Impacts of hydrothermal
dolomitization and
thermochemical sulfate
reduction on secondary porosity
creation in deeply buried
carbonates: A case study from
the Lower Saxony Basin,
northwest Germany
Bianca C. Biehl, Lars Reuning, Johannes Schoenherr,
Volker Lüders, and Peter A. Kukla
ABSTRACT
The role of deep-burial dissolution in the creation of porosity in
carbonates has been discussed controversially in the recent past.
We present a case study from the Upper Permian Zechstein
2 carbonate reservoirs of the Lower Saxony Basin in northwest
Germany. These reservoirs are locally characterized by high
amounts of carbon dioxide (CO2) and variable amounts of hydrogen sulfide (H2S), which are derived from thermochemical
sulfate reduction (TSR) and inorganic sources. To study the
contribution of these effects on porosity development, we combine petrography, stable isotope, and rare earth and yttrium (REY)
analyses of fracture cements with Raman spectroscopy and d 13C
analyses of fluid inclusions. It is shown that fluid migration along
deep fault zones created and redistributed porosity. Fluid inclusion analyses of vein cements demonstrate that hydrothermal
fluids transported inorganic CO2 into the reservoir, where it
mixed with minor amounts of TSR-derived organic CO2. The
likely source of inorganic CO2 is the thermal decomposition
of deeply buried Devonian carbonates. The REY distribution
patterns support a hydrothermal origin of ascending iron- and
CO2-rich fluids causing dolomitization of calcite and increasing
porosity by 10%–16% along fractures. This porosity increase
Copyright ©2016. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received March 27, 2015; provisional acceptance October 13, 2015; revised manuscript received
November 17, 2015; final acceptance January 14, 2016.
DOI:10.1306/01141615055
AAPG Bulletin, v. 100, no. 4 (April 2016), pp. 597–621
597
AUTHORS
Bianca C. Biehl ~ Energy and Mineral
Resources Group, Geological Institute, RWTH
Aachen University, Wüllnerstraße 2, 52056
Aachen, Germany; [email protected]
Bianca C. Biehl received her M.Sc. degree in
applied geosciences from RWTH Aachen
University in 2011. Her research interests
include carbonate sedimentology and
diagenesis, as well as salt tectonics. She is
currently working at the Geological Institute in
Aachen on her Ph.D. thesis, which will be
completed in 2016.
Lars Reuning ~ Energy and Mineral
Resources Group, Geological Institute, RWTH
Aachen University, Wüllnerstraße 2, 52056
Aachen, Germany; [email protected]
Lars Reuning worked at the GEOMAR Centre for
Ocean Research and received a Ph.D. from the
University of Kiel, Germany. In 2005, he joined
RWTH Aachen University, where he is currently
working as senior lecturer. His main research
interests are carbonate diagenesis,
carbonate–evaporate interactions, and threedimensional seismic geometries in carbonates.
Johannes Schoenherr ~ ExxonMobil
Production Germany, Riethorst 12, 30659
Hannover, Germany; johannes.schoenherr@
exxonmobil.com
Johannes Schoenherr received his M.Sc. in
structural geology from Technical University of
Darmstadt, Germany, and his Ph.D. in
carbonate reservoir quality and salt tectonics
from RWTH Aachen University, Germany. He is
currently working with ExxonMobil Production
Germany as a hydrocarbon geologist on
Zechstein carbonate reservoirs. Johannes’
research interests encompass basic and applied
aspects of hydrocarbon systems geology in
carbonate–evaporite settings.
Volker Lüders ~ GeoForschungsZentrum
Potsdam, Telegrafenberg, 14473 Potsdam,
Germany; [email protected]
Volker Lüders is a senior research scientist at
the Helmholtz Centre Potsdam. He received his
Ph.D. in geochemistry from the Free University
Berlin in 1988. His fields of research are fluid
inclusions, stable isotopes, and trace elements
in hydrothermal systems, including fluid and
oil/gas migration in sedimentary basins.
Peter A. Kukla ~ Energy and Mineral
Resources Group, Geological Institute, RWTH
Aachen University, Wüllnerstraße 2, 52056
Aachen, Germany; [email protected]
Peter A. Kukla has been a professor of geology
at RWTH Aachen University since 2000. He has
worked for Shell International (1991–2000) in
The Netherlands and Australia. Peter graduated
with degrees in geology from Wuerzburg
University, Germany, and Witwatersrand
University, South Africa (Ph.D., 1991). His
research interests include basin analysis,
reservoir characterization, unconventional gas,
geodynamics of salt basins, and pore pressures.
ACKNOWLEDGMENTS
We would like to thank ExxonMobil
Production Deutschland GmbH for
supporting us with core samples and thin
sections and for the opportunity to publish
the data, Philipp Binger (Energy and Mineral
Resources Group) for preparing further thin
sections for transmitted light microscopy and
cathodoluminescence, Uwe Wollenberg
(Energy and Mineral Resources Group) for
the help with the cathodoluminescence
microscope, Stefan Krüger (Leipzig
University) for the measurements of carbon
and oxygen isotopes, Harald Strauss
(Muenster University) for the sulfur isotope
analyses, Maike Leupold for help in drafting
figures, and Anna Lewin for helpful
discussions and her support. We thank our
reviewers Cathy Hollis and James (Jim)
Markello for their comments and suggestions
that greatly improved the quality of our
manuscript.
EDITOR’S NOTE
A color version of Figure 2 can be seen in the
online version of this paper.
598
results from hydrothermal dolomitization and dissolution by
acids generated from the reaction of Fe2+ with H2S to precipitate pyrite. In contrast, hydrothermal dolomite cements
reduced early diagenetic porosity in dolomitic intervals by
approximately 17%. However, the carbonate dissolution in the
predominantly calcitic host rock results in a net increase in
porosity and permeability in the vicinity of the fracture walls,
which has to be considered for modeling reservoir properties
and fluid migration pathways.
INTRODUCTION
The concept of carbonate dissolution in the deep-burial realm has
been widely applied in the interpretation of carbonate reservoir
quality. Different mechanisms for mesogenetic porosity creation
(Choquette and Pray, 1970) have been proposed, including
thermochemical sulfate reduction (TSR) (Ma et al., 2007),
dissolution by CO2-rich formation waters (Beavington-Penney
et al., 2008), and burial dolomitization (Davies and Smith,
2006). In contrast, Ehrenberg et al. (2012) claimed that porosity
creation during deep burial lacks supporting quantitative data. The
authors argued that burial dissolution in carbonates would not
contribute significantly to a net increase in carbonate reservoir
porosity. Furthermore, they stated that pore waters highly undersaturated with respect to calcite would in most cases be neutralized very rapidly during migration before reaching carbonate
reservoirs. Therefore, dissolution would be limited to narrow reaction fronts in case acidic waters would reach the reservoir level.
Many previous studies on porosity creation during burial focused
on petrographic evidence (Elliott, 1982; Mazzullo and Harris,
1992; Beavington-Penney et al., 2008; Zampetti, 2010), which
showed that it is challenging to distinguish deep-burial pores from
shallow-burial pores. Furthermore, Ehrenberg et al. (2012) suggested that fluids causing carbonate dissolution during burial could
in many cases be surface derived and migrated downward along
deep-reaching faults and fractures.
To evaluate porosity creation in deep-burial settings, we present
new data from an upper Permian (Zechstein) carbonate reservoir
in the Lower Saxony Basin, one of Germany’s most important gas
provinces. The reservoir gas in the Zechstein 2 (Z2) carbonate is
locally characterized by very large amounts of CO2, which is
thought to be derived from a mixture of organic and inorganic
sources (Fischer et al., 2006) and variable amounts of TSR-derived
hydrogen sulfide (H2S) (Mittag-Brendel, 2000). Nonfractured parts
of the studied reservoir are characterized by alternating porous
dolomitic and tight calcitic intervals. The almost complete loss of
primary and near-surface porosity in the calcitic intervals is
Secondary Porosity Creation in Deeply Buried Carbonates
resulting from calcite cementation and dedolomitization during shallow burial (Clark, 1980; Strohmenger
et al., 1996). This early burial porosity reduction helps
to define the creation of late-burial vuggy pores
caused by dolomitizing fluids ascending along
fractures from deeper parts of the basin. A contribution of descending surface-derived fluids can be
ruled out, as the carbonate reservoir is sealed by a thick
rock salt succession. This geological setting of the study
area makes it ideal to study modifications on burial
porosity development in carbonate reservoir systems.
