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Deformation of intraplate sedimentary basins essentially results from tectonic stresses that propagate from plate boundaries. Yet, the detailed characteristics of this far field propagation are often poorly constrained, especially in age. Recent developments of U/Pb dating of calcite precipitations in fault planes and veins hopefully greatly help to frame the schedule of deformation. This is the case for the Meso-Cenozoic Paris basin, liable to have recorded several Cenozoic events due to its location in the western Eurasian plate, like Pyrenean and Alpine collisions and Oligocene extension (ECRIS).Our study focusses on the southern Paris basin area, north of the Morvan massif, where Alpine deformations are expected. Field data reveal almost exclusively strike-slip deformations with variable σ1 direction. These results are about to be confirmed at the microscopic scale using Anisotropy of Magnetic Susceptibility and of P-Wave Velocity methods. Associated U-Pb dating of synkinematic calcites from Jurassic strata indicates distributed Eocene deformation and an absence of Oligocene or Miocene ages. Such ages suggest the record of Pyrenean and possibly ECRIS tectonic events. Strikingly, Alpine deformation and especially stress propagation leading to the growth of the Jura fold-and-thrust belt during Miocene does not imprint the studied area although the Alpine orogen is located closer than the Pyrenean belt.

Previous paleomagnetic studies performed in the central North-Algeria (Chellif and Mitidja basins) on Neogene formations pointed out tectonic clockwise blocks rotations. This deformation pattern was interpreted as resulting from a bookshelf neotectonics, consequence of the Africa-Eurasia plates convergence. A new paleomagnetic study was conducted on the Neogene volcanic rocks outcropping in the northwestern Algeria (Marset Ben Mhidi, Ain Temouchent, Tifaraouine area). The obtained stable remanent magnetization is mainly carried by Ti-poor titanomagnetite. The paleomagnetic data show that, since the lava emplacement, the northwestern Algeria underwent a mean moderate clockwise block rotation of 9.3° ± 4.5°. For the Algerian margin, this confirms a context of transpression and blocks rotations in a strike-slip tectonic setting. A decreasing deformation gradient from the E to the W affected the different basins of this margin, from strong rotations within the Mitidja, to the moderate ones in the Chellif and to Marset Ben Mhidi, Ain Temouchent, Tifaraouine area, where rotation magnitudes are significantly lower.

Sedimentary basins are subject to tectonic stresses that propagate from continental plate boundaries. The resulting strain can vary: the lithosphere can buckle and fold, brittle deformation can occur, often reactivating basement faults, and microstructure networks of faults, joints and veins can develop on a local scale. These intraplate deformations, either attributed to the Pyrenean Orogeny or to the Alpine Orogeny, are present in Tertiaryformations of the Paris Basin. Tectonic structures directly affect the stratigraphic architecture and facies of sedimentary deposits. However, when compared with the well-documented architecture of sedimentary deposits and tectonic structures of the Paris Basin, relatively little is known about the relationship between the two. Preliminary results of this PhD revisit the tectonic structures of Tertiary formations using new and existing data.Since 2000, the BRGM reprocessed thousands of kilometers of industrial seismic lines covering the entire Paris Basin. We propose an interpretation of the base of Tertiary formations in the Paris Basin, calibrated by hundreds of wells with velocity surveys. Strategically placed geological cross sections made with well data containing the top depth of different Tertiary stages, combined with information from geological maps (1/50 000 scale edited by the BRGM), reveal the Cenozoic tectonic structure of the Paris Basin.Interpreted seismic lines and geological cross sections make up the initial results of this PhD, illustrated by a synthetic structural map of Tertiary deformations in the Paris Basin. This map shows long wavelength and low amplitude post-Mesozoic undulations along with faults traversing the Tertiary base. The continuation of this PhD will involve fieldwork with structural observations (deformation directions, geometry, density, distribution) and sample collecting as well as laboratory work (magnetic and wave velocity anisotropy and in-situ U-Pb calcite dating) in order to constrainthe temporal and spatial evolution of paleo stresses propagating from tectonic plate limits.

In this study, we present preliminary results on paleomagnetic data collected in the Tunisian Tellian domain in both magmatic and sedimentary rocks of middle to lat Miocene ages from the Nefza-Mogods province, North-West of Tunisia. About 320 cores distributed over twenty one sites were collected both in magmatic rocks (16 sites) and in sedimentary rocks (5 sites), in order to obtain geometric constraints to establish a kinematic model along the North-East African margin. The sampled rocks are distributed between basanites, rhyodacites and microgranites. Some samples were taken from host sedimentary rocks host rocks in lacustrine limestones and jaspilites. Demagnetization process and Rock-Magnetism studies revealed a diversified magnetic mineralogy. In basalts, magnetite with an unblocking temperature of 580 °C is identified. In rhyodacites, the mineralogy is mixed with three types of minerals: a mineral with an unblocking temperature around 350°-400°C attributed either to a sulfide or to titanomagnetite, magnetite with unblocking temperature at 580°C, and a high temperature mineral with unblocking temperature between 600°C and 680°C attributed to hematite or titanohematite. The limestones, having a low magnetization intensity, are characterized by the presence of magnetite and the jaspilites by hematite. Basalts, which have been mainly demagnetized by AF process , show a characteristic component demagnetized between 20mT and 100mT. For rhyodacites, some sites have a characteristic component demagnetized between 400°C and 580°C and others up to 670°C. Although their low magnetization intensity, the lacustrine limestones show a magnetic component between 20mT and 140 mT. The first result indicate that the mean directions associated to the younger magmatic (basalts and rhyodacite) rocks (8 Ma, Tortonian) and their sedimentary host deposits are very close to the expected magnetic field after tilting in paleogeographic coordinates. By contrast, the older microgranites and rhyodacites(-12 Ma) display a vertical axis clockwise rotation of about 30°. This result suggests a significant tectonic phase between 12 Ma and 8 Ma, linked to the implementation of the Tell nappes.

