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Precipitation of fracture fillings and cements in theBuntsandstein (NW Germany)

S. NOLLET1, T. KOERNER2, U. KRAMM2 AND C. HILGERS1

1RWTH Aachen, Geologie-Endogene Dynamik, Aachen, Germany; 2RWTH Aachen, Institut fur Mineralogie und

Lagerstattenlehre, Aachen, Germany

ABSTRACT

The relationship between fracturing and fracture filling in opening-mode fractures in the Triassic Buntsandstein in

the Lower Saxony Basin (LSB; NW Germany) has been studied by an integration of petrographic and structural

analysis of core samples, strontium isotope analysis and microthermometry on fluid inclusions. This revealed the

relationship between the timing of the fracturing and the precipitation of different mineral phases in the fractures

by constraining the precipitation conditions and considering the possible fluid transport mechanisms. The core

was studied from four different boreholes, located in different structural settings across the LSB. In the core sam-

ples from the four boreholes, fractures filled with calcite, quartz and anhydrite were found, in addition to pore-

filling calcite cementation. In boreholes 2 and 3, calcite-filled fractures have a fibrous microstructure whereas in

borehole 1, fractures are filled with elongate-blocky calcite crystals. Anhydrite-filled fractures have, in all samples,

a blocky to elongate-blocky microstructure. Fractures that are filled with quartz are observed in borehole 2 only

where the quartz crystals are ‘stretched’ with an elongated habit. Fluid inclusion microthermometry of fracturing-

filling quartz crystals showed that quartz precipitation took place at temperatures of at least 140�C, from a fluid

with NaCl–CaCl2–H2O composition. Melting phases are meta-stable and suggest growth from high salinity for-

mation water. Strontium isotopes, measured in leached host rock, indicate that, in boreholes 2 and 3, the fluid

which precipitated the calcite cements and calcite-filled fractures is most likely locally derived whereas in borehole

1, the 87Sr ⁄ 86Sr ratios from the pore-filling cements and in the elongate-blocky calcite-filled fracture can only be

explained by mixing with externally derived fluids. The elongate-blocky anhydrite-filled fractures, present in bore-

holes 1, 3 and 4, precipitated from a mixture of locally derived pore fluids and a significant quantity of fluid with

a lower, less radiogenic, 87Sr ⁄ 86Sr ratio. Taking into account the structural evolution of the basin and accompany-

ing salt tectonics, it is likely that the underlying Zechstein is a source for the less radiogenic fluids. Based on the

samples in the LSB, it is probable that fibrous fracture fillings in sedimentary rocks most likely developed from

locally derived pore fluids whereas elongate-blocky fracture fillings with smooth walls developed from externally

derived pore fluids.

Key words: Buntsandstein, fluid inclusions, fractures, strontium isotopes, structural diagenesis

Received 18 May 2009; accepted 14 August 2009

Corresponding author: S. Nollet, ExxonMobil Upstream Research Company, Houston, TX, USA.

Email: sofie.nollet@exxonmobil. Tel: +1 713 431 7092. Fax: +1 713 431 6212.

Geofluids (2009) 9, 373–385

INTRODUCTION

During the evolution of a sedimentary basin, sedimentary

rocks often fracture due to the relation between stress,

rock strength and fluid pressure (e.g. Pollard & Aydin

1988). Depending on a combination of the effective stress

conditions and the availability of supersaturated fluids,

these fractures will stay open, collapse or will be sealed

with precipitating minerals (Laubach et al. 2004; Nollet

et al. 2005a; Laubach & Ward 2006). When such fractures

become sealed, the resulting structures are often called

‘veins’, although this term was originally used to describe

mineral precipitates in highly deformed metamorphic rocks

(Durney & Ramsay 1973; Ramsay & Huber 1983; Hilgers

& Sindern 2005). Because the fluid transport processes

and the mechanisms of opening and filling of fractures in

sedimentary rocks, especially when salt domes are present,

are probably somewhat different to those in metamorphic

Geofluids (2009) 9, 373–385 doi: 10.1111/j.1468-8123.2009.00261.x

� 2009 ExxonMobil Upstream Research Company

rocks (Bjorlykke 1993; McManus & Hanor 1993; Jamtveit

& Yardley 1997), here the term ‘sealed opening-mode

fractures’ is preferred to the term ‘veins’.

In many cases there is uncertainty about the timing of

active fracturing relative to their subsequent sealing via

supersaturated fluids (a process sometimes referred to as

‘structural diagenesis’) although it is important to know

this when evaluating fracture permeability in sedimentary

rocks (Lander & Walderhaug 1999; Laubach 2003).

Approaches involving stable isotopes, fluid inclusions and

strontium isotopes can be used to constrain the source of

the precipitated fluids in pore space (both cements and

fracture fill) and pressure and temperature conditions at

precipitation (Rieken & Gaupp 1991; Purvis & Okkerman,

1996; Muchez et al. 1998; Kenis et al. 2000; Hilgers &

Sindern 2005). Deriving the timing of the fracturing of

the sedimentary rocks is more challenging but using micro-

structures associated with the fracture fill can help to con-

strain the relationship between opening and filling (Ramsay

& Huber 1983; Urai et al. 1991; Bons & Jessell 1997;

Bons 2000; Hilgers et al. 2001). More specifically, frac-

tures with fibrous crystals reflect a precipitation rate that is

equal to, or greater than, opening rate. By contrast, elon-

gate-blocky crystals indicate either that the crystal growth

rate was slower than the opening rate or that sealing of the

fracture occurred after opening (Nollet et al. 2005b).

