The Pennsylvanian of the Ruhr Basin, Germany: Peat Deposition And

Home , Biphenyl

“The Pennsylvanian of the Ruhr Basin, Germany:

Peat deposition and characterisation of dispersed

kerogen, vitrinite, and coal at different levels of

maturation”

Von der Fakultät für Georessourcen und Materialtechnik der Rheinisch -Westfälischen Technischen Hochschule Aachen

zur Erlangung des akademischen Grades eines

Doktors der Naturwissenschaften

genehmigte Dissertation

vorgelegt von M.Sc. RWTH

Laura Zieger

aus Aachen

Berichter: Univ. Prof. Dr. rer. nat. Ralf Littke Univ. Prof. Dr. Reinhard Sachsenhofer

Tag der mündlichen Prüfung: 17. Januar 2020

Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek online verfügbar

Acknowledgements I would like to thank my supervisor Ralf Littke, who gave me the opportunity to face the adventure of doing a doctorate. For his continuous professional and mental support I am deeply thankful.

I sincerely want to thank Jan Schwarzbauer and Christoph Hartkopf-Fröder for their professional input during the production of two manuscripts and for the fruitful and pleasant collaboration in further projects.

I further would like to thank the staff members of the institute for creating a pleasant and supportive working atmosphere - at any time they made me feel most welcome and respected. My special thanks go to Donka Macherey and Annette Schneiderwind, who contributed to a great extent to the preparation of this thesis by providing technical and practical support.

At the early stage of my studies Olga Scheffler and later Michelle Evertz helped me a lot conquering bureaucratic challenges, for which I am very thankful.

My fellow (and former) doctoral students, especially Anna Kutovaya, Daniel Monhoff, Alireza Baniasad, Reinhard Fink, Sebastian Grohmann, Felix Froidl, Steffen Nolte, and Garri Gaus are given credit for making the time during, and occasionally after the working hours, entertaining or sometimes just interesting.

Last but not least, I would like to thank my family and friends, without whom I surely would not have been able to overcome particularly stressful times, for their love, patience and humour.

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Abstract This thesis deals with the depositional conditions during peat formation of Pennsylanian coals from the paralic German Ruhr Basin as well as the chemical and structural changes occurring in these coals, associated kerogen from clastic rocks, and vitrinite upon thermal maturation. For this purpose vitrinites of a natural maturity sequence of Pennsylvanian coals from the Ruhr Basin, consisting of eleven coal seams from subbituminous to semi-anthracite ranks (0.5 to 2.9% vitrinite reflectance), were handpicked, characterised concerning their elemental composition and analysed using Rock-Eval pyrolysis, attenuated total reflectance infrared spectroscopy (ATR FT- IR), and Curie Point-pyrolysis gas chromatography-mass spectroscopy (CP-Py-GC-MS) at two pyrolysis temperatures. Micro-FT-IR was used in order to assess variations in the proportions of functional groups in megaspores from five oil mature coal samples. Infrared spectra of vitrinites show a clear increase in aromaticity with increasing thermal maturity. Spectra of megaspores reveal a more aliphatic character than the vitrinites and the degree of aromaticity increases more slowly compared to that of the vitrinites with increasing maturity. Both types of macerals show a pronounced decrease in carboxyl/carbonyl functional groups. Vitrinites pyrolysed at 590 °C yield high amounts of aliphatic hydrocarbons, while those pyrolysed at 764 °C produce more aromatic compounds, including phenols, naphthalenes and phenanthrenes as well as sulphur-containing aromatics. The aromatic fraction relative to the aliphatic fraction increases upon maturation of the pyrolysates. The loss of phenolic moieties in favour of benzenes upon maturation evidences the loss of oxygen-containing functional groups and is in line with the findings obtained from the FT- IR measurements. Increasing yields of polyaromatic structures in the pyrolysed vitrinites show the evolution towards more condensed aromatic clusters as maturation proceeds. These general trends are also found for vitrinites of a narrower maturity window ranging from subbituminous to high volatile bituminous coals, as revealed by the analysis of coals and associated siliciclastic rocks.

These coals of Bolsovian age were also characterised regarding the environmental conditions prevailing during the deposition of the original peat mires. Eleven coal seams from the German Ruhr Coal Basin were analysed along depth profiles, on the one hand microscopically and on the other hand in terms of their ash yield, carbon and sulphur content as well as their Rock-Eval parameters. Established maceral indices and the newly introduced A/I ratio were employed to evaluate the depo-settings. Results show that the same basic depositional pathways of terrestrialisation and paludification exist as reported before for older (more mature) Duckmantian coal seams from the Ruhr Basin. The presence of coals formed as ombrotrophic peat further indicates a wet climate until the Upper Bolsovian. Very high sulphur values of seam Parsifal II

iv indicate a marine influence at the peat stage. A marine ingression event was not reported before for this stratigraphical level within the Ruhr Basin.

The influence of marine water on seam Parsifal II is recognisable on a molecular level as revealed by the results obtained by ATR FT-IR and CP-Py-GC-MS analyses on the Bolsovian samples. The same analytical methods as for the Pennsylvanian maturity series were applied in order to detect the influence on depositional environment and precursor maceral on the structural properties of coals, kerogen from adjacent clastic rocks, and pure vitrinite, while the maturity window was narrow (0.55 to 0.73% VRr). Higher Tmax values of siliciclastic rocks associated with the Bolsovian coals are explained by the mineral matrix effect, while the relatively higher oxygen index values of the siliciclastics and kerogen isolated from them are surprisingly not related to the abundance of carbonyl/carboxyl groups deduced from ATR FT-IR spectroscopy. FT-IR results imply a lower degree of aromaticity and at the same time higher degree of condensation for vitrinites originated from the sedimentary layers, as compared to vitrinites handpicked from the coal seams. Results from CP-Py-GC-MS at 590 °C show that the vitrinites originating from the siliciclastic rocks generate higher amounts of long-chained n-alkanes and - alkenes than the bulk samples or the vitrinites originating from the coals. In the narrow maturity range represented by the Bolsovian samples, only vitrinites of the coal seams show some correlation with depth. As for the natural maturity sequence of the Ruhr Basin, the relative abundance of phenols is decreasing with increasing maturity, as well as the ratio of benzenes over higher condensed aromatic moieties. The loss of oxygen-containing functional groups seems to be the most pronounced from the subbituminous to the high volatile bituminous coal ranks.

Kurzfassung Die vorliegende Arbeit befasst sich mit den während der Torfbildung herrschenden Ablagerungsbedingungen von Pennsylvanischen Kohlen aus dem paralischen Ruhr Becken sowie den chemisch strukturellen Veränderungen welche in diesen Kohlen, isolierten Vitriniten sowie Kerogenen aus begleitenden, klastischen Nebengesteinen im Zuge der thermischer Reifung auftreten. Zu diesem Zweck wurden isolierte Vitrinite einer natürlichen, aus dem Ruhr Becken stammenden, von Glanzbraunkohle bis Anthrazit reichenden Reifeserie (0,5 bis 2,9 % Vitrinitreflektion) hinsichtlich ihrer elementaren Zusammensetzung charakterisiert und mittels Rock-Eval Pyrolyse, abgeschwächter Totalreflexions-Infrarotspektroskopie (ATR FT-IR) und Curie Punkt-Pyrolyse-Gaschromatographie-Massenspektroskopie (CP-Py-GC-MS) bei zwei Pyrolysetemperaturen analysiert. Um Variationen in den Anteilen der funktionellen Gruppen in Megaporen von fünf Kohleproben im Bereich des Erdölfensters zu detektieren wurde Mikro-FT- IR verwendet. Infrarotspektren der begleitenden Vitrinite zeigen eine deutliche Steigerung der Aromatizität mit zunehmender thermischer Reife. Die Spektren der Megaporen zeigen im Vergleich zu den Vitriniten einen aliphatischeren Charakter, wobei der Grad der Aromatisierung mit zunehmender Reifung langsamer ansteigt als der der Vitrinite. Beide Kerogentypen zeigen einen deutlichen Verlust der funktionellen Gruppen der Carboxyle und Carbonyle mit der Reife. Die bei 590 °C pyrolysierten Vitrinite produzieren große Mengen an aliphatischen Kohlenwasserstoffen, während bei der Pyrolyse bei 764 °C mehr aromatische Verbindungen entstehen. Zu den aromatischen Verbindungen zählen Phenole, Naphthalene und Phenanthrene sowie schwefelhaltige Aromaten. Der aromatische im Verhältnis zum aliphatischen Anteil der Pyrolyseprodukte steigt mit zunehmender thermischer Reife an. Der Verlust an Phenolen zugunsten von Benzolen während der Reifung weist auf den Verlust von sauerstoffhaltigen funktionellen Gruppen hin, was die Ergebnisse der FT-IR-Messungen bestätigt. Größere Pyrolyseausbeuten an polyaromatischen Verbindungen deuten auf eine Entwicklung zu höher kondensierten aromatischen Clustern im Zuge der thermischen Reifung hin. Diese allgemeinen Reifetrends bestehen auch bei Vitriniten eines enger gefassten Reifefensters zwischen Glanzbraunkohlen und Gasflammkohlen, sowohl für Vitrinite, die aus Kohlen stammen, als auch für solche aus siliziklastischen Gesteinen.

Diese Kohlen Bolsovischen Alters wurden zudem hinsichtlich der Umweltbedingungen charakterisiert, die während der Ablagerung der ursprünglichen Torfmoore herrschten. Elf Kohleflöze aus dem Ruhrgebiet wurden entlang von Tiefenprofilen zum einen mikroskopisch und zum anderen hinsichtlich ihres Asche-, Kohlenstoff- und Schwefelgehalts sowie ihrer Rock-Eval Parameter analysiert. Zur Klassifizierung der Ablagerungsbedingungen wurden etablierte Mazeral-Indizes sowie das neu eingeführte A/I-Verhältnis herangezogen. Die Ergebnisse zeigen,

vi dass die gleichen grundlegenden Mechanismen der Terrestrisierung und Paludifizierung wie für ältere (und reifere) Duckmantische Kohleflöze aus dem Ruhrgebiet bestehen. Das Vorhandensein von Kohlen, deren Ablagerung als Torf unter ombrogenen Bedingungen stattfand, deutet zudem auf ein feuchtes Klima bis zum oberen Bolsovium hin. Sehr hohe Schwefelgehalte des Flözes Parsifal II werden als Resultat marinen Einflusses im Torfstadium interpretiert. Für dieses stratigraphische Alter war bisher kein marines Ingressionsereignis im Ruhrkarbon bekannt.

Ein mariner Einfluss auf Flöz Parsifal II ist auch auf molekularer Ebene erkennbar, wie Ergebnisse der ATR FT-IR- und CP-Py-GC-MS Analysen der Bolsovischen Proben zeigen. Mit den gleichen analytischen Methoden wie zuvor bei der Pennsylvanischen Reifenserie angewandt, wurde der Einfluss des Ablagerungsmilieus und der Mazeralzusammensetzung auf die chemisch- strukturelle Beschaffenheit von Kohlen, Kerogen aus assoziierten Gesteinen sowie reinen

Vitriniten aus einem engen Reifebereich (0,55 bis 0,73 VRr) während der thermischen Reifung analysiert. Höhere Tmax-Werte von mit den Bolsovischen Kohlen assoziierten siliziklastischen Gesteinen werden durch den Mineralmatrixeffekt erklärt, während die höheren OI-Werte der siliziklastischen Gesteine und der aus ihnen isolierten Kerogene überraschenderweise nicht durch die mittels ATR FT-IR-Spektroskopie gewonnenen Ergebnisse zu sauerstoffhaltigen Verbindungen erklärt werden können. Ergebnisse der FT-IR Messungen weisen auf einen geringeren Grad an Aromatizität und gleichzeitig einen höheren Grad an Kondensierung der aus den Sedimentschichten stammen Vitrinite verglichen mit den aus den Kohleflözen stammenden Vitriniten hin. Ergebnisse von CP-Pyrolyse bei 590 °C zeigen, dass die aus den siliziklastischen Gesteinen stammenden Vitrinite größere Mengen langkettiger n-Alkane und -Alkene erzeugen als die aus den Kohlen isolierten Vitrinite. Im durch die Bolsovischen Proben repräsentierten schmalen Reifebereich zeigen nur aus den Kohlen stammenden Vitrinite eine Korrelation ihrer Struktur mit der Tiefe. Was die natürliche Reifeserie des Ruhrgebiets betrifft, so nehmen sowohl die relative Häufigkeit an Phenolen wie auch das Verhältnis von Benzolen zu höher kondensierten aromatischen Verbindungen mit zunehmender Reife ab. Der Verlust sauerstoffhaltiger funktioneller Gruppen scheint am stärksten im Reifebereich zwischen Glanzbraunkohlen und Gasflammkohlen ausgeprägt zu sein.

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Content

Acknowledgements………………………………………….………………...……………...…iii Abstract……………………………………………………………………………………….....iv Kurzfassung…………………………………………………………………………….….....….vi Abbreviations………………………………………………………………………….……..….xi 1 Introduction…………………………………………………………………………...………..1 1.2 Geological background and motivation……………………………………………..….…1 1.3 Deposition of coal and terrigenous organic matter………………………………………..3 1.4 Formation and maturation of vitrinite………………………………………………….....6 1.5 Analytical techniques………………………………………………………………….….8

1.5.1 Overview……………………………………………………………………………………….8

1.5.2 Organic petrology……………………………………………………………………………...9

1.5.3 Pyrolysis experiments……………………………………………………………………..….10

1.5.4 Fourier-transform infrared spectroscopy………………………………………………..……11 1.6 Outline of this thesis……………………………………………………………………..12 2 Chemical and structural changes in vitrinites and megaspores from Carboniferous coals during maturation………………………………………………...……………………………13 2.1 Introduction………………………………………………………………………...……14 2.2 Geological setting……………………………………………………………………..…15 2.3 Material and methods……………………………………………………………...…….17

2.3.1 Samples………………………………………………………………………………….……17

2.3.2 Petrographic and bulk analyses…………………………………………………………….…17

2.3.3 FTIR………………………………………………………………………………………..…19

2.3.4 Curie Point-pyrolysis-GC-MS……………………………………………………………..…20 2.4 Results…………………………………………………………………………………...20

2.4.1 FT-IR……………………………………………………………………………………….…20

2.4.2 Curie Point-pyrolysis-GC-MS………………………………………………………………..22 2.5 Discussion………………………………………………………………………………..25

2.5.1 Aromatisation…………………………………………………………………………………25

2.5.2 Chain length and branching of aliphatic chains………………………………………………25

2.5.3 Condensation……………………………………………………………………………….…27

2.5.4 Oxygen-containing groups……………………………………………………………………30

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2.5.5 Comparison of megaspores and vitrinites………………………………………………….…31

2.6 Conclusions………………………………………………………………………………………32 3 Comparative geochemical and pyrolytic study of coals, associated kerogens, and isolated vitrinites at the limit between subbituminous and bituminous coal…………………………...33 3.1 Introduction……………………………………………………………………………...34 3.2 Geological setting……………………………………………………………………..…36 3.3 Methods……………………………………………………………………………….…38

3.3.1 Sample preparation…………………………………………………………………………...38

3.3.2 Elemental analyses and Rock-Eval pyrolysis………………………………………………...38

3.3.3 Microscopy…………………………………………………………………………………...39

3.3.4 Attenuated total reflectance Fourier-transform infrared spectroscopy……………………….39

3.3.5 Curie point-pyrolysis-gas chromatography-mass spectroscopy…………………………...…40 3.4 Results………………………………………………………………………………...…40

3.4.1 Elemental analyses and Rock-Eval pyrolysis……………………………………………...…40

3.4.2 Microscopy………………………………………………………………………………...…41

3.4.3 ATR FT-IR spectroscopy……………………………………………………………………..44

3.4.4 CP-Py-GC-MS………………………………………………………………………………..46 3.5 Discussion……………………………………………………………………………..…46

3.5.1 Differences in elemental composition, vitrinite reflectance and Rock-Eval parameters….….46

3.5.2 Variations in the chemical structure of bulk samples and vitrinites………………………….53

3.5.2.1 Polar compounds………………………………………………………………………...….54

3.5.2.2 Aromatic compounds…………………………………………………………………….…55

3.5.2.3 Aliphatic compounds……………………………………………………………………….58

3.5.3 Maturity trends………………………………………………………………………………..60 3.6 Conclusions……………………………………………………………………………...62 4 Bolsovian (Pennsylvanian) tropical peat depositional environments: The example of the Ruhr Basin, Germany……………………………………………….…………………………….…64 4.1 Introduction……………………………………………………………………………...64 4.2 Geological Setting…………………………………………………………………….…66 4.3 Samples and Methods…………………………………………………………………....69

4.3.1 Samples…………………………………………………………………………………….....69

4.3.2 Elemental and bulk analyses, Rock-Eval pyrolysis………………………………………..…69

4.3.3 Organic petrography………………………………………………………………………….71

4.4 Results………………………………………………………………………………...…72

4.4.1 Elemental and bulk analyses, Rock-Eval pyrolysis…………………………………………..72

4.4.2 Organic petrography………………………………………………………………………….72 4.5 Discussion……………………………………………………………………………..…79

4.5.1 Trends within the Bolsovian coal layers of the Ruhr Basin………………………………..…79

4.5.2 Depositional environments…………………………………………………………...... ……80

4.5.2.1 Dorsten Formation………………………………………………………………………………….83

4.5.2.2 Lembeck Formation………………………………………………………………………………...86

4.5.3 Comparison of Bolsovian and Duckmantian coals of the Ruhr Basin………………………..87 4.6 Conclusion……………………………………………………………………………….88 5 General discussion and outlook…………………………………………………………….…90 5.1 Peat deposition in the Ruhr Basin…………………………………………………….…90 5.2 General maturation trends……………………………………………………………….94 5.3 Characterisation of kerogen on a molecular level……………………………………….96 5.4 Outlook………………………………………………………………………………..…99 References………………………………………………………………………………….….100 List of figures………………………………………………………………………………….127 List of tables……………………………………………………………………………..…….131

Abbreviations a.u. Arbitrary unit µFT-IR Micro FT-IR A Ash yield ATR Attenuated total reflectance B Benzenes BNT Benzonaphthothiophenes C Carbon C=C Aromatic carbon bonds C=O Carbonyl or carboxyl group

CH2 Methylene group

CH3 Methyl group

CHx Methyl or methylene group

CO2 Carbon dioxide CP Curie Point DBT Dibenzothiophenes DOM Dispersed organic matter DTGS Deuterated triglycine sulphate FID Flame ionization detector FT-IR Fourier-transform infrared spectroscopy GC Gas chromatography

GWIAC Groundwater index H Hydrogen HC Hydrocarbon HCl Hydrochloric acid He Helium HF Hydrofluoric acid HI Hydrogen index I Inertinite Iso iso-alkanes L Liptinite

LRr Random liptinite reflectance

MCT Mercury cadmium telluride MIR Mid infrared MM Mineral matter MS Mass spectroscopy N Sum of n-alkanes and -alkenes N Nitrogen n Refraction index

Na Normal alkanes Na Naphthalenes

Ne Normal alkenes O Oxygen

Ocalc Calculated oxygen content OM Organic matter P Phenanthrenes Ph Phenols RB Ruhr Basin S Sulphur S1 Free hydrocarbons vaporised at 300 °C S2 Hydrocarbons generated between 300 °C and 650 °C

S3 CO2 generated between 300 °C and 400 °C T Thiophenes TC Total carbon TIC Total inorganic carbon

Tmax Temperature of maximum hydrocarbon yield TOC Total organic carbon TS Total sulphur UV Ultraviolet V Vitrinite VI Vegetation index VM Volatile matter vol% Volume %

VRr Random vitrinite reflectance

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1 Introduction

1.1 Geological background and motivation After several decades of intense hard coal production, the last subsurface coal mine in the western German Ruhr Area closed in 2018. The coal industry, whose rapid development in the 19th century led to the formation of today's metropolitan Ruhr region, enabled the access to hard coals of different maturities and lithotypes and thus their exploration. Particularly in the course of the 20th century, this led to some fundamental publications in the field of coal petrology and organic geochemistry. The Ruhr Basin forms part of the large Central European coal province, which stretches from England in the West to Poland and Ukraine in the East (Fig. 1.1a). During the Pennsylvanian the formation of vast tropical peat mires north of the emerging Variscan Orogen led to the deposition of more than 100 coal seams in a several km thick sedimentary succession, which is divided into several stratigraphic units (Hahne and Schmidt, 1982; Witzke, 1990). First peat mires formed during the Marsdenian, but most coals within the Ruhr Basin are of Langsettian and Duckmantian age, cyclically deposited between sediments of fluvio-deltaic, lacustrine and marine origin (Scheidt and Littke 1989; Drozdzewski and Wrede 1994; Juch et al., 1994; Süss et al., 1996). The northwards progradation of the Variscan Mountains south of the Ruhr Basin, caused large-scale folding of the Pennsylvanian layers (Fig. 1.1b). Because of the complex structural history of the paralic Ruhr Basin, hard coals cover a vast range of thermal maturities from high volatile bituminous to anthracite rank (Drozdzewski et al., 2009; Uffmann and Littke, 2011). This situation facilitated a series of fundamental research on indicators of thermal maturity of coals (e.g. Bartenstein and Teichmüller, 1974; Teichmüller and Teichmüller, 1979). The availability of coal seam profiles, cored from several exploration wells, further led to numerous studies of the depositional conditions of the palaeo-peat mires using the methods of organic petrology (e.g. Stach et al., 1982; Teichmüller, 1989; Dehmer, 2004 and sources herein), organic geochemistry (Radke et al., 1980; Littke and Ten Haven, 1989; Littke et al., 1990) and palynology (Grebe, 1962; Hartkopf-Fröder, 2005 and sources herein). With

1 the end of mining activities, accessibility of coals will become restricted, making further research more difficult or even impossible.

Fig. 1.1 a) The Variscan Foredeep in Central Europe and locations of Pennsylvanian coal mining areas (modified from Jasper et al., 2009), b) simplified profile of the Ruhr Basin (modified after Wrede and Ribbert, 2005).

In the context of this work, the structural changes occurring in coal and in particular those occurring in vitrinite upon thermal maturation will be investigated and put into context with the depositional conditions of the former peat mires and surrounding sediments. This is accomplished on the one hand by the structural analysis of a maturity series of coal and vitrinite samples covering the ranks of subbituminous to anthracite stages and on the other hand by a detailed petrographic and structural analysis of drilled profiles of Bolsovian coal seams of subbituminous to high volatile rank. In the following, the deposition of terrigenous organic material and especially that of peats as well as the processes occurring during their coalification and maturation are described. Subsequently, the methods used

2 in this thesis for the investigation of depositional conditions as well as for the elucidation of the chemical structural of the kerogen are outlined. Part of this introduction (chapter 1.2) is taken from a more extended review article of Littke and Zieger (2019a) and has been slightly modified.

1.2 Deposition of coal and terrigenous organic matter The depositional setting determines, among other factors, the amount and quality of organic material, i.e., total organic carbon (TOC) content and kerogen types. The deposition of sediments rich in organic matter is taking place in either terrestrial, lacustrine, or marine environments in which organic matter is produced faster than it can be destroyed (Tourtelot, 1979). These sediments are usually fine-grained and either dominated by silicate (clays and quartz) or carbonate minerals; they usually develop under a permanent water cover with bottom waters being commonly oxygen-depleted. An important exception to that is peat, which usually develops in humid climates and in areas with limited run-off of surface water leading to a high water level at or above the peat surface. Organic matter quantity and quality greatly varies even if environments favouring organic matter accumulation are compared. For example, deltaic and fluvial sediments as well as coals generally do not contain much organic matter derived from aquatic organisms. However, they often contain tissues of higher land plants in great quantity. This type of organic material (kerogen type III; Fig. 1.2a) is usually less hydrogen-rich and less oil-prone than the aquatic type. In contrast, marine and lacustrine sediments with high organic matter contents are commonly characterized by a predominance of aquatic organic matter, either of planktonic or benthic origin. This aquatic organic matter is usually rich in hydrogen, contains little oxygen and is classified as kerogen type I or II (Fig. 1.2a).

A major part of the terrestrial organic matter in sediments is preserved either in the form of dispersed particles in fluviatile siliciclastic rocks or in peats or coal, with the latter showing the highest concentrations in TOC (>50 wt%) of all organic matter-rich sediments. The formation of these sediments is restricted to regions proximate to or within areas of intense bioproductivity in humid climates with permanent freshwater and nutrient supply, where a large proportion of the produced biogenic material can be preserved under wet, oxygen-depleted conditions. Such prerequisites are given in peatlands, which

3 are either fed solely by rainwater (ombrotrophic mires) or by a combination of precipitation, flowing water and/or groundwater (rheotrophic mires). Ombrotrophic mires form raised, or more rarely blanked bogs, which are characterized by acidic pH values and low siliciclastic inputs, and thus have a relatively lower nutrient supply compared to mires that form under rheotrophic conditions (Moore, 1995). Ombrotrophic peats and the resultant coals are characterized by low ash/mineral contents. Depending on the height of their water table, rheotrophic mires are classified into fens, swamps, and marshes. Resultant peats are usually characterized by higher ash/mineral contents including higher sulfur contents as compared to ombrotrophic peats. Processes such as peat growth, subsidence, or eustatic sea level rise can affect the hydrological conditions in a peat-mire towards ombrotrophic or rheotrophic, respectively (Moore, 1995).

Fig. 1.2 a) Kerogen types and microscopy pictures (b-d) showing respective organic particles (macerals). a) van Krevelen Diagram with the atomic H/C vs. O/C ratios of some organic-rich rocks and kerogens (modified after van Krevelen, 1993, data from Tissot and Welte, 1984; Bandopadhyay and Mohanty, 2014; Zieger et al., 2018), b). Botryococcus algae (type I kerogen) under UV-light (from Rippen et al., 2013), c) Tasmanales algae (type I kerogen) under UV-light (from Stock et al., 2017), d) Carboniferous coal with V=vitrinite, C=cutinite, S=sporinite, and I=inertinite.

