**1. Introduction**

The goal of rock fabric characterization is to describe the spatial and geometric distribution of pore attributes as they impact petrophysical parameters such as porosity, permeability, and water saturation. We showed that multiple fabric domains exist in lithofacie at microscale instead of the traditional occurrence of a single fabric in a lithofacie. In this chapter, we expand and showed new insight into rock fabric characterization of *fabric domains* to pore size and porosity distribution, that is, the spatial distribution of pore size and porosity within the different fabric domain types, and show how their distribution can be linked to fabric domain types.

Petrographic investigation is one of the key techniques in sedimentary lithofacie fabric description/characterization, before advanced application of higher-resolution microscopy such as the scanning electron microscope (SEM) or Fourier transform scanning electron microscopy (FESEM) and transmission electron microscope (TEM) to visualize sedimentary rock fabric in their undisturbed state at the nanoscale (micrometer or nanometer) especially for argillaceous sediments [1]. It is essential to

improve on understanding of the rock fabric that controls overall reservoir behavior and to advance by deviating from the orthodox petrographic reservoir description. Petrographic characterization is marginally described as the different grain patterns (*viz*. grain assemblage (packing), grain shapes, minerals, rock fragments, grain orientation, grain sorting) are not completely visualized as a whole under the polarized microscope. Variations in the sandstone grain assemblage vary in sedimentary rock, and so in their petrophysical and engineering properties that potentially differ among overall response to stress, deformation, diagenesis, compaction, fracturing, and fluid flow at microscale level. This chapter presents scanning of petrographic slides (thin section) to have a wholly view or panoramic visualization of grain assemblage and pattern toward identification of different domain of fabric types before the conventional detail petrographic description or characterization. This act will extend knowledge of understanding macroscopic reservoir behavior as to what is responsible at the microscopic scale for petroleum geoscientist and engineers. Also, some technical question will be understood, for example, why do multiple hydraulic fluid units (HFU's) exist in a single sandstone lithofacie? And question such as why a characterized porous geologic material with a high porosity exhibits poor or low fluid flow potential (permeability) even when investigated with a high-pressure gas equipment, for example, helium porosimetry? This chapter relates rock fabric domain to pore sizes and pore volume (porosity) distribution as a most critical to hydrocarbon reservoir development and production efficiency.

### **1.1 Schemes for fabric classification**

Schemes for fabric classification began with carbonate classification by Archie [2] on carbonate reservoir rock in relation to pore size and fluid distribution (petrophysical properties). Choquette and Pray [3] present a classification of porosity which stresses interrelations between porosity and other geologic features in sedimentary carbonates. Rock-fabric and carbonate pore-space classification for reservoir characterization were developed by Lucia [4]. In argillaceous sediments, a quantitative scanning electron microscopic (SEM) methods are used for soil fabric analysis [5] and Sokolov and O'Brien [6] carried out fabric classification system for argillaceous rocks, sediment, and soils based upon the microfabric shown by the scanning electron microscopy (SEM) as it provides a frame of references in describing microfabrics. The nomenclature of pedological features and the various levels of structure and fabric are developed by Brewer and Sleeman [7]. In hard rocks, fabrics is used to characterize sediment transport, magma flow, and dynamo-metamorphic deformation using polar plots [8].

Not until the thin-section technique was accessible, there are no serious study of the fabric and composition of sandstones and utilization of their microscopical characteristics to elucidate the natural history of the rock [9]. Sedimentary rock fabric has been defined, studied, and documented by several renowned sedimentologists [10–12] as a mutual arrangement of grains in sediment including orientation of grains and their packing. Relationships between grains and matrix are examined and characterized as either of clasts-supported or matrix-supported. Earlier sedimentologists [10–12] are of the opinion that the fabric of a lithofacies can be characterized as aforementioned, while emphasizing on other rock fabric physical features such as grain orientation, packing, and sorting to better understand the description of the lithofacie fabric under investigation. The new insights, while upholding the present fabric classification by early sedimentologists, is to view entirely a lithofacie fabric, and to examine and visualize potential multiple fabrics instead of a single

fabric classification and description as earlier sedimentologist outlined in many books [10–12]. This approach in this chapter presents a systematic fabric classification for siliciclastic sandstone thereby maintaining existing classifications by earlier sedimentologists.
