Section 3: Computer Methods for Rock Composition Analysis from Logs
In this section of the manual, we consider methods for the compositional analysis of rocks using wireline logs. This goal is the logical extension of the techniques that have been described so far. Initially, interpretations were made with reference to a single log, which were then extended to the simultaneous consideration of several logs. The overlay method generally gives a good qualitative picture of lithology. However, a more precise appreciation of the mix of mineral components in any zone was found in log crossplots because zones are referenced to specific mineral points and lithology lines. Unfortunately, the depth information of the log overlay is lost in the crossplot, but this loss is offset by the ability to make semiquantitative estimates of mineral and porosity proportions.
These alternative ways of representing logging data are shown diagramatically in Figure 57. A mathematician would consider the alternatives to be different "mappings" of the zone log responses. As logs they are superimposed as continuous variables on a horizontal axis that form traces with respect to depth as the vertical axis. On a crossplot the log data are plotted in a Cartesian space, where the measurement logs are orthogonal reference axes. Finally if reference minerals are located in this log space, then the plotted zones can be remapped into a "composition space". Their coordinates in composition space are estimates of the proportions of the reference minerals.
All the operations have been described in terms of geometry, but the compositional proportions can be calculated from simple