SDScientific DrillingSDSci. Dril.1816-3459Copernicus PublicationsGöttingen, Germany10.5194/sd-23-13-2017Drilling into an active mofette: pilot-hole study of the impact of
CO2-rich mantle-derived fluids on the geo–bio interaction in the
western Eger Rift (Czech Republic)BussertRobertr.bussert@tu-berlin.deKämpfHorstFlechsigChristinaHesseKatjaNickschickTobiasLiuQiUmlauftJosefineVylitaTomášWagnerDirkWonikThomasFloresHortencia EstrellaAlawiMashalInstitute of Applied Geosciences, Technische Universität Berlin,
13355 Berlin, GermanyGFZ German Research Centre for Geosciences, Section 3.2: Organic
Geochemistry, 14473 Potsdam, GermanyInstitute for Geophysics and Geology, University of Leipzig, 04103
Leipzig, GermanyLeibniz Institute for Applied Geophysics, 30655 Hannover, GermanyGFZ German Research Centre for Geosciences, Section 5.3: Geomicrobiology,
14473 Potsdam, GermanyBalneological Institute, 360 01 Karlovy Vary, Czech RepublicRobert Bussert (r.bussert@tu-berlin.de)30November201723132719April201720July201715August2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://sd.copernicus.org/articles/23/13/2017/sd-23-13-2017.htmlThe full text article is available as a PDF file from https://sd.copernicus.org/articles/23/13/2017/sd-23-13-2017.pdf
Microbial life in the continental “deep biosphere” is closely
linked to geodynamic processes, yet this interaction is poorly studied. The
Cheb Basin in the western Eger Rift (Czech Republic) is an ideal place for
such a study because it displays almost permanent seismic activity along
active faults with earthquake swarms up to ML 4.5 and intense degassing of
mantle-derived CO2 in conduits that show up at the surface in form of
mofettes. We hypothesize that microbial life is significantly accelerated in
active fault zones and in CO2 conduits, due to increased fluid and
substrate flow. To test this hypothesis, pilot hole HJB-1 was drilled in
spring 2016 at the major mofette of the Hartoušov mofette field, after
extensive pre-drill surveys to optimize the well location. After drilling
through a thin caprock-like structure at 78.5 m, a CO2 blowout occurred
indicating a CO2 reservoir in the underlying sandy clay. A pumping test
revealed the presence of mineral water dominated by Na+, Ca2+,
HCO3-, SO42- (Na-Ca-HCO3-SO4 type) having a temperature
of 18.6 ∘C and a conductivity of 6760 µScm-1. The
high content of sulfate (1470 mg L-1) is typical of Carlsbad Spa
mineral waters. The hole penetrated about 90 m of Cenozoic sediments and
reached a final depth of 108.50 m in Palaeozoic schists. Core recovery was
about 85 %. The cored sediments are mudstones with minor carbonates,
sandstones and lignite coals that were deposited in a lacustrine environment.
Deformation structures and alteration features are abundant in the core.
Ongoing studies will show if they result from the flow of CO2-rich
fluids or not.
Introduction
Microbial processes in the “deep biosphere” and their interaction with
geological processes are a matter of ongoing debate. Microbial habitats
extend down to great depths beneath the earth's surface. However, cell counts
and detailed characterizations of the microbial community structure in the
continental deep biosphere are rare and mostly related to investigations in
oil reservoirs (Youssef et al., 2009), geothermal aquifers (Alawi et al.,
2011; Lerm et al., 2013) or gold mines (Deflaun et al., 2007; Takai et al.,
2001; Trimarco et al., 2006). A few continental drilling campaigns have
focused on the deep biosphere (Fredrickson et al., 1997; Onstott et al.,
1998; Zhang et al., 2005), though the state of knowledge reached is still in
the early stages. In deep saline aquifers intended for CO2 capture and
geological storage (CCS), changes in the microbial community caused by
injected CO2 can induce mineral dissolution and precipitation or the
formation of biofilms (Onstott, 2005; Mitchell et al., 2009), and might affect
long-term storage efficiency and reliability (Morozova et al., 2010; Gulliver
et al., 2016; Pellizzari et al., 2016).
In the scope of the International Continental Scientific Drilling Program (ICDP) project “Drilling the Eger
rift: Magmatic fluids driving the earthquake swarms and the deep biosphere”
(Dahm et al., 2013), one focus is to determine to which extent the microbial
communities are conditioned by the mantle-derived CO2 degassing and how
the microbial activity is potentially affected by seismic events such as
swarm earthquakes. As a pre-examination, a research project “Microbial
processes in the deep biosphere of the CO2-dominated active fault zone
in NW Bohemia” started in 2016 and included a 108.5 m deep drilling (Alawi
et al., 2015). We assume that in active fault zones, due to an intensified
substrate support, microbial processes are significantly accelerated compared
to other deep subsurface ecosystems. Similar to black smokers in the deep
sea, active fault zones might represent “hot spots” of microbial life in
the deep subsurface (Alawi et al., 2015). The combination of intense
CO2-rich mantle degassing, ongoing seismic activity including
earthquakes swarms and microbiological activity that occurs in the Cheb
Basin (western Eger Rift, Czech Republic; Fig. 1) is exceptional and allows
the study of bio–geo interactions as part of active geodynamic processes in
the lithosphere (Kämpf et al., 1989, 2005, 2007, 2013; Wagner et al.,
2007; Bräuer et al., 2007, 2008, 2011, 2014; Dahm et al., 2013; Fischer
et al., 2014, 2017; Alawi et al., 2015; Schuessler et al., 2016).
Geological map of the Cheb Basin and surroundings, based on Flechsig
et al. (2008) and Dahm et al. (2013).
About 10 km south of the Nový Kostel focal zone epicentral area
(Fig. 1), strong subcontinental mantle-dominated CO2 degassing occurs in
the Bublák and Hartoušov mofette fields (Bräuer et al., 2003,
2014; Kämpf et al., 2013; Nickschick et al., 2015, 2017). Mofettes are
places where geogenic CO2 ascends through conduits from the mantle to
the surface (Fig. 2). The CO2 conduits are regarded as important
structures of lithospheric mantle–crust interaction via mantle fluids.
