Sedimentary Rock Fabric Characterization

Abstract

The goal of rock fabric characterization is to describe the spatial and geometric distribution of pore attributes as they impact petrophysical parameter variations such as porosity, permeability, and water saturation. Pore attributes such as pore size and pore volumes are critical petrophysical properties as they change and vary over a short distance or side-by-side, and determine the exploitable capacity of sedimentary reservoir rocks. A proposed new insight is in the systematic characterization of fabrics by the combination of both digital petrographic scanning and conservative petrographic description, as we show that multiple fabrics occur in a single lithofacie in the form of a fabric domain. Characterization shows that these domain types determine the spatial and geometric distribution of the variable pore sizes and volume (porosity) within a certain lithofacie fabric. Based on this, we also infer that these fabric domain types are responsible for the multiple occurrences of hydraulic fluid units (HFU’s) and anomalous porosity-permeability relationships in siliciclastic sandstone sedimentary rocks since no long-range process aligns the microscopic internal fabric or microfabric architecture among grain aggregates in sedimentary rock.

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, 11, 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, 11, 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, 11, 12]. This approach in this chapter presents a systematic fabric classification for siliciclastic sandstone thereby maintaining existing classifications by earlier sedimentologists.

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, 17, 18, 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 close-packed 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 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.

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.

1.2.1 Fabric domain mapping

The proposed mapping of fabric domains includes scanning of prepared thin-section micrograph at a resolution scale of about 5 mm to have clearer image resolution (Figures 2–6). This will enable to visualize completely all the different grain assemblages or colonies at a single glance. It is also achievable when a thin-section micrograph is systematically gridded to capture images under a polarized microscope, but will be cumbersome when attempting to join all captured micrograph images at 500-μm magnification into a single micrograph due to edges-effect difference as can be easily be achieved using a high-resolution scanning machine (Figure 7). The scan micrograph image carefully visualizes all grain assemblages or pattern nature with reference to the dominant grain packing, contacts, sorting, orientation, matrix distribution, compaction and intergranular pore-spaces will be characterized into fabric-type domain (Figures 2–6).

Figure 2.

Showing fabric domain types present in the reservoir rock sedimentary facie; massive coarse grained sandstone (MCGS) from Offshore, Malaysia. In yellow dotted-line is the Random domain, in red dotted-lines in the Clasts-supported domain and in blue dotted-line is the Matrix- supported domain.

Figure 3.

Showing fabric domain types present in the reservoir rock sedimentary facie; massive Burrowed medium grained sandstone (BMGS) from the Offshore, Malaysia. In yellow dotted-line is the Laminar/matrix supported domain, in red dotted-lines in the Clasts-supported domain, in black dotted-line is the fractured-dominated domain and in purple dotted-line is the random domain.

Figure 4.

Showing fabric domain types present in the reservoir rock sedimentary facie; parallel laminated friable fine grained sandstone (PLFFGS) from Offshore, Malaysia. In yellow dotted-line is the Matrix supported domain, in red dotted-lines in the Clasts-supported/Fractured-dominated domain, and in purple dotted-line is the random domain.

Figure 5.

Showing fabric domain types present in the reservoir rock sedimentary facie faintly laminated fine grained sandstone (FLFGS) from, Offshore, Malaysia. In yellow dotted-line is the Random/fractured-dominated domain, in red dotted-lines in the matrix-supported domain and in deep blue dotted-line is the Clasts-supported domain.

Figure 6.

Showing fabric domain types present in the reservoir rock sedimentary facie massive very fine-grained sandstone (MVFGS) from Offshore, Malaysia. It exhibited only two (2) fabric type Matrix-supported and Fractured-dominated domains.

Figure 7.

Schematic diagram showing study area under the microscope and scan area for fabric domain type mapping.

In a practical approach to achieve mapping of the variable domains in a sedimentary facie, each representative micrograph (s) is scanned using a high-resolution scanning machine as earlier mentioned. The output gives a panoramic view or coverage of the entire aggregates of geometric and spatial configuration on the micrograph; thereby, a careful observation of the different domain fabric type will be mapped out clearly (Figures 2–6).

