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Characteristics and kinetics of catalytic degradation of immature kerogen in the presence of mineral and salt
PII:
Org. Geochem. Vol. 29, No. 5±7, pp. 1431±1439, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00154-5 0146-6380/98 $ - see front matter
Characteristics and kinetics of catalytic degradation of immature kerogen in the presence of mineral and salt SHUYUAN LI*, SHAOHUI GUO and XUEFEI TAN School of Chemical Engineering, University of Petroleum, Changping, 102200 Beijing, People's Republic of China AbstractÐIn the present work, a series of simulation experiments were carried out on isolated kerogen and mixtures of kerogen±montmorillonite and kerogen±potassium carbonate to investigate the eects of mineral and salt on the generation of various pyrolysates. A kinetic model of pyrolysate generation was developed which takes into account the eects of the added mineral and salt on kinetic parameters. It is found that montmorillonite and potassium carbonate (K2CO3) act as a catalyst to promote the degradation of kerogen, to increase the yield of gas and light hydrocarbons and to decrease the activation energies for the generation of gas and light hydrocarbons. In the generation of heavy hydrocarbons, however, K2CO3 exhibited feeble catalytic action. In contrast, montmorillonite decreased the yield of heavy hydrocarbons due to the retention on mineral surfaces. The catalytic eects of montmorillonite and K2CO3 become small and weak with increasing temperature. The catalytic action of montmorillonite and K2CO3 on the kerogen degradation can be tentatively accounted for by carbonium ion mechanism and electron transfer mechanism, respectively. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐkerogen degradation, pyrolysis, kinetics, catalytic eect, Shahejie Formation, Dongying Basin
INTRODUCTION
It is now widely accepted that petroleum is generated from the thermal degradation of kerogen in sedimentary rocks. The main factors aecting the amount of petroleum are the nature of kerogen, temperature, time, mineral composition, etc. Among them, the minerals and salt solutions existing in the sediments during kerogen maturation were known to have important eects on petroleum generation. They may catalyze or restrain thermal decomposition of kerogen, thereby altering the yield and composition of petroleum. Therefore, the work on this ®eld has been the subject of much research in recent years. As early as the beginning of the ®fties, Brooks (1948, 1952) reported the catalytic alteration of kerogen during petroleum formation. Soon afterwards, a number of workers have examined the eects of minerals on kerogen degradation and showed important catalytic action (Henderson et al., 1968; Johns and Shimoyama, 1972). Hors®eld and Douglas (1980) performed open-system simulation experiments and concluded that mineral matter decreased the yield of liquid hydrocarbon by increasing the yield of gas hydrocarbons. Espitalie et al. (1980) investigated the comparable pyrolysis on source rocks and related kerogens and indicated *To whom correspondence should be addressed. E-mail: [email protected]
that minerals decreased the hydrocarbon yield due to the retention of heavy hydrocarbon products on the mineral surface. Eglinton et al. (1986) investigated kerogen±mineral reactions in the presence of water. Dembicki (1990, 1992) reported the observations on the eects of the mineral matrix on the determination of kinetic parameters using modi®ed Rock-Eval pyrolysis. In China, there are also a few reports on the eects of minerals on kerogen degradation (Zhang, 1993; Liu, 1995). From the reviews above, it is found that there were some controversial views about the eects of mineral matter on kerogen degradation. The main reasons resulting in the controversies are simulation experiment methods including the experimental system, simulation conditions, the origin and type of kerogen, the activity of minerals as catalyst. Therefore, further systematic investigations will be necessary on this ®eld. The development of kinetic models of kerogen degradation is the main approach to quantitatively investigate the catalytic eects of minerals on petroleum generation. Much research work has been carried out on the kinetics of kerogen degradation (Burnham et al., 1987; Braun and Burnham, 1987; Reynolds and Burnham, 1995; Ritter et al., 1995). Little information, however, is available on the eects of minerals on kinetic parameters of kerogen degradation. By using kinetic parameters, one can model hydrocarbon generation from kerogen and thus predict the amounts of oil and gas formed in
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speci®c sediments (Ungerer and Pelet, 1987). This prediction should take into account the mineral eects on petroleum generation. In this case, the calculated amount of petroleum will be more practically accurate and more theoretically reasonable. It is emphasized that, based on early work of coal scientists (Juentgen and van Heek, 1970; Juentgen and Klein, 1975), and outlined in detail by Ungerer and Pelet (1987) and Braun and Burnham (1987), that the complex reaction system of kerogen degradation (and of petroleum generation) cannot satisfactorily be described by a single activation energy and frequency factor. Instead, they showed that realistic kinetic parameters for petroleum generation can only be derived from gross pyrolysis curves by the assumption of several parallel (®rst-order) reactions. This approach allowed the meaningful transfer of laboratory-derived kinetic data to geological ``heating-rates'' which are many orders of magnitude lower. This limitation in mind, the present paper focuses on kinetic observations, rather than the elaboration of activation energy distributions. Nevertheless, the kinetic parameters derived below, i.e. single activation energies and frequency factors, are used as a tool for the relative considerations. Because of various salt ions, such as, K+, Na+, 2ÿ Clÿ, CO2ÿ 3 , SO4 , etc. present in sea water or aqueous environments in the sedimentary rocks, they may aect the rate and extent of petroleum generation, as well as petroleum composition. Consequently, the research work on the catalytic eects of these salt ions becomes necessary. Up to now, however, there are few reports about the in¯uences of the salt ions on the process of petroleum generation. In the present work, a series of simulation experiments were carried out on isolated kerogen and mixtures of kerogen±montmorillonite and kerogen±potassium carbonate (K2CO3, a soluble salt in water), to tentatively investigate the eects of mineral and salt. On the basis of experimental data, a ®rst order reaction model was developed and kinetic parameters were obtained from the model. This paper summarizes the results of these experiments and discusses the eects of added minerals and salts on the kinetics of kerogen degradation. EXPERIMENTAL