Supported by empirical data, this study combines
classical carbonate petrography with geochemical
analyses, in particular carbon and oxygen stable isotope analysis of matrix and fracture-filling minerals,
and the analysis of fluid inclusions trapped in vein
cements. Additional information is provided for
mineral precipitation conditions and different gas
phases (methane [CH4], CO2, H2S) generated during
diagenesis. Information on fluorite formation is given
by analysis of rare earth and yttrium (REY) distribution
patterns. We aim to demonstrate that deep-burial,
fracture-controlled dissolution of carbonates can be
significant enough to convert a carbonate matrix reservoir with low porosity and permeability into a productive reservoir rock with moderate-to-good quality.
GEOLOGICAL SETTING
The study area is located in northwest Germany,
west of the city of Hannover and south of Bremen
(Figure 1A), and belongs structurally to the central
part of the Lower Saxony Basin, an intraplate
crustal segment in northwest Germany with a length
of 300 km (186 mi) and a width of 65 km (40 mi).
It represents a west–east striking trough and is
bordered by the Pompeckj block in the north and
the Rhenish Massif and the Harz Mountains in the
south. The sediments in the Lower Saxony Basin
experienced maximum burial and therefore maximum temperatures during the Early Cretaceous,
whereas a major inversion phase caused uplift of
the sediments during the Late Cretaceous (Betz
et al., 1987; Kockel et al., 1994; Petmecky et al.,
1999; Maystrenko et al., 2008). The Lower Saxony
Basin is part of the Southern Permian Basin, a landlocked depression that developed because of high
subsidence rates and underwent several flooding
periods in the Late Permian (Zechstein) (Pharaoh
et al., 2010). The evaporite sedimentation began
when the dry basins were flooded from the Boreal Sea
in the north by a combination of rifting and a rise in sea
level. Thick rock salt series are regionally interlayered
with thin sulfate beds occupying the basin center
(Biehl et al., 2014), with anhydrites and carbonates
deposited at the basin margins (Taylor, 1998). Cyclical, sea-level fluctuations controlled sedimentation
patterns in the Southern Permian Basin during the
Zechstein. In total, five to seven carbonate–evaporite
cycles are identified for the Southern Permian Basin,
whereof each transgression phase marks the base of a
new cycle (Peryt et al., 2010). In northern Germany,
the Zechstein Group comprises seven evaporite cycles
of formation rank. All cycles were described in detail
by Taylor (1998) and Peryt et al. (2010). The focus
of this study is the second Zechstein cycle, which is
composed of a basal carbonate unit (Z2 carbonate;
Stassfurt Carbonate), followed by anhydrite (A2), and
a thick halite interval (Figure 1B). The platform facies
of the Z2 carbonate hosts northwest Germany’s most
prolific gas play (Strohmenger et al., 1996). The Z2
carbonates are situated between two anhydrite layers
(A1 and A2), have a thickness of 20–80 m (66–262 ft)
on the platform, and show variation in the platform
facies from shallow subtidal to supratidal (Strohmenger
et al., 1996). In this study, core material from one
well of the Lower Saxony Basin was extensively
studied regarding rock types, diagenetic processes,
and the precipitation conditions of vein cements
within the Z2 carbonate, which was deposited in
intertidal (tidal flat) to shallow subtidal (oolite interbar; oolite bar) environments. The platform subfacies types were described in detail by Steinhoff and
Strohmenger (1999). The studied well is located close
to a deep-reaching fault system (Figure 2), which
most likely formed during a phase of rapid subsidence
during the Early Cretaceous and was reactivated
during Late Cretaceous inversion. The Zechstein
layers are underlain by lower Permian (Rotliegende)
volcanics, siliciclastic red beds, and Carboniferous
coals, which are the main source rocks for gas accumulations in the Lower Saxony Basin (Magri et al.,
2008; Kombrink et al., 2010).
MATERIALS AND METHODS
For this study, 27 core samples of the Z2 carbonate
member were collected in the core repository of
Biehl et al.
599
Figure 1. (A) Map of the study area in the Lower Saxony Basin between the cities of Hannover (H) and Bremen (B) in northwest Germany
with locations of gas fields and wells. The black circle indicates the location of the studied well for this study; black dots show the locations of
neighboring wells. (B) Stratigraphy of the lower Zechstein cycles and the underlying Rotliegende Formation. The studied carbonates belong
to the second Zechstein cycle (Z2) and are underlain and overlain by anhydrite deposits. Thick rock salt layers are present above the Z2
carbonate in the second and third Zechstein cycles. Z1 = first Zechstein cycle; Z3 = third Zechstein cycle.
ExxonMobil Production GmbH (EMPG), Nienhagen,
Germany. Thin sections for transmitted light microscopy were prepared from all core samples and further
103 thin sections, as well as the reservoir gas composition and its carbon isotopy were provided by EMPG.
All thin sections and all core samples were stained
with Alizarin red-S and potassium ferricyanide
where calcite is stained red, ferroan dolomite is
stained blue, and nonferroan dolomite remains
unstained. Cathodoluminescence microscopy was applied to 12 polished thin sections and was performed
using a hot cathode Lumic cathodoluminescence
600
Secondary Porosity Creation in Deeply Buried Carbonates
apparatus HC1-LM mounted on an Olympus polarizing microscope. The device was tuned to an
electron beam current of approximately 0.6 mA, an
excitation voltage of 14 kV, and a filament current of
2.5 A at 0.001 mbar vacuum.
Point counting on four samples was performed
with the software JMicroVision 1.2.7 with 1000
points each to evaluate the porosity evolution in
dolomitic and calcitic host rock samples.
Oxygen and carbon stable isotopes were measured for 41 samples. The measurements were carried
out at the Institute for Geophysics and Geology,
Figure 2. Geologic cross section based on seismic profile,
showing how close the studied
well is to deep-reaching, east–
west striking fault systems. A1 =
anhydrite layer of first Zechstein
cycle; A2 = anhydrite layer of
second Zechstein cycle; Ca1 =
carbonate layer of first Zechstein
cycle; Ca2 = carbonate layer of
second Zechstein cycle; TVDSS =
true vertical depth subsea.
Leipzig University, Germany. Carbonate powders were
reacted with 105% phosphoric acid at 70°C (158°F)
using a Kiel IV online carbonate preparation line connected to a MAT 253 mass spectrometer. All carbonate
values are reported in per mil, relative to Vienna
Peedee belemnite (VPDB). Reproducibility was
checked by replicate analysis of standard NBS19 and
was better than –0.023‰ (1s) for carbon (d 13C) and
better than –0.055‰ (1s) for oxygen isotopes (d 18O).
Sulfur isotope analysis (d 34S) was applied to four
pyrite samples. The measurements were carried out at
the Institute for Geology and Paleontology, Münster
University, Germany, using a ThermoFinnigan Delta
Plus equipped with an elemental analyzer isotope ratio
mass spectrometry. Approximately 100 mg of pyrite
powder were homogenously mixed with the same
amount of vanadium pentoxid and placed in a tin
capsule for subsequent automated combustion and
isotope analyses. All pyrite values are reported in per
mil, relative to Vienna Cañon Diablo troilite (V–CDT).
Reproducibility was checked by lab standards and
international reference material (IAEA S1, S2, S3,
NBS 127) and was better than 0.3‰ V–CDT.
Gas-bearing fluid inclusions hosted in anhydrite,
ferroan saddle dolomite, and fluorite were measured
in doubly polished thick sections for gas compositions. A Jobin–Yvon LabRam confocal Raman microspectrometer equipped with an Olympus optical
microscope was used. The excitation radiation used
was a 532.6 nm neodymium-doped yttrium aluminum
garnet laser (100 mW). For internal calibration, silicon
(520 cm-1) and diamond (1332 cm-1) were used.
Raman spectra of gaseous inclusions were collected
in the spectral range between 1200 and 3000 cm-1
with a Peltier cooled charge-coupled device collector.
A 2 · 45 sec acquisition time was used for all
CO2–CH4–bearing inclusions; CO2–CH4–nitrogen
(N2)–bearing inclusions were measured 3 · 60 sec to
improve the signal-to-noise ratio.
The carbon isotopic compositions of CH4 and
CO2 in fluid inclusions were measured using a
sample crusher connected via a gas chromatography
column to an elemental analyzer isotope ratio mass
spectrometry system. This analytical setup allows
online simultaneous measurements of stable isotope
ratios of N2, CH4, and CO2 in natural gas mixtures
released by crushing of fluid inclusions (Lüders
et al., 2012; Plessen and Lüders, 2012).
The REY measurements were performed on two
fluorite samples. Approximately 0.1 g of sample
powder was dissolved in 0.5 m nitric acid (HNO3)
and filled up to 50 ml volume with 0.5 mol-1 hydrogen chloride (HCl). Inductively coupled plasma
(ICP) mass spectrometry measurements were performed with an ELAN 5000A quadrupole ICP mass
spectrometer (Perkin–Elmer/SCIEX, Canada). The
precision of the method was checked by replicate
analyses of fluorite in-house standard C5-FL and was
better than 3% for REY (except for europium [Eu],
which was better than 5%).
