The Photoelectric Index
The photoelectric index (Pe) is a supplementary measurement by the latest generation of density logging tools, and records the absorption of low-energy gamma rays by the formation in units of barns per electron. The logged value is a direct function of the aggregate atomic number (Z) of the elements in the formation, and so is a sensitive indicator of mineralogy. For any given element, the index is given by the formula:
This relationship and the fact that the value is only m ldly affected by pore volume magnitude or fluid/gas content, means that the index is an excellent indicator of mineralogy. Basic mineral reference values are : quartz 1.81 ; dolomite 3.14 ; calcite 5.08 barns/electron. The table of logging properties in the Appendix lists Pe values for a wide variety of minerals.
The photoelectric index log is commonly scaled on a range between 0 and 10 barns/electron, and a generalized interpretation guide is given in Figure 41. On this scale, minerals such as calcite, dolomite and quartz have narrowly defined expected values, with a minor downward drift at increasing porosities in their host lithologies. The variable compositions of coals and clay minerals means that their position on the scale should only be taken as a broad indication. The ordering of clay minerals on the index is almost entirely a function of their likely content of iron. Ellis (1987) pointed out that the index to be expected for clay minerals which are free of iron should be very similar to the value for quartz.
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.