Validation of High Pressure Resin Impregnation Technique for High Resolution Confocal Imaging of Geological Samples

In this paper, we present a procedure for high pressure resin impregnation of microporous rock. This procedure produces the highquality pore casts that reveal the fine details of the complex pore space of micritic carbonates. We carefully test our resin impregnation procedure and demonstrate that it renders the high resolution, 3D confocal images of pore casts. In our work, we use silicon micromodels as a reference to validate the key parameters of high-pressure resin impregnation. We demonstrate possible artifacts and defects that might develop during rock impregnation with resin, e.g., the resin shrinkage and gas trapping. The main outcome of this paper is a robust protocol for obtaining the high-quality epoxy pore casts suitable for rock imaging with Confocal Laser Scanning Microscopy (CLSM). We have implemented this protocol and provided the high resolution, three-dimensional (3D) imagery and description of microporosity in micritic carbonates. Introduction Saturation of permeable rock samples with a pore-filling material, e.g., epoxy, that provides a contrast between the optical and/or electric properties of the void space and mineral grains has been reported by several researchers, for example (Waldo and Yuster, 1937, Pittman and Duschatko, 1970, Wardlaw, 1976, Yanguas and Paxton, 1986, Klaver et al., 2015, Jobe et al., 2018). The pore-filling epoxy resin supports fragile pores at different stages of thin-section preparation, such as grinding or polishing. Also, optical contrast between the dyed epoxy resin and rock helps in pore identification. For geological samples, epoxy resin casting is typically used to preserve the mechanical integrity of the sample after polishing it down to 15 microns. Blue dye enables porosity counting, and fluorescent dyes have been getting more attention recently in combination with confocal imaging. Several studies have reported applications of epoxy pore casts to gain better understanding of fluid flow capacity of carbonate rocks (Cantrell et al., 1999, Morrow and Buckley, 2006, Fullmer et al., 2014). Despite the broad importance of rock saturation with epoxy resin, currently there exists no standard approach for high pressure pore casting. The impregnation procedure in general consists of three steps: i) resin preparation, ii) rock sample degassing, and, and iii) resin injection into the sample. Depending on the subsequent characterization method, these three steps may differ significantly. For example, for optical microscopy a dye must be mixed with the resin, while electron microscopy does not require a dye. For confocal laser microscopy, an additional step of etching the rock is required to obtain the 3D rock images (Hassan et al., 2017). In this work, we have critically evaluated and optimized the resin impregnation procedure that generates the high-quality epoxy pore casts for high resolution 3D imaging with CLSM. Resin impregnation Vacuum impregnation is the most widely adopted method for producing thin sections of geological samples (Minoura and Conley, 1971) and other porous materials, such as cement (John, 1990). Alternative methods, such as high pressure impregnation and epoxysolvent replacement are also adopted for rock analysis (Wardlaw, 1976, Smith and Anderson, 1995). The preference for the vacuum process stems from is its low cost and general availability of laboratory equipment. Although vacuum impregnation is a simple and efficient method for preparation of the standard thin sections of rock, it has a number of limitations when the samples are too tight or microporous. Vacuum impregnation can only access a limited pore volume (Wong and Buenfeld, 2006), while excessive drying of the samples on vacuum may alter pore morphology (Hurst and Nadeau, 1995) or even induce cracks (Chen et al., 2002). For vacuum impregnation of tight samples, Kjellsen et al. (2003) showed that the epoxy penetration depth is limited to 120 μm. Thus, studies focused on the characterization of tight samples employed high pressure resin impregnation in their methodology to ensure effective impregnation throughout the sample (Yanguas and Dravis, 1985, Gies et al., 1987, Fredrich, 1999, Shah et al., 2014). Beckett