The sample used was collected from the very immature source rock of the forth member of
Shahejie Formation of the Dongying Basin in the east of China. The rock was powdered and extracted (Soxhlet method) to give bitumen. Kerogen was isolated from the mineral matrix by hydrochloric and hydro¯uoric acid treatment. The isolated kerogen was re-extracted by Soxhlet and then subjected to elemental analysis, Rock-Eval analysis, vitrinite re¯ectance (Ro) analysis, etc. The analytical data are listed in Table 1. The montmorillonite, a dominant mineral in source rocks, was selected as a potential catalyst. It was powdered, extracted and subjected to XRD analysis to show a purity of >99%. Potassium carbonate (K2CO3) was selected as salt catalyst which is an analytical grade reagent. Kerogen/montmorillonite mixture is in the ratio of 1:10. The montmorillonite, as a catalyst, was activated in the oven at temperature of 3508C for about 5 h. Then it was mixed with kerogen and fully ground to prepare the suitable mixture sample. Kerogen/K2CO3 mixture is in the ratio of 10:1. Aqueous potassium carbonate solution was slowly dropped in the kerogen sample. And then the mixture was dried in the stove and fully ground to obtain the sample. Flash pyrolysis gas chromatograph (Py±GC) with a CDS Pyroprobe 120 and a Varian 3700 GC was used to perform isothermal simulation experiments. Py±GC±MS with a HP5890 II GC and a HP5980A quadrupole MS was adopted to identify and analyze the kerogen pyrolysates. Pyrolysis temperatures are 450, 490, 530 and 5708C. Kerogen/montmorillonite mixture (6 mg) or kerogen (1.5 mg) in a quartz tube were introduced into the pyroprobe. The tube was quickly heated to the speci®ed temperature (450±5708C) in about 1 s under a continuous slow ¯ow of nitrogen. Liberated products were analyzed by the programmed GC equipped with 50 m SE-52 capillary column and were monitored by the calibrated FID (¯ame ionization detector). At each speci®ed temperature, the kerogen or kerogen±mineral (salt) mixture was subjected to four consecutive Py±GC runs. The time required for each run ranged from 5 to 20 s, and total pyrolysis time was from 50 to 80 s, depending on pyrolysis temperatures. Pyrolysis products were swept directly onto the front of the GC column. The composition and yield of pyrolysates were determined by the retention time and area of the chromatographic peaks.
Table 1. The analysis data of the source rock and isolated kerogen Elemental analysis of kerogen (wt%) C 69.77
Rock-Eval of source rock
H
O
N
H/C
O/C
TOC (%)
S1 (mg/g)
S2 (mg/g)
Tmax (8C)
HI
Ro (%)
Type
8.22
10.68
2.08
1.41
0.11
1.41
0.227
7.45
437
528
0.38
I
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Fig. 1. Gas hydrocarbon yield (C1±C4) vs time at a temperature of 5308C. RESULTS AND DISCUSSION
The yield of detectable pyrolysates In Py±GC experiments, total pyrolysates consist of detectable and nondetectable products in the gas
chromatograph. Most of saturated and aromatic hydrocarbons can be detected, since asphaltenes and resins are not detectable. For the convenience of discussion, total identi®ed pyrolysates were divided into three parts: hydrocarbon gas (C1±C4),
Fig. 2. Light hydrocarbon yield (C5±C14) vs time at a temperature of 5308C.
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Fig. 3. Heavy hydrocarbon yield (C15+) vs time at a temperature of 5308C.
light hydrocarbons (C5±C14) and heavy hydrocarbons (C15+). The relationship between the yields of various pyrolysates and time at a temperature of 5308C is shown in Figs 1±4. The data concerning the yield of pyrolysates are summarized in Table 2.
From these results, we can obtain the following conclusions. The presence of montmorillonite enhanced the evolution of gas, light hydrocarbons and total detectable hydrocarbons. But one interesting
Fig. 4. Total detectable hydrocarbon yield vs time at a temperature of 5308C.
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Table 3. The weight loss of kerogen (time is 30 s) Temperature (8C) 450 490 530 570
Kerogen mg/g TOC
Kerogen + montmorillonite (mg/g TOC)
Percentage increased
Kerogen + K2CO3 mg/g TOC
Percentage increased
225.02 383.71 625.64 792.13
291.36 500.77 667.26 900.09
29.5 30.5 6.7 13.6
310.08 484.24 708.85 899.06
37.8 26.2 13.3 13.5
phenomenon was observed that montmorillonite decreased the yield of heavy hydrocarbons (C15+) to a considerable extent (more than 50%). This may be attributed to the strong adsorption ability of montmorillonite grains for heavy hydrocarbons. The larger molecular weight of pyrolysate, the stronger adsorption of montmorillonite. The heavy hydrocarbon was retained and subjected to further reaction to produce the small molecular weight hydrocarbons. As a result, gas hydrocarbon has the higher yield in kerogen degradation in the presence of montmorillonite. Addition of potassium carbonate (K2CO3) promoted the generation of all pyrolysates. At a temperature of 5308C, K2CO3 increased the ®nal yield of total hydrocarbons by more than 30% while montmorillonite increased this yield by about 15%. Therefore, K2CO3 exhibited the stronger catalytic action than montmorillonite during the degradation of kerogen. As compared with the generation of light and heavy hydrocarbons, the generation of gas is more sensitive to catalysts. In the presence of K2CO3, for example, the gas yield increased by 69% while light and heavy hydrocarbons increased by 22 and 13%, respectively (see Table 2). The weight loss of kerogen Table 3 illustrates the results concerning the weight loss during isothermal degradation of kerogen for 30 s at dierent temperatures. The weight loss of kerogen represents the summary of all products, including both organic pyrolysates and inorganic gases, such as H2, CO2, CO, H2O, etc. The weight loss of kerogen refers to total kerogen degradation. For the weight loss correction, the montmorillonite was subjected to the pyrolysis under the same experimental conditions. The weight loss of kerogen was the dierence between the weight loss of kerogen±montmorillonite mixture and the weight loss of montmorillonite. Because K2CO3 does not decompose to release CO2 at the simulation temperatures,
