Methane Generation from Oil Cracking - American Chemical Society

Energy & Fuels 1999, 13, 471-481

471

Methane Generation from Oil Cracking: Kinetics of 9-Methylphenanthrene Cracking and Comparison with Other Pure Compounds and Oil Fractions F. Behar,*,† H. Budzinski,‡ M. Vandenbroucke,† and Y. Tang§ Geology-Geochemistry Division, IFP, 1-4 Avenue de Bois Pre´ au, 92506 Rueil-Malmaison Cedex, France, Laboratoire de Photophysique et Photochimie Mole´ culaire, Universite´ de Bordeaux 1, URA 348 CNRS, 351 Cours de la Libe´ ration, 33400 Talence, France, and Chevron Petroleum Technology, 1300 Beach Boulevard, Box 446, La Habra, California 90633-046 Received August 3, 1998. Revised Manuscript Received December 14, 1998

The aim of the study is to determine the apparent rate constants for methane generation from the 9-methylphenanthrene (9-MPh) thermal cracking in the temperature range 375-450 °C under a constant pressure of 120 bar to evaluate by extrapolation the thermal stability of methylated aromatics of crude oils in geological conditions. In the first step, based on the conversion of 9-MPh only, rate constants were determined and the generated products were identified: the overall kinetic scheme was compared to that proposed in the literature for the 1-methylpyrene thermal cracking (Smith, C. M.; Savage, P. E. Energy Fuels 1992, 6, 195-202). Then the methane generated during primary and secondary cracking of 9-MPh was quantified, and the corresponding apparent rate constants for its production were determined. The apparent kinetic parameters for the degradation of 9-MPh are 49.0 kcal/mol for E and 4.5 × 1010 s-1 for A, whereas E is 54.5 kcal/mol and A is 1.1 × 1012 s-1 for methane generation. When these results are extrapolated to geological conditions (T < 200 °C), the Arrhenius diagrams corresponding to the two reactions cross that of n-C25 between laboratory and geological conditions. Consequently, both the early transformation of the methylated compounds and the late methane production will take place before the occurrence of a significant cracking of the n-alkanes. The thermal cracking of 9-MPh was also compared to that of the lumped chemical class of methylated aromatics generated during artificial maturation of kerogens representative of the main types of organic matter (I, II, and III) in closed pyrolysis systems.

Introduction The main purpose of this work is related to the study of the natural process of thermal cracking of oils in sedimentary basins at temperatures generally below 200 °C and pressures ranging between 20 and 100 MPa. This process causes reservoir fluids evolution toward lighter and lighter oils and subsequently gas, when reservoirs are submitted to very high temperatures (>180 °C). One of the key parameters for explorationists is the prediction of the gas/oil ratio for a given reservoir, and thus, it is very important to know what chemical classes are responsible for oil and/or gas production. It is widely accepted1 that the thermal evolution of oils is controlled by the kinetics of cracking reactions. This allows petroleum geochemists to simulate experimentally the low-temperature, long residence time natural processes by operating at higher temperatures generally between 250 and 550 °C.2-14 At these temperatures, reactions are rapid enough to monitor crack†

Geology-Geochemistry Division. Universite´ de Bordeaux. Chevron Petroleum Technology. (1) Tissot, B. P.; Welte, D. H. Petroleum formation and occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (2) Tissot, B. P.; Espitalie, J. Rev. Inst. Fr. Pe´ t. 1975, 30, 743-777. (3) Monthioux, M.; Landais, P.; Monin, J. C. Org. Geochem. 1985, 8, 275-292. (4) Ungerer, P.; Pelet, R. Nature 1987, 327, 52-54. ‡ §

ing with an acceptable time, i.e., a few minutes to a couple of months. These experimental simulations are presently the best way to elaborate mathematical models that describe oil cracking and contribute to petroleum evaluation in natural reservoirs. Nevertheless, due to the diversity and the complexity of the chemical composition of oils, the development of a mathematical model can be made presently only on an empirical basis through lumping individual molecules having similar structures, thus similar thermal stabilities, into chemical classes. In previous papers,8-9 a kinetic model was proposed which is comprised of the following chemical classes defined by a given elementary composition: in the gas (5) Espitalie, J.; Ungerer, P.; Irwin, I.; Marquis, F. Org. Geochem. 1988, 13, 893-899. (6) Horsfield, B.; Disko, U.; Leibtner, F. Geol. Rundsch. 1989, 78/ 1, 361-374. (7) Ungerer, P. Org. Geochem. 1990, 16, 1-25. (8) Behar, F.; Kressmann, S.; Rudkiewicz, J. L.; Vandenbroucke, M. Org. Geochem. 1991, 19, 173-189. (9) Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L. Rev. Inst. Fr. Pet. 1991, 46, 151-181. (10) Burnham, A.; Braun, R. Org. Geochem. 1990, 16, 27-39. (11) Lewan, M. In Organic Geochemistry; Engel, M. H., Macko, S. A., Eds.; Plenum Publishing Corp.: New York, 1994; Chapter 18, pp 419-440. (12) Pepper, A. S.; Corvi, P. J. Mar. Pet. Geol. 1995, 12, 291-319. (13) Behar, F.; Vandenbroucke, M.; Tang, Y.; Marquis, F.; Espitalie, J. Org. Geochem. 1997, 26, 321-339. (14) Lewan, M. Geochim. Cosmochim. Acta 1997, 61, 3691-3723.

10.1021/ef980164p CCC: $18.00 © 1999 American Chemical Society Published on Web 02/12/1999

472 Energy & Fuels, Vol. 13, No. 2, 1999

fraction, methane, ethane, propane + butane; in the light hydrocarbon fraction, C9-C14 aromatics, C6-C14 saturates, and the mixture of aromatics benzene + toluene + xylenes + naphthalene (BTXN); in the heavy hydrocarbon fraction, the C14+ saturates, the C14+ unstable aromatics which is comprised of mainly alkyl and naphthenoaromatic structures, the more stable C14+ aromatics which comprises the methylated compounds; in a solid fraction, the precoke and the coke. In this kinetic scheme, methane, the BTXN fraction, and coke are considered as stable classes. Determination of the kinetic parameters for each unstable chemical class cracking (apparent activation energy Ei, preexponential factor Ai, and stoichiometric coefficients Ri) is made on the basis of a set of reference pyrolysis experiments. It consists of determining the minimum of an error function defined as the mean square residual of the model versus the experiments. To minimize the number of free parameters, it was decided to impose the same preexponential factor for all cracking reactions. This assumption is commonly used in kinetic models dealing with coal evolution15 or with other types of organic matter.4,16-19 It must be noticed, however, that such an assumption is arbitrary and not supported by any theoritical consideration. Moreover, a recent natural case study20 on the thermal stability of oils in deep natural reservoirs (190-200 °C) has demonstrated that the kinetic model proposed by Behar et al.9 was predictive for the saturates degradation but not correct for the methylated aromatics and methane. The aim of the present work is to gain a better understanding of the thermal degradation of these aromatic compounds in terms of reaction kinetics and the mechanism of methane generation. This new source of methane potential in natural reservoirs was suggested by McNeil and BeMent,21 but no kinetic parameters were calculated. The research strategy is the following: (1) Selection of an aromatic model compound representative of the methylated aromatics in an oil in order to follow its kinetic degradation in a closed system under a pressure similar to that of conventional natural reservoirs. We have selected 9-methylphenanthrene because it is in the average molecular weight of the aromatics found in natural crude oils and also because it is used together with its 1, 2, and 3 isomers for calculating maturity indicators in source rock extracts and oils.22-24 (2) Determination of the global rate constants for the thermal degradation of 9-methylphenanthrene and (15) Juntgen, H.; Klein, J. Erdoel Kohle, Erdgas, Petrochem. Brennst. Chem. 1975, 287, 65-73. (16) Campbell, J. H.; Gallegos, G.; Cregg, M. Fuel 1980, 59, 727732. (17) Sweeney, J. J.; Burnham, A. K.; Braun, R. L. AAPG Bull. 1987, 71, 967-985. (18) Horsfield, B.; Schenk, H. J.; Mills, N.; Welte, D. H. In Advances in Organic Geochemistry; Mattavelli, L., Novelli, L., Eds.; Pergamon Press: New York, 1991; pp 191-204. (19) Pepper, A. S.; Dodd, T. A. Mar. Pet. Geol. 1995, 12, 321-340. (20) Vandenbroucke, M.; Behar, F.; Rudkiewicz, J. L.; Wendebourg, J.; Gaulier, J. M.; Vear, A.; Duppenbecker, S.; Brigaud, F.; Grauls, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1998. (21) McNeil, R. I.; Bement, W. O. Energy Fuels 1996, 10, 60-67. (22) Radke, M.; Welte, D. H. In Advances in Organic Geochemistry; Bjoroy, M., et al., Eds.; Wiley and Sons: Chichester, 1983; pp 504511. (23) Radke, M.; Welte, D. H.; Willsch, H. In Advances in Organic Geochemistry; Leythauser, D., Rullkotter, J., Eds.; Pergamon Press: New York, 1986; pp 51-63. (24) Budzinski, H. The`se Doctorat, Universite Bordeaux I, 1993.

