High-Resolution Ultrasonic Logging of Sandstones

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Applications to Stratigraphy, Reservoir Characterization, and Dimension Stone Quality Evaluation

(Hochauflösende Ultraschallmessungen in Sandsteinen – Anwendungen zu stratigraphischen Fragestellungen, Reservoircharakterisierung und Bausteinqualität)

Der Naturwissenschaftlichen Fakultät der Friedrich-Alexander Universität Erlangen-Nürnberg

zur Erlangung des Doktorgrades Dr. rer. nat.

vorgelegt von

Dipl. Geol. Claudio Miro Filomena aus Tübingen

Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät der Friedrich-Alexander Universität Erlangen-Nürnberg

Tag der mündlichen Prüfung: 05.06.2012

Vorsitzender der Promotionskommission: Prof. Dr. R. Fink

Erstberichterstatter: Prof. Dr. Harald Stollhofen

Zweitberichterstatter: Prof. Dr. Roman Koch

für meine Oma

Table of contents

Zusammenfassung (deutsch) 8 Sommario (italiano) 10 Abstract (english) 12

Chapter I Introduction 1.1 Thesis rationale 14 1.2 Thesis outline 16 1.3 Geological background and stratigraphic sections 17

Chapter II Methodology 2.1 High-resolution ultrasonic logging (HRS) 22 2.1.1 Equipment 22 2.1.2 Controls on ultrasonic velocity patterns 24 2.1.3 Transmitter-receiver test assembly 26 2.2 Porosity 27 2.3 Permeability 29 2.4 Petrographic analysis and mineralogical composition 31 2.5 Micro-computed tomography (µ-CT) 31 2.6 Compressive strength 35 2.7 E-modulus 35

Chapter III Application of Ultrasonic Measurements to Stratigraphy: Detection of Cryptic Unconformities Abstract 36 3.1 Introduction 37 3.1.1 Geological setting 37 3.1.2 Study area and study sites 38

3.2 Identification of unconformities 39 3.3 Methods 41 3.4 Outcrop and well core sections 43 3.4.1 Well core: Guggenberg B28 (Northern Bavaria, Germany) 43 3.4.2 Outcrop: Gambach quarry (Northern Bavaria, Germany) 47 3.4.3 Well core: Rockensußra 209/83 (Thuringia, Germany) 50 3.5 Visualization of sub-H-unconformity pore networks 53 3.5.1 3D pore volume reconstruction 53 3.5.2 MIP versus numerical µ-CT pore analysis 57 3.6 Discussion 58 3.6.1 Petrophysical properties 58 3.6.2 Diagenesis and tectonic implications 61 3.7 Conclusions 63

Chapter IV Application of High-Resolution Ultrasonic Logging to Clastic Reservoir Heterogeneity Abstract 64 4.1 Introduction 65 4.2 The Middle Solling Sandstone Member (MSSM) 67 4.2.1 Lithology and facies types 67 4.2.2 Facies successions 71 4.3 Methods 72 4.4 Middle Solling Sandstone Member reservoir heterogeneity 73 4.4.1 MSSM petrology and porosity 73 4.4.2 Wireline log pattern 76 4.4.3 HRS logging – porosity relationships 76 4.4.4 HRS logging-derived porosity proxies 78 4.5 Conclusions 80

6 Table of contents

Chapter V Application of High-Resolution Ultrasonic Logging to Dimension Stone Quality Evaluation Abstract 82 5.1 Introduction 83 5.1.1 Dimension stone characteristics 83 5.1.2 Study site 85 5.2 Methods 86 5.3 Worzeldorf Sandstone characteristics 87 5.3.1 Lithology and facies 87 5.3.2 Composition 89 5.3.3 Diagenetic history outlined 90 5.3.4 Dimension sandstone qualities 93 5.4 Correlation between petrophysical parameters 96 5.4.1 Calculating E-moduli proxy values 96 5.4.2 Calculating compressive stength proxy values 97 5.4.3 Lateral tracing of dimension stone qualities 98 5.5 Conclusions 101

Chapter VI Synopsis 6.1 Résumé 102 6.2 Conclusions and Outlook 104

References 106

Table of figures 116

Danksagung 118

Zusammenfassung

Die petrophysikalischen Eigenschaften von Sandsteinkörpern und deren Heterogenitäten sind wichtige Faktoren bei der Beurteilung von Kohlenwasserstofflagerstätten und geo- thermischen Reservoiren. Standardmäßig angewendete Messverfahren zur Bestimmung der Porosität (z.B. Quecksilber- oder Heliumporosimetrie an Bohrkernproben oder Bohrlochmessungen) sind häufig sehr zeitaufwändig, zerstörerisch, oder sie haben eine nur geringe örtliche Auflösung im Meter- bis Dezimetermaßstab. In dieser Arbeit wird die Anwendung zerstörungsfreier, hochauflösender Ultraschallmessungen (p-Wellen) an Bohrkernen und Aufschlusswänden siliziklastischer Abfolgen als Porositäts-Proxy im Zentimetermaßstab getestet. Zusätzlich werden die Ultraschallmessungen mit petrographischen Analysemethoden wie Polarisations- und Elektronenmikroskopie von Dünn- schliffen und Röntgendiffraktometrie kombiniert und mit direkter Messung von petrophysikalischen Größen (Porosität, Permeabilität und Druckfestigkeit) und mit Daten aus der 3D Mikrocomputertomographie und Bohrlochmessungen korreliert. Drei Anwendungsbeispiele von hochauflösenden Ultraschallmessungen werden behandelt: A) zur Identifizierung von Diskordanzen, B) zur Reservoircharakterisierung und C) zur Qualitätsbeurteilung von Naturbausteinen.

A) In Sandsteinserien mit einheitlicher Korngröße sind Diskordanzen oftmals sehr schwer zu erkennen, da vor allem Oberflächenaufschlüsse nur begrenzt und über sehr kurze Strecken auftreten und typische Merkmale wie Paläoböden an der Grenzfläche fehlen können. Mittels hochauflösender Ultraschallmessung wurde die Hardegsen-Diskordanz des Mittleren Buntsandsteins (frühe Trias) an einem Aufschluss und zwei Bohrkern- abschnitten in Mittel- und Süddeutschland untersucht. Hierbei wurde festgestellt, dass Änderungen der Sandsteinzementation und -porosität über die Diskordanz hinweg durch einen erheblichen Sprung in der Ultraschalllaufzeit abgebildet werden. Die sich abrupt ändernden Laufzeitmerkmale der Hardegsen und Solling Formationen unter und über der Diskordanz trennen diese beiden Einheiten daher eindeutig voneinander. Der Einsatz von hochauflösenden Ultraschallmessungen kann somit dazu beitragen Diskordanzen zu lokalisieren, auch wenn kein Paläoboden ausgebildet ist. Geländegestützte Ultraschall- messungen stellen zudem eine nützliche Ergänzung zu herkömmlichen Aufschluss-

Loggingmethoden dar. Sie unterstützen die in-situ Bestimmung von Porositäts- bzw. Zementationsheterogenitäten in Sandsteinen und tragen zur Korrelation von Sequenz- grenzen zwischen Bohrungen und Aufschlussanalogen bei.

B) Die frühtriassische Mittlere Solling Sandstein Folge vor der niederländischen Küste ist ein gasführendes Reservoir, das im Rahmen dieser Dissertation als Testobjekt für die Anwendung von hochauflösenden Ultraschallogs bei der Reservoircharakterisierung dient. Der untersuchte Profilabschnitt besteht fast ausschließlich aus schräggeschichteten bis massigen Quarzsandsteinen, die als äolische Dünen in einer trockenen Sandebene abgelagert wurden und unterschiedlich starke Salzzementation aufweisen. Porositäten aus Plug-Kernproben und gemessene Ultraschalllaufzeiten zeigen eine gute lineare Korrelation zueinander, mit einem R² von 0.86 in massigen äolischen Sandsteinen mittlerer bis hoher Zementationsgrade. Mittels Ultraschallmessungen im Zentimetermaßstab können stark zementierte und hochporöse Bereiche innerhalb der Mittleren Solling Sandstein Folge verlässlich und hoch ortsauflösend dokumentiert und eindeutig voneinander unterschieden werden. Die Methode liefert wichtige zusätzliche Daten zur Reservoirheterogenität in ansonsten homogen erscheinenden Sandsteinserien.

C) Der Worzeldorfer Sandstein aus der späten Trias (Mittlerer Keuper), ein im Raum Nürnberg häufig verwendeter Naturbaustein, wurde für eine weitere Versuchsreihe für die Anwendung von hochauflösenden Ultraschallmessungen in der Qualitätsbewertung von Bausteinen getestet. Als Ergebnis werden hierbei zwei Qualitätsendglieder unterschieden: 1) Qualitativ hochwertige Sandsteine, die vor allem durch eine hohe Korndichte, ausge- prägte Quarzzementation und niedrige Tongehalte charakterisiert sind und bei Ultraschall- geschwindigkeiten von > 3.6 km/s eine hohe Druckfestigkeit von mehr als 50 MPa (bis zu 104 MPa) aufweisen. 2) Sandsteine geringer Qualität, die Druckfestigkeiten von weniger als 20 MPa besitzen und durch Ultraschallgeschwindigkeiten < 2.73 km/s, hohe Porosität, schlechte Zementation und niedrige Packungsdichte mit häufig losem Kornverband gekennzeichnet sind. Zwischen diesen beiden Endgliedern wurden Sandsteine aller Druckfestigkeiten untersucht. Wurden die Ultraschallgeschwindigkeiten einmal für eine bestimmte Lithologie und Korngröße an Messwerten der uniaxialen Druckfestigkeit kalibriert, stellen hochauflösende Ultraschallmessungen eine zeitsparende, zerstörungs- freie und hoch ortsauflösende Messmethode dar, die reproduzierbare quantitative Proxy- daten zu Gesteinsfestigkeiten potentieller Abbaubereiche liefert.

Sommario

Le proprietà petrofisiche e la eterogenità delle arenarie sono importanti fattori per la caratterizzazione di un reservoir petrolifero o geotermico. Molto spesso, i consueti metodi di rilevamento per la determinazione della porosità del pietrame (l’intrusione di mercurio e la porosimetria di elio eseguita su campioni di carote, o le misure in pozzo) richiedono molto tempo, sono distruttivi, e hanno una risoluzione limitata alla scala metrica o decimetrica. La presente tesi di dottorato utilizza misure di ultrasonica (onde p) ad alta risoluzione per effettuare un’ analisi non-distruttiva sulla porosità di rocce silicoclastiche in affioramento e carota. Le misure di ultrasonica sono integrate con analisi petrografiche su sezioni sottili usando diffrattometro ai raggi x e microscopio elettronico. Inoltre le misure petrofisiche (porosità, permeabilità e resistenza alla pressione), i dati provenienti da tomografia assiale computerizzata e le misure in pozzo sono correlati con le velocità di ultrasonica. Ci sono applicati tre studi di caso utilizzando le misure di ultrasonica esaminando A) l’identificazione delle discordanze, B) la caratterizzazione dei bacini petroliferi e C) l’indicazione della qualità delle arenarie da costruzione.

A) Il riconoscimento delle discordanze in sezioni di arenarie con uguale granulometria può risultare molto complicato, specialmente quando gli affioramenti non hanno un ottima esposizione, e le caratteristiche litologiche come i paleo-suoli non sono conservati. Utilizzando misurazioni di ultrasonica ad alta risoluzione, la Discordanza di Hardegsen nel Medio Buntsandstein (Triassico inferiore) è stata esaminata in un affioramento e in due carote della Germania meridionale e centrale. Attraverso questa discordanza, situata tra la Formazione Hardegsen al di sotto e la Formazione Solling al di sopra, si registra un cambiamento rilevante delle velocità di ultrasonica che può essere correlato ad un cambiamento della cementazione e della porosità all’interno delle arenarie. Dunque, le misure di ultrasonica eseguite sul campo rappresentano un buon supplemento alle misure effettuate in situ riguardanti il grado di cementazione e di porosità delle arenarie. Queste velocità aiutano a localizzare delle potenziali discordanze e contribuiscono alla correlazione stratigrafica sia in pozzo che in affioramento.

B) Il Membro delle Arenarie Medie del Solling (Triassico inferiore), un reservoir di gas naturale situato nel Mare del Nord di fronte la costa olandese, é stato usato come test per l’applicazione delle misure di ultrasonica ad alta risoluzione per caratterizzare bacini petroliferi. La successione consiste quasi esclusivamente di arenarie depositate come dune eoliche o piane sabbiose. Queste arenarie sono massive o con stratificazione incrociata e presentano diversi gradi di cementazione da parte del sale. La porosità misurata e le velocità di ultrasonica mostrano una buona correlazione, con un R² di 0.86 nelle arenarie massive cementati al medio e alto grado. I dati di ultrasonica a risoluzione centimetrica danno un proprio contributo alla ricerca delle eterogenità nelle arenarie ed indicano zone di alta porosità.

C) Nella zona di Norimberga, le Arenarie di Worzeldorf (Triassico superiore, Medio Keuper) sono usate spesso come materiale per la costruzione di edifici. Le misure di ultrasonica ad alta risoluzione sono testate nel campo dell’ esplorazione per materiali da costruzione come indicatore di qualità. Si possono suddividere in due categorie finali: 1) Arenarie di alta qualità sono caratterizzati da una forte cementazione quarzosa, con le velocità ultrasonica > 3.6 km/s e una resistenza alla pressione > 50 MPa (fino a 104 MPa). 2) Materiali di bassa qualità sono molto fragili, caratterizzati da alta porosità e poca cementazione, con le velocità di ultrasonica < 2.73 km/s e una resistenza alla pressione sotto le 20 MPa. Una volta calibrata la velocità ultrasonica per la resistenza uniassiale di una data litologia e granulometria, le misurazioni di ultrasonica permettono di risparmiare tempo sull’acquisizione dei dati di resistenza e di qualità della roccia evitandone la frantumazione.

Abstract

Petrophysical properties and their heterogeneity within sandstone bodies are key parameters in the evaluation of hydrocarbon and geothermal reservoirs. However, common tools applied to constrain porosity (e.g. mercury intrusion and helium porosimetry or borehole logging data) are often time consuming, destructive, or suffer from a resolution limited to the metre- to decimetre-scale. In this thesis, the applicability of non-destructive, high-resolution p-wave sonic (HRS) logging of well core and outcrop sections in the ultrasonic frequency range is examined as a method providing porosity proxy data at the cm-scale in clastic sedimentary sequences. Sonic measurements are integrated with petrographic analysis comprising microscopic thin section examination and x-ray diffractometry. In addition, petrophysical parameters (porosity, permeability, and compressive strength) as well as borehole logging data and 3D micro-computed tomography imaging are correlated to HRS data. Three key examples for the application of high-resolution ultrasonic measurements are examined: A) identification of unconformities, B) reservoir characterization, and C) dimension stone quality evaluation.

A) The recognition of unconformities in thick and monotonous sandstone successions of consistent grain size is often difficult in locally exposed surface outcrop sections and is even more challenging when typical lithological changes like paleosols are missing at the boundary surface. One outcrop and two well core sections incorporating the Early Triassic Middle Buntsandstein Hardegsen unconformity in southern and central Germany were investigated by HRS measurement. It has been documented, that relative changes in sandstone cementation and porosity across the unconformity correlate with significant offsets in sonic transit time, clearly separating the different sonic patterns of the Hardegsen and Solling Formations below and above the unconformity. Distinct sonic offsets in uniform sandstone successions may contribute to the localization of unconformities. Field-based ultrasonic measurement thus provides a useful addition to established outcrop logging methods. It helps in the in-situ identification of porosity/cementation heterogeneities in sandstones and in the correlation of borehole measured sequence boundaries to outcrop analogs.

B) The Early Triassic Middle Solling Sandstone Member (MSSM), a gas-bearing reservoir offshore from the Netherlands, is used as a test sample for HRS logging as a tool in hydrocarbon reservoir characterization. It consists almost entirely of cross-bedded to massive aeolian dune and dry sandflat deposits which are salt-plugged to variable degrees. Variations in sonic transit times are extracted as porosity proxies in the MSSM section. The correlation of sonic transit times with experimentally determined core plug porosities develops the highest correlation coefficient (R2 = 0.86) in structureless aeolian dune sandstones and achieves further optimized results at medium to high cementation indices. HRS logging thus contributes a reliable quantification of reservoir heterogeneities at the centimetre-scale, bridging between micro-scale thin section examination and macro-scale borehole logging or core plug sampling. Hence, it turns out to be a suitable tool for the detection of high-porosity zones in otherwise uniform sandstone successions.

C) The Worzeldorf Sandstone (Late Triassic Middle Keuper), a traditional dimension stone which has been widely used in the area of Nuremberg, southern Germany, provides the test sample for HRS logging as a quantitative method in dimension stone exploration. As a result of this study, two end-members within the range of quality grades are defined: 1) High-quality sandstones are characterized by high packing densities of detrital grains, strong quartz cementation and low clay mineral contents, providing high compressive strength values. High-quality Worzeldorf sandstone is allocated with a compressive strength exceeding 50 MPa (up to 104 MPa), which is realized at sonic velocities of > 3.6 km/s. 2) In contrast, low-quality sandstones only provide compressive strength of < 20 MPa and low p-wave velocities of < 2.73 km/s. They are highly porous and only fairly cemented with low packing densities and frequently floating grains. Once calibrated against uni-axial compressive strength values of a particular lithology and grain size, high-resolution sonic logging serves as a very time-efficient non-destructive tool to provide quantitative and reproducible proxy data for rock strength of mineable sandstone volumes.

13 ______

Chapter I

Introduction ______

1.1 Thesis rationale

Siliciclastic sediments, namely sandstones, play an important role in world’s economy and are, in one way or the other, part of our daily life. Providing a large percentage of all known reservoirs for natural oil and gas, they are major targets in hydrocarbon exploration. Fossil fuels are the backbone of any industrialized country. Sandstones are also known to provide host-rocks for groundwater or hydrothermal aquifers, which brings them into the focus of geothermal exploration projects. Besides their importance for the energy sector, sandstones are important building materials, and thus are major part of our cultural heritage. They are the most abundant natural dimension stones in Germany (Ehling, 2009), used for the construction of all kind of buildings, like castles and fortifications, houses, bridges, monuments, temples, cathedrals, and monasteries.

For the production and development of hydrocarbon or geothermal reservoirs, or the mining of a potential dimension stone prospect, a profound understanding and characterization of the occurring lithologies is of paramount importance. Thereby, various geological disciplines, their methods and tools are applied and integrated: Sedimentological facies analysis and analog studies, petrographic examination and diagenetic interpretation, and the definition of petrophysical key parameters are the basis for a precise evaluation. In times of decreasing non-renewable resources, current and future supply needs to be assured. To increase recovery from existing hydrocarbon fields and to develop small- sized or unconventional reservoirs, more sophisticated reservoir models are required.

14 Introduction

Petrophysical attributes qualify a rocks’ reservoir character or its suitability as a dimension stone. The most important parameters for reservoir rocks are porosity and permeability, as it is compressive strength for dimension stones. Established rock analysis methods directly measuring the above mentioned petrophysical parameters are often destructive to the applied material. Using individually rock samples, these methods also suffer from a limited spatial resolution and provide only punctual information. Non-destructive ultrasonic measurements therefore are applied as a proxy to describe the material composition or the porosity and compressibility of a solid (cf. Gassmann, 1951; Biot, 1956; Wyllie et al., 1956; Geertsma and Smit, 1961), for example of concrete (Popovics et al., 1990; Hernandez et al., 2000), mortar (Lafhaj et al., 2006), or sedimentary rocks (Hamilton and Bachmann, 1982; Han et al., 1986). Applied to sedimentary rocks, this offers the possibility to answer multiple questions in different fields of geological interest.

Sonic measurements can be either conducted in the laboratory at individual samples or in a borehole, using sonic logging devices. Though, the spatial resolution of downhole logging devices is limited to the decimetre to meter scale (Rider, 2002). For an appropriate characterization of thin reservoir intervals or highly heterogeneous lithologies, it is advantageous obtaining a higher data density.

This doctoral thesis presents three case studies to test the application of high-resolution ultrasonic logging (HRS) in siliciclastic sediments. HRS is integrated and combined with other petrophysical measurements, petrographic thin-section analysis and diagenetic interpretation.

15 Chapter I

1.2 Thesis outline

The thesis is arranged in six chapters, of which the first gives a general introduction to the topic and the second explains the applied analytical methodologies. Chapters III – V are presenting three case studies where HRS measurements are applied to different geological topics.

Chapter III illustrates a combined outcrop and well core study, examining the application of HRS measurements to solve stratigraphic problems. The recognition of so-called “cryptic unconformities” (Miall and Arush, 2001) in uniform sandstone successions is the focus of this study. The Early Triassic Hardegsen Unconformity of the Middle Buntsandstein Group in southern Germany is presented as a type-example. Changes in sonic transit time across the Hardegsen Unconformity are integrated with thin section analysis, porosity-permeability measurements, micro computed tomography scans, and gamma-density logging. The applicability of ultrasonic transit time trends as reliable proxies for the in-situ identification of porosity heterogeneities in otherwise monotonous sandstone sections and their addition to classic petrographic analysis are discussed.

In Chapter IV, the advantage of HRS logging in hydrocarbon reservoir characterization is discussed. The test sample is the Middle Solling Sandstone Member (de Jager and Geluk, 2007), a gas-bearing reservoir in the southern North Sea, offshore the Netherlands. HRS logging is applied as a method bridging between micro- and macro-scale reservoir analyses in salt-cemented aeolianites, contributing significant additional information to core plug and wireline log porosity data. The quantitative petrophysical evaluation of reservoir section heterogeneity on the cm-scale and precise separation of high and low- porosity zones within cored sections are major aspects of this chapter.

The application of HRS logging to dimension sandstone quality assessment is investigated in Chapter V. Continuous ultrasonic logging data, integrated with classic thin section examination, porosity, permeability, Young’s modulus of elasticity, and compressive strength measurements are presented for the Worzeldorf Sandstone, a Late Triassic dimension stone quarried near Nuremberg in Franconia (south Germany).

16 Introduction

Chapter VI provides a synopsis of the presented case studies. It gives an outlook to the successful application of high-resolution ultrasonic measurement and its combination with other petrophysical and sedimentological methods in outcrop and well core sample examination.

1.3 Geological background and examined stratigraphic sections

All case studies focus on Triassic rock sequences deposited in the Central European Basin System (CEBS), which covered a vast area limited by the United Kingdom in the west and eastern Poland and Lithuania in the east. It extended from the northern North Sea and Denmark to southern Germany, Switzerland, and eastern France in the south (Bachmann et al., 2010). At the end of the Paleozoic and through the beginning of the Mesozoic, tectonic pulses affected the evolving CEBS (Bachmann et al., 2008; Stollhofen et al., 2008), leading to the formation of syndepositional horst and graben structures (cf. Ziegler, 1990; Geluk and Röhling, 1999; Radies et al., 2005) and a number of pronounced unconformities (Röhling, 1991; Aigner and Bachmann, 1992). During this time, the development of the German Triassic (Fig. 1.1) was influenced by a repeatedly fluctuating sea level (Aigner et al., 1998). The depositional environments in south Germany changed from an arid to semiarid playa setting during the Upper Permian and Lower Buntsandstein, to largely fluvial-dominated of the Middle and Upper Buntsandstein, to a shallow marine shelf environment during the Muschelkalk period, and finally to fluvio- lacustrine to deltaic coastal setting in the Keuper (Aigner and Bachmann, 1992; Aigner et al., 1998; Beutler et al., 1999; Hauschke and Wilde, 1999; Bachmann and Kozur, 2004; Menning et al., 2005; Bachmann et al., 2010).

The stratigraphic sections investigated in this thesis comprise the Early Triassic siliciclastic series of the Middle Buntsandstein Hardegsen and Solling Formations, and the Late Triassic Arnstadt respectively Löwenstein Formation of the Middle Keuper (Fig. 1.1).

17 Chapter I

Figure 1.1: Stratigraphic overview of the German Triassic with age in million years (Ma) and the associated depositional environments, modified after Bachmann et al. (2010) and Beutler et al. (1999). Stratigraphic sections sampled and discussed in this thesis are marked in red.

