Sealing of fluid pathways in overpressure cells - Ged.rwth-aachen.de

Int J Earth Sci (Geol Rundsch) (2005) 94: 1039–1055
DOI 10.1007/s00531-005-0492-1
ORIGINAL PAPER
Sofie Nollet Æ Christoph Hilgers Æ Janos Urai
Sealing of fluid pathways in overpressure cells: a case study
from the Buntsandstein in the Lower Saxony Basin (NW Germany)
Received: 20 October 2004 / Accepted: 18 March 2005 / Published online: 30 June 2005
Ó Springer-Verlag 2005
Abstract We studied veins in the Triassic Buntsandstein
of the Lower Saxony Basin (NW Germany) with the aim
of quantifying the evolution of in-situ stress, fluids and
material transport. Different generations of veins are
observed. The first generation formed in weakly consolidated
rocks without a significant increase in fracture
permeability and was filled syntectonically with fibrous
calcite and blocky to elongate-blocky quartz. The stable
isotopic signature (d 18 O and d 13 C) indicates that the
calcite veins precipitated from connate water at temperatures
of 55–122°C. The second vein generation was
syntectonically filled with blocky anhydrite, which grew
in open fractures. Fluid inclusions indicate that the
anhydrite veins precipitated at a minimum temperature
of 150°C from hypersaline brines. Based on d 34 S measurements,
the source of the sulphate was found in the
underlying Zechstein evaporites. The macro- and microstructures
indicate that all veins were formed during
subsidence and that the anhydrite veins were formed
under conditions of overpressure, generated by inflation
rather than non-equilibrium compaction. The large
amount of fluids which are formed by the dehydrating
gypsum in the underlying Zechstein and are released into
the Buntsandstein during progressive burial form a likely
source of overpressures and the anhydrite forming fluids.
Keywords Veins Æ Lower Saxony Basin Æ Overpressure
Introduction
The role of fluids during the evolution of sedimentary
basins is partly recorded by cements in the rock matrix
and veins. During subsidence, porosity is reduced by
S. Nollet (&) Æ C. Hilgers Æ J. Urai
Geologie-Endogene Dynamik, RWTH Aachen,
Lochnerstr. 4-20, 52056 Aachen, Germany
E-mail: s.nollet@ged.rwth-aachen.de
Tel.: +49-241-8095416
Fax: +49-241-8092358
compaction. Connate and diagenetic/metamorphic fluids
are released by the dewatering of minerals—for
example the smectite–illite transition (Bruce 1984;
Bjørlykke et al. 1989; Bekins et al. 1994) or the dehydration
of gypsum (Hardie 1967; Shearman et al. 1972).
Additional fluids can be released by the maturation of
organic components producing hydrocarbons (Bredehoeft
et al. 1994; Law and Spencer 1998). Besides these
intrinsic processes, basin scale advective fluid flow of
meteoric waters is common, usually along fractures
(Gross et al. 1992, Mo¨ ller et al. 1997). In a sedimentary
basin, a number of processes may produce deviations
from hydrostatic pore pressure and thus drive fluid flow
along a hydraulic gradient, for example topography
(gravity-driven flow), compaction, thermal gradients
(e.g. around salt domes due to the higher thermal conductivity
of halite), or tectonic stresses (dilatancy and
tectonic pumping).
Fluid overpressures may be generated in the presence
of a seal (such as clays and evaporites), the processes
being either undercompaction (if the rock cannot drain
fast enough during subsidence), or inflation (if fluid
pressure rises after compaction by the introduction of
additional pore fluid; Law and Spencer 1998; Townend
and Zoback 2000). Extension fractures may form under
many different conditions, depending on the ratio of the
rock’s compressive strength and the effective stress
(Ingram and Urai 1999). Extension fracturing may occur
under conditions of overpressuring when fluid pressures
are as low as 0.6 times the total vertical stress (Ingebritsen
and Sanford 1998, p 104).
All of these processes could be relevant to the Triassic
Main Buntsandstein in the Lower Saxony Basin, which
is characterized by Permian to Jurassic rifting and
Cretaceous inversion, accompanied by salt tectonics
(Ziegler 1990; Brink et al. 1992; Scheck et al. 2003)
(Fig. 1).
Fluid activity may be recorded in a rock when
changes in parameters like temperature, pressure or fluid
chemistry cause supersaturation and the precipitation of
solute as cements or veins. Vein microstructures may