Microstructures with fibrous crystals that have host-rock

inclusions incorporated within their crystals are often called

‘crack-seal’ structures or ‘stretched’ crystals and the spacing

of the host-rock inclusions is assumed to reflect the crack

increments (Passchier & Trouw 1996, p. 135; Hilgers &

Urai 2002; Laubach et al. 2004).

In this study, we present an integrated approach to con-

strain the relationship between fracturing and sealing of

(predominantly) opening-mode fractures in Triassic sand-

stones of the Buntsandstein in the Lower Saxony Basin

(LSB; Germany), sampled in core from four boreholes.

Results of a previous analysis of the microstructures, fluid

inclusions in anhydrite-filled fractures and stable isotopes

in the same sample set have been described in Nollet et al.

(2005a). In this paper the results of the fluid inclusion

study of quartz-filled fractures and strontium isotope data

from calcite and anhydrite-filled fractures are presented and

integrated with the results of the previous study.

GEOLOGICAL SETTING

The LSB in NW Germany is part of the E–W-trending

southern Permian Basin, or Central European Basin Sys-

tem (CEB; Fig. 1). During the Permian, this basin was

filled with Rotliegend clastic sediments and cyclic deposits

of Zechstein evaporites (carbonates, sulphates and halite;

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 &

Clausen 2002; Szurlies et al. 2003). The Lower Triassic

Buntsandstein in northern Germany consists of three

units (Fig. 2): (i) shaley sediments in the Lower Bunt-

sandstein; (ii) four depositional sub-cycles in the Middle

Buntsandstein starting with a regressive sand package and

closing with transgressive shales; and (iii) marine shales,

sulphate and halite series in the Upper Buntsandstein

(Rot) (Herrmann et al. 1968; Ziegler 1990; Kovalevych

et al. 2002; Michelsen & Clausen 2002). Sedimentation

and subsidence continued at high rates until the Creta-

ceous.

During the time of rapid subsidence in the late Jurassic,

WNW–ESE-oriented normal faults were formed (Brink

et al. 1992; Kossow & Krawczyk 2002; Scheck et al.

2003) (Fig. 1). Around the Turonian, inversion started

and peaked during the Santonian and Campanian in the

LSB. The total maximum inversion-induced uplift is

estimated to be 2 km, although locally up to 8 km uplift

Fig. 1. Overview map with location of the case

study area and the main structures in the South-

ern Permian Basin (Gluckstadt Graben, Harz

Mountains, London Brabant Massif, Lower Sax-

ony Basin, Pompeckj Block, Rhenish Massif, West

Netherlands Basin). Inset shows an enlarged map

of the Lower Saxony Basin with the location of

the four sampled boreholes (BH1, BH2, BH3 and

BH4) and the main structural elements (normal

faults and reverse faults) (after Ziegler 1990 and

Baldschuhn et al. 2001).

374 S. NOLLET et al.

� 2009 ExxonMobil Upstream Research Company, Geofluids, 9, 373–385

has been reported. The inversion caused significant ero-

sion. In the southern part of the LSB, rocks were subjected

to higher burial and stronger inversion than in the north-

ern part (Petmecky et al. 1999).

Initial (passive) diapirism of Zechstein salt was initiated

in the late Triassic (Trusheim 1957; Brink 1984; Bald-

schuhn et al. 1998; Bayer et al. 1999; Mohr et al. 2005).

In the Upper Jurassic, salt mobilization continued and

broke through the weakest units of the Mesozoic cover

(Jaritz 1980). Salt-rim synclines indicate that salt diapirism

persisted from the early Cretaceous until early Cenozoic

(Scheck et al. 2003; Mohr et al. 2005).

In this study, opening-mode fractures were sampled in

the Middle Buntsandstein in four boreholes in the LSB

(Figs 1 and 3), in slightly different tectonic settings

(Fig. 3). Borehole 1 (BH1) is located in an inversion struc-

ture where faults have been reactivated during the inver-

sion in the late Cretaceous (Kockel 2003). Borehole 2

(BH2) is located in an anticlinal structure, related to salt

doming (Rieken & Gaupp 1991). The structure of bore-

hole 3 (BH3) 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 (BH4) is located above a salt pil-

low and at 4 km from a salt dome.

METHODS

A total of 56 rock samples containing filled opening-mode

fractures were selected from the cores for further analysis.

