Energy & Fuels 1987, 1, 507-513
507
Petrologic Chemistry of a Devonian Type I1 Kerogen T. L. Robl,* D. N. Taulbee, L. S. Barron, and W. C. Jones Kentucky Energy Cabinet Laboratory, Lexington, Kentucky 40512 Received June 12, 1987. Revised Manuscript Received August 31, 1987 The kerogen of the high-carbon interval of the Cleveland Member of the Ohio Shale in northeastern Kentucky is petrographically complex, being composed of liptinite, low-reflectance vitrinite, and inertinite macerals in the approximate proportion of two-thirds liptinites to one-third inertinite plus vitrinite. The liptinites are further divided into alginite and bituminite, which show an inverse relationship in abundance, suggesting that the bituminite is a product of bacterially reworked precursors, most likely algae. The inertinite and vitrinite were derived from terrestrial sources and were originally deposited as woody materials and charcoal. The kerogen was removed from the shale via demineralization and fractionated by density gradient centrifugation. The four maceral types clearly segregated according to density with the alginites in the 1.05-1.15 g/cm3 range, bituminite from 1.2 to 1.3 g/cm3, vitrinite from 1.3 to 1.35 g/cm3, and inertinite greater than 1.35 g/cm3. Relative N and 0 concentrations were lowest in the lightest fraction and highest in the bituminite and vitrinite density range. Hydrogen to carbon ratios showed a steady decline with density, indicative of the higher aromaticity of the heavier macerals. Organic sulfur was relatively constant across the gradient. Although pure fractions of the maceral types were not obtained, their concentration in selected fractions was sufficient to calculate the chemistry of individual maceral types. Ash-free densities of 1.01, 1.16, 1.25, and 1.40 g/cm3 were calculated for the alginite, bituminite, vitrinite, and inertinite, respectively. Molar H/C ratios were 1.48, 1.12, 1.03, and 0.65, respectively. The higher H/C ratios of the vitrinite and inertinite compared to that of their coal analogues of similar maturity support the view that the environment of deposition plays a major role in determining kerogen chemistry and ultimately the source rock potential of the materials.
Introduction to concentrate the maceral components, and to characterize them chemically. Upper Devonian and Lower Mississippian black shales occur as major stratigraphic units in the Illinois, AppaExperimental Section lachian, Michigan, and Black Warrior basins. Besides Sample Preparation and Kerogen Demineralization. The being sources of oil and gas, the more organically rich kerogen in the high-carbon interval of the Cleveland was studied intervals have potential as oil shales. The stratigraphic in an NX-size (2-inch diameter) core, which was drilled in Fleming interval of interest in this study is a high-carbon zone County, KY. The core was sawed in half, and samples were approximately 30 f t in thickness in the Cleveland Member composited on a two-foot interval basis, crushed to -20 mesh, and of the Ohio Shale of northeastern Kentucky. This zone made into pellets. The sample used for the kerogen separation is consistent stratigraphically and geochemically along was obtained from a bench cut in Lewis County, a few miles to strike from the Ohio River to the Irvine-Paint Creek fault the north of the core (Figure 1). On a volume basis (as determined by point counting), the kerogen consisted of 10% vitrinite, 12% zone in Estill County, KY (Figure l ) . l inertinite, 38% bituminite, and 41 % alginite. The earliest study of the kerogen from the black shales Two approaches have been used in preparing the maceral of Kentucky is that of Thiessen, who described the major isolates. The fist was to grind the shale to -200 mesh and extract components of the shales in thin sections.* The use of the bitumen before demineralization. Extractions were later made reflected light petrography to study these kerogens has to determine if bitumen, possibly trapped by the mineral matter, occurred only recently, (beginning in the late 1 9 7 0 ~ ) . ~ - ~ remained on the demineralized kerogen. Only an insignificant The latter studies generally utilized a modified Stopesamount of additional bitumen was extracted; however, evidence Heerlen system of maceral classification,which is also used for the presence of the surfactant used was found, indicating in this effort. inadequate washing. The second approach was to coarsely crush the shale (16 X 60 The objectives of this study were to determine the mamesh) and extract the bitumen from the kerogen after demincera1 composition of the kerogen in this zone, to