Organic petrographic investigations of the Kupferschiefer in northern Germany

ELSEVIER International Journal of Coal Geology 33 (1997) 301-316

Organic petrographic investigations of the Kupferschiefer in northern Germany

J. Koch Bundesanstalt,ftir Geowissenschaften und Rohstoffe (BGR), Postfach 510153, D-30631 Hannouer, Germany

Received 8 February 1996; accepted 24 August 1996

Abstract

Results of an organic petrographic study of the Kupferschiefer (Zechstein, Upper Permian) in northern Germany are presented. Because vitrinite occurs only sporadically in this black shale, the random reflectance (R,) of a relatively high reflecting, vitrinite-like variety of bituminite was measured as a maturity parameter and the problems connected with this procedure were investigated.

The main organic component is bituminite, generally with an abundance of more than 90%. Sporomorphs and algae are relatively scarce, inertinite is usually a trace component, and vitrinite is present only in some samples taken close to the coast of the Zechstein sea. Migrabitumen also occurs in small amounts in a few samples. The range of the bituminite reflectance values is often considerable and depends on rank, anisotropy, particle size, porosity, and shearing. Interpretation of the reflectance values is made difficult by differences of up to 0.4% R, between layers within the Kupferschiefer. Hydrothermal alteration may also be present. The difference between the reflectance of the light and normal varieties of bituminite disappears between 0.9 and 1.2% K,.

Bituminite behaves in a way very similarly to vitrinite during coalification. Their reflectance is identical between 2 and 4% R,. Below 2% R,, the reflectance of bituminite is lower than that of vi&mite. Linear regression curves valid up to 4% R, were calculated for R,,, versus R, and for R, versus R,,. Taking certain limitations into consideration, bituminite reflectance may be used as a coalification parameter.

Keywords: Upper Permian; Zechstein: black shale; rank; microscopic methods: macerals; bituminite: reflectance

1. Introduction

The investigations are part of the “Deep Gas Project” carried out by BGR from 1990 until 1994. Within the scope of this project, the coalification grade of the Kupferschiefer

0166-5162/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO166-5162(96)00047-X

302 J. Koch/International Journal of Coal Geology 33 (1997) 301-316

(Zechstein, Upper Permian) in northern Germany was determined for a detailed contour- ing of the areas influenced by post-Zechstein thermal events, especially those of the early Upper Cretaceous igneous massifs (Bramsche, Vlotho, etc.). This paper presents the results of general interest to organic petrography and the constraints and problems to be considered in maturity determinations for the Kupferschiefer and for maceral bituminite.

The Kupferschiefer was the most favourable lithostratigraphic horizon for the investigation because it is a rather uniform layer only a few decimeters thick, covering nearly all of northern Germany (Fig. 1); it is rich in organic matter and is intersected in numerous boreholes. According to a review by Vaughan et al. (19891, the Kupfer- schiefer is usually a thin bed of marine bituminous sediments deposited following a rapid transgression in an area stretching from Poland to northeast England. The facies is predominantly euxinic; lithologic variations can be related to paleohighs, paleorises and subbasins. In basin environments, the Kupferschiefer is a uniform, laminated, strongly bituminous, silty marlstone or shale of 0.3 to 0.5 m thickness. In the margin and rise environments, it is enriched in carbonates and has more sedimentation cycles of clayey to silty and calcareous bands, indicating more oxidizing conditions (Paul, 1982; Kulick et al., 1984; Vaughan et al., 1989) and may reach a thickness of as much as four meters. A rudimentary development of only one centimeter thickness and untypical organic matter, as well as complete wedge out, sometimes occurs on rises. Everywhere the Kupferschiefer is overlain by dolomitic limestones (Vaughan et al., 1989).

Fig. 1. Location map with boundary of the basal Zechstein (dashed line, after Briickner-Rihling et al., 1994).

.I. Koch /International Journal of Coal Geology 33 (1997) 301-316 303

But there is also a handicap for maturity determinations of the Kupferschiefer because vitrinite occurs at only a few sites. Therefore, the main objectives of the study were (a) investigation of the usually readily measurable reflectance of bituminite and metabiturninite, (b) testing their suitability for maturity determinations, (c) determination of the reflectance properties of the other main macerals present and cd> determination of the relationships between the reflectance parameters of the different macerals with increasing coalification.

2. Samples and methods

The study is based on samples from 347 localities in northwestern Germany - mainly cuttings, usually supplied by the German gas and oil companies - and on core-samples from 124 boreholes in northeastern Germany, supplied as polished sections by BGR’s Berlin branch (Fig. 1). Kupferschiefer fragments were handpicked from the cuttings under a stereomicroscope, sometimes after washing and drying at 30°C. In the case of cuttings, the micropetrographical investigations were carried out on polished grain mounts of < 1 mm grain-size. All the samples were examined at X 500 magnifica- tion under normal and fluorescing (blue light excitation) conditions.

