Identifying the composition of hydrocarbon inclusions in rock salt

Rock salt is one of the three potential host rocks for the final repository for high-level radioactive waste in Germany. For the site selection procedure and for safety assessments, the content and composition of trapped fluids can be crucial to evaluate the potential of gas formation. Certain salt formations in northern Germany are known to contain gases and hydrocarbons as fluid inclusions. Fluid inclusion studies can provide information on their composition or temperature- and pressure conditions of fluid migration. Investigating individual fluid inclusion assemblages has certain advantages compared to bulk analyses: (1) the possibility to differentiate between different fluid sources and/or (2) the reconstruction of a complex fluid history. Among the suitable methods for the analyses of the fluid inclusion composition is Raman spectroscopy. Here we used a Renishaw inVia QONTOR Raman spectrometer attached to a Leica microscope (DM3000) equipped with a 100x objective and a 532 nm laser. Raman spectra were obtained with a 2400 grating, at ~5 mW, 180–240 s acquisition time and 2 accumulations in the spectral range of 300–4000 cm^-1. The analyses of the hydrocarbon phase show peaks at ~940, at 2500–2700 and 2900–3100 cm^-1 indicating C-H stretching modes, potentially of alkanes and their derivatives (Dollish et al., 1974; Orange et al. 1996).