25th International Meeting on Organic Geochemistry IMOG 2011

P-366
Organic geochemical characterization and distribution of
unconventional hydrocarbon plays of the Lower Cretaceous of
NW Germany
Ulrich Berner, Matthias Heldt
Federal Institute for Geosciences and Natural Resources, Hannover, Germany (corresponding
author:ulrich.berner@bgr.de)
Within the Lower Saxony Basin (LSB) of NW
Germany sediments of the Lower Cretaceous are
known to contain abundant organic material at
different stratigraphic levels. We present a
geochemical high-resolution study of sediments of the
German Wealden (Berriasian) and compare the data
to Barremian and Aptian organic-rich sediments.
A complete succession comprising 360 m of the
German Wealden (Berriasian) has been investigated
at a drill site located in the western part of the LSB,
representing typical basin sediments of the larger
area. Thermally immature Barremian and Aptian
sediments where obtained from three wells of the
eastern part of the LSB. Samples have been collected
at all drill sites at a minimum spacing of 1 m in order
to better describe facies changes. Organic
geochemical investigations including pyrolysis
methods as well as molecular and carbon isotope
analyzes have been performed on the collected
sediments.
Our data combined with additional geochemical
information show that the Berriasian sediments have
been deposited in a predominantly lacustrine
environment that however experienced marine
ingressions. The numerous facies changes within the
Wealden are likely related to climatically driven
precipitation changes, which have lead to lake level
variations. The observed depositional changes are
associated with variations of the organic facies.
During periods of high lake levels, the organic matter
of the clay stones and marls is hydrogen-rich and
likely derived from aquatic precursors. The associated
anoxic to dysoxic water conditions are obvious from
the stable carbon isotopic composition of the organic
carbon which is highly enriched in 12 C suggesting that
a microbial methane cycle had established during the
deposition of the sediments and the organic matter
experienced a significant contribution from the
microbial pool. Low lake levels were associated with
the deposition of hydrogen-depleted organic matter,
which either relate to land plant and/or highly oxidized
material.
Our data of the Barremian to Lower Aptian organicrich
Dark Shale Sequence shows that the sediments
contain variable amounts of terrestrial and aquatic
organic matter deposited in a marine environment.
The Lower Aptian Fish Shale is dominated by aquatic
organic matter, and both Barremian and Lower Aptian
sediments contain rather high concentrations of C28steranes
(Fig. 1), which could be related to observed
higher abundances of coccolithophores.
Figure 1: Relative abundances of C27 to C29 ��steranes
of Lower Cretaceous sediments of the Lower
Saxony Basin.
Although, basin sediments of the Wealden are highly
variable the anoxic to dysoxic facies types containing
type I kerogens are excellent sources for in-situ oil
and gas (at higher maturities). Favorable targets for
the extraction of unconventional hydrocarbons in the
NW German Basin would likely be sections of
Wealden 3 and 4.
The Barremian and Aptian organic-rich shales
exceeding partly 100 m of thickness contain type II
kerogens, and would generate oil and gas, if the
maturities were sufficiently high in the basin.
The Lower Saxony Basin is tectonically segmented
with highly variable heat flow regimes, which set the
boundary conditions for the mapped distribution of
unconventional Lower Cretaceous plays.
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