Composition and origin of natural gases accumulated in the Polish and Ukrainian parts of the Carpathian region: Gaseous hydrocarbons, noble gases, carbon dioxide and nitrogen

Abstract

Ten natural gas samples associated and non-associated with oil in the Polish and Ukrainian Flysch Carpathians and Mesozoic Basement of the Carpathian Foredeep were analysed for molecular and stable isotope compositions as follows: 12, 13C in CH 4, C 2H 6, C 3H 8, and CO 2, 1, 2H in CH 4, 14, 15N in N 2, and 3, 4He, 20, 21, 22Ne, 36, 38, 40Ar, 84Kr and 132Xe in noble gases. The gaseous hydrocarbons both in the Silesian Unit of the Outer Flysch Carpathians and Mesozoic Basement are generally genetically related to thermogenic processes and occasionally to microbial carbon dioxide reduction. These gases were mainly generated from mixed type II/III kerogen during middle stage of the low-temperature thermogenic processes. Gaseous hydrocarbons from Starunia in the Boryslav-Pokuttya Unit were generated from type II kerogen during middle stage of the low-temperature thermogenic processes. He is generally of crustal origin with more than 99.2% of radiogenic 4He, but appreciable amount of 3He is probably derived from the upper mantle. The 3He might have been derived from the Pieniny Klippen Belt deep-seated fault system or Neogene and Pleistocene andesitic and basaltic volcanism occurred in the eastern Carpathian area. Nucleogenic 21Ne and 22Ne were analysed in all the samples. The nucleogenic 21Ne shows good correlation with radiogenic 4He with the production ratio of 4He/ 21Ne = (1–3) × 10 7 in the crust. These isotopic values indicate a common source for these gases, where both the nuclear reactions of α-decay and ( α, n) are occurring. Concentrations of radiogenic 4He and 40Ar and excess 21Ne in the samples from the Mesozoic Basement are generally higher and more scattered compared with those in the Silesian Unit. The larger concentrations of radiogenic and nucleogenic isotopes in the natural gases from the Mesozoic Basement is probably related to older geological formations than younger formations of the Silesian Unit. Carbon dioxide was generally generated during the thermal transformation of organic matter, and occasionally during microbial processes. Nitrogen was mainly generated during thermal transformation of organic matter and also originated from atmosphere. A least part of carbon dioxide and nitrogen may have been produced in crust.