Paleostress magnitudes can be estimated by using fault slip data, rupture and friction laws (Angelier, 1989) for dry conditions. However, their estimation is difficult if fluids are present during deformation, the lithostatic load and the fluid pressure being generally unknown. This paper shows that the quantitative estimation of the lithostatic load and the fluid pressure during a tectonic event can be derived from paleofluid analysis in fluid inclusion planes, (FIP). As FIP are healed mode I cracks oriented in a consistant manner relative to regional or local structures, stress and fluid features may be obtained for a given deformation event. This approach has been applied to a fault system affecting an Hercynian granite of the NW French Massif Central. A NW‐SE compression has been defined from a population of 51 faults characterized by orientations around N60°E to N110°E for dextral strike slips and N135°E to N175°E for sinistral movements. The stress ratio has been determined with fault slip data around 0.52±0.08 and by using Angelier's method, the ratio ψ (σ 3 /σ 1 ) was estimated around 0.27±0.03, and the friction coefficient μ was estimated around 0.58±0.1. FIP orientation were measured in a sample collected at 1 meter from the fault. The dominant trend is NW‐SE, vertical or dipping toward the SW. Inclusion fluids from FIP are characterized by homogenization temperatures in the 260°–380°C range with a mode around 300°C and melting temperatures from 0°C to −1.5°C (0–5% equivalent NaCl) with a mode around −1.0°C (1.7% equivalent NaCl). These data yield, assuming a geothermal gradient in the 60°–80°C/km range, and hydrostatic conditions, a fluid pressure estimate of about 50±10 MPa and a σ v around 132±10 MPa. Thus a major stress ratio σ 1 ‐σ 3 can be estimated in the 70–105 MPa range.
Soultz-sous-Forets is one of the designated deep geothermal Hot Dry Rocks test sites. Three boreholes have been drilled: GPK1, GPK2 which are the geothermal fluid transport system and a reference hole EPS1 which has been fully cored. A large database for EPS1 has been collected using cores as well as BHTV borehole wall imagery studies. These detailed data present an excellent opportunity to study the structural and mineralogical properties of the Soultz granite over a depth interval of 810 m. This granite has been strongly altered by fluid percolations which have completely sealed the majority of fractures. The purpose of this study is to describe and understand the mechanisms of fluid circulation that were responsible for the propagation of alteration through fractures (vein alterations). Observed correlations between the parameters: fracture orientation, thickness, density and nature of mineral phases occurring within fractures, demonstrate that: (1) the minerals which seal fractures define three general types of alteration (quartz-illite, calcite-chlorite, hematite). (2) fracturing, although generally occurring in two main systems (N005 degrees E, 70 degrees W et N170 degrees E, 70 degrees E) is not homogeneously distributed with depth. Ten other families of fractures have been described throughout the borehole. Clear relationships between the mineral assemblage of fractures, their depth occurrence and their orientation have been detected. Sixteen levels (of constant characteristics) have been thus identified by dividing the granite according to these criteria, into zones of different depth. It allows the calculation of the equivalent hydraulic properties of the rock. The palaeopermeabilities and characteristics of the three types of alteration are variable both within a single depth zone, and throughout the entire length of the EPS1 section. The average permeability of the whole core is 1.5.10 (super -6) m/s. The study of vein alteration in the Soultz EPS1 core shows that different fracture networks are present throughout the core, implying that the estimation of the average palaeopermeability is not representative of that on a local scale and not constant with time.
Abstract Introduction – Normal faults are part of the elements that control fluid flows in sedimentary basins. They can play the role of a barrier or a drain [Hippler, 1993]. These pathways are anisotropic. The aim of this study is to determine the fluid pathways and to characterise the pore network and its role in the transfer properties. Petrophysics, petrographics, geochemical and fluid inclusion studies allow us to characterise a Buntsandstein sandstone affected by a normal fault. This sandstone has a fluviatile origin, field evidenced by fluviatile channels, but also by some clay layers. The fault is located in the north east of France, in the Rhine Graben. The vertical displacement is about 3 meters, and the dip is 70o east. The fractured zone is composed of three compartments (the hanging wall and the footwall separated by a gouge) divided by three main faults (fig. 1). Oriented samples were taken from the three blocks and were studied following the procedure figure 2. Results – The petrographical and mineralogical composition of the three compartments were different. The gouge and the footwall were characterised by quartz overgrowths, authigenic kaolinite (30 to 40 % of the clay fraction) and diagenetic illite (40 to 60 % of the clay fraction). The hanging wall was characterised by 70 to 80 % diagenetic illite of the clay fraction (fig. 3). The isotopic composition of the footwall quartz overgrowth (fig. 4) was δ18O enriched ranging from 13,4 to 13,6 ‰ SMOW, compared to detritic quartz ranging from 10,7 to 11,8‰ SMOW. Such quartz precipitations originated from fluid circulations, with temperature ranging between 195oC and 225oC according to fluid inclusion data in quartz overgrowth. This occurred mainly in the hanging wall but also in the fault gouge. The isotopic study of minerals and the quartz overgrowth fluid inclusion study showed that these fluids were similar to present day fluids characterized by Pauwels et al. [1993] in the deep Upper Rhine Graben (tab. I). The fault gouge was first like a drain allowing the fluid to circulate from the deep graben and then it acted