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1 Final report of research project Fracture sealing of the Buntsandstein

1.1 Applicant (Antragsteller) Christoph Hilgers, Dr. rer. nat. wissenschaftlicher Assistent currently at Geologie-Endogene Dynamik RWTH Aachen, Lochnerstr. 4-20 52056 Aachen Tel. 0241-80-95780 Fax. 0241-80-92358 e-mail: c.hilgers@ged.rwth-aachen.de

1.2 Topic (Thema) Fracture sealing processes in the Buntsandstein – comparison of nature and see-through experiments / Bruch-Versiegelungs-Prozesse im Buntsandstein – Ein Vergleich von Natur und Durchlichtexperimenten

1.3 Project duration (Förderungszeitraum) This research project started on 02.09.2002 and was funded by the DFG for three years. The project ended on 31.08.2005.

1.4 Publication list of the project (Liste der Publikationen aus diesem Projekt)

Nollet, S. submitted. Fracture sealing processes in sedimentary basins – a multi-scale approach. PhD thesis, RWTH Aachen.

1.4.1 Articles Nollet, S., Hilgers, C., Urai, J.L. submitted. Experimental study of polycrystal growth from an advecting supersaturated fluid in a model fracture. Geofluids Nollet, S., Hilgers, C., Urai, J.L. 2005. Sealing of fluid pathways in overpressure cells - a case study from the Buntsandstein in the Lower Saxony Basin (NW Germany). International Journal of Earth Sciences DOI: 10.1007/s00531-005-0492-1 Nollet, S., Urai, J.L., Bons, P.D. Hilgers, C. 2005. Numerical simulations of polycrystal growth in veins. Journal of Structural Geology 27, 217-230

1.4.2 Abstracts in conference proceedings Nollet, S., Hilgers, C., Urai, J.L. 2005. Palaeo-overpressures in the Lower Saxony Basin (NW Germany) as derived from veins. DRT meeting, Zurich, 5/2005. Poster presentation

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Hilgers, C., Nollet, S., Urai, J.L. 2005. Some constraints on growth conditions drawn from microstructures in veins. DRT meeting, Zurich, 5/2005. Oral presentation Hilgers, C., Nollet, S., Kirschner, D., Urai, J.L. 2005. Fluid migration und Bildung von versiegelten Klueften in Ueberdruckzonen - Fallstudien von Sandstein- und Kalkstein-Reservoiren. DGMK meeting, Celle, 4/2005. Oral presentation Nollet, S., Hilgers, C., Urai, J.L. Sealing of fluid pathways in an overpressure cell – A case study in the Buntsandstein (NW Germany). DFG-SPP 1135 Meeting Eringerfeld, Germany (3. Rundgespräch)(1st-3rd Decemer 2004) Oral presentation Nollet, S., Urai, J.L., Hilgers, C. 2004. Microstructural observations of veins in the Buntsandstein (Lower Saxony Basin). GeoLeipzig, 2004. Oral presentation Nollet, S., Urai, J.L., Hilgers, C. 2004. Microstructural study of fracture selaing in the Middle Buntsandstein (Lower Saxony Basin, Germany). Gordon Research Conference on Rock Deformation, 8-13.08.2004, Mount Holyoke, Massachusetts, USA. Poster presentation Nollet, S., Urai, J.L., Hilgers, C. 2004. Experimental study of fracture sealing by advective flow. Geophysical Research Abstracts, Volume 6, 2004. EGU meeting, Nice, France. Oral presentation Nollet, S., Urai, J.L., Hilgers, C. 2004. Simulation of fracture sealing by advective flow using see-through experiments. Terra Nostra 2004/01 (TSK X 10. Symposium Tektonik, Struktur- und Kristallingeologie, Aachen, Germany, 31.03.-02.04.2004). Poster presentation Nollet, S., Urai, J.L., Hilgers, C. 2003. Fracture sealing processes in the Buntsandstein. Terra Nostra 2003/07. DFG -SPP 1135 (Dynamics of Sedimentary Systems under varying Stress Conditions by Example of the Central European Basin System), pp. 54-57. Poster presentation Nollet, S., Hilgers, C., Urai, J.L., 2003. Fracture sealing processes in the Buntsandstein - comparison of nature and see-through experiments. SPP 1135 Workshop: Temperaturentwicklung, KW-Systeme, Fluide und Salze, BGR, Hannover, 18.06.2003. Oral presentation Nollet, S., Hilgers, C., Urai, J.L., 2003. Numerical modelling of microstructures formed by fracture sealing processes. DRT conference, St. Malo, France, 14.-16.04.2003. Poster presentation Nollet, S., Hilgers, C., Urai, J.L., 2002. Fracture sealing processes in the Buntsandstein - project overview and first results. 1. Rundgespraech DFG-Schwerpunktprogramm, Schloss Eringerfeld, Geseke, 28.-29.11.2002. Poster presentation

