Understanding the physical-chemical features of liquid, gas and solid phases in natural analogue reservoirs of Carbon Capture and Sequestration (CCS) site is fundamental as they can provide key data for building up both conceptual and numerical modeling of reaction path for gas-water-rock interaction in high pCO2 systems. The aim of this work is improve the knowledge about these processes, by employing appropriate methods for compositional data on a case study, focusing on the solid (minerals) phases. In the early eighties, the PSS1 well (Eastern Tuscany, Central Italy), drilled down to almost 5,000 m for oil exploration by ENI (Italian National Agency of Hydrocarbons), intercepted a high pressure (≈700 bar) CO2 reservoir at the temperature of 120 °C. The reservoir rocks in the fertile horizon, located at about 3,800 m, consist of altered volcanic deposits interbedded with gypsumdolomite-bearing evaporites (“Burano Formation”). Surveys for determining the actual paragenesis of volcanic rocks, carried out on the drill core samples, corresponded to the top of CO2 reservoir (3,864-3,871 m depths from surface on the PSS1 bore-well log). Quartz, Ca-Fe-Mg carbonates, clay minerals (illite and chlorite series) and Fe-Ti oxides were found as principal mineralogical phases. Electron Microprobe Analysis on the carbonates has allowed to recognize the presence of ankerite and calcites. Compositional data, related to atomic % content of Ca, Fe, and Mg in carbonates minerals, were transformed by using Isometric Log-Ratio balances, whilst the variability affecting the data pattern was investigated in simple binary diagrams. The stoichiometric substitution processes governing the presence of Ca, Fe and Mg in carbonates were modeled by using regression techniques in the new space defined by ilrs coordinates. Results have evidenced the different role of Fe and Mg in substituting or not Ca in both carbonate minerals of these CO2-bearing reservoir rocks