18 Introduction

Hardegsen and Solling Formations: In southern and central Germany, the Hardegsen and Solling Formations are mainly fluvial dominated series of fine to medium-grained sandstones, alternating with argillaceous siltstones. Locally, a well-developed paleosol separates the Hardegsen and the Solling sandstones, highlighting the non-conformable contact between the two formations. This surface is known as the Hardegsen Unconformity. It is by far the most widespread Triassic unconformity in the Central European Basin, being identified throughout Germany, the Netherlands, the North Sea, northwest England, and the Irish Sea Basin (Cowan, 1993; Evans et al., 1993; Geluk, 1998; Lepper et al., 2005). In this thesis, the Hardegsen Unconformity has been examined in three locations (Gambach a.M., Guggenberg, and Rockensußra) in southern and central Germany (Fig. 1.2). Towards North Germany, the Netherlands and the southern North Sea, the fluvial series of the Solling Formation successively grade into semi-arid fluvio-aeolian dominated sandstones and finally into playa lake deposits (Geluk and Röhling, 1997; Geluk, 1998; Geluk and Röhling, 1999; Lepper et al., 2005; Geluk, 2007; Bachmann et al., 2010), see Figure 1.2. Synsedimentary tectonics and halokinetics result in fast subsidence, supporting the preservation of thick aeolian deposits there (de Jager, 2007; Geluk, 2007). The Middle Solling Sandstone Member offshore the Netherlands is such an example of well-preserved aeolianites (Fig. 1.2).

Arnstadt / Löwenstein Formation: The Arnstadt Formation of the Middle Keuper, also known as the “Steinmergelkeuper”, is a basinal series of fine-grained lacustrine deposits. In south Germany, the Löwenstein Formation is its proximal stratigraphic equivalent. It is largely composed of very coarse- grained fluvial siliciclastics which were derived from the rejuvenation of the Vindelician- Bohemian Massif (Mader, 1997). Conglomeratic sandstones alternate with clayey silt- and sandstones. The sandstone channel bodies embedded in floodplain and overbank deposits represent braided to meandering river environments (Mader, 1997; Beutler et al., 1999).

19 Chapter I

20 Introduction

Figure 1.2 (opposite page): Paleogeographic map and facies distributions of the Central European Basin System during the Middle Buntsandstein, modified after Bachmann et al. (2010). Displayed are clastic influx directions originating from structural high rejuvenation areas (brown arrows), the prevailing wind directions (blue arrows), the approximate orientation of the fluvial system (orange arrows), and the direction of marine ingression (bolt grey arrow). The study locations presented in this thesis are indicated: Hardegsen Unconformity (black dots), Middle Solling Sandstone Member (black square), and Worzeldorf Sandstone (black triangle, not time- equivalent).

21 ______

Chapter II

Methodology ______

2.1 High-resolution ultrasonic logging (HRS)

2.1.1 Equipment A GEOTRON USG 40 ultrasonic generator equipped with pointed stainless steel piézoelectric longitudinal and dilatation transmitter and receiver probes UPG-D and UPE- D was applied in all presented HRS case studies. As this device can be applied in the laboratory and in outcrop sections, it is also called a mobile ultrasonic tester (MUT). Considering core diameters and also for practical reasons, p-wave velocity measurements are conducted in the ultrasonic frequency range (Toksöz and Johnston, 1981).

The transmitter and receiver probes have been operated at signal frequencies of 46 and 80 kHz, and amplitudes of 50, 100, 200, 500 or 1000 mV, in each case adjusted to the particular sample specifications. Due to the probe tip geometry and the application of a 40 dB preamplifier (VV41), the use of signal-quality enhancing couplant materials is not obligatory.

The oscillograph curve of the ultrasonic signal is recorded with an accuracy of 12.5 nanoseconds. To obtain the sonic run time, the first arrival of the ultrasonic p-wave (also known as primary, longitudinal, or compressive wave) is picked from the oscillograph curve (Fig. 2.1). The picked run time then needs to be corrected according to the delay of the transmitter and receiver probes. The UPG-D / UPE-D probes have a total delay of 1.8 µs. In addition to run time measurement, the distance of transmitter and receiver probe (sample thickness) was determined with 0.01 mm accuracy. To obtain a reliable signal, at least one entire sonic wavelet should pass the sample. The higher the frequency, the shorter the wavelength and thereby the thinner the measured sample can be.

22 Methodology

In material science, sonic velocity data are provided in SI-units, in kilometres per second (km/s). In the hydrocarbon industry, however, sonic logs are displayed in microseconds per foot (µs/ft), called the interval transit time (cf. Rider, 2002). Considering the reservoir rock related focus, the interval transit time is used in Chapters III and IV. Sonic velocity (km/s) then is used in Chapter V as this study relates to the quality assessment of building stones dealing with terminologies and units common in material science.

In Chapter III, the GeoTec Multi Sensor Core Logger (MSCL) has been applied additionally to the MUT. It is operated at a frequency of 250 kHz. The difference in frequencies used by the MUT and MSCL does not affect the sonic transit time, higher frequencies simply allow the measurement of thinner rock samples.

Figure 2.1: Idealized example of an ultrasonic oscillograph curve, obtained from the UPG-D and UPE-D ultrasonic probes. The p-wave travel time through the sample is indicated by the first arrival signal, here at 19.5 µs. At all run time measurements, the delay of the sonic probes needs to be deducted from the measured run time.

23 Chapter II

2.1.2 Controls on ultrasonic velocity patterns Ultrasonic readings derived from outcrop or well core measurements depend on various control parameters of both primary and secondary relevance. Within the “primary” category, the original lithological parameters are focussed upon, such as grain size and grain type, grain contact types, content of lithic fragments, the type and grade of cementation and porosity, the presence of matrix and/or authigenic minerals filling the pore space, and variations in bedding style. “Secondary” parameters comprise rock water content as well as jointing of rock samples that may to variable degrees modify original sonic velocities, characterizing a particular rock type.

The p-wave velocity of sandstones is primarily constrained by the bulk porosity and the different velocities of the matrix material and the pore-filling fluids. This dependency is expressed by the time-average relationship of Wyllie et al. (1956) and the slightly modified equations of Gardner et al. (1974) and Raymer et al. (1980). Only in quartz dominated sandstones, these equations provide very reliable, almost linear p-wave velocity to porosity relations (Han et al., 1986; Vernik and Nur, 1992; Dvorkin and Nur, 1998), but they generally over-estimate porosities (Brereton and McCann, 1990) and fail to predict the porosity in polymineralic datasets (Kenter et al., 2007). According to Kenter et al. (2007), the influence of mineralogy on the absolute value of p-wave velocity is most important when intermediate porosities of 10-25% are considered. Below 10% sandstone porosity, p-wave velocity values are highest and the influence of mineralogy on the acoustic velocity is relatively low.

Clay contents affect p-wave velocities with an average of ~30% of the effect porosity has on p-wave velocity (Han et al., 1986; Gal et al., 1999), or ~ 8% in total (Kenter et al., 2007). In addition, grain size variations (Hamilton and Bachmann, 1982) and sonic anisotropy in sedimentary rocks need to be considered. This is largely dependent on the bedding style and expressed by higher velocities parallel to bedding and lower velocities normal to bedding. Sonic anisotropy is very pronounced in high-angle (trough) cross- bedded sandstones and almost negligible in plane bedded sedimentary rocks (Fitzner et al., 2003).

24 Methodology

Mainly during field application, but also in the laboratory, secondary parameters may vary and then influence the ultrasonic measurements. Common factors are: A variable degree of rock water content (due to precipitation), variable (non-standardized) contact pressure and transmitting quality between rock and transmitter/receiver probes, influence of rock surface weathering processes, and rock disintegration. The behaviour of ultrasonic velocities in sediments under different pore fluid saturation stages and variable hydrostatic pressures was previously examined in laboratory experiments (Schütt, 1992; Mayr and Burkhardt, 2006). In a completely air-filled pore-network, the Gassmann effect (Gassmann, 1951) leads to a lower p-wave velocity at full air-saturation, because the pores are filled with a gas (air) having a lower compression modulus compared to a fluid. Consequently, the transit time derived from HRS logging of a dry sample is higher for a given lithology than that from downhole sonic logs, where pore space is usually filled with formation water.

In-situ measurements in outcrop sections therefore need special attention to minimize these secondary factors, e.g. by monitoring rock water contents through a rock moisture measuring device which helps to correct the sonic transit time if necessary. Measuring across rock cracks should be avoided, and where weathered rock surfaces are developed, a fresh rock cut has to be prepared. Anisotropy effects due to differences in bedding style are reduced by keeping a consistent transmitter/receiver arrangement parallel to bedding.

With regard to the above secondary parameters, lab-based core measurements have several advantages: 1) variations in sample rock water content can be excluded by oven drying at 55°C over the duration of three days to one week (dependent on the sample size), 2) bedding-related anisotropy effects can be reduced by choosing the measuring orientation parallel to bedding planes, and 3) the contact pressure of testing device and rock surface can be standardized, e.g. to 3.0 bar, providing a constantly high transmitter strength.

Most of the disturbing secondary factors were eliminated or reduced to a minimum during the measurement programme. The most decisive of the remaining factors, affecting ultrasonic transit time, are then: I) the variable grade and type of cementation and porosity, II) variable grain contact types, and III) variable clay mineral content in the pore space and as grain coatings.

25 Chapter II

2.1.3 Transmitter-receiver test assembly Dependent on the sample geometry or to perform in-situ ultrasonic measurements on outcrop walls, different transmitter/receiver test assemblies are applied. In outcrop sections, an indirect test assembly has to be applied (Fig. 2.2A), positioning the ultrasonic probes parallel to each other, perpendicular to the outcrop wall, and parallel to the bedding. In the laboratory, the MUT can also be applied in a direct test assembly (Fig. 2.2B). Here, the measuring plane is oriented +/- parallel to bedding, normal to the well core surface. Sonic anisotropy effects due to bedding plane orientation thereby are almost eliminated.

Figure 2.2: Comparison of sonic logging methods providing sonic transit time and velocity values. Mobile ultrasonic tester (MUT) with test assemblies A) in the outcrop (indirect method) and B) in the laboratory (direct method). C) Downhole logging tool in a borehole with indirect transmitter / receiver arrangement and D) p-wave logging device of the multi sensor core logger (MSCL) with direct test assembly, modified after Filomena and Stollhofen (2011).

26 Methodology

Sonic logging tools applied to boreholes are vertically arranged, recording and integrating transit time across bedding planes and eventually across variable lithologies (Fig. 2.2C). This can affect the vertical resolution of measurements (Rider, 2002). In contrast, the transmitters/receivers of the Multi Sensor Core Logger (MSCL) are facing each other (Fig. 2.2D: direct test assembly),

It is important that one and the same transmitter/receiver setup is maintained during a measurement program to allow direct comparison of sonic velocity data in a section. As a consequence of transmitter/receiver arrangement, MUT measurements usually have higher resolution compared to borehole sonic logs, as transmitter and receiver are both located at one and the same sedimentary bed (Figs. 2.2B and 2.2C).

2.2 Porosity

The bulk porosity of solids can be subdivided into open and closed porosity. The open pore space, or effective porosity, comprises all connected and accessible cavities within a solid. Closed porosity describes non-accessible cavities, which can only be reached by destruction of the solid. In sedimentary rocks, closed porosity may reach a considerable percentage, for example in carbonate rocks. In sandstones, however, it appears only to a very low degree and can be largely neglected there. Anyway, considering the reservoir character of a sedimentary rock, only the open, connected “effective” porosity is of major interest. The open porosity of rock sample material can be determined by variable laboratory methods. In this thesis, helium porosity, isopropanol intrusion, and mercury intrusion porosimetry have been applied.

All methods require cleaned, oven-dried sample material (min. three days at 55°C).

Helium porosity: The ErgoTech Digital Modular Helium Expasion Volume Meter at the Geological Institute of RWTH Aachen University was applied to determine open porosity. Here, only ideal cylindrical sample plugs of 1 inch (2.54 cm) diameter and maximum 7 cm length can be measured. Grain volume and pore volume are measured using Boyle’s law of

27 Chapter II isothermal gas expansion of helium gas. The pore volumes are measured in a Hassler cell at confining pressures from 30 to 690 bar and porosity is calculated from these volume measurements. Grain density and bulk density are delivered by the relation of sample mass to grain volume and bulk volume, respectively.

Isopropanol vacuum impregnation: This method for porosity determination is based on the Archimedes principle of buoyancy. It does not require defined sample geometries. The average sample volume measured was 25 - 30 cm³, or 50 - 60 g per sample. After measuring the dry sample weight (mdry), the sample is entirely impregnated with isopropanol using vacuum intrusion. Afterwards, the pore volume (Volp), rock volume (Volr), and bulk volume

(Volb) are determined by applying the Archimedes principle: the weight of the saturated sample (msat) and its weight immerged in an isopropanol bath (mimm) are measured.

Additionally, the density of isopropanol (ρiso) at the current ambient temperature is required. The porosity in % is then determined as follows:

Pore volume: Volp = (msat - mdry) / ρiso

Rock volume: Volr = (mdry - mimm) / ρiso

Bulk volume: Volb = Volp + Volr

Porosity = (Volp / Volb) * 100

Mercury intrusion porosimetry (MIP) according to DIN 66133: The QUANTACHROME Poremaster 60 at the GeoZentrum Nordbayern was applied for porosity and pore size distribution measurements. Mercury is a non-wetting fluid and will not spontaneously penetrate pores by capillary action. Applying external pressure, it is forced into the pores. With increasing pressure it successively penetrates into smaller pores. The applied pressure (P) thereby is proportional to the pore radius (r), described by the Washburn equation (Washburn, 1921): Pr = - 2 γ cos θ with mercury surface tension γ = 480 N/m mercury contact angle θ = 140° results r = P / 0.736 At each pressure step one defined pore size is filled with mercury and the intruded volume is measured. As a result, the pore entry diameter (or radius) distribution is obtained after a series of pressure steps.

28 Methodology

2.3 Permeability

Darcy’s Law describes the horizontal, laminar flow of a fluid under steady-state conditions in porous media with the known length and area of the sample. The permeability (k) is given by: k = V η L / A ΔP (1 + ΔP/2) t with k: permeability of the medium in m² or mD (1 mD = 9.86923 * 10-16 m²) V: total discharge volume in m³ η: viscosity of the fluid in Pa*s L: length of the sample in m A: the area of the sample in m² ΔP: pressure difference between injection and outflow in Pa t: time to receive V in s

Gas slippage at low pressures or high velocity flow effects like turbulences, however, are neglected by this equation. In devices where the application of different pressure stages is possible, the Klinkenberg correction (Klinkenberg, 1941) is used to eliminate the gas slippage effect (intrinsic permeability). The intrinsic permeability is a theoretical permeability value, which is standardized to infinite backpressure.

Permeability measurements have been performed on core plug samples and directly on well core surfaces. The ErgoTech Digital Steady State Gas Permeameter at the Geological Institute of the RWTH Aachen University is equipped with a Hassler cell, exclusively designed for core plug samples. The New England Research TinyPerm II probe permeameter device at the GeoZentrum Nordbayern was applied on core plug samples and on well core surfaces.

Hassler cell (Fig. 2.3A): Hassler cells are designed for uni-variant sample geometries, allowing a variable core plug length but demanding a constant sample diameter. The sample plug is positioned in the centre of a double-walled steel cylinder, sealed to the side by rubber sleeves. The confining air or oil pressure is a multiple of the operating pressure. Permeability measurements can only be applied in the long-axis direction of the sample plug.

29 Chapter II

Therefore, the sample orientation is of basic importance, especially in heterogeneous or anisotropic rocks, where the measurement results are constrained by those sections with the lowest permeability. Inflow and outflow pressures can be regulated, allowing measurement at different backpressure steps. Thereby, the Klinkenberg correction can be accomplished, allowing the determination of intrinsic permeability.

Figure 2.3: Schematic constructions of A) a Hassler cell, and probe permeameters applying either B) air pressure or C) a vacuum. The sample in the Hassler cell is sealed to the side by a rubber sleeve and with an ambient pressure of the multiple of the operating pressure. The nozzle of a probe permeameter needs to be pressed against the sample surface to guarantee an adequate sealing.

30 Methodology

Probe permeameters: Mini- or probe permeameters may either use air pressure (Fig. 2.3B) or a vacuum (Fig. 2.3C) to determine rock permeability. The function of the here utilized TinyPerm device is based on vacuum application. Probe permeameter devices are small and handy, allowing permeability measurements in the laboratory and in field outcrop sections. They are equipped with a nozzle or probe tip, which is pressed against the surface of the sample material. An adequate sealing has to be provided to ensure that the measuring gas (air) passes only through the sample.

2.4 Petrographic analysis and mineralogical composition

Thin section analysis was performed by polarized light and scanning electron microscopy. The Tescan VEGA II scanning electron microscope (SEM) at the GeoZentrum Nordbayern is equipped with detectors for cathodoluminescence (CL) and backscatter electrons (BSE). For mineralogical composition analysis of rock samples, x-ray powder diffractometry (XRD) was applied using a Siemens D5000 X-ray diffractometer with a Cu α anode, operated at a voltage of 40 kV and a beam current of 30 mA. Rietveld analysis allows the quantification of mineral contents.

2.5 Micro-computed tomography (µ-CT)

High-resolution µ-CT is a newly established analytical tool in geosciences, greatly improving non-destructive 3D visualisation and quantification of mineral and porosity distributions in sedimentary rocks (Van Geet et al., 2000; Van Geet et al., 2003; Cnudde et al., 2009; Kahl and Holzheid, 2009; Long et al., 2009). The method of µ-CT imaging uses the effect of intensity attenuation of a focussed X-ray beam passing through a rock sample. The attenuated intensity, which is measured by a detector, results from the primary X-ray intensity, the thickness of the scanned object, and the linear attenuation coefficient. The latter value is largely controlled by the density and the effective atomic number of the test material (Wellington and Vinegar, 1987). For visualization, the linear attenuation coefficients are converted to grey scale values ranging from 0 (lowest

31 Chapter II attenuation coefficient – air in pores) to 255 (highest attenuation coefficient – high density) (Long et al., 2009), characterizing each pixel of a 2D image. Stacked and interpolated, the pixels (2D) become voxels (3D) and the cross section images are added to provide a 3D reconstruction of the object. Thus µ-CT imaging provides an ideal tool to identify materials of considerable differences in density, such as heavy minerals or (air-filled) pore space in quartz sandstones. The maximum resolution of µ-CT imaging predominantly depends on the sample size, the distance between source, sample, and detector, beam power and the resolution of the detector camera. More detailed information on the physical basis of µ-CT imaging is given by Van Geet et al. (2000) and Long et al. (2009).

A SkyScanTM 1172 desktop microfocus X-ray computer tomography device of RJL Micro & Analytic GmbH, Karlsdorf-Neuthard was applied in the study, presented in Chapter III. Operating conditions were at 100 kV beam energy and 100 µA source current, filtered by a 0.5 mm aluminium sheet. Artifact-reducing filters and corrections considering beam hardening, ring-artifact, frame averaging, and geometrical corrections have been applied during scanning. The detector of the camera has a size of 2672 x 4000 pixels. With a scanned interval of 6.4 mm length, an object-to-source distance of 64.7 mm, and a camera-to-source distance of 213.9 mm, results are at a maximum resolution of 2.6 µm in all three dimensions (voxel). A better resolution of 1.2 µm per voxel is achieved using a 2 mm sized sample with an object-to-source distance of 36.1 mm and a camera-to-source distance of 344.2 mm.

The SkyScanTM 3D CT-analyzer software (CTan) was applied for image processing, reconstruction, and 3D calculations of the pore network and pore diameters. To avoid boundary effects and to handle the enormous data volume, several regions of interest were defined in the inner part of a sample plug. The size of a region of interest (ROI) is variable and is customized to the different types of analyses being processed.

Figure 2.4 (next page): Examples of µ-CT images (minerals: grey scale, porosity: black) and visualization of pore space accessibility of the Gambach and the Guggenberg samples, executed by the “close” function of the CTan image analysing software. The numbers (0-5) indicate the amount of pixels added to the mineral structure during each computing step, leading to a pore throat narrowing. Step 0 represents the original pore space with pore space filling minerals. During each additional step pore throats were narrowed and thereby closed pores were identified. The remaining connected pore network is displayed in white. A schematic overview on pore throat narrowing is given in the third column. Scale bars: 100 µm.

32 Methodology

33 Chapter II

The 3D pore networks and the percentage of connected and isolated pore space of each sample were modelled from 16 cubic ROIs as the percent object volume of the binarized and segmented pore space. In CTan, the pore diameter can be displayed by the structure thickness calculation according to Hildebrand and Ruesegger (1997). The local thickness for a point in a solid thereby is defined as the diameter of the largest sphere which fulfils two conditions: (I) the sphere encloses the point (but the point is not necessarily the centre of the sphere), and (II) the sphere is entirely bounded within the solid volume.

In a binarized image, CTan recognizes white as solid and black as pore space. To apply this function to porosity and to visualize it in CTan, the binarized image was inverted, showing the porosity in white.

To evaluate the pore entry diameters and the degree of pore connectedness, the “close” function of the CTan morphological operation tool was used in combination with an extended 3D analysis. The “close” function increases the mineral diameter by a given number of pixels, eventually leading to the closure of narrow pore throats. Pore network sections which have been isolated due to this operation are then filled and recognized by the software as a solid. After the filling of these sections, the same pixel thickness is deducted again from the mineral surfaces to preserve the real volume of the non-closed part of the pore network.

The subsequent 3D analysis shows the remaining open pore space and contemporaneously indicates the percentage of pore space which is accessible only via pore throats larger than those which have been closed in the previous step. Two examples of the application of this method and a schematic overview are displayed in Figure 2.4. There, a low-porosity sample (Gambach) and a high-porosity sample (Guggenberg) are shown. Six computing steps (0-5), each with a solid thickening increase of 1 pixel, have been applied. The first step (0) was calculated with no thickening, using the original mineral surface; the last step (5) with 5 pixels thickening. In a 2D section, pore throats are always bounded from two sides, therefore a unilateral (from one side) thickening of the bounding minerals results in a narrowing of the pore diameter of 2 pixels (Fig. 2.4). Applied to the µ-CT dataset of the 10 mm plugs, the pore throat narrowing from one step to the next is equivalent to <5.2 µm.

34 Methodology

2.6 Compressive strength

The uni-axial compressive strength measurements presented in Chapter V have been performed at the Labor für Baudenkmalpflege Dr. Robert Sobott in Naumburg, using sample cubes of 4x4x4 cm. The compressive strength βD is defined by the vertically applied force F in Newton (N), which produces rock failure, divided by the area A0 of the sample cube (mm²), indicated as N/mm² or MPa (megapascal):

βD = F / A0

2.7 E-modulus

Young’s modulus of elasticity or E-modulus is a measure of the stiffness of an elastic material. In rock plug core samples, the E-modulus can be determined applying dilatation waves. Dilatation waves are generated in cylindrical samples of a length (l) greater and a diameter (d) smaller than the wave length of the p-wave (l/d ~ 2) (cf. Erfurt and Krompholz, 1996). The resonance frequency of the dilatation wave then can be measured.

The velocity of the dilatation wave (vD) is two-times the product of the resonance frequency and the sample length. The E-modulus (E) was then calculated from bulk density (b) and dilatation wave velocity (vD). 2 E = vD *b using the bulk density determined from isopropanol intrusion:

b = mdry / Volb (see Chapter 2.2)

35 ______

Chapter III

Application of Ultrasonic Measurements to Stratigraphy: Detection of a Cryptic Unconformity ______

Parts of this chapter are published in Filomena, C.M. and Stollhofen, H., 2011, Ultrasonic logging across unconformities – outcrop and core logger sonic patterns of the Early Triassic Middle Buntsandstein Hardegsen unconformity, southern Germany: Sedimentary Geology, v. 236, p. 185-196.

Abstract The recognition of unconformities in thick and monotonous sandstone successions of consistent grain size is often difficult in locally exposed surface outcrop sections and also poorly constrained by changes of petrophysical properties across the boundary surfaces. Here, a mobile ultrasonic device is tested as a time-efficient handheld logging tool to investigate relative changes in sandstone cementation and porosity across the Early Triassic Middle Buntsandstein Hardegsen unconformity in southern and central Germany. A persistent offset of 18-20 % sonic transit time occurs in all tested sections across the unconformity. This offset is not restricted to single peaks but clearly separates the different sonic patterns of the Hardegsen and Solling Formations below and above the unconformity. Integrated thin section analysis, porosity-permeability measurements, µ-CT imaging, and gamma-density logging suggest that these changes in sonic transit time are due to major variations in sandstone cementation and the formation of secondary porosity associated with the unconformity. Field-based ultrasonic measurements thus provide a useful addition to established outcrop logging methods and may help in the non-destructive in-situ identification of porosity/cementation heterogeneities in sandstones, in the localization of unconformities and in the correlation of borehole measured sequence boundaries to outcrop analogs.

