Biogenic Methane Formation in the Gas Field Atzbach-Schwanenstadt (Upper Austrian Molasse Basin)

Hans-Martin Schulz¹ and Wolfgang van Berk^2
1Dep. 4.3, GFZ Potsdam, D-14473 Potsdam, Germany
²Hydrogeology, TU Clausthal, D-38678 Clausthal, Germany

In the Austrian Molasse Basin biogenic methane gas accumulations occur in Upper Oligocene to early Miocene clastic deepwater sediments. Methane gas is produced from the Upper Puchkirchen Fm. (Aquitanian) in the gas field Atzbach-Schwanenstadt which is one of the largest gas fields in this basin. The diagenetic pathways of the reservoir cements were controlled by the conversion of metabolizable organic carbon incorporated in intercalated and overlying fine-grained sediments. Retracing these pathways by hydrogeochemical modeling enables the resolution of relative timing of the coupled processes gas generation, filling history and precipitation/dissolution of cements.

The labile kerogen type II-III organic material of claystone layers intercalating and sealing the reservoir sediments was decomposed via fermentation and subsequent CO₂ reduction. CO₂ released via these reactions was nearly completely fixed as carbonate cement. Methane generated during early diagenesis (10s to max. 100m sediment depth) was dissolved within pore water until saturation. After exsolution as a free gas phase, methane was 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 in the semi-closed system of Upper Puchkirchen Fm.

The presented approach offers (a) a tool to retrace the biogenic methane potential by analysis of diagenetic cement and (b) gas field water salinity and hydrochemistry as a tracer for gas hydrate formation.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas