Seismic stratigraphy of the Klints Bank east of Gotland (Baltic Sea): a giant drumlin sealing thermogenic hydrocarbons

Abstract

This work analyses six high-resolution multi-channel seismic profiles across the Klints Bank east of Gotland. The Klints Bank consists of a drop-shaped increase of the Quaternary thickness and is oriented in an approximately north-southern direction with a length of over 50 km, a width of about 15 km and a maximum thickness of 150 m. The glacial origin of the Klints Bank can be verified with the dataset presented in this study. We classify the feature as a (giant) drumlin due to its steep up-ice and tapered down-ice face in combination with an orientation parallel to the ice-flow direction of the Weichselian glaciation. The seismic image of the internal structure of the Quaternary unit shows no uniform stratification or deformation patterns; instead, local sub-parallel reflection patterns interlayered with transparent units are observed. The averaged seismic velocity of this unit is about 2000 m/s, which is interpreted as an autochthonous deposition of glaciogenic sediments. Signs of overprinting are interpreted based on the geometry of the flanks of the structure, which appear mostly in the form of collapse structures and lifted blocks due to compressional thrust faulting. Phase-reversed events within and beneath the Quaternary are perceived as strong evidence of fluid (hydrocarbon) presence within the Klints Bank. Organically enriched Palaeozoic shales in south-easterly direction of the Klints Bank presumably give the origin of these thermogenic hydrocarbons.

The Paleozoic Hydrocarbon System in the Gotland Basin (Central Baltic Sea) Leaks

“…In the sub-bottom profiler data (Figures 5 and 6b) we do not observe any typical signs for those layers or shallow biogenic hydrocarbons, such as increased seismic blanking or reverberations. Similar migration paths were also proposed by Sivhed et al (2004) and Schäfer et al (2021). The migration from the Cambrian and Ordovician source rocks toward the Silurian and Devonian layers-where we observe the hydrocarbons -can be facilitated through for example, small-scale fractures such as those observed within the Silurian strata in Figures 4a and 4c.…”

“…A similar assumption was made by Schäfer et al. (2021), who observed hydrocarbon accumulations underneath the Quaternary units of the Klints Bank. They suggested that these accumulations must have migrated there and based on the tectonic setting also assumed an updip migration from the southeast toward the northwest.…”

“…To its west, it is bounded by a morphologic high, the Klints Bank (Figure 1a). In a recent study, Schäfer et al (2021) showed that the Klints Bank is of glacial origin and suggested that it is an extraordinarily large submarine drumlin.…”

“…In a recent study, Schäfer et al. (2021) showed that the Klints Bank is of glacial origin and suggested that it is an extraordinarily large submarine drumlin.…”

Gas - escape features along the Trzebiatów Fault offshore Poland: evidence for a leaking petroleum system

“…Further north and east of Gotland Basin, Schäfer et al (2021) interpreted phase reversed seismic reflections beneath the Quaternary Klints Bank drumlin as evidence for hydrocarbon gas accumulation of thermogenic origin.…”