T43E-01
Mechanics of Clay Smear Formation Along Faults
Clay- or shale-rich smears developed along faults in interlayered clay and sand sediments are known to significantly change the hydraulic properties of the formation. Predicting the volume of clay or shale smeared along faults is therefore important for estimating the fluid connectivity in groundwater and hydrocarbon systems as well for the integrity of artificially sequestered CO2 reservoirs. Here we show how fault smears develop spontaneously in deforming layered systems, where the interlayered clay and sand have varying friction coefficients, and we present a quantitative dynamic model for this behavior. The model is based on Mohr-Coulomb failure theory, and with discrete element computations we demonstrate, how the model framework can be used for predicting the fault smear potential from soil friction angles and layer thicknesses. The modeled results show how friction angle variations not only control the amount of clay available for smearing, but also the along-bed offsetting of the fault planes - a phenomenon often recognized in outcrops. The modeled results are compared to observations of both the external and internal geometry of naturally formed clay smears in order to prove the validity of the dynamic model.
T43E-02
Two Types of Compaction Bands in Aztec Sandstone at Valley of Fire, NV
Deformation in porous and granular media tends to localize along planar bands of finite width, referred to as deformation bands. Based on their dominant relative displacement they have been subdivided into shear, compaction, and dilation bands, with compaction bands reported from laboratory experiments and two field sites. At one of these sites, Valley of Fire, NV, two types of compaction bands are observed, with distinct orientations and structural attributes: Pure compaction bands (PCBs) are composed of single strands that are 5-10 mm thick, are rarely planar and more commonly wavy or zigzag. PCBs are inferred to form perpendicular to the maximum compressive principal stress. Shear-enhanced compaction bands (SCBs) are planar, often composed of multiple strands, and form conjugate sets that are bisected by PCBs. SCBs are oriented at 32-53° relative to the inferred direction of the maximum compressive principal stress. Based on field observations and textural image analysis, SCBs are characterized by approximately equal magnitudes of band-perpendicular shortening (compaction) and shear, with shear not exceeding 2 mm. Shear-enhanced compaction bands are distinguished from shear bands with macroscopic shear in excess of 1-2 mm. Within the cross-bedded eolian sandstone, pure compaction bands, shear-enhanced compaction bands, and shear bands occur in two distinct styles within different parts of dune sets: Foresets contain one set of PCBs at high angle to bedding and two sets of SCBs in a strike-slip configuration, in addition to bedding-parallel SCBs. Bottom sets typically contain no PCBs, but two sets of SCBs in a reverse-slip configuration and bedding-parallel shear bands. Both styles can be shown to have formed concurrently. The absence of PCBs in bottom sets is attributed to slip along bedding-parallel shear bands in more clay-rich bottom sets. These observations suggest that the occurrence of PCBs is limited to loading conditions that approach uniaxial stress state. Zigzagging of PCBs is interpreted to result from a local increase in triaxiality ahead of a propagating PCB. Regional structural relations indicate that the compaction bands formed at estimated 300 m of burial in the vicinity of an approaching thrust sheet suggesting high deviatoric stresses, high porosity, and low induration at the time of deformation.
T43E-03 INVITED
Fragmentation and Localization Processes in 3D Simulations of Sheared Granular Systems
To better understand deformation processes relevant for sediments and sedimentary rocks, we investigate the grain scale processes operating in evolving granular systems under shear. Structural fabrics, such as deformation bands, are common in nature however the micro-scale processes responsible for their development are generally difficult to directly observe. Discrete numerical models of granular systems allow excellent visualization of grain scale interactions as well as tracking of macroscopic mechanical response. When combined with laboratory validation experiments and field observations, they become a powerful tool for investigating the dynamics of fault evolution. We present recent results from a new method that implements realistic grain evolution in 3D simulations of granular shear. The particle based model includes breakable bonds between individual particles allowing fracture of aggregate grains that are composed of many bonded particles. During simulations, particle motions and interactions as well as the mechanical behavior of the entire system are continuously monitored. We show that a model fault gouge initially characterized by mono-disperse spherical aggregate grains gradually evolves, with accumulated strain, to a wide size distribution composed of spherical and angular fragments. The comminution process yields a textural signature that is quantitatively comparable to natural and laboratory produced fault gouges. Mechanical behavior is comparable to a first order with relevant laboratory data. Simulations also reveal a strong correlation between regions of enhanced grain size reduction and localized strain. Thus in addition to producing realistic fault gouge textures, the model offers the possibility to explore direct links between strain partitioning and structural development in fault zones. This approach allows testing of fragmentation models and can reveal the sensitivity of different fragmentation processes to loading conditions, grain configurations and accumulated strain.
