Imaging Texture and Porosity in Mudstones and Shales: Comparison of Secondary and Ion-Milled Backscatter SEM Methods
Observations from a number of unconventional reservoirs lead us to conclude that four major pore types exist in fine-grained reservoir and non-reservoir rocks, that they are effectively connected, and that pore sizes from nanometers to microns must be considered when evaluating size distributions. This paper uses SEM imaging of Haynesville, Horn River, Barnett and Marcellus Shales to illustrate that pore types other than those hosted by organics are present in unconventional shale gas reservoirs, and that they are continuous and connected to kerogen-hosted pores. In addition, we present evidence that the maximum size of pores originating in organic matter is determined by the size of the kerogen mass (in the case of organic particles) or the geometry of enclosing crystals (in the case of amorphous, pore-filling kerogen). Pairs of secondary and ion-milled backscatter SEM images address the misconception that large pores observed in secondary electron images are grain pullouts.
2D image analysis and 3D volumetric reconstructions to study pore distributions should take rock microtexture and the various pore types into consideration. A combined method using thin section textural analysis, XRD, and SEM imaging is recommended to address scaling issues when choosing samples for 2D and 3D volumetric analysis.