Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue5.2000:
Organic Geochemistry 31 (2000) 453±461
www.elsevier.nl/locate/orggeochem
Comparison of Pliocene organic-rich lacustrine sediments in
twin craters
Alice Brukner-Wein a,*, CsanaÂd Sajgo b, Magdolna HeteÂnyi c
a
Geological Institute of Hungary, Budapest, StefaÂnia 14. H-1143, Hungary
Laboratory for Geochemical Research, Hungarian Academy of Science, Budapest, BudaoÈrsi uÂt 45. H-1112, Hungary
c
Institute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, Szeged POB 651 H-6701, Hungary
b
Abstract
The organic matter of Pliocene oil shales from maar-type twin craters (EgyhaÂzaskeszoÈ and VaÂrkeszoÈ) in Hungary
was studied by dierent analytical techniques (Rock-Eval pyrolysis, bitumen analysis, FTIR, elemental analysis and
pyrolysis of the insoluble material). The organic-rich, alginitic layers were deposited at the same time, under the same
palaeoclimatic conditions and have basically similar lithologies. Despite this, the oil shale deposits from each crater
show distinct dierences. Furthermore, within each crater, the older oil shale deposits are dierent from the younger.
This phenomenon can be explained both by variations in organic matter input and changes in the depositional environment. The principal source of the organic matter is the microalgae Botryococcus braunii, but the terrestrial contribution is also signi®cant. The prevalence of the algal material is supported by the elemental composition and
kerogen pyrolysis data. The pyrograms show that there is considerably more algal material in the VaÂrkeszoÈ samples.
Kerogens in the EgyhaÂzaskeszoÈ crater contain much more organic sulphur and pyrite is more abundant. The nominally
Type II kerogens in the twin craters are the products of diverse processes. VaÂrkeszoÈ kerogens are in fact mixtures of
Type I and Type III organic matter and are preserved relatively well. EgyhaÂzaskeszoÈ kerogens must have suered
biological degradation and chemical alteration during pyrite formation, resulting in medium sulphur-rich Type II
kerogen formation. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Oil shales; Pyrolysis; Kerogen elemental composition; Botryococcus braunii; Organic sulphur; Pyritic sulphur
1. Introduction
During the Pliocene several oil shales were deposited
in small maar-type volcanic craters in Hungary (Solti,
1990). Four to ®ve million years ago in the Pannonian
lake system, very intense and repeated volcanic eruptions disturbed the sedimentation. The basic magma
gave rise to stratavolcanoes and the formation of tu
rings. The craters formed by basalt volcanism ®lled with
water of the Pannonian Lake after volcanic activity had
ceased. The small, separate lakes were current free and
oligohaline. The depositional environment Ð warm
water caused by postvolcanic geysers and an abundant
nutrient supply due to the intense weathering of the
* Corresponding author.
E-mail address: brukner@ma®.hu (A. Brukner-Wein).
crater walls Ð was favorable for accumulation of
organic matter (JaÂmbor et al., 1982). Previous studies
revealed that the organic matter mainly consists of well
preserved fossil colonies of Botryococcus braunii algae
and that the selective preservation of the insoluble macromolecules of outer walls of B. braunii was the main
process in the formation of the organic matter (Derenne
et al., 1997).
Among these oil shales special attention has been paid
to those in twin craters named VaÂrkeszoÈ (Vkt) and
EgyhaÂzaskeszoÈ (Ekt). The craters are located along river
RaÂba (about 30 km from GyoÈr) North-western Hungary. The twin craters formed roughly at the same time
and are 1.5 km apart (Fig. 1). The organic-rich layers
were deposited on the same basalt tu base but their
thicknesses signi®cantly dier (28 m in borehole Vkt-1
and 6.6 m in borehole Ekt-34). The narrower Vkt crater
has only a very thin layer of basalt tu, while in the
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00012-7
454
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Fig. 1. Cross section of twin craters showing borehole locations (modi®ed after Solti, 1990).
wider Ekt crater, a thin layer of organic-rich sediment
was deposited on a relatively thick basalt tu deposit.
This basalt tu plays an important role as a reservoir
for the sulphur formed as a result of volcanic activity.
The lithology of both studied sequences is similar, consisting of clayey alginite.
2. Experimental
All geochemical analyses were carried out on core samples (Table 1) crushed and ground to less than 0.06 mm.
Rock-Eval analysis was carried out on a Rock-Eval II
instrument under a helium gas stream at 300 C for 4
min, followed by programmed pyrolysis at a rate of
25 C/min to 550 C (Espitalie et al., 1977). After
removal of inorganic carbonates total organic carbon
contents of the samples were determined by a LECO
Carbon analyzer. Bitumen extraction was carried out
with chloroform in a Soxhlet apparatus. After precipitating asphaltenes the extract was separated into
three fractions by column chromatography.
The FTIR spectra of bitumens and kerogens were
recorded with a Perkin±Elmer 1600 Series spectrophotometer using the KBr disc technique.
Gas chromatographic analysis of the non-aromatic
hydrocarbons was performed on an HP 5890A gas
chromatograph ®tted with a 25 m 0.2 mm id WCOT
fused-silica capillary column coated with OV-1, using
hydrogen as the carrier gas and ¯ame ionization detection (FID). The oven was programmed from 110 to
170 C at 25 C minÿ1 and from 170 to 320 C at 5 C
minÿ1 and the samples were injected in a 20:1 split
mode.
Kerogen was concentrated with repeated HCl and HF
treatments. Elemental analysis (N, C, H, S) of kerogens
was performed on an NA 1500 NCS analyser (Fisons
Instruments) at 1010 C. Kerogen concentrates were
pyrolysed in a Quantum device [MSSV pyrolyzer
directly connected to a Fisons 8000 GC; for details see
Hors®eld et al. (1989)]. 1±2 mg kerogen were sealed in
glass capillary tubes then heated from 300 to 530 C over
a 9 min period and held two min. before a single step
on-line GC-analysis. Following Larter (1984) the products of such pyrolyses (according to the heating rate)
are comparable to the products of ¯ash pyrolyses.
3. Results and discussion
The TOC content varies between 5.4 and 15.1% in the
Vkt-1 borehole and 2.0 and 12.4% in the Ekt-34 borehole. The amount of chloroform soluble organic matter
(bitumen) is 0.33±4.29% in Vkt-1 and 0.11±1.44% in
Ekt-34 borehole. The amount of HCl-insoluble residue
is nearly the same in samples studied in both craters
(60±80 wt%).
