Unstable Grains Research Articles

Potential Bias in Volcanic Paleomagnetic Records Due To Superimposed Chemical Remanent Magnetization

AbstractVolcanic rocks, preserving paleorecords of Earth's magnetic field, are essential to constrain the working of the geodynamo, provided their primary signal was not biased. Using a thermomagnetometer, we simulate a situation where a sample's primary record, carried by a thermoremanence (TRM, acquired by cooling in air from 600°C to room temperature), is partly overprinted by a chemical remanence (CRM, acquired by 200 hr of isothermal exposure at 400°C). This situation leads to two directional and intensity components (in the form of linear segments) in the Zijderveld and Arai‐Nagata diagrams. In the case of unstable titanomagnetite grains prior to CRM acquisition, we show that both components can be strongly biased by up to ∼50° for paleodirections and ∼50% for paleointensities. In such a worst‐case scenario, the secondary CRM strongly overprints the primary TRM, rendering the common interpretation of Zijderveld and Arai‐Nagata diagrams in terms of characteristic components invalid.

Diagenetic variability in Tertiary, syn-rift mixed siliciclastic‑carbonate depositional system (Lower Musayr Formation), Red Sea, Saudi Arabia

Mixed siliciclastic‑carbonate depositional systems are prone to differential diagenesis due to lithological heterogeneity. However, unlike pure carbonate and/or siliciclastic counterparts, the diagenesis within mixed clastic‑carbonate successions remains poorly known. This study integrates sedimentological and petrographic analysis with porosity-permeability measurements to understand diagenetic variability and its impact on fluid flow in the Oligocene-Miocene mixed clastic‑carbonate system (lower part of Musayr Fm) in Midyan Basin, Red Sea, Saudi Arabia. Sedimentological observations highlight that siliciclastic intervals comprise conglomerates, coarse- to fine-grained sandstones and subordinate mudstones whereas mixed siliciclastic‑carbonate intervals are composed of shell, ooids and microbialites-dominated facies. Petrographic analysis indicates presence of several diagenetic processes in siliciclastic intervals including dissolution of unstable silicate grains (feldspar), formation of pore filling and/or grain-replacive kaolinite, precipitation of calcite and/or dolomite, and replacement of dolomite by silica. The siliciclastic strata exhibit minimal compaction fabrics with no stylolites and are characterized by higher permeability (average = 1884 md) and porosity (average = 18.7 %). On the other hand, mixed siliciclastic‑carbonate intervals underwent micritization, sparry calcite formation, dolomitization of micrite and bioclasts, and replacement of sparry calcite by pyrite and Fe-oxides. Wavy stylolite seams were also observed in ooid facies reflecting moderate chemical compaction. In addition, the porosity (average = 6.6 %) and permeability (average = 93 md) are magnitudes lower than siliciclastic counterparts. Significant differences between porosity and permeability of siliciclastic and mixed intervals are partly linked to relatively intense diagenetic alteration (higher cementation and chemical compaction) in mixed units. Understanding such diagenetic heterogeneity in mixed systems has important implications for identification of reservoir and non-reservoir zones and may provide useful insights for hydrocarbon exploration and carbon sequestration in the analogous strata in the subsurface.

Facies and Hydrocarbon Reservoir Rock Characterization of the Paleozoic Rocks of Peshawar Basin, Northwest Pakistan

