Experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking

Advancesin OrganicC_,eochemhm'y1991 Org. Geochem. Vol. 19, Nos 1-3,pp. 173-189,1992 Printed in Great Britain.All fights reserved

0146-6380/92$5.00+ 0.00 Copyright © 1992PergamonPress Ltd

Experimental simulation in a confined system and kinetic modelling of kerogen and oil cracking F. BEHAR,1 S. KRESSMANN,2 J. L. RUDKIEWICZ1 and M. VANDENBROUCKEl qnstitut Frangais du P&role, D6partcment de G6ochimie, 1-4 avenue de Bois Pr6au 92506 Rueil-Malmalson Cedex and 2CEDI, BP 3, 69390 Vernaison, France Abstract--The purpose of this study is to experimentallysimulate both kerogen and oil cracking in a closed pyrolysis system and then, to model the kinetic scheme. Kerogens were isolated from their mineral matrix in order to obtain a complete recovery of the insoluble residue. Experiments were conducted under anhydrous conditions in order to calculate atomic balances for carbon, hydrogen, oxygen and sulfur. The pyrolysisproducts were fractionated by molecular weight and relative thermal stability. The kinetic scheme comprises four kinds of reactions: ----depolymerizationreactions for kerogen and heavy products such as resins and asphaltenes. This type of reaction involves the most labile compounds, which are likely to be generated by C-----Oor/and C---S bond cracking. It produces heavy soluble compounds but very small amounts of gases and pure liquid hydrocarbons. ----C--C bond cracking of C6+ saturated chains. These reactions occur at higher apparent activation energies than the previous ones. They generate shorter aliphatic chains but again, very low amounts of methane and ethane. --demethylation reactions of aromatic structures such as C9--C~3compounds, polycondensed C~4+ nuclei and the insoluble residue. The generated products are mainly gaseous compounds and coke. ----C42 bond cracking of C3--C5aliphatic chains which produces mainly methane and ethane, ethane being degraded into methane. Our results show that a unique kinetic scheme can be used for secondary cracking reactions either when oil is pyrolysed alone or when bitumen is first generated during kerogen pyrolysis. The kinetic scheme involves 11 chemical classes fractionated by molecular weight and thermal stability; among them 3 are stable (methane, a mixture comprising benzene/toluene/xylenes/naphthalene and coke), 8 are unstable (ethane, C3~5, C9~13 aromatics, C6--C]3saturates, Ci4+ unstable aromatics, Cj4+ condensed aromatics and precoke). Although the two kerogens selected in this study are degraded with the same apparent activation energy and preexponential factor, stoichiometriccoefficientsmust be specificallydetermined for each equation of the kinetic scheme in which kerogen cracking is included.

Key words--kerogen cracking, oil cracking, dosed pyrolysis system, kinetic modelling

INTRODUCTION Kerogen is defined as the fraction of sedimentary organic matter insoluble in usual organic solvents (Durand, 1980). It is a mixture of maeromolecules whose structures evolve under the influence of time and temperature due to increasing sediment burial. Thermal cracking of kerogen (primary cracking) leads to petroleum generation. Oil is formed first, then further thermal degradation of kerogen and oil (secondary cracking) produces gaseous compounds. The prediction of both the quantity and quality of hydrocarbons generated from a given kerogen buried in a given sedimentary basin is of paramount importance to exploration. Thus, the thermal degradation of organic matter is simulated in the laboratory in order to elaborate kinetic schemes that can be extrapolated to geological conditions, i.e. lower temperature and longer times. This extrapolation can be trusted only when the three following conditions are fulfilled: 1) the chemical structures of reactant and degradation products are known, oG 19/l-3---r~

2) the reactions relating reactants and products are written, 3) products are similar to natural products, suggesting similar mechanisms. In our study the reactants are, respectively, kerogen for primary cracking reactions and oil for secondary cracking processes. Thus products of kerogen cracking are reactants for secondary reactions. Although numerous studies exist on kerogen characterization (Largeau and de Leeuw, 1992; Behar and Vandenbroucke, 1987; Solomon et al., 1988, among others), accurate structural models are not yet available. Some general trends have been established: kerogen is likely to be a mixture of resistant biopolymers which have been more or less preserved during their incorporation in the sediment and of free molecules randomly combined during the first stages of sediment burial. Consequently, only bulk analyses or characterization of small pieces obtained by degradation techniques have allowed an insight into the molecular structure of these complex macromolecules. They can be seen as a mixture of polyaromatic

173

174

F. BEnARet al.

and/or naphthenic rings linked together with saturated chains of various lengths by functional groups containing oxygen, sulfur and nitrogen. The proportions of hydrocarbon moieties and distribution of functional groups depend strongly on both origin and maturity of the organic matter (Tissot and Welte, 1984). In contrast to kerogen, the chemical composition of oil which is the reactant for secondary cracking, can be accurately known. In fact, sophisticated analytical techniques enable: --the complete molecular characterization of oil constituents from C~ to C12, --the quantification of saturates, aromatics and NSO compounds in the heavy C~2+ fraction, --the quantification, among saturated compounds, of the relative proportions of n-, iso- and eycloalkanes. Inside cyclo-alkanes, it is possible to calculate the distribution of ring size (Hood and O'Neal, 1959), --the quantification among aromatics of the proportion of mono, di and triaromatic structures (Radke and Welte, 1983) and among each of them their group type (Fisher and Fischer, 1974) and their complete molecular distribution (Lumpkin and Aczel, 1964). --the quantification of sulfur-containing saturated and aromatic compounds and the identification of a large number of individual compounds (Sinninghe Damste and de Leeuw, 1990). Even if many individual compounds are now identified, it is still non realistic to determine specific kinetic parameters for each of them. Nevertheless, such an analytical approach is very useful, either for the selection of representative individual compounds in order to perform a real kinetic study (Rice, 1933; Rice and Herzfeld, 1934; Fabuss et al., 1964; Dominr, 1989; Kressmann, 1991 among others), or for the application of lumping techniques to construct empirical kinetic models. Although the chemical structure of a reactant such as kerogen is not yet known, several kinetic schemes are available in the literature for primary cracking. They assume either parallel reactions (Burnham and Braun, 1985; Ungerer and Pelet, 1987) or successive reactions (Tissot, 1969; Solomon et al., 1988), but it is impossible to discriminate between them because, as said previously, an accurate distribution of chemical bonds in kerogen is not yet available. Kerogen cracking can be simulated either in an open pyrolysis system swept by an inert carrier gas (Burnham and Braun, 1985; Espitalie et al., 1985a, b, 1986) or in closed pyrolysis systems (Winters et al., 1983; Monthioux, 1986; Tannenbaum et al., 1985; Horsfield et al., 1989, Castelli et al., 1990; Lewan, 1992). An open pyrolysis system is very convenient because it allows continuous detection of effluents as temperature increases. Thus, many samples can be studied in a short period. Effluents are generally