However, both rock-fluid and geo–bio interactions in these structures are
hardly understood.
The major wet mofette in Hartoušov is located in the direct
vicinity of the drill site. At the upper and lower part of the mofette
bubbles of CO2-rich gas are visible.
Microbial growth in the deep subsurface is limited by factors such as
temperature, pH, redox potential, gas concentration, water and substrate
availability (Kallmeyer and Wagner, 2014). The conditions for life in the
deep biosphere are extreme in comparison to most surface environments: no
light is available for photosynthesis, typically much higher temperatures
than in surface habitats prevail, the availability of water and of organic
carbon is severely restricted, and high gas pressures occur. The exchange of
substances is crucially reduced since it is coupled to water availability and
to diffusion processes. Therefore, microbial activities are slowed and
microbial turnover times are strongly decreased (Lomstein et al., 2012).
However, microbes can survive long periods of starvation by reducing their
metabolism or forming spores (Goldscheider et al., 2006; Hubert et al.,
2009). Interestingly, there are hints that the microbial turnover in active
fault zones is significantly accelerated in comparison to the ordinary
terrestrial deep biosphere. Eight weeks after an earthquake swarm occurred in
the northwestern part of the Cheb Basin, the amount of microbiologically
produced methane increased significantly and persisted for more than two
years (Bräuer et al., 2005, 2007). The authors assume that hydrogen was
released from a fractured granitic aquifer during seismic activity and became
available for microorganisms. In the same region, Schuessler et al. (2016)
explain recurring changes in iron isotope signatures of mineral spring water
by a combination of abiotic and biotic processes triggered by swarm
earthquakes. CO2 as the dominating gas in the Cheb Basin might liberate
organic compounds (Glombitza et al., 2009; Sauer et al., 2012); enhance the
dissolution, transformation and precipitation of minerals (Rempel et al.,
2011); and could thereby affect the availability of substrates for
microorganisms. These examples lead to the question of whether and how
geodynamic processes such as earthquakes can trigger microbiological activity
by transport of substrates and changes in environmental conditions. Our main
interests are to determine to which extend the microbial communities are
conditioned by the mantle-derived CO2, how the microbial activity is
potentially affected by seismic activity such as earthquake swarms, and if
active fault zones, due to an intensified substrate support, lead to
significantly accelerated microbial activity compared to other deep
subsurface ecosystems.
The present pilot-hole study that precedes the ICDP drilling
project “Drilling the Eger Rift”
(http://web.natur.cuni.cz/uhigug/icdp; Dahm et al., 2013) focuses on
the interaction between lithospheric geodynamic activity driven by magma
generation and magma/fluid escape beneath the Cheb Basin in the western Eger
Rift and the microbial communities of the deep biosphere in the upper crust.
Here we describe the 108.5 m deep drilling of well HJB-1 into an active
CO2 conduit in the Cheb Basin, located at the northern part of the
Hartoušov mofette field. It was intended to core as continuously as
possible down to the bottom of the lacustrine succession. The operational
challenges were manifold, starting with the logistics and technical
feasibility of the drilling to the high standards we require with regard to
assessing sample contamination through infiltration of drill mud. Because of
the potential risk of a spontaneous gas/fluid blowout during drilling, extra
safety measures including the application of a blowout gas preventer,
high-density bentonite-based drill mud and gas alarm techniques had to be
employed.
Geological background
The Cheb Basin represents the shallow western part of the Cenozoic
Ohře/Eger Rift, the easternmost segment of the European Cenozoic Rift
System that has developed in response to intraplate stresses exerted from the
Alps, and possibly to thermal doming (Malkovský, 1987; Rajchl et al.,
2009). The basin is located at the intersection of the ENE–WSW trending Eger
Rift with the N–S striking Regensburg–Leipzig–Rostock zone respectively its
major local segments, the Počatky–Plesná Fault zone and the
Mariánské Lázně Fault zone (Fig. 1; Bankwitz et al., 2003).
It is underlain by Palaeozoic metamorphics and granites (Hecht et al., 1997;
Fiala and Vejnar, 2004) and bounded on its eastern side by the
morphologically distinct scarp of the Mariánské Lázně Fault
zone (Peterek et al., 2011), and to the west and to the south by the Fichtel
(Smrčiny) Mountains and the Oberpfalz Forest.
The fill of the Cheb Basin consists of less than 300 m of continental
sediments. Sedimentation started in the Eocene with the local deposition of
clays and sands, possibly in maars, referred to as the Staré Sedlo
Formation (Fm.; Špičáková et al., 2000; Pešek, 2014).
Following a phase of uplift and erosion, sedimentation commenced with the
deposition of gravels, sands and clays of the Oligocene (Chattian) to Early
Miocene (Early Aquitanian) Lower Argillaceous-Sandy Fm. (Pešek et al.,
2014). In the Lower Miocene, the coal-bearing Main Seam Fm. formed in
an alluvial landscape enclosing extensive wetlands. Subsequently, a large
lake developed in which the clay-dominated Cypris Fm. was deposited
(Rojik, 2004). After a hiatus, sedimentation started again in the Pliocene
with lacustrine clays, sands and gravels of the Vildštejn Fm. and
continued without an obvious break into the Quaternary (Pešek et al., 2014).
In the surrounding of the Eger Rift, volcanism was temporarily active during
the Cenozoic. In the Quaternary, volcanic activity formed two small scoria
cones with lava flows and a just recently discovered explosive maar structure
(Mrlina et al., 2007, 2009; Flechsig et al., 2015; Ulrych et al., 2016).
