Role of Polar Compounds as Source of Hydrocarbons and Reactive

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Role of Polar Compounds as Source of Hydrocarbons and Reactive Medium during the Artificial Maturation of Mahakam Coal R. Michels,* V. Burkle, L. Mansuy, E. Langlois, O. Ruau, and P. Landais UMR7566 G2R, BP 23, 54501 Vandoeuvre Cedex, France Received March 9, 2000

To obtain a complete maturation set of strictly same precursor, an oil-prone tertiary coal from the Mahakam delta (Indonesia) is submitted to artificial maturation using confined pyrolysis. The analysis of the co-genetic phases (free hydrocarbons, resins, asphaltenes, and residual kerogen) reveals the composition and evolution of each fraction. It appears that the polars have a high oil potential and contribute significantly to the generation of hydrocarbons (up to 25% of the initial oil potential, and up to 50% at given maturation stages). During the main stage of bitumen generation, a great part of the initial oil potential is thus transferred to the polars. During the later stages of thermal breakdown, this potential is released and contributes to the generation of aliphatics. In addition to their contribution as hydrocarbon source, the polars of the Mahakam coal contain a significant amount of Dammar resin. This latter is revealed to be a source of hydronaphthalenics which are very efficient hydrogen donors. Thus, the polars also appear to be a significant source of hydrogen available to organic reactions. It is postulated that one important condition allowing the generation of oil from coal is the presence of abundant resins, similar to Dammar resin, able to yield hydronaphthalenics during maturation. These compounds avoid cross-linking of the coal structure and participate in the formation of a freeflowing oil. On the contrary, in the absence of resin able to liberate hydronaphthalenics (or if the quantity of such resin is too low) the formation of oil is hindered, despite the good quality of all other parameters (aliphaticity and hydrogen richness of the coal, geological setting, expulsion capabilities, etc.). During further maturation in-situ cracking of the bitumen occurs, leading primarily to the formation of gas and not of oil.

1. Introduction The existence of liquids present as trapped material inside the coal macromolecular network is an important concept in coal reactivity. The knowledge of the formation mechanisms of free molecules in coals during retort is of uppermost importance for the understanding and modelization of coal reactivity. Organic geochemistry aims at a similar goal as far as the prediction of oil generation from kerogen in natural conditions is strategic for petroleum exploration. This is especially true for type III organic matter (terrestrial-derived), for which the chemical conditions for oil generation and expulsion are still poorly understood. To gain knowledge on the thermal maturation mechanisms of kerogen, artificial maturation techniques using closed pyrolysis experiments were developed in order to reproduce in the laboratory the major steps of liquid formation from coals. Much attention has been paid to define pyrolysis conditions able to yield products and geochemical data consistent with natural. The identification of pyrolysis mechanisms is therefore a base for the understanding of the chemical mechanisms occurring in nature.1-10 * Author to whom correspondence should be addressed. (1) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985, 8, 275-292. (2) Monthioux, M.; Landais, P. Energy Fuels 1988, 2, 794-801.

Organic geochemistry defines among the solvent soluble organic material (e.g., oil, bitumen), the free hydrocarbons (aliphatic and aromatic), the asphaltenes, and the resins.11 These are complex macromolecular structures released by the kerogen during thermal stress. To predict the thermal transformation of the organic system, it is important to understand the complex interactions that exist between the various fractions. Their own chemical composition and structure as well as reactivity need to be studied. The study of asphaltenes and resins by open pyrolysis,12,13 closed pyrolysis,14,15 hydrous pyrolysis16-18 or hydrocracking19 reveals that these polar compounds (3) Monthioux, M. Fuel 1988, 67, 843-848. (4) Monthioux, M.; Landais, P. Fuel 1987, 66, 1703-1708. (5) Landais, P.; Michels, R.; Elie, M. Org. Geochem. 1994, 22, 617630. (6) Michels, R.; Landais, P.; Philp, R. P.; Torkelson, B. Energy Fuels 1995, 9, 204-215. (7) Michels, R.; Landais, P.; Torkelson, B. E.; Philp, R. P. Geochim. Cosmochim. Acta 1995, 59, 1589-1604. (8) Michels, R.; Langlois, E.; Ruau, O.; Mansuy, L.; Elie, M.; Landais, P. Energy Fuels 1996, 10, 39-48. (9) Mansuy, L.; Landais, P.; Ruau, O. Energy Fuels 1995, 9 (4), 691703. (10) Mansuy, L.; Landais, P. Energy Fuels 1995, 9 (5), 809-821. (11) Tissot, B. P.; Welte, D. H. Petroleum formation and occurrence, 2nd ed.; Springer-Verlag: Berlin, Heildelberg, New York, 1984; 699 pp. (12) Behar, F.; Pelet, R.; Roucache, J. Org. Geochem. 1984, 6, 587595.

