Origin and secondary processes in petroleum in the eastern part of the Polish Outer Carpathians

Abstract

A geochemical study of crude oil and natural gas from producing wells and seeps within the Lower Cretaceous–Oligocene sandstone reservoirs of the most eastern part of the Polish Outer Carpathians was conducted for the evaluation of their origin, migration pathways and the effects of secondary processes (biodegradation, water washing and evaporative fractionation). The petroleum was generated from mixed marine/terrigenous organic matter (type-II/III kerogen) occurring mostly in Oligocene Menilite beds in the Silesian and Skole nappes. Shales within Upper Cretaceous–Palaeocene Istebna and Lower Cretaceous Veřovice, Lgota and Spas beds are considered as an additional source of hydrocarbons (mainly gaseous). Biodegradation processes were recorded in numerous oils, especially occurring in seeps, but the extent of these processes is not severe, because steranes and terpanes were not affected. All oils are more-or-less water washed, while the evaporative fractionation processes are developed only on a minimal scale. The natural gases are primarily of thermogenic origin, usually co-genetic with oil and often contain components of secondary microbial degradation of both oil and natural gas (¹³C-enriched CO₂ and ¹³C-depleted CH₄). Gases with the highest microbial methane contribution have been recorded in petroleum accumulated in Dwernik and Łodyna fields.

Appendix: Description of the oil and gas fields and seeps in the study area

Silesian nappe—oil and gas field

In the Silesian nappe in the analysed area 13 petroleum accumulations were discovered: Wetlina, Rudawka Rymanowska, Mokre, Dwernik, Zatwarnica, Rajskie, Tarnawa–Wielopole, Czarna, Strachocina, Grabownica, Sanok–Zabłotce, Jurowce–Srogów and Turze Pole–Zmiennica (Karnkowski and Ozimkowski 1998; Karnkowski 1999).

The Wetlina gas field (discovered in 1965, abandoned in 2001) is a small (70 Mm³), field comprising accumulations in an imbricated fan complex of Krosno beds and sandstone intercalations within the Menilite beds on average depth ca. 2320 m. The trap is associated with a thrust fault, which is subdivided by transverse faults into several blocks. This field is located on the boundary of the Dukla and Silesian nappes on the most southern part of the study area (Figs. 1, 5).

The Rudawka Rymanowska oil field (discovered in 1889, abandoned in 2005) is located in the most SW part of the study area (Figs. 1, 4). The fold, where the deposit is located, is strongly sliced and overturned. The deposit occurs within horizons of Krosno beds at a depth of 200–800 m and Ciężkowice Sandstone at a depth of 45–500 m. In both reservoirs the trap is lithologically sealed by shales. Since the beginning of exploitation from the Krosno beds and Ciężkowice Sandstone ca. 6 thousand tons of oil and ca. 0.4 Mm³ of gas and ca. 4.4 Mm³of gas, respectively have been obtained.

The Mokre oil field (discovered in 1913, abandoned in 2005) is represented by Stefan-41 well (St-41) (Fig. 1). Oil is accumulated in Lower Krosno beds at two levels (60–1100 and 1200–2000 m). The field has produced over 86.5 Kt of oil and over 10 Mm³of associated gas. Hydrocarbons are accumulated in NW–SE oriented asymmetric duplex-type anticlinal structure cut by the SW–NE oriented transverse dislocations into several separated blocks which constitute the individual fracture type of reservoirs (Karnkowski 1999).

The Dwernik oil field (discovered in 1984, abandoned in 2001) and Zatwarnica oil field (discovered in 1962, in production) are connected to the same WNW-ESE oriented small anticline. Dwernik and Zatwarnica oil fields are located in central and in the western parts of this structure, respectively (Figs. 1, 5). The reservoirs occur at depths between 500 and 1000 m and 250 and 1135 m, respectively (Fig. 5). The reservoir for both of oil fields is Lower Krosno beds. Total production from Zatwarnica and Dwernik gas-driven deposits is ca 8 Kt of oil and ca. 2.6 Mm³and 3.8 Kt of oil and 0.3 Mm³ of associated gas, respectively.

Rajskie oil field (discovered in 1880, abandoned in 1999) was sampled in Rj-7 well (Fig. 1). The trap is associated with an asymmetric anticline with a gently dipping SE limb and a steep NE one. The reservoirs are Krosno beds. In total, three horizons were distinguished which can be correlated long distances along the strike of the anticline. Five other horizons are of local extent. The depths of the horizons vary from 150 to 475 m. The trap is of both structural and stratigraphic type (Karnkowski 1999). Total production from the field is 27.3 Kt of oil and 8.9 Mm³ of associated gas.

The Tarnawa–Wielopole oil field (discovered in 1900, abandoned in 2002) (sampled in TW-25 and TW-32 wells; Fig. 1) is accumulated in Lower and Middle Krosno beds, traced a long distance along WNW-ESE oriented asymmetrical fold. Additionally, the anticline is cut by transverse faults. Gas-driven oil production was obtained from three fractured horizons: 680 m, 765 m and 1225 m. Total production from the field is 286.8 Kt of oil and 43.6 Mm³ of associated gas.

The Czarna oil field (discovered in 1936) is still in production (Figs. 1, 5). The field is present in an asymmetrical anticlinal structure that is strongly disturbed tectonically. The deposit is accumulated in multiple horizons within Krosno Beds and is sealed lithologically and in part tectonically. The most productive reservoirs are zones adjacent to the faults, particularly the longitudinal ones. Eleven producing horizons are distinguished at depths from 130 to 1170 m. Total production from the field is: 263.2 Kt of oil and 21.3 Mm³ of associated gas.

