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Chloroform extracts of a series of coals from the Mahakam delta
Org. Geochem. Vol. 6, pp. 431-437, 1984
0146-6380/84 $I)3.(I0+tX).00 Copyright © 1984 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
Chloroform extracts of a series of coals from the M a h a k a m delta JEAN-PAUL BOUDOU* Universit6 d'Orl6ans C.N.R.S.-E.R.A. 601, 45046 Orleans, France
A b s t r a c t - - A detailed study of the changes that take place in the organic solvent extractable organic matter
during the early stages of thermal maturation has not been published to date. The present work describes several trends occurring with increasing diagenesis, observed in the chloroform extractable organic matter of a series of coals of Tertiary age from the Mahakam delta (Indonesia). with special consideration given to the lignite and sub-bituminous stages. Wide variations in the values of the total chloroform extracts were observed in this work and the extraction yields reached a maximum at the beginning of the sub-bituminous stage. This maximum is discussed and two hypotheses are offered which may provide an cxphmation for the trend. These are the formation of extractable compounds by defunctionalisation of kerogcn during the lignite stage, followed by a defunctionalisation of compounds in the chloroform extract itself during the sub-bituminous stage, and/or a migration from deeper horizons. Key words: coal, lignite, sub-bituminous coal, diagenesis, hydrocarbons, oil migration
INTRODUCTION
A great deal of work has been done on changes which occur in the organic solvent extractable fraction of sedimentary organic matter during kerogen and coal catagenesis. On the other hand, an accurate study of the changes that take place during the early stage of thermal maturation, or diagenesis, as defined by Tissot and Welte (1978), has not been made to date, since the supposedly low yield of hydrocarbons is of little economic interest. This work describes some trends which occur during diagenesis in the chloroform-extractable organic matter of a series of coals of Tertiary age from the Mahakam delta (Indonesia). Special consideration is given to the lignite and sub-bituminous stages of maturation, since the main diagenetic changes are probably thermal low-energy physico-chemical, rather than biochemical, reactions (Stach et al., 1975). Data were not related to the depth of absolute age of the formation, because of its overall heterogeneous nature and also because the maturity of coals of similar age and depth can be different, even if sampled from the same location (Durand and Oudin, 1979; Boudou, 1981). Instead, in this work, diagenetic trends were related to the organic carbon content as a rank parameter, since this internal parameter can be measured with a higher degree of accuracy (1%) than can vitrinite reflectance. GEOLOGICAL SETTING
The Mahakam delta (Indonesia) is situated on the eastern part of the Kutei Basin, in south-eastern Borneo (Kalimantan). The exact position of the ~Present address: Universit~ Pierre et Marie Curie, C.N.R.S., L.A, 196, Laboratoire de G6ochimie et M6tallog6nie, 4, Place Jussieu, Tour 16-26, 5° 6tage, 75230 Paris Cedex 05, France. A'II
basement is not known, nor is the age of the first deposits, but deltaic sedimentation is known to have occurred at least since the middle Miocene, and this eastward prograding series is more than 4000 m thick. One observes in the vertical profile (Allen et al., 1979) a succession of three deltaic deposits (Miocene, Plio-Pleistocene and Holocene deltas) described by Magnier et al. (1975). Since the middle Miocene the coal-generating flora has remained the same (Muller, 1964, 1972; Anderson and Muller, 1975; Morley, 1977; Caratini and Tissot, 1979). A number of organic geochemical studies of a series of coals from the Mahakam delta have already been published (Combaz and De Matharel, 1978; Durand and Oudin, 1979: Boudou, 1981: Dereppe et al., 1983; Schoell et al., 1983; Vandenbroucke et al., 1983; Hoffmann et al., 1984). In Table 1 petrographic characteristics of samples from two exploration fields representative of the Mahakam delta are given. EXPERIMENTAL
