Reflection seismic surveying for CCS geological characterization in the Trelleborg area, southern Sweden

Description

The project is a continuation of a collaboration within the framework of the Geological Survey of Sweden’s (SGU) government task to better understand the potential to store CO2 within Swedish territory. In 2023 reflection seismic surveys were performed on southern Gotland in the Sudret area where two boreholes had been previously drilled using the national research infrastructure for scientific drilling, Riksriggen, operated by Lund University (www.riksriggen.se). This combination of drilling and surface seismic data allows results from the boreholes to be extrapolated to a larger area. The Sudret and Trelleborg areas are of special interest since the geological conditions at these sites are expected to be similar to those offshore at their respective locations. Both places have sedimentary bedrock that is interpreted to be particularly suitable for CO2 sequestration. It is offshore where the real potential for geological storage of CO2 exists within Swedish territory since most of the land area consists of (unsuitable) Precambrian bedrock. Furthermore, there is a current limit on the amount of CO2 that is permitted to be injected into the subsurface on land, only 100 kilotons. It is significantly cheaper to drill onshore than offshore so the strategy is to do the preliminary drilling onshore with seismic data acquired both onshore (by UU) and offshore (by SGU). This will provide a better expectation model for the offshore conditions and the amount of CO2 that can be stored, probably on the order of one Gigaton. The alternative to offshore storage within Swedish territory is to the ship the CO2 to Norway for offshore storage there.

The seismic data acquired on Gotland proved highly useful, but were acquired over a relatively small area. We will now acquire data along several profiles ranging in length from 5 km to 25 km. Two profiles will cross the drill sites (where the drilling will also be done with Riksriggen) and two will cross a major fault zone. The target depth is also deeper in the Trelleborg area (1600 m) compared to Gotland (800 m). Therefore a more powerful source will be used, a mid-size vibrator, compared to a 400 kg weight drop on Gotland. Acquisition will start in the beginning of October with about 450 nodal recording units employed. They will be spaced at 20 m along the profiles and the source will be activated at as many of the recording sites as possible. A major challenge in the acquisition is that most of it will be along public roads, requiring careful planning and permissions to handle the logistics. On Gotland the acquisition was along private roads or in the bush. The seismic data will be processed at the Geophysics Program and interpreted together with SGU geologists. Lund University will perform geophysical logging once the boreholes have been drilled. The boreholes, which are to be fully cored, the logging data and the seismic data will form a sound basis for geological interpretation in the vicinity of the boreholes and further away. It is anticipated that borehole seismic surveys will also be performed at a later date. These measurements will allow better depth conversion of the seismic data and identification anomalous zones in the boreholes. Depending upon the methodology used, the borehole seismic data may also provide higher resolution images of the geological structure, and better identification of primary reflections versus multiples.