Energy & Fuels 1994,8, 1478-1488
1478
Chemical and Petrographic Classification of Kerogen/ Macerals Adrian Hutton* Department of Geology, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522, Australia
Sunil Bharati Geolab Nor, Hornberg vn 5, P.B. 5740, Fossegrenda, 7002, Trondheim, Norway
Thomas Rob1 CAER, University of Kentucky, 3572 Zron Works Pike, Lexington, Kentucky 4051 1 Received June 17, 1994. Revised Manuscript Received September 6, 1994@
The use of the term “kerogen”,originally described as the organic matter in oil shale, has been extended to include all nonsoluble solid organic matter in sedimentary rocks. Kerogen is divided into “types” based on elemental composition and potential maturation path. “Maceral” nomenclature, originally developed to describe the organic components of coal, has also been extended to source rocks and, later, oil shale. The extension and overlap of these two fields creates problems as attested to by the proliferation of vitrinite terminology and the application of the term vitrinite to organic matter that is not clearly derived from woody tissue. Other problems include the fundamental complex and wide-ranging nature of Type I1 kerogen which is now acknowledged to be a highly heterogeneous material. Present-day understanding of kerogen has exceeded the simple three- or fourfold division of kerogen, thus making the division (classification) outdated and deficient. Maceral terminology is based on the direct observation of the morphological and optical properties of the organic matter which ultimately reflects their internal chemistry. Problems associated with maceral terminology notwithstanding, the fundamental framework of organic matter in sedimentary rocks is best served by maceral nomenclature. This is consistent with observations that kerogen, or solid sedimentary organic matter, is clearly composed of macerals.
Introduction
Definition and Classification of Kerogen
Organic matter in rocks is studied by both geochemists and organic petrographers, whose training and backgrounds affect how kerogen and macerals are defined and classified. An organic petrographer views the organic matter as one or more separate entities having distinct and recognizable optical properties, each of which is classified using the optical properties along with morphology. The fundamental unit of classification is the “maceral”. A geochemist views the organic matter as a polymerized material formed from a long series of chemical reactions. The types of organic matter are classified by chemical composition and reactivity with respect to suitable reagents and conditions. They are grouped into loosely defined “types”. We wish to explore and review these two approaches and discuss where they are congruent and complementary and where either is problematic. In this paper, a review of the present status of kerogenlmacerals is given and three problems are discussed: (1)chemical classification of kerogen, especially Type 11; (2) identification and classification of vitrinite macerals; and (3) the nature and origin of the maceral bituminite.
Definition of Kerogen. The study of kerogen is at least 80 years old and has been closely related, during most of this time, to the study of oil shales. The American Geological Institute’s (AGI) Dictionary of Geological Terms defines kerogen as “the solid bituminous mineraloid substance in oil shales which yields oil when the shales undergo destructive distillation”.l The term kerogen was first used by Crum Brown2to denote the insoluble organic matter in Scottish oil shale. Since then, many authors have given the term kerogen such a variety of meanings that it is doubtful if it is still a useful term. Possibly as early as 1916, four years after the term was introduced, the meaning of kerogen may have been used incorrectly. Lyder et al.3 summarized most papers on kerogen, up to 1925, and cited Cunningham-Craigl as stating that Crum Brown defined kerogen as “the substance o r substances in Scottish shale that yield oil. It is neither petroleum, bitumen nor resin”. However, in contrast, when sum-
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Abstract published in Advance ACS Abstracts, October 1, 1994.
(1)Bates, R. L.; Jackson, J. A. Dictionary of Geological Terms; American Geological Institute, Anchor Press: Garden City, NY,1983. (2) Cane, R. F. In Oil Shale; Yen, T. F.; Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 1976; pp 27-60. (3) Lyder, E. E.; Goodwin, R. T.; McKee, R. H. In Shale Oil; McKee, R. H., Ed.; Chemical Catalog Co.: New York, 1925; pp 171-299. (4) Cunningham-Craig, E. H. Proc. R. Soc. 1915,36, 44-86.
0887-062419412508-1478~04.50~0 0 1994 American Chemical Society
Classification of Kerogen / Macerals marizing the same paper by Cunningham-Craig, Lyder et al. stated that “Evidence presented proves that oil once existed in Lothians and the present oil shales are a direct result of inspissation of the oil in shales.’’ On the one hand, kerogen was not petroleum but on the other it was the result of the thickening or condensing of oil which can only be assumed to be petroleum. McKee and Goodwin5 also alluded to the definition of Crum Brown and added that kerogen is a substance (pyrobitumen) yielding petroleum and nitrogenous compounds when distilled. Further, “Itis to be emphasized, however, that kerogen is not a definite chemical compound, but probably a mixture of complex compounds, and that the kerogen of different shales are dissimilar. The term is merely a convenient designation of the organic oil-yielding material of shales.” The summaries of Lyder et al. show that other authors had mixed interpretations of the term kerogen. For example, they stated that Bishop6described kerogen as an unfinished product of nature and quoted Jenson7 as saying that the word “petrogen” should be used instead of kerogen for the hydrocarbon substance from which shale oil is obtained; furthermore, the word “petro-shale”should be used to distinguish this rock, in which the oil is free, from oil shale and the word “shalene”for gasoline derived from shales. Lyder et al. also reported that Franks and Goddier8stated that the “oil forming part of kerogen is a complex mixture”. Clearly the implication was that kerogen was thought to have been composed of several parts. This was again repeated in a second paper;g in addition, a second component of kerogen was stated to be a coke-forming constituent. The use of the term, and concept, of kerogen was extended by BregerlO who included organic matter in carbonaceous shales as kerogen, as well as the organic matter in oil shales. Later, the term was further extended t o source rocks. Tissot and Welte11J2defined kerogen as “the organic constituent of sedimentary rocks that is neither soluble in aqueous alkaline solvents nor in the common organic solvents’’ stating that this is the most accepted definition. Durand13 also used the term in this sense in the book Kerogen. Tissot, in the Foreword to that volume, stated “This book is devoted to kerogen-the insoluble organic matter of sedimentary rocks and the source for most petroleum ...” There are problems when defining kerogen as the “insoluble organic matter in sedimentary rocks”. Solubility is a function of the solvent, and the solvents used have varied widely. For example, the procedure presented by Robinson14for kerogen isolation utilized only benzene to extract bitumen. Saxby15described a wide (5) McKee, R. H., Goodwin, R. T. In Shale Oil; Mckee, R. H., Ed.; Chemical Catalog Co.: NY,1925; pp 74-89. (6) Bishop, J. A.Railroad Red Book 1920,37, 979-985. (7) Jenson, J. B. Salt Lake Min. Rev. 1920,22, 23-27. ( 8 )Franks, A.J.; Goddier, B. D. Colo. School Mines Q. 1922,4, 17. (9) Franks, A. J.; Goddier, B. D. Oil Eng., Finance 1923, 4, 175177. (10)Breger, I. McGraw Hill Encyclopedia of Science and Technology; McGraw-Hill: New York, 1961. (11)Tissot, B.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1978. (12) Tissot, B.; Welte, D. H. Petroleum Formation and Occurrence; Springer-Verlag: Berlin, 1982. (13) Durand, B.In Kerogen; Durand, Ed.; Editions Technip: Paris, 1980; pp 13-34.