Sampling Samples, which range from lignite to bituminous coal, were provided by the Soci6t6 Total-Indon6sie and the Compagnie Franqaise des P6troles. They were obtained from cuttings taken at various depth intervals from many boreholes (maximum depth 3500 m). One hundred samples have been selected on the basis of kerogen analysis of 600 samples, which can be considered as statistically representative of the coal series, in spite of its overall intrinsic heterogeneity (Boudou, 1981; Boudou et al., 1984). Analytical methods Freeze-dried and crushed coals (< 100 mesh, 146 ixm) were extracted by boiling, and stirring, for 60 min in chloroform. Elemental analyses of coal extracts were according to the methods of Durand and
Depth (m)
Sulfides Clays Chloroform extract yield (mg/100 mg organic carbon)
Fusinite Scmifusinitc Sclerotinite
Sporinitc Cutinitc Resinitc Liptodctrinite
Maceral % (crude coal) Tcxtinitc Ulminitc Gelinite Corpohuminite Attrinitc Dcnsinite
Organic carbon % (d.a,f) Vitrinite reflectance %
100
180
2.4 4,9 4.4
4.8
0.0 9.7 0.0
1.6 0.8 0.8 1.6
0.8 57.7 57.7 4.1 12.2 0.o
62.9 0.20
3.1 111.3
0.0 7.9 0.0
0.0 0.8 5.5 2.4
0.79 16.6 19.8 5.5 24.6 2.3
56.7 (I.17
270
3.5
1.7 2.6
0.8 1.7 0.0
2.6 0.0 0.8 4.3
0.8 69,8 69.8 5.1 9.4 9,4
61.5 0.26
4.6
3.4 17.3
0.0 3.1 0.4
3.1 0.7 1.1 3.1
0.7 54.8 54.8 5.0 6.9 0.0
62.9 0.33
330
Handil 1270
4.0
4.9 2.1
0.0 4.1 1.4
1.4 2.1 3.5 0.0
4.5 74.6 74.6 1.2 0.0 0.0
67.4 0.45
156o
2.2
2.8 8.3
0.0 4.1 0.0
1.4 0.0 4.1 0.0
0.0 1.4 77.7 0.0 0.0 0.0
72.9 0.48
1.8
2.0 1.1
0,0 3.5 2.0
2.0 2.5 2.0 0.0
0.0 74.2 74.2 1.0 0.0 0.0
76.5 0.5(I
2080
2.0
6.3 2.5
0.7 7.8 1.2
1.5
4.5 1.0 3.2
0.0 11.5 31.9 4.0 18.9 10.0
60.3 0.26
579
2.1
6.7 1.2
0.0 3.7 1.7
7.9 4.6 5.4 2.9
0.0 0.4 21.8 1.2 20.2 21.8
59.7 0.33
840
2.3
3.7 2.2
I).11 5,9 1.1
2.9 2.2 7,4 11.3
0.0 1.5 33.1 1.8 14.8 21.9
60.1 0.33
89(1
3.1
4.2 5.5
0.0 4.7 1).7
4.11 3.4 0.9 1.3
0.2 1.1 50.9 1.9 9.7 10.0
60.5 0.30
845
2.2
9.1 14.6
I).5 2.5 11.2
1.7 11.5 t.1 0.5
0.0 0.5 64.7 2.2 1.7 0.11
60.6 0.31
941)
Bekapai
2.4
7.9 10.6
1).6
1.9
11.11
2.6 5.2 1/.6 (1.9
0.0 0.4 60.9 1.7 5.7 5.7
62.5 0.3(I
1095
Table 1. Maceral composition of Mahakam coals and relationships with rank and chloroform extract yields
1.9
7.4 4.2
0.0 3.6 0.8
5.8 2.5 1.2 0.6
0.1 11.1 63.8 0.3 9.1 11.0
64.8 0.32
1250
3.5
9.1 7.1
0.0 4.8 1.2
1.4 1.4 1.0 11.8
0.0 0.4 59.5 2.6 10.5 /).0
62.1 0.34
1640
©
t-
),
Chloroform extracts of Mahakam delta coals Monin (1980), 13C NMR and IH NMR spectra were obtained in CDCI3 on a Varian apparatus; i.r. spectra of the coal extracts (KBr pellets) were obtained by f.t.i.r, methods (Digilab). Hydrocarbons were separated by TLC on silica-gel with cyclohexane as the developer (Huc et al., 1976) and petrographic analysis were provided by B. Alpern. RESULTS
AND DISCUSSION
433
increase, in the chloroform extracts, may simply be that the proportion of nitrogen increases, because of the loss of more labile carbon and oxygen under the form of CO2. S/C atomic ratios do not show any clear trend but a slight tendency to increase with maturation. Since organic sulfur may come from allochthonous sources (Boulegue and Michard, 1974, 1976; Casagrande et al., 1977; Casagrande and Ng, 1979), it is difficult to draw any conclusions from these crude data.
