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Simulated thermal maturation of type I and III kerogens in the presence, and absence, of calcite and montmorillonite
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Pergamon
0146-6380(94)00120-0
Org. Geochem. Vol. 23, No. 2, pp. 121-127, 1995 Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0146-6380/95$9.50+ 0.00
Simulated thermal maturation of type I and III kerogens in the presence, and absence, of calcite and montmorillonite M A G D O L N A HETI~NYI Institute of Mineralogy, Geochemistry and Petrology, Attila Jrzsef University, H-6701 Szeged, P.O. Box 651, Hungary (Received 19 April 1994; revised 4 July 1994; accepted 31 October 1994)
Abstract--The effects of a mineral matrix on hydrocarbon generation from different types of kerogen were examined.Simulated thermal maturation of type I*, III/a and III/b kerogens, alone and mixedwith minerals, was investigated using anhydrous pyrolysis.Both calcite and montmorillonite modified the products of simulated thermal maturation of the kerogens. Montmorillonite slightly reduced, and calcite slightly enhanced the formation of oil-like products from the coals (type III/a and III/b kerogens). At higher levels of maturation, however, the catalytic effect of calcite resulted in cracking of the pyrolysates. For retention of the pyrolysis products of type I kerogen, montmorillonite proved to be a very active matrix whereas the products were less affected by calcite. The amount of soluble bitumen extracted from heated type I kerogen was higher in the presenceof minerals than in their absence. On the contrary, the amount of soluble bitumen obtained from heated type III/a kerogen was smaller in the presence of minerals. In the case of type III/b kerogens this effect of minerals changed as a function of the temperature of simulated thermal maturation. Key words--simulated maturation, mineral matrix, kerogen types, alginite, coal
INTRODUCTION It is well-known that both the amount and type of organic matter have an important role in the generation of oil and gas. The quality and quantity of petroleum formed in source rocks, as well as the kinetic parameters of hydrocarbon generation depend first of all, on the maturity and the type of kerogen (Tissot et al., 1974; Tissot and Welte, 1984; Dembicki, 1992). However, during diagenesis and catagenesis, the fate of the organic matter (dispersed O.M., oil shale, coal) may also be influenced by the inorganic matrix (Eglinton et al., 1986). Rock-Eval parameters also seem to be influenced by mineral matrix effects (Espitali6 et al., 1980, Espitali6 et al., 1984; Katz, 1983; Larter, 1984; Wilhelms et aL, 1991; Dembicki, 1992). Adsorption and the catalytic effect of minerals, or individual mineral components, are widely utilized in the chemical industry. For example montmorillonite and kaolinite have been used as catalysts for the cracking of petroleum. The effect of the interaction between inorganic and organic matter on the processes of kerogen evolution has been investigated. Some of this work examined the behaviour of individual compound classes (e.g. *Results of the effect of montmorillonite and calcite on the thermal maturation of kerogen of type I were presented at the 16th International Meeting on Organic Geochemistry, Stavanger, 1993,
carboxylic acids) with and without added minerals during pyrolysis (Jurg and Eisma, 1970; Shimoyama and Johns, 1972; Almon and Johns, 1977; Aizenshtat et al., 1984). Others illustrated the influence of minerals on the thermal alteration of the organic matter in sediments, by pyrolysis of the biological precursor material, such as the alga Botryococcus braunii, in the presence and in the absence of minerals (Douglas et al., 1970). Lu et al., (1989) attempted to recreate the diagenetic and catagenetic fates of biological markers by pyrolysis of kerogens with and without montmorillonite. Others studied the effects of minerals on the thermal maturation of organic carbon-rich samples such as oil shales and coals (Jovanricevi6 et al., 1992, Jovanricevi6 et al. 1993; Saxby et al., 1986, Saxby et al., 1992). Pyrolysis of coal macerals (vitrinite, sporinite, alginite) alone, and mixed with minerals, was used to demonstrate changes in the amount and composition of pyrolysates of different biological source materials under the influence of different minerals (Horsfield and Douglas, 1980). Numerous comparisons have been made between the pyrolysis products of whole source rocks and those of isolated kerogens (e.g. Arnosti and Mfiller, 1987; Espitali6 et al., 1980; Het6nyi, 1983; Wilhelms et al., 1991; Jovan~icevi6 et al., 1992). Most of the papers compared the results of artificial evolution of kerogen, and those of kerogen-mineral mixtures (e.g. Horsfield and Douglas, 1980; Espitali6 et al., 1984; Evans and
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Magdolna Het6nyi
Felbeck, 1983; Saxby et al., 1986; T a n n e n b a u m et al., 1986; Huizinga et al., 1987; Dembicki, 1992). M o s t a u t h o r s consider clays a n d especially m o n t m o r i l l o n i t e to be very active in terms of a d s o r p t i o n of products, a n d also in terms of their ability to crack organic matter at high temperatures. W i t h regard to calcite, opinions differ; for example Espitali6 et al. (1980); T a n n e n b a u m a n d K a p l a n (1985); T a n n e n b a u m et aL (1986) reported that calcite only slightly affected the evolutionary processes of kerogen, while others f o u n d its a d s o r p t i o n effect to be high (Evans and Felbeck, 1983; Wilhelms et al., 1991; Jovan6icevi6 et al., 1992). Applications of hydrous pyrolysis for oil generation studies, a n d artificial m a t u r a t i o n experiments are known, but a n h y d r o u s pyrolysis is also used to investigate the role of minerals in the t h e r m a l alteration of organic m a t t e r (Jovan6icevi6 et al., 1992, Jovan6icevi6 et al., 1993; Lu a n d Kaplan, 1989; T a n n e n b a u m a n d Kaplan, 1985). Interactions between a n inorganic matrix, a n d organic matter, can influence yields a n d p r o d u c t distributions which are a function of the type of kerogen, as well as the nature of the minerals. The aim of this work was to examine the p r o d u c t s of artificial m a t u r a t i o n of different types of kerogen, in the presence a n d absence of calcite a n d montmorillonite. The simulated thermal m a t u r a t i o n experiments were p l a n n e d to be performed by a n h y d r o u s a n d h y d r o u s pyrolysis. In this p a p e r the results o f the dry pyrolysis of kerogen of types I, III/a a n d I I I / b are presented.
