Abstract
The stable carbon isotopic ratios (δ 13C) of methane (CH 4) and carbon dioxide (CO 2) 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 CH 4 trapped in inclusions seems to be derived from different source rocks with different organic matter types. The δ 13C values of CH 4 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 δ 13C(CH 4) values of in co-genetic gas inclusions probably indicating different mtaturity of the potential source rocks at the time the fluids were released. The CO 2 isotopic composition of CH 4–CO 2-bearing inclusions shows slight negative or even positive δ 13C values indicating an inorganic source (e.g., water–rock interaction and dissolution of detrital, marine calcite) for CO 2 in inclusions. We conclude that the δ 13C isotopic ratios of CH 4–CO 2-bearing fluid inclusions can be used to trace migration pathways, sources of gases, and alteration processes. Furthermore, the δ 13C 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.
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