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contain structural organic moieties similar to those found in oils. This evidences also that the polars can bear a part of the hydrocarbon potential, as kerogens do. Numerous studies have pointed out the genetic link between polar compounds and kerogen12,20-22 and the similarities between kerogen and asphaltene structures.21,23 Therefore, asphaltenes are regarded as kerogen structural subunits. Although the fact that asphaltenes and resins contribute to the oil generation is widely admitted, the physical and chemical mechanisms and the actual role of the polars in the maturation processes remain largely unknown. Polars are thought to be released by the kerogen in the earlier stages of maturation by the thermal breakdown of the most labile bonds,11 a mechanism considered by some authors as similar to depolymerization.24,25 Because of their size and solubility properties, a specific part of them can be expelled with the hydrocarbons22,26 and migrate to the reservoirs. The polars remaining in the source rock continue to be submitted to thermal stress and are further thermally degraded. Their thermal behavior and reactivity are assumed to be the same as that of the kerogen from which they are derived. As a matter of fact, many oil generation models consider only the thermal alteration of the kerogen. The conversion processes developed to win hydrocarbons from oil resids and oil shales involve several important parameters such as proper thermal stress, the presence of catalysts, and a hydrogen source. It appears that hydrocarbon release from polar compounds is highly dependent upon hydrogen transfer reactions. Although numerous experiments were performed in order to simulate hydrogen transfer with hydrogen donor species,27-29 there is no evidence for these hydrogen sources to be effective during natural maturation. However, the importance of the hydrogenation reactions has been demonstrated by studies dealing with artificial maturation.6-10,30,31 The role of polars as a hydrocarbon source and a reaction medium during kerogen artificial (13) Huc, A. Y.; Behar, F.; Roussel, J. C. 1984 Geochemical Variety of Asphaltenes from crude oils. In Symposium on Characterization of Heavy Crude Oils and Petroleum Residues, Lyon, 25-27, june 1984, pp 99-108 Technip. (14) Cassani, F.; Eglinton, G. Chem. Geol. 1986, 56, 167-183. (15) Rubinstein, I.; Spyckerelle, C.; Strausz, O. P. Geochim. Cosmochim. Acta 1979, 43, 1-6. (16) Jones, D. M.; Douglas, A. G.; Connan, J. Org. Geochem. 1987, 13, 981-993. (17) Sofer, Z. Org. Geochem. 1987, 13, 939-945. (18) Lewan, M. D.; Williams J. A. A. A. P. G. Bull. 1987, 71, 207214. (19) Butz, T.; Oelert, H. H. Fuel 1995, 74, 1671-1676. (20) Bandurski, E. Energy Sources 1982, 6, 47-66. (21) Solli, H.; Leplat, P. Org. Geochem. 1985, 10, 313-329. (22) Pelet, R.; Behar, F.; Monin, J. C. Org. Geochem. 1986, 10, 481498. (23) Philp, R. P.; Gilbert, T. D. Geochim. Cosmochim. Acta 1985, 49, 1421-1432. (24) Larsen, J. W.; Li, S. Energy Fuels 1997, 11, 897-901. (25) Larsen, J. W.; Li, S. Org. Geochem. 1997, 26, 305-310. (26) Speight, J. G.; Moschopedis, S. E. Adv. Chem. Ser. 1981, 195, 1-15. (27) McMillen, D. F.; Malhotra, R.; Chang, S. J.; Ogier, W. C.; Nigenda, S. E.; Fleming, R. H. Fuel 1987, 66, 1611-1620. (28) Bedell, M. W.; Curtis, C. W.; Hool, J. H. Energy Fuels 1993, 7, 200-207. (29) Behar, F.; Pelet, R. Energy Fuels 1988, 2, 259-264. (30) Lewan, M. D. Laboratory simulation of petroleum formation: hydrous pyrolysis. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Publishing: New York, 1993; Chapter 18, pp 419442. (31) Lewan, M. D. Geochim. Cosmochim. Acta 1997, 61, 3691-3723.

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maturation has been demonstrated5-10 and revealed important interactions between free hydrocarbons, water, and polar compounds during artificial maturation.9,10 Optimal hydrocarbon release can only be obtained if the polar compounds form a solution with the free hydrocarbons and water. As a consequence, their chemical functions are highly available to organic reactions. The aim of the following study is therefore to compare the geochemistry of co-genetic liquid and solid phases produced by the artificial maturation of an oil-prone type III coal (Mahakam coal). This type of coal is considered as a major source of petroleum in South-East Asia and Australia. The experimental approach is based on the generation and collection of complete sets of cogenetic products (kerogen, resins, asphaltenes, hydrocarbons, gases). This avoids the difficulties related to the use of a natural multisourced set of samples to cover the whole maturation range, which always introduces some hints to the interpretations.32 It is expected that the geochemical characterization will give a deeper insight into the hydrocarbon potential and the structure of the kerogen, the asphaltenes, and the resins as well as to gain knowledge on the generation of oil from coal. 2. Experimental and Analytical Section Sample. The sample used in this study is a homogeneous vitrinite-rich coal from the Mahakam delta (Indonesia). Its main geochemical characteristics are the following: HI ) 197 mg of HC/g of C, OI ) 38 mg of CO2/g of C, Tmax ) 412 °C, mean vitrinite reflectance ) 0.5. It has been widely studied, and numerous geochemical and analytical data on both natural1,33-36 and artificial maturation are available. 1,2,9,10,36 Artificial Maturation. Amounts of 1 g aliquots of the coal were loaded in gold cells (L ) 5 cm; i.d. ) 1 cm) under inert atmosphere. Isothermal confined pyrolysis runs were conducted at temperatures ranging from 250 to 400 °C by 10 °C steps during 72 h at 700 bar pressure. The procedure and the pyrolysis apparatus are described elsewhere.37 Collection of the Different Fractions. After pyrolysis, the bitumen was extracted with a large excess of chloroform at 60 °C for 45 min. Asphaltenes were precipitated with 40 volumes of n-heptane at 40 °C for 15 min. The maltene fraction, soluble in n-heptane, was fractionated into saturates, aromatics, and resins using microcolumn liquid chromatography. Mass balance allowed quantitation of each fraction. Rock-Eval Analysis. Rock-Eval was performed on the extracted kerogen, the asphaltenes, and the resins. To allow measurements on the polars, a chloroform solution of asphaltenes and resins was deposited on silica sand. After evaporation of the solvent, the sand was submitted to the Rock-Eval analysis. Measurements were not possible for the experiments performed at T < 300 °C and T > 390 °C because the amounts of polars available were too low. Gas Chromatography-Mass Spectrometry (GC-MS). Saturates and aromatics were analyzed by gas chromatography-mass spectroscopy (HP 5890 Serie II GC coupled to a HP 5971 mass spectrometer), using an on-column injector, a 60 (32) Jenish, A.; Richnow, H. H.; Michaelis, W. Org. Geochem. 1990, 16, 917-929. (33) Boudou, J. P. Fuel 1984, 63, 430-431. (34) Monthioux, M.; Landais, P. Chem. Geol. 1989, 75, 209-226. (35) Monthioux, M.; Landais, P. Chem. Geol. 1989, 77, 71-85. (36) Benkhedda, Z.; Landais, P.; Kister, J.; Dereppe, J. M.; Monthioux, M. Energy Fuels 1992, 6, 166-172. (37) Landais, P.; Michels, R.; Poty, B. J. Anal. Appl. Pyrolysis 1989, 16, 103-115.