Biehl et al.
601
RESULTS AND INTERPRETATION
Petrography
Core Description
The studied core section consists of alternating calcitic
and dolomitic intervals (Figure 3). Calcitic intervals
dominate, whereas pure dolomitic intervals are only
found from 2898 to 2899 m (9508–9511 ft) and from
2926 to 2935 m (9600–9628 ft) depth. Two thicker
layers of dolomitic calcite are present from 2891 to
Figure 3. Lithology of the
Zechstein 2 carbonate of the
studied well, displaying the alternating layers of calcite, dolomite, and dolomitic calcite. Zones
with abundant pyrite mineralizations and zones with abundant
horizontal and vertical stylolites
are indicated. The major particles
are ooids and peloids. The positions of samples taken at the core
repository are shown with cross
reference to (micro)photographs
of samples. Fig = figure; G =
grainstone; M = mudstone; P =
packstone; W = wackestone.
602
Secondary Porosity Creation in Deeply Buried Carbonates
2895 m (9483–9498 ft) and from 2909 to 2915 m
(9544–9564 ft) depth. Because of the staining with
Alizarin red-S and potassium ferricyanide, calcite
and dolomite can be easily distinguished on all core
samples (Figure 4). The dolomite matrix was interpreted by Clark (1980) to be a result of reflux
dolomitization. The lower part of the core section
(2923–2936 m [9588–9633 ft]) is classified as
packstone, whereas the upper part (2878–2923 m
[9442–9588 ft]) is classified as grainstone. The
dominant grain types are ooids and peloids. Fractures
are very abundant in the studied core section (Figure 3)
and have widths of up to 3 cm (1 in.) and lengths of
several decimeters. Fractures with pyrite mineralization
occur in all intervals (Figure 4C–E). The fractures are
either completely cemented (veins) or partly open.
Several veins show significant dolomitic halos with
vuggy porosity in otherwise calcitic matrix (Figure 4B).
Abundant pressure solution features such as highamplitude horizontal and vertical stylolites are present in all core intervals. Horizontal stylolites are
commonly thought to form at burial depths greater
than 500 m (1640 ft; Machel, 2005) and are crosscut
by larger fractures. Vertical stylolites can either occur
during basin inversion or folding phases (Breesch
et al., 2007). In contrast to the horizontal stylolites,
large veins are crosscut by vertical stylolites, indicating that these veins formed during late burial,
after horizontal stylolitization had stopped but before
or simultaneously to the formation of vertical stylolites. Calcite-filled tension gashes originated along
vertical stylolites, such as in Figure 4D. Crack-seal
structures with inclusion trails are visible inside the
tension gashes, indicating that the cementation occurred contemporaneously with the formation of the
tension gashes. Triaxial stresses are responsible for
the formation of tension gashes perpendicular to the
Figure 4. Core samples stained with Alizarin red-S and potassium ferricyanide to distinguish calcite (red) from dolomite (white to gray) and
ferroan dolomite (blue). (A) Dolomite matrix (blue) shows large secondary vuggy pores and pyrite (arrow) along small fractures, compared
with tight calcite matrix (red). (B) A small fracture cemented with saddle dolomite (arrow) is surrounded by a zone where significant
dolomitization (blue) and pyrite mineralization occurred, in accordance with the creation of secondary vuggy pores (circle) along the fracture in
otherwise calcitic matrix (red). (C) Fracture filled with (1) nonferroan saddle dolomite (SD) (white), (2) ferroan saddle dolomite (blue), and (3)
ferroan calcite (CC) in the center (purple). Early moldic porosity in dolomite matrix is cemented with saddle dolomite and pyrite. (D) Calcite
cement-filled tension gash (arrow) along vertical stylolite with pyrite mineralization (P). (E) Interval of massive P alternating with saddle
dolomite 1. (F) Former anhydrite nodule, cemented with calcite 1 and pyrite (arrow) in dolomite matrix 1 (gray).
Biehl et al.
603
minimum principal stress axis s3, whereas vertical
stylolites develop perpendicular to the maximum
principal stress axis s1. This indicates that tension
gashes and vertical stylolites are typical for a compressional regime and developed at the same time
(Sibson, 1994). The burial and thermal history
(Figure 5), modeled by Petmecky et al. (1999) for a
neighboring well, shows that the Zechstein layers
were deeply buried during the Early Cretaceous and
underwent considerable uplift and cooling during the
Late Cretaceous.
Matrix Petrography
The matrix is either composed of finely to medium–
crystalline, sub- to euhedral dolomite (dolomite matrix
1) or of an anhedral, coarse-crystalline, calcite crystal
Figure 5. Burial and temperature history modeled for a well
approximately 4 km (2 mi)
southwest of the studied well. The
Zechstein layer is colored in gray.
The burial model is based on
borehole temperature and vitrinite reflectance data (modified
from Petmecky et al., 1999).
Neog. = Neogene; TDC = thermal
decomposition of carbonate;
TSR = thermochemical sulfate
reduction.
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Secondary Porosity Creation in Deeply Buried Carbonates
mosaic containing corroded dolomite inclusions
(calcite matrix). Plug measurements of nonfractured
samples show matrix porosities between 1% and 4%
in dominantly calcitic intervals and porosities up to
23% in dolomitic intervals (Figure 6). The dolomite
matrix 1 shows abundant moldic and intercrystal
porosity, whereas the calcite matrix is tightly packed,
and former molds (Figure 7A) are cemented with an
early calcite generation (calcite 1; Figure 7B). Because
of abundant dolomitic inclusions in calcite, we
assume the calcite matrix to be a replacement of
dolomite matrix 1 and refer to this replacement
calcite as dedolomite in the sense of Evamy (1967).
The same calcite generation also cements former molds.
Under cathodoluminescence, the dolomite matrix 1
appears bright red, whereas the calcite matrix is
Figure 6. Helium porosity and
permeability measured for matrix
plug samples. The diagram shows
much higher porosities and permeabilities in dolomitic intervals
(grain density 2.9 g/cm3) than in
pure calcitic intervals (grain density 2.71 g/cm3). Plug samples
with grain densities greater than
3 g/cm3 probably contain pyrite
derived from thermochemical
sulfate reduction.
nonluminescent. Such matrix types are typical for the
Z2 carbonate in the Southern Permian Basin and are
thought to be of early diagenetic origin, predating the
onset of burial stylolitization (Clark, 1980; Reijers,
2012). Large fractures crosscut horizontal stylolites,
indicating that those fractures formed after burial to at
least 500 m (1640 ft). Adjacent to larger fractures, the
calcite matrix is replaced by dolomite matrix 2, as indicated by halos visible on core sections (Figure 4A, B).
During burial, the matrix is affected by dolomitizing
fluids migrating through the extensive fracture system.
The dolomite matrix 1 and calcite matrix were thus
affected in different ways. As shown in Figure 7C, the
moldic porosity in dolomite matrix 1 is partly cemented
with pyrite and coarse crystalline ferroan dolomite (see
also Figure 4C) that is interpreted as saddle dolomite
based on its curved crystal boundaries and sweeping
extinction under cross-polarized light. The dolomitic
crystals within the matrix, however, have not been
altered by the late-burial dolomitizing fluids. In contrast, the calcite matrix is affected by the late-burial
dolomitization process, leading to a replacement of
former dedolomite by subhedral-to-anhedral, coarsecrystalline dolomite matrix 2 and pyrite. Additionally,
secondary intercrystal and vuggy porosity is created
(Figures 4, 7D). This replacement typically affects
several centimeters of the dolomite matrix 1 adjacent
to the veins (Figure 4A, B), creating secondary vuggy
porosity. Point counting of the dolomite matrix 2
revealed a porosity increase between 10% and 16% in
the former calcite matrix. In contrast, saddle dolomite
cements reduced primary porosity in dolomite by
approximately 17%. Former anhydrite nodules have
been completely replaced by calcite 1 (Figure 4F) and
are found in dolomitic and calcitic intervals.
Vein Petrography
Extensive fracturing occurs throughout the core
section. All mineral phases occurring in larger veins
are either cements or partial replacement products.
