Comparison of the microbial community composition of pristine rock cores and technical influenced well fluids from the Ketzin pilot site for CO₂ storage

Two geological formations at the CO₂ storage pilot site in Ketzin (Germany) were geochemically and microbiologically characterized to further evaluate changes resulting from CO₂ injection. Well fluids were collected from both Stuttgart (storage formation, ~650 m depth) and Exter Formations (~400 m depth, overlying the caprock) either through pump tests or downhole samplings. Rock samples were retrieved during a deep drilling into the Exter Formation and primarily comprised quartz, ferrous dolomite or ankerite, calcite, analcime, plagioclase and clay minerals, as determined through X-ray diffraction analyses. In the rocks, the total organic carbon (TOC), which potentially contributes to microbial growth, was mostly below 1000 mg kg⁻¹. The geochemical characterization of fluids revealed significant differences in the ionic composition between both formations. The microbial characterization was performed through fluorescence in situ hybridization and 16S rRNA gene fingerprinting. In the fluids obtained from the Stuttgart Formation, the microbial activity was affected by the relatively high TOC, introduced by the organic drill mud. The total cell counts were approximately 10⁶ cells mL⁻¹. The microbial community was characteristic of a saline deep biosphere environment enriched through increased carbon availability, with sulfate-reducing bacteria as the most abundant microorganisms (up to 60 % of total cells). Species belonging to halophilic/halotolerant Proteobacteria and Firmicutes were primarily detected. In Exter Formation rocks, Proteobacteria and Actinobacteria were detected. These data provide an explicit reference to further evaluate environmental changes and community shifts in the reservoir during CO₂ storage and provide information for evaluating the storage efficiency and reliability.