Behar et al.

identification of the generated products in order to compare our results to those of the literature. Many papers have been recently published on the cracking kinetics of methylated aromatics.25-40 In work done on 1-methylpyrene by Smith and Savage,38 except for the initiation reactions, the other reactions have activation energies lower than 46 kcal/mol and the corresponding frequency factors range from 108 to 1014 s-1. They are all lower than those of Behar and Vandenbroucke41 for the degradation of the n-C25, who proposed global kinetic parameters of 68.2 kcal/mol for the apparent activation energy and 6.1 × 1017 s-1 for the frequency factor. As we suspected, these results confirm that assuming the same global frequency factor for all lumped classes of our kinetic model,9 either saturates or aromatics, is incorrect. (3) Determination of the global kinetics for methane generation from both primary and secondary cracking of 9-methylphenanthrene. Again, on the basis of the work on 1-methylpyrene degradation, Smith and Savage38 have shown that direct production of methane through demethylation of this compound is only one of the possible reactions of the overall kinetic scheme and that many other methylated compounds (isomers, dimers, etc.) may be produced. (4) Quantitation of the methylated aromatics generated during cracking of kerogen and its degradation products in a closed pyrolysis system under various temperature/time conditions, from the early primary cracking to the end of secondary cracking of the NSO compounds, which are a source for aromatic production.9,13 Due to the complexity of the chemical structures in the total aromatic fraction of pyrolysates, a limited number of aromatic families was selected and in each of them only the low molecular weight compounds were identified. The total C14+ aromatics were fractionated by liquid chromatography in order to recover the triaromatic fractions separately. Quantification of individual compounds was performed by gas chromatography coupled to mass spectrometry using a liquid-crystalline stationary-phase column which enables one to separate and quantify the different isomers of the triaromatic compounds.42 (5) Comparison of apparent rate constants and methane yields from (25) Poutsma, M. L.; Dyer, C. W. J. Org. Chem. 1982, 47, 49034914. (26) Billmers, R.; Griffith, L. L.; Stein, S. E. J. Phys. Chem. 1986, 90, 517-523. (27) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 374376. (28) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987, 26, 488494. (29) Smith, C. M.; Savage, P. E. AIChE J. 1993, 39, 1355-1362. (30) Freund, H.; Olmstead, W. N. Int. J. Chem. Kinet. 1989, 21, 561574. (31) Billmers, R.; Brown, R. L.; Stein, S. E. Int. J. Chem. Kinet. 1989, 21, 375-386. (32) Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989, 28, 645-654. (33) Poutsma, M. L. Energy Fuels 1990, 4, 113-131. (34) Stein, S. E.; Brown, R. L. J. Am. Chem. Soc. 1991, 113, 787793. (35) Smith, C. M.; Savage, P. E. Ind. Eng. Chem. Res. 1991, 30, 331339. (36) Smith, C. M.; Savage, P. E. Energy Fuels 1991, 5, 146-155. (37) Smith, C. M.; Savage, P. E. AIChE J. 1991, 37, 1613-1624. (38) Smith, C. M.; Savage, P. E. Energy Fuels 1992, 6, 195-202. (39) Smith, C. M.; Savage, P. E. Chem. Eng. Sci. 1994, 49, 259270. (40) Savage, P. E. Energy Fuels 1995, 9, 590-598. (41) Behar, F.; Vandenbroucke, M. Energy Fuels 1996, 10, 932940.

Methane Generation from Oil Cracking

Energy & Fuels, Vol. 13, No. 2, 1999 473

Table 1. Geochemical Characterization of the Studied Kerogens type

origin

I II III

Green River Fm Paris Basin Mahakam Delta

Tmax HI (°C) (mg/g of C)

VR (%)

Corg (wt %) H/C

438 419 419

nd 0.55 0.57

66.88 55.33 76.14

918 600 194

O/C

1.55 0.081 1.27 0.145 0.84 0.152

cracking of the methylated aromatics and n-C2541 in natural conditions. Experimental Section Samples. Because 9-MPh could not be purchased, it was synthesized according to the analytical procedure proposed by McKinney.43 It was purified by liquid chromatograhy on silica gel and its purity checked by gas chromatography. Three kerogens representative of the main organic matter types at the end of the diagenesis stage1 were selected (Table 1): a Type I shale from the Green River Formation (Eocene), a Type II Toarcian shale from the Paris Basin (Toarcian), and a Type III coal from the Mahakam Delta (Miocene). For both shales and coal, kerogens were prepared by HF/ HCl digestion according to the standard procedure described by Durand and Nicaise,44 then extracted with dichloromethane. The analytical procedure has already been published for kerogen maturation;9 only that for 9-MPh is described. 9-MPh Experiments. Confined System Pyrolysis. Pyrolyses of 9-MPh were carried out in gold tubes (40 mm length, 5 mm i.d., and 0.5 mm thick) sealed by welding under an argon atmosphere and containing between 30 and 200 mg of initial sample.45 The gold tubes were placed in pressurized autoclaves in a furnace preheated at the chosen isothermal temperature and kept at a pressure of 14 MPa during the entire course of the experiment. At the end of the desired reaction time, the cells were taken out and cooled, the argon pressure was then vented, and the gold tube removed from the autoclave and weighed. The temperature was recorded during each experiment in order to have an accurate value when calculating the apparent rate constants. Gas Analysis. The gold tube was placed in a vacuum line at 10-5 MPa connected to a cold trap filled first with liquid nitrogen.45 After isolating the extraction line from the vacuum pump, the tube was pierced with a needle, allowing the permanent gases (H2, C1, and Ar) to be volatilized into the line and the condensable compounds to be trapped by liquid nitrogen. Permanent gases were concentrated by a Toepler pump into a calibrated volume in order to quantify their total yield and recover them for molecular analysis as described below. Then, the liquid nitrogen trap was heated to -100 °C, allowing condensable gases (C2-C4 alkanes) to be recovered and quantified by the same procedure as that used for permanent gases. Molecular characterization and quantification of the total gas fraction was performed by gas chromatography. Analysis of the C7+ Fraction. After gas analysis, the pierced gold tube was open and transferred into pentane. After extraction for 1 h by stirring under reflux and then filtration, the internal standard (squalane) was added and the solution was accurately divided by weighing into two fractions. The first one was injected as such, i.e., without solvent evaporation into an on-column gas chromatograph for identification and quantification of individual compounds. Both the internal standard and an external calibration of the FID with 9-MPh were used (42) Budzinski, H.; Radke, M.; Garrigues, P.; Wise, S. A.; Bellocq, J.; Willsch, H. J. Chromatogr. 1992, 627, 227-239. (43) McKinney, D. D. E. Ph.D. Thesis, Penn State University, 1998. (44) Durand, B.; Nicaise, N. In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 35-53. (45) Behar, F.; Saint-Paul, C.; Leblond, C. Rev. Inst. Fr. Pe´ t. 1989, 44, 387-397.

