Assessment of Pore Connectivity and Representative Elementary Volume Size in Marine-Continental Transitional Shale

Abstract

Directly obtaining representative pore structure information is important to understand subsurface shale gas storage and production, while acquiring large-scale and highresolution images using a single imaging tool is plausible as there is always a trade-off between the resolution and field-of-view. We report a new method to define representative elementary volumes (REV) of a shale pore system and define pore structural parameters of diameter, surface area, porosity, and other features. Automated ultrahigh resolution scanning electron microscopy, integrated with a modular automated processing system (MAPS), was used to image pore distribution in two dimensions. Focused ion beam (FIB) milling was further utilized to construct a true three-dimensional digital image, on which the REV analysis was then carried out. The results show that (i) pores are mainly developed in organic matter (OM) and as interparticle inorganic pores and (ii) the diameter of inorganic pores is slightly larger than those in OM. The pore network coordination number, representing the average number of pores that are connected to a specific pore, indicates that the pores can be either clustered within mainly OM pores or more widely connected by slit-like pores and throats in minerals. Extracting cubic sub-blocks, ranging from 500 to 5000 nm in edge dimension, defines the minimum REV as ∼4000 nm, as measured using minimum and maximum pore sizes, surface areas, and shape factors. Combined FIB and MAPS provide insight into pore morphology and connectivity at multiple scales with the reconstructed digital rock used to determine representative REV sizes. Such results are useful in understanding the pore structure in shales and for the rapid acquisition of pore structure distributions.