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
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.