Mineral replacement in long-term flooded porous carbonate rocks

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Mineral replacement in long-term flooded porous carbonate rocks Mona Wetrhus Minde a,b,⇑, Udo Zimmermann a,b, Merete Vadla Madland a,b Reidar Inge Korsnes a,b, Bernhard Schulz c, Sabine Gilbricht c b

a University of Stavanger, P.O. Box 8600 Forus, N-4036 Stavanger, Norway The National IOR Centre of Norway, University of Stavanger, P.O. Box 8600 Forus, N-4036 Stavanger, Norway c TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, D-09596 Freiberg, Saxony, Germany

Received 21 August 2018; accepted in revised form 9 September 2019; available online xxxx

Abstract This study reports mineralogical and physical property changes linked to geo-chemical alterations processes during three ultra-long-term tri-axial tests on outcrop-chalk from Lie`ge (Belgium). The test core plugs were flooded with MgCl2-brines for approximately one and a half, two and three years, mimicking effective reservoir stresses (9.5–12.5 MPa) and temperature (130 °C) of important hydrocarbon deposits at the Norwegian Continental Shelf. The flooded cores were studied using electron microscopy, whole-rock and stable isotope geochemical analyses, and ion chromatography of the effluent water. All tests show altered textures and mineralogy at the flow-inlet side of the approximately 7 cm long cores. With longer duration of flooding, these alterations moved further into the cores, and for the three-year-test, the entire core was altered. When studied at nano-scale, the newly formed crystals were found to be magnesite containing minor calcium impurities, together with clay-minerals. On the outlet side of the alteration-fronts in the two shorter tests, the mineralogy still mainly consists of calcite and primary clay-minerals, together with newly formed magnesite and secondary clay-minerals. Dolomite or low- and high-Mg-calcite are not observed. The textures of larger micro-fossils are often preserved, but the mineralogy of their shells is altered. A sharp, only 4 mm narrow transition zone at the border of the alteration front towards the less altered area for the two shorter tests, shows the highest porosity in the cores. This pattern resembles what is observed in single-crystal experiments, where the alterations are driven by phase dissolution and subsequent precipitation, the progression of high porosity zones and the state of equilibrium at the boundary between the primary and new mineral phase. This is also in line with observations in nature and models for transport driven mineral replacement in porous media, where differences in dissolution and precipitation rates may cause high porosity transitions zones. During the experiments, all cores underwent severe overall compaction between 10.1% and 18.2%. However, in the twoand three-year long test-cores, the permeability, and calculated porosity, started to increase after a primary phase of reduction. As magnesite precipitates at the expense of calcite, the density increase, but the solid volume decrease. As the bulk volume is constant, porosity and permeability are increased. The changes in ion-concentration of effluents, monitored throughout the experiments, balance the changes in mineralogy, compaction and permeability within the cores. Compositional variations of the injection fluid effectively control the amount of chemical reaction in chalk. This allows for control and predicting changes in geo-mechanical parameters induced by mineralogical replacement, which has significant impact on reservoir conditions. Ó 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/). Keywords: Chalk; MLA; Chemo-mechanical testing; Mineral replacement; Long-term testing; Alteration fronts

⇑ Corresponding author at: University of Stavanger, P.O. Box 8600 Forus, N-4036 Stavanger, Norway.

E-mail address: [email protected] (M.W. Minde). https://doi.org/10.1016/j.gca.2019.09.017 0016-7037/Ó 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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1. INTRODUCTION Studies of mineral replacement encompass a wide range of disciplines. A considerable amount of research has been carried out to understand the kinetics and reactions associated with mineral replacement, whether these are linked to metamorphism, metasomatism, diagenesis and/or weathering. Understanding these basic mineral replacement reactions is of importance not only as basic knowledge of rock-fluid interactions, but also to ‘‘(. . .) quantify and predict the response of Earth’s surface and crust to the disequilibria caused by the various natural and anthropic input of energy to our planet.” (Oelkers and Schott, 2009). Carbonate mineral kinetics dictate a wide range of processes in our world like preservation of large monuments and buildings, climate models, as well as the characteristics of petroleum reservoirs (Morse and Arvidson, 2002). Magnesite itself is often found as an alteration product of other carbonate minerals, such as calcite (CaCO3) and dolomite (CaMg(CO3)2), during e.g. hydrothermal alteration. Formation of magnesite through alteration of calcite by non-equilibrium solutions, has been studied in several contributions, both as batch reactions and at nano-scale (e.g. by AFM, review in Morse and Arvidson, 2002). These experiments also show that other compounds most likely also play significant roles in the alteration process interplay. The majority of experiments in the field of mineral replacement is carried out on single crystals, on powder and on single grains from natural samples. A thorough review of such experiments is given in Putnis (2009), concluding that in fluid-induced mineral replacement, porosity generation is the main driver for the progression of the alteration-front. However, observations of convection driven mineral replacement are observed in geological systems, where the process similarly seems to proceed in a front-like manner with a porous transition zone (e.g. Merino and Banerjee, 2008). Kondratiuk et al. (2015, 2017) propose chemical mechanisms and model such synchronized dissolution and precipitation fronts in porous rocks based on the chemical kinetics of the involved minerals species. They argue that volume-preserving replacement can be caused by shifts in the equilibrium for the dissolving and precipitated phases, rather than driven by the force of crystallization (Maliva and Siever, 1988). The model Kondratiuk et al. present allows for a stable system with self-regulating fronts to propagate through the system, preserving the volume. The processes involved differ slightly based on the state of equilibrium of the solution with regards to the primary and the secondary minerals in the transformation. As carbonate reservoirs hold over 50% of the world’s hydrocarbon reserves (Roehl and Choquette, 1985; Flu¨gel, 2004), research on carbonate mineralogy is important. In the southern part of the North Sea, particularly in Norwegian, British and Danish sectors, one of the major reservoir rocks is chalk, containing large hydrocarbon deposits like those in the carbonate-rich Ekofisk (Danian), Tor (Campanian to Maastrichtian) and Hod (Turonian to Campanian) Formations. More than a thousand carbonate EOR experiments have been performed at the University of Stavanger, along with

other petroleum laboratories world-wide. The time-span of such experiments commonly varies from days or weeks to several months, but rarely for years e.g. (Hellmann et al., 2002; Nermoen et al., 2015; Zimmermann et al., 2015). In this study, we present the results from experiments on Upper Cretaceous chalk from Lie`ge in Belgium with regard to its mineralogical alterations from the three mentioned ultra-long-term tests (516, 718 and 1072 days) flooded with MgCl2 at conditions matching important North Sea reservoirs. The length of these triaxial-cell experiments is novel and paramount to be able to better understand field-scale water-injection, which is commonly scheduled for years, not weeks or months. Oil-production from the chalk reservoir at the Ekofisk field started in 1971. A declining production curve along with severe compaction in the reservoir and seabed subsidence, initiated injection of seawater in the late 1980s (Teufel et al., 1991; Maury et al., 1996; Nagel, 1998; Hermansen et al., 1997, 2000). The rate of subsidence was reduced; but, further compaction was still observed. This indicates, together with decades of experimental work, that the interplay between injected fluids and the chalk itself plays an important role in the mechanical behaviour of chalk (Hellmann et al., 2002; Risnes et al., 2003; Heggheim et al., 2005; Risnes et al., 2005; Madland et al., 2006, 2008, 2011; Korsnes et al., 2008; Wang et al., 2016). This effect is commonly referred to as water weakening of chalk. The weakening may have a positive impact on the production of oil through reservoir compaction, and is also found to play an important role in erosion of carbonate coastal cliff formations occupying large areas of e.g. the British and French coastlines (Lawrence et al., 2013). Water-injection has been used successfully world-wide to (i) maintain reservoir pore-pressure and (ii) increase the recovery of hydrocarbons. Carbonate reservoirs are prone to be very reactive towards fluids and especially seawater e.g. (Tucker et al., 1990; Flu¨gel, 2004; Heggheim et al., 2005; Austad et al., 2008; Korsnes et al., 2008; Strand et al., 2006; Madland et al., 2011). Chalk is a carbonate rock, which due to its very fine-grained character, has a high specific surface area (often between 1.5 and 7 m2/g, Hjuler and Fabricius (2009)). Several parameters like the type of chalk, the composition of the fluid as well as the pressure and temperature conditions have been varied in numerous experiments to understand how these parameters impact fluid flow, rock-fluid interactions and compaction. Experiments reveal an extraordinary complexity of reactions even though the mineralogy of the rock itself is rather simple. These reactions have an effect on oil recovery through chemical and mineralogical alterations as well as changes in mineral surface complexes, surfacecharge and -potential of the rock (Borchardt et al., 1989; Zhang et al., 2007; Hiorth et al., 2010, 2013; Megawati et al., 2012; Jackson et al., 2016; Wang et al., 2016; Nermoen et al., 2018). Sakuma et al. (2014) showed that ion substitution in calcite, between Ca2+ and Mg2+, may alter the surface tension of calcite where Mg2+ incorporation renders the calcite surface more water wet, enough to increase the oil-production at macro-scale (Sakuma et al., 2014).