Globular structures in volcanic and hypabyssal rocks called varioles (or spherulites) have been described from a wide variety of places and rock types including komatiites, basalts, dacites, rhyolites as well as alkaline dykes, but the processes leading to their formation are still debated. Here, we present a field, petrological and geochemical study of variole-bearing lavas (variolites) in the Paleoproterozoic volcanic Hekpoort Formation in the southern part of the Transvaal sub-basin of South Africa. Variolites were only observed in a restricted area, corresponding to a ≥3.5 km long basin or depression. The variolites are locally up to 50 m thick and correspond to a succession of one-meter-thick lava flows. Most of the flows consist of two distinct layers, with an upper layer rich in spherical varioles (1–32 cm in diameter), overlying a layer generally devoid in varioles but often showing vertical jointing. Both layers show randomly oriented acicular textures of centimetre-sized skeletal clinopyroxene crystals (Al-rich augite) that precede the formation of varioles. The varioles themselves are mainly composed of radiating acicular augite (1 μm in diameter), the crystallization of which caused diffusion of incompatible elements, which became enriched in the groundmass around the varioles. The whole rock geochemistry of the Hekpoort variolites is indistinguishable from that of non-variolitic Hekpoort lavas with the same MgO content. This excludes liquid immiscibility as the origin of the varioles. The acicular texture (of Al-rich augite) in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification. Sub-aqueous deposition is thought to be the cause of undercooling.

This study presents the first analysis of the anisotropy of magnetic susceptibility (AMS) coupled with anisotropy of P-wave velocity (APV) in sedimentary rocks from a passive continental margin. In the Otway Ranges, Australia, the Lower Cretaceous sediments deposited during a period of subsidence and structural quiescence display triaxial-magnetic fabrics typical of extensional deformation. The extensional interpretation from AMS is also supported by the orientation of the pore fabric measured using APV. The inferred extensional azimuths from both AMS and APV analyses are in good agreement with axes for renewed Late Cretaceous rifting and the continental break up of Gondwana, a period of structural evolution characterised by abnormally-high levels of paleostress in the Otway Basin. Predicated on this we introduce the concept of layer parallel stretching (LPSt), a process that describes pore-scale structuring representative of the first phase of extensional deformation during periods of intense tectonic extension, in this case the LPSt was oriented NE-SW. These extensional structural fabrics in the Otway Basin were preserved during NE-SW oriented basin inversion, a process that involved reactivation of the local detachment, the partitioning of strain, and low levels of coupling between the basement and the cover. The results highlight the applicability of AMS and APV for characterising rock anisotropy in sedimentary basins at continental margins and have distinct outcomes with respect to the structural framework of the study area in the Otway Ranges.

International audience; Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

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Carbonate rocks are well-known to be tremendously heterogeneous. They mainly consist of component particles (from biological and non-biological origin) embedded in a lime-mud matrix and/or in a cement (composed of even smaller particles). The size, shape, density and spatial arrangement of those particles, alongside with natural fractures and cracks (although those are certainly not exclusive to carbonate rocks), define a microstructural pattern that is known to have a great influence on rock physical properties. Thus, to understand carbonate rock systems at large scales (formation, reservoir …), geophysicists have to study them at the pore scale, hoping to resolve the so-called “upscalling” problem. With this in mind, unravelling and identifying the relations between physical properties and carbonate rock microstructures is paramount for a global comprehension of a carbonate rock system. Since the late nineties, several research groups and authors have worked on documenting and providing significant insights into the microstructural parameters controlling the physical response of several rock properties (porosity, permeability, electrical conductivity, elastic, seismic and mechanical properties …) in carbonates. This article proposes a review of this specialized literature, from the early and recent contributions in rock physics, with emphasis on the recent studies on carbonate rocks from the Paris basin.

International audience; Knowledge of the paleo-stress distribution is crucial to understand the fracture set up and orientations during the tectonic evolution of a basin, and thus the corresponding fluid flow patterns in a reservoir. This study aims to predict the main stress orientations and evolution during the growth of a fold by using the limit analysis method. Fourteen different steps have been integrated as 2D cross sections from an early stage to an evolved stage of a schematic and balanced propagation fold. The stress evolution was followed during the time and burial of syn tectonic layers localized in front of the thrust. Numerical simulations were used to predict the occurrence and orientation of deformation bands, i.e., compaction and shear bands, by following the kinematic of a fault-propagation fold. The case study of the Sant-Corneli-Boixols anticline was selected, located in the South Central Pyrenees in the Tremp basin, to constrain the dimension of the starting models (or prototypes) used in our numerical simulations. The predictions of the numerical simulations were compared to field observations of an early occurrence of both pure compaction- and shear-enhanced compaction bands in the syn-tectonic Aren formation located in front of the fold, which are subjected to early layer parallel shortening during the burial history. Stress magnitude and stress ratio variations define the type of deformation band produced. Our results show that the band occurrence depends on the yield envelope of the host material and that a small yield envelope is required for these shallow depths, which can only be explained by the heterogeneity of the host rock facies. In our case, the heterogeneity can be explained by a significant contribution of carbonate bioclasts in the calcarenite rock, which change the mechanical behavior of the whole rock.

Previous paleomagnetic studies, performed in central North-Algeria (Chelif and Mitidja basins) on Neogene formations, pointed out some tectonic clockwise rotations of large blocks, sometimes of large amplitudes. Narrow zones represent also important shear zones with strong rotations. The preliminary results of a new paleomagnetic study undertaken in Northwestern Algeria (Ain Temouchent–Ghazaouet area) suggest the presence of clockwise block-rotations of lower magnitude. That confirms that the northern Algeria is organized as tectonic blocks, accommodating by transpression and blocks rotations, the Africa-Eurasia convergence. A gradient of “deformation” appears from the NNE to the SSW, i.e. from the Mitidja basin affected by strong rotations to Ain Temouchent–Ghazaouet area where the rotations are limited.