Because of the scarce occurrence of vitrinite in the Kupferschiefer (Section 3.31, the reflectance measurements were made on the bituminite (streaks and sometimes fragments of bituminous material) and at low coalification on a lighter, less porous vitrinite-like variety of bituminite. “Vitrinite-like macerals” present in minor amounts besides predominant amorphous material have been used by Buchardt et al. (1986) and Buchardt and Lewan (1990) for coalification determinations on Cambrian to Ordovician Alum Shales in southern Scandinavia. On the basis of the description of the organic matter given by Buchardt and coworkers and our own investigations, the present author assumes that in the Alum Shales the vitrinite-like matter, too, is a variety of bituminite. Hutton (1987) and Hutton et al. (1994) also classify vitrinite-like organic matter of marine oil shales as bituminite. Reflectance of vitrinite-like mace& was also used for determination of the maturity of high to overmature Early Paleozoic source rocks in China (Cheng Dingsheng et al., 1995). The use of bituminite reflectance measurements for determining degree of coalification is controversial: Durand et al. (1986) warned against using bituminite reflectance as a maturity parameter at all; Vigneresse (1993) commented positively on both publications from Scandinavia.

Random reflectance CR,) was usually measured with polarizer inserted (in contrast to the standard procedures). Only on polished sections of anisotropic material was the R, determination carried out with no polarizer to avoid the influence of the sample orientation. For example, the reflectance of samples that are cut perpendicular to the bedding plane and measured with inserted polarizer depends on the orientation of the bedding with respect to the E-W crosshairs. In general, the use of the polarizer is of advantage for recognizing weak anisotropy or the anisotropy of a fine-grained mosaic, for distinguishing vitrinite, bituminite and migrabitumen from semi-inertinite, sometimes for distinguishing organic from inorganic matter, as well as for reliably determining the minerals. The maximum reflectance (R,,,) was measured on selected samples that had


.I. Koch/International Journal of Coal Geology 33 (1997) 301-316 305

an R, value of at least 0.6%. Conditions of measurement: 50 X oil immersion objective, measuring field diameter 2 p.m.

3. Results and discussion

3.1. Modifications of the Kupferschiefer induced naturally or by drilling

The Kupferschiefer was first viewed as favourable for regional maturity studies; unfortunately the investigations brought some limitations to light. Modifications caused by the drilling or by natural influences made quite a number of samples unsuitable for measurements or led to results of limited quality and usefulness.

The drill cuttings often have a fine- to very fine-grained, irregular texture (Fig. 2a), i.e., the original texture of the rock is completely destroyed, resulting in an ultramy- lonite, which frequently accompanies shear planes. Transitions to a more granular (Fig. 2a) or a normal texture exist. Additionally, such samples may be oxidized, as best visible on larger relics of pyrite or chalcopyrite. The drilling apparently causes extensive attrition of the rock, followed by agglomeration. Besides shear planes, there is often micro-folding within these cuttings, which also might be caused by the drilling.

Natural oxidation of the organic matter must also be taken into account (Fig. 2b). Oxidative alteration of the synsedimentary sulfide mineralization and organic matter by ascending fluids occurs over large areas of northern Germany (areas of “Rote Faule” or hematite facies) or is associated with normal faults (Gerlach, 1989). The organic carbon is partially transformed to CO,, which reacts with the calcium in the fluids to form calcite. Traces of red iron oxides, as observed in some samples, may also indicate oxidation. According to Paul (19821, traces of iron oxides are typical of the more aerobic sedimentary facies on rises within the Kupferschiefer sea. Moreover, it cannot be excluded that numerous samples containing very fine streaks and/or fragments of bituminite with a corroded, porous texture are slightly oxidized. But for all supposedly oxidized samples it could not be determined whether there was an influence on the bituminite reflectance, because there may be considerable differences in the reflectance of different layers within the Kupferschiefer (Section 3.4.1). Speczik and Ptittmann (1987) found a higher “vitrinite” reflectance for Kupferschiefer samples from the Subsudetic Monocline in southwestern Poland, which, as proved by organic geochem- istry studies, had been oxidized by hydrothermal fluids enriched in metal ions. The reflectance was up to 0.7% higher than the normal range of 0.2-0.5% R,.

