SOLUTION MINING RESEARCH INSTITUTE Technical 105 Apple Valley Circle Conference Clarks Summit, PA 18411, USA Paper

Telephone: +1 570-585-8092 Fax: +1 570-585-8091 www.solutionmining.org

New insights in salt mining possibilities close to the

eastern Netherlands regional Boekelo fault zone

Tjeerd Koopmans, MWH, Arnhem, The Netherlands

Marinus den Hartogh, AkzoNobel Industrial Chemicals, Hengelo, The Netherlands

Maaike Broos, MWH, Arnhem, The Netherlands

SMRI Fall 2010 Technical Conference 3 - 6 October 2010 Leipzig, Germany

Solution Mining Research Institute Fall 2010 Technical Conference Leipzig, Germany, 3-6 October 2010

NEW INSIGHTS IN SALT MINING POSSIBILITIES CLOSE TO THE EASTERN NETHERLANDS REGIONAL BOEKELO FAULT ZONE Tjeerd Koopmans*, Marinus den Hartogh** and Maaike Broos*

* MWH, Arnhem, The Netherlands

** AkzoNobel Industrial Chemicals, Hengelo, The Netherlands

Abstract Since 1918, AkzoNobel has been mining salt from brine fields located in the eastern Netherlands near the cities of Hengelo and Enschede. Using solution mining, Triassic Röt salt is being mined from caverns at depths ranging from 300 to 500 meters. To secure the continuation of salt mining on the long term, AkzoNobel is investigating several expansion zones located close to the present brine field. Salt mining in these areas should cover the time span of approximately 5 years until salt mining in another concession area is anticipated to commence. A west-northwest to east-southeast trending regional fault zone, the so-called Boekelo fault zone, forms the south-western border of the present brine field. A detailed geological study has been conducted to obtain further insight into salt mining possibilities close to this zone. Geological models were developed using high quality source information including very accurate information from 60 boreholes and detailed interpretations of over 80 kilometers of seismic lines. During the seismic interpretation study specific attention was paid to the fault characteristics in order to delineate potential exclusion zones for salt mining. From the seismic study and the results of the following geological modeling it was concluded that the fault zone can be divided into three distinct parts, an easternmost part, a middle part and a western part, even though the length of the fault zone within the study area is only about 6 kilometers. Large differences are observed between these three areas, for example with respect to the width of the disturbed zone, the offset along the fault and the relative movement of the north-eastern and south- western fault blocks. Explanations for these strongly differing characteristics can be found in the presence of thickened Zechstein salt deposits (and adjacent rim-syncline) located several kilometers southwest of the fault zone. These salt deposits may have absorbed extensional and compressional movements during several tectonic events. The many fault activity cycles and the nearby presence of the major Gronau fault zone may have played a role as well. Based on the detailed study of the fault zone, it was concluded that salt mining too close to the fault zone should be avoided as salt deposits are disturbed. The fault zone was delineated, distinguishing an inner zone with heavily disturbed Röt salt and adjacent zones in which salt deposits still appear to be present in bedded form. The reliability of this delineation differs per area depending on seismic coverage. Future research, consisting of different geophysical observation techniques and drillings, will focus on further determining the extent of disturbed and undisturbed bedded salt deposits in those areas where Röt salt mining is deemed potentially interesting based on depth and thickness.

Key words: The Netherlands, Geology, Bedded Salt Deposits, Cavern Development, Seismic

2 Introduction In the eastern Netherlands, salt mining by AkzoNobel first started in 1918 near Boekelo. Since 1933, after the opening of the Twente Canal, AkzoNobel has been mining salt from the present brine field located near the cities of Hengelo and Enschede (figure 1). Using solution mining, Triassic Röt salt is being mined from caverns at depths ranging from 300 to 500 meters below surface level. All caverns within the present brine field are situated within the so-called Twenthe-Rijn and Uitbreiding Twenthe-Rijn concession areas. No salt mining is taking place yet within other AkzoNobel concession areas in eastern Netherlands, like Buurse and Weerselo. To secure the continuation of salt mining on the long term, AkzoNobel is investigating salt mining possibilities within as well as outside the present brine field and also from other formation layers, like the deeper Zechstein salt. In December 2008 AkzoNobel decided to pursue future salt mining activities in a new area, not far away from the present brine field and the salt factory in Hengelo. Preparation of future salt mining activities in this new area will take some time. Actual mining of Zechstein salt deposits in this area is expected to start in a few years.

