Origin of gases from the geothermal reservoir Groß Schönebeck (North German Basin)

Abstract

The operation of a geothermal facility is largely affected by the content and composition of gases dissolved in the formation fluids. Typically, the geothermal gas mixtures consists of different types of gases of different origin. In this study, the origin of various gas components from the Rotliegend – Permocarboniferous geothermal reservoir Groß Schönebeck (North German Basin) was investigated. Three formation fluid samples were collected from the depth of the production well Gt GrSk 04/05 (between 4120 and 4240m) at conditions of the Rotliegend/Permocarboniferus reservoir (about 140–150°C and 430–450bar). Identified and further investigated gases are N2, CH4, CO2, H2, higher gaseous alkanes, and noble gases. The stable isotope ratio of individual gas components range from 0.2 to 1.5‰ (δ15NAir) for N2, from −16.5 to −21.2‰ for CH4, from −18 to −26‰ for C2H6, from −24 to −30‰ for C3H8, and from −13.1 to −20.2‰ for CO2 (δ13CV-PDB for all hydrocarbons and carbon dioxide). Isotopic ratios measured for the noble gases He, Ne, and Ar indicated that 20Ne/22Ne and 38Ar/36Ar are indistinguishable from air, while all other isotope ratios show excess of radiogenic or nucleogenic isotopes.Results of δ15NAir suggest that the source of N2 are the highly matured Westphalian coals (Upper Carboniferous)l. Isotopic ratios of CH4 and CO2 confirmed thermogenic decomposition of terrestrial plants from a similar source as the most likely precursor for these gases. The inverse carbon isotope trend might be linked to heavy metal catalysed cracking of long-chain hydrocarbons and/or evolution of precipitates during fluid evaporation in the basin.The observed 3He/4He ratios of ≤0.0642±0.0026 Ra demonstrate a crustal origin of He. The N2/40Ar ratio (1648±480) indicates a crustal origin and practically without any influence of the air. 21Ne/22Ne is distinguished from the air and more enriched in nucleogenic 21Ne, relative to 22Ne, when compared to the average crustal Ne composition. The calculated residence time of fluids by means of in situ produced nuclides varies for 40Ar from 72 to 80Ma and for 4He from 365 to 401Ma. The apparent age discrepancy gives a hint for He migration from older stratigraphic layers or stratas enriched in parent nuclides (U, Th).