for quantification, allowing the determination of the respective response factors and then calculation of the real amount of 9-MPh in each experiment. This analysis showed that no compounds were formed in our experiments in the boiling point range C7-phenanthrene. The GC conditions were the following: on-column injector, apolar column (50 m, 0.2 mm i.d.), initial temperature of 20 °C, final temperature of 320 °C for 20 min, and heating rate from 20 to 150 °C at 5 °C/min, from 150 to 210 °C at 1 °C/min, and from 210 to 320 °C at 5 °C/min. The second part of the solution was evaporated, weighed, and fractionated by microcolumn liquid chromatography (MLC) in order to remove the internal standard and to recover the total aromatic fraction for mass balance. The amount of insoluble residue, if any, was determined by difference as follows:

residue ) 100% - % gas - % C7+ extract The relative experimental error is a 5 wt % maximum for the step of quantification of individual aromatic including the HPLC fractionation. It is a 10 wt % maximum if the n-pentane extraction and MLC fractionation are added. For the CH4 yield, the relative error does not exceed 5 wt % for a production of 1%. Aromatic Fractionation and Quantification. The analytical procedure was the same for the aromatics generated from kerogen pyrolysis and 9-MPh pyrolysates. HPLC Fractionation. The aromatic fraction obtained by microcolumn liquid chromatography (see above) was fractionated by high-pressure liquid chromatography (HPLC) on an aminosilane stationary phase as described elsewhere42,46 according to the number of aromatic rings. The triaromatic fraction was then analyzed by gas chromatography coupled to mass spectrometry (GC/MS). GC/MS Conditions. The gas chromatograph was a HewlettPackard 5890 series II equipped with a splitless injector (purge delay, 1 min; purge flow rate, 60 mL/min) and an electronic pressure controller (EPC). The column used was a liquidcrystalline stationary-phase capillary column, SB-Smectic (Dionex, Lee Scientific division) 50 m × 0.22 mm i.d. × 0.1 µm film thickness. The column was kept at 50 °C for 2 min, programmed to 140 °C at a 10 °C/min rate, kept at 140 °C for 2 min, then programmed to 250 °C at a 2 °C/min rate, and finally kept at 250 °C for 30 min. The injector and transfer line were kept at 250 °C. Helium was employed as the carrier gas at a 1.2 mL/min constant flow. The detector used was a Hewlett-Packard 5972 mass selective detector (MSD) (electron impact ionization (EI), 70 eV; voltage, 2000 V). It was operated under selected ion monitoring (SIM) mode using the molecular ions of the studied aromatic compounds at 2 scans/s. The identifications presented in Figure 1 were performed by injection of reference compounds according to Budzinski et al.48 The quantifications of the studied compounds were performed using perdeuterated phenanthrene added prior to the HPLC step.

Results and Discussion 9-Methylphenanthrene Pyrolysis. Mass Balances. Due to the low available amount of 9-MPh (650 mg) for all the experiments, the initial load in each gold tube could not exceed 25 mg. Pyrolysis conditions are given in Table 2. For experiments carried out at 375 °C, the gold tubes were very flat after pyrolysis and thus very difficult to open. Consequently, the C7+ fraction could not be (46) Garrigues, P.; Ewald, M. Org. Geochem. 1983, 5, 53-56.


Behar et al.

Figure 1. Examples of isomer separation on a Smectite column for the methyl-, dimethyl-, and trimethylphenanthrenes. Table 2. Pyrolysis Conditions for 9-MPh Thermal Cracking at 120 bara T (°C)

t (h)

375* 400 425 452

6-9-48-216 6-15-24-48-70-216 2-4-9-24-48-72 1-2-3-6-9

a An asterisk (*) indicates experiments for methane production only.

quantitatively recovered and these experiments were used only for quantifying methane production. The conversion of 9-MPh is defined as follows: conversion ) 1 - (residual reactant/initial reactant) and is expressed in weight percent. We have quantified by GC methane and the GC amenable peaks of the C7+ fraction (Figure 2), i.e., the phenanthrene, the remaining 9-MPh, the sum of the isomers 1-, 2-, and 3-methylphenanthrene (MPh), and the sum of the dimethylphenanthrene isomers (DMPh). The total amount of the C20+ aromatics is quantified by difference with the weight of the aromatic fraction from liquid chromatography because the heaviest compounds are not eluted through the GC column. The maximum amount of molecular hydrogen recovered in our experiments was always lower than 3 µmol, i.e., lower than 0.006 mg: its contribution is thus of negligible importance in the mass balance. Our analytical procedure for gas enables us to quantify amounts of methane higher than 1 µmol, i.e., 0.016 mg, thus its specific production will be more accurately measured than that of the C7+ products. Mass balances are given in Table 3: experiments are classified as a function of increasing conversion. As

Figure 2. 9-MPh thermal cracking: GC traces of the total C7+ fraction obtained at 452 °C for various heating times.

stated in the Experimental Section, the insoluble residue, i.e., char, was estimated by difference with the weight of initial reactant. However, when the gold tubes were open for C7+ fraction recovery, it was easy to see whether char was present. When no char was observed, the mass balance was directly normalized to 100 wt %. In that case, the hydrogen content of the C20+ aromatics can be easily calculated from the hydrogen balance; the corresponding values are reported in Table 3 together with the atomic ratio H/C. Results show that the conversion of 9-MPh ranges from 10 to ca. 100 wt % in our experimental conditions. The production of phenanthrene is observed as soon as 9-MPh starts to be degraded. Above 80% conversion, its yields remains constant and slowly decreases above 98%. As CH4 is a stable chemical class, its production increases continuously. However, only 2.8 wt % is generated below 81% conversion, whereas 2.9% is produced between 81% and 99%. This suggests, as already published by Smith and Savage on 1- and



Table 3. Mass Balances Obtained during Thermal Cracking of 9-MPh and Atomic Ratio of Heavy Aromatics and Chara T (°C)

t (h)

9-MPh conv (wt %)

400 425 452 400 425 452 400 425 452 400 400 452 425 452 425 400 425

6 2 1 15 4 2 24 9 3 48 70 6 24 9 48 216 72

9.9 11.7 20.6 21.3 21.4 38.1 42.4 47.5 55.0 59.4 78.5 81.0 84.0 93.8 97.7 98.6 99.4

a

CH4 0.22 0.24 0.42 0.59 0.57 1.22 1.22 1.62 2.04 2.10 2.77 3.83 3.55 4.52 5.10 5.25 5.68

Ph

1+2+3 MPh DMPh C20+ distribution of products (wt %)

1.8 2.2 4.3 6.1 6.1 14.0 17.8 19.5 25.0 23.0 35.5 37.1 38.0 38.8 37.3 36.7 36.1

0.0 0.0 0.0 0.0 0.0 0.6 1.5 1.8 2.9 2.8 9.9 8.5 9.9 9.3 8.6 9.1 6.3

0.8 0.8 1.1 1.8 1.5 3.1 4.8 4.7 4.8 5.1 5.3 3.3 3.5 2.0 1.2 0.8 0.5

char

7.1 8.4 14.8 12.8 13.1 16.9 15.9 13.5 16.2 21.0 18.1 16.4 12.1 15.3 13.7 13.1 12.5

0.0 0.0 0.0 0.0 0.0 2.3 1.2 6.5 4.2 5.5 7.0 11.8 17.0 23.9 31.8 33.6 38.2

C20+ aromatics C (wt %) H (wt %) 94.24 94.17 94.14 94.39 94.33 nd nd nd nd nd nd nd nd nd nd nd nd

5.76 5.83 5.86 5.61 5.67 nd nd nd nd nd nd nd nd nd nd nd nd

H/C 0.73 0.74 0.75 0.71 0.72 nd nd nd nd nd nd nd nd nd nd nd nd

The experiments are classified with increasing 9-MPh conversion.