separate eralization. Except where noted, the data discussed are from the the kerogen from the rock in a relatively unaltered state, (1) Robl, T. L.; Bland, A. E.; Koppenaal, D. W.; Barron, L. S. In Geochemistry and Chemistry of Oil Shales: Miknis, F. P., McKay, J. F., Eds.; ACS Symposium Series 230; American Chemical Society: Washington, DC, 1983; Chapter 9, pp 159-180. (2) Thiessen, R. Microscopic Examination of Oil Shales; Kentucky Geological Survey, Series 6; Kentucky Geological Survey: Frankfort, K Y , 1925; Vol. 21, pp 1-47. (3) Barrows, M. H.; Cluff, R. M.; Harvey, R. D. Proceedings, Third Eastern Gas Shales Symposium; METC/SP-7916; Morgantown Energy Technology Center: Morgantown, WV, 1979; pp 85-114. (4).Barrows, M. H.; Cluff, R. M.; Harvey, R. D. In Geology and Geochemical Studies of the New Albany Shale Group (Devonian-Mississippian) in Illinois; DOE/METC/12142-26; U. S. Dept. of Energy, Morgantown Energy Technology Center, 1980; Chapter 8, pp 63-76. (5) Barrows, M. H.; Cluff, R. M. Petroleum Geochemistry and Basin Evaluation; Demaison, G.; Murris, R. J., Eds.; AAPG Memoir 35; AAPG: Tulsa, OK, 1984; pp 111-138.
0887-0624/87/2501-0507$01.50/0
second approach. The shale was demineralized in an extraction device similar to that described by Durand and Nicaise.6 Two successive 24% H F solutions were used for a total reaction time of at least 16 h. This was followed by extraction with saturated H3B03to dissolve the neoformed mineral fluorides. The boric acid was followed by six washings with distilled water. The ash content of the kerogen was approximately 12% a t this point and consisted of pyrite, marcasite, and rutile as determined by X-ray diffraction. No attempt was made to remove the iron sulfides as effective reagents such as nitric acid or lithium aluminum hydride are known to alter the The use of hydrochloric acid to
(6) Durand, B.; Nicaise, G. In Kerogen: Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.; Editions Technip: Paris, 1980; Chapter 2, pp 35-53.
0 1987 American Chemical Society
Rob1 et al.
508 Energy & Fuels, Vol. 1, No. 6, 1987
LmA / (Dm + D J - D,) where Do is the ash free density of the organic matter, D,is the determined density of the maceral fractions, A is the weight fraction of the organic matter, and Dmis the density of the mineral matter. The mineral-matter-freedensities for the maceral fractions were then used to calculate the mineral-matter-free (MMF) densities of the pure macerals. This was done by solving simultaneous equations,using the maceral density fractions that had the highest concentration of maceral types. The calculated MMF densities for the “pure” kerogen macerals were 1.01 (&0.02),1.16 (*0.02), 1.25 (&0.03),and 1.40(h0.04)g/cm3 for the alginite, bituminite, vitrinite, and inertinite macerals, respectively. These MMF densities for the pure macerals were then used to convert the petrographic data, which is on a volume percent basis, to weight percent to match the chemical analyses. From the corrected maceral weight distributions, the elemental molar ratios of the pure macerals were calculated by simultaneous equations.
Results Petrography and Maceral Composition of the Cleveland Kerogen. The petrographic analysis of the kerogen in the Devonian black shale is complex in that it is composed of both terrestrially derived (allochthonous) organic matter and marine-derived (autochthonous) organic components. The terrestrial components consisted of pieces of wood, charcoal, and spores, which are preserved in the shale as vitrinite, inertinite, and sporinite, respectively. Algal- and bacterial-decay products comprise the bulk of the kerogen and are preserved as liptinites. For this study, the macerals have been placed into four groupings: alginite, bituminite, vitrinite, and inertinite. The liptinites have been subdivided into alginite and bituminite because they are chemically and petrographically dissimilar and are both present in the shale in large quantities. Alginite is present as both figured (telaginite of Hutton) and unfigured materials (lamalginite of Hutton).13J4 The figured materials primarily consist of Tasmanites and Leiosphaeridia. The unfigured material, which is the predominant form, are laminated and filamentous. The alginites fluoresce bright yellow to yellow-green under blue-light excitation (Figure 2b). Also included with the (12)Smith, J. D. Rep. Invest.-U.S. Bur. Mines 1969, No. 7248,14. (13)Hutton, A. C.;et al. Aust. Pet. Explor. Assoc. 1980, 20, 44-67. (14)Hutton, A. C. Proceedings of the First Australian Workshop on Oil Shale; CSIRO Division of Energy Chemistry: Sutherland, NSW, Australia, 1983;pp 31-34.