EXPERIMENTAL
Artificial m a t u r a t i o n of type I, l l I / a a n d I I I / b kerogens, as well as their admixture with calcite or m o n t m o r i l l o n i t e (1:3 ratio) was p e r f o r m e d between 325-500"C in six steps, each for 5 h on g r o u n d samples ( d < 0 . 2 m m ) . H e a t i n g was carried out in an open system, in a t e m p e r a t u r e - p r o g r a m m e d Hereaus-type
furnace, in a c o n t i n u o u s nitrogen flow. Liquid products were collected in air- and ice-cooled traps a n d the contents were combined. The organic c o m p o n e n t s were separated from the water by solvent extraction and were regarded as 'volatilized bitumen'. Soluble organic material was isolated by Soxhlet extraction of the heated residue (benzene/acetone/ m e t h a n o l 70:15:15 v/v/v) a n d was regarded as 'soluble bitumen'. Bitumens were concentrated by rotary e v a p o r a t i o n a n d were quantified. All data were normalized to the a m o u n t of organic c a r b o n measured on the u n h e a t e d samples. Duplicate artifical m a t u r a t i o n experiments were m a d e a n d the data in Tables 1 and 2 are the average values of the two series. H y d r o g e n index, m a t u r i t y p a r a m e t e r (Tmax) a n d petroleum potential (S1 + $2) were determined by the s t a n d a r d R o c k Eval m e t h o d (Oil Show Analyzer) using the following t e m p e r a t u r e program: pyrolysis of 30-40 m g samples at 300~C for 4 min. followed by p r o g r a m m e d pyrolysis at 25°C/min to 550~C in a helium atmosphere. Organic c a r b o n content (TOC) was measured by c o m b u s t i o n ( T = 1000"C, in an a t m o s p h e r e of oxygen). Type I kerogen was isolated by sink-float m e t h o d s from Pula alginite (Het6nyi a n d Vars~.nyi, 1976). The Pula alginite was deposited in a m a a r - t y p e crater, formed by the final basaltic volcanism (3-5 × 106 yr ago) in the P a n n o n i a n Basin (J~mbor a n d Solti, 1975). The special depositional e n v i r o n m e n t in the small, enclosed, current-free and n o n a g i t a t e d lake resulted in a highly oil-prone kerogen. The organic m a t t e r originates mainly from well-preserved colonies of Botryococcus braunii. The selected bulk geochemical data of the Pula kerogen are as follows: T O C = 80.2%, Tmax 441~C, H / C atomic ratio = 1.7, HI = 952 mg H C / g TOC. Organic geochemical data of the alginite: T O C = 14.2%, Tma~= 440°C, HI = 664 m g H C / g TOC. =
Table 1. Yields of the simulated thermal maturation of kerogens and kerogen/mineral mixtures (period = 5 h) Volatilized bitumen (mg/g TOC) Soluble bitumen (mg/g TOC) Yield of pyrolysis (mg/g TOC) Type of Temperature kerogen (C) 1 2 3 1 2 3 1 2 3 1 325 14.8 7.4 0.1 11.1 27.2 40.0 26.0 34.6 40,1 350 34.7 32.2 8.7 13.5 19.2 46.9 48.2 51.4 55,6 375 54.5 47.1 9.9 12.7 26.7 40.3 67.2 73.8 50,2 400 99.1 107.8 31.0 20,4 51.7 47.6 119.5 159.5 78.6 450 215.5 282.5 135.1 <5,0 65.3 50.4 220.5 347.8 185.5 500 588.4 558.6 50t .8 < 5~0 < 5.0 6.4 593.4 558.6 508.4 Ill/a 325 7.0 10.4 9.6 66,4 40,0 43.2 73.4 50.4 52.8 350 30.0 44.0 17.6 110.0 80,0 40.0 140.0 124.0 57.6 375 44.6 57.6 20.0 61.0 28,0 24.8 105.6 85.6 44.8 400 56.0 72.0 40.0 32.0 22.4 18,4 88.0 94.4 58.4 450 76.0 80.0 53.2 5.6 <5.0 8,8 81.6 85.0 62.0 500 142.0 72.0 68.8 < 5.0 < 5.0 < 5,0 147,6 77.0 73.8 Ill/b 325 10.9 9.3 7.9 37.0 23.0 19.4 47,9 32.3 27.3 350 15.1 23.7 10.8 33.2 51.0 55.3 48,3 74.7 66.1 375 16.2 26.6 10.1 33.0 14.4 10.7 39,2 41.0 20.8 400 21.5 35.9 14.4 21.5 12.2 10.0 43.0 48,1 24.4 450 28.7 52.4 23.7 6.3 10.0 9.3 35.0 62,4 33.0 500 52.4 37.3 27.3 < 5.0 < 5.0 < 5.0 57.4 42,3 32.3 1 = kerogen; 2 = kerogen + calcite (1 :3 ratio); 3 = kerogen + montmorillonite ( 1: 3 ratio).
Simulated thermal maturation of kerogens
123
Table 2. Some Rock-Evaldata of the unheated and heated samples T ~ (°C) Type of kerogen 1
Ill/a
lll/b
H1 ( m g H C / g TOC)
Ratio of HI heated/HI initial (%)
Temperature (°C)
1
2
3
1
3
1
2
3
Unheated sample 325 350 375 400 450 500 Unheated sample 325 350 375 400 450 500 Unheated sample 325 350 375 400 450
441 442 442 444 445 502 535 406 419 425 437 447 525 n.m. 397 424 433 441 452 515
443 444 446 447 448 448 553 408 427 429 438 451 532 n.m. 399 430 437 447 484 496
440 440 440 440 443 443 529 407 423 431 437 451 526 n.m. 399 432 437 444 490 500
952 925 812 798 797 22 3 214 152 145 97 52 27 24 113 94 68 59 42 35
1007 1008 950 952 915 85 4 210 102 72 49 29 17 14 115 49 47 44 24 20
1015 975 905 727 683 561 11 212 81 73 63 33 20 20 115 50 51 39 30 23
100 97 85 84 84 2 <1 100 71 68 45 24 13 11 100 83 60 52 37 31
100 100 94 94 91 8 < 1 100 49 34 23 14 8 7 100 43 41 38 21 17
100 96 89 72 67 55 I 100 38 34 30 16 9 9 100 44 44 34 26 20
500
n.m.
n.m.
n.m.
20
17
15
18
15
13
2
1 = kerogen; 2 = kerogen+calcite (1:3 ratio); 3 = kerogen+montmorillonite (1:3 ratio); n.m. = non measurable.
Type III/a and III/b kerogens were represented by an Eocene brown coal sample, and a Miocene lignite sample, respectively. The Middle Eocene sub-bituminous coal (North Hungary) was derived from semi-terrestrial tropical vegetation. The predominant members of the coal-forming plant assemblage were palms. Organic geochemical data of the examined brown coal are as follows: TOC = 50.02%, Tmax --- 406°C, HI = 214 mg HC/g TOC, Ro = 0.39%, H/C atomic ratio = 1.1. The Middle Miocene lignite, containing type III/b kerogen, accumulated in a limnic environment at the margin of the Pannonian Basin under warm climatic conditions. Remnants of Taxodiaceae-Cupressaceae paludal forests were identified by palynological examination. Selected bulk geochemical data are as follows: TOC = 55.7%, Tmax= 397°C, HI = 113 mg HC/g TOC, Ro = 0.26%, H/C atomic ratio = 0.9. Commercial bentonite, composed mainly of montmorillonite (more than 96%) with trace amounts of quartz and kaolinite, was used. Both the calcite and the bentonite were ground to the same mesh size as the alginite and coals (d<0.2 mm). Neither calcite nor bentonite contained organic carbon.