quantified by a flame ionization detector. The FID response versus thermal history is either proportional to the global yield of hydrocarbons or, if selective traps are set up between pyrolysis chamber and detector (Espitalie et al., 1988), to the yield of effluents as a function of increasing carbon number. Non-hydrocarbon compounds such as CO2, H2S, CO and H20 are only partly quantified thus mass balance or atomic balances for carbon, hydrogen, oxygen and sulfur cannot be accurately determined. Consequently, conversion calculations could be questionable if not impossible. Moreover, such experimental devices prevent any simulation of secondary cracking and thus any study combining primary and secondary reactions. In a closed pyrolysis system, primary cracking can be observed for low conversion yields, whereas secondary cracking will occur for higher conversions. However care must be taken regarding both wall effects (Kressmann, 1991) and the overlap of primary and secondary processes. Experimental simulations in closed system are generally performed on natural samples including both kerogen and mineral matrix under hydrous conditions (Winters et al., 1983; Tannenbaum et al., 1985; Tannenbaum and Kaplan, 1986; Castelli et al., 1990; Lewan, 1992; Chiaramonte et al., 1991). As the quantification of H20 released from organic matter is difficult and the complete recovery of the insoluble fraction is often not possible, mass balancing is often not accurate. An early empirical model for petroleum generation in sedimentary source rocks was based only on natural samples from the Paris Basin (Tissot, 1969). Primary cracking of kerogen into heteroatomic compounds was modelled by one reaction; secondary cracking of these compounds was modelled by two reactions, one forming C14+ oil and the other an insoluble residue. A second model was based on both experiments in sealed tubes and on natural extracts in geological series (Tissot and Espitalie, 1975). Here a set of activation energies for parallel reactions was imposed and the fractions of the kerogen corresponding to each primary cracking reaction were adjusted. Then, a similar scheme was followed for the transformation of oil into gas. At the present time, models for primary cracking are derived from Rock-Eval experimental data at several heating rates (Ungerer, 1990). However, as discussed previously, this apparatus cannot detect some of the degradation products such as coke and asphaltenes (Espitalie et al., 1988). Recent studies on secondary oil cracking (Espitalie et al., 1988; Behar et al., 1988; Ungerer et al., 1988) have shown that it could be possible to assign specific kinetic parameters to classes of compounds separated according to their molecular weight. Nevertheless this approach cannot be predictive because compounds in the same range of molecular weight may have different thermal stability; for example the C6-C13 fraction in the kinetic scheme of Behar et al., (1988) comprises

Simulation and modelling of kerogen and oil cracking Table I. Geochemicaldata on c

H

Sample (wt%) Type II 71.87 Type III 73.99 *Pyrite content: 23 wt%.

O

(wt%) 7.60 5.08

N

(wt%) 13.87 16.11

(wt%) 2.29 1.88

saturated compounds which definitively do not have the same thermal behavior as their aromatic counterparts. Consequently, the chemical composition of that total fraction changes with conversion, thereby preventing any predictive calculation of its specific kinetic parameters. In a later model (Behar et al., 1991), earlier model limitations have been partly overcome through a kinetic scheme of oil cracking comprising 10 compound classes fractionated by molecular weight and relative thermal stability. These are: Cl, C2, C3--C5, C6--C13 saturates, C9-C13 alkyl aromatics, Cr-C~0 mixture including, toluene, xylene and naphthalene, C14+ saturates, C14+ alkylated and naphtheno aromatic compounds, Cl4+ poorly alkylated and polycondensed aromatics and coke. This number of chemical classes is sufficient to define a unique kinetic model which satisfactorily accounts for the specific degradation kinetics of the two oils studied under the experimental conditions used. Nevertheless, some improvements are still needed such as fractionation, inside saturates between chains and cyclo-alkanes, or fractionation inside C~4+ aromatics between alkylated and naphtheno-aromatic compounds. Following previous work on compositional models for oil cracking (Behar et aL, 1991), we have used here the same experimental device described earlier and the same analytical procedure for the characterization of pyrolysis products in order to study both primary and secondary reactions on two different kerogens. This allows us to: ---describe pyrolysis products from kerogen cracking with the same compound classes as those previously defined by Behar et al. (1991): this means that a complete recovery of the pyrolysis

AR[

175

the kerogens studied Sorg*

(wt%) 4.46 2.70

H/C 1.27 0.80

O/C 0.14 0.10

HI 536 193

Tmax 416 426

products and the determination of both mass and atomic balances can be done, ----compare kinetic parameters obtained for bitumen degradation when generated from artificial maturation of kerogens with parameters obtained when oil is pyrolysed alone, ~ e f i n e common trends for primary cracking of Type II and Type III kerogen and construct a unique kinetic scheme for both of them, ~ i s c r i m i n a t e (at high kerogen conversions) between cracking products generated by residual kerogen and those generated during secondary cracking of the bitumen. As the aim of the present work is to elaborate a complete kinetic scheme of kerogen degradation including both primary and secondary cracking reactions, we have chosen to perform experiments in a closed pyrolysis system under anhydrous conditions. Kerogens were isolated from their mineral matrix in order to get a complete recovery of the organic insoluble residue (Durand, 1980).

SAMPLES

Two kerogens with the same maturity were selected (Table 1): a Type II kerogen from the Paris Basin and a Type III kerogen from the Mahakam Delta. Both were taken at the beginning of the oil window in order to simulate the complete generation of hydrocarbons. Nevertheless, these samples have already lost a major part of their heteroatom content through components such as CO2, CO, H2S and water during natural diagenesis. Consequently, specific kinetics for non-hydrocarbon compounds were not derived

==~PRESSUREREGULATORSETAT90BAR 3LTHERMOCOUPLE

SEALED fEN HEATEDATA IOSENTEMPERATURE IOM 25°CT0600°C

GOLDTUBEY OILORROCKS, Fig. 1. Pyrolysis device.

F. BEHARet al.

176

Table 2. Experimentalpyrolysisconditions Experimentduration(hr)

from the experiments although these compounds were recovered in all experiments in order to check both mass and atomic balances.