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Figure 1. Chloroform bitumen yield obtained by confined pyrolysis of Mahakam coal. m DB-5 J&W, 0.25 mm i.d, 0.1 mn film fused silica column. The temperature program was 40 to 300 °C at 3 °C/min followed by an isothermal stage at 300 °C for 15 min (constant helium flow of 25 cm/s) Flash Pyrolysis-Gas Chromatography-Mass Spectroscopy (Py-GC-MS). Py-GC-MS of asphaltenes, resins, and extracted residual kerogen were performed with a CDS 2000 pyroprobe. Samples were loaded in quartz tubes and heated at 620 °C for 15 s. The GCMS characteristics and temperature program were the same as described above. Fourier Transform Infrared Microspectroscopy (FTIR). The FT-IR spectra of asphaltenes, resins, and extracted residual kerogen were recorded in transmission mode on a Nicolet 800 spectrometer coupled with a Nick plan microscope and a nitrogen-cooled MCT detector. Amounts of 0.5 mg of powdered kerogen aliquots were placed on a diamond window compression cell.38 Samples were compressed to obtain a homogeneous pellet. The upper part of the anvil was retrieved and the sample placed under the microscope for FT-IR analysis.38 Asphaltenes and resins were solubilized in dichloromethane and dried on top of the diamond window cell, allowing the formation of a thin film. Background correction was performed with the air + diamond windows as reference spectrum. The size of the spot analyzed was 40-60 µm and the spectral resolution 4 cm-1. This technique is fast (no pellet preparation or special cells for liquid analysis are needed), avoids the usual problem of interferences with cell walls, solvents, or contamination by water adsorbed on the highly hydrophilic KBr. The spectra obtained needed no further signal treatment (smoothing, baseline correction). The high quality spectra obtained allowed deconvolution treatments. Valley-to-valley band integration allowed determination of the relative contribution of the aliphatic, aromatic, and oxygenated functions to the total spectrum. Deconvolution of the 2800-3000 cm-1 area was used to calculate the aliphatic CH3/CH2 ratio (asymmetric CH3/asymmetric CH2 vibration band areas). The area of the νCH aliphatic bands (2800-3000 cm-1) as well as the sum of the area of the oxygenated bands (νOH 3100-3600 cm-1, νCdO 1650-1800 cm-1, νC-O 10001300 cm-1) were ratioed to the area of the total spectrum (e.g., νCHali/S) in order to show relative contributions.38,39

3. Results 3.1. Effluent Yields. The bitumen yield increases with maturation, reaches a maximum close to 90 mg/g of coal at 340 °C, and decreases at higher temperatures (Figure 1). Polars are the most abundant fraction in the extracts obtained between 250 and 360 °C, and the resins are always more abundant then the asphaltenes (38) Ruau, O.; Landais, P.; Gardette, J. L. Fuel 1997, 76, 645-653. (39) Landais, P.; Rochdi, A. Fuel 1990, 4, 290-295.

Figure 2. Composition of the bitumen extracted from Mahakam coal as a function of confined pyrolysis temperature. Asphaltenes and resins were not distinguished in the 400 °C experiment.

Figure 3. Evolution of polars (asphaltenes + resins), asphaltenes, resins, C15+ saturates, and aromatics absolute yields of the fraction during confined pyrolysis of Mahakam coal. Asphaltenes and resins were not distinguished in the 400 °C experiment.

(Figure 2). With increasing maturation, the relative contribution of polars to the bitumen decreases first slowly between 250 and 350 °C and more sharply at higher temperatures (Figure 2). The proportion of aromatic hydrocarbons increases slowly with maturation until 360 °C. A stronger increase is observed at T > 360 °C, where they become dominant and represent up to 70% of the bitumen. The contribution of the C15+ aliphatic hydrocarbons increases steadily with maturation, reaches a maximum close to 35% at 370-380 °C, and decreases at higher temperatures. The absolute yields of asphaltenes is quite constant from 250 to 340 °C, then decreases (Figure 3). The resins are constantly generated until 350 °C, where their absolute amount decreases strongly. The amounts of aromatics generated increase constantly, while the aliphatics reach a maximum yield of 23 mg/g of coal at 370 °C before decreasing. 3.2. Overall Composition of the Hydrocarbons. The hydrocarbons generated from the Mahakam coal are characteristic of a terrestrial source. The saturates chromatograms (Figure 4) are rich in n-paraffins with a distribution exhibiting a strong odd predominance. Pristane is present in great abundance, and the biomarker content (especially hopanes) is high. With increasing maturation, the odd predominance in the n-paraffins decreases, and the maximum of distribution is shifted to lower-molecular-weight hydrocarbons. This effect is attributed to enhanced secondary cracking of

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Figure 4. Evolution of the C15+ saturates distributions (GCMS) obtained by artificial maturation of Mahakam coal. Some n-alkanes are indicated by their carbon number. T ) terpanes.

