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*Addresses of the authors:Dr. Ute Trautwein-Bruns ([email protected]), Dipl.-Geol. Stephan Becker ([email protected]), Prof. Peter A. Kukla, Ph.D. (corresponding author; [email protected]), Geological Institute, RWTH Aachen University, Wüllnerstrasse 2, 52056 Aachen, Germany;PD Dr. Christoph Hilgers ([email protected]), Reservoir-Petrology, RWTH Aachen University, Wüllnerstrasse 2, 52062 Aachen, Germany;Prof. Dr. Janos L. Urai ([email protected]), Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4–20, 52056 Aachen, Germany.

Fracture and fault systems characterising the intersection between the Lower Rhine Embayment and the Ardennes-Rhenish Massif – results from the RWTH-1 well, Aachen, Germany

Ute Trautwein-Bruns, Christoph Hilgers, Stephan Becker, Janos L. Urai & Peter A. Kukla*

Trautwein-Bruns, U., Hilgers, C., Becker, S., Urai, J.L. & Kukla, P.A. (2011): Fracture and fault systems characterising the intersection between the Lower Rhine Embayment and the Ardennes-Rhenish Massif – results from the RWTH-1 well, Aa-chen, Germany. [Bruch- und Störungssysteme in der Übergangszone zwischen der Niederrheinischen Bucht und dem Rhei-nischen Schiefergebirge im belgisch-deutschen Grenzbereich – Ergebnisse der Bohrung RWTH-1, Aachen, Deutschland.] – Z. dt. Ges. Geowiss., 162: 251–276, Stuttgart.

Abstract: The 2544 m deep geothermal well “RWTH-1”, drilled in 2004 in the city centre of Aachen, is located geologi-cally in a key position between the Variscan Ardennes-Rhenish Massif and the active rift zone of the Lower Rhine Embay-ment close to the early Palaeozoic London-Brabant Massif. The borehole was drilled in the footwall of the frontal Variscan Aachen-Midi Thrust and 300 m to the east of a major post-Variscan normal fault.

Downhole measurements and samples include wireline logs, image logs, 126 m of cores and continuous cuttings, thus providing a unique dataset from the Variscan Frontal Zone in the German-Belgian borderland. The borehole revealed an unprecedented stratigraphic sequence with a striking absence of Middle Devonian and Lower Carboniferous carbonate rocks, which form prominent outcrops only a few hundred metres SE of the borehole location.

Complex open and sealed fault and fracture systems can be discerned, refl ecting the complex tectonic history. Faulting and folding is observed at 240–1520 m depth, whilst rocks are much less deformed and dip gently towards the NE in the lower part of the well down to 2544 m. Low-angle NE–SW-trending faults refl ect Variscan compression. The superfi cially open fault and fracture system including joints and drilling enhanced fractures shows a NW–SE trend parallel to regional normal faults and the recent maximum horizontal stress. A prominent NNE–SSW-trending fracture set is interpreted as pa-laeojoints to have formed under NNE directed compression, which differs from the present in-situ stress orientation. Varying directions of borehole breakouts and drilling induced tensile fractures are related to a subordinate set of W–E oriented faults, located at depths of 1895 m, 2115 m and 2304 m. This orientation is aligned parallel to the maximum dextral in-situ shear stress, which is in accordance with a strike-slip faulting regime extending from the Variscan Zone in Belgium.

Kurzfassung: Im Jahr 2004 wurde im Aachener Stadtgebiet die geothermische Tiefbohrung „RWTH-1“ mit einer Endteufe von 2544 m gebohrt. Sie befi ndet sich in einer geologischen Schlüsselposition zwischen dem variszisch geprägten Rhei-nischen Schiefergebirge/Ardennen und der derzeit seismisch aktiven Riftzone der Niederrheinischen Bucht. Die Tiefboh-rung durchteufte das Liegende der variszischen Überschiebungsfront und liegt nur 300 m östlich einer postvariszischen Abschiebung.

Durch ein umfangreiches geophysikalisches Messprogramm, das auch bohrlochabbildende Verfahren umfasst, eine un-unterbrochene Beprobung des Bohrkleins und das Ziehen von 126 m Bohrkernen in drei Teufenintervallen bietet die Boh-rung RWTH-1 einen einmaligen Datensatz der variszischen Front im belgisch-deutschen Grenzbereich. Stratigrafi sch be-merkenswert ist das Fehlen der unterkarbonischen und mitteldevonischen Karbonate, welche nur wenige 100 m südlich der Bohrlokation in großer Mächtigkeit aufgeschlossen sind.

Entsprechend der lang anhaltenden tektonischen Geschichte wurde ein komplexes Bruch- und Störungssystem durch-teuft. Der variszische Deformationsgrad nimmt mit der Tiefe ab. Vor allem oberhalb 1520 m zeigt sich noch starke Faltungs- und Überschiebungsaktivität, wohingegen die Schichten unterhalb nur fl ach nach NE einfallen. Die variszischen Überschie-bungen streichen NE–SW und fallen fl ach nach SE ein. Offene Brüche und Störungen streichen vor allem NW–SE parallel zu den Abschiebungen der Niederrheinischen Bucht und der Richtung der maximalen Horizontalspannung. Oberfl ächlich

Z. dt. Ges. Geowiss., 162/3, p. 251–276, 11 figs., 2 tables ArticleStuttgart, September 2011

© 2011 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany DOI: 10.1127/1860-1804/2011/0162-0251

www.schweizerbart.de1860-1804/0162-0251 $ 11.25

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252 Ute Trautwein-Bruns et al.

1. Introduction

In 2004 the 2544 m deep RWTH-1 well was drilled in the city centre of Aachen (Herzog 2005, Lundershausen 2009). The borehole is located 500 m northwest of the frontal Variscan Aachen-Midi Thrust and 300 m east of a major NW–SE-trending normal fault of the Roer Valley Graben (Laurensberg Fault in Fig. 1A), which forms the SW part of the seismically active Lower Rhine Embayment. Numerous outcrop studies have been conducted in the area in the past. The RWTH-1 well however provided for the fi rst time deep subsurface data from this geodynamically interesting position. It offered the unique opportunity to study a fracture and fault system in an area where Variscan crustal shortening is overprinted by post-Var-iscan extension and strike-slip deformation.

In this study we present interpretations based on high-resolution wireline log data and image logs, which allow a detailed analysis of the natural fracture system and drilling-induced rock failure. Results support distinctive fault and fracture sets and lead to a revised tectonic and facies model in the footwall of the Aachen-Midi Thrust in the Aachen area.

2. Geological setting

The RWTH-1 well is located in the city centre of Aachen (Germany), geologically in the intersection area between the frontal zone of the Central European Variscan Orogen in NW Europe (Fielitz 1992, Fielitz & Mansy 1999, Hollmann & Winterfeld 1999, Hance et al. 1999, Oncken et al. 1999, Wal-ter 2010) and the northernmost adjacent Lower Rhine Em-bayment rift system (Geluk et al. 1994, Kooi et al. 1998, Garcia-Castellanos et al. 2000, Demoulin & Hallot 2009). The northwestern part of the Ardennes-Rhenish Massif forms part of the Variscan Orogen and extends from Ger-many to Belgium and Northern France (Fig. 1A). It is char-acterised by recent uplift, regional-scale NW-verging syn-clines and anticlines (Figs. 1B, 1C) and mainly SW-dipping thrust faults (Figs. 1B, 1C) (Muchez & Sintubin 1998, Mansy et al. 1999, Kenis et al. 2000). Variscan deformation and metamorphism took place between 336 and 300 Ma (Nier-hoff 1994).

The Aachen-Midi Thrust (Figs. 1A, 1B) is described as the frontal Variscan Thrust in the region and continues as

“Faille Du Midi” through Belgium towards the British chan-nel (Dittmar et al. 1994, Hance et al. 1999, Hollmann & Win-terfeld 1999, Mansy et al. 1999, Plesch & Oncken 1999, Leynaud et al. 2000, Walter 2010). At the Aachen-Midi Thrust the Ardennes allochthon was thrust over its foreland, the Brabant parautochthon (in Belgium) and the Variscan foreland (in Germany) during the latest (Asturian) stage of the Variscan Orogeny (Van Noten & Sintubin 2010). This fault zone is fringed at its footwall by numerous smaller thrust sheets and imbricate fans (Hance et al. 1999). Bal-anced cross-sections in the region suggest a Variscan short-ening of about 45 % with nappes in the hanging wall trans-ported over 17 km towards the NW (Hollmann & Walter 1995, Hollmann & Winterfeld 1999, Hance et al. 1999).

The Lower Palaeozoic Brabant Massif forms the south-eastern part of the Anglo-Brabant Deformation Belt, situated within the eastern Avalonia Terrane Assemblage (Debacker et al. 2005). Two major angular unconformities, which are generally considered to refl ect episodes of deformation, are observed in the region: the Ardennian and Brabantian uncon-formities (Sintubin et al. 2009). Recent studies integrating stratigraphic, metamorphic, sedimentological, geophysical and structural data show that the Brabantian Orogeny had a long-lived nature, lasting for at least 30 Ma, from the late Llandovery to the Emsian, possibly even continuing into the Eifelian (Debacker et al. 2005, Sintubin et al. 2009).

Coevally with the Brabantian Orogeny the Lower Devo-nian Ardennes-Eifel rift basin developed (Sintubin et al. 2009). Rifting started in Late Silurian times (420 Ma), pos-sibly documented by the Ardennian Unconformity (Van Bae-len & Sintubin 2008, Sintubin et al. 2009).