it is not necessary to check the weight loss of kerogen±K2CO3 mixture. The results of Table 3 show that the weight loss of kerogen increased with increasing temperature. When the mixtures of kerogen/mineral (salt) were pyrolyzed, the larger weight loss has been obtained due to the catalysis of mineral (salt). The presence of montmorillonite and K2CO3 increased the rate of kerogen pyrolysis and thus resulted in the considerable rise in weight loss of kerogen. As compared with montmorillonite, K2CO3 exhibited the stronger catalytic in¯uence on the whole degradation process of kerogen. The simulation temperature aected the catalytic degradation of kerogen. The catalytic action of montmorillonite and K2CO3 became small and weak with increasing pyrolysis temperatures. At a temperature of 4508C, for example, weight loss increased by 37.8%, due to the addition of K2CO3, while it increased by 13.5% at a temperature of 5708C. It can be expected that the ®nal yield of kerogen might be the same at the higher temperature when the kerogen is pyrolyzed to completion. This observation agrees with the fact that though a catalyst may speed up a reaction, it never changes the end point of a reaction (i.e. ®nal yield). At the present temperatures (<5708C), however, kerogen pyrolysis was not complete. Therefore, the yield of pyrolysates was aected by catalysts. It is imagined that at a higher temperature, the kerogen pyrolysis mainly follows the free radical reaction path and that the catalytic action of the mineral (salt) is negligible. Kinetics of pyrolysate generation There are numerous investigations on the kinetics of kerogen degradation by various methods and models. But few work was reported on the eects of minerals and salts on the kinetics of kerogen degradation. In this paper a tentative study on the subject was performed. A ®rst order reaction model was used to describe the kerogen degradation. The rate of kerogen degradation is often expressed by
Table 2. The yield of detectable pyrolysates at a temperature of 5308C Pyrolysates C1±C4 C5±C14 C15+ Total
Kerogen mg/g TOC
Kerogen + montmorillonite (mg/g TOC)
Percentage increased
Kerogen + K2CO3 mg/g TOC
Percentage increased
34.46 77.45 19.21 131.12
55.80 87.78 8.09 151.67
61.93 13.34 ÿ57.89 15.67
58.12 94.65 21.79 174.56
68.66 22.21 13.43 33.13
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Fig. 5. The relationship of ln(1 ÿ x) vs time for total hydrocarbon at dierent temperatures.
the general kinetic equation dx k 1 ÿ x dt
1
and Arrenhius equation
E k A exp ÿ RT
2
Under isothermal conditions, k is a constant and thus the integration of equation 1 gives following equation ln 1 ÿ x ÿkt
3
Taking the logarithm of equation 2, we obtain ln k ln A ÿ
E RT
4
Where t is the reaction time, k is reaction rate constant and x is the normalized mass fraction of total potential hydrocarbon generation, R is the gas constant (8.314 J/molK), T is the absolute temperature, and A and E are, respectively, the frequency factor and the activation energy. Based on the experimental data concerning x vs t at a constant temperature, rate constant k can be determined by the slope of a linear regression of equation 3. Using the values of k determined at dierent temperatures, we can obtain the A and E by linear regression of equation 4. The linear regression coecients of equations (3) and (4) are always larger than 0.99, which implies the little deviation of data treatment. The relationship curves of linear regression of equations 3) and (4 are
showed in Figs 5 and 6. In addition, the values of the rate constant (k) at a geological temperature (e.g. 1208C) were calculated by the Arrhenius equation. k120 represents the generation rate of various hydrocarbons from the kerogen at constant geological temperature (1208C). k120 can be approximately used to account for the generation process of hydrocarbons, to compare relative rate of various products in the presence or absence of mineral (salt) and thus to examine the eects of mineral and salt on the kerogen degradation in the geological conditions. The results concerning E, A and k120 for dierent pyrolysates are shown in Table 4. In the data of Table 4, several interesting trends are readily apparent. The activation energies estimated in Table 4 are in the low range, approximating to the reported parameters (Badzioch and Hawksley, 1970; Connan, 1974; Lu et al., 1995). According to previous work (Howard, 1981), kinetic parameters determined from isothermal experimental data are lower than those from nonisothermal data for the same sample. Frequency factors (A) are coupled with activation energies (E) such that when E is large or small, so is A. Therefore, the rate constant (k) is basically identical for the same sample. For the very complex system of kerogen degradation (and of petroleum generation) we are aware of the dierence between simpli®ed reaction kinetics (single activation energy and frequency factors) and the description of gross pyrolysis curves by a number of parallel (®rst-order) reactions. Juentgen and Klein (1975) showed, for example, that a gross pyrolysis curve, approximated by the kinetic par-
Catalytic degradation of immature kerogen
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Fig. 6. Arrhenius plots of the logarithm of the rate constant vs the reciprocal absolute temperature.
ameters activation energy Ea=84 kJ/mol and the frequency factor A = 1.7 102 sÿ1, is obtained, for instance, by the superposition of eight individual reactions with activation energy values increasing from 200 to 260 kJ/mol and a frequency factor of 1.7 1013 sÿ1. Using respectively 84 kJ/mol and 1.7 102 sÿ1 at a geological heating rate of 5.38C per million years, which is a typical value for a subsiding basin, would lead to totally unrealistic results in terms of the peak temperature of hydrocarbon generation (about 08C). The corresponding calculation based on the eight single reactions, however, revealed realistic temperature values, i.e. between 100 and 2408C for the main hydrocarbon generation phase in this example (Welte et al., 1988). In this respect, the kinetic data of the present investigation are mainly interpreted on a relative scale.
During the generation of gas and light hydrocarbons, the mixtures of kerogen±K2CO3 and kerogen±montmorillonite have the lower activation energies as compared to the isolated kerogen. This implies that montmorillonite and K2CO3 have acted as a catalyst in the generation process of gas and light hydrocarbon. In gas generation, the kerogen± montmorillonite mixture has the lowest energy, while in light hydrocarbon formation, the kerogen± K2CO3 mixture has the lowest energy. These trends indicate that montmorillonite is favorable for gas generation and K2CO3 for light hydrocarbon generation. In addition, the values of k120 in Table 4 increased with the decreasing activation energies, showing that the catalytic action of mineral (salt) on oil and gas formation is obvious in the geological temperature.
