Assignment Clays, Shales and Non-Clay Contributors to Low Resistivity
1. You are a geologist working on a multi-disciplinary team. Describe to your colleagues what is meant by the geological terms “shale” and “clay”.
Solution
“Shale” is a fine-grained, detrital sedimentary rock composed mostly of clay, silt, and mud that has been consolidated and hardened, always by compression and sometimes by cementation. It is normally characterized by a finely stratified structure of laminae, often ranging from 0.1 to 0.4 mm thick. Shale contains an appreciable content of clay minerals or derivatives from clay minerals, together with some detrital quartz.
“Clay” is a rock, mineral fragment or a detrital particle which has a diameter of less than 1/256 mm (4 microns). It contains a considerable amount of clay minerals, (hydrous aluminum silicates) derived by weathering or by precipitation from feldspars. Clay also normally contains significantly lesser amounts of finely divided quartz, decomposed feldspar, carbonates, ferruginous matter, and other impurities.
2. You are your team’s project geologist. Describe to your multi-disciplinary colleagues, any two of the four main types of clay minerals that can be found in the pore system of sandstone reservoirs.
Solution
The four main types of clay minerals commonly found within the pore system of sandstone reservoirs are montmorillonite/smectite, illite, kaolinite and chlorite.
Montmorillonite (smectite) 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 unusual 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.
Illite clays 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 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 are, therefore, often responsible for clogging pore throats.
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.
3. You are a petrophysicist for your company. Explain to your engineering colleagues, the broad, lithological models which are used for shaly sandstone reservoirs.
Solution
A reservoir sandstone can be fitted to one of four broad lithological distribution models, as follows:
- Clean sandstone, containing no significant clay content.
- Dispersed clay/shale throughout the sandstone reservoir. This is either a coating on the sand grains, or a filling of the pore spaces between the sand grains.
- Laminar clay/shale, with thin layers of clay between sandstone layers.
- Structural clay/shale, with clay grains, shale interclasts or nodules within the formation matrix.
Assessment Clays, Shales and Non-Clay Contributors to Low Resistivity
1. What content of shale formations has been identified as the leading cause of low resistivity pays?
A .Feldspars
B .Micas
C .Clay grains
D .Sand grains
2. Which of the following are characteristics of a shale formation? (Select all that apply.)
A .Coarse-grained
B .Fine-grained
C .Detrital sedimentary rock
D .Indurated
3. Clays that attach to the walls of pores and form a relatively continuous, thin mineral coating are referred to as ___________ clays.
4. What is the approximate lower limit of disseminated conductive minerals that are able to cause low logging while drilling (LWD) and wireline log resistivity readings?
A .10-20%
B .1-2%
C .50%
D .5-7%
5. What types of minerals are the primary constituents of clays?
A .Iron minerals
B .Alumino silicate minerals
C .Heavy minerals
D .Carbonate minerals
6. To be correctly termed a clay, what is the minimum percentage of clay minerals?
A .25%
B .75%
C .10%
D .50%
7. What are the three characteristics that are used to classify clays? (Select all that apply.)
A .Origin
B .Color
C .Grain size
D .Geological age
E .Mineralogy
8. Structural and laminated clays are examples of ___________ clays, while dispersed clays are ___________ .
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