Innovative characterization of organic nanopores in marine shale by the integration of HIM and SEM

Abstract

The characterization of organic nanopores across the entire range is essential for gas storage and productivity evaluation of marine shale. To achieve a precise and comprehensive analysis of organic nanopore properties, an integration method of helium ion microscope (HIM) and scanning electron microscope (SEM), as well as focused ion beam-HIM (FIB-HIM) and FIB-SEM, was proposed. This integration method extends the organic pore imaging and quantitative analysis to below 10 nm. Results demonstrate that organic pores below 10 nm significantly impact the evaluation of shale gas storage and productivity by contributing to the porosity and specific surface area and altering pore size distribution and connectivity characteristics. The surface porosity increases by 10.73%, while the specific surface area increases by 76% according to the integration method. The integrated pore size distribution converts the unimodal PSD from the stitched SEM mosaic to a multimodal PSD, supporting the multi-stage formation mode of organic nanopores: larger bubble pores appear with oil generation at the early stage, while smaller spongy pores appear with gas generation at the later stage. The organic pores revealed by FIB-HIM have higher coordination numbers and a higher proportion of inner-connected volume, showing the more complex connectivity characteristics. Despite limitations due to heterogeneity and inadequate resolution, these novel insights by the integration method highlight the importance of exploring organic pores at the nanoscale and have the potential to inform further research in this field.