Table 4. Kinetic parameters of decomposition of kerogen and mixtures of kerogen±mineral (salt) for dierent detectable pyrolysates Sample
Parameter*
C1±C4
C5±C14
C15+
Total hydrocarbon
Kerogen
E (kJ/mol) A (1/s) k120 (1/year)
59.67 142.00 52.47
79.87 3291.00 2.51
60.68 295.67 80.56
66.24 476.00 23.56
Kerogen + montmorillonite
E (kJ/mol) A (1/s) k120 (1/year)
43.50 11.10 575.63
72.62 1581.00 11.09
67.59 1150.00 37.60
54.44 113.90 208.60
Kerogen + K2Co3
E (kJ/mol) A (1/s) k120 (1/year)
52.17 212.90 780.88
49.96 12.10 87.26
56.31 415.30 429.13
46.68 38.78 763.54
*E, A and k120 represent activation energy, frequency factor and kinetic constant at temperature of 1208C, respectively.
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The catalytic action of K2CO3 on the generation of heavy hydrocarbon (C15+) is relatively lower than on other hydrocarbon generation. No appreciable decrease in activation energy was observed in the presence of K2CO3. In contrast to its catalytic properties, montmorillonite acts even as an inhibitor for the heavy hydrocarbon generation. In fact, heavy hydrocarbon retained on montmorillonite grains was subjected to further pyrolysis to produce gas and light hydrocarbons. As a result, the rate and amount of heavy hydrocarbon generation was cut down to some extent, and thus the determined activation energy was higher than other cases. As a whole, although montmorillonite inhibited heavy hydrocarbon evolution, it exhibited catalytic action on total hydrocarbon generation. This can be explained by the fact that the further cracking of retained heavy hydrocarbon gave a rise to the yield of total hydrocarbons. Catalytic mechanism Dierent mechanisms are operative for the thermal degradation and catalytic pyrolysis of kerogen. A free radical mechanism was proposed to explain the thermal degradation behavior of isolated kerogen in the absence of minerals (Gavalas, 1982; Petrakis and Grandy, 1983; Ungerer, 1990). By using ESR (electron spin resonance), the existence of many free radicals has been directly veri®ed in the macromolecules of kerogen. The concentration of free radicals ranges from 5 1016 to 2.5 1019/g TOC (Marchand and Conard, 1980; Wind et al., 1987). A carbonium ion mechanism was reasonably used to account for the catalytic properties of clay minerals in the degradation of kerogen±mineral mixtures (Brooks, 1948, 1952; Lao, 1989). The acid sites existing on the mineral surface are active to promote the formation of carbonium ions from kerogen. And the carbonium ions, as intermediates, are subjected to further processes, such as, cracking, isomerization, transformation, etc. and ®nally converted into hydrocarbons. In the presence of salt ions, however, the catalytic mechanism of kerogen degradation is not clear at present. In the processes of coal pyrolysis and combustion, salts are used as an industrial catalyst. Among these catalysts, K2CO3 is a most widely used catalyst because of its high catalytic activity. The catalytic eects of the salts on the pyrolysis and combustion of coals were tentatively interpreted by electron transfer mechanism (Essenhigh, 1981; Chen, 1993). In the general coal combustion, coal is ®rstly subjected to pyrolysis to release volatiles, and then the volatiles are combusted into gas. In this case, the catalytic eects of pyrolysis and the combustion of coal may be probably interpreted in terms of the same catalytic mechanism. It is well known that kerogen from source rock has the approximate physical and chemical properties with the
kerogen from coal. These two kinds of kerogens can reasonably well be compared with each other in many aspects, such as, composition, molecular structure, degradation characteristics, etc. Consequently, electron transfer mechanism can be used for reference to explain the catalytic eects of K2CO3 on kerogen degradation. The electron transfer mechanism emphasizes the salt ions±kerogen interaction. After salt ions enter into the intra- or intermolecule of kerogen, cationic ions (K+) and anionic ions (CO2ÿ 3 ) are adsorbed on the surface, or a neighboring site, of polar functional groups with negative and positive charges, respectively. As a result, the electronic con®guration and charge distribution in kerogen molecules are changed to some extent. These changes result in shifting of the charges from place to place, increase the polarity of covalent bonds (C±C or C±H) and polar bonds (C± O or C±S) and thus promote cleavage of some bonds. Furthermore, according to the electron transfer mechanism, it is supposed that the catalyst action induces a preferred distribution of p electrons whose mobility weakens the bond strength (Essenhigh, 1981). Further research on this subject is necessary in the future to understand more details of the mechanism of salt catalysis.
CONCLUSIONS
1. Montmorillonite and potassium carbonate (K2CO3) act as a catalyst to promote the degradation of kerogen, to increase the yield of gas and light hydrocarbons and to decrease the activation energies of the generation of gas and light hydrocarbons. 2. The catalytic action of K2CO3 on the generation of heavy hydrocarbon is relatively lower than on other hydrocarbon generation. No appreciable drop in activation energy was observed in the presence of K2CO3. 3. In contrast, montmorillonite acts even as an inhibitor for the heavy hydrocarbon to decrease the yield of heavy hydrocarbons to a considerable extent. 4. The catalytic eects of montmorillonite and K2CO3 become small and weak with increasing temperature. 5. The catalytic action of montmorillonite and K2CO3 on the kerogen degradation can be tentatively accounted for by carbonium ion mechanism and electron transfer mechanism, respectively.
AcknowledgementsÐWe thank the Natural Science Foundation of China (No.49402031) and Key Project of CNPC (quantitative model of reservoir formation in China, No. 960007) for the ®nancial support for this work. We also thank Dr Ruofu Liu and Mr. Guangjia Zhou to provide us immature source rock sample.
