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Micrinite and exudatinite in some Australian coals, and their relation to the generation of petroleum
Micrinite and exudatinite in some Australian coals, and their relation to the generation of petroleum Michio Shibaoka CSIRO Fuel Geoscience Unit, P.O. Box 136, North Ryde, NSW2113, (Received 21 December 1976)
Australia
The occurrence
in some Australian
reported.
and microscopic
features
of micrinite
connection
with
hydrocarbon
genesis.
In Australia,
minous coals which are very rich in inertinite, possible progenitors micrinite
was formed
liquid hydrocarbons high-volatile relation liquid
of micrinite from
between
is oxidized
‘2nd coalification
occurs in bituminous
and also in brown porigelinite.
during this process.
coals in the Gippsland
liquid hydrocarbons, from
micrinite
of their occurrence
There
solid liptinite
Basin.
and sub-bitu-
coal rich in inertinite.
One of the
is little reason to conclude
exudatinite,
Exudatinite There
micrinite
coals are
are discussed in
resinite and other macerals at an early stage of coalification
were formed
bituminous
hydrocarbons
so-called
and exudatinite
The origin of these macerals and also the significance
that
and that
occurs in sub-bituminous
and
is no positive evidence
of a genetic
and liptinite
The formation
macerals.
of
etc. may take place just before and during Teichmiiller’s
jump’.
is generally believed that oil is generated from organic material finely dispersed in a sediment at a certain stage of its geothermal alteration. However, not all authors are in agreement on the actual stage at which this occurs. Some suggest that a fairly low degree of organic diagenesis is sufficient for oil generation. Recently, M. Teichmuller132 stated that the appearance of two microscopic constituents of coal, micrinite and exudatinite, is indicative of the generation of oil. However, in Australia, micrinite commonly occurs in low-rank coals, and even in lignites. If petroleum is generated from organic matter of such a low degree of diagenesis, then some ideas on petroleum prospecting, particularly in the Australian context, should be revised. Several shallow basins, for example, could be reinvestigated. The purpose of this paper is, in the light of new evidence presented, to reconsider the genesis of micrinite and exudatinite and to assess the significance of their occurrence in the generation of petroleum. An explanation of the terms appearing in this paper and a summary of the macerals found in bituminous and brown coals are given in the Appendix. Following the recommendation of the International Committee for Coal Petrology3, the term ‘micrinite’ is used throughout the paper instead of ‘fine micrinite’. The term ‘exudatinite’ is used as defined by Teichmuller’.
It
PREVIOUS THEORIES OF THE ORIGIN OF MICRINITE AND EXUDATINITE The colour of micrinite in reflected li$t is pale grey to white and its reflectance is higher than that of vitrinite. It differs from other macerals of the inertinite group in shape and probably in chemical properties4. It has a characteristic rounded shape and a very small grain size, grain diameter being commonly less than one micrometre5. According to
Teichmuller’~‘, micrinite yields a larger volume of volatile matter than vitrinite, and has been shown to be sensitive to both heating6 and oxidation7. Micrinite occurs in the form of cell fillings in vitrinite, or finely dispersed in collinite. Sometimes agglomerations of micrinite grains form microscopically recognizable layers. The transition of resinite cell fillings to micrinite has been reported’. Micrinite is rarely present in lignite or peat’-‘*. occurring mainly in bituminous coals. It is especially common in ccrtain durites and trimacerites, such as sapropelite. and often appears in close proximity to sporinite”. It has been suggested that coals deposited under aerobic conditions are poor in micrinite*. Thiessen and Sprunk13 concluded that micrinite originated from peatification of cell walls. and Spackman and Barghoorn]’ and Cohen” suggested that semi-opaque granular substances in peats or brown coals could be possible precursors. Teichn~uller’32 considered that most micrinite was formed from the lipid compounds of different liptinites. particularly from resinite. bituminite and dcsmocollinite-degradinito, during the very early part of the high-volatile bituminous coal stage. She suggested that liquid bitumen and micrinite were formed simultaneously from these progenitors and. further. that exudatinite is secondary resinite occurring in vein-form or as fillings of bedding-plane joints. having exuded from liptinites and huminites into cavities. Exudatinite has markedly varied reflectivities and fluorescence properties and first makes its appearance in the rank sequence at the subbituminous coal boundary. On the basis of infrared spectral studies. Murchison’6,‘7 concluded that resinite bodies in peats. blown coals. li&cs and sub-bituminous coals are different from those ill lowand medium-rank bituminous coals. He found little evidence of the existence of material with spectra intermediate 111 character between these two groups, and suggested that two different petrographic entities liad been classified together
FUEL,
1978,
Vol 57, February
73
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka Table 1 Analysis of samples Ultimate Coal Seam and Coal Measures
analysis (w-t %, daf)
Whole coal
Vitrinite
Microlithotype
(age)
Location
C
H
C
Vit
Big Blair Athol (Permian)
Blair Athol Queensland
82.1-826
4.5-4.7
81.7-82.1
10-11
Unnamed Purni (Permian)
Padirka Basin South Australia
79.6-80.8
4.1-5.4
Callide Callide (Triassic)
Callide Oueensland
78.7-79.9
3.9-4.6
Unnamed Latrobe Valley (Tertiary)
Gippsland Victoria
72.5
4.8
Unnamed Leigh Creek (Triassic)