Figure 2. Photomicrographs of whole kerogen (a), alginite-rich density fraction under blue-light fluorescence (b), bituminite-rich fraction (c), vitrinite-rich fraction (d), and inertinite-rich fraction (e). Pyrite particles are the brightest objects in the fields; the large object in the center of the alginite photo (b) is a Tasmanites. The large bituminite in part c is aggregated with fusinite.
alginite group is sporinite, a minor (i.e. -1.0%) orangeyellow fluorescing component of the kerogen, present as collapsed spheres similar in shape to Tusmanites. We use the term “bituminite” for the sake of nomenclature uniformity, recognizing that it is not universally accepted. Bituminite is equivalent to the “sapropelic kerogen” of Barrows or “humic material” of Hutton, and is often referred to as “amorphous”or “sapropelic”material by petroleum palyn~logists.~J~J~ The term bituminite was applied by Teichmuller because it is known to produce considerable soluble bitumen and oil upon pyrolysis.16 Bituminite comprises -40% of the kerogen and has a grainy, irregular texture (Figure 2c) and is generally nonfluorescing, although a faint brown fluorescence has been noted in a few samples. It is distinguished from the vitrinite by lower reflectance (Ro 0.35), its irregular texture, amorphous nature, and lower relief upon polishing. Bituminite also commonly shows internal reflections. Vitrinite is primarily derived from woody tissues and is usually present as small particles Figure 2d, although vitrain bands and even entire compressed vitrinized logs are found. The reflectance of the vitrinite (R, = 0.45-0.55) is near the beginning of the oil generation window. Inertinite in the kerogen consists of fusinite and semifusinite, derived from charcoal (Figure 2e), and small amounts of micrinite, which is believed to be a dehydrated residue from oil and gas generation.17 The reflectances of these macerals are high and their morphologies distinctive. They are easily distinguished from vitrinite.
-
(15)Stach, E.,et al. Stach’s Textbook of Coal Petrology; Gebruder Borntraeger: Berlin, 1975. (16)Teichmuller, M. Fortschr. Geol. Rheinl. Westfalen 1974, 24, 65-112. (17)Teichmuller, M.;Ottenjahn, K. Erdoel Kohle, Erdgas, Petrochem., Sonderdruck 1977,30, 387-398.
Rob1 et al.
510 Energy & Fuels, Vol. 1, No. 6, 1987 60
f
20
8 :
0
40
CLEVELAND ASUNBURY
\\
50
0
10
20
30
40
50
60
70
80
% Bituminite
Figure 3. Plot of bituminite versus alginite and vitrinite plus inertinite for the kerogen of core no. KEP-5 of Fleming County, KY. Included are data for the Sunbury Shale, a black shale located stratigraphically above the Cleveland. All data are in volume percent of the organic fraction.
