Comparative studies of the reflectivity of vitrinite and sporinite

Org. Geochem. Vol. 14, No. 3. pp. 247 252. 1989

0146-6380/89 $3.00+ 0.00 Copyright ~-" 1989MaxwellPergamon Macmillanplc

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Comparative studies of the reflectivity of vitrinite and sporinite FRANCIST. C. TING and JEFFREYA'. SITLER* Department of Geology and Geography, West Virginia University, Morgantown, WV 26506, U.S.A. (Received 15 June 1988; accepted 7 November 1988)

Abstract--Reflectance of vitrinite and sporinite of 27 Upper Carboniferous and 31 Upper Cretaceous-Tertiary coal samples from the United States was measured and compared. Plots of vitrinite reflectance vs sporinite reflectance indicate that the two groups of coals follow separate trends. Differences in source plant material and plant evolutionary changes (conifers vs lycopods and articulates) are thought to be, in addition to other factors, the dictating factors controlling those variations. The convergence of vitrinite and sporinite upon coalification in Cretaceous-Tertiary coals appears to be "earlier", at a vitrinite reflectance 0.15q).20% lower than that of the Upper Carboniferous coals. Resin impregnation of coniferous wood may have retarded the measured reflectance value though the maturation level and process with respect to a time-temperature function are essentially the same for both Cretaceous-Tertiary and Upper Carboniferous coals. Key words--reflectance, vitrinite, sporinite, Cretaceous, Carboniferous, variation, convergence

INTRODUCTION Vitrinite reflectance is one of the parameters used to evaluate the maturation level of organic sediments and the thermal history of a sedimentary sequence. There are many factors that affect the vitrinite reflectance, particularly the temperature and the length of heating in the geologic past that may ultimately determine the maximum reflectance of vitrinite. Other factors acting at the peat stage, such as the degree of microbial decomposition, carbonization due to fire, and variation in the woody tissues of individual plants and among different comtemporaneous species, may play important roles in causing the scattering distribution of reflectance. The evolution of the plant kingdom and the change of swamp flora due to evolution, for example, conifers vs lycopods, may play an even more important role in altering some of the properties of coal. As source material of coal, variation in plants can affect the composition and properties of coal. Reflectance of maceral is expressed as a function of the index of refraction and the absorption coefficient of the maceral. It is related to the carbon and hydrogen content of the maceral, being that the higher the carbon content and the lower the hydrogen content, the higher the index of refraction and the absorption index. Low reflecting vitrinite tends to have, in general, a lower carbon and a higher hydrogen content than higher reflecting vitrinite. The same principle also applies to liptinite and inertinite. The *Present address: Virginia division of Mineral Resources, Charlottesville, VA 22901, U.S.A.

hydrogen content of a particular maceral is not only affected by rank, or level of thermal maturation, but potentially also by the inherent composition of the maceral. When subjecting to similar level of thermal maturation, plant wood tissues with a higher hydrogen content perhaps tend to exhibit lower reflectance than those with lower hydrogen content. Coal deposits of the geological past were derived from different plants occupying different stages of the plant evolution. Carboniferous coal-forming swamps were dominated by lycopods, articulates, and seed ferns. Most of them have long been extinct. Cretaceous and Early Tertiary coal swamps were dominated by conifers, ferns, and, to a lesser degree, angiosperms. Conifer wood tends to be resin-impregnated and contain more hydrogen. The vitrains observed in Cretaceous and Early Tertiary coals are essentially derived from conifer wood. Angiosperm wood, on the other hand, contains lesser amount of hydrogen and tends to be decomposed more rapidly. Very little angiosperm wood is found in the Cretaceous-Tertiary coals. Except for a few plants such as cordaites, an early conifer, Carboniferous peat-forming plants produced very little resinite, whereas Cretaceous, Tertiary, and extant conifers were, and still are, prodigious producers of resins. Old and non-functioning woods, which are referred to as heartwood by botanists, are filled with resins. The living and functioning woods, which are called sapwood, are not impregnated with resin (Esau, 1977; Sjorstrom, 1981). Conifer heartwood and sapwood are responsible for the differentiation of the dark huminite (huminite A) and the light huminite (huminite B) (Ting, 1977) rather than the