1.2.2 Fabric domain types

The reservoir sandstone facies of the West Baram Delta as an example is characterized into five fabric domains as in subsection 1.2.1 based on the dominant textural features (grain size, shape, grain sorting, grain-contact, grain orientation, pore space, and matrix content) namely the clasts-support, random, matrix-support, and laminar and fractured-dominated fabrics. The clasts-support (types 1 & 2), matrix-support (types 1 & 2), and random and fractured-dominated fabric (types 1 & 2). Subtype (type 1 & 2) for the clasts-support, matrix support, random and fractured-dominated are grouped based on their variances in percentage composition of matrix content (Table 1).

Lithofacies	Fabric types	Fabric type characteristics
		Grain size (%)	Grain shape	Grain sorting	Grain packing	Matrix (%)	Grain composition	Porosity
MCGS (15)	Clasts-supported (Type 1)	48.87	Angular-sub rounded	Moderate	Point, moderate long/concavo convex	Minor@7.78	Moderate	High
	Random	48.18–51-52	Angular-sub rounded	Poorly-moderate	Point, moderate long/concavo convex	Minor@6.10	Moderate	High
	Matrix-supported	48.32	sub angular-sub rounded	Moderate	Point, moderate long/concavo convex	Moderate @37.70	Very low	Very low
BMGS (24)	Laminar	Medium @ 32.30–53.20	sub angular-sub rounded	Moderate	Long & concavo-convex	High @42.30	Moderate	Moderate
	Clasts-supported (Type 2)	Medium @ 53.10	Moderate angular-sub rounded	Moderate - poorly	Float & point	Moderate @38.10	Moderate	Moderate
	Fractured-dominated (Type 1)	Medium @ 48.02	Moderate angular to Sub rounded	Well- sorted	Long, float & point	Minor @9.10	Minor	High
	Random	Medium @ 54.22	Sub rounded	Moderate – well- sorted	Long, float & point	Minor @5.60	Moderate	Moderate high
FLFGS (36)	Fractured-dominated (Type 2)	Fine @ 39.80	Sub rounded	Poorly	Long, float & point	Moderate @ 38.40	moderate	Moderate High
	Random	Fine @ 52.30	Sub rounded-rounded	Moderate - poorly	Long, float & point	Minor @56.60	low	high
	Clasts-supported (Type 2)	Fine – medium @ 37.60	Moderate angular to sub rounded	Poorly	Float, long – concavo convex	High @42.10	Moderate	High
	Matrix-supported	Fine medium @48.30	sub angular-sub rounded >100^o	poorly	Float, long – concavo convex	High @35.10	Moderate	Very High
	Laminar	Fine coarse @53.20	Sub angular - sub rounded	Well-sorted	Point, long-concavo-convex	Moderate@23.10	Moderate	low
PLFFGS (37)	Clasts-supported	Fine @33.20	Angular-sub angular	Very poorly	Long-concavo convex	Moderate@43.30	Moderate	Low
	Matrix-supported	Fine @ 26.12	Angular-sub angular	Very poorly	Float-concavo convex	High 47.80	Moderate	Very low
	Fractured-dominated	Fine @43.30	Sub rounded - rounded	Moderate	Float-point	High	Moderate	Moderate High
	Random	Fine @45.30	Angular-sub angular	Moderate	Point, long-concavo-convex	High	Moderate	Very low
MVFGS (20)	Matrix-supported	Fine @42.20	Angular-sub angular	Moderate	Point, long-concavo-convex	High	Moderate	Moderate
	Fractured-dominated	Fine medium @53.28	Angular - sub angular	Moderate	Float-point	Moderate	Moderate	High

Table 1.

Fabric types compositional characteristics in sandstone from Offshore, Malaysia.

From the above fabric domain-type mapping, it indicates that a single thin micrograph is not ideal to fully represent a single rock fabric type. This being that there are variable and discrete variations in physical grain assemblages and perhaps their properties within the micrograph. Thus, changes and variations will occur within the developed different fabric domains across the entire micrographs. It occurs that the multiple fabric domain type facilitates existence of multiple hydraulic fluid units (HFU’s) [22] orchestrated mainly by the variable difference or in some case similar [2] in pore size volume and pore throat in each of the different fabric domain types. It also shows that most abundant fabric domain type will be responsible to control the entire fluid flow dynamic at a microscopic scale [22] and thus their efficiency and responses of the rock to stress, pressure, and irreducible water saturation [21]. Each of the mapped fabric domain types is magnified at 500 μm (Figure 8) for detail petrographic descriptions, and the summaries are presented in Table 1.