Catalytic degradation of immature kerogen REFERENCES
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Org. Geochem. Vol. 29, No. 5±7, pp. 1431±1439, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00154-5 0146-6380/98 $ - see front matter
Characteristics and kinetics of catalytic degradation of immature kerogen in the presence of mineral and salt SHUYUAN LI*, SHAOHUI GUO and XUEFEI TAN School of Chemical Engineering, University of Petroleum, Changping, 102200 Beijing, People's Republic of China AbstractÐIn the present work, a series of simulation experiments were carried out on isolated kerogen and mixtures of kerogen±montmorillonite and kerogen±potassium carbonate to investigate the eects of mineral and salt on the generation of various pyrolysates. A kinetic model of pyrolysate generation was developed which takes into account the eects of the added mineral and salt on kinetic parameters. It is found that montmorillonite and potassium carbonate (K2CO3) act as a catalyst to promote the degradation of kerogen, to increase the yield of gas and light hydrocarbons and to decrease the activation energies for the generation of gas and light hydrocarbons. In the generation of heavy hydrocarbons, however, K2CO3 exhibited feeble catalytic action. In contrast, montmorillonite decreased the yield of heavy hydrocarbons due to the retention on mineral surfaces. The catalytic eects of montmorillonite and K2CO3 become small and weak with increasing temperature. The catalytic action of montmorillonite and K2CO3 on the kerogen degradation can be tentatively accounted for by carbonium ion mechanism and electron transfer mechanism, respectively. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐkerogen degradation, pyrolysis, kinetics, catalytic eect, Shahejie Formation, Dongying Basin
INTRODUCTION
It is now widely accepted that petroleum is generated from the thermal degradation of kerogen in sedimentary rocks. The main factors aecting the amount of petroleum are the nature of kerogen, temperature, time, mineral composition, etc. Among them, the minerals and salt solutions existing in the sediments during kerogen maturation were known to have important eects on petroleum generation. They may catalyze or restrain thermal decomposition of kerogen, thereby altering the yield and composition of petroleum. Therefore, the work on this ®eld has been the subject of much research in recent years. As early as the beginning of the ®fties, Brooks (1948, 1952) reported the catalytic alteration of kerogen during petroleum formation. Soon afterwards, a number of workers have examined the eects of minerals on kerogen degradation and showed important catalytic action (Henderson et al., 1968; Johns and Shimoyama, 1972). Hors®eld and Douglas (1980) performed open-system simulation experiments and concluded that mineral matter decreased the yield of liquid hydrocarbon by increasing the yield of gas hydrocarbons. Espitalie et al. (1980) investigated the comparable pyrolysis on source rocks and related kerogens and indicated *To whom correspondence should be addressed. E-mail: [email protected]
that minerals decreased the hydrocarbon yield due to the retention of heavy hydrocarbon products on the mineral surface. Eglinton et al. (1986) investigated kerogen±mineral reactions in the presence of water. Dembicki (1990, 1992) reported the observations on the eects of the mineral matrix on the determination of kinetic parameters using modi®ed Rock-Eval pyrolysis. In China, there are also a few reports on the eects of minerals on kerogen degradation (Zhang, 1993; Liu, 1995). From the reviews above, it is found that there were some controversial views about the eects of mineral matter on kerogen degradation. The main reasons resulting in the controversies are simulation experiment methods including the experimental system, simulation conditions, the origin and type of kerogen, the activity of minerals as catalyst. Therefore, further systematic investigations will be necessary on this ®eld. The development of kinetic models of kerogen degradation is the main approach to quantitatively investigate the catalytic eects of minerals on petroleum generation. Much research work has been carried out on the kinetics of kerogen degradation (Burnham et al., 1987; Braun and Burnham, 1987; Reynolds and Burnham, 1995; Ritter et al., 1995). Little information, however, is available on the eects of minerals on kinetic parameters of kerogen degradation. By using kinetic parameters, one can model hydrocarbon generation from kerogen and thus predict the amounts of oil and gas formed in
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speci®c sediments (Ungerer and Pelet, 1987). This prediction should take into account the mineral eects on petroleum generation. In this case, the calculated amount of petroleum will be more practically accurate and more theoretically reasonable. It is emphasized that, based on early work of coal scientists (Juentgen and van Heek, 1970; Juentgen and Klein, 1975), and outlined in detail by Ungerer and Pelet (1987) and Braun and Burnham (1987), that the complex reaction system of kerogen degradation (and of petroleum generation) cannot satisfactorily be described by a single activation energy and frequency factor. Instead, they showed that realistic kinetic parameters for petroleum generation can only be derived from gross pyrolysis curves by the assumption of several parallel (®rst-order) reactions. This approach allowed the meaningful transfer of laboratory-derived kinetic data to geological ``heating-rates'' which are many orders of magnitude lower. This limitation in mind, the present paper focuses on kinetic observations, rather than the elaboration of activation energy distributions. Nevertheless, the kinetic parameters derived below, i.e. single activation energies and frequency factors, are used as a tool for the relative considerations. Because of various salt ions, such as, K+, Na+, 2ÿ Clÿ, CO2ÿ 3 , SO4 , etc. present in sea water or aqueous environments in the sedimentary rocks, they may aect the rate and extent of petroleum generation, as well as petroleum composition. Consequently, the research work on the catalytic eects of these salt ions becomes necessary. Up to now, however, there are few reports about the in¯uences of the salt ions on the process of petroleum generation. In the present work, a series of simulation experiments were carried out on isolated kerogen and mixtures of kerogen±montmorillonite and kerogen±potassium carbonate (K2CO3, a soluble salt in water), to tentatively investigate the eects of mineral and salt. On the basis of experimental data, a ®rst order reaction model was developed and kinetic parameters were obtained from the model. This paper summarizes the results of these experiments and discusses the eects of added minerals and salts on the kinetics of kerogen degradation. EXPERIMENTAL
