Chemical and Isotope Composition of Drilling mud gas from the INFLUINS Borehole EF-FB 1/12 (Thuringian Basin, Germany)

Drilling mud gas was monitored and sampled during standard rotary and core drilling of the 1179 m deep INFLUINS borehole EF-FB 1/12 to gain information on the composition of gases and their distribution at depth of the Thuringian Syncline (Germany). The total abundance of formation gases in drilling mud was low. Methane, helium, hydrogen and carbon dioxide were detected in drilling mud when the drill hole encountered gas-rich strata, reaching maximum concentration of 55 ppmv He, 1400 ppmv of CH_4, 400 ppmv of hydrogen and 1.1 vol-% of CO₂. We therefore consider the INFLUINS borehole to be relatively dry. The drilling mud gas composition is linked with the drilled strata: Buntsandstein and Muschelkalk show different formation gas composition and are therefore hydraulically separated. The correlation between hydrogen and helium and the high relative helium abundance rules out any artificial origin of hydrogen and suggest a radiolytic origin. Ratios CH₄/(C₂H₆/C₃H₈) <50 imply that hydrocarbons derive from thermal degradation of organic matter. Carbon isotope studies are ongoing to clarify origin and provenance of hydrocarbons. Abundances of all noble gases and isotope ratios of He, Ne and Ar were determined on five samples. While Ne and Ar isotope ratios for all samples are indistinguishable from air, ³He/⁴He ratios are always lower than air, revealing the presence of a deep helium source in all samples. Two samples from shallower depth yield air-corrected ³He/⁴He ratios ≤0.03 Ra typical for radiogenic helium production in the crust. For two samples from greater depth, slightly higher air-corrected ³He/⁴He ratios of 0.188 ± 0.076 Ra and 0.220 ± 0.058 Ra, respectively, were observed, indicating small contributions of non-radiogenic helium.