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Permeability in tight crustal rocks is primarily controlled by the connected porosity, shape and orientation of microcracks, the preferred orientation of cross-bedding, and sedimentary features such as layering. This leads to a significant permeability anisotropy. Less well studied, however, are the effects of time and stress recovery on the evolution of the permeability hysteresis which is becoming increasingly important in areas ranging from fluid migration in ore-forming processes to enhanced resource extraction. Here, we report new data simulating spatio-temporal permeability changes induced using effective pressure, simulating burial depth, on a tight sandstone (Crab Orchard). We find an initially (measured at 5 MPa) anisotropy of 2.5% in P-wave velocity and 180% in permeability anisotropy is significantly affected by the direction of the effective pressure change and cyclicity; anisotropy values decrease to 1% and 10% respectively after 3 cycles to 90 MPa and back. Furthermore, we measure a steadily increasing recovery time (10–20 min) for flow parallel to cross-bedding, and a far slower recovery time (20–50 min) for flow normal to cross-bedding. These data are interpreted via strain anisotropy and accommodation models, similar to the “seasoning” process often used in dynamic reservoir extraction. [ABSTRACT FROM AUTHOR]

Inclination shallowing of detrital remanent magnetization in sedimentary strata has solely been constrained for the mechanical processes associated with mud deposition and shallow compaction of clay-rich sediment, even though a significant part of mud diagenesis involves chemical compaction. Here we report, for the first time, on the laboratory simulation of magnetic assemblage development in a chemically compacting illite shale powder of natural origin. The experimental procedure comprised three compaction stages that, when combined, simulate the diagenesis and low-grade metamorphism of illite mud. First, the full extent of load-sensitive mechanical compaction is simulated by room temperature dry axial compression. Subsequently, temperature controlled chemical compaction is initiated by exposing the sample in two stages to amphibolite or granulite facies conditions (temperature is 490 to 750°C and confining pressure is 170 or 300 MPa) both in the absence (confining pressure only) and presence of a deformation stress field (axial compression or confined torsion). Thermodynamic equilibrium in the last two compaction stages was not reached, but illite and mica dehydroxylation initiated, thus providing a wet environment. Magnetic properties were characterized by magnetic susceptibility and its anisotropy (AMS) in both high- and low-applied field. Acquisition of isothermal remanent magnetization (IRM), stepwise three-component thermal de-magnetization of IRM and first-order reversal curves were used to characterize the remanence-bearing minerals. During the chemical compaction experiments ferrimagnetic iron-sulphides formed after reduction of magnetite and detrital pyrite in a low sulphur fugacity environment. The degree of low-field AMS is unaffected by porosity reduction from 15 to ∼1 per cent, regardless of operating conditions and compaction history. High-field paramagnetic AMS increases with compaction for all employed stress regimes and conditions, and is attributed to illite transformation to iron-bearing mica. AMS of authigenic iron-sulphide minerals remained constant during compaction indicating an independence of ferrimagnetic fabric development to chemical compaction in illite shale powder. The decoupling of paramagnetic and ferrimagnetic AMS development during chemical compaction of pelite contrasts with findings from mechanical compaction studies. [ABSTRACT FROM AUTHOR]

A system of aligned vertical fractures produces azimuthal variations in stacking velocity and amplitude variation with offset, characteristics often reported in seismic reflection data for hydrocarbon exploration. Studies of associated attenuation anisotropy have been mostly theoretical, laboratory or vertical seismic profiling based. We used an 11 common-midpoint-long portion of each of four marine surface-seismic reflection profiles, intersecting each other at 45° within circa 100 m of a common location, to measure the azimuthal variation of effective attenuation, and stacking velocity, in a shallow interval, about 100 m thick, in which consistently orientated vertical fracturing was expected due to an underlying salt diapirism. We found qualitative and quantitative consistency between the azimuthal variation in the attenuation and stacking velocity, and published amplitude variation with offset results. The 135° azimuth line showed the least apparent attenuation and the fastest stacking velocity, hence we infer it to be closest to the fracture trend: the orthogonal 45° line showed the most apparent attenuation and slowest stacking velocity. The variation of with azimuth φ is well fitted by = 34 − 18cos[2(φ+40°)] giving a fracture direction of 140 ± 23° (±1SD, derived from ‘bootstrapping’ fits to all 114 combinations of individual common-midpoint/azimuth measurements), compared to 134 ± 47° from published amplitude variation with offset data. The effects of short-window spectral estimation and choices of spectral ratio bandwidth and offset ranges used in attenuation analysis, individually give uncertainties of up to ±13° in fracture direction. This magnitude of azimuthal variation can be produced by credible crack geometries (e.g., dry cracks, radius 6.5 m, aspect ratio 3 × 10−5, crack density 0.2) but we do not claim these to be the actual properties of the interval studied, because of the lack of well control (and its consequences for the choice of theoretical model and host rock physical properties) and the small number of azimuths available here. [ABSTRACT FROM AUTHOR]

We present an experimental and modelling study of pore fabric anisotropy using the method of anisotropy of magnetic susceptibility (AMS) applied to synthetic void spaces of known dimensions saturated with a high susceptibility magnetic ferrofluid. We analysed the data using the equivalent pore concept (EPC) proposed by Hrouda et al., who consider the theoretical demagnetization factors of an ellipsoid in order to relate physical pore fabric to magnetic measurements of lineation, foliation and bulk anisotropy. To test this theory, synthetic samples were prepared from cylindrical polycarbonate blanks, 25 mm in diameter by 22 mm long. A variety of ‘special fabrics’ were prepared by machining internal void spaces of: (a) a quasi-spherical fabric comprising a cylinder 10 mm in diameter by 8.8 mm long, (b) a capillary-like fabric comprising a set of 19 equally spaced holes, (c) a bedding-like fabric comprising a linear row of five larger diameter holes and (d) a crack-like fabric comprising a stack of four penny-shaped voids. A second set of quasi-spheroidal fabrics were prepared by machining a hemispherical cutter to different depths into the blanks. Eight samples were prepared with principal axial to radial axis ratios ( a/ r) from 0.75 to 1.3 (i.e. from oblateness through sphericity to prolateness). With the exception of the quasi-spherical fabric, the ‘special fabrics’ exhibit high anisotropy, with a maximum foliation of 1.41 and a maximum lineation of 1.29. Using a ferrofluid with a fixed intrinsic susceptibility of 1.09 SI, the quasi-spheroidal shape effect is investigated with change in value of the a/ r ratio. As the a/ r ratio increases, foliation decreases and lineation increases, reflecting the change from an oblate to a prolate fabric. The EPC is then used to estimate the physical void anisotropy from the magnetic measurements of lineation and foliation for direct comparison with the known geometry. Overall, the EPC method makes a reasonable job of estimating the void geometry, but it underestimates the physical void anisotropy by an average of about 8 per cent. We, therefore, report the effect of varying the intrinsic susceptibility of the ferrofluid on a void with a constant a/ r ratio of 1.2. As ferrofluid concentration is increased, the EPC predicted void geometry converges to the known physical void geometry. However, even for the highest intrinsic susceptibility ferrofluid used (3.34 SI) the EPC underpredicts the known void anisotropy. We, therefore, propose a simple, empirical correction factor that allows the EPC method accurately to predict real physical void space anisotropy from AMS measurements. [ABSTRACT FROM AUTHOR]