Orientation Of Clay Minerals Research Articles

Resistivity anisotropy analysis of Longmaxi shale by resistivity measurements, scanning electron microscope, and resistivity simulation

The resistivity anisotropy of shale has a non-negligible effect on resistivity logs. Many studies have shown that the macroscale factors of shale anisotropy are depositional environment, compaction process, and diagenesis process, and the microscale factors mainly include lamination, orientation of clay minerals, and microfractures. However, the mechanism of laminations, pores, organic matters, pyrite and clay minerals driving resistivity anisotropy remains unclear. In this research, experiments and simulations were performed to investigate the characteristics and mechanism of the resistivity anisotropy of shale. Two sets of shale samples were collected at different inclination angles from the Lower Silurian Longmaxi Formation in the Pengshui area, southeastern Sichuan Basin, China. Core sample resistivities were measured in several water saturation states established using imbibition, pressurized saturation and centrifuge methods. Scanning electron microscopy, thin sections, X-ray diffraction, the mercury injection capillary method, and N2 adsorption were also performed. Resistivity simulations were applied to three-dimensional models derived from the microimages. The results of the experiments and simulations show that Longmaxi shale has strong resistivity anisotropy. The resistivity anisotropy coefficient (λ) of Longmaxi shale ranged from 1.3699 to 2.1623. Longmaxi shale has a strong lamination structure. In the submillimeter scale, strawberry-like pyrite (3.8%–10.2%, v/v) aggregates in elongated strips with an extension direction consistent with the horizontal direction. Due to the differential compaction, the clay minerals (21.1%–26.4%, v/v) also tend to be parallel to the formation bedding. The most obvious directional distribution of pores occured in the organic pores. Most interparticle pores between detrital minerals did not exhibit anisotropic characteristics. Because of the horizontal distribution of lamination, pyrite, clay minerals and organic pores, a higher resistivity is measured in the vertical direction than in the horizontal direction.

Effect of mechanical and electro-chemical contacts on the particle orientation of clay minerals during swelling and sedimentation: A DEM simulation

The importance and quality of particle orientation on the performance of clay has been well-documented experimentally. This paper aims to quantitatively evaluate clayey mineral reorientation during swelling/sedimentation according to the balance of mechanical contacts and electrochemical forces. A DEM code has been developed and the behavior of montmorillonite, kaolinite and illite, each having different pore fluid characteristics, has been simulated as for platy-shaped particles. Nonlinear contact law was considered for the induced net force as it relates to the mechanical contacts, Van der Waals attractive force, and diffuse double-layer (DDL) repulsive force. Good compatibility was observed between the DEM simulation and DDL theory. The results showed that the particle weight, magnitude of the repulsive force, and number of DDL contacts were effective parameters controlling particle reorientation. At a low swelling pressure, the particles rotated easily such that an edge-to-face fabric was created and the potential for overlap between particles as well as the mechanical contacts increased. The number of the mechanical contacts versus the time of sedimentation was not linear, but bell-shaped. Kaolinite ranked higher on the DDL repulsive contact list; however, it had a higher deposition rate due to its higher particle weight and lower repulsion force.

The role of pre-subduction sediment diagenesis in a shallow tsunami-generated slip, Sunda Trench, south of Sumatra

Abstract The progressive development of microfabrics in sediments was investigated in an active subduction zone using cores recovered during the Integrated Ocean Drilling Program Expedition 362. A sequence of Paleocene–Holocene rock was recorded at Site U1480, which is located in the Sunda Trench, south of Sumatra. Hemipelagic clayey sediments from the drilled succession were recovered from a depth of 1327.23 m. Cardhouse fabrics were observed initially, consisting of clay flakes with edge-to-edge (E–E) or edge-to-face (E–F) contacts. These initial microfabrics developed into compacted bookhouse fabrics (which are random and consolidated), and consist of clay flakes with E–F or face-to-face (F–F) contacts as the domain. These microfabrics developed due to individual particle sliding. Bookhouse fabric developed into a shape-preferred orientation of clay minerals with F–F contacts as a shaly fabric due to domain sliding. Finally, clay flakes with F–F contacts deformed into an undulated shape due to further increased overburden pressure. These pervasive fabrics comprising clay flakes stacked with F–F contacts resisted shear deformation due to their strong cohesion. This strong cohesion in the undulated shaly fabric might be an important factor in the locking of shallow tsunami-generating slip in subduction zones.