as a barrier preventing the fluid from spreading in the hanging wall. This was confirmed by the study of thin sections, that revealed a cataclastic zone in samples located between the hanging wall and the gouge (fig. 5). The evolution of porosity was characterised along a profile crossing the fault. Porosity values evolved from 12 % in the hanging wall, to 6 % in the fault gouge, and 12 % in the footwall (fig. 6). Oriented mercury injection measurements were carried out on covered (fig. 7) and non covered (fig. 6) epoxy resin samples to compare permeability related to porous network. When the samples were covered with epoxy resin, mercury was injected only into the network which was connected to the injection surface (fig. 7). The process indicated a connectivity of the sample and it could be quantified. High differences between the two porosity values suggest that the porous network was not connected with the surface of the sample. The covered or not covered samples exhibited no porosity variations with orientation. The lowest mean permeability occurred in the fault gouge (0,1 mD). It increased in the hanging wall (100 mD) and in the footwall (200 mD). The maximum value of oriented permeability measurements occurred in the bedding plane (250 mD) (fig. 8). The direction of this maximum permeability varied in the two blocks with the direction of the fluviatile channels. The minimum permeability in the hanging wall (12 mD) and in the footwall (34 mD) were perpendicular to the bedding. This sedimentary permeability anisotropy disappeared in the gouge (fig. 8). Discussion – Fault zones are assumed to be fluid pathways and fluid barriers. This study has shown that the same fault can act as a barrier and a drain for fluid circulation. Permeability anisotropy is usually related to fracturation, but only in the case of short time fluids pathways. Indeed, when the fracture network is totally cemented, the matrix plays the role of pathway. The evolution of the porous network depends on the tectonics and on the fluid circulation. Permeability and permeability anisotropy decrease as the distance to the gouge decreases. We also noticed a decrease of pore threshold and connectivity of the porous network. In fact, permeability depends on tortuosity, connectivity, but also on porosity and pore threshold [Katz and Thompson, 1987]. In these sandstones, classical mercury injection did not indicate any significant variations. But oriented and resin covered mercury injection allowed us to distinguished three types of samples response (fig. 9) : – similar porosity and pore threshold in covered and non resin covered samples indicate a good connectivity , but no preferential orientation of the porous network ; – similar porosity but different pore threshold indicate a preferential orientation of the structures but also a good connectivity ; – different porosity and pore threshold indicate either a bad connectivity or a preferential orientation of the microstructures. In this study, we have clearly shown an evolution of the permeability due to tectonic events and fluid circulations. The decrease of permeability and permeability anisotropy near the fault is principally due to the tectonic event. This decrease was associated with a decrease of porosity and pore threshold due to compaction in the footwall because of the great number of stylolithes. In the hanging wall, the decrease of petrophysical properties was due to precipitation of cement around quartz grains. The permeability reduction near the fault accounted for the role of the microstructures in fluid pathways. They were horizontal in the undeformed rock and became vertical in the faulted rock.
Abstract Microthermometric characteristics of metamorphic to hydrothermal fluids and microfracturing were studied in a contact zone between metamorphic series and peraluminous granites, located in the southern part of the Mont Lozère pluton (Massif Central, France). Four major stages of fluid production or migration have been recognized: (1) N 2 -CH 4 (±CO 2 )-rich fluids related to the metamorphism of the C-bearing shales, occurring as fluid inclusion along the quartz grain boundaries; (2) CO 2 -CH 4 -H 2 O vapours or liquids, with homogenization temperatures of 400 ± 20 and 350 ± 50°C respectively, related to the first hydrothermal stage produced by the late peraluminuous intrusions; (3) aqueous fluids having low salinities and Th in the range 150–330°C; (4) low-temperature aqueous fluids. It is shown that the percolation of hydrothermal fluids occurs through a dense set of microfissures on a microscopic scale. The different stages of fluid percolation have been investigated by relating the deformational events to the observed fracturing. The nature of the hydrothermal fluid has been deduced by studying the trails of fluid inclusions. Analysis of the relationships of the fluid inclusion trails (F.I.T.) with structures associated with plastic deformation show that their propagation is independent of the intracrystalline anisotropies. Combined studies of their orientation in space and their microthermometric characteristics show that: (1) according to the direction, several generations of fluids are distinguished within each sample on the basis of their physical-chemical characteristics; they correspond to different stages of the hydrothermal activity and to different directions of micro-crack opening; (2) in bulk isotropic media (granite), fluid inclusion trails are essentially mode I cracks which can be used as excellent markers of paleostress fields; however, in bulk anisotropic media (quartz lenses in mica schists) the migration directions of the fluids are mostly dependent on the local reorientations of the stress fields. The study of the contact zone between granites and a metamorphic series submitted to local abnormal heat flows shows that fluid characteristics are significantly different in the two environments. Migration of carbonic fluids from mica schists towards granites occurred but is relatively limited, whilst aqueous fluids mixed in variable amounts with carbonic fluids in the metamorphic zone.