The present study details the facies description and hydrocarbon reservoir characterization of the Paleozoic rocks of Peshawar Basin, northwest Pakistan. The outcrop samples from the Cambrian-Devonian rocks along the famous Nowshera-Risalpur Road and Turlandi Village Section were investigated. The analysis of outcrop data revealed significant information regarding the facies and their depositional environments. Based on its detailed sedimentological data, it is believed that the Ambar Formation is deposited in shallow shelf-tidal flat settings, while the protolith of the Misri Banda Quartzite came from the wide beaches. The protolith of the Panjpir Formation was deposited in the shelf conditions, which shows an overall shallowing when moving up the section from the argillites and phyllites to crinoidal limestone. The Nowshera Formation was deposited as a reef complex on the shelf edge having reef core, reef breccia and a back-reef lagoon. The techniques of petrography, XRD and SEM were used for the bulk geochemical composition of the rocks focusing on their matrix, mineralogy, micro-porosity and pore-filling materials. The presence of micrite, goethite, kaolinite and illite as intergranular mass, dolomitization-induced porosity, twin cleavage plane and high dissolution porosity in the Ambar Formation can provide significant pore space to the reservoir fluids migration. Hematite with minor kaolinite, illite and chlorite as alteration products of unstable framework grains as intergranular mass and a deep seated burial diagenesis has minimized the reservoir potential in Misri Banda Quartzite. The coarse and fine calcite, dolomite, chlorite and kaolinite occur as pore-filling material between the framework grains while the high intensity of intragranular dissolution and micro intergranular connecting porosity in the Nowshera Formation makes it a promising reservoir.

The Atomic Observation of the Structural Change Process in Pt Networks in Air Using Environmental Cell Scanning Transmission Electron Microscopy.

The structural change in Pt networks composed of multiple chain connections among grains was observed in air at 1 atm using atomic-resolution environmental cell scanning transmission electron microscopy. An aberration-corrected incident electron probe with a wide convergence angle made it possible to increase the depth resolution that contributes to enhancing the signal-to-noise ratio of Pt network samples in air in an environmental cell, resulting in the achievement of atomic-resolution imaging. The exposure of the Pt networks to gas molecules under Brownian motion, stimulated by electron beams in the air, increases the collision probability between gas molecules and Pt networks, and the Pt networks are more intensely stressed from all directions than in a situation without electron irradiation. By increasing the electron beam dose rate, the structural change of the Pt networks became significant. Dynamic observation on an atomic scale suggested that the structural change of the networks was not attributed to the surface atomic-diffusion-induced step motion but mainly caused by the movement and deformation of unstable grains and grain boundaries. The oxidized surface layers may be one of the factors hindering the surface atomic step motion, mitigating the change in the size of the grains and grain boundaries.

Petrography, Diagenesis and Hydrocarbon Source Potential of the Barail and Disang Group of Rocks in Parts of the Naga Schuppen Belt and Inner Fold Belt of Assam-Arakan Basin

Abstract Rock-thin section study, scanning electron microscopy (SEM), X-ray diffraction (XRD), and Rock-Eval pyrolysis have been carried out on the Barail (Oligocene) and Disang (Eocene) groups of rocks of the Naga Schuppen Belt of Assam-Arakan Basin to understand the reservoir properties and hydrocarbon source potential. Analyses show that the Barail Group sandstones are moderately well-sorted, medium-grained, sublithic arenite type, and bear good porosity (avg. 14.48%). In contrast, the Disang Group sandstones are very hard and compact, fine-grained, sublihic arenite, and lithic greywacke types and show medium porosity (avg. 11.3%). The reservoir properties of the sandstones of both groups are controlled by various diagenetic alterations like mechanical compaction, cementation, overgrowth, authigenesis of clay, fracturing, dissolution, and alteration of unstable clastic grains. However, no single diagenetic process could be considered as the main controlling parameter in the evolution of the porosity of the studied sandstones. The Rock-Eval pyrolysis indicates that the carbonaceous shales of both the Barail and Disang groups are characterized by type III and type IV kerogen. A good number of shale samples show TOC (total organic carbon) values ranging from 0.88 to 10.71 wt%, which seems quite good. In most of the samples, S2 is recorded <2 mg HC/g of rock, indicating the potential to generate wet gas. The various binary plots mostly show the gas source potential of shales of both the Barail and Disang groups. Production Index (PI) values vary between 0.04-0.45 for the Barail shales and 0.04-1.0 for the Disang shales, indicating that the studied shale samples are in the last phase of catagenesis.

