The role of clays and shales in low resistivity log response

Clay has been identified as a leading cause of low-resistivity pays. To a certain extent, shale, by way of its intimate association with clay, also shares responsibility for low resistivity readings across pay zones. (Unfortunately, a good deal of this shared blame might be based on a loose and inappropriate tendency to use the terms clay and shale somewhat interchangeably.) However, while it may be difficult to distinguish clay from shale with common wireline measurements, it is clay that complicates the log analysis in low resistivity pay zones. In this section, we will look at clay and shale, and examine the various chemical, mineralogical, and structural factors which interact to lower the response of resistivity tools in pay zones.

Key Definitions

We will begin this discussion of clays and shales by defining these two key terms.

Shale

According to the American Geological Institute's Glossary of Geology, Shale is defined as a fine-grained, indurated detrital sedimentary rock formed by the consolidation
(by compression or cementation) of clay, silt, or mud.

It is characterized by a finely stratified structure of laminae ranging from 0.1 to 0.4 mm thick. Shale contains an appreciable content of clay minerals or derivatives from clay minerals, with a high content of detrital quartz; containing at least 50% silt, with 35% clay or mica fraction, and 15% chemical or authigenic materials (Krynine, 1948).

The term shale was originally applied to laminated clayey rock, but now applies to thinly laminated or fissile claystone, siltstone, or mudstone. The term is sometimes used without implication to composition, and has been loosely applied to massive or blocky indurated silts and clays that are not laminated, to laminated silts and clays that are not indurated, to fine-grained and thinly laminated sandstones, and to slates.

Clay

The AGI Glossary says that Clay is:
a rock or mineral fragment, or a detrital particle of any composition (often a crystalline fragment of a clay mineral), which has a diameter of less than 1/256 mm (4 microns). This size is approximately the upper limit at which a particle that can show colloidal properties.

It contains a considerable amount of clay minerals (hydrous aluminum silicates) derived by weathering (primarily decomposition) or by precipitation from feldspathic rocks, and also contains subordinate amounts of finely divided quartz, decomposed feldspar, carbonates, ferruginous matter, and other impurities. Furthermore, this composition should have more than 50% clay-sized particles (Twenhofel, 1937), and clay minerals must form at least 25% of the total (Pettijohn, 1957).

This term also applies to loose, earthy, extremely fine-grained natural sediment or soft rock composed primarily of clay-size or colloidal particles and characterized by plasticity. It is commonly applied to any soft, adhesive, fine-grained deposit (such as loam, or siliceous silt) and to earthy material.

Overview

From the above definitions, it is easy to see why shale and clay are so often used interchangeably. We can also see that shale is related to clay through its composition -shale is basically a rough mixture of about half clay and half silt. In the following sections we will move beyond the dictionary definitions for a broader discussion of clays and shales, and the ways in which they affect reservoir properties.

Shales

In shales, the silt (often mostly super-fine quartz grains or other materials) does not unduly affect formation properties or log response in itself (though silt might contribute to microporosity, which can lower resistivity because of bound water). The clay component does, however, affect log response. Any thorough log evaluation must account for the effects of shale and clay minerals. These effects reach beyond a simple reduction in resistivity response. Other petrophysical properties of shales and clays must be considered, such as density, amount of hydrogen, radioactivity, permeability, etc.

A number of log evaluation strategies have been devised to account for the effects of shales and clays. The extent to which log evaluation and the resulting estimation of Sw is affected by clay or shale will depend on the type and volume, as well as the distribution of shales and clays with respect to the pay sand.

Clays

We have just seen that clays generally consist of hydrous alumino-silicate minerals, mixed in with various other minerals and different proportions of silts. Any porosity found in clay will be affected by particle arrangement and rock compaction. These pore spaces most often contain water, but may also contain hydrocarbons.

As with other lithologies, the log response to clay will depend on its composition and porosity, as well as water-or hydrocarbon saturation. In a pore system, clay minerals are capable of exerting a response that is entirely out of line with their proportional volume to sand, and which is especially dramatic when applied to resistivity log response. Sandstones with large amounts of clay, such as Illite, often show very high values of log-calculated water saturations -but these sands often produce water-free. This log response is due to the presence of bound-water within the micropores of clay minerals, as well as surface conductivity of the clays and clay-water system (we will discuss these factors later).

