An example of a well logging event is the notice and recording of naturally-occurring phenomena in in-situ rocks. In further detail, an event could be the discovery of a conductive fluid within the borehole (rocks, by their nature, are not entirely solid. Some are porous and some indeed contain liquids.) Or something else entirely. The general idea with well logging is to find oil or gas between rocks, though of course these fossil fuels are not the only liquids that can become trapped.
The aim of well logging is indeed categorisation, but it is also discovery. A well log is an accurate record of what exactly lies underneath us, and there are a series of methods which are utilised in order to accurately and properly identify without the need of samples. Logging tools can (and do) measure electrical, acoustic, radioactive, and electromagnetic properties within a well, and a subsurface formation can be accurately interrogated and identified by the traits which are recorded.
Well logging is a much better form of recording and reporting than what was done in older times. Originally, drillers in the early 1800s simply wrote what happened at certain depths, which of course is rather inaccurate.
Some of the ways of which well logging differs from simply taking notes at certain depths is the measuring of depths of formation tops, the thickness of the formations, rock porosity, the water saturation and temperature, as well as the types of formations encountered. All of this can be recorded and categorised, in order to provide the most information possible to the interpreter.