Energy & Fuels, Vol. 8, No. 6,1994 1479 range of solvents and solvent mixes used in the extraction of bitumen, ranging from chloroform to benzeneacetone-methanol mixtures. The formal definition of bitumen in the AGI dictionary includes solubility in carbon disulfide as a criterion for defining bitumen. The presence or absence of a polar solvent is of particular importance, and more recent approaches generally use methanol and toluene or methanol and dichloromethane mixes for the extraction. Another related problem is that bulk rock analyses do not often include data for bitumen extraction. Analytical techniques may not discriminate between the soluble versus insoluble fractions even when demineralization is carried out. In other cases a pyrolysis parameter is substituted for the bitumen extraction step. Thus, many data presented for kerogens include at least some component of the bitumen. The definition for kerogen has grown t o clearly include all solid organic matter in all sedimentary rocks. Consequently,oil shales, coal, clastic sedimentary rocks, and metamorphic rocks which contain solid organic matter are included in the remainder of the discussion.
Classification of Kerogen Most classification schemes are based on the chemical properties of kerogen, usually isolated by demineralization techniques, and belong to one of the following types: (i) elemental analysis; (ii) “bitumen” or soluble fraction extraction; (iii) chemical degradation (including oxidation, hydrogenolysis, and pyrolysis);(iv) functional analysis; (v) electron spin resonance studies; and (vi) nuclear magnetic resonance studies. Many chemical classificationsof kerogen in shales are based on the elemental composition of organic matter. Grout as early as 1907 and later Ralston (see Durand and Monin, 1980, p 124; Himus, 1951, p 12416-18) used percentages of C, H, and 0 to plot the composition of coal. Later, van Krevelenlg used WC and O K ratios for the same purpose. Earlier classifications of coal (e.g., Mott and SeylerZ01 also used elemental percentages. These and other classifications are summarized by Francis and Breger.21,22 Durand and Moninl’ regarded the elemental ratio plot as more suitable than the elemental percentage plot because (i) it requires no preliminary standardization of C, H, and 0 values; (ii) uncertainties of WC and O/C are lower than those for C, H, and 0 values; (iii) an approximation of the structure of the carbon skeleton can be gained; and (ivl WC ratios give a preliminary idea of the aromaticity of the kerogen. (14) Robinson, W.E. Isolation Procedures for Kerogens and Associated Soluble Organic Materials In Organic Geochemistry; Eglinton, G., Murphy, M. T. J., Eds.; Springer Verlag: Berlin, 1969; Chapter 6, pp 181-195. (15) Saxby, J. D. Isolation of Kerogen in Sediments by Chemical Methods Chem. Geol. 1970, 6 , 173-184. (16) Grout, F. F. Econ. Geol. 1907, 2, 225-241. (17) Durand, B.; Monin, J. C. InKerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 113-142. (18)Himus Oil Shale and Cannel Coal; Institute of Petroleum; London, 1951; Vol. 2, pp 114-133. (19) Van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1961. (20) Mott, R. A.;Seyler, R. Oil Shale and Cannel Coal; Institute of Petroleum: London, 1951; Vol. 2, pp 134-149. (21) Francis, W. Coal, Its Formation and Composition; Edward Arnold: London, 1954. (22)Breger, I. Organic Geochemistry; Macmillan: New York, 1963; pp 50-86.
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1480 Energy & Fuels, Vol. 8, No. 6, 1994
’
1
o0
01
02
. 03
O K (ATOMIC RATIO)
04
05 h
Figure 1. Van Krevelen diagram showing Types I to I11 kerogen and the field in which most kerogen data plot (as Shaded area shows zone for shown by Durand and M~nin’~). most humic coals.
Based on WC and O/C ratios, four types of kerogen are recognized: Type I, Type 11, Type 111, and Type IV11J2J7(Figure 1). Type I is aliphatic kerogen with high initial WC (>1.5) and low initial O/C (O.l; Type N comprises organic matter containing abundant inertinite macerals. Several authors (for example, C ~ m b a zand ~ ~Tissot and Welte11J2) recognized that Types I, 11, and I11 kerogen plot in approximately the same zones as alginite, higher plant liptinite, and vitrinite in coal plots on the van Krevelen diagram. Where a small number of data points are plotted, the three-zoned kerogen model holds, but where large numbers of data are plotted, the fields overlap significantly as shown in Figures 1and 2 which have been compiled from diagrams given in Durand and Monin17 and Cook et al.24 This overlap of data reflects several factors including (i)preparation techniques; (ii)type of precursor organisms; (iii)environment of deposition; (iv) maturation; (v) weathering; and (vi) composition of the organic matter. Of the above factors, type of precursor organism is commonly overlooked. Many, if not most, kerogen samples contain a mixture of two or more maceral groups and any study on the bulk rock will reflect the relative abundance of each maceral group in the sample. Thus, plotting the com(23) Combaz, A. In Kerogen; Durand, B., Ed.; Editions Technip: Paris, 1980; pp 55-112. (24) Cook, A. C.; Hutton, A. C.; Sherwood, N. R. BUZZ.Centre Rech. Explor.-Prod.,Elf Aquitaine 1981,5 , 353-381.
position of kerogens, which commonly contain macerals from each maceral group, will produce plots over a wide field. This is shown with data for torbanites and Australian Tertiary oil shales (Figures 3 and 4) where the data for each are spread across the plot rather than plotting as discrete Type I as might be expected given that the principal oil-producing component is alginite. For both types of oil shales, the percentages of vitrinite and inertinite are variable; as the vitrinite and/or inertinite content increases, the sample plots further toward, or in, the Type I1 field. Another example showing the limitations of chemical classifications relating to organic matter type was given by Bitterli,25who plotted % carbon in various European oil shales and other rocks against both gasoline extract and organic carbon. His graphs show relatively poor separation of the rock types. Many authors clearly mean maceral when using the term kerogen. For example, Lyder et aL3 stated that microscopic examination revealed yellow bodies in shale called kerogen, and yield of oil “is proportional to their n ~ m b e r ” This . ~ is an early example, but the misrepresentation continues in modern work.
