Linking shallow gas occurrences and deeper structure offshore western Poland (Pomeranian Bight)

Presence of methane in the shallow sediments of the southern Baltic Sea area is a well-known phenomenon. In the Polish Exclusive Economic Zone (Polish EEZ), the observations were mostly based on hydroacoustic data, as well as sediment and water sampling. However, majority of the occurrences were reported in the eastern part of the EEZ (e.g., Gulf of Gdansk). Here we focus on the western part of the Polish EEZ (Pomeranian Bight) and combination of both hydroacoustic and seismic data to provide evidences of free gas in the shallow sediments.

Our study area is located in the inverted part of the Permo-Mesozoic Polish Basin, so called Mid-Polish Swell, with the main inversion-related fault zones: Adler-Kamień and Trzebiatów faults, rooted in the pre-Permian basement (Devonian, Carboniferous?).  Both the Permian and Paleozoic rocks are a proven hydrocarbon exploration play, with an ongoing exploration at the structural trend extending further onshore towards SE. We use seismic data acquired during the RV Maria S. Merian cruise in 2016 (cruise MSM52) with the co-located sub-bottom sediment profiler (Parasound) data.

Various signatures of shallow gas were identified across the seismic section including gas chimneys, shallow bright spots, seafloor polarity reversal and acoustic blanking. Seismic attributes were used to highlight and support interpretation of shallow gas anomalies. Anomalous zones in seismic data were observed in both the Cretaceous, Jurassic and Triassic section in the vicinity of the Adler-Kamień and Trzebiatów fault zones. Parasound data illustrated corresponding free gas accumulation in Pleistocene to Quaternary successions. Amplitude versus offset (AVO) analysis was carried out at two locations of the assumed gas chimney. The gradient analysis from angle gathers shows clearly amplitude variations with increasing offset due to existence of gas in the formation, in addition, free gas amplitude anomalies were highlight in the intercept vs gradient crossplot.

Our data indicate existence of potential fluid migration pathways from the Permian-Paleozoic reservoirs to shallow sediments below the seabed and helps in explanation of how this free gas escapes to the sea bottom.

This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316. Cruise MSM52 has been funded by German Science Foundation DFG and Federal Ministry of Education and Research (BMBF). We thank Federal Institute for Geosciences and Natural Resources (BGR) for their support during seismic data acquisition and sharing the data.