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| Titel |
Microstructures and composition of brittle faults in claystones: Constraints on the barrier behavior |
| VerfasserIn |
Tilo Kneuker, Jörg Hammer, Steffen Jahn, Gernold Zulauf |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141027
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| Publikation (Nr.) |
 EGU/EGU2017-4488.pdf |
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| Zusammenfassung |
| Investigations of fault rocks are crucial to evaluate the barrier properties of clay rich
formations used for the storage of hydrocarbons, carbon dioxide gas or for the storage of heat
generating radioactive waste. Claystones are considered as a geological barrier.
However, their barrier capability can be reduced if the claystones are cut by brittle
faults.
Our study is focusing on the microfabrics and element mobility of artificially and
naturally fractured claystones using a multi-method approach. Particular attention was paid to
small scale lithological heterogeneities occurring in the clayey sequence. The microfabrics
were investigated using SEM and optical microscopy. Geochemical and phase analyses were
carried out using XRD, XRF and ICP-MS. In addition, organic (TOC) and inorganic
carbon (TIC), total sulphur (TS) as well as the cation exchange capacity (CEC) were
determined.
Macroscopic observations of fault zones on outcrops and drill cores indicate closely
spaced planar and undulating discontinuities, including slickenside striations. The
investigated fault zones are often accompanied by calcite veins and calcite enriched zones.
The fault core is formed by a mm to cm thick clayey, fine grained, cohesionless fault gouge
including reworked calcite fragments. Duplex-like domains are separated by discrete
microshears, along which the rocks disintegrate. Calcareous fossils, common in undeformed
claystones, appear in these zones fragmented and rotated. In contrast to calcite, quartz is more
resistant to solution-precipitation processes. Rarely intracrystalline fracturing was
observed.
The calcite mineralization in veins, and solution-precipitation processes of calcite,
documented by stylolites, reflect enhanced palaeo-permeability and activity of Ca2+- and
CO2-rich fluids inside some of the fault zones, mainly along fault parallel shear planes.
Elevated Sr and Ba concentrations are bound to the tectonic, secondary calcite veins within
and outside the investigated fault zone. The geochemical data presented in form of isocon
diagrams suggest volume gain related to the opening of veins and pores, which are now filled
with calcite.
Our results do not provide evidence for presently open pores or fractures, which might be
related to non-artificial tectonic deformation. However, (micro)fractures as well as
mineralized veins represent inherited damage in the rock, and are prone to brittle reactivation
during fluid pressure increase or during the excavation of underground galleries. A complex,
polyphase deformation history including a possible reactivation of older structures is
supported by our observations. |
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