Definition and Classification of Macerals Reflected light analysis of coal was an accepted technique as early as 1913 and was used in the 1920s and early 1930s by the parents of petrography Stach, Thiessen, Stopes, and lesser k n o w n petrographers such as Duparque (France), Seyler (England), and Hoffman and Jenkner (Germany) who pioneered reflectance measurement and its use as a measure of rank. Two major advances came in 1935. Stach26published his textbook on coal petrology (which was updated 40 years later and reissued again in 1982). The second advance stemmed from a meeting of coal scientists at Heerlen (The Netherlands) who met to resolve some of the many dificulties that had arisen because of the many and varied approaches to coal analysis that had been adopted up to this time. Several ideas on classification had evolved and confusion had arisen over concepts and terminology. proposed that the term “maceral” designate the fundamental microscopic constituents of coal. As a consequence, the Stopes-Heerlen or International system of nomenclature was born and has been slowly evolving since. In the 1950s, the International Committee for Coal Petrology, now known as the International Committee for Coal and Organic Petrology (ICCP),was formed with the aim of advancing the science of coal petrology through standardization of methodology and terminology. Although there has been some standardization, there is still a long way to go and the field offers a moving target. Automated microscopy and quantitative fluorescence microscopy have, in recent years, added to the complexity and rapidity of what can be measured optically. Many of the problems associated with maceral terminology stem from the nature of coal which, essentially, is a heterogeneous rock that is composed of ~~
~
(25) Bitterli, P. Proc. 6th World Pet. Cong., FrankfurtlMam, Sect. 1 1963,Paper 30 (preprint). (26) Stach, E.; Mackowsky, M.-Th.; Teichmuller, M.; Taylor, G. H.; Chandra, G.; Teichmuller, R. Stuch’s Textbook of Coal Petrology; Gebruder Borntraeger: Berlin, 1982. (27) Stopes, M. Fuel 1935,1 4 , 4.
Classification of Kerogen I Macerals
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DOMAINS
-
Lamorlte Torbanlte/
0
Lamorlte
0
Kuckerrite
3
ATOMIC O/C Figure 2. Kerogen plots for various oil shales with (a)showing the raw data and (b) showing the domains for lamosite, torbanite and marinite oil shales.
0 0
0.1
0.2
0.3
OIC Figure 3. Kerogen plot for k”-Iite oil shales from ~arious parts of the world: large circles represent two O r more data points; shaded zone shows area for most humic coals (data from various sources, including LindneS4). organic matter derived from mostly terrestrial plant matter with subordinate mineral matter of either an authigenic or allochthonous origin. The definition of a maceral as given by the ICCPZ8is “Coal macerals evolve from the different organs O r tissues ofthe initial coalforming plant materials during the course of the first stage of carbonification. However, because of variable but severe alteration, it is not always possible to recognize the starting materials. Mace& are the microscopically recognizable individual constituents of coal and, depending on their quantitative participation (28) International Committee for Coal Petrology. Internatzonal Handbook of Coal Petrology, 2nd ed.; Centre National de la Recherche Scientific: Paris, 1963.
and their association,they control the chemical, physical and technological properties of a coal of given rank. In a sense macerals “may be likened to the minerals of rocks.” The emphasis is on “coal-forming”plants which are terrestrial or so-called “higher plants”. In reality, a maceral is the elementary optical microscopic constituent of coal and each can be recognized by its optical properties, that is, by its morphology, reflectance, and fluorescence where the latter occurs. Thus, a maceral is discrete organic matter that has a consistent, recognizable set of optical properties which are related to a range of chemical and physical properties. Maceral terminology was first developed for coal and then extended, primarily in the past 15 years, to include source rocks and oil shale. The inclusion of the latter categories has caused problems as, until their inclusion, maceral terminology was almost exclusively restricted to coal and carbonaceous shale. On the other hand, kerogen was restricted almost exclusively to oil shale and other organic-rich rocks not normally regarded as coal. Macerals are analogous to the minerals of rocks, are the elementary microscopic organic constituents of a rock, and form from plant cells or tissue during coalification. The type, quantity, and association of the floral components, as well as the environmental conditions and burial history, determine the chemical, physical and technological properties of a coal. Maceral Group. A maceral group comprises several macerals, each Of which has a set Of oPticallY and chemically similar properties. Thus, for macerals in any group, the properties Of the macerals are more closely related to each other than they are to the properties of in another maceral group. The analom with can be extended to the group ’eve’; mineral and contain minergroups such as als with similar properties. The three maceral groups into which the coal macerals fall are inertinite, liptinite exinitel, and vitrinite. Maceral group terms are used in two ways: formally, where used as
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1482 Energy & Fuels, Vol. 8, No. 6, 1994 a
b
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1 .o
9 I
I/
7 -
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/
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/
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c 1 % Vllrlnlle
A > 1% Vllrlnlle
0
0
0.1
OK
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0 . 50
0.3
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Figure 4. Kerogen plots for Australian lamosites: (a)D a t a from LindneS4 (squares) a n d other sources; large circles represent two or more d a t a points. (b)D a t a for samples for which petrographic d a t a are available; triangles show samples with ’1% circles show samples with < 1%vitrinite.