Sedimentological Controls on the Reservoir Characteristics of the Mid-Triassic Tredian Formation in the Salt and Trans-Indus Surghar Ranges, Pakistan: Integration of Outcrop, Petrographic, and SEM Analyses

The current study uses an integrated lithofacies, optical microscopy, and scanning electron microscopy (SEM) analysis to investigate the sedimentary processes, depositional architecture, and reservoir rock potential of the Tredian Formation’s (Mid-Triassic) mixed siliciclastic and carbonate succession in the Salt and Trans-Indus Ranges. The formation has been divided litho-stratigraphically into two components: the lower Landa Member, which consists of fine-grained sandstone and shale, and the upper Khatkiara Member, which consists of coarse-grained sandstone. Based on sedimentary structures and lithology, four distinct types of lithofacies are identified. Two lithofacies representing sandstones interbedded with shale (LF1) and thick-bedded sandstone (LF2) lithofacies suggestive of fluvio-deltaic settings are among them. Another two lithofacies of thin-bedded sandstone (LF3) and dolomite (LF4) suggest a tidal flat depositional environment, correspondingly. The petrographic examination of the Tredian sandstones indicates a lithology ranging from sub-feldspathic arenite to feldspathic arenite with moderate packing. The presence of primary calcite cement, silica cement, and iron oxide/hydroxide cements were shown by the diagenetic investigation, which was supported by SEM studies. In addition, secondary cements include ferroan-dolomite, chlorite, and illite, which is linked with chemical alteration of unstable grains. The paragenetic sequence depicts the diagenetic evolution of the Tredian sandstone from early to late diagenetic phases. The reservoir quality of the LF1 and LF4 lithofacies has been destroyed by early-stage calcite cementation, but the lithofacies LF2 and LF3 have a strong reservoir potential owing to the scarcity of calcite cement, dissolution of unstable feldspar grains, and grain fracture.

Hinterland environments of the Late Jurassic northern Weald Basin, England

Reconstructing provenance in sandstones can be challenging, especially when the hinterland palaeogeology is unknown due to burial, diagenesis or weathering of the original outcrops. As sedimentary processes alter the distribution of minerals in the depositional environment, the use of multiple provenance methods reduces uncertainties, because, together, they can account for depositionally‐controlled textural and mineralogical re‐distribution in a basin. Here, we have applied petrography, X‐ray fluorescence geochemistry and sedimentology to understand the provenance of shallow marine Corallian sandstones with the aim of deducing the palaeogeology, palaeoenvironment and sediment distribution within the northern parts of the Upper Jurassic Weald Basin, onshore UK. The Corallian sandstones had a mixed mafic‐felsic (intermediate), metasedimentary recycled orogen source. The hinterland experienced significant physical and chemical weathering under humid conditions. Corallian sandstones were relatively more chemically mature up‐dip and more texturally mature down‐dip. Chemically unstable grains and heavy minerals were relatively concentrated down‐dip. Heterogeneous, sedimentologically‐controlled mineral distribution patterns highlight potential errors which may be made in deriving source‐area maturity. This study is significant as it illustrates the combined roles of provenance and deposition in controlling primary mineral distribution that then influenced the style of burial diagenesis. The work presented here emphasizes the importance of a multi‐proxy approach to improve provenance analysis.

Petrography and Clay Mineral Variations Across the Cretaceous/Paleogene (K/Pg) Boundary, Sulaimani, Kurdistan Region, Iraq

The Cretaceous/Paleogene (K/Pg) boundary marks the global extinction of many life forms, this boundary around Sulaimani city coincides with the boundary between Tanjero and Kolosh formations. These two units are flysch deposits of the Zagros Foreland Basin. This study focuses on petrography and clay mineralogy variations between these two units. Petrographic study, X-Ray Diffraction analysis, and Scanning Electron Microscope analysis were conducted on samples from different lithologies. The petrographic study of fine-grain lithologies shows that they are mainly wackestone without variation across the boundary. Sandstone petrography shows variation in framework components by increasing quartz, feldspar, and igneous fragments while argillaceous, carbonates, chert, and chalcedony fragments are decreased from Tanjero to Kolosh formations. Obtained results from XRD and SEM show that the clay mineral assemblage is dominated by corrensite (regularly interstratified chlorite/smectite) with discrete chlorite, smectite and traces of illite. The existence of these clay assemblage suggests that corrensite is the diagenetic product of smectite as it is an intermediate stage of smectite chloritization. Enrichment of precursor smectite and high content of unstable grains in sandstones across the K/Pg boundary is the result of low-intensity weathering in arid and seasonal climates of the source area. The smaller size of detrital smectite than other clay minerals is behind its enrichment in the deep marine basin for both Tanjero and Kolosh formations. Sandstone enrichment with sedimentary fragments in both formations related to uplifted sedimentary terrain in its hinterland, while variation across the boundary indicates gradual uplift of Zagros ophiolite zone in the early Paleogene.

