SOLUTION MINING RESEARCH INSTITUTE Paper 105 Apple Valley Circle Clarks Summit, PA 18411, USA
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Technical Conference SOLUTION MINING RESEARCH INSTITUTE Paper 105 Apple Valley Circle Clarks Summit, PA 18411, USA Telephone: +1 570-585-8092 Fax: +1 505-585-8091 www.solutionmining.org The geological pathway towards the development of the Haaksbergen salt mining area in the eastern Netherlands T.P.F. Koopmans, AkzoNobel Industrial Chemicals, Hengelo, 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, Germany 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 Haaksbergen, 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 Boekelo. Since 1936, after the opening of the Twente 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 Enschede (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