Arbeitsbericht NAB 18-34

Preliminary horizon and structure mapping of the Nagra 3D seismics JO-15 (Jura Ost) in time domain

In the context of the "Sachplan Geologische Tiefenlager" ("Sectoral Plan for Deep Geological Repositories"; SGT), Nagra is currently investigating three siting regions in northern Switzerland: Jura Ost, Nördlich Lägern and Zürich Nordost. As part of these investigations, Nagra carried out three 3D seismic surveys over the years 2015 to 2017. This report summarises and illustrates the preliminary horizon and structure mapping of the newly available Nagra 3D seismics JO-15. The survey covered an area of 96.3 km2 in the Jura Ost siting region (JO).

The horizon and structure mapping of the JO-15 3D seismics presented here is carried out in time domain. It is considered preliminary as borehole interpretation control is presently limited to only one borehole, the pre-existing Riniken borehole, and the refinement processing of the seismic data including depth migration is still ongoing.

The present report summarises the current status of the work and is intended to provide a preliminary structural overview of the siting region, focusing on the Mesozoic sedimentary sequence and the larger-scale faults dissecting it. It will serve various planning aspects in SGT Stage 3, most importantly those related to the deep borehole campaign in the JO siting region (e.g. the definition of drilling work programmes) and the basic scoping of underground access facilities.

The synthetic seismogram of the Riniken borehole was revised during this study, considering the newly processed 3D seismic data. During seismic-to-well tie, a correlation coefficient of about 70 % was achieved, pointing to some remaining uncertainties. The further interpretation work was carried out using the Petrel software suite. Based on the seismic-to-well tie with the Riniken borehole, seven seismic marker horizons were defined. The important regional marker horizon Base Tertiary was not drilled by the Riniken borehole and had to be mapped conceptually on the basis of outcrop indications.

The horizon interpretation was done largely manually on a so-called seedline grid consisting of every 5th in- and crossline of the seismic dataset. Fault interpretation was done in the same context focusing on larger-scale faults with vertical throws of more than 10 ms. The interpretation of fault traces was additionally guided by seismic attribute volumes and horizon slices that were calculated and processed as part of the project. The horizon interpretation was completed by applying autotracking algorithms.

Most of the defined marker horizons were found to be well traceable throughout the entire survey area. Only locally, some marker horizons, including Top Opalinus Clay and Top Liassic, were more difficult to interpret. Time thickness maps of the intervals confined by the various seismic marker horizons show pronounced variations in some cases, most notably the Upper & Middle Dogger interval and the Opalinus Clay. These variations point towards lateral sedimentological changes (e.g. in facies) and subtle synsedimentary tectonic activity in the area, but must be treated with caution at this stage, with all seismic data being available in time domain only at present.

Among the previously known main faults, confirmed by the 3D seismic mapping are the Mandach Thrust north of the Jura Ost siting region, the Siggenthal Anticline, as well as the Jura Main Thrust marking the southern boundary of the siting region. The latter is only marginally covered by the JO-15 3D seismic survey. It is not clearly imaged, which is why a detailed interpretation was not undertaken as part of this study. The 3D seismics also allowed identification of a number of new larger-scale structures in the investigation area. These include the Effingen and Umiken Faults, both sub-vertical and trending N-S and NNE-SSW respectively, as well as the Villnachern Fault, a strongly segmented fault in the south-central part of the survey area striking E-W. At least locally, these faults affect or even dissect the potential repository host rock Opalinus Clay.

Besides the above-mentioned main structures, a number of structure zones that encompass areas characterised by less discrete deformation were identified. Their extent is mostly restricted to Triassic and lower strata but at least locally, these zones also have an attenuated effect on the above lying units. The Unterbözberg Structure Zone in the central part of the investigation area is characterised by a number of circular depressions and polygonal fault geometries at the Top Muschelkalk level, possibly related to dissolution processes and located above a NW-SE trending fault zone rooting in the basement. The Zeihen and Gallenkirch Structure Zones are contractional deformation zones characterised by thrusts and reverse faults as well as gentle folds. These zones mark a structural overprint of the area of interest, probably related to the same tectonics that drove the development of the adjacent Jura Fold-and-Thrust Belt. This overprint particularly concerns the lower part of the Triassic sequence hosting the thrust belt's basal décollement. However, more subtle indications for shallow dipping - and hence hard to identify - thrust faults are also found at higher stratigraphic levels. Since such structures apparently also affect the Opalinus Clay it is foreseen to carry out related follow-up interpretation work. The tectonic significance of the Brugg Structure Zone south of the siting region is still unclear as this pronounced structural depression may be related to the as yet not fully resolved static correction in the area of the Lower Aare Valley. In addition to these structural elements, a number of other structures can be identified on seismic attribute horizon slices. Their interpretation is to be further elaborated.

Overall, the preliminary horizon and structure mapping of the JO-15 3D seismic dataset confirmed the principal structural characteristics of the JO siting region, but also revealed a lot of new details. Looking ahead, further interpretation work on the JO-15 dataset is particularly needed to improve the understanding of the kinematic relationship between the various identified structures (e.g. apparent thrusts recognised at various stratigraphic levels and newly identified subvertical faults) and seismofacies phenomena (e.g. the apparent facies transition from the Klingnau to the Hauptrogenstein Formation in the west of the investigation area). The high-quality 3D seismic dataset also shows great potential for studying the Permo-Carboniferous Constance – Frick Trough underlying the siting region but not yet addressed by the 3D seismic mapping presented here. These and other aspects will be further investigated in the later course of the SGT when the presented seismic interpretation will be further refined considering currently ongoing refinement processing and additional geological information from deep boreholes.