The Lower Rhine Embayment to the north of the Varis-can NE–SW-trending fold-and-thrust belt has probably been active since the Permian and reactivated several times during the Meso-Cenozoic (Colbeaux et al. 1977, Wolf 1985, La-loux et al. 1997). It links the NNE–SSW-striking “Ober-rheingraben” structure with the NW–SE-striking West Neth-erlands Basin and the Sole Pit Basin in the North Sea (Zieg-ler 1990, Ziegler 1992, Ziegler & Dèzes 2007). Faults form horst and graben structures with migrating depocentres due to movement on different faults and block tilting (Dirkzwa-ger et al. 2000, Michon et al. 2003, Houtgast et al. 2005, Schäfer et al. 2005, Worum et al. 2005).

Since the early Oligocene, the ongoing extension coin-cides with a complex evolution of block tilting and stress

offene, NNE–SSW-streichende Klüfte werden als „palaeojoints“ gedeutet, entstanden unter NNE-gerichteter Kompression, die vom heutigen In-situ-Spannungsfeld abweicht. Abweichende Richtungen von Bohrlochrandausbrüchen und bohrungs-induzierten Rissen treten in der Nähe von offenen, W–E streichenden Störungen in einer Tiefe von 1895 m, 2115 m und 2304 m auf und werden als Indikatoren für rezente Aktivität gedeutet. Da die W–E-Streichen der Richtung maximaler In-situ-Scherspannung entspricht, liefern die Daten der Bohrung RWTH-1 eindeutige Beweise für die östliche Fortsetzung der im Bereich der belgischen Varisziden beschriebenen „strike-slip“-Deformationszone.

Keywords: RWTH-1 deep geothermal well, Variscan Deformation Front, Ardennes-Rhenish Massif, Lower Rhine Embay-ment, Roer Valley Graben, Aachen-Midi Thrust, borehole image logs, fracture analysis, strike-slip deformation

Schlüsselwörter: Geothermische Tiefbohrung RWTH-1, Variszische Deformationsfront, Rheinisches Schiefergebirge, Ardennen, Niederrheinische Bucht, Rurgraben, Aachener Überschiebung, bildgebende Bohrlochmessungen, Analyse des Bruch- und Störungssystems, Transversalverschiebungen


253Fracture and fault systems from the deep RWTH-1 well, Aachen

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Overcoring data

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Focal Mechanism:

In situ stressmeasurements:

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Fig. 1bFig. 1b

Stratigraphy:

Fig. 1: The RWTH-1 well (yellow star) is located in a geological key position at the intersection between the Ardennes-Rhenish Massif and the active rift zone of the Lower Rhine Embayment (LRE) close to the Brabant Massif (BM).(A) The seismotectonic situation of the Aachen area is shown on the map with the NE–SW-trending Variscan fold-and-thrust belt (red colour) in the south and the NW–SE-trending Lower Rhine Embayment with its normal fault system in the north. The orientation of the present-day stress fi eld is indicated by the earthquake focal mechanisms compiled from Camelbeeck et al. (2007), Hinzen (2003) and Dost & Haak (2007), and the in-situ stress measurements are taken from the World Stress Map (Heidbach et al. 2008). The data show a strike-slip regime in the present-day uplifting Ardennes-Rhenish Massif to the southwest, and a normal faulting regime in the younger, central Lower Rhine Embayment. The map is modifi ed after Verniers et al. (2001), Sintubin & Everaerts (2002) and Frisch & Meschede (2007). (B) De-tailed geological map of the Variscan fold-and-thrust belt of the Aachen region (modifi ed from Walter 2010). The RWTH-1 well is located in the city centre of Aachen, at the transition zone between the seismically active Lower Rhine Embayment with NW–SE-striking normal faults and the Variscan fold-and-thrust belt. (C) Geological cross-section across the northern part of the Variscan fold-and-thrust belt of the Aachen region (modifi ed from Sindern et al. 2008). Note that the RWTH-1 well is located approximately 500 m NW of the frontal Variscan Thrust and drilled through the folded and imbricated footwall of the thrust sheet.



rotation from NNE–SSW to NW–SE in Eocene-Miocene times (Breddin et al. 1963, Zagwijn 1989, Schreiber & Rotsch 1998, Michon et al. 2003, Worum et al. 2005).

The Lower Rhine Embayment is currently subsiding with rates of 2.0 mm/a (Campbell et al. 2002, Michon et al. 2003), whilst the Ardennes-Rhenish Massif to the south was up-lifted over the last 2 Ma (Mälzer et al. 1983, Garcia-Castel-lanos et al. 2000, Ziegler & Dèzes 2007). Active seismic deformation on the fault systems occurs in the Lower Rhine Embayment (Ahorner 1975, Leydecker 1986, Hinzen 2003, Skupin et al. 2008, Camelbeeck et al. 2007; Fig. 1A). Fault plane solutions from recent earthquakes show dip-slip move-ment at crustal shallow levels, grading into transtension to strike-slip at deeper crustal levels in the eastern part of the basin (Hinzen 2003, Reamer & Hinzen 2004). Normal and strike-slip movements have also been reported on Meso- to Cenozoic fault planes in Carboniferous rocks of the Ar-dennes-Rhenish Massif, which was uplifted since the Qua-ternary (Mälzer et al. 1983, Garcia-Castellanos et al. 2000, Meyer & Stets 1998, Demoulin & Hallot 2009).

The RWTH-1 well is located approximately 500 m NW of the Aachen Thrust in the north of the Aachen Anticline (Fig. 1). The lithologies around the borehole location are characterised by Quaternary, Tertiary and Cretaceous sedi-ments, which unconformably overlie deformed Carbonifer-ous (Pennsylvanian) and Devonian siliciclastics and carbon-ates (Figs. 1B, 1C).

3. Methods

The structural investigation of the RWTH-1 borehole is based on the interpretation of resistivity (STAR Imager – Baker Atlas™) and acoustic image logs (CBIL – Baker Atlas™) at depths of 1268–2528 m and hexagonal dipmeter data (242–1261 m; HDIP – Baker Atlas™), supplemented by the shear wave anisotropy (240–2528 m; Cross Multipole Array Acoustic Log [XMAC] – Baker Atlas™) and by stud-ies of the core material. Altogether 146 m of core were re-trieved from the borehole, cut in three geologically signifi -cant depth intervals at 1392–1515 m, 2128–2143 m and 2537–2544 m (= total depth). The cores from the fi rst and second core section were obtained by a coring-while-drilling system (CoreDrill – Baker Hughes™), which produces cores of 2 inch diameter. The core taken at the bottom of the well was cut by a conventional system and has a diameter of 4 inches. The cores were cut unoriented, but were subse-quently calibrated with the resistivity image log.

3.1 Acoustic and resistivity images

The Baker Atlas CBIL™ and STAR™ tools can be combined to allow simultaneous acquisition of acoustic and resistivity images of the borehole wall (Fig. 2). Resistivity is measured by arrays of electrodes on six pads, which were in contact with the borehole wall. The acoustic imaging device scans the entire circumference of the borehole by a rotating trans-

ducer operating in pulse-echo-mode. The acoustic data set can be displayed as amplitude or velocity image.

While resistivity images resolve stratigraphic details such as bedding and foliation, the acoustic images display the condition of the borehole wall (Davatzes & Hickman 2005) and were therefore used for the detection of open faults, fractures and breakouts (Fig. 2). The processing and interactive interpretation of the microresistivity and acoustic borehole image logs were performed within the RECALL software package provided by Baker Hughes Inc.

On the microresistivity images, bedding, cleavage, frac-tures and faults appear as conductive or resistive anomalies with a sinusoidal shape (Fig. 2). Bedding planes can easily be detected by resistivity contrasts. In the highly consoli-dated, low-porous Palaeozoic rocks, the resistivity image displays variations in mineralogy especially in clay content. While bedding planes are demarcated as zones of discernible resistivity changes, the cleavage planes occur as dark, regu-lar spaced lines crosscutting these planes. Natural fractures are also visualised, provided a resistivity contrast between the fracture and the formation exists (Fig. 2) or the rock fab-ric changes at the fracture plane. Sealed fractures are some-times demarcated by halos especially in the clay-rich Lower Devonian rocks.

The acoustic amplitude image is sensitive to the surface condition of the borehole wall (Davatzes & Hickman 2005). It reveals rock fabrics only in cases of a strong contrast in competence, which results in a different surface roughness, a different degree of wearing of the borehole wall or washouts at bedding planes. The acoustic amplitude image is very use-ful for the detection of open fractures and compressive bore-hole breakouts.

In the upper 130 m of the imaged borehole section (1260–1390 m) drilling problems resulted in a spiral-scratched borehole wall. Here the interpretation of the acous-tic image is impossible and the quality of the microresistivity image is reduced. Further down the hole, the STAR tool de-livers generally high quality images. The tool ran generally closer to the low side of the borehole (deviation up to 15°). This eccentricity results in dark and light traces on the acous-tic images (Fig. 2) but this does not affect their interpreta-tion. A groove pipe was scratched over long sections in the well, especially within the clay-rich formations at a higher deviation of the well between 1440 and 2100 m (e.g. in Figs. 5 and 6). This is clearly shown as a tool mark in the acoustic image, but only slightly reduces the quality of the circumfer-ential image.

3.2 Hexagonal dipmeter and shear wave anisotropy

The hexagonal dipmeter log is also based on six arm resistiv-ity measurements. Initially, a computer-aided dip detection (“autodip”) routine was run across the hexagonal dipmeter data in order to compute automated dip log results, which were subsequently verifi ed by manual picking. The signals were then used to generate a pseudo-resistivity image in or-



der to facilitate the interpretation by optical enhancement of the resistivity curves.

Within sections of slight to moderate dip and clear bed-ding contrast, the autodip routine produced reliable results. An increased amount of fractures decreases the quality of data, and it was in these zones where manual picking was used to trace bedding, fractures and fault planes in the bore-hole images.