Standard thin sections (25) with thickness of about 20 lm

were prepared for the microstructural analysis of the filled

fractures. Based on the analysis of the standard thin sec-

tions, seven double-polished, thick sections (thickness

150 lm) were prepared for fluid inclusion analyses in the

fracture-fill crystals with the method described in Muchez

et al. (1994). Microthermometry was carried out on a

Linkham stage (K.U. Leuven, Belgium), which was cali-

brated at )56.6, )21.2, 0.0 and 374.1�C with synthetic

Syn FlincTM inclusions. The rate of heating was monitored

in order to obtain an accuracy of ±0.2�C during freezing

and ±1�C when heating over the 25)250�C temperature

range.87Sr ⁄ 86Sr measurements were made on a total of 11

samples of calcite and anhydrite crystals in the fractures,

calcite pore cements in Buntsandstein rocks (leachate) and

also on leached Buntsandstein whole-rock samples. ICP-

MS analyses indicate that the calcite crystals in the fractures

contain around 1600 p.p.m. strontium, the anhydrite crys-

tals in the fractures around 2430 p.p.m. and the calcite

cements in the Buntsandstein host rocks around

Fig. 2. Stratigraphy of the Permian and Triassic

in the Lower Saxony Basin (LSB) and of the Mid-

dle Triassic in more detail in the LSB (anh, anhy-

drite; h, halite) (after Borchert & Muir 1964;

Baldschuhn et al. 2001).

Precipitation of fracture fillings and cements 375


300 p.p.m., indicating that enough strontium is available

to be able to determine a representative 87Sr ⁄ 86Sr ratio

analysis. Fractions of the fracture-filling minerals were

obtained by drilling small cylinders with a dental drill and

by mechanical separation, followed by careful hand-picking

under a binocular microscope. Buntsandstein host-rock

samples were crushed by hand in an agate mortar. To leach

the calcite cement out of the host rock, 0.17 N HCl was

added to these samples at room temperature and centri-

fuged for 15 minutes in order to separate the leachates

from the residual fraction.

Calcite samples of the fractures were dissolved in 6 N

HCl, anhydrite samples were dissolved in several steps

using 6 and 2.5 N HCl. The residual fraction of the host-

rock samples was decomposed using a HF–HNO3 mixture

and spiked with an 87Rb–84Sr-mixed spike. The leachate of

the host-rock samples as well as the solutions of the calcite

and anhydrite samples and of the host-rock residual frac-

tion were finally dried and re-dissolved in 2.5 N HCl.

Strontium from all samples and Rb from the spiked

host-rock residual fractions were separated by standard cat-

ion exchange techniques using DOWEX AG-50 W8 resin

and 2.5 N HCl as eluant. For the mass spectrometric analy-

ses, Sr was loaded as chloride with Ta on W filaments, Rb

as chloride on Ta filaments.

The measurements were performed on a VG Sector 54

multi-collector TIMS for spiked Sr in a dynamic mode, for

Rb in the static mode. Unspiked Sr samples were measured

on a FINIGAN Triton TIMS in dynamic configuration.

Corrections for mass fractionation were based on87Sr ⁄ 86Sr = 0.1194. Repeated analyses of the NBS 987 Sr

standard yielded a 87Sr ⁄ 86Sr ratio of 0.71030 ± 0.00004

(2r) (n = 22), whereas 10 analyses of this standard

show 0.71025 ± 0.00003 on the Triton machine, which is

identical within the limits of error with the result of the

VG mass spectrometer. Nevertheless, the data of the spiked

samples were adjusted to the 87Sr ⁄ 86Sr ratio of the Triton

standard mean. Blanks for Rb and Sr are below 100 and

50 pg respectively. The measurements were performed at

the isotope laboratory of the Institute of Mineralogy of the

WWU Munster, Germany. An overview of all samples and

associated analyses is given in Table 1.

FRACTURE AND FRACTURE-FILLOBSERVATIONS

In all sampled boreholes, the beds are horizontal to slightly

dipping, indicating that the rocks have undergone very

little tectonic disturbance in their geological history.

According to the original core descriptions, porosity in the

coarser grained intervals is very low in all boreholes

(3–5%), due to extensive cementation by calcite. The

fine-grained, more shaley intervals are less cemented. Grain

sizes in Buntsandstein sand intervals vary from very fine to

fine. Most of the fractures are present in the sand intervals,

although occasionally small, narrower, fractures are present

in the shales.

Representative samples of the fracture-fill structures with

calcite, anhydrite and quartz are shown in Fig. 4. Most of

the observed fractures dip perpendicular to bedding with

the dominant opening direction parallel to bedding

(Fig. 4B–D). In a few cases, a shear component has been

observed (Fig. 4A). The fracture opening width varies from

0.5 to 2 cm. The morphology of the fracture walls is

related not only to the grain size but also to fracture-fill

mineralogy. In the coarser grained sand intervals, fracture

walls are sharply defined and have a smooth morphology

and are filled with both calcite and anhydrite. By contrast,

(A) (C)

(B) (D)

Fig. 3. Simplified cross-sections illustrating the geological setting of the sampled areas with location of the sampled boreholes (after Baldschuhn et al. 2001).

(A) BH1, (B) BH2 (C) BH3 and (D) BH4.



Table 1 Overview of all samples and analyses described in this study (BH1, BH2, BH3 and BH4).

