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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]

SUMMARY Digital rock physics combines modern microscopic imaging with advanced numerical simulations to analyse the physical properties of rocks. Elastic-wave propagation modelling based on the microstructure images is used to estimate the effective elastic properties of the rock. The goal of this paper is to describe and understand how laboratory experiments compare with digital rock physics results using Berea sandstone. We experimentally measure pressure-dependent ultrasonic velocities and the pore size distribution. The effective elastic properties resulting from numerical simulations are based on microcomputed tomography (micro-CT) images, which are systematically stiffer than the laboratory measures. Because the tomographic images do not resolve the small-scale pore and crack network of the sample, we hypothesize that the numerical overprediction is attributable to the smallest pores and grain-to-grain contacts that are missing in the images. To reconcile the difference between numerical and experimental data, we suggest to use a grain boundary reconstruction algorithm. This allows to implement and approximate so far unresolved features in the virtual rock model. As a result, we can predict pressure-dependent effective velocity using micro-CT images. [ABSTRACT FROM AUTHOR]