### Resin Impregnation  
Despite the importance of rock saturation with epoxy resin, currently, there exists no standard approach for high-pressure pore casting. The impregnation procedure generally consists of three steps: (i) resin preparation, (ii) rock sample degassing, and (iii) resin injection into the sample. Vacuum impregnation or vacuum pore casting is the most widely adopted method of resin pore casting. It is used to produce thin sections of geological samples [15], cement [16] or reinforced wood. Alternative methods, such as high-pressure impregnation and epoxy-solvent replacement, are also adopted for rock analysis [8; 17]. The preference for the vacuum process stems from its low cost, safety, and general availability of laboratory equipment.  
Although vacuum impregnation is a simple and efficient method for preparing standard rock thin sections, it has some limitations when the samples are tight or microporous. A key limitation of vacuum impregnation is that the epoxy resin cannot access the pore volume associated with sub-micron scale pore throats, a major issue in microporous and tight rock samples. In addition, the application of a vacuum for a long time for a complete pore filling of a rock sample may deform the pore shapes [18] or even induce cracks [19].  
[PERSON] and [PERSON] [20] specified certain experimental parameters for impregnating carbonate samples at high pressure generated by nitrogen gas. However, their description did not address any artifacts or defects that may emerge during the impregnation process. For example, the impact of dissolved gas in the epoxy mixture was not considered a source of defects. The vacuum level at which the degassing step occurs prior to applying high pressure has varied significantly in the literature--even for the same material.  
Porous materials were reported to be impregnated at different vacuum levels. For example, unconsolidated samples were impregnated at 700-800 mbar [21; 22], while higher vacuum levels (8-30 mbar) were used for cementitious materials [19; 23]. Rock samples on the other hand are impregnated under more aggressive vacuum levels to ensure that the air is evacuated from the sub-micron scale pore throats. Carbonates and tight rocks were reported to be degassed under 1.33 mbar [20] and 0.067 mbar [24], respectively.  
Some studies, e.g., [15; 25; 26; 27], suggested that mixing solvents, such as acetone or toluene, into epoxy material to decrease its viscosity for more efficient impregnation. Such a technique negatively impacts the shrinkage of the epoxy. However, shrinkage has been considered irrelevant or \"negligible\" in earlier studies [15; 20; 24; 25; 28; 29]. While it may hold true for linear shrinkage determined according to the standard ASTM and low solvent fractions, we will show below that other types of shrinkage defects may develop using this technique. Unless prevented from occurring or properly identified in the samples, these defects may lead to false interpretations of the subsequent high-resolution images.  
All these variations and discrepancies regarding the resin impregnation procedure have resulted in the general lack of standard sample preparation protocol. In this study, we used custom-fabricated micromodels with well-defined pore geometry to validate the parameters of high-pressure resin impregnation. Based on these experiments, we attempted to address the above-mentioned concerns and provide guidelines for obtaining high-quality pore casts for high-resolution imaging. We applied the optimized resin-impregnation-procedure and produced quality epoxy pore casts for high-resolution 3D imaging with CLSM.