Extraction yields (Fig. 1)
The distribution of values for extraction yields was very scattered, and there was no evident trend other than a maximum at ca 70% organic carbon and a minimum at ca 78% organic carbon: the division between the sub-bituminous and the bituminous stage is near this last value, according to Stach et al. (1975).
.5O[r-
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-/
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• Coals from several origins
° • I
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5
10
Functional groups (Figs 3 and 4)
The proportion of aromatic hydrogen and aromatic carbon increased with maturation (Fig. 3). Similar trends have been shown by the measurement of u.v. fluorescence intensities of hydrocarbons and polar compounds isolated by liquid chromatography (Baud•u, 1981). So far, we have no clear explanation for the apparent aromatisation during the early stages of maturation although several hypotheses have been proposed (Streibl and Heroult, 1969; Spyckerelle, 1975; Wakeham et al., 1980; Corbet, 1980). The analysis of oxygenated groups by i.r. spectroscopy showed a decrease in the absorption intensity of hydroxyl groups and carbonyl groups. As shown in Fig. 4, the ester group bands (1770 and 1775 cm-~;Mosphocopis and Speight, 1976; Mosphocopis et al., 1976; Painter et al., 1981) decreased during the sub-bituminous stage. The surprising persistence of oxygenated groups in chloroform extract could be due to the low-electrophilic character of carbonyl and carboxylic substituents, which would prevent the extractable organic matter from quick thermal decarboxylation and decarbonylation.
Total chloroform e×troctyields (mg / 1 0 0 mg Org. C ) ( % )
Fig. 1. Changes in extraction yields with maturation (related to organic carbon of coal).
Some coal samples from several origins, extracted by the same method would show a minimum at ca 78% organic carbon as seen in Fig. 1. These irregularities of variation of extraction yields with coal maturation have been reported elsewhere, only once, by Hollerbach and Hageman (1981); however, the authors have not given any interpretation of this phenomenon. Elemental composition (Fig. 2)
H/C and O/C atomic ratios progressively decreased with increasing diagenesis. These changes, which are well known, imply a loss of oxygenated functions and an aromatisation. N/C atomic ratios increase with maturation, like they do in total coals (Boudou et al., 1984). This increase has never been reported in coal extracts at these stages of maturity. An explanation of this
Hydrocarbons yields (Figs 5 and 6)
Changes in hydrocarbon yields, expressed as a percentage of the organic carbon of the whole coal, showed a perceptible and regular increase during the sub-bituminous stage (Fig. 5). To compare hydrocarbon yields in coals and in Mahakam sediments, changes in the total chloroform extract yields were related to the hydrocarbon amounts taken as a maturity parameter (Fig. 6). We can distinguish in Fig. 6 three kinds of sediment samples: (i) The samples where the total extract yields are similar to those observed in coals. (ii) The samples where the total extract yields are high (up to 60 rag/100 mg of organic carbon) and the total hydrocarbon yields are also high (superior to 70% of the chloroform extract). (iii) The samples where the total extract yields are relatively high (superior to 10 rag/100 mg of organic carbon) and the total hydrocarbon yields are relatively close to those observed in coals (inferior to 50% of the chloroform extract).
434
JEAN-PAUL
BOUDOU
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60
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0.01
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Fig. 2. Changes in the elemental composmon of the extract with maturation (related to organic carbon of coal)•
• 13C • tH
6O q -o
o
NMR NMR
.V..\
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70
o
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catagenesis zone where migration of hydrocarbons starts (Durand and Oudin, 1979; Vandenbroucke et al., 1983) and impregnated samples enriched in hydrocarbons would be either from the top of the catagenesis zone or from the diagenesis zone where hydrocarbon deposition occurs (Combaz and De Matharel, 1978; Durand and Oudin, 1979).