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Certain ions have proven to play more important roles than others when chalk is exposed to seawater at elevated temperature and pressure, like Mg2+, Ca2+, and SO2
4 (Heggheim et al., 2005; Risnes et al., 2005; Strand et al., 2006; Puntervold and Austad, 2008). A complete understanding of the processes involved seems only feasible when the system is simplified. To especially study the interactions between Mg2+- and Ca2+-ions, core-scale flooding experiments injecting MgCl2, representing an abridged aqueous chemistry of seawater, have been performed. Based on these factors, the following reactions (Eqs. (1) and (2)) can be considered to be the two main reactions governing alteration through dissolution and precipitation: CaCO3 $ Ca2þ þ CO2
3

ð1Þ

MgCO3 $ Mg2þ þ CO2
3

ð2Þ

In addition, one may also consider the formation of dolomite as a possible reaction (Eq. (3)): CaMgðCO3 Þ2 $ CaCO3 þ MgO þ CO2

ð3Þ

Three chalk cores from ultra-long term flooding tests, 516, 718 and 1072 days long, have been studied with regards to textural and compositional alterations. Previous studies have shown alteration in flooded cores from calcitedominated mineralogy towards a magnesium-rich carbonate and possible precipitation of Mg-rich carbonate, talc and magnesite e.g. (Madland et al., 2011; Nermoen et al., 2015; Zimmermann et al., 2015; Wang et al., 2016). In two of the previous studies by Nermoen et al. (2015) and Zimmermann et al. (2015), both the 1072 and the 516 days test showed these above mentioned alterations towards Mgcarbonates, increasing the density of the rock. Interestingly, for the 1072 days test, an end-of-test porosity was calculated and showed to be similar to the porosity prior to any flooding, despite the high compaction of the core. Random tests by FEG-SEM-EDX along the 1072 days experiment (ULTT), all showed the similar composition, dominated by Mg-carbonates, indicative of a complete transformation of the mineralogy of the core (Nermoen et al., 2015). Data on the 516 days test (LTT) point to chemical alterations approximately half way into the core along the flooding axis (Zimmermann et al., 2015). In this study, we add a third long-term experiment (718 days), along with additional investigations for the two previous tests, to deepen the understanding of the mineralogical and coupled mechanical alterations over time. The mechanical flow-through test of the 718 days test is also discussed in Megawati et al. (2011). 2. LONG-TERM EXPERIMENTS UNDER RESERVOIR CONDITIONS The three cores from these experiments are named the Long-Term Test (LTT, flooded for 516 days), Medium Long-Term Test (MLTT, flooded for 718 days) and the Ultra Long-Term Test (ULTT, flooded for 1072 days). The cores were flooded in triaxial cells at reservoir conditions, similar to the conditions for a number of hydrocar-

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bon reservoirs at the NCS. After installing the cores in the triaxial cells, a minimum of three pore volumes distilled water were flooded for cleaning purposes, before the first injection brine (MgCl2 for LTT, and NaCl for MLTT and ULTT) was introduced at a rate of one pore volume per day (PV/day). In the next stage, the confining pressure and pore pressure were increased to 1.2 and 0.7 MPa, respectively, before the temperature was raised to 130 °C. Pore pressure and test temperature were kept constant throughout all experiments. A phase of hydrostatic loading then commenced until the cores passed yield strength, enabling estimation of elastic and plastic properties for the samples (for detailed experimental set up see Madland et al. (2011), Nermoen et al. (2015) and Zimmermann et al. (2015)). In the creep phase, following the hydrostatic loading, the LTT was in the entire test period flooded with 0.219 M MgCl2 at pH varying between 5.5 and 6.5, the same ionic strength as seawater, at effective stress of 12.5 MPa (Zimmermann et al., 2015). The MLTT had a slightly different flooding procedure in the creep phase. After the initial phase of flooding with NaCl, at effective stress 9.5 MPa, followed five flow sequences of 0.219 M MgCl2 with varying pH. Three flow sequences with 0.219 M MgCl2, with pH varying between 5.7 and 5.8, were performed in the time interval between 4–11, 22–33 and 57–102 days. In the time interval 11– 22 days, citric acid was added to the 0.219 MgCl2 to lower the pH to 2.73, and between 33 and 57 days NaOH was added to form pH 8.98. After 102 days, 0.130 M CaCl2 was added to the 0.219 M MgCl2 brine, and NaOH was added to adjust pH to 8.9. The reason for adding calcium into the brine, is to understand how this ion in particular affects the strength of chalk, compared to the weakening effect of magnesium ions (Madland et al., 2011). This injection period lasted from 102 to 165 days, before 0.219 M MgCl2 with pH close to 9, adjusted by the addition of NaOH, was injected for the rest of the experiment. The flooding sequences during creep for the ULTT (Nermoen et al., 2015), was first a period lasting 7 days with 0.657 M NaCl at 10.4 MPa effective stress. This flow sequence was followed by 0.219 M MgCl2, at pH between 5.6 and 6.2, from 6 to 57 days, and distilled water between 57 and 68 days. In the remaining part of the creep test ULTT was flooded with 0.219 M MgCl2 with rates varying between one and three PV/day. Variations in brine composition and flooding rate provide a possibility to understand correlations between injected brine, dissolution and precipitation and changes in strain, i.e. does the amount of dissolution change with altered flooding rate, and can this change be correlated to changes in strain rate? At the end of the flooding, the cores were all cleaned with distilled water and carefully removed from the triaxial cells. The cores were weighed and measured and left to dry at 100 °C until stable weight. In addition, test sleeves and drainage plates with chalk remains were also weighed and dried after testing to get a measurement of saturated and dry core mass after testing. Subsequently the cores were cut axially and into slices perpendicular to the flooding axis for further analyses (Fig. 1). Thin sections were produced to

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Fig. 1. Sketch to demonstrate the preparation of the core for mineralogical analyses. Each core was cut into five, six or seven slices to enable discrete analyses along the cores’ flooding axes.

cover half the diameter of the core, from core centre to the rim and along the entire core in the flooding direction. After dismantling, the cores were analysed by several methods to gain a complete picture of the alterations in texture and chemical composition. Different applications of electron microscopy, Field Emission Gun Scanning Electron Microscopy (FEG-SEM), Transmission Electron Microscopy (TEM and Mineral Liberation Analyser (MLA) were used, together with analyses of effluent water (Ion Chromatography, IC). Whole-rock geochemistry and stable isotope analyses (carbon and oxygen) were performed on each slice (Fig. 1) of the three tests. Detailed description of the applied techniques and the sample material can be found in Appendix A. 3. RESULTS 3.1. Permeability and strain evolution Permeability, porosity and strain evolution of the three experiments are described by Megawati et al. (2011), Nermoen et al. (2015) and Zimmermann et al. (2015) and are shortly discussed here. During the primary hydrostatic loading phase of the experiments, the three cores follow nearly the same behaviour in the relationship between axial strain and stress (Fig. 2). The LTT compacts more in total, but has a higher yield point and is loaded to higher stress. The yield points, i.e. the transition between the elastic and the plastic stress regimes, for the three cores are estimated to 9.0 MPa (LTT), 8.0 MPa (MLTT) and 7.5 MPa (ULTT). During all three experiments the difference in inlet and outlet pressure of the cores (DP) were logged, enabling calculation of permeability (k) through the cores from Darcy’s equation (Eq. (4)): k¼

QlL DP  A

ð4Þ

where Q is the flow rate, L is the length of the core, A is the cross-sectional area, and l is the brine viscosity. All three cores suffered severe reduction in permeability following compaction during the first 100–300 days of creep. The permeability of the LTT started at 0.50 mD and after the first 100 days continues to reduce together with increased axial strain, closing in on zero mD at the end of the experiment (Fig. 2b). The permeability curve flattens out after 160 days. At this point the pressure transducer was out of

range, however, as the inlet pressure of the flooding pump continues to increase, one can deduce that the permeability continues to decrease. The rate of axial creep strain, the percentage of shortening of the core in length direction, is high in the start of creep and gradually decreases. After 350 days, a small acceleration in the strain-rate is observed. The permeability at the start of the creep phase of the MLTT was 1.10 mD. This core followed a slightly different flooding sequence than the other two cores. During the period between 102 and 165 days, 0.130 M CaCl2 was added to the 0.219 M MgCl2 brine and this induced an immediate response in axial strain and permeability (Fig. 2c). Both curves flatten out at this point, indicating absence in rock-fluid interactions in the core. At the point where the CaCl2 is removed from the brine, the creep-rate returns to the same rate as before 102 days. After 300 days of flooding the permeability of the MLTT is at its lowest value, 0.09 mD, before it starts to increase continuously throughout the rest of the experiment, ending at 0.18 mD. The pH of the brine was altered during the flooding period, and small kinks in the strain vs creep may be observed. Especially at the point where the pH is changed from 5.68 to 8.97, the deformation rate is lowered, however no change in permeability is observed. During the final 400 days of flooding the axial strain of the MLTT starts to flatten out, however after 550 days of flooding a small acceleration in the strain is observed. The initial permeability of the third core, the ULTT, was 0.66 mD. After the first 102 days of injection the flooding rate of the ULTT was increased from one to three PVs/day. At this point the permeability reached a minimum of 0.07 mD and simultaneously with the change in flooding rate, the permeability starts to increase (Fig. 2d). After 400 days the permeability-curve stabilizes between 0.13 and 0.14 mD for the remaining period of the time, with minor fluctuations as flooding rate is changed and towards the end of the experiment. The strain of the ULTT is seen to be affected by the flooding rate, where an increase in flooding rate seems to slightly increase the deformation rate, i.e.% strain per day. Reduction in flooding rate is followed by a reduction in deformation rate. Between 57 and 68 days of flooding, distilled water was flooded, with an immediate reduction in deformation, before the deformation rate increased again when flooding with MgCl2.