Leigh Creek South Australia
69.6-74.6
3.6-4.5
No.1 Latrobe Valley (Tertiary)
Latrobe Victoria
60:2-69.7
4.7-4.9
a b
Basin
Valley
78-79
_
68.5
_
Vit: vitrite; Cl: clarite; In: intermediate; Du: durite; Fu: fusite Vitrinite: 65-89%; exinite: 4-28%; macrinite: 2-3%; and semi-fusinite:
under the one maceral type - resinite. Murchison is of the opinion that the resinite-like bodies that occur frequently in bituminous coals as veins or interconnected globules are, in fact, exudatinite which has originated from other macerals at the sub-bituminous level of coalification. He regarded micrinite as the residue left by resinite after the loss of exudatinite. Teichmuller’*2 has noted the effusion, under the microscope, of petroleum-like substances from coals of highvolatile bituminous C rank. She suggested that this is evidence of in situ generation of petroleum-type substances, and conjectured that, at about the same stage of diagenesis at which petroleum is formed in oil-source rocks, petroleum-type substances, micrinite and exudatinite, are also formed in coal seams. She regarded this as the beginning of what she calls the ‘1st coalification jump’ (or ‘bituminization range’). This jump covers quite a broad range extending from approximately the sub-bituminous coal stage (cu. 74% carbon, daf) up to the medium-volatile bituminous coal stage (cu. 87% carbon, daf) which is at the beginning of her ‘2nd coalification jump’. PRESENT WORK Sumples investigated Table I gives approximate petrographic and chemical analyses of the coal samples used in this study. The samples selected are rich in inertinite, with the exception of the Gippsland Basin coal. OCCURRENCE EXUDATINITE
AND FEATURES
OF MICRINITE
AND
Micrinite In the Pedirka Basin (80% carbon, daf) and Blair Athol
74
-
FUEL, 1978, Vol 57, February
Cl
analysisa (vol %) In
DU
Fu
1
27-39
36-40
11-2
12-41
4-6
35-51
5-20
8-l
13
Tr
9
64
26-60
bMaceral
12
analysis only
1 o-33
l-2
S-20
_
-
_
_
l-l
_
3-4%
Figure 7 Micrinite reflected light)
lenses in Pedirka Basin coal toil-immersion,
coals (82% carbon, daf) micrinite typically occurs as fillings in cell cavities. When the micrinite is compact and dense, it appears white, but some cell cavities contain relatively little micrinite (Figure I). Sometimes micrinite lenses with differing degrees of whiteness occur side by side in the same microscopic field (Figure I). Micrinite often reveals the fine detail in cell cavities, for example the shape of tracheids and bordered pits (Figure 2). In some cases it occurs as a matrix, embedding vitrinite fragments derived from disintegrated cell walls and, at times, semi-fusinite and exinite particles also. In other instances micrinite occurs as thin silky streaks, or is uniformly dispersed throughout relatively dark-coloured vitrinite (Figure 3). Occasionally it may be scattered in cavities of semi-fusinite and fusinite. Micrinite does not fluoresce under blue light excitation, nor is it associated with resinous cavity fillings. The occurrence and features of micrinite in Callide coal (79% carbon, daf) are somewhat similar to those in Pedirka
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka
of structured fluorescent material of unidentified botanical origin, an example of which is shown in Figure 6. It gives a strong yellow to orange-yellow fluorescence with blue light excitation. Vitrinite lenses associated with this unidentified material often contain fine cleats which are filled with another type of fluorescent material, corresponding to the exudatinite of Teichmtiller. Its fluorescence colour is a darker orange than the unidentified fluorescent material mentioned above. In Australia. exudatinite has so far been found only in Gippsland coals.
Figure 2 Cell cavities filled with micrinite, Pedirka Basin coal. Botanical structures similar to tracheids and bordered pits can be seen (oil-immersion, reflected light)
figure 4 Cell cavities filled with micrinite, immersion, reflected light)
Figure 3 Thinly dispersed micrinite, Big Seam coal. sporinite can be seen (oil-immersion, reflected light)
Callide coal (oil-
Light gray
and Blair Athol coals (Figure 4). Sometimes subangular particles of relatively high reflectance may be seen between vitrinite lenses. These are inertinite particles which are fine. but still coarser than micrinite aggregates. The mode of occurrence of this inertinite is similar to that of micrinite, and both may be present in the one sample. In Leigh Creek lignites (71% carbon, daf) micrinite commonly occurs as a groundmass in which resin bodies and vitrinite particles are embedded (F&-we 5). Cell cavities filled with micrinite also occur. When micrinite is densely packed in cell cavities, it appears white, similar to the compact micrinite in the Pedirka and Blair Athol coals. Coal seams deposited under sapropelic conditions are rare in Australia. However the Greta Seam (83% carbon daf) in the Sydney Basin does show some sapropelic properties. This seam does not contain typical micrinite, although very fine inertinite of relatively low reflectance is often present in bands of vitrinite B ‘* which are rich in sporinite. A small amount of micrinite-like material has been noted in the Crasstrees Seam (83% carbon, daf) in the Sydney Basin and in some of the seams of the Walloon Coal Measures in Queensland (80% carbon. daf). The former seam contains approximately 60% vitrinite, and the Walfoon seams 90%. on a mineral matterfree basis.
Figure 5 micrinite,
Resinite bodies and huminite fragments embedded Leigh Creek lignite (oil-immersion, reflected light)
in
Exudatinite
The Gippsland coals studied (73% carbon, daf) were found to be extremely rich in exinite. They contain various types
Figure 6 Unidentified fluorescent material and exudatinite vitrinite lens, Gippsland Basin coal (blue light excitation).
