Clay mineral a hydrous, aluminum silicate mineral with a layered crystal structure. Clay minerals usually occur in extremely tine-grained sizes in sedimentary roulvi and can be either allogenic Originally deposited with the other sediments) or authigenic (formed by chemical reactions In the subsurface). Clay minerals are composed of tetrahedral layers of surrounded by four oxygens, three of which are shared. They can also contain octahedral layers of aluminum or magnesium surrounded by six oxygens, whereby all the oxygens are snared and a layer of lousely bound atoms or molecules can be exchanged.
Authigenic clay minerals include neoformed clays that have precipitated out of subsurface waters and transformed clays that have formed by the alteration of a precursor mineral grain such as feldspar. The four common types of clay minerals are kaolinite.

Mite, sinectite, and chlorite. Smectite is the clay used in drilling mud to increase viscosity and density and to build the filter cake on the sides of the well. Clay minerals, because of their size, decrease the porosity and permeahiliry of a rock and increase irreducible water content. Clay minerals can trade (exchange) cations (positive charges) in their interlayer sites. The amount a particular clay mineral can exchange is called cation exchange capacity. Cations and fresh water can cause some clay minerals to swell (swelling clays) or disperse. This can cause the clay mineral to detach from mineral grains and clog pure throats in sedimentary rocks, reducing or destroying the rock’s permeability in a process called formation damage.

The smectite-type of clay mineral has the greatest cation exchange capacity. Some problems with clay minerals include a) migration of lines (kaoline and fibrous Mite), b) swelling (smectite, vermiculite and mixed layers), and c) HCl acid damage (chlorite and Fe illite).

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