Small-scale faulting in the Upper Cretaceous of the Groningen block (The Netherlands): 3D seismic interpretation, fault plane analysis and regional paleostress

摘要

Over the last years, field-based studies have shown that fault surfaces can exhibit a considerable self-affine topography. It is reasonable to assume that similar undulations are also present in fault interpretations from 3D reflection 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 km~2, 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.rnBased 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 NW-SE-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 field-based paleostress results from the UK, Belgium and France. A first 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.