Preferred Orientation Of Clay Minerals Research Articles

Anisotropy rock physics model for the Longmaxi shale gas reservoir, Sichuan Basin, China

The preferred orientation of clay minerals dominates the intrinsic anisotropy of shale. We introduce the clay lamination (CL) parameter to the Backus averaging method to describe the intrinsic shale anisotropy induced by the alignment of clay minerals. Then, we perform the inversion of CL and the Thomsen anisotropy parameters. The direct measurement of anisotropy is difficult because of the inability to measure the acoustic velocity in the vertical direction in boreholes and instrument limitations. By introducing the parameter CL, the inversion method provides reasonable estimates of the elastic anisotropy in the Longmaxi shale. The clay content is weakly correlated with the CL parameter. Moreover, the parameter CL is abnormally high at the bottom of the Longmaxi and Wufeng Formations, which are the target reservoirs. Finally, we construct rock physics templates to interpret well logging and reservoir properties.

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

Slickensided shear surfaces are ubiquitous in many fault zones. However the internal structure, the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity, grain-boundary sliding, microfracturing, solution-precipitation and mineral transformation under different conditions are subject of debate.We studied well- preserved core samples from the Main Fault, an up to 3 m wide zone of approximately 10 m offset in the Mont Terri Underground Research Laboratory (CH), a site to evaluate long-term safety of radioactive waste disposal. The drill core breaks easily along many slickensided shear surfaces indicating reverse slip, which form an anastomosing network connected by branch lines.Broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few μm thick shear zone, which acts as a crack guide for fracturing the samples. In this zone, a complex set of processes is inferred, leading to extreme localization of strain, development of strong particle preferred orientation, the formation of nanoparticles, and local precipitation of calcite veins in releasing sections. In lenses between shear zones, homogeneous gouge is formed with a well-developed oblique foliation and removal of calcite grains by pressure solution. We infer that with progressive deformation, the number and density of slickensided shear surfaces increases, generating tectonically derived scaly clay and more homogeneous gouge. In all deformed elements of the Main Fault, porosity is much smaller than in the undeformed Opalinus Clay. An interesting observation is the almost complete absence of cataclastic microstructures. Transmission electron microscopy (TEM) of focused ion beam lamellae of this micron-wide shear zone shows a strong preferred orientation of clay minerals, including nano-sized illite particles. In TEM, the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith.We propose that inter- and transgranular microcracking, pressure solution, clay neoformation, crystal plasticity and grain boundary sliding are important micro-scale processes during the early stages of faulting in Opalinus Clay and thus need to be considered in extrapolating laboratory results to long-term mechanical behavior.

Mineral Preferred Orientation and Microstructure in the Posidonia Shale in Relation to Different Degrees of Thermal Maturity

AbstractThe thermal maturity of samples of the Posidonia Shale collected from the Hils Syncline, northern Germany, varies significantly as a function of location indicating variations in local history. Synchrotron X-ray diffraction was used to document the composition and the preferred orientation of four samples of the Posidonia Shale with different degrees of maturity (0.68–1.45%, Ro) to determine possible effects on diagenesis and preferred orientation. Overall, the degree of preferred orientation of all clay minerals (illite-smectite, illite-mica, and kaolinite) and in all samples is similar, with (001) pole figure maxima ranging from 3.7 to 6.3 multiples of a random distribution (m.r.d.). Calcite displays weak preferred orientation, with c axes perpendicular to the bedding plane (1.1–1.3 m.r.d.). Other constituent phases such as quartz, feldspars, and pyrite have a random orientation distribution. The difference in thermal history, which causes significant changes in the maturity of organic matter, influenced the preferred orientation of clay minerals only marginally as most of the alignment seems to have evolved early in their history. Synchrotron X-ray microtomography was used to characterize the three-dimensional microstructure of a high-maturity sample. Low-density features, including porosity, fractures, and kerogen, were observed to be elongated and aligned roughly parallel to the bedding plane. The volume of low-density features was estimated to be ~7 vol.%, consistent with previous petrophysical measurements of porosity of 8–10 vol.%. Transmission electron microscopy analysis of samples with different degrees of maturity (0.74%Ro and 1.45%Ro) was used to document microstructures at the nanoscale as well as the presence of kerogen. In the high-maturity sample, pores were less abundant while minerals were more deformed as shown by fractured calcite and by kinked and folded illite. Some of the porosity was aligned with clay platelets.