Most modern peatlands are situated within the temperate climate zone and are concentrated in the northern hemisphere, where the largest areas of peat formation are in Russia, Canada, Fennoscandia, and NW Europe. Bioproductivity is generally higher in tropical regions (Field et al., 1998) leading to thicker peat layers within tropical mires. Although tropical peats only contribute little to the total area of modern wetlands (~10%), they account for 18–25% of the global peat volume (Page et al., 2011). Different from the peatlands of the northern hemisphere, in which the vegetation is dominated by Sphagnum and herbaceous plants, recent mires in tropical regions are characterized by a woody, rainforest vegetation, comparable to that of many palaeomires from the Carboniferous, Jurassic and Miocene that led to the formation of coal seams (Staub and Esterle, 1994).

Upon burial, peats grade into lignite, subbituminous and bituminous coal, anthracite, and finally graphite. The type of vegetal material plays an important role in the deposition and preservation of organic matter in sediments. While the polysaccharides cellulose and hemicellulose have a low resistance to microbial degradation, the wood forming substance lignin is relatively stable under anaerobic conditions and is an important precursor of vitrinite (Fig. 1.2a, d), a typical type III kerogen and abundant constituent of coal and the organic matter in most fluvio-deltaic sediments (van Krevelen, 1993; Hatcher et al., 1982). Cellulose is, however, still present in lignites (Fabbri et al., 2009; Stock et al., 2016). Lignin, on the other hand, can also be degraded substantially according to recent investigations (Waggoner et al., 2017). Other important macerals of similar resistance are sporinite, derived from spores and pollen of vascular plants, cutinite derived from waxy protective layers (cuticula) of higher land plants, and inertinite, oxidized, carbon-rich particles resulting from peat fires or fungal reduction (Fig. 1.2d).

Sites for deposition of dispersed organic matter on continents are, apart from lakes, lowlands flooded temporarily by rivers, but also backwaters, where conditions similar to those in lakes may exist. In humid climate zones, typical sites for the deposition of organic particles are overbank deposits and crevasse splays. Interestingly, organic matter in such fluvial systems is usually more degraded than that in peat, i.e., the petroleum generation capacity is much lower in fluvial sedimentary rocks than in the adjacent coals (Jasper et al., 2009).

Coal deposits and related plant fossils reflect very well the terrestrial plant evolution. The terrestrial plant species contributing to the organic matter preserved in sediments evolved

5 and diversified upon geologic times. First land plants appeared during the Middle Devonian and developed to vascular plants during the early Silurian (Edwards et al., 1983), delimitating the occurrence of sediments rich in terrestrial organic matter to later dates. During the late Devonian, spore producing pteridophyte and early gymnosperm trees populated the continents, leading to an adaptive radiation of land plants and to the formation of extended tropical peat mires. During the Pennsylvanian, these mires covered large areas of present-day North America and Europe. Coal-bearing sequences derived from such tropical, humid environments can reach thicknesses of several kilometres with numerous coal seams as well as dispersed terrigenous organic matter (Scheidt and Littke, 1989). Most Permian coals deposited on the former Gondwana continent at high southern latitudes in humid cool-temperate climates. The higher inertinite (see Fig. 1.2d) contents in these coals compared to coals that formed during the late Carboniferous are interpreted as indicators for seasonal changes (Taylor et al., 1989). Gymnosperms dominated the peat forming vegetation during the Triassic and Jurassic and angiosperms dominate the terrestrial vegetation since Cretaceous times (Niklas, 1986; Robinson, 1990). The relatively young C4 plants developed during the Oligocene (Christin et al., 2008; Vicentini et al., 2008) and expanded during Late Miocene to Pliocene times (Cerling et al., 1997). C4 species like grasses and sedges make up the ground cover of modern fens and marshes (Rydin and Jeglum, 2013) and have a 25% share in today’s terrestrial net bioproductivity (Still et al., 2003). Because angiospermous lignin is more easily degraded than gymnospermous or pteridophytal lignin, (Hedges et al., 1985; Hatcher et al., 1989), vitrinite particles of coals or fluvio-deltaic deposits that formed from these species tend to be more degraded/detrital compared to vitrinite from Carboniferous coals.

1.3 Formation and maturation of vitrinite After deposition, the original organic matter is subject to a series of processes that ultimately lead to the formation of fossil organic substance, recognizable as macerals. In particular, the processes that precede the formation of vitrinite will be discussed here. Vitrinite of Carboniferous coals is mainly derived from periderm of stems (bark of Lycophyta), roots (Stigmaria; lycopsids), and wood of Cordaites, and to a lesser content from leaves and twigs (Raistrick and Marschall, 1939; Taylor et al., 1998). Lapo (1978) stated that the composition of vitrinite of Carboniferous coals is less variable than that of

Jurassic vitrinites, which he explained by less complex plant communities and species richness during the Palaeozoic. The early phase of peatification of the vitrinite precursor material is characterized by the transformation of the primary humic substances cellulose, hemicellulose, and tannin into humic acids, fulvic acids and later to the largely insoluble macromolecule group of humins (Given and Dyrkacz, 1988). The rather resistant biopolymer lignin is preserved and undergoes some reorganization, including demethylation and dihydroxylation (Hatcher et al., 1982; Stout et al., 1988). A detailed review of these processes leading to the molecular transformation of lignin to huminite was published by Hatcher and Clifford (1997).

Fig. 1.3 a) Simplified molecular structure of vitrinite at different maturity stages, b) changes in aromaticity, ring condensation and dimension of aromatic clusters in vitrinite in relation to carbon content (redrawn from Taylor et al., 1998, based on Teichmüller and Teichmüller, 1968).

During this stage of biochemical gelification, the coal kerogen loses hydroxyl, carboxyl, methoxy, and carbonyl groups, leading to a relative increase of carbon (Taylor et al., 1998). Depending on the degree of decomposition and gelification, huminite is divided into the maceral subgroups telohuminite, detrohuminite or gelohuminite being the

7 precursors of telovitrinite, detrovitrinite and gelovitrinite at the beginning of the subbituminous stage, respectively (ICCP, 1998; Sýkorová et al., 2005). During further geochemical gelification, taking place at the interface between lignite and subbituminous coal ranks, further dehydroxylation and condensation reactions lead to formation of phenols at cost of catechol-like structures (Hatcher et al., 1989). As coal rank further progresses upon burial towards the high volatile bituminous stage, condensation of phenols to diaryl ethers and the new formation of aromatic moieties occurs (Hatcher et al., 1992). During this coalification stage, the process of bituminization, which describes the formation of petroleum-like substances from the lipid constituents of a coal, causes the impregnation of the kerogen structure with solvent extractable hydrocarbons, which are retained up to the beginning of the low volatile bituminous coal stages (Littke et al., 1989, 1990; Levine, 1993). By means of the elemental composition, type III kerogen progressively loses oxygen and hydrogen upon thermal maturation, following the pathway described by van Krevelen (1993) (Fig. 1a). The relative aromaticity, dimension of aromatic clusters and thus the reflectance of vitrinites steadily increase as the processes described prograde with increasing burial depth until the anthracite rank (Fig. 1.3, Teichmüller and Teichmüller, 1968). The further loss of hydrogen is a result of the formation and emission of light hydrocarbons, mainly methane from the low volatile bituminous stage on.

1.4 Analytical techniques 1.4.1 Overview For the classification and structural elucidation of kerogen, several methods based on different principles are available. The insoluble nature of kerogen requires the methods used to reveal its chemical structure either to be direct and non-destructive or to be destructive, allowing to interpret the products formed upon decomposition of the original substance. The elemental composition of kerogen holds information on both, type and maturity of the kerogen (van Krevelen, 1993). It is usually measured by detecting the amount of oxidation products that emerge during heating to certain temperatures under oxidative conditions. An overview on these techniques and the interpretation of the gained information is given in Durand and Monin (1980). The weight percentages of carbon (organic and inorganic), sulphur and hydrogen presented in this study are derived

8 by such oxidative methods and analytical conditions will be further described in the methodology parts of chapters 2 to 4. Some other chemical methods used to study the structure of kerogen, including oxidative degradation, reduction and hydrolysis using reactants are described elsewhere (Vitorović, 1980; Whelan and Thompson-Rizer, 1993). Destructive techniques used most frequently in turn of the analysis of kerogen are pyrolysis methods. In an inert atmosphere, kerogen is thermally cracked either in an open or in a closed system. The produced products can then be directly detected or further separated which allows for a more detailed analysis of the pyrolysates. A detailed review on the different pyrolysis methods employed in the field of organic geochemistry is given by Meuzelaar et al. (1982) and Larter and Horsfield (1993). Non-destructive, spectral methods based on the excitation of atomic nucleoids, electrons or molecular vibrations at different wavelength include Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy, electron spin spectroscopy and infrared spectroscopy (Whelan and Thompson-Rizer, 1993). Some of the techniques described, among others, can be combined with microscopy, allowing for the analysis of kerogen in-situ. The type of kerogen and its maturity is most often accessed by optical methods, with vitrinite reflectance being the most frequently used parameter (Hartkopf-Fröder et al., 2015). In the following, the methods employed in this work will be introduced and the principles they are based on described in more detail.

1.4.2 Organic petrology Microscopical methods, as employed in organic petrography, are one the one hand used to identify the type of kerogen (maceral analysis) and on the other hand to physically measure indicators for its thermal maturity (vitrinite reflectance). The microscopically visible kerogen can be differentiated into macerals (Fig. 1.2d). The basic concept of assigning the recognizable macerals to the original material either derived by higher land plants or algae was first picked up by Stopes (1935) and then expanded, further developed and standardized in the course of the 20th century (ICCP, 1998, 2001; Taylor et al., 1998; Sýkorová et al., 2005; Pickel et al., 2017). The maceral composition is determined by the point count method using incident white-light (Fig. 1.2d), and for the detection of liptinite macerals blue- or UV-light (Fig. 1.2b, c). On an equidistant grid, covering the whole polished section a minimum of 500 macerals are identified and counted, one on each grid point. Based on the relative composition of macerals, information on the conditions

9 during the deposition of the original organic matter can be gained in addition to information of industrial interest. Especially in the field of coal petrology, several indices based on maceral composition have been proposed and used in order to determine the dominant vegetation type, water levels, climatic and redox conditions during peat accumulation (e.g. von der Brelie and Wolf, 1981; Diessel, 1986; Calder et al., 1991; Kalkreuth et al., 1991; Petersen and Ratanasthien, 2011). Detailed maceral analysis is, however, only applicable for coal ranks below the low volatile stage, since the optical properties especially of the liptinite group macerals change considerably above this degree of maturity.

Vitrinite reflectance mirrors the irreversible chemical structural changes occurring upon thermal maturation. Its correlation with other maturity parameters is thoroughly studied (e.g. McCartney and Teichmüller, 1972; Burnham and Sweeney, 1989; van Krevelen, 1993; Mukhopadhyay, 1994 and sources herein). The standard procedure requires oil immersion and a magnification 500x. Particles are measured under incident, non- polarised light filtered to a wavelength of 546 nm. On coal samples, because of the usually high abundance of telovitrinites, 100 random particles are measured (Taylor et al., 1998).

1.4.3 Pyrolysis experiments

Another important maturity parameter is Tmax, the temperature of maximum hydrocarbon yield generated between 300 °C and 650 °C during Rock-Eval pyrolysis. This widely employed open system pyrolysis technique was first introduced by Espitalié et al. (1977) and a modified version was described by Behar et al. (2001). During the isothermal temperature of 300 °C (3 min) in a nitrogen atmosphere, the release of free hydrocarbons vaporized from the kerogen structure is monitored as the S1 peak by a flame ionization detector. Upon further heating of 25 °C/min to a final temperature of 650 °C, thermally cracked hydrocarbons are measured as S2 peak. Both, S1 and S2 peaks are expressed as mg HC/g rock. During the heating from 300 °C up to 400 °C, an infrared detector records the amount of CO2 produced and released from the kerogen as S3 peak, expressed as mg

CO2/g rock. By normalizing S2 and S3 peaks to the TOC content of the analysed sample, hydrogen (HI) and oxygen (OI) indices are obtained. These correlate to the atomic H/C and O/C ratios used in the van Krevelen diagram and thus allow for kerogen typing.

Another open system pyrolysis technique used in the framework of this study is the Curie Point (CP) pyrolysis. This method allows rapid thermal decomposition of the kerogen at

10 the chosen temperatures (within seconds) and has been employed for the characterisation of kerogen since the 1960s (Giacobbo and Simon, 1964; van Grass et al., 1980; Meuzelaar et al., 1984a, b; Nip et al., 1986, 1988). A small amount of sample is either attached to a ferromagnetic filament or packed into a metal foil with a certain CP and then inductively heated by a high frequency coil until the chosen metal loses its magnetic properties. The pyrolysates are then transported in a stream of inert gas, here helium, for further separation into a gas chromatograph coupled to a mass spectrometer (Meuzelaar et al., 1982). Because of the rapid heating, rather than step by step production of hydrocarbons during other pyrolysis experiments, the chosen CP temperature has a decisive effect on the moieties produced (al Sandouk-Lincke et al., 2014).

1.4.4 Fourier-transform infrared spectroscopy The principle function of Fourier-transform infrared spectroscopy (FT-IR) is based on the excitation of molecular vibrations by infrared radiation. Depending on the measuring mode, the energy lost during excitation is detected either as absorption, transmission or reflection. The mid-infrared range of the electromagnetic spectrum (2.5-25 µm) is used for the structural elucidation of organic substances. Depending on the type of oscillation, e.g. symmetric or asymmetric stretching vibration, scissoring or bending vibration, the different functional groups, e.g. methyl or hydroxyl groups, absorb energy at different but distinctive wavelengths. In the course of this work, the methods of attenuated total reflection and micro FT-IR operated in reflectance mode were used. Attenuated total reflectance (ATR) is a method that requires the samples contact to a crystal of defined refractive index and geometry. The infrared beam penetrates the sample and reflected radiation is directed to a detector after re-entering the crystal. Advantages of this technique for the analysis of kerogen are the lack of need for sample preparation and the good signal to noise ratio (Li et al., 2007). The micro FT-IR analysis using reflectance mode requires a smooth surface, so that the introduced infrared beam does not get diffracted. Using this method, the spectra of single particles within a matrix of minerals or kerogen of different type can be obtained in situ. A comprehensive review on the different techniques in the field of geological science was published by Chen et al. (2015).

1.5 Outline of this thesis Following this introductive first chapter, in chapter 2, the effect of thermal alteration on a set of vitrinites and sporinite from a natural maturity sequence of Pennsylvanian coals from within the Ruhr Basin is analysed and discussed. The samples maturities range from subbituminous to semi-anthracite coal ranks. Chemical and structural changes occurring in the kerogen are recorded by elemental analyses, vitrinite reflectance measurements, Rock-Eval pyrolysis, different FT-IR techniques and CP pyrolysis gas chromatography mass spectroscopy (CP-Py-GC-MS) performed at two temperatures. The chapter is based on the peer reviewed publication Zieger et al. (2018).

In chapter 3, the effect of depositional environment on the chemical structural conditions of dispersed organic matter, coals and isolated vitrinite of nine coal seam intervals from the Ruhr Basin of Bolsovian age is analysed. The samples lie within a narrow maturity range of subbituminous to high volatile bituminous coals. Analytic methods employed in this study are organic petrology, elemental analyses, Rock-Eval pyrolysis, ATR FT-IR and CP-Py-GC-MS at two CP temperatures. This chapter is based on the pee reviewed article Zieger et al. (2020).

Chapter 4 is based on the peer reviewed publication Zieger and Littke (2019). In this chapter, the depositional conditions of during the formation of the palaeo-peat mires of eleven coal seams from the Bolsovian Ruhr Basin are analysed by using ultimate and proximate analyses, Rock-Eval pyrolysis and organic petrology. The seams were sampled in profiles, allowing the reconstruction of the evolution of depositional conditions from base to top of the seams.

In the final chapter 5, the results obtained from the studies presented in chapters 2 to 4 are set in context to each other in terms of maturity trends and their relation to depositional conditions during the formation of the original organic matter as well as to the framework of depositional conditions prevailing during the Pennsylvanian of the Ruhr Basin. This chapter is partially based on the publication Littke and Zieger (2019b).

2 Chemical and structural changes in vitrinites and megaspores from Carboniferous coals during maturation Abstract Chemical and structural changes occurring in kerogen upon thermal alteration are identified and analysed based on a set of naturally matured Carboniferous coals from the Ruhr Basin (Germany). For this purpose, handpicked vitrinite from eleven samples comprising a maturity range from 0.55 to 2.86% VRr was analysed using attenuated total reflectance infrared spectroscopy (ATR FT-IR) and Curie Point pyrolysis gas chromatography/mass spectroscopy (CP-Py-GC-MS) at two pyrolysis temperatures. Additionally, reflectance µFT-IR was used to assess variations in the proportions of functional groups in megaspores from five oil mature coal samples. Infrared spectra of the vitrinites show a clear decrease in aliphatic CHx absorbance in favour of aromatic CH absorbance, pointing out an increase in aromaticity with increasing maturity. Spectra of megaspores are dominated by the absorbance of the aliphatic

CHx stretching region and reveal the loss of C=O groups with increasing maturity, while the degree of aromaticity (γCH/νCHx) increases slowly compared to that of the vitrinite spectra. Vitrinites pyrolysed at 590 °C show higher yields in aliphatic hydrocarbons than those pyrolysed at 764 °C, while at the higher pyrolysis temperature the yields in aromatic compounds, including phenols and sulphur-containing aromatics are higher. The aromatic fraction of the pyrolysates, in particular the relative amount of polyaromatics increases upon maturation, while the phenolic fraction decreases in favour of benzenes. Major processes leading to these structural and chemical changes in vitrinites and megaspores are defunctionalisation of oxygen-containing groups, the loss of aliphatic compounds and the formation of monoaromatic molecules. These prevail over the condensation of aromatic ring- structures, which is, however, evidenced by increasing proportions of polyaromatic fractions in the pyrolysed vitrinites.

2.1 Introduction Two of the most important maturity parameters employed for the classification of kerogen and for the reconstruction of burial and temperature histories of sedimentary rocks are the vitrinite reflectance (VRr) and the temperature of maximum Rock-Eval pyrolysis yield (Tmax). Throughout the last decades, many authors have investigated the relationship between these parameters and the concomitant structure and chemistry of organic material (Hartkopf-Fröder et al., 2015). In this context, the objective of this study is to contribute to this research and to identify and quantify chemical changes during thermal maturation in a representative, naturally matured sample set. For this purpose, a sequence of 11 coals from different strata within the northwest German Ruhr Basin, with vitrinite reflectance ranging widely between 0.56% and 2.86%, has been analysed regarding its chemical-structural composition. Geochemical data on kerogen structure and chemistry is often difficult to interpret, since kerogen is composed in most rocks of mixtures of different organic compounds of various biogenic origin. In order to characterize specific types of organic matter, i.e. specific macerals, either hand-picking or microspectroscopic techniques are required. Here, handpicked vitrain layers were studied using attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) and Curie Point-pyrolysis-gas chromatography-mass spectrometry (CP-Py-GC-MS). Vitrain/vitrinite is common in humic coals and comprises products of the chemical alteration of the complex macromolecule lignin, which features strong resistance against degradation under anaerobic conditions (Hatcher and Clifford, 1997). Its molecular composition is based on coniferyl- and p-coumaryl-alcohol units (Derenne and Largeau, 2001), undergoing dehydroxylation and demethylation (Hatcher, 1990), followed by the dehydroxylation of side chains and catechols during coalification (Hatcher and Clifford, 1997). Although changes in the relative distribution of chemical functional groups of vitrinite with increasing thermal maturity are thoroughly documented in several spectroscopical studies (Kuehn et al., 1982; Schenk et al., 1990; Mastalerz and Bustin, 1993, 1994, 1997; Chen et al., 2012; Li et al., 2013), more detailed information about changes in the structure of its macromolecules, derived by GC-MS methods is rather scarce (Schenck et al., 1981, 1983; Meuzelaar, 1984b; Radke et al., 1985; Senftle et al., 1986; Kruge and Bensley, 1994; Veld et al., 1994). In addition to vitrain layers, megaspores from some of the oil-mature samples the vitrain is derived from were characterised by reflectance µFT-IR spectroscopy. Sporinite, the petrographic representative of spores and pollen in sediments, represents kerogen type II and is the most common liptinite maceral in Westphalian coals from the Ruhr Basin (Littke, 1987). Due to their size of up to 2 mm, megaspores are easily assessable at a microscopic resolution. Fossil megaspores consist of the 14 macromolecule sporopollenin. This biopolymer is extremely resistant to degradation under anoxic conditions and can remain nearly unaltered throughout geologic times (Fraser et al., 2012). The molecular structure of sporopollenin is complex and still not fully resolved. While recent spore exines are built up by units derived by cinnamic acids, such as p-coumaric or ferulic acids, fossil sporopollenin additionally contains large amounts of aliphatic moieties (de Leeuw et al., 2006). The impact of artificial maturation on spore chemistry with FT-IR and other spectroscopic techniques has been subject to several studies in the past years (Fraser et al., 2014; Bernard et al., 2015). Al Sandouk-Lincke et al. (2013) combined FT-IR methods with CP-Py-GC-MS for the analyses of the alteration of sporopollenin from Lower Jurassic bisaccate pollen. With increasing maturity, sporopollenin undergoes decarboxylation in an immature stage, followed by the loss of aliphatic groups through thermal cracking and the associated formation of hydrocarbons (Yule et al., 2000).

2.2 Geological setting

The Ruhr Basin in Northwest Germany is bounded by the Rhenish Massif to the south and extends 80 km northwards. It stretches 150 km in southeast-northwest direction and holds one of the most important Carboniferous coal deposits of Western Europe, which has been intensively mined since the 19th century. The basin forms part of the fold and thrust belt in the northern part of the Variscan Orogenic Belt in Central Europe. The Late Palaeozoic fold belt extends for ca. 2500 km from Ireland in the northeast to Poland in the southwest (Ziegler, 1990). During the Silesian, up to 7000 m marine, deltaic and fluviatile sediments deposited within the strongly subsiding Ruhr foreland basin (Drozdzewski, 1993, 2005) beginning with the sedimentation of 1300 m fine grained material of marine origin during the Namurian A and B (Wrede, 2000; Wrede and Ribbert, 2005). In the southern part of the basin, these Carboniferous layers crop out, whereas they are discordantly overlain by up to 2000 m predominantly Cretaceous Mesozoic sequences in the central and northern part (Fig. 2.1a). The Carboniferous of the Ruhr Basin is characterised by complex fold tectonics with folds predominantly striking in southwest-northeast direction (Drozdzewski and Wrede, 1994). Throughout the Namurian C, sedimentary conditions changed towards paralic and the formation of in some instances thick peat (now coal seams) began, intercalated by deltaic to marine sands and silt (Süss, 1996; Drozdzewski, 2005; Suess et al., 2007). Favoured by a warm and humid climate in an equatorial setting (Ziegler, 1990), the development of coal forming swamps, primarily populated by lycopsid and cordaite trees (DiMichele and Phillips, 1994), peaked during the Westphalian A 15 and B (Scheidt and Littke, 1989; Drozdzewski and Wrede, 1994; Wrede and Ribbert, 2005). Throughout the Westphalian, within a time span of about 11.5 Ma (Menning et al., 2005), the Ruhr Basin was filled with more than 3000 m of alternating fine-grained and sandy sediments, containing more than 100 coal seams of varying thickness and maturities ranging from high volatile bituminous to anthracite due to deep burial at the end of the Carboniferous (Wrede and Ribbert, 2005; Uffmann and Littke, 2013).

Fig. 2.1 a) Profile and location of the Ruhr Basin (after Stancu-Kristoff and Stehn, 1984), and b) its Westphalian coal bearing stratigraphic units with the analysed coal seams highlighted (after Strehlau, 1990).

The northwards movement of the Variscan deformation front during the Westphalian is reflected by the successive shift of maximum coal content from southeast to northwest (Drozdzewski, 1992, 1993, 2005). Coal formation in Northwest Germany continued until the Stephanian and stagnated when climate progressively changed towards dryer conditions 16

(Roscher and Schneider, 2006; Izart et al., 2012; Michel et al., 2015). In the Ruhr area up to 2500 m of Palaeozoic sediment eroded (Littke et al., 1994, 2000), so that the youngest Carboniferous layers found within the basin are of Westphalian C age (Drozdzewski and Wrede, 1994; Uffmann and Littke, 2013).

2.3 Material and methods 2.3.1 Samples Eleven samples from different locations and coal seams within the Ruhr Basin were selected based on their thermal maturities. The sample set covers the period from Westphalian A1 (Witten Formation) to C1 (Lembeck Formation) (Fig. 2.1b) and vitrinite reflectance values

(VRr) range between 0.55% and 2.86% (Tab. 2.1). Vitrain layers from each coal sample were carefully handpicked and ground to powder prior to the elemental (TOC, S, C, H, N) and geochemical analyses (Rock-Eval, ATR FT-IR, CP-Py-GC-MS). Polished sections of the vitrain powders were prepared and their purity was determined microscopically (Tab. 2.1).

Polished blocks of the bulk coal samples were produced for the vitrinite, liptinite (LRr) and µFT-IR reflectance measurements.

2.3.2 Petrographic and bulk analyses The petrographic analyses of coals and vitrain powders were performed in 500x magnification using a Zeiss Axioplan incident light microscope equipped with an Epiplan-NEOFLUAR

50x/0.85 oil objective. VRr was measured on 100 points per sample on telocollinites, following a standard procedure. The determination of maceral groups was performed after the Stopes- Heerlen system as outlined in Taylor et al. (1998). Details of sample preparation and microscopic equipment are described in Littke et al. (2012) and Sachse et al. (2011). Rock-Eval Pyrolysis was performed based on the procedure specified in Espitalié et al. (1977) with a Rock- Eval 6 instrument (Vinci Technologies). The resulting peaks were normalised to the total organic carbon content (TOC), measured with a LiquiTOC II analyser (Elementar) along with the amount of total inorganic carbon (TIC). The elemental composition (N, H and S) was measured with a Leco CHN-628 elemental analyser following the procedure defined in DIN 51732 (2014) and a total evaporation analyser (Leco200), respectively. The oxygen content was calculated by difference.

