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