Estimating the elastic properties of mica and clay minerals

Abstract

The orientation distribution of the platelets of the phyllosilicate minerals, including mica clays, significantly affects the elastic velocities and anisotropy of shales. Despite a substantial number of publications on shale elasticity in general and, more specifically, on the elastic properties of clay minerals (phyllosilicates) and clay mineral aggregates with varying degree of the platelet preferred orientation, our understanding of these properties is still rather limited. Elastic properties of mudrocks, influenced by the presence of variously oriented phyllosilicates, such as muscovite, illite, and illite/smectite platelets, can be well-constrained if not accurately estimated by a combination of microstructural observations and semiempirical rock-physics models for conventional shale. The orientation distribution data from published literature are used to relate the maximum platelet pole orientation density to the double median orientation angle [Formula: see text] with respect to the bedding plane, both of which relate to the platelet alignment strength. The [Formula: see text] angle is an equally intuitive orientation distribution parameter, which we rely upon in a heuristic approach to model bedding-normal anisotropic elastic moduli and P- and S-wave velocities of zero-porosity clay (matrix) aggregates as a function of the median orientation angle of clay platelets composing these aggregates. It is demonstrated that zero-porosity illite-dominated aggregates with perfect platelet alignment ([Formula: see text]) should have the platelet-normal elastic moduli of [Formula: see text], [Formula: see text], and a [Formula: see text] ratio of approximately [Formula: see text]. The platelet-normal moduli and mineral density of muscovite are significantly greater than those of illite, whereas the respective moduli of the smectite-dominated clay composite are smaller (and the velocity ratio is greater) than those estimated for illite-dominated clays. Finally, it is also shown that the platelet-normal velocity ratio gradually decreases in the smectite, illite, and muscovite series.

Proposal for a mechanical model of mobile shales

“…5 c). These sketches illustrate common observations in deformed shales, like how: (i) clay platelets orientation with respect to bedding and the void space change progressively during consolidation and deformation (1 to 3) 25 , 66 , (ii) fractures are initiated in domains with organic-matter particles 67 ] and authigenic quartz cements 68 (3), and (iii) the previous fabric in sample is finally disrupted and a new set of anastomosing shear zones and fractures are formed when rock achieves critical state (4). …”

Rock Physics Model of Shale: Predictive Aspect

“…The assumption that the porosity change is solely caused by uniaxial mechanical compaction is reasonable at least in the early stages of mudstone compaction (Vernik & Anantharamu, 2020). However, the assumption is less likely when the shale porosity is reduced to less than 20%–25%, where the processes of chemical diagenesis become more important (Vernik & Anantharamu, 2020). Recrystallization of clay minerals during the late stages of diagenesis can either enhance or reduce the alignment of clay platelets depending on the stress history.…”

“…In practice, the compaction factor α in Equation 4 used in the distribution function in Equation 1 is tuning the distribution function (Figure 2), where Φ 0 (initial porosity) is chosen as a pragmatic tuning factor (with some link to the physical reality). Vernik and Anantharamu (2020) demonstrated that the ODF given by Equation 1 fits measured pole density orientation data on low porosity shale samples reasonably well by using the compaction factor as the single fitting parameter (note that their formula for the ODF is in slightly different form; their formula uses the inverse of the compaction factor as Z and the normalization factor of 1/(8 π 2 ) is not included; however, with the normalization factor, their formula gives the same result as Equation 1). In this study, we adopted simple manual adjustment of the initial porosity when necessary; this process will be described later.…”

“…The effective elastic properties of shale can therefore be calculated from the domain properties, porosity and shale content in this case. The assumption that the porosity change is solely caused by uniaxial mechanical compaction is reasonable at least in the early stages of mudstone compaction (Vernik & Anantharamu, 2020). However, the assumption is less likely when the shale porosity is reduced to less than 20%–25%, where the processes of chemical diagenesis become more important (Vernik & Anantharamu, 2020).…”

Mechanical properties of Indonesian organic-rich shales from the Central Sumatra and North Sumatra Basin, and comparison with US shales

“…Elastic stiffness of shales are strongly correlated to the soft component volume as well as deformational properties, including brittle strengths and ductile constitutive parameters (e.g., Sone and Zoback, 2013a, b). The orientation distribution of clay minerals also determines the degree of mechanical anisotropy (e.g., Sone and Zoback, 2013b;Vernik and Anantharamu, 2020). In addition, the use of nanoindentation in testing shale properties shows that the maturity of organic matter affects the elastic properties of organic-rich shales (Zargari et al, 2016;Abedi et al, 2016;Liu et al, 2018).…”