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2 Results (Arbeits-und Ergebnisbericht)

2.1 Introduction In this work, fracture sealing processes were analyzed at different scales, ranging from microstructures to basin-wide scale. The evolution of sedimentary basins is commonly associated with the generation and migration of fluids (Jamtveit and Yardley, 1997; Pedersen and Bjørlykke, 1994). There is an important interplay between fluids and the mechanical evolution of sedimentary rocks. When fluid pressures are higher than hydrostatic, overpressures are generated, which significantly reduce the effective stresses and result often in fracturing of the rock (Secor, 1965; Sibson, 2000; 2003). When different fractures are connected, fracture networks are generated and these can result in focused fluid flow, explaining important ore mineralizations (Cox et al., 2001). When fluids in fractures become supersaturated, they may precipitate and seal the fractures. This effect has important implications with respect to hydrocarbon flow pathways (Aydin, 2000). The microstructures of the resulting veins comprise a wide range of morphologies from dendrites, fibres, elongate-blocky to blocky crystals depending on boundary conditions. Such boundary conditions are (1) transport processes, (2) fluid properties, (3) nucleation sites and (4) available space (Durney and Ramsay, 1973; Bons, 2000; Hilgers et al., 2001).

2.2 Aim of the project This study was aimed at quantifying the advective and diffusive transport of material in fracture networks and the microstructural evolution of veins in the Buntsandstein of the Central European Basin System. There is already a good understanding of the influence of some boundary conditions on the resulting vein microstructures, but there are still unsolved problems. For example:

• What is the exact influence of the space in which crystals are growing in relation to the wall-rock morphology on the microstructure?

• Is there an effect of the transport mechanism on the vein microstructure and what are the characteristics of a typical advective transport microstructure?

• What is the influence of supersaturation on the microstructure and how can we recognize high paleo-supersaturations?

• What is the evolution of vein crystals over time? Do these crystals have a constant growth rate?

• What is the relation between veins and overpressures? And how do the overpressures evolve when material is precipitating?

• How can vein microstructures be related to the evolution of a sedimentary basin?

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2.3 Results

2.3.1 Real rock analysis General results Veins were studied 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. Samples were taken from four boreholes in the Lower Saxony Basin (Fig.1).

Figure 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 & RH=Ringkøbing-Fyn High). Insert shows an enlarged map of the Lower Saxony Basin with the location of the four sampled boreholes and the main structural elements (normal faults, reverse faults and salt domes) (after Ziegler, 1990 and Baldschuhn et al., 2001).

Different generations of veins are observed. The first generation formed in weakly consolidated rock without a significant increase of fracture permeability and was filled syntectonically with fibrous calcite and blocky to elongate blocky quartz. The stable isotopic signature (δ18O & δ13C) 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 minimum temperature of 150 °C from hypersaline brines. Based on δ34S 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 (Fig. 2). The large amount of fluids, which

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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.