T43E-04
Deformations of Sediments via Grain-Scale Simulations: A Quasi Static Approach
Deformation of granular materials such as sediments is highly nonlinear. Consequently, the macroscopic elastic moduli vary with deformation. We quantify these moduli by simulating numerically the deformations of a random disordered pack of spherical grains. In particular, we seek to quantify the impact of hydrate dissociation on the moduli of hydrate-bearing sediments. Our model is discrete, accounting for the interactions between individual grains. For each contact, we calculate the loads using the contact models of Hertz, Mindlin and Deresiewicz. We use a quasi-static model, where deformation is a sequence of static equilibrium configurations of the grains. We find these equilibrium configurations by minimizing a functional which is related to the total mechanical work in the pack. Minimization is obtained using an algorithm based on the conjugate gradient method. Starting with a loose configuration, we simulate a number of loading and unloading cycles. As contact forces develop, we calculate the macroscopic stresses and find the corresponding elastic moduli. The calculated bulk modulus matches published experimental data. We emphasize that this result requires no adjustment of material parameters. The bulk modulus does not vary substantially with different contact models. In contrast, the macroscopic shear modulus is sensitive to the inter-granular microscopic shear. Our simulations allow for analysis of microscopic features such as force chains and contact force statistics, which are extremely difficult to obtain in an experiment. Our model reveals the microscopic mechanisms that lead to the nonlinear, path- dependent stress-strain relations observed in experiments.
T43E-05
Experimental Investigation of the Dissolution of Quartz by Muscovite Mica Surfaces: Implications for Pressure Solution
Using the Surface Force Apparatus, which gives angstrom resolution of thin film and water layer thicknesses, we have measured dissolution of quartz sheets when muscovite mica surfaces are pressed against them in aqueous electrolyte solutions of CaCl2 (30 mM) at relatively low pressures (2-3 atm) and temperatures (25 °C). No detectable dissolution occurs in symmetrical systems (e.g. mica-mica or quartz-quartz) or in dry cases under similar pressures, indicating that the dissolution phenomena can not be attributed to simple pressure effects, slow aging effects, or plastic deformation of the quartz surface. In quartz-mica systems under pressure and in electrolyte solution, the onset of quartz dissolution is marked by a sudden, rapid, and spontaneous change in the quartz thickness. Upon the onset of quartz dissolution, we measure initial dissolution rates that range from 1 to 4 nm/min that gradually settle after several hours into a constant rate that is approximately 0.01 nm/min. The change in decay rate is interpreted as the gradual transformation of the quartz surface from an ordered crystalline lattice into a more amorphous and porous structure as material is removed by the dissolution process in a manner analogous to corrosion. We believe that the pressure solution is an electro-chemical phenomena driven by the electrostatic potential generated when two surfaces possessing dissimilar surface charge potentials are forced into close proximity in the presence of an electrolyte. We have measured up to 150 mV between quartz mica surfaces at the above conditions, confirming the presence of a electrical potential. Recent experiments examining dissolution in multi-faceted milled quartz particles (~ 1.0 μm diameter) compressed between two muscovite surfaces reveal a dissolution driven evolution of the particle geometry that suggests an asymmetry in the dissolution rates at different crystallographic planes. Dissimilarities in the surface charge potentials of different crystallographic planes and resultant differences in dissolution could give rise to the irregular patterns characteristic of stylolites in micaceous sandstones.