Effect of limited retained austenite on the strength–ductility trade-off in low-alloyed TRIP steel

A comprehensive experimental and numerical investigation of TRIP effect on mechanical behaviors, strain partitioning and fracture is carried out on a low-alloyed TRIP steel with and without subzero treatment. Load responses of individual phases and macroscopic mechanical properties are investigated through nanoindentation and uniaxial tensile tests. The increase of martensite volume fraction caused by subzero treatment improves tensile strength but appears detrimental to ductility. Two types of martensite transformation are observed: a twin-relationship variant selection to accommodate strain interior unstable austenite grains, and a variant selection based on the same crystal orientation as adjacent coarse ferrite grain reducing interfacial energy between ferrite and newly transformed martensite. A microstructure-based simulation model considering martensite transformation is developed with phenomenologically calibrated crystal plasticity parameters. The shift of strain localization from unstable austenite grain to neighboring ferrite grain is clarified to reduce the strain partitioning at interfaces. This effect is argued to be the origin of ductility enhancement in low-alloyed TRIP steel.

Diagenesis and reservoir quality of Oligocene Barail Group of Upper Assam Shelf, Assam and Assam Arakan basin, India

The Oligocene Barail deltaic sandstones are the main oil reservoirs in Upper Assam Basin. Nonetheless, the diagenetic alterations and their impact on reservoir quality on Barail sandstones are virtually unknown. The present study aims to know the ways diagenesis plays the role for the invariable hydrocarbon production from these clastic reservoirs within the Barail Group. Samples representing the lower unit of Barail Group known as Barail Main Sand (BMS) and the upper unit known as Barail Coal Shale (BCS) were studied using a combination of petrographic, scanning electron microscopy (SEM), X-ray diffraction (XRD) of clay and wireline log techniques. The average framework composition of BMS is Q49.42F4.18R14.75 while the BCS are composed of Q51.33 F3.89 R14.04. The BMS are moderately hard, fine to medium grained, sub angular to sub rounded, moderately well sorted, dominantly of lithic arenite type and porosity value ranges from 10 to 22.01%. The sandstones of BCS Unit are moderately hard, light grey, fine to very fine grained and usually have low porosity except the channel sands. Both the BMS and BCS have undergone various diagenetic alterations which are observed as mechanical compaction, authigenesis of cements and clays, dissolution and alteration of unstable clastic grains. The identified clay minerals are kaolinite, illite and chlorite. Smectite is occasionally observed as mixed layered smectite-illite clay. The diagenetic alterations along with the clay minerals occupying as pore-filling and lining cements have critically influenced the reservoir quality of both BMS and BCS units of Barail Group. Proper treatment of these clay minerals could aid in the prevention of formation damage to a large extent.

Viscous magnetization decrease in first-order reversal curves induced by rotatable magnetic anisotropy in polycrystalline exchange-biased bilayers

A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves. The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the hysteresis loop timescale.

