Bacterial methane in the Atzbach-Schwanenstadt gas field (Upper Austrian Molasse Basin), Part I: Geology

Bacterial methane gas accumulations occur in Upper Oligocene to Early Miocene clastic deepwater sediments in the Austrian Molasse Basin. Methane gas is produced from the Upper Puchkirchen Fm. (Aquitanian) in the Atzbach-Schwanenstadt gas field which is one of the largest gas fields in this basin.The bacterial gas is composed of >97 vol% methane (δ13C ∼ −62‰, δD ∼ −200‰). Connate pore water below the gas–water contact is characterized by low salinity (cCl−: ca. 0.25 mol/kgw or 8900 mg/kgw).The top of the main reservoir zone A4 is approximately 1600 m subsurface. Thickness of reservoir intervals in the A4 sands (the main reservoir) is variable and ranges from 8 to 30 m with decreasing thickness to the north. The porosity of the sand ranges from 13% to 20%, permeability from 6 mD to 70 mD. The reservoir temperature is 63 °C. Reservoir rocks of this field wedge out to the north and represent dam break-through (splay) sediment. This reservoir architecture enabled early gas accumulation in a stratigraphic trap.The source rocks and reservoir rocks are intimately connected. Claystone layers intercalate and seal the reservoir sediments. These fine-grained sediments with potential source rock characteristics contain immature organic matter of kerogen types II–III. The TOC contents of these fine-grained sediments vary from 0.34 to 0.94 wt.%.The cement inventory of the source and reservoir rocks is characteristic for diagenetic pathways of organic-rich sediments. Pyrite is preserved in the sediments as the first precipitation product due to rapidly developing anoxic pore water conditions. A following dolomite precipitation/dissolution phase was replaced by precipitation of calcite, ankerite and siderite. A last cement phase is chlorite.The about 700 m thick Upper Puchkirchen Fm. was deposited during a time interval of about 2.6 Ma (Aquitanian) in a deep-sea fan environment (water depth ≥1000 m, bottom water temperature of about 4 °C). Free gas phases may have been trapped as gas hydrate.Due to basin subsidence and high sedimentation rates, hydrates have been decomposed below the base of the gas hydrate stability zone (∼200–40 mbsf) still during the Aquitanian and, thus, still during or shortly after deposition of the Upper Puchkirchen Fm.