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Diagenetic Processes Near the Gas-Water Contact: The Case of the Austrian Alpine Foreland Basin

Abstract

The Alpine Foreland Basin extends along the N margin of the Alps from Geneva to Vienna and comprises Eocene to Miocene sediments. The Austrian part of the Molasse Basin hosts two petroleum systems: Thermogenic gas and oil from Lower Oligocene source rocks mainly occur within Cenomanian and Eocene reservoirs. In contrast Oligo-/Miocene sandstones contain basically biogenic gas. However, geochemical data suggest mixing of thermogenic and biogenic hydrocarbons as well as methane formation through biodegradation of oil in these horizons. For the better understanding of gas generation and alteration in the Molasse Basin, both gas and diagenesis analyses are performed within the frame of an ongoing project. In the present contribution we report data on rock-fluid interactions near gas/water contact (GWC) in Oligo-/Miocene reservoir sandstones hosting prevalent biogenic gas with subordinate thermogenic gas. Sediment petrographic techniques were used for determining mineralogy and diagenetic history, with special focus on reservoir quality development (cementation and dissolution). The main reservoir rocks are medium- to coarse-grained litharenites, intercalated by thin layers of calcareous shale. During early diagenesis a first cement generation with fibrous calcite and a second generation with micritic calcite formed. δ13C- and δ18O-isotope values of calcite cement vary around +1‰ and −5‰ [PDB], respectively, and are in agreement with marine setting. Based on paleo-PT conditions and the isotopy of accumulated gas, it is likely that gas hydrates formed simultaneously (cf. Schulz et al., 2009). Ongoing burial caused change in pH conditions and partial dissolution of the carbonate cement. In addition, gas hydrates became unstable and free gas accumulations formed. We speculate that the change in acidity may be related to the emplacement of minor thermogenic hydrocarbons. During late diagenesis a third carbonate cement generation precipitated near GWC resulting in porosity and permeability decrease. Alkaline conditions during carbonate cementation resulted in dissolution of siliciclastic minerals and a distinct reduction of quartz and feldspar percentages near the GWC. The δ18O of calcite cement (-8‰) shows a slight, but significant negative shift. This indicates that the calcite cement is recrystallized. It is supposed that previous dissolved calcium interacted with CO2 and precipitated at the GWC under higher formation temperature and/or alteration of pore water.