Objectives Current plans to mine Röt salt in the existing brine field southeast of Hengelo still run a couple of years and will cover the time span until the beginning of salt mining in the new area. Plans for the present brine field include the southern part of the recent Usseleres expansion. For the optimal development of this area the geological uncertainties have to be minimized. These uncertainties are caused by the presence of a nearby west-northwest to east-southeast trending regional fault zone, the so-called Boekelo fault zone (see figure 1). This fault zone forms the south-western border of the present brine field. To gain insight in salt mining possibilities close to the present brine field adjacent to the Boekelo Fault Zone, and to achieve optimal development of the southern part of the Usseleres expansion, a regional geological study of the fault zone and the surrounding area was conducted within AkzoNobels continuous research program for development of new brine fields and brine field extension.

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.

3 The study was commissioned and paid for by AkzoNobel. Seismic interpretation was performed by T&A Survey, MWH was responsible for overall project management, geological modeling, geological interpretation and reporting. The work started in July 2009 and the final report was delivered in February 2010. This study focused on three potential expansion zones that are located around the fault zone and to the south, west and southwest of the present brine field. These zones were named Zones IV-a, IV-b and V (see figure 2). Zones IV-a and IV-b both are dominated by the presence of the Boekelo fault zone. Zone IV-a included the soutern part of the Usseleres expansion. Zone V is located west of the present brine field, so away from the fault zone and is expected to be less effected by the Boekelo fault zone. The main objectives of this study were to offer reliable information on the location and orientation of the disturbed deposits within these zones as well as to create a detailed geological model. As these zones include the southern part of the Usseleres expansion, this study served several purposes: • offering supplementary geological insight in the salt mining possibilities of the southern part of the Usseleres expansion; • offering supplementary geological insight in the salt mining possibilities in the vicinity of the fault zone within the southern part of the Twenthe-Rijn concession area as well as more reliable estimates of the salt reserves present in this area.

Figure 2 Location of the Boekelo fault zone, the potential expansion zones IV-a, IV-b and V adjacent to the present brine field between the cities of Hengelo and Enschede and the geological and geophysical source data available for this study.

4 Triassic Röt salt in the eastern Netherlands Several studies describe Permian and Triassic salt geology in the eastern Netherlands (Ziegler, 1978; Geluk, 1994; De Jager, 1994; Geluk & Duin, 1997; TNO, 1998; De Jager, 2003; Kockel, 2003; Geluk, 2005 and Geluk, 2010). Here, we only present a summary of those aspects that have been important with respect to the presence of Röt salt deposits in the Hengelo area and the Boekelo Fault Zone. During Early Triassic times, the center of the Southern Permian Basin stretched from northern Germany to the North Sea and was surrounded by lake margins and alluvial planes with clastic influx from the south, where the basin was bordered by the Variscan London-Brabant-Massif, Rhenish Massif and Bohemian Massif. During the Middle Triassic, the basin area was restricted to northern Germany and Poland (North German Basin), and conditions were more marine due to the ingression of the Tethys from the south and southeast (see figure 3). Temporary interruption of this particular opening resulted in deposition of the Röt Evaporite during early Middle Triassic times and under dry climatic conditions a salt basin developed, covering large parts of the Netherlands (Geluk, 2005). At the beginning of the Middle Triassic the study area in the eastern part of the Netherlands was located in a bay-like area at the southern margin of the North German Basin salt pond during the deposition of the Röt (Geluk, 2005; see figure 3). As was described by Geluk (2005), predating the Triassic deposits the full cycle of Zechstein deposits is present in the Netherlands, followed by Triassic deposits of epicontinental character (Lower and Upper-Germanic Trias Group deposits). The Upper-Germanic Trias Group contains Middle to Late Triassic deposits and consists of epicontinental to marine deposits (Solling Formation, Röt Formation, Muschelkalk Formation and Keuper Formation). As can be seen in figure 4, deposition of the Röt Formation, one of the main salt containing Triassic formations in the Netherlands, was restricted to an area surrounding the Netherlands Swell and had a main depocenter in the Ems Low, north of the study area, where up to 300 meters of Röt Formation was deposited. Within the Röt Formation, four members can be distinguished (; see figure 5): the Main Röt Evaporite, the Intermediate Claystone, the Upper Röt Evaporite and the Upper Röt Claystone. Within the Main Röt Evaporite, four evaporitic cycles can be distinguished, from oldest to youngest named Salt A to Salt D (Kovalevych et al., 2002). At the base of the Main Röt Evaporite, a thin layer of anhydrite (several decimeters) is found. The overlying four salt layers have thicknesses of several meters (layers B and D) up to 15 to 50 meters (layers A and C). All salt layers are separated by layers consisting predominantly of claystone, some dolomite and anhydrite. The upper part of the Main Röt Evaporite is formed by a 10 to 15 meters thick layer of anhydrite with interlayering claystone. Altogether, the thickness of the Main Röt Evaporite may be over 100 meters within the Ems Low and southeast of the Gronau Fault Zone (located just north of Hengelo). Within these thick Main Röt Evaporites rock salt may be present, although this presence is restricted to a distinct area (Geluk, 2005).

