**1.2 Concept of fabric domain characterization**

The fabric of a rock includes the complete spatial and geometric configuration of all its components and encompasses (micro-) structure and crystallographic preferred orientation of the ensemble of minerals [13]. Microfabric is referred to as arrangement of soil aggregates [14]. The importance of microfabric in the development of sedimentary deposit was suggested by Sorby (1908). Terzaghi (1925) and Casagrande (1932) proposed primitive fabric model to help explain the relationship and sensitivity of cohesive sediment. In Argillaceous sediments (shale), fabric produced by modern sedimentary environment has been recorded by using scanning electron microscope (SEM), transmission electron microscope (TEM), and thin section technique [15]. Electron microscopy observations [16–19] have revealed the presence of multiple particles (domains) as the predominant fundamental particle type rather than the thin, single-plate particles proposed in early models [1]. A domain is considered to have significant structural integrity and to behave in a functional sense as a unit particle for a finite period of time under an applied stress regime. Thus, it is important to note that "a particle" can be defined in terms of its morphology as well as its function [15]. By an online dictionary, a *fabric domain* is the three-dimensional area of the rock occupied by uniform fabric and is delineated by some kind of boundary such as structural or compositional discontinuity (http://encyclopedia2.thefreedictionary.com/fabric+domain).

In sandstones, microfabrics are mixtures of close-packed domains and packing flaws [20]. Close-packed domains contain relatively smaller pores and pore throats, and so have affinity for the wetting phase, while the losses-packed domain have been demonstrated to possess larger pores connected by larger pore throat, for they form pathways for fluid flow and virtually all the non-wetting phases [20]. The closepacked zones have smaller pores and pore throats, and along with microporosity, which retains irreducible water. The aforesaid description of a fabric domain is in sandstone is further extended to share more insights to the present day understanding and challenge to provide technical explanation for reservoir rocks efficiency or under performances that are constantly linked to rock fabrics. It is undisputed that the fabrics of reservoir rock determine storage capacity and fluid displacement within the framework. As rock physical properties *viz.* grain size, shape, sorting, and orientations to mention but few are the essential elements that shape the behaviors and effectiveness of any sandstone reservoir rock.

A fabric domain in this context is considered as an aggregate unit that exhibits a functional physical textural configuration (variable grain shape, orientation, sorting, contact, matrix composition, and compaction) and pore attribute properties (pore volume, pore size, pore throat, pore structure, and pore interconnectivity) that controls fluid flow dynamic and other petrophysical properties (e.g., water saturation, wettability, thermal conductivity). This description of a domain in this context, however, upholds earlier given domain characteristic for both the close-packed and packing flaws [21]. Considering thin-section micrographs for conservative description and characterization will not give a full or wholly view of the entire rock fabric (**Figure 1**), such that some vital information will be lost to understand the macroscopic behavior

#### **Figure 1.**

*Schematic diagram of scan thin section micrographs depicting both homogeneous and heterogeneous fabric domains, while the black square boxes depicted polarized microscope pin-hole captured image for orthodox description and characterization into clasts-supported fabrics.*

or efficiency of the rock. The above schematic diagram (**Figure 1**) reveals two fabric domain types, instead of the conservative characterization as a clasts-supported fabric types on which a detail characterization of fabric physical properties such as grain sorting, packing, contacts can be carried upon for better description.
