SHALE BASICS
There are many petrophysical models for calculating shale volume from well logs. For a summary, see Crain's Usage Rules for Selecting Shale Volune Methods.

In petrophysical analysis, shale volume is one of the key answers used later to correct porosity and water saturation for the effects of clay bound water, (CBW).

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz, dolomite, and calcite. The ratio of clay to other minerals varies.

 

 

 

 

 

 

Core photo of black shale with minor silt and laminations
and partings between layers

Shale is characterized by breaks along thin laminations, parallel to the bedding. Mudstones are similar in composition but do not usually show layering within the zone.

Geologists define clay as any mineral in a rock with a grain size less than 4 microns, even though the mineral may not be a clay mineral. Silt is defined as a rock with particle size between 4 and 62 microns. Silt sized particles are usually non-clay minerals and clay sized particles are usually clay minerals, although non-clay minerals may also fall into this category.

From a petrophysical analysis point of view, clay-rich shales have traditionally  been called “shales” and non-clay shales have been called “silts”. Petrophysical analysis deals with minerals, not particle size, so it is confusing to us when a zone is called a shale when the logs show little clay is present.

 

An example is the Montney shale in northeast British Columbia. Ir is roughly 45% quartz, 45% dolomite, 10% other minerals (few of them are clay). The zone is radioactive due to uranium (not due to clay), so it looks a lot like shale on quick look log analysis; density neutron separation and PE values are also close to shale values. This kind of reservoir needs to be treated as a tight sand.

 

Resistivity scanner image of a shale with open (dark colour)
 and healed fractures (white)

 

Other so-called "shales", such as the Monterey Shale, the Niobrara, and Milk River, are laminated shaly sands. These sands need to be analyzed with a Laminated Shaly Sand Model, not a Shaly Sand Model.  The sand laminations have good porosity and permeability. The shale laminations contain very little.

 

Others are radioactive silts with clay and kerogen, such as the Haynesville Shale, which is 50% clay and 50% quartz and calcite. This shale has low effective porosity and very poor permeability. Total organic content is moderately high and there is adsorbed gas, so it gets treated as a true gas shale.

 

 

 

 

 

XRD analysis of a silty shale. Notice clay-quartz ratio averages about 50:50.

 

Using the wrong log analysis model, or the wrong assumption as to the character of the zone,  will produce silly results, so be sure to understand what type of "gas shale" you are dealing with.

Natural fractures in gas shales are an important component in assessing productivity. Fracture analysis using formation resistivity images and acoustic televiewer images is covered elsewhere in this Handbook.

Below is a series of core photos of a gas shale showing the laminated nature of shale. Gas is adsorbed in the microporosity on the clay surfaces. The natural fractures along the shale partings help move gas to the well bore when well bore pressure is below formation pressure.

Core photo of gas shale - about 50% clay, 50% quartz plus calcite, 10 - 15% total porosity, 3 - 6% effective porosity, < 0.001 mD permeability.

CLAY BASICS
The following is from http://en.wikipedia.org/wiki/Clay_minerals  and other sources (edited). These descriptions are best suited to discussions of the dispersed clay in shaly sands, but may also be useful in describing real shales.

Clay minerals are hydrous aluminum silicates, with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to mica and form flat hexagonal sheets. Clay minerals are common weathering products of feldspar and low temperature hydrothermal alteration of granite. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate.

Clay minerals include the following groups:

• Kaolin group which includes the minerals kaolinite, dickite, halloysite and nacrite
  • Some sources include the serpentine group due to structural similarities.
• Smectite group which includes dioctahedral smectites such as montmorillonite and nontronite, and trioctahedral smectites for example saponite.
• Illite group which includes the clay-micas. Illite is the only common mineral.
• Chlorite group includes a wide variety of similar minerals with considerable chemical variation.

Clay minerals are characterized by two-dimensional sheets of corner sharing SiO₄ and AlO₄ tetrahedra. These tetrahedral sheets have the chemical composition (Al,Si)₃O₄, and each tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra, forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra point in the same direction, that is, all of the unshared vertices are on the same side of the sheet).

   
Tetrahedron and Octahedron molecular structures

  In clays, the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminum or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form part of one side of the octahedral sheet, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorized depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.

    
1:1 Clay (Kaolinite) and 2:1 Clay (Mica)

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged, this charge is balanced by interlayer cations such as Na⁺ or K⁺.

In each case the interlayer can also contain water; this is part or all of the so-called "clay-bound water" found in shales. The empirical chemical formula for the four main clay groups are shown below,
   Montmorillonite (Smectite): Al₂ Si₄ O₁₀ (OH)₂ · n H₂O
   Illite K₂ Al₄ (Si₆ Al₂) O₂₀ (OH)
   Kaolinite Al₄ Si₄ O₁₀ (OH)₈
   Chlorite (Mg, Fe)₅ (Al, Fe¹¹¹)₂ Si₃ O₁₀ (OH)₈

The crystal structure of a clay is formed from a stack of layers interspaced with the interlayers, as shown in the illustration at the right.

Unit layer arrangement for the four common clay minerals: montmorillonite, illite, chlorite, and kaolinite. The water in the interlayer of mont-morillomite makes this a "serlling clay".

When montmorillonite (smectite) clay imbibes fresh water, it swells to several times its original (dry) volume and retains a good deal of water between layers in its mineral structure. This change in volume can cause montmorillonite clays to dislodge and migrate within the pore system, thus resulting in plugged pore throats.

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

Chlorite 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.

Kaolinite 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 become dislodged, they are usually too large to fit through pore throat openings and so are often responsible for clogging pore throats.

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

SHALE DISTRIBUTION
Shale can be distributed in several different ways, as shown below.

How Shale is Distributed in a Shaly Sand

Shale corrections are applied to porosity logs to determine effective porosity, as shown in the illustration above. Since shale contains some water, this water must be subtracted from the total porosity as measured by conventional logging tools. The mathematical method for finding shale volume is the same for all the shale distribution types, but the method for applying the shale correction to the porosity varies.