1040
Fig. 1 Overview map of the
central and western part of the
southern Permian Basin with
main structures (LBM London
Brabant Massif, RM Rhenish
Massif, LSB Lower Saxony
Basin, CG Central Graben, PB
Pompeckj Block and RH
Ringkøbing-Fyn High). Inset
shows an enlarged map of the
LSB with the location of the
four sampled boreholes and the
main structural elements
(normal faults, reverse faults
and salt domes) (after Ziegler
1990; Baldschuhn et al. 2001)
UK
NL
London Amsterdam
B
CG RH
LSB
LBM RM
D
Borehole 1
Ems
Hamburg
Hannover
Borehole 3
Osnabrück
PB
LSB
Weser
Weser
Borehole 4
Leine
Hannover
Borehole 2
N
53°
52°
Legend:
7° 8°
9° 10°
normal fault salt structure
reverse fault
25 km
vary widely with crystal habits ranging from dendritic,
fibrous, elongate-blocky to blocky crystals, depending
on the boundary conditions of crystal growth (Ramsay
and Huber 1983; Sunagawa 1984; Bons and Jessell 1997;
Hilgers et al. 2001).
Syntectonic vein microstructures can be divided into
antitaxial, syntaxial, stretched (or ataxial crystals) and
blocky veins. Antitaxial veins are often fibrous and grow
from the vein centre towards both sides of the wall.
Fibres are optically continuous even when they are
curved and these fibres may sometimes be used to infer
the fracture’s opening kinematics (Ramsay and Huber
1983; Uraietal.1991; Hilgers et al. 2001). Syntaxial
veins are overgrowths of wall rock material, growing
from the walls towards the centre of the vein (Durney
and Ramsay 1973). Their microstructure is usually
elongate-blocky rather than truly fibrous (Bons 2000).
Stretched veins may contain fibrous crystals and connect
fractured wall rock grains. Although they are assumed
to have formed by delocalized fracturing (Passchier and
Trouw 1996, p 135), at least some veins show clear evidence
for accretion at the vein-wall interface (Hilgers
and Urai 2002). From the vein microstructure, the
dimensions of the fluid pathways can be estimated. For
example, euhedral crystals grew in open fractures or
voids, while fibrous veins, even when they are centimetre-wide,
can only be formed when the incremental crack
openings are less than about 10 lm (Hilgers et al. 2001).
Thus, microstructures contain significant information
on the boundary conditions during vein emplacement.
Precipitation from fluids as intergranular cements in
the Triassic sandstones in the Central European Basin
was studied by a number of researchers (Laier and
Nielsen 1989; Rieken and Gaupp 1991; Purvis and
Okkerman 1996; Weibel 1998, Putnis and Mauthe 2001;
Weibel and Friis 2004). Laier and Nielsen (1989) showed
that the Bunter Sandstone formation in Denmark,
corresponding to the Buntsandstein in Germany, is
cemented with halite, which is mainly present in the
sandstone layers. They suggested that the cement was
formed very late in the diagenetic history and explain the
halite cementation by downward micro-filtration of
connate fluids. Halite cementation is located above lowpermeable
layers of shale, which these authors interpreted
as semi-permeable membranes.
Purvis and Okkerman (1996) observed that the reservoir
properties in the Main Buntsandstein (offshore
the Netherlands) are reduced by cementation with
dolomite, anhydrite and halite. Sulphur isotopes indicate
that the source for anhydrite and halite was the underlying
Zechstein. In North-West Germany, halite
cementation is also observed in the Solling Formation
within the Buntsandstein (Putnis and Mauthe 2001).
Rieken and Gaupp (1991) showed that in the Tho¨ nse
gasfield area, dolomite, anhydrite and quartz precipitated
in veins in the Buntsandstein at temperatures of
140–230°C. Based on vitrinite reflectance data, they
suggested that the veins were formed during short episodes
of large-scale vertical fluid flow during the Upper
Cretaceous (Rieken and Gaupp 1991).
In this study, we focus on sealed fractures in the Main
Buntsandstein in the Lower Saxony Basin (LSB). The
aim is to unravel the complex fluid history, in particular
the fluid source, transport mechanisms and the evolution
of in-situ stress. This has important consequences for the
total flow rate and the bulk volume of fluid flowing in a
sedimentary basin. We use stable isotopes and microthermometry
to constrain p–T conditions and timing
of vein formation, and we will deduce the transport
mechanism of anhydrite and the structural control
during vein emplacement.
Geological setting
The LSB is part of the E–W trending southern Permian
basin, bordered in the north by the Ringkøbing–Fynn
High (Scheck and Bayer 1999; Kossow and Krawczyk

1041
2002) (Fig. 1). It was filled with Rotliegend clastic sediments
and cyclic deposits of Zechstein evaporites (carbonates,
sulphates and halite). Rotliegend sandstones
and Zechstein carbonates are important reservoir rocks,
with the Zechstein evaporites acting as a regional seal
(Ziegler 1990). During the Triassic, NNE–SSW oriented
rifting took place and continental, brackish-marine red
beds, shallow marine carbonates, sulphates and halite
were deposited in an arid to semi-arid climate
(Michelsen and Clausen 2002; Szurlies et al. 2003). The
Lower Triassic Buntsandstein in northern Germany
consists of three units (Fig. 2): (1) shaly sediments in the
Lower Buntsandstein; (2) four depositional sub-cycles in
the Middle Buntsandstein that start with a regressive
sand package formed by braided streams and sheet
floods, and close with transgressive shales, deposited
under ephemeral playa lake conditions and (3) marine
shales, sulphate and halite series in the Upper Buntsandstein
(Ro¨ t) (Herrmann et al. 1968; Ziegler 1990;
Kovalevych et al. 2002; Michelsen and Clausen 2002).
During the time of rapid subsidence in the Late
Jurassic, WNW–ESE oriented normal faults were
formed, for example in the Glu¨ ckstadt Graben (Brink
et al. 1992; Scheck et al. 2003). In the LSB, inversion
started in the Turonian and peaked during the Santonian
and Campanian. Total maximum inversion uplift is
estimated at 2 km to locally even 8 km, causing significant
erosion. The inversion created thrusts along the
basin margins and flower structures (de Jager 2003;
Kockel 2003). In the southern part of the LSB, rocks
were subjected to higher burial and stronger inversion,
then in the northern part (Petmecky et al. 1999).
Abnormally high maturities are observed at the time of
inversion in the Bramsche area (western part of LSB),
which are explained by some authors by abnormally
high heat flow due to magmatic intrusions, while others
suggest that tectonic events caused deep burial and the
high temperatures (Brink et al. 1992; Petmecky et al.
1999).
Initial (passive) diapirism of Zechstein salt was initiated
in the Late Triassic (Trusheim 1957; Brink 1984;
Brink et al. 1992; Baldschuhn et al. 1998; Bayer et al.
1999; Scheck et al. 2003). In the Upper Jurassic, salt
mobilization continued and broke through the weakest
units of the Mesozoic cover (Jaritz 1980). From the
Early Cretaceous until Early Cenozoic, salt diapirism
persisted as indicated by salt-rim synclines (Scheck et al.
2003; Mohr et al. this volume).
In this study, the Middle Buntsandstein was sampled
in four boreholes in the LSB (Fig. 1), because this
formation was affected by the structural and fluid history
starting from early salt movement to basin inversion.
The four boreholes are located in different
tectonic settings (Fig. 3). Borehole 1 is located in an
inversion structure where faults have been reactivated
during the inversion in the Late Cretaceous (Kockel
2003). Borehole 2 is located in an anticlinal structure,
related to salt doming (Rieken and Gaupp 1991). The
structure of borehole 3 in the southern part of LSB is
also located in an inversion structure where Zechstein
salt intruded into the overlying Triassic sediments
along faults (Baldschuhn et al. 2001). Borehole 4 is
Fig. 2 Stratigraphic column of
the Middle Triassic (anh
anhydrite, h halite) (after
Borchert and Muir 1964;
Baldschuhn et al. 2001) and
table showing the
characteristics of the observed
veins for each formation in the
Middle Buntsandstein (cc
calcite, anh anhydrite, h halite, d
dolomite, qtz quartz)

