Auke Barnhoorn

Dissolution-reprecipitation of zircon at low-temperature, high-pressure conditions (Lanzo Massif, Italy)

Abstract An eclogite facies meta-plagiogranite from the Lanzo massif (western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2° misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light- to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 ± 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite.

Stress- and fluid-driven failure during fracture array growth: Implications for coupled deformation and fluid flow in the crust

Brittle experimental deformation on dolomite rocks shows for the fi rst time the differ- ence in growth of fracture networks by ordinary percolation and invasion percolation processes. Stress-driven fracture growth, in the absence of fl uid pressure, is an ordinary percolation process characterized by distributed nucleation and growth of microfractures, which coalesce with increasing strain to form a connected fracture network. Fluid pres- sure-driven fracture growth is more akin to an invasion percolation process characterized by preferential fracture growth occurring initially at the high fl uid pressure part of the rock. With progressive deformation, the network propagates rapidly through the sample and away from the high fl uid pressure reservoir. X-ray microtomography analysis suggests that the fracture network in three dimensions (3-D) is probably a fully connected network at peak stress conditions, whereas conventional 2-D analysis suggests that connectivity only occurs at shear failure. The development of 3-D fracture connectivity prior to shear failure has important implications for fl uid fl ow and fl uid pressure changes immediately prior to rupture nucleation in active fault zones, for fl uid migration in ore-producing hydrothermal systems, and for reservoir integrity in hydrocarbon systems.

Grain size-sensitive viscoelastic relaxation and seismic properties of polycrystalline MgO

Torsional forced-oscillation experiments on a suite of synthetic MgO polycrystals, of high-purity and average grain sizes of 1–100 µm, reveal strongly viscoelastic behavior at temperatures of 800–1300°C and periods between 1 and 1000 s. The measured shear modulus and associated strain energy dissipation both display monotonic variations with oscillation period, temperature, and grain size. The data for the specimens of intermediate grain size have been fitted to a generalized Burgers creep function model that is also broadly consistent with the results for the most coarse-grained specimen. The mild grain size sensitivity for the relaxation time τL, defining the lower end of the anelastic absorption band, is consistent with the onset of elastically accommodated grain boundary sliding. The upper end of the anelastic absorption band, evident in the highest-temperature data for one specimen only, is associated with the Maxwell relaxation time τM marking the transition toward viscous behavior, conventionally ascribed a stronger grain size sensitivity. Similarly pronounced viscoelastic behavior was observed in complementary torsional microcreep tests, which confirm that the nonelastic strains are mainly recoverable, i.e., anelastic. With an estimated activation volume for the viscoelastic relaxation, the experimentally constrained Burgers model has been extrapolated to the conditions of pressure and temperature prevailing in the Earths uppermost lower mantle. For a plausible grain size of 10 mm, the predicted dissipation Q−1 ranges from ~10−3 to ~10−2 for periods of 3–3000 s. Broad consistency with seismological observations suggests that the lower mantle ferropericlase phase might account for much of its observed attenuation.

Microstructural Investigation of the Whitby Mudstone (UK) As an Analog for Posidonia Shale (NL)

As part of a study to investigate methods to enhance pore/crack connectivity between the shale matrix and the induced fractures, we have investigated the matrix microstructure of an exposed analogue of the Jurassic Posidonia shales in the Dutch sub-surface. A combination of Precision-Ion-Polishing and Scanning Electron Microscopy has been used to image the in-situ porosity and mineralogy in shale samples from Whitby (UK), which are an analogue for the Dutch Posidonia shale. First results show a fine-grained mudstone with cm-sub-mm scale stratification. The section at Whitby can be divided into a clay matrix dominated upper half and a coarser grained, calcite-rich, lower half of the section. Commonly occurring minerals are pyrite, calcite (fossils, grains and cement), quartz, mica and dolomite. Organic matter content varies from 0 – 2 % in the calcite dominated layers to 5 – 18% in the clay matrix dominated layers. The most porous phases are the clay matrix and calcite fossils. This microstructural study shows which intervals within Posidonia shale contain the largest porosity and organic matter contents for sweet spot analyses and forms a basis for future research on enhancing connectivity between the pores and induced fractures

Determining Individual Particle Magnetizations in Assemblages of Micrograins

Obtaining reliable information from even the most challenging paleomagnetic recorders, such as the oldest igneous rocks and meteorites, is paramount to open new windows into Earths history. Currently, such information is acquired by simultaneously sensing millions of particles in small samples or single crystals using superconducting quantum interference device magnetometers. The obtained rock-magnetic signal is a statistical ensemble of grains potentially differing in reliability as paleomagnetic recorder due to variations in physical dimensions, chemistry, and magnetic behavior. Here we go beyond bulk magnetic measurements and combine computed tomography and scanning magnetometry to uniquely invert for the magnetic moments of individual grains. This enables us to select and consider contributions of subsets of grains as a function of particle-specific selection criteria and avoid contributions that arise from particles that are altered or contain unreliable magnetic carriers. This new, nondestructive, method unlocks information from complex paleomagnetic recorders that until now goes obscured.

