The geophysical characteristics of the carbonate-dominated Ordovician succession is described using wire-line logging data from exploration wells located within the Swedish part of the Baltic Basin, both offshore and from the island of Gotland. The petrophysical properties and log-motifs are compared and correlated with the lithology of cores from the Hamra-10, Skåls-1 and Grötlingbo-1 wells on southern Gotland. The 80–125-m-thick Ordovician succession is divided into five log stratigraphic units O_a–O_e, which are correlated throughout the study area. The proposed log stratigraphy and wire-line log characteristics are evaluated and compared with the established Ordovician stratigraphy from the adjacent areas of Öland, Östergötland and South Estonia. The newly established log stratigraphy is also linked to the existing seismic stratigraphic framework for the study area and exemplified with a selection of interpreted seismic type sections from Gotland and the south Baltic Sea. The presented characterization, division and correlation provide a basis for understanding the lateral and vertical variation of the petrophysical properties, which are essential in assessing the sealing capacity of the Ordovician succession, in conjunction with storage of CO₂ in the underlying Cambrian sandstone reservoir.

The Fennoscandian transition zone, including the Sorgenfrei–Tornquist Zone, constitutes the weakened and faulted bedrock between a craton, including the ancient continent Baltica to the north, and the boundary between Baltica and Avalonia along the Trans-European Fault Zone to the south. Early Permian subsidence in this transition zone resulted in the development of various basins and the initiation of a more or less continuous Permian–Paleogene depositional cycle. In southwestern Sweden, magmatic activity associated with transtensional deformation along the Sorgenfrei–Tornquist Zone prevailed during the Late Carboniferous–Permian. However, the transition zone is dominated by a Mesozoic sedimentary rock succession displaying both hiatuses and great lateral variability in composition and thickness, which can be related to several tectonic events including the progressive break-up of Pangaea. Much of the deposition took place in continental, coastal and shallow-marine settings. Early–Middle Jurassic block faulting and basanitic or melanephelinitic volcanism, as well as Late Cretaceous tectonic inversion along the Sorgenfrei–Tornquist Zone, related to a changeover to a predominantly compressive tectonic regime coeval with the Alpine orogeny, significantly influenced the depositional setting. Subsequent Paleogene–Neogene regional uplift of the southwestern margin of Baltica resulted in significant erosion of the bedrock.

Reflection seismic data were acquired in the Sudret area of Gotland between 6 and 13 November 2023. Objectives of the survey were to obtain images of the subsurface down to the Precambrian basement in the vicinity of two cored boreholes that had been drilled earlier down to about 800 m. The seismic profiles were positioned to provide a better understanding of the sedimentary strata and local structure near the two boreholes. It was also hoped that they could be used to correlate the properties of the geological formations offshore for studying the potential of future geological storage of CO₂ within Swedish waters. For these purposes a sparse 3D survey was acquired that covered a ca. 300 m by 700 m rectangular area with high fold, including the locations where the boreholes were drilled. A longer ca. 2.8 km 2D profile was also acquired adjacent to the 3D survey that ran roughly in a N–S direction. In addition, distributed acoustic sensing (DAS) measurements were performed in the two cored boreholes. We report here on some results from the 2D and 3D surveys and from the DAS measurements, incorporating information from the core and sonic logs. Numerous semi-continuous reflection horizons are observed in the ca. upper 500 ms after stacking. A particularly strong reflection at about 330 ms likely originates from the top of Ordovician limestones. Generation of synthetic seismograms based on the acquired sonic logs in the two boreholes confirms this interpretation. Cambrian sandstones are also reflective, as well as shallow sandstone layers in the upper 150 ms. Normal moveout (NMO) velocities are relatively constant at about 3500 m s⁻¹. However, depth conversion using this velocity places the reflectivity deeper than is expected from the well data. In comparison, using the DAS data, the vertically propagating P-wave velocity can be measured at an average 3100 m s⁻¹ from the surface to 580 m depth. Using this velocity for depth conversion provides more reasonable depths to the main horizons. Since the NMO velocities are largely controlled by the horizontal velocity of the rock the difference between these and the DAS velocity can be explained by the rocks in the area having significant anisotropy. Seismic modeling indicates that a horizontal velocity of about 3500 m s⁻¹ is necessary to explain the difference between the NMO velocity and the vertical velocity. This corresponds to an anisotropy of about 13 %. This may be important to take into account when acquiring and processing future, or legacy, offshore seismic data for the purpose of mapping potential structures or formations for CO₂ storage.

