# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 3 Results and Discussion
### SEM-EDS Analysis  
The SEM micrographs acquired at magnifications of 100\(\times\) and 200\(\times\) reveal substantial morphological differences between untreated and hydrogen-exposed carbonate core samples. As illustrated in Figure 5, Core A (dolomite, untreated) displays a relatively compact granular structure, composed of well-defined dolomite crystals with sharp boundaries and tight grain contact features typical of a preserved stoichiometric carbonate phase with a uniform crystalline framework [40]. Conversely, the hydrogen-exposed counterpart (Core AH) exhibits a more developed porous texture, with partially eroded grains, irregular margins, and expanded intergranular voids. Fine particles are observed to accumulate between larger crystals, indicating localized dolomite dissolution and the possible reprecipitation of secondary phases [17; 30; 31; 32]. These morphological observations corroborate the BET/BJH adsorption data and confirm the partial reorganization of the pore network following hydrogen exposure. Figure 5: Scanning electron microscopy (SEM) images of core samples A and AH. A similar trend is observed for calcite samples. As shown in Figure 6, the SEM images of Core B reveal a relatively dense microstructure formed by large, well-faceted calcite crystals with regular faces and compact intergranular contacts. The pore network is poorly developed, with minimal connectivity. After hydrogen treatment (Core BH), the microstructure becomes noticeably irregular, characterized by fragmented crystal surfaces, fine fissures, and increased grain boundary roughness. While intercrystalline voids are more apparent, they do not translate into enhanced effective porosity, likely due to partial pore clogging by redeposited carbonate phases (Figure 6). Quartz-based samples exhibit a markedly different behavior. As seen in Figure 7, Core C shows compact granular architecture with large, smooth-faced quartz crystals and a limited intergranular space. Following hydrogen exposure, Core CH maintains this structure with only minor surface roughening and the appearance of microcracks. The pore network remains largely unaffected, with no signs of structural collapse or significant  
Figure 6: Scanning electron microscopy (SEM) images of core samples B and BH. mineralogical alteration, confirming the high chemical and structural stability of quartz under reductive conditions [15; 20; 37; 39]. The EDS analysis provides further insights into the chemical evolution associated with hydrogen treatment. For dolomitic cores, the EDS spectra and elemental maps, in Figure 8, show that Core A is dominated by calcium and magnesium with uniform distribution, consistent with stoichiometric CaMg(CO\({}_{3}\))\({}_{2}\). Iron is present only in trace amounts. After hydrogen exposure, Core AH exhibits a notable increase in iron concentration and a relative decrease in magnesium. The heterogeneous distribution of Fe suggests cation exchange, where Fe\({}^{2+}\) likely sourced from siderite reduction partially replaces Mg\({}^{2+}\) in the dolomite lattice, leading to ankerite-type phase formation [31]. Other elements (Si, Al, K) show no significant compositional variation between A and AH, indicating that these elements are not actively involved in hydrogen-induced reactions (Figure 8). This SEM-EDS evidence confirms that dolomite is susceptible to both structural and chemical alterations under hydrogen exposure, primarily via selective dissolution, Fe for Mg substitution, and secondary carbonate reprecipitation. These changes reshape the  
Figure 7: Scanning electron microscopy (SEM) images of core samples C and CH. pore framework and influence the rock's petrophysical behavior, which is critical when assessing its suitability for hydrogen storage applications. The EDS analysis of the calcite cores (Figure 9) reinforces these trends. Core B exhibits a composition dominated by Ca and O with a uniform distribution of minor elements.