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Hydrocarbon generation potential of some Hungarian low-rank coals
Advances in Organic Geochemistry 1989
0146-6380/90 $3.00 + 0.00 Copyright© 1990Pergamon Press pie
Org. Geoehem. Vol. 16, Nos 4---6,pp. 907-916, 1990 Printed in Great Britain.All rights reserved
Hydrocarbon generation potential of some Hungarian low-rank coals M. HETI~NYII and Cs. SAJG6 2 tDepartment of Mineralogy, Geochemistry and Petrography, Attila J6zsef University, P.O. Box 651, Szeged, H6701, Hungary 2Laboratory for Geochemical Research, Buda6rsitit 45, Budapest, HIll2, Hungary
(Received 8 November 1989; accepted 7 March 1990)
Abstract--Several Hungarian lignite and brown coal samples were studied by coal petrographical, palynological and organic geochemical methods. Three of these were chosen for a series of pyrolysis experiments. Thermal treatment was carried out on two H-rich Eocene brown coals (kerogen: Type II) and a H-poor Miocene lignite (kerogen Type III) between 200 and 500°C. The products of experiments (insoluble residue, chloroform soluble bitumen and volatilized bitumen) were investigated. During diagenesis the hydrocarbon potential of lignite decreased by 75% and that of the coals diminished approximately 50%. The zone of the catagenesis was reached at 350°C by lignite and at 375°C by coals. The coal-2 is somewhat more resistant to thermal degradation than coal-l. Various hydrocarbon classes (alkanes, alkenes, phyllocladanes, isoprenoids) were measured in nonaromatic hydrocarbon fractions. Volatile bitumens contained much more unsaturated hydrocarbons than the bitumens extracted after pyrolysis. Prist-l-ene and prist-2-ene were measurable only in the volatile yields. 16~(H)-phyllocladane was present among the products and its generation stability and isomerization were also studied. The ratios between different hydrocarbon products were found variable in the case of different samples as a function of increasing temperature and time (e.g. n-alkenes to n-alkanes). Key words--thermal degradation, soluble and volatile bitumens, catagenesis, low-rank coals, phyllocladanes, Rock-Eval
INTRODUCTION During the last decade certain coals and coal macerais have been recognized as source rocks for petroleum. The generative potential of coals has a great similarity to Type III kerogen which yields gas rather than oil, but may generate commercial amounts of crude oil depending on the liptinite content (Tissot and Welte, 1984; Saxby and Shibaoka, 1986). On the basis of the results of the examination performed by electron-microscopy, evolution paths of coal and that of the Type III disseminated organic matter were found to be nearly the same (Oberlin et al,, 1980). At the same time Durand and Paratte (1983) observed that the coals rich in exinite could be found between the kerogen of Type II and Type III in the evolution field. Generally the hydrogen index of coals does not exceed 300mgHC/gTOC (Bertrand, 1984; Durand and Paratte, 1983; Espitali6 et al., 1985, 1986; Johns et al., 1984; Leplat and Paulet, 1985; Monthioux et al., 1985; Peters et al., 1981; Peters, 1986; Verheyen et al., 1984). Thus, coals can contain organic matter of both Type II and III. There were two main objects of this study: (i) to compare the evolution paths of coals containing different types of organic matter and OG 161ll6-.-Q
different hydrocarbon potentials as a consequence of the different precursors, i.e. different peat-forming plant communities and different swamp-types; (ii) to compare the evolution paths of coals containing the same type of organic matter with similar hydrocarbon potentials, i.e. the samples which came from the same swamp-type but their precursor materials were partly different. The evolution paths were traced in "bulk flow" pyrolysis experiments. The products of experiments: insoluble residue, extracted and volatilized bitumens were investigated. SAMPLES The most important parameters of the samples studied are summarized in Table 1. The two Eocene sub-bituminous coals (referred to as coal-I and coal-2) were derived from tropical vegetation of semi-terrestrial ecological conditions. Plant microfossils of these two coals were found to be partially destroyed. On the basis of palynological examinations, the predominant members of the coalforming plant assemblage were Palms in the case of coal-1 and Myricaceae shrubs in the case of coal-2. In coal-1 remnants of coniferous woods were 907
M. HET~NYIand Cs. SAJC,6
908
Table 1. Some parameters of the samples chosen for thermal degradation Parameters
Coal-I
Locality
Dorog North Hungary Eocene Tropical Semiterrestric Partially destroyed 50.02 60.88 0.39 0.44 402 412 212 187 II II
Age Climate Zonation of the vegetation Preservation of microfossils TOC (%)
Ro (%) Tmax(°c) HI (mgHC/gTOC) Type of kerogen HC-pot = SI + S2 (kgHC/ton of sample)
$2/$3 Chloroform soluble bitumen (mg/gTOC)
Coal-2
112.16 6.4
Lignite Borsod NE Hungary Miocene Subtropical Open swamp Very poor 55.44 0.26 375 136 III
119.42 5.4
61
86.63 3.7
54
76
observed. The considerable fungal remnants, identified in coal-2, indicated biological (enzymatical) activity during the sedimentation (Kedves, personal communication, 1988). On comparing the microlithotypes (Stach et al., 1982; Rigby et al., 1981; Alpern, 1980), telite dominated over gelite (42.6 and 39.0%) in coal-l; in coal-2 the telite was somewhat less than gelite (24.0 and 31.4%). Probably, the relatively high hydrogen content of coals was a consequence of their clarite concentration: in the H-richer coal-1 the clarite was 41.2% and in the H-poorer coal-2 the clarite was 29.1%. The Miocene lignite was deposited in an open swamp. The preservation of plant microfossils was poor. Remnants of deciduous forest predominate over the remnants of Taxodiaceae-Cupressaceae paludal forests on the basis of palynological examinations (Kedves, personal communication, 1988).
EXPERIMENTAL
The samples were ground to size d < 0.2 ram. The total organic carbon content (TOC) was measured by means of combustion at 1000°C under intense oxygen flow before and after heating. The thermal degradation of the samples was carried out in a temperature-programmed Hereaus-type furnace under continuous nitrogen flow (Het6nyi, 1980, 1987). The products were collected in two traps. The firstcollector was air-cooled and the second one was cooled by salted ice. The unified bitumen content of the traps was regarded as volatilized bitumen. After thermal degradation the bitumen was extracted by chloroform in Soxhlet apparatus and regarded as soluble bitumen. Hydrocarbon potential, type and thermal maturity of the unheated and the degraded samples were determined by a Rock-Eval II pyroanalyser (Espitali~ et al., 1977). Pyrolysis of of 30-40 mg of samples at 300°C for 4 min was followed by programmed pyrolysis at 25°C/min to 550°C, in an atmosphere of helium. After precipitating the asphaltenes with light petroleum (40-70°C), the bitumens were chromatographed on a column packed with 1:4 alumina over silica gel. Successive elution with hexane, benzene and benzene-methanol (1:1, v:v) afforded non-aromatic HC, aromatic HC and resin fraction, respectively.The non-aromatic HCs were analyzed on a capillary column (20 m x 0.23 mm i.d.) coated with OV-101 and temperature programmed from 90 to 330°C at 5°C/min, 15min at 330°C isothermal. In the identification work of ~- and /~-phyllocladanes, reference compounds were used for the coelution with the samples.
300
C-1 C
• l~jr~te (L)
2°q°~ ~ z,oo"
i ~o
I
'
~o
'
IMMATURE
'
x cooL-2 (C-Z)
, '
~o
O,t ZONE I
'
'
'
~o
'
aS ZONE
'
'
I
Fig. 1. HI-Tm,~ plots of the thermally degraded samples (degradation period = 5 h).