vitrinite,
Table 1. Maceral Terminology for Coals, Stopes-Heerlen System maceral group huminite
maceral subgroup humotelinite humodetrinite humocollinite
vitrinite
liptinite
inertinite
maceral
precursors
textinite ulminite attrinite densinite gelinite corpohuminite
cell walls gelified homogeneous layers derived from cell walls small plant fragments densely-packed small plant fragments gels homogeneous sphaeroidal or ovoid gels
telinite collinite telocollinite corpocollinite gelocollinite desmocollinite
cell walls gelified homogeneous layers derived from cell walls homogeneous sphaeroidal or ovoid gels gels homogeneous groundmass from gelified cell walls and contents
alginite telalginite lamalginite cutinite resinite suberinite bituminite exsudatinite fluorinite liptodetrinite
large colonial or unicellular algae small colonial or unicellular algae cuticle resins, fats, oils, waxes suberinized walls of cork tissue altered algae and humic materials secondary sweated “resinite” secondary resinite fragments of lipid-rich material
fusinite semifusinite sclerotinite macrinite micrinite inertodetrinite
well-preserved cells walls, charcoal variously preserved cell walls fungal tissue and spores homogeneous bodies “granular” inertinite inertinite fragments, less than one cell
the maceral group name, and less formally, where used as a general term where specificity of the macerals is not required (for example “vitrinite-rich”, “trace liptinite”). Vitrinite Terminology. The terminology for vitrinite is not absolute, that is, some forms of vitrinite are covered by more than one term. The reason for this is simply that several classifications or terminologies have been developed for vitrinite and different words are used for the same maceral. A change of word or term does not change either the maceral or its optical, chemical, or physical properties. Confusion arises where the terminology from one system is mixed indiscriminately
with terminology from another. Most nomenclature systems use the same liptinite and inertinite terminology but differ in the vitrinite terminology. A brief summary of three vitrinite terminologies follows. ICCP (Stopes-Heerlen or International) System. The ICCP nomenclature (Table 1) is used in most parts of the world, even in the USA and Australia where it is used in addition to other systems. The ICCP system separates huminite (vitrinite) macerals in brown coals and vitrinite macerals in bituminous coals. It is commonly argued that the ICCP vitrinite terminology is based on rank parameters rather than type parameters. Several of the brown coal macerals are the precursors
Classification of Kerogen I Macerals
Energy & Fuels, Vol. 8, No. 6,1994 1483
Table 2. Terminology of Spackman and Thompson30 maceral suites
maceral series
maceral groups
lignogene maceral suite
vitrinite
anthrinoid meta-vitrinoid vitrinoid xylinoid micrinoid semi-micrinoid fusinoid semi-fusinoid
micrinite fusinite leptogene maceral suite
exinite resinite
anthra-exinoid meta-exinoid exinoid anthra-resinoid meta-resinoid resinoid
Table 3. Australian Standards Association Maceral System39 maceral textiniten texto-ulminitea eu-ulminitea telocollinite attriniten densiniten desmocollinite corpogelinite porigelinitea eugelinite alginite telalginite lamalginite bituminite cutinite exsudatinite fluorinite liptodetrinite resinite sporinite suberinite
maceral subgroup telovitrinite
maceral group vitrinite
detrovitrinite gelovitrinite
liptinite
of vitrinite macerals in bituminous coal. For example, textinite is used for “structured vitrinite” in brown coal but telinite is the term used for “structured vitrinite” in bituminous coal. The textural or morphological properties of textinite are the same as those of telinite; the optical (especially reflectance and fluorescence)and fusinite telo-inertinite inertinite chemical properties are different. But are these differsemifusinite ences sufficient to separate the two macerals? sclerotinite The dual terminology for vitrinite is one of the major inertodetrinite detro-inertinite difficulties with the ICCP system. This is especially micrinite gelo-inertinite evident for coals within the rank range between 0.35% macrinite and 0.6% vitrinite reflectance. The change in rank is a Commonly occur mostly in low-rank coals. gradational and it is difficult to see any significant change in the optical, chemical or physical properties the current knowledge of the various properties of the of vitrinite macerals in many suites of samples over this different macerals; and (iv) can be easily adapted for range. However, at 0.5%vitrinite reflectance, a differspecial purposes, such as paleobotany, by the use of ent term must be used even though the optical properproper adjectives. ties of the maceral do not change. It should be noted Australian Standards Association System. The “Gondthat when the ICCP terminology was first detailed, only wana coals” of Australia, South Africa, and India are of European coals had been examined and there was Permian age and are highly variable and somewhat evidence of a coalificationjump or a change in chemical different from the Paleozoic coals of the Northern and physical properties between 0.4 and 0.6% for some Hemisphere. Although many of the ICCP terms apply coals.26 to Gondwana coals, “intermediate forms of the organic Spackman Terminology. Whereas Stopes introduced constituents” were thought not to be adequately covered the term maceral to distinguish the organic components by the existing t e r m i n ~ l o g y .Consequently, ~~ a number of coal from the inorganic components, S p a ~ k m a n ~ ~of the original terms were amended to suit research defined macerals as “... organic substances, or optically needs or applications to industry, and a proliferation homogeneous aggregates of organic substances, posin terminology resulted; standardization of terminology, sessing distinctive physical and chemical properties, and as well as results, was difficult. occurring naturally in the sedimentary, metamorphic, In response to these difficulties with maceral termiand igneous materials of the earth.” nology, as experienced by the Australian coal industry, Spackman terminology3O is based on the concept that the Standards Association of Australia33 set up a each maceral is a substance with a distinctive set of to prepare a standard for use in Austrasubcommittee properties which changes not only because of type but of meetings which attempted to lia. After a number as a result of rank as well (Table 2). Consequently, the modify the ICCP system, the subcommittee decided to maceral concept of Spackman is based on the concept adopt a new system. The liptinite and inertinite groups that macerals have a narrow range of properties. Thus, were essentially unaltered, but the vitrinite group was vitrinite in bituminous coal is a different maceral to revised. The results were a significant departure from vitrinite in anthracite; the two have different properties. the commonly-accepted ICCP maceral terminology, but Crelling and Dutcher31compared the ICCP and Spackof the system (Table 3 were stated to the advantages man systems as form versus substance. The practical be (i)it provides a more cohesive description of macerals significance of the Spackman concept is it (i) avoids the than the ICCP system; (ii) it is applicable to coals proliferation of maceral names based on morphology; ranging from low brown coal rank t o the low volatile (ii) takes into account changes in macerals with rank; bituminous coals; and (iii) the number of maceral terms (iii) provides a classification framework that can express is significantly reduced. (29)Spackman, W.Trans New York Acad. Sci. Ser. II 1968,20(51, 411. (30)Spackman, W.; Thompson, L. Trans. New York Acad, Sci. Ser. ZI 1958,20,411-425. (31)Crelling, J. C.; Dutcher, R. R. Principles and Applications of Coal Petrology; S.E. P. M. Short Course Notes, 1980. ~~
(32)Falcon, R. M.;Snyman, C. P. An Introduction to Coal Petrog raphy: Atlas of Petrographic Constituents in the Bituminous Coals of South Africa; GeologicaI Society of South Africa: Johannesburg, 1986. (33)Standards Association of Australia, Australian Standard AS 2856,1986.
