Stable carbon isotopic ratios of CH4–CO2-bearing fluid inclusions in fracture-fill mineralization from the Lower Saxony Basin (Germany) – A tool for tracing gas sources and maturity

The stable carbon isotopic ratios (δ13C) of methane (CH4) and carbon dioxide (CO2) of gas-rich fluid inclusions hosted in fracture-fill mineralization from the southern part of the Lower Saxony Basin, Germany have been measured online using a crushing device interfaced to an isotopic ratio mass spectrometer (IRMS). The data reveal that CH4 trapped in inclusions seems to be derived from different source rocks with different organic matter types. The δ13C values of CH4 in inclusions in quartz hosted by Carboniferous rocks range between −25 and −19‰, suggesting high-maturity coals as the source of methane. Methane in fluid inclusions in minerals hosted by Mesozoic strata has more negative carbon isotope ratios (−45 to −31‰) and appears to represent primary cracking products from type II kerogens, i.e., marine shales. There is a positive correlation between increasing homogenization temperatures of aqueous fluid inclusions and less negative δ13C(CH4) values of in co-genetic gas inclusions probably indicating different mtaturity of the potential source rocks at the time the fluids were released. The CO2 isotopic composition of CH4–CO2-bearing inclusions shows slight negative or even positive δ13C values indicating an inorganic source (e.g., water–rock interaction and dissolution of detrital, marine calcite) for CO2 in inclusions. We conclude that the δ13C isotopic ratios of CH4–CO2-bearing fluid inclusions can be used to trace migration pathways, sources of gases, and alteration processes. Furthermore, the δ13C values of methane can be used to estimate the maturity of the rocks from which it was sourced. Results presented here are further supported by organic geochemical analysis of surface bitumens which coexist with the gas inclusion-rich fracture-fill mineralization and confirm the isotopic interpretations with respect to fluid source, type and maturity.