1042
Fig. 3 Simplified cross-sections
of the sampled boreholes (after
Baldschuhn et al. 2001).
aBorehole 1, b Borehole 2,
c Borehole 3 and d Borehole 4
located above a salt pillow and at 4 km from a salt
dome.
Methods
Standard thin sections (20–30 lm) were used for
microstructural analyses and cathodoluminescence.
Cathodoluminescence was carried out on a Technosyn
Cold Cathodo Luminescence Model 8200 MkI under
8.5–10.5 kV and 260–350 mA. Double polished, 150-
lm-thick sections were prepared for fluid inclusion
analyses in anhydrite, according to the method described
in Muchez et al. (1994). Microthermometry was carried
out on a Linkham stage (K.U.Leuven, Belgium), which
was calibrated at 56.6, 21.2, 0.0 and 374.1°C with
synthetic Syn Flinc inclusions. Homogenization temperatures
of inclusions were measured before freezing.
Freezing may cause a volume increase related to the
liquid water–ice transition, which induces deformation
of the crystal and a volume change in the fluid inclusions.
This volume change results in incorrect homogenization
temperatures (Reynolds and Goldstein 1990).
For d 18 O V-PDB /d 13 C V-PDB measurements, carbonate
powders reacted with 100% phosphoric acid at 75°C
using a Kiel III online carbonate preparation line connected
to a ThermoFinnigan 252 mass spectrometer
(Institut fu¨ r Geologie und Mineralogie, Universität Erlangen-Nu¨
rnberg, Germany). All values are reported in
per million relative to V-PDB by assigning a d 18 O value
of 2.20& and a d 13 C value of +1.95& to NBS19.
Reproducibility was checked by replicate analysis of
laboratory standards and is better than ± 0.03 for d 13 C
and ± 0.06 for d 18 O. Sulphur isotopes were measured in
an EA-ConfloII-Finnigan Delta+ mass spectrometer
after mixing of the SO 2 gas with V 2 O 5 at 1,050°C
(Department of Geology-Westfa¨ lische Wilhelms-Universita¨
tMu¨ nster, Germany). Results are reported in the
standard delta notation (d 34 S) as per mil difference relative
to Canyon Diablo Troilite (CDT). Analytical
reproducibility was generally better than ±0.4&.
Macroscopic observations
In boreholes 1, 3 and 4, the Middle Buntsandstein is
located at depths of around 2,500 m whereas in borehole
2, the Middle Buntsandstein is at 3,400 m. All the
boreholes are vertical and although they are located in
different tectonic settings and up to 200 km apart, the
vein types and the vein-filling minerals are very similar in
the cores studied (Fig. 2).
In all boreholes, the Solling Formation is composed
of red–brown coloured, fine-grained silt- to claystone,
cemented with calcite and dolomite. Green reduction
spots with diameters of 1 cm are very often observed.
Veins are frequent in the Solling Formation, with
approximately one vein in every 2 m core. They are filled
mainly with anhydrite and calcite and rarely with only
one of the two minerals. Occasionally, veins with calcite
and halite were found in borehole 2. Veins are oriented
70° to 90° to bedding. The thickness of the veins ranges
from 0.5 cm up to 3 cm. Veins are longer than the
section exposed by the cores, and in one case where the
vein is parallel to the core axis, a vein length of 3 m
was observed. The fracture morphology of the calciteanhydrite
and anhydrite veins is very regular (Fig. 4a).
When the vein consists of calcite and anhydrite, the
calcite is located at the vein walls whereas the anhydrite
is located in the centre of the vein. In the veins where
halite and calcite are observed, the calcite is also located
at the vein wall and the halite in the centre. The veins
with only calcite filling are thin (0.5 cm), have a smooth
vein-wall interface in claystone and a more irregular
morphology in coarser-grained host rock. When crossing
a layer boundary into coarser-grained host rock,
they split into different mm-thin branches (Fig. 4b).
The Hardegsen Formation is only present in borehole
1 and consists of coarse-grained, grey-green coloured
sandstone with alternations of red–brown
coloured clay-rich layers. It is often cemented with
quartz and pyrite concretions are widely present in this
formation. Veins are only occasionally observed