Borehole

name

Sample

name

Top sample

depth (m)

Base sample

depth (m) Stratigraphic unit

Fracture-fill

minerals

Fluid inclusion

analysis

Sr isotope analysis

sample name

BH1 BH1-01 2576.1 2576.2 Solling-Folge Anhydrite

BH1-02 2578 2579 Solling-Folge Anhydrite ·*

Anhydrite

BH1-03 2580.4 2581 Solling-Folge Calcite ·* SN01 (cements leached)

Calcite SN01b (host rock)

SN04 (anhydrite)

SN07 (calcite-filled fracture)

SN10 (calcite fracture wall)

BH1-04 2664.5 2664.6 Detfurth Calcite

BH1-05 2665 2665.2 Detfurth Anhydrite

BH1-06 2672.25 2672.4 Detfurth Calcite ·*

BH1-07 2674.55 2674.6 Detfurth Anhydrite

BH1-08 2795.3 2795.4 Volpriehausen-Folge Anhydrite

BH1-09 2796.2 2796.3 Volpriehausen-Folge Calcite


BH1-11 2612.2 2612.3 Solling-Folge Calcite


BH2 BH2-01 3327.5 3327.6 Solling-Folge Calcite + halite

BH2-02 3335.7 3336 Solling-Folge Calcite + halite

BH2-03 3383 3383.1 Solling-Folge Calcite + halite

BH2-04 3383.4 3383.5 Solling-Folge Calcite + halite

BH2-05 3408.6 3408.7 Detfurth Quartz ·BH2-06 3334.3 3334.4 Solling-Folge Halite + calcite

BH2-07 3334.35 3334.5 Solling-Folge Halite + calcite SN02 (cements leached)

SN02b (host rock)

SN08 (calcite-filled fracture)


BH3 BH3-01 2499.55 2499.6 Upper Bunter ⁄ Rot Anhydrite

BH3-02 2499.8 2499.9 Upper Bunter ⁄ Rot Anhydrite

BH3-03 2503 2503.2 Upper Bunter ⁄ Rot Anhydrite

BH3-04 2504 2504.2 Upper Bunter ⁄ Rot Anhydrite


BH3-06 2551.95 2552 Upper Bunter ⁄ Rot Anhydrite




BH3-10 2652.6 2652.7 Solling-Folge Anhydrite



BH3-13 2719.7 2719.8 Solling-Folge Anhydrite ·*

BH3-14 2722.85 2722.9 Solling-Folge Anhydrite ·*



BH3-17 2736.1 2736.2 Solling-Folge Anhydrite ·* SN03 (cements leached)

SN03b (host rock)

SN05 (anhydrite)


BH3-19 2768.8 2769 Detfurth Calcite + anhydrite ·* SN09 (calcite-filled fracture)

SN11 (calcite fracture wall)




BH4 BH4-01 2357.7 2357.8 Solling-Folge Calcite



Anhydrite


BH4-05 2377 2377 Solling-Folge Calcite



the thinner fractures in the more mudstone-rich intervals

are less sharply defined, have a more irregular morphology

and are filled with calcite.

All fractures are filled to various degrees with combina-

tions of calcite, anhydrite, quartz and halite. The calcite-

filled fractures (cc1) in BH2 and BH3 have a fibrous

microstructure and are antitaxial, indicating that crystals

started growing in the middle of the fracture towards both

sides (Fig. 5A). Fracture walls are very irregular. In BH1,

syntaxial calcite-filled fractures (cc1) with an elongate-

blocky microstructure are predominantly observed, indicat-

ing that crystals grew on the fracture walls towards the

centre and that the fracture opening rate was greater than

the crystal growth rate (Fig. 5B). In BH1, BH3 and BH4,

syntaxial, anhydrite-filled fractures are observed with a

blocky- to elongate-blocky microstructure and locally with

rosette-type crystals (Fig. 5C,D). Cross-cutting relations

between the different fracture fillings indicate that the

anhydrite-filled fractures formed later than the calcite-filled

fractures. In some samples (e.g. BH1 and BH3), the anhy-

drite crystals are partly replaced by a second generation of

calcite (cc2; Fig. 5C). In BH2, a fracture filled with quartz

crystals growing from one side of the fracture wall towards

the other side of the fracture wall was observed (Fig. 6A).

The quartz crystals contain solid inclusions of host-rock

material in some places, which can be interpreted as trap-

ping by quartz growing from fracture walls during crack-

seal increments (Fig. 6A). This structure is similar to the

‘mineral bridges’ described by Laubach et al. (2004).

Locally, calcite is present in between the quartz crystals,

suggesting co-precipitation of calcite and quartz (Fig. 6B).

Isolated anhydrite crystals are found locally within fractures

(Fig. 6B). Such fractures are in some places not completely

filled and therefore have some porosity preserved (see

Fig. 4C). Possible explanations for differential fracture fill

along the fracture length are (i) differences in opening rate

along the fracture or (ii) differences in the factors control-

ling the supersaturation of the fluid.

FLUID INCLUSIONS

In the fracture with stretched quartz crystals in BH2, fluid

inclusions with diameters around 20 lm are common

within growth zones in the quartz crystals. The inclusions

are characterized by subhedral and elongated morphology

and are interpreted as being primary in origin. A total of

19 inclusions have been measured in one quartz crystal.