Fig. 2. Some microscopic characteristics of the Kupferschiefer. (a) Left, normal rock texture with highly reflecting meta-bituminite bands CR, = 2.68%); top right, tine-grained, very weakly stratified texture; bottom right, extremely fine-grained, unstratified texture. Scale bar = 50 pm; valid for Fig. 2a-d. (b) Oxidized Kupferschiefer with surrounding and impregnating iron-oxides. (c) Lighter variety of bituminite with smooth surface (center; light grey) and darker, porous normal bituminite (medium to dark-grey) in a marly mineral matrix; white mineral is pyrite. (d) Inertinitic chitinous, tube-like fragment of a fossil and thin, porous, grey bituminite bands in a clayey to silty matrix. The light-colored mineral in the center is sphalerite.

306 J. Koch/Inremationul Journal of Coal Geology 33 (1997) 301-316

3.2. Sedimentary petrography

The investigated samples are predominantly silty, marly shales to silty marlstones (to some extent due to selection by handpicking of the cuttings). A minor number of samples are silty shales or marly limestones. Epigenetic calcite is frequent, in some cases formed by recrystallization of calcareous microfossils.

In general, the proportion of syngenetic pyrite (framboids and very small particles) in the Kupferschiefer is very high. A grain-size of < 10 p_rn prevails and often < 2 pm. The most important epigenetic ore minerals, which usually occur within thin bands, are sphalerite and chalcopyrite; less common are bornite, pyrite/markasite, galenite, chal- cocite and covellite. But marcasite is present in almost all samples as crystals of usually < 10 p,rn size. As already pointed out by Kulick et al. (19841, there is no relationship between the organic content, coalification and the mineralization with metal ore. Highly reflecting inertinitic haloes surrounding pitchblende(?) were observed in a few samples, for example, in and around the Harz Mountains, where the Kupferschiefer directly overlies Lower Carboniferous sediments. Phosphoritic fossils (e.g. fish scales) are present in traces in only a few samples.

3.3. Organic petrography

The organic matter in the Kupferschiefer has a uniform composition and is enriched in layers. Bituminite (type II kerogen), or metabituminite in the case of higher maturity, is the main component with an abundance of mostly > 90%. The term bituminite is used here in the sense of a maceral group rather than as a maceml of the liptinite group. “Metabituminite” is not a defined maceral; the term is used in this study for bituminite with a high degree of maturation, beginning with the disappearance of the palynomorph fluorescence. The coalification residues of algae and sporomorphs cannot be distinguished in it.

Below about 0.9% R,, bituminite is usually present as a lower reflecting predominant variety and as a substantially scarcer, higher reflecting vitrinite-like variety (Fig. 2~). The reflectance measurements for the maturity determination were carried out on the latter. The lighter variety never showed fluorescence and the darker one showed a weak brownish fluorescence in a few samples only. Additionally, some samples contained large bituminite bands with an even higher reflectance than the lighter type and showed a very fine-grained anisotropic pattern at a relatively low coalification. For example, this third type had a reflectance of 0.92% R, in contrast to 0.61% R, of the typical lighter variety of bituminite. Sporadically, a distinct coke-like structure with fine-grained anisotropy is associated with hydrothermal influences. Highly variable reflectances have also been observed for different forms of bituminite in a single sample by Stasiuk and Goodarzi (1988). These many types of bituminite were one of the main points why Durand et al. (1986) considered reflectance determinations on bituminous sediments as risky. The variability of the optical and chemical properties of bituminite in marine dysoxic-anoxic sediment environments depends, according to present knowledge, on the mode of degradation and type of source material. The source material is predominantly phytoplanctonic and bacterial; to a minor extent the bituminite is derived from zooplank-

J. Koch/International Journal of Coal Geology 33 (1997) 301-316 307

ton, cyanobacteria and other organic material (ICCP, 1993; Hutton et al., 1994; Tyson, 1995). Nearly all samples of low maturity contain algal akinete cells (Stasiuk, 19931, rounded resting spores up to 15 Km in diameter, which have a reflectance similar to that of the lighter variety of bituminite.

The lighter, non-fluorescing vitrinite-like bituminite varieties of the Kupferschiefer and similar sediments are not included in the internationally accepted classification of bituminite (Teichmliller and Ottenjann, 1977; ICCP, 1993). Despite fluorescing only occasionally at very low maturity, the darker variety corresponds more or less to type I of the classification proposed by Teichmiiller and Ottenjann (1977). During diagenesis the bituminite material becomes increasingly thin banded or fine to very fine grained (depending on the original texture or on tectonic deformation) and increasingly vitrinite-like (Fig. 2a, left; Section 3.4.2), but not micrinite-like (Teichmiiller and Ottenjann, 1977; ICCP, 1993). This finding is supported by numerous studies by the author on moderately to very highly coalified Lower Carboniferous and other black shales as well as on Fischer Assay residues of Toarcian oil shales. The term metabituminite, introduced by Teichmiiller and Ottenjann, might be more accurate for the character- ization of the coalification residues of bituminite.

