Bacterial methane in the Atzbach-Schwanenstadt gas field (upper Austrian Molasse Basin), Part II: Retracing gas generation and filling history by mass balancing of organic carbon conversion applying hydrogeochemical modelling

In the Austrian Molasse Basin bacterial methane gas accumulations occur in Upper Oligocene to early Miocene deepwater clastic sediments. Gas is produced from the Upper Puchkirchen Formation (Aquitanian) in the Atzbach-Schwanenstadt gas field.The diagenetic pathway of the reservoir cements was controlled by the conversion of metabolizable organic carbon incorporated in the fine-grained sediments. These pathways have been retraced by hydrogeochemical modelling the redox-conversion of metabolizable organic carbon. This approach unravels the relative timing of the coupled processes gas generation and precipitation/dissolution of cements.The labile organic material was decomposed by fermentation and subsequent CO2 reduction. CO2 released via these reactions was nearly completely fixed as carbonate cement. Methane generated during early diagenesis (10 s to max., 100 m sediment depth) was dissolved in pore water until saturation. After exsolution as a free gas phase, methane could have been fixed as hydrate within pore space due to the prevailing paleoceanographic conditions (sediment/water interface ∼1000 mbsl; ∼4 °C). The temperature increase in consequence of the rapid basin subsidence and high sedimentation rates led to the decomposition of the hydrates and to charging of the reservoir by methane still during the Aquitanian. A further consequence of hydrate decomposition was dilution of pore water salinity.According to the results, the presented approach offers (a) a tool to retrace the bacterial methane potential by analysis of diagenetic cement as a quantitative indicator, and (b) to predict the interaction between porosity development and reservoir charging by methane. Additionally, (c) gas field water salinity and hydrochemistry may be applied in similar architectural elements of deepwater channels as tracers for fossil gas hydrate formation as low chloride concentrations are result of sediment compaction and dilution by pure H2O due to gas hydrate dissociation.