Abstract
Pore characterization in shales is challenging owing to the wide range of pore sizes and types present. Haynesville-Bossier shale (USA) was sampled as a typical clay-bearing siliceous, organic-rich, gas-mature shale and characterized over pore diameters ranging 2 nm to 3000 nm. Three advanced imaging techniques were utilized correlatively, including the application of Xe+ plasma focused ion beam scanning electron microscopy (plasma FIB or PFIB), complemented by the Ga+ FIB method which is now frequently used to characterise porosity and organic/inorganic phases, together with transmission electron microscope tomography of the nano-scale pores (voxel size 0.6 nm; resolution 1–2 nm). The three pore-size scales each contribute differently to the pore network. Those <10 nm (greatest number), 10 nm to 100 nm (best-connected hence controls transport properties), and >100 nm (greatest total volume hence determines fluid storativity). Four distinct pore types were found: intra-organic, organic-mineral interface, inter-mineral and intra-mineral pores were recognized, with characteristic geometries. The whole pore network comprises a globally-connected system between phyllosilicate mineral grains (diameter: 6–50 nm), and locally-clustered connected pores within porous organic matter (diameter: 200–800 nm). Integrated predictions of pore geometry, connectivity, and roles in controlling petrophysical properties were verified through experimental permeability measurements.
Highlights
Shales constitute two-thirds of the volume of sedimentary rocks yet are arguably the least well understood rock type of the sedimentary record[1]
This study aims to build an integrated geometric and network model of a representative shale sample based on pore occurrence using a principal component analysis (PCA) method to reconstruct the pore system, and to construct models incorporating properties of size, geometry and network connectivity
We explore the potential of Scanning (S)transmission electron microscope (TEM) tomography to provide nanometre resolution images for nano-pores and present the first-time application of Xe+ Plasma focused ion beam system (FIB) (PFIB) slice-and-view imaging in shale to provide volumes with a large field of view, further combining these with Ga+ FIB slice-and-view imaging to bridge the associated length scales
Summary
Shales constitute two-thirds of the volume of sedimentary rocks yet are arguably the least well understood rock type of the sedimentary record[1]. High-resolution (nanometre to sub-nanometre) techniques are required to image nano-pores, whilst a large field of view (tens of microns) is needed to provide a representative imaging volume. This requires high-resolution electron microscopy (EM) such as scanning electron microscope (SEM) and transmission electron microscope (TEM) instead of X-ray tomography, as they are able to provide mineral and organic matter information which cannot be acquired using present X-ray tomography techniques, due to their lower contrast and resolution[20]. There is a need for a combination of electron microscopy techniques that span a wide range of scales to visualize the geometry and networking of all pore types
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