Carbon isotope composition and origin of natural gas from the Petišovci-Dolina oil and gas field (Mura-Zala Basin, Pannonian Basin System, NE Slovenia) – a preliminary study

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The carbon isotope composition of natural gas from the Petišovci-Dolina oil and gas field (Kerčmar, 2018; Markič et al., 2020) was measured for the first time in August and September 2021 (Figs. 1 and 2). It was sampled at the Petišovci gas station (Fig. 3). According to gas chromatography analysis (by INA Zagreb, 2019), the natural gas consists predominantly of methane (85–92 %).

Fig.1: Geographical position of the Petišovci (Pet)-Dolina (Dol) oil and gas field. See also Markič et al. (2020).

In our sampling campaign for carbon isotope analysis of methane in 2021 the natural gas was sampled from three wells: D-5/1943, Pg-5/1987 and Pg-10/2011. The »deep« gas from wells Pg-10 (obtained from a depth interval 3102–3104 m) and from Pg-5 (obtained from a depth interval 2772–2795 m) showed less negative δ13CCH4 values, namely -36.7 ‰ and -36.6 ‰, respectively than the gas from well D-5 (from a depth interval 1212 – 1250 m) with the δ13CCH4 value of -38.6 ‰. All these values clearly indicate a thermogenic origin of methane in the sampled gases (Fig. 4 with explanation). According to available data (provided by the Petrol Geo company in Petišovci), the “deep” gases with methane (C1) content of about 85 % contain in addition about 8 % of ethane and propane (C2 and C3), 4 % of carbon dioxide (CO2), up to 3 % of butane (C4) and higher alkanes (C5, C6+), and up to 0.5 % of nitrogen (N2). The “upper” gases contain even more than 90 % of methane, about 7 % of C2 and C3, and up to 3 % of higher alkanes. CO2 content is negligible. Increasing methane content and decreasing CO2 content upwards from deeper to shallower gases is well understandable as an effect of fractionation that methane (smaller and lighter molecule) migrates faster than heavier CO2. In connection with this process also carbon isotopic composition was fractionated with lighter 12C being more abundant in the “upper” gas than in the “deep” one. We conclude that such a fractionation of the gas components and isotopic composition in reservoir sands is due to an origin from the same source rock in a greater depth of which we still do not have enough of knowledge because of not deep enough wells. Additional investigations of gas composition and isotopic carbon composition of methane and of CO2 are planned in the continuation of the research. 

Fig. 2: “Classical” geological profile of the Petišovci-Dolina field – note three positions (black circles) of the sampled two “deep” gases and one shallower “upper” gas.


Fig. 3: Sampling of natural gas from a gas pipeline (yellow) into 0.5 l gas bomb (left) at Petišovci gas station (right) in August 2021.


Fig. 4: Well-known basic presentation of generation of natural oil and gas in correlation with depth, temperature, and thermal maturity considering normal temperature gradient 33 °C/1 km depth (after e.g., Bjørlykke, 1989).  


Thermogenic gas in Petišovci was formed in source rock(s) of Lower Miocene Karpatian sediments prevailingly composed of organic matter rich marls and shales. Vitrinite reflectance measurements of organic clasts in deepest shales and marls (Sachsenhofer et al., 2001) showed %R maturity of up to 2.66 what correspond to a maximal depth of gas formation of about 5.5 km. Maybe it was even formed in carbonates of pre-tertiary basement but these were not drilled-through and investigated yet.

Because the Petišovci area was gradually lifted into antiform (Djurasek, 1988; Mioč and Marković, 1998) the initially formed gas migrated via fractures upwards into reservoir sands termed as “deep” reservoirs (or “deep” gases), and “upper” reservoirs (or “upper” gases).     




Miloš Markič
Geological Survey of Slovenia

Tjaša Kanduč
Institute Jožef Stefan, Slovenia   



References:

Markič, M., Šram, D., Kerčmar, J., 2020: Oil and gas geomanifestations in the Mura-Zala Basin (Petišovci-Dolina; NE Slovenia). GeoConnect3D Posts: https://geoera.eu/blog/oil-and-gas-geomanifestations-mura-zala/

Bjørlykke. K., 1989: Sedimentology and Petroleum Geology. Springer-Verlag, xii + 363 pp.

Sachsenhofer, R. F., Jelen, B., Hasenhüttl C., Dunkl, I. & Rainer, T. 2001: Thermal history of Tertiary basins in Slovenia (Alpine-Dinaride-Pannonian junction). Tectonophysics, 334/2: 77-99. ISSN 0040-1951. 

Djurasek, S. 1988: Results of geophysical exploration in Slovenia (1985-1987) (in Croatian, Engl. abstract). Nafta, 39, 311-326.

Mioč, P. & Marković, S. 1998: Guidebook to the Geological map – Sheet Čakovec, 1:100 000; in Slovene, Engl. abstract). Inštitut za geologijo, geotehniko in geofiziko Ljubljana in Institut za geološka istraživanja Zagreb, 84 p.

Kerčmar, J., 2018: Natural gas reservoirs on the oil-gas field Petišovci (in Slovene, Engl. abstract). Geologija 61/2, 163-176, https://doi.org/10.5474/geologija.2018.011

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