A component very similar (identical?) to micrinite of bituminous coals was found in traces and as small, irregular aggregates in the Kupferschiefer samples of all ranks. Rare, massive lenses of micrinite were also described by Sherwood and Cook (1986) from Early Cretaceous oil shales from the Eromanga Basin in Australia. Such macerals were interpreted by Stasiuk (1993) “as products of the degradation and microbial alteration... of marine organic matter, both within the water column and the sediment”.

Palynomorphs and palynomorph fragments are normally visible only up to a random bituminite reflectance of O&0.9% and make up only a small percentage of the total organic matter. According to semi-quantitative estimates they scarcely exceed 10% by volume. Small unicellular algae prevail over sporomorphs. In contrast to the findings of Alisch (19911, colonial algae were not found in any sample. But Kulick et al. (1984) and Wolf et al. (1989) also did not mention colonial algae in their publications about the Kupferschiefer in Germany.

Migrabitumen (usually impsonite) was present in small amounts in only a few samples. It occurs sometimes between epigenetic calcite crystals or inside recrystallized calcareous microfossils. However, distinguishing between migrabitumen and bituminite may be a problem, especially if the rock matrix is strongly calcareous. As a result of the growth of calcite crystals through bituminite bands these are divided, leaving relics of bituminite between the crystals. These relics then occupy the typical intergranular position, which is a major criterion for the microscopic identification of migrabitumen.

Even more problematic is the occurrence of vitrinite in the Kupferschiefer. It could be determined reliably only in samples from shafts of bituminous coal mines in the Lower Rhine coal district, which contained large coalified plant fossils and fossil fish remains, as well as in samples from a few other places. All these sites are close to the shoreline of the Kuperschiefer sea. The main sedimentation area contains no vitrinite. Despite the bituminous facies, there was no microscopic evidence for bitumen impregna- tion of the vitrinite.

Inertinite, too, is rare in most of the samples, and is mainly present as inertodetrinite,


50-

40.

30.

20.

10.

7 i.01

r- r 0 0.20 0.40 0.60 0.80 1 .oo

Rr (W Fig. 3. Example of the reflectance difference between the normal (left) and the lighter variety of bituminite. Numbers from top to bottom = mean, standard deviation, number of measurements.

in many samples it also has features typical of chitinous insect remains (Fig. 2d). The minor terrestrial input is also evident from the scarcety of true inertinite. Graphite, which is chemically inert, is a trace component in numerous samples. Its presence is always accompanied by the occurrence of zircon.

3.4. Reflectance studies

3.4.1. General results The distinct difference between the darker and lighter varieties of bituminite of low

rank samples is shown in Fig. 3. The difference disappears rapidly with increasing maturity (Section 3.4.2). The range of the reflectance values of bituminite or metabituminite is frequently very broad; the standard deviation is about k 10% of the mean. Moreover, the bituminite bands are mostly very thin (close to the diameter of the measuring field) and sometimes slightly porous or in case of higher coalification more or less fine-grained anisotropic. Therefore, the reliability of the reflectance values is not indicated only by the mean, number of measurements and standard deviation, but the quality of the polished surface must also be taken into consideration. The standard deviation also depends on whether the reflectance measurements were done with a polarizer. As evident in Table 1, the coefficient of variation for samples with at least ten measurements is usually lower when no polarizer is used.

Like for other macerals, the range of random bituminite reflectance increases

Table 1 Percentage of samples with a standard deviation less than, equal to, or more than 10% of the mean (i.e., coefficient of variation) of reflectance measurements on the lighter variety of bituminite with and without uolarizer

s < 10% z 10% of the mean > 10%

With polarizer (234 samples) Without polarizer (64 samples)

47.9% 7.1% 44.4% 73.4% 10.9% 15.6%


50 a) 1.70 0.073 25

40 G L

$

s s i? L

30 DIL-

20

10

p.00 2.00 3.00

Rmin (x)

50

1 40

G L 2 30

5

$ 20 I=

10

Lo too 1.50 2.

1.93 0.128 25

b, ) 2.50 3.00

Rr (Xl Rr (X)

2.07 0.426

0.00 2.00 4.00 6.00

Rmln (x)

501

40.

