Mineral Reorientation Research Articles

Effect of mechanical and electro-chemical contacts on the particle orientation of clay minerals during swelling and sedimentation: A DEM simulation

The importance and quality of particle orientation on the performance of clay has been well-documented experimentally. This paper aims to quantitatively evaluate clayey mineral reorientation during swelling/sedimentation according to the balance of mechanical contacts and electrochemical forces. A DEM code has been developed and the behavior of montmorillonite, kaolinite and illite, each having different pore fluid characteristics, has been simulated as for platy-shaped particles. Nonlinear contact law was considered for the induced net force as it relates to the mechanical contacts, Van der Waals attractive force, and diffuse double-layer (DDL) repulsive force. Good compatibility was observed between the DEM simulation and DDL theory. The results showed that the particle weight, magnitude of the repulsive force, and number of DDL contacts were effective parameters controlling particle reorientation. At a low swelling pressure, the particles rotated easily such that an edge-to-face fabric was created and the potential for overlap between particles as well as the mechanical contacts increased. The number of the mechanical contacts versus the time of sedimentation was not linear, but bell-shaped. Kaolinite ranked higher on the DDL repulsive contact list; however, it had a higher deposition rate due to its higher particle weight and lower repulsion force.

Controle litoestrutural e caracterização geofísica do Complexo Alcalino Planalto da Serra, Mato Grosso

O Complexo Alcalino Planalto da Serra está alojado nas rochas metassedimentares do Grupo Cuiabá, na Zona Interna da Faixa Paraguai, sendo composto de rochas ultramáficas alcalinas, como peridotitos, piroxenitos, carbonatitos, apatititos e bebedouritos. Os trabalhos iniciais indicavam formação desse complexo durante o Cretáceo, correlacionada ao magmatismo alcalino associado ao Lineamento Azimute 125º. Novos dados geocronológicos, obtidos pelos métodos de Rb/Sr, Ar-Ar e Sm/Nd, indicaram, no entanto, a idade de formação do complexo alcalino próximo a 600 Ma, remontando assim a um modelo de intrusão de diques e stocks diferente do já descrito. Com o intuito de compreender a relação entre as rochas alcalinas e o Grupo Cuiabá, tanto na relação temporal quanto ao modelo de intrusão, foram utilizados dois métodos de trabalho: o emprego de dados aerogeofísicos, gamaespectometria e magnetometria, e o mapeamento litoestrutural nos sítios de ocorrência das intrusões alcalinas (Chibata e Lau-Massao). O alto contraste entre características magnéticas dos metassedimentos do Grupo Cuiabá e as rochas alcalinas permitiu a identificação e a delimitação de sete anomalias magnéticas relacionadas às intrusões alcalinas do Complexo Planalto da Serra. Baixos teores de potássio nos dados de gamaespectometria permitiram delimitação dos corpos alcalinos. Feições como deformação penetrativa e reorientação mineral, presentes nas bordas de diques alcalinos, corroboram com modelo de intrusão sin a tardi-tectônica. A relação entre as idades, características deformacionais e metamórficas indica que os registros tectônicos verificados nas rochas alcalinas são correlatos aos observados nas rochas metamórficas do Grupo Cuiabá, Faixa Paraguai.

Structural characterization of the metamorphic basement from North Dobrogean Promontory (Romania) using geophysical well logging

Metamorphic processes, leading to mineralogical and structural changes of the rocks in response to physical (pressure, temperature) and chemical conditions, can be associated with the development of sinmetamorphic or post-metamorphic fractures. The post-metamorphic ones are directly related to tectonic stress. In addition, tectonic stress may cause the reorientation of minerals on a direction perpendicular to the stress direction, generating foliations (schistosity). This paper presents a structural analysis of the pre-Alpine metamorphic basement and its relations with the sedimentary cover by using geophysical data recorded in an exploration well located in the North Dobrogean Promontory (Romania). The analysis was based on quad-combo wireline logs, spectral gamma ray, sonic cross-dipole and borehole electrical imaging data. The imaging analysis allowed the identification and characterization of metamorphic foliations, sedimentary bedding, natural fractures and the determination of tectonic stress orientation. A brittleness index was computed by means of elastic parameters derived from density and sonic compressional and shear logs. Also, a fracture intensity characterization by using fracture area per volume of rock was conducted. The integration of geophysical well loging with mud logging and drilling data allowed us to carry out a detailed analysis of the metamorphic basement in the studied area.

Elastic wave velocity evolution of shales deformed under uppermost crustal conditions

AbstractConventional triaxial tests were performed on a series of samples of Tournemire shale along different orientations relative to bedding (0°, 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 80 MPa, and at strain rates ranging between 3 × 10−7 s−1 and 3 × 10−5 s−1. During each experiment, P and S wave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress. This extensive velocity measurement setup allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation. The evolution of elastic anisotropy was quantified using Thomsen's parameters which were directly inverted from measurement of elastic wave velocity. Brittle failure was preceded by a change in P wave anisotropy, due to both crack growth and mineral reorientation. Anisotropy variations were largest for samples deformed perpendicular to bedding, at the onset of rupture. Anisotropy reversal was observed at the highest confining pressures. For samples deformed parallel to bedding, the P wave anisotropy change is weaker.

Depth variations of P-wave azimuthal anisotropy beneath Mainland China.

