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The paper presents an attempt to integrate the results of surface geochemical survey (“free gas” method) and soil magnetic susceptibility carried out in "Brzesko-Wojnicz" area, the SE part of Poland. The measurement profile is located at the border of two regional tectonic units: the central part of the Carpathian Foredeep and the Outer Carpathians. . The purpose of the article was to find out a relationship between different indicators of hydrocarbon microseepage, which can provide useful information, especially in petroleum prospecting. A total of 38 soil and 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 in soil gas samples reached: 36.1 ppm, 0.95 ppm and 0.23 ppm, respectively. Very low values of magnetic susceptibility of soil ranged between 0.8 and 12.9 10-8 m³ /kg. Moreover, the results of hydrocarbon concentrations and magnetic susceptibility measurements of soil gas samples were presented in a linear plot and then integrated with seismic image. The method presented in this paper is a complementary one and its results allow to better understanding of the complexity of hydrocarbon migration mechanisms in the studied area. [ABSTRACT FROM AUTHOR]

Light hydrocarbons migrate from the petroleum accumulations to the Earth’s surface through faults and fractures. The occurrence of these hydrocarbons in the near-surface zone forms the basis for the surface geochemical methods. The purpose of these methods is to record and analyze the distributions of micro-concentration of gaseous hydrocarbons. The paper presents the results of surface geochemical survey carried out in the “Kosina” hydrocarbon concession block in SE part of the Carpathian Foredeep. The surveys were conducted along 6 measurements profiles. A total of 270 soil gas samples were collected and analyzed for methane and higher gaseous hydrocarbons. The maximum concentrations of CH4, total alkanes C2-C5, total alkenes C2-C4 reached: 2.6 vol.%, 0.532 ppm and 8.25 ppm, respectively. These analytical results enabled us to calculate the geochemical indicators CH4/∑C2-C5 and C2H6/C3H8 which allowed to evaluate the character of deep hydrocarbon accumulations. The first indicator suggest that observed surface geochemical signature represents the hydrocarbon migration mostly from deep-seated petroleum accumulations. The values of CH4/∑C2-C5 varied from 6.02 – 141318.7 ppm with mean value – 845.7 ppm. Additionally, the statistical parameters and histogram distributions of C2H6/C3H8 indicator values suggest the dominance of condensate and condensate-gas accumulations at depths. Furthermore, normalized and filtered values of methane, total alkanes C2-C5 and total alkenes C2-C4 were placed on the structural map of the base of the Miocene strata. Small zones of anomalous concentrations of total alkanes C2-C5 are concentrated in the northern part of the area and occur above elevation of Miocene base as well as in the adjacent subsided zone. Measured concentrations and their normalized values plotted and then integrated with the seismic image. The results of study show the possibility of occurrence of relatively small accumulation of conventional gas trapped in Miocene compact structure, formed above the elevation of the Miocene strata. The condensate gas mostly migrated from deep-seated parts of the Miocene horizons. [ABSTRACT FROM AUTHOR]

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]