Alkanes (Fig. 7)
In Fig. 7 pristane yields, pristane/phytane ratios and pristane/n-C17 ratios were plotted against the organic carbon content• Pristane and phytane are 1 2 5 4. 5 6 i' fa(IH NMR)(%) present in low abundance as free hydrocarbons I I I I I I ] 1 1 during early diagenesis, but generally increase in 0 20 40 fa ( 13C NMR}(%) abundance with time. This observation concurs with Number of oromotic C or H xIO0 to= Totol numberof C o r H current ideas that pristane and phytane are normal Fig. 3. Changes in the aromaticity of the total extract with products of early diagenesis of isoprenoids and maturation (related to organic carbon of coal). phytol. The pristane/phytane curve reached a maximum at the end of diagenesis. Such a maximum has often been reported but not exactly at the same degree of maturity (Brooks and Smith, 1969; ConSediments low in organic carbon are more sensitive nan, 1973). The pristane/n-C17 curve also showed to impregnation as a result of migration; this con- the same trend as the pristane/phytane curve• A decrease in the values of these ratios during catagenetrasts with coals which provide poor "reservoirs". Impregnated samples (rich in total extract), which sis can be explained as being the result of thermal are depleted in hydrocarbons, would be from the cracking during catagenesis (Tissot and Welte, 1978).
Chloroform extracts of Mahakam delta coals
435
Peat ( C = 4 9 % )
Peat (C = 55%)
Lignite
(C= 6 1 % )
Lignite
(C = 6 7 % )
Sub-bituminous cool (C= 7 8 % )
I
1850
1770 17351710
1635 1600
1550
Wavenumber (cm-~) Fig. 4. Partial i.r. spectra of total chloroform extract showing changes of oxygenated functional groups.
I
=° (..-./ 2 6o
"a
't §80 o
~
O
Lignite stage
}-..iX" V. \ . -~...:\ 'k
50
Sub- bituminous stage
"
100
Total hydrocarbon yields ( m g / 1 0 0 mg chloroform e x t r a c t ) ( % )
Fig. 5. Changes in hydrocarbon yields with maturation (related to organic carbon in coal). Pristane/phytane, pristane/n-Cl7 have similar trends in coals and in sediment samples not perturbed by impregnation (Boudou, 1981). CONCLUSIONS
Although wide variations in the values of the total chloroform extract were observed, the extraction
yields reached a maximum at the beginning of the sub-bituminous stage (i.e. mean vitrinite reflectance in oil of c a 0.45%). These changes, which have not been reported previously, would not result from petrographic variations: they seem to occur in some other coal series. The formation of extractable compounds, by defunctionalisation of humic-like compounds of the kerogen, followed by defunctionalisation of compounds in chloroform extract itself during the subbituminous stage, may provide an explanation for this trend. This hypothesis seems consistent with changes in the properties of the chloroform extract (elemental composition, functional groups, alkanes) which changes markedly during the sub-bituminous stage. This implies that the sub-bituminous stage represents a period of intense defunctionalisation, as seen by losses of oxygen and oxygenated group, formation of pristane, and also a period of aromatisation, as seen by the relative increase in aromatic hydrogen and aromatic carbon of the extractable organic m a t t e r . These transformations would account for the sharp decrease in the total extract yields during the sub-bituminous stage and might account for some changes which occur beyond this stage.
436
JEAN-PAUL BOUDOU
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T o t a l c h l o r o f o r m extract yields ( m g / 1 Q O m g O r g . C ) ( % )
Fig. 6. Comparison of hydrocarbon yields in coals and Mahakam sediments• Changes of total extract yields are related to the hydrocarbon weight percentages of the total chloroform extract.
A
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Fig. 7. Pristanc yields, pristane/phytane and pristane/n-CI7 plotted against the organic carbon content of the coal.
Another explanation may be that variations in extract yield are due to migration from deeper horizons as described by Durand and Oudin (1979) and by Vandenbroucke et al. (1983) for the Mahakam delta, or by Philp et aL (1983) for diterpenoids in crude oils and coals from south-eastern Australia. This explanation is not confirmed by changes of hydrocarbon yields in coal, that is, we did not observe any hydrocarbon depletion or hydrocarbon enrichment of the chloroform extract of coals, as is readily observed in sediments where migration phenomena occur. A more accurate study describing changes of solvent extractable organic matter of coals with depth from each borehole should lead to an answer to this question.