Both organic-rich sequences can be divided into two
sections on the basis of the quantity and bulk composition of the bitumen and the Rock-Eval pyrolysis results
(Table 1). This phenomenon is due to the variation of
the organic matter input during sedimentation and also
changes in the depositional environment (redox conditions, sedimentation rate, microbial activities). The
upper part in both craters has lower TOC, bitumen
contents and bitumen/TOC ratios than in the sections
below. The higher HI values and the higher hydrocarbon potential (HCpot) in the latter sections re¯ect
455
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 1
Rock-Eval pyrolysis and geochemical parameters of organic mattera
Depth (m)
TOC (%)
Tmax ( C)
HI
HCpot
Bitumen (%)
bit/TOC (mg/g)
tHC (%)
tNSO (%)
CPI
VaÂrkeszoÈ-1
43.0±43.9
43.9±45.0
45.0±46.2
46.2±47.0
47.0±48.0
48.0±49.0
49.0±50.0
50.0±51.5
51.5±53.0
53.0±54.0
54.0±55.0
55.0±56.0
56.0±57.0
57.0±58.0
58.0±59.0
59.0±60.0
60.0±60.7
60.7±61.7
61.7±62.7
62.7±63.5
63.5±64.5
64.5±65.5
65.5±66.0
66.0±67.0
67.0±68.0
68.0±68.8
68.8±70.0
70.0±71.0
10.38
8.12
7.16
5.44
5.98
8.9
9.01
11.23
10.88
8.51
8.94
10.14
10.98
11.4
10.34
11.46
8.6
7.55
11.57
13.86
12.3
8.6
8.47
10.41
12.79
15.09
11.21
17.8
436
435
437
433
434
434
434
406
428
433
435
436
430
438
432
416
427
420
420
432
431
433
430
430
433
434
429
431
416
503
498
465
493
604
564
580
582
572
485
602
581
635
497
550
420
401
551
618
491
581
440
485
607
567
566
635
45.09
42.51
37.19
26.65
31.18
59.22
54.26
78.41
73.27
52.96
46.27
66.16
74.33
76.91
55.77
77.08
41.12
34.15
77.13
94.7
70.85
55.52
36.69
56.57
94.3
114.34
69.04
127.71
0.53
0.33
0.39
0.38
0.58
1.61
n.d.
3.00
2.82
1.17
0.64
1.20
1.65
1.19
1.04
4.12
0.92
1.17
4.29
2.69
3.22
1.59
1.01
1.32
1.47
2.44
1.28
2.15
50.6
40.4
54.7
69.4
96.4
181.1
n.d.
267.1
259.3
137.6
71.4
118.1
150.4
104.6
100.4
359.1
106.7
154.5
370.7
194.0
261.9
185.3
118.9
126.5
114.7
161.6
114.6
120.9
9.4
12.4
13.4
12.1
11.8
6.4
n.d.
6.7
3.8
7.6
11.0
7.2
9.7
9.2
9.4
3.2
12.1
9.0
3.0
18.5
5.8
6.6
7.0
7.4
8.9
5.6
6.8
5.3
90.6
87.6
86.6
87.9
88.2
93.6
n.d.
93.3
96.2
92.4
89.0
92.8
90.3
90.8
90.6
96.8
87.9
91.0
97.0
81.5
94.2
93.4
93.0
92.6
91.1
94.4
93.2
94.7
10.7
11.9
12.5
8.7
10.1
9.2
n.d.
5.7
5.9
9.6
11.6
7.2
13.2
12.7
14.2
5.3
11.9
12.4
6.7
n.d.
12.8
10.0
8.8
8.9
11.5
10.2
9.1
7.6
EqyhaÂzaskeszoÈ-34
34.2±34.5
35.0±35.5
35.5±36.0
36.0±36.5
36.5±37.0
37.0±37.5
37.5±38.0
38.0±38.5
38.5±39.0
39.0±39.5
39.5±40.0
40.0±40.5
40.5±40.8
2.7
5.4
5.7
2.3
2.0
2.2
2.5
4.3
10.5
12.4
11.2
9.3
9.1
423
431
430
430
432
429
431
423
419
430
429
432
430
317
465
155
264
231
262
301
317
369
525
536
564
474
13.12
26.01
9.53
6.60
4.99
6.32
8.17
13.12
42.95
71.81
65.63
55.71
44.33
0.11
0.26
0.16
0.15
0.13
0.20
0.17
0.25
0.69
1.44
0.83
0.75
0.72
41
49
29
68
65
94
70
58
65
116
73
81
79
10.3
5.3
16.9
10.0
11.2
10.7
10.0
11.8
10.0
5.6
8.6
7.4
5.5
89.7
94.7
83.1
90.0
88.8
89.3
90.0
88.2
90.0
94.4
91.4
92.6
94.5
3.0
4.3
9.6
6.1
4.9
5.4
5.1
3.7
6.2
9.3
10.4
10.1
8.2
Vkt. (ave.)
43.0±48.0
48.0±71.0
7.4
10.8
435
429
475
546
36.5
69.0
0.44
1.93
62.3
179.9
11.8
7.2
88.2
92.7
10.8
9.3
Ekt (ave.)
34.2±38.5
38.5±40.8
3.4
10.5
429
428
288
494
11.0
56.1
0.18
0.88
21.0
83.0
10.8
7.4
89.2
92.6
5.2
8.8
a
TOC: total organic carbon; HI: mg HC/g TOC; HCpot: kg HC/t rock; bitumen: soluble organic matter; bit/TOC: mg bitumen/ g
TOC; tHC=HCsaturated+HCaromatic; tNSO=resin+asphalthene; CPI: carbon preference index in n-C22 and n-C32 range; n.d.: not
determined. Data averaged over depth ranges indicated.
456
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
hydrogen-richness. Beyond that the dierences in HI
values and hydrocarbon potentials reveal a more oilprone organic matter type in Vkt crater. According to
the Rock-Eval data the organic matter in each case is
Type II, but their dierences can readily be seen on
Fig. 2. There is practically no dierence between the
average Tmax values calculated for the two craters (430
and 428 C). The thermal evolution of all the studied
samples has already passed the early diagenesis stage
and reached diagenesis sensu stricto (Vandenbroucke et
al., 1993).
A previous simulated thermal maturation study on a
VaÂrkeszoÈ kerogen suggested that the kerogen is derived
from B. braunii algae and higher plant debris (HeteÂnyi,
1983). The principal algal source of organic matter is B.
braunii microalgae but other algal input cannot be
excluded. The contribution of higher terrestrial plants is
supported by the gas chromatographic data of the aliphatic fractions of bitumens. These fractions show a
strong predominance of odd-carbon numbered C27 to
C31 n-alkanes in both craters and the concentration
of C15±C19 n-alkanes is negligible, similarly to other
immature lacustrine sediments (Meyers et al., 1979;
Ishiwatari et al., 1980; Cranwell, 1984). These odd,
straight hydrocarbons with chain length ranging from
C25 to C33 are widespread in higher plants, however
lipids from B. braunii algae also have a similar hydrocarbon distribution (Largeau et al., 1980; Brukner-Wein
Fig. 2. HI versus Tmax evolution diagram showing averaged
data.
and HeteÂnyi, 1993). The carbon preference index (CPI)
calculated over the C22±C32 range shows the dominance
of odd-carbon numbered, long chain, n-alkane homologues (CPI=5.3±14.2 in Vkt samples and 3.0±10.4 in
Ekt samples in Table 1). The average CPI values of
upper sections display a large dierence (10.8 in Vkt and
5.2 in Ekt) indicating changes in source organisms in
Ekt crater. This observation is corroborated by a considerably lower average HI in the same section.