Hydrocarbon generation potential of some Hungarian low-rank coals RESULTS AND DISCUSSION
Type of the organic matter On the basis of Rock-Eval pyrolysis the organic matter of the lignite proved to be of Type III kerogen. Within this type the sample was expected to have good hydrocarbon generation features, because its hydrogen index was near the upper limit of Type III. The organic matter of coal-I and coal-2 found to be located in the field of the disseminated organic matter of Type II on a HI vs Ymax diagram (Fig. 1). The difference in HI values of the two coals was very small. The coal-1 was poorer in organic carbon ( T O C = 5 0 % ) and richer in hydrogen (HI = 212mgHC/gTOC), than coal-2 (TOC = 61% and HI = 187mgHC/gTOC) (Table 1). The type of the organic matter of the samples was also examined by their experimental thermal evolution path. Immaturity of organic matter offers a possibility to simulate the catagenetic and partly the diagenetic pathways by laboratory thermal degradation. The artificial evolution paths of the samples (Fig. 1) demonstrated very well the general trend of the maturation of organic matter: the variations inherited from the young sediment become progressively weaker with increasing evolution (Tissot and Welte, 1984). The three HI-Tm~x plots converged at the boundary of oil zone and gas zone, where the Tmax was about 460°C (Ro ~ 1.3%). However, the experimental conditions under which the three samples reached this Tm~ value were different. It was 400°C, 5 h in the case of lignite and 500°C, 5 h for the coals (Fig. 1). Furthermore, the lignite entered the zone of catagenesis at 350°C and the zone of metagenesis at 450°C. Coals entered the
909
zone of catagenesis at 375°C and remained within this zone following heating at 500°C for 2 h. The zone of metagenesis was reached by only the coal-I after 5 h thermal degradation at 500°C, its carbonization rank corresponded to that of semi-anthracite state (Tmax = 542°C). However, under these experimental conditions the maturity of the organic matter of coal-2 did not enter the zone of metagenesis within the used conditions (Table 2). Owing to the different types of the organic matter the slope of the HI-Tmax plots was also dissimilar. The HI of lignite containing Type III O M decreased very quickly during diagenesis, but only a little during catagenesis, whereas the HI of the coals decreased similarly in both of the evolution zones (Fig. 1).
Hydrocarbon potential Hydrocarbon potential of samples of similar maturity depends on quantity and the type of their organic matter. The HC-potential of the lignite, which had a TOC content (55%) between that of the two coals (50 and 61%), was about 72% of the HC-potential of the coals (Table 1). The lignite of Type III and the coals of Type II differed from each other not only on the basis of their original HC-potential. These ratios also showed dissimilar changes during the artificial evolution (Fig. 2). Decreasing of HC-potential of lignite was considerable even under the mildest experimental conditions (200°C, 1 h = 37%). A rapid decrease could be observed between 200 and 350°C. Namely, in the case of lignite 75% of the HC-potential reduction took place in the diagenetic phase. During catagenesis the slope of the decrease was far smaller. At the same
Table 2. Hydrocarbon potential, Tmax and hydrogen index of the thermallydegraded samples Thermal degradation HI Residue HC-potential(%) Tm,~(°C) (mgHC/gTOC) Temperature Period (°C) (h) Coal-I C o a l - 2 Lignite C o a l - I C o a l - 2 Lignite C o a l - I C o a l - 2 Lignite Unheated sample 100.0 100.0 100.0 402 412 375 212 187 136 200 90.6 98.0 62.6 409 414 392 183 174 77 2 92.7 98.0 65.3 409 414 389 183 167 83 5 89.6 97.2 60.7 405 411 391 174 168 78 300 87.6 88.3 61.2 409 417 396 173 151 71 84.8 88.6 56.6 410 417 410 165 152 70 -87.2 26.1 -417 413 -153 35 350 77.5 84.8 40.4 412 420 418 149 147 58 80.7 80.4 32.2 415 420 420 148 137 38 72.9 75.2 26.5 417 423 429 141 130 31 375 68.1 76.3 38.0 420 422 419 132 130 47 58.7 57.3 28.1 424 426 429 I11 93 34 50.7 58.9 18.2 428 431 439 99 98 19 400 64.6 58.5 21.7 420 432 438 122 102 26 57.8 63.6 19.9 423 428 444 115 106 23 41.7 51.3 13.3 430 433 466 80 82 15 450 26.4 36.4 10.0 441 438 525 47 61 II 25.9 39.6 10.0 431 428 526 48 63 11 29.5 37.0 9.1 433 438 543 55 60 8 500 18.3 12.1 6.7 429 436 543 31 28 7 12.6 12.7 5.7 442 444 546 20 21 6 5.9 12.6 3.6 542 487 549 lO 14 3 - - , Not determined.
910
M. HETI~NYIand Cs. S~C,6 Oc~'rc~sing of the HC-pof ( % 1 0 50
Table 3. Experimentalconditionsnecessaryto reach 10, 50 and 90% decrease in hydrocarbon potential
100
I
[] coat-1
i
-....
0
Fig. 2. Decreasing of the hydrocarbon potential in function of the temperature of thermal degradation (degradation period = 5 h). time, the HC-potential of coals decreased by only 25% degraded at 350°C for 5 h. At the beginning of catagenesis, which was simulated by thermal degradation at 375°C, the change was 40 and 50% in case of coal-I and coal-2, respectively (Table 2). Concerning the reduction of HC-potential a slight difference was observed between the two coals having the same original HC-potential and different biological precursor material. The HC-potential of coal-I decreased by 10% during thermal degradation performed at 200°C. The change of HC-potential of coal-2 was less than 3 % under the same experimental conditions. In each stage of the artificial evolution a small dissimilarity could be observed between the two coals (Fig. 2 and Table 2). The residue potential of the samples decreased not only as a function of temperature, but with the heating period. The effect of the degradation period was highest at 300 and 350°C in the case of lignite, at 375°C in the case of coal-2 and at 400°C in the case of coal-1. The close relationship between the HC-potential and the type of the organic matter could also be demonstrated by the results mentioned above. While the hydrocarbon production of the lignite took place mostly in the first zone of the evolution, the hydrocarbon potential in the coals changed in similar degree during both the diagenesis and the catagenesis. Comparing the lignite with the coals, the experimental conditions necessary to reach the same decrease of their HC-potential were very different. However, a smaller difference was also found between the two coals (Table 3). For example, 50% reduction of the HC-potential took place at 300, 375 and 400°C in the case of lignite, coal-1 and coal-2, respectively. At the same time 90% reduction could be detected at 450°C (lignite) and at 500°C coals. The coal samples differed from each other only slightly. Namely,
Decrease of HC potential 10% 50% 90% Lignite < 200°C* 300°C, 2 h 450°C, 1h Coal-I 200°C, 5h 375°C, 5h 500°C, 1h Coal-2 300°C, l h 400°C, 5 h 500°C, 5 h *At 200°C the decreasingis about 40%. the temperatures were the same (500°C), only the degradation periods were different (1 and 5 h).
Quantity of bitumen The change in the quantity of bitumen during thermal evolution also reflected the slight difference of hydrocarbon generation features of the two coals and a strong one between the lignite and coal samples (Table 4). On the basis of the change of the soluble bitumen content, dissimilarities were observed mainly during diagenesis. In this zone not only the lignite of Type III differed from the coals of Type II but the coals also differed from each other substantially (Fig. 3). Under the mildest experimental conditions bitumen (mO/g TOC) 20
60
100
140
180
220
20
60
100
140
180
220
.~
"~
!!!!!!!!i!i!i!iiii!%
Z "~ 5OO
300-
I"=1 c ~ o b ' m sotu~ ~ tOO-
~latitimd bflum~
S0¢ Fig. 3. The change of the quantity of bitumen during the thermal degradation (degradation period = 5 h).
911
Hydrocarbon generation potential of some Hungarian low-rank coals Table 4. Quantity of bitumen Thermal degradation
Temperature (°C)
Period (h) sample
Chloroform soluble bitumen (mg/gTOC)
Coal-I
Coal-2
Volatilized bitumen (mg/gTOC)
Lignite Coal-I
Coal-2
Total bitumen (mg/gTOC)
Lignite Coal-I
Coal-2
Lignite
61 34 32 27
54 64 53 49
76 75 81 73
n.m. n.m. n.m.
n.m. n.m. n.m.
4 4 4
34 32 27
64 53 49
79 85 77
300
21 23 30
46 43 47
38 36 15
n.m. n.m. n.m.
n.m. n.m. 4
9 7 7
21 23 30
46 43 51
47 43 22
350
65 61 40
57 47 47
I1 7 4
14 19 17
8 15 16
13 16 16
79 80 57
65 62 63
24 23 20
375
19 41 25
51 43 31
11 II 4
37 52 49
27 38 53
22 23 22
56 93 74
78 81 84
33 44 26
400
38 32 26
45 28 20
6 4 2
34 57 73
33 66 76
27 45 40
72 89 99
78 94 96
33 49 42
450
16 10 13
21 12 11
2 2 2
100 87 116
79 80 104
49 51 67
116 97 129
100 92 115
51 53 69
500
11 13 2
4 3 1
2 2
140 145 183
71 100 170
104 118 78
151 158 185
75 103 171
106 120 79
Unheated
200
l
2 5
n.m., non measurable.