1 :o
1.1
1.2
1.3
1.4
Density gm/cm3
Other macerals such as resinite are rarely found but are not present in significant quantities and are not discussed further. The petrographic composition of the kerogen for the entire interval of the Cleveland Member of the Ohio Shale was determined in core material from Fleming County, KY. In the core, a relatively constant amount of inertinite and vitrinite was noted, together averaging 28% by volume and ranging from 21 to 34%,with the approximate proportion of one-third vitrinite and two-thirds inertinite. Bituminite and alginite concentrations were found to be more variable and displayed an inverse relationship, (Figure 3), with the bituminite increasing upward, ranging in concentration from 9 to 53%. The alginite ranged from 21 to 59% by volume of the total organic matter. The proportion of vitrinite plus inertinite in the kerogen suggests a relatively constant source of terrigenous material throughout the period of deposition of the Cleveland. The vitrinite was derived from logs, stems, and other woody tissues of plants transported from Devonian forests, probably located in what is now the Northeastern US.and Canada by river systems leading into the Catskill Delta of present-day New York and Pennsylvania. The fusinite and semifusinite were most likely derived from plant debris, which was oxidized through fire or fungal/bacterial activity and transported to the ocean as charcoal or charcoal-like material. Often the shard-like form of the charcoal is preserved (Figure 2e). The alginite is most likely derived from the lipid-rich remains of algae. It has been suggested that the bituminite may be derived from an algal/bacterial mat that grew on the surface of sediment beneath the oxic/anoxic interface within the water column.ls Other possible contributors to the bituminite include faunal plankton, terrestrial organic matter, and fragments and bodies of higher animals (fish, crustacean^).'^ The strong inverse relationship ex-
hibited by alginite and bituminite suggests that the bituminite was derived from the bacterial reworking of the alginite precursors. Chemistry and Petrography of the Density Fractions. The four maceral types were clearly segregated according to density, with the alginites concentrating in the 1.05-1.15 g/cm3 range, the bulk of the bituminite from 1.2 to 1.3 g/cm3, vitrinite from 1.3 to 1.35 g/cm3, and inertinites in the >1.35 g/cm3 density fraction (Figure 4). Separations were not as good with the 16 X 60 mesh shale as with the pulverized shale. For example, an alginite tail of 10% was present in the 1.4 g/cm3 fraction but was essentially absent in the separates of the kerogen from the finely pulverized shale. This is due to a greater degree of aggregation of the kerogen components (e.g. Figure 2c). However, the retention of the morphologic information in larger kerogen particles greatly aided and improved the accuracy of the petrographic identification of the macerals. The elemental analyses were used to calculate molar ratios (Figure 5). Relative N and 0 concentrations were found to be lowest for the lighter, alginite-rich fractions, reaching a maximum in the bituminite-rich fractions and decreasing slightly at higher density. Hydrogen/carbon ratios exhibited a steady decline with increasing density indicative of the greater aromaticity of the heavier fractions. The organic sulfur concentration remained relatively constant at about 1.9% across the density gradient, indicating no particular maceral preference. Kerogen Typing and Petrographic Correlation. Kerogens have been typed by elemental composition. Plots of molar H/C vs O/C (after van Krevelen) have been used extensively for this purpose.m Three major groupings are discernible on this type of plot: type I field, which is
(18)Sherwood, N. R.; Cooke, A. C. Low Rank Oil Shales: Part I-Organic Petrography; University of Wollongong Printery; Keiraville, NSW, Australia, 1984.
(19) Powell, T. G.; Creaney, S.; Snowden, L. R. AAPG Bull. 1982,66, 430-435. (20) van Krevelen, D. W. Coal; Elsevier: New York, 1981.
Figure 4. Distribution of maceral type versus density for the Cleveland kerogen. The calculated densities of the pure inertinite (I),vitrinite (V), bituminite (B), and alginite (A) are also indicated.
Energy & Fuels, Vol. 1, No. 6, 1987 511
Petrologic Chemistry of Kerogens
..
l.6$ 1.4 1.2
I .o 0.8 0.040 0.030
..-
Table I. Density, High-Temperature Ash (HTA), and Elemental Analysis of Kerogen Density Fractions
’
+
D,
H/C Ratio
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--
0
0
..
N/C Ratio
0.020i
*
e 2 0
I
0.010
0.016 0.012 A AAAA4
A -A A AA
A A
A
S/C Ratio
0.004
0.000
,
,
.
1 .oo
.
.
.
.
.
1.30
1.20
1.10
.
.