The presence of a-phyllocladane derived from the
Coniferopsida class of Gymnosperm (Noble et al., 1985,
1986) corroborates the terrestrial input. There are
dierences between the two craters in the occurrence of
a-phyllocladane. On gas chromatograms of Vkt samples, a-phyllocladane is ubiquitous although relative
quantities vary. In samples from Ekt borehole, a-phyllocladane occurs only in those from the lower section.
Above 38 m it is not detected, which can be attributed to some variation of precursors (Fig. 3). The 16a
Fig. 3. Gas chromatograms of saturated hydrocarbon fractions
of bitumen from Ekt samples (20, 21: n-alkanes; phy: a-phyllocladane; CPI: carbon preference index in C22±C32 range).
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
457
(H)-phyllocladane is a characteristic component of
Taxodiaceae±Cupressaceae origin in Neogene coals in
Hungary. We suggest that the input to Ekt crater of
these two gymnospermous family ceased or was reduced
during the deposition of the upper section. Another
possible explanation is that the upper sections were not
deposited simultaneously: Vkt samples accumulated ®rst
and then ¯oral changes took place.
The FTIR spectra of kerogens from Vkt samples
show strong absorption in the aliphatic stretching and
deformation vibration range (2960±2850 cmÿ1 and 1470
show the precmÿ1, respectively). Kerogens marked
sence of long-chain aliphatic compounds (720 cm ÿ1) in
accordance with their higher HI values (604, 635 and
581, respectively; Fig. 4). FTIR spectra of kerogens of
Ekt samples indicate the presence of considerable
amounts of sulphur (550±420 cmÿ1). The band intensities arising from aromatic (1610 cmÿ1) and carbonyl
(1710 cmÿ1) stretching vibrations are similar in each
sample (Fig. 5).
The elemental composition and atomic ratios of the
kerogens isolated from selected samples in both craters
are summarised in Table 2. There are dierences in
kerogen elemental composition between the two craters.
In the Ekt crater, the high sulphur contents of kerogen
concentrates (S) are the consequence of the presence
of a considerable amount of pyrite beside a substantial
organic sulphur content. The relatively low pyrite content in the Vkt crater can be explained by the thin basalt
tu deposit under the thick alginite layer. This basalt
tu would have been an appropriate reservoir for the
sulphur formed by volcanic activity.
The H, N and C contents are higher in samples from
Vkt whereas organic sulphur content is higher in Ekt
samples (Table 2). It is also instructive to compare the
atomic ratios for the kerogens, because of their widely
varying pyrite contents. The elemental ratios of the
kerogens are consistent with a prevalence of algal
material (H/C=1.2±1.6; Table 2). However, the low N/
C ratios suggest vascular plant origin (Meyers, 1994),
although the low ratios could also be due to dierent
factors [depositional setting, microbial reworking, early
diagenetic reactions (Patience et al., 1992)]. The low N/
C ratios might accompany high rates of aquatic productivity, possibly under conditions of limited nitrogen
availability (Meyers, 1992). The studied samples are
examples of lipid-rich and nitrogen-poor kerogens.
There are some dierences between the H/C and N/C
ratios not only between the two craters but within the
sections de®ned in Table 1. In Vkt kerogens the H/C
and N/C ratios are 1.44 and 0.024 in the upper section
and they vary between 1.41 and 1.64 and between 0.018
and 0.024 in the lower section, respectively. In the upper
section of Ekt crater the H/C ratios show extreme
Fig. 4. FTIR spectra of kerogens from Vkt-1 samples.
Fig. 5. FTIR spectra of kerogens from Ekt-34 samples.
458
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 2
Elemental composition and atomic ratios of kerogensa
Depth (m)
N (%)
C (%)
H (%)
VaÂrkeszoÈ-1
43.0±43.9
1.53
53.9
6.50
48.0±49.0
57.0±58.0
60.0±60.7
63.5±64.5
64.5±65.5
1.55
1.30
1.98
1.35
1.70
72.6
58.4
67.9
48.4
69.2
9.93
7.83
7.98
6.31
8.43
EgyhaÂzaskeszoÈ-34
35.0±35.5
35.5±36.0
39.0±39.5
40.5±40.8
1.06
1.49
1.22
1.40
53.9
54.0
49.3
54.6
6.79
5.05
6.04
6.38
a
S (%)
Sorg (%)
H/C
N/C
Sorg/C
1.04
0.17
1.44
0.024
0.001
0.45
0.97
1.66
0.98
4.22
0.35
0.18
1.47
0.26
3.56
1.64
1.61
1.41
1.56
1.46
0.018
0.018
0.024
0.023
0.021
0.002
0.001
0.009
0.001
0.020
11.30
7.87
10.44
12.37
3.67
2.70
4.42
4.43
1.51
1.12
1.47
1.40
0.016
0.023
0.021
0.022
0.026
0.019
0.034
0.030
S: total S in kerogen concentrates; Sorg=SÿSpyritic.
values: 1.51 and 1.12, while in the lower section these
values are much moderate in their dierences. The same
trend is to be seen in the N/C ratios (Table 2). The
higher value of H/C ratios shows more algal contribution to the kerogen, also corroborated by HI values.
The relative N-content of kerogens (N/C ratios in Table
2) is controlled by the nitrogen availability in the lake
water and by land-plant contribution.
There are striking dierences in Sorg/C ratios between
the kerogens from the two craters. In the case of Vkt
crater the Sorg/C ratios are an order of magnitude lower
(except for the deepest sample), than those from Ekt
crater (Table 2). The ratio also shows a relatively strong
dierence between the upper and lower section of the
Ekt-34 borehole (0.022 and 0.032, respectively). Di Primio and Hors®eld (1996) studied and divided kerogens
on the basis of suphur contents into three groups. On
the basis of their classi®cation, kerogens in Vkt are low
sulphur Type II while those in Ekt are medium sulphur
Type II. The dierent Sorg/C ratios in the craters indicate very dierent sulphate contents in the lake water of
the craters. The Ekt crater has a thick basalt tu
(40 m) and a basalt tute (40 m) deposits while in
Vkt crater 11 m basalt tu was only accumulated. The
relative sulphur availability for alginite (6.6 m) in Ekt
crater was much higher than that for alginite (28 m) in
Vkt crater. Comparing sulphur-richness in the craters
on the basis of mass ratios, sulphur content in Ekt crater
might have been 20±30 times more than that in Vkt
crater. The pyritic sulphur content varies between 0.1
and 0.84% in Vkt samples and between 5.17 and 7.94%
in Ekt samples. In the case of Ekt kerogens the formation of the reduced sulphur consumed a considerable
amount of organic matter and the sulphate-reducing
bacteria (Desulfovibrio) must have contributed to the
organic matter. Despite the reducing (anaerobic) conditions
during pyrite formation a part of the organic matter was
thus oxidised (consumed) by bacteria.
