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Surface geochemical surveys were carried out in the Pawłosiów area in the eastern part of the Carpathian Foredeep. The research included an analysis of the molecular composition of soil gas samples as well as measurements of the calcite content, magnetic susceptibility and pH of soil samples. The results obtained were integrated with a geological-seismic model. Changes in the concentrations of total alkanes C 2 –C 5 in relation to changes in indirect geochemical indicators showed "compensatory" character. This means that in places where alkane concentrations were increasing, negative anomalies of magnetic susceptibility and a decrease in pH values were observed. It may be related to the intensity of the oxidation processes or reduction of the magnetic minerals in, as well as the reduced permeability of, the Miocene layers in the zones analysed. Moreover, the complex geochemical-geophysical interpretation showed a zone of anomalous values of the total C 2 –C 5 alkanes in the Pawłosiów area, which is suitable for prospecting. The research performed proved that a thorough analysis of the seismic record allowing subtle DHI anomalies appearing within the Miocene formations to be located, combined with the use of direct and indirect geochemical indicators, can help in identifying an area in terms of hydrocarbon potential. This applies in particular to this type of foreland basin, within which geological formations lie flat, and positive structures (anticlines) are hardly visible in seismic sections. In this type of structure, hydrocarbons can be accumulated in traps of a hybrid character (conventional and unconventional). The extensive zone of geochemical and geophysical anomalies found in the area of Pawłosiów may indicate the presence of this type of hydrocarbon accumulation. Comprehensive research, including direct and indirect geochemical methods in combination with a seismic survey, allows for a better assessment of the hydrocarbon potential of the studied area. • Direct and indirect surface geochemical indicators help interpret DHI seismic anomalies. • Alkane anomalies are associated with a decrease in magnetic susceptibility. • CH 4 and C 2 H 6 mainly migrate to the surface from shallow Miocene horizons, while C 3 H 8 and C 4 H 10 from deeper Miocene layers. • Integrated analysis of geochemical and geophysical indicators can help in the search for hybrid natural gas accumulations. [ABSTRACT FROM AUTHOR]

Abstract: In 2001 a surface geochemical survey was carried out in the Carpathian Foredeep, in the area between Jarosław and Radymno (SE Poland) where multihorizon gas deposits were discovered. These deposits accumulate microbial CH4 with small amounts of N2 and higher molecular weight gaseous hydrocarbons. Soil–gas composition in the hydrocarbon fields in the study area is relatively different from the original composition of natural gas occurring in the subsurface reservoir. In 449 analyzed soil gas samples collected from 1.2m depth relatively low concentrations were found for CH4 (median value 2.2ppm) and its homologues (median value of total alkanes C2–C4 – 0.02ppm). Alkenes were encountered in 36.3% of the analyzed samples (mean value of total alkenes C2–C4 – 0.015ppm) together with distinctly higher concentrations of H2 (maximum value – 544ppm, mean value – 42ppm) and CO2 (maximum value – 10.26 vol.%, mean value – 2.27 vol.%). Individual, very high concentrations of CH4 (up to about 35 vol.%) resulted from sub-surface biochemical reactions whereas higher alkanes detected in soil gases (up to about 68ppm) originated from deep gas accumulations. Both the H2 and alkenes may be indirect indicators of deep hydrocarbon accumulations. Carbon dioxide may also be useful for hydrocarbon exploration, revealing increased concentrations in those sampling sites where CH4 concentrations are strongly depleted, presumably due to bacterial oxidation. These relationships are valid only for the study area and should not be extended as an universal principle. The interpreted zones of cumulative anomalies of CH4, higher molecular weight gaseous hydrocarbons, H2 and CO2 related to the results of geological and geophysical studies directly or indirectly point to the presence of undiscovered, deep gas accumulations and suggest larger sizes for known deposits. The character of the anomalies along with the relationships between the components of soil gases support the opinion that gas accumulations occur at various depths. Moreover, the results indicate that the surface geochemical pattern is significantly influenced by tectonic discontinuities. [Copyright &y& Elsevier]

Abstract: The paper reports on the methods of mathematical processing of the results of surface geochemical survey applied to petroleum exploration. The results include concentrations of methane analyzed in 1251 soil gas samples collected from the area over the Wierzchowice gas field (SW Poland). The methods were presented of determining the geochemical background, the anomalous threshold as well as of data normalization and filtration. The advantages were presented of iterative method for determination of background values proposed by the authors. In the authors'' opinion, selection of the background should be preceded by statistical analysis of each set received during the iteration process. The method presented relies on the assumption of normal distribution of this dataset. The spatial distributions were compared of raw (measured) concentrations and mathematically process values. The applied mathematical data processing enables the researcher to eliminate the “noise” and to select objectively the anomalous values. Thus, such an attempt facilitates the comprehensive interpretation of geochemical data referred to geological model of a field. Results of the studies have demonstrated that the distribution of surface anomalies of methane over the Wierzchowice gas field has been disturbed by long-lasting exploitation. The anomalies appear mainly over dislocation zones, and in the SE part of the area they take the “halo” form around exploited portions of the field due to unexploited areas off the field. [Copyright &y& Elsevier]

The goal of the study is to investigate the relationships between soil gas composition and geological structures in a structurally complex area of the Carpathian fold and thrust belt. Surface geochemical surveys, aimed at determining the molecular and isotopic composition of soil gas samples, were carried out in the central part of the Polish Outer Carpathians in two major tectonic units, the Magura Unit and the Fore-Magura Group of units, which outcrop at the front of the former and within it as a tectonic window (the so-called Mszana Dolna Tectonic Window, MDTW). The tectonic structure of the area has numerous deformations, such as folds, thrusts, and strike-slip faults, that are accompanied by broad zones of mélange. In all 572 soil gas samples, the concentrations of light alkanes, gaseous alkenes as well as non-hydrocarbon (e.g. hydrogen) components were determined, and part of the samples were analysed to test the carbon isotope composition of methane and carbon dioxide. The results of the surface geochemical survey showed significant differences in the recorded microconcentrations of hydrocarbons within the tectonic units of the Outer Carpathians. Significantly higher concentrations of total alkanes C 2 -C 5 were mainly recorded in the MDTW, although the distribution of total alkane values within the Window shows strong connections with the arrangement of strata and the strike of faults and thrusts. Generally, the sealing is insufficient within the MDTW to support the existence of conventional economically viable hydrocarbon accumulations. Our data confirms the sealing role of the Magura Unit in the Carpathian oil system. Therefore, hydrocarbon sources (i.e. potential hydrocarbon accumulations) may be expected within the Fore-Magura Group beneath the cover of the Magura Unit. Our results are also of key importance for the interpretation of surface geochemistry data in other oil and gas provinces with a complex geological structure. • Surface geochemical signal varies depending on the tectonic unit in the Carpathians. • Geochemical signal indicates gas migration from Oligocene Menilite formation. • Surface geochemistry strongly supports hydrocarbon exploration in complex orogens. • C 2 H 6 /C 2 H 4 ratio determines the relative strength or activity of alkane migration. [ABSTRACT FROM AUTHOR]