36 Application to Stratigraphy

3.1 Introduction

3.1.1 Geological setting From Permian times onwards, the evolving Central European Basin System (CEBS) was repeatedly affected by tectonic pulses (Bachmann et al., 2008; Stollhofen et al., 2008) associated with the earliest phases of Tethyan and Arctic-North Atlantic rifting (cf. Ziegler, 1982; Radies et al., 2005b; 2005a). This led to the formation of syndepositional horst and graben structures (Röhling, 1991; Geluk and Röhling, 1997; Radies et al., 2005a; 2005b), commonly termed "swells" or "highs" (e.g. Eichsfeld-Altmark Swell, Hunte, Hannover, and Brandenburg High) and intervening areas of enhanced subsidence (e.g. Viking, Central, Horn and Glückstadt Graben, Hessian, Weser, Franconian, and Thuringia-Westbrandenburg Depression) (Wolburg, 1962; Ziegler, 1990; Freundenberger, pers. comm. 2010) (Fig. 3.1). Contemporaneous to tectonism, sea level fluctuations affected the epicontinental CEBS and its sub-basins (Ziegler, 1990; Aigner and Bachmann, 1992; Legler et al., 2005; Feist-Burkhardt et al., 2008).

During the Early Triassic (Olenekian) Middle Buntsandstein (Menning and Hendrich, 2002) this interaction of tectonic and eustatic base level controls is recorded by the fluvial sandstone dominated series of the Volpriehausen, Detfurth, Hardegsen, and Solling Formations (Boigk, 1959; Trusheim, 1961; Röhling, 1991; Aigner et al., 1998; Lepper et al., 2005) (Fig. 3.2). Each of these formations, except for the Hardegsen Formation, is floored by a distinct unconformity (Röhling, 1991), used as important boundaries within the sequence stratigraphic framework of the German Triassic (Aigner and Bachmann, 1992).

Of the Triassic unconformities, the Hardegsen unconformity (H-unconformity) is by far the most widespread. It has been identified in a huge area covering the south German Black Forest area (Ortlam, 1974; Aigner and Bachmann, 1992), Central Germany (Beutler, 1991; Lepper et al., 2005), northern Germany and the Netherlands (Geluk and Röhling, 1997; Geluk, 2005), the North Sea (Geluk, 1998), northwest England (Evans et al., 1993) and the Irish Sea Basin (Cowan, 1993). Mostly, the H-unconformity classifies as a paraconformity or disconformity. Associated erosion may cut deeply into the underlying formations, occasionally causing the Solling Formation to sit directly on Lower Buntsandstein (Figs. 2.1 and 2.2) or even Upper Permian Zechstein (Röhling,

37 Chapter III

1991; Geyer, 2002; Geyer and Schmidt-Kaler, 2009). Calculated stratigraphic loss associated with the unconformity achieves values of up to 300 m at the Eichsfeld-Altmark Swell and up to 600 m at the Hunte High (Trusheim, 1963).

3.1.2 Study area and study sites The H-unconformity in southern Germany is the main target of this study. Examined outcrop and well core sections are located in the Franconian and Thuringian sub-basins of the Central European Basin (Fig. 3.1). The first test site where in-situ ultrasonic measurements have been performed is Gambach (Fig. 3.1) where the H-unconformity is well exposed in an abandoned quarry. Complementary ultrasonic measurements in the laboratory were accomplished with the well core B28 from the Guggenberg disposal site near Miltenberg (Fig. 3.1), located ~50 km southwest of Gambach. Both, well core and outcrop section show a well-developed paleosol associated with the H-unconformity.

Figure 3.1: Location map of the well sites Guggenberg B28 and Rockensußra 209/83, and of the Gambach surface outcrop study sites in southern Germany. Also illustrated is the distribution of Lower Triassic strata underlying the Hardegsen unconformity and of major syndepositional highs and depressions, compiled from Laemmlen (1966), Diederich and Hickethier (1975), Röhling (1991), Beutler (1991), Dersch-Hansmann and Hug (2004), and Freudenberger (pers. comm. 2010).

38 Application to Stratigraphy

Due to non-deposition or erosion, the paleosol may laterally pinch out and may not be present at other localities. This is the case at our third test-site, the well core Rockensußra from the Thuringian Basin (Fig. 3.1), where the exact location of the H-unconformity within the well core section is still controversial. Here, the ultrasonic survey is applied to provide a petrophysical basis for the positioning of the unconformity. The sandstone successions of the Hardegsen and Solling formations that enclose the H-unconformity are interpreted as having formed in ephemeral braided fluvial to sheet flood environments (Ziegler, 1990; Bindig, 1994; Geyer, 2002). The maximum post-Buntsandstein overburden in the area of interest can be calculated to achieve 1500-2200 m, based on available paleogeographic and isopach maps (Freudenberger and Schwerd, 1996; Geyer, 2002).

3.2 Identification of unconformities

Established laboratory and field criteria to identify unconformities in siliciclastic systems comprise changes in the assemblage of detrital minerals (Ingersoll, 1978) or heavy minerals (cf. Krumbein, 1942), in situ paleosols (Ortlam, 1974; , 1980; Marriott and Wright, 1993; McCarthy et al., 1999) and associated changes in rock color or fabric (e.g. mottling, nodules) (Mack and James, 1992), remains of reworked paleosols known as pedogenic mud aggregates (cf. Gierlowski-Kordesch and Gibling, 2002), and changes in mean paleocurrent vectors (Miall and Arush, 2001). To pinpoint the exact location of unconformities in boreholes, abrupt changes in vitrinite reflectance (Hertle and Littke, 2000) or offsets of shale compaction trends are frequently used (Sclater and Christie, 1980; Henk, 1992). The latter method is based on the principle that "normal" sediment compaction gradually increases during burial, whereas porosities record the opposite trend (Magara, 1976). In lithologically uniform and conformable strata, sonic velocity is mainly a function of porosity, thus a gradual increase can be expected with depth (cf. Mayr and Burkhardt, 2006). Abrupt offsets of sonic velocities then provide reliable proxies for the positioning of unconformities and even for the calculation of associated stratigraphic loss (Henk, 1992).

39 Chapter III

The recognition of unconformities in continental settings may be challenging when classic macroscopic features like paleosols are missing. Amalgamated on structural highs, unconformities may laterally split only in areas of enhanced synsedimentary subsidence (Aigner and Bachmann, 1992; Radies et al., 2005b). However, the sediment source area may not change across the unconformity, leading to a uniform sandstone succession of similar granulometry and detrital components. Resulting unconformities have been termed cryptic sequence boundaries and may only become obvious through increased proportions of cement and inversely porosity immediately below the boundary surfaces (Miall and Arush, 2001).

Weathering processes rather similar to those occurring at paleo-land surfaces and associated disconformities can also be observed on the surfaces of building stones and/or monuments, although exposure to the atmosphere there rarely exceeds several hundreds of years and weathering effects are commonly restricted to depths of a few cm. To quantify and map the intensity of weathering damage on building stones, ultrasonic testing is a reliable and well-established method (Fitzner et al., 2003). Likewise, ultrasonic velocities are used as a tool for porosity quantification of building elements and mortar in the concrete construction industry (Lafhaj et al., 2006). Schmidt rebound hammer, ultrasonic tester, or a combination of both devices are also used in engineering to quantify concrete and rock hardness (ASTM D5873 standard). In sedimentary studies, however, the determination of the degrees of rock cementation, decementation and weathering is commonly purely based on thin section analyses.

In this study, changing ultrasonic patterns across unconformities in monotonous sandstone successions are presented, which compare well to the cryptic sequence boundaries of Miall and Arush (2001). The method requires, that lithologically similar rocks, but with contrasting burial and diagenetic history are compared to one another. A further pre- requisite are “fresh” rock cuts (e.g. quarry walls, cores) without recent weathering surfaces that may overprint the unconformity (see Chapter 2.1.2). At this point, it is clearly pointed out that this study is not aiming to compare absolute sonic velocity values but relative changes and ultrasonic trends, derived from a representative number of measurements across one and the same unconformity surface.

40 Application to Stratigraphy

Figure 3.2: A) Regional unconformities of the Lower Triassic Buntsandstein Group in Germany (modified from Aigner and Bachmann, 1992) within the framework of third order base level cycles (Aigner et al., 1998) and tectonic pulses (Bachmann et al. 2008). Stratigraphy and numerical ages after Menning and Hendrich (2002). B) Stratigraphic details of the outcrop and well core sections considered in this study.

3.3 Methods

To evaluate functionality, performance capability, and reliability of ultrasonic tests, a combination of methods has been applied: I) a Geotron UKS-40 mobile ultrasonic tester (MUT), II) a lab-based GeoTek multi sensor core logger (MSCL), III) an ErgoTech gas porosimeter and permeameter, IV) isopropanol vacuum impregnation porosimetry, V) conventional polarizing light microscopy of thin sections and electron (SEM) / backscatter electron (SEM-BSE) microscopy, and VI) microfocus X-ray computer tomography (µ-CT) of selected samples using a SkyScanTM 1172 device. The mobile ultrasonic tester (MUT) was operated at a signal frequency of 46 kHz. To enhance performance, a fixed distance of 30 cm between transmitter and receiver was used in the outcrop section, which enables direct identification of exceptional records and easy repetition of measurements, if necessary. At each measuring point, the ultrasonic

41 Chapter III transit time was recorded 10 times, resulting in a median value that characterizes a particular part of the section. To cross-check the reliability of the MUT against readings derived from the lab-based multi sensor core logger (MSCL), the Guggenberg well core section was taken for comparison. The p-wave transmitter of the MSCL is operated at a frequency of 250 kHz. High-resolution ultrasonic data were sampled with the MUT at a spacing of 0.5 cm and then compared to 901 p-wave velocity data points from the same core interval, measured with the MSCL. The partial regression line of the cross-plot (Fig. 3.3) is nearly a bisectrix, with a standard error of 3.26, suggesting that the MUT and MSCL values are almost identical. The coefficient of determination (R²) is 0.9109, which is equivalent to >91 % accordance of the two devices. As this study focuses on general sonic trends and not absolute values, the minor differences of MUT and MSCL devices have a negligible affect on the interpretation of the recorded sonic logs. Additional gamma density data were recorded with the MSCL and porosity and permeability measurements of plug core samples from the outcrop and well core sections were used to test the relationship between changes in ultrasonic velocities and variations in the degree of cementation and remaining porosity. Thin sections from key positions in the measured sections have been analyzed with the polarizing microscope to record grain size, mineral content, type and intensity of cementation. µ-CT was applied to two cylindrical plugs of 10 mm diameter and two samples of 2 mm size.

Figure 3.3: Scatter-plot of 901 sonic transit time measurements (Δt) taken with mobile ultrasonic tester (MUT) and lab-based multi sensor core logger (MSCL) devices of the Guggenberg B28 well core section. The R² value indicates a 91 % accordance of the measuring values.

42 Application to Stratigraphy

3.4 Outcrop and well core sections

3.4.1 Well core: Guggenberg B28 (Northern Bavaria, Germany) The Guggenberg B28 well core (Fig. 3.4) has a total length of 57.40 m, of which the lowermost 9.00 m cover strata of the Middle Buntsandstein Group. The upper part of the Hardegsen Formation is encountered in the well core between 52.90 m and 57.40 m and the VH2a paleosol (Karneol-Dolomit-Horizont) which marks the H-unconformity extends from 51.80 m to 52.90 m. The Solling Formation includes Solling Sandstone, VH2b paleosol, and Chirotheriensandstein, extending from 48.30 m to 51.80 m.

Figure 3.4: Combined sedimentological and petrophysical logs of the Guggenberg B28 well core section (48.30-57.30 m), including gamma-density, sonic transit time (Δt), and porosity- permeability measurements from plug core samples. The H-unconformity and the associated VH2a paleosol separate relatively porous and permeable sandstones of the Hardegsen Formation (high sonic transit times) from tight low-porosity sandstones of the overlying Solling Formation (lower sonic transit times).

43 Chapter III

The upper part of the Hardegsen Formation is dominated by trough cross-stratified, medium-grained sandstone beds with clay rip-up clasts at their erosive bases. Few micro- root traces can be found throughout the sandstone sections. The sandstone units are separated by minor interbeds of tabular plane bedded mudstones, 20-50 cm thick, occasionally with convolute bedding. The thinly Fe-oxide coated quartz grains of the sandstone intervals show mainly tangential and long contacts, only few grains are sutured and intergrown (Fig. 3.5A). Early diagenetic, euhedral quartz overgrowths occupy almost the entire original intergranular pore volume (Fig. 3.5A). Macroporosity predominantly traces the shapes of dissolved feldspar and quartz grains, suggesting secondary leaching processes (Fig. 3.5B). The sandstone bed directly below the VH2a paleosol is particularly affected by Fe-oxide cementation around corroded quartz and feldspar grains (Fig. 3.5C). Clay mineral (illite) and muscovite content is increased in this section.

Lithology The H-unconformity is represented by the 110 cm thick VH2a in situ paleosol. Its lower part consists of discontinuously plane bedded sandy siltstones, with two thin clean sandstone inter-beds. Small carbonate nodules and few macro-root traces filled with carbonate, Fe-oxides, and clay minerals can be found throughout the paleosol section (Fig. 3.5D). The top of the paleosol comprises a strongly cemented, 35 cm thick unit of large carbonate concretions.

Overlying is the "Solling Sandstone" (Fig. 3.4), consisting of plane to trough cross- bedded medium-grained sandstones alternating with mudstone intervals. The detrital quartz and feldspar grains show only few quartz overgrowths. Quartz and feldspar grains and quartz overgrowths are largely corroded and illite coated. Mica minerals, primarily muscovite, are deformed or crushed due to mechanical compaction. Carbonate and mesh illite infill the secondary pore space created by feldspar dissolution (Fig. 3.5E).

The successive VH2b paleosol unit above is formed by inter-layered fine-grained sandstones and mudstones comprising carbonate nodules of a few mm in diameter, whitish rooting-traces and occasional water escape structures. Fine, downward branching fracture-like networks are filled with Fe-oxides, interpreted as micro-root traces.

44 Application to Stratigraphy

The upper section of the Solling Formation then is characterized by several 10 to 20 cm thick medium-grained sandstone beds with large clay rip-up clasts at their bases and a fining upward trend towards their top. This interval is interpreted to represent the Thüringer Chirotheriensandstein (cf. Fig. 3.2B). Here, the thinly Fe-oxide and illite coated quartz grains show again an increasing percentage of quartz overgrowths and leached feldspar grains providing secondary porosity.

Figure 3.5: Thin section microphotographs of the Guggenberg B28 well core section. Open pore space (p, sp) is indicated by blue epoxy. A) Hardegsen Formation (57.30m), euhedral quartz overgrowths (eq) indicating early diagenetic cementation of open pore space. Note the sutured and intergrown quartz grains (s) indicating chemical compaction, crossed polars. B) Hardegsen Formation sandstone (54.12 m), showing secondary porosity (sp) due to quartz and feldspar dissolution and mixed iron-oxide and clay minerals (Fe-cm) partly filling the pore space. C) Hardegsen Formation (53.12 m) immediately below paleosol VH2a with increased mixed iron- oxide and clay mineral content (Fe-cm) and largely reduced porosity (p). D) Sandstone section within the paleosol VH2a (52.10m) showing root traces (r) filled with carbonate, iron-oxides, and clay minerals. E) Solling Formation (50.80 m), carbonate (c) and illite (i) pore space fillings, mica (m) deformation due to compaction.

45 Chapter III

Petrophysical properties Figure 3.4 summarizes lithological properties (grain size, sedimentary structures, components) and logs of gamma-density, sonic transit time, and porosity-permeability of the Guggenberg well core. In general, the sonic log behaves in reverse to the gamma- density log, resulting from lower transit times (higher ultrasonic velocities) in denser materials. Differences between sandstone and mudstone intervals are very pronounced in the sonic log. The laminated mudstones show transit times of 75-85 μs/ft, whereas sandstone transit times record a broad range of 90-130 μs/ft, dependent upon their grain size, cementation and stratigraphic position.

The Hardegsen Formation is characterized by high sonic transit times (mean 115.0 μs/ft) and low densities (mean 2.327 g/cm³). The sonic and density values of the Fe-oxide rich sandstone bed below the VH2a paleosol (53.11-53.65 m) differ from this mean values; here lower transit times and higher densities, related to enhanced cementation and a decrease in porosity occur. Within the VH2a paleosol section the mean sonic and density values of the sandstones are 97.7 μs/ft and 2.528 g/cm³, wheras 95.5 μs/ft and 2.450 g/cm³ were measured in the overlying Solling Formation. Ultrasonic transit times across the H-unconformity thus reveal significant changes. The mean t value of the sub VH2a sandstones of the Hardegsen Formation is about 18% higher compared to equivalent lithologies of the overlying Solling Formation. This obvious sonic offset coincides with 6.5 % rise in mean gamma density.

Porosity and permeability measurements have been performed on 27 sandstone plug core samples from selected key-positions of the well core (Fig. 3.4). High porosities are characteristic for the Hardegsen Formation, ranging at 11.0-17.8 %. In contrast, lower porosity values of 7.2-10.2% were recorded in the Solling Formation and the paleosol VH2a. This change in porosity corresponds very well with changes in sonic and density measurements. Considerable permeability is developed solely within the Hardegsen Formation sandstones, with average values of 43.05 mD, varying between 0.02 mD minimum and a maximum of 214 mD. The sandstones directly below and above the H-unconformity are usually tight, with permeabilities < 0.1 mD.

46 Application to Stratigraphy

3.4.2 Outcrop: Gambach quarry (Northern Bavaria, Germany) In the abandoned quarry at Gambach (Fig. 3.1), the sandstones of the Hardegsen and Solling Formations with the intervening H-unconformity are exposed over 15 m length with > 4.5 m thickness (Fig. 3.6). This outcrop was first described in detail by Schuster (1933) and was later further elaborated by Laemmlen (1966), Backhaus (1968), Lepper (1970), Wittmann (1972), and Joos (2000). According to Joos (2000) neither sediment sources nor detritus composition and heavy mineral assemblage changed significantly across the unconformity.

Figure 3.6: Combined sedimentological logs, petrophysical logs and photographs of the Gambach outcrop section illustrating the position of the H-unconformity. A) Photograph of the Hardegsen Formation overlain by the well-developed paleosol VH2a which marks the H- unconformity, B) Photograph of the Solling Formation including the paleosol VH2b. See Fig. 3.4 for key to sedimentary structures. Note the abrupt change in sonic velocities associated with the H-unconformity.

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Lithology The lowermost 1.10 m of the measured section (Fig. 3.6) belong to the Hardegsen Formation, including a thin pedogenically overprinted mudstone unit, some 10-20 cm thick with a violet coloration near its base. The remaining exposure of the Hardegsen Formation consists of plane-bedded to low-angle cross-bedded, medium-grained sandstones, transected by large subvertical root traces. The sandstone unit directly below the H-unconformity shows an abundance of euhedral, intergranular pore volume-filling quartz overgrowths, reducing the open porosity to a large extent (Figs. 3.7A and 3.7B). The Fe-oxide coated quartz and feldspar grains have predominantly tangential contacts. Thin illite rims partly cover the remaining pore space (Fig. 3.7A). Violet colored 50 cm thick mudstones overly the sandstones of the Hardegsen Formation. Micro-root traces, microcrystalline quartz (carneol) and dolomite concretions record considerable pedogenic modification of the mudstones. This unit is finally capped by a strongly cemented, 25 cm thick horizon made up of large amalgamated carbonate nodules, as described above from a time-stratigraphic equivalent of the Guggenberg B28 well core (Fig. 3.4). The tripartite section described, represents the VH2a paleosol, sitting directly below the H- unconformity. The overlying Solling Formation comprises medium-grained, trough cross-bedded to lowangle bedded sandstones, in part with clay rip-up clasts. The VH2b paleosol is developed in a 30 cm thin interval of greenish colored silty fine sandstones capped by a horizon of amalgamated carbonate nodules. The sandstone components are slightly corroded quartz and feldspar grains, and few lithic fragments (Figs. 3.7C and 3.7D). The detrital grains are largely coated by illite and Fe-oxides and some lithic fragments are entirely illitized (Fig. 3.7D). The intergranular pore space is partly filled with carbonate cement, illite, and Fe-oxides, but a considerable amount of open pore space is still preserved (Fig. 3.7C).

Petrophysical Properties Within the sandstones of the Hardegsen Formation, sonic transit times and porosities consistently decrease from the base of the measured section towards the VH2a paleosol section (Fig. 3.6). Porosities reduce from a maximum of 20.7 % and mean ∆t values of 163.4 µs/ft to a minimum porosity of 7.3 % and 141.8 μs/ft. The mean sonic transit time of the exposed Hardegsen Formation is 153.8 µs/ft. Sonic measurements within the VH2a paleosol section have not been considered due to the strong alteration of the mudstones.

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In the Solling Formation the sonic transit time suddenly increases to mean Δt values of 197.2 μs/ft, correlative to a porosity increase to 12.7-20.2 % maximum. The observed change of mean ultrasonic transit time across the H-unconformity is equal to a positive sonic offset of 25 % towards the Solling Formation.

Figure 3.7: Thin section microphotographs of Gambach outcrop samples: A) Hardegsen Formation (1.00 m), characterized by euhedral quartz overgrowths (eq) with original grain shapes preserved by thin Fe-oxide coatings (Fe). Small illite nests (i) are developed in remnant pore space. B) same as A, crossed polars. C) Solling Formation (2.50 m) with corroded, illite coated quartz (i) and illitized feldspar (i/Fe) D) same as C, crossed polars.

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3.4.3 Well core: Rockensußra 209/83 (Thuringia, Germany) The Rockensußra well core 209/83 was recovered from the Thuringian Depression (Fig. 3.1). The well (Fig. 3.8) intersects the H-unconformity, but its exact position has not yet been unequivocally ascertained. A paleosol is not preserved and sandstones of similar grain size are present below and above the unconformity. Based on differences in coloration and a slight increase of grain size, Grumbt et al. (1997) interpreted the position of the H-unconformity at a well core depth of 449.20 m. By means of cyclic analysis Lang (2001) then proposed the location of the unconformity 0.70 m below the previously suggested position.

Figure 3.8: Combined sedimentological logs, petrophysical logs and photographs of the Rockensußra 209/83 well core section (446.50-451.50 m). Suggested positions of the H- unconformity are numbered: 1. Lang (2001), 2. Grumbt et al. (1997), 3. this study. Well core photographs of A) Hardegsen Formation including a sand-dyke water escape structure. B) An erosive surface associated with rock bleaching and clay rip-up clasts marking the position of the H-unconformity suggested in this study. C) Solling Formation. See Fig. 3.4 for key to sedimentary structures. Note the abrupt change in sonic velocities associated with the H-unconformity.

50 Application to Stratigraphy

A new analysis of the core section combines the lithological description with an ultrasonic log (Fig. 3.8) comprising 218 measuring points covering 2.5 m of section above and below the previously suggested unconformity positions.

Lithology The examined well-core section of the Hardegsen Formation comprises fine- to medium grained sandstone beds alternating with 3-20 cm thick mudstones intervals. The sandstones are horizontal to trough cross-bedded, and in parts also structureless. Water escape structures are documented by sand dykes (Fig. 3.8A). Isolated small clay rip-up clasts are common near to erosive bases. Thin sections show predominantly illite coated quartz and feldspar grains, but quartz overgrowths occur only subordinately (Figs. 3.9A, 3.9B). The open pore space is mainly of a secondary nature, associated with corroded quartz and leached feldspar grains (Fig. 3.9B). The position of the H-unconformity suggested here is characterized by an erosive surface cutting down into horizontally bedded, bleached sandstones (Fig. 3.8B) with an enhanced content of clay rip-up clasts. The sandstones of the Solling Formation (Fig. 3.8C) are moderately cemented by euhedral quartz overgrowths, growing into primary open pore space (Fig. 3.9C). Secondary porosity is largely restricted to dissolved feldspar grains, indicated by ghost structures of the remaining Fe-oxide coatings (Fig. 3.9D). Bituminous remains are common in all samples of the Rockensußra well core section (Figs. 3.9A to 3.9D).

