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Mudrock Components and the Genesis of Bulk Rock Properties: Review of Current Advances and Challenges

Kitty L. Milliken1 and Nicholas W. Hayman2

1 John A. and Katherine G. Jackson School of Geosciences, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX, USA

2 John A. and Katherine G. Jackson School of Geosciences, Institute for Geophysics, The University of Texas at Austin, Austin, TX, USA

ABSTRACT

Fine-grained sediment (mud) and lithified equivalents (mudrock, mudstone, and shale) contain components similar to ones in coarser sedimentary materials, albeit of such small size that high-resolution imaging is required to observe them. Such imaging reveals that fine-grained sedimentary rocks display diversities of grains, pores, and diagenetic features that actually exceed the variations of components in common sandstones and limestones. Mudrock diversity reflects the extraordinary range of grain and pore sizes, which extend from detrital grains and authigenic crystals in the <1-100?µm fraction to the nanomaterials (crystals and pores) in the matrix surrounding the silt-size fraction. Prediction of bulk-property evolution in fine-grained materials lags current process understanding in coarse materials but a view is emerging that while there are similarities, there are also contrasts between the responses of coarse and fine materials to changing conditions in the subsurface. Mechanical and chemical processes that operate on submicrometer pores and crystals very likely proceed to different limits, at different rates, and even by entirely different mechanisms than do comparable processes in coarser materials. This paper reviews current knowledge about mudrock components, and explores some of the gaps that exist in our understanding of microscale properties and processes in Earth's most abundant sedimentary material.

1.1. INTRODUCTION

Mudrocks, the overall class of fine-grained sedimentary rocks, contain grain assemblages composed by weight or volume of >50% particles that are <62.5?µm. Mudrocks include fissile shales, nonfissile mudstones, siltstones, and even claystones; bulk mineralogies of mudrocks span nearly the entire range of compositions displayed by sandstones and limestones. At the scale of a core or a hand specimen, it is easy to gain the impression that many mudrocks are homogeneous and uniform (Fig. 1.1). Data at both small and large scales challenge this view (e.g., Ilgen et al., 2017; Milliken and Curtis, 2016). For example, permeability in shale varies over 13-15 orders of magnitude, perhaps the largest range of any measured natural property (Bryant, 2002; Dewhurst et al., 1999). Mechanical behavior in fine-grained sediments also varies tremendously as the combined effects of compaction and cementation drive lithification of mud to mudstone (Storvoll and Bjorlykke, 2004; Storvoll et al., 2005), ultimately producing rocks with bulk mechanical properties that approach those of solid crystals of quartz or feldspar (Kumar et al., 2015). The wide range of mudrock properties is expressed in the spatial and temporal complexity of hydrocarbon production from them; 30-year economic tiers in the Barnett Shale of the Fort Worth Basin vary around eightfold in production and more productive wells show strong spatial clustering (Gulen et al., 2013).

Figure 1.1 Homogeneous black shale. Although subtle variations in texture and color can be discerned in this short section of black shale, describing cores of this type from visual description alone is challenging.

Microscopic examination of mudrocks readily shows that their profound heterogeneities are rooted in large variations in the abundance and sizes of basic mudrock components such as grains, cements, and pores. Imaging based on field-emission scanning electron microscopy, for example, including high-resolution secondary electron imaging, elemental X-ray mapping, and scanned cathdoluminescence imaging have expanded the scale at which mudrock components can be imaged well into the submicrometer range, opening new potentials for both qualitative and quantitative characterization. Newer methods for the preparation of flat surfaces with minimal mechanical damage (specifically ion-milling, by broad-beam and focused-beam methodologies) have figured prominently in imaging advances (Curtis et al., 2011; Desbois et al., 2009; Loucks et al., 2009; Milliken and Curtis, 2016). Several advances have also had significant impact on the examination of mudrock components at the submicron scale. These include: (i) advances in SEM automation (Goergen et al., 2014; Saif et al., 2017), (ii) imaging with X-ray tomography (Dewers et al., 2012), (iii) and neutron scattering (Radlinski et al., 2004a), (iv) novel applications of rock-mechanics experimentation and porosity and permeability analysis (Bhandari et al., 2015; Daigle et al., 2017; Yang et al., 2017), and (v) machine learning (Hutton, 1994; Jianhua et al., 2015; Mlynarczuk and Skiba, 2017; Petrelli and Perugini, 2016; Xiao and Daigle, 2018).

In the following sections, we describe some of the component arrangements (fabrics) documented at the micro to nanometer scale in mudrocks. Components are the basic categories of solids and pores that compose mudrocks at the microscale, and thus we take this systematic "component" approach to provide a useful review of basic mudrock fabrics for the community of researchers who do not routinely make use of petrographic inspection in their workflows. Geophysicists, petrophysicists, and flow modelers are among the groups who increasingly seek to link bulk properties to the fundamental causes of mudrock heterogeneity. This paper illustrates for nonpetrographers the general realities of mudrock heterogeneity at the scale of grains and pores and describes some of the petrographic insights on small-scale processes that control the evolution of properties such as porosity, permeability, seismic velocity, and mechanical moduli. This paper also is relevant to those who study pelitic metamorphic rock systems, as the diagenetic evolution that mudrocks undergo up to burial depths and temperatures amenable to petroleum production (to around 200?°C) set the stage for higher-grade processes during metamorphism.

1.2. COMPONENTS IN MUDROCKS

Table 1.1 summarizes 18 mudrock units we have studied and notes age, tectonic setting, and grain assemblage composition. Mudrock components are broadly similar to component categories known in sandstones and limestones (Table 1.2). A key point is that particular minerals (e.g., quartz and clays), and also the organic and other nonsolid rock components (pores), may appear in multiple component categories, for example, silt grains, clay-size matrix grains, cements, and grain replacements, in any given rock. Note that we use the term "grain" as synonymous with a "particle" that existed in the sedimentary environment. Such primary particles include polycrystalline and polymineralogic aggregates and lithic grains and also submicron particles that are challenging to image. At the highest magnifications it is not trivial to determine if a solid component is a grain. The multiscale, multicomponent nature of mudrocks means that they cannot be fully assessed by bulk analytical methods; petrographic inspection is required. Fortunately, atlas resources are available that provide instruction on petrographic identification of sedimentary rock components, including muds and mudstones (Adams and MacKenzie, 1998; Marsaglia et al., 2013, 2015; Milliken and Choh, 2011; Milliken et al., 2007b; Scholle, 1979; Scholle and Ulmer-Scholle, 2003; Ulmer-Scholle et al., 2014). We describe some broad characterizations of mudrock components below.

Table 1.1 List of Mudrock Units Studied by the Authors.