Table 4. Relative Distribution (wt %) of the 3-, 2-, and 1-Methylphenanthrene Isomers T (°C)

t (h)

9-MPh conv

452 400 425 452 400 400 452 425 452 425 400 425

2 24 9 3 48 70 6 24 9 48 216 72

38.1 42.4 47.5 55.0 59.4 78.5 81.0 84.0 93.8 97.7 98.6 99.4

relative distribution of methylphenanthrenes (wt %) 3 2 1 3/1 3/2 0.27 0.26 0.28 0.29 0.29 0.32 0.34 0.35 0.38 0.43 0.45 0.46

0.20 0.19 0.20 0.22 0.22 0.25 0.27 0.27 0.30 0.36 0.40 0.42

0.53 0.55 0.52 0.49 0.49 0.43 0.39 0.38 0.32 0.21 0.15 0.12

0.51 0.47 0.54 0.59 0.59 0.74 0.87 0.92 1.19 2.05 3.00 3.83

1.35 1.37 1.40 1.32 1.32 1.28 1.26 1.30 1.26 1.19 1.13 1.10

9-methylanthracene and 1-methylpyrene, that methane is produced from both primary and secondary reactions. Table 4 reports the relative abundances of the methylphenanthrene isomers and the ratios of isomers 3/1 and 3/2. The generation of 1-, 2-, and 3-methylphenanthrene isomers appear for conversions higher than 21 wt %, and as expected,47-48 4-MPh, which is very unstable, is not produced. The production of MPh isomers does not occur at the onset of 9-MPh cracking since the isomers are present above 400 °C/15 h, 425 °C/4 h, or 452 °C/1 h. This is in good agreement with the kinetic parameters proposed by Smith and Savage;38 apparent rate constants are slower for methane generation compared to that of the phenanthrene. Once produced, the amount of the MPh mixture increases and reaches a maximum yield of 10 wt %. Then the mixture starts to undergo secondary cracking when 9-MPh is totally converted, but 6.3 wt % of these isomers are still present in the most severe conditions. The 1-MPh is the major compound produced at low severity. Then, the two isomers 3- and 2-MPh increase and predominate for high 9-MPh conversion. As indicated by the ratios between the isomer abundances, the 2-MPh is the most stable of all isomers. The relative high thermal stability of this isomer was confirmed by (47) Garrigues, P.; Ewald, M. Anal. Chem. 1983, 55, 2155-2159. (48) Budzinski, H.; Garrigues, P.; Radke, M.; Connan, J.; Oudin, J. L. Org. Geochem. 1993, 20, 917-926.

Table 5. Comparison of Thermal Stability of 2-MPh and 9-MPh Isomers conversion (wt %) T (°C)

t (h)

2-MPh

9-MPh

425 425 425 450 425

6 9 24 9 48

3.6 7.2 21.5 25.3 43.9

36.0 47.5 84.0 93.8 97.7

performing a subset of pyrolysis experiments on this model compound, as indicated in Table 5. Consequently, the order of decreasing thermal stability for the methylphenanthrene isomers is 2-MPh > 3-MPh > 1-MPh > 9-MPh. These results confirm the observations already done on natural rock extracts49-52 and oils.47,52-53 They are in good agreement with the Dewar reactivity numbers54,55 of these compounds, respectively, 2.18, 2.04, 1.86, and 1.79, and with the molecular mechanics calculations proposed by Budzinski et al.48 The DMPh mixture does not exceed 5% and undergoes secondary cracking above 80% conversion. The absolute amounts of the different isomers are given in Table 6. For 9-MPh conversion below 42.4 wt %, the 3-10, 1-9, 2-9 isomers are predominant. As they are not the most stable compounds,48 they undergo secondary cracking above 59.4 wt % conversion. At higher severities, the 2-3, 2-6, 2-7, 3-6 isomers, which are the most stable structures, start to be cracked. The C20+ aromatics are produced as soon as 9-MPh starts to crack. At the very early stages of their production, their average hydrogen content calculated from the hydrogen atomic balance (5.75 wt %) is very (49) Radke, M.; Willsch, H.; Leythauser, D. Geochim. Cosmochim. Acta 1982, 46, 1831-1848. (50) Radke, M.; Willsch, H.; Welte, D. H. Geochim. Cosmochim. Acta 1982, 46, 1-10. (51) Garrigues, P.; Connan, J.; Parlanti, E.; Bellocq, J.; Ewald, M. Geochim. Cosmochim. Acta 1988, 52, 375-384. (52) Cassani, F.; Gallango, O.; Talukdar, S.; Vallejos, C.; Ehrmann, U. In Advances in Organic Geochemistry; Mattavelli, L., Novelli, L., Eds.; Pergamon Press: New York, 1988; pp 73-80. (53) Radke, M.; Garrigues, P.; Willsch, H. Org. Geochem. 1990, 15, 17-34. (54) Dewar, M. J. S. J. Am. Chem. Soc. 1952, 74, 3357-3363. (55) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 48994907.


Behar et al.

Table 6. Absolute Amounts (wt %) of Dimethylphenanthrene Isomers T (°C)

t (h)

9-MPh conv (wt %)

9-Et

425 400 452 400 425 452 400 400 452 425 425

2 15 2 24 9 3 48 70 6 24 48

11.7 21.3 38.1 42.4 47.5 55.0 59.4 78.5 81.0 84.0 97.7

2.99 3.75 3.19 3.76 1.56 1.70 1.65 0.00 0.46 0.20 0.31

3-6

3-10

3-9

1-3

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.41 1.06 0.25

0.54 1.44 3.76 6.01 6.76 7.52 7.68 8.82 5.49 5.88 2.34

0.94 2.25 4.54 7.31 8.01 8.86 8.78 7.83 4.93 4.78 1.58

0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.31 1.63 0.99 0.12

Figure 3. Overall kinetic scheme proposed by Smith and Savage38 for the 1-methylpyrene thermal cracking.

close to that of the aromatics labeled as β-β compounds (Figure 3) defined by Smith and Savage:38 the molecular formula we can propose is C30H22 with a hydrogen content at 5.76 wt %. A maximum production at 21% for these heavy aromatics is obtained for 60% 9-MPh conversion, then as for the phenanthrene, they start to be slowly degraded above 97.7% conversion. Char is not produced at the very early stages of 9-MPh degradation and is considered as a secondary product by Smith and Savage.38 Its yield increases strongly in the most severe conditions with a maximum reached at almost 40%. In conclusion, our mass balances show that only methane and phenanthrene have a molecular weight lower than that of the reactant; all other pyrolysis products generated from 9-MPh are heavier. This confirms the results published by Smith and Savage.38 Without going into detail, the main reactions of the overall kinetic scheme (Figure 3) proposed by Smith and

absolute amounts (wt %) 2-Et 2-6 1-9 2-10 1-6 2-3 1.01 4.94 8.74 14.84 15.62 14.98 17.20 13.20 6.59 6.59 0.50