Figure 2. (a) Subsidence curve for the Lower Saxony Basin (after Petmecky et al. 1999) including the

p-T-t conditions of vein formation. Based on the δ18O data, calcite veins formed at depths between 1 and 2.5 km. The fluid inclusions data indicate that the anhydrite veins precipitated at a minimum depth of 3 km. Veins have formed during the basin´s subsidence rather then during uplift because (i) their orientation suggest that the maximum principal stress acted vertically and (ii) stretching of the fluid inclusions requires an increase in p-T conditions after vein formation. (b) The porosity evolution in the Buntsandstein is plotted against the depth for comparison of the porosity evolution with depth of vein formation. This function is calculated from the initial porosity (typical 50% for sedimentary rocks) and the present porosity of the Buntsandstein (5%), reached at the maximum burial.

Strontium isotopes In addition to the stable istopes, strontium isotopes were measured in calcite and anhydrite veins, host rock and in leachate of the host rock samples (corresponding to calcite cements) (Fig. 3). Strontium isotope analysis is a very reliable method to find the source of the fluid and to check relations between the different vein generations. In the samples where the calcite veins have a fibrous microstructure, the fluids are derived from the remobilized pore fluids only. When the calcite veins have an

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elongate-blocky microstructure, the signature in the vein can not be explained by remobilized pore fluids and mixing with isotopic lighter fluid is required. Another possibility is that the vein precipitated before calcite cementation of the host rock and that the cements are remobilized vein material. The anhydrite veins can not be explained by precipitation from local pore fluids only and mixing with isotopic light fluids is required. The underlying Zechstein evaporites are a possible source for isotopic light fluids, although influence of overlying Röt evaporites can not be excluded, based on strontium isotope measurements. The differences in the data between the different boreholes indicate that the veins are not the result from one basin-wide fluid event.

Figure 3. Strontium isotopic signature (87Sr/86Sr) in leachate of the host rock, unleached host rock,

anhydrite veins, calcite veins and TSR calcite. Data are presented for the four sampled boreholes and can be compared with the Phanerozoic seawater signature (Cr=Cretaceous, Ju=Jurassic, Tr=Triassic, Ze=Zechstein) (Data based on Kramm and Bless, 1986).

EBSD analysis A detailed analysis of the fabric of characteristic Buntsandstein veins was carried out using EBSD analysis. Fibrous and blocky calcite veins show a random texture. The latter result is unexpected as one would assume that crystals tend to compete in growth, finally forming a preferred growth texture. In contrast to fibrous calcite, fibrous anhydrite shows a preferred orientation. Thus, the microstructure itself cannot be used to infer the growth conditions during fracture sealing.

2.3.2 See-through experiments The aim of the see-through experiments was the observation and analysis of ongoing fracture sealing, in order to allow conclusions on the boundary conditions of natural veins. An improved experimental set-up was built and carefully calibrated before experiments were carried out. It is composed of an accurately controlled flow system and experiments are done with the well-characterized analogue material alum (KAl(SO4)2 x 12 H2O).

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Figure 4. Series of polycrystal growth experiments. Four experiments are run with same flow rate and

different supersaturation. The identical seed crystals are used in the different experiments (a) Evolution of the microstructures over time. (b) Growth competition due to crystallographic orientation. (c) Influence of crystal location on growth competition. The crystal, which is located on the ridge, outgrows the crystal located in a depression. (d) Growth rate evolution shown by traced grain boundaries in time intervals of 60 minutes in experiment 1 and 30 minutes in experiment 2, 3 and 4. The arrow indicates a facet that changes its orientation during the evolution, due to influence of high-index facets, which are outgrown in the latest stages.

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A first series of experiments shows the evolution of a polycrystal at supersaturations between 0.095 and 0.263 (Fig. 4). The average growth rate of the crystals was measured and we observed that it is influenced by growth competition and the depletion of the fluid along fracture length (Fig. 5). Growth competition is controlled by crystallographic orientation, crystal size and crystal location. In addition, the growth rate of an individual crystal facet also shows variations depending on the facet index, facet size and flow velocity. These variations can influence the morphology of the grain boundaries and the microstructures.

Figure 5. Growth rate calculated based on grown area measurements. The growth rates are plotted

against time and for three different sectors: inlet (in), center (mid) and outlet (end) of the reaction cell. Growth rate for (a) experiment 1 (supersaturation = 0.095), (b) experiment 2 (supersaturation = 0.135), (c) experiment 3 (supersaturation = 0.176) and (d) experiment 4 (supersaturation = 0.263). Error bars are based on the error measured in the area measurements, followed by error propagation for the further calculations.