Figure 8.

Showing the mapped different fabric domain types at 100 μm and 500 μm magnification (a) Clasts-supported grains @ depth 987.3 m; (b) Random domain grains are pushed apart with minor matrix, sample @ depth 1943 m (c) Laminar @ 972.6 m (d) Matrix-support, matrix filling intergranular pore space; sample collected @ depth 1509.1 m (c-d) Fractured - dominated fabric (type 1) @ 2364.8 m & (type 2), grain-matrix are fracture for pore spaces @ depth of 1882.2 m.

Table 1 present fabric domain-type compositional characteristics as studied at magnified 500-μm resolution. The percentage composition of each of the physical textural characteristics is given in the table. It is observable within the table instances of subtypes 1 and 2. These represent a subfabric domain of same fabric-type domain and thus will vary their petrophysical parameters. In both instances, the type 1 represent the domain with lesser matrix composition (<10%), while the type 2 is above 10%. These differences required as already known from the literature that matrix content fill intergranular pore space and will potentially have impact on pore volume distribution, ability to transmit fluid and detainments of irreducible water saturation within the domain of higher matrix composition. It is clear that a particular lithofacies could exhibit a multiple or more than one fabric-type domain(s).

1.3 Fabric domain and porosity distribution

Pore volume is a critical physical parameter that changes and varies within a short distance or space in a porous medium, associated in this context with the different and variable fabric domain type present side-by-side. In order to relate fabric domain type to pore size or porosity distribution, it is important to determine which among the fabric domain exhibits abundant pore space volume within reservoir rock. It is logical that such fabric domain type would have a significant impact on fluid dynamic and retained irreducible water saturation at microscale. Each fabric domain types are expected to have different pore space volume or porosity, and thus pore size and pore throats [21]. From the five (5) sandstone lithofacies where the fabric domain types were mapped (Figures 2–6) and summarized in Table 1, pore volume or porosity in each of the domain types is quantified and compared with similar domain (in either subclass 1 or 2) in another sandstone lithofacie to infer porosity range that can be associated with each of the domain type. In doing so, predominantly considering the most impactful fabric domain (positively or negatively), one can predict expected macroscopic behavior of the reservoir rock in terms of its irreducible water saturation, potential production rate, and total reservoir efficiency.

Generally, in the clasts-support fabric domain across the three (3) different sandstone lithofacies (Figure 8a–c), porosity distribution varies in Figure 8a between 10% and 32%, 20% and 34% in Figure 8b, and 21–25% in Figure 8c, respectively. Porosity in this fabric domain for the subtype 1 is mainly intergranular porosities that vary from 25 to 34%, because it shows moderate- to well-sorted grain sorting, slight compaction, grain-contacts that are point, long, and moderate in concavo-convex, with matrix content less than 10% composition. While in the subclass type 2, characterized by the moderate high-matrix content above 10% obliterating the intergranular pore space, porosity varies from 10 to 21%. This domain will develop smaller pore sizes and pore throats, and will retain high irreducible water saturation, and subtype 1 will develop moderate- to high-conducive pathways for fluid flow due to their larger pore spaces, pore throat, and pore volume potentials [21].

In the random fabric domain (Figure 8a–d) characterized with abundance of float and point grain contact, minor to moderate compaction with variable percentage of matrix content, porosity distribution in Figure 8a is 11–27%, and in Figure 7b is between 21% and 33%, also in Figure 8c the porosity varies from 18 to 33%, and from 21 to 24% in Figure 8d within the four (4) different sandstone lithofacies. This is associated with the favorable physical rock parameters that enhance the development of potentially more larger pore sizes, pore throats with less matrix composition to retain irreducible water saturation in them, while segment with low porosity values are associated with the slight increase in their matrix composition within this domain as visible in above-discussed domain.