Before any temperature changes, all inclusions contain two

phases: liquid and vapour. After freezing, the first melt was

observed at a temperature (Tfm) of around )67.8�C, which

is much lower than the H2O–NaCl system eutectic temper-

ature ()21.5�C) or the H2O–NaCl–CaCl2 ()52�C)n

(Table 2). Such a low eutectic melting temperature can be

explained by (i) a metastable eutectic point in the H2O–

NaCl–CaCl2 system of around )70�C (Davis et al. 1990;

Williams-Jones & Samson 1990; Samson & Walker 2000)

and ⁄ or (ii) the presence of additional components such as

LiCl (with very low eutectic temperature, around )84�C)

in a H2O–NaCl–CaCl2 type of fluid (Zwart & Touret

1994). Low first melt temperatures were typically observed

in inclusions with moderate to high CaCl2 concentrations

(Davis et al. 1990). During further heating, cotectic melt-

ing of hydrohalite and ice melt was observed in the inclu-

sions. A last melting temperature between )53.7 and

)37.3�C (mode )39.2�C) was measured and there is

uncertainty about identification of this last melting phase

(hydrohalite versus ice) (Table 2). At a temperature

between 14.4 and 30.7�C (mode 18.6�C), the sudden

nucleation of a cubic halite daughter crystal was observed

in all inclusions. During further heating, homogenization

of the vapour bubble in the liquid phase occurred in most

cases around 143.8�C with some exceptions between

143.8 and 222.7�C (Fig. 7), followed by dissolution of the

halite crystal around 170.7�C (range 50.3–200.5�C).

However, in four inclusions, dissolution of the halite

daughter crystal occurred before or contemporaneous with

homogenization of the vapour bubble. The large range of

Table 1 (Continued).

Borehole

name

Sample

name

Top sample

depth (m)

Base sample

depth (m) Stratigraphic unit

Fracture-fill

minerals

Fluid inclusion

analysis

Sr isotope analysis

sample name




BH4-09 2424.5 2442.5 Volpriehausen-Folge Anhydrite


BH4-11 2357.6 2375.6 Solling-Folge Anhydrite ⁄ gypsum SN06 (anhydrite)





*Analyses described in Nollet et al. (2005a).



homogenization temperatures could point to (i) growth

of quartz throughout this large temperature range,

(ii) re-equilibration due to stretching of the quartz crystals

during further evolution of the basin after quartz precipita-

tion or (iii) leakage.

To calculate the salinities in the inclusions in a NaCl–

CaCl2–H2O system, the melting temperatures of at least

two phases are required together with identification of

those phases (Shepherd et al. 1985; Bakker 2003). We

were able to measure melting temperature around )40�Cbut there is uncertainty on identification of the phase

(hydrohalite versus ice). Also, the extremely high melting

temperature of halite indicates that the melting stage in

these fluid inclusions was probably metastable. Repeating

the freezing-melting cycle could not resolve the uncer-

tainty and therefore, we choose not to calculate salinities

of the fluids but to conclude that the observations of the

melting process indicate high salinity fluids.

STRONTIUM ISOTOPES

The results of the strontium isotope analysis are shown in

Table 1 and in Fig. 8. The 87Rb ⁄ 86Sr ratio was monitored

only for the leached host-rock samples because Rb concen-

trations in the calcite and anhydrite diagenetic phases are

very low resulting in negligible Rb ⁄ Sr ratios.

Comparison of calcite in fracture-fill and calcite cements

As mentioned earlier, significant cementation with calcite

was observed in the sand intervals of the Buntsandstein.

To find out if there is a relation between the calcite

cement and calcite fill in the fractures, host rocks were

leached and the assumption was made that the leachate

represents the calcite cement. In none of the samples,

strontium shows identical isotope signatures for calcite in

the fractures and the respective leachates of the neigh-

bouring host rocks (Fig. 8, Table 3). In BH1, 87Sr ⁄ 86Sr

of the elongate-blocky calcite fracture filling (0.71020) is

lower compared with the leachate of the respective host

rock (0.71054). In BH2 and BH3, both samples from

fibrous calcite crystals, the 87Sr ⁄ 86Sr ratio of the calcite

fracture fill is higher than the respective calcite cements in

the host rock.

Comparison of anhydrite and calcite in the fractures

In the anhydrite-filled fractures, the measured 87Sr ⁄ 86Sr

signatures show a variation between 0.70988 (BH1) and

0.71094 (BH3; Table 3). Both calcite and anhydrite were

observed in the fractures in BH1 and BH3, which allows

comparison of the strontium isotopic signature between

both fracture-filling minerals. In both boreholes, the87Sr ⁄ 86Sr ratio of the anhydrite is lower than in the calcite.

Replacement of anhydrite by calcite was observed in

BH1 and BH3. Thermochemical sulphate reduction

(TSR) could explain this replacement, as suggested by

Nollet et al. (2005a). To find out if this process occurred

in the anhydrite-filled fractures, the 87Sr ⁄ 86Sr signatures of

these late-stage calcites have been compared to the signa-

tures of the associated anhydrite samples. In both bore-

holes, the 87Sr ⁄ 86Sr values of the late-stage calcite are

slightly greater than the values of the anhydrite and are

closer to the signatures of the calcite fracture fillings.

Therefore, we conclude that the fluid which precipitated

the second stage of calcite was not directly related to

anhydrite dissolution.

(A) (B)

(C) (D)

Fig. 4. Core photographs showing some of the observed fracture struc-

tures. (A) Calcite-filled fracture, (B) anhydrite-filled fracture, (C) quartz-

filled fracture (D) anhydrite-filled fracture. All observed fractures are normal

to subnormal to the bedding.



(A) (B)

(D)(C)

Fig. 5. Microstructures of the different observed fracture fillings. (A) Fibrous calcite-filled fracture in calcite cemented clay-rich host rock (sample BH2-07).

