25th International Meeting on Organic Geochemistry IMOG 2011

P-088
Crude oil in the Alpine Foreland Basin of Austria: a known
petroleum system revisited
Achim Bechtel 1 , Reinhard Gratzer 1 , Reinhard F. Sachsenhofer 1 , Hans-Gert Linzer 2 ,
Doris Reischenbacher 1 , Hans-Martin Schulz 3
1 Montanuniversitaet, Leoben, Austria, 2 Rohoel-Aufsuchungs AG, Vienna, Austria, 3 GFZ German Research
Centre for Geosciences, Potsdam, Germany (corresponding author:Achim.Bechtel@mu-leoben.at)
The Alpine Foreland Basin is a minor oil and
moderate gas province in central Europe. The
majority of the fields is located in southeastern
Germany and Austria. In the Austrian part of the
Alpine Foreland Basin, oil and minor thermal gas are
thought to be predominantly sourced from lower
Oligocene horizons (i.e. the Schöneck, Dynow, and
Eggerding formations). The source rocks are
immature where the oil fields are located and enters
the oil window at ca. 4 km depth beneath the Alpine
nappes indicating long-distance lateral migration.
Most important reservoirs are Upper Cretaceous and
Eocene basal sandstones.
Stable carbon isotope and biomarker ratios of oils
from different reservoirs indicate compositional trends
in W-E direction which reflect differences in source,
depositional environment (facies), and maturity of
potential source rocks (Fig. 1). Thermal maturity
parameters from oils of different fields are only in the
western part consistent with northward displacement
of immature oils by subsequently generated oils. In
the eastern part of the basin different migration
pathways must be assumed (Fig. 1c). The trend in
S/(S+R) isomerisation of ��� C29 steranes versus the
��� (20R)/��� (20R) C29 steranes ratio from oil
samples can be explained by differences in thermal
maturation without invovling long-distance migration.
The results argues for hydrocarbon migration through
highly permeable carrier beds or open faults rather
than relatively short migration distances from the
source. The lateral distance of oil fields to the position
of mature source rocks beneath the Alpine nappes in
the south suggests migration distances between less
than 20 km and more than 50 km.
Biomarker compositions of the oils suggest Oligocene
shaly to marly succcesions (i.e. Schoeneck, Dynow,
and Eggerding formations) as potential source rocks,
taking into account their immature character. Best
matches are obtained between the oils and units a/b
(marly shale) and c (black shale) of the ―normal‖
Schöneck Formation, as well as with the so-called
―Oberhofen facies‖. Results from open system
pyrolysis-gas chromtography of potential source rocks
indicate sligthly higher sulfur content of the resulting
pyrolysate from unit b. The enhanced dibenzo–
thiophene/phenanthrene ratios of oils from the
western part of the basin would be consistent with a
higher contribution of unit b to hydrocarbon expulsion
in this area. Differences in the relative contribution of
sedimentary units to oil generation are inherited from
thickness variations of respective units in the
overthrusted sediments. The observed trend toward
lighter � 13 C values of hydrocarbon fractions from oil
fields in a W-E direction are consistent with lower
� 13 C values of organic matter in unit c.
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