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

Abstract

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.Hydrogeochemical modelling of methane generation, fate and behaviour showed that methane formed during the early diagenesis got lost from Posidonia Shale, the mainly investigated shale in this study. As the mature Posidonia Shale is according to the incubation experiments still sensitive for gas generation, a production scenario of generically generated methane due to oil degradation is simulated. The basic conditions resemble the conditions in the Antrim Shale in the Michigan Basin. Gas production due to dewatering and pressure decrease results in an increase of the gas volumes coupled to a progressive enrichment of carbon dioxide in the produced gas. Moreover, calcite scaling may be the consequence during progressive production similar to the Antrim Shale in the Michigan Basin. Such calcite precipitation has been demonstrated to predominantly occur in natural samples at oil window maturity.