T (°C) 250 260 280 300 350 380 400 445 450 500

EXPERIMENTAL The reactor was a gold tube with 5 mm o.d., 40 mm length and 0.5 mm thickness, sealed at both ends by welding under argon atmosphere. Around 50 mg of sample were placed in each gold cell. The total set of gold cells was put in one single oven kept at a pressure of 12 MPa at a given temperature. The cells were then successfully removed from the oven as they reached their heating time; this reduced considerably the uncertainty on temperature measurement. The temperature, measured with an accuracy estimated to be __.2°C, by a thermocouple penetrating in an empty cell inside the autoclave (Fig. 1), was recorded for each experiment. Cooling the autoclave in a liquid nitrogen bath took less than 2 min and this time was consequently of negligible importance. Pyrolysis

Pressure gauge

II

Type II kerogen 24--,216 24--,216 1--.1296 0--*648 0~216

Type III kerogen 1--*2000

24 0--,216 3---,216

1--,216 1--,216 1~ 648 24 1--,216 3--,216

conditions for Type II and Type III kerogen cracking are summarized on Table 2. Pyrolysis products were first fractionated according to their molecular weight (Behar et al., 1989). The gold tube was placed in a vacuum line at 10 -5 MPa (Fig. 2) and connected to a liquid nitrogen trap. Then, the gold tube was heated up to 60°C and this temperature was held constant until the volatiles were totally vaporized. After isolating the extraction

Pressure gauge

Pressure gauge

Plercer

Sample[I (.)

t

Stopcock Cold trap

Sampling port Cold trap Calibrated I manometer

~

TOEPLER PUMP TURBO MOLECULAR PUMP

Fig. 2. Vacuum line and Toepler pump used for quantification of non-hydrocarbon gases and C~425 compounds, and total recovery of C6-C~3 fraction.

Simulation and modelling of kerogen and oil cracking line from the vacuum pump, the gold tube was pierced with a needle, allowing the permanent gases to be volatilized into the line and condensable compounds to be trapped into liquid nitrogen. Permanent gases (H2, CO, CI and Ar) were concentrated by a Toepler pump into a calibrated volume in order to be quantified and recovered. Then, the liquid nitrogen (T = - 173°C) in the trap was replaced by a mixture of ethanol and liquid nitrogen (T = - 106°C), allowing condensable gases (COs, H2S and C2--C5) to be recovered and quantified by the same procedure. Then, molecular characterization and quantification of permanent gases on the one hand and condensable gases on the other hand, were performed by gas chromatography with specific capillary columns. The C6-C13 fraction was dissolved in I ml of pentane injected through a self-sealing device, then recovered by disconnecting the trap from the vacuum line and quantified in a gas chromatograph (GC) equipped with an on-column injector. Then, the gold cell was cut into small pieces, extracted with chloroform and filtered. The insoluble fraction was recovered, mixed with the gold pieces on the filter and weighed. The filter and gold tube being previously weighed, the amount of insoluble residue was calculated by difference. The soluble Cl4+ fraction was weighed after evaporation of the solvent, then fractionated. Asphaltenes (ASP) were precipitated with n-heptane, filtered and weighed. The soluble fraction was fractionated by microcolumn liquid chromatography into saturates (SAT), unsaturates (UNSAT), aromatics (ARO) and resins (RES). The whole analytical procedure is summarized on Fig. 3. This procedure did not allow recovery of the amount of water present in the gold cell because of its strong adsorption on glass walls of the vacuum line. Therefore TG/FTIR analyses were performed (G. Whelan et al., 1988). The apparatus comprises a thermogravimetric chamber (open system with fast helium sweeping and temperature programming) connected to an i.r. spectroscopic detector. It allowed

C02, H2S C2-C5

I

restdms

[

Fig. 3. Analytical procedure used for fractionation of kerogen pyrolysis products.

177

us to follow water release during kerogen pyrolysis at different heating rates and thus to calculate a corresponding rate constant for water production from kerogen. Consequently, it was possible to estimate the water amount produced in each of our closed pyrolysis experiments. Atomic balances were calculated as follows. Direct elemental analyses were carried out on both the C,4+ extract and the insoluble residue. The molecular characterization of all individual gaseous species allowed us to calculate their complete carbon, hydrogen, oxygen and sulfur balances. The C6--C,3 fraction was first fractionated by liquid chromatography into saturated + unsaturated and aromatic hydrocarbons and then quantified by gas chromatography. Due to the difficulty of identifying each saturated compound by GC-MS, the percentage of hydrogen for C6~[~13 saturates was estimated to be 15.50 wt% corresponding to a linear C9 aliphatic structure, representative of that fraction. In contrast to the saturated fraction, each aromatic compound of the C6-C]3 mixture could be identified unambiguously by combination of GC and GC-MS analyses, allowing accurate calculations of atomic balances. Once all complete atomic balances were performed, an independent check of mass balance was possible: the weighted average of a given atom obtained in all recovered fractions must fit with the corresponding atomic percentage in the initial sample. Moreover, in order to test the reproducibility of the analyses, all pyrolysis experiments for most were duplicated.

RESULTS AND DISCUSSION

Thirty-five experiments were performed on Type II kerogen and twenty-seven on Type III kerogen; almost all were duplicated. In order to enable comparisons of thermal behavior between the two kerogens, mass balances are calculated on total organic matter for all the experiments: pyrite and products released from pyrite degradation during pyrolysis are not taken into account for the Type II kerogen. A subset of mass balances, ordered according to increasing time and temperature is given in Tables 3 and 4 respectively for Type II and Type III kerogens. Some general trends for the cracking of both samples can be noted: --as maturity increases, the amount of residue (the insoluble fraction in CHCi3) decreases down to 46.9 wt% then increases for Type II kerogen while it still decreases for Type III, --the production of both C:--C5 and C6-C13 fractions is at a maximum during the decrease of the C,4+ extract, --methane is produced in very small amount at low cracking severity, but is the main volatile

178

F. BEI-~R

et al.