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Fig. 2. (a) Stress (MPa) vs axial strain in % for the three cores. The LTT (red curve) compacted approximately 0.5% more than the other two in total during loading, while the MLTT (black curve) and the ULTT (green curve) compacted more at an earlier stage of the experiments. Evolution of permeability and axial strain vs creep time for the three cores: (b) LTT: Permeability (black curve) reduces significantly during the first 150 days, before the reduction rate is reduced. This follows the increase in strain (red curve). (c) MLTT: Permeability and strain (coloured curve) follow a similar behaviour as the LTT, however with a break in reactivity during CaCl2 injection between 102 and 165 days, where both curves flatten out. After 300 days, the permeability starts to increase after reaching its lowest point of 0.09 mD. (d) ULTT: Minor fluctuations can be observed on the strain curve (coloured curve) with rates of deformation linked to the flooding rate. The permeability reduces significantly during the first 100 days, before an increase in permeability is seen when flooding rate is increased from one to three pore volumes per day (PVs/day). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.2. Field Emission Gun Scanning Electron Microscopy (FEG-SEM) Analyses of the samples were performed with FEGSEM to gain a first impression of the texture and mineralogy of the samples, as well as imaging and identification of areas suitable for further analyses by MLA and TEM. The surfaces of the rock fragments show a clear difference in the rock texture before and after flooding. In unflooded samples, recognizable coccolithophore rings with rounded grains are easily identifiable (Fig. 3a) with the main constituents being calcite and minor occurrences of clay (Fig. 3b). In addition, authigenic calcite is observed (Fig. 3b) (see also Zimmermann et al. (2015) and Nermoen et al. (2015)). In the flooded cores, several different textures and mineralogical compositions can be observed. At the inlet of all three cores, the material (Figs. 4a and 5a and c) display an altered texture and is dominated by smaller rhombic crystals, mostly in the range from 100 nm to 1 mm, along with flaky clay minerals. The average size of the grains and crystals is reduced and the shape has become more angular in flooded material, compared to grains occurring

in unflooded samples. In these heavily altered parts of the samples, the composition is dominated by magnesium carbonate and silicate phases (Fig. 4). The clay minerals are too small (commonly below 500 nm) to allow for reliable chemical measurements by FEG-SEM-EDX. However, the main constituents are definitely magnesium and silicon along with aluminium and with occurrences of sodium, potassium, calcium and iron (Fig. 4c and d). In cases, assemblages of magnesium carbonate crystals are observed in larger pore-spaces and these crystals assemblages may grow up to 10 mm in size (Figs. 4 and 8). The composition is in accordance with XRD analyses in Zimmermann et al. (2015), which identified magnesite and showed the occurrences of chrysotile and tilleyite. The texture and mineralogy observed in Fig. 4 are characteristic for the entire ULTT core, while towards the outlets of the two other cores, the rock gradually resembles unflooded material again, though with a higher content of magnesium and occurrences of magnesite crystals in larger pore-spaces. Further along the flooding axis from the completely altered areas of the LTT and the MLTT, a millimetresized transition zone is observed (Figs. 6 and 7), where

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Fig. 3. FEG-SEM secondary electron (SE) micrographs of unflooded Lie`ge chalk. (a) Recognizable rings (coccoliths) from coccolithophores along with fragments and decoupled grains from micro- and nano-fossils. (b) Occurrences of clay minerals (black arrows) and authigenic calcite crystals (white arrow).

Fig. 4. Secondary electron (SE) images from the completely altered part in the ULTT. (a) Typical texture of the newly formed crystals with rhombic shape and sizes mostly between 100 nm and 1 mm. (b) Close-up of crystals in (a). (c) Remnants of foraminifera fossil in the ULTT (slice 5) filled with silicates. (d) Assemblages of larger magnesite crystals filling the mould of what seems to have been a foraminifera fossil. These crystals are larger than the surrounding matrix, typically 6–10 mm. For EDX-spectrum and quantification, please see Appendix B, Fig. B.1 and B.2).

the porosity seems to be increased compared to the remaining part of the sample, and there is an abrupt change in mineralogy. This zone is characterized by the same Mgbearing rhombic crystals and clay-minerals as observed in the completely altered areas along with rare occurrences of calcite. Given the increased porosity, the clay-minerals are more visible in this zone.

Using a Back-Scattered Electron (BSE) detector, which provides different grey-scale values for different average atom number of minerals, the transition between the two mineral regimes could be imaged (Fig. 7). The brighter phases represent calcite (higher AAN), while the darker are Mg-carbonate and clay-minerals. Fig. 7a shows clearly how abrupt this change is. The calcite stops on micrometre

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Fig. 5. FEG-SEM secondary electron (SE) micrographs of the MLTT and LTT. (a) Image of the completely altered area in slice 2, MLTT (b) close-up of area in unaltered zone in slice 4, MLTT; newly formed magnesite, clay minerals and original calcitic coccolith microfossils are observed. (c) The completely altered area of the LTT (slice1), and (d) close-up of the chalk matrix at the outlet of the LTT core (slice 6) with similar texture and composition as unflooded chalk, however, more compacted.

Fig. 6. FEG-SEM micrographs of the transition zone in slice 2 in the LTT with increased porosity. Flooding direction from bottom to top of image. Notice the abrupt change in porosity from the lower to the upper part of the section, separated by the stippled line.

scale and in the transitions zone, shells of microfossils are preserved in calcite, while the surrounding matrix is altered to lighter phases; magnesite and clay. The magnesite seems to spread into the original calcite, with a more transitional character opposed to the abrupt depletion of calcite.

Towards the outlet of the two cores, LTT and MLTT, the texture and mineralogy resemble unflooded chalk, however with visible Mg-carbonate crystals and higher abundances of clay-minerals (Figs. 5b, d and 8). Compared to unflooded chalk, the rock-material towards the outlet of

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Fig. 7. SEM-BSE micrographs of the transition zone between completely altered parts of the LTT core and the area with considerable amounts of calcite. (a) Overview and (b) close-up: Light phases represents calcite with a higher average atom number (AAN), while the darker phases are magnesite and clay-minerals with lower AAN. Notice the abrupt (mm-sized) change in mineralogy. Area with higher porosity in the transition zone. White arrows indicate the flooding direction.

Fig. 8. FEG-SEM micrographs of the calcitic part of the LTT (outlet of slice 2). (a) Foraminifera fossil inside the chalk matrix and (b) closeup of one of the chambers in the fossil with newly precipitated magnesite crystals (black arrows in (a) and (b)).

LTT and MLTT is more compacted, with seemingly lower porosity (Fig. 5d). In Fig. 8 large Mg-carbonate crystals are seen to precipitate in larger pore-spaces of foraminifera fossils, similar to the magnesite assemblages observed in the completely altered mineralogy in Fig. 4b. 3.3. Mineral Liberation Analyzer (SEM-MLA) 3.3.1. Unflooded material MLA analyses of unflooded chalk confirm the observations by FEG-SEM (Fig. 3). The majority of the sample is dominated by calcite (red), while minor occurrences of quartz, clays and feldspar, mainly albite, are observed (not shown in Fig. 9). In the detailed image (to the right, Fig. 9a), clay minerals (green) are observable in the porespaces of the sample. In both the overview and the detailed map, larger macro-fossils of foraminifera and shell fragments can be observed, mostly with calcitic (red) exoskeleton containing the original internal compartments of the fossil preserved. These are often left open as pore-space or are in many cases filled with clay minerals.

3.3.2. Long Term Test (LTT) sample (516 days) This core contains two chemical ‘‘fronts”, which seem to have altered the core at different rates. The first front is recognized by an increase of clay minerals and precipitated magnesium-bearing carbonates or magnesite, and reaches all the way to the outlet of the core. Clay minerals are indicated by green (different types of clays), while Mgcarbonates are shown as blue pixels in the core map (Fig. 9b). Similar observations have been made in earlier studies, however with only selected, representative, areas analysed (Madland et al., 2011; Zimmermann et al., 2015). In the area towards the outlet of the core, where the first front is observed, large amounts of calcite, 85% (red in Fig. 9) are still present. Macro-fossils are still abundant, as observed in unflooded material. These are often filled with clay minerals, Mg-carbonate and in cases a combination of those. The amount of fine-grained silicates, interpreted as clay minerals (green pixels), appears in the MLA analyses to be largest close to the secondary front, indicated by the sharp transition to blue (magnesite) from red (calcite) colours.

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Fig. 9. SEM-MLA-EDX spectral maps (GXMAP) of (a) unflooded chalk. The mineralogy is dominated by calcite (red) with minor occurrences of non-carbonate material (green). Detailed map to the right shows clay minerals (green) in pore-spaces. (b) Mapping of the LTT sample, flooded from the left side with MgCl2 for 516 days. Two fronts of alterations are observed in the core. The minor blue and green pixels inside the calcite indicate a clay and magnesite precipitation front, while the blue area displays a front of complete transformation of the mineralogy from calcite dominated to magnesite. Note: A hole (white) and its apparent clay coating are artefacts of the sample preparation. (c) Mapping of the MLTT, flooded with MgCl2 and a certain period of time with CaCl2 and MgCl2 for together 718 days. Two fronts of alterations seem to have affected the core as observed in the LTT. (d) Mapping of the ULTT. The two fronts of alteration have assumingly moved through the core and a chemical process from calcite to magnesite has completely changed the core. The white circular holes are cutouts from the sample for further detailed analyses. Top: Legend for all SEM-MLA-EDX spectral images. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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However, occurrences of clay precipitation can be observed along the entire core sample towards the outlet (Fig. 9b, right) visualized by decreasing abundances of green pixels. The same can be observed for magnesite. The second front is observed in the inlet part of the flooded core (blue area towards the left, Fig. 9). This front is characterized by a high magnesium content and appears, at this scale, to consist of nearly only magnesite together with minute amounts of clay minerals, and is not observed as far into the core as the first front of clay/magnesite. The mineralogy seems to have been completely altered up until 21 mm into the core measured from the inlet (approximately 35% of the sample). Calcite cannot be identified in this specific area, indicative of a complete transformation from the primary mineralogy of calcite to new-grown phases; magnesite and clay. For the whole core, the latter has increased in abundance compared to unflooded material, from originally approximately 5%. The exact composition of these clay minerals is difficult to determine because of the small dimension of the grains (as described in Section 3.2). Additionally, the abundancy is too low to be detected by XRD. However, the composition is similar in all findings of these fine-grained silicates, with main constituents of silicon, oxygen, aluminium, magnesium and minor occurrences of iron, as also observed by FEMSEM-EDX (see Appendix B, Fig. B.2). Based on the semi-quantitative measurements of their relative concentrations, the clay is interpreted to be either talc (Mg3Si4O10(OH)2) or saponite (Ca0.25(Mg, Fe)3((Si, Al)4O10)(OH)2n (H2O)). In addition, a new grown phase consisting of oxygen, magnesium and silicon only, devoid of aluminium, has been observed in the flooded material (dark green,). These grains are small, and may be occurrences of talc, or hydrous silica together with the magnesite. The magnesite contains, in all crystals that were measured by SEM-EDS, small amounts (1–7.5 wt%) of calcium and smaller amounts of silicon. The transition between the second front and the remaining core material towards the outlet is sharp, but interfingering can be observed between the two mineral regimes (Fig. 9), the strongly altered consisting mainly of magnesite (blue) and the lightly altered with high amount of original calcite (red). 3.3.3. Medium Long Term Test (MLTT) sample (718 days) The MLTT was flooded with MgCl2-brine and for a certain period of time with a combination of MgCl2 and CaCl2 (see section 2 for details). Additionally, the pH of the brine was varied over the flooding sequence. The core from this experiment, the MLTT, shows a similar core-scale frontalteration pattern as for the LTT (516 days). The part furthest away from the inlet still contains significant amounts of calcite (red pixels), from 65% (slice 4) to 95% (slice 5) at the outlet, with decreasing abundances of highmagnesium carbonate and clay minerals towards the outlet (Fig. 9c, blue and green pixels). Again, as for the LTT; the 516 days’ test, the abundance of clay minerals is at maximum close to the transition zone between the calcium-dominated area and the magnesium-dominated area (green area).