AOI

Figure 3 Present-day distribution and facies map of the Röt (Middle Trias; based on Geluk, 2010). Study area indicated (AOI).

5 Salt D Salt C Salt B Salt A

AOI

Figure 4 Isopach map (in meters) of the Röt Figure 5 Stratigraphic subdivision of the Triassic in the Netherlands. Formation. The map also shows the Based on Geluk, 2010. limits of salt distribution. Based on Geluk (2010).

Tectonic activity affecting Röt salt deposits During the Late Triassic, the extensional Early Kimmerian phase resulted in relatively gentle faulting, uplift and swell formation and erosion in the Netherlands (Ziegler, 1978; Geluk, 2005). Extension was east-west with minor strike-slip movements along a number of fault zones. This also affected the thickness and salt distribution of the Röt formation, as subsidence took place during the Röt. In the Ems Low, near growth faults of which reactivation was often facilitated by Zechstein salt, thickened Röt deposits were deposited. Early Kimmerian phase basement faulting during much of the Triassic also caused widespread mobilization of Zechstein salt with thick Muschelkalk deposits occurring in rim synclines of salt diapirs. Following Triassic tectonic activity, tectonic activity also occurred during Jurassic and Cretaceous times (extensional Late Kimmerian tectonic phase), resulting in normal faulting along reactivated fault systems and movement of Zechstein salt, causing the formation of synclinal and anticlinal structures within the overlying strata. During the Late Cretaceous and Early Tertiary compression took place in four tectonic phases and erosion at the end of the Cretaceous and the beginning of the Tertiary caused the major Laramide unconformity at the base of the Tertiary (Base North Sea Group deposits). During these phases, the pre-existing rift-basins were inverted during reactivation of normal faults and N-S or NNW-SSE oriented compression caused dextral strike-slip movements and creation of flower structures. During these compressional phases, salt from Zechstein salt pillows or diapirs may have been squeezed into fault zones and even into overlying sequences, preferentially into Triassic salt layers. Here, post-salt deposits have been uplifted, forming anticlinal structures, while faulting and halokinesis took place at larger depths. Finally, during the Tertiary a more extensional regime came into existence and regional subsidence may have taken place. In the eastern Netherlands subsidence was probably related to the formation of the Cenozoic Rhine Graben system and probably caused reactivation of the major NW-SE trending fault zones (de Jager, 2003; Kockel, 2003).

6 The Gronau and Boekelo fault systems In the eastern part of the Netherlands an active system of WNW-ESE trending faults is present, of which the major Gronau Fault Zone is one of the largest faults (see figure 1). During the Early Kimmerian phase, this fault was reactivated (TNO, 1998). Many new individual faults were formed and salt accumulation occurred, especially along a southern branch of this fault zone. Also during the Late Kimmerian phase the Gronau Fault Zone was reactivated, showing normal faulting, dextral strike-slip faulting and growth-faulting. During the Late Cretaceous and Early Tertiary compressional phases reactivation occurred again and inversion led to thrust faulting. During these compressional phases, probably most of the earlier Kimmerian vertical movements along the Gronau Fault Zone were compensated. The Boekelo Fault Zone, which forms the southwestern border of the present AkzoNobel brine field, is located only several kilometers south of the Gronau Fault Zone. It is assumed to be directly related to the Gronau Fault Zone and may be a separate branch of it.