The original organic input from biota living in the
crater lakes and from the organic transport of the surrounding vegetation must have been similar during
deposition of lower sections. The organic inputs to the
craters were dierent in the upper sections. The compositional dierences were partly caused by microbial and
chemical alterations during pyrite formation, in proportion of pyrite content.
Some pyrolysis±gas chromatographic data are summarized in Table 3. Two pyrograms of Vkt and Ekt
kerogens are shown in Fig. 6A and B, respectively. The
relative amounts of ®ve alkane/alk-1-ene doublets
represent the distribution of n-alkanes/n-alkenes within
each GC traces. The four Ekt samples and the deepest
Vkt sample are the richest in the C5 doublet and the
poorest in the C25 one. These observations suggest that
these samples are more gas prone and less paranic
than the other 5 Vkt samples. The normalized relative
abundances of n-octene, (m+p)-xylenes and phenol in
the kerogen pyrolysates are also shown in Table 3.
Larter (1984) used the relative concentrations of these
compounds in a ternary diagram for kerogen characterization. The phenol content of kerogen was interpreted
to represent terrestrial contribution, dominantly vascular plant input. The relative phenol concentrations in
the crater samples are higher, than it would be expected
on the basis of HI indices and of H/C ratios. The relatively higher n-octene concentrations in Vkt samples
indicate greater algal organic matter contents compared
to Ekt samples, in good agreement with the HI and
HCpot values (Table 1). This is also expressed by the noctene/(m+p)-xylenes ratio (Table 3). The relatively
higher xylene concentrations in Ekt samples indicate
that they contain Type II kerogens predominantly,
459
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 3
Some relative concentrations and indices calculated from pyrolysis±gas chromatography tracesa
Depth (m)
C5 (rel%) C10 (rel%) C15 (rel%) C20 (rel%) C25 (rel%) o (rel%) x (rel%) ph (rel%) o/x
mt/t
VaÂrkeszoÈ-1
43.0±43.9
16.3
19.8
27.0
22.1
14.8
30.0
27.5
42.5
1.09 0.131 0.81
48.0±49.0
57.0±58.0
60.0±60.7
63.5±64.5
64.5±65.5
16.1
26.4
20.0
18.4
31.8
20.5
28.1
23.5
18.6
30.4
21.0
22.6
19.6
17.6
23.2
18.8
11.8
16.3
22.0
11.6
23.6
11.1
20.6
23.4
3.0
42.6
46.0
21.3
32.2
25.7
22.8
11.3
27.7
20.6
33.9
34.6
42.7
51.0
47.2
40.4
1.87
4.07
0.77
1.56
0.76
0.213
0.236
0.174
0.135
0.193
0.33
0.37
0.41
0.65
0.37
EgyhaÂzaskeszoÈ-34
35.0±35.5
36.7
35.5±36.0
31.1
39.0±39.5
32.2
40.5-40.8
34.6
23.6
34.9
34.2
23.9
29.6
29.5
27.7
22.1
8.1
4.5
6.1
17.4
2.0
±
±
2.0
19.1
6.9
32.4
18.8
45.1
42.3
24.6
36.7
35.8
50.8
43.0
44.5
0.42
0.16
1.32
0.51
0.243
0.182
0.148
0.208
0.74
1.07
0.21
0.99
pr/17
a
C5, C10, C15, C20 and C25 rel%: normalized distribution of the given n-alk-1-ene/n-alkane doublets; o, x and ph rel%: normalized distribution of oct-1-ene, (m+p)-xylenes and phenol; o/x:ratio . oct-1-ene/(m+p)-xylenes; mt/t: 2-methylthiophene/toluene;
pr/17: (pristane+prist-1-ene+prist-2-ene)/heptadec-1-ene+n-heptadecane).
Fig. 6. A and B: pyrograms of kerogens from Vkt-1 and Ekt-34 boreholes [numbers indicate the carbon number of the n-alk-1-ene/nalkane doublets (*,*, respectively); b=benzene; t=toluene; x=(m+p)-xylenes; ph=phenol; PrI±II=prist-1-ene and prist-2-ene].
while Vkt samples consist of mixed Type I and Type III
kerogens, resulting in a pseudo-Type II kerogen.
The 2-methylthiophene/toluene ratio displays some
variation, but it does not re¯ect the dierences found in
the case of Sorg/C ratio between the craters. Consequently,
an important part of organic sulphur is not present in
thiophenic form, but in other organic sulphur-containing moieties.
460
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
The pr/17 ratio is somewhat higher in Ekt samples
than in those from Vkt. The same time the ratio is
higher in the upper sections of both craters, indicating
more chlorophyll-rich precursor contributions, than in
lower sections. This observation is in accord with the
changes of N/C ratios, because both nitrogen and phytol are units of chlorophyll.
4. Conclusions
Despite being roughly the same age, deposited under
the same climatic conditions and of similar basic lithology, the alginite deposits in the twin craters developed in
dierent ways. Vkt kerogens contain more aliphatic
compounds, have higher H/C ratios and lower Sorg
contents. Ekt kerogens contain relatively less aliphatic
compounds and have higher organic and pyritic sulphur
quantities. The principal source of organic matter is B.
braunii microalgae but other algal input cannot be
excluded. The terrestrial contribution is signi®cant but
its relative amount and nature are variable. As a result
of volcanic activity a signi®cant amount of sulphur was
deposited in the twin craters, but in Vkt there was no
mineral sink for binding sulphur due to a lack of abundant basaltic tu. Assuming a similar nutrient supply,
the conditions for accumulation and preservation of
organic matter (oxicity, biological activity, sedimentation rate and delivery of remobilised basalt tu) were
more favorable in the narrow and deep Vkt crater, than
in Ekt. The thinner layer of organic matter sedimented
in the wider Ekt crater might have suered more chemical and biological oxidation as well.
The Type II kerogens in the twin craters are the products of dierent processes. Vkt kerogens are mixtures
of Type I and Type III organic matter predominantly
and are preserved relatively better. Ekt kerogens must
have suered more severe biological and chemical
alteration during microbial sulphate reduction. During
oxidation, the proportion of the more reactive lipid-rich
organic matter decreased and the residual organic matter was enriched in land-derived components. During
the reduction of sulphates, a portion of the sulphur
reacted with organic matter and was incorporated into
the macromolecular network of kerogen, producing
medium sulphur-rich Type II kerogen in the Ekt crater.
Acknowledgements
This work was funded through grant OTKA-025541
and AKP 96/2-558 2.5/35 from the Hungarian National
Science Foundation. The authors thank Dr. Michael
Kruge and Dr. FrancËois Baudin for helpful comments.