30. I n

3.41 0.60 40

20

10

i, It.00 2.00 4.00 6.00

50

I 2.14 50 4.66 0.124 0.296 25 20

40 40 s v

i.00 2.00 3.00 0.00 2.00 4.00 6.00

Rmar (X) Rmax (X)

Fig. 4. Comparison of frequency distributions of R,,,, R, and R,,, of metabituminite: (a) medium degree of coalification, (b) high degree of coalification. Numbers represent the same parameters as in Fig. 3.

distinctly with increasing anisotropy. In the case of relatively high coalification, the range of R,;,, is narrower than that of R, and the standard deviation is relatively small (Fig. 4a, b). At very low maturity, R, has a range of less than 0.2%, increases up to about 0.4% within the low maturity field (end at about 1% R,), then increases rapidly with increasing anisotropy at medium maturity (Fig. 4a) and even more so at high maturity (Fig. 4b), where the frequency distributions are sometimes very erratic.

The range of the measurements is probably sometimes influenced by the heat caused by the drilling. This was assumed for the sample shown in Fig. 5a, for which the mode


40 b) 2.02 0.277 40

:.OO 0.50 1.00 1.50 2.00 2.50 3.00 t.00 0.50 1.00 1.50 2.00 2.50 3.00

Rr (%I Rr (%I

40 c)

g 30

E g 20 I

1.67 0.223 20

40- d)

30.

l-l 20 .

2.31 0.456 22

b L

E lo- 10.

i.00 -I l-l 1.50 too , 0.50 1.00 2.00 2.50 3.00 1.00 2.00 3.00 n 4.00 5.00

Rr (%I Rr W Fig. 5. Examples of a very broad range of R, values caused (a) possibly by heating during drilling; (b) by fine-grained organic matter at a medium degree of coalification; (c) by folding and slight shearing at a medium degree of coalification; (d) by strong shearing at a high degree of coalification. Numbers represent the same parameters as in Fig. 3.

of approximately 1.0% R, was used instead of the mean for the regional maturity studies. Moreover, fine-grained organic matter often produces a broad range of R, values (Fig. 5b). Tectonic deformation, like microfolding and shearing, may lead - depending on the extent of the movements - to very broad ranges even at medium rank (Fig. 5c) and still b roader ones at high rank (Fig. 5d), when the R,,, values, too, show a broader range than usual.

A problem for the interpretation of the data is reflectance differences of unknown origin that occur between layers of the Kupferschiefer and which are sometimes of considerable size. This was the case at several sampling sites. In the case of cuttings, the result is a broad range of values if there is no clear separation of the peak for the lighter population in the R, frequency plots. Therefore, in northwest Germany this phenomenon could be demonstrated only in very few boreholes. But in polished sections the differences can be clearly seen. Measurements on two polished sections from different layers of the Kupferschiefer intersected in 25 boreholes in northeast Germany sometimes resulted in considerable differences, with a maximum difference of 0.38% R, (Table 2). The lowest value was used for the maturity evaluations.

Orientation of polished sections parallel to the bedding or a preference for this orientation in polished grain mounts, which sometimes occurs, may also be a source of values that are too high. For anisotropic organic matter this plane is less suitable for the


Table 2 Differences between the reflectances of layers within Kupferschiefer cores from 25 boreholes in eastern Germany

A%R, n -

< 0.05 8 0.05-0.09 3 0.10-0.19 6 0.20-0.29 5 0.30-0.39 3

determination of R, than sections cut perpendicular to the bedding because (also for measurements without polarizer) R,,, is generally determined because of the normally optically uniaxial negative properties of vitrinite and correspondingly of bituminite. This is illustrated by the following sample pairs:

K 19061 (whose surface is perpendicular to the bedding) with R, = 1.38% and K 19062 (parallel to bedding) with R, = 1.48%, with only a small reflectance difference, and K 19066 (perpendicular to bedding) with R, = 1.75% and K 19064 (parallel to bedding) with R, = 2.00%, with a larger difference at a higher degree of coalification.

Usually there is no fluorescing liptinite in the Kupferschiefer samples above 0.90% R, of the bituminite. But in some samples from eastern Germany, rather strongly fluorescing liptinite (mostly alginite and alginite fragments) was visible, for example, at 0.97% and 1.17% R,. This may be due to rapid local heating, which could have altered the bituminite and had little or no influence on the slower reacting liptinite, like in the early phases of the Fischer test. Such reflectance values may have only very local significance. Other examples of local significance are values that are higher than the vitrinite reflectance trend-line of an entire borehole or higher than the reflectance of the directly underlying Carboniferous coal measures. These alterations were probably caused by hydrothermal and/or tectonic influences on this highly reactive sediment rich in organic matter.