The sample used was collected from the very immature source rock of the forth member of
Shahejie Formation of the Dongying Basin in the east of China. The rock was powdered and extracted (Soxhlet method) to give bitumen. Kerogen was isolated from the mineral matrix by hydrochloric and hydro¯uoric acid treatment. The isolated kerogen was re-extracted by Soxhlet and then subjected to elemental analysis, Rock-Eval analysis, vitrinite re¯ectance (Ro) analysis, etc. The analytical data are listed in Table 1. The montmorillonite, a dominant mineral in source rocks, was selected as a potential catalyst. It was powdered, extracted and subjected to XRD analysis to show a purity of >99%. Potassium carbonate (K2CO3) was selected as salt catalyst which is an analytical grade reagent. Kerogen/montmorillonite mixture is in the ratio of 1:10. The montmorillonite, as a catalyst, was activated in the oven at temperature of 3508C for about 5 h. Then it was mixed with kerogen and fully ground to prepare the suitable mixture sample. Kerogen/K2CO3 mixture is in the ratio of 10:1. Aqueous potassium carbonate solution was slowly dropped in the kerogen sample. And then the mixture was dried in the stove and fully ground to obtain the sample. Flash pyrolysis gas chromatograph (Py±GC) with a CDS Pyroprobe 120 and a Varian 3700 GC was used to perform isothermal simulation experiments. Py±GC±MS with a HP5890 II GC and a HP5980A quadrupole MS was adopted to identify and analyze the kerogen pyrolysates. Pyrolysis temperatures are 450, 490, 530 and 5708C. Kerogen/montmorillonite mixture (6 mg) or kerogen (1.5 mg) in a quartz tube were introduced into the pyroprobe. The tube was quickly heated to the speci®ed temperature (450±5708C) in about 1 s under a continuous slow ¯ow of nitrogen. Liberated products were analyzed by the programmed GC equipped with 50 m SE-52 capillary column and were monitored by the calibrated FID (¯ame ionization detector). At each speci®ed temperature, the kerogen or kerogen±mineral (salt) mixture was subjected to four consecutive Py±GC runs. The time required for each run ranged from 5 to 20 s, and total pyrolysis time was from 50 to 80 s, depending on pyrolysis temperatures. Pyrolysis products were swept directly onto the front of the GC column. The composition and yield of pyrolysates were determined by the retention time and area of the chromatographic peaks.
Table 1. The analysis data of the source rock and isolated kerogen Elemental analysis of kerogen (wt%) C 69.77
Rock-Eval of source rock
H
O
N
H/C
O/C
TOC (%)
S1 (mg/g)
S2 (mg/g)
Tmax (8C)
HI
Ro (%)
Type
8.22
10.68
2.08
1.41
0.11
1.41
0.227
7.45
437
528
0.38
I
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Fig. 1. Gas hydrocarbon yield (C1±C4) vs time at a temperature of 5308C. RESULTS AND DISCUSSION
The yield of detectable pyrolysates In Py±GC experiments, total pyrolysates consist of detectable and nondetectable products in the gas
chromatograph. Most of saturated and aromatic hydrocarbons can be detected, since asphaltenes and resins are not detectable. For the convenience of discussion, total identi®ed pyrolysates were divided into three parts: hydrocarbon gas (C1±C4),
Fig. 2. Light hydrocarbon yield (C5±C14) vs time at a temperature of 5308C.
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Fig. 3. Heavy hydrocarbon yield (C15+) vs time at a temperature of 5308C.
light hydrocarbons (C5±C14) and heavy hydrocarbons (C15+). The relationship between the yields of various pyrolysates and time at a temperature of 5308C is shown in Figs 1±4. The data concerning the yield of pyrolysates are summarized in Table 2.
From these results, we can obtain the following conclusions. The presence of montmorillonite enhanced the evolution of gas, light hydrocarbons and total detectable hydrocarbons. But one interesting
Fig. 4. Total detectable hydrocarbon yield vs time at a temperature of 5308C.
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Table 3. The weight loss of kerogen (time is 30 s) Temperature (8C) 450 490 530 570
Kerogen mg/g TOC
Kerogen + montmorillonite (mg/g TOC)
Percentage increased
Kerogen + K2CO3 mg/g TOC
Percentage increased
225.02 383.71 625.64 792.13
291.36 500.77 667.26 900.09
29.5 30.5 6.7 13.6
310.08 484.24 708.85 899.06
37.8 26.2 13.3 13.5
phenomenon was observed that montmorillonite decreased the yield of heavy hydrocarbons (C15+) to a considerable extent (more than 50%). This may be attributed to the strong adsorption ability of montmorillonite grains for heavy hydrocarbons. The larger molecular weight of pyrolysate, the stronger adsorption of montmorillonite. The heavy hydrocarbon was retained and subjected to further reaction to produce the small molecular weight hydrocarbons. As a result, gas hydrocarbon has the higher yield in kerogen degradation in the presence of montmorillonite. Addition of potassium carbonate (K2CO3) promoted the generation of all pyrolysates. At a temperature of 5308C, K2CO3 increased the ®nal yield of total hydrocarbons by more than 30% while montmorillonite increased this yield by about 15%. Therefore, K2CO3 exhibited the stronger catalytic action than montmorillonite during the degradation of kerogen. As compared with the generation of light and heavy hydrocarbons, the generation of gas is more sensitive to catalysts. In the presence of K2CO3, for example, the gas yield increased by 69% while light and heavy hydrocarbons increased by 22 and 13%, respectively (see Table 2). The weight loss of kerogen Table 3 illustrates the results concerning the weight loss during isothermal degradation of kerogen for 30 s at dierent temperatures. The weight loss of kerogen represents the summary of all products, including both organic pyrolysates and inorganic gases, such as H2, CO2, CO, H2O, etc. The weight loss of kerogen refers to total kerogen degradation. For the weight loss correction, the montmorillonite was subjected to the pyrolysis under the same experimental conditions. The weight loss of kerogen was the dierence between the weight loss of kerogen±montmorillonite mixture and the weight loss of montmorillonite. Because K2CO3 does not decompose to release CO2 at the simulation temperatures,
