Neutron-Density Porosity Overlay Case Studies
The following examples of the geological interpretation of neutron-density log overlays are taken from a variety of stratigraphic intervals and geographic locations. Up to this point in the manual, the major emphasis has been on the discrimination of shales and the resolution of carbonate and siliceous rocks that are composed of quartz, calcite, and dolomite. The emphasis is appropriate, because these lithologies make up the bulk of rock encountered in drilling through sedimentary successions. However, the interpretation technique can be extended to the recognition of other minerals and lithologies as shown in some of these examples.
A cautionary note: always keep an eye on the caliper log. The caliper measures the diameter of the hole and so shows washout zones. The depth of investigation by both neutron and density tools is very shallow, so that they are adversely affected by the borehole mud within thin washout zones. The effect is usually obvious because of the recording of excessively high neutron and density porosity values. In most cases, the washouts are caused by thin shales and this possibility can be checked out by looking at the gamma ray trace. There is generally less of a problem where thick zones have been washed out, because the tool is able to maintain contact between its pads and the borehole wall.
In some instances of complex geology, the caliper log can even be a useful geological aid in its own right (see, for instance, page 103). The caliper trace shows a profile of
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.