Sanok–Zabłotce (discovered in 1951, in production), Jurowce–Srogów (discovered in 1921, in production) and Strachocina (discovered in 1928, gas storage from 2011) gas fields are located in a long anticlinal structure (Fig. 1). Sanok–Zabłotce is the smallest field with cumulative production ca. 860 Mm³ from an average depth ca. 950 m (Fig. 4) and is located in eastern part of the fold. To the west the Jurowce–Srogów gas field is located with total production to date of 6.6 Mm³ of gas. The Strachocina gas field is the biggest gas field in the Polish Outer Carpathians and has produced more than 4.5 Bcm of gas from average depth of 935 m (Fig. 3). The Sanok–Zabłotce, Jurowce–Srogów and Strachocina structures form a normal anticline in the Istebna beds (Figs. 3, 4). The main producing horizons are within the Istebna sandstone that is sealed by the Istebna shales (Table 1).

The Turze Pole–Zmiennica (discovered in 1883, in production) oil field is located in the western part of the above described structure (Figs. 1, 3). The total production of oil to date is ca. 267.8 Kt and over 63 Mm³ of gas and over 4.5 Kt of light oil. Oil is accumulated in sandstones of the Lgota, Istebna, Ciężkowice and Hieroglyphic beds. The traps are stratigraphic in Ciężkowice and Hieroglyphic sandstones or tectono-stratigraphic (in Lgota and Istebna sandstones) and hosted in a strongly folded and tilted to SW anticlinal structure. Hydrocarbons are accumulated at the depth of 100–860 m.

In the most northwestern part of the Silesian nappe, the Grabownica oil field (Figs. 1, 3) was discovered in 1898 (still in production). Reservoir rocks are Lower Cretaceous (Lgota beds and Grodziszcze Sandstone). The trap is located in a steep symmetrical anticline cut by NW–SE oriented faults into several blocks. In the axial part of the anticline a lower order steep fault is present, which additionally divides the anticline on two longitudinal blocks: southern and northern. Oil is accumulated in five horizons at an average depth of 730 m with total production to date of over 1 Mt and over 375.4 Mm³ of associated gas.

Skole nappe—oil and gas field

In the Skole nappe in the analysed area, five petroleum accumulations were discovered: Tyrawa Solna, Leszczowate–Wańkowa–Paszowa, Łodyna, Brzegi Dolne and Słonne.

The Tyrawa Solna oil field (discovered in 1900, abandoned in 2003) is a small deposit discovered in Kliwa Sandstone enclosed in Menilite beds (Figs. 1, 3). Since the beginning of exploitation ca. 54 Kt of oil and over 11 Mm³ of gas has been obtained from average depth 230 m. Hydrocarbons are accumulated in a sliced and overturned limb of an anticlinal structure in a lithological trap.

Leszczowate–Wańkowa–Paszowa, Łodyna and Brzegi Dolne oil fields were discovered in 1885, 1880 and 1881, respectively. These three oil fields are located in the southeastern part of the Skole nappe (Figs. 1, 6, 7). Total production from Leszczowate–Wańkowa–Paszowa oil field is ca. 1.446 Mt of oil and over 142 Mm³ of gas; in Łodyna oil field is ca. 307.5 Kt of oil and ca. 42 Mm³ of gas; in Brzegi Dolne oil field is ca. 13.5 Kt of oil and ca. 0.14 Mm³ of gas. All of these fields have a very complex, structural and stratigraphic trap type (Figs. 6, 7). Oil and associated gas accumulations occur in all of the deposits in the southern limb of the structure, in several pinch-out traps of the Kliwa Sandstone, which shows in many cases flexural narrowing [the result of backthrust faulting(?)] and tilting to the southwest. The average depth of hydrocarbons accumulation is 500 m (Dziadzio et al. 2006).

Słonne oil field is the smallest and most northeast-located oil field in the Skole nappe (Fig. 1); it was discovered in 1951 and abandoned in 1961. Oil is accumulated in the Upper Cretaceous Inoceramian beds (Fig. 3). The trap is connected to anticline cut in central part by small thrust fault. Total production was ca. 0.5 Kt of oil from the depth of 50–220 m.

Sampled oil and gas seeps

In the Polish (also in Ukrainian) Outer Carpathians oil and gas seeps are a common phenomenon (e.g. Kuśmierek and Machowski 2008; Kotarba et al. 2009a). They are the result of natural petroleum outflows from shallow deposits through fissures (mostly gas, Sechman and Dzieniewicz 2009), but mostly are a remnant of the improperly abandoned wells (Kotarba et al. 2009b).

Five oil seeps from the study area have been collected and analysed (Fig. 1), those are: Łopienka (reservoir: Krosno beds—old abandoned well); Zmiennica (reservoir: Ciężkowice Ss.—connected to the oil field); Płowce (reservoir: Krosno beds—old abandoned well); Żłobek (reservoir: Krosno beds—old abandoned dug well) belonging to the Silesian nappe and Wara (reservoir: Inoceramian Beds—an old abandoned well). Geochemical data of oil collected from a seep in Bandrów (reservoir: Kliwa Ss.—old abandoned dug well) belonging to the Skole nappe presented by Curtis et al. (2004) were used for discussion.