Conference PaperPDF Available

Fossil Multiphase Normal Faults -Prime Targets for Geothermal Drilling in the Bavarian Molasse Basin?

Authors:

Abstract and Figures

Keywords: fault zone drilling, carbonate reservoirs, foreland basin exploration, stratigraphy related fault throw analysis ABSTRACT Foreland basins with their increasing depth towards the orogenic belt are ideal geologic systems as host for geothermal resources. The Bavarian Molasse Basin is the only basin worldwide where geothermal energy is being successfully developed by industry in conduction dominated heat transport environment. However, the predicted productivity is not achieved in all projects because either temperature or flow rate or both are lower than expected. The utilized reservoir rock consists of Upper Jurassic fractured carbonates, and reservoir quality is governed by fracture and fault frequency, dolomitisation degree, karstification and facies type. From these controlling factors only fracture density can be reliably assessed before drilling because generally highest fracture density is found in the vicinity of faults which can be in turn clearly identified in reflection seismic. In contrast facies and dolomitic domains can be reliably detected only after drilling and seismic-well log correlation. From this perspective faults seem to be the most reliable target in geothermal exploration in the Molasse Basin. A particular fault system, however, might have experienced a complex kinematic history from Jurassic to the present-day state, eliciting the question which fault among many detected faults might be the best drilling target. The geothermal site Mauerstetten in the Western Bavarian Molasse Basin is an example where drilling into a fault zone did not result in high flow rates. This article tries to identify the effects on the lacking reservoir permeability by detailed geological well data and advanced structural geological interpretation. The results are set into context with successful geothermal drilling projects in the Molasse basin where high flow rates were achieved by fault zone drilling. 1. INTRODUCTION The Bavarian Molasse Basin is one of seven Alpine foreland basins in Europe (Fig. 1). The 3-5 km deep Upper Jurassic Malm formation of the Molasse Basin (Fig. 1) is explored as an fractured and faulted carbonate reservoir rock recently for geothermal and-to a minor degree-in the last decades for hydrocarbon resources. Faults play a major role in reservoir exploration of carbonate reservoirs. Critical questions are addressed to the hydraulic properties of faults and their ability to channel fluids while matrix properties are interactively affected by depositional environment, karst evolution, pressure solution, early and late diagenetic processes, metasomatic dolomitisation and changes in diagenetic grade during basin subsidence. The different respond of faults on paleostress regimes causes open (mechanically or secondary opened by dissolution), cemented sealed (dissolution and precipitation), kaciritic cohesion less, tectoclastic (kataclastic) lithified, authigenic clay mineral or fault gouge filled, and discrete decollated or disperse brecciated, fossilized or migration active cuts. The fault style has thus a strong controlling factor on the hydrostatic, hydro-pressured (gas over-pressured), hydrothermal and telethermal setting of the fluids (volatiles) involved. The Upper Jurassic Malm formation in particular is well studied for facies types (Kott, 1989; Meyer & Schmidt-Kaler, 1996; Koch et al. 2010), karst formation, diagenesis and dolomitisation (e.g. Michel 1999), paleontology including microfossils and paleogeography (e.g. Pomoni-Papaioannou et al., 1989) in outcropping sections of the Franconian Alp.
Content may be subject to copyright.
Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015
1
Fossil Multiphase Normal Faults - Prime Targets for Geothermal Drilling in the Bavarian
Molasse Basin?
Inga S. MOECK
1
, Stephan UHLIG
2
, Bernd LOSKE
3
, Anna JENTSCH
4
, Rafael FERREIRO MÄHLMANN
5
, Stephan
HILD
6
1
University of Alberta, Dep. Earth and Atmospheric Sciences, Edmonton, T6G 2E3 Alberta, Canada
2
GeoTec Consult, Markt Schwaben, Germany
3
Loske Geosciences, Zeche Eiberg 77, 45279 Essen, Germany
4
GFZ - Helmholtz Centre Potsdam, Telegrafenberg, 14473 Potsdam, Germany
5
Technische Universität Darmstadt, Schnittsphanstr. 9, 64287 Darmstadt, Germany
6
Exorka, Bavariafilmplatz, Grünwald-Munich, Germany
E-mail: moeck@ualberta.ca
Keywords: fault zone drilling, carbonate reservoirs, foreland basin exploration, stratigraphy related fault throw analysis
ABSTRACT
Foreland basins with their increasing depth towards the orogenic belt are ideal geologic systems as host for geothermal resources.
The Bavarian Molasse Basin is the only basin worldwide where geothermal energy is being successfully developed by industry in
conduction dominated heat transport environment. However, the predicted productivity is not achieved in all projects because either
temperature or flow rate or both are lower than expected. The utilized reservoir rock consists of Upper Jurassic fractured
carbonates, and reservoir quality is governed by fracture and fault frequency, dolomitisation degree, karstification and facies type.
From these controlling factors only fracture density can be reliably assessed before drilling because generally highest fracture
density is found in the vicinity of faults which can be in turn clearly identified in reflection seismic. In contrast facies and dolomitic
domains can be reliably detected only after drilling and seismic-well log correlation. From this perspective faults seem to be the
most reliable target in geothermal exploration in the Molasse Basin. A particular fault system, however, might have experienced a
complex kinematic history from Jurassic to the present-day state, eliciting the question which fault among many detected faults
might be the best drilling target. The geothermal site Mauerstetten in the Western Bavarian Molasse Basin is an example where
drilling into a fault zone did not result in high flow rates. This article tries to identify the effects on the lacking reservoir
permeability by detailed geological well data and advanced structural geological interpretation. The results are set into context with
successful geothermal drilling projects in the Molasse basin where high flow rates were achieved by fault zone drilling.
1. INTRODUCTION
The Bavarian Molasse Basin is one of seven Alpine foreland basins in Europe (Fig. 1). The 3-5 km deep Upper Jurassic Malm
formation of the Molasse Basin (Fig. 1) is explored as an fractured and faulted carbonate reservoir rock recently for geothermal and
- to a minor degree - in the last decades for hydrocarbon resources. Faults play a major role in reservoir exploration of carbonate
reservoirs. Critical questions are addressed to the hydraulic properties of faults and their ability to channel fluids while matrix
properties are interactively affected by depositional environment, karst evolution, pressure solution, early and late diagenetic
processes, metasomatic dolomitisation and changes in diagenetic grade during basin subsidence. The different respond of faults on
paleostress regimes causes open (mechanically or secondary opened by dissolution), cemented sealed (dissolution and
precipitation), kaciritic cohesion less, tectoclastic (kataclastic) lithified, authigenic clay mineral or fault gouge filled, and discrete
decollated or disperse brecciated, fossilized or migration active cuts. The fault style has thus a strong controlling factor on the
hydrostatic, hydro-pressured (gas over-pressured), hydrothermal and telethermal setting of the fluids (volatiles) involved. The
Upper Jurassic Malm formation in particular is well studied for facies types (Kott, 1989; Meyer & Schmidt-Kaler, 1996; Koch et al.
2010), karst formation, diagenesis and dolomitisation (e.g. Michel 1999), paleontology including microfossils and paleogeography
(e.g. Pomoni-Papaioannou et al., 1989) in outcropping sections of the Franconian Alp.