Acknowledgements--We acknowledge Total Indonrsie and the Compagnie Franqaise des Prtroles for providing samples and useful information. We also thank the Institut Franqais de Prtrole for funding and for performing most of the analyses•
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0146-6380/84 $I)3.(I0+tX).00 Copyright © 1984 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
Chloroform extracts of a series of coals from the M a h a k a m delta JEAN-PAUL BOUDOU* Universit6 d'Orl6ans C.N.R.S.-E.R.A. 601, 45046 Orleans, France
A b s t r a c t - - A detailed study of the changes that take place in the organic solvent extractable organic matter
during the early stages of thermal maturation has not been published to date. The present work describes several trends occurring with increasing diagenesis, observed in the chloroform extractable organic matter of a series of coals of Tertiary age from the Mahakam delta (Indonesia). with special consideration given to the lignite and sub-bituminous stages. Wide variations in the values of the total chloroform extracts were observed in this work and the extraction yields reached a maximum at the beginning of the sub-bituminous stage. This maximum is discussed and two hypotheses are offered which may provide an cxphmation for the trend. These are the formation of extractable compounds by defunctionalisation of kerogcn during the lignite stage, followed by a defunctionalisation of compounds in the chloroform extract itself during the sub-bituminous stage, and/or a migration from deeper horizons. Key words: coal, lignite, sub-bituminous coal, diagenesis, hydrocarbons, oil migration
INTRODUCTION
A great deal of work has been done on changes which occur in the organic solvent extractable fraction of sedimentary organic matter during kerogen and coal catagenesis. On the other hand, an accurate study of the changes that take place during the early stage of thermal maturation, or diagenesis, as defined by Tissot and Welte (1978), has not been made to date, since the supposedly low yield of hydrocarbons is of little economic interest. This work describes some trends which occur during diagenesis in the chloroform-extractable organic matter of a series of coals of Tertiary age from the Mahakam delta (Indonesia). Special consideration is given to the lignite and sub-bituminous stages of maturation, since the main diagenetic changes are probably thermal low-energy physico-chemical, rather than biochemical, reactions (Stach et al., 1975). Data were not related to the depth of absolute age of the formation, because of its overall heterogeneous nature and also because the maturity of coals of similar age and depth can be different, even if sampled from the same location (Durand and Oudin, 1979; Boudou, 1981). Instead, in this work, diagenetic trends were related to the organic carbon content as a rank parameter, since this internal parameter can be measured with a higher degree of accuracy (1%) than can vitrinite reflectance. GEOLOGICAL SETTING
The Mahakam delta (Indonesia) is situated on the eastern part of the Kutei Basin, in south-eastern Borneo (Kalimantan). The exact position of the ~Present address: Universit~ Pierre et Marie Curie, C.N.R.S., L.A, 196, Laboratoire de G6ochimie et M6tallog6nie, 4, Place Jussieu, Tour 16-26, 5° 6tage, 75230 Paris Cedex 05, France. A'II