Fractal analysis and spatial disposition of porosity in major indian gas shales using low-pressure nitrogen adsorption and advanced image segmentation

Pore surface heterogeneities and their associated lacunarity of major Indian shale gas reservoirs were analyzed using both gas adsorption and advanced image analysis. The experiments were carried on samples from priority Indian gas shales namely Cambay, Cauvery, Krishna-Godavari and Damodar Valley. An advanced watershed segmentation was adopted on the SEM images to understand the control of pore size on the porosity of shale formations. Further, 3D as well as 2D surface intensity maps were plotted from the SEM images applying the distance transformation technique. The fractal dimension D ranged from 2.55 to 2.78 for LPN2GA suggesting a high degree of complexity in their fractal pattern. The 3D surface intensity map is found to be useful in picturizing the anisotropy in shale brought about by preferential orientation of clay minerals. The results can be extended in understanding the behavior of gas flow within the complex pore systems as exhibited by shale.

Computation of effective elastic properties of clay from X-ray texture goniometry data

We have developed a new method for estimating the contribution of a pure clay fraction (i.e., devoid of organic matter) to the total effective rock stiffness. The method is based on published clay mineral stiffness data and on an original preferred clay mineral orientation data set obtained by X-ray texture goniometry on 56 samples of Kimmeridgian and Devonian age from two North American shale plays. We find that (1) large variability in preferred orientation of clay results in moderate variability in effective clay elastic anisotropy and (2) the effect of variations in the preferred orientation on effective rock properties is small compared with the effects of variations in clay abundance. As a result, a single clay elastic tensor is computed to be used in effective medium models. In addition, to account for various degrees of hydration, water is incorporated into the dry clay tensor through inclusion models. In situations in which isotropic approximations are necessary, we also provide apparent bulk and shear moduli for a hydrated clay fraction as a function of porosity and propagation angle.

Anisotropy rock physics model for the Longmaxi shale gas reservoir, Sichuan Basin, China

The preferred orientation of clay minerals dominates the intrinsic anisotropy of shale. We introduce the clay lamination (CL) parameter to the Backus averaging method to describe the intrinsic shale anisotropy induced by the alignment of clay minerals. Then, we perform the inversion of CL and the Thomsen anisotropy parameters. The direct measurement of anisotropy is difficult because of the inability to measure the acoustic velocity in the vertical direction in boreholes and instrument limitations. By introducing the parameter CL, the inversion method provides reasonable estimates of the elastic anisotropy in the Longmaxi shale. The clay content is weakly correlated with the CL parameter. Moreover, the parameter CL is abnormally high at the bottom of the Longmaxi and Wufeng Formations, which are the target reservoirs. Finally, we construct rock physics templates to interpret well logging and reservoir properties.

Characterization of typical 3D pore networks of Jiulaodong formation shale using nano-transmission X-ray microscopy

The microscopic pore structure is one of the most important factors to understand shale gas reservoirs. Recognized as a non-destructive method, nano-transmission X-ray microscopy (Nano-TXM) is sufficiently powerful to resolve nanometer pore structures and to quantify the effective network in shale. In this work, three dimensional (3D) pore networks of typical pore structures, such as organic matter pores (OM pores), interparticle pores (InterP pores), intraplatelet pores within clay aggregates (IntraP pores) and intercrystalline pores within pyrite (InterC pores), developing in Jiulaodong (JLD) formation shale in the Weiyuan 201 well (W201) in Sichuan Basin were reconstructed by Nano-TXM. Meanwhile, the pore morphology, pore size, porosity and interconnectivity were analyzed using Pore Network Modeling (PNM). The results indicated that the pore shape, pore size distribution, porosity and interconnectivity varied between the four pore types. Nanometer pores ranging from 150nm to 1000nm dominate the OM pores in the samples. However, pores with a sheet-like structure that are larger than 1000nm are mainly found in InterP and IntraP pores. OM pores and InterP pores have larger porosities (35% and 23.7%, respectively) than the other two pore types. OM pores, InterP pores and InterC pores exhibit good and homogenous 3D connectivity, whereas IntraP pores have good extensity parallel to the clay mineral orientation but have no connectivity perpendicular to it. The 3D morphology and pore parameters suggest that the nano-pores in OM and InterC pores store absorbed gas and might be connected by micropores that existed in InterP and IntraP pores. The characterization of the pore structure in shale samples provides useful information for shale gas development.