Saddle dolomite and ferroan calcite (calcite 2) are the
two most important vein cements. Saddle dolomite
(Figure 8A) is either present as rare, milky-white
nonferroan saddle dolomite (saddle dolomite 1) or
abundant ferroan saddle dolomite (saddle dolomite
2), as revealed by the blue staining with potassium
ferricyanide (Figure 4C). Saddle dolomite 1 predates
saddle dolomite 2, if both types are present. Under
cathodoluminescence, saddle dolomite 2 is nearly
nonluminescent, whereas saddle dolomite 1 shows a
dull red color (Figure 8B). The coarse saddle dolomite
crystals with a size up to 2000 mm host many aqueous
and gaseous fluid inclusions and solid mineral phases
such as calcite, anhydrite, pyrite, or bitumen along
crystal growth zones, probably indicating alternating
mineral precipitation (Figure 8C, D), in which a
phase of saddle dolomite growth is followed by
other mineral phases. Saddle dolomite 2 is abundant
in veins and occurs either as a single mineral phase or
with pyrite and calcite 2 (Figure 8C). Calcite 2
replaces or postdates saddle dolomite 2 and occurs
simultaneously with pyrite. Staining reveals a purple
color with partly extensive zonation of the crystal
Biehl et al.
605
Figure 7. Microphotographs of
different matrix types under plane
polarized light. Pores appear blue
due to blue epoxy. (A) Dolomitic
matrix 1 with intercrystal porosity
between euhedral crystals and
oomoldic porosity. Coarser dolomite cements are present in the
molds (arrow). (B) Calcite matrix
with anhedral calcite crystals,
corroded dolomite inclusions
(dedolomite), and oomolds cemented with calcite 1. (C) Cementation of oomoldic porosity
with saddle dolomite 2 (SD).
Porosity is reduced by approximately 17% (see Figure 4C). (D)
Former calcite matrix replaced by
dolomite matrix 2 and pyrite (P)
under creation of secondary
vuggy pores (Figure 4B). Simultaneously, some porosity is cemented again by SD. However,
porosity is increased by
10%–16% in these intervals.
(Figure 8E). Calcite 2 shows bright yellow to bright
orange colors in cathodoluminescence (Figure 8F), with
well-developed zonation in some crystals (Figure 9B, C).
Fluorite postdates saddle dolomite 2 and pyrite
(Figure 9D), and it shows clear uniaxial crystals with
fluid inclusions of primary and secondary origin
(Figure 9E). Fluorite is known to precipitate from
cooling fluids (Richardson and Holland, 1979;
Esteban and Taberner, 2003) and is therefore assumed to be the youngest vein mineral phase of the
Z2 carbonate; however, the relationship to calcite 2 is
uncertain. In addition to the mineral phases mentioned above, minor amounts of anhydrite occur,
hosting abundant fluid inclusions (Figure 9F).
All observations regarding the matrix evolution
and vein cements are summarized in the paragenetic
sequence shown in Figure 10.
Fluid Inclusion Analyses
Gaseous inclusions are contained in studied samples
of saddle dolomite 2, anhydrite, and fluorite. As
shown in Figure 9F, the inclusions have a rounded
or rectangular shape with a size less than 20 mm.
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Secondary Porosity Creation in Deeply Buried Carbonates
Because of the high number of fluid inclusions, a welldefined classification of a primary or secondary origin
of the fluid inclusions using the criteria of Roedder
(1984) is often vague or even impossible (see
Table 1). Primary fluid inclusions are often orientated
parallel to the crystal planes. Raman spectroscopic
analyses revealed different molecular compositions of
gaseous inclusions, which can be grouped as follows:
(1) CH4-rich inclusions with CH4 content greater
than 66 mol % and similar CO2 and N2 content
(sample 4), (2) CO2-rich inclusions with CO2 contents
ranging from 60 to 100 mol % (samples 1, 2, and 6), and
(3) N2-rich inclusions with N2 content greater than
42 mol % with still-high CO2 and CH4 contents
(sample 3). All studied gas inclusions show no or only
low H2S contents (maximum 2.3 mol % in sample 3B).
Inclusions rich in H2S were reported from other
Zechstein carbonates in the northern part of the
Lower Saxony Basin (Gerling et al., 1997) but were not
observed in the studied samples. The results show
that primary inclusions hosted in saddle dolomite 2
are CO2 rich and contain no H2S (samples 1 and 2)
compared with fluorite (samples 3 and 4) and that
the CO2 concentrations decrease in younger mineral phases (Table 1; Figure 11A). Secondary fluid
Figure 8. Microphotographs of veins and vein cements taken under plane polarized light and cathodoluminescence (CL). (A) Picture of
polished section (plane polarized light) showing large saddle dolomite crystals, which are nonferroan in the center (saddle dolomite 1). The
outer zone (arrow) is ferroan and shows small pyrite inclusions (saddle dolomite 2). Note that the polished section is unstained (Figure 4C).
(B) CL picture of Figure 8A reveals zonation of saddle dolomite. The outer zones are nonluminescent, reflecting higher iron concentrations.
The dashed line shows the transition between outer crystal zone and pore (Figure 4C). (C) Saddle dolomite 2 (SD) with curved crystal
boundaries postdated by calcite 2 (Cc). (D) Saddle dolomite 2 under crossed polarized light showing anhydrite inclusions along growth bands
(arrow). (E) Fracture cemented with zoned calcite 2. The intense purple staining in the crystal center reflects higher iron concentrations. (F) CL
picture of vein cements with bright-orange luminescent calcite 2 in the center postdating nonluminescent saddle dolomite 2 at rims.
Biehl et al.
607
Figure 9. Microphotographs of
veins and vein cements taken
under plane polarized light and
cathodoluminescence (CL). (A)
Tension gash filled with calcite 2
and an inclusion band indicating a
crack-seal structure (arrow) in
dolomite matrix 1. The tension
gash is syngenetic with a vertical
stylolite (Figure 4D), and the
crack-seal structure shows that
cementation was simultaneous
with crack opening and stylolite
formation. (B) CL picture of
coarse crystalline calcite 2 vein
cement in dolomite matrix 1. (C)
CL picture of calcite 2 (Cc)
postdating nonluminescent saddle dolomite 2 (SD). Nonluminescent zones in calcite 2
(arrow) are associated with fluid
inclusion trails. (D) Saddle dolomite 2 (SD) with pyrite (P) and
calcite 1 (arrow) inclusions predating fluorite (F). The pyrite
occurs both as inclusions in saddle dolomite 2 and as stand-alone
crystals postdating saddle dolomite 2. (E) Picture of fluorite with
fluid inclusion trails (arrow)
postdating pyrite and calcite 1. (F)
Fluid inclusion trails with gas
bubbles in anhydrite.
inclusions measured in fluorite (sample 4) reflect
the composition of the wells’ reservoir gas, which is
63.8% CH4, 20.4% CO2, 14.3% N2, and 2.3% H2S.
The d 13CCO2 and d 13CCH4 values of fluid inclusion gases were measured in two samples of saddle
dolomite 2 and four samples of fluorite, and they
range from -4.2‰ to -0.8‰ VPDB for CO2 and
from -20‰ to -11‰ VPDB for CH4 (Figure 11B;
Table 2). The reservoir gas has a d 13CCO2 of -1.4‰
VPDB. The d 13C values for CO2 in fluid inclusions
are in the range of d 13C values for saddle dolomite 2
and calcite 2 (Figures 11, 12).
Stable Isotopes of Carbonates
Stable oxygen and carbon isotopes were measured for
dolomite matrix 1, calcite matrix, saddle dolomite 1,
608
Secondary Porosity Creation in Deeply Buried Carbonates
saddle dolomite 2, dolomite matrix 2, and calcite 2
(Table 3; Figure 12). Saddle dolomite 2 and calcite 2
show d 13C values that range between +0.9‰
and -6‰ VPDB and d 18O values that range between -8.8‰ and -13‰ VPDB. In contrast, the
calcite matrix and dolomite matrix 1 have positive
d 13C values, ranging from +2.8‰ to +6.1‰ VPDB
(Figure 12). Dolomite matrix 1 displays d 18O values
between -5.6‰ and -2.9‰ VPDB.
Sulfur isotopes (d 34S) were determined for four
pyrite samples (Table 4). The measured values show
a range from +0.1‰ to +10.2‰ V–CDT. The
lowest d 34S value of 0.1‰ V–CDT was measured
for the massive pyrite mineralization occurring together with saddle dolomite 2, as shown in Figure 4E.
The highest d 34S value of 10.2‰ V–CDT was
measured for pyrite associated with calcite 2.
Figure 10. Paragenetic sequence of observed mineral phases for the studied well. Dashed lines indicate an uncertain timing or a longterm event.