1043
Fig. 4 Rock samples showing
the different vein generations.
a Blocky calcite-anhydrite vein
with very regular wall rock
interface. Calcite (cc) is located
at the vein wall and anhydrite
(anh) in the vein centre (sample
B3-14). b Fibrous calcite vein
with smooth wall-rock interface
in the claystone and irregular
vein branches in the coarsegrained
sandstone (sample
B2-08). S 0 =bedding plane
(approximately 1 vein in every 10 m core), have
thicknesses smaller than 1 mm and are filled with
anhydrite or very rarely halite. They are oriented 85° to
90° to the bedding.
The Detfurth Formation consists of red-brown,
coarse-grained sandstone with clay-rich intercalations.
Veins are rare, approximately one vein in a 5 m core and
are filled with anhydrite and calcite. Thicknesses vary
between 0.5 cm and 1 cm and lengths are at least 30 cm
(veins rarely terminate in the core). The veins are oriented
between 65° and 90° to the bedding. In borehole 4,
very thin (1 mm or smaller) halite veins are occasionally
observed in this formation.
The Volpriehausen Formation is composed of red- to
grey-coloured sandstone with clay-rich layers. Veins are
rare (approximately 1 in every 30 m core) and are very
thin (

1044
Fig. 5 aMicrostructure showing a calcite vein in claystone. The
vein splits in different branches in the coarse-grained siltstone (thin
section s78, normal to bedding, sample B2-08). b Calcite overgrowth
in sandstone showing that the vein is delocalized and does
not cross the quartz grains of the host rock. Quartz grains are
partly dissolved by pressure solution (thin section s81, normal to
bedding, sample B3-19). c Antitaxial calcite vein in claystone
towards the vein centre. Syntaxial growth resulted in
significant growth competition at the vein–wall interface,
indicating that the vein grew syntectonically with a
significant opening (>10 lm) to the opposing wall.
Microstructures suggest that some stretched calcite
crystals grew contemporaneously with the quartz
(Fig. 6b). Stretched crystals of quartz are clear evidence
for syntectonic growth. The lateral variation from
stretched crystals to elongate-blocky syntaxial growth
suggests that the open cavity growth with euhedral
crystal terminations in the veins is also syntectonic.
showing small crystals in the centre, which grew towards both sides
of the wall. The vein grew asymmetrically (thin section s78, normal
to bedding, sample B2-08). d Cathodoluminescence image of a
calcite vein showing that the cements in the host rock have the same
luminescence colour as the vein. Note that fibres connect bedding
on both sides of the vein in this example (thin section s78, normal
to bedding, sample B2-08). cc calcite, qtz quartz
Locally, anhydrite fills open vugs in the syntaxial veins,
indicating that quartz–calcite grew at an earlier phase.
Anhydrite veins
Anhydrite is found in extension veins oriented normal to
bedding. Fracture surfaces are more regular, in contrast
to the irregular morphology of the calcite veins,
indicating that the rocks were more consolidated during
this phase of vein formation.
Fig. 6 a Microstructures
showing a stretched quartz vein
with few solid inclusions
incorporated in the quartz
crystals indicating syntectonic
growth (thin section s61_a,
normal to bedding, sample
B2-05). b Stretched quartz and
calcite crystals suggesting that
both calcite and quartz
precipitated
contemporaneously. Faceted
quartz with euhedral crystal
terminations are also observed
in the same vein (thin section
s61_b, normal to bedding,
sample B2-05). cc calcite, qtz
quartz

1045
Anhydrite veins contain aggregates of clear needlelike
radiating crystals (rosettes) or single, thin needles,
with well-defined cleavage (Murray 1964; Holliday 1970)
(Fig. 7a). The rosettes radiate from a single blocky
anhydrite crystal. Veins may have a blocky or elongateblocky
microstructure, with growth competition at the
vein–wall interface (Fig. 7b).
Cross-cutting relationships of calcite veins or branches
and anhydrite veins indicate that calcite veins
re-opened and were filled with anhydrite (Fig.7c). This
indicates that anhydrite veining postdates calcite
veining.
Halite veins
In contrast to the other boreholes, borehole 2 contains
centimetre thick halite veins in the Solling Formation.
Halite is located in the centre of a calcite vein, which is
oriented normal to bedding (Fig. 8a). The vein microstructure
varies laterally from fibrous to blocky within a
single vein, indicating syntectonic growth at different
opening or growth rates (Hilgers et al. 2001). Halite
contains mainly primary subgrains, recognized by their
fibrous shape. Few grains show polygonal subgrains,
which are indicative of deformation due to dislocation
creep (Fig. 8b) (see also Schleder and Urai this volume).
The subgrain size is then inversely proportional to the
differential stress (Carter et al. 1993). The subgrain size
was measured and results suggest low differential stresses
of around 1.4 MPa.
Sulphate reduction
Green reduction spots and pyrite concretions are frequently
observed in the shaly host rock close to anhydrite
veins. In these veins, calcite is locally present at the
vein–wall interface and within the vein. The texture in
these veins points to replacement of the anhydrite crystals
by calcite at a later stage (Fig. 7d). This may indicate
that anhydrite was dissolved and thermochemical
sulphate reduction took place with methane to form
calcite and H 2 S according to the following reaction
(Machel 1987, 2001;Worden et al. 1995):
CaSO 4 þ CH 4 ) CaCO 3 þ H 2 S þ H 2 O:
Stable isotopes
Oxygen and carbon isotopes
The d 18 O and d 13 C isotope compositions were measured
in the calcite veins and in dolomite and calcite cements
in the host rock (Table 1, Fig. 9). Values for d 18 O vary
between 11.98& and 6.11&, whereas d 13 C values
range between 2.81& and 1.87& (Table 1). Signatures
for veins and cements are very similar, indicating that
the vein minerals have the same source as the cements in
the host rock. This is also suggested by the cathodoluminescence
colours in the calcite cements and veins.
The d 18 O signature in the calcite veins depends on the
origin of the precipitating fluids and on the precipitation
Fig. 7 a Microstructures
showing anhydrite rosette
crystal indicating that the
crystals grew in an open space
(thin section s67/34, normal to
bedding, sample B1-03).
b Growth competition between
anhydrite crystals at the vein
wall pointing to syntaxial
growth in an open void (thin
section s67_a, normal to
bedding, sample B1-03).
c Cross-cutting relationship
between elongate-blocky calcite
vein and blocky anhydrite
indicating that the calcite vein
was reactivated and filled with
anhydrite (thin section s66_a,
normal to bedding, sample B1-
02). d Anhydrite replacement by
calcite, indicated by anhydrite
remnants in a blocky calcite
matrix (thin section s67_1,
normal to bedding, sample B1-
03). cc calcite, anh anhydrite

