Consideration of the distinction between rocks and minerals is of vital importance in the petrophysical task of determining porosity and hence, water saturation. Given the significance of this task it is surprising how much confusion there is across the industry over this issue. The confusion between clay and shale is the most common, to the extent that many books, papers, training courses and software products still do not differentiate, or explain these clearly or correctly.
Shale is a rock, typically defined as an indurated, finely laminated, sedimentary rock, composed primarily of clay, mud and silt. The important feature to note is that this definition does not describe the mineralogy but rather the grain size. In this definition, clay refers to clay sized particles, i.e. < 1/256 mm.
Whilst clay can refer to grain size it can also refer to clay minerals, and it is the dual meaning of the word clay that is at the heart of the confusion in the industry. Clay minerals are a group of hydrous aluminium silicates with a sheet-like structure (phyllosilicates), which adsorb water on their surfaces. It is these clay minerals that we are concerned about when determining porosity and water saturation.
In shale, most of the clay sized particles are composed of clay minerals. When computing porosity we need to account for the clay minerals, for example when their density is different from the matrix density. When computing water saturation we need to account for the excess conductivity due to the clay minerals. In both cases it is the volume of clay minerals we need to correct for, not the volume of shale.
Shale models are technically correct when they include a relationship between volume of shale and either volume of clay or volume of silt.
It is very important not to confuse clay and shale models as this simple error can lead to many problems: misunderstanding the difference between effective porosity and total porosity; the erroneous use of structural, laminated and dispersed clay models when really it is a change in the clay content of the shale; treating silt as a mineral when it is a grain size; shale computed with zero porosity; other low quality reservoir layers, such as siltstones, computed with zero porosity leading to them being modelled as zero permeability baffles, when in reality they can store hydrocarbons and be considered reservoir when they are high enough above the free water level.
This paper seeks to lift the fog of confusion.