A geophysical investigation of carbonate build-ups in the Baltic Basin using reflection seismic and well data

Abstract: During the Late Ordovician, the region around Gotland was part of a shallow epicratonic basin in the southern subtropics. Low latitudes, relatively warm sea temperatures and the presence of a shallow marine environment promoted algae to flourish and diverse carbonate build-ups such as carbonate mounds and reefs developed on the southern margin of Baltica. Locations where these build-ups can be found today include the Palaeozoic sequence beneath the island of Gotland, Sweden and surrounding areas offshore Gotland. Ordovician mud mounds on Gotland were exploited for their hydrocarbon potential during the 1970’s and 1980’s, with large amounts of seismic and well data being acquired by the oil company Oljeprospecketering AB (OPAB). In recent years this largely unpublished dataset has become available for research purposes. Furthermore, the islands of Gotland and Öland have been the target of helicopter-borne electromagnetic investigations conducted by the Geological Survey of Sweden (SGU) (SkyTEM and VLF). Moreover, new seismic reflection data acquired on a research vessel during 2017 complement the OPAB data over certain areas. In published scientific literature, carbonate mounds and reefs have mainly been identified based on outcrops, cuttings samples, cores and wireline logs. Therefore, the extensive seismic and well dataset used in this study provides an opportunity to showcase how large amounts of vintage data can be utilised to generate regional scale attribute maps which can describe geological systems.In this thesis, the combination of historic seismic and well data, helicopter-borne resistivity data and newly acquired marine seismic data is utilised firstly, to investigate the geometry, distribution and reservoir characteristics of carbonate build-up structures in the Gotland area. Secondly, we generate detailed depth and thickness maps of the Ordovician formation in the subsurface of Gotland based on the seismic data. Thirdly, a detailed interpretation of a 3D seismic dataset acquired over a mound structure on Gotland and a scoping assessment of the potential to utilize these mud mounds for subsurface compressed air energy storage (CAES) is performed. Finally, an automating refraction velocity analysis in the marine seismic data is used to generate a basement velocity map over a large portion of the Swedish sector of the Baltic Sea.