Matrix-supported domain is the most prevalent domain type across the five (5) different lithofacies, because influx of sediments during deposition cannot be controlled or restricted. Porosity distribution within the domain is generally lower compared with aforesaid in the group (Figure 8a–e). Intergranular pore spaces are obliterated and reduced by variation in their matrix composition within this domain. Porosity within the domain varies in order of 10–28%, 17–20%, 15–22%, 16–24%, 7–27%, respectively (Figure 8a–e). Lower porosity values in this domain that are between 7% and 16% are from the subdomain that exhibit matrix content above 40%, while those that have shown porosity that varies from 20 to 28% are subdomain with matrix composition above 40% both characterized as subdomain types 1 and 2, respectively. Variations in composition of the matrix filling intergranular pore spaces determine distribution of pore volume present within the domain types.

In the laminar domain associated with laminated sandstone lithofacie, fine rock fragments somewhat align forming a visible lamination at microscale. In general, porosity varies in order of 18%–28%, 17%–18%, and 16%–28%, respectively (Figure 7b–d). Regions characterized with moderately low matrix composition reveals porosity values between 18% and 28%, and the grains aligning resulted into the development of favorable grain packing and contacts such as the point, and long and moderate concavo-convex that enhances moderate occurrence of intergranular pore spaces within the domain, while that with a moderate increase in matrix content exhibits porosity variation from 16 to 17%. In this domain considering their nearness in porosity, it could be considered as single domain type with no subdomain types despite their variations in the matrix composition. Other textural characteristics are presented in Table 1.

The fractured-dominated domain is visible in sandstone lithofacie associated with the occurrence of matrix-fracture porosity [23, 24] during rapid sedimentation [25] probably occurring during a regime of an abnormally high pore-pressure that exceeded hydrostatic pressure at a given depth [26] in the delta. Other textural characteristics (Table 1) contribute to development variable occurrence and distribution of porosity within the domain. Porosity distribution in the domain is in order of 22–33%, 18–26%, 22–33%, and 19–29% (Figure 8b–e) across studied sandstone lithofacies.

It has indicated that the clasts-supported and random domain types as the most prevalent fabric in reservoir sandstone associated with a high percentage composition of intergranular porosities or pore volumes considering favorable fabric physical parameters such low matrix content, grain sorting, packing/contacts, and moderate compaction contributing to abundance of intergranular porosities. The laminar and matrix-supported domain exhibits next moderate porosities as a result of a high matrix composition filling intergranular pore spaces. At a point, these variable domain types exhibit close to similar volume of porosity [2] within them, but varies at most instance. The fabric domain with the highest pore volume is the fracture-dominated domain that exhibits high occurrence of matrix-fracture pores associated with overpressure at depth as a result of a rapid sedimentation in some sedimentary basin. Furthermore, a lithofacie that exhibit poor intergranular pore-space distribution, termed poorly porous, may experience high permeability values and sudden decline in reservoir pressure due to the occurrence and abundance of the fractured-dominated domains that are not fully noted during petrographic description using polarized microscope only. But if such micrographs are scanned as proposed here, it will be deduce that such reservoir behavior was in response to possible abundance of the fracture-dominated domain presence within the lithofacies.

1.4 Fabric domain and pore size distribution

Pore diameter or pore size is another important reservoir parameter as the width of pores where hydrocarbon is stored. Pore sizes and their interconnectivity determine the rate of fluid transmitivity (permeability) across the fabric as their sizes also vary across the fabric domain types. Some domain may exhibit abundance of large pore diameters [21] especially those with moderate matrix composition, mechanical compaction, and favorable physical rock parameter such as grain sorting, grain size, packing/contacts, and moderate diagenetic alteration. For instance, where a particular domain types exhibit increase in aforementioned physical rock parameters, pore size across that domain will be greatly affected through complete occlusion of intergranular pore space or reduction in their pore size diameter across entire rock fabrics. This aspect links fabric domain to variable pore size distribution (Figure 9) within mapped fabric domain types to further accentuate that a lithofacie or a single micrograph exhibits multiple fabric domain responsible for the occurrence of multiple hydraulic fluid units (HFU’s) in a reservoir [22]. However, it is visible during characterization for hydraulic fluid units (HFU’s), where similar in range porosity values alongside their corresponding permeability values from the different domains in a particular or certain lithofacies are plotted or grouped as in reservoir quality index versus porosity in z-direction (RQI Vs. ϕz) to form a Flow Zones Indicator (FZI) unit from different lithofacie successions within a well. The fabric domain types close to similar reservoir quality index ranges in spite of existing in different lithofacies within a succession are plotted as single Flow Zones Indicator (FZI) units that translate to form high fluid pathways for fluid conductivity within a well bore. Thus, the most dominant and abundant fabric domain types will control the Flow Zones Indicator (FZI) r-squared (r²) factor values [22]. This interprets that increase or nearness to 1 in r-squared (r²) factor value is a function of the dominant fabric domain type (s) within investigated lithofacie succession. For an example, when an r-squared (r²) factor value is 0.9, it indicates that there are more abundance of positive and favorable domain (s) that exhibits larger pore sizes and pore volume with favorable physical rock parameters such as grain size, mechanical compaction, grain contacts; float, long, point moderate in concavo-convex, less in matrix composition, and diagenetic features within their domain. And in an instance with more abundance of the opposite physical rock parameters in a particular domain, the Flow Zones Indicator (FZI) r-squared (r²) factor value will decrease far from 1 and closer to zero depending on domination of the domain(s).