it is not necessary to check the weight loss of kerogen±K2CO3 mixture. The results of Table 3 show that the weight loss of kerogen increased with increasing temperature. When the mixtures of kerogen/mineral (salt) were pyrolyzed, the larger weight loss has been obtained due to the catalysis of mineral (salt). The presence of montmorillonite and K2CO3 increased the rate of kerogen pyrolysis and thus resulted in the considerable rise in weight loss of kerogen. As compared with montmorillonite, K2CO3 exhibited the stronger catalytic in¯uence on the whole degradation process of kerogen. The simulation temperature aected the catalytic degradation of kerogen. The catalytic action of montmorillonite and K2CO3 became small and weak with increasing pyrolysis temperatures. At a temperature of 4508C, for example, weight loss increased by 37.8%, due to the addition of K2CO3, while it increased by 13.5% at a temperature of 5708C. It can be expected that the ®nal yield of kerogen might be the same at the higher temperature when the kerogen is pyrolyzed to completion. This observation agrees with the fact that though a catalyst may speed up a reaction, it never changes the end point of a reaction (i.e. ®nal yield). At the present temperatures (<5708C), however, kerogen pyrolysis was not complete. Therefore, the yield of pyrolysates was aected by catalysts. It is imagined that at a higher temperature, the kerogen pyrolysis mainly follows the free radical reaction path and that the catalytic action of the mineral (salt) is negligible. Kinetics of pyrolysate generation There are numerous investigations on the kinetics of kerogen degradation by various methods and models. But few work was reported on the eects of minerals and salts on the kinetics of kerogen degradation. In this paper a tentative study on the subject was performed. A ®rst order reaction model was used to describe the kerogen degradation. The rate of kerogen degradation is often expressed by
Table 2. The yield of detectable pyrolysates at a temperature of 5308C Pyrolysates C1±C4 C5±C14 C15+ Total
Kerogen mg/g TOC
Kerogen + montmorillonite (mg/g TOC)
Percentage increased
Kerogen + K2CO3 mg/g TOC
Percentage increased
34.46 77.45 19.21 131.12
55.80 87.78 8.09 151.67
61.93 13.34 ÿ57.89 15.67
58.12 94.65 21.79 174.56
68.66 22.21 13.43 33.13
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Fig. 5. The relationship of ln(1 ÿ x) vs time for total hydrocarbon at dierent temperatures.
the general kinetic equation dx k 1 ÿ x dt
1
and Arrenhius equation
E k A exp ÿ RT
2
Under isothermal conditions, k is a constant and thus the integration of equation 1 gives following equation ln 1 ÿ x ÿkt
3
Taking the logarithm of equation 2, we obtain ln k ln A ÿ
E RT
4
Where t is the reaction time, k is reaction rate constant and x is the normalized mass fraction of total potential hydrocarbon generation, R is the gas constant (8.314 J/molK), T is the absolute temperature, and A and E are, respectively, the frequency factor and the activation energy. Based on the experimental data concerning x vs t at a constant temperature, rate constant k can be determined by the slope of a linear regression of equation 3. Using the values of k determined at dierent temperatures, we can obtain the A and E by linear regression of equation 4. The linear regression coecients of equations (3) and (4) are always larger than 0.99, which implies the little deviation of data treatment. The relationship curves of linear regression of equations 3) and (4 are
showed in Figs 5 and 6. In addition, the values of the rate constant (k) at a geological temperature (e.g. 1208C) were calculated by the Arrhenius equation. k120 represents the generation rate of various hydrocarbons from the kerogen at constant geological temperature (1208C). k120 can be approximately used to account for the generation process of hydrocarbons, to compare relative rate of various products in the presence or absence of mineral (salt) and thus to examine the eects of mineral and salt on the kerogen degradation in the geological conditions. The results concerning E, A and k120 for dierent pyrolysates are shown in Table 4. In the data of Table 4, several interesting trends are readily apparent. The activation energies estimated in Table 4 are in the low range, approximating to the reported parameters (Badzioch and Hawksley, 1970; Connan, 1974; Lu et al., 1995). According to previous work (Howard, 1981), kinetic parameters determined from isothermal experimental data are lower than those from nonisothermal data for the same sample. Frequency factors (A) are coupled with activation energies (E) such that when E is large or small, so is A. Therefore, the rate constant (k) is basically identical for the same sample. For the very complex system of kerogen degradation (and of petroleum generation) we are aware of the dierence between simpli®ed reaction kinetics (single activation energy and frequency factors) and the description of gross pyrolysis curves by a number of parallel (®rst-order) reactions. Juentgen and Klein (1975) showed, for example, that a gross pyrolysis curve, approximated by the kinetic par-
Catalytic degradation of immature kerogen
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Fig. 6. Arrhenius plots of the logarithm of the rate constant vs the reciprocal absolute temperature.
ameters activation energy Ea=84 kJ/mol and the frequency factor A = 1.7 102 sÿ1, is obtained, for instance, by the superposition of eight individual reactions with activation energy values increasing from 200 to 260 kJ/mol and a frequency factor of 1.7 1013 sÿ1. Using respectively 84 kJ/mol and 1.7 102 sÿ1 at a geological heating rate of 5.38C per million years, which is a typical value for a subsiding basin, would lead to totally unrealistic results in terms of the peak temperature of hydrocarbon generation (about 08C). The corresponding calculation based on the eight single reactions, however, revealed realistic temperature values, i.e. between 100 and 2408C for the main hydrocarbon generation phase in this example (Welte et al., 1988). In this respect, the kinetic data of the present investigation are mainly interpreted on a relative scale.
During the generation of gas and light hydrocarbons, the mixtures of kerogen±K2CO3 and kerogen±montmorillonite have the lower activation energies as compared to the isolated kerogen. This implies that montmorillonite and K2CO3 have acted as a catalyst in the generation process of gas and light hydrocarbon. In gas generation, the kerogen± montmorillonite mixture has the lowest energy, while in light hydrocarbon formation, the kerogen± K2CO3 mixture has the lowest energy. These trends indicate that montmorillonite is favorable for gas generation and K2CO3 for light hydrocarbon generation. In addition, the values of k120 in Table 4 increased with the decreasing activation energies, showing that the catalytic action of mineral (salt) on oil and gas formation is obvious in the geological temperature.