(B) Elongate-blocky calcite-filled fracture in calcite cemented sandstone (cc1) (sample BH1-03). (C) Replacement of anhydrite by calcite (cc2) (sample BH1-

03) and needle-like anhydrite crystals grown in rosettes. (D) Elongate-blocky anhydrite-filled fracture with calcite (cc1) at the fracture wall (sample BH1-03).

(A) (B)

Fig. 6. (A) Microstructures showing a stretched quartz-filled fracture with few solid inclusions incorporated in the quartz crystals indicating syntectonic

growth (sample BH2-05). (B) Stretched quartz and calcite crystals. Faceted quartz with euhedral crystal terminations are also observed in the same fracture

(sample BH2-05). Anh, anhydrite; cc, calcite; qtz, quartz.



DISCUSSION

Fluid source of fracture-filling minerals

Buntsandstein sediments were deposited in a dominantly

non-marine setting and therefore there is no a priori rea-

son that the strontium isotopic signature of the diagenetic

phases should correspond to the contemporary sea water.

Nevertheless, the strontium isotopic signature of the pre-

cipitated diagenetic phases could correspond to the sea

water 87Sr ⁄ 86Sr signature from later times. A comparison

with literature data (Burke et al. 1982; Kramm & Wed-

epohl 1991) shows that the strontium isotopic signature of

all analysed samples in this study is significantly higher than

any Phanerozoic sea water signature (Fig. 8). Elevated

strontium ratios are common in shales and impure sand-

stones, which contain detrital micas, clay minerals and

K-feldspar. With ageing these minerals accumulate 87Sr,

because of the radioactive decay of 87Rb, and during

K-feldspar dissolution, clay diagenesis or pressure solution,

they may thus produce pore fluids with a high 87Sr ⁄ 86Sr

ratio. Both K-feldspar dissolution and pressure solution are

described in the Buntsandstein, offshore the Netherlands,

indicating that pore fluids with a high 87Sr ⁄ 86Sr ratio could

be released after deposition of the sediments (Purvis &

Okkerman 1996; Weber & Lepper 2002).

Based on the measured 87Sr ⁄ 86Sr ratios, the measured87Rb ⁄ 86Sr ratios, the time since deposition and the known

decay constant, the 87Sr ⁄ 86Sr ratio at the time of deposi-

tion of the sediments can now be calculated based on the

following formula:

87Sr86Sr

¼87Sr86Sr

� �0

þ87Rb86Sr

ðe�t � 1Þ

with ð87Sr=86SrÞ0 equal to the measured strontium signa-

ture, 87Sr=86Sr the strontium signature at the time t of

deposition, k the decay constant (Faure 2001, p. 6; Fig. 9,

Table 3). The slopes of the curves in Fig. 9 are different

for the three measured boreholes because the 87Rb ⁄ 86Sr

ratios in the present-day measured samples are also slightly

different.

In BH1, the calculated isotopic signature of the whole

rock samples at the time of Middle Buntsandstein deposi-

tion is higher than the signature measured in the leachate

from the host-rock (representing the calcite cements) and

calcite-filled fractures (Fig. 9, Table 3). This indicates that

the isotopic signature of the calcite cements cannot be

explained by precipitation from indigenous (local diage-

netic) pore fluids alone and that, even when the cementa-

tion occurred immediately after deposition, mixing with an

additional, isotopically lighter fluid occurred before calcite

cementation. The later cementation took place, the more

fluids with low 87Sr ⁄ 86Sr ratio are required.

By contrast, in BH2 and BH3, the back-calculated isoto-

pic signature of host-rock samples at the time of deposition

is lower than the signature measured in the calcite cements

(leachate of host rock) and in calcite-filled fractures

(Fig. 9, Table 4). Therefore, these Sr signatures could be

explained by cementation which occurred shortly after

deposition from locally derived pore fluids, without any

influence of externally derived sea water. The time at which

Table 2 Result of microthermometry on fluid inclusions in quartz in BH2-

05.

No. Tfm Tm(HH or ice) TxH Th TmH

1 )70.0 )41.1

2 )69.8 )40.1

3 )67.1 )39.1

4 )68.6 )39.2 18.2 146.7 146.7

5 )67.1 )37.8 23.0 175.5

6 )67.1 )39.2 30.7 143.8 170.7

7 )67.1 )41.2 14.4 143.8 170.7

8 )67.1 )39.2 24.9 143.8 174.5

9 )78.5 )53.7 22.6 168.2 200.5

10 )68.6 )41.0 22.9 153.4 179.1

11 )63.6 )38.6 18.6 196.2 182.1

12 )68.6 )39.4 21.4 182.1 182.1

13 )68.6 )39.1 23.6 50.3

14 )68.6 )38.8 23.2 222.7 125.6

15 )68.6 )39.5 19.0 172.2 179.1

16 )68.6 )37.3 23.6 158.3

17 )63.6 )40.3 25.6 144.4 171.2

18 )63.6 )40.3 18.6 144.4 176.1

19 )63.6 )38.8 28.5 149.4 154.3

Average )67.8 )40.2 22.4 162.1 162.6

Mode )68.6 )39.2 18.6 143.8 170.7

Min )78.5 )53.7 14.4 143.8 50.3

Max )63.6 )37.3 30.7 222.7 200.5

Th, homogenization temperature; Tfm, temperature of first melting; Tm(HHor ice), last melting temperature of hydrohalite; TxH, crystallization temper-

ature of halite; TmH, melting ⁄ dissolution temperature of halite crystal.