247

248

FRANCIS T. C. TING and JEFFREY A. SITLER

result of differing degree of humification (Stach et aL, 1982). Most angiosperm wood, if preserved, is also converted to huminite B. Resins are terpenoids ranging in molecular size from sesquiterpenes (Ct5) to tetraterpenes (C40). They are made up of isoprenoids, the basic building blocks of many natural materials and the sources of petroleum liquids (Hunt, 1979). Although some angiosperms produce resins, most of them secrete gum. The function of gum production by angiosperms is similar to the production of resins by conifers, a response to wounding and infectious disease. Most gums tend to decompose more rapidly than resins. Resins and tannins are also preservatives deposited in and acting to protect the old, defunct wood (heartwood). It has long been stated by many coal scientists that western coals (Rocky Mountain coals) are different from eastern coals (Appalachian coals). Backed by a large data bank, Given et al. (1986) claimed that western coals are different in many aspects from eastern coals. Fuel technologists have long known that Rocky Mountain coking coals do not behave the same way as Appalachian coking coals (R. Gray, personal communication). Nandi et al. (1977, 1981) had observed that the Canadian Rocky Mountain coals have different combustion properties when compared with Appalachian coals of similar rank, and they attribute this to the presence of abundant "semi-vitrinite" in the Canadian coals. What has not been pointed out in all these statements is the potential effect of the different vegetation that produced the eastern and western coals. Plants evolve throughout the geologic past (Carboniferous vs Cretaceous-Tertiary) and change in type (lycopods and articulates vs conifers). This systematic change in source material may exhibit different thermal maturation behavior. One must also keep in mind that the early development of coal petrography and coal chemistry in Europe and in the United States was based on studies and examinations of Carboniferous coals. Standards of characterization and analysis were well established before any serious studies of younger coals were undertaken. Basic assumptions are that all coals can be examined and analyzed by the same established techniques. Therefore, a coal should behave the way the analysis had indicated. The petrography of low rank, younger coals--for example, the Miocene brown coal of Germany--is so different from the Carboniferous coals of western Europe, a set of different descriptive terms had to be developed (Teichmiiller, 1974; ICCP, 1971, 1975) in order not to be confused with hard coal petrography. High rank younger coals, on the other hand, are treated and characterized as if they are high rank Carboniferous coals. No serious consideration has been given perhaps to the effects of the differences of the source material (the plants) as a potential variance affecting the properties of coal.

SAMPLING AND PROCEDURE OF MEASUREMENT

In view of the above mentioned potential variabilities we undertook the present research in an attempt to ascertain whether variations in source material (plants) can affect the physical and chemical properties of coal. For convenience and simplicity, we selected reflectance as the basic tool for this study. Fifty eight coal samples were collected with 27 from the eastern coal fields and 31 from western coal fields (Tables 1 and 2). Depending upon the homogeneity and availability of vitrinite and sporinite, 50 or 100 readings were made on vitrinite and 20 or 40 readings were made on sporinite. Of the 27 eastern coals, 25 samples came from Appalachian basin and 2 from Illinois basin. The Western coals came from Colorado, Utah, Arizona, New Mexico, and North Dakota. An effort was made to collect a suite of coal samples that comprises all coal ranks. The two Illinois coals were collected because they represent high volatile bituminous C rank which is not readily available in the Appalachian coal fields. Both dull and bright coal samples were collected from each locality in order to assure the presence of sporinite and rank vitrinite (banded vitrinite) for measurement. The samples were prepared into pellets and polished according to standard procedure. The maximum reflectance of sporinite and vitrinite was measured according to Ting's 3P technique (Ting, 1982; Ting and Lo, 1978). The 3P technique was developed for measuring small particles such as sporinite and dispersed vitrinite. In order to minimize instrumentation bias, we also applied this technique in the measurement of large vitrinite particles, though they can be measured more conveniently by the conventional technique of rotating the mechanical stage. The 3P technique differs from the conventional technique in which the polarizer is rotated, instead of the microscope stage. Calibration of the system was made with the polarizer placed at the 45 ° position as in normal routine analysis of maximum reflectance. Three readings were measured and one each at 0 °, 45 ° and 90 ° positions. Readings made at 0 ° and 90 ° positions were converted to what they should be if the polarizer is placed at the 45 ° position by two predetermined conversion factors. The two conversion factors, K0 and Kg0, were determined by dividing the reading on glass standard at 45 ° position by readings at 0 ° and 90 ° positions, respectively. g 0 = R45/Ro;

