Gas generation, alteration and migration in the Austrian Alpine Foreland Basin and their interaction with rock diagenesis
The Alpine Foreland Basin, extending from Geneva to Vienna, is the northern foreland basin of the Alps and a minor oil and gas province. It formed due to overthrusting of the Alpine nappes on the European foreland and was filled with shallow to deep marine sediments since Eocene time. Within the Austrian sector two petroleum systems can be distinguished: A thermal petroleum system comprising Lower Oligocene source and Cenomanian and Eocene reservoirs rocks; A biogenic (bacterial) gas system in Oligocene and Miocene strata. However, geochemical data suggest mixing of thermogenic and biogenic hydrocarbons as well as methane formation through biodegradation of oil in these horizons.
The aim of the present study is to understand the generation, migration and alteration of gaseous hydrocarbons and it’s implication to reservoir qualityin theAustrian Alpine Foreland Basin. To reach this goal, both analysis of hydrocarbon gases and CO 2 and diagenetic studies on potential reservoir rocks have been performed.
(1) The molecular composition and stable carbon of gas and condensate samples from Oligocene/Miocene and Cenomanian/Eocene reservoirs have been determined.These data show that some gas deposits, considered bacterial in origin, contain wet gas and locally even liquid hydrocarbons. The results from GC and IRMS show mixing of thermogenic and biogenic gases, biodegradation of oil and wet gas and fractionation processes. The vertical wetness distribution suggests that migration occurs mostly as bulk volume flow along faults and fractures.
(2) The study of diagenetic history and hydrocarbon emplacement is an important tool to retrace processes which control pore space quality (cementation and dissolution). In this research microbial and thermogenic hydrocarbon generation and migration in interaction with authigenic mineral formation is examined. On the basis of Upper Eocene and Oligo/Miocene reservoir rocks of the North Alpine Foreland Basin the strong relationship of carbonate cement and hydrocarbon is exemplified.