Comparison of Pliocene organic-rich lacustrine sediments in
twin craters
Alice Brukner-Wein
a,*, CsanaÂd SajgoÂ
b, Magdolna HeteÂnyi
c aGeological Institute of Hungary, Budapest, StefaÂnia 14. H-1143, HungarybLaboratory for Geochemical Research, Hungarian Academy of Science, Budapest, BudaoÈrsi uÂt 45. H-1112, Hungary cInstitute of Mineralogy, Geochemistry and Petrography, JoÂzsef Attila University, Szeged POB 651 H-6701, Hungary
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 envir-onment. The principal source of the organic matter is the microalgae Botryococcus braunii, but the terrestrial con-tribution 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 erup-tions 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
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 mac-romolecules of outer walls of B. brauniiwas 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 Hun-gary. 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
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* Corresponding author.
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, con-sisting of clayey alginite.
2. Experimental
All geochemical analyses were carried out on core sam-ples (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 300C for 4
min, followed by programmed pyrolysis at a rate of 25C/min to 550C (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 pre-cipitating 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 spectro-photometer 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 m0.2 mm id WCOT
fused-silica capillary column coated with OV-1, using hydrogen as the carrier gas and ¯ame ionization detec-tion (FID). The oven was programmed from 110 to 170 C at 25C minÿ1 and from 170 to 320C at 5C
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 1010C. 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 530C over
a 9 min period and held two min. before a single step on-line GC-analysis. Following Larter (1984) the pro-ducts 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 bore-hole. 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%).
Table 1
Rock-Eval pyrolysis and geochemical parameters of organic mattera
Depth (m) TOC (%) Tmax(C) HI HC
pot Bitumen (%) bit/TOC (mg/g) tHC (%) tNSO (%) CPI
VaÂrkeszoÈ-1
43.0±43.9 10.38 436 416 45.09 0.53 50.6 9.4 90.6 10.7
43.9±45.0 8.12 435 503 42.51 0.33 40.4 12.4 87.6 11.9
45.0±46.2 7.16 437 498 37.19 0.39 54.7 13.4 86.6 12.5
46.2±47.0 5.44 433 465 26.65 0.38 69.4 12.1 87.9 8.7
47.0±48.0 5.98 434 493 31.18 0.58 96.4 11.8 88.2 10.1
48.0±49.0 8.9 434 604 59.22 1.61 181.1 6.4 93.6 9.2
49.0±50.0 9.01 434 564 54.26 n.d. n.d. n.d. n.d. n.d.
50.0±51.5 11.23 406 580 78.41 3.00 267.1 6.7 93.3 5.7
51.5±53.0 10.88 428 582 73.27 2.82 259.3 3.8 96.2 5.9
53.0±54.0 8.51 433 572 52.96 1.17 137.6 7.6 92.4 9.6
54.0±55.0 8.94 435 485 46.27 0.64 71.4 11.0 89.0 11.6
55.0±56.0 10.14 436 602 66.16 1.20 118.1 7.2 92.8 7.2
56.0±57.0 10.98 430 581 74.33 1.65 150.4 9.7 90.3 13.2
57.0±58.0 11.4 438 635 76.91 1.19 104.6 9.2 90.8 12.7
58.0±59.0 10.34 432 497 55.77 1.04 100.4 9.4 90.6 14.2
59.0±60.0 11.46 416 550 77.08 4.12 359.1 3.2 96.8 5.3
60.0±60.7 8.6 427 420 41.12 0.92 106.7 12.1 87.9 11.9
60.7±61.7 7.55 420 401 34.15 1.17 154.5 9.0 91.0 12.4
61.7±62.7 11.57 420 551 77.13 4.29 370.7 3.0 97.0 6.7
62.7±63.5 13.86 432 618 94.7 2.69 194.0 18.5 81.5 n.d.