Since geothermal exploration has been started in the Upper Jurassic Malm Fm., open questions address physico-chemical properties
of rock type, reservoir quality, the impact of faults on deep hydrogeology, fluid flow patterns, discrimination of meteoric advective
and deep thermal convective cycles, rock-water interaction and cement precipitation as also authigenic clay-mineral formation and
reaction progress, dolomitisation and karstification processes with the ultimate goal to determine optimal drilling targets for
geothermal production wells. It is known from fractured carbonate reservoirs that a cluster of faults channelling fluids and an
enhanced matrix porosity increases the storage capacity for fluids thus leading to favourable reservoir rock qualities (Lian and Ma,
2012). Due to the deep burial setting of the Malm reservoir in contrast to lithofacies changes and the occurrence of high porosity
domains, only faults can be reliably detected in the dataset of seismic sections during green field exploration, i.e. before drilling
without well-log and seismic correlation. Thus, the characterisation of faults, the better knowledge about their kinematic and
diagenetic evolution together with the better understanding of syn-kinematic processes controlling permeability structures obtain a
primary role in exploration. The dominating E-W to ENE-WSW striking normal faults in the carbonate reservoir (i.e. the Malm
formation) must have been generated in a stress regime with a minimum horizontal stress direction Sh N-S (Moeck et al.,
submitted). The present-day stress field is however 9rotated to this normal faulting stress regime with a present-day minimum
horizontal stress direction Sh E-W and a maximum horizontal stress direction SH N-S (Fig. 1) (Reinecker et al., 2010). This
inconsistency between the fault geometry and the recent stress regime indicates the existence of fossil normal faults in a present-
day stress regime.
Moeck et al.
2
This article addresses the question on normal faults in the Upper Jurassic formation of the Bavarian Molasse Basin. The elementary
focus relates to the formation time, the time spans of a potential tectonic reactivation and the implications on permeability structure
in faulted carbonate reservoirs.
Figure 1: (a): The seven Alpine foreland basins with 1-Aquitaine Basin, 2-Molasse Basin, 3-Carpathian Basin, 4-Apenninic
basins and Po Basin, 5-Atlas Basin,6-North Bethic Basin, 7-South Pyrenean and Ebro Basin. (modified from Allen et
al., 1986). 1(b): Andersonian stress field/faulting regime relation for normal faults. 3(c): Map of the Upper Jurassic
(Malm) carbonate formation. Red polygone: well GT1, red solid lines: 2D seismic profiles used in this study, dashed
red lines: 2D seismic profiles used for 3D geological modeling but not used for the stratigraphy related fault throw
analysis. Question marks indicate the transition region from prolific carbonate facies to Helvetian facies. Back dots:
neighboring wells with depth range of borehole breakouts indicating the direction of SH from the present day stress
field. Green Sh-SH arrows: stress field causing the normal faults, blue Sh-SH arrows: present day stress field
causing the Alpine frontal fault (modified from Moeck et al., submitted; Reinecker et al., 2010; Bayerischer
Geothermie-Atlas, 2010).
2. GEOLOGICAL SETTING
The Molasse Basin and its substratum underwent four major evolutionary stages, termed as syn-rift (Permo-Carboniferous),
epicontinental (Triassic-Middle Jurassic), passive margin (Middle Jurassic to Late Cretaceous-Palaeocene) and Alpine foredeep
(Oligocene to Pliocene), illustrated in an updated standard stratigraphic profile of the basin. At the basis of the Molasse sediments
(Oligocene to Miocene) a large hiatus is testifying a basal unconformity evolving from the Helvetic European shelf domain to the
Molasse Basin (Schmid et al., 1996, 2008) from the south in the Palaeocene to the north in the Oligocene (Chattian). The Molasse
is underlain by 500-1,000 m thick Mesozoic shelf sediments that represent the passive margin basin of the Neotethys (Stampfli &
Borel, 2004). The predominantly Mid to Late Jurassic carbonate beds are deposited on Variscan basement that is locally segmented
by Permo-Carboniferous troughs containing clastic sediments of largely unknown thickness and composition (Lemcke, 1988;
Lüschen et al., 2011). After several marine transgression and regression phases from Permian to Early Cretaceous, prior to
continuous deepening due to increased shelf subsidence evidenced by neritic to pelagic sediments (mostly eroded in the Palaeocene
to Eocene emersion phase, Trümpy 1960) in Late Cretaceous and progressing to Early Eocene, the deposition of marine and
freshwater Molasse from Late Eocene to Late Miocene represents the foreland basin period where sedimentation was controlled by
erosion and uplift cycles of the Alpine fold and thrust belt (Lemcke, 1977; Kuhlemann & Kempf, 2002).
The involved faulting processes may be identified by a quantitative fault zone analysis in seismic sections measuring specifically
the fault throw on individual seismo-stratigraphic horizon cut-offs on seismic profiles. The six 2D seismic sections of the
Mauerstetten prospect cover a major ENE-WSW trending discontinue offset of normal fault array with an absolute throw of 270±10
m and a length of 20 km (Fig. 2). The fault zone truncates from Upper Jurassic to Miocene strata. However, the fault throw seems
larger in the Mesozoic strata than in the Cenozoic strata and declines towards the Miocene in the upper section. A stratigraphy
related fault throw analysis is therefore chosen to identify intra-formational fault throws to detect a fault activity at a certain time
slot.
3. GEOTHERMAL DRLLING AND ADVANCED STRUCTURAL INTERPRETATION OF SEISMIC DATA
The well Mauerstetten GT1 hits the fault zone of the afore mentioned undulating E-W striking normal fault array at a depth of
3.763 m TVD (i.e. below ground level) encountering the Malm aquifer (Figs. 1 and 2). The well is deviated along ESE direction
(120° azimuth), inclined with an angle of 50° and has a total length of 4.523 m MD (4.085 m TVD). The well path is placed into
the hanging wall block of the fault zone and truncates three branches of synthetic normal faults evidenced by repeating strata in the
lithostratigraphic section of well GT-1. The sidetrack GT1a is placed into the hanging wall of the second fault branch and is drilled
Moeck et al.
3
in fault dip direction along ESE-WNW (108° azimuth) with an inclination of 57° to 4.052 m MD (3.572 m TVD) depth with a
lateral distance of 513 m to GT1 in 289° azimuth. The temperature in 3,675 m depth TVD is 132°C shortly after reaching the target
horizon in the side track GT1a. The drilled imbricated normal fault zone is visible in the 2D seismic sections as one fault zone,
obviously with the synthetic parasitic fault branches not detected below seismic resolution. The seismic sections originate from
2007 and were adjoined to older seismic sections from 1989, 1990 and 2003. A total of 12 seismic sections were processed or re-
processed and interpreted to build a 3D geological model (Loske and Witte, 2008).
3.1 Stratigraphy related fault through analysis
Two methods were employed for advanced structural interpretation of the 2D seismic data to determine the growth history of the
studied fault zone (Fig. 3): (i) the fault throw was measured sequentially through all mapped seismic horizon from the uppermost
faulted layer with an identifiable cut-off in the lower Neuhofener Formation down to the Purbeck Formation that represents the top
carbonate geothermal reservoir. The throw measured on each seismic horizon was subtracted from the throw of the next lower
horizon in order to detect the differential offset for each layer instead of only measuring the cumulative offset. The differential
offset is referred as to Quantitative Fault Expansion Index (QFEI) and is given in meter; (ii) the thickness variation of seismic
stratigraphic intervals were measured in the footwall and hanging wall adjacent to the fault dislocation plane. The thickness of the
seismic horizon in the hanging wall was compared with the thickness of the same layer in the footwall at the intersection of the
horizons with the fault dislocation plane. The thickness variation from hanging wall to footwall is expressed in percent and referred
as to Expansion Index. In the case that the horizons have the same thickness in hanging and foot-wall, the Expansion Index is 1.
