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| Titel |
Characterization of CO2-induced (?) bleaching phenomena in German red bed sediments by combined geochemical and evolved gas analysis |
| VerfasserIn |
Ulrike Hilse, Andreas Goepel, Dieter Pudlo, Klaus Heide, Reinhard Gaupp  |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032884
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| Zusammenfassung |
| We investigated varied coloured Buntsandstein and Rotliegend sandstones in Central
Germany (Thuringian Vorderrhön, Altmark) by thermogravimetric/pyrolytic (DEGAS–
directly coupled evolved gas analysis) and geochemical (ICP-MS/OES) means
to evaluate geochemical/mineralogical characteristics of red bed rocks and their
presumably altered, bleached modifications. Commonly bleaching of primary red
bed sediments is regarded as a result of fluid-rock reactions by the participation of
CO2.
This study is performed in the framework of the special research program
“GEOTECHNOLOGIEN” (funded by the German Ministry of Education and Research –
BMBF) and is part of two BMBF sponsored projects – “COMICOR”, an analogue study on
potential effects of CO2-bearing fluids on Buntsandstein and Rotliegend deposits in Hesse
and Thuringia and “CLEAN”, an enhanced gas recovery (EGR) pilot project in cooperation
with GDF SUEZ E&P Deutschland GmbH. The intention of CLEAN is to evaluate the
feasibility of EGR techniques and the suitability of depleted natural gas reservoirs for
potential industrial CO2 sequestration projects.
According to rock colour variations two slices of handspecimens (M49, A1) were split
into 12 and 15 equally sized samples for analytical work.
The medium grained Lower Buntsandstein sample M49 from Thuringia is of fluvial
origin and partially bleached with transitions from red (unbleached) to light colours
(bleached). Bulk rock geochemistry of red bed and bleached subsamples of M49
are almost similar, including rare earth element (REE) content. Only the content
of iron and related metals is depleted in bleached samples compared to the red
bed types. All PAAS normalized pattern of M49 show positive Eu and slightly
negative Ce anomalies, most likely caused by the presence of apatite and illite in the
rocks.
The degassing behavior observed by DEGAS of M49 subsamples is mainly controlled by
the breakdown of sheet silicates, hydroxides and hydrates, as well as of carbonates and
sulphates. DEGAS pattern show no obvious systematic differences between the varied
coloured zones of this specimen.
Sample A1 consists of totally bleached medium grained, lithoclast rich Rotliegend
sandstone which was deposited on a flood plain with braided rivers and aeolian dunes.
Subsamples of A1 are grouped into three zones - all are bleached, with colours ranging from
white to dark grey.
Grey and dark grey zones (A1-1 to A1-6, A1-15) are cemented by Ca-rich carbonates and
contain microscopically identified bitumina. In contrast the pore space of white zones (A1-7
to A1-14) is filled by anhydrite. These mineralogical differences are also reflected in the bulk
rock geochemistry. In comparison to grey rocks white subsamples are depleted in iron and
related elements as well as in REEs.
Moreover, correlations between rock colour and degassing behavior exist. White samples
display typical degassing signatures of sulphates, whereas dark grey zones reveal minor
sulphate content, but also the presence of an additional S-species (sulphide) and
CO2 (carbonate). Similar features were obtained regarding the specification and
abundance of hydrocarbon components. In all samples of A1 methane, ethane and
carbonylsulphide were detected, with higher contents in the more whitish parts.
In grey rocks an additional, long-chained hydrocarbon component occurs. The
relevance of this species is not yet resolved and will be investigated further in more
detail.
Mass spectrometric gas analytical and related geochemical data confirm major differences
in rock composition of Buntsandstein and Rotliegend samples, mainly caused by primary
rock composition and by the involvement of variable fluid composition during burial
diagenetic alteration.
In this study DEGAS was applied for the first time to characterize sandstone
geochemistry. Our results constrain that this method might be a complementary analytical
tool appropriate for petrological sedimentary research. |
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