Rare Earth and Yttrium Pattern of Fluorite
DISCUSSION
Rare earth elements (REE) were determined for
two fluorite samples. Since there is a close association between REE and yttrium (Y), the normalized REE patterns are extended to the normalized
REY pattern, where Y is inserted between dysprosium (Dy) and holmium (Ho). The parts per
million concentrations (Table 5) were normalized to
post-Archaean Australian Shale after McLennan
(1989). The fluorite samples show bell-shaped patterns with high-middle REE (MREE; samarium,
Eu, gadolinium, terbium) compared with light REE
(LREE; lanthanum, cerium (Ce), praseodymium,
neodymium) and heavy REE (Dy, Ho, erbium, thulium, ytterbium, lutetium). The LREE especially
show very low concentrations, with a strong increase toward MREE. Both samples display a
strong positive normalized-Y anomaly (Figure 13)
but no normalized-Eu anomaly.
Effects of Thermochemical Sulfate Reduction
and Hydrothermal Fluids
It has been proposed that TSR is a process causing
dissolution of carbonates and a net increase in porosity
(Ma et al., 2007; Cai et al., 2014), but many case
studies do not indicate such significant changes in
porosity (Machel, 2001). In the Lower Saxony Basin,
TSR took place in reservoirs that have experienced
burial temperatures greater than 125°C (257°F),
whereas the contribution of bacterial sulfate reduction (BSR) to H2S generation was shown to be
negligible (Mittag-Brendel, 2000). The well used in
this study reached burial temperatures exceeding
125°C (257°F) at the end of the Jurassic (~150–140
Ma) and experienced a maximum burial temperature
of up to 270°C (518°F) during the Late Cretaceous
(ca. 90 Ma; Figure 5). Independent evidence for TSR
is given by relatively small differences in the sulfur
Biehl et al.
609
Table 1. Molecular Compositions of Primary and Secondary
Gaseous Inclusions Hosted by the Vein Cements Saddle Dolomite
2, Anhydrite, and Fluorite
Gas Phase (mol %)
Sample
1A
1B
1C
1D
2A
2B
2C
2D
2E
2F
3A
3B
3C
3D
3E
3F
4A
4B
4C
4D
4E
5A
5B
5C
5D
5E
6A
6B
6C
6D
6E
6F
Mineral
SDol 2
SDol 2
Fluorite
Fluorite
Anhydrite
Anhydrite
CH4
CO2
N2
H2S
Type
0
0
0
0
36.3
26.8
21.6
24.9
20.1
28.4
20.9
17.5
19.3
18.5
20.1
19.5
67.4
66.5
66.9
67
66.8
45.5
45.1
45.4
45.6
45.3
27
39.9
40
27.2
33.4
37.5
100
100
100
100
49.5
73.2
64.6
72.6
69.1
58.7
35.2
36.7
34.9
36.5
34.9
36.1
15.9
16.6
15.7
16
16.3
35.5
38.2
38
38.2
37.1
66
60.1
60
64.4
62.9
60.5
0
0
0
0
14.2
0
13.8
2.5
10.8
12.9
42.3
43.5
43.7
43
43.2
42.5
14.9
14.9
15.8
15.2
14.9
17
14.8
14.6
14.3
16.6
7
0
0
7.9
3.2
2
0
0
0
0
0
0
0
0
0
0
1.6
2.3
2.1
2
1.8
1.9
1.8
2
1.6
1.8
2
2
1.9
2
1.9
2
0
0
0
0.5
0.5
0
n.p.
n.p.
n.p.
n.p.
P
P
P
P
P
P
P
P
P
P
P
P
S
S
S
S
S
P
P
P
P
P
S
S
S
S
S
S
Abbreviations: CH4 = methane; CO2 = carbon dioxide; H2S = hydrogen sulfide; N2 =
nitrogen; n.p. = not possible to distinguish between primary and secondary fluid
inclusions (see Figure 11A); P = primary; S = secondary; SDol 2 = saddle dolomite 2.
isotope composition between pyrite and anhydrite.
The typical d 34S value for sedimentary anhydrite
above and below the carbonate reservoir is 10‰–12‰
V–CDT (Mittag-Brendel, 2000), which is 2%–12%
higher than the measured isotopic composition of
the pyrite samples (Table 4). This is consistent with
a typical fractionation of only 20‰–10‰ between
610
Secondary Porosity Creation in Deeply Buried Carbonates
100°C and 200°C (212°F and 392°F) during TSR
and some degree of Rayleigh fractionation (Machel
et al., 1995). A stronger separation of up to 65‰
between the d 34S values of anhydrite and pyrite
would be expected for BSR (Ohmoto and Rye,
1979; Machel et al., 1995). The complete replacement of all anhydrite nodules by calcite and pyrite
(Figure 4F) demonstrates that TSR has been extensive (Worden and Smalley, 1996). The high
dryness coefficient (C1/C1–3) of 0.998 in the reservoir gas is also consistent with extensive TSR, because high-carbon-number hydrocarbon molecules,
such as ethane or propane, are preferentially removed
during TSR (Krouse et al., 1988). Once these heavier
hydrocarbons are almost depleted in the reservoir and
the dryness coefficient is greater than 0.95, methane
becomes involved in TSR. The relatively high d 13CCH4
values of up to -11‰ VPDB of fluid inclusion gases
(Figure 11B) confirm that methane was involved in
TSR, because lighter carbon isotopes are preferentially
consumed during TSR, leading to a 13C-rich residual
carbon pool. The first methane generated from type III
kerogen in the Carboniferous source rock of the
Lower Saxony Basin can be assumed to have d 13C
values of -30‰ VPDB (Lüders et al., 2012). With
increasing maturity during burial, the isotopic composition of methane shifts toward less negative values.
Considering a vitrinite reflectance between 3.5 and 4
for the studied well (Petmecky et al., 1999), methane
with d 13CCH4 values between -23‰ and -20‰
VPDB can be expected for late gas being generated
during maximum burial (Lüders et al., 2012;
Figure 6). The difference between these d 13CCH4
values and the measured d 13CCH4 values of fluid inclusion gases in the studied well amounts to 9‰–19‰.
Such a positive shift of the d 13CCH4 values can be attributed to TSR and has also been described by several
other authors (Krouse et al., 1988; Worden and Smalley,
1996; Heydari, 1997; Worden et al., 2000; Cai et al.,
2004). A calculation introduced by Cai et al. (2013)
using the above d 13CCH4 values shows that approximately 17%–50% of reactive methane was used during
TSR. Lu et al. (2012) showed that, besides a 13C enrichment in methane, 13C-depleted CO2 is also diagnostic of TSR production, because 13C-depleted CO2 is
formed by the oxidation of isotopically light methane.
Consequently, the residual methane should be isotopically less negative than newly generated CO2. Our results, however, show a 13C enrichment in CH4 but less
Figure 11. (A) Crossplot
showing relationship between
carbon dioxide (CO2) and hydrogen sulfide (H2S) gas composition
(in percent) measured in fluid
inclusions hosted by saddle dolomite 2, anhydrite, and fluorite.
Secondary fluorite inclusions
show a comparable gas composition to present-day reservoir gas.
(B) Carbon isotope ratios (d 13C)
of CO2 and methane (CH4) (in ‰
VPDB) measured in fluid inclusions hosted by saddle dolomite 2 and fluorite. Carbon
isotopes of CH4 indicate thermochemical sulfate reduction, dominated by CH4. The carbon isotopes
of CO2 reflect a mixing of organic
and inorganic CO2. VPDB = Vienna
Peedee belemnite.
negative d 13C values for CO2 compared with methane
(Figure 11B), indicating that the CO2 was not exclusively derived from TSR but also derived from an
additional source.
Fracture filling calcite 2 cement shows d 13C
values as depleted as -6‰ VPDB (Figure 12).
Comparing these with the host-rock depleted carbon
isotope values points to an incorporation of TSRderived carbon during calcite cement formation.
Despite this evidence for extensive TSR, the
reservoir gas of the studied well only shows 2.3%
H2S, and the concentrations of H2S in fluid inclusions
do not exceed 2.3%, either. In particular, fracture
cements, such as saddle dolomite 2, do not contain H2S
in fluid inclusions (Table 1). Typically, TSR is associated with H2S concentrations greater than 3% (Orr,
1977). However, it is known that the H2S accumulation in a reservoir can be limited by the presence of
metal ions, such as Fe2+ (Machel, 2001). Veins with
massive pyrite mineralizations are present in the
studied well (Figure 4E), and we assume that large
amounts of H2S were removed from the reservoir
during pyrite formation. This process has also been
described by Orr (1977). Pyrite is often included in
vein cements, such as saddle dolomite 2 and calcite 2,
indicating that iron-rich fluids ascended along fractures
into the reservoir, where Fe2+ reacted with TSRderived H2S to form pyrite. Further evidence for ascending fluids is given by the REY analyses of two
fluorite samples. The fluorites show enrichment in
intermediate REE, resulting in bell-shaped patterns
with strong positive normalized-yttrium anomalies
(Figure 13), which are typical for pore waters that
mobilized REE from clay minerals, biogenic phosphates, and/or iron–manganese (Fe–Mn) oxides from
siltstones or shales. Bell-shaped REE patterns are
typically observed in biogenic phosphate debris that
derived MREE-enrichment from post-depositional
Biehl et al.
