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The geological pathway towards the development of the

Haaksbergen salt mining area in the eastern

T.P.F. Koopmans, AkzoNobel Industrial Chemicals, , The Netherlands M.J. Broos, MWH, Arnhem, The Netherlands M.C. van den Heuvel-Albers, AkzoNobel Industrial Chemicals, Hengelo, The Netherlands A.M. den Hartogh, AkzoNobel Industrial Chemicals, Hengelo, The Netherlands

SMRI Fall 2012 Technical Conference 1-2 October 2012 Bremen, Solution Mining Research Institute Fall 2012 Technical Conference Bremen, Germany, 30 Sept. ̶ 3 October 2012

THE GEOLOGICAL PATHWAY TOWARDS THE DEVELOPMENT OF A NEW SALT MINING AREA IN THE EASTERN NETHERLANDS

Tjeerd Koopmans*, Maaike Broos**, Mirjam van den Heuvel-Albers* and Marinus den Hartogh*

* AkzoNobel Industrial Chemicals, Hengelo, The Netherlands, ** MWH, Arnhem, The Netherlands

Abstract A long term strategic study in 2005 revealed that the remaining salt reserves in the current AkzoNobel brine field in the eastern Netherlands would become increasingly sparse and expensive to extract during the coming decades. Therefore, AkzoNobel has been investigating new salt mining possibilities since 2006, also looking at the deeper Permian Zechstein salt formations. Initial studies in 2006 and 2007 indicated that the best prospects for future salt mining were located in an area with relatively thick salt deposits at relatively shallow depths near the village of , 10 kilometers southwest of AkzoNobel’s current brine field and salt plant. In 2008 AkzoNobel initiated a more detailed study of the geological situation in the Haaksbergen area to support the decision-making process for further exploratory research. This study by MWH was based on all available and usable seismic and borehole data in and around the area. Results indicated the presence of Zechstein salt resources with an elongated, pillow-like geometry at relatively shallow depths with a maximum thickness of the Zechstein Z1 Halite salt layers of almost 400 meters. In early 2011 an exploration well was drilled near Haaksbergen. The top of the Zechstein Z1 Halite was found at a depth of approximately 600 meters and the thickness of the Zechstein Z1 Halite deposits measured over 300 meters. These results closely matched the depth and thickness previously modeled by MWH. Several well logs, cuttings and cores from the borehole were made, offering a lot of additional insight into the composition and characteristics of the salt pillow. In anticipation of receiving a mining permit for this new area, AkzoNobel decided to conduct a seismic survey along two seismic lines of approximately 3 and 6 kilometers length to acquire higher resolution information, especially in areas with low data coverage. The objective of this additional study was to obtain more detailed knowledge on the location, geometry and thickness of the salt resources and on the presence of faults. The newly acquired seismic data were interpreted and integrated into the framework of previously interpreted seismic lines. Using seismic velocity and other information from the available wells, the seismic velocity model was updated and depth data were generated. Also, clearly visible faults were identified in the seismic profiles as they present a potential risk for salt mining and may create hydrocarbon traps. The interpreted depth data were integrated into the existing geological model. The final geological model shows a narrow, elongated, slightly carved salt structure. Along its crest the modeled thickness of the Zechstein salt exceeds 400 meters and the top of the crest is located at a depth of less than 550 meters below sea level (approximately 580 meters below surface). This new geological model strongly reduces the level of uncertainty in estimates of salt resources and reserves which facilitates the decision-making process with respect to salt mining and planning. Estimated salt reserves in this area suffice for several decades of salt production. Finally, additional insight was gained with respect to the risk at encountering hydrocarbons when drilling wells to develop new caverns. Key words: The Netherlands, Geology, Computer Modeling, Seismic, Permian Basin, Zechstein

1 Introduction In the eastern Netherlands, salt mining by AkzoNobel first started in 1919 near . Since 1936, after the opening of the Canal, AkzoNobel has been mining salt from the present brine field within the Twenthe-Rijn, Twenthe-Rijn Uitbreiding and Helmerzijde concession areas located near the cities of Hengelo and (figure 1). Using solution mining, Triassic Röt salt is mined from caverns at depths ranging from 300 to 500 meters below surface. As remaining salt reserves in the current AkzoNobel brine field will become increasingly sparse and expensive to extract during the coming decades, AkzoNobel has been investigating new salt mining possibilities since 2006 in new areas in the eastern Netherlands, also looking at other geological formations, such as the deeper, Permian, Zechstein salt. This paper describes the different steps within the geological research pathway, leading towards the development of the Haaksbergen brine field in the near future.