K45 = R45/R45 -- 1;

/(90 = R45/ Rgo in which R0, R45, and Rg0 were measured on glass standard. The conversion factor at the 45 ° position is 1 and no conversion is needed at this position. True values of the three readings, which are designated as R 1, R2, and R3 measured at 0 °, 45 °, and 90 ° positions, are obtained by dividing the conversion

249

Comparative studies of the reflectivity of vitrinite and sporinite Table 1. Sample locations and reflectance data of Upper Carboniferous coals Seam L. Freeport Fire Creek Sewell "B" Upper Eager U. Kittanning Sewell "B" U. Kittanning Sewell "'B" Brush C r e e k Gilbert No. 2 Gas U. Freeport Hernshaw U. Kittanning Redstone L. Kittanning Cedar Grove Stockton Stockton Pittsburg Pittsburg Redstone Pittsburgh Pittsburgh Pittsburgh ---

Location Davis WV Duo, WV Fayettesville, WV Clear Creek, WV Cuzzart, WV Clifftop, WV Gladysvile, WV Leivasy, WV Friendsville, MD Randolph Co, WV Clear Clreak, WV Tunnelton, WV Clear Creek, WV Glydsville, WV Elk City, WV Harewood, WV Gilbert, WV Lundale, WV Lorado, WV Four State, WV Burnsville, WV Wolf Summit, WV Armstrong Mills, OH Eldersville, PA Powhanton, OH Illinois Illinois

Vitfinite

Sporinite

Rmas

Rma,

1.43 1.36 1.21 1.17 1.17 1.17 1.17 1.15 1.13 1.11 1.09 1.02 0.99 0.96 0.93 0.90 0.89 0.89 0.85 0.76 0.75 0.73 0.72 0.72 0.65 0.48 0.45

--" 1.12 0.76 1.06 0.92 0.88 0.77 0.67 0.82 0.62 0.65 0.68 0.55 0.53 0.25 0.23 0.30 0.27 0.29 0.28 0.18 0.22 0.20 0.18 0.21 0.15 0.14

"Sporinite is not visible due to convergence with vitrinite. Table 2. Sample location and reflectance data of Upper Cretaceous Tertiary coals Vitrinite Seam

Location

Rm~x

Sunny Ridge Silt, CO Coal Basin "'B" Carbondale, CO Coal Basin Somerset, CO Dutch C r e e k Carbondale, CO Dutch C r e e k Carbondale, CO "'A" Seam Carbondale, CO Anderson Carbondale, CO York Canyon Raton, NM AI Seam Durango, CO Mesa Verde Fm Gunnison Co., CO "B'" Seam Chimney Rock, CO Lower Sunnyside Emery, UT0.710.14 "'E" Seam Sommerset, CO Sunny Ridge Silt, CO Cameo "'B" Cameo, CO Soldier Canyon Soldier Canyon, UT "'B" and "C'" Seam Somerset, CO Mueller Hesperus, CO "'B'" Seam Somerset, CO Castle Gate Helper, UT (unknown) Orangeville, UT Gordon Creek No. 3 Price, UT Hiawatha Hiawatha, UT B, C. D Seams Paonia, CO Wattis Price, UT Black Diamond Clear Creek, UT (unknown) Salina, UT "I" Seam Emery, UT Belina No. 1 Scofield, UT Wepo Fm Kayenta, AZ -North Dakota