Modeling anisotropic elasticity in an unconventional reservoir

During the last two decades, several studies have shown that organic-rich shales are characterized by strong velocity anisotropy. It is well known that these types of rocks exhibit strong polar anisotropy, also known as vertical transverse isotropy (VTI), with vertical (or bedding-perpendicular) velocities being much slower than horizontal (or bedding-parallel) velocities. However, the amount of elastic anisotropy can vary significantly from one formation to another because the individual clay minerals, the volume concentration and geometric distribution of organic matter, and other geologic characteristics (such as texture and nonclay mineralogy) vary by geologic formation. It is therefore important to quantify the amount of elastic anisotropy in our formations of interest, for various geophysical and geomechanical applications related to reservoir characterization. Strongly anisotropic compressional and shear velocities in two formations of interest in an unconventional reservoir are analyzed. Appropriate rock-physics models explain the anisotropic elastic response of these rocks. Recognizing that there might be several possible sources of anisotropy, three sources are focused on: mineral orientation, layering, and microcracks. Preferred orientation of clay minerals is the largest and most significant source of elastic anisotropy in the samples.

Carbon dioxide sealing capacity: Textural or compositional controls? A case study from the Oklahoma Panhandle

One of the challenges confronting carbon dioxide capture and sequestration (CCS) in geologic media over extended periods of time is determining the caprock sealing capacity. If the pressure of supercritical carbon dioxide injected in the repository overcomes the caprock sealing capacity, leaking of may enter other porous formations, compromising the storage formation, or even may go back to the atmosphere, and thus the process of sequestration becomes futile. Carbon dioxide sealing capacity is controlled by two groups of parameters: (1) texture (e.g., the pore-throat size, distribution, geometry, and sorting; median grain size, porosity, degree of bioturbation, specific surface area, preferred orientation of matrix clay minerals, orientation, and aspect of ratio of organic particles) and (2) composition (mineralogical content, proportion of soft, deformable mineral grains to rigid grains, organic matter content, carbonate content, silt content, cementation, ductility, compaction, and ash content). The primary goal of this study was to investigate several parameters listed above and to estimate their respective contributions to sealing capacity to better understand its role in shale and carbonates. To assess the effect of textural and compositional properties on maximum retention column height, we collected 30 representative core samples from caprock formations in three counties (Cimarron, Texas, and Beaver) in the Oklahoma Panhandle. The study area was chosen because it hosts three depleted gas fields with a storage capacity of more than 35 million bbl and is situated at a crossroad leading to some significant stationary sources from North Texas, South Kansas, and northern Oklahoma. We used mercury injection porosimetry, scanning electron microscopy (SEM), Sedigraph energy dispersive spectra (EDS), x-ray diffraction (XRD), Brunauer–Emmett–Teller-specific surface area, and total organic carbon (TOC) measurements to assess textural and compositional properties of collected samples. The range of column height for the samples used in this study is between 0.2 and 1358 m (0.66 and 4455 ft). The average column height is 351 m (1152 ft). The depth interval approximately 1400 m (4593 ft) could reach relatively high values of column height, up to 1200 m (3937 ft). The above-mentioned interval is composed of mainly Cherokee and Morrowan Formations (shale seals). Principal component analysis (PCA) was carried out to infer the possible relationships between textural and compositional parameters. Generally, composition of our samples (shales vs. carbonates and sandstones) indicates a relatively stronger control on caprock sealing capacity, although individual mineral makeup of shale samples seems not correlated with retention column heights. In the same time, many textural parameters play a significant role in determining the sealing capacity of carbonate caprocks.

