Study on the distribution and connectivity of organic matter pores in Longmaxi shale based on 2D and 3D FIB-SEM

Knowledge of shale pore structure characteristics is crucial to understand gas storage and seepage mechanisms. Organic matter (OM) pores are considered the most important pore type in shale, and one of the currently significant research questions focuses on the spatial distribution and connectivity of OM pores. To answer this question, typical OM-rich siliceous shale samples from the Lower Silurian Longmaxi Formation were comprehensively investigated using focused ion beam scanning electron microscopy. A three-dimensional model of the OM-rich region of interest was segmented and reconstructed based on numerous two-dimensional slices. The types of OM were found to control the development of organic pores, and OM pores including honeycomb-shaped pores, spongy-shaped pores, and slit-like irregular pores are mainly formed in the pyrobitumen. The pore structure parameters of the OM-rich ROI revealed that the pore size distribution of honeycomb-shaped OM pores formed in the pyrobitumen was mainly distributed in the range of 10-50 and 80-100 nm, while the throat equivalent diameter distribution demonstrated a unimodal curve with the main peak located at approximately 30 nm. Pore connectivity analysis further indicated that pyrobitumen also contained several isolated nano-pores, and pores with diameters smaller than 40 nm were poorly connected. Furthermore, permeability simulation revealed clear discrepancies in different directions owing to the heterogeneity of the OM pores. These findings provide experimental evidence for the assessment of shale gas resources and their development potential.

Automated summary

Knowledge of shale pore structure is crucial to understand gas storage and seepage mechanisms. The study investigated the spatial distribution and connectivity of organic matter (OM) pores in shale samples using scanning electron microscopy. The findings showed that the type of OM controlled the development of organic pores, with honeycomb-shaped, spongy-shaped, and slit-like irregular pores mainly formed in pyrobitumen. The pore size distribution of honeycomb-shaped OM pores was mainly in the range of 10-50 and 80-100 nm. The permeability simulation revealed discrepancies in different directions due to the heterogeneity of the OM pores.