0.65 1.21 3.14 4.67 4.29 4.97 5.06 6.23 3.76 3.70 1.42

0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.03 1.57 1.51 0.53

0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.63 1.49 3.18 1.91

2-9

1-8

1-7

1-2

2-7

1.86 4.41 7.63 11.42 10.77 9.96 10.63 8.19 4.66 4.89 1.97

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.60 0.45 0.26 0.00

0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.26 0.92 1.02 0.37

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.49 0.30 0.14 0.00

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.42 0.34 0.78 0.68

Savage38 applied by analogy to 9-MPh are the following (the reaction numbers in parentheses refer to the reactions numbered in Figure 3): DMPh formation from addition of a methyl radical to methylphenanthrene (R9), phenanthrene formation by addition of a H atom to methylphenanthrene (R8) and subsequent elimination of the methyl group (R11), C20+ compound formation by further condensation of alkylphenanthrenyl radicals (R14), production of CH4 and H2 by hydrogen abstraction (R12 and 13), production of the MPh isomers by elimination reactions (R10). The easy abstraction and recombination of methyl radicals explains the great chemical variety of aromatics in source rock extracts and oils. However, among natural aromatic fractions as well as in pyrolysates of 9-MPh, there is neither anthracene nor anthracenederived compounds generated. As in the other published studies on cracking of various aromatics, our results show that condensation into coke is a very late reaction occurring only at high conversion. It is interesting to note that methane generation results from a combination of primary and secondary cracking reactions: methane is still increasing although some of the reaction products (C20+, DMPh, and Ph in a lower extent) are decreasing. Kinetic Parameters for 9-MPh Degradation. Firstorder plots for 9-MPh degradation at 400, 425, and 452 °C are given in Figure 4. They show a good linearity at the three temperatures, allowing rate constants to be calculated. Then the apparent rate constants were plotted in an Arrhenius diagram (Figure 4). The kinetic parameters obtained with these experiments are 49.0 kcal/mol for the activation energy and 4.5 × 1010 s-1 for the frequency factor. We have compared our results to those of Smith and Savage38 for 1-methylpyrene by reporting the conversions obtained at 400, 425, and 450 °C. Although the authors estimated the uncertainty on the conversion values to 15 mol %, data reported in Table 7 clearly indicate that 9-MPh is more stable than 1-methylpyrene. These results are in good agreement with the Dewar reactivity numbers for these two model compounds (respectively, 1.90 and 1.51). To predict the amounts of products lumped into stability classes in compositional kinetic schemes of integrated basin models, it is necessary to establish the kinetics of average degradation reactions on results from model compounds. The kinetic parameters obtained here for 9-MPh degradation have been compared to those obtained for n-C25 thermal cracking in our previous work.41 The two Arrhenius diagrams are plotted in



Figure 4. First-order plots and Arrhenius diagram for 9-MPh thermal degradation. Table 7. Comparison of the Conversion Obtained in the Same Pyrolysis Conditions for 9-MPh and 1-Methylpyrene (after Smith and Savage38)

Table 8. Comparison of Apparent Rate Constants for 9-MPh and n-C25 Thermal Cracking in Natural and Laboratory Conditions

conv (%) T (°C)

t (h)

9-MPh

1-MPyr

400

4 5 1 2 4 5 1 2 4

8 9 7 14 25 31 22 39 63

13 15 10 20 44 47 35 58 65

425

450

Figure 5. Comparison of the two Arrhenius plots obtained for 9-MPh and n-C25 thermal cracking.

Figure 5, and the corresponding apparent rate constants were calculated both in geological and laboratory temperature conditions (Table 8). Results show that the two Arrhenius diagrams cross each other if extrapolated from laboratory to geological conditions for a temperature at 315 °C. This means that 9-MPh should be more stable at higher temperatures than n-C25 but becomes more unstable below 200 °C. This reverse behavior is amplified by a mixture effect on the rate constants for these two models compounds. In fact, a recent study56 has clearly demonstrated that

T (°C) 140 160 180 200 300 350 400 425

9-MPh k1 (s-1) 10-16

5.87 × 9.15 × 10-15 1.12 × 10-13 1.11 × 10-12 9.55 × 10-9 2.98 × 10-7 5.57 × 10-6 2.06 × 10-5

n-C25 k2 (s-1) 10-19

4.96 × 2.30 × 10-17 7.63 × 10-16 1.88 × 10-14 5.94 × 10-9 7.27 × 10-7 4.36 × 10-5 2.71 × 10-4

k1/k2 1183 397 147 59 1.61 0.41 0.13 0.08

Table 9. Relative Ratio of n-C25/9-MPh in Natural Oils with Increasing Maturity crude oil

location

maturity

n-C25/9-MPh

Boscan Safanyia Elgin

Venezuela Saudi Arabia North Sea

low medium high

1.81 2.53 26.34

n-C25 degradation is delayed when mixed with either a marine oil or a terrestrial one. Another work of McKinney43 has shown that in contrast to n-C25, the degradation rate of 9-MPh seems to be accelerated. Thus, the difference in terms of thermal stability for these two model compounds will be bigger when mixed in a reservoir oil than that observed in neat conditions. These calculated results have been compared to analyses from three oils generated by the same Type II organic matter and differing only by their increasing maturity. Table 9 shows the relative ratios of n-C25 to 9-MPh measured by GC-MS peak areas in m/z fragmentograms, respectively, 71 and 192. The very high ratio observed in the Elgin oil located in the high-pressure/high-temperature zone of the Central Graben is consistent with the sudden burial of 1000 m during the last million years, resulting in an increase from 160 to 190 °C of the reservoir temperature. Our kinetic parameters applied in comparable conditions (180 °C, 1 My) indicate 2% conversion for n-C25 and 97% for 9-MPh, thus supporting the extrapolation of these experimental results to geological conditions. The results on 9-MPh, if they can be applied to all methylated (56) McKinney, D. D. E.; Behar, F.; Hatcher, P. G. Org. Geochem. 1998, 29, 119-136.


Behar et al.

Figure 6. First-order plots and Arrhenius diagram for methane production during 9-MPh thermal degradation. Table 10. Apparent Rate Constants for Methane Production in the Temperature Range 375-452 °C T (°C)

k (s-1)

375 400 425 452

4.24 × 10-7 2.21 × 10-6 8.70 × 10-6 3.92 × 10-5

aromatics generated in source rocks, then accumulated in reservoirs, have important implications in petroleum exploration. They show that in kinetic schemes of integrated basin models, it is not possible to assume, as done currently, the same preexponential factor for all chemical classes of compounds. In this case, the higher stability of aromatics relative to saturates in the laboratory conditions will not be extended to geological conditions. Kinetic Parameters for Methane Generation. The key problem for determining the apparent rate constants for the specific production of methane is calculation of the conversion because it cannot be based on 9-MPh, which has totally disappeared although the methane yield still increases. The only way to overcome this difficulty is to assume that methane production follows a first-order reaction. Then, through a nonlinear least-squares fit of the methane vs time curve, one can obtain the methane yield for an infinite reaction time (Af value), which was found to be 6.4%. The conversion can be calculated based on Af, and the expected linear relationship of the logarithm of conversion vs pyrolysis time can be plotted. Then, the activation energy and preexponential factor for this reaction can be calculated. Figure 6 shows the four first-order plots obtained in our temperature range between 375 and 452 °C for methane production, and the corresponding apparent rate constants are reported in Table 10. The Arrhenius diagram obtained is plotted in Figure 6 and shows very good linearity (R2 ) 0.9996). The resulting kinetic parameters are 54.5 kcal/mol for the activation energy and 1.1 × 1012 s-1 for the frequency factor. From this Arrhenius diagram, the apparent rate constants at geological and laboratory temperatures were calculated and compared to those of n-C25 (Table 11). As for 9-MPh degradation, the two Arrhenius diagrams cross each other between laboratory and natural conditions at a temperature of 250 °C.