The second front of the complete mineralogical transformation from mainly calcite to magnesium carbonate and clay minerals has a similar shape as in the LTT (516 days), however it seems to have propagated further into the core from the inlet, 36 mm compared to <21 mm in LTT (Fig. 9b and c). The composition is the same, magnesite with calcium impurities, clay minerals and talc, and an unaltered area towards the sleeve can be observed. 3.3.4. Ultra-Long Term Test (ULTT) sample (1072 days) The flooding experiments longing for 1072 days, and displays no fronts of different mineralogy throughout the core. The ultra-long time-span of the experiment and the fact that the flooding rate was varied between one and three PVs per day, have most likely enhanced the alteration processes. The entire core consists of a magnesium dominated carbonate, most likely magnesite (Fig. 9d). Occurrences of calcium and silicon can be observed in minerals, considered to be impurities in the magnesite. Calcite is absent in this core, similar to the areas of total transformation observed in the two earlier described experiments. It is assumed that the alteration front passed completely throughout the entire core (ULTT). 3.3.5. Detailed EDX spectral maps (GXMAP) All thin-sections were scanned in detail in a strip with a width between 400 and 500 mm along the centre of the flooded cores. In the transition-zone of LTT the mineralogy changes from the secondary front (blue, magnesite) through a zone with high density of silicates and/or higher porosity (Fig. 10a). Clay minerals are very fine-grained and the signal may also be the result of higher porosity in the area, but have in this mapping been classified as clay minerals (green). In the transition zone, calcite (red pixels) starts to occur, mostly related to the shells and fragments of microfossils, increasing in abundance away from the magnesite front. The size of the grains or crystals made up of the original mineral calcite also increases in the same matter, before the primary mineralogy takes over with the main constituent of calcite. The transition zone in MLTT (Fig. 10b), is similar to the one seen in the LTT, however with a more gradational trend from magnesite (blue) to the original calcite (red), as also seen in the overview image (Fig. 9b). In Fig. 10(c) a detailed section from the ULTT taken from the outlet-side of the core, is shown. No calcite (red) is observed, only magnesite (blue) together with different silicates, clay-minerals (containing Al, green) and talc (O, Mg and Si only, dark green). Talc is only found in the area of completely altered mineralogy (the secondary front), and not together with the original calcite, secondary Mgcarbonate and the clay-minerals in the primary front. Equal for all cores is the observation that micro-fossils such as foraminifera and shell fragments are preserved in the texture. These are observed with open pore-space inside, e.g. lower part of Fig. 10b), or filled with fine-grained silicates, such as in the centre part of Fig. 10c). In the transition-zones, and onwards in the cores of LTT and MLTT, the skeletons of these macro-fossils have kept their original calcite mineralogy, while the surrounding matrix

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Fig. 10. Detailed SEM-MLA-EDX spectral maps from the three flooded cores. The dashed lines and the T indicate the start of the transition zones progressing towards the top of the images with the flooding direction. (a) The transition-zone of the LTT from the secondary front (blue, magnesite) through a zone enhanced with silica (clay, green) into the primary front with the main constituent of calcite (red). (b) The transition zone from MLTT, similar to the LTT in (a), however with a more gradational nature from magnesite (blue) to the original calcite (red). (c) A detailed section from the ULTT taken from the outlet-side of the core. No calcite (red) is observed, only magnesite (blue) together with different silicates (green and dark green). For legend see Fig. 9. T = transition zone. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

has been altered to magnesium-bearing carbonates (e.g. top of Fig. 10b). Even though the transition zone of the two cores flooded for the shortest time cannot be seen as abrupt on mm scale as in mm and cm scale, the mineralogy changes from nearly pure magnesite to high amounts of clay and calcite over a short length, hundreds of mm. 3.4. Transmission Electron Microscopy (TEM) Seven samples were fabricated from the three cores using FIB-milling. This is undoubtedly not enough to analyse the grain-scale mineralogy of the entire cores, but allows for studying the true changes in mineralogy in these fine-grained rocks; one of the major objectives. Mineralogical alterations or incorporation of ions can even on grainscale and on smaller scale change the adsorption energy of the surface, thus altering the wettability (Andersson et al., 2016). Also, by studying the mineralogy at nano-scale, the scale of pseudomorphism may also be determined. STEM-EDX analyses from all studied samples show similar results. The areas which were strongest affected by chemical changes in all three tests are characterised by a complete alteration from calcite to magnesite (Fig. 11) and all analysed grains (images from the MLTT are presented here) display comparatively the same composition: magnesium (Mg, red pixels) together with carbon and

oxygen. In addition, smaller amounts of calcium (Ca, blue pixels) can be found, in the range of 1–4 wt% CaO. TEMEDX quantification-analyses (see Appendix B, Fig. B.3) were not done with the use of standards at University of Stavanger. However, the content of 1–4 wt% CaO was confirmed by standardized TEM-EDX analyses at the Institute of Planetary Materials (IPM), Okayama University, Japan (Egeland et al., 2017). The calcium is homogenously distributed within the crystals, and does not appear in separate grains or crystals. The calcium exists at random places within the magnesium-dominated grains. The newly formed Mg-rich crystals have an angular shape, and appear in varying sizes, from approximately 100 nm to one mm. The mineral is interpreted to be magnesite with calcium impurities, also identified by bulk X-ray diffraction and imaged by nano Secondary Ion Mass Spectroscopy (nanoSIMS, Zimmermann et al., 2015). Crystals with the same elemental composition can also be found inside the primary front towards the outlet of the LTT and the MLTT (Fig. 11c and d), where only small abundances of this Mg-rich mineral are identified. Magnesite-grains containing 1–4 wt% CaO are also present, together with a large amount of calcite grains. The calcite grains are more rounded, compared to the angular magnesite crystals. The size of these rounded grains is typically between one and two mm. The calcite grains appear in cases to be partly dissolved, with etch marks at the edges of crys-

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Fig. 11. (a) Bright field-STEM image inside the secondary front in the MLTT, slice 2. Newly precipitated magnesite grains (red) with an angular shape, containing small impurities of calcium (blue). For EDX spectra, see Appendix B, Fig. B.3. (b) the distribution of magnesium (red) and calcium (blue) in the crystals in (a). (c) Bright field-STEM image of the primary front in the MLTT, slice 2. Original calcite grains (blue) are rounded (marked with white arrows) and resemble the form of coccolithophore rings. To the right of the image, partly dissolved grains are observed (marked with black arrow). Newly precipitated magnesite grains (red) have a more angular shape and contain small impurities of calcium. (d) The distribution of magnesium (red) and calcium (blue) in the crystals in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1 d13CVPDB and d18OVPDB values along with calculated temperatures from the measured samples. d13C ratios does not seem to be disturbed by flooding, while d18O ratios are significantly altered and the calculated formation temperature often approximates the temperature of the flooded brine (see text for further discussion). LTT

MLTT

ULTT

Slice

d13C ‰

d18O ‰

Temp °C

Slice

d13C ‰

d18O ‰

Temp °C

Slice

d13C ‰

d18O ‰

Temp °C

Unflooded Slice 1 Slice 2 Slice 3 Slice 4 Slice 5 Slice 6

1.73 1.70 1.81 1.77 1.92 1.67 1.93


1.52
17.66
2.24
2.26
1.87
2.18
1.78

18.19 121.05 21.33 21.42 19.70 21.07 19.31

Unflooded In Unflooded Out Slice 1_M Slice 2_M Slice 3_M Slice 4_M Slice 5_M Slice 1_R Slice 2_R Slice 3_R Slice 4_R Slice 5_R

1.72 1.66 1.33 1.36 1.51 1.73 1.65 1.22 1.38 1.60 1.74 1.72


1.31
1.50
17.97
19.30
18.35
1.81
1.78
18.60
19.18
4.86
2.05
1.70

17.28 18.10 123.70 135.32 127.00 19.44 19.29 129.13 134.27 33.92 20.48 18.96

Unflooded In Unflooded Out Slice 1_M Slice 2_M Slice 3_M Slice 4_M Slice 5_M Slice 6_M Slice 1_R Slice 2_R Slice 3_R Slice 4_R Slice 5_R Slice 6_R

2.02 1.93 1.32 1.29 1.58 1.32 1.47 1.51 0.65 0.96 0.78 1.34 1.22 1.13


3.33
3.51
18.95
19.72
20.78
20.15
21.09
21.24
20.27
19.57
20.06
20.91
21.47
21.34

26.33 27.19 132.24 139.02 148.71 143.00 151.68 153.07 144.01 137.67 142.10 149.95 155.25 153.98