Ongoing tectonic activity in the Cheb Basin is manifested by earthquake
swarms that concentrate along the northern segment of the Mariánské
Lázně Fault zone (Fig. 1). The strongest registered earthquakes
reached local magnitudes of ML 4.5 (Fischer et al., 2014). Active fault zones
very likely represent migration pathways for the degassing of mantle-derived
CO2 that causes intense mofette activity (Kämpf et al., 2013;
Nickschick et al., 2015, 2017), while the ascent of magmas and the fluid
activity probably constitute the forcing mechanisms of the seismic activity
(Bräuer et al., 2003, 2008, 2011, 2014; Dahm et al., 2008; Fischer et
al., 2014).
Hydrogeological background
Numerous mineral water springs occur in the Cheb Basin. In spa towns such as
Františkovy Lázně the springs are used for illness prevention, and
rehabilitation and consequently their catchment areas are safeguarded as
protection zones. The springs are linked to gas-saturated and highly
mineralized waters of an aquifer located at the eastern margin of the
Mariánské Lázně Fault zone. Most mineral waters are of the
Carlsbad Spa type, i.e., Na-HCO3(SO4Cl) to
Na(Ca)-HCO3(SO4). Total dissolved solids are highly variable
and range from a few mg L-1 to over 20 g L-1. The components are
of a complex origin with both exogenous (oxidative and hydrolytic) and
endogenous (hydrolytic and possibly fossil and evaporitic) contributions
(Egeter et al., 1984; Dvořák, 1998; Paces and Smejkal, 2004).
Microbiological background
Microorganisms involved in all major global biogeochemical cycles exist in
the deep biosphere. They are capable of catalyzing reactions between gases,
fluids, sediments and rocks, thus enhancing mineral alteration as well as
precipitation. Depending on respective subsurface environmental conditions,
(hyper-)thermophilic and halotolerant microorganisms were identified.
Abundant metabolic groups are for example methanogenic archaea and
sulfate-reducing bacteria, and strains from both of these taxa are able to
obtain their carbon solely from CO2 (Alawi et al., 2011). McMahon and
Chapelle (1991) highlight that more than 90 % of the 16S ribosomal DNA
sequences recovered from hydrothermal waters circulating through deeply
buried igneous rocks in Idaho are related to hydrogenotrophic methanogenic
microorganisms. Geochemical characterization indicates that hydrogen is the
primary energy source for this methanogen-dominated microbial community.
These results demonstrate that hydrogen-based microbial communities do occur
in earth's deep biosphere. Considering increased hydrogen concentrations
during seismic periods in the Cheb Basin (Bräuer et al., 2005) one might
conclude that the microbial activity is potentially positively correlated to
hydrogen availability, and therefore increased seismicity. We assume that a
proliferating primary production based on methanotrophic archaea might
provide the starting point for a secondary heterotrophic microbial community.
As Alawi (2014) has shown elsewhere, such microorganisms produce energy-rich
organic polymers that might be subsequently degraded by fermentative
processes and thereby can close the carbon cycle by the emission of CO2
as well as H2. Acetate which is produced by acetogenic bacteria may then
become a valuable substrate for FeIII, MnIII,IV and
SO42- reducing microorganisms as well as acetoclastic methanogens
(Alawi, 2014). In addition, first analyses of the microbial communities in
wetland soils of the Bublák mofette field in the Cheb Basin show that
both bacteria and archaea are able to incorporate 13C-labeled CO2
(Beulig et al., 2015, 2016). Hence, an effect of the increased CO2
concentrations on the composition of the microbial community seems very
likely. Despite various indicators for geo–bio interactions in the deep biosphere, it remains to be understood precisely how geological processes
influence microbial activities in the deep subsurface and what role these
processes have played in the geological evolution of the earth through time.
Pre-drilling site surveys
During the last years several geophysical, soil gas and gas flux analyses
were performed to understand the patterns of CO2 degassing at the
Hartoušov mofette field. Well sections of prior boreholes drilled in the
region, made available by the Czech Geological Survey
(http://www.geology.cz/extranet-eng/services/data), provided provisional
information on the near sub-surface sediments but detailed data on the
mofette field were first acquired in a scientific drill campaign in 2007
(Flechsig et al., 2008). The objective of the pre-drill surveys was to
understand the structural and sedimentological control of CO2 degassing
and to determine an optimal drill site of intense degassing underlain by a
conduit.
CO2 mapping
Mapping of the temporal and spatial pattern of mantle-related CO2
degassing in the Hartoušov mofette field (Fig. 3) revealed distinct
differences in the spatial pattern of emitted CO2, with low emission
rates in a north–south trending zone in the southern part of the mofette
field and heavy degassing in the central and northern part (Schütze et
al., 2012; Kämpf et al., 2013; Rennert and Pfanz, 2016; Nickschick et
al., 2015, 2017). This led Nickschick et al. (2015) to hypothesize that
sinistral strike-slip fault movement causes the opening of pull-apart
structures, in which intense mantle-derived CO2 degassing occurs in
conduits. For the total area of 0.35 km2 Nickschick et al. (2015)
estimate that between 23 and 97 t of CO2 are emitted each day.
Location of the ICDP drill HJB-1 (black star) in the Hartoušov
mofette field, with CO2 gas flux mapping results of Nickschick et al.
(2015). The black rectangle shows the area of passive seismic noise
measurements (Fig. 6 and Sect. 5.3; matched field processing, MFP). The location of
the geoelectrical profile in Fig. 5 is indicated by the orange line.
Coordinates are in UTM zone 33N.
The results of Nickschick et al. (2015) formed a major basis for the
selection of the drill site of HJB-1 (Fig. 3). Located in the central part of
the Hartoušov mofette field, CO2 emission rates here can vary
considerably, but are generally high (Nickschick et al., 2015). During a test
study in 2012, we measured daily CO2 gas fluxes on the spot that later
became the drill site. Emission rates varied between ∼ 14 and
43 kg m-2 d-1 (Fig. 4) with a mean rate of 27.5±9.5 kg m-2 d-1 in the observation period. Because of the
continuously high CO2 gas fluxes, the site proved an ideal location for
HJB-1.