1.40
. 1.50
GRADIENT DENSITY
Figure 5. Elemental ratios for the organic matter recovered as a function of gradient density. 1.6
1
i
/
C
fraction g/cm3
I
f
1.050 1.090 1.110 1.130 1.150 1.170 1.190 1.205 1.215 1.225 1.235 1.245 1.255 1.265 1.275 1.285 1.295 1.305 1.315 1.325 1.340 1.360 1.385 1.420
1
L
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18. 19 20 21 22 23 24
79.38 79.71 77.87 77.51 76.34 75.73 75.47 75.29 74.72 72.94 73.47 72.09 71.59 70.68 71.12 70.81 70.22 70.38 70.70 69.88 69.42 68.72 69.00 68.09 66.79
kerd
anal.: w t % % H N 0 SWb S ( O ) ~HTA 9.66 9.28 8.73 8.44 8.06 7.82 7.29 6.99 6.99 7.04 7.08 6.79 6.75 6.67 6.68 6.41 6.35 6.10 6.22 6.13 6.09 5.79 5.58 5.13 6.12
1.44 1.71 1.95 2.00 2.17 2.47 2.80 2.88 2.91 2.86 2.93 2.74 2.64 2.76 2.58 2.57 2.44 2.49 2.34 2.41 2.30 2.51 2.29 2.00 2.31
5.12 5.61 7.12 6.65 7.84 8.65 9.38 8.37 9.34 9.12 8.87 8.92 8.65 9.08 9.65 9.29 9.24 9.06 9.38 8.30 8.76 8.53 7.74 7.36 7.00
3.44
1.74
3.31 3.11 3.29 3.81 3.66 4.25 4.39 4.30 4.73 4.61 5.02 5.40 5.47 5.54 5.67 6.20 6.23 6.30 7.15 8.17
1.90 1.85 1.89 2.14 1.75 2.11 1.97 1.63 1.74 1.39 1.60 1.87 1.74 1.70 1.65 1.95 1.52 1.39 1.61
Moisture-free basis. Total sulfur. Demineralized whole kerogen.
2.5 2.0 2.5 3.0 3.0 3.0 3.0 3.0 3.0 3.5 4.5 5.0 6.0 6.5 7.0 7.5 7.5 8.0 8.0 8.5 9.5 10.0 10.5 11.0 11.3
Organic sulfur.
TYPE I
I I
7
/
0
Alginite Field
1
0
1.2
Il .o. 1
______----- -TYPE /I1
I . _
,
1.4
\
1.04
,.**
_ _ - - - .--
_ / - -
/’
f
/
I /’
I
0.4
I
o.2 0.0
I
I
1 I
0
I
4 WHOLE KEROGEN 0 DENSITY SEPARATES I 0.05
I
I
0.10
0.15
r
i
I
OK
Figure 6. Plot of the kerogen density fraction compositions on a van Krevelen diagram. Also shown is the demineralized whole
kerogen.
representative of highly aliphatic kerogens; type I1 field, typical of kerogens mainly derived from aquatic organic matter; type I11 field, characteristic of terrestrial organic matter.21 The mean evolutionary pathways for these fields were derived from the kerogens of the Green River shale (type I), the Lower Toarcian shale of the Paris basin (type 11),and the Upper Cretaceous shale of the Douala basin (type 111). The Type I11 field is also compositionally very close to van Krevelen’s “vitrinisation band”.20 A type IV field plotting below the type I11 field is also defined, within ~
(21)Durand, B.;Monin, J. C. In Kerogen: Insoluble Organic Matter from Sedimentary Rocks; Durand, B., Ed.;Editions Technip: Paris, 1980, Chapter 4,pp 113-142.
(22)Demaison, G. J.; Holck, A. A. J.; Jones, R. W.; Moore, G . T., Preprints, Eleuenth World Petroleum Congress;Publication No. PDl(2); Wiley: London, 1983; p 13.
Rob1 et al.
512 Energy &Fuels, Vol. 1, No. 6,1987
Table 11. Organic Elemental Composition of the Pure Macerals" aleinite bituminite vitrinite
inertinite
I
D,g/cm3 anal.,b wt % C H N 0
S(dC H/C
o/c
1.01
1.16
1.25
1.40
81.7 (81.7-81.9) 10.1 (10.1-10.1) 1.4 (1.1-1.4) 5.0 (5.0-5.2) 1.8 (1.7-1.8) 1.48 (1.47-1.48) 0.046 (0.046-0.048)
78.6 (77.2-78.6) 7.4 (7.4-8.0) 3.1 (2.6-3.2) 8.9 (8.9-9.7) 2.0 (1.9-2.1) 1.12 (1.12-1.23) 0.085 (0.085-0.100)
75.3 (74.7-78.9) 6.5 (6.4-7.0) 2.4 (2.4-3.7) 13.5 (10.1-13.5) 2.3 (1.9-2.3) 1.03 (0.97-1.11) 0.133 (0.094-0.133)
82.9 (79.5-82.9) 4.6 (4.4-5.5) 2.2 (2.1-3.5) 8.5 (8.5-10.2) 1.8 (1.3-2.0) 0.65 (0.63-0.82) 0.076 (0.067-0.096)
Data are calculated from the four kerogen density fractions that had the highest concentration of macerals. The values in parentheses represent the range of "end-member" calculations (see text). Ash- and moisture-free basis. Organic sulfur.