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tal planes. They are also often arranged in circular structures, resembling the original coccolithophore ring textures of chalk. 3.5. Carbon and oxygen isotopes Unflooded rock-material from the core display d13C and d O values (Table 1) as expected for onshore Lie`ge chalk (Hjuler and Fabricius, 2009), varying from 1.66 to 2.02‰ for d13C and
1.31 and
3.51‰ for d18O. However, samples from the flooded cores show extremely disturbed oxygen isotope values, down to
21.47‰, compared to the values for unflooded material (Table 1). The two cores flooded for the least amount of time, LTT and MLTT, show a trend from very low d18O values towards values comparable to unflooded material at the outlets. Again, as seen with MLA, there is an abrupt change in values from one slice to another, indicating a rather sharp transition between the significantly altered material (d18O values around
20.00‰) and values in the range of unaltered material. For the core flooded for 1072 days, the ULTT, all slices show severely altered d18O values varying between
18.95 and
21.47‰. For cores MLTT and ULTT, flooded for 718 and 1072 days, respectively, isotope measurements were performed on samples from the core centre (denoted _M) and the rim of the cores (denoted _R). The ULTT display similar values at the centre and at the rim, while the MLTT have significantly different values in slice 3,
18.35‰ in the centre and
4.85‰ at the rim, matching very well the pattern observed in SEM-MLA-EDX spectral maps of the core. Although calculations for paleo-water temperatures using oxygen isotopes are based on fractionation in open ocean seawater, and may not necessarily be directly applicable to laboratory experiments such as the ones performed here, it is interesting to estimate the formation temperature of the newly formed minerals in the flooded cores (Table 1). Based on Eq. (A.1), Section 1.4 in Appendix A (Anderson and Arthur, 1983), temperature values in the range of the experimental temperature are observed in the inlet (slice 1) of all cores. The values decrease towards the outlet as the d18O values, reaching temperatures expected of Cretaceous chalk in the region (Surlyk et al., 2010) towards the outlet for the two cores flooded for the least amount of time. Carbon isotope ratios does not seem to be disturbed for any of the samples measured and all values match those for Late Cretaceous chalk (Jørgensen, 1987). 18

3.6. Whole-rock geochemistry Whole rock geochemistry analyses has been performed on all slices of the three cores except for slice 2 of LTT, of which it was not possible to sample sufficient representative material. The overview over the constituents can be found in Table 2. Main observations include significant changes in CaO and MgO values. In slice 1 of all the tested cores, the MgO values have significantly increased from average

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value of 0.34 weight% (wt%) to values of 33.1, 38.0 and 42.1 wt% for LTT, MLTT and ULTT, respectively. A corresponding decrease in values for CaO follows this MgO increase, from an average value of unflooded material at 52.12 wt% to as low as 14.4 (LTT), 8.7 (MLTT) and 3.2 wt% (ULTT). For the LTT and the MLTT MgO content decreases together with an increase of CaO towards the outlet of the core, while in ULTT, the content of MgO and CaO stays close to constant throughout the core. SiO2 and Al2O3 values are relatively low in all measured samples with no repeating trends along the flooding axis of the three cores. For LTT, the highest SiO2 value, 4.7 wt%, is found in the first slice, a value in the range of unflooded material (4.39 wt%). The rest of the slices in this core have a SiO2 content ranging between 3.9 and 4.2 wt%. The Al2O3 content are in the first slice slightly higher than in the unflooded pieces (0.9 wt% compared to 0.7 wt%) and decreases towards the outlet of the core. Sr values follow the values of CaO, with lowered values in the first slices, increasing towards the outlet reaching similar values as unflooded material. MLTT has low contents of SiO2 and Al2O3 compared to unflooded material (4.39 and 0.7 wt%, respectively). The content fluctuates slightly in the flooded material, but the values are still within the natural variation of the rock. As for LTT, the Sr values correlate well with the CaO content. In the ULTT, the SiO2 and Al2O3 content vary slightly up and down along the flooded core but in general lie a little higher than unflooded material (SiO2: Unflooded: 4.39 wt %, flooded: 4.7–5.6 wt%, Al2O3: Unflooded: 0.7 wt%, flooded: 0.8–0.9 wt%). Again, Sr content follows the values of CaO. 3.7. Effluent composition During the three experiments, the effluents were sampled regularly to produce profiles based on the concentrations of the following ions: Ca2+ and Mg2+. Na+ and Cl
were also measured as NaCl was used as flooding brine for shorter periods. Cl
was used as calibration, as Cl
is considered inert to the system. The concentration of Cl
-ions in the effluent should therefore match the concentration injected. In the profile of the LTT, also discussed in Zimmerman et al. (2015), Cl
-concentrations are stable throughout the experiment fluctuating slightly, with values between 0.435 and 0.450 mol/L. Fig. 12 shows how the concentration of Ca2+ and Mg2+ varies over the time of the experiment. For the first 20 days of flooding a calcium-peak is observed, starting at a concentration of 0.065 mol/L before the production of calcium decreases and stabilize at 0.020 mol/L. This concentration increases slightly towards the end of the experiment, where the value is 0.22 mol/L. Complemtary to the primary peak in calcium, a loss of magnesium is found, where the concentration starts at 0.15 mol/L and increases over the next 20 days to 0.19 mol/L. This value is constant throughout the experiment, so that the sum of the Ca2+- and Mg2+-concentrations adds up to match the injected concentrations of magnesium, 0.219 mol/L. The MLLT displays a similar effluent profile as the LTT. However, this test was flooded with NaCl for the first

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Unit

SiO2 %

Al2O3 %

MgO %

CaO %

Sr ppm

TOT/C %

Rb ppm

Zr ppm

Y ppm

MDL

0.01

0.01

0.01

0.01

0.50

0.02

0.10

0.10

0.10

Unflooded Average

4.39

0.70

0.34

52.12

1032.38

11.69

6.00

13.48

7.75

LTT 516 days Slice 1 Slice 3 Slice 4 Slice 5 Slice 6

4.68 4.05 3.94 3.93 4.22

0.89 0.68 0.71 0.71 0.73

33.03 6.99 3.88 3.18 3.03

14.43 45.58 49.42 49.87 50.15

244.20 815.60 876.00 1019.10 1015.60

12.53 11.61 11.47 11.48 11.55

3.70 2.70 2.90 2.80 3.00

18.00 15.60 14.90 14.00 13.90

8.70 8.30 6.90 8.40 8.90

MLTT 718 days Slice 1 Slice 2 Slice 3 Slice 4 Slice 5

2.71 2.53 2.34 2.00 1.87

0.47 0.44 0.40 0.36 0.36

38.02 35.00 24.86 7.98 2.27

8.70 12.78 25.02 45.52 52.17

113.70 149.50 370.70 803.20 955.60

13.68 13.38 13.18 12.19 12.14

1.70 1.10 1.00 0.90 0.90

8.00 8.00 8.70 9.00 8.40

9.50 9.80 7.80 7.60 7.80

ULTT 1072 days Slice 1 Slice 2 Slice 3 Slice 4 Slice 5 Slice 6

4.85 5.15 5.58 4.97 5.04 4.74

0.87 0.82 0.87 0.81 0.86 0.80

42.12 41.96 40.95 41.82 41.00 41.62

3.15 3.57 4.19 4.00 4.30 3.95

4.80 4.50 5.60 5.90 6.40 6.30

13.38 12.84 12.88 12.95 13.05 13.10

3.10 2.10 1.90 1.90 1.80 2.10

17.10 16.50 18.30 16.80 16.20 16.70

9.50 8.40 11.10 10.40 9.70 9.80

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Table 2 ICP-MS measurements of all slices of the three cores, except for slice 2 of the LTT. All major elements are relatively undisturbed by the flooding processes, except for MgO and CaO. MgO increases to levels from 33.0 and 42.1 wt% in the first slice of the flooded cores, with a corresponding decrease for CaO. For the two cores flooded for the shortest time, values change towards the outlet of the core, to values similar to what is found in unflooded material. For the ULTT, the heavily altered MgO and CaO values are more or less constant throughout the core.

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Fig. 12. Effluent profile with time for a) the LTT, showing the concentration of Ca2+ (yellow curve), Mg2+ (red curve), Cl
(green curve) along with the sum of Ca2+ and Mg2+ (blue curve), (b) the MLTT, showing the concentration of Ca2+, Mg2+ along with the sum of Ca2+ and Mg2+ and Cl
. Modified after Megawati et al. (2011), and (c) the ULTT, showing the concentration of Ca2+, Mg2+along with the sum of Ca2+ and Mg2+ and Cl
. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5 days of creep. Therefore, a small calcium peak is found at the start of the NaCl flooding, due to cation exchange (Fig. 12b). This peak has a value of 0.04 mol/L and goes quickly down to zero during the five days of flooding. When the MgCl2-flooding commenced, a new and larger peak in the calcium production can be observed, with a maximum value of 0.039 mol/L. The concentration Ca2+ in the effluent then gradually reduces over the next 35 days, to a minimum of 0.016 mol/L before it starts to increase. The increase continues throughout the experiment, ending at a value of 0.027 mol/l. During the period between 102 and 165 days, 0.130 M CaCl2 was added to the flooding brine, and there is an immediate response in the calcium production. The Ca2+ starts to increase, but does not reach 0.130 mol/l before 105 days. At 165 days, when the brine is switched back to 0.219 M MgCl2 the Ca2+ concentrations reduces to 0.022 mol/L and continues the increasing trend as seen prior to day 102. Again, as seen for the LTT, the Mg2+ concentration profile mirrors the profile of Ca2+, but with opposite directions. The magnesium production starts at 0.201 mol/L, and with smaller fluctuations, increases over the next 35 days to a level of 0.203 mol/L. The concentration then gradually decreases towards 102 days, when the addition of 0.130 M CaCl2 commenced. At this stage, a small peak in the Mg2+-concentration is observed, matching the delay in Ca2+ production. This peak has a maximum value of 0.262 mol/L. During the period as CaCl2 is added to the MgCl2 brine, the magnesium production lies steady on the same value as in the injection-brine; 0.219 mol/L. When this period is finished, at 165 days, an immediate drop in Mg2+ concentration is observed (0.178 mol/L), before the value continues the decreasing trend as seen prior to the first 102 days, starting at 0.197 and ending at 0.193 mol/ L. Again, the sum of Ca2+ and Mg2+ in the effluent, matches the sum of these ions in the injected brine. The test which lasted for the longest period of time, the ULTT (1072 days, also discussed in Nermoen et al. (2015)), also follows the trends of LTT, with Ca2+ peaks initially as NaCl and MgCl2 flooding commenced, at values 0.130 (0 days) and 0.076 mol/L (7 days), respectively (Fig. 12c). The concentration then decreases until it stabilizes at 0.028 mol/L after 20 days. The concentration is quite stable until 56 days of flooding where an 11 day period of

