G. Nover

Petrophysical properties of the 9‐km‐deep crustal section at KTB

Petrophysical properties of drill core and drill cuttings samples from both bore holes of the German Continental Deep Drilling Program (KTB) measured at atmospheric pressure and room temperature in the field laboratory are presented, along with data of core samples measured at simulated in situ conditions by other laboratories. Most of the petrophysical properties show a bimodal frequency distribution corresponding to the two main lithologies (gneiss and metabasite), except electrical resitivity and Th/U ratio which are lithology independent (monomodal distribution). Low resistivities are mainly associated with fractures zones enriched in fluids and graphite. The most abundant ferrimagnetic mineral is monoclinic pyrrhotite. Below 8600 m, hexagonal pyrrhotite with a Curie temperature of 260°C is the stable phase. Thus the Curie isotherm of the predominant pyrrhotite was reached (bottom hole temperature about 265°C). The highest values of magnetic susceptibility are linked with magnetite. Microcracks grow due to pressure and temperature release during core uplift. This process continues after recovery and is documented by the anelastic strain relaxation and acoustic emissions. The crystalline rocks exhibit marked reversible hydration swelling. Anisotropy of electrical resistivity, permeability, P and S wave velocity is reduced significantly by applying confining pressure, due to closing of microcracks. Fluids within the microcracks also reduce the P wave velocity anisotropy and P wave attenuation. Anisotropy and shear wave splitting observed in the field seismic experiments is caused by the foliation of rocks, as confirmed by laboratory measurements under simulated in situ conditions. The petrophysical studies provide evidence that microfracturing has an important influence on many physical rock properties in situ.

Relationship between low-frequency electrical properties and hydraulic permeability of low-permeability sandstones

The relationship between low-frequency electrical properties and hydraulic permeability of rocks has been the focus of geophysical investigations for a long time because it offers a possibility for an in situ and noninvasive permeability estimation of rocks. We examined the hydraulic and low-frequency (10 mHz to 100 Hz) electrical properties as well as the anisotropic properties of low-permeability sandstones from a tight gas reservoir. Single-frequency electrical properties were found to be of low value for the determination of permeability for the studied samples, whereas a strong link between the spectral-induced polarization (SIP) response and permeability was found. The SIP response was transformed into a relaxation-time distribution using a Debye decomposition procedure. We observed a strong positive correlation in form of a power law between median relaxation time of the distribution and permeability, suggesting that relaxation time is a good measure of the effective hydraulic length scale. From a ...

The crystal structure of nizn3.r

Abstract The phase NiZn3.r has an orthorhombic end-centred cell with the content 2(34Ni+104Zn) and lattice constants a 1 = 33.326 A , a 2 = 8.869 A and a 3 = 12.499 A ; the structure is closely related to the γ-brass type. In contrast with the W-type substructure, there are 12 unoccupied sites in the cell and the atoms surrounding these are displaced towards the vacancy, causing further displacements of atoms that are not adjacent to a vacancy. The structure obeys the quasi-linear dependence of occupancy on valence electron concentration first found by Bradley in NiAl. This phenomenon may be interpreted by the two-correlations model which also gives arguments for the different deformation homeotypes and the extra vacancy formation at high valence electron concentrations.

High pressure phase transformations of cubic boron nitride from amorphous boron nitride using magnesium boron nitride as the catalyst

Abstract Results are described of high pressure phase transformations of amorphous boron nitride (aBN) to cubic boron nitride (cBN) using magnesium boron nitride (Mg 3 B 2 N 4 ) as a catalyst-solvent. It was observed that amorphous boron nitride undergoes various structural modifications under high pressures and high temperatures leading to the formation of hexagonal, cubic and wurtzitic phases of boron nitride. The minimum pressure at which aBN starts transforming into cBN was found to be 25 kbar at 1800°C. This is the lowest pressure for cBN formation employing the catalyst-solvent process and is reported here for the first time.

In-Situ Electrical Conductivity and Permeability of Mid-Crustal Rocks from the Ktb Drilling: Consequences for High Conductive Layers in the Earth Crust

Freshly cored samples from a microprofile (7011–7013m in depth) of the German Continental Deep Drilling Project (KTB) were taken to measure the complex electrical conductivity σ (1 kHz up to 1 MHz), porosity, BET-surface, permeability and density. The porosity ranged about 1 vol%, while the permeability k varied from 16.05 µD to > 0.01 µD for in-situ pressure conditions. The permeability decreased about 2 orders in magnitude up to pressures of 200 MPa. Conductivity σ was measured in the same pressure range on 1 M NaCl saturated samples. Thin sections and SEM analysis revealed an enrichment of carbon and ilmenite (about 1 vol%) on inner cleavage cracks of mica, thus causing an unusual high σ (ranging from 4.2 × 10-3 S/m to 67 × 10-3 S/m) being orders of magnitude higher than normally measured on such types of rocks (about 300 × 10-6 S/m). An inverse pressure dependence of σ was detected on some of the samples. Electronic conduction was confirmed by least-squares-fits of model data to the frequency dispersion of the conductivity and by measuring the time dependence of the volume conductivity and its frequency dispersion. Thus the dominating role of the reconnected network of carbon and ilmenite on the enhanced volume conductivity was proved. An increase of the conductivity due to hydrofracturing by high pore fluid pressures plays a less important role.

