The mineral compositions in the sedimentary rocks are related closely to the various geological factors ; much work has been attempted to correlate these clay minerals or specific accessory minerals to the petroleum geology in terms of the depositional environments, zonations and supplemental consideration for the oil exploration.
In the followings, the writer will introduce many of the reports on the applications to petroleum geology, and describe some studies made in Japan. Among them, zonation by mineral composition was particularly applicable to oil exploration and can be utilized in the areas where there is not sufficient paleontological data. Consideration of the depositional environments and their correlation to mineral compositions seems to be suitable for the detailed investigation in a limited sedimentary basin.

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Since the term ‘illite’ was first proposed in 1937 as a general term for the “clay mineral constituent of argillaceous sediments belonging to the mica group”, many investigations of these minerals have been reported. Confusion has developed from attempts to make the term more specific than originally was proposed.
It is emphasized here that the term ‘illite’ remains a useful and convenient abbreviation for “clay-grade, mica-like material”, if one does not make the mistake of regarding the characteristics of any one clay as being the characteristics of a specific mineral, illite.
Any further definition of the term only limits its use to circumstances where the particular characteristics can be proved.
Micaceous clays in sediments are not all the same; illite is a non-specific term which can be applied to these minerals which are variable in composition, in degree of crystalline order and in their related physical and chemical properties.
Much information on illites may be either wrong or misleading, in that insufficient attention has been given to clean-up treatments to remove contaminants, to fractionation into well-sized fractions, and to conditions under which low-temperature water is removed.
Furthermore, possible inhomogeneity of the particles must be considered in examining such high-surface-area minerals as illites.
When the surface becomes a considerable part of the whole, it cannot be neglected when trying to interpret the properties of the material. A disordered, potassium deficient external surface surrounding a structurally coherent core is a possible explanation for the low K₂O and source of “extra”-water in some illites, but certainly does not explain the properties of all illites.
The nature and amount of water related to illites and to finegrained micas generally is a complex problem not yet solved but the surface properties are clearly important. There is little hope of interpreting illite analyses unless the experimental conditions for determining water are greatly refined over what is customarily done very careful decisions will be necessary for distinguishing water originating from surfaces and from the volume.

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