Table 3. Mass balances for Type II kerogen pyrolysis at various temperature/time combinations (wt%oof initial sample) T Time C642,3 C642,3 (°C) (h)* CO2 H2S H20 C1 C2~[~5 SAT ARO Ct4+ Residue 350 I 39 9 27 4 7 4 2 208 699 350 1 39 9 28 6 7 5 2 199 707 350 3 44 11 32 7 13 13 5 330 544 350 3 44 11 32 6 13 13 5 340 536 350 9 50 20 38 11 24 22 8 368 459 350 9 49 20 38 10 24 22 8 360 469 350 24 54 24 48 13 31 46 21 293 471 350 24 53 24 48 13 32 52 24 290 466 500 48 72 34 46 192 32 0 16 0 608 500 48 71 34 46 193 37 0 17 0 602 500 216 78 34 44 212 2 0 2 0 631 500 216 80 35 44 210 2 0 2 0 628 *Experiment duration does not comprise time elapsed during heat-up stage.

p r o d u c t w h e n C6 + c o m p o u n d s have completely disappeared. It is w o r t h noticing t h a t m a x i m u m p r o d u c t i o n o f the different classes involved in Type II a n d Type III kerogen d e g r a d a t i o n seems to occur for the same t e m p e r a t u r e / t i m e combinations: C t 4 + b i t u m e n (350°C/9 h), C2-C5 (445°C/24 h), C~ (500°C/216h). This suggests t h a t p r o d u c t i o n or d e g r a d a t i o n rates are likely to be in the same range for a given fraction whatever the kerogen type. In order to m a k e the description of cracking m e c h a n i s m s easier, all experiments were sorted with a n a r b i t r a r y severity r a n k (Table 5 a n d 6). As b o t h kerogens start to crack into C~4+ soluble compounds, the p r o d u c t i o n , a n d the decrease of the C~4 + fraction were used as a first cracking severity indicator. W h e n this fraction was n o longer present, the p r o d u c t i o n a n d decrease of C6-C~3 fraction were used. M e t h a n e yield which increased continuously with increasing degree o f reaction allowed us to crosscheck this classification. Each t e m p e r a t u r e / t i m e c o m b i n a t i o n could be easily r a n k e d for Type II kerogen experiments because the C~4+ fraction represented the m a j o r p a r t o f pyrolysis products. As the Type III kerogen C I , + yields were too low to classify the results this way, c o r r e s p o n d i n g experiments were classified according to the equivalent experiments o b t a i n e d with Type II kerogen.

Non-hydrocarbon gaseous compounds A t low severity, n o n - h y d r o c a r b o n species are d o m i n a n t c o m p o u n d s a n d are generated as soon as kerogen cracking starts. In all experiments neither h y d r o g e n n o r c a r b o n m o n o x i d e were detected. As water was only estimated by T G / F T I R and was s h o w n to be less t h a n 5 w t % for the kerogens analysed here, we did n o t a t t e m p t to establish its kinetics of generation. CO2 is the m a j o r c o m p o n e n t o f the total gaseous fraction generated. F o r m e d i u m severity experiments, C O 2 yields reach a m a x i m u m a n d then remain c o n s t a n t for Type II kerogen whereas it is still produced in the most severe conditions for Type III kerogen (Table 6). This suggests t h a t CO2 does n o t come from the same functional groups in these kerogen structures. This can be confirmed by T G / F T I R determinations of C O / C O 2 yields which are in equilibrium in a n open pyrolysis system (Table 7): for Type II kerogen, CO/CO2 is mainly produced below 450°C while for Type III kerogen, CO/CO2 is generated up to 900°C. H2S is p r o d u c e d in small a m o u n t for b o t h kerogens and is negligible for Type III kerogen pyrolysis t h o u g h low yields m a y be related to surface absorption problems. In conclusion, total n o n - h y d r o c a r b o n c o m p o u n d s could represent up to 1 0 w t % of the total products. It is clear t h a t a specific kinetic study is still needed to d o c u m e n t the c o n t r i b u t i o n s of the various

Table 4. Mass balances for Type III kerogen pyrolysis at various temperature/time combinations (wt%o of initial sample) T Time (°C) (h)* CO2 H2S H20 Ci C2--C5 C642,3 CL4+ Residue 350 1 36 7 35 3 4 4 46 865 350 1 36 7 36 3 4 2 51 861 350 9 49 9 43 3 8 5 52 831 350 9 45 9 43 4 7 4 44 844 350 24 51 7 50 6 8 7 44 827 350 24 53 8 49 5 9 5 51 820 445 24 73 8 53 60 18 3 26 760 445 24 71 8 53 60 17 3 24 765 500 24 129 6 51 108 10 0 0 694 500 24 106 5 50 102 10 0 0 726 500 216 142 6 48 124 0 0 0 680 500 216 125 5 48 108 0 0 0 713 *Experiment duration does not comprise time elapsed during heating stage.

Simulation and modelling of kerogen and oil cracking

179

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180

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Simulation and modelling of kerogen and oil cracking

181

Table 7. Production of both CO/CO2 observed by TG/FTIR on the two kerogens: yield expressed as rag/1000 mg of initial sample Kerogen

25-320°C

320-550°C

550-650°C

650-900°C

CO + CO s

Type II Type III

13 12

31 39

9 34

14 51

67 136

oxygenated functional groups present in the kerogen structure to the non-hydrocarbon gases produced.

Hydrocarbon gases Methane is produced in very low amounts during Cl4+ fraction generation in both samples. In fact, when maximum production of the C14+ fraction is reached, only 5 wt% of total methane is produced. Then, this yield remains fairly constant during both C~4+ degradation and C¢-C~3 production. Thus, methane is produced mainly at high severity conditions. For Type II kerogen, a significant production of methane occurs as soon as C : C s hydrocarbons are cracked. When these compounds have completely disappeared, a strong increase of methane yield is observed which can be related to the decrease of hydrogen content in insoluble residue (Table 8). For Type III kerogen, although C2--C5 compounds are produced in small amounts, a strong increase of methane yield is only observed for the most severe conditions. As for Type II kerogen, it can be related to the decrease of H/C atomic ratio in the insoluble residue (Table 6). Around 30 wt% of the C : C 5 fraction is produced during C~4+ generation in both samples. For Type II kerogen, C2-Cs production remains fairly constant when C~4+ degradation starts and then increases continuously during production and degradation of the C6-C~3 fraction. For Type III kerogen, as C¢-C~3 production is very low, C2-Cs yield remains constant for a large range of severity. Complete degradation of the C : C s fraction is observed under the most severe conditions. Molecular distributions are given on Tables 5 and 6. In both samples, olefins are produced at very low kerogen conversion, then these compounds rapidly disappear. During C:-C5 production, chemical composition is constant except for C5 which starts to decrease before maximum production of the C2--C5 fraction: for Type II kerogen, C2, Ca and C4 are

present in equal amount while C2 is dominant in Type III. When C2--C5 production is decreasing, C4 is first degraded followed by C3, leading to a pure ethane fraction under very severe conditions. In conclusion: --methane produced in very low amount during oil generation is generated mainly from the C2-C5 fraction and insoluble residue degradation, --the C2-C5 fraction mainly derives from oil cracking. In this fraction, ethane is the most stable compound.