In fact, many problems attributed to clays are caused by its interaction with water. Water in the molecular lattice of clay causes it to expand as water molecules squeeze between lattice layers. This increases the volume of the clay particle, reduces its density, and often causes a reduction in rock permeability. Clay expansion in water may also be responsible for shale sloughing and for movement of fines.

Clay Classification

We can take three approaches to the way we classify clay:

    Mineralogy,

    Grain Size, and

    Origin

We will discuss each of these in turn below.

Classification According to Mineralogy

Common clay minerals can be classified in 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: Crystal structure of common clay minerals). We shall see that the most important aspect of these minerals is their ability to hold adsorbed water on their grain surfaces.



Figure 1
Figure 1


The four major types of clay minerals are commonly found within the pore system of sandstones (Figure 2:SEM photos of four common clay minerals): Smectite, Illite, Kaolinite, and Chlorite.

Figure 2
Figure 2


    Montmorillonite (Smectite): Al2 Si4 O10 (OH)2 n H2O
Figure 3: Montmorillonite is a well-known member of the Smectite family of swelling clay minerals.

Figure 3
Figure 3


Smectites often occur as frilly minerals within a pore system. When this 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:K2 Al4 (Si6 Al2) O20 (OH)4
Figure 4: 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 often associated with Illite clays.



Figure 4
Figure 4


    Kaolinite: Al4 Si4 O10 (OH)8
Figure 5: 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.

Figure 5
Figure 5


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

    Chlorite: (Mg, Fe)5 (Al, Fe111)2 Si3 O10 (OH)8
Figure 6: 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.



Figure 6
Figure 6


When clays are found to contain a combination of clay minerals, they are called mixed-layer clays. Table 1 (Properties of various clays) lists a number of petrophysical properties associated with four common clay types.

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 1/256 mm. Recall that the Wentworth Scale classifies particles by setting a standard definition of grain size (based on diameter). The following table presents that portion of the Wentworth Scale which applies most to particles encountered in logging :

Table 2: Wentworth Classification of Sands -Clays

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

 

1000to 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

 

It is important to recognize that this classification scheme does not distinguish between grains of clay minerals and other similarly sized grains which are not composed of clay minerals. Therefore, this classification does not account for the unique petrophysical properties that set clay minerals apart from other particles of the same size.

Classification According to Origin

Clays can be further classified to reflect their mode of origin.

    Allogenic (detrital) clays: deposited with the sandstone at the time the sediments are laid down. Examples of such clays are found in the structural and the laminated clays (described below).

    Authigenic clays: rather than being transported, these clays precipitate from solution at a later time. Dispersed clays (described below) fit into this class of authigenic clays.

McDonald and Schmidt (1992) point out an important distinction between detrital and authigenic clays in sandstones. Detrital clays tend to be trapped between grains and are not part of the effective pore network. Authigenic (dispersed) clays are situated within the pore system and, by virtue of their immense surface area relative to the detrital framework grains (Table 3), have an impact on the chemical sensitivity and petrophysical properties of the sandstone which is greatly out of proportion to their volumetric contribution.

Table 3: Nitrogen absorption measurements comparing specific surface areas of quartz
and various clay minerals
(From Asquith, 1989, modified after Almon, 1979).

Mineral

 

Surface Area

 

Quartz 

 

0.15 cm2/gm*

 

Smectite 

 

752 m2/gm

 

Illite 

 

113 m2/gm

 

Chlorite

 

42 m2/gm

 

Kaolinite

 

23 m2/gm

 

* Depends on grain size and distribution

 

Distribution of Clays and Shales in the Reservoir

As stated previously, the manner in which clays are distributed throughout the reservoir plays a key role in the approach that should be used for evaluating the reservoir. Do the logs show a clean sand, with massive shales on either side, or more likely, intervals of shaly sands? In this section, we will describe the modes of distribution for each scenario.

A reservoir sand can display any of four modes of distribution (Figure 7: Modes of distribution, after Schlumberger)



Figure 7
Figure 7


    Clean sand: essentially no distribution of clay

    Dispersed clay: interspersed throughout the sand; either as a coating on the sand grains or by filling pore spaces between the sand grains

    Laminar clay: thin layers of clay between layers of sand

    Structural clay: clay grains, shale interclasts, or nodules in the formation matrix.

We will discuss each of these modes below.