Sample

Tab. K H G D A C B E F J I

2.1

Westphalian

Origin, petrographic characteri A1 A2 A2 B1 B1 B1 B1 B2 B2 C1 C2

Geological age and origin and age Geological

Formation Lembeck Bochum Bochum Dorsten Witten Essen Essen Essen Essen Horst Horst

Ibbenbüren 74

Girondelle 5 Girondelle Zollverein 2 Criemhild Rübezahl Karl 1/2 Karl 2 Albert Seam

H N P I

stics (V=vitrinite, L=liptinite, I=inertinite, and MM=mineral matter), Rock 2.86 1.44 1.07 1.04 0.99 0.99 0.86 0.85 0.76 0.56 VR (%) 0.9 r

0.64 0.33 0.20 0.21 0.19 LR (%) ------

elemental composition of isolated vitrain layers. (vol%) 96 98 82 98 95 94 91 99 89 96 93 V Petrography

(vol%) L 0 0 5 0 2 3 3 1 5 2 3

(vol%) 4 2 9 2 3 3 6 0 5 2 4 I

(vol%) MM 0 0 0 0 4 0 0 0 1 0 0

T 607 493 459 456 458 453 447 445 435 452 428 (°C) max max

(mg HC g HC (mg 181 183 173 174 166 142 185 182 103 Rock HI HI 10 80 -

TOC) - Eval data Eval

(mg CO (mg

OI 15 2 5 4 3 4 5 5 5 4 5 3

TOC)

(wt%) TOC TOC 91.3 89.3 84.5 85.4 84.9 83.8 84.7 81.7 82.7 85.2 72.1 - Eval data of the coal samples and

(wt%) 91.3 89.3 84.6 85.4 85.2 83.8 84.7 81.7 82.7 85.3 72.3 TC TC Elemental composition Elemental

(wt%) 3.1 4.7 5.4 5.6 5.3 4.9 H - - - - -

(wt%) 1.3 1.7 1.6 1.8 1.9 1.2 N - - - - -

(wt%)

0.9 1.1 1.2 1.5 1.0 1.1 S - - - - -

(wt%) 20.4 O 3.5 3.2 8.1 8.4 6.6 - - - - - calc

2.3.3 FT-IR All infrared spectra were collected within the mid infrared (MIR) range of 4000 cm-1-650 cm-1 using a Perkin Elmer Spectrum 400 system. The ATR FT-IR spectra of the vitrains were obtained using a Frontier FT-IR/NIR spectrometer (PerkinElmer) equipped with an ATR device (diamond crystal). Spectra were recorded by a DTGS (deuterated triglycine sulphate) detector. Ca. 2 mg of pulverized sample was placed on the ATR crystal and fixated with a pressure arm. Each spectrum was collected in 32 scans with a resolution of 4 cm-1. The obtained spectra were ATR corrected by adjusting the absorbance intensities at higher wavelengths software Spectrum (Perkin Elmer). The µFT-IR spectra of megaspores from 5 bulk coal samples with maturities within the oil window were collected with a Spotlight 400 microscope (PerkinElmer) coupled to the same spectrometer. The spectra were detected by a MCT (mercury cadmium telluride) detector were processed with the software Spotlight (PerkinElmer).

Tab. 2.2 Frequency regions and band assignment of the FT-IR spectra.

Region (cm-1) Modes of functional groups Acronym 3100-3000 aromatic CH stretching νCH

3000-2800 aliphatic CHx stretching νCHx

3000-2950 asymmetric CH3 stretching νasCH3

2950-2900 asymmetric CH2 stretching νasCH2

2900-2800 symmetric CHx stretching νsCHx 1780-1650 Carbonyl/Carboxyl C=O stretching νC=O 1650-1520 aromatic C=C ring stretching νC=C 900-700 aromatic CH out of plane bending γCH

The measurement was performed on polished blocks of the bulk coals in reflectance mode with an aperture of 20x20 µm, each. Prior to each record, the microscope was calibrated with a gold plate. Per specimen, 3-4 measurements were performed and then averaged. All µFT-IR spectra were recorded in 128 scans with a resolution of 16 cm-1. The resulting µFT-IR reflectance spectra were converted into absorbance spectra by use of the Kramers-Kroenig transformation. All spectra (ATR and µFT-IR) were baseline corrected. The baseline correction and peak area calculation was performed with the Spectrum software. Peak areas were determined after baseline correction by integrating the area between absorbance intensities and a consistent, manually set linear baseline for the specific regions. The assignment of absorbance bands used in this study is based on Socrates (1980) as employed in Lin and Ritz (1993) and Painter et al. (1981, 1985) (Tab. 2.2). To enable comparability between the samples, the calculated peak

19 areas were finally normalised to the sum of all considered absorbance areas within each spectrum.

2.3.4 Curie Point-pyrolysis-GC-MS Ca. 1-3 mg of 6 selected vitrain samples were pyrolysed at two temperatures (590 °C and 764 °C) in a Curie Point Fischer GSG CPP 1040 PSC pyrolyser. The samples were chosen based on their vitrinite reflectances (Tab. 2.1). The temperatures were chosen based on the findings of al Sandouk-Lincke et al. (2014) to provoke a maximum yield of aliphatic and polyaromatic pyrolysis products, respectively. After a pyrolysis time of 10 s, the pyrolysates were introduced directly into the coupled GC-MS system (Fisons 8065; Thermoquest MD 800). The procedure is explained in detail by al Sandouk-Lincke et al. (2013). Peak identification was performed by comparison with mass spectral databases (NIST spectral library, 2014) and published retention information as well as by comparison with authentic reference material. Full scan peak areas for quantification were calculated using the software Excalibur. The pyrolysates of the highly mature sample (VRr=2.86%, measured at both temperatures) and of sample B (measured at a pyrolysis temperature of 590 °C) were found not to be analysable because of low abundances accompanied by the superimposition of artefacts and are therefore excluded from interpretation.

2.4 Results The results of the Rock-Eval pyrolysis and the elemental analysis of the vitrain samples are listed in Tab. 2.1. TOC and Tmax are indicative of coal maturity and all data increase with increasing VRr. The calculated oxygen content of five samples is coherent with their oxygen index (OI), although VRr and OI of the sample set show no correlation.

2.4.1 FT-IR Fig. 2.2a shows the ATR FT-IR spectra of the vitrinite powders with the bands assigned in this study. Although the overall absorbance of the vitrinite powders fades in the more mature samples, there is no systematic relationship between the total absorbance and thermal maturity of the coal. This observation is most probably method-related e.g. due to variations in grain size of the powders and the contact pressure. Because of the very low overall absorbance of the vitrinite of sample K (Fig. 2.2a), a distinction between νasCH2 and CH3 bands is not possible.

The spectra of the vitrinites with reflectances between 0.56% and 1.44% VRr show a significant increase in absorbance in the γCH region as well as a slight increase in the νCH region with -1 increasing maturities. The absorbance of the νCHx bands at 3000-2800 cm initially increases

20 until 1.07% VRr and then decreases significantly with higher maturity. The absorbance of the

νC=O modes clearly weakens with increasing VRr while the νC=C region is more or less stable over the maturity range (Tab. 2.3). The spectra obtained in reflectance mode are characterised by a lower overall absorbance and resolution of the aliphatic stretching regions compared to spectra obtained in transmission mode or recorded with the ATR technique (Mastalerz and Bustin, 1995). A direct comparison between these spectra is therefore not possible.

Fig. 2.2 a) ATR FT-IR spectra of the vitrinite samples and b) reflectance µFT-IR spectra of the megaspores.

The νasCH2 and νasCH3 signals are not visibly resolved in most of the recorded spectra and the resolution of the νCH band is poor (Fig. 2.2b). The overall absorbance of the reflectance spectra correlates well with the VRr of the coals containing the Megaspores, while the IR absorbance and the LRr of the measured specimens are not associated. Both, the γCH region at 900-700

-1 cm and the νCHx region show increasing absorbance with higher maturity. The absorbance of oxygen-containing compounds in the νC=O band lineally decreases from VRr 0.56% to 1.07% (Tab. 2.3). Because of the lower intensities of the νCH bands, the γCH region was chosen as a more representative signal for the aromatic fractions of both, vitrains and megaspores.

Tab. 2.3 Normalised absorbance of the considered functional groups in the vitrinite and megaspore spectra (in %).

* Sample νCH νCHx νsCHx νasCHx νC=O νC=C γCH

νasCH3 νasCH2 A 0.9 13.1 6.1 2.1 4.9 19.6 50.2 16.2 B 1.8 21.0 9.8 3.1 8.1 4.6 35.5 37.1 C 1.6 23.8 10.3 3.3 10.2 3.6 38.8 32.2 D 1.2 20.8 9.8 2.9 8.2 5.0 42.8 30.3 E 1.7 22.4 10.1 2.7 9.6 3.9 37.3 34.7 F 1.7 20.0 8.4 3.0 8.6 4.6 36.2 37.5 G 1.7 20.5 9.2 2.5 8.8 7.0 37.2 33.6 H 2.4 22.7 10.3 2.8 9.6 2.4 31.4 41.1 I 1.8 23.7 10.5 2.9 10.3 2.5 30.5 41.5 J 3.6 13.1 6.2 1.5 5.5 3.1 28.1 52.1 K 4.0 1.4 - - - 5.9 27.9 60.8 A 5.4 51.8 - - - 19.0 21.3 2.5 B 5.6 54.2 - - - 14.4 17.5 8.3 D 2.1 60.0 - - - 10.9 22.3 4.7 E 3.4 61.3 - - - 5.6 16.5 13.1 I 8.9 57.4 - - - 3.6 11.5 18.7

* υCHx is the sum of the areas υsCHx and υ asCHx

It should be noted that absorbance in the γCH region can be overlapped by rocking bands of aliphatic moieties (Kuehn et al., 1982; Sobkowiak et al., 1984) or the presence of absorbance bands of silicates and carbonates in the 900-700 cm-1 region (Riesser et al., 1984; Chen et al., 2015). Here, the error caused by the probable contribution of artefactual absorbance is considered to be negligible because absorbance of aliphatic moieties is weak (Iglesias et al., 1995) and because the only microscopically detected mineral species in the handpicked vitrinite samples was pyrite (Tab. 2.1).

2.4.2 Curie Point-pyrolysis-GC-MS

The aliphatic compounds detected in the pyrolysates are n-alkanes and n-alkenes up to C28 as well as cyclic and branched alkanes and alkenes. Benzene, toluene, ethylbenzene, xylenes, styrene and higher alkylated benzenes represent the monoaromatic fraction. The detected compounds with two or three aromatic rings are naphthalene, fluorene, biphenyl phenanthrene,

22 anthracene, fluoranthene, terphenyl, benzylnaphthalene and their alkylated derivates. The 4- ring aromatics detected in the pyrolysates include pyrenes, tetracenes, chrysenes and triphenylenes. Phenols, benzaldehydes, furans and dibenzofurans represent the aromatic oxygen-containing compounds. Oxygen-containing aliphatic hydrocarbons such as aldehydes, ketones, alcohols and carboxylic acids were detected particularly in the least mature sample. Additionally, the sulphur-containing heteroaromatic compounds thiophene (only alkylated), dibenzothiophene and benzonaphthothiophene (and their alkylated derivates) were identified in the samples pyrolysed at 764 °C. Fig. 2.3 shows the chromatograms of two vitrain samples of different maturities with the most important compounds highlighted. The pyrolysis products comprise a large proportion of aliphatic hydrocarbons, in particular from the samples pyrolysed at 590 °C. The vitrain samples, pyrolysed at 590 °C show an inverse trend in the yields of aromatic compounds. Specifically, the fraction of polyaromatic pyrolysis products increases with increasing VRr of the sample. The vitrinites pyrolysed at 764 °C show a considerable loss of oxygen-containing components with increasing thermal maturity of the samples. Because of the high complexity of the pyrograms and the in some cases only sporadically abundant chemical compounds only the most regular and frequently detected molecules are discussed here. For further interpretation, n-alkanes (Na) and 1-alkenes (Ne) (both up to C28), as well as iso-alkanes (Iso) were chosen for representing the aliphatic fraction of the pyrolysates. Benzene

(B), phenol (Ph), naphthalene (Na) and phenanthrene (P) (alkylated up to C4) represent the aromatic moieties and heteroaromatic compounds are represented by thiophenes (t), dibenzothiophenes (DBT) and benzonaphthothiophenes (BNT) (alkylated up to C3). The proportions of the above-named molecules in the pyrolysates are listed in Tab. 2.4.

Tab. 2.4 Semi-quantitative amounts of the considered chemical compounds of vitrinite at both pyrolysis temperatures (in %).

Sample Na Ne Iso B Ph Na P T DBT BNT Pyrolysis temperature = 590 °C A 18.2 33.4 2.4 19.4 22.2 4.5 0.0 - - - C 21.4 13.8 7.9 25.1 26.5 5.0 0.4 - - - F 15.8 12.2 8.6 39.8 14.2 7.9 1.6 - - - J 3.4 2.7 1.7 22.3 10.5 27.8 31.7 - - - Pyrolysis temperature = 764 °C A 8.1 4.2 5.8 13.8 60.9 6.9 0.4 1.5 0.0 0.0 B 8.7 5.3 6.5 21.4 45.1 11.8 1.2 0.2 0.0 0.0 C 7.8 4.3 4.0 25.8 50.4 7.2 0.4 1.6 0.0 0.0 F 12.4 7.0 11.4 22.6 27.2 16.4 3.0 0.5 0.1 0.0 J 1.8 1.2 0.6 36.4 27.9 28.8 3.3 0.0 0.4 0.2

Fig. 2.3 Total ion chromatograms of vitrain sample C (VRr=0.85%) at pyrolysis temperatures of a) 590 °C and b) 764 °C and of sample J (VRr=1.44%) at pyrolysis temperatures of c) 590 °C and d) 764 °C. 24

2.5 Discussion 2.5.1 Aromatisation The relative abundance of functional groups of several macerals in coals of different thermal maturities have been scrutinised using µFT-IR spectroscopy (Mastalerz and Bustin, 1993; Lis et al., 2005; Chen et al., 2012, 2013). For the evaluation of changes in aromaticity in coals as revealed by FT-IR spectroscopy, several ratios between bands assigned to aliphatic and aromatic constituents have been applied (Lis et al., 2005). Here, we express the degree of aromaticity as the ratio of the areas of absorbance in the νCHx to the γCH regions. Both, vitrinite and megaspore spectra, show trends towards an increasing aromaticity as VRr increases although in the megaspore samples absorbance in the aliphatic CHx stretching region clearly dominates. This is evident from the very low index values as compared to those of the vitrinite samples (Fig. 2.4a). The aliphatic character of fossil spores reported is known to decrease as thermal alteration proceeds (Yule et al., 2000; Fraser et al., 2014). This increase in aromaticity is either caused by the loss of aliphatic components or the new formation of aromatic rings. The trend towards higher aromaticity in the vitrinite samples as indicated by the γCH/υCHx ratios is supported by the results of the CP-Py-GC-MS experiments. The aliphatic character of vitrinite samples pyrolysed at 590 °C decreases considerably with thermal maturity, while this trend is less clear for the products of samples pyrolysed at 764 °C (Fig. 2.5a, b). The yields of aliphatic constituents at this pyrolysis temperature are always much lower than those from the pyrolysis conducted at 590 °C. In any case, even at the high pyrolysis temperature of 764 °C, the highest ratio of aromatic over aliphatic compounds was observed for the most mature sample. This indicates that a lower pyrolysis temperature is more suitable for the analysis of aliphatic products while a higher temperature favours the release of aromatic products, confirming the findings of al Sandouk-Lincke et al. (2014).

2.5.2 Chain length and branching of aliphatic chains

The ratio of the areas of the νasCH2 over the νasCH3 peaks in the aliphatic stretching region is indicative of the chain length and/or the degree of branching of aliphatic moieties (Painter et al., 1985; Wang and Griffiths, 1985; Lin and Ritz, 1993). Here, a higher ratio indicates either less branched or longer aliphatic chains or refers to the occurrence of cyclic hydrocarbons. The vitrinite samples show a weak linear trend towards higher νasCH2/CH3 with increasing maturity (Fig. 2.4b), indicating either the presence of longer aliphatic chains or lesser branching of the aliphatic components. In previous studies, however, either a decrease of this parameter with

25 progressed maturation (Chen et al., 2012) or no correlation (Lin and Ritz, 1993) has been reported for vitrinite macerals.

Fig. 2.4 a) Aromaticity index of vitrinite and megaspore samples plotted against vitrinite reflectance, b)

νasCH2/νasCH3 ratios of the vitrinite samples plotted against vitrinite reflectance, c) condensation index of vitrinite and megaspore samples plotted against vitrinite reflectance, d) relative yields of benzenes, naphthalenes and phenanthrenes in the vitrinite samples pyrolysed at 764 °C.

Fig. 2.5 Yields of aliphatic compounds (Na, Ne, Iso) in relation to aromatic compounds (B, Na, Ph, P) of the vitrinite pyrolysed at a) 590 °C and b) 764 °C, c) relative distribution of the chain lengths of n-alkanes and 1- alkenes and d) n-alkanes and 1-alkenes in relation to iso-alkanes in the vitrinite samples pyrolysed at 590 °C.

The relative distribution of chain lengths of the n-alkanes and -alkenes in the pyrolysed vitrinite samples shows a vague trend towards shorter (C7-C10) aliphatic chains as thermal maturation increases (Fig. 2.5c). At a pyrolysis temperature of 590 °C, the ratio of iso-alkanes over n- alkanes and-alkenes increases (Fig. 2.5d). This contradicts the observations from FT-IR measurements, as it implies an increase in branching of the aliphatic moieties. The pyrolysis results are more in line with the expected shortening of aliphatic moieties. The FT-IR results may reflect a decrease in percentage of the branched alkanes relative to straight alkanes in the kerogen structure or an increase in cyclic hydrocarbons. The divergence from observations made by other authors (Lin and Ritz, 1993; Chen et al., 2012) needs to be further explored in future studies.

2.5.3 Condensation Sun (2005), Wang et al. (2011a), Chen et al. (2012) and Mastalerz et al. (2013) employed ratios of the γCH areas over the areas of the νC=C regions in order to characterize the degree of aromatic ring condensation. The absorbance in the νCH region is caused by the out of plane deformation vibration of hydrogen attached to aromatic ring structures. With increasing substitution, caused for example by the condensation of aromatic rings, the νCH absorbance should weaken relative to the C=C absorbance bands. Here, an increasing index indicates that

27 the formation of isolated aromatic rings prevails the process of condensation in both, vitrinite and megaspore samples. This trend, however, levels off in the higher mature vitrinite samples (Fig. 2.4c). This observation is in line with findings made by van Krevelen (1993), who reported that higher aromatic structures in vitrinite form by condensation of aromatic moieties from the semi-anthracite stage onwards. Another indicator for the prevailing of relative aromatisation over condensation processes is the atomic H/C ratio variation relative to the TC content of vitrinite (Hatcher et al., 1982). This ratio seems to decrease only slightly until the total carbon content reaches approximately 90% (Fig. 2.6a). The relative yields in benzenes, naphthalenes and phenanthrenes of the vitrinite powder pyrolysed at 764 °C all increase linearly with maturity (Fig. 2.4d). Kruge and Bensley (1994) reported an increase in tri- and tetra-aromatic hydrocarbons with increasing maturity of vitrinites, which they explained by the condensation of phenolic structures, but they recorded only minor changes in the yields of mono- and diaromatic hydrocarbons. Senftle et al. (1986) reported the absence of a correlation between vitrinite reflectance and the relative amount of alkylbenzenes in pyrolysates of vitrinites.

Fig. 2.6 a) H/C atomic ratios and b) O/C atomic ratios in relation to TC of the vitrain samples from the Ruhr Basin compared to published data of vitrinite samples. 28

In this study, the amount of alkylbenzenes clearly increases with maturity for the samples pyrolysed at 764 °C. As mentioned above, different pyrolysis temperatures are necessary in order to investigate aliphatic (590 °C) and aromatic moieties (al Sandouk-Lincke et al., 2014). However, the results presented here are only semi-quantitative and therefore the increase of aromatic moieties may be the result of the loss of aliphatic groups. The observation of the distribution of the different species within the aromatic fraction, however, reveals an increase in polyaromatic compounds at the expense of the monoaromatic compounds, in particular phenols (Fig. 2.7a). This is clearly related to the process of aromatic ring condensation, especially at maturity stages beyond peak oil generation. Veld et al. (1994) report an increase in the absolute pyrolysis yields of polyaromatics from vitrinite concentrates accompanied by decreasing yields of alkylnaphthalenes with increasing maturity. In the samples studied here, the proportion of alkylnaphthalenes in the aromatic fraction increases, which is in line with observations made by Meuzelaar et al. (1984b). Another indicator for an on-going condensation of aromatic moieties is the relative distribution of the yields of heteroaromatic compounds. While sulphur-containing heteroaromatics are represented solely by rare alkylated thiophenes in the samples with maturities ranging from 0.56% to 0.85%, dibenzothiophenes and benzonaphthothiophenes are detected in vitrinite sample F (VRr=0.99%) and are the dominant heteroaromatic compounds in the vitrinite from the medium volatile bituminous coal

(VRr=1.44%) (Tab. 2.4).

Fig. 2.7 a) Composition of the aromatic constituents and b) ratio of benzenes/phenols of vitrinite samples pyrolysed at 764 °C.

2.5.4 Oxygen-containing groups A loss of oxygen-containing groups is indicated by a linear decrease in carboxyl/carbonyl groups in the megaspore samples with increasing maturity, as evidenced by diminished signals in the 1830-1660 cm-1 region. This rapid loss of C=O groups and the distinct linear trend is less pronounced in the vitrinite spectra (Fig. 2.8a), although the least mature sample has the highest oxygen content. The continuous decrease of oxygen-containing compounds in spore samples has been reported by several authors (Dutta et al., 2013). Al Sandouk-Lincke et al. (2013) explained the decrease in C=O functional groups by the process of defunctionalisation and the concomitant loss of oxygen in sporopollenin due to thermal alteration. Ganz and Robinson (1985) and Ganz and Kalkreuth (1987, 1991) introduced the C-factor, later modified by Guo and Bustin (1998a), depicting modifications of the C=C stretching region relative to regions representing oxygen-containing moieties. The C-Factor is used to represent changes in the carboxyl/carbonyl stretching regions and indicates the degree of oxidation in type 1 and 2 kerogens. Yule et al. (2000) showed that a sudden drop of the C-factor in spore samples at the spore oil window between 0.8% and 1.0% VRr coincides with a change in spore colour and

LRr. A similar trend is observed in the megaspores from the Ruhr Basin coals (Fig. 2.8b), but it is not explicitly related to the measured reflectances of the spores (see Tab. 2.1). Previous studies reported a loss of phenol and its alkylated representatives in pyrolysed vitrinite samples as maturity increases (Meuzelaar et al., 1984b; Senftle et al., 1986; Veld et al., 1994), and attributed it to the loss of oxygen-containing groups due to defunctionalisation of phenolic moieties during maturation. A steady loss of oxygen-containing functional groups in vitrinite is furthermore indicated by the noticeable decrease of the atomic O/C ratio with increasing TC (Fig. 2,6b) and was reported by several authors (Dormans et al., 1957; Senftle et al., 1986; van Krevelen, 1993; Bandopadhyay and Mohanty, 2014). In the vitrinites studied here, this process is indicated by a linear decrease of phenol yields in the samples pyrolysed at 764 °C. In this study, phenols and benzenes represent the monoaromatic fraction of the pyrolysed vitrinites. The balance between phenols and benzenes in the vitrinites pyrolysed at 764 °C (Fig. 2.7b) suggests an ongoing dehydroxylation of the phenolic moieties in favour of benzenes upon thermal maturation.

Fig. 2.8 a) Relative absorbance of the C=O region of vitrinite and spore samples plotted against vitrinite reflectance, b) C-factor of the spore samples plotted against vitrinite reflectance.

2.5.5 Comparison of megaspores and vitrinites In the above staged subchapters, we discussed some of the features upon maturation of both, megaspore and vitrinite samples. Here, we would like to summarise the most important similarities and differences between the different kerogens. The µFT-IR spectra of the megaspores reveal a clear dominance of aliphatic over aromatic structures compared to the vitrinite samples, even though the spectra have been recorded in reflectance mode and thus have stronger signals in the 900-700 cm-1 region compared to transmittance or ATR spectra (Chen et al., 2012). Like the vitrain samples, the megaspores show an incremental aromatic character upon thermal maturation. The γCH/νC=C ratios of the spores indicate that aromatisation, either caused by the loss of aliphatic groups or the new formation of aromatic rings, prevails condensation processes that lead to the formation of higher aromatic clusters. The same trend is observed for the vitrinite samples, which show a stagnation of this ratio in the sample with anthracite maturity. The main difference between the analysed megaspore and vitrinite samples is the evolution of carbonyl- and carboxyl-groups with ongoing maturation (Fig. 2.8a). This can be explained by the different prevailing oxygen-containing species within the two kerogen types. Clearly the methods used in this study for characterising megaspore and vitrain samples 31 differ. In future work, more megaspores should be handpicked so that pyrolytic studies can be performed, and results can be compared.

2.6 Conclusions For a better understanding of the processes leading to chemical and structural changes in kerogen types II and III during thermal alteration, isolated vitrinites and megaspores from coals of a maturity range between 0.55% and 2.86% VRr are analysed using infrared spectroscopic methods. The FT-IR results of the vitrinite samples, here representing type III kerogen, are combined with pyrolysis GC-MS experiments, providing insight into changes at the molecular level. The main processes occurring during the thermal maturation of the vitrinites and megaspores studied here are aromatisation due to the loss of aliphatic groups and the new formation of monoaromatic species, the condensation of aromatic rings and the defunctionalisation of oxygen-containing moieties. FT-IR spectra of vitrinite and megaspore samples both show an increase in the aromatic relative to the aliphatic fraction with advanced thermal alteration, caused by either the loss of aliphatic compounds or the new formation of aromatic rings. This increase in aromaticity with ongoing maturation is noticeably more intense in the vitrinite samples than the megaspores. An increase in aromaticity of the vitrinites is furthermore supported by the yields in the aliphatic relative to the aromatic fractions generated at a pyrolysis temperature of 590 °C. The condensation of aromatic ring structures is evidenced by the higher yields in di- and polyaromatic molecules in the vitrinite pyrolysates of the thermally more mature samples. However, for the sample set analysed in this study, the new formation of isolated aromatic rings prevails this process as indicated by the ratio of the absorbance in the γCH over that of the C=C regions. The linear decrease in absorbance of the C=O region in the µFT-IR spectra of the megaspores as maturity rises is explained by defunctionalisation of carbonyl/carboxyl groups. Vitrinites pyrolysed at 764 °C reveal a significant loss of hydroxyl groups as the yields in alkylated phenols become lower in favour of alkylated benzenes.

