Arbeitsbericht NAB 20-30

Petrophysical Log Analyses of Deep and Shallow Boreholes: Methodology Report

During stage 2 of the "Sachplan Geologische Tiefenlager" (SGT), three potential siting regions (Jura Ost, Nördlich Lägern and Zürich Nordost) have been chosen for a detailed site investigation programme during stage 3 of the SGT to assess their suitability for the safe disposal of radioactive waste in Switzerland. Currently, a large-scale investigation programme is in place including drilling several deep boreholes to characterise the rocks of the Mesozoic (and partly also the upper Paleozoic) sedimentary sequence within the siting regions. An elaborate measurement and sampling programme will be performed, including the acquisition of petrophysical logs in the borehole and along cores and samples for laboratory measurements. In addition, logging and laboratory data from several already existing boreholes (deep boreholes as well as shallow geothermal boreholes so-called "Erdwärmesondenbohrungen" EWS) are available.
Next to other parameters, the mineralogy (and here especially the clay mineral content) and porosity of the rocks are of main importance for the site characterisation. This report presents the methodology used for the analysis of petrophysical logs for all existing and newly acquired data from boreholes to calculate the aforementioned mineralogical content and continuous porosity profiles. 

The workflow presented here will be used for any subsequent log interpretation performed for Nagra in SGT stage 3 and will be referred to in each documentation of the interpretation results. If any deviation from this workflow is necessary, it will be explained in detail in the report(s) accompanying the log interpretation results.

The aim of the petrophysical log analysis is the elaboration of a continuous profile for formation porosities and detailed mineralogical compositions. The reliability of the interpreted data will depend on the quantity and quality of log datasets or any other continuous or non-continuous measurements suitable for the workflow presented here.

The log interpretation is calibrated on core data (mineralogy, porosity) – where available – which will also be explained in detail in the following chapters.

This methodology report will use data from a previous study from the Benken borehole to provide examples of the workflow. As such, it is not intended to deliver the results of the Benken log analysis results. Also note that the stratigraphy, especially formation names, used in the composite plots shown in this report in the examples from the Benken borehole has previously been revised and hence represents slightly outdated formation and member names. 

Finally, most abbreviations used in this report refer to Schlumberger (SLB) mnemonics. Other logging contractors may use different abbreviations for their tools.