Oil and Gas geomanifestations in the Mura-Zala Basin (Petišovci-Dolina; NE Slovenia)

Oil in a well in Peklenica (CRO) – field museum.

The most prospective geological area for oil and gas in Slovenia is the Neogene Mura-Zala Basin situated in SW part of the Pannonian Basin System. Within the Mura-Zala Basin, Middle-to-Upper Miocene strata of the Petišovci area have a considerable potential for oil and gas reserves and resources. Oil and gas were discovered in this area in 1942 – as a continuation of the already known Lovaszi field in the neighbouring Hungary. Also, in Croatia, there were known oil and gas fields at Selnica and Peklenica, having been exploited since the mid of 19th century.


Oil and gas-bearing reservoirs of the Petišovci area (7 x 2 km wide) are divided into two bed-sets – the shallow and the deep one. Both consist of alternating 10-40 m thick impermeable marls and porous oil-and/or-gas-bearing sandstones of low porosity – below 15 %, decreasing with depth to ca. 7% only. The Petišovci hydrocarbons are therefore characterized as the tight gas and oil.


The shallow reservoirs occur in 4 main horizons in a depth interval from 1000 to 1800 m. They were exploited in the 1950s (mostly oil) and 1960s (mostly gas). Nowadays. they are depleted – offering a possibility for e.g. storage of imported natural gas or e.g. CO2. The deep reservoirs start at a depth of 2200 m and are developed as about 17 tight gas-bearing (“A-Q“) layers down to a depth of 3550 m (deepest wells), maybe even more. The deepest well was Mg-6/1985 (3858 m). It was the only one reaching the pre-Tertiary carbonate basement.


The whole set of the Petišovci reservoirs is a typical antiform setting formed by lifting between the Donat and Ljutomer reverse faults. It is interpreted that hydrocarbons were generated in an originally synform setting (to reach the oil and gas generation windows), which was later lifted. Lifting has lasted still in the Pontian (Uppermost Miocene) times as known from the coal measures antiform structure in the uppermost Mura Formation. The lifting with accompanying fissure system enabled migration of hydrocarbons upwards into porous sandstone reservoirs in which hydrocarbons were trapped by impermeable marl layers.

Map showing general geology of NE Slovenia, profiles (two shown below) used in regional geothermal studies, areas of potential HC generation, wells of organic petrological measurements, and a depth of pre-Tertiary basement marked by 1.5 km isoline. Basement N of the Ljutomer fault is mainly magmatic and metamorphic, S of it is predominantly carbonate.
In 2001, two basic studies on thermal history, source rocks and HC generation in Tertiary basins of Slovenia were published by Sachsenhofer et al. (2001), and by Hasenhüuttl et al. (2001). Studies revealed in which sediments organic matter reached beginning of oil and gas windows, what are the prospective areas for HC generation and how was the thermal history.
Schematic column of Petišovci field (Geomega, 2012).
1-1’ and NW-SE profiles (Šram et al., 2015) with data (black dots along wells) of maturity of OM by vitrinite reflectance (data from Hasenhüttl et al., 2001). OM reached gas window (%Rr>0,9) only in the Lendava area.
In 2016, EU Unconventional Oil and Gas Assessment Project has been carried out by Geological Surveys of Europe. Areas and volumes of potential oil and/or gas generation strata (OGS and GGS) were defined on the basis of TOC, OM maturity, HI, kerogen type.
General geological profile NW-SE showing Ptuj-Ljutomer synform and Ormož-Selnica antiform, the latter being a host structure for tight hydrocarbons. Note a recent Low Volume Hidraulic Fracturing test in 2011 (LVHF). Coal measures (Pontian – Mura Fm.) are well proved in the Lendava area but are only presumed in the rest of NE Slovenia (masterlog data – well cuttings).
An example of a volumetric cross-section of a layer from the Petišovci „deep“ reservoirs.



Miloš Markič, Dejan Šram
GeoZS – Geological Survey of Slovenia

Jernej Kerčmar
PetrolGeo


* Content based on Markič et al. (2019): Oil and Gas in the Mura-Zala Basin (Petišovci-Dolina; NE Slovenia), presented at the AAPG Regional Meeting in Vienna, April 2019.



References:

Hasenhüttl, C., Kraljič, M., Sachsenhofer, R.F., Jelen, B., Rieger, R., 2001: Source rocks and hydrocarbon generation in Slovenia (Mura Depression, Pannonian Basin). Marine and Petroleum Geology, 18, 115-132.

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, 77-99.

Šram, D., Rman, N., Rižnar, I., Lapanje, A., 2015: The three-dimensional regional geological model of the Mura-Zala Basin, NE Slovenia. Geologija, 58/2, 139-154.

Markič, M., Lapanje, A., Rajver, D., Rman, N., Šram, D., Kumelj, Š., 2016: Geological evaluation of potential unconventional oil and gas resources in Europe – Evaluation of the potential in Slovenia. Geological Survey of Slovenia – Study for the EUOGA project (H2020 call B.2.9), 35p.

Kerčmar, J., 2018: Natural gas reservoirs on the oil-gas field Petišovci (in Slovene). Geologija, 61/2, 163-176.




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