Petrophysical properties The sandstones of the Hardegsen Formation reveal a mean sonic transit time of 132.3 μs/ft (Fig. 3.8) and a mean porosity of 18.3 % with a total range of 17.2-20.7 %. A lower mean t value of 106.7 μs/ft was recorded in the overlying Solling Formation, accompanied by reduced mean porosities of 13.1 % (total range 10.9-16.5%). The sharp drop in sonic transit time achieves ~18.5 % at 448.75 m, with sonic transit time values kept reduced for the remaining upper part of the measured section. At the position of the sonic offset, the grain size remains constant, but an erosive base is developed. This boundary most probably coincides with the true position of the H-unconformity, fixing it 0.45 m higher in the section than previously suggested by Grumbt et al. (1997) and consequently 1.15 m higher than suggested by Lang (2001).

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Figure 3.9: Thin section microphotographs of the Rockensußra well core samples: A) Hardegsen Formation (450.60 m), secondary porosity (sp) due to quartz and feldspar corrosion, partly bituminous (b) pore fills. B) Hardegsen Formation (449.60 m) with secondary porosity (sp) development due to quartz and feldspar dissolution. C) Solling Formation (447.70 m) with euhedral quartz overgrowths (eq) and primary pore space preserved. D) same as C, ghost structure (g), resulting from feldspar dissolution and preserved by thin Fe-oxide coating. Bituminous remains (b) are present. Note differences in scale!

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3.5 Visualization of sub-H-unconformity pore networks

3.5.1 3D pore volume reconstruction Micro-CT imaging provides 2D sections of scanned specimens and allows their 3D volume reconstruction. Two samples from the Guggenberg (Fig. 3.10A) and Gambach (Fig. 3.10B) locations have been selected to visualize the sandstone pore networks occurring below the H-unconformity. Contrasting proportions and spatial distributions of porosity in the Hardegsen Formation sandstones from the Guggenberg and Gambach localities have been documented by thin section polarized light and SEM microscopy. At Guggenberg, the pore space is rather homogeneously distributed throughout the entire sample. In contrast, the Gambach sample reveals a heterogeneous porosity pattern with selected patches developing macropores and adjacent regions being largely quartz cemented (see Chapter 3.4).

Figure 3.11 illustrates two 1.3x2 mm sized cylindrical region of interest (ROI) volumes of the Guggenberg (Fig. 3.11A) and the Gambach (Fig. 3.11B) samples, reconstructed from the µ-CT datasets of the 10 mm samples (Fig. 3.10). 2D sections of the µ-CT ROI volumes (Figs. 3.11C, 3.11D) and of the segmented pore space (Figs. 3.11E, 3.11F) are accompanied by 3D pore network reconstructions from top (Figs. 3.11G, 3.11H) and side views (Figs. 3.11I, 3.11J). Similar to classic thin sections, the µ-CT 2D sections show different amounts of pore volume present in the two samples. The 3D reconstructions visualize the contrasting pore networks of the two Hardegsen sandstones. Whilst a large and well-connected pore network is developed within the Guggenberg sample (Figs. 3.11G, 3.11I), small and fairly connected pores are typical for the Gambach sample (Figs. 3.11H, 3.11J).

Secondary porosity development, associated with intensive feldspar leaching was observed in the Guggenberg sample by SEM and BSE microscopy (Figs. 3.12A and 3.12B), which can be also detected by µ-CT imaging (Figs. 3.12C to 3.12E). These intra- granular porosities are resolved best in µ-CT datasets using 2 mm sized sandstone samples. Three-dimensional µ-CT reconstructions further document the distribution of intra-granular porosity within individual feldspar grains.

53 Chapter III

Figure 3.10: Sedimentary litho- and grain size logs of A) the well core section at the Guggenberg disposal, and B) the outcrop section at Gambach (Main), both southern Germany. Indicated are the positions of two cylindrical sandstone samples (10 mm diameter and 6.4 mm length) from the Hardegsen Formation. The grey-scale µ-CT scans illustrate different framework minerals of the sandstone samples, where quartz appears in dark grey, feldspar in slightly lighter grey, and heavy minerals in almost white. Porosity is indicated in black.

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Figure 3.11 (this page): Cylindrical, 1.3 x 2 mm µ-CT ROI volumes of the Guggenberg (A) and Gambach (B) samples. 2D grey-scale µ- CT slices (C, D) showing porosity (black) and minerals (grey). Binarized and black / white swapped 2D µ-CT slices show porosity in white (E, F). Top and sideview of 3D reconstructions, visualizing the pore systems (white) of the Guggenberg (G, I) and Gambach samples (H, J).

Figure 3.12 (next page): SEM-BSE (A) and SEM (B) images of quartz [Q] and partly dissolved feldspar [Fd] grains of the Guggenberg sample. Horizontal 2D µ-CT slices of different positions within one and the same feldspar grain (C, D, E). F) 3D reconstruction of the intra-granular secondary pore network, view from above. Note the linear arrangement of pore space parallel to the feldspar cleavage. G) Close-up sideview of the intra-granular pore network of a degraded feldspar grain.

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56 Application to Stratigraphy

Feldspars affected by moderate dissolution tend to develop a linear, subparallel aligned pore-network (Fig. 3.12F) with up to 15 % intra-granular porosity, which suggests preferred leaching along the mineral cleavage planes. These linear cavities extend throughout the entire mineral grain and develop only a few inter-connections between each other (Fig. 3.12G). Enhanced dissolution of feldspars results in the formation of large cavities and intra-granular porosities approaching > 40 %. The pores are partly filled with fibrous clay minerals causing porosity reduction. Though, the exact identification of the clay mineral type is not possible on the basis of µ-CT imaging due to limited resolution.

3.5.2 MIP versus numerical µ-CT pore analysis After µ-CT scanning, the presented samples have been analyzed by mercury intrusion porosimetry (MIP), defining open porosity and pore entry diameters. The Guggenberg sample is characterized by a mercury intrusion porosity of 16.4 %, showing nearly a Gaussian distribution of pore entry diameters, predominantly ranging in the macropore class (Fig. 3.13A). The sample of the Gambach location reaches a mercury intrusion porosity of 5.7 %, covering the entire pore spectrum from micro- to macropores (Fig. 3.13A). The Gambach sample has a strong bimodal distribution of pore diameters, with almost 56 % macropores and 44 % meso- and micropores. Besides the two major peaks, also three smaller peaks are recorded: one of each in the upper macro- and mesopore and in the micropore sections.

The pore volumes of the Hardegsen Formation sandstone samples were calculated and reconstructed using the CTan software. Due to resolution limitations of µ-CT, pore diameters < 2.6 µm (10 mm samples), or <2 µm (2 mm samples), could not be detected. Consequently, µ-CT porosity calculations are mainly restricted to the macropore system. The open porosity of the Guggenberg sandstone sample was calculated to 9.9 % (MIP: 16.4 %), whereas the Gambach sample provides only 2.5 % porosity (MIP: 5.7 %).

The calculation of the pore entry diameter distribution and the accessibility of the pore network can be also determined from µ-CT datasets, provided by an iterative process of the CTan close function combined with a 3D pore volume analysis. The resulting histogram (Fig. 3.13B) illustrates the different pore diameter classes and the respective

57 Chapter III percentile of accessible pore space (where the total porosity is 100%). The majority of the pore volume in the Gambach sample (black columns, Fig. 3.13B) is connected via the 15.6 µm pore diameter class but it is generally dominated by smaller pore throats, whilst the Guggenberg sample is dominated by larger pore throats, with a maximum percentile at 20.8 µm (grey columns, Fig. 3.13B).

Figure 3.13 A) Results of mercury intrusion porosimetry (MIP), showing the pore throat diameter distributions of the Gambach (solid line) and Guggenberg samples (dashed line). For comparison of measuring ranges, the µ-CT imaging range is marked in grey. B) Calculated percentage of accessible pore space (100%) via defined pore diameter classes, using the close function and 3D analysis of CTan, applied to the µ-CT datasets of the Gambach (black columns) and Guggenberg samples (grey columns).

3.6 Discussion

3.6.1 Petrophysical properties In the well core sections, the mudstone intervals show in general lower t values than the sandstones (Fig. 3.4), which is contrary to common experience from downhole sonic logging (e.g. Rider 2002). This can be explained by the strong anisotropy of mudstones (Vernik and Nur 1992) and the different test assembly applied in this study: Downhole ultrasonic velocities are measured normal to bedding (Fig. 3.3C) and are extremely low, whereas they are much higher when transmitter and receiver probes are positioned according to our test assembly within the same stratigraphic horizon (Figs. 2.3A and 2.3B), measuring ultrasonic velocities parallel to bedding. Here, faster p-wave propagation along the aligned long-axes of mica and clay minerals results in lower Δt values in horizontally bedded mudstones. However, in many sections the mudstone

58 Application to Stratigraphy intervals were slightly disintegrated, resulting in a low ultrasonic signal quality. Thus, only mean sonic transit time and mean gamma density measurements derived from medium-grained sandstone intervals have been used to characterize the petrophysical properties of the studied sections.

Most of the sonic transit time variations observed throughout the studied outcrop (Fig. 3.6) and well core sections (Figs. 3.4 and 3.8) are short-lasting peaks associated with minor grain size changes, local variations in cementation and/or rock weathering, which are considered as “noise”, not representative of the general ultrasonic trend. This “background noise” signal variations contrast to a persistent offset of 18-20 % of the sonic transit time in all tested sandstones across the H-unconformity. This offset is not restricted to single peaks but clearly separates the different sonic patterns of the Hardegsen and the Solling Formations.

Lower sonic transit times in well core sections above the H-unconformity (Guggenberg, Rockensußra) suggest tighter rocks with lower porosities within the Solling Formation. Porosity (Figs. 3.4 and 3.8) and permeability measurements (Fig. 3.4) confirm this interpretation: sandstones below the H-unconformity achieve average values of >19% porosity and 200 mD permeability, whereas sandstones above record porosities of <10%- 12% and <0.1mD effective permeability, classifying the basal Solling Formation at the Guggenberg site as tight. In the Rockensußra well core section (Fig. 3.8), the lower sonic transit times of the Solling Formation result from tighter grain contacts bridged by quartz overgrowths (Figs. 3.9C, 3.9D) and a largely reduced clay mineral content compared to the Hardegsen Formation. However, the polarity of porosity-permeability reduction is not consistent: At the Gambach outcrop section, sonic transit time increases above the unconformity (Fig. 3.6), correlating with enhanced porosity in the basal Solling Formation. The contrasting pore network design occurring in the sandstones below the H- unconformity has been visualized by 3D µ-CT scanning of samples from the Guggenberg and the Gambach locations (Fig. 3.11), basically supporting the data from core plug porosity measurements and thin section analysis. However, porosity volume calculations from µ-CT datasets (here: 9.9 and 2.5 %) in general underestimate the real porosity measured with MIP (16.4 and 5.7 %), since pore sizes < 2.6 µm are beyond the resolution limit of µ-CT imaging.

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Figure 3.14: Petrophysical plots of core plug sample material of the Guggenberg B28 well core section (Fig. 3.4). A) Sonic (Δt) versus porosity plot. B) Sonic (Δt) versus permeability plot, note the semi-logarithmic scale.

The experimental studies of Vernik and Nur (1992) imply a linear relationship of ultrasonic velocities and porosities in sandstones of consistent grain size and mineralogy. Although measurements performed during this study appear rather scattered in t versus porosity (Fig. 3.14A) and permeability plots (Fig. 3.14B) on first sight, they confirm the general trend of increasing sonic transit times with increasing porosities and permeabilities. The gamma-density versus sonic transit time diagram (Fig. 3.15) then establishes distinct fields, which allow a direct allocation to particular stratigraphic units in the Guggenberg well core section. Sandstones of the Hardegsen Formation with higher porosities consequently plot at lower densities and higher sonic transit times than the tighter Solling sandstones. The coefficient of determination (R²) thereby is only 0.4284 (equal to about 43 % accordance), which underlines that porosity and conversely density, is not the only factor influencing sonic transit time. Remarkable is the shift towards lower sonic transit times (Fig. 3.15) for the sandstone bed directly below the paleosol VH2a in the Guggenberg well core (Fig. 3.4: 53.10-53.65 m), most probably caused by a slightly decreasing grain size and a stronger cementation with quartz and Fe-oxides.

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Figure 3.15: Density versus sonic (Δt) plot of the Guggenberg well core section samples. The sandstones of the Solling and Hardegsen Formations plot within distinctive fields. Note the shift towards lower sonic transit times of the sandstone bed directly below the paleosol VH2a.

3.6.2 Diagenesis and tectonic implications Transmission light photomicrographs of thin sections reveal contrasting cementation types and cementation intensities above and below the H-unconformity at the three test sites (Figs. 3.5, 3.7, and 3.9). The sandstones of the Hardegsen Formation are characterized by tangential- and long- grain contacts, both in the Gambach outcrop section and the Guggenberg well core. The high proportion of original intergranular pore volume is almost entirely cemented by euhedral quartz overgrowths which is evident for early diagenetic pre-compaction cementation (Summerfield, 1983; Bromley, 1991). At Gambach, the sandstones of the Hardegsen Formation act as silica-importers, since no appreciable solution of silica was observed in this section. Illite coatings have only been observed in the remaining pore space, but not around the primary detrital grains. Therefore, this cementation must have been taken place before the formation of the paleosol VH2a, which otherwise would have infiltrated the underlying sandstones with detrital clay minerals.

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From the Guggenberg well core section, enhanced iron oxide cementation and few carbonate nodules are documented in the Hardegsen sandstones directly below the H- unconformity. Iron oxide precipitation is considered as evidence of a water saturated soil environment (Price and Velbel, 2000). Here, partly dissolved feldspar grains (Figs. 3.5B, 3.9B) suggest vadose leaching of the sandstone units during unconformity formation, providing an explanation for enhanced secondary porosity and permeability values (Bjørlykke, 1983; Shanmugam and Higgins, 1988). However, at the Gambach outcrop section neither Fe-oxide cementations nor major secondary porosities have been observed below the paleosol, which may be indicative for local erosion and uplift prior to the formation of the paleosol. The paleosol would then represent a semi-stable surface developed during the waning phase of H-unconformity related erosion. In contrast, capping Solling sandstones above the paleosol show only subordinate quartz overgrowths and a lower percentage of intergranular pore volume filled by carbonate cements and clay minerals (illite). The quartz grains have corroded rims or are partly to entirely dissolved. This suggests compaction of the Solling sandstones prior to cementation and enhanced pH values during burial causing corrosion of quartz.

In the Franconian Depression, a post-Buntsandstein burial to a depth of at least several hundreds of meters is indicated by mechanical compaction parameters involving deformed muscovite grains (Fig. 3.5E), tangential- to long-grain quartz contacts (Figs. 3.5B and 3.7B), and few examples of initial chemical compaction recorded by sutured quartz grains (Fig. 3.5A). This corresponds well to a calculated post-Buntsandstein overburden of 1500 – 2200 m, based on available paleogeographic and isopach maps of Northern Bavaria (Freudenberger and Schwerd, 1996). Thin sections from the Rockensußra well core suggest a different burial history of the Thuringian Basin. There, the sandstones of the Hardegsen Formation are much more affected by quartz and feldspar corrosion and secondary leaching (Figs. 3.9A and 3.9B), whereas the Solling Formation sandstones show increased quartz cementation (Fig. 3.9C).

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3.7 Conclusions

High-resolution ultrasonic readings of sandstones conducted in this study demonstrate significant changes in sonic transit time across the H-unconformity of the Middle Buntsandstein Group in southern Germany. Integrated thin section analysis, porosity- permeability measurements, µ-CT imaging, and gamma-density logging suggest that these changes in sonic transit time are due to major variations in sandstone cementation and the formation of secondary porosity above and below the unconformity. The latter most probably favour channelized fluid flow parallel to the unconformity surface. If grain size, grain contact and bedding types, detrital and authigenic components and rock water content are fairly constant, a linear coincidence of increasing t values with increasing porosities (Fig. 3.14A) and decreasing gamma-densities (Fig. 3.15) exists. This implies an applicability of ultrasonic transit time trends as a reliable proxy for the in-situ identification of porosity heterogeneities in otherwise monotonous sandstone sections.

The in-situ measurements of ultrasonic transit times in outcrop sections are considered as a useful addition to classic petrographic analysis and hitherto applied outcrop logging methods, such as gamma-ray, magnetic susceptibility and Miniperm measurements. The handheld ultrasonic tool may aid the solution of stratigraphic problems by the in situ identification of unconformities in monotonous sandstone successions. Moreover it helps in the identification of variable cementation and porosity within a sandstone body which may be decisive for the mining of high-quality monument stone as well as the development of unconventional hydrocarbon and geothermal reservoirs and prospectives for underground CO2 storages.

Dissolution dominated unconformities favor the creation of secondary porosity and resulting high porosity-permeability channels may provide flow paths and reservoirs for hydrocarbons. In contrast, unconformities with enhanced cementation tend to create flow- barriers, ultimately leading to reservoir compartmentalization. Outcrop sonic logs may be compared to borehole measurements and support the correlation of subsurface and outcrop equivalents. Because of their applicability to field outcrop analogs they may help to extend the very localized information on sonic facies and porosity distribution in well cores to outcrop scale architecture.

63 ______

Chapter IV

Application of High-Resolution Ultrasonic Logging to Clastic Reservoir Characterization ______

This chapter is largely based on Filomena, C.M., Stollhofen, H., van Ojik, K., in press, High-resolution ultrasonic measurements as proxies to resolve clastic reservoir heterogeneity in a salt-cemented gas reservoir: American Association of Petroleum Geologists (AAPG) Bulletin.

Abstract Petrophysical properties and their heterogeneity within sandstone bodies are key parameters in the evaluation of hydrocarbon and geothermal reservoirs. However, common tools applied to constrain porosity distribution pattern in borehole cores are often time consuming, destructive or suffer from a resolution limited to the metre- to decimetre- scale. The applicability of non-destructive, high-resolution sonic (HRS) logging of well core sections in the ultrasonic frequency range is examined as a method providing porosity proxy data at the cm-scale in a clastic sedimentary sequence. The Middle Solling Sandstone Member (MSSM), a gas-bearing reservoir offshore from the Netherlands, is used as a test sample. It consists almost entirely of "clean", cross-bedded to massive aeolian dune and dry sandflat deposits which are salt-plugged to variable degrees. Plots of HRS logging data versus core plug porosity values show a positive linear relationship which develops the highest correlation coefficient (R² = 0.86) in structureless aeolian dune sands, most probably due to the lack of bedding-related anisotropies there. Once calibrated for a particular facies type, this correlation enables the calculation of porosity proxy data from sonic transit time values, acquired at centimetric steps. HRS logging thus contributes a reliable and time-efficient, highly spatially resolving quantification of reservoir heterogeneities at cm-scale and turns out to be a suitable tool for the non- destructive in situ detection of high-porosity zones in otherwise uniform sandstone successions. Also, plots of closely spaced HRS logging derived porosity proxy data significantly improve interpolation between single, wider spaced core plug porosity data points.

64 Application to Reservoir Characterization

4.1 Introduction

Over the last few decades, a spectrum of new technologies has been developed for non- destructive laboratory testing of sediment cores, to produce high-quality, closely spaced, co-located downcore measurements of physical properties (Rothwell and Rack, 2006; Zinszner and Pellerin, 2007). In particular, laboratory measurements of seismic velocities are of fundamental interest for seismic attribute analysis and the lithological interpretation of seismic surveys (e.g. Bourbié et al., 1987; Burkhardt et al., 1990; Vernik, 1996). Considering core diameters and also for practical reasons most laboratory measurements are conducted in the ultrasonic frequency range (Toksöz and Johnston, 1981).

Seismic velocities and attenuations are controlled by lithology, porosity, permeability, the nature and quantity of pore fluids, as well as the macro- and microstructure (e.g. distribution, density, dimension, geometry of microcracks and the constitution of grain contacts) of rocks (Geertsma and Smit, 1961; Freund, 1992; Mayr and Burkhardt, 2006). Additional precaution is therefore required when laboratory measurement are taken with no confinement pressure, as any cracks present have a considerable impact on the measurement (Zinszner and Pellerin, 2007).

Experimental studies suggest, that relatively simple relations between seismic velocities and important petrophysical parameters such as porosity and clay content exist (Wyllie et al., 1956; Wyllie et al., 1958; Raymer et al., 1980; Minear, 1982; Tosaya and Nur, 1982; Kowallis et al., 1984; Han et al., 1986; Marion et al., 1992; Vernik and Nur, 1992). At low to medium porosities the time-average equations of Wyllie (1956), Gardner et al. (1974), and Raymer et al. (1980) suggest almost linear p-wave velocity to porosity relations (Vernik and Nur, 1992; Dvorkin and Nur, 1998). However, the direct sonic velocity/porosity relationship and its comparability is only given if other parameters that influence sonic values such as pore filling fluid types, grain size and grain composition, clay contents and cement phases are kept reasonably constant (Hamilton and Bachmann, 1982; Han et al., 1986; Kenter et al., 2007).

Vernik and Nur (1992) examined I.) clean arenites, II.) altered arenites and arkoses, III.) wackes, and IV.) sandy shales and found the compressional velocity-porosity relation for each of these lithologies to be linear with very high correlation coefficients. This led them

65 Chapter IV to conclude, that careful combination of petrographic observations with measurements can yield remarkably accurate porosity estimates or lithology predictions from sonic logs (Vernik and Nur, 1992).

Compared to previously published studies, this study samples sonic velocity data points at a much narrower spacing along a cored sequence and applies a transmitter/receiver arrangement in the same plane as sedimentary bedding to enhance resolution, implemented as continuous (HRS: high-resolution sonic) core logging. The aim is to test HRS logging of well cores as a time-efficient, non-destructive proxy method for the in- situ identification of heterogeneities in porosity at the centimetre- to decimetre-scale. Such information is important in upscaling rock reservoir parameters from outcrop or core to the reservoir scale (Liu et al., 2002) and significantly improves the resolution of reservoir characterization.

Figure 4.1: Location map showing the position of block L9 offshore the Netherlands, southern North Sea. The L9-FF gas field and the location of well L9-FF-101 are displayed enlarged.

66 Application to Reservoir Characterization

4.2 The Middle Solling Sandstone Member (MSSM)

4.2.1 Lithology and facies types In order to extract porosity changes as the dominant variable we were aiming to confine the spectrum of other, competing control factors on measured p-wave velocities as much as possible. The fluvio-aeolian, largely salt-cemented Middle Triassic (Anisian) Middle Solling Sandstone Member (Geluk, 2007; Bachmann et al., 2010) was chosen as a test sample, because of its rather "simple" detrital and authigenic mineral assemblages, rather monotonous grain size, high structural and compositional maturity and poorly developed bedding anisotropies. The examined 159 m core is derived from well L9-FF-101, drilled in 1994 at block L9, approximately 140 km north of the city of Amsterdam, southern North Sea offshore from the Netherlands (Fig. 4.1).

The Solling Formation comprises the Basal Solling Sandstone and the Lower and Upper Solling Claystone Member (Fig. 4.2) which bracket the Middle Solling Sandstone Member (MSSM) (Geluk, 2005). The latter is the main gas producing reservoir of the L9- FF field (de Jager and Geluk, 2007), covering an area of approximately 10x4 km (Fig. 4.1). The MSSM is made up of 5 major lithofacies, labelled in Figures 3.2 and 3.3 and listed in Table 3.1.

Dominant are:

I.) Very well to well sorted, horizontal to cross-bedded, in parts also massive, very fine- to fine, rarely medium "clean" sandstones. These sandstones commonly reveal gamma-ray values of ~20-35 API and form sets of 0.3 to 2.0 m thickness. The depositional environment for this lithology (AeD-x) is interpreted to record an aeolian dune setting, preserving small, barchanoid dune forms.

II.) With rather equivalent abundance, similar aeolian grain characteristics and gamma-ray signatures, mottled, almost massive sandstones are present. Their homogenized texture is interpreted to result from pedogenic overprinting and bioturbation of original aeolian dune sands (AeD-h), predominantly by rooting and occasional burrowing. Some of the root traces are well-preserved by root casts or calcareous rhizocretions.

67 Chapter IV

68 Application to Reservoir Characterization

Other facies are only subordinately present:

III.) Fine- to medium, plane bedded and well-sorted grey sandstones (30-35 API) are interpreted as alternating shallow braided sheet flood deposits that recycled aeolian sands and wind-blown sand sheets of a dry sandflat environment (Sdr).

IV.) Very fine- to fine, plane to wavy bedded, slightly clayey reddish sandstones (35-45 API), occasionally containing small clay intraclasts and/or pin-head-sized carbonate and anhydrite nodules are deposits of a damp aeolian sand flat (Sda). V.) A less abundant lithology is then characterized by plane-bedded clay-rich fine sandstones (<75 API) of a mudflat environment (Md), locally developing a mottled texture and large carbonate and anhydrite nodules.