Effect of laser remelting on the surface characteristics of 316L stainless steel fabricated via directed energy deposition

Herein, laser remelting (LRM) technology was adopted for the stainless steel 316L deposited using the directed energy deposition (DED) process. We focused on examining the scanning direction, laser power, and overlap, which are the main parameters of LRM, and changes in the surface characteristics after LRM. The LRM could remove the traces of the spatter, unmelted powder, and deposited tracks remaining on the DEDed surface. Moreover, overlap was the major factor affecting the surface roughness and waviness. The roughness and waviness of the LRM-treated surface decreased as the overlap increased. An interaction was observed between the laser power and overlap in terms of the surface roughness, and when the laser power was high and the overlap was large, the surface roughness was low. Furthermore, the laser power of 1200 W transferred the excess energy to the surface, reducing the surface improvement. After performing LRM, the thermodynamically unstable crystal grains recrystallized. From the X-ray diffraction results, face-centered cubic and body-centered cubic phases were observed; however, no difference was observed in the phase fraction ratio before and after the LRM treatment. Regarding residual stress measurements, no obvious effect was observed on the removal of tensile residual stress formed on the as-prepared surface. For this reason, the change in hardness after LRM was not significant (increased by 10.7%). However, the costs of postprocessing, such as machining and grinding, can be partially reduced using LRM after DED processing.

Development of grain size/ texture graded microstructures through friction stir processing and subsequent cold compression of a rare earth bearing magnesium alloy

The present work deals with the effect of mechanical post-treatment on the microstructure and texture evolution of friction stir processed Mg-4.37 wt%Y-2.9 wt%RE-0.3 wt%Zr alloy. In this respect, the stir processed specimens were compressed down to engineering strain of 10 and 20%, and the resultant microstructures were characterized. Through applying subsequent compressive loading, fraction of low angle boundaries was increased, substructure was developed, the grain size was refined, and specified misorientation angles/texture components were faded or appeared. The degrees of such evolutions were different between the surface layer and stir zone, and resulted in development of graded microstructures. The fraction of strain-less and fresh grains was considerably decreased; but the fraction of coarse grains holding high grain orientation speared (GOS) was remarkably increased specially by moving away from material surface. Schmid analysis indicated the higher probability of non-basal slip within the unstable coarse grain in the course of subsequent loading. This was led to the occurrence of post-dynamic recrystallization which was traced through the sequence of substructure development and transformation of the low angle boundaries in to the high angle ones. Such gradual microstructure/texture evolution indicate the stir processing plus compression loading as capable method for fabrication of micro-scale functionally graded materials. • Mechanical post-treatment of friction stir processed WE43 magnesium alloy. • Fabricating graded microstructures regarding grain size, substructure and texture. • Contribution of continuous dynamic recrystallization during friction stir processing. • High potential of unstable coarse grains for refinement during subsequent loading. • Post-dynamic/Meta-dynamic recrystallization activates during subsequent compression.

Effect of Irradiation with Si+ Ions on Phase Transformations in Ti–Al System during Thermal Annealing

The article deals with the effect of irradiation with Si+ ions on phase transformations in the Ti–Al system during thermal annealing. An aluminum film with a thickness of 500 nm was deposited on VT1-00 titanium samples by magnetron sputtering, followed by ion implantation. Samples before and after irradiation with Si ions were annealed in a vacuum of 10−4 Pa in the temperature range 600–1000 °C. It was established that ion implantation reduces the dissolution of Al in α-Ti with the formation of titanium silicides (TiSi2, Ti5Si3) and stabilizes aluminide phases Ti3Al rich in aluminum. As a result, a composite structure based on titanium silicide/aluminide was obtained on the surface of the sample synthesized by complex treatment: deposition, irradiation with Si+, and thermal annealing at the near-surface layers. The formation of the phase-structural state of the implanted layers is associated with the displacement of atoms of the crystal lattice, a result that is reflected in an increase in the size of the crystal lattice and a decrease in microdistortion of the lattice. The opposite effect is observed with increasing temperature. This fact is explained by the relaxation of unstable large grains with an excess of internal energies. At the annealing temperature of 900–1000 °C, a significant increase in microhardness was observed due to silicide phases.