Table 3 Linear relationships between the reflectance parameters of Kupferschiefer macerals

Reflectance parameters n Linear Correl. 95% confidence

?’ x regression, coeff., r limits of r y=ax+b

bit(light)R,,, bitilight)R,

bit(normal)R, impsoniteR,., semiin R,,, inertinite R,,, inertinite R,, n vitriniteR, impsoniteR, semiin R, semiin R,,,

bit(light) R, impsoniteR, semiin R, inertinite R, inertinite R, bit(light)R, bit(light)R, bit(light)R, bit(li~ht)R,,,

_ see text

22 \’ =0.859x -0.150 11 $ = 1.142.x -0.043 9 v = 1.041.x +0.125

25 ; = 1.014x +0.165 24 ; = 1.014x -0.282 10 ;1=0.942x+O.195 15 ; = 1.187~ +0.076 9 1 = 1.521.x +0.286 9 Y = 1.322 x + 0.426

_

0.950 0.995 0.991 0.972 0.982 0.98 1 0.824 0.936 0.920

0.877-0.978 0.976-0.998 0.952-0.998 0.934-0.987 0.955-0.992 0.911-0.995 0.526-0.936 0.698-0.985 0.638-0.981


7.0

6.0

0 Rmax-Rmln

5.0

x

9) 4.0 : 0

t P) 3.0

F m

2.0

1.0

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Rr (%)

Fig. 6. R,i,, R,,, and bireflectance vs. R, for the lighter variety of bituminite.

3.4.2. Reflectance of Kupferschiefer macerals During the early phase of these studies, a set of 49 samples with bituminite

reflectances between 0.18% and 3.41% R, was selected for determinations of R,, R,,, and Rmin on bituminite (both the normal and the lighter varieties), migrabitumen, inertinite (semi-inertinite and more or less fusinitic types), and, when present, vitrinite. The reflectance parameters, including bireflectance ( Rmax-Rmin), were correlated using a correlation matrix. Of the numerous possible correlations, only the correlations that were appropriate and more or less statistically sound (on the basis of the number of data, the correlation coefficients and the confidence limits of the correlation coefficients) were evaluated further (Table 3). In the case of relationships that are non-linear with respect to the coalification range as a whole, for example between the maximum and random reflectance, the results are valid only within the approximately linear part of the relations.

3.4.2.1. Bituminite. Differences between Rmin, R, and R,,, of bituminite at medium and high degree of coalification have been shown already in Fig. 4. Bituminite tends to behave like vitrinite during coalification (Fig. 61, i.e., increase of R,, R,,, and at first also R,, (and therefore, the bireflectance). The reversal to lower Rmin-values accompanied by an accelerated increase of the bireflectance begins between 2.3 and 2.5% R, (Fig. 6). The correlation between the reflectance parameters of the lighter variety of


0.0 1.0 2.0 3.0 4.0 5.0 6.0

Rr (X)

Fig. 7. Linear regression curve for R,,, vs. R, of the lighter variety of bituminite.

bituminite is good to very good except for the correlation of bireflectance with respect to R m,n because of the decreasing R,,,-values (Fig. 6).

Numerous additional measurements of R, and R,,,, besides those used for the correlation matrix, were used to obtain even more reliable regression lines for R,,, versus R, and vice versa (Fig. 7): R *ax = 1.2272 R, - 0.0468

R, = 0.7841 R,,, + 0.1033 Correlation coefficient r = 0.98.

These linear equations may be applied for the lighter variety of bituminite up to about 4% R,. Above this reflectance, they cannot be used owing to the exponential increase of R max (further limitation: R,,, cannot be smaller than R,). For moderate degrees of coalification these relationships correspond nearly to those found for vitrinite (Koch and Gunther, 1995).

3.4.2.2. Migrabitumen (impsonite). The data base is markedly smaller than for bituminite. This maceral group, too, behaves like vitrinite and bituminite during coalification and the reflectance parameters develop correspondingly. The slope of the regression curve R,,, = 1.142 R, - 0.043 (r = 0.99; Table 3) is smaller than those of the curves for vitrinite and bituminite and correspondingly the anisotropy develops at a slower rate during coalification. The onset of the R,,, decrease is at about 3% R,.

3.4.2.3. Semi-inertinite and inertinite. The relationships between the reflectance parameters for the semi-inertinite are similar to those for bituminite and migrabitumen (Table 3). R,, R,,, and Rmin correlate very well for typical inertinite (measurements made on


6.0

1.0

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Rr (X)

Fig. 8. R,,,,,, R,,, and bireflectance vs. R, for inertinite

fusinite and sharp inertodetrinite fragments; Table 3; Fig. 81, but the regression curves

for LX and Rmin are parallel, i.e., the bireflectance is almost constant, in contrast to the other mace&.