In summary, the facies association of the MSSM, sampled by the L9-FF-101 well core, registers a prevailing barchanoid sand dune environment with minor proportions of dry to damp sandflats and mudflats as they are known from interdune and erg margin areas. Considerable depositional hiati favoured stabilization and intense rooting of dune sands, that enhanced homogenization and finally led to a complete loss of original depositional textures.

These processes are well known from fossil (e.g. Loope, 1988; Marzolf, 1988; Herries, 1993) and various modern aeolian environments (e.g. Glennie and Evamy, 1968; Sharp and Hawk, 1977; Koster, 1988). Although convincing evidence for rooting is present, other factors such as sediment-gravity flows (Loope et al., 1999; Benan and Kocurek, 2000) or flood recycling of dune sands (Stanistreet and Stollhofen, 2002; Svendsen et al., 2003) may have contributed to the frequently structureless, homogenous nature of the MSSM dune sands as well.

Figure 4.2 (opposite page): Overview section of the aeolian facies dominated Middle Solling Sandstone Member (MSSM) located between the Lower and the Upper Solling Claystone Members of well L9-FF-101. Displayed are wireline gamma-ray and sonic logs, lithology and grain size distribution, interpreted lithofacies and sedimentary facies successions. See text and Table 3.1 for further explanation of lithofacies codes. The total drilling depth is indicated in metres drilling depth.

69 Chapter IV

Figure 4.3: Well core photographs of Middle Solling Sandstone Member (MSSM) lithofacies types: A) Aeolian dune, cross-stratified (AeD-x), B) Aeolian dune, homogenized (AeD-h), C) Aeolian dune, homogenized (AeD-h), with bioturbation (rhizocretions), (note: well core slab was wetted to enhance contrast), D) dry sandflat (Sdr), E) damp sandflat (Sda), and F) mudflat (Md) deposits. Note ~11° structural dip of strata, best visible at horizontally deposited facies (D, E, F).

70 Application to Reservoir Characterization

Table 4.1: Lithofacies, sedimentary fabrics and process-related interpretation of fluvio- aeolian deposits of the Middle Solling Sandstone Member (MSSM) in well L9-FF-101, offshore the Netherlands. Marked in grey are clay-bearing lithologies with strong bedding related anisotropies, which are less suitable for the calculation of HRS logging derived porosity proxies.

Code Grain size Characteristics GR Interpretation

Fine to Very well to well sorted, cross- 20-35 Aeolian dune AeD-x medium bedded, high structural and API deposits sandstones compositional maturity

Fine to Very well to well sorted, 20-35 Strongly bioturbated AeD-h medium massive, high structural and API (homogenized) sandstones compositional maturity, aeolian sandstone mottling, root casts and/or calcareous rhizocretions Fine to Well sorted, weakly plane to 30-35 Sandflat deposits Sdr medium low-angle cross-bedded, API (alternating aeolian sandstones rippled, in parts shallow erosive sandsheet and bases shallow braided sheet flood deposits) Very fine to Horizontally to wavy bedded, 35-45 Damp aeolian sand Sda fine slightly clayey, small clay API flat deposits sandstones intraclasts, carbonate and/or anhydrite nodules Clay-rich fine Plane bedded, mottled texture, <75 Mudflat deposits Md sandstones large carbonate and anhydrite API nodules

4.2.2 Facies successions Throughout the MSSM in well Log L9-FF-101, aeolian dune deposits are the most abundant sedimentary facies. They dominate particularly in the lower and in the upper part of the MSSM (Fig. 4.2), whilst damp sandflat and mudflat deposits, well indicated by their elevated gamma-ray values become gradually more frequent in the middle part. The stepwise increasing occurrence of damp playa deposits is interpreted to indicate a wettening trend, associated with transgression and expansion of the playa lake located farther to the basin. The following drying-upward trend then reflects the regressive counterpart. Maximum playa lake flooding and the transgressive-regressive turn-around point are marked by the thickest and most extensive occurrence of damp mudflat facies at about 3157 m drilling depth, causing a pronounced gamma-ray peak (Fig. 4.2). This superimposed wettening/drying trend of the MSSM splits into 15 nested smaller- scale drying-upward successions (Fig. 4.2), each 3.0 to 20.5 m thick. An idealized succession starts with mudflat and/or damp sandflat deposits, followed by dry sandflat and finally aeolian dune deposits. However, most of these successions do not record the

71 Chapter IV full facies spectrum, which is probably due to the lower preservation potential of aeolian deposits towards the cycle top. Throughout the entire MSSM no major changes in grain size or detrital components can be observed, except for a few, sporadically occurring, medium sandstone beds. The coincidence of enhanced bioturbation with the top parts of drying-upward successions, suggests a stabilization of aeolian dunes, most probably caused by slightly wetter climatic conditions in association with the "moist" onset of the overlying drying-upward succession. The superimposed wettening/drying trend of the MSSM may correspond to 100 ka eccentricity cycles, well known from the Solling Formation in the Netherlands and northern Germany (Menning et al., 2005; Szurlies, 2007).

4.3 Methods

The ultrasonic device used for HRS logging was operated at a frequency of 80 kHz. In contrast to the usual transmitter/receiver arrangement of downhole sonic tools, the HRS probes are arranged in a direct measuring assemblage, normal to well core elongation (Fig. 2.2). Here, HRS logs were conducted with a sampling rate of 1 cm. Almost all known causes for bad ultrasonic log quality were excluded by performing measurements under controlled and reproducible lab conditions. To exclude disturbing effects of variable pore water types and pore water saturation stages, the HRS measurements were only undertaken on oven-dried well core material (>24h at 60°C) with a homogeneous gas (air) saturated pore-network. A total of 30 regularly sampled thin sections were petrographically examined and results were integrated with 160 XRD analyses (provided by NAM) to control grain and cement compositions of sandstone samples. Detailed mineral identification on selected samples was performed by a Tescan Vega II SEM. A backscatter electron (BSE) detector combined with an energy dispersive X-ray spectroscopy device (EDX) has been applied to distinguish different framework grains and to identify element compositions of selected mineral phases. Porosity determination (uncorrected for subsurface stress conditions) is based on buoyancy measurements (Archimedes principle) on 160 plug core samples which were fully saturated with heptane (courtesy of NAM). A Quantachrome PoreMaster 4 mercury intrusion porosimeter was then used to determine pore throat sizes of selected samples.

72 Application to Reservoir Characterization

4.4 Middle Solling Sandstone Member reservoir heterogeneity

4.4.1 MSSM petrology and porosity The MSSM aeolian (Aed-x and Aed-h) and fluvially recycled aeolian units (Sdr) exhibit a comparatively simple mineralogy in that modal analyses for all thin sections examined show an average composition of detrital components of 92% quartz, 6% feldspar and 2% rock fragments. Mica and interstitial clay minerals, predominantly illite, are only found in traces <1%. It is clear therefore that the aeolian sandstone units can be classified as "clean" quartz sandstones. This contrasts to argillaceous sandstones deposited in damp sandflat environments (Sda) where total clay mineral contents may achieve up to 12%. Prevailing and, in terms of their effects on reservoir quality, the most important cement- building minerals are halite and subordinate carbonate (dolomite), although cementation intensity varies considerably throughout the measured MSSM section. Its upper part is showing an overall higher degree and its lower part an overall lower degree of salt cementation. Carbonate cements are most common in damp sandflat (Sda) and mudflat (Md) deposits, locally associated by traces of anhydrite. The test of HRS measurements as proxies for sandstone porosities is thus focussed on "clean" sandstone lithologies (Aed-x, Aed-h, Sdr). This has the additional advantage in that these lithologies are bearing halite as the only cementing phase. The majority of halite cemented sandstones shows tangential to long grain contacts, suggesting an early diagenetic salt cementation, prior to the onset of significant compaction effects.

Figure 4.4 provides a detail of the section shown in Figure 4.2, illustrating that the measured MSSM core interval is rather monotonous in terms of grain size variations (very fine to fine sandstone) but bears a rather heterogeneous distribution of core plug derived porosity values within a range of 5-27%. BSE images illustrate contrasting samples P99 and P188 with strong salt cementation and low porosities < 8 % (Figs. 4.5A and 4.5B) and highly porous samples (> 20 % porosity) P102 and P189, where salt cementation is only moderate to weak (Figs. 4.5C and 4.5D). Thin section and BSE analyses identify two pore families: Intergranular macropores and both, intergranular and intragranular micropores. One of the latter corresponds to leaching of intergranular salt cements, the other (pore diameters <0.006 µm) mainly to degradation of potassium feldspars (cf. sample P188, Fig. 4.5B) which occurred prior to salt cementation. Mercury porosimetry data (histograms of access diameters) also reveal contrasting pore throat sizes: Pore access

73 Chapter IV diameter maxima of 0.04-2.0 µm in strongly cemented samples (Figs. 4.5A and 4.5B) differ markedly from maxima at 15 µm in moderately cemented plug P102 (Fig. 4.5C) and 20 µm in weakly cemented plug sample P189 (Fig. 4.5D). The latter, less cemented samples are clearly unimodal macroporous with low microporosity proportions whereas the strongly cemented ones are unimodal (P99) or slightly bimodal (P188) microporous. This suggests that the throats to the few intergranular macropores of samples P99 and P188 are only "microaccessible" due to extensive salt cementation.

Figure 4.4: Enlarged section of the Middle Solling Sandstone Member (MSSM) covering the interval between 3112 – 3148 m drilling depth (see Figure 2 for stratigraphic position). Comparison of downhole gamma-ray and "porosity" log pattern (sonic, bulk density, neutron porosity) with HRS log and core plug porosity data, suggesting that the HRS data mirror porosity variations. P99, P102, P188 and P189 refer to sampling positions of core plugs examined by BSE and mercury intrusion (Fig. 4.5).

74 Application to Reservoir Characterization

Figure 4.5: Backscatter electron (BSE) images and mercury intrusion porosity data from selected core plug samples of the MSSM. Pore diameter distributions are displayed as log differential mercury intrusion volume (dV/dLogD), which is mercury volume intruded per gram sample in cm³/g, plotted versus pore diameter. The stratigraphic positions of the plug samples are indicated in Figure 4. A and B: Plugs 99 and 188, strongly halite (H) cemented, quartz (Q) dominated sandstones with largely degraded feldspars (Fd), low porosities (5.1 % and 7.9 %), and pore access diameter maxima of 0.04 - 2.0 µm. C: Plug 102, moderate halite cementation (H) and open primary (P) and secondary pore space totalling 21.7 % with a pore access diameter maxima of 15 µm. D: Plug 189, weakly cemented sandstone with high porosity (24.1 %) and a pore access diameter maxima of 20 µm.

75 Chapter IV

4.4.2 Wireline log pattern The downhole sonic log (Fig. 4.4) indicates multiple, but in terms of their amplitude only minor variations in sonic transit time throughout the MSSM section which at least roughly mirror amplitudes and pattern of porosity variations derived from plug core measurements. A general offset of mean sonic run time values is evident just below a drilling depth of 3128 m (Fig. 4.4). Above this depth point, mean sonic transit times show low values of 70-80 µs/ft and below they increase to 80-100 µs/ft. This offset separates the upper, stronger salt-cemented part of the MSSM from the lower part, where the overall salt cementation is weaker. Bulk density and neutron porosity log values (Fig. 4.4) vary from 2.15-2.50 g/cm³ and from 2-17 % respectively. Neutron porosity log patterns only weakly reflect the core plug porosity variations and largely ignore short-term porosity peaks, whereas bulk density log patterns provide a much closer match and enhanced resolution. However, the resolutions of the downhole sonic, neutron porosity, and bulk density logs of the L9-FF-101 well are generally limited to the decimetre- to metre-scale. Consequently, porosity heterogeneities beyond that scale cannot be adequately identified within the MSSM section.

4.4.3 HRS logging - porosity relationships As much as 4.000 HRS data points have been recorded along the L9-FF-101 well core section at 1 cm spacing (Fig. 4.4). The general offset of mean sonic transit time values, observed at 3128 m depth in the wireline sonic log, is less evident but a much more pronounced heterogeneity of HRS transit times becomes obvious below this depth. The HRS log clearly resolves at the cm-scale “fast” core sections with low sonic transit times (<100 µs/ft) and “slow” sections, characterized by high transit times (>110, max. 220 µs/ft). "Fast" core intervals with low sonic transit times correlate with increased bulk densities in well logs and low core plug porosities as do high sonic transit times, reduced bulk densities and high core plug porosities in "slow" sections. BSE images (Figs. 4.5A and 4.5B: samples P99 and P188) confirm strong salt cementation and low porosities (<8%) within the "fast" sections. In contrast, "slow" sections with high sonic transit times are characterized by reduced (Fig. 4.5C: sample P102) or almost absent salt cementation (Fig. 4.5D: sample P189) and high porosities (>20%).

76 Application to Reservoir Characterization

Figure 4.6 plots a set of 160 HRS transit time data of various MSSM facies types versus mercury intrusion porosity values, suggesting on first sight a roughly linear relationship. The degree of correlation between HRS transit time data is described by a correlation coefficient of R2 = 0.73. However, this degree of correlation varies considerably within the dataset, dependent on the respective facies type: The highest degree of positive linear correlation between sonic transit times and porosity values (R2 = 0.86) is given for homogenized aeolian dune deposits (AeD-h). Crossbedded aeolian dune (AeD-x) and dry sandflat deposits (Sdr) display weaker correlation coefficients of R2 = 0.57 and R2 = 0.59.

As the homogenized (AeD-h) and cross-bedded dune facies (AeD-x) are completely identical in terms of their composition it is assumed, that the lower degree of correlation is caused by crossbedding related anisotropy effects, where sonic transit time is highest perpendicular and lowest parallel to bedding planes (Fitzner et al., 2003). Since the homogenized sandstones are essentially structureless, anisotropy effects are largely excluded, favouring a higher degree of correlation.

Figure 4.6: Scatter diagram of HRS transit time data plotted versus experimentally determined porosity data of 160 MSSM core plugs from well L9- FF-101, testing facies related anisotropy effects on the degree of correlation. See Table 1 for explanation of lithofacies codes. The highest degree of correlation (R2 = 0.86) between sonic transit time and porosity values refers to structureless, homogenized aeolian dune facies AeD-h (bold regression line). Lithofacies AeD-x and Sdr reveal lower R2 values of 0.57 and 0.59, which is most probably due to sedimentary bedding induced anisotropy effects.

Although salt-cemented "clean" very fine to fine-grained sandstones dominate the section, minor variations in grain size, clay and/or contents may have some influence on sonic wave propagation in the measured MSSM sandstones. Enhanced clay mineral contents, for example, may result in an increased transit time (Han et al., 1986), whereas the presence of carbonate cements may have the opposite effect, leading to slightly reduced sonic transit times.

77 Chapter IV

4.4.4 HRS logging-derived porosity proxies Considering the above, facies-dependent sonic transit time/porosity relationships, the most reliable linear relationship should exist for homogenized aeolian dune deposits (AeD-h) at R2 = 0.86 (Fig. 4.6) with the resulting regression line being defined by y = 2.7784 x + 69.815

Applying this equation, a theoretical porosity (x) can be calculated for a given sonic transit time (y) and vice versa. Figure 4.7 further zooms into the MSSM section introduced by Figure 4.5 and illustrate the degree of correlation between porosity values calculated from HRS transit time data and experimentally determined porosity values. Within the porosity range of <15% the degree of correspondence in terms of porosity trends and absolute porosity values appears to be almost perfect for homogenized aeolian dune deposits (AeD-h) and still rather satisfyingly for crossbedded aeolian dune deposits (AeD-x).

Figure 4.7 thus illustrates a further advantage of HRS logging: Porosity data calculated from 1 cm spaced HRS transit times significantly improve the interpolation between the 25-30 cm spaced plug porosity data points and provide a much more detailed picture of reservoir heterogeneity. Their high resolution bears the potential to identify even small- scale porosity "channels" in otherwise low-porosity lithologies, examples of which are labelled by grey bars in Figure 4.8.

78 Application to Reservoir Characterization

Figure 4.7: Details of the MSSM section at A) 3113-3121 m and B) 3137-3145 m drilling depth, illustrating the match between calculated porosity data (black squares) derived from HRS logging of facies Aed-h and AeD-x and experimentally determined core plug porosity data (yellow bars). Red squares represent "calibration" points where porosity has been determined by both methods I) direct measurement of core plug porosity and II) calculation from sonic transit time data of the same sample. Calculated porosity values x are based on the equation y = 2.7784 x + 69.815 with y representing measured HRS transit times. HRS log data improve the resolution of reservoir heterogeneities in providing additional porosity proxy data that enable the demarcation of potential porosity zones (marked by grey bars), even in sections with rather widely spaced core plug porosity data cover. P99, P102, P188 and P189 refer to sampling positions of core plugs examined by BSE (Fig. 4.5).

79 Chapter IV

4.5 Conclusions

HRS logging at centimetric acquisition steps is regarded as a fast and non-destructive method that provides reliable porosity proxies to improve interpolation between more widely spaced core plug or wireline log porosity data. Results are optimized once the sonic transit time to porosity conversion is calibrated for a particular lithology. Thus the method bridges between micro- and macro-scale reservoir analyses since it improves a quantitative evaluation of the reservoir section heterogeneity on the cm-scale and allows a precise separation of high and low-porosity zones within cored sections.

However, a detailed petrographic study of the rocks to be tested is of paramount importance for a successful application of HRS logging. A positive linear relationship between sonic transit time and porosity of sandstones is only given for a sedimentary section (Wyllie et al., 1956; Gardner et al., 1974; Raymer et al., 1980), if other factors that influence sonic transit time values (e.g. grain size, detrital composition, cementation characteristics and type of pore space fill) are not showing pronounced variations (Han et al., 1986; Kenter et al., 2007). Possible effects of bedding-related anisotropies have to be considered as well.

In this regard, the fairly constant grain size and compositionally mature detrital grain composition of the structureless "clean" aeolian MSSM quartz sandstones as well as their mono-mineralic salt cementation is a major advantage of the test sample. All samples were sufficiently dried to assure a uniformly air-filled pore space. Given these conditions, variations in sonic transit times are extracted as porosity proxies in the MSSM section. The correlation of sonic transit times with experimentally determined core plug porosities develops the highest correlation coefficient (R2 = 0.86) in structureless aeolian dune sandstones and achieves further optimized results at medium to high cementation indices (low to medium porosities). The latter is most probably due to improved grain-to-grain coupling, with the result that the loss of acoustic energy is minimized while it is transmitted through the rock sample. Even if the pre-requisites outlined above are not entirely given, HRS logging data at least provide a quick and non-destructive test of relative variation of porosity proxies in a cored section.

80 Application to Reservoir Characterization

81 ______

Chapter V

Application of High-Resolution Ultrasonic Logging to Dimension Stone Quality Evaluation ______

Abstract Key petrophysical parameters used for quality assessment of dimension stones are uniaxial compressive strength and E-moduli. However, common methods to determine these parameters usually comprise destructive sampling and rather time-consuming laboratory measurements which only provide punctual information on quality grades. High-resolution ultrasonic (HRS) logging of entire borehole cores is tested as a time- efficient quantitative method in dimension stone exploration and as a reliable, non- destructive tool for sandstone quality assessment. The Worzeldorf Sandstone, a traditional dimension stone (Late Triassic Middle Keuper) which has been widely used in the area of Nuremberg, southern Germany provides the test sample.

Two endmembers within the range of quality grades are defined: High-quality sandstones are characterized by high packing densities of detrital grains, frequent occurrences of sutured grain-to-grain contacts, a strong quartz cementation and low clay mineral contents, providing high compressive strength values. High-quality Worzeldorf sandstone is allocated with a compressive strength exceeding 50 MPa, which is realized at sonic velocities of >3.6 km/s. Young’s modulus of elasticity is >20 kN/mm², porosity values are commonly <15% and permeabilities are <500 mD. In contrast, low-quality sandstones only provide compressive strength of <20 MPa and low low p-wave velocities of <2.73 km/s. They are highly porous and only fairly cemented with low packing densities and frequently floating grains. Various types of inter- and intraparticle secondary porosity occur. The positive correlation between sonic velocity, compressive strength and Young’s modulus of elasticity allows to calculate elastic properties from non-destructive p-wave sonic measurements which can be achieved at high resolution. As p-wave velocities in

82 Application to Dimension Stone Quality Evaluation rocks are dependent on various variables such as grain size, porosity, permeability, the nature and quantity of pore fluids as well as rock fabrics it is required that careful petrological investigations accompany HRS logging. Once calibrated for a particular lithology and grain size, high-resolution sonic (HRS) logging then serves as a very time- efficient non-destructive tool to provide quantitative and reproducible proxy data which constrain elastic rock properties of mineable sandstone qualities.

5.1 Introduction

5.1.1 Dimension stone characteristics Many historic buildings and monuments such as castles, city walls, churches and monasteries, and public or residential houses are built from natural dimension stones. During the pre-industrial age, dimension stones were usually quarried in the closer surroundings of the building site to keep transportation costs low. Frequently it is the unique coloring and structure of the utilized dimension stones which give distinction to a particular village or city and reflect the style of the respective architectural epoch. But also in recent times, natural building stones are a major element of modern constructions. To match new buildings with old city quarters, origin and durability of the historically used construction material is frequently of special interest.

Today, the adequate restoration of historic buildings is a challenging task. Very often the original material is no longer available since local sources are depleted or the original quarries have been closed. Imported dimension stones and replacement materials which come from various source areas frequently do not match the original material, neither in color nor in quality (Blows et al., 2003; Prikryl, 2006). Therefore, the local re-exploration for suitable traditional replacement material can be essential for restoration projects of cultural and historical heritage in a certain region (Ashurst and Dimes, 2004; Carta et al., 2005; Werner and Hoffmann, 2007; Prikryl and Török, 2010; Stück et al., 2011).

For the economic evaluation of a dimension stone deposit, raw block prospection is one of the most important factors (Lepper, 2007). Over the last few years, geotechnical investigations comprising various petrological and petrophysical analysis have become increasingly important for dimension stone exploration and quality assessment (Hoffmann

83 Chapter V and Siegesmund, 2007). Dimension stone quality is mainly controlled by the mechanical properties of a rock and its resistance against physical and chemical weathering processes (cf. Stück et al., 2011). These properties are largely determined by the mineral composition of the rock and the degree and type of cementation (Smith et al., 2001; Smith and Prikryl, 2007). Petrophysical key parameters for the evaluation of dimension stone quality are uniaxial compressive strength, porosity, and permeability. The latter two parameters control water uptake and saturation, which are ultimately responsible for weathering-resistance.

Classical methods for the determination of these parameters include destructive sampling and rather time-consuming laboratory measurements which are partly also destructive (Bortz and Wonneberger, 1997). Non-destructive in-situ ultrasonic measurements are already established as a convenient method to test the unifomity of concrete (cf. Popovics et al., 1990; Hernandez et al., 2000) and to quantify weathering damage of natural building stones in historic buildings and monuments (Bortz and Wonneberger, 1997; Siegesmund et al., 1997; Dürrast et al., 1999; Fitzner et al., 2003). Ultrasonic testing is based on measurements of pulse p-wave velocities in a solid material which in turn are related to its density and its elastic properties. Sobott and Koch (2009) have already outlined the positive linear correlation between ultrasonic velocities and E- moduli values of some Worzeldorf Sandstone samples. This study will examine to closer detail the application of ultrasonic measurements to obtain proxy values of rock-specific elastic properties, such as E-moduli and compressive strength (Allison, 1988; Mosch and Siegesmund, 2007). We test "continuous" high-resolution ultrasonic (HRS) logging of entire borehole cores as a time-efficient quantitative method in dimension stone exploration and as a reliable, non-destructive tool for sandstone quality assessment.

84 Application to Dimension Stone Quality Evaluation

5.1.2 Study site The historic buildings of the city center of Nuremberg (Germany) are constructed of diverse Late Triassic Middle Keuper sandstones outcropping in the vicinity of the city (Fig. 5.1). These sandstones provide a great variability in quality. Some very well- preserved sections of the 11th century fortification, like the “Fünfeckturm” and also the 19th century opera and hall of justice, are constructed from a very robust dimension stone derived from the Upper Burgsandstein (Baier, 1998), which stratigraphically is part of the Arnstadt Formation, respectively the stratigraphic equivalent proximal facies of the Löwenstein Formation (Freudenberger, 2005) (Figs. 1.1 and 5.1A). This sandstone is traded as so-called "Worzeldorf Quarzit" or "Wendelstein Quarzit". Technically, the term “quartzite” is not appropriate, since it is not a metamorphic rock but a hydrothermally silicified version of the Upper Burgsandstein (Dorn, 1926; von Gehlen, 1956; Berger, 1978; Koch and Zinkernagel, 2004). Multiphase cementation of the Worzeldorf Sandstone is pronounced in a ~300 m halo fringing a WNW-ESE striking (Fig. 5.1) fault zone (Knetsch, 1929; Grimm, 1990; Koch et al., 2003; Sobott and Koch, 2009) and most probably has an Upper Cretaceous age (von Gehlen, 1956). Thus we use the term "Worzeldorf Sandstone". A strong quartz cementation makes it little susceptible to weathering and highly durable, with an uniaxial pressure resistance of up to 180 MPa (Sobott and Koch, 2009).