the higher-molecular-weight hydrocarbons.1,40 The m/z 191 fragmentogram (Figure 5; Table 1) of the saturates shows a distribution dominated by the 17R(H),21β(H)hopane, the 22R-17R(H),21β(H)-30 homohopane, and olean-1,3-ene. Some other oleanenes and oleananes are also present.41 The overall hopanoid trace is progressively modified as maturation proceeds and several biomarker indexes can be used to monitor the degree of thermal evolution; the R/S ratio of the homohopane increases significantly during maturation.42,43 The aromatic chromatograms (Figure 6) are dominated by alkylnaphthalenes, especially by 1,6-dimethyl4-isopropylnaphthalene (cadalene) and 1,6-dimethylnaphthalene, which both originate from the Dammar resin polymer, characteristic of the South-East Asia tertiary coals.44-47 During maturation, the relative abundance of cadalene gradually decreases while the (40) Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L. Rev. Inst. Fr. Pet. 1991, 46, 151-187. (41) ten Haven, H. L.; Rullkoter, J. Geochim. Cosmochim. Acta 1988, 52, 2543-2548. (42) Seifert, W. K.; Moldowan, J. M. In Physics and Chemistry of the Earth; Douglas, A. G., Maxwell, J. R., Eds.; Pergamon: Elmsford, NY, 1980; Vol. 12, pp 229-237. (43) Peters, K. E.; Moldowan, J. M. The biomarker guide; Prentice Hall: New York, 1993. (44) Van Aarssen, B. K. G.; Cox, H. C.; Hoogendoorn, P.; De Leeuw, J. W. Geochim. Cosmochim. Acta 1990, 54, 3021-3031. (45) Van Aarssen, B. K. G.; De Leeuw, J. W.; Horsfield, B. J. Anal. Appl. Pyrolysis 1991, 20, 125-139.

Figure 5. Evolution of the terpanes (m/z ) 191) obtained during artificial maturation of Mahakam coal. For peak assignments refer to Table 1.

abundance of 1,6-dimethylnaphthalene increases to become the dominant peak (Figure 6), which suggests a genetic relationship between these two compounds (a feature already observed elsewhere45). The other naphthalenes remain in similar relative abundance. 3.3. Rock-Eval of Kerogen and Polar Compounds. Figure 7 presents the evolution of the Hydrogen Index obtained on the unextracted and extracted kerogen, the asphaltenes, and the resins as a function of pyrolysis temperature. The initial Hydrogen Index of the coal is 197 mg of HC/g of C, and decreases progressively with maturation to reach about 30 mg/g of C at 420 °C. The Hydrogen Index of the asphaltenes (46) Singh, R. K.; Alexander, R.; Kagi, R. I. Org. Geochem. 1994, 21, 249-256. (47) Simoneit, B. R. T.; Grimalt, J. O.; Wang, T. G.; Cox, R. E.; Hatcher, P. G.; Nissenbaum, A. Org. Geochem. 1986, 10, 877-889.



Figure 7. Rock-Eval Hydrogen Index as a function of pyrolysis temperature for the unextracted coal, the chloroformextracted coal, as well as co-genetic resins and asphaltenes. The values for immature coal are plotted at 270 °C.

Figure 8. Evolution of Rock-Eval Tmax as a function of pyrolysis temperature for the unextracted coal, the chloroformextracted coal, and co-genetic resins and asphaltenes. The values for immature coal are plotted at 270 °C. Table 1: Peak Assignments for Terpanes Chromatograms

Figure 6. Evolution of the aromatic hydrocarbons distribution (GC-MS) generated by Mahakam coal during confined pyrolysis. 1: C2-naphthalenes; 2: C3-naphthalenes; 3: C4-naphthalenes*, 1,6 dimethylnaphthalene**, cadalene. Note the opposite evolution of 1,6 dimethylnaphthalene and cadalene as a function of maturity.

is 300 mg/g of C at 310 °C and reaches 220 mg of H/g of C at 340 °C. The Hydrogen Index for the resins is strongest with an average value of about 450 mg of HC/g of C. For both resins and asphaltenes, the Hydrogen Index is highest for the 310 and the 390 °C experiments. The Rock-Eval Tmax for the unextracted coal samples (Figure 8) increases constantly from 412 °C (initial sample) to 442 °C (aliquot pyrolyzed at 370 °C). A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

18R(H)-22,29,30-trisnorhopane (Ts) 17R(H)-22,29,30-trisnorneohopane (Tm) olean-13(18)-ene 17R(H),21β(H)-30-norhopane olean-12-ene 17β(H),21R(H)-30-normoretane 18R(H)- or 18β(H)-oleanane 17R(H),21β(H)-hopane 17β(H),21R(H)-moretane 22S-17R(H),21β(H)-30-homohopane 22R-17R(H),21β(H)-30-homohopane 17β(H),21R(H)-homomoretane 22S-17R(H),21β(H)-30-bishomohopane 22R-17R(H),21β(H)-30-bishomohopane 17β(H),21R(H)-bishomomoretane 22S-17R(H),21β(H)-30-trishomohopane 22R-17R(H),21β(H)-30-trishomohopane

sudden jump from 442 to 508 °C in the Tmax values is observed for the experiment performed at 380 °C, and is followed by a constant increase to a value of 550 °C for the most mature sample. Similar evolution is observed for the extracted coal; however, the Tmax increases faster. This is simply explained by the removal of the polars, which add to the broadness of the S2 peak when the sample is analyzed unextracted (the maximum of the S2 peak is thus shifted to lower temperatures). The Tmax values for the asphaltenes and the resins are fairly constant and range between 432 and 446 °C.

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Figure 9. Examples of Py-GC-MS chromatograms obtained for the chloroform-extracted coal and the corresponding co-genetic resins and asphaltenes. Chromatograms are presented at two maturation stages :° alkene-alkane doublets B1: C1 benzenes; B2: C2 benzenes; B3: C3 benzenes; N: naphthalene; N1: C1-naphthalenes; N2: C2 naphthalenes; N3: C3 naphthalenes; φ: phenol; φ1: C1 phenol; φ2: C2 phenol; HD: hydronaphthalenics*, 1,6-dimethylnaphthalene**, cadalene; T: terpanes.