1.41 1.32 1.31 1.21 1.16 1.12 0.94 0.84 0.76 0.71 0.71 0.71 0.71 0.69 0.66 0.66 0.65 0.64 0.63 0.62 0.60 0.59 0.57 0.56 0.55 0.54 0.53 0.52 0.51 0.44 0.30

Sporinite Rmax --° --" --" 0.97 --° 0.90 0.62 0.28 0.17 0.13 0.27 0.14 0.15 0.12 0.13 0.15 0.15 0.14 0.11 0.11 0.16 0.14 0.13 0.13 0.12 0.12 0.13 0.11 0.13 0.09 0.12

"Sporinite is not visible due to convergence with vitrinite. f a c t o r into the p h o t o m e t e r r e a d i n g s at these positions, R1 = R o / K o,

R 2 = R45,

R 3 --- R90/K9o,

respectively. T h e m a x i m u m reflectance, Rm,x, f o r each m a c e r a l g r a i n is t h e n calculated by the following equation:

Rm. . = (R 1 + R 3 ) / 2 + (((R 1 - R 2 ) 2 + ( R 2 - R3)2)/2) I/2

(1)

T h e a p p a r e n t m i n i m u m reflectance, R ~ n , c a n be calculated by the f o l l o w i n g e q u a t i o n : Rm~ . = (RI + R3)/2 - (((RI - R2) 2 + (R2-

R3)2)/2) ~'2 (2)

250

FRANCIST. C. TINGand JEFFREYA. SITLER

This technique is particularly useful for measuring very small grains because rotation of the stage becomes increasingly impractical due to difficulties in alignment of the axis of rotation with respect to the sample. We also used the conventional technique of rotating the microscope stage to measure the maximum reflectance, Rmax. It was done occasionally to verify the accuracy of the vitrinite reflectance by the Ting's 3P technique. Sporinite grains are too small and impractical to be measured by the conventional method in large numbers.

REFLECTANCE DIFFERENCES RELATED TO AGE 1,40 -

0 ~

v---S.--

1.20 '~

E

1.00 0.80.

g

0.60 ' 0.40 f

RESULTS

AND

DISCUSSIONS

Our intitiai goal was to investigate the effect of variation of source plants on the physical properties (reflectance in this case) of vitrinite. Vitrinite reflectance is a measured quantity and it changes progressively with increasing level of thermal maturation or coalification. Thus vitrinite reflectance alone cannot provide the necessary answer of the effect due to source material without comparing it with another independent maturation parameter. Our task would be made easy if an independent and precise parameter is available to express the thermal-maturation level of the location or close vicinity where each of the coal samples was collected. We therefore selected the reflectance of sporinite for comparison. The goal was to isolate the effect of variation due to differences in source material by assuming the composition of one component to remain the same throughout the geologic past whereas allowing the other component to vary, or vice versa. Theoretically, a plot of the reflectance of vitrinite vs sporinite should be a single straight line if the composition and maturation behavior of both macerals remain similar throughout the geologic past. Any differential changes in one maceral would result a change of the shape of the plot. Figure 1 shows the result of reflectance measurements of Carboniferous and Cretaceous-Tertiary vitrinite and sporinite samples covering a wide range of rank before the convergence of vitrinite and sporinite. Instead of a straight line it shows a curvilinear path, suggesting a differential response to thermal maturation by sporinite and vitrinite. The apparent separate paths of the Carboniferous and Cretaceous-Tertiary samples also suggest some potential differences in the composition and/or thermal maturation behavior of the source material from the two geologic ages. Sporinite reflectance remains essentially unchanged until vitrinite reflectance reaches 0.6%. Beyond this point sporinite reflectance increases at a much faster rate than vitrinite until vitrinite and sporinite reflectance converge. Upon convergence the two macerals can no longer be distinguished microscopically under normal oil immersion (n = 1.518) conditions. What is intriguing on Fig. 1 is that the plot of Cretaceous-Tertiary coals