Methods The main parts of this investigation consisted of a detailed seismic study followed by the development of geological models for the three potential expansion zones adjacent to the current brine field using high quality geological and geophysical source information such as very accurate borehole information and detailed seismic interpretations. In total over 80 kilometers of seismic lines were studied in extreme detail supported by information from 60 boreholes that were studied in detail by the geological consulting company Geowulf in 2008 and 2009 (see figure 2). Also, a new seismic velocity model was created during the seismic study and was used for time-to-depth conversion of the interpreted seismic data. During the seismic study specific attention was paid to the fault zone that runs across the study area just southwest of the present brine field. This seismic study provided useful input information for the geological modeling and was also used to delineate exclusion zones for salt mining around fault lines. Using the time-to-depth converted data and the Geowulf borehole data, the main geological horizons were modeled: the top and base of the Main Röt Evaporite and the base of the Tertiary. Modeling was performed using different types of interpolation methods like geostatistical kriging and IDW (Inverse Distance Weighting). Interpolation of the depth of the base of the Main Röt Evaporite was also done manually, to be able to support interpolation with geological knowledge and expertise. Other horizons, such as the top and base of the usable Röt salt, were derived from these models. The results of the seismic study together with the geological models were then used to evaluate the actual potential of the three studied expansion zones for future Röt salt mining, to delineate potential exclusion zones for salt mining and to produce reliable estimates of the amount of safely, economically and technically extractable Röt salt reserves in each of them.

Seismic study and geological modeling: available data For the seismic study a total of 78 kilometers of seismic lines were selected from the available seismic data of TNO (former Dutch Geological Survey). The available lines (see figure 2) cover the potential expansion zones VI-a, VI-b en V and the southwestern part of the present brine field quite well. They cross-cut the Boekelo Fault Zone at many places, which makes them useful to gain insight in the geological situation within and around the fault zone. Nevertheless, large quality differences can be observed between the different seismic lines, with the 87-AK-series having the best quality, especially with respect to the shallow subsurface, down to the Early Triassic deposits. The 85EN(V)-series is of high quality too, although these lines have their focus on the deeper subsurface, meaning that Röt deposits are less clearly visible, but still good enough for seismic interpretation. As the lines of the 85EN(V)-series are very long, these lines offer a good regional picture of the geological situation. These lines provide an understanding of the regional setting and were used to connect different parts of the area. The older lines of the 7060 and 7160 series and line 6082, are off lower quality, but useful to fill in areas where no other data are present. These lines also have their focus on the deeper subsurface, but Röt deposits are clearly identifiable. Seismic coverage is high for the southern part of the Usseleres expansion zone (Zone IV-a) and also the area west of the present brine field (Zone V) is quite well covered. Zone IV-b clearly has less coverage.

7 In addition to seismic data, a lot of boreholes are present in the area, providing very useful information about the depth of various geological horizons, both of the salt deposits itself, as well as horizons above and below the salt deposits of interest, like the Base Tertiary, top and base of the Main Röt Evaporite, top Zechstein, etc. Borehole data are available through TNO as well. As a result of almost a century of salt mining, information for many deep boreholes are available, although the vast majority of boreholes are located within the existing brine field. Only a few boreholes (BKM-01, DED-02, OZG-01, OZG-02 and BUU-boreholes) are located outside the brine field in or near the potential expansion zones and even fewer are located south of the Boekelo Fault Zone (BKM-01 and BUU-boreholes). Furthermore, several relatively shallow boreholes (up to 100 m below surface level) are located in this area. These boreholes (TW-I - TW-XII) do not reach the Röt salt deposits, but may be used to determine the depth of the Tertiary deposits. Borehole measurements (like analog and digital sonic logs, Gamma-ray-logs and resistivity logs) and lithological interpretations are available for several wells in the existing AkzoNobel brine field. Data from borehole 399 were used. For 64 selected boreholes AkzoNobel provided detailed stratigraphic information, available from a recent investigation of the lateral variation in the Röt salt deposits and the overlying deposits of the Hengelo brine field by Geowulf Laboratories (GEOWULF Laboratories, 2008; 2009a; 2009b; 2009c). In addition, boreholes BKM-01, BOR-01, DED-01, DED-02, OZG-01, OZG-02, BUU-02, BUU-03, BUU-06, BUU-06, TW-II and TW-IV were studied by Geowulf especially for this investigation of salt mining possibilities in the vicinity of the Boekelo fault zone.