For a long time, sources of the Worzeldorf Sandstone were thought to be exhausted and appropriate replacement material was lacking. Only recently, a new source of highly durable dimension stone was discovered near the village of Worzeldorf, providing perfect replacement material of Worzeldorf Sandstone. The Worzeldorf quarry (49.3761° N, 11.1058° E or R3652900, H5473030), is located ~8 km southwest of the city centre of Nuremberg (Fig. 5.1B). Though, the quality of the Worzeldorf Sandstone varies, areas of enhanced or reduced quartz cementation alternate vertically and laterally, controlled by sedimentary facies, the presence of detrital and authigenic clay minerals, and the vicinity to hydraulically active faults (Sobott and Koch, 2009). Four well cores of an exploration campaign in the Worzeldorf quarry provide our test samples, representing transects through the prospective area of the deposit from east to west (wells 11 to 7) and from north to south (wells 7, 8, and 9) (Fig. 5.1C). Well core No. 8 covers a vertical thickness of 20 meters of Upper Burgsandstein whereas cores No. 7, 9, and 11 were drilled to a depth of 10 meters.

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Figure 5.1: A) Stratigraphy of the Late Triassic Middle Keuper in southern Germany (Freudenberger, 2005), B) Simplified geological map showing the location of the Worzeldorf quarry and surface geology (based on Berger, 1978) south of the city of Nuremberg, Germany. The upper Burgsandstein (kmBo) stratigraphic unit is the test sample used in this paper and derived from the active Worzeldorf quarry (framed). C) Active and inactive mining sections of the Worzeldorf quarry, potential prospects, and locations of boreholes No. 7, 8, 9, and 11.

5.2 Methods

X-ray diffraction (XRD) and electron microscope cathodoluminescence (SEM-CL) analysis integrated with thin section examination and laboratory testing of p-wave velocities, porosity, permeability and compressive strength are applied to constrain the vertical and lateral distribution of dimension stone quality.

For quantitative mineral identification, 16 representative samples of well core No. 8 have been examined by both, XRD and Rietveld analysis. Thin section polarized light microscopy and SEM-CL analysis are then used to confirm and further refine mineral composition, type and intensity of cementation, and diagenetic history. The rock samples were previously impregnated with blue-dyed epoxy to improve visualization of the open pore space.

86 Application to Dimension Stone Quality Evaluation

High-resolution sonic logging was performed under controlled laboratory conditions with a total of 1300 measuring points along the Worzeldorf well cores at 2 cm spacing and a direct probe test assembly (Fig. 2.2B). In order to provide an uniformly air-filled pore network, the well core sections have been oven-dried at 55°C for three days prior to sonic logging. Porosity and bulk density were determined by measuring buoyancy (Archimedes principle) on 48 plug core samples which were fully saturated with isopropanol. Permeability measurements have been performed directly on the well core surface and also on single core plug samples using a New England Research TinyPerm II minipermeameter probe. Uniaxial compressive strength tests according to EN 1926 standard have been applied to 25 Worzeldorf Sandstone sample cubes of 4 cm edge length, involving the full range of available sandstone qualities. Additionally, 31 plug core samples were used for the determination of Young’s modulus of elasticity (E-modulus).

5.3 Worzeldorf Sandstone characteristics

5.3.1 Lithology and facies The core material recovered from wells 7, 8, 9, and 11 is predominantly made up of reddish grey, very coarse-grained, in parts pebbly, trough cross-bedded sandstones, with set thicknesses of several decimeters to meters. Subordinately, fine- to medium-grained sandstones occur. Claystone rip-up clasts of up to 7 cm size are commonly associated with the sharp erosive bases of stacked sandstone bodies. An example of the facies architecture is provided by the measured section of core 8 (Fig. 5.2), illustrating that grain size variations relate to weakly developed fining-upward successions. On average the sections comprise ~85 % sandstones and ~15 % mudstones. The latter form thin tabular units that can be traced throughout the entire quarry, thus defining maximum sandstone block thicknesses. Worzeldorf Sandstone equivalents have been interpreted to represent fluvial channel deposits of a semi-arid terrestrial environment (von Gehlen, 1956). The spread of palaeocurrent indicators, the proportions of mudstones present in the fining upward sections, and channel width/depth ratios suggest deposition by high sinuosity meandering river systems (Koch et al., 2003), similar to facies associations outlined by Beutler et al. (1999).

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Figure 5.2: Combined sedimentological-petrophysical logs of the Upper Burgsandstein in borehole core No. 8, Worzeldorf quarry. Measured p-wave sonic velocities Vp correlate well with E-moduli, porosity, and permeability values. Integrated petrological-petrophysical assessment of dimension stones distinguishes high (green), moderate (yellow), and low (red) qualities. Encircled numbers 1-16 indicate XRD sample positions. Yellow squares demarcate samples where both, E- moduli and sonic velocities have been measured.

88 Application to Dimension Stone Quality Evaluation

5.3.2 Composition XRD and Rietveld analysis of representative samples, collected from well No. 8, reveal a rather simple composition (Fig. 5.3) of > 80 % quartz, 9-17 % feldspar (microcline), 0-6 % mica (muscovite) and clay minerals (illite, kaolinite). Close to the bases of fluvial fining upward units enhanced feldspar contents of up to 22 % may occur. Thin section microscopy reveals that the detrital quartz components are comprised of 74-80 % monocrystalline quartz, 15-20 % polycrystalline quartz (>3 subgrains) and 4-6% chert. Thus the average Worzeldorf Sandstone classifies as a lithic subarkose (McBride, 1963) to arkosic litharenite (Folk, 1974). Clay contents (detrital matrix and authigenic) and sorting vary within the section, as do cementation intensities: In the uppermost and in the middle part of the borehole No. 8 section (Fig. 5.2: 0.50-3.10 m and 7.70-9.40 m drilling depth), the well-sorted, clay-free sandstones are strongly quartz cemented (Fig. 5.4A), whereas the section between 3.10 m to 7.70 m and the entire section below 9.40 m drilling depth are characterized by moderately to poorly sorted, partly clay-bearing sandstones (Fig. 5.4B).

Figure 5.3: Mineralogical composition of well core No. 8 samples (see Fig. 5.2 for positions), based on XRD and Rietveld analysis.

In addition to enhanced cementation, some sandstone packages also show enhanced compaction features in areas proximal to the fault zone. This is expressed by an improved packing density of detrital grains and frequent occurrences of sutured grain-to-grain contacts (Sobott and Koch, 2009), whereas elsewhere most of the framework grains show tangential to long grain contacts.

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Some quartz (Fig. 5.4C) and feldspar grains (Fig. 5.4D) develop fractures which radiate out from the original point of contact between the grains. This suggests that these sandstones experienced considerable compaction and pressure solution before the grain framework became supported by cement precipitation (Dickinson and Milliken, 1995; Sobott and Koch, 2009). The fractures became healed by quartz, probably derived from presssure solution during shallow burial, prior to the first generation of quartz cementation.

5.3.3 Diagenetic history outlined Strongly cemented sandstones are extremely hard and durable and comprise almost exclusively quartz overgrowths of at least three successive generations: The first (Qe1) and the third generation of overgrowths (Qe3) consists of clean authigenic quartz, whilst the second phase (Qe2) is characterized by a high amount of fluid inclusions (Fig. 5.4E).

Cathodoluminescence (CL) imaging reveals a distinct internal zonation of Qe1 and Qe3 generations, suggesting multiple phases of quartz overgrowths, whereas Qe2 appears darker and patchy (Fig. 5.4F). A previous CL study on Worzeldorf Sandstone samples (Koch and Zinkernagel, 2004) relates the abundant quartz cementation to a hydrothermal origin. Following quartz overgrowths, a further diagentic phase is registered by the formation of kaolinite booklets and fibrous illite in the remaining pore space (Fig. 5.4G). Tangential hematite-rich illite coatings around crushed feldspar grains and quartz overgrowths indicate the late timing of clay authigenesis (Figs. 5.4D, 5.4G, 5.4H).

Figure 5.4 (next page): Thin section microphotographs of the Worzeldorf Sandstone, core No. 8. A) Strongly silicified quartz sandstone (Q) with minor clay mineral content of tangential illite (It) and hematite-rich clay minerals (Ih), in contrast to B) clay mineral rich, poorly sorted sandstone containing quartz (Q) and feldspar (F). C) Fissured and crushed quartz grains due to high punctual pressures during burial, D) moderately to intensively corroded quartz grains (Qc), partly with quartz overgrowths (Qe). The illitized feldspars are often crushed (Fc) or have few feldspar overgrowths (Fo). Tangential illite coatings (It) are present on quartz and feldspar grains. E) Three generations of quartz overgrowths (Qe1-3) on the original quartz grain (Qg) surface in strongly cemented sections. The first (Qe1) and the third (Qe3) overgrowths consist of clear quartz, whilst the second (Qe2) is rich in inclusions. F) Cathodoluminescence microphotograph of Fig. 5.4E, rotated to the right by 45°, identifying crushing of quartz grains prior to the three phases of quartz cementation. G) Pore space between quartz (Q) and illitized feldspars (F) filled with authigeneous kaolinite (K) and hematite-rich illite (Ih) in clay-mineral rich sandstone sections. H) Quartz grains with euhedral quartz overgrowths (Qe) and crushed feldspar grains (Fc).

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Incomplete quartz overgrowths are preferentially but not exclusively associated with feldspar grains, coated by thin clay rims that might have acted as inhibitors to further quartz nucleation (Cecil and Heald, 1971; Dewers and Ortoleva, 1991; Ehrenberg, 1993). In such less cemented portions, detrital feldspar grains are generally poorer preserved compared to well cemented portions of the Worzeldorf Sandstone (Dorn, 1926; Sobott and Koch, 2009). They are partly or completely dissolved, leading to frequent occurrences of honeycombed relict feldspar. Intergranular porosity between corroded or crushed framework grains (Figs. 5.4F, 5.4H) and intragranular porosity in feldspars resulting from feldspar degradation (Fig. 5.4G) are all of secondary origin.

Another type of poorly cemented sandstones is characterized by a high degree of interparticle porosity, floating and/or corroded quartz grains (Fig. 5.4D). Such features suggest secondary porosity (Schmidt and McDonald, 1979), most probably formed by dissolution of a pre-existing carbonate cement (Sobott and Koch, 2009). The latter could represent the earliest diagenetic phase during which detrital quartz grains became partly dissolved. Several possibilities for quartz dissolution are discussed in the literature, either by very alkaline pore waters as congruent dissolution, provoked by the cristallization of carbonate (cf. Pettijohn et al., 1972), or at nearly neutral pH conditions by effects of organic acids and NaCl (Blake and Walter, 1999). Low pH values during subsequent precipitation of silica (cf. Blatt et al., 1980) then might have caused leaching and degradation of feldspar but also an almost complete dissolution of early carbonate cements.

92 Application to Dimension Stone Quality Evaluation

5.3.4 Dimension sandstone qualities The quality assessment of the Worzeldorf Sandstones involves the measurement of a spectrum of petrophysical properties, such as p-wave sonic velocity (Vp), compressive strength (βD), Youngs modulus of elasticity (E), porosity, and permeability (Fig. 5.2). Selective sampling of sandstone intervals of wells 7, 8, 9, and 11 provide sonic velocities in a range of 1.4-4.3 km/s and uniaxial compressive strengths ranging from 2 - 104 MPa (Table 5.1). Measured core plug porosity values vary from 9-24 % and permeabilities vary over four orders of magnitude, from 6-5000 mD.

To obtain consistent information about the occurring sandstone qualities, petrographic information from thin section analysis is integrated with petrophysical data collected along the core sections. This integrated dataset allows the quantitative assessment of distinct sandstone intervals, allocating to them high, medium, or low dimension stone qualities.

High compressive strength class: High-quality dimension stones are characterized by high packing densities of detrital grains, frequent occurrences of sutured grain-to-grain contacts, a strong quartz cementation (Figs. 5.4A, 5.4E) and low clay mineral contents, providing high compressive strength values. In this study, high-quality Worzeldorf sandstone is allocated with a compressive strength βD exceeding 50 MPa (up to 104 MPa), which is realized at sonic velocities of > 3.6 km/s (Fig. 5.5). Young’s modulus of elasticity is > 20 kN/mm², porosity values are commonly < 15 % and permeabilities are < 500 mD. In well core No. 8, high-quality dimension stones predominantly occur in the uppermost part of the section at 0.50-2.80 m and in the middle part at 7.70-9.40 m drilling depth. This high-quality class provides suitable dimension stones for the construction of supporting elements in buildings, where high compressive strength values > 50 MPa are required.

Moderate compressive strength class: Ultrasonic p-wave velocities within the range of 2.7-3.6 km/s correlate with intermediate compressive strength values of 20-50 MPa (Fig. 5.5) and define a moderate dimension stone quality. Associated porosities are 15-21 % and permeabilities are significantly higher than 1000 mD. Common are moderate packing densities of detrital grains and

93 Chapter V tangential to long grain-to-grain contacts. In well core No. 8, this moderate quality is concentrated at 2.15-3.10 m and at 6.75-9.40 m drilling depth (Fig. 5.2). Dimension stones with intermediate compressive strength values between 20 and 50 MPa may be used as decorative or non-supporting construction elements.

Table 5.1: P-wave sonic velocity and compressive strength of 25 sample cubes from well cores 7, 8, 9, and 11. The grain size classes are medium to coarse sand (sm-sc), coarse to very coarse sand (sc-vc), and granules and pebbles (gr-p).

Well Borehole P-wave Sonic Compressive Grain Core Depth Velocity strength (MPa) Size (m) (km/s) Class 1.50 3.82 74.90 sc-vc Core 7 2.50 3.77 63.75 sc-vc 4.50 1.52 2.00 gr-p 1.20 4.15 77.10 sc-vc Core 8 1.90 3.75 52.40 sc-vc 2.90 3.48 36.30 sc-vc 6.90 2.59 28.90 sm-sc 7.85 3.62 25.70 gr-p 8.70 4.08 54.00 gr-p 9.80 2.61 16.80 sc-vc 11.8 3.21 23.40 sc-vc 14.30 1.89 10.40 sc-vc 18.70 1.52 6.30 sc-vc 18.80 1.98 19.00 sm-sc 0.40 3.10 52.46 Sm-sc Core 9 0.70 4.22 104.10 sc-vc 5.75 2.21 5.83 gr-p 6.50 1.75 10.26 sc-vc 7.10 1.67 6.96 sc-vc 8.40 3.94 82.02 sc-vc 9.60 2.63 13.69 sc-vc 10.00 3.06 21.36 sc-vc 7.23 4.01 79.01 sc-vc Core 11 9.00 3.42 48.25 sc-vc 10.00 4.11 73.65 sc-vc

94 Application to Dimension Stone Quality Evaluation

Low compressive strength class: Compressive strength of < 20 MPa and low p-wave velocities of < 2.73 km/s (Fig. 5.5) characterize low-quality dimension sandstones. They are highly porous and only fairly cemented with low packing densities and frequently floating grains. Various types of inter- and intraparticle secondary porosity occur. However, high porosities are not the only reason for reduced ultrasonic velocities. Clay minerals make up to 20 % of the whole rock volume (Fig. 5.3, XRD-sample 16), which also contributes to reduced p-wave velocities. As a consequence of high porosity, permeability, and high clay mineral content, these sandstones are highly friable. They are susceptible towards water infiltration, which results in a strong vulnerability through progressive weathering.

In borehole No. 8, low dimension stone quality is assigned to the interval between 3.50 m to 5.80 m depth and to almost the entire borehole section below 9.40 m depth (Fig. 5.2). Sandstones of this class with compressive strength values < 20 MPa do not represent suitable dimension stones and thus are considered as excavation material.

Figure 5.5: Correlation of p-wave sonic velocity (Vp) versus compressive strength (βD) measured at 25 4x4x4 cm sample cubes. The best-fit curve and correlation coefficient R² were calculated on the basis of coarse to very coarse grained sandstone samples (medium sized symbols). Major aberrations from the best-fit curve are due to different grain sizes, symbolized by smaller or larger triangles, squares, and diamonds.

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5.4 Correlation between petrophysical parameters

5.4.1 Calculating E-moduli proxy values Plots of petrophysical properties (Fig. 5.2) suggest a visual correlation of increased sonic velocities with high E-moduli but reduced porosity and permeability values. Figure 5.6 constrains the positive linear correlation of sonic velocities and E-moduli (E) of 31 plug core samples at a correlation coefficient of R2 = 0.9572 with the resulting regression line being defined by: Equation 5.1: E = 12.356 Vp - 20.516

Figure 5.6: Scatter diagram of sonic velocity (Vp) versus E-moduli of 23 measured cylindrical sample plugs, showing a positive linear correlation at R2 = 0.9572.

The positive linear correlation of p-wave sonic velocities and experimentally determined E-moduli (Fig. 5.6) constrained by equation 5.1, allows the calculation of elasticity from undestructive Vp measurements acquired at high resolution. However, applying equation 5.1 to sonic velocities <1.66 km/s results in negative E-moduli, which in reality do not exist. An E-modulus of 0 kN/mm² implies a non-elastic behaviour of the sample, which is the case when a rock is largely de-cemented and the framework grains are no longer connected by rigid quartz or carbonate cements. Related to well core No. 8 (Fig. 5.2), sandstone sections with p-wave velocities <1.7 km/s are characterized by high porosities (~25 %) and are partly rich in illite and kaolinite. Such friable, de-cemented sandstone horizons can be easily detected at high resolution on the basis of low E-moduli values, calculated from ultrasonic velocity data.

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5.4.2 Calculating compressive strengths proxy values One of the most important parameters in dimension stone quality assessment is compressive strength. Experimentally determined uniaxial compressive strength (βD) of Worzeldorf sandstone samples and measured p-wave velocities (Vp) are listed in Table 5.1 and plotted in Figure 5.5. Although the overall correlation of compressive strength values with sonic velocities works very well, some sandstone samples of rather similar compressive strengths show "exotic" sonic transit times, causing deviations from the best- fit curve (Fig. 5.5). As porosity variations are minimal (+/- 2 %) as does the modal composition of the sandstones, other reasons for these deviations in sonic velocity need to be considered.

Indeed, the only obvious feature which makes the samples concerned different from the coarse- to very coarse-grained pebbly sandstones of the best-fit curve are deviations in grain size. If all other variables are kept constant, finer grained or clay-bearing sandstones correspond to lower sonic velocities than their coarse-grained equivalents as the sonic wave signal is more strongly absorbed in finer grained lithologies. There, the sonic signal needs to trespass much more grain-to-grain and grain-to-cement contacts compared to their coarser grained equivalents. An improved degree of correlation is therefore reached, if samples of only one particular grain size interval are selected. Considering sonic velocity and compressive strength values only of coarse- to very coarse-grained pebbly sandstones then reveals a correlation coefficient of R² = 0.96, defined by: 0.9918 Vp Equation 5.2: βD = 1.3574 e

The integrated analysis of well core No. 8 serves as a calibration test to extract the degree of correlation between compressive strength, HRS p-wave velocities, porosity, permeability and dimension stone quality for a particular lithology, in this case the Worzeldorf Sandstone. Measured sonic velocities and measured compressive strength show a positive linear correlation at R2 = 0.96, defined by equation 5.2. This equation is then used to calculate continuous compressive strength proxy values for well cores 7, 8, 9 and 11 (Fig. 5.7), solely on the basis of non-destructive, in situ acquired HRS velocity data. Results allow spatial evaluation of dimension stone qualities and thus predictions of mineable rock volumes, with well core 8 acting as a baseline.

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5.4.3 Lateral tracing of dimension stone qualities The uppermost section of well No. 8 (0.75 - 3,1 m drilling depth) represents high to moderate dimension stone qualities, indicated by high sonic velocities of 3-4 km/s and compressive strengths ranging between 25 and 104 MPa. This high-velocity section can be traced at a rather constant thickness of ~2,4 m over a N-S distance of 22 m to boreholes 7 and 9 (Fig. 5.7), suggesting equivalent high to moderate dimension stone qualities there. The calculated compressive strengths vary from 30 to 55 MPa, locally also exceeding 70 MPa (Fig. 5.7).

The second high-velocity interval identified in well core No. 8 at 7.70 - 9.40 m drilling depth is also present in all other cores, although its thickness remains not constant. Towards the north (well No. 7) the high-velocity interval thickens to 2.10 m, whilst it is reduced to < 1 m thickness towards the south (well No. 9).

The remaining part of the well section, sandwiched between the high- to moderate quality sandstone bodies reveals reduced sonic velocities, down to values < 2 km/s, which is associated with increasing porosity values. This low-velocity zone is most pronounced in boreholes No 7 and 9. Calculated compressive strength values of < 10 MPa suggest low dimension stone qualities for the sandstones in this interval. Instead, the stratigraphic equivalent section in borehole No. 8 is characterized by slightly enhanced sonic velocities of ~2.8 km/s, indicating moderate dimension stone qualities there. Towards the east, borehole core No. 11 provides the most “homogeneous” dimension stone quality distribution. Even though only moderate dimension stone qualities are attributed to almost the entire section, compressive strength values are largely balanced at +/- 50 MPa, ranging between 30 to 60 MPa. Only two thin intervals of ~20 cm thickness provide low dimension stone qualities at 15-20 MPa. Overall, the available dataset suggests a predominance of reduced dimension stone qualities in the southern part of the well transect, whilst high-quality dimension stones can be expected towards its northern and eastern part.

Figure 5.7: Dimension sandstone quality evaluation from boreholes No. 7, 8, 9, and 11 using HRS logs, empirical porosity (yellow squares), and compressive strength data from individual sample cubes (blue diamonds). Additionally, compressive strength is calculated from HRS logs using equation 5.1. Positive and negative grain-size related aberrations have been considered for quality assessment, see Fig. 5.5. For easy comparison, the 20-50 MPa range for moderate compressive strength is marked in yellow. The borehole cores are levelled to 296 m above sea level (asl), or “über Normalnull (üNN)”. See Fig. 5.1C for borehole locations.

Application to Dimension Stone Quality Evaluation

5.5 Conclusions

Variations in grain size, porosity, clay mineral content, mineral compositions and their resulting effects on the ultrasonic signal need to be taken into account when ultrasonic velocity measurements are used to calculate compressive strength, and ultimately to assess dimension stone qualities.

Low-porosity, essentially clay-free, highly silicified sandstones with high packing densities of detrital grains are the requirements for high dimension stone qualities. In contrast, enhanced porosities and clay mineral contents and low packing densities of detrital grains are both responsible for reduced compressive strengths and thus for lower dimension stone qualities.

Once it is calibrated for a particular lithology and grain size, high-resolution sonic (HRS) logging is a time-efficient non-destructive method to quantify the degree of cementation or de-cementation in building stones. Thus it provides a useful tool for a reliable assessment of building stone qualities, either directly in the quarry wall, from raw blocks or from drill cores. In contrast to commonly applied punctual ultrasonic measurements, the application of "continuous" HRS well core logging provides much more detailed information on the vertical and lateral distribution of occurring sandstone qualities. Thereby, distinct target zones for high-quality dimension stone mining can be identified and traced laterally, also showing the maximum raw block thickness distribution.

Using HRS data logs, a much more precise calculation of exploitable rock volumes can be performed. Considering environmental restrictions, future reserves of high-quality dimension stones may largely depend on sophisticated exploration and ongoing enlargement of presently existing quarries. A detailed mapping of dimension stone qualities in place, on the basis of high-resolution sonic logging therefore helps to identify prospective areas for a potential enlargement of the mining area and contributes to reduce financial risks.