(200°C) the soluble bitumen content of the residue and that of the unheated samples was the same in the case of lignite and coal-2. At the same time the original soluble bitumen content of coal-I fell to about one half. At the boundary of diagenesis and catagencsis the quantity of bitumen extracted from the thermally degraded lignitewas about 1/20 of that of the unheated sample. Simultaneously volatilized bitumen of insignificantquantity developed (Table 4). The original soluble bitumen content of coal-2 remained unchanged in the totalzone of itsdiagencsis. In this zone the quantity of bitumen extracted from coal-I firstdropped to a minimum value at 200 and 300°C, after that it increased a little.However, it did not reach the soluble bitumen content of the unheated sample. At 350°C insignificant quantity of volatilized bitumen developed from the three samples. At this temperature the soluble and the volatilized bitumen content of the two coals and consequently the total bitumen content was the same. However, the lignite which yielded the same quantity of volatilized bitumen, has only an insignificant amount of soluble bitumen compared to the coals. The ratio of soluble and volatilizedbitumen developed from the lignitewas 2: I at 300°C and it was 1:4 at the beginning of itscatagencsis (350°C). In the case of coals the ratios mentioned above were about 1:2 at the boundary of the two evolution zones (375°C). During catagenesis both the coals and the lignite proved to be fairly good sources of volatilized bitumen. The quantity of volatilized bitumen developed from the three samples showed a rapid increase in the total zone of the catagencsis (Fig. 3). As a consequence of the different types of organic matter the
ratio o f t h e o r g a n i c c a r b o n c o n t e n t c o n v e r t e d to volatilized b i t u m e n was less in the case o f lignite o f T y p e III t h a n in the case o f coals o f T y p e II. A p p r o x i m a t e l y twice the q u a n t i t y o f the volatilized b i t u m e n d e v e l o p e d f r o m coals t h a n f r o m lignite (Table 4). In the catagenesis z o n e a f u r t h e r c h a r a c t e r istic difference was f o u n d b e t w e e n the lignite a n d the t w o coals. T h e q u a n t i t y o f b i t u m e n e x t r a c t e d f r o m lignite s e e m e d to be a m i n i m a l value, less t h a n 10 m g H C / g T O C . A t the s a m e time, the q u a n t i t y o f the soluble b i t u m e n o r i g i n a t i n g f r o m coals d e c r e a s e d c o n t i n u o u s l y a n d fell to less t h a n 10 m g H C / g T O C at 500°C.
J
iol
coat-1 (o)
y- B6.9'7- 0.53x ra,O,9'7 y,93.~-O.S6x r~,O.~
co~-2 (R) coo1-1* co¢.-2
y,90.52-O.~x ra,O.90
r so
a: \ .
i ,
,
,
V~Wf'lliZid
,
a
\
,
)
biSumml
,
(molg
,~
,
TOC]
Fig. 4. Correlation between the S2-value of the thermally degraded samples and the quantity of the volatilized bitumen developed from them (degradation periods -- 1, 2 and 5 h).
912
M. HE~NY] and Cs. S~dc,6 more favourable hydrocarbon genetic features, than it was in the case of coal-2 (r2= 0.86). Unlike the coals the lignite showed no direct proportion between the volatile bitumen content and the S2-value (Fig. 4).
The bitumen content of the lignite which developed in its natural evolution was converted to hydrocarbon gas mainly during its artificial diagenesis. The original bitumen content of the two coals was converted partly to hydrocarbon gas and partly to volatilized bitumen mostly during their artificial catagenesis. The different hydrocarbon genetic features of the examined samples were revealed in the correlation which was found between the quantity of volatilized bitumen and the amount of hydrocarbons as well as hydrocarbon-like compounds ($2) generated by pyrolysis of insoluble organic matter (Fig. 4). The S2-values concerning the coals proved to be directly proportional to the quantity of volatilized bitumen, the correlation coefficient was 0.90. The correlation was better in the case of coal-I (r 2 = 0.97) which had
Quality o f bitumen
The nature of volatilized and soluble bitumens is different. In the volatilized bitumens during simulation the quantity of the NSO fraction rises as a function of increasing temperature, i.e. the increasing temperature produces more and more free radicals which leave the reactor rapidly in sweeping gas and react in the trap yielding higher and higher amount of the polar compounds than in lower temperature experiments. Figure 5 shows the change of bulk composition during simulation. Three diagrams are
(O)
HCor
O-9o
6 X / ~)
300°C 325°C x 3500C .37soc
74
~,~
~"
/
\
,
/
(60)
26
32
38
\
41.
(b)
50
56
~
• soooc
62
68
74
HCor (66 1/3)
//~60 17 / 2~"~ / 30 /
(61 1/11
,oooc
• ,sooc
40
B - 20o ~ ~ %50 ~ ~ ~40
60
80
Fig. 5. (a), (b) Caption on facing page.
" 300"C x 3500C o 3750C v 400"C • ~,500C . 5000C
Hydrocarbon generation potential of some Hungarian low-rank coals
(C)
913
HCor (305) ~ .
/
D -lSp
\
/
.,~-e
~
/
v 400"C
\
Ill
•
.o.¢
91 l ' l
/ ?
'2<. +\
HCnon -at / (iS'k)
/ 80
,' 90
~ Z NSO(S'k)
Fig. 5(c)
Fig. 5. Ternary diagram depicting relative distributions of non-aromatic hydrocarbons, aromatic hydrocarbons and NSO compounds (asphaltenes and resins) for volatilized bitumens of the coal-I (a) and the lignite (b) and extracted bitumen after experiment of the coal-2 (c). displayed: volatilized bitumens of the coal-1 and the lignite and the soluble bitumen of coal-2. The distributions were only plotted when the yield was over 15 rag. In cases of the coal-I sample and the lignite the above-mentioned reverse maturation trend is obviously recognizable. In the case of coal-1 the 350°C for I h and the two 500°C for 5 h experiments are significantly separated from the others. The first is in the zone of diagenesis, the second is in the zone of metagenesis and the rest is in the phase of hydrocarbon generation. In the case of the lignite the distribution is more widespread than in the cases of the coals in accordance with the Fig. 1. ~r o ,r--' ~"
In the case of extracted bitumens the normal maturation trend (the higher temperature is, the more abundant hydrocarbons are present in soluble phase) was observed. The bitumens of immature stage (200-350°C, see Fig. l) are separated from the more mature bitumens which show a considerable dispersion indicating the complexity of the process. The non-aromatic hydrocarbon fractions were analysed by GC, Fig. 6 shows the ratio of n-alkenes to n-alkanes in the volatilized bitumens. In the soluble bitumens unsaturates were only present in negligible extent. The ratio in the case of coal-I demonstrates a solid temperature dependence. In the
The ratio of n-aikenes to n-o|kanes
0.70-
O,SO-
0,30-OZ'O 'O ' ,ZO.
~
lignite
0.10o o-
0.200,1o. oo, 0.10o-
coo| - 2
1'~
r l:ivv~ h
lh~Sh
,
300oC 325oC 3sooe 375oc 400oc 4Gooc 500oc
Fig. 6. The ratio ofY Cj5 + n-alkenes to • C~ + n-alkanes in volatilized bitumens of the coals and lignite.
914
M. HEI"I~NYIand Cs. SAJC__,6 Ratio
of p r i s t - l - e n e , p r l s t - 2 - e n e
t o ~r n - o l k o n e s
prl,prn s. n 0.10 •
0-
0,10.