Has the hanging wall horizon a 10% higher thickness than in the footwall, the expansion index is added by 10% resulting in 1.1,
with a 20% higher thickness from hanging to footwall layer the expansion index is 1.2. The thickness is measured vertically to the
horizon base, respectively, so that an apparent thickness in rotated fault blocks can be excluded. This second method aims to
discriminate syn-sedimentary growth faulting from post-sedimentary faulting while the first method aims to identify the fault
activity at a certain period.
Figure 2: (a) 2D seismic lines of the Mauerstetten prospect, gray shaded is the dominating multiphase normal fault. (b) 3D
geological model developed from 2D seismic sections with well path of main well and side track in the hanging wall
of the fault zone (modified from Moeck et al., submitted).
4. RESULTS
The QFEI derived from the fault throw analysis indicates the inactivity of the fault since the Middle Miocene (17 Mio years)
confirmed by the absence of natural seismicity in the present-day stress field (Barnikel & Geiss, 2008). Depending on strike and dip
azimuth fault segments exhibit a different fault kinematic evolution. While E-W oriented segments underwent a multiphase normal
faulting history from Jurassic to Miocene due to rifting in the Helvetic-European shelf and subsequent lithospheric bending in the
foreland of the evolving orogeny during Alpine nappe thrusting (Moeck et al., submitted), NE-SW trending fault segments acted as
Jurassic normal faults presumably reactivated first as normal faults in the lower Cretaceous and as strike-slip faults in the Upper
Cretaceous due to subduction in the Valais ocean (e.g. Stampfli, 1994). The culmination of normal faulting was in the Lower
Oligocene obviously related to lithospheric bending associated with the lithospheric load of the Alpine fault-thrust belt. Normal
faulting continued on the Mauerstetten fault to Miocene and died out in the Burdigalian (Moeck et al., submitted) (Fig. 3a).
The present-day stress state of the faults and its likelihood for fault slip can be estimated by the slip tendency and derived fault
reactivation potential (Morris, 1996; Moeck et al., 2009a) following the concept of limiting stress ratios and the Mohr-Coloumb
failure criterion extended by the Hoek-Brown parameters (Moeck et al., 2009b; Cacace et al., 2013). Two important facts can be
addressed: (i) the orientation of fault segments with high slip ratio; (ii) the conditions for slip referring to rock strength and fluid
pressure. Assuming a present-day strike-slip stress regime in the Mesozoic succession (Cacace et al., 2013; Reinecker et al., 2010)
with a maximum horizontal stress direction SH 170° (±15°) (Moeck, 2011), an estimated fair rock mass quality with fracture
spacing at 0.3-1 m (Moeck et al., 2009a) accounting for the fractured fault damage zone as indicated by the lithology of the well
GT1, and hydrostatic conditions of 35 MPa for the Malm reservoir in Mauerstetten, the slip tendency on any segment of the normal
fault is below the friction coefficient of the rock mass. This result is another indicator for a fossil normal fault with no probability of
reactivation in the current stress field. E-W trending fault segments with dip angles >42° cannot be reactivated as reverse faults in
Moeck et al.
4
the current stress field. With a dip angle <42° these faults could be re-activated as reverse faults. NNE and NNW trending faults
could be reactivated as sinistral and dextral strike slip faults, respectively.
The observed normal fault with an E-NE trend in the Mauerstetten area has significant steeper dips (>45°). Thus, these fault
segments undergo a frictional blockade and are unlikely to be reactivated. Therefore unusual high horizontal stresses would be
necessary to reactivate these faults as reverse faults. Obviously this is not the case because no indication for reverse faulting
reactivation (such as anticlinal bending of the hanging wall formations) is observed in the 3D geological model. An additional fluid
pressure of 33 MPa would be required to reactivate NE-SW segments of the normal fault in fairly fractured rock (fracture spacing
0.3-1 m) indicating an inactive normal fault exhibiting fossil multiphase activity.
5. IMPLICATIONS ON FAULT ZONATION AND PERMEABILITY STRUCTURE
The hydraulic properties of the fault zone can be estimated from the well tests and the geologic profiles of the well GT1 and the
sidetrack GT1a. The well GT1 has a minimum distance of 173 m to the mapped major fault surface while GT1a has a distance of
337 m to the main fault. The estimated flow rate from GT1a is 24 l/sec derived from an injectivity index of 25.2 m
3
/h*MPa.
However, injection tests always opened fractures to a larger extend than under production conditions resulting in smaller flow rates.
GT1a provides 7 l/sec derived from a productivity index of 7.2 m
3
/h*MPa. The well GT1 is located closer to the major normal fault
and transects possibly synthetic minor faults, which cause a flow rate that is about 3 times higher than in the sidetrack GT1a (Fig.
4). Another reason for the higher flow rate derived from GT1 is that the injectivity can be three to five times higher than the
productivity due to poro-elastic effects (Grant and Bixley, 2011). Garg and Comb (1997) suggest a 1:1 ratio of productivity to
injectivity and assuming the linear relationship of injectivity to productivity GT1 is significantly more productive than GT1a. GT1
is located in the damage zones of the minor faults identified by the repeated lithologic and facies type sections from the well
geological profile. The sidetrack GT1a is located out of the damage zone and is influenced by the matrix permeability of the host
rock rather than the fracture permeability of the damage zone. The multiple phases of normal faulting may have generated a fault
damage zone with higher fracture density with a positive effect on the permeability of the fault core and fault damage zone.
Moreover, the Mauerstetten fault was active as normal fault up to the Miocene when the thrusting and compression in the Alpine
orogeny started to decay in the Alpine front. Obviously this fault preserved its dilative character as an inherited fossil structure in
the recent compressional stress field due to local extension and lithospheric bending in the Upper Eocene to Lower Miocene with
possibly positive effects on its hydraulic properties in the damage zone containing parasitic synthetic normal faults indicated by the
geologic profile of the well.
Since no image logs are available from the well, only suggestions can be drawn to fracture characteristics in fault core and damage
zone. Calcite mineralisation in cuttings indicates healed fractures or slip planes acting as fluid barriers. No breccia is reported from
cuttings as someone would expect from drilling along a multiphase normal fault zone. One reason might be that the spatial
relationship between fault and borehole is not accurately defined because the fault geometry is mapped from 2D seismic profiles
instead of a 3D seismic survey. A 3D seismic survey might deliver a modified structural pattern with rather a segmented normal
fault than one coherent fault surface. The fault dip and bed thickness in the 2D seismic sections might be correct as the fault throw
analysis is, however the uncertainty of strike and dip of the fault surface increases with distance to the seismic sections and
interpolation effects from 3D model building influence the mapping result. Another reason for missing breccia in the carbonate
section is that the repetition of the lithofacies types in the well profile can also be related to facies boundaries and not only to
faulted boundaries. In this case the well GT1 is only crossing one minor synthetic fault with a small damage zone thickness. Under
this circumstance a fractured reservoir should be combined with a stratiform lithofacies reservoir with hybrid pore types as vuggy
fractures (Nelson, 2001). Vugs are identified in the cuttings from the well GT1. These vugs are associated with Cretaceous
siliciclastics indicating a sediment filled karst in the Malm limestone obviously formed in the Lower Cretaceous during a terrestrial
period posterior to the carbonate platform formation.
