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Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
Exploring High Temperature reservoirs: new challenges for geothermal energy - Volterra, Italy, Workshop2
1-4 April 2007 Volterra, Tuscany, Italy
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Clay mineral occurrences in volcanic and granitic geothermal contexts: signatures of high temperature fluid circulations in natural permeable fractures
Based on well and surface data, hydrothermal alteration and namely clay 
minerals have been investigated in the volcanic area of Bouillante (Guadeloupe) 
and in the European EGS wells penetrating the Soultz granite (France). 

In the volcanic area of Bouillante, a surface sampling zone of about 40 km² 
surrounding the geothermal field of Bouillante, which produced 15 MWe, was 
investigated. Several mineral associations were outlined: (1) dioctahedral 
smectites with calcite ± quartz ± kaolinite; (2) ordered I/S clay with adularia, 
silica ± calcite and (3) kaolinite smectite ± halloysite ± kaolinite ± smectite ± 
silica. The argillaceous signature of the present-day surface geothermal activity 
(i.e. dioctahedral smectites) can be distinguished from argillization due to 
weathering, which is dominated by kaolinite smectite mixed-layers and 
halloysite. Clay minerals thus provide a reliable tool to distinguish mineral 
associations derived from weathering processes or hydrothermal fluids. 

Moreover, the drilling of 3 geothermal deviated wells in 2001 offered the 
opportunity to investigate the clay content related to the hydrothermal activity 
within the reservoir between 0 and 1,5 km depth. Special attention has been 
paid to the clay signature of the fractured zones which channel the present 
geothermal fluids. Three successive zones, dominated, respectively by 
dioctahedral smectite, illite and chlorite were identified at increasing depths. 
Alteration petrography indicates that these mineralogical clay zones result from 
the spatial superimposition of at least two successive hydrothermal alteration 
stages. The first one, assimilated to a propylitic alteration stage, consisted of 
crystallization of chlorite or corrensite, zeolite and epidote. The later stage of 
alteration is related to the circulation of the present geothermal fluids (T=250°
C) and is assimilated to argillic or phyllic alteration. It consists of a more or less 
intense argillization which results from the crystallization of aluminous 
dioctahedral clay phases (smectite, illite ± I/S mixed layers, and accessory 
kaolinite) associated with quartz, calcite, hematite or pyrite. At Bouillante, the 
permeable zones which channel most of the present geothermal fluids are 
fracture controlled and do not contain specific clay paragenesis. However the 
illite ± I/S mixed layers minerals differ from those of the surroundings by specific 
properties including both crystal structure and texture. Being mainly a product 
of the earlier propylitic alteration stage, chlorites are much less informative on 
the fracture controlled permeable levels.

From the EGS Soultz site located in the Upper Rhine graben (France), clay 
minerals have been investigated from cuttings collected within the granitic 
sections of the Soultz wells between 1,5 km and 5 km depth. Two main 
hydrothermal assemblages have been distinguished: (1) an illite ± quartz ± 
calcite ± hematite assemblage which characterise vein alteration related to fluid 
flow circulation in fractures. Locally, the association of tosudite (a regular 
dioctahedral mixed layered chlorite/smectite bearing lithium) with the illite 
assemblage characterises permeable fracture zones. (2) A chlorite ± corrensite 
± calcite ± epidote assemblage which evidences earlier hydrothermal event 
defined as propylitic alteration. This assemblage which corresponds to poorly 
fractured massive granite occurs at the scale of the granite body. It is 
interpreted as an early hydrothermal event related to small scale fractures and 
characterises low permeability. On the opposite, the illite-secondary quartz 
assemblage characterises later hydrothermal events related to the tectonic 
activity of the Rhine graben. It occurs within some localized rather permeable 
fluid pathways corresponding to large-scale normal faults that support natural 
fluid flow (brines, 100g/l). 

Clay minerals appear to be rather good indicators of geothermal fluid 
circulations in natural permeable fractures in volcanic and granitic reservoir 
Id: 15
Place: Volterra, Tuscany, Italy
Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy
Starting date:
02-Apr-2007   11:59
Duration: 03'
Primary Authors: Mr. GENTER, Albert (BRGM)
Co-Authors: Mrs. PATRIER-MAS, Patricia (UMR 6532 CNRS Hydrasa Poitiers Univ.)
Mr. BEAUFORT, Daniel (UMR 6532 CNRS Hydrasa Poitiers Univ.)
Mrs. DEZAYES, Chrystel (BRGM)
Ms. GUISSEAU, Delphine (UMR 6532 CNRS Hydrasa Poitiers Univ.)
Mrs. LEDESERT, Beatrice (Cergy-Pontoise University)
Mr. MAS, Antoine (UMR 6532 CNRS Hydrasa Poitiers Univ.)
Mr. TRAINEAU, Hervé (CFG Services)
Presenters: Mr. GENTER, Albert
Material: slides Slides
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