Philippe Robion

Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions

Abstract This article presents a systematic analysis of the anisotropic behaviours of the Bentheim and Rothbach sandstones using ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility measurements. For each sandstone, the data were obtained from three core samples drilled perpendicularly to each other and tested in dry- and water-saturated conditions. For acoustic and magnetic investigations, the same statistical analysis was applied in order to present the data on comparable stereoplots. Surprisingly, the Bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the Rothbach sandstone in which cross-laminations were clearly identified, as confirmed by a sedimentary magnetic fabric. A discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies. First, by comparing electrical and acoustic properties in the Bentheim sandstone, we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape: the inclusion model developed by Kachanov (Kachanov, M., 1993. Elastic solids with many cracks and related problems. Advances in Applied Mechanics, vol. 30. Academic Press, Boston, MA, pp. 259–445) accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio. Second, acoustic, electrical and magnetic properties indicate that the observed anisotropy in the Rothbach sandstone can be attributed to the matrix, and more specifically to cementation: we modified the Dvorkin and Nur (Geophysics 61 (5) (1996) 1363) model of cemented granular media by introducing a spatially variable contact length, and the model suggests that a very small variability of cemented contact length is enough to account for the observed P-wave velocity anisotropy. We emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks.

Magnetic fabric and petrographic investigation of hematite-bearing sandstones within ramp-related folds: examples from the South Atlas Front (Morocco)

An investigation of the anisotropy of magnetic susceptibility (AMS) of hematite-bearing sandstones has been carried out along two fault-propagation folds located at the front of the High Atlas Belt (Goulmima area, Morocco). It reveals two types of unusual magnetic fabrics. The first one is always located in the forelimb of the anticlines and is characterised by an oblique magnetic foliation with respect to bedding. The relationship between bedding and magnetic foliation indicates a sense of shear, which is consistent with the one expected from kinematic modelling. It is proposed that such an oblique magnetic foliation is the result of a weak strain that develops during folding. The other striking feature of the AMS results is that 40% of the magnetic lineations are parallel to the regional shortening direction. From structural and petrographical data, it is suggested that this magnetic lineation corresponds to the intersection lineation between two planar elements that are, respectively, the bedding and iron oxides mineralised veins. The latter result from iron-rich fluid circulations through a fracturing network.

Offshore Frontal Part of the Makran Accretionary Prism: The Chamak Survey (Pakistan)

The Makran accretionary prism developed in the north-western part of the Indian Ocean as a consequence of the subduction of the Arabian Sea since Late Cretaceous times. It extends from southern Iran to the Baluchistan region of Pakistan where it joins the Chaman-Ornach-Nal left-lateral strike-slip fault systems to the north and the Owen Fracture Zone-Murray Ridge transtensional (right-lateral) system to the south in a complex triple junction near the city of Karachi. In September to October of 2004, we surveyed most of the accretionary complex off Pakistan with R/V Marion Dufresne. We achieved a nearly continuous bathymetric mapping of the prism and the subduction trench from 62°30′E to the triple junction near 62°30′E together with nearly 1000 km of seismic reflection (13 lines) and we took 18 piston cores in different geological settings. One of the main results is that the frontal part of the Makran accretionary prism is less two-dimensional than previously expected. We interpret the along-strike tectonic variation as a consequence of lateral variations in sediment deposition as well as a consequence of the under-thrusting of a series of basement highs and finally of the vicinity to the triple junction.

The anisotropy of magnetic susceptibility (AMS) in low-grade, cleaved pelitic rocks: influence of cleavage/bedding angle and type and relative orientation of magnetic carriers

Abstract Cambrian and Silurian, low-grade, pelitic rocks of the single-phase deformed Brabant Massif consistently have a maximum magnetic susceptibility axis (K1) parallel to the cleavage/bedding intersection. In contrast, the minimum susceptibility axis (K3) either coincides with the bedding pole, with the cleavage pole or occupies an intermediate position. Anisotropy of anhysteretic remanence (AARM) and X-ray pole figure goniometry allow the distinguishing of the orientation distributions of the ferromagnetic and paramagnetic (white mica and chlorite) carriers, respectively. Mismatches between K3 and the poles to the macroscopic fabric elements (i.e. bedding and cleavage) are attributed to different orientations of the different magnetic (s.l.) carriers. A strong relationship exists between the cleavage/bedding angle and the shape parameter: low, respectively high angles leading to oblate, respectively prolate susceptibility ellipsoids. However, differences are observed between the Cambrian and Silurian samples in terms of the shape parameter and the behaviour of the degree of anisotropy with changing cleavage/bedding angle. This is tentatively attributed to differences in relative orientation and mineralogy of the magnetic (s.l.) carriers. These results demonstrate the influence of the relative orientation of the different carriers on AMS and suggest that, although being a petrofabric tool, AMS cannot be used as a strain gauge in the case of composite magnetic fabrics.

Early record of tectonic magnetic fabric during inversion of a sedimentary basin Short review and examples from the Corbières transfer zone (France)

An integrated study combining anisotropy of magnetic susceptibility (magnetic fabric) and analysis of meso-scale structures is presented. The studied sites, which belong to the Corbieres transfer zone (France), suffered a clear polyphase deformation. The magnetic fabric is of tectonic origin and records an early layer parallel shortening. In general, this primary fabric is not altered by later deformations including folding. However, secondary fabrics have been observed in the vicinity of major faults. They are probably related to tectonically driven fluids migrating along these planes. Consequences on interpretation and use of magnetic fabrics in inverted sedimentary basins are discussed.

