Page 159 - Well Logging and Formation Evaluation

Production Geology Issues 149

Authigenic clays are those that have formed at the same time or after
deposition. Three processes are possible that lead to their presence:

• Allogenic clays may be transformed into new types through the effect
of temperature, pressure, and pH.
• Clay mineral may be formed by the diagenesis of nonclay minerals such
as feldspars, pyroxenes, amphiboles, and micas.
• Clay mineral may be precipitated from porefluids.

Principal clay minerals are as follows:

• Kaolinite, Al 2Si 2 O 5 (OH) 4. The clay is usually in the form of hexago-
nal crystals, which may be stacked to form accordion-type shapes.
These structures may fill the pores and have an impact on permeabil-
ity. However, the cation exchange capacity (CEC) of kaolinite is low,
meaning that it absorbs relatively little water.
• Chlorite, (Mg, Fe)6 AlSi 3O 10(OH) 8 . Chlorite may form in many dif-
ferent shapes, such as plates, rosettes, honeycombs, or round growths.
Typically it will coat the sand grains and pore throats, having a nega-
tive effect on permeability. However, unlike some other clays, it does
react to acid, so permeability around the wellbore may be increased via
acid stimulation. CEC is also relatively low.
• Illite, (H 3 O, K) (Al 4Fe 4Mg 4Mg 6)(Si 7Al)O 22 (OH) 4. Illite can be par-
ticularly damaging to permeability when it takes the form of hairlike
structures that may block the pore throats. It does react partially to
strong acid. The CEC is higher than that of chlorite or kaolinite but
lower than that of montmorillonite.
• Montmorillonite, (Na, K, Mg, Ca) Al 2 Si 4 O 10 (OH) 2 H 2 O. Montmoril-
lonite has the highest CEC and has therefore the greatest effect on sat-
uration through clay-bound water. Swelling of the clay can also cause
problems when drilling with freshwater mud. The clay takes the form
of crinkly coatings on detrital grains, or a cellular structure similar to
honeycomb chlorite.

10.2.2 Carbonate Reservoirs

Carbonates originate from the calcareous skeletons of organisms, forming
bioclastic sediments. These fragments are cemented by carbonate precipitat-
ing from water. Most of the organisms lived on the bottom in shallow marine
water, where algae were present. However, after dying, the organisms may