The Porosity Logs
In the following pages, we will review three types of measurement which are collectively referred to as "porosity logs" when used by the oil industry. Although none of these logs actually measures pore volume directly, the marked contrast between the physical characteristics of water and rock-forming minerals is such that bulk rock properties are influenced strongly by water content and so, by implication, pore volume. Simple equations can be used to calculate porosity from log measurements when the matrix mineralogy is known. Essentially, these calculations are equivalent to the interpolation of a zone reading between the two extremes of matrix mineral property (zero porosity) and that of water (100% porosity). The depth of investigation of all three tools is a matter of inches, so that the pore fluid that is "seen" is mostly invading mud filtrate, together with any residual oil or gas.
Some minerals or lithologies have such distinctive characteristics that they can be recognized in their own right and differentiated from others, either qualitatively or quantitatively with the use of a single porosity log. Examples include coal beds, metallic minerals, sulfur, etc. In other instances, differences in the properties of common rock-forming minerals in a mixed lithology may be more subtle, but they can be resolved separately through the application of overlay methods and pattern recognition techniques to porosity log combinations.