Acoustic and reservoir properties of microporous carbonate rocks: Implication of micrite particle size and morphology

This integrated study provides significant insight into parameters controlling the acoustic and reservoir properties of microporous limestones, improving the knowledge of the relationships among petrophysic and microstructural content. Petrophysical properties measured from laboratory and logging tools (porosity, permeability, electrical conductivity, and acoustic properties) have been coupled with thin section and scanning electron microscope observations on the EST205 borehole from the Oxfordian limestone aquifer of the eastern part of the Paris Basin. A major achievement is the establishment of the link between micrite microtexture types (particle morphology and nature of intercrystal contacts) and the physical response, introducing a new effective and interesting rock-typing approach for microporous reservoirs. Fluid-flow properties are enhanced by the progressive augmentation of intercrystalline microporosity and associated pore throat diameter, as the coalescence of micrite particles decreases. Concerning acoustic properties, the slow increase of P wave velocity can be seen as a reflection of crystal size and growing contact cementation leading to a more cohesive and stiffer micrite microtexture. By applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates. This dispersion analysis highlights the presence of microcracks in the rocks, and their overall effect on acoustic and transport properties. The presence of microcracks is also confirmed with observations and permeability measurements under high confining pressure. Finally, a possible origin of high porous levels in neritic limestones is a mineralogical transformation of carbonates through freshwater-related diagenesis during subaerial exposure time. Finally, by applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates.

Fabric development and metamorphic evolution of lower Palaeozoic slaty rocks from the Rocroi massif (French–Belgian Ardennes): new constraints from magnetic fabrics, phyllosilicate preferred orientation and illite crystallinity data

Abstract This paper presents new results of a petrofabric study from Cambrian slates cropping out within the anchi–epimetamorphic core of the Rocroi massif (Ardennes Variscan fold-thrust belt of northern France and southern Belgium). This study includes measurements of the Anisotropy of the Isothermal Remanent Magnetisation (AIRM) and the phyllosilicate preferred orientation. It completes the previous results of Anisotropy of Magnetic Susceptibility (AMS) and of magnetic mineralogy which were obtained in 47 sites situated in and around this massif. These new data allow the characterisation of the different subfabrics carried by the ferromagnetic minerals (metamorphic pyrrhotite and magnetite) and the paramagnetic matrix (Fe-rich phyllosilicates). Combined with some illite crystallinity data measured throughout the massif, the petrofabric measurements evidence a polyphase tectonometamorphic evolution. A major Variscan compressional event is evidenced inducing the growth of highly anisotropic pyrrhotite, coarse-grained hematite and phyllosilicates. By contrast, the magnetite grains display a less well-organised fabric due to the existence of an inherited orientation pattern. In agreement with recent metamorphic studies, it argues for an early diastathermal metamorphic event developed during a Devonian crustal-scale extension.

Tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks: an example from the Ardennes Massif (France-Belgium)

Abstract The discrimination of the respective contribution of strain and mineralogy of the bulk Anisotropy of low field Magnetic Susceptibility (AMS) of rocks is addressed by a study of the Ardennes Variscan Thrust Belt. Unmetamorphosed to low-medium-grade rocks in the study area define a succession of magnetic parageneses: hematite + maghemite (?), hematite + maghemite (?) + magnetite, magnetite + pyrrhotite. A direct control of the magnetic mineralogy on the magnitude of AMS axes is shown. In addition, in all cases, the interpretation of the directions of AMS axes as kinematic indicators also applies to these rocks. We show a progressive change in shearing direction from the south of the Rocroi massif, where the nappe transport is N-directed; in the north, the vergence changes to northwestward.

Cryptic magnetic fabric of tectonic origin revealed by heating of sedimentary samples from the Corbières, France

Abstract It is largely recognized that heating enhances magnetic signal and that this is probably due to crystallization in a pre-existing petrofabric. This paper presents an example where heating not only enhances the magnetic fabric, but reveals a primary tectonic fabric overprinted by a late crystallization. We focus on a site of red Paleocene silts from the Lagrasse area (Aude, France), which exhibits, in two superimposed beds, primary (due to layer parallel shortening) and secondary (due to late crystallization) fabrics. Partial anisotropy of anhysteretic remanent magnetization and magnetic mineralogy investigations led us to consider that heating allows the growth of oxides, enhancing a cryptic pre-existing fabric.

Magnetic mineralogy and metamorphic zonation in the Ardennes Massif (France-Belgium)

In the Ardennes area, pelitic schists of Cambrian to Early Devonian age have experienced a low-grade polyphase metamorphism which can be characterised by changes in magnetic mineralogy. Using mainly rock magnetic tools we have defined various iron oxides and sulphide assemblages allowing a magnetic mineralogy “zoneography” of the Ardennes metamorphism to be constructed. Low-temperature and three-axial IRM measurements identify pyrrhotite in the southern part of the massif. This mineral had not been previously recognised in the Ardennes area. In the Devonian cover, toward the metamorphic apex, the appearance of coarse-grained haematite is characterised by low coercivity values. The trends of low field and anhysteretic susceptibilities allow a discrimination between the lithologic and metamorphic effects to be made. Additionally, the relative contributions of an early metamorphic event and the classical metamorphism linked to the Variscan shortening are discussed.