Variations in the CO2 gas flux at the drill site of HJB-1
measured in 2012.
Geoelectrical near-surface surveys
The main objective of geophysical surveys in the Hartoušov mofette field
was to detect and characterize subsurface structures that potentially
represent fluid pathways or domains of fluid–rock interaction. We used
preferentially electrical resistivity tomography (ERT), since the
resistivity of rocks is notably sensitive to the presence of fluids. To
retrieve a detailed conductivity image of the mofette field, modern ERT
inversion and modeling techniques were applied (Günther et al., 2006;
Günther and Rücker, 2009).
Combined with sedimentological studies and CO2 soil gas measurements,
the ERT surveys provided an image of near-surface structures down to a depth
of ∼ 100 m underneath the mofette field (Flechsig et al., 2008, 2010;
Schütze et al., 2012; Nickschick et al., 2015, 2017). The detected
structures are, most probably, directly or indirectly caused by CO2 flow
because the geophysical subsurface anomalies not only correlate positively
with areas of high CO2 but also with sediment properties such as
elevated organic carbon (Corg) and pyrite contents, and with the occurrence of
dispersed quartz pebbles in fine-grained sediments. The sedimentological
properties are likely related to chemical and physical conditions caused by
high concentrations of CO2 in the sediments, and to high gas pressure.
Near-surface features such as low-permeability beds seem to influence primarily
the variation in CO2 degassing linked to meteorological conditions. The
presence of such beds could cause the temporal accumulation of CO2 in
underlying porous sand layers.
ERT was also used in time-lapse mode to detect temporal changes in subsurface
resistivity, caused by fluctuating relations of the gaseous to liquid phase,
and was combined with repeated soil gas measurements (Nickschick et al.,
2017). To evaluate the stability of subsurface degassing structures over
time, repeated measurements were carried out (Nickschick et al., 2017). Two
repeated 2D-ERT surveys in 2007 and 2016 at the central mofette (Fig. 5)
indicated small-scale near-surface (< 2 m depth) variations in the
resistivity, caused by meteorological and seasonal influences, while a
distinct anomaly below the central mofette at 45–55 m correlates positively
with a zone of high-intensity soil degassing (Fig. 3). The detected changes
suggest that CO2 degassing sites are not steady structures; instead,
their architecture changes over time spans of days to years (Nickschick et
al., 2017).
Results of repeated 2-D resistivity surveys at the central mofette in
Hartoušov, (a) inversion model of the resistivity (Ωm) distribution
measured in May 2007, (b) inversion model
of the resistivity distribution measured in October 2016 (inversion code
DC2DInvRes) and (c) relative change in the subsurface resistivity by
inversion of the ratio of data from the initial (2006) and later (2016) data
sets. A variation of 1 means that the resistivity has not changed, a
variation of 1.2 represents an increase of 20 %.
matched field processing (MFP) results (normalized) for the array
deployed in May 2015 in the Hartouŝov mofette field. (a) Surface
plot. The white triangles indicate the position of stations. The black star shows
the location of HJB-1. (b) 3-D depth plot (0–30 m).
Matched Field Processing (MFP) for noise source localization
Subsurface CO2 flow is often accompanied by gas bubble collapses that
act as ambient noise sources and produce seismic signals. With the help of
dense small-aperture instrumental arrays and matched field processing (MFP)
techniques, the noise sources can be located (Vandemeulebrouck et al., 2010;
Corciulo et al., 2012; Flores Estrella et al., 2016). The data are typically
displayed as a 3-D probability map that illustrates the distribution of noise sources within and beneath the
array. In this way, degassing spots such as mofettes can be detected, and
their corresponding subsurface feeding channels can be imaged (Flores
Estrella et al., 2016).
In May 2015 we measured continuous seismic noise with an instrumental array
of 25 stations (vertical geophones connected to REF TEK Texan recorders)
covering 1 ha surface area in the Hartoušov mofette field. The normalized MFP
output shows three clearly defined surface maxima (values ∼ 1; Fig. 6a).
Including areas with medium MFP amplitude (values between 0.35 and
0.6), the sources form a NW–SE trending zone of increased noise activity.
While the northernmost source is only visible down to a depth of 10 m
(Fig. 6b), the other two sources are still recognizable in the 10–18 m
depth interval but continuously decay below 18 m in amplitude and they
widen. In the depth interval 18–30 m another source appears in between, but
contrary to the other ones it shows a steady amplitude increase with depth.
The measurements suggest the presence of two major fluid channels to the NE
of the array that reach from the surface down to a depth of at least 30 m.
Another channel in the northernmost part of the array is limited to the
uppermost 10 m. A fourth channel seems to exist between the two major
channels starting at 18 m depth and continuously increases in MFP amplitude
with depth. The deep-seated channel might form the main feeder channel for the
near-surface channels.
Shallow wells
The borehole HJB-1 was placed in an area in which the two exploratory
drillings HJ-3 and HJ-4 were conducted in 1993, exploring the presence of
groundwater of deeper aquifers closer to the crossing of faults. Next, direct
information on the shallow subsurface sediments in the Hartoušov mofette
field was gained in an exploratory drill campaign in 2007 when five shallow
wells reached depths of up to 9 m (Flechsig et al., 2008). The main
objective of the campaign was to compare near-surface sediments underneath an
active mofette with sediments in reference sites likely not affected by
CO2 emissions. The authors assumed that sediments in mofette sites are
influenced by accelerated silicate weathering due to acidifying and leaching
effects of CO2, decelerated decomposition of organic matter, and
enhanced preservation and possibly formation of sulfides and sulfates.
Another purpose of the pilot–hole was to corroborate the interpretation of
geoelectrical lines measured at that time.