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www.elsevier.nl/locate/orggeochem
Comparison of Pliocene organic-rich lacustrine sediments in
twin craters
Alice Brukner-Wein a,*, CsanaÂd Sajgo b, Magdolna HeteÂnyi c
a
Geological Institute of Hungary, Budapest, StefaÂnia 14. H-1143, Hungary
Laboratory for Geochemical Research, Hungarian Academy of Science, Budapest, BudaoÈrsi uÂt 45. H-1112, Hungary
c
Institute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, Szeged POB 651 H-6701, Hungary
b
Abstract
The organic matter of Pliocene oil shales from maar-type twin craters (EgyhaÂzaskeszoÈ and VaÂrkeszoÈ) in Hungary
was studied by dierent analytical techniques (Rock-Eval pyrolysis, bitumen analysis, FTIR, elemental analysis and
pyrolysis of the insoluble material). The organic-rich, alginitic layers were deposited at the same time, under the same
palaeoclimatic conditions and have basically similar lithologies. Despite this, the oil shale deposits from each crater
show distinct dierences. Furthermore, within each crater, the older oil shale deposits are dierent from the younger.
This phenomenon can be explained both by variations in organic matter input and changes in the depositional environment. The principal source of the organic matter is the microalgae Botryococcus braunii, but the terrestrial contribution is also signi®cant. The prevalence of the algal material is supported by the elemental composition and
kerogen pyrolysis data. The pyrograms show that there is considerably more algal material in the VaÂrkeszoÈ samples.
Kerogens in the EgyhaÂzaskeszoÈ crater contain much more organic sulphur and pyrite is more abundant. The nominally
Type II kerogens in the twin craters are the products of diverse processes. VaÂrkeszoÈ kerogens are in fact mixtures of
Type I and Type III organic matter and are preserved relatively well. EgyhaÂzaskeszoÈ kerogens must have suered
biological degradation and chemical alteration during pyrite formation, resulting in medium sulphur-rich Type II
kerogen formation. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Oil shales; Pyrolysis; Kerogen elemental composition; Botryococcus braunii; Organic sulphur; Pyritic sulphur
1. Introduction
During the Pliocene several oil shales were deposited
in small maar-type volcanic craters in Hungary (Solti,
1990). Four to ®ve million years ago in the Pannonian
lake system, very intense and repeated volcanic eruptions disturbed the sedimentation. The basic magma
gave rise to stratavolcanoes and the formation of tu
rings. The craters formed by basalt volcanism ®lled with
water of the Pannonian Lake after volcanic activity had
ceased. The small, separate lakes were current free and
oligohaline. The depositional environment Ð warm
water caused by postvolcanic geysers and an abundant
nutrient supply due to the intense weathering of the
* Corresponding author.
E-mail address: brukner@ma®.hu (A. Brukner-Wein).
crater walls Ð was favorable for accumulation of
organic matter (JaÂmbor et al., 1982). Previous studies
revealed that the organic matter mainly consists of well
preserved fossil colonies of Botryococcus braunii algae
and that the selective preservation of the insoluble macromolecules of outer walls of B. braunii was the main
process in the formation of the organic matter (Derenne
et al., 1997).
Among these oil shales special attention has been paid
to those in twin craters named VaÂrkeszoÈ (Vkt) and
EgyhaÂzaskeszoÈ (Ekt). The craters are located along river
RaÂba (about 30 km from GyoÈr) North-western Hungary. The twin craters formed roughly at the same time
and are 1.5 km apart (Fig. 1). The organic-rich layers
were deposited on the same basalt tu base but their
thicknesses signi®cantly dier (28 m in borehole Vkt-1
and 6.6 m in borehole Ekt-34). The narrower Vkt crater
has only a very thin layer of basalt tu, while in the
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00012-7
454
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Fig. 1. Cross section of twin craters showing borehole locations (modi®ed after Solti, 1990).
wider Ekt crater, a thin layer of organic-rich sediment
was deposited on a relatively thick basalt tu deposit.
This basalt tu plays an important role as a reservoir
for the sulphur formed as a result of volcanic activity.
The lithology of both studied sequences is similar, consisting of clayey alginite.
2. Experimental
All geochemical analyses were carried out on core samples (Table 1) crushed and ground to less than 0.06 mm.
Rock-Eval analysis was carried out on a Rock-Eval II
instrument under a helium gas stream at 300 C for 4
min, followed by programmed pyrolysis at a rate of
25 C/min to 550 C (Espitalie et al., 1977). After
removal of inorganic carbonates total organic carbon
contents of the samples were determined by a LECO
Carbon analyzer. Bitumen extraction was carried out
with chloroform in a Soxhlet apparatus. After precipitating asphaltenes the extract was separated into
three fractions by column chromatography.
The FTIR spectra of bitumens and kerogens were
recorded with a Perkin±Elmer 1600 Series spectrophotometer using the KBr disc technique.
Gas chromatographic analysis of the non-aromatic
hydrocarbons was performed on an HP 5890A gas
chromatograph ®tted with a 25 m 0.2 mm id WCOT
fused-silica capillary column coated with OV-1, using
hydrogen as the carrier gas and ¯ame ionization detection (FID). The oven was programmed from 110 to
170 C at 25 C minÿ1 and from 170 to 320 C at 5 C
minÿ1 and the samples were injected in a 20:1 split
mode.
Kerogen was concentrated with repeated HCl and HF
treatments. Elemental analysis (N, C, H, S) of kerogens
was performed on an NA 1500 NCS analyser (Fisons
Instruments) at 1010 C. Kerogen concentrates were
pyrolysed in a Quantum device [MSSV pyrolyzer
directly connected to a Fisons 8000 GC; for details see
Hors®eld et al. (1989)]. 1±2 mg kerogen were sealed in
glass capillary tubes then heated from 300 to 530 C over
a 9 min period and held two min. before a single step
on-line GC-analysis. Following Larter (1984) the products of such pyrolyses (according to the heating rate)
are comparable to the products of ¯ash pyrolyses.
3. Results and discussion
The TOC content varies between 5.4 and 15.1% in the
Vkt-1 borehole and 2.0 and 12.4% in the Ekt-34 borehole. The amount of chloroform soluble organic matter
(bitumen) is 0.33±4.29% in Vkt-1 and 0.11±1.44% in
Ekt-34 borehole. The amount of HCl-insoluble residue
is nearly the same in samples studied in both craters
(60±80 wt%).