m Elm Upper Cretaceous 0 m CerbonHeroue

0.20 ' 0.00 0.00

i

i

i

i

i

i

i

0.20

0.40

o.eo

0.s0

1.00

t.20

1.40

RMA x EXINITE (SPORINITE) %

Fig. l. Plot of maximum vitrinite reflectance vs maximum sporinite reflectance for Upper Cretaceous-Lower Tertiary and Carboniferous coals showing separate trends presumably related to age and plant evolution. Note the lower reflectance values for Cretaceous-Tertiary vitrinite when sporinite reflectance remains the same. The low readings are resulted probably from resin-impregnated coniferous wood that was dominant in Cretaceous and Tertiary peat-forming swamps. and that of Carboniferous coals take on different paths after 0.6--0.7% vitrinite reflectance. If one considers sporinite reflectance to be constant, the changes in vitrinite reflectance may range upward to 0.20% If one holds vitrinite reflectance constant, the variation in sporinite reflectance may range up to 0.35%. The separation of the two curves suggests that the thermal maturation behavior of vitrinite of Carboniferous coals differs from that of Cretaceous-Tertiary coals. Carboniferous vitrinite tends to increase its reflectance more rapidly by as much as 0.2%. This may be explained by the differences in composition (mainly hydrogen content) of the source material (lycopods and articulates vs conifers) of Carboniferous vitrinite and Cretaceous-Tertiary vitrinite. Cretaceous-Tertiary conifer woods are enriched with hydrogen due to resin impregnation and therefore alter perhaps more slowly upon maturation. Figure 2 is a plot of vitrinite reflectance vs the difference between vitrinite and sporinite reflectance based on values obtained from the regression lines in Fig. I. It provides a good expression of the relative changes of reflectance of vitrinite and sporinite along the path of thermal maturation. The differences in reflectance between vitrinite and sporinite increase rapidly as rank increases and they attend maxima when vitrinite reflectance reaches 0.8% for Cretaceous-Tertiary coals and 0.9% for Carboniferous coals. The differences then decrease rapidly until vitrinite and sporinite converge. The two curves are almost identical in shape, except the vitrinite reflectance is higher for Carboniferous coals when

Comparative studies of the reflectivity of vitrinite and sporinite REFLECTANCE

vitrinite reflectance 1.20%. Thus the differences of vitrinite reflectances between Carboniferous and Cretaceous-Tertiary coals is at least 0.16% ( = 1.36-1.20%) and higher. We estimated the value from Fig. 1 to be 0.15-0.20%. The positions of the highest value on Fig. 1 for both Carboniferous and Cretaceous-Tertiary samples are plots of vitrinite reflectance and assumed sporinite reflectance of equal value. Upon convergence, sporinite reflectance are assumed to have the same values as vitrinite. They represent the readings next to the highest readings of visible sporinite.

DISTRIBUTION

1.40 - "~* oniferous

a~

i .oo -

~7

o.80-

251

Up

0.60 -

0.40 -

0.20 CONCLUSIONS O.O0 O.O0

~ O. 0

(RMA x VITRINITE)

I 0.40 -

I 0.60

(RMA x SPORINITE)

i

0.80 %

Fig. 2. Plots of maximum vitrinite reflectance vs the difference between vitrinite and sporinite maximum reflectance. Note the differences between vitrinite and sporinite reach a maximum of 0.5 and 0.6% for Upper Cretaceous and Carboniferous coals, respectively. Data were obtained regression lines in Fig. 1. If the two curves are brought together, a difference of about 0.2% vitrinite reflectance can be obtained.