0-
0.10
0
ahsh I"~S h Sh lh~s h 1"2%h I"~'S" lh2% h lh~s h 200oc 300oc 325°(: 350oc 375oc 400oc 450°C 50O°C
Fig. 7. The ratio of prist-l-ene + prist-2-ene to ~ C~5+ n-alkanes in the volatilized bitumens under the given experimental conditions. cases of the lignite and coal-2 the thermal dependence is not genuine, albeit some extent of dependence is recognizable in the case of coal-2. In the volatilized bitumens, prist-l-ene and prist-2-ene are present in a considerable amount. Their relative concentrations to n-alkanes are shown in Fig. 7. The variation of the ratio is akin to, in some way, the case of the two coals, but it illustrates a rather complex process in the case of lignite (3maxima). The considerable concentration of pristenes indicates that they must have a different precursor or precursors from tocopherols (Goossens et aL, 1984). The most probable source is the phytyl side chain of chlorophyll (Ikan et al., 1975), but the Archaebacterial lipids (diphytanyl ethers; Chappe et al., 1982) are not negligible as sources. Perhaps all the mentioned sources have contributed to the yields and their varying contributions caused the complexity of the figure (different contributors produce pristenes at different temperatures). The differences in mineral composition also can cause variances (Lao et al., 1989). The amount of ash was as follows: 19.8% in coal-l, 6.62% in coal-2 and 6.49% in lignite. The samples all contain kaolinite as the predominant clay mineral beyond the less important deviations. In the non-aromatic hydrocarbon fraction of the unheated lignite there is an overwhelming component i.e. 93% 16~ (H)-phyllocladane. During simulation its concentration decreases drastically in extracted bitumens and not so severely in the volatilized bitumens (Fig. 8). The great difference starts from 350°C temperature. At least two effects take place: (i) the higher the temperature is, the more important the resident time is; (ii) the higher the temperature is, the more volatile the compound is. The first is the more important factor. There is another significant divergence in the cases of the two bitumens types: the isomerization of 16~ (H)-phyllocladane to 16IT(H)phyllocladane occurs only in the extracted bitumens between 300 and 400°C. In the yields of the 200, 450 and 500°C experiments 16~(H)-phyllocladane was
only present, i.e. at 200°C experiments the isomerization has not started yet and in the 450 and 500°C experiment the degradation of 16fl(H)-phyllocladane was more rapid than its generation. The second fact was probed by the change of the rate of the isomerization as a function of duration, i.e. the rate decreased in the 2-h experiments in proportion to l-h ones and it also decreased in the 5-h experiment as compared to 1- and 5-h ones. In other words the generation of 16/3(H)-phyllocladane is a member of consecutive reaction series. This isomerization has been studied by Noble et al. (1985) and Alexander et al. (1987). The apparent rate parameters were calculated from the l-h experiments using Arrhenius' law to relate the reaction rates to the temperature. We have assumed that the progress of the reaction could be described by unimolecular first-order reaction laws (AH* = 63.66-70.22 kJ/mol and A = 11.26--42.1 s-l). We have started a new series of simulation experiments, further data on this reaction together with other bitumen analysis results will be published in the future.
CONCLUSIONS
(1) The two coals containing Type II OM have higher hydrocarbon potentials and yield more bitumen than the lignite sample containing Type III OM. (2) During pyrolysis experiments at 350°C the lignite, and at 375°C the coals entered the zone of catagenesis. The boundary of the oil and gas zone (T~,x = 460°C) was reached at 400°C by lignite and at 500°C by the coals. (3) During diagenesis the HC-potential of lignite decreased by 75% and that of the coals diminished approximately 50%. (4) During catagenesis the reduction of HCpotential was 20% in lignite and about 45% in the coals.
Hydrocarbon generation potential of some Hungarian low-rank coals (a)
Lignite volotlll zed bitumen
rl,{./J phyllocladone % I ~ z n - alko.es % I l z n-alkenes %
(b)
Lignite exrocted
r-i~c./3 phyllocladane % I~]Z n * alkones %
bttumen
IIIz n - alkenes %
I'~'IZ the rest %
I
90"/, 80%
70%60%-
!
915
I~Z the rest 't, )0% [~ )0%
!
~0'1, ~0"I, 4:"
~D'/,
i
50%-
i0%
40%-
~,O*l,
30%-
30'1,
20*/r
20'1,
10%-
10"1,
A-.
O-
lh2hs h sample 200oC 300oC
unheated 2h
lh2h$1h th2h5 h 3500C 375°C
1h~:~5h lh2h5 h lh2h5 h 400*C 4500C 500"C
O unheated sample
lhzhsh 200"C
300*C
~hZ~5h lhZ 350°C 375°C
4000C
4500C 5000C
Fig. 8. The relative distributions of the most significant components in the non-aromatic hydrocarbon fraction of the volatilized and extracted bitumens of the lignite. (The rest consists of any non-aromatic hydrocarbons except of Cts + n-alkanes, Ct~ + n-alkenes and phyllocladanes.) (5) The good correlation found between $2 and the quantity of volatilized bitumen reflects the oil prone nature of these coals. Such a correlation was not recognized in the case of the lignite which is gas prone. (6) The two coals differ from each other only slightly, their original HC-potentials are nearly the same. Differences were found during the maturation simulation. By the end of diagenesis the residual HC-potential of coal-1 is 50% and that of coal-2 is 60%. Coal-I entered the zone of metagenesis at 500°C in 5 h experiment, but coal-2 still remained in the zone of catagenesis, consequently coal-2 is somewhat more resistant to thermal degradation than coal-l. (7) Unsaturates were present only in volatilized bitumens. The non-aromatic HC fraction of the lignite was predominated by the 16~(H)phyllocladane. During the experiments phyllocladanes were only severely destroyed in the extracted bitumens over 400°C. The isomerization of 16~(H)-phyllocladane to 16fl(H)phyllocladane was only observed in extracted bitumens from the range of the 300 and 400°C experiments. Acknowledgements--This work would not have been possible without the support of the KFH (Central Geological Office of Hungary). We thank Dr V. Dank for the permission to publish. We also thank Drs M. Kedves, I. Elek and Z. A. Horv~ith for palynological, microlithotype and vitrinite reflectance analyses, respectively. We are grateful
to Mr J. Kiss, Mrs E. Pfipay, Mrs I. Szederk6nyi, Mrs A. Mar&, Ms K. D6me and Mrs V. Csontos for technical assistance. Cs. S thanks Dr B. Durand and the Local Committee for benefits which have rendered possible his participation in the Meeting. We are indebted to Dr J. Zumberge of Ruska Labs for his review and constructive criticism of the manuscript.
REFERENCES
Alexander G., Hazai I., Grimalt J. and Albaig6s J. (1987) Occurrence and transformation of phyllocladanes in brown coals from N6gnld basin, Hungary, Geochim. Cosmochim. Acta 51, 2065-2073. Alpern B. (1980) P6trographie du k6rogdne. In Kerogen (Edited by Durand B.), pp. 339-371. Technip, Paris. Bertrand P. (1984) Geochemical and petrographic characterization of humic coals considered as possible oil source rocks. In Advances in Organic Geochemistry 1983(Edited by Schenk P. A., Leeuw J. W. de and Lijmbach G. W. M.). Org. Geochem. 6, 481--488. Pergamon Press, Oxford. Chappe P., Albrecht P. and Michaelis W. (1982) Polar lipids of Archaebacteria in sediments and petroleums. Science 217, 65-66. Durand B. and Paratte M. (1983) Oil potential of coals: A geochemical approach, In Petroleum Geochemistry and Exploration of Europe (Edited by Brooks J.), pp. 255-265. Blackwell, Oxford. Espitali6 J., Deroo G. and Marquis F. (1985) La pyrolyse Rock-Eval et ses applications. Rev. Inst. Ft. Pet. 40, 563-579 and 755-784. Espitali6 J., Deroo G. and Marquis F. (1986) La pyrolyse Roek-Eval et ses applications. Rev. Inst. Ft. Pet. 41, 73-89. Espitali6 J., Madec M., Tissot B. P., Menning J. J. and Leplat P. (I 977) Source rock characterization method for exploration. Offshore Technology Conf. Paper No. 2935, 11th Annual OTC, Houston.