63.5±64.5 12.3 431 491 70.85 3.22 261.9 5.8 94.2 12.8
64.5±65.5 8.6 433 581 55.52 1.59 185.3 6.6 93.4 10.0
65.5±66.0 8.47 430 440 36.69 1.01 118.9 7.0 93.0 8.8
66.0±67.0 10.41 430 485 56.57 1.32 126.5 7.4 92.6 8.9
67.0±68.0 12.79 433 607 94.3 1.47 114.7 8.9 91.1 11.5
68.0±68.8 15.09 434 567 114.34 2.44 161.6 5.6 94.4 10.2
68.8±70.0 11.21 429 566 69.04 1.28 114.6 6.8 93.2 9.1
70.0±71.0 17.8 431 635 127.71 2.15 120.9 5.3 94.7 7.6
EqyhaÂzaskeszoÈ-34
34.2±34.5 2.7 423 317 13.12 0.11 41 10.3 89.7 3.0
35.0±35.5 5.4 431 465 26.01 0.26 49 5.3 94.7 4.3
35.5±36.0 5.7 430 155 9.53 0.16 29 16.9 83.1 9.6
36.0±36.5 2.3 430 264 6.60 0.15 68 10.0 90.0 6.1
36.5±37.0 2.0 432 231 4.99 0.13 65 11.2 88.8 4.9
37.0±37.5 2.2 429 262 6.32 0.20 94 10.7 89.3 5.4
37.5±38.0 2.5 431 301 8.17 0.17 70 10.0 90.0 5.1
38.0±38.5 4.3 423 317 13.12 0.25 58 11.8 88.2 3.7
38.5±39.0 10.5 419 369 42.95 0.69 65 10.0 90.0 6.2
39.0±39.5 12.4 430 525 71.81 1.44 116 5.6 94.4 9.3
39.5±40.0 11.2 429 536 65.63 0.83 73 8.6 91.4 10.4
40.0±40.5 9.3 432 564 55.71 0.75 81 7.4 92.6 10.1
40.5±40.8 9.1 430 474 44.33 0.72 79 5.5 94.5 8.2
Vkt. (ave.)
43.0±48.0 7.4 435 475 36.5 0.44 62.3 11.8 88.2 10.8
48.0±71.0 10.8 429 546 69.0 1.93 179.9 7.2 92.7 9.3
Ekt (ave.)
34.2±38.5 3.4 429 288 11.0 0.18 21.0 10.8 89.2 5.2
38.5±40.8 10.5 428 494 56.1 0.88 83.0 7.4 92.6 8.8
a TOC: total organic carbon; HI: mg HC/g TOC; HC
hydrogen-richness. Beyond that the dierences in HI values and hydrocarbon potentials reveal a more oil-prone 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 averageTmaxvalues calculated for the two craters (430
and 428C). 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 fromB. brauniialgae and higher plant debris (HeteÂnyi, 1983). The principal algal source of organic matter isB. braunii microalgae but other algal input cannot be excluded. The contribution of higher terrestrial plants is supported by the gas chromatographic data of the ali-phatic 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 hydro-carbon distribution (Largeau et al., 1980; Brukner-Wein
and HeteÂnyi, 1993). The carbon preference index (CPI) calculated over the C22±C32range shows the dominance
of odd-carbon numbered, long chain, n-alkane homo-logues (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 con-siderably 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
sam-ples, a-phyllocladane is ubiquitous although relative
quantities vary. In samples from Ekt borehole,a
-phyl-locladane occurs only in those from the lower section. Above 38 m it is not detected, which can be attrib-uted to some variation of precursors (Fig. 3). The 16a
Fig. 2. HI versus Tmax evolution diagram showing averaged data.
Fig. 3. Gas chromatograms of saturated hydrocarbon fractions of bitumen from Ekt samples (20, 21:n-alkanes; phy:a
(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 cmÿ1and 1470
cmÿ1, respectively). Kerogens marked show the
pre-sence of long-chain aliphatic compounds (720 cmÿ1) 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
inten-sities 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 con-tent 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 pro-ductivity, 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
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 contribu-tion 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 Pri-mio 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
indi-cate 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 crater11 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 forma-tion 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 sur-rounding vegetation must have been similar during deposition of lower sections. The organic inputs to the craters were dierent in the upper sections. The compo-sitional dierences were partly caused by microbial and chemical alterations during pyrite formation, in pro-portion of pyrite content.
