LIGHT HYDROCARBONS IN SOIL GAS ABOVE PROSPECTIVE OIL- AND GAS-BEARING STRUCTURES: POMERANIAN SYNCLINORIUM, NW POLAND

H. Sechman ^a, M. Dzieniewicz ^a and A. Nowicka ^b

^a AGH University of Science and Technology, Faculty of Geology, Geophysics and Environment Protection, Department of Fossil Fuels, al. A. Mickiewicza 30, 30-059 Krakow, Poland.

^b PGNiG – Polish Oil & Gas Company, Zielona Góra Branch, Pl. Staszica 9, 64-920 Piła, Poland.

Corresponding author, email: sechman@agh.edu.pl

The occurrence of methane and heavier homologues and unsaturated hydrocarbons was recorded in 226 soil gas samples recovered from a study area in the Pomeranian Synclinorium, NW Poland. Samples were collected at stations located along three survey lines (I-I’, II-II’ and III-III’) at a 200 m spacing. Concentrations of methane, total alkanes (C₂-C₄) and total alkenes (C₂-C₄) reached 13.7 vol. %, 18.4 ppm and 0.555 ppm, respectively. Soil gas alkanes heavier than methane were interpreted to be derived from subsurface hydrocarbon accumulations. These hydrocarbons migrated up into the near-surface zone along structural discontinuities and fractures which were observed on seismic profiles.

The migration rate of hydrocarbons from subsurface accumulations towards the surface was determined by the ethane/ethylene (C₂/C₂=) ratio. The statistical distribution of the ethane:propane (C₂/C₃) ratio and plots of the C₂/C₃+C₄ and C₁/C₂+C₃ ratios indicated that accumulations of condensate, gas or oil with a gas cap probably occur in the study area. Variations in normalized values of total alkane C₂-C₄ concentrations allowed surface geochemical anomaly zones to be identified. These anomaly zones were evidence for the occurrence of subsurface hydrocarbon accumulations.

Hydrocarbon accumulations are most likely to occur beneath the central part of profile II-II’ and may also occur in the SW and NE parts of profile III-III’ where both the Carboniferous and the Zechstein Main Dolomite are prospective. In these areas, hydrocarbon accumulations may occur in fault-bounded anticlinal highs.

Surface geochemical anomalies also confirm the presence of a non-commercial hydrocarbon accumulation in the Main Dolomite which was discovered by the D-1 well, and the possibility of another subsurface accumulation in an adjacent tectonic block. Soil gas analyses combined with seismic data provide evidence for the hydrocarbon prospectivity of the study area.

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