DISTRIBUTION OF LIGHT HYDROCARBONS IN THE SELECTED AREA OF THE CARPATHIAN FOREDEEP – CASE STUDY FROM SE POLAND.

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]