Table 11. Comparison of Apparent Rate Constants for Methane Production and n-C25 Thermal Cracking in Both Natural and Laboratory Conditions T (°C)

C1 production k1 (s-1)

n-C25 k2 (s-1)

k1/k2

140 160 180 200 250 300 350 400 425

1.57 × 10-17 3.40 × 10-16 5.55 × 10-15 7.15 × 10-14 1.81 × 10-11 1.74 × 10-9 8.06 × 10-8 2.11 × 10-6 9.06 × 10-6

4.96 × 10-19 2.30 × 10-17 7.63 × 10-16 1.88 × 10-14 1.93 × 10-11 5.94 × 10-9 7.27 × 10-7 4.36 × 10-5 2.71 × 10-4

31 15 7 4 0.94 0.29 0.11 0.05 0.03

Consequently, both the early transformation of the methylated compounds and the late methane production will take place before the occurrence of a significant cracking of the n-alkanes. In a recent study on the Elgin field in a deep prospect of the North Sea, Vandenbroucke et al.20 have shown that when using the kinetic parameters of Behar et al.,9 the C14+ n-alkanes start to crack at 190 °C and 1 million years whereas for the same conditions the methylated aromatics are stable. These results fit with the observed crude oil composition at the present time for saturates but not for both the aromatics, which are overestimated, and the methane, which is underestimated. The kinetic parameters obtained in the present study predict that under the same geological conditions, 9-MPh degradation is completed and the conversion for methane production is already 40 wt %. As stated in the Introduction, the discrepancy should be explained by the assumption of considering a unique frequency factor for all chemical classes of our lumped kinetic scheme. In the latter, the kinetic parameters were optimized from experiments carried out on two distillation cuts and one heavy residue; these three fractions were enriched in saturates. Although the calculated frequency factor was an average value, it was shifted toward the one of the saturates which make a large part of the total oil compounds and, at least for the n-alkanes, are generally higher than those of the aromatics. Moreover, by fixing this frequency factor and in order to be consistent with the experimental data, i.e., the methylated aromatics are more stable than the nalkanes in the laboratory conditions, our previous



Table 12. Mass Balances (mg/g of C) Obtained during Artificial Maturation of Kerogens in Closed System for Various T/t Conditions C14+ fraction T (°C)

t (h)

C1-C5

350 350 350 350 350 350 350 375 400

1 2 3 6 9 15 24 15 24

1 2 nd nd nd 25 32 84 193

350 350 350 350 350 350 375 400

1 2 6 9 15 24 15 24

350 350 350 350 350 375

2 3 9 15 24 24

C6-C14

sat.

arom

NSO

total

Type I 15 10 21 23 32 38 44 51 50 63 68 85 90 125 133 161 231 49

45 82 122 161 178 229 228 211 58

137 245 401 536 601 640 502 238 107

192 350 561 748 842 954 855 610 215

11 15 30 37 41 57 109 178

Type II 10 14 29 37 45 60 83 109

14 16 32 37 40 55 69 10

71 92 108 120 122 127 137 33

354 350 315 293 271 258 199 76

439 458 455 450 433 440 405 119

7 8 11 12 13 28

Type III 13 15 19 21 21 21

5 5 7 7 8 7

5 5 5 6 6 4

19 20 18 19 15 15

29 30 30 32 29 26

optimization led us to propose a very high activation energy (63 kcal/mol) for these aromatics. For writing and calculating the stoichiometric coefficients corresponding to methane production, we have plotted the yields of Ph, ΣMPh, and the C20+ aromatics with increasing yields and extrapolated the mass balance for a total yield of methane at 6.4 wt %, corresponding to the Af value obtained from the first-order plots. Because the DMPh yield is already very low at 425 °C/72 h, their contribution is negligible. The resulting equation is as follows:

9-MPh f 6.4% C1 + 34.6% Ph + 11.2% C20+ arom + 47.8% char Quantitation of Selected Aromatics during Kerogen Artificial Maturation. Artificial maturation of the three kerogens described in the Experimental Section was carried out by closed-system pyrolysis in order to follow the generation of saturates and aromatics and their subsequent degradation. From our previous work13 a subset of experiments was selected for these analyses, and the corresponding mass balances are reported in Table 12. The Type I kerogen generates the highest amount of extractable products, followed by the Type II sample; the Type III coal having the lowest yield. For all samples, the pyrolysis products are dominated by the NSO compounds. The maximum of extractable products occurs at 350 °C/15 h for the Type I, 350 °C/2 h for the Type II, and is not marked for the Type III sample, the hydrocarbon production compensating for the NSO degradation. The absolute amounts of gases, light hydrocarbons, C14+ saturates, and aromatics increase up to 375 °C/24 h. In the most severe conditions (400 °C/24 h), the C14+ hydrocarbons begin to crack. The aromatic fraction contains a very complex mixture of

naphtheno, methylated, and alkylated structures together with sulfur compounds.1 In such a mixture, because it is not possible to determine quantitatively the complete molecular distribution, a limited number of aromatic families was selected and in each of them only the low molecular weight compounds were identified. Table 13 gives the absolute amounts (µg/g of C) of phenanthrene and the methyl- (MPh), dimethyl- (DMPh), and trimethylphenanthrene (TMPh) isomers. When present, the anthracenic structures were also quantified. For the experiment at 375 °C, correct identification of all the DMPh isomers could not be done for the Type I kerogen because the separation was done on another smectite column which does not allow resolution as good as for the other experiments. However, the total amount of that fraction was calculated. As expected, data in Table 13 show that the amounts of phenanthrene increase continuously with increasing severity. Anthracenic structures are also present, mainly in the lower maturity stages. As they were never formed during pyrolysis of 9-MPh, this indicates that either they are incorporated as such in the kerogen structure or they form by isomerization/aromatization during kerogen cracking; in both cases, bondings of these structures are more labile than those of phenanthrenic structures. Other anthracenic compounds were found as methylated compounds only. They could not be analyzed in the dimethyl and trimethyl mixtures because the standards for calibrating their retention times are not available. Kinetic parameters of the neat 9-MPh indicate that its conversion will be around 3% at 350 °C/24 h, 11% at 375 °C/24 h, and 38% at 400 °C/24 h. The predicted decrease of 9-MPh is observed at 400 °C/24 h for type II kerogen but with some delay because the intensive secondary cracking of the NSOs generates methylated aromatics and thus compensates for the amount of those already degraded. The total amount of analyzed compounds represents, even at 400 °C, 1.4-1.7 mg/g of C, which is very low compared to the total C14+ aromatics reported in Table 12 (respectively, 58 and 33 mg/g of C). In terms of relative distribution, the MPh and DMPh isomers predominate over the phenanthrene and TMPh isomers. The low amount quantified is due to the fact that only low molecular weight molecules can be separated without coeluting compounds. However, when the comparison is done with the average yield of individual n-alkanes, these amounts are on the same order of magnitude.57 A more exact estimation of the total proportion of methylated aromatics in the C14+ aromatic fraction could be performed based on quantification of aromatic families by low-voltage mass spectrometry. The present results show that the methylated aromatic compounds may be considered as a significant chemical class in source rock extracts and especially in oils which are generally enriched in saturates and aromatics. Due to their very different kinetic parameters, they are expected to have a large contribution in the variations of the ratio of saturates over aromatics with maturity. (57) Tang, Y.; Behar, F. Energy Fuels 1996, 9, 507-512.