Strain localization in a porous calcarenite facies of the Aren formation in the Tremp basin was studied. This Maastrichtian syn-tectonic formation exposed in front of the Boixols thrust, in the Central South Pyrenean Zone, hosts bedding perpendicular deformation bands. These bands are organized in two major band sets, striking East-West and N-020 respectively. Both populations formed during early deformation stages linked to the growth of the fold and thrust. A magnetic fabric study (Anisotropy of Magnetic Susceptibility, AMS) was carried out to constrain the shortening direction responsible for the deformation bands development during the upper Cretaceous-Paleocene N-S contraction in the region, which allowed us to define populations of Pure Compaction Bands (PCB) and Shear Enhanced Compaction Bands (SECB) regarding their orientations compared to the shortening direction. Both sets are formed by cataclastic deformation, but more intense in the case of SECBs, which are also thinner than PCBs. The initial pore space is both mechanically reduced and chemically filled by several cementation phases. We propose a geomechanical model based on the regional context of layer parallel shortening, thrusting and strike-slip tectonics considering the burial history of the formation, in order to explain the development of both types of bands at remarkably shallow depths.

International audience; Dans le cadre de son Plan Climat, la Communauté d’Agglomération de Cergy-Pontoise (CACP) souhaite développer les énergies renouvelables sur son territoire. Ainsi elle s’est associée au laboratoire " Géoscience et Environnement Cergy " (GEC) afin d’évaluer le potentiel géothermique des couches profondes de la zone. Cette étude s’appuie sur les données de puits d’exploitation géothermique du Dogger à Cergy-Pontoise des années 1980 et les différentes connaissances du bassin de Paris déjà publiées. Elle utilise également les études de la ressource géothermique du Trias dans le bassin de Paris effectuées par le BRGM. Ainsi, l’objectif final de cette étude est de modéliser les structures tectoniques profondes grâce à l’étude de profils sismiques, et de caractériser les réservoirs du Trias de la région de Cergy en menant une étude stratigraphique, sédimentologique et pétrophysique des réservoirs à partir de données de forages, de carottes et de diagraphies.Les questions scientifiques qui se posent sont :- Comment les paléo-environnements de dépôt et le contexte structural impactent-ils la qualité et la continuité des réservoirs ?- Ces résultats permettent-ils d’estimer le potentiel géothermique de Cergy-Pontoise ?Au Trias supérieur, l’extrême ouest du bassin de Paris enregistre les derniers dépôts continentaux issus de l’érosion des massifs hercyniens, avant la phase transgressive du Lias. Ces dépôts triasiques se biseautent vers le NW du bassin, jusqu’en Normandie. Ainsi, la zone d’étude se situe au sein de ce biseau, dans le graben du Vexin, entre les failles du Bray et de la Seine. Elle contient 17 puits d’exploration pétrolière datant des années 1970-80, sur un rayon de 50km autour de Cergy-Pontoise. Ces puits ont fait l’objet de corrélations diagraphiques afin de préciser l’étendue des réservoirs et leur architecture en 3D. En parallèle, 47m de carottes en continu du Trias ont été étudiés. Une étude de détail des macrofaciès a permis de reconstituer différents environnements de dépôt ainsi que l’estimation de la porosité en 2D (point-counting) sur les 27 lames minces de cette même carotte. De plus, différentes cimentations de la matrice ont été identifiées. Les propriétés pétrophysiques de ces échantillons a été caractérisé par des essais de perméabilité et porosité à l’eau. Enfin, l’étude de lignes sismiques retraitées au GEC a permis de préciser les failles qui affectent les réservoirs du Trias.

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We propose a simple method based on the normalized eigenvalues of the tensor of anisotropy of magnetic susceptibility (AMS) to estimate horizontal ductile strain in detrital clay-rich sedimentary rocks subjected to layer parallel shortening (LPS). This method is mainly based on two assumptions: (1) The magnetic carriers are contacting clay-mineral platelets, and (2) The platelets respond to LPS by crenulation and formation of a zone axis of unchanged magnetic susceptibility perpendicular to the plane of deformation. We further simplify the problem by considering, as the exact distribution of particles orientations is unknown, that the AMS tensor arises from a single equivalent clay platelet, of which the angle with the bedding plane directly reflects the amount of horizontal strain. The proposed method is applied to data from samples retrieved during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) complex drilling project. Sample-wise finite strain values assist AMS data interpretation, which is traditionally based on the AMS ellipsoid shape parameter, and allow comparison with strain estimates obtained by other means. In the Nankai Accretionary Prism, AMS-derived strain values range from 0 to 25% and suggest substantial mechanical compartmentalization with inter-unit strain contrasts larger than 5 percentage points, including the penetrated shallow portion of a megasplay fault. At a local scale, AMS strain also appears to be affected by thrusting, what can be interpreted as a record of a ductile strain component away for the brittle zone or as an ‘interseismic’ ductile strain within otherwise brecciated material.

Models for basin evolution and natural fracture development often contain many uncertainties. Multiscale approaches to structural analysis assist in reducing these by providing checkpoints for structural evolution to better constrain the development of paleostress phases through time. In this study, we integrate the analysis of calcite twins, magnetic fabrics, stylolites and natural fractures at Castle Cove in the eastern Otway Basin, producing a five-phase model for stress evolution consisting of: phase 1 ~NW–SE Mid-Cretaceous strike-slip or compression; phases 2 and 3 Late Cretaceous extension, coinciding with the development of ~NW–SE and ~NE–SW striking extensional fracture sets; phase 4 ~NE–SW strike-slip and compression, representing an enigmatic period of stress evolution with respect to the current understanding of the Otway Basin; and phase 5, present day ~NW–SE strike-slip stress. The results contribute to a 4D structural history construction for the eastern Otway Basin and suggest that the evolution of the region may require reassessing in order to determine the timing and nature of the detected ~NE–SW oriented compressional event. This study also demonstrates how the use of a calcite stress inversion technique can assist in providing mechanical checkpoints for the evolution of complex natural fracture networks, which can easily be expanded within the sub-surface.

