Rev. B 1972, 5, 4951. (32) Hiura, H.; Ebbesen, T. W.; Tanigaki, K.; Takahashi, H. Chem. Phys. Lett. 1993, 202, 509.
Maturity Trends in Raman Spectra from Kerogen and Coal
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Figure 2. Raman spectrum of a bituminous coal. Figure 4. Raman amorphous/graphitic band ratio, R ) fA/fG, for fresh and laboratory matured coals. A logarithmic trend line is shown for reference.
Figure 3. Raman band separation (G-A) for fresh and laboratory matured coals. A linear trend line is shown for reference. Table 1. Raman Spectroscopy Results of Carbon Bands for Argonne Premium (AP) and Penn State Coals
sample Pocahontas (AP) Upper Freeport (AP) Lewiston (AP) Pittsburgh #8 (AP) Blind Canyon (AP) Illinois #6 (AP) Wyodak (AP) Beulah Zap (AP) Penn State #201489 Penn State #201483 Penn State #201487 Penn Sate #201488
vitrinite Raman band Raman reflectance separation (G-A) ratio (R0) (cm-1) R ) (fA/fG) 1.68 1.16 0.89 0.81 0.57 0.46 0.32 0.25 5.72 5.19 2.80 1.88
240 237 232 234 226 218 218 223 282 273 245 240
2.60 2.63 2.79 2.75 2.71 2.76 3.00 2.90 2.36 2.40 2.53 2.50
broad amorphous A band around 1310-1360 cm-1 and a graphite-like G band at 1580-1590 cm-1. A typical Raman spectrum of a bituminous coal is shown in Figure 2. Table 1 lists the band separation (G-A) and the Raman intensity ratio R ) fA/fG for these two bands. The peak maximum of the amorphous band tends to shift to lower frequencies with increasing R0/maturity and the shift can exceed 50 cm-1. The position of the G band shifts slightly toward higher frequency with increasing R0/maturity but, the shift does not exceed 10 cm-1. The energy separation between G and A bands appears to be related to vitrinite reflectance (Figure 3) for fresh, and laboratory matured coals to be discussed (Section III.C). The increase of the band separation is caused by a narrowing of the amorphous bandwidth as
Figure 5. Raman amorphous (A) and graphitic (G) bandwidths vs R0 for fresh coals.
R0 increases. Figure 4 shows the dependence of the Raman R ratio (the ratio of the integrated areas between A and G bands) on vitrinite reflectance. For catagenesis stage coals, the Raman ratios drop rapidly with increasing vitrinite reflectance, but levels off for metagenesis stage coals. Again, this trend is followed for fresh, and laboratory matured coals to be discussed (Section III.C). Figure 5 shows the Raman G and A bandwidths as a function of R0. The G band narrows with increasing R0. Even at high R0 the A bandwidth is substantially wider than the A bandwidths observed with graphite samples. The most noticeable maturityrelated changes for fresh coals are a drop in the Raman intensity ratio (R) from 3.2 to 2.4 and a narrowing and shift to lower frequencies for the A band. C. Laboratory Matured Coals. Samples were heated in the laboratory under well-defined conditions of time and temperature in order to simulate natural thermal maturation processes. The exact pyrolysis conditions and the conversion to a laboratory R0 scale are shown in Table A-1 of the Appendix. The Raman spectra before and after laboratory maturation were measured and compared to the starting coal. Table 2 shows these results. The Raman G-A band separation increases and the fA/fG intensity ratio decreases with increasing laboratory maturation of coals. Figure 3 shows the Raman G-A band separation data for laboratory ma-
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Table 2. Raman Spectroscopy Results for Argonne Premium Coal Samples before and after Laboratory Maturation
sample
treatmenta
Pocahontas Pocahontas Beulah Zap Beulah Zap Beulah Zap Beulah Zap Beulah Zap
fresh lab maturation fresh lab maturation lab maturation lab maturation lab maturation
calculated Raman band vitrinite Raman separation reflectance ratio (G-A) (R0) R ) (fA/fG) in cm-1 1.68 3.73 0.23 0.68 1.34 1.42 3.73
240 242 223 222 224 222 236
2.64 2.38 2.90 2.76 2.70 2.64 2.45
a The detailed conditions for laboratory maturation are described in the Appendix.
