A. Twarog, J. Przybytek, M. Ochman, P. Guzy, A. Gora
Thursday 11 October 2018 by Libadmin2018


Light hydrocarbons migrate from the petroleum accumulations to the Earth’s surface through faults, fractures and horizons with increased permeability. The occurrence of light hydrocarbons in the near-surface zone forms the basis for the surface geochemical methods. These methods are divided into two groups: direct and indirect. The paper presents the results of “free-gas” method (concentrations of light hydrocarbons and non-hydrocarbon components in the soil gas samples), and also values of pH and calcite content measured in the soil samples. The aim of this paper was to recognize zones of the highest hydrocarbon potential using direct and indirect surface geochemical methods.
The studies were carried out along the AB profile within the Biecz Anticline located in the eastern part of the Polish Outer Carpathians. Concentrations of methane, total alkanes C2-C5, total alkenes C2-C4, H2 and CO2 varies in ranges: 1.50 – 9.82 ppm, b.d.l. - 0.347 ppm, b.d.l. - 0.181 ppm, b.d.l. – 621.2 ppm, 0.01 – 3.19 vol.%, respectively. Along the AB profile the values of pH change from 4.34 to 8.14, whereas the values of calcite content vary from 0.04 to 23.14 vol.%. The highest anomalous zone for methane, total alkanes C2-C5, total alkenes C2-C4 and H2 was determined above the anticline core. In this zone we also recorded inverted anomaly of pH, which values were below the calculated background level; whereas the values of calcite content were above the calculated background value. The second, lower anomalous zone for light hydrocarbons and hydrogen is located in the southern limb of the anticline, where the Strzeszyn gas deposit have been discovered. In this area we have observed high similarity between pH and total alkanes C2-C5 anomalies, but we have not noticed anomalous values for calcite content.
In the zones where the intensity of hydrocarbons migration is high, the results obtained by the direct and indirect methods confirm the mutual connections between the concentrations of the light hydrocarbons and the values of pH and calcite content. Therefore, indirect method is a reliable supplement for direct method, and both of these surface geochemical methods may be helpful for hydrocarbon exploration.

Keywords: soil gas, light hydrocarbons, surface geochemical survey, Polish Outer Carpathians

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