3 Comparative geochemical and pyrolytic study of coals, associated kerogens, and isolated vitrinites at the limit between subbituminous and bituminous coal

Abstract A sample set consisting of coal, overlying and intercalating siliciclastic rocks and kerogen concentrated from these rocks of nine Pennsylvanian subbituminous to bituminous coal seam intervals from the German Ruhr Basin was analysed in order to detect chemical structural differences of the organic matter. Vitrinites from the coal seams and the kerogen concentrates were isolated to study effects of maceral group composition on chemical properties of typical type III kerogen. Rock-Eval Tmax values of sediment samples are higher than both those of the kerogen concentrated from them and associated coal samples. This is a result of the mineral matrix effect. Higher OI values of the sediment and kerogen samples as compared to those of the bulk coal samples cannot be explained by the abundance of carbonyl/carboxyl groups as reflected by results obtained by attenuated total reflectance (ATR) FT-IR spectroscopy. Ratios of the areas of the aromatic γCH out of plane, the aliphatic νCHx stretching, and the aromatic νC=C ring stretch regions imply a lower degree of aromaticity and at the same time higher degree of condensation of vitrinites originated from the sedimentary layers, respectively. Upon Curie Point (CP) pyrolysis at 590 °C, these vitrinite samples generate higher amounts of long-chained normal alkanes and n-alkenes than the bulk samples or the vitrinite originating from the coals. While reflectance values of vitrinites from coals and sedimentary layers are within the standard deviation of values expected for the narrow maturity window, only the vitrinites of the coal seams show some correlation with depth. Clearly, the relative abundance of phenols is decreasing with increasing maturity, as well as the ratio of benzenes over higher condensed aromatic moieties as revealed by their pyrolysis products generated at 764 °C.

3.1 Introduction While the maturation pathways of the different kerogen types are basically well studied, only little is known about the mechanisms causing anomalies in maturity parameters of organic matter (OM) in association with the surrounding substrate (Goodarzi et al., 1988; et al., 1993; Gentzis et al., 1993; Murchison et al., 1991). However, it has been frequently observed and described that the surrounding material of kerogen has an influence on maturation processes. A study from Horsfield and Douglas (1980) showed that the amount and composition of hydrocarbons generated from source rocks during pyrolysis experiments do not only depend on maturity and type of OM but also on the mineralogy of source rocks. This observation has been confirmed by Espitalié et al. (1984), who showed that in the presence of clay minerals with large specific surface areas, hydrocarbon yield generated during Rock-Eval pyrolysis was retarded in both intensity and timing. Huizinga et al. (1987) reported the same effects for type I and II kerogen isolated from the Monterey and Green River Formations and explained them by strong adsorption capacities and thermo-catalytic properties of the clay minerals. Such insights gained from artificial heating experiments allow conclusions to be drawn for maturation of kerogen under natural conditions. For example, Rock-Eval Tmax values tend to be higher for OM in organic-rich siliciclastic rocks dominated by type III kerogen, as compared to those of adjacent coals (Littke et al., 1989; Jasper et al., 2009). Besides Tmax, vitrinite reflectance (VRr) is the by far most frequently used thermal maturity indicator (Hartkopf-Fröder et al., 2015). This parameter is reported to be affected not only by the stratigraphically induced maximum temperatures reached but also by the type of surrounding material. A comparative study published by Goodarzi et al. (1988) found lower reflectance values for vitrinite particles within carbonates as compared to dispersed vitrinite particles matured in shale for the same depth intervals. Another matrix-related parameter discussed to potentially influence VRr values is the grain size of the siliciclastics surrounding the OM. Random vitrinite reflectance measured for samples of the German Ibbenbüren area was found to be consistently higher for particles found within claystones and lower for those within silt- and sandstones as compared to high maturity coals from the same depth intervals (Bruns and Littke, 2015). Bostick and Foster (1975) made similar observations on Pennsylvanian coals and kerogens isolated from associated sedimentary rocks from the Illinois Basin, but found in general lower values for dispersed vitrinite as compared to that of the associated coals. The same trend was

34 found for coals and dispersed OM (DOM) from layers of the Ruhr Basin by Scheidt and Littke (1989). Jones et al. (1971) explained the gradual decrease of reflectance values from base to top of coal seams overlain by sandstones by the higher thermal conductivity of sandstones as compared to that of shales. Few observations on the relationship between reflection properties of vitrinites and depositional environment have been published. Veld et al. (1996) reported significantly lower VRr values in samples derived from a depositional environment influenced by rivers as compared to samples derived from peat formed in a deltaic setting, while other authors (Veld et al., 1991; Diessel and Gammidge, 1998; Littke and Zieger, 2019) found a lower reflectance in coal seams affected by marine waters. These variations are explained by either differences in redox conditions or in vegetational input. A further factor seemingly influencing the chemical and thermal alteration of kerogen is the composition of OM itself. Here, not only type/composition of OM but also its abundance is reported to take influence on maturity parameters (Huc et al., 1986; Goodarzi et al., 1993). For example, “suppression” of VRr values occurs in source rocks with high liptinite contents (Diessel and Gammidge, 1998; Raymond and Murchison, 1991) and especially in the presence of liptinites of algal origin (Wenger and Baker, 1987). The structure of vitrinite and thus its reflectance is further related to their floral origin. Floras producing rather hydrogen- or aliphatic rich tissue may result in the presence of lower reflecting vitrinite (Carr, 2000). Although the aforementioned variations in maturity parameters are widely observed and described, detailed information on the associated differences in the chemical and structural properties of kerogen is rarely available. For this purpose, the chemical and structural constitution of kerogen and vitrinite, concentrated and isolated from sedimentary rocks and that of bulk coal seams and handpicked vitrinite from the same depth intervals within a narrow maturity range is comparatively analysed. Based on elemental, pyrolytic, and optical analyses of several Bolsovian coal seam intervals from within the German Ruhr Basin, the aim is to detect and to quantify differences in chemical and optical properties of the kerogen, in order to elucidate to which extent and how the amount of OM in a source rock influences maturity parameters.

3.2 Geological setting The prevalence of a tropical humid climate during the Pennsylvanian promoted the extensive formation of extended peat accumulations in several basins of the Euramerican continent, situated close to the equator (Ziegler, 2012). The Ruhr Basin is located in western Germany north of the Rhenish Massif and is one of several paralic coal bearing basins stretching from the United Kingdom in the west to Poland in the east (Fig. 1a). Because of glacio-eustatic sea level fluctuations, these basins, evolving just north of the emerging Variscan Mountains, were repeatedly subjected to marine ingressions (Dusar et al., 2000; Krull, 2005; Roscher and Schneider, 2006). The Pennsylvanian succession of the Ruhr Basin composes of up to 4000 m sedimentary rocks, consisting of cyclically deposited mudstones, siltstones and sandstones of predominantly deltaic to fluvial origin and containing more than 150 coal seams with thicknesses ranging between several cm and 2 m (Drozdzewski, 1993; Suess et al., 2007). The onset of coal-forming peat deposition occurred during the Marsdenian (Namurian C), with the occasional occurrence of seams of minor thickness in the Sprockhövel Formation. Economically most important coal-bearing strata with coal contents of up to 5% developed during the Langsettian (Westphalian A; Witten and Bochum formations) and the Duckmantian (Westphalian B; Essen and Horst formations) (Hahne and Schmidt, 1982; Scheidt and Littke, 1989). During the Westphalian, the northward movement of the Variscan Deformation Front shifted the centre of deposition and thus maximum coal formation towards the northwest (Drozdzewski, 1993). The youngest coals within the Ruhr Basin are of Bolsovian (Westphalian C; Dorsten and Lembeck formations) age, but some Asturian (Westphalian D) coal seams of high rank are present in the Ibbenbüren area further to the north (Fig. 1a). Climatic conditions changed towards drier and more seasonal, so that they no longer allowed the formation of extensive peat swamps from the Stephanian onwards (Selter, 1989; David, 1990). The Pennsylvanian successions of the basin are characterized by complex deformation structures, with large-scale folds striking in southwest-northeast direction (Fig. 1b) (Drozdzewski, 1993; Drozdzewski and Wrede, 1994). This pattern is further reflected by variations in coal rank throughout the basin (Teichmüller and Teichmüller, 1979; Uffmann and Littke, 2013). While Marsdenian and Yeadonian coal bearing layers crop out in the southern part of the basin, the Bolsovian succession is unconformably overlain by mainly Cretaceous and younger sedimentary rocks in the northern part of the basin.

Fig. 3.1 a) Location of the Ruhr Basin within the Variscan Foredeep in Central Europe (modified from Jasper et al., 2009), b) main structural features of the Ruhr area and sampling site (modified from Wrede, 2005), c) sampled seam successions and sample numbers. K and V in index letters indicate kerogen concentrate and isolated vitrinite, respectively. Ash yields are taken from Zieger and Littke (2019).

3.3 Methods 3.3.1 Sample preparation Coal and sediment samples are derived from nine Bolsovian coal seam successions, encountered in borehole Hervest 5 between the cities Dorsten and Haltern in the northern Ruhr Basin area in a depth interval between 670 and 1124 m (Fig. 1b). Coals from these cores have been sampled during a previous study (Zieger and Littke, 2019) and composite samples from the seams were prepared. In addition, vitrain layers from the coal seams have been handpicked and powdered in a mortar. Organic-rich sedimentary rocks, in most cases clayey siltstones, were sampled right above the seams and for some seams also from intercalating sedimentary layers (Fig. 1c). The samples were pulverised using a rotation mill. Part of the powdered sediment samples were then used for the preparation of kerogen concentrates. Carbonates were first dissolved in 32% hydrochloric acid (HCl) and silicates were removed by a treatment with 70-75% hydrofluoric acid (HF), respectively. In order to dissolve newly formed silica gel, the samples were once more treated with 32% HCl. After cleaning and neutralising with water after each concentration step, the samples were dried at room temperature. Proportions of the kerogen concentrates were used for the selection and isolation of single macerals. Vitrinites of each depth interval were handpicked from the kerogen concentrates and powdered in a mortar. Polished sections of the bulk kerogen concentrates and coals were prepared according to the standard procedures described in Taylor et al. (1998) for microscopic analyses.

3.3.2 Elemental analyses and Rock-Eval pyrolysis Total carbon (TC) contents of bulk coal seams, sediment, and kerogen samples were measured directly as the sum of total organic (TOC) and total inorganic (TIC) carbon using an elemental analyser (LiquiTOC, Elementar). The device was calibrated with a soil standard (47.1 wt% TC). 20 mg of pulverised coal or kerogen concentrate and approximately 50 mg sediment were first heated to 550 °C and then to 1000 °C applying the temperature ramp method using oxygen as carrier gas. Total sulphur (TS) content was measured using a Leco S-200 total evaporation analyser, calibrated with a Leco sulphur standard (0.31 wt% TS). 20 mg coal and kerogen concentrate powder and 100 mg pulverised sediment sample, mixed with iron fillings were incinerated in a stream of oxygen at 1800 °C. For further information on elemental analyses, see Grohmann et al. (2018). 50 mg sediment powder, and 10 mg of coal and kerogen concentrate powders

(mixed with sand) were pyrolysed using a Vinci Technologies Rock-Eval 6 apparatus, calibrated with a sediment standard (TOC=7.65 wt%, Tmax=432 °C) according to the procedure described in Behar et al. (2001).

3.3.3 Microscopy Vitrinite reflectance of the bulk coal seams and kerogen concentrate samples was measured on 100 random telocollinites particles per sample using a Zeiss Axioplan incident light microscope equipped with a Zeiss Epiplan-Neofluar oil immersion objective (50x magnification). The software Fossil (Hilgers) was used for measurement and data collection. The system was two-point calibrated with a leuco-sapphire (0.592% reflectance) and an yttrium-aluminium-garnet (0.889% reflectance) standard prior to each measurement. Maceral group composition was determined by employing the two-scan method (2x 500 points on an equidistant grid; white and blue incident light) using an Axio Imager.M2m incident light microscope (Zeiss). More information on the preparation of polished sections and devices can be found in Zieger and Littke (2019).

3.3.4 Attenuated total reflectance Fourier-transform infrared spectroscopy Infrared spectra of bulk coal seam samples, kerogen concentrates as well as from pure vitrinite were collected with a Frontier Fourier-transform infrared (FT-IR) spectrometer (Perkin Elmer) equipped with a universal attenuated total reflectance (ATR) module. A small amount (1-3 mg) of pulverised sample was placed on the ATR crystal (diamond) and fixed with a pressure arm applying 30 ±4 N. Reflectance spectra were recorded by an DTGS (deuterated triglycine sulphate) detector in mid infrared between 4000 cm-1 and 650 cm-1. Each sample was measured in triplets in 16 scans with a resolution of 4 cm-1. A background spectrum was collected prior to each measurement. All spectra were then ATR and baseline corrected using the software Spectrum (Perkin Elmer). Peak fitting was performed using Voight profiles (convolutions of Lorenz and Gaussian distributions) applying the nonlinear method of least squares. Areas between peaks and linear baselines, set individually for the three considered regions between 3000 cm-1 and 2800 cm-1, 1790 cm-1 and 1520 cm-1, and 900 cm-1 and 700 cm-1, were calculated. Peak assignment and baseline endpoint setting was performed in accordance with the findings of previously published work (Painter et al., 1981, 1983; Ibarra et al., 1996). Peak areas were then normalised to the sum of areas of all considered peaks to enable comparability between the samples.

3.3.5 Curie point-pyrolysis-gas chromatography-mass spectroscopy A small amount (0.5-3 mg) of pulverised kerogen, bulk coal seam samples, and vitrinite concentrates was packed into ferromagnetic foil crucibles having Curie point temperatures of 590 °C and 764 °C, respectively. Each sample was pyrolysed at both temperatures for 10 s in a Fischer Curie point pyrolyser (CP-Py). The pyrolysates were then introduced into a Fisions GC 8000 gas chromatograph (GC), equipped with a Zebron ZB-1 non-polar capillary column (30 m, 0.25 mm inner diameter, 0.25 µm film thickness) via splitless injection (60 s at 270 °C). Helium was used as carrier gas at a velocity of 40 cm/s. The initial oven temperature was set to 40 °C and the pyrolysates were initially cooled down by a cryogenic trap (-70 °C). The GC temperature was set to increase to 310 °C (held for 20 min) with a heating rate of 3 °C/min. A Trace MS ion trap mass spectrometer (MS) with an electron ionisation of 70 eV and a source temperature of 200 °C measured a scan range of 35 to 700 m/z. Peaks were identified on the basis of known retention times and MS data of mass spectroscopic libraries (NIST Mass Spectral Library, V. 2.2, 2014) and reference material. The program Excalibur was used to calculate peak areas of the relevant compounds from the total ion chromatogram. The same set of compounds was evaluated for all samples and normalised to the sum of areas of all compounds considered.

3.4 Results 3.4.1 Elemental analyses and Rock-Eval pyrolysis The ash yields of the bulk coal seam samples are reflected in their TOC contents (Fig. 1c, Tab. 1). The bulk coal seam samples of the mineral matter rich seam Midgard II show the lowest TOC values, especially sample MII6, which represents the basal part of the seam. The amount of OM in the sediment samples from above and within the coal seams are quite variable. Sample PI1, having a TOC content of 53.9 wt% is per definition a coal but will here act as a reference for possible influences of kerogen concentration. The kerogen concentrates show TOC values varying between 56.5 wt% and 76.2 wt%, depending on their pyrite contents. With 0.0-0.1 wt%, TIC values of the bulk coal seam samples are very low and even beyond the accuracy of measurement. Some sediment samples overlying the coal seams show higher values of up to 0.7 wt%. TS values among the bulk

40 coal seam samples are quite variable, ranging from 1.4 wt% (OV3) to 6.2 wt% (PII2). Total sulphur contents of the kerogen concentrates are clearly higher than in the samples from which they originate, which is explained by its relative concentration due to the insolubility of pyrite in HCl and HF. Rock-Eval Tmax values of the bulk coal seam samples are in the range between 420 °C and 428 °C. A clear maturity trend with depth is not reflected by the data. Tmax values of the sediment samples vary between 423 °C and 437

°C. In most cases, Tmax values of the kerogens are somewhat lower than those of the material they are concentrated from and are similar to the values obtained for the associated coal seams (Tab. 1). Hydrogen index (HI) values are variable for the whole sample set, without any trend with depth or sample type. Values range from 65 to 240 mg HC/g TOC. Oxygen index (OI) values on the other hand are usually higher in the sediment samples as compared to the bulk coal seam samples from the same depth intervals, and in most cases also higher in the kerogen concentrates.

3.4.2 Microscopy Random vitrinite reflectance of bulk coal seam and kerogen concentrate samples varies between 0.55% and 0.73% (Tab. 2), without showing any pronounced trend with depth or sample type. In all bulk coal seam samples, vitrinite is the dominant maceral group (Tab. 2). Liptinite contents are relatively low (<14 vol%). Inertinite contents vary between 3 vol% and 29 vol% (MII2). Mineral matter (MM) content varies between 2 vol% and 8 vol%, with exception of the bulk sample originating from the lowermost part of seam Midgard II, which has 20 vol% MM, reflecting the high ash content of the sample

(MII6; Fig. 1c). The kerogen concentrates show higher variability in their maceral composition. For examples, those derived from the siliciclastic layer within seam Parsifal

I (PI3K) and those from the sediments overlying seams Odin III (OIII1K) and Erda (E1K) are dominated by liptinite, the latter containing only 3 vol% vitrinite. The amount of mineral matter in the kerogen samples of up to 4 vol% directly reflects the amount of pyrite.

Tab. 3.1 Results of elemental analyses and Rock-Eval pyrolysis data of sediment, bulk seam samples (in italics) and kerogen concentrates.

Depth TOC TIC TS Tmax HI OI Seam Sample (mg CO /g (m) (wt%) (wt%) (wt%) (°C) (mg HC/g TOC) 2 TOC) R1 27.9 0.2 2.9 429 65 13

Rübezahl R1K 670 64.8 0.0 3.9 424 123 16 R2 70.5 0.0 2.0 424 161 5

PI1 53.9 0.7 4.7 427 72 11

PI1K 56.5 0.0 8.9 428 105 16

PI2 58.2 0.0 3.0 424 132 6 Parsifal I 726 PI3 3.7 0.1 0.1 432 166 14

PI3K 72.9 0.0 1.6 419 210 16

PI4 66.0 0.1 2.4 420 150 5

PII1 3.7 0.1 0.5 430 100 12 Parsifal PII1K 735 76.2 0.0 2.2 423 150 6 II PII2 56.8 0.1 6.2 420 240 5

OIII1 11.3 0.1 0.5 423 203 10

Odin III OIII1K 802 72.9 0.0 1.6 418 195 11

OIII2 70.1 0.0 1.9 421 179 4

OIV1 33.0 0.6 0.5 426 109 9

Odin IV OIV1K 804 72.3 0.0 1.2 424 115 14

OIV2 55.3 0.0 1.6 424 196 5

OV1 27.2 0.5 0.6 433 101 12

OV1K 71.7 0.0 1.1 429 101 10

Odin V OV2 810 2.6 0.5 0.1 430 107 19

OV2K 68.5 0.0 1.6 424 110 17

OV3 59.9 0.0 1.4 428 93 7

MII1 18.0 0.2 1.5 428 117 10

MII1K 70.5 0.1 2.2 422 101 16

MII2 62.8 0.1 1.9 426 176 5

MII3 23.9 0.2 2.1 428 96 12 Midgard MII3K 930 68.9 0.0 4.0 424 108 12 II MII4 52.8 0.0 4.0 423 141 7

MII5 14.8 0.1 0.4 426 195 10

MII5K 66.2 0.0 4.2 427 169 7

MII6 43.1 0.0 2.9 424 165 7

HII1 10.4 0.1 0.6 433 162 7

Hagen II HII1K 1063 74.4 0.0 1.4 425 98 7

HII2 76.2 0.0 1.5 427 160 4 E1 11.4 0.1 1.0 437 110 6

Erda E1K 1124 75.9 0.0 1.6 424 179 9 E2 73.3 0.0 1.5 426 236 4

Tab. 3.2 Vitrinite reflectance values and maceral composition of the bulk seam samples (in italics) and concentrated kerogens. V=vitrinite, L=liptinite, I=inertinite, MM=mineral matter.

VRr V L I MM Seam Sample (%) (vol%) (vol%) (vol%) (vol%)

R1K 0.62 74 15 7 4 Rübezahl R2 0.58 74 6 16 4

PI1K 0.61 78 10 2 10

PI2 0.59 76 7 11 6 Parsifal I PI3K 0.59 34 38 26 2

PI4 0.65 82 6 7 5

PII1K 0.57 3 20 74 3 Parsifal II PII2 0.55 76 5 11 8

OIII1K 0.59 24 44 31 1 Odin III OIII2 0.6 71 9 18 2

OIV1K 0.6 71 16 13 0 Odin IV OIV2 0.67 74 4 18 4

OV1K 0.69 85 6 9 0

Odin V OV2K 0.57 83 7 8 2

OV3 0.72 93 3 4 0

MII1K 0.64 69 27 3 1

MII2 0.67 53 14 29 4

MII3K 0.64 74 12 10 4 Midgard II MII4 0.66 84 7 3 6

MII5K 0.66 59 33 6 2

MII6 0.7 51 10 19 20

HII1K 0.73 73 16 10 1 Hagen II HII2 0.66 73 7 18 2

E1K 0.65 3 65 31 1 Erda E2 0.73 59 13 24 4

Fig. 3.2 ATR FT-IR spectra of the bulk Rübezahl coal seam, the concentrated kerogen from the overlying sediment layer, and vitrinite handpicked from the seam and isolated from the kerogen concentrate. Relevant peaks are marked. See text for explanation of the abbreviations.

3.4.3 ATR FT-IR spectroscopy The position of considered peaks in the FT-IR spectra and a comparison of spectra for samples of an exemplary coal seam interval are shown in Fig. 2. All samples show strong absorption in the region around 3300 cm-1, which is assigned to the stretching of hydrogen bonded oxygen. This band was not considered in this study due to its proneness to errors

(Painter et al., 1981). Stretching modes of the asymmetric (νCH3as, νCH2as) and symmetric (νCHxs) aliphatic C-H bonds are here representative for the aliphatic fraction of the samples. The values given in Tab. 3 represent the peak areas relative to the total area of all peaks considered for each spectrum. All samples are characterised by strong absorption in the C=O stretching region of carbonyl and carboxyl groups (νC=O) between 1770 cm-1 and 1660 cm-1 and in the C=C aromatic ring stretching region (νC=C) around 1600 cm-1 (Fig. 2, Tab. 3). In the region between 720 cm-1 and 900 cm-1, the aromatic C- H out of plane vibrations (γCH) are represented by three distinct peaks at 870 cm-1, 815 cm-1, and 750 cm-1, representing isolated hydrogen (870), isolated hydrogen or two neighbouring H-atoms (815), or 1,2 substituted H-atoms (750) attached to aromatic rings, respectively (Painter et al., 1981). The absorbance in this region is rather weak in the vitrinite samples isolated from the kerogen concentrates (Fig. 2, Tab. 3).

3.4.4 CP-Py-GC-MS Compounds representing the aliphatic fraction of the samples are n-alkanes and n-alkenes from C7 to C30, which are well resolved in all spectra (Fig. 3). The sum of n-alkanes and n-alkenes (N) is quite variable but highest (21.3%-63.9%) for the bulk coal seam and kerogen samples pyrolyzed at 590 °C (Fig. 3, Tab. 4). Benzenes (B), naphthalenes (Na) and phenanthrenes (P) methylated up to +C3 are chosen here as representatives of aromatic compounds. This fraction is clearly dominated by benzenes (Fig. 3, Tab. 4) in all pyrolysates and at both pyrolysis temperatures. Especially in the samples pyrolyzed at

764 °C and in the vitrinite samples, phenols (Ph, methylated up to +C3), representing oxygen containing molecules, are very abundant (Figs. 3, 4). Thiophenes (T, methylated up to +C3) serve as a measure of the relative organic sulphur content with values ranging between 0.1% (E1K at 590 °C) and 4.5% (OV2KV at 764 °C) and without showing any clear relation to sample type or pyrolysis temperature.

3.5 Discussion 3.5.1 Differences in elemental composition, vitrinite reflectance and Rock-Eval parameters Results of Rock-Eval pyrolysis show in part substantial differences between the organic- rich sediments and the kerogen concentrated from them. In order to illustrate the effect of original TOC content and maceral composition on the different Rock-Eval parameters prior to and after demineralisation, three sample pairs are marked by different symbols in

Fig. 5a-c. The sample pair PI1(K) reflects the influence of kerogen concentration on coal, dominated by vitrinite (original TOC=53.9 wt%), while the sample pairs PII1(K) and E1(K) represent kerogen dominated by inertinite and liptinite, respectively (Tab. 2). Hydrogen index values of demineralised and original samples do not correlate well (Fig. 5a), i.e. there is no trend towards higher or lower values in the kerogen concentrates. A systematic decrease of HI values with decreasing TOC values for kerogen was reported by several authors (e.g. Stein et al., 1989; Spiro, 1991). However, for the sample pairs analysed here, neither TOC content nor maceral composition seem to have any influence on this parameter. In their experimental study on the influence of mineral matter on type III kerogen, Jasper et al. (2009) found a strong inhibitory effect of pyrite and smectite-type clay minerals on the release of cracked hydrocarbons during Rock-Eval pyrolysis.

Fig. 3.3 Total ion chromatograms of a) bulk coal seam, b) coal seam vitrinite, c) the bulk kerogen concentrate, and d) isolated vitrinite from the kerogen concentrate from seam Rübezahl pyrolysed at 590 °C.

