Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system...

Pergamon Org. Geochem. Vol. 26, Nos. 11-12, pp. 705-720, 1997

© 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

PII: S0146-6380(97)00038-7 0146-6380/97 $17.00 + 0.00

Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolyses

D A N I E L D E S S O R T ~, J A C Q U E S C O N N A W , SYLVIE D E R E N N E 2 and C L A U D E L A R G E A U 2

~Elf Aquitaine, CSTJF, 64018 Pau, Cedex, France and 2Laboratoire de Chimie Bioorganique et Organique Physique, UA CNRS 1381, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris, Cedex 05,

France

(Received 9 July 1996; returned to author for revision 14 October 1996; accepted 27 May 1997)

A~traet--Kinetic parameters were determined for the first time, via open-system pyrolyses, on algae- hans (highly resistant biomacromolecules that are selectively preserved during kerogen formation) isolated from extant microalgae. Parallel studies were also carried out on 10 kerogens exhibiting, with one exception, a low level of maturity. These kerogens included samples chiefly derived from the selective preservation of the above algaenans and samples mainly, or almost exclusively, derived from the "natural vulcanization" pathway. Important differences in activation energy (Ea) distributions were observed between the four algaenans investigated and correlated with their chemical structures. The kerogens predominantly derived from algaenan-selective preservation (Pula alginite, NE 70 and BJ 248 Torba- nites, Rundle Oil Shale) also exhibited pronounced differences in Ea distributions. These distributions provided: (i) information on the diversity of the source materials; and (ii) reflected the occurrence of important differences in chemical structures and thermal behaviour between three of the tested kerogens, even though they are all classified as low maturity type I. The Kimmeridge Clay samples and the Lorca Oil Shale showed broad E~ distributions shifted to low energies when compared with the above algaenans and kerogens. Such shifts reflect an important (or even almost exclusive for some of these kerogens) contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Finally, the kinetic data derived for the nine low maturity fossil samples were extrapolated to a very low, geological heating rate of 3°C Ma -l and the generation rate curves and cumulative yield curves thus obtained were compared. © 1997 Elsevier Science Ltd

Key words--open-system pyrolyses, algaenans, kerogens, kinetic parameters, activation energy distribution, selective preservation, natural vulcanization

INTRODUCTION

The kinetic parameters governing the thermal breakdown of kerogens have been extensively studied via laboratory experiments so as to derive values that can be subsequently used in basin mod- elling, for simulating kerogen maturat ion and predicting the timing and extent of oil generation (e.g. Ungerer and Pelet, 1987; Hunt et al., 1991). To this end various heating methods have been used, including open-system pyrolyses (Braun et al., 1991; Tegelaar and Noble, 1994; Peters et al., 1995; Reynolds et al., 1995; Reynolds and Burnham, 1995; Schmidt, 1995). These experiments permit the calculation of activation energy distributions and generation curves for an assumed geological heating rate. Kerogens of different types have been studied in the last few years using such an approach. In some cases, the kinetic parameters thus derived were discussed in relation to the kerogen's chemical structure (Tegelaar and Noble, 1994), maceral composition (Schmidt, 1995) or the influence of associ-

ated mineral matter (Reynolds and Burnham, 1995).

Recent studies (reviewed by de Leeuw and Largeau, 1993; Largeau, 1995; Largeau and de Leeuw, 1995) led to the recognition of a new type of biomacromolecule, characterized by an unusually high resistance to non-oxidative chemical degradation. Such non-hydrolysable macromolecular constituents were identified in the cell walls or the protective layers of a number of organisms, including microalgae. The resistant macromolecules, shown to comprise the outer walls occurring in numerous microalgal species, were termed "algaenans" (Tegelaar et aL, 1989). As reported in the three reviews mentioned above: (i) the chemical structure of several algaenans was elucidated via a combination of spectroscopic and pyrolytic methods: (ii) high molecular weight lipids implicated as building blocks in algaenan formation were identified in a few cases; and (iii) parallel comparative studies with low maturity kerogens revealed an important role of algaenans in kerogen formation. In fact, this

705

706 D. Dessort et al.

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type of biomacromolecule appears also to exhibit an extremely high resistance to bacterial degradation and, as a result, they can escape diagenetic alterations to be retained nearly unaltered in kerogens. Algaenan fossilization, therefore, occurs through the so-called "selective preservation" pathway and such macromolecules, thus, afforded a major contribution to a number of algal kerogens (Largeau and Derenne, 1993). It cannot be completely excluded that algaenans undergo some alterations in chemical structure during the drastic treatments required for isolation from extant microalgae and/or during the first steps of diagenesis. Nevertheless, the tight relationships based not only on bulk chemical features but also on extended comparative examinations of pyrolysis products observed between isolated algaenans and low maturity kerogens indicate that the occurrence of major alterations can be ruled out.

In spite of the important geochemical role of algaenans, no study dealing with the kinetic parameters of these non-hyrolysable diagenetically-resistant biomacromolecules has so far been reported to the best of our knowledge, In the present work, open-system pyrolyses were carried out on four algaenans isolated from extant microalgae and on kerogens known to be chiefly derived from the selective preservation of such highly resistant biomacromolecules. For comparison, kerogens having only a relatively low (or even negligible) contribution of selectively-preserved components, whereas the "natural vulcanization" pathway played a substantial to virtually exclusive role, were also examined; namely five Upper Jurassic samples from the Kimmeridge Clay Formation and a Messinian kerogen from the Lorca Basin. In recent years the so- called "natural vulcanization" pathway has been shown to play a major role in the formation of a number of kerogens (reviewed in Sinninghe Damst6 and de Leeuw, 1990). This pathway results in the formation of high molecular weight, diagenetically- resistant, sedimentary organic matter via the incorporation of sulphur into various functionalized lipids.