The drillings showed that the uppermost sediments in the mofette field
consist of Quaternary fluviatile channel and flood plain deposits of the
Plesná river, while the lower section is made up of Pliocene lacustrine
clays. Drilling in the central mofette site revealed the occurrence of
dispersed pebbles in fine-grained sediments and confirmed the presence of a
domal uplift of the Pliocene clays already recognized in geoelectrical data
(Flechsig et al., 2008). Laboratory analyses showed increased
Corg and pyrite contents of the sediments in areas of high
CO2 degassing. The results of the drilling campaign supported the
hypothesis that in areas of high CO2 degassing, such as mofettes,
physical and mineralogical properties of sediments can be significantly
influenced by CO2.
Drilling and coring, hydraulic and geochemical analyses, logging
operations, and sampling
The drill site of HJB-1 (Fig. 7) is located in the protection zone of natural
healing resources of the spa town of Františkovy Lázně
(Franzensbad) but likewise in the protected area of natural water
accumulation of the Cheb Basin and the Slavkovský Forest. Thus, drilling
permissions had to be obtained from the State Land Office of the Czech
Republic, Ministry of Public Health, Regional Authority of the Karlovy Vary
Region (OŽPaZ) and the District Mining Bureau for the region of Karlovy
Vary. Because of difficult terrain conditions, it was necessary to build a
gravel track to allow the heavy drilling lorry to access the drilling
location. The drilling operator was the company SG Geoprůzkum České
Budĕjovice.
(a) Drill site of HJB-1 in Hartouŝov; (b) core
barrel display after recovery; (c) geophysical logging operations.
Drilling and coring
Drilling started on 30 March 2016 and lasted until 27 April 2016. First, a
steel casing with a diameter of 324 mm was cemented down to a depth of 8 m.
Next, a conductor pipe of diameter 219 mm was fitted and cemented down to
15.50 m below surface level. On the inner casing (219 mm) a preventive
slide valve (preventer) DN 300 mm was installed. Then the hole was drilled
near-vertically and reached a final depth of 108.50 m utilizing a core
drilling system Drillmec G-25 with a cutting diameter from 450 to 275 mm
installed on a Tatra 815 drilling lorry. For the coring, a Terracore S Geobor
wireline core barrel system (Atlas Copco) was used (inner diameter 96.1 mm).
Cores were retrieved in PVC liners of 3 m length. Pure bentonite was used as
a drilling mud additive. Drilling was conducted under strict contamination
control using fluorescein in the drilling mud as a contamination tracer, as
tested before at the CO2-sequestration site in Ketzin (Brandenburg,
Germany) (Pellizzari et al., 2013). The concentration of fluorescein in the
drilling mud was kept constant at 5 mg L-1.
Drilling was performed through the gate valve so that the drilling crew was
able to overcome problems of pressurized CO2 in the drilling shaft.
Pressure signs, which were expected in the project, occurred in the form of
smaller and larger gas eruptions after drilling through the ceiling formed by
the Cypris Fm. The first eruption of CO2 occurred at a depth of 78.5 m when
about 0.3 m3 of clay was flushed to surface, and acoustic signs of
CO2 emission became loud. Afterwards, until the final depth was reached,
dense bentonite mud (∼ 1150 kg m-3) was used, while the
drilling mud initially had a density of ∼ 1100 kg m-3. Mud loss
was high in the Quaternary deposits and in the claystones of the Cypris Fm.,
and particularly in the interval of 27 to 45 m.
On the 28 April 2016 the drilling string was pulled out of the hole and
geophysical logging was performed. On the 2 May 2016 the first borehole
cleaning was realized by using airlift of debris; an annulus was used for filling the
stem with drinking water and the air was pushed using airlift tubes with a
diameter of 72 mm. The stem was purified from the residual mud and dressed
with a PVC-U casing with a diameter of 114 mm. In sections 58.50 to
63.50 m, 68.50 to 83.50 m and 88.50 to 103.50 m a perforated PVC-U casing
was fitted. The bottom of the borehole was equipped with a full casing with a
length of 5 m. Subsequently, the casing was backfilled using washed gravel.
Pressure on the gridiron is monitored online (on average 510 kPa or 5.1 bar)
by H. Woith (GFZ Potsdam). Today, the wellhead is closed by a gas-tight seal
and flange mounted gauge. During the drilling, the groundwater level was
irregularly monitored. A very shallow aquifer was reached at 0.60 m below
surface, then an aquifer in the upper part of the Cypris Fm., and finally a
deep aquifer below 81.50 m in the Main Seam Fm. 24 h after reaching the
final depth, the groundwater level in the well was at 4.20 m below surface.
Hydraulic properties of the aquifer system were checked using short-time
pumping and recovery tests. The borehole was left open for long-term online
monitoring of the casing-head pressure. Until today, the borehole forms an
active ascent path for gases. Since no cores were recovered in the uppermost
9 m, a dry drilling (HAR-2R) was performed immediately after HJB-1 was drilled. The dry
drilling allowed the retrieval of uncontaminated shallow-core samples.
Comparison of the chemical parameters of the recovered mineral water in HJB-1. The sampling
date was 10 May 2016 (20:00 LT, end of recovery test). A comparison to the
chemistry of two typical mineral waters from western Bohemia is shown
(Karlovy Vary: spring Vřídlo (Sprudel), borehole BJ-35, 55.2 m
deep and Františkovy Lázně: spring Adler, borehole 33 m deep).
Water sourceK Na Ca Mg Cl SO4HCO3Fe Mineralization (mg L-1)(mg L-1)(mg L-1)(mg L-1)(mg L-1)(mg L-1)(mg L-1)(mg L-1)(mg L-1)HJB-144.2108041185.12291470242013.75870Karlovy Vary90.2167011842.9592161021801.36422Františkovy Lázně21.992148.411.2398111967114.33306Hydraulic and geochemical analyses
A 24 h pumping test was performed after a mechanical gas-separator and an
automatic measuring station were fitted to the wellhead to monitor
groundwater level and flow, conductivity and temperature of pumped water.