611
Figure 12. Crossplot showing
relationship between d13C and
d 18O isotopic ratio values of dolomite matrix 1, calcite matrix,
dolomite matrix 2, and vein cements. VPDB = Vienna Peedee
belemnite.
alteration (i.e., diagenesis; Lécuyer et al., 1998;
Reynard et al., 1999). Fluorites that originate from
marine carbonate source rocks are characterized by
inherited negative Ce anomalies (Bau et al., 2003).
The fluorite samples have Y/Ho ratios of 140 for
fluorite sample 1 and 196 for fluorite sample 2. According to Bau and Dulski (1995), fluorites with
Y/Ho ratios ranging from 40 to 200 are of hydrothermal origin. The absence of negative Eu
anomalies in the REY patterns of fluorite indicates that
the fluorite-forming fluids have not experienced temperatures above 250°C (482°F; Bau and Möller, 1992)
and also excludes fluid origin and/or significant fluidrock interaction with felsic magmatic rocks (Lüders
et al., 1993). In addition to our data, Fischer et al.
(2006) measured REY for calcite cements of the same
well, which also show a bell-shaped pattern with high
MREE and depleted LREE concentrations, reflecting
carbonate formation under hydrothermal conditions.
Timing of Vein Formation
The studied well shows a great degree of fracturing, in
which several veins crosscut horizontal stylolites but
predate vertical stylolites (Figure 14). This observation
restricts the vein formation to a minimum burial depth
of 500 m (1640 ft) and the end of basin inversion.
Further petrographic evidence for the timing of
vein formation is given in Figure 4D, where a calcite
2–cemented tension gash originates at a vertical stylolite, indicating that they formed at the same time
(Sibson, 1994). The calcite 2 cements within the
tension gash show a crack-seal structure (Figure 9A),
612
Secondary Porosity Creation in Deeply Buried Carbonates
which commonly is the result of episodic crack
opening, where the cracks are rapidly cemented with
crystalline material before the next cracking and
closing phase follows (Ramsay, 1980; Urai et al.,
1991). The observed tension gash textures are interpreted to be formed during basin inversion, simultaneously with the cementation by calcite 2.
The isotopic signature of the calcite 2 cementing the
tension gash is in the same range as all other calcite 2
cements (Table 3; Figure 12), indicating a formation
from similar fluids and under similar temperatures.
Calcite 2 often appears together with saddle dolomite
2 and pyrite, so we assume that this mineral assemblage precipitated during tectonic inversion in the
Late Cretaceous (ca. 70–60 Ma), as they occur in
tension gashes associated with vertical stylolites. This
is supported by the study of Fischer et al. (2006),
who determined absolute ages based on rubidium/
Table 2. Carbon Isotopic Compositions of Methane and Carbon
Dioxide Trapped in Fluid Inclusions Hosted by Saddle Dolomite 2
and Fluorite
Mineral
d 13CCH4 (‰VPDB)
d 13CCO2 (‰VPDB)
SDol 2
SDol 2
Fluorite
Fluorite
Fluorite
Fluorite
-12.5
-11
-20
-17.1
-12.6
-11.7
-4.2
-2
-0.8
-2.4
-0.8
-2
See Figure 11B.
Abbreviations: CH4 = methane; CO2 = carbon dioxide; d 13C = carbon isotopic ratio;
SDol 2 = saddle dolomite 2; VPDB = Vienna Peedee belemnite.
Table 3. Results of Carbon and Oxygen Stable Isotope Analyses of Matrix and Vein Filling Minerals
Sample
Sample Type
Mineralogy
d 13C (‰ VPDB)
d18O (‰ VPDB)
2-I
2-II
2-III
2-IV
2-V
4-II
6-I
6-II
6-III
6-IV
7-II
7-III
8-I
9-I
11-I
11-II
11-MI
12-I
12-II
14-I
14-II
15-I
15-II
15-III
16-I
16-II
16-III
16-IV
17-I
17-II
17-III
17-IV
17-V
20-II
21-I
21-II
22-I
26-II
27-I
27-II
27-III
27-IV
Fracture
Matrix
Matrix
Matrix
Matrix
Fracture
Fracture
Fracture
Matrix
Fracture
Fracture
Fracture
Fracture
Fracture
Fracture
Matrix
Matrix
Fracture
Fracture
Fracture
Matrix
Fracture
Matrix
Matrix
Fracture
Matrix
Matrix
Matrix
Concretion
Concretion
Concretion
Matrix
Matrix
Fracture
Matrix
Matrix
Fracture
Fracture
Fracture
Fracture
Matrix
Matrix
Saddle dolomite 2
Dolomite matrix 2
Calcite matrix
Dolomite matrix 2
Dolomite matrix 2
Saddle dolomite 1
Calcite 2
Saddle dolomite 2
Calcite matrix
Calcite 2
Calcite 2
Calcite 2
Calcite 2
Calcite 2
Saddle dolomite 2
Dolomite matrix 2
Calcite matrix
Saddle dolomite 2
Saddle dolomite 2
Saddle dolomite 2
Dolomite matrix 1
Saddle dolomite 2
Dolomite matrix 2
Calcite matrix
Saddle dolomite 2
Dolomite matrix 2
Dolomite matrix 2
Dolomite matrix 2
Mix of calcite 1 + 2
Mix of calcite 1 + 2
Mix of calcite 1 + 2
Calcite matrix
Dolomite matrix 1
Saddle dolomite 2
Calcite matrix
Dolomite matrix 2
Saddle dolomite 2
Calcite 2
Saddle dolomite 2
Saddle dolomite 2
Dolomite matrix 1
Dolomite matrix 2
0.878
1.445
4.372
1.587
1.152
-1.741
-4.150
-1.594
5.028
0.141
-3.346
-4.765
-5.967
-0.821
-1.741
-0.067
4.848
-0.347
-0.558
-0.658
2.815
-1.335
-0.058
5.197
-1.415
-1.305
-0.364
-1.109
-0.125
0.179
0.113
6.089
5.029
-0.444
5.309
0.334
0.421
-2.250
-1.264
-0.361
5.342
0.401
-8.819
-8.236
-11.920
-7.754
-8.160
-9.357
-13.011
-11.073
-11.509
-9.311
-12.568
-12.602
-12.946
-9.448
-9.357
-8.199
-6.673
-9.542
-9.767
-10.376
-5.582
-10.707
-9.409
-9.488
-9.172
-9.174
-8.409
-8.944
-9.202
-9.083
-9.332
-7.983
-4.758
-9.699
-8.906
-9.182
-8.865
-12.486
-10.488
-9.612
-2.909
-8.710
See Figure 12.
Abbreviation: VPDB = Vienna Peedee belemnite.
Biehl et al.
613
Table 4. Results of Sulfur Stable Isotope Analyses of Pyrite
Sample
4-I
10-I
20-I
26-I
Sample Type
Mineralogy
d 34S (‰ V–CDT)
Fracture
Fracture
Fracture
Fracture
Pyrite
Pyrite
Pyrite
Pyrite
0.1
7.2
8.5
10.2
Abbreviation: V-CDT = Vienna Cañon Diablo troilite.
strontium (Rb/Sr) and uranium/lead (U/Pb) isotope
analyses of fracture-hosted carbonates, sphalerite,
and galena as Late Cretaceous. The burial history
(Figure 5) shows temperatures of approximately
270°C (518°F) prevailing at the onset of inversion
(ca. 80 Ma), with temperatures decreasing to approximately 100°C (212°F) at the end of inversion
(ca. 60 Ma). This temperature range is consistent
with saddle dolomite as the main vein cement, which
is known to precipitate from fluids with temperatures
ranging from 80°C to 235°C (176°F to 455°F; Davies
and Smith, 2006). Variations in the iron content of
saddle dolomite can be correlated to the abundance
Table 5. Results of Rare Earth and Yttrium Analyses of Two
Fluorite Samples and Values Used for Post-Archaean Australian
Shale Normalization
REY
Fluorite 1 (ppm)
Fluorite 2 (ppm)
PAAS (ppm)*
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Y
Ho
Er
Tm
Yb
Lu
0.006
0.028
0.013
0.155
0.223
0.104
0.609
0.074
0.378
7.910
0.056
0.098
0.006
0.022
0.002
0.006
0.013
0.005
0.058
0.107
0.056
0.376
0.049
0.274
8.790
0.045
0.081
0.004
0.016
0.001
38.2
79.6
8.83
33.9
5.55
1.08
4.66
0.774
4.68
27
0.991
2.85
0.405
2.82
0.433
Abbreviations: Ce = cerium; Dy = dysprosium; Er = erbium; Eu = europium;
Fe = iron; Gd = gadolinium; Ho = holmium; La = lanthanum; Lu = lutetium;
Nd = neodymium; PAAS = Post-Archaean Australian Shale; Pm = promethium;
Pr = praseodymium; REY = rare earth and yttrium; Sm = samarium; Tb = terbium;
Tm = thulium; Y = yttrium; Yb = ytterbium.
