IMPORTANCE OF CLAY IN PETROLEUM GEOLOGY

IMPORTANCE OF CLAY IN PETROLEUM GEOLOGY

Clay is the common name for a number of fine-grained, earthy materials that become plastic when wet. Chemically, clays are hydrous aluminums silicates, usually containing minor amounts of impurities such as potassium, sodium, calcium, magnesium, or iron.

One of the commonest processes of clay formation is the chemical decomposition of feldspar. Clay consists of sheets of interconnected silicates combined with a second sheet-like grouping of metallic atoms, oxygen, and hydroxyl, forming a two-layer mineral such as kaolinite. Sometimes the latter sheet like structure is found sandwiched between two silica sheets, forming a three-layer mineral such as vermiculite. In the lithification process, compacted clay layers can be transformed into shale. Under the intense heat and pressure that may develop in the layers, the shale can be metamorphosed into slate.

Properties of clay minerals include plasticity, shrinkage under firing and air drying, fineness of grain, color after firing, hardness, cohesion, and capacity of the surface to take decoration. On the basis of such qualities, clays are variously divided into classes or groups.

Individual clay particles are always smaller than 0.004 mm. Clays often form colloidal suspensions when immersed in water, but the clay particles flocculate (clump) and settle quickly in saline water. Clays are easily moulded into a form that they retain when dry, and they become hard and lose their plasticity when subjected to heat.

Clays are divided into two classes:

1.    Residual clay – found in the place of origin

2.    Transported clay, also known as sedimentary clay, removed from the place of origin by an agent of erosion and deposited in a new and possibly distant position.

Residual clays are most commonly formed by surface weathering, which gives rise to clay in three ways:

1.    Chemical decomposition of rocks, such as granite, containing silica and aluminia

2.    Solution of rocks, such as limestone, containing clayey impurities, which, being insoluble, are deposited as clay

3.    Disintegration and solution of shale.

Clay rocks can be identified by their very fine grain size of < 0.002 mm, and have different properties depending on which particular clay minerals they contain.

There are three main groups of clay minerals, each with its own particular properties:

1.    Kaolinite

2.    Illite

3.    Montmorillonite

Clay rocks may contain a mixture of these minerals, so they have very variable properties, giving rise to a number of different uses. The most abundant use of clay is in brick making.

Granite is made up of quartz, mica and feldspar. As quartz is resistant to chemical weathering, it may be removed only as mineral grains of quartz. Feldspars and micas are susceptible to chemical weathering and break down to form clay minerals.

Some of the original elements contained in the micas and feldspars are carried away in solution as ions (Na+, Ca+, and K+), and so the clays formed are relatively enriched in aluminums and silicon.

The main groups of clay minerals are kaolinite, illite and montmorillonite. The layers in kaolinite are held together by fairly weak bonds, whereas there is strong bonding in illite and montmorillonite because of the presence of positively charged metal ions; potassium in the case of illite, and calcium and sodium in the case of montmorillonite.

Generally, potassium feldspar breaks down to form kaolinite; micas weather to give illite, and ferromagnesian minerals break down to form montmorillonite.

Clay uses

Clay has been used since the very beginnings of civilization, for making cooking pots, bricks, porcelain, and also drainage pipes. Both brick clays and other clays are used for other purposes, such as the manufacture of clay pipes, and for floor and wall tiles. Fireclays are used for more refractory purposes such as heat-resistant tiles or bricks. Ball clays are used for ceramics. China clay, predominantly kaolinite, is used in ceramics, as a filer and in drug manufacture. Expanded clays are used as a lightweight aggregate in the manufacture of expanded clay blocks used for insulation. However, the major use of clay, after brick manufacture, is in the manufacture of cement.

 Clays and subsidence

One important aspect of clays as far as construction goes, is the possibility of subsidence where a building has been constructed on clay. This was a big problem in the hot dry summers of the late 1970s and 1980s in southern England. Most clay rocks contain a high proportion of natural water (up to 40%), filling all the minute pore spaces between the mineral grains, but because these spaces are so small, water cannot pass through the rock, and it is impermeable. Clay top soils dry out in a long hot summer, shrinking as they lose water, and develop cracks in the surface layer. If the ground is also being dried out at depth by tree roots – that is, the underlying clay is being dewatered – the clay can lose enough water in a long, hot summer for it to shrink enough to cause part of the building to settle or subside, especially if the house foundations are shallow. Cracks may then develop in the building, requiring expensive remedial work to underpin the foundations. In some cases it is possible to stop the damage by cutting down the tree, and so allow the clay below the house to rehydrate. However, if the tree has been there a long time, rehydration of the clay as it reabsorbs water and swells up can itself cause further ground movements and damage to the house, a process known as heave. The moral of this tale is don’t plant large thirsty trees near your house!