Figure 9.

Porosity distribution in all fabric types within (a) in massive coarse grained sandstone (b) in burrowed medium grained sandstone (c) in faintly laminated fine grained sandstone (d) in parallel laminated friable fine grained sandstone(e) massive very fine grained sandstone.

In Figure 9a–c, the clasts-supported domain-type pore size values vary from 62 μm to 248 μm within region of minimum matrix composition, compaction, and diagenetic alteration. In the region with moderately high matrix content, pore size diameter varies between 8 μm and 81 μm within the domain. This domain exhibits the moderate large pore size diameters within it. The regions with large pore sizes presumably will also have large pore throats and good interconnectivity for fluid conductivity less potential to retain irreducible water saturation, while region with a moderate increase in matrix content filling intergranular pore spaces experiences reduction in pore size diameter and certainly will exhibit smaller pore sizes and pore throats, and will retain more irreducible water saturation as result of the matrix composition.

In the random-supported domain (Figure 10a–d), pore sizes also vary between 10 μm and 58 μm for the region with small pore sizes, as region with large pore sizes exhibit pore diameter ranging from 65 μm to 330 μm. The domain exhibits the maximum pore size diameter or oversized pores as compared to the clasts-supported domain. This is ascribed to the abundance of intergranular pore space orchestrated by favorable grain sorting and grain packing, and float, point, and long with less occurrence of concavo-convex and sutured contact as result of mechanical compaction due to overburden. The region with an abundant of clasts-to-clasts intergranular pore space with lesser matrix contents and favorable physical rock properties will show larger pore attributes (pore sizes and pore throat) and potentially will retain slighter irreducible water saturation. Conversely, regions with moderate high matrix content potentially characterize with smaller pore attributes and high possibility of retention of irreducible water saturation as a result of higher matrix composition.

In the matrix-supported domain, generally matrix compositions are moderately very high across the entire domain (Figure 10a–e), as smaller pore diameters vary between 8 μm and 80 μm, while the larger pore sizes range from 81 μm to 140 μm as their maximum diameters within the domain. In general, this domain exhibits more of medium to smaller pore size and pore throats, because the entire domain is a matrix supported. It will show a high affinity to retain irreducible water saturation within it, and their abundance within a lithofacies will greatly reduce or influence their ability to enhance fluid transmitivity (permeability) and required intergranular pore interconnectivity in fabrics.

Figure 10.

Pore sizes distribution in fabric types a) in massive coarse grained sandstone (b) in burrowed medium grained sandstone (c) in faintly laminated fine grained sandstone (d) in parallel laminated friable fine grained sandstone(e) massive very fine grained sandstone.

The fracture-dominated domain (Figure 10b–e) also reveals occurrence of smaller and large pore diameters. In this domain, the large pore sizes vary from 52 μm to 105 μm and those with small pore diameters vary between 25 μm and 49 μm, respectively. In this domain, their pores are extensive and dominant. Thus, this will have an enormous influence on mass fluid flow and conductivity. The moderate smaller diameter pores are characterized with high matrix content termed subclass type 2.