We present a study integrating petrophysical measurements along a general N–S section of the Corbieres–Minervois fold and thrust belt (NE Pyrenees, France) in order to study the petrofabric of weakly deformed sediments. We focus on the relationship between the evolution of deformation measured with the anisotropy of various physical properties (anisotropy of magnetic susceptibility (AMS), anisotropy of P-waves velocity (APV) and anisotropy of electrical conductivity (AEC)) and the distribution of the porous space. The origin of AMS fabric is interpreted as recording a layer parallel shortening oriented N145, steadily throughout the studied section with local gradient associated with folding as sites close to the forelimbs. AEC is in good agreement with AMS results and records the same shortening direction. The most probable source of anisotropy for AMS and AEC is the preferred orientation of phyllosilicate minerals. APV, which reflects the distribution of porous space, is more complex and is controlled by the tectonic style of the magnetic fabric. The porosity is a flat-lying-bedding porosity, where the magnetic fabric is poorly evolved and it becomes a flat-lying-pressure-solution-cleavage porosity where the magnetic fabric is tectonic. At regional scale, we show that changes in anisotropy of petrophysical properties are controlled by individual structures, like regional fold or thrust fault and occurred mainly before folding as a consequence of a well-expressed NNE–SSW layer parallel shortening. The folds and thrust faults are developed subsequently from inherited E–W basement faults. This explains why the structural grain of the sedimentary cover, expressed notably by the magnetic lineation, is oblique to the major structures of the cross-section: the Alaric Anticline and the North-Mouthoumet Fault.

This paper presents a thorough magnetic study of centimeter-sized rock samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP). Fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization. We infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks. Using other evidence from ferrofluid saturation experiments, we conclude that this network of microcracks is not sealed but open in situ, and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the TCDP area.

Petrophysical properties were measured on oolitic limestone from the Oolithe Blanche formation (middle Jurassic) in the Paris Basin. Eighteen oriented blocks were collected from three outcrops regarding of the three main facies, oolitic shoal facies, tide dominated facies and prograding oolitic facies. We investigated the relationship between both compressional wave and electrical conductivity with different petrophysical properties such as porosity (water porosity and mercury porosity), permeability and capillarity imbibition. These have led us to relate the variations of petrophysical properties to several microstructural parameters, among them the microporous structure is the most important. Concerning P wave velocities, the general trend observed is as expected a decrease of velocities as the porosity increases but with a significant fluctuation of velocities for a given value of porosity. We have used two distinct effective medium approximations to describe the velocity variations: the self-consistent (SC) approximation and the cemented contact theory (CCT) but no unique model can simply explain our velocity behaviours. The main parameter that controls the P wave velocities is the distribution of microporosity inside oolites: for samples with velocities higher than 4 km/s the microporosity is mainly located in the rim of the ooids while for samples with velocity lower than 4 km/s the microporosity is homogeneous across the ooids. Acoustical fabrics, which are controlled by the facies, indicate that sedimentary textural components such as the amount of cement and arrangement of elements within the oolitic limestones (bioclasts, pellets and ooids) and their degree of connectivity may have some influences on the acoustic velocities. In contrast with acoustic properties, the electrical conductivity data are not so clustered by facies indicating that transport properties are more influenced by microstructural heterogeneities. One single Archie's law can well account for all the data sets but these sets also show a large range of tortuosity factor related to the microporosity arrangement inside oolites. Some good predictive permeability modeling were found using mercury injection spectra. Our investigations reveal that, the best way for fluids to flow is to pass through the microporosity in the oolites, and that permeability depends mainly of the connectivity of the ooids, which is certainly related to the facies.

International audience; The Paris Basin seems a suitable location for CO2 capture and storage given both the amount of CO2 produced and the availability of depleted fields and deep saline aquifers. This study investigates the petrophysical properties in relation to the sedimentary and diagenetic environment of the Oolithe Blanche Formation (Middle Jurassic), a deep saline aquifer considered a potential candidate for CO2 storage. Because of the scarcity of core data in the Paris Basin, our investigation was based on the study of field analogues in the south-east part of the Paris Basin. The Oolithe Blanche Formation (Bathonian) is 70-80 m thick. Eighteen oriented blocks were collected from three outcrops selected for the different sedimentological facies identified in the formation. Their petrophysical properties were measured: porosity, pore size distribution derived from mercury injection tests, permeability and capillary imbibition parameters. The sedimentary and diagenetic facies were defined by using optical microscopy coupled with image analysis on thin sections and SEM. The Oolithe Blanche Formation is composed of oolitic and bioclastic limestones, deposited in a shallow marine setting. Three main sedimentological facies were defined: the oolitic shoal facies, the tide dominated facies and the prograding oolitic facies. They exhibit subtle variations in composition and in the distribution of carbonate textures. Porosity, measured using the water saturation triple weight method, ranges from 6% to 34%. Permeability values are low, between 0.1 mD and 9 mD. The data from mercury intrusion porosimetry show that the distribution of the pore throat diameter is either unimodal (microporosity only) or bimodal (macro- and microporosity). Microporosity is intraparticle (intraooid) and macroporosity is interparticle and is related to dolo-dedolomitization. Variations of petrophysical parameters can be explained by the microstructure. Now, these latter results calibrated by studying field analogues must be extended to borehole data and core data to optimize our knowledge of the deep saline aquifer of the Oolithe Blanche Formation. Without such investigations, it will be impossible to estimate the suitability of this deep saline aquifer for CO2 storage.

SUMMARY The Sheep Mountain anticline (Wyoming, USA) is a well-exposed asymmetric, basementcored anticline that formed during the Laramide orogeny in the early Tertiary. In order to unravel the history of strain during folding, we carried out combined anisotropy of magnetic susceptibility (AMS), anisotropy of P-wave velocity (APWV) and Fry strain analyses. The results are compared to previously published stress–strain data from calcite twins at the microscopic scale and from fracture sets at the mesoscopic scale, and are used to discuss the kinematics and mechanics of forced folding. The results obtained in sandstone and carbonate lithologies demonstrate a good agreement between (1) the principal axes of the AMS and APWV tensors, (2) stress–strain tensors derived from calcite twins, (3) Fry strain axes and mesoscopic fracture sets. Furthermore, these tensors are coaxial with the main structural trends of the anticline. The differences between AMS and APWV fabrics on one hand, and the differential stress values of the forelimb and the backlimb on the other hand, emphasize how the macroscopic asymmetry of Sheep Mountain anticline affects the strain pattern at the microscopic scale. The data set presented in this paper offers a consistent mechanical scenario for the development of Sheep Mountain anticline.