Figure 1. Location of the AkzoNobel concession areas in the eastern part of the Netherlands (‘Twente’) and the present AkzoNobel brine field area (indicated by numerous boreholes) in-between the cities of Hengelo and Enschede.

Initial studies Since 2006, AkzoNobel has been investigating the geological situation in the area around Hengelo and Enschede to gain insight in its future salt mining possibilities. Initial studies in 2006 and 2007 on the geological situation in the area around Hengelo and Enschede (Respec, 2006; MWH, 2007b) were combined with previously gathered information on spatial planning possibilities and obstacles (Oranjewoud, 2007). This allowed for a first selection of several so-called ‘areas of interest’ that appeared to offer good opportunities for future salt mining, both from bedded Triassic Röt sat deposits as well as

2 from deeper Zechstein salt occurences. The Zechstein areas included three known salt pillows just northeast of Hengelo, near Rossum and near Haaksbergen. The evaluation of these areas of interest indicated that the best prospects for future salt mining were located in the Haaksbergen area of interest, just west of the village of Haaksbergen (MWH, 2008a). In this area, that is located approximately 10 kilometers southwest of AkzoNobel’s present brine field and the Hengelo salt plant, salt mining from relatively shallow Zechstein salt resources seemed very promising.

Study of the geological situation near Haaksbergen In 2008 AkzoNobel initiated a more detailed study of the geological situation in the Haaksbergen area of interest in support of the decision-making process for further exploratory research (MWH, 2008b). This study was based on all available and usable (mainly seismic) subsurface data in and around the Haaksbergen area of interest. The results of this study indicated that, within the Haaksbergen area, Zechstein salt resources are present with an elongated, pillow-like geometry at relatively shallow depths ranging from 600 to 900 meters below sea level (630 to 930 meters below surface). In the thickest parts of the salt pillow the thickness of the Zechstein Z1 Halite salt layers adds up to almost 400 meters. Based on the information available at that time, the top of the salt pillow was modeled as having two local highs (peaks), a western and an eastern one, with a lower area (saddle) in-between as can be seen in figure 2.

Figure 2. The Haaksbergen salt pillow. Map showing the modeled thickness of the Zechstein Z1 Halite, according to the 2008 geological study (MWH, 2008b). Darker shades of grey indicate thicker deposits, while lighter shades of grey indicate thinner deposits.The two local peaks (thickness over 350 meters) with a saddle in-between are clearly visible.

Zechstein salt deposits in the eastern Netherlands The 2008 geological study helped to gain further insight in the geological history of the Zechstein salt deposits in the eastern Netherlands following earlier studies mainly by Geluk (2005). Within the different deposits of the Zechstein group, salt is found within four different formations, numbered from the oldest formation (Z1 or Werra formation) to the youngest formation (Z4 or Aller formation). The youngest salt formation is overlain by the Zechstein upper claystone. For salt mining purposes all Zechstein formations may be of interest, although the Z1 or Werra formation is of most interest as in this area it has the largest thickness and contains salt of the highest quality with hardly any impurities.

3 The Z1 formation can be further subdivided into five layers, which are named (from oldest to youngest): the Copper Shale layer, the Z1 Carbonate layer, the Z1 Lower Anhydrite layer, the Z1 Salt layer and the Z1 Upper-Anhydrite layer. The Z1 Salt layer is of interest for salt mining. Table 1 shows the division of the Zechstein group into various formations and layers. Like most other salt resources, the Zechstein salt layers were deposited in a shallow marine environment at the bottom of a large shallow sea that formed along the southern edge of the Southern Permian basin during the late Permian (Geluk, 2005; see figure 3). In the eastern Netherlands, a small sea formed that was sometimes land-locked. In this area a condensed succession of layers of carbonates and evaporates was deposited under starved conditions and rock salt deposition occurred, sometimes related to synsedimentary faulting.

Table 1. Division of the Permian in Zechstein and Rotliegendes group and into Zechstein formations and Z1 layers.

System Group Formation Layers Zechstein Upper Claystone Z4 Formation (Aller) Z3 Formation (Leine) Z2 Formation (Stassfurt) Zechstein Z1 Upper Anhydrite Permian Z1 Salt (Halite) Z1 Formation (Werra) Z1 Basal Anhydrite Z1 Carbonate Copper Shale (Kupferschiefer) Rotliegendes

*

Figure 3. Combined facies and present-day distribution map of the Zechstein 1. The central part of the Southern Permian Basin was characterized by deeper water evaporites, whereas sabkh’a and anhydrite walls delineated these deeper basins. In tectonically-controlled intra-platform depressions (like the Central Netherlands Basin that stretched from east tip of England into western Germany) rock salt was deposited. Unpublished map from Geluk 2011. The Haaksbergen area is indicated with an asterisk (*).