C¢-C13fraction This fraction is produced in too low a concentration to derive clear conclusions for the Type III kerogen. For Type II kerogens, it starts to be produced at low kerogen conversion and reaches its maximum yield when around 60 wt% of the oil has already disappeared. This suggests that the major part of this fraction originates from oil cracking. In terms of molecular composition, the relative amount of saturated and aromatic hydrocarbons does not change up to the maximum production. During the degradation of total C6-C13 an opposite thermal behavior is noted for saturated and aromatic counterparts: saturates are degraded for a medium range of severity leading to a corresponding increase of C2425 yield while aromatics remain stable and finally disappear in most severe conditions. Among aromatic compounds, alkylated compounds are more labile leading to a final fraction comprising only benzene, toluene, xylenes and naphthalene (Table 8). This fraction can be considered as a stable chemical end class for the complete range of experimental conditions investigated here. In conclusion, --the C6-C13 fraction is mainly generated from oil cracking and partly from kerogen cracking. The maximum yield can reach 110 mg/g in

Table 8. Methane yield, proportion of the BTXN class in the total Cr--Cjj aromatics and atomic ratio H/C of the insoluble residue from pyrolysis of both Type II kerogen and C~4+ fraction. Data are sorted by increasing pyrolysis severity T °C

t (h)

350

648

500 500

48 216

Ci4+ (wt%o of initial kerogen)

CI

Atomic ratio H/C residue

BTXN in C6-C13 ARO

97 70 73 51 56 42 47 20 0 0

26 38 35 41 45 76 99 143 192 210

0.63 0.58 0.58 0.55 0.55 0.50 0.44 0.32 0.29 0.28

38 42 39 42 38 61 59 80 90 100

182

F. BEHARet al.

Type II kerogen whereas only 10 mg/g could be produced by Type III kerogen, ~6--C13 saturates are degraded first into C2-C5 hydrocarbons, then aromatics undergo a demethylation reaction at high temperature. As previously observed (Behar et al., 1991) this reaction leads to methane production on the one hand and formation of polyaromatic compounds on the other hand.

Cz4+ fraction For Type III kerogen, the C143i- bitumen yield does not exceed 5 wt%. In contrast, for Type II kerogen, it represents the major fraction among pyrolysis products and reaches a maximum of 37 wt% which means 517 mg/gCorg. This figure added to C,3-fraction yield leads to a total oil yield of 43.5 wt% or 596 mg/gCorg. It is worth noticing that during the continuous increase of oil production, the bitumen has a constant chemical composition where resins and asphaltenes are dominant (90 wt%). Secondary cracking started just before maximum product yield as indicated by a small decrease of asphaltenes which are thus the most thermally labile compounds of the extract. When oil cracking occurs, asphaltenes and resins are degraded first while the absolute yield of saturates increases. These latter are then degraded and a residual fraction containing mainly aromatic structures is still present, except in the most severe conditions. In conclusion, --the C~4+ bitumen is the first fraction generated during kerogen cracking and comprises mainly polar compounds, --the Ct4+ fraction is present over a fairly large domain of time/temperature conditions, -~luring further secondary cracking, asphaltenes are cracked first followed by saturated hydrocarbons and finally by condensed polyaromatic species.

Insoluble fraction For Type III kerogen, continuous variations in fraction composition are observed along the complete cracking severity scale. As bitumen production does not account for more than 50 mg/g, its contribution to coke formation is likely to be low. Consequently, the insoluble fraction is not a mixture of residual kerogen and coke coming from secondary cracking, but is primarily only residual kerogen, for which the specific thermal evolution is easy to follow. When secondary cracking is completely achieved, a decrease of the residue yield is still observed together with a strong increase of methane, suggesting that the residual kerogen is undergoing demethylation reaction confirmed by the sharp increase of its atomic H/C ratio (Table 6).

For Type II kerogen, the residue yield starts to decrease until the point of maximum yield of bitumen and then increases during oil cracking. This increase can be related to asphaltene degradation which produees coke as demonstrated in our previous work (Behar et al., 1991). Thus, as soon as secondary cracking starts, it is no longer possible to discriminate between oil and kerogen cracking. In the most severe conditions, as for Type III kerogen, methane is produced whereas the atomic H/C ratio decreases in the insoluble residue (Table 8). In conclusion, --kerogen cracking can be described by two cracking reactions; one occurring for low conversion and leading to C,4+ compounds with a high proportion of NSO compounds, the other one occurring at very high degrees of degradation and leading to methane and coke production, ---overlap of kerogen and oil cracking is clearly shown for Type II kerogen at the point at which the maximum production of bitumen occurs.

KINETIC MODELLING

The principles of the empirical model have already been described (Ungerer et al., 1988). It comprises labile chemical classes, which undergo first-order degradation and stable chemical classes. Noting Xl Armthe unstable classes and X~ + ~... X, the stable classes, the scheme involving the main degradation products is as follows: •

.

.

Xm km,~%l + ct.~2X2+ ... + ~,..X. where: ~j~= stoichiometric coefficient of the pyrolysis products for the class Xj decomposition; kj=first order Arrbenius rate constant for degradation of class ~ . kj = Aje Rr(t) The determination of the kinetic parameters of the model (activation energies Ej, preexponential factors Aj and stoichiometric coefficients ~j~) is based on a set of reference pyrolysis experiments. It consists in searching the minimum of an error function F (i.e. optimizing), defined as the mean square residual of the model result versus the experimental measurements• F(~,Aj,~tj,) = ~ ~ (XobS.kt-- Xcomp.k~Ej,Aj,0t~))2 k=ll=l

Simulation and modelling of kerogen and oil cracking where Xobs.kt and Xcomp.kt are respectively the observed and computed concentrations of the lth class for the kth experiment and p is the number of reference experiments. Moreover in order to constrain the system, stoichiometric coefficients must satisfy conservation equations for both total mass and atomic balances. This means that elemental analysis of each chemical class selected for the model must be constant in the complete set of experiments. As a consequence, it is not possible to determine stoichiometric coefficients in cracking reactions when a given class includes compounds with various hydrogen contents: for example saturated and aromatic hydrocarbons C6.-~13 fraction must be separated as the composition of the total fraction changes from hydrogen rich compounds (15.5 wt%) for low severity to hydrogen poor compounds (8.6wt%) for high severity (Table 3 and 4). These constraints reduce the effective number of degrees of freedom in the optimization problem and therefore make the determination of kinetic parameters easier. Moreover, in order to reduce the number of free parameters, we have chosen to keep all preexponential factors constant. Several sets of kinetic parameters can be found, that fit the experimental data with equivalent mean errors. Although they are all mathematically valid as a minimum of the previously defined error function, they have to be re-evaluated with respect to the chemical consistency of the kinetic schemes and to fit to the experimental data. The apparent activation energies must reflect the relative thermal stability of the chemical classes. In addition, the distribution of major pyrolysis products obtained in each reaction must be supported by corresponding experiments. Finally the selected set of kinetic parameters must be compared with other available data from the literature.