The horizontal shear wave anisotropy (XMAC tool) is measured using the principle of shear wave splitting. The tool provides the magnitude and orientation of the fast and slow shear waves and quantifi es the presence, amount and orientation of formation anisotropy. This anisotropy may arise from intrinsic structural effects, such as fractures par-allel or inclined to the borehole, rock fabrics, or from une-qual stresses within the formation (Brie et al. 1998). These effects lead to horizontal differences in formation elastic properties, and if they are on a smaller scale than the sonic wave-length, then sonic wave propagation can be used to detect and quantify the anisotropy (Brie et al. 1998). There-fore, shear-wave anisotropy measurements are sensitive to anisotropic stress and fracture density and directions (Arm-strong et al. 1994).

3.3 Detection of deformation from borehole images

Different borehole image tools were applied to determine the orientation and evolution of the fracture- and fault inventory at the drilling location in the vicinity of the frontal Variscan Aachen-Midi Thrust. The acoustic and electric resistivity tools respond to different petrophysical properties and thus are sensitive to different fracture sets.

Acoustic images display the condition of the borehole wall because of differences in the acoustic slowness or am-plitude and are therefore sensitive to open fractures, faults and breakouts (Fig. 2). Analyses of these types of logs allow the interpretation of different types of fractures detected along the borehole wall (Zoback 2007): (i) sinusoidal struc-tures which might represent traces of inclined natural frac-tures or fault planes; (ii) drilling induced tensile wall frac-tures which form parallel to the maximum horizontal stress and propagate only a few centimetres into the wall (Brudy & Zoback 1999); (iii) breakouts which form parallel to the min-imum horizontal stress direction and result from compres-sive stresses at 90° to drilling induced tensile wall fractures (Fig. 2).

2300

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th [m

]

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0 90 180 270 360 0 90 180 270 360

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Fig. 2: Resistivity image (left) and acoustic am-plitude image (right) from the RWTH-1 borehole (dark colours = low resistive and low amplitudes, respectively). The typical example shows an open fault plane (F) at 2304 m, joints (J), drilling enhanced fractures (DENF), drilling induced ten-sile fractures (DIF) and compressive borehole breakouts (BO). Joints are pure tensile, com-pletely open fractures formed in direction of the maximum horizontal stress, while the drilling en-hanced fractures are pre-existing fractures that have been partially enhanced during the drilling process in the tensile quadrant. Drilling induced tensile fractures (DIF), that form parallel to the maximum horizontal stress at opposite sites of the borehole wall, are vertical in a normal or strike slip regime, while compressive borehole breakouts (BO) form in minimum horizontal stress direction.



The natural fracture and fault system which can be inter-preted in relation to in-situ stress include joints, drilling en-hanced fractures and faults (Fig. 2). Zones of increased de-formation are characterised by visible drag folds in image logs, increased anisotropy and anomalies in the caliper and/or velocity measurements.

Joints are pure tensile fractures following the direction of the maximum horizontal stress. In contrast to these subverti-cal, completely open fractures introduced or reactivated in the recently active stress fi eld, the drilling enhanced frac-tures are pre-existing fractures having formed in an earlier stress fi eld but trend nearly parallel to the recent maximum horizontal stress. This type of fractures is partially enhanced (conductive in the resistivity image) in the tensile quadrant of the borehole wall.

Potential fault planes can be detected by a clearly visible offset of bedding structures. Although the slip direction of fault planes is rarely visible on the image logs, a small-scale displacement of bedding planes can often be observed. Fault plane interpretation therefore has to be accompanied by in-dependent lines of evidence derived from other borehole measurements. For example faults are usually located in sec-tions with high fracture density, disturbed bedding (e.g. scat-tered or increased bedding dip), an enlarged caliper due to breakouts and washouts at the borehole wall, a decreased sonic velocity and an increased shear wave anisotropy.

Recently active faults possibly yield to local disturbances of the in-situ stress fi eld as could be observed by differing directions of drilling induced tensile fractures and borehole breakouts (Davatzes & Hickmann 2005, Zoback 2007). The abrupt disappearance of borehole failure next to a fault plane without changing rock properties can be attri-buted to a stress release at a recently active fault (Zoback 2007).

Completely closed or cemented fractures and faults are not observed in the acoustic image logs because the borehole wall is mechanically intact. Nevertheless, these structural planes can be observed in the resistivity image if a resistivity contrast of the fracture fi lling and the surrounding rock exists or in cases where the rock fabric changes or is disturbed (i.e. by a fault gouge). Sealed fractures are sometimes demar-cated by halos especially in the clay-rich Lower Devonian rocks. Fracture planes are highlighted as faults in the resis-tivity image if a displacement of bedding planes, increased bedding dip interpreted as drag folds or brecciated rock ma-terial can be observed.

Cleavage planes are picked in the resistivity image, where a more or less regular lineation of the rock is observed, which is clearly distinguishable from bedding planes. Whilst bedding planes are characterised by lithologic changes clearly demarcated by a resistivity variation especially in sil-iciclastic rocks, the cleavage planes occur as dark, regular spaced lines crosscutting the bedding planes.

In this study we applied these different tools to determine the orientation of the fracture and fault system in the foot-wall of the frontal Variscan Thrust.

4. Results

4.1 Lithological data

The lithology observed in the well was determined based on drill cuttings, core and log interpretation (i.e. natural gamma radiation, velocity, resistivity). The well penetrated a 20 m thick cover of Quaternary and Cretaceous sediments, under-lain by strongly compacted siliciclastic rocks (claystones, siltstones and sandstones) of Carboniferous (Pennsylvanian) and Devonian age (Fig. 3). Upper Carboniferous rocks (Pennsylvanian, 20–1012 m depth) consist of coal bearing siliciclastic rocks, similar to the one exposed in the Aachen coal mine district a few kilometres further to the north, and are dated as Namurian stages A–C and Westfalian A (Lun-dershausen et al. 2005, Ribbert 2006, Ribbert et al. 2008). Intercalated between the silty shales are thin packages of coarse-grained, quartz- or carbonate-cemented sandstone layers.

Upper Devonian (Famennian) lithologies were encoun-tered at 1012–1260 m depth consisting of grey, interbedded silt- and fi ne-grained sandstones (Condroz Formation). The Upper Famennian shales, from 1260 m to 1408 m, are grey shaly siltstones with interbedded, fi ne-grained, quartz- or carbonate-cemented thin sandstones. The section between 1408 m and 1438 m comprises two distinct carbonate layers, separated by a siliciclastic unit. The upper carbonate unit (1408–1413 m) is a grey Fe-rich dolomite, the interbedded siliciclastic unit comprises dolomitic silty shales and the sec-tion between 1428 m and 1438 m is a grey to light green-blue, nodular, partly fossil-rich limestone. Conodont data from the nodular limestone section indicate the Crepida Zone of the Upper Devonian Nehden stage (Piecha 2006). This formation is correlated with the Cheiloceras Limestones (Lower Famennian) exposed in the hanging wall of the Aachen Thrust and is analogous to layers in cores of the Bol-land and Soumagne boreholes located in Belgium to the west of Aachen close to the Aachen-Midi Thrust (Graulich 1975, 1977, Lögering 2008). At 1438 m the well penetrated a 2–3 m thick sequence of black, tectonically deformed shales (see also Fig. 6), followed by a sequence of red-green or grey, silty shales in the upper part (>1895 m) and grey, silty sandstones with intercalated thin layers of red, green and grey silty shales between 1895–2400 m (Fig. 3). Spores dat-able by micropalaeontology were found at 1489.75 m and suggest Lower Devonian age (Ribbert & Hartkopf-Fröder 2006). Below 2400 m the well intersected a quartz-cemented, micaceous grey sandstone. Spores retrieved from the 3rd core close to bottom hole most likely confi rm Lower Devo-nian age (Ribbert 2006). The fi nal section (2539–2544.5 m) of the borehole revealed a partly conglomeratic coarse-grained sandstone, possibly an equivalent of the Lower De-vonian (Pragian) “Upper Arkose” (Ribbert et al. 2008).

It should be noted from the above that Lower Carbonifer-ous (Mississippian) limestones and Middle Devonian strata, which both crop out in quarries in the vicinity of Aachen are completely absent in the RWTH-1 borehole.



4.2 Well log data

4.2.1 Caliper

The caliper log shows the variation of borehole diameter with depth (Cal in Fig. 4). The borehole was drilled with a 23 inch bit in the upper 240 m, 17.5 inch bit between 240–1263 m and 8.5 inch bit from 1263 m to fi nal depth. The caliper log corresponds in most borehole sections with the drilled diameter and correlates with the competency contrast of the encountered lithologies. It varies between 8.10 and 13.06 inches within the 8.5 inch section and 17.38–21.09 inches within the 17.5 inch section. In the uppermost part of the well, the enlarged caliper corresponds to washouts of or-ganic shales and coal beds. Wide calipers at 276 m, 299 m, 1090 m, 1595 m, 1895 m and 2425 m match with discontinu-ity surfaces observed in the image logs (see below). Below 1895 m the caliper log is continuous in the low porosity silt- and sandstones.

4.2.2 Natural gamma radiation

The gamma ray log (GR in Fig. 4) shows a rather uniform siliciclastic composition. Gamma radiation ranges from 15 API in the Lower Devonian quartzitic sandstones to 180 API in Upper Carboniferous organic shales. Coarsening upward cycles, which are typical for the Upper Carboniferous fl uvio-deltaic sequences in Central Europe (Elliot 1976), can be observed from 240–1012 m depth. This is interrupted how-ever between 400 m and 550 m where a fi ning upward cycle can be observed.

From 1012 m–1260 m the gamma ray log shows a rather chaotic signature, indicating a zone of thinly interbedded sandstones and shales (Famennian “Condroz”, Upper Devo-nian). It is underlain by a homogeneous zone of high gamma radiation from 1260–1408 m (Upper Famennian shales, Up-per Devonian). A small zone of low gamma radiation from 1408–1438 m characterises the limestones (Cheiloceras Limestones, Famennian, Upper Devonian), which were also observed in the fi rst core section.