4 Geological history of the Haaksbergen salt pillow Based on the general knowledge of the Permian basin and the formation of salt deposits and salt pillows (Geluk, 2005; De Jager, 2003 & 2007 and Ziegler, 1978), and the results from the 2008 geological modeling study, we can sketch the following geological history of the Haaksbergen salt pillow. The Haaksbergen salt pillow is underlain by a heavily faulted Carboniferous horst block. Horsts and adjacent grabens were formed during Permian time. The tectonic regime during Late Permian deposition of the Zechstein is unclear. The different faults were probably reactivated during the Variscan Orogeny, as the northernmost occurrence of Variscan thrusts, the so-called Variscan Front, was situated just south of the area (Geluk, 2005). During Late Permian times a more extensional tectonic regime was present during which expansion of the Central Netherlands Basin and rifting of the Variscan front occurred. According to Geluk (2005) tectonics during the early Zechstein (Tubantian I) along the southeastern margin of the Central Netherlands Basin were extensional too, causing halfgrabens, grabens and graben shoulders to form.During this time, deposition of rock salt took place in tectonically-controlled, mosty NWN-ESE trending intra-platform depressions. This WNW-ESE oriented lineament in the Netherlands is assumed to have originated from minor transtensional movements. Within the Haaksbergen area of interest, Z1 salt was deposited filling up these grabens and half grabens. Rapid deposition and loading of evaporates like salt and anhydrite may have amplified fault relief (i.e. synsedimentary faulting). Some examples of synsedimentary faulting are present within the Haaksbergen area. Although this type of tectonics may explain the presence of thick Z1 deposits, it does not explain all thick salt deposits, as most of these thick deposits are not present adjacent to major faults. Therefore it is thought that most other thick salt deposits originate from the process of halokinesis (salt movement). In the Haaksbergen area, Zechstein and Triassic deposits overlying the salt pillow exhibit the same folded structure, indicating halokinesis took place after deposition of these younger sediments. From the presence of thickened Röt salt formations in the Beckum synclinal structure (MWH, 2007) we can deduce that halokinesis already started during Triassic times and continued until the end of Triassic times and probably even into Jurassic and Cretaceous times as, except for Tertiary deposits, all salt overlying deposits display the same arched structure. The Triassic start of halokinesis is assumed to be related to the extensional regime during Triassic and Jurassic times. This triggered wide-spread mobilization of Zechstein salt as sediment cover is shown in figure 4. Nevertheless, it is assumed that extensive salt movement within this area is related to salt compressional tectonics that did not take place until Late substrate Cretaceous to Early Tertiary times (Subhercynian tectonic phase; Geluk, 2005). During compression deep (Carboniferous) fault zones were re-activated enabling shortening. Salt layers could adapt to this shortening in a different way, causing further halokinesis, especially near WNW-ESE trending faults, like the fault zones that intersect the area around Haaksbergen.

Figure 4. The development of salt structures (from http://home.kpn.nl/mark.geluk/Salt_Index.htm based on RGD, 1993).

5 Halokinesis is assumed to occur only within thick salt successions over a major fault at the base of the salt. Here, due to its lower density, the salt moves up the northeastern slope of the Carboniferous horst block. Although a thick salt pillow is present, salt movement has not been powerful enough to have caused salt diapirs to break through the overlying layers, probably due to the relatively shallow burial depth. This depth further decreased as Tertiary erosion removed a big part of the sediment load above, causing any further halokinesis to end.

Exploratory well Following the 2008 geological modeling study, AkzoNobel applied for an exploration permit, which was granted in 2010. In early 2011 an exploratory well (ISH-01) was drilled near Haaksbergen (see figure 5). The top of the Zechstein Z1 Halite was found at a depth of approximately 600 meters and the thickness of the Zechstein Z1 Halite deposits measured approximately 350 meters (DEEP, 2011). These results closely matched the depth and thickness previously modeled by MWH (MWH, 2008b). Several well logs, cuttings and cores from the borehole were taken, offering a lot of additional insight into the composition and characteristics of the salt pillow. The Zechstein Z1 Halite was found to consist mainly of rock salt with a low anhydrite content of less than 5% for more than three-quarters of the exposed salt interval. Anhydrite intercalations mainly occur in the upper part and, although less frequently, in the lowermost section. Both are thought to be characteristic sedimentary features for bedded salt deposits (DEEP, 2011). Bromide analysis of samples of the Z1 Halite revealed a considerably lower bromide content than known from Z2 (Strassfurt) Salt deposits (DEEP, 2011).