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Synopsis ______

6.1 Résumé

In this doctoral thesis, high-resolution ultrasonic (HRS) logging of sandstones has been applied to various problems in sedimentary geology. The measurement of sonic velocity or sonic transit time in sandstones is primarily used as a tool to gather porosity proxy data (Fig. 6.1). As sonic transit time not only depends on porosity, but also on grain size, grain contact and cementation, detrital and authigenic components, bedding types, and rock water content, these parameters have to be considered when HRS measurements are used to constrain porosity proxies in siliciclastic sediments (Fig. 6.1). Through careful monitoring in the laboratory, most of these “disturbing” factors can be largely eliminated or at least reduced to a minimum. Three case studies from Triassic sandstones in Central Europe have been presented, contributing to questions in stratigraphy, hydrocarbon reservoir characterization, and dimension stone prospection and quality assessment (Fig. 6.1). Compositionally mature sandstone series with a rather simple mineralogical composition have been selected to provide best testing conditions. Sonic velocity or sonic transit time measurements are additionally integrated with petrographic analysis comprising microscopic thin section examination and x-ray diffractometry, petrophysical parameters such as porosity, permeability, and compressive strength, as well as wireline logging data, and 3D µ-CT scans.

Significant changes in sonic transit time across the Hardegsen Unconformity of the Middle Buntsandstein Group in southern and central Germany have been documented. In monotonous sandstone sections, where definite unconformity indicators like paleosols are missing, a potential unconformity may remain cryptic by first sight. Though, distinct offsets in sonic transit time can help to identify its exact position. One outcrop section and two well core intervals incorporating the Hardegsen Unconformity document remarkable sonic offsets there. HRS transit time measurements document different porosity/

102 Synopsis cementation indices in the Solling and Hardegsen Formations above and below the Hardegsen Unconformity. However, the polarity of the sonic offset is not identical in all three locations, suggesting different cementation and dissolution processes taking place below the Hardegsen Unconformity.

HRS logging was further applied as a meso-scale reservoir analysis tool, exemplary tested on well core sections of an aeolian reservoir rock in the southern North Sea. The Early Triassic Middle Solling Sandstone Member is a suite of compositionally mature aeolian quartz sandstones, which are salt-cemented to variable degrees. Conventional core plug porosity and wireline log data in combination with HRS logging data provide a much more detailed picture of vertical reservoir architecture. Thin interlayers of high-porosity or high-cementation at sub-metre scale can be precisely outlined, which largely contributes to an improved evaluation of potential migration pathways.

The Worzeldorf Sandstone of the Middle Keuper in Franconia (south Germany) was subject of a dimension stone exploration campaign. HRS measurements of well core material were tested for dimension stone quality assessment. Sonic velocity and compressive strength values are correlated, defining three quality classes: 1) High-quality dimension sandstones are characterized by high packing densities of detrital grains, strong silification, and low clay mineral contents, providing high compressive strength values. High-quality Worzeldorf sandstone is allocated with a compressive strength exceeding 50 MPa (up to 104 MPa), which is realized at sonic velocities of > 3.6 km/s. Such sandstones are suitable for structural elements in buildings, like load-bearing walls and supporting pillars. 2) Moderately quartz-cemented, porous sandstones with sonic velocities ranging from 2.7 to 3.6 km/s and compressive strength values between 20 and 50 MPa characterize dimension stone qualities suitable for non-supporting construction purposes. 3) Sandstone intervals characterized by low sonic velocities < 2.7 km/s and low compressive strength values < 20 MPa, however, are not considered as appropriate dimension stone material. In contrast to commonly applied punctual ultrasonic measurements, the application of HRS well core logging provides much more detailed information on the vertical and lateral distribution of occurring sandstone qualities. Thereby, distinct target zones for high-quality dimension stone mining can be identified and traced laterally, showing the maximum raw block thickness distribution.

103 Chapter VI

6.2 Conclusions and Outlook

High-resolution ultrasonic logging at centimetric acquisition steps is regarded as a fast and non-destructive method that provides reliable porosity proxies to improve interpolation between more widely spaced core plug or wireline log porosity data. Results are optimized once the sonic transit time to porosity conversion is calibrated for a particular lithology. Thus the method bridges between micro- and macro-scale reservoir analyses since it improves a quantitative evaluation of the reservoir section heterogeneity on the cm-scale and allows a precise separation of high and low-porosity zones in otherwise monotonous sandstone sections.

The in-situ measurements of ultrasonic transit times in outcrop sections are considered as a useful addition to classic petrographic analysis and hitherto applied outcrop logging methods, such as gamma-ray, magnetic susceptibility and Miniperm measurements. Outcrop sonic logs may be compared to borehole measurements and support the correlation of subsurface and outcrop equivalents. Because of their applicability to field outcrop analogs they help to extend the very localized information on sonic facies and porosity distribution in well cores to outcrop scale architecture.

High-resolution ultrasonic measurement turns out to be a useful tool in applied sedimentary geology, which may aid the solution of stratigraphic problems by the in situ identification of unconformities in monotonous sandstone successions. Moreover it helps in the identification of variable cementation and porosity within a sandstone body which may be decisive for the mining of high-quality monument stone. Using HRS data logs, a precise calculation of exploitable rock volumes and maximum raw block size can be performed. HRS logging-derived heterogeneity data on porosity significantly contributes to the development of unconventional hydrocarbon and geothermal reservoirs and prospectives for underground CO2 storages.

The application of HRS logging as a standard tool in well core and outcrop examination is strongly recommended for future projects dealing with petrophysical properties of sedimentary rocks at high-resolution scales. Thereby, much more detailed information about heterogeneity distributions can be provided, greatly improving predictions and resulting models.

104 Synopsis

Figure 6.1: Principle of high-resolution sonic (HRS) logging, factors influencing sonic transit time or velocity, and fields of application in geosciences contributing to questions in stratigraphy, reservoir characterization, and dimension stone exploration.

105 References

References

Aigner, T., and G. H. Bachmann, 1992, Sequence-stratigraphic framework of the German Triassic: Sedimentary Geology, v. 80, p. 115-135. Aigner, T., J. Hornung, W.-D. Junghans, and M. Pöppelreiter, 1998, Baselevel cycles in the Triassic of the South-German Basin: a short progress report: Zentralblatt für Geologie und Paläontologie, v. 1998, p. 537-544. Allison, R. J., 1988, A non-destructive method of determining rock strength: Earth Surface Processes and Landforms, v. 13, p. 729-736. Ashurst, J., and F. G. Dimes, 2004, Conservation of Building and Decorative Stone: Amsterdam, Elsevier. Bachmann, G. H., M. Geluk, G. Warrington, A. Becker-Roman, G. Beutler, H. Hagdorn, M. Hounslow, E. Nitsch, H.-G. Röhling, T. Simon, and A. Szulc, 2010, Triassic, in H. Doornenbal, and A. Stevenson, eds., Petroleum Geological Atlas of the Southern Permian Basin Area: Houten, EAGE Publications BV, p. 149-173. Bachmann, G. H., and H. W. Kozur, 2004, The Germanic Triassic: correlations with the international chronostratigraphic scale, numerical ages and Milankovitch cyclicity: Hallesches Jahrbuch für Geowissenschaften B, v. 26, p. 17-62. Bachmann, G. H., T. Voigt, U. Bayer, H. von Eynatten, B. Legler, and R. Littke, 2008, Depositional history and sedimentary cycles in the Central European Basin System, in R. Littke, U. Bayer, D. Gajewski, and S. Nelskamp, eds., Dynamics of Complex Intracontinental Basins, The Central European Basin System: Heidelberg, Germany, Springer Verlag, p. 157-172. Backhaus, E., 1968, Fazies, Stratigraphie und Paläogeographie der Solling-Folge (Oberer Buntsandstein) zwischen Odenwald/Rhön und Thüringer Wald: Oberrheinische geologische Abhandlungen, v. 17, p. 1-164. Baier, A., 1998, Zur Geschichte, Geologie und Hydrogeologie des Burgberges zu Nürnberg: Geologische Blätter Nordost-Bayern, v. 48, p. 277-300. Benan, C. A. A., and G. Kocurek, 2000, Catastrophic flooding of an aeolian dune field: Jurassic Entrada and Todilto Formations, Ghost Ranch, New Mexico, USA: Sedimentology, v. 47, p. 1069-1080. Berger, K., 1978, Erläuterungen zur Geologischen Karte Nürnberg-Fürth-Erlangen und Umgebung 1:50000: München, Bayerisches Geologisches Landesamt. Beutler, G., 1991, Zur Frage der Eichsfeld-Schwelle im Keuper: Zeitschrift für Geologische Wissenschaften, v. 19, p. 79-89. Beutler, G., N. Hauschke, and E. Nitsch, 1999, Faziesentwicklung des Keupers im Germanischen Becken, in N. Hauschke, and V. Wilde, eds., Trias - Eine ganz andere Welt: München, Pfeil, p. 129-174. Bindig, M., 1994, Die Architektur der fluviatilen Environments der Solling-Formation (Buntsandstein): Zentralblatt für Geologie und Paläontologie I, v. 1992, p. 1167- 1187. Biot, M. A., 1956, Theory of propagation of elastic waves in a fluid-saturated porous solid, part I: Journal of the Acoustical Society of America, v. 28, p. 168. Bjørlykke, K., 1983, Diagenetic reactions in sandstones, in A. Parker, and B. W. Sellwood, eds., Sediment Diagenesis: NATO ASI Series: Dodrecht / Boston / Lancaster, D. Reidel Publishing Company, p. 169-213.

106 References

Blake, R. E., and L. M. Walter, 1999, Kinetics of feldspar and quartz dissolution at 70– 80°C and near-neutral pH: Effects of organic acids and NaCl: Geochimica Cosmochimica Acta, v. 63, p. 2043-2059. Blatt, H., F. Middleton, and R. Murray, 1980, Origin of sedimentary rocks: Englewood Cliffs, New Jersey, Prentice-Hall. Blows, J. F., P. J. Carey, and A. B. Poole, 2003, Prelimnary investigations into Caen Stone in the U.K.; its use, weathering and comparison with repair stone: Building and Environment, v. 38, p. 1143-1149. Boigk, H., 1959, Zur Gliederung und Fazies des Buntsandsteins zwischen Harz und Emsland: Geologisches Jahrbuch, v. 76, p. 597-636. Bortz, S. A., and B. Wonneberger, 1997, Laboratory evaluation of building stone weathering, in J. F. Labuz, ed., Degradation of Natural Building Stone: Geotechnical Special Publications, v. 72: Reston (USA), ASCE, p. 85-104. Bourbié, T., O. Coussy, and B. Zinszner, 1987, Acoustics of porous media: Houston, Gulf Publishing. Brereton, N.R. and D. M. McCann, 1990, A fresh look at predictive equations for compressional wave velocity–porosity, in: Worthington (ed.), Advances in Core Evaluation Accuracy and Precision in Reserves Estimation. Proceedings of the First Society of Core Analysts European Core Analysis Symposium, London, May 1990, Gordon and Breach Scientific Publishers, pp. 270–298. Bromley, M. H., 1991, Architectural Features of the Kayenta Formation (Lower Jurassic), Colorado Plateau, USA - Relationship to Salt Tectonics in the Paradox Basin: Sedimentary Geology, v. 73, p. 77-99. Burkhardt, H., R. Mörig, and R. Schütt, 1990, Experimental and theoretical investigations of seismic wave absorption mechanisms in sedimentary rocks, Application of Absorption of Seismic Waves in Hydrocarbon Exploration: DGMK-report, v. 397: Hamburg, German Society for Petroleum Sciences and Coal Chemistry, p. 141- 199. Carta, L., D. Calcaterra, P. Cappelletti, A. Langella, and M. De Gennaro, 2005, The stone materials in the historical architecture of the ancient center of Sassari: distribution and state of conservation: Journal of Cultural Heritage, v. 6, p. 277-286. Cecil, C. B., and M. T. Heald, 1971, Experimental investigation of the effects of grain- coatings on quartz over-growths: Journal of Sedimentary Petrology, v. 41, p. 582- 584. Cnudde, V., A. Cwirzen, B. Masschaele, and P. Jacobs, 2009, Porosity and microstructure characterization of building stones and concretes: Engineering Geology, v. 103, p. 76-83. Cowan, G., 1993, Identification and significance of aeolian deposits within the dominantly fluvial Sherwood Sandstone Group of the East Irish Sea Basin UK, in C. P. North, and D. J. Prosser, eds., Characterization of Fluvial and Aeolian Reservoirs: Geological Society Special Publications, v. 73: London, Geological Society, p. 231-245. Davis, J. M., J. L. Wilson, and F. M. Phillips, 1994, A portable air-minipermeameter for rapid in situ field measurements: Ground Water, v. 32, p. 258-266. de Jager, J., 2007, Geological Development, in T. E. Wong, D. A. J. Batjes, and J. de Jager, eds., Geology of the Netherlands, Royal Netherlands Academy of Arts and Sciences, p. 5-26. de Jager, J., and M. C. Geluk, 2007, Petroleum Geology, in T. E. Wong, D. A. J. Batjes, and J. de Jager, eds., Geology of the Netherlands, Royal Netherlands Academy of Arts and Sciences, p. 241-264.

107 References

Dersch-Hansmann, M., and N. Hug, 2004, Oberer und Mittlerer Buntsandstein im Untergrund des Dieburger Beckens: Geologisches Jahrbuch Hessen, v. 131, p. 81- 95. Dewers, T., and P. Ortoleva, 1991, Influences of Clay-Minerals on Sandstone Cementation and Pressure Solution: Geology, v. 19, p. 1045-1048. Dickinson, W. W., and K. Milliken, 1995, The diagenetic role of brittle deformation in compaction and pressure solution, Etjo Sandstone, Namibia: Journal of Geology, v. 103, p. 339-347. Diederich, G., and H. Hickethier, 1975, Der Buntsandstein am Südwestrand des Vogelsberges: Notizblatt des Hessischen Landesamtes für Bodenforschung, v. 97, p. 195-205. Dorn, P., 1926, Geologie des Wendelsteiner Höhenzuges: Zeitschrift der deutschen geologischen Gesellschaft, v. 78, p. 522-546. Dürrast, H., S. Siegesmund, and M. Prasad, 1999, Die Schadensanalyse von Naturwerksteinen mittels Ultraschalldiagnostik: Möglichkeiten und Grenzen: Zeitschrift der deutschen Gesellschaft für Geowissenschaften, v. 150, p. 359-374. Dvorkin, J., and A. Nur, 1998, Time-average equation revisited: Geophysics, v. 63, p. 460-464. Ehling, 2009, Bausandsteine in Deutschland Band 1: Stuttgart, Schweizerbart'sche Verlagsbuchhandlung. Ehrenberg, S. N., 1993, Preservation of anomalously high-porosity in deeply buried sandstones by grain-coating chlorite - examples form the Norwegian continental- shelf: AAPG Bulletin, v. 77, p. 1260-1286. Erfurt, W., and R. Krompholz, 1996, Anwendung von Dehnwellenmessungen für Baustoffuntersuchungen: Wissenschaftliche Zeitschrift der Hochschule für Architektur und Bauwesen Weimar, v. 42, p. 95-100. Evans, D. J., J. G. Rees, and S. Holloway, 1993, The Permian to Jurassic stratigraphy and structural evolution of the central Cheshire Basin: Journal of the Geological Society, v. 150, p. 857-870. Feist-Burkhardt, S., A. E. Götz, J. Szulc, R. Borkhataria, M. Geluk, J. Haas, J. Hornung, P. Jordan, O. Kempf, J. Michalík, J. Nawrocki, L. Reinhardt, W. Ricken, H.-G. Röhling, T. Rüffer, Á. Török, and R. Zühlke, 2008, Triassic, in T. McCann, ed., The Geology of Central Europe, v. Volume 2: Mesozoic and Cenozoic, Geological Society of London, p. 749-821. Filomena, C. M., and H. Stollhofen, 2011, Ultrasonic logging across unconformities – outcrop and core logger sonic patterns of the Early Triassic Middle Buntsandstein Hardegsen unconformity, southern Germany: Sedimentary Geology, v. 236, p. 185-196. Fitzner, B., K. Heinrichs, and D. La Bouchardiere, 2003, Weathering damage on Pharaonic sandstone monuments in Luxor-Egypt: Building and Environment, v. 38, p. 1089-1103. Folk, R. L., 1974, Petrology of sedimentary rocks: Austin, Texas, Hemphill Publishing. Freudenberger, W., 2005, Der Keuper in Franken und der Oberpfalz (Bayern), in G. Beutler, D. Dittrich, J. Dockter, R. Ernst, A. Etzold, J. Farrenschon, W. Freudenberger, C. Heunisch, K.-P. Kelber, G. Knapp, M. Lutz, E. Nitsch, K. Oppermann, J. Schubert, E. Schulz, V. Schweizer, D. Seegis, R. Tessin, and U. Vath, eds., Stratigraphie von Deutschland IV - Keuper - Stratigraphische Kommission Deutschland: Frankfurt a.M., Cour. Forsch.-Inst. Senckenberg, p. 203-213. Freudenberger, W., and K. Schwerd, 1996, Erläuterungen zur Geologischen Karte von Bayern 1:500 000: München, Germany, Bayerisches Geologisches Landesamt.

108 References

Freund, D., 1992, Ultrasonic compressional and shear velocities in dry clastic rocks as a function of porosity, clay content, and confining pressure: Geophysical Journal International, v. 108, p. 125-135. Gal, D., J. Dvorkin, and A. Nur, 1999, Elastic-wave velocities in sandstones with non- load-bearing clay: Geophysical Research Letters, v. 26, p. 939-942. Gardner, G. H. F., L. W. Gardner, and A. R. Gregory, 1974, Formation velocity and density - the diagnostic basics for stratigraphic traps: Geophysics, v. 39, p. 770- 780. Gassmann, F., 1951a, Elastic waves through a packing of spheres: Geophysics, v. 26, p. 673-685. Gassmann, F., 1951b, Über die Elastizität poröser Medien: Vierteljahresschrift der Naturforschenden Gesellschaft in Zürich, v. 96, p. 1-23. Geertsma, J., and D. C. Smit, 1961, Some aspects of elastic wave propagation in fluid- saturated porous solids: Geophysics, v. 26, p. 169-181. Geluk, M., 1998, Paleogeographic and structural development of the Triassic in the Netherlands - new insights, in G. H. Bachmann, and I. Lerche, eds., Epicontinental Triassic: Zentralblatt für Geologie und Paläontologie, v. 1998: Stuttgart, p. 545- 570. Geluk, M., 2005, Stratigraphy and tectonics of Permo-Triassic basins in the Netherlands and surrounding areas: PhD thesis, Utrecht University, Utrecht, Netherlands. Geluk, M. C., 2007, Triassic, in T. E. Wong, D. A. J. Batjes, and J. de Jager, eds., Geology of the Netherlands, Royal Netherlands Academy of Arts and Sciences, p. 85-106. Geluk, M. C., and H. G. Röhling, 1997, High-resolution sequence stratigraphy of the Lower Triassic 'Buntsandstein' in the Netherlands and northwestern Germany: Geologie En Mijnbouw-Netherlands Journal of Geosciences, v. 76, p. 227-246. Geluk, M. C., and H. G. Röhling, 1999, High-resolution sequence stratigraphy of the Lower Triassic Buntsandstein: a new tool for basin analysis, in G. H. Bachmann, and I. Lerche, eds., The Epicontinental Triassic: Zentralblatt für Geologie und Paläontologie, v. 7-8, p. 727-745. Geyer, G., 2002, Geologie von Unterfranken und angrenzenden Regionen: Gotha, Stuttgart, Klett-Perthes. Geyer, G., and H. Schmidt-Kaler, 2009, Den Main entlang durch das Fränkische Schichtstufenland: Wanderungen in die Erdgeschichte, v. 23: Munich, Friedrich Pfeil. Gierlowski-Kordesch, E. H., and M. R. Gibling, 2002, Pedogenic mud aggregates in rift sedimentation, in R. W. Renault, and G. M. Ashley, eds., Sedimentation in continental rifts, SEPM Special Publication, v. 73: Tulsa, OK, SEPM, p. 195-206. Glennie, K. W., and B. D. Evamy, 1968, Dikaka: Plants and plant-root structures associated with aeolian sand: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 4, p. 77-87. Grimm, W. D., 1990, Bildatlas wichtiger Denkmalgesteine der Bundesrepublik Deutschland: Arbeitshefte, v. 50: München, Bayerisches Landesamt für Denkmalpflege. Grumbt, E., H. Kästner, and H. Lützner, 1997, Erläuterungen zur Geologischen Karte 1:25000 von Thüringen Blatt Schlotheim, Nr. 4729, Blatt Ebeleben, Nr. 4730: Thüringer Landesanstalt für Geologie. Hamilton, E. L., and R. T. Bachmann, 1982, Sound velocity and related properties of marine sediments: Journal of the Acoustical Society of America, v. 72, p. 1891- 1904.

109 References

Han, D.-h., A. Nur, and D. Morgan, 1986, Effects of porosity and clay content on wave velocities in sandstone: Geophysics, v. 51, p. 2093-2107. Hauschke, N., and V. Wilde, 1999, Trias - Eine ganz andere Welt. Mitteleuropa im Erdmittelalter: München, Pfeil. Henk, A., 1992, Mächtigkeit und Alter der erodierten Sedimente im Saar-Nahe-Becken (SW-Deutschland): Geologische Rundschau, v. 81, p. 323-331. Hernandez, M. G., M. A. G. Izquierdo, A. Ibanez, J. j. Anaya, and L. G. Gomez-Ullate, 2000, Porosity estimation of concrete by ultrasonic NDT: Ultrasonics, v. 38, p. 531-536. Herries, R. D., 1993, Contrasting styles of fluvial-aeolian interaction at a downwind erg margin: Jurassic Kayenta-Navajo transition, northeastern Arizona, USA, in C. P. North, and D. J. Prosser, eds., Characterization of fluvial and aeolian reservoirs: Special Publication, v. 73: London, Geological Society, p. 199-218. Hertle, M., and R. Littke, 2000, Coalification pattern and thermal modelling of the Permo- Carboniferous Saar Basin (SW-Germany): International Journal of Coal Geology, v. 42, p. 273-296. Hildebrand, T., and P. Ruesegger, 1997, A new method for the model independent assessment of thickness in three dimensional images: Journal of Microscopy, v. 185, p. 67-75. Hoffmann, A., and S. Siegesmund, 2007, Investigation of dimension stones in Thailand: an approach to a methodology for the assessment of stone deposits: Zeitschrift der deutschen Gesellschaft für Geowissenschaften, v. 158, p. 375-416. Ingersoll, R. V., 1978, Petrofacies and Petrologic Evolution of Late Cretaceous Fore-Arc Basin, Northern and Central California: Journal of Geology, v. 86, p. 335-352. Joos, S., 2000, Der Paläoboden (VH 2a) an der Hardegsen-Diskordanz in Gambach/Unterfranken: Diploma thesis, Institut für Geologie, University of Würzburg, Würzburg. Kahl, W.-A., and A. Holzheid, 2009, Quantifying porosities in sandstones: How to assess accuracy?, SkyScan User Meeting 2009. Kenter, J. A. M., H. Braaksma, K. Verwer, and X. M. T. van Lanen, 2007, Acoustic behaviour of sedimentary rocks; geologic properties versus Poisson's ratio: The Leading Edge, v. 26, p. 436-444. Klinkenberg, L. J., 1941, The permeability of porous media to liquids and gases, API Drilling and Production Practice: New York, American Petroleum Institute, p. 200-213. Knetsch, G., 1929, Der Keuper in der bayerischen Oberpfalz: Neues Jahrbuch für Mineralogie, Beilagen Band, v. 61, p. 83-150. Koch, R., A. Baier, H. Lorenz, and A. Fritsch, 2003, Sandsteine des Keupers als Naturwerksteine in und um Nürnberg (Exkursion B am 22. April 2003): Jahresbericht Mitteilungen des oberrheinischen geologischen Vereins, v. 85, p. 45- 64. Koch, R., and U. Zinkernagel, 2004, The relationship between spatial distribution of hydrothermal silicification in Keuper sandstone, primary facies and early diagenesis (The Wendelstein-Quarzit near Nuremberg), in: R. Prikryl, ed., Dimension Stone 2004 - New perspectives for a traditional building material, p. 57-62, Proceedings International Conference on dimension stone 2004, Prague. Koster, E. A., 1988, Ancient and modern cold-climate aeolian sand deposition: a review: Journal of Quaternary Science, v. 3, p. 69-83. Kowallis, B., L. E. A. Jones, and H. F. Wang, 1984, Velocity-porosity-clay content; systematics of poorly consolidated sandstones: Journal of Geophysical Research, v. 89, p. 10355-10368.