5.1 Controlling factors on permeability anisotropy in carbonate rock
Fluid flow in carbonate rock is however not only fractured controlled but is affected by a complex interplay between deposition,
early and late stage alteration by diagenesis, hydrology including dissolution and karst, and tectonic overprint including healing of
fractures. The challenges in carbonate reservoir exploration are associated with a complexity of multi-scale porosity and
permeability distribution related to a wide range of biosediments on the carbonate platform (Ahr, 2008). Geothermal exploration
has shown that highest recovery from the Malm formation in the Bavarian Molasse Basin are from reef detritus limestone of the
profilic and highly bio-diversified carbonate factory of the outer carbonate platform with barrier reefs. In contrast low-diversity
carbonate factory dominated by algo-microbal associations are less prospective (Ahr, 2008). According to the cutting analysis the
Mauerstetten wells are located in the latter facies type exhibiting a minor degree on dolomitisation and vuggy pores generated by
karst and dissolution. Dolomitisation and karstification seems to be facies-selective, however mechanisms on fabric-selective,
texture-selective and facies-selective porosity and permeability development or pore space structure are not fully understood yet for
the Upper Jurassic of the Molasse Basin.
The polyphase activity of the normal fault may have a positive impact on the fracture density however the low recovery might be
related to poorly connected fractures, sealed fractures and/or low interstitial porosity in the low-biodiversity limestone. Fracture
porosity scale is not only related to fracture spacing, width and length but also to the area size or reservoir thickness and
interconnectivity of the fractures (Ahr, 2008; Youn and Gutierrez; 2011). The latter could be the critical factor for the Mauerstetten
fault. Only synthetic normal faults are identified by the lithologic profile of the well and the 2D seismic sections. Mapped fractures
and parasitic faults are parallel and may undulate in strike possibly generating steep or vertical intersection lines that could channel
fluids. A higher degree in interconnectivity is however given by an array of synthetic and antithetic fractures forming an X-
geometry. X-fractures are typically generated in Y-shaped graben situations or along listric normal faults. At an analogue outcrop in
the front ranges of the Rocky Mountains in Alberta open channels could have been observed in the damage zone of a reactivated
normal fault in carbonate rock. The channels were bound to intersections of X-shaped fractures (Fig. 3b). Intersection lines of X-
shaped fractures or crossing normal faults are lateral and provide optimal channels for fluid flow along fault strike (Ferrill et al.,
2009). Transferred to reservoir depth, these channels might be kept open even in a compressional stress field. Along the
Moeck et al.
5
Mauerstetten well only synthetic fractures are observed, X-fractures are lacking thus no lateral fluid channels can be expected with
reduced fracture-assisted permeability and resulting low flow rate from the well GT1.
Reservoir productivity cannot be simply attributed to fast communication along faults cutting the top of the reservoir. However,
faults are the only structural element that can be reliably detected through reflection seismic before drilling while diagenetic and
depositional heterogeneities can only be identified after drilling and extensive well log core analysis seismic data correlation of
a minimum of 20 wells in a 20 km
2
reservoir block (Spina et al., 2014). Before the 3D stratigraphic and sequential architecture of a
carbonate platform can be simulated from information of a number of wells and 3D seismic data, identifiable seismic-scale faults
may play a primary role in targeting of geothermal wells in a new prospect. The stratigraphy related fault throw analysis may help
to select favourable faults bearing fluids from a complex fault pattern for the following reasons inferred from the hydraulic well
tests from fault zone and off the fault zone in Mauerstetten.
(a)
Figure 3: (a) Multiphase normal fault identified in the
Mauerstetten prospect in the Bavarian Molasse
basin. (b) Multiphase normal fault in the Middle
Cambrian Cathedral carbonate formation in the Front Ranges of the Rocky Mountains, Alberta. Red are slip planes
in the fault core zone, green are fracture planes in the damage zone, and blue are intersection points of X-fractures.
6. CONCLUSION
The geothermal prospect Mauerstetten in the southwestern Bavarian Molasse Basin is one of the industry projects where high
temperature of over 150°C but insufficient flow rate due to tight carbonates in the Malm unit (i.e. Upper Jurassic) dragged down the
overall project performance. In a subsequent research project, a detailed structural geological analysis of twelve 2D seismic profiles
with a total length of 155 km has been performed to better understand fault kinematics with possible implications on fault core and
damage zone formation over time. The applied approach for fault plane analysis is a stratigraphy related fault throw analysis to
identify multiphase reactivation of detected normal faults. The normal faults are striking about E-W and obviously generated in a
normal faulting stress regime with a minimum principal stress S3 in N-S. The present-day stress field has a maximum principal
stress direction S1 in N-S, indicating fossil normal faults in a current stress field.
The results of seismic fault throw analysis show that the throw varies in different stratigraphic layers indicating different periods of
faulting activity. A major fault activity occurred after deposition of the Upper Jurassic Malm unit presumable in the Lower
Cretaceous. A second major faulting period occurred in the late Oligocene. All fault throws indicate normal faulting in the
Mauerstetten prospect until Miocene period when the main fault-and-thrust activity in the Alpine orogen has terminated. Due to
lithospheric bending these normal faults preserved obviously their dilative character in a plate tectonic compressional regime. Flow
test data from different wells indicate that these multiphase fossil normal faults are loci of increased permeability due to high
interconnected fracture density in a well-evolved fault damage zone. This example shows that stratigraphy related fault throw
analysis in combination with stress field determination is an important addition in advanced seismic interpretation to identify
reactivated faults.
Insufficient flow rates might be explained by three reasons: (I) Crossing X-fractures and their intersections are prime flow channels
for flow along faults that are compressional in the present-day stress field. The dominating fault at the Mauerstetten prospect is a
major southward dipping normal fault with an offset of more than 200 m. The fault damage zone commonly a high permeability
zone in carbonate rock fault zones is built up by synthetic fault parallel secondary normal faults. Crossing X-fractures are lacking;
(II) multiphase normal and presumably strike-slip faulting activity of the drilled NE-oriented fault segment started in Jurassic and
decayed in the Miocene. The multiple fault activity might have caused sealed the fault zone by fault gouge; (III) low interstitial
matrix porosity as effect of the formation of the carbonates at the outer slope of the carbonate platform.
However, technological treatments and reservoir engineering may help to develop sites as Mauerstetten provided the carbonate
platform at depth is geologically characterized. Reservoir engineering can only be developed for carbonate reservoirs if the
depositional, diagenetic and tectonic history and its impact on pore space, permeability distribution and the overall fluid-rock
system are identified. In so far the case study Mauerstetten is an example that the Malm unit of the Bavarian Molasse Basin is not
comprehensively understood yet.
ACKNOWLEDGEMENTS
This research was partly funded by the German Federal Ministry BMU under the project ID 032567B, and the Alberta Innovation
Program of Alberta Innovates Energy and Environment Solutions (project ID RES0010033). Thank is given to Exorka for data
Moeck et al.
6
release. The 3D geological model was developed by Petrel (Schlumberger) and earthVision (DGI). The stratigraphy related fault
throw analysis was conducted with Petrel.
REFERENCES
Ahr, WM (2008) Geology of carbonate reservoirs. Wiley & Sons, Hoboken, New Jersey, USA, 277 pp
Allen PA, Homewood P, Williams GD (1986) Foreland basins: an introduction. In PA Allen, P Homewood (eds): Foreland Basins.