Behar et al.

Table 13. Quantitation of Triaromatic Compounds Produced during Kerogen Pyrolysis at Various T/t Conditions (µg/g of C)a type I compounds phenanthrene anthracene ratio of Ph/An MPI1 MPI2 MPR1 MPR2 MPR3 MPR9 3-MP 9-MP 1-MA 1-MP 2-MP 2-MA MPh MAn

type II

type III

350 °C, 350 °C, 350 °C, 375 °C, 400 °C, 350 °C, 350 °C, 350 °C, 375 °C, 400 °C, 325 °C, 350 °C, 350 °C, 350 °C, 375 °C, 2h 24 h 48 h 15 h 15 h 2h 24 h 48 h 24 h 24 h 3h 6h 15 h 24 h 15 h 2 2 1.2 0.91 1.25 0.58 0.92 0.42 0.62 0.99 1.46 1.95 1.36 2.16 2.33 6 4

14 6 2.1 0.80 1.02 0.49 0.74 0.42 0.69 5.85 9.50 6.26 6.80 10.22 8.41 32 15

23 8 3.1 0.90 1.13 0.62 0.90 0.54 0.77 12.54 17.78 10.42 14.29 20.81 14.10 65 25

107 45 2.4 0.83 1.02 0.31 0.58 0.36 0.39 38.28 41.93 18.23 32.81 61.98 40.11 175 58

167 60 2.8 1.43 1.88 0.53 1.16 0.60 0.32 100.26 53.80 16.14 88.03 193.18 92.92 435 109

14 11 1.3 0.94 1.19 0.73 0.97 0.56 0.72 8.11 10.49 8.72 10.59 14.12 14.41 43 23

44 15 2.9 0.95 1.13 0.78 0.99 0.67 0.85 29.41 37.52 19.88 34.20 43.73 31.13 145 51

98 23 4.2 0.93 1.18 0.72 1.01 0.59 0.85 57.62 83.81 30.73 70.54 99.18 59.72 311 90

153 44 3.4 1.11 1.32 0.67 1.01 0.70 0.64 106.63 98.53 26.27 102.03 155.24 88.46 462 115

226 54 4.1 1.30 1.44 0.52 0.91 0.73 0.37 163.99 82.58 32.86 118.17 204.65 82.58 569 115

4 5 0.9 1.35 1.64 0.71 1.21 0.78 0.50 3.27 2.08 2.38 2.98 5.06 10.42 13 13

8 4 2.0 1.31 1.59 0.82 1.37 0.90 0.76 6.95 5.90 4.65 6.40 10.61 11.03 30 16

11 3 3.3 1.24 1.43 0.71 1.14 0.83 0.67 9.31 7.48 4.07 7.94 12.72 9.06 37 13

27 4 7.7 1.22 1.40 0.59 1.00 0.75 0.57 20.28 15.38 3.63 15.82 27.06 9.09 79 13

3,6-DMP 3,10-DMP 3,9-DMP 1,3-DMP 1,9-DMP 2,10-DMP 1,6-DMP + 2-EtP 2,6- + 2,3-DMP 2,9-DMP 1,8-DMP 1,7-DMP 1,2-DMP 2,7-DMP SDMP

0.00 0.44 0.36 0.30 0.26 0.28 0.56 0.71 0.54 0.39 1.79 0.36 0.44 6

0.00 2.41 2.00 1.70 1.55 1.44 3.13 3.49 2.28 2.64 10.24 2.32 2.41 36

0.00 4.92 4.57 3.65 3.15 3.52 7.09 9.08 4.59 5.75 24.21 5.61 6.28 82

3.40 8.32 10.40 0.00 0.00 0.00 0.00 0.00 0.00 8.32 34.03 8.89 13.04 169

16.23 20.03 19.53 25.56 10.90 19.53 50.46 84.19 23.08 20.79 94.26 24.47 50.46 459

0.00 4.00 2.60 2.04 1.13 2.10 3.60 3.87 3.70 3.40 11.28 4.14 2.20 44

0.00 14.67 10.74 10.86 8.28 14.97 9.45 19.02 12.64 10.37 38.48 10.92 9.63 170

0.00 28.25 19.92 23.18 17.02 33.32 22.82 50.70 28.25 21.37 89.81 23.18 24.99 383

18.17 40.64 34.29 33.15 20.66 33.83 52.23 80.38 41.55 27.48 106.49 27.70 41.78 558

31.49 32.86 31.49 37.96 10.20 29.62 58.04 104.09 33.59 19.59 85.15 14.73 58.52 547

0.90 1.60 1.27 1.39 1.11 1.27 2.56 4.60 1.85 1.02 3.18 1.10 1.41 23

0.00 3.00 2.81 2.76 1.99 2.51 4.68 8.69 3.48 1.75 6.02 1.43 3.27 42

0.00 3.13 2.60 2.79 1.76 2.54 4.47 8.63 3.35 1.40 4.76 1.11 2.79 39

0.00 6.26 5.55 6.08 4.01 5.78 8.85 17.55 6.77 2.39 8.02 1.68 5.97 79

12 13.02 13.33 11.84 16.39 9.64 15.53 17.80 45.56 17.17 5.88 20.54 4.63 16.86 208

1,3,10-TMP 1,3,6-TMP 1,3,9-TMP 2,3,6- + 1,6,9-TMP 2,3,10-TMP 2,6,9-TMP 2,6,10-TMP 1,7,9- + 1,3,8-TMP 1,3,7-TMP 2,7,9-TMP 1,6,7-TMP 2,3,7-TMP 1,2,6-TMP 1,2,8-TMP 1,2,7-TMP STMP

0.22 0.11 0.16 0.38 0.39 0.41 0.30 0.51 0.30 0.22 0.33 0.31 0.45 3.54 0.52 8

0.55 0.35 0.75 1.39 1.35 1.25 0.77 1.87 0.97 0.80 2.47 0.72 1.08 13.33 1.85 30

1.22 1.04 2.02 3.68 2.41 2.73 2.05 4.92 2.93 2.73 1.92 1.54 1.45 23.87 4.35 59

2.65 1.46 1.80 1.29 2.63 3.06 1.76 1.95 2.31 6.31 2.95 3.84 1.29 14.75 3.40 51

7.10 8.11 3.93 16.99 5.58 10.65 5.83 16.99 18.00 8.37 12.93 15.98 7.86 19.53 14.71 173

2.10 0.70 1.17 2.47 2.14 3.37 1.63 2.97 1.43 1.67 3.30 2.60 2.50 45.08 4.00 77

5.89 2.88 3.13 7.55 7.00 8.84 3.68 10.62 5.58 7.36 7.18 6.57 3.31 43.26 9.51 132

11.59 5.79 6.52 17.38 13.40 18.83 9.05 25.35 10.86 17.38 18.83 12.68 9.78 77.50 22.82 278

14.76 11.58 8.86 18.17 10.90 25.43 12.49 31.79 20.66 16.58 19.98 19.75 5.45 43.82 15.67 276

10.20 11.90 10.20 25.98 6.23 15.06 11.09 20.80 19.18 13.19 11.90 19.83 4.94 7.28 7.93 196