3.4. Compounds Distribution in the Kerogen, Asphaltenes, Resins, and Free Hydrocarbons. Compounds distribution in the kerogen, asphaltenes, and resins was investigated using Py-GC-MS. Figure 9 shows typical pyrograms of co-genetic material. Relative Contributions of Aliphatics, Aromatics, and NSO Compounds in the Pyrograms. The geochemical signatures of the extracted residual kerogen, the asphaltenes, and resins have some common features, typical of Mahakam coal: high n-paraffin content with many biomarkers (prist-1-ene, cadalene, hopanes) and a significant contribution of aromatic hydrocarbons (alkylbenzenes, alkylnaphthalenes, and alkylphenanthrenes) as well as alkylphenols. However, the pyrograms of the co-genetic fractions exhibit striking differences in the relative contributions of the various dominant hydrocarbon families as a function of increasing maturation (Figures 9 and 10). Below 310 °C, the kerogen pyrograms show aromatics, aliphatics, and NSO compounds (mainly phenols), with a significant contribution of lower-molecular-weight material (nC6-nC12 range). With higher temperatures, there is a reduction of the aliphatic contribution and to a lesser extent of the NSO compounds. Correlatively, the aromatic contribution increases. At 370 °C, the pyrograms are strongly dominated by aromatic compounds. The distribution of the hydrocarbons in the Py-GCMS chromatograms of the asphaltenes and resins is dominated by the aliphatics of the C15-C35 range and a group of aromatics eluting in the nC20 range. The

relative abundance of cadalene and its thermal byproducts and precursors (hydronaphthalenes, hydrocadalenes, Dammar resin byproducts) is low in the kerogen, but important in asphaltenes and resins generated between 250 and 320 °C (Figure 9) (the most dominant aromatic hydrocarbons are the 1,6-dimethyl-4-isopropylnaphthalene (cadalene) and 1,6-dimethylnaphthalene, as well as hydroaromatics). With increasing maturation, there is no increase of the contribution of aromatic hydrocarbons, while the high-molecularweight aliphatics remain dominant in the chromatograms. Alkanes Distribution. To follow the evolution of the aliphatic content in the kerogen and polars, the m/z ) 57 fragmentograms of the pyrograms are compared to the chromatograms of the free saturates extracted by chloroform (Figure 10). It must be kept in mind, when such comparison is made, that flash pyrolysis has an effect on the alkanes distribution: alkene-alkane doublets are generated (which are not present in the free hydrocarbons of the chloroform extract) and the detailed aliphatic distribution can be affected. However, the general features can be compared, especially among the resins, asphaltenes, and kerogen (all submitted to the same analytical conditions). For a given maturation temperature, the aliphatics distribution in the resins pyrograms is quite similar to that of the co-genetic free saturates. The same oddalkanes predominance is characteristic. On the contrary, the aliphatic distribution in the asphaltenes and in the kerogen is different from that of the free saturates.


Figure 10. m/z ) 57 chromatograms (GC-MS) obtained on the saturates as well as m/z ) 57 chromatograms obtained by Py-GC-MS of resins, asphaltenes, extracted coal. Two pyrolysis stages are compared (260 and 330 °C) to show maturity impact. Chromatograms are focused on the C15+ alkanes in order to allow comparison with the saturates fraction. Some n-alkanes and alkene-alkane doublets are identified by their carbon number. Pr: pristane and prist-1-ene.

Figure 11. Evolution of the pristane/nC17 measured on GCMS chromatograms for saturates and (pristane + prist-1-ene)/ (nC17alkane + nC17alkene) measured on Py-GC-MS of cogenetic resins, asphaltenes, and residual kerogen as a function of confined pyrolysis temperature. Values for the immature coal sample are plotted at 230 °C.

Especially, the odd-alkanes predominance observed in the free saturates is less pronounced in the asphaltenes and kerogen m/z ) 57 traces, and the maximum of the distribution is shifted toward the lower-molecularweight hydrocarbons (this is especially true for the kerogen). The evolution of the relative maturity of each fraction can be followed by parameters such as the pristane/nC17 and the prist-1-ene/n-C17 ratios (Figure 11). Although the pristane/n-C17 ratio is lower for resins than for the free saturates, both have similar behavior with


an increase followed by a decrease. The ratio obtained for the asphaltenes shows a milder evolution, with a slight increase until 290 °C followed by a very gradual decrease. For the kerogen, the ratio is the lowest, and decreases continually with maturation. Also, the evolution of the pristane/nC17 ratio in the various fractions occurs at different rates during maturation, thus indicating different apparent maturation degrees for identical thermal histories. Distribution of the Hopanoids. The terpanes distribution in the kerogen, polars, and free hydrocarbons is presented in Figure 12. In the 260-300 °C temperature range, the m/z 191 mass chromatogram of the kerogen and corresponding asphaltenes show similar distribution, whereas the resins chromatograms show distributions intermediate between the asphaltenes and the free saturates. This intermediate composition is partly due to the flash pyrolysis process: the saturated hopanoids distribution in the Py-GC-MS chromatograms is similar to that of the free saturates (the R/S epimer ratios is here a good marker), while the unsaturated compounds distribution generated by flash pyrolysis is similar to that of the asphaltenes. Despite the flash pyrolysis artifact it is interesting to note that the hopanoids signature disappear at 330 °C from the kerogen, while they remain in the polars. 3.5. FT-IR Spectroscopy on Kerogen, Asphaltenes, and Resins. Typical FT-IR spectra of co-genetic kerogen, asphaltenes, and resins are presented in Figure 13 (see Table 2 for band identification). These differences in general features are similar at all maturation temperatures. The overall spectra of the kerogen are dominated by the bands corresponding to oxygenated functions (1000-1300 cm-1 region as well as 16501800 cm-1 and 3100-3700 cm-1), to CdC aromatic (1590-1620 cm-1), and CH aromatic (700-900 cm-1 as well as 3000-3100 cm-1). The asphaltenes also show significant bands of oxygenated, aromatic functions as well as the OH band (3100-3700 cm-1). The relative contribution of the CH aliphatic (2800-3000 cm-1) is, however, far stronger than for the kerogen. The spectra obtained with resins are always characterized by a very strong contribution of the CH aliphatic bands (28003000cm-1 and 1140-1470 cm-1), all other FT-IR bands being minor. The relative contribution of the νCHaliphatic bands to the total FT-IR spectra are measured by the νCHaliphatic/S ratio (Figure 14). The aliphatic content of the kerogen, asphaltenes, and resins are very different. The kerogen has the lowest aliphatic content and the ratio starts to decrease at 340 °C. The asphaltenes show intermediate values, which decrease significantly along the maturation profile. The resins display the highest νCHaliphatic/S values which decrease slowly as maturation increases. The average chain length in the structures is measured by the νCH3as/νCH2as ratio (Figure 15). The kerogen contains aliphatic chains that are shorter compared to the asphaltenes and resins. In addition, with increasing maturation the average chains length decreases. The ratio in the asphaltenes is fairly constant between 240 and 340 °C and lower than for the kerogen. At higher temperatures, an increase is noticed, indicating a shortening of the average chain length. Concerning