Some pyrolysis±gas chromatographic data are sum-marized 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 ofn-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 C25one. These observations suggest that
these samples are more gas prone and less paranic than the other 5 Vkt samples. The normalized relative abundances ofn-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 character-ization. The phenol content of kerogen was interpreted to represent terrestrial contribution, dominantly vas-cular 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 rela-tively higher n-octene concentrations in Vkt samples indicate greater algal organic matter contents compared to Ekt samples, in good agreement with the HI and HCpotvalues (Table 1). This is also expressed by then
-octene/(m+p)-xylenes ratio (Table 3). The relatively higher xylene concentrations in Ekt samples indicate that they contain Type II kerogens predominantly,
Table 2
Elemental composition and atomic ratios of kerogensa
Depth (m) N (%) C (%) H (%) S (%) Sorg(%) H/C N/C Sorg/C
VaÂrkeszoÈ-1
43.0±43.9 1.53 53.9 6.50 1.04 0.17 1.44 0.024 0.001
48.0±49.0 1.55 72.6 9.93 0.45 0.35 1.64 0.018 0.002
57.0±58.0 1.30 58.4 7.83 0.97 0.18 1.61 0.018 0.001
60.0±60.7 1.98 67.9 7.98 1.66 1.47 1.41 0.024 0.009
63.5±64.5 1.35 48.4 6.31 0.98 0.26 1.56 0.023 0.001
64.5±65.5 1.70 69.2 8.43 4.22 3.56 1.46 0.021 0.020
EgyhaÂzaskeszoÈ-34
35.0±35.5 1.06 53.9 6.79 11.30 3.67 1.51 0.016 0.026
35.5±36.0 1.49 54.0 5.05 7.87 2.70 1.12 0.023 0.019
39.0±39.5 1.22 49.3 6.04 10.44 4.42 1.47 0.021 0.034
40.5±40.8 1.40 54.6 6.38 12.37 4.43 1.40 0.022 0.030
a
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-contain-ing moieties.
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 pr/17
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 16.1 20.5 21.0 18.8 23.6 42.6 22.8 34.6 1.87 0.213 0.33 57.0±58.0 26.4 28.1 22.6 11.8 11.1 46.0 11.3 42.7 4.07 0.236 0.37 60.0±60.7 20.0 23.5 19.6 16.3 20.6 21.3 27.7 51.0 0.77 0.174 0.41 63.5±64.5 18.4 18.6 17.6 22.0 23.4 32.2 20.6 47.2 1.56 0.135 0.65 64.5±65.5 31.8 30.4 23.2 11.6 3.0 25.7 33.9 40.4 0.76 0.193 0.37
EgyhaÂzaskeszoÈ-34
35.0±35.5 36.7 23.6 29.6 8.1 2.0 19.1 45.1 35.8 0.42 0.243 0.74
35.5±36.0 31.1 34.9 29.5 4.5 ± 6.9 42.3 50.8 0.16 0.182 1.07
39.0±39.5 32.2 34.2 27.7 6.1 ± 32.4 24.6 43.0 1.32 0.148 0.21
40.5-40.8 34.6 23.9 22.1 17.4 2.0 18.8 36.7 44.5 0.51 0.208 0.99
a C5, C10, C15, C20 and C25 rel%: normalized distribution of the givenn-alk-1-ene/n-alkane doublets; o, x and ph rel%: nor-malized 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).
Thepr/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 phy-tol are units of chlorophyll.
4. Conclusions
Despite being roughly the same age, deposited under the same climatic conditions and of similar basic lithol-ogy, 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 isB. 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 abun-dant basaltic tu. Assuming a similar nutrient supply, the conditions for accumulation and preservation of organic matter (oxicity, biological activity, sedimenta-tion 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 che-mical and biological oxidation as well.
The Type II kerogens in the twin craters are the pro-ducts 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 mat-ter 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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