Fig. 3.4 Total ion chromatograms of a) bulk coal seam, b) coal seam vitrinite, c) the bulk kerogen concentrate, and d) isolated vitrinite from the kerogen concentrate from seam Rübezahl pyrolysed at 764 °C.

Tab. 3.4 Relative abundance of n-alkanes and n-alkenes, aromatic constituents, phenols, and thiophenes of bulk seam and kerogen samples (left) and vitrinites (right) pyrolysed at 590 °C and 764 °C. N=n- alkanes and –alkenes, B=benzenes, Na=naphthalenes, P=phenanthrenes, Ph=phenols, and T=thiophenes. Results of bulk seam samples and coal seam vitrinites are written in italics.

590 °C Seam Sample N B Na P Ph T Sample N B Na P Ph T R1 61 13.4 9.3 4.1 10 2.2 R1 16 28.8 8 2.6 41.2 3.3 Rübezahl K KV R2 40 17.6 8.3 1.4 30.1 2.5 R2V 20 13.6 10.1 2.1 52.1 2

PI1 K 52.7 11.8 10.9 4.4 18.6 1.6 PI1 KV 27 18.9 9.7 0.4 41.6 2.4 P 2 34 16.3 11.4 1.7 34.7 2 P 32.9 14.6 14.4 2.8 32.1 3.2 Parsifal I I I2V PI3 K 62.3 7.7 8.9 6.7 12.6 1.8 PI3 KV 40.8 22.2 7 1.7 25.1 3.3

PI4 36.8 12.7 11.9 1.8 35.2 1.6 PI4V 14.7 10.3 10.6 3.1 59.7 1.6 Parsifal PII1 K 39.7 9.1 16.6 9.9 22.8 1.8 PII1 KV 45.6 22.9 7.7 1.3 19.8 2.8 II PII2 21.3 17.1 13.9 1.5 43.5 2.7 PII2V 24.9 17 15.7 5.5 34 2.8 O 1 63.9 9.2 8.4 3.7 13.2 1.6 O 1 31.1 20.6 12.9 1 31.5 3 Odin III III K III KV OIII2 35 14.6 10.7 1.9 35.7 2 OIII2 V 43.4 16 7.7 1.4 28.6 3 O 1 50.3 10 10.7 2.6 25 1.5 O 1 31.6 31 16.4 1.2 16.4 3.3 Odin IV IV K IV KV OIV2 38.3 15.9 12.2 2.2 29.9 1.5 OIV2 V 37.1 14 12.9 1.9 31.3 2.8

OV1 K 40 10.9 9.3 4 34.4 1.5 Odin V OV2 K 44.2 9 9.8 2.4 33.8 0.9 OV2 KV 21.9 24.9 9.7 0.5 38.6 4.5

OV3 27.2 18.7 11.7 0.3 39.9 2.3 OV3 V 35.7 14.2 11.3 2.1 33.8 2.9

MII1 K 49.4 7 17.6 6.7 18.3 1.1 MII1 KV 32.2 23.4 12.1 2.2 26.4 3.7

MII2 47.6 16.5 12.4 1.7 19.9 1.9 MII2V 45.5 13.2 8.1 2.9 28.3 2 Midgard MII3 K 56.4 8.5 14.4 7.4 11.8 1.4 MII3 KV 22.5 22.6 14.5 0.4 37.5 2.5 II MII4 33.6 17 13.3 1.6 32 2.5 MII4V 39 10 15.5 6.3 27.5 1.6

MII5 K 51.7 5.1 14.1 5.3 23 0.7 MII5 KV 15.7 40 9.2 1.2 29.3 4.5

MII6 44.2 16.5 10.9 3.1 22.8 2.5 MII6V 42.4 12.8 19.4 5.6 17.7 2.1 H 1 26.4 11.9 16.8 6.6 37.3 1 H 1 30.9 22.3 11 1.2 32.4 2.2 Hagen II II K II KV HII2 34 17 10.3 1.8 35.2 1.7 HII2 V 26.6 17.7 11 2 39.9 2.8 E1 53.8 13.2 10.1 1.9 20.7 0.4 E1 34.3 47.3 9.7 0 6 2.7 Erda K KV E2 44.8 11.8 13.6 3.6 25.5 0.8 E2V 20.8 13 18.8 6.6 38.1 2.7 764 °C Seam Sample N B Na P Ph T Sample N B Na P Ph T R1 33.3 19.2 6.6 1.2 39.5 0.2 R1 11.3 22.1 12.3 0.1 52.6 1.7 Rübezahl K KV R2 26.8 23.5 9.3 2.2 35.7 2.5 R2V 19.5 16.5 9.8 1.1 50.9 2.2

PI1 K 29.3 15.9 9.8 2.6 41.1 1.4 PI1 KV 17.8 18.6 11.7 0.3 50 1.6 P 2 27.9 17.5 13 1.8 36.9 2.8 P 26.7 10.9 12.3 1.9 46.3 2 Parsifal I I I2V PI3 K 49.6 13.4 8.5 3.4 23.3 1.9 PI3 KV 48 33.6 4.2 0.1 10.7 3.4

PI4 28.3 16.5 16.1 1.7 35.2 2.2 PI4V 11.7 16.7 14.3 3.8 51.7 1.9 Parsifal PII1 K 23.5 19.7 16 4.6 34.6 1.5 PII1 KV 50.4 19.7 10 1.3 17.8 0.9 II PII2 27 15.8 15.3 2.7 36.6 2.6 PII2V 21.2 14.2 16.7 6 40.2 1.7 O 1 37.7 19.9 9.5 1.1 29.3 2.5 O 1 29.2 26.9 9.1 3.5 29.8 1.4 Odin III III K III KV OIII2 32.6 14.2 13.3 1.6 36.4 1.9 OIII2 V 20.9 15.8 10.2 1.3 48.6 3.2 O 1 24.6 27.1 7.2 1.7 38.1 1.4 O 1 20.4 35.5 11.7 4.6 25.2 2.6 Odin IV IV K IV KV OIV2 29.4 21.4 12.9 0.3 33.4 2.7 OIV2 V 32.7 16 11.4 2 35.7 2.2

OV1 K 23.1 24.8 5.4 3.1 41 2.6 Odin V OV2 K 20.9 23 10.4 1.8 42 1.9 OV2 KV 13.3 28.2 11.2 0.7 43.6 3

OV3 24.9 21.5 13.5 1.7 37 1.4 OV3 V 23.8 13.1 13.8 2.5 44.8 2

MII1 K 36.5 10.3 21.2 13.5 16.9 1.6 MII1 KV 28.4 24.6 11.3 2.3 31.5 1.8

MII2 40.7 18.9 12.2 2.4 23.7 2 MII2V 33.2 20.2 9.1 0.4 35.2 2 Midgard MII3 K 30.1 13.9 14.7 2.2 37 2.2 MII3 KV 15.5 30.2 14.7 2 35.9 1.6 II MII4 35.5 13.7 15.3 2.1 31.9 1.5 MII4V 27.6 14.9 12 2.8 40.6 2.2

MII5 K 11.5 15.1 14.5 2.3 55.1 1.5 MII5 KV 12.5 41.6 10.8 1.1 31.2 2.7

MII6 37 16.4 14.5 2.2 27 2.8 MII6V 29.1 17.8 16.9 4 30.3 1.9 H 1 39.9 13.7 12.6 1.2 30.5 2.2 H 1 17.9 22.5 14.3 1.9 42.4 1 Hagen II II K II KV HII2 28.3 17.8 13.1 1.5 38.2 1 HII2 V 27.8 12.2 16.9 4 37.5 1.6 E1 44 3.6 36.8 11.6 3.8 0.1 E1 9 19.7 20 3 46.9 1.4 Erda K KV E2 26.2 20 13.1 0.7 37.9 2.1 E2V 23.4 14.2 27.9 15.9 17.5 1.2

Related to this effect, HI values of kerogen concentrates rich in pyrite were consistently lower as compared to the original sedimentary rocks except for some samples with HI of less than 50 mg HC/g TOC for which sorption on clay minerals is the dominant effect reducing HI values. This trend cannot be seen in the sample set analysed here, even though concentrated kerogens are clearly enriched in pyrite, as indicated by high TS values after demineralisation. A similar, incoherent pattern as for HI values is obtained when comparing the oxygen indices of kerogen and original sample (Fig. 5b). OI values of the sediment samples might be overestimated by the presence of carbonates (Katz, 1983), especially siderite (Jasper et al., 2009; Ordoñez et al., 2019), which add to the S3 peak and this effect is reported to even increase in the presence of smectite-type clay minerals (Espitalié et al., 1984). However, for the samples with the highest TIC values within the sample set, OI values even increased as mineral matter was removed. Jasper et al. (2009) explained decreased OI values after mineral removal by the contribution of

CO2 from the mineral matrix to the S3 peak. This observation was made by other authors as well (Spiro, 1991; Dembicki, 1992; Grohmann et al., 2018), but an increase in the presence of minerals and with decreasing TOC content is not observed for the analysed samples, possibly due to the overall low carbonate contents. The temperature of maximum hydrocarbon generation, on the other hand, is generally higher for the samples from the mineral-rich layers from above or within the coal seams (Fig. 5c), while the values of the kerogen concentrates are in line with those of the corresponding bulk coal seam samples (Tab. 1). Such elevated Tmax values of DOM in siliciclastics is well documented (Dembicki, 1992; Jasper et al., 2009; Rahman et al., 2017) and can be explained by the retention of hydrocarbons by argillaceous mineral matter (Espitalié et al., 1984). As an exception, the sample pair PI1(K), as reference for the effect of mineral dissolution from coal, nearly plots on the 1:1 line in Fig. 5c, whereas almost all TOC- poor samples plot below the line. When comparing the results of pyrolysis and optical experiments of kerogen concentrates with those of the associated bulk coal seam samples, one important factor is the maceral composition, which differs between the two sample sets. The maceral composition of the bulk coal seam samples is rather homogenous with vitrinite being the dominant maceral group while that of the DOM concentrated from the overlaying or intercalating mineral rich-layers is quite variable.

Fig. 3.5 Comparison of the different Rock-Eval parameters of sediment samples and corresponding kerogen concentrates. a) Hydrogen index, b) oxygen index, and c) Tmax. d), e), and f) show the same parameters for the kerogen concentrates and associated bulk seam samples. The dashed line marks the 1:1 ratio. Three sample pairs are marked for better differentiation of the effect of kerogen concentration on samples with different maceral composition and/or original TOC content. See text for explanation.

Opposed to the relationship between sediment and kerogen concentrates, differences between the Tmax values of kerogen and bulk coal samples (Fig. 5d) are within the range of natural samples of similar maturities (Teichmüller and Durand, 1983) and show no preference towards higher or lower temperatures.

The bulk coal seam samples in general show higher HI values than the associated DOM (Fig. 5e). Notable exceptions to this general pattern are the sample sets from seam Odin III and the mineral rich layer within seam Parsifal I which are both characterised by high liptinite contents in the sedimentary rocks/kerogen concentrates but not in the coals. Here, kerogen concentrates as well as the original non-demineralised samples show higher HI values. For samples in the same maturity range, Veld et al. (1993) found a positive correlation between liptinite content and HI. Littke et al. (1989), on the other hand found a correlation of HI values and vitrinite content, which they explained by the loss of early generated hydrocarbons from sporinite in clastic rocks. In case of the samples analysed here, maceral composition does not seem to have strong influence on this Rock-Eval parameter (see sample pairs E1K/E2 and PII1K/PII2 in Fig. 5e). On the other hand, the OI values of the concentrated DOM are systematically higher than those of the bulk coal seam samples, implying a greater proportion of oxygen-containing functional groups in the kerogens (Fig. 5f). Much higher oxygen indices for dispersed type III kerogen than for coals have been reported for a Duckmantian coal-bearing sequence by Jasper et al. (2009), who explained this effect by a stronger degradation of the vitrinite precursor material in clastic environments. The fact that sample PI1K from the uppermost part of seam Parsifal I shows an elevated OI as compared to the other bulk coal seam samples might either indicate oxidization of the OM in this part of the seam as a result of insufficient water availability causing the termination of peat formation or, less likely, an oxidative effect of the acid treatment during the demineralisation process. Increased OI values, lower HI values as well as lower average vitrinite reflectance values have been previously reported for naturally weathered coals (Lo and Cardott, 1995; Copard et al., 2002). Durand et al. (1977) analysed the effect of chemical mineral dissolution on solvent extracted coals and did not detect any changes in the kerogen structure after the treatment. For immature OM in soil, peat, and lignite, however, hydrolyzing and oxidizing effects of HCl and to a lesser extend HF are reported, reflected by higher intensities in the νC=O stretching vibration regions as compared to untreated material (Durand et al., 1977; Durand and Nicaise, 1980), the increase of acid as compared to aldehyde functional

52 groups (Rumpel et al., 2006), or losses of organic carbon affecting the average composition (Saxby, 1976). In contrast to spectral fluorescence, which is shifted towards higher wavelengths, vitrinite reflectance was found not to be affected by conventional kerogen isolation using HCl and HF (Mendonça Filho et al., 2010). The differences in vitrinite reflectance values between bulk coal seam samples and DOM are in most cases minor and lower than the range of values reported for coals sampled in cm distances within the same coal seams (see Diessel and Gammidge, 1998; Jasper et al., 2009; Zieger and Littke, 2019 among others). Further, the standard deviations resulting from the reflectance measurements are not higher for the isolated kerogens than for the bulk coal seam samples. The rather slight variations in VRr values between bulk coal seam samples and DOM from the same seam intervals seem to be independent from maceral composition (Tab. 2).

Concisely, the higher Tmax values of the sediment samples are most probably a result of the mineral matrix effect, retarding the release of hydrocarbons during pyrolysis in the presence of argillaceous minerals. The lower HI values as well as the high OI values of the samples from the mineral rich layers overlying and intercalating the coal seams as compared to the TOC-rich isolated kerogen and bulk coal seam samples of the concentrated kerogens cannot be explained by this effect. Changes of the OM upon acid treatment, even though not expected, cannot be ruled out for the latter effect.

3.5.2 Variations in the chemical structure of bulk samples and vitrinites Data obtained by FT-IR spectroscopy provide information on the bulk chemical structure of the kerogen. Semiquantitative ratios employed for coals and single macerals (Lis et al., 2005; Chen et al., 2015 and references therein) are used here to reveal differences between the sample sets. Pyrolysis data, on the other hand, is representative for those parts of the kerogen structure, which is accessible by the fast heating, and thus only reveals parts of the complex macromolecules building up the kerogen matrix. Following the suggestions of al Sandouk-Lincke et al. (2014), for analysis of the aliphatic fraction of type III kerogen, i.e. the n-alkanes and n-alkenes up to a carbon number of 30, results of pyrolysis at a temperature of 590 °C are taken into account, while aromatic polar compounds (phenols and thiophenes) as well as aromatic hydrocarbon moieties are accessed by interpreting results of the pyrolysates produced at 764 °C.

3.5.2.1 Polar compounds The relative abundance of oxygen-containing functional groups is reflected by the percentage of absorbance in the carbonyl/carboxyl stretching area in the FT-IR spectra. In this region carbonyl- and carboxyl groups of esters, ketones, aldehydes and carboxylic acids absorb at around 1700 cm-1 (Painter et al., 1981). No systematic differences in C=O groups between kerogen concentrates and bulk coal seam samples are evident (Fig. 6a), i.e., the relative share in absorbance of carboxyl- and carbonyl groups of the two sample sets shows some variance but within the same range, which is in line with the expected values for samples in this maturity range (Zieger et al., 2018). Pure vitrinite samples show higher values as compared to their parent bulk material as observed in previous studies (Dyrkacz et al., 1984) and vitrinites originating from coal seams and the DOM show percentages, even though scattering, within the same range. During the weathering of vitrinite, a loss of hydrogen and the formation of carbonyl and other oxygenated functional groups are reported (Lo and Cardott, 1995). Experimental FT-IR studies showed that artificial low temperature air oxygenation of coals results in an increase of carbonyl and carboxyl functional groups and at the same time in a slight increase of phenolic OH (Painter et al., 1981; Landais et al., 1984).

This effect, however, is not reflected by FT-IR data for the analysed kerogen nor the dispersed vitrinite samples. The relative abundance of phenols in the bulk coal and kerogen samples pyrolyzed at 764 °C is rather similar (Tab. 4). The pyrolysate of sample

E1K yields the lowest amount of phenols, which is likely a result of the sample’s maceral composition dominated by liptinite, generating lower amounts of these molecules upon pyrolysis (Meuzelaar et al., 1984; Nip et al., 1988). The relative amount of phenols is, as expected, higher for the pure vitrinites than for the bulk samples. In vitrinite, alkylated phenols are common and directly derived from the ligno-cellulose rich precursor material, but simple phenols might get generated upon pyrolysis (Larter and Douglas, 1980; Senftle et al., 1986). However, any severe differences in the abundance of oxygen-containing functional groups between the samples derived from the coal seams and those concentrated and isolated from the mineral-rich layers are not self-evident from the results obtained either from ATR FT-IR or CP pyrolysis analyses.

Fig. 3.6 Comparison of different FT-IR derived indicators of bulk samples (bulk seam and coal) and associated vitrinites. a) relative areas of the νC=O stretching region, b) aromaticity ratio, c) degree of condensation ratio, c) chain length/branching ratio.

Thiophenes, representing organic sulphur in the samples, are detected in all samples, with no major changes with sampling depth or maceral composition recognisable. Organic sulphur is reported to be higher in sporinite than in vitrinite and higher in vitrinite than in macerals of the inertinite group, respectively, although variations in the organic sulphur contents within a maceral group are common (Demir and Harvey, 1991). For the samples analysed here, no trend between the thiophene yield in the pyrolysates and maceral composition can be found, and overall thiophene contents are low (Tab. 4).

3.5.2.2 Aromatic compounds A parameter employed to assess the degree of aromaticity based on FT-IR data is the ratio of areas of the aliphatic νCHx stretching region and the aromatic out of plane deformation

55 between 900 and 700 cm-1 (Lis et al., 2005; Chen et al., 2012). As for the proportion of carbonyl/carboxyl groups, the aromaticity values expressed as γCH/νCHx (Fig. 6b) of bulk coal seam samples are similar to those of the isolated kerogens in terms of their ranges. Two kerogen concentrates, however, show much higher aromaticity.

Interestingly, sample E1k does not show a rather aliphatic character as expected for a sample containing mainly liptinite (Lin and Ritz, 1993; Guo and Bustin, 1998). The maceral composition of the sample with the highest ratio (HII1K) is similar to that of most bulk coal seam samples. The γCH/νCHx ratio is indicative of maturity, as aromaticity increases with vitrinite reflectance for type III kerogen (van Krevelen, 1993; Veld et al.,

1994; Mastalerz and Bustin, 1993). The reflectance of this sample (VRr=0.73%), though the highest among the kerogen concentrates, cannot explain its high aromaticity, since other samples with similar reflectance values show no such high ratios (see example highlighted in Fig. 6b). Vitrinites originating from the coal seams show a more aromatic character than the bulk coal samples, while the vitrinite samples from the kerogen concentrates show the lowest γCH/νCHx ratio values among all sample sets. A possible explanation for this finding might be the incorporation of early formed mobile hydrocarbons, which migrated through the rather permeable siliciclastic sediments into the pore system of the dispersed vitrinites. Pyrolysates of the kerogen concentrated from the sedimentary intervals above or within the coal seams show higher amounts of aliphatic compounds than the bulk coal seam samples (except seam Hagen II, which showed high aromaticity as indicated by the FT-IR results) and also than the vitrinites isolated from them, except for the samples derived from seams Parsifal II and Hagen II (Fig. 7b, d). This is true for both pyrolysis temperatures (Tab. 4). In case of the kerogen isolated from the sediment overlying seam Parsifal II, this can be explained by its maceral composition, with a volumetric contribution of inertinite macerals of more than 70%, leading to relatively higher amount of aromatic constituents in the kerogen sample. The chemical structure of inertinite is in general characterised by lower hydrogen contents and thus aliphatic functional groups and by a higher degree of aromaticity (Meuzelaar et al., 1984; Nip et al., 1992; Maroto-Valer et al., 1998).

The comparison between monoaromatic moieties and polyaromatic ring structures can further elucidate the molecular structure of the sample sets. The ratio of areas of the aromatic C=C stretching region and the aromatic CH out of plain bending, for example, gives insight into the degree of aromatic ring condensation. The ratio here expressed as

νC=C/γCHx increases as the amount of higher aromatic ring systems increases relative to that of lower condensed aromatic ring systems. The values show no substantial nor systematic differences in the aromatic group composition between kerogen concentrates and bulk coal seam samples (Fig. 6c). While the degree of condensation of the vitrinites isolated from the coal seams does not seem to differ much from that of the bulk coal seam samples, vitrinites originating from the siliciclastic layers show in part much higher values of this ratio, even if they originate from kerogen concentrates which are petrographically similar to the vitrinite-rich coals. Furthermore, in the bulk samples from seams Rübezahl, Parsifal I as well as Odin III and V, as opposed to the others, the combined percentage of naphthalenes and 3-ring aromatic compounds is slightly higher than that of benzenes and phenols at a pyrolysis temperature of 764 °C, indicating a more condensed structure of the bulk coal seam samples (Fig. 7c). Interestingly, the pyrolysate of the kerogen sampled from above seam Parsifal II shows no increased degree of condensation, as would be expected for a sample mainly composed of inertinite (Meuzelaar et al., 1984; Nip et al., 1988). The extremely high amount of naphthalenes in sample E1K supports the high aromaticity of this sample (Figs. 6b, 7a, c) and is a result of its high content of liptinite group macerals, reported to generate high amounts of naphthalenes upon pyrolysis at high temperatures (al Sandouk-Lincke et al., 2014). The composition of the aromatic fraction in general, however, seems to be more condensed in vitrinites sampled from the coals, as the relative amounts of benzenes and phenols versus naphthalenes and phenanthrenes of their pyrolysates (764 °C) imply (Fig. 7e), thus contradicting the results obtained by FT-IR analysis. Benzenes are more abundant in the pyrolysis products of the dispersed vitrinites, while the monoaromatic fraction is clearly dominated by phenols in the coal seam vitrinites (Fig. 7e). Whereas FT-IR analysis results imply a higher degree of aromatic ring condensation and a lower level of aromaticity of the vitrinite isolated from the sedimentary layers, CP pyrolysis data reveals a dependence of the constitution of this fraction to maceral composition.

Fig. 3.7 Relative abundance of pyrolysis products from bulk seam and kerogen concentrate samples (a, b, c) and the associated vitrinites (d, e) showing the maceral composition of the bulk seam and kerogen concentrates (in vol%, mineral matter free basis), n-alkanes and n-alkenes (N) versus aromatics (A=B (benzenes), Ph (phenols), Na (naphthalenes), and P (phenanthrenes)) from the pyrolysis at 590 °C and (b, d) relative abundance of aromatic moieties of the same samples pyrolysed at 764 °C (c, e).

3.5.2.3 Aliphatic compounds The aliphatic fraction of kerogen can be described in terms of chain lengths and degree of branching of the aliphatic chains, as reflected by the relative proportion of the -1 -1 asymmetric CH2 and CH3 stretching regions between 3000 cm and 2900 cm

(Mastalerz and Bustin, 1993; Lis et al., 2005; Chen et al., 2012, 2013). The CH2/CH3 ratio varies strongly in all sample sets, with the inertinite rich sample PII1K showing the highest value, indicating either a higher amount of long relative to short aliphatic chains

58 or lesser branching of the aliphatic fraction (Fig. 6d). The vitrinites of both, kerogen and coal samples, show either similar or lower ratios as compared to the bulk samples, which might be attributed to the absence of liptinite group macerals, the latter usually showing higher values for this parameter (Lin and Ritz, 1993; Chen et al., 2013). Vitrinites isolated from the kerogen concentrates show a weak trend towards lower ratios as compared to the vitrinites from the coal seams.

The pyrograms of the samples, however, show some clear trends of the composition of the aliphatic fraction, which are systematic and cannot be explained by differences in maceral composition or maturity effects. For example, the kerogen concentrates pyrolysed at 590 °C form a higher amount of long-chained n-alkanes and n-alkenes (≥C19) than the corresponding coals (Fig. 8). At the same time, the pyrolysates of coal vitrinites show a higher proportion of longer n-alkanes and n-alkenes (C19-C30) over C7-C18 n- alkanes/n-alkenes than those of the kerogen concentrates, implying relatively longer aliphatic chain-length than their original samples. The variations in chain-lengths within the aliphatic compounds of each sample set are, however, high. A high variation in the amount of normal hydrocarbons in pyrolysates from low-rank coals as well as an independence of the amount of waxy normal hydrocarbons and liptinite content has been reported (Powell et al., 1991). Indeed, the liptinite content of the samples analysed here does not seem to either control chain lengths or relative amount of the aliphatic moieties.

The fact that the coal derived kerogen concentrate (PI1K) shows a much higher C19-

C30/C7-C18 ratio than the bulk coal seam samples points towards changes in the aliphatic fraction upon the acid treatment. For the aliphatic fraction it can therefore be summarised, that the kerogen concentrates as well as vitrinites isolated from these are characterised by a higher proportion of aliphatic moieties and that the latter are composed of a larger amount of long-chained normal alkanes and alkenes.

Fig. 3.8 Comparison of the proportion of long- over short-chained n-alkanes and n-alkenes (C19-C30/C7-

C18) of bulk seam and kerogen samples and their corresponding vitrinites pyrolysed at 590 °C.

3.5.3 Maturity trends Vitrinite reflectance, as the most commonly used maturity indicator, is supposed to show an increase with depth. While the reflectance values of the dispersed vitrinites from the overlying and intercalated siliciclastic layers show no trend with depth, the coal vitrinites show increasing values. Two samples, however, are clear outliers, showing either too low

(PII2) or higher than expected values (OV3). If these two samples are excluded, a positive 2 correlation of VRr and depth is evident for the coal samples (r =0.64, Fig. 9a). The bulk coal seam sample PII2 has a high TS content, suggesting marine influence on the former peat (Zieger and Littke, 2019) and also the highest HI value among the coals (240 mg

HC/g TOC). Li et al. (2010) reported that telocollinites (with relatively low VRr) of high sulphur coals showed lower absorbance in the νC=C aromatic ring stretching region and at the same time higher absorbance in the aliphatic νCHx stretching region than low- sulphur coals of the same maturity interval. A more aliphatic character, however, of samples PII2 and PII2V is not indicated by their γCH/νCHx ratios. In contrast, sample OV3 has the lowest HI value of all bulk coal seam samples but does not show any noticeable differences of its chemical structure as revealed by FT-IR indicators nor do its pyrolysis products show any substantial differences to the other coal samples.