RESULTS AND DISCUSSION
Type I kerogen The organic matter, isolated from the alginite, proved to be a highly oil-prone kerogen. The pyrolysate yield (regarded as volatilized bitumen) gradually increased as a function of temperature up to 450°C. Between 450 and 500°C a rapid increase was detected, and at 500°C the amount of the volatilized bitumen reached 588 mg/g TOC (Table 1). On the basis of these results, the montmorillonite essentially influenced the thermal degradation of the type I
kerogen. About 2-4 times more bitumen could be extracted from 1 g kerogen heated with montmorillonite, than without it (Table 1). As can be seen in Fig. 1 the amount of bitumen trapped on the kerogen/ montmorillonite mixture was practically unchanged up to 450°C. At a higher temperature, as a consequence of the thermocatalytic effect of montmorillonite, the difference between the amount of soluble bitumen produced from kerogen, with and without admixed montmorillonite, disappeared. At all pyrolysis temperatures, the presence of montmorillonite reduced the amount of volatilized bitumen. However, as evolution advanced the difference was smaller, than at the beginning of maturation (Fig. 1). At 325 and 350°C the thermal degradation of type I kerogen resulted in higher total yields (soluble plus volatilized bitumen) in the presence of montmorillonite than in its absence. On the other hand, between 375 and 500°C, yields from the artificial maturation of kerogen alone exceeded that of the kerogen-montmorillonite mixture (Table 1). Calcite proved to be a less efficient thermocatalytic mineral than montmorillonite. Simulated thermal maturation of type I kerogen resulted in similar quantities of volatilized bitumen in the absence and presence of calcite. On the other hand, the increased amount of soluble bitumen shows the moderate retention effect of calcium carbonate, which seems to depend strongly on the pyrolysis temperature (Table 1 and Fig. 1). The most pronounced effect is observed at 450°C, where a substantial difference can be seen in the amount of soluble bitumen obtained from kerogen with and without calcite (Fig. 1). T ~ measured on heated pure kerogen remained roughly constant up to 400°C and somewhat increased from 400 to 500°C. At the same time, a moderate decrease in the hydrogen index up to 400°C (26%) was followed by a very rapid depletion between 400 and
Magdolna Het6nyi
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450'~C (82%) (Table 2). The trend of HI vs Tm~, obtained by simulated thermal maturation of kerogen (Fig. 2) seemed to be correlated with the HI vs T~,~ diagram of a natural maturation series of type I kerogen (Bordenave et al., 1993). Both minerals only slightly modified the trend of HI vs Tm,~ (Fig. 2).
Type lll/a kerogen The Middle Eocene sub-bituminous coal contains kerogen with moderate petroleum potential (ll0.6mg/g). It is composed of type III/a kerogen (HI = 214 mg HC/g TOC, Tm~x= 406°C, H/C atomic ratio = 1.1). Simulated maturation resulted in a smaller amount of the volatilized bitumen, than from type I kerogen and this effect was most pronounced at higher temperatures (Table 1). Between 325 and 375°C amounts differed slightly. The ratios of the volatilized bitumens, formed by heating type I and III/a kerogens were 1.8, 2.8 and 4.l at 400, 450 and 500°C,
respectively. Conversely the quantity of soluble bitumen extracted from type III/a kerogen exceeded that of type I. An especially high amount of soluble bitumen ( l l 0 m g / g TOC) was measured after simulated thermal maturation at 350°C. Montmorillonite influenced the yields from pyrolysis of type III/a kerogen significantly, reducing the amount of both volatilized and soluble bitumen (Fig. 1). During heating at 350-450°C, 1.4-2.2 times more oil-like products were obtained from 1 g kerogen alone than in the presence of montmorillonite (Table 1). The ratios of the amount of soluble bitumen extracted from 1 g kerogen, alone and with montmorillonite, were 1.5, 2.8, 2.5 and 1.8 at 325,350, 375 and 400°C, respectively. Below 500°C calcite seemed to promote slightly the formation of volatilized bitumen, and at all temperatures smaller amount of soluble bitumen was obtained in the presence of calcite than in its absence
Yield of thermal degradation (mg/g TOC) 100 300 5O0 100 I00 I lll/b III/a
400 ' ! ~ ~ o
o ~
~
',~i/
~f 500 -- ' " ~ ' ~
~
'
400 .o
500
500 ~ m
"Solublebitumen" [ ' ~ "Volatilizedbitumen"
Fig. 1. Yields of the simulated maturation products of type I, IIl/a, Ill/b kerogens in the absence (1) and in the presence of calcite (2) and montmorillonite (3).
Simulated thermal maturation of kerogens Type I kerogen
1000
~x ' { . ~
--.~ ~x~ Kerogen ;~1 - - e - • Kerogen + calcite Et%lN~ . . . . ~ - - - Kerogen + bentonite i - - I \ (450°) = temperature of the -- ? ' I ' . . _ ~ simulated maturation 500 I450")~ ""..N~. v
i
500
550
Fig. 2. Experimental evolution path (HI vs Tm,,diagram) of type I kerogen in the presence and in the absence of minerals.
(Fig. 1). At 500°C twice as much volatilized bitumen was produced from 1 g of type III/a kerogen without minerals, than with calcite or montmoriUonite. Both calcite and montmorillonite modified the pyrolysis products of type III/a kerogen to a lesser extent than that of type I kerogen. The artifical evolution path (HI vs Tmaxdiagram) of pure kerogen, and that of kerogen/mineral mixtures, hardly differ from each other (Fig. 3).In contrast to type I kerogen, heating type III/a kerogen gave decreasing hydrogen indices at the beginning of the experiment (325°C).
Type III/b kerogen The Middle Miocene lignite contains kerogen with a low petroleum potential (67.1 mg/g), and thermal maturation resulted in a small a m o u n t of the oil-like products. The quantity of the volatilized bitumen gradually increased, as a function of the temperature, up to 450°C and almost doubled between 450 and 500°C (Table 1 and Fig. 1). At the same time, less soluble bitumen could be extracted from the heated lignite (type III/b kerogen), than from the sub-bituminous coal (type III/a kerogen). After initial (practically) constant values ( T < 375°C) the quantity Type lll/a kerogen
(490o) ~L..
7
100 --
~:--Kerogen ..... Kerogen + calcite . . . . m.--Kerogen + bentonite , )¢ ? \(450\ ) = temperature of the simulated maturation °. "x
"~,,~ -e12
I
I
J'~""×
400
450
500
Tmax (°C) Fig. 4. Experimental evolution path (HI vs Tm~ diagram) of type IlI/b kerogen in the presence and in the absence of minerals.
of soluble bitumen decreased abruptly with increasing temperature (400-500°C) (Fig. 1). The volatile and soluble bitumen yields ofpyrolysed type III/b kerogen was modified by both of the minerals. The quantity of soluble bitumen extracted from 1 g kerogen changed as a function of temperature, and was practically independent of the nature of the minerals (Fig. 1). The moderate retention effect detected at 350°C, was followed between 350 and 375°C by a decrease in the soluble material content which thereafter remained roughly unchanged until 450°C. Nevertheless, some slight difference was noted between the effect of the two minerals. While during simulated maturation with montmorillonite at 375, 400 and 450°C the amounts of the soluble bitumen were practically unchanged, with calcite a slight decrease was detected (Fig. 1). The role of the two minerals was reversed concerning the quantity of volatilized bitumen. In the zone of catagenesis montmorillonite showed a moderate retention, whereas calcite promoted the formation of the volatilized bitumen (Fig. 1). The H1 vs Tm~xdiagram for heated type III/b kerogen revealed that the minerals slightly accelerated the simulated thermal maturation (Fig. 4).