in a
FUEL, 1978, Vol 57, February
75
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka
Teichmuller has reported that Upper Silesian vitrite, which is relatively rich in micrinite, yielded larger amounts of volatile matter than vitrinite. If this is not due to resinite, which may have occurred in association with micrinite, then the micrinite may still have an exinite-like composition. This may mean that hydrocarbons have not as yet been released to any extent. Finally, even if the progenitors are actually altered at such an early stage of coalification, there is no reason to believe that the co-product of micrinite is liquid hydrocarbons. Water, carbon dioxide and other oxygen-containing volatile compounds are more likely fluid products. Too little research has been done to permit us to assume that the formation of micrinite is related to liquid hydrocarbon generation. Figure 7 Porigelinite and corpohuminite filling cell cavities, Morwell brown coal foil-immersion, reflected light)
DISCUSSION Micrinite in Australian coals
in Australia, unlike the Northern Hemisphere, micrinite occurs commonly in inertinite-rich coal seams. These seams were deposited in sedimentary basins formed on stable cratonic basement rocks separated from a main deep subsiding zone in Permo-Triassic ages - the Bowen-Sydney Basins. During the formation of these seams, subsidence and deposition in these basins were, in general, slow, and coal seams rich in inertinite were formed in a comparatively aerobic environment”. The overburden on these seams was thin and consequently the rank of the coals is relatively low. Outside Australia, only a few occurrences of micrinite in brown coal and lignite have been reported. Examples are Rhineland brown coal (Ref. 20, Fig. 3, Plate D) and the lignite of North Dakota and Saskatchewan (Ref. 2, Ting’s personal communication). However, micrinite is commonly present in Leigh Creek lignite. In this respect the occurrence of micrinite in Australian coals is different from that in Northern Hemisphere coals. Some organic material of granular texture in peat and brown coal has been suggested as a precursor of micrinite13-15. Porigelinite in brown coal also has a microgranular appearance’, Its occurrence and texture in Morwell brown coal (Figure 7) resemble those of micrinite in Leigh Creek lignite. It is possible that porigelinite, because of its very fine granular texture, may have been preferentially oxidized in an aerobic environment prior to burial and have become inertinite. However, the present fragmentary data on micrinite do not permit conclusions on its origin to be drawn, and a more systematic investigation is necessary. Genesis of micrinite in Northern Hemisphere coals Teichmuller’s hypothesis’>2 of micrinite genesis raises some problems, for example, how and why resinite and other progenitors gained a fine granular texture during coalification. The mere coexistence of micrinite and resinous cell filfings need not be evidence of a transition from one to the other1y2’8. As discussed later, resinite globules in bituminous coals are not likely to be a secondary form of exudatinite. If this were the case, it appears rather strange that resinous material in cell cavities should be preferentially altered to micrinite at an early coalification stage (approximately 74% carbon, daf), even though isolated resinite globules in the same coals show no change until they reach the medium-volatile bituminous coal stage (approximately 87% carbon, daf).
76
FUEL, 1978, Vol 57, February
Exudatinite Resinous material filling veins in bituminous coals could be called exudatinite, as Teichmdller”2 and Murchison’7 have suggested. However, rounded resinous globules appear to have been formed without interference from surrounding material, and are most likely to be of primary origin. The shape of interconnecting veins between resinous globules indicates that when migration of resinous material took place, that is, formation of exudatinite, vitrinite was already so brittle that migrating resinous material could not have become rounded. Murchison’s idea” is based mainly on the marked differences in the infrared spectra of resinites in lower-rank coals and those in higher-rank coals. All his lignite samples16’21 were obtained from Tertiary deposits in the Halle-Leipzig area, East Germany, and the resinites were relatively large lumps from which it is easy to obtain 100% resin samples. The bituminous coal samples, on the other hand, were taken from Carboniferous deposits in Northumberland and Yorkshire, UK, and resinites in these samples were small lenses, patches, globules and thin films. Consequently, the resinite contents of isolated samples ranged from 39 to 88%. The infrared spectra of the lignite resins and the bituminous coal resins are different. Another possible explanation is that they have a different botanical origin and also that they could have been formed by different processes. There is no positive evidence to show that exudatinite and micrinite originated from a common source material, or to show that exudatinite is a residual material, similar to petroleum asphalt, resulting from the loss of a liquid fraction. Consequently the occurrence of exudatinite in Gippsland coals does not necessarily indicate the generation of liquid hydrocarbons in these coals. It may merely indicate that some organic material containing fluorescent compounds migrated into cleats in vitrinite. The formation of exudatinite may not be directly related to chemical changes (coalitication) of lipids, but the movement of mobile material into cleats does show that a force such as compaction pressure was applied to the coal seam. Thick sediments were deposited in this basin over a relatively short period of geological time. The rank of Gippsland Tertiary coals is therefore lower than normal, although they have been subjected to the pressure of a heavy overburden. Diffusion of petroleum-like materi& During the actual microscopic observation of Gippsland coals using oil-immersion techniques, some fluorescent material diffused into the immersion oil from cracks and pores in the coals, particularly from cavities in fusinite and semifusinite. Since these samples were taken from coal seams just
Relating
below oil reservoirs, it is quite possible that the fluorescent material is crude oil which migrated from a deeper part of the basin and was retained within these cavities and cracks. As crude oil can migrate freely for long distances, and as coals tend to retain migrated oil, such a phenomenon may be common in coals taken from oilfields’. It is preferable not to use such a phenomenon as evidence of in situ 011 generation. If polyester resin is used to mount coal particles for microscopic observation”‘. some part of the coaly material may be dissolved in the monostyrene of the polyester resin, and thus be trapped in cleats and pores in the coal particles. If. as reported by Teichmuller the low-rank coals contain so much liquid substance that it can be seen microscopically as droplets, then it should be possible to examine the liquid by solvent extraction/gas chromatographic analysis and other techniques. If the droplets are natural petroleum-like substances, the content of liquid hydrocarbons in the extracts should be high. However. this point is not clear in Teichmuller’s papers. Generation
ofhydrocarbons
Teichmuller’s ‘1st coalification jump’ covers quite a broad range of rank (74-87s carbon, daf). She maintains that oil is generated early in this process and that the formation of micrinite and exudatinite is closely related to this oil generation. However, there is no firm evidence to suggest that liquid hydrocarbons are generated at such an early stage of coalification, although bituminous material (mainly solid), formed from various types of proto-material, may have accumulated. This process may be characterized by the breakdown of oxygen-containing groups and the loss of oxygencontaining compounds. The present author suggests that this is not true oil generation. Accumulated bituminous material could have decomposed, forming liquid and gaseous hydrocarbons as the subsurface temperature increased. The hydrocarbons would have formed just before and during the ‘2nd coalification jump’ of Teichmuller. This generation of liquid hydrocarbons is real oil generation. characterized by the breaking of C-C bonds.