Both organic-rich sequences can be divided into two
sections on the basis of the quantity and bulk composition of the bitumen and the Rock-Eval pyrolysis results
(Table 1). This phenomenon is due to the variation of
the organic matter input during sedimentation and also
changes in the depositional environment (redox conditions, sedimentation rate, microbial activities). The
upper part in both craters has lower TOC, bitumen
contents and bitumen/TOC ratios than in the sections
below. The higher HI values and the higher hydrocarbon potential (HCpot) in the latter sections re¯ect
455
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 1
Rock-Eval pyrolysis and geochemical parameters of organic mattera
Depth (m)
TOC (%)
Tmax ( C)
HI
HCpot
Bitumen (%)
bit/TOC (mg/g)
tHC (%)
tNSO (%)
CPI
VaÂrkeszoÈ-1
43.0±43.9
43.9±45.0
45.0±46.2
46.2±47.0
47.0±48.0
48.0±49.0
49.0±50.0
50.0±51.5
51.5±53.0
53.0±54.0
54.0±55.0
55.0±56.0
56.0±57.0
57.0±58.0
58.0±59.0
59.0±60.0
60.0±60.7
60.7±61.7
61.7±62.7
62.7±63.5
63.5±64.5
64.5±65.5
65.5±66.0
66.0±67.0
67.0±68.0
68.0±68.8
68.8±70.0
70.0±71.0
10.38
8.12
7.16
5.44
5.98
8.9
9.01
11.23
10.88
8.51
8.94
10.14
10.98
11.4
10.34
11.46
8.6
7.55
11.57
13.86
12.3
8.6
8.47
10.41
12.79
15.09
11.21
17.8
436
435
437
433
434
434
434
406
428
433
435
436
430
438
432
416
427
420
420
432
431
433
430
430
433
434
429
431
416
503
498
465
493
604
564
580
582
572
485
602
581
635
497
550
420
401
551
618
491
581
440
485
607
567
566
635
45.09
42.51
37.19
26.65
31.18
59.22
54.26
78.41
73.27
52.96
46.27
66.16
74.33
76.91
55.77
77.08
41.12
34.15
77.13
94.7
70.85
55.52
36.69
56.57
94.3
114.34
69.04
127.71
0.53
0.33
0.39
0.38
0.58
1.61
n.d.
3.00
2.82
1.17
0.64
1.20
1.65
1.19
1.04
4.12
0.92
1.17
4.29
2.69
3.22
1.59
1.01
1.32
1.47
2.44
1.28
2.15
50.6
40.4
54.7
69.4
96.4
181.1
n.d.
267.1
259.3
137.6
71.4
118.1
150.4
104.6
100.4
359.1
106.7
154.5
370.7
194.0
261.9
185.3
118.9
126.5
114.7
161.6
114.6
120.9
9.4
12.4
13.4
12.1
11.8
6.4
n.d.
6.7
3.8
7.6
11.0
7.2
9.7
9.2
9.4
3.2
12.1
9.0
3.0
18.5
5.8
6.6
7.0
7.4
8.9
5.6
6.8
5.3
90.6
87.6
86.6
87.9
88.2
93.6
n.d.
93.3
96.2
92.4
89.0
92.8
90.3
90.8
90.6
96.8
87.9
91.0
97.0
81.5
94.2
93.4
93.0
92.6
91.1
94.4
93.2
94.7
10.7
11.9
12.5
8.7
10.1
9.2
n.d.
5.7
5.9
9.6
11.6
7.2
13.2
12.7
14.2
5.3
11.9
12.4
6.7
n.d.
12.8
10.0
8.8
8.9
11.5
10.2
9.1
7.6
EqyhaÂzaskeszoÈ-34
34.2±34.5
35.0±35.5
35.5±36.0
36.0±36.5
36.5±37.0
37.0±37.5
37.5±38.0
38.0±38.5
38.5±39.0
39.0±39.5
39.5±40.0
40.0±40.5
40.5±40.8
2.7
5.4
5.7
2.3
2.0
2.2
2.5
4.3
10.5
12.4
11.2
9.3
9.1
423
431
430
430
432
429
431
423
419
430
429
432
430
317
465
155
264
231
262
301
317
369
525
536
564
474
13.12
26.01
9.53
6.60
4.99
6.32
8.17
13.12
42.95
71.81
65.63
55.71
44.33
0.11
0.26
0.16
0.15
0.13
0.20
0.17
0.25
0.69
1.44
0.83
0.75
0.72
41
49
29
68
65
94
70
58
65
116
73
81
79
10.3
5.3
16.9
10.0
11.2
10.7
10.0
11.8
10.0
5.6
8.6
7.4
5.5
89.7
94.7
83.1
90.0
88.8
89.3
90.0
88.2
90.0
94.4
91.4
92.6
94.5
3.0
4.3
9.6
6.1
4.9
5.4
5.1
3.7
6.2
9.3
10.4
10.1
8.2
Vkt. (ave.)
43.0±48.0
48.0±71.0
7.4
10.8
435
429
475
546
36.5
69.0
0.44
1.93
62.3
179.9
11.8
7.2
88.2
92.7
10.8
9.3
Ekt (ave.)
34.2±38.5
38.5±40.8
3.4
10.5
429
428
288
494
11.0
56.1
0.18
0.88
21.0
83.0
10.8
7.4
89.2
92.6
5.2
8.8
a
TOC: total organic carbon; HI: mg HC/g TOC; HCpot: kg HC/t rock; bitumen: soluble organic matter; bit/TOC: mg bitumen/ g
TOC; tHC=HCsaturated+HCaromatic; tNSO=resin+asphalthene; CPI: carbon preference index in n-C22 and n-C32 range; n.d.: not
determined. Data averaged over depth ranges indicated.
456
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
hydrogen-richness. Beyond that the dierences in HI
values and hydrocarbon potentials reveal a more oilprone organic matter type in Vkt crater. According to
the Rock-Eval data the organic matter in each case is
Type II, but their dierences can readily be seen on
Fig. 2. There is practically no dierence between the
average Tmax values calculated for the two craters (430
and 428 C). The thermal evolution of all the studied
samples has already passed the early diagenesis stage
and reached diagenesis sensu stricto (Vandenbroucke et
al., 1993).
A previous simulated thermal maturation study on a
VaÂrkeszoÈ kerogen suggested that the kerogen is derived
from B. braunii algae and higher plant debris (HeteÂnyi,
1983). The principal algal source of organic matter is B.
braunii microalgae but other algal input cannot be
excluded. The contribution of higher terrestrial plants is
supported by the gas chromatographic data of the aliphatic fractions of bitumens. These fractions show a
strong predominance of odd-carbon numbered C27 to
C31 n-alkanes in both craters and the concentration
of C15±C19 n-alkanes is negligible, similarly to other
immature lacustrine sediments (Meyers et al., 1979;
Ishiwatari et al., 1980; Cranwell, 1984). These odd,
straight hydrocarbons with chain length ranging from
C25 to C33 are widespread in higher plants, however
lipids from B. braunii algae also have a similar hydrocarbon distribution (Largeau et al., 1980; Brukner-Wein
Fig. 2. HI versus Tmax evolution diagram showing averaged
data.
and HeteÂnyi, 1993). The carbon preference index (CPI)
calculated over the C22±C32 range shows the dominance
of odd-carbon numbered, long chain, n-alkane homologues (CPI=5.3±14.2 in Vkt samples and 3.0±10.4 in
Ekt samples in Table 1). The average CPI values of
upper sections display a large dierence (10.8 in Vkt and
5.2 in Ekt) indicating changes in source organisms in
Ekt crater. This observation is corroborated by a considerably lower average HI in the same section.