The influence of diagenetic heterogeneity on tight-reservoir properties in the Upper Triassic Yanchang Formation, Southeastern Ordos Basin, China

ABSTRACT The Chang 7–8 sections of Yanchang Formation in the Ordos Basin are dominated by lacustrine tight sandstones but characterized by high hydrocarbon production. This setting provided an opportunity to study reservoir characteristics and diagenetic processes of tight sandstones, to facilitate future oil exploration. In this study, it was found that reservoir qualities were mainly controlled by the diagenetic heterogeneity at different parts of sandstones, based on a comprehensive analysis of thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), pressure-controlled mercury injection, X-ray computed tomography (XCT), and well log data. Carbonate cements tend to occur at the edges of sandstone bodies. Sandstone layers with a distance less than 0.5 m from the sandstone–mudstone interface, or with a thickness of less than 1.5 m, were rich in carbonate cements (carbonate content was more than 15%). Lower porosity (less than 4%) and permeability (less than 0.01 mD) trend to distribute in sandstones that have higher content of carbonate cements. The source of carbonate cements was most likely from adjacent mudstones or from the dissolution of unstable grains. Samples collected from the center of sandstone layers have higher porosity, pore connectivity, and permeability but with low carbonate cement content. Hydrocarbon accumulation was facilitated through vertical fractures between the layers. The dissolution of feldspar and lithic fragments by organic acid associated with oil charging improved reservoir properties in the center of sandstone layers, and generated material for carbonate cement precipitation at the edge of sandstone layers. The results of this study demonstrated that the dominance of different diagenetic processes led to a reduction of reservoir quality at the edge of sandstone layers but an increase in the center of sandstone layers. This research offered a new perspective on the study of reservoir properties and the diagenesis mechanism of tight sandstones, which could be conducive to tight oil exploration and development.

Reservoir Characteristic of Triassic Sandstone, Eastern Seram, Maluku, Indonesia

The Triassic aged sandstone from Kanikeh Formation, are spread across Seram, Kesui and Teor Island. The Kanikeh formation has been known as excellent source rock, however, reservoir characteristics of this formation are unknown. This paper aimed at the diagenetic influence on reservoir characteristics was investigated for the Carnian-Norian sandstone sequence in the Eastern Seram. This study comprises two methods which are petrographic analysis and Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS). The petrographic analysis shows the sandstones of the Carnian sequence in the Eastern Seram are lithic wacke, litharenite, arkose. SEM-EDS observations showed that the minerals belong to the silica group derived from the silica minerals, feldspar, clay, and mica minerals. The silica mineral is a quartz grain. The observed feldspar mineral is K-Feldspar in the form of granules. The most dominant clay minerals are smectite, illite, chlorite, kaolinite, halloysite. The observed mica group is muscovite. Diagenetic identified in these sandstones include compaction, cementation by calcite, quartz, clay minerals, iron oxides, dissolution, and alteration of unstable clastic grains, tectonically induced grain fracturing. Unstable clastic grains like feldspars suffered considerable alteration to clay minerals. Based on characteristics of the Diagenetic, the Carnian-Norian sandstones in the study area have a negligible reservoir characteristic.

The interfacial lattice origin of anatase titanium dioxide by hydrothermal architecture

The interfacial microstructure and defects formed during the growth of anatase TiO2 nanocrystalline by oriented attachment (OA) in the process of hydrothermal synthesis have been studied by using high-resolution transmission electron microscope (HRTEM) combined with theoretical simulation. In the {101} crystal plane of anatase TiO2 nanoparticles, it is easy to observe that the nanocrystal grains are attached to form large single crystals or twin structure through high resolution energy filtered images. At the initial stage of synthesis, obvious changes of interplanar spacing between crystal planes, lattice distortions or dislocations could be observed at the interfaces of those unstable grains. The analyses on the Burgers vector loop of dislocations conclude that the Burgers vectors are 1/2〈101〉. Furthermore, the simulated images and the atomic structure models based on the captured images are in perfect agreement with the original ones. Meanwhile, the OA of anatase TiO2 nanoparticles on {112} crystal plane is also observed and twin crystals are formed along 〈311〉 crystallographic zone axis including dislocations at the interfaces. However, due to the damages of electron irradiation, few high-resolution images of these grains have been obtained. Analysis of these interfacial structures provide a new idea to understand the formation and transformation of the microstructure of anatase TiO2 nanoparticles during hydrothermal synthesis.

