# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 3 Results and Discussion
### X-Ray Diffraction (XRD) Analysis
In Table 1, the dolomitic core (A) exhibits a clear reduction in dolomite content after hydrogen treatment (from 12.20% to 10.53% for the dominant (104) plane, and complete disappearance of other dolomite components), confirming the selective dissolution of CaMg(CO\({}_{3}\))\({}_{2}\). This process is facilitated by hydrogen in the presence of moisture, which triggers the partial decarbonation and release of Ca\({}^{2+}\) and Mg\({}^{2+}\) ions. Concurrently, the complete loss of calcite (from 1.32% to 0%) and albite (2.03% to 0%) suggests a destabilization of accessory phases, while the increase in the (116) dolomite plane (from 2.68% to 2.84%) may indicate local recrystallization or lattice reorganization, as mentioned above. These compositional shifts align with the dissolution-reprecipitation mechanism and support the formation of secondary Fe-rich carbonates (ankerite), as also confirmed by EDS. In Table 2, the limestone core (B) demonstrates a sharp decrease in the vaterite content (e.g., from 6.05% to 4.82% at 27\({}^{\circ}\), and complete loss at 20.9\({}^{\circ}\)), as well as the disappearance of several calcite planes (e.g., from 2.35% to 0% at 31.2\({}^{\circ}\) and from 1.11% to 0% at 43.1\({}^{\circ}\)). These losses indicate intensive carbonate dissolution, likely promoted by pore fluid acidification due to hydrogen-carbonate interactions. The slight increase in some vaterite and calcite \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Crystal**} & & \\
**Mineral Phase** & **Plane—Miller** & **2\(\theta\) (\({}^{\circ}\))** & **Content \% in** & **Content \% in** \\  & **Index (hkl)** & & **Core B** & **Core BH** \\ \hline Vaterite (V) & 004 & 20.9 & 2.61 & - \\ Vaterite (V) & 110 & 24.8 & 3.87 & 3.43 \\ Vaterite (V) & 112 & 27.1 & 6.05 & 4.82 \\ Calcite (C) & 104 & 29.3 & 6.42 & 6.37 \\ Calcite (C) & 006 & 31.2 & 2.35 & - \\ Vaterite (V) & 114 & 32.7 & 5.13 & 5.44 \\ Calcite (C) & 113 & 39.38 & 1.66 & - \\ Calcite (C) & 202 & 43.2 & 1.11 & - \\ Vaterite (C) & 300 & 43.8 & 2.98 & 3.50 \\ Calcite (C) & 018 & 47.4 & 1.75 & 1.75 \\ Calcite (C) & 116 & 48.5 & 2.68 & 2.94 \\ Vaterite (V) & 224 & 55.7 & 1.27 & 1.64 \\ Calcite (C) & 122 & 57.3 & 0.52 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristic XRD reflections and variations in mineralogical composition of Core B and Core BH samples. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Crystal**} & & \\
**Mineral Phase** & **Plane—Miller** & **2\(\theta\) (\({}^{\circ}\))** & **Content \% in** & **Content \% in** \\  & **Index (hkl)** & & **Core B** & **Core BH** \\ \hline Chlorite (Ch) & 001 & 6.2 & 6.87 & 5.94 \\ Illite (I) & 001 & 8.9 & 2.62 & - \\ Chlorite (Ch) & 002 & 12.5 & 2.19 & 3.22 \\ Quartz (Q) & 100 & 20.8 & 3.57 & 3.78 \\ Quartz (Q) & 101 & 26.7 & 14.64 & 13.87 \\ Albite (A) & 112 & 27.92 & 2.58 & - \\ Calcite (C) & 104 & 29.4 & 3.90 & - \\ Quartz (Q) & 110 & 36.45 & 1.52 & 1.77 \\ Quartz (Q) & 102 & 39.5 & 1.92 & 1.68 \\ Quartz (Q) & 200 & 42.5 & 1.16 & 1.39 \\ Quartz (Q) & 201 & 45.9 & 1.30 & 1.52 \\ Quartz (Q) & 112 & 50.2 & 2.02 & 2.88 \\ Quartz (Q) & 211 & 60.2 & 1.50 & 1.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristic XRD reflections and variations in mineralogical composition of Core C and Core CH samples. reflections (e.g., 43.8\({}^{\circ}\) vaterite from 2.98% to 3.50%, 50\({}^{\circ}\) calcite from 2.68% to 2.94%) suggests secondary carbonate precipitation possibly clogging pore throats and contributing to the observed drop in porosity and permeability. This mineralogical behavior reinforces the idea that calcite formations are structurally vulnerable under reductive hydrogen environments. Table 3 shows minimal compositional changes in the siliceous core (C), particularly for quartz, whose dominant reflections remain almost unchanged (e.g., 14.64% to 13.87% for 26.7\({}^{\circ}\), and 3.57% to 3.78% for 20.9\({}^{\circ}\)).