plotted against similar differences between vitrinite reflectance and sporinite reflectance. The maximum difference between vitrinite and sporinite reflectance is 0.5% for Cretaceous-Tertiary coals and 0.6% for Carboniferous coals. The curves also indicate that the vitrinite reflectance of Cretaceous-Tertiary coals increases slower than that of Carboniferous coals during coalification. Two possible explanations may be advanced: (1) The Cretaceous-Tertiary source material, the conifier wood, is enriched in hydrogen and its reflectance increases slowly. (2) The high rank Rocky Mountain coals are more isotropic than their Appalachian counterparts. Almost all of the high rank Rocky Mountain coals come from the Coal Basin in central Colorado where igneous intrusions, rather than deep burial, was the primary mechanism of coalification. The differences in anisotropy may not be able to account for all the differences in maximum vitrinite reflectance, which at times is greater than 0.2%. The beginning of the rapid increase of sporinite reflectance appears to correspond with the maturation level of maximum oil generation and the first coalification jump (Teichm/iller, 1974). The maximum sporinite reflectance measured in our suite of samples is 1.12% for a Carboniferous coal sample which has a mean maximum vitrinite reflectance of i.36%. The maximum sporinite reflectance for an Upper Cretaceous sample is 0.90% corresponding to its vitrinite reflectance of 1.12%. Sporinite is no longer visible in the Upper Cretaceous coals at

In view of the above-mentioned observations and measurements we conclude that: (1) The reflectance of vitrinite derived from Cretaceous-Tertiary coniferous source wood exhibits lower values than Carboniferous vitrinite when subjected to similar levels (sum of time temperature effect) of thermal maturation or coalification. (2) The rate of increase of sporinite reflectance is slower than that of vitrinite below vitrinite reflectance of 0.8% for Cretaceous--Tertiary coals and 0.9% for Carboniferous coals. After that the rate of sporinite reflectance increases rapidly until the convergence of sporinite and vitrinite reflectance. (3) In view of the coincidence of the first coalification jump and the beginning of the sharp increase of sporinite reflectance at vitrinite reflectance 0.6% and sporinite reflectance about 0.2% it signifies the intense generation of oil at this maturation level. Oil generation is perhaps essentially completed after sporinite reflectance increases beyond 0.20%. Acknowledgement--This work was supported by a National Science Foundation grant EAR-7912533.

REFERENCES

Brooks J., Grant P. R., Muir M. D., Gijzel P. van and Shaw G. (1971) Sporopollenin. Academic Press, London. Esau K. (1977) Anatomy of Seed Plant, 2nd edn. Wiley, New York. Given P. H., Weldon D. and Zoeller J. H. (1986) Calculation of calorific values of coals from elemental analysis. Fuel 65, 849-854. Hunt J. M. (1979) Petroleum Geochemistry and Geology. W. H. Freeman, San Francisco. ICCP (1971) International Handbook of Coal Petrology, 2nd edn, Suppl. CNRS, Paris. ICCP (1975) International Handbook of Coal Petrology, 2nd edn, 2nd Suppl. CNRS, Paris. Nandi B. N., Brown T. D. and Lee G. K. (1977) Inert coal macerals in combustion. Fuel 56, 125-130. Nandi B. N., Macphee J. A., Ciavaglia L. A. and Chornet E. (1981) Upgrading of inert-rich oxidized coal from western Canada with reducing gases to improve combustion performance. Proc. 64th CIC Coal Symposium, Vol. 1, pp. 319-323.

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FRANCIST. C. TING and JEFFREYA. SITLER

Sjorstrom E. (1981) Wood Chemistry. Academic Press, New York. Stach E., Mackowsky M.-Th,, Teichmfiller M., Taylor G. H. Chandra D. and Teichmiiller R. (1982) Stach's Text book o f Coal Petrology, 2nd end. Gebruder Borntraeger, Berlin. Teichmiiller M. (1974) Entstehung und Veranderung bituminoser Substanzen in Kohlen in besiehung zur Entstehung und Umwanderung des Erd61s. Fortschr. Geol. RheinM u. Wes(f. 24, 65-112.

Ting F. T. C. (1977) Microscopical investigation of the transformation (diagenesis) from 'peat to lignite. J. Microsc. 109 (Pt 2), 75-83. Ting F. T. C. (1982) Coal macerals. In Coal Structure (Edited by Meyers R. A.), pp. 7--49. Academic Press, New York. Ting F. T. C. and Lo. H. B. (1978) New techniques for measuring maximum reflectance of vitrinite and dispersed vitrinite in sediments. Fuel 57, 717 721.