Description:

_Journal of Structural Geology 33 (2011) 292-311_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _The internal geometry of salt structures - A first look using 3D seismic data from the Zechstein of the Netherlands_ _Heijn van Gent a,*, Janos L. Urai a, Martin de Keijzer b Structural Geology, Tectonics and Geomechanics, Geological Institute, RWTH Aachen University, Lochnerstra?e 4-20, D-52056, Aachen, Germany b Nederlandse Aardolie Maatschappij b.v. (N.A.M.), Postbus 28000, 9400 HH, Assen, The Netherlands_ _article info_ _Article history: Received 14 January 2010; Received in revised form 18 June 2010; Accepted 11 July 2010; Available online 16 July 2010_ _Keywords: Zechstein Salt tectonics 3D reflection seismic Folding Boudinage_ _abstract_ _We present a first look at the large-scale, complexly folded and faulted internal structure of Zechstein salt bodies in NW Europe using 3D reflection seismic data from two surveys on the Groningen High and the Cleaver Bank High. We focus on a relatively brittle, folded and boudinaged, claystone–carbonate–gypsum layer (the Z3 stringer) enclosed in ductile salt. A first classification of the structures is presented and compared with observations from salt mines and analogue and numerical models._ _Z3 stringers not only are reservoirs for hydrocarbons but can also present a serious drilling problem in some areas. Results of this study could provide the basis for better prediction of zones of drilling problems. More generally, the techniques presented here can be used to predict the internal structure of salt bodies, to estimate the geometry of economic deposits of all kinds and locate zones suitable for storage caverns._ _Structures observed include an extensive network of zones with increased thickness of the stringer. These we infer to have formed by early diagenesis, karstification, gravitational sliding and associated local sedimentation. Later, this template was deformed into large-scale folds and boudins during salt tectonics. Salt flow was rarely plane strain, producing complex fold and boudin geometries. Deformation was further complicated by the stronger zones of increased thickness, which led to strongly noncylindrical structures. We present some indications that the thicker zones also influence the locations of later suprasalt structures, suggesting a feedback between the early internal evolution of this salt giant and later salt tectonics._ _This study opens the possibility to study the internal structure of the Zechstein and other salt giants in 3D using this technique, exposing a previously poorly known structure which is comparable in size and complexity to the internal parts of some orogens._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The sedimentary basins of NW Europe are classic areas of salt tectonics (Fulda, 1928; Richter-Bernburg, 1953a; Ziegler, 1982; Taylor, 1998; Mohr et al., 2005; Geluk, 2007; Geluk et al., 2007; H?bscher et al., 2007). The Dutch part of the Central European Basin contains five evaporite cycles of the Late Permian Zechstein Group (Z1eZ5, see: Fulda, 1928; Ziegler, 1982; Best, 1989; Taylor, 1998; Geluk, 2000; De Mulder et al., 2003; TNO-NITG, 2004; Wong et al., 2007), including a relatively brittle layer consisting of anhydrite, carbonate and clay (the “Z3 stringers”1)._ _? Corresponding author. Tel.: ?49 241 80 98441; fax: ?49 241 80 92358. E-mail address: h.vangent@ged.rwth-aachen.de (H. Van Gent)._ _1 The terms raft and floater are sometimes used to specify layered, competent inclusions in salt. The names raft and floater imply buoyancy, but carbonate, dolomite and anhydrite have a higher density than halite and are expected to sink under the influence of gravity over geological timescales._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.07.005_ _1.1. On the importance of stringers_ _A large part of the world’s hydrocarbon reserve is associated with evaporitic deposits (Warren, 2006), for example, in the Central European Basin, the Caspian Sea, the Gulf of Mexico, offshore Brazil, and the basins of the Middle East. Prediction of the thickness, porosity, geometry and fluid fill of stringers is of significant economic importance. In some settings in Europe as well as in