Figure 5. The exploratory well in January 2011.

6 2011 seismic study Following the positive results from the exploratory well and in anticipation of receiving a mining permit for this new area, AkzoNobel decided to conduct a seismic survey along two seismic lines of approximately 3 and 6 kilometers length to acquire higher resolution information, especially in areas with low data coverage in previous studies (see figure 6). The objective of this additional study was to obtain more detailed knowledge on the location, geometry and thickness of the salt resources and on the presence of faults. This would reduce the level of uncertainty in estimates of salt resources and reserves to a level that facilitates the decision-making process with respect to mining of the salt and planning thereof. In particular, additional insight was gained with respect to the risk at encountering hydrocarbons when drilling wells to develop caverns. This knowledge is not only necessary for further investment decisions, but also for the mining plan (detailing how the salt will be mined such that the risks to the environment are minimized), for brine field design and to predict subsidence induced by salt mining. The 2D reflection seismic survey was carried out in late July/early August 2011 by the German company DMT GmbH & Co. KG (DMT), using ground vibrators as energy source. Following data acquisition, data processing was done by DMT as well.

Figure 6. Survey area with location of the seismic lines. Contour lines indicate the thickness of the Haaksbergen salt pillow as obtained from the 2008 geological study (MWH, 2008b).

7 The newly acquired seismic lines 11-AK-ISH-01 (ISH-01) and 11-AK-ISH-02 (ISH-02) were interpreted by the Dutch geophysical company T&A Survey and the results were integrated into the framework of previously interpreted seismic lines. Seismic velocity and other information from exploration well Isidorushoeve-1 (ISH-01) was incorporated as well. Together with the wells Hengevelde-1 (HGV-01) and Haaksbergen-1 (HKS-01), well ISH-01 was used to tie in and check the seismic data interpretation, and to convert the seismic interpretations of the different formations from the time domain to a depth below surface. Also, clearly visible faults were identified in the seismic profiles as they may create hydrocarbon traps. Figure 7 displays the seismic profile for seismic line ISH-01. This seismic line, located in a longitudinal direction of the elongated salt pillow, clearly shows the presence of the thickened salt deposits.

Northwest Southeast

Figure 7. Seismic profile for seismic line ISH-01 (MWH, 2011). Top and base of the Zechstein Z1 Halite, the salt pillow itself and the base of the North Sea Group deposits (Base Tertiary) are clearly visible.

Update of the geological model From the seismic interpretation in the Haaksbergen area of interest, depth data for boundaries between six geological formations were obtained to be used to model the depth and thickness of the salt layer of the Zechstein Z1 Halite and other important geological horizons. Geological modeling of the salt pillow then focused on the Zechstein Z1 Halite deposits and was done in two consecutive steps. First, manual interpretation of the depth of the top and base of the Zechstein Z1 Halite was done in order to get a good sense of the geology in the area, to trace important geological faults running through the area, and to find outliers in the depth data. Following further analysis of the orientation of the faults, several adjustments were made to the data in order to optimize the results from computer modeling, without compromising the integrity of the data, like removal of outliers. Several data interpolation techniques were tested to optimize the fit with the interpreted seismic data for each of the geological horizons, focusing on the area with the thickest salt deposits as this area is of most interest for future salt mining. Using state-of-the-art methods the most important geological horizons, representing major formation boundaries, were then reconstructed in the form of surfaces at different depths in the subsurface, thus obtaining a geological model in three dimensions of the subsurface in the Haaksbergen area of interest.