The main purposes of the present study were: (i) to examine the relationships between the chemical structure of various algaenans and their kinetic parameters as obtained from open-system pyrolyses; (ii) to compare the above kinetic parameters with those of algaenan-derived kerogens so as to deter- mine the chemical features that control the thermal behaviour of these fossil materials; (iii) to compare the kinetic parameters of kerogen fractions derived from the selective preservation and the "natural vulcanization" pathways; and (iv) to examine the natural evolution of these kerogens via extrapolation of laboratory kinetic data to very low, geological heating rates.

Kinetic parameters of algaenous and kerogens 707

EXPERIMENTAL

Samples

The studied algaenans were isolated from Botryococcus braunii and Scenedesmus quadricauda. Laboratory cultures of these green microalgae were grown under air-lift conditions (Casadevall et al., 1985); algaenan isolation via successive, drastic, base and acid hydrolyses on the lipid-free biomass was carried out as previously described (Berkaloff et al., 1983).

The bulk chemical features of the tested fossil samples are reported in Table 1. Three lacustrine organic-rich deposits chiefly composed of the accumulation of fossil colonies of B. braunii, including a sample from the massive section of the Pula Oil Shale (Pliocene, Hungary), two Torbanites, NE 70 (Permian, Winkelhoak mine, Evander area, South Africa) and BJ 248 (Permian, Glen Davis, Sydney Basin, New South Wales, Australia) (Derenne et al., 1994) were used. A sample of the Rundle Oil Shale (Eocene, lacustrine, Kerosene Creek seam, Rundle, Queensland, Australia), known to be formed mainly by the accumulation of ultralaminae (Largeau et al., 1990), five samples from the Kimmeridge Clay Formation (Upper Jurassic, marine, Marton 87 borehole, Pickering Vale, Cleveland Basin, Yorkshire, U.K.) previously studied by Boussafir et al. (1995a) and a sample of the Lorca Oil Shale (Messinian, marine, pre-evaporitic, Lorca, Murcia, Spain) (Derenne et al., 1995) were also used.

Kinetic studies

Previous pyrolysis (Pyromat) studies have shown generally good agreement between the kinetic parameters obtained from bitumen-free whole rocks and the corresponding isolated kerogens, provided that samples with TOC > 1% were examined (Tegelaar and Noble, 1994; Reynolds and Burnham, 1995). The present study was, therefore, carried out on pre-extracted samples (finely pow- dered material stirred at room temperature over- night in CH2CI2/MeOH, 2/1, v/v). Kinetic parameters were determined via open-system, programmed temperature pyrolyses. These experiments were carried out under a flow of helium from 250 to 650°C, with a Pyromat II micropyrolyser (Lab Instruments, Kenwood, CA, U.S.A.). Kinetic data were obtained from multiple runs with at least three different constant heating rates between 1- 25°C min -1 for each sample. Heating rates below I°C were not used because they resulted in low sig- nal to noise ratios and to avoid problems related to the lower volatility of the pyrolysis products (Tegelaar and Noble, 1994). The bitumen-free samples (2-3 mg of organic matter), were loaded in a 3 mm i.d. quartz crucible. In situ temperature measurements were carried out in the centre of the

material with a calibrated thermocouple. The evolution of the total volatile products thus generated (except CO2 and H20) was measured using a flame ionization detector (FID).

These experiments afforded several profiles from a given kerogen or algaenan corresponding to the rate of evolution of pyrolysis products vs temperature for the different heating conditions. Each profile was characterized by its Tmax value (temperature at which the maximum rate of evolution takes place) and its width at half-height (Reynolds et al., 1995). Kinetic parameters of generation of volatile products were calculated for each sample from the above profiles using a discrete model. The multiplicity of the reactions implicated in the pyrolysis of such materials was replaced at each heating rate by 30 independent reactions. Further simplification was obtained by considering that all these parallel, non-isothermal, first-order pseudo-reactions have the same frequency factor (Ungerer and Pelet, 1987). Indeed, due to the com- plexity of pyrolysis reactions this factor would merely correspond to a mathematical optimization parameter bearing no relationship with molecular vibrational frequencies (Schaefer et al., 1990). The validity of the parallel reaction model with a single frequency factor was recently tested (Burnham et al., 1995), and such approximations were shown to be adequate except for very immature samples with high oxygen levels.

Thirty-one parameters were, therefore, considered and optimized by an iteration method to obtain the best-fit between the measured and calculated rate curves. The quality of the fit for these curves was judged from the difference between experimental and calculated Tmax values. A discrete distribution of activation energies (Ea), with a regular spacing of 1 kcal mol - l in the range 40 to 70 kcal mo1-1 and one frequency factor were thus obtained for each kerogen and algaenan. Calculations of kinetic parameters were carried out by using the regression analysis program KINETICS (Burnham et aL, 1987). The reproducibility was typically +2 kcal mo1-1 for Ea and +I°C for Tmax. Comparison of kinetic parameters can be biased when the frequency factors calculated for the studied samples are sharply different. This is due to possible compensation effects, since a low (high) frequency factor can compensate for low (high) activation energies. To avoid such a problem, rate constants were calculated for each sample at 450°C (temperature close of the averaged Tmax observed in these laboratory heating experiments).

As stressed by Tegelaar and Noble (1994), comparisons of the kinetic data for different samples are easier from cumulative yield curves (transformation ratios into volatile pyrolysis products vs temperature). These generation profiles were, therefore, established for the different laboratory heating rates


P R B A

i I % A = 7 . 1 E+15 s-1

80 . . . . . . . . . . . . .

60 . . . . . . . . . . . . . . . .

40 . . . . . . . . . . . . . . . .

20 . . . . . . . . . . . . .

(DqD

Kca l I m o l e

P R B B 00

% , A = 3 . 0 E + l T s - 1

80 . . . . . . . . . . . . . . . . . . . .

60 . . . . . . . . . . . . . . .

40 . . . . . . . . . . . . . . . . . . .

20 . . . . . . . . . . . . . . .