~'x
R
£ r~
"o',~.x~ 400
450
(450°) 500
rmax (°C) Fig. 3. Experimental evolution path (HI vs T ~ diagram) of type Ill/a kerogen in the presence and in the absence of minerals. 1243 2 3 / 2 ~ "
'~
(4?0°)
Tmax (°C)
"r.
5o -
X "'/*.
440
200-
L) R
~
Type lll/b kerogen
~x~ Kerogen • x v. % - - e - . Kerogen + calcite v,\°° °°°-[~,-°° Kerogen + bentonite v.\*. X (400°) = temperature of the \.. \ simulated maturation
~
%,,,,o°,
450°)1
100 -
125
CONCLUSIONS Both calcite and montmorillonite modified the processes, and yields, from the thermal maturation of kerogens, but their effects differed from each other and depended on the type of kerogens : 1. Montmorillonite proved to be a very active mineral which essentially influenced the yield, and evolution, of organic matter. Owing to its retention effect, smaller yields of both soluble and volatilized bitumen were measured during the pyrolysis of type III/a and III/b kerogens in the presence of montmorillonite than with pure kerogen. On the other hand, with type I kerogen, a higher a m o u n t of soluble
Magdolna Het6nyi
126
bitumen, a n d a smaller a m o u n t o f volatilized bitumen, was o b t a i n e d from type I kerogen plus m o n t m o r i l lonite t h a n with kerogen alone. 2. Calcite seemed to be a less efficient mineral with respect to type I kerogen, t h a n montmorillonite. Pyrolysis resulted in volatilized b i t u m e n of similar quantity, a n d soluble b i t u m e n o f s o m e w h a t higher quantity, in the presence of calcite t h a n in its absence. F o r type III/a a n d I I I / b kerogens, in c o n t r a s t to m o n t m o r i l l o n i t e , calcite p r o m o t e d the f o r m a t i o n o f the volatilized b i t u m e n up to 450°C. A t 500°C, however, the a m o u n t of volatilized b i t u m e n o b t a i n e d from type I I l / a kerogen was less ( - 5 0 % ) in the presence o f b o t h minerals, t h a n in their absence. In the case of type I I I / b kerogen the two minerals acted differently. While the a m o u n t of volatilized b i t u m e n o b t a i n e d from type I I I / b kerogen, at 500°C, was reduced by 50% in the presence of m o n t m o r i l l o n i t e , it was only 2 9 % in the presence of calcite. 3. C o m p a r i n g the role of the two minerals in controlling the a m o u n t of the soluble b i t u m e n produced o n pyrolysis it can be stated that, (a) for a given kerogen, their retention or catalytic effects were similar, but differed in extent, which, (b) d e p e n d e d o n the type of kerogen. Minerals increased the a m o u n t of soluble b i t u m e n o b t a i n e d after t h e r m a l d e g r a d a t i o n of type I kerogen, a n d decreased it for type III/a kerogen. F o r type I I I / b kerogen this mineral effect changed as a function o f temperature. Associate E d i t o r - - M . Vandenbroucke Acknowledgements--This work was supported in part by the Hungarian Scientific Research Foundation (OTKA), grant No. T 007429. The author would like to thank to the reviewers of this manuscript, Dr A. Lewis and one other, for their constructive and helpful comments. Thanks are expressed for the useful suggestions of Dr M. Vandenbroucke. REFERENCES
Aizenshtat Z., Miloslavsky 1. and Heller-Kallai L.(1984)The effect of montmorillonite on the thermal decomposition of fatty acids under 'bulk flow' conditions. Org. Geochem. 7, 85-90. Almon W. R. and Johns W. D. (1977) Petroleum forming reactions: the mechanism and rate of clay catalyzed fatty acid decarboxylation. In Advances in Organic Geochemistry 1975 (Edited by Campos R. and Goni J.), pp. 157-171. ENADIMSA, Madrid. Arnosti C. and Miiller P. J. (1987) Pyrolysis-GC characterization of whole rock and kerogen-concentrate samples of immature Jurassic source rocks from NW-Germany. Org. Geochem. 11, 505-512. Bordenave M. L., Espitali6 J., Leplat P., Oudin J. L. and Vandenbroucke M. (1993) Screening Techniques for Source Rock Evaluation. In Applied Petroleum Geochemistry (Edited by Bordenave M. L.), pp. 217-278. Editions Technip, Paris. Dembicki H, Jr (1992) The effects of the mineral matrix on the determination of kinetic parameters using modified Rock Eval pyrolysis. Org. Geoehem. 18, 531-539. Douglas A. G., Eglinton G. and Henderson W. (1970) Thermal alteration of the organic matter in sediments.