T43E-06
Experimental study of the effect of mica on pressure solution of single crystal calcite
Field and experimental studies suggest that clays and micas accelerate the rate of pressure solution in various geomaterials. It is not clear however whether the "clay effect" is purely mechanical (i.e., maintaining a thick conduit for fluids at the contact) or whether its surface chemistry plays a critical role. A case in point is the insoluble clay filling of stylolites, which are thought by some to be merely an inert byproduct of dissolution, or by others to be a necessary feature for the propagation of the seam. To study the effect of mica on carbonate pressure solution, the corner of a cleaved calcite single crystal rhomb was polished into a triangular face (edge length ~ 200micron) and pressed against either muscovite or quartz discs to yield a nominal stress of 10-20MPa. Immersing the contact in pre-saturated (with respect to microcrystalline calcite) solutions of distilled water or 0.25M NH4Cl caused axial shortening of the crystal. This axial strain was measured with a capacitance sensor (<0.5nm/h resolution) while the contact morphology was imaged in situ with a confocal microscope (3micron spatial resolution). In pre-saturated water solution the axial shortening of calcite loaded against muscovite is ~1nm/h, and no significant changes in contact morphology are detected. In pre-saturated NH4Cl solution however, both calcite- quartz and calcite-muscovite contacts evolve in two stages: the first stage is characterized by low axial strain rates (<5nm/h) during which the original contact area inside the triangle (as determined by interference fringes) shrinks and its perimeter roughens. The second stage is distinguished by high axial strain rates (~40nm/h) and changes in the size and spatial position of isolated contacts (diameter< 10 microns) in a dynamic channel-island morphology covering the entire triangular region. Post-experiment SEM analysis suggests dissolution in this region and precipitation on the free faces adjacent to it. At this point we see no significant difference between the calcite quartz and calcite muscovite experiments under similar load conditions.
T43E-07
Scaling of Natural Stylolites and Their use as Stress-Depth Gauges
Stylolites are rough dissolution pairs of surfaces that are generated by stress induced solution in sedimentary rocks. These irregular structures that can be found in various rock types are filled with fine-grained insoluble material. Stylolites have been shown to be self-affine fractal surfaces that have characteristic statistical properties (Schmittbuhl et al., 2004), which allow the characterisation of the roughness in a quantitative way, namely by the Hurst-exponent. With this approach it can be demonstrated that stylolites exhibit two scaling regimes with different roughness exponents, separated by a well-defined crossover-length (L) between small and large scales, respectively. According to the analytical solution of Schmittbuhl et al. these regions correspond to two thermodynamic regimes that are either dominated by surface or elastic energies, L being a function of the state of stress during stylolite formation. The aim of this contribution is to test the analytically predicted scaling of stylolites on a natural system of bedding-parallel stylolites. We show a correlation between L and the stress during stylolite formation and thus unravel the depth of stylolitization. For this, we sampled bedding-parallel stylolites from the Cirque de Navacelles (southern France), where a 300m succession of flat-lying Jurassic limestones crops out. The collected samples where cut normal to the mean stylolitic surface and the 1D stylolite profiles were digitized using a camera and image analysis techniques. For every sample, the roughness exponent and the corresponding crossover-length L of the 1D rough interface was calculated. To compare the results with the analytical solution, we calculated the elastic properties of typical samples from P- and S-wave velocities. Our results show that L decreases from 2 mm at the top of the profile to 0.7mm at the base. Plotting depth as a linear function of L allows the comparison of the slopes of the analytical solution and the natural samples. The present investigation reveals a validation for the analytical solution given by Schmittbuhl et al. We further conclude that our results utilise the construction of a robust and potentially very powerful new use of bedding- parallel stylolites as stress/depth gauges. Schmittbuhl, J., Renard, F., Gratier, J.-P., Toussaint, R., 2004. The roughness of stylolites: Implications of 3D high resolution topography measurements, Phys. Rev. Lett., 93, 238501.
T43E-08
Are stylolitic surfaces inherently unstable surfaces? Insights from shape minimization considerations
Non-planar solid-fluid-solid interfaces under stress are very common in many materials, and particularly in the rocks of the Earth's crust. Such patterns are observed in many rocks in a wide range of spatial scales, from undulate grain boundaries at the micrometer scale, to stylolite dissolution interface at the meter scale. It is proposed here that these initially flat rock-fluid interfaces become rough by a morphological instability triggered by elastic stress. A model for the formation of stylolitic patterns at all scales is thus presented. It is shown that such instability is inherently present due to the uniaxial stress that promotes them, owing to the gain in the total elastic energy: the intrinsic elastic energy plus the work of the external forces. This is shown explicitly by solving the elastic problem in a linear stability analysis, and proved more generally without having resort to the computation of the elastic field.