Fig. 7. Histogram with homogenization temperatures measured in fluid

inclusions in the quartz-filled fracture in sample BH2-05. The homogeniza-

tion temperatures show a large range (140–230�C) with the highest

frequency between 140 and 150�C.



the precipitated fluids were in equilibrium with the pore

fluids is 226 Ma in BH2 and 210 Ma in BH3 (Fig. 9).

Because these ages are maximum ages, we cannot exclude

the possibility that calcite cementation in the Middle Bunt-

sandstein occurred later. However, in that case, mixing

with fluids with a lower 87Sr ⁄ 86Sr ratio is required to

explain the measured signature in the leachate of the host

rock.

Based on previous observations and calculations, we can

conclude that the fluid source for the calcite cements and

calcite in the fractures in BH1 is a mixture between pore

fluids and fluids with a lower 87Sr ⁄ 86Sr ratio, related to

Phanerozoic sea water. In BH2 and BH3, calcite cement

and calcite in fractures could have precipitated from locally

derived pore fluids. Anhydrite fracture fillings in BH1 and

in BH4 precipitated from a fluid source with a relatively

low 87Sr ⁄ 86Sr ratio. Likely sources are fluids derived from

underlying Zechstein evaporites and overlying Rot evapor-

ites. In BH3, the anhydrite in the fractures does not show

a significantly lower 87Sr ⁄ 86Sr ratio.

Based on the fluid inclusion measurements, quartz pre-

cipitated in the fracture at a minimum temperature of

143.8�C and in a range up to 200�C, more or less con-

temporaneous with the anhydrite precipitation in the other

boreholes (Nollet et al. 2005b). Data providing direct

information on the source of the fluids are not available,

but the melting temperatures in the fluid inclusions sug-

gest highly saline fluids.

Timing of fracturing versus fracture filling

In a previous study, it was assumed that precipitation of

the minerals in the fractures of the Middle Buntsandstein

occurred contemporaneously with or shortly after the

Fig. 8. Strontium isotopic signature measured in

the leachate of the host rock (representing cal-

cite cement), residual host rock (after leaching),

anhydrite-filled fractures, first generation of

calcite-filled fractures (cc1), second generation of

calcite-filled fractures (cc2) in four boreholes

(BH1, BH2, BH3 and BH4). For comparison, the

isotopic signatures of sea water at Cretaceous

(Cr), Jurassic (Ju), Triassic (Tr) and Zechstein (Ze)

times are represented by the dashed lines (data

based on Kramm & Bless 1986). The measured

strontium isotope signature is for all samples

higher than any Phanerozoic sea water signa-

ture.

Table 3 87Sr ⁄ 86Sr and 87Rb ⁄ 86Sr ratios of calcite

and anhydrite fracture fillings, their Buntsand-

stein host rocks (leached h.r.) and respective

leachates (leachate of h.r.) representing calcite

cement.

Borehole

Laboratory

sample number Type 87Sr ⁄ 86Sr 2r 87Rb ⁄ 86Sr 2r

BH1 SN01 Leachate of h.r.

(calcite pore cements)

0.71054 0.00002

SN04 Anh fracture filling 0.70988 0.00003

SN07 cc1 0.71020 0.00001

SN10 cc2 0.71023 0.00001

SN01b Leached h.r. 0.71630 0.00001 1.49 0.05



0.71133 0.00002

SN08 cc1 0.71186 0.00002

SN02b Leached h.r. 0.71647 0.00001 1.60 0.05



0.71074 0.00002

SN05 Anh fracture filling 0.71094 0.00003

SN03b Leached h.r. 0.72179 0.00002 3.7 0.1

SN09 cc1 0.71105 0.00001

SN11 cc2 0.71117 0.00007

BH4 SN06 Anh fracture filling 0.71043 0.00002

h.r., host rock; anh, anhydrite; cc1, first stage of calcite precipitation in fracture; cc2, second stage ofcalcite precipitation in fracture.



fracturing (Nollet et al. 2005a). Employing this assump-

tion and by using an existing burial history curve (Pet-

mecky et al., 1999), stress conditions in the rocks at the

time of fracturing were constrained and it was concluded

that fracturing of the anhydrite-filled fractures most likely

occurred at burial depths between 3 and 5 km, as a result

of fluid overpressures. One likely cause of these fluid over-

pressures was an influx of fluids from the underlying Zech-

stein in the Buntsandstein at the initial stages of salt

movement. The data presented here and more specifically,

the fluid source discussion above, are in overall agreement

with this model for both the anhydrite and quartz precipi-

tation. The calcite signature in both the cements and the

fractures can be explained by precipitation of local pore flu-

ids in BH2 and BH3, whereas in BH1 influx of external

fluid must have occurred.

