Spatial distribution and evolution of porosity in a heterogeneous clay-rich fault core

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Title: Spatial distribution and evolution of porosity in a heterogeneous clay-rich fault core
Author: Nenonen, Ville
Contributor: University of Helsinki, Faculty of Science, Department of Geosciences and Geography
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
Thesis level: master's thesis
Discipline: Geology
Abstract: Sedimentary clay rocks present impermeable, low-porosity formations, often having a seal function to hydrocarbon reservoirs and geological repositories. Due to their impermeable properties, these shale beds can act as a barrier to fluid flow. However, shale formations are intruded by fault zones with permeabilities that can differ of several orders of magnitudes with respect to the undeformed host rock. The fault core comprises of several structures, including breccias, cataclasites and one or several slip surfaces. The slip surface of a fault consists of clay gouge that is heterogeneous material with anisotropic properties in terms of porosity and permeability. The fault core with clay-gouge can act as a barrier or as a lengthwise conduit to fluid flow, depending on physical and chemical properties of a fault. For these reasons, the distribution of porosity in fault core is a key parameter for many applications, including hydrocarbon reservoir capacity, geothermal energy projects and geological repositories for CO2 or high-level radioactive waste. The objective of this work is to combine information on spatial distribution of porosity provided by the 14C-PMMA autoradiography method with the mineralogical information provided by a comprehensive SEM-EDS elemental mapping and to define the porosity variations in and around fault gouges and connect them to processes in a fault system. The samples used in this study were taken from a small-scale vertical strike-slip fault in an argillaceous shale formation at the Tournemire underground research laboratory, Southern France. The results display significant variations in porosity and mineralogy along the studied gouge zone, most probably due to a polyphased tectonic history and paleo-fluid migrations. The studied gouge expresses low porosities in the central part, but porosity values increase significantly in the margins of the gouge. Moreover, the mineralogical changes indicate sealing/healing effects and past hydrothermal activities within the fault zone. The sealing effect is displayed by distribution of calcium as calcite, which is concurrent with lower porosities around the gouge zone. The EDS analysis reveals the zonality of iron and existence of zinc sulphide and barium sulphate inside the gouge, further suggesting hydrothermal activity in the past. Furthermore, the zonality of iron is coherent with different porosity areas as it has formed concentration bands around the areas of low-porosity gouge. Even though the observed porosity variations occur in only a centimetre-wide gouge zone, the higher porosity sections may imply pathways for fluid flow if the fault is reactivated.

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