1046
d 18 O SMOW isotopic composition of 4&, precipitation
temperatures were between 84°C and 122°C (Fig. 10).
The range of d 13 C around 0& in our samples can be
the result of mixing of bacterial carbon with carbonaterich
pore fluids (Irwin et al. 1977; Harwood and Coleman
1983). If the carbon had its origin only in organic
material, the d 13 C values would be extremely negative,
which is not the case in our data.
Sulphur isotopes
Fig. 8 a Halite vein in the Solling Formation with calcite (cc) on
both sides of the wall. b Halite contains primary subgrains, and is
almost free of deformation (1.4 MPa differential stress)
temperature (Faure 1998) (Fig. 10). The Buntsandstein
was deposited in an arid climate and under these conditions,
meteoric water has d 18 O SMOW values between
0& and 5& (Harwood and Coleman 1983). If the
meteoric water in the pores is mixed with connate
waters, resulting from mineral reactions during the early
compaction of the sediments, the pore fluids will have a
more positive d 18 O signature (Suchecki and Land 1983).
Assuming that the pore fluids in the Buntsandstein have
a d 18 O SMOW composition of 0& during vein growth, the
calcite veins precipitated at a temperature of 55–84°C
(Fig. 10). If the pore fluids had a more connate
In borehole 1, 3 and 4 we analysed 14 samples from
anhydrite veins sampled in the Solling Formation. Values
for d 34 S range between 11.9& and 13.4& CDT (Table 2),
and do not show a relation with depth. This indicates that
the veins precipitated from a fluid with a relative uniform
d 34 S composition. Lower Triassic seawater had a sulphur
isotopic composition of 16–21.5& CDT, which is significantly
higher than the values in our samples (Claypool
et al. 1980; Kramm and Wedepohl 1991; Kampschulte
and Strauss 2004) (Fig.11). The Upper Permian seawater
had a lighter sulphur isotopic composition of 12.1–14.4&
CDT, which corresponds very well with our data. This
suggests that the sulphate in the anhydrite veins was remobilized
from the underlying Zechstein.
Fluid inclusions
Fluid inclusions with diameters between 5 lm and
20 lm are common in growth zones in the large, blocky
anhydrite crystals. Most inclusions contain two phases,
liquid and vapour, although some inclusions with only
one phase (liquid) were observed. These could indicate a
formation below 50°C. However, it is more likely that
the absence of a gas bubble may be due to nucleation
problems (metastable one-phase inclusions), as these
inclusions are small (

1047
-12 -10 -8 -6 -4 -2
Legend:
veins
cements
0 0 1
Fig. 9 Carbon isotopic signature (d 13 C) plotted against oxygen
isotopic signature (d 18 O) for the calcite veins and cements in
borehole 1–4. The data of the cements are very similar to the data
of the veins, indicating that they precipitated from the same fluids.
All values are in PDB (based on d 18 O (SMOW) =1.03086 · d 18 O (PDB)
+ 30.86
hole 1, a wide range of fluid inclusion homogenization
temperatures is observed (Fig. 12a). Such variation can
be the result of re-equilibration (stretching) of the primary
inclusions during further burial. In this case, the
higher temperatures do not represent the homogenization
of the primary inclusions (Goldstein 1986; Burruss
1987; Muchez et al. 1991). However, the lowest
homogenization temperatures (between 110°C and
150°C) indicate the minimum homogenization temperature
of the original fluid inclusions. The highest frequency
of homogenization temperatures is observed
around 150°C (Fig. 12a). After freezing, the first melting
(T fm ) is observed around 52°C, which suggests that the
fluid is of the NaCl–CaCl 2 –H 2 O type (Roedder 1984;
Fig. 10 Oxygen isotopic signature against the temperature for different
isotopic compositions of the precipitating water (d 18 O (SMOW)
water), assuming that the fluid and the calcite veins were in
equilibrium. When the precipitating fluid had a composition of
0–4& SMOW, the calcite veins and cements precipitated between
55°C and 122°C. Calculations based on O’Neil et al. (1969)
2
-1
-2
-3
Shepherd et al. 1985; Reynolds and Goldstein 1990).
The last melting of ice is between 28.6°C and 13.9°C,
pointing to salinities 2.3–3.3wt% NaCl and 14.9–
21.5wt% CaCl 2 (Oakes et al. 1990) (Table 3).
In borehole 3, homogenization temperatures range
between 100°C and 150°C, with the highest frequency at
140°C (Fig. 12b). Higher homogenization temperatures
are also observed but have a smaller frequency
(Fig. 12b). This suggests, as in borehole 1, stretching of
the primary inclusions during further basin evolution.
The temperature of first melting indicates fluids from a
NaCl–CaCl 2 –H 2 O type, as in borehole 1. However,
further heating resulted (in some cases) in final melting
of hydrohalite crystals around 24°C, after all ice was
melted around 35°C, which points to salinities of 3.6–
4.4wt% NaCl and 24.9–27.0wt% CaCl 2 (Oakes et al.
1990; Zwart and Touret 1994). In some inclusions, a
halite crystal was present, which dissolved between 7°C
and 24°C, pointing to salinities of 39.0–40.5wt% CaCl 2
(Oakes et al. 1990; Zwart and Touret 1994) (Table 3).
Inclusions in anhydrite in borehole 2 and 4 could not be
measured, because anhydrite crystals were not present or
too small, respectively.
Discussion
Timing and paragenesis of the veins
The extensional structures in this study are characterized
by different generations of vein formation. The first
generation of veins consists of calcite and quartz–calcite
veins. Their fibrous and stretched-crystal microstructure
points to syntectonic vein formation. The intergranular
fracture morphology, the rough shape of the vein–wall
interface, and the wall-rock inclusions of the calcite
veins indicate an emplacement in a slightly consolidated
rock. Microstructural observations suggest that the
anhydrite veins formed later and in a more consolidated
rock than the calcite veins. This age relationship is
derived from the fracture morphology (which in turn
depends on rock strength) and is also supported by
cross-cutting relationships of calcite and anhydrite veins.
The d 18 Oandd 13 C signature in the calcite veins indicates
that they precipitated between 55°C and 122°C from
meteoric fluids mixed with connate fluids. Homogenization
temperatures in fluid inclusions show that the
anhydrite veins precipitated at around 150°C. Rieken
and Gaupp (1991) measured homogenization temperatures
around 140–230°C in quartz and calcite veins in
the same area, and we therefore propose that the
different vein generations gradually change from calcite–
quartz to anhydrite veins.
The extension fractures are consistently oriented at a
high angle to the bedding, which suggests a near vertical
maximum principal stress in the rock. This indicates that
the veins formed during basin subsidence. Petmecky
et al. (1999) modelled subsidence curves in the LSB and
show that the basin subsided rapidly from the Late

