Detailed compositional analysis of hydrocarbons in soil gases above multi-horizon petroleum deposits – A case study from western Poland

Abstract

A surface geochemical survey was carried on in an area with a cluster of petroleum deposits in western Poland. The largest of these accumulations is the Paproć-Cicha Góra dry gas deposit reservoired in the Rotliegend sediments. Other, similar gas deposits are: the Paproć gas field discovered in the top part of the Carboniferous formation and the Paproć W, Paproć N, and Nowy Tomyśl gas fields localized in the Zechstein Limestone reef structures. Moreover, the Paproć gas-condensate and the Jastrzębsko oil deposits were found at shallower depths, in the Main Dolomite formation. This paper aims to analyze the variability of composition and distribution of near-surface gaseous hydrocarbons in relation to hydrocarbon accumulations reservoired in various stratigraphic units at depths. The occurrence of CH4, its heavier homologues and unsaturated hydrocarbons was measured in 267 soil gas samples. The sampling sites were located along six survey lines (A–A, B–B, C–C, D–D, E–E and F–F) at a 200 m spacing. Concentrations of CH4, total alkanes (C2–C4) and total alkenes (C2–C3) reached maxima of 0.26 vol.%, 5.32 ppm and 5.88 ppm, respectively. Soil–gas alkanes heavier than CH4 were interpreted as being derived from subsurface hydrocarbon accumulations. These compounds penetrated to the near-surface zone by means of active and passive microseepage. Source strength of alkanes microseepage from subsurface accumulations to the land surface (determined by the C2H6/C2H4 ratio) is not a factor that substantially changes the patterns of surface anomalies. Analysis of the values of CH4/C2H6 and C2H6/C3H8 coefficients confirmed the presence of compositionally diversified, deep hydrocarbon accumulations. The study revealed that even small accumulations of gas condensate produce very strong surface geochemical signatures, contrasting with dry gas deposits. The reason may be a specific effect of hydrocarbon migration within the existing petroleum system. The Main Dolomite, which is both the source and the reservoir rock in the Fore-sudetic monocline, plays the role of a “filter”, which modifies the flux of gases ascending from deep-seated reservoirs. As a result of such “filtering”, the geochemical signature of hydrocarbons recorded at the surface indicates the dominance of gas condensate and oil–gas accumulations at depths, giving rise to obviously incorrect interpretations. These interpretations were evidenced by compositional analysis of soil gases, which did not explicitly confirm the existence of a dry gas deposit in the Rotliegend reservoir (Paproć-Cicha Góra gas field). On the other hand, the identified anomalies generally reflect the outline of this accumulation. It supports the applicability of surface geochemical surveys to hydrocarbon exploration in the Fore-sudetic Monocline. The presence of the Main Dolomite at shallower depth must be taken into account in the interpretation of data used for positioning of seismically identified Rotliegend structures. The results of this project indicate that the Main Dolomite may obscure the geochemical signature of near-surface gases derived from dry gas accumulations in both the deeper Rotliegend and Carboniferous reservoirs.