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

Slickensided shear surfaces are ubiquitous in many fault zones. However the internal structure, the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity, grain-boundary sliding, microfracturing, solution-precipitation and mineral transformation under different conditions are subject of debate.We studied well- preserved core samples from the Main Fault, an up to 3 m wide zone of approximately 10 m offset in the Mont Terri Underground Research Laboratory (CH), a site to evaluate long-term safety of radioactive waste disposal. The drill core breaks easily along many slickensided shear surfaces indicating reverse slip, which form an anastomosing network connected by branch lines.Broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few μm thick shear zone, which acts as a crack guide for fracturing the samples. In this zone, a complex set of processes is inferred, leading to extreme localization of strain, development of strong particle preferred orientation, the formation of nanoparticles, and local precipitation of calcite veins in releasing sections. In lenses between shear zones, homogeneous gouge is formed with a well-developed oblique foliation and removal of calcite grains by pressure solution. We infer that with progressive deformation, the number and density of slickensided shear surfaces increases, generating tectonically derived scaly clay and more homogeneous gouge. In all deformed elements of the Main Fault, porosity is much smaller than in the undeformed Opalinus Clay. An interesting observation is the almost complete absence of cataclastic microstructures. Transmission electron microscopy (TEM) of focused ion beam lamellae of this micron-wide shear zone shows a strong preferred orientation of clay minerals, including nano-sized illite particles. In TEM, the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith.We propose that inter- and transgranular microcracking, pressure solution, clay neoformation, crystal plasticity and grain boundary sliding are important micro-scale processes during the early stages of faulting in Opalinus Clay and thus need to be considered in extrapolating laboratory results to long-term mechanical behavior.

Texture and diagenesis of Ordovician shale from the Canning Basin, Western Australia: Implications for elastic anisotropy and geomechanical properties

Microstructural and textural measurements from two Ordovician shale units (Goldwyer and Bongabinni formations) within the Palaeozoic–Mesozoic Canning Basin indicate that the former unit was affected by mechanical compaction and clay mineral transformation whereas the latter preserves an early fabric due to syn-depositional precipitation of authigenic dolomite and anhydrite. Conventional petrographic analysis coupled with quantitative mineralogy, electron micro probe analyses, X-ray Texture goniometry (XTG) and cathodoluminescence spectroscopy of core samples were used to decipher the post-depositional evolution of marine and supratidal facies in the Goldwyer and Bongabinni formations, respectively. Differences in diagenesis are strongly reflected in the orientation of clay minerals as quantified by XTG: in both cases the c-axes of illite diffract strongest normal to the bedding plane but the measurements clearly illustrate that shale in the Goldwyer Formation has a stronger preferred orientation relative to the Bongabinni Formation, with multiple of random distributions (m.r.d.) values of 5.77 and 2.54, respectively.Laboratory measurements conducted at 10 MPa effective stress also indicate distinct rock physics signatures: the Bongabinni Formation shows very low anisotropy, whereas the Goldwyer Formation displays a higher degree of elastic anisotropy in terms of both P- and S- waves. The crystallographic preferred orientation of illite, highlighted by the XTG, is likely to contribute to the significant difference in elastic anisotropy observed in the two units. Therefore, the Bongabinni Formation is mechanically stronger and stiffer than the Goldwyer Formation, likely due to the early dolomite and anhydrite cementation of the former providing a rigid microstructure framework.

Mineral Preferred Orientation and Microstructure in the Posidonia Shale in Relation to Different Degrees of Thermal Maturity

AbstractThe thermal maturity of samples of the Posidonia Shale collected from the Hils Syncline, northern Germany, varies significantly as a function of location indicating variations in local history. Synchrotron X-ray diffraction was used to document the composition and the preferred orientation of four samples of the Posidonia Shale with different degrees of maturity (0.68–1.45%, Ro) to determine possible effects on diagenesis and preferred orientation. Overall, the degree of preferred orientation of all clay minerals (illite-smectite, illite-mica, and kaolinite) and in all samples is similar, with (001) pole figure maxima ranging from 3.7 to 6.3 multiples of a random distribution (m.r.d.). Calcite displays weak preferred orientation, with c axes perpendicular to the bedding plane (1.1–1.3 m.r.d.). Other constituent phases such as quartz, feldspars, and pyrite have a random orientation distribution. The difference in thermal history, which causes significant changes in the maturity of organic matter, influenced the preferred orientation of clay minerals only marginally as most of the alignment seems to have evolved early in their history. Synchrotron X-ray microtomography was used to characterize the three-dimensional microstructure of a high-maturity sample. Low-density features, including porosity, fractures, and kerogen, were observed to be elongated and aligned roughly parallel to the bedding plane. The volume of low-density features was estimated to be ~7 vol.%, consistent with previous petrophysical measurements of porosity of 8–10 vol.%. Transmission electron microscopy analysis of samples with different degrees of maturity (0.74%Ro and 1.45%Ro) was used to document microstructures at the nanoscale as well as the presence of kerogen. In the high-maturity sample, pores were less abundant while minerals were more deformed as shown by fractured calcite and by kinked and folded illite. Some of the porosity was aligned with clay platelets.

