Clays, Shales and Non-Clay Contributors to Low Resistivity

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Clay Classifications

Contents

Clay Classifications Clay Classification According to Mineralogy The Four Major Types of Clay Minerals Montmorillonite (Smectite)Illite Kaolinite Chlorite Clay Classification According to Grain Size Clay Classification According to Origin

Clay can be classified by examining the following three characteristics:

Mineralogy
Grain size
Origin

Clay Classification According to Mineralogy

Common clay minerals can be classified into broad types, each having a similar base structure while exhibiting a wide range of compositions. On a molecular level, these minerals are arranged into sheets of alumina-octahedron and silica-tetrahedron lattices (Figure 1). The four major types of clay minerals that are commonly found within the pore system of sandstones are smectite (montmorillonite), illite, kaolinite and chlorite. Figure 2 shows microscopic photos of the four types. The most important aspect of these minerals with regard to petrophysical interpretation, is their ability to hold adsorbed water on their grain surfaces, which complicates the determination of the fluid saturation.

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The Four Major Types of Clay Minerals

Montmorillonite (Smectite)

Montmorillonite (Figure 3) is a member of the smectite family of swelling clay minerals. Smectites have a large chemically active surface area, a high cation exchange capacity and inter-lamellar surfaces with extreme hydration characteristics. When this clay imbibes fresh water, it swells to several times its original volume and retains a significant amount of water between the layers in its mineral structure. This change in volume can cause montmorillonite clays to dislodge and migrate within the pore system, resulting in plugged pore throats, which usually cause a dramatic permeability reduction.

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Illite

Illite clays (Figure 4) commonly appear as fibrous masses of fine crystals. Illite is often associated with the migration of fines, along with a reduction in permeability. High values of microporosity and immobile bound water saturation are often associated with illite clays.

Kaolinite

Kaolinite (Figure 5) commonly occurs in the pore system as discrete particles in the form of large flaky booklets, which do not attach securely to sand grain surfaces. When kaolinite booklets become dislodged, they are usually too large to fit through pore throat openings, and therefore, are often responsible for clogging pore throats.

Chlorite

Chlorite (Figure 6) commonly occurs as a pore lining around individual sand grains or in clusters. Chlorite often contains significant amounts of iron and magnesium within its structure.

When clays are found to contain a combination of clay minerals, they are termed mixed-layer clays.

Clay Classification According to Grain Size

Clay can also be defined as a sedimentary particle of any composition that is smaller than a very fine silt grain, having a diameter of less than 1256 mm (4 microns). The Wentworth Scale classifies particles by setting a standard definition of grain size, based on its diameter. Table 1 shows the relevant parts of the Wentworth Scale.

Particle Type	Grade Limits (mm)	Grade Limits (microns)	Aggregate
Sand	Range: from 2 to 0.625 mm	Range: from 2000 to 50 microns	Sand / Sandstone
Very coarse sand	2 to 1 mm	2000 to 1000 microns	Sand / Sandstone
Coarse sand	1 to 1/2 mm	1000 to 500 microns	Sand / Sandstone
Medium sand	1/2 to 1/4 to mm	500 to 250 microns	Sand / Sandstone
Fine sand	1/4 to 1/8 mm	250 to 125 microns	Sand / Sandstone
Very fine sand	1/8 to 1/16 mm	125 to 63 microns	Sand / Sandstone
Silt	Range: from 0.05 to 0.004 mm	Range: from 50 to 4 microns	Silt / Siltstone
Coarse silt	1/16 to 1/32 mm	63 to 31 microns	Silt / Siltstone
Medium silt	1/32 to 1/64 mm	31 to 16 microns	Silt / Siltstone
Fine silt	1/64 to 1/128 mm	16 to 8 microns	Silt / Siltstone
Very fine silt	1/128 to 1/256 mm	8 to 4 microns	Silt / Siltstone
Clay	Range: less than 0.004 mm	Range: less than 4 microns	Clay / Shale
Coarse clay	1/256 to 1/512 mm	4 to 2 microns	Clay / Shale
Medium clay	1/512 to 1/1024 mm	2 to 1 microns	Clay / Shale
Fine clay	1/1024 to 1/2048 mm	1 to 0.5 micron	Clay / Shale

Table 1: Wentworth Classification of Sands, Silts, and Clays

This classification scheme does not distinguish between grains of clay minerals and other similarly sized grains that are not composed of clay minerals. Therefore, the Wentworth classification does not account for the unique petrophysical properties that set clay minerals apart from other particles of the same size.

Clay Classification According to Origin

Clays can be classified to reflect their mode of origin, either allogenic (detrital) or authigenic:

Allogenic, or detrital, clays are deposited with the sandstone at the same time that the sediments are laid down. Examples of such clays are found in structural and laminated clays.
Authigenic clays precipitate from solution at a later time, rather than being transported. Dispersed clays fit into this class of authigenic clays.

There is an important distinction between detrital and authigenic clays in sandstones. Detrital clays tend to be trapped between the grains and are not part of the effective pore network. Authigenic, dispersed clays are located within the pore system and, due to their very large surface area relative to the detrital framework grains (Table 2), have an impact on the chemical sensitivity and petrophysical properties of the sandstone, which is disproportionate to their volumetric contribution.

Mineral	Surface Area
Quartz	0.15 $\tfrac{cm^2}{gm}^*$
Smectite	752 $\tfrac{m^2}{gm}$
Illite	113 $\tfrac{m^2}{gm}$
Chlorite	42 $\tfrac{m^2}{gm}$
Kaolinite	23 $\tfrac{m^2}{gm}$

* Depends on grain size and distribution
Table 2: Nitrogen Absorption Measurements Comparing
Specific Surface Areas of Quartz and Various Clay Minerals