A Numerical Study on the Effect of Anisotropy on Hydraulic Fractures

In this paper, we present a two-dimensional numerical model for modelling of hydraulic fracturing in anisotropic media. The numerical model is based on extended finite element method. The saturated porous medium is modelled using Biot’s theory of poroelasticity. An enhanced local pressure model is used for modelling the pressure within the fracture, taking into account the external fluid injection and the leak-off. Directional dependence of all the rock properties, both elastic and flow related, is taken into account. A combination of the Tsai–Hill failure criterion and Camacho–Ortiz propagation criterion is proposed to determine the fracture propagation. We study the impact on fracture propagation (in both magnitude and direction) due to anisotropies induced by various parameters, namely ultimate tensile strength, Young’s modulus, permeability and overburden pressure. The influence of several combinations of all these anisotropies along with different grain orientations and initial fracture directions on the fracture propagation direction is studied. Different regimes are identified where the fracture propagation direction is controlled by the degree of material anisotropy instead of the stress anisotropy.

Mechanical Factors Controlling the Development of Orthogonal and Nested Fracture Network Geometries

Orthogonal fracture networks form an arrangement of open well-connected fractures which have perpendicular abutment angles and sometimes show topological relations by which fracture sets abut against each other, thus forming a nested network. Previous modelling studies have shown that orthogonal fractures may be caused by a local stress perturbation rather than a rotation in remote stresses. In this study, we expand on the implications of these local stress perturbations using a static finite element approach. The derived stress field is examined to assess the development of implemented microfractures. The results show that the continuous infill of fractures leads to a gradual decrease in the local tensile stresses and strain energies, and, therefore, results in the development of a saturated network, at which further fracture placement is inhibit. The geometry of this fully developed network is dependent on the remote effective stresses and partly on the material properties. Saturated networks range from: (1) a set of closely spaced parallel fractures; (2) a ladder-like geometry; and (3) an interconnected nested arrangement. Finally, we show that our modelling results at which we apply effective tension, are equivalent to having a uniformly distributed internal pore fluid pressure, when assuming static steady state conditions and no dynamic fluid behaviour.

Hydraulic fracturing in anisotropic and heterogeneous rocks

In this paper, we present a two dimensional model for modelling the hydraulic fracture process in anisotropic and heterogeneous rocks. The model is formulated using extended finite elements (XFEM) in combination with Newton-Raphson method for spatial and Eulers implicit scheme for time. The fracture is modelled with the help of cohesive zone method (CZM). Anisotropy arising due to orientation of grains in a specific direction in rocks is modelled to understand the influence of the degree of anisotropy and the grain orientation direction on fracture propagation. A combination of Tsai-Hill failure criterion and Camacho-Ortiz propagation criteria is used to predict mixed mode fracture propagation in anisotropic media. Effect of heterogeneities due to material inclusions, on fractures are modelled.

The Effect of Temperature and Pressure on the Rock Mechanical Behaviour of the Whitby Mudstone Formation, UK

We studied the rock mechanical behaviour of the outcropping Whitby Mudstone Formation shales (Toarcian Age) under varying temperature and confining pressure conditions, focusing on strength and elastic moduli. We compared the rock mechanical properties of the Whitby shale to published data of time and depositional equivalent shales from Northern Europe and producing shales from the US. We performed uniaxial and triaxial tests in order to constrain the effect of temperature and pressure on the mechanical behaviour of the shale. We loaded the samples normal to the bedding plane under room humidity conditions. We performed uniaxial tests at room temperature and zero confining pressure, whereas we applied confining pressure in several steps up to 50 MPa and considered temperature values between 20-150°C during triaxial tests. Ultimate strength and elastic moduli are strongly influenced by mineralogy, temperature and pressure. An increase in temperature enhances the deformability, whereby the Young’s Modulus and ultimate strength decreases. With an increase in confining pressure, Young’s Modulus and ultimate strength increases, whereas the Poisson’s ratio decreases.

Matrix to Fracture Flow Inferences from Core Measurements and Structural Fabric (Whitby UK)

The Early Jurassic (Toarcian) Shales in Northern Europe are investigated as possible unconventional sources for gas, where gas in shales is trapped in poorly connected micro pores and is sorbed within particles of organic material and clay minerals in the matrix of the host rock. Having a dual permeable medium consisting of a high permeable fracture network together with a tight shale matrix will improve gas flow rates from matrix to well. The Whitby Mudstone is currently outcropping on the Yorkshire coast hence getting sufficient sample material for permeability experiments is easily available, in combination with mapping of the natural occurring fractures in the cliffs and pavements along the coast this area is an ideal natural analogue to investigate matrix characteristics in combination with the natural fracture network. The studies show that fracture spacing on average is in the order of 10 centimeters and that in combination with a matrix permeability of 1∙10-18 m2 results in gas residue times in the matrix in the order of hours to tens of days depending on the input parameters used.