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The Australian Standards Association system uses the same maceral group and maceral terms as the ICCP system, but maceral subgroup terminology (telovitrinite, detrovitrinite, and gelovitrinite) was thought to be based on genetic parameters. Telovitrinite comprises phyterals or phytoclasts of plant tissue, especially woody plants, which have undergone only minor change to the cell wall structure during sedimentation, peatification, and coalification. Detrovitrinite comprises finely comminuted plant debris. This debris may have been comminuted during transport or in situ. Gelovitrinite is derived from gels which may have been produced by the living plant, by diagenesis during peatification or metamorphism after burial. Gelovitrinite is commonly found in fractures and pores, commonly assumes the form of the host, and may be porous or massive.
Problems in the Application of the Terms Maceral and Kerogen Oil Shales. Elemental analyses of oil shale kerogen are subject to error, bias, and lack of precision. Sources of error and bias include contamination, dilution by mineral matter, faulty laboratory techniques, and faulty data analysis. Error and bias can be minimized if petrographic control is exercised over samples and if results are d~p1icated.l~~ 24 Figure 2 shows that it is difficult to divide kerogen data into clearly defined domains, even where the oil shale type is the same, although this is sometimes attempted and is implied by demarcation of Types I, 11, and 111. Considerable overlap of data is evident; this is not a problem for oil shales if one accepts that oil shales are heterogeneous, that is, have varying compositions, and that the position at which an oil shale plots depends on the kerogen composition (that is, the type and abundance of constituent macerals) and maturity (vitrinite reflectance). Rapid petrographic examination usually clarifies apparently anomalous kerogen plots. Classifications of oil shales based on elemental composition group the oil shales according to organic matter types which have the same bulk elemental distribution even though the precursors may be different. In addition, petrographically-different oil shales may be placed in the same chemical group. The geochemistry of oil shales and the derived oils have been studied for a much longer period than has the petrography of oil shales. This is logical given the need for characterization prior t o and during use of retort oils. The major limitation of the van Krevelen diagram for oil shale, as it is with any technique that uses bulk rock properties, lies in the very nature of the kerogen (and thus the nature of the oil shales themselves, when the diagram is used for oil shales). Oil shale kerogen generally is derived from at least two, and commonly many more, chemically different components. Almost without exception, all oil shales contain liptinite, vitrinite, and inertinite, the latter two groups being minor components in many oil shales. The relative hydrogen content of vitrinite is much lower than that of liptinite and, correspondingly, the oxygen content of vitrinite is higher; the hydrogen content of inertinite is lower than that of vitrinite. Therefore, it is to be expected that the relative abundance of hydrogen and oxygen in any kerogen will be dependent on the relative proportions
Hutton et al.
of vitrinite, liptinite, and inertinite, in addition to factors such as maturation level and weathering. Australian Tertiary Lamosites. A lamosite is a lacustrine oil shale that contains predominantly lamalginite. Tertiary lamosites generally plot as Type I kerogen, and in the alginite field. However, exceptions are common (Figure 3). In the study of Lindner34 on Australian lamosites as an example, some samples plot near the Type I1 kerogen field and some samples plot as Type I11 (Figure 4a). Figure 4b shows that for Australian Tertiary lamosites, samples with 1% vitrinite plot below the Type I kerogen field. Samples 1-3 have >5% vitrinite. These samples are resinite-rich cannel coals of brown coal rank with high oil yields. The liptinite content is high enough to give oil yields similar to low to moderately high grade, alginite-dominated oil shales. It is, therefore, logical that these brown coals with abundant resinite would plot between Types I1 and 111, or, if the resinite content is low, they would plot as Type 111. The apparently anomalous plots for carbonaceous lamosites from the Rundle, Stuart, Nagoorin, and Lowmead deposits of Queensland (Australia) shales, defined previously as lamosites containing > 5 vol % vitrinite andlor inertinite, is one of the major reasons for introducing the term carbonaceous oil shale.35 Most of these oil shales plot as Type I1 kerogen, and this is clearly misleading, as these lamosites, lamosites by virtue of the abundant alginite, contain an assemblage of liptinite and vitrinite. Carbonaceous lamosites must plot as Type I1 kerogen because of the high vitrinite content which gives a lower than expected WC ratio. Reference to Type I1 kerogen, and possibly Type 111, in carbonaceous lamosite is uninformative and is of little value as, clearly, there are at least two types of quite different organic matter. A van Krevelen diagram shows the average chemical composition of the lamosite maceral assemblage rather than the chemistry of a single kerogen type. Subdivision of Tertiary lamosite on the basis of elemental composition is quite arbitrary because of the wide range of chemical compositions; each group is separated by a thin line which represents insignificant changes in chemical compositions. Torbanites. Most torbanite (or boghead coal) kerogen plots as a classic Type I kerogen (Figure 5 ) . However, petrographic analysis of torbanite shows that this oil shale type contains vitrinite and inertinite. Alginite abundance ranges from 5 to '95 ~ 0 1 % .Kerogen plots reflect this composition with plots ranging from Type I (predominantly alginite) to Type I1 and even Type I11 possible. Examination of data for torbanites in Crisp et al.36showed that as the relative abundance of humic macerals increases, the plot shifts from a position of high WC and low O/C to one of lower WC and higher O/C (Figure 5). Marine Oil Shales. Kerogen derived from marine oil shales mostly plots as Type I1 or between Types I and I1 (Figure 6 ) . Petrographic studies show that marinites (34) Lindner, A. W. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1983,28(31, 10-19. (35)Hutton, A. C. Int. J . Coal Geol. 1987, 8, 203-231. (36) Crisp, P. T.; Hutton, A. C.; Ellis, J.;Korth, J.;Martin, F.; Saxby, J. D. Australian Oil Shales: A Compendium of Geological and Chemical Data; University of Wollongong: Wollongong, Australia, 1987.