916
M. H~r,~tt and Cs. S~JC_,6
Goossens H., De Leeuw J. W., Schenck P. A. and Brassel S. C. (1984) Tocopherols as likely precursors of pristane in ancient sediments and crude oils. Nature 312, 440--442. Hetrnyi M. (1980) Thermal degradation of the organic matter of oil shale of Pula (Hungary) at 573-773 K. Acta Mineral Petrogr. Szeged. XXIV/2, 301-314. Hetrnyi M. (1987) Methods for measuring the maturity of organic matter in diagenesis stage. Acta Mineral Petrogr. Szeged. XXIX, 107-118. Ikan R., Baedecker M. J. and Kaplan I. R. (1975) Thermal alteration experiments on organic matter in recent marine sediment--II. Isoprenoids. Geochim. Cosmochim. Acta 39, 187-194. Johns R. B., Chaffee A. L. and Verheyen T. V. (1984) Chemical variation as a function of lithotype and depth in Victorian brown coal. In The Chemistry of Low-Rank Coals (Edited by Schobert H. H.), ACS Symposium Series, Vol. 264, pp. 109-132. ACS Washington. Kedves M. (1988) JATE, Univ. of Szeged, Dept of Botany, personal communication. Lao Y., Korth J., Ellis J. and Crips P. T. (1989) Heterogeneous reactions of 1-pristene catalysed by clays under simulated geological conditions. Org. Geochem. 14, 375-379. Leplat P. and Paulet J. (1985) Study of diagenesis, catagenesis and metagenesis of coals and dispersed organic matter with high-temperature modified Rock-Eval. In Petroleum Geochemistry in Exploration of the Norwegian Shelf(Edited by Thomas B. M. et al.), 28, 319-326. Monthioux M., Landais P. and Monin J. (1985) Comparison between natural and artificial maturation
series of humic coals from the Mahakam delta, Indonesia. Org. Geochem. 8, 275-292. Noble R. A., Alexander R., Kagi R. I. and Knox J. (1985) Tetracyclic ditzrpenoid hydrocarbons in some Australian coals, sediments and crude oils. Geochim. Cosmochim. Acta 49, 2141-2147. Oberlin A., Boulmier J. L. and Villey M. (1980) Electron microscopic study of kerogen mierotexture. In Kerogen (Edited by Durand B.), pp. 191-240. Technip, Paris. Peters K. E. (1986) Guidelines for evaluating petroleum source rock using programmed pyrolysis. Bull. Am. Assoc. Petr. Geol. 70, 318-329. Peters K. E., Rohrback B. G. and Kaplan I. R. (1981) Geochemistry of artificially heated humic and sapropelic sediments---I: Protokerogen. Bull. Am. Assoc. Petr. Geol. 65, 688-705. Rigby D., Batts B. D. and Smith J. W. (1981) The effect of maturation on the isotopic composition of fossil fuels. Org. Geochem. 3, 29-36. Saxby J. and Shibaoka M. (1986) Coal and coal macerals as source rocks for oil and gas. Appl. Geochem. 1, 25-36. Stach E., Mackowsky M-Th., Teichmtiller M., Taylor G. H., Chandra D. and Teichmiiller R. (1982) Coal Petrology, 3rd edn. Gebrfider Borntraeger, Berlin. Tissot B. P. and Welte D. H. (1984) Petroleum Formation and Occurrence, pp. 69-184, 229-253. Springer, Berlin. Verheyen T. V., Johns R. B. and Espitali6 J. (1984) An evaluation of Rock-Eval pyrolysis for study of Australian coals including their kerogen and humic acid fractions. Geochim. Cosmochim. Acta 48, 63-71.
0146-6380/90 $3.00 + 0.00 Copyright© 1990Pergamon Press pie
Org. Geoehem. Vol. 16, Nos 4---6,pp. 907-916, 1990 Printed in Great Britain.All rights reserved
Hydrocarbon generation potential of some Hungarian low-rank coals M. HETI~NYII and Cs. SAJG6 2 tDepartment of Mineralogy, Geochemistry and Petrography, Attila J6zsef University, P.O. Box 651, Szeged, H6701, Hungary 2Laboratory for Geochemical Research, Buda6rsitit 45, Budapest, HIll2, Hungary
(Received 8 November 1989; accepted 7 March 1990)
Abstract--Several Hungarian lignite and brown coal samples were studied by coal petrographical, palynological and organic geochemical methods. Three of these were chosen for a series of pyrolysis experiments. Thermal treatment was carried out on two H-rich Eocene brown coals (kerogen: Type II) and a H-poor Miocene lignite (kerogen Type III) between 200 and 500°C. The products of experiments (insoluble residue, chloroform soluble bitumen and volatilized bitumen) were investigated. During diagenesis the hydrocarbon potential of lignite decreased by 75% and that of the coals diminished approximately 50%. The zone of the catagenesis was reached at 350°C by lignite and at 375°C by coals. The coal-2 is somewhat more resistant to thermal degradation than coal-l. Various hydrocarbon classes (alkanes, alkenes, phyllocladanes, isoprenoids) were measured in nonaromatic hydrocarbon fractions. Volatile bitumens contained much more unsaturated hydrocarbons than the bitumens extracted after pyrolysis. Prist-l-ene and prist-2-ene were measurable only in the volatile yields. 16~(H)-phyllocladane was present among the products and its generation stability and isomerization were also studied. The ratios between different hydrocarbon products were found variable in the case of different samples as a function of increasing temperature and time (e.g. n-alkenes to n-alkanes). Key words--thermal degradation, soluble and volatile bitumens, catagenesis, low-rank coals, phyllocladanes, Rock-Eval
INTRODUCTION During the last decade certain coals and coal macerais have been recognized as source rocks for petroleum. The generative potential of coals has a great similarity to Type III kerogen which yields gas rather than oil, but may generate commercial amounts of crude oil depending on the liptinite content (Tissot and Welte, 1984; Saxby and Shibaoka, 1986). On the basis of the results of the examination performed by electron-microscopy, evolution paths of coal and that of the Type III disseminated organic matter were found to be nearly the same (Oberlin et al,, 1980). At the same time Durand and Paratte (1983) observed that the coals rich in exinite could be found between the kerogen of Type II and Type III in the evolution field. Generally the hydrogen index of coals does not exceed 300mgHC/gTOC (Bertrand, 1984; Durand and Paratte, 1983; Espitali6 et al., 1985, 1986; Johns et al., 1984; Leplat and Paulet, 1985; Monthioux et al., 1985; Peters et al., 1981; Peters, 1986; Verheyen et al., 1984). Thus, coals can contain organic matter of both Type II and III. There were two main objects of this study: (i) to compare the evolution paths of coals containing different types of organic matter and OG 161ll6-.-Q
different hydrocarbon potentials as a consequence of the different precursors, i.e. different peat-forming plant communities and different swamp-types; (ii) to compare the evolution paths of coals containing the same type of organic matter with similar hydrocarbon potentials, i.e. the samples which came from the same swamp-type but their precursor materials were partly different. The evolution paths were traced in "bulk flow" pyrolysis experiments. The products of experiments: insoluble residue, extracted and volatilized bitumens were investigated. SAMPLES The most important parameters of the samples studied are summarized in Table 1. The two Eocene sub-bituminous coals (referred to as coal-I and coal-2) were derived from tropical vegetation of semi-terrestrial ecological conditions. Plant microfossils of these two coals were found to be partially destroyed. On the basis of palynological examinations, the predominant members of the coalforming plant assemblage were Palms in the case of coal-1 and Myricaceae shrubs in the case of coal-2. In coal-1 remnants of coniferous woods were 907
M. HET~NYIand Cs. SAJC,6
908
Table 1. Some parameters of the samples chosen for thermal degradation Parameters
Coal-I
Locality
Dorog North Hungary Eocene Tropical Semiterrestric Partially destroyed 50.02 60.88 0.39 0.44 402 412 212 187 II II
Age Climate Zonation of the vegetation Preservation of microfossils TOC (%)
Ro (%) Tmax(°c) HI (mgHC/gTOC) Type of kerogen HC-pot = SI + S2 (kgHC/ton of sample)
$2/$3 Chloroform soluble bitumen (mg/gTOC)
Coal-2
112.16 6.4
Lignite Borsod NE Hungary Miocene Subtropical Open swamp Very poor 55.44 0.26 375 136 III
119.42 5.4
61
86.63 3.7
54
76
observed. The considerable fungal remnants, identified in coal-2, indicated biological (enzymatical) activity during the sedimentation (Kedves, personal communication, 1988). On comparing the microlithotypes (Stach et al., 1982; Rigby et al., 1981; Alpern, 1980), telite dominated over gelite (42.6 and 39.0%) in coal-l; in coal-2 the telite was somewhat less than gelite (24.0 and 31.4%). Probably, the relatively high hydrogen content of coals was a consequence of their clarite concentration: in the H-richer coal-1 the clarite was 41.2% and in the H-poorer coal-2 the clarite was 29.1%. The Miocene lignite was deposited in an open swamp. The preservation of plant microfossils was poor. Remnants of deciduous forest predominate over the remnants of Taxodiaceae-Cupressaceae paludal forests on the basis of palynological examinations (Kedves, personal communication, 1988).