The effect of pressure on the electrical conductivity of KTB rocks

Complex electrical resistivity and permeability were measured on two gneiss samples and nine amphibolites (originally located at a depth of 4150 m to 5012 m) from the main drilling of the German deep drilling project (KTB). Measurements were performed as a function of hydrostatic pressures up to 240 MPa on core samples (30 mm in diameter and 10–20 mm high). For each measurement, two samples were used, one being parallel, and one perpendicular to the borehole axis. At low pressures and again at maximum pressure the frequency dispersion (1 kHz up to 1 MHz) of the complex resistivity was measured using a two electrode device. An unusual pressure effect was detected on some of the samples and was established to be due to the oriented deposition of good conducting phases in the foliation. Rock fabric and the orientation of ore mineralization was measured on thin sections and polished sections prepared from the same samples.

Influence of oxygen partial pressure on the Mg/Fe distribution in olivines

The dependence of Mg/Fe ordering on oxygen partial pressure in natural olivine crystals of volcanic origin has been studied by X-ray diffraction. Two natural crystals with 10% and 12% fayalite have been investigated and the atomic positions, anisotropic temperature factors, extinction coefficients and site occupancies have been refined, reaching R-values of 2.2%. After subjecting the crystals to oxygen partial pressures of 10−16 bar and 10−21 bar the crystals were studied again. In total six crystals were studied and the distribution coefficients KD determined. The natural untreated crystals had KD=1.09 and 1.06, e.g., a slight preference of Fe in (M1). p(O2) of 10−16 bar increased the ordering of Fe in (M1) to KD=1.2, while p(O2)=10−21 bar reversed KD to 0.8 with ordering of Fe in (M2).These experiments suggest that Mg/Fe ordering in olivines is primarily determined by the prevailing oxygen partial pressure.

In-situ investigations of the reversible hBN-cBN-hBN-transformation in the Li3N-BN catalyst system using synchrotron radiation

Abstract The kinetics of the transformation of boron nitrides hexagonal form (hBN) into the polymorphic cubic high pressure phase (cBN) were studied in the Li3N-BN catalyst system under in-situ pressure and temperature conditions. Energy-dispersive X-ray experiments were performed in a MAX 80 high pressure cell at HASYLAB, DESY, Hamburg using synchroton radiation. The transformation of hBN into cBN and the reverse transformation of cBN into hBN within the same experimental run were examined. Simultaneously the kinetics of the transformation were determined in the pressure range 0.65-6.5 GPa and at temperatures between 600–1400 °C. It could be shown that, in most cases, the transformation was rather fast and it was completed in less than 5 min. The observed data confirm the experimental results and the phase diagram as given by Bundy and Wentorf, but are in contrast to the experimental results and thermodynamic calculations by Solozhenko and by Maki.

Anisotropy of compressional wave velocities, complex electrical resistivity and magnetic susceptibility of mylonites from the deeper crust and their relation to the rock fabric

Abstract Laboratory measurements of compressional wave velocities ( V P ) complex electrical resistivity and magnetic susceptibility have been carried out on a mylonitic clinopyroxene amphibolite (MCA) and an epidote-muscovite-quartz ultramylonite (MQU) from the Ivrea Zone. Both mylonites exhibit significant anisotropies that are related to the rock fabric. At low pressure of about 50 MPa the anisotropy of V P for the MCA and MQU is 17 and 34%, respectively. It is due to oriented microcracks (intragranular, transgranular, grain boundary cracks and cleavage cracks), in addition to preferred orientation of anisotropic rock-forming minerals. At high pressure (600 MPa) where most of the cracks are closed a residual anisotropy of about 10 and 12%, respectively, is observed, that is mainly controlled by preferred orientations of mineral constituents. The anisotropies of the complex electrical resistivity measured with an electrolyte (tap water) at 0.1 MPa are 40 and 36% for the MCA and MQU, whereas at 80 MPa these values increase to 44% for both samples. In contrast, measured with an electrolyte of 0.1 M NaCl solution the resistivity is lower and gives anisotropies of 71% (MCA) and 63% (MQU) at 0.1 MPa. The anisotropies of the magnetic susceptibility (AMS) are about 27% (MCA) and 14% (MQU). For both mylonites the axes of the AMS ellipsoid coincide with the macroscopic fabric elements, where the main axis of magnetic susceptibility ( K max ) lies approximately parallel to the lineation. An intercorrelation of the different physical properties in terms of rock composition and anisotropic rock fabric is difficult. Nevertheless, the laboratory determination of anisotropic in-situ physical properties provide more objective data for the modelling of in-situ crustal conditions.

Pressure induced phase transition in Mg2GeO4 as determined by frequency dependent complex electrical resistivity measurements

The phase transition of Mg2GeO4 from the olivine structure (α-phase) to the spinel structure (γ-phase) was determined by complex electrical resistivity measurements in the frequency range 10 Hz up to 100 kHz. The stability fields of the α- and the γ-phase were confirmed up to 2.05 GPa in the temperature range 845° C–1400° C. Based on volume resistivity data, a decrease of about a factor of 5 was found at the α-γ phase transition. Activation energies of electrical conduction Eaat 1.05 GPa and 2.05 GPa were calculated using the volume resistivities (ϱ) and the relaxation times (τ). The values range from 1.98 eV up to 2.78 eV. The relative dielectric permeability increases with increasing temperature. This is is due to crystal defects and charge transport mechanisms.