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
environments.
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