THE RHOmaa - Umaa CROSSPLOT
The interpretation of simple minerals and lithologies is generally straightforward from a visual inspection of the photoelectric index curve with the neutron/density overlay. In cases of mixed lithologies or mysterious minerals, a crossplot of a zone log responses can be helpful both for purposes of identification and for semi-quantitative estimations of volumetric proportions.
The RHOmaa - Umaa crossplot was introduced to utilize measurements of the photoelectric index, neutron porosity, and bulk density for matrix mineral evaluation. The two dimensions of the crossplot require the three log variables to be condensed in some manner. This is done with a similar approach to the M-N plot methodology, through the suppression of porosity as a variable and calculation of the apparent properties of the rock matrix. The log variables that are needed for the calculation are shown diagramatically in Figure 50.
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.