EXPERIMENTAL
The samples were ground to size d < 0.2 ram. The total organic carbon content (TOC) was measured by means of combustion at 1000°C under intense oxygen flow before and after heating. The thermal degradation of the samples was carried out in a temperature-programmed Hereaus-type furnace under continuous nitrogen flow (Het6nyi, 1980, 1987). The products were collected in two traps. The firstcollector was air-cooled and the second one was cooled by salted ice. The unified bitumen content of the traps was regarded as volatilized bitumen. After thermal degradation the bitumen was extracted by chloroform in Soxhlet apparatus and regarded as soluble bitumen. Hydrocarbon potential, type and thermal maturity of the unheated and the degraded samples were determined by a Rock-Eval II pyroanalyser (Espitali~ et al., 1977). Pyrolysis of of 30-40 mg of samples at 300°C for 4 min was followed by programmed pyrolysis at 25°C/min to 550°C, in an atmosphere of helium. After precipitating the asphaltenes with light petroleum (40-70°C), the bitumens were chromatographed on a column packed with 1:4 alumina over silica gel. Successive elution with hexane, benzene and benzene-methanol (1:1, v:v) afforded non-aromatic HC, aromatic HC and resin fraction, respectively.The non-aromatic HCs were analyzed on a capillary column (20 m x 0.23 mm i.d.) coated with OV-101 and temperature programmed from 90 to 330°C at 5°C/min, 15min at 330°C isothermal. In the identification work of ~- and /~-phyllocladanes, reference compounds were used for the coelution with the samples.
300
C-1 C
• l~jr~te (L)
2°q°~ ~ z,oo"
i ~o
I
'
~o
'
IMMATURE
'
x cooL-2 (C-Z)
, '
~o
O,t ZONE I
'
'
'
~o
'
aS ZONE
'
'
I
Fig. 1. HI-Tm,~ plots of the thermally degraded samples (degradation period = 5 h).
Hydrocarbon generation potential of some Hungarian low-rank coals RESULTS AND DISCUSSION
Type of the organic matter On the basis of Rock-Eval pyrolysis the organic matter of the lignite proved to be of Type III kerogen. Within this type the sample was expected to have good hydrocarbon generation features, because its hydrogen index was near the upper limit of Type III. The organic matter of coal-I and coal-2 found to be located in the field of the disseminated organic matter of Type II on a HI vs Ymax diagram (Fig. 1). The difference in HI values of the two coals was very small. The coal-1 was poorer in organic carbon ( T O C = 5 0 % ) and richer in hydrogen (HI = 212mgHC/gTOC), than coal-2 (TOC = 61% and HI = 187mgHC/gTOC) (Table 1). The type of the organic matter of the samples was also examined by their experimental thermal evolution path. Immaturity of organic matter offers a possibility to simulate the catagenetic and partly the diagenetic pathways by laboratory thermal degradation. The artificial evolution paths of the samples (Fig. 1) demonstrated very well the general trend of the maturation of organic matter: the variations inherited from the young sediment become progressively weaker with increasing evolution (Tissot and Welte, 1984). The three HI-Tm~x plots converged at the boundary of oil zone and gas zone, where the Tmax was about 460°C (Ro ~ 1.3%). However, the experimental conditions under which the three samples reached this Tm~ value were different. It was 400°C, 5 h in the case of lignite and 500°C, 5 h for the coals (Fig. 1). Furthermore, the lignite entered the zone of catagenesis at 350°C and the zone of metagenesis at 450°C. Coals entered the
909
zone of catagenesis at 375°C and remained within this zone following heating at 500°C for 2 h. The zone of metagenesis was reached by only the coal-I after 5 h thermal degradation at 500°C, its carbonization rank corresponded to that of semi-anthracite state (Tmax = 542°C). However, under these experimental conditions the maturity of the organic matter of coal-2 did not enter the zone of metagenesis within the used conditions (Table 2). Owing to the different types of the organic matter the slope of the HI-Tmax plots was also dissimilar. The HI of lignite containing Type III O M decreased very quickly during diagenesis, but only a little during catagenesis, whereas the HI of the coals decreased similarly in both of the evolution zones (Fig. 1).
Hydrocarbon potential Hydrocarbon potential of samples of similar maturity depends on quantity and the type of their organic matter. The HC-potential of the lignite, which had a TOC content (55%) between that of the two coals (50 and 61%), was about 72% of the HC-potential of the coals (Table 1). The lignite of Type III and the coals of Type II differed from each other not only on the basis of their original HC-potential. These ratios also showed dissimilar changes during the artificial evolution (Fig. 2). Decreasing of HC-potential of lignite was considerable even under the mildest experimental conditions (200°C, 1 h = 37%). A rapid decrease could be observed between 200 and 350°C. Namely, in the case of lignite 75% of the HC-potential reduction took place in the diagenetic phase. During catagenesis the slope of the decrease was far smaller. At the same
Table 2. Hydrocarbon potential, Tmax and hydrogen index of the thermallydegraded samples Thermal degradation HI Residue HC-potential(%) Tm,~(°C) (mgHC/gTOC) Temperature Period (°C) (h) Coal-I C o a l - 2 Lignite C o a l - I C o a l - 2 Lignite C o a l - I C o a l - 2 Lignite Unheated sample 100.0 100.0 100.0 402 412 375 212 187 136 200 90.6 98.0 62.6 409 414 392 183 174 77 2 92.7 98.0 65.3 409 414 389 183 167 83 5 89.6 97.2 60.7 405 411 391 174 168 78 300 87.6 88.3 61.2 409 417 396 173 151 71 84.8 88.6 56.6 410 417 410 165 152 70 -87.2 26.1 -417 413 -153 35 350 77.5 84.8 40.4 412 420 418 149 147 58 80.7 80.4 32.2 415 420 420 148 137 38 72.9 75.2 26.5 417 423 429 141 130 31 375 68.1 76.3 38.0 420 422 419 132 130 47 58.7 57.3 28.1 424 426 429 I11 93 34 50.7 58.9 18.2 428 431 439 99 98 19 400 64.6 58.5 21.7 420 432 438 122 102 26 57.8 63.6 19.9 423 428 444 115 106 23 41.7 51.3 13.3 430 433 466 80 82 15 450 26.4 36.4 10.0 441 438 525 47 61 II 25.9 39.6 10.0 431 428 526 48 63 11 29.5 37.0 9.1 433 438 543 55 60 8 500 18.3 12.1 6.7 429 436 543 31 28 7 12.6 12.7 5.7 442 444 546 20 21 6 5.9 12.6 3.6 542 487 549 lO 14 3 - - , Not determined.
910
M. HETI~NYIand Cs. S~C,6 Oc~'rc~sing of the HC-pof ( % 1 0 50
Table 3. Experimentalconditionsnecessaryto reach 10, 50 and 90% decrease in hydrocarbon potential
100
I
[] coat-1
i
-....
0
Fig. 2. Decreasing of the hydrocarbon potential in function of the temperature of thermal degradation (degradation period = 5 h). time, the HC-potential of coals decreased by only 25% degraded at 350°C for 5 h. At the beginning of catagenesis, which was simulated by thermal degradation at 375°C, the change was 40 and 50% in case of coal-I and coal-2, respectively (Table 2). Concerning the reduction of HC-potential a slight difference was observed between the two coals having the same original HC-potential and different biological precursor material. The HC-potential of coal-I decreased by 10% during thermal degradation performed at 200°C. The change of HC-potential of coal-2 was less than 3 % under the same experimental conditions. In each stage of the artificial evolution a small dissimilarity could be observed between the two coals (Fig. 2 and Table 2). The residue potential of the samples decreased not only as a function of temperature, but with the heating period. The effect of the degradation period was highest at 300 and 350°C in the case of lignite, at 375°C in the case of coal-2 and at 400°C in the case of coal-1. The close relationship between the HC-potential and the type of the organic matter could also be demonstrated by the results mentioned above. While the hydrocarbon production of the lignite took place mostly in the first zone of the evolution, the hydrocarbon potential in the coals changed in similar degree during both the diagenesis and the catagenesis. Comparing the lignite with the coals, the experimental conditions necessary to reach the same decrease of their HC-potential were very different. However, a smaller difference was also found between the two coals (Table 3). For example, 50% reduction of the HC-potential took place at 300, 375 and 400°C in the case of lignite, coal-1 and coal-2, respectively. At the same time 90% reduction could be detected at 450°C (lignite) and at 500°C coals. The coal samples differed from each other only slightly. Namely,
Decrease of HC potential 10% 50% 90% Lignite < 200°C* 300°C, 2 h 450°C, 1h Coal-I 200°C, 5h 375°C, 5h 500°C, 1h Coal-2 300°C, l h 400°C, 5 h 500°C, 5 h *At 200°C the decreasingis about 40%. the temperatures were the same (500°C), only the degradation periods were different (1 and 5 h).