Below 1438 m the rocks show again a high variation of gamma radiation indicating thinly interbedded siliciclastic rocks. This can be subdivided into a shale-dominated upper zone (1438–1895 m) and a more sandy lower zone (1895–2544 m).

500

1000

1500

2000

unconformity

1s

t cor

e se

ctio

n

2nd core section

14,80 m

20,00 m

1012 m

artificial aggradationPleistocene

hiatus

1438 m

1408 m

Car

boni

fero

usD

evon

ian

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er C

arbo

nife

rous

Upp

er D

evon

ian

Low

er D

evon

ian

(Con

droz

)

ChL

Upper Cretaceous (Hergenrath - beds)

hiatus

depth [m]

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(Nam

ur A

- C

+

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estfa

l A)

(Z

wei

fall

- bed

s)

UFS

Pra

gian

and

Em

sian

Fam

enni

an

3rd core section

2500 2544,5 m (final depth)

sandstone

siltstone

shale (green)

shale (red)

shale (grey) organic shale / coaldolomite

nodular limestone

Penn

sylv

ania

nUp

per

Lowe

r

Fig. 3: Lithostratigraphic log of the RWTH-1 well (modifi ed from Ribbert 2006). Three core sections were cut at 1392–1515 m, 2128–2142 m and 2536–2544 m depth (yellow squares on the right hand side of the lithocolumn). The Mississippian “Kohlenkalk” and Mid-dle Devonian strata are absent at the borehole location (see discus-sion, Chap. 6.2).UFS = Upper Fammenian Shales, ChL = Cheiloceras Limestone.



DipBedding

17U

pper

Car

boni

fero

usU

pper

Dev

onia

nLo

wer

Dev

onia

nA

ge Depth[m]

Cal [inch] GR[GAPI]

DTS[μs/f]

ANIA[%]

Bedding[degree]

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2100

2300

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20

50 150 95145 5 15 25 657 10

< 1260 m

> 1260 m

zone

s of

hig

hde

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atio

n

Fig. 4: Composite log of the borehole caliper (Cal), natural gamma radiation (GR), slowness of shear waves (DTS), the averaged horizon-tal shear wave velocity anisotropy (ANIA), the bedding dip from auto correlation of hexagonal dipmeter data and the resistivity image as tadpole profi le (autodip). Note that the tadpole indicates the dip and the tail of the tadpole shows the dip direction. Stereographic projection (lower hemisphere) of poles to bedding planes generally show NE–SW-striking beds in the upper 1260 m and slightly NE-dipping beds below 1520 m.Logging data clearly show “high deformation zones” at 240–327 m, 368–534 m, 585–719 m, 758–795 m, 825–900 m, 915–985 m, 1028–1034 m, 1080–1115 m, 1268–1620 m, 1872–1950 m, 2240–2335 m and 2402–2428 m depth by increased or scattered dip data, anisotropy peaks and anomalies in the caliper and/or velocity measurements.



4.2.3 Shear wave velocity and horizontal shear wave anisotropy

The travel time of shear waves (DTS in fi gure 4, reciprocal of the shear wave velocity, measured in microseconds per foot [ft] with the XMAC tool) ranges from 170 μs/ft in the uppermost part of the borehole to 80 μs/ft in the quartzitic sandstones of the lower borehole section. The acoustic travel times (compressional and shear waves) and the reciprocal velocities are a measure of rock mechanical properties and lithology, if the infl uence of rock porosity can be neglected. In the RWTH-1 well, rock cuttings show no visual porosity and core samples measured by helium expansion technique reveal porosities below 0.7 %. Rock mechanical laboratory tests on core plugs indicate a brittle behaviour even of the mudstones. The unconfi ned compressive strength reaches 336 MPa for the quartzitic sandstone at bottom hole (Traut-wein-Bruns et al. 2010). Thus, porosity and pore fl uids are unlikely to infl uence sonic velocities.

Similar to the gamma radiation, the upper part of the DTS log is characterised by a cyclic v ariation of shear wave velocities down to 1012 m depth. This is followed by a zone of high shear wave velocities at 1012–1438 m depth, which refl ects the increased carbonate content in rock cements. An increased amount of dolomite content is also confi rmed by XRD measurements (Becker 2008). The two limestone lay-ers can be clearly distinguished between 1408–1413 m and 1428–1438 m.

Below 1438 m the shear wave shows strong variations at a constant high velocity level indicating a small-scale inter-calation of strongly compacted mudstones, siltstones and sandstones. Overall, the shear wave velocities increase downwards due to more competent rocks. Anomalously long travel times are observed at 275 m, 300 m, 1595 m, 1895 m and 2425 m and match with an increased caliper as well as with open fractures in the image and pseudo-image logs. The compressional and shear wave velocities are described in more detail by Pechnig (2005).

The mean velocity anisotropy (ANIA in Fig. 4) of the slow and the fast horizontal shear wave reaches 20 % in the upper part of the borehole and decreases with depth. Up to 15 % velocity anisotropy is observed below 1263 m. In the RWTH-1 borehole, zones of high anisotropy are caused by high bedding dip and/or a high fracture density, thus the hor-izontal shear wave anisotropy is rather structural than stress induced. We observed a zone of high anisotropy in the up-permost part of the well down to 505 m depth. This corre-lates well with the inverse gamma ray trend (fi ning-up se-quence) and steeply dipping bedding planes observed in the dipmeter log. Further peaks of high velocity anisotropy are clearly visible at 533 m, 624 m, 670–850 m, 1085 m and 1300–1430 m. Below 1440 m the anisotropy level is quite low with the exception of a couple of distinct peaks at 1595 m, 1709 m, 1895 m, 2115 m, in a zone between 2240 and 2335 m and at 2425 m, which all correlate with zones of disturbed or increased bedding dip.

5. Interpretation of RWTH-1 borehole data

The drilled section can roughly be subdivided into a gener-ally SE-dipping upper part (above 1520 m) and a gently NE-dipping lower part (dip


A B C

0° 360°180° 360°180° 0° 360°180° 360°180° 0° 360°180° 360°180° 0° 360°0° 360°0° 360°

D E

0° 360°180° 360°180° 0° 360° 0° 360°180° 360°180° 0° 360°

no c

ore

sect

ion

no c

ore

sect

ion

Fig. 5: Examples of faults shown by the resistivity image log (left, dark colours = low resistive), the acoustic amplitude image (middle, dark colours = low amplitudes) and the 360° core scan (right). The quality of the acoustic image is reduced due to a groove pipe scratched on the downhole side of the borehole (vertical dark and light traces).(A) Thrust fault (1404 m): Bedding dip is increased close to the thrust fault (yellow arrow, 186°/25°), clearly displayed by the resistivity image. Cleav-age and fractures are abundant in the whole section (1403.6–1405.9 m). (B) Partly open fault zone (1454 m): The strongly deformed zone between 1453.4–1455.7 m is characterised by disturbed bedding and several fractures of different orienta-tion displayed by the resistivity image. A partly open fracture (yellow arrow) is displayed by the acoustic amplitude image at 1454.3 m (036°/69°). The core is strongly broken within this section and shows several fractures with different orientations and basically down-dipping slickensides. (C) Fault (1516 m): The resistivity image shows a strongly faulted interval between 1514.0–1516.3 m. A pseudotachylith was observed in thin-sections from this fault zone (Scholz et al. 2006, Lögering 2008). As fractures and faults are completely closed, no features are detected by the acoustic amplitude image. (D) Fault (1709 m): The resistiv-

ity image shows a clear lithology change from siltstones to sandstones at the fault plane which strikes NW–SE and dips to the SW (217/69). Breakouts and stronger wearing in the lower competent siltstones especially close to the fault plane within the 20 cm broad deformation zone map the fault in the acoustic amplitude image, but the fault itself is completely closed (1706–1711 m). (E) Fault (1893–1898 m): The most prominent fault zone in the lower borehole section strikes W–E (184/70) and is characterised by an increased caliper due to open fractures and breakouts. A high shear wave velocity anisotropy and anomalously long sonic travel times also highlight the fault. Varying directions of breakouts and drilling induced tensile fractures indicate recent slip.



1440

1436

1437

1438

1439

1441

1442

1443

1444

Up

per

Dev

on

ian

Lo

we

r D

ev

on

ian

Age0° 180° 360° 0° 180° 360° 0° 180° 360°

Depth[m]

Resistivity Image Amplitude Image 360˚- Core Scan

Fig. 6: The section including the litho-stratigraphic gap between Upper and Lower Devonian strata is displayed in the resistivity image, the acoustic amplitude image and in the 360° cores scan. The in-tensively fractured and slated organic-rich shales below 1439.5 m (partly bro-ken and therefore not preserved by the core scan) mark the lithostratigraphic gap in the core. Open fractures at 1439.5 m and 1443.5 m depth (46°/55°) indicate potential normal faults.

rock colour. The general bedding dip is 30–50° to SE. Sev-eral intervals with increased, sometimes changing bedding dips are observed, which represent small-scale folding and faulting. Very typical for this section, as well as for the sec-tion covered by the image logs above, are zones of steeply dipping beds sheared-off on small scale fault planes and the abundance of cleavage (Fig. 5A). These structures are in-terpreted as drag folds and together with the features de-scribed above from petrographic and geochemical analysis provide strong evidence for the occurrence of a thrust fold zone intersecting the borehole section between 1269–1452 m.

Within the argillaceous and silty siliciclastic rocks below 1440 m bedding is completely overprinted by tectonic defor-mation (Fig. 5). The few bedding planes, which are detecta-ble, show strongly varying dip angles between 10° and 30° and dip directions to the NNE–SE.