Seismic study and subsequent study of the fault zone A detailed seismic study was carried out by T&A Survey. Eighty kilometers of seismic lines were interpreted followed by time-to-depth conversion. The following geological horizons were interpreted (from shallow to deep): base North Sea Supergroup, base Niedersachsen Group, Intra Muschelkalk Formation, top and base Main Röt Evaporite Member, base Lower Germanic Trias Group (=top Zechstein; only used for structural reasons) and base Zechstein Group (only used for structural reasons too). Following seismic interpretation, time-to-depth conversion was carried out based on velocity model that was created using velocity information from well B399 and depth information from nearby boreholes (see table 1). The results of the seismic interpretation showed that the Röt formation is present in almost the entire study area. It thickens towards the northeast, although this does not apply for the Main Röt Evaporite member. This thickening is assumed to be related to the fact that the depocenter was located northeast of the study area (Ems Low; see figure 4). The Muschelkalk formation can also be found almost in the entire study area. Other formations, like the Altena and Niedersachsen Group formations, are only present in parts of the area, partly due to erosion at the base of the North Sea Super Group (Laramide unconformity). The study of the northwest-southeast trending Boekelo fault zone formed a major part of the seismic interpretation. As it was impossible to interpret individual faults within this zone, instead the edges of a so-called ‘disturbed zone’ were indicated within the seismic profiles. This zone was determined as the area where Röt salt deposits were clearly disturbed and were not present in a layered form anymore. Subsequently, a very detailed study of the Boekelo Fault Zone was performed by taking a close look at all seismic profiles crossing it. In each seismic line the disturbed zone was determined in three different ways.

Table 1: Seismic velocity model used for time-to-depth conversion of the seismically interpreted data within the study area Layer Interval velocity NAP to Base North Sea Supergroup (N) 1,670 m/sec Base North Sea Supergroup to Base Niedersachsen Group (SK) 3,000 m/sec Base Niedersachsen Group to Intra Muschelkalk reflector (RNMU) 3,600 m/sec Intra Muschelkalk reflector (RNMU-) to Top Main Röt Evaporite Member or Base 3,100 m/sec Intermediate Röt Claystone Member (RNROM) Base Intermediate Röt Claystone Member to Base Main Röt Evaporite Member (RNRO1) 4,500 m/sec

8 In the inner zone (zone 1; see figure 6) Röt salt deposits are clearly disturbed and the top and base of the Main Röt Evaporite cannot be traced in the seismic section anymore. Salt mining in this area is discouraged as salt will probably not be present at the expected depth and with the expected thickness. Outside this disturbed zone Röt deposits may have been disturbed by a normal fault, changing depth or dip of the deposits, but layered deposits can be identified in the seismic sections. The outermost normal faults at the base of the Main Röt Evaporite and at the base of the Tertiary respectively determine the borders of the second and third zone (see figure 6). A borehole in this area penetrates disturbed overlying strata and will meet a normal fault at a certain depth, but this is not assumed to be a major problem as long as this fault is not directly above a salt cavern. Consequently, salt mining may be possible between the inner and outer disturbed zone boundaries. The width of the inner fault zone is narrow in the extreme southeast, widens to about 2 kilometers south of the present brine field, then narrows again southwest of the present brine field and widens once more towards the northwestern tip of the study area. Both eastward and westward of the study area, the fault zone is expected to diminish. In general, the fault zone consists of a set of normal faults with the hanging wall in the center of the zone and the footwall on either side of the fault zone. There appear to be three distinct parts of the fault zone within the study area: a northwestern part and a southeastern part, both with very different fault characteristics and possibly with different originating mechanisms, and a middle part in-between. Precisely the transition zone (the middle part) where the northwestern part changes into the southeastern part has the poorest seismic coverage. In the southeastern part of the Boekelo Fault Zone, the northeastern side has moved down relative to the southwestern side, while in the northwest it has moved up. In the middle hardly any relative movement is observed. The offset ranges from 200 meters in the southeast to no offset in the middle to 60 meters in the northwest. There are a lot of possible explanations for these locally strongly differing fault characteristics. As characteristics change over very short distances, differences are probably caused by a combination of things. The main aspect that may have influenced fault activity and characteristics is the presence and thickness of Zechstein salt. Zechstein salt is known to have a large influence on faulting within the overlying strata, especially during basin inversion. Only a little is known about Zechstein salt thickness in this area. Overall, the main Zechstein salt (i.e. the Zechstein Z1 or Werra formation) is assumed to be around 100 meters thick, but locally salt pillows may have developed by halokinesis.

