DISTRIBUTIONS OF LIGHT HYDROCARBONS IN THE NEAR SURFACE ZONE OF THE MARGINAL PART OF THE CARPATHIAN FOREDEEP AND OUTER CARPATHIANS - CASE STUDY FROM SE POLAND.

The paper presents the results of surface geochemical surveys carried out using "free gas method" in "Brzesko-Wojnicz" area in the SE part of the Poland. The surveys were conducted along 2 measurement profiles. A total of 341 soil gas samples were collected from 1.2 m depth below the surface. Maximum concentrations of CH4, total alkanes C₂- C₅, total alkenes C₂-C₄ reached: 27,1 vol.%., 45,3 ppm and 0,174 ppm, respectively. In total, in the analyzed area there are anomalous concentration of total C₂-C₅ alkanes (over 3σ above calculated background mean) in 121 measurement points, comprising 9 zones ranging from 200 m to a 2 kilometres. Calculated hydrocarbon ratios CH₄/ΣC₂-C₅ (C₁/ΣC₂-C₅) and C₂H₆/C₃H₈ (C₂/C₃) allow to define the presence of compositionally diversified, deep hydrocarbon accumulations. The statistical parameters and histogram distributions of the above-mentioned ratios show dominance of condensate-oil character of deep-seated sources with the presence, in some samples, of the effects of current biochemical processes. The highest number of anomalies of total alkanes C₂-C₅ was determined over the Wojnicz thrust slice, particularly in the area of significant increase in thickness of autochthonous miocene sediments. The Wojnicz thrust slice is bordered to the south by the Dębno thrust slice, over where a decay of anomalous indications of total alkanes C₂-C₅ was observed. Besides, the thrust fault zone of this thrust slice collectively with the marginal part of the Outer Carpathian may create barriers to the potential migration and microseepage of hydrocarbons from deeper miocene horizons and from flysch sediments. The conducted analysis indicates that the number and distribution of surface geochemical anomalies point to the possibility of the occurrence of deep-seated accumulations, but the penetration of gas ingredients to the surface is determined predominantly by the tectonics of the area. [ABSTRACT FROM AUTHOR]