basement is not known, nor is the age of the first deposits, but deltaic sedimentation is known to have occurred at least since the middle Miocene, and this eastward prograding series is more than 4000 m thick. One observes in the vertical profile (Allen et al., 1979) a succession of three deltaic deposits (Miocene, Plio-Pleistocene and Holocene deltas) described by Magnier et al. (1975). Since the middle Miocene the coal-generating flora has remained the same (Muller, 1964, 1972; Anderson and Muller, 1975; Morley, 1977; Caratini and Tissot, 1979). A number of organic geochemical studies of a series of coals from the Mahakam delta have already been published (Combaz and De Matharel, 1978; Durand and Oudin, 1979: Boudou, 1981: Dereppe et al., 1983; Schoell et al., 1983; Vandenbroucke et al., 1983; Hoffmann et al., 1984). In Table 1 petrographic characteristics of samples from two exploration fields representative of the Mahakam delta are given. EXPERIMENTAL
Sampling Samples, which range from lignite to bituminous coal, were provided by the Soci6t6 Total-Indon6sie and the Compagnie Franqaise des P6troles. They were obtained from cuttings taken at various depth intervals from many boreholes (maximum depth 3500 m). One hundred samples have been selected on the basis of kerogen analysis of 600 samples, which can be considered as statistically representative of the coal series, in spite of its overall intrinsic heterogeneity (Boudou, 1981; Boudou et al., 1984). Analytical methods Freeze-dried and crushed coals (< 100 mesh, 146 ixm) were extracted by boiling, and stirring, for 60 min in chloroform. Elemental analyses of coal extracts were according to the methods of Durand and
Depth (m)
Sulfides Clays Chloroform extract yield (mg/100 mg organic carbon)
Fusinite Scmifusinitc Sclerotinite
Sporinitc Cutinitc Resinitc Liptodctrinite
Maceral % (crude coal) Tcxtinitc Ulminitc Gelinite Corpohuminite Attrinitc Dcnsinite
Organic carbon % (d.a,f) Vitrinite reflectance %
100
180
2.4 4,9 4.4
4.8
0.0 9.7 0.0
1.6 0.8 0.8 1.6
0.8 57.7 57.7 4.1 12.2 0.o
62.9 0.20
3.1 111.3
0.0 7.9 0.0
0.0 0.8 5.5 2.4
0.79 16.6 19.8 5.5 24.6 2.3
56.7 (I.17
270
3.5
1.7 2.6
0.8 1.7 0.0
2.6 0.0 0.8 4.3
0.8 69,8 69.8 5.1 9.4 9,4
61.5 0.26
4.6
3.4 17.3
0.0 3.1 0.4
3.1 0.7 1.1 3.1
0.7 54.8 54.8 5.0 6.9 0.0
62.9 0.33
330
Handil 1270
4.0
4.9 2.1
0.0 4.1 1.4
1.4 2.1 3.5 0.0
4.5 74.6 74.6 1.2 0.0 0.0
67.4 0.45
156o
2.2
2.8 8.3
0.0 4.1 0.0
1.4 0.0 4.1 0.0
0.0 1.4 77.7 0.0 0.0 0.0
72.9 0.48
1.8
2.0 1.1
0,0 3.5 2.0
2.0 2.5 2.0 0.0
0.0 74.2 74.2 1.0 0.0 0.0
76.5 0.5(I
2080
2.0
6.3 2.5
0.7 7.8 1.2
1.5
4.5 1.0 3.2
0.0 11.5 31.9 4.0 18.9 10.0
60.3 0.26
579
2.1
6.7 1.2
0.0 3.7 1.7
7.9 4.6 5.4 2.9
0.0 0.4 21.8 1.2 20.2 21.8
59.7 0.33
840
2.3
3.7 2.2
I).11 5,9 1.1
2.9 2.2 7,4 11.3
0.0 1.5 33.1 1.8 14.8 21.9
60.1 0.33
89(1
3.1
4.2 5.5
0.0 4.7 1).7
4.11 3.4 0.9 1.3
0.2 1.1 50.9 1.9 9.7 10.0
60.5 0.30
845
2.2
9.1 14.6
I).5 2.5 11.2
1.7 11.5 t.1 0.5
0.0 0.5 64.7 2.2 1.7 0.11
60.6 0.31
941)
Bekapai
2.4
7.9 10.6
1).6
1.9
11.11
2.6 5.2 1/.6 (1.9
0.0 0.4 60.9 1.7 5.7 5.7
62.5 0.3(I
1095
Table 1. Maceral composition of Mahakam coals and relationships with rank and chloroform extract yields
1.9
7.4 4.2
0.0 3.6 0.8
5.8 2.5 1.2 0.6
0.1 11.1 63.8 0.3 9.1 11.0
64.8 0.32
1250
3.5
9.1 7.1
0.0 4.8 1.2
1.4 1.4 1.0 11.8
0.0 0.4 59.5 2.6 10.5 /).0
62.1 0.34
1640
©
t-
),
Chloroform extracts of Mahakam delta coals Monin (1980), 13C NMR and IH NMR spectra were obtained in CDCI3 on a Varian apparatus; i.r. spectra of the coal extracts (KBr pellets) were obtained by f.t.i.r, methods (Digilab). Hydrocarbons were separated by TLC on silica-gel with cyclohexane as the developer (Huc et al., 1976) and petrographic analysis were provided by B. Alpern. RESULTS
AND DISCUSSION
433
increase, in the chloroform extracts, may simply be that the proportion of nitrogen increases, because of the loss of more labile carbon and oxygen under the form of CO2. S/C atomic ratios do not show any clear trend but a slight tendency to increase with maturation. Since organic sulfur may come from allochthonous sources (Boulegue and Michard, 1974, 1976; Casagrande et al., 1977; Casagrande and Ng, 1979), it is difficult to draw any conclusions from these crude data.