Fig.3.9 a) Depth, b) relative amount of phenols, c) ratio of phenols over benzenes, and d) ratio of benzenes over naphthalenes and phenanthrenes of vitrinites from coal seams and isolated from the kerogen concentrates pyrolysed at 764 °C compared to vitrinite reflectance.

There is also no correlation of Tmax values and depth for coal or kerogen samples. When comparing the vitrinite reflectance with the indicators derived from FT-IR data, no correlation can be found, even though vitrinite reflectance is a function of its chemical structure. This might be explained by the narrow maturity range of the sample set. Some of the CP pyrolysis data, on the other hand, show evident maturity trends, when the two samples described before are excluded. There is, for example, a negative linear trend (r2=0.77) for the relative amount of phenols in vitrinites derived from the coal seams with

61 increasing vitrinite reflectance, when neglecting samples PII2 and OV3 (Fig. 9b). The loss or defunctionalisation of phenols upon maturation is expected and well observed (Schenck et al., 1981; Senftle et al., 1986; Powell et al., 1991). The same trend is visible for the ratio of phenols over benzenes, which linearly decreases with vitrinite reflectance (r2=0.71) if these two samples are excluded (Fig. 9c), indicating defunctionalisation of phenols in favour of benzenes during the narrow maturity range of the coals. Without the two aforementioned samples, a linear correlation between the amount of benzenes relative to that of naphthalenes and phenanthrenes can be found with vitrinite reflectance (Fig. 9d). The correlation is, however, weak, and strong condensation of aromatic rings upon thermal maturation in type III kerogen only occurs at maturity levels above VRr of 2% (van Krevelen, 1993; Chen et al., 2012; Zieger et al., 2018).

The fact that all other parameters and indicators do not show any relationship to vitrinite reflectance, is most probably the result of the rather small reflectance window and further a complex interplay of the different indicators such as aliphatic character, aromaticity and condensation.

3.6 Conclusions Optical and pyrolytic analysis of the coal-bearing sequence at the limit between subbituminous and bituminous rank from within the Ruhr Basin revealed some systematic differences between OM dispersed in siliciclastic rocks and OM sampled from coal seams. The Rock-Eval data show higher Tmax and lower HI values for the mineral rich layers overlying and intercalating the coal seams as compared to the TOC-rich isolated kerogen and bulk coal seam samples. For Tmax, this is a result of retention of hydrocarbons on clay minerals. The relatively high OI values of the kerogen concentrates cannot be explained by the results obtained from ATR FT-IR or CP pyrolysis analyses, as these do not reveal any substantial differences in the relative abundance of carboxyl/carbonyl groups or phenols. The effect of kerogen concentration by dissolution of carbonates and silicates with acids on the chemical structure of coal and DOM cannot be ruled out and therefore should be systematically analysed in future studies, in order to avoid misinterpretation.

FT-IR data of vitrinites isolated from the sedimentary layers imply a higher degree of aromatic ring condensation but at the same time a lower level of aromaticity as compared to vitrinites sampled from the coal seams of the same depth intervals. A higher abundance of aliphatic compounds in the kerogen concentrates is further confirmed by results of the CP pyrolysis. This might be explained by sorption of early formed hydrocarbons on OM dispersed in the relatively higher permeable siliciclastic matrix.

A closer examination of the aliphatic fraction of the pyrolysates obtained at 590 °C reveals that the kerogen concentrates in general generate higher amounts of long-chained

(>C18) normal hydrocarbons than associated bulk coal seam and vitrinite samples. The variability of vitrinite reflectance within the sample set is within the range of expected values for a coal-bearing sequence, but vitrinites isolated from the kerogen concentrates show a less pronounced trend towards increasing values with depth than vitrinites from the coal seams. For the latter samples, a correlation with depth is evident when excluding two samples from the coal seams characterised by high sulphur content or a higher relative abundance of phenols deviating from the normal maturity trend. Correlations with vitrinite reflectance of these selected samples are further found for the abundance of phenols as well as the degree of condensation as reflected by the ratios of benzenes over naphthalenes and phenanthrenes in their pyrolysis products.

4 Bolsovian (Pennsylvanian) tropical peat depositional environments: The example of the Ruhr Basin, Germany Abstract A set of 11 Bolsovian coal seams from the German Ruhr Coal Basin were analyzed in profiles for their organic geochemical and petrographic composition, aiming at the reconstruction of depositional environments, mainly in terms of water supply and redox conditions. For this purpose, established maceral indices and A/I ratio were employed. The seams show the same basic depositional pathways of terrestrialisation and paludification as reported for Duckmantian coal seams from the same basin. The presence of coals formed as ombrotrophic peat indicates that climate was still wet enough for the formation of domed bogs until the Upper Bolsovian, although the typical densosporite facies does not occur within the uppermost third of the analyzed succession. Very high TS values for seam Parsifal II indicate the influence of marine waters at the peat stage. A marine ingression event was not reported before for this stratigraphical level within the Ruhr Basin. Data also reveal some variability in vitrinite reflectance (0.65-0.74%) within a given coal seam which cannot be due to thermal maturation. For seams with decreased

VRr values, a correlation with high liptinite contents was found. Rock-Eval HI values do not show any strong variation with depths in the recorded maturity interval. Tmax values vary between 418 °C and 444 °C showing dependence on both, thermal maturity and liptinite content. Decreasing contents of inertinite macerals throughout the Pennsylvanian in the Ruhr Basin suggest climatic changes during that time span.

4.1 Introduction Coal deposits, as relics of earlier mires, swamps and bogs hold a wealth of information on peat depositional environments. Methods employed to gain information about the conditions prevailing during peat deposition include palaeobotany (Phillips and DiMichele, 1990; Willard and Phillips, 1993; DiMichele and Phillips, 1994; DiMichele et al., 1996;), palynology (Cleal, 1997; Hartkopf-Fröder, 2005; Jasper et al., 2010),

64 sedimentology and sequence stratigraphy (Styan an Bustin, 1983; Falcon-Lang, 2004; Wang et al., 2011b), mineralogy (Ward, 2002; Shao et al., 2003), geochemistry (Dai et al., 2002; Bechtel et al., 2003; Auras et al., 2006; Izart et al., 2012), and organic petrology (Taylor et al., 1989; Teichmüller, 1989; Kalkreuth et al., 1991). In the paralic basins of western Europe, the Bolsovian period represents a transition from extensive tropical mires towards the end of coal formation, when climate gradually changed from tropical humid to more seasonal and finally semi-arid at the Upper Westphalian – Stephanian boundary, causing changes in water supply and vegetation (Roscher and Schneider, 2006; Cleal et al., 2012). The elevation of the Variscan Orogen during that time further changed depositional patterns and thus sedimentation (Leeder, 1988; Suess et al., 2007).

While the coal basins of Eastern Europe are thoroughly investigated in terms of petrographic composition and palynology (see Nowak, 2004 and sources herein; Opluštil and Cleal, 2007; Opluštil et al., 2013; Uglik and Nowak, 2015; Opluštil and Sýkorová, 2018), published petrographical data especially on the paralic coals of Central and Western Europe are rare. Although many studies have been made on the palaeobotany and palynology of British, French, Belgian and Dutch coal deposits (e.g. Fulton, 1987; Phillips and DiMichele, 1990; Dimitrova et al., 2005; Cleal et al., 2012; Cleal, 2018), little has been published on their petrographic composition (e.g. Smith, 1962; Veld et al., 1993, 1996). Some petrographical work has been performed on Spanish coals from the Bolsovian. Piedad-Sánchez et al. (2004) for example studied the organic petrology of Bolsovian and Westphalian D bulk coal samples from several locations within the Central Asturian Basin. For the reconstruction of peat-forming environments, however, the study of whole seam profiles is more reasonable than the examination of composite samples since peat mires can evolve from topogenous to ombrogenous and vise versa, affecting the composition of microscopical constituents, caused by changes of climatic or depositional conditions. For coals of the German Ruhr Basin, once of great economic interest, only a few petrographical surveys have been conducted. Littke (1987) investigated the organic petrography and geochemistry of a sequence of 21 seams from the Westphalian B1 to C1 of the Ruhr Basin. He differentiated 3 main types of depositional environments for the palaeo-peat, with the end members topogenous and ombrogenous coals being characterized by high proportions of vitrinite and minerals and high proportions of inertinite with low mineral contents, respectively. In contrast, Strehlau (1990) interpreted the so-called densosporite facies, with a high proportion of

65 inertinite and micro-petrographically recognizable thick-walled, dumbbell-shaped sporinite, to be originated from open mires with a high groundwater level. Jasper et al. (2010) recognized a cyclic structure within Duckmantian coal seams from the Ruhr Basin, evolving from topogenous to ombrogenic and finally back to a topogenous facies. They found that the groundwater index (GWI) of Duckmantian coals shows good correlation with palynological data collected for the same samples. Johnston et al. (2017), who combined palynological and petrographical data of seam successions from the Appalachian Basin (USA), used a similar approach. Both found that in coals interpreted as former domed mires (densosporite facies), the proportion of spores originating from smaller lycopods increased. Eble et al. (1994) also confirmed that densospores are related to small lycopods, which they found to be present both in coals of high and low ash content. Some studies have shown that the facies models and maceral indices designed for Carboniferous and Permian coals (Diessel, 1986; Calder et al., 1991; Kalkreuth et al., 1991), are not applicable for the identification of recent mire environments (Dehmer, 1995; Wüst et al., 2001; Moore and Shearer, 2003). Amijaya and Littke (2005) argued that because of extreme differences in original plant material throughout geologic time, the applicability of these depositional models originally designed for Palaeozoic coals might not be suitable for the reconstruction of Mesozoic or Cenozoic peat environments. For Carboniferous coals, on the other hand, they provide a useful tool and the systematic record of the petrographic composition of a coal can contribute significantly to the elucidation of palaeoenvironmental conditions during peat formation, especially if combined with organic geochemical and/or palaeontological methods. With the coal petrographic and organic geochemical data presented here, set in context to published data on sedimentology and palynology, we would like to contribute to the clarification of the depositional conditions prevailing during the Bolsovian in the paralic basins of Western Europe using the Ruhr Foreland Basin as an example.

4.2 Geological Setting During the Pennsylvanian, Euramerican landmasses were located close to the equator, with a tropical humid climate promoting the formation of extensive peat accumulation (Ziegler, 1990). The German Ruhr Basin is one of the largest of several foreland basins that formed north of the emerging Variscan Mountains from the early Carboniferous

66 onwards (Fig. 4.1a). It is situated north of the Rhenish Massif, from where it expands 80 km towards the north, having a southeastern-northwestern dimension of 150 km (Fig. 4.1b).

Fig. 4.1 a) The Variscan Foredeep in Central Europe and locations of Pennsylvanian coal mining areas (modified from Jasper et al., 2009) b) Map of the Ruhr Basin with major tectonic structures (simplified after Wrede and Ribbert, 2005), c) Cross-section of the Ruhr Basin (after Drozdzewski and Wrede, 1994) with L. Fm = Lembeck Formation, D.=Dorsten, H. = Horst, E. = Essen, B.=Bochum, W.=Witten.

The Palaeozoic strata within the Ruhr Basin crop-out in the southernmost part of the basin, dipping northwards with an average angle of approximately 3° where the layers are unconformably overlain by up to 2000 m of Cretaceous and younger sediments (Drozdzewski and Wrede, 1994; Juch et al., 1994) (Fig. 4.1c). The basin contains more than 150 coal seams, occurring within more than 4000 m of cyclically deposited clay-, silt-, and sandstone of deltaic to fluviatile-lacustrine origin (Drozdzewski, 1993; Wrede

67 and Ribbert, 2005; Suess et al., 2007). The coal-bearing formations of the Subvariscan basins were repeatedly subjected to marine ingressions, likely caused by glacio-eustatic sea level fluctuations, most probably coming from the west (Krull, 2005; Ziegler, 2012). The first peat mires in the Ruhr Basin formed during the Marsdenian with coal formation peaking during the upper Langsettian (Westphalian A) and Duckmantian (Westphalian B) (Drozdzewski, 2005). The Langsettian formations have thicknesses of up to 1210 m (Wrede and Ribbert, 2005) and sediments between the coal-bearing layers are predominantly of marine, deltaic, and in the Upper Westphalian A2 of lacustrine origin (Suess et al., 2007). The marine Katharina Horizon marks the transition to the Duckmantian Essen and Horst Formations, which are separated by the marine Domina Horizon, having a combined thickness of 800 m (Wrede and Rippert, 2005). Depositional conditions varied between upper and lower delta plain (Süss, 1996; Suess et al., 2007). Due to the northward migration of the Variscan deformation front, the center of deposition and maximum coal formation shifted to a northwestern direction during the Westphalian (Drozdzewski, 1993). The marine Ägir Horizon marks the transition from the Duckmantian to the Bolsovian (Westphalian C) and is the last, fully marine horizon deposited in the Ruhr Basin during the Carboniferous. Bolsovian strata are, based on the occurrence of a marine influenced horizon above seam Nibelung, subdivided into the Dorsten (C1) and the Lembeck (C2) Formations (Bless et al., 1977; Fiebig and Groscurth, 1984). From the late Bolsovian onwards, the areas of sedimentary supply were uplifted, which is reflected by an increase in the proportion of sandstones and rooted soils in the sediments deposited during Bolsovian and Asturian (Westphalian C and D) (David, 1990). Sedimentary conditions changed from upper delta plain to advancing alluvial conditions (Suess et al., 2007). Asturian and younger Palaeozoic layers are not present in the Ruhr Basin, but can be found in the nearby Ibbenbüren area. Littke et al. (2000) calculated an erosion of up to 2500 m of Palaeozoic sediments prior to the Upper Permian. Apart from sedimentary conditions, a change from tropical-humid to semi-arid climate from the Asturian onwards is documented by the occurrence of caliche horizons in the Ruhr Basin (Selter, 1989; David, 1990) and the absence of pseudomatrix consisting of altered and compacted silicate fragments, which is indicative for strong meteoric leaching and typically occurs in Bolsovian sandstones of the adjacent Campine Basin (Bertier et al., 2008).

4.3 Samples and Methods 4.3.1 Samples The coal seams analyzed in this study were cored vertically at depths between 671 and 1224 m in the course of the exploratory drill Hervest 5. The borehole was drilled between Marl and Hervest in the northern Ruhr area, in the center of the Lippe Syncline (Fig. 4.1a). Here, Bolsovian strata were reached at a depth of 638 m below Cretaceous and younger overburden. Out of the 56 seams accessed by the drilling, 11 seams from the Bolsovian (5 seams from the Dorsten and 6 seams from the Lembeck Formation) with thicknesses ranging between 25 cm and 165 cm, were selected and sampled in intervals of 1-12 cm. An overview of the sampled seams, number of samples and coring depth is shown in Fig. 4.2. Half of the vertically cut sample sections were prepared for microscopy, while the remaining sample was ground to powder for the bulk and elemental analyses using a rotation mill.

4.3.2 Elemental and bulk analyses, Rock-Eval pyrolysis The ash yield was determined in accordance with the guidelines of DIN 51719 (1997) in triple measurements by combustion of 1 g pulverized, dry sample between 500 and 815 °C. See Prinz et al. (2017) for a more detailed description of the procedure.

Total carbon content was determined as the sum of total organic (TOC) and total inorganic carbon, measured with a LiquiTOC II analyzer (Elementar). 20 mg of pulverized coal and between 50-100 mg mudstone were heated in a steam of oxygen to release organic-bound and inorganic-bound carbon. A detailed description of the measuring process and in particular the temperature program is described in Grohmann et al. (2018). The device was calibrated at an interval of 6 measurements with a soil standard (47.1% TC).

Total sulphur content (TS) was determined using a Leco total evaporation analyzer (S- 200). A quantity of 20 mg sample powder, mixed with iron fillings, was introduced to the oxygen-flooded analyzer and incinerated at 1800 °C. The device was calibrated periodically with a Leco standard (0.31% TS).

For determination of hydrogen index (HI) and temperature of maximum hydrocarbon formation (Tmax) 10 mg coal (mixed with sand) and 50 mg organic-rich mudstone, respectively, were pyrolyzed using a Rock-Eval 6 apparatus (Vinci Technologies),

69 calibrated with a sediment standard (7.65% TOC; Tmax = 432 °C). Pyrolysis was performed according to the basic method as described in Behar et al. (2001) within a temperature range of 300 °C to 650 °C.

Fig. 4.2 Simplified stratigraphic section of the Lembeck (left) and Dorsten Formations (right) as cored by the Hervest 5 well in the Lippe Syncline. Seam groups are shown, and sampled seams are indicated by arrows and seam thicknesses and number of samples (n) are given for these.

4.3.3 Organic petrography Pieces of coal and organic-rich claystones were cut wet into pieces of 2 x 1-9 cm perpendicular to bedding with a diamond circular saw. After drying, the pieces were transferred into silicone embedding molds, which were then filled with a two-phase epoxy resin (Araldite®). After curing of the resin at 45 °C (12 h), the sections were ground and polished manually on a Tegra Pol-21 polishing device (Struers), first at 300 r/min using silicon carbide grinding papers with increasingly finer grain sizes (120 µm, 25 µm, 15 µm) and water as a lubricant. Polishing was performed on the same device at 150 r/min for 3-4 min per step using Struers polishing cloth and water-based diamond suspensions with 9 µm (Dia Pro Plan 9) and 1 µm (Dia Pro Nap B) abrasive particles, respectively. In a final polishing step, a colloidal silica suspension (OP-U, 0.04 µm) with the addition of a small amount of rinsing agent and water was used.

Vitrinite reflectance was measured with an incident light microscope (Zeiss Axioplan) fitted with a HAL 100 illuminator (12 V, 100 W) with a quartz collector lamp mount and heat-recycling filter coupled to a Basler monochrome digital camera (1392x1040 px) with an integrated interference filter for 546 nm wavelength. The system is connected to a pc implementing the Diskus-Fossil software (Hilgers Technisches Büro) for reflection measurement and evaluation. Vitrinite reflectance was measured with a Zeiss Epiplan- Neofluar objective (50x) in oil immersion (Zeiss Immersol®, n=1.518 at 23 °C) on 100 telocollinites per sample with a measurement spot size of 1143.67 µm2. Prior to each measurement, the microscope was calibrated with a leuco-sapphire standard (Klein & Becker) with a reflectance of 0.592%.

Maceral analysis was performed with an Axio Imager.M2m incident light microscope (Zeiss) equipped with a VIS/LED illuminator for white light and an HXP 120 C illuminator for UV/blue light excitation, respectively, and an Epiplan-Neofluar oil immersion objective. Images taken with a Basler Scout color digital camera were directly processed by the program Fossil (Hilgers Technisches Büro), which also controls the automatic sample stage. Fluorescence was excited by ultraviolet light with a beam splitter at 460 nm and a barrier filter at 470 nm. The stage was programmed to move on an equidistant grid. Between 500 and 1200 points per sample and measurement (white- and UV light) were counted employing the two-scan method as described in Taylor et al. (1998). Macerals counted in white light were normalized to the liptinite content counted

71 in fluorescence mode. Macerals were distinguished following the nomenclature of the Stopes-Heerlen system. Identification was based on the definitions and descriptions given in ICCP (1998), ICCP (2001), Pickel et al. (2017), and Taylor et al. (1998). Collotelinite was further differentiated into collotelinite bands smaller and larger than 50 µm, respectively. Sporinite macerals were either counted as sporangia, thin-walled tenuisporinite, thick-walled crassisporinite, and densosporinite. Densosporinite has been identified based on its typical dumbbell-shaped form (see Taylor et al., 1998).

4.4 Results 4.4.1 Elemental and bulk analyses, Rock-Eval pyrolysis The seams Baldur, Hagen, Odin III and IV, and the youngest seam Rübezahl I have low ash yields of 5.2 wt% to 8.4 wt% on average (Tab. 4.1). The two seams of the Parsifal group and especially seam Midgard II are characterized by several intercalations of organic-rich, fine-grained claystones. Seam Chriemhild I has higher ash yields in its upper section. The average TOC contents of the seams vary between 40.4 wt% and 77.2 wt% (Tab. 4.1) and are lower in seams intercalated by thin layers of claystone or in those which are grading into the overlying sediments. For a better overview and because of the good correlation between ash yields and TOC contents (r2=0.96), both values are expressed as the ratio TOC/A (TOC/ash yield) in Figs. 4.3 and 4.4. TIC contents are very low, although seam Rübezahl I shows some higher values of up to 2.0 wt% (Tab. 4.1). Total sulphur contents are generally low (Ø 1.9 wt%). Samples originating from the Parsifal group show higher values of up to 7.2 wt% and 14.9 wt%, respectively. Average hydrogen indices (HI) of the seams vary between 91.7 mg HC/g TOC (Odin IV) and 220.4 mg HC/g TOC (Erda), respectively, with a mean value of 164.4 mg HC/g TOC for all samples analyzed.

Average Tmax values range between 419 °C and 430 °C.

4.4.2 Organic petrography Average vitrinite reflectance values vary between 0.58% for the youngest seam (Rübezahl I) and 0.79% for the stratigraphically oldest seam (Baldur), following the expected maturity trend within the succession from high volatile bituminous C to A coal. Slightly deviating from this trend, the Parsifal II and Odin III seams show lower average values of 0.59% and 0.61%, respectively (Tab. 4.1, Figs. 4.3, 4.4).

Tab. 4.1 Minimum (Min), maximum (Max) and average (Ø) values of vitrinite reflectance, elemental and bulk analyses and Rock-Eval parameters.

VRr TOC TIC TS Ash yield HI Tmax % wt.% wt.% wt.% wt.% mg HC/g TOC °C Rübezahl I Min 0.52 63.37 0.00 0.57 1.19 89 418 Max 0.63 78.81 2.01 6.01 16.39 261 427 Ø 0.58 72.49 1.52 1.51 5.65 137 423 Parsifal I Min 0.55 10.07 0.00 0.39 3.27 74 416 Max 0.68 72.14 0.16 6.96 79.50 279 431 Ø 0.61 52.76 0.04 2.80 23.48 154 422 Parsifal II Min 0.55 34.56 0.01 1.23 6.62 111 416 Max 0.65 69.81 0.67 14.56 50.33 273 425 Ø 0.59 55.32 0.13 6.80 19.29 189 419 Odin III Min 0.55 61.19 0.02 1.26 2.02 110 417 Max 0.66 76.26 0.25 2.52 26.57 282 426 Ø 0.61 70.50 0.11 1.67 8.42 170 421 Odin IV Min 0.64 47.53 0.08 1.01 9.25 125 426 Max 0.74 67.96 0.34 1.50 33.39 213 427 Ø 0.68 56.21 0.17 1.37 22.46 167 426 Odin V Min 0.69 58.56 0.00 1.23 1.65 71 426 Max 0.75 73.18 0.03 1.39 18.21 121 434 Ø 0.72 68.93 0.01 1.31 5.87 92 430 Midgard II Min 0.70 6.01 0.01 0.30 9.79 93 423 Max 0.73 69.35 2.39 9.32 84.25 256 435 Ø 0.72 40.41 0.20 1.75 43.80 145 426 Hagen II Min 0.62 67.26 0.00 0.72 2.12 115 425 Max 0.75 81.70 0.97 2.65 14.47 216 431 Ø 0.69 77.17 0.16 1.14 5.24 169 428 Erda Min 0.60 0.55 0.00 0.03 2.05 125 408 Max 0.76 81.11 0.01 2.58 85.80 325 444 Ø 0.71 62.24 0.00 0.86 17.78 220 427 Chriemhild Min 0.74 35.31 0.00 0.58 1.35 100 423 Max 0.79 80.34 0.02 1.41 52.28 208 430 Ø 0.77 54.95 0.01 1.03 30.24 144 426 Baldur Min 0.76 61.13 0.00 0.79 1.84 69 425 Max 0.84 80.54 0.12 1.35 24.18 346 438 Ø 0.79 74.28 0.01 1.07 8.74 220 429

Fig. 4.3 Profiles of the seams from the Lembeck Formation showing the maceral composition (left, representing 100 vol%) and selected results from elemental, bulk, and microscopic analyses and Rock-Eval pyrolysis. For explanations, see text.