CONCLUSIONS 1. In Australia, micrinite occurs mainly in coal seams rich in inertinite. These seams were deposited in a comparatively aerobic environment in some sedimentary basins formed on stable cratonic basement rocks. Micrinite is rarely found in coal seams which are rich in vitrinite and exinite, in contrast to Northern Hemisphere coals. 2. Micrinite, which is rarely found in Northern Hemisphere peats and Iignites, is quite common in Leigh Creek lignite. 3. A possible progenitor of micrinite is oxidized porigelinite, although further investigation is needed to substantiate this.
micrinite
and exudatinite
to petroleum
generation:
M. Shibaoka
It is premature to conclude that micrinite in Northern Hemisphere coals was formed from resinite and other macerals at an early stage of coalification, and that liquid hydrocarbons were formed during this process. 4. There is no positive evidence to link the formation of exudatinite with that of micrinite and liquid hydrocarbons and. consequently. the occurrence ofexudatinite does not necessarily indicate the generation of hydrocarbons. Exudatinite may be nothing more than fluorescent organic compounds which have migrated into cleats and pores in coal particles. 5. Because petroleum is so mobiie, the presence of petrolcumlike substances at a particular site does not necessarily mean that liquid hydrocarbons were generated at that site. 6. Liquid hydrocarbons may not be generated at an early stage of Teichmuller’s ‘1 st coalification jump’. However. it is possible that. during this period, solid bituminous material formed and accumulated. Petroleum generation, in the accepted sense. is the formation of liquid hydrocarbons resulting from rhe decomposition of mainly solid liptinite material just before and during Teichmuller’s ‘2nd coalification jump’.
REFERENCES 1
2 3
4 5
Teichmulkr, M., in Advances itz Organic (Geochemistry, 19 73 (Eds. B. Tissot and F. Bienner), Technip, Paris, lP74, pp 319-407 Teichmuller, M., in Stach‘s Textbook of Coal Petrology, Gebruder Borntraeger, Berlin, 1975, pp 176-238 lnternational Committee for Coal Petrology, Ititerrlational Handbook of Coal Petrology, Supplement to 2nd edn, Centre National de la Recherche Scientifique. Paris, 1971 Teichmuller, M. and Teichmuller, R., in Stach’s Textbook of’ Coal Petrology, Gebrtider Borntraeger, Berlin, 1975, pp 5-54 Alpern, B. and Prdgermain, S. C. r. Acad. Sci., Paris 1965,
261, 2693 6 I 8 9 10 11 12 13 14
15 16
17 18 19 20 21
Nandi, B. N. and Montgomery, D. S. Fltel 1967,46, 394 Stach, E.Abh. preuss. geol. L. A., N. F. 1936, 171, 83 Teichmuller, M. Zeitschr. firr praktische Geologie 1944, 52, 1 Koch, J. Diss. Techn. Hochsch. Aacherl. 1966 Koch, J. Geol. Mitt. 1970, 10, 113 Teichmuller, M. Geol. Mitt. 1968, 8, 65 Stach, E. Fortschr. Geol. Rheinld. u Westf. 1964, 12,403 Theissen, R. and Sprunk, G. C. Fuel 1936, 15, 304 Spackman, W. and Barghoorn, E. S., in ‘Coal Science’. Adv. in Chem. Series, No. 55, Amer. Chem. Sot., Washington, D. C., 1966, pp 695-707 Cohen, A. D., Thesis, Perznsylvatzia State Univ. (Dept. Geol. Geophys.), 1968, p 352 Murchison, D. G., in ‘Coal Science’ (Ed. R. F. Gould),Adv. irz Chem. Series, No. 55, Amer. Chem. Sot., Washington, D. C., 1966, p 307 Murchison, D. G. Fuel 1976,55, 79 Brown, H. R., Cook, A. C. and Taylor, G. H. Fuel 1964.43, 111 Shibaoka, M. and Smyth, M. Econ. Geol. 1975, 70, 1463 Teichmtiller, M. Geol. Jb. 1950,64,429 Murchison. D. G. and Jones, J. M. Ftlel 1963,42, 141
FUEL, 1978, Vol 57, February
77
Relating micrinite and exudatinite
to petroleum
generation:
M. Shibaoka
APPENDIX (a) Summary of macerals of hard coals and brown coals
Group Mace& ~__
Maceral
Submaceral
Vitrinite*
Telinite
Telinite 1 Telinite 2 Telocollinite Gelocollinite Desmocollinite* Corpocollinite
Collinite” Vitrodetrinite
Group Maceral
Maceral Subgroup
Humotelinite
Maceral Texinite Ulminite
Submaceral
Texto-Ulminite Eu-Ulminite
Attrinite Humodetrinite
Huminite*
Densinite Exinite*
Sporinite* Gelinite
Cutinite Resinite* Alginite Liptodetrinite Inertinite*
Humocollinite
____ Corpohuminite
Porigelinite* Levigelinite Phlobaphinite Pseudophlobaphinite
Sporinite”
Micrinite* Macrinite* Semifusinite* Fusinite
Cutinite Resinite”
Pyrofusinite Degrddofusinite Sclerotinite Inertodetrinite
Suberinite
Liptinite*
Alginite Liptodetrinite Chlorophyllinite Fusinite Semifusinite” Macrinite
lnertinite*
Sclerotinite Inertodetrinite -.* Terms appear in this paper (From International Hundbook of CoalPetrology,
by Int. Committee
(b) Explanation of terms Degradinite
resembles vitrinite but has no definite form.
Durite is composed
of inertinite and exinite. Each must be present in a proportion greater than that of vitrinite, and neither inertinite nor exinite must exceed 95%.
Sapropefic coals is a collective term for cannel and boghead
coals. Trimacetite consists of an association
The content
of three maceral groups. of each maceral group must be greater than 5%.
78
1978, Vol 57, February
FUEL,
for Coal Petrology,
1971)
Vitrinite is classified into vltrinite A and vitrinite B. Vitrinite B, which is of lower reflectance than A, is typically intimately associated with other macerals, and lacks coherent botanical structure. Bituminite is a liptinite
maceral which has no definite form, but often appears in the form of streaks and shows relatively weak brownish fluorescence.