Quantity of bitumen The change in the quantity of bitumen during thermal evolution also reflected the slight difference of hydrocarbon generation features of the two coals and a strong one between the lignite and coal samples (Table 4). On the basis of the change of the soluble bitumen content, dissimilarities were observed mainly during diagenesis. In this zone not only the lignite of Type III differed from the coals of Type II but the coals also differed from each other substantially (Fig. 3). Under the mildest experimental conditions bitumen (mO/g TOC) 20
60
100
140
180
220
20
60
100
140
180
220
.~
"~
!!!!!!!!i!i!i!iiii!%
Z "~ 5OO
300-
I"=1 c ~ o b ' m sotu~ ~ tOO-
~latitimd bflum~
S0¢ Fig. 3. The change of the quantity of bitumen during the thermal degradation (degradation period = 5 h).
911
Hydrocarbon generation potential of some Hungarian low-rank coals Table 4. Quantity of bitumen Thermal degradation
Temperature (°C)
Period (h) sample
Chloroform soluble bitumen (mg/gTOC)
Coal-I
Coal-2
Volatilized bitumen (mg/gTOC)
Lignite Coal-I
Coal-2
Total bitumen (mg/gTOC)
Lignite Coal-I
Coal-2
Lignite
61 34 32 27
54 64 53 49
76 75 81 73
n.m. n.m. n.m.
n.m. n.m. n.m.
4 4 4
34 32 27
64 53 49
79 85 77
300
21 23 30
46 43 47
38 36 15
n.m. n.m. n.m.
n.m. n.m. 4
9 7 7
21 23 30
46 43 51
47 43 22
350
65 61 40
57 47 47
I1 7 4
14 19 17
8 15 16
13 16 16
79 80 57
65 62 63
24 23 20
375
19 41 25
51 43 31
11 II 4
37 52 49
27 38 53
22 23 22
56 93 74
78 81 84
33 44 26
400
38 32 26
45 28 20
6 4 2
34 57 73
33 66 76
27 45 40
72 89 99
78 94 96
33 49 42
450
16 10 13
21 12 11
2 2 2
100 87 116
79 80 104
49 51 67
116 97 129
100 92 115
51 53 69
500
11 13 2
4 3 1
2 2
140 145 183
71 100 170
104 118 78
151 158 185
75 103 171
106 120 79
Unheated
200
l
2 5
n.m., non measurable.
(200°C) the soluble bitumen content of the residue and that of the unheated samples was the same in the case of lignite and coal-2. At the same time the original soluble bitumen content of coal-I fell to about one half. At the boundary of diagenesis and catagencsis the quantity of bitumen extracted from the thermally degraded lignitewas about 1/20 of that of the unheated sample. Simultaneously volatilized bitumen of insignificantquantity developed (Table 4). The original soluble bitumen content of coal-2 remained unchanged in the totalzone of itsdiagencsis. In this zone the quantity of bitumen extracted from coal-I firstdropped to a minimum value at 200 and 300°C, after that it increased a little.However, it did not reach the soluble bitumen content of the unheated sample. At 350°C insignificant quantity of volatilized bitumen developed from the three samples. At this temperature the soluble and the volatilized bitumen content of the two coals and consequently the total bitumen content was the same. However, the lignite which yielded the same quantity of volatilized bitumen, has only an insignificant amount of soluble bitumen compared to the coals. The ratio of soluble and volatilizedbitumen developed from the lignitewas 2: I at 300°C and it was 1:4 at the beginning of itscatagencsis (350°C). In the case of coals the ratios mentioned above were about 1:2 at the boundary of the two evolution zones (375°C). During catagenesis both the coals and the lignite proved to be fairly good sources of volatilized bitumen. The quantity of volatilized bitumen developed from the three samples showed a rapid increase in the total zone of the catagencsis (Fig. 3). As a consequence of the different types of organic matter the
ratio o f t h e o r g a n i c c a r b o n c o n t e n t c o n v e r t e d to volatilized b i t u m e n was less in the case o f lignite o f T y p e III t h a n in the case o f coals o f T y p e II. A p p r o x i m a t e l y twice the q u a n t i t y o f the volatilized b i t u m e n d e v e l o p e d f r o m coals t h a n f r o m lignite (Table 4). In the catagenesis z o n e a f u r t h e r c h a r a c t e r istic difference was f o u n d b e t w e e n the lignite a n d the t w o coals. T h e q u a n t i t y o f b i t u m e n e x t r a c t e d f r o m lignite s e e m e d to be a m i n i m a l value, less t h a n 10 m g H C / g T O C . A t the s a m e time, the q u a n t i t y o f the soluble b i t u m e n o r i g i n a t i n g f r o m coals d e c r e a s e d c o n t i n u o u s l y a n d fell to less t h a n 10 m g H C / g T O C at 500°C.
J
iol
coat-1 (o)
y- B6.9'7- 0.53x ra,O,9'7 y,93.~-O.S6x r~,O.~
co~-2 (R) coo1-1* co¢.-2
y,90.52-O.~x ra,O.90
r so
a: \ .
i ,
,
,
V~Wf'lliZid
,
a
\
,
)
biSumml
,
(molg
,~
,
TOC]
Fig. 4. Correlation between the S2-value of the thermally degraded samples and the quantity of the volatilized bitumen developed from them (degradation periods -- 1, 2 and 5 h).
912
M. HE~NY] and Cs. S~dc,6 more favourable hydrocarbon genetic features, than it was in the case of coal-2 (r2= 0.86). Unlike the coals the lignite showed no direct proportion between the volatile bitumen content and the S2-value (Fig. 4).
The bitumen content of the lignite which developed in its natural evolution was converted to hydrocarbon gas mainly during its artificial diagenesis. The original bitumen content of the two coals was converted partly to hydrocarbon gas and partly to volatilized bitumen mostly during their artificial catagenesis. The different hydrocarbon genetic features of the examined samples were revealed in the correlation which was found between the quantity of volatilized bitumen and the amount of hydrocarbons as well as hydrocarbon-like compounds ($2) generated by pyrolysis of insoluble organic matter (Fig. 4). The S2-values concerning the coals proved to be directly proportional to the quantity of volatilized bitumen, the correlation coefficient was 0.90. The correlation was better in the case of coal-I (r 2 = 0.97) which had
Quality o f bitumen
The nature of volatilized and soluble bitumens is different. In the volatilized bitumens during simulation the quantity of the NSO fraction rises as a function of increasing temperature, i.e. the increasing temperature produces more and more free radicals which leave the reactor rapidly in sweeping gas and react in the trap yielding higher and higher amount of the polar compounds than in lower temperature experiments. Figure 5 shows the change of bulk composition during simulation. Three diagrams are
(O)
HCor
O-9o
6 X / ~)
300°C 325°C x 3500C .37soc
74
~,~
~"
/
\
,
/
(60)
26
32
38
\
41.
(b)
50
56
~
• soooc
62
68
74
HCor (66 1/3)
//~60 17 / 2~"~ / 30 /
(61 1/11
,oooc
• ,sooc
40
B - 20o ~ ~ %50 ~ ~ ~40
60
80
Fig. 5. (a), (b) Caption on facing page.
" 300"C x 3500C o 3750C v 400"C • ~,500C . 5000C
Hydrocarbon generation potential of some Hungarian low-rank coals
(C)
913
HCor (305) ~ .
/
D -lSp
\
/
.,~-e
~
/
v 400"C
\
Ill
•
.o.¢
91 l ' l
/ ?