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Classification of Kerogen IMacerals
1.5.
1.o.
2
r
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0
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Figure 5. Kerogen plot for torbanite (data from Crisp et al.36
and other sources). are composed predominantly of bituminite, lamalginite, and telalginite with minor vitrinite, sporinite, and i n e ~ t i n i t e .Marinites ~~ thus contain a mixed marine liptinite assemblage, with minor humic components; again, the kerogen plots reflect this. Two factors emerge from elemental data and WC and O X ratios for oil shales and other sedimentary rocks. Firstly, the chemical composition of kerogen reflects the maceral assemblage and, secondly, the simple concept of three geochemical reference types is no longer adequate. As stated by Durand and Monin,17“the three types serve only as references and other types may exist”. The kerogen types provide a framework for comparison and work best where the types of the organic matter remain constant, and more importantly, where changes in elemental ratios are brought about by levels of maturation. To illustrate this, marine shales, which contain kerogen that plots within the Type I1 field, commonly contain organic matter that is heterogeneous in nature (Figure 6). The marine Upper Devonian Cleveland member of the Ohio shale was found to be composed of primarily alginite (which plots as Type I) bituminite (which plots as Type 11,) vitrinite (which plots as Type 111) and inertinite (which plot as Type I11 or Type IV depending on the maceral type) and “vitrinite”-like organic matter of problematic origin which plots as Type 1137,38 (Figure 7). Shales from the Toarcian Paris Basin (France and Luxemburg), Posidonia Basin (Germany), and the Cretaceous Toolebuc oil shale from the Eromanga Basin (Australia) also show similar varied composition^.^^ The individual components of these kerogens, that is, the individual macerals, therefore, should not and do not (37)Robl, T. L.; Taulbee, D. N.; Barron, L. S. Energy Fuels 1987,1, 507-513. (38)Taulbee, D.N.;Seibert, E. D.; Barron, L. S.; Robl, T. L. Energy Fuels 1990,4, 254-263. (39)Bustin, R. M.;Cameron, A. R.; Grieve, D. A.; Kalkreuth, W. Coal Petrology Its Principles, Methods, and Applications; Geol. Assoc. Canada, Short Course Notes 3, 1983. (40)Buchardt, B.; Lewan, M. D. AAPG Bull. 1990, 74, 394-406. (41)Bharati, S.;Larter, S. in Manning, D. A. C. Organic Geochemistry - Advances and Applications in the Natural Environment; Manchester University Press: Chichester, U.K., 1991.
follow the same maturation paths. The concept of Type I1 kerogen, if applied to these oil shales, is neither reasonable nor adequate. In some marine shales, tasmanitid telalginite is more abundant than either lamalginite or bituminite. The kerogen for samples which have lamalginite and/or telalginite in greater abundance than bituminite would obviously plot either as Type I or close to Type I, whereas samples in which bituminite is the more abundant maceral would plot nearer to Type 11. Vitrinite in Marine Oil Rocks. Most of the macerals were defined when the dominant coal studies related to coke and carbonization studies, with the definitions given for examination of polished surfaces in reflected white light. However, over the past two decades, organic petrography has broadened in its fields of interest, with petroleum source rock and oil shale studies becoming increasingly important. Along with this, organic petrographers have recognized organic matter that does not fit the macerals as defined. Consequently, a number of new macerals have been defined, but, in some cases, not always accepted by some, or the majority of, petrographers; alternatively the organic matter has been placed in the maceral or maceral group of best fit. Vitrinite has suffered this fate as well. Stach et a1.,26Bustin et al.,39and Falcon and Snyman31 stated that vitrinite is derived from the woody tissue of plants (trunks, branches, twigs, roots, and leaf tissue) especially cellulose, lignins, and possibly tannins, with the two most significant processes involved being humification and gelification during the peat and brown coal stages. These standard definitions imply or state that vitrinite is derived from terrestrial plants. This raises the question as to whether vitrinite can be found in rocks of Early Ordovician age or older. If vitrinite is derived from terrestrial plants, then vitrinite cannot be found in rocks of these ages. However, if vitrinite is a chemical substance that can be formed during coalification from non-terrestrial plants, then it is a different scenario. Even if nonterrestrial precursors give rise to vitrinite, the coalification or maturation paths of the various “vitrinites” would not be the same and, therefore, it is unlikely that exact coalification paths would be followed by the different vitrinites. Whereas it may be purely academic as to whether vitrinite is vitrinite or not, the practical implications are exceedingly important. In marine oil shales, some vitrinite-like material clearly is not derived from terrestrial plants. Older Paleozoic rocks commonly contain graptolites, bitumen, and other organic matter that appears to be vitrinite.40,41 Added t o this, it can be shown that the reflectance of such organic matter increases with maturity, indicating, or perhaps suggesting at least, a correlation between maturity of the sediments and reflectance of vitrinite-like macerals. This has significant implications for source rock studies, as these rocks, or lateral equivalents, may be the source rocks for some marine crude oils. In many cases, all vitrinite-like organic matter is counted as vitrinite; reflectance measurements are taken and equated to vitrinite reflectance values. Marine shales generally contain vitrinite and inertinite, commonly less than 1 vol % but as high as 5 ~ 0 1 % .Reflectance studies
1486 Energy & Fuels, Vol. 8, No. 6,1994
Hutton et al. a
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OlTUMlNlTE
Figure 6. Kerogen and petrographic data for selected marinite samples: (a) Kerogen data; large circles represent two or more
data points (data from various sources). (b) Petrographic data.
y
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. 0.51
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.-
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Figure 7. Van Krevelen diagram showingTypes I to I11 (after
Durand and Monin17)and the main field for coal maceral groups (after Tissot and Welte=). Shaded areas show compositional error limits for macerals separated from a kerogen derived from the Ohio hale.^^^ 38 invariably show a wide range of reflectance values. Alpern and Cheym01~~ found a trimodal reflectance range, with mean reflectance values of 0.26, 0.58, and 0.92%, for the Toarcian Paris Basin marine oil shale. Similar results have been found for other marine oil shales.43 Clearly, before any interpretation is made, the data need to be reviewed, and variables such as depth and lateral position need to be determined. For example, (42) Alpern, B.; Cheymol, D. Rev. Z’Znst. Franc. Pet. 1978,33,515-
536. (43) Hutton, A. C. Organic Petrology of Oil Shales. Ph.D. Thesis, University of Wollongong, Wollongong, 1982.