Advances in Organic Geochemistry 1966 (Edited by Hobson G. D. and Speers G. C.), 369-388. Pergamon Press, Oxford. Eglinton T. J., Rowland S. J., Curtis C. D. and Douglas A. G. (1986) Kerogen-mineral reactions at raised temperatures in the presence of water. Org.Geochem. 10, 1041-1052. Espitali6 J., Madec M, and Tissot B. (1980) Role of mineral matrix in kerogen pyrolysis: influence on petroleum generation and migration. Bull. Am. Assoc. Pet. Geol., 64, 59-66. Espitali6 J., Senga Makadi K. and Trichet J. (1984) Role of the mineral matrix during kerogen pyrolysis. Org.Geochem. 6, 365-382. Evans R. J. and Felbeck G. T. Jr (1983) High temperature simulation of petroleum formation-II. Effect of inorganic sedimentary constituents of hydrocarbon formation. Org.Geochem. 4, 145-152. Het6nyi M. and Vars~inyi I. (1976) Contribution to the isolation of the kerogen in Hungarian oil shales. Acta Miner.Petr. Szeged XXII/2, 231-239. Het6nyi M. (1983) Experimental evolution of oil shales and kerogen isolated from them. Acta Miner.Petr. Szeged XXVI/1, 73-85. Horsfield B. and Douglas A. G. (1980) The influence of minerals on the pyrolysis of kerogens. Geochim. Cosmochim. Acta 44, 1119-1131. Huizinga B. J., Tannenbaum E. and Kaplan 1. R. (1987) The role of minerals in the thermal alteration of organic matter III. Generation of bitumen in laboratory experiments. Org. Geochem. 11, 591-604. J~imbor ,~. and Solti G. (1975) Geological conditions of the Upper Pannonian oil shale deposit recovered in the Balaton Higland and at Kemenesh~it. Acta Miner. Petr. Szeged XXII, 9-28. Jovan~icevi6 B., Vitorovi6 D., Saban M. and Wehner H. (1992) Evaluation of the effects of native minerals on the organic matter of Aleksinac oil shale based on the composition of free and bound bitumens. Org. Geochem. 18, 511-519. Jovan6icevi6 B., Vu6eli6 D., Saban M., Wehner H. and Vitorovi6 D. (1993) Investigation of the catalytic effects of indigenous minerals in the pyrolysis of Aleksinac oil shale substrates: steranes, triterpenes and triaromatic steroids in the pyrolysates. Org. Geochem. 20, 69-76. Jurg J.W. and Eisma E. (1970) The mechanism of the generation of petroleum hydrocarbons from a fatty acid. In Advances in Organic Geochemist o, 1966 (Edited by Hobson G. D. and Speers G. C.), pp. 367-368. Pergamon Press, Oxford. Katz B.J. (1983) Limitation of Rock Eval pyrolysis for typing organic matter. Org. Geochem. 4, 195-199. Larter S. (1984) Application of analytical pyrolysis techniques to kerogen characterization and fossil fuel exploration/exploitation. In Analytical Pyrolysis: Methods and Application (Edited by Voorhees K.), pp. 212-275. Butterworths, London. Lu, S.-T., Ruth, E. and Kaplan, I. R., (1989) Pyrolysis of kerogens in the absence and presence of montmorillonite -I. The generation, degradation and isomerisation of steranes and triterpanes at 200 and 300C. Org. Geochem. 14, 491-499. Lu S.-T. and Kaplan i. R. (1989) Pyrolysis of kerogens in the absence and presence of montmorillonite-II. Aromatic hydrocarbons generated at 200 and 300:'C. Org. Geochem. 14, 501-510. Saxby J. D., Bennett A. J. R., Corcoran J.F., Lambert D. E. and Riley K. W. (1986) Petroleum generation: simulation over six years of hydrocarbon formation from torbanite and brown coal in a subsiding basin. Org. Geochem. 9, 69-81. Saxby J. D., Chatfield P., Taylor G. H., Fitzgerald J. D., Kaplan I. R. and Lu S.-T. (1992) Effect of clay minerals on
Simulated thermal maturation of kerogens products from coal maturation. Org. Geochem. 18, 373-383. Shimoyama A. and Johns W. D. (1972) Formation ofalkanes from fatty acids in the presence of CaCO3. Geochim. Cosmochim. Acta 36, 87-91. Tannenbaum E. and Kaplan I. R. (1985) Role of minerals in the thermal alteration of organic matter-I. Generation of gases and condensates under dry condition. Geochim. Cosmochim. Aeta 49, 2589-2604. Tannenbaum E., Huizinga B. J. and Kaplan I. R. (1986) Role of minerals in thermal alteration of organic matter--II: A
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1156-1165. Tissot B., Durand B., Espitali6 J. and Combaz A. (1974) Influence of the nature and diagenesis of organic matter in formation of petroleum. Bull. Am. Assoc. Pet. Geol. 58, 499-506. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, 2nd Edn. Springer, Berlin. Wilhelms A., Larter S. R. and Leythaeuser D. (1991) Influence of bitumen-2 on Rock Eval pyrolysis. Org. Geochem. 17, 351-354.
Pergamon
0146-6380(94)00120-0
Org. Geochem. Vol. 23, No. 2, pp. 121-127, 1995 Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0146-6380/95$9.50+ 0.00
Simulated thermal maturation of type I and III kerogens in the presence, and absence, of calcite and montmorillonite M A G D O L N A HETI~NYI Institute of Mineralogy, Geochemistry and Petrology, Attila Jrzsef University, H-6701 Szeged, P.O. Box 651, Hungary (Received 19 April 1994; revised 4 July 1994; accepted 31 October 1994)
Abstract--The effects of a mineral matrix on hydrocarbon generation from different types of kerogen were examined.Simulated thermal maturation of type I*, III/a and III/b kerogens, alone and mixedwith minerals, was investigated using anhydrous pyrolysis.Both calcite and montmorillonite modified the products of simulated thermal maturation of the kerogens. Montmorillonite slightly reduced, and calcite slightly enhanced the formation of oil-like products from the coals (type III/a and III/b kerogens). At higher levels of maturation, however, the catalytic effect of calcite resulted in cracking of the pyrolysates. For retention of the pyrolysis products of type I kerogen, montmorillonite proved to be a very active matrix whereas the products were less affected by calcite. The amount of soluble bitumen extracted from heated type I kerogen was higher in the presenceof minerals than in their absence. On the contrary, the amount of soluble bitumen obtained from heated type III/a kerogen was smaller in the presence of minerals. In the case of type III/b kerogens this effect of minerals changed as a function of the temperature of simulated thermal maturation. Key words--simulated maturation, mineral matrix, kerogen types, alginite, coal
INTRODUCTION It is well-known that both the amount and type of organic matter have an important role in the generation of oil and gas. The quality and quantity of petroleum formed in source rocks, as well as the kinetic parameters of hydrocarbon generation depend first of all, on the maturity and the type of kerogen (Tissot et al., 1974; Tissot and Welte, 1984; Dembicki, 1992). However, during diagenesis and catagenesis, the fate of the organic matter (dispersed O.M., oil shale, coal) may also be influenced by the inorganic matrix (Eglinton et al., 1986). Rock-Eval parameters also seem to be influenced by mineral matrix effects (Espitali6 et al., 1980, Espitali6 et al., 1984; Katz, 1983; Larter, 1984; Wilhelms et aL, 1991; Dembicki, 1992). Adsorption and the catalytic effect of minerals, or individual mineral components, are widely utilized in the chemical industry. For example montmorillonite and kaolinite have been used as catalysts for the cracking of petroleum. The effect of the interaction between inorganic and organic matter on the processes of kerogen evolution has been investigated. Some of this work examined the behaviour of individual compound classes (e.g. *Results of the effect of montmorillonite and calcite on the thermal maturation of kerogen of type I were presented at the 16th International Meeting on Organic Geochemistry, Stavanger, 1993,