Australia
The occurrence
in some Australian
reported.
and microscopic
features
of micrinite
connection
with
hydrocarbon
genesis.
In Australia,
minous coals which are very rich in inertinite, possible progenitors micrinite
was formed
liquid hydrocarbons high-volatile relation liquid
of micrinite from
between
is oxidized
‘2nd coalification
occurs in bituminous
and also in brown porigelinite.
during this process.
coals in the Gippsland
liquid hydrocarbons, from
micrinite
of their occurrence
There
solid liptinite
Basin.
and sub-bitu-
coal rich in inertinite.
One of the
is little reason to conclude
exudatinite,
Exudatinite There
micrinite
coals are
are discussed in
resinite and other macerals at an early stage of coalification
were formed
bituminous
hydrocarbons
so-called
and exudatinite
The origin of these macerals and also the significance
that
and that
occurs in sub-bituminous
and
is no positive evidence
of a genetic
and liptinite
The formation
macerals.
of
etc. may take place just before and during Teichmiiller’s
jump’.
is generally believed that oil is generated from organic material finely dispersed in a sediment at a certain stage of its geothermal alteration. However, not all authors are in agreement on the actual stage at which this occurs. Some suggest that a fairly low degree of organic diagenesis is sufficient for oil generation. Recently, M. Teichmuller132 stated that the appearance of two microscopic constituents of coal, micrinite and exudatinite, is indicative of the generation of oil. However, in Australia, micrinite commonly occurs in low-rank coals, and even in lignites. If petroleum is generated from organic matter of such a low degree of diagenesis, then some ideas on petroleum prospecting, particularly in the Australian context, should be revised. Several shallow basins, for example, could be reinvestigated. The purpose of this paper is, in the light of new evidence presented, to reconsider the genesis of micrinite and exudatinite and to assess the significance of their occurrence in the generation of petroleum. An explanation of the terms appearing in this paper and a summary of the macerals found in bituminous and brown coals are given in the Appendix. Following the recommendation of the International Committee for Coal Petrology3, the term ‘micrinite’ is used throughout the paper instead of ‘fine micrinite’. The term ‘exudatinite’ is used as defined by Teichmuller’.
It
PREVIOUS THEORIES OF THE ORIGIN OF MICRINITE AND EXUDATINITE The colour of micrinite in reflected li$t is pale grey to white and its reflectance is higher than that of vitrinite. It differs from other macerals of the inertinite group in shape and probably in chemical properties4. It has a characteristic rounded shape and a very small grain size, grain diameter being commonly less than one micrometre5. According to
Teichmuller’~‘, micrinite yields a larger volume of volatile matter than vitrinite, and has been shown to be sensitive to both heating6 and oxidation7. Micrinite occurs in the form of cell fillings in vitrinite, or finely dispersed in collinite. Sometimes agglomerations of micrinite grains form microscopically recognizable layers. The transition of resinite cell fillings to micrinite has been reported’. Micrinite is rarely present in lignite or peat’-‘*. occurring mainly in bituminous coals. It is especially common in ccrtain durites and trimacerites, such as sapropelite. and often appears in close proximity to sporinite”. It has been suggested that coals deposited under aerobic conditions are poor in micrinite*. Thiessen and Sprunk13 concluded that micrinite originated from peatification of cell walls. and Spackman and Barghoorn]’ and Cohen” suggested that semi-opaque granular substances in peats or brown coals could be possible precursors. Teichn~uller’32 considered that most micrinite was formed from the lipid compounds of different liptinites. particularly from resinite. bituminite and dcsmocollinite-degradinito, during the very early part of the high-volatile bituminous coal stage. She suggested that liquid bitumen and micrinite were formed simultaneously from these progenitors and. further. that exudatinite is secondary resinite occurring in vein-form or as fillings of bedding-plane joints. having exuded from liptinites and huminites into cavities. Exudatinite has markedly varied reflectivities and fluorescence properties and first makes its appearance in the rank sequence at the subbituminous coal boundary. On the basis of infrared spectral studies. Murchison’6,‘7 concluded that resinite bodies in peats. blown coals. li&cs and sub-bituminous coals are different from those ill lowand medium-rank bituminous coals. He found little evidence of the existence of material with spectra intermediate 111 character between these two groups, and suggested that two different petrographic entities liad been classified together
FUEL,
1978,
Vol 57, February
73
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka Table 1 Analysis of samples Ultimate Coal Seam and Coal Measures
analysis (w-t %, daf)
Whole coal
Vitrinite
Microlithotype
(age)
Location
C
H
C
Vit
Big Blair Athol (Permian)
Blair Athol Queensland
82.1-826
4.5-4.7
81.7-82.1
10-11
Unnamed Purni (Permian)
Padirka Basin South Australia
79.6-80.8
4.1-5.4
Callide Callide (Triassic)
Callide Oueensland
78.7-79.9
3.9-4.6
Unnamed Latrobe Valley (Tertiary)
Gippsland Victoria
72.5
4.8
Unnamed Leigh Creek (Triassic)
Leigh Creek South Australia
69.6-74.6
3.6-4.5
No.1 Latrobe Valley (Tertiary)
Latrobe Victoria
60:2-69.7
4.7-4.9
a b
Basin
Valley
78-79
_
68.5
_
