Technischer Bericht NTB 86-11

The concentrations of hydrocarbon gas, carbon dioxide and hydrogen sulphide measured during the drilling of the Nagra boreholes are correlated with various drilling and drilling fluid parameters and presented in both graphic and tabular form.

The Permocarboniferous in the Weiach borehole and the Muschelkalk in the Schafisheim borehole show clear positive gas anomalies. The hydrocarbon gas concentrations in the Crystalline were many times higher than those in the overlying sediments.

Based on gas measurements (gas content and composition) for deep groundwaters and rock samples, it can be concluded for the Crystalline that gases penetrated from the rock formation through the borehole wall and into the drilling fluid and are also released from the crushed rock during the drilling process.

Measurements of hydrogen and carbon isotopes on the gases from the Crystalline of the Weiach and Schafisheim boreholes indicate that the gases originated thermocatalytically from organic fossil material; their presence could therefore be explained by, e.g., migration from the Permocarboniferous. On the other hand, the higher deuterium and ¹³C values in the Crystalline zone of Böttstein and Leuggern boreholes indicate rather that the hydrocarbon gases have an abiotic origin.