carboxylic acids) with and without added minerals during pyrolysis (Jurg and Eisma, 1970; Shimoyama and Johns, 1972; Almon and Johns, 1977; Aizenshtat et al., 1984). Others illustrated the influence of minerals on the thermal alteration of the organic matter in sediments, by pyrolysis of the biological precursor material, such as the alga Botryococcus braunii, in the presence and in the absence of minerals (Douglas et al., 1970). Lu et al., (1989) attempted to recreate the diagenetic and catagenetic fates of biological markers by pyrolysis of kerogens with and without montmorillonite. Others studied the effects of minerals on the thermal maturation of organic carbon-rich samples such as oil shales and coals (Jovanricevi6 et al., 1992, Jovanricevi6 et al. 1993; Saxby et al., 1986, Saxby et al., 1992). Pyrolysis of coal macerals (vitrinite, sporinite, alginite) alone, and mixed with minerals, was used to demonstrate changes in the amount and composition of pyrolysates of different biological source materials under the influence of different minerals (Horsfield and Douglas, 1980). Numerous comparisons have been made between the pyrolysis products of whole source rocks and those of isolated kerogens (e.g. Arnosti and Mfiller, 1987; Espitali6 et al., 1980; Het6nyi, 1983; Wilhelms et al., 1991; Jovan~icevi6 et al., 1992). Most of the papers compared the results of artificial evolution of kerogen, and those of kerogen-mineral mixtures (e.g. Horsfield and Douglas, 1980; Espitali6 et al., 1984; Evans and
121
122
Magdolna Het6nyi
Felbeck, 1983; Saxby et al., 1986; T a n n e n b a u m et al., 1986; Huizinga et al., 1987; Dembicki, 1992). M o s t a u t h o r s consider clays a n d especially m o n t m o r i l l o n i t e to be very active in terms of a d s o r p t i o n of products, a n d also in terms of their ability to crack organic matter at high temperatures. W i t h regard to calcite, opinions differ; for example Espitali6 et al. (1980); T a n n e n b a u m a n d K a p l a n (1985); T a n n e n b a u m et aL (1986) reported that calcite only slightly affected the evolutionary processes of kerogen, while others f o u n d its a d s o r p t i o n effect to be high (Evans and Felbeck, 1983; Wilhelms et al., 1991; Jovan6icevi6 et al., 1992). Applications of hydrous pyrolysis for oil generation studies, a n d artificial m a t u r a t i o n experiments are known, but a n h y d r o u s pyrolysis is also used to investigate the role of minerals in the t h e r m a l alteration of organic m a t t e r (Jovan6icevi6 et al., 1992, Jovan6icevi6 et al., 1993; Lu a n d Kaplan, 1989; T a n n e n b a u m a n d Kaplan, 1985). Interactions between a n inorganic matrix, a n d organic matter, can influence yields a n d p r o d u c t distributions which are a function of the type of kerogen, as well as the nature of the minerals. The aim of this work was to examine the p r o d u c t s of artificial m a t u r a t i o n of different types of kerogen, in the presence a n d absence of calcite a n d montmorillonite. The simulated thermal m a t u r a t i o n experiments were p l a n n e d to be performed by a n h y d r o u s a n d h y d r o u s pyrolysis. In this p a p e r the results o f the dry pyrolysis of kerogen of types I, III/a a n d I I I / b are presented.
EXPERIMENTAL
Artificial m a t u r a t i o n of type I, l l I / a a n d I I I / b kerogens, as well as their admixture with calcite or m o n t m o r i l l o n i t e (1:3 ratio) was p e r f o r m e d between 325-500"C in six steps, each for 5 h on g r o u n d samples ( d < 0 . 2 m m ) . H e a t i n g was carried out in an open system, in a t e m p e r a t u r e - p r o g r a m m e d Hereaus-type
furnace, in a c o n t i n u o u s nitrogen flow. Liquid products were collected in air- and ice-cooled traps a n d the contents were combined. The organic c o m p o n e n t s were separated from the water by solvent extraction and were regarded as 'volatilized bitumen'. Soluble organic material was isolated by Soxhlet extraction of the heated residue (benzene/acetone/ m e t h a n o l 70:15:15 v/v/v) a n d was regarded as 'soluble bitumen'. Bitumens were concentrated by rotary e v a p o r a t i o n a n d were quantified. All data were normalized to the a m o u n t of organic c a r b o n measured on the u n h e a t e d samples. Duplicate artifical m a t u r a t i o n experiments were m a d e a n d the data in Tables 1 and 2 are the average values of the two series. H y d r o g e n index, m a t u r i t y p a r a m e t e r (Tmax) a n d petroleum potential (S1 + $2) were determined by the s t a n d a r d R o c k Eval m e t h o d (Oil Show Analyzer) using the following t e m p e r a t u r e program: pyrolysis of 30-40 m g samples at 300~C for 4 min. followed by p r o g r a m m e d pyrolysis at 25°C/min to 550~C in a helium atmosphere. Organic c a r b o n content (TOC) was measured by c o m b u s t i o n ( T = 1000"C, in an a t m o s p h e r e of oxygen). Type I kerogen was isolated by sink-float m e t h o d s from Pula alginite (Het6nyi a n d Vars~.nyi, 1976). The Pula alginite was deposited in a m a a r - t y p e crater, formed by the final basaltic volcanism (3-5 × 106 yr ago) in the P a n n o n i a n Basin (J~mbor a n d Solti, 1975). The special depositional e n v i r o n m e n t in the small, enclosed, current-free and n o n a g i t a t e d lake resulted in a highly oil-prone kerogen. The organic m a t t e r originates mainly from well-preserved colonies of Botryococcus braunii. The selected bulk geochemical data of the Pula kerogen are as follows: T O C = 80.2%, Tmax 441~C, H / C atomic ratio = 1.7, HI = 952 mg H C / g TOC. Organic geochemical data of the alginite: T O C = 14.2%, Tma~= 440°C, HI = 664 m g H C / g TOC. =
Table 1. Yields of the simulated thermal maturation of kerogens and kerogen/mineral mixtures (period = 5 h) Volatilized bitumen (mg/g TOC) Soluble bitumen (mg/g TOC) Yield of pyrolysis (mg/g TOC) Type of Temperature kerogen (C) 1 2 3 1 2 3 1 2 3 1 325 14.8 7.4 0.1 11.1 27.2 40.0 26.0 34.6 40,1 350 34.7 32.2 8.7 13.5 19.2 46.9 48.2 51.4 55,6 375 54.5 47.1 9.9 12.7 26.7 40.3 67.2 73.8 50,2 400 99.1 107.8 31.0 20,4 51.7 47.6 119.5 159.5 78.6 450 215.5 282.5 135.1 <5,0 65.3 50.4 220.5 347.8 185.5 500 588.4 558.6 50t .8 < 5~0 < 5.0 6.4 593.4 558.6 508.4 Ill/a 325 7.0 10.4 9.6 66,4 40,0 43.2 73.4 50.4 52.8 350 30.0 44.0 17.6 110.0 80,0 40.0 140.0 124.0 57.6 375 44.6 57.6 20.0 61.0 28,0 24.8 105.6 85.6 44.8 400 56.0 72.0 40.0 32.0 22.4 18,4 88.0 94.4 58.4 450 76.0 80.0 53.2 5.6 <5.0 8,8 81.6 85.0 62.0 500 142.0 72.0 68.8 < 5.0 < 5.0 < 5,0 147,6 77.0 73.8 Ill/b 325 10.9 9.3 7.9 37.0 23.0 19.4 47,9 32.3 27.3 350 15.1 23.7 10.8 33.2 51.0 55.3 48,3 74.7 66.1 375 16.2 26.6 10.1 33.0 14.4 10.7 39,2 41.0 20.8 400 21.5 35.9 14.4 21.5 12.2 10.0 43.0 48,1 24.4 450 28.7 52.4 23.7 6.3 10.0 9.3 35.0 62,4 33.0 500 52.4 37.3 27.3 < 5.0 < 5.0 < 5.0 57.4 42,3 32.3 1 = kerogen; 2 = kerogen + calcite (1 :3 ratio); 3 = kerogen + montmorillonite ( 1: 3 ratio).