Fig. 4.4 Profiles of the seams from the Dorsten Formation showing the maceral composition (left, representing 100 vol%) and selected results from elemental, bulk and microscopic analyses and Rock-Eval pyrolysis.

the 11 Bolsovianthe coal seams. Collotelinites were separately counted as withbands thicknesses ≤ and 50 µm ≥ (see subscripts). Resinites were distingu Tab. Resinite Fluorinite Mineral Matter Mineral ∑Liptinite Exsudatinite Liptodetrinite ∑Resinite Resinite Cutinite ∑ Sporinite Sporangia Tenuisporinite Densosporinite Crassisporinite Megasporinite ∑Inertinite Funginite Micrinite Macrinite Inertodetrinite Semifusinite Fusinite ∑Vitrinite ∑Gelovitrinite Gelinite Corpogelinite Detrovitrinite ∑Telovitrinite Collotelinite Collotelinite Telinite Maceral 4.2

cell corp

Minimum (Min), maximum (Max) and average (Ø) percentages volume of macerals and maceral groups counted for

>50µ <50µ

56.1 33.3 Min 0.0 0.0 0.0 2.4 0.0 1.2 0.0 0.0 0.0 0.4 0.0 0.1 0.0 0.0 0.0 2.1 0.0 0.0 0.0 0.1 0.3 0.2 4.2 2.8 0.4 5.8 8.9 8.5 0.2

Rübezahl I Rübezahl

21.9 14.8 32.0 17.3 90.8 16.9 11.0 24.5 74.3 55.1 32.8 35.0 Max ished according to their occurrence either as cell fillings (resinite 7.2 0.4 6.2 1.9 1.4 1.5 1.0 4.3 1.1 4.9 1.0 9.0 2.8 0.6 2.9 2.0 6.3 7.8 8.7

10.4 12.7 75.1 11.6 54.8 26.7 20.7

0.2 0.3 1.8 0.1 3.1 0.5 0.3 1.6 4.9 0.3 1.1 0.1 2.5 1.0 0.2 1.1 0.6 2.1 3.1 5.6 8.7 5.7 3.0 7.3 Ø

22.5 13.9 10.2 Min 0.0 0.0 0.4 2.3 0.0 0.8 0.0 0.0 0.2 0.2 0.0 0.2 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.0 0.0 2.7 1.5 1.2 5.3 0.8 0.3

Parsifal I Parsifal 71.8 24.4 22.6 15.5 89.5 12.8 20.0 74.9 40.7 43.3 Max 1.4 2.8 0.6 8.1 1.5 1.1 5.8 8.7 1.3 3.9 0.3 4.8 1.1 0.6 1.2 1.3 4.2 7.9 5.2 8.6 6.1

12.6 10.2 70.7 13.6 48.6 18.7 27.3 0.2 0.6 1.8 3.8 0.2 2.1 0.0 1.1 0.3 6.5 0.1 0.4 0.3 1.4 1.2 3.3 8.5 3.2 5.2 2.6 0.1 3.5 0.5 0.3 Ø

19.0 12.7 Min 0.0 0.0 0.6 2.0 0.0 0.3 0.0 0.2 0.0 1.1 0.0 0.2 0.0 0.4 0.2 0.2 0.7 0.4 0.4 5.6 3.6 8.8 0.2 0.6 5.3 0.0 1.8 0.0 0.0

Parsifal II Parsifal 14.5 23.5 30.6 26.8 13.2 92.6 14.9 12.9 18.0 71.7 31.9 32.1 30.0 Max 0.4 8.3 1.0 0.3 0.7 1.4 2.9 0.8 4.7 0.3 6.5 2.2 0.5 0.6 1.1 8.5 1.8 5.6

17.9 24.3 13.1 70.2 12.3 48.1 0.8 7.4 5.9 9.2 0.2 3.9 0.3 0.1 0.2 0.5 1.9 6.2 0.2 2.6 0.1 2.6 0.1 0.4 0.5 2.3 0.8 3.3 9.9 3.1 6.8 Ø

36.0 19.8 14.9 Min 0.6 6.5 0.0 1.5 0.0 0.0 0.0 0.0 0.9 2.9 0.0 0.5 0.0 0.8 0.1 6.6 0.0 0.2 0.1 1.8 1.5 2.2 4.0 1.9 1.4 5.1 4.6 0.2

Odin III Odin 11.3 25.2 10.3 13.6 39.6 10.7 14.2 18.8 74.3 14.2 12.9 51.3 29.1 30.8 19.4 Max 1.1 2.3 1.2 1.7 1.0 4.1 0.6 4.3 0.8 6.7 4.1 0.2 1.7 3.9 8.1 9.7

14.5 22.2 59.9 41.6 14.1 22.4 3.4 0.3 3.8 0.8 0.5 0.3 0.5 1.9 7.2 0.2 1.9 0.1 3.4 1.6 0.0 0.9 1.2 4.8 6.7 8.5 9.9 4.3 5.7 8.3 5.1 Ø

cell 10.5 11.4 39.2 11.2 22.8 Min 1.4 7.2 0.0 3.0 0.2 0.2 0.0 0.2 1.1 1.3 0.0 0.9 0.0 0.0 0.0 9.0 0.0 0.2 0.0 2.8 2.4 2.1 4.7 3.5 5.2 0.0 ) isolated or particles (resinite

Odin IV 21.3 17.2 22.3 11.2 13.8 71.6 15.0 10.3 10.5 48.5 24.2 32.4 Max 0.0 7.2 0.8 0.8 0.0 2.1 4.6 3.7 0.7 1.2 0.0 2.1 0.2 0.0 0.5 0.7 9.6 8.1 0.9

12.4 17.8 61.3 13.2 41.0 17.6 22.8 8.4 0.0 5.2 0.5 0.5 0.0 1.1 3.3 2.3 0.4 1.0 0.0 0.9 0.0 0.0 0.3 0.4 4.9 5.1 7.2 6.4 6.8 7.2 0.6 Ø

76.8 63.2 18.8 22.3 Min 0.0 3.0 0.0 1.0 0.0 0.0 0.0 0.0 0.6 1.3 0.0 0.4 0.0 0.5 0.0 0.6 0.0 0.0 0.0 0.2 0.0 0.0 7.1 1.8 4.4 6.4 7.1

Odin V 12.0 95.6 14.7 12.4 12.1 72.2 33.4 37.2 22.5 Max 1.9 0.2 6.9 0.1 0.1 0.0 0.2 1.8 4.3 0.2 2.6 0.2 1.9 0.4 9.4 0.0 0.5 0.2 1.9 2.1 5.4 8.3

89.7 11.6 69.3 25.2 30.1 14.0 0.5 6.2 0.1 2.5 0.0 0.0 0.0 0.1 1.0 2.5 0.1 1.0 0.0 1.1 0.2 3.5 0.0 0.2 0.1 0.7 0.7 1.9 3.8 7.8 8.8 Ø

corp

Ø 3.8 0.1 0.1 7.9 3.1 0.0 3.1 5.6 3.2 3.8 0.4 0.8 0.0 2.5 0.8 3.3 1.1 0.3 8.0 1.8 0.1 0.3 0.0 0.3 2.7 0.0 58.3 58.6 69.6 13.8 12.8

0.8 1.4 8.4 0.0 8.4 7.4 9.4 1.0 1.4 0.2 6.0 2.0 7.7 2.5 1.4 3.9 0.4 0.8 0.2 1.0 5.3 0.0 83.1 13.9 84.4 16.4 89.9 14.6 32.2 15.0 20.8 Max Baldur

0.0 0.0 2.5 0.8 0.0 0.8 1.1 0.6 1.0 0.0 0.2 0.0 3.4 0.4 0.0 0.2 0.2 0.0 2.5 0.4 0.0 0.0 0.0 0.0 0.8 0.0 4.4 0.2 Min 41.7 41.9 45.8

Ø 0.1 1.2 5.2 5.1 0.0 5.1 4.4 2.0 2.6 4.2 1.3 0.3 0.5 0.6 1.0 1.2 0.1 3.5 2.0 0.4 0.7 0.0 0.7 0.8 0.0 8.2

55.3 56.6 66.9 14.9 20.9

0.4 6.1 0.0 2.7 2.3 0.8 0.9 0.0 1.9 2.5 2.2 1.9 0.4 6.3 3.2 0.8 0.9 0.0 0.9 2.0 0.0 69.7 75.7 10.8 10.9 10.9 90.4 11.6 17.9 12.5 50.9 Max Criemhild Criemhild

0.2 0.0 0.0 1.1 2.1 0.0 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 0.0 0.0 0.2 0.0 1.7 0.8 0.0 0.2 0.0 0.2 0.2 0.0 3.5 Min 35.9 35.9 39.1

Ø 7.7 5.3 2.0 7.3 8.3 3.6 3.6 7.6 2.9 0.2 1.5 1.5 0.4 2.4 1.2 6.9 2.5 0.6 0.5 0.5 1.0 4.1 0.7 8.8 11.7 12.3 31.7 10.2 49.2 26.2 15.8

4.3 1.0 8.1 9.5 6.5 4.0 2.9 1.1 4.0 2.5 5.5 2.3 2.5 1.2 3.6 1.6 23.9 31.2 48.9 16.2 11.0 15.0 73.4 19.8 35.3 42.1 14.1 55.9 28.0 12.8 39.2 Max Erda

0.0 1.5 0.6 2.8 2.6 0.6 0.6 1.1 7.1 0.0 0.6 0.2 0.6 0.0 0.0 0.1 0.2 0.0 0.2 0.0 0.6 0.2 0.0 0.0 0.0 0.0 0.2 0.0 0.9 1.6 Min 13.0 continued.

4.2 Ø 9.0 1.9 0.8 2.7 2.2 2.2 0.8 2.5 0.5 0.8 3.3 0.6 1.3 0.6 6.4 3.8 0.2 1.1 1.0 2.1 0.9 0.1 2.4 65.6 10.2 13.9 12.5 27.5 53.9 18.5 13.5

Tab. Tab. 4.1 3.3 7.4 6.3 5.4 2.1 6.2 1.8 1.8 5.2 1.4 2.2 1.9 6.4 1.0 4.4 2.0 4.6 2.1 1.6 8.8 88.7 25.4 44.1 20.7 39.7 79.0 21.2 35.8 11.7 21.6 Max Hagen II

1.3 0.6 0.0 0.0 0.2 0.7 2.9 6.3 3.8 4.8 0.2 0.0 0.2 0.3 0.0 1.1 0.0 3.4 0.0 1.2 0.0 0.4 0.0 3.1 1.1 0.0 0.0 7.0 0.7 Min 40.2 48.4

Ø 0.2 2.1 5.1 2.8 0.1 2.8 4.9 1.3 1.7 0.2 0.1 0.0 8.2 0.5 1.3 0.7 1.6 0.2 4.3 3.4 0.3 0.5 0.0 0.6 1.5 0.0

39.6 41.9 49.8 10.1 31.9

6.7 1.7 1.5 0.2 1.5 1.2 7.2 0.4 7.4 1.1 0.6 0.7 1.9 6.0 2.1 8.3 0.8 6.0 0.2 84.2 36.2 76.8 16.1 77.5 18.6 11.8 45.3 17.6 12.5 33.6 83.6 Max

Midgard II

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.2 0.0 Min 13.6 13.6 15.3

Maceral Telinite Collotelinite<50µ Collotelinite>50µ Telovitrinite ∑ Detrovitrinite Corpogelinite Gelinite Gelovitrinite ∑ Vitrinite ∑ Fusinite Semifusinite Inertodetrinite Macrinite Micrinite Funginite Inertinite ∑ Megasporinite Crassisporinite Densosporinite Tenuisporinite Sporangia Sporinite ∑ Cutinite Fluorinite Resinitecell Resinitecorp Resinite ∑ Liptodetrinite Exsudatinite Liptinite ∑ Mineral Matter

Fig. 4.5 Microphotographs (270x190 µm) showing a) telinite surrounding resinitecell (Rübezahl I), b) typical association of detrovitrinite with liptinite and inertodetrinite between collotelinite bands (Parsifal II), c) sporangia in seam Rübezahl I in incident white and d) UV light, e) densosporinite in a matrix of inertinite in incident white and f) UV light (Chriemhild I), g) fluorinite associated with cutinite, h) the same picture in UV light (Parsifal II), i) different forms of cutinite in white, and j) UV light (Parsifal II).

Petrographically, all seams are dominated by vitrinite with collotelinite being the most abundant maceral, in most coal samples occurring as bands thicker than 50 µm

(collotelinite>50µm). Telinite, surrounding corpogelinite and more rarely resinitic cell fillings (resinitecell), is a common feature in most of the studied coal seams (Tab. 4.2, Fig. 4.5a). Gelinite, in significant proportions, only occurs in the seams of the Lembeck layers, here ranging between 3.1 vol% and 6.4 vol%. Detrovitrinite is in most cases associated with sporinite and inertodetrinite, intercalated between layers of collotelinite (Fig. 4.5b). Inertinite contents are especially high in seams Odin III (Ø 22.2 vol%) and Erda (Ø 26.2 vol%) (Figs. 4.3, 4.4). The most abundant macerals of the inertinite group are fusinite, semifusinite, and inertodetrinite (Tab. 4.2). Seams Erda and Chriemhild I show significant amounts of macrinite, 7.6 vol% and 4.2 vol%, respectively, while this maceral is less abundant in the other seams. Funginite is very rare and micrinite accounts for up to 2.9% (Ø Erda). Liptinites on average account for 6.2 vol% to 15.8 vol% of the macerals in the different seams (Figs. 4.3, 4.4), with either sporinite or liptodetrinite (Odin IV and V) being most abundant. Sporangia are present in all seams analyzed (Fig. 4.5c, d). Densosporinite occurs mainly in seams of the Dorsten layers in percentages of up to 3.3 vol% (Ø Baldur, Fig. 4.5e, f) and is either rare or absent in seams of the Lembeck layers. Cutinite occurs in different forms, either thick-walled, with an orange fluorescence or as fine streaks usually surrounding aggregates of bright fluorescing fluorinite (Fig. 4.5g, h). It is the second most abundant liptinitic maceral in some seams with average percentages between 1.0 and 3.8 vol% (Tab. 4.2). Average resinite contents are generally <1 vol%, except for seam Hagen II (Ø 2.1 vol%).

4.5 Discussion 4.5.1 Trends within the Bolsovian coal layers of the Ruhr Basin In general, vitrinite reflectance increases with depth for the seams analyzed in this study.

Coal samples from the Parsifal II and Odin III seams, however, show slightly lower VRr and Tmax values than their stratigraphic level within the seam succession would lead one to expect (Fig. 4.6). Suppressed vitrinite reflectance is reported to occur in source rocks with high liptinite contents (Raymond and Murchison, 1991; Diessel and Gammidge, 1998). The presence of lower reflecting vitrinite might also be explained by deposition in

79 anoxic or alkaline depositional environments, or by floras producing rather hydrogen- or aliphatic rich vitrinite (Carr, 2000). Comparison of the average VRr values and liptinite contents shows that both parameters seem to be related (Fig. 4.6). Average TS values are higher in coals from the Lembeck as compared to those of the Dorsten Formation. A correlation between TS and VRr values is not implied (Fig. 4.6).

Fig. 4.6 Average values of vitrinite reflectance, liptinite content (L) and total sulphur plotted against depth. Grey lines mark seams of the Parsifal (upper) and Odin Groups (lower).

4.5.2 Depositional environments To deduce the depositional environment of palaeo-peat mires several indicators have been used ranging from organic geochemical to petrographic parameters. Maceral analysis, by determining the composition of organic constituents, provides information about water and oxygen availability and the generation of coal facies (Teichmüller, 1989). The

80 groundwater index (GWI), introduced by Calder et al. (1991), compares constituents indicative of a high water table. Vitrinite, not affected by gelification, is interpreted to develop at an early stage after deposition as the result of biochemical processes and is thus a proxy for hydrological conditions during peat formation (Taylor et al., 1998; ICCP,

1998). Here, we use the GWIAC expressed as:

+ 2 = 𝑎𝑎𝑎𝑎ℎ 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 𝐺𝐺𝐺𝐺𝐺𝐺𝐴𝐴𝐴𝐴 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 − 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔

The ratio, modified by Stock et al. (2016) by using the ash yield instead of counted minerals to help with the difficulty of microscopically assessing the mineral matter content, is here applied for bituminous coals. Ash yield is divided by two in Stock’s equation, because of the on average about two times greater density of minerals as compared to organic matter, leading to roughly two times lower volume percentages.

Variations of GWIAC values calculated for the coal profiles analyzed are shown in Figs. 4.3 and 4.4. In his peat depositional model, Calder (1991) used the ratio of macerals indicative for more arborescent plant species to those that are typical for a rather herbaceous flora to take into account not only water availability and thus nutrient supply but also possible changes in vegetation of palaeo-peat mires. This vegetation index (VI) is calculated as:

+ ( ) + = + + + + + 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑟𝑟𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑉𝑉𝑉𝑉 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐

The use of such indices for the assessment of palaeoenvironments is, however, controversial (Scott, 2002; Moore and Shearer, 2003). They can also be misleading, because they might, based on high telovitrinite content, suggest woody or arborescent vegetation, although most plants were small and vegetation in peats contained little wood. Littke (1985) published twenty-one coal seam profiles for Duckmantian and Bolsovian coals demonstrating that vitrinite (vitrain) layers are rarely more than 0.5 cm thick; thus, any trees were quite diminutive in size. In general, we share some of these reservations,

81 especially the assumptions that certain macerals are exclusively assigned to certain depositional features (all lower vascular flora reproduced via the spore habit, regardless of size), but at the same time the use of these indices enables a direct comparison to other coal deposits. This is especially useful to detect variations within coal-bearing strata of the same basin or between these strata of different depositional settings. GWIAC and VI values for the coals of the Dorsten and Lembeck Formations are shown in Fig. 4.7. According to Diessel et al. (2000) and Diessel (2007), there are two basic styles of paralic peat formation. Either peat formation follows the pathway of terrestrialisation, i.e. a mire develops from topogenous to ombrogenous and finally stops because of insufficient water availability, or the reverse pathway of paludification.

Fig. 4.7 Diagram after Calder et al. (1991) showing GWIAC and VI values of individual samples from the Bolsovian seam profiles. The evolution of seams Baldur (grey) and Midgard (light grey) is indicated by shaded areas.

Often, these pathways alternate. For most coals, exceptions are coals containing tonstein bands or high amounts of epigenetic precipitates, the ash yield is highly indicative for topogenous conditions during peat formation. Most common components of coal mineral matter are quartz and silicates (mainly clay minerals), transported into the peat mire mainly by water, or sulfides (mainly pyrite) of early diagenetic origin (Taylor et al., 1998; Ward, 2016). On the other hand, high percentages of inertinite are indicative of oxidation processes and thus subaerial oxidation. Especially fusinite and semifusinite are thought to be originated from oxidation through fires, desiccation, incomplete combustion or subaeral oxidation (Diessel, 1992; Guo and Bustin, 1998b; ICCP, 2001), whereas other inertinite macerals such as macrinite can also be oxidized by microbial or fungal degradation, thus not being exclusively indicative for redox conditions (Hower et al., 2013). Usually (semi-) fusinite contents increase towards more ombrogenous depositional conditions (Eble and Grady, 1993; Hower et al., 1996). In an attempt to better describe the evolution of the coal peats from base to top, we employ the ratio of ash to semifusinite and fusinite (A/I):

/ = + ( ) 𝑎𝑎𝑎𝑎ℎ 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝐴𝐴 𝐼𝐼 𝑎𝑎𝑎𝑎ℎ 𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦𝑦 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 − 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓

The A/I ratio serves as a simple indicator of distinction between the two depositional end- members being topotelmites with high water tables and intermittently dry ombrotelmites (Fig. 4.8).

4.5.2.1 Dorsten Formation Seam Baldur, in its basal part, shows features of a rather mesotrophic peat, evolving towards ombrotrophic peat from a most probably forested bog with main organic material input derived from small, arborescent plants as indicated by high amounts of relatively thick collotelinite bands (Tab. 4.2, Fig. 4.7). In Fig. 4.7, the general depositional pathway of Baldur, indicating a more herbaceous vegetational input in the central part of the seam, is depicted by the grey areas (I-III). This shift towards lower VI values can be explained by an increasing amount of sporinite in this section of the seam, dominated by densosporinite. The densosporinite content is highest for all seams analyzed, and the

83 occurrence of high inertinite percentages at the same time is reported to be typical for the central parts of raised bogs (Littke, 1987; Taylor et al., 1998). A/I ratio show mainly values <0.5 and suggest that ombrogenous, rather dry conditions persisted over most of the time of peat accumulation (Fig. 4.8). It is highly probable that peat growth under such rather extreme ombrogenic conditions was slow. Therefore, seam Baldur represents a much longer time span than other coals of similar thickness. Furthermore, low TS values support ombrotrophic conditions in the central and upper parts of the seam. Interestingly, Syngenetic pyrite, formed by sulfate reducing bacteria, is rarely found in coals which formed as raised bogs, as the conditions there are usually too acidic (Diessel, 1992). The overlying sandstones are interpreted by Strehlau and David (1989) as distributary channel sediments.

Fig. 4.8 Variations in A/I ratios of seams from the Lembeck (left) and Dorsten Formations (right) from base (left) to top (right). See text for explanations.

Seam Chriemhild I shows similar indicators for deposition as ombrogenic peat in its central part but rapidly increasing A/I and GWIAC values show an evolution of the peat mire towards paludification and finally drowning at the top (Figs. 4.4, 4.8).

GWIAC and VI values indicate that deposition of seam Erda started at topogenic to mesotrophic conditions, interrupted by an inundation or flooding event as reflected by an approximately 10 cm thick layer of fine-grained siliciclastic sediment above the base (see Fig. 4.3). Relatively low ash yields and increasing inertinite (mainly composed of fusinite and semifusinite) and liptinite contents depict two cycles towards more ombrotrophic, oxidative conditions (Figs. 4.3, 4.8). As opposed to seams Chriemhild and Baldur, densosporinite is very rare, even though the overall low ash yields and sulphur contents, accompanied by high proportions in fusinite and semifusinite in the central and upper part of the seam, are typical for peats originated from raised bogs. The absence of a densosporinite facies in German Upper Bolsovian coals has been previously described by Strehlau (1990) and Littke (1987), who explained its fate upwards in the Pennsylvanian Ruhr Coal Basin with changes in drainage patterns and drier climate, respectively.

Seam Hagen is the thickest continuous (non-intercalated) coal seam analyzed in this study. It is especially rich in resinite and the overall low GWIAC (<0.2), caused by low amounts of mineral matter and gelified vitrinite, clearly indicating peat deposition under ombrotrophic conditions according to the palaeoenvironmental plot after Calder et al. (1991). Similar to seam Baldur, it can be assumed that it represents a long time span due to slow growth rates of the ombrogenic peat. Overall, the coal is especially rich in cutinite and crassisporinite, both indicative of a rather herbaceous and leafy vegetation. The profile indicates two cycles of decreasing vitrinite contents in favor of inertinite macerals (Fig. 4.3). This can be explained by gradually increasing and subsequently decreasing oxic conditions or by periodic changes of the groundwater level.

The high mineral matter and at the same time very low inertinite content of the lower part of seam Midgard, which is repeatedly intercalated by cm to dm thick silty sediments, are typical features of peats deposited in topogenous mires under meso- to rheotrophic conditions (Diessel, 1992). TS values are slightly higher than for the other seams analyzed (Ø 1.75 wt%), which further indicates rheotrophic conditions. In the uppermost 20 cm of seam Midgard, vitrinite content (mainly thin telocollinite bands and detrovitrinite) increases, while the amounts of inertinite and liptinite decrease. In the palaeoenvironmental model of Calder et al. (1991), seam Midgard represents a typical fen with an indicated shift towards a more herbaceous vegetation in its upper part (see

85 light grey areas IM-IIIM in Fig. 4.7). The A/I ratio indicates that paludification occurred at the end before peat accumulation stopped due to drowning (Fig. 4.8).

4.5.2.2 Lembeck Formation As compared to the Dorsten coal profiles, classification of seams of the Lower Odin Group, regarding their depositional environment, is less clear. This is especially true for Odin V and IV, for which the applicability of the A/I ratio seems to be limited, simply because ash yields and volumetric (semi-) fusinite percentages show positive correlations. Possible explanations for this could be a non-authigenic origin of inertinite, wind blown into the mires. Moderate TS values and high ash yields of samples from the top of Odin

V and the central and upper parts of Odin IV and III, as well as GWIAC values indicate deposition under mesotrophic conditions (Tab. 4.2).

Seams Parsifal II and I are, compared to the other seams of the Lembeck Formation, rich in mineral matter and detrovitrinite (Tab. 4.2). In the depositional diagram of Calder et al. (1991) they plot as mesotrophic to partly ombrotrophic mires with variable vegetation patterns (Fig. 4.7). The A/I ratios are high for the thin Parsifal II and the lower part of Parsifal I, suggesting that high water tables prevailed during deposition of the peats (Fig. 4.8). Both seams of the Parsifal Group, and in particular Parsifal II, show exceptionally high TS values compared to the other seams from the Dorsten and Lembeck Formations. High TS values are typical for coal seams influenced by marine waters or overlain by marine roofs. The petrographic analysis of this seam reveals high proportions of pyrite framboids, which form during early diagenesis as a result of bacterial reduction of sulfate- rich waters (Chou et al., 2012; Stock et al., 2016; Ward et al., 2016). A further indication for brackish/marine influence is the distribution of these framboidal pyrites within the seam succession. TS values are higher at the base and top of Parsifal II. Such a distribution was described as typical for coals deposited in a transgressive system, in proximity to the sea (Diessel, 1992; Banerjee et al., 1996). Strehlau and David (1989) reported brackish fossils within sediments of the Upper Odin Group. These were later correlated to the Top Marine Band of the Pennines and South Wales Basins of England by Dusar et al. (2000). Although Jankowski (1991) mentioned the presence of foraminiferous fossils in rooted soils within the Parsifal Group in the Ibbenbüren and north of the Osnabrück area, there are no other reported indications of at least brackish conditions during the Upper Bolsovian in the Ruhr Basin nor other continental Euramerican basins (Bless et al., 1972;

Köwing and Rabitz, 2005). We argue that the observations on seam Parsifal II are a clear indication of marine influence, probably the last one or at least one of the last for almost 50 million years until the Zechstein Sea ingressed into large parts of present-day central Europe.

Seam Rübezahl shows several cycles of changing depositional conditions (Figs. 4.7, 4.8) but in general rather low ash yields, which make large changes of the groundwater table during deposition unlikely. Opluštil and Sýkorová (2018) attributed the cyclic alternation of ombrogenous and topogenous facies to fluctuations between continuous rain-rich and rather seasonal climates, which may be a possible explanation for the cyclic changes of

GWIAC and A/I for the Rübezahl profile. However, most of the peat accumulation in its upper part was related to mesotrophic to ombrogenous conditions, proving that humid climate still persisted during the late Bolsovian.