*After McLennan (1989); see Figure 13.
614
Secondary Porosity Creation in Deeply Buried Carbonates
of pyrite. Iron-depleted saddle dolomites 1 are
directly bound to massive pyrite mineralization zones
(Figure 4E). Here, we assume that iron is incorporated
in the pyrite crystals. In contrast, less pyrite is present in
samples with saddle dolomite 2. Calcite 2 postdates
saddle dolomite 2 and shows less variation in iron
concentration. The high amounts of iron were probably derived from fluids migrating upward through
underlying Carboniferous clays and Rotliegende
(lower Permian) volcanic and siliciclastic red beds
into Zechstein reservoirs. Iron-rich minerals present
in Rotliegende intervals are, among others, chlorite,
ankerite, and siderite, which have released iron during diagenetic processes (Gaupp et al., 1993). Overpressures in these clastic successions could control
the upward flow of basinal fluids (Davies and Smith,
2006). Downward-migrating fluids seem very unlikely in the case of the Z2 carbonate, because it is
overlain by more than 150 m (492 ft) of salt
(Figure 1B) that acts as a seal for reservoir gases and
lacks brittle fractures. Therefore, the system can be
regarded as closed, prohibiting a further ascent of
fluids, as well. As shown in Figure 2, the studied well
is located near a deep-reaching fault system. Considering the burial history (Figure 5) and the results of
REY analysis, it is evident that fluid mobilization occurred after the maximum burial and during basin inversion between 6500 and 2000 m (21,325 to 6562 ft).
Sources of Carbon Dioxide
The isotopic composition of CO2 measured in fluid
inclusions suggests that the high amount of CO2 in
reservoir gas and fluid inclusions has been mixed from
at least two sources. The carbon isotope values of
CO 2 measured in fluid inclusions range between
-4‰ and -1‰ VPDB (Table 2; Figure 12), and the
d 13CCO2 value for reservoir gas is in the same range
(-1.4‰ VPDB). Such carbon isotope values are
known for CO2 originating from inorganic sources,
such as volcanic origins, mantle degassing, or carbonate decomposition processes (Clayton et al., 1990;
Wycherley et al., 1999). Dai et al. (1996) showed
that CO2 derived from a magmatic source or metamorphism of carbonate rocks is a likely origin for
inorganic CO2 in sedimentary basins and should not
be neglected. The Permian strata is underlain by
Devonian carbonates (Kombrink et al., 2010), which
Figure 13. Post-Archaean Australian Shale (PAAS)-normalized
rare earth and yttrium (REY)
pattern of two fluorite samples
displaying a bell-shaped pattern.
Ce = cerium; Dy = dysprosium;
Er = erbium; Eu = europium;
Fe = iron; Gd = gadolinium; Ho =
holmium; La = lanthanum;
Lu = lutetium; Nd = neodymium;
Pm = promethium; Pr = praseodymium; Sm = samarium; Tb =
terbium; Tm = thulium; Y =
yttrium; Yb = ytterbium.
experienced much higher temperatures resulting
from a greater burial depth (>310°C [590°F] for
Carboniferous; Figure 5) than the Z2 carbonate.
Temperatures greater than 330°C (626°F) are sufficient to produce CO2 from thermogenic breakdown
of carbonates, as modeled by Cathles and Schoell
(2007). Devonian carbonates have d 13C values between 1‰ and 4‰ (Buggisch and Joachimski, 2006),
and the d 13C values of the produced CO2 should be
similar (Sharp et al., 2003) or slightly heavier
compared with the precursor carbonate (Santos
et al., 2009). Nevertheless, CO2 derived from a
volcanic source cannot be ruled out completely,
because vitrinite reflectances up to 6% Ro for the
Carboniferous observed for other wells in the study
area were interpreted to result from a deep-seated
magmatic intrusion (Stadler and Teichmüller, 1971).
In contrast, Petmecky et al. (1999) stated that the
extremely deep subsidence in the Lower Saxony Basin
is responsible for high vitrinite reflectances, which was
confirmed by recent modeling studies (Bruns et al.,
2013). Alternatively, Fischer et al. (2006) suggested
that the high amounts of CO2 could also be derived
from pressure solution within the Z2 carbonate itself.
Frequent high-amplitude stylolites are observed in
the studied core. However, we do not assume that
13
C-rich CO2 generated by pressure solution is
sufficient enough to explain the high amounts of
CO2 found in fluid inclusions and reservoir gas. Fluid
inclusions measured from the Upper Carboniferous
in one of the neighboring wells show CO2 concentrations up to 88 mol %, supporting the idea of externally derived CO2-rich fluids. The d 13C values of
CO2 measured in fluid inclusions in this study, as well
as in the reservoir gas, are much more depleted than
would be expected by decarbonation. We therefore
suggest that the inorganic CO2 derived from the
thermal decomposition of carbonates was mixed with
isotopically more negative organic CO2 derived from
TSR, resulting in the d 13C values of fluid inclusions
and reservoir gas.
Fluid inclusions trapped in saddle dolomite 2 show
high CO2 concentrations (49.5–100 mol %) but no H2S
content (Table 1). In contrast, fluid inclusions trapped
in younger vein cements, such as fluorite, show higher
CH4 concentrations (17.5–67.4 mol %) and moderate
CO2 concentrations (15.7–36.7 mol %; Figure 11A),
likely indicating that ascending CO2-rich fluids mixed
with reservoir gases containing H2S and methane.
Fluid Drive Mechanisms
The migration of externally derived fluids was facilitated by the deep faults and associated dense fracture system (Figure 2). The studied well is situated
Biehl et al.
615
Figure 14. Idealized three-step
sketch of matrix and vein paragenesis for dolomite matrix 1
(left) and calcite matrix (right) to
display porosity creation and redistribution, fluid migration, and
the fracture–stylolite relationship.
Veins are partially cemented with
saddle dolomite 2, pyrite, and
calcite 2. Fluorite is the youngest
vein cement precipitating in accordance with falling temperatures (T). Hydrothermal
dolomitizing fluids cause cementation of former molds in
dolomite matrix 1, whereas secondary porosity is created in tight
calcite matrix. CO2 = carbon dioxide; CH4 = methane; H2S =
hydrogen sulfide.
approximately 500 m (1640 ft) away from a deep
reaching, east–west striking fault system. We assume
that these faults and associated fracture networks,
which are below seismic resolution, acted as preferred
pathways for CO2-bearing fluids ascending into the
Z2 carbonate reservoir. Baldschuhn et al. (1991) and
Urai et al. (2008) have shown that basement-hosted
faults in the Lower Saxony Basin can be open and thus
act as fluid conduits for deep-reaching fluid flow. It is
assumed that overpressures developed due to the rapid
subsidence and sedimentation during the Early Cretaceous. This is also confirmed by a pore-pressure and
overpressure calculation made by Fischer et al. (2006),
which shows that overpressure developed directly
after precipitation of the Zechstein, due to the good
616
Secondary Porosity Creation in Deeply Buried Carbonates
evaporitic seal, followed by a rapid increase during
burial. Due to the impermeable evaporitic Zechstein
units above the Z2 carbonate, overpressures could be
maintained for a long time period. Local pressure release
as a result of basin inversion triggered the ascent of CO2rich fluids and carbonate precipitation resulting from
CO2 degassing in open fractures (Leach et al., 1991).
Creation of Secondary Porosity
In intervals with the highly porous dolomite matrix 1,
the dominantly moldic pores are partly cemented by
saddle dolomite 2 (Figure 7C). In the tight calcite
matrix (dedolomite), the almost complete loss of
primary and near-surface porosity caused by calcite
cementation and dedolomitization during shallow
burial (Clark, 1980; Strohmenger et al., 1996) helps
to differentiate the fracture-related dolomitization
from its associated vuggy porosity. Many veins are
surrounded by a dolomitic, chemically blue-stained
halo (Figure 4), indicating a significant influence of
the ascending basinal brines into the host rock, with
different effects on porosity depending on matrix
mineralogy. Point counting reveals an overall porosity
increase of 10%–16% in the former calcite matrix
after late dolomitization and a porosity reduction of
17% in dolomite matrix 1 from a few millimeters up
to several centimeters along the fractures. However,
the dominance of calcite matrix in the studied core
section (Figure 3) results in a net increase in porosity
around fractures. Porosity was increased only in dedolomitic layers affected by hydrothermal alteration
(Figure 14). Davies and Smith (2006) pointed out
that hydrothermal dolomitization is often accompanied by a porosity increase. A porosity increase up
to 13% could be expected if the dolomitization follows mole-per-mole replacement of calcite by dolomite (Machel, 2004). However, the presence of
saddle dolomite 2 cements within the vuggy pores
indicates that some of the newly created porosity was
closed again by a continuous supply of fluids capable of precipitating saddle dolomite 2 in open vugs
(Figure 14). However, because not all porosity is
closed and the porosity increase lies between 10% and
16% after dolomitization of the former calcite matrix,
other processes likely contributed in combination with
the dolomitization to the creation of burial porosity.