Origin and behavior of clay minerals in the Bogd fault gouge, Mongolia

We analyzed twelve fault gouge samples from the Bogd fault in south-western Mongolia to understand the origin and behavior of clay minerals. The investigation relies on x-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high energy synchrotron x-ray diffraction methods to investigate microstructure and preferred orientation. Smectite (montmorillonite), illite-smectite mixed layers, illite-mica and kaolinite are the major clay components, in addition to quartz and feldspars, which are present in all samples. The observations suggest that the protoliths and the fault rocks were highly altered by fluids. The fluid-rock interactions allow clay minerals to form, as well as alter feldspars to precipitate kaolinite and montmorillonite. Thus, newly formed clay minerals are heterogeneously distributed in the fault zone. The decrease of montmorillonite component of some of the highly deformed samples also suggests that dehydration processes during deformation were leading to illite precipitation. Based on synchrotron x-ray diffraction data, the degree of preferred orientation of constituent clay minerals is weak, with maxima for (001) ranging from 1.3 to 2.6 multiples of a random distribution (m.r.d). Co-existing quartz and feldspars have random orientation distributions. Microstructure and texture observations of the gouges from the foliated microscopic zone, alternating with micrometric isotropic clay-rich area, also indicate that the Bogd fault experienced brittle and ductile deformation episodes. The clay minerals may contribute to a slip weakening behavior of the fault.

Preferred orientation of phyllosilicates: Effects of composition and stress on resedimented mudstone microfabrics

Development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material (Boston Blue Clay). Boston Blue Clay (BBC), an illitic glacio-marine clay composed of 57% clay-sized particles (<2 μm) was resedimented and step-loaded to a vertical effective stress of 0.1 MPa, 1 MPa, 6 MPa, and 10 MPa. These four samples show a small increase in preferred orientation of mica and chlorite with increasing vertical effective stress. Furthermore, pure BBC powder was admixed with silica (silt-sized quartz) in five different ratios of BBC:silica to form sediments with 57%, 52%, 48%, 44%, and 36% < 2 μm particles and were loaded to a maximum vertical effective stress of 2.4 MPa. Mica preferred orientation decreases significantly with decreasing clay content at one stress state. We used and compared three different techniques to characterize and quantify the preferred orientation of phyllosilicates: transmission-mode X-ray texture goniometry, reflection-mode X-ray texture goniometry, and grain and pore networks imaged using secondary and backscattered electron micrographs gathered on argon-ion-milled surfaces. Preferred orientation of the clay minerals shows good agreement between transmission-mode X-ray texture goniometry and reflection-mode X-ray texture goniometry. Mercury porosimetry further illuminates vertical effective stress and compositional controls on microfabric.

Preferred orientation of phyllosilicates: Comparison of fault gouge, shale and schist

Samples of fault gouge from the San Andreas Fault drill hole (SAFOD), a shale from the North Sea sedimentary basin and schists from metamorphic rocks in the Alps have been analyzed with high energy synchrotron X-rays to determine preferred orientation of mica and clay minerals. The method relies on obtaining 2D diffraction images which are then processed with the crystallographic Rietveld method, implemented in the software MAUD, allowing for deconvolution of phases and extraction of their orientation distributions. It is possible to distinguish between detrital illite/muscovite and authigenic illite/smectite, kaolinite and chlorite, and muscovite and biotite, with strongly overlapping peaks in the diffraction pattern. The results demonstrate that phyllosilicates show large texture variations in various environments, where different mechanisms produce the rock microfabrics: fault gouge fabrics are quite weak and asymmetric with maxima for (001) in the range of 1.5–2.5 multiples of random distribution (m.r.d.). This is attributed to heterogeneous deformation with randomization, as well as dissolution–precipitation reactions. Shale fabrics have maxima ranging from 3 to 9 m.r.d. and this is due to sedimentation and compaction. The strongest fabrics were observed in metamorphic schists (10–14 m.r.d.) and developed by deformation as well as recrystallization in a stress field. In the analyzed samples, fabrics of co-existing quartz are weak. All phyllosilicate textures can be explained by orientation of (001) platelets, with no additional constraints on a-axes.