1048
Table 2 Sulphur isotopic data
for all boreholes containing
anhydrite veins. (a) Borehole 1,
(b) borehole 3 and (c) borehole
4
Borehole Sample name Sample depth (m) Stratigraphic interval d 34 S(& CDT)
Borehole 1 B1-01 2576.15 Solling formation 12.3
B1-02 2578 Solling formation 13.4
B1-03a 2580.4 Solling formation 12.7
B1-03b 2580.42 Solling formation 12
B1-03c 2580.44 Solling formation 12.8
B1-03d 2580.46 Solling formation 11.9
B1-03e 2580.48 Solling formation 13.4
Borehole 3 B3-14 2722.85 Solling Formation 11.7
B3-17 2736.1 Solling Formation 12.6
B3-19 2768.8 Solling Formation 11.9
Borehole 4 B4-03 2360 Solling formation 12.4
B4-04 2360.5 Solling formation 12.6
B4-11 2362 Solling formation 12.8
B4-12 2362 Solling formation 12.1
Jurassic to the Early Cretaceous. The Buntsandstein
reached a maximum burial of up to 7-km depth during
the Mid Cretaceous and was uplifted to 2–3-km depth
by inversion during the Late Cretaceous. Although
maximum burial and inversion differ in various parts of
the LSB, similar tectonic events and fluids affected all
parts of the basin. We thus take one curve, representing
borehole 3, for the fluid history of the basin (Fig. 13a).
The temperature data imply that the calcite veins formed
at depths of 1–3 km. We did not observe compaction
halos (e.g. Price and Cosgrove 1990, p 421; Urai et al.
2001) around these early veins and this implies that the
porosity was already reduced significantly (see Appendix).
The minimum depth of vein formation should
therefore correspond with a porosity of around 20%,
which fits very well with a minimum depth of 1–2 km
(Fig. 13b). The anhydrite veins formed at minimum
depths of 3 km during the upper Jurassic when the basin
subsided rapidly (Fig. 13a). The wide range of homogenization
temperatures in the fluid inclusions is explained
by stretching, which suggests that veins were
brought to higher temperatures after their formation.
Thus, veins were buried to even greater depths during
further basin subsidence.
a
5
Borehole 1
4
35
30
25
20
15
10
5
Perm. Triassic Jurassic
Rot Zec Bu Mu Keuper Lias Dogger
11.9-13.4 ‰
0
260 210 160
Age [Ma]
Legend
moving average of SSS (structurally substituted sulfate)
sulfate in evaporites
vein sulfate in the Buntsandstein
Fig. 11 Sulphur isotopic signature during Permian to Jurassic time
(after Kampschulte and Strauss 2004) compared with results of
veins in the Buntsandstein. The data correlate with Zechstein
sulphur isotopes, which suggests that the anhydrite layers in the
Zechstein are the source rock for the anhydrite veins in the
Buntsandstein (Rot Rotliegend, Zec Zechstein, Bu Buntsandstein,
Mu Muschelkalk)
Frequency
b
Frequency
3
2
1
0
100 120 140 160 180 200 220 240
Homogenization temperature [˚C]
15
10
5
Borehole 3
0
100 120 140 160 180 200 220 240
Homogenization temperature [˚C]
Fig. 12 Homogenization temperatures of fluid inclusions in anhydrite
represented in a histogram for the two analysed boreholes: a
Borehole 1 (n=21, three samples) and b Borehole 3 (n=57, four
samples). The wide range in homogenization temperature (in the
same sample) suggests that the primary inclusions in the vein
crystals are re-equilibrated during further basin evolution. This
means that only the smallest values of T h represent the minimum
temperature of vein formation