Spec Publ Int Assoc Sediment 8: 3-12
Anderson EM (1951) The dynamics of faulting and dyke formation with applications to Britain. Oliver & Boyd, Edinburgh
Barnikel F, Geiss E (2008) The BASE Project an open-source catalogue for earthquakes in Bavaria, Germany. Nat Hazards Earth
Sci 8: 1395-1401
Bayerischer Geothermieatlas (2010) Bayerischer Geothermieatlas Hydrothermale Energiegewinnung. Bayerisches
Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie, 93 p., München, Germany
Cacace M, Bloecher G, Watanabe N, Moeck I, Boersing N, Scheck-Wenderoth M, Kolditz O, Huenges E (2013) Modelling of
fractured carbonate reservoirs: outline of a novel technique via a case study from the Molasse Basin, southern Bavaria,
Germany. Environ Earth Sci, 70(8): 3585-3602
Ferrill DA, Morris AP, McGinnis RN (2009) Crossing normal faults in field exposures and seismic data. AAPG Bulletin 95(11):
1471-1488
Garg SK, Combs J (1997) Use of slim holes with liquid feedzones for geothermal reservoir assessment. Geothermics, 26-2: 153-
178
Grant MA, Bixley PF (2011) Geothermal reservoir engineering, II Edition. Academic Press, New York
Koch R, Bachmann GH, ller M (2010) Fazies des Oberen Jura (Malm) der Bohrungen Scherstetten 1 und 2 (Molasse-Becken,
Süddeutschland) und ihre Bedeutung für die geothermische Exploration. Z geol Wiss 38: 327-351, Berlin
Kott R (1989) Fazies und Geochemie des Treuchtlinger Marmors (Unter- und Mittel-Kimmeridge, Südliche Frankenalb). Berliner
Geowiss. Abh A 111: 115 p., Berlin
Kuhlemann J, Kempf O (2002) Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics.
Sedimentary Geology 152: 45-78
Lemcke K (1988) Geologie von Bayern.I. Teil: Das bayerische Alpenvorland vor der Eiszeit. I E. Schweizerbart, Stuttgart, 175
pp
Lemcke K (1977) Erdölgeologisch wichtige Vorgänge in der Geschichte des süddeutschen Alpenvorlandes. Erdöl-Erdgas-
Zeitschrift 93: 5056
Lian PQ, Ma, CY (2012) The characteristics of relative permeability curves in naturally fractured carbonate reservoirs. SPE Journal
of Canadian Petroleum Technology 3: 137-142
Loske B, Witte C (2008) 2D seismic survey for the geothermal permit areas Mauerstetten, Marktoberdorf, Bidingen and Weilheim
Data Processing and Interpretation. Final Report DMT, Exorka, unpubl. report, 87 p
Lüschen E, Dussel M, Thomas R, Schultz R (2011) 3D seismic survey for geothermal exploration at Unterhaching, Muncih,
Germany. First Break 29: 45-54
Meyer RKF, Schmidt-Kaler H (1996) Jura. In: Erläuterungen zur Geologischen Karte von Bayern 1:500000, 112-125; Munich
Michel U (1999) Gesteinsphysikalische Eigenschaften und fazielle Ausbildung der oberjurassischen Massenfazies (Kimmeridge)
der südlichen Frankenalb (Stammham). GSF Bericht Oberschleißheim 4(99): 48-56
Moeck I, Maehlmann RF, Loske B, Jentsch A, Uhlig S, Hild S (submitted) Multiphase fossil normal fault characterization for
geothermal exploration in the Bavarian Molasse Basin. Submitted to International Journal of Earth Sciences.
Moeck I, Backers T (2011) Fault reactivation potential as critical factor during reservoir stimulation. First Break 29: 67-74
Moeck I, Schandelmeier H, Holl HG (2009a) The stress regime in Rotliegend reservoir of the Northeast German Basin. Int J Earth
Sci (Geol Rundsch) 98/7: 1643-1654
Moeck I, Kwiatek G, Zimmermann G (2009b) Slip tendency, fault reactivation potential and induced seismicity in a deep
geothermal reservoir. J Struct Geol 31: 1174-1182
Morris A, Ferrill DA, and Henderson DB (1996) Slip-tendency analysis and fault reactivation. Geology 24(3): 275-278
Nelson RA ( 2001 ) Geologic Analysis of Naturally Fractured Reservoirs , 2nd ed. Gulf Publishing , Houston , 332 pp.
Pomoni-Papaioannou F, Flügel E, Koch R (1989) Depositional Environments and Diagenesis of Upper Jurassic Subsurface
Sponge- and Tubiphytes Reef Limestones: Altensteig 1 well, Western Molasse Basin, Southern Germany. Facies 21: 263-284
Reinecker J, Tingay M, ller B, Heidbach O (2010) Present-day stress orientation in the Molasse Basin. Tectonophysics, 482:
129-138
Schmidt SM, Pfiffner OA, Froitzheim N, Schönborn G, Kissling E (1996) Geophysical-geological transect and tectonic evolution
of the Swiss-Italien Alps. Tectonics 15: 1036-1064
Moeck et al.
7
Schmid SM, Bernoulli D, Fügenschuh B, Matenco L, Schefer S, Schuster R, Tischler M, Ustaszewski K (2008) The Alpine-
Carpathian-Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss Journal of Geosciences 101: 139-183
Spina V, Borgomano J, Nely G, Shuchukina N, Irving A, Neumann C, Neillo V (2014) The evolution of the Kharyaga carbonate
platform during the Devonian - Carbonate response to the structural heritage. 76
th
EAGE Conference & Exhibition, 16-19 June
2014, Amsterdam, proceedings, 5 p
Stampfli GM, Borel G (2004) The TRANSMED transects in space and time: contraints on the paleotectonic evolution of the
Mediterranean domain. In: Cavazza W, Roure FM, Spakman W, Stampfli GM, Ziegler PA (Eds): The TRANSMED Atlas:
The Mediterranean Region from Crust to Mantle. Springer, Berlin and Heidelberg, 53-80
Stampfli, G (1994) Exotic terrains in the Alps: a solution for a single Jurassic ocean. Schweizerische Mineralalogische und
Petrographische Mitteilungen 74(3): 449-452
Trümpy R (1960) Paleotectonic evolution of the Central and Western Alps. Bull GSA 71: 843-908
Youn D, Gutierrez M (2011) Effect of fracture distribution on permeability of fractured rock masses. 45
th
US Rock
Mechanics/Geomechanics Symposium, San Francisco, CA, June 26-29, 2011, ARMA paper 11-329, 7 p
... Synsedimentary normal faulting during the Cretaceous is documented from the growth of faults in the frontal part of the French Alps at the western part of the Alpine Molasse Basin (e.g., Welbon 1988), however little is known about synsedimentary normal faulting in the central or eastern basin part. Multiphase normal faulting and synsedimentary faulting during Tertiary is indicated from fault throw analysis in 2D seismic data in the central Alpine Molasse Basin (Moeck et al. 2015a). In the study area, located in the eastern central basin part, the major movements incorporating the highest throws took place during the Upper Cretaceous, e.g., after the deposition of Malm and Purbeck sediments (cf. ...
... The Coniacian/Santonian (Upper Cretaceous) marked the onset of tectonic upheaval in response to initiation of the Alpine orogeny, followed by synsedimentary subsidence, overall increase of faulting and the development of north dipping, antithetic secondary faults (Freudenberger and Schwerd 1996). Various seismic data from the German Molasse Basin covering the central and part of the eastern part of the Alpine Molasse Basin reveal normal faulting until at least the Lower Miocene (Moeck et al. 2015a). Mechanisms for normal faulting in foreland basins contemporary to thrusting in the adjacent orogenic belt is described, e.g., by Blisnuik et al. (1998) from the Himalayan thrust front development. ...
