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_Journal of Structural Geology 32 (2010) 537-553_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Small-scale faulting in the Upper Cretaceous of the Groningen block (The Netherlands): 3D seismic interpretation, fault plane analysis and regional paleostress Heijn van Gent a,*, Stefan Back b, Janos L. Urai a, Peter Kuklab Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstra?e 4-20, Haus A, D-52056 Aachen, Germany b Geological Institute, RWTH Aachen University, W?llnerstra?e 2, D-52056 Aachen, Germany Article info Article history: Received 7 July 2009 Received in revised form 24 February 2010 Accepted 9 March 2010 Available online 16 March 2010 Keywords: Upper Cretaceous chalk Paleostress Fault surface undulations Seismic interpretation Fault plane analysis Abstract Over the last years, ?eld-based studies have shown that fault surfaces can exhibit a considerable selfaf?ne topography. It is reasonable to assume that similar undulations are also present in fault interpretations from 3D re?ection seismic data; however, both the interpretation uncertainty and geophysical resolution limits hinder their analysis. This study analyses a set of small-scale, non-reactivated faults in the Upper Cretaceous Chalk Group (Upper Ommelanden Formation) of the NW-part of the Groningen Block, the Netherlands, in a high quality Pre Stack Depth Migrated 3D seismic data set. The studied faults are fully contained inside the Chalk Group, in an area located between the major tectonic-bounding faults of the NW Groningen Block. Over 200 faults, with offsets in the order of 30-50 m, were interpreted across an area of ca. 150 km², showing a clear preferential orientation for strike, dip and dip-direction. Detailed interpretations and 3D fault plane analyses show undulations on the fault plane. We show that these undulations are not an interpretation or gridding artefact, and interpret these to indicate direction of fault slip. These results were used to calculate a paleostress tensor, using all faults to calculate a single stress tensor for the entire study area by Numerical Dynamic Analysis. Based on the orientation, position and a thickness analysis, it is interpreted that these faults formed due to the tectonic reactivation of salt structures in the Latest Cretaceous. The calculated paleostress state shows a general NWeSE-extension, with a vertical maximum principle stress, and a stress ratio of about 0.3, indicating that the studied faults are not the result of dewatering. This interpretation agrees both with a nearby salt-tectonic reconstruction, as well as ?eld-based paleostress results from the UK, Belgium and France. A ?rst look at other surveys from the Dutch sector indicates that similar faults are present in other areas, with different orientations. We propose that a dedicated analysis of these faults across on- and offshore Europe would allow extending the stress map of the Late Cretaceous into areas where the Chalk is not outcropping. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction This work presents a detailed analysis of a set of small-scale faults interpreted on high-quality 3D seismic data of the Upper Cretaceous Chalk Group of the NW Groningen Block, the Netherlands (Fig. 1a). The interpretation results are compared with existing analyses of faults in the chalk of NW Europe, and used for paleostress analysis. Previous studies on small-scale faults in chalk strata have been controversial concerning the interpretation of the origin of faulting. Hibsch et al. (1995) and Hibsch et al. (2003) * Corresponding author. Fax: ?49 241 80 92358. E-mail address: H.vangent@ged.rwth-aachen.de (H. van Gent). 0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.03.003 interpreted intra-Chalk faults to have formed by compaction. In contrast, Vandycke (2002) argued for tectonic deformation as the main cause of faulting observed in Chalk outcrops. The study presented here will help to distinguish between the two models. Paleostress analyses provide information on the tectonic evolution of the crust and help to predict the location and possible orientations of fracture and fault systems below the resolution of seismic observation. In hydrocarbon exploration, these fracture systems can have economically viable permeabilities (Koestler and Ehrmann, 1991; Arnott and van Wunnik, 1996; van Konijnenburg et al., 2000; Smith and McGarrity, 2001; Otrtuno-Arzate et al., 2003; Casabianca et al., 2007); thus, the seismic-based paleostress-analysis approach can potentially impact oil and gas exploration and production in carbonate 538 H. van Gent et al. Journal of Structural Geology 32 (2010) 537-553 Fig. 1. (a) Location of the study area in the NW of the Groningen High, at the border of the Lauwerszee Trough. Image courtesy of NAM. (b) Simple stratigraphic column for the northern Netherlands. Modi?ed from Herngreen and Wong (2007). Also indicated are the approximate stratigraphic positions of the four internal re?ectors (AeD), see Table 1. provinces in general. Paleostress analyses can be used to estimate the timing of the opening and closing of faults and fractures, and for analyzing and modelling the migration of geo?uids (du Rouchet, 1981; Sapra, 1997). Paleostress analyses are usually based on maps of fault systems at km-scale (e.g. Anderson, 1942; Michon et al., 2003), on the detailed mapping of fault surfaces and slip directions in outcrops at m-scale (Bergerat, 1987; Kleinspehn et al., 1989; Angelier, 1994; Hibsch et al., 1995; Delvaux, 1997; Saintot and Angelier, 2002; Vandycke, 2002; Caiazzo et al., 2006; Sippel et al., 2009), or on the analysis of calcite twins at mm-scale (Turner, 1953; Spang, 1972; Larroque and Laurant, 1988; Rocher et al., 2004). With the increased availability of industrial 3D seismic data for the scienti?c community, several attempts have been made to extract (paleo-) stress tensors from 3D seismic data (this does not include papers on seismic processing that constrain the orientation of either fractures or the present-day stress tensor, such as Neves et al., 2003). Seismic extraction of paleostress has the advantage that direct access to rocks is no longer required, so that