Hindawi Geofluids Volume 2018, Article ID 7014324, 20 pages https://doi.org/10.1155/2018/7014324 Research Article Near-Surface Geochemical Anomalies Integrated with Seismic and Well Data over the Contact of the Outer Carpathians and the Carpathian Foredeep (SE Poland) Henryk Sechman , Adrianna Góra, Anna Twaróg, Piotr Guzy, Ewelina Górska-Mruk, and Wojciech Górecki Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Krakow, Poland Correspondence should be addressed to Henryk Sechman; [email protected] Received 28 February 2018; Accepted 15 May 2018; Published 14 June 2018 Academic Editor: Julie K. Pearce Copyright © 2018 Henryk Sechman et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The objective of this study is to interpret the regularities in distribution of surface geochemical anomalies observed in a fragment of the contact zone between the Carpathian Fold-and-Thrust Belt and the Carpathian Foredeep (SE Poland). The surface geochemical survey included a total of 670 soil gas samples analyzed by gas chromatography, integrated with seismic cross sections, and supplemented by production data from wells. Maximum concentrations of methane, total alkanes C2-C5, total alkenes C2-C4, H2, and CO2 measured in soil gas samples were 27.1 vol.%, 45.3 ppm, 0.49 ppm, 0.232 vol.%, and 6.29 vol.%, respectively. Traces of alkanes detected in samples are the result of microseepage from a few accumulations of various compositions. Methane and ethane migrate primarily from relative shallow Miocene strata, and higher alkanes from deeper Miocene strata and from Paleozoic-Mesozoic basement. We found relatively high positive correlation between the distribution of surface geochemical anomalies and the relative intensity of hydrocarbon shows recorded in the wells. The location and range of 13 anomalous zones are controlled by the patterns of both the Dębno and the Wojnicz slice-folds. Most of the anomalies occur over the Wojnicz Trough, suggesting the presence of conventional accumulations and/or the sites where the Miocene mudstone and heterolithic formations are saturated with gas. 1. Introduction data. It is not surprising that all the petroleum deposits discovered with the direct hydrocarbon indicators are In the Polish part of the Carpathian Foredeep, 84 gas and located outside the Carpathian Overthrust [2, 3]. In the 6 oil deposits have been discovered up to the present [1]. contact zone of the Carpathian Fold-and-Thrust Belt with The gas deposits are most common in the Miocene forma- the Carpathian Foredeep, the readability of seismic records tions whereas the oil ones occur in the Paleozoic-Mesozoic is limited [4], which practically precludes the identification basement of the Miocene succession. These hydrocarbon of anomalous seismic zones. Hence, the integration of seis- accumulations were discovered and assessed using the data mic and surface geochemical data may be instrumental in from the seismic survey, particularly the interpretation of the localization of hydrocarbon accumulations. direct hydrocarbon indicators in seismic sections, which gave The surface geochemical survey applied to the explora- rise to the identification of anomalous seismic records tion of deep hydrocarbon accumulations is based upon the resulted from saturation of sandstone horizons with gas premise that both the concentration and the pressure gradi- [2, 3]. However, both the studies and the interpretation ents enable the seepage of hydrocarbons from accumulating of seismic anomalies deadly need the high-quality seismic into the enclosing rock formations. Generally, it is valid for 2 Geofluids light gaseous hydrocarbons, which primarily migrate, verti- Carpathian Foredeep and the Outer Carpathians (see cally, first due to diffusion and then (if conditions allow) also Figure 1(a)). The Carpathian Foredeep is an asymmetric by effusion towards the Earth’s surface and finally become peripheral foreland basin filled mostly with Miocene clastic dispersed in the Earth’s atmosphere [4–7]. Apart from diffu- sediments (molasses), up to 3 km thick, underlain by plat- sional/effusional migration from deep accumulations to the form successions. The northern margin of the foredeep is surface, some other mechanisms of secondary vertical hydro- erosional whereas the southern one is determined by the carbon migration are quoted, for example, hydrodynamic front of the Outer (Flysch) Carpathians thrusted at a low migration in water solutions and colloids [8, 9]. Other migra- angle over the Miocene formations filling the foredeep tion mechanism can be the hydrostatic lift of microbubbles of [22–25]. The front of the Flysch Carpathians is marked