Work package 1 : Characterisation of alteration

**Work package 1 : Caractérisation de l’altération**

Objectives
This WP aims at providing a continuous alteration profile using state-of-the-art techniques. At Nojima, it will provide a continuous description of the interplay between alteration reactions, deformation and petrophysical proxies at borehole and core scales. At Soultz-sous-Forêt, special focus will be on the timing between alteration phases previously identified in the geothermal site.

Work Program :
We will quantify alteration from geological studies on samples (mineralogy [Task 1.1], structure [Task 1.2] and timing [Task 1.3]) and calibrating its intensity with petrophysical proxies [Task 1.4]. Petrophysical properties from cores will be extrapolated with borehole logs [Task 1.5] to provide a continuous alteration profile.

Partner’s contribution and interactions :
GM provides the Nojima samples, ISTerre the Nojima thin sections and ITES the Soultz samples. During the kick-off meeting at Montpellier [Task 0.1], all partners will select about 5 high priority Nojima samples for the most expensive and time-consuming analyses and agree on their sequential processing. All Nojima samples will be otherwise systematically processed and split in 4 batches that will rotate between partners, who will process in parallel the samples. ITES will provide selected Soultz samples either fresh or with superimposed alteration phases. ISTerre will track the WP1 workflow by maintaining a database [Task 0.3]. The partners will compile the results of the analysis during the Year 1 meeting and combine their contribution during the Year 2 meeting. Specific contributions of the partners are listed in the tables under each specific tasks.

Task 1.1 - Mineralogical quantification of alteration and alteration petrography [IC2MP, ISTerre]

Objectives :
Identify and quantify the mineral phases on the samples to characterize (1) the alteration reactions at stake and (2) the advancement of these reactions.
Description of planned work :
Data concerning the geothermal site of Soultz were obtained during the PhD of C. Glaas, which involved several partners of this project.
On Nojima, this work will build up on the thin sections collected by Anne-Marie Boullier. Thin sections corresponding to the selected sample will be (…)

Task 1-2 : Microstructural analysis [ISTerre, IC2MP]

Objectives :
Quantify the damage (fracturing intensity) and deformation experienced by the analysed samples.
Description of planned work :
Damage is more pervasive on Nojima, which will be the main focus of this work. Fracturing and fracture offsets will be quantified on the selected thin sections. X-Ray CT scan will allow identify fractures in 3D at the voxel size (10μm-100μm size depending on the sample size) and through segmentation, identify interconnected fracture network and (…)

Task 1-3 : Geochemistry and datation of the sequencing of alteration phases [ISTerre, IC2MP]

Objectives :
Date precisely the phases of alteration at Nojima and Soultz to understand the P-T conditions and tectonic context of these alterations
Description of planned work :
We plan to determine the fluid chemistry and its temperature using 13C and 18O isotopes whereas calcite U/Pb geochronology will give date the carbonate phases and provide insight on the fault activity history. This work is peculiarly important for Soultz, where illite is associated with carbonate in veins. The (…)

Task 1-4 : Petrophysical characterization [GM, ISTerre, IC2MP]

Objectives :
Measure petrophysical properties of the samples in Nojima and Soultz characterized in tasks T1.1-T1.3, to derive a petrophysical proxy of alteration. The main target property will be electrical resistivity, also measured by borehole geophysics [T1.5]
Description of planned work :
(1) A first part of this work will extend the preliminary work undertaken by Ph. Pezard on electrical resistivity, to target more specifically the main fault zone. Complex resistivity (…)

Task 1-5 : Interpretation of borehole geophysics [GM, ITES]

Objectives :
Invert borehole geophysical logs to extrapolate a continuous alteration profile.
Description of planned work :
Logs provide a continuous in situ recording of the petrophysical properties at decametric to metric scale. Hence, they could provide a continuous alteration profile, after 3 steps of processing : Correction of geophysical logs from environmental effects. For instance, true resistivity can be inverted from electrical resistivity measurements at different depths of (…)

ANR Alteration in Active Faults

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