Cleavage is observed in the clay-rich to silty rocks espe-cially in high strain zones between 1390–1455 m and 1490–1512 m depth. In parts, a second cleavage is oriented oblique to S1 indicating shear deformation (Sindern et al. 2007).

Interfaces with slickensides were picked from the cores and were oriented in comparison to the resistivity image in order to indicate the relative movement within the deformed fi rst core section (Fig. 7). Slickensides with dominantly up-dipping movement appear at 1390–1452 m, 1463–1467 m and 1507–1516 m. The associated planes strike ENE to WNW and mainly dip with 20 to 45° to the southeast (Fig. 7: left panel). In particular in the uppermost part the slickensides are ob-served on the SE-dipping bedding planes indicating that the stress has been released by sliding on bedding planes.

The orientations of planes with down-dipping move-ment are more scattered due to the complex tectonic his-tory. These fractures mostly occur below 1440 m (Fig. 7: right panel).

The core intervals from 2128–2143 m and 2537–2545 m represent an undisturbed stratigraphic succession of Lower Devonian rocks, which appear unaffected or only weakly overprinted by Variscan deformation and veining (Sindern et al. 2010). Lögering (2008) emphasises the similarity of a conglomeratic layer close to the fi nal depth with a conglom-erate in the borehole Bolland where it is interpreted to repre-sent the base of the Devonian strata.

5.2 Structural interpretation from log data

5.2.1 Hexagonal dipmeter log (260–1260 m)

From 260–405 m bedding is characterised by a constant gently dip (


ESE to NNW) indicate open fractures or faults. Similarly, more intervals with low-resistive fractures and disturbed bedding as well as smaller anisotropy peaks are observed be-tween 369 m and 397 m (Table 1, Fig. 4).

The interval from 405–740 m is characterised by more scattered to random dips with small scale changes between fl at to steep dips (Fig. 4). The steep SE-dip from 410–530 m is interpreted as the overturned NW-limb of the Aachen An-ticline (Fig. 1) supported by the inverse GR trend in the in-terval described above. Shear wave anisotropy is high in this part and refl ects the steeply dipping beds. At the top of the overturned strata between 405 and 430 m a (Variscan?) fault (Table 1) is assumed. An individual fault plane could not be picked because of the overall low quality and disturbed bed-ding dip in this section, but is proposed at a depth of 420 m. At this position the peak dip trend occurs, it is marked by a variation in the azimuth of the fast shear wave, a change in the anisotropy level (high in the overturned section) and a change from generally wider to smaller caliper without an observable change in lithology (Fig. 4). The suggested fault is positioned between two sections of coals/organic shale layers which might be doubled by thrusting.

At 534 m depth the bedding dip changes from steeply SE to moderately NE- and N-dipping beds (Fig. 4). Here the core of the overturned fold structure is proposed, also marked by a peak in shear wave velocity anisotropy. At 675 m the general bedding dip changes again from N to SE marked by the core of another fold structure. Again a peak of formation anisotropy is observed at that depth (Fig. 4). Fracture zones in this interval emerge at 517 m, 561–568 m and 585–600 m (Table 1).

The interval between 740 and 1260 m is characterised by a constant SE dip between 10 and 30° (Fig. 4). Several sections of disturbed and increased dip angles occur that correlate with peaks in shear wave velocity anisotropy, e.g. at 709–717 m, 792–796 m and 1085–1098 m (Fig. 4). For the fi rst two sections, sedimentary reasons (e.g. cross-bed-ding) are excluded and thrust-related drag folds are inter-preted, similar to those observed within the fi rst core sec-tion and in the resistivity image. Within the interval 1085–1098 m one partly open fracture is documented by a “technical” core sample provided by a reverse circulation tool during a hole cleaning operation at 1093 m (Table 1). Several zones of disturbed bedding and potential open frac-

lithologicalprofile

organic shale / coal

sandstone siltstone shale (grey)

dolomite nodular limestone

shaly siltstone(grey-green)

shaly siltstone(red-green)

up-dipping slickensides

0˚ 90˚45˚ 0˚ 90˚45˚

planes with slickensidesindicating up-dip movement

planes with slickensidesindicating down-dip movement

planes with slickensidesindicating up-dip movement

planes with slickensidesindicating down-dip movement

down-dipping slickensides

UF

SC

heilo

cera

sLi

mes

tone

Pra

gian

and

Em

sian

(Zw

eifa

ll -

beds

)

Upp

er D

evon

ian

Low

er D

evon

ian

N

E

S

W

N

E

S

W

agedepth

[m]

1410

1430

1450

1470

1490

1510

1410

1430

1450

1470

1490

1510

Fig. 7: The fi rst core section is strongly tectonically deformed, indicated by numerous fractures, cleavage and many slickensides. The Up-per Devonian strata above 1438 m show preferentially slickensides indicating an up-dipping movement, whereas the Lower Devonian strata below 1438 m show preferentially slickensides indicating a down-dipping movement.UFS = Upper Fammenian Shales.



Table 1: Potential faults interpreted from the resistivity pseudo image (hexagonal dipmeter data) and in the downhole measurements of the caliper, sonic velocity and shear wave velocity anisotropy. “+” and “–” indicate if the fault was detected or not detected in the respective log. The lithology was derived from gamma radiation.sst = sandstone, ust = siltstone, tst = claystone. DF = assumed drag fold, LRF = low resistive fracturesloggers depth[m]

lithology caliper velocity anisotropy bedding dip comment

260 ust – – + + DF273 tst – – + + DF276 sst/ust + + + + LRF299 sst + + + + LRF317 ust + – + + LRF377 ust – – + + LRF383 ust – – + + LRF388 ust – – + + LRF397 ust – – + + LRF420 (405–430) coal, organic shale + – + + 517 ust – + – + LRF559–565 ust – – – + LRF585–600 ust – – (+) + LRF625 tst + – – + LRF660 ust (+) – (+) + LRF681 sst/ust (+) – + + 706 tst – – – + 707–717 tst – – + + DF731 tst + – – + 771 tst/coal + + – + 788 tst/coal + + – + 794 tst/sst + – + + DF915.5–918.5 ust/tst – – – + LRF920.5–922 ust – – – + LRF935–938.5 ust – – – + DF at 935.0,

937.0 & 938.5942–961.5 ust/tst – – – + LRF963.5–979.5 tst/ust – – – + LRF981–984.5 ust/tst – – – + LRF1009 tst – – – + 1030 (1028–1034) tst/ust – – – + 1080–1082.5 ust + + + + technical core1085–1093 sst – – – + 1159–1161 ust (+) + + + LRF

tures from 915–1035 m and at 1159–1161 m indicate pos-sible fractures or faults. A list of all interpreted faults from the upper borehole section (260–1260 m) is provided in table 1.

5.2.2 Image logs (1260–2520 m)

5.2.2.1 Bedding and cleavage

The upper part of the section (1260–1520 m), which includes the fi rst core section, is again characterised by partly scattered and strongly varying dips. An increased velocity anisotropy



and a high fracture density suggest folding and faulting (Figs. 4 and 8). Zones of increased bedding dips are interpreted as fault related folds along thrust faults and correlate with the peaks of high shear wave anisotropy (Fig. 8).

Bedding planes below 1520 m are slightly dipping to the NE with a mean dip of 10.8° (median 9.8°). Increased dip angles of bedding planes up to 30° can be locally observed, i.e. at 1575–1593 m, 2114–2149 m and at 2218–2358 m (Fig. 8). Both the low and constant dips in the lower borehole sec-tion as well as the quite small shear wave velocity anisotropy level suggest that folding is restricted to the upper 1500 m of the borehole.

Cleavage planes are detected in the resistivity image log at 1263–1440 m, 1515–1585 m and 2250–2300 m depth. Cleavage (foliation) planes strike WSW–ENE with S to

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1:5000METRES MD

CALIN

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%0 15

ANIA 900 AUTODIP_5M_15:DIPCOMPUTED >.590 >.60

OTHER >.590 >.60

SUM_C0 35

SUM_C

SUM_C0 35

SUM_C 0 FAULTS_AUTODIP_F:DIP

C3STAR >.30 >.60

S3FAULT >.30 >.60

BRKAZIDEG

0 360

TNSAZIDEG

0 360

Depth

[m] GR

[GAPI]DTS[μs/f]

ANIA[%]

Fractures[per 1m]

Open

Cal[inch]

OrientationBreakouts

and Induced Fractures Closed

Faults(degree)

25 6550 150 115 65 4 11 5 15 2 4 8 10 90 360

=Dip Direction (in this case 45∞ = NE) Closed Fault= Open Fault= Induced Fracture= Breakout=

Bedding(degree)

25 65

Low

er D

evon

ian

Uppe

rDe

voni

anA

ge

Fig. 8: Composite log of structural results from the borehole image analysis in the deeper RWTH-1 borehole below 1263 m including the number of closed fractures (from the resistivity image), superfi cially open fractures (from the acoustic amplitude image), closed and open faults (white and black tad poles, respectively) and the orientation of stress induced borehole failure, e.g. compressive borehole breakouts (triangles) as well as drilling induced tensile fractures (circles). Note that the direction of the minimum horizontal stress is indicated by the orientation of the breakouts (NE–SW), whereas the direction of the maximum horizontal stress is indicated by orientation of the drilling induced fractures (SE–NW; last column). Variations in the direction of borehole failure can be caused by stress perturbation due to slip on recently active faults. The number of closed fractures is high at 1268–1590 m and decreases downhole. Note that no open fractures could be derived from acoustic borehole images between 1268–1390 m because of the bad image quality due to technical reasons (see 3.1). See text for details.GR = natural gamma radiation, DTS = shear wave slowness, ANIA = averaged horizontal shear wave velocity anisotropy, Cal = borehole caliper.

SSE-dipping planes. Their 30–50° dip correlates well with the NW-verging Variscan folds, whilst the second maximum of subhorizontal planes can be explained by a synsedimen-tary shale fabric subparallel to bedding.

