CO2 storage possibility through the formation of calcite in volcanic rocks (Case study: Mejdar basaltic rocks, southeast of Ardabil)

The ever-increasing global demand for energy is associated with the more consumption of fossil fuels, which has caused the widespread release of CO2 gas into the earth's atmosphere. The process of carbonation in geological formations is one of the safest and most promising approaches for CO2 storage. Majdar area is located in the northwest of Iran on the Alborz-Azerbaijan magmatic belt. Eocene volcanic units consisting of andesite, basaltic andesite, basalt, agglomerate and tuff form the thickest and widest rock units in the study area. Basalt and basaltic andesite rocks have been studied in this area. Petrographic studies showed that plagioclase, pyroxene and some olivine minerals are the main phenocrysts of host rock placed in a glassy to microcrystalline matrix. The XRF results showed that Majder basalt samples contain adequate amounts of Ca oxides (10.29–12.27 wt%), Fe (9.83–12.66 wt%) and Mg (5.75–7.45 wt% ) that can react with fluids containing CO2 to form stable carbonate minerals. According to the XRD results, three carbonate minerals including calcite, manganocalcite and ankrite are formed along with secondary alteration minerals clinochlore, clinoptilolite, illite, montmorillonite, glauconite and quartz. The study of microscopic sections showed that the alteration of calcium and magnesium minerals such as augite and anorthite by CO2-rich fluids has caused the release of cations needed for the formation of carbonate minerals. The value of δ18O isotopes in the studied samples ranges from -10.14 to -12.54. The calculations based on the results of δ18Ocalcite show the temperature range of 71.35–88.35 °C for the formation of calcite, which corresponds to a depth of 2.4-3 km. According to the measured average porosity of rocks, the possibility of storing 154,000 tons of CO2 in a block measuring 1000 meters long, 1000 meters wide and 70 meters high in the studied area can be estimated.