110 References

Krumbein, W. C., 1942, Criteria for subsurface recognition of unconformities: AAPG Bulletin, v. 26, p. 36-62. Laemmlen, 1966, Der Mittlere Buntsandstein und die Solling-Folge in Südhessen und in den südlich angrenzenden Nachbargebieten: Zeitschrift der deutschen geologischen Gesellschaft, v. 116, p. 908-949. Lafhaj, Z., M. Goueygou, A. Djerbi, and M. Kaczmarek, 2006, Correlation between porosity, permeability and ultrasonic parameters of mortar with variable water/cement ratio and water content: Cement and Concrete Research, v. 36, p. 625-633. Lang, S., 2001, Zur Sedimentologie der Solling-Formation (Buntsandstein) im Thüringer Becken, Institut für Geowissenschaften, Friedrich-Schiller-Universität, Jena, Germany. Legler, B., U. Gebhardt, and J. W. Schneider, 2005, Late Permian non-marine-marine transitional profiles in the central Southern Permian Basin, northern Germany: International Journal of Earth Sciences, v. 94, p. 851-862. Lepper, J., 1970, Neue Ergebnisse lithostratigraphisch-fazieller Detailuntersuchungen im Grenzbereich Mittlerer/Oberer Buntsandstein zwischen Fulda und Neckar: PhD thesis, University of Würzburg. Lepper, J., 2007, Dimension stones - a unique construction material: Zeitschrift der deutschen Gesellschaft für Geowissenschaften, v. 158, p. 685-693. Lepper, J., D. Rambow, and H.-G. Röhling, 2005, Der Buntsandstein in der Stratigraphischen Tabelle von Deutschland 2002: Newsletters on Stratigraphy, v. 41, p. 129-142. Liu, K. Y., L. Paterson, P. Wong, and D. S. Qi, 2002, A sedimentological approach to upscaling: Transport in Porous Media, v. 46, p. 285-310. Long, H., R. Swennen, A. Foubert, M. Dierick, and P. Jacobs, 2009, 3D quantification of mineral components and porosity distribution in Westphalian C sandstone by microfocus X-ray computed tomography: Sedimentary Geology, v. 220, p. 116- 125. Loope, D. B., 1988, Rhizoliths in ancient eolianites: Sedimentary Geology, v. 56, p. 301- 314. Loope, D. B., J. A. Mason, and L. Dingus, 1999, Lethal sandslides from aeolian dunes: Journal of Geology, v. 107, p. 707-713. Mack, G. H., and W. C. James, 1992, Calcic Paleosols of the Plio-Pleistocene Camp Rice and Palomas Formations, Southern Rio-Grande Rift, USA: Sedimentary Geology, v. 77, p. 89-109. Mader, D., 1997, Palaeoenvironmental evolution and bibliography of the Keuper (Upper Triassic) in Germany, Poland and other parts of Europe: 1st edition, Cologne, Sven von Loga. Magara, K., 1976, Thickness of Removed Sedimentary-Rocks, Paleopore Pressure, and Paleotemperature, Southwestern Part of Western Canada Basin: AAPG Bulletin, v. 60, p. 554-565. Marion, D., A. Nur, H. Yin, and D.-h. Han, 1992, Compressional velocity and porosity in sand clay mixtures: Geophysics, v. 57, p. 554-563. Marriott, S. B., and V. P. Wright, 1993, Paleosols as Indicators of Geomorphic Stability in two Old Red Sandstone Alluvial Suites, South Wales: Journal of the Geological Society, v. 150, p. 1109-1120. Marzolf, J. E., 1988, Episodic deposition and preservation of eolian sands: A late Paleozoic example from southeastern Utah: Sedimentary Geology, v. 56, p. 167- 191.

111 References

Mayr, S. I., and H. Burkhardt, 2006, Ultrasonic properties of sedimentary rocks: effect of pressure, saturation, frequency and microcracks: Geophysical Journal International, v. 164, p. 246-258. McBride, E. F., 1963, A classification of common sandstones: Journal of Sedimentary Petrology, v. 33, p. 664-669. McCarthy, P. J., U. F. Faccini, and A. G. Plint, 1999, Evolution of an ancient coastal plain: palaeosols, interfluves and alluvial architecture in a sequence stratigraphic framework, Cenomanian Dunvegan Formation, NE British Columbia, Canada: Sedimentology, v. 46, p. 861-891. Menning, M., R. Gast, H. Hagdorn, K.-C. Käding, T. Simon, M. Szurlies, and E. Nitsch, 2005, Zeitskala für Perm und Trias in der Stratigraphischen Tabelle von Deutschland 2002, zyklostratigraphische Kalibrierung von höherer Dyas und Germanischer Trias und das Alter der Stufen Roadium bis Rhaetium 2005: Newsletters on Stratigraphy, v. 41, p. 173-210. Menning, M., and A. Hendrich, 2002, Stratigraphic Table of Germany 2002, Potsdam, Germany, German Stratigraphic Commission. Miall, A. D., and M. Arush, 2001, Cryptic sequence boundaries in braided fluvial successions: Sedimentology, v. 48, p. 971-985. Minear, M. J., 1982, Clay models and acoustic velocities, Society of Petroleum Engineers of the American Institute of Mining, Metallurgical, and Petroleum Engineers, 57th Annual Meeting, New Orleans, SPE paper 11031, p. 11. Mosch, S., and S. Siegesmund, 2007, Petrophysical and technical properties of dimensional stones: a statistical approach: Zeitschrift der deutschen Gesellschaft für Geowissenschaften, v. 158, p. 821-868. Ortlam, D., 1974, Inhalt und Bedeutung fossiler Bodenkomplexe in Perm und Trias von Mitteleuropa: Int J Earth Sci (Geol Rundsch), v. 63, p. 850-884. Ortlam, D., 1980, Erkennung und Bedeutung fossiler Bodenkomplexe in Locker- und Festgesteinen: Int J Earth Sci (Geol Rundsch), v. 69, p. 581-593. Pettijohn, F. J., P. E. Potter, and R. Siever, 1972, Sand and Sandstone, Springer-Verlag. Popovics, S., L. Joseph, and S. John, 1990, The behavior of ultrasonic pulses in concrete: Cement and Concrete Research, v. 20, p. 259-270. Price, J. R., and M. A. Velbel, 2000, Weathering of the Eaton Sandstone (Pennsylvanian), Grand Ledge, Michigan: Geochemical mass-balance and implications for reservoir properties beneath unconformities: Journal of Sedimentary Research, v. 70, p. 1118-1128. Prikryl, R., 2006, "New natural stone" for the reconstruction of Charles Bridge in Prague, in R. Fort, M. Alvarez De Buergo, M. Gomez-Heras, and C. Vazquez-Calvo, eds., Heritage, Weathering and Conservation, v. 1: London, Taylor & Francis Group, p. 23-29. Prikryl, R., and Á. Török, 2010, Natural stones for monuments: their availability for restoration and evaluation, in R. Prikryl, and Á. Török, eds., Natural Stone Ressources for Historical Monuments, v. 333: London, Geological Society, p. 1-9. Radies, D., H. Stollhofen, G. Hollmann, and P. Kukla, 2005a, Syn-tectonic generation of potential reservoir-sandstones in arid low-gradient depositional systems - the Lower Triassic Middle Buntsandstein at the western margin of the Ems Trough, NW Germany.: Oil Gas European Magazine, v. 4, p. 201-205. Radies, D., H. Stollhofen, G. Hollmann, and P. Kukla, 2005b, Synsedimentary faults and amalgamated unconformities: Insights from 3D-seismic and core analysis of the Lower Triassic Middle Buntsandstein, Ems Trough, north-western Germany: International Journal of Earth Sciences, v. 94, p. 863-875.

112 References

Raymer, D. S., E. R. Hunt, and J. S. Gardner, 1980, An improved sonic transit time to porosity transform: Society of Petrophysicists and Well Log Analysts, 21st Annual Meeting, Lafayette, Los Angeles, Paper P, p. 1-12. Rider, M., ed., 2002, The geological interpretation of well logs: Sutherland (UK), Rider- French Consulting Ltd. Röhling, H.-G., 1991, A lithostratigraphy subdivision of the Lower Triassic in the Northwest German Lowland and the German Sector of the North Sea, based on Gamma-Ray and Sonic Logs: Geologisches Jahrbuch, v. A, p. 3-24. Rothwell, R. G., and F. R. Rack, 2006, New techniques in sediment core analysis: an introduction, in R. G. Rothwell, ed., New Techniques in Sediment Core Analysis: Special Publications, v. 267: London, Geological Society, p. 1-29. Schmidt, V., and D. A. McDonald, 1979, Texture and recognition of secondary porosity in sandstones, in P. A. Scholle, and P. R. Schluger, eds., Aspects of diagenesis: SEPM Special Publication, v. 26, p. 209-225. Schuster, M., 1933, Die Gliederung des Unterfränkischen Buntsandsteins. II. Der Obere Buntsandstein oder das Röt. a) Die Grenzschichten zwischen Mittlerem und Oberem Buntsandstein: Abhandlungen geologischer Landesuntersuchungen Bayerisches Oberbergamt, v. 9, p. 58. Schütt, R., 1992, Experimentelle und theoretische Untersuchungen zur Dämpfung und Geschwindigkeiten von Kompressions- und Scherwellen in Sedimentgesteinen bei Ultraschallfrequenzen: PhD thesis, Department of Applied Geosciences, Technical University of Berlin, Berlin. Sclater, J. G., and P. A. F. Christie, 1980, Continental Stretching - an Explanation of the Post-Mid-Cretaceous Subsidence of the Central North-Sea Basin: Journal of Geophysical Research, v. 85, p. 3711-3739. Shanmugam, G., and J. B. Higgins, 1988, Porosity Enhancement from Chert Dissolution beneath Neocomian Unconformity - Ivishak Formation, North Slope, Alaska: AAPG Bulletin, v. 72, p. 523-535. Sharp, W. C., and V. B. Hawk, 1977, Establishment of woody plants for secondary and tertiary dune stabilization along the Mid-Atlantic coast: International Journal of Biometeorology, v. 21, p. 245-255. Siegesmund, S., A. Vollbrecht, K. Ullemeyer, T. Weiss, and R. Sobott, 1997, Application of geological fabric analyses for the characterisation of natural building stone - case study Kaufung marble: International Journal for Restoration of Buildings and Monuments, v. 3, p. 269-292. Smith, B. J., and R. Prikryl, 2007, Diagnosing decay: the value of medical analogy in understanding the weathering of building stones, in R. Prikryl, and B. J. Smith, eds., Building stone decay: from diagnosis to conservation: Special Publications, v. 271: London, Geological Society, p. 1-8. Smith, B. J., A. V. Turkington, and J. M. Curran, 2001, Calcium loading of quartz sandstones during construction: implications for future decay: Earth Surface Processes and Landforms, v. 26, p. 877-883. Sobott, R., and R. Koch, 2009, Die Qualität von Naturwerksteinen aus dem Steinbruch Worzeldorf bei Nürnberg, Petrographie, Diagenese und gesteinsphysikalische Kenndaten (Mittlerer Keuper, Oberer Burgsandstein): Geologische Blätter Nordost-Bayern, v. 59, p. 419-444. Stanistreet, I. G., and H. Stollhofen, 2002, Hoanib River flood deposits of Namib Desert interdunes as analogues for thin permeability barrier mudstone layers in aeolianite reservoirs: Sedimentology, v. 49, p. 719-736. Stollhofen, H., G. H. Bachmann, J. Barnasch, U. Bayer, G. Beutler, M. Franz, M. Kästner, B. Legler, J. Mutterlose, and D. Radies, 2008, Upper Rotliegend to Early

113 References

Cretaceous basin development, in R. Littke, U. Bayer, D. Gajewski, and S. Nelskamp, eds., Dynamics of complex intracontinental basins. The Central European Basin System, p. 181-210. Stück, H., S. Siegesmund, and J. Rüdrich, 2011, Weathering behaviour and construction suitability of dimension stones from the Drei Gleichen area (Thuringia, Germany): Environmental Earth Sciences, v. 63, p. 1763-1786. Summerfield, M. A., 1983, Petrography and Diagenesis of Silcrete from the Kalahari Basin and Cape Coastal Zone, Southern-Africa: Journal of Sedimentary Petrology, v. 53, p. 895-909. Svendsen, J., H. Stollhofen, C. B. E. Krapf, and I. G. Stanistreet, 2003, Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia: Sedimentary Geology, v. 160, p. 7-31. Szurlies, M., 2007, Latest Permian to Middle Triassic cyclo-magnetostratigraphy from the Central European Basin, Germany: Implications for the geomagnetic polarity timescale: Earth and Planetary Science Letters, v. 261, p. 602-619. Toksöz, M. N., and D. H. Johnston, 1981, Seismik wave attenuation, v. 2: Tulsa, Society of Exploration Geophysicists. Tosaya, C., and A. Nur, 1982, Effects of diagenesis and clays on compressional velocities in rocks: Geophysical Research Letters, v. 9, p. 5-8. Trusheim, F., 1961, Über Diskordanzen im Mittleren Buntsandstein Nordwest- deutschlands zwischen Ems und Weser: Erdöl-Zeitschrift, v. 77, p. 361-367. Trusheim, F., 1963, Zur Gliederung des Buntsandsteins: Erdöl-Zeitschrift, v. 79, p. 277- 292. van der Zwan, C. J., and P. Spaak, 1992, Lower to middle Triassic sequence stratigraphy and climatology of the Netherlands, a model: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 91, p. 277-290. Van Geet, M., D. Lagrou, and R. Swennen, 2003, Porosity measurements of sedimentary rocks by means of microfocus X-ray computed tomography (µCT), in F. Mees, R. Swennen, M. Van Geet, and P. Jacobs, eds., Applications of X-ray computed tomography in the geosciences: Special Publications, v. 215, Geological Society of London, p. 51-60. Van Geet, M., R. Swennen, and M. Wevers, 2000, Quantitative analysis of reservoir rocks by microfocus X-ray computerized tomography: Sedimentary Geology, v. 132, p. 25-36. Vernik, L., 1996, Predicting porosity from acoustic velocities in siliciclastics: a new look: Geophysics, v. 62, p. 118-128. Vernik, L., and A. Nur, 1992, Petrophysical classification of siliciclastics for lithology and porosity prediction from seismic velocities: AAPG Bulletin, v. 76, p. 1295- 1309. von Gehlen, K., 1956, Sekundär-hydrothermale Mineralisation im Burgsandstein des Wendelsteiner Höhenzuges bei Nürnberg: Geologische Blätter Nordost-Bayern, v. 6, p. 12-21. Washburn, E. W., 1921, The dynamics of capillary flow: Physical Review, v. 17, p. 273- 383. Wellington, S. L., and H. J. Vinegar, 1987, X-ray computerized tomography: Journal of Petroleum Technology, v. 39, p. 885-898. Werner, W., and B. Hoffmann, 2007, Dimension sandstones of Southwest Germany: occurrence, composition, use and exploration: Zeitschrift der deutschen Gesellschaft für Geowissenschaften, v. 158, p. 737-750.

114 References

Wittmann, O., 1972, Geologische Karte von Bayern 1:25000. Erläuterungen zum Blatt Nr. 6022 Rothenbuch, München, Geologisches Landesamt Bayern. Wolburg, J., 1962, Über Schwellenbildung im Mittleren Buntsandstein des Weser-Ems Gebietes: Erdöl-Zeitschrift, v. 78, p. 183-190. Wyllie, M. R. J., A. R. Gregory, and G. H. F. Gardner, 1958, An experimental investigation of factors affecting elastic wave velocities in porous media: Geophysics, v. 23, p. 459-493. Wyllie, M. R. J., A. R. Gregory, and L. W. Gardner, 1956, Elastic wave velocities in heterogeneous and porous media: Geophysics, v. 21, p. 41-70. Ziegler, P. A., 1982, Geological Atlas of Western and Central Europe: The Hague, Shell. Ziegler, P. A., 1990, Geological Atlas of Western and Central Europe: The Hague, Netherlands, Shell International Petroleum Maatschappij BV. Zinszner, B., and F. M. Pellerin, 2007, A Geoscientist's Guide to Petrophysics: Éditions Technip, IFP Publications.

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Table of figures

Figure number & short description page Chapter I 1.1: Stratigraphic chart of the German Triassic 18 1.2: Paleogeographic map of the Central European Basin System of the Olenekian to Anisian and 20 well core or outcrop positions elaborated in this thesis

Chapter II 2.1: Example of an ultrasonic oscillograph curve highlighting the first arrival of the p-wave 23 2.2: Test assemblies of ultrasonic probes realized in different logging devices 26 2.3: Schematic constructions of different gas-permeameter devices 30 2.4: Principle of pore diameter distribution calculation using µ-CT datasets and examples from 33 two sandstones with different cementation intensities.

Chapter III 3.1: Location map of southern Germany indicating positions of outcrop and well core sections at 38 Gambach a.M., Guggenberg, and Rockensußra 3.2: General stratigraphy and unconformities of the Buntsandstein Group in Germany 41 3.3: Scatter-plot of sonic logging data derived from a multi sensor core logger and a mobile tester 42 3.4: Petrophysical and grain size logs of the Guggenberg well core section 43 3.5: Thin section microphotographs from the Guggenberg well core section 45 3.6: Petrophysical and grain size logs of the Gambach outcrop section 47 3.7: Thin section microphotographs from the Gambach outcrop section 49 3.8: Petrophysical and grain size logs of the Rockensußra well core section 50 3.9: Thin section microphotographs from the Rockensußra well core section 52 3.10: µ-CT samples from the Hardegsen Formation 54 3.11: Three-dimensional µ-CT pore network reconstructions from Hardegsen sandstones 55 3.12: SEM/BSC images and µ-CT 2D and 3D reconstructions of secondary porosity in a leached 56 feldspar grain 3.13: Comparison of pore diameter distributions derived from mercury intrusion and µ-CT 58 3.14: Ultrasonic p-wave transit time plotted versus core plug porosity and permeability 60 3.15: Ultrasonic p-wave transit time plotted versus density data of the Guggenberg well core 61 section

Chapter IV 4.1: Location map of the Netherlands and the southern North Sea indicating the position of the 66 offshore well L9-101-FF 4.2: The Middle Solling Sandstone Member (MSSM): Wireline logs, grain size, lithofacies, and 68 facies successions in well L9-FF-101

116 Table of Figures

4.3: Core slab photographs of MSSM lithofacies in well L9-FF-101 70 4.4: Well L9-FF-101, 3112-3148 m: High-resolution sonic log compared to wireline logs, 74 lithology, and core plug porosity data 4.5: SEM-BSE images and pore diameter distributions of selected MSSM samples 75 4.6: Core plug sample porosity data plotted against sonic transit time of different lithofacies types 77 4.7: Meso-scale reservoir heterogeneity: High-resolution sonic logs and core plug porosity data of 79 well L9-FF-101, 3113-3121 m and 3137-3145 m

Chapter V 5.1: Stratigraphy of the Middle Keuper and location map indicating the position of the 86 Worzeldorf quarry and local mining areas 5.2: Lithology and petrophysical logs of Worzeldorf borehole No. 8 88 5.3: Mineral content of selected Worzeldorf samples determined by XRD and Rietveld analysis 89 5.4: Thin section microphotographs and SEM-CL image of the Worzeldorf Sandstone 91 5.5: P-wave sonic velocity versus compressive strength of Worzeldorf Sandstone samples 95 5.6: P-wave sonic velocity versus E-moduli of Worzeldorf Sandstone samples 96 5.7: Comparison of Worzeldorf borehole cores 7, 8, 9, and 11: Grain size, high-resolution sonic 99 logs, calculated and measured compressive strength, and core plug porosity data.

Chapter VI 6.1: Principle of high-resolution sonic logging and fields of application in geosciences 105

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Danksagung

Mein größter Dank gilt Professor Dr. Harald Stollhofen, der die Idee zu meinem Promotionsthema hatte und mich während der letzten vier Jahre mit immer wieder neuen Denkanstößen und Ideen gefördert und unterstützt hat. Ich bedanke mich für seine große Hilfsbereitschaft, seinen wissenschaftlichen und persönlichen Rat, sowie für unvergessliche Geländeaufenthalte bei denen abends immer bestens für das leibliche Wohl gesorgt war.

Prof. Dr. Roman Koch möchte ich herzlich danken für anregende Diskussionen über Sedimentologie und Diagenese, sowie für seine Unterstützung bei der Beschaffung der Bohrkernproben aus dem Steinbruch Worzeldorf.

Prof. Dr. Thomas Aigner, Dr. Helmut Bock und Prof. Dr. Vincenzo Pascucci danke ich dafür, dass sie während meines Studiums mein Interesse für Sedimentologie und insbesondere für Siliziklastika geweckt haben.

Besonders bedanken möchte ich mich bei Mario Kittel, Henning Schulz, Maria Fensterer und Sergio Pampana die mir als Promotions-Mitstreiter im Büro, in der Espresso-Pause und in der Freizeit zu guten Freunden geworden sind. Ich bedanke mich für eine schöne Zeit bei Matthias Alberti, Philipp Brandl, Anne Brauers, Patrick Chellouche, Sarah Freund, Manja Hethke, Melanie Meyer, Anssi Myrttinen, Inga Osbahr, Stefan Schöbel und allen meinen Freunden und Kollegen am GeoZentrum Nordbayern.

Konrad Kunz bin ich zu allertiefstem Dank verpflichtet für seine Fähigkeit jegliche Probleme technischer Art schnell und unkompliziert zu lösen. Ohne sein Zutun wären viele Projekte überhaupt nicht möglich, oder nur schwer zu verwirklichen gewesen. Ebenso danke ich Dipl.-Ing. Michael Miller, Erich Meyer und Sebastian Zametzer von der Mechanik- und Elektronikwerkstatt der Technischen Fakultät.

Dr. Robert Schöner danke ich für zahlreiche Diskussionen und seine Hilfe bei petrographischen Fragestellungen. Ich bedanke mich bei Dr. Robert Sobott für seine Unterstützung bei petrophysikalischen Messungen und deren Interpretation. Ich danke Prof. Dr. Michael Joachimski für seine Unterstützung bei Vorbereitungen zu Lehrveranstaltungen und Geländekursen.

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Dr. Stefan Krumm danke ich für die Durchführung von Rietveld Analysen und für Hilfe bei Fragen zu Soft- und Hardware und ich danke Christian Schulbert für die Einweisung ins Rasterelektronenmikroskop.

Daniele Lutz, Friederike Urban und Melanie Hertel danke ich für ihre Unterstützung im Labor und bei der Probenpräparation.

Ich bedanke mich bei Veronika Kühnert, die mich stets mit guter Laune empfangen und sicher durch den Bürokratie-Dschungel geführt hat. Gudrun Klein sei gedankt für ständige Nachsicht, auch wenn einige Fachbücher erst nach vielen Wochen den Weg zurück von meinem Schreibtisch in die Bibliothek gefunden haben.

Cornelia Lutter … ti ringrazio per tutto!

Ich bedanke mich bei Prof. Peter Kukla, Prof. Dr. Christoph Clauser und Dr. Jens Hornung für die Benutzung ihrer Labore und Messgeräte. Prof. Dr. Reihard Gaupp möchte ich danken für konstruktive und sehr hilfreiche Kritik zu meinen Forschungsvorhaben.

Für finanzielle Unterstützung danke ich der Deutschen Forschungsgemeinschaft im Schwerpunktprogramm 1135 Sedimentbecken-Dynamik, sowie der International Association of Sedimentologists (IAS) für einen Post-Graduate Grant.

Herzlichen Dank an Dr. Jürgen Grötsch, John Marshall, Kees van Ojik, Joris Graaf und das gesamte Bohrkernlager-Team der Nederlandse Aardolie Maatschappij BV (NAM) für ihre Unterstützung mit Bohrlochdaten und Bohrkernen. Ich danke Dr. Walter Freudenberger vom Landesamt für Umwelt in Hof, Dr. Lutz Katzschmann und Dr. Hermann Huckriede von der Thüringer Landesanstalt für Umwelt und Geologie, sowie GS Schenk GmbH & Co. KG in Fürth für die Bereitstellung von Probenmaterial.

Meinen Eltern Jutta und Giovanni Filomena, meiner Schwester Angela, meiner Oma Margarete Proksch und meiner ganzen Familie danke ich für Ihre ständige Unterstützung bei allen meinen Vorhaben. Euch allen, sowie meinen guten Freunden Christian Bäßler und Harald Haakh, danke ich dafür, dass ihr es mir nicht übel genommen habt, dass ich mich oft nicht gemeldet habe und wir uns lang nicht gesehen haben.

Zu guter Letzt danke ich der Firma Steinbach Bräu Erlangen für würziges Storchenbier, das als Treibstoff bei abendlichen Fachgesprächen für anhaltenden Ideenfluss sorgte.

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