Characteristics of mixed-layer smectite/illite density separates during burial diagenesis

Oblique dextral motion on the central Alpine Fault in the last circa 5 Ma has exhumed garnetoligoclase facies mylonitic fault rocks from 35 km depth. During exhumation, deformation, accompanied by fluid infiltration, has generated complex lithological variations in fault-related rocks retrieved during Deep Fault Drilling Project (DFDP-1) drilling at Gaunt Creek, South Island, New Zealand. Lithological, geochemical, and mineralogical results reveal that the fault comprises a core of highly comminuted cataclasites and fault gouges bounded by a damage zone containing cataclasites, protocataclasites, and fractured mylonites. The fault core-alteration zone extends 20-30 m from the principal slip zone (PSZ) and is characterized by alteration of primary phases to phyllosilicate minerals. Alteration associated with distinct mineral phases occurred proximal the brittle-to-plastic transition (T 300-4008C, 6-10 km depth) and at shallow depths (T 5 20-1508C, 0-3 km depth). Within the fault core-alteration zone, fractures have been sealed by precipitation of calcite and phyllosilicates. This sealing has decreased fault normal permeability and increased rock mass competency, potentially promoting interseismic strain buildup.

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“…[]. Note the density of phyllosilicate minerals can vary with cation ratio and interlayer hydration state [e.g., Totten et al ., ].…”

Section: Discussionmentioning

confidence: 99%

Geochemical and microstructural evidence for interseismic changes in fault zone permeability and strength, Alpine Fault, New Zealand

Boulton

Menzies

Toy

et al. 2017

Geochem Geophys Geosyst

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“…As can be seen, smectite and illite are the main components at wells 1 and 3, respectively. The elastic properties of smectite and illite are given in Table 1 (Carcione, 2000;Totten et al, 2002;Carcione et al, 2011;Pinna et al, 2011). Due to compaction effects (closing of micropores), we assume that the density of kerogen is 1.2 g∕cm 3 at well 1 and 1.3 g∕cm 3 at well 3 (a simple calculation indicates a 1/13 volume reduction).…”

Section: Model Calibrationmentioning

confidence: 99%

Rock-physics templates for clay-rich source rocks

Carcione

Avseth

2015

GEOPHYSICS

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Shale source rocks are composed of various minerals, mainly smectite and illite, depending on the burial depth, and they can be described as transversely isotropic media. The "pore space" may contain kerogen, water, oil, and gas determined by the in situ conditions. A petroelastic description is based on the following: Smectite-illite transformation as a function of depth is described by a fifth-order kinetic reaction. Backus averaging to "mix" isotropic smectite and anisotropic illite is then used to obtain the elasticity constants of the mineral composing the frame. Porosity is obtained from density, and water is part of the mineral, whose elasticity constants are obtained from Gassmann equations. Oil and gas generated from kerogen are assumed to saturate the kerogen phase. The bulk modulus of the oil-gas mixture is calculated by a mesoscopic-loss model, and the stiffnesses of the kerogen/fluid mixture are obtained with the Kuster and Toksöz model, assuming that the fluid is included in a kerogen matrix. Two models are considered to obtain the seismic velocities of the shale, namely, Backus averaging and the Gassmann equation generalized to the anisotropic case with a solid pore infill. We built rock-physics templates (RPTs) containing only kerogen (immature) and kerogen plus hydrocarbons (mature). Pore-pressure effects were modeled and used as templates. We considered the Kimmeridge Shale at different depths. To model kerogen-oil and oil-gas conversions, we assumed a basin-evolution model with a constant sedimentation rate, geothermal gradient, and a first-order kinetic (Arrhenius) reaction. The detection of the hydrocarbons was investigated from RPTs, built with wave velocities, impedances, Lamé constants, density, Poisson ratio, Young modulus, and anisotropy parameters for varying kerogen content, fluid saturations, and pore pressure. Moreover, the amplitude variation with offset intercept and gradients were computed, corresponding to the seismic response of a source rock layer for varying kerogen content and fluid saturation.

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“…X-ray CT investigations of geomaterials often focus on elastic properties, porosity and permeability, as well as wettability and/or particle packing (e.g., Song et al, 2001;Totten et al, 2002;Naveed et al, 2013;Yang et al, 2017;Glatz et al, 2018;Ju et al, 2018;Marone et al, 2020;Chenlu et al, 2021). Since the physical and mechanical properties of an object are dictated by the nature and distribution of its constituents, it is necessary to be able to differentiate between them.…”

Section: Grating-based Synchrotron Radiation Computed Tomography On A...mentioning

confidence: 99%

Grating-Based X-Ray Computed Tomography for Improved Contrast on a Heterogeneous Geomaterial

et al. 2022

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Phase-contrast X-ray imaging has shown its potential to capture at micrometric scale and in three dimensions (3D), the structure and deformation of poorly-absorbing soft materials such as polymers and biological structures. When composed of constituents that attenuate X-rays differently, heterogeneous hard materials are often well resolved in 3D using absorption-based X-ray computed tomography (CT) techniques. As a result, phase-contrast techniques have been less frequently used to image such materials. However, many geomaterials contain similarly X-ray attenuating constituents, thereby complicating the use of absorption-based X-ray CT methods. Here we present the innovative use of grating-based synchrotron radiation computed tomography, a phase-sensitive technique, in order to better identify the distribution of constituents within geomaterials. We show that this approach enhances the contrast between similarly X-ray absorbing constituents, and can be used to identify spatially small structures such as pores or mineral grains that are typically poorly resolved with conventional absorption-based X-ray CT.

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Characteristics of mixed-layer smectite/illite density separates during burial diagenesis

Cited by 20 publications

References 19 publications

Geochemical and microstructural evidence for interseismic changes in fault zone permeability and strength, Alpine Fault, New Zealand

Geochemical and microstructural evidence for interseismic changes in fault zone permeability and strength, Alpine Fault, New Zealand

Rock-physics templates for clay-rich source rocks

Grating-Based X-Ray Computed Tomography for Improved Contrast on a Heterogeneous Geomaterial

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