5.2.2.2 Borehole failure

The orientation of compressive borehole breakouts and drill-ing induced tensile fractures, investigated in the image logs between 1268–2528 m, displays the orientation of minimum and maximum horizontal stress along the RWTH-1 well (Fig. 8: last track). Compressive borehole breakouts are formed in the direction of minimum horizontal stress and are oriented perpendicular (90°) to the drilling induced tensile fractures. Data show a NW–SE directed maximum horizon-



tal stress, which is in accordance with the recent stress fi eld in the region (Müller et al. 1997, Heidbach et al. 2008, Hinzen 2003).

A change in the azimuth of induced fractures and bore-hole breakouts indicates disturbances in the recently active stress fi eld, which can be caused by recent slip on active faults (Zoback 2007). Prominent sections displaying varia-ble orientations of stress induced borehole failures are visi-ble at depths of 1895 m, 2115 m and 2304 m depth. These depths match with anomalies observed in the caliper and anisotropy logs and with faults detected on the acoustic im-age log (Fig. 8).

Two sections with only a low number of borehole failures are visible between 1890 m and 2163 m as well as below 2382 m, although the bulk petrophysical properties (GR, ve-locity, resistivity) do not indicate any signifi cant rock strength variations. These two zones maybe attributed to stress release at fault zones rather than to a lithological change.

Breakouts from 1268–1440 m are rotated by approxi-mately 20° anticlockwise (Fig. 8). The acoustic image deliv-ers no clear data in this section due to the borehole shape (see 3.1), thus open fault planes cannot be traced. However, the

breakouts in this section are assumed to be caused by steeply dipping bedding planes and high fracture and cleavage den-sity, based on log analysis, and thus do not refl ect the orienta-tion of the principal stresses in this section.

The borehole failure analysis suggests in-situ minimum and maximum horizontal stress gradients of 0.02 MPa/m and 0.04 MPa/m, respectively in a strike-slip faulting regime with the maximum principal stress oriented subhorizontal at 126º ± 15° (NW–SE; Trautwein-Bruns et al. 2010; Fig. 9).

5.2.2.3 Natural fractures

Natural fractures have been picked using electrical (STAR) and acoustic (CBIL) image logs (Fig. 8). While fractures de-tected by the electrical tool are indicative of a closed fracture system, it is the acoustic log, which detects fractures which are superfi cially open at the borehole wall (Davatzes & Hickman 2005). The number of closed fractures is high be-tween 1268 and 1590 m depth with up to 16 picked fractures per metre (5 fractures per metre on average) and decreases downhole. The open fractures become dominant below 1590 m with up to 7 fractures per metre (Fig. 8). No open

Borehole Breakouts

Drilling Induced FracturesB

1

3

1

A

Width

45° (+/- 14°)

44° (+/- 19°)

119° (+/- 12°)

Azimuth [degree] Azimuth [degree]

Width [degree]

45 90 135 180 0 45 90 135 180

45 90 135 180

num

ber

00

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00

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num

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2 2Orientation Orientation

0

Orientation

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Width

Borehole

45°

44°

119°

+/- 14° +/- 12°Deviation

DrillingInduced

FracturesBreakoutsFig. 9: The analysis of stress induced borehole failure in the image logs below 1263 m displays the orientation of the re-cent in-situ horizontal stress. (A) Compressive borehole breakouts form in direction of minimum horizontal stress at 45° ± 14° and have a mean width (defi ned by the angle spanned by the breakout zone at the borehole wall) of 44° ± 19°. (B) Drilling induced tensile fractures form in direction of maximum horizontal stress and have a mean trend of 119° ± 12°



fractures could be retrieved from the acoustic borehole im-ages between 1268 and 1390 m because of the bad image quality due to the borehole shape (see 3.1).

The open fractures show two prominent maxima, one set striking NW–SE with a majority of dips towards the NE, and the second set striking NNE–SSW with a majority of dips towards the ESE (Fig. 10A). The subvertical frac-tures (joints) show a strike-azimuth of 133° (Fig. 10C) and follow the direction of maximum horizontal stress in the well as determined by drilling induced tensile fractures and compressive borehole breakouts (i.e. the minimum hori-zontal stress plus 90°). Most of these mode I fractures show high dip angles (74 and 88° to NE). The mean trend of the drilling enhanced fractures is 123° and is slightly shifted anticlockwise compared to the mean trend of the joints (Fig. 10D).

A large amount of closed fractures on electrical images has been detected in the resistivity image and is strongly scattered over the whole dip and azimuth range (Fig. 11A). The dominant strike trend is NE–SW. Generally, high frac-ture densities correlate with peaks of high shear wave aniso-tropy and variations of dip data, and are thus interpreted to refl ect deformation zones (Fig. 8).

5.2.2.4 Faults

Several faults can be traced along the borehole below 1260 m following the criteria described in 3.3 (Fig. 8). A list of the faults of the lower borehole part (1260–2520 m) is given in table 2. Two fault types can be discerned, one with shallower dip angles and apparently completely closed fault planes (Fig. 11B), and one with steeper dip angles corresponding to potentially open fault systems (Fig. 10B).

The closed-fault system is typically located in zones of high fracture density in the resistivity image log, scattered or increased bedding dip and associated with peaks in the shear wave velocity anisotropy associated with drag folds. A thrust-related fault zone has been cored in the fi rst core sec-tion which reveals typical drag folds (see Fig. 5A). The steeply dipping bedding planes and the observed cleavage can clearly be picked from the resistivity image log between 1260 and 1440 m (within the cored interval and above).The increased dip of bedding is always associated with a domi-nant peak in the shear wave velocity anisotropy log. Similar patterns can be found in the 17 ½ inch section (260–1260 m), where only HDIP data are available, at 291–300 m, 709–719 m and 789–792 m (Tab. 1).

Below 1440 m the deformation style changes, maybe due to the lower competence associated with the high clay con-

Open Fractures

Joints

Open Faults

Drilling Enhanced Fractures

N

S

EW

S

EW

N

S

EW

N

S

EW

A B

C Dn = 268 n = 12

n = 210n = 19

N

Fig. 10: Stereographic projections of open struc-tures picked in the acoustic amplitude image.(A) Open fractures show two prominent trends striking NW–SE and NNE–SSW. The majority of the NW–SE-trending fractures dip towards NE, whilst the NNE–SSW fracture dip preferen-tially towards ESE. (B) Open faults trend NW–SE and ENE–WSW to ESE–WNW. Varying di-rections of breakouts and drilling induced tensile fractures, which are assumed to indicate a poten-tial stress disturbance have been observed at 1893 m, 2115 m and 2304 m depth and indicate recent activity of these faults. (C) Open joints are subvertical fractures with NW–SE direction. (D) The mean strike of drilling induced fractures is WNW–ESE (maximum = 119° ± 12°).



tent (between 1440–1895 m) and the generally diminishing deformation level (below 1668 m).

The faults between 1454 and 1635 m (Table 2) are lo-cated within a strongly deformed borehole section, but here it is diffi cult to separate and defi ne fault planes from the abundant fractures and microfaults.

One prominent fault is observed in the resistivity image at 1709 m (Table 2, Fig. 5D). It is demarcated by a clear li-thology change from siltstones to sandstones and varying bedding dips close to the structure. It strikes NW–SE and dips towards the SW parallel to the extensive faults of the Lower Rhine Embayment. However, this fault orientation is less abundant in the deeper borehole section (below 1260 m; Fig. 11B).

The increased bedding dips at 2210–2300 m and 2460–2500 m in combination with the detection of cleavage in the resistivity image are also attributed to Variscan tecton-ics. Several potential fault planes are detected within this section at 2240 m, 2300 m, 2468 m and 2500 m depth (Ta-ble 2). Generally lower importance is assumed for the faults below 1668 m, because deformation zones are smaller and less apparent and fault related folding is almost not devel-oped.

The closed faults have a prominent NE–SW to ENE–WSW strike (Fig. 11B), which is in general concordance with the Variscan thrusts and reverse faults known from many fi eld and coal mining observations. The majority of these structures dip 30–60° towards the SSE, whilst some dip towards the NNW (Figs. 8, 11). These trends can be correlated with Varis-can thrusts and backthrusts, respectively. This is in line with the SE-dipping zone of Variscan imbricates (Graulich 1984, Wrede 1985, Muchez & Sintubin 1998).

The open fault planes detected by the acoustic log dis-play dips of 53–76° (Fig. 10B). Most faults strike NW–SE and dip to the NE, i.e. at 1595 m, 1764 m, 1798 m, 2303 m and 2424 m depth. A few faults detected in acoustic images (CBIL) strike W–E and dip to the S, i.e. at 1636 m, 1895 m,

1920 m and 2115 m depth. The E–W strike corresponds to the direction of maximum dextral shear stress within the cur-rent stress fi eld.

Recently active faults can be identifi ed among potentially open faults by a variation of orientations of stress induced borehole failure (breakouts and induced fractures). Varying directions of breakouts and drilling induced tensile fractures, which are assumed to indicate a potential stress disturbance, have been observed at 1895 m, 2115 m and 2304 m depth. These depths match with faults detected on the acoustic im-age log, and are thus interpreted to represent recently active faults.

Two sections with only a low number of borehole fail-ures are visible between 1890–2163 m as well as below 2382 m, although the bulk petrophysical properties (GR, ve-locity, resistivity) do not indicate any signifi cant rock strength variations (Fig. 8). These two zones maybe attrib-uted to stress release at fault zones rather than to a litho-logical change.

The most prominent W–E-striking and southward dip-ping fault zone is observed at 1893–1898 m depth (Fig. 5E), and is characterised by an increased caliper, open fractures, increased anisotropy and reduced sonic velocity (see Fig. 8). This fault also marks the boundary between an upper clay-dominated and a lower sand-dominated part of the Lower Devonian as well as the change in the dominant rock colours from red-green to grey.