Zone 3 Zone 2 Zone 1

Figure 7 Seismic time section of line 87-AK-15, showing the Boekelo Fault Zone and its various normal faults, and the different types of disturbed zone that were determined based upon the seismic interpretation of the behavior of the seismic horizons on either side of those faults.

9 Several kilometers southwest of the Boekelo Fault Zone, just west of the village of Haaksbergen, one such a salt pillow is present, with a thickness of over 300 meters. A so-called rim-syncline has formed in between the salt pillow and the Boekelo Fault Zone. The formation and presence of thick salt deposits nearby may have influenced local fault activity as salt is able to absorb compression and extension locally (Kockel, 2003). Other salt pillows may be present in the area surrounding Hengelo. A second aspect that may have influenced fault activity and characteristics is the fact that fault activity occurred in many cycles. Although the Boekelo fault zone is assumed to date back to pre-Permian times, the main tectonic events probably were the Jurassic to Early Cretaceous extensional phases, followed by Late Cretaceous to Early Tertiary compressional phases, which caused fault reactivation and basin inversion. This may even have been followed by relaxation and extension during the Tertiary. As orientation of these different tectonic phases was not always perpendicular to the main fault direction, transtensional and transpressional regimes may have developed. Especially during the Late Cretaceous to Early Tertiary compression, transpression and right lateral movement along the pre-existing faults may have occurred. This may have led to local differences in compression and extension and to the formation of local side branches to the main fault zone system, of which some are visible in the seismic profiles. A third aspect that may have influenced fault characteristics is the presence of the major Gronau fault zone. This fault zone, running from SE to NW through Twente, about 10 kilometers northeast of Hengelo, shows major offset and has been highly active since the Late Carboniferous until recent, i.e. during all tectonic phases described above (TNO, 1998). This fault zone may have led to local differences in compression and extension too as it may have absorbed major tectonic movements.

Geological modeling method Using the seismic depth data and the depth data from the 64 nearby detailed borehole descriptions, geological models have been created for the areas north-east and south-west of the fault zone. In order to get the best possible representation of the Main Röt Evaporite deposits, special emphasis was placed on modeling the base of the Main Röt Evaporite. This horizon could be easily traced in the studied seismic profiles and determines cavern height during salt mining activity to a large extend. Furthermore, the top of the Main Röt Evaporite was modeled to be able to identify any lateral thickness variations, as well as the base Tertiary, which is an important horizon when determining safe maximum cavern heights for salt mining. The top of the usable Röt salt, defined as the top of Röt salt layer C, and the base of the usable Röt salt, defined as the base of Röt salt layer A, were derived from the modeled top and base of the Main Röt Evaporite. The presence of the Boekelo fault zone, running through the area from west to east, required special treatment. For modeling purposes the borders of the disturbed zone were delineated by connecting the edges of the disturbed zone (zone 1). The disturbed zone itself was extended in a northwesterly and a southeasterly direction towards the border of the modeling area (following the general orientation of the Boekelo Fault Zone), thereby effectively splitting the modeling area into three parts: a north-eastern zone, a south-western zone and a disturbed zone. As the processes that formed the geological structures may have differed strongly on either side of the fault zone modeling of all horizons was done separately for the south-western and the north-eastern areas. Manual interpolation of the base of the Main Röt Evaporite was performed to bring in geological expertise during the interpolation of the seismic data. The results of the manual interpolation were used to check and improve computer modeling results. The top and base of the Main Röt Evaporite were modeled by computer using the IDW interpolation technique north of the fault zone and Ordinary Kriging south of it. The base of the Tertiary deposits was modeled by computer using the IDW interpolation technique both north and south of the fault zone.

