Procedures of Shaly Sand Analysis

The process of shaly sand analysis consists of three main steps, each of which should be accomplished in a specific order. These three steps include:

1. Determining volume of shale (Vsh) in the zone of interest.

2. Correcting porosity for the presence of clays (determining effective porosity of the zone of interest).

3. Determining effective water saturation (Swe) of the zone of interest (water

saturation of the effective pore network).

Each of these three processes will be examined in more detail in the paragraphs that follow. Realize that the overall goal of shaly sand analysis is to essentially lower the value of water saturation (Sw) to what it would be if clay minerals did not exist in the

reservoir.

Determining Volume of Shale (V

)

The first step in shaly sand analysis is to determine the amount of clay minerals present in the formation. There are many methods that are used to determine this, and several are discussed below. It should be remembered that these determinations are simply

estimates of the volume, or percentage, of clay minerals within a reservoir, and do not

consider the type of clay present or its distribution.

When the volume of shale (Vsh) is determined to be less than 15% of the bulk rock

volume, then it is safe to assume that clay minerals are not having a significant effect on log responses, and analysis may be pursued by conventional means (i.e., Archie equation). Where Vsh exceeds 15% bulk rock volume, shaly sand analysis should be

performed to obtain more accurate values of water saturation.

Many types of logs, used either alone or in combination with others, are used to indicate shale content, although none of them is consistently reliable. Each method discussed below is designed to give either a good estimation of Vsh in conditions

favorable to the particular tool, or to give an upper limit of Vsh.

V

from Gamma Ray

In a formation containing clay minerals or shale of a constant radioactivity level and no other radioactive minerals, the volume of shale (Vsh) is expressed as a linear function of

the borehole-corrected gamma ray response. GR = A + N + Vsh

This condition is usually not the case, and an alternative method of determining Vsh

must be used. This alternative method requires that the Gamma Ray Index (IGR) be

calculated. By using the Gamma Ray Index as an indicator of clay content, we are simply normalizing the gamma ray response to estimate the percentage of shale present in a reservoir.

Using Gamma Ray Index (IGR) as a linear expression of Vsh is most suitable for

laminated shales. In this case, the resulting ratio reflects the percentage of clay minerals contained in the reservoir. Again, when this ratio exceeds 15%, then it should be assumed that the formation is indeed a shaly sand and that the Archie equation should be abandoned for a technique that will yield better results of water saturation in the presence of clay minerals.

Some analysts prefer to use Gamma Ray Index (IGR) as a shale indicator in all types of

shales; however, the relationship between IGR and Vsh becomes non-linear for both

structural clays and dispersed clays. There is a wide variety of non-linear relationships between IGR and Vsh, but none of these is universally accepted. A summary of these

non-linear relationships is illustrated below.

The choice of which equation to use depends mainly upon local knowledge. Generally speaking, if the bulk density (?b) of the clean formation did not change as clay minerals

were added, then the linear equation will work. If the addition of clay minerals resulted in an increase in bulk density, then the Clavier equation should be considered. For great increases in bulk density, the Steiber equation should be used.

Notice from the previous paragraph that choosing between the Clavier and Steiber equations requires that bulk density of the clay-bearing formation be referenced back to the bulk density of that formation if it is considered to be clay-free. This is the same type of approach used in determining Gamma Ray Index (IGR) itself, but can be a very

difficult and confusing task. Therefore, it is more common for analysts to calculate Vsh

using the Western Atlas equations illustrated below. Typically, Tertiary sands such as those encountered along the Gulf Coast are unconsolidated. Formations of other ages may be considered consolidated.

V

from Spontaneous Potential

In water-bearing sandstones of low to medium resistivity, Vsh may be calculated from

the SP response by using the equation below. Though this equation works best for laminated shales, it is commonly employed where reservoirs contain structural or dispersed clays as well. Pseudo-SP (PSP) represents the amount of SP deflection in the zone of interest, whereas Static-SP (SSP) represents the maximum SP deflection in a clean formation.

When using the SP response to determine Vsh, the limitations of the SP measurement

should be kept in mind. Shaly sands may exhibit extremely suppressed SP responses where there is very little difference between the SP response in the shaly sand and the shale baseline. In this situation, the volume of shale (Vsh) determined from the SP

method may very easily be overestimated.

V

from Neutron-Density Logs

Another common method of estimating Vsh in a potentially clay-bearing formation is to

use a combination of porosity measurements from neutron and density logs as follows:

Recall that neutron and density responses are both influenced by the presence of clays. When used in estimating Vsh, neutron and density responses tend toexaggerate the

influence of kaolinite and chlorite. Kaolinite and chlorite have high neutron porosities in comparison to montmorillonite and illite (Figure 6.1).

When gas is present in a formation, Vsh should not be determined from the neutron-