... The QFEI gives information about the fault movement at a given time (e.g., during deposition of the horizon's sediments). This approach has been applied, e.g., by Moeck et al. (2015a) and Tvedt et al. (2013). ...
Article
Full-text available
The structural evolution of faults in foreland basins is linked to a complex basin history ranging from extension to contraction and inversion tectonics. Faults in the Upper Jurassic of the German Molasse Basin, a Cenozoic Alpine foreland basin, play a significant role for geothermal exploration and are therefore imaged, interpreted and studied by 3D seismic reflection data. Beyond this applied aspect, the analysis of these seismic data help to better understand the temporal evolution of faults and respective stress fields. In 2009, a 27 km2 3D seismic reflection survey was conducted around the Unterhaching Gt 2 well, south of Munich. The main focus of this study is an in-depth analysis of a prominent v-shaped fault block structure located at the center of the 3D seismic survey. Two methods were used to study the periodic fault activity and its relative age of the detected faults: (1) horizon flattening and (2) analysis of incremental fault throws. Slip and dilation tendency analyses were conducted afterwards to determine the stresses resolved on the faults in the current stress field. Two possible kinematic models explain the structural evolution: One model assumes a left-lateral strike slip fault in a transpressional regime resulting in a positive flower structure. The other model incorporates crossing conjugate normal faults within a transtensional regime. The interpreted successive fault formation prefers the latter model. The episodic fault activity may enhance fault zone permeability hence reservoir productivity implying that the analysis of periodically active faults represents an important part in successfully targeting geothermal wells.
... But depending on the previously mentioned factors these two elements can drastically vary in their hydraulic properties. Therefore, it is possible that fault zones, viewed as an integrative area from a hydraulic point of view, behave either transparent to fluid flow, as highly permeable conduits or as a barrier (Cacace et al. 2013;Caine et al. 1996;Micarelli et al. 2006;Michie et al. 2014;Moeck et al. 2015). Even though there is a high uncertainty involved in predicting this, for the exploration of the Upper Jurassic reservoir fault zones are generally thought of as an area with possibly increased hydraulic permeability and as an enhancement for the connection between well and aquifer (Böhm et al. 2013). ...
... The dip angle of the majority of faults in the Molasse Basin varies between 60° and 85°. This is based on available seismic data from five different locations (Weilheim, Dürrnhaar, Kirchstockach, Sauerlach, Geretsried) and also on the work of Moeck et al. (2015), von Hartmann et al. (2016 and Lüschen et al. (2014). Throw values of these ENE-WSW-trending normal faults are known to range from 25 to 300 m (Bachmann et al. 1987(Bachmann et al. , 1982Moeck et al. 2015;Schneider and Thomas 2012). ...
... This is based on available seismic data from five different locations (Weilheim, Dürrnhaar, Kirchstockach, Sauerlach, Geretsried) and also on the work of Moeck et al. (2015), von Hartmann et al. (2016 and Lüschen et al. (2014). Throw values of these ENE-WSW-trending normal faults are known to range from 25 to 300 m (Bachmann et al. 1987(Bachmann et al. , 1982Moeck et al. 2015;Schneider and Thomas 2012). On the other hand, few data are available to quantify their lateral extent. ...
Article
Full-text available
Abstract Fault zones in the Upper Jurassic aquifer of the North Alpine Foreland Basin are generally regions with possibly increased hydraulic properties. They are consequently often part of the geothermal exploration concepts in this area and a primary target for the drilling operation. Data from this aquifer, gathered in pump tests, however, show that only four out of 41 successful wells exhibit hydraulic proof for the presence of such a fault zone in terms of a bi-/linear flow regime. Besides technical effects, also the contrast in hydraulic properties itself, between fault zone and surrounding host rock, can prevent the detection of a fault zone in pump test data. This means a certain threshold has to be surpassed until its effects become clearly visible. A simplified realistic numerical model was constructed and calibrated with pressure data from an exploration site in the south of Munich. This model was then used to observe the presence of linear and bilinear flows in dependence on the Malm aquifers parameter space. Sampling the possible hydraulic property combinations with the help of an HPC (high-performance computing) cluster and automating the detection of the corresponding main flow type allowed to quantify the areas in parameter space where the fault zone-related flow regimes of interest are present. Through the investigation of more than 30,000 combinations between fault zone permeability, matrix permeability, fault zone storage, matrix storage and fault zone thickness, it was found that, in the parameter space of the Malm aquifer, a bilinear flow can be observed for the first time only if the matrix permeability is lower than 2.0 × 10−13 m2, and a linear flow for matrix permeability values below 6.0 × 10−14 m2. Additionally, it was shown that fault zones, which have better hydraulic properties than the surrounding matrix, can indeed be hidden in pumping tests due to the parameter setting.
... Dussel et al. (2016) e.g., determined mechanically altered zones along main faults with a width of 50-150 m. In contrast, facies and dolomitic domains can be reliably detected only after drilling and seismic-well log correlation (Moeck et al. 2015), or can be assessed from high resolution 3D-seismics, usually available only in advanced development stages and in project size areas (cf. Diepolder and GeoMol Team 2015). ...
... Diepolder and GeoMol Team 2015). From this perspective faults seem to be the most reliable target in geothermal exploration in the deep carbonate rocks (Moeck et al. 2015). Many successful drillings for geothermal installations over the last decade, specifically in the Molasse Basin (#1), have proved this approach but recent failures of ultra-deep explorations (> 5,500 m) show that it is not inherently propitious. ...
Conference Paper
Full-text available
Hydrothermal systems in deep carbonate bedrock are among the most promising low-enthalpy geothermal plays. Across Europe, apart from a few areas where viability of hydrothermal heat and power generation has been proved, most deep carbonate bedrock has received relatively little attention, because such rocks are perceived as ‘tight’. Exploration and development of the deep subsurface is an acknowledged high-risk investment, particularly in low-enthalpy systems, where tapping suitable temperatures for geothermal energy commonly requires drilling to depths of more than 3 km. In order to de-risk this geothermal exploration it is crucial to improve our understanding of generic geological conditions that determine the distribution and technical recoverability of their potential resources, specifically the possible groundwater yield controlled by fracture conduits and karstification. HotLime is one of 15 projects under the GeoERA umbrella that has received funding from the European Union's Horizon 2020 research and innovation programme. From July 2018 to June 2021 mapping, characterization and comparison of geological situations, the structural inventory of hydrothermal plays in deep carbonate rocks and their petro- and hydro-physical characteristics is carried out in 11 different target areas across Europe in order to identify the generic structural controls of geothermal plays in carbonates. The consistent assessment and the sharing of knowledge among the 15 European partners are geared towards uniformly applicable best practice workflows for estimation, comparison and prospect ranking of these hydrothermal resources. The principal outcomes of HotLime presently prepared will be spatial representations of the areas under investigation (3D models, 2D map series) on the principal geological features and properties relevant for geothermal exploration and production, supplemented by glossaries and a knowledge base including methods and tools which can be transferred and adapted to other carbonate rock suites.
