One of the most important discoveries of crustal mechanics in the past 15 years is,
according to K. Evans (2006), the realisation that the Earth's crust is generally
close to failure, even in tectonically quiet areas. This deduction results especially
from induced seismicity recorded during fluid injection in Enhanced Geothermal
Systems (EGS) like in Soultz and from in situ measurements (Evans et al., 2005).
Monitoring of seismic events shows the progression of microshearing along potential
structures perpendicular to the minimum stress direction. However, deformation
mechanisms and strain regime remains poorly constrained and hinder prediction and
repeatability of stimulation of reservoir by hydrofracturing. Thus, if the link is
well established between fluid injection and microseismicity, the resulting increase
in permeability of the system after injection and its distribution in 3D remains debated.
The in situ observation of the induced effects of hydrofracturing should be obviously
the best way to increase our knowledge of the processes occurring during fluid
injection. This is of course difficult as it requires re-drilling or sophisticated
imaging and monitoring. Another way could be to look for a natural analogue for which
shearing is caused by an increase in fluid pressure. To explore this way, it is
proposed to establish a comparison between EGS and mineralisations systems.
Pervasive alteration, vein systems, stockwork and dissemination are among the most
obvious signatures of fluid transport in Earth's crust. Within metallogenic Archaean
provinces of Canada or Australia, processes of hydrothermal alteration have been
widely documented through structural analysis, mineralogical, isotopic and fluid
inclusion studies. Concepts like seismic pumping and fault valve systems have been
proposed that defines a genetic link between shear zones, fluid pressure and
mineralisations (Sibson et al., 1975; Sibson, 1992). Moreover, a continuum is
established through the crust from catazone lode deposits to epizone vein networks
(Groves, 1993; Groves et al. 1998; Cuaig and Kerrich, 1998) showing that a whole
range of mineralisations are formed at depth that are presently explored in EGS.
Finally, Soultz is a Naturally Fractured and Mineralized Systems that has been
intensively studied. All natural fractures are filled by minerals of hydrothermal
origin mainly calcite, clay, quartz, sulfides. Thus, although economic
mineralisations are absent, the link between hydrothermal alteration and
paleogeothermal system is implicit.
A first topic for advanced studies could be a comparison between the geometry of
these mineralized systems (that can be easily studied within underground mines) with
the available dataset and imagery for EGS. Thus, Boullier and Robert (1992) have
shown that gold-quartz vein networks in Archean can be used to reconstitute
paleoseismic events and cyclic fluid pressure fluctuations. It is considered that
such fault-valve mechanisms present many similarities with suspected fluid
circulation within the fracture network during injection.
On another hand, a better understanding of the fluid redistribution related to
transitional tectonic stress field constitutes another way of research (Sibson,
2000). For example, changes in fluid circulation during uplift and exhumation is a
widespread phenomenon identified in many tectonic environments that could be of great
interest to understand the role of pre-existing structures (Bouchot et al., 2005),
Pressure-Temperature conditions and lithological composition for fluid channelling
and seismic activity of the faults.
Boullier A.M., Robert F. (1992). Paleoseismic events recorded in Archaean gold-quartz
vein networks, Val d’Or, Abitibi, Quebec, Canada. Journal of Structural geology, 14,
McCuaig T. C., Kerrich R. (1998). P-T-t-deformation-fluid characteristics of lode
gold deposits: evidence from alteration systematics. Ore Geology Reviews, 12, 381-453
Groves D.I. (1993). The crustal continuum model for late-Archaean lode gold deposits
of the Yilgarn Block, Western Australia. Mineralium Deposita 28, 366– 374.
Groves D.I., Goldfarb R.J., Gebre-Mariam M., Hagemann S.G., Robert, F. (1998).
Orogenic gold deposits: a proposed classification in the context of the crustal
distribution and relationship to other gold deposit types. Ore Geology Reviews 13, 7 –27.
Evans K., Genter A., Sausse J. (2005). Permeability creation and damage dur to
massive fluid injections into granite at 3.5 km at Soultz : 1. Borehole observations.
Journal Geophysical research, 10.
Evans K. (2006). The importance of natural rock stress in the stimulation process,
and the difficulty of its characterisation. In : Baujard C., Kohl T., Eds.
Proceedings of the Workshop 3 "Stimulation of reservoir and induced microseismicity",
June 29-July 1 2006, Karthause Ittingen, Switzerland., p. 56.
Sibson R H., Moore J. M. M., Rankin A. H. (1975) Seismic pumping--a hydrothermal
fluid transport mechanism. Journal Geological Society of London, 131, 653-659.
Sibson R.H. (1992). Implication of fault-valve behaviour for rupture nucleation and
recurrence. Tectonophysics, 211, 283-293.
Sibson R.H. (2000). Tectonic controls on maximum sustainable over pressure: fluid
redistribution from stress transitions. Journal of Geochemical Exploration, 69-70,