Assessing the optimal placement and design of a large-scale high temperature energy storage system in crystalline bedrock is a challenging task. This study applies and evaluates various methods and strategies for pre-site investigation for a potential high temperature borehole thermal energy storage (HT-BTES) system at Linköping in Sweden. The storage is required to shift approximately 70 GWh of excess heat generated from a waste incineration plant during the summer to the winter season. Ideally, the site for the HT-BTES system should be able to accommodate up to 1400 wells to 300 m depth. The presence of major fracture zones, high groundwater flow, anisotropic thermal properties, and thick Quaternary overburden are all factors that play an important role in the performance of an HT-BTES system. Inadequate input data to the modeling and design increases the risk of unsatisfactory performance, unwanted thermal impact on the surroundings, and suboptimal placement of the HT-BTES system, especially in a complex crystalline bedrock setting. Hence, it is crucial that the subsurface geological conditions and associated thermal properties are suitably characterized as part of pre-investigation work. In this study, we utilize a range of methods for pre-site investigation in the greater Distorp area, in the vicinity of Linköping. Ground geophysical methods, including magnetic and Very Low-Frequency (VLF) measurements, are collected across the study area together with outcrop observations and lab analysis on rock samples. Borehole investigations are conducted, including Thermal Response Test (TRT) and Distributed Thermal Response Test (DTRT) measurements, as well as geophysical wireline logging. Drone-based photogrammetry is also applied to characterize the fracture distribution and orientation in outcrops. In the case of the Distorp site, these methods have proven to give useful information to optimize the placement of the HT-BTES system and to inform design and modeling work. Furthermore, many of the methods applied in the study have proven to require only a fraction of the resources required to drill a single well, and hence, can be considered relatively efficient.

With an aggregated thickness of c.100 m, a porosity of up to 15 % and a permeability above hundred millidarcy, previous studies have assessed the three widely distributed Cambrian sandstone members in the Swedish sector of the Baltic Sea as the most potential CO2-storage candidates in Sweden. Existing models indicate an effective storage capacity between 450–1500 Mt CO2. However, these rough numbers are uncertain as they are related to vintage and partly inadequate data sets, especially regarding physical property values needed for a more certain evaluation of the storage capacity. Hence, as part of a larger screening and evaluation programme, launched by the Swedish government to identify and quantify potential storage sites in Sweden, two scientific core drillings were completed in 2023 on the southernmost part of the island of Gotland in the Baltic Sea. The primary aim was to collect complementary and missing data on the Lower Palaeozoic succession including both caprocks and reservoirs. The scientific evaluations and results of the core drillings on south Gotland will together with geophysical logging of the boreholes, new seismic data and 3D models constitute an essential part in improving the models of the effective storage capacity of the Cambrian reservoirs in the Swedish sector of the Baltic Sea. The coring, monitoring and investigations were managed by the Geological Survey of Sweden and the Swedish national research infrastructure for scientific drilling, “Riksriggen”, operated by the department of Engineering Geology at Lund university. H-dimension triple tube coring (96/61 mm hole/core diameter) was successfully performed with an Atlas Copco CT20C rig. The two wells, Nore-1 and Nore-2, penetrate 470 m of Silurian marlstone and claystone, 85 m of Ordovician argillaceous limestone and 225 m of Cambrian sandstone, siltstone and shale before finishing in the Precambrian crystalline basement of potassium porphyritic granite at 791.5 m and 787.7 m, respectively. The operation managed to reach the set goals despite challenges of over pressured formations and up to metre-thick bentonites. Initial results show a thick, tight, and homogenous caprock and that the Faludden Sandstone, one of the three Cambrian sandstone members, have hydraulic properties that qualifies it as a possible storage reservoir. The preliminary results from the two wells are here presented together with evaluations of drilling performance, monitoring programme, logging and test operations.