Section 4: Geochemical Logging
The Schlumberger Geochemical Logging Tool (GLT) uses three types of nuclear measurement on a single string to estimate concentrations of ten elements: potassium, thorium, uranium (from the natural gamma ray spectrum); aluminum (by delayed neutron activation analysis); and silicon, calcium, iron, sulfur, titanium and gadolinium (from the prompt capture gamma ray spectrum measured after a 14 Mev neutron burst).
The geochemical logs have been used for a variety of purposes, but a major effort has been made to transform them to realistic mineral assemblages, using computer methods that are extensions of those described in the last section. The notion of using chemical analysis data to build hypothetical minerals is by no means new and dates back to Cross and others (1902). Such hypothetical minerals are often called "normative" to distinguish them from "modal" minerals that are actually observed in the rock. However, the aim of workers with geochemical logging data is to achieve as close a match as reasonably possible between the normative calculations and modal reality.
Harvey and others (1990) have described both the advantages and limitations of this approach as applied to sequences logged in the Ocean Drilling Program (ODP). Herron and Herron (1990) pointed out that the potentially overwhelming complexity of possible mineral associations in sedimentary sequences is offset to the degree that typically only ten minerals occur in the bulk of most rocks. Herron (1988) also discussed the possible need to introduce new methods of classification in order to integrate these new data with associations defined
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.