Fluid inclusion analyses revealed high CO2 concentrations of 50%–100% in primary inclusions
hosted by saddle dolomite 2. Retrograde dissolution
of carbonates resulting from cooling of CO2-rich
brines during the ascent of hydrothermal fluids or
uplift could contribute to porosity creation (Davies
and Smith, 2006). However, this seems unlikely, as
no dissolution of vein carbonates such as saddle dolomite 2 and calcite 2 was observed petrographically.
Instead, dolomitization to form dolomite matrix 2
was favored by such high initial CO2 concentrations,
as proposed by Leach et al. (1991), who stated that a
sudden release of initially high amounts of CO2
(“CO2 effervescence”) leads to high dolomitization
rates and dissolution of calcite. They demonstrated by
geochemical modeling that continued dolomitization
causes a later supersaturation of the pore water with
respect to calcite, leading to a phase of calcite cementation (Leach et al., 1991; Figure 5), which is in
accordance with our petrographic observations. The
rapid ascent of the CO2-rich fluids therefore facilitated the vuggy porosity creation in calcitic intervals;
however, it also led to cementation of some of the
vuggy and fracture porosity by saddle dolomite 2,
pyrite, and calcite 2.
It has been proposed that TSR creates late burial
porosity in carbonate reservoirs either directly
through the generation of H2S (Ma et al., 2007) or
through TSR reactions involving magnesium sulfate
formed previously by the reaction of dolomite with
anhydrite (Cai et al., 2014). However, despite clear
evidence of TSR, there is no indication of deep-burial
porosity increase in the host rock unaffected by
fractures. On the other hand, a significant porosity
increase has been observed associated with burial
dolomitization and pyrite formation (Figure 6). Significant volumes of pyrite crystals occur in areas
where the matrix has been affected by hydrothermal
fluids (Figures 4B; 7C, D). The pyrites are particularly
abundant at the boundary of the calcite matrix and
the zone of fracture-related dolomitization (Figure
14), because pyrite is precipitated along the stationary reaction front by diffusion of Fe2+ from the
dolomitizing fluid and H2S from the reservoir gas.
Acidification of fluids by metal sulfide deposition is an
accepted mechanism for deep-burial dissolution of
carbonates and the creation of secondary porosity,
which has been reported from Mississippi Valley type
deposits (Moore and Druckman, 1981; Qing and
Mountjoy, 1994) and was modeled by Corbella et al.
(2004). Carbonates are dissolved under acidic conditions when metal sulfides are precipitated and H+ is
released from TSR-derived H2S. For the presented
study, the reaction would be the following:
2H2 S + Fe2 + →FeS2 + 4H+
Machel (2001) claimed that this process is often
negligible in deep-burial carbonates because metal
ions, such as Fe2+, are limited and that the removal of
H2S by Fe2+ is more typical of BSR settings. However, in
the studied well, the high volumes of ferroan carbonate
cements and pyrite provide evidence of a high Fe2+
concentration, which was introduced into the system
from the underlying clastic strata. Consequently, the
formation of pyrite by removal of TSR-derived H2S
Biehl et al.
617
likely contributed to the creation of secondary vuggy
porosity in the studied carbonates. The dissolved
material is partially incorporated into newly formed
dolomite cements in veins and pores. Due to acidification by the formation of pyrite, additional calcium carbonate can be absorbed in the fluid of the
closed reservoir system. Ehrenberg et al. (2012) argued that the rapid kinetics of carbonate dissolution
would cause the nearly instantaneous buffering of
undersaturated water chemistry during upward flow,
leaving virtually no potential for porosity creation.
However, the above reaction would generate the
acidity necessary for calcite dissolution within
the reservoir itself, without the need to assume migration of undersaturated waters from below. The
ascent of iron-rich, dolomitizing, basinal fluids into
carbonate reservoirs affected by TSR is not uncommon and therefore has to be considered as an
important setting for deep-burial porosity creation
(Gregg, 1985; Davies and Smith, 2006). Thus, CO2rich fluids in combination with hydrothermal dolomitization of the calcite matrix and the reaction of
H2S with Fe2+ to produce pyrite are the processes
leading to the creation of secondary burial pores in
the calcite matrix and to partial cementation of moldic
pores in dolomite matrix 1 and fractures (Figure 14).
In accordance with Ehrenberg et al. (2012), the net
increase in porosity is limited to a relatively narrow
reaction zone along the length of the fractures,
likely exerting a stronger control on permeability than
total net porosity. More than 80% of matrix permeability measurements from plugs of the studied
well range between 0.003 and 0.7 mD, with an
average value of 0.03 mD (Figure 6). However,
production inflow performance tests from the same
well indicate a much higher average dynamic permeability (calculated at 2 mD), representing the in situ
effective flow capacity for fluids averaged over the entire
reservoir away from the wellbore. It is likely that this
dynamic permeability better displays an additional
contribution to reservoir performance from a hydrothermally altered carbonate matrix, which often shows
permeabilities greater than 1 mD (Figure 6) close to
the conduit faults. Small volumes in the direct vicinity
of fractures therefore influence reservoir permeabilities
to a large extent, despite their small effect on total reservoir
porosity. Such an increase in permeability and redistribution in porosity have to be considered for modeling reservoir properties and fluid migration pathways.
618
Secondary Porosity Creation in Deeply Buried Carbonates
CONCLUSIONS
1. The Z2 carbonate in the studied well consists of
alternating porous dolomitic and tight calcitic
(dedolomitic) intervals, which have been affected
by late diagenetic, iron-rich hydrothermal fluids.
The fluids ascended along a deep-reaching fault
system into the reservoir, which is covered by
anhydrites and thick rock salt layers. These layers
prohibit a fluid descent from shallower stratigraphic units into the reservoir.
2. Sudden release of CO2 from the ascending fluids
facilitated fracture-controlled burial dolomitization
of the calcite matrix. The CO2 was mainly sourced
from an inorganic origin, most likely from the thermal decomposition of carbonates, such as underlying
deeply buried Devonian carbonates. The d 13C values
of CO2 gas within fluid inclusions of saddle dolomite
and fluorite and reservoir gas reflect a mixture of
inorganic CO2 and TSR-derived, organic CO2.
3. Calcitized anhydrite nodules and abundant pyrite
mineralizations indicate that TSR occurred to a
great extent in the studied well. High-carbonnumber hydrocarbons and up to 50% of reactive
methane were consumed during TSR, as evidenced
by the positive shift in the d 13C values of methane.
4. The TSR-derived H2S reacting with Fe2+ from
iron-rich, ascending fluids led to the precipitation
of pyrite and acidification of formation waters.
This, in combination with hydrothermal dolomitization, caused the replacement of the calcite
matrix with dolomite, accompanied by a 10%–16%
increase in porosity in a several-centimeter-wide
halo around fractures. In contrast, hydrothermal
saddle dolomite cements reduced moldic porosity in the early diagenetic dolomite matrix by
approximately 17% and led to partial-to-complete
cementation of fractures. However, the carbonate
dissolution in the dominantly calcite matrix in the
reservoir results in a net increase in porosity and
permeability in the vicinity of the fracture walls,
which has to be considered for modeling reservoir
properties and fluid migration pathways. This
process generates the acidity necessary for calcite
dissolution within the reservoir itself, without the
need to assume migration of undersaturated waters from below. The co-occurrence of TSR with
hydrothermal alteration is reported for many
deeply buried carbonate basins and therefore could
be a common mechanism for the redistribution and
creation of porosity in the deep-burial realm.
5. All processes contributing to fracture cementation
and secondary porosity creation occur in depths
greater than 2000 m (6562 ft) and were caused by
fluids circulating upward along deep-reaching fault
systems during basin inversion in the Late Cretaceous (ca. 70–60 Ma). The link between fault
zones and fluid migration has a major impact on
the distribution of fracture-related hydrothermal
dolomitization, the creation or destruction of burial
porosity, and, hence, the reservoir quality.
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