Preferred orientation of phyllosilicates: Effects of composition and stress on resedimented mudstone microfabrics

Development of a preferred orientation of clay minerals is investigated in response to changes in vertical effective stress and composition in resedimented material (Boston Blue Clay). Boston Blue Clay (BBC), an illitic glacio-marine clay composed of 57% clay-sized particles (<2 μm) was resedimented and step-loaded to a vertical effective stress of 0.1 MPa, 1 MPa, 6 MPa, and 10 MPa. These four samples show a small increase in preferred orientation of mica and chlorite with increasing vertical effective stress. Furthermore, pure BBC powder was admixed with silica (silt-sized quartz) in five different ratios of BBC:silica to form sediments with 57%, 52%, 48%, 44%, and 36% < 2 μm particles and were loaded to a maximum vertical effective stress of 2.4 MPa. Mica preferred orientation decreases significantly with decreasing clay content at one stress state. We used and compared three different techniques to characterize and quantify the preferred orientation of phyllosilicates: transmission-mode X-ray texture goniometry, reflection-mode X-ray texture goniometry, and grain and pore networks imaged using secondary and backscattered electron micrographs gathered on argon-ion-milled surfaces. Preferred orientation of the clay minerals shows good agreement between transmission-mode X-ray texture goniometry and reflection-mode X-ray texture goniometry. Mercury porosimetry further illuminates vertical effective stress and compositional controls on microfabric.

Dilatant clayey microstructure in the Barbados décollement zone

Dilatant clayey microstructures in cores from ODP Leg 156, which drilled the basal décollement zone of the northern Barbados accretionary prism, are obtained from freeze-dried samples. The microstructure in the scaly fabric portion shows heterogeneous strain characterized by domains with strong preferred orientation of clay minerals and with sigmoidal, spindle, or tube-shaped coarse pores in fabric domains with random particle orientation. These dilatant microstructures have not been observed in clayey sediments from the Barbados décollement zone or other active shear zones. The dilatancy of the microstructure might be formed under the condition of high pore-fluid pressure in the course of shearing clays and due to tectonic stress.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating. The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.

Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history

ABSTRACTThe degree of preferred orientation of clay minerals in a series of fifteen fine‐grained sediments has been precisely measured by transmitted X‐ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid‐point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine‐scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present‐day muddy sediments. On this basis it is concluded that preferred orientation in clay‐rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.

Clay-Ironstone Concretion Preserving Fabrics Due to Progressive Compaction

Fabrics arising from preferred orientation of clay minerals are presented in a sequence of samples taken from the center of a diagenetic siderite concretion into the enclosing shale sediment. Compactional histories, as deduced directly from estimated porosity values before cementation with siderite, and as deduced from the rotation of mineral flakes to produce the observed fabric, confirm each other. It is concluded that the particular carbonate concretion studied started to grow within the pore space of a flocculated clay sediment soon after deposition. An early period of rapid growth is distinguished from a second, protracted phase which extended effectively until the sediment achieved its present degree of compaction.

Clay Fabric and its Relation to Structural Properties in Mid-Continent Pennsylvanian Sediments

ABSTRACT Many structural properties of argillaceous sediments correlate closely with clay fabric; the degree of preferred orientation of clay particles. Shaly structure is associated with a significant degree of clay particle orientation, massive structure with a more random clay particle arrangement. Fissile structure occurs in shales where there is a high degree of orientation of clay minerals and other flaky constituents. It is best developed in shales containing a high percentage of organic material and where organic material is concentrated in bands. Organic material and in some cases, carbonate minerals, exert a secondary influence on fabric and structural properties. In ancient sediments that contain a significant percentage of organic material, the preferred orientation of clay minerals generally increases as the percentage of organic matter increases. Sediments with more than 10 percent carbonate minerals commonly show a poor degree of clay particle orientation. Little relation appears to exist between fabric or structural properties and clay mineral composition, except perhaps in sediments with appreciable montmorillonite. Poor particle orientation has been shown to exist in sediments in which montmorillonite is the major clay mineral component. No apparent relation was found between fabric or structural properties and total clay content (above 20 to 30 percent), geologic age, or depth of burial. Variations in orientation of clay particles, from essentially parallel to nearly random, occur within a few vertical inches in some thin stratigraphic sections. This, and the close relation of particle orientation to structural properties suggest that the orientation of clay minerals (fabric) in argillaceous sediments is not dominantly controlled by compaction caused by the weight of overlying sediments. These relations appear to reflect physicochemical conditions in the environment of deposition that influence the mode of clay mineral sedimentation, and/or geochemical factors that may promote particle rearrangement soon after deposition.