RESULTS

From our experimental data, the stable fractions are methane, a mixture of benzene/toluene/xylenes/ naphthalene (BTXN), and coke. The complete set of stable and unstable classes is given in Table 9 with the corresponding hydrogen content. Most of these classes were previously described by Behar et al. (1991), except precoke. In fact, from Tables 6 and 8, hydrogen content (i.e. H/C ratio) decreases with maturity for residue, suggesting that the residue undergoes demethylation reactions. Thus, we have

1. 2. 3. 4. 5. 6.

183

Table 9. List of stable and unstable chemicalclasses and their correspondinghydrogencontents selectedfor kinetic modellingof primary and secondarycracking reactions Hydrogen content Chemical classes (wt%) methane 25.00 ethane 18.50 C~-C~ 17.35 C6-C~3saturates 15.50 C6--CI3stable aromatics* 8.57 C9--C13unstablearomatics* 9.01 Ci4+ saturates 14.50 C~4+ unstablearomatics* 9.60 Ci4+ condensedaromatics* 6.90 Type II kerogen 7.60 Type III kerogen 5.08 Type II precoke 5.30 Type III precoke 4.80 Type II coke 2.20 Type III coke 1.90 *C6~CI3 stable aromatics: mixture benzene/toluene/ xylenes/naphthalene, C9-C~3 unstable aromatic: alkylatedC9-C13aromatics,C14+ unstablearomatics: mixturecomprisingalkyl and/or naphthenoaromatics, resins and asphaltenes, C~4+ condensed aromatics: mixture of methylated and poorly hydrogenated aromatics,

assumed that coke production occurs through an intermediate residue which contains enough hydrogen to be demethylated. This intermediate fraction corresponds to the insoluble residue obtained at the maximum production of C14+ bitumen generated from initial kerogen. At this stage, the corresponding hydrogen content (i.e. H/(C + H)) is respectively 5.5 and 4.8 wt% for Type II and Type III kerogens. Consequently, coke is defined as the insoluble residue obtained in the most severe conditions i.e. 500°C and 216 hours. The corresponding hydrogen contents are respectively 2.2 and 1.9 wt% for Type II and type III kerogens. Optimization was done separately on each kerogen. In order to get a correct degradation scheme for C6-C13 saturated and aromatic hydrocarbons, C2-C5 and C2, it was necessary to include experiments previously performed on Boscan and Pematang condensates (Behar et al., 1991) with the data sets coming from kerogen experiments. For the two kerogens, the model fits the experimental data with a mean error of 2.5wt% equally distributed for each individual chemical class, and the discrepancy between experimental and model data lies inside the range of experimental errors as shown in Fig. 4 (Type II sample). The kinetic scheme obtained for both Type II and Type III samples can be summarized as follows:

KEROGEN-~CI4+ ARO.U + Cl4+ SAT. + PRECOKE P R E C O K E ~ C 1+ C2 + COKE C I 4 + A R O . U ~ C : C 5 + Cr-Ct3 SAT. + CI4+SAT. + CI4+ARO.C + PRECOKE CI4+ARO.C-~CI + C: + PRECOKE CI4+SAT.~Ca-C 5 + C6-C13 SAT. + CI4+ARO.C C9-C13 ARO.-+CI + BTXN g- PRECOKE

184

F. BESARet al. 7. C 6 - C 1 3 SAT.--*C3--C 5 + BTXN + Cz4+ARO.C 8. C3-'-C5">C1 + C 2 "1-C6-C13 SAT. + C9-'-C13 A R O 9. C2-'1'C1 "~ C3-'C 5 + C6-C13 SAT.

where CI4+ARO.U = C]4 + unstable aromatics C]4+ARO.C = Ct4 + condensed aromatics BTXN = mixture benzene/toluene/xylenes/naphthalene

The corresponding apparent activation energies, obtained with a similar preexponential factor, are given in Table 10. They are compared to those found in our previous work (Behar et al., 1991) for Boscan oil, which has a chemical composition close to that of Cl

the bitumen generated from our Type II kerogen. It is worth noticing that the apparent activation energies are very similar, whether optimization is done only on oil experiments, or on both oil and kerogen C2

100.

.t/ 75

C3-C5

i z, ////iii I

/, /z/"

/if/I zzl x/// /

////

C9-C13 ARO.

C6-C13 SAT.

'00/ .~

/1~II

z/

7sl

BTXN

//~/// L//

iz~II

z~ z

/ z / / / ¢/

C14+ SAT.

1°° I

.1/"

C14+ ARO.U

/~',

/z~/,/

KEROGEN

/z, /z// /t//it//

C14+ ARO.C

////////

/t/// t///

PRECOKE

/ 25

//j/zZ"/

////

COKE

/z//

/z///" 5O

7S

100

25 00

/d/C// t// 75

100

0

25

50

75

100

DATA CONCENTRATION (weight%) Stable Aro (C 6 - C 13) = Benzene+Toluene+Xylenes+ Naphthalene C 14+Aro U = Alkyl Aromatics+Resins+Asphaltenes CI 4+Aro C = Condensed Aromatics Fig. 4. Computed concentrations from the kinetic model plotted vs those observed for the Type II kerogen together with condensate pyrolysis (Behar et al., 1991) [(---) represent the 5% error range.].

Simulation and modelling of kerogen and oil cracking Table 10. Distribution of apparent activation energies (kcal/mol) obtained in the kinetic scheme for cracking of both Type II and Type III kerogens compared with those obtained on Boscan oil

from which they are generated as shown by the interval of 5 kcal/mol in their respective apparent activation energies of degradation. - - C - C bond cracking in C6+ saturated aikyl chains (equations 5 and 7). This reaction occurs with higher apparent activation energies than the previous ones. It generates shorter aliphatic chains but again, very low amounts of methane and ethane. --demethylation of C~++ ARO.C, C9----C13 ARO. and precoke (equations 2, 4 and 6). This occurs under severe cracking conditions and the range of corresponding activation energies is very narrow, which is consistent with the fact that the same degradation mechanism is probably effective for the three reactions. The generated products are mainly gaseous compounds and coke. - - C - C bond cracking of C3-C5 aliphatic chains (equations 8 and 9) which produces mainly methane and ethane; the latter is partly degraded into methane.

