Mineralogical and geochemical analysis of Fe-phases in drill-cores from the Triassic Stuttgart Formation at Ketzin CO2 storage site before CO2 arrival

Reactive iron (Fe) oxides and sheet silicatebound Fe in reservoir rocks may affect the subsurface storage of CO2 through several processes by changing the capacity to buffer the acidification by CO2 and the permeability of the reservoir rock: (1) the reduction of threevalent Fe in anoxic environments can lead to an increase in pH, (2) under sulphidic conditions, Fe may drive sulphur cycling and lead to the formation of pyrite, and (3) the leaching of Fe from sheet silicates may affect silicate diagenesis. In order to evaluate the importance of Fe-reduction on the CO2 reservoir, we analysed the Fe geochemistry in drill-cores from the Triassic Stuttgart Formation (Schilfsandstein) recovered from the monitoring well at the CO2 test injection site near Ketzin, Germany. The reservoir rock is a porous, poorly to moderately cohesive fluvial sandstone containing up to 2–4 wt% reactive Fe. Based on a sequential extraction, most Fe falls into the dithionite-extractable Fe-fraction and Fe bound to sheet silicates, whereby some Fe in the dithionite-extractable Fe-fraction may have been leached from illite and smectite. Illite and smectite were detected in core samples by X-ray diffraction and confirmed as the main Fe-containing mineral phases by X-ray absorption spectroscopy. Chlorite is also present, but likely does not contribute much to the high amount of Fe in the silicate-bound fraction. The organic carbon content of the reservoir rock is extremely low (\0.3 wt%), thus likely limiting microbial Fe-reduction or sulphate reduction despite relatively high concentrations of reactive Fe-mineral phases in the reservoir rock and sulphate in the reservoir fluid. Both processes could, however, be fuelled by organic matter that is mobilized by This article is part of a Topical Collection in Environmental Earth Sciences on ‘‘Subsurface Energy storage’’, guest-edited by Sebastian Bauer, Andreas Dahmke and Olaf Kolditz. & Patrick Meister [email protected] 1 Section 5.3 Geomicrobiology, GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Telegrafenberg, 14473 Potsdam, Germany 2 Institute of Geological Sciences, Jagiellonian University, Gronostajowa 3a, 30-387 Kraków, Poland 3 Institute of Geosciences, Friedrich Schiller University of Jena, Burgweg 11, 07737 Jena, Germany 4 Department of Engineering and Natural Sciences, University of Applied Science Merseburg, 06217 Merseburg, Germany 5 Max-Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany 6 Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA 7 National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK 8 School of Civil Engineering and Geoscience, Drummond Building, Newcastle University, Newcastle-upon-Tyne NE1 7RU, UK 9 School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794-5000, USA 10 Center for Crystallography and Applied Material Sciences, Department of Geosciences, University of Bremen, Bibliothekstraße 1, 28359 Bremen, Germany 11 Department of Geodynamics and Sedimentology, University of Vienna, Althanstr. 14, 1090 Vienna, Austria 123 Environ Earth Sci (2017) 76:161 DOI 10.1007/s12665-017-6460-9