Results of the geological modeling and derived horizons Modeled horizons of the depth of the base (see figure 7) and top of the Main Röt Evaporite provide information about the depth and geological structures of these deposits and also offer insight in its thickness. In addition to the Boekelo Fault Zone, the so-called Stepelo depression represents the main geological structure, forming an oval-shaped, elongated structure in the southwestern part of the area. A less pronounced depression is located in the most northwestern part of the area (the so-called Delden depression). The modeled thickness of the Main Röt Evaporite is quite constant throughout the area: between 60 and 80 meters in more than 90% of the area.

10 The base of the usable salt, defined as the base of Röt salt layer A, was put 1 meter above the base of the Main Röt Evaporite. The top of the usable salt, defined as the top of Röt salt layer C, was put 18 meters below the top of the Main Röt Evaporite. Consequently, both horizons show the same structures as the top and base of the Main Röt Evaporite and the thickness of the usable salt shows the same variation as the Main Röt Evaporite, although it is 19 meters thinner. The base of the Tertiary (i.e. Base North Sea Supergroup) was also modeled to be able to calculate safely extractable maximum cavern heights. In general, the base of the Tertiary shallows in an east- southeastern direction from about 150 meters in the northwest of the study area to about 25 meters in the most southeastern part. Within the fault zone a local depression of the base of the Tertiary may indicate a pull-apart basin, caused by Tertiary strike-slip fault activity along the Boekelo Fault Zone. Five geological profiles were drawn across the study area, three perpendicular to the fault zone and two parallel to the fault zone, one north of it and one south of it. These profiles provide excellent insight in the Röt salt occurrences in the study area. Figure 8 shows Profile B-B’, i.e. the SW-NE profile through the middle part of the Boekelo Fault Zone.

B’

B

Figure 7 Modeled depth of the base of the Main Röt Evaporite horizon. Location of profile B-B’ (see figure 9) is indicated.

11 Quarternary and Tertiary deposits

Cretaceous, Jurassic and Upper Triassic and deposits

Boekelo Fault Zone Unextractable and unusable Main Röt Evaporite

Lower Triassic and Permian deposits Extractable Main Röt Evaporite (cavern height)

Figure 8 SW-NE profile through the middle part of the Boekelo Fault Zone

Salt mining resources and reserves in the vicinity of the Boekelo fault zone Based upon the three originally determined zones (IV-a, IV-b and V) and the geological modeling results, five so-called resource zones were identified, referred to as zones A to E (see figure 9). Zones A, B and C are located north of the fault zone, zones D and E south of it. Within these five zones, mineral resources , defined as the volume of usable Röt salt, range from 27 to 137 million m 3 (59 to 366 million metric tons) and add up to 458 million m 3 (986 million metric tons). Both safely extractable salt reserves and economically extractable salt reserves have been determined. First, safe maximum cavern heights were determined, taking into account the Röt salt mining safety regulations and a minimum hanging safety of 5 meter. Surface area of the resource zones and safe maximum cavern height together determine the safely extractable salt reserves within each zone. These are found to be large but vary largely due to differences in surface area and safe maximum cavern height. Economically extractable salt reserves are smaller as only caverns with a significant height are deemed economically feasible. Finally, technically extractable salt reserves were determined for a given potential cavern layout, Technically extractable salt reserves are found to be potentially interesting within most resource zones, especially within Zone C (the southern part of the Usseleres expansion).