0 , . . , . % . , , , . . . , , , , , , , ,

Kca l / m o l e

P R B L 100

% A = 3.6 E+16 s-1

80

40

20

0

Kca l I m o l e

Fig. 1. Distribution of activation energies and frequency factor (A) of the algaenans of the three races of B. braunii.

used and extrapolated to a low geological value of 3°C Ma -1. The latter value was also used for simulating rate profiles under geological conditions.

RESULTS AND DISCUSSION

B. braunii algaenans and Botryococcus-derived kerogens

B. braunii is characterized by a high efficiency for biosynthesizing very large amounts of lipids, and three distinct chemical races producing different types of hydrocarbons have been identified in this species (Metzger et al., 1991); namely A race (n- alkenes), B race (botryococcenes), L race (a lycopa-

diene). The algaenans isolated from the three races were termed PRB A, PRB B and PRB L, respectively.

The discrete distributions of activation energies and the frequency factors obtained for the non- hydrolysable macromolecular materials isolated from the different races of B. braunii are reported in Fig. 1. As an example, the experimental and calculated rate profiles corresponding to the different heating rates used for determining E~ distribution are shown for PRB A in Fig. 2. The activation energy distributions of B. braunii algaenans are all characterized by one predominant energy in the high range. Such a predominance is especially pronounced for PRB A, for which the distribution is

1°C / m n 2 ° C / m n

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250 3so 45o 550 650

T ( ° C )

Fig. 2. Experimental and calculated generation rate curves obtained for PRB A at different laboratory heating rates.


almost exclusively (95%) represented by a single Ea value. The principal energy accounts for about 85 and 60% of the reaction in the case of PRB B and PRB L, respectively. For the latter algaenan, about one-third of the reaction is also described by relatively low activation energies in the 45- 60 kcal mol - l range. These kinetic parameters and their differences are correlated with the chemical structure of the algaenans.

The monoenergetic distribution of PRB A reflects an homogeneous structure based on a single type of building block exhibiting a relatively simple structure, so that the thermolysis of the macromolecular structure would originate from the primary cleavage of the same type of bond with a similar environment. Indeed, it was recently established (Gelin et al., 1994a) that the major building block of PRB A is an aliphatic polyaldehyde. Further condensation and/or reticulation of this soluble material leads to the formation of the insoluble and non-hydrolysable algaenan.

Similar bulk chemical features (H/C and O/C atomic ratios, FTIR and solid state 13C N M R spec- tra) were observed for PRB A and PRB B (Kadouri et al., 1988), and their pyrolysates are both dominated by n-alkane/n-alk- l -ene doublets originating from the thermal cleavage of polymethylene chains. In addition, recent studies on the high molecular weight lipids of the B race (Metzger et al., unpublished data) indicate that: (i) the aliphatic polyaldehyde, mentioned above for PRB A, is also likely to play an important role in PRB B structure; and (ii) tetramethylsqualene is probably implicated, via ketal formation with these polyaldehyde units. Spectroscopic and pyrolytic examination of the algaenan of the L race (Derenne et al., 1989; Derenne et al., 1990) revealed substantial differences in bulk chemical features when compared with those of the A and B races. Isoprenoid chains with a C4o lycopane-type skeleton were, thus, shown to be implicated in the formation of PRB L, instead of unbranched chains for PRB A and PRB B. Some of these lycopane chains in PRB L are probably cross- linked by ether bridges (Gelin et al., 1994b; Behar et al., 1995). More recently, it was observed (Metzger et al., unpublished data) that: (i) the aliphatic polyaldehyde also affords a substantial contribution to PRB L; and (ii) some polyaldehydic units are probably linked to lycopadiene chains via ketal formation. The involvement of the same polyaldehyde in the formation of the algaenans of the three races of B. braunii is consistent with the presence of principal energies around the same value in their corresponding activation energy distributions. The relatively broad Ea distributions observed for the algaenans of the B and L races likely reflects a more complicated macromolecular structure due to the occurrence of tetramethylsqualene and lycopane-type chains, respectively, and of ketal bridges.

MARTON U . I ~ MARTON I1-1 ~ ~ l ~ " - - ~ \ KWm~ndpT ~ ¥ / ~ A . ~ , • \ KImo~dg* • _ ~ - - ~ W " ~ L , i l l i p l ~ 111 \ Klmrneddge M J'w,~ -- J~lBm,-'-,w~,Ll~,~ \

~ , o, . , , , , ~ s ~ " ~ m r ~ P ~ j P ~ [ I . l r J H /7 /-80.0 RunaZe e3 l~S - ' l P m Y . , I g i l l m w ~ ~ l l l u ~ l [ i l j r I/- so.o

p . , . . ,o f . . .~ fP~v-~s~p . - -~ j / / -40 .0 -.... _ ~ ~ , d ~ ' ~ , , m ~ t l * m J F |/-20.0 PR s, ] ~ " - ~ ~ t m ~ " A d t , A g . ; PRB L I L W ~ a ~ P ~ - ~ ~ ....

k4S0

Fig. 3. Rate constants calculated at 450°C for the studied algaenans and fossil materials.

In addition, the presence of ether bridges between some lycopane chains was noted in PRB L and all the carbon atoms in the central part of the lycopane skeleton are implicated in the formation of such bridges (Behar et al., 1995). The PRB L structure, thus, comprises both tertiary and quaternary ether links and the latter would be expected to exhibit a lower thermal stability. The presence of these bridges could, therefore, account, at least in part, for the low energy range portion in the activation energy distribution of PRB L. The latter feature may also be related to the contribution of isoprenoid chains in this algaenan, since it is well docu- mented that such branched chains are more sensitive to thermal stress when compared with polymethylene chains. The lower thermal stability of PRB L is also illustrated by the higher reaction rate at 450°C (Fig. 3) when compared with those of PRB A and PRB B.