Vit: vitrite; Cl: clarite; In: intermediate; Du: durite; Fu: fusite Vitrinite: 65-89%; exinite: 4-28%; macrinite: 2-3%; and semi-fusinite:
under the one maceral type - resinite. Murchison is of the opinion that the resinite-like bodies that occur frequently in bituminous coals as veins or interconnected globules are, in fact, exudatinite which has originated from other macerals at the sub-bituminous level of coalification. He regarded micrinite as the residue left by resinite after the loss of exudatinite. Teichmuller’*2 has noted the effusion, under the microscope, of petroleum-like substances from coals of highvolatile bituminous C rank. She suggested that this is evidence of in situ generation of petroleum-type substances, and conjectured that, at about the same stage of diagenesis at which petroleum is formed in oil-source rocks, petroleum-type substances, micrinite and exudatinite, are also formed in coal seams. She regarded this as the beginning of what she calls the ‘1st coalification jump’ (or ‘bituminization range’). This jump covers quite a broad range extending from approximately the sub-bituminous coal stage (cu. 74% carbon, daf) up to the medium-volatile bituminous coal stage (cu. 87% carbon, daf) which is at the beginning of her ‘2nd coalification jump’. PRESENT WORK Sumples investigated Table I gives approximate petrographic and chemical analyses of the coal samples used in this study. The samples selected are rich in inertinite, with the exception of the Gippsland Basin coal. OCCURRENCE EXUDATINITE
AND FEATURES
OF MICRINITE
AND
Micrinite In the Pedirka Basin (80% carbon, daf) and Blair Athol
74
-
FUEL, 1978, Vol 57, February
Cl
analysisa (vol %) In
DU
Fu
1
27-39
36-40
11-2
12-41
4-6
35-51
5-20
8-l
13
Tr
9
64
26-60
bMaceral
12
analysis only
1 o-33
l-2
S-20
_
-
_
_
l-l
_
3-4%
Figure 7 Micrinite reflected light)
lenses in Pedirka Basin coal toil-immersion,
coals (82% carbon, daf) micrinite typically occurs as fillings in cell cavities. When the micrinite is compact and dense, it appears white, but some cell cavities contain relatively little micrinite (Figure I). Sometimes micrinite lenses with differing degrees of whiteness occur side by side in the same microscopic field (Figure I). Micrinite often reveals the fine detail in cell cavities, for example the shape of tracheids and bordered pits (Figure 2). In some cases it occurs as a matrix, embedding vitrinite fragments derived from disintegrated cell walls and, at times, semi-fusinite and exinite particles also. In other instances micrinite occurs as thin silky streaks, or is uniformly dispersed throughout relatively dark-coloured vitrinite (Figure 3). Occasionally it may be scattered in cavities of semi-fusinite and fusinite. Micrinite does not fluoresce under blue light excitation, nor is it associated with resinous cavity fillings. The occurrence and features of micrinite in Callide coal (79% carbon, daf) are somewhat similar to those in Pedirka
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka
of structured fluorescent material of unidentified botanical origin, an example of which is shown in Figure 6. It gives a strong yellow to orange-yellow fluorescence with blue light excitation. Vitrinite lenses associated with this unidentified material often contain fine cleats which are filled with another type of fluorescent material, corresponding to the exudatinite of Teichmtiller. Its fluorescence colour is a darker orange than the unidentified fluorescent material mentioned above. In Australia. exudatinite has so far been found only in Gippsland coals.
Figure 2 Cell cavities filled with micrinite, Pedirka Basin coal. Botanical structures similar to tracheids and bordered pits can be seen (oil-immersion, reflected light)
figure 4 Cell cavities filled with micrinite, immersion, reflected light)
Figure 3 Thinly dispersed micrinite, Big Seam coal. sporinite can be seen (oil-immersion, reflected light)
Callide coal (oil-
Light gray
and Blair Athol coals (Figure 4). Sometimes subangular particles of relatively high reflectance may be seen between vitrinite lenses. These are inertinite particles which are fine. but still coarser than micrinite aggregates. The mode of occurrence of this inertinite is similar to that of micrinite, and both may be present in the one sample. In Leigh Creek lignites (71% carbon, daf) micrinite commonly occurs as a groundmass in which resin bodies and vitrinite particles are embedded (F&-we 5). Cell cavities filled with micrinite also occur. When micrinite is densely packed in cell cavities, it appears white, similar to the compact micrinite in the Pedirka and Blair Athol coals. Coal seams deposited under sapropelic conditions are rare in Australia. However the Greta Seam (83% carbon daf) in the Sydney Basin does show some sapropelic properties. This seam does not contain typical micrinite, although very fine inertinite of relatively low reflectance is often present in bands of vitrinite B ‘* which are rich in sporinite. A small amount of micrinite-like material has been noted in the Crasstrees Seam (83% carbon, daf) in the Sydney Basin and in some of the seams of the Walloon Coal Measures in Queensland (80% carbon. daf). The former seam contains approximately 60% vitrinite, and the Walfoon seams 90%. on a mineral matterfree basis.
Figure 5 micrinite,
Resinite bodies and huminite fragments embedded Leigh Creek lignite (oil-immersion, reflected light)
in
Exudatinite
The Gippsland coals studied (73% carbon, daf) were found to be extremely rich in exinite. They contain various types
Figure 6 Unidentified fluorescent material and exudatinite vitrinite lens, Gippsland Basin coal (blue light excitation).
