Jean-Pierre Sizun

The importance of the degree of cataclasis in shear bands for fluid flow in porous sandstone, Provence, France

Determination of the membrane seal capacity of deformation bands is critical for managing geologic reservoirs in porous sandstones. In this study, we have analyzed a cataclastic shear-band network developed in uncemented porous sandstone in Provence, France. Geometrical analyses of the bands show significant differences between three types of bands (single strand, multistrand, and band cluster), sorted by their number of strands, their amount of shear displacement, and their thicknesses. At the microscopic scale, the image-analysis porosities and the grain-size distributions allow definition of three different types of microstructural deformation: damage zone, protocataclastic, and cataclastic. Whereas damage zone and protocataclastic deformations are observed in each type of band, cataclastic strands are observed in clusters and, sometimes, in multistrands. Cataclastic strands are characterized by a porosity reduction of 10 to 25% and a permeability reduction of three to five orders of magnitude compared to the host rock. Field observations of iron hydroxide precipitations around the bands suggest that cataclastic strands were membrane seals to water flow under vadose condition. This study therefore highlights the importance of the degree of cataclasis in shear bands as membrane seals to subsurface fluid flows in sandstone reservoirs.

Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, delta 18 O) evolution of the clay minerals from faulted, carbonate-rich, passive paleomargin of southeastern Massif Central, France

ABSTRACT Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, d18O) data of < 0.2 µm clay fractions from core samples have been used to enhance our understanding of the evolution of the faulted, carbonate-rich, passive paleomargin in the southeastern French Massif Central. The fault system described at the lower part of the sedimentary sequence was active at 190 ± 20 Ma during a rifting tectono-thermal activity with important migration of hot fluids. It was active again afterwards, probably after maximum burial occurring at 110-120 Ma, as a result of the compressive Alpine activity in the region. This late shearing induced an overall throw of at least 1160 m and subsequent erosion of as much as 2000 m of s diments. However, it probably did not induce major fluid movements, which means in turn that the mineralizing fluids responsible for economic ore deposits in the margin relate to the 190-Ma-old Liassic hydrothermal event. The Rb-Sr, K-Ar, and oxygen isotopic systematics of the clay fractions suggest that late faulting activity occurred under very low water-to-rock conditions, which did not favor widespread fluid migration. The clay material of the fault yields unreasonably widely scattered Rb-Sr and K-Ar dates, from 70 to 183 Ma, because of variable contents of radiogenic 40Ar and some unrealistic initial 87Sr/86Sr ratios, even below 0.700. The 190 Ma illites that crystallized in the fault at 200-210°C also have abnormally low d18O values of about +13.3, leading to a d18O value of the pa eofluids as high as +8.1. The 87Sr/86Sr ratios and REE distributions of acid-leached clay residues delineate two types of fluids that have probably interacted with the clay minerals. One, with an 87Sr/86Sr ratio of 0.7250 and a humped REE distribution pattern, is believed to be of hydrothermal origin, and the other, with an 87Sr/86Sr ratio of 0.7082 and a flat REE pattern, might be of recent continental origin.

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Gres d'Annot, southeastern France)

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195C to 268C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Volumetric matrix strain related to intraformational faulting in argillaceous sediments

Soft-sediment deformation involves complex interactions between discrete fracturing and diffuse bulk strain, described in terms of volume change and shear strain in a critical state mechanics framework. This study reports on a mesoscale normal fault zone, intraformational in Oligocene argillaceous sediments from the Boom Formation (Belgium), containing several metre-scale normal fault strands. They form either discrete fault planes or decimetre-wide shear zones with internal fabric. The faults have been subjected to microtectonic and petrophysical analysis. Small but significant changes occur in the porous network of the argillaceous matrix approaching a fault or shear zone, indicating compactional strain in both footwall and hanging wall. Internal compaction associated with faulting is put forward as a ductile–brittle feedback mechanism in the kinematics of intraformational fracture systems. Small differential stress induced by compaction and minor regional tectonic forces (differential uplift and tilt) and subsequent gravitational forces (downslope shear stress) induce small shear bands in nearly critically stressed weak mud. Shear banding is accompanied by layer-parallel shortening and bulk volume loss. This provides an additional extension of endogenous origin, accommodated by further deformation. This ductile–brittle feedback mechanism eventually leads to commonly observed intraformational collapse structures called polygonal fault systems.

Changes in reservoir quality determined from the diagenetic evolution of Triassic and Lower Lias sedimentary successions (Balazuc borehole, Ardèche, France)

Abstract Diagenetic mineral sequences in Triassic and Lower Lias sedimentary successions studied in the Balazuc borehole are seen to have modified the initial reservoir qualities of the formations. The upper part of the Triassic Lower Sandstone unit and the Hettangian Lower and Upper Calcarenite units were initially good quality reservoirs, whereas the Triassic Upper Sandstone unit and the Rhaetian Sandstone unit were initially average quality reservoirs, temporarily improved by pedogenesis. Early burial induced partial plugging of the initially porous bodies as a result of fluid exudation from proximal sediment bodies; the circulation of these fluids brought about alteration, dissolution and mineral deposition. Tectonic events associated with the formation of the Southeast Basin palaeomargin caused movement along the Uzer Fault and imparted a fracture porosity to the buried sedimentary formations. However, increasing subsidence and burial reactivated fluid circulation, dissolution and mineralization with further plugging of pore spaces. Present reservoir quality, as assessed from measured water porosity and neutron density well logs shows a changed reservoir distribution. Apart from the main Uzer Fault and one secondary fault (1644–1647 m) that today form drainage channels, the Triassic Lower Sandstone unit and its topmost breccia are now non-reservoirs, the Triassic Upper Sandstone, Rhaetian Sandstone and Hettangian Calcarenite units form a succession of residual-matrix-porosity reservoirs isolated by non-porous units.

Water content of limestones submitted to realistic wet deposition: a CIME2 chamber simulation

An experimental chamber (CIME2) has been specially designed to simulate wet atmospheric deposition on limestones used in Paris cultural heritage. This instrument is a complementary tool to CIME, a previously developed chamber dedicated to the simulation of dry atmospheric deposition on monuments and artifacts. The aim of this paper is to describe CIME2 and characterize the wet deposits produced inside it. Mist (fog), drizzle, and rainfall are differentiated in order to document their ability to saturate the limestones most currently used in Paris monuments: The Saint-Maximin’s limestone, the Liais of Saint-Maximin, and the Chauvigny’s limestone are tested. The comparison between normalized and environmental petrophysical data shows that in the wet deposition simulations, limestones are not systematically water-saturated. Moreover, the realistic experimental conditions chosen favor a more rapid evaporation of the stone water. The quantification of the non-saturation state is a first step that has to be taken into account to improve the geochemical models used to predict the alteration.