# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 3 Results and Discussion
### X-Ray Diffraction (XRD) Analysis
Following hydrogen exposure, the treated sample Core BH (Figure 2b) exhibited the complete disappearance of several calcite-associated peaks, including those at 31.3\({}^{\circ}\), 43.2\({}^{\circ}\), and 57.3\({}^{\circ}\) (the (006), (202), and (122) planes), along with the 20.9\({}^{\circ}\) peak corresponding  
Figure 2: XRD diffractograms of raw core samples before and after hydrogen exposure at 100 bar: (a) dolomite (A/AH), (b) limestone (B/BH), and (c) siliceous rock (C/CH). to the (004) plane of vaterite. These changes imply strong reactivity between hydrogen and carbonate phases, particularly calcite, resulting in progressive dissolution [17]. The reduction of carbonate by H\({}_{2}\) likely increases the acidity of the pore fluid [9; 15], thereby accelerating the dissolution process and possibly leading to methane formation, as outlined in reaction (3) [32]. \[CaCO_{3}+5H_{2}\leftrightarrow Ca^{2+}+CH_{4}+3H_{2}O+2e^{-} \tag{3}\]  
The siliceous rock sample (Core C), prior to hydrogen exposure, exhibited a mineralogical composition dominated by quartz, accompanied by minor amounts of chlorite and albite, as shown in Figure 2c. The diffractogram revealed prominent quartz peaks at 20.8\({}^{\circ}\), 26.7\({}^{\circ}\), 36.45\({}^{\circ}\), 39.5\({}^{\circ}\), 42.5\({}^{\circ}\), 45.9\({}^{\circ}\), 50.2\({}^{\circ}\), 54.9, and 60.2\({}^{\circ}\), corresponding to the (100), (101), (110), (102), (200), (201), (112), (202), and (211) planes [28; 33]. In addition, chlorite reflections were noted at 6.2\({}^{\circ}\) and 12.5\({}^{\circ}\) (the (001) and (002) planes), with a distinct illite signal at 8.9\({}^{\circ}\) (100) [29], and peaks at 27.92\({}^{\circ}\) (albite (112)) [33] and 29.4\({}^{\circ}\) (calcite (104)) [27], indicating minor accessory phases. Following hydrogen exposure, the XRD analysis of the siliceous rock sample Core CH (Figure 2c) showed that the diffraction peaks associated with quartz remained unchanged after hydrogen exposure compared to other minerals such as feldspars [30]. This observation confirms the exceptional chemical stability of quartz in reactive hydrogen-rich environments, thereby reinforcing its suitability as a long-term host matrix for underground hydrogen storage. These findings establish the baseline mineralogical composition of the three core samples, serving as a reference for comparative analysis after hydrogen exposure. Tables 1-3 provide quantitative insights into the compositional changes in core samples A, B, and C before and after hydrogen exposure, highlighting the extent of mineral dissolution and secondary transformations as reflected in the variation in mineral content (%). These variations, more than peak position shifts, directly illustrate the chemical reactivity and structural resilience of different lithologies under high-pressure hydrogen conditions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Mineral Phase**} & \multicolumn{2}{c}{**Crystal**} & \multirow{2}{*}{**2\(\theta\) (\({}^{\circ}\))**} & **Content \% in** & **Content \% in** \\  & **Plane—Miller** & & **Core A** & **Core AH** \\ \hline Chlorite (Ch) & 001 & 6.2 & 5.62 & - \\ Illite (I) & 001 & 8.9 & 2.84 & 2.41 \\ Quartz (Q) & 100 & 20.8 & 2.49 & 2.10 \\ Dolomite (D) & 012 & 24.1 & 1.92 & - \\ Quartz (Q) & 101 & 26.6 & 4.13 & 3.46 \\ Albite (A) & 112 & 27.92 & 2.03 & - \\ Calcite (C) & 104 & 29.4 & 1.32 & - \\ Dolomite (D) & 104 & 30.88 & 12.20 & 10.53 \\ Dolomite (D) & 110 & 37.25 & 1.31 & - \\ Dolomite (D) & 113 & 41.05 & 3.17 & 3.13 \\ Dolomite (D) & 202 & 44.85 & 2.04 & - \\ Dolomite (D) & 018 & 50.4 & 2.19 & 2.21 \\ Dolomite (D) & 116 & 50.9 & 2.68 & 2.84 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristic XRD reflections and variations in mineralogical composition of Core A and Core AH samples.