'2<. +\
HCnon -at / (iS'k)
/ 80
,' 90
~ Z NSO(S'k)
Fig. 5(c)
Fig. 5. Ternary diagram depicting relative distributions of non-aromatic hydrocarbons, aromatic hydrocarbons and NSO compounds (asphaltenes and resins) for volatilized bitumens of the coal-I (a) and the lignite (b) and extracted bitumen after experiment of the coal-2 (c). displayed: volatilized bitumens of the coal-1 and the lignite and the soluble bitumen of coal-2. The distributions were only plotted when the yield was over 15 rag. In cases of the coal-I sample and the lignite the above-mentioned reverse maturation trend is obviously recognizable. In the case of coal-1 the 350°C for I h and the two 500°C for 5 h experiments are significantly separated from the others. The first is in the zone of diagenesis, the second is in the zone of metagenesis and the rest is in the phase of hydrocarbon generation. In the case of the lignite the distribution is more widespread than in the cases of the coals in accordance with the Fig. 1. ~r o ,r--' ~"
In the case of extracted bitumens the normal maturation trend (the higher temperature is, the more abundant hydrocarbons are present in soluble phase) was observed. The bitumens of immature stage (200-350°C, see Fig. l) are separated from the more mature bitumens which show a considerable dispersion indicating the complexity of the process. The non-aromatic hydrocarbon fractions were analysed by GC, Fig. 6 shows the ratio of n-alkenes to n-alkanes in the volatilized bitumens. In the soluble bitumens unsaturates were only present in negligible extent. The ratio in the case of coal-I demonstrates a solid temperature dependence. In the
The ratio of n-aikenes to n-o|kanes
0.70-
O,SO-
0,30-OZ'O 'O ' ,ZO.
~
lignite
0.10o o-
0.200,1o. oo, 0.10o-
coo| - 2
1'~
r l:ivv~ h
lh~Sh
,
300oC 325oC 3sooe 375oc 400oc 4Gooc 500oc
Fig. 6. The ratio ofY Cj5 + n-alkenes to • C~ + n-alkanes in volatilized bitumens of the coals and lignite.
914
M. HEI"I~NYIand Cs. SAJC__,6 Ratio
of p r i s t - l - e n e , p r l s t - 2 - e n e
t o ~r n - o l k o n e s
prl,prn s. n 0.10 •
0-
0,10.
0-
0.10
0
ahsh I"~S h Sh lh~s h 1"2%h I"~'S" lh2% h lh~s h 200oc 300oc 325°(: 350oc 375oc 400oc 450°C 50O°C
Fig. 7. The ratio of prist-l-ene + prist-2-ene to ~ C~5+ n-alkanes in the volatilized bitumens under the given experimental conditions. cases of the lignite and coal-2 the thermal dependence is not genuine, albeit some extent of dependence is recognizable in the case of coal-2. In the volatilized bitumens, prist-l-ene and prist-2-ene are present in a considerable amount. Their relative concentrations to n-alkanes are shown in Fig. 7. The variation of the ratio is akin to, in some way, the case of the two coals, but it illustrates a rather complex process in the case of lignite (3maxima). The considerable concentration of pristenes indicates that they must have a different precursor or precursors from tocopherols (Goossens et aL, 1984). The most probable source is the phytyl side chain of chlorophyll (Ikan et al., 1975), but the Archaebacterial lipids (diphytanyl ethers; Chappe et al., 1982) are not negligible as sources. Perhaps all the mentioned sources have contributed to the yields and their varying contributions caused the complexity of the figure (different contributors produce pristenes at different temperatures). The differences in mineral composition also can cause variances (Lao et al., 1989). The amount of ash was as follows: 19.8% in coal-l, 6.62% in coal-2 and 6.49% in lignite. The samples all contain kaolinite as the predominant clay mineral beyond the less important deviations. In the non-aromatic hydrocarbon fraction of the unheated lignite there is an overwhelming component i.e. 93% 16~ (H)-phyllocladane. During simulation its concentration decreases drastically in extracted bitumens and not so severely in the volatilized bitumens (Fig. 8). The great difference starts from 350°C temperature. At least two effects take place: (i) the higher the temperature is, the more important the resident time is; (ii) the higher the temperature is, the more volatile the compound is. The first is the more important factor. There is another significant divergence in the cases of the two bitumens types: the isomerization of 16~ (H)-phyllocladane to 16IT(H)phyllocladane occurs only in the extracted bitumens between 300 and 400°C. In the yields of the 200, 450 and 500°C experiments 16~(H)-phyllocladane was
only present, i.e. at 200°C experiments the isomerization has not started yet and in the 450 and 500°C experiment the degradation of 16fl(H)-phyllocladane was more rapid than its generation. The second fact was probed by the change of the rate of the isomerization as a function of duration, i.e. the rate decreased in the 2-h experiments in proportion to l-h ones and it also decreased in the 5-h experiment as compared to 1- and 5-h ones. In other words the generation of 16/3(H)-phyllocladane is a member of consecutive reaction series. This isomerization has been studied by Noble et al. (1985) and Alexander et al. (1987). The apparent rate parameters were calculated from the l-h experiments using Arrhenius' law to relate the reaction rates to the temperature. We have assumed that the progress of the reaction could be described by unimolecular first-order reaction laws (AH* = 63.66-70.22 kJ/mol and A = 11.26--42.1 s-l). We have started a new series of simulation experiments, further data on this reaction together with other bitumen analysis results will be published in the future.
CONCLUSIONS
(1) The two coals containing Type II OM have higher hydrocarbon potentials and yield more bitumen than the lignite sample containing Type III OM. (2) During pyrolysis experiments at 350°C the lignite, and at 375°C the coals entered the zone of catagenesis. The boundary of the oil and gas zone (T~,x = 460°C) was reached at 400°C by lignite and at 500°C by the coals. (3) During diagenesis the HC-potential of lignite decreased by 75% and that of the coals diminished approximately 50%. (4) During catagenesis the reduction of HCpotential was 20% in lignite and about 45% in the coals.
Hydrocarbon generation potential of some Hungarian low-rank coals (a)
Lignite volotlll zed bitumen
rl,{./J phyllocladone % I ~ z n - alko.es % I l z n-alkenes %
(b)
Lignite exrocted
r-i~c./3 phyllocladane % I~]Z n * alkones %
bttumen
IIIz n - alkenes %
I'~'IZ the rest %
I
90"/, 80%
70%60%-
!
915
I~Z the rest 't, )0% [~ )0%
!
~0'1, ~0"I, 4:"
~D'/,
i
50%-
i0%
40%-
~,O*l,
30%-
30'1,
20*/r
20'1,
10%-
10"1,
A-.
O-
lh2hs h sample 200oC 300oC
unheated 2h
lh2h$1h th2h5 h 3500C 375°C
1h~:~5h lh2h5 h lh2h5 h 400*C 4500C 500"C
O unheated sample
lhzhsh 200"C
300*C
~hZ~5h lhZ 350°C 375°C
4000C
4500C 5000C
Fig. 8. The relative distributions of the most significant components in the non-aromatic hydrocarbon fraction of the volatilized and extracted bitumens of the lignite. (The rest consists of any non-aromatic hydrocarbons except of Cts + n-alkanes, Ct~ + n-alkenes and phyllocladanes.) (5) The good correlation found between $2 and the quantity of volatilized bitumen reflects the oil prone nature of these coals. Such a correlation was not recognized in the case of the lignite which is gas prone. (6) The two coals differ from each other only slightly, their original HC-potentials are nearly the same. Differences were found during the maturation simulation. By the end of diagenesis the residual HC-potential of coal-1 is 50% and that of coal-2 is 60%. Coal-I entered the zone of metagenesis at 500°C in 5 h experiment, but coal-2 still remained in the zone of catagenesis, consequently coal-2 is somewhat more resistant to thermal degradation than coal-l. (7) Unsaturates were present only in volatilized bitumens. The non-aromatic HC fraction of the lignite was predominated by the 16~(H)phyllocladane. During the experiments phyllocladanes were only severely destroyed in the extracted bitumens over 400°C. The isomerization of 16~(H)-phyllocladane to 16fl(H)phyllocladane was only observed in extracted bitumens from the range of the 300 and 400°C experiments. Acknowledgements--This work would not have been possible without the support of the KFH (Central Geological Office of Hungary). We thank Dr V. Dank for the permission to publish. We also thank Drs M. Kedves, I. Elek and Z. A. Horv~ith for palynological, microlithotype and vitrinite reflectance analyses, respectively. We are grateful
to Mr J. Kiss, Mrs E. Pfipay, Mrs I. Szederk6nyi, Mrs A. Mar&, Ms K. D6me and Mrs V. Csontos for technical assistance. Cs. S thanks Dr B. Durand and the Local Committee for benefits which have rendered possible his participation in the Meeting. We are indebted to Dr J. Zumberge of Ruska Labs for his review and constructive criticism of the manuscript.
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M. H~r,~tt and Cs. S~JC_,6
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