The lower stability of PRB L relative to PRB B was previously observed in artificial maturation experiments carried out in sealed gold tubes (Behar et al., 1993). While no kinetic parameters were derived from the above study it was found, for example, that conversion yields were of 48 and 16 wt% under isothermal conditions (300°C 3 h -1) from PRB L and PRB B, respectively. Interestingly, it was noted that the conversion yields calculated for the two algaenans from their energy distributions are lower than the experimental values obtained from the closed-system pyrolyses. This is likely due to the differences between open and closed pyrolyses, especially the nature of the detected products and the mechanism of the primary cracking reactions.


Pule Alginite NE 70 Torbanite BJ 248 Torbanite

% / A=4.4E+15s-1 % A=3.9E+14s-1

80 t . . . . . . . . . . . . . . . . . . . . . 8o . . . . . . . . . . . . . . . . . 8o . . . . . . . . . . . . . . . . . . . . . . .

60 - 60. 60 . . . . . . . . . . . . .

, o . , o . , o . . . . . . . . . . . . . . . . . .

20 - 20 • 20

o

m

o . . . ~ . ~ . ; . . a . . . . . L . . . , , . . . . . o.

Kcal I mole Kcal / mole Kcal I mole

Fig. 4. Distribution of activation energies and frequency factor (A) of Botryococcus-derived kerogens.

Pyromat pyrolyses were also carried on Botryococcus-derived kerogens in order to compare their kinetic features with those of the algaenans discussed above. The first sample examined was the maar-type Pliocene oil shale from Pula. Previous palynologieal observations (Jambor and Solti, 1975) revealed an abundant contribution of fossil colonies of Botryococcus to Pula alginite and scanning electron microscopy indicated excellent morphological preservation of the colonies in this low maturity type I material (Solti, 1985). Prolific growth of Botryoeoccus took place in the Pula crater and the total thickness of the organic-rich alginite deposit is about of 40m (Brukner-Wein et al., 1991). While the discrete distribution of activation energies obtained from the bitumen-free Pula alginite appears relatively broad (Fig. 4) it is dominated (75%) by a single value (60 kcal tool-l). These kinetic parameters were compared with those obtained for the algaenans of the three races of B. braunii, taking into account the information derived from recent organic geochemical studies on the Pula deposit (Derenne et al., 1994, 1997). The kinetic features of the Pula sample, thus, appear to be fully consistent with these studies indicating that: (i) both the A and L races of Botryococcus were present, with the A race providing a predominant input; and (ii) that PRB A and (to a lesser extent) PRB L played a major role in the formation of this alginite by selective preservation.

The NE 70 Torbanite deposit is extremely rich in organic matter (ash content below 2%) composed of low maturity type I kerogen. Previous examination by UV fluorescence microscopy showed that the bulk of this material corresponds to tightly packed colonies of Botryococcus cemented by a see- mingly amorphous organo-mineral matrix. Bulk spectroscopic examinations, identification of kero-

gen pyrolysis products and bitumen analyses (Largeau et al., 1984, 1986; Derenne et al., 1987, 1988, 1994; Kister et al., 1990) indicated that the NE 70 Torbanite was formed via the fossilization of the A and/or B race(s). The activation energy distribution from this sample (Fig. 4) is characterized by the presence of a single energy in the high value range, as observed for PRB A. However, the kinetic parameters of the NE 70 Torbanite could also be related to PRB B based on: (i) the reproducibility typically observed for Ea (_+2 kcal mol -]) in these experiments; and (ii) the possible occurrence of compensating effects originating from large differences in frequency factors. Close k450 values are also observed for the Torbanite and the two resistant biopolymers (Fig. 3). Accordingly, the kinetic parameters do not allow clear discrimination between the two algaenans as the source of the Torbanite, although the presence of a single activation energy makes an input from the A race more likely and indicates an overwhelming role for algaenan selective preservation in the genesis of this extremely organic matter-rich deposit. Such a monoenergetic distribution was quite unexpected since kerogens usually exhibit a highly heterogeneous structure, even when sharply dominated by a single source organism. Thus, micro-FTIR examination of a set of low maturity Torbanites (Landais et al., 1993) (not including the NE 70) revealed a completely different composition of the organic matter in the cementing organo-mineral matrix when compared with the colonies. Furthermore, this matrix comprises two markedly distinct types of organic matter corresponding to degraded algal and bacterial constituents. In sharp contrast, based on the present kinetic observations, the degree of heterogeneity appears negligible in the NE 70 sample. Tegelaar and Noble (1994) also

Kinetic parameters of algaenous and kerogens

PR sq 40 %

4O %

Rundle 0il shale

711

30 30

20 20

10 10

0 0

Kcal I mole Kcal I mole

Fig. 5. Distribution of activation energies and frequency factor (A) of the algaenan of S. quadricauda and of the Rundle Oil Shale.

noted a single activation energy for the Green River Shale kerogen and considered this to reflect a homogeneous polymethylenic structure. Likewise, narrow distributions consisting of only three adja- cent energies were observed from alginite-rich lacustrine sediments (Horsfield et al., 1994). As emphasized by Tegelaar and Noble (1994), comparison of kinetic parameters with literature data can only be made if the same sample is analysed. It is interesting to note, however, that a single, high activation energy value was also previously obtained from an Australian low maturity Permian Torbanite via a discrete model (Tegelaar and Noble, 1994), whereas a substantially lower Ea value was calculated via a Gaussian model.