sedimentary cover, or seawater coverage in offshore settings does not hinder paleostress analysis. Furthermore, the fact that seismic data is often available in areas of hydrocarbon exploration or production means that the results are directly applicable to aid the local exploration production strategy (du Rouchet, 1981; Gartrell and Lisk, 2005; Henk, 2005; Lohr, 2007; Van Gent et al., 2009). For example, Gartrell and Lisk (2005) have used 3D seismic data to calculate the present-day stress ?eld in the Timor Sea (N Australia). Lohr (2007) used 3D seismic data to constrain the stresses that caused deformation of the Top Rotliegend in the Central European Basin. Finally, Van Gent et al. (2009) showed how reactivated faults in re?ection seismic data can be used to calculate paleostress stratigraphy in the NW part of the Groningen Block (Fig. 1) by using structural reconstructions, matching of horizon shapes across faults, and the analysis of undulations of fault planes. In this study, a set of small-scale (on a seismic scale, the faults are actually roughly the same size as structures used in ?eld-based paleostress study) faults (<50 m offset) of the Upper Cretaceous Chalk Group is interpreted and analyzed in detail (Figs. 2 and 3). These faults have low offset, are fully contained inside the Chalk Group, and not reactivated by later tectonic phases. To differentiate these small-scale faults from large, long-living, cross-formational faults, we use the term “Intra-Chalk faults”. This term reflects that the studied faults do not penetrate Top or Base of the Chalk Group; but is not meant to imply syn-sedimentary faulting. Using several overlapping and detailed interpretations of a number of these faults, it will be shown that these faults commonly have a down-dip oriented undulation, which is not the result of imaging or interpretation artefacts. These undulations can be used to constrain the slip direction in the down-dip-direction (pure normal faulting). Assuming that all faults slipped in a similar fashion as the faults studied in detail, we used the orientation and related slip direction of all faults spread over the 10-15 km study area to calculate the regional paleostress tensor at the time of development of these faults. This approach differs from “normal” field-based paleostress H. van Gent et al. Journal of Structural Geology 32 (2010) 537-553 539 Fig. 2. Four seismic crosssections (aed) of part of the Groningen high. Although some meso-faults are interpreted, a high number of small-throw faults are observed between Base Upper North Sea and Base Chalk re?ectors. Orientations of the crosssections are indicated in the inset. Indicated with “Salt structure bound Graben” is the Graben that is also indicated in Figs. 3f and 5f and discussed in the text. studies in two important aspects: Firstly, this approach does not use direct fault observations from the ?eld, where usually slickenlines or slicken?bers are used to constrain slip-direction (Means, 1987). Since these are much too small (in the order of 1-5 mm) to be observed in seismic data, a different approach of constraining the slip-direction as is required (also see: Gartrell and Lisk, 2005; Lohr, 2007; Van Gent et al., 2009). The second aspect deals with the size of the study area. In ?eld-based paleostress studies, it is a common approach to compare a number of outcrop-scale (3-300 m) paleostress tensors with each other to gain insight into the regional (10-100 km) differences in stress state. In this work we use all visible faults (faults above seismic resolution) in the study area to calculate a single regional paleostress tensor for this area._ Ключевые слова: groningen, base chalk, study area, harlingen field, method, detailed interpretation, wong, goulty, extension, society, set, tensor, thickness, aapg, petroleum geology, roberts, break, plane, ?eld, wa, chalk group, north sea, study, re?ector, southern england, vandycke, geological society, based, angelier, upper cretaceous, variance map, studied fault, sperner, figs, paleostress, grabens, interpretation, fully contained, base, regional, high number, fault undulation, groningen high, yellow box, netherlands, analysis, cretaceous, faulting, cenozoic mudrocks, geometry kinematics, stress ratio, nda-calculation, advances fourth, der, geology, late, groningen block, michon, international, stress elds, wide range, sea, seismic, cartwright, petroleum, salt movement, journal, van der, orientation, nw, elsevier, area, herngreen wong, signicant difference, fault stick, collapse graben, default setting, paleostress tensor, intra-chalk fault, salt pillow, journal structural, upper, bruhn renard, observed, formation, major fault, letters, block, paleostress result, hancock, fault plane, fault surface, deformation, variance, van gent, deposit, structural geology, tectonics, normal, pollard, polygonal faulting, data, fault, massive chalk, fault orientation, geophysics, geophysical, groningen area, slip, principle stress, hydrocarbon exploration, ommelanden formation, direction, graben, geological, reservoir, eld study, undulation, dip, seismic study, interpreted, nda, basic, interpretation fault, jones, herngreen, seismic data, bulletin, scholz power, paleostress analysis, chalk interval, amsterdam, model, point, north, group, map, salt structure, inverted basin, calculated, tectonic, cretaceous chalk, interpretation prole, scale, small-scale fault, interpretation direction, fault interpretation, polygonal fault, interpretation uncertainty, concentric gridlines, stress, calculated paleostress, mist angle, interpretation noise, oxford, surface, angle, spaced count, mist histogram, internal friction, journal structural geology, corner, gridding algorithm, high ribbon, thickness analysis, interpreted fault, renard, scheck, data set, stress tensor, ax, interpreted point, gent, santonian deposit, calculate, tectonophysics, structure, isochore, ha, detailed, reactivated fault, high, result, studied, chalk deposit, salt, ekosk formation, chalk, slip direction, oblique view, mohr, structural, gent journal, paleostress calculation, hibsch, late cretaceous, lohr, eds, glennie, fault studied, van, basin, inversion, interpretation plane, interpolation method