Simulated thermal maturation of kerogens
123
Table 2. Some Rock-Evaldata of the unheated and heated samples T ~ (°C) Type of kerogen 1
Ill/a
lll/b
H1 ( m g H C / g TOC)
Ratio of HI heated/HI initial (%)
Temperature (°C)
1
2
3
1
3
1
2
3
Unheated sample 325 350 375 400 450 500 Unheated sample 325 350 375 400 450 500 Unheated sample 325 350 375 400 450
441 442 442 444 445 502 535 406 419 425 437 447 525 n.m. 397 424 433 441 452 515
443 444 446 447 448 448 553 408 427 429 438 451 532 n.m. 399 430 437 447 484 496
440 440 440 440 443 443 529 407 423 431 437 451 526 n.m. 399 432 437 444 490 500
952 925 812 798 797 22 3 214 152 145 97 52 27 24 113 94 68 59 42 35
1007 1008 950 952 915 85 4 210 102 72 49 29 17 14 115 49 47 44 24 20
1015 975 905 727 683 561 11 212 81 73 63 33 20 20 115 50 51 39 30 23
100 97 85 84 84 2 <1 100 71 68 45 24 13 11 100 83 60 52 37 31
100 100 94 94 91 8 < 1 100 49 34 23 14 8 7 100 43 41 38 21 17
100 96 89 72 67 55 I 100 38 34 30 16 9 9 100 44 44 34 26 20
500
n.m.
n.m.
n.m.
20
17
15
18
15
13
2
1 = kerogen; 2 = kerogen+calcite (1:3 ratio); 3 = kerogen+montmorillonite (1:3 ratio); n.m. = non measurable.
Type III/a and III/b kerogens were represented by an Eocene brown coal sample, and a Miocene lignite sample, respectively. The Middle Eocene sub-bituminous coal (North Hungary) was derived from semi-terrestrial tropical vegetation. The predominant members of the coal-forming plant assemblage were palms. Organic geochemical data of the examined brown coal are as follows: TOC = 50.02%, Tmax --- 406°C, HI = 214 mg HC/g TOC, Ro = 0.39%, H/C atomic ratio = 1.1. The Middle Miocene lignite, containing type III/b kerogen, accumulated in a limnic environment at the margin of the Pannonian Basin under warm climatic conditions. Remnants of Taxodiaceae-Cupressaceae paludal forests were identified by palynological examination. Selected bulk geochemical data are as follows: TOC = 55.7%, Tmax= 397°C, HI = 113 mg HC/g TOC, Ro = 0.26%, H/C atomic ratio = 0.9. Commercial bentonite, composed mainly of montmorillonite (more than 96%) with trace amounts of quartz and kaolinite, was used. Both the calcite and the bentonite were ground to the same mesh size as the alginite and coals (d<0.2 mm). Neither calcite nor bentonite contained organic carbon.
RESULTS AND DISCUSSION
Type I kerogen The organic matter, isolated from the alginite, proved to be a highly oil-prone kerogen. The pyrolysate yield (regarded as volatilized bitumen) gradually increased as a function of temperature up to 450°C. Between 450 and 500°C a rapid increase was detected, and at 500°C the amount of the volatilized bitumen reached 588 mg/g TOC (Table 1). On the basis of these results, the montmorillonite essentially influenced the thermal degradation of the type I
kerogen. About 2-4 times more bitumen could be extracted from 1 g kerogen heated with montmorillonite, than without it (Table 1). As can be seen in Fig. 1 the amount of bitumen trapped on the kerogen/ montmorillonite mixture was practically unchanged up to 450°C. At a higher temperature, as a consequence of the thermocatalytic effect of montmorillonite, the difference between the amount of soluble bitumen produced from kerogen, with and without admixed montmorillonite, disappeared. At all pyrolysis temperatures, the presence of montmorillonite reduced the amount of volatilized bitumen. However, as evolution advanced the difference was smaller, than at the beginning of maturation (Fig. 1). At 325 and 350°C the thermal degradation of type I kerogen resulted in higher total yields (soluble plus volatilized bitumen) in the presence of montmorillonite than in its absence. On the other hand, between 375 and 500°C, yields from the artificial maturation of kerogen alone exceeded that of the kerogen-montmorillonite mixture (Table 1). Calcite proved to be a less efficient thermocatalytic mineral than montmorillonite. Simulated thermal maturation of type I kerogen resulted in similar quantities of volatilized bitumen in the absence and presence of calcite. On the other hand, the increased amount of soluble bitumen shows the moderate retention effect of calcium carbonate, which seems to depend strongly on the pyrolysis temperature (Table 1 and Fig. 1). The most pronounced effect is observed at 450°C, where a substantial difference can be seen in the amount of soluble bitumen obtained from kerogen with and without calcite (Fig. 1). T ~ measured on heated pure kerogen remained roughly constant up to 400°C and somewhat increased from 400 to 500°C. At the same time, a moderate decrease in the hydrogen index up to 400°C (26%) was followed by a very rapid depletion between 400 and
Magdolna Het6nyi
124
450'~C (82%) (Table 2). The trend of HI vs Tm~, obtained by simulated thermal maturation of kerogen (Fig. 2) seemed to be correlated with the HI vs T~,~ diagram of a natural maturation series of type I kerogen (Bordenave et al., 1993). Both minerals only slightly modified the trend of HI vs Tm,~ (Fig. 2).
Type lll/a kerogen The Middle Eocene sub-bituminous coal contains kerogen with moderate petroleum potential (ll0.6mg/g). It is composed of type III/a kerogen (HI = 214 mg HC/g TOC, Tm~x= 406°C, H/C atomic ratio = 1.1). Simulated maturation resulted in a smaller amount of the volatilized bitumen, than from type I kerogen and this effect was most pronounced at higher temperatures (Table 1). Between 325 and 375°C amounts differed slightly. The ratios of the volatilized bitumens, formed by heating type I and III/a kerogens were 1.8, 2.8 and 4.l at 400, 450 and 500°C,
respectively. Conversely the quantity of soluble bitumen extracted from type III/a kerogen exceeded that of type I. An especially high amount of soluble bitumen ( l l 0 m g / g TOC) was measured after simulated thermal maturation at 350°C. Montmorillonite influenced the yields from pyrolysis of type III/a kerogen significantly, reducing the amount of both volatilized and soluble bitumen (Fig. 1). During heating at 350-450°C, 1.4-2.2 times more oil-like products were obtained from 1 g kerogen alone than in the presence of montmorillonite (Table 1). The ratios of the amount of soluble bitumen extracted from 1 g kerogen, alone and with montmorillonite, were 1.5, 2.8, 2.5 and 1.8 at 325,350, 375 and 400°C, respectively. Below 500°C calcite seemed to promote slightly the formation of volatilized bitumen, and at all temperatures smaller amount of soluble bitumen was obtained in the presence of calcite than in its absence
Yield of thermal degradation (mg/g TOC) 100 300 5O0 100 I00 I lll/b III/a
400 ' ! ~ ~ o
o ~
~
',~i/
~f 500 -- ' " ~ ' ~
~
'
400 .o
500
500 ~ m
"Solublebitumen" [ ' ~ "Volatilizedbitumen"
Fig. 1. Yields of the simulated maturation products of type I, IIl/a, Ill/b kerogens in the absence (1) and in the presence of calcite (2) and montmorillonite (3).