The second tested Torbanite, BJ 248, also appears to be composed of an accumulation of Botryococcus colonies when observed by UV fluorescence microscopy and, based on kerogen and bitumen analyses (Derenne et al., 1987, 1988), the A and/or B race(s) of B. braunii was (were) implicated in its formation. In contrast to the NE 70 deposit, however, BJ 248 is mature enough to be in the oil window. Comparative spectroscopic and pyrolytic studies of these two Torbanites (Derenne et al., 1987, 1988; Kister et al., 1990) showed that, as expected, thermal evolution resulted in: (i) a partial elimination of the oxygen groups; (ii) a progressive increase in aromaticity (aromaticity fraction, fa, of 0.58 determined by solid-state 13C NMR); and (iii) the shortening and release of some of the hydrocarbon chains. Nevertheless, as demonstrated by its spectroscopic features and by the study of its pyrolysis products the BJ 248 Torbanite retains a substantial oil potential and long hydrocarbon chains still afford an important contribution to the kerogen's macromolecular structure. The activation

energy distribution for BJ 248 (Fig. 4) extends from 45 to 67 kcal mo1-1 and the principal energy, at 56 kcal mol -~, accounts for about 60% of the reaction. The difference between the principal E~ of the BJ 248 and NE 70 Torbanites might not be as high as suggested by the calculated distributions and might reflect compensation due to frequency factors. Indeed, it is well known that a lower activation energy can be compensated by a lower frequency factor and both Ea and A values are much lower for the BJ 248 sample. It should, nevertheless, be noted that: (i) the rate constants calculated from these kinetic parameters for the two Torbanites are sharply different (Fig. 3); and (ii) the main point here is the pronounced difference in Ea distributions, that is, an extended range for BJ 248 instead of a single energy for the NE 70 sample. The latter feature cannot be accounted for by compensation effects. Comparison of activation energy distributions was recently carried out on bitumen- free source rock samples from the Lias e formation (Lower Toarcian, Germany) with different levels of maturity (Schaefer et al., 1990). Their study showed a selective cleavage of more thermally labile bonds during natural evolution with the mature samples exhibiting a narrower distribution and a shift to higher energy values. In sharp contrast, reverse trends were observed here on comparison of the BJ248 and NE 70 samples. Accordingly, the observed differences between the kinetic parameters of these two Torbanites should not chiefly reflect their relative levels of evolution but some intrinsic differences in their source materials. As stressed above, the NE 70 sample shows a quite unusual homogeneity and is almost exclusively composed of selectively preserved B. braunii algaenan. In contrast, the BJ 248 Torbanite exhibits a higher degree


of heterogeneity (Landais et al., 1993). Various sources were, thus, probably important contributors along with PRB A and/or PRB B in the formation of the BJ 248 kerogen. Indeed, comparison of the fatty acids released on pyrolysis by the NE 70 and BJ 248 samples (Derenne et al., 1988) indicated that diagenetic incorporation of some degraded lipids occurred during the formation of the latter Torbanite.

S. quadricauda algaenan (PRsq) and Rundle Oil Shale

No predominantly single value was observed in the discrete distribution of activation energies obtained for the algaenan of the ubiquitous freshwater microalga S. quadricauda (Fig. 5). This distribution extends from 46 to 67 kcal tool -~, and the principal energy at 60 kcal tool -1 only accounts for approximately 35% of the reaction. Based on such kinetic features, a rather heterogeneous structure can be inferred for PRsq. To date, the high molecular weight lipids of S. quadricauda have not been examined; accordingly, no precise information is available on the building block of the a|gaenan. Nevertheless, spectroscopic and pyrolytic studies on PRsq (Derenne et al., 1991) indicated, as in the case of the algaenans of the A and B races of B. braunii, a highly aliphatic nature and a macromolecular structure based on long unbranched hydrocarbon chains, which are probably cross-linked at least in part by ether bridges. An important difference con- cerned nitrogen functions in PRB A and PRB B, on the one hand, and S. quadricauda algaenan, on the other hand. The latter, which contains 2.4 wt % of nitrogen instead of approximately 0.5% for the PRBs, comprises alkylamide groups. The thermal cleavage of these groups results in the formation of n-alkylnitriles with a bimodal distribution (Derenne et al., 1991, 1993). In sharp contrast, no nitrile production was observed, even in trace amounts, on pyrolysis of B. braunii algaenans. The presence of these amide functions should explain, in part, the far more heterogeneous structure of PRsq when compared with the latter algaenans, as reflected by its discrete Ea distribution.

The Rundle Oil Shale, when observed by transmission electron microscopy at high magnification, appears to be composed of accumulations of very thin lamellar structures associated into bundles. Such very thin (approximately 15nm) structures ("ultralaminae", Largeau et al., 1990) were also detected in a number of other kerogens. Parallel studies have shown that numerous extant microalgae, among them S. quadricauda, contain very thin, algaenan composed outer walls (reviewed in Derenne et al., 1992a). In addition, examination of the algaenans isolated from some of the above species (Derenne et al., 1992b,c) revealed the same chemical features as in the case of PRsq, including

the formation of n-alkylnitriles on pyrolysis. Indeed, the occurrence of tight morphological and chemical correlations (Derenne et al., 1991, 1992b,c) showed that fossil ultralaminae were formed via the selective preservation of algaenan composed very thin outer walls of green microalgae like S, quadricauda. The kerogen from the Rundle Oil Shale is characterized by a broad distribution of activation energies from 45 to 67 kcal mol -t , with the principal energy only accounting for about one- third of the reaction (Fig. 5). Very close features were previously observed in studies of various Rundle Oil Shale samples (Tegelaar and Noble, 1994; Reynolds and Burnham, 1995) for the general shape of E a distributions, the principal activation energy (57 kcal mo1-1 instead of 58), the frequency factor (8.2 × 1014 S I instead of 1.8 x 1015) and Tm~ (481.5°C instead of 482 for a heating rate of 25"C rain-l).

Comparison of the Rundle Oil Shale sample with PRsq (Fig. 5) also showed close similarites in Ea distributions. This agreement is consistent with the above-described mode of formation of ultralaminae and with a major contribution of such structures in the kerogen of the Rundle Oil Shale. Comparisons of the activation energy distributions showed important differences between the kerogen of the Rundle Oil Shale on the one hand, and Botryococcus-derived kerogens (Pula alginite and NE 70 Torbanite), on the other. Nevertheless, all these kerogens are characterized, based on bulk chemical composition, as low maturity type I. Accordingly, as already stressed by Tegelaar and Noble (1994), such similarities do not preclude large differences in chemical structures that are revealed by kinetic studies and should have major implications for the timing of oil formation.