Alpern4 stated that paleogeographic position affected vitrinite reflectance. Another important question is whether all the vitrinite is terrestrially-derived vitrinite. It is not difficult to extrapolate that if the reflectance values are not for vitrinite, any source rock models may well be far from the mark. Some researchers have attempted to overcome the vitrinite and maturation difficulty by using other maturation indices. Geochemists commonly employ the methylphenanthrene index (MPI)?5 Crick46used properties of alginite, including reflectance, as an indicator of maturation for the Macarthur River Basin, Australia, and Cole (this volume) has shown the association between graptolite reflectance and maturation for Saudi Arabian source rocks. Apart from the “is it vitrinite” question, other researchers have shown that maceral association also affects vitrinite reflectance. This is also a problem that impinges upon source rock maturation studies. Hutton and and Hutton41 found that in some coal and all torbanite samples there is a very strong correlation between depressed vitrinite reflectance and increased alginite content. Cox et al.48found a probable relationship between anomalous coking properties and vitrinite reflectance; these authors assumed that vitrinite was impregnated with liptinite. A related problem is bitumen impregnation of vitrinite, especially samples in the early maturation stage. Bitumen impregnation depresses vitrinite reflectance. The warning for vitrinite reflectance studies is clear. Much research remains before the origin and coalificatiodmaturation of vitrinite macerals are fully understood. Bituminite. Bituminite was defined by Teichmullefig (44)Alpern, B. Znd. Miner. 1979, August-September, 1-9. (45) Radke, M.; Welte, D. H. Adu. Gkochem. 1983,504-512. (46) Crick, I. Aust. J. Earth Sci. 1992,39, 501-520. (47) Hutton, A. C.; Cook, A. C. Fuel 1980,59,711-714. (48)Cox, R.; O’Dea, T. R.; Graylin, R. K. Proc. Aust. Znst. Min. Metall. 1980,273, 1-12. (49) Teichmuller, M. Fortsch. Geolog. Rhein. Westf. 1974,24, 65112.
Classification of Kerogen IMacerals (translated from German) as ‘%based on its optical properties bituminite falls between that of sporinite and vitrinite ...but has a reflectivity clearly less than vitrinite ... its fluorescence is initially dark orange-brown but alters (increases) with irradiation ... its most typical characteristic is its unfigured “amorphousnform... under white light bituminite appears dark to black with brownish internal reflectance... bituminite typically exhibits fine grainy structure ...” Bituminite is a t least as perplexing, if not more so, than vitrinite. In marine oil shales, where it commonly is the dominant constituent in many samples, bituminite may have identical properties to vitrinite and nonfluorescing bitumen. Bituminite (i) occurs as layers, lenses, or pods or is intimately mixed with the mineral matrix, and from this association comes matrix bituminite; (ii) has yellow to brown fluorescence from part or all of the bituminite; some bituminite has positive fluorescence alteration, especially if irradiated with blue- W light for periods of 10 min or more; (iii) may have inclusions of liptodetrinite and small alginite; and (iv) may be nonfluorescing, in which case it has similar optical properties to vitrinite. Teichmullef1° and Ottenjahn and TeichmulleP identified three types of bituminite (not analogous to the three types of kerogen) in Toarcian and Posidonia oil shales. Type I comprises ”lenses and streaks of 20200 pm length”, is black to dark gray, and has a weak brown fluorescence. This bituminite is the common constituent in many marine oil shales and is also the type of bituminite recognized in coals. Type I1 is generally “more rounded” than Type I, has internal reflectance and moderately intense fluorescence. Type I11 has a “fine granular structure” and low reflectance; micrinite and clay-sized mineral matter are probably mistaken for this bituminite. C r e a n e recognized ~~~ two forms of Type I bituminite, as well as matrix bituminite, in samples from the Boundary Creek Formation of the Canadian BeaufortMackenzie Basin. Creaney regarded bituminite as a degradation product of marine algae. Earlier, Teichmullefl and Ottenjahn and Teichmullee suggested the same origin. Hirst and Dunham53ascribed alternating bituminous and nonbituminous layers to annual layering with alternations of marine and terrestrial conditions. However, Creaney4’ stated that bituminite accumulated continuously, with layering being a function of the input of mineral matter. The origin of bituminite is a vexing problem because of the variable shape and lack of structure (hence, the use of the terms “nonfigurated” and “amorphous” for maceral). Bituminite has virtually no optical property that sheds light on the origin question. At worst, the origin of bituminite is uncertain. At best, it could be (i) bacterially- or biologically-degraded terrestrial plant matter; (ii) bacterially- or biologically-degraded planktonic algae; (iii) bacterially- or biologically-degraded benthonic algae and/or algal mats; (iv) physicochemically degraded algal or terrestrial plant matter; and/or (v) gelified algal or terrestrial plant matter. (50) Teichmuller, M.; Ottenjahn, K. Souder. Erdol Kohle 1977,30, 387-398. (51) Teichmuller, M.Address to International Committee for Coal Petrology, 1982. (52)Creaney, S. Bull. Can. Pet. Geol. 1980,28,112-129. (53)Hirst, D.M.;Dunham, K. C. Econ. Geol. 1963,58, 912.
Energy & Fuels, Vol. 8, No. 6, 1994 1487 Abundant bituminite is found in samples that probably had very little terrestrial input.38 The weak brown to moderately intense fluorescence of some forms of bituminite indicate a relatively hydrogen-rich precursor or a precursor that was degraded to a hydrogen-rich material. An algal source appears to be more likely. It is possible to shed some light on the nature and origin of bituminite using WC and OIC ratios. As most marinites which contain abundant bituminite plot as Type 11, or close to this field, bituminite obviously has higher oxygen and lower hydrogen contents than lamalginite. One explanation for the high oxygen-low hydrogen content is that bituminite is derived from alginite, planktonic, or benthonic (although the former appears to be more plausible), which, after deposition, was degraded by mechanical, physicochemical or biogenic means such as bacteria and blue-green algae. This process would directly alter the relative amounts of oxygen and hydrogen or, indirectly, through oxidation or selective removal of hydrogen-rich components. This degradation would also account for bacterial biomarkers in marine oil shales.