flooding with distilled water started. During this time, effluents were not collected. When the MgCl2-flooding commenced again, at 67 days, a small peak (0.042 mol/L) is observed, before the production of calcium again stabilizes around 0.028 mol/L. This trend continues the next 400 days, but during this time, the flooding rate was changed twice, and this change has an immediate response on the calcium production. After 112 days, the flooding rate was changed from one initial PV/day to three initial PVs/day. The Ca2+ concentration is reduced to 0.023 mol/L and continues at this level until the flooding rate is changed again after 368 days of flooding. Changing the rate from three PVs/day to one PV/day increases the Ca2+ concentration to 0.030 mol/L and shows how sensitive the calcium production is to any changes in flooding rate. From 400 days the Ca2+ concentration steadily decreases over the next 500 days, where the concentration is near zero (900 days). From this point, until the end of the experiment (1072 days) the calcium production is negligible. Even though the calcium concentration is reduced in the samples from the period of flooding with three PVs/day, this does not mean that the calcite dissolution is less per day. On the contrary, if added up, the net production of calcium per day increases during this period, as the flow rate of the effluent is multiplied by three, and the cumulative production during one day of flooding adds up to a higher number (Nermoen et al., 2015). The strain rate (Fig. 2) also increases at the same point where the increase in production of calcium can be observed. After 800 days, the flooding rate is again increased to three PVs/day. At this point, a slight fall in the calcium production may be observed, however, as the production already is very low, this is hard to measure. As in the other two experiments, the magnesium concentration in the effluent from the ULTT, follows the production of calcium, but in an opposite manner. However, the same dependency on flow-rate is not observed for Mg2+ (Nermoen et al., 2015). The overall trend throughout the experiment is an increased magnesium-production, matching the decrease in calcium production. At the end of the experiment, the Mg2+ concentration in the effluent, matches the concentration in the injected brine, indicating no significant dissolution and precipitation or other surface processes taking place in the core.

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4. DISCUSSION This study clearly confirms that there are two alterationfronts moving through the cores during such long-term injections of non-equilibrium brines under reservoir conditions. The seemingly first front, which has progressed furthest into the cores, consists of a combination partially dissolved calcite grains and newly grown magnesite grains. Additionally, silicates, possibly chrysotile, tilleyite (Zimmermann et al., 2015), talc, chlorite or saponite, are observed together with primary clay minerals. For the cores LTT and MLTT, this primary front has affected the cores until the outlet of the samples. At core scale, this transition zone can appear to have a dolomitic composition, however neither TEM-studies, Raman-analyses (Borromeo et al., 2018) nor XRD analyses (Zimmermann et al., 2015) can confirm this observation. On the contrary, only calcite and magnesite were identified at the nano-scale, indicating that the observation of dolomite is rather an effect of limitations in resolution of the analytic technique. As such, this study finds no gradual transitional phases, such as dolomite, between calcite and magnesite in the studied samples. Dolomite can be considered as alternating layers of CaCO3 and MgCO3 at atomic level, either ordered or disordered, but should at the resolution it has been analysed at in this study, appear as uniform mineral with the formula CaMg (CO3)2. Dolomite is often observed in reservoir chalk (Hjuler and Fabricius, 2009), however, seldom in outcrop chalk, and has proven very difficult to produce in laboratory experiments conducted below 150 °C (Anovitz and Essene, 1987). In addition, for dolomite to form the Ca/ Mg ratio has to be high enough, meaning that the dissolution of calcite has to provide more Ca2+ in the solution than the access of Mg2+. When the Ca/Mg ratio is low, magnesite (MgCO3) primarily forms (Jonas et al., 2015), and the high porosity of these chalks may allow for constant supply of Mg2+ in the solution during flooding, keeping Ca/Mg ratios low. When considering the stoichiometric equations for the given carbonate minerals (Eqs. (1)–(3)), the given temperature and pressure conditions thermodynamically favour dissolution of calcite (CaCO3) and precipitation of magnesite (MgCO3), not dolomite (CaMg(CO3)2). No crystals were observed with gradual changes in calcium and magnesium content, all studied crystals have a homogenous composition dominated by magnesium carbonate with calcium impurities, in accordance with Egeland et al. (2017). As the crystals were precipitated out of a fluid with high Ca-concentration after dissolution of calcite, and the mentioned Ca impurities are therefore not unlikely to be found in the crystal lattice. The observations point to a process of mineralogical transformation that is not driven by solid-state diffusion into the original calcite grains, but rather a process of dissolution and precipitation also described by Putnis and Parsons (2002), Putnis (2009), Kasioptas et al. (2011), Ruiz-Agudo et al. (2013) and Jonas et al. (2014). Oxygen isotopes are severely disturbed for the carbonates in the magnesite dominated mineralogy. This indicates considerable exchange of oxygen isotopes with addition of the lighter 16O on the expense of 18O. Subsequently, it

can be implied that; (i) during the transformation from calcite to magnesite, the carbonate ion (CO
3 ) is free to exchange atoms from the fluid, whether the transition happens through dissolution and precipitation or by diffusion; (ii) the oxygen isotope values are altered either due to fractionation processes related to temperature, or by the initial 18 O/16O ratio of the injected water, which decreases when adding salts such as MgCl2 to pure water (Hoefs, 2015). The severe changes in the stable oxygen isotopes provide evidence that the mineralogical alterations are part of the rock-fluid interactions, and as the amount of altered isotope values follow the amount of MgO and CaO observed in whole-rock geochemistry analyses, they may provide a tool to determine the degree of mineral transformation in a carbonate. The substitution between calcium and magnesium in the cores as a net value is confirmed by all the analytical methods in this study. Selected values from whole-rock geochemistry and stable isotope analysis (d18O) are plotted in Fig. 13, superimposed on the MLA maps of the three tests. For all cores, the values of CaO (red curve) and MgO (black curve) have a negative correlation and the crossover between the two values moves further towards the outlet of the cores with longer flooding time. For ULTT it is assumed that this ‘‘front” has travelled completely through the core, and no cross-over is observed. As data for slice 2 in LTT is missing, this part of the curve has been fitted with a dashed line. The isotope data (green curve) follows mostly the trend as the MgO, with more negative d18O isotopes, indicative of higher formation temperatures, matching the amount of MgO along the flooding axis of the cores. These observations, along with the effluents of the ULTT indicate that before the flooding with MgCl2 was ended, all of the calcite was dissolved, and the core transformed completely to magnesite with minor impurities of calcium. After 900 days of flooding, the calcium concentration in the effluent is close to zero, and this should therefore be the point where the calcite is gone, taken flooding rate into account. The formation of the secondary front is, based on the data from these experiments, dependent on dissolution of CaCO3 and precipitation of MgCO3. This implies that the fluid has been under-saturated with respect to CaCO3 and yield over-saturation with respect to MgCO3. The sum of Ca2+ and Mg2+ at the outlet is throughout the experiment equivalent to the concentration of injected Mg2+, consistent with a one-to-one exchange between the two ions. The values vary with changes in flooding rate. Measurements of carbonate content in the effluents were not possible with the used methodology, but core-scale modelling by e.g. PHREEQC performed on several short-term experiments over few months (e.g. Madland et al., 2011; Andersen et al., 2018; Andersen and Berawala, 2018) shows that the effluents are not in equilibrium at the outlet of the cores under the experimental conditions. Calculations on massbalance, which are consistent with the models and changes in chemical composition in the tested cores have also been performed (e.g. Nermoen et al., 2015; Andersen et al., 2012, 2018). The rate of propagation of the two fronts is difficult to quantify exactly, due to the differences in the flooding pro-

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Fig. 13. Graphs showing the chemical alterations along the flooding axis of the three long-term tests, superimposed on the MLA-maps of the respective cores. MgO-values (black curves) are increased and CaO-levels (red curves) and d18O isotopes (green curves) decreased accordingly in the completely altered areas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

cedure of the three cores. Moreover, such long-term studies are rare, and it is difficult to find alternative experiments for calculating these rates in porous media. We present therefore here some estimates. The calculations are done, based on the position of the secondary magnesite front, measured at the centre of the core (see Figs. 1 and 9). In the LTT, flooded for 516 days, the secondary front has travelled approximately 21 mm into the core from the inlet. This is equivalent of 40.1 mm per day. For the MLTT these numbers are 36 mm over 718 days, corresponding to a rate of 50.1 mm per day. However, one could assume that during

the period of adding 0.130 M CaCl2 to the brine, no mineralogical alterations took place in the core, and that the rate therefore is higher. For the ULTT the two fronts have completely passed through the core (at least 62.47 mm over 1072 days, approximately 58.3 mm per day). As this core was exposed to varying flooding rates, alternating between one and three PVs/day, it can be assumed that the fronts have travelled at a higher spatially rate in this core than in the other ones. These rates can therefore only be considered as guidelines. The fronts most probably do not propagate through the cores in a linear manner, and the two

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fronts display different alteration behaviour. The exact mechanisms of which parameters control this propagation rate should therefore be discussed in detail in future work. One of the most compelling findings in these analyses are the sharp fronts between the two mineral regimes in cores LTT and MLTT. In the MLA-maps (Fig. 9b and c), this transition is abrupt, with a transitional zone covering less than 4 mm. Such sharp transitions have also been observed by others in single-crystal-experiments without fluid-flow e.g. (Putnis and Parsons, 2002; Jonas et al., 2014, 2015) and described in transport driven transformation in mineral systems sharing a common ion (Kondratiuk et al., 2015, 2017), like in our case. Reaction fronts or transition zones are also observed in nature during processes such as metasomatism (Merino and Banerjee, 2008). The transition zones in our study are characterised by a gradual decrease of magnesite together with an increase of calcite. Such a transition is expected, but an increase in clay and/or porosity however, was not anticipated, and further investigations are thus needed. This seemingly accumulation of Si-rich phases could originate from several sources. One could speculate in a process involving precipitation of clays when an excess of magnesium is available in the fluid due to a decrease of magnesite formation. Alternatively, again the phase may be hydrous silica or most likely a result of an increase in porosity in the area, causing the clay in this area to be more visible. Complementary analyses by whole-rock geochemistry does not show an increase in SiO2 in the relevant slice of the two cores, as such the most likely explanation may therefore be an increased porosity in the area in question. When handled in the laboratory, the slices of the tested cores, have a habit of dismantling exactly at the observed transition zones. Similar high porosity zones are also observed by e.g. Putnis (2009) and Jonas et al. (2015) and is the explanation of how mineral replacement through dissolution and precipitation is possible in non-porous crystals. As discussed above, Kondratiuk et al. (2015, 2017) present a model for transformations with a sharp front between primary and secondary mineralogy, which also explains the presence of a zone with higher porosity, caused by differences in rates of dissolution of the primary mineral and precipitation of the secondary mineral. The alterations observed in the three long-term tests fit very well with this model, and in the case of these tests, clay minerals also seem to play a role in this interplay. Putnis and Putnis (2007) describe the mechanisms of pseudomorphism, where the transformation of the crystal is driven by a thin, porous layer surrounding the crystal with a different equilibrium state than the surrounding fluid which enhances the coupled dissolution and precipitation process locally. Throughout all of the analysed cores, remnants of macro-fossils such as foraminifera and shellfragments commonly found in chalk, can still be observed. These are, in the transition zone and the primary front, still consisting of original calcite, and, if not containing open pore-space inside, filled with fine-grained silicates and/or magnesite. In the secondary front, the mineralogy of these fossils has been altered into magnesite. However, the fossil shape has in many cases been preserved. As found in the