Kimmeridge Clay kerogens

The Kimmeridge Clay is a low maturity marine deposit, composed of type II and I-S kerogens according to its bulk features and considered to be a lateral equivalent of the main source rocks of the North Sea. This formation is characterized by short-term, parallel, wave-shaped cyclic variations (microcycles), corresponding to about 30,000y, which are reflected in both kerogen quantity (TOC) and quality (hydrogen index) (Herbin et al., 1991, 1993). The origin of such variations was recently determined via a combination of morphological and chemical studies (Boussafir et al., 1995a). Kimmeridge Clay kerogens were, thus, shown to be chiefly composed of three distinct types of organic matter: (i) "orange OM" is nanoscopically amorphous and originates from the sulphurization of lipids, mostly of algal origin; (ii) "brown OM" is chiefly composed of ultralaminae and, as discussed in the previous section, it originates from the selective preservation of very thin resistant outer walls


Kimmeridge Clay B 50,

30

2O

10

0

Kcal / mole

Kimmeridge Clay M

40

30

20

10

0

Kcal I mole

Klmmerldge Clay ! 5O

~ o = "

30

20

10

0

5O % 40

30

20

Kesl I mole

Lores oli shale

A = 1.8 E+15 s-1 . . . . . . . . . . . . . . . . . . . . . . . . .

10

Kesl I mole

Fig. 6. Distribution of activation energies and frequency factor (A) of the Kimmeridge Clay samples from micro-

cycle III and of the Lorca Oil Shale.

of microalgae; and (iii) "black OM" mainly comprises minute ligneous debris associated with a nanoscopically amorphous matrix, probably derived from bacterial constituents. The relative abundances

713

of these different types of organic matter exhibit large changes along each microcycle. The contribution of "orange OM" increases with TOC and HI. In microcycle III from the Marton 87 borehole, this type of OM accounts for about 75% of the total kerogen at the top of the microcycle, whereas the low TOC samples at the beginning of the microcycle are dominated by the "brown" and the "black" OM. Such changes in relative composition are thought to reflect a varying primary pro- ductivity and they account for the variations in kerogen quality also occurring in the microcycles.

Three Kimmeridge Clay samples corresponding to the beginning (B, lower TOC and HI), the middle (M) and the top (T, higher TOC and HI) of microcycle III were examined for their kinetic features. The resulting distributions of activation energies are shown in Fig. 6. Relatively broad distributions were obtained for these three kerogens and the principal energy at 54-55kcalmol -~ accounts for only 30-40% of the reaction. When compared with the algaenans and the other low maturity kerogens tested above, these Kimmeridge Clay kerogens show a distribution shifted to lower Ea values, especially in the case of sample T. Such a shift is consistent with the presence in these kerogens of sulphur-rich "orange OM" and with the increase in the relative contribution of this type of OM along the microcycle. Jensen e t al. (1997) using a Geoelf Sulphur Analyser (GSA) showed that the "sulphur index", which reflects the abundance of the pyrolysable organic sulphur relative to TOC, increases from sample B to M and to T (Table 1).

Pin-point analyses (Boussafir et al., 1995b) showed that the "orange OM" in Kimmeridge Clay kerogens contains high amounts of organic sulphur (atomic S/C ratios of 0.09 to 0.11). It is well docu- mented that S-C bonds are weaker than C-C bonds and, therefore, that S-rich kerogens tend to exhibit a lower thermal stability (e.g. Orr, 1986; Sinninghe Damst6 et al. , 1989; Di Primio and Horsfield, 1995; Reynolds et al. , 1995; Soldan et al. , 1995; Tomic et al., 1995). Indeed, studies on the products generated on closed pyrolyses of an organic- rich Kimmeridge Clay sample (Monin et al., 1990) and on some North Sea oils expelled from the Kimmeridge Clay Formation (Chung e t al. , 1995) illustrated such low stability. Accordingly, the shift to lower energies observed from these three Kimmeridge Clay samples, especially from T, likely reflects the origin of the "orange OM" and the variations in its relative abundance along microcycle III. A low contribution of energies >60 kcal tool -~ (approximately 4%) is noted for sample B, and such a contribution is still weaker for M and negligible for T. This may reflect the presence of "brown OM" in the former sample. As already stressed: (i) this "brown OM" is dominated by ultralaminae and is, thus, chiefly derived by the selective preservation of

714 D. Dessort et aL

MARTON 11-16 MARTON 11-1 . . . . 60 60 % [ A:2.1E+13S-1 I % / A = s'8 E+13 s'l 5 0 . . . . . . . . . . . . . . . . . . . . . 5 0 . . . . . . . . . . . . . . . . . . . . . . . .

40 . . . . . . . . . . . . . . . . . . . . . . 40 . . . . . . . . . . . . . . . . . . . . . . . .

30 . . . . . . . . . . . . . . . . . . .

30 . . . . . . . . . . . . . . . . . . . . . . i . . . . . 20 . . . . . . . . . . . . . . . . . . . . . 20 . . . . . . . . . . . . . . . . . . . i

11 1 i - - ,

Kcal I mole Kcal I mole

Fig. 7. Distribution of activation energies and frequency factor (A) of the Kimmeridge Clay samples from microcycle II.

algaenan; and (ii) algaenans and related kerogens are characterized by high range values for Ea.

Previous kinetic studies of Kimmeridge Clay samples, exhibiting similar TOC values (Tegelaar and Noble, 1994; Reynolds and Burnham, 1995) when compared with T showed activation energy distributions and frequency factors similar to those observed here for the latter material.

Two additional Kimmeridge Clay samples corresponding to the beginning (Marton-1) and the top (Marton-16) of another microcycle, Marton-87-II, were also examined (Fig. 7). When compared with Marton-87-III, this microcycle is characterized by much higher TOC values (Table 1). The "sulphur index" of these two samples is 37 and approximately 200, respectively, indicating that the "natural vulcanization" pathway played a major role in the formation of the latter. Indeed: (i) the activation energy distribution of Marton-1 is similar to that observed for sample T; and (ii) the distribution of Marton-16 is still shifted to lower values and the principal energy at 51 kcalmol -~ accounts for approximately 50% of the reaction.