pulverized kerogen as well. The least dense alginite-rich maceral fractions plot in the type I field, and the remainder of the material plot in the type I1 field. Chemistry of the "Pure"Macerals. The calculated elemental concentrations and molar ratios are presented in Table 11, and the molar ratios were plotted in Figure 7. The accuracy of the deconvoluted data is greatly dependent on both the maceral purity in the fractions selected for the calculation and the accuracy of the petrographic determination. In the first set of runs, finely pulverizing the kerogen resulted in better separations but reduced the reliability of the petrographic determinations. The second approach resulted in more reliable petrographic determinations but did not achieve as clean a separation (e.g., maximum inertinite of 52 versus 76 w t 90). A meaningful statistical evaluation of the deconvoluted data is difficult to perform. The data in the table were calculated from the maceral density fractions that had the highest concentration of the maceral types from the second run (i.e. fractions 1, 8, 17, and 24 with 95% alginite, 89% bituminite, 38% vitrinite, and 52% inertinite by weight respectively). Also presented are "end member" values, which were carried through the algorithms from other maceral density fractions that also had high concentrations of the four maceral types, end members of replicate chemical determinations, and also data from the first set of runs. In other terms, the presented data are the authors' selections as the best of the data along with boundary conditions that represent a "no-worse-than" case. The plots in Figure 7 were constructed from this data and represent fields bounded by the end member values. The size of the fields are approximately inversely related to the level of purity of the respective maceral concentrates used for the calculation. The chemistry of the pure macerals is approximately congruent with the observations of trends for the maceral density fractions; hydrogen was highest in the alginite, nitrogen was highest in the bituminite, oxygen was highest in the vitrinite, and carbon was highest in inertinite. The alginite plotted in the type I field and the bituminite in the type I1 field. The allochthonous materials were clearly outside the range of the plots of the maceral fractions, however, with the inertinite plotting in the type I11 field and vitrinite located between the type I1 and type I11 fields. For the sake of clarity, it is pointed out that kerogen "type" is a sedimentological concept, not a petrographic one; for example, alginite is not a type I kerogen, but a type I kerogen may be composed entirely of alginite. Discussion One of the most interesting aspects of the calculated maceral density and chemistry is the composition of the terrestrial kerogen macerals compared to their coal analogues. From the carbon versus density relationship developed by van Krevelen for vitrinite from coal, a density in the range of 1.35-1.40 g/cm3 would be expected for this
1.0
.4
,
A.
i
0
I
I
I
I
I
I
I
I
I
1
2
3
4
5
6
7
8
9
10
Hydrogen wt%
Figure 8. Plot of relationship between hydrogen calculated for the pure macerals and specificvolume. The regression line is from Franklin.24
vitrinite based on its carbon content, much higher than the 1.25 g/cm3 calculated.20 Although the information available is not as reliable for the coal inertinites-as they are more difficult to isolate in high purity-the average density for this maceral also appears to be low compared to that for coal-derived materials (1.40 g/cm3 here versus -1.5 g/cm3 for f~sinite).'~ Differences are also evident when the H/C ratios are compared. Van Krevelen's inertinite and vitrinite compositional bands for coal are also plotted in Figure 7.20 The marine deposited vitrinites have a much higher H/C ratio by a level of 0.2 to 0.3 compared to that for a typical coal vitrinite. Although not as clear due to the greater uncertainty, the inertinites also appear to be somewhat more hydrogen rich with an average calculated H/C of about 0.7, -0.15 higher than that for coal inertinites. Franklin studied the relationships between hydrogen and ash-corrected d e n ~ i t y . ~This ~ , ~work ~ included a wide range of carbonaceous materials-from lignites to anthracite coals-and was not limited to vitrinites as incorrectly cited by van Krevelen and later by S t a ~ h . ~A~ J ~ strong linear correlation was found between specific volume (density-l) and hydrogen weight percent. Her correlation agreed well (Figure 8) with the data for the derived pure maceral compositions of Table 11. Although the authors do not accept the contention that the structure of carbonaceous material is approaching something other than (23) Franklin, R. E. Fuel 1948, 27, 46-49. (24) Franklin, R. E. Trans. Faraday SOC.1949, 45, 274-286.