Variation of Preferred Orientation in Oriented Clay Mounts as a Result of Sample Preparation and Composition

AbstractIn X-ray diffraction (XRD) analysis, preparation of oriented clay specimens enhances their 00l reflections by arranging basal surfaces parallel to the specimen surface. In one-dimensional modeling of XRD intensities, degree of preferred orientation is one of the variable parameters and a user may choose different σ* values for different minerals. The usual assumption is, however, that the layers of all clay minerals that are present exhibit a similar degree of preferred orientation to that of the clay mineral flakes parallel to the basal plane. If the orientation of individual clay minerals is significantly different, and if this is not taken into account, the relative proportions of the constituent minerals cannot be modeled accurately. The actual or so-called ‘preferred’ orientation is a potentially large source of error in any attempt at quantitative XRD analysis because it cannot be assumed to be constant among different minerals and may also vary as a result of pretreatment. In the present study the influence of sample composition and sample pretreatment on the degree of preferred orientation was determined using the parameter σ*. A statistical parameter was calculated to determine and ensure the reproducibility of σ* measurements. The most important result was that, when mixed together, clay minerals influence each other in terms of the degree of preferred orientation. Among individual samples, the degree of preferred orientation can be different for each clay mineral. The power of sonication used in sample pretreatment of a pure kaolinite and a pure illite had no significant influence on the degree of preferred orientation. The changes in intensities upon variation of the tilting angle (χ) allowed for calculation of σ* of smectites in pure samples, in admixtures, and in samples treated in two different ways (air-dried and glycerol-intercalated), which is reported here for the first time. Smectites are very fine grained with flexible morphology which is believed to be the reason for their tendency to exhibit poor orientation (σ* = 22°); further research is required to establish whether this is a general feature of smectites. After glycerol treatment a soil smectite showed a slightly better orientation compared to the air-dried pattern. The results of the study illustrate the difficulty of predicting changes in preferred orientation of clay mineral admixtures, even if non-platy minerals such as clay-sized quartz are added. In general, σ* decreased when non-platy minerals were added, which is explained by changes in geometry of the specimen. Not all clay minerals, however, showed simultaneous changes in their orientation behavior.

Microfabric and rock properties of experimentally compressed silt-clay mixtures

Oedometric mechanical compaction tests were performed on brine-saturated synthetic samples consisting of silt-clay mixtures to study changes in microfabric and rock properties as a function of effective stress. The silt consisted of crushed quartz (∼100%) with grain size range between 4 and 40 μm, whereas the clay consisted of 81% kaolinite, 14% mica/illite and 5% microcline of grain size between 0.4 and 30 μm. Five sample pairs ranging in composition from pure silt to pure clay were compacted to 5 and 50 MPa effective stress respectively. SEM studies were carried out to investigate microfabric changes in the mechanically compacted silt-clay mixtures. The degree of alignment of the different minerals present (quartz, mica/illite and kaolinite) were computed by using an image analysis software. Experimental compaction have measured the changes in the rock properties such as porosity and velocity as a function of effective stress for different mixtures of clay and silt. Clay-rich samples showed a higher degree of mineral orientation and lower porosity compared to silt-dominated samples as a function of effective stress. Pure clay sample had 11% porosity at 50 MPa effective stress whereas the pure silt sample retained about 29% porosity at the same effective stress. The experiments showed that low porosity down to 11% is possible by mechanical compaction only. A systematic increase in strain was observed in the silt-clay mixtures with increasing clay content but the porosity values found for the 50:50 silt-clay mixture were lower than that of 25:75 silt-clay mixture. No preferential mineral orientation is expected before compaction owing to the high initial porosity suggesting that the final fabric is a direct result of the effective stress. Both P- and S-wave velocities increased in all silt-clay mixtures with increasing effective stress. The maximum P- and S-wave velocities were observed in the 25:75 silt-clay mixture whereas the minimum Vp and Vs were recorded in the pure silt mixture. At 50 MPa effective stress P- wave velocities as high as 3 km/s resulted from experimental mechanical compaction alone. The results show that fine-grained sediment porosity and velocity are dependent on microfabric, which in turn is a function of grain size distribution, particle shape, sediment composition and stress. At 5 MPa effective stress, quartz orientation increased as a function of the amount of clay indicating that clay facilitate rotation of angular quartz grains. Adding clay from 25% to 75% in the silt-clay mixtures at 50 MPa effective stress decreased the quartz alignment. The clay mineral orientation increased by increasing both the amount of clay and the effective stress, the mica/illite fabric alignment being systematically higher than that of kaolinite. Even small amount of silt (25%) added to pure clay reduced the degree of clay alignment significantly. This study demonstrates that experimental compaction of well characterized synthetic mudstones can be a useful tool to understand microfabric and rock properties of shallow natural mudstones where mechanical compaction is the dominant process.