Also in the laminar domain (Figure 10b–d), the smaller pore diameter ranges from 0.5 μm to 25 μm, while the large pore diameters vary between 58 μm and 66 μm, respectively. It is therefore logical that the most predominant fabric domain types in a lithofacie control occurrence or development of certain pore sizes and pore volume. Variations in matrix composition and diagenetic features play major roles in pore volume and pore size variation and distributions within same fabric domain type. According to [Archie [2]] pore size distribution does not necessarily define the type of rock, for actually several types of rock may have essentially the same pore size distribution and thus in this context for the different fabric domain types. Therefore, pore size and porosity distribution will vary or comparable within the variable fabric domain types and are responsible for occurrence of multiple hydraulic fluid units (HFU’s) and anomalous porosity-permeability relationship in sandstone reservoir rock.

A variation in the fabric shows that on a particular lithofacie could exhibits variable fabric domains spatially and temporally. And also at any given point, one dominant fabric domain could be determined over other domain type (s). Therefore, this proposed concept of a fabric domain needs to be incorporated in detail sedimentological analysis of reservoir rocks. The concept has shown existence of a multiple fabric domains in a lithofacie thin section or micrographs and thus exhibits variation in both their pore size and porosity distribution within the different fabric domain types. These that no fabric of a sedimentary rock is entirely homogeneous thus will have potential impact on pore attributes distribution (pore volume, pore size, pore structure, and pore throat) and irreducible water saturation distribution that determines the distribution of hydraulic fluid unit (HFU’s) and entire reservoir fluid dynamics behaviors at a macroscopic scale.

However, there are no long-range processes existing to align the internal rock fabrics as pore volume distribution has dominant influence upon the macroscopic phase separation of mutually immiscible fluids [20]. The proposed concept on existence of multiple fabric domains is visible as pore volume distribution (porosity) and pore sizes within them differ and at some point closer in ranges. Understanding a characterized porous geologic (highly porosity), but low or poor fluid flow transmitivity (permeability) using concept of fabric domain, it could occur that abundance of non-similar domain (differs pore attributes) networks has possibly out numbers the positively similar domain that will form radial intergranular network connectivity for flawless fluid conductivity in lithofacie. Therefore, when investigating, describing variability, or predicting a reservoir macroscopic behavior, the type of heterogeneity should be defined in terms of fabric characteristic and the presence or absence of certain dominant fabric domain type(s).

2. Conclusions

The goal of rock fabric characterization is to describe the spatial and geometric distribution of pore attributes as they impact on petrophysical parameter variations such as porosity, permeability, and saturation. The new insights propose a systematic fabric characterization for siliciclastic sandstone thereby maintaining existing fabric classifications by earlier sedimentologists through the combinations of both digital petrographic scanning and thin-section description to view entirely a lithofacie fabric, such as to examine and visualize potential multiple fabrics instead of a single fabric classification and description as earlier sedimentologist outline in many books.

Fabric domain mapping is achievable through scanning micrographs into high-resolution images at 5-mm scale and carefully visualizes all the different grain assemblages or patterns with reference to their nature of their grain packing, contacts, sorting, orientation, matrix distribution, mechanical compaction, and intergranular pore-spaces into fabric-type domain type (s). It indicates that a single thin micrograph is not ideal to fully a representation of lithofacies rock fabric, and that there are multiple fabric domains existing within a single lithofacies fabric. Five fabric domain types were mapped into clasts-supported, matrix-supported, random, and laminar and fracture-dominated domains some with subclass types 1 and 2. The subclass types 1 and 2 are distinguishable with respect to their individual matrix compositions filling intergranular pore space as in <10% or > 10% respectively across the domain types.

The concept of fabric domain types has shown existence of a multiple fabric domain in a lithofacie and pore volume distribution (porosity) and pore sizes within them differ and at some point are closer in ranges. However, a lithofacie that exhibits poor intergranular pore-space distribution, termed poorly porous, may experience high permeability values and sudden decline in reservoir pressure due to the occurrence and abundance of the fracture-dominated domains or oversize pore size within the different domain types that are not fully noted during petrographic description using a polarized microscope only. But if such micrographs are scanned as propose here, it will be understood from the reservoir behavior that it was a response due to the abundance of the fracture-dominated domain or an oversize pores presence within the lithofacies. Therefore, when investigating, describing variability, or predicting a reservoir macroscopic behavior, the type of heterogeneity should be defined in terms of the fabric characteristic and the presence or absence of certain dominant fabric domain types needed to be considered critically in sedimentary rock characterization.