Table 3. Raman Spectroscopy Data before and after Solvent Extraction Raman band separation (G-A) (cm-1 ) sample
before solvent extraction
after solvent extraction
Fruitland #1 Fruitland #2 Fruitland #3 Fruitland #4 Fruitland #5 Fruitland #6 Fruitland #7
220 218 220 230 242 240 247
218 218 222 228 246 238 246
Figure 6. Raman band separation (G-A) for Fruitland and Duvernay kerogen samples. (The data for Argonne Premium and Penn State Coals with a linear trend line are included for reference).
Table 4. Raman Spectroscopy, Vitrinite Reflectance, and Rock-Eval Tmax Results from the Duvernay (type II) and Fruitland (type III) Kerogen Samples
sample Fruitland #1 Fruitland #2 Fruitland #3 Fruitland #4 Fruitland #5 Fruitland #6 Fruitland #7 Duvernay #1 Duvernay #2 Duvernay #3 Duvernay #4 Duvernay #5 Duvernay #6 Duvernay #7
Rock-Eval vitrinite Tmax reflectance (°C) (R0) 424 440 442 451 482 476 487 417 430 440 444 447 451 467
0.63 0.69 0.79 1.13 1.42 1.56 1.57 0.42 0.48 0.52 0.60 0.62 0.78 1.50
Raman band separation Raman (G-A) ratio (cm-1) R ) (fA/fG) 220 218 220 230 242 240 242 206 228 224 232 231 235 239
2.75 2.65 2.52 2.54 2.50
Figure 7. Raman amorphous/graphitic band ratio, R ) fA/fG, for Fruitland and Duvernay Kerogen samples. (The data for Argonne Premium and Penn State Coals with a logarithmic trend line are included for reference).
3.18 2.89 2.99 2.82 3.16 2.80 2.76
significant changes occurred in Raman spectra after the bitumen was removed with solvent extraction. Table 3 shows that there are no significant changes in the G-A separation after solvent extraction. However, a significant reduction in the fluorescence background was observed. Table 4 contains the Raman data for Fruitland and the Duvernay kerogens maturation suites of samples. The available vitrinite reflectance data and Rock-Eval Tmax data are included in Table 4. Figure 6 shows the Raman G-A band separation and Figure 7 shows the fA/fG ratio for both maturation suites of kerogen samples as a function of R0. The data for coal is included for reference along with their trend lines. The data from the Duvernay and Fruitland kerogen samples scatter closely about the trend lines found for the coal samples. Immature kerogens were heated under well-controlled conditions of temperature and heating rate to simulate the natural thermal maturation process. The Raman spectra before and after maturation were measured for Green River (type I) and Bakken (type II) kerogens. Table 5 shows these results. The directional trends in the Raman spectra of laboratory matured kerogens are similar to coal. There is an increase of G-A band separation and the decrease
tured and fresh coals plotted versus R0. The data points for the laboratory matured samples fall below the trend line for coal. Figure 4 also contains a plot of the Raman fA/fG intensity ratio for laboratory matured coal along with fresh coal results. The laboratory maturation data points at high R0 lie near the trend line found for high R0 coal. D. Kerogens. Kerogen is the portion of sedimentary organic matter insoluble in common organic solvents and is composed of a covalently bound cross-linked, polymer-like macromolecular network in which solventsoluble/-extractable bitumen may be trapped. The network consists of aromatic structures linked by aliphatic and heteroatomic moieties. Bitumen trapped in the kerogen network can be extracted with solvents.7,8,33 For the Fruitland samples, a check was made to see if any (33) Van Krevelen, D. W. Coal, 3rd ed.; Elsevier: Amsterdam, 1993; p 293.
Maturity Trends in Raman Spectra from Kerogen and Coal
Energy & Fuels, Vol. 15, No. 3, 2001 657 Table 6. Raman Spectroscopy Results after Laboratory Maturation
Figure 8. Raman band separation (G-A) for laboratory matured type I and type II kerogen samples. (The data for Argonne Premium and Penn State Coals with a linear trend line are included for reference).