4.5.3 Comparison of Bolsovian and Duckmantian coals of the Ruhr Basin The Bolsovian seams studied here show much higher VI but only slightly lower GWI values than Duckmantian coal seams from the Ruhr Basin analyzed by Jasper et al. (2010). One reason for lower VI values seems to be the relatively high detrovitrinite contents (13.64 to 39.13 vol% on average) reported for the 8 coal seams analyzed by Jasper et al. (2010), which are denominators in the VI ratio. This may be explained by a larger contribution of detrital vitrinite-precursor plant material to the peat during the Duckmantian (ICCP, 1998). In comparison, Bolsovian peats were characterized by higher contribution of distinct but thin stems and rootlets leading to deposition of telovitrinite. It should be noted, that this precursor telovitrinite represents rather small plants, usually with diameters less than 5 cm. Palynological data points towards a vegetational change between the Duckmantian and the Bolsovian in the Ruhr Basin, characterized mainly by a notable decrease in Sigillarian and Sphenopteroid fern species in favor of non- Sigillarian Lycophhytes and medullosalean Pteridosperms (DiMichele and Phillips, 1994; Cleal et al., 2009, 2012; Uhl and Cleal, 2010). The noticeable decline of densosporinite content in seams younger than Chriemhild might rather be an indicator for such general vegetational changes than for fundamental changes in peat formation, since other indicators show a variety of depositional environments for these seams ranging from clearly topogenic to ombrogenous. However, these youngest layers represent times close to the terminus of densosporinites anyway. The extent to which the reported plant

87 diversity and species changes are related to the micro-petrographic composition of the coals needs to be investigated in future research. Littke (1987), by analyzing Ruhr Basin coals of Duckmantian and Lower Bolsovian age found a depletion of inertinite and an enrichment in mineral matter for the younger coal seams. This trend is verified by our data as compared to data published in Littke (1987) and Jasper et al. (2010). A global decline of inertinite contents in coals from the Upper Pennsylvanian towards the Permian is also reported by Diessel (2010) who found a positive correlation for this process with decreasing global temperatures as inferred by δ18O values. Another factor is related to the northward movement of central Europe out of fully humid tropical conditions towards dryer conditions, which were clearly established in the Asturian and Stephanian stages and in the Permian Rotliegend Formation in the Ruhr area and further north (Selter, 1989; David, 1990; Bertier et al., 2008). However, our data clearly prove that ombrogenic conditions could still persist until the Upper Bolsovian which clearly require humid conditions.

4.6 Conclusion The peat depositional settings during the Bolsovian are represented in coal seams from the Dorsten and Lembeck Formations. They can only be fully assessed by studying complete coal profiles, from base to top, applying a comprehensive set of organic geochemical and petrographical analyses. Our results show the fundamental patterns of variation between topogenous and ombrogenous conditions as previously found for other Pennsylvanian coals. Some Bolsovian coals of the Ruhr Basin formed as topogenous mires and are mainly characterized by high ash yields and moderate TS values. Other coals are ash- and sulphur-poor, appearing to have formed from domed peat bogs. However, there is no clear trend upwards in the successions recorded. Typical ombrogenous peat is indicated for seams Baldur, Erda and Hagen, which in part show a typical densosporinite facies, mainly recognizable by the presence of high volume- percentages of thick-walled spores and abundant inertinite. The A/I ratio of these seams is usually low (<0.5). Densosporinite content, while being high in the aforementioned seams, decreases markedly in the coals from the Middle Bolsovian onwards. High TS values and the occurrence of abundant framboidal pyrite in seams of the Parsifal Group indicate the influence of marine or brackish waters, which was not reported previously

88 for the Bolsovian in the Ruhr Basin at this stratigraphic level. An overall decrease in inertinite content of Pennsylvanian coal seams of the Ruhr Basin from Duckmantian to Bolsovian times indicates either changes towards a colder climate as suggested by Diessel (2010) based on increased glaciation in Upper Pennsylvanian times. Alternatively, and more probably this change was triggered by the progressing northwards movement of the Euramerican landmasses, resulting in a less humid climate effecting the depositional conditions during that time. In addition, this study revealed an influence of high liptinite content on the maturity parameters VRr and Tmax towards lower values.

5 General discussion and outlook

5.1 Peat deposition in the Ruhr Basin A comparison of the maceral composition between the Bolsovian samples analysed in the course of this study and published data from Ruhr Basin coals allows for an overview of peat formation of the coals deposited during the Pennsylvanian. For that purpose, this and the following subchapters will present some general trends regarding depositional conditions that prevailed during peat formation and maturity of coals of Duckmantian to the Bolsovian age. The data presented here is compiled from Littke (1985; 1987), Jasper et al. (2010), and Littke and Zieger (2019b).

Fig. 5.1 Triangular diagram of vitrinite, inertinite, and liptinite contents in coals and associated claystones, siltstones and sandstones (Littke and Zieger, 2019b).

The maceral composition of coals and dispersed organic matter of the Pennsylvanian layers are summarised in Fig. 5.1. Coals are clearly dominated by vitrinite, which accounts on average for 60 to 80 vol% organic matter. Inertinite content of the coals varies between 10 and 30 vol% and liptinite content between 10 and 20 vol%. The maceral group composition of organic matter within sediments associated with these coal seams, however, is much more variable. Sediments underlying the coals show the highest variability. Dispersed organic matter from sediments overlying the coal seams are either dominated by vitrinite or liptinite.

Fig. 5.2 Seam thickness, ash yield, inertinite content and HI values of coal seams from the Ruhr Basin. Rübezahl is the youngest coal seam, Zollverein is the oldest one (from Littke and Zieger, 2019b).

The mineral matter content can be expressed as ash yield, since content of inorganic components in coal is generally low. As shown in Fig. 5.2, the ash contents vary strongly from seam to seam, with no dependence to seam thickness being noticeable. There is a general trend of higher ash yields and lower inertinite contents towards the younger coal seams, although these trends are not very pronounced. The HI values first increase and then decrease with stratigraphic age. This trend is related to thermal maturity of the coals rather than depositional conditions and implies the formation and retention of hydrocarbons in the high volatile bituminous coal stages.

Fig. 5.3 TOC versus total sulphur diagram for the average composition of the coal seams presented in Figs. 5.1 and 5.2 as well as for several samples from marine-influenced coal seam Katharina, and Parsifal II (from Littke and Zieger, 2019b).

Most coal seams poor in mineral matter (or ash yield) are characterised by high inertinite contents. Such low mineral matter contents are typical for coals derived from peat mires deposited as ombrogenous mires or raised bogs. Because of a relative elevation to the groundwater table, transport of authigenic sediments into the peat is impeded. The low groundwater table also allows oxidation of the organic matter, leading to higher inertinite

92 contents. Under ombrogenic conditions, the water supply is provided by rain, leading to a low pH in such mires, prohibiting the extensive formation of pyrite (Moore, 1995). The correlation of organic matter content and abundance of pyrite to mire facies was utilised by Jasper et al. (2010). In the diagram shown in Fig. 5.3, TOC and TS are used as proxies for organic-matter richness and pyrite content, whereas the impact of coalification on TOC content is neglected, because all coals presented are of high volatile bituminous rank. The average composition of most Duckmantian and Bolsovian coal seams reflect ombrogenous or transitional facies. As two examples for a typical seam of the transitional facies and one of clearly ombrotrophic facies, TOC and TS values determined for seams Parsifal I and Baldur are shown in detail. This assignment to mire facies, however, finds its limits in case of coal seams influenced later by marine ingression. As an example, the Langsettian coal seam Katharina which is overlain by the marine Katharina Horizon is plotted in Figs. 5.3 and 5.4.

Fig. 5.4 Comparison of total sulphur depth trends for a) a Miocene lignite below marine sand (seam Frimmersdorf), b) the marine influenced Pennsylvanian Katharina coal profile, and c) seam Parsifal II (modified from Littke and Zieger, 2019b). For better comparison, the thicknesses of the Pennsylvanian coals were multiplied by 4.5.

Syngenetic pyrite is one of the most common minerals found during the microscopical assessment of the coals studied here. In general, coal seams deposited in a paralic setting which have been subjected to marine ingressions shortly after peat formation, are especially rich in syngenetic, framboidal pyrite. In previous studies on the Miocene lignite seam Frimmersdorf (Lower Rhine Embayment, Germany), huge amounts of syngenetic pyrite and high total sulphur contents are found only for the uppermost part of the profile overlain by marine sands (see Stock et al., 2016; Prinz et al., 2017) (Fig. 5.4a). In case of seam Katharina, total sulphur contents are continuously high within the entire coal seam without showing any clear trend with depth (Fig. 5.4b). Such uniform distribution of high sulphur contents may indicate that the marine influence on the seam was rather long term. Total sulphur contents in the profile of the Bolsovian seam Parsifal II first show a decrease from the base towards the middle part of the seam and then increases again towards the top (Fig. 5.4c). This distribution is typical for coals deposited in a transgressive system (Diessel, 1992; Holz et al., 2002).

5.2 General maturation trends The relationship between vitrinite reflectance and Tmax is well established (Teichmüller and Durand, 1983). The data of Pennsylvanian coals and dispersed organic matter from the Ruhr Basin shows a distinct linear trend of increasing Tmax values with increasing

VRr although with some scatter between 10 to 20 °C Tmax at a given VRr value (Fig. 5.5). For the maturity window from subbituminous to low volatile bituminous coals, a good correlation of these two maturity parameters for coals exists (r2=0.72), while the dispersed organic matter from associated sediments show a less clear correlation (r2=0.58). The HI values of the coals at maturities between 0.5 and 1.0 %VRr are quite variable, from about 60 to 350 mg HC/g TOC, and clearly decrease above this maturity threshold (Fig. 5.6). HI values of dispersed organic matter from the associated clastic rocks tend to be much lower as compared to those of the coals, especially in the samples with VRr values exceeding 1.0%. While HI values of marine or lacustrine petroleum source rocks decrease with advancing thermal maturity (Tissot and Welte, 1984; Rullkötter et al., 1988), a big proportion of newly formed mobile hydrocarbons the high-medium volatile bituminous coals studied here, is retained in the structure of vitrinite, so that HI values do not only

94 represent aliphatic moieties associated with the kerogen, like methyl groups or longer aliphatic chains connecting aromatic clusters (Vandenbroucke, 1980).

Fig. 5.5 Correlation of vitrinite reflectance and Rock-Eval Tmax values for subbituminous to medium volatile bituminous coals (from Littke and Zieger, 2019b).

Fig. 5.6 Hydrogen index versus vitrinite reflectance values of Duckmantian and Bolsovian coals and associated sediments from the Ruhr Basin (from Littke and Zieger, 2019b).

This phenomenon was already explained by Littke and Leythaeuser (1993), who calculated that a great amount of petroleum is generated in coals but is not expelled at an early stage, as it occurs in clastic, kerogen types I and II bearing source rocks. The preservation high HI values in coals can be explained by the production of rather polar, and heavier bitumen and its entrapment in the ultrafine pore system of coals (0.2-0.02 nm) (Amarasekera et al., 1995; Clarkson and Bustin, 1996, 1999). In the medium to low volatile bituminous coal stages, these hydrocarbons, after further defunctionalisation and thermal decomposition, are expelled as natural gas.

5.3 Characterisation of kerogen on a molecular level The results obtained using different FT-IR techniques and Curie Point pyrolysis GC-MS allow to draw some conclusions on differences between the different constituents of coal and the changes occurring upon thermal maturation as indicated by vitrinite reflectance and Rock-Eval Tmax. For example, the FT-IR derived A- and C-factors are used as tool for kerogen typing and maturity estimation. As shown in the interpretive diagram developed by Ganz and Robinson (1985) (Fig. 5.7), the maceral composition of the Bolsovian samples (bulk samples) does not seem to affect the C-factor, which is indicative for kerogen type. The presence of liptinites in these coal and kerogen samples, does not seem to have significant influence. The analysed spores, on the other hand plot at much higher C-factors (as suspected for kerogen I-II, Ganz and Kalkreuth, 1991), and the trend with maturity towards a less aliphatic character (decreasing A-factor) is well pronounced. The dependency of the depositional environment on molecular characteristics of pure vitrinite are not apparent, at least based on the dataset collected in the course of this study. One possible explanation might be that the advancing maturation, even in the narrow maturity window between subbituminous and high volatile bituminous coal, overwrites molecular variations that may be caused by differences in plant precursors or depositional environment.

Fig. 5.7 Interpretational plot of FT-IR derived A-and C-factors for the characterization of kerogen and maturity after Ganz and Kalkreuth (1991). Arrows indicate the path of maturation.

As thermal maturity emerges, an increase in aromaticity is clearly seen for the maturity series from the Ruhr Basin (Fig. 5.8). On a large maturity scale, the major molecular changes occurring in coals and vitrinites upon increasing thermal maturity seem to be independent from depositional setting and age of the coals, even though during the geological history, plant communities in peat mires changed tremendously and therefore the composition of the original humic material contributing to the coals (Hedges et al., 1985; Hatcher et al., 1989). For example, Chen et al. (2012), by employing FT-IR derived ratios for coals of a similar maturity range to those presented here derived from different ages and locations worldwide, made similar findings and conclusions regarding the chemical structure of vitrinites. A closer look into a narrower maturity range, however, reveals quite some variability of the data (Fig. 5.8).

Fig. 5.8 Ratio of the absorbance of aromatic (γCH) versus aliphatic (νCHx) stretching vibrations for vitrinite samples from the maturity series (Ruhr Basin) and the Bolsovian sample set from the Ruhr Basin.

Fig. 5.9 Relative area of the absorbance in the carbonyl/carboxyl region (νC=O) of vitrinites from the maturity series (Ruhr Basin) and the Bolsovian sample set from the Ruhr Basin.

The most important and distinct change occurring on a molecular level in the range of subbituminous to high volatile bituminous coals is the loss of oxygen-containing functional groups of vitrinites and spores. This is evident by a sharp decrease in the absorption in the carboxyl/carbonyl region in FT-IR spectrograms (Fig. 5.9) and by a pronounced decrease in phenols in favour of benzenes in the pyrolysis products.

5.4 Outlook The contribution of early formed hydrocarbons to the aliphatic fraction in the kerogen in the maturity range between sub-bituminous and high volatile bituminous coal is evident by the increase of HI values of the Ruhr coals in this maturity window. At the same time, spores from the investigated maturity series show a decrease in aliphatic constituents with increasing vitrinite reflectance. To further elucidate the contribution of these early formed mobile hydrocarbons to vitrinite reflectance and to chemical structural properties of vitrinites, micro spectroscopic investigations of liptinites can be useful. Further, repetition of the FT-IR and CP-Py-GC-MS analyses on the same samples after solvent extraction could provide insight into the extent of this aforementioned contribution.

The variations of optical and molecular maturity parameters in the Bolsovian samples might also be a result of differences in the original plant organic matter, which cannot be resolved by the subdivision into different depo-settings, as revealed by maceral indices alone. For a better understanding of the depositional environments and especially the plant communities that contributed to the formation of vitrinites, biomarker analysis and the analysis of the palynological composition of the Bolsovian coals and the dispersed organic matter might give some more insight into the rank-independent variations of the chemical composition of the vitrinites analysed.

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List of figures

1.1 a) The Variscan Foredeep in Central Europe and locations of Pennsylvanian coal mining areas (modified from Jasper et al., 2009), b) simplified profile of the Ruhr Basin (modified after Wrede and Ribbert, 2005). 2 1.2 a) Kerogen types and microscopy pictures (b-d) showing respective organic particles (macerals). a) van Krevelen Diagram with the atomic H/C vs. O/C ratios of some organic-rich rocks and kerogens (modified after van Krevelen, 1993, data from Tissot and Welte, 1984; Bandopadhyay and Mohanty, 2014; Zieger et al., 2018), b). Botryococcus algae (type I kerogen) under UV-light (from Rippen et al., 2013), c) Tasmanales algae (type I kerogen) under UV-light (from Stock et al., 2017), d) Carboniferous coal with V=vitrinite, C=cutinite, S=sporinite, and I=inertinite. 4 1.3 a) Simplified molecular structure of vitrinite at different maturity stages, b) changes in aromaticity, ring condensation and dimension of aromatic clusters in vitrinite in relation to carbon content (redrawn from Taylor et al., 1998, based on Teichmüller and Teichmüller, 1968). 7 2.1 a) Profile and location of the Ruhr Basin (after Stancu-Kristoff and Stehn, 1984), and b) its Westphalian coal bearing stratigraphic units with the analysed coal seams highlighted (after Strehlau, 1990). 16 2.2 a) ATR FT-IR spectra of the vitrinite samples and b) reflectance µFT-IR spectra of the megaspores. 21

2.3 Total ion chromatograms of vitrain sample C (VRr=0.85%) at pyrolysis

temperatures of a) 590 °C and b) 764 °C and of sample J (VRr=1.44%) at pyrolysis temperatures of c) 590 °C and d) 764 °C. 24 2.4 a) Aromaticity index of vitrinite and megaspore samples plotted against vitrinite

reflectance, b) νasCH2/νasCH3 ratios of the vitrinite samples plotted against vitrinite reflectance, c) condensation index of vitrinite and megaspore samples plotted against vitrinite reflectance, d) relative yields of benzenes, naphthalenes and phenanthrenes in the vitrinite samples pyrolysed at 764 °C. 26

2.5 Yields of aliphatic compounds (Na, Ne, Iso) in relation to aromatic compounds (B, Na, Ph, P) of the vitrinite pyrolysed at a) 590 °C and b) 764 °C, c) relative distribution of the chain lengths of n-alkanes and 1-alkenes and d) n-alkanes and 1-alkenes in relation to iso-alkanes in the vitrinite samples pyrolysed at 590 °C. 27 2.6 a) H/C atomic ratios and b) O/C atomic ratios in relation to TC of the vitrain samples from the Ruhr Basin compared to published data of vitrinite samples. 28

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2.7 a) Composition of the aromatic constituents and b) ratio of benzenes/phenols of vitrinite samples pyrolysed at 764 °C. 29 2.8 a) Relative absorbance of the C=O region of vitrinite and spore samples plotted against vitrinite reflectance, b) C-factor of the spore samples plotted against vitrinite reflectance. 31 3.1 a) Location of the Ruhr Basin within the Variscan Foredeep in Central Europe (modified from Jasper et al., 2009), b) main structural features of the Ruhr area and sampling site (modified from Wrede, 2005), c) sampled seam successions and sample numbers. K and V in index letters indicate kerogen concentrate and isolated vitrinite, respectively. Ash yields are taken from Zieger and Littke (2019). 37 3.2 ATR FT-IR spectra of the bulk Rübezahl coal seam, the concentrated kerogen from the overlying sediment layer, and vitrinite handpicked from the seam and isolated from the kerogen concentrate. Relevant peaks are marked. See text for explanation of the abbreviations. 44 3.3 Total ion chromatograms of a) bulk coal seam, b) coal seam vitrinite, c) the bulk kerogen concentrate, and d) isolated vitrinite from the kerogen concentrate from seam Rübezahl pyrolysed at 590 °C. 47 3.4 Total ion chromatograms of a) bulk coal seam, b) coal seam vitrinite, c) the bulk kerogen concentrate, and d) isolated vitrinite from the kerogen concentrate from seam Rübezahl pyrolysed at 764 °C. 48 3.5 Comparison of the different Rock-Eval parameters of sediment samples and corresponding kerogen concentrates. a) Hydrogen index, b) oxygen index, and c)

Tmax. d), e), and f) show the same parameters for the kerogen concentrates and associated bulk seam samples. The dashed line marks the 1:1 ratio. Three sample pairs are marked for better differentiation of the effect of kerogen concentration on samples with different maceral composition and/or original TOC content. See text for explanation. 51 3.6 Comparison of different FT-IR derived indicators of bulk samples (bulk seam and coal) and associated vitrinites. a) relative areas of the νC=O stretching region, b) aromaticity ratio, c) degree of condensation ratio, c) chain length/branching ratio. 55

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3.7 Relative abundance of pyrolysis products from bulk seam and kerogen concentrate samples (a, b, c) and the associated vitrinites (d, e) showing the maceral composition of the bulk seam and kerogen concentrates (in vol%, mineral matter free basis), n-alkanes and n-alkenes (N) versus aromatics (A=B (benzenes), Ph (phenols), Na (naphthalenes), and P (phenanthrenes)) from the pyrolysis at 590 °C and (b, d) relative abundance of aromatic moieties of the same samples pyrolysed at 764 °C (c, e). 58 3.8 Comparison of the proportion of long- over short-chained n-alkanes and n-alkenes

(C19-C30/C7-C18) of bulk seam and kerogen samples and their corresponding vitrinites pyrolysed at 590 °C. 60 3.9 a) Depth, b) relative amount of phenols, c) ratio of phenols over benzenes, and d) ratio of benzenes over naphthalenes and phenanthrenes of vitrinites from coal seams and isolated from the kerogen concentrates pyrolysed at 764 °C compared to vitrinite reflectance. 61 4.1 a) The Variscan Foredeep in Central Europe and locations of Pennsylvanian coal mining areas (modified from Jasper et al., 2009) b) Map of the Ruhr Basin with major tectonic structures (simplified after Wrede and Ribbert, 2005), c) Cross- section of the Ruhr Basin (after Drozdzewski and Wrede, 1994) with L. Fm = Lembeck Formation, D.=Dorsten, H. = Horst, E. = Essen, B.=Bochum, W.=Witten. 67 4.2 Simplified stratigraphic section of the Lembeck (left) and Dorsten Formations (right) as cored by the Hervest 5 well in the Lippe Syncline. Seam groups are shown, and sampled seams are indicated by arrows and seam thicknesses and number of samples (n) are given for these. 70 4.3 Profiles of the seams from the Lembeck Formation showing the maceral composition (left, representing 100 vol%) and selected results from elemental, bulk, and microscopic analyses and Rock-Eval pyrolysis. For explanations, see text. 74 4.4 Profiles of the seams from the Dorsten Formation showing the maceral composition (left, representing 100 vol%) and selected results from elemental, bulk and microscopic analyses and Rock-Eval pyrolysis. 75

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4.5 Microphotographs (270x190 µm) showing a) telinite surrounding resinitecell (Rübezahl I), b) typical association of detrovitrinite with liptinite and inertodetrinite between collotelinite bands (Parsifal II), c) sporangia in seam Rübezahl I in incident white and d) UV light, e) densosporinite in a matrix of inertinite in incident white and f) UV light (Chriemhild I), g) fluorinite associated with cutinite, h) the same picture in UV light (Parsifal II), i) different forms of cutinite in white, and j) UV light (Parsifal II). 78 4.6 Average values of vitrinite reflectance, liptinite content (L) and total sulphur plotted against depth. Grey lines mark seams of the Parsifal (upper) and Odin Groups (lower). 80

4.7 Diagram after Calder et al. (1991) showing GWIAC and VI values of individual samples from the Bolsovian seam profiles. The evolution of seams Baldur (grey) and Midgard (light grey) is indicated by shaded areas. 82 4.8 Variations in A/I ratios of seams from the Lembeck (left) and Dorsten Formations (right) from base (left) to top (right). See text for explanations. 84 5.1 Triangular diagram of vitrinite, inertinite, and liptinite contents in coals and associated claystones, siltstones and sandstones (Littke and Zieger, 2019b). 90 5.2 Seam thickness, ash yield, inertinite content and HI values of coal seams from the Ruhr Basin. Rübezahl is the youngest coal seam, Zollverein is the oldest one (from Littke and Zieger, 2019b). 91 5.3 TOC versus total sulphur diagram for the average composition of the coal seams presented in Figs. 5.1 and 5.2 as well as for several samples from marine- influenced coal seam Katharina, and Parsifal II (from Littke and Zieger, 2019b). 92 5.4 Comparison of total sulphur depth trends for a) a Miocene lignite below marine sand (seam Frimmersdorf), b) the marine influenced Pennsylvanian Katharina coal profile, and c) seam Parsifal II (modified from Littke and Zieger, 2019b). For better comparison, the thicknesses of the Pennsylvanian coals were multiplied by 4.5. 93

5.5 Correlation of vitrinite reflectance and Rock-Eval Tmax values for subbituminous to medium volatile bituminous coals (from Littke and Zieger, 2019b). 95 5.6 Hydrogen index versus vitrinite reflectance values of Duckmantian and Bolsovian coals and associated sediments from the Ruhr Basin (from Littke and Zieger, 2019b). 95 5.7 Interpretational plot of FT-IR derived A-and C-factors for the characterization of kerogen and maturity after Ganz and Kalkreuth (1991). Arrows indicate the path of maturation. 97

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5.8 Ratio of the absorbance of aromatic (γCH) versus aliphatic (νCHx) stretching vibrations for vitrinite samples from the maturity series (Ruhr Basin) and the Bolsovian sample set from the Ruhr Basin. 98 5.9 Relative area of the absorbance in the carbonyl/carboxyl region (νC=O) of vitrinites from the maturity series (Ruhr Basin) and the Bolsovian sample set from the Ruhr Basin. 98

List of tables

2.1 Origin, petrographic characteristics (V=vitrinite, L=liptinite, I=inertinite, and MM=mineral matter), Rock-Eval data of the coal samples and elemental composition of isolated vitrain layers. 18 2.2 Frequency regions and band assignment of the FT-IR spectra. 19 2.3 Normalised absorbance of the considered functional groups in the vitrinite and megaspore spectra (in %). 22 2.4 Semi-quantitative amounts of the considered chemical compounds of vitrinite at both pyrolysis temperatures (in %). 23 3.1 Results of elemental analyses and Rock-Eval pyrolysis data of sediment, bulk seam samples (in italics) and kerogen concentrates. 42 3.2 Vitrinite reflectance values and maceral composition of the bulk seam samples (in italics) and concentrated kerogens. V=vitrinite, L=liptinite, I=inertinite, MM=mineral matter. 43 3.3 FT-IR peak areas of the considered functional groups for bulk coal and kerogen samples (left) and handpicked and isolated vitrinite samples (right). All values are given in % relative to the total area of all considered peaks of each spectrum. Results of bulk seam samples and coal seam vitrinite are written in italics. See text for abbreviations of functional groups. 45 3.4 Relative abundance of n-alkanes and n-alkenes, aromatic constituents, phenols, and thiophenes of bulk seam and kerogen samples (left) and vitrinites (right) pyrolysed at 590 °C and 764 °C. N=n-alkanes and –alkenes, B=benzenes, Na=naphthalenes, P=phenanthrenes, Ph=phenols, and T=thiophenes. Results of bulk seam samples and coal seam vitrinites are written in italics. 49 4.1 Minimum (Min), maximum (Max) and average (Ø) values of vitrinite reflectance, elemental and bulk analyses and Rock-Eval parameters. 73

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4.2 Minimum (Min), maximum (Max) and average (Ø) value percentages of macerals and maceral groups counted for the 11 Bolsovian coal seams. Collotelinites were separately counted as bands with thicknesses ≤ and ≥ 50 µm (see subscripts). Resinites were distinguished according to their occurrence either as cell fillings

(resinitecell) or isolated particles (resinitecorp). 76

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