The presence of a-phyllocladane derived from the
Coniferopsida class of Gymnosperm (Noble et al., 1985,
1986) corroborates the terrestrial input. There are
dierences between the two craters in the occurrence of
a-phyllocladane. On gas chromatograms of Vkt samples, a-phyllocladane is ubiquitous although relative
quantities vary. In samples from Ekt borehole, a-phyllocladane occurs only in those from the lower section.
Above 38 m it is not detected, which can be attributed to some variation of precursors (Fig. 3). The 16a
Fig. 3. Gas chromatograms of saturated hydrocarbon fractions
of bitumen from Ekt samples (20, 21: n-alkanes; phy: a-phyllocladane; CPI: carbon preference index in C22±C32 range).
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
457
(H)-phyllocladane is a characteristic component of
Taxodiaceae±Cupressaceae origin in Neogene coals in
Hungary. We suggest that the input to Ekt crater of
these two gymnospermous family ceased or was reduced
during the deposition of the upper section. Another
possible explanation is that the upper sections were not
deposited simultaneously: Vkt samples accumulated ®rst
and then ¯oral changes took place.
The FTIR spectra of kerogens from Vkt samples
show strong absorption in the aliphatic stretching and
deformation vibration range (2960±2850 cmÿ1 and 1470
show the precmÿ1, respectively). Kerogens marked
sence of long-chain aliphatic compounds (720 cm ÿ1) in
accordance with their higher HI values (604, 635 and
581, respectively; Fig. 4). FTIR spectra of kerogens of
Ekt samples indicate the presence of considerable
amounts of sulphur (550±420 cmÿ1). The band intensities arising from aromatic (1610 cmÿ1) and carbonyl
(1710 cmÿ1) stretching vibrations are similar in each
sample (Fig. 5).
The elemental composition and atomic ratios of the
kerogens isolated from selected samples in both craters
are summarised in Table 2. There are dierences in
kerogen elemental composition between the two craters.
In the Ekt crater, the high sulphur contents of kerogen
concentrates (S) are the consequence of the presence
of a considerable amount of pyrite beside a substantial
organic sulphur content. The relatively low pyrite content in the Vkt crater can be explained by the thin basalt
tu deposit under the thick alginite layer. This basalt
tu would have been an appropriate reservoir for the
sulphur formed by volcanic activity.
The H, N and C contents are higher in samples from
Vkt whereas organic sulphur content is higher in Ekt
samples (Table 2). It is also instructive to compare the
atomic ratios for the kerogens, because of their widely
varying pyrite contents. The elemental ratios of the
kerogens are consistent with a prevalence of algal
material (H/C=1.2±1.6; Table 2). However, the low N/
C ratios suggest vascular plant origin (Meyers, 1994),
although the low ratios could also be due to dierent
factors [depositional setting, microbial reworking, early
diagenetic reactions (Patience et al., 1992)]. The low N/
C ratios might accompany high rates of aquatic productivity, possibly under conditions of limited nitrogen
availability (Meyers, 1992). The studied samples are
examples of lipid-rich and nitrogen-poor kerogens.
There are some dierences between the H/C and N/C
ratios not only between the two craters but within the
sections de®ned in Table 1. In Vkt kerogens the H/C
and N/C ratios are 1.44 and 0.024 in the upper section
and they vary between 1.41 and 1.64 and between 0.018
and 0.024 in the lower section, respectively. In the upper
section of Ekt crater the H/C ratios show extreme
Fig. 4. FTIR spectra of kerogens from Vkt-1 samples.
Fig. 5. FTIR spectra of kerogens from Ekt-34 samples.
458
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 2
Elemental composition and atomic ratios of kerogensa
Depth (m)
N (%)
C (%)
H (%)
VaÂrkeszoÈ-1
43.0±43.9
1.53
53.9
6.50
48.0±49.0
57.0±58.0
60.0±60.7
63.5±64.5
64.5±65.5
1.55
1.30
1.98
1.35
1.70
72.6
58.4
67.9
48.4
69.2
9.93
7.83
7.98
6.31
8.43
EgyhaÂzaskeszoÈ-34
35.0±35.5
35.5±36.0
39.0±39.5
40.5±40.8
1.06
1.49
1.22
1.40
53.9
54.0
49.3
54.6
6.79
5.05
6.04
6.38
a
S (%)
Sorg (%)
H/C
N/C
Sorg/C
1.04
0.17
1.44
0.024
0.001
0.45
0.97
1.66
0.98
4.22
0.35
0.18
1.47
0.26
3.56
1.64
1.61
1.41
1.56
1.46
0.018
0.018
0.024
0.023
0.021
0.002
0.001
0.009
0.001
0.020
11.30
7.87
10.44
12.37
3.67
2.70
4.42
4.43
1.51
1.12
1.47
1.40
0.016
0.023
0.021
0.022
0.026
0.019
0.034
0.030
S: total S in kerogen concentrates; Sorg=SÿSpyritic.
values: 1.51 and 1.12, while in the lower section these
values are much moderate in their dierences. The same
trend is to be seen in the N/C ratios (Table 2). The
higher value of H/C ratios shows more algal contribution to the kerogen, also corroborated by HI values.
The relative N-content of kerogens (N/C ratios in Table
2) is controlled by the nitrogen availability in the lake
water and by land-plant contribution.
There are striking dierences in Sorg/C ratios between
the kerogens from the two craters. In the case of Vkt
crater the Sorg/C ratios are an order of magnitude lower
(except for the deepest sample), than those from Ekt
crater (Table 2). The ratio also shows a relatively strong
dierence between the upper and lower section of the
Ekt-34 borehole (0.022 and 0.032, respectively). Di Primio and Hors®eld (1996) studied and divided kerogens
on the basis of suphur contents into three groups. On
the basis of their classi®cation, kerogens in Vkt are low
sulphur Type II while those in Ekt are medium sulphur
Type II. The dierent Sorg/C ratios in the craters indicate very dierent sulphate contents in the lake water of
the craters. The Ekt crater has a thick basalt tu
(40 m) and a basalt tute (40 m) deposits while in
Vkt crater 11 m basalt tu was only accumulated. The
relative sulphur availability for alginite (6.6 m) in Ekt
crater was much higher than that for alginite (28 m) in
Vkt crater. Comparing sulphur-richness in the craters
on the basis of mass ratios, sulphur content in Ekt crater
might have been 20±30 times more than that in Vkt
crater. The pyritic sulphur content varies between 0.1
and 0.84% in Vkt samples and between 5.17 and 7.94%
in Ekt samples. In the case of Ekt kerogens the formation of the reduced sulphur consumed a considerable
amount of organic matter and the sulphate-reducing
bacteria (Desulfovibrio) must have contributed to the
organic matter. Despite the reducing (anaerobic) conditions
during pyrite formation a part of the organic matter was
thus oxidised (consumed) by bacteria.