1049
Table 3 Fluid inclusion data in anhydrite for the two boreholes
Borehole Th (°C) Tfm (°C) TmI (°C) TmHH (°C) TmH (°C) wt% NaCl wt% CaCl 2
Borehole 1 110–250 52 Between 28.6 and 13.9 37 – 2.3–3.3 14.9–21.5
Borehole 3 105–200 52 Between 25 and 22.9 35 – 3.6–3.78 18.7–19.6
105–200 52 Between 41.1 and 36.7 Between 25.7 and 22.7 – 3.6–4.4 24.9–27.0
105–200 52 Between 41.3 and 29.0 Between 25.7 and 22.9 7–5 24.2 39.0–40.5
Th homogenization temperature, Tfm temperature of first melting, TmI temperature of ice melting, TmHH temperature of hydrohalite
melting, TmH=temperature of halite dissolution. Salinities are calculated with the program Aqso2e (Bakker 2003)
Mechanical aspects
All veins are extensional mode I fractures, which form
when the differential stress r 1 r 3

1050
failure stress conditions at a particular depth z and
varying fluid pressures. With this plot, we can derive the
minimum fluid overpressure required to form the
anhydrite veins, because (1) we only observed extension
veins and (2) we estimated the depth of vein formation
from fluid inclusion data and a published subsidence
curve (Fig. 13). At 10 MPa tensile strength of the rock
(typical for Buntsandstein sandstones) and a minimum
depth of 3.2 km for the onset of anhydrite veining,
extension fractures only form at differential stresses
r 1 r 3

1051
cipitate a vein with this mass from a fluid which was 5%
supersaturated at 150°C, 3.4·10 5 kg H 2 O is necessary,
corresponding to a volume of 348 m 3 , assuming a density
of 977.7 kg/m 3 (Blount and Dickson 1969; Freyer
and Voigt 2004). When the supersaturation is 20%, only
87 m 3 of fluid is necessary. Therefore, the required fluid
volume to precipitate the observed veins is, even for 20%
supersaturation, very high.
The d 34 S isotopes in the anhydrite veins have the
same signature as the Zechstein anhydrite. Assuming
that the vein-precipitating fluids came from the Zechstein,
we can calculate how much fluid can be produced
in the Zechstein. In the anhydrite of the
Zechstein 4 (‘‘Grenzanhydrit’’), the gypsum-anhydrite
transition occurs around 70°C and releases about 60%
water per unit volume gypsum (Borchert and Muir
1964, p 133; Hardie 1967). These amounts of fluids
could be a source for the precipitated anhydrite veins
in the overlying sediments. However, based on the
temperature in the subsidence curve, the fluid release
from the ‘‘Grenzanhydrit’’ occurred before anhydrite
vein formation in the Buntsandstein (during Early to
Mid Jurassic). The fluids in gypsum layers deeper in
the Zechstein sequence were produced earlier than
those in the uppermost Zechstein and must have
drained upwards through evaporite layers with very
low permeabilities. Here, it is interesting to speculate
about the possible retention of these fluids in the
Zechstein over geological time scales, starting to flow
upwards at a later stage at deeper burial. This scenario
would require extremely low permeabilities of the halite
layers over geologically long periods. Although
halite has very low permeabilities under non-dilatant
conditions (dilatancy only occurs at close to lithostatic
fluid pressures; Urai et al. 1986; Fokker ; Peach and
Spiers 1996; Popp et al. 2001), little is known about
the exact value of permeability and its anisotropy. Of
particular importance here is the topology of the small
amount of grain boundary fluids present in the halite
(Lewis and Holness 1996; Schenk and Urai 2004) and
the orientation of the principal stress under geological
conditions, which affects the way fluids are distributed
in grain boundaries during dynamic recrystallization.
Stress in halite is near-isotropic (Schleder and Urai
this volume) but little is known about the orientation
of the principal stress which may be an important
factor in controlling permeability anisotropy. With
these arguments in mind (although much more work is
needed here), it is possible that under suitable conditions,
the brine released from gypsum can be retained
in the halite for long periods, and released to higher
levels in the basin during small changes in the state of
stress in the halite. The underlying Rotliegend and
Carboniferous sediments also contain large volumes of
pore fluids (e.g. Gaupp et al. 1993). If these fluids are
able to migrate through the Zechstein evaporites, they
may have transported Zechstein components into the
Buntsandstein (van Bergen and de Leeuw 2001).
Irrespective of the mechanism of overpressuring, the
claystone layers of the Upper Buntsandstein and the Ro¨ t
evaporites provide a seal for the overpressure cell in the
Buntsandstein. The increase in fluid pressure was associated
with extensional fracturing in the Middle Buntsandstein
(Fig. 15). The solubility of anhydrite decreases
Fig. 15 Model of the pressure versus the depth during vein
formation. a Pore pressure versus depth. The Middle Buntsandstein
forms a transitional zone between close to lithostatic and
hydrostatic pressure. One possible scenario for the pressure
decrease is by stepwise pressure drops in the different Buntsandstein
sequences with the Ro¨ t evaporite and the top claystone layers
of each sequence acting as a seal. The shaded area shows the range
of overpressure, which is at least 18 MPa above the hydrostatic
fluid pressure in the Buntsandstein. In the Ro¨ t evaporite, the fluid
pressure is between hydrostatic and 18 MPa above hydrostatic. b
Horizontal pressure versus depth. The differential stress to form
extension fractures can reach a maximum of 40 MPa and the
shaded area displays this range. Extension fractures will form when
the fluid pressure is 10 MPa above the horizontal stress. In the
evaporite layers, the differential stress is low and therefore the
horizontal stress is close to lithostatic. (anh anhydrite layer, S
Solling formation, H Hardegsen formation, D Detfurth formation
and V Volpriehausen formation, r V vertical stress, r H =horizontal
stress)