Theoretically, within a given pelitic rock in which the main paramagnetic carriers are white mica and chlorite and in which, judging from the AMS parameters, AMS is controlled by the paramagnetic carriers, the results of X-ray pole figure goniometry should show a qualitative relationship with AMS. This idea is tested on single-phase deformed, low-grade pelites of the Lower Palaeozoic Brabant Massif (Belgium), representing the southeastern part of the Anglo–Brabant Deformation Belt. For the vast majority of the samples investigated, the different LF-AMS parameters suggest a paramagnetic control on LF-AMS. Different parameters of LF-AMS measured at 77 K (low-T LF-AMS) and a comparison of these with the LF-AMS parameters indicate that LF-AMS is controlled virtually entirely by paramagnetic carriers. High-field torque magnetometry (HF-AMS) indicates further that AMS is controlled entirely by paramagnetic carriers for the vast majority of the samples. Two apparent discrepancies are observed, however. These are 1) a frequent mismatch between the LF-AMS fabric on the one hand and the (paramagnetic) fabric indicated by low-T LF-AMS and HF-AMS on the other hand, and 2) a rather common mismatch between the preferred orientation of chlorite and white mica suggested by X-ray pole figure goniometry on the one hand and the LF-AMS fabric, low-T LF-AMS fabric and HF-AMS paramagnetic fabric on the other hand. These apparent discrepancies may partly be attributed to the composite nature of both the LF-AMS fabric and the paramagnetic susceptibility fabric. The presence of different populations of paramagnetic carriers, possibly in combination with minor traces of ferromagnetic (s.l.) carriers which are not picked up by the methods applied, may explain some of the mismatches observed and are compatible with the basin evolution history of the Brabant Massif. In addition, it is possible that the low temperatures (low-T LF-AMS) and high fields (HF-AMS) are capable of enhancing one of the paramagnetic phases, thus leading to discrepancies between methods that define the mineral and the magnetic fabrics. The results of this study show that, even when the LF-AMS data suggest that the LF-AMS of a given pelitic rock in which phyllosilicates form the main paramagnetic carriers, is controlled entirely by paramagnetic carriers, alternative methods, such as phyllosilicate X-ray pole figure goniometry, will not necessarily reflect the paramagnetic carrier orientation. Although a number of cases have successfully correlated LF-AMS with mineral fabric and occasionally even strain, this study serves as an example for how methods to define mineral and magnetic fabrics can be sensitive in detecting different paramagnetic phases.

The Minervois basin recorded the Pyrenean compression in the Languedoc region, at the northeastern tip of the Pyrenees (France). It exhibits tectonic magnetic fabrics characterized by a complete loss of the sedimentary heritage. By contrast, the Potwar basin, which is located at the front of the southwestern Himalaya (Pakistan), shows magnetic fabrics with preservation of the sedimentary record. Two factors can explain these differences. The first is a tectonic factor and is related to the degree of coupling between cover and basement, which is notably higher in Minervois than in Potwar. The second and probably main factor is the degree of initial anisotropy which is low in Minervois, due to the importance of pedological processes erasing the sedimentary heritage and hardening rocks before burial in the basin. By contrast, the Potwar exhibits a more classical evolution where the sedimentary signature is related to compaction during burial.

The mechanical behavior of oolithic carbonate rocks was investigated for selected rocks with two different microstructural attributes: uniform (UP) and rimmed (RP) distribution of microporosity within ooids. These oolithic carbonate rocks are from the Oolithe Blanche formation, a deep saline aquifer in the Paris Basin, and a possible target for CO2 sequestration and geothermal production. Samples of similar physical properties (porosity, grain diameter, cement content) but different microporosity textures were deformed under triaxial configuration, in water saturated conditions, at 28 MPa of confining pressure, 5 MPa of pore pressure and at a temperature of 55 °C. During the experiments, acoustic velocities were monitored, and permeability was measured. The results show that the mechanical behavior of these microporous carbonates are strongly controlled by the microporosity distribution within the grains, at the origin of variations in elastic properties, mechanical strength and failure mode. The lower velocities measured in UP samples indicate a larger compliance of the whole structure. The mechanical response indicates that UP samples are characterized by a ductile behavior whereas RP samples display a brittle behavior. Using a conceptual model for the failure envelope of both rocks, our observations can be accounted for if one considers a significant variation of the critical pressure P ∗ , with UP samples having a lower P ∗ than RP samples. The permeability evolution under stress was interpreted using a revised Kozeny–Carman equation, showing that fluid flow is strongly affected by the tortuosity of the pore space, which is controlled by the microporosity distribution within the ooids. This study brings new insight into the parameters controlling the physical and mechanical response of oolithic carbonates, and the possible impact on production of geothermal energy at depth or storativity for CO2 sequestration operations.

International audience; 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.

Microstructural analysis of two undeformed sandstones, the Bentheim and Rothbach sandstones, supports the origin of the macroscopic anisotropies previously investigated (ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility). The shape and spatial distribution of both grains and voids were determined by computer image processing, measurement of magnetic anisotropy coupled with ferrofluids and stereological analysis. In addition, data on permeability measured in different directions are also provided for the two sandstones. The elastic models proposed in a previous paper are confirmed by the present study. In the Bentheim sandstone, the anisotropic behaviour was attributed to the non-spherical shape of the porosity, while in the Rothbach sandstone, an orientation-dependent cement radius at the intergranular contacts controls the observed anisotropy. The methods used for analysing the rock microstructure and its anisotropy are shown to be very useful and appropriate for detecting features such as pore or grain shape anisotropy, or the spatial distribution of grain contacts.