3.4.2.4. Relationships between the reflectance parameters of different macerals. A regression curve with the desired reliability could not be obtained for the relationship of the most interest for maturity determination, namely that for vitrinite R, versus bituminite R,, owing to the already discussed problems related to vitrinite and the small amount of available data (Table 3). The equation for this regression curve ( y = 0.942~ + 0.195) indicates that during coalification the random vitrinite reflectance is at first distinctly, then moderately higher than R, of the lighter variety of bituminite. Beginning at about 2% R,, the two reflectance parameters are approximately equal. This is in agreement with our experience with coalification measurements on sequences containing bituminous sediments other than Kupferschiefer. According to the equation, the vitrinite reflectance would be lower than the bituminite reflectance beginning at about 5% bituminite R,. But there are indications (Koch, 1997) that the equation probably cannot be used above 4% bituminite R,. Between 0.4 and 0.6% random vitrinite reflectance, the difference from the R, values of the bituminite would amount to 0.17-0.18% R,. This is in agreement with the mean difference of 0.17% R, between the values for vitrinite reflectance and kerogen matrix (bituminite) reflectance published by Alpem et al. (1994) for Tertiary sediments of the same coalification range from Angola that are not impregnated with hydrocarbons. This could mean that the Kupferschiefer samples are also not impregnated with hydrocarbons and that there is no influence on the reflectance measurements.


The regression equation for R, of the normal bituminite type versus R, of the lighter variety ( y = 0.859~ - 0.15; Table 3) is valid only up to 1.2% R, of the lighter variety. Above this value, the two varieties can no longer be distinguished. The slope for the random reflectance of migrabitumen is clearly steeper than that of the lighter variety of bituminite ( y = 1.187x + 0.076). The reflectance of the semiinertinite increases distinctly with increasing bituminite reflectance and is considerably higher than the bituminite reflectance (Table 31, whereas the reflectance of the typical high-reflecting inertinite is nearly constant.

4. Conclusions

The Kupferschiefer has some characteristics (e.g., mylonitization caused by the drilling, natural oxidation, reflectance differences between layers) that cause problems for maturity determinations. These characteristics are related to the position of this very thin, tectonically and geochemically highly reactive layer between such different forma- tions, for example, the underlying Lower Permian Rotliegendes or the Carboniferous and the overlying carbonates, anhydrites and salts. Such characteristics might also be found for other similar bituminous sediments and might be restrictive factors for maturity determinations.

At a very low grade of diagenesis the lighter variety of bituminite reflects distinctly lower than vitrinite, but beginning at 1.25% bituminite-R, the difference is small and the bituminite values may be used instead of vitrinite reflectance. The two are more or less identical between 2 and 4% R,.

On the whole, the Kupferschiefer and similar bituminous rocks and the lighter, vitrinite-like variety of bituminite may be useful for maturity determinations if the problematic factors are taken into account and if a sufficient number of samples from the bituminous horizon are measured or if the data can be cross-checked with data from the overlying and/or underlying strata.

Acknowledgements

The research was supported by the Federal Ministry of Education, Science, Research and Technology, Project Nos. 032 668 A and 032 7105 A (“Deep Gas Project”). The author would like to thank the German hydrocarbon industry for providing most of the samples and for the permission to publish the results. Dr. F. Gramann from the Lower Saxonian Geological Survey, Hannover, is thanked for his help with the identification of the insect remains and Dr. Kockel, BGR, for providing an unpublished map containing the boundary of the basal Zechstein. The publication benefited significantly from the comments of Prof. Dr. Alpern and an unknown reviewer.

References

Alisch, O., 199 I. Die Fluoreszenz der orgdnischen Substanzen des Kupferschiefers - ein Beispiel ftiir die Rangdiagnose sapropelitischer Gesteine. Z. Angew. Geol., 37: 11 l-l 15.


Alpem, B., Oudin, J.-L., Pinheiro, J. Pittion, J.-L. and Zhu, X., 1994. MCthode d’6mde optique des hydrocarbures extraits et fix&s dans la r&sine des sections polies de roches. Influence de la richesse en huile sur la riflectance des kgrogtnes. Bull. Centres Rech. Explor..Prod. Elf Aquitaine, Publ. Sptc., 18: 15-35.

Briickner-RGhling, S., Hoffmann, N., Kockel, F., Krull, P. and Stumm, M., 1994. Die Struktur- und Mkhtigkeitskarten des Nordeurop%ischen Permbeckens und seiner Rtider 1: 1.5 Mio. BGR-Rep., 111 921, 33 pp. (Unpubl.)