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Figure 12. Examples of m/z ) 191 chromatogram of saturates (GC-MS) and co-genetic resins, asphaltenes, and extracted coal (Py-GC-MS). Two maturation stages are presented. For peak assignments refer to Table 1.

the resins, the evolution is opposite to that of the kerogen and the asphaltenes: the initial material (raw coal and 250-260 °C pyrolysis temperatures) displays a νCH3as/νCH2as ratio significantly higher, but with increasing maturation this ratio decreases, indicating an enrichment in compounds of longer average chain length. The relative aromaticity is measured by the νCHaro/ νCHali ratio (Figure 16). The relative aromatic content of the kerogen is by far the strongest (the values are 5-fold that of the asphaltenes and 10-fold that of the resins). However, the ratio increases fairly similarly for the three co-genetic fractions with increasing maturation. The contribution of the oxygenated bands in the spectra is measured using the (νOH + νC ) O + νCO)/S ratio (Figure 17). The kerogen displays the highest relative oxygenated function content, while the asphaltenes are intermediate, and the resins show the lowest values. For the kerogen, the relative oxygen content

decreases significantly during maturation. Values remain about constant for the resins, while those for the asphaltenes increase constantly with maturation, a behavior opposite to the kerogen. Discussion Hydrocarbon Potential of Kerogen, Asphaltenes, and Resins. The comparison of the Rock-Eval Hydrogen Index values allows us to roughly estimate the relative contribution of the polars to the initial oil potential in the coal (HI0) or to the residual oil potential present in the kerogen + polars assemblage during maturation. Figure 18 presents [(HIun-HIext)*100/HI0)] as a function of temperature and indicates that part of the oil potential is transferred from the coal to the polars until about 350 °C (maximum of bitumen generation, which also corresponds to the maximum for resins). Thus, up to 25% of the original oil potential is transferred to the polars. Normalization of this same param-


Figure 13. Examples of FT-IR spectra obtained on co-genetic resins, asphaltenes, and extracted coal pyrolyzed at 270 °C. For peak assignments refer to Table 2.

Figure 14. Evolution of the νCHaliphatic (2800-3000cm-1) over the total FT-IR spectrum ratio for extracted kerogen, resins, and asphaltenes as a function of confined pyrolysis temperature. Immature sample values are plotted at 200 °C.


Figure 15. Evolution of the νCH3asymmetric/νCH2asymmetric ratio (2955 cm-1/2920 cm-1 band ratio measured on deconvoluted spectra) for extracted kerogen, resins, and asphaltenes as a function of confined pyrolysis temperature. Immature sample values are plotted at 200 °C.

Figure 16. Evolution of the νCHaromatic/νCHaliphatic (30003100 cm-1/2800-3000 cm-1) ratio for extracted kerogen, resins, and asphaltenes as a function of confined pyrolysis temperature. Values for resins and asphaltenes are, respectively, multiplied by 10 and 5 for better comparison. Immature sample values are plotted at 200 °C.

Table 2: Band Assignments for FT-IR Spectra νCHaliphatic νCH3asymmetric νCH2asymmetric δ(CH3 + CH2) γCHaromatic νCHaromatic νCdCaromatic νOH νCdO νC-O

2800-3000 cm-1 2955 cm-1 2920 cm-1 1440-1470 cm-1 700-900 cm-1 3000-3100 cm-1 1590-1620 cm-1 3100-3700 cm-1 1650-1800 cm-1 1000-1300 cm-1

eter to the residual oil potential at each maturation temperature [(HIun - HIext)*100/HIun] as presented in Figure 18 shows a similar trend, with a maximum reached at 380 °C. The polars can thus represent up to 45% of the residual oil potential present in the kerogen + polars assemblage at a given temperature. The composition of the bitumen, mostly composed of resins for temperatures up to 350 °C (Figure 2), as well as the high Hydrogen Index of the resins (450 mg of HC/g of C in average; Figure 7) suggests that a significant part of the oil potential is transferred to this fraction during maturation. Asphaltenes, however, are not to be forgotten as far as their HI is significant (an average of 190 mgHC/gC) compared to the kerogen (HI0 ) 200 mg of HC/g of C).

Figure 17. Evolution of the contribution of oxygenated bands to the total spectrum (νOH: 3100-3700 cm-1; νCdO: 16501800 cm-1; νC-O: 1000-1300 cm-1) as a function of pyrolysis temperature for extracted kerogen, resins, and asphaltenes. Immature sample values are plotted at 200 °C.