Petrologic Chemistry of Kerogens
graphite-as proposed by Franklin from the extrapolation of the data to zero-it does appear that the hydrogendensity relationship is a more fundamental one than that of carbon-density, as it seems to hold true for a wider range of types of organic materials, at least for materials with hydrogen contents of 3% or more. There are two contrasting viewpoints relative to the provenance of kerogen in relationship to ita potential as a hydrocarbon generator. One viewpoint is that the source of the kerogen, i.e., nature of plant precursors, is of major importance. For example, Breger and Brown, in a study of the Devonian Chattanooga shale of Tennessee, concluded that the kerogen was composed of a mixture of sapropelic material of marine origin and terrestrially derived humic material, the latter being in too high a concentration for the shale to serve as a good petroleum source rock and that the Devonian black shale kerogen should be considered a "coaly" material.25 This concept is repeated in later works on Devonian shale from various locations in the Midwest as ell.^^*^' The second viewpoint is that the environment of deposition determines, to a large degree, the chemistry and subsequent hydrocarbon-generating potential. This viewpoint is exemplified by the work of Demaison, who related kerogen type primarily to the concentration of oxygen in the waters overlaying the sediment and to sedimentation rate.22*28 (25) Breaer, I. A.; Brown, I. Science (Washington, DC) 1962, 137, 221-224. (26) Breger, I. A. Proceedings, Third Eastern Gas shale8 Symposium; METC/SP-79/6; Morgantown Energy Technology Center: Morgantown, WV, 1976; p 26. (27) Breger, I. A.; Hatcher, P. G.; Romankiw, L. A.; Miknis, F. P.; Maciel, G. E. In Geochemistry and Chemistry of Oil Shale; Miknis, F. P.; McKay, J. F., Eds.; ACS Symposium Series 230; American Chemical Society: Washington, DC, 1983; Chapter 10, pp 181-198.
Energy &Fuels, Vol. 1, No. 6,1987 513
The comparison of the kerogen maceral chemistry with the coal maceral chemistry indicates that it is dangerous to draw many conclusions from petrographic data alone. The presence of "coal-like" organic matter does not necessarily indicate that it is primarily a gas generator, as the van Krevelen plots clearly show. The higher H/C ratios found for the vitrinite and inertinite fractions and the inverse relationship between alginite and bituminite tend to support the viewpoint that the environment of deposition has a very strong affect on the chemistry of the kerogen. The mechanism for this hydrogen enrichment in the terrigenously sourced kerogen macerals is not known; however, it has been suggested to us by Senftle that humic matter may be capable of adsorbing and/or absorbing lipid-rich materials during transportation and deposition in the marine e n ~ i r o n m e n t . ~Another ~ suggestion, by Durand, is that the hydrogen enrichment may take place during the vitrinite gel formation phase in the sediment shortly after d e p o ~ i t i o n . ~ ~
Acknowledgment. This work was supported by funding from the Kentucky Energy Cabinet, Commonwealth of Kentucky, and in part by grants from the Mobil Foundation. We would also like to thank Gerard Damaison for his encouragement and suggestions for this work and Bernard Durand, Boris Alpern, Wallace DOW, and Tina Tsui for their reviews and comments upon versions of this manuscript. We would also like to thank our colleague James Hower for his assistance with the petrography. (28) Demaison, G. J.; Moore, G. T. AAPG Bull. 1980,64,1179-1209. (29) Senftle, J. T., personal communication, 1983. (30) Durand, B., personal communication, 1985.