A duplex located at the lateral tip of a thrust fault is identified and this paper aims to understand why and how such a structure developed at this particular place. The “La Cagaliere” duplex is a well-exposed example of a second-order structure developed firstly as an “autochthonous roof duplex” at the front of the Lagrasse ramp-related fold (NE Pyrenees, France). During a second stage of deformation, an out-of-sequence thrusting allows the emplacement of the overturned forelimb of Lagrasse fold as an allochthonous roof above the duplex. Using geometric analysis, magnetic fabric and structural measurements, it is shown that the “La Cagaliere” duplex developed as a response to the progressive inhibition of the propagation of the Lagrasse fold frontal thrust. This study provides the first natural example of a duplex acting as a “stopping structure” at a thrust tip.

Heating produces changes, which does not always correspond to simple enhancement of the magnetic fabric. Two methods are proposed to determine the anisotropy of magnetic susceptibility of the ferrimagnetic minerals formed or that have disappeared by chemical change during successive heating. The first diagonalizes the tensor from the difference between each tensor term before and after heating. The second employs linear regression for each tensor term made with the values obtained throughout a thermal treatment. When the same magnetic fabric is obtained from several thermal steps, it cannot be related to randomly oriented ferrimagnetic minerals. Instead, the newly formed fabric must be related to characteristics of the pre-existing rock. By comparing this ferrimagnetic minerals fabric with the initial whole rock fabric, we can distinguish cases where heating simply enhances pre-existing fabric from those where thermal treatment induces a different fabric. Relative to the pre-heating fabric, this different fabric may simply be an inverse fabric or one whose principal susceptibility axes are oriented in a different direction, related to petrostructural elements other than those defining the initial fabric.

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 Corbières 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.

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.

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.

14 pages; International audience; he ferrofluid impregnation technique combined with anisotropy of magnetic susceptibility measurements (AMSff) is one of the ways to analyze the 3-D geometry of the pore space in a rock and indirectly to infer the anisotropy of permeability. We applied this method on different types of rocks (sandstones and carbonates) with a range of different porosity values (10-30%) and permeability (1 mD to 1 D). To get additional information on both the pore aspect ratio and the directional anisotropy we used another technique, measuring the anisotropy of P-waves velocity (APV) in dry and water saturated conditions. Comparing between both methods shows that despite the good agreement in directional data, inferring the true shape of the porosity is not straightforward. Modeling the presence of an elastic anisotropy in the solid matrix for sandstones allows one to get more consistent values for the pore aspect ratio obtained from both APV and AMSff. However for the carbonate rocks, due to an intricate distribution of microstructures, the aspect ratios obtained show significant discrepancies between the two methods. The ferrofluid method is very sensitive to the quality of the impregnation and suffers from a major drawback which is the threshold size of investigation, limited by the size of the magnetite nanoparticles (10 nm) and probably this method doesn't see all the porosity. On the other hand with acoustic methods, the range of porosity investigated is probably larger but several microstructural attributes can contribute to the elastic anisotropy which makes the pore shape effect more difficult to decipher. Therefore, we promote the combined use of both methods in order to get more reliable information on the pore shape in porous media.

SUMMARY Stress-induced remagnetization has been applied to multidomain pyrrhotite‐calcite synthetic aggregates in a triaxial rig. Experimental deformation used 150 MPa confining pressure, a constant strain rate of 10 x5 s x1 and applied differential stresses of up to 70 MPa. New components of magnetization, parallel to the direction of the pressure vessel field, were added to the pre-deformational magnetization. The intensity of remagnetization (MkxM0) increases with the intensity of the applied differential stress and affects the coercivity fraction below 15 mT. Bulk shortening is less than 8 per cent, thus grain rotation cannot explain selective remagnetization of the low-coercivity fraction. Remagnetization is thus attributed to deformational viscous remanent magnetization (DVRM). It is observed that high-coercivity (>15 mT) grains do not remagnetize. There is, however, slight progressive rotation of pre-deformational magnetization with increasing strain up to 8 per cent of bulk shortening. The lack of piezoremanent magnetization in the high-coercivity range may be due to defects introduced in pyrrhotite during sample preparation. Experiments using synthetic pyrrhotite, expected to show low dislocation densities, would be necessary to test this effect.

18 pages; International audience; The Oolithe Blanche Formation was studied in three quarries, located at the south-eastern edge of the Paris Basin (France). Heterogeneities in reservoir properties were assessed through a sedimentological, diagenetic and petrophysical study. The relationships between depositional settings, diagenesis and petrophysical properties were analysed using detailed petrographic studies, image analysis, Nano CTscans and petrophysical measurements. The carbonate reservoir pore network is mainly controlled by intraparticle microporosity which ensures the connectivity with interparticle meso- and macroporosity. Early cementation vs. early compaction processes (mainly grain interpenetration) may have considerable influence on fluid-flow properties and parameters such as permeability, acoustic velocities and tortuosity. Better reservoir properties are found when compaction processes begin before cementation. From statistical analyses, e.g. Principal Component Analysis and Linear Discriminant Analysis, a sedimentological/diagenetic and petrophysical model is proposed that is in a good agreement with the geological model developed from field work.

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.

The present study provides laboratory constrains on the in situ source of shear wave birefringence in siltstone and sandstone lithologies from the Taiwan Chelungpu Fault Drilling Project (TCDP). Bench top measurements at ambient pressure and temperature are performed across the end faces of vertically drilled samples while the polarization angle of the source and receiver is rotated from 0 to 360 degrees in azimuth. Shear wave birefringence in siltstone is low (similar to 3.5%) and controlled by low angle (similar to 30 degrees) bedding dip. The sandstone samples exhibit moderate (similar to 15%) shear wave anisotropy due to a network of vertical microcracks with normal about the bedding strike direction. As related studies infer that the microcracks responsible for shear wave birefringence in sandstone are also open at depth, our results suggest that anisotropy in borehole measurements can simply be accounted for by the observed siltstone and sandstone microstructures, i.e., without invoking larger scale objects, nor randomly oriented cracks/fractures subjected to triaxial stress state. (C) 2012 Elsevier Ltd. All rights reserved.