Lorca Oi l Shale

This low maturity deposit, up to 100m thick, comprises calcareous organic-rich layers overlain by gypsiferous porous limestone partly filled with native sulphur (Benalioulhaj et aL, 1994). The activation energy distribution of the Lorca sample (Fig. 6) is rather broad and the principal energy at 57 kcalmol -l accounts only for ca. 40% of the reaction. An important contribution of relatively low values is also noted, the energies <52 kcalmol -t describing 16% of the reaction. A previous examination based on bulk features (Permanyer et al., 1994) showed that the kerogen of the Lorca Oil Shale can be classified as a type I-S

material, as recently defined by Sinninghe Damst6 et al. (1993). In fact, the Ea distribution of the Lorca sample, with substantial contributions of relatively low energies recalls the distribution observed (Tegelaar and Noble, 1994) for the Ribesalbes Oil Shale, a typical Type I-S kerogen (Sinninghe Damst6 et al., 1993). Previous studies on the Lorca material (Derenne et al., 1995) demonstrated that sulphur incorporation into lipids over- whelmingly contributed to the formation of this kerogen, Such an origin was indicated by: (i) the exclusive presence of nanoscopically amorphous OM; (ii) an extremely high content of organic sulphur (16.3 wt%, atomic S/C ratio of 0.1), whereas pyrite only occurs in trace amounts; and (iii) the nature of the pyrolysis products. Indeed, recent analyses (Jensen et al., 1997) revealed an extremely high "sulphur index" of approximately 290 (Table 1) for this oil shale. The Lorca sample, thus, provides an example of a kerogen formed virtually exclusively by the "natural vulcanization" pathway. As stressed above, this process leads to the formation of geomacromolecules exhibiting a relatively low thermal stability. This is consistent with: (i) recent observations on early release of bitumen from the Lorca Oil Shale, during artificial maturation (Landais et al., 1995); and (ii) the shift to lower activation energy values observed here for this shale when compared with the low maturity type I kerogens (Pula alginite, NE 70 Torbanite, Rundle Oil Shale).

In agreement with previous pyrolytic studies on sulphur-rich kerogens (Baskins and Peters, 1992 and references therein) it, therefore, appears that the involvement of the "natural vulcanization" pathway in kerogen formation is associated with a decrease in activation energies. Nevertheless, the sulphur content is not the only factor that controls


0.15 Thermal history: 3"C/MY 0.14 - - - Rundle Oil Shale 0.13 0.12 Pula Alginite 0.11

0.1 0.09 Lorca Oil Shale 0.08 ""

m 0.07 NE 70 Torbanite 0.06

--0.050.04 ~MARTONII-16 / I-" \~/',\ \

0.020"03 MARTON II-1 , " t " ¢ . " " / ~ , ~ '! t

0.01 ..... - 0

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 T (°C)

Fig. 8. Simulated generation rate curves (geological heating rate of 3°C Ma -l) for the low maturity fossil samples. The curves corresponding to the Kimmeridge Clay samples from microcycle IIl, relatively

close to the curve obtained for the Marton II-1 sample, are not shown.

715

the extent of such a shift. Indeed, while the Lorca Oil Shale is characterized by a higher "sulphur index" than the Marton-16 sample, the shift to lower activation energies is less pronounced for the former material. Again, this may reflect compensating effects associated with the large differences in frequency factors observed for these two samples. Nevertheless, in agreement with Ea distributions, the 450°C rate constant is markedly lower for the Lorca Oil Shale (Fig. 3). In fact, several factors suggest themselves to account for the relative ther-

mal stabilities of the Lorca and Marton-16 samples, such as." (i) the nature of the lipids where sulphur incorporation took place; (ii) the nature of the sulphur groups implicated in lipid cross-linking, i.e. the relative importance of intramolecular (formation of moieties containing only one sulphur atom) and intermolecular links (formation of bridges that can contain a large number of sulphur atoms); and (iii) differences in the environment of the sulphur groups (influence of the neighbouring functions on their thermal stability).

1 Thermal history: 3°C/MY

°' I "un0eO "el O 0.8 / Pula Alginite ,=o., I 1 / . 7 / 0"61--'-- L°rca Oll Shale / / / / ,'/ / ~ 0 . 5 =: / NE'°r°r"n" r l " l . . - I I , 'O' 0"4 /--MARTONII'16 / flY.. I Io.3 } . - ' ] l . . J / ~0 .2

0 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

T (°C)

Fig. 9. Simulated cumulative yield curves (geological heating rate of 3°C Ma -l) for the low maturity fossil samples. The curves corresponding to the Kimmeridge Clay samples from microcycle III relatively

close to the curve obtained for the Marton II-l sample, are not shown.


Table 2. Tmax and width at half height (IV) from the simulated generation rate curves (geological heating rate of 3°C Ma -1) for the low maturity fossil samples

Pula Alginite NE 70 Rundle Kimmeridge Kimmeridge Kimmeridge Kimmeridge Kimmeridge Lorca Oil Torbanite Oil Shale Clay B Clay M Clay T Clay II-1 Clay I1-16 Shale

Tmax 170 184 164 147 152 145 146 136 151 W (°C) 22 20 27 28 30 29 29 25 36

Extrapolation to geological conditions

The data obtained from the laboratory experiments can also be used for calculating kinetic features at very low heating rates, similar to those commonly assumed to occur in geological environments. There are uncertainties in the extrapolation of such data to the much lower (approximately 1019-fold) heating rates experienced by kerogens during natural maturation (Schaefer et al., 1990; Tegelaar and Noble, 1994); howeve:, the general trends so predicted should be reliable. The kinetic results derived from open-system pyrolyses, for the low maturity fossil materials (Pula alginite, NE 70 Torbanite, Rundle Oil Shale, Kimmeridge Clay and Lorca Oil Shale) were extrapolated at a geological heating rate of 3°C Ma - l , with an initial temperature of 10°C (Fig. 8). Width at half-height and Tmax were determined from the simulated generation rate curves thus obtained (Table 2). Cumulative yield curves (transformation ratio of kerogens into volatile pyrolysis products vs temperature) were also constructed (Fig. 9). Such curves are commonly referred to in the literature as "oil" or "hydrocarbon" generation profiles. It should, however, be noted that the information provided by Pyromat pyrolyses reflects the sum of the different types of volatile products swept away by the inert gas flow (including gaseous compounds and non-hydrocarbon constituents). Accordingly, the derived results cannot be considered, sensu stricto, as repre- sentative of oil or hydrocarbon generation.