Role of cementation and compaction in controlling the reservoir quality of the Middle to Late Jurassic Sandstones, Jara dome, Kachchh Basin, western India

Thin‐section petrography, scanning electron microscopy (SEM) and X‐ray diffraction (XRD) analyses are applied to investigate the various diagenetic controls on the reservoir quality of the Ridge and Athleta Sandstone members (Chari Formation) of the Jara dome, Kachchh Basin, western India. The results show that the sandstones of Jara dome are quartz‐arenite to sub‐arkose, medium‐ to coarse‐grained, sub‐angular to sub‐rounded, poorly to moderately well‐sorted and having low‐ to medium‐sphericity. Diagenetic features such as mechanical compaction, pressure solution, cementation, and dissolution of unstable framework grains are analysed to synthesize the reservoir characteristics. The principal cementing materials are carbonates, iron oxides, silica overgrowth, and authigenic clays, mainly chlorite and kaolinite. Compaction and cementation are the controlling factors for the reduction in porosity and permeability of the sandstones. Dissolution of detrital feldspars is the main cause of porosity enhancement in these sandstones. The relationship between the intergranular volume (IGV) and cement volume indicates that cementation had played a more important role in the destruction of primary porosity than that of compaction. On the one hand, the ubiquitous development of carbonate cement advocates the loss in primary porosity at the early stage of diagenesis, while on the other hand it also prevented the further compaction of sandstone, thus preserving unfilled voids as primary porosity. Precipitation of carbonate cement is followed by development of pore‐lining and pore‐filling clayey cements. The pore‐filling kaolinite resulted in a considerable loss of porosity while the pore‐lining chlorite has helped in retaining the porosity by preventing syntaxial silica overgrowths. The reservoir quality of the studied sandstone is reduced by the authigenic clay cements, early carbonate precipitation and silica overgrowths, nonetheless, it is improved by the dissolution and alteration of unstable grains, in addition to partial dissolution of carbonate cements. The potential of the studied sandstone to act as a reservoir is strongly related to sandstone diagenesis.

Visualization of microstructural mechanisms in nanocrystalline ferrite during grinding

Microstructural changes in the near-surface regions of a material determine its mechanical properties and consequently also its tribological behavior. This work is a study of the microstructural development of nanocrystalline ferrite subjected to a grinding process using molecular dynamics simulations. We visualize the work piece by producing various types of computational tomographic sections that are colored according to the grain orientation, the local temperature, the stress in grinding direction, as well as the atomic flow velocities. In particular, we introduce “differential EBSD” tomographs to highlight the changes to the microstructure caused by the grinding process, allowing us to detect even subtle differences in lattice orientation and small distances in grain boundary migration. We use our visualization approach to discuss the acting microstructural mechanisms in a load- and time-resolved fashion, spanning a wide range of grinding conditions from mild to severe. In addition to removed matter, we observe lattice rotation originating at the surface and advancing deeper into the work piece with increasing load, grain growth by grain boundary migration, and the transient formation of unstable small new grains.

Inducing high resonant frequency in exchange-coupled bilayer via fast annealing

The reversed magnetization of NiFe (10 nm)/FeMn (8 nm) bilayers was carried out in an applied external magnetic field (Ha) with the direction opposite to the deposition magnetic field (Hd). The exchange bias field (He), static magnetic anisotropic field (Hksta), and resonant frequency (fr) of the bilayers before and after annealing were characterized. Part of the AFM moments were fixed at an opposite direction to another part by controlling the capacitor voltage. At the critical capacitor voltages 37 and 43 V, the AFM moments and corresponding FM moments in Ha direction were almost the same as in Hd direction, respectively, and were responsible for the lower Hksta and He. This demonstrates that the strongest competitive effect between the AFM/FM exchange coupled with opposite direction, and thus more unstable AFM grains occurred, causing the rotational anisotropy field (Hrot), fr, and the dynamic magnetic anisotropic field (Hkdyn) to reach their maximum.