Structural Evolution of Kerogen, Asphaltenes, and Resins. Pyrolysis GC-MS and FT-IR show that the co-genetic kerogen-asphaltenes and resins do not have identical composition. Kerogen is more aromatic than asphaltenes and resins (Figures 9, 13, 14, 16). The

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Figure 18. Contribution of the oil potential of polars (as measured by Rock-Eval Hydrogen Index) normalized to the Hydrogen Index of extracted coal or to immature coal as a function of confined pyrolysis temperature. ∆HI ) (HI of unextracted coal - HI of extracted coal). Unheated sample values are plotted at 270 °C.

resins are by far the most aliphatic material, and bear alkanes distributions that are closest to the free aliphatics (Figures 9, 10). Therefore, in addition to having a significant oil potential (Figures 7 and 18), the polars are a source of aliphatic hydrocarbons that are liberated during cracking, especially at temperatures over 350 °C (Figures 3 and 18). Tracking the structural changes of the polars and kerogen also allows us to get information on their reactivity. The kerogen loses its aliphatic material, especially the moieties containing the long chains, as indicated by the significant increase of the νCH3as/νCH2as ratio (Figure 15) and the evolution of the Py-GC-MS traces (the alkenes-alkanes distribution in the chromatograms is shifted to the lower-molecular-weight material; Figures 9 and 10). A significant part of these long-chain aliphatic moieties are transferred to the polars (Figures 10, 14, 15), especially the resins. It is interesting to note that part of the oxygenated material lost by the kerogen is transferred specifically to the asphaltenes (Figure 17), as the FT-IR parameter (OH + C ) O + C-O)/S of both moieties evolve in opposite trends. This phenomenon is not recorded by the resins, which keep a very aliphatic structure. In the meantime, the kerogen undergoes aromatization (Figure 16) accompanied by a sudden increase in thermal stability (jump in Tmax values; Figure 8) corresponding with the maximum of bitumen generation (Figure 1). The asphaltenes and resins also show an evolution with maturity, as reflected by the decrease of HI values (Figure 7) and νCHaliphatics/S ratio (Figure 14) as well as the increase in relative aromaticity (Figure 16). However, the changes remain fairly limited compared to that of the kerogen (especially for the resins). This shows that the polars do not undergo major structural transformation during maturation, but are simply cracked partly into smaller units (i.e., gas and hydrocarbons) as well as incorporated into pyrobitumen (for the most heavy and aromatic material). Molecular Signatures of Co-genetic Kerogen, Polars, and Asphaltenes. As shown previously, the oil potential as well as structural features are different for the kerogen and the co-genetic polars. Such differences extend into the molecular signature as shown by Py-GC-MS.

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It is striking to observe that, at a given pyrolysis temperature, the distribution of the alkanes in the free saturates and resins (Figure 10) is different from that of the cogenetic asphaltenes and residual kerogen. An increased apparent maturity is therefore observed in the following order: the free saturates and resins look less mature than the asphaltenes, which look less mature than the residual kerogen. These results show that although the coal has experienced the same thermal history, the compound distribution in each fraction does not reflect the degree of maturation in the same manner. The evolution of the m/z ) 57 chromatograms with maturation is also different for each fraction (i.e., the maturity seems to evolve faster in the residual kerogen and asphaltenes than in the resins and free saturates). This confirms that major changes occur in the kerogen during maturation, while the polars, and especially the resins, have a rather conservative effect, mainly acting as a “transfer medium” for hydrocarbons from the kerogen to the free hydrocarbons. At low maturation stage, the geochemical signature of the polars bears similar general information than the kerogen. However, with increasing maturity, the composition of the polars and the kerogen diverge. As far as the polars have a specific thermal behavior, they cannot just be considered as simple kerogen subunits, but must be considered as having the potential of participating differently to the generation of free hydrocarbons. Thus, it is expected that a careful investigation of their behavior during maturation can give us some information on the hydrocarbons release process. Polars as Source of Hydrocarbons during the Maturation of Mahakam Coal. Between 250 and 350 °C, the bitumen is mainly composed of polars and of aromatic hydrocarbons (Figure 2). As far as the polars contribute to a significant part of the oil potential and aliphaticity of the system, it can be considered that, during this stage of maturation, part of the aliphatic potential is transferred from the kerogen to the polars without significant generation of aliphatic hydrocarbons (Figures 2, 3). This process is going on until about 340350 °C, where the amounts of polars (especially resins) decreases strongly in the system (Figure 3): the kerogen undergoes aromatization and thermostabilization (Figures 8, 16) while the cracking of the polars occurs. At this stage of maturation, the kerogen and the polars have very different geochemical properties: the polars contribute up to 45% of the residual oil potential (Figure 18), they are highly aliphatic (Figure 14, 15), and show a compound distribution similar to the free hydrocarbons (Figure 10). On the contrary, despite its similar contribution to the total oil potential, the kerogen exhibits a lower HI, and contains far less aliphatic material which has a distribution that does not resemble the free aliphatics. In addition, the kerogen exhibits a strong Tmax, which indicates that its overall thermal stability is far higher than that of the polars; its ability to undergo cracking is therefore lower. In regards to all these parameters, it can be considered that the cracking of the polars generates a significant part of the free aliphatic hydrocarbons, and at a rate that is probably far different from the kerogen. It may be suggested that the kerogen, in the 340-380 °C maturation stage, rather generates gas and lower-molecular-weight hy-



Figure 19. Expanded section of the Py-GC-MS chromatograms obtained on co-genetic resins, asphaltenes, and coal generated at 270 °C and showing the contribution of Dammar resin markers. Compounds are norcadalene-type structures and hydronaphthalenics derived from cadalene. Some structures are shown. (*) Dammar resin-derived structures. (°) Alkene-alkane doublets.