Clean Sands

Clean sands are made up of relatively pure, well washed sand. As shown in Figure 7, they contain essentially no clay minerals or shales, and consist solely of sand grains. These sands were deposited as a result of a single-energy level flow regime.

The standard Archie equation would be suitable for log analysis of clean sands.

Dispersed Clay

Dispersed clays generally occur as a pore-filling component of the rock, and have a variety of crystal sizes and shapes. They are able to produce a broad spectrum of adverse effects on fluid flow and fluid saturation properties without necessarily having much effect on the total pore volume of the rock.

Overgrowths and Distinct Particles

The two key criteria used to define and contrast types of dispersed clay in sandstone are clay crystal structure and location. Dispersed clays are distributed throughout the sand in either of two distinct forms:

    As clay overgrowths that adhere to or coat the surface of the sand grains (typically seen in the case of Chlorite).

    As distinct particles of clay that fill some portion of the interstices between sand grains. In this case, the minutely smaller size of the clay particles allows them to line or fill the pore throats between the comparatively larger sand grains.

Three Common Forms of Dispersed Clay

Neasham (1979) has identified three common forms of dispersed clays seen in sandstone reservoirs.

    Pore-filling clays are the most common mode of occurrence for Kaolinite, which typically develops as pseudohexagonal, platy crystals that attach loosely to pore walls or occupy intergranular pores (Figure 8).

Figure 8
Figure 8


The crystal platelets may be stacked face-to-face, forming long crystal aggregates or "booklets." Kaolinite crystals of either single or stacked platelet morphology are characteristically scattered in a "patchy" manner throughout the pore system. These Kaolinite crystals are usually not well attached to each other or to pore walls, and can be mobilized by fast-flowing pore fluids. Kaolinite crystals that extensively fill pores have a random arrangement with respect to one another and affect rock petrophysical properties primarily by reducing intergranular pore volume, and by behaving as migrating "fines" within the pore system.

    Pore-lining clays attach to pore walls and form a relatively continuous, thin mineral coating (Figure 9).

Figure 9
Figure 9


Crystals attached perpendicularly to the pore wall surface are usually intergrown to form a continuous clay layer that contains abundant micropore space (pore diameters of less than 2 m). Illite, Chlorite and Smectite typically occur as pore linings.

    Pore-bridging clays are essentially similar to pore-lining clays, except that they not only line pore walls, but they also extend far into or completely across a pore or pore throat to create a bridging effect (Figure 10).

Figure 10
Figure 10


Pore-bridging clays exhibit extensive development of intertwined plates and fibers that produce an intricate network with abundant microporosity and tortuous fluid flow pathway. Smectite, Chlorite and Illite all display this morphology, although it is most typical of Illite.

Sand intervals that contain dispersed clays are deposited under a single flow regime, and the dispersed clays are subsequently formed within the sand as a result of authigenesis or through post-depositional bioturbation or diagenesis. It is not uncommon for theses clays to develop as a result of precipitation or by alteration of preexisting silicate minerals.

Dispersed clays are able to increase total water saturation while significantly reducing the resistivity, porosity and permeability of the sand. Therefore, when dispersed clay content exceeds about 40% of a sand's pore space, it can severely impact the producibility of the sand.

    When dispersed clays take the form of a coating on the sand grains, an increase in irreducible water saturation takes place, along with a substantial reduction in well log resistivity.

Completions in shaly sands containing dispersed clays often produce water-free because of their high irreducible water saturation.

    When dispersed clays fill the pores between sand grains, they take up space that would normally serve as a channel for fluid movement between pores. Furthermore, the wettability of such clays tends to be greater than that of the surrounding quartz grains. The sum total of these factors is a reduction in porosity and permeability, along with an increase in water saturation.

Juhasz (1986) describes the significant impact which dispersed shale can have on producibility: "A certain amount of dispersed (pore filling) shale has a far more detrimental effect on the permeability of the sand than the same amount of shale concentrated into shale laminae between clean sand laminae. The permeability of a 33% porosity clean sand for instance would be reduced to practically zero if its pore-space is filled with shale (that is: Vsh -33%), but it would retain two-thirds of its permeability if this shale is present in laminations only."