Oman, stringers enclosed in the salt are hydrocarbon reservoirs (Mattes and Conway Morris, 1990; Geluk, 1997, 2000; Peters et al., 2003; Al-Siyabi, 2005; Reuning et al., 2009; Schoenherr et al., 2009a,b). Better understanding stringers in NW Europe can help the interpretation of the complex geometry and history of the hydrocarbon-bearing stringers in the Ara Salt in Oman._ _In addition, in most cases the Z3 stringer is considered a drilling hazard by operators in the Central European Basin. The Carbonate Member of Z3 stringer can be significantly overpressured, with_ _H. Van Gent et al. Journal of Structural Geology 33 (2011) 292-311_ _293_ _pressures up to lithostatic (Williamson et al., 1997). Overpressures in stringers are difficult to predict, therefore when planning the well path, stringers are avoided where possible and not drilled when strongly folded and or faulted. Zechstein salt is also used for different kinds of geological storage or solution mining (Hofrichter, 1974; Coelewij et al., 1978; Bornemann, 1991; Fokker et al., 1995; Geluk et al., 2000; Van Eijs and Breunse, 2003; Evans and Chadwick, 2009) and prediction of internal structure is of major relevance in these fields (see Bornemann, 1991; Koyi, 2001; Chemia et al., 2008)._ _1.2. Internal and external salt tectonics_ _In the literature, salt structures are typically shown in two strikingly different ways. In studies using 3D seismic and well data that focus on the subsalt or suprasalt sediments and are typically hydrocarbon-related, the evaporites are invariably shown as structureless bodies (for example: Jackson and Vendeville, 1994; Jackson, 1995; Scheck et al., 2003; Mohr et al., 2005; Maystrenko et al., 2006)._ _On the other hand, studies of the internal structure of salt are typically salt-mining or storage-related and are based on observations from mine galleries and borehole data (for example: Krische, 1928; Richter-Bernburg, 1953a; Siemeister, 1969; Hofrichter, 1974; Richter-Bernburg, 1980; Jackson, 1985; Richter-Bernburg, 1987; Bornemann, 1991; Smith, 1996; Behlau and Mingerzahn, 2001; Schl?der et al., 2008). These studies show the extremely complex internal geometry with less attention to the structure of the surrounding sediments._ _Detailed observations of salt mines and drill holes (with cm-tom resolution) display a variety of deformation structures in the salt on a wide range of scales (Krische, 1928; Richter-Bernburg, 1953a; Lotze, 1957; Borchert and Muir, 1964; Kupfer, 1968; Muehlberger, 1968; Richter-Bernburg, 1980; Schwerdtner and Van Kranendonk, 1984; Richter-Bernburg, 1987; Talbot and Jackson, 1987; Best, 1989; Jackson et al., 1990; Bornemann, 1991; Zirngast, 1991, 1996; Geluk, 1995; Burliga, 1996; Smith, 1996; Behlau and Mingerzahn, 2001; Siemann and Ellendorff, 2001; Schl?der et al., 2008). Observations (typically 2D to 3D in salt mines and 1D in storage or solution mining) include boudins and folds together with shear zones (Bornemann, 1991; Geluk, 1995, 2000; Burliga, 1996; Taylor, 1998; Behlau and Mingerzahn, 2001). The folds have curved, open-to-isoclinal fold axes, and boudins from millimeter (Schl?der et al., 2008) to kilometer scale (Burliga, 1996) are common. Cross-sections through the Zechstein in the Gorleben and Morsleben salt domes (Bornemann, 1991; Behlau and Mingerzahn, 2001) show isoclinal folding of the Z3. Richter-Bernburg (1980) further describes several examples of fold structures with amplitudes over half the height of the salt structures._ _Field studies, from Iran and Oman, have also shown the internal complexities of surface-piercing salt domes, such as the distribution of different age salt, the position and internal deformation of solid inclusions, the microstructures and, by inference, the deformation mechanisms (among others: Jackson et al., 1990; Talbot and Aftabi, 2004; Talbot, 2008; Reuning et al., 2009; Schoenherr et al., 2009a,b; Desbois et al., 2010)._ _Numerical as well as analogue centrifuge and extrusion models of salt tectonics tend to assume relatively homogeneous rheological properties (although mechanical stratigraphy is used), and consequently produce relatively simple salt structures (for example: Jackson and Talbot, 1989; Van Keken et al., 1993; Koyi, 