1052
with decreasing pressure and increases with decreasing
temperature. Therefore, we assume that the supersaturation
of the anhydrite fluids is caused by a pressure
drop during fracturing.
The arguments above can be illustrated by a simple
1D-model in which the complete Buntsandstein acted as
an overpressure cell during progressive burial at 3–5-km
depth (Fig. 15a). This model may be constrained for the
pore fluid pressure by the following parameters: (1) nearlithostatic
fluid pressures in the Zechstein because of the
low permeable evaporites, (2) hydrostatic fluid pressures
(assumed) above Ro¨ t because of the highly permeable
Keuper and Jurassic, (3) minimum fluid pressure of
18 MPa above hydrostatic at a depth of 3.2 km to cause
extensional fracturing, (4) pressure drops where
anhydrite precipitated (18 MPa pressure drop =20%
supersaturation), (5) near-hydrostatic fluid pressure
gradients along the open fractures until sealed by
anhydrite.
The horizontal stress depends linearly on the effective
vertical stress and on the lithology. In the permeable
layers above the Buntsandstein, the horizontal
stress can be calculated by r h ¢=K 0·r v ¢ taking the
coefficient of the earth pressure at rest K 0 as 0.4 (Mandl
2000, p 177) (Fig. 15b). In the Buntsandstein, we cannot
determine the horizontal stress exactly, because
there are uncertainties about the fluid pressure and we
only know the fluid pressure range. However, as the
minimum differential stress is 40 MPa, the range of the
horizontal stress is known. Extension fractures are
expected to form in the Buntsandstein where the fluid
pressure is 10 MPa above the horizontal stress. In the
Ro¨ t and Zechstein evaporites, the horizontal stress is
expected to be close to the lithostatic stress, because the
differential stress is low in these layers (Schleder and
Urai, this volume).
In cores from the four studied boreholes, despite their
distance of 200 km and noticeable differences in tectonics,
similar extensional veins and vein microstructures
are observed. There is clear evidence for at least
two generations of vein formation, associated with different
parts of the burial path. The first generation of
veins formed syntectonically in subvertical fractures and
is filled with fibrous calcite. Stable isotopes indicate that
these veins precipitated from connate water (d 18 O=0–
4& SMOW) at temperatures between 55°C and 122°C.
A generation of quartz–calcite veins is associated with
these calcite veins. No significant increase of fracture
permeability was created during the generation of these
veins.
The second generation of veins is also syntectonic,
formed in steeply oriented open fractures, which are
usually reactivated calcite veins. These veins are filled
with blocky anhydrite. In this second generation of
veins, fluid inclusions in anhydrite indicate precipitation
around 150°C, from hypersaline brines. Sulphur
isotopes from anhydrites indicate that the origin of the
sulphate is the underlying Zechstein. The first generation
calcite veins formed in a much less consolidated
rock than the second generation anhydrite veins, although
the cohesion of the wall rock remained relatively
low (no transgranular fractures were observed
during the whole evolution). We interpret the first
generation calcite veins to have formed during the early
stages of burial, between 1 km and 3 km depth, and
the second generation to have formed during progressive
burial at a depth of minimum 3 km. However, all
vein generations formed in a continuum. The second
generation of veins formed in a phase when the Buntsandstein
was overpressured, the fluid pressure at least
18 MPa at 3.2 km. Pore pressures in Zechstein were
near-lithostatic, dropping progressively to hydrostatic
above the Ro¨ t, providing the pressure decrease to
precipitate anhydrite veins. All mechanisms that generate
overpressures in sedimentary basins could have
contributed to the overpressure in the Buntsandstein
but inflation by Zechstein fluids, released from dehydrating
gypsum (enclosed in halite), is the only required
process.
Acknowledgements We thank Dr. Jentsch (EMPG) for access to
Buntsandstein cores. Werner Kraus is acknowledged for the
preparation of thin sections and the fluid inclusion wafers. Philippe
Muchez (K.U.Leuven) is thanked for the use of the Linkham stage
and discussions on the fluid inclusions. We are very thankful to
Prof. Strauss (Mu¨ nster) for the analyses of the S isotopes and to
Dr. Joachimski (Erlangen) for the d 18 O and d 13 C isotopes. Comments
by Anne-Marie Bouiller and an anonymous reviewer significantly
improved the manuscript. This project is funded by the
DFG (Hi 816/1–2) and is part of the SPP 1135 ‘‘Dynamics of
Sedimentary Systems under varying Stress Conditions by Example
of the Central European Basin System’’.
Conclusions
Appendix
Strain due to compaction of clays
We consider a fluid-saturated sediment, compacting
uniaxially. Taking the usual notation:
V t ¼ V s þ V f ;
V t ¼ total volume ½m 3 Š; V s ¼ solid volume ½m 3 Š; V f
¼ fluid volume ½m 3 Š
Porosity / ¼ V f
V t
It can be shown that when the sediment changes its
porosity from / 1 to / 2 , the volumetric stretch (s v ) and
the volumetric extension (e v ) are:
s v ¼ V t1
¼ V f1 þ V s 1 / 2
and e v ¼ V t2 V t1
¼ s v 1
V t2 V f2 þ V s 1 / 1 V t1

1053
In uniaxial compaction, this leads to a finite strain
tensor:
1 0 0
D ¼
0 1 0
0 0
1 / 2
1 / 1
If the veins are formed shortly after deposition (assume
/ 1 = 50%), and compaction continues to / 2 =5%
during the maximum burial at 5,000 m, V t2 is then 52%
of V t1 and this should be visible in compaction halos
around the veins (which would then act as rigid struts).
As these structures are not observed, the initial porosity
was smaller.
Taking / 1 = 20%, / 2 = 5%, V t2 is reduced to 84% of
V t1 . This minor volume reduction is probably not observable
and this fits very well with the observations of
the veins in the Buntsandstein rocks.
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