Sulfur isotopic analysis of sedimentary pyrite at micro.nano scale and its implications to oil/ gas generation in the Dongying Depression

Secondary Ion Mass Spectrometry (SIMS)has been widely applied to in situ sulfur isotopic analysis for the study of mineralization mechanisms, paleoenvironmental reconstruction, atmospheric sulfur source tracing and planetary evolution. The application of nano scale SIMS (CAMECA Nano SIMS 50L) has achieved the sulfur isotope spot analysis method with a spatial resolution of 1 similar to 2 mu m. Thus,sufficient signal amount cannot be achieved due to limited analytical depths,resulting in low analytical precision (1. 5%). Recently,we report a new method that simultaneously improves spatial resolution and precision of sulfur isotopic analysis based on the NanoSIMS imaging mode. This method uses a long acquisition time (e. g.,3h)for each analytical area to obtain sufficient signal amount. The optimal spatial resolution of similar to 100nm for sulfur isotopic analysis can be implemented with an analytical precision of similar to 1% (1SD). In order to further optimize the analysis process to extend the application of Nano SIMS sulfur isotopic analytical method,this study systematically compares the characteristics and analysis process of NanoSIMS sulfur isotopic spot analytical methods and imaging analysis methods. We then applied the above methods to study pyrites in the Dongying Depression. Sub.micron sulfur isotopic variations were observed,which were used to reveal the genesis of pyrite in the reservoir and yield implications for oil charging