The prospects for petroleum exploration in the eastern sector of Southern Baltic as revealed by sea bottom geochemical survey correlated with seismic data

In the Polish offshore Łeba (B) tectonic block in the southeastern part of the Baltic Sea the oil and gas fields are accumulated in Middle Cambrian quartzose sandstone, often fractured and diagenetically sealed at depth by advanced silification developed in reservoir around the petroleum deposit. Petroleum traps aremainly of structure-tectonic type, i.e., anticlines closed with strike-slip faults. At least four gas-condensate and four oil deposits of total reserves more than 10 Gm3 gas and 30 Mt oil were discovered by the “Petrobaltic” Co. in the Polish Baltic sector. The subsurface petroleum deposits in the Cambrian reservoir are the source of secondary vertical hydrocarbon migration to the surface which produces surface microseepages and hydrocarbon anomalies. Geochemical survey of the sea bottom sediments and waters run along seismic profiles was completed in 1999–2002 within a joint project of “Petrobaltic” Co. Gdańsk and the Fossil Fuels Dept., AGH University of Science and Technology, Kraków, approved by theMinistry of Environmental Protection, Natural Resources and Forestry. It was found that seafloor hydrocarbon anomalies are closely related to subsurface geologic structure and location of petroleum deposits. Particularly the faults as principal venues for vertical hydrocarbon migration are reflected in high-magnitude seafloor anomalies. Above petroleum field there occurs a “halo” effect of high-magnitude anomalies contouring the deposit with damping related to productive zone situated inbetween. Thus, the section of sea bottom anomalies over a petroleum deposit resembles the shape of a volcanic caldera. Positive subsurface structures manifest themselves as neotectonic features in the sea-floor morphology and as petrological variations of the bottom sediments. Along the contours of petroleum field, the sea-floor seeps of gas and submarine springs of subsurface water occur. These are seismically recognizable as gas chimneys, geysers, craters and effusive cones. The sea-floor geysers and springs disturb thermal and density stratification of sea water column. The submarine geochemical studies strictly correlated with seismic profiles may contribute greatly to offshore petroleum exploration and marine environmental protection.