Following this analysis, it is now possible to relate the

fluid source to the fracture morphology and microstructure

in the fracture fillings. In BH2 and BH3, it was observed

that fibrous crystals and irregular fracture walls were calcite

filled (Fig. 5A), whereas in BH1 the calcite in the fractures

was more blocky and fracture walls smoother (Fig. 5B),

although, locally, fibrous calcite fillings can be found

(Fig. 5D). This suggests that the fracture opening rate ver-

sus crystal growth rate was lower when fluids were locally

derived (case of BH2 and BH3), whereas the fracture

opening rate versus crystal growth rate was significantly

greater when fluids were externally derived (BH1). When

fluids were locally derived, fracture opening probably hap-

pened as a slow, more gradual process whereas when a

major fluid influx occurred, fracture opening happened

more dramatically as a single major event. The association

of elongate-blocky anhydrite fill in fractures with smooth

walls (all boreholes) in association with externally derived

fluids seems to confirm this conclusion. The microstructure

in the fracture filled with quartz suggests a more gradual

opening of the fracture, but the fluid inclusions suggest a

very saline fluid source. Here, we do not have enough data

to confirm this model.

Fig. 9. Diagram showing the back-calculated

values of 87Sr ⁄ 86Sr ratio in Buntsandstein sedi-

ments, based on present-day measurements in

the residual host rock after leaching, versus time

and measured values in host-rock leachates (rep-

resenting calcite cement) for BH1, BH2 and BH3.

In BH1, the values in the cements correspond to

the values in the residual host rock after leaching

at t = 272 Ma, which is before the Middle Bunt-

sandstein was deposited. In BH2 and BH3, the

values in the cements correspond to the values

in the residual host rock after leaching at t = 226

and 210 Ma, respectively, indicating that these

ages are maximum ages of calcite cement pre-

cipitation.

Table 4 Back-calculation of the 87Sr ⁄ 86Sr in the host rock at different times,

based on the present-day measured 87Sr ⁄ 86Sr ratio of the host rock.

Period

Time

(Ma)

(87Sr ⁄ 86Sr)0

BH1 BH2 BH3

Buntsandstein 249 0.71102 0.71080 0.70868

Muschelkalk 245 0.7111 0.71089 0.70890

Keuper 235 0.71132 0.71112 0.70942

Lias 200 0.71206 0.71192 071127

Dogger 180 0.71249 0.71238 071232

Malm 155 0.71302 0.71294 0.71364

Lower Cretaceous 140 0.71333 0.71329 0.71443

Upper Cretaceous 100 0.71418 0.71420 0.71653

Leachate of the host

rock (calcite cement)

0.71054 0.71133 0.71074

Zechstein sea water 0.7068



The original observation that fibrous fracture fillings were

associated with the shalier intervals and more elongate-

blocky fracture fillings were associated with coarser grained

intervals fits very well with this fluid source model. More-

over, it is more likely that external fluids will find their way

through the high-permeable coarser grained intervals,

increase fluid pressure and fracture the (eventually cemen-

ted) rocks, followed by precipitation at a later stage, whereas

local pore fluids gradually open rocks in the low-permeable

shaley sections followed by immediate precipitation.

CONCLUSIONS

(1) Calcite cementation and fracture filling with calcite,

anhydrite and quartz are observed in borehole cores

from the Triassic Middle Buntsandstein in the LSB.

(2) Different methods were used to constrain the bound-

ary conditions during precipitation of these minerals

including micro-structural analysis, fluid inclusion mic-

rothermometry and strontium isotope analysis.

(3) Microthermometry results from fluid inclusions in

quartz suggest that precipitation occurred at high tem-

peratures (143�C) from fluids with very high salinities.

(4) The strontium isotopic signature was measured in cal-

cite and anhydrite fracture fillings, leached host rock

and in leachates of the host-rock samples (correspond-

ing to calcite cements). The fluid source of the calcite

cements and calcite fracture fillings in two boreholes

can be explained by precipitation of locally derived

pore fluids, i.e. a mixture of non-marine fluids with

components resulting from diagenetic reactions. In a

third borehole, however, the fluids that led to the pre-

cipitation of the calcite cement and fracture fillings

were mixed with a fluid with a lower strontium isotope

ratio. Also, the anhydrite fracture fillings precipitated

from a fluid that was influenced by this lighter stron-

tium isotope source.

(5) Based on the strontium isotope signature, the fluid

inclusion salinities and the geological history, it is

likely that the underlying Zechstein evaporite was the

source for the isotopically light fluid. In addition to

constraining the fluid source in more detail, we were

also able to relate fluid source to fracturing opening

mechanism and fracture-fill microstructure.

ACKNOWLEDGEMENTS

We thank Dr Jentsch and Dr Achilles (ExxonMobil Pro-

duction Germany) for granting permission to publish data.

Werner Kraus is acknowledged for the preparation of thin

sections and the fluid inclusion wafers. We thank Miss Jak-

obi for assistance during the sample preparation for the

strontium isotopes and Philippe Muchez for the use of the

cryogenic Linkham stage at K.U. Leuven and for valuable

discussions on fluid inclusion results. Marlina Elburg is

thanked for comments on an earlier version on the manu-

script. We acknowledge three anonymous reviewers and

Professor Richard Worden for the excellent editorial han-

dling. This project was funded by the DFG (Hi 816 ⁄ 1-2)

and was part of the SPP 1135 project ‘Dynamics of Sedi-

mentary Systems under varying Stress Conditions by

Example of the Central European Basin System’.

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