Despite the fact that numerous superfi cially open faults and fractures are inferred from the acoustic image log, fl uid and pressure testing in the well using the Reservoir Charac-terization Instrument (RCI – Baker Atlas™) failed to retrieve formation fl uids. More than 50 positions from 1268–2528 m were tested. The pressure build-up indicated a permeability of 0.8 darcy for a fractured interval between 1593 and 1601 m (1595 m fault).

Closed Fractures Closed Faults CleavageN

S

EW

N

S

EW

N

S

EW

A B C

n = 2046 n = 204 n = 629

1709 m

Fig. 11: Stereographic projections of the closed interfaces mapped in the resistivity images.(A) The closed fractures are strongly scattered over the whole dip and azimuth range and roughly show a dominant NE–SW trend. (B) Closed faults strike NE–SW to WSW–ENE. Fault planes with clear offset of bedding plane (black dots) are marked in fi gure 8 (white tad poles). The 1709 m fault occurs as prominent structural discontinuity in the resistivity image; it is demarcated by a clear lithology change from siltstones to sandstones and varying bedding dips close to the structure. (C) Cleavage planes are oriented WSW–ENE and slightly oblique to the closed faults shown in (B).



Table 2: Faults observed in the resistivity image (STAR), acoustic amplitude image (CBIL) and in the downhole measurements of the caliper, sonic velocity and shear wave velocity anisotropy. Rotated borehole breakouts (BO) and induced fractures (IF) are potential indica-tion for stress perturbation due to recent slip on faults. The lithology was derived from gamma radiation.sst = sandstone, ust = siltstone, tst = claystone. LQ = low quality, DF = assumed drag fold, LRF = low resistive fractures, FG = fault gauge.

loggersdepth [m]

dip direction

STAR CBIL lithology caliper velocity aniso-tropy

bedding dip

rotated BO/IF

comment

1269 SE + LQ ust/sst LQ – – + – DF1280 SE + LQ ust/sst LQ – (+) + – DF1306 SE + LQ ust/sst LQ – + + – DF1335 SSE + LQ ust LQ – + + – DF1379 SE + LQ ust LQ – + + – DF1404 SSE + – tst/sst – – + + – DF1437 SE + – carbonate – – – + – DF1454 ENE – + tst/sst – + – + – core1516 SE + – tst/sst – – – + – core, pseudotachylith1546 NW + – tst – – – + – 1583 SE + – ust/tst – – – + (+) 1595 NE – + sst + + + + – reservoir characterisation

instrument1635 S + – ust>sst – – – (+) (+) BO–rotation not

stress induced1636 S – + ust – – – (+) (+) unconformity?

BO–rotation not stress induced

1709 SW + (+) ust>sst – – – + – F1762 NE – + tst/ust – – – + – FG1798 NE – + tst/ust – (+) – (+) (+) 2 m broad zone1801 SE + – tst/ust – – – + – 1895 S – + sst + + + + +

Faul

t zo

ne

1920 SE – + sst – – – + +1932 NE – + tst – (+) – + +2012 SE + – sst – + – + – 2061 SE + – sst – – – + – 2115 SSW – + sst – (+) + + + 2158 NE – + sst – – – + –2240 SE + – ust – – + + – DF?2300 SE + – ust – – + + – DF?2304 NNE – + (t)ust – – + + + 2402 N + – tst – – – + + (lack) 2414 NW? + – tst – – – + – FG2425 N – + tst/ust + (+) + + + (lack) 2468 SE + – ust/sst – – – + – 2489 NE – + sst – – – (+) – 2500 SE + – ust/sst – – – + –



6. Integration of results into regional setting

6.1 Structural data

Aachen is located in a structural “triple junction” between the Brabant Massif, the Ardennes-Rhenish Massif and the Lower Rhine Embayment. Major crustal weakness zones can be expected to separate these units against each other. The Brabant Massif and the Ardennes-Rhenish Massif are delim-ited by the Belgium (Brabant) dextral shear zone (Ahorner 1975, Leynaud et al. 2000), while the SW border of the Lower Rhine Embayment is marked by the faults of the Roer Valley Graben System (e.g. Feldbiss Fault). The latter com-pensates the antipodal movement of uplift in the Ardennes-Rhenish Massif against subsidence of the Lower Rhine Em-bayment and is characterised by normal faulting activity (Camelbeeck et al. 2007).

The case has been made above, that the structural data image both the NE–SW oriented Variscan strike trend and the NW–SE trend of the Lower Rhine Embayment faults. The analysis of borehole data indicates however the presence of additional structural fault and fracture systems.

The Variscan Thrust Front is characterised by regional-scale synclines, anticlines and thrust faults striking NE–SW related to the Variscan deformation (Kenis et al. 2000, Muchez & Sintubin 1998). Wrede (1985) describes an inten-sively imbricated structure for the southern Wurm Syncline (Variscan foreland north of Aachen), associated with many thrust faults of different displacement.

More important thrust faults in the borehole are inter-preted from geophysical borehole and core data at depths of 291–300 m, 709–792 m and 1260–1440 m, all of which are characterised by potential drag folds in the resistivity and pseudo-resistivity image. Below 1520 m, low-angle NE–SW-trending faults are observed as well, but have caused more moderate changes of bedding dips, e.g. at 2240–2300 m, and thus seem to have minor importance. Sindern et al. (2007, 2008) observed geochemical anomalies due to hy-drothermal fl uid-fl ow along faults within the fi rst core sec-tion between 1450 and 1484 m and in cuttings at 1096 m, 1936 m and between 2269–2275 m.

The bedding dips confi rm the absence of the prominent NE–SW Variscan strike below 1520 m (Figs. 4 and 8) where the strike direction changes downhole towards NW–SE. Folding and the more important thrust faults are observed above this depth. In addition the amount of closed fractures observed in the resistivity image log, as well as the shear wave velocity anisotropy decreases with depth indicating that the intensity of Variscan deformation diminishes with distance from the Aachen-Midi Thrust.

All fractures in the resistivity image are cemented and were therefore assumed to be inactive in the recently active stress fi eld. The strongly varying dips display the complex geodynamic history of the location. The general trend fol-lows the Variscan NE–SW-striking structures, but fractures striking parallel to post-Variscan trends are also included and cannot be separated by the image analysis.

Using the acoustic image logs helped to detect fractures, which are open along the borehole walls but gives no infor-mation about the three-dimensional extension of the frac-tures or about their age of formation.

NW–SE-trending joints and faults are in line with the maximum horizontal stress as indicated by borehole failure analysis (Trautwein-Bruns et al. 2010). These faults basi-cally dip to NE in accordance to the SW border faults of the Roer Valley Graben System and are probably active since the Permian and reactivated several times during the Meso-Cenozoic (Colbeaux et al. 1977, Wolf 1985, Laloux et al. 1997).

Wrede (1985) postulates a development of this fault set as cross (ac) faults to the major Variscan fold structures, which are frequently reactivated and overprinted by post-Variscan tectonics. The cross (NW–SE-trending) faults dom-inate the fault pattern observed in the Wurm coal mining area north of Aachen, and are basically developed as normal faults (Wrede 1985). Frequently, WNW–ESE-trending branches or transfer faults split off, which partly show con-siderable horizontal displacement. NNW–SSE oriented transverse faults cross-cut the Verviers Synclinorium and are related to the formation of the Roer Valley Graben System, originated during the Permian and were repeatedly reacti-vated in the Mesozoic and the Cenozoic age (Demoulin 1988, Laloux et al. 1997, Schroyen & Muchez 2000).

A dextral (W–E) strike slip fault system in the Variscan Front Complex is investigated by Muchez & Sintubin (1998) further to the west in Belgium. Such W–E trend of faults is also displayed by the acoustic image log from the RWTH-1 borehole, e.g. at 1893 m. Since varying orienta-tions of borehole failure breakouts in the proximity of these faults are interpreted as the result of stress perturbations due to recent slip, we conclude that W–E-trending strike-slip faults have been recently active in the deeper subsur-face. This is confi rmed by the borehole failure analysis per-formed by Trautwein-Bruns et al. (2010), which supports a strike-slip faulting regime for the borehole location below 1440 m.

However, the strike of open fractures observed in acous-tic images (Fig. 10) in the borehole not only show a NW–SE strike being in line with the in-situ stress fi eld, but also a prominent second NNE–SSW trend, which was previously unexpected because it is not within the sector of shear defor-mation (± 30° apart from the direction of maximum horizon-tal stress).

The NNE–SSW-striking fractures occur in highly com-petent sandstones. The steeply dipping, open or partly open fractures show no displacement of bedding planes and are therefore assumed to be pure tensile fractures (Mode I, joints). According to theory, joints should strike parallel to the direction of the maximum compressive stress and should be open in direction of minimum principal stress. Therefore recent joints in the Aachen borehole should trend NW–SE as described above. If the NNE–SSW directed fractures are joints, then these should be initiated under NNE directed compression and may indicate a subrecent anticlockwise ro-tation in horizontal stress orientations.



Stress rotation has been reported for the Tertiary in the area, as indicated by migrating sedimentary depocentres (e.g. Van den Berg 1994) and palaeomagnetic measurements (Schreiber & Rotsch 1998). Since the early Oligocene the ongoing extension of the Lower Rhine Embayment is ac-companied by a complex evolution of block tilting and stress rotation from NNE–SSW to NW–SE in Eocene to Miocene times (Breddin et al. 1963, Zagwijn 1989, Schreiber & Rotsch 1998, Michon et al. 2003, Worum et al. 2005). Van den Berg (1994) postulates that a switch in the stress fi eld has happened several times in the recent past as is indicated by the observation of terraces of the river Maas and depocen-tres within the Roer Valley Graben System.

