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