8 Modeling results As a result of the two newly acquired seismic lines data coverage increased significantly, especially in the center of the salt pillow where data coverage previously was too low to make accurate predictions. Consequently, the new modeled depth, geometry and thickness of the formations in the subsurface have become more reliable, particularly in the area where salt deposits are thickest (over 150 m). The new geological model indicates that the base of the Zechstein Z1 Halite is generally located deeper than predicted by the previous model (MWH, 2008b), especially in areas where it was predicted to occur at relatively large depths already (due to the new seismic velocity model used). Essentially, the same is true for the top of the Zechstein Z1 Halite (figure 8). However, in-between the two highs of the salt pillow, the depth of the top of the Zechstein Z1 Halite is now predicted to lie shallower as a result of the interpretation of the newly acquired seismic data. The two individual highs that were present in the modeled top of the Z1 Halite in the previous model (see figure 2) are now “connected” to form one elongated salt ridge instead, with only minor indications for multiple individual highs. Furthermore, the presence and inclusion of faults in the model has caused local changes in the modeled depth of the base of the Zechstein Z1 Halite horizon, resulting in some further adjustment of the Zechstein Z1 Halite thickness. A thickness model of the Zechstein Z1 Halite was created by subtracting the Top Z1 Halite horizon from the base Zechstein Z1 Halite horizon. Compared to the previous geological model, the modeled thickness of the Zechstein Z1 Halite has changed significantly south of the salt pillow, where a major growth fault has led to another, yet smaller, area with thickened salt deposits. More importantly, because the new model predicts one elongated salt ridge instead of two local highs with a low in-between, the predicted amount of salt present in the structure has increased significantly (see figure 9).

Figure 8. Modeled depth of the top of the Zechstein Z1 Halite (MWH, 2011). Lighter shades of grey indicate shallower deposits, while darker shades of grey indicate deeper deposits.

9 Figure 9. Modeled thickness of the Zechstein Z1 (MWH, 2011). Darker shades of grey indicate thicker deposits, while lighter shades of grey indicate thinner deposits.

Hydrocarbon risks During a final step in the investigation, the new geological model was used to review the risk at encountering hydrocarbons when drilling salt mining wells, which was previously investigated in preparation of drilling of well ISH-01 during the winter of 2010-2011 (PanTerra Geoconsultants BV, 2010a, 2010b and 2010c). It can be concluded that there is a very small chance at encountering hydrocarbons (chance ≈ 1%) in the upper Zechstein (Z2 and Z3) carbonates, which are located just above the Zechstein Z1 Halite deposits. Risks are considered highest when drilling in local highs in the Zechstein Z2 and Z3 formations, which are located in approximately the same places as the highs in the top of the overall Zechstein formation. These highs are located above the thickest parts of the salt pillow in a narrow band that runs east to southeast from the village of Sint Isidorushoeve (see the dark-colored areas in figure 10). Other areas with local highs in the upper Zechstein (Z2 and Z3) carbonates might be located just north and south of the salt ridge, where small size local faults may have led to local highs in the top Zechstein (see figure 10).

Conclusions and further steps The final geological model shows a narrow, elongated, slightly carved salt structure. Along its crest the modeled thickness of the Zechstein salt exceeds 400 meters and the top of the crest is located at a depth of less than 550 meters below sea level (i.e. 580 meters below surface). This new geological model strongly reduces the level of uncertainty in estimates of salt resources and reserves which facilitates the decision-making process with respect to salt mining and planning. Estimated salt reserves in this area suffice for several decades of salt production. Finally, additional insight was gained with respect to the risk at encountering hydrocarbons when drilling wells to develop new caverns. Following this pathway of extensive geological research, AkzoNobel is now able to finish the planning and design of its new Haaksbergen brine field (Den Hartogh & Pinkse-Brommer, 2012) and apply for further necessary permits. Further preparations and infrastructure construction (scheduled for 2015) will then result in the first brine production from the Haaksbergen salt pillow in 2016.

10 Figure 10. Map of the slope of the top of the Zechstein (darker shades of grey indicate lowest slope, while lighter shades of grey indicate steepest slope), isopachs for the depth of the top of the Zechstein and possible top Zechstein faults (MWH, 2011). Low slope and shallowest depth areas indicate local highs with relatively high potential hydrocarbon risks. Local faults (dashed lines) may lead to local highs with relatively high potential hydrocarbon risks as well.

11 References DEEP, 2011. Development of a new brine cavern field for the Hengelo salt plant: Geological end-of-well report for exploration wel Isidorushoeve-01 (ISH-01). Unpublished report.

Geluk, M.C., 2005. Stratigraphy and tectonics of Permo-Triassic basins in the Netherlands and surrounding areas. PhD Thesis.

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MWH, 2011. Seismic survey and geological model update of the Haaksbergen area of interest. By: Koopmans, T.P.F. and Broos, M.J. Unpublished report.

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PanTerra Geoconsultants BV, 2010c. Evaluation of some questions on the drilling program for exploration well Isidorishoeve - 1 (ISH-01), PanTerra Geoconsultants BV, Report No. G824_bis. Unpublished report.

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