Petrological characterization of the Cambrian sandstone reservoirs in the Baltic Basin, Sweden

Abstract: The Cambrian sandstone reservoirs of the Baltic Basin have been identified by the Geological Survey of Sweden (SGU) as potential CO2 storage sites in line with regulations regarding Carbon Capture and Storage (CCS). In order to understand the petrological and petrophysical characteristics of the sandstone units, 54 thin sections from the three main Cambrian sandstone units were studied. The description of the thin sections involved the use of scanning electron microscopy, backscatter electron and cathodeluminescence. The sandstone units are the Faludden, När and Viklau sandstones. The Faludden sandstone forms part of the Borgholm Formation whereas the Viklau and När form the File Haidar Formation. In addition to thin sections, core samples were also studied and this data was synthesized with petrophyisical data available at the SGU-archive in Lund. Of the 54 thin sections, 19 from the Faludden sandstone unit were sent to the Geological Survey of Denmark and Greenland (GEUS) for measurement of porosity and permeability, to add to the data already available at the SGU archive in Lund. The purpose is to provide a comprehensive petrological report on the variations of porosity, permeability, mineralogy, texture and cementation with depth. The results show that it is difficult to establish trends with depth in the study area because it is located in the basin peripheries at relatively shallow depth. However, the results showed that calcite and authigenic quartz are significant cement material which may reduce or obliterate porosity. The authigenic quartz is partly euhedral towards the empty pore space, partly filling the pore space and sometimes completely around the detrital quartz grain. The carbonate cements exhibit poikolotopic texture and where it occurs, the detrital grains show a low grain packing density. The results of the modal analyses showed that the Faludden sandstone, the youngest of the reservoirs, has the highest average porosities of 17% and the Viklau and När sandstones have 6.7 and 11.4% respectively. The deeper lying Viklau sandstone unit is made up of coarse grained sandstone which is not homogenous. The study also shows apatite as an early cementation phase and it has not been found elsewhere in the Cambrian reservoirs of the Baltic Basin. Therefore, it is important to carry out further studies of this apatite in order to determine its source and the diagenetic processes behind its formation.