A high-resolution model of P-wave anisotropic tomography beneath Mainland China and surrounding regions is determined using a large number of arrival-time data recorded by the China seismic network, the International Seismological Centre (ISC) and temporary seismic arrays deployed on the Tibetan Plateau. Our results provide important new insights into the subducted Indian plate and mantle dynamics in East Asia. Our tomographic images show that the northern limit of the subducting Indian plate has reached the Jinsha River suture in eastern Tibet. A striking variation of P-wave azimuthal anisotropy is revealed in the Indian lithosphere: the fast velocity direction (FVD) is NE-SW beneath the Indian continent, whereas the FVD is arc parallel beneath the Himalaya and Tibetan Plateau, which may reflect re-orientation of minerals due to lithospheric extension, in response to the India-Eurasia collision. There are multiple anisotropic layers with variable FVDs in some parts of the Tibetan Plateau, which may be the cause of the dominant null splitting measurements in these regions. A circular pattern of FVDs is revealed around the Philippine Sea slab beneath SE China, which reflects asthenospheric strain caused by toroidal mantle flow around the edge of the subducting slab.

Mudstone diversity: Origin and implications for source, seal, and reservoir properties in petroleum systems

Mudstone is the most abundant sedimentary rock and variously acts as sources, seals, and shale gas reservoirs in petroleum systems. Many important physicochemical properties of mudstones are strongly influenced by the mineralogy and size of deposited grains, and by diagenetic changes (precompaction and postcompaction); these are commonly predictable. The diverse composition of mudstones reflects input and hydrodynamic segregation of detrital materials to basins, primary production within basins, and diagenetic processes (both precipitation and dissolution) in the sediment. High-magnification observations both in modern and ancient sediments demonstrate that mudstones are texturally and mineralogically heterogeneous; this variability is not always readily apparent. Although some mud is indeed deposited by suspension settling out of low-energy buoyant plumes, textural analyses reveal that it is commonly dispersed by a combination of waves, gravity-driven processes, and unidirectional currents driven variously by storms and tides. Such dispersal mechanisms mean that muddy successions are typically organized into packages that can be interpreted using sequence stratigraphy. Early bioturbation homogenizes mud, whereas early chemical diagenesis can result in highly cemented zones developing, especially at stratal surfaces. The nature of deeper burial diagenesis, which involves compaction, mineral dissolution, recrystallization, mineral reorientation and lithification, and petroleum generation, is preconditioned by depositional and early diagenetic characteristics of the mud. Although the petrophysical properties of homogeneous mudstones are reasonably well known, the quantitative implications of heterogeneity for petroleum expulsion, retention, petroleum migration, seal capacity, acoustic anisotropy, and identification of shale gas reservoir sweet spots are essentially unexplored. Future work should seek to redress this position.

Seismic anisotropy near source region in subduction zones around Japan

Broad-band seismographs have been distributed widely over the Japanese islands and have provided us with high-quality digital waveform data in recent years. We have investigated splitting of ScS waves from deep earthquakes in the Kuril and Izu-Bonin subduction zones recorded on the STS seismograms. We used ScS waves from 13 events observed in Japan and recognized shear-wave splitting which is mainly caused by anisotropy in the lithospheric slab. The anisotropy was confirmed around the source region by comparing ScS-wave splitting from deep earthquakes in different regions and at different depths with S-wave splitting from deep events just beneath central Japan. We have found that the anisotropic regions exist within the subducting slab, although they are distributed locally in and around the source, possibly as patches with a diameter of 100 km or less. Discrepancy in the direction of the fast polarized shear waves between two nearby events is observed in the Izu-Bonin subduction zone: the splitting of the shallower event (289 km) shows its fast polarization direction to be parallel to the fossil motion of the Pacific plate, but that of the deeper event (361 km) immediately beneath is parallel to the current motion. These observations suggest that the change in shear-wave splitting with depth comes from the modification of preferred orientation of minerals depending on depth. We propose in this study that the reorientation of minerals (olivine) occurs owing to the change in physical conditions associated with the phase transformation of olivine (α phase) to modified spinel (β phase).

Mineral Reorientation and Slaty Cleavage in the Martinsburg Formation, Lehigh Gap, Pennsylvania

The outcrop of the Martinsburg and Shawangunk Formations at Lehigh Gap, Pennsylvania, is an important locality for studying the development of slaty cleavage because a gradual transition from shale to slate exists within a distance of 50 m. Field, thin-section, and x-ray preferred-orientation studies show that in the uncleaved rock in the Martinsburg Formation near the contact with the overlying Shawangunk Formation, white mica is parallel to bedding whereas chlorite is about 20° off bedding. In the transition region farther from the contact, a bimodal orientation results from mica becoming oriented parallel to an incipient cleavage, nearly perpendicular to bedding, whereas chlorite retains its 20° off-bedding orientation. Farther from the contact, where the cleavage is best developed, the chlorite also is reoriented. The distinct difference in the locations at which these two platy minerals are reoriented argues strongly against their preferred orientations being produced by mechanical rotation of pre-existing tabular grains, whether during tectonic dewatering or at high temperatures. Petrographic observations of crystals parallel to cleavage that transect crystals parallel to bedding strengthen this argument. The preferred orientation associated with the cleavage appears to be a result of some solution and recrystallization process that is poorly understood. This study makes it clear that caution is necessary in using models of mechanical rotation in relating strain to preferred orientations. The fact that other recent work suggests that such models are useful may mean either that slate may form in many different ways or that the fit of such models is fortuitous.