sample
0.23 °C/s heat-up final temperature (°C)
Raman band separation (G-A) (cm-1)
Raman ratio R ) (fA/fG)
Pocahontas Pocahontas Beulah Zap Beulah Zap Bakken Bakken Green River Green River
690 810 690 810 690 810 690 810
242 255 236 246 246 246 236 240
2.38 2.20 2.45 2.30 2.56 2.40 2.64 2.61
present in kerogen and coal begin to convert into larger polynuclear aromatic (PNA) structures.9,35-37 Table 6 shows the Raman spectra of coal and kerogen change very little after pyrolysis up to 810 °C. Despite significant anticipated structural changes to the bulk of the organic matter only small changes are observed in the Raman spectra. The Raman intensity ratio fA/fG drops only slightly from 2.4 and appears to reach an asymtotic value of 2.2. 4. Discussion
Figure 9. Raman amorphous/graphitic band ratio, R ) fA/fG, for laboratory matured type I and type II kerogen samples. (The data for Argonne Premium and Penn State Coals with a logarithmic trend line are included for reference). Table 5. Raman Spectroscopy Data of Bakken and Green River Kerogens before and after Laboratory Maturation
sample
treatment
Bakken Bakken Bakken Bakken Bakken Green River Green River Green River
fresh lab maturation lab maturation lab maturation lab maturation fresh lab maturation lab maturation
calculated Raman band vitrinite separation Raman reflectance (G-A) ratio -1 (R0) (cm ) R ) (fA/fG) 0.68 1.03 1.34 3.73
216 215 230 242 246
2.71 2.80 2.74 2.67 2.56
1.34 3.73
236 240
2.64 2.61
of intensity ratio of fA/fG with laboratory maturation. The Raman G-A band separation data for laboratory matured samples are plotted as a function of R0 in Figure 8. Figure 9 contains a plot of the Raman fA/fG intensity ratio for laboratory matured kerogens as a function of R0. The fresh coal data and trend lines are plotted in each figure for reference. The data scatter near the trend lines found for coal. The greatest change in the Raman spectra of coal and kerogens occurs up to R0 ) 2.4%. Under severe pyrolysis conditions (slow heat-up, T g 690 °C) most of the 1-4 aromatic ring units originally
It is generally accepted that most of the aromatic organic matter in coal and kerogen is composed of small 1- to 4-ring aromatic structures,7,8,33 however, traces of graphitic domains appear in all of the coals studied. It is well-known that the Raman spectra from coal have bands associated with amorphous and graphitic carbon.1-6,38-40 The Raman spectra of coals are similar to those of polycrystalline carbons. It was thought33 that the initial Raman spectra from coals38-40 were a result of artifacts induced during the Raman measurement. However, recent findings1-6 indicate that Raman data may provide maturity information for metagenesis stage samples (vitrinite reflectance values greater than 2.0%). The present Raman spectroscopy investigation has explored lower maturity samples and has considered other kerogen types. As coal rank increases and R0 increases, the most noticeable changes in the first-order Raman spectra are a narrowing and a shift to lower frequencies of the A band and a decrease in R ) fA/fG from 3.2 to 2.2. On the basis of the subsequent discussion, it is unlikely that these changes can be interpreted to be a result of an increase in the size of graphitic domains. The laboratory pyrolysis of coal and kerogen using the Rock-Eval experiment divides the progressive results into several stages.34 The Rock-Eval S1 peak is associated with the evolution of low molecular weight hydrocarbons present/trapped in the sample that evolve below 200 °C. Most of this material is removed by solvent extraction. At higher temperatures some organic oxygen species are decomposed into CO, CO2, and H2O and these gases appear in the Rock-Eval S3 peak. The (34) Espitalie, J.; Laporte, J. L.; Madec, M.; Marquis, F.; Leplat, P.; Boutefeu, A. Inst. Fr. Pet. 1977, 32, 23. (35) Kelemen, S. R.; Gorbaty, M. L.; Kwiaterk, P. J.; Fletcher, T. H.; Watt, M.; Solum, M. S.; Pugmire, R. J. Energy Fuels 1998, 12, 159. (36) Kelemen, S. R.; Freund, H.; Gorbaty, M. L.; Kwiaterk, P. Energy Fuels 1999, 13, 529. (37) Perry, S.; Hambly, E. M.; Fletcher, T. H.; Solum, M. S.; Pugmire, R. J. Proc. Combust. Inst. 2000, 28. (38) Friedel, R. A.; Carlson, G. L. Chem. Ind. 1971, 40, 1128. (39) Friedel, R. A.; Carlson, G. L. Fuel 1972, 51, 194. (40) Zerda, T. W.; John, A.; Chmura, K. Fuel 1981, 60, 375.