The original organic input from biota living in the
crater lakes and from the organic transport of the surrounding vegetation must have been similar during
deposition of lower sections. The organic inputs to the
craters were dierent in the upper sections. The compositional dierences were partly caused by microbial and
chemical alterations during pyrite formation, in proportion of pyrite content.
Some pyrolysis±gas chromatographic data are summarized in Table 3. Two pyrograms of Vkt and Ekt
kerogens are shown in Fig. 6A and B, respectively. The
relative amounts of ®ve alkane/alk-1-ene doublets
represent the distribution of n-alkanes/n-alkenes within
each GC traces. The four Ekt samples and the deepest
Vkt sample are the richest in the C5 doublet and the
poorest in the C25 one. These observations suggest that
these samples are more gas prone and less paranic
than the other 5 Vkt samples. The normalized relative
abundances of n-octene, (m+p)-xylenes and phenol in
the kerogen pyrolysates are also shown in Table 3.
Larter (1984) used the relative concentrations of these
compounds in a ternary diagram for kerogen characterization. The phenol content of kerogen was interpreted
to represent terrestrial contribution, dominantly vascular plant input. The relative phenol concentrations in
the crater samples are higher, than it would be expected
on the basis of HI indices and of H/C ratios. The relatively higher n-octene concentrations in Vkt samples
indicate greater algal organic matter contents compared
to Ekt samples, in good agreement with the HI and
HCpot values (Table 1). This is also expressed by the noctene/(m+p)-xylenes ratio (Table 3). The relatively
higher xylene concentrations in Ekt samples indicate
that they contain Type II kerogens predominantly,
459
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
Table 3
Some relative concentrations and indices calculated from pyrolysis±gas chromatography tracesa
Depth (m)
C5 (rel%) C10 (rel%) C15 (rel%) C20 (rel%) C25 (rel%) o (rel%) x (rel%) ph (rel%) o/x
mt/t
VaÂrkeszoÈ-1
43.0±43.9
16.3
19.8
27.0
22.1
14.8
30.0
27.5
42.5
1.09 0.131 0.81
48.0±49.0
57.0±58.0
60.0±60.7
63.5±64.5
64.5±65.5
16.1
26.4
20.0
18.4
31.8
20.5
28.1
23.5
18.6
30.4
21.0
22.6
19.6
17.6
23.2
18.8
11.8
16.3
22.0
11.6
23.6
11.1
20.6
23.4
3.0
42.6
46.0
21.3
32.2
25.7
22.8
11.3
27.7
20.6
33.9
34.6
42.7
51.0
47.2
40.4
1.87
4.07
0.77
1.56
0.76
0.213
0.236
0.174
0.135
0.193
0.33
0.37
0.41
0.65
0.37
EgyhaÂzaskeszoÈ-34
35.0±35.5
36.7
35.5±36.0
31.1
39.0±39.5
32.2
40.5-40.8
34.6
23.6
34.9
34.2
23.9
29.6
29.5
27.7
22.1
8.1
4.5
6.1
17.4
2.0
±
±
2.0
19.1
6.9
32.4
18.8
45.1
42.3
24.6
36.7
35.8
50.8
43.0
44.5
0.42
0.16
1.32
0.51
0.243
0.182
0.148
0.208
0.74
1.07
0.21
0.99
pr/17
a
C5, C10, C15, C20 and C25 rel%: normalized distribution of the given n-alk-1-ene/n-alkane doublets; o, x and ph rel%: normalized distribution of oct-1-ene, (m+p)-xylenes and phenol; o/x:ratio . oct-1-ene/(m+p)-xylenes; mt/t: 2-methylthiophene/toluene;
pr/17: (pristane+prist-1-ene+prist-2-ene)/heptadec-1-ene+n-heptadecane).
Fig. 6. A and B: pyrograms of kerogens from Vkt-1 and Ekt-34 boreholes [numbers indicate the carbon number of the n-alk-1-ene/nalkane doublets (*,*, respectively); b=benzene; t=toluene; x=(m+p)-xylenes; ph=phenol; PrI±II=prist-1-ene and prist-2-ene].
while Vkt samples consist of mixed Type I and Type III
kerogens, resulting in a pseudo-Type II kerogen.
The 2-methylthiophene/toluene ratio displays some
variation, but it does not re¯ect the dierences found in
the case of Sorg/C ratio between the craters. Consequently,
an important part of organic sulphur is not present in
thiophenic form, but in other organic sulphur-containing moieties.
460
A. Brukner-Wein et al. / Organic Geochemistry 31 (2000) 453±461
The pr/17 ratio is somewhat higher in Ekt samples
than in those from Vkt. The same time the ratio is
higher in the upper sections of both craters, indicating
more chlorophyll-rich precursor contributions, than in
lower sections. This observation is in accord with the
changes of N/C ratios, because both nitrogen and phytol are units of chlorophyll.
4. Conclusions
Despite being roughly the same age, deposited under
the same climatic conditions and of similar basic lithology, the alginite deposits in the twin craters developed in
dierent ways. Vkt kerogens contain more aliphatic
compounds, have higher H/C ratios and lower Sorg
contents. Ekt kerogens contain relatively less aliphatic
compounds and have higher organic and pyritic sulphur
quantities. The principal source of organic matter is B.
braunii microalgae but other algal input cannot be
excluded. The terrestrial contribution is signi®cant but
its relative amount and nature are variable. As a result
of volcanic activity a signi®cant amount of sulphur was
deposited in the twin craters, but in Vkt there was no
mineral sink for binding sulphur due to a lack of abundant basaltic tu. Assuming a similar nutrient supply,
the conditions for accumulation and preservation of
organic matter (oxicity, biological activity, sedimentation rate and delivery of remobilised basalt tu) were
more favorable in the narrow and deep Vkt crater, than
in Ekt. The thinner layer of organic matter sedimented
in the wider Ekt crater might have suered more chemical and biological oxidation as well.
The Type II kerogens in the twin craters are the products of dierent processes. Vkt kerogens are mixtures
of Type I and Type III organic matter predominantly
and are preserved relatively better. Ekt kerogens must
have suered more severe biological and chemical
alteration during microbial sulphate reduction. During
oxidation, the proportion of the more reactive lipid-rich
organic matter decreased and the residual organic matter was enriched in land-derived components. During
the reduction of sulphates, a portion of the sulphur
reacted with organic matter and was incorporated into
the macromolecular network of kerogen, producing
medium sulphur-rich Type II kerogen in the Ekt crater.
Acknowledgements
This work was funded through grant OTKA-025541
and AKP 96/2-558 2.5/35 from the Hungarian National
Science Foundation. The authors thank Dr. Michael
Kruge and Dr. FrancËois Baudin for helpful comments.
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