Extraction yields (Fig. 1)
The distribution of values for extraction yields was very scattered, and there was no evident trend other than a maximum at ca 70% organic carbon and a minimum at ca 78% organic carbon: the division between the sub-bituminous and the bituminous stage is near this last value, according to Stach et al. (1975).
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Functional groups (Figs 3 and 4)
The proportion of aromatic hydrogen and aromatic carbon increased with maturation (Fig. 3). Similar trends have been shown by the measurement of u.v. fluorescence intensities of hydrocarbons and polar compounds isolated by liquid chromatography (Baud•u, 1981). So far, we have no clear explanation for the apparent aromatisation during the early stages of maturation although several hypotheses have been proposed (Streibl and Heroult, 1969; Spyckerelle, 1975; Wakeham et al., 1980; Corbet, 1980). The analysis of oxygenated groups by i.r. spectroscopy showed a decrease in the absorption intensity of hydroxyl groups and carbonyl groups. As shown in Fig. 4, the ester group bands (1770 and 1775 cm-~;Mosphocopis and Speight, 1976; Mosphocopis et al., 1976; Painter et al., 1981) decreased during the sub-bituminous stage. The surprising persistence of oxygenated groups in chloroform extract could be due to the low-electrophilic character of carbonyl and carboxylic substituents, which would prevent the extractable organic matter from quick thermal decarboxylation and decarbonylation.
Total chloroform e×troctyields (mg / 1 0 0 mg Org. C ) ( % )
Fig. 1. Changes in extraction yields with maturation (related to organic carbon of coal).
Some coal samples from several origins, extracted by the same method would show a minimum at ca 78% organic carbon as seen in Fig. 1. These irregularities of variation of extraction yields with coal maturation have been reported elsewhere, only once, by Hollerbach and Hageman (1981); however, the authors have not given any interpretation of this phenomenon. Elemental composition (Fig. 2)
H/C and O/C atomic ratios progressively decreased with increasing diagenesis. These changes, which are well known, imply a loss of oxygenated functions and an aromatisation. N/C atomic ratios increase with maturation, like they do in total coals (Boudou et al., 1984). This increase has never been reported in coal extracts at these stages of maturity. An explanation of this
Hydrocarbons yields (Figs 5 and 6)
Changes in hydrocarbon yields, expressed as a percentage of the organic carbon of the whole coal, showed a perceptible and regular increase during the sub-bituminous stage (Fig. 5). To compare hydrocarbon yields in coals and in Mahakam sediments, changes in the total chloroform extract yields were related to the hydrocarbon amounts taken as a maturity parameter (Fig. 6). We can distinguish in Fig. 6 three kinds of sediment samples: (i) The samples where the total extract yields are similar to those observed in coals. (ii) The samples where the total extract yields are high (up to 60 rag/100 mg of organic carbon) and the total hydrocarbon yields are also high (superior to 70% of the chloroform extract). (iii) The samples where the total extract yields are relatively high (superior to 10 rag/100 mg of organic carbon) and the total hydrocarbon yields are relatively close to those observed in coals (inferior to 50% of the chloroform extract).