The Classification of Organic-Rich Rocks Over the past 20 years, the domains of the organic petrographer and geochemist have been extended, and most researchers study organic matter in all types of sedimentary rocks. For example, devised a classification of organic matter in source rocks; it is applicable to coal, oil shale, and any rock containing organic matter. The classification contained three groups-primary identifiable biological constituents, primary shapeless matter, and secondary products produced by the thermal transformation of organic matter belonging to the first two groups. The primary identifiable biological constituents comprise most macerals with the exception of alginite and liptinite of secondary origin. Algal components were listed as “microscopic algae” and included Botryococcus. The secondary products included solid bitumen and macerals known to be of secondary origin, such as exsudatinite. The primary shapeless matter group contained bituminite, alginite, sapropelic groundmass, and humic groundmass. Although the classification recognized that organic matter in coal and source rocks is derived from a number of precursors, it placed algal components in two groups. Recognition of the term primary shapeless matter, equivalent to amorphous alginite, is contradictory to the definition of alginite as originally defined by the ICCP, which based the definition on algae with well-preserved structure. A l ~ e r advocated n~~ a “universal classification of solid fuels” and suggested a classification that “deals with fossil fuels as geological products”. The classification was basically a classification of coal, including washed and raw coal, but also included oil shales and source rocks. Oil shales were divided into two groups, one of which was further subdivided into low, medium, and high grade, whereas the second contained bogheads, sapropelites, and cannels. Features of the classification (54) Robert, P. Bull. Centre Rech. Exp1or.-Prod.,EZfAquitaine 1979, 3,223-263. (55) Robert, P.Int. J. Coal Geol. 1981,I , 101-137. (56)Alpern, B.Bull. Centre Rech. ExpZor.-Prod.,ElfAquituine 1981, 5,319-352.
1488 Energy & Fuels, Vol. 8, No. 6, 1994
were (i) it groups rocks on the basis of type, rank, and facies; (ii) the three categories of rank are lignite, bituminous, and anthracite, with each further subdivided into hypo-, meso-, and meta; and (iii) the three facies divisions are coal, mixtures (“mixte”),and shales. The major problem with this classification is the subdivision of oil shales into grade categories. Any classification based on grade must, inherently, separate oil shales which have the same organic assemblages. It is possible that two adjacent samples from any deposit, whether it be Green River oil shale, a torbanite or a marine oil shale, would be placed at either end of the oil shale group despite having the same maceral assemblage and relative abundances of each organic constituent. published a classification of organic rocks, with an expanded classification of oil shales, that “develops a systematic treatment of occurrence and chemistry of the organic matter in oil shales”. Both organic petrography and chemistry (elemental data) were used as discriminatory criteria in this classification which covered both coals and oil shales. Any classification,as this one, using discriminatory criteria from different disciplines introduces a large number of categories. This makes use of the classification difficult and, at times, academic at best. Organic constituents are just part of a rock; in coal, the organic matter is, by definition, the most abundant part of the rock; in oil shale, the organic matter constitutes much less than 50% (by volume) of the bulk rock; in petroleum source rocks other than coal, the organic matter constitutes as little as 5% (by weight) of the rock. Any sedimentary rock is composed of one or more of six, easily recognized components, some of which are organic and others of which are inorganic: vitrinite: organic matter derived from woody tissue; inertinite: organic matter derived from woody tissue in an oxidizing environment; primary liptinite: organic matter derived from hydrogen-rich plant tissue; secondary liptinite: liptinite derived from primary organic components; includes solid bitumen and oil; allochthonous minerals: clastic mineral matter, commonly from outside a basin; autochthonous minerals: authigenic mineral matter precipitated in a basin. Many rocks contain organic matter of uncertain origin, which is commonly regarded as vitrinite or bituminite. T e i c h m ~ l l e rsuggested ~~ these and other areas of organic petrography for further study. She included methods and techniques (such as high resolution transmission electron microscopy, characteristics of chemical and physical causes of maceral fluorescence), (57) Cook, A. C . SOC.Org. Petrol. Abstr. 1987, 15-25. (58)Teichmuller, M. Int. J. Coal Geol. 1993, 22-24.
Hutton et al.
causes of coalification, the coalification process (for example, bituminization, inertinization and others in which coals are sources of oil), and application of coalification studies to geology. Vitrinite and bituminite should be added to this list, as both macerals are less than well understood. Geochemistry also has a significant part to play in these studies.
Conclusions The concept of kerogen is rooted in oil shale nomenclature and has been refined into categories and extended to include other forms of sedimentary matter (including coal). In a similar manner, maceral nomenclature was rooted in coal studies and has been extended to other forms of organic matter (including oil shale). These extensions have run into operational problems. Vitrinite was originally defined as having been derived from woody tissue. However, it is identified by reflectance and morphology. Thus, when applied to noncoal rocks where the origin of the matter is, in many cases uncertain, its application is problematic. In practice, the maceral bituminite is problematical. It is not common in coal, where it was originally defined, but is of great importance in oil shale and source rocks. Its broad range of characteristics, which can overlap that of vitrinite, lack distinctive morphology, and uncertain origin makes its application difficult. The purpose of kerogen classification was t o provide a framework for comparison. Since 1961 and the popularization of the “van Krevelen diagram”, the understanding of kerogen has advanced and the single concept of three geochemical reference types is no longer adequate. Each kerogen type does not define a physical or chemical entity but rather a broad compositional field which defines a path taken by the organic matter type during diagenesis and catagenesis. The problems in this system are best illustrated by Type I1 kerogen which is often found to consist of many different kinds of organic matter, each of which has a different maturation path. The entities of which kerogen is composed are best defined by the concept of macerals. Macerals represent the highest level of chemical organization of organic matter in sedimentary rocks. Thus, macerals should be used as the basis to classify organic matter as it is rooted in a physical and chemical entity. Thus, the solution t o the classification of organic matter is not a choice between kerogen type and maceral, but rather the use of both, that is, maceral nomenclature to describe the physical entities which compose kerogen. This does not mean that the suggested solution will not pose problems; however, it appears to be more rational and logical as it takes into account the individual components of kerogen, a complex heterogeneous material.
