The biogenic methane potential of European gas shale analogues: Results from incubation experiments and thermodynamic modelling

Core samples from the Lower Jurassic Posidonia Shale (Lower Toarcian), Wealden black shales (both from the Lower Saxony Basin, northern Germany), and from the Cambrian–Ordovician Alum Shale (southern Sweden) have been investigated for their biogenic gas potential in incubation experiments. The sample set represents all stages of thermal maturity (from immature to overmature). Methane and to a higher degree carbon dioxide are generated from all samples by added microorganisms. In general, overmature samples (Rr>1.5%) show lowest gas generation rates. The highest generation rates for produced methane (up to 89μmolg−1day−1) and carbon dioxide (139μmolg−1day−1) were detected in immature Alum Shale samples, when different hydrocarbon-degrading inocula where added; whereas the lowest methane production rates were detected in Wealden sediments. The lightest δ13C values for produced methane and carbon dioxide have been detected in Posidonia Shale samples of oil window maturity. Such samples are some of the most sensitive for gas generation and exhibit the strongest decreases of hydrogen indices after incubation.