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majority of hydrocarbons are produced between 400 and 550 °C and they are associated with the Rock-Eval S2 peak. Before the evolution of hydrocarbons associated with S2 there is little change in the Raman spectrum. This indicates that the loss of many of the organic oxygen functionalities have little impact on the Raman A and G bands. During the hydrocarbon evolution associated with S2, many of the alkyl side chains and aliphatic carbon linkages are lost. Most of the changes in the Raman spectrum of kerogen and coal are found over this pyrolysis interval. It is known that during laboratory pyrolysis above 690 °C many of the original 1- to 4-ring aromatic carbon clusters develop into larger PNA structures.35-37 The larger PNA domain size may be associated with a narrowing of the Raman A band. However, there is no evidence in the Raman spectra for 3D layering. The apparent asymptotic value for the Raman ratio fA/fG ) 2.2 reflects the lack of long-range ordering among the aromatic rings. The development of graphitic layed stacking is inhibited by defects and boundary structures (including H, N, S, O). Even graphitizing carbons in a turbostratic configuration contain wrinkled layers (in diameter La) with defected graphitic-aromatic domains. The mean number of layers per stack is relatively low (Lc < 3) and shows no welldefined graphitic domain structure. Gruber et al.15 have applied Raman and X-ray diffraction to probe the in-plane dimension of graphitic microcrystallites in carbon black under laboratory thermal treatment. Raman was used to estimate La (the twodimensional domain size) based on an empirical formula13,15,19,30 that relates the Raman ratio R to La (La ) 43.5 Å /R). X-ray diffraction was used to probe both La and Lc (the layer-to-layer domain size) based on the Scherrer equation. The La values determined by X-ray diffraction and Raman were similar. The range of La was 25 to 300 Å. Samples were treated from 300 °K to 3000 °K. They concluded that the major changes of Raman bands were caused by the formation of microcrystallites (2-D rearrangement of existing layer plane and not due to a growth of 3-D graphitic structures). If we assume that Raman ratio R relationship to La is also (La ) 43.5 Å /R) for coals then the 3.0 to 2.2 range for R corresponds to the 15-20 Å range for La. The results suggest that Raman signal from coals graphitic domains involve ∼10 PNA units. There is still not enough evidence to define the exact underlying chemical and
Kelemen and Fang
structural changes associated with the apparent changes in the Raman A and G bands from coals and kerogens. Furthermore, more work is needed to explain the geochemical basis for the presence of Raman A and G bands in categenesis stage organic matter. 5. Summary The results presented in this paper demonstrate that the Raman spectra of catagenesis stage samples vary in regular ways with fresh sample and laboratorymatured sample maturities. The most noticeable maturity-related changes for fresh coals are a drop in the Raman intensity ratio (R) from 3.2 to 2.4 and a narrowing and shift to lower frequencies for the A band. The Raman G-A band separation increases and the fA/ fG intensity ratio decreases with increasing laboratory maturation of coals. The directional trends in the Raman spectra of laboratory matured kerogens are similar to coal. There is an increase of G-A band separation and the decrease of intensity ratio of fA/fG with laboratory maturation. Acknowledgment. We thank P. J. Kwiatek for his technical help with the pyrolysis experiments and H. Freund, R. Chimenti, M. L. Gorbaty, and M. Siskin for many helpful discussions. Appendix Detailed conditions for laboratory maturation are found in Table A-1. Table A.1. Pyrolysis Conditions and Conversion to a Laboratory R0 Scale sample
reaction
conditions
calculated R0
Pocahontas Pocahontas Beulah Zap Beulah Zap Beulah Zap Beulah Zap Green River Green River Green River Bakken Bakken Bakken Bakken
non-isothermal non-isothermal isothermal non-isothermal isothermal non-isothermal isothermal non-isothermal non-isothermal isothermal isothermal non-isothermal non-isothermal
0.23 °C/s to 520 °C 0.23 °C/s to 690 °C 400 °C for 300 s 0.23 °C/s to 520 °C 380 °C for 18000 s 0.23 °C/s to 690 °C 400 °C for 300 s 0.23 °C/s to 520 °C 0.23 °C/s to 690 °C 400 °C for 300 s 365 °C for 57600 s 0.23 °C/s to 520 °C 0.23 °C/s to 690 °C
1.34 3.73 0.68 1.34 1.42 3.73 0.68 1.34 3.73 0.68 1.03 1.34 3.73
EF0002039