Subsequently, short-time hydrodynamic tests with a maximum tapping phase of
22 h were completed using a submersible pump (Grundfos SQ 2 70). The
effective yield value of the liquid phase was 0.15 L s-1, while the
yield of the gaseous phase was significantly higher, on average
0.5 L s-1, with a maximum of 2.92 L s-1. The coefficient of
determination between the water level and the yield of the gaseous phase
amounted to 0.67; the coefficient of determination between the yields of
liquid phase and gaseous phase yields was at 0.89 during the short-term
tests. The specific yield value amounted to 2.9×10-5 m2 s-1. Transmissivity was around 7.8×10-6 m2 s-1 during non-steady flow at the end of the
short-term hydrodynamic test, just before eruption. Thus, the Variscan mica
schists and basal Cenozoic sediments seem to be weakly to slightly permeable.
Less permeable rocks were reached after approximately 10 h of pumping. The
tests suggest the presence of a major fluid ascending channel located in a
permeable fault zone. Reaching a significant fault in basal layers of the
basin and its fundament is also supported by a large spreading of the gas
phase after penetrating the ceiling formed by less permeable strata of the
Cypris Fm.
Starting at a depth of approximately 78.5 m, after penetrating a
carbonate-rich layer approximately 30–40 cm in thickness, significant
circulation of gas-saturated groundwater was observed, and bursts of gaseous
CO2 and water occurred at the wellhead. Gas analysis measured
99.90 vol. % CO2, 0.0831 vol. % N2 and
0.00558 vol. % O2. We assume that the sand-bearing sediments
directly underneath this caprock-like structure form a reservoir for
mantle-derived CO2. Physicochemical parameters of groundwater recovered
from the basal aquifer (approximately at a depth of 79–85 m) were measured
during the pumping tests (Table 1). After cleaning the borehole, samples were
taken for chemical and microbiological analysis of liquid and or gaseous
phases. The subthermal mineral water had a temperature of up to
18.6 ∘C. The water was highly gas-saturated with about
1892 mg L-1 of free dissolved CO2, and heavily mineralized with
an electrical conductivity of around 6800 µScm-1. The pH was
about 6.4.
The content of H2SiO3 in the groundwater of the basal aquifer is
very high, with up to 112 mg L-1, as is the Fe content with up to 13.7 mg L-1.
The groundwater of the basal aquifer is of the
Na-Ca-HCO3-SO4 type, typical of the Františkovy Lázně
Spa and its mineral waters. Its composition is mainly influenced by intense
hydrolysis of aluminosilicate minerals probably caused by CO2, and,
given the relatively high concentration of chloride ions, by a fossil
component (Na-Cl) related to an evaporitic closed lacustrine basin
represented by the claystones of the Cyris Fm. (unit 3 in Fig. 8). A high
content of sulfate is typical of Karlovy Vary Spa mineral waters and might
reflect, at least partly, the dissolution of evaporites such as
Na2SO4 (Paces and Smejkal, 2004).
Logging operations
Logging data were acquired in eight runs to ensure complete well coverage
without losing data due to sensor offsets in stacked tool strings. Most tools
were combined with a gamma ray (GR) probe to allow an accurate depth
alignment. Since borehole stability was a concern, the spectral GR was run
before pulling out the drill pipe, logging downwards at 2 m min-1. A bismuth
germanate (BGO) scintillation crystal within the tool allowed us to determine
the amount of natural radioactivity within the formation and additionally a
splitting into thorium (Th), potassium (K) and uranium (U) based on discrete
energy peaks. After run 1, the drill pipe was pulled and all consecutive
measurements were acquired in open hole. Run 2 consisted of a probe
recording the environmental parameters of the borehole fluid including
temperature, pressure, conductivity, salinity, pH, oxygen saturation and
chloride content. A magnetic susceptibility probe was run in combination
with GR in runs 3 and 4 in two intervals. An overlap was logged to allow for
proper depth alignment and splicing of the two runs. In run 4, a different gain
was used for the susceptibility probe. To compensate for the difference
between both measurements, a multiplier was utilized to homogenize the data
before splicing. Due to the presence of a conductor pipe at approximately 15 m,
no open hole data could be gained above that depth. Since the data of run
3 and 4 were acquired logging upwards and in open hole, they were used as
depth reference. All other measurements were matched with these runs using
either a linear or an interactive depth shift. Run 5 provided a focused
electric resistivity measurement followed by formation velocity in run 6.
The sonic data were reprocessed by picking the first arrivals of the near
and far detectors and recalculating delta time compressional and primary
velocity (Vp).
A dipmeter probe was logged in run 7 providing four pad
conductivities in four directions, caliper data and borehole navigation data.
Microsusceptibility was the last measurement proving to be a higher-resolution log
than the standard susceptibility in runs 3 and 4, with a vertical resolution
of about 2 cm. It will be used in the future for a better correlation with
core data. To complete the logging suite, Prague University acquired gamma
density and neutron porosity data.
The composite log is presented in Fig. 8 together with a lithology obtained
from core data. The core depth and lithological boundaries were adjusted on
the basis of the logging data, especially in areas of low core recovery. The
borehole was drilled vertically with a slight north trend and a deviation of
1–2∘. The GR log indicates a gradual decrease in clay content
towards the surface. Especially the weathered schists near total depth feature
high thorium contents. Susceptibility is generally low but shows several
peaks throughout Miocene deposits. Within the weathered Paleozoic schist an
increase in gamma ray, sonic velocity and resistivity can be noted indicating
higher clay content and likely a higher compaction. The dipmeter shows
several conductivity spikes within the Miocene that could be related to minor
fractures or cracks. The sonic wavelets indicate several chevrons that
probably developed for the same reason.
Logging data of well HJB-1 and summary stratigraphy based on initial
core description results. Positions of cores in Fig. 9 are indicated by
corresponding labels (a–h) and arrows.