... Henceforth, we generally refer to the Molasse sediments simply as Molasse. As the only one of seven Alpine foreland basins in Europe (Allen et al. 1986;Moeck et al. 2015b), the NAFB was partially covered by piedmont lobes of glaciers during the Quaternary glacial periods, in particular the last (Würm) glaciation, which lasted from about 115 to 11.7 ka (Seguinot et al. 2018;Van Husen 1987). Figures 1 and 2 show the extent of Würm glaciers in the MB at the Last Glacial Maximum (LGM) (Doppler et al. 2011;Gibbard et al. 2011;Van Husen 1987). ...
Article
Full-text available
The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field, more specifically the time-varying recharge and discharge governing groundwater and heat flow, are still debated. Within the Upper Jurassic (Malm) carbonate aquifer as the main geothermal reservoir in the Molasse Basin, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying fluid and heat transport processes are yet poorly understood. We delineate the two end members of thermal–hydraulic regimes in the Molasse Basin by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer along a model section through the Wasserburg Trough anomaly by means of two-dimensional numerical thermal-hydraulic modelling. We test the sensitivity of the thermal-hydraulic regime with regard to paleoclimate by computing the two Malm permeability scenarios both with a constant surface temperature of 9 °C and with the impact of paleo-temperature changes during the last 130 ka including the Würm Glaciation. Accordingly, we consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the subsurface targets of geothermal interest, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism.
... 3D seismic data and their attributes have been utilized in exploring reefs and karst systems (Lüschen et al., 2014;von Hartmann et al., 2012). Others have pointed out the importance of analyzing the permeability behavior of the E-W oriented faults zones (Moeck et al., 2015;Rojas et al., 2017;Mraz et al., 2018). Seithel et al. (2015) have emphasized the importance of applying geomechanical approaches in defining the structural permeability of the Upper Jurassic carbonates. ...
Conference Paper
Full-text available
Geothermal energy exploitation of the Upper Jurassic carbonates underlying the South German Molasse Basin is challenging due to its heterogeneous geological and structural characteristics (karst, fault zones, fractures, and lithofacies). Our approach is based on analyzing depth migrated 3D seismic data for mapping reef growth and areas of intensified karstification, as well as borehole data for correlation and calibration. A 3D facies model created by three seismic attributes displays the distribution of chaotic/massive and laminated/bedded facies of the Upper Jurassic carbonates. Geomechanical-geometrical considerations constrain the in situ stress state and the maximum horizontal stress orientation which found to be matching with the regional in situ stress field and observed open fracture orientation. The integration of these methodologies minimizes exploration risks related to the heterogeneous permeability distribution within the Upper Jurassic carbonates. They derive their validity from nearby projects results, well data, and geological observation, where the ease of their application decreases the overall exploration costs for the development of geothermal projects in the Molasse Basin.
... Intersection lines of "x-shaped" fault and fracture sets produce dilatational zones under various tectonic stress regimes and provide lateral and vertical optimal channels for fluid flow (e.g., [11,[43][44][45]). Such structural intersections represent thus "pipe-like" structures that can act as fluid preferential pathways and may be kept open even in compressional stress field [46]. Therefore, the current and fossil fluid outflows at the different working sites highlight that this local "x-crossing" structural pattern produces sufficient relative vertical fracture permeability to drive significant amount of fluids, in regard to other locations along the TBFS without structural intersection. ...
Article
Full-text available
Regional fault structures along rift basins play a crucial role in focusing fluid circulation in the upper crust. The major Toro-Bunyoro fault system, bounding to the east of the Albertine Rift in western Uganda, hosts local fluid outflow zones within the faulted basement rocks, one of which is the Kibiro geothermal prospect. This major fault system represents a reliable example to investigate the hydrogeological properties of such regional faults, including the local structural setting of the fluid outflow zones. This study investigated five sites, where current (i.e., geothermal springs, hydrocarbon seeps) and fossil (i.e., carbonate veins) fluid circulation is recognized. This work used a multidisciplinary approach (structural interpretation of remote sensing images, field work, and geochemistry) to determine the role of the different macroscale structural features that may control each studied fluid outflow zones, as well as the nature and the source of the different fluids. The local macroscale structural setting of each of these sites systematically corresponds to the intersection between the main Toro-Bunyoro fault system and subsidiary oblique structures. Inputs from three types of fluid reservoirs are recognized within this fault-hosted hydrogeological system, with “external basin fluids” (i.e., meteoric waters), “internal basin fluids” (i.e., hydrocarbons and sediment formation waters), and deep-seated crustal fluids. This study therefore documents the complexity of a composite hydrogeological system hosted by a major rift-bounding fault system. Structural intersections act as local relative permeable areas, in which significant economic amounts of fluids preferentially converge and show surface manifestations. The rift-bounding Toro-Bunyoro fault system represents a discontinuous barrier for fluids where intersections with subsidiary oblique structures control preferential outflow zones and channel fluid transfers from the rift shoulder to the basin, and vice versa. Finally, this work contributes to the recognition of structural intersections as prime targets for exploration of fault-controlled geothermal systems.
Article
In the North Alpine Foreland Basin, especially in the greater Munich area, several geothermal plants exploit a deep hydrothermal reservoir. So far at three sites events with ML>2.0 were detected, in an otherwise seismically inactive region. In this study we investigate a site east of Munich, where moderate seismicity started to appear around five years after the beginning of geothermal circulation. Two larger events (ML2.1/1.8) occurred in December 2016, followed by an additional ML2.1 event about 10 months later in September 2017. All three events occurred at about 3 km depth and were felt by the population. As this is a densely inhabited area, the estimation of the maximum ground motion and its distribution are of great importance for the authorities and the public. We perform 3D seismic simulations of the main events using the spectral element code SALVUS. The results can supplement recorded and macroseismic data in order to estimate the possible seismic impact in the area. In addition, we evaluate the influence of the uncertainties contained in the event parameters and in the subsurface model on the maximum peak ground velocity (PGV) values to calibrate the simulations. The simulated waveforms are mostly in good agreement with the ground motion recordings. Furthermore, the ground motion distribution coincides with the macroseismic data. According to the simulation results, even the largest event in the area did not exceed the critical PGV value of 5 mm/s defined by German norms and therefore had no damage potential. Such a numerical approach can help to improve the seismic monitoring network, identify affected zones and mitigate the seismic risk.
Article
Fault zones (FZ) are major components of geothermal exploration concepts for the Southern German Upper Jurassic aquifer (UJA). Because these sections of possibly favorable hydraulic properties can be hidden in pumping test data, their explorational importance with respect to well productivity is still debated. In this work, the effect of hydraulically active FZ on the well-productivity-index (PI) is quantified in dependency of the UJA parameter space using numerical modeling and the Reduced Basis method. Results suggest that hidden FZ can increase PI significantly, while a hydraulically active FZ thickness of ≥ 100 m is unlikely for the Upper Jurassic aquifer.
Preprint
Full-text available
The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field are still debated. Especially seismic and deep well data from extensive oil and gas exploration in the Molasse Basin led to conceptual hydrogeological and thermal-hydraulic models. Corrected borehole-temperature data helped to constrain subsurface temperatures by geostatistical interpolation and facilitated the set-up of 3D temperature models. However, within the geothermally used Upper Jurassic (Malm) carbonate aquifer, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying physical processes are yet poorly understood. From other foreland basins like the Alberta Basin in Western Canada, it is known that climate during the last ice age has a considerable effect even on subsurface temperatures up to two kilometres depth. Therefore, we study the impact of paleoclimatic changes on the Molasse Basin during the last 130 ka including the Würm glaciation. We consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. The major difference between the thermal-hydraulic regime in the western and eastern parts of the Southern German Molasse Basin are delineated by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the geothermally drillable subsurface, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism. Practically, the results might help to reduce the exploration risk for geothermal energy projects in the Molasse Basin. More importantly, this study serves as a reference for the comparison and understanding of the interplay of high permeability aquifers, gravity-driven groundwater flow and paleoclimate in other orogenic foreland basins worldwide.
