| Summary: | <p>Geochemical characteristics in subsurface fluid systems provide a wealth of information about fluid sources, migration and storage conditions. Determining the extent of fluid communication in an aquifer system is complicated by multiphase systems containing oil, gas, water or supercritical fluids. For example, a buoyant gas phase may have a different transport pathway than the residual water, and thus there is potential to change geochemical signals over time. The residence time of fluids is critical in such systems and can vary from tens of thousands of years to billions of years. Our understanding of the length-scales in multiphase systems, while equally important, is more limited. Noble gas data from the Rotliegend natural gas field, northern Germany are used here to determine the length-scale and isolation-age of the combined water-gas system. We show that geologically bound volume estimates (i.e., gas-to-water volume ratios) match closed-system noble gas model predictions, suggesting the Rotliegend system has remained isolated as a closed system since hydrocarbon formation. Radiogenic helium data reveal the age of reservoir filling and corroborate long-term geologic isolation (63–129Ma). It is critical that we have the ability to distinguish between fluid systems that – despite phase separation – have remained closed to fluid loss, from those that have lost oil or gas phases. These findings are the first to demonstrate that such systems remain isolated and fully gas-retentive on timescales >100Ma over km length scales and have broad implications for saline aquifer CO2 disposal site viability and hydrocarbon resource prediction, which both require an understanding of the length-scales of crustal fluid transport pathways.</p>
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