in a
FUEL, 1978, Vol 57, February
75
Relating micrinite and exudatinite to petroleum generation: M. Shibaoka
Teichmuller has reported that Upper Silesian vitrite, which is relatively rich in micrinite, yielded larger amounts of volatile matter than vitrinite. If this is not due to resinite, which may have occurred in association with micrinite, then the micrinite may still have an exinite-like composition. This may mean that hydrocarbons have not as yet been released to any extent. Finally, even if the progenitors are actually altered at such an early stage of coalification, there is no reason to believe that the co-product of micrinite is liquid hydrocarbons. Water, carbon dioxide and other oxygen-containing volatile compounds are more likely fluid products. Too little research has been done to permit us to assume that the formation of micrinite is related to liquid hydrocarbon generation. Figure 7 Porigelinite and corpohuminite filling cell cavities, Morwell brown coal foil-immersion, reflected light)
DISCUSSION Micrinite in Australian coals
in Australia, unlike the Northern Hemisphere, micrinite occurs commonly in inertinite-rich coal seams. These seams were deposited in sedimentary basins formed on stable cratonic basement rocks separated from a main deep subsiding zone in Permo-Triassic ages - the Bowen-Sydney Basins. During the formation of these seams, subsidence and deposition in these basins were, in general, slow, and coal seams rich in inertinite were formed in a comparatively aerobic environment”. The overburden on these seams was thin and consequently the rank of the coals is relatively low. Outside Australia, only a few occurrences of micrinite in brown coal and lignite have been reported. Examples are Rhineland brown coal (Ref. 20, Fig. 3, Plate D) and the lignite of North Dakota and Saskatchewan (Ref. 2, Ting’s personal communication). However, micrinite is commonly present in Leigh Creek lignite. In this respect the occurrence of micrinite in Australian coals is different from that in Northern Hemisphere coals. Some organic material of granular texture in peat and brown coal has been suggested as a precursor of micrinite13-15. Porigelinite in brown coal also has a microgranular appearance’, Its occurrence and texture in Morwell brown coal (Figure 7) resemble those of micrinite in Leigh Creek lignite. It is possible that porigelinite, because of its very fine granular texture, may have been preferentially oxidized in an aerobic environment prior to burial and have become inertinite. However, the present fragmentary data on micrinite do not permit conclusions on its origin to be drawn, and a more systematic investigation is necessary. Genesis of micrinite in Northern Hemisphere coals Teichmuller’s hypothesis’>2 of micrinite genesis raises some problems, for example, how and why resinite and other progenitors gained a fine granular texture during coalification. The mere coexistence of micrinite and resinous cell filfings need not be evidence of a transition from one to the other1y2’8. As discussed later, resinite globules in bituminous coals are not likely to be a secondary form of exudatinite. If this were the case, it appears rather strange that resinous material in cell cavities should be preferentially altered to micrinite at an early coalification stage (approximately 74% carbon, daf), even though isolated resinite globules in the same coals show no change until they reach the medium-volatile bituminous coal stage (approximately 87% carbon, daf).
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FUEL, 1978, Vol 57, February
Exudatinite Resinous material filling veins in bituminous coals could be called exudatinite, as Teichmdller”2 and Murchison’7 have suggested. However, rounded resinous globules appear to have been formed without interference from surrounding material, and are most likely to be of primary origin. The shape of interconnecting veins between resinous globules indicates that when migration of resinous material took place, that is, formation of exudatinite, vitrinite was already so brittle that migrating resinous material could not have become rounded. Murchison’s idea” is based mainly on the marked differences in the infrared spectra of resinites in lower-rank coals and those in higher-rank coals. All his lignite samples16’21 were obtained from Tertiary deposits in the Halle-Leipzig area, East Germany, and the resinites were relatively large lumps from which it is easy to obtain 100% resin samples. The bituminous coal samples, on the other hand, were taken from Carboniferous deposits in Northumberland and Yorkshire, UK, and resinites in these samples were small lenses, patches, globules and thin films. Consequently, the resinite contents of isolated samples ranged from 39 to 88%. The infrared spectra of the lignite resins and the bituminous coal resins are different. Another possible explanation is that they have a different botanical origin and also that they could have been formed by different processes. There is no positive evidence to show that exudatinite and micrinite originated from a common source material, or to show that exudatinite is a residual material, similar to petroleum asphalt, resulting from the loss of a liquid fraction. Consequently the occurrence of exudatinite in Gippsland coals does not necessarily indicate the generation of liquid hydrocarbons in these coals. It may merely indicate that some organic material containing fluorescent compounds migrated into cleats in vitrinite. The formation of exudatinite may not be directly related to chemical changes (coalitication) of lipids, but the movement of mobile material into cleats does show that a force such as compaction pressure was applied to the coal seam. Thick sediments were deposited in this basin over a relatively short period of geological time. The rank of Gippsland Tertiary coals is therefore lower than normal, although they have been subjected to the pressure of a heavy overburden. Diffusion of petroleum-like materi& During the actual microscopic observation of Gippsland coals using oil-immersion techniques, some fluorescent material diffused into the immersion oil from cracks and pores in the coals, particularly from cavities in fusinite and semifusinite. Since these samples were taken from coal seams just
Relating
below oil reservoirs, it is quite possible that the fluorescent material is crude oil which migrated from a deeper part of the basin and was retained within these cavities and cracks. As crude oil can migrate freely for long distances, and as coals tend to retain migrated oil, such a phenomenon may be common in coals taken from oilfields’. It is preferable not to use such a phenomenon as evidence of in situ 011 generation. If polyester resin is used to mount coal particles for microscopic observation”‘. some part of the coaly material may be dissolved in the monostyrene of the polyester resin, and thus be trapped in cleats and pores in the coal particles. If. as reported by Teichmuller the low-rank coals contain so much liquid substance that it can be seen microscopically as droplets, then it should be possible to examine the liquid by solvent extraction/gas chromatographic analysis and other techniques. If the droplets are natural petroleum-like substances, the content of liquid hydrocarbons in the extracts should be high. However. this point is not clear in Teichmuller’s papers. Generation
ofhydrocarbons
Teichmuller’s ‘1st coalification jump’ covers quite a broad range of rank (74-87s carbon, daf). She maintains that oil is generated early in this process and that the formation of micrinite and exudatinite is closely related to this oil generation. However, there is no firm evidence to suggest that liquid hydrocarbons are generated at such an early stage of coalification, although bituminous material (mainly solid), formed from various types of proto-material, may have accumulated. This process may be characterized by the breakdown of oxygen-containing groups and the loss of oxygencontaining compounds. The present author suggests that this is not true oil generation. Accumulated bituminous material could have decomposed, forming liquid and gaseous hydrocarbons as the subsurface temperature increased. The hydrocarbons would have formed just before and during the ‘2nd coalification jump’ of Teichmuller. This generation of liquid hydrocarbons is real oil generation. characterized by the breaking of C-C bonds.