Representative lithologies in core HJB-1. (a) Palaeozoic schists
with siderite concretions (104.16–104.66 m); (b) sandy mudstone with lignite
coal (79.50–80.00 m); (c) interbedded sandy mudstones, calcareous mudstones
and carbonates (78.50–79.00 m); (d) calcareous mudstones with gypsum layers
and injection structures (77.61–78.11 m); (e) interbedded laminated
mudstones and peloidal to bioclastic carbonates (69.56–70.06 m); (f) “false
bedded” mudstones (light colored regular banding represents injected
drilling mud) with natural injection structures (64.55–65.05 m);
(g) laminated mudstones with natural deformation structures and “false bedding”
(∼ 42.00–42.46 m and (h) massive mudstones (∼ 35.15–35.60 m).
Scale is in centimeters.
Sampling
At the well site, the laboratory container of GFZ Potsdam (BUGLAB) was
installed to allow subsampling under optimal conditions. Equipped with
refrigerators as well as with -80 ∘C freezers, the container
permitted optimal storage conditions for biological samples. Below 20 m
depth, the sediment was too consolidated for subsampling with cutoff syringes and
consequently whole round cores (8 cm long, still in plastic liner) were cut.
To preserve the samples, different techniques were used. Core material
assigned for cultivation-based analyses were stored in CO2-flushed
gas-tight bags, while samples intended for geochemical analyses were stored
in N2-flushed bags, and samples reserved for molecular biological
studies were frozen in the field at -80 ∘C. By now, segments of the
core have been transferred to project partners to analyze the microbiology,
sedimentology and mineralogy as well as to perform geochemical analyses.
Because the perimeter of the core most likely was contaminated by drilling
mud, it was discarded. This so-called inner coring was performed under
aseptic conditions at the GFZ Potsdam. More than 300 samples are currently
processed in this way. From each core meter about 60 cm were retained as
whole round cores for sedimentological and mineralogical analyses and to
perform core logging. All cores were photographed at the GFZ (Fig. 9).
Contamination control
To assess drill-mud penetration into cores the tracer fluorescein was
extracted from the cut-off rim and the inner core according to the protocol
from Pellizzari et al. (2013; Fig. 10). Sediment samples were ground using a
mortar so that 0.250 g of the powder was mixed with 600 µL buffer (50 mM TRIS,
pH 9) in a 2 mL reaction tube. The tubes were placed on a vortex and mixed
for 30 min at maximum speed. Then, the sediment samples were centrifuged at
20 800 ×g for 10 min and the supernatant was transferred to a 1.5 mL
reaction tube. The extraction procedure was then repeated. The supernatants
were combined, centrifuged and transferred to a clean tube. The fluorescein
content was measured in triplicate using 96-well plates processed using a
filter fluorometer (CLARIOstar® OPTIMA, BMG LABTECH, Germany).
The quantification of fluorescein indicates that 5 out of 45 inner core
samples (after inner-coring) were contaminated by drill mud. Generally sandy
(highly permeable) samples showed a higher degree of contamination in
comparison to clay-rich samples.
Initial core description
In total, 85 % of the 108.5 m hole was available in the form of core halves
for inspection and sampling. Major gaps exist primarily in the uppermost 30 m.
The core was split in half lengthwise and subsequently photographed at the
GFZ Potsdam, followed by a visual core description. The recovered core is
severely affected by drilling disturbance. Partly, it shows a conspicuous
banding composed of relatively dark mudstones typically 2–5 cm thick and of
lighter colored homogeneous and comparatively soft mud mostly 0.5–2.5 cm
thick (Fig. 9g and f). Although the banding resembles rhythmic bedding, close
inspection reveals that it results from the injection of drilling mud along
preexisting bedding planes. Drilling mud was also injected into the
core along sub-vertical, natural and drilling-induced fractures, while in the
basal interval of the core the mud has intruded highly-altered or weathered
mica schists. Thus, careful examination of the core is vital to identify
artificial “false bedding” and to differentiate natural and
drilling-induced deformation structures.
The core is composed of five units (Fig. 9):
The lowermost unit 1 from ∼ 108.5 to ∼ 89.8 m consists of highly
altered or weathered Palaeozoic mica schists (Fig. 9a). According to
preliminary XRD measurements, the schists are principally composed of
kaolinite, muscovite/illite, siderite and quartz. Kaolinite likely has formed
under near-surface weathering conditions during Mesozoic-Early Cenozoic time
(Störr, 1976). The common presence of siderite might either be related to
an alteration under reducing conditions and elevated pCO2, hence possibly
to fluids rich in CO2, or to a formation during the influence of an
overlying freshwater swamp.
Unit 2 from ∼ 89.8 to ∼ 79.0 m is made up primarily of massive
to crudely bedded grey to brown and sandy to peaty mudstones
with abundant mottles and nodules (Fig. 9b). The mineralogy is dominated by
kaolinite, siderite, quartz and anatase. Thin lignite layers and abundant
lignite coal fragments as well as the presence of root structures and
possible soil horizons suggest deposition in a swamp environment. The unit
might represent the Main Seam Fm. (Lower Miocene).
Contamination assessment of drill mud in cores. The grey area
indicates the background noise of the fluorometric measurements of
fluorescein (evaluated with core material obtained at the same site but
without addition of fluorescein). 5 out of 45 inner core samples (red squares) are
contaminated by drill mud.