0.90 0.83 0.34 1.94 0.59 1.70 1.02 2.25 1.54 1.14 1.60 1.20 0.64 1.41 0.70 18

2.04 1.83 1.63 4.37 1.76 2.72 2.47 3.93 2.54 2.24 1.36 1.59 1.37 1.86 2.74 34

1.73 1.53 1.33 3.35 0.69 2.02 1.82 3.08 1.81 1.60 0.93 2.63 0.68 0.79 0.92 25

3.11 2.75 2.48 5.86 1.98 3.19 3.05 4.84 3.14 3.03 3.19 3.20 0.81 0.82 1.55 43

6.19 6.90 3.37 10.51 3.84 8.08 7.14 10.98 8.70 7.53 5.88 7.68 2.67 1.65 1.65 93

29

132

262

605

1403

213

557

1183

1609

1708

76

134

129

244

554

total

56 5 10.7 1.31 1.43 0.61 1.06 0.88 0.62 49.68 34.63 0.83 34.48 59.73 11.64 179

MPI1 ) 1.5(2-MPh + 3-MPh)/(P + 1-MPh + 9-MPh). MPI2 ) 3(2-MPh)/(P + 1-MPh + 9-MPh). MPR1 ) 1-MPh/Ph. MPR2 ) 2-MPh/ Ph. MPR3 ) 3-MPh/Ph. MPR9 ) 9-MPh/Ph. a

In the kerogen pyrolyses, anthracene (An) is produced under laboratory conditions. As it was not observed in similar experiments on the neat 9-MPh, this suggests that the anthracene is not a pyrolysis artifact. Although detected in very small amounts, this compound and its methylated homologues were found in natural samples.58-59 It is interesting to note that the ratio 9-MPh/ Ph and 9-MPh/(Ph+An) do not decrease with maturity: for Type I and Type II kerogen pyrolysis, these ratios reach their maximum at 375 °C, whereas for the Type III coal, they do not seem to decrease. The same is true for the ratio of two isomers 9-MPh/2-MPh, except that the maximum value is reached earlier for the Type I and II experiments. This means that there are different sources for each individual compound during pyrolysis. The first one is the kerogen itself, followed by the NSOs (58) Carruthers, W. J. Chem. Soc. 1956, 603-607. (59) Soclo, H. 1986. Universite´ de Bordeaux I, n°50, 158 pp.

between 2 and 24 h at 350 °C, and finally the C14+ aromatics at higher temperature. We have calculated some of the maturity parameters proposed by Radke et al.23 using the yield of phenanthrene and that of the methylphenanthrenes, i.e., the MPI1, MPI2, MPR1, MPR2, MPR3, and MPR9 (Table 13). On the basis of the work of Behar et al.13 for the three kerogens under study, the experiment at 350 °C/ 48 h correspond to the end of the primary cracking together with a substantial contribution of secondary cracking of the NSO compounds. Results in Table 13 show that the selected maturity parameters do not change significantly along severity. This confirms Radke’s23 conclusions and those of other studies:24,52 these parameters can be used only for very severe conditions, i.e., at vitrinite reflectance higher than 1.5% under geological conditions and above 375 °C/15 h in laboratory conditions.


Conclusion 9-Methylphenanthrene was selected as the methylated aromatic compound representative of that of source rock extracts and oils. It was pyrolyzed in a closed system at various temperature/time conditions in order to determine the order of reaction for its decomposition. From 400 to 450 °C, the degradation of 9-MPh follows a first-order reaction. The corresponding kinetic parameters are 49.0 kcal/mol for E and 4.5 × 1010 s-1 for A. The main chemical classes produced with increasing reaction severity are phenanthrene, the dimethylphenanthrenes, and a mixture of heavy aromatics, then methylphenanthrene isomers and an insoluble residue appears only for a conversion above 21 wt %. 9-MPh decomposition leads to the generation of a great variety of other methylated compounds. This production may explain the high complexity of aromatic compound mixtures observed in natural conditions for source rock extracts and oils and in laboratory conditions for kerogen pyrolysates. For the same pyrolysis conditions, the conversion was found to be lower than that of 1-methylpyrene published by Smith and Savage:38 this is in good agreement with the Dewar reactivity number,54,55 which is generally used for predicting the thermal stability of a given aromatic structure. Methane is generated from both primary and secondary reactions. Its production may be represented by a first-order kinetic law, and the global stoichiometric equation is

9-MPh f 6.4% C1 + 34.6% Ph + 11.2% C20+ arom + 47.8% char The apparent activation energy is 54.5 kcal/mol, and the frequency factor is 1.1 × 1012 s-1. For the two reactions, 9-MPh decomposition and methane production, the frequency factors were found to be very low, ranging from 1010 to 1012 s-1. They are very different from those found for n-alkane thermal cracking (1017 s-1). This result clearly shows that it is not possible to take a single-frequency factor when optimizing the kinetic parameters for the overall scheme of oil cracking. We propose to use at least two frequency factors: one for the aromatics and one for the saturates. In the long term, it should be better to use one frequency factor measured on representative model compounds for each defined chemical class. However, so far the assumption of considering first-order reactions for primary and secondary cracking still seems to be valid. In fact, with the condition of taking correct preexponential factors based on model compounds, the kinetic schemes can be extrapolated to geological conditions and their results fit with the observed compositions of petroleum fluids.


Results obtained in our previous work on n-C25 thermal cracking enable us to compare the relative stability of the methylated aromatics and n-alkanes in both laboratory and natural conditions. In laboratory conditions, 9-MPh is more stable than n-C25 but the two Arrhenius diagrams cross each other at 315 °C, and the reverse behavior is expected in geological conditions. This behavior is amplified significantly when these two model compounds are mixed either with a marine or a terrestrial oil.56 This means these methylated aromatics will be even more unstable in geological conditions than what was predicted from the neat experiments. When both n-C25 degradation and methane production from 9-MPh are compared, a crosspoint of the two Arrhenius diagrams is also found but at 250 °C instead of 315 °C. This is the first time that such a behavior is shown when using the extrapolation of kinetic parameters to geological conditions. The implications for petroleum exploration are obvious. The lower stability of methylated aromatics at T > 170 °C and 20 million years explains why the oils found in deep reservoirs are more and more depleted in aromatics and consequently enriched in n-alkanes. In terms of absolute methane production, the Af value for 9-MPh was found at 6.4 wt %: it is 3 times more than that of n-C25.41 The methylated aromatics may be considered as a new source for natural gas. In fact, the methylated compounds are primary products during kerogen cracking, whatever the initial organic matter type. These compounds are mixed to either naphtheno aromatics or alkylated aromatics. Although these two chemical classes may be predominant, their thermal cracking will lead to the production of methylated aromatics, as suggested by McNeil and BeMent.21 Consequently, the relative percentage of the methylated aromatics in the reservoir oils will increase with maturity before being cracked. Thus, it is important to quantify the relative proportion of three main aromatic fractions, i.e., naphtheno/ alkylated/methylated, in the total C14+ aromatics of both source rocks extracts and oils. A study aimed at adapting mass spectrometric methods to aromatic fractions from oils and rock extracts is currently in progress for this analysis.60 Acknowledgment. We thank D. McKinney for 9-methylphenanthrene preparation, I. Merdrignac for purification of 9-MPh and GC/MS analysis of crude oils, J. Bellocq for HPLC fractionation, and T. Lesage for technical assistance and drawing of figures. EF980164P (60) Vandenbroucke, M.; Behar, F.; Bence, A. E. Abstract submitted for the EAOG meeting: Istanbul, 6-10 September, 1999.

Methane Generation from Oil Cracking - American Chemical Society

Recommend Documents