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.
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.