drocarbons (C1-C12 range), while the polars release more of the nC15+ range material. Attemps to test such hypothesis have been done in separating the polars from the kerogen and doing pyrolysis of single fractions.9,10 However, the separate pyrolysis of kerogen and polars leads to the suppression

of bitumen generation from the kerogen at a very early maturation stage (300 °C). This work clearly underlines the importance of chemical interactions between polars and kerogen in order to properly release the free hydrocarbons from kerogen. Further questions ask why the removal of polars (and especially resins) from the

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Figure 20. Example of reactions involving Dammar resinderived molecules. The formation of cadalene can be explained by the generation of molecules liberated by the maturation of Dammar resin and reactions through hydrogen transfer mechanisms. In a parallel manner, 1,6-dimethylnaphthalene is formed by cracking of the isopropyl side chain of cadalene.

coal has such strong impact on the bitumen generation? A partial answer may rise from the present work. Chemical Role of Polars in the Generation of Hydrocarbons from Mahakam Coal. Aromatic hydrocarbons are constantly released by the coal + polars during maturation, and no clear relationship can be made with the source of aromatics. At the initial stage of maturation (250-350 °C) it can be considered that the aromatics mostly are derived from the kerogen (this moiety is by far the most aromatic). This may also be true at temperatures over 350 °C. However, the GCMS analysis of the aromatic fraction (Figure 6) reveals that the composition is strongly dominated by 1,6dimethylnaphthalene and cadalene, two compounds specifically derived from the Dammar resin, a natural polymer. It is also interesting to note that the relative abundance of cadalene and 1,6-dimethylnaphthalene seem to evolve in an opposite trend, suggesting a genetic relationship. Closer inspection of the Py-GC-MS chromatograms obtained on the kerogen and polars reveal the presence of Dammar resin derivatives (Figure 19), which have a

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significant relative abundance in the asphaltenes and resins chromatograms. Dammar resin is composed of aliphatic monomers of the cadinane structure. These compounds, under PyGC-MS are released as cadalene, 1,6-dimethylnaphthalene, as well as methylnaphthalene and hydronaphthalenics (Figure 19). The Mahakam kerogen and polars thus contain a noticeable amount of Dammar resin. In regards to the data presented in this paper, we propose a set of chemical reactions that are related to the artificial maturation of Dammar resin (Figure 20). During maturation, Dammar resin undergoes first a depolymerization process which allows the liberation of the natural polymer either as monomer into the bitumen, or as polymer into the polars. Of course, part of the Dammar resin remains in the kerogen, and will follow the proposed reactions within the kerogen network (as it does within the polars molecular structures). Parallel and/or successive reactions of isomerization, hydrogen transfer, and cracking lead from the cadinane monomer to hydrocadalene, cadalene, and 1,6-dimethylnaphthalene (Figure 20). Most of these reactions lead to hydrogen transfer from the Dammar resin derivatives to carbon radicals. These are proposed reactions that explain the compounds observed; however, they may not be the only existing. Dammar resin, present in the kerogen and the polars, has therefore a significant role as reactant during maturation. The removal of the polars (especially the resins) from the coal during maturation9,10 implies the removal of these hydrogen donors from the system. In addition, Dammar resin derivatives are not found in the aromatic fraction of the experiments in which bitumen generation is suppressed by removal of polars.9,10 The importance of hydrogen sources and hydrogen transfer reactions in the generation of hydrocarbons from kerogen has been widely demonstrated.6,7,9,10,27,30,31,48 We therefore suggest that Dammar resin (especially the fraction linked to the resins and to a lesser extent to the asphaltenes and kerogen) plays a major role in the generation of hydrocarbons from Mahakam coal by avoiding the cross-linking of the structure and promoting the formation of hydrogen-saturated hydrocarbons. Conclusion This work shows that the composition, oil potential, and reactivity of the resins and asphaltenes differ significantly from the residual kerogen. Polars are mainly generated by this oil-prone coal in the early stages of maturation. As a consequence, the generation of free hydrocarbons depends not only on the kerogen at a given maturation stage, but also on the composition of the polars. In addition, the polars present in Mahakam coal contain a specific type of reactant which is Dammar resin. This latter, through thermal degradation, generates compounds which have hydrogen transfer properties analogous to tetralin.27,48 In addition, polars and kerogen form a co-solution promotings together with the free hydrocarbonssthe formation of a plastic phase in the coal.49-51 Thus, the high hydrogen (48) Poutsma, M. L. Energy Fuels 1990, 4, 113-131. (49) Fortin, F.; Rouzaud, J. N. Fuel 1993, 72, 245-250. (50) Grint, A.; Mehani, S.; Trewhella, M.; Crook, M. J. Fuel 1985, 64, 1355-1360.


donor potential of the polars (especially the resins) and their high mobility within the coal network ensures a proper medium to allow the generation of additional polars and hydrocarbons. Indeed, when the polar phase is removed from the system by selective solvent extraction, the generation potential of the coal is suppressed.9,10 Many gas-prone coals contain significant amounts of n-alkanes in the nC10-nC35 range, and however, do not generate oil. From the published data9,10 and those presented in this work, we postulate that the generation of oil from coal is also strongly linked to the presence of hydrogen-donating compounds such as the Dammar resin. In the absence of resins that are able to liberate (51) Butterfield, I. M.; Thomas, K. M. Fuel 1995, 74, 1780-1785.


hydronaphthalenic material (or if the quantity of such resin is too low) the generation of oil is hindered, all other parameters being favorable (aliphaticity and hydrogen richness of the coal, geological setting, expulsion capabilities, etc.). In such case, the coal will only generate gas. It is, however, needed to perform additional work on different coals, as well as to gain a more in-depth knowledge of the reactivity of Dammar resin and other types of resins in order to generalize this hypothesis. Acknowledgment. The authors thank Jean-Louis Oudin and Total-Fina-Elf for providing the Mahakam coal sample and Rock-Eval analysis. EF000046D

Role of Polar Compounds as Source of Hydrocarbons and Reactive

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