Buchardt, B. and Lewan, M.D., 1990. Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian-Ordovician Alum Shale, Southern Scandinavia. Am. Assoc. Petrol. Geol. Bull., 74: 394-406.

Buchardt, B., Clausen, J. and Thomsen, E., 1986. Carbon isotope composition of Lower Palaeozoic kerogen: Effects of maturation. Org. Geochem., 10: 127-134.

Cheng Dingsheng, Hao Shisheng and Wang Feiyu, 1995. Reflectance of vitrinite-like macerals, a possible thermal maturity index for highly/overmatured source rocks of the Lower Paleozoic. Shiyou Kantan Yu Kaifa, 22: 25-28 (from Chem. Abstr., 123, 1995, No. 60785~).

Durand, B., Alpem, B., Pittion, J.L. and Pradier, B., 1986. Reflectance of vitrinite as a control of thermal history of sediments. In: J. Burms (Editor), Thermal Modelling in Sedimentary Basins. Technip, Paris, pp. 441-474.

Gerlach, R., 1989. Zur mineral&h-stofflichen Typisierung von Kupferreicherzen im Lagerstittentyp Kupfer- schiefer (SE-Harzvorland) mit dem Versuch einer genetischen Interpretation. Freiberg. Forschungsh., C 437: 59-72.

Hutton, A., 1987. Petrographic classification of oil shales. Int. J. Coal Geol., 8: 203-231. Hutton, A., Bharati, S. and Robl, T., 1994. Chemical and petrographic classification of kerogen/macerals.

Energy Fuels, 8: 1478-1488. International Committee for Coal Petrology, ICCP. 1993. Bituminite in Rocks Other than Coal. International

Handbook of Coal Petrology, 3rd suppl. 2nd ed., 15 pp., Newcastle. Koch, J., 1997. Upper limits for vitrinite and bituminite reflectance as coalification parameters. Int. J. Coal

Geol., in press. Koch, J. and Giinther, M., 1995. Relationship between random and maximum vitrinite reflectance. Fuel, 74:

1687-1691. Kulick, J., Leifeld, D., Meisl, S., Piischl, W., Stellmacher, R., Strecker, G., Theuerjahr, K.-A. and Wolf, M.,

1984. Petrofazielle und chemische Erkundung des Kupferschiefers der Hessischen Senke und des Harz- Westrandes. Geol. Jahrb., D68: 3-223.

Paul, J., 1982. Zur Rand- und Schwellenfazies des Kupferschiefers. Z. Dtsch. Geol. Ges., 133: 571-605. Sherwood, N.R. and Cook, A.C., 1986. Organic matter in theToolebuc Formation. In: D.I. Gravestock, P.S.

Moore and G.M. Pitt (Editors), Contributions to the Geology and Hydrocarbon Potential of the Eromanga Basin. Geol. Sot. Australia Spec. Publ., 12: 255-265.

Speczik, S. and Pllttmann, W., 1987. Origin of Kupferschiefer mineralization as suggested by coal petrology and organic geochemical studies. Acta Geol. Pol., 37: 168-186.

Stasiuk, L.D., 1993. Algal bloom episodes and the formation of bituminite and micrinite in hydrocarbon source rocks: evidence from the Devonian and Mississippian, northern Williston Basin, Canada. Int. J. Coal Geol., 24: 195-210.

Stasiuk, L.D. and Goodarzi, F., 1988. Organic petrology of Second White Speckled Shale, Saskatchewan, Canada - A possible link between bituminite and biogenic gas? Bull. Can. Petrol. Geol., 36: 397-406.

Teichmiiller, M. and Ottenjann, K., 1977. Liptinite und lipoide Stoffe in einem Erdiilmuttergestein. ErdGl und Kohle Erdgas Petrochem., 30: 387-398.

Tyson, R.V., 1995. Sedimentary Organic Matter. Chapman and Hall, London, 615 pp. Vaughan, D.J., Sweeney, M., Friedrich, G., Diedel, R. and Haranczyk, C., 1989. The Kupferschiefer: An

overview with an appraisal of the different types of mineralization. Econ. Geol., 84: 1003-1027. Vigneresse, J.L., 1993. Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian-Ordo-

vician Alum Shale, Southern Scandinavia: Discussion. Am. Assoc. Petrol. Geol. Bull., 77: 509-5 11. Wolf, M., David, P. Eckardt, C.B., Hagemann, H.W. and Piittmann, W., 1989. Facies and rank of Permian

Kupferschiefer from the Lower Rhine Basin and NW Germany. Int. J. Coal Geol., 14: 119-136.

Documents

Organic petrographic investigations of the Kupferschiefer in northern Germany