Geologisk karaktärisering av den kambriska Faluddensandstenen i Östersjön och dess lämplighet för koldioxidlagring

Sammanfattning: The Geological Survey of Sweden (SGU) has spent several years investigating the Swedish bedrock to find suitable bedrock for geological storage of carbon dioxide. Deep saline aquifers are considered to have the greatest potential, and the Cambrian Faludden Sandstone subcropping in the Baltic Sea east of Gotland, shows great potential as a storage aquifer. In this study, the drill core St. Sutarve-2018 from southern Gotland has been studied to investigate the mineralogy and texture of the Faludden Sandstone, with the aim of more accurately investigating its suitability for carbon dioxide storage. Three thin sections from different depths (547.3 m, 552.4 m and 559.35 m) from St. Sutarve-2018 were studied under a polarizing microscope. Two additionally thin sections from drill cores B7 (862 m) and B9 (1030 m), which are drilled in the Baltic Sea, were also used in the study to determine whether there are differences in the sandstone depending on burial depth. By point counting (modal analysis), the mineralogical composition of the sandstone was investigated. Additionally, an ocular description of the Faludden Sandsten in St. Sutarve-2018 was made, resulting in a log. The results show that the sandstone mainly consists of white to light grey, fine-grained and well-sorted quartz sand. Thin horizontal layers of clay, as well as precipitates of pyrite are common. Vertical fissures are also observed. In addition to quartz, the accessory minerals consist mainly of pyrite and weathered feldspar. Pressure dissolution and compaction become more noticeable at greater depths, while porosity decreases. Siliceous cement is most common, often as quartz overgrowths, but carbonate cement is also observed. The modal analyses in this study are comparable to previously performed analyses and give largely the same results. The porosity of the sandstone varies slightly between different studies, but overall, it corresponds well with the requirements for carbon dioxide storage. The permeability, thickness and extent of the sandstone also indicate that it is suitable for carbon dioxide storage. Further investigations should focus on mapping heterogeneities in the bedrock, such as fractures, impermeable layers and variations in porosity and permeability, as these can have a major impact on carbon dioxide diffusion paths.