Anisotropy of experimentally compressed kaolinite-illite-quartz mixtures

The anisotropy of physical properties is a well-known characteristic of many clay-bearing rocks. This anisotropy has important implications for elastic properties of rocks and must be considered in seismic modeling. Preferred orientation of clay minerals is an important factor causing anisotropy in clay-bearing rocks such as shales and mudstones that are the main cap rocks of oil reservoirs. The preferred orientation of clays depends mostly on the amount of clays and the degree of compaction. To study the effect of these parameters, we prepared several samples compressing (at two effective vertical stresses) a mixture of clays (illite and kaolinite) and quartz (silt) with different clay/quartz ratios. The preferred orientation of the phases was quantified with Rietveld analysis on synchrotron hard X-ray images. Pole figures for kaolinite and illite display a preferred orientationof clay platelets perpendicular to the compaction direction, increasing in strength with clay content and compaction pressure. Quartz particles have a random orientation distribution. Aggregate elastic properties can be estimated by averaging the single-crystal properties over the orientation distribution obtained from the diffraction data analysis. Calculated P-wave velocity anisotropy ranges from 0% (pure quartz sample) to 44% (pure clay sample, highly compacted), but calculated velocities are much higher than measured velocities. This is attributed to uncertainties about single-crystal elastic properties and oriented micropores and limited grain contacts that are not accounted for in the model. In this work, we present an effective method to obtain quantitative data, helping to evaluate the role of clay percentage and compaction pressure on the anisotropy of elastic properties of clay-bearing rocks.

Large-scale landslides strongly governed by the Himekawa melange

The Himekawa melange is composed of serpentine sheets overthrusted into Mesozoic-Paleozoic rock. This melange has been classified as geotectonic and is associated to a serpentine matrix. The O'dokoro landslide is found within this geological setting. The slip surface of this landslide was formed by transformation of the existing deep seated overthrust fault at a depth of about 60 to 100 m from ground surface. The slickensided slip zone surface is formed by a mixture of the original fault-clay gouge and the clay resulting from transformation along the fractured zone. Fault-clay material is dominating and showed a low residual strength in the presence of high effective normal stress, at preferred clay mineral orientation. Fractured zone-clay in turn, exhibited high residual strength. The presence of preferred-oriented clay minerals originated from nonserpentinized rocks is considered indispensable to the occurrence of large-scale landslides.

Dilatant clayey microstructure in the Barbados décollement zone

Dilatant clayey microstructures in cores from ODP Leg 156, which drilled the basal décollement zone of the northern Barbados accretionary prism, are obtained from freeze-dried samples. The microstructure in the scaly fabric portion shows heterogeneous strain characterized by domains with strong preferred orientation of clay minerals and with sigmoidal, spindle, or tube-shaped coarse pores in fabric domains with random particle orientation. These dilatant microstructures have not been observed in clayey sediments from the Barbados décollement zone or other active shear zones. The dilatancy of the microstructure might be formed under the condition of high pore-fluid pressure in the course of shearing clays and due to tectonic stress.