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Source rock geochemistry, petrography of reservoir horizons and origin of natural gas in the Devonian of the Lublin and Lviv basins (SE Poland and western Ukraine)

Radkovets N. Y., Kotarba M. J., Wójcik K.

Geological Quarterly

2017

Vol. 61, No. 3

569--589

The Rock-Eval source rock characteristics, mineral composition and type-porosity of reservoir horizons, and origin of natural gas in the Devonian of the Lublin and Lviv basins are described. In the Lower Devonian, the TOC content ranges from 0.01 to 1.82 wt.% in the Lublin Basin, and from 0.01 to 0.45 wt.% in the Lviv Basin. Transformation of organic matter varies from immature in the Lochkovian (Lviv Basin) to mature and overmature in the Emsian (Lublin Basin). The organic matter contains mainly Type-II kerogen, and underwent primary and/or secondary oxidation processes. In the Middle Devonian, the TOC content varies from 0.00 to 1.63 wt.% in the Lublin Basin, and from 0.02 to 0.64 to 2.35 wt.% in the Lviv Basin. The organic matter contains mainly Type-II kerogen and is immature in the Givetian of the Lviv Basin and mature in the Eifelian of the Lviv Basin and in the Eifelian and Givetian in the Lublin Basin. In the Upper Devonian, the TOC content is from 0.02 to 2.62 wt.% in the Lublin Basin, and from 0.04 to 1.43 wt.% in the Lviv Basin. Type-II kerogen dominates in both basins. Organic matter is mature in the Upper Devonian in the Lublin Basin and in the Famennian of the Lviv Basin and overmature in the Frasnian of the Lviv Basin. The reservoir horizons in the Devonian of the Lublin and Lviv basins are developed in clastic, carbonate and sulphate rocks. Terrigenous rocks form several separate horizons in the Lower and Middle Devonian of the Lviv Basin, and in the Upper Devonian (Famennian) of the Lublin Basin. Their filtration properties relate to intergranular porosity, while the fracture space has subordinate significance. Carbonate rocks form thick saturated horizons in the Givetian in the Lviv Basin, and in the Eifelian, Givetian and Frasnian in the Lublin Basin. Their filtration properties are produced by fracture porosity. Sulphates and carbonate-sulphate rocks with fracture and cavern porosity play a role as reservoir horizons in the Middle Devonian of the Lublin Basin. The natural gas collected from the Upper Devonian of the Lublin Basin was generated mainly during low-temperature thermogenic processes, from Ordovician–Silurian Type-II kerogen. The gas from the Middle Devonian reservoirs of the Lviv Basin was produced from Ordovician–Silurian Type-II kerogen and partly from the Middle and Upper Devonian mixed Type-III/II kerogen with maturity from about 0.9 to 1.4%. Carbon dioxide was formed by both thermogenic and microbial processes. Molecular nitrogen was generated mainly through thermal transformation of organic matter and also from destruction of NH4-rich illite of the clayey facies of the Ordovician–Silurian strata.

Origin and migration of oil from the Ukrainian Outer Carpathians to their Mesozoic basement: a case of Lopushna traps

Radkovets N. Y., Kotarba M., Koltun Y., Kowalski A., Kosakowski P., Więclaw D.

Geological Quarterly

2016

Vol. 60, No. 1

133--148

Lopushna oil field has been discovered in Mesozoic-Paleogene strata of the platform basement under the Ukrainian Outer (Flysch) Carpathians. Oils from two main accumulations, occurring in the Upper Cretaceous sandstone and Upper Jurassic limestone reservoirs, have been analysed in order to identify their origin and relation to the known petroleum systems in the region of the Outer Carpathians and the basement of the Carpathian Foredeep. Results of geochemical investigations of oils, including biomarker and isotopic data, show a good correlation with oils, accumulated in the Carpathian flysch strata. Oils are low-sulphur and did not undergo significant degradation. They were generated from Type II or II/III kerogen, deposited in anoxic environments in clastic sediments. Generation occurred at the peak of “oil window”, probably in the temperature range of 122 to 126oC. The presence of oleanane indicates the Cretaceous or younger source rocks. Good correlation on biomarker and isotopic data with the organic matter dispersed in Oligocene Menilite Beds, occurring at the top of the Carpathian flysch succession, allowed authors to identify them as source rocks for oils of the Lopushna field, which thus can be regarded as an oil family of the Outer Carpathians. The migration distance for both Lopushna oils and oils accumulated in the Carpathian flysch strata was similar. A number of sub-vertical dislocations, on which the subsided flysch strata occur at the same depth level with the Jurassic and Cretaceous rocks of the platform, can be assumed as possible migration pathways from the mature source rocks of the Oligocene Menilite Beds of Boryslav-Pokuttya, Skyba or Krosno units to the Lopushna type traps, which suggests the possibility of existence of other hydrocarbon accumulations of this type. The Lopushna field so far is unique containing this type of oil found in the basement of the Ukrainian Carpathian Foredeep.