Upward, following unit 3 from about 79.0 to ∼ 15.5 m
is dominated by grey to green mudstones. The relatively heterogeneous lower
part of the unit consists of calcareous, sandy or peaty mudstones that are
interbedded with thin peloidal or bioclastic carbonates, dolomite beds and
gypsum layers (Fig. 9c, d, e). The overlying part up to ∼ 37.5 m
consists primarily of laminated or thin bedded mudstones (Fig. 8, while the uppermost part of
the unit up to ∼ 15.5 m is made up of massive to crudely bedded
mudstones. Laminated mudstones were most likely deposited in a relatively
deep lake with dysoxic to anoxic bottom-water conditions. The planar
lamination from a few millimeters up to 2 cm thick might have formed due to
seasonal changes in the bioproductivity or in the water stratification,
whereas thin detrital carbonate beds up to 5 cm thick possibly represent
event layers such as turbidites. The mudstones of the whole core interval are
made up of clay minerals such as muscovite/illite, kaolinite, smectite and
mixed-layers, and of quartz, K-feldspars, pyrite, zeolites, gypsum and
analcime. Greigite occurs in several layers in the middle part of the
mudstone interval according to petrophysical well site observation (magnetic
susceptibility), but its presence is not yet confirmed by XRD. The presence
of gypsum and analcime suggests a saline-alkaline lacustrine depositional
environment, while the common occurrence of pyrite implies suboxic conditions
during early diagenesis. However, the formation of these minerals might also
be influenced by CO2-rich fluids or microbial activity during diagenesis
(e.g., Chen et al., 2016). The whole mudstone-dominated unit 3 likely
correlates to the Cypris Fm.
Unit 4 between ∼ 15.5 and ∼ 7.2 m consists of green to brown
clay and of minor gravel probably correlative to the Vildštejn Fm.
Unit 5, representing the uppermost sediments down to a depth of ∼ 7.2 m,
is made up of predominantly moderately to poorly sorted sand to gravel
with minor peat, most likely Quaternary channel and floodplain deposits of
the nearby Plesná river.
Natural deformation structures such as microfaults, dikes and sills as well
as irregular intrusions are abundant in several core intervals but are best
visible in the laminated mudstones of unit 3. Most of the structures seem to
have formed by hydrofracturing, sediment fluidization and injection as a
result of excessive pore fluid pressure. Carbonate and gypsum cements are
infrequently associated with the deformation structures and the host rock in
the surrounding partly has changed its color or is bleached. Whether the
deformation structures, cements and color changes are related to the flow
of CO2-rich fluids is the subject of ongoing investigations.
Conclusions, ongoing studies and open questions
Pilot hole HJB-1 drilled into an active mofette has proved as a successful
test for upcoming well projects planned in the scope of the
ICDP project “Drilling the Eger Rift”
(http://web.natur.cuni.cz/uhigug/icdp). First microbiological investigations
including activity tests for microbial methane production, DNA extractions
and cultivation experiments are ongoing. Microbial DNA was extractable from
all samples and is currently analyzed through Illumina MiSeq 16S rDNA
sequencing and quantitative PCR. A new procedure for the recovery of DNA
from deep subsurface sediment samples was recently established by Alawi et
al. (2014). With this method, DNA can be extracted from sediments with a low
bacterial abundance, where commercial DNA extraction kits fail. Furthermore,
with this technique it is possible to separate extracellular and
intracellular DNA, and therefore to distinguish between fossil and modern
microbial communities. Additionally, it is planned to perform community
analyses based on Shotgun metagenomic sequencing for selected samples (in
cooperation with P. Kyslik, Academy of Sciences of the Czech Republic). This
method allows the identification of genes from all organisms present in the
sediment, regardless of their taxa and specificity of PCR primers. For
anaerobic culturing, the focus is set on methanogenic archaea and
sulfate-reducing bacteria. Both metabolic groups are cultivated in a liquid
anaerobic media and are inoculated inside an anaerobic chamber (glovebox).
Growth is monitored by methane production and changing sulfate
concentrations. Using different media compositions and
temperatures, defined enrichment cultures have already been obtained and
will be further characterized physiologically in detail. Pure cultures are a
prerequisite for further laboratory experiments to gain deeper insights into
the link to mineralogical processes such as mineral precipitation and
alteration under varying conditions. Further on molecular biological
analyses will be complemented by biomarker analyses at the Deutsches
GeoForschungsZentrum GFZ (K. Mangelsdorf). Diversity of soil fungal
communities will be analyzed by P. Baldrian (Academy of Sciences of the
Czech Republic).
The core shows many features that might result from the flow of
mantle-derived CO2 e.g., deformation structures and alteration features,
which are the subject of upcoming petrographic and geochemical studies. Further
pending questions are to distinguish between Mesozoic–Cenozoic deep chemical
weathering of Palaeozoic mica schists and alteration due to CO2 flow or
hydrothermal influence, the contribution of pyroclastics to the basin fill,
and the palaeoenvironmental information contained in the Cenozoic lake
sediments, in particular in the finely laminated (varved) interval of the
Cypris Fm.
Ongoing geophysical studies, notably seismic (TU Freiberg), geoelectric
(University of Leipzig) and magnetotelluric surveys (GFZ Potsdam) focus on
examining the subsurface structure beneath the Hartoušov degassing area.
The drill core from well HJB-1 provides essential information on the basin's
sediments for these studies whereas the geophysical surveys will help to
better understand the findings of the drill.
The data used in this paper that stems from pilot hole
HJB-1 are in the process of being interpreted in detail for various research
targets. When these analyses are finished, and the related publications are
submitted, the data will be deposited in reliable public depositories for
access. However, at the current state of the projects the raw data cannot be
made accessible for public use. When available,
details of the data depositories can be obtained by
contacting the corresponding author.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank the German Research Foundation (DFG) for financial support of the
project (grants AL 1898/1 and KA 902/16). The project AL 1898/1 was funded by
the DFG Priority Programme 1006, International Continental Scientific Drilling Program
(ICDP). Technical expertise and supervision of drilling operations by Bernhard Prevedel (GFZ Potsdam) is highly appreciated. Special thanks go to the
drilling, logging and sampling crews in the field, in particular to Thomas
Grelle and Carlos Lehne (both LIAG) and to Maria Börger, Susanne Boteck,
Oliver Burckhardt, Patryk Krauze, Kai Mangelsdorf and Mareike Noah (all
GFZ Potsdam). Alexander Wendt (University of Greifswald) photographed the
core and preliminary XRD analyses were done by Sandro Wobeser (University
of Potsdam) in the scope of his Bachelor thesis. Edited by: Thomas Wiersberg Reviewed by: Luca
Pizzino and one anonymous referee
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