International audience; This study provides an original example of exploitation of Anisotropy of Magnetic Susceptibility (AMS) for rocks with weak magnetic susceptibility. Within the upper Weber Sandstone at Split Mountain, Utah, 430 cores from 31 sites were collected for magnetic characterization. The magnetic susceptibility ranges from -10 to 10μSI, indicating a mostly diamagnetic matrix, with degree of anisotropy up to 1.6. Specific treatment of magnetic susceptibility allows using diamagnetic data. The fabrics are fairly clustered and triaxial. Sedimentary magnetic fabrics show a foliation plane parallel to the lamina of the sand dunes, without defined lineation. Apart from sedimentary fabrics (70%). Magnetic fabric patterns averaged for sites distributed on the anticline are well defined in sub-groups related to the major structural domains of the anticline. The fracture network at Split Mountain is composed of a dominant N120 set and a secondary N035 set. A scenario of strain record is proposed based on the correlation of (1) fracture sets orientation, (2) diagenetic cementation, (3) paleostresses and (4) distribution of magnetic susceptibility anisotropy. Following the Sevier orogeny and N120 fracture set emplacement, the N035 fracture network and AMS signal were recorded during the Laramide Layer Parallel Shortening phase, with local deviation along pre-existing structures, and recorded a partitioning of the strain during early folding, with a maximum horizontal stress axis perpendicular to the fold bounding faults within the fold.

The description of folded/fractured reservoirs requires the understanding of mechanical behaviour of rocks during deformation. In folds, at the decametric scale, deformation is accommodated by flexural slip, faulting and formation of macroscopic fracture sets. The role of LPS has been recognized for years using magnetic fabric analysis which shows that in siliciclastic deposits the deformation is mainly accommodated by pretilting strain. This paper thus combines analyses of calcite twins, Anisotropy of Magnetic Susceptibility and Anisotropy of P-wave Velocity, together with petrological study to investigate the relationship between fold development and stress/strain. The results are compared to already available and newly collected mesoscale fracture and fault slip data. The Sheep Mountain anticline was chosen as a natural laboratory.Our results show a good agreement between calcite twinning and anisotropy of rocks properties, and with macroscopic fracturing on the other hand. Our study points toward a better description of deformation mechanisms of folded strata. A major result is the consistency of the record of deformation at microscopic and macroscopic scales, emphasizing that core scale data can be relevant to fold-scale structuring. This study yields important constraints for forthcoming modeling of stress distribution during fold development.

The anisotropy of magnetic susceptibility (AMS) and anisotropy of P-wave velocity (APV) were characterized in core samples retrieved from the TCDP Hole A at depths ranging from 600m to 1400m, thus cutting across the thrust fault that ruptured during the 1999 Chi-chi earthquake. Our data show that the AMS was similar for all the samples regardless of the lithologic contrasts, with a magnetic fabric characteristic of weakly deformed sedimentary rocks that had undergone layer parallel shortening. This robust observation applies to both the siltstones and sandstones at all depths investigated. In contrast the APV data reflect fundamental differences between the elastic fabrics in the two facies. While APV of the siltstone samples is controlled by a preferred orientation of the matrix minerals, the APV of the sandstones is greatly influenced by microcrack porosity with a preferred orientation parallel to the maximum principal direction of paleostress and tectonic deformation. These AMS and APV data obtained in TCDP Hole A at locations not directly impacted by the coseismic rupture provide a first order petrofabric framework for the characterization of hydromechanical anisotropy in the Chelungpu fault system.

International audience; Abstract The influence of temperature on magnetic and elastic properties of the Callovo-Oxfordian argillite formation, host of the French underground research laboratory for studying radioactive waste repository, has been investigated following two types of experiments. In the first experiment, rock samples were heated at 95 °C in an oven for 41 days, and in the second, heating–cooling cycles at increasing maximum temperatures were applied. Magnetic susceptibility and P-wave velocity measurements were conducted to investigate the effect of heating not only on the absolute values but also on the anisotropy. Whereas little variation was observed in the first experiment, the results of the heating–cooling cycles showed that different stages can be identified in the evolution of P-wave velocity and magnetic susceptibility with maximum temperature applied. At temperatures below 200 °C, the velocity increases sharply whereas the magnetic susceptibility remains unchanged: this is interpreted as being the result of mineralogical transformations, probably associated with dewatering of the clays. Above 200 °C, a large increase in susceptibility associated with a levelling off (or even a decreasing trend) of the velocity results from the competition between thermal cracking processes and another kind of mineralogical transformation affecting both properties. A magnetic mineralogical study showed that the main ferromagnetic mineral present is magnetite. An interesting result of the study is that in the temperature range investigated, the elastic properties are enhanced and the anisotropy is increased by raising the temperature, which has important consequences for the seismic monitoring of the storage site.

We investigated the anisotropic behaviour of the Callovo-Oxfordian argillite in laboratory experiments, and analysed the rock microstructure using different imaging techniques. The starting material was a core retrieved at 460 m depth in well EST204 at the site of the French Underground Research Laboratory in the Meuse/Haute-Marne region, from which 17 small subcores were prepared in three orthogonal directions. The anisotropy of magnetic susceptibility (AMS) was studied using a KLY3S kappabridge. The technique allows one to determine the susceptibility tensor which can be represented by an ellipsoid with principal axes K max , K int and K min in decreasing order. We found that the AMS ellipsoid is consistent with an oblate fabric. We have also investigated the elastic anisotropy through the measurement of P-wave velocity in different directions, following the procedure described in our previous works [Louis, L., David, C., Robion, P., 2003. Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions. Tectonophysics 370, 193–212; Louis, L., Robion, P., David, C., 2004. A single method for the inversion of anisotropic data sets with application to structural studies. Journal of Structural Geology 26, 2065–2072]. The anisotropy of P-wave velocity is significantly larger than the anisotropy of magnetic susceptibility. Interestingly, we found an excellent correlation between AMS and the anisotropy of P-wave velocity, with a minimum velocity V min and susceptibility K min fairly well constrained in the direction perpendicular to bedding. Both from the elastic and magnetic viewpoint, the argillite behave as a transverse isotropic medium, which is typical of a sedimentary fabric with no clear sign of tectonic fabric. Our data set on P- and S-wave velocity measurements allowed for the full determination of the transverse isotropy stiffness tensor as well as the Thomsen’s parameters within the framework of weak anisotropy. We also present some results of a study of the rock microstructure and its structural anisotropy using stereological techniques and X-ray microtomography. The microtomography technique revealed the existence of tubular structures made of dense material (pyrite), the origin of which is not clearly defined but has probably to do with bioturbation during the formation of the sediment.