It was previously noted from cumulative generation curves simulating the transformation of oil into gas in reservoirs that the relative thermal stability of some fossil materials can be inverted when curves corresponding to laboratory and geological heating rates are compared (Schenk and Horsfield, 1995). Such changes should reflect the effects of both activation energy distribution and frequency factors in reactivity control. In fact, at very low geological heating rates, the influence of Ea prevails and possible compensating effects due to differences in frequency factors are then negligible. In the present case no inversion was observed in the profiles corresponding to the nine tested samples on comparison of the cumulative yield curves obtained at laboratory and geological heating rates. Such a feature, therefore, suggests that the influence of Ea still prevails for the heating rates used in our laboratory

experiments. In agreement with the observations on 450°C rate constants, major compensation effects associated with frequency factors are, therefore, unlikely to occur in these experiments.

Comparison of the extrapolated kinetic features revealed substantial differences between the two Botryococcus-derived kerogens (Figs 8 and 9; Table 2). However, in agreement with a previous study (Tegelaar and Noble, 1994) showing generation at higher temperatures for an Australian Torbanite when compared with other type I kerogens, relatively high values of Tm~x were noted for both samples.

The five Kimmeridge Clay samples show relatively close features for simulated maturation (Figs 8 and 9; Table 2). Their Tmax is shifted to lower values when compared with Botryococcus-derived kerogens and to the Rundle Oil Shale. This shift appears especially pronounced in the case of the sulphur-rich Marton 11-16 sample. These observations are consistent with previous studies (Tegelaar and Noble, 1994; Peters et al., 1995) indicating a shift of generation curves for sulphur-rich kerogens.

The Lorca Oil Shale, with an extremely high sulphur content, also shows a lower value for Tmax when compared with the type I kerogens (Fig. 8). Although the Lorca Oil Shale kerogen, like the "orange OM" in the Kimmeridge Clay samples, originates from natural sulphurization the rate and generation profiles obtained for the former kerogen exhibit markedly different shapes. The rate profile of the Lorca material is unsymmetrical and shows both a pronounced shoulder, reflecting the release of an important part of its potential at low temperatures in the 90 to 120°C range, and a maximum at the relatively high value of 151°C. Accordingly, as already discussed in the previous section on activation energy distributions, the kinetic differences between the Kimmeridge Clay samples and the Lorca Oil Shale are unlikely to be related merely to differences in the extent of "vulcanization" and in the abundance of organic sulphur. The maturation of the Lorca Oil Shale is characterized by a substantially earlier onset of generation and a broader generation range (Figs 8 and 9). Catagenesis of this material should, thus, begin at a lower thermal stress and the oil window should correspond to a wider temperature range.


CONCLUSIONS

The determination for the first time of the kinetic paramaters of algaenans showed important differences between the four samples studied. The activation energies thus obtained were correlated with chemical structures. Two extremes were observed, first, the algaenan from the A race of B. braunii showed an E~ distribution virtually corresponding to a single high value energy, reflecting an homogeneous structure with a relatively simple aliphatic polyaldehyde as building block. Secondly, on the contrary the algaenan from S. quadricauda showed a broad distribution and the principal Ea accounts for only one-third of the reaction, thus reflecting a more complex macromolecular structure.

The E~ distributions of the four kerogens chiefly derived from the selective preservation of algaenan also exhibited pronounced differences. The kinetic features of the Pula Oil Shale are consistent with a contribution of both the A (predominant) and the L races of B. braunii. The distribution of the NE 70 Torbanite revealed an unusually homogeneous nature for a fossil material, with a major contribution of selectively preserved algaenan. The differences in kinetic parameters between the NE 70 and BJ 248 Torbanites do not chiefly reflect the different levels of maturity, but rather intrinsic differences due to the diversity of the materials implicated along with the selectively preserved algaenan in the formation of the latter Torbanite. The Rundle Oil Shale showed a broad distribution similar to the one obtained from S. quadricauda algaenan. Important differences in the chemical structure and thermal behaviour, not revealed by type classification since these three kerogens are classified as low maturity type I are, therefore, reflected by the Pyromat-derived kinetic parameters of the Rundle, Pula and N E 70 samples.

The Kimmeridge Clay and Lorca samples showed broad Ea distributions shifted to low energies. Such shifts reflect a more or less important contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Nevertheless, as shown by comparison of the Lorca Oil Shale and of the sulphur-rich Kimmeridge Clay sample, the sulphur content is not the only factor that controls the shift to lower energies.

Finally, the kinetic data derived from Pyromat pyrolysis were extrapolated to a very low, geological heating rate of 3°C Ma -1 for the nine low maturity fossil samples. The simulated rate and generation profiles showed: (i) substantial differences between the Botryococcus-derived kerogens, although they both exhibited relatively high Tmax; (ii) a shift of Tmax to lower values for the samples with a high contribution of the "vulcanizat ion" pathway; and (iii) a markedly different shape for

the profiles obtained from the Lorca sample, reflecting the release of an important part of its potential at low temperatures, probably due to differences in the nature and/or the environment of the cross-linking sulphur groups.

Associate Ed i to r - - J . S. Sinninghe Damste

Acknowledgements--This work was supported by Elf Aquitaine and by CNRS. Publication approval by Elf Aquitaine is also acknowledged.

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