Kerogen Chemical classes

Types II/III

Boscan oil

46.7 60.1 51.4 59.1 54.6 60.0 56.4 61.1 65.1

--

Kerogen Precoke C1++ A R O . U C1++ A R O . C C1++ SAT.

C9--Ci3 ARO. C6-C 13 SAT. C3~ 5 C:

nd

52.4 57.1 54.9 60.1 57.2 62.5 65.9

experiments. This similar behavior suggests that chemical classes as defined in the present kinetic scheme seem to behave and decompose independently as previously demonstrated by Villalba (1988). Thus, the kinetic scheme elaborated in our previous work on oils can be used to predict secondary cracking occurring in experimental simulation of kerogen pyrolysis, provided that the stoichiometric coefficients of kerogen degradation are known. The differences in the activation energies of CI++ARO.U and Ct4+ ARO.C are probably due to the limited experimental range of temperature (350--400°C) used in the oil cracking simulation. These conditions were too severe to accurately follow the degradation of the C~++ fraction which is better constrained in kerogen cracking experiments where the temperature range is 260--400°C. The kinetic scheme presented above comprises four types of reactions:

All of these results confirm previous experimental observations regarding both the description of the successive reactions with kerogen maturity and the chemical distribution of pyrolysis products in each cracking reaction. This means that the relative values of apparent activation energies and stoichiometric coefficients are chemically consistent. However, some improvements are needed, especially for equation 1 in which non-HC species are not taken into account, although their presence is well shown in experiments (Tables 3 and 4). The same observation can be made at least for Type III precoke degradation which generates some CO2 (Table 6). When comparing the kinetic scheme presented here with previous kerogen cracking models elaborated in our Institute (Espitalie et al., 1988; Ungerer et al., 1988 and Behar et al., 1988), one can note that each of the four types of cracking reactions involve chemical classes with various molecular weights. For example, the demethylation reaction also occurs in light aromatics (C6--C~3)as well as in heavy insoluble residues such as precoke. Conversely, different chemical classes as defined by their thermal stability may be in the same range of carbon numbers (Cc-Cl3

---depolymerization reactions (equations 1 and 3) in kerogen and heavy products including asphaltenes. This reaction involves the most labile moieties which are likely to be generated by C--g) or/and C--S bond cracking. It produces soluble NSO compounds but very small amounts of gases and liquid hydrocarbons. This is consistent with the first kinetic scheme proposed by Tissot (1969) and with the recent work of Burnham and Braun (1990) and fits with the models of chemical structure of kerogens (Behar and Vandenbroucke, 1987). These soluble NSO compounds are definitely more stable than the kerogen

Table 11. Distribution of apparent activation energies of chemical classes in each molecular weight range Molecular weight range

Chemical classes

Thermal stability

Kerogen Precoke Coke

E = 46.7 kcal/mol E = 60.1 kcal/mol stable

Saturates Unstable aromatics Condensed aromatics

E = 54.6 kcal/mol E ffi 51.4 kcal/mol E = 59.1 kcal/mol

C6-CI3

Saturates Unstable aromatics Stable aromatics

E = 56.4 kcal/mol E = 60.0 keal/mol stable

C2-C5

C3-C 5 C2

E ffi 61.1 kcal/mol E = 65.1 kcal/mol

Ci

stable

Insoluble residue C~++ bitumen

C~

185

F. BEHARet al.

186

saturates, C6--C13 aromatics and BTXN etc). This means that if reactants are defined only by a carbon number range i.e., by molecular weight classes, the average value of the apparent activation energy of the resulting fraction will be somewhere between the lowest and the highest value of the chemical classes previously defined (Table 11). If these average values are used for extrapolation to geological conditions, the degradation of the most labile fraction (C~4+ ARO.U and C6-C~3 saturates) will be delayed, whereas the degradation of the most stable fraction (Cl4÷ ARO.C and C9--C13 aromatics) will occur earlier. The amplitude of this effect depends strongly on the interval between the lowest and the highest activation energies inside a given carbon number range. In addition, as the hydrogen content of the total fraction in a given carbon range may vary dramatically with maturity, calculations of stoichiometric coefficients by optimization procedure can give data chemically inconsistent with the experiments. Finally, regarding the absolute significance of the values of apparent activation energies and preexponential factors in our kinetic scheme, a systematic bias could occur due to the choice of constant

Table 12. Comparison between reaction rates obtained on chemical classes generated from kerogen pyrolysis with those available in the literature for individual hydrocarbons: TI is representative of average temperature in sedimentary basins, whereas T2 is representative of our experimental range

C6HI2 (2) C6H12 (2) CIoHI8 (2)

Pressure (MPa) 10 14-18 0.1 0.1 0.1 15 0.1 2.8-3.5 0.1

Logk (T1) -13.4 - 12.4 - 14.7 -13.7 - 13.4 -12.8 -14.9 --15.0 -14.6

Logk (T2) -4.2 -3.6 -4.8 -4.1 -3.9 -3.2 -4.3 -4.9 --4.3

C3~C5 (1) C5H12 (2)

10 0.1

-15.7 -15.1

-5.7 -5.0

Compound

C6~13 SAT (1) n-C6 (2) n-C6 (2) n-C10(2) n-Ci3(2) nC13(2)

C2 (1) 10 -17.2 -6.7 C2 (2) 0.1 -18.2 -7.0 (1) Behar et al. (presentwork). (2) Fabusset al. (1964).

preexponential factors. Nevertheless, we have calculated the rate constants for some degradation reactions at two temperatures and compared them to literature data (Fabuss e t al., 1964). Data presented in Table 12 show that our rate constants

SATURATED COMPOUNDS

C6-C13 Sat ca4+Sat

•~ -140

-120

-100

O

C3-C5\,

~,

-80 -60 Time in Ma

~

C

-40

y 2

-20

0

-20

0

O

AROMATIC COMPOUNDS

"4

i

t

C14+ Aro U

-140

-120

-100

-80 -60 Time in Ma

-40

Fig. 5. Composition of hydrocarbons as a function of depth: simulation for Type II and Type III kerogen degradation using the derived kinetic scheme.