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.

In this paper, anisotropies developed in silicoclastic continental deposits during the building of the Chaudrons anticline (Corbieres, France) are studied. A microstructural analysis of the deformational features in three different panels within the fold (crest, hinge, and forelimb, respectively) is reported and compared with early field observations (distribution and orientation of cleavage) and laboratory measurements (estimation of magnetic and acoustic anisotropies). The main finding of this investigation is the preservation of unwelded joints between grains of calcite promoted by the presence of quartz grains. These joints, which appear as discontinuities in a matrix of calcite, are analyzed in orientation and composition. In the three panels of the fold that are investigated, a range of dip angles is observed with at least two major generations of joints, the average orientation of which is found to be consistent with both macroscopic cleavage and magnetic and acoustic fabrics. To account for the multimodal distribution of the joints orientation, we suggest an original scenario in which they are successively generated by sets. Two processes have operated simultaneously during the development of the fold: (1) horizontal rock mass compaction inducing pressure solution and twinning in calcite; (2) preservation of unwelded calcite/calcite grain joints due to stress heterogeneities associated with quartz inclusions. From these results, we suggest that microstructural processes are the same before and during folding, ruling out a passive shearing of cleavage plane formed during a first step of layer parallel shortening.

The anisotropy of magnetic susceptibility (AMS) is often interpreted in terms of strain. However, since AMS is controlled by all magnetic (s.l.) carriers present, an AMS interpretation is not straightforward, especially in the presence of composite magnetic fabrics. Considering the large number of factors that may influence rock mineralogy (e.g. sediment source area, metamorphism), it becomes clear that one cannot interpret AMS in terms of strain without applying additional techniques that allow determining the nature and preferred orientation of all magnetic (s.l.) carriers likely influencing the measured AMS. This is discussed using samples from the Brabant Massif (Belgium) and the Moesian Platform (E-Romania).

In this paper, a single method for analysing magnetic susceptibility and P-wave velocity data is proposed, assuming that both measurement sets can be described by a second rank tensor, which is usually assumed for magnetic data only. For the velocity data, we estimate the discrepancy this hypothesis implies with respect to the theoretical P-wave velocity formulation for transverse isotropic media, with generalization for orthorhombic media. We find that the associated error in the determination of the principal values mostly lies below 1% when computed for published experimental data on sandstones and limestones. This result promotes the use of a unique and simple method for analysing anisotropy revealed by both physical properties, the advantage of which appears clearly in structural studies. We also check on our strategy for data sampling in order to get reliable outputs, and our statistical analysis shows that the measurement design used is suitable. The method is finally applied to a ramp-related fold structure in Corbieres (France): we emphasize that combining data sets for different physical properties and using a single inversion scheme leads to a better understanding of the deformation processes at the microstructural scale, which can be related to structural features and tectonic settings.

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.

Major oil discoveries in the foothills of the Venezuelan and Colombian Andes have recently focused the interest of exploration companies toward sub-Andean basins. Seismic, well, and core data from the El Furrial (Venezuela) and Cusiana (Colombia) productive fields have been integrated herein with other regional information to document the evolution of the thrust belt and the history of the petroleum systems, and to propose practical guidelines for prediction of sandstone reservoir quality in such a complex geodynamic environment. Although timing of deformation is slightly different in these areas of eastern Venezuela and Colombia, sedimentary and tectonic burial of the foreland autochthon in both regions led to the maturation of prolific Cretaceous marine source rocks, resulting in successive and diachronous hydrocarbon migration and trapping episodes. Early sedimentary burial at the current location of the Serrana del Interior (Venezuela) and the Eastern Cordillera (Colombia) resulted in long-range migration of early-generated hydrocarbons toward the foreland, forming the large accumulation of hydrocarbon along the Faja Petrolifera (Eastern Venezuela). Early entrapped hydrocarbons also have been preserved in pre-Andean prospects of the Andean foothills, as evidenced by the complex charge history of the Cusiana field. However, wide areas of source rocks in the Andean foothills and adjacent foreland reached the oil window only during the late Neogene and Pliocene-Quaternary, when maximum burial was attained. This produced a second migration episode, coeval with the growth of frontal anticlinal prospects. The main reservoir in Cusiana is fluvial sandstone of the Mirador Formation (Eocene); in El Furrial, it is sandstone of the Naricual-Merecure Formation (Oligocene). Pressure solution and quartz cementation decreased permeability of these sandstones. Results of studies of the anisotropy of the magnetic susceptibility (AMS), coupled with studies of fluid inclusions in quartz overgrowths and thermal modeling, demonstrate that sandstone reservoirs of these oil fields were compacted both vertically, by the load of the synflexural sequence, and horizontally, by tectonic stress (layer-parallel shortening) prior to being tectonically emplaced into the allochthon. Layer-parallel shortening by pressure solution is a major source of silica in the underthrust foreland. Venezuelan and Colombian sandstones still have reasonably good reservoir characteristics, although they have been buried to great depths. Overpressure that developed in these reservoirs as a result of rapid foredeep sedimentation probably caused a delay in compaction. Early carbonate cements also may have contributed locally to prevent compaction until secondary porosity developed as a result of dissolution of this early diagenetic phase. Finally, development of structural closures and hydrocarbon trapping has resulted progressively in the shutting down of the hydraulic system, preventing the transport of exotic silica by regional fluid flow.