Origin of hydrocarbon and noble gases, carbon dioxide and molecular nitrogen in Devonian, Pennsylvanian and Miocene strata of the Polish Lublin and Ukrainian Lviv basins, southern part of the Upper Silesian Coal Basin and western part of the Carpathian Foredeep (Poland)

Abstract

Sixteen natural gas samples were collected from Devonian, Pennsylvanian and Miocene strata of the Polish Lublin and Ukrainian Lviv basins, from the southern area of the Upper Silesian Coal Basin (USCB) and western part of the Carpathian Foredeep (CF) in Poland. The samples were analysed for the molecular composition and stable isotope compositions of gaseous hydrocarbons (CH4, C2H6 and C3H8) and CO2,N2, and noble gases (He, Ne, Ar, Kr, and Xe) in order to reveal the origin and migration pathways of these gases. Hydrocarbon gases and the majority of the of CO2 were generated during two stages of thermogenic processes from Type-II and II/III kerogens in the Lublin Basin, and during both thermogenic and microbial processes in the Lviv Basin. The thermogenic component was generated from Type-II kerogen. Coal-bed hydrocarbon gases accumulated in the Pennsylvanian strata of the southern part of the USCB, originated from both thermogenic and microbial processes. Isotopically light microbial methane may have migrated from Miocene (Upper Badenian) Skawina Formation of the CF to Carboniferous coal-bearing strata and/or might result from diffusion and adsorption-desorption processes during the migration of thermogenic gases from deeper lying coal seams through the microporous coal structure. Hydrocarbon gases and carbon dioxide which accumulated in the sandstone reservoirs of Upper Badenian Skawina Formation of the western part of the CF, most probably were generated from dispersed organic matter within the claystones and mudstones of these strata and at least partly migrated from the Carboniferous coal-bearing strata of the USCB. N2 could have been produced as a result of various biogenic and abiogenic processes, and contributed to the atmospheric component. Characteristic radiogenic 4He composition of the analysed gases suggest the accumulation of α-particles formed from U and Th radioactive decay in the crustal gas reservoirs. Radiogenic 40Ar and nucleogenic 21Ne with abundance ratios characteristic of crustal reservoirs were also observed. Addition of mantle-derived He to the gases from Miocene strata is evident from elevated 3He/4He ratios ranging between 0.2 and 1.6 × 10−6. A small contribution of Ne from the mantle along with He is also suggested by the slightly elevated 20Ne/22Ne ratios relative to atmosphere. The mantle component is most likely to have migrated from the upper mantle to the gas reservoirs through the deep-seated Bzie-Czechowice Fault Zone.