--> Mixing of primary microbial gas and thermogenic hydrocarbons; the Austrian Molasse Basin case study

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Mixing of primary microbial gas and thermogenic hydrocarbons; the Austrian Molasse Basin case study


The Molasse Basin, extending from Geneva to Vienna, is the classical asymmetric foredeep and comprises Eocene to Miocene shallow to deep marine sediments. Within the Austrian sector two petroleum systems can be distinguished: a thermal petroleum system comprising Lower Oligocene source and Cenomanian/Eocene reservoirs rocks and a microbial gas system in Oligocene and Miocene strata. Nevertheless, results of geochemical research suggest an admixture of thermogenic to prevailing microbial hydrocarbons in these horizons.

The purpose of the present study is to understand the origin, migration and alteration of natural gas and condensate in the Molasse Basin. To reach this goal more than 50 natural gas samples were analyzed for molecular and stable carbon isotopic composition of individual hydrocarbons and CO2, additionally stable hydrogen isotope compositions of CH4 were determined. Moreover, twelve condensate samples were examined for biomarkers and specific compounds composition as well as carbon isotopic composition of n-alkanes and acyclic isoprenoids.

The primary biogenic gas from Oligo/Miocene deposits consists almost exclusively of methane (96.2-98.7 vol %) with carbon isotopic composition ranging between -48.5‰ PDB and -65.3‰ PDB. Isotopically heavy carbon in methane, occurrence of higher hydrocarbons and locally even liquid hydrocarbons in these Oligo/Miocene reservoir rocks indicate mixing with thermogenic hydrocarbons from deeper stratigraphic units.

Hopane and sterane biomarker patterns in associated condensates as well as δ13C of n-alkanes and acyclic isoprenoids suggest the same origin as that of oils produced from Cenomanian/Eocene deposits. Parameters derived from short-chain hydrocarbons (e.g. Halpern and Thompson parameters) show that extensive biodegradation and diffusive fractionation processes occurred, influencing molecular composition. The coexistence of strongly altered condensates with primary microbial dry gas in reservoir rocks at relatively low temperature (below 60°C) suggests that secondary microbial methane formed by the biodegradation of condensate also contributes to the total gas volume.

The results of the on-going project point out that geochemical characteristics of gas fields in the Molasse Basin are triggered by the interaction of different processes like primary gas generation (from solid organic matter) and secondary microbial gas generation (biodegradation of thermogenic hydrocarbons), migration and mixing processes. Occurrence of oil-derived condensates in the western part of study area may be related to undiscovered oil reservoirs in deeper layers.