Table 4. Kinetic parameters of decomposition of kerogen and mixtures of kerogen±mineral (salt) for dierent detectable pyrolysates Sample
Parameter*
C1±C4
C5±C14
C15+
Total hydrocarbon
Kerogen
E (kJ/mol) A (1/s) k120 (1/year)
59.67 142.00 52.47
79.87 3291.00 2.51
60.68 295.67 80.56
66.24 476.00 23.56
Kerogen + montmorillonite
E (kJ/mol) A (1/s) k120 (1/year)
43.50 11.10 575.63
72.62 1581.00 11.09
67.59 1150.00 37.60
54.44 113.90 208.60
Kerogen + K2Co3
E (kJ/mol) A (1/s) k120 (1/year)
52.17 212.90 780.88
49.96 12.10 87.26
56.31 415.30 429.13
46.68 38.78 763.54
*E, A and k120 represent activation energy, frequency factor and kinetic constant at temperature of 1208C, respectively.
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The catalytic action of K2CO3 on the generation of heavy hydrocarbon (C15+) is relatively lower than on other hydrocarbon generation. No appreciable decrease in activation energy was observed in the presence of K2CO3. In contrast to its catalytic properties, montmorillonite acts even as an inhibitor for the heavy hydrocarbon generation. In fact, heavy hydrocarbon retained on montmorillonite grains was subjected to further pyrolysis to produce gas and light hydrocarbons. As a result, the rate and amount of heavy hydrocarbon generation was cut down to some extent, and thus the determined activation energy was higher than other cases. As a whole, although montmorillonite inhibited heavy hydrocarbon evolution, it exhibited catalytic action on total hydrocarbon generation. This can be explained by the fact that the further cracking of retained heavy hydrocarbon gave a rise to the yield of total hydrocarbons. Catalytic mechanism Dierent mechanisms are operative for the thermal degradation and catalytic pyrolysis of kerogen. A free radical mechanism was proposed to explain the thermal degradation behavior of isolated kerogen in the absence of minerals (Gavalas, 1982; Petrakis and Grandy, 1983; Ungerer, 1990). By using ESR (electron spin resonance), the existence of many free radicals has been directly veri®ed in the macromolecules of kerogen. The concentration of free radicals ranges from 5 1016 to 2.5 1019/g TOC (Marchand and Conard, 1980; Wind et al., 1987). A carbonium ion mechanism was reasonably used to account for the catalytic properties of clay minerals in the degradation of kerogen±mineral mixtures (Brooks, 1948, 1952; Lao, 1989). The acid sites existing on the mineral surface are active to promote the formation of carbonium ions from kerogen. And the carbonium ions, as intermediates, are subjected to further processes, such as, cracking, isomerization, transformation, etc. and ®nally converted into hydrocarbons. In the presence of salt ions, however, the catalytic mechanism of kerogen degradation is not clear at present. In the processes of coal pyrolysis and combustion, salts are used as an industrial catalyst. Among these catalysts, K2CO3 is a most widely used catalyst because of its high catalytic activity. The catalytic eects of the salts on the pyrolysis and combustion of coals were tentatively interpreted by electron transfer mechanism (Essenhigh, 1981; Chen, 1993). In the general coal combustion, coal is ®rstly subjected to pyrolysis to release volatiles, and then the volatiles are combusted into gas. In this case, the catalytic eects of pyrolysis and the combustion of coal may be probably interpreted in terms of the same catalytic mechanism. It is well known that kerogen from source rock has the approximate physical and chemical properties with the
kerogen from coal. These two kinds of kerogens can reasonably well be compared with each other in many aspects, such as, composition, molecular structure, degradation characteristics, etc. Consequently, electron transfer mechanism can be used for reference to explain the catalytic eects of K2CO3 on kerogen degradation. The electron transfer mechanism emphasizes the salt ions±kerogen interaction. After salt ions enter into the intra- or intermolecule of kerogen, cationic ions (K+) and anionic ions (CO2ÿ 3 ) are adsorbed on the surface, or a neighboring site, of polar functional groups with negative and positive charges, respectively. As a result, the electronic con®guration and charge distribution in kerogen molecules are changed to some extent. These changes result in shifting of the charges from place to place, increase the polarity of covalent bonds (C±C or C±H) and polar bonds (C± O or C±S) and thus promote cleavage of some bonds. Furthermore, according to the electron transfer mechanism, it is supposed that the catalyst action induces a preferred distribution of p electrons whose mobility weakens the bond strength (Essenhigh, 1981). Further research on this subject is necessary in the future to understand more details of the mechanism of salt catalysis.
CONCLUSIONS
1. Montmorillonite and potassium carbonate (K2CO3) act as a catalyst to promote the degradation of kerogen, to increase the yield of gas and light hydrocarbons and to decrease the activation energies of the generation of gas and light hydrocarbons. 2. The catalytic action of K2CO3 on the generation of heavy hydrocarbon is relatively lower than on other hydrocarbon generation. No appreciable drop in activation energy was observed in the presence of K2CO3. 3. In contrast, montmorillonite acts even as an inhibitor for the heavy hydrocarbon to decrease the yield of heavy hydrocarbons to a considerable extent. 4. The catalytic eects of montmorillonite and K2CO3 become small and weak with increasing temperature. 5. The catalytic action of montmorillonite and K2CO3 on the kerogen degradation can be tentatively accounted for by carbonium ion mechanism and electron transfer mechanism, respectively.
AcknowledgementsÐWe thank the Natural Science Foundation of China (No.49402031) and Key Project of CNPC (quantitative model of reservoir formation in China, No. 960007) for the ®nancial support for this work. We also thank Dr Ruofu Liu and Mr. Guangjia Zhou to provide us immature source rock sample.
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