434
JEAN-PAUL
BOUDOU
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Fig. 2. Changes in the elemental composmon of the extract with maturation (related to organic carbon of coal)•
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Alkanes (Fig. 7)
In Fig. 7 pristane yields, pristane/phytane ratios and pristane/n-C17 ratios were plotted against the organic carbon content• Pristane and phytane are 1 2 5 4. 5 6 i' fa(IH NMR)(%) present in low abundance as free hydrocarbons I I I I I I ] 1 1 during early diagenesis, but generally increase in 0 20 40 fa ( 13C NMR}(%) abundance with time. This observation concurs with Number of oromotic C or H xIO0 to= Totol numberof C o r H current ideas that pristane and phytane are normal Fig. 3. Changes in the aromaticity of the total extract with products of early diagenesis of isoprenoids and maturation (related to organic carbon of coal). phytol. The pristane/phytane curve reached a maximum at the end of diagenesis. Such a maximum has often been reported but not exactly at the same degree of maturity (Brooks and Smith, 1969; ConSediments low in organic carbon are more sensitive nan, 1973). The pristane/n-C17 curve also showed to impregnation as a result of migration; this con- the same trend as the pristane/phytane curve• A decrease in the values of these ratios during catagenetrasts with coals which provide poor "reservoirs". Impregnated samples (rich in total extract), which sis can be explained as being the result of thermal are depleted in hydrocarbons, would be from the cracking during catagenesis (Tissot and Welte, 1978).
Chloroform extracts of Mahakam delta coals
435
Peat ( C = 4 9 % )
Peat (C = 55%)
Lignite
(C= 6 1 % )
Lignite
(C = 6 7 % )
Sub-bituminous cool (C= 7 8 % )
I
1850
1770 17351710
1635 1600
1550
Wavenumber (cm-~) Fig. 4. Partial i.r. spectra of total chloroform extract showing changes of oxygenated functional groups.
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Total hydrocarbon yields ( m g / 1 0 0 mg chloroform e x t r a c t ) ( % )
Fig. 5. Changes in hydrocarbon yields with maturation (related to organic carbon in coal). Pristane/phytane, pristane/n-Cl7 have similar trends in coals and in sediment samples not perturbed by impregnation (Boudou, 1981). CONCLUSIONS
Although wide variations in the values of the total chloroform extract were observed, the extraction
yields reached a maximum at the beginning of the sub-bituminous stage (i.e. mean vitrinite reflectance in oil of c a 0.45%). These changes, which have not been reported previously, would not result from petrographic variations: they seem to occur in some other coal series. The formation of extractable compounds, by defunctionalisation of humic-like compounds of the kerogen, followed by defunctionalisation of compounds in chloroform extract itself during the subbituminous stage, may provide an explanation for this trend. This hypothesis seems consistent with changes in the properties of the chloroform extract (elemental composition, functional groups, alkanes) which changes markedly during the sub-bituminous stage. This implies that the sub-bituminous stage represents a period of intense defunctionalisation, as seen by losses of oxygen and oxygenated group, formation of pristane, and also a period of aromatisation, as seen by the relative increase in aromatic hydrogen and aromatic carbon of the extractable organic m a t t e r . These transformations would account for the sharp decrease in the total extract yields during the sub-bituminous stage and might account for some changes which occur beyond this stage.
436
JEAN-PAUL BOUDOU
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Fig. 6. Comparison of hydrocarbon yields in coals and Mahakam sediments• Changes of total extract yields are related to the hydrocarbon weight percentages of the total chloroform extract.
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Fig. 7. Pristanc yields, pristane/phytane and pristane/n-CI7 plotted against the organic carbon content of the coal.
Another explanation may be that variations in extract yield are due to migration from deeper horizons as described by Durand and Oudin (1979) and by Vandenbroucke et al. (1983) for the Mahakam delta, or by Philp et aL (1983) for diterpenoids in crude oils and coals from south-eastern Australia. This explanation is not confirmed by changes of hydrocarbon yields in coal, that is, we did not observe any hydrocarbon depletion or hydrocarbon enrichment of the chloroform extract of coals, as is readily observed in sediments where migration phenomena occur. A more accurate study describing changes of solvent extractable organic matter of coals with depth from each borehole should lead to an answer to this question.
Acknowledgements--We acknowledge Total Indonrsie and the Compagnie Franqaise des Prtroles for providing samples and useful information. We also thank the Institut Franqais de Prtrole for funding and for performing most of the analyses•
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