Possibilities for future salt mining near the Boekelo fault zone The detailed study of the fault zone has led to a useful classification of this zone into a zone with disturbed Röt salt deposits and bordering zones which have been subjected to faulting, but still show layered Röt salt. In most locations the border of the disturbed zone, which indicates the area where salt mining is deemed impossible, is quite reliable, as it is supported by many seismic lines as well as by information from several boreholes. For salt mining purposes, several geological aspects are important. As mentioned previously, salt mining may be possible within the disturbed zone, namely within the area between the inner boundary and the outer boundaries, although a borehole in this area will penetrate disturbed overlying strata and will meet a normal fault at a certain depth. A second aspect is the reliability of the delineation of the disturbed zone, which depends on the availability of seismic data and boreholes. These two things should be kept in mind when determining salt mining possibilities close to the Boekelo fault zone.

12 Figure 9 Resource zones A to E in the vicinity of the Boekelo fault zone

In the extreme southeast (i.e. the Usseleres area) narrowing of the fault zone is observed like in the middle part. The results for the southeast are regarded reliable as they are supported by several seismic lines. Special attention was paid to determining salt mining possibilities within this area and especially in the southern part of the Usseleres expansion zone (the first mining area to be developed in the near future). Salt mining in this area is assumed to be very well possible. Here, the nearby Boekelo Fault Zone does not seem to present any major problems as the fault zone is observed to make a southward bend, away from the planned phase 3 expansion. The geological knowledge about this area and the nearby Boekelo Fault Zone, obtained during this study, is assumed to be more than sufficient to develop this expansion of the brine field.

Quality and reliability The quality and reliability of the geological models varies significantly throughout the study area. Model reliability depends on many aspects like the distance to the closest source data points, the density of the available data points, the quality of the source data and the modeling techniques. Furthermore, the reliability of the modeled horizons (the top and base of the Main Röt Evaporite and the base of the Tertiary) is higher than the reliability of information that is derived from these horizons (like the thickness of the Main Röt Evaporite and top, base and thickness of the usable Röt salt). The reliability of the geological models also varies for the various resource zones due to differences in density of seismic lines.

13 Recommendations for further research Future research should focus on further determining the extent of disturbed and undisturbed layered Röt salt deposits in those areas where Röt salt mining is deemed potentially interesting based on depth and thickness. In the southern part of the Usseleres expansion zone (resource zone C), that offers good prospects for future salt mining, gathering extra seismic data is not deemed necessary For other expansion areas a 2D-seismic survey in combination with a borehole at a strategically selected location, is recommended to provide additional information on the depth and thickness of the salt deposits. Alternatively, a 3D visualization can be achieved by ZeroTEM in combination with one or two boreholes.

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

Geluk, 2010. Website: http://home.kpn.nl/mark.geluk/Index.htm

GEOWULF Laboratories, 2008.Geological Framework TWR Area Compilation Report volumes I & II, report no. GL08.502.

GEOWULF Laboratories, 2009a.TWR Area Evaluation of historical well data, Lithological Study DED- 02, report no. GL09.401.

GEOWULF Laboratories, 2009b.TWR Area Structural Setting of B346 top B351, report no. GL09.402.

GEOWULF Laboratories, 2009c.TWR Area Series 275, report no. GL09.801.

Jager, J. de, 2003. Inverted basins in the Netherlands, similarities and differences. Netherlands Journal of Geosciences / Geologie en Mijnbouw 82 (4): 355-366.

Kockel, F., 2003. Inversion structures in Central Europe – Expressions and reasons, an open dis- cussion. Netherlands Journal of Geosciences / Geologie en Mijnbouw 82 (4): 367-382.

Kovalevych, V., T. Marek Peryt, W. Beer, M. Geluk and S. Halas, 2002. Geochemistry of Early Triassic seawater as indicated by study of the Röt halite in the Netherlands, Germany and Poland. Chemical Geology 182: 549-563.

MWH, 2010. Salt mining possibilities in areas adjacent to the Hengelo brine field, final report. By: Koopmans, T.P.F. and Broos, M.J.

T&A Survey BV, 2009. Seismic interpretation of Röt salt deposits in the subsurface of the region Hengelo Enschede in the Netherlands. Project number: 0309-OEM1623

TNO, 1998. Geological Atlas of the Subsurface of the Netherlands-Onshore. Sheet X.

Ziegler, P.A., 1978. North-Western Europe: Tectonics and Basin Development, Geologie en Mijn- bouw 57 (4), p. 589-626

14