2001; Schultz-Ela and Walsh, 2002; Talbot and Aftabi, 2004; Chemia et al., 2008). It must be noted, however, that most of these models do provide a way to study structural evolution due to deforming meshes or the use of multicoloured analogue materials._ _Despite the relatively simple rheological models, analogue and numerical models with mechanical stratigraphy have shown the complex deformation associated with (brittle) inclusions in deforming ductile media (see, for example: Escher and Kuenen, 1929; Koyi, 2001; Goscombe and Passchier, 2003; Goscombe et al., 2004; Zulauf and Zulauf, 2005; Chemia et al., 2008; Zulauf et al., 2009; Schmid et al., 2009)._ _1.3. Aim of this work_ _In this paper we aim to contribute to the understanding of the structural style and geometry of the Z3 stringer, by describing detailed interpretations of two 3D reflection seismic surveys from the Dutch onshore and offshore. We focus on thickness variations and structural style at the scale of 30 m20 km, to draw inferences about sedimentary and diagenetic evolution and salt tectonic processes._ _2. The stratigraphy of the Zechstein_ _In the Dutch subsurface, the Zechstein can be subdivided into a marine lower part (Z1eZ3) and a playa-type upper part (Z4 and Z5) with more clastic deposits (Geluk, 1997, 2000). Z1eZ3 follow the classic carbonate-evaporite cycle: claystone–carbonate–gypsum–halite–potassium and magnesium salts consecutively (Warren, 20_ Ключевые слова: surface, geologische, structure formed, groningen, western offshore, formation, zone, drilling, observation, cloud, salt structure, van gent, surrounding stringer, eds geology, deformation mechanism, internal, thicker zone, numerical modeling, netherlands, large-scale fold, stringer, zirngast, basement fault, early, van adrichem-boogaert, extensive network, primary thickness, richter bernburg, eds, geluk, geometry, talbot, point, complex, horizontal resolution, tectonophysics, zulauf, youtube comuserstrucgeology, jackson, permian, natural anhydrite, detailed observation, folding, high, point cloud, salt pillow, fault, data, salt dome, cross, groningen high, isoclinal fold, carbonate, bernburg, location, central, scale, type, kupfer, brittle, europe, map, boudinage, noncylindrical structure, carbonate member, van, main anhydrite, study area, salt tectonics, geology, geoarabia, eew diapir, smith, thicker, youtube channel, groningen area, model, parallel, saint martin, figs, ax, analogue model, fault interpretation, gravity-induced sinking, anvis, coeval folding, manual interpretation, basement, structural geology, ?rst, shear zone, dark grey, burliga, observed, paul buerle, bornemann, grey, richter-bernburg, relative viscosity, brittle inclusion, gent, taylor, interpretation software, messinian, tzs, central area, deformation, williamson, journal structural, layer, international, mingerzahn schlder, result, evolution, structure, basin, memoir, ramsay, depth, bulletin, basal claystone, vertical exaggeration, increased thickness, constriction fold, depth map, wa, diapir, oblique view, inverted block, grujic, und, dome, large area, journal structural geology, local deviation, der, large, off-platform high, interpretation, block, created, tectonics, complex geometry, science, enveloping surface, fold structure, numerical model, aspect ratio, salt body, seismic data, salt wall, folded surface, offshore area, seismic, anhydrite, inverted basin, vertical sampling, urai, local excursion, gent journal, gure legend, discussion, tz, geological, visible, academic, note, zechstein, internal structure, structural, passchier goscombe, non-cylindrical fold, ged rwth-aachen, regional scale, underlying halite, aapg, suprasalt deposit, technique exposing, western, complex structure, teil, boudin, thickness variation, cross-section, concentric boudins, richter, salt, halite, early life, case study, tno, berlin, layer-parallel extension, plane strain, offshore, journal, wong, offset, boudins, folded, elsevier, petroleum, fold, sleep, schl?der, tectonic, internal thrust, observed structure, trend, koyi, interpreted, thickness, vertical, marble schenk, study, detailed view, hill strohmenger, structural style, base, mechanical stratigraphy, drilling problem, map view, salt giant, area