light gaseous hydrocarbons, which drives their episodic seep- by a narrow zone of folded Miocene strata [26]. North age towards the surface along the systems of fissures and of the folded Miocene zone, the autochthonous Miocene microfractures or through the tectonic zones cutting the cap- sequences occupy the outer part of the Carpathian Fore- rocks of hydrocarbon accumulations [8, 10–12]. Recent stud- deep [23, 26, 27]. The basement of the central portion of ies carried on in the Carpathian Foredeep [13] demonstrated the foredeep is built of Mesozoic sediments whereas its top that such migration can be recorded in the seismic sections as surface is shaped by the morphology of Badenian evaporites so-called “gas chimneys.” One chimney was confirmed by and shows characteristic, significant topographic promi- surface geochemical survey by Marzec et al. [13]. Chimneys nences interpreted as a system of paleovalleys, which merge may play a key role in gas saturation of Miocene heterolithic near Brzesko town into a single valley and then plunge successions composed of thin, isolated, commonly poorly beneath the Carpathian Overthrust [28]. lithified sandstone/mudstone alternations [14–16]. The oldest rocks encountered in the study area are Pre- The gas accumulations in Miocene reservoirs are domi- cambrian sediments [29]. Higher in the sequence, the Lower nated by methane of microbial provenance generated pri- Carboniferous carbonates appear (see Figure 2) followed by marily by reduction of carbon dioxide [17, 18]. In addition Lower Triassic deposits recognized only in the vicinity of to methane, ethane and propane are also present, produced Brzesko town [31]. The overlying Jurassic strata include ter- by diagenetic processes or low-temperature thermogenic rigenous sediments (Dogger) and carbonates (Malm). The reactions at greater depths. Such microbial generation is strongly dissected post-Jurassic erosional surface is covered active even recently [19]. The deep geochemical indicators by an Upper Cretaceous succession. generally point to relatively short migration pathways of The Miocene succession of the Carpathian Foredeep gases between the “generation kitchen” and the traps [19]. includes a number of formal and informal stratigraphic units According to Hunt [20], only a few percent of hydrocarbons [28]. In the Brzesko-Wojnicz area, the Miocene sediments generated in source rocks can be accumulated in traps are traditionally classified into the Badenian and the Sarma- whereas the rest become dispersed or trapped within the tian (see Figure 2). In the early Badenian, the so-called “outer source rocks. Consequently, a significant volume of hydro- basin” of the Carpathian Foredeep was formed [26]. In its carbons generated in the Carpathian Foredeep sediments southern part, the clayey-sandy sediments of the Skawina might have been trapped also in Miocene heterolithic or Formation, up to 1000 m thick, were deposited [30, 32, 33] mudstone-claystone sequences, forming the “hybrid” deposits whereas in the northeastern part, the stratigraphic equivalent [21]. This opinion is confirmed by numerous hydrocarbon of the Skawina beds are clayey-sandy deposits of the Baranów shows observed in wells completed in the study area. Hence, beds [26, 34]. Up the sequence, evaporites appear, which pro- we suggest that the surface geochemical anomalies may vide the main correlation horizon of the foredeep [35]. These reflect the microseepage of gaseous hydrocarbons not only are followed by the Chodenice beds developed as claystones from conventional accumulations but also from the zones with minor sandy and marly interbeds and by the clayey- of intensive hydrocarbon generation within the mudstone- sandy Grabowiec beds with the Bogucice sands at the bottom claystone formations. In the Carpathian Foredeep, such for- [36] (see Figure 2). mations host hydrocarbon source rocks, but when thin sand- The folded Miocene succession includes two units: Steb- stone/mudstone intercalations and/or alternations appear nik and Zgłobice. The Stebnik Units comprise Lower and within the formations, they may accumulate the gas, as well. Middle Miocene strata encountered only at the front and The research seeks to explain the reasons of observed beneath the flysch Skole Unit [35]. In the central part of the regularities in the distribution of surface geochemical Carpathian Foredeep, the deformed Miocene strata are named anomalies in a selected contact zone of the Carpathian the Zgłobice Unit (see Figure 2). Usually, it forms a narrow orogen and the Carpathian Foredeep. Analysis of the sur- (some hundreds of meters) zone of Badenian-Sarmatian sed- face