Article
Full-text available
The Brianconnais area is explained as a large scale exotic terrain separating from Europe during the opening of the Valais ocean. Its displacement history during the Alpine evolution replaces older concepts of multiple oceans separating narrow strips of continental crust. -Author
Chapter
Full-text available
The Phanerozoic evolution of the western Tethyan region was dominated by terrane collisions and accretions, during the Variscan, Cimmerian and Alpine cycles. Most terranes were derived from Gondwana and present a similar early Palaeozoic evolution. Subsequently, they were detached from Gondwana and affected by different deformation and metamorphic events, which permit to decipher their geodynamic history. Lithospheric scale peri-Mediterranean transects show the present-day juxtaposition of these terranes, but do not allow to unravel their exotic nature or their duplication. To create a reliable palinspastic model around these transects, plate tectonics constraints must be taken into consideration in order to assess the magnitude of lateral displacements. For most of the transects and their different segments, thousand km scale differential transport can be demonstrated.
Article
The geometry of rock discontinuities plays a crucial role in permeability of fractured rock. However, exact determination of the fracture system is not possible due to the high scale dependency of the system. Often only limited data are available from core samples, outcrop analogues and seismic surveys. This research is intended to evaluate the scale effects on permeability in fractured rock mass using Oda's permeability tensor and Monte Carlo simulation. Power law and Fisher distributions are used to generate realistic fractured rock sample in the simulations. Fracture distribution is related with permeability distribution in the first part of the research, and analysis on permeability variations in different volume scales and fracture sampling ratios is conducted. The above comparison and analysis results yield the possible fracture geometry and permeability relationship without requiring redundant calculation. The permeability distribution from different sampling volume ratios shows the expected tendency in the specific range of volume ratio, determined by intensity of fracture. In the last part of the report, proportional contribution of fracture length distribution is also examined, and compared with the above result.
Article
In naturally fractured reservoirs, fractures are the main flowing channels, while matrix is the dominant storage space. The oil/water relative permeability curve for the fracture in this kind of reservoir is very important for water-injection field development. In this study, we conducted experiments on the oil/water relative permeability of carbonate cores from Kenkiyak oil field and compared the differences in relative permeability curves between natural matrix cores and artificial-fractured cores. After the fracturing process, the two-phase flow area of tested cores becomes narrower, the permeability of the equal-permeability point gets higher, the relative permeability curve rises or drops more rapidly, and the displacement recovery efficiency decreases. The stress-sensitivity characteristics of the relative permeability curves were also studied on the basis of experiments on naturally fractured cores. With increasing effective confining pressure, the irreducible water saturation increases, the residual-oil saturation changes slightly, the equal-permeability point moves downward, and the displacement recovery efficiency declines. Numerical-simulation results indicate that for a given recovery factor, the water cut would increase more slowly but ultimate recovery factor would decrease using the relative permeability curve under higher confining pressure. Therefore, the water injection should be operated when the reservoir pressure is relatively higher to maintain formation pressure during waterflooding and lower the impact of stress sensitivity accordingly.
Chapter
foreland basins - an introduction;foreland basins - sedimentary basins' mountain chain front and adjacent craton;lithospheric behaviour;subsidence history and depocentre mobility;foreland basin deposit petrography;molasse deposits - analysis of late orogenic histories;foreland basin evolution;foreland basins - model with respect to sedimentation and tectonics
Article
An increasing number of low-enthalpy geothermal power plants are being implemented in the Munich area with holes drilled into the Malm Formation (Upper Jurassic carbonates) to depths greater than 3000 m, where temperatures in excess of 1000C are encountered. Optimal development of geothermal productivity requires exploration of the geological structure, as well as information on the karstification of the Malm. Seismic surveys, including reprocessed older 2D lines from hydrocarbon exploration, new 2D lines filling gaps, and 3D surveys, are increasingly used for geothermal exploration. A geothermal power plant at Unterhaching, near Munich, has been producing heat and electrical power since 2009. A high resolution 3D seismic volume has been acquired over an area of 27 km2 for hydrological modelling purposes at the Gt2 reinjection well, located within the southern suburbs of Munich. First results show preferential hydraulic pathways along reactivated fault zones within the Malm reservoir which were partly active until the Aquitanian (Early Miocene). Additionally, a great number of sinkholes have been identified at the top Malm, located along fault zones which constitute new possible targets for geothermal exploration.
Article
Hydraulic stimulation is frequently used to enhance reservoir productivity. The aim of hydraulic stimulation is to increase the formation pressure by fluid injection to create artificial fractures that act as additional fluid pathways. But large-scale fluid injection as applied in hydrocarbon and geothermal reservoirs can also induce seismicity and fault reactivation depending on the reservoir geomechanics and stress regime. Recent case studies in stimulation of geothermal reservoirs have shown induced seismicity as an undesirable side effect which needs to be understood prior to massive fluid injection. Slip tendency analysis has been successfully used to characterize fault slip likelihood and fault slip directions in any stress regime. In our study, we applied slip tendency analysis to assess the reactivation potential of shear and dilational fractures in a deep geothermal reservoir in the North-East German Basin, based on the notion that slip on faults is controlled by the ratio of shear to normal effective stress acting on the plane of weakness. The results from slip tendency analysis are supported by the spatial distribution of recorded microseismicity, which indicates slip rather than extension along a presumed NE-striking failure plane.
Article
Crossing conjugate normal faults are common in many hydrocarbon-producing basins. In these settings, they exert a range of influences from trapping hydrocarbon accumulations to producing permeability anisotropy by preferentially enhancing or reducing permeability, and reducing effective thicknesses of seal and reservoir units. The fault intersection region is typically poorly imaged with seismic data, and consequently, developing a coherent interpretation of deformation in the intersection region is difficult. In this article, we explore crossing conjugate normal faults across two orders of magnitude of displacement using clear field exposures from the western United States and subsurface examples from the Jeanne d'Arc Basin, offshore Newfoundland. We demonstrate common structural elements and potential pitfalls associated with interpretation of crossing conjugate normal faults, and emphasize the widespread and often unrecognized occurrence of these structures.
Book
As nations alike struggle to diversify and secure their power portfolios, geothermal energy, the essentially limitless heat emanating from the earth itself, is being harnessed at an unprecedented rate. For the last 25 years, engineers around the world tasked with taming this raw power have used Geothermal Reservoir Engineering as both a training manual and a professional reference. This long-awaited second edition of Geothermal Reservoir Engineering is a practical guide to the issues and tasks geothermal engineers encounter in the course of their daily jobs. The book focuses particularly on the evaluation of potential sites and provides detailed guidance on the field management of the power plants built on them. With over 100 pages of new material informed by the breakthroughs of the last 25 years, Geothermal Reservoir Engineering remains the only training tool and professional reference dedicated to advising both new and experienced geothermal reservoir engineers. The only resource available to help geothermal professionals make smart choices in field site selection and reservoir management Practical focus eschews theory and basics- getting right to the heart of the important issues encountered in the field Updates include coverage of advances in EGS (enhanced geothermal systems), well stimulation, well modeling, extensive field histories and preparing data for reservoir simulation Case studies provide cautionary tales and best practices that can only be imparted by a seasoned expert.