fossils still consisting of original calcite, these ‘‘pseudomorphic fossils” are also filled with either open pore-space or silicates and/or magnesite. These observations are similar to what was observed in Minde et al. (2016), where the original texture of the chalk is proved to have a significant effect on the fluid-flow. This seems to also be the case in these long-term experiments. An example of this is found in the ULTT, where a foraminifera-fossil is preserved, but the composition is magnesite with calcium impurities, the same as the surrounding matrix (Fig. 14). Pseudomorphism is not uncommon in geological processes, often seen in e.g. stromatolites (Kremer et al., 2012), and is described by several authors based on inorganic-crystal experiments in still-standing fluids, e.g. (Jonas et al., 2015; Putnis, 2009). Our three experiments were carried out under continuous fluid-flow through the cores and can therefore not be directly compared to the results from e.g. Putnis and Putnis (2007) and Jonas et al. (2015). However, there are certain similarities. Jonas et al. (2015) describes diffusion and transport rates of the ions in the fluid to be the controlling factor on how the mineral replacement takes place, which is also interpreted to be a factor here in this study. In the first front, with only minor precipitation of magnesite within the original calcite, it seems that the diffusion- and transport-rates are sufficiently high to limit a local coupled dissolution and precipitation processes In the secondary front, the process seems to be different from the primary front. Here, macrofossils have an altered mineralogy, while the texture is preserved, described as pseudomorphism. Xia et al. (2009) have an explanation of this phenomenon: As long as the dissolution rate of the primary mineralogy is the rate-limiting factor, nanometrescale pseudomorphism is possible, enabling precipitation of the secondary mineral as soon as there is dissolution.

Fig. 14. SEM-BSE micrograph of a shell of a foraminifera preserved in the ULTT, app. 0.5 cm from the outlet (see Fig. 1). The shape of the fossil is preserved, but the mineralogy consists of magnesite with calcium impurities. The chambers are partially filled with silicate and magnesium bearing mineral phases.

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If the precipitation-rate of the secondary mineral is the ratelimiting factor, pseudomorphism is visible at a larger scale, typically around 10 mm, which is also the case in the three flooded cores. It may be that the processes at play in the flooded chalk cores are the same as in single-crystal experiments. However, the chalk cores being porous media, the alteration happens in two different stages and the transition between the primary and the new mineral regime is more gradational than seen in a non-porous crystal. A complete understanding of these alteration-mechanisms by analysing the cores after flooding is difficult, and further experiments, where in-situ observation is possible, would greatly contribute to a complete understanding. Last, but not least, it needs to be evaluated how the changes in mineralogy affect the geo-mechanical properties of the cores. It is clear that injection of MgCl2 enhances the strain of chalk, tested at core-scale, also proved in many earlier studies (Hellmann et al., 2002; Madland et al., 2006, 2008, 2011; Korsnes et al., 2008; Wang et al., 2016; Andersen et al., 2018; Minde et al., 2018). There are most likely several processes at play affecting the compaction of these cores, but dissolution and precipitation definitely play a major role. Magnesite precipitates at the expense of calcite, and magnesite has a higher density than calcite, thus filling less volume in the core. This could enhance compaction, as well as increase porosity and permeability if the bulk volume is kept constant. This is consistent with what is observed in the two cores flooded for the longest time-periods. For both, the permeability is significantly reduced during the primary phase of creep, the first 300 days and 100 days for the MLTT and the ULTT, respectively. Thereafter, the permeability starts to increase again. This change in behaviour may be explained by different processes playing major roles at different times, (see Nermoen et al., 2015). During the primary phase the permeability is reduced due to compaction of the core. Several factors may cause the subsequent increase in permeability, however, we believe the main factor is mineral replacement through dissolution of calcite and subsequent precipitation of the denser magnesite. The MLTT was for a period between 102 and 165 days flooded with a combination of 0.219 M MgCl2 and 0.130 M CaCl2. During these 63 days, the permeability and strain evolution flattened out together with loss of additional calcium production and magnesium retention in the core. These are all evidences that the mineralogical processes, which enhance compaction and permeability increase, were non-existent in the core during the mention period. When comparing the two cores flooded for 2 (MLTT) and 3 (ULTT) years it can be observed that addition of Ca2+ to the non-equilibrium brine postpones the chemical reactions in the core, while increasing the flooding rate from one to three PVs/day accelerates these reactions, also provoking additional creep, displaying a correlation between dissolution of calcite and the strain rate. During the long time period of these two experiments, the processes of dissolution/precipitation and compaction seem to even out, leaving the permeability at a stable level. All these observations show that by manipulating the composition of the flooding brine, it is possible to control the chemical reactions in chalk, hence the evolution of geo-mechanical properties.

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Any changes in mineralogy, composition of effluent fluids and stress-strain relations together with the rate these changes occur are used to build core-scale models, which in turn are used when up-scaling to full field scale. 5. CONCLUSIONS This study shows that during long-term (1.5–3 years) flooding experiments studying the interaction between non-equilibrium brines (MgCl2 and mixture of MgCl2 and CaCl2) and chalk, two different fronts of alteration travel through cores of Cretaceous onshore chalk. The first front seems to affect the core samples rather rapidly, and is seen as partial dissolution of calcite and precipitation of magnesite crystals, containing minute calcium impurities (1–4 wt% CaO). However, because of the very fine-grained nature of chalk, the true composition of the minerals can only be verified by studies of higher spatial resolution. Additionally, minor occurrences of secondary clay-minerals are seen. The first front has been observed to affect the entire core in short time spans, such as months in previous studies (Madland et al., 2011; Megawati et al., 2015; Andersen et al., 2018). The second front travels much more slowly and induces a complete transformation from calcite to magnesite with the mentioned impurities. The transition between these two fronts is sharp and below 4 mm wide, in all samples where it occurs. Additionally, this front is not homogenous within the diameter of the core, which is of utmost importance for modelling purposes. The front of complete transformation from calcite to magnesite travels at a rate between 40 and 60 mm per day, however, the propagation of this front is not linear and must be linked to geo-mechanical behaviour. Interfingering between the magnesium-dominated and the calcium-dominated areas occurs, and the inhomogeneous boundary may be explained by the micro-scale texture in chalk in terms of orientation, geometry and form of calcite grains. Use of data from flowthrough experiments should therefore be interpreted with care when used for modelling and up-scaling. However, analyses of the chemical fronts, both axially and laterally will help in constraining models and simulations of such experiments. The transformation from calcite to magnesite cannot, based on the data in this study, be described as having a transitional nature, e.g. through stages of dolomite and high magnesium calcite. Additionally, no diffusion of magnesium into calcite crystals or vice versa can be observed, hence the processes of alteration must be driven by dissolution and subsequent precipitation, not solid-state diffusion. The rate of precipitation seems to be the limiting factor, thus forming pseudomorphic texture on micron scale (Xia et al., 2009). As such, macro-fossil shells, and not nanofossils, are preserved in texture. Macro-fossils also seem to be the last constituents to dissolve in the core, most likely due to differences in primary composition, structure, shape and/or crystal orientation. Between the two mineral regimes of partly altered and completely altered mineralogy, the transition zone is sharp, with a higher porosity than the rest of the cores. This is similar to observations in single crystal experiments, as described by Putnis and

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Putnis (2007) and in systems with fluid-flow (Kondratiuk et al., 2015, 2017). Additionally, oxygen isotope ratios are severely disturbed by rock-fluid interaction and are closely linked to the amount of MgO and CaO present after mineral alteration, and may therefore be used as a tool to evaluate the degree of mineral transformation in chalk. The mineralogical changes can be directly linked to the changes in the geo-mechanical behaviour of the cores. The transformation of mineralogy from calcite to magnesite enhances the permeability after a primary phase of compaction and reduced permeability of the MLTT and the ULTT. Additionally, changes in brine composition, e.g. addition of CaCl2, directly impact the compaction, dissolution and precipitation, thus the permeability evolution during flooding. Changes in flooding rate also impact these factors. These results, together with data from previous studies, show that by manipulating the composition of the flooding brine, the chemical reactions in chalk may be controlled, hence the evolution of geo-mechanical properties. The major implications of our results are that the identification of mineralogical changes and their rates and the distribution of these alterations can be directly inserted into poreand core-scale models to constrain models and aid in finding the mechanisms, which are of high importance during upscaling, playing a significant role for field-scale EOR-behaviour. It is shown that several scales need to be studied to achieve these results. A combination of different methods, such as MLA and TEM, give important results for such exercises. ACKNOWLEDGEMENT We thank the editor and the four reviewers for thorough comments to enhance the manuscript. The authors acknowledge the Research Council of Norway and the industry partners, ConocoPhillips Skandinavia AS, Aker BP ASA, Va˚r Energi AS, Equinor ASA, Neptune Energy Norge AS, Lundin Norway AS, Halliburton AS, Schlumberger Norge AS, Wintershall Norge AS, and DEA Norge AS, of The National IOR Centre of Norway for support. In addition, the authors would like to thank Pricille Cuvillier for help with TEM studies. The research presented is integral part of the PhD thesis of Mona Wetrhus Minde. FUNDING This work was funded by The National IOR Centre of Norway and the Research Council of Norway.