The aim of the second series of experiments was to investigate the growth evolution of rough/dissolved facets in detail. The growth distance required for the development of facets is around 15 µm. In all the experiments, we observe that the measured growth rates are in a much larger range than predicted by alum single crystal growth kinetics. This is due to the combined effect of the facet index and the crystal size. Furthermore, at high supersaturations, the facet growth rate measurements do not longer fit with the same growth rate equation as for the experiments at lower supersaturation (< 0.176). This can be explained by a change in the growth mechanism at high supersaturation with more influence of the volume diffusion of the bulk solution on the growth rate. This effect can also cause a more homogeneous sealing pattern over fracture length.

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At high supersaturation, the larger crystals in these experiments incorporate regularly spaced fluid inclusion bands and we propose that these can be used as indicators for paleo-supersaturations (Fig. 6). The final microstructures of the experiments show no asymmetry with respect to the flow direction.

Figure 6. Fluid inclusions in experimentally grown alum crystals in experiment 3 (supersaturation =

0.176) & experiment 4 (supersaturations = 0.263). The inclusions are regularly spaced and contain one phase. In experiment 3, inclusions show a band spacing of 50-150 µm, whereas in experiment 4, the band spacing is around 20 µm.

2.3.3 Numerical simulations Vein microstructures contain a wealth of information on conditions during vein growth but correct interpretation requires an improved understanding of the processes involved. In this part of the project we investigated the parameters controlling vein microstructures using numerical simulations of anisotropic crystal growth with two different simulation algorithms: Vein Growth (Bons, 2001; Hilgers et al. 2001) & FACET (Zhang and Adams, 2002). We focused on the effects of crystal growth rate anisotropy on growth competition and on the effects of the wall rock on vein microstructure during crack-seal growth. Growth competition in a free fluid is controlled by the crystallographic orientation and growth anisotropy of the crystals. We discussed the merits and limitations of the different algorithms to simulate free growth in veins, based on a detailed study of the crystal facets and grain boundaries produced (Fig. 7).

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Figure 7. Simulations of a polycrystal with square anisotropy. The polycrystal contains four crystals.

(a) Simulation of the polycrystals with the growth rate function for square growth that was originally implemented in Vein Growth. (b) Simulations of the polycrystals with the new growth rate function for square crystal growth in Vein Growth. (c) Simulations of the polycrystals with the suggested improved method for the triple junction movement, according to the FACET algorithm. Notice that the facets now make the correct 90° angle for the square crystal model. The same initial settings were used as in (a) and (b). Dashed line for grain tip line; full line for grain boundary line. At growth increment (1), a grain tip line meets a grain boundary line and the result is that one crystal facet disappears and the grain boundary line changes its orientation. At growth increment (2), a grain boundary line meets another grain boundary line and the result is that one grain is overgrown and disappears.

Microstructures in crack-seal veins are influenced by additional parameters, such as the width of individual crack-seal increments and the fracture morphology. We present a detailed study of the transition between free-fluid growth and crack-seal growth as a function of the relative rates of crack opening and crystal growth, to illustrate how this induces the microstructural transition between fibrous and blocky veins (Fig. 8).

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Figure 8. Overview of numerical simulations with varying opening distances as a function of time in

Vein Growth. In these simulations, the link between the touching of peaks by the crystals before an opening increment and the magnitude of opening increment was studied. In all simulations, the same wall morphology and seed crystals with the same properties (square anisotropy) were used. The wall rock has peaks with three different amplitudes: 40, 20 and 10 pixels. Magnitude of opening increment is given in the Y-axes for each simulation. (a)–(c) show the same final result, the wall morphology has no influence on the growth competition. From simulation (d) on, the wall morphology has influence on the growth competition and the competition effects based on anisotropic growth decreases. In simulation (g), the crystals grow completely isotropic. The orientation of the surviving crystals is given in a rose diagram at the right hand side of each simulation.