On the other hand, tensional NW–SE-trending normal faults in the Cenozoic sediments are described to become dextral WNW and sinistral NNE-striking wrench faults with a minor dip-slip component in the eastern part of the Rhenish Massif (Pilger 1957, Hinzen 2003, Schreiber & Rotsch 1998; see also Fig. 1). Similar observations were made in the coal mines of the Wurm district of the western Rhenish Massif, north of the well (Wrede 1985). There, NNE–SSW-trending faults are interpreted as oblique (hk0) faults in association to the Variscan fold structures.

The geological subcrop maps of the Brabant Massif (Legrand 1968, De Vos et al. 1993) show some NE–SW-trending transverse faults segmenting the entire Lower Pa-laeozoic basement (Piessens et al. 2002). Specifi c NE–SW-trending faults have also been described from the coal mines in the Wurm area north of Aachen, which are interpreted to have initiated pre-Variscan and to have been active through-out the Variscan deformation phase (Wrede 1985).

Local fault orientations differing from the regional trend could be expected because of the imprint of the Brabant Massif. A change of Variscan fold axes from NE–SW to N–S is known from the Aachen-Erkelenz mines, interpreted to have formed by the London Brabant Massif acting as a re-gional indenter (Wrede 1985).

Again, the age of the NNE–SSW fracture set cannot be resolved by the image analysis, but as these fractures basi-cally occur in the more competent sandstones and show no displacement of the bedding planes, we assume that this fracture set represents joints formed at a NNE-directed max-imum horizontal stress different from today’s in-situ stress regime.

6.2 Lithology and facies changes

The most prominent stratigraphic outcome is the absence of Middle Devonian and Lower Carboniferous (Mississippian) carbonate rocks in the RWTH-1 borehole. These are exposed in outcrops and were intersected by several wells in the past only a few hundred metres SE of the RWTH-1 in the hanging wall of the Variscan Aachen-Midi Thrust (Beissel 1886, Dantz 1893, Breddin 1962, Kasig 1980, Reissner 1990, Pommerening 1993, Oesterreich & Lundershausen 2002; Fig. 3). This hiatus of Mississippian platform carbonates im-mediately in front of the frontal Variscan Thrust correlates

well with the hiatus between Namurian shales (Pennsylva-nian) and Upper Devonian sandstones in Val Dieu, Belgium (Ancion et al. 1943, Ribbert 2006). The hiatuses can be ei-ther interpreted by a facies change and non-deposition in the vicinity of the Brabant Massif or by erosion. On the other hand the resistivity pseudo-image shows disturbed bedding and potential open fractures in several zones between 915 m and 1035 m, which potentially indicates a zone of normal faults responsible for the loss of Mississippian strata.

The lithostratigraphic gap between Upper Devonian (manifested by conodont data; Piecha 2006) and Lower Dev-onian age (spores at 1490 m; Ribbert & Hartkopf-Fröder 2006) is covered by the fi rst core section and the image logs. These document a clay-rich interval between 1440 and 1479 m, which includes the large stratigraphic gap covering more than the complete Middle Devonian sequence. This clay-rich interval is tectonically highly stressed documented by cleav-age development and numerous fracture and fault planes completely overprinting the bedding structure.

At the 1440 m boundary itself and four metres below, open fractures are observed in the acoustic image log (CBIL) strik-ing NW–SE and dipping moderately to NE (Fig. 6). These fractures/faults are not preserved in the core due to core loss. Because of the strong disturbance with several fracture orien-tation and slickensides indicating different directions it is as-sumed, that the clay-rich section below the 1440 m boundary marks a zone of structural weakness. However, available core and logging data did not resolve whether or not faults may have caused the lithostratigraphic gap.

Due to the absence of clear structural evidence we there-fore assume that non deposition or erosion on a palaeogeo-graphic high caused by early active geodynamic phases (e.g. Brabantian Orogeny of the Lower Devonian; Sintubin et al. 2009) is the more plausible explanation.

6.3 Style of the Variscan Front

Different geological models have been proposed for the de-velopment of the Variscan fold and thrust belt at the borehole location: the “thrust model” published by Winterfeld (1994) and the “fold model” published by Wrede (e.g. Wrede et al. 1993 and Wrede 2000).

The balanced cross-section by Winterfeld (1994) is based on detailed fi eld work and the interpretation of seismic data. Following this model, the borehole was expected to intersect an imbricated fan of thrust sheets consisting of Namurian (Pennsylvanian) to Upper Devonian strata in normal strati-graphic position that unconformably overlies an unfolded basement.

The “fold model” (Wrede 1985) is also based on surface and mining exposures especially in the Variscan Foreland north of Aachen. It predicts a folded, steeply dipping or over-turned Upper Carboniferous (Pennsylvanian) to Devonian rock sequence for the borehole location. In contrast to the balanced “thrust model”, the “fold model” does not predict nappe structures. Further to the west in Belgium, the “nappe” character is confi rmed by surface data covering a large area,



seismic sections (Dejonghe et al. 1989, DEKORP Research Group 1991, Hollmann & Walter 1995) and deep boreholes (Graulich 1984). The boreholes Bolland and Soumagne (Graulich 1984, Muchez & Sintubin 1998) are located in a similar position to the Variscan Front Thrust as the RWTH-1 borehole and show several thrust faults and a multiply stacked strata (Graulich 1984, Muchez & Sintubin 1998).

The log data from the RWTH-1 well confi rm that the Var-iscan deformation rapidly diminishes in the footwall of the Aachen Midi Thrust that crops out only a few hundred me-tres south of the borehole site. The inverse gamma ray trend between 400 and 550 m documents an overturned position of this sequence that matches the concept of an overturned N fl ank of a large anticline as postulated by Wrede (1985). However, most likely this is the only overturned section of the borehole. Below 1520 m the Lower Devonian rocks are only slightly affected by Variscan deformation. In accord-ance to the fold model the slightly NE-dipping beds below 1520 m would represent the core of an anticline with fl at bedding dips directed towards the northeast according to the axial dip of the structure (Wrede 2008). The direct detection of large-scale thrust faults in the borehole proved problem-atic due to the uniform lithology and missing stratigraphic marker horizons (carbonates, conglomerates), which would allow the detection of lithological or stratigraphic duplica-tions. Nevertheless, deformation is documented by the nu-merous fracture and faults zones. Heterogeneous dip orienta-tion, overprint by cleavage and increased dip angles due to drag folds are observed on the image logs (below 1263 m) and can be interpreted from the hexagonal dipmeter log (above 1263 m). Variscan Thrust fault zones are interpreted at 260–273 m, 405–430 m, 700–717 m, 798 m, 935–939 m, 1269–1437 m and probably (with minor importance) at 2240–2300 m.

In summary we observed folding in the upper part of the borehole, which is manifested by the overturned sequence between 400 and 550 m depth. Thrust faults are documented by the occurrence of drag folds. The strong tectonically dis-turbed sequence between 1440 and 1520 m comes along with the change from generally SE-dipping to gently NE-dipping beds. This section also includes the lithostratigraphic gap between Upper and Lower Devonian rocks. Further be-low only a slight Variscan imprint is documented by the low angle NE–SW-striking faults. We conclude from the RWTH-1 well data analysis that results are consistent with an interpretation of a severely deformed sequence organised in high- and low-strain zones.

7. Conclusions

The RWTH-1 well is located at the intersection between the Lower Rhine Embayment and the Ardennes-Rhenish Massif. It has penetrated Cenozoic and Cretaceous sediments and a thick sequence of Palaeozoic rocks in the footwall next to the frontal Variscan Midi-Aachen Thrust. The well contains folded and faulted Upper Carboniferous (Pennsylvanian) coal bearing rocks in the upper part. Lower Carboniferous

(Mississippian) and Middle Devonian rocks are absent in the well, constituting pronounced hiatuses of most likely deposi-tional origin (non-deposition, eroded or facies change).

Structural data indicate a Variscan NE–SW oriented structural trend containing folds (overturned between 400–550 m), several fracture zones and faults. Drag folds indicate that thrust faulting activity takes place at 260–273 m, 405–430 m, 700–717 m, 798 m, 935–939 m, 1269–437 m and probably (with minor importance) at 2240–2300 m.

The intensity of Variscan deformation diminishes with depth, e.g. with distance to the Midi-Aachen Thrust. The NE–SW trend is most prominent in the upper 1.5 km of the borehole, while below the strike of bedding changes towards NW–SE with overall gentle dips towards the NE further downhole.

A complex fracture system is displayed by the image logs (run below 1263 m) confi rming the complex geodynamic history at the borehole location. Generally, the Variscan trend is visible in the cemented fractures, but post-Variscan, sealed fractures cannot be separated by image analysis and are responsible for the strongly varying dips of closed frac-tures. Fractures, which are superfi cially open at the borehole wall are detected by the acoustic image log and show two prominent maxima: one which includes joints and drilling enhanced fractures and follows the orientation of the NW–SE-trending recent maximum horizontal stress, and a second maximum, trending NNE–SSW. The NNE strike is inter-preted to represent an older set of joints formed at a NNE-directed compression, which implies an anticlockwise rota-tion of the maximum horizontal stress.

Superfi cially open faults are visible throughout the well and trend NW–SE to W–E. The NW–SE set generally dips to NE in accordance to the SW border faults of the Roer Valley Graben System. Varying directions of borehole breakouts and drilling induced tensile fractures are related to the W–E oriented faults (1895 m, 2115 m and 2304 m) and are pro-posed to indicate recent tectonic activity. As the W–E-orien-tation is aligned parallel to the maximum dextral in-situ shear stress, this represents a strong further line of evidence for the presence of a strike-slip faulting regime also in this part of the Variscan Deformation Zone.

Results shown in this study, combined with information gathered using geochemical and mineralogical metho

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