APPENDIX A. SAMPLE MATERIAL AND METHODOLOGY 1. Sample material The cores that were sampled for this study are all taken from the Gulpen Formation exposed near Lie`ge in Belgium and belongs to the basal succession, the Zeven Wegen Member. This chalk is one of the outcrop chalks considered to be a representative geo-mechanical analogue to the reser-

voir chalk found at Ekofisk (Hjuler and Fabricius, 2009). It is of Campanian to late Early Maastrichtian age (Molenaar and Zijlstra, 1997) and has clean nature in mineralogical terms. The non-carbonate content is low, approximately 5 wt% and consists of quartz, smectite/mixed smectiteillite layer, mica and clinoptilolite as well as apatite, feldspar, pyroxene and titanium oxide (Hjuler and Fabricius, 2009). The chalk commonly displays medium preservation of coccolithophores and good preservation of pore-space. No calcite cementation can be observed, however, scarce amounts of contact cement is commonly found in studies on this type of chalk (Hjuler and Fabricius, 2009), pointing to a low degree of diagenesis. The initial porosity of the samples was between 40 and 43%. 1.1. Field Emission Gun Scanning Electron Microscopy (FEG-SEM) FEG-SEM analyses were performed at the University of Stavanger (Norway) with a Zeiss Supra VP 35. Pieces of freshly chipped off rock from core-slices of the three cores together with unflooded chalk were used to study unaltered surfaces of rock. Additionally, to map the extent of chemical alteration, thin sections were studied. The samples were coated with palladium or carbon and parameters of the microscope were set to an acceleration voltage of 15 kV, 30 mm aperture and a working distance between 10 and 11 mm. High current setting was used. For imaging of the samples the Secondary Electron (SE) detector was used, while a Back-Scattered Electron (BSE) detector was used to provide an image with different grey-scale values based on the Average Atom Number (AAN) of each mineral. A mineral with higher AAN will yield a brighter grey in the image. For Energy Dispersive X-ray spectroscopy (EDX), providing semi-quantitative elemental analyses, an EDAX detector was used. To calibrate the EDX, an Island spar calcite crystal was used, along with a dolomite standard (provided by Astimex). 1.2. Mineral Liberation Analyzer (SEM-MLA) The SEM-MLA analyses were carried out at Technische Universita¨t (TU) Bergakademie Freiberg, Germany. The system was set up with a SEM-type FEI Quanta 650 together with two Bruker X-flash EDX detectors. The software controlling the EDX-analyses is Quantax Esprite 1.8. Imaging and analyses were performed using 15 kV acceleration voltage and 12 mm working distance. The mineral liberation analysis software version MLA 2.9 by FEI was used for the automated SEM analyses. MLA is a technique that combines imaging by Scanning Electron Microscopy-Backscattered Electron (SEM-BSE) analyses and X-ray mineral identification by Energy Dispersive X-ray Spectroscopy (EDX) to perform a spectral mapping of polished samples, creating color-coded maps of the mineralogy. The minerals in the samples are identified and characterised through comparison of their EDX spectra to a list of reference spectra by a ‘‘best match” algorithm. The prepared samples analysed were polished thin sections, coated with carbon. Each single EDX spectrum was

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classified based on elemental composition as a certain mineral or mix of minerals, labelled by generic names and assigned to a colour, and added to the project database. This EDX-spectrum/mineral is coupled to the average greyscale value of the BSE image and as the average atomic number of each mineral phase corresponds to the number of backscattered electrons from a sample, the average greyscale-value will therefore be unique to this mineral (Fandrich et al., 2007). To ensure good measurements of the chalk samples with predominant calcite, the greyscale was calibrated with a copper-standard. After the BSE images were scanned, processing of the data is required. The first step was particulation; removal of background based on a minimum BSE greyscale level. Anything below this threshold, in this case, the epoxy resin or air bubbles, was removed from the image. Subsequently, segmentation took place, where grain boundaries and internal structures were defined based on BSE characteristics. After segmentation, classification of the minerals present in the area of interest was performed. The measurement mode used for this project was GXMAP mode, based on a primary identification of particles through BSE imaging, then X-ray mapping of each particle in a pre-defined grid, collecting the spectra of characteristic X-rays at each point within this grid. This allows for high spatial resolution and avoids limitation by poorly defined grain boundaries in BSE images caused by similar average atomic number of minerals (Fandrich et al., 2007). The scanned areas were coupled to the EDX-spectra database for classification of minerals in the sample. In this way, color-coded maps of the mineralogical distribution of the surface were produced, enabling evaluation of spatial textural and chemical composition in one process. The resolution of the SEM-MLA scanning depends on the size of the area scanned and the time used, but will always be constrained by the electron-beam spot-size (1– 2 mm) and the corresponding excitation volume. In this case, all cores were scanned at two resolution settings, one for an overview scan, with a single EDX analysis step-size of 12 mm, and a second for more detailed investigation with a step-size of 1.2 mm. A certain number of grains or crystals will not be identified based on the mineral database used, and are counted as ‘‘unknown”. To produce an adjusted image of the mineral distribution in the sample, the different spectra assigned to clay minerals (e.g. with EDX peaks for Al and Si) have been grouped into one colour and the measured dolomite has been grouped together with magnesite. 1.3. Transmission Electron Microscopy (TEM) Sample preparation of brittle chalk for TEM is a challenging task, because of the demands for extreme thin samples and possible undesired smearing effect if polishing of such small samples is used. The samples were therefore produced by Focused Ion Beam (FIB) milling in a scanning electron microscope (FIB-SEM) at Saarland University, Germany. Samples of approximately 20  10 mm were cut by a gallium ion-source gun, thinned to approximately 100 nm thickness by the same ion source, and welded with gallium to a copper grid.

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For the LTT and the MLTT (flooded for 516 and 718 days, respectively), two samples were collected from each core. From the LTT, one sample was picked from slice 1 and one from slice 4, near the centre of the core (Fig. 1), while from the MLTT two samples were taken from slice 2; one inside the completely transformed area, and one outside (Fig. 9c, second slice from the left, green area). Additionally, several FIB-lamellas were collected from the ULTT (flooded for 1072 days). A JEOL JEM-2100 TEM at the University of Stavanger was used with acceleration voltage of 200 kV together with a convergent beam in Scanning Transmission Electron Microscopy (STEM) mode. For X-ray elemental identification, an EDX detector (EDAX) was applied to create colorcoded maps of the elemental composition of the samples. The high magnification and small beam-size of TEM enables imaging and EDX-mapping below grainsizeresolution, at a resolution below 20 nm. 1.4. Carbon and oxygen isotopes Carbon and oxygen stable isotopes were analysed at the John Murray Laboratories at the University of Edinburgh. For each sample, a fresh surface was drilled twice to produce two samples of fine powder, one at the centre of the core and one at the rim of the core. Oxygen and carbon stable isotope analyses were performed on crushed 1.0– 3.0 mg sub-samples. The carbonate powder was reacted with 100% orthophosphoric acid at 90 °C in an ISOCARB automatic carbonate preparation system. The resulting CO2 was then analysed on a VG Isogas PRISM III stable isotope ratio mass spectrometer. The standard deviation (n = 41) of a powdered coral laboratory standard (COR1D, d13C =
0.648, 18 d O =
4.920) run as a sample on the same days as the study samples, was ±0.04‰ for d13C and ±0.06‰ for d18O. All carbonate isotopic values are quoted relative to the Vienna Pee Dee Belemnite (VPDB). Based on d18O, the formation temperature can be estimated according to the empirical relation described in Anderson and Arthur (1983) and Go`mez et al. (2008), T ¼ 16:0
4:14ðdc
dw Þ þ 0:13ðdc
dw Þ2

ðA:1Þ

where dc attains the values of the oxygen isotope (d18 O), dw ¼
1‰ according to the assumption Standard Mean Ocean Water (SMOW) of the ambient Cretaceous seawater (Wright, 1987) (‘normal’ salinity). The estimated temperature is given in °C. 1.5. Whole-rock geochemistry (Inductive Coupled Plasma Mass Spectrometry, ICP-MS) Samples of minimum 5 g were carefully separated from all slices from the three cores. For the LTT and the ULTT samples were milled in an ultraclean agate mill to a fine mesh, while for the MLTT, the samples were milled at Bureau Veritas Acme labs (Canada), where the ICP-MS was performed on all three cores. The milled sample was mixed with LiBO2/Li2B4O7 flux in crucibles and fused in a furnace. The cooled bead was dissolved in ACS grade nitric acid and analysed by ICP-MS. Loss on ignition (LOI) was determined by igniting a sample split then mea-

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suring the weight loss, as the 1 g sample was weighed into a tarred crucible and ignited to 1000 °C for one hour, and then cooled and weighed again. The loss in weight is the LOI of the sample. Total carbon and sulphur were determined by the Leco method. Here, induction flux was added to the prepared sample and then ignited in an induction furnace. A carrier gas sweeps up released carbon to be measured by adsorption in an infrared spectrometric cell. Results are total concentrations and attributed to the presence of carbon and sulphur in all components. An additional 14 elements were measured after dilution in Aqua Regia. The prepared sample was digested with a modified Aqua Regia solution of equal parts concentrated HCl, HNO3, and DI-H2O for one hour in a heating block or hot water bath. The sample volume was increased with dilute HCl-solutions and splits of 0.5 g were analysed. None of the measured concentrations were too far above the possible detection limit, but in standard range, and accuracy and precision are between 2 and 3%. 1.6. Ion chromatography Effluents from the three experiments were collected at regular intervals (on average every second day) during the experiment period to compare effluent composition over the time of the experiment. The ionic concentrations were analysed with a Dionex Ion Chromatography System (ICS)-3000 ion-exchange chromatograph. The analyses were performed with ICS 3000CD Conductivity Detector and IonPac AS16 and IonPac CS12A were used as anion and cation exchange columns, respectively. The sampled effluents were diluted (Gilson, GX-271) to stay in the linear region of the calibration curve and ionic concentrations were calculated based on an external standard method. The following ion concentrations were quantified: Mg2+, Na+, Cl
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