Different Perspectives

Asquith (1990) notes that any log analysis in pay sands which contain dispersed clays must consider the authigenic origin of this clay. Asquith feels that most dispersed clay is diagenetically formed in place after deposition of the sand. Because they form under different conditions than those which formed adjacent shale beds, these dispersed clays may differ in composition and more importantly, may exhibit different resistivities from that of the adjacent shales. This difference between the resistivity of dispersed clay in the reservoir and the resistivity of the adjacent shale is especially critical in cases where the resistivity of the adjacent shale is greater than the resistivity of the shaly sands. Any log analysis through such zones should therefore use an equation that does not require a reference resistivity (Rsh) taken from adjacent shales.

Thomas and Stieber (1975) built a model (discussed later) which assumes that "within the interval being investigated, there is no change in shale type and the shale mixed in the sand is mineralogically the same as the "pure" shale sections above and below the sand." In their view, this similarity stems from the fact that both sand and shale facies "are derived from the same source material, carried by the same river and emptied into the same basin. The differentiation between sands and shales begins as the particles settle at differing rates according to their size and transport energy and not mineral type." Thus, they feel that "the porosity destroying material introduced into a sand stratum will be of the same composition as the shales above and below the sand stratum. Of course, this will not be true for the diagenetic alteration of feldspars into clay within the sand stratum."

Laminar Clays and Shales

Laminar clays are distributed in a reservoir as relatively thin layers of allogenic clay or shale that have been deposited between otherwise clean layers of sand. (Figure 11).

Figure 11
Figure 11


Each lamination of compacted clay, mudstone, or siltstone is a distinct layer, and each such layer can vary in thickness as well as proportion of sand, silt and clay. The overall sand and laminated clay interval reflects multiple cycles of deposition under a dual flow regime characterized by fluctuations in energy levels. This regime requires higher energies for deposition of sand grains than are required for the lighter shale constituents of muds, clay minerals, and silts.

Reservoirs containing laminated shales alternate between layers of reservoir-quality sands and thinner layers of shales and clays having zero effective porosity. These shale laminations do not affect the resistivity, porosity or permeability of the surrounding sand streaks themselves, and a shale fraction of up to 60% can therefore be tolerated in the reservoir. However, many logging tools lack the vertical resolution to differentiate between individual thin beds of sand and shale (Figure: Tool resolution versus bed thickness). This lack of vertical resolution causes many standard logging tools to average their readings over such alternating sequences of sand and shale.

Though laminated shale layers are generally thinner than the adjacent sand layers, the clay constituents contribute a disproportionate change in resistivity and porosity for their thickness. Petrophysical and reservoir properties between each layer may vary because of changing proportions of clays within each lamination. However, Asquith (1990) reasons that because of their detrital origin, shale laminations between sands normally have the same clays and water content as adjacent thick shale beds. This similarity leads to the assumption that resistivities of laminated shale will be similar to those of adjacent thick shales. Therefore, it is safe to use log analysis equations that require clay resistivities, and in such equations, the resistivity of adjacent shales is used to represent that of the shale in the shaly sand.

The problem of finely layered sands and shales is fairly common. In the Gulf Coast region of the United States, laminar shales have been found in about half of the low resistivity zones.

Structural Clays and Shales

Structural clays or shales consist of shale nodules or lithified clay fragments that have been intermixed with grains of sand to form part of the sandstone rock matrix. Unlike dispersed clays, whose grain size is so small that they occupy the interstices between framework grains, the structural clays have a grain size that is at least as large as the sand grains, therefore placing the structural clay fragments into the framework of the matrix. Figure 12 shows clasts of structural shale intermixed with grains of sand.

Figure 12
Figure 12


Structural clays or shales occur by way of three different modes:

    As reworked fragments of lithified shale that have been deposited simultaneously with sand grains of comparable size.

    As nodules that replace selected grains through diagenesis (as when feldspar is transformed into clay).

    As nodules introduced through bioturbation.

Sand intervals that contain structural shales are deposited under a single flow regime, with the shale fragments being deposited simultaneously along with the sand grains. Because of their size, structural shales act as framework grains, and so do not alter reservoir properties through clogging interstitial spaces between grains. Structural shale does not commonly occur in quantities to affect reservoir quality. However, when evaluating reservoirs that contain structural shales, the approach must account for the way in which the clay grains will affect log response, as opposed to simply trying to evaluate the response through a homogeneous sand. According to Visser, et al. (1988), structural and laminated shales produce similar log responses. These detrital clays can be compared to adjacent thick shale beds for resistivity, thus enabling shaly sand equations that require a shale resistivity value to be used.