Surface geochemical anomalies in the vicinity of the Wańkowa Oil Field (SE Polish Carpathians)

Abstract

The oil accumulations discovered in the Skole Synclinorium (eastern part of Polish Outer Carpathians) are located in zones of pinch-out of the Kliva Sandstone. An example is the Wankowa Oil Field, which contains the largest oil reserves in the region. As the seismic identification of this type of hydrocarbon trap is ambiguous, a surface geochemical survey was carried out in the vicinity of the Wankowa Oil Field along an experimental line perpendicular to the fold axes. A traverse across the zones with anomalous seismic records indicated the presence of undiscovered lithological traps. During the surface geochemical survey, 94 samples of soil gas were collected from a depth of 1.2 m and then investigated chromatographically. The spacing of sampling sites was 100 m, which was reduced to 50 m in the Wankowa Oil Field area. The maximum concentrations of CH4 and total alkanes C 2 -C 5 detected in samples were: 4250.0 ppm (0.425 vol. %) and 0.43 ppm, respectively. The first of these was detected at measurement point no. 86, located over the Wankowa Field and the second at point no. 59, about 1,300 m south of the Wankowa Field. The chemical analyses also detected maximum values of total alkenes C 2 -C 4 , H 2 and CO 2 : 0.147 ppm, 0.042 vol. % and 4.4 vol. %, respectively. The results of the surface geochemical survey were integrated with observations on subsurface geological structures, which were interpreted on the basis of seismic data. This procedure permitted the documentation of anomalous concentrations of alkanes in the near-surface zone and contributed to an understanding of the tectonics of the hydrocarbon reservoirs in depth. The pattern of geochemical anomalies here is controlled by anticlines made up of Early Oligocene–Paleocene sediments and by overthrusts that displace these structures. The hydrocarbons migrated from condensate and/or oil accumulations located at various depths. The character of the anomalous zone discovered over the Wankowa Oil Field is related to the effective sealing of hydrocarbon traps and/or the relatively low pressure caused by the production of oil for 130 years. However, this anomaly also may be the result of hydrocarbon migration from deeper, as yet undiscovered gas or gas-condensate accumulations, hosted in older reservoirs forming the hinge of the Ropienka-Łodyna Mine Anticline. The most pronounced anomalies were detected over the Wankowa Village - Bandrow and Grabownica-Zaluz anticlines. The active hydrocarbon dispersion in this zone may have resulted from the presence of overthrusts displacing the structures. Moreover, these anomalies may indicate the presence of shallow, maybe poorly sealed hydrocarbon accumulations.