Simulated thermal maturation of kerogens Type I kerogen
1000
~x ' { . ~
--.~ ~x~ Kerogen ;~1 - - e - • Kerogen + calcite Et%lN~ . . . . ~ - - - Kerogen + bentonite i - - I \ (450°) = temperature of the -- ? ' I ' . . _ ~ simulated maturation 500 I450")~ ""..N~. v
i
500
550
Fig. 2. Experimental evolution path (HI vs Tm,,diagram) of type I kerogen in the presence and in the absence of minerals.
(Fig. 1). At 500°C twice as much volatilized bitumen was produced from 1 g of type III/a kerogen without minerals, than with calcite or montmoriUonite. Both calcite and montmorillonite modified the pyrolysis products of type III/a kerogen to a lesser extent than that of type I kerogen. The artifical evolution path (HI vs Tmaxdiagram) of pure kerogen, and that of kerogen/mineral mixtures, hardly differ from each other (Fig. 3).In contrast to type I kerogen, heating type III/a kerogen gave decreasing hydrogen indices at the beginning of the experiment (325°C).
Type III/b kerogen The Middle Miocene lignite contains kerogen with a low petroleum potential (67.1 mg/g), and thermal maturation resulted in a small a m o u n t of the oil-like products. The quantity of the volatilized bitumen gradually increased, as a function of the temperature, up to 450°C and almost doubled between 450 and 500°C (Table 1 and Fig. 1). At the same time, less soluble bitumen could be extracted from the heated lignite (type III/b kerogen), than from the sub-bituminous coal (type III/a kerogen). After initial (practically) constant values ( T < 375°C) the quantity Type lll/a kerogen
(490o) ~L..
7
100 --
~:--Kerogen ..... Kerogen + calcite . . . . m.--Kerogen + bentonite , )¢ ? \(450\ ) = temperature of the simulated maturation °. "x
"~,,~ -e12
I
I
J'~""×
400
450
500
Tmax (°C) Fig. 4. Experimental evolution path (HI vs Tm~ diagram) of type IlI/b kerogen in the presence and in the absence of minerals.
of soluble bitumen decreased abruptly with increasing temperature (400-500°C) (Fig. 1). The volatile and soluble bitumen yields ofpyrolysed type III/b kerogen was modified by both of the minerals. The quantity of soluble bitumen extracted from 1 g kerogen changed as a function of temperature, and was practically independent of the nature of the minerals (Fig. 1). The moderate retention effect detected at 350°C, was followed between 350 and 375°C by a decrease in the soluble material content which thereafter remained roughly unchanged until 450°C. Nevertheless, some slight difference was noted between the effect of the two minerals. While during simulated maturation with montmorillonite at 375, 400 and 450°C the amounts of the soluble bitumen were practically unchanged, with calcite a slight decrease was detected (Fig. 1). The role of the two minerals was reversed concerning the quantity of volatilized bitumen. In the zone of catagenesis montmorillonite showed a moderate retention, whereas calcite promoted the formation of the volatilized bitumen (Fig. 1). The H1 vs Tm~xdiagram for heated type III/b kerogen revealed that the minerals slightly accelerated the simulated thermal maturation (Fig. 4).
~'x
R
£ r~
"o',~.x~ 400
450
(450°) 500
rmax (°C) Fig. 3. Experimental evolution path (HI vs T ~ diagram) of type Ill/a kerogen in the presence and in the absence of minerals. 1243 2 3 / 2 ~ "
'~
(4?0°)
Tmax (°C)
"r.
5o -
X "'/*.
440
200-
L) R
~
Type lll/b kerogen
~x~ Kerogen • x v. % - - e - . Kerogen + calcite v,\°° °°°-[~,-°° Kerogen + bentonite v.\*. X (400°) = temperature of the \.. \ simulated maturation
~
%,,,,o°,
450°)1
100 -
125
CONCLUSIONS Both calcite and montmorillonite modified the processes, and yields, from the thermal maturation of kerogens, but their effects differed from each other and depended on the type of kerogens : 1. Montmorillonite proved to be a very active mineral which essentially influenced the yield, and evolution, of organic matter. Owing to its retention effect, smaller yields of both soluble and volatilized bitumen were measured during the pyrolysis of type III/a and III/b kerogens in the presence of montmorillonite than with pure kerogen. On the other hand, with type I kerogen, a higher a m o u n t of soluble
Magdolna Het6nyi
126
bitumen, a n d a smaller a m o u n t o f volatilized bitumen, was o b t a i n e d from type I kerogen plus m o n t m o r i l lonite t h a n with kerogen alone. 2. Calcite seemed to be a less efficient mineral with respect to type I kerogen, t h a n montmorillonite. Pyrolysis resulted in volatilized b i t u m e n of similar quantity, a n d soluble b i t u m e n o f s o m e w h a t higher quantity, in the presence of calcite t h a n in its absence. F o r type III/a a n d I I I / b kerogens, in c o n t r a s t to m o n t m o r i l l o n i t e , calcite p r o m o t e d the f o r m a t i o n o f the volatilized b i t u m e n up to 450°C. A t 500°C, however, the a m o u n t of volatilized b i t u m e n o b t a i n e d from type I I l / a kerogen was less ( - 5 0 % ) in the presence o f b o t h minerals, t h a n in their absence. In the case of type I I I / b kerogen the two minerals acted differently. While the a m o u n t of volatilized b i t u m e n o b t a i n e d from type I I I / b kerogen, at 500°C, was reduced by 50% in the presence of m o n t m o r i l l o n i t e , it was only 2 9 % in the presence of calcite. 3. C o m p a r i n g the role of the two minerals in controlling the a m o u n t of the soluble b i t u m e n produced o n pyrolysis it can be stated that, (a) for a given kerogen, their retention or catalytic effects were similar, but differed in extent, which, (b) d e p e n d e d o n the type of kerogen. Minerals increased the a m o u n t of soluble b i t u m e n o b t a i n e d after t h e r m a l d e g r a d a t i o n of type I kerogen, a n d decreased it for type III/a kerogen. F o r type I I I / b kerogen this mineral effect changed as a function o f temperature. Associate E d i t o r - - M . Vandenbroucke Acknowledgements--This work was supported in part by the Hungarian Scientific Research Foundation (OTKA), grant No. T 007429. The author would like to thank to the reviewers of this manuscript, Dr A. Lewis and one other, for their constructive and helpful comments. Thanks are expressed for the useful suggestions of Dr M. Vandenbroucke. REFERENCES
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