CONCLUSIONS 1. In Australia, micrinite occurs mainly in coal seams rich in inertinite. These seams were deposited in a comparatively aerobic environment in some sedimentary basins formed on stable cratonic basement rocks. Micrinite is rarely found in coal seams which are rich in vitrinite and exinite, in contrast to Northern Hemisphere coals. 2. Micrinite, which is rarely found in Northern Hemisphere peats and Iignites, is quite common in Leigh Creek lignite. 3. A possible progenitor of micrinite is oxidized porigelinite, although further investigation is needed to substantiate this.
micrinite
and exudatinite
to petroleum
generation:
M. Shibaoka
It is premature to conclude that micrinite in Northern Hemisphere coals was formed from resinite and other macerals at an early stage of coalification, and that liquid hydrocarbons were formed during this process. 4. There is no positive evidence to link the formation of exudatinite with that of micrinite and liquid hydrocarbons and. consequently. the occurrence ofexudatinite does not necessarily indicate the generation of hydrocarbons. Exudatinite may be nothing more than fluorescent organic compounds which have migrated into cleats and pores in coal particles. 5. Because petroleum is so mobiie, the presence of petrolcumlike substances at a particular site does not necessarily mean that liquid hydrocarbons were generated at that site. 6. Liquid hydrocarbons may not be generated at an early stage of Teichmuller’s ‘1 st coalification jump’. However. it is possible that. during this period, solid bituminous material formed and accumulated. Petroleum generation, in the accepted sense. is the formation of liquid hydrocarbons resulting from rhe decomposition of mainly solid liptinite material just before and during Teichmuller’s ‘2nd coalification jump’.
REFERENCES 1
2 3
4 5
Teichmulkr, M., in Advances itz Organic (Geochemistry, 19 73 (Eds. B. Tissot and F. Bienner), Technip, Paris, lP74, pp 319-407 Teichmuller, M., in Stach‘s Textbook of Coal Petrology, Gebruder Borntraeger, Berlin, 1975, pp 176-238 lnternational Committee for Coal Petrology, Ititerrlational Handbook of Coal Petrology, Supplement to 2nd edn, Centre National de la Recherche Scientifique. Paris, 1971 Teichmuller, M. and Teichmuller, R., in Stach’s Textbook of’ Coal Petrology, Gebrtider Borntraeger, Berlin, 1975, pp 5-54 Alpern, B. and Prdgermain, S. C. r. Acad. Sci., Paris 1965,
261, 2693 6 I 8 9 10 11 12 13 14
15 16
17 18 19 20 21
Nandi, B. N. and Montgomery, D. S. Fltel 1967,46, 394 Stach, E.Abh. preuss. geol. L. A., N. F. 1936, 171, 83 Teichmuller, M. Zeitschr. firr praktische Geologie 1944, 52, 1 Koch, J. Diss. Techn. Hochsch. Aacherl. 1966 Koch, J. Geol. Mitt. 1970, 10, 113 Teichmuller, M. Geol. Mitt. 1968, 8, 65 Stach, E. Fortschr. Geol. Rheinld. u Westf. 1964, 12,403 Theissen, R. and Sprunk, G. C. Fuel 1936, 15, 304 Spackman, W. and Barghoorn, E. S., in ‘Coal Science’. Adv. in Chem. Series, No. 55, Amer. Chem. Sot., Washington, D. C., 1966, pp 695-707 Cohen, A. D., Thesis, Perznsylvatzia State Univ. (Dept. Geol. Geophys.), 1968, p 352 Murchison, D. G., in ‘Coal Science’ (Ed. R. F. Gould),Adv. irz Chem. Series, No. 55, Amer. Chem. Sot., Washington, D. C., 1966, p 307 Murchison, D. G. Fuel 1976,55, 79 Brown, H. R., Cook, A. C. and Taylor, G. H. Fuel 1964.43, 111 Shibaoka, M. and Smyth, M. Econ. Geol. 1975, 70, 1463 Teichmtiller, M. Geol. Jb. 1950,64,429 Murchison. D. G. and Jones, J. M. Ftlel 1963,42, 141
FUEL, 1978, Vol 57, February
77
Relating micrinite and exudatinite
to petroleum
generation:
M. Shibaoka
APPENDIX (a) Summary of macerals of hard coals and brown coals
Group Mace& ~__
Maceral
Submaceral
Vitrinite*
Telinite
Telinite 1 Telinite 2 Telocollinite Gelocollinite Desmocollinite* Corpocollinite
Collinite” Vitrodetrinite
Group Maceral
Maceral Subgroup
Humotelinite
Maceral Texinite Ulminite
Submaceral
Texto-Ulminite Eu-Ulminite
Attrinite Humodetrinite
Huminite*
Densinite Exinite*
Sporinite* Gelinite
Cutinite Resinite* Alginite Liptodetrinite Inertinite*
Humocollinite
____ Corpohuminite
Porigelinite* Levigelinite Phlobaphinite Pseudophlobaphinite
Sporinite”
Micrinite* Macrinite* Semifusinite* Fusinite
Cutinite Resinite”
Pyrofusinite Degrddofusinite Sclerotinite Inertodetrinite
Suberinite
Liptinite*
Alginite Liptodetrinite Chlorophyllinite Fusinite Semifusinite” Macrinite
lnertinite*
Sclerotinite Inertodetrinite -.* Terms appear in this paper (From International Hundbook of CoalPetrology,
by Int. Committee
(b) Explanation of terms Degradinite
resembles vitrinite but has no definite form.
Durite is composed
of inertinite and exinite. Each must be present in a proportion greater than that of vitrinite, and neither inertinite nor exinite must exceed 95%.
Sapropefic coals is a collective term for cannel and boghead
coals. Trimacetite consists of an association
The content
of three maceral groups. of each maceral group must be greater than 5%.
78
1978, Vol 57, February
FUEL,
for Coal Petrology,
1971)
Vitrinite is classified into vltrinite A and vitrinite B. Vitrinite B, which is of lower reflectance than A, is typically intimately associated with other macerals, and lacks coherent botanical structure. Bituminite is a liptinite
maceral which has no definite form, but often appears in the form of streaks and shows relatively weak brownish fluorescence.