Scale dependence of textural alignment in shales quantified using electron microscopy

Highlights

• Novel method, combining electron microscopy and image analysis, to quantify clay alignment in sediments.

• In tested samples, alignment increases with clay content.

• Alignment intensity decreases with decreasing field of view.

• To determine alignment in shales, optimal ranges are suggested for imaging fields of view and resolution.

Abstract

Textural alignment is the primary cause of anisotropy associated with the permeability and mechanical and acoustic properties of rocks. Although particle alignment in shales is known to be influenced by burial depth, mineralogical composition, and diagenetic history, understanding how these influence image-based measures of compaction at the micrometer to nanometer scale is not well understood. Here, we use a novel method, which combines scanning electron microscopy and image analysis algorithms, to quantify clay alignment in sedimentary rocks. For a given field of view, the alignment intensity in shales from the Podhale Basin in Poland is positively correlated with clay content based on image analysis. However, our analysis also shows that the intensity is strongly dependent on the spatial scale of imaging: as magnification increases and the field of view in the SEM images decreases, the alignment intensity drops. Therefore, in images with small fields of view, clay alignment is determined by the presence of localized equant silicate mineral grains, which effectively hinder the alignment process at the micrometer scale. By contrast, in images with larger fields of view, alignment intensity is determined by the overall compaction state. We also show that when the field of view is constant, alignment intensity decreases with pixel density, and we suggest a range of operating parameters required for obtaining optimum values for alignment intensity from SEM images of shales.

Introduction

The orientation of clay minerals in sediments can strongly influence rock strength and the way in which fluids and acoustic waves move through geological media (Sivakumar et al., 2002; Sarout and Guéguen, 2008a, 2008b; Kanitpanyacharoen et al., 2012; Ougier-Simonin et al., 2016; Sayers and den Boer, 2016). This is because clay minerals can develop a high degree of preferential alignment, which induces anisotropy in the physical properties of rocks, including their permeability, mechanical properties, and velocity (Bolton et al., 2000; Kwon et al., 2004; Mondol et al., 2007; Bobko and Ulm, 2008; Wenk et al., 2008, 2010; Militzer et al., 2011; Sayers and den Boer, 2018).

Burial depth and mineralogical composition are usually considered to play critical roles in the development of clay mineral alignment in sedimentary rocks (e.g., Bjørkum et al., 1998; Paxton et al., 2002; Dutton and Loucks, 2010). As burial depth increases, so does lithostatic stress, causing both detrital and diagenetic clay minerals to align themselves orthogonally to the direction of primary stress (Marshak and Engelder, 1985; Yang and Aplin, 1998; Kawamura and Ogawa, 2004; Aplin et al., 2006; Pluijm and Marshak, 2010; Piane et al., 2011). However, the presence of equant quartz grains is thought to disrupt this alignment process, so that sediments with high quartz/clay ratios are expected to show a lower degree of alignment than sediments with low ratios (Curtis et al., 1980; Day-Stirrat et al., 2008; Voltolini et al., 2009). Additional factors affecting alignment include bioturbation (e.g., Kanitpanyacharoen et al., 2011), fluid salinity (e.g., Bergsaker et al., 2016), and organic matter (e.g., Vanorio et al., 2008).

While such processes can be understood intuitively, studies aiming to isolate the effects of burial depth and mineralogy on clay alignment have provided conflicting results. Day-Stirrat et al. (2008) used X-ray texture goniometry (XRTG) to demonstrate a relationship between maximal burial depth and clay mineral alignment in mudstones from the Podhale Basin in Poland. However, despite a range of mineral compositions in the samples of that study, no correlation was found between the quartz/clay ratio and clay alignment on a per sample basis, and instead the preferred orientation in these sediments was understood to be driven by the degree of clay mineral diagenesis. By contrast, other studies have reported a strong influence of quartz grains on clay alignment (e.g., Curtis et al., 1980; Louis et al., 2018). Developing new approaches to quantifying clay mineral orientation could provide a way to resolve such apparent disagreements.

In this paper, we use a novel method, which combines scanning electron microscopy and image analysis algorithms, to quantify clay alignment in sedimentary rocks. We use our method to test the way clay alignment is affected by clay content, and to test the dependence of alignment on imaging scale. From our results, we infer that different physical processes are recorded at different spatial scales, and we discuss the implications this has for fabric analysis in shales and other rocks.