Simulation and modelling of kerogen and oil cracking Table 13. Evolution of the chemical composition for natural Type II bitumens (Toarcian Shales---Paris Basin) H/C kerogen 1.28 1.23 1.14 0.98 0.67

SAT 8 10 12 19 53

(wt% bitumen) ARO NSO 14 22 30 37 20

78 68 58 44 27

are in the same range as those of the literature. This is encouraging for extrapolation of our empirical kinetic parameters to lower temperatures corresponding to sedimentary basin conditions. An ultimate goal of a kinetic model of kerogen and oil cracking is to be used as part of a basin model, in order to compute the production of HC in response to varying thermal conditions. An extrapolation to specific geological conditions is shown in Fig. 5. It aims at describing the complete evolution of kerogen and associated products given: a. a uniform subsidence rate: 50 m/million years. b. a constant geothermal gradient through time: 30°C/km. c. the assumption that the source rock behaves like a closed system, i.e. that no expulsion occurs. Under these assumptions, some general features encountered in sedimentary basins can be seen: --kerogen starts to be cracked at 2500 m producing CI4+ bitumen rich in NSO compounds mixed with a poorly hydrogenated insoluble residue. The bitumen content is very low for Type III kerogen when compared to Type II for the same organic content. However, in natural conditions, TOC for Type II source rocks drops down to 7wt% leading to a bitumen yield around 40 mg/g of rock while for Type III coals TOC is around 70 wt%. Therefore, bitumen yields in both cases may have similar values. Production of bitumen occurs between 2500 m and 4700 m, thus defining an oil window ranging over 2200m. NSO compounds represent two thirds of this bitumen fraction and exhibit a higher stability than kerogen. Together with kerogen cracking, production of C~4+ saturates can be observed for the Type II sample. This fraction, due to its higher thermal stability, will increase in the total bitumen with additional burial. These observations are in good

187

agreement with those in natural samples (see Table 13 for Type II samples of the Toarcian Shales). Consequently, both the amount and chemical composition of bitumens of natural samples are good parameters to check kinetic data extrapolated to geological conditions. --secondary cracking starts with degradation of NSO compounds producing a mixture of hydrocarbons containing mainly C6+ compounds. These latter are stable down to 5500m and are likely to represent the first candidates for efficient primary migration. Associated with these saturated hydrocarbons, light aromatic hydrocarbons are stable down to 7000 m. --the gas window zone comprises two main zones; first, wet gas predominates, then dry gas mixed with light stable aromatics such as benzene, toluene, xylenes and naphthalene. The C342s fraction is present from 4000 to 7500m. Methane is generated from demethylation of insoluble residue and C--C bond cracking of C3--C 5 gaseous compounds. In conclusion, results obtained from describing the successive thermal events by modelling of kerogen and oil cracking are in good agreement with data available either on natural samples or artificial samples from previous work (Louis and Tissot, 1967; Tissot, 1969; Bjoroy et al., 1988). They are significantly different from those obtained by deriving kinetic parameters of kerogen cracking from open pyrolysis devices, for which apparent activation energies are usually higher than 50 kcal/mol (Ungerer and Pelet, 1987; Burnham and Braun, 1990). This can be explained by the kinetic scheme proposed here in which two successive reactions describe kerogen cracking: the first one (equation 1), occurring with a 46 kcal/mol activation energy, which generates Ct4 + bitumen and the second one, with 60kcal/mol (equation 2), which generates methane. We have separately computed conversions for open pyrolysis conditions, i.e. from 25 to 600°C and heating rates of 2 and 20°C/min. Results presented in Table 14 clearly show that the initial kerogen is completely degraded with the conditions 500°C, 20°C/min; precoke is partly degraded at 600°C, 20°C/min and completely degraded for 2°C/min, leading to an overlap of the two reactions during heating of the sample. Thus, resulting distribution of apparent energies gives an average value between 46 and

Table 14. Simulation of Rock-Eval pyrolysis for kerogen, Ci4+ unstable aromatics and precoke degradation using kinetic parameters derived from kerogen pyrolysis (this study) Temperature range (°C) 25450 25-500 25~fi00 25~00

Heating rate (°C/rain)

Kerogen

CI4+ ARO.U (conversion wt%)

Precoke

2 20 2 20

100 100 100 100

65 100 100 100

0 5 100 60

F. BEHAR et al.

188

60 kcal/mol, depending on the relative contribution of the reactants involved in equations 1 and 2. Moreover, as said previously, asphaltenes and some resins, when formed, cannot be vaporized and thus are not or are only partly quantified in an open pyrolysis system. As these last ones are major compounds generated in equation 1, they will be cracked together with precoke as temperature rises. Consequently, they contribute to the increase in the average apparent activation energies found for kerogen cracking in an open pyrolysis system.

CONCLUSION We have carried out experimental simulation of cracking in a closed pyrolysis system in order to elaborate the kinetic modelling of both kerogen and oil degradation. The pyrolysis products were fractionated according to the analytical procedure used in our previous work (Behar et al., 1991). The kinetic scheme comprises 12 chemical classes fractionated by molecular weight and thermal stability; among them three are stable (methane, coke and a mixture comprising benzene/toluene/xylenes/naphthalene), nine are unstable (ethane, C3-C5, C9-Ct3 aromatics, C6--C13 saturates, C~4+ saturates, C~4+ unstable aromatics, Cl4÷ condensed aromatics, kerogen and precoke). To model kerogen cracking with first order reactions, identical values of both apparent activation energies and preexponential factor can be used for the two kerogens selected in this study, but stoichiometric coefficients must be specifically determined for each kerogen. However, although both the Type II and III kerogens studied here seem to decompose with the same set of activation energies, using these values for any other kerogen remains an open question. Therefore, it is necessary to confirm our results on a larger set of reference kerogens. For secondary cracking reactions, our kinetic results show that a unique kinetic scheme can be used either when oil is pyrolysed alone or when bitumen is first generated from kerogen pyrolysis. Moreover, the similarity of rate constants calculated on global chemical classes as described in this study and those of individual compounds found in the literature supports the use of our empirical model as a predictive tool under geological conditions. Nevertheless, some improvements are needed such as a specific study of non-hydrocarbon gas generation and the influence of pressure on cracking rates and reaction selectivity. Acknowledgements--The authors wish to thank Dr Del

Bianco (Eniricerche) for gas analyses, C. Leblond and C. Saint-Paul for performing the experiments and N. Schoellkopf for helpful comments.

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