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2.4 References Aydin, A. 2000. Fractures, faults, and hydrocarbon entrapment, migration and flow.

Marine and Petroleum Geology 17(7 SU -), 797-814. Bons, P. D. 2000. The formation of veins and their microstructures. In: Stress, strain

and structure, a volume in honour of W.D. Means. Journal of the Virtual Explorer 2 (edited by Jessell, M. W. & Urai, J. L.).

Bons, P. D. 2001. Development of crystal morphology during unitaxial growth in a progressively widening vein: I. The numerical model. Journal of Structural Geology 23, 865-872.

Cox, S. F., Knackstedt, M. A. & Braun, J. 2001. Principles of structural control on permeability and fluid flow in hydrothermal systems. Society of Economic Geologists Reviews 14, 1-24.

Durney, D. W. & Ramsay, J. G. 1973. Incremental strains measured by syntectonic crystal growth. In: Gravity and tectonics (edited by de Jong, K. A. & Scholten, R.). Wiley, New York, 67-96.

Hilgers, C., Koehn, D., Bons, P. D. & Urai, J. L. 2001. Development of crystal morphology during unitaxial growth in a progressively widening vein: II. Numerical simulations of the evolution of antitaxial fibrous veins. Journal of Structural Geology 23, 873-885.

Jamtveit, B. & Yardley, W. D. 1997. Fluid flow and transport in rocks: An overview. In: Fluid flow and transport in rocks. Mechanisms and effects (edited by Jamtveit, B. & Yardley, B. W. D.). Chapman & Hall, London, 1-14.

Pedersen, T. & Bjorlykke, K. 1994. Fluid flow in sedimentary basins: model of pore water flow in a vertical fracture. Basin Research 6, 1-16.

Secor, D. T. 1965. Role of fluid pressure in jointing. American Journal of Science 263, 633-646.

Sibson, R. H. 2000. Fluid involvement in normal faulting. Journal of Geodynamics 29, 469-499.

Sibson, R. H. 2003. Brittle-failure controls on maximum sustainable overpressure in different tectonic regimes. AAPG Bulletin 87(6), 901-908.

Zhang, J. & Adams, J. B. 2002. FACET: a novel model of simulation and visualisation of polycrystalline thin film growth. Modelling and Simulation in Materials Science and Engineering 10, 381-401.

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3 Summary of the project (Zusammenfassung) This project improved our understanding of fracture sealing processes in sedimentary

basins, based on a general approach covering real rock analysis, analogue experiments

and numerical modeling at different scales (microstructures to sedimentary basin).

The analysis of rocks in the Lower Saxony basin, part of the Central European Basin

System, allowed a better understanding of the fluid evolution during basin formation

since the Buntsandstein. Although precipitates in fractures and vein microstructures

are consistent across the basin, isotope analyses reveal the complexity and difference

of similar veins and vein microstructures at different locations. Fibrous calcite veins

formed from connate waters, while anhydrite veins precipitated from a mixed fluid.

The veins are a result of multiple fluid events, with the anhydrite most likely derived

from the underlying Zechstein evaporites.

See-through experiments delivered new tools to establish boundary conditions from

natural vein microstructures. Incorporation of fluid inclusions in elongate-blocky

grains depends on supersaturation and the grain’s location in an advective

environment. This may be used to establish flow directions and paleosupersaturation

from natural vein microstructures. Very rough fluid-wall interfaces result in more

constant growth rate of polyhedral vein crystals along-fracture at high supersaturation,

possibly caused by a change of growth kinetics at high fluid flow velocity.

Modeling of vein microstructures better constrained the conditions of growth

competition and the formation of fibrous veins. The width of individual fracture

opening increments governs the final vein microstructure, and the fracture roughness

also controls the number of growing grains. Results of two programmes Facet and

Vein Growth were critically compared and the improvements were suggested with

respect to a better microstructural code.

Result were published in international peer-reviewed journals.

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4 Acknowledgement We thank ExxonMobil and Wintershall providing data.