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

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IMMATURE

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x cooL-2 (C-Z)

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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

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i

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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

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Z "~ 5OO

300-

I"=1 c ~ o b ' m sotu~ ~ tOO-

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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